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Dr. Mugurel TOLEA

Scientific Researcher II

Laboratory of Theoretical Physics and Computational Modeling

tzolea[AT]infim[DOT]ro

Education

PhD in Theoretical Physics (at NIMP, Bucharest, Romania / Faculty of Physics, University of Bucharest). Title of the thesis : “Spin dependent transport in mesoscopic systems”. The thesis was defended on November 2010, receiving “summa cum laude” distinction.

Positions

Professional experience : Since 2001 employed at NIMP, currently as Scientific Researcher II (since 2017). May-October 2005 employed as a researcher at the Institute of Molecular Physics, Polish Academy of Science, Poznan, Poland (during a 6 months Marie-Curie Fellowship).

 

Experience as a group leader: Won a Young Teams (TE) project in the 2011 within the PN-II competition, contract no. TE 90/05.10.2011. This resulted in coordinating a young team of 5 scientists for three years to achieve the proposed goals of the project (7 ISI papers were published in highly ranked journals). Project webpage: http://www.infim.ro/projects/theoretical-studies-open-questions-mesoscopic-phase-measurement-problem.

Member of research teams in over 10 national projects.

Expertize

Quantum Physics, Computational Physics, Mesoscopic Physics, Strongly Correlated Electrons, Keldysh Formalism, Configuration Interaction, Condensed matter theory, Modeling of Solid state Phase Transitions.

Hirsch index: 8

Google scholar link: https://scholar.google.com/citations?view_op=new_articles&hl=ro&imq=Mugurel+Tolea#

Researchgate link: https://www.researchgate.net/profile/Mugurel_Tolea

Brainmap ID: U-1700-039W-8388

 Reviewer for: Phys.Rev.Lett, Phys.Rev.A/B, Physica E, Physics Status Solidi

Publications

1. Kinetics and the Effect of Thermal Treatments on the Martensitic Transformation and Magnetic Properties in Ni49Mn32Ga19 Ferromagnetic Shape Memory Ribbons
Authors: Tolea, F; Popescu, B; Bartha, C; Enculescu, M; Tolea, M; Sofronie, M

Published: JAN 2023, MAGNETOCHEMISTRY, 9, 7, DOI: 10.3390/magnetochemistry9010007

In our work, the kinetics of martensitic transformations and the influence of thermal treatments on martensitic transformations, as well as the related magnetic properties of the Ni49Mn32Ga19 ferromagnetic shape memory melt-spun ribbons, have been investigated. Thermal treatments at 673 K for 1, 4 and 8 h can be considered an instrument for fine-tuning the performance parameters of alloys. One-hour thermal treatments promote an improvement in the crystallinity of these otherwise highly textured ribbons, reducing internal defects and stress induced by the melt-spinning technique. Longer thermal treatments induce an important magnetization rise concomitantly with a slight and continuous increase in martensitic temperatures and transformation enthalpy. The activation energy, evaluated from differential scanning calorimeter experimental data with a Friedman model, significantly increases after thermal treatments as a result of the multi-phase coexistence and stabilization of the non-modulated martensitic phase, which increases the reverse martensitic transformation hindrance.
2. Kinetics and the Effect of Thermal Treatments on the Martensitic Transformation and Magnetic Properties in Ni49Mn32Ga19 Ferromagnetic Shape Memory Ribbons
Authors: Tolea, F; Popescu, B; Bartha, C; Enculescu, M; Tolea, M; Sofronie, M

Published: JAN 2023, MAGNETOCHEMISTRY, 9, 7, DOI: 10.3390/magnetochemistry9010007

In our work, the kinetics of martensitic transformations and the influence of thermal treatments on martensitic transformations, as well as the related magnetic properties of the Ni49Mn32Ga19 ferromagnetic shape memory melt-spun ribbons, have been investigated. Thermal treatments at 673 K for 1, 4 and 8 h can be considered an instrument for fine-tuning the performance parameters of alloys. One-hour thermal treatments promote an improvement in the crystallinity of these otherwise highly textured ribbons, reducing internal defects and stress induced by the melt-spinning technique. Longer thermal treatments induce an important magnetization rise concomitantly with a slight and continuous increase in martensitic temperatures and transformation enthalpy. The activation energy, evaluated from differential scanning calorimeter experimental data with a Friedman model, significantly increases after thermal treatments as a result of the multi-phase coexistence and stabilization of the non-modulated martensitic phase, which increases the reverse martensitic transformation hindrance.
3. Unidirectional Magnetic Anisotropy in Molybdenum Dioxide-Hematite Mixed-Oxide Nanostructures
Authors: Tolea, F; Sorescu, M; Diamandescu, L; Iacob, N; Tolea, M; Kuncser, V

Published: MAR 2022, NANOMATERIALS, 12, 938, DOI: 10.3390/nano12060938

MoO2-Fe2O3 nanoparticle systems were successfully synthesized by mechanochemical activation of MoO2 and alpha-Fe2O3 equimolar mixtures throughout 0-12 h of ball-milling. The role of the long-range ferromagnetism of MoO2 on a fraction of more defect hematite nanoparticles supporting a defect antiferromagnetic phase down to the lowest temperatures was investigated in this work. The structure and the size evolution of the nanoparticles were investigated by X-ray diffraction, whereas the magnetic properties were investigated by SQUID magnetometry. The local electronic structure and the specific phase evolution in the analyzed system versus the milling time were investigated by temperature-dependent Mossbauer spectroscopy. The substantially shifted magnetic hysteresis loops were interpreted in terms of the unidirectional anisotropy induced by pinning the long-range ferromagnetic order of the local net magnetic moments in the defect antiferromagnetic phase, as mediated by the diluted magnetic oxide phase of MoO2, to those less defect hematite nanoparticles supporting Morin transition. The specific evolutions of the exchange bias and of the coercive field versus temperature in the samples were interpreted in the frame of the specific phase evolution pointed out by Mossbauer spectroscopy. Depending on the milling time, a different fraction of defect hematite nanoparticles is formed. Less nanoparticles supporting the Morin transition are formed for samples exposed to a longer milling time, with a direct influence on the induced unidirectional anisotropy and related effects.
4. Conductance zeros in complex molecules and lattices from the interference set method
Authors: Nita, M; Tolea, M; Marinescu, DC

Published: MAR 23 2021, PHYSICAL REVIEW B, 103, DOI: 10.1103/PhysRevB.103.125307

Destructive quantum interference (DQI) and its effects on electron transport are studied in chemical molecules and finite physical lattices that can be described by a discrete Hamiltonian. Starting from a bipartite system whose conductance zeros are known to exist between any two points of a specially designated set, the interference set, we use the Dyson equation to develop a general algorithm for determining the zero conductance points in complex systems, which are not necessarily bipartite. We illustrate this procedure as it applies to the fulvene molecule. The stability of the conductance zeros is analyzed with respect to external perturbations.
5. Magnetic and Magnetostrictive Properties of Ni50Mn20Ga27Cu3 Rapidly Quenched Ribbons
Authors: Sofronie, M; Tolea, M; Popescu, B; Enculescu, M; Tolea, F

Published: SEP 2021, MATERIALS, 14, DOI: 10.3390/ma14185126

The influence of the rapid solidification technique and heat treatment on the martensitic transformation, magnetic properties, thermo- and magnetic induced strain and electrical resistivity is investigated for the Cu doped NiMnGa Heusler-based ferromagnetic shape memory ribbons. The martensitic transformation temperatures are unexpectedly low (below 90 K-which can be attributed to the disordered texture as well as to the uncertainty in the elements substituted by the Cu), preceded by a premartensitic transformation (starting at around 190 K). A thermal treatment slightly increases the transformation as well as the Curie temperatures. Additionally, the thermal treatment promotes a higher magnetization value of the austenite phase and a lower one in the martensite. The shift of the martensitic transformation temperatures induced by the applied magnetic field, quantified from thermo-magnetic and thermo-magnetic induced strain measurements, is measured to have a positive value of about 1 K/T, and is then used to calculate the transformation entropy of the ribbons. The magnetostriction measurements suggest a rotational mechanism in low fields for the thermal treated samples and a saturation tendency at higher magnetic fields, except for the temperatures close to the phase transition temperatures (saturation is not reached at 5 T), where a linear volume magnetostriction cannot be ruled out. Resistivity and magnetoresistance properties have also been measured for all the samples.
6. Martensitic Transformation and Magnetic Properties of Ni57Fe18Ga25 Shape Memory Alloy Subjected to Severe Plastic Deformation
Authors: Popescu, B; Gurau, C; Gurau, G; Tolea, M; Sofronie, M; Tolea, F

Published: OCT 2021, TRANSACTIONS OF THE INDIAN INSTITUTE OF METALS, 74, 2498, DOI: 10.1007/s12666-021-02293-8

The effects of severe plastic deformation (SPD) process via high-speed high-pressure torsion technique on martensitic transformation of Ni-Fe-Ga Heusler shape memory alloy are the subject of this work. The results show that moderate degrees of deformation lead to a decrease in the martensitic transformation temperatures, while the heat of reaction is enhanced only for the sample processed with the lowest degree of deformation. The results are explained by the interplay between the constituent tetragonal L10 and the cubic gamma crystal structures and the evolution of the samples morphology with the severity of deformation. The reduction in the samples granulation due to the progressive increase in the SPD is reflected by the magnetic properties of the samples with decreasing coercivity and Curie temperatures. At the highest applied degree of deformation, sample nanostructuring and a possible amorphization might explain the vanishing of MT.
7. Quantum dot exciton dephasing by Coulomb interaction: A fermionic analog of the independent boson model
Authors: Dinu, IV; Tolea, M; Gartner, P

Published: FEB 18 2020, PHYSICAL REVIEW B, 101, DOI: 10.1103/PhysRevB.101.085304

The time evolution of a quantum dot exciton in Coulomb interaction with wetting layer carriers is treated using an approach similar to the independent boson model. The role of the polaronic unitary transform is played by the scattering matrix, for which a diagrammatic, linked cluster expansion is available Similarities and differences to the independent boson model are discussed. A numerical example is presented.
8. Robust conductance zeros in graphene quantum dots and other bipartite systems
Authors: Nita, M; Tolea, M; Marinescu, DC

Published: JUN 30 2020, PHYSICAL REVIEW B, 101, DOI: 10.1103/PhysRevB.101.235318

Within the Landauer transport formalism we demonstrate that conductance zeros are possible in bipartite systems at half-filling when leads are contacted to different sublattice sites. In particular, we investigate the application of this theory to graphene quantum dots with leads in the armchair configuration. The obtained conductance cancellation is robust in the presence of any single-site impurity.
9. Magnetic and magnetostrictive properties of the ternary Fe67.5Pd30.5Ga2 ferromagnetic shape memory ribbons
Authors: Sofronie, M; Tolea, F; Tolea, M; Popescu, B; Valeanu, M

Published: JUL 2020, JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, 142, DOI: 10.1016/j.jpcs.2020.109446

Specific functional characteristics of the ferromagnetic shape memory alloy Fe67.5Pd30.5Ga2 prepared as ribbons by rapid quenching technique are reported. The shift of the martensitic transformation temperature induced by the applied magnetic field was determined from the thermomagnetic measurements at various fields up to 5T being proved to be in good agreement with the result obtained from calorimetric data via the Clapeyron - Claussius relation. Magnetostriction measurements reveal a pure spin rotation mechanism under low applied magnetic fields, allowed by the reduced magnetic anisotropy. In high magnetic fields and at temperatures close to martensitic transformation, the magnetostriction has a linear increase up to the maximum considered magnetic induction of 3T. This behavior has been discussed in connection with the forced magnetostiction aspects.
10. Localization Properties of Zig-Zag Edge States in Disordered Phosphorene
Authors: Nita, M; Ostahie, B; Tolea, M; Aldea, A

Published: JUL 2018, PHYSICA STATUS SOLIDI-RAPID RESEARCH LETTERS, 12, DOI: 10.1002/pssr.201800051

Herein, the spectral and transport properties in the finite phosphorene lattice are investigated using the tight-binding model in the presence of Anderson disorder potential. The focus is on the zig-zag edge states localization, provided by the numerically calculated inverse participation number. At low disorder, the zig-zag states undergo a localization process, keeping their 1D character, while further increasing the disorder leads to delocalization due to hybridization with the extended 2D states. The disorder-induced changes in the electronic conductance, from one zig-zag edge to the other, are also discussed.
11. In-gap corner states in core-shell polygonal quantum rings
Authors: Sitek, A; Tolea, M; Nita, M; Serra, L; Gudmundsson, V; Manolescu, A

Published: JAN 10 2017, SCIENTIFIC REPORTS, 7, DOI: 10.1038/srep40197

We study Coulomb interacting electrons confined in polygonal quantum rings. We focus on the interplay of localization at the polygon corners and Coulomb repulsion. Remarkably, the Coulomb repulsion allows the formation of in-gap states, i.e., corner-localized states of electron pairs or clusters shifted to energies that were forbidden for non-interacting electrons, but below the energies of corner-side-localized states. We specify conditions allowing optical excitation to those states.
12. Thermal memory fading by heating to a lower temperature: Experimental data on polycrystalline NiFeGa ribbons and 2D statistical model predictions
Authors: Tolea, F; Tolea, M; Valeanu, M

Published: MAY 2017, SOLID STATE COMMUNICATIONS, 257, 41, DOI: 10.1016/j.ssc.2017.04.003

Shape memory alloys are known to memorise one -or several-temperatures at which the martensite-austenite transformation was stopped before completion in the past, the memory manifesting as specific dips in subsequent calorimetric scans. Previous studies have shown that this memory can be erased by heating to higher temperatures than the ones previously recorded. In this paper, we study a distinct memory fading effect which takes place by heating to a lower temperature. This effect is reported in NiFeGa as polycrystalline ribbons, the alloy being initially studied as bulk for which the thermal memory effect was not found. If, after an initial incomplete heating up to T-1 one performs a second incomplete heating up to T-2 < T-1, a new calorimetric dip appears at T-2, as expected, while less expected was that the dip corresponding to T-1 reduces in amplitude or even vanishes ( if the arrest at T-2 is repeated). The memory fading effect is more clear for small differences T-1-T-2 and less obvious or absent for large ones. The second part of the paper employs a statistical 2D model, which associates the memorized temperatures with a depletion of certain martensite plates sizes, and also supports the memory fading effect.
13. Specific Changes in the Magnetoresistance of Ni-Fe-Ga Heusler Alloys Induced by Cu, Co, and Al Substitutions
Authors: Tolea, F; Tolea, M; Sofronie, M; Popescu, B; Crisan, A; Leca, A; Valeanu, M

Published: APR 2017, IEEE TRANSACTIONS ON MAGNETICS, 53, DOI: 10.1109/TMAG.2016.2628386

In this paper, we investigate the influence of Cu, Co, and Al substitutions on the transport properties, and in particular, the magnetoresistive effect in Ni-Fe-Ga ferromagnetic shape memory alloys (FSMAs) prepared as ribbons by melt spinning method and subjected to different thermal treatments. X-ray diffraction, differential scanning calorimetry, magnetometry, and magnetoresistive characterizations were performed. In the range of the martensitic transformation (MT), different FSMA compositions show a rich spectrum of different behaviors. For one of the compositions (Ni52Fe20Co2Ga23Al3), the magnetoresistance (MR) showed a local minimum or, on the contrary, a local maxima of reduced amplitude on cooling, in the range of the MT, depending on the performed thermal treatments. In the same composition, by replacing one Al atom with a Co one, no local extremes are seen, the alloy having a concomitant magneto-structural transition. When the magnetic field was varied, the MR showed a nonmonotonic variation in the martensite phase for some compounds, possibly due to the dynamics of the martensite variants realignment. From the studied compositions, the highest MR found on the MT of -9% for 5 T is for Ni50Fe20Ga27Cu3.
14. Tuning the transmission phase by the dot size
Authors: Tolea, F; Tolea, M

Published: JAN 2017, PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, 85, 173, DOI: 10.1016/j.physe.2016.08.034

The phase shift of the electron's wave function after a tunneling event (i.e. the transmission phase) was at first measured for its fundamental or applicative relevance for quantum circuitry, but later the phase study self-motivated due to a number of unexpected results. One such result was the reduced increment of the phase on some resonances - with only fractions of pi - in the few-electrons "mesoscopic" regime. In this paper we address such a regime for a rectangular quantum dot and compute the total phase increase on the first four resonances by means of accurate configuration-interaction method and a generalized Friedel sum rule as proposed by Rontani (2006) [17]. Our findings confirm that the electronic correlations reduce the on-resonance phase growth which is also found to decrease quasi-linearly with the dot size, the decrease being more pronounced as the number of electrons on the dot is raised. Sudden jumps (of small amplitude) of the phase are found to accompany ground states spin transitions. (C) 2016 Elsevier B.V. All rights reserved.
15. Hund and anti-Hund rules in circular molecules
Authors: Nita, M; Tolea, M; Marinescu, DC; Manolescu, A

Published: DEC 1 2017, PHYSICAL REVIEW B, 96, DOI: 10.1103/PhysRevB.96.235101

We study the validity of Hund's first rule for the spin multiplicity in circular molecules-made of real or artificial atoms such as quantum dots-by considering a perturbative approach in the Coulomb interaction in the extended Hubbard model with both on-site and long-range interactions. In this approximation, we show that an anti-Hund rule always defines the ground state in a molecule with 4N atoms at half-filling. In all other cases (i.e., number of atoms not a multiple of four, or a 4N molecule away from half-filling) both the singlet and the triplet outcomes are possible, as determined primarily by the total number of electrons in the system. In some instances, the Hund rule is always obeyed and the triplet ground state is realized mathematically for any values of the on-site and long-range interactions, while for other filling situations the singlet is also possible but only if the long-range interactions exceed a certain threshold, relatively to the on-site interaction.
16. Ground state spin and excitation energies in half-filled Lieb lattices
Authors: Tolea, M; Nita, M

Published: OCT 3 2016, PHYSICAL REVIEW B, 94, DOI: 10.1103/PhysRevB.94.165103

We present detailed spectral calculations for small Lieb lattices having up to N = 4 number of cells, in the regime of half-filling, an instance of particular relevance for the nanomagnetism of discrete systems such as quantum dot arrays, due to the degenerate levels at midspectrum. While for the Hubbard interaction model-and even number of sites-the ground state spin is given by the Lieb theorem, the inclusion of long-range interaction-or odd number of sites-makes the spin state not known a priori, which justifies our approach. We calculate also the excitation energies, which are of experimental importance, and find significant variation induced by the interaction potential. One obtains insights on the mechanisms involved that impose as ground state the Lieb state with lower spin rather than the Hund one with maximum spin for the degenerate levels, showing this in the first and second orders of the interaction potential for the smaller lattices. The analytical results agree with the numerical ones, which are performed by exact diagonalization calculations or by a combined mean-field and configuration interaction method. While the Lieb state is always lower in energy than the Hund state, for strong long-range interaction, when possible, another minimal spin state is imposed as ground state.
17. Effect of thermal treatments in Ni-Fe-Ga with Co substitutions and Ni-Mn-Ga melt spun ribbons
Authors: Tolea, F; Sofronie, M; Crisan, AD; Popescu, B; Tolea, M; Valeanu, M

Published: 2016, 21ST EUROPEAN CONFERENCE ON FRACTURE, (ECF21), 2, 1480, DOI: 10.1016/j.prostr.2016.06.187

The effect of "in situ" thermal treatments (by DSC measurements) on the martensitic transformation in two representative Ni-Fe-Ga and Ni-Mn-Ga alloys has been studied and discussed by correlating the structural and magnetic properties. The alloys were prepared from high purity elements, by arc melting under argon protective atmosphere as bulk and also as melt-spun ribbons - an alternative preparation route that also allows to assess the influences of grains size and strain induced by this processing method. All samples presented reversible thermo-elastic transformations. The thermal treatments promote a reduction of the martensitic transformation temperatures in the Ni-Fe-Ga investigated samples, as opposed to the stoichiometric Ni2MnGa where the temperatures increase with increasing the annealing temperatures. Interestingly however, the off-stoichiometric Ni-Mn-Ga with increased Ni content recovers the behaviour with reduction of transformation temperatures by thermal treatments. The precipitation of the secondary FCC (gamma) phase is inherently found in Ni-Fe-Ga alloys with Ga <= 27% at, and also-although in lower amounts- in the off-stoichiometric Ni-Mn-Ga. The gamma phase is considered to contribute to the decrease of the MT temperatures (via valence electrons concentration depletion of the main matrix) and of the transformation heat as well as to the final structural degradation if the temperature of the thermal treatments is further increased. In addition, this phase, located mainly at the grain boundaries, is responsible for the improved ductility of Ni-Fe-Ga based alloys. Changes in the transformation heat due to thermal treatments are observed and discussed in both types of alloys, the maxima of the transformation heat being associated with the highest atomic order. Thermo-magnetic measurements show that Ni-Fe-Ga alloys have close magnetic and structural transitions temperatures, with promising applications for magnetic refrigeration. Copyright (C) 2016 The Authors. Published by Elsevier B.V.
18. Distribution of plates' sizes tell the thermal history in a simulated martensitic-like phase transition
Authors: Tolea, F; Tolea, M; Sofronie, M; Valeanu, M

Published: JUL 2015, SOLID STATE COMMUNICATIONS, 213, 41, DOI: 10.1016/j.ssc.2015.04.016

A phenomenological 2D model, simulating the martensitic transformation, is built upon existing experimental observations that the size of the formed plates - in direct transformation - decreases as the temperature is lowered; then they transform back in reversed order. As such, if a reverse transformation is incomplete (arrested), the subsequent direct one will show anomalously a large number of big size plates - old plus newly formed - but consequentially a depletion of intermediate sizes, due to geometrical constraints, phenomenon that generates thermal memory. (C) 2015 Elsevier Ltd. All rights reserved.
19. Magnetic and Martensitic Transformations in the bulk and melt spun ribbons of Ni57-xNdxFe18Ga25 Ferromagnetic Shape Memory Alloys
Authors: Tolea, F; Crisan, AD; Sofronie, M; Tolea, M; Valeanu, M

Published: 2015, MATERIALS TODAY-PROCEEDINGS, 2, 878, DOI: 10.1016/j.matpr.2015.07.421

Influence of Nd substitution for Ni on the magnetic properties and the martensitic transformation (MT) characteristics are investigated on Ni57-xNdxFe18Ga25 (x=0 divided by 4) ferromagnetic shape memory alloys (FSMAs) in bulk and also in ribbons prepared by melt spinning method and subjected to different thermal treatments. Increasing the Nd content induces a decrease of both the Curie and the MT temperatures. (C) 2015 The Authors. Published by Elsevier Ltd.
20. Hearing shapes of few electrons quantum drums: A configuration-interaction study
Authors: Tolea, F; Tolea, M

Published: FEB 1 2015, PHYSICA B-CONDENSED MATTER, 458, 91, DOI: 10.1016/j.physb.2014.11.013

The - highly remarkable - existence of non-congruent yet vibrationally isospectral shapes has been first proved theoretically and then also tested experimentally - by using electromagnetic waves in cavities, vibrating smectic films or electrons in nanostructures. In this context, we address the question whether isospectrality holds if two or more electrons interact electrostatically, using the accurate configuration-interaction method, in a discrete representation of the Bilby and Hawk shapes. Isospectral pairs offer an unique possibility to test how identical sets of single-particle energies may combine differently in the few-electrons eigenmodes, due to different wave functions spatial distributions. Our results point towards the break down of isospectrality in the presence of interactions. Thus one should be able to "hear" the shapes of few electrons quantum drums. Interestingly however, for the analyzed two and three electrons cases, there exists an interaction strength (which can be tuned by changing the size of the shapes), for which the ground states energies of Bilby and Hawk coincide, but not the excited states as well Wigner localization is studied and shown to occur at about the same size for both Bilby and Hawk shapes. Next, an exercise is proposed to use the two-electrons charge density of the Bilby and Hawk ground states in the phase extraction scheme as proposed by Moon et al. (2008). Results show that out-of-phase regions appear if the linear size of the shapes exceeds the Bohr radius as occupation of higher Slater determinants becomes significant. (C) 2014 Elsevier B.V. All rights reserved.
21. SHAPE MEMORY PROPERTIES OF FeNiCoTi RIBBONS EVIDENCED BY MAGNETIC MEASUREMENTS
Authors: Tolea, F; Sofronie, M; Tolea, M; Kuncser, V; Valeanu, M

Published: APR-JUN 2015, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 10, 575, DOI:

The present work addresses the shape memory and (ferro) magnetic properties of Fe52Ni29-xCo15+xTi4 (with x=0, 3 and 6) alloys. The analysed samples were prepared as ribbons by the melt spinning method and subjected to thermal treatments. X-ray diffraction, DSC, thermomagnetic measurements and Mossbauer spectroscopy were used for a complete structural and magnetic characterization. Both the preparation route and the different Co addition induce specific effects which are discussed in detail. The sample with x=0 sustains an irreversible transformation, while a partial reversible transformation and a relatively increased Curie temperature were observed for sample with x=3. However, further increasing the Co content to x= 6 leads to a loss of the martensitic transformation.
22. Electron Reconfiguration and Enhanced Phonon Activation in the Superconducting State of a FeSe0.3Te0.7 Single Crystal, as Evidenced by Mossbauer Spectroscopy
Authors: Greculeasa, S; Miu, L; Badica, P; Nie, J; Tolea, M; Kuncser, V

Published: JAN 15 2015, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 84, DOI: 10.7566/JPSJ.84.014701

The Mossbauer spectra of a FeSe0.3Te0.7 single crystal grown by the Bridgman method were analysed across the superconducting transition by considering the interplay between the structure and electron configuration of the transition metal. The magnetically determined superconducting critical temperature is T-C similar to 14 K. The Fe-57 Mossbauer spectra collected in the temperature range from 5 to 200K mainly have an asymmetric doublet pattern, which was conveniently fitted by the full Hamiltonian method. No effective magnetic moment ascribed to the superconducting phase was observed down to 5K. The unusual behaviour observed below similar to 17K for the chemical isomer shift and quadrupole splitting may be associated with an electron reconfiguration process intimately related to an unusual lattice distortion accompanying the superconducting transition. The decreasing trend of the total absorption spectral area and second-order Doppler shift during cooling the sample below the critical temperature, point to enhanced phonon activation in the superconducting state.
23. Magnetocaloric effect in Ni-Fe-Ga Heusler alloys with Co and Al substitutions
Authors: Tolea, F; Sofronie, M; Crisan, AD; Tolea, M; Valeanu, M

Published: 2015, ESOMAT 2015 - 10TH EUROPEAN SYMPOSIUM ON MARTENSITIC TRANSFORMATIONS, 33, DOI: 10.1051/matecconf/20153305008

The functionality of the ferromagnetic shape memory alloys is related to the martensitic and magnetic order-disorder transformations, both of which may be tailored by doping with other elements or by suitable thermal treatments, so that alloys with concomitant (or sequential but close) structural and magnetic phase transitions may be obtained. Concerning the magnetocaloric applications, it is assumed that the thin melt-spun ribbons assure a more efficient heat transfer. In the present work we investigate the influence of Co and Al substitutions on magnetocaloric effect characteristics of NiFeGa in bulk and also in ribbons prepared by melt spinning method and subjected to different thermal treatments. X-ray diffraction, differential scanning calorimetry, magnetocaloric and magnetoresistive characterizations have been performed. The results highlight the differences between the bulk and the ribbons (both as prepared and annealed) and the role of substitutions.
24. Transmission phase lapses at zero energy in graphene quantum dots
Authors: Nita, M; Tolea, M; Ostahie, B

Published: SEP 2014, PHYSICA STATUS SOLIDI-RAPID RESEARCH LETTERS, 8, 793, DOI: 10.1002/pssr.201409228

We address theoretically the electronic transport through graphene quantum dots with the emphasis on the transmission phase. Analytical and numerical results are presented regarding the existence - or not - of a lapse of the transmittance phase (and, consequentially, a Fano zero in the transmittance) at the charge neutrality point. A simple universal criterium is found, the phase lapses being always present if the contact sites belong to the same sub-lattice. ((c) 2014 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)
25. Analysis of the phase lapse problem in closed interferometers
Authors: Tolea, M; Moldoveanu, V; Dinu, IV; Tanatar, B

Published: OCT 1 2012, PHYSICS LETTERS A, 376, 3234, DOI: 10.1016/j.physleta.2012.07.007

We investigate the connection between the asymmetry of the Fano resonances in a mesoscopic interferometer with an embedded quantum dot and the pi lapses in the phase of the "bare" dot transmittance. Consecutive Fano resonances with the same (opposite) sign of the Fano parameter imply the presence (absence) of a phase lapse with pi between the corresponding resonances of the dot. Our results suggest that the famous "phase lapse" problem, first reported by Schuster et al. [R. Schuster, E. Buks, M. Heiblum, D. Mahalu, V. Umansky, H. Shtrikman, Nature 385 (1997) 417], can therefore be experimentally addressed in closed interferometers. It is also proposed that the Fano effect can be used to extract the phase distributions of the eigenfunctions for a mesoscopic 20 shape, via the parity of the resonances. In the presence of electron-electron interaction, one can calculate the phases of the T-matrix elements. The numerical results lead to the same conclusions as for the non-interacting case. (C) 2012 Elsevier B.V. All rights reserved.
26. Phase extraction in disordered isospectral shapes
Authors: Tolea, M; Ostahie, B; Nita, M; Tolea, F; Aldea, A

Published: MAR 14 2012, PHYSICAL REVIEW E, 85, DOI: 10.1103/PhysRevE.85.036604

The phase of the electronic wave function is not directly measurable but, quite remarkably, it becomes accessible in pairs of isospectral shapes, as recently proposed in the experiment by Moon et al. [Science 319, 782 (2008)]. The method is based on a special property, called transplantation, which relates the eigenfunctions of the isospectral pairs, and allows us to extract the phase distributions, if the amplitude distributions are known. We numerically simulate such a phase extraction procedure in the presence of disorder, which is introduced both as Anderson disorder and as roughness at edges. With disorder, the transplantation can no longer lead to a perfect fit of the wave functions, however we show that a phase can still be extracted-defined as the phase that minimizes the misfit. Interestingly, this extracted phase coincides with (or differs negligibly from) the phase of the disorder-free system, up to a certain disorder amplitude, and a misfit of the wave functions as high as similar to 5%, proving a robustness of the phase extraction method against disorder. However, if the disorder is increased further, the extracted phase shows a puzzle structure, no longer correlated with the phase of the disorder-free system. A discrete model is used, which is the natural approach for disorder analysis. We provide a proof that discretization preserves isospectrality and the transplantation can be adapted to the discrete systems.
27. Electronic transmittance phase extracted from mesoscopic interferometers
Authors: Tolea, M; Moldoveanu, V; Dinu, IV; Tanatar, B

Published: OCT 13 2012, NANOSCALE RESEARCH LETTERS, 7, 7, DOI: 10.1186/1556-276X-7-568

The usual experimental set-up for measuring the wave function phase shift of electrons tunneling through a quantum dot (QD) embedded in a ring (i.e., the transmittance phase) is the so-called 'open' interferometer as first proposed by Schuster et al. in 1997, in which the electrons back-scattered at source and the drain contacts are absorbed by additional leads in order to exclude multiple interference. While in this case one can conveniently use a simple two-path interference formula to extract the QD transmittance phase, the open interferometer has also a number of draw-backs, such as a reduced signal and some uncertainty regarding the effects of the extra leads. Here we present a meaningful theoretical study of the QD transmittance phase in 'closed' interferometers (i.e., connected only to source and drain leads). By putting together data from existing literature and giving some new proofs, we show both analytically and by numerical simulations that the existence of phase lapses between consecutive resonances of the 'bare' QD is related to the signs of the corresponding Fano parameters - of the QD + ring system. More precisely, if the Fano parameters have the same sign, the transmittance phase of the QD exhibits a I lapse. Therefore, closed mesoscopic interferometers can be used to address the 'universal phase lapse' problem. Moreover, the data from already existing Fano interference experiments from Kobayashi et al. in 2003 can be used to infer the phase lapses.
28. Mesoscopic Fano effect in a spin splitter with a side-coupled quantum dot
Authors: Moldoveanu, V; Tolea, M; Tanatar, B

Published: FEB 20 2012, PHYSICS LETTERS A, 376, 1083, DOI: 10.1016/j.physleta.2012.02.017

We investigate the interplay between the spin interference and the Fano effect in a three-lead mesoscopic ring with a side-coupled quantum dot (QD). A uniform Rashba spin-orbit coupling and a perpendicular magnetic field are tuned such that the ring operates as a spin splitter in the absence of the QD: one lead is used to inject unpolarized electrons and the remaining (output) leads collect almost polarized spin currents. By applying a gate potential to the quantum dot a pair of spin-split levels sweeps the bias window and leads to Fano interference. The steady-state spin and charge currents in the leads are calculated for a finite bias applied across the ring via the non-equilibrium Green's function formalism. When the QD levels participate to transport we find that the spin currents exhibit peaks and dips whereas the charge currents present Fano lineshapes. The location of the side-coupled quantum dot and the spin splitting of its levels also affect the interference and the output currents. The opposite response of output currents to the variation of the gate potential allows one to use this system as a single parameter current switch. We also analyze the dependence of the splitter efficiency on the spin splitting on the QD. (C) 2012 Elsevier B.V. All rights reserved.
29. Fano-Kondo oscillations of the conductance and thermopower in a mesoscopic transistor
Authors: Aldea, A; Tolea, M; Dinu, IV

Published: 2012, 14TH INTERNATIONAL CONFERENCE ON TRANSPORT IN INTERACTING DISORDERED SYSTEMS (TIDS-14), 376, DOI: 10.1088/1742-6596/376/1/012019

We study the conductance G and Seebeck coefficient S of a side-coupled double quantum dot system. The transport properties are the result of the competition between the Fano interference and Kondo correlations, being also controlled by the Coulomb blockade of the multilevel side-dot. The external parameters are the gate potential V-g applied on the side-dot and the temperature. When V-g is varied continuously the blockade is switched on and off periodically, resulting in oscillations of the transport coefficients. The profile of the oscillations and the temperature dependence are studied. An extended Anderson model and the Keldysh transport formalism are used.
30. Zero bias anomalies in the Kondo regime of single and double quantum dots
Authors: Aldea, A; Tolea, M; Dinu, IV

Published: 2012, ADVANCED MANY-BODY AND STATISTICAL METHODS IN MESOSCOPIC SYSTEMS, 338, DOI: 10.1088/1742-6596/338/1/012001

The zero bias anomaly of the differential conductance of mesoscopic systems is a fingerprint of the electron-electron interaction. The common example is the peak of the differential conductance in Kondo mesoscopic systems. We show that in complex mesoscopic systems, in particular in the side-coupled double dot, the dip of the differential conductance at zero bias is also possible due to simultaneous effects of interference and correlations. The external parameters that control this effect are the temperature and the gate potential on the lateral dot. We argue that even in single dots, in a multiple lead configuration, the shift from suppression to enhancement of dI/dV with increasing bias is allowed if the bias applied on the leads is not symmetric. The differential conductance exhibits a peak-dip crossover, the effect being controlled by the strength of the asymmetry and the ratio of the dot-lead couplings. The scaling of the differential conductance for the double-dot system is also discussed. The results are obtained using an extended Anderson model, the Keldysh transport formalism and the equation of motion technique extended to the non-equilibrium.
31. Coulomb oscillations in the Fano-Kondo effect and zero-bias anomalies in a double-dot mesotransistor
Authors: Aldea, A; Tolea, M; Dinu, IV

Published: JUN 27 2011, PHYSICAL REVIEW B, 83, DOI: 10.1103/PhysRevB.83.245317

We investigate theoretically the transport properties of the side-coupled double quantum dots in connection with the experimental study of Sasaki et al. [Phys. Rev. Lett. 103, 266806 (2009)]. The setup consists of connecting the Kondo dot directly to the leads, while the side dot provides an interference path which affects the Kondo correlations. We analyze the oscillations of the source-drain current due to the periodical Coulomb blockade of the many-level side dot at the variation of the gate potential applied on it. The Fano profile of these oscillations may be controlled by the interdot coupling, level spacing of the side dot, and also by the temperature. The nonequilibrium conductance of the double-dot system exhibits zero-bias anomaly which, besides the usual enhancement, may show also a suppression (a diplike aspect) which occurs around the Fano zero. In the same region, the weak temperature dependence of the conductance indicates the suppression of the Kondo effect. Scaling properties of the nonequilibrium conductance in the Fano-Kondo regime are discussed. Since the Kondo temperature of the single-impurity Anderson model is no longer the proper scaling parameter, we look for an alternative specific to the double dot. The extended Anderson model, Keldysh formalism, and equation-of-motion technique are used.
32. Analyzing the measured phase in the multichannel Aharonov-Bohm interferometer
Authors: Tolea, M; Nita, M; Aldea, A

Published: JUL 2010, PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, 42, 2236, DOI: 10.1016/j.physe.2010.04.022

We address the quantum dot phase measurement problem in an open Aharonov-Bohm interferometer, assuming multiple transport channels. In such a case, the quantum dot is characterized by more than one intrinsic phase for the electrons transmission. It is shown that the phase which would be extracted by the usual experimental method (i.e. by monitoring the shift of the Aharonov-Bohm oscillations, as in Schuster et al., Nature 385 (1997) 417) does not coincide with any of the dot intrinsic phases, but is a combination of them. The formula of the measured phase is given. The particular case of a quantum dot containing a S = 1/2 spin is discussed and variations of the measured phase with <pi are found, as a consequence of the multichannel transport. (C) 2010 Elsevier B.V. All rights reserved.
33. Kondo peaks and dips in the differential conductance of a multi-lead quantum dot: Dependence on bias conditions
Authors: Tolea, M; Dinu, IV; Aldea, A

Published: JAN 2009, PHYSICAL REVIEW B, 79, DOI: 10.1103/PhysRevB.79.033306

We study the differential conductance in the Kondo regime of a quantum dot coupled to multiple leads. When the bias is applied symmetrically on two of the leads (V and -V, as usual in experiments) while the others are grounded, the conductance through the biased leads always shows the expected enhancement at zero bias. However, under asymmetrically applied bias (V and lambda V, with lambda>0), a suppression-dip-appears in the differential conductance if the asymmetry coefficient lambda is beyond a given threshold lambda(0)=3 root 1+r determined by the ratio r of the dot-lead couplings. This is a recipe to determine experimentally this ratio which is important for the quantum-dot devices. This finding is a direct result of the Keldysh transport formalism. For the illustration we use a many-lead Anderson Hamiltonian, the Green's functions being calculated in the Lacroix approximation, which is generalized to the case of nonequilibrium.
34. Peak-dip crossover of the differential conductance in mesoscopic systems with quantum impurities
Authors: Aldea, A; Tolea, M; Dinu, IV

Published: 2009, 25TH INTERNATIONAL CONFERENCE ON LOW TEMPERATURE PHYSICS (LT25), PART 2, 150, DOI: 10.1088/1742-6596/150/2/022001

We investigate the differential conductance dI/dV for the interacting T-shape model, using an approach based on the Keldysh formalism and the Lacroix solution for the equation of motion. A peak-dip crossover has been noticed by changing the hybridization between the two dots. For the same model, the combined interaction and interference processes give rise to the Fano-Kondo effect with an interesting crossing point of the isoterms below the Kondo temperature A tentative explanation of these effects is given in terms of the many-body spectral properties of the system.
35. Transport through quantum dots with magnetic impurities
Authors: Tolea, M; Aldea, A; Bulka, BR

Published: JUL 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 1758, DOI:

We analyze the electronic transport through a quantum dot that contains a magnetic impurity. The coherent transport of electrons is governed by the quantum confinement inside the dot, but is also influenced by the exchange interaction with the impurity. The interplay between the two gives raise to the singlet-triplet splitting of the energy levels available for the tunneling electron. In this paper, we focus on the charge fluctuations and, more precisely, the height of the conductance peaks. We show that the conductance peaks corresponding to the triplet levels are three times higher than those corresponding to singlet levels, if electronic correlations are neglected (for non-interacting dots, when an exact solution can be obtained). Next, we consider the Coulomb repulsion and the many-body correlations. In this case, the singlet/triplet peak height ratio has a complex behavior. Usually the highest peak corresponds to the state that is lowest in energy (ground state), regardless if it is singlet or triplet. In the end, we get an insight on the Kondo regime for such a system, and show the formation of three Kondo peaks. We use the equation of motion method with appropriate decoupling.
36. Coherent and incoherent transport through T-shaped double quantum dots
Authors: Moldoveanu, V; Tolea, M; Tanatar, B

Published: MAR 2008, PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, 40, 1104, DOI: 10.1016/j.physe.2007.08.012

We investigate the measurement induced dephasing of the Fano effect in the electronic transport through a double quantum dot mesoscopic interferometer coupled to a charge detector. The current and the differential conductance are computed within the Keldysh formalism, taking into account of the inelastic processes due to the dot-detector interaction. We show that the visibility of the Fano lineshape is reduced by applying a finite bias on the charge detector. (C) 2007 Elsevier B.V. All rights reserved.
37. Charge fluctuations in quantum point contact
Authors: Bulka, BR; Dinu, IV; Tolea, M

Published: MAY 2008, PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, 40, 2617, DOI: 10.1016/j.physe.2007.08.001

We present studies of the role of charge fluctuations in transport through a quantum point contact (QPC). Our model of QPC is based on assumption of a specific electronic energy structure with a resonant level below the electronic sub-band. We show that charge fluctuations lead to a dynamical Coulomb blockade effect, which is responsible for reduction of the conductance to the value 2/3 x (2e(2)/h). The evolution of conductance with a magnetic field and as a function of source-drain voltage is presented as well. The conductance plateau at 2/3 x (2e(2)/h) evolves continuously with an increase of the voltage to a 0.8 or 0.4 plateau, depending on the relative position of the resonant state with respect to the Fermi energy at zero bias. Our simple model shows many similarities with experimental characteristics. (C) 2007 Elsevier B.V. All rights reserved.
38. Measurement-induced decoherence in electronic interferometry at nanoscale
Authors: Moldoveanu, V; Tanatar, B; Tolea, M

Published: SEP 29 2008, PHYSICS LETTERS A, 372, 6196, DOI: 10.1016/j.physleta.2008.07.079

We introduce a theoretical formalism describing a wide class of 'Which Path' experiments in mesoscopic/nanoscopic transport. The physical system involves a mesoscopic interferometer (e.g. an Aharonov-Bohm ring with embedded dots or a side-coupled quantum dot) which is electrostatically coupled to a nearby quantum point constriction. Due to the charge sensing effect the latter acts as a charge detector. Therefore the interference pattern can be monitored indirectly by looking at the current characteristics of the detector as shown in the experimental work of Buks et al. [E. Buks, R. Schuster, M. Heiblum, D. Mahalu, V. Umansky, Nature (London) 391 (1998) 871]. We use the non-equilibrium Green-Keldysh formalism and a second order perturbative treatment of the Coulomb interaction in order to compute the relevant transport properties. it is shown that in the presence of the Coulomb interaction the current through the detector exhibits oscillations as a function of the magnetic field applied on a single-dot AB interferometer. We also discuss the dependence of the visibility of the Aharonov-Bohm oscillations on the gate potential applied to the dot. (C) 2008 Elsevier B.V. All rights reserved.
39. Transport through a multiply connected interacting mesoscopic system using the Keldysh formalism
Authors: Dinu, IV; Tolea, M; Aldea, A

Published: SEP 2007, PHYSICAL REVIEW B, 76, DOI: 10.1103/PhysRevB.76.113302

We apply the Keldysh formalism in order to derive a current formula easy to use for a system with many sites, one of which is interacting. The main technical challenge is to deal with the lesser Green's function. It turns out that, in the case of the left-right symmetry, the knowledge of the lesser Green's function is not necessary and an exact current formula can be expressed in terms of retarded Green's functions only. The application is done for a triangular interferometer which gives a good account of the Fano-Kondo effect. It is found that the interference effects, in the context of Kondo correlations, give rise to a point in the parameter space where the conductance is temperature independent. We include a comparison with the results from Ng's ansatz [Phys. Rev. Lett. 70, 3635 (1993)], which are less accurate, but can be used also in the absence of the above mentioned symmetry.
40. Controlled dephasing in single-dot Aharonov-Bohm interferometers
Authors: Moldoveanu, V; Tolea, M; Tanatar, B

Published: JAN 2007, PHYSICAL REVIEW B, 75, DOI: 10.1103/PhysRevB.75.045309

We study the Fano effect and the visibility of the Aharonov-Bohm oscillations for a mesoscopic interferometer with an embedded quantum dot in the presence of a nearby second dot. When the electron-electron interaction between the two dots is considered the nearby dot acts as a charge detector. We compute the currents through the interferometer and detector within the Keldysh formalism and the self-energy of the nonequilibrium Green's functions is found up to the second order in the interaction strength. The current formula contains a correction to the Landauer-Buttiker formula. Its contribution to transport and dephasing is discussed. As the bias applied on the detector is increased, the amplitude of both the Fano resonance and Aharonov-Bohm oscillations are considerably reduced due to controlled dephasing. This result is explained by analyzing the behavior of the imaginary part of the interaction self-energy as a function of energy and bias. We emphasize as well the role of the ring-dot coupling. Our theoretical results are consistent with the experimental observation of Buks [Nature 391, 871 (1998)].
41. Theoretical study of electronic transport through a small quantum dot with a magnetic impurity
Authors: Tolea, M; Bulka, BR

Published: MAR 2007, PHYSICAL REVIEW B, 75, DOI: 10.1103/PhysRevB.75.125301

We model a small quantum dot with a magnetic impurity by the Anderson Hamiltonian with a supplementary exchange interaction term. The transport calculations are performed by means of the Green functions within the equation of motion scheme, in which two decoupling procedures are proposed, for high and low temperatures, respectively. The paper focuses on the charge fluctuations for such a system, an aspect not addressed before, as well as on the Kondo resonance. We show a specific role of the excited state, which can be observed in transport and in spin-spin correlations. Our studies show a many-body feature of the phase shift of transmitted electrons, which is manifested in a specific dip. In the Kondo regime, our calculations complement existing theoretical results. The system shows three Kondo peaks in the density of states: one at the Fermi energy and two side peaks, at a distance corresponding to the singlet-triplet level spacing. The existence of the central peak is conditioned by a degenerate state (the triplet) below the Fermi energy.
42. Mesoscopic Fano effect in an Aharonov-Bohm interferometer Coulomb-coupled to a nearby quantum dot
Authors: Tolea, M; Moldoveanu, V; Tanatar, B

Published: 2007, PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 4, NO 2, 4, +, DOI: 10.1002/pssc.200673304

Motivated by the pionieering experiments of Buks et al. [Nature 391, 871 (1998)] we investigate the visibility of the Fano effect in a single-dot Aharonov-Bohm interferometer which is Coulomb-coupled to a nearby quantum dot. The latter acts as a 'Which Path Detector' and is coupled to two leads on which a finite bias is applied. Using the non-equilibrium Keldysh-Green function formalism we compute the currents through the detector and the interferometer. We take into account the first two contributions to the interaction self energy and emphasize the correction to the Landauer formula which appears beyond the single-particle approximation. Particular attention is given to the coherence properties of the interferometer in the presence of the electron-electron interaction between the embedded dot and the detector. We show that when the detector is subjected to a finite bias the amplitude of Aharonov-Bohm oscillations of the current through the interferometer decreases. The Fano line is in turn rather stable under interaction. Our results generalize an earlier work of Silva and Levit [Phys. Rev. B 63, 201309 (2001)] and complement the existing description of the controlled dephasing. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
43. Correlated electrons and transport in a quantum point contact and in a double-quantum-dot system
Authors: Bulka, BR; Kostyrko, T; Tolea, M; Dinu, IV

Published: JUN 27 2007, JOURNAL OF PHYSICS-CONDENSED MATTER, 19, DOI: 10.1088/0953-8984/19/25/255211

A problem of electronic correlations is considered for two specific mesoscopic systems: the quantum point contact (QPC) and the double quantum dot (2QD) system. The systems are described using a generalized Anderson Hamiltonian. We show that charge fluctuations are relevant for electronic transport. In the QPC a local accumulation of charge and the dynamical Coulomb blockade effect lead to the 0.7 structure in the conductance characteristics. The evolution of the conductance with a magnetic field and in non-equilibrium situations is presented as well. The double quantum dot is studied in the approach, in which correlations within the 2QD are treated exactly, whereas the coupling of the 2QD to the leads is considered in the approximation valid at temperatures above the Kondo temperature. We analyse the evolution of the gate voltage dependence of the spin correlation functions and the conductance with the change of the interdot hopping. For the hopping parameter greater than a threshold value of the on-dot repulsion the physics of the device is dominated by the ground state eigenstates of the 2QD and antiferromagnetic correlations in the case of the doubly occupied 2QD. With a decrease of the interdot hopping repulsion below the threshold we observe a significant reduction of the antiferromagnetic coupling between the dots together with an enhanced occupation of the triplet states.
44. Mesoscopic Fano effect in Aharonov-Bohm rings with an embedded double dot
Authors: Tanatar, B; Moldoveanu, V; Tolea, M; Aldea, A

Published: 2006, LOW TEMPERATURE PHYSICS, PTS A AND B, 850, +, DOI:

We investigate theoretically in a tight-binding model the transport properties of the Aharonov-Bohm interferometer (ABI) with one dot embedded in each of its arms. For weak interdot coupling the model Hamiltonian describes the system considered in the experiments of Holleitner et al. [Phys. Rev. Lett. 87, 256802 (2001)]. The electronic transmittance of the interferometer is computed within the Landauer-Buttiker formalism while the coexistence of resonant and coherent transport is explicitly emphasized by using the Feschbach formula. The latter produces effective Hamiltonians whose spectral properties describe the tunneling processes through each dot. We reproduce numerically the stability charging diagrams reported in the experiments of Holleitner et al. When the magnetic flux is fixed and one dot is set to resonance the interferometer transmittance shows Fano, lineshapes as a function of the gate voltage applied to the other dot. Our model includes the effect of the magnetic field on the dot levels and explains the change of the asymmetric tail as the magnetic flux is varied. The transmittance assigned to the Fano, dips located in the almost crossing point of the charging diagrams shows Aharonov-Bohm oscillations.
45. Fano interference in mesoscopic rings with quantum dots
Authors: Moldoveanu, V; Aldea, A; Tolea, M

Published: 2006, PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 3, NO 2, 3, +, DOI: 10.1002/pssc.200562736

We propose a general formalism for describing the coexistence of coherent and resonant transport in hybrid mesoscopic structures. The approach is based on Landauer- Buttiker formula for the electronic transmittance and on an old formula of Feshbach. The latter gives the complete Green function of coupled subsystems in terms of effective Green functions of the disconnected parts and provides informations about the individual contributions of each subsystem to transport. Motivated by the experiments of Kobayashi et al.(Phys. Rev. Lett. 88, 256806 (2002)) and Holleitner et al. (Phys. Rev. Lett. 87, 256802 (2001)) we apply the formalism to study transport in Aharonov-Bohm interferometers (ABI) containing one or two coupled two-dimensional quantum dots (QD). In the single dot case, we reproduce and explain the magnetic field control of the Fano interference and investigate the interaction effects in a self-consistent approach. In the double dot case, we obtain the charging diagrams and establish precise criteria for the observation of mesoscopic Fano effect and AB oscillations. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
46. Criteria for the occurrence of negative resonant magnetoresistance in magnetic tunnel junctions
Authors: Tolea, M; Aldea, A; Tolea, F

Published: SEP 2006, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 243, R86, DOI: 10.1002/pssb.200642347

The dependence of the driven-current through a tunnel junction on the relative orientation of the magnetization in the electrodes deviates from the Julliere formula when the current is resonant through impurity levels. For asymmetric coupling of the impurities to the electrodes, the magnetoresistance can even be negative on the resonance. Such inversion in sign is due to the fact that not only the density of states in the electrodes is important (for both orientations of spins), but also the density of states at the impurity position, which has a specific behavior. We find the exact conditions under which the negative magnetoresistance occurs, as function of both polarization and coupling asymmetry. The resonant magnetoresistance may have single or double-dip aspect, depending on the mentioned parameters.
47. Interference in a system of coupled quantum wires: Theoretical study of electronic transport
Authors: Bulka, BR; Tolea, M; Dinu, IV

Published: NOV 2006, PHYSICAL REVIEW B, 74, DOI: 10.1103/PhysRevB.74.205301

Theoretical studies of the coherent electronic transport in a system of coupled quantum wires show that switching on the conducting channel in one wire can be manifested in the other coupled wires. Interference processes and electronic correlations are taken into account in our studies on the same footing. The conductance changes depend on the interference conditions of a transmitted wave with that one reflected from the wires that indirectly influence the transport. We show that electronic correlations lead to a dynamical Coulomb blockade effect, which changes the conductance response quantitatively but its shape is still kept the same. Our results are discussed in correspondence with an experiment recently performed by Morimoto [Appl. Phys. Lett. 82, 3952 (2003)] on a system of coupled quantum wires.
48. Tunnelling magnetoresistance of a T-shaped Fano dot
Authors: Tolea, M; Bulka, BR

Published: JAN 2006, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 243, 254, DOI: 10.1002/pssb.200562462

We show the role of the Fano interference on the magnetoresistance properties of a T-shaped double-dot placed between ferromagnetic electrodes. In the linear voltage regime, the magnetoresistance is a non-monotonic function of the gate voltage. We find high values of the magnetoresistance at the Fano dip and far from it. For resonant transmission, the magnetoresistance is lower and it can be negative for asymmetric couplings. Particularities of the system in nonequilibrium transport are also analysed. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
49. Fano regime of one-dot Aharonov-Bohm interferometers
Authors: Moldoveanu, V; Tolea, M; Gudmundsson, V; Manolescu, A

Published: AUG 2005, PHYSICAL REVIEW B, 72, DOI: 10.1103/PhysRevB.72.085338

We use the Landauer-Buttiker formalism to study the mesoscopic Fano effect in Aharonov-Bohm rings with an embedded two-dimensional noninteracting dot. The magnetic field dependence of the dot levels leads to a global shift of the Fano lines which becomes important for small ring/dot area ratios. As the magnetic field is varied the Fano dips move periodically from one side of the peak to the other, as reported by Kobayashi [Phys. Rev. Lett. 88, 256806 (2002)]. We show that this effect appears due to a specific magnetic control of the difference between the phase of the single nonresonant path via the free arm of the ring and the global phase of all trajectories involving resonant tunnelings through the dot.
50. Resonant and coherent transport through Aharonov-Bohm interferometers with coupled quantum dots
Authors: Moldoveanu, V; Tolea, M; Aldea, A; Tanatar, B

Published: MAR 2005, PHYSICAL REVIEW B, 71, DOI: 10.1103/PhysRevB.71.125338

A detailed description of the tunneling processes, within Aharonov-Bohm (AB) rings containing two-dimensional quantum dots is presented. We show that the electronic propagation through the interferometer is controlled by the spectral properties of the embedded dots and by their coupling with the ring. The transmittance of the interferometer is computed by the Landauer-Buttiker formula. Numerical results are presented for an AB interferometer containing two coupled dots. The charging diagrams for a double-dot interferometer and the Aharonov-Bohm oscillations are obtained, in agreement with the recent experimental results of Holleitner et al. [Phys. Rev. Lett. 87, 256802 (2001)] We identify conditions in which the system shows Fano line shapes. The direction of the asymetric tail depends on the capacitive coupling and on the magnetic field. We discuss our results in connection with the experiments of Kobayashi et al. [Phys. Rev. Lett. 88, 256806 (2002)] in the case of a single dot.
51. Spin magnetoconductance in the mesoscopic spin-interferometer
Authors: Aldea, A; Tolea, M; Zittartz, J

Published: JUL 2005, PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, 28, 202, DOI: 10.1016/j.physe.2005.03.013

We investigate the properties of the electron spin-transmission through an Aharonov-Bohm interferometer with an embedded two-dimensional (2D) multilevel quantum dot containing magnetic impurities. A suitable formalism is developed. The amplitude and the phase of the flip- and non-flip-transmittances are calculated numerically as function of the magnetic field. The effects induced by the exchange interaction and those arising from the 2D character of the dot are shown. (c) 2005 Elsevier B.V. All rights reserved.
52. Disorder effects on the properties of two-dimensional finite systems in perpendicular magnetic field: the tight binding approach
Authors: Aldea, A; Nita, M; Dinu, V; Tolea, M

Published: JUL 2004, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 241, 2096, DOI: 10.1002/pssb.200404781

We review some aspects concerning the disorder effects on the spectral properties of 2D electronic systems with relevance for the transport and orbital magnetism in perpendicular magnetic field. The lattice model and random Anderson disorder are used. The discussion is carried out separately for periodic and hard wall boundary conditions. The degree of localization is evidenced by the calculation of the inverse participation number of the eigenstates of the Hamiltonian and a striking asymmetry of the localization effect of states in the Landau band is reported. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
53. Martensitic Transformation and Magnetic Properties of Ni57Fe18Ga25 Shape Memory Alloy Subjected to Severe Plastic Deformation
Authors: Popescu, B; Gurau, C; Gurau, G; Tolea, M; Sofronie, M; Tolea, F

Published: , TRANSACTIONS OF THE INDIAN INSTITUTE OF METALS, DOI: 10.1007/s12666-021-02293-8

The effects of severe plastic deformation (SPD) process via high-speed high-pressure torsion technique on martensitic transformation of Ni-Fe-Ga Heusler shape memory alloy are the subject of this work. The results show that moderate degrees of deformation lead to a decrease in the martensitic transformation temperatures, while the heat of reaction is enhanced only for the sample processed with the lowest degree of deformation. The results are explained by the interplay between the constituent tetragonal L10 and the cubic gamma crystal structures and the evolution of the samples morphology with the severity of deformation. The reduction in the samples granulation due to the progressive increase in the SPD is reflected by the magnetic properties of the samples with decreasing coercivity and Curie temperatures. At the highest applied degree of deformation, sample nanostructuring and a possible amorphization might explain the vanishing of MT.
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