National Institute Of Materials Physics - Romania

We investigate the properties of Dirac electrons in a finite graphene sample under a perpendicular magnetic field that emerge when an in-plane electric bias is also applied. The numerical analysis of the Hofstadter spectrum and of the edge-type wave functions evidence the presence of shortcut edge states that appear under the influence of the electric field. The states are characterized by a specific spatial distribution, which follows only partially the perimeter, and exhibit ridges that connect opposite sides of the graphene plaquette. Two kinds of such states have been found in different regions of the spectrum, and their particular spatial localization is shown along with the diamagnetic moments that reveal their chirality. By simulating a four-lead Hall device, we investigate the transport properties and observe unconventional plateaus of the integer quantum Hall effect that are associated with the presence of the shortcut edge states. We show the contributions of the novel states to the conductance matrix that determine the new transport properties. The shortcut edge states resulting from the splitting of the n = 0 Landau level represent a special case, giving rise to nontrivial transverse and longitudinal resistance.

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.

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.

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.

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.

We study the generation of p-shell excitons in optically active disk-shaped quantum dots subjected to ultrafast optical pulses. The single-particle spectral properties are obtained from the four-band kp theory, whereas the Coulomb interaction is taken into account within the configuration-interaction approach, particular attention being payed to configuration mixing due to electron-hole correlations. The effect of intraband relaxation processes is included in the non-unitary dynamics derived from the von-Neumann Lindblad equation. We find that the fast hole relaxation processes drive the p-shell excitons to intermediate states which eventually evolve to s-shell excitons via slower electron relaxation.

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.

We investigate the equilibrium and nonequilibrium transport through a quantum dot in the Kondo regime, embedded between a normal metal and a topological superconductor supportingMajorana bound states at its end points. We find that the Kondo physics is significantly modified by the presence of theMajorana modes. When the Majorana modes are coupled, aside from the Kondo scale T-K, a new energy scale T* << T-K emerges, that controls the low-energy physics of the system. At lowtemperatures, the ac conductance is suppressed for frequencies below T*, while the noise spectrum acquires a similar to omega(3) dependence. At high temperatures, T >> T-K, the regular logarithmic dependence in the differential conductance is also affected. Under nonequilibrium conditions, and in particular in the {T, B}-> 0 limit, the differential conductance becomes negative. These findings indicate that the changes in transport may serve as clues for detecting the Majorana bound states in such systems. In terms of methods used, we characterize the transport by using a combination of perturbative and renormalization-group approaches.

We address the existence of steady state Green-Keldysh correlation functions of interacting fermions in mesoscopic systems for both the partitioning and partition-free scenarios. Under some spectral assumptions on the non-interacting model and for sufficiently small interaction strength, we show that the system evolves to a NESS which does not depend on the profile of the time-dependent coupling strength/bias. For the partitioned setting we also show that the steady state is independent of the initial state of the inner sample. Closed formulae for the NESS two-point correlation functions (Green-Keldysh functions), in the form of a convergent expansion, are derived. In the partitioning approach, we show that the 0(th) order term in the interaction strength of the charge current leads to the Landauer-Buttiker formula, while the 1(st) order correction contains the mean-field (Hartree-Fock) results.

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)