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Dr. Marian NITA

Scientific Researcher III

Laboratory of Theoretical Physics and Computational Modeling

nitza[AT]infim[DOT]ro

Education

-1997-2004 PhD Degree in Theoretical Physics at. University of Physics.

Thesis: “Magnetotransport in two-dimensional mesoscopic systems”. Supervisor: Prof. A. Aldea.

-1990-1995 Faculty of Physics, University of Bucharest.

Title of diploma: “Integer Quantum Hall Effect”. Supervisor: Prof. G. Nenciu.

Positions

Senior Researcher 3rd degree from 2001 until present at National Institute of Material Physics (NIMP)

Research interests

Quantum Hall Effect and topological invariants, models of mesoscopic systems, quantum transport in nano and meso systems, quantum physics and computation.

Expertize

Research in the field of theory of condensed matter physics.

Publications

1. A general algorithm for determining the conductivity zeros in large molecular nanostructures: applications to rectangular graphene sheets
Authors: Nita, M; Tolea, M; Marinescu, DC

Published: JUN 9 2025, JOURNAL OF PHYSICS-CONDENSED MATTER, 37, 235301, DOI: 10.1088/1361-648X/add77f

We propose an algorithm for determining the zeros of the electric conductivity in large molecular nanonstructures such as graphene sheets. To this end, we employ the inverse graph method, whereby non-zeros of the Green's functions are represented graphically by a segment connecting two atomic sites, to visually signal the existence of a conductance zero as a line that is missing. In rectangular graphene structures the topological properties of the inverse graph determine the existence of two types of Green's function zeros that correspond to absolute conductance cancellations with distinct behavior in the presence of external disorder. We discuss these findings and their potential applications in some particular cases.
Open Access
2. Physicochemical Characterization of Ca- and Cu-Decorated TiO2 Microparticles and Investigation of Their Antimicrobial Properties
Authors: Neacsu, A; Chihaia, V; Bucuresteanu, R; Ficai, A; Trusca, RD; Surdu, VA; Nicolaev, A; Cojocaru, B; Ionita, M; Calinescu, I; Parvulescu, V; Ditu, LM

Published: SEP 2024, MATERIALS, 17, 4483, DOI: 10.3390/ma17184483

Ca- and Cu-decorated TiO2 microparticles are titanium dioxide nanoparticles that have been decorated with calcium and copper ions. TiO2, CaO, and CuO are low-cost, non-toxic, and non-hazardous materials. The aim of the present study was the physicochemical characterization of Ca- and Cu-decorated TiO2 microparticles and the evaluation of their antimicrobial activity. Thus, Ca2+ and Cu2+ species were incorporated onto TiO2 surfaces by a two-step wet method. The obtained TiO2-CaO-CuO composites were characterized by several experimental techniques. The electronic structure and charge properties of the composites were investigated by density functional theory calculations. Furthermore, the composites were successfully tested for inhibitory effects on Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, and Candida albicans standard strains. The zeta potential data indicate that the physiological condition of investigated microbial strains was strongly affected in presence of a dispersion of 10 mu g/L of composites in a saline phosphate buffer also, the recorded SEM images show a damaged microbial cell surface in the presence of composites.
3. Memory of incomplete phase transitions from a random squares model
Authors: Tolea, F; Sofronie, M; Nita, M; Tolea, M

Published: DEC 26 2023, PHYSICAL REVIEW E, 108, 064134, DOI: 10.1103/PhysRevE.108.064134

We present a simple two-dimensional model for a phase transition, then study its predictions, in particular the memory properties. The direct transformation is modeled by randomly placing small squares, "nuclei", on an initially empty surface. Then, the nuclei expand ("grow") up to finite final sizes which are randomly chosen in a given range, while keeping their square shape. An important issue is the "interaction" which forces some squares to remain at smaller sizes if the surrounding squares get in the way of their growth. Interestingly, this naturally leads to quasiequal total area covered by the squares of each size after a complete direct transformation. Next, it is shown that the system "remembers" incomplete ("arrested") reverse transformations taking place in reversed order of the squares sizes. The memory is "encrypted" in the distribution of the squares sizes after a next direct transformation and manifests as a significant imbalance between the areas covered by the "big" and "small" (relative to the arrest size) squares. We are able to also reproduce the so-called "hammer effect" and the memorizing of multiple arrest points. Our model is particularly relevant for the thermal memory effect in shape memory alloys, and we actually borrowed many features from existing thermodynamic models addressing this effect. However, here we eliminate the explicit thermodynamics and end up with a statistical geometry model, presumably easier to reproduce.
4. Molecular OR and AND logic gates: A theoretical proposal
Authors: Nita, M; Tolea, M; Marinescu, DC

Published: DEC 11 2023, PHYSICAL REVIEW B, 108, 235307, DOI: 10.1103/PhysRevB.108.235307

A conductance zero that results from the destructive quantum interference of the electron states in quantum transport between two given sites of a molecular system persists or disappears depending on the location of an externally applied perturbation. The a priori knowledge of the perturbation site that destroys or preserves a zero is the basis of an algorithm that outlines the creation of logic gates having external perturbations as inputs and a given conductance as output. Using a graph of the possible conductance paths between the various sites, we showcase the several different scenarios that correspond to AND/OR/XOR logical functions for a given set of contacts. This setup is shown to be independent of the strength of the coupling to the leads and magnitude of the perturbation. We illustrate this approach in the case of bipartite and nonbipartite single carbon cycle molecules (fulvene and benzene) and double carbon cycle molecules (naphthalene and biphenyl).
5. Persistent destructive quantum interference in the inverted graph method
Authors: Nita, M; Marinescu, DC

Published: APR 8 2022, PHYSICAL REVIEW B, 105, 155303, DOI: 10.1103/PhysRevB.105.155303

We formulate a general criterion that underlies the persistence of conductance zeros induced by destructive quantum interference under the application of external perturbations in physical systems described by a discrete nonsingular Hamiltonian H. Our approach uses nonzero matrix elements of H???1 between two lattice points as edges of a graph to indicate the existence of a nonzero conductance between the same points. A given conductance zero, or a missing edge in the inverted graph, is preserved when the perturbation, in the form of on-site or additional interpoint hopping energies, is applied only to points outside the set of first-order graph neighbors of either the entry lead or the exit lead. We discuss the application of these results to a study of the robustness of the conductance zeros in the fulvene and benzene molecules.
Open Access
6. Antimicrobial Properties of TiO2 Microparticles Coated with Ca- and Cu-Based Composite Layers
Authors: Bucuresteanu, R; Ionita, M; Chihaia, V; Ficai, A; Trusca, RD; Ilie, CI; Kuncser, A; Holban, AM; Mihaescu, G; Petcu, G; Nicolaev, A; Costescu, RM; Husch, M; Parvulescu, V; Ditu, LM

Published: JUL 2022, INTERNATIONAL JOURNAL OF MOLECULAR SCIENCES, 23, 6888, DOI: 10.3390/ijms23136888

The ability of TiO2 to generate reactive oxygen species under UV radiation makes it an efficient candidate in antimicrobial studies. In this context, the preparation of TiO2 microparticles coated with Ca- and Cu-based composite layers over which Cu(II), Cu(I), and Cu(0) species were identified is presented here. The obtained materials were characterized by a wide range of analytical methods, such as X-ray diffraction, electron microscopy (TEM, SEM), X-ray photoelectron (XPS), and UV-VIS spectroscopy. The antimicrobial efficiency was evaluated using qualitative and quantitative standard methods and standard clinical microbial strains. A significant aspect of this composite is that the antimicrobial properties were evidenced both in the presence and absence of the light, as result of competition between photo and electrical effects. However, the antibacterial effect was similar in darkness and light for all samples. Because no photocatalytic properties were found in the absence of copper, the results sustain the antibacterial effect of the electric field (generated by the electrostatic potential of the composite layer) both under the dark and in light conditions. In this way, the composite layers supported on the TiO2 microparticles' surface can offer continuous antibacterial protection and do not require the presence of a permanent light source for activation. However, the antimicrobial effect in the dark is more significant and is considered to be the result of the electric field effect generated on the composite layer.
Open Access
7. Preliminary Study on Light-Activated Antimicrobial Agents as Photocatalytic Method for Protection of Surfaces with Increased Risk of Infections
Authors: Bucuresteanu, R; Ditu, LM; Ionita, M; Calinescu, I; Raditoiu, V; Cojocaru, B; Cinteza, LO; Curutiu, C; Holban, AM; Enachescu, M; Enache, LB; Mustatea, G; Chihaia, V; Nicolaev, A; Borcan, EL; Mihaescu, G

Published: SEP 2021, MATERIALS, 14, 5307, DOI: 10.3390/ma14185307

Preventing and controlling the spread of multidrug-resistant (MDR) bacteria implicated in healthcare-associated infections is the greatest challenge of the health systems. In recent decades, research has shown the need for passive antibacterial protection of surfaces in order to reduce the microbial load and microbial biofilm development, frequently associated with transmission of infections. The aim of the present study is to analyze the efficiency of photocatalytic antimicrobial protection methods of surfaces using the new photocatalytic paint activated by light in the visible spectrum. The new composition is characterized by a wide range of analytical methods, such as UV-VIS spectroscopy, electron microscopy (SEM), X-ray powder diffraction (PXRD) or X-ray photoelectron spectroscopy (XPS). The photocatalytic activity in the UV-A was compared with the one in the visible light spectrum using an internal method developed on the basis of DIN 52980: 2008-10 standard and ISO 10678-2010 standard. Migration of metal ions in the composition was tested based on SR EN1186-3: 2003 standard. The new photocatalytic antimicrobial method uses a type of photocatalytic paint that is active in the visible spectral range and generates reactive oxygen species with inhibitory effect against all tested microbial strains.
Open Access
8. 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, 125307, 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.
Open Access
9. 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, 235318, 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.
10. Deposition temperature influence on the wear behaviour of carbon-based coatings deposited on hardened steel
Authors: Feldiorean, D; Cristea, D; Tierean, M; Croitoru, C; Gabor, C; Jakab-Farkas, L; Cunha, L; Barradas, NP; Alves, E; Craciun, V; Marin, A; Moura, C; Leme, J; Socol, M; Craciun, D; Cosnita, M; Munteanu, D

Published: MAY 1 2019, APPLIED SURFACE SCIENCE, 475, 773, DOI: 10.1016/j.apsusc.2019.01.028

This paper presents an evaluation regarding the influence of substrate material characteristics and deposition parameters on the tribological behaviour of carbon-based coatings. Chromium nitride ceramic interlayers and carbon-based thin films were deposited by magnetron sputtering on hardened AISI 5115 (16MnCr5) case hardening steel. The physical vapour deposition (PVD) deposition was performed at three different temperatures: 180 degrees C, 200 degrees C and 250 degrees C. The chemical composition of the samples was assessed by Rutherford Backscattering Spectroscopy (RBS), the structure by X-ray Diffraction (XRD), and the surface morphology by Atomic Force Microscopy (AFM). The surface chemistry was analysed by X-ray Photoelectron Spectroscopy (XPS) and Raman Spectroscopy. The coatings are homogeneous, amorphous, with a smooth surface. The mechanical behaviour has been assessed on a pin-on disk rotational tribometer (wear characteristics), on a micro scratch tester (adhesion to the substrate), by ball-cratering (film thickness) and by nanoindentation (hardness and the modulus of elasticity). A strong correlation has been observed between the substrate characteristics and, more importantly, the deposition temperature, and the mechanical properties of the assembly. The fracture toughness is positively influenced by the presence of the ceramic chromium nitride interlayer. The modulus of elasticity and friction coefficient (both in dry and lubricated conditions) are decreased for higher deposition temperatures, however the higher deposition temperature negatively affects the mechanical characteristics of the steel substrate.
11. Internal and external surface features of newly developed porous ceramics with random interconnected 3D channels by a fibrous sacrificial porogen method
Authors: Mocanu, AC; Miculescu, M; Machedon-Pisu, T; Maidaniuc, A; Ciocoiu, RC; Ionita, M; Pasuk, J; Stan, GE; Miculescu, F

Published: SEP 30 2019, APPLIED SURFACE SCIENCE, 489, 238, DOI: 10.1016/j.apsusc.2019.05.354

The bone remodeling research field has shifted focus towards sustainable, eco-friendly and reproducible manufacturing technologies of 3D structures. It is now accepted that a suitable internal architecture and an active interface between the 3D structure and host bone-tissue constitute the two most critical traits for a successful bone tissue engineering application. A completely reproducible synthesis set-up was recently developed for calcium phosphate (CaP) bioceramics preparation from natural highly available marble and seashells. The influence of the pressing force in the fabrication process of porous 3D scaffolds derived from such CaPs by a sacrificial porogen method using natural fibers is here investigated. The fiber-ceramic based-products underwent thermal processing, followed by surface and volume features characterization. After fibers' thermal removal, interconnected 3D channels were obtained, which could allow a suitable in vivo irrigation and implant-associated negative side-effects prevention. This method provides the prospect of tunable HA/beta-TCP content in the case of both precursors' derived-scaffolds. The morphological results revealed the internal and external pores dimensions, modulated through different pressing forces that led to a controlled total porosity, evidenced by computed tomography techniques. Further, the wettability and mechanical features supported the advance of the novel porous-ceramic-structure designs as reliable bone reconstruction alternatives.
12. Edge-state mechanism for the anomalous quantum Hall effect in a diatomic square lattice
Authors: Ostahie, B; Nita, M; Aldea, A

Published: SEP 4 2018, PHYSICAL REVIEW B, 98, DOI: 10.1103/PhysRevB.98.125403

We investigate the spectral and transport properties of a diatomic square lattice with hopping to the next-nearest-neighbors and broken time-reversal symmetry, which behaves as a Chern insulator. In a finite-size approach, the attention is paid to the formation of chiral edge states in the topological insulating phase, but also in the semimetallic one. The edge states are revealed in the ribbon and plaquette geometries by analytical and numerical methods, significant differences being produced by the specific atomic connectivity at the boundary. The Hall resistance R-H is calculated in the plaquette geometry using the Landauer-Mittiker approach. The chiral edge states located in the unique gap of the energy spectrum manifest themselves by quantized values R-H = +/- h/e(2) specific to the Chern insulator. The semimetallic system containing chiral edge states embedded in the quasicontinuum of bulk states shows a disorder-driven AQHE as a consequence of the Anderson localization process.
13. 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.
14. 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.
15. Aging phenomena and wettability control of plasma deposited carbon nanowall layers
Authors: Vizireanu, S; Ionita, MD; Ionita, ER; Stoica, SD; Teodorescu, CM; Husanu, MA; Apostol, NG; Baibarac, M; Panaitescu, D; Dinescu, G

Published: NOV 2017, PLASMA PROCESSES AND POLYMERS, 14, DOI: 10.1002/ppap.201700023

This paper reports on the aging of carbon nanowalls (CNW) and modification of their wettability by the storage time, growth conditions, and post-fabrication plasma treatments. The as-deposited CNW initially exhibit marked hydrophilic behavior (fresh CNW), but within a few days they become highly hydrophobic (aged CNW). Their final hydrophobicity is closely related to their topography which is controlled by the deposition parameters. In addition, subsequent fluorinated plasma treatments result in super-hydrophobic CNW layers, irrespective of the hydrophilic or hydrophobic character of the pre-treated samples. To explain this, we show that the CNW edges contain many defects initially, but such defects become passivated in time. As a result, the surfaces become highly hydrophobic after aging or fluorination, having inert stable terminations.
16. 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.
17. 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.
18. Functionalization of carbon nanowalls by plasma jet in liquid treatment
Authors: Ionita, MD; Vizireanu, S; Stoica, SD; Ionita, M; Pandele, AM; Cucu, A; Stamatin, I; Nistor, LC; Dinescu, G

Published: FEB 9 2016, EUROPEAN PHYSICAL JOURNAL D, 70, DOI: 10.1140/epjd/e2016-60499-8

Submerged in liquid plasma treatment is a new approach for nanomaterials functionalization. This paper presents a surfactant free method for functionalization of graphene nano-platelets derived from carbon nanowalls through plasma jet treatment of their water suspensions. The untreated and under-liquid plasma treated suspensions were characterized in terms of their UV-Vis absorption, zeta-size, zeta-potential, pH, and conductivity. Investigation of dried material revealed that the graphene nano-sheets morphology and structure have been preserved, showing also new oxygen functional groups bonded to the carbon network after in liquid plasma treatment. The results demonstrate the efficiency of this technique in changing the properties of carbon nanowalls suspensions and also in getting functionalized multilayered graphene sheets.
19. Non-Hermitian approach of edge states and quantum transport in a magnetic field
Authors: Ostahie, B; Nita, M; Aldea, A

Published: NOV 21 2016, PHYSICAL REVIEW B, 94, DOI: 10.1103/PhysRevB.94.195431

We develop a manifest non-Hermitian approach of spectral and transport properties of two-dimensional mesoscopic systems in a strong magnetic field. The finite system to which several terminals are attached constitutes an open system that can be described by an effective Hamiltonian. The lifetime of the quantum states expressed by the energy imaginary part depends specifically on the lead-system coupling and makes the difference among three regimes: resonant, integer quantum Hall effect, and superradiant. The discussion is carried on in terms of edge state lifetime in different gaps, channel formation, role of hybridization, and transmission coefficients quantization. A toy model helps in understanding non-Hermitian aspects in open systems.
20. Electrical manipulation of edge states in graphene and the effect on quantum Hall transport
Authors: Ostahie, B; Nita, M; Aldea, A

Published: APR 9 2015, PHYSICAL REVIEW B, 91, DOI: 10.1103/PhysRevB.91.155409

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.
21. 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)
22. Topological properties of the mesoscopic graphene plaquette: Quantum spin Hall effect due to spin imbalance
Authors: Ostahie, B; Nita, M; Aldea, A

Published: APR 15 2014, PHYSICAL REVIEW B, 89, DOI: 10.1103/PhysRevB.89.165412

We study the electronic properties of the confined honeycomb lattice in the presence of the intrinsic spin-orbit (ISO) interaction and perpendicular magnetic field, and report on uncommon aspects of the quantum spin Hall conductance corroborated by peculiar properties of the edge states. The ISO interaction induces two specific gaps in the Hofstadter spectrum, namely the "weak" topological gap defined by Beugeling et al. [Phys. Rev. B 86, 075118 (2012)], and spin-imbalanced gaps in the relativistic range of the energy spectrum. We analyze the evolution of the helical states with the magnetic field and with increasing Anderson disorder. The "edge" localization of the spin-dependent states and its dependence on the disorder strength is shown. The quantum transport, treated in the Landauer-Buttiker formalism, reveals interesting new plateaus of the quantum spin Hall effect (QSHE), and also of the integer quantum Hall effect (IQHE), in the energy ranges corresponding to the spin-imbalanced gaps. The properties of the spin-dependent transmittance matrix that determine the symmetries with respect to the spin, energy, and magnetic field of the longitudinal and transverse resistance are shown.
23. Spectral and transport properties of the two-dimensional Lieb lattice
Authors: Nita, M; Ostahie, B; Aldea, A

Published: MAR 29 2013, PHYSICAL REVIEW B, 87, DOI: 10.1103/PhysRevB.87.125428

The specific topology of the line-centered square lattice (known also as the Lieb lattice) induces remarkable spectral properties such as the macroscopically degenerated zero-energy flat band, the Dirac cone in the low-energy spectrum, and the peculiar Hofstadter-type spectrum in a magnetic field. We study here the properties of the finite Lieb lattice with periodic and vanishing boundary conditions. We investigate the behavior of the flat band induced by disorder and external magnetic and electric fields. We show that in the confined Lieb plaquette threaded by a perpendicular magnetic flux there are edge states with nontrivial behavior. The specific class of twisted edge states, which have alternating chirality, are sensitive to disorder and do not support integer quantum Hall effect (IQHE), but contribute to the longitudinal resistance. The symmetry of the transmittance matrix in the energy range where these states are located is revealed. The diamagnetic moments of the bulk and edge states in the Dirac-Landau domain, and also of the flat states in crossed magnetic and electric fields are shown. DOI: 10.1103/PhysRevB.87.125428
24. Persistent oscillatory currents in a 1D ring with Rashba and Dresselhaus spin-orbit interactions excited by a terahertz pulse
Authors: Nita, M; Marinescu, DC; Manolescu, A; Ostahie, B; Gudmundsson, V

Published: SEP 2012, PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, 46, 20, DOI: 10.1016/j.physe.2012.08.017

Persistent, oscillatory charge and spin currents are shown to be driven by a two-component terahertz laser pulse in a one-dimensional mesoscopic ring with Rashba and Dresselhaus spin-orbit interactions (SOI) linear in the electron momentum. The characteristic interference effects result from the opposite precession directions imposed on the electron spin by the two SOI couplings. The time dependence of the currents is obtained by solving numerically the equation of motion for the density operator, which is later employed in calculating statistical averages of quantum operators on few electron eigenstates. The parameterization of the problem is done in terms of the SOI coupling constants and of the phase difference between the two laser components. Our results indicate that the amplitude of the oscillations is controlled by the relative strength of the two SOI's, while their frequency is determined by the difference between the excitation energies of the electron states. Furthermore, the oscillations of the spin current acquire a beating pattern of higher frequency that we associate with the nutation of the electron spin between the quantization axes of the two SOI couplings. This phenomenon disappears at equal SOI strengths, whereby the opposite precessions occur with the same probability. (c) 2012 Elsevier B.V. All rights reserved.
25. 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.
26. Nonadiabatic generation of spin currents in a quantum ring with Rashba and Dresselhaus spin-orbit interactions
Authors: Nita, M; Marinescu, DC; Ostahie, B; Manolescu, A; Gudmundsson, V

Published: 2012, ADVANCED MANY-BODY AND STATISTICAL METHODS IN MESOSCOPIC SYSTEMS, 338, DOI: 10.1088/1742-6596/338/1/012013

When subjected to a linearly polarized terahertz pulse, a mesoscopic ring endowed with spin-orbit interaction (SOT) of the Rashba-Dresselhaus type exhibits non-uniform azimuthal charge and spin distributions. Both types of SOT couplings are considered linear in the electron momentum. Our results are obtained within a formalism based on the equation of motion satisfied by the density operator which is solved numerically for different values of the angle 0, the angle determining the polarization direction of the laser pulse. Solutions thus obtained are later employed in determining the time-dependent charge and spin currents, whose values are calculated in the stationary limit. Both these currents exhibit an oscillatory behavior complicated in the case of the spin current by a beating pattern. We explain this occurrence on account of the two spin-orbit interactions which force the electron spin to oscillate between the two spin quantization axes corresponding to Rashba and Dresselhaus interactions. The oscillation frequencies are explained using the single particle spectrum.
27. Post-synthesis Carbon Nanowalls Transformation under Hydrogen, Oxygen, Nitrogen, Tetrafluoroethane and Sulfur Hexafluoride Plasma Treatments
Authors: Vizireanu, S; Ionita, MD; Dinescu, G; Enculescu, I; Baibarac, M; Baltog, I

Published: APR 2012, PLASMA PROCESSES AND POLYMERS, 9, 370, DOI: 10.1002/ppap.201100153

Carbon nanowalls (CNWs) functionalization allows the tailoring of some of their properties after they have been synthesized. Herewith we report on post-synthesis CNWs functionalization by plasma treatments. Radiofrequency plasmas generated in hydrogen, oxygen, nitrogen, tetrafluoroethane, and sulfur hexafluoride admixed to argon, were used. The changes of morphology, structure, chemical composition were determined by SEM, FTIR, Raman Spectroscopy, specifically for each gas mixture. The results point out to the surface erosion, the functional groups attachment, or to thin film coverage of CNWs material. Such processes strongly influence the material properties, as exemplified by the wettability: superhydrophilic or superhydrophobic surfaces are obtained starting from the same base CNWs material.
28. Nonadiabatic generation of a pure spin current in a one-dimensional quantum ring with spin-orbit interaction
Authors: Nita, M; Marinescu, DC; Manolescu, A; Gudmundsson, V

Published: APR 18 2011, PHYSICAL REVIEW B, 83, DOI: 10.1103/PhysRevB.83.155427

We demonstrate the theoretical possibility of obtaining a pure spin current in a 1D ring with spin-orbit interaction by irradiation with a nonadiabatic, two-component terahertz laser pulse, whose spatial asymmetry is reflected by an internal phase difference phi. The solutions of the equation of motion for the density operator are obtained for a spin-orbit coupling linear in the electron momentum (Rashba) and they are used to calculate the time-dependent charge and spin currents. We find that there are critical values of phi at which the charge current disappears, while the spin current reaches a maximum or a minimum value.
29. 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.
30. Optica and structural properties of polythiophene-like films deposited by plasma polymerization
Authors: Galca, AC; Satulu, V; Ionita, MD; Bercu, M; Barna, E; Dumitru, M; Mitu, B; Dinescu, G

Published: AUG 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 2037, DOI:

Polymer-like organic thin films obtained by plasma techniques are of wide interest for various applications. Polythiophene-like thin films were deposited by plasma polymerization in a parallel plate radiofrequency reactor starting from thiophene vapors. The optical properties of the films were obtained from spectroscopic ellipsometry, in the 250-1250 nm range. The calculated band gap is in agreement with a polythiophene-like material consisting of cross-linking of oligomers. The presence of functional groups and film stability against oxidation is analyzed by FTIR spectrometry. The monitoring of functional groups incorporation during the film deposition was done by analyzing the integral intensities of specific absorption bands and led to the conclusion that homogeneous films are obtained whatever the process duration.
31. Materials processing with radiofrequency plasmas at low and atmospheric pressure
Authors: Dinescu, G; Mitu, B; Vizireanu, S; Ionita, ER; Luciu, I; Ionita, MD; Stancu, C; Stancu, CE; Acsente, T; Nistor, L; Kravets, L

Published: 2008, ROMANIAN REPORTS IN PHYSICS, 60, 690, DOI:

The paper presents a few examples relevant for the application of radiofrequency plasmas for processing of materials and surfaces. The examples have been chosen in order to match the trends in plasma processing science, namely processing at atmospheric pressure, growth of nanostructured materials and design of filtration systems responsive to external stimuli.
32. Asymmetric localization in disordered Landau bands
Authors: Nita, M; Aldea, A; Zittartz, J

Published: JUN 6 2007, JOURNAL OF PHYSICS-CONDENSED MATTER, 19, DOI: 10.1088/0953-8984/19/22/226217

We show that, due to band mixing, the eigenstate localization within the disordered Landau bands gets an asymmetric structure: the degree of localization increases in the lower part of the band and decreases in the upper one. The calculation is performed for a two-dimensional lattice with the Anderson disorder potential and we prove that this effect is related to the upper shift of the extended states within the band and is enhanced by the disorder strength. The asymmetric localization and the energy shift disappear when the interband coupling is switched off.
33. Mean-field theory for Bose-Hubbard model under a magnetic field
Authors: Oktel, MO; Nita, M; Tanatar, B

Published: JAN 2007, PHYSICAL REVIEW B, 75, DOI: 10.1103/PhysRevB.75.045133

We consider the superfluid-insulator transition for cold bosons under an effective magnetic field. We investigate how the applied magnetic field affects the Mott transition within mean-field theory and find that the critical hopping strength (t/U)(c) increases with the applied field. The increase in the critical hopping follows the bandwidth of the Hofstadter butterfly at the given value of the magnetic field. We also calculate the magnetization and superfluid density within mean-field theory.
34. Net current generation in a 1D quantum ring at zero magnetic field
Authors: Gylfadottir, SS; Nita, M; Gudmundsson, V; Manolescu, A

Published: MAR 2005, PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, 27, 283, DOI: 10.1016/j.physe.2004.12.002

We study a non-adiabatic excitation of an electron system in a 1D quantum ring radiated by a short terahertz pulse. The response of two models, a continuous and a discrete, is explored. By introducing a spatial asymmetry in the external perturbation, a net current can be generated in the ring at a zero magnetic field. Effect of impurities and ratchets are investigated in combination with symmetric and asymmetric external excitation. (c) 2005 Elsevier B.V. All rights reserved.
35. 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.
36. Spin magnetization of a strongly correlated electron gas confined in a two-dimensional finite lattice
Authors: Nita, M; Dinu, V; Aldea, A; Tanatar, B

Published: FEB 2004, PHYSICAL REVIEW B, 69, DOI: 10.1103/PhysRevB.69.073106

The influence of disorder and interaction effects on the ground state polarization of the two-dimensional correlated electron gas is studied by numerical investigations of the unrestricted Hartree-Fock approach. With the model of Anderson disorder a continuous increase of the spin magnetization until the fully polarized regime is obtained. The ferromagnetic ground state is found to be favorable when the electron number is lowered and the interaction and disorder parameters are suitably chosen.
37. Orbital magnetization of single and double quantum dots in a tight-binding model
Authors: Aldea, A; Moldoveanu, V; Nita, M; Manolescu, A; Gudmundsson, V; Tanatar, B

Published: JAN 15 2003, PHYSICAL REVIEW B, 67, DOI: 10.1103/PhysRevB.67.035324

We calculate the orbital magnetization of single and double quantum dots coupled both by Coulomb interaction and by electron tunneling. The electronic states of the quantum dots are calculated in a tight-binding model, and the magnetization is discussed in relation to the energy spectrum and to the edge and bulk states. We identify effects of chirality of the electronic orbits and of the anticrossing of the energy levels when the magnetic field is varied. We also consider the effects of detuning the energy spectra of the quantum dots by an external gate potential. We compare our results with the recent experiments of Oosterkamp [Phys. Rev. Lett. 80, 4951 (1998)].
38. Coulomb effects on the transport properties of quantum dots in a strong magnetic field
Authors: Moldoveanu, V; Aldea, A; Manolescu, A; Nita, M

Published: JAN 15 2001, PHYSICAL REVIEW B, 63, DOI: 10.1103/PhysRevB.63.045301

We investigate the transport properties of quantum dots placed in a strong magnetic field using a quantum-mechanical approach based on the two-dimensional tight-binding Hamiltonian with direct Coulomb interaction and the Landauer-Buttiker formalism. The electronic transmittance and the Hall resistance show Coulomb oscillations and also prove multiple addition processes. We identify this feature as the "bunching" of electrons observed in recent experiments and give an elementary explanation in terms of spectral characteristics of the dot. The spatial distribution of the added electrons may distinguish between the edge and bulk states and it has specific features for bunched electrons. The dependence of the charging energy on the number of electrons is discussed for a strong magnetic field. The crossover from the tunneling to quantum Hall regime is analyzed in terms of dot-lead coupling.
39. Magnetization and level statistics at the quantum Hall liquid-insulator transition in the lattice model
Authors: Nita, M; Aldea, A; Zittartz, J

Published: DEC 15 2000, PHYSICAL REVIEW B, 62, 15370, DOI: 10.1103/PhysRevB.62.15367

Statistics of level spacing and magnetization are studied for the phase diagram of the integer quantum Hall effect in a two-dimensional finite lattice model with Anderson disorder.
40. Quantum analog of channeled electron trajectories in periodic magnetic and electric fields
Authors: Manolescu, A; Zwerschke, SDM; Nita, M; Gossmann, UJ; Gerhardts, RR

Published: DEC 1998, PHYSICA B-CONDENSED MATTER, 256, 379, DOI: 10.1016/S0921-4526(98)00564-X

We calculate the quantum states corresponding to the drifting and channeled classical orbits in a two-dimensional electron gas (2DEG) with strong magnetic and electric modulations along one spatial direction, x. The channeled states carry high, concentrated currents along the y-axis, and are confined in an effective potential well. The quantum and the classical states are compared. (C) 1998 Elsevier Science B.V. All rights reserved.
41. QHE, magnetoresistance and disordered transport on 2D mesoscopic plaquettes
Authors: Aldea, A; Gartner, P; Nita, M

Published: JAN 1998, PHYSICA STATUS SOLIDI B-BASIC RESEARCH, 205, 356, DOI: 10.1002/(SICI)1521-3951(199801)205:1<351::AID-PSSB351>3.3.CO;2-Z

The transport properties of a rectangular mesoscopic plaquette in the presence of a perpendicular magnetic field are studied in a tight-binding model with randomly distributed traps. The longitudinal and Hall resistances are calculated in the four-probe Landauer-Buttiker formalism which accounts automatically both for the quantum coherence and the trapping-induced localization. The localized character of eigenvectors and the specific aspect of the density of states at a given magnetic flux are correlated with the behaviour of the mentioned resistances as function of the Fermi energy. The Hall insulator and quantum Hall regimes are evidenced. The magnetic field dependence of the configurational averages of the longitudinal and Hal; resistance is studied in a purely quantum-mechanical approach. Both, negative and positive magnetoresistances are found.
42. Specific plateaus of the quantum Hall effect induced by an applied bias
Authors: Aldea, A; Gartner, P; Manolescu, A; Nita, M

Published: MAY 15 1997, PHYSICAL REVIEW B, 55, 13392, DOI: 10.1103/PhysRevB.55.R13389

The spectrum and the eigenstates of a finite two-dimensional tight-binding electronic system, with Dirichlet boundary conditions, in a magnetic field and with an external linear potential are studied. The eigenstates show an equipotential character, and may cross the plaquette in the direction perpendicular to the electric field. When leads are added to the plaquette, the channels carrying the current may be shortcut by equipotentials, resulting in additional plateaus situated between the usual integer-quantum-Hall-effect plateaus. This ideals confirmed by a numerical calculation within the four-terminal Landauer-Buttiker approach.
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