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M. Nita

Publications and source records attributed to M. Nita.

17 recordsLinked to original sources
arXiv2021

Conductance zeros in complex molecules and lattices from the interference set method

Destructive quantum interference (DQI) and its effects on electron transport is 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 of 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 in respect with external perturbations.

cond-mat.mes-hall↗
arXiv2020

Robust conductance zeroes in graphene quantum dots and other bipartite systems

Within the Landauer transport formalism we demonstrate that conductance zeroes 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.

cond-mat.mes-hall↗
arXiv2018

Edge states mechanism for the anomalous quantum Hall effect in diatomic square lattice

The understanding of the Chern insulator and anomalous quantum Hall effect (AQHE) in terms of chiral edge states in confined systems is the first aim of the paper. The model we use consists in a diatomic square lattice with hopping to the next-nearest-neighbors and broken time-reversal symmetry, which exhibits edge states in the absence of an external magnetic field. The question of chiral edge states is approached in the ribbon and plaquette geometries with different atomic connectivities at the boundaries. Insulating and semi-metallic phases are revealed, the resulting phase diagram being richer than in Haldane's model. The transmission coefficients and the Hall resistance $R_H$ are calculated for the finite size system in the Landauer-Büttiker formalism. The quantized values $R_H=\pm h/e^2$, specific to the Chern insulator, are manifest in the energy range occupied by the chiral edge states, corresponding to the unique gap existing in the energy spectrum. Our second aim is to examine the disorder-induced properties of the model, and, as a novelty, we prove the disorder-driven AQHE in the semi-metallic phase.

cond-mat.mes-hall↗
arXiv2017

Hund and anti-Hund rules in circular molecules

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 {\it always} defines the ground state in a molecule with $4N$ atoms at half-filling. In all other cases (i.e. number of atoms {\it not} 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 {\it 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.

cond-mat.mes-hall↗
arXiv2016

Non-Hermitian approach of edge states and quantum transport in a magnetic field

We develop a manifest non-Hermitian approach of spectral and transport properties of two- dimensional mesoscopic systems in strong magnetic field. The finite system to which several ter- minals are attached constitutes an open system that can be described by an effective Hamiltonian. The life time 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 quan- tum Hall effect and superradiant. The discussion is carried on in terms of edge state life time in different gaps, channel formation, role of hybridization, transmission coefficients quantization. A toy model helps in understanding non-Hermitian aspects in open systems.

cond-mat.mes-hall↗
arXiv2016

Ground state spin and excitation energies in half-filled Lieb lattices

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 nano-magnetism of discrete systems such as quantum dot arrays, due to the degenerate levels at mid-spectrum. 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- make the spin state not a priori known, 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 order of the interaction potential for the smaller lattices. The analytical results concorde 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.

cond-mat.mes-hall↗
arXiv2015

Electrical manipulation of the edge states in graphene and the effect on the quantum Hall transport

We investigate new properties of the Dirac electrons in the finite graphene sample under 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 evidentiate 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 shortcut opposite sides of the graphene plaquette. Two kinds of such states have been found in different regions of the spectrum, their particular spatial localization being shown along with the diamagnetic moments that reveal their chirality. By simulating a four-lead Hall device, we investigate the transport properties and observe new, unconventional plateaus of the integer quantum Hall effect, which are associated with the presence of the shortcut edge states. The contributions of the novel states to the transmittance matrix that determine the new transport properties are shown. The shortcut edge states resulting from the splitting of the n=0 Landau level represent a special case, giving rise to non-trivial transverse and longitudinal resistance.

cond-mat.mes-hall↗
arXiv2014

Topological properties of the mesoscopic graphene plaquette: QSHE due to spin imbalance

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-Büttiker 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.

cond-mat.mes-hall↗
arXiv2007

Mean-field theory for Bose-Hubbard Model under a magnetic field

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.

cond-mat.stat-mech↗
arXiv2006

Asymmetric localization in disordered Landau bands

We show that due to the Landau band mixing the eigenstate localization within the disordered bands get 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 2D 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 dissapear when the interband coupling is switched off.

cond-mat.mes-hall↗
arXiv2003

Spin magnetization of strongly correlated electron gas confined in a two-dimensional finite lattice

The influence of disorder and interaction on the ground state polarization of the two-dimensional (2D) correlated electron gas is studied by numerical investigations of unrestricted Hartree-Fock equations. The ferromagnetic ground state is found to be plausible when the electron number is lowered and the interaction and disorder parameters are suitably chosen. For a finite system at constant electronic density the disorder induced spin polarization is cut off when the electron orbitals become strongly localized to the individual network sites. The fluctuations of the interaction matrix elements are calculated and brought out as favoring the ferromagnetic instability in the extended and weak localization regime. The localization effect of the Hubbard interaction term is discussed.

cond-mat.str-el↗
arXiv2002

The orbital magnetization of single and double quantum dots in a tight binding model

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 anti-crossing 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 et al., Phys. Rev. Lett. 80, 4951 (1998).

cond-mat.mes-hall↗
arXiv2000

Coulomb effects on the transport properties of quantum dots in strong magnetic field

We investigate the transport properties of quantum dots placed in strong magnetic field using a quantum-mechanical ' approach based on the 2D tight-binding Hamiltonian with direct Coulomb interaction and the Landauer-Büttiker (LB) 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 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 strong and vanishing magnetic field. The crossover from the tunneling to quantum Hall regime is analyzed in terms of dot-lead coupling.

cond-mat.mes-hall↗
arXiv1998

Quantum analog of channeled electron trajectories in periodic magnetic and electric fields

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.

cond-mat.mes-hall↗
arXiv1998

QHE, magnetoresistance and disordered transport on 2D mesoscopic plaquettes

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 calculted in the four-probe Landauer-Büttiker 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 evidentiated. The dependence on magnetic field of the configurational averages of the longitudinal and Halll resistance is studied in a purely quantum-mechanical approach. Both negative and positive magnetoresistances are found.

cond-mat.mes-hall↗
arXiv1997

Specific Plateaus of the Quantum Hall Effect Induced by an Applied Bias

The spectrum and the eigenstates of a finite 2D tight-binding electronic system, with Dirichlet boundary conditions, in magnetic field and 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 inbetween the usual IQHE plateaus. This idea is confirmed by a numerical calculation within the four-terminal Landauer-Büttiker approach.

cond-mat.mes-hall↗