arXiv subjects

George Kirczenow

Publications and source records attributed to George Kirczenow.

At least 19 recordsLinked to original sources

arXiv2026

Symmetry, Disorder and Transport Through Altermagnetic Quantum Dots and Their Antiferromagnetic Twins

Altermagnetic crystals resemble antiferromagnets in that they have no macroscopic magnetization, but unlike antiferromagnets they exhibit spin-split band structures. Here the transport properties of altermagnetic quantum dots and their antiferromagnetic twins are explored theoretically with the help of Landauer-Buttiker theory, symmetry considerations and tight-binding models. The influence of the symmetries of the quantum dots, their parent crystal lattices, their shapes and edges, lead arrangements and disorder on the anomalous Hall effect, the spin-Hall effect and spin filtering by the quantum dots are investigated.

cond-mat.mes-hall↗

arXiv2025

How to Make a Pristine Tellurium Atomic Helix

Tellurium atomic helices are currently attracting increasing attention. However, to date individual tellurium atomic helices have only been grown encapsulated in host materials. A bare free-standing tellurium atomic helix has yet to be realized experimentally despite the fundamental interest of these systems. Here DFT-based simulations are presented that show that a pristine tellurium atomic helix can be drawn from a tellurium crystal with the help of a gold STM tip, paving the way to a better understanding of tellurium atomic helices.

cond-mat.mes-hall↗

arXiv2024

On the Nature of Tellurium Atomic Helices

A theoreticsal study of single, double and triple hydrogen-terminated chains of tellurium atoms is presented. Surprisingly, H-terminated single chains with 3 Te atoms per unit cell (as in Te crystals) are found to be unstable. They relax to helices with lower energies and smaller twist angles. However, some compact disordered Te chains of mixed chirality are found to have still lower energies. Pairs of H-terminated Te atomic chains are found to form DNA-like double helices with lower energies than compact disordered structures of the two chains. Triplets of H-terminated Te atomic chains are found to form triple helices. The single, double and triple Te helices reported here are beyond the scope of previously studied periodic models with small unit cells.

cond-mat.mes-hall↗

arXiv2022

Strong Spin Filtering by Silicon Nanoparticles with Adsorbed Bismuth Atom Clusters: Role of Symmetry and Electrostatic Control of the Direction of the Spin Polarization

We present a theoretical study, based on density functional theory and tight binding modeling, of the electronic structure and spin transport properties of silicon nanoparticles with adsorbed bismuth atoms. We find the bismuth atoms to form clusters separated by quantum tunnel barriers. We predict strong spin filtering by these nanostructures in the high conductance regime when the source and drain leads are connected to the same bismuth cluster and in the low conductance regime when the source and drain leads are connected to different bismuth clusters. We relate the spin filtering to a symmetry obeyed by the spin transmission probability matrix of the system. We also predict that for such systems the direction of the spin polarization in the drain lead can be tuned through large angles and even reversed electrostatically simply by varying the voltage applied to a gate. Realization of these silicon-bismuth nanostructures in the laboratory is feasible. We expect the predicted spin filtering to be experimentally accessible and potentially relevant for device applications.

cond-mat.mes-hall↗

arXiv2021

Nearly perfect spin-filtering in curved two-dimensional topological insulators

The spintronic properties of curved nanostructures derived from two-dimensional topological insulators (2DTI's) are explored theoretically with density functional theory-based (DFT) calculations and tight-binding models. We show that curved geometries make it possible to manipulate electron spins in ways that are not available for planar 2DTI devices. We predict that, unlike planar 2DTI devices, curved 2DTI-related nanostructures can function as highly effective {\em two}-terminal spin filters even in the absence of magnetic fields. We construct a generalization to curved geometries of our previous tight binding model of the wide band gap planar 2DTI bismuthene on SiC. The resulting model, applied to an ideal dome geometry with a free edge, is shown to exhibit quantum spin Hall physics, including spin polarized edge states. The model predicts nearly perfect spin filtering by the dome for a particular two-terminal geometry in the absence of magnetic fields. Our DFT calculations predict a Bi$_{105}$Si$_{105}$H$_{15}$ dome of bismuthene with adsorbed silicon and hydrogen atoms to be stable. Our tight binding model, adjusted to match density of states given by DFT calculations, predicts that the Bi$_{105}$Si$_{105}$H$_{15}$ dome should exhibit quantum spin Hall physics and very effective spin filtering in a two-terminal arrangement.

cond-mat.mes-hall↗

arXiv2020

Mechanisms of jump to contact and conductance plateau formation in copper atomic junctions in vacuum and aqueous environments

The interplay between groups of water molecules and single-atom contacts, as reflected in the electrical conductances and mechanical forces of copper atomic junctions, is explored by means of first-principles theory and semi-empirical calculations. We study the influence of the atomic geometries of copper electrodes with pyramidal and non-crystalline structures in the presence and absence of water on the conductance profiles as the electrodes approach each other. It is shown that the atomic arrangements of nano-contacts have crucial effects on the formation of plateaus and the conductance values. Groups of hydrogen bonded water molecules bridge the junction electrodes before a direct Cu-Cu contact between the electrodes is made. However, the bridging of the two copper electrodes by a single H$_{2}$O molecule only occurs in the junctions with pyramidal electrodes. Our findings reveal that the presence of H$_{2}$O molecules modifies strongly the conductance profile of these junctions. In the absence of water molecules, the pyramidal junctions exhibit continuous transitions between integer conductance plateaus, while in the presence of H$_{2}$O molecules, these junctions show abrupt jump to contact behavior and no well-defined conductance plateaus. By contrast, in the absence of H$_{2}$O molecules, the non-crystalline junctions display jump to contact behavior and no well-defined plateaus, while in the presence of H$_{2}$O molecules they exhibit a jump to contact and abrupt transitions between fractional and integer plateaus.

cond-mat.mes-hall↗

arXiv2020

Systematic study of low energy geometries of copper nano-junctions exposed to water and to species that can result from dissociation of water

A detailed computational analysis has been performed, considering copper atomic contacts that are exposed directly to water molecules, hydroxyl groups, and monatomic as well as molecular hydrogen and oxygen species. The optimized physical bonding structure, electrical conductance and inelastic tunneling spectra (IETS) have been determined theoretically for moderately large structures by performing appropriate ab-initio and semi-empirical calculations. By considering the aforementioned properties, it has been possible to determine that some of the molecular bridging structures may be regarded as being highly-probable outcomes, resulting from the exposure of copper electrodes to the atomic/molecular contaminants. We specifically identify the conductance properties of a variety of configurations including examples with very high and very low conductance values. This is done in order to identify junction geometries that may be realized experimentally and their conductance and IETS signatures. By reporting geometries with very high and very low conductance values here, we intend to provide a wider perspective view than previous studies of copper-molecular junctions that have focused on high conductance structures. In addition, we explore the properties of metal junctions with multiple molecules, a class of systems for which little theoretical work has been available in the molecular electronics literature. We find that water molecules surrounding the junction can influence the bonding geometry of the molecules within the junction and consequently can affect strongly the calculated conductances of such junctions.

physics.chem-ph↗

arXiv2019

Valley filters, accumulators, and switches induced in graphene quantum dots by lines of adsorbed hydrogen atoms

We present electronic structure and quantum transport calculations that predict conducting channels induced in graphene quantum dots by lines of adsorbed hydrogen atoms to function as highly efficient, experimentally realizable valley filters, accumulators and switches. The underlying physics is a novel property of graphene Dirac point resonances (DPRs) that is revealed here, namely, that an electric current passing through a DPR-mediated conducting channel in a given direction is carried by electrons of em only one of the two graphene valleys. Our predictions apply to lines of hydrogen atoms adsorbed on graphene quantum dots that are either free standing or supported on a hexagonal boron nitride substrate.

cond-mat.mes-hall↗

arXiv2018

Thermoelectric voltage switching in gold atomic wire junctions

We explore the thermoelectric properties of gold atomic chains bridging gold electrodes by means of \textit{ab initio} and semi-empirical calculations, and heuristic reasoning. We predict the thermoelectric voltage induced by a temperature difference across such junctions to oscillate, repeatedly changing sign, as a function of the number of atoms $N$ making up the atomic chain. We also predict the amplitude of the oscillations to be proportional to $N$ for long atomic chains. Further we predict the thermoelectric voltage to change sign, in some cases, if the junction is stretched without changing $N$. Our predictions apply regardless of whether regular or irregular electrode grain boundaries are present and whether the electrodes are symmetric or asymmetric. Our findings may pave the way to the realization of new mechanically controllable voltage switches enabling direct conversion of heat into electricity in energy harvesting applications.

cond-mat.mes-hall↗

arXiv2017

Gate-tunable valley currents, non-local resistances and valley accumulation in bilayer graphene nanostructures

Using the Büttiker-Landauer formulation of transport theory in the linear response regime, the valley currents and non-local resistances of bilayer graphene nanostructures with broken inversion symmetry are calculated. It is shown that broken inversion symmetry in bilayer graphene nanostructures leads to striking enhancement of the non-local 4-terminal resistance and to valley currents several times stronger than the conventional electric current when the Fermi energy is in the spectral gap close to the energy of Dirac point. The scaling relation between local and non-local resistances is investigated as the gate voltage varies at zero Fermi energy and a power-law is found to be satisfied. The valley velocity field and valley accumulation in four-terminal bilayer graphene nanostructures are evaluated in the presence of inversion symmetry breaking. The valley velocity and non-local resistance are found to scale differently with the applied gate voltage. The unit cell-averaged valley accumulation is found to exhibit a dipolar spatial distribution consistent with the accumulation arising from the valley currents. We define and calculate a {\em valley capacitance} that characterizes the valley accumulation response to voltages applied to the nanostructure's contacts.

cond-mat.mes-hall↗

arXiv2017

Malleability at the extreme nanoscale: Slow and fast quakes of few-body systems

We explore the malleability of ultra-small metal nanoparticles by means of ab initio calculations. It is revealed that, when strained, such nanoparticles exhibit complex behavior, including bifurcation between slow and fast quakes of their atomic structure, despite being few-body systems. We show the bifurcation to arise from the collapse of the nanoparticle's stiffness and a broken soft mode symmetry, and that whether a slow or fast quake occurs can be controlled by varying the amplitude of the externally applied strains. We predict that while energy is released abruptly in a fast quake, surprisingly, it continues to build up during a slow quake and that, in common with slow-slip geological earthquakes, the slow nanoparticle quake is a silent precursor to a fast "seismic" quake. We show that electrical conductance and force measurements can detect and distinguish between slow and fast quakes, opening the way for experiments and potential applications of these phenomena.

cond-mat.mes-hall↗

arXiv2017

Controlling the thermoelectric effect by mechanical manipulation of the electron's quantum phase in atomic junctions

The thermoelectric voltage developed across an atomic metal junction (i.e., a nanostructure in which one or a few atoms connect two metal electrodes) in response to a temperature difference between the electrodes, results from the quantum interference of electrons that pass through the junction multiple times after being scattered by the surrounding defects. Here we report successfully tuning this quantum interference and thus controlling the magnitude and sign of the thermoelectric voltage by applying a mechanical force that deforms the junction. The observed switching of the thermoelectric voltage is reversible and can be cycled many times. Our ab initio and semi-empirical calculations elucidate the detailed mechanism by which the quantum interference is tuned. We show that the applied strain alters the quantum phases of electrons passing through the narrowest part of the junction and hence modifies the electronic quantum interference in the device. Tuning the quantum interference causes the energies of electronic transport resonances to shift, which affects the thermoelectric voltage. These experimental and theoretical studies reveal that Au atomic junctions can be made to exhibit both positive and negative thermoelectric voltages on demand, and demonstrate the importance and tunability of the quantum interference effect in the atomic-scale metal nanostructures.

cond-mat.mtrl-sci↗

arXiv2016

Rashba-Dirac cones at the tungsten surface: Insights from a tight-binding model and thin film subband structure

A tight-binding model of bcc tungsten that includes spin-orbit coupling is developed and applied to the surface states of (110) tungsten thin films. The model describes accurately the anisotropic Dirac cone-like dispersion and Rashba-like spin polarization of the surface states, including the crucial effect of the relaxation of the surface atomic layer of the tungsten towards the bulk. It is shown that the surface relaxation affects the tungsten surface states because it results in increased overlaps between atomic orbitals of the surface atomic layer and nearby layers whereas electric fields that are due to charge transfer between the tungsten and the vacuum near the surface or between the bulk and surface layers do not significantly affect the Rashba-Dirac surface states. It is found that hybridization with bulk modes has differing strengths for thin film surface states belonging to the upper and lower Rashba-Dirac cones and results in $reversal$ of the directions of travel of spin $\uparrow$ and $\downarrow$ electrons in most of the upper Rashba-Dirac cone relative to those expected from phenomenology. It is also shown that intrasite (not intersite) matrix elements of the spin-orbit Hamiltonian are primarily responsible for the formation of the Rashba-Dirac cones, and their spin polarization. This finding should be considered when modeling topological insulators, the spin Hall effect and related phenomena.

cond-mat.mes-hall↗

arXiv2015

Valley currents and non-local resistances of graphene nanostructures with broken inversion symmetry from the perspective of scattering theory

Valley currents and non-local resistances of graphene nanostructures with broken inversion symmetry are considered theoretically in the linear response regime. Scattering state wave functions of electrons entering the nanostructure from the contacts represented by groups of ideal leads are calculated by solving the Lippmann- Schwinger equation and are projected onto the valley state subspaces to obtain the valley velocity fields and total valley currents in the nanostructures. In the tunneling regime when the Fermi energy is in the spectral gap around the Dirac point energy, inversion symmetry breaking is found to result in strong enhancement of the nonlocal 4 terminal Buttiker-Landauer resistance and in valley currents several times stronger than the conventional electric current. These strong valley currents are the direct result of the injection of electrons from a contact into the graphene in the tunneling regime. They are chiral and occur near contacts from which electrons are injected into the nanostructure whether or not a net electric current flows through the contact. It is also pointed out that enhanced non-local resistances in the linear response regime are not a signature of valley currents arising from the combined effect of the electric field and Berry curvature on the velocities of electrons.

cond-mat.mes-hall↗

arXiv2015

Coulomb bound states and resonances due to groups of Ca dimers adsorbed on suspended graphene

The electronic bound states and resonances in the vicinity of the Dirac point energy due to the adsorption of calcium dimers on a suspended graphene monolayer are explored theoretically using density functional theory (DFT) and an improved extended Hückel model that includes electrostatic potentials. The Mulliken atomic charges and the electrostatic potentials are obtained from DFT calculations and reveal charge transfer from the Ca dimers to the graphene which is responsible for the emergence of resonant states in the electronic spectrum. The number of resonant states increases as the number of adsorbed dimers is increased. We find a bound "atomic-collapse" state in the graphene local density of states, as has been observed experimentally [Wang \textit{et al.}, Science {\bf 340}, 734 (2013)]. We find the formation of the atomic-collapse state and its population with electrons to require fewer adsorbed Ca dimers than in the experiment, possibly due to the different spacing between dimers and the dielectric screening by a boron nitride substrate in the experiment. We also predict the onset of filling of a second atomic-collapse state with electrons when six Ca dimers are adsorbed on the suspended graphene monolayer. Experiments testing these predictions would be of interest.

cond-mat.mes-hall↗

arXiv2014

Mechanism of the enhanced conductance of a molecular junction under tensile stress

Despite its fundamental importance for nano physics and chemistry and potential device applications, the relationship between atomic structure and electronic transport in molecular nanostructures is not well understood. Thus the experimentally observed increase of the conductance of some molecular nano junctions when they are stretched continues to be counterintuitive and controversial. Here we explore this phenomenon in propanedithiolate molecules bridging gold electrodes by means of {\em ab initio} computations and semi-empirical modeling. We show that in this system it is due to changes in Au-S-C bond angles and strains in the gold electrodes, rather than to the previously proposed mechanisms of Au-S bond stretching and an associated energy shift of the highest occupied molecular orbital and/or Au atomic chain formation. Our findings indicate that conductance enhancement in response to the application of tensile stress should be a generic property of molecular junctions in which the molecule is thiol-bonded in a similar way to gold electrodes.

cond-mat.mes-hall↗

arXiv2013

Voltage-controlled spin injection with an endohedral fullerene CoC$_{60}$ dimer

Spin-dependent transport through an endohedral fullerene Co@C$_{60}$ dimer with gold electrodes is explored theoretically using density functional and extended Hückel theory. Density of states spin polarizations up to 95%, due to spin-splitting of Co 3d orbitals, are found by varying the gate and/or bias voltage. The current-voltage characteristics and strong (up to 100%) spin polarization of the current indicate that the device can be utilized for highly efficient spin injection into nonmagnetic conductors. This finding opens the way to the realization of electrostatically tuned spintronic nano devices less than 2 nanometers in size, without ferromagnetic electrodes.

cond-mat.mes-hall↗

arXiv2013

Switching of a Quantum Dot Spin Valve by Single Molecule Magnets

We explore theoretically the spin transport in nanostructures consisting of a gold quantum dot bridging nonmagnetic electrodes and two Mn12-Ph single molecule magnets (SMMs) that are thiol-bonded to the dot but are not in direct contact with the electrodes. We find that reversal of the magnetic moment of either SMM by the application of a magnetic field leads to a large change in the resistance of the dot, i.e., a strong spin valve effect. We show that this phenomenon arises from a novel physical principle: The spin-dependent molecular orbitals that extend over the dot and both SMMs change drastically when the magnetic moment of either SMM is reversed, resulting in a large change in the conduction via those orbitals. The same physics may also be responsible for the spin valve phenomena discovered recently in carbon nanotube quantum dots with rare earth SMMs by Urdampilleta, Klyatskaya, Cleuziou, Ruben and Wernsdorfer [Nature Mater. 10, 502506 (2011)]

cond-mat.mes-hall↗