arXiv subjects

M. I. Katsnelson

Publications and source records attributed to M. I. Katsnelson.

At least 163 records · Page 9Linked to original sources

arXiv2015

Transport and Optical Properties of Single- and Bilayer Black Phosphorus with Defects

We study the electronic and optical properties of single- and bilayer black phosphorus with shortand long-range defects by using the tight-binding propagation method. Both types of defect states are localized and induce a strong scattering of conduction states reducing significantly the charge carrier mobility. In contrast to properties of pristine samples, the anisotropy of defect-induced optical excitations is suppressed due to the isotropic nature of the defects. We also investigate the Landau level spectrum and magneto-optical conductivity, and find that the discrete Landau levels are sublinearly dependent on the magnetic field and energy level index, even at low defect concentrations.

cond-mat.mes-hall↗

arXiv2015

Moir{é} patterns as a probe of interplanar interactions: graphene on h-BN

By atomistic modeling of moir{é} patterns of graphene on a substrate with a small lattice mismatch, we find qualitatively different strain distributions for small and large misorientation angles, corresponding to the commensurate-incommensurate transition recently observed in graphene on hexagonal BN. We find that the ratio of C-N and C-B interactions is the main parameter determining the different bond lengths in the center and edges of the moir{é} pattern. Agreement with experimental data is obtained only by assuming that the C-B interactions are at least twice weaker than the C-N interactions. The correspondence between the strain distribution in the nanoscale moir{é} pattern and the potential energy surface at the atomic scale found in our calculations, makes the moir{é} pattern a tool to study details of dispersive forces in van der Waals heterostructures.

cond-mat.mes-hall↗

arXiv2015

Melting temperature of graphene

We present an approach to the melting of graphene based on nucleation theory for a first order phase transition from the 2D solid to the 3D liquid via an intermediate quasi-2D liquid. The applicability of nucleation theory, supported by the results of systematic atomistic Monte Carlo simulations, provides an intrinsic definition of the melting temperature of graphene, $ T_m $, and allows us to determine it. We find $T_m \simeq 4510$ K, about 250 K higher than that of graphite using the same interatomic interaction model. The found melting temperature is shown to be in good agreement with the asymptotic results of melting simulations for finite disks and ribbons of graphene. Our results strongly suggest that graphene is the most refractory of all known materials.

cond-mat.mes-hall↗

arXiv2015

Exchange parameters of strongly correlated materials: extraction from spin-polarised density functional theory plus dynamical mean field theory

In this paper we present an accurate numerical scheme for extracting inter-atomic exchange parameters ($J_{ij}$) of strongly correlated systems, based on first-principles full-potential electronic structure theory. The electronic structure is modelled with the help of a full-potential linear muffin-tin orbital method. The effects of strong electron correlations are considered within the charge self-consistent density functional theory plus dynamical mean-field theory (DFT+DMFT). The exchange parameters are then extracted using the magnetic force theorem, hence all the calculations are performed within a single computational framework. The method allows to investigate how the $J_{ij}$-parameters are affected by dynamical electron correlations. In addition to describing the formalism and details of the implementation, we also present magnetic properties of a few commonly discussed systems, characterised by different degrees of electron localisation. In bcc Fe we found a minor renormalisation of the $J_{ij}$ interactions once the dynamical correlations are introduced. However, generally, if the magnetic coupling has several competing contributions from different orbitals, the redistribution of the spectral weight and changes in the exchange splitting of these states can lead to a dramatic modification of the total interaction parameter. In NiO we found that both static and dynamical mean-field results provide an adequate description of the exchange interactions, which is somewhat surprising given the fact that these two methods result in quite different electronic structures. By employing Hubbard-I approximation for the treatment of the $4f$ states in hcp Gd we reproduce the experimentally observed multiplet structure. The calculated exchange parameters result to be rather close to the ones obtained by treating the $4f$ electrons as non-interacting core states.

cond-mat.str-el↗

arXiv2015

Relaxation of moiré patterns for slightly misaligned identical lattices: graphene on graphite

We study the effect of atomic relaxation on the structure of moiré patterns in twisted graphene on graphite and double layer graphene by large scale atomistic simulations. The reconstructed structure can be described as a superlattice of `hot spots' with vortex-like behaviour of in-plane atomic displacements and increasing out-of-plane displacements with decreasing angle. These lattice distortions affect both scalar and vector potential and the resulting electronic properties. At low misorientation angles (<$\sim$1$^\circ$) the optimized structures deviate drastically from the sinusoidal modulation which is often assumed in calculations of the electronic properties. The proposed structure might be verified by scanning probe microscopy measurements.

cond-mat.mes-hall↗

arXiv2015

Effective Hamiltonians for fastly driven many-body lattice systems

We consider 1D lattices described by Hubbard or Bose-Hubbard models, in the presence of periodic high-frequency perturbations, such as uniform ac force or modulation of hopping coefficients. Effective Hamiltonians for interacting particles are derived using an averaging method resembling classical canonical perturbation theory. As is known, a high-frequency force may renormalize hopping coefficients, causing interesting phenomena such as coherent destruction of tunnelling and creation of artificial gauge fields. We find explicitly additional corrections to the effective Hamiltonians due to interactions, corresponding to non-trivial processes such as single-particle density-dependent tunnelling, correlated pair hoppings, nearest neighbour interactions, etc. Some of these processes arise also in multiband lattice models, and are capable to give rise to a rich variety of quantum phases. The apparent contradiction with other methods, e.g. Floquet-Magnus expansion, is explained. The results may be useful for designing effective Hamiltonian models in experiments with ultracold atoms, as well as in the field of ultrafast nonequilibrium magnetism. An example of manipulating exchange interaction in a Mott-Hubbard insulator is considered, where our corrections play an essential role.

cond-mat.other↗

arXiv2015

Modeling Klein tunneling and caustics of electron waves in graphene

We employ the tight-binding propagation method to study Klein tunneling and quantum interference in large graphene systems. With this efficient numerical scheme, we model the propagation of a wave packet through a potential barrier and determine the tunneling probability for different incidence angles. We consider both sharp and smooth potential barriers in n-p-n and n-n' junctions and find good agreement with analytical and semiclassical predictions. When we go outside the Dirac regime, we observe that sharp n-p junctions no longer show Klein tunneling because of intervalley scattering. However, this effect can be suppressed by considering a smooth potential. Klein tunneling holds for potentials changing on the scale much larger than the interatomic distance. When the energies of both the electrons and holes are above the Van Hove singularity, we observe total reflection for both sharp and smooth potential barriers. Furthermore, we consider caustic formation by a two-dimensional Gaussian potential. For sufficiently broad potentials we find a good agreement between the simulated wave density and the classical electron trajectories.

cond-mat.mes-hall↗

arXiv2014

Plasmons in strongly correlated systems: spectral weight transfer and renormalized dispersion

We study the charge-density dynamics within the two-dimensional extended Hubbard model in the presence of long-range Coulomb interaction across the metal-insulator transition point. To take into account strong correlations we start from self-consistent extended dynamical mean-field theory and include non-local dynamical vertex corrections through a ladder approximation to the polarization operator. This is necessary to fulfill charge conservation and to describe plasmons in the correlated state. The calculated plasmon spectra are qualitatively different from those in the random-phase approximation: they exhibit a spectral density transfer and a renormalized dispersion with enhanced deviation from the canonical $\sqrt{q}$-behavior. Both features are reminiscent of interaction induced changes found in single-electron spectra of strongly correlated systems.

cond-mat.str-el↗

arXiv2014

Inverse magneto-refraction as a mechanism for laser modification of spin-spin exchange parameters and subsequent terahertz emission from iron oxides

Ultrafast non-thermal manipulation of magnetization by light relies on either indirect coupling of the electric field component of the light with spins via spin-orbit interaction or direct coupling between the magnetic field component and spins. Here we propose a novel scenario for coupling between the electric field of light and spins via optical modification of the exchange interaction, one of the strongest quantum effects, the strength of which can reach 1000 Tesla. We demonstrate that this isotropic opto-magnetic effect, which can be called the inverse magneto-refraction, is allowed in a material of any symmetry. Its existence is corroborated by the experimental observation of THz emission by magnetic-dipole active spin resonances optically excited in a broad class of iron oxides with a canted spin configuration. From its strength we estimate that a sub-picosecond laser pulse with a moderate fluence of ~ 1 mJ/cm^2 acts as a pulsed effective magnetic field of 0.01 Tesla, arising from the optically perturbed balance between the exchange parameters. Our findings are supported by a low-energy theory for the microscopic magnetic interactions between non-equilibrium electrons subjected to an optical field which suggests a possibility to modify the exchange interactions by light over 1 %.

cond-mat.str-el↗

arXiv2014

Proton transport through one atom thick crystals

Graphene is impermeable to all gases and liquids, and even such a small atom as hydrogen is not expected to penetrate through graphene's dense electronic cloud within billions of years. Here we show that monolayers of graphene and hexagonal boron nitride (hBN) are unexpectedly permeable to thermal protons, hydrogen ions under ambient conditions. As a reference, no proton transport could be detected for a monolayer of molybdenum disulfide, bilayer graphene or multilayer hBN. At room temperature, monolayer hBN exhibits the highest proton conductivity with a low activation energy of about 0.3 eV but graphene becomes a better conductor at elevated temperatures such that its resistivity to proton flow is estimated to fall below 10^-3 Ohm per cm2 above 250 C. The proton barriers can be further reduced by decorating monolayers with catalytic nanoparticles. These atomically thin proton conductors could be of interest for many hydrogen-based technologies.

cond-mat.mtrl-sci↗

arXiv2014

Reply to 'Comment on "Thermodynamics of quantum crystalline membranes"'

In this note, we reply to the comment made by E.I.Kats and V.V.Lebedev [arXiv:1407.4298] on our recent work "Thermodynamics of quantum crystalline membranes" [Phys. Rev. B 89, 224307 (2014)]. Kats and Lebedev question the validity of the calculation presented in our work, in particular on the use of a Debye momentum as a ultra-violet regulator for the theory. We address and counter argue the criticisms made by Kats and Lebedev to our work.

cond-mat.mes-hall↗

arXiv2014

Role of magnetic degrees of freedom in a scenario of phase transformations in steel

The diversity of mesostructures formed in steel at cooling from a high-temperature austenite ("gamma") phase is determined by the interplay of shear reconstructions of crystal lattice and diffusion of carbon. Combining first-principles calculations with large-scale phase-field simulations we demonstrate a decisive role of magnetic degrees of freedom in the formation of energy relief along the Bain path of "gamma"-"alpha" transformation and, thus, in this interplay. We show that there is the main factor, namely, the magnetic state of iron and its evolution with temperature which controls the change in character of the transformation. Based on the computational results we propose a simple model which reproduces, in good agreement with experiment, the most important curves of the phase transformation in Fe-C, namely, the lines relevant to a start of ferrite, bainite, and martensite transformations. Phase-field simulations within the model describe qualitatively typical patterns at these transformations.

cond-mat.mtrl-sci↗

arXiv2014

Metal-Insulator Transition in Graphene on Boron Nitride

Electrons in graphene aligned with hexagonal boron nitride are modelled by Dirac fermions in a correlated random-mass landscape subject to a scalar- and vector-potential disorder. We find that the system is insulating in the commensurate phase since the average mass deviates from zero. At the transition the mean mass is vanishing and electronic conduction in a finite sample can be described by a critical percolation along zero-mass lines. In this case graphene at the Dirac point is in a critical state with the conductivity $\sqrt{3}e^2/h$. In the incommensurate phase the system behaves as a symplectic metal.

cond-mat.mes-hall↗

arXiv2014

Motion of domain walls and the dynamics of kinks in the magnetic Peierls potential

We study the dynamics of magnetic domain walls in the Peierls potential due to the discreteness of the crystal lattice. The propagation of a narrow domain wall (comparable to the lattice parameter) under the effect of a magnetic field proceeds through the formation of kinks in its profile. We predict that, despite the discreteness of the system, such kinks can behave like sine-Gordon solitons in thin films of materials such as yttrium iron garnets, and we derive general conditions for other materials. In our simulations we also observe long-lived breathers. We provide analytical expressions for the effective mass and limiting velocity of the kink in excellent agreement with our numerical results.

cond-mat.mes-hall↗

arXiv2014

Electron correlations in Mn$_x$Ga$_{1-x}$As as seen by resonant electron spectroscopy and dynamical mean field theory

After two decades from the discovery of ferromagnetism in Mn-doped GaAs, its origin is still debated, and many doubts are related to the electronic structure. Here we report an experimental and theoretical study of the valence electron spectrum of Mn-doped GaAs. The experimental data are obtained through the differences between off- and on-resonance photo-emission data. The theoretical spectrum is calculated by means of a combination of density-functional theory in the local density approximation and dynamical mean-field theory (LDA+DMFT), using exact diagonalisation as impurity solver. Theory is found to accurately reproduce measured data, and illustrates the importance of correlation effects. Our results demonstrate that the Mn states extend over a broad range of energy, including the top of the valence band, and that no impurity band splits off from the valence band edge, while the induced holes seem located primarily around the Mn impurity.

cond-mat.str-el↗

arXiv2014

Probing of valley polarization in graphene via optical second-harmonic generation

Valley polarization in graphene breaks inversion symmetry and therefore leads to second-harmonic generation. We present a complete theory of this effect within a single-particle approximation. It is shown that this may be a sensitive tool to measure the valley polarization created, e.g., by polarized light and, thus, can be used for a development of ultrafast valleytronics in graphene.

cond-mat.mes-hall↗

arXiv2014

Effect of Point Defects on the Optical and Transport Properties of MoS2 and WS2

Imperfections in the crystal structure, such as point defects, can strongly modify the optical and transport properties of materials. Here, we study the effect of point defects on the optical and DC conductivities of single layers of semiconducting transition metal dichalcogenides with the form $M$S$_2$, where $M$=Mo or W. The electronic structure is considered within a six bands tight-binding model, which accounts for the relevant combination of $d$ orbitals of the metal $M$ and $p$ orbitals of the chalcogen $S$. We use the Kubo formula for the calculation of the conductivity in samples with different distributions of disorder. We find that $M$ and/or S defects create mid-gap states that localize charge carriers around the defects and which modify the optical and transport properties of the material, in agreement with recent experiments. Furthermore, our results indicate a much higher mobility for $p$-doped WS$_2$ in comparison to MoS$_2$.

cond-mat.mes-hall↗

arXiv2014

Thermodynamics of quantum crystalline membranes

We investigate the thermodynamic properties and the lattice stability of two-dimensional crystalline membranes, such as graphene and related compounds, in the low temperature quantum regime $T\rightarrow0$. A key role is played by the anharmonic coupling between in-plane and out-of plane lattice modes that, in the quantum limit, has very different consequences than in the classical regime. The role of retardation, namely of the frequency dependence, in the effective anharmonic interactions turns out to be crucial in the quantum regime. We identify a crossover temperature, $T^{*}$, between classical and quantum regimes, which is $\sim 70 - 90$ K for graphene. Below $T^{*}$, the heat capacity and thermal expansion coefficient decrease as power laws with decreasing temperature, tending to zero for $T\rightarrow0$ as required by the third law of thermodynamics.

cond-mat.mes-hall↗