arXiv subjects

M. I. Katsnelson

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

At least 145 records · Page 8Linked to original sources

arXiv2016

Self-consistent Dual Boson approach to single-particle and collective excitations in correlated systems

We propose an efficient dual boson scheme, which extends the DMFT paradigm to collective excitations in correlated systems. The theory is fully self-consistent both on the one- and on the two-particle level, thus describing the formation of collective modes as well as the renormalization of electronic and bosonic spectra on equal footing. The method employs an effective impurity model comprising both fermionic and bosonic hybridization functions. Only single- and two-electron Green's functions of the reference problem enter the theory, due to the optimal choice of the self-consistency condition for the effective bosonic bath. We show that the theory is naturally described by a dual Luttinger-Ward functional and obeys the relevant conservation laws.

cond-mat.str-el↗

arXiv2015

Phonon-assisted resonant tunneling of electrons in graphene-boron nitride transistors

We observe a series of sharp resonant features in the differential conductance of graphene-hexagonal boron nitride-graphene tunnel transistors over a wide range of bias voltages between $\sim$10 and 200 mV. We attribute them to electron tunneling assisted by the emission of phonons of well-defined energy. The bias voltages at which they occur are insensitive to the applied gate voltage and hence independent of the carrier densities in the graphene electrodes, so plasmonic effects can be ruled out. The phonon energies corresponding to the resonances are compared with the lattice dispersion curves of graphene-boron nitride heterostructures and are close to peaks in the single phonon density of states.

cond-mat.mes-hall↗

arXiv2015

Geometric, electronic and magnetic structure of Fe$_{x}$O$_{y}^{+}$ clusters

Correlation between geometry, electronic structure and magnetism of solids is both intriguing and elusive. This is particularly strongly manifested in small clusters, where a vast number of unusual structures appear. Here, we employ density functional theory in combination with a genetic search algorithm, GGA$+U$ and a hybrid functional to determine the structure of gas phase Fe$_{x}$O$_{y}^{+/0}$ clusters. For Fe$_{x}$O$_{y}$ cation clusters we also calculate the corresponding vibration spectra and compare them with experiments. We successfully identify Fe$_{3}$O$_{4}^{+}$, Fe$_{4}$O$_{5}^{+}$, Fe$_{4}$O$_{6}^{+}$, Fe$_{5}$O$_{7}^{+}$ and propose structures for Fe$_{6}$O$_{8}^{+}$. Within the triangular geometric structure of Fe$_{3}$O$_{4}^{+}$ a non-collinear, ferrimagnetic and ferromagnetic state are comparable in energy. Fe$_{4}$O$_{5}^{+}$ and Fe$_{4}$O$_{6}^{+}$ are ferrimagnetic with a residual magnetic moment of 1~\muB{} due to ionization. Fe$_{5}$O$_{7}^{+}$ is ferrimagnetic due to the odd number of Fe atoms. We compare the electronic structure with bulk magnetite and find Fe$_{4}$O$_{5}^{+}$, Fe$_{4}$O$_{6}^{+}$, Fe$_{6}$O$_{8}^{+}$ to be mixed valence clusters. In contrast, in Fe$_{3}$O$_{4}^{+}$ and Fe$_{5}$O$_{7}^{+}$ all Fe are found to be trivalent.

physics.atm-clus↗

arXiv2015

Misfit stabilized embedded nanoparticles in metallic alloys

Nanoscale inhomogeneities are typical for numerous metallic alloys and crucially important for their practical applications. At the same time, stabilization mechanisms of such a state are poorly understood. We present a general overview of the problem, together with a more detailed discussion of the prototype example, namely, Guinier-Preston zones in Al-based alloys. It is shown that coherent strain due to a misfit between inclusion and host crystal lattices plays a decisive role in the emergence of the inhomogeneous state. We suggest a model explaining formation of ultrathin plates (with the thickness of a few lattice constants) typical for Al-Cu alloys. Discreteness of the array of misfit dislocations and long-ranged elastic interactions between them are the key ingredients of the model. This opens a way to a general understanding of the nature of (meta)stable embedded nanoparticles in practically important systems.

cond-mat.mtrl-sci↗

arXiv2015

Racah materials: role of atomic multiplets in intermediate valence systems

We address the long-standing mystery of the nonmagnetic insulating state of the intermediate valence compound SmB$_6$. Within a combination of the local density approximation (LDA) and an exact diagonalization (ED) of an effective discrete Anderson impurity model, the intermediate valence ground state with the $f$-shell occupation $\langle n_{4f} \rangle=5.6$ is found for the Sm atom in SmB$_6$. This ground state is a singlet, and the first excited triplet state $\sim 3$ meV higher in the energy. SmB$_6$ is a narrow band insulator already in LDA, with the direct band gap of $\sim 10$ meV. The electron correlations increase the band gap which now becomes indirect. Thus, the many-body effects are relevant to form the indirect band gap, crucial for the idea of ``topological Kondo insulator" in SmB$_6$. Also, an actinide analog PuB$_6$ is considered, and the intermediate valence singlet ground state is found for the Pu atom. We propose that [Sm,Pu]B$_6$ belong to a new class of the intermediate valence materials with the multi-orbital ``Kondo-like" singlet ground-state. Crucial role of complex spin-orbital $f^n$-$f^{n+1}$ multiplet structure differently hybridized with ligand states in such Racah materials is discussed.

cond-mat.str-el↗

arXiv2015

Layer-resolved magnetic exchange interactions of surfaces of late 3d elements: effects of electronic correlations

We present the results of an ab initio study of magnetic properties of Fe, Co and Ni surfaces. In particular, we discuss their electronic structure and magnetic exchange interactions (Jij), as obtained by means of a combination of density functional theory and dynamical mean-field theory. All studied systems have a pronounced tendency to ferromagnetism both for bulk and surface atoms. The presence of narrow-band surface states is shown to enhance the magnetic moment as well as the exchange couplings. The most interesting results were obtained for the Fe surface where the atoms have a tendency to couple antiferromagnetically with each other. This interaction is relatively small, when compared to interlayer ferromagnetic interaction, and strongly depends on the lattice parameter. Local correlation effects are shown to lead to strong changes of the overall shape of the spectral functions. However, they seem to not play a decisive role on the overall picture of the magnetic couplings studied here. We have also investigated the influence of correlations on the spin and orbital moments of the bulk-like and surface atoms. We found that dynamical correlations in general lead to enhanced values of the orbital moment.

physics.comp-ph↗

arXiv2015

Microscopic origin of Heisenberg and non-Heisenberg exchange interactions in ferromagnetic bcc Fe

By means of first principles calculations we investigate the nature of exchange coupling in ferromagnetic bcc Fe on a microscopic level. Analyzing the basic electronic structure reveals a drastic difference between the $3d$ orbitals of $E_g$ and $T_{2g}$ symmetries. The latter ones define the shape of the Fermi surface, while the former ones form weakly-interacting impurity levels. We demonstrate that, as a result of this, in Fe the $T_{2g}$ orbitals participate in exchange interactions, which are only weakly dependent on the configuration of the spin moments and thus can be classified as Heisenberg-like. These couplings are shown to be driven by Fermi surface nesting. In contrast, for the $E_g$ states the Heisenberg picture breaks down, since the corresponding contribution to the exchange interactions is shown to strongly depend on the reference state they are extracted from. Our analysis of the nearest-neighbour coupling indicates that the interactions among $E_g$ states are mainly proportional to the corresponding hopping integral and thus can be attributed to be of double-exchange origin.

cond-mat.mtrl-sci↗

arXiv2015

Scaling behavior and strain dependence of in-plane elastic properties of graphene

We show by atomistic simulations that, in the thermodynamic limit, the in-plane elastic moduli of graphene at finite temperature vanish with system size $ L $ as a power law $ ~ L^{-η_u} $ with $ η_u \simeq 0.325 $, in agreement with the membrane theory. Our simulations clearly reveal the size and strain dependence of graphene's elastic moduli, allowing comparison to experimental data. Although the recently measured difference of a factor 2 between the asymptotic value of the Young modulus for tensilely strained systems and the value from {\it ab initio} calculations remains unsolved, our results do explain the experimentally observed increase of more than a factor 2 for a tensile strain of only a few permille. We also discuss the scaling of the Poisson ratio, for which our simulations disagree with the predictions of the self-consistent screening approximation.

cond-mat.mes-hall↗

arXiv2015

Toward a realistic description of multilayer black phosphorus: from $GW$ approximation to large-scale tight-binding simulations

We provide a tight-binding model parametrization for black phosphorus (BP) with an arbitrary number of layers. The model is derived from partially self-consistent $GW_0$ approach, where the screened Coulomb interaction $W_0$ is calculated within the random phase approximation on the basis of density functional theory. We thoroughly validate the model by performing a series of benchmark calculations, and determine the limits of its applicability. The application of the model to the calculations of electronic and optical properties of multilayer BP demonstrates good quantitative agreement with \emph{ab initio} results in a wide energy range. We also show that the proposed model can be easily extended for the case of external fields, yielding the results consistent with those obtained from first principles. The model is expected to be suitable for a variety of realistic problems related to the electronic properties of multilayer BP including different kinds of disorder, external fields, and many-body effects.

cond-mat.mtrl-sci↗

arXiv2015

Chiral tunneling through generic one-dimensional potential barriers in bilayer graphene

We study tunneling of charge carriers in single- and bilayer graphene. We propose an explanation for non-zero "magic angles" with 100% transmission for the case of symmetric potential barrier, as well as for their almost-survival for slightly asymmetric barrier in the bilayer graphene known previously from numerical simulations. Most importantly, we demonstrate that these magic angles are not protected in the case of bilayer and give an explicit example of a barrier with very small electron transmission probability for any angles. This means that one can lock charge carriers by a p-n-p (or n-p-n) junction without opening energy gap. This creates new opportunities for the construction of graphene transistors.

cond-mat.mes-hall↗

arXiv2015

Fingerprints of disorder in graphene

We present a systematic study of the electronic, transport and optical properties of disordered graphene including the next-nearest-neighbor hopping. We show that this hopping has a non-negligible effect on resonant scattering but is of minor importance for long-range disorder such as charged impurities, random potentials or hoppings induced by strain fluctuations. Different types of disorders can be recognized by their fingerprints appearing in the profiles of dc conductivity, carrier mobility, optical spectroscopy and Landau level spectrum. The minimum conductivity $4e^{2}/h$ found in the experiments is dominated by long-range disorder and the value of $4e^{2}/πh$ is due to resonant scatterers only.

cond-mat.mes-hall↗

arXiv2015

Effect of structural relaxation on the electronic structure of graphene on hexagonal boron nitride

We performed calculations of electronic, optical and transport properties of graphene on hBN with realistic moiré patterns. The latter are produced by structural relaxation using a fully atomistic model. This relaxation turns out to be crucially important for electronic properties. We describe experimentally observed features such as additional Dirac points and the "Hofstadter butterfly" structure of energy levels in a magnetic field. We find that the electronic structure is sensitive to many-body renormalization of the local energy gap.

cond-mat.mes-hall↗

arXiv2015

Dirac electrons and domain walls: a realization in junctions of ferromagnets and topological insulators

We study a system of Dirac electrons with finite density of charge carriers coupled to an external electromagnetic field in two spatial dimensions, with a domain wall (DW) mass term. The interface between a thin-film ferromagnet and a three-dimensional topological insulator provides a condensed-matter realization of this model, when an out-of-plane domain wall magnetization is coupled to the TI surface states. We show how, for films with very weak intrinsic in-plane anisotropies, the torque generated by the edge electronic current flowing along the DW competes with an effective in-plane anisotropy energy, induced by quantum fluctuations of the chiral electrons bound to the wall, in a mission to drive the internal angle of the DW from a Bloch configuration towards a Néel configuration. Both the edge current and the induced anisotropy contribute to stabilize the internal angle, so that for weak intrinsic in-plane anisotropies DW motion is still possible without suffering from an extremely early Walker breakdown.

cond-mat.mes-hall↗

arXiv2015

Quantum theory as a description of robust experiments: Application to Stern-Gerlach and Einstein-Podolsky-Rosen-Bohm experiments

We propose and develop the thesis that the quantum theoretical description of experiments emerges from the desire to organize experimental data such that the description of the system under scrutiny and the one used to acquire the data are separated as much as possible. Application to the Stern-Gerlach and Einstein-Podolsky-Rosen-Bohm experiments are shown to support this thesis. General principles of logical inference which have been shown to lead to the Schrödinger and Pauli equation and the probabilistic descriptions of the Stern-Gerlach and Einstein-Podolsky-Rosen-Bohm experiments, are used to demonstrate that the condition for the separation procedure to yield the quantum theoretical description is intimately related to the assumptions that the observed events are independent and that the data generated by these experiments is robust with respect to small changes of the conditions under which the experiment is carried out.

quant-ph↗

arXiv2015

Many-body effects in graphene beyond the Dirac model with Coulomb interaction

This paper is devoted to development of perturbation theory for studying the properties of graphene sheet of finite size, at nonzero temperature and chemical potential. The perturbation theory is based on the tight-binding Hamiltonian and arbitrary interaction potential between electrons, which is considered as a perturbation. One-loop corrections to the electron propagator and to the interaction potential at nonzero temperature and chemical potential are calculated. One-loop formulas for the energy spectrum of electrons in graphene, for the renormalized Fermi velocity and also for the dielectric permittivity are derived.

cond-mat.str-el↗

arXiv2015

Magnetism and interaction-induced gap opening in graphene with vacancies or hydrogen adatoms: Quantum Monte Carlo study

We study electronic properties of graphene with finite concentration of vacancies or other resonant scatterers by a straightforward lattice Quantum Monte Carlo calculations. Taking into account realistic long-range Coulomb interaction we calculate distribution of spin density associated to midgap states and demonstrate antiferromagnetic ordering. Energy gaps are open due to the interaction effects, both in the bare graphene spectrum and in the vacancy/impurity bands. In the case of 5 % concentration of resonant scatterers the latter gap is estimated as 0.7 eV and 1.1 eV for graphene on boron nitride and freely suspended graphene, respectively.

cond-mat.str-el↗

arXiv2015

Chemical modifications and stability of phosphorene with impurities: A first principles study

We perform a systematic first-principles study of phosphorene in the presence of typical monovalent (hydrogen, fluorine) and divalent (oxygen) impurities. The results of our modeling suggest a decomposition of phosphorene into weakly bonded one-dimensional (1D) chains upon single- and double-side hydrogenation and fluorination. In spite of a sizable quasiparticle band gap (2.29 eV), fully hydrogenated phosphorene found to be dynamically unstable. In contrast, full fluorination of phosphorene gives rise to a stable structure, being an indirect gap semiconductor with the band gap of 2.27 eV. We also show that fluorination of phosphorene from the gas phase is significantly more likely than hydrogenation due to the relatively low energy barrier for the dissociative adsorption of F2 (0.19 eV) compared to H2 (2.54 eV). At low concentrations, monovalent impurities tend to form regular atomic rows phosphorene, though such patterns do not seem to be easily achievable due to high migration barriers (1.09 and 2.81 eV for H2 and F2, respectively). Oxidation of phosphorene is shown to be a qualitatively different process. Particularly, we observe instability of phosphorene upon oxidation, leading to the formation of disordered amorphous-like structures at high concentrations of impurities.

cond-mat.mtrl-sci↗

arXiv2015

Landau levels of single layer and bilayer phosphorene

In this work we introduce a low-energy Hamiltonian for single layer and bilayer black phosphorus that describes the electronic states at the vicinity of the gamma point. The model is based on a recently proposed tight-binding description for electron and hole bands close to the Fermi level. We calculate expressions for the Landau level spectrum as function of magnetic field and in the case of bilayer black phosphorus we investigate the effect of an external bias on the electronic band gap. The results showcase the highly anisotropic character of black phosphorus and in particular for bilayer BP, the presence of bias allows for a field-induced semiconductor-metal transition.

cond-mat.mes-hall↗