Effect of long-range structural corrugations on magnetotransport properties of phosphorene in tilted magnetic field

Rippling is an inherent quality of two-dimensional materials playing an important role in determining their properties. Here, we study the effect of structural corrugations on the electronic and transport properties of monolayer black phosphorus (phosphorene) in the presence of tilted magnetic field. We follow a perturbative approach to obtain analytical corrections to the spectrum of Landau levels induced by a long-wavelength corrugation potential. We show that surface corrugations have a non-negligible effect on the electronic spectrum of phosphorene in tilted magnetic field. Particularly, the Landau levels are shown to exhibit deviations from the linear field dependence. The observed effect become especially pronounced at large tilt angles and corrugation amplitudes. Magnetotransport properties are further examined in the low temperature regime taking into account impurity scattering. We calculate magnetic field dependence of the longitudinal and Hall resistivities and find that the nonlinear effects reflecting the corrugation might be observed even in moderate fields (\mbox{$B<10$ T})

An ab-initio study of the electron-phonon coupling within a Cr(001)-surface

It is experimentally well established that the Cr(001)-surface exhibits a sharp resonance around the Fermi level. However, there is no consensus about its physical origin. It is proposed to be either due to a single particle dz2 surface state renormalised by electron-phonon coupling or the orbital Kondo effect involving the degenerate dxz/dyz states. In this work we examine the electron-phonon coupling of the Cr(001)-surface by means of ab-initio calculations in the form of density functional perturbation theory. More precisely, the electron-phonon mass-enhancement factor of the surface layer is investigated for the 3d states. For the majority and minority spin dz2 surface states we find values of 0.19 and 0.16. We show that these calculated electron-phonon mass-enhancement factors are not in agreement with the experimental data even if we use realistic values for the temperature range and surface Debye frequency for the fit of the experimental data. More precisely, then experimentally an electron-phonon mass-enhancement factor of 0.70~0.10 is obtained, which is not in agreement with our calculated values of 0.19 and 0.16. Therefore, we conclude that the experimentally observed resonance at the Cr(001)-surface is not due to polaronic effects, but due to electron-electron correlation effects

Toward a realistic description of multilayer black phosphorus: from GWGW 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.Comment: 10 pages, 9 figures, 2 tables (final version, minor changes

Exchange interactions in transition metal oxides: The role of oxygen spin polarization

Magnetism of transition metal (TM) oxides is usually described in terms of the Heisenberg model, with orientation-independent interactions between the spins. However, the applicability of such a model is not fully justified for TM oxides because spin polarization of oxygen is usually ignored. In the conventional model based on the Anderson principle, oxygen effects are considered as a property of the TM ion and only TM interactions are relevant. Here, we perform a systematic comparison between two approaches for spin polarization on oxygen in typical TM oxides. To this end, we calculate the exchange interactions in NiO, MnO, and hematite (Fe2O3) for different magnetic configurations using the magnetic force theorem. We consider the full spin Hamiltonian including oxygen sites, and also derive an effective model where the spin polarization on oxygen renormalizes the exchange interactions between TM sites. Surprisingly, the exchange interactions in NiO depend on the magnetic state if spin polarization on oxygen is neglected, resulting in non-Heisenberg behavior. In contrast, the inclusion of spin polarization in NiO makes the Heisenberg model more applicable. Just the opposite, MnO behaves as a Heisenberg magnet when oxygen spin polarization is neglected, but shows strong non-Heisenberg effects when spin polarization on oxygen is included. In hematite, both models result in non-Heisenberg behavior. General applicability of the magnetic force theorem as well as the Heisenberg model to TM oxides is discussed.Comment: 19 pages, 2 figure

Polaronic effects in monolayer black phosphorus on polar substrates

We investigate the effect of charge carrier interaction with surface optical phonons on the band properties of monolayer black phosphorus induced by polar substrates. We develop an analytical method based on the Lee-Low-Pines theory to calculate the spectrum of Fr\"ohlich type continuum Hamiltonian in the long-wavelength limit. We examine the modification of a band gap and renormalization of effective masses due to the substrate-related polaronic effect. Our results show that an energy gap in supported monolayer black phosphorus is enlarged depending on a particular substrate and the interlayer distance, $z$. Among the substrate considered, the largest gap broadening at $z=2.5$ \AA{} is observed for the Al$_{2}$O$_{3}$ substrate, which is found to be $\sim 50$ meV. Carrier-phonon coupling also renormalizes the effective masses which is more pronounced along the zigzag direction. Anisotropy of the effective masses becomes stronger by the influence of the polaronic effect corresponding to direction-dependent carrier-phonon coupling. We conclude that substrate phonons have a non-negligible effect on the static band properties of monolayer black phosphorus, which may be further exploited in its experimental and theoretical studies

Excitonic Instability and Pseudogap Formation in Nodal Line Semimetal ZrSiS

Electron correlation effects are studied in ZrSiS using a combination of first-principles and model approaches. We show that basic electronic properties of ZrSiS can be described within a two-dimensional lattice model of two nested square lattices. High degree of electron-hole symmetry characteristic for ZrSiS is one of the key features of this model. Having determined model parameters from first-principles calculations, we then explicitly take electron-electron interactions into account and show that at moderately low temperatures ZrSiS exhibits excitonic instability, leading to the formation of a pseudogap in the electronic spectrum. The results can be understood in terms of Coulomb-interaction-assisted pairing of electrons and holes reminiscent to that of an excitonic insulator. Our finding allows us to provide a physical interpretation to the unusual mass enhancement of charge carriers in ZrSiS recently observed experimentally.Comment: 6 pages, 4 figures. Final versio

Adsorption of cobalt on graphene: Electron correlation effects from a quantum chemical perspective

In this work, we investigate the adsorption of a single cobalt atom (Co) on graphene by means of the complete active space self-consistent field approach, additionally corrected by the second-order perturbation theory. The local structure of graphene is modeled by a planar hydrocarbon cluster (C$_{24}$H$_{12}$). Systematic treatment of the electron correlations and the possibility to study excited states allow us to reproduce the potential energy curves for different electronic configurations of Co. We find that upon approaching the surface, the ground-state configuration of Co undergoes several transitions, giving rise to two stable states. The first corresponds to the physisorption of the adatom in the high-spin $3d^74s^2$ ($S=3/2$) configuration, while the second results from the chemical bonding formed by strong orbital hybridization, leading to the low-spin $3d^9$ ($S=1/2$) state. Due to the instability of the $3d^9$ configuration, the adsorption energy of Co is small in both cases and does not exceed 0.35 eV. We analyze the obtained results in terms of a simple model Hamiltonian that involves Coulomb repulsion ($U$) and exchange coupling ($J$) parameters for the 3$d$ shell of Co, which we estimate from first-principles calculations. We show that while the exchange interaction remains constant upon adsorption ($\simeq1.1$ eV), the Coulomb repulsion significantly reduces for decreasing distances (from 5.3 to 2.6$\pm$0.2 eV). The screening of $U$ favors higher occupations of the 3$d$ shell and thus is largely responsible for the interconfigurational transitions of Co. Finally, we discuss the limitations of the approaches that are based on density functional theory with respect to transition metal atoms on graphene, and we conclude that a proper account of the electron correlations is crucial for the description of adsorption in such systems.Comment: 12 pages, 6 figures, 2 table