We derive the improved estimators for general interactions and employ these for the continuous-time quantum Monte Carlo method. Using a worm algorithm we show how measuring higher-ordered correlators leads to an improved high-frequency behavior in irreducible quantities such as the one-particle self-energy or the irreducible two-particle vertex for non-density-density interactions. A good knowledge of the asymptotics of the two-particle vertex is essential for calculating nonlocal electronic correlations using diagrammatic extensions to the dynamical mean field theory as well as for calculating susceptibilities. We test our algorithm against analytic results for the multiorbital atomic limit and the Falicov-Kimball model.

Multiple-band electronic structure and proximity to antiferromagnetic (AF) instability are the key properties of iron-based superconductors. We explore the influence of scattering by the AF spin fluctuations on transport of multiple-band metals above the magnetic transition. A salient feature of scattering on the AF fluctuations is that it is strongly enhanced at the Fermi surface locations where the nesting is perfect (“hot spots” or “hot lines”). We review derivation of the collision integral for the Boltzmann equation due to AF-fluctuations scattering. In the paramagnetic state, the enhanced scattering rate near the hot lines leads to anomalous behavior of electronic transport in magnetic field. We explore this behavior by analytically solving the Boltzmann transport equation with approximate transition rates. This approach accounts for return scattering events and is more accurate than the relaxation-time approximation. The magnetic-field dependences are characterized by two very different field scales: the lower scale is set by the hot-spot width and the higher scale is set by the total scattering amplitude. A conventional magnetotransport behavior is limited to magnetic fields below the lower scale. In the wide range in-between these two scales, the longitudinal conductivity has linear dependence on the magnetic field and the Hall conductivity has quadratic dependence. The linear dependence of the diagonal component reflects growth of the Fermi-surface area affected by the hot spots proportional to the magnetic field. We discuss applicability of this theoretical framework for describing of anomalous magnetotransport properties in different iron pnictides and chalcogenides in the paramagnetic state.

We determine the elastic properties of the layered thermoelectrics BiOCuSe and LaOCuSe using first-principles density functional theory calculations. To predict their stability, we calculate six distinct elastic constants, where all of them are positive, and suggest mechanically stable tetragonal crystals. As elastic properties relate to the nature and the strength of the chemical bond, the latter is analyzed by means of real-space descriptors, such as the electron localization function (ELF) and Bader charge. From elastic constants, a set of related properties, namely, bulk modulus, shear modulus, Young's modulus, sound velocity, Debye temperature, Grüneisen parameter, and thermal conductivity, are evaluated. Both materials are found to be ductile in nature and not brittle. We find BiOCuSe to have a smaller sound velocity and, hence, within the accuracy of the used Slack's model, a smaller thermal conductivity than LaOCuSe. Our calculations also reveal that the elastic properties and the related lattice thermal transport of both materials exhibit a much larger anisotropy than their electronic band properties that are known to be moderately anisotropic because of a moderate effective-electron-mass anisotropy. Finally, we determine the lattice dynamical properties, such as phonon dispersion, atomic displacement, and mode Grüneisen parameters, in order to correlate the elastic response, chemical bonding, and lattice dynamics.

We introduce a hybrid method for dielectric-metal composites that describes the dynamics of the metallic system classically while retaining a quantum description of the dielectric. The time-dependent dipole moment of the classical system is mimicked by the introduction of projected equations of motion (PEOM), and the coupling between the two systems is achieved through an effective dipole-dipole interaction. To benchmark this method, we model a test system (semiconducting quantum dot-metal nanoparticle hybrid). We begin by examining the energy absorption rate, showing agreement between the PEOM method and the analytical rotating wave approximation (RWA) solution. We then investigate population inversion and show that the PEOM method provides an accurate model for the interaction under ultrashort pulse excitation where the traditional RWA breaks down.

We present results for time-dependent electron transport in a ballistic graphene field-effect transistor with an ac-driven gate. Nonlinear response to the ac drive is derived utilizing Floquet theory for scattering states in combination with Landauer-Büttiker theory for transport. We identify two regimes that can be useful for applications: (i) low and (ii) high doping of graphene under source and drain contacts, relative to the doping level in the graphene channel, which in an experiment can be varied by a back gate. In both regimes, inelastic scattering induced by the ac drive can excite quasibound states in the channel that leads to resonance promotion of higher-order sidebands. Already for weak to intermediate ac drive strength, this leads to a substantial change in the direct current between source and drain. For strong ac drive with frequency $\mathrm{\Omega }$, we compute the higher harmonics of frequencies $n\mathrm{\Omega }$ ($n$ integer) in the source-drain conductance. In regime (ii), we show that particular harmonics (for instance, $n=6$) can be selectively enhanced by tuning the doping level in the channel or by tuning the drive strength. We propose that the device operated in the weak-drive regime can be used to detect THz radiation, while in the strong-drive regime, it can be used as a frequency multiplier.

The influence of defect dipoles on the electrocaloric effect (ECE) in acceptor doped ${\mathrm{BaTiO}}_3$ is studied by means of lattice-based Monte-Carlo simulations using a Ginzburg-Landau type effective Hamiltonian. Oxygen vacancy-acceptor associates are described by fixed local dipoles with orientation parallel or antiparallel to the external field. By a combination of canonical and microcanonical simulations the ECE is directly evaluated. Our results reveal that in the case of antiparallel defect dipoles the ECE can be positive or negative depending on the dipole density. Moreover, a transition from a negative to positive ECE can be observed when the external field increases. These transitions are due to the delicate interplay of internal and external fields and are explained by the domain structure evolution and related field-induced entropy changes. The results are in good qualitative agreement to those obtained by molecular dynamics simulations employing an ab initio based effective Hamiltonian. Finally, a modified electrocaloric cycle, which makes use of the negative ECE in the presence of defect dipoles, is proposed to enhance the cooling effect.

Reversible polymorphism of monolayer transition-metal dichalcogenides (TMDC) has currently attracted much attention from both academic and applied perspectives. Of special interest is ${\mathrm{MoTe}}_2$, where the stable semiconducting $2H$ and metastable (semi)metallic $1T^{\prime }$ phases have a rather small energy difference implying the low-energy cost of such a transition. In this work, using first-principles calculations, we demonstrate that there exists a previously unknown phase of ${\mathrm{MoTe}}_2$, namely a distorted trigonal prismatic phase with alternating shorter and longer bonds and bond angles, that is formed in the electronically excited state due to population inversion. This phase, which is unstable and decays to the ground $2H$ state after cessation of the excitation, is metallic and can act to lower the energy barrier on the way to the metastable $1T^{\prime }$ phase. Our findings indicate that there exists a previously unexplored route of ultrafast local and selective band-structure control in monolayer TMDC using electronic excitation, which will significantly broaden the application spectrum of these materials.

A long-standing question for avant-garde data storage technology concerns the nature of the ultrafast photoinduced phase transformations in the wide class of chalcogenide phase-change materials (PCMs). Overall, a comprehensive understanding of the microstructural evolution and the relevant kinetics mechanisms accompanying the out-of-equilibrium phases is still missing. Here, after overheating a phase-change chalcogenide superlattice by an ultrafast laser pulse, we indirectly track the lattice relaxation by time resolved x-ray absorption spectroscopy (tr-XAS) with a sub-ns time resolution. The approach to the tr-XAS experimental results reported in this work provides an atomistic insight of the mechanism that takes place during the cooling process; meanwhile a first-principles model mimicking the microscopic distortions accounts for a straightforward representation of the observed dynamics. Finally, we envisage that our approach can be applied in future studies addressing the role of dynamical structural strain in PCMs.

The square lattice antiferromagnet with frustrating next-nearest-neighbor coupling continues to generate tremendous interest, with an elusive quantum disordered phase in the vicinity of $J_2=J_1/2$. At this precise value of frustration, the classical model has a very large degeneracy, which makes the problem difficult to handle. We show that introducing a ferromagnetic $J_3$ coupling partially lifts this degeneracy. It gives rise to a four-site magnetic unit cell with the constraint that the spins on every square must add to zero. This leads to a two-parameter family of ground states and an emergent vector order parameter. We reinterpret this family of ground states as coexistence states of three spirals. Using spin wave analysis, we show that thermal and quantum fluctuations break this degeneracy differently. Thermal fluctuations break it down to a threefold degeneracy with one Néel phase and two stripe phases. This threefold symmetry is restored via a ${\mathbb{Z}}_3$ thermal transition, as we demonstrate using classical Monte Carlo simulations. On the other hand, quantum fluctuations select the Néel state. In the extreme quantum limit of spin $1/2$, we use exact diagonalization to demonstrate Néel ordering beyond a critical $J_3$ coupling. For weak $J_3$, a variational approach suggests an $s$-wave plaquette-RVB state. Away from the $J_2=J_1/2$ line, we show that quantum fluctuations favor Néel ordering strongly enough to stabilize it within the classical stripe region. Our results shed light on the origin of the quantum disordered phase in the $J_1-J_2$ model.

The aim of this paper is to clarify details of the projectional analysis for atomic vibrations in vitreous silica presented in a paper by Taraskin and Elliott [Phys. Rev. B 56, 8605 (1997)]. The description of the stretching and bending modes has been discussed in detail while a description of the rotational modes and the procedure of their calculation have not been given in full detail. We specify the method for calculating the rotational modes omitted in the paper. We also correct a misprint in the expression for the ${\mathbf{B}}_2^{\left(E\right)}$ mode which, unfortunately, has been reproduced in the vibrational analysis of the $E$ mode in some publications.

An improved thermodynamic cycle is validated in ferroelectric single crystals, where the cooling effect of an electrocaloric refrigerant is enhanced by applying a reversed electric field. In contrast to the conventional adiabatic heating or cooling by on-off cycles of the external electric field, applying a reversed field is significantly improving the cooling efficiency, since the variation in configurational entropy is increased. By comparing results from computer simulations using Monte Carlo algorithms and experiments using direct electrocaloric measurements, we show that the electrocaloric cooling efficiency can be enhanced by more than 20% in standard ferroelectrics and also relaxor ferroelectrics, like $\mathrm{Pb}({\mathrm{Mg}}_{1/3}$/${\mathrm{Nb}}_{2/3}$)${}_{0.71}{\mathrm{Ti}}_{0.29}{\mathrm{O}}_3$.

We report low-temperature heat transport measurements on superconducting iron sulfide FeS with $T_c\approx 5$ K, which has the same crystal structure and similar electronic band structure to the superconducting iron selenide FeSe. In zero magnetic field, a significant residual linear term ${\kappa }_0/T$ is observed. At low field, ${\kappa }_0/T$ increases rapidly with increasing field. These results suggest a nodal superconducting gap in FeS. We compare it with the sister compound FeSe and other iron-based superconductors with nodal gaps.

The general method for the calculation of $n\mathrm{th}$ ($n\ge 2$) order elastic constants of the loaded crystal is given in the framework of the nonlinear elasticity theory. For the crystals of cubic symmetry under hydrostatic compression, the two schemes of calculation of the elastic constants of second, third, and fourth order from energy–finite strain relations and stress–finite strain relations are implemented. Both techniques are applied for the calculation of elastic constants of orders from second to fourth to the bcc phase of tungsten at a 0–600 GPa pressure range. The energy and stress at the various pressures and deformations are obtained ab initio in the framework of projector augmented wave+generalized gradient approximation (PAW+GGA) method, as implemented in Vienna Ab initio Simulation Package (VASP) code. Using the obtained results, we found the pressure dependence of Grüneisen parameters for long-wave acoustic modes in this interval. The Lamé constants of second and third order were estimated for polycrystalline tungsten. The proposed method is applicable for crystals with arbitrary symmetry.

High-resolution powder x-ray diffraction and single-crystal neutron diffraction were used to investigate the crystal structure and magnetic ordering of the compound $\mathrm{P}{\mathrm{r}}_{1-x}\mathrm{C}{\mathrm{a}}_x\mathrm{V}{\mathrm{O}}_3$ ($0\le x\le 0.3$), which undergoes an insulator-to-metal transition for $x\sim 0.23$. Since the ionic radii of $\mathrm{P}{\mathrm{r}}^{3+}$ and $\mathrm{C}{\mathrm{a}}^{2+}$ are almost identical and structural disorder is minimal, $\mathrm{P}{\mathrm{r}}_{1-x}\mathrm{C}{\mathrm{a}}_x\mathrm{V}{\mathrm{O}}_3$ is a good model system for the influence of hole doping on the spin and orbital correlations in transition metal oxides. The end member $\mathrm{PrV}{\mathrm{O}}_3$ is a Mott-Hubbard insulator, which exhibits a structural phase transition at $T_{\mathrm{S}}=180\mathrm{K}$ from an orthorhombic to a monoclinic structure with space groups Pbnm and $P{2}_1/b$, respectively. This transition is associated with the onset of orbital ordering and strong Jahn-Teller distortions of the $\mathrm{V}{\mathrm{O}}_6$ octahedra. Antiferromagnetic $C$-type order with vanadium moments oriented in the $ab$ plane is observed below $T_{\mathrm{N}}=140\mathrm{K}$. Upon cooling, the vanadium moments induce a progressive magnetic polarization of the praseodymium sublattice, resulting in a ferrimagnetic structure with coexisting modes ($C_x, F_y$) and ($F_x, C_y$). In the insulating range of the $\mathrm{P}{\mathrm{r}}_{1-x}\mathrm{C}{\mathrm{a}}_x\mathrm{V}{\mathrm{O}}_3$ phase diagram, Ca doping reduces both the orbital and magnetic transition temperatures so that $T_{\mathrm{S}}=108\mathrm{K}$ and $T_{\mathrm{N}}=95\mathrm{K}$ for $x=0.20$. The Jahn-Teller distortions and ordered vanadium moments also decrease upon doping. In a metallic sample with $x=0.30$, Jahn-Teller distortions and long-range orbital ordering are no longer observable, and the average crystal structure remains orthorhombic down to low temperature. However, broadening of some lattice Bragg reflections indicate a significant increase in lattice strain. Antiferromagnetic short-range order with a weak ordered moment of 0.14(3) ${\mu }_{\mathrm{B}}$ per vanadium atom could still be observed on the vanadium site below $T\sim 60\mathrm{K}$. We discuss these observations in terms of doping-induced spin-orbital polaron formation.

Motivated by the recent discovery of a new family of chromium-based superconductors, we consider a two-band model, where a band of electrons dispersing only in one direction interacts with a band of electrons dispersing in all three directions. Strong $2k_f$ density fluctuations in the one-dimensional band induces attractive interactions between the three-dimensional electrons, which, in turn, makes the system superconducting. Solving the associated Eliashberg equations, we obtain a gap function which is peaked at the “poles” of the three-dimensional Fermi sphere, and decreases towards the “equator.” When strong enough local repulsion is included, the gap actually changes sign around the equator and nodal rings are formed. These nodal rings manifest themselves in several experimentally observable quantities, some of which resemble unconventional observations in the newly discovered superconductors which motivated this work.

The honeycomb-lattice ruthenate ${\mathrm{Li}}_2{\mathrm{RuO}}_3$ is made heavily Li deficient by chemical oxidation by iodine. The delithiation induces a different phase ${\mathrm{Li}}_{2-x}{\mathrm{RuO}}_3$, the “D phase,” with superlattice. For the first time we disclose the magnetic and structural properties of the D phase in the dimer-solid state. The low-temperature magnetic susceptibility and the bond lengths indicate a bonding configuration consisting of both ${\mathrm{Ru}}^{4+}\text{−}{\mathrm{Ru}}^{4+}$ and ${\mathrm{Ru}}^{5+}\text{−}{\mathrm{Ru}}^{5+}$ dimers.

There have been several experimental reports showing high-order harmonic generation from solids, but there has been no unifying theory presented as of yet for all these experiments. Here we report on the systematic investigation of high-order harmonic generation within the semiconductor Bloch equations, taking into account multiple bands and relaxation processes phenomenologically. In addition to reproducing key experiments, we show the following: (i) Electronic excitations, direct-indirect excitation pathways, and relaxation processes are responsible for high-order harmonic generation and control using midinfrared drivers in zinc oxide. We describe an intuitive picture explaining a two-color experiment involving noninversion symmetric crystals. (ii) High-order harmonic generation can be considered as a general feature of ultrafast strong-field-driven electronic dynamics in solids. We demonstrate this statement by predicting high-order harmonic spectra of solids that have not been studied yet.

Domain walls in three-dimensional Weyl semimetals, formed by localized magnetic moments, are investigated. There appear bound states around the domain wall with the discrete spectrum, among which we find “Fermi arc” states with the linear dispersion. The Fermi arc modes contribute to the electric charge and current localized at the domain wall, which reveal a universal behavior depending only on chemical potential and the splitting of the Weyl nodes. This equilibrium current can be traced back to the chiral magnetic effect, or the edge counterpart of the anomalous Hall effect in the bulk. We propose a way to manipulate the motion of the domain wall, accompanied with the localized charge, by applying an external electric field.

We report growth and characterization of a coupled quantum dot structure that utilizes nanowire templates for selective epitaxy of radial heterostructures. The starting point is a zinc blende InAs nanowire with thin segments of wurtzite structure. These segments have dual roles: they act as tunnel barriers for electron transport in the InAs core, and they also locally suppress growth of a GaSb shell, resulting in coaxial InAs-GaSb quantum dots with integrated electrical probes. The parallel quantum dot structure hosts spatially separated electrons and holes that interact due to the type-II broken gap of InAs-GaSb heterojunctions. The Coulomb blockade in the electron and hole transport is studied, and periodic interactions of electrons and holes are observed and can be reproduced by modeling. Distorted Coulomb diamonds indicate voltage-induced ground-state transitions, possibly a result of changes in the spatial distribution of holes in the thin GaSb shell.

Using first-principles calculations, we report a large band-gap opening in the van der Waals heterostructure of graphene and graphane (hydrogenated graphene) under normal compressive (NC) strain. In the presence of graphane, interlayer charge transfer from graphene to graphane triggers the intralayer charge redistribution in graphene, breaking the equivalence of the two sublattices. This chiral symmetry breaking, however, is not strong enough to split the Dirac cone. The application of the NC strain enhances the inter- and intralayer charge transfer leading to a splitting of the Dirac cone, reflected as a redshift of the $G$ peak in Raman spectra. With strain, the band gap increases monotonically and attains a maximum of 0.74 eV at 20% strain, which is the largest ever reported splitting of a Dirac cone in graphene. Tight-binding analysis demonstrates that the applied strain changes the on-site interactions of carbon atoms belonging to a particular sublattice of graphene, thereby breaking the chiral symmetry leading to the opening of a band gap. A sufficiently large band gap with linear dispersion of Dirac bands in the graphene/graphane heterostructure constitutes promising features for room-temperature electronic and optical devices.