In low-temperature antiferromagnetic ${\mathrm{LaMnO}}_3$, strong and localized electronic interactions among Mn $3d$ electrons prevent a satisfactory description from standard local density and generalized gradient approximations in density functional theory calculations. Here, we show that the strong on-site electronic interactions are described well only by using direct and exchange corrections to the intraorbital Coulomb potential. Only DFT + U calculations with explicit exchange corrections produce a balanced picture of electronic, magnetic, and structural observables in agreement with experiment. To understand the reason, a rewriting of the functional form of the  + U corrections is presented that leads to a more physical and transparent understanding of the effect of these correction terms. The approach highlights the importance of Hund's coupling (intraorbital exchange) in providing anisotropy across the occupation and energy eigenvalues of the Mn $d$ states. This intraorbital exchange is the key to fully activating the Jahn-Teller distortion, reproducing the experimental band gap and stabilizing the correct magnetic ground state in ${\mathrm{LaMnO}}_3$. The best parameter values for ${\mathrm{LaMnO}}_3$ within the DFT (PBEsol)  + U framework are determined to be $U=8$ eV and $J=1.9$ eV.

We investigate the excitonic optical properties of carbon nanotubes modulated by an electrostatic field applied in a direction transversal to the carbon nanotubes' axis. We find that excitation energies are redshifted while absorption peaks split due to symmetry breaking. Furthermore, from analysis of the electron-hole wave function calculated in the Wannier approximation, it is seen that the exciton wave function tends to polarize, with a separation of the electron and hole in the large-field limit. This has the effect of reducing the excitonic intrinsic radiative decay rates due to reduced electron-hole overlap. We compute thermalized effective decay rates for comparison with experiments, reflecting the interplay between exciton energy shifts and reduced intrinsic decay rates. Hence, these results suggest several possibilities for modulating the optical response of carbon nanotubes by the application of an electrostatic field in gatelike configurations.

We have constructed the surface phase diagram of the ${\mathrm{BaTiO}}_3$(001) surface by employing an evolutionary algorithm for surface structure prediction, where the ferroelectric polarization is included as a degree of freedom. Among over 1000 candidate structures explored, a surface reconstruction of $(2\times 1)-\mathrm{TiO}$ is discovered to be thermodynamically stable and have the p2mm plane group symmetry as observed experimentally. We find that the influence of ferroelectric polarization on the surface free energy can be either negligibly small or sizably large [over 1 eV per $\left(2\times 1\right)$ supercell], depending strongly on the surface structure and resulting in a significant distinction of surface phase diagram with varying ferroelectric polarization. It is therefore feasible to control the surface stability by applying an external electric field. Our results may have important implications in understanding the surface reconstruction of ferroelectric materials and tuning surface properties.

Ferroelectricity in strain-free and strained double perovskite fluorides, ${\mathrm{Na}}_3{\mathrm{ScF}}_6$ and ${\mathrm{K}}_2{\mathrm{NaScF}}_6$, is investigated using first-principles density functional theory. Although the experimental room temperature crystal structures of these fluoroscandates are centrosymmetric, i.e., ${\mathrm{Na}}_3{\mathrm{ScF}}_6$ ($P{2}_1/n$) and ${\mathrm{K}}_2{\mathrm{NaScF}}_6$ ($Fm\overline{3}m$), lattice dynamical calculations reveal that soft polar instabilities exist in each prototypical cubic phase and that the modes harden as the tolerance factor approaches unity. Thus the double fluoroperovskites bear some similarities to $AB{\mathrm{O}}_3$ perovskite oxides; however, in contrast, these fluorides exhibit large acentric displacements of alkali metal cations (Na, K) rather than polar displacements of the transition metal cations. Biaxial strain investigations of the centrosymmetric and polar ${\text{Na}}_3{\text{ScF}}_6$ and ${\text{K}}_2{\text{NaScF}}_6$ phases reveal that the paraelectric structures are favored under compressive strain, whereas polar structures with in-plane electric polarizations ($\sim 5–18\mu \mathrm{C}{\mathrm{cm}}^{-2}$) are realized at sufficiently large tensile strains. The electric polarization and stability of the polar structures for both chemistries are found to be further enhanced and stabilized by a coexisting single octahedral tilt system. Our results suggest that polar double perovskite fluorides may be realized by suppression of octahedral rotations about more than one Cartesian axis; structures exhibiting in- or out-of-phase octahedral rotations about the $c$ axis are more susceptible to polar symmetries.

The hexagonal phase of $\mathrm{LuFe}{\mathrm{O}}_3$ is a rare example of a multiferroic material possessing a weak ferromagnetic moment, which is predicted to be switchable by an electric field. We stabilize this structure in bulk form though Mn and Sc doping, and determine the complete magnetic and crystallographic structures using neutron-scattering and magnetometry techniques. The ferroelectric $P{6}_{3}cm$ space group is found to be stable over a wide concentration range, ordering antiferromagnetically with Néel temperatures that smoothly increase following the ratio of $c$ to $a(c/a)$ lattice parameters up to 172 K, the highest found in this class of materials to date. The magnetic structure for a range of temperatures and dopings is consistent with recent studies of high quality epitaxial films of pure hexagonal $\mathrm{LuFe}{\mathrm{O}}_3$ including a ferromagnetic moment parallel to the ferroelectric axis. We propose a mechanism by which room-temperature multiferroicity could be achieved in this class of materials.

We study the temperature dependence of the longitudinal spin Seebeck effect (LSSE) in an yttrium iron garnet ${\mathrm{Y}}_3\mathrm{F}{\mathrm{e}}_5{\mathrm{O}}_{12}$ (YIG)/Pt system for samples of different thicknesses. In this system, the thermal spin torque is magnon driven. The LSSE signal peaks at a specific temperature that depends on the YIG sample thickness. We also observe freeze-out of the LSSE signal at high magnetic fields, which we attribute to the opening of an energy gap in the magnon dispersion. We observe partial freeze-out of the LSSE signal even at room temperature, where $k_{B}T$ is much larger than the gap. This suggests that a subset of the magnon population with an energy below $k_B{T}_C \left(T_C\sim 40\mathrm{K}\right)$ contributes disproportionately to the LSSE; at temperatures above $T_C$, we label these magnons subthermal magnons. The $T$ dependence of the LSSE at temperatures below the maximum is interpreted in terms of an empirical model that ascribes most of the temperature dependence to that of the thermally driven magnon flux, which is related to the details of the magnon dispersion.

Charge order has emerged as a generic feature of doped cuprates, leading to important questions about its origin and its relation to superconductivity. Recent experiments on two classes of hole doped cuprates indicate a novel $d$-wave symmetry for the order. These were motivated by earlier spin fluctuation theoretical studies based on an expansion about hot spots in the Brillouin zone that indicated such an order would be competitive with $d$-wave superconductivity. Here, we reexamine this problem by solving strong coupling equations in the full Brillouin zone for experimentally relevant parameters. We find that bond-oriented order, as seen experimentally, is strongly suppressed. We also include coupling to $B_{1g}$ phonons and do not see any qualitative change. Our results argue against an itinerant model for the charge order, implying instead that such order is likely due to Coulombic phase separation of the doped holes.

The recent reports on high-temperature superconductivity above 190 K in hydrogen sulfide at 200 GPa pressure, exceeding all previously discovered superconductors, has greatly invigorated interest in dense hydrogen-rich solids. Here, we investigate a possible way to optimize the critical temperature in these compounds by using first-principles linear-response calculations. We construct hypothetical alchemical atoms to smoothly interpolate between elements of the chalcogen group and study their bonding and superconducting properties. Our results show that the already remarkable critical temperatures of ${\mathrm{H}}_3\mathrm{S}$ could be improved even further by increasing the ionic character of the relevant bonds, i.e., partially replacing sulfur with more electronegative elements.

Information about the pairing mechanism for superconductivity is contained in the spectral weight for the anomalous (Gorkov) Green's function. In the most general case, this spectral weight can change sign on the positive real axis or even be complex in the presence of broken time-reversal symmetry. This renders impossible the direct analytic continuation with maximum entropy methods of numerical results obtained for the anomalous Green's function either in Matsubara frequency or in imaginary time. Here we show that one can define auxiliary Green's functions that allow one to extract the anomalous spectral weight in the most general multiorbital spin-singlet or triplet cases with no particular symmetry. As a simple example, we treat the case of lead in Eliashberg theory: Maximum entropy analytic continuation with auxiliary Green's functions (MaxEntAux) allows us to recover spectral functions that change sign on the positive real axis. The approach can be extended to transport quantities.

Thirty-five years after its synthesis, magnetic observables are calculated for the molecular nanomagnet ${\mathrm{Mn}}_{12}$-acetate using a spin Hamiltonian that contains all spins. Starting from a very advanced density functional theory parametrization [V. V. Mazurenko et al., Phys. Rev. B 89, 214422 (2014)], we evaluate magnetization and specific heat for this anisotropic system of 12 manganese ions with a staggering Hilbert space dimension of 100 000 000 using the finite-temperature Lanczos method. We compare the results with those obtained from other parametrizations. Our investigations demonstrate that it is now possible to assess the quality of parametrizations of effective spin Hamiltonians for rather large magnetic molecules.

The optical signature of niobium in the low-temperature photoluminescence spectra of three common polytypes of SiC (4H, 6H, and 15R) is observed and confirms the previously suggested concept that Nb occupies preferably the Si-C divacancy with both Si and C at hexagonal sites. Using this concept we propose a model considering a Nb-bound exciton, the recombination of which is responsible for the observed luminescence. The exciton energy is estimated using first-principles calculation and the result is in very good agreement with the experimentally observed photon energy in 4H SiC at low temperature. The appearance of six Nb-related lines in the spectra of the hexagonal 4H and 6H polytypes at higher temperatures is tentatively explained on the grounds of the proposed model and the concept that the Nb center can exist in both ${\mathrm{C}}_{1h}$ and ${\mathrm{C}}_{3v}$ symmetries. The Zeeman splitting of the photoluminescence lines is also recorded in two different experimental geometries and the results are compared with theory based on phenomenological Hamiltonians. Our results show that Nb occupying the divacancy at the hexagonal site in the studied SiC polytypes behaves like a deep acceptor.

We discuss methods of quantum state tomography for solid-state systems with a large nuclear spin $I=3/2$ in nanometer-scale semiconductors devices based on a quantum well. Due to quadrupolar interactions, the Zeeman levels of these nuclear-spin devices become nonequidistant, forming a controllable four-level quantum system (known as quartit or ququart). The occupation of these levels can be selectively and coherently manipulated by multiphoton transitions using the techniques of nuclear magnetic resonance (NMR) [Yusa et al., Nature (London) 434, 1001 (2005)]. These methods are based on an unconventional approach to NMR, where the longitudinal magnetization $M_z$ is directly measured. This is in contrast to the standard NMR experiments and tomographic methods, where the transverse magnetization $M_{xy}$ is detected. The robustness against errors in the measured data is analyzed by using the condition number based on the spectral norm. We propose several methods with optimized sets of rotations yielding the highest robustness against errors, as described by the condition number equal to 1, assuming an ideal experimental detection. This robustness is only slightly deteriorated, as given by the condition number equal to 1.05, for a more realistic “noisy” $M_z$ detection based on the standard cyclically ordered phase sequence (CYCLOPS) method.

By taking into account the possibility of all the intralayer as well as the interlayer current orderings, we derive an eight-band model for interacting electrons in bilayer graphene. With the numerical solution to the model, we show that only the current orderings between the same sublattice sites can exist within the range of the physical interacting strength. This result confirms our previous model of spin-polarized-current phase for the ground-state of interacting electrons in bilayer graphene that resolves a number of experimental puzzles.

The $\varepsilon$ and $\zeta$ phases of solid oxygen are studied with periodic hybrid Kohn-Sham calculations and atomic basis sets. The evolution of the lattice parameters and transition pressure are in excellent agreement with experiment. The greater stability of the $\zeta$ phase at high pressure is mainly due to thermal contributions. Phonon calculations indicate that the experimental $C2/m {\left({\mathrm{O}}_2\right)}_4$ unit cell for the $\zeta$ phase actually arises as a time average of $P1$ reduced-symmetry ${\left({\mathrm{O}}_2\right)}_{16}$ unit cells. The structural distortion of the reduced-symmetry $\zeta$ structure gives rise to a mechanism that could explain the superconductivity of the $\zeta$ phase.

We explore a material design strategy to optimize the thermoelectric power factor. The approach is based on screening the band structure changes upon a controlled volume change. The methodology is applied to the binary silicides and germanides. We first confirm the effect in antifluorite ${\mathrm{Mg}}_2\mathrm{Si}$ and ${\mathrm{Mg}}_2\mathrm{Ge}$ where an increased power factor by alloying with ${\mathrm{Mg}}_2\mathrm{Sn}$ is experimentally established. Within a high-throughput formalism we identify six previously unreported binaries that exhibit an improvement in their transport properties with volume. Among these, hexagonal ${\mathrm{MoSi}}_2$ and orthorhombic ${\mathrm{Ca}}_2\mathrm{Si}$ and ${\mathrm{Ca}}_2\mathrm{Ge}$ have the highest increment in $zT$ with volume. We then perform supercell calculations on special quasirandom structures to investigate the possibility of obtaining thermodynamically stable alloy systems which would produce the necessary volume changes. We find that for ${\mathrm{Ca}}_2\mathrm{Si}$ and ${\mathrm{Ca}}_2\mathrm{Ge}$ the solid solutions with the isostructural ${\mathrm{Ca}}_2\mathrm{Sn}$ readily forms even at low temperatures.

We give a detailed comparison of the hierarchical quantum master equation (HQME) method to a continuous-time quantum Monte Carlo (CT-QMC) approach, assessing the usability of these numerically exact schemes as impurity solvers in practical nonequilibrium calculations. We review the main characteristics of the methods and discuss the scaling of the associated numerical effort. We substantiate our discussion with explicit numerical results for the nonequilibrium transport properties of a single-site Anderson impurity. The numerical effort of the HQME scheme scales linearly with the simulation time but increases (at worst exponentially) with decreasing temperature. In contrast, CT-QMC is less restricted by temperature at short times, but in general the cost of going to longer times is also exponential. After establishing the numerical exactness of the HQME scheme, we use it to elucidate the influence of different ways to induce transport through the impurity on the initial dynamics, discuss the phenomenon of coherent current oscillations, known as current ringing, and explain the nonmonotonic temperature dependence of the steady-state magnetization as a result of competing broadening effects. We also elucidate the pronounced nonlinear magnetization dynamics, which appears on intermediate time scales in the presence of an asymmetric coupling to the electrodes.

The lattice structure and symmetry of two-dimensional (2D) layered materials are of key importance to their fundamental mechanical, thermal, electronic, and optical properties. Raman spectroscopy, as a convenient and nondestructive tool, however, has its limitations in identifying all symmetry allowing Raman modes and determining the corresponding crystal structure of 2D layered materials with high symmetry, such as graphene and $\mathrm{Mo}{\mathrm{S}}_2$. Due to the lower structural symmetry and extraordinary weak interlayer coupling of $\mathrm{Re}{\mathrm{S}}_2$, we successfully identify all 18 first-order Raman active modes for bulk and monolayer $\mathrm{Re}{\mathrm{S}}_2$. Without a van der Waals correction, our local density approximation (LDA) calculations successfully reproduce all the Raman modes. Our calculations also suggest no surface reconstruction effect and the absence of low frequency rigid-layer Raman modes below $100\mathrm{c}{\mathrm{m}}^{-1}$. Combining Raman spectroscopy and LDA thus provides a general approach for studying the vibrational and structural properties of 2D layered materials with lower symmetry.

Motivated by the recent discovery of superconductivity in the iron-based ladder compound ${\mathrm{BaFe}}_2{\mathrm{S}}_3$ under high pressure, we derive low-energy effective Hamiltonians from first principles. We show that the complex band structure around the Fermi level is represented only by the Fe $3d_{xz}$ (mixed with $3d_{xy}$) and $3d_{x^2-y^2}$ orbitals. The characteristic band degeneracy allows us to construct a four-band model with the band unfolding approach. We also estimate the interaction parameters and show that the system is more correlated than the 1111 family of iron-based superconductors. Provided the superconductivity is mediated by spin fluctuations, the $3d_{xz}$-like band plays an essential role, and the gap function changes its sign between the Fermi surface around the $\mathrm{\Gamma }$ point and that around the Brillouin-zone boundary.

Polarization switching kinetics is one of the key issues for future development of nanoelectronic devices based on ferroelectrics. Up to now, such kinetics still remains poorly studied despite its crucial impact on the device performances. Here, the switching mechanism in 11-nm-thick ferroelectric films of pure homopolymer of polyvinylidene fluoride is investigated. While the usual mechanism involves nucleation and growth of domains, a homogeneous ferroelectric switching is evidenced in such ultrathin films. Indeed, the dependence of the switching rate on applied voltage reveals a critical behavior with the existence of a true threshold field (of $\sim 0.26\mathrm{GV}/\mathrm{m}$) which is required to overcome the energy barrier to reverse the whole polarization homogeneously as suggested by Landau-Ginzburg mean-field theory. Such finding not only supports few previous works but also raises the question on the general aspect of such homogeneous mechanism that might exist in any other nanoscale ferroelectric materials.

We study degeneracy and entropy in the ground state of artificial square ice. In theoretical models, individual nanomagnets are typically treated as single spins with only two degrees of freedom, leading to a twofold degenerate ground state with intensive entropy and thus no zero-point entropy. Here, we show that the internal degrees of freedom of the nanostructures can result, through edge bending of the magnetization and breaking of local magnetic symmetry at the vertices, in a transition to a highly degenerate ground state with finite zero-point entropy, similar to that of the pyrochlore spin ices. We find that these additional degrees of freedom have observable consequences in the resonant spectrum of the lattice, and predict the occurrence of edge “melting” above a critical temperature at which the magnetic symmetry is restored.