The band gaps of rare-earth titanates are commonly reported to be 0.2–0.7 eV. These values are based on optical reflectivity measurements, from which the onset of optical absorption is derived. Here we report experimental and theoretical results on ${\mathrm{GdTiO}}_3$ (GTO) indicating that the gap is significantly larger. Photoluminescence (PL) measurements show a strong peak near 1.8 eV, consistent with an observed onset in PL excitation (PLE) at about the same energy. First-principles calculations, based either on density-functional theory (DFT) with a hybrid functional or on DFT+$U$, consistently show that the gap is close to 2 eV. We also propose an interpretation of the previously reported optical absorption spectra. Given the similarities in electronic structure between the rare-earth titanates, our results for GTO have repercussions for the other members of the series. The results also affect the design of complex-oxide heterostructures involving these materials.

We study a system of weakly interacting electrons described by the energy dispersion $\xi \left(\mathbf{k}\right)=k_x^2-k_y^2-\mu$ in two dimensions within a renormalization group approach. This energy dispersion exhibits a neck-narrowing Lifshitz transition at the critical chemical potential ${\mu }_c=0$ where a van Hove singularity develops. Implementing a systematic renormalization group analysis of this system has long been hampered by the appearance of nonlocal terms in the Wilsonian effective action. We demonstrate that nonlocality at the critical point is intrinsic, and the locality of the effective action can be maintained only away from the critical point. We also point out that it is crucial to introduce a large momentum cutoff to keep locality even away from the critical point. Based on a local renormalization group scheme employed near the critical point, we show that, as the energy scale $E$ is lowered, an attractive four-fermion interaction grows as ${log}^{2}E$ for $E>\mu$, whereas it retains the usual BCS growth, $-logE$, for $E<\mu$. Starting away from the critical point, this fast growth of the pairing interaction suggests that the system becomes unstable toward a superconducting state well before the critical point is reached.

We investigate dipolar donor materials mixed with a ${\mathrm{C}}_{70}$ acceptor in an organic photovoltaic (OPV) cell. Dipolar donors that have donor-acceptor-acceptor (d-a-a′) structure result in high conductivity pathways due to close coupling between neighboring molecules in the mixed films. We analyze the charge transfer properties of the dipolar donor:${\mathrm{C}}_{70}$ mixtures and corresponding neat donors using a combination of time-resolved electroluminescence from intermolecular polaron pair states and conductive tip atomic force microscopy, from which we infer that dimers of the d-a-a′ donors tend to form a continuous network of nanocrystalline clusters within the blends. Additional insights are provided by quantum-mechanical calculations of hole transfer coupling and hopping rates between donor molecules using nearest-neighbor donor packing motifs taken from crystal structural data. The approximation using only nearest-neighbor interactions leads to good agreement between donor hole hopping rates and the conductive properties of the donor:${\mathrm{C}}_{70}$ blends. This represents a significant simplification from requiring details of the nano- and mesoscale morphologies of thin films to estimate their electronic characteristics. Using these dipolar donors, we obtain a maximum power conversion efficiency of $9.6\pm 0.5\%$ under 1 sun, AM1.5G simulated illumination for an OPV comprised of an active layer containing a dipolar donor mixed with ${\mathrm{C}}_{70}$.

Topologically ordered quantum systems have robust physical properties, such as quasiparticle statistics, and ground-state degeneracy, which do not depend on the microscopic details of the Hamiltonian. We consider topological phase transitions under a deformation such as an effective string tension on a ${\mathbb{Z}}_3$ topological state. This is studied in terms of the gauge-symmetry-preserved quantum state renormalization group, first proposed by He, Moradi, and Wen [Phys. Rev. B 90, 205114 (2014)]. In this approach modular matrices $S$ and $T$ can be obtained and used as order parameters to characterize the topological properties of the phase and determine phase transitions. From a mapping to a classical two-dimensional Potts model on the square lattice, the critical string tension, at which the transition to a topologically trivial phase takes place, can be obtained analytically and agrees with the numerically determined value. Such a transition can be generalized to a ${\mathbb{Z}}_N$ topological model under a string tension and determined in the same way. With different deformations, the ${\mathbb{Z}}_3$ topological phase can also be driven to a critical phase, which contains, in the large deformation limit, the wave function analogous to the Rokhsar-Kivelson point in the quantum dimer model in one case and the fully packed loop model in another case.

The group-IV monochalcogenides SnS, SnSe, GeS, and GeSe form a family within the wider group of semiconductor “phosphorene analogues.” Here, we used first-principles calculations to investigate systematically their structural, electronic, and optical properties, analyzing the changes associated with the reduction of dimensionality, from bulk to monolayer or bilayer form. We show that all those binary phosphorene analogues are semiconducting, with band-gap energies covering part of the infrared and visible range, and in most cases higher than phosphorene. Further, we found that they have multiple valleys in the valence and conduction band, the latter with spin-orbit splitting of the order of 19–86 meV.

Efficient nonradiative carrier recombination strongly counteracts the appearance of optical gain in graphene. Based on a microscopic and fully quantum-mechanical study of the coupled carrier, phonon, and photon dynamics in graphene, we present a strategy to obtain a long-lived gain: Integrating graphene into a high quality photonic crystal nanocavity and applying a high-dielectric substrate suppresses the nonradiative recombination channels and gives rise to pronounced coherent light emission. This suggests the design of graphene-based laser devices covering a broad spectral range.

Biaxial strain in coherent epitaxial thin films can have a pronounced effect on the point-defect profile in the films' material. Detailed fundamental knowledge of the interaction of strain with point defects is crucial to understanding the stoichiometry and resulting properties of strained thin films. Here we investigate the effect of biaxial strain on the formation energy of cation and anion vacancies using MnO as a model system. Our density-functional-theory calculations show that, as expected from local volume arguments, compressive strain favors the formation of cation vacancies. Interestingly, we find that small compressive and tensile strains lead to ordering of the resulting holes along the in-plane and normal direction, respectively, which should manifest in different anisotropic properties in the two strain states.

Using the dynamical matrix of a crystal obtained from ab initio calculations, we have evaluated the strength of the dynamic flexoelectric effect and found it comparable to that of the static bulk flexoelectric effect, in agreement with earlier order-of-magnitude estimates. We also propose a method of evaluation of these effects directly from the simulated phonon spectra. This method can also be applied to the analysis of the experimental phonon spectra, currently the only one enabling experimental characterization of the static bulk flexoelectric effect.

We present here a first-principles study of the ternary compounds formed by Ni, In, and As, a material of great importance for self-aligned metallic contacts in next-generation InAs-based MOS transistors. The approach we outline is general and can be applied to study the crystal structure and properties of a host of other new interface compounds. Using the ab initio random structure searching approach we find the previously unknown low-energy structures of ${\text{Ni}}_x\text{InAs}$ and assess their stability with respect to the known binary compounds of Ni, In, and As. Guided by experiments, we focus on ${\text{Ni}}_3\text{InAs}$ and find a rich energy landscape for this stoichiometry. We consider the five lowest-energy structures, with space groups $Pmmn, Pbcm, P{2}_1/m, Cmcm$, and $R\overline{3}$. The five low-energy structures for ${\text{Ni}}_3\text{InAs}$ are all found to be metallic and nonmagnetic. By comparison to previously published TEM results we identify the crystal structure observed in experiments to be $Cmcm {\text{Ni}}_3\text{InAs}$. We calculate the work function for $Cmcm {\text{Ni}}_3\text{InAs}$ and, according to the Schottky-Mott model, expect the material to form an Ohmic contact with InAs. We further explicitly consider the interface between $Cmcm {\text{Ni}}_3\text{InAs}$ and InAs and find it to be Ohmic with an $n\text{-type}$ Schottky barrier height of $-0.55\text{eV}$.

We study the magnetic damping in the simplest of synthetic antiferromagnets, i.e., antiferromagnetically exchange-coupled spin valves, in the presence of applied magnetic fields that enforce noncolliear magnetic configurations. We formulate the dynamic exchange of spin currents in a noncollinear texture based on the spin-diffusion theory with quantum mechanical boundary conditions at the ferrromagnet/normal-metal interfaces and derive the Landau-Lifshitz-Gilbert equations coupled by the interlayer static and dynamic exchange interactions. We predict noncollinearity-induced additional damping that is modulated by an applied magnetic field. We compare theoretical results with published experiments.

Bloch domain walls are the most common type of transition between two out-of-plane magnetized domains (one magnetized upwards, one downwards) in films with perpendicular magnetic anisotropy. The rotation of the spins of such domain walls in the plane of the film requires energy, which is described by an effective anisotropy, the so-called transverse or hard axis anisotropy $K_{\perp }$. This anisotropy and the related Döring mass density of the domain wall are key parameters of the one-dimensional model to describe the motion of magnetic domain walls. In particular, the critical field strength or current density where oscillatory domain wall motion sets in (Walker breakdown) is directly proportional to $K_{\perp }$. So far, no general framework is available to determine $K_{\perp }$ from static characterizations such as magnetometry measurements. Here, we derive a universal analytical expression to calculate the transverse anisotropy constant for the important class of perpendicular magnetic multilayers. All the required input parameters of the model, such as the number of repeats, the thickness of a single magnetic layer, and the layer periodicity, as well as the effective perpendicular anisotropy, the saturation magnetization, and the static domain wall width are accessible by static sample characterizations. We apply our model to a widely used multilayer system and find that the effective transverse anisotropy constant is a factor of seven different from that when using the conventional approximations, showing the importance of using our analysis scheme. Our model is also applicable to domain walls in materials with Dzyaloshinskii-Moriya interaction (DMI). The accurate knowledge of $K_{\perp }$ is needed to determine other unknown parameters from measurements, such as the DMI strength or the spin polarization of the spin current in current-induced domain wall motion experiments.

Motivated by the recent heat transport experiments in two-dimensional antiferromagnets, such as ${\mathrm{La}}_2{\mathrm{CuO}}_4$, where the exchange coupling $J$ is larger than the Debye energy ${\mathrm{\Theta }}_D$, we discuss different types of relaxation processes for magnon heat current with a particular focus on coupling to three-dimensional phonons. We study thermal conductivity by these in-plane magnetic excitations using two distinct techniques: Boltzmann formalism within the relaxation-time approximation and memory-function approach. Within these approaches, a close consideration is given to the scattering of magnons by both acoustic and optical branches of phonons. A remarkable accord between the two methods with regards to the asymptotic behavior of the effective relaxation rates is demonstrated. Additional scattering mechanisms, due to grain boundaries, impurities, and finite correlation length in the paramagnetic phase, are discussed and included in the calculations of the thermal conductivity $\kappa \left(T\right)$. Again, we demonstrate a close similarity of the results from the two techniques of calculating $\kappa \left(T\right)$. Our complementary approach strongly suggests that scattering from optical or zone-boundary phonons is important for magnon heat current relaxation in a high-temperature window of ${\mathrm{\Theta }}_D\lesssim T\ll J$.

Recent experiments have unveiled important properties of the ground state of the elusive heavy fermion ${\mathrm{URu}}_2{\mathrm{Si}}_2$. While tetragonal symmetry-breaking was reported below the hidden-order (HO) transition at $T_{\mathrm{HO}}\approx 17.5\mathrm{K}$, time-reversal symmetry breaking was observed below the superconducting transition temperature $T_c<T_{\mathrm{HO}}$. Although the latter results have been used to argue in favor of a chiral $d+id$ superconducting state, such an order parameter is incompatible with broken tetragonal symmetry. Here, we employ a phenomenological model to investigate the properties of a chiral superconducting state that develops inside the hidden-order phase. In this case, there are actually two superconducting transition temperatures: while $T_c$ marks a normal-state to superconducting transition, $T_c^{*}<T_c$ signals a superconducting-to-superconducting transition in which time-reversal symmetry is broken. In the phase $T_c^{*}<T<T_c$, the low-energy density of states $\rho \left(\omega \right)$ is enhanced due to the crossing of two nodal lines, giving rise to an unusual $\omega ln\left(\omega \right)$ dependence of $\rho \left(\omega \right)$, which is manifested in several thermodynamic properties. We also investigate the emergence of a soft amplitude gap mode near $T_c^{*}$. In contrast to the usual amplitude mode near a regular normal-state to superconducting transition, this mode becomes soft near a superconducting-to-superconducting transition, which in principle allows for its detection by Raman spectroscopy. Finally, we investigate the impact of twin domains on the anisotropic properties of the superconducting state, and propose experiments in mechanically strained samples to explore the interplay between hidden order and superconductivity in ${\mathrm{URu}}_2{\mathrm{Si}}_2$.

We report on theoretical investigations of scattering asymmetry vs incidence of carriers through exchange barriers and magnetic tunnel junctions made of semiconductors involving spin-orbit interaction. By an analytical $2\times 2$ spin model, we show that when Dresselhaus interaction is included in the conduction band of antiparallel magnetized electrodes, the electrons can undergo a large difference of transmission depending on the sign of their incident in-plane wave vector. In particular, the transmission is fully quenched at some points of the Brillouin zone for specific in-plane wave vectors and not for the opposite. Moreover, the asymmetry is universally scaled by a unique function independent of the spin-orbit strength. This particular feature is reproduced by a $14\times 14$ band $\mathbf{k}·\mathbf{p}$ model showing, in addition, corresponding effects in the valence band and highlighting the robustness of the phenomenon, which even persists for a single magnetic electrode. Upon tunneling, electrons undergo an asymmetrical deflection which results in the occurrence of a transverse current, giving rise to a so-called tunnel Hall effect.

Ferroelectric thin films are considered to be among the top candidates for room-temperature electrocaloric materials as they exhibit excellent electric properties and allow application of record high electric fields. At the same time, downsizing of ferroelectric electrocalorics brings about an unwanted but unavoidable depolarizing field that could critically alter or even destroy the films' electrocaloric properties. We use an atomistic first-principles-based computational approach that does not rely on the use of Maxwell relations (i) to reveal the critical role of the depolarizing field on the electrocaloric properties of ferroelectric ultrathin films, (ii) to demonstrate the contribution of nanodomains to the electrocaloric effect in such films, and (iii) to revisit the potential limitations of the indirect approach to study electrocaloric effect in nanoscale ferroelectrics.

We report on the electronic ground state of a layered perovskite vanadium oxide ${\mathrm{Sr}}_2{\mathrm{VO}}_4$ studied by the combined use of synchrotron radiation x-ray diffraction (SR-XRD) and muon spin rotation/relaxation $(\mu \mathrm{SR})$ techniques, where $\mu \mathrm{SR}$ measurements were extended down to 30 mK. We found an intermediate orthorhombic phase between $T_{\mathrm{c}2}\sim 130$ K and $T_{\mathrm{c}1}\sim 100$ K, whereas a tetragonal phase appears for $T>T_{\mathrm{c}2}$ and $T<T_{\mathrm{c}1}$. The absence of long-range magnetic order was confirmed by $\mu \mathrm{SR}$ at the reentrant tetragonal phase below $T_{\mathrm{c}1}$, where the relative enhancement in the $c$-axis length versus that of the $a$-axis length was observed. However, no clear indication of the lowering of the tetragonal lattice symmetry with superlattice modulation, which is expected in the orbital order state with superstructure of $d_{yz}$ and $d_{zx}$ orbitals, was observed by SR-XRD below $T_{\mathrm{c}1}$. Instead, it was inferred from $\mu \mathrm{SR}$ that a magnetic state developed below $T_{\mathrm{c}0}\sim 10$ K, which was characterized by the highly inhomogeneous and fluctuating local magnetic fields down to 30 mK. We argue that the anomalous magnetic ground state below $T_{\mathrm{c}0}$ originates from the coexistence of ferromagnetic and antiferromagnetic correlations.

We investigate the composition and temperature (10–450 K) dependence of the Yb valence in ${\mathrm{YbMn}}_6{\mathrm{Ge}}_{6-x}{\mathrm{Sn}}_x$ ($x=0.0$, 3.8, 4.2, 4.4, and 5.5) using resonant inelastic x-ray scattering (RIXS). The observed change in the Yb valence with composition (from $\upsilon \sim 3$ for $x=0$ to $\upsilon \sim 2.7$ for $x=5.5$) is likely driven by negative chemical pressure effects due to the Sn for Ge substitution. While Yb is nonmagnetic in ${\mathrm{YbMn}}_6{\mathrm{Ge}}_{0.5}{\mathrm{Sn}}_{5.5}$, intermediate valent Yb magnetically orders at unexpectedly high temperature (up to 90 K) in the alloys with $x=3.8$, 4.2, and 4.4. The three latter alloys further exhibit an increase in the Yb valence upon cooling, which is opposite to the usual behavior. Interactions with the magnetically ordered Mn sublattice are invoked to account for these unprecedented phenomena through a simplified model based on an Anderson Hamiltonian with a Zeeman term mimicking the Mn-Yb exchange interactions. We further show that the Yb magnetic behavior in this series can be interpreted based on Doniach's picture. Compared with standard intermediate valent materials where Yb is embedded in a nonmagnetic matrix, the strong Mn-Yb exchange interaction enhances the intermediate valent Yb magnetic ordering temperature and allows for extending the stability domain of the Yb magnetic order towards lower Yb valence.

We propose a pseudopotential for the electron-electron Coulomb interaction to improve the efficiency of many-body electronic structure calculations. The pseudopotential accurately replicates the scattering properties of the Coulomb interaction, and recovers the analytical solution for two electrons in a parabolic trap. A case study for the homogeneous electron gas using the diffusion Monte Carlo and configuration interaction methods recovers highly accurate values for the ground state energy, and the smoother potential reduces the computational cost by a factor of $\sim 30$. Finally, we demonstrate the use of the pseudopotential to study isolated lithium and beryllium atoms.

Three-dimensional (3D) topological Dirac materials have been under intensive study recently. The layered compound ${\mathrm{ZrTe}}_5$ has been suggested to be one such material as a result of transport and angle-resolved photoemission spectroscopy experiments. Here, we perform infrared reflectivity measurements to investigate the underlying physics of this material. The derived optical conductivity increases linearly with frequency below normal interband transitions, which provides optical spectroscopic proof of a 3D Dirac semimetal. In addition, the plasma edge shifts dramatically to lower energy upon temperature cooling, which might be due to the shrinking of the lattice parameters. In addition, an extremely sharp peak shows up in the frequency-dependent optical conductivity, indicating the presence of a Van Hove singularity in the joint density of state.

We derive exact formulas for bipartite von Neumann entanglement entropy after partial projective local measurement in $(1+1)$-dimensional conformal field theories with periodic and open boundary conditions. After defining the setup we will check numerically the validity of our results in the case of Klein-Gordon field theory (coupled harmonic oscillators) and spin-$1/2 XX$ chain in a magnetic field. The agreement between analytical results and the numerical calculations is very good. We also find a lower bound for localizable entanglement in coupled harmonic oscillators.