The field-induced polarization reversal in the $c$-oriented ferroelectric phase of strained perovskite film has been studied. We show that in addition to the conventional longitudinal switching mechanism, when the $c$-oriented polarization vector changes its modulus, the longitudinal-transversal and transversal mechanisms when the perpendicular component of polarization is dynamically admixed are possible. The latter process can occur either via the straight-abrupt or initially continues polarization turnover scenario. We specified the obtained results for the case of ${\mathrm{PbTiO}}_3$ and ${\mathrm{BaTiO}}_3$ ferroelectrics and propose the experimental methods for their investigation.

The specific heat, magnetization, and thermal expansion of single crystals of the antiferromagnetic insulator EuTe, measured at temperatures down to 2 K and in magnetic fields up to 90 kOe, demonstrate nontrivial properties. The Néel temperature, being $\sim 9.8$ at $H=0$, decreases with magnetic field and tends to zero at $\sim 76 \mathrm{kOe}$, therefore forming a quantum critical point. The heat capacity and thermal expansion coefficient reveal $\lambda$-type anomalies at the second order magnetic phase transition at low magnetic fields, evolving into simple jumps at high magnetic fields and low temperatures; these are well described in a fluctuation-free mean-field theory. The experimental data and the corresponding analysis favor the quantum concept of an effective increase in spatial dimensionality at low temperatures that suppresses a fluctuation-driven divergence at a second-order phase transition.

Ternary oxide ${\mathrm{Ba}}_x{\mathrm{Ti}}_2{\mathrm{O}}_3$ ultrathin films on Au(111) substrates $(x\le 2/3)$ have been studied using a joint experimental and theoretical approach, including the use of scanning tunneling microscopy (STM), first principles calculations, and Monte Carlo simulations. The films are created by first covering the Au(111) substrate with a ${\mathrm{Ti}}_2{\mathrm{O}}_3(2\times 2)$ honeycomb (HC) network and then evaporating low concentrations of Ba atoms onto this film. STM imaging shows that the Ba atoms adsorb individually in the hollow sites of the HC network. Depending on the Ba coverage $x$, which ranges from 0 to 2/3, two ordered phases can be identified at $x=1/3$ and $x=2/3$. A disordered labyrinthlike phase is observed for values of $x$ between 1/3 and 2/3. Theoretical modeling shows that the structural character of these films is driven by the charge transfer that occurs between the electropositive Ba atoms and the electronegative ${\mathrm{Ti}}_2{\mathrm{O}}_3$/Au substrate. This results in a number of calculated effects including an increase in the film rumpling and a reduction of the film work function with increasing $x$. The evolution of the structure of the thin films as a function of Ba coverage can be described by a lattice gas model with first-, second-, and third-neighbor Ba-Ba repulsive interactions. The range of the dipolar interactions is a key factor in understanding the behavior of Ba ordering. The structural and electronic flexibility, which can be engineered through stoichiometry, temperature, or support control, makes these ultrathin films promising materials for applications related to adsorption or reactivity, or as template supports for the growth of size-selected clusters.

The specific heat, the susceptibility under pressure, and the dielectric constant were measured for single crystals ${\mathrm{Y}}_{1-x}{\mathrm{La}}_x{\mathrm{TiO}}_3$. The observed $T^2$-dependent specific heat at low temperatures for $0.17\le x\le 0.3$ samples shows a spin-orbital liquid state between the ferromagnetic/orbital ordering $(x<0.17)$ and antiferromagnetic/possible orbital liquid phase $(x>0.3)$. The nonmonotonous pressure dependence of $T_{\text{C}}$ and the glassy behavior of the dielectric loss for the $x=0.23$ sample suggest that it is approaching a possible quantum critical point. All these properties result from the coupling between the strong spin and orbital fluctuations while approaching the phase boundary.

The connection between noncentrosymmetric materials' structure, electronic structure, and bulk photovoltaic performance remains not well understood. In particular, it is still unclear which photovoltaic (PV) mechanisms are relevant for the recently demonstrated visible-light ferroelectric photovoltaic (K,Ba)(Ni,Nb)${\mathrm{O}}_{3-\delta }$. In this paper, we study the bulk photovoltaic effect (BPVE) of (K,Ba)(Ni,Nb)${\mathrm{O}}_{3-\delta }$ and ${\mathrm{KNbO}}_3$ by calculating the shift current from first principles. The effects of structural phase, lattice distortion, oxygen vacancies, cation arrangement, composition, and strain on BPVE are systematically studied. We find that (K,Ba)(Ni,Nb)${\mathrm{O}}_{3-\delta }$ has a comparable shift current with that of the broadly explored ${\mathrm{BiFeO}}_3$, but for a much lower photon energy. In particular, the Glass coefficient of (K,Ba)(Ni,Nb)${\mathrm{O}}_5$ in a simple layered structure can be as large as 12 times that of ${\mathrm{BiFeO}}_3$. Furthermore, the nature of the wavefunctions dictates the eventual shift current yield, which can be significantly affected and engineered by changing the O vacancy location, cation arrangement, and strain. This is not only helpful for understanding other PV mechanisms that relate to the motion of the photocurrent carriers, but also provides guidelines for the design and optimization of PV materials.

Deviations from Landau Fermi-liquid behavior are ubiquitous features of the normal state of unconventional superconductors. Despite several decades of investigation, the underlying mechanisms of these properties are still not completely understood. In this work, we show that two-dimensional electron liquids at $\mathrm{SrTi}{\mathrm{O}}_3/R\mathrm{Ti}{\mathrm{O}}_3$ $(R=\mathrm{Gd}$ or Sm) interfaces reveal strikingly similar physics. Analysis of Hall and resistivity data shows a clear separation of transport and Hall scattering rates, also known as “two-lifetime” behavior. This framework gives a remarkably simple and general description of the temperature dependence of the Hall coefficient. Distinct transport lifetimes accurately describe the transport phenomena irrespective of the nature of incipient magnetic ordering, the degree of disorder, confinement, or the emergence of non–Fermi-liquid behavior. The Hall scattering rate diverges at a critical quantum well thickness, coinciding with a quantum phase transition. Collectively, these results introduce constraints on the existing microscopic theories of lifetime separation and point to the need for unified understanding.

The high index (441) surface of bismuth has been studied using scanning tunneling microscopy (STM), angle resolved photoemission spectroscopy (APRES), and spin-resolved ARPES. The surface is strongly corrugated, exposing a regular array of (110)-like terraces. Two surface localized states are observed, both of which are linearly dispersing in one in-plane direction ($k_x$), and dispersionless in the orthogonal in-plane direction ($k_y$), and both of which have a Dirac-like crossing at $k_x=0$. Spin ARPES reveals a strong in-plane polarization, consistent with Rashba-like spin-orbit coupling. One state has a strong out-of-plane spin component, which matches with the miscut angle, suggesting its possible origin as an edge state. The electronic structure of Bi(441) has significant similarities with topological insulator surface states and is expected to support one-dimensional quantum spin Hall-like coupled spin-charge transport properties with inhibited backscattering, without requiring a topological insulator bulk.

We report on theoretical and experimental investigation of $A$-site-driven ferroelectricity in ferromagnetic ${\mathrm{La}}_2{\mathrm{NiMnO}}_6$ thin films grown on ${\mathrm{SrTiO}}_3$ substrates. Structural analysis and density-functional theory calculations show that epitaxial strain stretches the rhombohedral ${\mathrm{La}}_2{\mathrm{NiMnO}}_6$ crystal lattice along the $\mathrm{[}111{\mathrm{]}}_{\mathrm{cubic}}$ direction, triggering a displacement of the $A$-site La ions in the double-perovskite lattice. The lattice distortion and the $A$-site displacements stabilize a ferroelectric polar state in ferromagnetic ${\mathrm{La}}_2{\mathrm{NiMnO}}_6$ crystals. The ferroelectric state only appears in the rhombohedral ${\mathrm{La}}_2{\mathrm{NiMnO}}_6$ phase, where ${\mathrm{MnO}}_6$ and ${\mathrm{NiO}}_6$ octahedral tilting is inhibited by the threefold crystal symmetry. Electron localization mapping showed that covalent bonding with oxygen and $6s$ orbital lone-pair formation are negligible in this material.

The phase behavior of multicomponent metallic liquids is exceedingly complex because of the convoluted many-body and many-elemental interactions. Herein, we present systematic studies of the dynamical aspects of a model ternary metallic liquid ${\mathrm{Cu}}_{40}{\mathrm{Zr}}_{51}{\mathrm{Al}}_9$ using molecular dynamics simulations with embedded atom method. We observed a dynamical crossover from Arrhenius to super-Arrhenius behavior in the transport properties (self diffusion coefficient, self relaxation time, and shear viscosity) bordered at $T_x\sim 1300$ K. Unlike in many molecular and macromolecular liquids, this crossover phenomenon occurs well above the melting point of the system $(T_m\sim 900$ K) in the equilibrium liquid state; and the crossover temperature $T_x$ is roughly twice of the glass-transition temperature of the system $\left(T_g\right)$. Below $T_x$, we found the elemental dynamics decoupled and the Stokes-Einstein relation broke down, indicating the onset of heterogeneous spatially correlated dynamics in the system mediated by dynamic communications among local configurational excitations. To directly characterize and visualize the correlated dynamics, we employed a nonparametric, unsupervised machine learning technique and identified dynamical clusters of atoms with similar atomic mobility. The revealed average dynamical cluster size shows an accelerated increase below $T_x$ and mimics the trend observed in other ensemble averaged quantities that are commonly used to quantify the spatially heterogeneous dynamics such as the non-Gaussian parameter ${\alpha }_2$ and the four-point correlation function ${\chi }_4$.

We examine the distribution of the magnetic anisotropy experienced by a magnetic impurity embedded in a metallic nanograin. As an example of a generic magnetic impurity with a partially filled $d$ shell, we study the case of $d^1$ impurities embedded into ordered and disordered Au nanograins, described in terms of a realistic band structure. Confinement of the electrons induces a magnetic anisotropy that is large, and can be characterized by five real parameters, coupling to the quadrupolar moments of the spin. In ordered (spherical) nanograins, these parameters exhibit symmetrical structures and reflect the symmetry of the underlying lattice, while for disordered grains they are randomly distributed and, for stronger disorder, their distribution is found to be characterized by random matrix theory. As a result, the probability of having small magnetic anisotropies $K_L$ is suppressed below a characteristic scale ${\Delta }_E$, which we predict to scale with the number of atoms $N$ as ${\Delta }_E\sim 1/N^{3/2}$. This gives rise to anomalies in the specific heat and the susceptibility at temperatures $T\sim {\Delta }_E$ and produces distinct structures in the magnetic excitation spectrum of the clusters that should be possible to detect experimentally.

We combine point contact spectroscopy with specific heat measurements to probe the superconducting state in noncentrosymmetric ${\mathrm{Nb}}_{0.18}{\mathrm{Re}}_{0.82}$ single crystals. The conductance spectra clearly exhibit a two-peak structure that is well reproduced within a two-band model with isotropic gaps in the spectrum. Such an observation is confirmed by distinct features of the specific heat both at low temperatures and in the range approaching the transition to the normal state. The analyses provide convincing evidence that the two-gap superconducting pairing is a robust feature of ${\mathrm{Nb}}_{0.18}{\mathrm{Re}}_{0.82}$.

For two-dimensional single-valley quadratic band crossing systems with weak repulsive electron-electron interactions, we show that upon introducing a chemical potential, particle-hole order is suppressed and superconductivity becomes the leading instability. In contrast to the two-valley case realized in bilayer graphene, the single-valley quadratic band touching is protected by crystal symmetries, and the different symmetries and number of fermion flavors can lead to distinct phase instabilities. Our results are obtained using a weak-coupling Wilsonian renormalization group procedure on a low-energy effective Hamiltonian relevant for describing electrons on checkerboard or kagome lattices. In fourfold-symmetric systems, we find that $d$- and $s$-wave superconductivity are realized for short-ranged (Hubbard) and longer-ranged (forward scattering) interactions, respectively. In the sixfold-symmetric case, we find either $s$-wave superconductivity or no superconducting instability.

In light of recent proposals to realize a topological superconductor on the surface of strong topological insulators, we study impurity and vortex scattering in two-dimensional topological superconductivity. We develop a theory of quasiparticle interference in a model of the surface of a three-dimensional strong topological insulator with a pairing term added. We consider a variety of different scatterers, including magnetic and nonmagnetic impurity as well as a local pairing order parameter suppression associated with the presence of a vortex core. Similar to the case of a surface of a three-dimensional topological insulator without pairing, our results for nonmagnetic impurity can be explained by the absence of back scattering, as expected for a Dirac cone structure. In the superconducting case, doping away from the Dirac point leads to a doubling of the contours of constant energy. This is in contrast to the unpaired case where the chemical potential simply adds to the bias voltage and shifts the energy. This doubling of contours results in multiplying the number of possible scattering processes in each energy. Interestingly, we find that some processes are dominant in the impurity case while others are dominant in the vortex case. Moreover, the two types of processes lead to a different dependence on the chemical potential.

By means of low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS), we have investigated the adsorption of single Au atoms on a PTCDA monolayer physisorbed on the Au(111) surface. A chemical reaction between the Au atom and the PTCDA molecule leads to the formation of a radical that has an unpaired electron in its highest occupied orbital. This orbital is a $\pi$ orbital that extends over the whole Au-PTCDA complex. Because of the large Coulomb repulsion in this orbital, the unpaired electron generates a local moment when the molecule is adsorbed on the Au(111) surface. We demonstrate the formation of the radical and the existence of the local moment after adsorption by observing a zero-bias differential conductance peak that originates from the Kondo effect. By temperature dependent measurements of the zero-bias differential conductance, we determine the Kondo temperature to be $T_{\mathrm{K}}=(38\pm 8)\mathrm{K}$. For the theoretical description of the properties of the Au-PTCDA complex we use a hierarchy of methods, ranging from density functional theory (DFT) including a van der Waals correction to many-body perturbation theory (MBPT) and the numerical renormalization group (NRG) approach. Regarding the high-energy orbital spectrum, we obtain an excellent agreement with experiments by both spin-polarized DFT/MBPT and NRG. Moreover, the NRG provides an accurate description of the low-energy excitation spectrum of the spin degree of freedom, predicting a Kondo temperature very close to the experimental value. This is achieved by a detailed analysis of the universality of various definitions of $T_{\mathrm{K}}$ and by taking into account the full energy dependence of the coupling function between the molecule-metal complex and the metallic substrate.

We apply the diagrammatic Monte Carlo approach to three-dimensional Fermi-polaron systems with mass imbalance, where an impurity interacts resonantly with a noninteracting Fermi sea whose atoms have a different mass. This method allows us to go beyond frequently used variational techniques by stochastically summing all relevant impurity Feynman diagrams up to a maximum expansion order limited by the sign problem. Polaron energy and quasiparticle residue can be accurately determined over a broad range of impurity masses. Furthermore, the spectral function of an imbalanced polaron demonstrates the stability of the quasiparticle and allows us to locate in addition also the repulsive polaron as an excited state. The quantitative exactness of two-particle-hole wave functions is investigated, resulting in a relative lowering of polaronic energies in the mass-imbalance phase diagram. Tan's contact coefficient for the mass-balanced polaron system is found in good agreement with variational methods. Mass-imbalanced systems can be studied experimentally by ultracold atom mixtures such as ${}^6\mathrm{Li}-{}^{40}\mathrm{K}$.

Electronic spin-orbit coupling (SOC) is essential for various newly discovered phenomena in condensed-matter systems. In particular, one-dimensional topological heterostructures with SOC have been widely investigated in both theory and experiment for their distinct transport signatures indicating the presence of emergent Majorana fermions. However, a general framework for the SOC-affected transport in superconducting heterostructures, especially with the consideration of interfacial effects, has not been developed even regardless of the topological aspects. We hereby provide one for an effectively one-dimensional superconductor-normal heterostructure with nonuniform magnitude and direction of both Rashba and Dresselhaus SOC as well as a spin-orbit-active interface. We extend the Blonder-Tinkham-Klapwijk treatment to analyze the current-voltage relation and obtain a rich range of transport behaviors. Our work provides a basis for characterizing fundamental physics arising from spin-orbit interactions in heterostructures and its implications for topological systems, spintronic applications, and a whole variety of experimental setups.

We have investigated the evolution of the electronic properties of $\mathrm{L}{\mathrm{a}}_{1-x}\mathrm{S}{\mathrm{r}}_x\mathrm{Cr}{\mathrm{O}}_3(0\le x\le 1)$ epitaxial films deposited by molecular beam epitaxy (MBE) using x-ray diffraction, x-ray photoemission spectroscopy, Rutherford backscattering spectrometry, x-ray absorption spectroscopy, electrical transport, and ab initio modeling. $\mathrm{LaCr}{\mathrm{O}}_3$ is an antiferromagnetic insulator, whereas $\mathrm{SrCr}{\mathrm{O}}_3$ is a metal. Substituting $\mathrm{S}{\mathrm{r}}^{2+}$ for $\mathrm{L}{\mathrm{a}}^{3+}$ in $\mathrm{LaCr}{\mathrm{O}}_3$ effectively dopes holes into the top of valence band, leading to $\mathrm{C}{\mathrm{r}}^{4+}$ (${3d}^2$) local electron configurations. Core-level and valence-band features monotonically shift to lower binding energy with increasing $x$, indicating downward movement of the Fermi level toward the valence band maximum. The material becomes a $p$-type semiconductor at lower doping levels and an insulator-to-metal transition is observed at $x\ge 0.65$, but only when the films are deposited with in-plane compression via lattice-mismatched heteroepitaxy. Valence-band x-ray photoemission spectroscopy reveals diminution of electronic state density at the $\mathrm{Cr}d{t}_{2g}$-derived top of the valence band, while O K-edge x-ray absorption spectroscopy shows the development of a new unoccupied state above the Fermi level as holes are doped into $\mathrm{LaCr}{\mathrm{O}}_3$. The evolution of these bands with Sr concentration is accurately captured using density functional theory (DFT) with a Hubbard U correction of 3.0 eV $(\mathrm{DFT}+U)$. Resistivity data in the semiconducting regime $(x\le 0.50)$ do not fit perfectly well to either a polaron hopping or band conduction model but are best interpreted in terms of a hybrid model. The activation energies extracted from these fits are well reproduced by $\mathrm{DFT}+U$.

We investigate exciton-polariton condensation in a magnetic field in a single high-quality semiconductor micropillar cavity. We observe successive polariton condensation of each spin component for two distinct threshold powers. Pronounced and nonmonotonous variations of both the Zeeman splitting and the circular polarization of the emission are measured across these two condensation thresholds. This unexpected behavior deeply deviates from the so-called spin-Meissner effect predicted for a fully thermalized system. Our measurements can be understood in a kinetic approach taking into account spin-anisotropic interactions within the entire system: the polariton condensate and the cloud of uncondensed excitons.