In recent years, there has been an increased interest in octahedral rotations in perovskite materials, particularly on their response to strain in epitaxial thin films. The current theoretical framework assumes that rotations are affected primarily through the change in in-plane lattice parameters imposed by coherent heteroepitaxy on a substrate of different lattice constant. This model, which permits prediction of the thin-film rotational pattern using first-principles density functional theory, has not been tested quantitatively over a range of strain states. To assess the validity of this picture, coherent LaAlO${}_3$ thin films were grown on SrTiO${}_3$, NdGaO${}_3$, LaSrAlO${}_4$, NdAlO${}_3$, and YAlO${}_3$ substrates to achieve strain states ranging from $+$3.03$\%$ to $-$2.35$\%$. The out-of-plane and in-plane octahedral rotation angles were extracted from the intensity of superlattice reflections measured using synchrotron x-ray diffraction. Density functional calculations show that no measurable change in intrinsic defect concentration should occur throughout the range of accessible strain states. Thus, the measured rotation angles were compared with those calculated previously for defect-free films. [Hatt and Spaldin, Phys. Rev. B 82, 195402 (2010)]. Good agreement between theory and experiment was found, suggesting that the current framework correctly captures the appropriate physics in LaAlO${}_3$.

We report the results of a search for spontaneous magnetism due to a time-reversal symmetry breaking phase in the superconducting state of (110)-oriented YBa${}_2$Cu${}_3$O${}_{7-\delta }$ films, expected near the surface in this geometry. Zero field and weak transverse field measurements performed using the low-energy muon spin rotation technique with muons implanted a few nm inside optimally doped YBa${}_2$Cu${}_3$O${}_{7-\delta }$-(110) films show no appearance of spontaneous magnetic fields below the superconducting temperature down to 2.9 K. Our results give an upper limit of $\sim$0.02 mT for putative spontaneous internal fields.

We study, both numerically and analytically, the finite-size scaling of the fidelity susceptibility ${\chi }_J$ with respect to the charge or spin current in one-dimensional lattice models and relate it to the low-frequency behavior of the corresponding conductivity. It is shown that in gapless systems with open boundary conditions the leading dependence on the system size $L$ stems from the singular part of the conductivity and is quadratic, with a universal form ${\chi }_J=[7\zeta \left(3\right)/2{\pi }^4]K{L}^2$, where $K$ is the Luttinger liquid parameter and $\zeta \left(x\right)$ is the Riemann $\zeta$ function. In contrast to that for periodic boundary conditions the leading system size dependence is directly connected to the regular part of the conductivity and is subquadratic, ${\chi }_J\propto L^{\gamma }$, where the $K$-dependent exponent $\gamma$ is equal to 1 in most situations (as a side effect, this relation provides an alternative way to study the low-frequency behavior of the regular part of the conductivity). For open boundary conditions, we also study another current-related quantity, the fidelity susceptibility to the lattice tilt ${\chi }_{\mathcal{P}}$, and show that it scales as the quartic power of the system size, ${\chi }_{\mathcal{P}}=[31\zeta \left(5\right)/8{\pi }^6](K{L}^4/u^2)$, where $u$ is the sound velocity. Thus, the ratio $L^2{\chi }_J/{\chi }_{\mathcal{P}}$ directly measures the sound velocity in open chains. The behavior of the current fidelity susceptibility in gapped phases is discussed, particularly in the topologically ordered Haldane state.

A tree tensor network variational method is proposed to simulate quantum many-body systems with global symmetries where the optimization is reduced to individual charge configurations. A computational scheme is presented, namely how to extract the entanglement spectra in a bipartite splitting of a loopless tensor network across multiple links of the network, by constructing a matrix product operator for the reduced density operator and simulating its eigenstates. The entanglement spectra of $2\times L$, $3\times L$, and $4\times L$ with either open or periodic boundary conditions on the rungs are studied using the presented methods, where it is found that the entanglement spectrum depends not only on the subsystem but also on the boundaries between the subsystems.

Detailed ${}^{195}$Pt and ${}^{171}$Yb nuclear magnetic resonance (NMR) studies on the heterogeneous mixed valence system YbPtGe${}_2$ are reported. The temperature dependence of the ${}^{195}$Pt-NMR shift ${}^{195}K\left(T\right)$ indicates the opening of an unusual magnetic gap below 200 K. ${}^{195}K\left(T\right)$ was analyzed by a thermal activation model which yields an isotropic gap $\Delta /k_B\approx 200$ K. In contrast, the spin-lattice relaxation rate ${}^{195}$($1/T_1$) does not provide evidence for the gap. Therefore, an intermediate-valence picture is proposed while a Kondo-insulator scenario can be excluded. Moreover, ${}^{195}$($1/T_1$) follows a simple metallic behavior, similar to the reference compound YPtGe${}_2$. A well-resolved NMR line with small shift is assigned to divalent ${}^{171}$Yb. This finding supports the proposed model with two subsets of Yb species (di- and trivalent) located on the Yb2 and Yb1 site of the YbPtGe${}_2$ lattice.

The recent theoretical discovery of fractional Chern insulators (FCIs) has provided an important new way to realize topologically ordered states in lattice models. In earlier works, on-site and nearest-neighbor Hubbard-like interactions have been used extensively to stabilize Abelian FCIs in systems with nearly flat, topologically nontrivial bands. However, attempts to use two-body interactions to stabilize non-Abelian FCIs, where the ground state in the presence of impurities can be massively degenerate and manipulated through anyon braiding, have proven very difficult in uniform lattice systems. Here, we study the remarkable effect of long-range interactions in a lattice model that possesses an exactly flat lowest band with a unit Chern number. When spinless bosons with two-body long-range interactions partially fill the lowest Chern band, we find convincing evidence of gapped, bosonic Read-Rezayi (RR) phases with non-Abelian anyon statistics. We characterize these states through studying topological degeneracies, the overlap between the ground states of two-body interactions and the exact RR ground states of three- and four-body interactions, and state counting in the particle-cut entanglement spectrum. Moreover, we demonstrate how an approximate lattice form of Haldane's pseudopotentials, analogous to that in the continuum, can be used as an efficient guiding principle in the search for lattice models with stable non-Abelian phases.

We present a coarse-grained Monte Carlo method designed for the simulation of vacancy-mediated diffusion phenomena. A coarse-grained master equation is derived from the atomic scale master equation using an assumption of local equilibrium within each coarse-grained cell. Atomic kinetic Monte Carlo (AKMC) simulations are used to perform both the thermodynamic and the kinetic parametrization of the coarse-grained simulations. Quantitative reproduction of flux couplings is achieved in the coarse-grained simulation. The ability of the CKMC method to simulate kinetics controlled by diffusion such as a precipitate dissolution and the decay of sinusoidal modulations of the concentration field is illustrated on body-centered-cubic (bcc) model alloys with a clustering tendency. It is shown on the case of a model of the Fe-Cu alloy previously developed that the use of this method can reduce the computational cost of a kinetic simulation by several orders of magnitude.

We report low-temperature specific heat and positive muon spin relaxation/rotation ($\mu$SR) measurements on both polycrystalline and single-crystal samples of the pyrochlore magnet Yb${}_2$Ti${}_2$O${}_7$. This material is believed to possess a spin Hamiltonian able to support a quantum spin ice (QSI) ground state. Yb${}_2$Ti${}_2$O${}_7$ displays sample variation in its low-temperature heat capacity and, while our two samples exhibit extremes of this variation, our $\mu$SR measurements indicate a similar disordered low-temperature state down to 16 mK in both. We report little temperature dependence to the muon spin relaxation and no evidence for ferromagnetic order, in contrast to reports by Chang et al. [Nat. Comm. 3, 992 (2012)] and Yasui et al. [J. Phys. Soc. Japan. 72, 11 (2003)]. Transverse field (TF) $\mu$SR measurements show changes in the temperature dependence of the muon Knight shift that coincide with heat capacity anomalies, which, incidentally, prove that the implanted muons are not diffusing in Yb${}_2$Ti${}_2$O${}_7$. From these results, we are led to propose that Yb${}_2$Ti${}_2$O${}_7$ enters an unconventional ground state below $T_c\sim 265$ mK. As found for all the current leading experimental candidates for a quantum spin liquid state, the precise nature of the state below $T_c$ in Yb${}_2$Ti${}_2$O${}_7$ remains unknown and, at this time, defined by what is not as opposed to what it is: lacking simple periodic long-range order or a frozen spin glass state.

Spin-filter tunneling is a promising way to generate highly spin-polarized current, a key component for spintronics applications. In this paper we explore the tunneling conductance across the spin-filter material CoFe${}_2$O${}_4$ interfaced with Au electrodes, a geometry which provides nearly perfect lattice matching at the CoFe${}_2$O${}_4$/Au(001) interface. Using density functional theory calculations we demonstrate that interface states play a decisive role in controlling the transport spin polarization in this tunnel junction. For a realistic CoFe${}_2$O${}_4$ barrier thickness, we predict a tunneling spin polarization of about $-60\%$. We show that this value is lower than what is expected based solely on considerations of the spin-polarized band structure of CoFe${}_2$O${}_4$, and therefore that these interface states can play a detrimental role. We argue that this is a rather general feature of ferrimagnetic ferrites and could make an important impact on spin-filter tunneling applications.

The total spectral weight $S$ of the emergent low-energy quasiparticles in high-temperature superconductors (HTSCs) is explored by x-ray absorption spectroscopy. In order to examine the applicability of the Hubbard model, regimes that cover from zero doping to overdoping are investigated. In contrast to results of mean-field theory, we found that $S$ deviates from linear dependence on the doping level $p$. The slope of $S$ versus $p$ changes continuously throughout the whole doping range with no sign of saturation up to $p=0.23$. Therefore, the picture of the Zhang-Rice singlet remains intact within the most prominent doping regimes of HTSCs.

For a mobile spin-1/2 impurity, coupled antiferromagnetically to a one-dimensional gas of fermions, perturbative ideas have been used to argue in favor of two-channel Kondo behavior of the impurity spin. Here we combine general considerations and extensive numerical simulations to show that the problem displays a novel quantum phase transition between two-channel and one-channel Kondo screening upon increasing the Kondo coupling. We construct a ground-state phase diagram and discuss the various nontrivial crossovers as well as possible experimental realizations.

We present an exchange bias (EB) effect of variable size in La${}_{0.67}$Sr${}_{0.33}$MnO${}_{3-\delta }$ single and La${}_{0.67}$Sr${}_{0.33}$MnO${}_{3-\delta }$/SrTiO${}_3$ (LSMO/STO) bilayers grown on STO substrates. Samples have been prepared by pulsed laser deposition (PLD) and high-pressure sputter deposition (HSD) in oxygen atmosphere at different oxygen pressures. Increased out-of-plane lattice parameters of the LSMO layers and reduced Curie temperatures indicate oxygen deficiencies in the samples grown at lower oxygen pressures. We found an increase of the EB and the coercive fields with decreasing oxygen pressure, whereas the growth at high pressures does not lead to an EB effect. The depth-sensitive method of polarized neutron reflectometry (PNR) reveals a region with drastically reduced, maybe even completely vanishing net magnetization in the LSMO layer at the interface to STO in the exchange-biased samples, but not in the non-exchanged-biased one. It is suggested that an antiferromagnetic structure is formed in this part of the LSMO thin film, which causes the EB effect in this system. An explanation based on the strain-doping phase diagram of LSMO is provided. Thus our findings open a route for tuning the EB effect by changing the oxygen vacancy concentrations.

$\alpha$-MnO${}_2$, an active catalyst for oxygen reduction and evolution reactions, has been investigated using ab initio calculations with different exchange-correlation functionals: the generalized-gradient approximation in the version of Perdew, Burke, and Ernzerhof (PBE), PBE$+U$, and hybrid functionals. Both hybrid functionals and PBE$+U$ ($U\ge 2.0$ eV) fail to capture the antiferromagnetic (AFM) ground state found experimentally, and a ferromagnetic configuration has the lowest energy. An AFM ground state is then recovered when using PBE or PBE$+U$ ($U\le 1.6$ eV). Interestingly, a reduction of the gap is observed at increasing values of the $U$ parameter. We offer a qualitative explanation for the change in the calculated ground state employing the results for the electronic structure and physical arguments similar to those exposed in the Goodenough-Kanamori-Anderson rules. It is argued that the $p$${}_z$ orbital of oxygen atoms with $sp$${}^2$ hybridization plays a fundamental role in the superexchange AFM interaction and in the reduction of the gap.

Following the theoretical approach by J. Xiao et al. [Phys. Rev. B 81, 214418 (2010)] to the spin Seebeck effect, we calculate the mean value of the total spin current flowing through a normal metal/ferromagnet interface. The spin current emitted from the ferromagnet to the normal metal is evaluated in the framework of the Fokker-Planck approach for the stochastic Landau-Lifshitz-Gilbert equation. We show that the total spin current depends not only on the temperature difference between the electron and the magnon baths, but also on the external magnetic field and magnetic anisotropy. Apart from this, the spin current is shown to saturate with increasing magnon temperature, and the saturation temperature increases with increasing magnetic field and/or magnetic anisotropy.

The magnetic proximity effect is a fundamental feature of heterostructures composed of layers of topological insulators and magnetic materials since it underlies many potential applications in devices with novel quantum functionality. Within density functional theory we study magnetic proximity effect at the three-dimensional topological insulator/magnetic insulator (TI/MI) interface in the Bi${}_2$Se${}_3$/MnSe(111) system as an example. We demonstrate that a gapped ordinary bound state caused by the interface potential arises in the immediate region of the interface. The gapped topological Dirac state also arises in the system owing to relocation to deeper atomic layers of topological insulator. The gap in the Dirac cone originates from an overlapping of the topological and ordinary interfacial states. This result being also corroborated by the analytic model, is a key aspect of the magnetic proximity effect mechanism in the TI/MI structures.

We present the results of a theoretical study of current-phase relations (CPRs) $J_S\left(\phi \right)$ in Josephson junctions of SIsFS type, where S is a bulk superconductor and IsF is a complex weak link consisting of a superconducting film s, a metallic ferromagnet F, and an insulating barrier I. At temperatures close to critical, $T\lesssim T_C$, calculations are performed analytically in the frame of the Ginsburg-Landau equations. At low temperatures a numerical method is developed to solve self-consistently the Usadel equations in the structure. We demonstrate that SIsFS junctions have several distinct regimes of supercurrent transport and we examine spatial distributions of the pair potential across the structure in different regimes. We study the crossover between these regimes, which is caused by shifting the location of a weak link from the tunnel barrier I to the F layer. We show that strong deviations of the CPR from sinusoidal shape occur even in the vicinity of $T_C$, and these deviations are strongest in the crossover regime. We demonstrate the existence of temperature-induced crossover between 0 and $\pi$ states in the contact and show that the smoothness of this transition strongly depends on the CPR shape.

We report the angular dependence of three distinct de Haas–van Alphen (dHvA) frequencies of the torque magnetization in the itinerant antiferromagnet CrB${}_2$ at temperatures down to 0.3 K and magnetic fields up to 14 T. Comparison with the Fermi surface calculations considering an incommensurate cycloidal magnetic order suggests that two of the observed dHvA oscillations arise from electronlike Fermi surface sheets formed by bands with strong B-$p_{\text{x,y}}$ character. The third orbit could correspond to a Cr-$d$ derived Fermi surface sheet. The measured effective masses of these Fermi surface sheets display strong enhancements of a factor of about 2 over the calculated band masses which can be attributed to electron-phonon coupling and electronic correlations. Signatures of further heavy $d$-electron bands that are predicted by the calculations are not observed in the temperature and field range studied. In the view that the B-$p$ bands are at the heart of conventional high-temperature superconductivity in the isostructural MgB${}_2$, we consider possible implications of our findings for nonmagnetic CrB${}_2$ and an interplay of itinerant antiferromagnetism with superconductivity.

Organic charge-transfer salts based on the molecule Pd(dmit)${}_2$ display strong electronic correlations and geometrical frustration, leading to spin-liquid, valence bond solid, and superconducting states, among other interesting phases. The low-energy electronic degrees of freedom of these materials are often described by a single band model: a triangular lattice with a molecular orbital representing a Pd(dmit)${}_2$ dimer on each site. We use ab initio electronic structure calculations to construct and parametrize low-energy effective model Hamiltonians for a class of Me${}_{4-n}$ Et${}_{n}X$[Pd(dmit)${}_2$]${}_2$ ($X=$ As, P, N, Sb) salts and investigate how best to model these systems by using variational Monte Carlo simulations. Our findings suggest that the prevailing model of these systems as a $t-t^{\prime }$ triangular lattice is incomplete and that a fully anisotropic triangular lattice description produces importantly different results, including a significant lowering of the critical $U$ of the spin-liquid phase.

Thermal noise can destroy topological insulators (TI). However, we demonstrate how TIs can be made stable in dissipative systems. To that aim, we introduce the notion of band Liouvillian as the dissipative counterpart of band Hamiltonian, and show a method to evaluate the topological order of its steady state. This is based on a generalization of the Chern number valid for general mixed states (referred to as density-matrix Chern value), which witnesses topological order in a system coupled to external noise. Additionally, we study its relation with the electrical conductivity at finite temperature, which is not a topological property. Nonetheless, the density-matrix Chern value represents the part of the conductivity which is topological due to the presence of quantum mixed edge states at finite temperature. To make our formalism concrete, we apply these concepts to the two-dimensional Haldane model in the presence of thermal dissipation, but our results hold for arbitrary dimensions and density matrices.

We develop a theory for the interaction of classical light fields within a chain of coupled quantum dots (QDs), in the strong-coupling regime, taking into account the local-field effects. The QD chain is modeled by a one-dimensional periodic array of two-level quantum particles with tunnel coupling between adjacent ones. The local-field effect is taken into regard as QD depolarization in the Hartree-Fock-Bogoliubov approximation. The dynamics of the chain is described by a system of two discrete nonlinear Schrödinger (DNLS) equations for local amplitudes of the probabilities of the ground and first excited states. The two equations are coupled by cross-phase-modulation cubic terms, produced by the local-field action, and by linear terms also. In comparison to previously studied DNLS systems, an essentially new feature is a phase shift between the intersite-hopping constants in the two equations. By means of numerical solutions, we demonstrate that, in this QD chain, Rabi oscillations (RO) self-trap into stable bright Rabi solitons or Rabi breathers. The mobility of the solitons is considered as well. The related behavior of the observable quantities, such as energy, inversion, and electric-current density, is given a physical interpretation. The results apply to a realistic region of physical parameters.