FeSe is a unique member of the family of iron-based superconductors, not only because of the high values of $T_c$ in FeSe monolayer, but also because in bulk FeSe superconductivity emerges inside a nematic phase without competing with long-range magnetic order. Near $T_c$, superconducting order necessarily has $s+d$ symmetry, because nematic order couples linearly the $s$-wave and $d$-wave harmonics of the superconducting order parameter. Here we argue that the near-degeneracy between $s$-wave and $d$-wave pairing instabilities in FeSe, combined with the sign-change of the nematic order parameter between hole and electron pockets, allows the superconducting order to break time-reversal symmetry at a temperature $T^{*}<T_c$. The transition from an $s+d$ state to an $s+{\mathrm{e}}^{i\alpha }d$ state should give rise to a peak in the specific heat and to the emergence of a soft collective mode that can be potentially detected by Raman spectroscopy.

We report temporal changes in Raman-scattering spectra of detwinned ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ single crystals under exposure to red laser light polarized along the crystallographic $a$ axis. A recent publication by Bakr et al. [Phys. Rev. B 88, 214517 (2013)] identified new Raman-active modes that appear upon cooling below $T\sim 200$ K and attributed these modes to charge ordering phenomena observed in x-ray scattering and nuclear magnetic resonance experiments on the same materials. Here we report that the intensities of these modes depend not only on temperature and oxygen content but also on the cumulative photon dose absorbed by the ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ samples. The light-induced changes in the Raman spectra exhibit a stretched-exponential time dependence, with a characteristic time that increases strongly upon cooling. They also depend strongly on doping and are most pronounced for $x\sim 0.6$. In fully illuminated samples, the mode intensities exhibit a monotonic temperature dependence indicative of a second-order phase transition. The findings indicate a metastable state generated by light-induced rearrangement of the oxygen dopants. We hypothesize that the new Raman phonons are associated with a three-dimensional charge-ordered state induced by light illumination, analogous to a state that was recently observed by resonant x-ray scattering in oxygen-disordered ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ films.

Some $AB{X}_3$ perovskites exhibit different local environments (DLE) for the same $B$ atoms in the lattice, an effect referred to as disproportionation, distinguishing such compounds from common perovskites that have single local environments (SLE). The basic phenomenology associated with such disproportionation involves the absence of $B$-atom charge ordering, the creation of different B-X bond length (“bond alternation”) for different local environments, the appearance of metal (in SLE) to insulator (in DLE) transitions, and the formation of ligand holes. We point out that this phenomenology is common to a broad range of chemical bonding patterns in $AB{X}_3$ compounds, either with s-p electron $B$-metal cations (${\mathrm{BaBiO}}_3$, ${\mathrm{CsTlF}}_3$) or with noble-metal cations (${\mathrm{CsAuCl}}_3$), as well as with $d$-electron cations (${\mathrm{SmNiO}}_3$, ${\mathrm{CaFeO}}_3$). We show that underlying much of this phenomenology is the “self-regulating response,” whereby in strongly bonded metal-ligand systems with high-lying ligand orbitals, the system protects itself from creating highly charged cations by transferring ligand electrons to the metal, thus preserving a nearly constant metal charge in different local environments, while creating $B$-ligand bond alternation and ligand-like conduction band (“ligand hole” states). We are asking what are the minimal theory ingredients needed to explain the main features of this SLE-to-DLE phenomenology, such as its energetic driving force, bond length changes, possible modifications in charge density, and density of state changes. Using as a guide the lowering of the total energy in DLE relative to SLE, we show that density functional calculations describe this phenomenology across the whole chemical bonding range without resort to special strong correlation effects, beyond what DFT naturally contains. In particular, lower total energy configurations (DLE) naturally develop bond alternation, gapping of the metallic SLE state, and absence of charge ordering with ligand hole formation.

We discuss the possibility of significantly enhancing the nonlinear electro-optical response in strained perovskite $\mathrm{BaTi}{\mathrm{O}}_3$. First-principles calculations predict the enhancement for both compressive and tensile strain. The physical origin can be traced to strain-induced phonon softening that results in diverging first-order susceptibility. Within the Landau-Ginzburg-Devonshire formalism we demonstrate how, in turn, this divergence results in a diverging second-order susceptibility and Pockels coefficient. Our results suggest a way to optimize $\mathrm{BaTi}{\mathrm{O}}_3$ films for use in silicon nanophotonics.

The thermal excitation of electrons to higher, unoccupied states leads in certain cases to the paradox situation that the chemical potential needs to be shifted to lower energies. Here, a manipulation of Dirac fermions through the temperature dependence of the chemical potential is analyzed while maintaining sufficient insulating character in the bulk. The appearance of a bulk conduction band and consequently, a remarkable energy shift of the Dirac point is observed in ${\mathrm{Bi}}_2{\mathrm{Te}}_2\mathrm{Se}$ at low temperatures (20 K), compared to the high temperatures (200–300 K), as revealed by hard x-ray photoelectron spectroscopy as well as momentum-resolved photoelectron microscopy. The temperature-induced shift of the Dirac cone and the appearance of bulk bands are related to the paradoxical shift of the chemical potential. The experiments are completely reversible, i.e., repeated cooling and heating of the sample up to room temperature recovers the original spectra. Such bulk related energy shifts must be considered not only in photoelectron spectroscopy but also in analysis of other measurable physical quantities and while designing devices for applications.

This work presents both the parametrization procedure of a self-consistent charge density functional based tight-binding scheme (SCC-DFTB) for hafnium oxide and the structural as well as the electronic results of the simulations of amorphous hafnium oxide $a\text{-HfO}{}_2$ using the data resulting during the parametrization. While aiming at describing amorphous hafnium oxide, the created parameters could be successfully compared to ab initio DFT data for hafnium and hafnium oxide crystalline structures. Furthermore the atomic structure and electronic properties obtained in the DFTB simulations of $a\text{-HfO}{}_2$ compare well to other theoretical studies using classical many-body as well as quantum-mechanical approaches.

Bethe ansatz and bosonization procedures are used to describe the thermodynamics of the strong-coupled Hubbard chain in the spin-incoherent Luttinger liquid (LL) regime: $J(\equiv 4t^2/U)\ll k_{B}T\ll E_F$, where $t$ is the hopping amplitude, $U(\gg t)$ is the repulsive on-site Coulomb interaction, and $k_{B}T(E_F\sim t)$ is the thermal (Fermi) energy. We introduce a fractional Landau LL approach, whose $U=\infty$ fixed point is exactly mapped onto an ideal gas with two species obeying the Haldane-Wu exclusion fractional statistics. This phenomenological approach sheds light on the behavior of several thermodynamic properties in the spin-incoherent LL regime: specific heat, charge compressibility, magnetic susceptibility, and Drude weight. In fact, besides the hopping (mass) renormalization, the fractional Landau LL parameters, due to quasiparticle interaction, are determined and relationships with velocities of holons and spinons are unveiled. The specific heat thus obtained is in very good agreement with previous density matrix renormalization group (DMRG) simulations of the $t\text{−}J$ model in the spin-incoherent regime. A phase diagram is provided and two thermodynamic paths to access this regime clarifies both the numerical and analytical procedures. Further, we show that the high-$T$ limit of the fractional Landau LL entropy and chemical potential exhibit the expected results of the $t\text{−}J$ model, under the condition $U\gg k_{B}T$. Lastly, finite-temperature Lanczos simulations of the single-particle distribution function confirm the characteristics of the spin-incoherent regime and the high-$T$ limit observed in previous DMRG studies.

The hyper-kagome lattice iridate ${\mathrm{Na}}_3{\mathrm{Ir}}_3{\mathrm{O}}_8$, closely related to spin liquid candidate ${\mathrm{Na}}_4{\mathrm{Ir}}_3{\mathrm{O}}_8$, is unusual in that spin-orbit interactions acting on the 1/3-filled $J_{\mathrm{eff}}=1/2$ state lead to a semimetallic ground state, in contrast to the conventional insulating Mott state stabilized by S-O interactions in the 1/2-filled $J_{\mathrm{eff}}=1/2$ state of other iridates including ${\mathrm{Na}}_4{\mathrm{Ir}}_3{\mathrm{O}}_8$. We have studied the evolution of crystal structure, electronic structure, and transport in ${\mathrm{Na}}_3{\mathrm{Ir}}_3{\mathrm{O}}_8$ under high pressure using x-ray diffraction, x-ray absorption near edge structure, and electrical resistance measurements in a diamond anvil cell. The study was augmented by the use of ab initio calculations, which provided insight into pressure-induced changes in crystal and electronic structure. We found that ${\mathrm{Na}}_3{\mathrm{Ir}}_3{\mathrm{O}}_8$ transforms from a semimetal to an insulator under pressure, with an estimated energy gap that increases to about 130 meV at $P\sim$ 9 GPa. At approximately 10 GPa, a cubic-to-monoclinic structural transition takes place between two insulating phases. This structural transition features a sizable volume collapse and a high-pressure phase characterized by apparent dimerization of Ir-Ir distances, wide dispersion of Ir-O-Ir bond angles, and an increase in the occupation of $5d$ states. Although the energy gap is reduced in the high-pressure phase, insulating behavior remains to the highest pressures $\sim 1$ Mbar. The strongly pressure-dependent insulating gap shows a positive correlation with the expectation value of the angular part of the S-O interaction, $〈\mathbf{L}·\mathbf{S}〉$, which may indicate an active role of S-O interactions in stabilizing the insulating state that emerges in the compressed ${\mathrm{Na}}_3{\mathrm{Ir}}_3{\mathrm{O}}_8$ structure.

Multiferroic hexagonal manganites ${\mathrm{RMnO}}_3$ have a very high ferroelectric transition temperature around and above 1200 K, depending on the rare-earth elements, and a reasonably large electric polarization of about $5.5\mu \mathrm{C}/\mathrm{c}{\mathrm{m}}^2$ at room temperature. It is generally believed that the ferroelectric transition is driven by the combination of $\text{R-}{\mathrm{O}}_{\mathrm{P}}$ displacement and ${\mathrm{MnO}}_5$ tilting, and hence called improper ferroelectric. In order to better understand the improper ferroelectric transition, we studied doping effects, using two elements with the same valence but a different ionic size: Al and Ga on the Mn site of ${\mathrm{YMnO}}_3$. Through detailed structural studies and nanoscale measurements of piezoresponse force microscopy (PFM) we conclude that there is a drastic doping effect for Al, whose ionic size is much smaller than Mn. It is in stark contrast with our observation that Ga, having a slightly smaller ionic size with Mn, does not change the ferroelectric transition up to 50% doping. This drastic difference in the doping effect is due to local strain induced by the difference in the ionic size of Al and Mn as compared with that of Mn, and sheds light on the intriguing nature of the improper ferroelectric transition.

Recently, the doping of topological insulators has attracted significant interest as a potential route towards topological superconductivity. Because many experimental techniques lack sufficient surface sensitivity, however, definite proof of the coexistence of topological surface states and surface superconductivity is still outstanding. Here we report on highly surface sensitive scanning tunneling microscopy and spectroscopy experiments performed on Tl-doped ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, a three-dimensional topological insulator which becomes superconducting in the bulk at $T_{\mathrm{C}}=2.3$ K. Landau level spectroscopy as well as quasiparticle interference mapping clearly demonstrated the presence of a topological surface state with a Dirac point energy $E_{\mathrm{D}}=-(118\pm 1)$ meV and a Dirac velocity $v_{\mathrm{D}}=(4.7\pm 0.1)\times {10}^5$ m/s. Tunneling spectra often show a superconducting gap, but temperature- and field-dependent measurements show that both $T_{\mathrm{C}}$ and ${\mu }_0{H}_{\mathrm{C}}$ strongly deviate from the corresponding bulk values. Furthermore, in spite of a critical field value which clearly points to type-II superconductivity, no Abrikosov lattice could be observed. Experiments performed on normal-metallic Ag(111) prove that the gapped spectrum is caused only by superconducting tips, probably caused by a gentle crash with the sample surface during approach. Nearly identical results were found for the intrinsically $n$-type compound Nb-doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. Our results suggest that the superconductivity in superconducting-doped V-VI topological insulators does not extend to the surface where the topological surface state is located.

A topological boundary can be formed at the interface between a trivial and a topological insulator. The difference in the topological index across the junction leads to robust gapless surface states. Optical studies of these states are scarce in the literature, the reason being the difficulty in isolating their response from that of the bulk. In this work, we propose to deposit a $\delta$ layer of donor impurities in close proximity to a topological boundary to help in detecting gapless surface states. As we will show, gapless surface states are robust against this perturbation and they enhance intraband optical transitions as measured by the oscillator strength. These results help us to understand the interplay of surface and bulk states in topological insulators.

The study shows that presence of the quantum coherent, unitary component of the evolution of the system can improve constancy of heat engines, i.e., decrease fluctuations of the output power, in comparison with purely stochastic setups. This enables us to overcome the recently derived trade-off between efficiency, power, and constancy, which applies to classical Markovian steady-state heat engines. The concept is demonstrated using a model system consisting of two tunnel-coupled orbitals (i.e., electronic levels), each attached to a separate electronic reservoir; such a setup can be realized, for example, using quantum dots. Electronic transport is studied by means of the exact Levitov-Lesovik formula in the case without the Coulomb interaction between electrons, as well as applying a quantum master equation in the interacting case. Constancy of the analyzed thermoelectric generator is increased due to the fact that tunneling between the orbitals is associated with a unitary evolution of the electron state instead of a stochastic Poisson transition. This reduces stochasticity of the system, thus suppressing the current and power fluctuations. Moreover, noise can be further reduced by the Coulomb interaction between electrons, which prevents the double occupancy of the system.

While edge pinning is known to play an important role in sub-$\mu \mathrm{m}$ wires, we demonstrate that strong deviations from the universal creep law can occur in 1 to $20\mu \mathrm{m}$ wide wires. Magnetic imaging shows that edge pinning translates into a marked bending of domain walls at low drive and is found to depend on the wire fabrication process and aging. Edge pinning introduces a reduction of domain wall velocity with respect to full films which increasingly dominates the creep dynamics as the wire width decreases. We show that the deviations from the creep law can be described by a simple model including a counter magnetic field which links the width of the wire to the edge dependent pinning strength. This counter field defines a key nonuniversal contribution to creep motion in patterned structures.

We examine the anisotropic properties of the exchange stiffness constant $\mathcal{A}$ for a rare-earth permanent magnet, ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$, by connecting analyses with two different scales of length, i.e., Monte Carlo (MC) method with an atomistic spin model and Landau-Lifshitz-Gilbert (LLG) equation with a continuous magnetic model. The atomistic MC simulations are performed on the spin model of ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$ constructed from ab initio calculations, and the LLG micromagnetics simulations are performed with the parameters obtained by the MC simulations. We clarify that the amplitude and the thermal property of $\mathcal{A}$ depend on the orientation in the crystal, which are attributed to the layered structure of Nd atoms and weak exchange couplings between Nd and Fe atoms. We also confirm that the anisotropy of $\mathcal{A}$ significantly affects the threshold field for the magnetization reversal (coercivity) given by the depinning process.

We report the $\mathbf{H}-T$ phase diagram of $S=1/2$ strongly frustrated anisotropic spin chain material linarite ${\mathrm{PbCuSO}}_4{\left(\mathrm{OH}\right)}_2$ in tilted magnetic fields up to 10 T and temperatures down to 0.2 K. By means of torque magnetometry we investigate the phase diagram evolution as the magnetic field undergoes rotation in ${\mathbf{ba}}^{*}$ and $\mathbf{bc}$ planes. The key finding is the robustness of the high field spin density wavelike phase, which may persist even as the external field goes orthogonal to the chain direction $\mathbf{b}$. In contrast, the intermediate collinear antiferromagnetic phase collapses at moderate deflection angles with respect to $\mathbf{b}$ axis.

Gapped interfaces (and boundaries) of two-dimensional (2D) Abelian topological phases are shown to support a remarkably rich sequence of 1D symmetry-protected topological (SPT) states. We show that such interfaces can provide a physical interpretation for the corrections to the topological entanglement entropy of a 2D state with Abelian topological order found by J. Cano, T. L. Hughes, and M. Mulligan [Phys. Rev. B 92, 075104 (2015)]. The topological entanglement entropy decomposes as $\gamma ={\gamma }_a+{\gamma }_s$, where ${\gamma }_a>0$ only depends on universal topological properties of the 2D state, while a correction ${\gamma }_s>0$ signals the emergence of the 1D SPT state that is produced by interactions along the entanglement cut and provides a direct measure of the stabilizing symmetry of the resulting SPT state. A correspondence is established between the possible values of ${\gamma }_s$ associated with a given interface—which is named the “boundary topological entanglement sequence”—and classes of 1D SPT states. We show that symmetry-preserving domain walls along such 1D interfaces (or boundaries) generally host localized parafermion-like excitations that are stable to local symmetry-preserving perturbations.

By using solid-state reactions, we successfully synthesize new oxyselenides ${\mathrm{CsV}}_2{\mathrm{Se}}_{2-x}\mathrm{O}$ ($x=$ 0, 0.5). These compounds containing ${\mathrm{V}}_2\mathrm{O}$ planar layers with a square lattice crystallize in the ${\mathrm{CeCr}}_2{\mathrm{Si}}_2\mathrm{C}$ structure with the space group of $P4/mmm$. Another new compound, ${\mathrm{V}}_2{\mathrm{Se}}_2\mathrm{O}$, which crystallizes in space group $I4/mmm$ is fabricated by topochemical deintercalation of cesium from ${\mathrm{CsV}}_2{\mathrm{Se}}_2\mathrm{O}$ powder with iodine in tetrahydrofuran. Resistivity measurements show a semiconducting behavior for ${\mathrm{CsV}}_2{\mathrm{Se}}_2\mathrm{O}$, while a metallic behavior for ${\mathrm{CsV}}_2{\mathrm{Se}}_{1.5}\mathrm{O}$, and an insulating feature for ${\mathrm{V}}_2{\mathrm{Se}}_2\mathrm{O}$. A charge- or spin-density-wave like anomaly has been observed at 168 K for ${\mathrm{CsV}}_2{\mathrm{Se}}_2\mathrm{O}$ and 150 K for ${\mathrm{CsV}}_2{\mathrm{Se}}_{1.5}\mathrm{O}$, respectively. These anomalies are also confirmed by the magnetic susceptibility measurements. The resistivity in ${\mathrm{V}}_2{\mathrm{Se}}_2\mathrm{O}$ exhibits an anomalous log(1/$T$) temperature dependence, which is similar to the case in parent phase or very underdoped cuprates indicating the involvement of strong correlation. Magnetic susceptibility measurements show that the magnetic moment per V site in ${\mathrm{V}}_2{\mathrm{Se}}_2\mathrm{O}$ is much larger than that of ${\mathrm{CsV}}_2{\mathrm{Se}}_{2-x}\mathrm{O}$, which again suggests the correlation-induced localization effect in the former.

Quantum Hall matrix models are simple, solvable quantum mechanical systems which capture the physics of certain fractional quantum Hall states. Recently it was shown that the Hall viscosity can be extracted from the matrix model for Laughlin states. Here we extend this calculation to the matrix models for a class of non-Abelian quantum Hall states. These states, which were previously introduced by Blok and Wen, arise from the conformal blocks of Wess-Zumino-Witten conformal field theory models. We show that the Hall viscosity computed from the matrix model coincides with a result of Read, in which the Hall viscosity is determined in terms of the weights of primary operators of an associated conformal field theory.

Adsorbate engineering offers a seemingly simple approach to tailor spin-orbit interactions in atomically thin materials and thus to unlock the much sought-after topological insulating phases in two dimensions. However, the observation of an Anderson topological transition induced by heavy adatoms has proved extremely challenging despite substantial experimental efforts. Here, we present a multiscale approach combining advanced first-principles methods and accurate single-electron descriptions of adatom-host interactions using graphene as a prototypical system. Our study reveals a surprisingly complex structure in the interactions mediated by random adatoms, including hitherto neglected hopping processes leading to strong valley mixing. We argue that the unexpected intervalley scattering strongly impacts the ground state at low adatom coverage, which would provide a compelling explanation for the absence of a topological gap in recent experimental reports on graphene. Our conjecture is confirmed by real-space Chern number calculations and large-scale quantum transport simulations in disordered samples. This resolves an important controversy and suggests that a detectable topological gap can be achieved by increasing the spatial range of the induced spin-orbit interactions on graphene, e.g., using nanoparticles.

We study the combined impact of random disorder and electron-electron and electron-hole interactions on the absorption spectra of a three-dimensional Hubbard Hamiltonian. We determine the single-particle Green's function within the typical medium dynamical cluster approximation. We solve the Bethe-Salpeter equation (BSE) to obtain the dynamical conductivity. Our results show that increasing disorder strength at a given interaction strength leads to decreased absorption with the dynamical conductivity, systematically going to zero at all frequencies, a fingerprint of a correlation-mediated electron localization. Surprisingly, our data reveal that taking into account the effects of electron-hole interactions through the BSE significantly changes the oscillator strength with a concomitant reduction in the critical disorder strengths $W_c^U$. We attribute this behavior to enhanced quantum correction induced by electron-hole interactions.