We present electrical resistivity and ac-susceptibility measurements of ${\text{GdTe}}_3, {\text{TbTe}}_3$, and ${\text{DyTe}}_3$ performed under pressure. An upper charge-density-wave (CDW) is suppressed at a rate of $d{T}_{\mathrm{CDW},1}/dP\sim -85\mathrm{K}/\mathrm{GPa}$. For ${\text{TbTe}}_3$ and ${\text{DyTe}}_3$, a second CDW below $T_{\mathrm{CDW},2}$ increases with pressure until it reaches the $T_{\mathrm{CDW},1}$($P$) line. For ${\text{GdTe}}_3$, the lower CDW emerges as pressure is increased above $\sim 1\text{GPa}$. As these two CDW states are suppressed with pressure, superconductivity (SC) appears in the three compounds at lower temperatures. Ac-susceptibility experiments performed on ${\text{TbTe}}_3$ provide compelling evidence for bulk SC in the low-pressure region of the phase diagram. We provide measurements of superconducting critical fields and discuss the origin of a high-pressure superconducting phase occurring above 5 GPa.

Interface confinement plays a central role in charge carrier accumulation and transport along the channel of organic field-effect transistors. Understanding the relevant interfacial interactions that affect the energy landscape experienced by carriers in the channel is of fundamental interest. Here we investigate charge transport in the submonolayer regime of pentacene transistors in which confinement arises due to the finite size of the interconnected semiconducting islands. In situ real-time electrical characterization is used to monitor the formation and evolution of the accumulation layer at the very early stages of growth. The morphology of the confining interfaces is controlled by growth conditions and pentacene coverage. Charge transport occurs when percolation pathways connecting source and drain electrodes are formed at a critical coverage. The displacement current across the oxide/semiconductor interface is observed starting from the onset of percolation (0.69 monolayer coverage). The analysis of the characteristics shows that already the submonolayer film fully screens the gate field and accumulates higher charge carrier density as compared to the monolayer film. We propose an electrostatic model to correlate the charge density to the characteristic length scale of the submonolayer film and the thickness of the dielectric layer. This explains charge mobility and threshold voltage of thin-film transistors in the submonolayer regime.

We analyze classical dimer models on a square and a triangular lattice using a tensor network representation of the dimers. The correlation functions are numerically calculated using the recently developed “tensor renormalization group” (TRG) technique. The partition function for the dimer problem can be calculated exactly by the Pfaffian method, which is used here as a platform for comparing the numerical results. The TRG approach turns out to be a powerful tool for describing gapped systems with exponentially decaying correlations very efficiently due to its fast convergence. This is the case for the dimer model on the triangular lattice. However, the convergence becomes very slow and unstable in the case of the square lattice where the model has algebraically decaying correlations. We highlight these aspects with numerical simulations and critically appraise the robustness of the TRG approach by contrasting the results for small and large system sizes against the exact calculations. Furthermore, we benchmark our TRG results with the classical Monte Carlo method.

Field enhancement behavior of a ${\mathrm{SiO}}_2$/Au nanoshell is studied in the framework of strong-field physics. Localized plasmonic fields induce local electric field enhancement with the potential to lead to the strong-field regime without the use of costly amplified lasers. In this framework, electrons are tunnel ionized from the nanoshell and accelerated by the local field being spatially inhomogeneous in terms of spectral and polarization properties. These processes are happening within a single laser shot, and thermal effects are therefore neglected. We show that the localized response to ultrashort femtosecond pulses can be investigated by extending Mie theory to multilayer spherical particles. Nanoshell plasmonic resonances can be easily tuned depending on the volume ratio between core and whole particle. Optimum geometric parameters of the nanoshell are selected for use with ultrashort pulses centered at 800 nm where most femtosecond oscillators operate. It is shown that phase and amplitude reshaping of the incident pulse by the plasmonic resonance can be partially corrected using active shaping leading to ultrashort response of the medium. Finally, free electron classical trajectories are calculated to highlight the inhomogeneous nature of the local enhancement in a strong-field picture.

The electron spin resonance doublet indicating the width of the two-spinon continuum in a spin-$\frac{1}{2}$ triangular lattice Heisenberg antiferromagnet ${\mathrm{Cs}}_2{\mathrm{CuCl}}_4$ was studied in high magnetic field. The doublet was found to collapse in a magnetic field of one-half of the saturation field. The collapse of the doublet occurs via vanishing of the high-frequency component in a qualitative agreement with the theoretical prediction for the $S=\frac{1}{2}$ chain. The field of the collapse is, however, much lower than expected for the $S=\frac{1}{2}$ chain. This is proposed to be due to the destruction of frustration of interchain exchange bonds in a magnetic field, which restores the 2D character of this spin system. In the saturated phase the mode with the Larmor frequency and a much weaker mode downshifted for 119 GHz are observed. The weak mode is of exchange origin; it demonstrates a positive frequency shift at heating corresponding to the repulsion of magnons in the saturated phase.

We study the stability of topologically protected zero-energy flat bands at the surface of nodal noncentrosymmetric superconductors, accounting for the alteration of the gap near the surface. Within a self-consistent mean-field theory, we show that the flat bands survive in a broad temperature range below the bulk transition temperature. There is a second transition at a lower temperature, however, below which the system spontaneously breaks time-reversal symmetry. The surface bands are shifted away from zero energy and become weakly dispersive. Simultaneously, a spin polarization and an equilibrium charge current develop in the surface region.

Searching for unconventional Cooper pairing states has been at the heart of superconductivity research since the discovery of BCS superconductors. In particular, spin-triplet odd-parity pairing states were recently revisited due to the possibility of tuning towards topological superconductors. In this context, it is interesting to note a recent proposal that such a spin-triplet pairing instability occurs when the band filling is near van Hove singularities (vHS) associated with momenta away from time-reversal invariant momenta named type-II vHS. However, this result was obtained within a weak coupling renormalization group with Fermi surface patch approximation. To explore superconducting instabilities beyond this weak coupling Fermi surface patch approximation, we perform systematic study on the Hubbard model in a two-dimensional square lattice using three different methods: random phase approximation, large-scale dynamical mean-field theory simulations with continuous time quantum Monte Carlo (CTQMC) impurity solver, and large-scale dynamical cluster simulations with the CTQMC cluster solver. We find, in a wide doping range centered around the type-II van Hove filling, a twofold degenerate, spin-triplet, odd-parity $p$-wave pairing state emerges due to repulsive interaction, when the Fermi surface is not sufficiently nested. Possible relevance of our findings to the recently discovered superconductors ${\mathrm{LaO}}_{1-x}{\mathrm{F}}_x{\mathrm{BiS}}_2,{\mathrm{Ir}}_{1-x}{\mathrm{Pt}}_x{\mathrm{Te}}_2$, and proposed doped ${\mathrm{BC}}_3$ are also discussed.

One of the most dominant candidates for the paired quantum Hall (QH) state at filling factor $\nu =5/2$ is the Moore-Read (MR) Pfaffian state. A salient problem, however, is that it does not occur exactly at the Coulomb interaction, but rather at a modified interaction, which favors particle-hole-symmetry breaking. In an effort to find a better state, in this work, we investigate the possible connection between the paired QH state and the antisymmetrized bilayer ground state, which is inspired by the intriguing identity that the MR Pfaffian state is entirely equivalent to the antisymmetrized projection of the bilayer QH state called the Halperin (331) state, which is valid at interlayer distance, $d$, roughly equal to the magnetic length, $l_{\mathrm{B}}$. Specifically, by using exact diagonalization in the torus geometry, we show that the exact $5/2$ state at a given Haldane pseudopotential variation is intimately connected with the antisymmetrized bilayer ground state at a corresponding $d/l_{\mathrm{B}}$ via one-to-one mapping, which we call the bilayer mapping. One of the most important discoveries in this work is that the paired QH state occurring at the Coulomb interaction is mapped onto the antisymmetrized bilayer ground state at $d\gg l_{\mathrm{B}}$, which is equivalent to the antisymmetrized product state of two composite fermion seas at quarter filling, not the MR Pfaffian state. While maintaining high overlap with the paired QH state, the antisymmetrized bilayer ground state at $d\gg l_{\mathrm{B}}$ exhibits an abrupt change under the influence of small anisotropy. This suggests that the paired QH state occurring at the Coulomb interaction might be susceptible to anisotropic instability, opening up the possibility of anisotropic $p_x$- or $p_y$-wave pairing instead of $p_x\pm i{p}_y$-wave pairing in the MR Pfaffian/anti-Pfaffian state.

It has been well established that covalency significantly enhances the electric polarization produced by the ionic displacement for ferroelectric perovskite transition metal oxides (TMO). Furthermore, recent experimental and theoretical works on the organic ferroelectrics TTF-CA $(\text{tetrathiafulvalene}-p-\text{chloranil})$ have revealed that the covalency-induced polarization is one to two orders of magnitude larger than that of the ionic polarization and that the two contributions are in the opposite direction. Here we propose a formulation to analyze the detailed mechanism of the covalency-induced polarization within the framework of maximally localized Wannier orbitals and apply it to an organic exotic ferroelectrics TTF-CA and typical ferroelectric perovskite TMOs, ${\mathrm{BaTiO}}_3$, and ${\mathrm{PbTiO}}_3$. This formulation discriminates three components in the electronic contribution to the polarization. The first one corresponds to the point charge model, the second to the intra-atomic or molecular polarization, and the third comes from the electron transfer between unit cells. The framework of the present formulation is the same as the one proposed by Bhattacharjee and Waghmare [Phys. Chem. Chem. Phys. 12, 1564 (2010)], but we give a more explicit expression of each component and discuss fundamental aspects of the formulation.

We study the phase diagram of electronic nematic instability in the presence of $xy$ anisotropy. While a second-order transition cannot occur in this case, mean-field theory predicts that a first-order transition occurs near Van Hove filling and its phase boundary forms a wing structure, which we term a Griffiths wing, referring to his original work of ${\mathrm{He}}^3-{\mathrm{He}}^4$ mixtures. When crossing the wing, the anisotropy of the electronic system exhibits a discontinuous change, leading to a metanematic transition, i.e., the analog to a metamagnetic transition in a magnetic system. The upper edge of the wing corresponds to a critical end line. It shows a nonmonotonic temperature dependence as a function of the external anisotropy and vanishes at a quantum critical end point for a strong anisotropy. The mean-field phase diagram is found to be very sensitive to fluctuations of the nematic order parameter, yielding a topologically different phase diagram. The Griffiths wing is broken into two pieces. A tiny wing appears close to zero anisotropy and the other is realized for a strong anisotropy. Consequently three quantum critical end points are realized. We discuss that these results can be related to various materials including a cold atom system.

Accuracy of the first-principles $GW$+Bethe-Salpeter equation (BSE) method is examined for low-energy excited states of small molecules. The standard formalism, which is based on the one-shot $GW$ approximation and the Tamm-Dancoff approximation (TDA), is found to underestimate the optical gap of ${\mathrm{N}}_2$, CO, ${\mathrm{H}}_2\mathrm{O},{\mathrm{C}}_2{\mathrm{H}}_4$, and ${\mathrm{CH}}_2\mathrm{O}$ by about 1 eV. Possible origins are investigated separately for the effect of TDA and for the approximate schemes of the self-energy operator, which are known to cause overbinding of the electron-hole pair and overscreening of the interaction. By applying the known correction formula, we find the amount of the correction is too small to overcome the underestimated excitation energy. This result indicates a need for fundamental revision of the $GW$+BSE method rather than adjustment of the standard one. We expect that this study makes the problems in the current $GW$+BSE formalism clearer and provides useful information for further intrinsic development beyond the current framework.

The structural, electronic, and optical properties of InAs, InSb, and their ternary alloys ${\mathrm{InAs}}_x{\mathrm{Sb}}_{1-x}$ ($x=0.25$, 0.5, 0.75) are investigated within density functional theory utilizing the wien2k package. We find that the lattice constants and bulk moduli as a function of $x$ are in best agreement with Vegard's linear rule. When computing the electronic band structures with the modified Becke-Johnson exchange-correlation functional (mBJLDA), our results for the band gaps of InAs, InSb, and their ternary alloys are in good agreement with the available experimental results while the conventional Wu-Cohen generalized gradient approximation (GGA) functional leads to zero or close to zero band gaps. In particular, our mBJLDA results confirm experimental evidence that the minimum band gap occurs for As concentrations around $x\approx 0.3$. Furthermore, we investigate the dielectric function of these compounds within the random phase approximation using both the Wu-Cohen GGA and the mBJLDA functionals. While the mBJLDA results of our fully first-principles calculations show good agreement of the peak positions in ${\epsilon }_2\left(\omega \right)$ with experiments, the peaks in the optical spectra based on the Wu-Cohen GGA band structure appear redshifted compared to experiment. We further identify the interband transitions responsible for the structures in the spectra. Looking at the optical matrix element, we note that the major peaks are dominated by transition from the Sb $5p$ (As $4p$) states to In $s$ states for InSb and ${\mathrm{InAs}}_{0.25}{\mathrm{Sb}}_{0.75}$ (InAs, ${\mathrm{InAs}}_{0.75}{\mathrm{Sb}}_{0.25}$, and ${\mathrm{InAs}}_{0.5}{\mathrm{Sb}}_{0.5}$).

We consider a quantum wire connected to the leads and subjected to dissipation along its length. The dissipation manifests as tunneling into (out of) the chain from (to) a memoryless environment. The evolution of the system is described by the Lindblad equation. Already infinitesimally small dissipation along the chain induces a quantum phase transition (QPT). This is a decoherence QPT: the reduced density matrix of a subsystem in the nonequilibrium steady state (far from the ends of the chain) can be represented as the tensor product of single-site density matrices. The QPT is identified from the jump of the current and the entropy per site as the dissipation becomes nonzero. We also explore the properties of the boundaries of the chain close to the transition point and observe that the boundaries behave as if they undergo a second-order phase transition as a function of the dissipation strength: the particle-particle correlation functions and the response to the electric field exhibit a power-law divergence. Disorder is known to localize one-dimensional systems, but the coupling to the memoryless environment pushes the system back into the delocalized state even in the presence of disorder. Interestingly, we observe a similar transition in the classical dissipative counterflow model: the current has a jump at the ends of the chain introducing an infinitely small dissipation.

The coexistence pressure of two phases is a well-defined point at fixed temperature. In experiment, however, due to nonhydrostatic stresses and a stress-dependent potential energy barrier, different measurements yield different ranges of pressure with a hysteresis. Accounting for these effects, we propose an inequality for comparison of the theoretical value to a plurality of measured intervals. We revisit decades of pressure experiments on the $\text{bcc}\leftrightarrow \text{hcp}$ transformations in iron, which are sensitive to nonhydrostatic conditions and sample size. From electronic-structure calculations, we find a $\text{bcc}\leftrightarrow \text{hcp}$ coexistence pressure of $8.4\mathrm{GPa}$. We construct the equation of state for competing phases under hydrostatic pressure, compare to experiments and other calculations, and address the observed pressure hysteresis and range of onset pressures of the nucleating phase.

We fabricated epitaxial exchange biased (EB) IrMn/FeGa bilayers by oblique deposition and systematically investigated their magnetization reversal. Two different configurations with the uniaxial magnetic anisotropy $K_u$ parallel and perpendicular to the unidirectional anisotropy $K_{eb}$ were obtained by controlling the orientation of the incident FeGa beam during deposition. A large ratio of $K_u/K_{eb}$ was obtained by obliquely depositing the FeGa layer to achieve a large $K_u$ while reducing the IrMn thickness to obtain a small $K_{eb}$. Besides the previously reported square loops, conventional asymmetrically shaped loops, and one-sided and two-sided two-step loops, unusual asymmetrically shaped loops with a three-step magnetic transition for the descending branch and a two-step transition for the ascending branch and biased three-step loops were observed at various field orientations in the films of both IrMn ($t_{\mathrm{IrMn}}=1.5$ to 20 nm)/FeGa (10 nm) with $K_u\perp K_{eb}$ and IrMn ($t_{\mathrm{IrMn}}\le 2$ nm)/FeGa (10 nm) with $K_u\left|\right|K_{eb}$. Considering the geometries of anisotropies, a model based on domain wall nucleation and propagation was employed to quantitatively describe the angular dependent behaviors of IrMn/FeGa bilayers. The biased three-step magnetic switching was predicted to take place when $|K_u|>{\varepsilon }_{{90}^{\circ }}+K_{eb}$, where ${\varepsilon }_{{90}^{\circ }}$ is the 90° domain wall nucleation energy, and the EB leads to the appearance of the unusual asymmetrically shaped hysteresis loops.

We present temperature-dependent in-plane resistivity measurements on FeSe single crystals under He-gas pressure up to 800 MPa and magnetic fields $B\le 10$ T. A sharp phase transition anomaly is revealed at the tetragonal-to-orthorhombic transition at $T_s$ slightly below 90 K. $T_s$ becomes reduced with increasing pressure in a linear fashion at a rate $d{T}_s/dP\simeq -31$ K/GPa. This is accompanied by a $P$-linear increase of the superconducting transition temperature at $T_c\sim 8.6$ K with $d{T}_c/dP\simeq +5.8$ K/GPa. Pressure studies of the normal-state resistivity highlight two distinctly different regimes: for $T>T_s$, i.e., in the tetragonal phase, the in-plane resistivity changes strongly with pressure. This contrasts with the state deep in the orthorhombic phase at $T\ll T_s$, preceding the superconducting transition. Here, a $T$-linear resistivity is observed the slope of which does not change with pressure, pointing against a spin-fluctuation origin of this term. Resistivity studies in varying magnetic fields both at ambient and finite pressure reveal clear changes of the magnetoresistance, $\Delta \rho \propto B^2$, upon cooling through $T_s$. Our data are consistent with a reconstruction of the Fermi surface accompanying the structural transition.

We report the role of long- and short-range order on the thermal conductivity and mode relaxation times of a model ${\mathrm{Si}}_{0.5}{\mathrm{Ge}}_{0.5}$ alloy using molecular dynamics simulation. All interactions used the Stillinger-Weber potential and the Si and Ge atoms differed only by their mass. The simulated alloys were generated using a Monte Carlo approach to decouple the short-range order from the long-range order. The thermal conductivity is almost entirely determined by the alloy's nearest-neighbor short-range order. Changes to the mode relaxation times between $\sim 3$ and $\sim 6\mathrm{THz}$ upon short-range ordering, and the observed $f^{-2}$ power law trend, suggest that short-range ordering reduces the anharmonic scattering rate of low frequency modes. The trend of thermal conductivity with short-range order may be transferred to real ${\mathrm{Si}}_{0.5}{\mathrm{Ge}}_{0.5}$ and other semiconductor alloys to the extent that scattering from mass disorder dominates their thermal conductivities.

Solid hydrogen sulfide is a typical molecular crystal, but its stability under pressure remains controversial. In particular, the recent experimental discovery of high-pressure superconductivity at 190 K in an ${\mathrm{H}}_2\mathrm{S}$ sample (arXiv:1412.0460) inspired efforts to revalidate this controversial issue, the pressure at which ${\mathrm{H}}_2\mathrm{S}$ decomposes and the resultant decomposition products urgent need to be evaluated. In this paper we performed an extensive structural study on different stoichiometries of H_nS with $n>1$ under high pressure using ab initio calculations. Our results show that ${\mathrm{H}}_2\mathrm{S}$ is stable below 43 GPa and at elevated pressure it decomposes into ${\mathrm{H}}_3\mathrm{S}$ and sulfur. ${\mathrm{H}}_3\mathrm{S}$ is stable at least up to 300 GPa, while other H-rich compounds, including ${\mathrm{H}}_4\mathrm{S}, {\mathrm{H}}_5\mathrm{S}$, and ${\mathrm{H}}_6\mathrm{S}$, are unstable in the pressure range of this study.