The structural evolution with pressure in bulk $\mathrm{Mo}{\mathrm{S}}_2$ has been investigated by high-pressure x-ray diffraction using synchrotron radiation. We found that the out-of-plane S-Mo-S bond angle θ increases and that in in-plane angle $\varphi$ decreases linearly with increasing pressure across the known semiconducting-to-metal phase transition, whereas the Mo-S bond length and the S-Mo-S trilayer thickness display only little change. Extrapolating the experimental result along the in-plane lattice parameter with pressure, both S-Mo-S bond angles trend to those found in monolayer $\mathrm{Mo}{\mathrm{S}}_2$, which manifests as a structural characteristic closely correlating the electronic band gap of $\mathrm{Mo}{\mathrm{S}}_2$ to its physical forms and phases, e.g., monolayer as direct band gap semiconductor, multilayer or bulk as indirect band gap semiconductor, and high-pressure $\left(>19\mathrm{GPa}\right)$ bulk form as metal. Combined with the effects of bond strength and van der Waals interlayer interactions, the structural correlations between the characteristic bond angle and electronic band gaps are readily extendible to other transition metal dichalcogenide systems ($M{X}_2$, where $M=\mathrm{Mo}$, W and $X=\mathrm{S}$, Se, Te).

Temperature dependence of resistivity on $\mathrm{KF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ single crystals down to 20 mK was measured under various hydrostatic pressures up to 17.5 GPa generated in a cubic-anvil cell. With increasing the pressure, the superconducting transition of tetragonal $\mathrm{KF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ was suppressed gradually and disappears completely at ∼11 GPa, which was related to the weakening of electronic correlations and/or critical fluctuations under pressure. In sharp contrast to previous reports, no superconducting phase emerges upon further increasing pressures until the collapsed tetragonal $\mathrm{KF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ forms. We argue that such a discrepancy can be attributed to the different pressure apparatus or homogeneity.

We report synchrotron x-ray diffraction investigations of interfacially engineered oxygen octahedral rotations and their impact on strain relief in perovskite $\mathrm{SrRu}{\mathrm{O}}_3$ films. We show that octahedral rotations with distinct patterns and magnitudes can be accommodated into coherently grown films. The $\mathrm{SrRu}{\mathrm{O}}_3$ film grown directly on the $\mathrm{GdSc}{\mathrm{O}}_3$ substrate has the $\mathrm{Ru}{\mathrm{O}}_6$ octahedral rotation with the $a^{-}{b}^{+}{c}^{-}$ pattern in the Glazer notation and the rotation angles of ${\alpha }_{\mathrm{rot}}=6.6\pm 0.2^{\circ }, {\beta }_{\mathrm{rot}}=5.5\pm 0.2^{\circ }$, and ${\gamma }_{\mathrm{rot}}=3.6\pm 0.2^{\circ }$. On the other hand, when a 1-nm-thick $\mathrm{BaTi}{\mathrm{O}}_3$ layer without $\mathrm{Ti}{\mathrm{O}}_6$ rotations is inserted between the $\mathrm{SrRu}{\mathrm{O}}_3$ and $\mathrm{GdSc}{\mathrm{O}}_3$, the $\mathrm{SrRu}{\mathrm{O}}_3$ has the $\mathrm{Ru}{\mathrm{O}}_6$ rotation with $a^{-}{b}^0{c}^{+}$, and ${\alpha }_{\mathrm{rot}}=5.6\pm 0.8^{\circ }$ and ${\gamma }_{\mathrm{rot}}=3.6\pm 0.8^{\circ }$. These results indicate that there are some degrees of freedom in the octahedral rotations accommodated in $\mathrm{SrRu}{\mathrm{O}}_3$ depending on the interface structure and that the ${\gamma }_{\mathrm{rot}}$ rotations play the important roles in the film's structural properties when the rotation about the ${\left[010\right]}_{\mathrm{pc}}$ axis is blocked. We also found that the strain relief in the film is influenced by the interfacially engineered octahedral rotations. The interfacial $\mathrm{BaTi}{\mathrm{O}}_3$ layer results in the in-plane periodic lattice modulation in the t-SRO film, allowing for the anisotropic relief of the substrate-induced strain. The results highlight the importance of the interface structure as a factor, determining not only octahedral rotations in coherently grown SRO films but also the strain reliefs in them.

We implement a pseudolikelihood approach with $l_1$ and $l_2$ regularizations as well as the recently introduced pseudolikelihood with decimation procedure to the inverse problem in continuous spin models on arbitrary networks, with arbitrarily disordered couplings. Performances of the approaches are tested against data produced by Monte Carlo numerical simulations and compared also to previously studied fully connected mean-field-based inference techniques. The results clearly show that the best network reconstruction is obtained through the decimation scheme, which also allows us to make the inference down to lower temperature regimes. Possible applications to phasor models for light propagation in random media are proposed and discussed.

We studied experimentally the high-temperature magnetoelectric ${\left({\mathrm{Ba}}_x{\mathrm{Sr}}_{1-x}\right)}_3{\mathrm{Co}}_2{\mathrm{Fe}}_{24}{\mathrm{O}}_{41}$ prepared as ceramics ($x=0$, 0.2) and a single crystal ($x=0.5$) using inelastic neutron scattering, THz time-domain, Raman, and far-infrared spectroscopies. The spectra, measured with varying temperature and magnetic field, reveal rich information about the collective spin and lattice excitations. In the ceramics, we observed an infrared-active magnon which is absent in ${\mathbf{E}}^{\omega }\perp z$ polarized THz spectra of the crystal, and we assume that it is an electromagnon active in ${\mathbf{E}}^{\omega }\parallel z$ polarized spectra. On heating from 7 to 250 K, the frequency of this electromagnon drops from 36 to $25{\mathrm{cm}}^{-1}$ and its damping gradually increases, so it becomes overdamped at room temperature. Applying external magnetic field has a similar effect on the damping and frequency of the electromagnon, and the mode is no longer observable in the THz spectra above 2 T, as the transverse-conical magnetic structure transforms into a collinear one. Raman spectra reveal another spin excitation with a slightly different frequency and much higher damping. Upon applying magnetic field higher than 3 T, in the low-frequency part of the THz spectra, a narrow excitation appears whose frequency linearly increases with magnetic field. We interpret this feature as the ferromagnetic resonance.

We perform transport measurements and band structure calculations of electron-doped $\mathrm{LaFeAs}{\mathrm{O}}_{1-x}{\mathrm{H}}_x$ over a wide range of $x$ from 0.01 to 0.66. The $T^2$ and $\surd T$ dependency of the resistivity are observed at $x\sim 0.17$ and 0.41, respectively. The sign change of $R_{\mathrm{H}}$ without opening of the spin-density-wave gap for $0.45\le x\le 0.58$ and $T<T_{\mathrm{N}}$ as well as the calculated non-nested Fermi surface at $x=0.5$ indicate the more localized nature of the AF2 as compared to spin-density-wave phase at the nondoped sample. Considering the results from band calculations and the finite size of Hund's rule coupling, the change of the normal conducting state with $x$ is reasonably explained by a strong depression of the coherent scale owing to the increased effective Coulomb repulsion in the narrow antibonding $3d_{xy}$ band that approaches the half-filled regime. The following transition from the paramagnet to AF2 is understood as the quenching or ordering of the less-screened spins with a large entropy at low temperature. These results suggest that the normal conducting properties over a wide doping range for the $\mathrm{LaFeAs}{\mathrm{O}}_{1-x}{\mathrm{H}}_x$ are strongly influenced by the local spins in the incoherent region. Thus, we conclude that the parent phase in $\mathrm{LaFeAs}{\mathrm{O}}_{1-x}{\mathrm{H}}_x$ is not the spin-density wave but the AF2, which may be primarily responsible for the singular superconducting properties of the 1111 type compared with other iron pnictides.

The transmission of THz electromagnetic waves via a slab of layered superconductor in the presence of dc magnetic field $H_0$ is theoretically studied. We demonstrate that the external dc field turns the layered superconductor into nonuniform medium with spatially and frequency-dependent dielectric permittivity. Even a relatively weak dc magnetic field, when the superconductor is in the Meissner state, significantly affects the transmittance of the layered superconductor. Moreover, the proper choice of $H_0$ can provide the perfect transparency of the slab. In addition, the dc magnetic field changes the dependence of the transmittance on the slab thickness, the frequency, and the incident angle of the wave. Thus, it can serve as an effective tool to control the transmissivity of layered superconductors.

We present numerical studies of fermion and boson models with random all-to-all interactions (the Sachdev-Ye-Kitaev models). The high-temperature expansion and exact diagonalization of the $N$-site fermion model are used to compute the entropy density: our results are consistent with the numerical solution of $N=\infty$ saddle-point equations, and the presence of a nonzero entropy density in the limit of vanishing temperature. The exact-diagonalization results for the fermion Green's function also appear to converge well to the $N=\infty$ solution. For the hard-core boson model, the exact-diagonalization study indicates spin-glass order. Some results on the entanglement entropy and the out-of-time-order correlators are also presented.

The Pauli spin blockade effect in transport through two, coupled in series, single molecular magnets weakly attached to external leads is considered theoretically. By using the real-time diagrammatic technique in the lowest-order perturbation theory with respect to the coupling strength, the behavior of the current and the shot noise is studied in the nonlinear response regime. It is shown that the current suppression occurs due to the occupation of highest-weight spin states of the system. Moreover, transport properties are found to strongly depend on parameters of the double molecular magnet, such as the magnitude of spin, internal exchange interaction and the hopping between the molecules. It is also demonstrated that the current suppression may be accompanied by negative differential conductance and a large super-Poissonian shot noise. The mechanisms leading to those effects are discussed.

We propose four different thermodynamically stable structural phases of arsenic monolayers based on ab initio density functional theory calculations, all of which undergo a topological phase transition on application of a perpendicular electric field. All four arsenic monolayer allotropes have a wide band gap, varying from 1.21 to 3.0 eV (based on GW calculations), and in general they undergo a metal-insulator quantum phase transition on application of uniaxial in-layer strain. Additionally, an increasing transverse electric field induces band inversion at the $\mathrm{\Gamma }$ point in all four monolayer allotropes, leading to a nontrivial topological phase (insulating for three allotropes and metallic for one allotrope), characterized by the switching of the $Z_2$ index from 0 (before band inversion) to 1 (after band inversion). The topological phase tuned by the transverse electric field should support spin-separated gapless edge states which should manifest in the quantum spin Hall effect.

We numerically compute the low-energy spectrum of a gate-controlled superconducting topological nanowire segmented into two islands, each Josephson coupled to a bulk superconductor. This device may host two pairs of Majorana bound states and could provide a platform for testing Majorana fusion rules. We analyze the crossover between (i) a charge-dominated regime utilizable for initialization and readout of Majorana bound states, (ii) a single-island regime for dominating interisland Majorana coupling, (iii) a Josephson-plasmon regime for large coupling to the bulk superconductors, and (iv) a regime of four Majorana bound states allowing for topologically protected Majorana manipulations. From the energy spectrum, we derive conservative estimates for the time scales of a fusion-rule testing protocol proposed recently (D. Aasen et al., arXiv:1511.05153). We also analyze the steps needed for basic Majorana braiding operations in branched nanowire structures.

We propose that the simultaneous presence of both Rashba and band inversion can lead to a Rashba-like spin splitting formed by two bands with the same in-plane helical spin texture. Because of this unconventional spin texture, the backscattering is forbidden in edge and bulk conductivity channels. We propose a noncentrosymmetric honeycomb-lattice quantum spin Hall (QSH) insulator family formed by the IV, V, and VII elements with this property. The system formed by Bi, Pb, and I atoms is mechanically stable and has both a large Rashba spin splitting of 60 meV and a large nontrivial band gap of 0.14 eV. Since the edge and the bulk states are protected by the time-reversal (TR) symmetry, contrary to what happens in most doped QSH insulators, the bulk states do not contribute to the backscattering in the electronic transport, allowing the construction of a spintronic device with less energy loss.

We report on spin transport features which are unique to high quality bilayer graphene, in the absence of magnetic contaminants and strong intervalley mixing. The time-dependent spin polarization of a propagating wave packet is computed using an efficient quantum transport method. In the limit of vanishing effects of substrate and disorder, the energy dependence of the spin lifetime is similar to monolayer graphene with an $\mathsf{M}$-shaped profile and minimum value at the charge neutrality point, but with an electron-hole asymmetry fingerprint. In sharp contrast, the incorporation of substrate-induced electron-hole puddles (characteristics of supported graphene either on ${\mathrm{SiO}}_2$ or $\mathrm{hBN})$ surprisingly results in a large enhancement of the low-energy spin lifetime and a lowering of its high-energy values. Such a feature, unique to the bilayer, is explained in terms of a reinforced Dyakonov-Perel mechanism at the Dirac point, whereas spin relaxation at higher energies is driven by pure dephasing effects. This suggests further electrostatic control of the spin transport length scales in graphene devices.

A so-called anti-optic-null medium (anti-ONM), which can be utilized to cancel the optic-null medium (ONM) and create many novel optical illusions, is introduced and designed by transformation optics (TO). Optical separation illusions can be achieved with an anti-ONM. With the help of the anti-ONM, we can achieve the same optical illusions where ONM is required via a shelled structure filled with low anisotropic medium, which is easier to realize for some novel optical devices designed by TO and optical surface transformation. The special function of the anti-ONM will lead to a new way to design optical devices or simplify the material requirements. Overlapping illusions, and wave-front reshapers are designed to demonstrate the function of the proposed method.

The key challenge in superconductivity research is to go beyond the historical mode of discovery-driven research. We put forth a new strategy, which is to combine theoretical developments in the weak-coupling renormalization-group approach with the experimental developments in lattice-strain-driven Fermi surface engineering. For concreteness we theoretically investigate how superconducting tendencies will be affected by strain engineering of ruthenates' Fermi surface. We first demonstrate that our approach qualitatively reproduces recent experiments under uniaxial strain. We then note that the order of a few percent strain, readily accessible to epitaxial thin films, can bring the Fermi surface close to van Hove singularity. Using the experimental observation of the change in the Fermi surface under biaxial epitaxial strain and ab initio calculations, we predict $T_{\mathrm{c}}$ for triplet pairing to be maximized by getting close to the van Hove singularities without tuning on to the singularity.

To clarify the dynamics of polarons in doped Mott insulators, we performed pump-probe reflectivity measurements for two barium titanates, ${\mathrm{Ba}}_2{\mathrm{Ti}}_{13}{\mathrm{O}}_{22}$ and ${\mathrm{Ba}}_x{\mathrm{Ti}}_8{\mathrm{O}}_{16}$, with a noninteger average number of $d$ electrons per Ti. We found that the photoinduced dynamics in ${\mathrm{Ba}}_2{\mathrm{Ti}}_{13}{\mathrm{O}}_{22}$ with a three-dimensional electronic structure is characterized by a broadening of the polaron-excitation peak immediately after photoirradiation, whereas that in ${\mathrm{Ba}}_x{\mathrm{Ti}}_8{\mathrm{O}}_{16}$ with a one-dimensional electronic structure is characterized by a change in the intensities of the polaron-excitation peak and charge-gap-excitation peak with time. This indicates that a difference in dimensionality results in different photoinduced dynamics of polarons in doped Mott insulators.

A unique nanolithography technique compatible with conducting oxide interfaces, which requires a single lithographic step with no additional amorphous deposition or etching, is presented. It is demonstrated on a ${\mathrm{SrTiO}}_3/{\mathrm{LaAlO}}_3$ interface where a constriction is patterned in the electron liquid. We find that an additional backgating can further confine the electron liquid into an isolated island. Conductance and differential conductance measurements show resonant tunneling through the island. The data at various temperatures and magnetic fields are analyzed and the effective island size is found to be of the order of 10 nm. The magnetic field dependence suggests the absence of spin degeneracy in the island. Our method is suitable for creating superconducting and oxide-interface-based electronic devices.

Oxides have many potentially desirable characteristics for thermoelectric applications, including low cost and stability at high temperatures, but thus far there are few known high $zTn$-type oxide thermoelectrics. In this work, we use high-throughput first-principles calculations to screen transition metal oxides, nitrides, and sulfides for candidate materials with high power factors and low thermal conductivity. We find a variety of promising materials, and we investigate these materials in detail in order to understand the mechanisms that cause them to have high power factors. These materials all combine a high density of states near the Fermi level with dispersive bands, reducing the trade-off between the Seebeck coefficient and the electrical conductivity, but they do so for several different reasons. In addition, our calculations indicate that many of our candidate materials have low thermal conductivity.

By way of the nonequilibrium Green's function simulations and analytical expressions, the quantum thermal conductance of stanene is studied. We find that, due to the existence of Dirac fermion in stanene, the ratio of electron thermal conductance and electric conductance becomes a chemical-potential-dependent quantity, violating the Wiedemann-Franz law. This finding is applicable to any two-dimensional (2D) materials that possess massless Dirac fermions. In strong contrast to the negligible electronic contribution in graphene, surprisingly, the electrons and phonons in stanene carry a comparable heat current. The unusual behaviors in stanene widen our knowledge of quantum thermal transport in 2D materials.