We have measured shear flow and residual structural anisotropy of $\mathrm{Si}{\mathrm{O}}_2$ glass uniaxially compressed in a diamond-anvil cell at room temperature with in situ optical-microscope and ex situ x-ray diffraction techniques, respectively. Large shear flow began at $8-10\mathrm{GPa}$ and continued at least up to 20 GPa, where the macroscopic differential strain reached 70%. Recovered samples after shear flow were in the fully densified state and showed a large microscopic differential strain of 3% only in the intermediate-range network structure. These phenomena may be attributable to the changes in the Si-O bond covalency and the Si-O-Si bond angle with pressure and stresses.

We use a tunneling model for two-level systems in insulators that takes into account the interaction between them and a slow power-law dependence of their density of states. We show that the predictions of this model are in a perfect agreement with the recent studies of the noise in high quality superconducting resonators. The predictions also agree with the temperature dependence of the TLS dephasing rates observed in phase qubits. Both observations are impossible to explain in the framework of the standard tunneling model of TLS. We discuss the origin of the universal dimensionless parameter that controls the interaction between TLS in glasses and show that it is consistent with the assumptions of the model.

We studied the vibrational dynamics of a single crystal of ${\mathrm{LaFe}}_4{\mathrm{Sb}}_{12}$ by three-axis inelastic neutron spectroscopy. The dispersion of phonons with wave vectors $\mathbf{q}$ along $\left[xx0\right]$ and $\left[xxx\right]$ directions in the energy range of eigenmodes with high amplitudes of lanthanum vibrations, i.e., at $\hslash \omega \lesssim 12$ meV is identified. Symmetry-avoided anticrossing dispersion of phonons is established in both monitored directions and distinct eigenstates at high-symmetry points and at the Brillouin-zone center are discriminated. The experimentally derived phonon dispersion and intensities are compared with and backed up by ab initio lattice dynamics calculations. Results of the computer model match well with the experimental data.

Through the use of Heisenberg spin-spin interactions, we provide analytical representations for inelastic neutron scattering eigenstates and excitation cross sections of the general $S_1-S_2$ spin dimeric systems. Using an exact diagonalization approach to the spin Hamiltonian, we analyze various spin coefficients to provide general representations for the neutron scattering cross sections of two interacting spins. We also provide a generalized method for the determination of $S_z$-polarized excitations, which produces an approximation for the spin excitations within an applied $z$-axis magnetic field. These calculations enhance the general understanding of the interactions between two individual or compound spin systems, which can help provide insight into condensed matter systems such as molecular magnets, quantum dots, and spintronic systems, as well as particle physics investigations into quark matter and meson interactions.

A thorough analysis of the ground state of the relativistic magnetic insulator ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ is performed. The results are in accordance with the small antiferromagnetic moment and gapped state found in experiment. The solution, obtained using the DFT+SO+$U$ methodology, is thoroughly analyzed in terms of Landau theory. We find that the ordered magnetic moment only forms a secondary order parameter while the primary order parameter is a higher order magnetic multipole of rank five. It is further observed that the electronic structure in the presence of this order parameter is related to the earlier proposed $j_{\mathrm{eff}}=1/2$ model, but in contrast to that model, the present picture can exactly explain the small magnitude of the ordered magnetic moments.

We investigate the antiferromagnetic canting instability of the spin-1/2 kagome ferromagnet, as realized in the layered cuprates ${\mathrm{Cu}}_3\mathrm{Bi}({\mathrm{SeO}}_3{)}_2{\mathrm{O}}_{2}X (X = \mathrm{Br}, \mathrm{Cl})$. While the local canting can be explained in terms of competing exchange interactions, the direction of the ferrimagnetic order parameter fluctuates strongly even at short distances on account of frustration which gives rise to an infinite ground state degeneracy at the classical level. In analogy with the kagome antiferromagnet, the accidental degeneracy is fully lifted only by nonlinear $1/S$ corrections, rendering the selected uniform canted phase very fragile even for spins-1/2, as shown explicitly by coupled-cluster calculations. To account for the observed ordering, we show that the minimal description of these systems must include the microscopic Dzyaloshinsky-Moriya interactions, which we obtain from density-functional band-structure calculations. The model explains all qualitative properties of the kagome francisites, including the detailed nature of the ground state and the anisotropic response under a magnetic field. The predicted magnon excitation spectrum and quantitative features of the magnetization process call for further experimental investigations of these compounds.

The Tomasch effect (TE) is due to quasiparticle interference (QPI) as induced by a nonuniform superconducting order parameter, which results in oscillations in the density of states (DOS) at energies above the superconducting gap. Quantum confinement in nanoscale superconductors leads to an inhomogenerous distribution of the Cooper-pair condensate, which, as we found, triggers the manifestation of a new TE. We investigate the electronic structure of nanoscale superconductors by solving the Bogoliubov-de Gennes (BdG) equations self-consistently and describe the TE determined by two types of processes, involving two- or three-subband QPIs. Both types of QPIs result in additional BCS-like Bogoliubov-quasiparticles and BCS-like energy gaps leading to oscillations in the DOS and modulated wave patterns in the local density of states. These effects are strongly related to the symmetries of the system. A reduced $4\times 4$ inter-subband BdG Hamiltonian is established in order to describe analytically the TE of two-subband QPIs. Our study is relevant to nanoscale superconductors, either nanowires or thin films, Bose-Einsten condensates, and confined systems such as two-dimensional electron gas interface superconductivity.

Polycrystalline ${\mathrm{Hf}\mathrm{Pd}}_2\mathrm{Al}$ has been synthesized using the arc-melting method and studied under ambient-pressure conditions by x-ray diffraction from room temperature up to 450 °C. High-pressure x-ray diffraction up to 23 GPa was also performed using Diacell-type membrane diamond anvil cells. The estimated linear thermal expansion coefficient was found to be $\alpha =1.40\left(3\right)\times {10}^{-5}{\mathrm{K}}^{-1}$, and the bulk modulus derived from the fit to the third-order Birch-Murnaghan equation of state is $B_0=97\left(2\right)$ GPa. Resistivity studies under applied pressure $\left(p\le 7.49\mathrm{GPa}\right)$ showed a linear decrease of superconducting critical temperature with increasing pressure and the slope $d{T}_c/dp=-0.13\left(1\right)\mathrm{K}{\mathrm{GPa}}^{-1}$. The same behavior is observed for the electron-phonon coupling constant ${\lambda }_{\mathrm{ep}}\left(p\right)$ that changes from 0.67 to 0.6, estimated for $p=0.05$ and 7.49 GPa, respectively. First-principles electronic structure and phonon calculation results are presented and used to estimate the magnitude of electron-phonon interaction ${\lambda }_{\mathrm{ep}}$ and its evolution with pressure. Theoretical results explain the experimentally observed decrease in $T_c$ due to considerable lattice stiffening.

A two-dimensional compound made of one monolayer of Tl and one monolayer of Sn on Si(111) has been found to have a sandwichlike structure in which the Sn layer (having the milk-stool arrangement) resides on the bulklike terminated Si(111) surface and the Tl layer (having the honeycomb-chained-trimer arrangement) is located above the Sn layer. The electronic band structure of the compound contains two spin-split surface-state bands, of which one is nonmetallic and the other is metallic. Near the Fermi level the metallic band is split with the momentum splitting $\Delta k_{\parallel }=0.037$ Å${}^{-1}$ and energy splitting $\Delta E_{\mathrm{F}}=167$ meV. The steep dispersion of the band when crossing the Fermi level corresponds to an electron velocity of $\approx 8.5\times {10}^5$ m/s, which is comparable to the value reported for graphene. The 2D Fermi contours have almost circular shape with spin texture typical for hexagonal surfaces.

We calculate the magnetic response of a buckled honeycomb lattice with intrinsic spin-orbit coupling (such as silicene) which supports valley-spin polarized energy bands when subjected to a perpendicular electric field $E_z$. By changing the magnitude of the external electric field, the size of the two band gaps involved can be tuned, and a transition from a topological insulator (TI) to a trivial band insulator (BI) is induced as one of the gaps becomes zero, and the system enters a valley-spin polarized metallic state (VSPM). In an external magnetic field $\left(B\right)$, a distinct signature of the transition is seen in the derivative of the magnetization with respect to chemical potential $\mu$, which gives the quantization of the Hall plateaus through the Streda relation. When plotted as a function of the external electric field, the magnetization has an abrupt change in slope at its minimum, which signals the VSPM state. The magnetic susceptibility $\chi$ shows jumps as a function of $\mu$ when a band gap is crossed, which provides a measure of the gaps' variation as a function of external electric field. Alternatively, at fixed $\mu$, the susceptibility displays an increasingly large diamagnetic response as the electric field approaches the critical value of the VSPM phase. In the VSPM state, magnetic oscillations exist for any value of chemical potential while for the TI and BI states, $\mu$ must be larger than the minimum gap in the system. When $\mu$ is larger than both gaps, there are two fundamental cyclotron frequencies (which can also be tuned by $E_z$) involved in the de-Haas van-Alphen oscillations that are close in magnitude. This causes a prominent beating pattern to emerge.

Polariton condensation of one-dimensional multimode whispering gallery mode exciton polaritons is investigated in hexagonal ZnO microwires at cryogenic temperatures. At threshold, stimulated scattering is fed by the resonant emission from highly populated defect-bound excitons. With further increasing excitation power, condensates relax within the multimode whispering gallery mode polariton ladder, leading to their effective evaporative cooling. The relaxation is a parametric polariton-polariton scattering process evidenced by its intensity, energy, and momentum evolution. The experimental observations are in good agreement with numerical simulations based on a model for polariton-polariton scattering extended to the multimodal whispering gallery mode system.

We study the low-temperature properties of the classical three-dimensional compass or $t_{2g}$ orbital model on simple-cubic lattices by means of comprehensive large-scale Monte Carlo simulations. Our numerical results give evidence for a directionally ordered phase that is reached via a first-order transition at the temperature $T_0=0.098328\left(3\right)J/k_{\mathrm{B}}$. To obtain our results, we employ local and cluster update algorithms, parallel tempering and multiple histogram reweighting as well as model-specific screw-periodic boundary conditions, which help counteract severe finite-size effects.

We present a general frame to extend functional renormalization group (fRG) based computational schemes by using an exactly solvable interacting reference problem as starting point for the RG flow. The systematic expansion around this solution accounts for a nonperturbative inclusion of correlations. Introducing auxiliary fermionic fields by means of a Hubbard-Stratonovich transformation, we derive the flow equations for the auxiliary fields and determine the relation to the conventional weak-coupling truncation of the hierarchy of flow equations. As a specific example we consider the dynamical mean field theory (DMFT) solution as reference system, and discuss the relation to the recently introduced ${\mathrm{DMF}}^2\mathrm{RG}$ and the dual-fermion formalism.

We reexamine the one-dimensional spin-1 $XXZ$ model with on-site uniaxial single-ion anisotropy as to the appearance and characterization of the symmetry-protected topological Haldane phase. By means of large-scale density-matrix renormalization group (DMRG) calculations the central charge can be determined numerically via the von Neumann entropy, from which the ground-state phase diagram of the model can be derived with high precision. The nontrivial gapped Haldane phase shows up in between the trivial gapped even Haldane and Néel phases, appearing at large single-ion and spin-exchange interaction anisotropies, respectively. We furthermore carve out a characteristic degeneracy of the lowest entanglement level in the topological Haldane phase, which is determined using a conventional finite-system DMRG technique with both periodic and open boundary conditions. Defining the spin and neutral gaps in analogy to the single-particle and neutral gaps in the intimately connected extended Bose-Hubbard model, we show that the excitation gaps in the spin model qualitatively behave just as for the bosonic system. We finally compute the dynamical spin structure factor in the three different gapped phases and find significant differences in the intensity maximum which might be used to distinguish these phases experimentally.

We use cellular dynamical mean-field theory with the continuous-time quantum Monte Carlo solver to study the Kane-Mele-Hubbard model supplemented with an additional third-neighbor hopping term. For weak interactions, the third-neighbor hopping term drives a topological phase transition between a topological insulator and a trivial insulator, consistent with previous fermion sign-free quantum Monte Carlo results [Hung et al., Phys. Rev. B 89, 235104 (2014)]. At finite temperatures, the Dirac cones of the zero-temperature topological phase boundary give rise to a metallic regime of finite width in the third-neighbor hopping. Furthermore, we extend the range of interactions into the strong-coupling regime and find an easy-plane antiferromagnetic insulating state across a wide range of third-neighbor hopping at zero temperature. In contrast to the weak-coupling regime, no topological phase transition occurs at strong coupling, and the ground state is a trivial antiferromagnetic insulating state. A comprehensive finite-temperature phase diagram in the interaction-third-neighbor hopping plane is provided.

The radial confining potential in a semiconductor nanowire plays a key role in determining its quantum transport properties. Previous reports have shown that an axial magnetic field induces flux-periodic conductance oscillations when the electronic states are confined to a shell. This effect is due to the coupling of orbital angular momentum to the magnetic flux. Here, we perform calculations of the energy level structure, and consequently the conductance, for more general cases ranging from a flat potential to strong surface band bending. The transverse states are not confined to a shell, but are distributed across the nanowire. It is found that, in general, the subband energy spectrum is aperiodic as a function of both gate voltage and magnetic field. In principle, this allows for precise identification of the occupied subbands from the magnetoconductance patterns of quasiballistic devices. The aperiodicity becomes more apparent as the potential flattens. A quantitative method is introduced for matching features in the conductance data to the subband structure resulting from a particular radial potential, where a functional form for the potential is used that depends on two free parameters. Finally, a short-channel InAs nanowire field-effect transistor device is measured at low temperature in search of conductance features that reveal the subband structure. Features are identified and shown to be consistent with three specific subbands. The experiment is analyzed in the context of the weak localization regime; however, we find that the subband effects predicted for ballistic transport should remain visible when backscattering dominates over interband scattering, as is expected for this device.

We clarify the long-standing controversy concerning the behavior of the ground-state fidelity in the vicinity of a quantum phase transition of the Berezinskii-Kosterlitz-Thouless type in one-dimensional systems. Contrary to the prediction based on the Gaussian approximation of the Luttinger-liquid approach, it is shown that the fidelity susceptibility does not diverge at the transition but has a cusplike peak ${\chi }_c-\chi \left(\lambda \right)\sim \sqrt{|{\lambda }_c-\lambda |}$, where $\lambda$ is a parameter driving the transition and ${\chi }_c$ is the peak value at the transition point $\lambda ={\lambda }_c$. Numerical claims of the logarithmic divergence of fidelity susceptibility with the system size (or temperature) are explained by logarithmic corrections due to marginal operators, which is supported by numerical calculations for large systems.

The applied magnetic field and temperature phase diagram of the archetypal frustrated magnet, ${\mathrm{Gd}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$, has been reinvestigated using single-crystal magnetometry and polarized neutron diffraction. The updated phase diagram is substantially more complicated than previously reported and can be understood in terms of competing interactions with loops of spins, trimers, and decagons, in addition to competition and interplay between antiferromagnetic, incommensurate, and ferromagnetic order. Several additional distinct phase boundaries are presented. The phase diagram centers around a multiphase convergence to a single point at 0.9 T and $\sim 0.35$ K, below which, in temperature, a very narrow magnetically disordered region exists. These data illustrate the richness and diversity that arise from frustrated exchange on the three-dimensional hyperkagome lattice.

$\mathrm{FeV}{\mathrm{O}}_4$ is a multiferroic that undergoes two antiferromagnetic transitions at $T_{\mathrm{N}1}\sim 21\mathrm{K}$ and $T_{\mathrm{N}2}\sim 15\mathrm{K}$, with a small ferroelectric polarization developing at the 15-K transition. We have studied the effect of magnetic and nonmagnetic transition metal dopants on these magnetic phase transitions in order to investigate the microscopic mechanisms for magnetoelectric coupling in this system as well as probing the evolution of the phase transitions. We have synthesized polycrystalline $\mathrm{F}{\mathrm{e}}_{1-\mathrm{x}}T{M}_{\mathrm{x}}\mathrm{V}{\mathrm{O}}_4(TM=\mathrm{Zn},\mathrm{Cr},\mathrm{Mn})$ samples up to $x=0.2$ and studied dynamics of these two ferroic phase transitions using thermodynamic, magnetic, and pyrocurrent measurements. We find that the magnetic ordering temperatures in $\mathrm{FeV}{\mathrm{O}}_4$ are remarkably stable, showing only a minimal suppression in transition temperatures even for larger doping fractions. We also observe clear reversible polarization at $x=0.05$ for all samples, which also persists to the largest doping for Zn, suggesting that the multiferroic order persists over a large range of compositions.