Thermoelectric effects in silicene nanoribbons are analyzed theoretically and numerically. The main focus is on the influence of topological edge states and transition between the topological-insulator and conventional gap-insulator states on the thermoelectric properties, and especially on the spin-related thermoelectric effects. The model includes a staggered exchange field and also an external electric field normal to the atomic plane. Both fields separately open a gap in the edge states and therefore lead to a nonzero thermopower in the vicinity of the gap edges. Interplay of both fields leads to a spin-dependent gap, and thus gives rise to a spin thermopower in addition to the conventional Seebeck effect. The role of the Coulomb interaction taken in the form of the Hubbard term in the mean-field approximation is also analyzed. This interaction leads to antiparallel configuration of the edge magnetic moments in the ground state. It is shown that the Coulomb interaction significantly modifies topological properties of the materials, and thus also their transport and thermoelectric properties.

We demonstrate an innovative multifunctional artificial material that combines exotic metamaterial properties and the environmentally responsive nature of phase-change media. The tunable metamaterial is designed with the aid of two interwoven coordinate-transformation equations and implemented with a network of thin-film resistors and vanadium dioxide (${\mathrm{VO}}_2$). The strong temperature dependence of ${\mathrm{VO}}_2$ electrical conductivity results in a significant modification of the resistor network behavior, and we provide experimental evidence for a reconfigurable metamaterial electric circuit that not only mimics a continuous medium, but is also capable of responding to thermal stimulation through dynamic variation of its spatial anisotropy. Upon external temperature change, the overall effective functionality of the material switches between a “truncated cloak” and a “concentrator” for electric currents. Possible applications may include adaptive matching resistor networks, multifunctional electronic devices, and equivalent artificial materials in the magnetic domain. Additionally, the proposed technology could also be relevant for thermal management of integrated circuits.

The origin of the relaxor behavior in ${\mathrm{K}}_{1-x}{\mathrm{Li}}_x{\mathrm{TaO}}_3\left(\mathrm{KLT}\right)$ and other disordered perovskites is now recognized to be due to the reorientation of the polar nanodomains formed by the correlated dipoles of off-center ions. The collective dynamics of these systems evolve through several temperature stages. On decreasing temperature below the so-called Burns temperature $T_B$, individual dipoles become correlated within nanosized regions. On further cooling, the slow dynamics of these polar regions allows local lattice distortions to take place and the formation of polar nanodomains at $T^{*}<T_B$. At still lower temperature, some relaxors undergo a phase transition while others do not. In KLT, there is a critical Li concentration $x_c=0.022$ above which the system undergoes a structural transition at $T_c$, and below which it freezes in a dipole glass state at $T_f$. To better understand the nature of this critical concentration, the changes that occur upon crossing it and the nature of the dipole glass state, the collective dynamics of KLT have been studied by dielectric spectroscopy and neutron diffraction for two Li concentrations $(x=0.026$ and $0.018)$, close to but straddling the critical concentration $x_c$. Two very different transitional behaviors are observed. Just below this critical concentration, KLT displays critical slowing down and the onset of freezing as seen in hydrogen-bonded molecular ferroelectrics, while just above this concentration, KLT undergoes a first-order structural transition.

We study the quantum-spin-liquid phase in a variant of the Kitaev model where the bonds of the honeycomb lattice are distributed in a Kekulé pattern. The system supports gapped and gapless $Z_2$ quantum spin liquids with interesting differences from the original Kitaev model, the most notable being a gapped $Z_2$ spin liquid on a kagome lattice. Perturbing the exactly solvable model with antiferromagnetic Heisenberg perturbations, we find a magnetically ordered phase stabilized by a quantum “order by disorder” mechanism, as well as an exotic continuous quantum phase transition between the topological spin liquid and this magnetically ordered phase. Using a combination of field theory and Monte Carlo simulations, we find that the transition likely belongs to the $3D\text{−}XY\times Z_2$ universality class.

Andreev reflection of thermal quasiparticles from quantized vortices is an important technique to visualize quantum turbulence in low temperature ${}^3\mathrm{He}\text{−}B$. We revisit a problem of Andreev reflection from the isolated, rectilinear vortex line. For quasiparticle excitations whose impact parameters, defined as distances of the closest approach to the vortex core, do not exceed some arbitrary value $b$, we calculate exactly the reflected fraction of the total flux of excitations incident upon the vortex in the direction orthogonal to the vortex line. We then define and calculate exactly, as a function of $b$, the scattering length, that is the scattering cross section per unit length of the vortex line. We also define and calculate the scattering lengths for the flux of energy carried by thermal excitations, and for the net energy flux resulting from a (small) temperature gradient, and analyze the dependence of these scattering lengths on temperature.

In systems with broken $U\left(1\right)$ symmetry, such as superfluids, superconductors, or magnets, the symmetry restoration is driven by the proliferation of topological defects in the form of vortex loops (unless the phase transition is strongly first order). Here we discuss that the proliferation of topological defects can, by contrast, lead to the breakdown of an additional symmetry. We demonstrate that this effect should take place in $s+is$ superconductors, which are widely discussed in connection with iron-based materials (although the mechanism is much more general). In these systems a vortex excitation can create a “bubble” of fluctuating $Z_2$ order parameter. The thermal excitation of vortices then leads to the breakdown of $Z_2$ time-reversal symmetry when the temperature is increased.

We have measured the spin torques of $\beta \text{−}\mathrm{tantalum}/\mathrm{C}{\mathrm{o}}_{20}\mathrm{F}{\mathrm{e}}_{60}{\mathrm{B}}_{20}$ bilayers versus Ta thickness at room temperature using a ferromagnetic resonance (FMR) technique. A significant fieldlike spin torque originating from Ta was identified, which is constant and independent of Ta thickness. Because of this constant torque, the spin Hall coefficient ${\theta }_{\mathrm{SH}}$ needs to be calculated from the ratio of the symmetric component of the FMR signal to the slope of the antisymmetric component with Ta thickness, from which a value of $-0.11\pm 0.01$ was determined. The saturation magnetization of the CoFeB layers for samples deposited with Ta was found to be smaller than that of a single CoFeB layer, with values of $1.84\pm 0.01$ and $1.92\pm 0.01$ T, respectively. The origin of the fieldlike torque is ascribed to an interface spin-orbit coupling, or Rashba effect, due to the strength and constancy of the torque with Ta thickness. From fitting measured data to a semiclassical diffusion model that includes interface spin-orbit coupling, we have determined the spin diffusion length for β-tantalum to be 2.5 nm.

We present measurements of the magnetization $M$, specific heat $C$, resistivity $\rho$, and magnetoresistance MR of single-crystalline hexagonal CeAuSn for temperature down to $T=1.6 \mathrm{K}$ and in magnetic field up to $B=12 \mathrm{T}$. Antiferromagnetic ordering at $T_{\mathrm{N}}=4.4 \mathrm{K}$ is observed as previously found for polycrystalline samples. A strong magnetic easy-plane anisotropy of $M$ for $B$ perpendicular and parallel to the $c$ direction is found with $M_{\perp }/M_{\left|\right|}\approx 15$ in $B=0.1 \mathrm{T}$ around $T_{\mathrm{N}}$, which is attributed to crystal-electric-field anisotropy. The analysis of the magnetic susceptibility indicates ferromagnetic correlations above $T_{\mathrm{N}}$. Measurements of $M\left(T\right)$ under hydrostatic pressure $P$ show that $T_{\mathrm{N}}\left(P\right)$ increases linearly with $P$ at a small rate of 0.035 K/kbar up to 4 kbar and gradually saturates approaching $P=16\mathrm{kbar}$. Zero-field $\Delta C/T$, the phonon contribution to $C$ being subtracted, is proportional to $T^2$ below $T_{\mathrm{N}}$ indicating a gapless spin-wave spectrum. It is found that all $\Delta C(T,B)/T$ curves for $B=0–9 \mathrm{T}$ cross at the same temperature, providing an example of a particularly well defined isosbestic point in a very narrow region around $T_{\mathrm{iso}}=6.6 \mathrm{K}$. Finally, $\rho \left(T\right)$ and MR experiments with current perpendicular and parallel to $B$ allow us to separate orbital effects from the Zeeman splitting.

We investigate the eigenstate thermalization hypothesis (ETH) in integrable models, focusing on the $\text{spin}-\frac{1}{2}$ isotropic Heisenberg $\left(XXX\right)$ chain. We provide numerical evidence that the ETH holds for typical eigenstates (weak ETH scenario). Specifically, using a numerical implementation of state-of-the-art Bethe ansatz results, we study the finite-size scaling of the eigenstate-to-eigenstate fluctuations of the reduced density matrix. We find that fluctuations are normally distributed, and their standard deviation decays in the thermodynamic limit as $L^{-1/2}$, with $L$ the size of the chain. This is in contrast with the exponential decay that is found in generic nonintegrable systems. Based on our results, it is natural to expect that this scenario holds in other integrable spin models and for typical local observables. Finally, we investigate the entanglement properties of the excited states of the $XXX$ chain. We numerically verify that typical midspectrum eigenstates exhibit extensive entanglement entropy (i.e., volume-law scaling).

Superconductivity evolves as functions of pressure or doping from charge-ordered phases in a variety of strongly correlated systems, suggesting that there may be universal characteristics associated with the competition between superconductivity and charge order in these materials. We present an inelastic light (Raman) scattering study of the structural changes that precede the pressure-tuned charge-density-wave (CDW) to superconductor transition in one such system, ${\mathrm{ZrTe}}_3$. In certain phonon bands, we observe dramatic linewidth reductions that accompany CDW formation, indicating that these phonons couple strongly to the electronic degrees of freedom associated with the CDW. The same phonon bands, which represent internal vibrations of ${\mathrm{ZrTe}}_3$ prismatic rods, are suppressed at pressures above $\sim 10\mathrm{kbar}$, indicating a loss of long-range order within the rods, specifically amongst intrarod Zr-Te bonds. These results suggest that the pressure-induced suppression of CDW order observed in ${\mathrm{ZrTe}}_3$ is structurally driven and provide insights into the origin of pressure-induced superconductivity in this material.

The energy dispersion of the tunnel spin polarization of ferromagnetic tunnel contacts $(\mathrm{C}{\mathrm{o}}_{70}\mathrm{F}{\mathrm{e}}_{30}/\mathrm{MgO})$ to semiconductors (silicon and germanium) has been extracted from thermal and electrical spin currents. It is demonstrated that the variation of thermal and electrical spin currents with bias voltage can be described simultaneously using the same energy dispersion of the tunnel spin polarization, whereby the electrical spin current is determined by the value of the tunnel spin polarization, whereas the energy derivative of the polarization determines the thermal spin current. Variations in the behavior of the spin currents for different devices can be correlated to the degree of energy dispersion of the tunnel spin polarization. It is also shown that the application of a modest voltage can switch the thermal spin current to zero and can even reverse its sign. The characterization of the energy dispersion of the tunnel spin polarization provides a tool for the engineering of the spin-dependent thermoelectric properties of ferromagnetic tunnel contacts.

We investigate the properties of the molecular quantum dot (Holstein-Anderson) model using numerical and analytical techniques. Path integral Monte Carlo simulations for the cumulants of the distribution function of the phonon coordinate reveal that at intermediate temperatures the effective potential for the oscillator exhibits two minima rather than a single one, which can be understood as a signature of a bistability effect. A straightforward adiabatic approximation turns out to adequately describe the properties of the system in this regime. Upon lowering the temperature the two potential energy minima of the oscillator merge to a single one at the equilibrium position of the uncoupled system. Using the parallels to the x-ray edge problem in metals we derive the oscillator partition function. It turns out to be identical to that of the Kondo model, which is known to possess a universal low-energy fixed point characterized by a single parameter—the Kondo temperature $T_K$. We derive an analog of $T_K$ for the molecular quantum dot model, present numerical evidence pointing towards the appearance of the Kondo physics, and discuss experimental implications of the discovered phenomena.

We have investigated the electronic structure and carrier dynamics of the topological insulator $\mathrm{Bi}{}_{2-x}\mathrm{Sb}{}_x\mathrm{Te}{}_{3-y}\mathrm{Se}{}_y$, for $x=0.5,y=1.3$ and $x=1,y=2$, using infrared spectroscopy. Our results show that both of these BSTS alloys are highly insulating in the bulk, with analysis of the infrared data indicating an upper limit to the carrier density of $4.4\times {10}^{17}$ ${\mathrm{cm}}^{-3}$. Furthermore, analysis of the interband transitions of $\mathrm{Bi}{}_{1.5}\mathrm{Sb}{}_{0.5}\mathrm{Te}{}_{1.7}\mathrm{Se}{}_{1.3}$ revealed distinct band-structure critical points, which suggest high crystallographic order of our crystals. Motivated by the low carrier density and crystallographic order identified in these compounds, we searched for the Landau level transitions associated with the surface states through magneto-optical measurements in the far infrared range. We failed to observe any indications of the Landau level resonances at fields up to 8 T in sharp contrast with our earlier finding for a related $\mathrm{Bi}{}_{1-x}\mathrm{Sb}{}_x$ alloy. We discuss factors that may be responsible for suppressed magneto-optics response of these single crystals.

We propose that low-strain Si/O superlattices can be constructed by connecting reconstructed Si{001} surfaces by Si-O-Si bridges. Ab initio calculations show that our models are energetically more favorable than all the models proposed so far. The part of our Si/O superlattice model is experimentally accessible just by oxidizing a Si(001) substrate. To complete our Si/O superlattice model, we propose a three-step method. We also explore the potential of our Si/O superlattice models for new materials used in future Si electronics. We find that the location of the channel where the carriers travel can be controlled between the interfaces and the Si layers by the insertion of O atoms into the Si-Si dimers. By revealing the origins of the interface electron and hole states, we find that similar interface states should be easily achieved for Si slabs and Si substrates. Interestingly, the interface electrons and holes have small effective masses in the direction parallel to the channel and large effective masses in the direction normal to the channel, which makes the Si/O superlattices attractive to be used for channel materials. We also find that the valley splitting of Si is enhanced by the formation of the Si/O/Si interfaces, which is ideal for developing Si-based qubits. Our findings open new perspectives to design and control the electronic properties of Si.

In one-dimensional quantum systems with strong long-range repulsion particles arrange in a quasiperiodic chain, the Wigner crystal. We demonstrate that besides the familiar phonons, such one-dimensional Wigner crystal supports an additional mode of elementary excitations, which can be identified with solitons in the classical limit. We compute the corresponding excitation spectrum and argue that the solitons have a parametrically small decay rate at low energies. We discuss implications of our results for the behavior of the dynamic structure factor.

The scaling technique, often used to study magnetic properties of nanostructures, is based on a reduction in the geometrical lengths of the system via a scaling factor $x$ and a scaling exponent $\eta$ [Phys. Rev. Lett. 88, 237202 (2002)]. At the same time, to keep competition among the energy terms involved, the exchange constant $J$ is reduced throughout the system in accordance with the scaling factor $x$. Different values for the scaling exponent $\eta$ have been used in several studies, and the discussion of whether there is just one or several values is resolved here. In this study we revise the scaling method and propose a different approach than the above. In this generalized approach, the scaling scheme is applied to the energy term as a whole, instead of only to the exchange coupling $J$. We apply this proposed methodology to typical iron cylindrical samples and find that the application of the scaling technique to systems in the mesoscopic range does not depend on the selection of the value of a scaling exponent. We show that this generalized approach gives the same results when it is applied under both discrete and continuous frameworks, and, therefore, it represents a solution for this pending issue.

We report the results of high-field, low-temperature $\mu \mathrm{SR}$ measurements of the quasi-two-dimensional organic superconductors $\kappa \text{−}{\left(\mathrm{ET}\right)}_2\mathrm{Cu}{\left(\mathrm{NCS}\right)}_2$, and $\kappa \text{−}{\left(\mathrm{ET}\right)}_2\mathrm{Cu}\left[\text{N}{\left(\mathrm{CN}\right)}_2\right]\mathrm{Br}$. The $\mu \mathrm{SR}$ lineshapes for these compounds indicate the existence of partially ordered vortex lattice phases in the high-magnetic-field regime, up to 2.5 T for the former compound and 4 T for the latter compound. The observed sharp loss of order is found to be consistent with a vortex-lattice melting transition that is predicted by numerical simulations of weakly coupled layers of pancake vortices. It is argued that the robustness of the partially ordered vortex lattice phases could be due to strong flux-line pinning by a dilute ensemble of defects.

Investigation of the properties and phase behavior of noncrystalline water is hampered by rapid crystallization in the so-called “no man's land.” We here show that it is possible to shrink the no man's land by lifting its low-temperature boundary, i.e., the pressure-dependent crystallization temperature $T_x\left(p\right)$. In particular, we investigate two types of high-density amorphous ice (HDA) in the pressure range of $0.10–0.50\mathrm{GPa}$ and show that the commonly studied unannealed state, uHDA, is up to 11 K less stable against crystallization than a pressure-annealed state called eHDA. We interpret this finding based on our previously established microscopic picture of uHDA and eHDA, respectively [M. Seidl et al., Phys. Rev. B 88, 174105 (2013)]. In this picture the glassy uHDA matrix contains ice ${\mathrm{I}}_{\mathrm{h}}$-like nanocrystals, which simply grow upon heating uHDA at pressures $\le 0.20\mathrm{GPa}$. By contrast, they experience a polymorphic phase transition followed by subsequent crystal growth at higher pressures. In comparison, upon heating purely glassy eHDA, ice nuclei of a critical size have to form in the first step of crystallization, resulting in a lifted $T_x\left(p\right)$. Accordingly, utilizing eHDA enables the study of amorphous ice at significantly higher temperatures at which we regard it to be in the ultraviscous liquid state. This will boost experiments aiming at investigating the proposed liquid-liquid phase transition.

We study the lattice thermal conductivity $\left(\kappa \right)$ of fully filled skutterudites $\mathrm{YbFe}{}_4{\text{Sb}}_{12}$ and ${\text{BaFe}}_4{\text{Sb}}_{12}$ from first principles. The calculated $\kappa$ of ${\text{YbFe}}_4{\text{Sb}}_{12}$ is ten times lower than that calculated for $\mathrm{BaFe}{}_4{\text{Sb}}_{12}$, and much lower than any values ever reported for other skutterudites. The ultralow $\kappa$ of $\mathrm{YbFe}{}_4{\text{Sb}}_{12}$ is closely related to the flat Yb-dominated modes appearing in the frequency range from 5 to 7.2 rad/ps. The flat modes significantly increase the three-phonon scattering channels, which is reflected by the unique characteristics of weighted phase space and leads to much stronger anharmonic scattering of intermediate-frequency optical phonons. Although those flat modes are dominated by the Yb atoms, the filler-related anharmonic interaction does not play a role in the increased phonon scattering. This underlines the importance of the hybridization of the filler and the host matrix in those flat modes, which can be guaranteed by the avoided crossing with acoustic phonons. The depressed phonon spectrum mechanism, common to other skutterudites, also plays a role in the ultralow $\kappa$. Our work presents a reduction mechanism of $\kappa$ by the filler in cage-like structures such as skutterudites and clathrates, and demonstrates the nonuniqueness of the reduction mechanism in these materials.

It has been known that an antiunitary symmetry such as time-reversal or charge conjugation is needed to realize ${\mathit{Z}}_2$ topological phases in noninteracting systems. Topological insulators and superconducting nanowires are representative examples of such ${\mathit{Z}}_2$ topological matters. Here we report the ${\mathit{Z}}_2$ topological phase protected by only unitary symmetries. We show that the presence of a nonsymmorphic space group symmetry opens a possibility to realize ${\mathit{Z}}_2$ topological phases without assuming any antiunitary symmetry. The ${\mathit{Z}}_2$ topological phases are constructed in various dimensions, which are closely related to each other by Hamiltonian mapping. In two and three dimensions, the ${\mathit{Z}}_2$ phases have a surface consistent with the nonsymmorphic space group symmetry, and thus they support topological gapless surface states. Remarkably, the surface states have a unique energy dispersion with the Möbius twist, which identifies the ${\mathit{Z}}_2$ phases experimentally. We also provide the relevant structure in the $K$ theory.