We study formation and superfluidity of dipolar excitons in double layer heterostructures formed by two transition metal dichalcogenide (TMDC) atomically thin layers. Considering screening effects for an electron-hole interaction via the harmonic oscillator approximation for the Keldysh potential, the analytical expressions for the exciton energy spectrum and the mean field critical temperature $T_c$ for the superfluidity are obtained. It is shown that binding energies of $A$ excitons are larger than for $B$ excitons. The mean field critical temperature for a two-component dilute exciton system in a TMDC double layer is analyzed and shown that the latter is an increasing function of the factor $Q$, determined by the effective masses of $A$ and $B$ excitons and their reduced mass. Comparison of the calculations for $T_c$ performed by employing the Coulomb and Keldysh interactions demonstrates the importance of screening effects in TMDC.

Strongly anharmonic phonon properties of CuCl are investigated with inelastic neutron-scattering measurements and first-principles simulations. An unusual quasiparticle spectral peak emerges in the phonon density of states with increasing temperature, in both simulations and measurements, emanating from exceptionally strong coupling between conventional phonon modes. Associated with this strong anharmonicity, the lattice thermal conductivity of CuCl is extremely low and exhibits anomalous, nonmonotonic pressure dependence. We show how this behavior arises from the structure of the phonon dispersions augmenting the phase space available for anharmonic three-phonon scattering processes, and contrast this mechanism with common arguments based on negative Grüneisen parameters. These results demonstrate the importance of considering intrinsic phonon-dispersion structure toward understanding scattering processes and designing new ultralow thermal conductivity materials.

We experimentally demonstrate by time-resolved scanning magneto-optical Kerr microscopy the possibility to locally excite multiple spin-wave beams in the dipolar-dominated regime in metallic NiFe films. For this purpose we employ differently shaped nonuniform microwave antennas consisting of several coplanar waveguide sections different in size, thereby adapting an approach for the generation of spin-wave beams in the exchange-dominated regime suggested by Gruszecki et al. [Sci. Rep. 6, 22367 (2016)]. The occurring spin-wave beams are diffractive and we show that the width of the beam and its widening as it propagates can be tailored by the shape and the length of the nonuniformity. Moreover, the propagation direction of the diffractive beams can be manipulated by changing the bias field direction.

We develop a comprehensive description of static and dynamic spin-transfer torque at interfaces between a normal metal and a magnetic material. Specific examples of the latter include ferromagnets, collinear and noncollinear antiferromagnets, general ferrimagnets, and spin glasses. We study the limit of the exchange-dominated interactions, so that the full system is isotropic in spin space, apart from a possible symmetry-breaking order. A general such interface yields three coefficients (corresponding to three independent generators of rotations) generalizing the well-established notion of the spin-mixing conductance, which pertains to the collinear case. We develop a nonequilibrium thermodynamic description of the emerging interfacial spin transfer and its effect on the collective spin dynamics, while circumventing the usual discussion of spin currents and net spin dynamics. Instead, our focus is on the dissipation and work effectuated by the interface. Microscopic scattering-matrix based expressions are derived for the generalized spin-transfer coefficients.

We report measurements on yttrium iron garnet (YIG) thin films grown on both gadolinium gallium garnet (GGG) and yttrium aluminum garnet (YAG) substrates, with and without thin Pt top layers. We provide three principal results: the observation of an interfacial region at the Pt/YIG interface, we place a limit on the induced magnetism of the Pt layer, and confirm the existence of an interfacial layer at the GGG/YIG interface. Polarized neutron reflectometry (PNR) was used to give depth dependence of both the structure and magnetism of these structures. We find that a thin film of YIG on GGG is best described by three distinct layers: an interfacial layer near the GGG, around 5 nm thick and nonmagnetic, a magnetic “bulk” phase, and a nonmagnetic and compositionally distinct thin layer near the surface. We theorize that the bottom layer, which is independent of the film thickness, is caused by Gd diffusion. The top layer is likely to be extremely important in inverse spin Hall effect measurements, and is most likely ${\mathrm{Y}}_2{\mathrm{O}}_3$ or very similar. Magnetic sensitivity in the PNR to any induced moment in the Pt is increased by the existence of the ${\mathrm{Y}}_2{\mathrm{O}}_3$ layer; any moment is found to be less than $0.02{\mu }_{\mathrm{B}}/\mathrm{atom}$.

The zero-temperature angles of magnetic moments in a helix or sinusoidal fan confined to the $xy$ plane, with respect to an in-plane magnetic field $H_x$ applied perpendicular to the $z$ axis of a helix or fan, are calculated for commensurate helices and fans with field-independent turn angles $kd$ between moments in adjacent layers of the helix or fan using the classical $J_0\text{−}{J}_1\text{−}{J}_2$ Heisenberg model. For $0<kd<4\pi /9$, first-order transitions from helix to a fan structure occur at fields $H_{\mathrm{t}}$ as previously inferred, where the fan is found to be approximately sinusoidal. However, for $4\pi /9\le kd\le \pi$, different behaviors are found depending on the value of $kd$ and these properties vary nonmonotonically with $kd$. In this $kd$ range, the change from helix to fanlike structure is usually a crossover with no phase transition between them, although first-order transitions are found for $kd=3\pi /5$ and $8\pi /11$ and a second-order transition for $kd=3\pi /4$. At a critical field $H_{\mathrm{c}}$, the fan or fanlike structures exhibit a second-order transition to the paramagnetic state. The $H_{\mathrm{c}}$ for a helix undergoing a field-induced change to a fan or fanlike structure is found to be the same as for a sinusoidal fan with the same $kd$ and interlayer interactions. Analytical expressions for $H_{\mathrm{c}}$ versus $kd$ are presented. We also calculated the average $x$-axis moment per spin ${\mu }_{x\mathrm{ave}}$ versus $H_x$ for helices and fans with crossovers and phase transitions between them. When smooth helix to fanlike crossovers occur in the range $4\pi /9\le kd\le \pi ,{\mu }_{x\mathrm{ave}}$ exhibits an S-shape behavior with increasing $H_x$. This predicted behavior is consistent with ${\mu }_{x\mathrm{ave}}\left(H_x\right)$ data previously reported by Sangeetha et al. [Phys. Rev. B 94, 014422 (2016)] for single-crystal ${\mathrm{EuCo}}_2{\mathrm{P}}_2$ possessing a helix ground state with $kd\approx 0.85\pi$. The low-field magnetic susceptibility and the ratio $H_{\mathrm{t}}/H_{\mathrm{c}}$ are calculated analytically or numerically versus $kd$ for helices, and are shown to approach the respective known limits for $kd\to 0$.

We introduce an inhomogeneous bosonic mixture composed of two kinds of hard-core and semi-hard-core bosons with different nilpotency conditions and demonstrate that in contrast with the standard hard-core Bose-Hubbard model, our bosonic mixture with nearest- and next-nearest-neighbor interactions on a square lattice develops the checkerboard supersolid phase characterized by the simultaneous superfluid and checkerboard solid orders. Our bosonic mixture is created from a two-orbital Bose-Hubbard model including two kinds of bosons: a single-orbital boson and a two-orbital boson. By mapping the bosonic mixture to an anisotropic inhomogeneous spin model in the presence of a magnetic field, we study the ground-state phase diagram of the model by means of cluster mean field theory and linear spin-wave theory and show that various phases such as solid, superfluid, supersolid, and Mott insulator appear in the phase diagram of the mixture. Competition between the interactions and magnetic field causes the mixture to undergo different kinds of first- and second-order phase transitions. By studying the behavior of the spin-wave excitations, we find the reasons of all first- and second-order phase transitions. We also obtain the temperature phase diagram of the system using cluster mean field theory. We show that the checkerboard supersolid phase persists at finite temperature comparable with the interaction energies of bosons.

The In-doped topological crystalline insulator ${\mathrm{Sn}}_{1-x}{\mathrm{In}}_x\mathrm{Te}$ is a candidate for a topological superconductor, where a pseudo-spin-triplet state has been proposed. To clarify the spin symmetry of ${\mathrm{Sn}}_{1-x}{\mathrm{In}}_x\mathrm{Te}$, we perform ${}^{125}\mathrm{Te}$-nuclear magnetic resonance (NMR) measurements in polycrystalline samples with $0\le x\le 0.15$. The penetration depth calculated from the NMR line width is $T$ independent below half the superconducting transition temperature ($T_c$) in polycrystalline ${\mathrm{Sn}}_{0.96}{\mathrm{In}}_{0.04}\mathrm{Te}$, which indicates a fully opened superconducting gap. In this sample, the spin susceptibility measured by the spin Knight shift ($K_{\mathrm{s}}$) at an external magnetic field of ${\mu }_0{H}_0=0.0872$ T decreases below $T_c$, and $K_{\mathrm{s}}(T=0)/K_{\mathrm{s}}(T=T_c)$ reaches $0.36\pm 0.10$, which is far below the limiting value 2/3 expected for a spin-triplet state for a cubic crystal structure. Our result indicates that polycrystalline ${\mathrm{Sn}}_{0.96}{\mathrm{In}}_{0.04}\mathrm{Te}$ is a spin-singlet superconductor.

We give a semiclassical analysis of the average photon number as well as photon number variance (Fano factor $F)$ for a Josephson junction (JJ) embedded microwave cavity system, where the JJ is subject to a fluctuating (i.e., noisy) bias voltage with finite dc average. Through the ac Josephson effect, the dc voltage bias drives the effectively nonlinear microwave cavity mode into an amplitude squeezed state $\left(F<1\right)$, as has been established previously [Armour et al., Phys. Rev. Lett. 111, 247001 (2013)], but bias noise acts to degrade this squeezing. We find that the sensitivity of the Fano factor to bias voltage noise depends qualitatively on which stable fixed point regime the system is in for the corresponding classical nonlinear steady-state dynamics. Furthermore, we show that the impact of voltage bias noise is most significant when the cavity is excited to states with large average photon number.

The superconducting properties of the TiO epitaxial thin films were systematically investigated under hydrostatic pressures $\left(P\right)$ up to 2.13 GPa. At ambient pressure, the normal state resistivity increases with decreasing temperature, and steeply increases below $T_{\mathrm{kink}}\sim 115\mathrm{K}$. With further reducing temperature to $T_c\sim 5.99\mathrm{K}$, the thin film enters into a superconducting state. Interestingly, the superconducting temperature $T_c$ gradually decreases upon increasing $P$, and the decreasing rate of $T_c$ with $P$ is much larger than the McMillan theoretical expectation. In contrast, $T_{\mathrm{kink}}$ increases with $P$ and a remarkable resistivity enhancement was observed in the temperature range between $T_{\mathrm{kink}}$ and $T_c$. The variations of $T_c,T_{\mathrm{kink}}$, and normal state resistivity under high pressure may be induced by the charge localization related to the atomic vacancies rearrangement in TiO thin film. Furthermore, the temperature dependencies of the upper critical field $H_{c2}\left(T\right)$ indicate that both the orbital and Pauli-paramagnetic pair-breaking effects should be taken into account. Finally, the thermally activated flux flow investigations under different pressures suggest that the pressure will suppress the thermal activate energy.

Recent theoretical models and experiments suggest that $\mathrm{Sm}{\mathrm{B}}_6$ is a topological Kondo insulator. Here we report on heat capacity $C\left(T\right)$ investigations of three single-crystalline $\mathrm{Sm}{\mathrm{B}}_6$ samples having different quality, of $\mathrm{Sm}{\mathrm{B}}_6$ single crystals doped with various concentration of trivalent lanthanum ions ($\mathrm{S}{\mathrm{m}}_{1–x}\mathrm{L}{\mathrm{a}}_x{\mathrm{B}}_6$ solid solutions) and divalent ytterbium ions ($\mathrm{S}{\mathrm{m}}_{1–x}\mathrm{Y}{\mathrm{b}}_x{\mathrm{B}}_6$), and of samarium-deficient sintered samples ($\mathrm{S}{\mathrm{m}}_{1–x}{\mathrm{B}}_6$). Isosbestic (crossing) points, which can be considered as a feature of universality, were observed on $C$($T, x$) dependencies for all investigated types of samples at $T_{\mathrm{iso}}\left(\mathrm{S}{\mathrm{m}}_{1–x}\mathrm{L}{\mathrm{a}}_x{\mathrm{B}}_6\right)\approx 17\mathrm{K}, T_{\mathrm{iso}}\left(\mathrm{S}{\mathrm{m}}_{1–x}\mathrm{Y}{\mathrm{b}}_x{\mathrm{B}}_6\right)\approx 31\mathrm{K}$, and $T_{\mathrm{iso}}\left(\mathrm{S}{\mathrm{m}}_{1–x}{\mathrm{B}}_6\right)\approx 21\mathrm{K}$. In connection with this as well as with the detailed analysis of the low-temperature $C\left(T\right)$ dependencies it can be assumed that by appropriate doping the properties of $\mathrm{Sm}{\mathrm{B}}_6$ may be changed/tuned. To obtain the evolution of the gap structure of $\mathrm{Sm}{\mathrm{B}}_6$ with changing doping, a simple phenomenological model based on rectangular densities of states was proposed. Additionally, for $\mathrm{S}{\mathrm{m}}_{1–x}\mathrm{L}{\mathrm{a}}_x{\mathrm{B}}_6$ solid solutions which were available with a high La concentration, it was shown that the insulator-metal transition appears around $x_{\mathrm{La}}\approx 0.40$.

We investigate the finite-temperature screening behavior of three-dimensional $U\left(1\right)$ spin-liquid phases with fracton excitations. Several features are shared with the conventional $U\left(1\right)$ spin liquid. The system can exhibit spin-liquid physics over macroscopic length scales at low temperatures, but screening effects eventually lead to a smooth finite-temperature crossover to a trivial phase at sufficiently large distances. However, unlike more conventional $U\left(1\right)$ spin liquids, we find that complete low-temperature screening of fractons requires not only very large distances, but also very long time scales. At the longest time scales, a charged disturbance (fracton) will acquire a screening cloud of other fractons, resulting in only short-range correlations in the system. At intermediate time scales, on the other hand, a fracton can only be partially screened by a cloud of mobile excitations, leaving weak power-law correlations in the system. Such residual power-law correlations may be a useful diagnostic in an experimental search for $U\left(1\right)$ fracton phases.

We study spin-wave excitations in $\alpha -{\mathrm{RuCl}}_3$ by the spin-wave theory. Starting from the five-orbital Hubbard model and the perturbation theory, we derive an effective isospin-$1/2$ model in the large Hubbard ($U$) limit. Based on the energy-band structure calculated from the first-principles method, we find that the effective model can be further reduced to the $K-\mathrm{\Gamma }$ model containing a ferromagnetic nearest-neighbor (NN) Kitaev interaction ($K$) and a NN off-diagonal exchange interaction ($\mathrm{\Gamma }$). With the spin-wave theory, we find that the $K-\mathrm{\Gamma }$ model can give magnetic excitations, which is consistent with recent neutron-scattering experiments.

In this paper we investigate the effect of strong electronic interactions on the thermoelectric properties of a simple generic system, consisting of a single correlated layer sandwiched between two metallic leads. Results will be given for the linear response regime as well as beyond, for which a full nonequilibrium many-body calculation is performed, based on nonequilibrium dynamical mean-field theory (DMFT). As impurity solver we use the auxiliary master equation approach, which addresses the impurity problem within a finite auxiliary system consisting of a correlated impurity, a small number of uncorrelated bath sites, and Markovian environments. For the linear response regime, results will be presented for the Seebeck coefficient, the electrical conductance, and the electronic contribution to the thermal conductance. Beyond linear response, i.e., for finite differences in the temperatures and/or the bias voltages in the leads, we study the dependence of the current on various model parameters, such as gate voltage and Hubbard interaction of the central layer. We will give a detailed parameter study as far as the thermoelectric efficiency is concerned. We find that strong correlations can indeed increase the thermopower of the device. In addition, some general theoretical requirements for an efficient thermoelectric device will be given.

We report on high frequency resolution coherent nonlinear optical spectroscopy on an ensemble of InGaN disks in GaN nanowires at room temperature. Sub-$\mu \mathrm{eV}$ resonances in the inhomogeneously broadened third order $\left({\chi }^{\left(3\right)}\right)$ absorption spectrum show asymmetric line shapes, where the degree of asymmetry depends on the wavelength of the excitation beams. Theory based on the optical Bloch equations (OBE) indicates that the line shape asymmetry is a result of fast decoherence in the system and the narrow resonances originate from coherent population pulsations that are induced by decoherence in the system. Using the OBE, we estimate that the decoherence time of the optically induced dipole (formed between the unexcited ground state and the excited electron-hole pair) at room temperature is 125 fs, corresponding to a linewidth of $\sim 10$ meV. The decay time of the excitation is $\sim 5–10$ ns, depending on the excitation energy. The line shapes are well fit with the OBE indicating that the resonances are characterized by discrete levels with no evidence of many body physics.

We investigate thermoelectric properties of a ferromagnet-superconductor hybrid structure with Rashba spin-orbit interaction and delta function potential barrier at the interfacial layer. The exponential rise of thermal conductance with temperature manifests a crossover temperature scale separating two opposite behaviors of it with the change of polarization in the ferromagnet whereas the inclusion of an interfacial Rashba spin-orbit field results in a nonmonotonic behavior of it with the strength of the Rashba field. We employ scattering matrix approach to determine the amplitudes of all the scattering processes possible at the interface to explain the thermoelectric properties of the device. We examine Seebeck effect and show that higher thermopower can be achieved when the polarization of the ferromagnet tends towards the half-metallic limit. It can be enhanced even for lower polarization in the presence of the finite potential barrier. In the presence of interfacial Rashba spin-orbit interaction, the Seebeck coefficient rises with the increase of barrier strength and polarization at weak or moderate interfacial Rashba field. From the application perspective, we compute the figure of merit and show that $zT\sim 4–5$ with higher polarization of the ferromagnet both in absence and presence of weak or moderate Rashba spin-orbit interaction along with the scalar potential barrier.

The phase-resolved sum-frequency-generation (SFG) spectrum for the basal face of hexagonal ice is reported and is interpreted by molecular dynamics simulations combined with ab initio quantum calculations. Here, we demonstrate that the line shape of the SFG spectra of isotope-diluted OH chromophores is a sensitive indicator of structural rumpling uniquely emerging at the subsurface of hexagonal ice. In the outermost subsurface between the first (B1) and second (B2) bilayer, the hydrogen bond of ${\mathrm{O}}_{\mathrm{B}1}-\mathrm{H}\cdots {\mathrm{O}}_{\mathrm{B}2}$ is weaker than that of ${\mathrm{O}}_{\mathrm{B}1}\cdots \mathrm{H}-{\mathrm{O}}_{\mathrm{B}2}$ . This implies that subsurface O-O distance is laterally altered, depending on the direction of O-H bond along the surface normal: H-up or H-down, which is in stark contrast to bulk hydrogen bonds. This new finding uncovers how water molecules undercoordinated at the topmost surface influence on the subsurface structural rumpling associated with orientational frustration inherent in water ice.

We propose and analyze a scheme for performing a long-range entangling gate for qubits encoded in electron spins trapped in semiconductor quantum dots. Our coupling makes use of an electrostatic interaction between the state-dependent charge configurations of a singlet-triplet qubit and the edge modes of a quantum Hall droplet. We show that distant singlet-triplet qubits can be selectively coupled, with gate times that can be much shorter than qubit dephasing times and faster than decoherence due to coupling to the edge modes. Based on parameters from recent experiments, we argue that fidelities above $99\%$ could in principle be achieved for a two-qubit entangling gate taking as little as 20 ns.

Two qubits form a quantum four-level system. The golden-rule based transition rates between these states are determined by the coupling of the qubits to noise sources. We demonstrate that depending on whether the noise acting on the two qubits is correlated or not, these transitions are governed by different selection rules. In particular, we find that for fully correlated or anticorrelated noise, there is a protected state, and the dynamics of the system depends then on its initialization. For nearly (anti)correlated noise, there is a long time scale determining the temporal evolution of the qubits. We apply our results to a quantum Otto refrigerator based on two qubits coupled to hot and cold baths. The steady-state power does not scale with the number $(=2$ here) of the qubits when there is a strong correlation of noise acting on them; under driven conditions the highest cooling power of the refrigerator is achieved for fully uncorrelated baths.