According to Clausius formulation of the second law of thermodynamics, for any thermal machine withdrawing heats $Q_{1,2}$ from two heat reservoirs at temperatures $T_{1,2}$, it holds that $Q_1/T_1+Q_2/T_2\le 0$. Combined with the observation that the quantity $Q_1+Q_2$ is the work $W$ done by the system, that inequality tells us that only four operation modes are possible for the thermal machine, namely, heat engine, refrigerator, thermal accelerator, and heater. We illustrate their emergence in the finite time operation of a quantum Otto engine realized with a single qubit. We first focus on the ideal case when isochoric and thermally insulated strokes are well separated and give general results as well as results pertaining to the specific finite-time Landau-Zener dynamics. We then present realistic results pertaining to the solid-state experimental implementation proposed by Karimi and Pekola [Phys. Rev. B 94, 184503 (2016)]. That device is nonadiabatic both in the quantum mechanical sense and in the thermodynamical sense. Oscillations in the power extracted from the baths due to coherent LZ tunneling at too low of temperatures are observed that might hinder the robustness of the operation of the device against experimental noise on the control parameters.

The superconducting gap symmetry of $\beta \text{−}{\mathrm{PdBi}}_2$ is studied by measuring the London penetration depth $\mathrm{\Delta }\lambda \left(T\right)$ as well as the heat capacity $C_P(T,B)$. In the low temperature limit, both the penetration depth $\mathrm{\Delta }\lambda \left(T\right)$ and the electronic specific heat $C_e\left(T\right)/T$ follow exponential-type temperature dependence, providing evidence for fully gapped superconductivity in this compound. Analysis of the superfluid density ${\rho }_s\left(T\right)$ suggests single-gap $s$-wave superconductivity for $\beta \text{−}{\mathrm{PdBi}}_2$, which is further supported by the linear field dependence of the Sommerfeld coefficient ${\gamma }_0\left(B\right)$ determined in the superconducting state.

Domains and domain walls are among the key factors that determine the performance of ferroelectric materials. In recent years, a unique type of domain walls, i.e., the sawtooth-shaped domain walls, has been observed in ${\mathrm{BiFeO}}_3$ and ${\mathrm{PbTiO}}_3$. Here, we build a minimal model to reveal the origin of these sawtooth-shaped domain walls. Incorporating this model into Monte Carlo simulations shows that (i) the competition between the long-range Coulomb interaction (due to bound charges) and short-range interaction (due to opposite dipoles) is responsible for the formation of these peculiar domain walls and (ii) their relative strength is critical in determining the periodicity of these sawtooth-shaped domain walls. Necessary conditions to form such domain walls are also discussed.

LaSb has attracted intense interest due to the recent discovery of its extreme magnetoresistance (XMR) and controversial topology states. Motivated by the existing structural phase transition and the possible topological phase transition, the electrical transport properties of a LaSb single crystal under pressure up to 35 GPa were investigated. Superconductivity appears at 10.8 GPa, and the critical temperature ($T_c$) rapidly reaches a maximum of 5.3 K at 13.7 GPa and then linearly decreases with further loading pressure, which can be represented by a typical dome-shaped superconducting phase diagram. The superconductivity is believed to originate from the significant increase of the density of states near the Fermi level $\left[N\left(E_F\right)\right]$, which is induced by the volume collapse due to the structural transition from cubic to tetragonal at approximately 10.8 GPa. This result has inspired systematic studies of the $\mathrm{Ln}X$ (Ln = lanthanide and $X=\mathrm{P}$, As, Sb, Bi) family under high pressure, especially superconductivity investigations.

The Berry curvature for magnons in ferromagnetic films gives rise to new phenomena such as thermal Hall effect and a shift of a magnon wave packet at the reflection at the edge of the magnetic film. In this paper, we calculate the Berry curvature of magnetoelastic waves in ferromagnets. In order to calculate the Berry curvature, we first formulate the eigenvalue equation into a Hermitian form from the dynamical equation of motion. We find that the Berry curvature of the magnetoelastic waves shows a peak at the crossing point of the dispersions of magnons and elastic waves, and its peak value is dependent on the hybridization gap at the crossing point. In addition, the behavior of the Berry curvature in the long-wavelength limit changes drastically by the magnetoelastic interaction. We calculate the effect of dipolar interactions in a magnetic film on the Berry curvature, and we find a sign change by increasing the film thickness. The effect of the Berry curvature may be detected by measuring the shift of a wave packet for magnetoelastic waves at a reflection at the edge of a magnet and we find that a shift shows an abrupt change when the wave vector changes across the crossing point.

Detailed investigation of the incommensurate magnetic ordering in a single crystal of multiferroic $\mathrm{NdM}{\mathrm{n}}_2{\mathrm{O}}_5$ has been performed using both nonpolarized and polarized neutron-diffraction techniques. Below $T_N\approx 30.5\mathrm{K}$ magnetic Bragg reflections corresponding to the nonchiral-type magnetic structure with propagation vector ${\mathit{k}}_1=\left(0.50k_{z1}\right)$ occurs. Below about 27 K a distorted magnetic modulation with a similar vector $k_{z2}$ occurs, which is attributed to the magnetization of the $\mathrm{N}{\mathrm{d}}^{3+}$ ions by the Mn sublattice. Strong temperature hysteresis in the occurrence of the incommensurate magnetic phases in $\mathrm{NdM}{\mathrm{n}}_2{\mathrm{O}}_5$ was observed depending on the cooling or heating history of the sample. Below about 20 K the magnetic structure became of a chiral type. From spherical neutron polarimetry measurements, the resulting low-temperature magnetic structure $k_{z3}$ was approximated by the general elliptic helix. The parameters of the magnetic helixlike ellipticity and helical plane orientation in regard to the crystal structure were determined. A reorientation of the helix occurs at an intermediate temperature between 4 and 18 K. A difference between the population of right- and left-handed chiral domains of about 0.2 was observed in the as-grown crystal when cooling without an external electric field. The magnetic chiral ratio can be changed by the application of an external electric field of a few kV/cm, revealing strong magnetoelectric coupling. A linear dependence of the magnetic chirality on the applied electric field in $\mathrm{NdM}{\mathrm{n}}_2{\mathrm{O}}_5$ was found. The results are discussed within the frame of the antisymmetric superexchange model for Dzyaloshinskii-Moriya interaction.

We study the interaction of electromagnetic waves with a cluster composed of gyromagnetic and dielectric spheres. We expand the electromagnetic fields in terms of the vector multipole fields and relate the expansion of the scattered field to that of the incident field. We apply this generalized multisphere Mie theory to dimers and hexamers containing one dielectric sphere and similar gyromagnetic spheres. We observe multiple Fano resonances in the extinction, circular extinction, and optical activity spectra of the oligomers. Fano resonances can be engineered by adjusting the geometrical and structural parameters of the system. More importantly, Fano resonances can be dynamically tuned by an external magnetic field. A Fano resonance frequency shift as large as 0.5–1 GHz for a magnetic field increase of $200\mathrm{G}$ is achievable.

Magnetic Weyl semimetals (WSMs) have been presumed to be immune to proximity-induced spin-singlet superconducting pairing due to a chirality blockade of the regular Andreev reflection. In this work, we study all possible superconducting pairing induced in a WSM sandwiched between two conventional superconductors in a Josephson junction setup. We confirm that, while conventional intraorbital $s-\mathrm{wave}$ pairing is present on the surface of the WSM, it cannot propagate into the bulk due to the chirality blockade. However, interorbital $s-\mathrm{wave}$ pairing, with both even-frequency spin-singlet and odd-frequency mixed-spin-triplet symmetry, propagates into the bulk of the WSM, as do several $p-\mathrm{wave}$ symmetries. To demonstrate the importance of these finite interorbital and $p-\mathrm{wave}$ pair amplitudes in an experimental setup, we calculate the Josephson current and find a finite and even increasing current when the chirality blockade effect for the conventional intraorbital pairing is enhanced.

Metastable ${\mathrm{He}}_2^{*}$ excimer molecules have been utilized as tracer particles of the normal component in superfluid ${}^4\mathrm{He}$ (He II) which can be imaged via laser-induced fluorescence. These excimer molecules form tiny bubbles in He II and can bind to quantized vortices at sufficiently low temperatures, thereby allowing for direct visualization of vortex dynamics in an inviscid superfluid. However, the $a^3{\mathrm{\Sigma }}_u^{+}\to c^3{\mathrm{\Sigma }}_g^{+}$ optical absorption line, which is responsible for the fluorescence imaging of the ${\mathrm{He}}_2^{*}$ molecules, is controlled by fluctuations on the bubble shape, and its exact line profile is not known at low temperatures. In this paper, we present a bubble model for evaluating the surface fluctuation eigenmodes of the excimers in He II. The line profile of the $a^3{\mathrm{\Sigma }}_u^{+}\to c^3{\mathrm{\Sigma }}_g^{+}$ transition is calculated at different temperatures by considering both the zero-point and thermal fluctuations on the bubble shape. We show that as the temperature drops from 2 K to 20 mK, the peak absorption strength is enhanced by a factor of about five, accompanying a blueshift of the peak location by about 2 nm. A double-peak line profile due to the rotational levels of the molecular core can be resolved. This bubble model also allows us to evaluate the stiffness of the ${\mathrm{He}}_2^{*}$ bubbles and hence their diffusion constant in He II due to scattering off thermal phonons. Our results will aid the design of future experiments on imaging quantized vortices in He II using ${\mathrm{He}}_2^{*}$ tracers.

${\mathrm{AgSnSe}}_2$, by formal electron count, should have Sn in a highly unusual $3+$ valence state and was therefore suggested to be a valence-skipping compound with potential for negative-$U$ centers and local electron pairing. It has been proposed that the latter may be the mechanism beyond seemingly conventional superconductivity in this compound. We report NMR measurements and first-principles calculations that agree with each other perfectly, and both indicate that valence skipping does not take place and the highly unusual ${\mathrm{Sn}}^{3+}$ state is realized instead, likely because of geometrical constraint prohibiting a breathing distortion that could screen the on-site Coulomb repulsion.

The tensor network (TN) has recently triggered extensive interests in developing machine learning models defined in quantum Hilbert space. Here, we propose a generative TN classification (GTNC) model for supervised machine learning. The strategy is first to map the classical data onto the states in a many-body Hilbert space and then to capture these states with tensor network schemes. We adopt the TN in the form of matrix product states as an example to implement GTNC where the testing images are classified by comparing the fidelities between different states. Our results show that GTNC has a very impressive performance on benchmark datasets in comparison to several well-known machine learning models. The advantage of GTNC is reflected from the facts that the samples are naturally clustering in the many-body Hilbert space, and it relies much less on hyperparameters. These characters make GTNC an adaptive and universal quantum-inspired method, which would have important applications in quantum computation and quantum information.

The prospect of using semiconductor quantum dots as an experimental tool to distinguish Majorana zero modes (MZMs) from other zero-energy excitations such as Kondo resonances has brought up the fundamental question of whether topological superconductivity and the Kondo effect can coexist in these systems. Here, we study the Kondo effect in a quantum dot coupled to a metallic contact and to a pair of MZMs. We consider a situation in which the MZMs are spin polarized in opposite directions. By using numerical renormalization group calculations and scaling analysis of the renormalization group equations, we show that the Kondo effect takes place at low temperatures, regardless of the coupling to the MZMs. Interestingly, we find that the Kondo singlet essentially decouples from the MZMs such that the residual impurity entropy can show local non-Fermi-liquid properties characteristic of the single Majorana excitations. This offers the possibility of tuning between Fermi-liquid and non-Fermi-liquid regimes simply by changing the quantum-dot-MZM couplings.

Motivated by the recent discovery of higher-order topological insulators, we study their counterparts in strongly interacting bosons: “higher-order symmetry-protected topological (HOSPT) phases.” While the usual (first-order) SPT phases in $d$ spatial dimensions support anomalous $(d-1)$-dimensional surface states, HOSPT phases in $d$ dimensions are characterized by topological boundary states of dimension $(d-2)$ or smaller, protected by certain global symmetries and robust against disorders. Based on a dimensional reduction analysis, we show that HOSPT phases can be built from lower-dimensional SPT phases in a way that preserves the associated crystalline symmetries. When the total symmetry is a direct product of global and crystalline symmetry groups, we are able to classify the HOSPT phases using the Künneth formula of group cohomology. Based on a decorated domain-wall picture of the Künneth formula, we show how to systematically construct the HOSPT phases, and demonstrate our construction with many examples in two and three dimensions.

Impurity doping in atomic chains provides a potentially effective way to adjust its electronic excitation. Recently, it was reported that doping transition-metal atoms into small gold atomic chains induces a local dipolar plasmon mode which corresponds to charge oscillations around the impurity atoms, whereas our studies indicate that such a local mode does not exist. In this paper, using time-dependent density functional theory, we extensively investigate the excitation properties of plasmons in doped gold nanostructures. Our results show that all the plasmon peaks are affected by impurity, and the corresponding plasmon modes mutate into different forms compared to pure gold atomic chains. The propagation of charge oscillations in some plasmon modes is blocked in the doped chain, so that the charges oscillate mainly in half of the chain. Furthermore, the $5d$ electrons of gold atoms reduce the energies and intensities of plasmon resonances for both the transverse and longitudinal modes, and cause the end and center features of the induced charge distributions to nearly vanish in the transverse plasmon modes.

We develop a method for finding the time evolution of exactly solvable models by Bethe ansatz, with Hilbert space linear in excitation number. The dynamical Bethe wave function takes the same form as the stationary Bethe wave function, except for time-varying Bethe parameters and a complex phase prefactor. From this, we derive a set of first-order nonlinear coupled differential equations for the Bethe parameters, called the dynamical Bethe equations. We find that this gives the exact solution to particular types of exactly solvable models, including the Bose-Hubbard dimer. The developed formalism allows us to demonstrate analytically that time dynamics of the detuning-quenched Bose-Hubbard dimer occurs in the subspace which dimensionality is less than that of the physical Hilbert space. This allows for calculation of the time dynamics within a reduced problem dimensionality.

The Dzyaloshinskii-Moriya interaction gives rise to a chiral exchange between neighboring spins in the technologically relevant class of perpendicularly magnetized ultrathin film materials. In this paper, we study the temperature dependence of the Dzyaloshinskii-Moriya interaction based on extensive characterization of a thin film which hosts a skyrmion state using both bulk magnetometry and x-ray magnetic circular dichroism photoemission electron microscopy. A version of the Bloch law explicitly for thin film geometries is derived to extract the exchange stiffness. The strength of the Dzyaloshinskii-Moriya interaction, $D$, is found to have a dependence on the saturation magnetization, $M_s$ of $D\propto M_s^{1.86\pm 0.16}$. Further, by extracting the uniaxial anisotropy $K_u$ and the exchange stiffness $A$, we find that $D\propto K_u^{1.02\pm 0.11}$ and $D\propto A^{0.95\pm 0.07}$. Skyrmion radii are also used to extract the strength of the Dzyaloshinskii-Moriya interaction which is compared to that derived from measurements of stripe domains. The origins of the correlations between material parameters are discussed and consequences of these relationships for skyrmion devices are considered.

The magnetization process of the breathing pyrochlore magnet ${\mathrm{CuInCr}}_4{\mathrm{S}}_8$ has been investigated in ultrahigh magnetic fields up to 150 T. Successive phase transitions characterized by a substantially wide 1/2-plateau from 65 T to 112 T are observed in this system, resembling those reported in chromium spinel oxides. In addition to the 1/2-plateau phase, the magnetization is found to exhibit two inherent behaviors: A slight change in the slope of the $M\text{−}H$ curve at $\sim 85$ T and a shoulderlike shape at $\sim 135$ T prior to the saturation. Both of them are accompanied by a hysteresis, suggesting first-order transitions. The theoretical calculation applicable to ${\mathrm{CuInCr}}_4{\mathrm{S}}_8$ is also shown, based on the microscopic model with the spin-lattice coupling. The calculation fairly well reproduces the main features of the experimentally observed magnetization process, including a relatively wide cant 2:1:1 phase clearly observed in the previous work [Y. Okamoto et al., J. Phys. Soc. Jpn. 87, 034709 (2018)]. The robust 1/2-plateau on ${\mathrm{CuInCr}}_4{\mathrm{S}}_8$ seems to be originated from the dominant antiferromagnetic exchange interactions and the strong spin-lattice coupling.

Josephson junctions with three or more superconducting leads have been predicted to exhibit topological effects in the presence of few conducting modes within the interstitial normal material. Such behavior, of relevance for topologically protected quantum bits, would lead to specific transport features measured between terminals, with topological phase transitions occurring as a function of phase and voltage bias. Although conventional, two-terminal Josephson junctions have been studied extensively, multiterminal devices have received relatively little attention to date. Motivated in part by the possibility to ultimately observe topological phenomena in multiterminal Josephson devices, as well as their potential for coupling gatemon qubits, here we describe the superconducting features of a top-gated mesoscopic three-terminal Josephson device. The device is based on an InAs two-dimensional electron gas proximitized by epitaxial aluminum. We map out the transport properties of the device as a function of bias currents, top gate voltage, and magnetic field. We find a very good agreement between the zero-field experimental phase diagram and a resistively and capacitively shunted junction computational model.

The generation and propagation of photoexcited elastic waves in crystalline $\mathrm{G}{\mathrm{e}}_2\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_5$ were analyzed by picosecond time-resolved x-ray diffraction using a femtosecond-laser pump and an x-ray free-electron laser probe technique. The crystalline lattice anisotropically expanded initially in approximately 20 ps after the excitation. This was followed by a periodic oscillation of the lattice strain. The elastic stiffness along the cubic $\langle 111\rangle$ direction had significantly softened during the initial expansion, and the strain magnitude was the largest in the $\langle 100\rangle$ and $\langle 110\rangle$ directions. This indicates that femtosecond-laser excitation creates a shallower interlayer potential between the Te and Ge-Sb layers and eventually leads to softening of the elastic stiffness along the cubic $\langle 111\rangle$ direction. Furthermore, this softened state increases the system's sensitivity to an external stress field. This residual internal stress in a thin film enhances the selective formation of a particular type of variant during the symmetry change from cubic to rhombohedra. This causes the subsequent anisotropic expansion. These phenomena are quite interesting and align with the ultrafast amorphization of this material.