We present an approximate analytical solution to the dynamic equation of two Ising-coupled qubits with oscillating classical control fields that are nonperpendicular to the static fields. This is a situation that has recently arisen in some solid-state experiments. With our solution, we derive the analytical expressions for the local invariants as well as the local rotations needed to isolate a purely nonlocal gate. This determines the set of parameters that are required to generate any entangling gate. Moreover, we use our results to describe a recent experimental work on capacitively coupled singlet-triplet qubits in GaAs and discuss possible differences for a similar device in silicon.

Two-dimensional (2D) topological insulators (TIs) have attracted tremendous research interest from both the theoretical and the experimental fields in recent years. However, it is much less investigated in realizing node line (NL) semimetals in 2D materials. Combining first-principles calculations and symmetry analysis, we find that NL phases emerge in $p\text{−}{\mathrm{CS}}_2$ and $p\text{−}{\mathrm{SiS}}_2$, as well as other pentagonal ${\mathrm{IVX}}_2$ films, i.e., $p\text{−}{\mathrm{IVX}}_2$ (IV= C, Si, Ge, Sn, Pb; X=S, Se, Te) in the absence of spin-orbit coupling (SOC). The NLs in $p\text{−}{\mathrm{IVX}}_2$ consist of symbolic Fermi loops centered around the $\mathrm{\Gamma }$ point and are protected by mirror reflection symmetry. As the atomic number is downward shifted, the NL semimetals are driven into 2D TIs with the large bulk gap up to 0.715 eV induced by the remarkable SOC effect. The nontrivial bulk gap can be tunable under external biaxial strain and uniaxial strain. Moreover, we also propose a quantum well by sandwiching a $p\text{−}{\mathrm{PbTe}}_2$ crystal between two NaI sheets in which $p\text{−}{\mathrm{PbTe}}_2$ still keeps its nontrivial topology with a sizable band gap ($\sim 0.5$ eV). These findings provide a new 2D material platform for exploring fascinating physics in both NL semimetals and TIs.

Huygens' metasurfaces are electrically thin devices which allow arbitrary field transformations. Beam refraction is among the first demonstrations of realized metasurfaces. As previously shown for extreme-angle refraction, control over only the electric impedance and magnetic admittance of the Huygens' metasurface proved insufficient to produce the desired reflectionless field transformation. To maintain zero reflections for wide refraction angles, magnetoelectric coupling between the electric and magnetic response of the metasurface, leading to bianisotropy, can be introduced. In this paper, we report the theory, design, and experimental characterization of a reflectionless bianisotropic metasurface for extreme-angle refraction of a normally incident plane wave towards $71.8^{\circ }$ at 20 GHz. The theory and design of three-layer asymmetric bianisotropic unit cells are discussed. The realized printed circuit board structure was tested via full-wave simulations as well as experimental characterization. To experimentally verify the prototype, two setups were used. A quasi-optical experiment was conducted to assess the specular reflections of the metasurface, while a far-field antenna measurement characterized its refraction nature. The measurements verify that the fabricated metasurface has negligible reflections and the majority of the scattered power is refracted to the desired Floquet mode. This provides an experimental demonstration of a reflectionless wide-angle refracting metasurface using a bianisotropic Huygens' metasurface at microwave frequencies.

Topological materials provide an exclusive platform to study the dynamics of relativistic particles in table-top experiments and offer the possibility of wide-scale technological applications. ZrSiS is a newly discovered topological nodal-line semimetal and has drawn enormous interests. In this paper, we have investigated the lattice dynamics and electron-phonon interaction in single-crystalline ZrSiS using Raman spectroscopy. Polarization and angle-resolved Raman data have been analyzed using crystal symmetries and theoretically calculated atomic vibrational patterns along with phonon dispersion spectra. Wavelength- and temperature-dependent measurements show the complex interplay of electron and phonon degrees of freedom, resulting in resonant phonon and quasielastic electron scattering through interband transition. Our high-pressure Raman studies reveal vibrational anomalies, which are the signature of structural phase transitions. Further investigations through high-pressure synchrotron x-ray diffraction clearly show pressure-induced structural transitions and coexistence of multiple phases, which also indicate possible electronic topological transitions in ZrSiS. This study not only provides the fundamental information on the phonon subsystem, but also sheds some light in understanding the topological nodal-line phase in ZrSiS and other isostructural systems.

The electrocaloric effect in ferroics is considered a powerful solid-state cooling technology. Its potential is enhanced by correlation to the inverse electrocaloric effect and leads into mechanisms of decreasing or increasing dipolar entropy under applied electric field. Nevertheless, the mechanism underlying the increase of the dipolar entropy with applied electric field remains unclear and controversial. This study investigates the electrocaloric response of the antiferroelectric $\mathrm{P}{\mathrm{b}}_{0.99}\mathrm{N}{\mathrm{b}}_{0.02}{\left[{\left(\mathrm{Z}{\mathrm{r}}_{0.58}\mathrm{S}{\mathrm{n}}_{0.43}\right)}_{0.92}\mathrm{T}{\mathrm{i}}_{0.08}\right]}_{0.98}{\mathrm{O}}_3$ in which the critical electric field is low enough to induce the ferroelectric phase over a broad temperature range. Utilizing temperature- and electric-field-dependent dielectric measurements, direct electrocaloric measurements, and in situ transmission electron microscopy, a crossover from conventional to inverse electrocaloric response is demonstrated. The origin of the inverse electrocaloric effect is rationalized by investigating the field-induced phase transition between antiferroelectric and ferroelectric phases. The disappearance of the latent heat at field-induced transition coincides with the crossover of the electrocaloric effect and demonstrates that the overall electrocaloric response is an interplay of different entropy contributions. This opens new opportunities for highly efficient, environmentally friendly cooling devices based on ferroic materials.

The paper investigates the role of spin-orbit interaction in the prediction of structural stability, lattice dynamics, elasticity, thermodynamic and transport properties (electrical resistivity and thermal conductivity) of lead under pressure with the FP-LMTO (full-potential linear-muffin-tin orbital) method for the first-principles band structure calculations. Our calculations were carried out for three polymorphous lead modifications (fcc, hcp, and bcc) in generalized gradient approximation with the exchange-correlation functional PBEsol. They suggest that compared to the scalar-relativistic calculation, the account for the SO effects insignificantly influences the compressibility of Pb. At the same time, in the calculation of phonon spectra and transport properties, the role of SO interaction is important, at least, for $P\lesssim 150$ GPa. At higher pressures, the contribution from SO interaction reduces but not vanishes. As for the relative structural stability, our studies show that SO effects influence weakly the pressure of the $\mathrm{fcc}\to \mathrm{hcp}$ transition and much higher the pressure of the $\mathrm{hcp}\to \mathrm{bcc}$ transition.

Raman scattering spectroscopy is used to monitor the transition of reduced niobium oxyfluoride ${\mathrm{Nb}}_2{\mathrm{O}}_2{\mathrm{F}}_3$ single crystals from their high-temperature monoclinic (I2/a) phase into the low-temperature triclinic ($P\overline{1}$) phase at $T_c\approx 90\text{K}$ due to charge disproportionation of (${\mathrm{Nb}\text{−}\mathrm{Nb})}^{7+}$ dimers and creation of crystallographically nonequivalent dimers with long and short Nb-Nb bonds. The group-theoretic analysis of lattice vibrations is performed, and the assignment of observed phonon lines to the specific lattice eigenmodes of the two phases is achieved based on a comparison with the results of density functional lattice dynamics calculations. Nb-Nb dimers are found to possess stretching vibrational frequencies as high as 382 ${\mathrm{cm}}^{-1}$. Strikingly, the kinetics of the monoclinic-to-triclinic structural transformation and the volume fraction of corresponding phases below $T_c$ are shown to strongly depend upon the sample cooling rate. Fast cooling results in a “freezing” of the high-temperature monoclinic phase and allows us to observe a spin-ordered state below ${\mathrm{T}}_N\approx$ 49 K.

We report results on electronic transport properties of liquid Fe-S alloys at conditions of planetary cores, computed using first-principle techniques in the Kubo-Greenwood formalism. We describe a combined effect of resistivity saturation due to temperature, compression, and chemistry by comparing the electron mean free path from the Drude response of optical conductivity to the mean interatomic distance. At high compression and high sulfur concentration the Ioffe-Regel condition is satisfied, and the temperature coefficient of resistivity changes sign from positive to negative. We show that this happens due to a decrease in the $d$ density of states at the Fermi level in response to thermal broadening.

We study dynamical properties at finite temperature $\left(T\right)$ of Heisenberg spin chains with random antiferromagnetic exchange couplings, which realize the random singlet phase in the low-energy limit, using three complementary numerical methods: exact diagonalization, matrix-product-state algorithms, and stochastic analytic continuation of quantum Monte Carlo results in imaginary time. Specifically, we investigate the dynamic spin structure factor $S(q,\omega )$ and its $\omega \to 0$ limit, which are closely related to inelastic neutron scattering and nuclear magnetic resonance (NMR) experiments (through the spin-lattice relaxation rate $1/T_1)$. Our study reveals a continuous narrow band of low-energy excitations in $S(q,\omega )$, extending throughout the $q$ space, instead of being restricted to $q\approx 0$ and $q\approx \pi$ as found in the uniform system. Close to $q=\pi$, the scaling properties of these excitations are well captured by the random-singlet theory, but disagreements also exist with some aspects of the predicted $q$ dependence further away from $q=\pi$. Furthermore we also find spin diffusion effects close to $q=0$ that are not contained within the random-singlet theory but give non-negligible contributions to the mean $1/T_1$. To compare with NMR experiments, we consider the distribution of the local relaxation rates $1/T_1$. We show that the local $1/T_1$ values are broadly distributed, approximately according to a stretched exponential. The mean $1/T_1$ first decreases with $T$, but below a crossover temperature it starts to increase and likely diverges in the limit of a small nuclear resonance frequency ${\omega }_0$. Although a similar divergent behavior has been predicted and experimentally observed for the static uniform susceptibility, this divergent behavior of the mean $1/T_1$ has never been experimentally observed. Indeed, we show that the divergence of the mean $1/T_1$ is due to rare events in the disordered chains and is concealed in experiments, where the typical $1/T_1$ value is accessed.

An antiferromagnet is a promising material for spin-orbit torque generation. Earlier studies of the spin-orbit torque in an antiferromagnet were limited to collinear spin configurations. We calculate the spin-orbit torque in an antiferromagnet whose spin ordering is weakly noncollinear. Such noncollinearity may be induced spontaneously during the magnetization dynamics even when the equilibrium spin configuration is perfectly collinear. It is shown that deviation from perfect collinearity can modify the properties of the spin-orbit torque since noncollinearity generates extra Berry phase contributions to the spin-orbit torque, which are forbidden for collinear spin configurations. In sufficiently clean antiferromagnets, this modification can be significant. We estimate this effect to be of relevance for fast antiferromagnetic domain-wall motion.

In most strongly correlated electron systems superconductivity appears nearby a magnetic quantum critical point (QCP) which is believed to cause unconventional behaviors. In order to explore this physics, we present here a study of the heavy-fermion superconductors ${\mathrm{CeIrSi}}_3$ and ${\mathrm{CeRhSi}}_3$ carried out using a newly developed system for high-resolution magnetic penetration-depth measurements under pressure. Superconductivity in ${\mathrm{CeIrSi}}_3$ shows a change from an excitation spectrum with a line-nodal gap to one which is entirely gapful when pressure is close but not yet at the QCP. In contrast, ${\mathrm{CeRhSi}}_3$ does not possess a $T=0$ quantum phase transition and the superconducting phase remains for all accessible pressures with a nodal gap. Combining both results suggests that in these compounds unconventional superconducting behaviors are rather connected with the coexisting antiferromagnetic order. This study provides another viewpoint on the interplay of superconductivity, magnetism, and quantum criticality in ${\mathrm{CeIrSi}}_3$ and ${\mathrm{CeRhSi}}_3$ and maybe in other heavy fermions.

We propose a theoretical model for a gapless spin liquid phase that may have been observed in a recent experiment on ${\mathrm{H}}_3{\mathrm{LiIr}}_2{\mathrm{O}}_6$. Despite the insulating and nonmagnetic nature of the material, the specific heat coefficient $C/T\sim 1/\sqrt{T}$ in zero magnetic field and $C/T\sim T/B^{3/2}$ with finite magnetic field $B$ have been observed. In addition, the NMR relaxation rate shows $1/\left(T_{1}T\right)\sim {(C/T)}^2$. Motivated by the fact that the interlayer/in-plane lattice parameters are reduced/elongated by the hydrogen intercalation of the parent compound ${\mathrm{Li}}_2{\mathrm{IrO}}_3$, we consider four layers of the Kitaev honeycomb lattice model with additional interlayer exchange interactions. It is shown that the resulting spin liquid excitations reside mostly in the top and bottom layers of such a layered structure and possess a quartic dispersion. In an applied magnetic field, each quartic mode is split into four Majorana cones with the velocity $v\sim B^{3/4}$. We suggest that the spin liquid phase in these “defect” layers, placed between different stacking patterns of the honeycomb layers, can explain the major phenomenology of the experiment, which can be taken as evidence that the Kitaev interaction plays the primary role in the formation of a quantum spin liquid in this material.

Through Bi deposition on the single-crystalline $\mathrm{Mo}{\mathrm{S}}_2$ surface, we find that the density of the sulfur vacancy is a critical parameter for the growth of the crystalline Bi overlayer or cluster at room temperature. Also, the $\mathrm{Mo}{\mathrm{S}}_2$ band structure is significantly modified near Γ due to the orbital hybridization with an adsorbed Bi monolayer. Our experimental observations and analysis in combination with density functional theory calculation suggest the importance of controlling the sulfur vacancy concentration in realizing an exotic quantum phase based on the van der Waals interface of Bi and $\mathrm{Mo}{\mathrm{S}}_2$.

We present stationary and transient absorption measurements on 3D Au nanoparticle (NP)-decorated $\mathrm{Si}{\mathrm{O}}_2$ nanowire arrays. The 3D NP array has been produced by the dewetting of a thin Au film deposited on silica nanowires produced by oxidation of silicon nanowires. The experimental behaviors of the spectral and temporal dynamics observed in the experiment are accurately described by a two-step, three-temperature model. Using an arbitrary set of Au NPs with different aspect ratios, we demonstrate that the width of the experimental spectra, the energy shift of their position with time, and the asymmetry between the two positive wings in the dynamical variation of absorption can all be attributed to the nonuniform shape distribution of the Au NPs in the sample.

Using an analytically tractable kinetic model of a two-dimensional Fermi liquid of electrons, we characterize the crossovers among zero sound, first sound, and plasmons. For experimentally realized Fermi liquids in a hydrodynamic limit, both zero and first sound waves are essentially replaced by plasmons. The plasmon dispersion relation is robust against hydrodynamic effects up to acquiring the viscous-limited decay rate of a first sound wave in the hydrodynamic limit. We discuss implications for experiments in clean two-dimensional electron gases.

The wave-function-matching (WFM) technique for first-principles transport-property calculations was bettered by Sørensen et al. so as to exclude rapidly decreasing evanescent waves [Sørensen et al., Phys. Rev. B 77, 155301 (2008)]. In their method, the translational invariance of the transmission probability is not preserved when moving the matching planes between electrode and transition regions, and the sum of transmission and reflection probabilities does not agree with the number of transport channels in the transition region. The lack of the translational invariance is caused by the overlap of the layers between the electrode and transition regions. We reformulate the WFM method by removing the layer overlap, and the translational invariance of the transmission probability becomes preserved. On the other hand, the error in the sum of transmission and reflection probabilities is attributed to using pseudoinverses that is accompanied by the exclusion of rapidly decreasing evanescent waves. We introduce a formulation to calculate the transmission/reflection probability without the pseudoinverses, resulting in that the sum of the transmission and reflection probabilities exactly agrees with the number of channels, and the accuracy is largely improved. In addition, we prove that the accuracy in the transmission probability obtained by our WFM technique is comparable to that obtained by a nonequilibrium Green's function method. Furthermore, we carry out electron transport calculations on two-dimensional graphene sheets embedded with B-N line defects sandwiched between a pair of semi-infinite graphene electrodes and find the dependence of the electron transmission on the transverse momentum perpendicular to the transport direction.

Electron confinement in fractal Sierpiński triangles (STs) on Ag(111) is investigated using scanning tunneling spectroscopy and theoretically simulated by employing an improved two-dimensional (2D) multiple scattering theory in which the energy-dependent phase shifts are explicitly calculated from the electrostatic potentials of the molecular building block of STs. Well-defined bound surface states are observed in three kinds of triangular cavities with their sides changing at a scale factor of 2. The decrease in length of the cavities results in an upshift of the resonances that deviates from an expected inverse quadratic dependence on the cavity length due to the less efficient confinement of smaller triangular cavities. Differential conductance maps at some specific biases present a series of alternative bright and dark rounded triangles preserving the symmetry of the boundary. Our improved 2D multiple scattering model reproduces the characteristics of the standing wave patterns and all features in the differential conductance spectra measured in experiments, illustrating that the elastic loss boundary scattering dominates the resonance broadening in these ST quantum corrals. Moreover, the self-similar structure of STs, that a larger central cavity is surrounded by three smaller ones with a half side length, gives rise to interactions of surface states confined in neighboring cavities, which are helpful for the suppression of the linewidth in differential conductance spectra.

The Majorana fermion can be described by a real wave function with only two phases (zero and $\pi$) which provide a controllable degree of freedom. We propose a strategy to regulate the phase of the chiral Majorana state by coupling with a scanning tunneling microscope tip in a system consisting of a quantum anomalous Hall insulator coupled with a superconductor. With the change in the chemical potential, the chiral Majorana state can be tuned alternately between zero and $\pi$, in which the perfect normal tunneling and perfect crossed Andreev reflection appear, respectively. The perfect crossed Andreev reflection, by which a Cooper pair can be split into two electrons going into different terminals completely, leads to a pumping current and distinct quantized resistances. These findings may provide a signature of Majorana fermions and pave a feasible avenue to regulate the phase of the Majorana state.