We report herein the results of magnetotransport measurements and ab initio calculations on single crystalline $\mathrm{CaA}{\mathrm{l}}_2\mathrm{S}{\mathrm{i}}_2$ semimetal. The transport properties could be understood in connection with the two-band model, agreeing well with the theoretical calculations indicating four main sheets of Fermi surface consisting of three hole pockets centered at the $\mathrm{\Gamma }$ point and one electron pocket centered at the $M$ point in the Brillouin zone. Magnetotransport measurements showed striking Shubnikov–de Haas oscillations associated with a nontrivial Berry phase, which originate from a hole Fermi pocket indicated by the ab initio calculations. The calculations also unveiled that the hole Fermi surface encloses a nodal line setting around the $\mathrm{\Gamma }$ point close to the Fermi level without considering the spin-orbit coupling (SOC). Once the SOC is included, the fragile nodal-line will be gapped and a pair of Dirac points emerge along the high symmetric $\mathrm{\Gamma }\text{−}A$ direction located at the Brillouin zone coordinates (0, 0, $k_z^D\approx \pm 0.278\times \frac{2\pi }{c}$), about 1.22 eV below the Fermi level. In addition, the SOC can also induce a topological insulator state along the $\mathrm{\Gamma }\text{−}A$ direction with a gap of about 3 meV. The results demonstrate $\mathrm{CaA}{\mathrm{l}}_2\mathrm{S}{\mathrm{i}}_2$ as an excellent platform for the study of novel topological physics with multiple nontrivial topological states.

The effect of electron-phonon coupling on Dirac fermions was recently explored numerically on a honeycomb lattice, leading to precise quantitative values for the finite temperature and quantum critical points. In this paper, we use the unbiased determinant quantum Monte Carlo method to study the Holstein model on a half-filled staggered-flux square lattice and compare the results with the honeycomb lattice geometry, presenting results for a range of phonon frequencies $0.1⩽\omega ⩽2.0$. We find that the interactions give rise to charge density wave order, but only above a finite coupling strength ${\lambda }_{\mathrm{crit}}$. The transition temperature is evaluated and presented in a $T_c\text{−}\lambda$ phase diagram. An accompanying mean-field theory calculation also predicts the existence of a quantum phase transition, but at a substantially smaller coupling strength.

Exact expressions for ensemble averaged Madelung energies of finite volumes are derived. The extrapolation to the thermodynamic limit converges unconditionally and can be used as a parameter-free real-space summation method of Madelung constants. In the large volume limit, the surface term of the ensemble averaged Madelung energy has a universal form, independent of the crystal structure. The scaling of the Madelung energy with system size provides a simple explanation for the structural phase transition observed in cesium halide clusters.

Results of structural, magnetic, thermodynamic, and electron transport measurements on single-crystalline ${\mathrm{NdPt}}_2{\mathrm{Si}}_2$ are reported. Temperature variations of crystal structure show two different charge density wave states. The structure modulation with the propagation vector ${\mathit{q}}_{\mathbf{1}}=(0.323,0,0)$ arises below $T=300$ K. An additional modulation, with propagation vector ${\mathit{q}}_{\mathbf{2}}=(0.163,0.163,0.500)$, appears below $T=50$ K in a first-order phase transition. This situation was found analogous to isostructural ${\mathrm{LaPt}}_2{\mathrm{Si}}_2$ with similar (but not identical) propagation vectors and characteristic temperatures. Nd magnetic moments order antiferromagnetically at $T_N=1.5$ K. Temperature dependence of electrical resistivity for current j applied along the $a$, $b$, and $c$ axes is strongly affected by the first-order transition, inducing a Fermi surface gapping.

The scattering cancellation technique (SCT) has proved to be an effective way to render static objects invisible to electromagnetic and acoustic waves. However, rotating cylindrical or spherical objects possess additional peculiar scattering features that cannot be canceled by regular SCT-based cloaks. Here, a generalized SCT theory to cloak spinning objects, and hide them from static observers, based on rotating shells with different angular velocity is discussed. This concept is analytically and numerically demonstrated in the case of cylinders, showing that the generalized SCT operates efficiently in making rotating objects appear static to an external observer. Our proposal extends the realm of SCT and brings it one step closer to its practical realization that involves moving objects.

The three-dimensional Dirac semimetal $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$ has attracted intensive attention recently for its exotic properties. Understanding the hot carrier excitation and its subsequent relaxation is a prerequisite for $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$-based optoelectronics functional in the infrared (IR) and terahertz (THz) frequency range. In this work, the photoexcited hot carrier dynamics of a bulk single crystal $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$ in the mid-IR has been explored by using the time-resolved pump-probe reflectance ($\mathrm{\Delta }R/R$) measurement at room temperature under a strong optical pumping. By combining the experimental $\mathrm{\Delta }R/R$ results and theoretical modeling, we analyzed the transient hot carrier redistribution upon a strong photoexcitation and the subsequent interband transitions. We show that the $\mathrm{\Delta }R/R$ response of $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$ has a complex behavior, due to the interplay of transitions between Dirac bands and transitions involving both the Dirac and non-Dirac bands. Throughout the mid-IR region measured, we find that $\mathrm{\Delta }R/R$ is contributed primarily by changes in the refractive index, rather than by changes in the extinction coefficient. Our findings can help understanding the transient hot electron excitation and relaxation in $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$ under an intense optical pumping, and provide a valuable reference for the $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$-based ultrafast optoelectronics application.

The ${\mathrm{Y}}_3\mathrm{F}{\mathrm{e}}_5{\mathrm{O}}_{12}$ (YIG) films with perpendicular magnetic anisotropy (PMA) have recently attracted a great deal of attention for spintronics applications. Here, we report the induced PMA in the YIG films grown on $\left(\mathrm{G}{\mathrm{d}}_{2.6}\mathrm{C}{\mathrm{a}}_{0.4}\right)\left(\mathrm{G}{\mathrm{a}}_{4.1}\mathrm{M}{\mathrm{g}}_{0.25}\mathrm{Z}{\mathrm{r}}_{0.65}\right){\mathrm{O}}_{12}$ (SGGG) substrates by epitaxial strain without preprocessing. Reciprocal space mapping shows that the films are lattice matched to the substrates without strain relaxation. Through ferromagnetic resonance and polarized neutron reflectometry measurements, we find that these YIG films have an ultralow Gilbert damping constant ($\alpha <1\times {10}^{-5}$) and a magnetic dead layer, which is negligible at the YIG/SGGG interfaces. Moreover, the transport behavior of Pt/YIG/SGGG films reveals an enhancement of spin mixing conductance and a large unusual anomalous Hall effect (UAHE) as compared with $\mathrm{Pt}/\mathrm{YIG}/\mathrm{G}{\mathrm{d}}_3\mathrm{G}{\mathrm{a}}_5{\mathrm{O}}_{12}$ (GGG) films. Although the UAHE in Pt/YIG/SGGG films show different characteristics with varying YIG thickness, they are all ascribed to the possible noncollinear magnetic order at the Pt/YIG interfaces induced by epitaxial strain.

We have investigated the structural, magnetic, and dielectric properties of the Pb-based langasite compound ${\mathrm{Pb}}_3{\mathrm{TeMn}}_3{\mathrm{P}}_2{\mathrm{O}}_{14}$ both experimentally and theoretically in light of metal-oxygen covalency, and the consequent generation of multiferroicity. It is known that the large covalency between Pb $6p$ and O $2p$ plays an instrumental role behind the stereochemical lone pair activity of Pb. The same happens here, but a subtle structural phase transition above room temperature changes the degree of such lone pair activity and the system becomes ferroelectric below 310 K. Interestingly, this structural change also modulates the charge densities on different constituent atoms and consequently the overall magnetic response of the system while maintaining the global paramagnetism behavior of the compound. This single origin of modulation in polarity and paramagnetism inherently connects both functionalities and the system exhibits mutiferroicity at room temperature.

A highly anticipated application for quantum computers is as a universal simulator of quantum many-body systems, as was conjectured by Richard Feynman in the 1980s. The last decade has witnessed the growing success of quantum computing for simulating static properties of quantum systems, i.e., the ground-state energy of small molecules. However, it remains a challenge to simulate quantum many-body dynamics on present to near-future noisy intermediate-scale quantum computers. Here, we demonstrate successful simulation of nontrivial quantum dynamics on IBM's Q16 Melbourne quantum processor and Rigetti's Aspen quantum processor; namely, ultrafast control of emergent magnetism by terahertz radiation in an atomically thin two-dimensional material. The full code and step-by-step tutorials for performing such simulations are included to lower the barrier to access for future research on these two quantum computers. As such, this work lays a foundation for the promising study of a wide variety of quantum dynamics on near-future quantum computers, including dynamic localization of Floquet states and topological protection of qubits in noisy environments.

We study noise-induced creation and annihilation of magnetic droplet solitons in experimental parameter regions in which they are linearly stable against drift. Exploiting the rotational symmetry of the problem, we transform to the reference frame rotating with the droplet soliton and introduce an effective potential energy that accounts for the work done against spin transfer torque to rotate the magnetization between two different orientations. We use this function to compute the activation barrier in both directions between the uniform magnetization state and the droplet soliton state for a variety of nanocontact radii and currents. We investigate droplet soliton structures with both zero and nonzero spin-torque asymmetry parameters. Our approach can be applied to estimate activation barriers for dynamical systems where nongradient terms can be absorbed by changes of reference frames, and suggests a technique applicable to extended systems that may not be uniformly magnetized.

We comparatively investigated the transport properties for S- and Te-substituted FeSe thin films under magnetic fields to clarify the origin of the contrasting behavior of the superconducting transition temperature in S and Te substitution. A classical two-carrier analysis revealed that the carrier densities of the films increased with increasing Te content, while no significant change was observed for the S substitution, which suggests a correlation between $T_{\mathrm{c}}$ and the carrier densities. These observations suggest that the structural transition affects the electronic structure in a different manner between Fe(Se,S) and Fe(Se,Te) and that this fact is the direct cause of the difference in the $T_{\mathrm{c}}$ behaviors at the end point of the structural transition.

A drastic enhancement of the thermal Hall angle in $d$-wave superconductors was observed experimentally in a cuprate superconductor and in ${\mathrm{CeCoIn}}_5$ at low temperatures and very weak magnetic field [Phys. Rev. Lett. 86, 890 (2001); Phys. Rev. B 72, 214515 (2005)]. However, to the best of our knowledge, its microscopic calculation has not been performed yet. To study this microscopically, we derive the thermal Hall coefficient in extreme type-II superconductors with an isolated pinned vortex based on the augmented quasiclassical equations of superconductivity with the Lorentz force. Using it, we can confirm that the quasiparticle relaxation time and the thermal Hall angle are enhanced in $d$-wave superconductors without impurities of the resonant scattering because quasiparticles around the gap nodes which become dominant near zero temperature are restricted to the momentum in a specific orientation. This enhancement of the thermal Hall angle may also be observed in other nodal superconductors with large magnetic-penetration depth.

We explore the connection between the area law for entanglement and geometry by representing the entanglement entropies corresponding to all $2^N$ bipartitions of an $N$-party pure quantum system by means of a (generalized) adjacency matrix. In the cases where the representation is exact, the elements of that matrix coincide with the mutual information between pairs of sites. In others, it provides a very good approximation, and in all the cases it yields a natural entanglement contour which is similar to previous proposals. Moreover, for one-dimensional conformal invariant systems, the generalized adjacency matrix is given by the two-point correlator of an entanglement current operator. We conjecture how this entanglement current may give rise to a metric entirely built from entanglement.

We investigate the thermal-driven charge density wave (CDW) transition of two cubic superconducting intermetallic systems $\mathrm{Lu}{\left({\mathrm{Pt}}_{1-x}{\mathrm{Pd}}_x\right)}_2\mathrm{In}$ and (${\mathrm{Sr}}_{1-x}{\mathrm{Ca}}_x{)}_3{\mathrm{Ir}}_4{\mathrm{Sn}}_{13}$ by means of x-ray diffraction technique. A detailed analysis of the CDW modulation superlattice peaks as function of temperature is performed for both systems as the CDW transition temperature $T_{\mathrm{CDW}}$ is suppressed to zero by an nonthermal control parameter. Our results indicate an interesting crossover of the classical thermal-driven CDW order parameter critical exponent from a three-dimensional universality class to a mean-field tendency, as $T_{\mathrm{CDW}}$ vanishes. Such behavior might be associated with the presence of quantum fluctuations which influences the classical second-order phase transition, strongly suggesting the presence of a quantum critical point (QCP) at $T_{\mathrm{CDW}}=0$. This also provides experimental evidence that the effective dimensionality exceeds its upper critical dimension due to a quantum phase transition.

Photoinduced nonequilibrium states can provide new insight into dynamical properties of strongly correlated electron systems. One of the typical and extensively studied systems is the half-filled one-dimensional extended Hubbard model (1DEHM). Here, we propose that the supervised machine learning (ML) can provide useful information for characterizing photoexcited states in 1DEHM. Using entanglement spectra as a training dataset, we construct a neural network. Judging from the trained network, we find that bond-spin-density wave (BSDW) order can be enhanced in photoexcited states if the frequency of a driving pulse nearly resonates with the gap. We separately calculate the time evolution of local and nonlocal order parameters and confirm that the correlation functions of BSDW are actually enhanced by photoexcitation as predicted by ML. The successful prediction of BSDW demonstrates the advantage of ML to assist characterizing photoexcited quantum states.

Small-wave-vector excitations in Coulomb-interacting systems can be decomposed into high-energy collective longitudinal plasmon and low-energy single-electron excitations. At the critical wave vector and corresponding frequency where the plasmon branch merges with the single-electron excitation region, the collective energy of the plasmon dissipates into single electron-hole excitations. The jellium model provides a reasonable description of the electron-energy-loss spectrum of metals close to the free electron limit. The random phase approximation (RPA) is exact in the high-density limit but can capture the plasmonic dispersion reasonably even for densities with $r_s>1$. The RPA and all beyond-RPA methods investigated here result in a wrong infinite plasmon lifetime for a wave vector smaller than the critical one where the plasmon dispersion curve runs into particle-hole excitations. Exchange-correlation kernel corrections to the RPA modify the plasmon dispersion curve. There is, however, a large difference in the construction and form of the kernels investigated earlier. Our current work introduces recent model exchange-only and exchange-correlation kernels and discusses the relevance of some exact constraints in the construction of the kernel. We show that, because the plasmon dispersion samples a range of wave vectors smaller than the range sampled by the correlation energy, different kernels can make a strong difference for the correlation energy and a weak difference for the plasmon dispersion. This work completes our understanding of the plasmon dispersion in realistic metals, such as Cs, where a negative plasmon dispersion has been observed. We find only positive plasmon dispersion in jellium at the density for Cs.

An inverse spin Hall effect (ISHE) in $n$-type silicon was observed experimentally when conduction electrons were scattered on the spin-orbit potential of bismuth. The spin current in the silicon layer was generated by excitation of the magnetization precession during ferromagnetic resonance in a thin Permalloy (Py) layer deposited on a Si layer doped by phosphor and bismuth. From the angular dependences of the dc voltage for different $\mathrm{Py}/n$-Si:Bi structures aligned along the [011] or [100] crystal axes, we were able to distinguish the planar Hall effect (PHE) and ISHE contributions. The ISHE dc voltage signal was proportional to the $sin\theta sin2\theta$ product for the structure aligned to the [011] crystal axis and to $sin\theta cos2\theta$ for the [100] direction. In addition, the PHE dc voltage was observed for the angles corresponding to the $sin2\theta$ dependence. This means that for silicon as a many-valley semiconductor, the scattering of spins due to the spin-orbit potential induced by shallow donor in $n$-type material is dependent on the orientation of the valley axes relative to the direction of the magnetic field.

In semiconductor physics, the Urbach rule describes an exponential dependence of the UV-Vis absorption coefficient on photon energy at the band edge. The rule is so ubiquitously applied that a single common mechanism, variability in the local potential, is generally considered to be the cause of exponential band edges. Variability in the local potential may be caused by alloy inhomogeneity, structural defects, impurities, thermal excitation, or any other mechanism that interrupts ideal crystallinity and creates disorder. A crucial point of the Urbach rule is its development for band edges which approach linearity when plotted on a logarithmic scale. The rule was not developed in the context of nonlinear band edges with complex fine structure. Even so, this model has been applied extensively to band edges with such features. Using differential analysis and deconvolution, we develop an analytical technique capable of separating the components responsible for the fine structure observed. This approach reduces challenging signal-artifact convolution problems to a simple linear combination. Furthermore, we develop a model that can be applied to the nonlinear band-edge signals extracted. We show that in the limiting case of linearity, our model is consistent with and produces the same results as the linear Urbach model. Our model is capable of bypassing “tailing” as an indirect measure of the spread in the local potential and gives a direct measure of this quantity.

The recent proposals of devices with overlapping Andreev bound states (ABS) open up opportunities to control and fine tune their spectrum that can be used in various applications in quantum sensing and manipulation. In this paper, we study the ABS in a device consisting of a semiconducting nanowire covered with three superconducting leads. The ABS are formed at two junctions where the wire is not covered. They overlap in the wire where the electron propagation is 1D and in one of the leads where the propagation is 3D. We identify a number of regimes where these two overlaps either dominate or compete, depending on the junction separation $L$ as compared to the correlation lengths ${\xi }_{\mathrm{w}},{\xi }_{\mathrm{s}}$ in the wire and in the lead, respectively. We utilize a simple model of 1D electron spectrum in the nanowire and take into account the quality of the contact between the nanowire and the superconducting lead. We present the spectra for different $L$, detailing the transition from a single ABS in the regime of strong 1D hybridization to two almost independent ABS hybridized at the degeneracy points, in the regime of weak 1D hybridization. We present the details of merging the upper ABS with the continuous spectrum upon decreasing $L$. We study in detail the effect of quantum interference due to the phase accumulated during the electron passage between the junctions. We develop a perturbation theory for analytical treatment of hybridization. We address an interesting separate case of fully transparent junctions. We derive and exemplify a perturbation theory suitable for the competition regime demonstrating the interference of 1D and two 3D transmission amplitudes.