In order to establish the reliable $GW$ scheme that can be consistently applied to post-transition-metal oxides (post-TMOs), we carry out comprehensive $GW$ calculations on electronic structures of ZnO, ${\text{Ga}}_2$${\text{O}}_3$, ${\text{In}}_2$${\text{O}}_3$, and ${\text{SnO}}_2$, the four representative post-TMOs. Various levels of self-consistency ($G_0{W}_0$, $G{W}_0$, and $QPG{W}_0$) and different starting functionals (GGA, GGA + $U$, and hybrid functional) are tested and their influence on the resulting electronic structure is closely analyzed. It is found that the $G{W}_0$ scheme with GGA + $U$ as the initial functional turns out to give the best agreement with experiment, implying that describing the position of metal-$d$ level precisely in the ground state plays a critical role for the accurate dielectric property and quasiparticle band gap. Nevertheless, the computation on ZnO still suffers from the shallow Zn-$d$ level and we propose a modified approach ($G{W}_0+U_d$) that additionally considers an effective Hubbard $U$ term during $G{W}_0$ iterations and thereby significantly improves the band gap. It is also shown that a GGA + $U$-based $G{W}_0$(+$U_d$) scheme produces an accurate energy gap of crystalline ${\text{InGaZnO}}_4$, implying that this can serve as a standard scheme that can be applied to general structures of post-TMOs.

We report on a signature indicating a transition to the orbital-selective Mott phase in superconducting ${\mathrm{K}}_x$${\mathrm{Fe}}_{2-y}$${\mathrm{Se}}_2$ single crystals by employing dual-color pump-probe spectroscopy. In addition to the multiexponential-decay recovery dynamics of photoinduced quasiparticles, a damped oscillatory component caused by coherent acoustic phonons emerges when the superconducting phase is suppressed at an increased temperature or excitation power. Upon raising the temperature to 150–170 K, the oscillatory component diminishes alongside a significant enhancement of the slow decay component in the recovery traces. These results can be understood as a gap opening in certain $k$ directions, indicating that a vital role is played by electron correlation in iron-based superconductors.

It is shown that a stable gyrotropic motion of a vortex core in a vortex-state free layer of a magnetic nanopillar driven by a spin-polarized current can be quantitatively described in the framework of a standard model of a nonisochronous auto-oscillator. The nonisochronous parameters of a vortex auto-oscillator, determining its nonautonomous dynamics and synchronization properties, can be found from the experimentally measured linewidths of higher harmonics of the generated microwave signal. The presented results demonstrate that vortex spin-torque nano-oscillators, having low generation linewidth and relatively high output power, can be strongly nonisochronous, and, therefore, could be prospective candidates for mutual synchronization in large oscillator arrays. At the same time, the optimization of their working parameters can be done using the traditional auto-oscillator theory.

We present a method to approximate the Hessian matrix of the Born-Oppenheimer energy landscape by using a simple force field model whose parameters are fitted to on-the-flight ab-initio results. The inversed Hessian matrix is used as the preconditioner of conjugate gradient algorithms to speed up the atomic structure relaxation, resulting in a speedup factor of 2 to 5 on systems of bulk, slab, sheets, and atomic clusters. Because the force field model employed is simple and general, the parameter fitting is straightforward; the method is applicable to a variety of complicated systems for minimum structure relaxation. In the metal cluster new structure search, the new method yields better structures than the one obtained before with conventional algorithms.

We investigate the capabilities of hyperbolic metamaterials (HMs) to couple near-fields (i.e., evanescent waves) emitted by a two-dimensional periodic array of electric dipoles to propagating waves. In particular, large-order Floquet harmonics with transverse-magnetic polarization that would be evanescent in free space, and therefore confined near the array surface, are transformed into a propagating spectrum inside the HM and thus carry power away. Because of this property, independent of the finite or infinite extent of the HM, the power generated by an array of elementary electric dipoles is strongly enhanced and is mostly directed into the HM when the array is located near a HM surface. In particular, the power coupled to the HM exhibits narrow frequency features that can be employed in detection applications. The results shown in this paper provide a clear signature on wave dynamics in HMs. A link between the results pertaining to the case of an isolated dipole on top of HM and the planar array is found to be convenient in explaining both wave dynamics and spectral power distribution. The narrow frequency emission features appear in the array case only; they depend on its spatial periodicity and remarkably on the HM thickness.

Pulsed ultrasonic experiments performed on the parent compounds of FeAs based superconductors $A$Fe${}_2$${\mathrm{As}}_2$ ($A=\text{Ba}$, Ca) revealed elastic anomalies that agree with a two-step process for the structural/magnetic transitions. Upon cooling, a pronounced velocity softening of longitudinal phonons propagating along the $\mathit{c}$ axis is observed at the structural transition $T_{\mathrm{s}}$ due to a distortion of the lattice. Below a slightly lower temperature $T_{\mathrm{N}}$, a biquadratic coupling between the distortion and an antiferromagnetic order parameter produces a steep velocity stiffening upon further cooling, the stiffening being five times larger in ${\mathrm{CaFe}}_2$${\mathrm{As}}_2$ than in ${\mathrm{BaFe}}_2$${\mathrm{As}}_2$. Microwave resistivity measurements at 16.5 GHz suggest that, upon cooling, the lattice distortion at $T_{\mathrm{s}}$ is driven by magnetic fluctuations in ${\mathrm{BaFe}}_2$${\mathrm{As}}_2$, while it is rather due to lattice fluctuations in ${\mathrm{CaFe}}_2$${\mathrm{As}}_2$. This suggestion appears consistent with a second-order character of the transitions in ${\mathrm{BaFe}}_2$${\mathrm{As}}_2$ and a first-order one in ${\mathrm{CaFe}}_2$${\mathrm{As}}_2$.

Majorana fermions, quantum particles with non-Abelian exchange statistics, are not only of fundamental importance, but also building blocks for fault-tolerant quantum computation. Although certain experimental breakthroughs for observing Majorana fermions have been made recently, their conclusive detection is still challenging due to the lack of proper material properties of the underlined experimental systems. Here we propose a platform for Majorana fermions based on edge states of certain nontopological two-dimensional semiconductors with strong spin-orbit coupling, such as monolayer group-VI transition-metal dichalcogenides (TMDs). Using first-principles calculations and tight-binding modeling, we show that zigzag edges of monolayer TMD can host a well isolated single edge band with strong spin-orbit-coupling energy. Combining with proximity induced $s$-wave superconductivity and in-plane magnetic fields, the zigzag edge supports robust topological Majorana bound states at the edge ends, although the two-dimensional bulk itself is nontopological.

Based on the scattering cancellation, we provide a method not only making a nanoparticle nearly invisible, but also hiding its interior region from the outside probing matter wave. By applying the interplay among the nodal points of partial waves along with the concept of streamline in fluid dynamics for probability flux, a quantum invisible cloak to the electron transport in a host semiconductor is demonstrated by simultaneously guiding the probability flux outside a hidden region and keeping the total scattering cross section negligible. As the probability flux vanishes in the interior region, one can embed any materials inside a multiple core-shell nanoparticle without affecting physical observables from the outside. Our results reveal the possibility to design a protection shield layer for fragile interior parts from the impact of transport electrons.

We extend the range of possibilities for the charge response in the quantum critical regime in 2 + 1D using holography, and compare them with field theory and recent quantum Monte Carlo results. We show that a family of (infinitely many) higher derivative terms in the gravitational bulk leads to behavior far richer than what was previously obtained. For example, we prove that the conductivity becomes unbounded, undermining previously obtained constraints. We further find a nontrivial and infinite set of theories that have a self-dual conductivity. Particle-vortex or S duality plays a key role; notably, it maps theories with a finite number of bulk terms to ones with an infinite number. Many properties, such as sum rules and stability conditions, are proven.

Generally strong charge and magnetic inhomogeneities are observed in NQR/NMR experiments on underdoped cuprates. It is not the case for the underdoped ${\mathrm{HgBa}}_2$${\mathrm{CuO}}_{4+\delta }$, the most symmetric and highest $T_c$ single layer cuprate, whose magnetic inhomogeneity is strongly suppressed. Also neutron scattering experiments reveal a unique pair of weakly dispersive magnetic modes in this material. We propose that these special properties stem from the symmetric positioning of the O dopants between adjacent ${\mathrm{CuO}}_2$ layers that lead to a strong superexchange interaction between a pair of hole spins. In this Rapid Communication we present a theoretical model, which gives a consistent explanation to the anomalous magnetic properties of this material.

A broadband asymmetric transmission of linearly polarized waves with totally suppressed copolarization transmission is experimentally demonstrated in ultrathin 90°-twisted Babinet-inverted metasurfaces constructed by an array of asymmetrically split ring apertures. The only accessible direction-dependent cross-polarization transmission is allowed in this anisotropic chiral metamaterial. Through full-wave simulation and experiment results, the bilayered Babinet-inverted metasurface reveals broadband artificial chirality and asymmetric transmission, with a transmission contrast that is better than 17.7 dB within a 50% relative bandwidth for two opposite directions. In particular, we can modify polarization conversion efficiency and the bandwidth of asymmetric transmission via parametric study.

Electronic transport is theoretically investigated in laterally confined semiconductor superlattices using the formalism of nonequilibrium Green's functions. Velocity-field characteristics are calculated for nanowire superlattices of varying diameters, from the quantum dot superlattice regime to the quantum well superlattice regime. Scattering processes due to electron-phonon couplings, phonon anharmonicity, charged impurities, surface and interface roughness, and alloy disorder are included on a microscopic basis. Elastic scattering mechanisms are treated in a partial coherent way beyond the self-consistent Born approximation. The nature of transport along the superlattice is shown to depend dramatically on the lateral dimensionality. In the quantum wire regime, the electron velocity-field characteristics are predicted to deviate strongly from the standard Esaki-Tsu form. The standard peak of negative differential velocity is shifted to lower electric fields, while additional current peaks appear due to integer and fractional resonances with optical phonons.

We propose a dynamically tunable nonreciprocal response for wave propagations by employing nonlinear Fano resonances. We demonstrate that the transmission contrast between waves propagating in opposite directions can be controlled by an excitation signal. In particular, the unidirectional transmission can be flipped at different time of a pulse, resembling a Fano diode with dynamical reconfigurable nonreciprocity. The key mechanism is the interaction between a linear and a nonlinear Fano resonance that allows for the tunable unidirectional wave propagation and ultrahigh transmission contrast ratio. We further present a realistic photonic example which demonstrates the properties of nonreciprocity can be dynamically manipulated using a pump pulse, based on the general theoretical model.

The response of physical systems to external perturbations can be used to probe both their equilibrium and nonequilibrium dynamics. While response and correlation functions are related in equilibrium by fluctuation-dissipation theorems, out of equilibrium they provide complementary information on the dynamics. In the past years, a method has been devised to map the quantum dynamics of an isolated extended system after a quench onto a static theory with boundaries in imaginary time; up to now, however, the focus was entirely on symmetrized correlation functions. Here we provide a prescription which, in principle, allows one to retrieve the whole set of relevant dynamical quantities characterizing the evolution, including linear response functions. We illustrate this construction with some relevant examples, showing in the process the emergence of light-cone effects similar to those observed in correlation functions.

We propose a reliable and efficient computational method for predicting elastic and thermal expansion properties in crystals, particularly complex anisotropic molecular solids, and we apply it to the room-temperature orthorhombic $Pnma$ phase of ${\mathrm{LiBH}}_4$. Using density-functional theory, we find thermal expansion coefficients at finite temperature, and we confirm them by temperature-dependent, in situ x-ray diffraction measurements. We also consider the effects of volume and pressure, as well as energy barriers for ${\mathrm{BH}}_4$${}^{-}$ rotations and collective motions. Our combined study validates the theory and provides a better understanding of the structural behavior of ${\mathrm{LiBH}}_4$.

A single crystal of PrSi was grown by the Czochralski method in a tetra-arc furnace. Powder x-ray diffraction of the as-grown crystal revealed that PrSi crystallizes in an FeB-type structure with space group Pnma (No. 62). The anisotropic magnetic properties were investigated by means of magnetic susceptibility, isothermal magnetization, electrical transport, and heat capacity measurements. Magnetic susceptibility data clearly indicate the ferromagnetic transition in PrSi with a $T_C$ of 52 K. The relative easy axis of magnetization was found to be the [010] direction. Heat capacity data confirm the bulk nature of the transition at 52 K and exhibit a huge anomaly at the transition. A sharp rise in the low-temperature heat capacity has been observed (below 5 K) which is attributed to the ${}^{141}$Pr nuclear Schottky heat capacity arising from the hyperfine field of the Pr moment. The estimated Pr magnetic moment 2.88 ${\mu }_{\mathrm{B}}$/Pr from the hyperfine splitting is in agreement with the saturation magnetization value obtained from the magnetization data measured at 2 K. From the crystal electric field analysis of the magnetic susceptibility, magnetization, and heat capacity data it is found that the degenerate $J=4$ Hund's rule derived state of the Pr${}^{3+}$ ion splits into nine singlets with an overall splitting of 284 K, the first excited singlet state separated by just 9 K from the ground state. The magnetic ordering in PrGe appears to be due to the exchange-generated admixture of low-lying crystal field levels. The magnetocaloric effect (MCE) has been investigated from magnetization data along all three principal crystallographic directions. The large magnetic entropy change, $-\Delta S_M=$22.2 J/kg K, and the relative cooling power, RCP = 460 J/kg, characteristic of the giant magnetocaloric effect are achieved near the transition temperature ($T_C$ = 52 K) for $H=$ 70 kOe along $\left[010\right]$. Furthermore, the PrSi single crystal exhibits a giant MCE anisotropy.

In this work we examine the low-temperature magnetic properties of the two double-perovskite compounds ${\mathrm{Sr}}_2$${\mathrm{CuWO}}_6$ and ${\mathrm{Sr}}_2$${\mathrm{CuMoO}}_6$ using magnetic susceptibility, muon spin rotation and relaxation, and neutron powder diffraction measurements. Additionally, the most relevant spin exchange interaction constants are derived from ab initio electronic structure calculations, aided by x-ray absorption spectroscopy. The compounds exhibit quasi-two-dimensional magnetic properties, with broad maxima at $T_{\max }$ = 83 and 95 K for ${\mathrm{Sr}}_2$${\mathrm{CuWO}}_6$ and ${\mathrm{Sr}}_2$${\mathrm{CuMoO}}_6$, respectively. However, three-dimensional long-range order takes place below $T_{\mathrm{N}}$ = 24(1) and 28(2) K for ${\mathrm{Sr}}_2$${\mathrm{CuWO}}_6$ and ${\mathrm{Sr}}_2$${\mathrm{CuMoO}}_6$, respectively. Our results show that the low-dimensional magnetic correlations are mainly due to the significant next-nearest-neighbor interactions in the $ab$ plane of the double-perovskite structure, whereas three-dimensional long-range magnetic order is caused by weaker next-nearest-neighbor interactions along the $c$ axis. Next-nearest-neighbor interactions are also slightly frustrated by weaker nearest-neighbor interactions within the $ab$ plane. Based on these results we predict the low-temperature magnetic structure in these compounds to be type-II antiferromagnetic order of the double-perovskite lattice.

Two-component dipolar fermions in zigzag optical lattices allow for the engineering of spin-orbital models. We show that dipolar lattice fermions permit the exploration of a regime typically unavailable in solid-state compounds that is characterized by a spin-liquid phase with a finite magnetization and spontaneously broken SU(2) symmetry. This peculiar spin liquid may be understood as the Luttinger liquid of composite particles consisting of bound states of spin waves and orbital domain walls moving in an unsaturated ferromagnetic background. In addition, we show that the system exhibits a boundary phase transitions involving nonlocal entanglement of edge spins.

In superconductor-topological insulator-superconductor hybrid junctions, the barrier edge states are expected to be protected against backscattering, to generate unconventional proximity effects, and, possibly, to signal the presence of Majorana fermions. The standards of proximity modes for these types of structures have to be settled for a neat identification of possible new entities. Through a systematic and complete set of measurements of the Josephson properties we find evidence of ballistic transport in coplanar $\mathrm{Al}-{Bi}_2{\mathrm{Se}}_3-\mathrm{Al}$ junctions that we attribute to a coherent transport through the topological edge state. The shunting effect of the bulk only influences the normal transport. This behavior, which can be considered to some extent universal, is fairly independent of the specific features of superconducting electrodes. A comparative study of Shubnikov–de Haas oscillations and scanning tunneling spectroscopy gave an experimental signature compatible with a two-dimensional electron transport channel with a Dirac dispersion relation. A reduction of the size of the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ flakes to the nanoscale is an unavoidable step to drive Josephson junctions in the proper regime to detect possible distinctive features of Majorana fermions.

We present a study of Josephson junctions arrays with two-band superconducting elements in the high-capacitance limit. We consider two particular geometries for these arrays: a single rhombus and a rhombi chain with two-band superconducting elements at the spinal positions. We show that the rhombus shaped JJ circuit and the rhombi chain can be mapped onto a triangular JJ circuit and a JJ two-leg ladder, respectively, with zero effective magnetic flux, but with Josephson couplings that are magnetic flux dependent. If the two-band superconductors are in a sign-reversed pairing state, one observes transitions to or from chiral phase configurations in the mapped superconducting arrays when magnetic flux or temperature are varied. The phase diagram for these chiral configurations is discussed. When half-flux quantum threads each rhombus plaquette, new phase configurations of the rhombi chain appear that are characterized by the doubling of the periodicity of the energy density along the chain, with every other two-band superconductor locked in a sign-reversed state. In the case of identical Josephson couplings, the energy of these phase configurations becomes independent of the inner flux in the rhombi chain and the supercurrent along the rhombi chain is zero.