We study superconductivity and surface Andreev bound states in helical crystals. We consider the interlayer pairings along the helical hopping and investigate the surface local density of states on the (001) and zigzag surfaces for all the possible irreducible representations. There are three and four irreducible representations exhibiting the zero-energy peaks in the local density of states at the (001) and zigzag surfaces of helical lattices, respectively. By calculating the one-dimensional winging number, we show that these appearances of the zero-energy peaks stem from the surface Andreev bound states.

We argue that the layer electric polarization of noncentrosymmetric layered heterostructures can be generically controlled by light yielding a layer photovoltaic effect (LPE). The LPE possesses a rich phenomenology and can arise from myriad distinct mechanisms displaying strong sensitivity to symmetry (e.g., point group and time reversal) as well as the presence/absence of a Fermi surface. We systematically classify these and unveil how LPE manifests for a range of light polarizations. Strikingly, LPE manifests even for unpolarized light in rotationally symmetric heterostructures, sharply contrasting with that of in-plane photocurrent responses. These unusual layer photoresponses can be realized in a range of layered heterostructures such as bilayer graphene aligned on hexagonal boron nitride and manifest sizable layer polarization susceptibilities in the terahertz frequency range that can be used as a means of bulk photodetection.

Persistent spin helix (PSH) manifests itself as an effective knob to tackle spin decay inevitably occurring in disordered two-dimensional electron gases. Here, for ordinary (110)-oriented two-subband GaInAs wells subjected to top and back gate voltages, we theoretically achieve adjusting the Dresselhaus terms of the two bands while meanwhile consistently pinning the system at symmetric configuration [i.e., locking the Rashba spin-orbit (SO) terms to zero], thus enabling simultaneous formation of two copies of PSHs of flexible control. Strikingly, we are able to stretch the pitch—spin density wave length—of PSH by far more than one period, enabling helix-stretch functional spin field-effect transistor (FET), with both on and off states protected by the PSH symmetry. Moreover, we attain a scenario in which the helicities of the two copies of PSHs are sufficiently compensated. This makes possible a new concept: “orbit (band) filter,” which resembles spin FET while with novel functionality of orbit filtering, opening up a new route towards spintronic and orbitronic combined applications.

The magnonic crystal, which has a spatial modulation wave vector $q$, couples the spin wave with wave vector $k$ to the one with wave vector $k-q$. For a conventional magnonic crystal with direct current (dc) supply, the spin waves around $q/2$ are resonantly coupled to the waves near $-q/2$, and a band gap is opened at $k=\pm q/2$. If instead of the dc current the magnonic crystal is supplied with an alternating current (ac), then the band gap is shifted to $k$ satisfying $|{\omega }_s\left(k\right)-{\omega }_s(k-q)|={\omega }_{\mathrm{ac}}$; here ${\omega }_s\left(k\right)$ is the dispersion of the spin wave, while ${\omega }_{\mathrm{ac}}$ is the frequency of the ac modulation. The resulting gap in the case of the ac magnonic crystal is the half of the one caused by the dc with the same amplitude of modulation. The time evolution of the resonantly coupled spin waves controlled by properly suited ac pulses can be well interpreted as the motion on a Bloch sphere. The tunability of the ac magnonic crystal broadens the perspective of spin-wave computing.

For a long time, it has been thought that two-dimensional (2D) Fermi gases could support long-lived excitations, owing to the collinear quasiparticle scattering controlled by phase-space constraints at a 2D Fermi surface. We present a direct calculation that reveals such excitations. The excitation lifetimes are found to exceed the fundamental bound set by the Landau Fermi-liquid theory by a factor as large as ${(T_F/T)}^{\alpha }$ with $\alpha \approx 2$. These excitations represent Fermi-surface modulations of an odd parity, one per each odd angular momentum. To explain this surprising behavior, we employ a connection between the linearized quantum kinetic equation and the dynamics of a fictitious quantum particle moving in a 1D reflectionless ${\mathrm{sech}}^2$ potential. In this framework, we identify the long-lived excitations in Fermi gases as zero modes that arise from supersymmetry.

Based on first-principles molecular dynamics simulations, the equation of state and Hugoniot curve of a methane-water mixture are investigated at relevant conditions in Uranus and Neptune. Under a strong shock wave, the compression ratio of the methane-water mixture is significantly higher than that of pure methane. Calculations of the pair-correlation function show that methane molecules are dissociated by ionized water above 4000 K. Above 6000 K, the methane and water molecules are completely dissociated, and many short-lived chemical species such as ${\mathrm{H}}_3\mathrm{O}$ and $\mathrm{OH}$ ions, C–O bonds, and free H atoms are formed. These results may provide a reasonable material basis for the unusual magnetic fields and the thin-shell dynamo models in Uranus and Neptune. The temperature and density ranges of our simulations are also relevant to dynamic shock experiments, and the results may provide reasonable models for future laboratory studies.

Two-dimensional lead halide perovskites are promising materials for optoelectronics due to the tunability of their properties with the number of lead halide layers and the choice of an organic spacer. Physical understanding for the rational design of materials primarily requires knowledge of crystal structure. Two-dimensional lead halide perovskites are usually prepared in the form of films, complicating the experimental determination of structure. To enable theoretical studies of experimentally unresolvable structures as well as high-throughput virtual screening, we present an algorithm for crystal structure prediction of lead halide perovskites. Using an automatically prepared classical potential we show that our algorithm enables fast access to a structure that can be used for further first-principles studies.

The dielectric properties of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, a layered compound crystallizing in a rhombohedral structure, are investigated by means of first-principles calculations at the random phase approximation level. A special attention is devoted to the anisotropy in the dielectric function and to the local field effects that strongly renormalize the optical properties in the UV-visible range when the electric field is polarized along the stacking axis. Furthermore, both the Born effective charges for each atom and the zone center phonon frequencies and eigenvectors needed to describe the dielectric response in the infrared range are computed. Our theoretical near-normal incidence reflectivity spectra in both the UV-visible and infrared range are in fairly good agreement with the experimental spectra, provided that the free carriers Drude contribution arising from defects is included in the infrared response. The anisotropic plasmon frequencies entering the Drude model are computed within the rigid band approximation, suggesting that a measurement of the reflectivity in the infrared range for both polarizations might allow one to infer not only the type of doping but also the level of doping.

The magneto-optical Kerr effect (MOKE) is widely exploited in laboratory-based setups for the study of thin films and nanostructures, providing magnetic characterization with good spatial and temporal resolutions. Due to the complex coupling of light with a magnetic sample, conventional MOKE magnetometers normally work by selecting a small range of incident wave-vector values, focusing the incident light beam to a small spot, and recording the reflected intensity at that angular range by means of photodetectors. Using this approach, additional methodologies and measurements are required for full vectorial magnetic characterization. Here, we computationally investigate a Fourier-space MOKE setup, where a focused beam ellipsometer using high numerical aperture optics and a camera detector is employed to simultaneously map the intensity distribution for a wide range of incident and reflected wave vectors. We employ circularly incident polarized light and no analyzing optics, in combination with a fitting procedure of the light intensity maps to the analytical expression of the Kerr effect under linear approximation. In this way, we are able to retrieve the three unknown components of the magnetization vector as well as the material' s optical and magneto-optical constants with high accuracy and short acquisition times, with the possibility of single-shot measurements. Fourier MOKE is thus proposed as a powerful method to perform generalized magneto-optical ellipsometry for a wide range of magnetic materials and devices.

The interaction of a femtosecond laser pulse with magnetic materials has been intensely studied for more than two decades in order to understand ultrafast demagnetization in single magnetic layers or terahertz emission from their bilayers with nonmagnetic spin-orbit (SO) materials. However, in contrast to well-understood spin and charge pumping by dynamical magnetization in spintronic systems driven by microwaves or current injection, analogous processes in light-driven magnets and radiation emitted by them remain largely unexplained due to the multiscale nature of the problem. Here we develop a multiscale quantum-classical formalism—where conduction electrons are described by quantum master equation (QME) of the Lindblad type, classical dynamics of local magnetization is described by the Landau-Lifshitz-Gilbert (LLG) equation, and incoming light is described by classical vector potential, while outgoing electromagnetic radiation is computed using the Jefimenko equations for retarded electric and magnetic fields—and apply it to a bilayer of antiferromagnetic Weyl semimetal ${\mathrm{Mn}}_3\mathrm{Sn}$, hosting noncollinear local magnetization, and SO-coupled nonmagnetic material. Our $\mathrm{QME}+\mathrm{LLG}+$Jefimenko scheme makes it possible to understand how a femtosecond laser pulse directly generates spin and charge pumping and electromagnetic radiation by the ${\mathrm{Mn}}_3\mathrm{Sn}$ layer, including both odd and even high harmonics (of the pulse center frequency) up to order $n\le 7$. The directly pumped spin current then exerts spin torque on local magnetization whose dynamics, in turn, pumps additional spin and charge currents radiating in the terahertz range. By switching on and off LLG dynamics and SO couplings, we unravel which microscopic mechanism contributes the most to emitted terahertz radiation—charge pumping by local magnetization of ${\mathrm{Mn}}_3\mathrm{Sn}$ in the presence of its own SO coupling is far more important than standardly assumed (for other types of magnetic layers) spin pumping and subsequent spin-to-charge conversion within the adjacent nonmagnetic SO-coupled material.

We successfully synthesize a verdazyl-based complex, ${\left(p\text{−}\mathrm{Py}\text{−}\mathrm{V}\text{−}p\text{−}\mathrm{F}\right)}_2\left[\mathrm{Co}{\left(\mathrm{hfac}\right)}_2\right]$. Molecular-orbital calculations of this complex indicate that spin ladders composed of radical spins are coupled to Co spins via intramolecular interactions, forming a ladder-based two-dimensional (2D) spin model. The complex exhibits paramagnetic behavior associated with Co spins and the 3/5 magnetization plateau; numerical analysis demonstrates that this originates from a rung-singlet-like ground state with fully polarized Co spins. The peculiar behavior of the linear magnetization curve after the magnetization plateau is attributed to the nature of the 2D quantum spin system, which demonstrates a quantum phase transition from the gapped quantum state to the 2D magnetic state. Furthermore, we examine the magnetic anisotropy originating from ${\mathrm{Co}}^{2+}$ in the distorted octahedral environment of the complex. The anisotropic $g$ values are determined by electron-spin resonance (ESR) powder pattern analysis. Considering the spin-orbit coupling and crystal-field distortion of the ${\mathrm{Co}}^{2+}$ ion, we explain the anisotropic $g$ values of the fictitious spin-1/2 on the ${\mathrm{Co}}^{2+}$ ion. We also evaluate the anisotropy of the effective exchange interaction between the radical spin and the fictitious spin on the ${\mathrm{Co}}^{2+}$ ion, thus revealing the existence of an Ising-like exchange interaction.

We present triple-axis neutron scattering studies of low-energy magnetic fluctuations in strongly underdoped ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ with $x=0.05$, 0.06 and 0.07, providing quantitative evidence for a direct competition between these fluctuations and superconductivity. At dopings $x=0.06$ and $x=0.07$, three-dimensional superconductivity is found, while only a very weak signature of two-dimensional superconductivity residing in the ${\mathrm{CuO}}_2$ planes is detectable for $x=0.05$. We find a surprising suppression of the low-energy fluctuations by an external magnetic field at all three dopings. This implies that the response of two-dimensional superconductivity to a magnetic field is similar to that of a bulk superconductor. Our results provide direct evidence of a very gradual onset of superconductivity in cuprates.

Superconductivity in the family of spinel oxides is very rare owing to their robust Mott-insulating nature. About half a century ago, ${\mathrm{LiTi}}_2{\mathrm{O}}_4$ became the first reported spinel compound to show superconductivity with a 12 K transition temperature. Since then, several unsuccessful attempts were made to enhance the $T_{\mathrm{c}}$ of this family of materials. However, a very recent experiment [A. Rahaman et al., arXiv:2209.02053] has reported superconductivity at a higher temperature (below $\sim 16$ K) in the V-doped ${\mathrm{Mg}}_{1-\delta }{\mathrm{Ti}}_2{\mathrm{O}}_4$ thin surface layer while its bulk counterpart remains Mott insulating. The superconducting $T_{\mathrm{c}}$ of this material is significantly higher compared to other engineered ${\mathrm{MgTi}}_2{\mathrm{O}}_4$ thin films grown on different substrates. From our first-principles analysis, we have identified that Mg depletion significantly reduces Jahn-Teller (JT) activity and antiferromagnetic superexchange at the surface layer of V-doped ${\mathrm{Mg}}_{1-\delta }{\mathrm{Ti}}_2{\mathrm{O}}_4$ due to considerable charge transfer between various ions. The combined effect of a degraded antiferromagnetic order and reduced JT activity weakens the “Mottness” of the system, leading to the emergence of superconductivity at higher temperatures.

Accurate Au $L$-edge EXAFS (extended x-ray absorption fine structure) spectra of elemental Au have been collected and analyzed in a wide range of temperatures from 80 to 1400 K in the solid and liquid phases. Suitable samples for high-temperature measurements in the liquid phase were obtained by in situ reduction of an organic precursor. The data analysis was carried out using reliable multiple-scattering calculations including relativistic corrections (GnXAS), especially important for this heavy element. Simultaneous $L_1$-, $L_2$-, and $L_3$-edge refinements were performed and local structural results were compared with those obtained in previous studies based mainly on x-ray diffraction experiments. In the solid phase, an increased asymmetry of the first-neighbor distribution is observed for increasing temperatures while the average distance is found to be compatible with present thermal expansion data. The structure refinement of liquid Au was carried out considering the short-range contribution to the pair distribution function $g\left(r\right)$, using available x-ray diffraction data as a constraint for long-range order. We have shown that EXAFS is extremely sensitive to the local structure and that the $g\left(r\right)$ of liquid Au is characterized by a clear shift of the foot of the $g\left(r\right)$ and a slight shortening of the position of the maximum as compared with the previous data. The present data put to a test advanced computer simulations based on embedded atom models.

Non-Hermitian skin effect (NHSE) is a novel phenomenon appearing in non-Hermitian systems. Here, we report the experimental observation of NHSE. Different from the previous nonreciprocal circuit implementation scheme using active components, we construct our one-dimensional circuits using linear components only. Besides, we achieve the nonreciprocity by proportionally varying the parameter value of the components. By measuring the voltage response of each site, the information of eigenstates can be mapped out. The results show that the voltage response is always larger on one end of the circuit no matter on which end voltage driving is applied, indicating clearly the presence of the NHSE. Furthermore, we also simulate the interplay of NHSE and Anderson localization when additional disorder is introduced. Upon increasing the disorder strength, we observe the transformation from the skin effect phase to the localized phase. In the regime of skin effect phase, the eigenstates are all localized at one edge whereas eigenstates are affected by the voltage supply input in the localized phase. Our findings unveil a possible new route for simulation of topological phenomena in non-Hermitian systems.

The quasi-two-dimensional $1T\text{−}{\mathrm{TaS}}_2$ displays a unique two-step charge density wave (CDW) transition, from incommensurate (ICDW) to nearly commensurate (NCDW), and from NCDW to commensurate (CCDW), which is reflected in the stepwise resistivity behavior. In this work, we show that the hysteresis observed in the resistivity across the NCDW-to-CCDW transition is coupled to a local structure anomaly, evident from the pair density function analysis of neutron and x-ray diffraction data. We find that upon cycling the system from high to low temperatures (through the NCDW-CCDW transition) and collecting data on warming, local distortions in the in-star Ta and out-of-plane S atoms become evident, disrupting the trigonal symmetry of the star of David structures, without breaking the lattice periodicity, and preserving the underlying $\mathrm{P}\overline{3}$ symmetry. When the system is warmed up from the NCDW to the ICDW state, the local distortions are absent and the stars are symmetric, indicating that it is the thermal cycling through the NCDW-CCDW that locks in the distortions. Furthermore, we verify the temperature dependence of the two types of stacking along the $c$ axis: at low temperatures, the layer stacking exhibits a $13{\mathrm{c}}_o$ order that disappears upon warming through the NCDW transition, while across the NCDW and ICDW phases, the layer stacking is $3{\mathrm{c}}_o$. The $3{\mathrm{c}}_o$ order gradually disappears in the ICDW state.

The atomic structure of the recently discovered antiferromagnetic (AFM) ${\mathrm{Ga}}_{50}{\mathrm{Pd}}_{35.5}{\mathrm{Tb}}_{14.5}$ 2/1 approximant to quasicrystal with the space group of $Pa\overline{3}$ (No. 205), $a=23.1449\left(0\right)\AA$ was determined by means of single-crystal x-ray diffraction. The refined structure model revealed two main building units, namely, a Tsai-type rhombic triacontahedron (RTH) cluster with three concentric inner shells and an acute rhombohedron filling the gaps in between the RTH clusters. One of the interesting findings was a very low number of chemically mixed sites in the structure, which amount to only 7.40% of the all the atomic sites within an RTH cluster. A disorder-free environment was noticed within a nearest neighbor of an isolated ${\mathrm{Tb}}^{3+}$ ion, which is presumably one of the main contributors in enhancing AFM order in the present compound. The second significant finding was the observance of an orientationally ordered trigonal pyramidlike unit with a height of $4.2441\left(7\right)\AA$ at the center of the RTH cluster. Such a unit is noticed to bring structural distortion to outer shells, particularly to the surrounding dodecahedron cage being another possible contributor to the AFM order establishment in the present compound. The results, therefore, are suggestive of a possible link between chemical/positional order and the AFM order establishment.

A hyperbolic lattice allows for any $p$-fold rotational symmetry, in stark contrast to a two-dimensional crystalline material, where only 2-fold, 3-fold, 4-fold, or 6-fold rotational symmetry is permitted. This unique feature motivates us to ask whether the enriched rotational symmetry in a hyperbolic lattice can lead to any new topological phases beyond a crystalline material. Here, by constructing and exploring tight-binding models in hyperbolic lattices, we theoretically demonstrate the existence of higher-order topological phases in hyperbolic lattices with 8-fold, 12-fold, 16-fold or 20-fold rotational symmetry, which is not allowed in a crystalline lattice. Since such models respect the combination of time-reversal symmetry and $p$-fold ($p=8$, 12, 16, or 20) rotational symmetry, $p$ zero-energy corner modes are protected. For the hyperbolic {8,3} lattice, we find a higher-order topological phase with a finite edge energy gap and a gapless phase. Our results thus open the door to studying higher-order topological phases in hyperbolic lattices.

Transition metal dichalcogenides (TMDs) are known to have a wide variety of magnetic structures by hosting other transition metal atoms in the van der Waals gaps. To understand the chemical trend of the magnetic properties of the intercalated TMDs, we perform a systematic first-principles study for 48 compounds with different hosts, guests, and composition ratios. Starting with calculations based on spin density functional theory, we derive classical spin models by applying the local force method to the ab initio Wannier-based tight-binding model. We show that the calculated exchange couplings are overall consistent with the experiments, and the chemical trend can be understood in terms of the occupation of the $3d$ orbital in the intercalated transition metal. The present results give us a useful guiding principle to predict the magnetic structure of compounds that are yet to be synthesized.