In this paper, we present systematic improvements to a method that allows us to determine the surface phase diagrams based on ab initio calculations and atomistic thermodynamics. In particular, we include a contribution derived from thermal vibrations of the surface, which is usually omitted in such analysis. On the basis of first-principles phonon calculations, we determine a zero-point vibrational energy and thermal dependencies of vibrational entropy and free energy for atoms and molecules adsorbed on the surface. A series of polar GaN(0001) surfaces are examined. Significant changes are observed in the surface phase diagrams compared to the diagrams obtained by standard density-functional theory calculations without phononic contributions.

In the conventional paradigm of the spin Hall effect, the side-jump conductivity due to electron-phonon scattering is regarded to be temperature independent. In contrast, we draw the distinction that, while this side-jump conductivity is temperature independent in the classical equipartition regime where the longitudinal resistivity is linear in temperature, it can be temperature dependent below the equipartition regime. The mechanism resulting in this temperature dependence differs from the familiar one of the longitudinal resistivity. In the concrete example of Pt, we show that the change of the spin Hall conductivity with temperature can be as high as 50%. Experimentally accessible high-purity Pt is proposed to be suitable for observing this prominent variation below 80 K.

Metasurfaces, the two-dimensional (2D) counterpart of metamaterials, have recently attracted a great deal of attention due to their amazing properties, including negative refraction, hyperbolic dispersion, and manipulation of the evanescent spectrum. In this work, a theory model is proposed for the near field radiative heat transfer (NFRHT) between two nanoparticles in the presence of an anisotropic metasurface. Specifically, the metasurface is modeled as an array of graphene strips (GS), which is an ideal platform to implement any metasurface topology, ranging from isotropic to hyperbolic propagation. The NFRHT between two nanoparticles are significantly amplified when they are placed in the proximity of the GS, and regulated over several orders of magnitude. In this configuration, the anisotropic surface plasmon polaritons (SPPs) supported by the GS are excited and provide a new channel for the near-field energy transport. The dependence of conductance between two nanoparticles on the orientation, the structure parameters, the chemical potential of the GS, and the interparticle or the particle-surface distances are analyzed by clearly identifying the characteristics of the anisotropic SPPs such as dispersion relations, propagation length, and decay length. These results demonstrate a powerful method to regulate the energy transport in particle systems, and create a way to explore the anisotropic optical properties of the metasurface based on the measured heat transfer properties.

We identify and investigate two classes of non-Hermitian systems, i.e., one resulting from a Lorentz-symmetry violation (LSV) and the other from a complex mass (CM) with Lorentz invariance. The mechanisms to break, and approaches to restore, the bulk-boundary correspondence in these two types of non-Hermitian systems are clarified. The non-Hermitian system with LSV shows a non-Hermitian skin effect, and its topological phase can be characterized by mapping it to the Hermitian system via a noncompact $U\left(1\right)$ gauge transformation. In contrast, there exists no non-Hermitian skin effect for the non-Hermitian system with CM. Moreover, the conventional bulk-boundary correspondence holds in this (CM) system. We also consider a general non-Hermitian system in the presence of both LSV and CM, and we generalize its bulk-boundary correspondence.

The melting curve of elemental sulfur was measured to pressures of 65 GPa in a laser-heated diamond-anvil cell using ex situ textural analyses combined with spectroradiometry and benchmarked with laser-power-temperature functions. The melting curve reaches temperatures of $\sim 1800\mathrm{K}$ by 65 GPa and is smooth in the range of 23–65 GPa with a Clapeyron slope of $\sim 14\mathrm{K}/\mathrm{GPa}$ at 23 GPa. This is consistent with melting of a single tetragonal sulfur structure in this range, which is confirmed by in situ x-ray diffraction. An updated equation of state for tetragonal sulfur is determined, and the high-pressure, high-temperature stability region of tetragonal sulfur is reassessed.

The tetragonal-to-orthorhombic phase transition at $T_s$, which precedes the antiferromagnetic phase transition at $T_N$ in many iron-based superconductors, is considered one of the manifestations of electronic nematic order. By constructing temperature-pressure phase diagrams of pure and Co-doped ${\mathrm{BaFe}}_2{\mathrm{As}}_2$, we study the relation of $T_s$ and $T_N$ under pressure $p$. Our data disclose two qualitatively different regimes in which $\mathrm{\Delta }T=T_s-T_N$ either increases or decreases with $p$. We provide experimental evidence that the transition between the two regimes may be associated with sudden changes of the Fermi surface topology. Therefore, our results not only support the electronic origin of the structural order, but also emphasize the importance of details of the Fermi surface for the evolution of nematic order under pressure.

Deploying an analytical atomistic model of the bulk band structure of the IV-VI crystalline topological insulators, we connect the spin structure of the surface state to the crystal field and spin-orbit coupling parameters of the bulk material. While the Dirac-Weyl-type topological surface state is often assumed to be universal, we show that the physics of the surface state is strikingly nonuniversal, carrying a profound imprint of the bulk physics. To see this explicitly we calculate the Ruderman-Kittel-Kasuya-Yosida interaction, which may be viewed as a probe of this surface state spin structure, finding that its qualitative form depends on the values the bulk spin-orbit and crystal field parameters take. This opens the way to tune the spin interaction on the surface of a IV-VI topological insulator by, for instance, varying the composition of the IV-VI ternary compounds.

An ultrathin active polarization-selective metasurface is designed and optimized for operation in the X band. The metasurface leverages the design of a previously reported passive polarization converting metasurface. Similar to the passive polarization converter, the proposed active metasurface consists of three patterned metallic sheets. Its bottom sheet is modified and populated with ultrawideband, unconditionally stable amplifiers and corresponding biasing networks. The metasurface provides over 35 dB of input-output isolation achieved through orthogonal polarizations, which ensures stable operation. It was fabricated with a low-cost, printed-circuit-board and pick-and-place process. The performance of the metasurface was measured and its stability confirmed. The desired effects of amplification, polarization conversion, polarization selectivity, and nonreciprocal behavior were successfully demonstrated in both full-wave simulations and measurements. Due to its extremely small thickness (0.66 mm or $0.02\lambda$ at 10 GHz), the metasurface shows excellent performance for a wide range of incident angles, which exceeds $\pm {30}^{\circ }$. It achieves a perfect (100%) polarization conversion ratio in simulation, and 98.4% in measurement.

We have performed a systematic study of the physical properties and electronic structure of ${\mathrm{SrTiO}}_3, {\mathrm{SrTi}}_{0.5}{\mathrm{Ru}}_{0.5}{\mathrm{O}}_3$, and ${\mathrm{SrRuO}}_3$. For the mixed compound, the temperature dependence of the magnetization is consistent with the occurrence of $\mathrm{Ru}Ti$ and $\mathrm{Ti}Ru$ defects. Despite being a semiconductor, the behavior of the electrical resistivity as a function of temperature is compatible with the emergence of small metallic regions richer in Ru concentration, a feature supported by the finite spectral weight at the Fermi level observed in the valence-band x-ray photoemission spectroscopy spectrum. The x-ray photoemission and absorption spectra of the ${\mathrm{SrTi}}_{0.5}{\mathrm{Ru}}_{0.5}{\mathrm{O}}_3$ compound were simulated by double cluster model calculations, which include the ${\mathrm{TiO}}_6-{\mathrm{RuO}}_6$ interaction, and also by the linear combination of single cluster calculations for ${\mathrm{SrTiO}}_3$ and ${\mathrm{SrRuO}}_3$. The results indicate that the interaction between different octahedra may give rise to distinct peak characters, depending on the experimental spectrum being calculated. We argue that these effects are only captured with the explicit inclusion of the Ti-Ru interaction.

We experimentally study quantized conductance in an electrostatically defined constriction in a high-mobility InAs two-dimensional electron gas. A parallel magnetic field lifts the spin degeneracy and allows for the observation of plateaus in integer multiples of $e^2/h$. Upon the application of a perpendicular magnetic field, spin-resolved magnetoelectric subbands are visible. Through finite bias spectroscopy we measure the subband spacings in both parallel and perpendicular direction of the magnetic field and determine the $g$ factor.

We performed measurements of the gradient of the Casimir force between Au-coated surfaces of a sphere and a plate by means of a significantly upgraded dynamic atomic force microscope (AFM)-based technique. By introducing a combined cleaning procedure of interior surfaces of the vacuum chamber and the test bodies by means of UV light and Ar ions, we reached higher vacuum and eliminated the role of electrostatic patches. Furthermore, the use of a much softer cantilever allowed a sixfold decrease of the systematic error in measuring the force gradient. The experimental data are compared with the theoretical predictions of the Lifshitz theory taking into account corrections due to the inaccuracy of the proximity force approximation and that due to surface roughness. It is shown that the theoretical approach accounting for the relaxation properties of free electrons is excluded by the data up to a larger than previous sphere-plate separation of 820 nm, whereas an alternative approach is found in very good agreement with the data. The importance of these results in connection with the foundations of quantum statistical physics is discussed.

$\mathrm{Zn}{M}_2^{III}{\mathrm{O}}_4$ ($M^{III}=\text{Co}$, Rh, Ir) spinels have been recently identified as promising $p$-type semiconductors for transparent electronics. However, discrepancies exist in the literature regarding their fundamental optoelectronic properties. In this paper, the electronic structures of these spinels are directly investigated using soft/hard x-ray photoelectron and x-ray absorption spectroscopies in conjunction with density functional theory calculations. In contrast to previous results, ${\mathrm{ZnCo}}_2{\mathrm{O}}_4$ is found to have a small electronic band gap with forbidden optical transitions between the true band edges, allowing for both bipolar doping and high optical transparency. Furthermore, increased $d-d$ splitting combined with a concomitant lowering of Zn $s/p$ conduction states is found to result in a ${\mathrm{ZnCo}}_2{\mathrm{O}}_4\left(\text{ZCO}\right)<{\mathrm{ZnRh}}_2{\mathrm{O}}_4\left(\text{ZRO}\right)\approx {\mathrm{ZnIr}}_2{\mathrm{O}}_4\left(\text{ZIO}\right)$ band gap trend, finally resolving long-standing discrepancies in the literature.

In this work, density functional computations and Monte Carlo simulations are performed to investigate the structural, electronic, magnetic, and thermodynamic properties of a $\mathrm{Co}/{\mathrm{WS}}_2$ nanolayer, a semiconductor ${\mathrm{WS}}_2$ monolayer covered by a ferromagnetic cobalt monolayer. In addition to a conventional semilocal exchange-correlation functional, three nonlocal functionals, including the novel Agapito-Curtarolo-Buongiorno Nardelli (ACBN0) scheme, are applied to obtain reliable electronic and magnetic properties. It is argued that the ACBN0 scheme is very efficient for first-principles description of the $\mathrm{Co}/{\mathrm{WS}}_2$ nanolayer. The obtained electronic structures evidence a trustworthy half-metallic gap in the majority-spin channel of the lowest-energy configuration of the nanolayer, which is promising for spintronic applications. The obtained magnetic thermodynamics properties from Monte Carlo simulations predict a Curie temperature of about 110 K, which is far small for device applications of this nanolayer. The electronic and magnetic properties of the system are calculated under various compressive and tensile strains, and it is shown that a tensile strain of about 4% may effectively improve the thermal stability of half-metallic ferromagnetism in the $\mathrm{Co}/{\mathrm{WS}}_2$ nanolayer.

We study longitudinal transport in Weyl semimetal nanowires, both in the absence and in the presence of a magnetic flux threading the nanowires. We identify two qualitatively different regimes of transport with respect to the chemical potential in the nanowires. In the “surface regime,” for low doping, most of the conductance occurs through the Fermi arc surface states, and it rises in steps of one quantum of conductance as a function of the chemical potential; furthermore, with varying flux the conductance changes in steps of one quantum of conductance with characteristic Fabry-Pérot interference oscillations. In the “bulk-surface regime,” for highly doped samples, the dominant contribution to the conductance is quadratic in the chemical potential, and mostly conditioned by the bulk states; the flux dependence shows clearly that both the surface and the bulk states contribute. The two aforementioned regimes prove that the contribution of Fermi arc surface states is salient and, therefore, crucial for understanding transport properties of finite-size Weyl semimetal systems. Last but not least, we demonstrate that both regimes are robust to disorder.

We calculate transverse spin susceptibility in the linear response method based on the ground states determined in the quasiparticle self-consistent $GW$ (QSGW) method. Then we extract spin-wave (SW) dispersions from the susceptibility. We treat bcc Fe, hcp Co, fcc Ni, and B2-type FeCo. Because of the better description of the independent-particle picture in QSGW, calculated spin stiffness constants for Fe, Co, and Ni give much better agreement with experiments in QSGW than those in the local density approximation (LDA); the stiffness for Ni in LDA is two times greater than in experiment. For Co, both acoustic and optical branches of SWs agree with experiment. As for FeCo, we have some discrepancy between the spin stiffness in QSGW and that in experiment. We may need further theoretical and experimental investigations on the discrepancy.

We demonstrate anisotropic tunnel magnetoconductance by controllably engineering charging islands in the layered semiconducting ferromagnet ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$. This is achieved by assembling vertical van der Waals heterostructures comprised of graphene electrodes separated by crystals of ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$. Carefully applying vertical electric fields in the region of $(E\sim 25$–50 mV/nm) across the ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ causes its dielectric breakdown at cryogenic temperatures. This breakdown process has the effect of introducing subgap defect states within the otherwise semiconducting ferromagnetic material. Low-temperature electron transport through charging islands reveals Coulomb blockade behavior with a strongly gate-tuneable anisotropic magnetoconductance, which persists up to $T\sim 60$ K. We report average tunnel magnetoresistance values of $100\%$. This work opens new avenues and material systems for the development of nanometer-scale electrically controlled spintronic devices.

Exploiting the possibility to switch from antiferromagnetic (AFM) and ferromagnetic (FM) ground states (GSs) in out-stoichiometric branches of ${\mathrm{Ce}}_2{\mathrm{Pd}}_2\mathrm{In}$ alloys, the stability of Shastry-Sutherland (ShSu) phase of ${\mathrm{Ce}}_2{\mathrm{Pd}}_2\mathrm{Sn}$ as a function of Sn/In electron doping was studied. Magnetic and specific-heat measurements show that the Ce-rich compositions stabilize the FM-GS throughout the Sn/In-FM substitution, allowing to extend the formation of the ShSu phase up to its collapse in a tricritical point around $y_{cr}=0.5$. On the other hand, this behavior is quite different from that reported in a recent investigation on the AFM branch where atomic disorder at intermediate Sn/In-AFM concentrations inhibits the formation of the ShSu phase.

The study of universal critical behavior is a crucial issue in a continuous phase transition, which groups various critical phenomena into universality classes for revealing microscopic electronic behaviors. The understanding of the nature of magnetism in Eu-based ferromagnetic superconductors is largely impeded by the infeasibility of performing inelastic neutron scattering measurements to deduce the microscopic magnetic behaviors and the effects on the superconductivity, due to the significant neutron absorption effect of natural ${}^{152}\mathrm{Eu}$ and unavailability of large single crystals. However, by systematically combining the neutron diffraction experiment, the first-principles calculations, and the quantum Monte Carlo simulations, we have obtained a perfectly consistent universal critical exponent value of $\beta =0.385\left(13\right)$ experimentally and theoretically for $\mathrm{Eu}{\left({\mathrm{Fe}}_{0.75}{\mathrm{Ru}}_{0.25}\right)}_2{\mathrm{As}}_2$, from which the magnetism in the Eu-based ferromagnetic superconductors is identified as the universal class of a three-dimensional anisotropic quantum Heisenberg model with long-range magnetic exchange coupling. This systematic study points out a suitable microscopic theoretical model for describing the nature of magnetism in the intriguing Eu-based ferromagnetic superconductors.

The FeSe-based superconductors exhibit a wide range of critical temperature $T_c$ under a variety of material and physical conditions, but extensive studies to date have yet to produce a consensus view on the underlying mechanism. Here we report on a systematic Raman-scattering work on intercalated FeSe superconductors ${\mathrm{Li}}_x{\left({\mathrm{NH}}_3\right)}_y{\mathrm{Fe}}_2{\mathrm{Se}}_2$ and (Li,Fe)OHFeSe compared to pristine FeSe. All three crystals show an anomalous power-law temperature dependence of phonon linewidths, deviating from the standard anharmonic behavior. This intriguing phenomenon is attributed to electron-phonon coupling effects enhanced by electron correlation, as evidenced by the evolution of the $A_{1g}$ Raman mode. Meanwhile, an analysis of the $B_{1g}$ mode, which probes the out-of-plane vibration of Fe, reveals a lack of influence by previously suggested structural parameters, and instead indicates a crucial role of the joint density of states in determining $T_c$. These findings identify carrier doping as the direct factor driving and modulating superconductivity in FeSe-based compounds.

Although the topic of intermittent plastic flow manifesting as load fluctuations or displacement jumps in nanoindentation (depths less than 100 nm) has attracted considerable attention, the existence of steps on load-indentation ($F\text{−}z$) curves reported in microindentation (depths of several microns) of samples of dilute alloys of varying concentrations and load dates, has received little attention from a modeling point of view. Following our earlier approaches to nanoindentation instabilities and indentation size effect, we develop a minimal dislocation mechanism based model that predicts all the generic experimental features by setting up time-evolution equations for the mobile, the forest, dislocations with solute atmosphere, and the geometrically necessary dislocation densities. The model includes basic dislocation mechanisms common to most plastic deformations, such as dislocation multiplication, storage, and recovery mechanisms. We model the indentation instability as a variant of the standard Portevin–Le Chatelier (PLC) effect seen in the constant strain rate condition by including collective pinning and unpinning of dislocations from solute atmosphere. The instability mechanism is further generalized to include concentration-dependent dislocation-solute interaction to capture both concentration dependence of the indentation instability and strengthening of alloy samples. Based on recent experimental observations that show small misorientation at small depths suggesting limited geometrically necessary dislocation density, we model the growth of the geometrically necessary dislocation density by the number of loops that can be activated under the contact area and the mean strain gradient. The equations are then coupled to the load rate equation. The model predicts all the generic experimental features, such as (a) the stepped nature of the $F\text{−}z$ curves, (b) the existence of a critical load and critical indentation depth for the onset of the instability, (c) the decreasing dependence of the maximum indentation depth with concentration of the alloying element, (d) the mean critical indentation depth $z^{*}$ for the onset of the instability increasing with decreasing concentration with a concomitant increase in levels of fluctuations, (e) the decreasing power-law dependence of the critical indentation depth with concentration, (f) the manifestation of intermittent stepped response in a window of load rates, and (g) the magnitude of the load steps scaling linearly with the load. In essence, the basic physical mechanisms responsible for predicting all the experimental results (a)–(g) are the generalization of pinning and unpinning mechanism (of dislocations from solute atmosphere) to include concentration-dependent dislocation-solute interaction and solution hardening of alloy samples with concentration together with the inherent rate-dependent nature of the PLC instability.