The high carrier mobility of graphene makes it an attractive material for electronics, however, graphene's application for optoelectronic systems is limited due to its low optical absorption. We present a cavity-coupled nanopatterned graphene absorber designed to sustain temporal and spatial overlap between localized surface plasmon resonance and cavity modes, thereby resulting in enhanced absorption up to an unprecedented value of theoretically $(60\%)$ and experimentally measured $(45\%)$ monolayer graphene in the technologically relevant 8–12-μm atmospheric transparent infrared imaging band. We demonstrate a wide electrostatic tunability of the absorption band $(\sim 2\mu \mathrm{m})$ by modifying the Fermi energy. The proposed device design allows enhanced absorption and dynamic tunability of chemical vapor deposition grown low carrier mobility graphene which provides a significant advantage over previous strategies where absorption enhancement was limited to exfoliated high carrier mobility graphene. We developed an analytical model that incorporates the coupling of the graphene electron and substrate phonons, providing valuable and instructive insights into the modified plasmon-phonon dispersion relation necessary to interpret the experimental observations. Such gate voltage and cavity tunable enhanced absorption in chemical vapor deposited large area monolayer graphene paves the path towards the scalable development of ultrasensitive infrared photodetectors, modulators, and other optoelectronic devices.

Bulk Au-Fe alloys separate into Au-based fcc and Fe-based bcc phases, but ${\mathrm{L}1}_0$ and ${\mathrm{L}1}_2$ orderings were reported in single-phase Au-Fe nanoparticles. Motivated by these observations, we study the structural and ordering energetics in this alloy by combining density functional theory (DFT) calculations with effective Hamiltonian techniques: a cluster expansion with structural filters, and the configuration-dependent lattice deformation model. The phase separation tendency in Au-Fe persists even if the fcc-bcc decomposition is suppressed. The relative stability of disordered bcc and fcc phases observed in nanoparticles is reproduced, but the fully ordered ${\mathrm{L}1}_0$ AuFe, ${\mathrm{L}1}_2{\mathrm{Au}}_3\mathrm{Fe}$, and ${\mathrm{L}1}_2{\mathrm{AuFe}}_3$ structures are unstable in DFT. However, a tendency to form concentration waves at the corresponding [001] ordering vector is revealed in nearly random alloys in a certain range of concentrations. This incipient ordering requires enrichment by Fe relative to the equiatomic composition, which may occur in the core of a nanoparticle due to the segregation of Au to the surface. Effects of magnetism on the chemical ordering are also discussed.

We present in situ high-pressure synchrotron x-ray diffraction (XRD) and electrical transport measurements on quasi-one-dimensional single-crystal $\mathrm{Ti}{\mathrm{S}}_3$ up to 29.9–39.0 GPa in diamond-anvil cells, coupled with first-principles calculations. Counterintuitively, the conductive behavior of semiconductor $\mathrm{Ti}{\mathrm{S}}_3$ becomes increasingly insulating with pressure until $P_{C1}\sim 12\mathrm{GPa}$, where extremes in all three axial ratios are observed. Upon further compression to $P_{C2}\sim 22\mathrm{GPa}$, the XRD data evidence a structural phase transition. Based on our theoretical calculations, this structural transition is determined to be isosymmetric, i.e., without change of the structural symmetry ($P{2}_1/m$), mainly resulting from rearrangement of the dangling ${\mathrm{S}}_2$ pair along the $a$ axis.

We present an analytical theory of domain-wall tilt due to a transverse in-plane magnetic field in a ferromagnetic nanostrip with out-of-plane anisotropy and Dzyaloshinskii-Moriya interaction (DMI). The theory treats the domain walls as one-dimensional objects with orientation-dependent energy, which interact with the sample edges. We show that under an applied field the domain wall remains straight, but tilts at an angle to the direction of the magnetic field that is proportional to the field strength for moderate fields and sufficiently strong DMI. Furthermore, we obtain a nonlinear dependence of the tilt angle on the applied field at weaker DMI. Our analytical results are corroborated by micromagnetic simulations.

We study angular-dependent magnetoresistance in iron-based superconductors ${\mathrm{Ba}}_{1-x}{\mathrm{Na}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ and ${\mathrm{FeTe}}_{1-x}{\mathrm{Se}}_x$. Both superconductors have relatively small anisotropies $\gamma \sim 2$ and exhibit a three-dimensional (3D) behavior at low temperatures. However, we observe that they start to exhibit a profound two-dimensional behavior at elevated temperatures and in applied magnetic field parallel to the surface. We conclude that the unexpected two-dimensional (2D) behavior of the studied low-anisotropic superconductors is not related to layeredness of the materials, but is caused by appearance of surface superconductivity when magnetic field exceeds the upper critical field $H_{c2}\left(T\right)$ for destruction of bulk superconductivity. We argue that the corresponding 3D-2D bulk-to-surface dimensional transition can be used for accurate determination of the upper critical field.

We derive the theoretical and numerical framework for investigating nonequilibrium properties of spin-orbit coupled wires with Zeeman splitting proximized by a superconductor in the nonlinear diffusive regime. We demonstrate that the anisotropic behavior of triplet Cooper pairs in this system leads to novel spin accumulation profiles tunable by the magnetic field and strength of applied voltage bias. This paves the way for enhanced manipulation of superconducting spintronic devices, and it enables further investigation of nonequilibrium effects in proximity-coupled superconducting structures more generally.

We report our ab initio pseudopotential results for the structural, electronic, vibrational, and electron-phonon interaction properties of the cubic Laves-phase compounds ${\mathrm{CaIr}}_2$ and ${\mathrm{CaRh}}_2$. While the spin-orbit coupling (SOC) does not result in any appreciable changes in structural parameters, it lifts the degeneracies of some bands near the Fermi level, albeit with a much smaller amount for ${\mathrm{CaRh}}_2$. The effect of SOC on the vibrational properties of both materials is considerable. The SOC results in a slight decrease in the electronic density of states near the Fermi level $N\left(E_F\right)$ and makes low-frequency phonon branches harder, and the electron-phonon coupling parameter $\lambda$ is lowered from 1.43 to 1.05 for ${\mathrm{CaIr}}_2$ and from 1.17 to 0.96 for ${\mathrm{CaRh}}_2$. On the other hand, the logarithmically averaged phonon frequency ${\omega }_{ln}$ is enhanced from 79.60 to 100.97 K for ${\mathrm{CaIr}}_2$ and from 120.20 to 140.80 K for ${\mathrm{CaRh}}_2$ with the inclusion of SOC. Using the calculated values of $\lambda$ and ${\omega }_{ln}$, the superconducting critical temperature is determined to be 5.94 K (7.34 K without SOC) for ${\mathrm{CaIr}}_2$ and 6.97 K (9.08 K without SOC) for ${\mathrm{CaRh}}_2$. The superconducting critical-temperature values with SOC compare very well with corresponding experimental values of 5.80 and 6.40 K, indicating the importance of SOC for the physical properties of both materials.

Artificial ice systems have unique physical properties that are promising for potential applications. One of the most challenging issues in this field is to find novel ice systems that allow precise control over the geometries and many-body interactions. Superconducting vortex matter has been proposed as a very suitable candidate to study artificial ice, mainly due to the availability of tunable vortex-vortex interactions and the possibility to fabricate a variety of nanoscale pinning potential geometries. So far, a detailed imaging of the local configurations in a vortex-based artificial ice system is still lacking. Here we present a direct visualization of the vortex-ice state in a nanostructured superconductor. By using scanning Hall probe microscopy, a large area with the vortex-ice ground-state configuration has been detected, which confirms the recent theoretical predictions for this ice system. Besides the defects analogous to artificial spin-ice systems, other types of defects have been visualized and identified. We also demonstrate the possibility to realize different types of defects by varying the magnetic field.

Scandium hydrides at high pressures have been investigated by using ab initio density functional calculations. Although the stable scandium hydride so far known to have the highest content rate of hydrogen is ${\mathrm{ScH}}_3$, other more hydrogen-rich compounds are found to be possible at high pressures. These are ${\mathrm{ScH}}_4$ in the $I4/mmm$ structure above 160 GPa, ${\mathrm{ScH}}_6$ in the $P{6}_3/mmc$ structure from 135 to 265 GPa, and ${\mathrm{ScH}}_6$ in the $Im\overline{3}m$ structure above 265 GPa. The three phases are all metallic, and the superconducting transition temperatures estimated from the extended McMillan equation are 67 K in the $I4/mmm{\mathrm{ScH}}_4$ at 195 GPa, 63 K in the $P{6}_3/mmc{\mathrm{ScH}}_6$ at 145 GPa, and 130 K in the $Im\overline{3}m{\mathrm{ScH}}_6$ at 285 GPa. While the $I4/mmm$ tetrahydride and the $Im\overline{3}m$ hexahydride were similarly predicted for yttrium (another group-3 element), the $P{6}_3/mmc$ hexahydride is possible only for scandium. The smaller atomic size of scandium stabilizes the $P{6}_3/mmc$ structure, and other nearby $d$-block elements, whose atomic sizes are smaller or comparable, might be likewise capable of forming such polyhydrides.

Using all-electron density functional theory, we have performed an ab initio study on x-ray absorption spectra of highly compressed polystyrene (CH). We found that the $K$-edge shifts in strongly coupled, degenerate polystyrene cannot be explained by existing continuum-lowering models adopted in traditional plasma physics. To gain insights into the $K$-edge shift in warm, dense CH, we have developed a model designated as “single mixture in a box” (SMIAB), which incorporates both the lowering of the continuum and the rising of the Fermi surface resulting from high compression. This simple SMIAB model correctly predicts the $K$-edge shift of carbon in highly compressed CH in good agreement with results from quantum molecular dynamics (QMD) calculations. Traditional opacity models failed to give the proper $K$-edge shifts as the CH density increased. Based on QMD calculations, we have established a first-principles opacity table (FPOT) for CH in a wide range of densities and temperatures [$\rho =0.1-100\mathrm{g}/\mathrm{c}{\mathrm{m}}^3$ and $T=2000-1000000\mathrm{K}$]. The FPOT gives much higher Rosseland mean opacity compared to the cold-opacity–patched astrophysics opacity table for warm, dense CH and favorably compares to the newly improved Los Alamos atomic model for moderately compressed CH (${\rho }_{\mathrm{CH}}\le 10\mathrm{g}/\mathrm{c}{\mathrm{m}}^3$), but remains a factor of 2 to 3 higher at extremely high densities (${\rho }_{\mathrm{CH}}\ge 50\mathrm{g}/\mathrm{c}{\mathrm{m}}^3$). We anticipate the established FPOT of CH will find important applications to reliable designs of high-energy-density experiments. Moreover, the understanding of $K$-edge shifting revealed in this study could provide guides for improving the traditional opacity models to properly handle the strongly coupled and degenerate conditions.

We report on THz time-domain spectroscopy on multiferroic ${\mathrm{GeV}}_4{\mathrm{S}}_8$, which undergoes orbital ordering at a Jahn-Teller transition at 30.5 K and exhibits antiferromagnetic order below 14.6 K. The THz experiments are complemented by dielectric experiments at audio and radio frequencies. We identify a low-lying excitation close to 0.5 THz, which is only weakly temperature dependent and probably corresponds to a molecular excitation within the electronic level scheme of the ${\mathrm{V}}_4$ clusters. In addition, we detect complex temperature-dependent behavior of a low-lying phononic excitation, closely linked to the onset of orbitally driven ferroelectricity. In the high-temperature cubic phase, which is paramagnetic and orbitally disordered, this excitation is of relaxational character becomes an overdamped Lorentzian mode in the orbitally ordered phase below the Jahn-Teller transition, and finally appears as well-defined phonon excitation in the antiferromagnetic state. Abrupt changes in the real and imaginary parts of the complex dielectric permittivity show that orbital ordering appears via a structural phase transition with strong first-order character and that the onset of antiferromagnetic order is accompanied by significant structural changes, which are of first-order character, too. Dielectric spectroscopy documents that at low frequencies, significant dipolar relaxations are present in the orbitally ordered, paramagnetic phase only. In contrast to the closely related ${\mathrm{GaV}}_4{\mathrm{S}}_8$, this relaxation dynamics that most likely mirrors coupled orbital and polar fluctuations does not seem to be related to the dynamic processes detected in the THz regime.

We analyze quantum criticality at finite temperature for a class of non-Fermi liquids with massless bosons. Finite temperature gives rise to new infrared singularities that invalidate standard perturbative treatments. We show how such divergences are resolved at a nonperturbative level, and obtain the resulting fermion self-energy. This leads to a new “thermal” non-Fermi liquid regime that extends over a wide range of frequencies, and which violates finite temperature scaling laws near quantum critical points. We analyze the resulting quantum critical region and properties of the retarded Green's function. More generally, such effects dominate in the nearly static limit and are expected to have a nontrivial impact on superconductivity and transport.

To understand the rich electronic phase diagram in FeSe under pressure that vividly demonstrates the strong interplay between the nematicity, magnetism, and superconductivity, we analyze the electronic states by including the higher-order many-body effects called the vertex correction (VC). We predict the pressure-induced emergence of $xy$-orbital hole pocket based on the first-principles analysis. Due to this pressure-induced Lifshitz transition, the spin fluctuations on the $xy$ orbital are enhanced, whereas those on $xz,yz$ orbitals are gradually reduced. For this reason, nonmagnetic orbital order $O=n_{xz}-n_{yz}$, which is driven by the spin fluctuations on $xz,yz$ orbitals through the intraorbital VCs, is suppressed, and it is replaced with the magnetism of $xy$-orbital $d$ electrons. The nodal $s$-wave state at ambient pressure ($O\ne 0$) and the enhancement of $T_{\mathrm{c}}$ under pressure are driven by the cooperation between the spin and orbital fluctuations.

Inspired by recent experiments, we study a short superconducting junction of length $L\ll \xi$ (coherence length) inserted in a dc-SQUID containing an ancillary Josephson tunnel junction. We evaluate the nonequilibrium occupation of the Andreev bound states (ABS) for the case of a conventional junction and a topological junction, with the latter case of ABS corresponding to a Majorana mode. We take into account small phase fluctuations of the Josephson tunnel junction, acting as a damped LC resonator, and analyze the role of the distribution of the quasiparticles of the continuum assuming that these quasiparticles are in thermal distribution with an effective temperature different from the environmental temperature. We also discuss the effect of strong photon irradiation in the junction leading to a nonequilibrium occupation of the ABS. We systematically compare the occupations of the bound states and the supercurrents carried by these states for conventional and topological junctions.

Isolated Weyl cones in a disordered environment do not show the phenomenon of Anderson localization due to the absence of backscattering processes. However, besides the conventional three dimensional diffusive metal, an additional semimetallic phase can form. In this paper we study the effect of tilt on the physics of disorder in isolated Weyl cones. Our main finding is that, while the semimetallic phase remains a stable phase, tilt decreases the size of the semimetallic region. Conversely, disorder increases the effective tilt of the quasiparticle excitations.

Chern insulators exhibit fascinating properties, which originate from the topologically nontrivial state characterized by the Chern number. How these properties change if the system is quenched between topologically distinct phases is, however, not fully understood. In this paper, we investigate the quench dynamics of the prototypical massive Dirac model for topological insulators in two dimensions. We consider both dissipationless dynamics and the effect of electron-phonon interactions, and ask how the transient dynamics and nonequilibrium steady states affect simple observables. Specifically, we discuss a time-dependent generalization of the Hall effect and the dichroism of the photoexcitation probability between left and right circularly polarized light. We present optimized schemes based on these observables, which can reveal the evolution of the topological state of the quenched system.

Topological ordered phases are related to changes in the properties of their quasiparticle excitations (anyons). We study these relations in the framework of projected entanglement pair states and show how condensing and confining anyons reduces a local gauge symmetry to a global on-site symmetry. We also study the action of this global symmetry over the quasiparticle excitations. As a byproduct, we observe that this symmetry reduction effect can be applied to one-dimensional systems as well and brings about appealing physical interpretations on the classification of phases with symmetries using matrix product states. The case of ${\mathbb{Z}}_2$ on-site symmetry is studied in detail.

Hexagonal FeSe thin films were grown on $\mathrm{SrTi}{\mathrm{O}}_3$ substrates and the temperature and thickness dependence of their electronic structures were studied. The hexagonal FeSe is found to be metallic, with a Fermi surface consisting of six elliptical electron pockets. With decreased temperature, parts of the bands shift downward to high binding energy while some bands shift upward to $E_{\mathrm{F}}$. The shifts of these bands begin around 300 K and saturate at low temperature, indicating a magnetic phase-transition temperature of about 300 K. With increased film thickness, the Fermi surface topology and band structure show no obvious change. Our paper reports the electronic structure of hexagonal FeSe, and shows that the possible magnetic transition is driven by large-scale electronic structure reconstruction.

We show that the fermionic and bosonic spectrum of $d=2$ fermions at finite density coupled to a critical boson can be determined nonperturbatively in the combined limit $k_F\to \infty ,N_f\to 0$ with $N_f{k}_F$ fixed. In this double scaling limit, the boson two-point function is corrected but only at one loop. This double scaling limit therefore incorporates the leading effect of Landau damping. The fermion two-point function is determined analytically in real space and numerically in (Euclidean) momentum space. The resulting spectrum is discontinuously connected to the quenched $N_f\to 0$ result. For $\omega \to 0$ with $k$ fixed the spectrum exhibits the distinct non-Fermi-liquid behavior previously surmised from the RPA approximation. However, the exact answer obtained here shows that the RPA result does not fully capture the IR of the theory.

Contact of a spherical tip with a flat elastic substrate is simulated with a Green's-function method that includes atomic structure at the interface while capturing elastic deformation in a semi-infinite substrate. The tip and substrate have identical crystal structures with nearest-neighbor spacing $d$ and are aligned in registry. Purely repulsive interactions between surface atoms lead to a local shear strength that is the local pressure times a constant local friction coefficient $\alpha$. The total friction between tip and substrate is calculated as a function of contact radius $a$ and sphere radius $R$, with $a$ up to ${10}^{3}d$ and $R$ up to $4\times {10}^{4}d$. Three regimes are identified depending on the ratio of $a$ to the core width of edge dislocations in the center of the contact. This ratio is proportional to $\alpha a^2/Rd$. In small contacts, all atoms move coherently and the total friction coefficient $\mu =\alpha$. When the contact radius exceeds the core width, a dislocation nucleates at the edge of the contact and rapidly advances to the center where it annihilates. The friction coefficient falls as $\mu \sim \alpha {(\alpha a^2/Rd)}^{-2/3}$. An array of dislocations forms in very large contacts and the friction is determined by the Peierls stress for dislocation motion. The Peierls stress rises with pressure, and $\mu$ rises with increasing load.