The proximity effect, which arises at the interface between two fermionic superfluids with different critical temperatures, is examined with a nonlocal (integral) equation whose kernel contains information about the size of Cooper pairs that leak across the interface. This integral approach avoids reference to the boundary conditions at the interface that would be required with a differential approach. The temperature dependence of the pair penetration depth on the normal side of the interface is determined over a wide temperature range also varying the interparticle coupling along the BCS side of the BCS-BEC crossover independently on both sides of the interface. In this way, the size of Cooper pairs evolves from being much larger than (BCS limit) the interparticle distance to being comparable with (unitarity limit, halfway between the BCS and BEC limits) the interparticle distance. Conditions are then found for which the proximity effect is optimized in terms of the extension of the pair penetration depth.

We predict that superconductivity in mesoscopic thin films can be stabilized in high magnetic fields if the superconductor is driven out of equilibrium by a DC voltage bias. For realistic material parameters and temperatures, we show that superconductivity is restored in fields many times larger than the Chandrasekhar-Clogston limit. After motivating the effect analytically, we perform rigorous numerical calculations to corroborate these findings and present concrete experimental signatures. On the technical side, we also introduce a formulation of the nonequilibrium kinetic equations that both generalizes and simplifies previous results.

We study the quasiparticle spectrum in an underscreened Kondo-lattice (KL) model that involves a single spin degenerate conduction band and two crystalline-electric-field (CEF) split Kramers doublets coupled by both orbital-diagonal and nondiagonal exchange interactions. We find the three quasiparticle bands of the model using a constrained fermionic mean field approach. While two bands are similar to the one-orbital model, a new genuinely heavy band inside the main hybridization gap appears in the quasiquartet model. Its dispersion is due to effective hybridization with conduction states, but the bandwidth is controlled by the size of the CEF splitting. Furthermore, several new indirect and direct hybridization gaps may be identified. By solving the self-consistency equation, we calculate the CEF splitting and exchange dependence of effective Kondo low-energy scale, hybridization gaps, and bandwidths. We also derive the quasiparticle spectral densities and their partial orbital contributions. We suggest that the two-orbital KL model can exhibit mixed CEF/Kondo excitonic magnetism.

Ultraviolet (UV)/visible/infrared (IR) transmission spectroscopy, Hall effect measurements and synchrotron x-ray photoelectron spectroscopy were used to determine the limits of Sb doping on the electrical and optical properties of single crystal $\mathrm{Sn}{\mathrm{O}}_2$ (101) films grown by molecular beam epitaxy. The resistivity (ρ) and carrier density ($n$) of these $\text{Sb:Sn}{\mathrm{O}}_2$ films could be varied by more than seven orders of magnitude, covering the insulating, semiconducting, and semimetallic regimes, by adjusting the Sb fraction in the incident metal flux from 0 to 3.0 at. %, corresponding to a Sb incorporation of up to $8\times {10}^{20}\mathrm{atoms}/\mathrm{c}{\mathrm{m}}^3$. A maximum conductivity limit of $\rho \approx 6\times {10}^{-4}\mathrm{\Omega }\mathrm{cm}$ and $n\approx 4\times {10}^{20}\mathrm{c}{\mathrm{m}}^{-3}$ was determined at approximately $3\times {10}^{20}\mathrm{Sb}\mathrm{atoms}/\mathrm{c}{\mathrm{m}}^3$, after which an increase in ρ (and decrease in $n$) with increasing Sb flux was observed, most likely due to charge compensation arising from the preferential formation of $\mathrm{S}{\mathrm{b}}^{3+}$ acceptors. The limits for the coexistence of transparency and electrical conductivity were found to be governed by a Burstein-Moss shift of the optical gap ($E_{\mathrm{opt}}$) in the UV spectrum and a much larger blue shift of the free-carrier adsorption edge in the IR region due to an increasing plasma frequency (${\omega }_{\mathrm{p}}$), with both effects scaling with the carrier concentration of the $\mathrm{Sb}:\mathrm{Sn}{\mathrm{O}}_2$ films. Despite these effects, the transparency window in the visible region was almost unaffected, with more than 90% transmission in the 350–1500 nm range for 150-nm-thick $\mathrm{Sb}:\mathrm{Sn}{\mathrm{O}}_2$ films at the conductivity limit. The density of states reduced effective mass and the conductivity effective electron mass were determined from the variation of ${\omega }_{\mathrm{p}}$ and $E_{\mathrm{opt}}$ with n, resulting in values of $(0.46\pm 0.12)m_e$ and $(0.31\pm 0.10)m_e$, with the difference consistent with a shrinkage of the fundamental gap due to many-body interactions. Finally, the correlation between the changes in $E_{\mathrm{opt}}$ and $n$ with increasing Sb incorporation was used to provide an estimate for the charge neutrality level of $\mathrm{Sn}{\mathrm{O}}_2$ of +0.49 eV above the conduction band minimum.

Graphene/${\mathrm{MoS}}_2$ van der Waals (vdW) heterostructures have promising technological applications due to their unique properties and functionalities. Many experimental and theoretical research groups across the globe have made outstanding contributions to benchmark the properties of graphene/${\mathrm{MoS}}_2$ heterostructures. Even though some research groups have modeled the graphene/${\mathrm{MoS}}_2$ heterostructures using first-principles calculations, there exist several discrepancies in the results from different theoretical research groups and the experimental findings. In the present work, we revisit this problem by means of first-principles calculations and address the existing discrepancies about the interlayer spacing between graphene and ${\mathrm{MoS}}_2$ monolayers in graphene/${\mathrm{MoS}}_2$ heterostructures and about the location of Dirac points near the Fermi level. We further investigate the electronic, mechanical, and vibrational properties of the optimized graphene/${\mathrm{MoS}}_2$ heterostructures created using $5\times 5/4\times 4$ and $4\times 4/3\times 3$ supercell geometries having different magnitudes of lattice mismatch. The effect of the varying interlayer spacing on the electronic properties of heterostructures is discussed. Our phonon calculations reveal that the interlayer shear and breathing phonon modes, which are very sensitive to the weak vdW interactions, play a vital role in describing the thermal properties of the studied systems. The thermodynamic and elastic properties of heterostructures are further discussed. A systematic comparison between our results and the results reported from other research groups is presented.

We present both the experimental and theoretical investigations of a nontrivial electron Landau-level shift in magnetic field in wide ($\sim 20$ nm) HgTe quantum wells: Landau levels split under magnetic fields but become degenerate again when magnetic field increases. We reproduced this behavior qualitatively within an isotropic six-band Kane model; then using semiclassical calculations, we showed this behavior is due to the mixing of the conduction band with a total spin of 3/2 with the next well subband with a spin of 1/2, which reduces the average vertical spin from 3/2 to around 1. This change in the average spin changes the Berry phase, explaining the evolution of Landau levels under magnetic field.

The Su-Schrieffer-Heeger (SSH) model is likely the simplest one-dimensional concept to study nontrivial topological phases and topological excitations. Originally developed to explain the electric conductivity of polyacetylene, it has become a platform for the study of topological effects in electronics, photonics, and ultracold atomic systems. Here, we propose an experimentally feasible implementation of the SSH model based on coupled one-dimensional acoustic nanoresonators working in the gigahertz-terahertz range. In this simulator it is possible to implement different signs of the nearest-neighbor interaction terms, showing full tunability of all parameters in the SSH model. Based on this concept we construct topological transition points generating nanophononic edge and interface states and propose an easy experimental scheme to directly probe their spatial complex amplitude distribution by well-established optical pump-probe techniques.

Searching for exotic transport properties in new topological states of matter is an active topic. One of the most fascinating transport phenomena in recently discovered Weyl semimetals (WSMs) is the negative longitudinal magnetoresistivity (nLMR) in the presence of a magnetic field $B$ parallel to an electric field $E$ , which is arguably attributed to the chiral anomaly. Another predicted key effect closely related to the chiral anomaly and the nonzero Berry curvatures in WSMs is the planar Hall effect (PHE), which has not been identified so far. Here we carry out the planar Hall measurements on ${\mathrm{Cd}}_3{\mathrm{As}}_2$ nanoplates, and find that, accompanied by the large nLMR, a PHE with nonzero transverse voltage can be developed while tilting the in-plane magnetic field $\mathit{B}$ away from the electric field $\mathit{E}$. Further experiments reveal that both the PHE and the nLMR can be suppressed synchronously by increasing the temperature, but still visible at room temperature, indicating the same origin of these two effects. Our experiments indicate that PHE may provide another key transport evidence for topologically nontrivial electronic structures in topological materials.

We propose a bulk topological invariant for one-dimensional Floquet systems with chiral symmetry which quantifies the particle transport on each sublattice during the evolution. This chiral flow is physically motivated, locally computable, and improves on existing topological invariants by being applicable to systems with disorder. We derive a bulk-edge correspondence which relates the chiral flow to the number of protected dynamical edge modes present on a boundary at the end of the evolution. In the process, we introduce two real-space edge invariants which classify the dynamical topological boundary behavior at various points during the evolution. Our results provide an explicit bulk-boundary correspondence for Floquet systems in this symmetry class.

We propose a lattice model for strongly interacting electrons with the potential to explain the main phenomenology of the strange metal phase in the cuprate high-temperature superconductors. Our model is motivated by the recently developed “tetrahedron” rank-3 tensor model that mimics much of the physics of the better-known Sachdev-Ye-Kitaev (SYK) model. Our electron model has the following advantageous properties: (1) it needs only one orbital per site on the square lattice. (2) It does not require any quenched random interaction. (3) It has local interactions and respects all the symmetries of the system. (4) The soluble limit of this model has a longitudinal dc resistivity that scales linearly with temperature within a finite temperature window. (5) Again, the soluble limit of this model has a fermion pairing instability in the infrared, which can lead to either superconductivity or a “pseudogap” phase. The linear-$T$ longitudinal resistivity and the pairing instability originate from the generic scaling feature of the SYK model and the tetrahedron tensor model.

We discuss possible approaches to the problem of the ${\mathrm{URu}}_2{\mathrm{Si}}_2$ “hidden order” (HO) which remains unsolved after tremendous research efforts. Suppose there is no spatial symmetry breaking at the HO transition temperature and solely the time-reversal symmetry breaking emerges owing to some sort of magnetic order. As a result of its $4/mmm$ symmetry, each uranium atom is a three-dimensional magnetic vortex; its intra-atomic magnetization $\mathbf{M}\left(\mathbf{r}\right)$ is intrinsically noncollinear, so that its dipole, quadrupole, and toroidal moments vanish, thus making the vortex “hidden.” The first nonzero magnetic multipole of the uranium vortex is the toroidal quadrupole. In the unit cell, two uranium vortices can have either the same or opposite signs of $\mathbf{M}\left(\mathbf{r}\right)$; this corresponds to either ferrovortex or antiferrovortex structures with $I4/mmm$ or $P_{I}4/mmm$ magnetic space groups, respectively. Our first-principles calculations suggest that the vortex magnetic order of ${\mathrm{URu}}_2{\mathrm{Si}}_2$ is rather strong: the total absolute magnetization $\left|\mathbf{M}\right(\mathbf{r}\left)\right|$ is about $0.9{\mu }_B$ per U atom, detectable by neutron scattering in spite of the unusual form factor. The ferrovortex and antiferrovortex phases have almost the same energy and they are energetically favorable compared to the nonmagnetic phase.

Feedback control is a powerful tool to stabilize systems for which precision control is difficult to impose directly, such as the environment of an open quantum system. Reduction of noise from the environment is a major challenge on the road to harnessing delicate quantum effects such as superposition and entanglement. In particular, spin states of defects and quantum dots in semiconductors display promising coherence properties for future applications, often being limited by disturbance from disordered nuclear spins in their environment. Here we show how optical coherent population trapping (CPT) of the spin of localized semiconductor electrons stabilizes the surrounding nuclear spins via feedback control. We find distinct control regimes for different signs of laser detuning and examine the transition from an unpolarized, narrowed state to a polarized state possessing a bistability. The narrowing of the state protects the electron spin against dephasing and yields self-improving CPT. Our analysis is relevant for a variety of solid-state systems where hyperfine-induced dephasing is a limitation for using electron spin coherence.

A theory is presented for surface polaritons that may propagate along the electrostatically doped phosphorene layer (black-phosphorus monolayer) surrounded by two insulating media of different dielectric constants. It is shown that the phosphorene layer in such a structure supports three types of strongly localized surface polariton modes: the transverse magnetic (TM), the transverse electric (TE), and the hybrid TE-TM. Both the TM and TE modes can propagate only along the phosphorene's “armchair” and “zigzag” crystallographic axes, while the in-plane wave vector $\mathbf{q}$ of the hybrid TE-TM modes makes a finite angle with respect to both the abovementioned axes. It is found that there exists such a critical value $q_C$ of the polariton mode wave number $q$, peculiar to each type of mode, that the mode ceases from propagating at $q>q_C$. The critical value of polariton frequency, corresponding to this critical wave number, lies either below the phosphorene's interband absorption threshold or slightly above it, depending on the type of the polariton mode, its propagation direction, and the doping level of the phosphorene. Our findings suggest that the phosphorene, in which the charge carrier density is tuned by an external gate voltage, could be promising for polariton waveguiding applications in future planar nanophotonic devices operating at near-infrared wavelengths.

Zirconium-hydrogen compounds under pressure are investigated by using ab initio density functional calculations. In addition to the high-pressure phases so far predicted, other new metallic phases of ${\mathrm{ZrH}}_3$, ${\mathrm{ZrH}}_4$, and ${\mathrm{ZrH}}_6$ are found above about 170, 110, and 150 GPa, respectively. The estimated superconducting transition temperatures of the ${\mathrm{ZrH}}_4$ and ${\mathrm{ZrH}}_6$ phases are as follows: 78 K in the $Fddd{\mathrm{ZrH}}_4$ (140 GPa), 47 K in the $I4/mmm{\mathrm{ZrH}}_4$ (230 GPa), 55 K in the $Cmc{2}_1{\mathrm{ZrH}}_6$ (160 GPa), 153 K in the $P{2}_1/c{\mathrm{ZrH}}_6$ (295 GPa), and 114 K in the $I4/mmm{\mathrm{ZrH}}_6$ (295 GPa). The $I4/mmm$ structure of ${\mathrm{ZrH}}_4$ is actually the structure similarly predicted for ${\mathrm{MgH}}_4$ and ${\mathrm{ScH}}_4$. Also, the $I4/mmm$ structure of ${\mathrm{ZrH}}_6$ is a close modification of the $Im\overline{3}m$ structure proposed for ${\mathrm{MgH}}_6$ and ${\mathrm{ScH}}_6$. The structural resemblances observed in these magnesium, scandium, and zirconium hydrides suggest that diagonally adjacent elements tend to have similar chemical trends, namely, diagonal relationships, which are expected to serve as a helpful guide to searching for new metallic hydrides.

The circular dichroic charge-magnetic interference effect in resonant x-ray diffraction (RXD) is reported for antiferromagnetic (AFM) materials. The polarization dependence of the scattering cross-section was formulated, and the conditions that the interference gives rise to circular dichroic RXD signals for AFM materials were clarified. By using the interference effect, a pair of AFM domains can be distinguished in certain conditions. We applied this effect to observe collinear chirality AFM domains in a high-temperature magnetoelectric hexaferrite, $\mathrm{B}{\mathrm{a}}_{1.3}\mathrm{S}{\mathrm{r}}_{0.7}\mathrm{CoZnF}{\mathrm{e}}_{11}\mathrm{Al}{\mathrm{O}}_{22}$. Despite its collinear spin arrangement and absence of macroscopic magnetization, the domains are clearly visualized by a circularly polarized x-ray beam. The present result shows that various types of AFM domains including collinear ones can be examined by utilizing the charge-magnetic interference effect, together with the well-established circular dichroism on RXD emerged from pure magnetic scattering.

 ${\mathrm{Sr}}_2{\mathrm{Cr}}_3{\mathrm{As}}_2{\mathrm{O}}_2$ is composed of alternating square-lattice ${\mathrm{CrO}}_2$ and ${\mathrm{Cr}}_2{\mathrm{As}}_2$ stacking layers, where ${\mathrm{CrO}}_2$ is isostructural to the ${\mathrm{CuO}}_2$ building-block of cuprate high-$T_c$ superconductors and ${\mathrm{Cr}}_2{\mathrm{As}}_2$ to ${\mathrm{Fe}}_2{\mathrm{As}}_2$ of Fe-based superconductors. Current interest in this material is raised by theoretic prediction of possible superconductivity. In this neutron powder diffraction study, we discovered that magnetic moments of Cr(II) ions in the ${\mathrm{Cr}}_2{\mathrm{As}}_2$ sublattice develop a C-type antiferromagnetic structure below 590 K, and the moments of Cr(I) in the ${\mathrm{CrO}}_2$ sublattice form the ${\mathrm{La}}_2{\mathrm{CuO}}_4$-like antiferromagnetic order below 291 K. The staggered magnetic moment $2.19\left(4\right){\mu }_B$/Cr(II) in the more itinerant ${\mathrm{Cr}}_2{\mathrm{As}}_2$ layer is smaller than $3.10\left(6\right){\mu }_B$/Cr(I) in the more localized ${\mathrm{CrO}}_2$ layer. Different from previous expectations, a spin-flop transition of the Cr(II) magnetic order observed at 291 K indicates a strong coupling between the ${\mathrm{CrO}}_2$ and ${\mathrm{Cr}}_2{\mathrm{As}}_2$ magnetic subsystems.

In this work we present the magnetic structures and spin reorientation (SR) transitions of the mixed orthochromite-orthoferrite perovskites $R\mathrm{F}{\mathrm{e}}_{0.5}\mathrm{C}{\mathrm{r}}_{0.5}{\mathrm{O}}_3$ ($R=\mathrm{Tb}$, Dy, Ho, Er). Magnetization as a function of temperature and external magnetic field as well as neutron powder diffraction measurements were used to characterize the magnetic transitions, including the SR transitions in the transition metal sublattice and the ordering of the rare earth sublattice. The studied compounds order antiferromagnetically below 270 K in a ${\mathbf{G}}_{\mathbf{x}}$ configuration compatible with the ${\mathrm{\Gamma }}_4$ representation. As temperature decreases, all the compounds show a SR transition from ${\mathbf{G}}_{\mathbf{x}}$ (${\mathrm{\Gamma }}_4$) to ${\mathbf{G}}_{\mathbf{z}}$ (${\mathrm{\Gamma }}_2$). This transition occurs in a wide temperature range, where both magnetic configurations coexist. Below this SR, the behavior in each case depends on the rare earth. $\mathrm{HoF}{\mathrm{e}}_{0.5}\mathrm{C}{\mathrm{r}}_{0.5}{\mathrm{O}}_3$ shows $\mathrm{H}{\mathrm{o}}^{3+}$ sublattice ordering at a relatively high temperature (45 K). $\mathrm{DyF}{\mathrm{e}}_{0.5}\mathrm{C}{\mathrm{r}}_{0.5}{\mathrm{O}}_3$ and $\mathrm{ErF}{\mathrm{e}}_{0.5}\mathrm{C}{\mathrm{r}}_{0.5}{\mathrm{O}}_3$ show a second SR transition of the transition metal sublattice, from ${\mathbf{G}}_{\mathbf{z}}$ (${\mathrm{\Gamma }}_2$) to ${\mathbf{G}}_{\mathbf{y}}$ (${\mathrm{\Gamma }}_1$) at low temperatures (15 and 8 K, respectively). Below these temperatures a metamagnetic (MM) transition is observed for these two compounds at an external magnetic field of H ≈ 7 kOe. The fact that this is only observed for compounds showing ${\mathbf{G}}_{\mathbf{z}}$ (${\mathrm{\Gamma }}_2$) to ${\mathbf{G}}_{\mathbf{y}}$ (${\mathrm{\Gamma }}_1$) transition suggests that there is a correlation between the ${\mathbf{G}}_{\mathbf{y}}$ (${\mathrm{\Gamma }}_1$) order and the MM transition. Finally, $\mathrm{TbF}{\mathrm{e}}_{0.5}\mathrm{C}{\mathrm{r}}_{0.5}{\mathrm{O}}_3$ is a peculiar case, since it is the only compound in this family that shows a re-entrant SR to ${\mathbf{G}}_{\mathbf{x}}$ (${\mathrm{\Gamma }}_4$) at very low temperatures. By combining these results with previous reports on $R\mathrm{Fe}{\mathrm{O}}_3$, $R\mathrm{Cr}{\mathrm{O}}_3$, and $R\mathrm{F}{\mathrm{e}}_{0.5}\mathrm{C}{\mathrm{r}}_{0.5}{\mathrm{O}}_3$, we develop a method to qualitatively estimate the SR temperature and the type of transition. We also propose a complete magnetic phase diagram containing the SR transitions and $R^{3+}$ ordering temperatures for all the $R\mathrm{F}{\mathrm{e}}_{0.5}\mathrm{C}{\mathrm{r}}_{0.5}{\mathrm{O}}_3$ ($R=\mathrm{Tb}$, Dy, Ho, Er, Tm, Yb, and Lu) compounds.