The equilibrium dynamics of the spin-$\frac{1}{2}$ XX chain is reexamined within a recently developed formalism based on the quantum transfer matrix and a thermal form-factor expansion. The transversal correlation function is evaluated in real time and space. The high-accuracy calculation reproduces several exact results in limiting cases as well as the well-known asymptotic formulas obtained by the matrix Riemann-Hilbert approach. Furthermore, comparisons to numerical data based on a direct evaluation of the Pfaffian as well as to asymptotic formulas obtained within nonlinear Luttinger liquid theory are presented.

A nonhelical spin texture was observed in the centrosymmetric superconductor $\beta \text{−}{\mathrm{PdBi}}_2$ using an imaging-type spin- and angle-resolved spectroscopy. The observed surface and bulk states near the Fermi energy are found to be spin polarized with nonhelical textures. First-principles calculations and effective models show that the inter- and intralayer hoppings account for the complex spin texture and thus play a significant role in the spatial distribution of the layer-locked spin states. Our work not only provides insights into the local spin states in inversion-symmetric systems but also paves the way to the identification of the nature of superconducting pairing in the presence of a complex spin texture in $\beta \text{−}{\mathrm{PdBi}}_2$ and the related centrosymmetric superconductors.

We experimentally demonstrate gold microdisc structures that produce confined Tamm plasmons (CTPs)— interface modes between a metal layer and a distributed Bragg reflector— resonant around 1.3 μm. Quantum dots grown within the structures show an order of magnitude increase in the photoluminescence emitted at room temperature. Varying the disc diameter, we show spectral tuning of the resonance and measure the dispersion relation as evidence of mode confinement. The simplicity of fabrication and tuneability of these structures make CTPs an ideal platform for making scalable telecom devices based on quantum dots.

The valley and spin polarizations associated with electronic transport in quantum dots of the large-gap topological insulator (TI) monolayer bismuthene on SiC are investigated in the linear response regime using a minimal tight-binding model that accurately describes the low-energy electronic band structure of this TI. It is found that for zigzag edges the electronic edge states are strongly valley polarized if the Fermi energy lies in the bulk energy band gap. We predict the edge-state valley polarizations to switch between valleys $K$ and $K^{\prime }$ as the Fermi energy varies from the top of the valence band to the bottom of the conduction band or if the direction of electric current through the dot is reversed. If the electrostatic potential in the dot is nonuniform, we predict that the valley polarization of an electron can reverse as it travels through the dot. The valley polarization reversal is due to the zigzag edge-state dispersion crossing the center of the Brillouin zone that separates valleys $K$ and $K^{\prime }$ and is therefore predicted to be a general phenomenon. Although the spin polarization within the edge states is ferromagnetic, as expected for spin Hall devices, our calculations reveal the out-of-plane component of the spin polarization of the bulk valence band scattering states to be antiferromagnetic, and the direction of the out-of-plane component of the Neel vector to depend on whether the electronic accumulation belongs primarily to valley $K$ or $K^{\prime }$.

We explore the use of cylindrical metasurfaces in providing several illusion mechanisms including scattering cancellation and creating fictitious line sources. We present the general synthesis approach that leads to such phenomena by modeling the metasurface with effective polarizability tensors and by applying boundary conditions to connect the tangential components of the desired fields to the required surface polarization current densities that generate such fields. We then use these required surface polarizations to obtain the effective polarizabilities for the synthesis of the metasurface. We demonstrate the use of this general method for the synthesis of metasurfaces that lead to scattering cancellation and illusion effects and discuss practical scenarios by using loaded dipole antennas to realize the discretized polarization current densities. This study is the first fundamental step that may lead to interesting electromagnetic applications, like stealth technology, antenna synthesis, wireless power transfer, sensors, cylindrical absorbers, etc.

Hydrogen at high temperature and pressure undergoes a phase transition from a liquid molecular phase to a conductive atomic state, or liquid metallic hydrogen, sometimes referred to as the plasma phase transition (PPT). The PPT phase line was observed in a recent experiment studying laser-pulse heated hydrogen in a diamond anvil cell in the pressure range $\sim 100-170$ GPa for temperatures up to $\sim 2000\mathrm{K}$. The experimental signatures of the transition are (i) a negative pressure-temperature slope, (ii) a plateau in the heating curve, assumed to be related to the latent heat of transformation, and (iii) an abrupt increase in the reflectance of the sample. We present a finite-element simulation that accurately takes into account the position and time dependence of the heat deposited by the laser pulse. We calculate the heating curves and the sample reflectance and transmittance. This simulation confirms that the observed plateaus are related to the phase transition; however, we find that large values of latent heat are needed and may indicate that dynamics at the transition are more complex than considered in current models. Finally, experiments are proposed that can distinguish between a change in optical properties due to a transition to a metallic state or due to closure of the band gap in molecular hydrogen.

We present a scattering theory of transport through noncollinear disordered magnetic insulators. For concreteness, we study and compare the random field model (RFM) and the random anisotropy model (RAM). The RFM and RAM are used to model random spin disorder systems and amorphous materials, respectively. We utilize the Landauer-Büttiker formalism to compute the transmission probability and spin conductance of one-dimensional disordered spin chains. The RFM and the RAM both exhibit Anderson localization, which means that the transmission probability and spin conductance decay exponentially with the system length. We define two localization lengths based on the transmission probability and the spin conductance, respectively. Next, we numerically determine the relationship between the localization lengths and the strength of the disorder. In the limit of weak disorder, we find that the localization lengths obey power laws and determine the critical exponents. Our results are expressed via the universal exchange length and are therefore expected to be general.

We report a comparative analysis and theoretical description of the superconducting properties of two spin-valve-valve structures containing the Heusler alloy ${\mathrm{Co}}_2{\mathrm{Cr}}_{1-x}{\mathrm{Fe}}_x{\mathrm{Al}}_y$ as one of two ferromagnetic (F1 or F2) layers of the F1/F2/S structure, where S stands for the superconducting Pb layer. In our experiments, we used the Heusler alloy layer in two roles: as a weak ferromagnet on the place of the F2 layer and as a half-metal on the place of the F1 layer. In the first case, we obtained a large ordinary superconducting spin-valve effect $\mathrm{\Delta }{T}_c$ assisted by the triplet superconducting spin-valve effect $\mathrm{\Delta }{T}_c^{\text{trip}}$. In the second case, we observed a giant magnitude of $\mathrm{\Delta }{T}_c^{\text{trip}}$ reaching 0.5 K. An underlying theory based on the solution of the Usadel equations using Kupriyanov-Lukichev boundary conditions with arbitrary material parameters for all layers and arbitrary boundary parameters for all interfaces is presented in the Appendix. We find a good agreement between our experimental data and theoretical results.

We report low-temperature muon spin relaxation/rotation ($\mu \mathrm{SR}$) measurements on single crystals of the actinide superconductor ${\mathrm{UTe}}_2$. Below 5 K we observe a continuous slowing down of magnetic fluctuations that persists through the superconducting transition temperature ($T_c=1.6$ K), but we find no evidence of long-range or local magnetic order down to 0.025 K. The temperature dependence of the dynamic relaxation rate down to 0.4 K agrees with the self-consistent renormalization theory of spin fluctuations for a three-dimensional weak itinerant ferromagnetic metal. Our $\mu \mathrm{SR}$ measurements also indicate that the superconductivity coexists with the magnetic fluctuations.

We address what seemed to be a contradiction between the lanthanide series ${\mathrm{REBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_y$ (RE123) and the charge-compensated series $\left({\mathrm{Ca}}_x{\mathrm{La}}_{1-x}\right)\left({\mathrm{Ba}}_{1.75-x}{\mathrm{La}}_{0.25+x}\right){\mathrm{Cu}}_3{\mathrm{O}}_y$ (CLBLCO) regarding the superexchange ($J$) dependence of the maximum superconductivity (SC) critical temperature $T_c^{\max }\left(J\right)$; RE and $x$ are implicit variables. This is done by measuring the Néel temperature and the temperature dependence of the magnetic order parameter for RE = Nd, Sm, Eu, Gd, Dy, Yb, Y, and for $\mathrm{Y}\left(\mathrm{BaSr}\right){\mathrm{Cu}}_3{\mathrm{O}}_y$, at various very light dopings. The doping is determined by thermopower and the magnetic properties by muon spin rotation. We find that the normalized-temperature dependence of the order parameter is identical for all RE123 in the undoped limit (with the exception of Gd123) implying identical out-of-plane magnetic coupling. The extrapolation of $T_N$ to zero doping suggests that, despite the variations in ionic radii, $J$ varies too weakly in this system to test the relation between SC and magnetism. This stands in contrast to CLBLCO, where both $T_c^{\max }$ and $T_N^{\max }$ vary considerably in the undoped limit and a positive correlation between the two quantities was observed.

Recent experimental progress in the fields of cold quantum gases and ultrafast optical spectroscopy of quantum materials allows us to controllably induce and probe nonadiabatic dynamics of superconductors and superfluids. The time evolution of the gap function before relaxation with the lattice is determined by the superposition of coherently evolving individual Cooper pairs within the manifold of the Bardeen-Cooper-Schrieffer (BCS) wave function. While dynamics following an abrupt quench of the pairing interaction strength in the single-band BCS model has been exactly solved due to the integrability of the model, the dynamics of postquench multiband superconductors remain under scrutiny. Here, we develop a generalization of the Volkov-Kogan Laplace-space perturbative method that allows us to determine the nonadiabatic gap dynamics of two-band fully gapped superconductors for a wide range of quench amplitudes. Our approach expands the long-time dynamics around the steady-state asymptotic value of the gap, which is self-consistently determined, rather than around the equilibrium value of the gap. We explicitly demonstrate that this method recovers the exact solution of the long-time gap dynamics in the single-band case and perfectly agrees with a numerical solution of the two-band model. We discover that dephasing of Cooper pairs from different bands leads to faster collisionless relaxation of the gap oscillation with a power law of $t^{-3/2}$ instead of the well-known $t^{-1/2}$ behavior found in the single-band case. Furthermore, the gap oscillations display beating patterns arising from the existence of two different asymptotic gap values. Our results have important implications to a variety of two-band superconductors driven out of equilibrium, such as iron-based superconductors, ${\mathrm{MgB}}_2$, and ${\mathrm{SrTiO}}_3$.

Quasiparticles represent an intrinsic source of perturbation for superconducting qubits, leading to both dissipation of the qubit energy and dephasing. Recently, it has been shown that normal-metal traps may efficiently reduce the quasiparticle population and improve the qubit lifetime, provided the trap surpasses a certain characteristic size. Moreover, while the trap itself introduces new relaxation mechanisms, they are not expected to harm state-of-the-art transmon qubits under the condition that the traps are not placed too close to extremal positions where electric fields are high. Here we study a different type of trap, realized through gap engineering. We find that gap-engineered traps relax the remaining constraints imposed on normal metal traps. First, the characteristic trap size, above which the trap is efficient, is reduced with respect to normal metal traps, such that here, strong traps are possible in smaller devices. Second, the losses caused by the trap are now greatly reduced, providing more flexibility in trap placement. The latter point is of particular importance, since for efficient protection from quasiparticles, the traps ideally should be placed close to the active parts of the qubit device, where electric fields are typically high.

The $S$-matrix invariant is known to be complete for translation invariant topological stabilizer models in two spatial dimensions, as such models are phase equivalent to some number of copies of toric code. In three dimensions, much less is understood about translation invariant topological stabilizer models due to the existence of fracton topological order. Here we introduce bulk commutation quantities inspired by the 2D $S$-matrix invariant that can be employed to coarsely sort 3D topological stabilizer models into qualitatively distinct types of phases: topological quantum field theories, foliated or fractal type-I models with rigid string operators, or type-II models with no string operators.

One salient feature of systems with moiré superlattice is that the Chern number of “minibands” originating from each valley of the original graphene Brillouin zone becomes a well-defined quantized number because the miniband from each valley can be isolated from the rest of the spectrum due to the moiré potential. Then a moiré system with a well-defined valley Chern number can become a nonchiral topological insulator with $\mathrm{U}\left(1\right)\times Z_3$ symmetry and a $\mathbb{Z}$ classification at the free fermion level. Here we demonstrate that the strongly interacting nature of the moiré system reduces the classification of the valley Chern insulator from $\mathbb{Z}$ to ${\mathbb{Z}}_3$, and it is topologically equivalent to a bosonic symmetry-protected topological state made of local boson operators. We also demonstrate that even if the system becomes a superconductor when doped away from the valley Chern insulator, the valley Chern insulator still leaves a topological imprint as the localized Majorana fermion zero mode in certain geometric configuration.

The realization of the unusual properties of two-dimensional (2D) materials requires the formation of large domains of single-layer thickness, extending over the mesoscale. It is found that the formation of uniform graphene on SiC, contrary to textbook diffraction, is signaled by a strong bell-shaped component (BSC) around the (00) and G(10) spots (but not around the substrate spots). The BCS is also seen on graphene grown on metals, because a single uniform graphene layer can be also grown with large lateral size. It is only seen by electron diffraction but not with x-ray or He scattering. Although the origin of such an intriguing result is unclear, its presence in the earlier literature (but never mentioned) points to its robustness and significance. A likely mechanism relates to the the spatial confinement of the graphene electrons, within a single layer. This leads to large spread in their wave vector which is transferred by electron-electron interactions to the elastically scattered electrons to generate the BSC.

Investigation of strongly interacting, nonlinear quantum field theories (QFTs) remains one of the outstanding challenges of modern physics. Here, we describe analog quantum simulators for nonlinear QFTs using mesoscopic superconducting circuit lattices. Using the Josephson effect as the source of nonlinear interaction, we investigate generalizations of the quantum sine-Gordon model. In particular, we consider a two-field generalization, the double sine-Gordon model. In contrast to the sine-Gordon model, this model can be purely quantum integrable, when it does not admit a semiclassical description—a property that is generic to many multifield QFTs. The primary goal of this work is to investigate different thermodynamic properties of the double sine-Gordon model and propose experiments that can capture its subtle quantum integrability. First, we analytically compute the mass spectrum and the ground-state energy in the presence of an external “magnetic” field using Bethe ansatz and conformal perturbation theory. Second, we calculate the thermodynamic Bethe ansatz equations for the model and analyze its finite temperature properties. Third, we propose experiments to verify the theoretical predictions.

A periodic network of connected rhombi, mimicking a spintronic device, is shown to exhibit an intriguing spin-selective extreme localization, when submerged in a uniform out-of plane electric field. The topological Aharonov-Casher phase acquired by a traveling spin is seen to induce a complete caging, triggered at a special strength of the spin-orbit coupling, for half-odd integer spins $s\ge n\hslash /2$, with $n$ odd, sparing the integer spins. The observation finds exciting experimental parallels in recent literature on caged, extreme localized modes in analogous photonic lattices. Our results are exact.

Valley-contrasting Berry curvature and orbital magnetic moment have led to highly selective circular polarization of direct excitons at the K valleys in transition-metal dichalcogenides. In addition to K valleys, Q valleys, another critical point in the conduction band, also possess well-defined but distinct magnetic moment. Being akin to the direct excitons at K valleys, indirect excitons associated with Q (K) valleys in the conduction (valence) band could allow circular polarization in principle. Here, we report an experimental observation of the circular polarization of indirect Q-K transitions in noncentrosymmetric bilayer $3R-\mathrm{W}{\mathrm{S}}_2$. In stark contrast to the circular polarization of direct excitons which depolarizes with increasing lattice temperature, the circular polarization of indirect Q-K excitons is extremely robust and independent on the temperature. Such robust circular polarization can be understood as follows: the spin-orbit coupling in the Q valley is much stronger than that in the K point of the conduction band, significantly suppressing the temperature induced valley depolarization. Our results open up opportunities for exotic valleytronics and quantum information processing applications.

We report an observation of metamagnetism in the $5f$-electron heavy-fermion antiferromagnets ${\mathrm{UNi}}_2{\mathrm{Al}}_3$ at 78 T and $\mathrm{U}{\left({\mathrm{Pd}}_{1-x}{\mathrm{Ni}}_x\right)}_2{\mathrm{Al}}_3$ at 30 T (42 T) for $x=0.5$ ($x=0.75$) using single-crystalline samples. The magnetization curves for $\mathrm{U}{\left({\mathrm{Pd}}_{1-x}{\mathrm{Ni}}_x\right)}_2{\mathrm{Al}}_3$ ($x=0.5$ and 0.75) show a sharp increase at the metamagnetic transition (MMT) field $B_{\mathrm{m}}$, strongly indicating their first-order nature. The obtained $B-T$ phase diagram for $x=0.5$ suggests the possible presence of the tricritical point, while the MMT still occurs at high temperatures above $T_{\mathrm{N}}$ from paramagnetic to polarized-paramagnetic states as a crossover. The results indicate that the antiferromagnetic order is closely connected to the MMT in $\mathrm{U}{\left({\mathrm{Pd}}_{1-x}{\mathrm{Ni}}_x\right)}_2{\mathrm{Al}}_3$, but the metamagnetic behavior cannot be explained by the conventional spin flip of antiferromagnetic ordered moments. Our data for $\mathrm{U}{\left({\mathrm{Pd}}_{1-x}{\mathrm{Ni}}_x\right)}_2{\mathrm{Al}}_3$ have revealed an empirical linear relation of $B_{\mathrm{m}}\left(x\right)\propto T_{{\chi }_{\max }}\left(x\right)$ and a scaling behavior in magnetic susceptibility of $\chi (T/T_{\chi \max })/{\chi }_{\max }$, where $T_{\chi \max }$ is the temperature of the susceptibility maximum [${\chi }_{\max }\equiv \chi \left(T_{\chi \max }\right)$]. Importantly, the critical value of the magnetic polarization at the MMT $M_{\mathrm{cr}}$ is roughly unchanged, although the value of susceptibility is notably suppressed with increasing $x$.

The mixed-valent homometallic ludwigite $\left({\mathrm{Co}}_2^{2+}{\mathrm{Co}}^{3+}\right){\mathrm{O}}_2{\mathrm{BO}}_3$ is investigated above the ferrimagnetic ordering temperature $T_C=43\mathrm{K}$ through structural, thermal, magnetic, electric, and spectroscopic probes. X-ray absorption at the Co $L_{2,3}$ edges is consistent with the coexistence of ${\mathrm{Co}}^{2+}$ and ${\mathrm{Co}}^{3+}$ ions, as expected by the sample stoichiometry. Magnetic susceptibility shows a relatively large net paramagnetic moment per Co ion above room temperature, $p=4.87$, indicating that the ${\mathrm{Co}}^{3+}$ ions are not in a pure low-spin configuration at high temperatures, also showing a non-Curie-Weiss behavior below 300 K. Electrical conductivity and differential scanning calorimetry measurements on single crystals indicate two phase transitions at $\sim 475$ and $\sim 495\mathrm{K}$. X-ray powder diffraction shows substantial lattice parameter anomalies below 500 K. These results indicate phase transitions associated with changes in the Co oxidation state in each of its four crystallographic sites. Such transitions are possibly dictated by a competition between (i) an ordered ground state with all ${\mathrm{Co}}^{3+}$ ions occupying the same crystallographic site and (ii) either partially or totally charge-disordered states that are favored at high temperatures due to their higher entropy.