By numerically solving the Bogoliubov–de Gennes equations for the single vortex state in a two-band superconductor, we demonstrate that the disparity between the healing lengths of two contributing condensates is strongly affected by the band Fermi velocities, even in the presence of the magnetic field and far beyond the regime of nearly zero Josephson-like coupling between bands. Changing the ratio of the band Fermi velocities alters the temperature dependence of the condensate lengths and significantly shifts parameters of the “length-scales locking” regime at which the two characteristic lengths approach one another.

We study the dynamical process of the vortex tangle development under a spherically symmetric thermal counterflow around a heat source submerged into a bulk superfluid ${}^4\mathrm{He}$. We reveal a peculiar vortex dynamics that is unique to this geometry, which is greatly diverse from the vortex dynamics in a homogeneous counterflow. Two types of heater are considered here, namely, a spherical heater with a solid wall and a pointlike heater. In both cases, a spherical vortex tangle is formed surrounding the heater. The mechanism of vortex tangle development in the vicinity of a solid wall is strongly governed by the Donnelly-Glaberson instability; while, far away from the heater or around a point heater, the mechanism is governed by the dynamics of polarized vortex loops in radial counterflow. The decay process of such localized vortex tangles is also investigated and is compared with that of homogeneous vortex tangles.

Skyrmions hold great promise for low-energy consumption and stable high-density information storage, and stabilization of the skyrmion lattice (SkX) phase at or above room temperature is greatly desired for practical use. The topological Hall effect can be used to identify candidate systems above room temperature, a challenging regime for direct observation by Lorentz electron microscopy. Atomically ordered FeGe thin films are grown epitaxially on Ge(111) substrates with $\sim 4\%$ tensile strain. Magnetic characterization reveals enhancement of the Curie temperature to 350 K due to strain, well above the bulk value of 278 K. A strong topological Hall effect was observed between 10 and 330 K, with a significant increase in magnitude observed at 330 K. The increase in magnitude occurs just below the Curie temperature, similar to the onset of the SkX phase in bulk FeGe. The results suggest that strained FeGe films may host a SkX phase above room temperature when significant tensile strain is applied.

There is intense controversy around the unconventional superconductivity in strontium ruthenate, where the various theoretical and experimental studies suggest diverse and mutually exclusive pairing symmetries. Currently, the investigation is solely focused on only one material, ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, and the field suffers from the lack of comparison targets. Here, employing a density-functional-theory-based analysis, we show that the heterostructure composed of ${\mathrm{SrRuO}}_3$ and ${\mathrm{SrTiO}}_3$ is endowed with all the key characteristics of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, and, in principle, can host superconductivity. Furthermore, we show that competing magnetic phases and associated frustration, naturally affecting the superconducting state, can be tuned by epitaxial strain engineering. This system thus offers an excellent platform for gaining more insight into superconductivity in ruthenates.

We observe proximity-induced superconductivity in in situ prepared heterostructures constructed by topological insulator ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ thin films and high-temperature cuprate superconductors ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$. The superconducting gap maximum is about 7.6 meV on the surface of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ thin films with a thickness of two quintuple layers, and the gap value decreases with an increase in the film thickness. Moreover, the quasiparticle interference data show clear evidence of a twofold symmetric superconducting gap with gap minima along one pair of the principal crystalline axes of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. This gap form is consistent with the ${\mathrm{\Delta }}_{4y}$ notation of the topological superconductivity proposed in such systems. Our results provide fruitful information on the possible topological superconductivity induced by the proximity effect in high-temperature superconducting cuprates.

The incommensurate stacking of multilayered two-dimensional materials is a challenging problem from a theoretical perspective and an intriguing avenue for manipulating their physical properties. Here we present a multiscale model to obtain the mechanical relaxation pattern of twisted trilayer van der Waals (vdW) heterostructures with two independent twist angles, a generally incommensurate system without a supercell description. We adopt the configuration space as a natural description of such incommensurate layered materials, based on the local environment of atomic positions, bypassing the need for commensurate approximations. To obtain the relaxation pattern, we perform energy minimization with respect to the relaxation displacement vectors. We use a continuum model in combination with the generalized stacking fault energy to describe the interlayer coupling, obtained from first-principles calculations based on density functional theory. We show that the relaxation patterns of twisted trilayer graphene and ${\mathrm{WSe}}_2$ are “moiré of moiré,” as a result of the incommensurate coupling two bilayer moiré patterns. We also show that, in contrast to the symmetry-preserving in-plane relaxation in twisted bilayers, trilayer relaxation can break the two fold rotational symmetry about the $xy$ plane when the two twist angles are equal.

Given the challenges in experimental studies of uranium, the heaviest naturally occurring metal, we present first-principles calculation for the spin-dependent transport. Showing the largest atomic spin-orbit coupling we explore the ability of various crystal phases to maximize the charge-to-spin conversion using a fully relativistic Korringa-Kohn-Rostoker Green's function method. The transport theory is based on a semiclassical description where intrinsic and extrinsic, skew scattering, contributions can be separated easily. In addition to the various crystal phases we analyze the effect of substitutional impurities for $\gamma$, hcp, as well as the $\alpha$ phase. We predict a very high, ${10}^4{\left(\mathrm{\Omega }\text{cm}\right)}^{-1}$, spin Hall conductivity for the metastable hcp-U phase, a giant value five times larger than for the conventional spin Hall material Pt. We estimated an efficiency of charge-to-spin current conversion of up to 30%. The spin diffusion length, a crucial parameter in any application, is predicted to be in the range from 3 to 6.5 nm, compatible with other charge-to-spin conversion materials. Relating our work to the sparse experimental results, our calculations suggest a $\gamma$ phase in the thin film rather than the experimentally expected $\alpha$ phase.

The electronic band-structure controlled ultrafast demagnetization mechanism in $\mathrm{C}{\mathrm{o}}_2\mathrm{F}{\mathrm{e}}_x\mathrm{M}{\mathrm{n}}_{1-x}\mathrm{Si}$ Heusler alloy is underpinned by systematic variation of composition. We find the spin-flip scattering rate controlled by spin density of states at the Fermi level is responsible for nonmonotonic variation of ultrafast demagnetization time (${\tau }_{\mathrm{M}}$) with $x$ with a maximum at $x=0.4$. Furthermore, Gilbert damping constant exhibits an inverse relationship with ${\tau }_{\mathrm{M}}$ due to the dominance of interband scattering mechanism. This demonstrates a connection between ultrafast demagnetization and magnetic damping based on Fermi-level position in this Heusler alloy system.

Heavy fermion compounds consisting of two or more inequivalent local moment sites per unit cell have been a promising platform of investigating the interplay between distinct Kondo screenings that is absent in the conventional systems containing only one rare-earth ion per unit cell. We report a remarkable enhancement of the antiferromagnetic ordering tendency in the staggered periodic Anderson model (PAM) with two alternating inequivalent local moments if their hybridization strengths reside separately in the Kondo singlet and antiferromagnetic insulator regime of the phase diagram of homogeneous PAM. Our results uncover the rich physics induced by the interplay of multiple energy scales in the staggered PAM and furthermore imply the ubiquitous existence of the enhancement of physical quantities in general inhomogeneous systems.

On the basis of the “molecular-orbital” representation which describes generic flat-band models, we propose a systematic way to construct a class of flat-band models with finite-range hoppings that have topological natures. In these models, the topological natures are encoded not into the flat band itself but into the dispersive bands touching the flat band. Such a band structure may become a source of exotic phenomena arising from the combination of flat bands, topology, and correlations.

It has been known that encoding Boltzmann weights of a classical spin model in amplitudes of a many-body wave function can provide quantum models whose phase structure is characterized by using classical phase transitions. In particular, such correspondence can lead to finding new quantum phases corresponding to well-known classical phases. Here, we investigate this problem for the Kosterlitz-Thouless (KT) phase in the $d$-state clock model, where we find a corresponding quantum model constructed by applying a local invertible transformation on a $d$-level version of Kitaev's toric code. In particular, we show the ground-state fidelity in such a quantum model is mapped to the heat capacity of the clock model. Accordingly, we identify an extended topological phase transition in our model in the sense that, for $d\ge 5$, a KT-like quantum phase emerges between a $Z_d$ topological phase and a trivial phase. Then, using a mapping to the correlation function in the clock model, we introduce a nonlocal (string) observable for the quantum model which exponentially decays in terms of distance between two end points of the corresponding string in the $Z_d$ topological phase, while it shows a power law behavior in the KT-like phase. Finally, using well-known transition temperatures for the $d$-state clock model, we give evidence to show that while the stability of both the $Z_d$ topological phase and the KT-like phase increases by increasing $d$, the KT-like phase is even more stable than the $Z_d$ topological phase for large $d$.

We calculate the entanglement and the universal boundary entropy (BE) in critical quantum spin chains, such as the transverse field Ising chain and the XXZ chain, with arbitrary direction of the boundary magnetic field (ADBMF). We determine the boundary universality class that an ADBMF induces. In particular, we show that the induced boundary conformal field theory depends on the point on the Bloch sphere where the boundary magnetic field directs. We show that the classification of the directions boils down to the simple fact that the boundary field breaks the bulk symmetry or does not. We present a procedure to estimate the universal BE, based on the finite-size corrections of the entanglement entropy, which apply to ADBMF. To calculate the universal BE in the XXZ chain, we use the density matrix renormalization group. The transverse field XY chain with ADBMF after Jordan-Wigner transformation is not a quadratic free fermion Hamiltonian. We map this model to a quadratic free fermion chain by introducing two extra ancillary spins coupled to the main chain at the boundaries, which makes the problem integrable. The eigenstates of the transverse field XY chain can be obtained by proper projection in the enlarged chain. Using this mapping, we are able to calculate the entanglement entropy of the transverse field XY chain using the usual correlation matrix technique up to relatively large sizes.

We measure the electronic structure of FeSe from within individual orthorhombic domains. Enabled by an angle-resolved photoemission spectroscopy beamline with a highly focused beam spot (nano-ARPES), we identify clear stripelike orthorhombic domains in FeSe with a length scale of approximately 1–5 $\mu \mathrm{m}$. Our photoemission measurements of the Fermi surface and band structure within individual domains reveal a single electron pocket at the Brillouin zone corner. This result provides clear evidence for a one-electron-pocket electronic structure of FeSe, observed without the application of uniaxial strain, and calls for further theoretical insight into this unusual Fermi surface topology. Our results also showcase the potential of nano-ARPES for the study of correlated materials with local domain structures.

In standard doping, adding charge carrier to a compound results in a shift of the Fermi level towards the conduction band for electron doping and towards the valence band for hole doping. We discuss the curious case of antidoping, where the direction of band movements in response to doping is reversed. Specifically, $p$-type antidoping moves the previously occupied bands to the principal conduction band resulting in an increase of band gap energy and reduction of electronic conductivity. We find that this is a generic behavior for a class of materials: early transition and rare-earth metal (e.g., Ti, Ce) oxides where the sum of composition-weighed formal oxidation states is positive; such compounds tend to form the well-known electron-trapped intermediate bands localized on the reduced cation orbitals. What is less known is that doping by a hole annihilates a single trapped electron on a cation. The latter thus becomes electronically inequivalent with respect to the normal cation in the undoped lattice, thus representing a symmetry-breaking effect. We give specific theoretical predictions for target compounds where hole antidoping might be observed experimentally: Magnéli-like phases (i.e., $\mathrm{Ce}{\mathrm{O}}_{2\text{−}x}$ and $\mathrm{Ti}{\mathrm{O}}_{2\text{−}x}$) and ternary compounds (i.e., $\mathrm{B}{\mathrm{a}}_2\mathrm{T}{\mathrm{i}}_6{\mathrm{O}}_{13}$ and $\mathrm{B}{\mathrm{a}}_4\mathrm{T}{\mathrm{i}}_{12}{\mathrm{O}}_{27}$), and note that this unique behavior opens the possibility of unconventional control of materials conductivity by doping.

A three-dimensional (3D) photonic band gap crystal is an ideal tool to completely inhibit the local density of optical states (LDOS) at every position in the crystal throughout the band gap. This notion, however, pertains to ideal infinite crystals, whereas any real crystal device is necessarily finite. This raises the question as to how the LDOS in the gap depends on the position and orientation inside a finite-size crystal. Therefore, we employ rigorous numerical calculations using finite-difference time domain simulations of 3D silicon inverse woodpile crystals filled with air or with toluene, as previously studied in experiments. We find that the LDOS versus position decreases exponentially into the bulk of the crystal. From the dependence on dipole orientation, we infer that the characteristic LDOS decay length ${\ell }_{\rho }$ is mostly related to far-field dipolar radiation effects, whereas the prefactor is mostly related to near-field dipolar effects. The LDOS decay length has a remarkably similar magnitude to the Bragg length for directional transport, which suggests that the LDOS in the crystal is dominated by vacuum states that tunnel from the closest interface toward the position of interest. Our work leads to design rules for applications of 3D photonic band gaps in emission control and lighting, quantum information processing, and in photovoltaics.

The recent ability of plasmonic nanostructures to probe subnanometer and even atomic scales demands theories that can account for the nonlocal dynamics of the electron gas. The hydrodynamic Drude model (HDM) captures much of the microscopic dynamics of the quantum mechanical effects when additional boundary conditions are considered. Here, we revisit the HDM under the Madelung formalism to reexpress its coupled system of equations as a single nonlinear Schrödinger equation in order to have a natural quantum mechanical description of plasmonics. Specifically, we study the response of two overlapping nanowires with this formalism. We ensure that an proposed frame concurs with classical electrodynamics when the local response approximation holds in the plasmonic system by finding the correction needed.

We put forward a gauge-invariant theoretical framework for studying time-resolved thermoelectric transport in an arbitrary multiterminal electronic quantum system described by a noninteracting tight-binding model. The system is driven out of equilibrium by an external time-dependent electromagnetic field (switched on at time $t_0$) and possibly by static temperature or electrochemical potential biases applied (from the remote past) between the electronic reservoirs. Numerical simulations are conducted by extending to energy transport the wave-function approach developed by Gaury et al. and implemented in the t-Kwant library. We provide a module that allows us to compute the time-resolved heat currents and powers in addition to the (already implemented) charge currents, and thus to simulate dynamical thermoelectric transport through realistic devices, when electron-electron and electron-phonon interactions can be neglected. We apply our method to the noninteracting resonant level model and verify that we recover the results reported in the literature for the time-resolved heat currents in the expected limits. Finally, we showcase the versatility of the library by simulating dynamical thermal transport in a quantum point contact subjected to voltage pulses.