Magnetic multiple-$Q$ states consisting of multiple spin density waves are a source of unconventional topological spin textures, such as skyrmion and hedgehog. We theoretically investigate a topologically nontrivial double-$Q$ state with a net spin scalar chirality on a two-dimensional square lattice. We find that a double-$Q$ spiral superposition of the ordering wave vectors located at the Brillouin zone boundary gives rise to unconventional noncoplanar spin textures distinct from the skyrmion crystal, the latter of which is defined in the continuum limit. We show that such a double-$Q$ state is stabilized by the interplay among the easy-axis anisotropic interaction, high-harmonic wave-vector interaction, and external magnetic field. Furthermore, the obtained double-$Q$ state becomes a Chern insulating state with a quantum Hall conductivity when the Fermi level is located in the band gaps. Our present results provide another platform to realize topological magnetic states other than skyrmion crystals by focusing on the symmetry of constituent ordering wave vectors in momentum space.

The molecular compound ${\left({\mathrm{NH}}_4\right)}_2\left[{\mathrm{FeCl}}_5\left({\mathrm{H}}_2\mathrm{O}\right)\right]$ is a type-II multiferroic material, in which incommensurate cycloidal order directly induces ferroelectric polarization. The multiferroic domain kinetics in ${\left({\mathrm{NH}}_4\right)}_2\left[{\mathrm{FeCl}}_5\left({\mathrm{H}}_2\mathrm{O}\right)\right]$ were studied by time-resolved neutron-diffraction experiments utilizing neutron polarization analysis. The temperature- and electric-field-dependent multiferroic relaxation obeys the simple combined Arrhenius-Merz law, which was reported to describe domain kinetics in the prototype multiferroics ${\mathrm{TbMnO}}_3$ and ${\mathrm{NaFeGe}}_2{\mathrm{O}}_6$. However, the characteristic time scale of the multiferroic relaxation is considerably larger than those in ${\mathrm{TbMnO}}_3$ or ${\mathrm{NaFeGe}}_2{\mathrm{O}}_6$. Temperature-dependent diffraction on ${\left({\mathrm{NH}}_4\right)}_2\left[{\mathrm{FeCl}}_5\left({\mathrm{H}}_2\mathrm{O}\right)\right]$ reveals the emergence of higher-order and commensurate magnetic contributions upon cooling in the multiferroic phase in zero field. The good agreement with studies of higher-harmonic contributions in the deuterated material indicates that the isotopes only possess a minor impact on the magnetic ordering. However, in contrast to similar observations in multiferroic ${\mathrm{MnWO}}_4$, this anharmonic modification of magnetic ordering does not depin multiferroic domain walls or alter the temperature dependence of the multiferroic relaxation.

Light-matter coupling in a planar optical cavity substantially modifies the transport regimes in the system in the presence of a short-range excitonic disorder. Based on a master equation for a resonantly coupled exciton-photon system, and treating disorder scattering in the Born-Markov approximation, we demonstrate the onset of ballistic and diffusive transport regimes in the limits of weak and strong disorder, respectively. We show that transport parameters governing the crossover between these two regimes strongly depend on the parameters characterizing light-matter coupling, in particular Rabi energy and detuning between excitonic and photonic modes. The presented theory agrees with recent experimental data on transport in disordered organic microcavities.

We study analytic solutions to the Bardeen-Cooper-Schrieffer (BCS) gap equation for isotropic superconductors with finite-range interaction potentials over the full range of temperatures from absolute zero to the superconducting critical temperature $0\le T\le T_c$. Using these solutions $\mathrm{\Delta }(\epsilon ,T)$, we provide a proof of the universality of the temperature dependence of the BCS gap ratio at the Fermi level $\mathrm{\Delta }(\epsilon =0,T)/T_c$. Moreover, by examining the behavior of this ratio as a function of energy $\epsilon$, we find that nonuniversal features emerge away from the Fermi level, and these features take the form of a temperature-independent multiplicative factor $F\left(\epsilon \right)$, which is equal to $\mathrm{\Delta }(\epsilon ,T)/\mathrm{\Delta }(\epsilon =0,T)$ up to exponentially small corrections, i.e., the error terms vanish like $e^{-1/\lambda }$ in the weak-coupling limit $\lambda \to 0$. We discuss the model-dependent features of both $F\left(\epsilon \right)$ and $T_c$, and we illustrate their behavior focusing on several concrete examples of physically relevant finite-range potentials. Comparing these cases for fixed coupling constants, we highlight the importance of the functional form of the interaction potential in determining the size of the critical temperature and provide guidelines for choosing potentials which lead to higher values of $T_c$. We also propose experimental signatures which could be used to probe the energy dependence of the gap and potentially shed light on the underlying mechanisms giving rise to superconductivity.

We have performed angle-resolved photoemission spectroscopy on NdSb, which exhibits the type-I antiferromagnetism below $T_{\mathrm{N}}=16\mathrm{K}$. By utilizing microfocused photons, we succeeded in selectively observing the band structure for all three types of $\text{single-}q$ antiferromagnetic (AF) domains at the surface. We found that two of the three surfaces whose AF-ordering vector lies within the surface plane commonly show twofold-symmetric surface states (SSs) around the bulk-band edges, whereas the other surface with an out-of-plane AF-ordering vector displays fourfold-symmetric shallow electronlike SS at the Brillouin-zone center. We suggest that these SSs commonly originate from the combination of the $\mathcal{PT}$- (space-inversion and time-reversal) symmetry breaking at the surface and the band folding due to the AF order. The present results pave a pathway toward understanding the relationship between the symmetry and the surface electronic states in antiferromagnets.

In a recent paper [Horowitz et al., Phys. Rev. B 107, 195120 (2023)], an alternative route has been proposed to construct the so-called exchange-correlation (xc) enhancement factor $F_{xc}$ of density-functional theory, defined as the enhancement of a realistic xc energy density over its local exchange-only counterpart. This new route, based on the ab initio calculation of the exact exchange energy density of a family of electron-density profiles, was implemented on the basis of jellium-slab exact-exchange self-consistent calculations. Here, we follow this route to construct a meta-generalized-gradient approximation (MGGA) for exchange from a nonuniform one-dimensional coordinate scaling, which we implement on the basis of a number of calculations performed for model densities of electrons confined by infinite-barrier walls, as the electron system is shrunk from three to two dimensions. Our MGGA yields exchange energies that approach in the two-dimensional (2D) limit the exact exchange energy of a 2D electron gas, by appealing to a scaling of the MGGA exchange enhancement factor.

Energy transport in quantum many-body systems with well defined quasiparticles has recently attracted interest across different fields, including out of equilibrium conformal field theories, one-dimensional quantum lattice models, and holographic matter. Here we study energy transport between two interacting quantum baths without quasiparticles made by two Sachdev-Ye-Kitaev (SYK) models at temperatures $T_L\ne T_R$ and connected by a Fermi-liquid system. We obtain an exact expression for the nonequilibrium energy current, valid in the limit of large bath and system size and for any system-bath coupling $V$. We show that the peculiar criticality of the SYK baths has direct consequences on the thermal conductance, which above a temperature $T^{*}\left(V\right)\sim V^4$ is parametrically enhanced with respect to the linear-$T$ behavior expected in systems with quasiparticles. Interestingly, below $T^{*}\left(V\right)$ the linear thermal conductance behavior is restored, yet transport is not due to quasiparticles. Rather the system gets strongly renormalized by the bath and becomes non-Fermi liquid and maximally chaotic. Finally, we discuss the full nonequilibrium energy current and show that its form is compatible with the structure $\mathcal{J}=\mathrm{\Phi }\left(T_L\right)-\mathrm{\Phi }\left(T_R\right)$, with $\mathrm{\Phi }\left(T\right)\sim T^{\gamma }$ and power law crossing over from $\gamma =3/2$ to $\gamma =2$ below $T^{*}$.

Heavy-fermion materials are compounds in which localized $f$ orbitals hybridize with delocalized $d$ ones, leading to quasiparticles with large renormalized masses. The presence of strongly correlated $f$ electrons at the Fermi level may also lead to long-range order, such as magnetism, or unconventional superconductivity. From a theoretical point of view, the “standard model” for heavy-fermion compounds is the periodic Anderson model (PAM). Despite being extensively scrutinized, its thermodynamic properties in three-dimensional (3D) lattices have not been carefully addressed by unbiased methodologies. Here we investigate the 3D PAM employing state-of-the-art finite-temperature auxiliary-field quantum Monte Carlo simulations. We present the behavior of the kinetic energy, the entropy, the specific heat, and the double occupancy as functions of the temperature and the hybridization strength. From these quantities, and by the analysis of the spin-spin correlation functions, we investigate the occurrence of magnetic phase transitions at finite temperatures, and we determine the phase diagram of the model, including the behavior of the Néel temperature as a function of the external parameters.

Barium titanate, $\mathrm{BaTi}{\mathrm{O}}_3$ (BTO), has emerged as a promising electro-optic material with applications in silicon photonics. It boasts one of the largest known electro-optic coefficients; however, the origin of this giant electro-optic response has not been investigated in detail and is poorly understood. Here we report on a first-principles study of the electro-optic or Pockels tensor in tetragonal P4mm BTO. We find good agreement with experiment if the P4mm structure is viewed as a dynamic average of four lower symmetry $\mathit{Cm}$ structures. The large value of the Raman component of the EO coefficient is attributed to a low frequency and strong electron-phonon coupling of the lowest optical mode, and we trace the equally large piezoelectric contribution to the large components of the piezoelectric and elasto-optic tensors.

A predictive model of scattering processes in semiconductor lasers is derived, enabling us to model relaxation processes starting from well-known parameters such as the dielectric constant. The resulting effective mode interaction terms are explicitly calculated for an (InGa)N quantum well using Coulomb scattering. In contrast to the method used so far to model mode competition phenomena in Fabry-Pérot-type laser diodes, the model correctly includes, e.g., accelerated scattering at higher densities or temperatures and eliminates the scattering rate as an unknown parameter. The effective mode interaction term derived in this work can be used for the simulation of the mode dynamics in various laser diode types, for example, broad area laser diodes, where multiple transversal and longitudinal modes are active. Thus, our model offers an increased predictability and improved modeling of switch-on behavior.

Electrostatically gating various two-dimensional materials has shown different phase transitions upon changing their charge carrier density, which could provide an additional understanding about the fundamental properties of solids. Moreover, the gate voltage or electric field control of electron spin in materials with high spin-orbit coupling (SOC) is a key factor in the field of spintronics. In this paper, we performed electrostatic doping in $p$-type tellurium (Te) thin flakes using electric double-layer transistors (EDLTs). Our results show the possibility of gate tuning the insulator-metal phase transition in Te by effectively changing its charge carrier density. Furthermore, a crossover from weak localization (WL) to weak antilocalization (WAL) was observed, suggesting an increased Rashba-like SOC in the material created by a strong electric field restricted to the solid/electrolyte interface in EDLTs. The WAL becomes more pronounced as we increase the conductivity of the sample. The temperature dependence of the WAL further showed an $e\text{−}e$ interaction to be the main scattering mechanism of the quantum decoherence and dephasing in the material. More interestingly, we have demonstrated the ability to control the electron spin and increase the Rashba parameter by a factor of 4 through ionic gating $p$-type Te, which holds an intriguing potential for spintronics applications.

Green's function zeros, which can emerge only if correlation is strong, have been for long overlooked and believed to be devoid of any physical meaning, unlike Green's function poles. Here, we prove that Green's function zeros instead contribute on the same footing as poles to determine the topological character of an insulator. The key to the proof, worked out explicitly in two dimensions but easily extendable in three dimensions, is to express the topological invariant in terms of a quasiparticlethermal Green's function matrix $G_{*}(i\epsilon ,\mathbf{k})=1/[i\epsilon -H_{*}(\epsilon ,\mathbf{k})]$, with Hermitian $H_{*}(\epsilon ,\mathbf{k})$, by filtering out the positive-definite quasiparticle residue. In that way, the topological invariant is easily found to reduce to the Thouless, Kohmoto, Nightingale, and den Nijs formula for quasiparticles described by the noninteracting Hamiltonian $H_{*}(0,\mathbf{k})$. Since the poles of the quasiparticle Green's function $G_{*}(\epsilon ,\mathbf{k})$ on the real frequency axis correspond to poles and zeros of the physical-particle Green's function $G(\epsilon ,\mathbf{k})$, both of them equally determine the topological character of an insulator.

We consider the topological protection of entanglement and particle fluctuations for a general one-dimensional chiral topological insulator with winding number $\mathcal{I}$. We prove, in particular, that when the periodic system is divided spatially into two equal halves, the single-particle entanglement spectrum has $2\left|\mathcal{I}\right|$ protected eigenvalues at $1/2$. Therefore the number fluctuations are bounded from below by $\mathrm{\Delta }{N}^2\ge \left|\mathcal{I}\right|/2$ and the entanglement entropy by $S\ge 2\left|\mathcal{I}\right|ln2$. We note that our results are obtained by applying directly an index theorem to the microscopic model and do not rely on an equivalence to a continuum model or a bulk-boundary correspondence for a slowly varying boundary.

The relentless miniaturization of transistors drives the search for alternative metals to copper for low-dimension interconnects. Indeed, some elementary metals, like ruthenium, become less resistive than copper at low dimensions, leading to smaller losses in the connection lines. For most parts, such knowledge is still lacking for binary metals and other alloys. In this work, we carry such an investigation based on first principles, combining electron-phonon and surface scatterings in the relaxation time approximation of Boltzmann's transport equation. We discuss the validity of different proxies of the resistivity at low dimensions (both for thin films and rectangular nanowires), including the so-called $\rho \lambda$ product, that do not require the computation of the electron-phonon relaxation time. We then identify a few promising binary systems that can in principle compete with copper at low dimensions, namely NiAl, RuAl, and ${\mathrm{Cu}}_3\mathrm{Al}$. Finally, we derive on the way the analytical solution of Boltzmann's transport equation for rectangular nanowires.

We report finite-size topology in the quintessential time-reversal (TR) invariant systems, the quantum spin Hall insulator (QSHI) and the three-dimensional, strong topological insulator (STI)—previously-identified helical or Dirac cone boundary states of these phases hybridize in wire or slab geometries with one open boundary condition for finite system size, and additional, topologically protected, lower-dimensional boundary modes appear for open boundary conditions in two or more directions and coexist with the response signatures of the higher-dimensional topological bulk. We explicitly demonstrate this coexistence for both the QSHI in a ribbon geometry and the STI in a slab geometry. For the quasi-one-dimensional (q(2-1)D) QSHI, we find topologically protected, quasi-zero-dimensional (q(2-2)D) boundary states within the hybridization gap of the helical edge states, determined from q(2-1)D bulk topology characterized by topologically nontrivial Wilson loop spectra. We show this finite-size topology furthermore occurs in $\text{1T"-}{\mathrm{WTe}}_2$ in ribbon geometries with sawtooth edges, based on analysis of a tight-binding model derived from density-functional theory calculations, motivating experimental investigation of our results. In addition, we find quasi-two-dimensional (q(3-1)D) finite-size topological phases occur for the STI, yielding helical boundary modes distinguished from those of the QSHI by a nontrivial magneto-electric polarizability linked to the original 3D bulk STI. Finite-size topological phases therefore exhibit signatures associated with the nontrivial topological invariant of a higher-dimensional bulk, clearly distinguishing them from previously-known topological phases. Finally, we find the q(3-2)D STI also exhibits finite-size topological phases, finding the first signs of topologically protected boundary modes of codimension greater than one due to finite-size topology. Finite-size topology of four- or higher-dimensional systems is therefore possible in experimental settings without recourse to thermodynamically large synthetic dimensions.

High magnetic fields up to 105 T have been utilized in deriving the upper critical field $B_{\mathrm{c}2}$ of ${\mathrm{LaFeAsO}}_{1-x}{\mathrm{H}}_x$ throughout the wide temperature range below $T_{\mathrm{c}}$. Resistivity measurements demonstrate that $B_{\mathrm{c}2}$ behaves differently in samples with $x=0.12$ (SC1) from those with 0.32 (SC2). In SC1, the two-band model assuming $s$-wave pairing gives a good fitting with a repulsive intraband interaction and dominant interband coupling. In SC2, we have to assume an attractive intraband interaction with weak interband coupling, which in fact suggests a non-$s$-wave pairing in view of the strong Coulomb repulsion. These results support the possibility that SC1 and SC2 have different pairing symmetries.

$2H\text{-NbS}{\mathrm{e}}_2$ is an archetypal system in which superconductivity and charge density wave (CDW) coexist and compete macroscopically with each other. In particular, this interplay also manifests in their dynamical fluctuations. As a result, the superconducting amplitude fluctuation (i.e., Higgs mode) is pushed below the quasiparticle continuum, allowing it to become a coherent excitation observable by Raman scattering. In the present study, we coherently drive the collective oscillations of the two orders and visualize their interplay in the time domain. We find that both collective modes contribute to terahertz third-harmonic generation (THG) and their THG signals interfere below $T_{\mathrm{c}}$, leading to an antiresonance of the integrated THG signal. The dynamical Ginzburg-Landau model suggests that around the antiresonance a periodic energy transfer between the driven Higgs oscillations and the driven CDW oscillations is possible. Our results illustrate the roles of collective modes in the terahertz THG process, revealing a close connection of this technique to Raman scattering. In systems where the different collective modes are coupled, our experimental scheme also illustrates a paradigm for realizing coherent control via such couplings.

We study the quantum valley Hall effect and related domain wall modes in twisted bilayer graphene at a large commensurate angle. Due to the quantum valley and subvalley Hall effect, a small deviation from the commensurate angle generates two-dimensional conducting network patterns composed of one-dimensional domain walls, which can host non-Fermi-liquid behavior within an accessible temperature range. The domain wall modes can be manipulated using an external electric field and layer shifting, manifesting the physics of the celebrated Haldane and Semenoff models for the subvalley Dirac cones, living on the domain walls. These findings open up an alternative direction towards realizing the emergence and manipulation of topological quantum valley and subvalley Hall states and possible applications in valleytronics. Our theory can be generalized to many twisted bilayer systems, including twisted graphene, twisted $\alpha$-graphene, and twisted kagome bilayers at large-angle rotation.

The rate of charge-parity switching in a full-shell superconductor-semiconductor nanowire qubit is measured by directly monitoring the dispersive shift of a readout resonator. At zero magnetic field, the measured switching time scale $T_P$ is on the order of $100\mathrm{m}\mathrm{s}$. Two-tone spectroscopy data post-selected on charge parity is demonstrated. With increasing temperature or magnetic field, $T_P$ is at first constant, then exponentially suppressed, consistent with a model that includes both nonequilibrium and thermally activated quasiparticles. As $T_P$ is suppressed, qubit lifetime $T_1$ also decreases. The long $T_P\sim 0.1\mathrm{s}$ at zero field is promising for future development of qubits based on hybrid nanowires.