A striking general bound on the energy gap in topological matter was recently discovered by Y. Onishi et al. [Phys. Rev. X 14, 011052 (2024)]. A nontrivial indirect derivation builds on the properties of optical conductivity at an arbitrary frequency. We propose a simpler derivation, allowing multiple generalizations, such as a universal bound on a gap in anisotropic systems, systems with multiple charge carrier types, and topological systems with zero Hall conductance. The derivation builds on the observation that the bound equals $\hslash$ times the ratio of the force by the external electric field on the charge carriers and their total kinematic momentum in the direction perpendicular to the force.

The electronic excitation spectrum of a material characterizes the response to external electromagnetic perturbations through its energy loss function (ELF), which is obtained from several experimental sources that usually do not completely agree among them. In this paper, we assess the available ELF of three metals, namely chromium, palladium, and samarium, by using the dielectric formalism to calculate relevant stopping quantities, such as the stopping cross sections for protons and alpha particles, as well as the corresponding electron inelastic mean free paths. The comparison of these quantities (as calculated from different sets of ELF) with the available experimental data for each of the analyzed metals highlights the promising capability of the recently proposed reverse Monte Carlo method for the determination of the ELF. This study also analyzes the contribution of different electronic shells to the electronic excitation spectra of these materials, and reveals the important role that the excitation of “semi-core” bands plays on the energy loss mechanism for these metals.

We report the observation of a pronounced third-order nonlinear Hall effect (NLHE) in 1T-phase ${\mathrm{VSe}}_2$ nanosheets, synthesized using chemical vapor deposition (CVD). The nanosheets exhibit a charge density wave (CDW) transition at ∼77 K. Detailed angle-resolved and temperature-dependent measurements reveal a strong cubic relationship between the third-harmonic Hall voltage $V_{\perp }^{3\omega }$ and the bias current $I^{\omega }$, persisting up to room temperature. Notably, the third-order NLHE demonstrates a twofold angular dependence and significant enhancement below the CDW transition temperature, indicative of threefold symmetry breaking in the CDW phase. Scaling analysis suggests that the intrinsic contribution from the Berry connection polarizability tensor is substantially increased in the CDW phase, while extrinsic effects dominate at higher temperatures. Our findings highlight the critical role of CDW-induced symmetry breaking in modulating quantum geometric properties and nonlinear transport phenomena in ${\mathrm{VSe}}_2$, paving the way for future explorations in low-dimensional quantum materials.

Spin-spin exchange interactions between semiconductor spin qubits allow for fast single- and two-qubit gates. During exchange, coupling of the qubits to a surrounding phonon bath may cause errors in the resulting gate. Here, the fidelities of exchange operations with semiconductor double quantum dot spin qubits in a Si-SiGe heterostructure coupled to a finite-temperature phonon bath are considered. By employing a master-equation approach, the isolated effect of each spin-phonon coupling term may be resolved, as well as leakage errors of encoded qubit operations. As the temperature is increased, a crossover is observed from where the primary source of error is due to a phonon-induced perturbation of the two-electron spin states, to one where the phonon-induced coupling to an excited orbital state becomes the dominant error. Additionally, it is shown that a simple tradeoff in pulse shape and length can improve robustness to spin-phonon-induced errors during gate operations by up to an order of magnitude. Our results suggest that for elevated temperatures within $200–300\mathrm{mK}$, exchange gate operations are not currently limited by bulk phonons. This is consistent with recent experiments.

A family of V-based kagome metals $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A=\text{Cs}$, Rb, K) presents an intriguing platform for exploring the interplay of time-reversal symmetry breaking, nontrivial topological bands, and electron correlations, resulting in a range of exotic quantum states, including the anomalous Hall effect, unconventional charge density waves, and superconductivity. These features prompt critical questions regarding the roles of magnetism and spin-orbit coupling (SOC) in these systems. Our density functional theory (DFT) calculations demonstrate a notable sensitivity of the magnetic properties to the choice of $k$-point mesh used in Brillouin zone integrations. Specifically, we find that using a dense $k$-point mesh yields a nonmagnetic pristine phase characterized by paramagnetic susceptibility, consistent with the recently observed Pauli paramagnetic behavior in single-crystalline samples at high temperatures. In contrast, a coarser $k$-point mesh significantly increases the density of states at the Fermi level, inducing a ferromagnetic instability that satisfies the Stoner criterion. Moreover, our results show that the effect of SOC on both the geometric and electronic structures is minimal, with only a slight gap opening at the Dirac points, indicating a weak SOC influence in these materials. Importantly, our DFT band-structure calculations closely align with angle-resolved photoemission spectroscopy data, reinforcing the notion of weak electron correlations in these kagome metals. This refined understanding challenges recent theoretical assertions that the interplay of magnetism, SOC, and electron correlations is essential for determining the nature of charge density waves in $A{\mathrm{V}}_3{\mathrm{Sb}}_5$.

We report thermodynamic and transport properties of ${\mathrm{LaCu}}_x{\mathrm{Sb}}_2$ $(0.92\le x\le 1.12)$, synthesized by controlling the initial loading composition and investigated by magnetization, electrical resistivity, and specific heat measurements. The physical properties of this system are highly dependent on Cu-site occupancy $x$, where residual resistivity ratio (RRR), magnetoresistance (MR), superconducting transition temperature $\left(T_c\right)$, and electronic specific heat coefficient $\left(\gamma \right)$ indicate a systematic variation as a function of $x$. The Shubnikov-de Haas quantum oscillations are observed in magnetoresistance measurements for samples close to the Cu stoichiometry $x\sim 1$, while the de Haas-van Alphen oscillations are detected in a wide range of $x$ $(0.92\le x\le 1.12)$. For $H\parallel c$, the oscillation frequency indicates a clear $x$-dependence, implying a systematic change of Fermi surface. DFT calculations for the sample closest to ideal Cu stoichiometry reveal electronic structures with a common feature of the square-net-based semimetals, which is in good agreement with the experimental observations. The magnetic response of ${\mathrm{LaCu}}_x{\mathrm{Sb}}_2$ to magnetic fields is anisotropic owing to the Fermi surface anisotropy. Our results show how the physical properties are influenced by the Cu-site occupancy $x$, linked to the electronic bands arising from the Sb square net.

We theoretically propose a design for two-dimensional Dirac semimetals using a bilayer modified Bernevig-Hughes-Zhang (BHZ) model. By introducing new sites into the BHZ model, we engineer flat bands at the Fermi energy. In the bilayer system, interlayer coupling separates these flat bands, resulting in two Dirac points that preserve time-reversal and inversion symmetries. Two Dirac points are connected by one-dimensional Fermi arc edge states, whose bound nature is confirmed by quantized transmission resonance peaks. Notably, the position of the Dirac points can be precisely tuned by adjusting interlayer coupling strengths and symmetries.

We investigate some possible symmetries of the superconducting state that emerges in three-dimensional altermagnets in the presence of spin-orbit coupling. We demonstrate within a weak-coupling approach that these altermagnets, which naturally possess an order modulated by a vector form factor ${\mathit{g}}_{\mathbf{k}}$, favor spin-triplet superconductivity described by gap functions given by $\mathit{d}\left(\mathbf{k}\right)=\mathit{u}\left(\mathbf{k}\right)\times {\mathit{g}}_{\mathbf{k}}$, where $\mathit{u}\left(\mathbf{k}\right)=-\mathit{u}(-\mathbf{k})$. Consequently, this singles out $f$-wave spin-triplet superconductivity as the most favorable pairing state to appear in the vicinity of $d$-wave altermagnetism. Furthermore, we obtain that the combination of spin-singlet superconducting states with altermagnetism gives rise to Bogoliubov-Fermi surfaces, which are protected by a ${\mathbb{Z}}_2$ topological invariant. Using a Ginzburg-Landau analysis, we show that, for a class of spin-orbit coupled altermagnetic models, a superconducting phase is expected to appear at low temperatures as an intertwined $d+if$ state, thus breaking time-reversal symmetry spontaneously.

The analysis of relationships among resistivity, temperature, and doping in the electron-doped cuprate ${\mathrm{La}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_{4+\delta }$ (LCCO) has yielded profound insights into its low-temperature transport behavior. Within the intermediate-temperature range of 70–200 K, a ubiquitous $T^2$ resistivity behavior emerges, distinct from the linear resistivity characteristic of the strange metal state below 50 K. Despite its prominence, the origin of the $T^2$ resistivity at various doping levels remains elusive, prompting debates on the sufficiency of Fermi-liquid theory. Here, a systematic electrical transport on epitaxial composition-spread films reveals a doping-independent intermediate-temperature region of $T^2$ behavior in the phase diagram of overdoped LCCO. This observation suggests a topologically stable Fermi surface with strong electron-phonon coupling. The Fermi liquid of polaronic quasiparticles and their interactions provides a plausible framework to understand the intermediate-temperature $T^2$ behavior in cuprate superconductors.

Magnetically doped topological insulators have been extensively studied over the past decade as a material platform to exhibit quantum anomalous Hall effect. Most material realizations are magnetically doped and despite material advances suffer from large disorder effects. In such systems, it is believed that magnetic disorder leads to a spatially varying Dirac mass gap and chemical potential fluctuations, and hence quantized conductance is only observed at very low temperatures. Here, we use a recently developed high-precision time-domain terahertz (THz) polarimeter to study the low-energy electrodynamic response of Cr-doped $({\mathrm{Bi},\mathrm{Sb})}_2{\mathrm{Te}}_3$ thin films. These films have been recently shown to exhibit a dc quantized anomalous Hall response up to the high temperature of $T=2 \mathrm{K}$ at zero gate voltage. In contrast, the real part of the THz range Hall conductance ${\sigma }_{xy}\left(\omega \right)$ is slightly smaller than $e^2/h$ down to $T=2 \mathrm{K}$ with an unconventional decreasing dependence on frequency. The imaginary (dissipative) part of ${\sigma }_{xy}\left(\omega \right)$ is small but increasing as a function of frequency. We connect both aspects of our data to a simple model for effective magnetic gap disorder. The extracted distribution of gap values shows rough consistency with those from dc transport, but different than tunneling. This remains to be definitively understood.

We report on an observation and investigation of a broad central line (BCL) in electron-spin resonance (ESR) studies of hydrogen and tritium atoms embedded in solid molecular films of hydrogen isotopes (${\mathrm{H}}_2$ and ${\mathrm{T}}_2$) at a temperature of 0.1 K and in a high magnetic field $B=4.6\mathrm{T}$. We observed that this broad central line remains in the ESR spectrum after evaporating the hydrogen films and even after heating to $T=150\mathrm{K}$ but disappears after heating to room temperature. We suggest possible nature of the ESR active centers forming this line.

Understanding the nature of vortices in type-II superconductors is crucial for comprehending exotic superconductors and advancing the application of superconducting materials in future electronic devices. This study uses spectroscopic scanning tunneling microscopy to visualize Josephson vortices along crystalline domain boundaries in the superconducting spinel oxide $\mathrm{LiT}{\mathrm{i}}_2{\mathrm{O}}_4$. Our experimental results reveal that the local curvature of the Josephson junction dictates the positioning of Josephson vortices. Self-consistent solutions of the Bogoliubov–de Gennes and gap equations theoretically corroborate this observation. In addition to enhancing our understanding of the physics of Josephson vortex formation, this study offers potential guidelines for developing vortex-based superconducting applications.

We report results of magnetization and ${}^{19}\mathrm{F}$ NMR measurements in the normal state of as-grown vacuum-annealed ${\mathrm{LaO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$. The magnetization is dominated by a temperature-independent diamagnetic component and a field- and temperature-dependent paramagnetic contribution $M_{\mu }(H,T)$ from a $\sim 1000$ ppm concentration of local moments, an order of magnitude higher than can be accounted for by measured rare-earth impurity concentrations. $M_{\mu }(H,T)$ can be fit by the Brillouin function $B_J\left(x\right)$ or, perhaps more realistically, a two-level $tanh\left(x\right)$ model for magnetic Bi $6p$ ions in defect crystal fields. Both fits require a phenomenological Curie-Weiss argument $x={\mu }_{\mathrm{eff}}H/(T+T_W), T_W\approx 1.7$ K. There is no evidence for magnetic order down to 2 K, and the origin of $T_W$ is not clear. ${}^{19}\mathrm{F}$ frequency shifts, linewidths, and spin-lattice relaxation rates are consistent with purely dipolar ${}^{19}\mathrm{F}$/defect-spin interactions. The defect-spin correlation time ${\tau }_c\left(T\right)$ obtained from ${}^{19}\mathrm{F}$ spin-lattice relaxation rates obeys the Korringa relation ${\tau }_{c}T=\text{const}$, indicating the relaxation is dominated by conduction-band fluctuations.

A trigonal semiconductor Si allotrope with 22 atoms in the primitive cell, named as Tri-${\mathrm{Si}}_{22}$, is predicted by structural searching. The Tri-${\mathrm{Si}}_{22}$ is the most stable phase among previous predicted structures with the same atomic number. Our state-of-the-art first-principles calculation for the band structure and optical properties shows that compared with the diamond silicon, the Tri-${\mathrm{Si}}_{22}$ has a more suitable quasidirect band gap of 1.68 eV and improved compatibility to the solar spectral irradiance and therefore shows great advantage in photovoltaic application. The phonon transport of Tri-${\mathrm{Si}}_{22}$ is also investigated. Due to the symmetry, the Tri-${\mathrm{Si}}_{22}$ exhibits a weakly anisotropic heat transport and the lattice thermal conductivities at room temperature along $x$ and $z$ directions are about 18.6 and 17.9 ${\mathrm{W}\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$, respectively. The detailed mechanism for the phonon transport of Tri-${\mathrm{Si}}_{22}$ is also discussed.

Collective spin excitations were utilized to probe many-particle energies of strongly correlated quantum Hall ferromagnets (FMs) at $\frac{1}{3}\le \nu \le 1$. For this purpose, experiments on inelastic light scattering and polarized photoluminescence (PL) were performed on a MgZnO/ZnO heterostructure, containing a high-mobility two-dimensional electron system with the Wigner-Seitz parameter $r_s\approx 11$. The dispersion of the Larmor spin exciton was utilized to extract the spin stiffness, which was found to have maxima at $\nu =\frac{1}{3}, \nu =1$, and to be strongly suppressed at intermediate filling factors. Measurements of circularly polarized PL helped to confirm a FM configuration of the system. In the close vicinity of $\nu =\frac{1}{3}$, a hybrid spin-density excitation was discovered, the energy of which enables us to probe the correlation energy of the Laughlin liquid. The measured correlation and exchange energies at $\nu =\frac{1}{3}$ and $r_s\gg 1$ turned out to be much less suppressed by the Landau level mixing than the exchange energy at integer filling factors. The same answer follows from the calculations performed using the exact diagonalization method.

Interference among surface plasmon polaritons (SPPs) is a well-established phenomenon and has been experimentally demonstrated across both classical and quantum realms. However, actively controlling SPP interference within subcycle time intervals (typically less than several femtoseconds in the visible range) remains challenging, hindering the ultimate manipulation of SPP interference on an ultrafast timescale. Here, we manipulated the interference between surface plasmon polaritons (SPPs) launched by a hole dimer excited by a grazing incident free-electron beam without direct contact. Specifically, using cathodoluminescence spectroscopy, the emergence of higher-order interference orders was obtained by increasing separation distances of the dimer. Meanwhile, the observation of peak-valley-peak evolution at a specific wavelength by varying the accelerating voltages is attributed to the controlled initial phase difference of hole-launched SPPs. Our findings suggest that fast electron beams can serve as an efficient tool for controlling polariton interference on a subcycle temporal scale, potentially enabling applications in ultrafast optical processing or sensing.

Computer simulations were conducted to explore the equilibrium states of a planar crystalline membrane as a function of external strain and temperature. The employed models include the continuous elastic Nelson-Peliti Hamiltonian, which describes the anharmonic coupling between in-plane and out-of-plane membrane fluctuations, and the atomistic long-range carbon bond-order potential (LCBOPII), which is designed to describe carbon interactions. The primary focus of the simulations is on studying the elastic constants of graphene: the compressibility modulus $B$ and the Lamé coefficient $\mu$. By analyzing the stability conditions in terms of the elastic constants, we can determine the region of thermodynamic stability for the planar morphology of the membrane as a function of external strain and temperature. The instability is associated with the vanishing of one of the elastic coefficients. Depending on the elastic response of the system, thermodynamic instability may be caused either by the condition $B=0$ or $\mu =0.$ The symmetry of the intrinsic anharmonicity of the model determines its elastic response to decreasing tensile stress. The two studied models provide different predictions for the membrane properties near the critical point. The continuous Nelson-Peliti model predicts auxetic behavior, with a Poisson's ratio of $\nu =-1$ at the critical point. In contrast, the atomistic LCBOPII model exhibits a positive Poisson's ratio, which becomes $\nu =1$ at the critical point. Our findings challenge the generally accepted view that a Poisson's ratio of $\nu =-1/3$ is a universal property of unstrained crystalline membranes. Instead, our results highlight that anharmonic effects may cause significant differences in the mechanical responses, especially near the critical point of instability.

We explore the phase diagram of a twisted bilayer of strongly interacting electrons on a honeycomb lattice close to half filling using the slave-boson mean-field theory. Our analysis indicates that a variety of new phases can be realized as a function of chemical doping and twist angle. In particular, we find a nonmagnetic excitonic insulating phase that breaks the translational symmetry of the underlying moiré pattern. This phase results from the interplay of strong Coulomb interactions and the twist angle. In addition, we show that the features of the renormalized dispersion, such as the magic angles, depend significantly on the interactions. Our results highlight the rich physics arising in doped moiré superlattices of Mott insulators.