Planar Josephson junctions provide a platform to host topological superconductivity, which, through manipulating Majorana bound states (MBS), could enable fault-tolerant quantum computing. However, what constitutes experimental signatures of topological superconductivity and how MBS can be detected remains strongly debated. In addition to spurious effects that mimic MBS, there is a challenge to discern the inherent topological signals in realistic systems with many topologically trivial Andreev bound states, determining the transport properties of Josephson junctions. Guided by the advances in microwave spectroscopy, we theoretically study Al/InAs-based planar Josephson junction embedded into a radio-frequency superconducting quantum interference device to identify microwave signatures of topological superconductivity. Remarkably, by exploring the closing and reopening of a topological gap, we show that even in a wide planar Josephson junction with many Andreev bound states, such a topological signature is distinguishable in the resonance frequency shift of a microwave drive and the half-slope feature of the microwave absorption spectrum. Our findings provide an important step towards experimental detection of non-Abelian statistics and implementing scalable topological quantum computing.

We revisit the pyrochlore spin liquid materials ${\mathrm{Ce}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$ and ${\mathrm{Ce}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$ by examining the existing experiments. We continue to rely on the special properties of the dipole-octupole nature of the ${\mathrm{Ce}}^{3+}$ moment. The inelastic neutron scattering (INS) measurement in the octupolar U(1) spin liquid selects the (gapped) spinon continuum, and thus has suppressed spectral weights below the energy threshold of two spinon gaps. This measurement, however, includes all other emergent excitations at lower energies in the dipolar U(1) spin liquid, in particular, the gapless photon and the continuum of the electric monopoles . Although the electric monopole continuum is weakly gapped (compared to the larger spinon gap), the energy scale is actually close to the gauge photons, and the spectrum largely overlaps with the photons. Due to the background dual $\pi$ flux for the electric monopoles, the density of states is enhanced at lower energies, creating peak structures. This can be contrasted with the linearly suppressed spectral weight of the gauge photons at low energies. We propose that the electric monopole continuum should be mostly responsible for the low-energy spectrum in the INS measurement in the dipolar U(1) spin liquid. With these understanding and calculation, we discuss the available experimental results and predict further experiments for ${\mathrm{Ce}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$ and ${\mathrm{Ce}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$.

Recent angle-resolved photoemission spectroscopy (ARPES) experiments [Phys. Rev. Lett. 130, 046402 (2023)] on PdTe, a 3D-Dirac semimetal and a superconductor with the transition temperature $T_{\mathrm{c}}\sim 4.3$ K, have revealed compelling evidence of the presence of bulk nodes in the superconducting order parameter. To investigate the validity of this proposition, here we present a detailed investigation of the magnetic field dependence of the specific heat of PdTe down to temperatures $\sim 58$ mK. We observed that the low-temperature specific heat of PdTe with an externally applied magnetic field exhibits a power-law field dependence, a characteristic of unconventional superconductivity. Furthermore, the zero-field low-temperature electronic specific heat follows a cubic temperature dependence, which is a signature of the presence of bulk point nodes in PdTe. These intriguing observations suggest that PdTe is a rare and fascinating topological material that exhibits both Dirac semimetallic properties and superconductivity with point nodal gap symmetry.

We present a general ab initio method based on Wannier functions using the covariant derivative for simulating the photocurrent in solids. The method is widely applicable to charge/spin dc and ac photocurrent at any perturbation levels in both semiconductors and metals for both linearly and circularly polarized light. This is because the method is theoretically complete (within the relaxation-time approximation), that is to say, it includes all intraband, interband, and their cross terms. Specifically for the second-order dc photocurrent, it includes all of the following contributions—shift current, gyration current, (magnetic) injection current, Berry curvature dipole, and other Fermi surface contributions, instead of only a part of them as in most previous ab initio methods. It is also free from the degeneracy issue, i.e., applicable to arbitrary band structures with arbitrary numbers of degenerate bands. We apply the method to simulate the charge/spin dc and ac photocurrent of various semiconductors and metals, including GaAs, graphene-hBN heterostructure, monolayer ${\mathrm{WS}}_2$, monolayer GeS, bilayer antiferromagnetic ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$, and topological Weyl semimetal RhSi. Our theoretical results are in agreement with previous theoretical works. Our numerical tests of GaAs, ${\mathrm{WS}}_2$, and GeS suggest setting the degeneracy threshold in the conventional method as $\hslash {\mathrm{\Gamma }}^{\left(2\right)}$, with ${\mathrm{\Gamma }}^{\left(2\right)}$ the relaxation rate of the off-diagonal elements of the density matrix between two states with close energies. We find that compared with the conventional Wannier-function-based method using nondegenerate perturbation theory, the numerical errors of optical susceptibilities of bilayer antiferromagnetic ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ with the $\mathcal{PT}$ symmetry can be reduced by one to two orders of magnitude by our method for circularly polarized light. Our method provides a universal computational tool for reliable and accurate predictions of abundant weak-field photocurrent phenomena in disparate materials.

The magnetization manipulation by spin-orbit torques (SOTs) in nonmagnetic-metal (NM)–ferromagnet (FM) heterostructures has provided great opportunities for spin devices. Besides the conventional spin Hall effect (SHE) in heavy metals with strong spin-orbit coupling, the orbital currents have been proposed as another promising approach to generate strong SOTs. Here, we systematically study the SOT efficiency and its dependence on the FM thickness and different NM–FM interfaces in two prototypical Pt/Py and Ta/Py systems by inserting an ultrathin magnetic layer (ML), (0.4-nm-thick $\mathrm{ML}=\mathrm{Co}$, Fe, and Ni). The dampinglike (DL) torque efficiency ${\xi }_{\mathrm{DL}}$ is significantly enhanced by inserting ultrathin ML layers. Moreover, the Ni insertion results in a sign change of the fieldlike (FL) torque in Pt/Py and substantially reduces ${\xi }_{\mathrm{DL}}$ in Ta/Py. These results are likely related to the additional spin currents generated by combining the orbital Hall effect in the NM and orbital-to-spin conversion in the ML insertion layer and/or their interfaces, especially for the Ni insertion. Our results demonstrate that inserting ultrathin ML can effectively manipulate the strength and sign of the SOTs, which would be helpful for spintronics applications.

We study the phase diagram of the Rashba-Hubbard model by employing the Hartree-Fock mean-field theory and thereby establish the existence of an antiferromagnetically ordered Weyl semimetallic state with in-plane magnetic moments. This phase is found to be sandwiched between the antiferromagnetic insulator and Rashba metal in the interaction vs spin-orbit coupling phase diagram. The antiferromagnetically ordered topological semimetallic state exists in the presence of combined time-reversal and inversion symmetry, although individually both are broken. The study of the static magnetic susceptibility indicates the robustness of the antiferromagnetic order within a realistic range of interaction and spin-orbit coupling parameters. In addition to the edge states associated with the Weyl points, we also investigate the spin-resolved quasiparticle interference, which provides important insight into the possible spin texture of the bands, especially in the vicinity of Weyl points.

We consider double perforated slabs (membranes) that support a symmetry-protected bound state in the continuum (BIC). These slabs are immersed into a liquid at room temperature. Laser excitation of the BIC generates giant optical forces that strongly affect the Brownian motion of nanoparticles in a colloidal solution. By solving of the Fokker-Planck equation we show that a single membrane can localize only larger nanoparticles. However, system of two parallel membranes can trap extremely small nanometer-sized nanoparticles by a resonant excitation of quasi-BICs, with varying intensities in each membrane.

In this work, we propose an electronic Lévy glass built from graphene nanoribbons in the presence of regions with enhanced spin-orbit coupling. Although electrons in graphene nanoribbons present a low spin-orbit coupling strength, it can be increased by a proximity effect with an appropriate substrate. We consider graphene nanoribbons with different edge types, which contain circular regions with a tunable Rashba spin-orbit coupling, whose diameter follows a power-law distribution. We find that spin-orbital clusters induce a transition from superdiffusive to diffusive charge transport, similar to what we recently reported for nanoribbons with electrostatic clusters [Phys. Rev. B 107, 155432 (2023)]. We also investigate spin polarization in the spin-orbital Lévy glasses, and show that a finite spin polarization can be found only in the superdiffusive regime. In contrast, the spin polarization vanishes in the diffusive regime, making the electronic Lévy glass a useful device whose electronic transmission and spin polarization can be controlled by its Fermi energy. Finally, we apply a multifractal analysis to charge transmission and spin polarization, and find that the transmission time series in the superdiffusive regime are multifractal, while they tend to be monofractal in the diffusive regime. In contrast, spin polarization time series are multifractal in both regimes, characterizing a marked difference between mesoscopic fluctuations of charge transport and spin polarization in the proposed electronic Lévy glass.

Topological pumps have many implications in physics; for instance, it enables coherent transfer of energy, charge, and spin. However, a topological pump would fail for signal and information transmission since the underlying adiabatic condition is unavoidably violated with rapid modulation. Here, we construct a nonadiabatic pump in a two-color Floquet setting of topological acoustics and demonstrate a dynamical topological pumping for delivering signal pulse in both real time ($t$) and propagation axis ($z$, synthetic time). The pulse transfer indicates a direct detection of anomalous topological invariants of periodically driven systems. Using our fabricated acoustic waveguide arrays, we demonstrate a topological pump transport of both continuous and pulsed sound waves, proving the utility of our pump array for dynamical signal transmission and wave manipulation. Our findings can advance both fundamentals and implementations of dynamical topological pumps in driven systems.

In this study, we have explored simultaneously two distinctive physical aspects of electron transport within a semiconductor superlattice subjected to both randomly distributed barrier scattering strengths and in-plane interface-roughness scattering of electrons within each barrier layer. To include interface-roughness scattering of electrons within a single barrier layer, we adopt the reduced Boltzmann transport equation to compute the nonequilibrium occupation function of electrons. In the presence of randomness within a superlattice, we apply a quantum-mechanical transfer-matrix approach for an electron wave to take into account all barrier-layer scattering within the superlattice. Consequently, the single-electron group velocity has been replaced by a mean group velocity assisted with a randomness-averaged transmission coefficient. For the interface-roughness scattering of electrons by each barrier layer, we numerically solve the reduced Boltzmann transport equation exactly and acquire the electrical current by another weighted average of obtained mean group velocities over the numerically calculated transient nonequilibrium occupation function of electrons from the reduced Boltzmann transport equation.

We apply the Nakajima-Zwanzig approach to open quantum systems to study steady-state transport across generic multilevel junctions coupled to bosonic or fermionic reservoirs. The method allows for a unified diagrammatic formulation in Liouville space, with diagrams being classified according to an expansion in the coupling strength between the reservoirs and the junction. Analytical, approximate expressions are provided up to the fourth order for the steady-state boson transport that generalizes to multilevel systems the known results for the low-temperature thermal conductance in the spin-boson model. The formalism is applied to the problem of heat transport in a qubit-resonator junction modeled by the quantum Rabi model. Nontrivial transport features emerge as a result of the interplay between the qubit-oscillator detuning and coupling strength. For quasidegenerate spectra, nonvanishing steady-state coherences cause a suppression of the thermal conductance.

The stable and metastable configurations of interstitial Mg in GaN and its migration energy barriers are studied from first-principles calculations. In addition to the conventional octahedral (O, global energy minimum) and tetrahedral (T, metastable) interstitial sites, we have discovered two metastable interstitial complexes with formation energy lower than or close to that of the T configuration but higher than O. Except for Mg at O site, which only has $+2$ charge state, all other configurations also permit charge states $+1$ or 0. The minimum migration energy barrier for ${\mathrm{Mg}}^{++}$ between O sites is found to be $1.95\mathrm{eV}$. We further find that, when Fermi energy is close to the conduction band, the migration between O sites via metastable configurations occurs through a recombination-enhanced mechanism in which the charge state changes from $+2$ at O site to 0 at metastable sites by consecutive capture of two electrons during the migration. This process greatly reduces the migration energy barrier to as low as $1.47\mathrm{eV}$. This value is consistent with experiments, and we also discuss the role of intrinsic defects in the migration of Mg.

The emergent structure under high pressure introduces a distinctive lattice arrangement, deviating from conventional configurations and offering a unique landscape for exploring intriguing properties, especially in the domain of superconductivity. Here, we have systematically investigated the structural evolution and electrical properties of α-SnS under high pressure. The x-ray diffraction results indicate that SnS undergoes two phase transitions at 16.8 and 49.7 GPa, representing a second-order Pnma to Cmcm transition and a first-order Cmcm to $Pm\overline{3}m$ transition. In addition, single-crystal in situ high-pressure Raman spectroscopy experiments have confirmed it. Electrical transport measurements demonstrate the transition of SnS from a semiconductor to a metal and then to a superconductor under high pressure, intricately linked to a structural transition and an increase in the hole carrier concentration. Combined with our first principles calculation, it is determined that the topological properties and superconductivity coexist in SnS with the $Pm\overline{3}m$ phase. The nonmonotonic variation of $T_c$ in SnS with pressure is attributed to the electron-phonon coupling effect. Additionally, this study marks a confirmation of the structural phase transitions in SnS under high pressure. Our study contributes to a deeper understanding of the intricate relationship between structure and performance in extreme conditions.

Antiferromagnetic (AFM) skyrmions have been in the spotlight as ideal topological magnetic bits. Although they are topologically protected, they do not exhibit the skyrmion Hall effect unlike the ferromagnetic ones. Thus, AFM skyrmions are considered to provide a better control of the skyrmion's motion due to the absence of the skyrmion Magnus effect. In this work, we propose a possible realization of controllable AFM skyrmions in a twisted moiré magnet. The tunability of moiré materials is not only a good platform for the provision of rich phases, but also for the stabilization of the skyrmion phase. We investigate the ground state of twisted bilayer AFM system by solving the Landau-Lifshitz-Gilbert equation in a continuum model. We show that the AFM skyrmions are stabilized even in the absence of the external/dipolar magnetic field, as a consequence of the interplay of interlayer coupling, Dzyaloshinskii-Moriya (DM) interaction and Ising anisotropy. More interestingly, due to the magnetoelectric effect, the application of an external electric field locally stabilizes the skyrmions in the twisted bilayer AFM systems, even in the absence of DM interaction. It also allows the skyrmion helicity to change continuously when both the DM interaction and an electric field are present. We show the phase diagram with respect to the strength of interlayer coupling, the DM interaction and an electric field. Our results suggest the possibility of using AFM skyrmions as stable, controllable topological magnetic bits.

The interaction between two impurity spins provides vital information about the host system and has been suggested to form a building block in quantum computation and spintronic devices. We here determine this Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in recently discovered altermagnetic materials, including the presence of a Zeeman field, in a two- and three-dimensional altermagnet. In two dimensions, we also study the effect of Rashba spin-orbit coupling. Our results reveal that the momentum-resolved spin polarization of the itinerant carriers in the altermagnet changes the RKKY interaction qualitatively from the isotropic spin splitting in a ferromagnet. The Ising-contribution directed parallel with the altermagnetism is found to exhibit a beating pattern reflecting the shape of the Fermi surface of the system and may thus be further influenced by an out-of-plane Zeeman field. However, the exchange interaction for in-plane impurity spins deviates only slightly from that of a normal metal. On shorter length scales, we find that the Ising, the Dzyaloshinskii-Moriya terms, and other noncollinear interaction terms in the RKKY interaction acquire a rapidly oscillating behavior as a function of the relative angle between the impurity spins, which is not present in the ferromagnetic case. We determine how this new length scale depends on the system parameters analytically. Our results show that the RKKY interaction in altermagnets is qualitatively different from that of ferromagnets despite both breaking time-reversal symmetry.

The $S=1/2$ hyperkagome-lattice Heisenberg antiferromagnet allows us to study the interplay of geometrical frustration and quantum as well as thermal fluctuations in three dimensions. We use 16 terms of a high-temperature series expansion complemented by the entropy-method interpolation to examine the specific heat and the uniform susceptibility of this model. We obtain thermodynamic quantities for several possible scenarios determined by the behavior of the specific heat as $T\to 0$: A power-law decay with the exponent $\alpha =1, 2$, and also 3 (gapless energy spectrum) or an exponential decay (gapped energy spectrum). All scenarios give rise to a low-temperature peak in $c\left(T\right)$ (almost a shoulder for $\alpha =1$) at $T<0.05$, i.e., well below the main high-temperature peak. The functional form of the uniform susceptibility $\chi \left(T\right)$ below about $T=0.5$ depends strongly not only on the chosen scenario but also on an input parameter ${\chi }_0\equiv \chi (T=0)$. An estimate for the ground-state energy $e_0$ depends on the adopted specific scenario but is expected to lie between $-0.441$ and $-0.435$. In addition to the entropy-method interpolation we use the finite-temperature Lanczos method to calculate $c\left(T\right)$ and $\chi \left(T\right)$ for finite lattices of $N=24$ and 36 sites. A combined view on both methods leads us to favor the gapless scenario with $\alpha =2$ (but $\alpha =1$ cannot be excluded) and finite ${\chi }_0$ around 0.1.

Using first-principles simulations combined with many-body calculations, we show that two-dimensional free-standing quintuple-layer (QL) ${\mathrm{Bi}}_2{\mathrm{Se}}_2\mathrm{Te}$ is an inversion-symmetric monolayer expected to achieve spatially indirect exciton with large exciton radius, small exciton effective mass, and long exciton lifetime. Such system is theoretically predicted to be a promising platform for realizing excitonic Bose-Einstein condensation and superfluid due to its high phase transition temperatures of $\sim 257$ and $\sim 64.25$ K for the excitonic degeneracy and superfluid, respectively. The importance of spin-orbit coupling is revealed, and the angular momentum selection rules for photon absorption are discussed. This finding suggests the potential of QL ${\mathrm{Bi}}_2{\mathrm{Se}}_2\mathrm{Te}$ monolayer with exotic bosonic bound states provides as a tantalizing high-temperature platform to probe excitonic physics.

It remains an open question whether the two-dimensional single-band pure Hubbard model and its related pure $t\text{−}J$ model truly capture the superconducting order in cuprates. Recent numerical studies on this issue have raised a notable disparity in superconducting order between the pure Hubbard model and the pure $t\text{−}J$ model. Inspired by these works, we investigate the role of the three-site hopping term in $d$-wave superconductivity, such a term is usually neglected in the effective Hamiltonian of the Hubbard model, although its amplitude is of the same order as the superexchange coupling $J$ in the $t\text{−}J$ model. Our slave-boson mean-field solution demonstrates the suppression of $d$-wave superconducting order by incorporating the three-site hopping term, consistent with numerical observations by the density-matrix renormalization group. This suppression could be understood as a result of competition between superexchange interaction and three-site hopping, the former favors $d$-wave pairing while the latter favors $s$-wave pairing. We also discussed its role in quasiparticle dispersion and boson-condensation temperature. Our findings may offer an alternative understanding of the recent numerical contrasting findings in the strong-coupling regime: the absent or weak superconductivity in the pure Hubbard model, while the robust superconductivity in the $t\text{−}J$ model without including the three-site hopping term.