We have calculated the lattice and electronic thermal conductivity of arsenic between 50 and $500\mathrm{K}$ by using the first-principles Boltzmann transport equation. We find that the lattice thermal conductivity (${\kappa }_{\mathrm{ph}}$) exhibits strong anisotropy: the calculated room-temperature ${\kappa }_{\mathrm{ph}}$ is 18.6 and $5.8{\mathrm{W}\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ along the binary and trigonal directions, respectively. The anisotropy of ${\kappa }_{\mathrm{ph}}$ is mainly ascribed to the longitudinal acoustic phonon branch. ${\kappa }_{\mathrm{ph}}$ in the binary direction is larger than those reported for most of the other elemental metals. The electronic thermal conductivity (${\kappa }_{\mathrm{e}}$) is almost isotropic, and almost independent of temperature above $150\mathrm{K}$, with a room-temperature value of $\sim 26{\mathrm{W}\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$. ${\kappa }_{\mathrm{e}}$ dominates over ${\kappa }_{\mathrm{ph}}$ in the binary direction above $200\mathrm{K}$. Compared to the neighboring element Ge, As has comparable harmonic but much stronger anharmonic interatomic interactions. These strong anharmonic interactions in As are the dominant mechanism limiting ${\kappa }_{\mathrm{ph}}$. Despite a larger carrier concentration in As than in Bi or Sb, the phonon-electron interactions hardly affect ${\kappa }_{\mathrm{ph}}$ at room temperature. The calculated total thermal conductivity is in good agreement with the experimental values. Our work provides more insights into the thermal transport in elemental semimetals.

We calculate the finite temperature thermal conductivity of a time-reversal invariant chiral ${\mathbb{Z}}_3$ clock model along an integrable line in the parameter space using the time-dependent density matrix renormalization group. The thermal current itself is not a conserved charge, unlike in the case of XXZ model, but has a finite overlap with a local conserved charge $Q^{\left(2\right)}$ obtained from the transfer matrix. We find that the Drude weight is finite at nonzero temperature, and the Mazur bound from $Q^{\left(2\right)}$ saturates the Drude weight, allowing us to obtain an asymptotic expression for the Drude weight at high temperatures. The numerical estimates of the conductivity are validated using a sum rule for thermal conductivity. On the computational side, we also explore the effectiveness of the ancilla disentangler in the integrable and nonintegrable regimes of the model. We find that at high temperature, the disentangler helps by localizing the entanglement growth around the quench location allowing us to obtain the Drude weight in the integrable model. The improvement is insufficient to extract the asymptotic value of the correlator in the nonintegrable regime. The ancilla disentangler also provides no additional advantage at low temperatures.

Magnetic double perovskite (DP) compounds provide a fertile playground to explore interesting electronic and magnetic properties. By complementary macroscopic characterizations, neutron powder diffraction measurements, and first-principles calculations, we have performed comprehensive studies on the magnetic ordering in the DP compound ${\mathrm{Sr}}_2{\mathrm{NiWO}}_6$. It is found by neutron diffraction to order magnetically in a collinear type-II antiferromagnetic (AFM) structure in a tetragonal lattice with $k=(0.5,0,0.5)$ below $T_{\mathrm{N}}=56\mathrm{K}$. In the ground state, the ordered moment of the spin-1 ${\mathrm{Ni}}^{2+}$ ions is determined to be $1.9\left(2\right){\mu }_{\mathrm{B}}$, indicating a significant quenching of the orbital moment. The ${\mathrm{Ni}}^{2+}$ moments in ${\mathrm{Sr}}_2{\mathrm{NiWO}}_6$ are revealed to cant off the $c$ axis by $29.2^{\circ }$, which is well supported by the first-principles magnetic anisotropy energy calculations. Furthermore, the in-plane and out-of-plane next-nearest-neighbor superexchange couplings ($J_2$ and $J_{2\mathrm{c}}$) are found to play a dominant role in the spin Hamiltonian of ${\mathrm{Sr}}_2{\mathrm{NiWO}}_6$, which accounts for the stabilization of the type-II AFM structure as its magnetic ground state.

Superconducting nanostrip single-photon detectors (SNSPDs) are widespread tools in photonic quantum technologies. Here, we study the afterpulsing phenomenon in commercial SNSPDs exhibiting significant levels of afterpulses even at low counting rates. We find different contributions, where the probability of an afterpulse is not a constant but depends on the mean number of photons per light pulse including mean numbers much less than 1. Our observations exclude the electrical circuit as the primary cause of the main contribution to the observed afterpulsing probability, which exhibits a strong dependence on mean photon number. We propose a phenomenological model that qualitatively explains our findings via the introduction of slowly relaxing “afterpulsing centers,” storing the absorbed photons' energy for several tens of nanoseconds. We conjecture that two-level systems in amorphous materials are physical candidates for the role of such afterpulsing centers.

Phase transitions of a specific type in a magnetic crystal that take the form of the appearance of regions where the direction of the magnetization is opposite to that of the rest of the bulk are studied. The phase transition arises because of the strong exchange interaction between spins in the magnetic crystal upon meeting the conditions of (1) the presence of external pumping, which generates an excess of magnons as compared with their equilibrium number, and (2) the contribution of diffusion processes into the relaxation processes of the magnetization (nonlocal relaxation). The processes of formation of the phases with the inverse direction of magnetization intensify upon rapid cooling of the sample. It is shown that, for a certain set of the parameters, such phases can arise at rapid cooling even in the absence of the magnon pumping.

${}^{139}\mathrm{La}$ nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements have been performed to investigate the magnetic properties of the itinerant magnet ${\mathrm{La}}_2{\mathrm{Ni}}_7$, which shows a series of antiferromagnetic (AFM) phase transitions at $T_{\mathrm{N}1}=61$ K, $T_{\mathrm{N}2}=56$ K, and $T_{\mathrm{N}3}=42$ K under zero magnetic field. Two distinct La NMR signals were observed due to the two crystallographically inequivalent La sites in ${\mathrm{La}}_2{\mathrm{Ni}}_7$ (La1 and La2 in the ${\mathrm{La}}_2{\mathrm{Ni}}_4$ and the ${\mathrm{LaNi}}_5$ subunits of the ${\mathrm{La}}_2{\mathrm{Ni}}_7$ unit cell, respectively). From the ${}^{139}\mathrm{La}$ NQR spectrum in the AFM state below $T_{\mathrm{N}3}$, the AFM state was revealed to be a commensurate state where Ni-ordered moments align along the crystalline $c$ axis. Owing to the two different La sites, we were able to estimate the average values of the Ni-ordered moments ($0.09–0.10$ ${\mu }_{\mathrm{B}}/\mathrm{Ni}$ and $0.17 {\mu }_{\mathrm{B}}/\mathrm{Ni}$ around La1 and La2, respectively) from ${}^{139}\mathrm{La}$ NMR spectrum measurements in the AFM state below $T_{\mathrm{N}3}$, suggesting a nonuniform distribution of the Ni-ordered moments in the AFM state. In contrast, a more uniform distribution of the Ni-ordered moments in the saturated paramagnetic state induced by the application of high magnetic fields is observed. The temperature dependence of the sublattice magnetization measured by the internal magnetic induction at the La2 site in the AFM state was reproduced by a local-moment model better than the self-consistent renormalization (SCR) theory for weak itinerant antiferromagnets. Given the small Ni-ordered moments in the magnetically ordered state, our results suggest that ${\mathrm{La}}_2{\mathrm{Ni}}_7$ has characteristics of both itinerant nature and localized nature in its magnetism. With this in mind, it is noteworthy that the temperature dependence of nuclear spin-relaxation rates ($1/T_1$) in the paramagnetic state above $T_{\mathrm{N}1}$ measured at zero magnetic field can be explained qualitatively by both the SCR theory and the local-moment model.

We report on our terahertz spectroscopic investigation of the van der Waals insulator $\mathrm{Ni}{\mathrm{I}}_2$, exhibiting antiferromagnetism below $T_{\mathrm{N}1}\simeq 78\mathrm{K}$ and multiferroicity below $T_{\mathrm{N}2}\simeq 59.5\mathrm{K}$. Two electromagnon modes were detected at 34 and $37\mathrm{c}{\mathrm{m}}^{-1}$ below $T_{\mathrm{N}2}$ where the material is in the helimagnetic-multiferroic phase. Our transmission measurement shows that the electromagnon resonance modes redshift with increasing temperature (at zero magnetic field) but blueshift with increasing magnetic field (at $1.5\mathrm{K}$). A separate reflection measurement confirms the electric dipole active nature of the two electromagnon modes. The polarization, temperature, and magnetic field dependences show that these electromagnon modes are closely linked with the helimagnetic ordering in the multiferroic phase of $\mathrm{Ni}{\mathrm{I}}_2$. The electromagnon energies are also consistent with the energy scale of the two-magnon sideband excitation around the Zhang-Rice exciton mode recently discovered in $\mathrm{Ni}{\mathrm{I}}_2$.

On-chip cavity magnomechanics is an emerging field exploring acoustic and magnonic functionalities of various ferromagnetic materials and structures using strongly confined phonons. It is expected that such cavity magnomechanics can be extended to multilayer ferromagnets, especially synthetic antiferromagnets (SAFs) that exhibit zero net magnetization through interlayer exchange coupling. However, the conventional theoretical framework for a single ferromagnet cannot be used directly because of the antiferromagnetic magnetization dynamics associated with the interlayer exchange coupling. In this paper, we theoretically investigate phonon-magnon coupling with a three-layer SAF. Our formulation of the phonon-magnon coupling constants reveals that the acoustic (optical) magnon mode dominantly couples to the cavity phonon when the magnetization angles in the two ferromagnetic layers are antiparallel (near orthogonal). Moreover, numerical calculations including the effects of dipole-dipole interactions and in-plane uniaxial magnetic anisotropy allow us to predict phonon frequency shifts and linewidth broadening that can be detected in experiments. These theoretical insights would greatly help us to make a strategy for bringing the system into the strong coupling regime and to devise control protocols in analogy to cavity quantum electrodynamics and cavity optomechanics.

We perform a systematic ab initio density functional study of the superconductivity, electronic and phononic band structures, electron-phonon coupling, and elastic constants of all four possible structures of niobium nitride $\beta \text{−}{\mathrm{Nb}}_2\mathrm{N}$ as well as Nb-rich $\gamma \text{−}{\mathrm{Nb}}_4{\mathrm{N}}_3$ and N-rich ${\beta }^{\prime }\text{−}{\mathrm{Nb}}_4{\mathrm{N}}_5$. First of all, we find that all four structures of $\beta \text{−}{\mathrm{Nb}}_2\mathrm{N}$ are superconductors with superconducting transition temperatures ($T_c$) ranging from 0.6 to 6.1 K, depending on the structure. This explains why previous experiments reported contradicting $T_c$ values for $\beta \text{−}{\mathrm{Nb}}_2\mathrm{N}$. Furthermore, both $\gamma \text{−}{\mathrm{Nb}}_4{\mathrm{N}}_3$ and ${\beta }^{\prime }\text{−}{\mathrm{Nb}}_4{\mathrm{N}}_5$ are predicted to be superconductors with rather high $T_c$ of 8.5 and 15.3 K, respectively. Second, the calculated elastic constants and phonon dispersion relations show that all the considered niobium nitride structures are mechanically and dynamically stable. Moreover, the calculated elastic moduli demonstrate that all the niobium nitrides are hard materials with bulk moduli and hardness being comparable to or larger than the well-known hard sapphire. Third, the calculated band structures reveal that the nitrides possess both type I and type II Dirac nodal points and are thus topological metals. Finally, the calculated electron-phonon coupling strength, superconductivity, and mechanical properties of the niobium nitrides are discussed in terms of their underlying electronic structures and also Debye temperatures. The present ab initio study thus indicates that $\beta \text{−}{\mathrm{Nb}}_2\mathrm{N}$, $\gamma \text{−}{\mathrm{Nb}}_4{\mathrm{N}}_3$ and ${\beta }^{\prime }\text{−}{\mathrm{Nb}}_4{\mathrm{N}}_5$ are hard superconductors with nontrivial band topology and are promising materials for exploring exotic phenomena due to the interplay of hardness, superconductivity, and nontrivial band topology.

We analyze an analog of the $t\text{−}J\text{−}U$ model as applied to the description of a single moiré flat band of twisted ${\mathrm{WSe}}_2$ bilayer. To take into account the correlation effects induced by a significant strength of the Coulomb repulsion, we use the Gutzwiller approach and compare it with the results obtained by the Hartree-Fock method. We discuss in detail the graduate appearance of a two-dome structure of the superconducting state in the phase diagram by systematically increasing the Coulomb repulsion integral $U$. The two superconducting domes residing on both sides of a Mott insulating state can be reproduced for a realistic parameter range, in agreement with the available experimental data. According to our analysis, the paired state has a highly unconventional character with a mixed $d+id$ (singlet) and $p-ip$ (triplet) symmetry. Both components of the mixed paired state are of comparable amplitudes. However, as shown here, a transition between pure singlet and pure triplet pairing should be possible in the considered system by tuning the top and bottom gate voltage, which controls the magnitude of valley-dependent spin splitting in the system.

We demonstrate that a single layer of graphene subject to a superlattice potential nearly commensurate to a $\sqrt{3}\times \sqrt{3}$ supercell exactly maps to the chiral model of twisted bilayer graphene, albeit with half as many degrees of freedom. We comprehensively review the properties of this “half-chiral model,” including the interacting phases stabilized at integer fillings and the effects of substrate-induced symmetry breaking. We list candidate substrates that could produce a superlattice potential on graphene with the correct periodicity to access the flat-band limit. Experimental measurements on a half-chiral moiré heterostructure, in which valley skyrmions cannot form, could yield insights on the physics they mediate in twisted bilayer graphene.

Nowadays, it is recognized that semiconductors are prospective candidates for promising thermoelectric materials and the gapless topological phonon modes can result in a high phonon scattering rate. Therefore it is necessary to identify the topological phonons in semiconductors, which will aid future research aimed at gaining a better understanding of the thermoelectric properties of semiconductors. Using first-principles calculations and symmetry analysis, we propose a series of semiconductors as excellent candidates for the presence of exotic topological phonons. Remarkably, almost all the types of topological phonons, including various cases of Weyl/Dirac/triple point phonons, sextuple point phonons, nodal line phonons with different shapes and degenerates, and one-, two-, and three-nodal surface phonons can be observed in the phonon curves of these proposed semiconductors, revealing the ubiquitous existence of topological phonon modes in semiconductors. Moreover, the diverse types of topological phonons induce rich types of phononic surface modes in the surface orientations of semiconductors, which is advantageous to surface physics research.

Time-crystalline behavior has been predicted and observed in quantum central-spin systems with periodic driving and Ising interactions. Here, we theoretically show that it can also arise in central-spin systems with Heisenberg interactions. We present two methods to achieve this: application of a sufficiently large Zeeman splitting on the central spin compared to the satellite spins, or else by applying additional pulses to the central spin every Floquet period. In both cases, we show that the system exhibits a subharmonic response in spin magnetizations in the presence of disorder for both pure Heisenberg and XXZ interactions. Our results pertain to any XXZ central-spin system, including hyperfine-coupled electron-nuclear systems in quantum dots or color centers.

We investigate the emergence and corresponding nature of exceptional points located on exceptional hypersurfaces of non-Hermitian transfer matrices for finite-range one-dimensional lattice models. We unravel the nontrivial role of these exceptional points in determining the system-size scaling of electrical conductance in nonequilibrium steady state. We observe that the band edges of the system always correspond to the transfer matrix exceptional points. Interestingly, albeit the lower band edge always occurs at wave vector $k=0$, the upper band edge may or may not correspond to $k=\pi$. Nonetheless, in all the cases, the system exhibits universal subdiffusive transport for conductance at every band edge with scaling $N^{-b}$ with scaling exponent $b=2$. However, for cases when the upper band edge is not located at $k=\pi$, the conductance features interesting oscillations with overall $N^{-2}$ scaling. Our work further reveals that this setup is uniquely suited to generate higher-order transfer matrix exceptional points at upper band edge when one considers finite-range hoppings beyond nearest neighbor. Additional exceptional points other than those at band edges are shown to occur, although interestingly, these do not give rise to anomalous transport.

Thermoelectric effects allow replacing mechanical work with electric power, and the resulting devices can be used as heat engines or refrigerators. In the former, a voltage is generated by connecting the device to contacts at different temperatures, while in the latter electric power induces the heat flow from cold to hot reservoirs. Thermoelectric nanoscale devices hold great technological potential due to the ability to integrate them into chips. So far, however, their practical use is hindered by their low performance. We suggest here a design for thermoelectric devices based on chiral organic molecules connected to two magnetic electrodes. Our device utilizes the chiral-induced spin selectivity exhibited by such chiral systems and specifically its main manifestation of strong spin-dependent transport. We analyze the figure of merit and generated power of chiral-molecule-based heat engines and show that both can be significant with the potential to exceed other existing designs.

The Halperin $(m^{\prime },m,n)$ fractional quantum Hall effects of two-component quantum particles are studied in topological checkerboard lattice models. Here for $m\ne m^{\prime }$, we demonstrate the emergence of fractional quantum hall effects with the associated $\mathbf{K}=\left(\begin{array}{cc}m+2& 1\\ 1& m\end{array}\right)$ matrix (even $m=2$ for boson and odd $m=3$ for fermion) in the presence of both strong intercomponent and intracomponent repulsions. Through exact diagonalization and density-matrix renormalization group calculations, we elucidate their topological fractionalizations, including (i) the $det\left|\mathbf{K}\right|=(m^2+2m-1)$-fold ground-state degeneracies and (ii) fractionally quantized topological Chern number matrix $\mathbf{C}={\mathbf{K}}^{-1}$. Our flat band model provides a paradigmatic example of a microscopic Hamiltonian featuring fractional quantum Hall effect with partial spinpolarization.

The antiferromagnetic topological insulator ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ was believed to have a topological surface state (TSS) with large band gap due to the ferromagnetic (FM) order on the surface and to be able to host the long-sought axion states. However, recent angle-resolved photoemission spectroscopy experiments indicate that the TSS is gapless, contradicting the theoretical predictions. Meanwhile, several experiments have suggested that there is robust out-of-plane FM order on the surface of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$. To understand these seemingly contradictory results, we carry out comprehensive first-principles calculations to investigate the interplay between the surface magnetism and the TSS. Our calculations provide direct evidence that in a wide range of parameters, the (nearly) gapless TSS can coexist with the surface FM order, therefore solving the paradox of the surface magnetism and the gapless TSS. We further show that proximity effects can be a promising route to open the gap in the TSS of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$. Our research deepens the understanding of the relationship between surface magnetism and TSSs.

In topological semimetals, the bands can cross at points or lines with different dimensionality and connectivity in momentum space. For graphene and other systems hosting zero-dimensional band touching points, inhomogeneous strain is used to shift the nodal points to mimic gauge fields, whereas the one-dimensional nodal lines can transit between topologically distinct structures in strain fields. Such a nodal-line transition can provide a powerful way to engineer the electronic properties. Here we study the strain-induced Landau quantization for diamond lattices, where nodal chains split into separate lines. The nodal-line transition opens a finite Landau gap for the critical chain point with a vanishing Fermi velocity, which is impossible to be opened in the scenario of magnetic fields or nodal-point systems. Besides the unconventional energy quantization near the chain point, the strained diamond lattices exhibit perfect flat bands in three dimensions with a $\sqrt{n}$-scaling $(n$ is an integer). We also investigate the associated edge states and the line-dependent Hall response. Our work provides an avenue towards understanding the profound roles of nodal-line transition in topological matter and paves the way to study the interplay between strain and higher-dimensional nodal manifolds in arbitrary dimensions.

Motivated by a recent breakthrough transport experiment [Phys. Rev. B 107, 245423 (2023)] in Majorana nanowires, we theoretically investigate local and nonlocal transport in Majorana nanowires in various disorder regimes, correlating the transport properties with the corresponding local and total density of states as well as various topological diagnostics. We find three distinct disorder regimes, with weak (strong)-disorder regimes manifesting (not manifesting) topological superconductivity with clear end Majorana zero modes for longer (but not necessarily for shorter) wires. The intermediate-disorder regime is both interesting and challenging because the topology depends on many details in addition to the strength of disorder, such as the precise disorder configuration and the wire length. The intermediate-disorder regime often manifests multiple effective transitions between topological and nontopological phases as a function of system parameters (e.g., the Zeeman field), and is consistent with the recent Microsoft experiment reflecting small topological gaps and narrow topological regimes in the parameter space.

We consider theoretically the physics of bulk topological superconductivity accompanied by boundary non-Abelian Majorana zero modes in semiconductor-superconductor (SM-SC) hybrid systems consisting of finite wires in the presence of correlated disorder arising from random charged impurities. We find the system to manifest a highly complex behavior due to the subtle interplay between finite wire length and finite disorder, leading to copious low-energy in-gap states throughout the wire and considerably complicating the interpretation of tunneling spectroscopic transport measurements used extensively to search for Majorana modes. The presence of disorder-induced low-energy states may lead to the nonexistence of end Majorana zero modes even when tunneling spectroscopy manifests zero-bias conductance peaks in local tunneling and signatures of bulk gap closing/reopening in the nonlocal transport. In short wires within the intermediate disorder regime, apparent topology may manifest in small ranges (“patches”) of parameter values, which may or may not survive the long-wire limit depending on various details. Because of the dominance of disorder-induced in-gap states, the system may even occasionally have an appropriate topological invariant without manifesting isolated end Majorana zero modes. We discuss our findings in the context of a recent breakthrough experiment from Microsoft reporting the simultaneous observations of zero-bias conductance peaks in local tunneling and gap opening in nonlocal transport within small patches of parameter space. Based on our analysis, we believe that the disorder strength to SC-gap ratio must decrease further for the definitive realization of non-Abelian Majorana zero modes in SM-SC devices.