We report continuous measurements of quasiparticles trapping and clearing from Andreev bound states in aluminum nanobridge Josephson junctions integrated into a superconducting-qubit-like device. We find that trapping is well modeled by independent spontaneous emission events. Above 80 mK the clearing process is well described by the absorption of thermal phonons, but other temperature-independent mechanisms dominate at low temperature. We find a complex structure in the dependence of the low-temperature clearing rate on the Andreev bound state energy. Our results shed light on quasiparticle behavior in qubitlike circuits.

We address the outstanding problem of electron pairing in the presence of strong Coulomb repulsion at small to moderate values of the Coulomb parameter, $r_s\lesssim 2$, and demonstrate that the pseudopotential framework is fundamentally biased and uncontrolled. Instead, one has to break the net result into two distinctively different effects: the Fermi liquid renormalization factor and the change in the effective low-energy coupling. The latter quantity is shown to behave nonmonotonically with an extremum at $r_s\approx 0.75$. Within the random-phase approximation, Coulomb interaction starts to enhance the effective pairing coupling at $r_s>2$, and the suppression of the critical temperature is entirely due to the renormalized Fermi liquid properties. Leading vertex corrections change this picture only quantitatively. Our results call for radical reconsideration of the widely accepted repulsive pseudopotential approach and show the need for precise microscopic treatment of Coulomb interactions in the problem of superconducting instability.

The two-dimensional (2D) to three-dimensional (3D) structural evolution of transition metal dichalcogenides (TMDCs) under high pressure is a significant subject since the change of dimensionality could induce a drastic change in various physical properties. A more compact 3D structure of TMDCs could result in metallization and even superconductivity. In this work, we report an unusual 3D state of an $\mathrm{Hf}{\mathrm{S}}_2$ crystal that can still maintain its semiconducting nature over a wide pressure range. X-ray diffraction results show that the layered $\mathrm{Hf}{\mathrm{S}}_2$ undergoes a reversible transformation to a 3D $Immm$ structure at $\sim 12$ GPa, accompanied by a semiconductor-to-semiconductor transition. The Raman and optical absorption measurements show that the phase transition may occur as early as $\sim 9.2$ GPa. The semiconducting state is stable up to $\sim 65$ GPa, and the metallization occurs at 68.6 GPa, the maximum pressure in this work. This work not only reports the 2D-to-3D structural phase transition in an $\mathrm{Hf}{\mathrm{S}}_2$ crystal driven by pressure, but also gives a clear physical picture that shows the intrinsic electrical transport properties and electronic behavior that enriches our understanding of the electronic behavior of TMDCs and sheds light on the future design of novel optoelectronic devices.

We present a theory and a calculation scheme of structural optimization at finite temperatures within the quasiharmonic approximation (QHA). The theory is based on an efficient scheme of updating the interatomic force constants (IFCs) with the change of crystal structures, which we call the IFC renormalization. The cell shape and the atomic coordinates are treated equally and simultaneously optimized. We apply the theory to the thermal expansion and the pyroelectricity of wurtzite GaN and ZnO, which accurately reproduces the experimentally observed behaviors. Furthermore, we point out a general scheme to obtain correct $T$ dependence at the lowest order in constrained optimizations that reduce the number of effective degrees of freedom, which is helpful to perform efficient QHA calculations with little sacrificing of accuracy. We show that the scheme works properly for GaN and ZnO by comparing with the optimization of all the degrees of freedom.

We report a class of spin gapless semiconductors (SGSs) hosting topological nontrivial features along with a fully compensated ferrimagnetic behavior in VTaNbAl, a quaternary Heusler alloy. Unlike conventional SGSs, this class of compounds acquires a unique band structure with opposite spin characters in the valence and conduction band edges, making them potential candidates for spin valves and a large anomalous Nernst effect. Interestingly, despite compensated ferrimagnetic (CF) behavior, VTaNbAl shows a reasonably large anomalous Hall effect possibly arising from the intrinsic nonvanishing Berry curvature. The CF state breaks the time-reversal symmetry and hence opens the possibility of Weyl nodes. We found four pairs of Weyl nodes located near the Fermi level leading to the nonvanishing Berry curvature, and hence a large anomalous Hall conductivity ($\sim 100{\mathrm{\Omega }}^{-1}{\mathrm{cm}}^{-1}$). Our experimental synthesis confirms VTaNbAl to crystallize in a cubic Heusler structure with $A2$-type disorder. A magnetization measurement shows a very small saturation moment ($\sim {10}^{-3}{\mu }_B/\text{f.u.}$), which agrees fairly well with our theoretical findings of fully compensated ferrimagnetism in the alloy. Resistivity shows a metalliclike nature, with the signature of disorder-induced weak localization in the low-temperature limit. The overall variation with varying temperature, however, is extremely weak.

Entangling a pair of far-distant qubits in many-body systems has been a challenging task in quantum computing. A robust entanglement was predicted in the rainbow states and generating nonlocal Bell pairs protected by a mirror symmetry was recently proposed. We investigate a way to create entangled Majorana fermions in the spin-$\frac{1}{2}$ Kitaev chain with open boundary conditions. The spin-$\frac{1}{2}$ Kitaev chain, a one-dimensional version of the honeycomb lattice with bond-dependent Ising interactions, has a macroscopic degeneracy related to the zero modes containing nonlocal spin strings. We show that applying a pair pulse sequence on the central unit cell of the chain promotes long-distance spin correlations and maximal bipartite entanglement entropy. We extend this method to the generalized bond-dependent spin-$\frac{1}{2}$ chain by introducing another set of Majorana fermions and make a comparison to the entangled Bell pairs. The time to reach maximal bipartite entanglement entropy is shorter in the Kitaev chain as the zero modes do not participate in the entangled pairs. An application of our results to a recently proposed twisted Kitaev chain, ${\mathrm{CoNb}}_2{\mathrm{O}}_6$, is presented and future directions are also discussed.

We study microscopic pairing mechanisms of superconductivity in doped Weyl semimetals. Depending on the nature of the election scattering, there exist three possible superconducting pairings. We evaluate the condensation energy required for electrons to pair up. The ground state of superconducting Weyl semimetals is determined by the fermionic interaction. The scattering between two Weyl nodes plays an important role in pairing mechanisms. It is shown that the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase is the ground state if the electron scattering is dominated by the intranode scattering. On the other hand, if the internode scattering is strong, the ground state is with the BCS pairing. More importantly, we show that the state with FFLO/BCS hybrid pairing cannot be the ground state of superconducting Weyl semimetals, thus our result provides a definitive answer to an unresolved question.

In this study, topologically nontrivial edge and vortex bound states are described in the coexistence phase of chiral spin-singlet superconductivity and noncollinear spin ordering on a triangular lattice in the presence of few (up to four) vortices. We consider the topological phase transition induced by the magnetic order between the phase hosting Majorana modes and the initial phase of the chiral $d$-wave superconductivity supporting non-Majorana modes which is also topologically nontrivial. The change of the excitation spectrum at the critical point is obtained in both cases of open and periodic boundary conditions in the presence of vortices. It is proved that zero-energy Majorana modes localized at vortex cores are caused by noncollinear long-range magnetic ordering. Even though nearby excitation energies of subgap states including the edge-localized and vortex-localized states are very close to each other, the energy difference between different vortex bound states is an order of magnitude higher. This difference determines the energy gap for Majorana vortex modes separating them from other vortex bound states. It is found that even in the presence of noncollinear spin ordering its value can be estimated from the excitation energy of vortex bound states in the pure chiral $d$-wave state for the nonmagnetic case. By studying local density of states near the vortex cores the possibility to experimentally detect the described Majorana vortex modes by scanning tunneling microscopy is discussed. It is demonstrated that Majorana vortex modes and Majorana antivortex modes induced by noncollinear magnetism have different features in energy and spatially resolved density of states due to the chiral symmetry on the superconducting order parameter.

We study the hydrodynamics of superconductors within the framework of Schwinger-Keldysh effective field theory (EFT). We show that in the vicinity of the superconducting phase transition the most general leading-order EFT satisfying the local Kubo-Martin-Schwinger condition is described by a version of the time-dependent Ginzburg-Landau (TDGL) equations augmented with stochastic terms. This version of TDGL is applicable in the gapless regime independent of any microscopic details. Within this approach, it is possible to include systematically the effects of nonuniform temperature and heat conductivity, as well as explicit or spontaneous breaking of time reversal. We also introduce a thermal version of the Josephson relation and use it to construct an exotic hydrodynamics describing a phase of matter where heat can flow without dissipation.

We study the quantum-classical correspondence for systems with interacting spin particles that are strongly chaotic in the classical limit. This is done in the presence of constants of motion associated with the fixed angular momenta of individual spins. Our analysis of the Lyapunov spectra reveals that the largest Lyapunov exponent agrees with the Lyapunov exponent that determines the local instability of each individual spin moving under the influence of all other spins. Within this picture, we introduce a rigorous and simple test of ergodicity for the spin motion, and use it to identify when classical chaos is both strong and global in phase space. In the quantum domain, our analysis of the Hamiltonian matrix in a proper representation allows us to obtain the conditions for the onset of quantum chaos as a function of the model parameters. From the comparison between the quantum and classical domains, we demonstrate that quantum quantities, such as the local density of states (LDoS) and the shape of the chaotic eigenfunctions written in the noninteracting many-body basis, have well-defined classical counterparts. We also find a relationship between the Kolmogorov-Sinai entropy and the width of the LDoS, which is useful for studies of many-body dynamics.

We theoretically investigate the spectral properties of a quantum impurity (QI) hosting the here proposed Majorana-Ising-type quasiparticle excitation. It arises from the coupling between a finite topological superconductor (TSC) based on a chain of magnetic adatoms–superconducting hybrid system and an integer large spin $S$ flanking the QI. It is noteworthy that the spin $S$ couples to the QI via the Ising-type exchange interaction. As the Majorana zero modes (MZMs) at the edges of the TSC chain are overlapped, we counterintuitively find a regime wherein the Ising term modulates the localization of a fractionalized and resonant MZM at the QI site. Interestingly enough, the fermionic nature of this state is revealed as purely of the electron tunneling type and, most astonishingly, it has the Andreev conductance completely null in its birth. Therefore, we find that a resonant edge state appears as a zero mode and discuss it in terms of a poor man's Majorana [Dvir et al., Nature 614, 445 (2023)].

We investigate the electronic excitation of silicon by hydrogen, helium, nitrogen, and neon ions for ion energies ranging from several tens to a few hundred kiloelectronvolts. Experiments are carried out in transmission geometry using a time-of-flight medium energy ion scattering system. The targets are self-supporting, single-crystalline Si (100) foils with nominal thicknesses of 50 and 200 nm. Stopping cross-sections (SCSs) are derived and compared with datasets available from the literature and predictions from theory. The results for H projectiles reveal good agreement with literature datasets, within quoted uncertainties. For He projectiles, the results show good agreement with most of the literature data. For N ions, higher values than reported in the literature are measured. For Ne, where literature data are scarce, we extend the velocity regime for which data exist by a factor of two toward higher velocities. The electronic SCS is found to be proportional to ion velocity for all impinging ions, for velocities below the Bohr velocity $\left(v<v_0\right)$. Comparison with theoretical predictions for a homogeneous free electron gas indicates strong contributions of local energy loss processes different from those expected for electron-hole pair excitation in binary collisions for all ions different from protons.

The $su\left(2\right)$ coherent-state path-integral technique is employed to study lattice electrons strongly coupled to a quantum spin background. In the large-spin limit it is replaced by its classical counterpart that breaks the time-reversal symmetry. The fermions propagating through a classical large-spin texture may then exhibit the topological Hall effect which arises even for a zero scalar spin chirality of the underlying spin background.

Tetralayer graphene has recently become a new addition to the family of few-layer graphenes with versatile electronic properties. This material can be realized in three distinctive stacking configurations for which we determine spectroscopic signatures in angle-resolved photoemission spectroscopy, dynamical optical conductivity, and Raman spectra of interband excitations. The reported library of spectral features of tetralayer graphenes can be used for the noninvasive identification of the stacking order realized in a particular film.

Recently, it was discovered that certain non-Hermitian systems can exhibit qualitative different properties at different system sizes, such as being gapless at small sizes and having topological edge modes at large sizes $L$. This dramatic system-size sensitivity is known as the critical non-Hermitian skin effect (cNHSE), and occurs due to the competition between two or more non-Hermitian pumping channels. In this work, we rigorously develop the notion of a size-dependent generalized Brillouin zone (GBZ) in a general multicomponent cNHSE model ansatz, and found that the GBZ exhibits a universal $a+b^{1/(L+1)}$ scaling behavior. In particular, we provided analytical estimates of the scaling rate $b$ in terms of model parameters, and demonstrated their good empirical fit with two paradigmatic models, the coupled Hatano-Nelson model with offset, and the topologically coupled chain model with offset. We also provided analytic result for the critical size $L_c$, below which cNHSE scaling is frozen. The cNHSE represents the result of juxtaposing different channels for bulk-boundary correspondence breaking, and can be readily demonstrated in non-Hermitian metamaterials and circuit arrays.

We present here a formalism based on time-dependent density functional theory to describe characteristics of both intra- and intervalley excitons in semiconductors, the latter of which had remained a challenge. Through the usage of an appropriate exchange-correlation kernel (nanoquanta), we trace the energy difference between the intra- and intervalley dark excitons in monolayer (1L) $\mathrm{W}{\mathrm{Se}}_2$ to the domination of the exchange part in the exchange-correlation energies of these states. Furthermore, our calculated transition contribution maps establish the momentum resolved weights of the electron-hole excitations in both bright and dark excitons thereby providing a comprehensive understanding of excitonic properties of 1L $\mathrm{W}{\mathrm{Se}}_2$. We find that the states consist of hybridized excitations around the corresponding valleys which leads to brightening of the dark excitons, i.e., significantly decreasing their lifetime which is reflected in the PL spectrum. Using many-body perturbation theory, we calculate the phonon contribution to the energy band gap and the linewidths of the excited electrons, holes, and (bright) exciton to find that as the temperature increases the band gap significantly decreases, while the linewidths increase. Our work paves for describing the ultrafast charge dynamics of transition metal dichalcogenide within an ab initio framework.

In this paper we propose an electronic Lévy glass, analogous to a recent optical realization. To that end, we investigate the transmission of electrons in graphene nanoribbons in the presence of circular electrostatic clusters, whose diameter follow a power-law distribution. We analyze the effect of the electrostatic clusters on the electronic transport regime of the nanoribbons, in terms of its diffusion behavior. Our numerical calculations show that the presence of circular electrostatic clusters induces a transition from Lévy (superdiffusive) to diffusive transport as the energy increases. Furthermore, we argue that in our electronic Lévy glass, superdiffusive transport is an exclusive feature of the low-energy quantum regime, while diffusive transport is a feature of the semiclassical regime. We thus attribute the observed transition to the chiral symmetry breaking, once the energy moves away from the Dirac point of graphene.

Neural network quantum states are a promising tool to analyze complex quantum systems given their representative power. It can however be difficult to optimize efficiently and effectively the parameters of this type of ansatz. Here we propose a local optimization procedure which, when integrated with stochastic reconfiguration, outperforms previously used global optimization approaches. Specifically, we analyze both the ground state energy and the correlations for the nonintegrable tilted Ising model with restricted Boltzmann machines. We find that sequential local updates can lead to faster convergence to states which have energy and correlations closer to those of the ground state, depending on the size of the portion of the neural network which is locally updated. To show the generality of the approach we apply it to both 1D and 2D nonintegrable spin systems.

Using a unified method of the extended first-principles molecular dynamics, we report a wide-range (density $\rho =2.329–18.632$ $\mathrm{g}/{\mathrm{cm}}^3$, temperature $T=1.00\times {10}^4–1.29\times {10}^8$ K) equation of state (EOS) and principal Hugoniot of silicon, which agree well with experimental results, and then evaluate the precision of the results of first-principles calculations and a universal database. Moreover, we investigate the effects of finite-temperature exchange-correlation functionals and emphasize their importance and necessity in the warm dense matter regime, although it vanishes in the nondegenerate and fully degenerate limits. Finally, we show the ionic radial distribution function and density of electronic states to study the evolution of warm dense Si towards the classic plasma limit along its principal Hugoniot. The established standard theoretical EOS table of Si provides a benchmark for the development of high-precision first-principles methods and may improve radiation-hydrodynamics simulations of inertial confinement fusion and deepen the understanding of high-energy-density physics.