We study the transient phenomena appearing in a subgap region of the double quantum dot coupled in series between the superconducting and normal metallic leads, focusing on the development of the superconducting proximity effect. For the uncorrelated nanostructure we derive explicit expressions of the time-dependent occupancies in both quantum dots, charge currents, and electron pairing induced on individual dots and between them. We show that the initial configurations substantially affect the dynamical processes, in which the in-gap bound states emerge upon coupling the double quantum dot to the superconducting reservoir. In particular, the superconducting proximity effect would be temporarily blocked whenever the quantum dots are initially singly occupied. Such triplet/Andreev blockade has been recently reported experimentally for double quantum dots embedded in the Josephson [Bouman et al., Phys. Rev. B 102, 220505 (2020)] and Andreev [Zhang et al., arXiv:2102.03283 (2021)] junctions. We also address the role of correlation effects within the lowest-order decoupling scheme and by the time-dependent numerical renormalization group calculations. Competition of the repulsive Coulomb interactions with the superconducting proximity effect leads to renormalization of the in-gap quasiparticles, speeding up the quantum oscillations and narrowing a region of transient phenomena, whereas the dynamical Andreev blockade is well pronounced in the weak interdot coupling limit. We propose feasible methods for detecting the characteristic timescales that could be observable by the Andreev spectroscopy.

We show that many-body localization (MBL) effects can be observed in a finite chain of exchange-coupled spin qubits in the presence of both exchange and magnetic noise, a system that has been experimentally realized in semiconductors and is a potential solid-state quantum computing platform. In addition to established measures of MBL, the level spacing ratio and the entanglement entropy, we propose another quantity, the spin-spin correlation function, that can be measured experimentally and is particularly well-suited to experiments in semiconductor-based electron spin qubit systems. We show that, in cases that the established measures detect as delocalized “phases,” the spin-spin correlation functions retain no memory of the system's initial state (i.e., the long-time value deviates significantly from the initial value), but that they do retain memory in cases that the established measures detect as localized “phases.” We also discover an interesting counterintuitive result that there is no clear tendency towards localization with increasing charge noise in small systems ($3–10$ spins). The proposed experiments should be feasible in the existing semiconductor spin qubit systems.

We present exact analytical results for the differential conductance of a finite Kitaev chain in an N-S-N configuration, where the topological superconductor is contacted on both sides with normal leads. Our results are obtained with the Keldysh nonequilibrium Green's function technique, using the full spectrum of the Kitaev chain without resorting to minimal models. A closed formula for the linear conductance is given, and the analytical procedure to obtain the differential conductance for the transport mediated by higher excitations is described. The linear conductance attains the maximum value of $e^2/h$ only for the exact zero-energy states. Also, the differential conductance exhibits a complex pattern created by numerous crossings and anticrossings in the excitation spectrum. We reveal the crossings to be protected by inversion symmetry, while the anticrossings result from a pairing-induced hybridization of particlelike and holelike solutions with the same inversion character. Our comprehensive treatment of the Kitaev chain allows us also to identify the contributions of both local and nonlocal transmission processes to transport at arbitrary bias voltage. Local Andreev reflection processes dominate the transport within the bulk gap and diminish for higher excited states but reemerge when the bias voltage probes the avoided crossings. The nonlocal direct transmission is enhanced above the bulk gap but contributes also to the transport mediated by the topological states.

To overcome the brittleness of metallic glasses (MGs), their structure, chemistry, or loading conditions are usually controlled. Here, the local stress state in MGs was modulated without affecting their structure. The elastically designed MG heterostructures provide enhanced ductility together with strain hardening during loading. The stress heterogeneity leads to shear band multiplication that consequently enhances the macroscopic ductility of MGs. In addition, the residual compressive stress significantly increases the strength of the glass and is responsible for the observed strain hardening.

Electron spin dynamics are studied in Ga-doped ZnO single crystals by time-resolved Faraday and Kerr rotation spectroscopies. Long-lived spin coherence with two dephasing processes is discovered where the characteristic time is up to 5.2 ns at room temperature. Through the dependence measurements of laser wavelength and temperature, the room-temperature long-lived spin signal is attributed to localized electrons. The spin dephasing (relaxation) processes are independent of transverse (longitudinal) magnetic fields, indicating the spin dephasing not resulting from the $g$-factor inhomogeneity and electron-nuclear hyperfine interaction. Phonon-induced spin dephasing/relaxation mechanism is also excluded by the magnetic field and temperature dependence measurements. It reveals that the two spin dephasing processes originate from two types of localized electrons, both of which are dominated by the anisotropic exchange Dzyaloshinskii-Moriya interaction between adjacent localized electrons.

We report electrically tunable spin singlet and triplet exciton emission from atomically aligned transition metal dichalcogenide (TMD) heterostructures. The observation of these states in both 0° and 60° heterostructures provides the stacking orientation degree of freedom for polarization switching in interlayer excitons. We confirm the spin configurations of the light-emitting excitons employing magnetic fields to measure effective exciton $g$ factors. The interlayer tunneling current across the TMD heterostructure enables the electrical generation of singlet and triplet exciton emission in this atomically thin p-n junction. We demonstrate tunability between the singlet and triplet exciton photoluminescence via electrostatic gates and excitation power. By tuning the gates and interlayer bias voltage, the electroluminescence of the singlet and triplet can be switched with ratios of 10:1. Atomically thin TMD heterostructure light-emitting diodes thus enable a route for optoelectronic devices that can configure spin and valley quantum states independently.

High pressure properties of silicon subhydrides SiH, ${\mathrm{SiH}}_2$, and ${\mathrm{SiH}}_3$ are investigated and compared with those of ${\mathrm{SiH}}_4$ by using ab initio methods. Although the subhydrides are thermodynamically unstable at lower pressures, SiH, ${\mathrm{SiH}}_2$, and ${\mathrm{SiH}}_3$ are found to be stabilized above 486, 386, and 430 GPa, respectively. The predicted phases of SiH, ${\mathrm{SiH}}_2$, and ${\mathrm{SiH}}_3$ have $Fm\overline{3}m, I4/mmm$, and $I4/m$ symmetries, respectively, and are metallic. The density of states suggests that the electronic states at the Fermi energy are more delocalized in the subhydrides than in ${\mathrm{SiH}}_4$. This fact is well reflected in the superconducting transition temperature ($T_c$) estimated for ${\mathrm{SiH}}_2$ and ${\mathrm{SiH}}_3$. Indeed, $T_c$ reaches 83 K in ${\mathrm{SiH}}_2$ at 400 GPa and 88 K in ${\mathrm{SiH}}_3$ at 450 GPa while that is 28 K in ${\mathrm{SiH}}_4$ at 400 GPa. Although $T_c$ of ${\mathrm{SiH}}_4$ is expected to exceed 100 K above 503 GPa, the current capability of diamond anvil cells suggests that high-$T_c$ silicon hydride is more likely to be attained experimentally in subhydrides.

We analytically determine the non-Hermitian mobility edges of a one-dimensional quasiperiodic lattice model with exponentially decaying, hopping, and complex potentials as well as its dual model, which is just a non-Hermitian generalization of the Ganeshan-Pixley–Das Sarma model with nonreciprocal nearest-neighboring hopping. The presence of the non-Hermitian term destroys the self-duality symmetry and, thus, prevents us exploring the localization-delocalization point through looking for self-dual points. Nevertheless, by applying Avila's global theory, the Lyapunov exponent of the Ganeshan-Pixley–Das Sarma model can be exactly derived, which enables us to get an analytical expression of mobility edge of the non-Hermitian dual model. Consequently, the mobility edge of the original model is obtained by using the dual transformation, which creates exact mappings between the spectra and the wave functions of these two models.

Magnetic resonance is a widely established phenomenon that probes magnetic properties such as magnetic damping and anisotropy. Even though the typical resonance frequency of a magnet ranges from gigahertz to terahertz, experiments also report the resonance near zero frequency in a large class of magnets. Here we revisit this phenomenon by analyzing the symmetry of the system and find that the resonance frequency $\omega$ follows a universal power law $\omega \propto |H-H_c{|}^p$, where $H_c$ is the critical field at which the resonance frequency is zero. When the magnet preserves the rotational symmetry around the external field $H, p=1$. Otherwise, $p=1/2$. The magnon excitations are gapped above $H_c$, gapless at $H_c$, and gapped again below $H_c$. The zero frequency is often accompanied by a reorientation transition in the magnetization. For the case in which $p=1/2$, this transition is described by a Landau theory for second-order phase transitions. We further show that the spin current driven by thermal gradient and spin-orbit effects can be significantly enhanced when the resonance frequency is close to zero, which can be measured electrically by converting the spin current into electric signals. This may provide an experimentally accessible way to characterize the critical field. Our findings provide a unified understanding of the magnetization dynamics near the critical field and may, furthermore, inspire the study of magnon transport near magnetic transitions.

To clarify a mechanism of a giant magnetoresistance (MR), we study the resistivity of a paramagnetic metal in which a magnetic-ion pair is embedded as impurities. In this pair, magnetic ions at the positions ${\mathbf{r}}_1$ and ${\mathbf{r}}_2$, respectively, are assumed to have localized spins ${\mathbf{S}}_1$ and ${\mathbf{S}}_2$, which are called $d$ spins. Conducting electrons, on the other hand, are assumed to have a light mass, being called $s$ electrons. The $s$ electrons are scattered by both the $d$ spins through the $s-d$ exchange interaction. Since interferences occur in the wave functions of electrons scattered by both the $d$ spins, transition probabilities between different electronic states are shown to depend strongly on $\mathbf{R}={\mathbf{r}}_2-{\mathbf{r}}_1$ and the spin configuration in the pair $({\mathbf{S}}_1·{\mathbf{S}}_2)$. The steady state of the $s$ electron system under an external electric field is obtained on the basis of the Boltzmann equation and the relaxation-time approximation. By using the obtained relaxation times, the resistivity of the metal is calculated to find its $\mathbf{R}$ and $({\mathbf{S}}_1·{\mathbf{S}}_2)$ dependencies. MRs are shown to appear when an antiparallel spin configuration realized by the Ruderman-Kittel-Kasuya-Yoshida interaction is changed to be a parallel one by a magnetic field. The maximum MR ratio ${[({\rho }_{\mathrm{ap}}-{\rho }_{\mathrm{p}})/{\rho }_{\mathrm{p}}]}_{\mathrm{MR}}$ is found to give a giant MR (GMR) of the order of ${10}^2\%, {\rho }_{\mathrm{ap}}$ and ${\rho }_{\mathrm{p}}$ being the resistivities in the metals with the antiparallel and parallel spin pairs, respectively. These results are extensively applied to the GMR in the metallic magnetic multilayers by taking into account possible magnetic-ion pairs in their nonmagnetic layers. This application shows that Mott's two-fluid model can exhibit a GMR of magnetic multilayers also in the case of the current in planes (CIP).

We study the role of time reversal symmetry (TRS) in the circular photogalvanic (CPG) responses considering a chiral Weyl semimetal (WSM), while a quantized CPG response is guaranteed by both the broken inversion symmetry and broken mirror symmetries. The TRS broken WSM yields one left and one right chiral Weyl node (WN), while there are two left and right chiral WNs for a TRS invariant WSM. We show that these features can potentially cause the quantization of a CPG response at higher values compared to the topological charge of the underlying WSM. This is further supported by the fact that Berry curvature and velocity behave differently depending on whether the system preserves or breaks the TRS. We find the CPG responses for a TRS invariant type-II WSM to be quantized at two and four times the topological charge of the activated WNs while the chemical potentials are, respectively, chosen in the vicinity of the energies associated with the left and right chiral WNs. By contrast, irrespective of the above choice of the chemical potential, the quantization in the CPG response is directly given by the topological charge of the activated WNs for the TRS broken case. Interestingly, we notice a nonquantized peak in the CPG response when the energies of the WNs associated with opposite chiralities are close to each other, as is the case for the TRS invariant type-I WSM considered here. Moreover, we show that the tilt can significantly modify the CPG response as the velocity in the tilt direction changes, which enters into the CPG tensor through the Fermi distribution function. Given these exciting outcomes, the second-order CPG response emerges as a useful indicator to characterize the system under consideration. Furthermore, we investigate the momentum resolved structure of the CPG response to relate with the final results and strengthen our analysis from the perspective of the lattice models.

Valley pseudospins, as quantum states of energy extrema in momentum space, have been introduced from condensed-matter systems into classical sound systems, and several valley sonic crystals (VSCs) have been realized experimentally. However, in the existing VSCs, topological kink states generally appear in a single band gap, which apparently has become an obstacle for multiband topological sound devices. To overcome this challenge, we here experimentally demonstrate dual-band VSCs, in which robust valley kink states exist in two separated bulk band gaps. More interestingly, two opposite valleys separately located in two band gaps are locked to a single propagation direction, which arises from the fact that the bands below two band gaps show opposite valley Chern numbers at the K/K′ valley. This double valley-locking phenomenon has been demonstrated via measuring the topological refraction of the kink states into the ambient space at a zigzag termination. We observe positive refraction at the lower band gap, whereas the coexistence of positive and negative refraction at the higher band gap. Additionally, we observe the robust valley transport through the sharp corners at two band gaps. The designed VSCs with the dual-band topological refraction and robust valley transport could find potential applications in multiband and multidirectional devices.

We study theoretically the transport through a finite-size sawtooth lattice coupled to two fermionic reservoirs kept in the superfluid state. We focus on the DC Josephson effect and find that the flat-band states of the sawtooth lattice can support larger critical current and a higher temperature than the dispersive band states. However, for this to occur the boundary states of the finite-size lattice need to be tuned at resonance with the bulk flat-band states by means of additional boundary potentials. We show that transport in a two-terminal configuration can reveal the salient features of the geometric contribution of flat-band superconductivity, namely the linear dependence of key quantities, such as the critical current and critical temperature, on the interaction. Our results are based on parameters of a realistic experimental lattice potential, and we discuss the conditions one needs to reach to observe the predicted effects experimentally.

We study the tunneling magnetoresistance and the spin-valley transport in silicene-based magnetic superlattices. The superlattice profile is obtained by the periodic modulation of the exchange field induced by ferromagnetic electrodes. The silicene band gap in the superlattice structure is also modulated through an external perpendicular electric field. The concept of parallel and antiparallel magnetization configurations of single magnetic junctions is extended to the periodic case by switching the magnetization orientation of the adjacent magnetic barriers in parallel and antiparallel fashion. The transfer matrix method and the Landauer-Büttiker formalism are used to obtain the transmission and transport properties, respectively. We find an oscillating conductance once the periodic modulation is incorporated. By tuning the external perpendicular electric field a conductance gap is obtained for the antiparallel configuration, which results in an enhancement of the tunneling magnetoresistance with respect to single magnetic junctions. In the case of the spin-valley polarization it is not possible to obtain two well-defined polarization states by simply switching the magnetization orientation, as in single magnetic junctions, due to the equivalence of the spin-valley conductance components. However, by inducing structural asymmetry in the width of barriers-wells, two well-defined polarization states can be reached. Moreover, an additional enhancement of the tunneling magnetoresistance is induced by the structural asymmetry. Our findings indicate that magnetic periodic modulation can be an option to improve the tunneling magnetoresistance and the spin-valley polarization of silicene-based structures.

Using first-principles and ab initio molecular dynamics calculations, effects of temperatures on ferroelastic transition, band gap engineering, anisotropic carrier effective masses, and stability of transition metal adatom magnetization in phosphorene under tensile strains are investigated, which determine mainly its unique properties for designing novel low-dimensional electronic and spintronic devices. We find that (i) the ferroelastic transition of phosphorene under tensile strains predicted at 0 K is replaced by developments of a series of twinning structures above the room temperature (300 K), which will modify predictions on the band structure, carrier mobility, and thermal conductivity of phosphorene in strain engineering processes; (ii) the emerging twinning structure brings about strong ductility in phosphorene, which can sustain stretching up to a tensile strain of 73% at 300 K before the structural breakdown; (iii) the stable sites for the adsorbed TM atoms (V, Cr, and Mn) on phosphorene are higher in energy compared to those of the (interstitially) doped TM atoms in phosphorene. However, the potential barriers between the adsorbed and doped states are high enough to stabilize the adsorbed TM atoms on phosphorene even at high temperatures (450 K); (iv) under moderate tensile strains ($\sim 10\%$) at room temperature, the adsorbed TM atoms start to move into phosphorene as interstitial dopants. Such room or high temperature stretching may provide a possible way to prepare dilute magnetic semiconducting phosphorene using adsorbed TM atoms from simple chemical vapor depositions; (v) for some TM atoms (V, Cr), whose magnetic moments come from the $3d$ electron states near the Fermi level, their magnetic moments decrease substantially as they move into phosphorene, while for other TM atoms (Mn), whose magnetic moments come from the $3d$ electron states deep below the Fermi level, their magnetic moments are stable as their positions change. These results will deepen our understandings of phosphorene and broaden its technological applications as a versatile two-dimensional material.

In the heavy-fermion materials, the characteristic energy scales of many exotic strongly correlated phenomena (Kondo effect, magnetic order, superconductivity, etc.) are at millielectron-volt order, implying that the heavy-fermion materials are surface sensitive. Here, we investigate the electronic structures for Si- and Ce-terminated surfaces of ${\mathrm{CeRh}}_2{\mathrm{Si}}_2$ by first-principles methods. Our research reveals three notable impacts of surface effects on electronic structures, which are consistent with recent angle-resolved photoemission spectroscopy experiments. First, the relaxation of surface crystal structures changes the relative position of Fermi level, adjusts the dispersion of bands, and enhances the Kondo resonance. Second, the decrease of the hybridization between the $\mathrm{Ce}\text{−}4f$ and conduction electrons in the surface layer leads to a weaker Kondo resonance peak and the shift of spin-orbit bands. Third, the variation of crystal electric field around surface Ce atoms affects the splitting of Kondo resonance peaks, and also pushes down the lower Hubbard bands of surface $4f$ electrons. Moreover, we find the characteristic of bulk's lower Hubbard bands, which was overlooked in previous works. Our investigation suggests that these surface effects are potentially important and highlighted in the future researches on properties of strongly correlated materials.

Recent experimental measurements of light absorption in few-layer black phosphorus (BP) revealed a series of high and sharp peaks, interspersed by pairs of lower and broader features. Here, we propose a theoretical model for these excitonic states in few-layer BP within a continuum approach for the in-plane degrees of freedom and a tight-binding approximation that accounts for interlayer couplings. This yields excitonic transitions between different combinations of the subbands created by the coupled BP layers, which leads to a series of high and low oscillator strength excitonic states, consistent with the experimentally observed bright and dark exciton peaks, respectively. The main characteristics of such subband exciton states, as well as the possibility to control their energies and oscillator strengths via applied electric and magnetic fields, are discussed, towards a full understanding of the excitonic spectrum of few-layer BP and its tunability.

We generalize Wentzel-Kramers-Brillouin (WKB) semiclassical equations for pseudospin-1 $\alpha -{\mathcal{T}}_3$ materials with arbitrary hopping parameter $0<\alpha <1$, which includes the dice lattice and graphene as two limiting cases. In conjunction with a series-expansion method in powers of Planck constant $\hslash$, we acquired and solved a system of recurrent differential equations for semiclassical electron wave functions in $\alpha -{\mathcal{T}}_3$. Making use of these obtained wave functions, we analyzed the physics-related mechanism and quantified the transmission of pseudospin-1 Dirac electrons across nonrectangular potential barriers in $\alpha -{\mathcal{T}}_3$ materials with both zero and finite band gaps. Our studies reveal several unique features, including the way in which the electron transmission depends on the energy gap, the slope of the potential barrier profile and the transverse momentum of incoming electrons. Specifically, we have found a strong dependence of the obtained transmission amplitude on the geometry-phase $\varphi ={tan}^{-1}\alpha$ of $\alpha -{\mathcal{T}}_3$ lattices. We believe our current findings can be applied to Dirac cone-based tunneling transistors in ultrafast analog RF devices, as well as to tunneling-current control by a potential barrier through a one-dimensional array of scatters.

Electrical control of magnetism in a two-dimensional (2D) semiconductor is of great interest for emerging nanoscale low-dissipation spintronic devices. Here, we propose a general approach of tuning magnetic coupling and anisotropy of a van der Waals (vdW) 2D magnetic semiconductor via a built-in electric field generated by the adsorption of superatomic ions. Using first-principles calculations, we predict a significant enhancement of ferromagnetic coupling and a great change of magnetic anisotropy in 2D semiconductors when they are sandwiched between superatomic cations and anions. The magnetic coupling is directly affected by the built-in electric field, which lifts the energy levels of mediated ligands' orbitals and enhances the superexchange interactions. These findings will be of interest for ionic gating controlled ferromagnets and magnetoelectronics based on vdW 2D semiconductors.

Spontaneous current orders due to odd-parity order parameters have attracted increasing attention in various strongly correlated metals. we discover a spin-fluctuation-driven charge loop current (cLC) mechanism based on the functional renormalization group theory. The present mechanism leads to the ferro-cLC order in a simple frustrated chain Hubbard model. The cLC appears between the antiferromagnetic and $d$-wave superconducting ($d\mathrm{S}\mathrm{C}$) phases. While the microscopic origin of the cLC has a close similarity to that of the $d\mathrm{S}\mathrm{C}$, the cLC transition temperature $T_{\mathrm{cLC}}$ can be higher than the $d\mathrm{S}\mathrm{C}$ one for a wide parameter range. Furthermore, we reveal that the ferro-cLC order is driven by the strong enhancement of the forward scatterings $g_2$ and $g_4$ owing to the two dimensionality based on the $g$-ology language. The present study indicates that the cLC can emerge in metals near the magnetic criticality with geometrical frustration.