The interplay of topology and superconductivity has become a subject of intense research in condensed-matter physics for the pursuit of topologically nontrivial forms of superconducting pairing. An intrinsically normal-conducting material can inherit superconductivity via electrical contact to a parent superconductor via the proximity effect, usually understood as Andreev reflection at the interface between the distinct electronic structures of two separate conductors. However, at high interface transparency, strong coupling inevitably leads to changes in the band structure, locally, owing to hybridization of electronic states. Here, we investigate such strongly proximity-coupled heterostructures of monolayer $1T^{\prime }\text{−}\mathrm{W}{\mathrm{Te}}_2$, grown on $\mathrm{Nb}{\mathrm{Se}}_2$ by van der Waals epitaxy. The superconducting local density of states, resolved in scanning tunneling spectroscopy down to 500 mK, reflects a hybrid electronic structure well described by a multiband framework based on the McMillan equations which captures the multiband superconductivity inherent to the $\mathrm{Nb}{\mathrm{Se}}_2$ substrate and that is induced by proximity to $\mathrm{W}{\mathrm{Te}}_2$, self-consistently. Our material-specific tight-binding model captures the hybridized heterostructure quantitatively and confirms that strong interlayer hopping gives rise to a semimetallic density of states in the two-dimensional $\mathrm{W}{\mathrm{Te}}_2$ bulk, even for nominally band-insulating crystals. The model further accurately predicts the measured order parameter $\mathrm{\Delta }\simeq 0.6$ meV induced in the $\mathrm{W}{\mathrm{Te}}_2$ monolayer bulk, stable beyond a 2 T magnetic field. We believe that our detailed multiband analysis of the hybrid electronic structure provides a useful tool for sensitive spatial mapping of induced order parameters in proximitized atomically thin topological materials.

Geometrical frustration leads to novel quantum phenomena such as the spin-liquid phase in triangular and kagome lattices. Intraband and interband Fermi surface (FS) nesting can drive unique superconducting (SC) ground states with $d$-wave and $s^{\pm }$-pairing symmetries, respectively, according to the criterion that the SC gap changes sign across the nesting wave vector. For an odd number of FSs, when multiple interband nesting is of comparable strength, the sign-reversal criterion between different FS sheets can lead to frustration, which promotes novel SC order parameters. Here, we report the experimental observation of a time-reversal symmetry breaking pairing state in ${\mathrm{Re}}_2\mathrm{Hf}$ resulting from FS nesting frustration. Furthermore, our electronic specific heat and transverse-field muon spin rotation experiments suggest a fully gapped pairing symmetry. The first-principles electronic structure calculation reveals multiple Fermi surface sheets with comparable interband nesting strength. Implementing the ab initio band structure, we compute spin-fluctuation mediated SC pairing symmetry which reveals an $s+i{s}^{\prime }$-pairing state—consistent with experimental observations. Our investigation demonstrates an alternative SC state which provides a putative setting for both applied and fundamental study.

Using an adaptive strategy which enables the study of quenched disordered system via the density-matrix renormalization-group method, we compute the various ground-state spin-spin correlation measures of the spin-$\frac{1}{2}$ antiferromagnetic Heisenberg chain with random coupling constants, namely, the mean values of the bulk and of the end-to-end correlations, the typical value of the bulk correlations, and the distribution of the bulk correlations. Our results are in agreement with the predictions of the strong-disorder renormalization-group method. We do not find any hint of logarithmic corrections either in the bulk average correlations, which were recently reported by Shu et al. [Phys. Rev. B 94, 174442 (2016)], or in the end-to-end average correlations. We report the existence of a logarithmic correction on the end-to-end correlations of the clean chain. Finally, we have determined that the distribution of the bulk correlations, when properly rescaled by an associated Lyapunov exponent, is a narrow and universal (disorder-independent) probability function.

It is shown that in organic semiconductors where organic magnetoresistance (OMAR) is observed, the exchange interaction between electrons and holes localized at different molecules leads to dynamic spin polarization in the direction of the applied magnetic field. The polarization appears even at room temperature due to the nonequilibrium conditions. The strong spin polarization requires the exchange energy to be comparable to the Zeeman energy in the external field and to be larger than or comparable to the energy of hyperfine interaction of electron and nuclear spins. The exchange interaction also modifies the shape of OMAR.

Three symmetries prevent a twisted bilayer of graphene from developing an in-plane spontaneous magnetization in the absence of a magnetic field: time-reversal symmetry, $C_2$ symmetry to $\pi$ rotation, and $C_3$ symmetry to $2\pi /3$ rotation. In contrast, there are experimental and theoretical indications that, at certain electron densities, time-reversal and $C_2$ symmetries are broken spontaneously, while the $C_3$ symmetry is often broken due to strain. We show that in-plane orbital magnetization is a very sensitive probe to the simultaneous breaking of these three symmetries, exhibiting surprisingly large values (of the order of one Bohr magneton per moiré unit cell) for valley polarized states at rather small values of heterostrain. We attribute these large values to the large magnitude of the characteristic magnetization of individual Bloch states, which we find to reflect the fast Dirac velocity of single-layer graphene, rather than the slow velocity of the twisted bilayer. These large values are forced to mutually cancel in valley-symmetric states, but the cancellation does not occur in valley-polarized states. Our analysis is carried out both for noninteracting electrons and within a simplified Hartree-Fock framework.

Ensembles of quantum chaotic systems are expected to exhibit energy eigenvalues with random-matrix-like level repulsion between pairs of energies separated by less than the inverse Thouless time. Recent research has shown that exact and approximate global symmetries of a system have clear signatures in these spectral statistics, enhancing the spectral form factor or correspondingly weakening level repulsion. This paper extends those results to the case of spontaneous symmetry breaking and shows that, surprisingly, spontaneously breaking a symmetry further enhances the spectral form factor. For both random-matrix-theory–inspired toy models and models where the symmetry breaking has a description in terms of fluctuating hydrodynamics, we obtain formulas for this enhancement for arbitrary symmetry breaking patterns, including broken Abelian symmetries $Z_n$ and $U\left(1\right)$ and partially or fully broken non-Abelian symmetries.

Nonmagnetic impurities in iron-based superconductors can provide an important tool to understand the pair symmetry and they can significantly influence the transport and the superconducting behavior. Here, we present a study of the role of strong impurity potential in the Fe plane, induced by Cu substitution, on the electronic and superconducting properties of single crystals of FeSe. The addition of Cu quickly suppresses both the nematic and superconducting states and increases the residual resistivity due to enhanced impurity scattering. Using magnetotransport data up to 35 T for a small amount of Cu impurity, we detect a significant reduction in the mobility of the charge carriers by a factor of $\sim 3$. While the electronic conduction is strongly disrupted by Cu substitution, we identify additional signatures of anisotropic scattering which manifest in linear resistivity at low temperatures and $H^{1.6}$ dependence of magnetoresistance. The suppression of superconductivity by Cu substitution is consistent with a sign-changing $s_{\pm }$ order parameter. Additionally, in the presence of compressive strain, the superconductivity is enhanced, similar to FeSe.

Besides spin, the valley degree of freedom is also a promising candidate as a carrier of information. Spintronics has come a long way, and spin modulation can be realized by quantum interference and the spin-orbit coupling effect. However, how to control the valley degree of freedom using quantum interference is still a problem to be explored. Here, we discover a mechanism for producing valley polarization in a monolayer transition metal dichalcogenide nanoribbon with attached stubs, in which valley-resolved Fano resonance is formed due to the quantum interference of intervalley backscattering. When the quantum interference occurs between the localized states at the edge of the stubs and the continuous channels in the nanoribbon, the transmission dips of the Fano effect are valley polarized. As the number of stubs increases, the valley-polarized transmission dips will split, and valley-resolved minigaps are formed by Fano resonance with intervalley backscattering in the stub superlattice. When the electron incident energy is in these valley-resolved gaps of the superlattice, even with several stubs, the transmission can have a significant valley polarization. Our findings point to an opportunity to realize valley functionalities by quantum interference.

The quantum dynamics of the electron's spin and orbital angular momenta in semiconductor quantum rings is analyzed. Both Rashba and Dresselhaus spin-orbit interactions (SOIs) in their quasi-two-dimensional forms are taken into account. The narrow quantum rings are treated with models including one and two radial modes. We find that when either Rashba or Dresselhaus SOI acts alone, the different angular momentum states are coupled in blocks of two (for a single radial mode) or four (for two radial modes). We also show that the full Hilbert space splits into two disjoint subspaces, which are not coupled by either of the two SOIs, thereby decoupling accordingly the state evolution. When both SOI mechanisms are present, in principle infinitely many states are coupled, but we find by numerical computation of the quantum dynamics that for typical evolution times in practice only a few neighboring states are involved in the dynamics. Thus the exchange of angular momenta proceeds only via very few states. Furthermore, we find a trend that when initially high orbital momenta are prepared, the time evolution of spin and orbital momenta is almost unaffected by the availability of a second radial mode, in sharp contrast to the case of preparing the system in low orbital angular momentum states. The implications of our findings for the coherent control of angular momentum in quantum rings are pointed out.

Terahertz scanning tunneling microscopy (THz-STM) enables ultrafast measurements of surfaces, single molecules, and nanostructures with simultaneous subpicosecond temporal resolution and atomic spatial resolution. In pump-probe THz-STM experiments employing femtosecond optical pump pulses, lightwave-driven tunneling by a time-delayed THz probe pulse accesses the evolving differential conductance of the tunnel junction following photoexcitation. However, a general theoretical approach to extract the time- and voltage-dependent differential conductance from THz-STM measurements is lacking. Here, we introduce an algorithm for pump-probe THz scanning tunneling spectroscopy (THz-STS) analysis. Our approach allows us to reliably reconstruct the tunnel junction's differential conductance in steady-state or time-dependent scenarios from simulated THz-STS data. The algorithm achieves subcycle time resolution, which we demonstrate by retrieving dynamics faster than the bandwidth of the input THz voltage transient. Subcycle THz-STS will make lightwave-driven microscopy yet more powerful as a tool for characterizing ångström-scale ultrafast dynamics in novel materials.

We present a nonperturbative and formally exact approach for the charge transport in interacting nanojunctions based on a real-time path-integral formulation of the reduced system dynamics. For reservoirs of noninteracting fermions, the exact trace over the leads' degrees of freedom results in the time-nonlocal Feynman-Vernon influence functional, a functional of the Grassmann-valued paths of the nanojunction, which induces correlations among the tunneling transitions in and out of the nanojunction. An expansion of the influence functional in terms of the number of tunneling transitions, and integration of the Grassmann variables between the tunneling times, allows us to obtain a still exact generalized master equation for the populations of the reduced density matrix in the occupation-number representation, as well as a formally exact expression for the current. By borrowing the nomenclature of the famous spin-boson model, we parametrize the two-state dynamics of each single-particle fermionic degree of freedom, in the occupation-number representation, in terms of blips and sojourns. We apply our formalism to the exactly solvable resonant level model (RLM) and to the single-impurity Anderson model (SIAM), the latter being a prototype system for studying strong correlations. For both systems, we demonstrate a hierarchical diagrammatic structure. While the hierarchy closes at the second tier for the RLM, this is not the case for the interacting SIAM. Upon inspection of the current kernel, known results from various perturbative and nonperturbative approximation schemes to quantum transport in the SIAM are recovered. Finally, a noncrossing approximation for the hierarchical kernel is developed, which enables us to systematically decrease temperature at each next level of the approximation. Analytical results for a simplified fourth-tier scheme are presented both in equilibrium and nonequilibrium and with an applied magnetic field.

A magnetic field can generally induce circular phonon dichroism based on the formation of Landau levels of electrons. Here, we study the magnetization-induced circular phonon dichroism in transition metal dichalcogenides, without forming Landau levels. We find that, instead of the conventional deformation potential coupling, pseudogauge-type electron-phonon coupling plays an essential role in the emergence of the phenomenon. As a concrete example, a large dichroism signal is obtained in monolayer ${\mathrm{MoTe}}_2$ on a EuO substrate, even without considering Rashba spin-orbit coupling. Due to the two-dimensional spin-valley-coupled band structure, ${\mathrm{MoTe}}_2$ shows a reciprocal and nonreciprocal absorption of circularly polarized acoustic phonons upon reversing the direction of phonon propagation and magnetization, respectively. By varying the gate voltage, a tunable circular phonon dichroism can be realized, which paves a way toward different physics and applications of two-dimensional acoustoelectronics.

The synthesis of germanene on semiconducting substrates such as ${\mathrm{MoS}}_2$ has revealed that it exhibits a V-shaped density of states around the Fermi level, indicating the presence of Dirac electrons. Further experiments demonstrated that charge inhomogeneities of germanene on ${\mathrm{MoS}}_2$ appear as $n$- and $p$-type doped regions, the origin of which is not well understood. In this paper, by means of density functional theory calculations, we study germanene deposited on ${\mathrm{MoS}}_2$ considering various defect types and a variety of stacking configurations. We find that some native defects typical to ${\mathrm{MoS}}_2$ samples lead to the charge transfer between germanene and ${\mathrm{MoS}}_2$. Unlike vacancies and antisite defects, substitution of molybdenum by group IV-V and VII transition metal atoms does not lead to any midgap states, and appears as a plausible explanation for the experimentally observed charge puddles. Our results shed light on the mechanism of breaking the charge neutrality in Dirac materials without altering their electronic properties, which could be important for the realization of lateral $p\text{−}n$ junctions based on two-dimensional materials.

The emergent behaviors in thin films of a multiaxial ferroelectric (FE) due to electrochemical coupling between the rotating polarization and surface ions are explored within the framework of the 2–4 Landau-Ginzburg-Devonshire (LGD) thermodynamic potential combined with the Stephenson-Highland (SH) approach. The combined LGD-SH approach allows us to describe the electrochemical switching and rotation of a polarization vector in a multiaxial ferroelectric film covered by surface ions with a charge density defined by the oxygen pressure. We calculate phase diagrams, analyze the dependence of polarization components on the applied voltage, and discuss the peculiarities of quasistatic ferroelectric, dielectric, and piezoelectric hysteresis loops in thin strained multiaxial ferroelectric films. The nonlinear surface screening by oxygen ions makes the diagrams very different from the known diagrams of, e.g., strained ${\mathrm{BaTiO}}_3$ films. Quite unexpectedly, we predict the appearance of ferroelectric reentrant phases. The obtained results point to the possibility to control the appearance and features of ferroelectric, dielectric, and piezoelectric hysteresis in multiaxial FE films covered with surface ions by varying their concentration via the partial oxygen pressure. The LGD-SH description of a multiaxial FE film can be further implemented within the Bayesian optimization framework, paving the way toward predictive materials optimization.

We report the ${}^{63}\mathrm{Cu}$ and ${}^{65}\mathrm{Cu}$ nuclear spin-lattice relaxation rate measurements of cuprous oxide ${\mathrm{Cu}}_2\mathrm{O}$ in a zero field Cu nuclear quadrupole resonance at $T=77–325\mathrm{K}$. From the detailed isotopic measurements of the relaxation rates, we successfully estimated a finite magnetic relaxation rate ${}^{63}W_M$ and a predominant nuclear quadrupole relaxation rate ${}^{63}W_Q$. ${}^{63}W_Q$ changed as $T^{2.1}$, whereas ${}^{63}W_M$ changed as $T^{1.6}$ or $T^{\beta }\exp (-\mathrm{\Delta }/\mathit{T})$ with $\beta =0.6\left(3\right)$ and $\mathrm{\Delta }=190\left(62\right)\mathrm{K}$. The nuclear spin scattering process due to a nondegenerate Fermi gas was discussed as a possible candidate of the magnetic relaxation.

We theoretically study the Josephson effect in a superconductor/normal metal/superconductor junction based on Kekulé patterned graphene. For the Kekulé-O patterned junctions, a Fermi momentum-splitting Andreev reflection at the interface can be induced by the off-resonant circularly polarized light applied in the normal region, which results in the possible $\pi$ state. In contrast, for the Kekulé-Y patterned junctions, the Fermi momentum-splitting Andreev reflection is strongly suppressed due to the valley-momentum locking, and the junction always exhibits the 0 state. The dependence of the critical current on the junction length and the illumination parameter of the light field is also presented in detail.

We analyze dynamics of the order parameter in superconductor open nanotubes under a strong transport current in an external homogeneous magnetic field using the time-dependent Ginzburg-Landau equation. Near the critical transport current, the dissipation processes are driven by vortex and phase-slip dynamics. A transition between the vortex and phase-slip regimes is found to depend on the external magnetic field only weakly if the magnetic field and/or transport current are switched on gradually. In the case of an abrupt switch-on of the magnetic field or transport current, the system can be triggered to the stable phase-slip regime within a certain window of parameters. Finally, a hysteresis effect in the current-voltage characteristics is predicted in superconductor open nanotubes.

We theoretically study the magnetization relaxation of Ising spins distributed randomly in a $d$-dimension homogeneous and Gaussian profile under a soft-core two-body interaction potential $\propto 1/[1+{(r/R_c)}^{\alpha }]$ $(\alpha \ge d)$, where $r$ is the interspin distance and $R_c$ is the soft-core radius. The dynamics starts with all spins polarized in the transverse direction. In the homogeneous case, an analytic expression is derived at the thermodynamic limit, which starts as $\propto exp(-k{t}^2)$ with a constant $k$ and follows a stretched-exponential law at long time with an exponent $\beta =d/\alpha$. In between an oscillating behavior is observed with a damping amplitude. For Gaussian samples, the degree of disorder in the system can be controlled by the ratio $l_{\rho }/R_c$, with $l_{\rho }$ the mean interspin distance and the magnetization dynamics is investigated numerically. In the limit of $l_{\rho }/R_c\ll 1$, a coherent many-body dynamics is recovered for the total magnetization despite the position disorder of spins. In the opposite limit of $l_{\rho }/R_c\gg 1$, a similar dynamics as that in the homogeneous case emerges at a later time after a initial fast decay of the magnetization. We obtain a stretched exponent of $\beta \approx 0.18$ for the asymptotic evolution with $d=3,\alpha =6$, which is different from that in the homogeneous case $(\beta =0.5)$.