We report intrinsic tunneling data for mesa structures fabricated on three over- and optimally-doped ${\mathrm{Bi}}_{2.15}{\mathrm{Sr}}_{1.85}{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ crystals with transition temperatures of 86–78 K and 0.16–0.19 holes per ${\mathrm{CuO}}_2$ unit, for a wide range of temperature ($T$) and applied magnetic field ($H$), primarily focusing on one overdoped crystal (OD80). The differential conductance above the gap edge shows a clear dip structure which is highly suggestive of strong coupling to a narrow boson mode. Data below the gap edge suggest that tunnelling is weaker near the nodes of the $d$-wave gap and give clear evidence for strong $T$-dependent pair breaking. These findings could help theorists make a detailed Eliashberg analysis and thereby contribute towards understanding the pairing mechanism. We show that for our OD80 crystal the gap above $T_c$, although large, is reasonably consistent with the theory of superconducting fluctuations.

We report that the spin-orbit torque (SOT) in a Co/Pt structure is significantly enhanced by partial oxidation of the Co-side interface. The interface-oxidized and unoxidized Co/Pt samples were prepared to compare the SOTs, which were determined using the harmonic Hall measurement. Even though an insulating ${\mathrm{CoO}}_x$ layer exists at the Co/Pt interface, the dampinglike and fieldlike SOT efficiencies in the interface-oxidized sample were severalfold and nearly an order of magnitude larger than those of the unoxidized sample, respectively. The enhancement of the SOT is most likely attributed to the efficient spin current transmission across the ${\mathrm{CoO}}_x$ layer and the appropriately modulated interfacial Rashba effect.

We report on the magnetic, thermodynamic, dielectric, and pyroelectric measurements on the hitherto unreported $\mathrm{Fe}{}_4\mathrm{Ta}{}_2{\mathrm{O}}_9$. This system is seen to exhibit a series of magnetic transitions, many of which are coupled to the emergence of ferroelectric order, making $\mathrm{Fe}{}_4\mathrm{Ta}{}_2{\mathrm{O}}_9$ the only genuine multiferroic in its material class. We suggest that the observed properties arise as a consequence of an effective reduction in the dimensionality of the magnetic lattice, with the magnetically active $\mathrm{Fe}{}^{2+}$ ions preferentially occupying a quasi-two-dimensional buckled honeycomb structure. The low-temperature $H\text{−}T$ phase diagram of $\mathrm{Fe}{}_4\mathrm{Ta}{}_2{\mathrm{O}}_9$ reveals a rich variety of coupled magnetic and ferroelectric phases similar to that observed in the distorted kagome systems.

Using random-phase approximation spin-fluctuation theory, we study the influence of the hybridization between iron $d$ orbitals and pnictide $p$ orbitals on the superconducting pairing state in iron-based superconductors. The calculations are performed for a 16-orbital Hubbard-Hund tight-binding model of ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ that includes the As-$p$ orbital degrees of freedom in addition to the Fe-$d$ orbitals and compared to calculations for a 10-orbital Fe-$d$ only model. In both models we find a leading $s^{\pm }$ pairing state and a subleading $d_{x^2-y^2}$-wave state in the parent compound. Upon doping, we find that the $s^{\pm }$ state remains the leading state in the 16-orbital model up to a doping level of 0.475 electrons per unit cell, at which the hole Fermi-surface pockets at the zone center start to disappear. This is in contrast to the 10-orbital model, where the $d$-wave state becomes the leading state at a doping of less than 0.2 electrons. This improved stability of $s^{\pm }$ pairing is found to arise from a decrease of $d_{xy}$ orbital weight on the electron pockets due to hybridization with the As-$p$ orbitals and the resulting reduction of near $(\pi ,\pi )$ spin-fluctuation scattering which favors the competing $d$-wave state. These results show that the orbital dependent hybridization of Fermi-surface Bloch states with the usually neglected $p$-orbital states is an important ingredient in an improved itinerant pairing theory.

We perform a comparative study of the electronic structures and electrical resistivity properties of black $\left(A17\right)$ and of pressurized gray $\left(A7\right)$ phosphorus, showing band-selective Kondoesque electronic reconstruction in these layered $p$-band phosphorus allotropes. Based on density functional dynamical mean-field theory calculations, we show that gray phosphorus can host a three-dimensional Kondo semimetal to a Fermi liquid phase crossover under anisotropic compression, which lifts in-layer orbital degeneracy. Therein, the $3p$ spectrum is almost unaffected both in the semiconducting and semimetallic Kondo phases, however in the Fermi liquid regime strong electronic reconstruction is predicted to exist in the orbital nematic phase of compressed gray phosphorus. These findings contribute to the microscopic understanding of the role played by dynamical multiorbital electronic interactions in the low energy spectrum of correlated semiconductors and topological semimetals.

We show that the boundary charge and spin can be used as alternative signatures of the topological phase transition in topological models such as semiconducting nanowires with strong Rashba spin-orbit interaction in the presence of a magnetic field and in proximity to an $s$-wave superconductor. We identify signatures of the topological phase transition that do not rely on the presence of Majorana zero-energy modes and, thus, can serve as independent probes of topological properties. The boundary spin component along the magnetic field, obtained by summing contributions from all states below the Fermi level, has a pronounced peak at the topological phase transition point. Generally such signatures can be observed at boundaries between topological and trivial sections in nanowires and are stable against disorder.

Employing the self-learning quantum Monte Carlo algorithm, we investigate the frustrated transverse-field triangle-lattice Ising model coupled to a Fermi surface. Without fermions, the spin degrees of freedom undergo a second-order quantum phase transition between paramagnetic and clock-ordered phases. This quantum critical point (QCP) has an emergent U(1) symmetry and thus belongs to the (2+1)D XY universality class. In the presence of fermions, spin fluctuations introduce effective interactions among fermions and distort the bare Fermi surface towards an interacting one with hot spots and Fermi pockets. Near the QCP, non-Fermi-liquid behaviors are observed at the hot spots, and the QCP is rendered into a different universality with Hertz-Millis–type exponents. The detailed properties of this QCP and possibly related experimental systems are also discussed.

With the aim of constructing an electronic structure approach that systematically goes beyond the GW and random phase approximation (RPA) we introduce a vertex correction based on the exact-exchange (EXX) potential of time-dependent density functional theory. The EXX vertex function is constrained to be local but is expected to capture similar physics as the Hartree-Fock vertex. With the EXX vertex, we then unify different beyond-RPA approaches such as the various resummations of RPA with exchange and the second-order screened exchange approximation. The theoretical analysis is supported by numerical studies on the hydrogen dimer and the electron gas, and we discuss the role of including the vertex correction in both the screened interaction and the self-energy. Finally, we give details on our implementation within the plane-wave pseudo potential framework and demonstrate the excellent performance of the different RPA with exchange methods in describing the energetics of hydrogen and van der Waals bonds.

Nonideal topological insulator (TI) films in which the bulk states are not insulating due to unintentional doping exhibit strong surface–bulk coupling. Such surface–bulk coupling can further induce intersurface coupling that affects the electrical conductivity of the TI films through a quantum interference effect known as weak antilocalization. Increased understanding and control of intersurface coupling is therefore crucial for the use of TI-based quantum devices. In this report on the transport properties of doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ films under perpendicular and parallel magnetic fields, we observe a crossover between coupled and decoupled surface channels that is mediated by intentional disorder controlled by a post-annealing process. The intentional disorder causes the surface state carriers to rapidly lose their quantum phase and coherence, and as a result, more disordered ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ films exhibit a shorter penetration depth of the surface state into the bulk states and weaker intersurface coupling, even though stronger surface–bulk coupling is expected. In previous studies, the role of disorder has generally been considered by determining its effect on surface–bulk scattering, but our results indicate that the role of disorder must be considered as a source of decoherence.

We present simulations of quantum transport in graphene p-n junctions $\left(pn\mathrm{Js}\right)$ in which moiré superlattice potentials are incorporated to demonstrate the interplay between $pn\mathrm{Js}$ and moiré superlattice potentials. It is shown that the longitudinal and Hall resistivity maps can be strongly modulated by the $pn\mathrm{J}$ profile, junction height, and moiré potentials. Device resistance measurements are subsequently performed on graphene/hexagonal-boron-nitride heterostructure samples with accurate alignment of crystallographic orientations to complement and support the simulation results.

Spin-crossover (SCO) solids have been studied from magnetic and structural viewpoints under the form of powder, single crystals, and nanoparticles. Recently, an important progress made in chemistry allowed the design of spin-crossover nanocomposites, combining the properties of two types of spin-crossover solids having different properties, for example, different transition temperatures, ligand fields, etc. In this work, we sketch theoretical investigations based on an elastic description of a SCO nanocomposite, aiming to reproduce the experimental results of literature which exhibited two-step and even three-step spin transitions. Our results not only reproduce the experimental features but also allow a better understanding of the interplay between the elastic and electronic structures of the two components of the SCO nanocomposite. The obtained data offer serious possibilities of extensions to several configurations of nanocomposites according to the shape of the lattice and the relative sizes of the two components under study.

A degeneracy line in parameter space, at which the energy gap closes up, can be the boundary separating either two topological quantum phases or two conventional phases, depending on the surface containing the line. We study the topological feature of degeneracy line in parameter space via a dimerized Kitaev spin chain with staggered transverse field, which can be mapped onto the system of dimerized spinless fermions with $p$-wave superconductivity. The quantum phase boundaries are straight crossing degeneracy lines in three-dimensional (3D) parameter space. We show that the degeneracy line acts as a vortex filament associated with a vector field, which is generated from the Zak phase of the Bogoliubov–de Gennes band. We also investigate the topological invariant in Majorana fermion representation for open chains. The edge modes are not the zero mode but are still protected by the energy gap. The exact midgap states of the Majorana lattice allow us to obtain the corresponding Majorana probability distribution for the whole 3D parameter space, which can establish a scalar field to identify the topological feature of the degeneracy lines. Furthermore, when we switch on a weak tunneling between two ends of the Majorana lattice, the topological invariants of the degeneracy lines can be obtained by the pumped Majorana probability from an adiabatic passage encircling the lines. Numerical simulations of quasiadiabatic time evolution are performed in a small system. We compute the current to monitor the transport of the Majorana fermion, which exhibits an evident character of topological pumping. Our results link the bulk topological invariant to the dynamical behavior of the edge mode, extending the concept of bulk-edge correspondence.

The high critical temperature superconductor lanthanum barium copper oxide (${\mathrm{La}}_{2-x}{\mathrm{Ba}}_x{\mathrm{CuO}}_4$) exhibits a strong anomaly in critical temperature at 1/8th doping, nematicity, and other interesting properties. We report here scanning superconducting quantum interference device imaging of the magnetic fields and susceptibility in a number of thin film ${\mathrm{La}}_{2-x}{\mathrm{Ba}}_x{\mathrm{CuO}}_4$ samples with doping in the vicinity of the 1/8th anomaly. Spatially resolved measurements of the critical temperatures of these samples do not show a pronounced depression at 1/8th doping. They do, however, exhibit strong, nearly linear modulations of the susceptibility (“striae”) of multiple samples with surprisingly long periods of 1–4 $\mu \mathrm{m}$. Counterintuitively, vortices trap in positions of largest diamagnetic susceptibility in these striae. Given the rich interplay of different orders in this material system and its known sensitivity to epitaxial strain, we propose phase separation as a possible origin of these features and discuss scenarios in which that might arise.

We provide a detailed formulation of the recently proposed variational approach [Y. Ashida, T. Shi, M.-C. Bañuls, J. I. Cirac, and E. Demler, Phys. Rev. Lett. 121, 026805 (2018)] to study ground-state properties and out-of-equilibrium dynamics for generic quantum spin-impurity systems. Motivated by the original ideas of Tomonaga, Lee, Low, and Pines, we construct a canonical transformation that completely decouples the impurity from the bath degrees of freedom. By combining this transformation with a Gaussian ansatz for the fermionic bath, we obtain a family of variational many-body states that can efficiently encode the strong entanglement between the impurity and fermions of the bath. We give a detailed derivation of equations of motions in the imaginary- and real-time evolutions on the variational manifold. We benchmark our approach by applying it to investigate ground-state and dynamical properties of the anisotropic Kondo model and compare results with those obtained using the matrix-product state (MPS) ansatz. We show that our approach can achieve an accuracy comparable to MPS-based methods with several orders of magnitude fewer variational parameters than the corresponding MPS ansatz. Comparisons to the Yosida ansatz and the exact solution from the Bethe ansatz are also discussed. We use our approach to investigate the two-lead Kondo model and analyze its long-time spatiotemporal behavior and the conductance behavior at finite bias and magnetic fields. The obtained results are consistent with the previous findings in the Anderson model and the exact solutions at the Toulouse point.

Disorder-induced Anderson localization in quasiparticle transport is a challenging problem to address, even more so in the presence of dissipation, as the symptoms of disorder-induced localization are very closely simulated by the absorption in a system. Following up on recent experimental studies, we numerically study the occurrence of Anderson localization in plasmonic systems at terahertz frequencies. The low losses in the material at these frequencies allow us to separately quantify the localization length and the loss length in the system. We measure a nonmonotonic variation of loss length as a function of disorder, and attribute it to the participation ratio of the localized modes and resulting light occupancy in the metal. Next, we identify a unique behavior of the gap-state frequencies and the density of states under disorder. We observe that the maximally displaced gap-state frequencies have a propensity to remain pinned to the frequency of the gap center. Even under strong disorder, the gap does not close, and the density of states profile continues to remain peaked in the gap, unlike in conventionally studied disordered systems. The origins of this behavior are traced to the nature of the quasiparticle dispersion. In our case, the quasiparticles are identified to be hybrid plasmons generated due to the hybridization of surface plasmon polaritons at a metal-dielectric interface and cavity resonances at subwavelength apertures thereon. This situation is akin to the Kondo systems, where dispersive conduction electrons hybridze with a localized impurity state opening a hybridization gap. Our results provide new insights on the elusive problem of the interplay of loss and localization, and underlines interesting physics at the hybridization gap in hybrid plasmonic systems.

It is shown that the temperature of the resistive transition $T_r$ of a superconducting film can be increased by a thin superconducting or normal overlayer. For instance, deposition of a highly conductive thin overlayer onto a dirty superconducting film can give rise to an “antiproximity effect,” which manifests itself in an initial increase of $T_r\left(d_2\right)$ with the overlayer thickness $d_2$ followed by a decrease of $T_r\left(d_2\right)$ at larger $d_2$. Such a nonmonotonic thickness dependence of $T_r\left(d_2\right)$ results from the interplay of the increase of a net superfluid density mitigating phase fluctuations and the suppression of the critical temperature $T_c$ due to the conventional proximity effect. This behavior of $T_r\left(d_2\right)$ is obtained by solving the Usadel equations to calculate the temperature of the Berezinskii-Kosterlitz-Thouless transition, and the temperature of the resistive transition due to thermally activated hopping of single vortices in dirty bilayers. The theory incorporates relevant material parameters such as thicknesses and conductivities of the layers, the interface contact resistance between them, and the subgap quasiparticle states, which affect both phase fluctuations and the proximity effect suppression of $T_c$. The transition temperature $T_r$ can be optimized by tuning the overlayer parameters, which can significantly weaken vortex fluctuations and nearly restore the mean-field critical temperature. The calculated behavior of $T_r\left(d_2\right)$ may explain the nonmonotonic dependence of $T_r\left(d_2\right)$ observed on (Ag,Au,Mg,Zn)-coated Bi films, Ag-coated Ga and Pb films, or NbN and NbTiN films on AlN buffer layers. These results suggest that bilayers can be used as model systems for systematic investigations of optimization of fluctuations in superconductors.

We study the photoexcitation dynamics of correlated band insulators, using nonequilibrium dynamical mean-field theory for the ionic Hubbard model. We find two distinct behaviors, depending on the ratio of the on-site interaction $U$ and the bare band gap $\mathrm{\Delta }$. For small interactions, the relaxation is characterized by intraband carrier scattering in relatively rigid bands, leading to a nonthermal intermediate state with separate thermal distributions of electrons and holes. This behavior can be viewed as typical for a band insulator with weak interactions. For larger interaction, on the other hand, we observe a strong modification of the electronic spectrum and a filling-in of the gap after photoexcitation, along with a rapid thermalization of the system. The two behaviors therefore provide a dynamical distinction of a correlated band insulator and a band insulator, which can differ even when the spectra of the two systems are similar in equilibrium. The crossover happens when the interaction $U$ is comparable to $\mathrm{\Delta }$.

The unique zero-energy Landau level of graphene has a particle-hole symmetry in the bulk, which is lifted at the boundary leading to a splitting into two chiral edge modes. It has long been theoretically predicted that the splitting of the zero-energy Landau level inside the bulk can lead to many interesting physics, such as quantum spin Hall effect, Dirac-type singular points of the chiral edge modes, and others. However, so far the obtained splitting with high magnetic field even on a $h$-BN substrate is not amenable to experimental detection, and functionality. Guided by theoretical calculations, here we produce a large-gap zero-energy Landau-level splitting ($\sim 150\mathrm{meV}$) with the usage of a one-dimensional (1D) superlattice potential. We have created tunable 1D superlattice in a $h$-BN encapsulated graphene device using an array of metal gates with a period of $\sim 100\mathrm{nm}$ and carried out magnetocapacitance spectroscopy as a function of superlattice potential. At zero magnetic field we observe the modification of the density of states in our capacitance measurement which is consistent with the existing literature. At finite perpendicular magnetic field, we monitor the splitting of the zeroth Landau level as a function of superlattice potential. The observed splitting energy is an order higher in magnitude compared to the previous studies of splitting due to the symmetry breaking in pristine graphene. The origin of such large Landau-level splitting in 1D potential is explained with a degenerate perturbation theory. We find that owing to the periodic potential, the Landau level becomes dispersive, and acquires sharp peaks at the tunable band edges. Our study will pave the way to create the tunable 1D periodic structure for multifunctionalization and device application like graphene electronic circuits from appropriately engineered periodic patterns in near future.

The self-learning Monte Carlo method (SLMC), using a trained effective model to guide Monte Carlo sampling processes, is a powerful general-purpose numerical method recently introduced to speed up simulations in (quantum) many-body systems. In this Rapid Communication, we further improve the efficiency of SLMC by enforcing physical symmetries on the effective model. We demonstrate its effectiveness in the Holstein Hamiltonian, one of the most fundamental many-body descriptions of electron-phonon coupling. Simulations of the Holstein model are notoriously difficult due to a combination of the typical cubic scaling of fermionic Monte Carlo and the presence of extremely long autocorrelation times. Our method addresses both bottlenecks. This enables simulations on large lattices in the most difficult parameter regions, and an evaluation of the critical point for the charge density wave transition at half filling with high precision. We argue that our work opens a research area of quantum Monte Carlo, providing a general procedure to deal with ergodicity in situations involving Hamiltonians with multiple, distinct low-energy states.

The recent discovery of Weyl and Dirac semimetals is one of the most important progresses in condensed matter physics. Among the very few available tools to characterize Weyl semimetals through electrical transport, negative magnetoresistance is most commonly used. Considering the shortcomings of this method, new tools to characterize the chiral anomaly in Weyl semimetals are desirable. We employ the planar Hall effect (PHE) as an effective technique in the half Heusler Weyl semimetal GdPtBi to study the chiral anomaly. This compound exhibits a large value of 1.5 $\mathrm{m}\mathrm{\Omega }\mathrm{cm}$ planar Hall resistivity at 2 K and in 9 T. Our analysis reveals that the observed amplitude is dominated by Berry curvature and chiral anomaly contributions. Through angle-dependent transport studies we establish that GdPtBi with relatively small orbital magnetoresistance is an ideal candidate to observe the large PHE.