Magnetoplasmons on graphene edges in quantizing magnetic fields are investigated at different Landau-level filling factors. To find the mode frequency, the optical conductivity tensor of disordered graphene in a magnetic field is calculated in the self-consistent Born approximation, and the nonlocal electromagnetic problem is solved using the Wiener-Hopf method. Magnetoplasmon dispersion relations, velocities, and attenuation lengths are studied numerically and analytically while taking into account the screening by metallic gate and the energy dissipation in graphene. The magnetoplasmon velocity decreases in the presence of nearby gates and oscillates as a function of the filling factor because of the dissipation-induced frequency suppression occurring when the Fermi level is located near the centers of Landau levels, in agreement with recent experiments.

Bilayer $\mathrm{Mo}{\mathrm{S}}_2$ is a centrosymmetric semiconductor with degenerate spin states in the six valleys at the corners of the Brillouin zone. It has been proposed that breaking of this inversion symmetry by an out-of-plane electric field breaks this degeneracy, allowing for spin and valley lifetimes to be manipulated electrically in bilayer $\mathrm{Mo}{\mathrm{S}}_2$ with an electric field. In this work, we report phase coherent transport properties of double-gated mono-, bi-, and trilayer $\mathrm{Mo}{\mathrm{S}}_2$. We observe a similar crossover from weak localization to weak antilocalization, from which we extract the spin relaxation time as a function of both electric field and temperature. We find that the spin relaxation time is inversely proportional to momentum relaxation time, indicating that the D’yakonov-Perel mechanism is dominant in all devices despite the centrosymmetry of the bilayer device. Further, we found no evidence of electric-field-induced changes in spin-orbit coupling strength. This suggests that the interlayer coupling is sufficiently weak and that electron-doped dichalcogenide multilayers behave electrically as decoupled monolayers.

It was recently proposed that the interface between a graphene nanoribbon in the canted antiferromagnetic quantum Hall state and a $s$-wave superconductor may present topological superconductivity, resulting in the appearance of Majorana zero modes [San-Jose et al., Phys. Rev. X 5, 041042 (2015)]. However, a description of the low-energy physics in terms of experimentally controllable parameters was still missing. Starting from a mean-field continuum model for graphene in proximity to a superconductor, we derive the low-energy effective Hamiltonian describing the interface of this heterojunction from first principles. A comparison between tight-binding simulations and analytical calculations with effective masses suggests that normal reflections at the interface must be considered in order to fully describe the low-energy physics.

Using resistance fluctuation spectroscopy, we observe current-induced narrow-band noise (NBN) in the magnetic skyrmion-lattice phase of micrometer-sized MnSi. The NBN appears only when electric-current density exceeds a threshold value, indicating that the current-driven motion of the skyrmion lattice triggers the NBN. The observed NBN frequency is $10–{10}^4$ Hz at $\sim {10}^9\mathrm{A}/{\mathrm{m}}^2$, implying a skyrmion steady-flow velocity of $1–100\mu \mathrm{m}/\mathrm{s}$, 3–5 orders of magnitude slower than previously reported. The temperature evolution of the NBN frequency suggests that the steady flow entails thermally activated processes, which are most likely due to skyrmion creation and annihilation at the sample edges. This scenario is qualitatively supported by our numerical simulations considering boundary effects, which reveals that the edges limit the steady flow of skyrmions, especially at low temperatures. We discuss a mechanism that dramatically slows the skyrmion steady flow in a microfabricated specimen.

We report a systematic NMR study on $\left[{\mathrm{Sr}}_4{\mathrm{Sc}}_2{\mathrm{O}}_6\right]{\mathrm{Fe}}_2{\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right)}_2$, for which the local lattice parameters of the iron-pnictogen ($\mathrm{Fe}Pn$) layer are similar to those of the series $\mathrm{LaFe}\left({\mathrm{As}}_{1-x^{\prime }}{\mathrm{P}}_{x^{\prime }}\right)\mathrm{O}$, which exhibits two segregated antiferromagnetic (AFM) order phases, AFM1 at $x^{\prime }=0-0.2$ and AFM2 at $x^{\prime }=0.4-0.7$. Our results reveal that the parent AFM1 phase at $x=0$ disappears at $x=0.3-0.4$, corresponding to a pnictogen height ($h_{pn}$) from the Fe plane of 1.3–1.32 Å, which is similar to that of $\mathrm{LaFe}\left({\mathrm{As}}_{1-x^{\prime }}{\mathrm{P}}_{x^{\prime }}\right)\mathrm{O}$ and various parent Fe pnictides. By contrast, the AFM2 order reported for $\mathrm{LaFe}\left({\mathrm{As}}_{0.4}{\mathrm{P}}_{0.6}\right)\mathrm{O}$ does not appear at $x\sim 0.8$, although the local lattice parameters of the $\mathrm{Fe}Pn$ layer and the microscopic electronic states are quite similar. Despite the absence of the static AFM2 order, reemergent dynamical AFM spin fluctuations were observed at approximately $x\sim 0.8$, which can be attributed to the instability of the AFM2 phase. We suggest this re-enhancement of AFM spin fluctuations plays a significant role in enhancing the $T_c$ to 17 K for $x=0.8-1$. Finally, we discuss the universality and diversity of the complicated magnetic ground states from a microscopic point of view, including the difference in the origins of the AFM1 and AFM2 phases, and their relations with the high superconducting transitions in Fe pnictides.

In this work, we utilize semiclassical kinetic equations to investigate the order parameter collective modes of a class of two-dimensional superfluids. Extending the known results for $p$-wave superfluids, we show that for any chiral ground state of angular momentum $L\ge 1$, there exists a subgap mode with a mass $\sqrt{2}\mathrm{\Delta }$ in the BCS limit, where $\mathrm{\Delta }$ is the magnitude of the ground-state gap. We determine the most significant Landau parameter that contributes to the mass renormalization and show explicitly that the renormalized modes become massless at the Pomeranchuk instability of the fermion vacuum. Particularly for $L=1$, we propose a continuous field theory to include the Fermi liquid effect in a quadrupolar channel and produce the same result under consistent approximations. They provide potential diagnostics for distinguishing two-dimensional chiral ground states of different angular momenta with the order parameter collective modes and reveal another low-energy degree of freedom near the nematic transition.

By measuring point-contact Andreev reflection (PCAR) spectra in the newly discovered chromium-based quasi-one-dimensional superconductor ${\mathrm{RbCr}}_3{\mathrm{As}}_3$, we find clear evidence of two superconducting components, i.e., one having a gap value of about 1.8 meV and another with a gap value of about 5 meV. Since the current injection may have components in both the directions parallel and perpendicular to the ${\left[{\left({\mathrm{Cr}}_3{\mathrm{As}}_3\right)}^{-}\right]}_{\infty }$ chains in the PCAR measurements, it naturally explains the two-component feature observed in this multiband superconductor. Detailed analysis shows that the larger gap may have an $s$-wave nature. We then carry out the single-particle tunneling measurements based on a scanning tunneling spectroscope by using the needlelike sample as the tip, and in this case the measured current is mainly parallel to the ${\left[{\left({\mathrm{Cr}}_3{\mathrm{As}}_3\right)}^{-}\right]}_{\infty }$ chains. The single-particle tunneling spectra show only one gap feature with a gap value of about 1.8 meV. Fitting to the single-particle tunneling spectra indicates that the gap should have a large anisotropy or even node(s). We argue that the absence of the larger gap may be related to the direction of the injecting current. Therefore, our combined experiments show the multiband superconductivity with one gap being nodal or highly anisotropic.

The superconducting proximity effect is a long-standing topic of great importance in condensed matter physics. A crucial but unresolved issue is which interfacial and material details determine the efficiency of the proximity effect. In this paper, we study an epitaxially grown superconductor/normal metal (SC-NM) heterostructure (Pb/Ag) and find a spatially constant superconducting gap determined by local tunneling spectroscopy and magnetoresponse measurements, despite the highly mismatched Fermi surfaces between individual Pb and Ag epitaxial layers and the large differences in the lattice constants and electronic densities of states in the separate components. The uniform superconducting gap is in contrast to the spatially varying pair potential with a discontinuity at the interface theoretically predicted for an ideal SC-NM junction and experimentally observed previously in several lateral SC-NM junctions. We experimentally verify that the transmission of electrons across the interface in the vertical Pb/Ag heterostructure is high enough that a new band structure emerges even at the single-particle level. Our experimental results call for further theoretical work in order to develop predictive power for the proximity effect starting from realistic, materially relevant microscopic models.

Spin liquids may host emergent quasiparticles, collective excitations of the spin degrees of freedom with characteristic features of Majorana fermions, which experimentally are detectable by broad excitation continua due to spin fractionalization. The latter is predicted for the Kitaev spin liquid, an exactly solvable model with bond-dependent interactions on a two-dimensional honeycomb lattice. Here we report on detailed terahertz experiments in $\alpha \text{−}\mathrm{RuC}{\mathrm{l}}_3$, identifying these characteristic fingerprints of Majorana fermions. The continuum intensity decreases and finally vanishes on increasing temperature. It partly overlaps with phonon modes, representing characteristic sliding and compression modes of the van der Waals bonded molecular layers.

As an inversely designed artificial device, meta-surface usually means densely arranged meta-atoms with complex substructures. In acoustics, those meta-atoms are usually constructed by multifolded channels or multiconnected cavities of a deep subwavelength feature, which limits their implementation in pragmatic applications. We propose here a comprehensive concept of high-efficiency anomalous splitter based on an acoustic meta-grating. The beam splitter is designed by etching only two or four straight-walled grooves per period on a planar hard surface. Different from the recently reported reflectors or splitters, our device can split an incident wave into different desired directions with arbitrary power flow partition. In addition, because ultrathin substructures with thin walls and narrow channels are avoided in our design procedure, the proposed beam splitter can be used for waves with much shorter wavelength compared to the previous suggested systems. The design is established by rigorous formulas developed under the framework of the grating theory and a genetic optimization algorithm. Numerical simulation and experimental evidence are provided to discuss the involved physical mechanism and to give the proof of concept for the proposed high-efficiency anomalous acoustic splitter.

Density functional theory has been used to investigate the elastic properties of various Bi-based alloy materials as potential replacements of lead-based solders. We compare calculated quantities which can be used to determine the effectiveness of our proposed replacements, such as the bulk $\left(K\right)$, shear $\left(G\right)$, and Young's $\left(E\right)$ moduli. We also computed the Pugh ratio $(\gamma =K/G)$, a quantity that is used to estimate the ductility of a solid, for each of the alloys. The effect of spin-orbit coupling on these materials has also been investigated. By analyzing the changes in charge density as Bi is substituted, we link the predicted changes in ductility in these materials to atomistic descriptors; specifically, delocalization of the charge density is known to be a signature of increased ductility, whereas more localized charge density is typically associated with brittleness. Through Bader charge analysis we provide quantitative insight into the uniformity of charge distribution and how this alters the ductility of these alloys. By using density functional theory to calculate the elastic properties of different ${\mathrm{Bi}}_{1-x}{Z}_x$ alloys $(Z$ = Sb, Te, In, Sn; $0\le x\le 1)$, we correlate changes in ductility and corresponding changes in the electronic structure of a material. We have found that low concentrations of Sn and Te, substituted into the hexagonal crystal structure of Bi, induce significant increases in ductility.

By employing resonant x-ray microdiffraction, we image the magnetization and magnetic polarity domains of the $Y$-type hexaferrite ${\mathrm{Ba}}_{0.5}{\mathrm{Sr}}_{1.5}{\mathrm{Mg}}_2{\mathrm{Fe}}_{12}{\mathrm{O}}_{22}$. We show that the magnetic polarity domain structure can be controlled by both magnetic and electric fields and that full inversion of these domains can be achieved simply by reversal of an applied magnetic field in the absence of an electric field bias. Furthermore, we demonstrate that the diffraction intensity measured in different x-ray polarization channels cannot be reproduced by the accepted model for the polar magnetic structure, known as the 2-fan transverse conical (TC) model. We propose a modification to this model, which achieves good quantitative agreement with all of our data. We show that the deviations from the TC model are large and may be the result of an internal magnetic chirality, most likely inherited from the parent helical (nonpolar) phase.

At zero temperature, a two-dimensional lattice of Majorana zero modes on mesoscopic superconducting islands exhibits a topologically ordered toric code phase. Recently, a Landau field theory was used to describe the different phases of the aforementioned system and the phase transitions separating them. While the field theory provides details on the properties of the system close to the phase transitions, signatures of topological ordering in the different phases have not been computed. This is the primary goal of this work. We describe a lattice gauge theory of the Majorana toric code in terms of $\mathrm{U}\left(1\right)$ matter fields coupled to an emergent ${\mathbb{Z}}_2$ gauge field. Subsequently, we use a generalized Wilson-loop order parameter, the equal-time Fredenhagen-Marcu order parameter, to characterize the topological ordering in the different phases. Our computation provides evidence of the toric code phase both in the Mott-insulator and the charge-$2e$ superconductor phases, while showing that the toric code phase disappears in the charge-$e$ superconductor phase. In addition, we perturbatively analyze the influence of Cooper pair tunneling on the topological gap of the toric code in the limit of strong charging energy and show that the toric code phase is, in fact, stabilized by the Cooper pair tunneling. Our results are relevant for experimental realizations of the Majorana toric code.

Double perovskites (DPs) are a large family of compounds that exhibit a wide range of properties of both fundamental and potential technological interest. Due to the presence of $3d,4d$, or $5d$ transition metal atoms with narrow $t_{2g}$ and $e_g$ bands in DPs, the correlation effects play an important role for the properties of these materials, leading to diverse physical phenomena, such as colossal magnetoresistance, ferroelectricity, magnetism, and superconductivity. By employing the constrained random-phase approximation within the full-potential linearized augmented-plane-wave method, we have calculated the effective on-site Coulomb interaction parameters between localized $d$ electrons in ${\mathrm{Sr}}_{2}AB{\mathrm{O}}_6(A=\mathrm{Cr},\text{Mn},\text{Fe},\text{Co},\text{Ni}$, and $B=\mathrm{Mo},\text{W})$ DPs. We find that the correlated subspace can be defined to contain only the $e_g$ states in Ni-based compounds, leading to a simple two-band low-energy model, whereas at least an eight-orbital $\left(d+t_{2g}\right)$ model is necessary for the other compounds. Except for Ni, the $U$ values for $A$ sites in Mo (W) based compounds are around 4 eV (4.5 eV), and they are almost independent of the $3d$ electron number, while the $U$ for Mo (W) $t_{2g}$ electrons slightly decreases with increasing $3d$ electron number, from 3 to 2.5 eV. Moreover, our calculations reveal that the contribution of the $3d\to 3d$ channel to the total electronic screening is larger in DPs than the corresponding contribution in elementary transition metals.

Phonon scattering by electrons, or “phonon-electron scattering,” has been recognized as a significant scattering channel for phonons in materials with high electron concentration, such as thermoelectrics and nanoelectronics, even at room temperature. Despite the abundant previous studies of phonon-electron scattering in different types of three-dimensional bulk materials, its impact on the phonon transport, and thus the heat transfer properties, of two-dimensional (2D) materials has not been understood. In this work, we apply ab initio methods to calculate the phonon-electron scattering rates in two representative 2D materials, silicene and phosphorene, and examine the potential of controlling the thermal conductivity of these materials via externally induced phonon-electron scattering by electrostatic gating. We also develop an analytical model to explain the impact of reduced dimensionality and distinct electron and phonon dispersions in two dimensions on phonon-electron scattering processes. We find that over 40% reduction of the lattice thermal conductivity can be achieved in silicene with an induced charge-carrier concentration around ${10}^{13}{\mathrm{cm}}^{-2}$, which is experimentally achievable. Our study not only generates new fundamental insights into phonon transport in 2D materials but also provides practical guidelines to search for 2D materials with strong phonon-electron scattering for potential thermal switching applications.

Atomic disorder is a common limiting factor for the low-temperature mobility in monolayer transition-metal dichalcogenides (TMDs; $M{X}_2$). Here, we study the effect of often occurring atomic vacancies on carrier scattering and transport in $p$- and $n$-type monolayer ${\mathrm{MoS}}_2$. Due to charge trapping in vacancy-induced in-gap states, both neutral and charged vacancies resembling, respectively, short-range and combined short-range and long-range Coulomb scatterers must be considered. Using the $T$-matrix formalism, we demonstrate a strong renormalization of the Born description of short-range scattering, manifested in a pronounced reduction and a characteristic energy dependence of the scattering rate. As a consequence, carrier scattering in TMDs with charged vacancies is dominated by the long-range Coulomb disorder scattering, giving rise to a strong screening-induced temperature and density dependence of the low-temperature carrier mobility. For TMDs with neutral vacancies, the absence of intrinsic Coulomb disorder results in significantly higher mobilities as well as an unusual density dependence of the mobility which decreases with the carrier density. Our work illuminates the transport-limiting effects of atomic-vacancy scattering relevant for high-mobility TMD devices.

We examine the time evolution of the entanglement spectrum of a small subsystem of a nonintegrable spin chain following a quench from a product state. We identify signatures in this entanglement spectrum of the distinct dynamical velocities (related to entanglement and operator spreading) that control thermalization. We show that the onset of level repulsion in the entanglement spectrum occurs on different timescales depending on the “entanglement energy,” and that this dependence reflects the shape of the operator front. Level repulsion spreads across the entire entanglement spectrum on a timescale that is parametrically shorter than that for full thermalization of the subsystem. This timescale is also close to when the mutual information between individual spins at the ends of the subsystem reaches its maximum. We provide an analytical understanding of this phenomenon and show supporting numerical data for both random unitary circuits and a microscopic Hamiltonian.

Both insulating and conducting electronic behaviors have been experimentally seen in clean bilayer graphene samples at low temperature, and there is still no consensus on the nature of the interacting ground state at half filling and in the absence of a magnetic field. Theoretically, several possibilities for the insulating ground states have been predicted for weak interaction strength. However, a recent renormalization-group calculation on a Hubbard model for charge-neutral bilayer graphene with short-range interactions suggests the emergence of low-energy Dirac fermions that would stabilize the metallic phase for weak interactions. Using a nonperturbative projective quantum Monte Carlo, we calculate the ground state for bilayer graphene using a realistic model for the Coulomb interaction that includes both short-range and long-range contributions. We find that a finite critical on-site interaction is needed to gap bilayer graphene and the transition belongs to the Gross-Neveu universality class, thereby confirming the Hubbard model expectations even in the presence of a long-range Coulomb potential, in agreement with our theoretical analysis. In addition, we also find that the critical on-site interactions necessary to destabilize the metallic ground state decrease with increasing interlayer coupling.

Recent tunneling conductance measurements on semiconductor-superconductor nanowires find zero-bias peaks to be ubiquitous across wide ranges of chemical potential and Zeeman energy J. Chen et al., Phys. Rev. Lett. 123, 107703 (2019). Motivated by this, we demonstrate that topologically trivial Andreev abound states (ABSs) pinned near zero energy are produced rather generically in inhomogeneous systems with multiband occupancy in the presence of interband coupling. We first investigate the interband coupling mechanism responsible for the pinning within a multiband one-dimensional toy model, then we confirm the findings using a three-dimensional Schrödinger-Poisson approach that incorporates the geometric and electrostatic details of the actual device. Our analysis shows that level repulsion generated by interband coupling can lead to a rather spectacular pinning of the lowest-energy mode near zero energy in systems (or regions) characterized by very short length scales ($\sim 100\mathrm{nm}$). We show that level repulsion between the lowest-energy levels can mimic the gap opening feature (simultaneous with the emergence of a near-zero-energy mode) predicted to occur in Majorana hybrid systems. We also demonstrate that nearly zero-bias differential conductance features exhibiting particle-hole asymmetry are due to the presence of (topologically trivial) ABSs pinned near zero energy by level repulsion, not to Majorana zero modes, quasi-Majoranas, or any other low-energy mode that involves (partially) separated Majorana bound states. Our findings demonstrate the importance of understanding in detail multiband physics and electrostatic effects in the context of the ongoing search for Majorana modes in semiconductor-superconductor heterostructures.

We investigate phase transitions in the two-dimensional dipolar Heisenberg model with uniaxial anisotropy with a specific ratio between the exchange and dipolar constants, $\delta =1$. We obtain the $\eta -T$ (anisotropy vs temperature) phase diagrams for typical values of magnetic field by a Monte Carlo method with an $O\left(N\right)$ algorithm. We find that at lower fields, the $\eta -T$ phase diagram consists of the planar ferromagnetic (F), perpendicular stripe-ordered (SO), and paramagnetic (P) phases, and is characterized by the triple point. In the SO phase realized at larger $\eta$ and smaller $T$, the SO pattern changes depending on the field. On the other hand, we find that at higher fields, the SO phase does not exist, while the planar F phase robustly remains. We study the properties of the phase boundaries in detail. We find that the slope of the spin-reorientation-transition line is positive with and without field, i.e., $\frac{d\eta }{dT}>0$, which implies that the planar F phase changes to the SO phase with lowering temperature. In the phase diagrams we observe a characteristic shape of the P–planar F phase-transition line, whose maximum point of $\eta$ is located at an intermediate temperature. This structure leads to the temperature-induced reentrant transition associated with P and planar F phases, which appears in successive phase transitions with lowering temperature: P $\to$ planar F $\to$ P $\to$ SO phase at lower fields and P $\to$ planar F $\to$ P phases at higher fields.