We report on a strongly coupled bilayer graphene-${\mathrm{Bi}}_2{\mathrm{Se}}_3$ device with a junction resistance of less than 1.5 $\mathrm{k}\mathrm{\Omega }\mu {\mathrm{m}}^2$. This device exhibits unique behavior at the interface, which cannot be attributed to either material in absence of the other. We observe quantum oscillations in the magnetoresistance of the junction, indicating the presence of well-resolved Landau levels due to hole carriers of unknown origin with a very large Fermi surface. These carriers, found only at the interface, could conceivably arise due to significant hole doping of the bilayer graphene with charge transfer on the order of $2\times {10}^{13}{\mathrm{cm}}^{-2}$, or due to twist-angle-dependent miniband transport.

Motivated by the recent proposal that several candidate materials such as ${\mathrm{YbMgGaO}}_4$ could be spinon Fermi surface spin liquids, we explore the experimental consequences of the external magnetic fields on this exotic state. Specifically, we focus on the weak field regime where the spin-liquid state is well preserved and the spinon remains to be a good description of the magnetic excitations. From the spin-1/2 nature of the spinon excitation, we predict the unique features of the spinon continuum when the weak magnetic field is applied to the system. Due to the small energy scale of the exchange interactions between the local moments in the spin-liquid candidate like ${\mathrm{YbMgGaO}}_4$, our proposal for the spectral weight shifts and spectral crossing in the magnetic fields can be immediately tested by inelastic neutron scattering experiments. Several other experimental aspects about the spinon Fermi surface and the spinon excitations are discussed and proposed. Our work provides an experimental scheme to examine the fractionalized spinon excitation and the candidate spin-liquid states in ${\mathrm{YbMgGaO}}_4$, the 6H-B phase of ${\mathrm{Ba}}_3{\mathrm{NiSb}}_2{\mathrm{O}}_9$, and other relevant materials.

The rise of graphene marks the advent of two-dimensional atomic crystals, which have exhibited a cornucopia of intriguing properties, such as the integer and fractional quantum Hall effects, valley Hall effect, charge density waves, and superconductivity, to name a few. Yet, magnetism, a property of extreme importance in both science and technology, remains elusive. There is a paramount need for magnetic two-dimensional crystals. With the availability of many magnetic materials consisting of van der Waals coupled two-dimensional layers, it thus boils down to the question of how the magnetic order will evolve with reducing thickness. Here we investigate the effect of thickness on the magnetic ordering in nanothick ${\mathrm{V}}_5{\mathrm{S}}_8$. We uncover an anomalous Hall effect, by which the magnetic ordering in ${\mathrm{V}}_5{\mathrm{S}}_8$ down to 3.2 nm is probed. With decreasing thickness, a breakdown of antiferromagnetism is evident, followed by a spin-glass-like state. For thinnest samples, a weak ferromagnetic ordering emerges. The results not only show an interesting effect of reducing thickness on the magnetic ordering in a potential candidate for magnetic two-dimensional crystals, but demonstrate the anomalous Hall effect as a useful characterization tool for magnetic orderings in two-dimensional systems.

We consider a system of two one-dimensional nanowires coupled via an $s$-wave superconducting strip, a geometry that is capable of supporting Kramers pairs of Majorana fermions. By performing an exact analytical diagonalization of a tunneling Hamiltonian describing the proximity effect (via a Bogoliubov transformation), we show that the excitation gap of the system varies periodically on the scale of the Fermi wavelength in the limit where the interwire separation is shorter than the superconducting coherence length. Comparing with the excitation gaps in similar geometries containing only direct pairing, where one wire is decoupled from the superconductor, or only crossed Andreev pairing, where each nanowire is considered as a spin-polarized edge of a quantum Hall state, we find that the gap is always reduced, by orders of magnitude in certain cases, when both types of pairing are present. Our analytical results are further supported by numerical calculations on a tight-binding lattice. Finally, we show that treating the proximity effect by integrating out the superconductor using the bulk Green's function does not reproduce the results of our exact diagonalization.

We report on the transition from magnetic edge to electric edge transport in a split magnetic gate device which applies a notch magnetic field to a two-dimensional electron gas. The gate bias allows tuning the overlap of magnetic and electric edge wave functions on the scale of the magnetic length. Conduction at the magnetic edges, in the two-dimensional bulk, is found to compete with conduction at the electric edges until the magnetic edges become depleted. Current lines then move to the electrostatic edges as in the conventional quantum Hall picture. The conductivity was modeled using the quantum Boltzmann equation in the exact hybrid potential. The theory predicts the features of the bulk-edge crossover, in good agreement with experiment.

We have studied the $\nu =5/2$ fractional quantum Hall state in a density-tunable sample at extremely low electron densities. For the densities accessed in our experiment, the Landau level mixing parameter $\kappa$ spans the $2.52<\kappa <2.82$ range. In the vicinity of $5.8\times {10}^{10}{\mathrm{cm}}^{-2}$ or $\kappa =2.6$ an anomalously large change in the density dependence of the energy gap is observed. Possible origins of such an anomaly are discussed, including a topological phase transition in the $\nu =5/2$ fractional quantum Hall state.

Magnetic skyrmions are particlelike objects with topologically protected stability which can be set into motion with an applied current. Using a particle-based model we simulate current-driven magnetic skyrmions interacting with random quenched disorder and examine the skyrmion velocity fluctuations parallel and perpendicular to the direction of motion as a function of increasing drive. We show that the Magnus force contribution to skyrmion dynamics combined with the random pinning produces an isotropic effective shaking temperature. As a result, the skyrmions form a moving crystal at large drives instead of the moving smectic state observed in systems with a negligible Magnus force where the effective shaking temperature is anisotropic. We demonstrate that spectral analysis of the velocity noise fluctuations can be used to identify dynamical phase transitions and to extract information about the different dynamic phases, and show how the velocity noise fluctuations are correlated with changes in the skyrmion Hall angle, transport features, and skyrmion lattice structure.

In this ab initio study we demonstrate that molecular adsorption on a surface Rashba system can be used to modulate the surface electronic structure in different momentum space directions, i.e., to create anisotropic spin splittings in $\mathbf{k}$ space. This effect is rooted in the asymmetric adsorption of the molecules on the surface in a hollow site which breaks the surface symmetry. More specifically, we demonstrate that the physisorbed ${\mathrm{NH}}_3$ has a small influence on the surface Rashba states and only gives rise to variations of the surface state Rashba parameters up to a factor of 1.4 over the surface Brillouin zone. In contrast, the chemisorption of ${\mathrm{BH}}_3$ leads to variations of the Rashba parameter by more than a factor of 2.5. Consequently, the anisotropy of the Rashba-split-surface states induced by molecular adsorption also gives rise to a modulation of the surface state spin texture, i.e., the out-of-plane spin polarization varies along different $\mathbf{k}$ directions by up to 70% for the occupied states. This offers the possibility to change the spin direction from in-plane to predominantly out-of-plane by modifying the electronic momentum by 90°.

We measure the spin-resolved transport of dipolar excitons in a biased GaAs double quantum well structure. From these measurements we extract both spin lifetime and mobility of the excitons. We find that below a temperature of $4.8\mathrm{K}$ there is a sharp increase in the spin lifetime of the excitons, together with a sharp reduction in their mobility. Below a critical power the spin lifetime increases with increasing mobility and density, while above the critical power the opposite trend is observed. We interpret this transition as evidence of the interplay between two different spin dephasing mechanisms: at low mobility the dephasing is dominated by the hyperfine interaction with the lattice nuclei spins, while at higher mobility the spin-orbit interaction dominates and a Dyakonov-Perel spin relaxation takes over. The excitation power and temperature regime where the hyperfine interaction induced spin dephasing is observed correlates with the regime where a dark dipolar quantum liquid was reported recently on a similar sample.

Low thermal polarization of nuclear spins is a primary sensitivity limitation for nuclear magnetic resonance. Here we demonstrate optically pumped (microwave-free) nuclear spin polarization of ${}^{13}\mathrm{C}$ and ${}^{15}\mathrm{N}$ in ${}^{15}\mathrm{N}$-doped diamond. ${}^{15}\mathrm{N}$ polarization enhancements up to $-2000$ above thermal equilibrium are observed in the paramagnetic system ${{\mathrm{N}}_{\mathrm{s}}}^0$ . Nuclear spin polarization is shown to diffuse to bulk ${}^{13}\mathrm{C}$ with NMR enhancements of $-200$ at room temperature and $-500$ at 240 K, enabling a route to microwave-free high-sensitivity NMR study of biological samples in ambient conditions.

We revisit here the issue of thermally assisted Quantum Annealing by a detailed study of the dissipative Landau-Zener problem in the presence of a Caldeira-Leggett bath of harmonic oscillators, using both a weak-coupling quantum master equation and a quasiadiabatic path-integral approach. Building on the known zero-temperature exact results [Wubs et al., Phys. Rev. Lett. 97, 200404 (2006)], we show that a finite temperature bath can have a beneficial effect on the ground-state probability only if it couples also to a spin direction that is transverse with respect to the driving field, while no improvement is obtained for the more commonly studied purely longitudinal coupling. In particular, we also highlight that, for a transverse coupling, raising the bath temperature further improves the ground-state probability in the fast-driving regime. We discuss the relevance of these findings for the current quantum-annealing flux qubit chips.

Fractionalization is a hallmark of spin-liquid behavior; it typically leads to response functions consisting of continua instead of sharp modes. However, microscopic processes can enable the formation of short-distance bound states of fractionalized excitations, despite asymptotic deconfinement. Here we study such bound-state formation for the ${\mathbb{Z}}_2$ spin liquid realized in Kitaev's honeycomb compass model, supplemented by a kekulé distortion of the lattice. Bound states between flux pairs and Majorana fermions form in the Majorana band gaps. We calculate the dynamic spin susceptibility and show that bound states lead to sharp modes in the magnetic response of the spin liquid, with the momentum dependence of the corresponding spectral weight encoding the internal symmetry of the bound state. As a byproduct, we also show that isolated fluxes may produce Majorana bound states at exactly zero energy. Generalizations and implications of the results are discussed.

Spin-orbit coupling in transition metal dichalcogenides (TMDCs) causes spin-valley locking, giving rise to unconventional optical, transport, and superconducting properties. In this paper, we propose exotic superconductivity in bilayer group-IV TMDCs by symmetry control. The sublattice-dependent “hidden” spin-orbit coupling arising from local inversion symmetry breaking in the crystal structure may stabilize the odd-parity superconductivity by purely $s$-wave local pairing interaction. The stability of the odd-parity superconducting state depends on the bilayer stacking. The $2{\mathrm{H}}_b$ stacking in $\text{Mo}{X}_2$ and $\text{W}{X}_2$ ($X=\mathrm{S}$,Se) favors the odd-parity superconductivity due to interlayer quantum interference. On the other hand, the odd-parity superconductivity is suppressed by the $2{\mathrm{H}}_a$ stacking of ${\mathrm{NbSe}}_2$. Calculating the phase diagram of the tight-binding model derived from first-principles band calculations, we conclude that the intercalated bilayer ${\mathrm{MoS}}_2$ and ${\mathrm{WS}}_2$ are candidates for a new class of odd-parity superconductors by spin-orbit coupling.

One of the most unique and robust experimental facts about iron-based superconductors is the renormalization of the electronic band dispersion by factor of 3 and more near the Fermi level. Obviously related to the electron pairing, this prominent deviation from the band theory lacks understanding. Experimentally studying the entire spectrum of the valence electrons in iron arsenides, we have found an unexpected depletion of the spectral weight in the middle of the iron-derived band, which is accompanied by a drastic increase of the scattering rate. At the same time, the measured arsenic-derived band exhibits very good agreement with theoretical calculations. We show that the low-energy Fermi velocity renormalization should be viewed as a part of the modification of the spectral function by a strong electronic interaction. Such an interaction with an energy scale of the whole $d$ band appears to be a hallmark of many families of unconventional superconductors.

Charge doping would inevitably induce strain, which can significantly influence device performance but cannot be directly predicted by classical mechanical laws. Here we present a set of equations of states for deformable cubic crystals subjected to charge doping by introducing the quantum electronic stress at fixed lattice as equivalent mechanical pressure into the classical hydrostatic pressure-vs-deformation equations. The equations are proved to be efficient for all the cubic crystals considered in this work (diamond, Si, Ge, GaAs, Al, and ${\mathrm{ZrO}}_2$) by first-principles calculations. The proposed method and presented equations should pave a convenient way to predict doping effects on device performance.

Fivefold symmetry is supposed to have an important role in suppressing crystallization and promoting glass transition due to its structural incompatibility with crystal. In this paper, we study the correlation between the fivefold symmetry and the dynamical slowing down in glass-forming Cu-Zr liquids using the single-particle dynamics method based on molecular dynamics simulations. The dynamics of the glass-forming liquids is microscopically characterized by the jump cage motion for individual atoms; moreover, the cooperative jumps become more pronounced upon approaching the glass transition temperature. We find that the role of fivefold symmetry in the dynamical slowing down does not lie in caging atomic motion but, more importantly, in suppressing cooperative jumps. The atoms with a high degree of fivefold symmetry and involved in jump motions appear more sluggish compared to other jumps. This behavior significantly suppresses the cooperative jumps around them, leading to the slowing down of fast dynamics. The degree of suppression has a close relation to the glass-forming ability and contributes to the “strong” character of liquids.

We report the angular dependence of magnetoresistance in the two-dimensional electron gas at the ${\mathrm{LaAlO}}_3$/${\mathrm{SrTiO}}_3$ interface. We find that this interfacial magnetoresistance exhibits a similar angular dependence to the spin Hall magnetoresistance observed in ferromagnet/heavy metal bilayers, which has been so far discussed in the framework of the bulk spin Hall effect of the heavy metal layer. The observed magnetoresistance is in qualitative agreement with a theoretical model calculation including both Rashba spin-orbit coupling and an exchange interaction. Our result suggests that magnetic interfaces subject to spin-orbit coupling can generate a non-negligible contribution to the spin Hall magnetoresistance, and the interfacial spin-orbit coupling effect is therefore key to the understanding of various spin-orbit-coupling-related phenomena in magnetic/nonmagnetic bilayers.

Implementation of antiferromagnetic compounds as active elements in spintronics has been hindered by their insensitive nature against external perturbations which causes difficulties in switching among different antiferromagnetic spin configurations. Electrically controllable strain gradient can become a key parameter to tune the antiferromagnetic states of multiferroic materials. We have discovered a correlation between an electrically written straight-stripe mixed-phase boundary and an in-plane antiferromagnetic spin axis in highly elongated La-5%-doped $\mathrm{BiFe}{\mathrm{O}}_3$ thin films by performing polarization-dependent photoemission electron microscopy in conjunction with cluster model calculations. A model Hamiltonian calculation for the single-ion anisotropy including the spin-orbit interaction has been performed to figure out the physical origin of the link between the strain gradient present in the mixed-phase area and its antiferromagnetic spin axis. Our findings enable estimation of the strain-gradient-induced magnetic anisotropy energy per Fe ion at around $5\times {10}^{-12}\mathrm{eV}\mathrm{m}$, and provide a pathway toward an electric-field-induced 90° rotation of antiferromagnetic spin axis at room temperature by flexomagnetism.