We consider a spinless fermionic $p\pm ip$ superfluid living on a two-dimensional sphere. Using superfluid hydrodynamics, we show that the ground state necessarily exhibits topological defects: either a pair of elementary vortices or a domain wall between $p\pm ip$ phases. In the topologically nontrivial BCS phase we identify the chiral fermion modes localized on the topological defects and compute their low-energy spectrum.

Impurities act as in situ probes of nontrivial electronic structure, causing real-space modulations in the density of states detected by scanning tunneling spectroscopy on the sample surface. We show that distinctive topological features of Weyl semimetals can be revealed in the Fourier transform of this map, interpreted in terms of quasiparticle interference (QPI). We develop an exact Green's function formalism and apply it to generalized models of Weyl semimetals with an explicit surface. The type of perturbation lifting the Dirac node degeneracy to produce the three-dimensional bulk Weyl phase determines the specific QPI signatures appearing on the surface. QPI Fermi arcs may or may not appear, depending on the relative surface orientation and quantum interference effects. Line nodes give rise to tube projections of width controlled by the bias voltage. We consider the effect of crystal warping, distinguishing dispersive arclike features from true Fermi arcs. Finally, we demonstrate that the commonly used joint-density-of-states approach fails qualitatively, and cannot describe QPI extinction.

The atomic structure of single-crystalline black phosphorus is studied using high-resolution synchrotron-based photoelectron diffraction (XPD). The results show that the topmost phosphorene layer in the black phosphorus is slightly displaced compared to the bulk structure and presents a small contraction in the direction perpendicular to the surface. Furthermore, the XPD results show the presence of a small buckling among the surface atoms, in agreement with previously reported scanning tunneling microscopy results. The contraction of the surface layer added to the presence of the buckling indicates a uniformity in the size of the $s{p}^3$ bonds between P atoms at the surface.

We study the interaction effect in a three-dimensional Dirac semimetal and find that two competing orders, charge-density-wave orders and nematic orders, can be induced to gap the Dirac points. Applying a magnetic field can further induce an instability towards forming these ordered phases. The charge-density-wave phase is similar to that of a Weyl semimetal, while the nematic phase is unique for Dirac semimetals. Gapless zero modes are found in the vortex core formed by nematic order parameters, indicating the topological nature of nematic phases. The nematic phase can be observed experimentally using scanning tunneling microscopy.

Weyl semimetals constitute a newly discovered class of three-dimensional topological materials with linear touchings of valence and conduction bands in the bulk. The most striking property of topological origin in these materials, so far unequivocally observed only in photoemission experiments, is the presence of open constant-energy contours at the boundary— the so-called Fermi arcs. In this Rapid Communication, we establish the universal characteristics of Fermi-arc contributions to surface quasiparticle interference. Using a general phenomenological model, we determine the defining interference patterns stemming from the existence of Fermi arcs in a surface band structure. We then trace these patterns in both simple tight-binding models and realistic ab initio calculations. Our results show that definitive signatures of Fermi arcs can be observed in existing and proposed Weyl semimetals using scanning tunneling spectroscopy.

We investigate electron transport through the interface between a niobium superconductor and the edge of a two-dimensional semimetal, realized in a 20-nm-wide HgTe quantum well. Experimentally, we observe that typical behavior of a single Andreev contact is complicated by both a pronounced zero-bias resistance anomaly and shallow subgap resistance oscillations with $1/n$ periodicity. These results are demonstrated to be independent of the superconducting material and should be regarded as specific to a two-dimensional semimetal in proximity with a superconductor. We interpret these effects to originate from the Andreev-like correlated process at the edge of a two-dimensional semimetal.

We report distinctive magnetotransport properties of a graphene $p\text{−}n\text{−}p$ junction prepared by controlled diffusion of metallic contacts. In most cases, materials deposited on a graphene surface introduce substantial carrier scattering, which greatly reduces the high mobility of intrinsic graphene. However, we show that an oxide layer only weakly perturbs the carrier transport, which enables fabrication of a high-quality graphene $p\text{−}n\text{−}p$ junction through a one-step and resist-free method. The measured conductance-gate voltage $\left(G\text{−}{V}_G\right)$ curves can be well described by a metal contact model, which confirms the charge-density depinning due to the oxide layer. The graphene $p\text{−}n\text{−}p$ junction samples exhibit a pronounced quantum Hall (QH) effect, a well-defined transition point of the zeroth Landau level (LL), and scaling behavior. The scaling exponent obtained from the evolution of the zeroth LL width as a function of temperature exhibits a relatively low value of $\kappa =0.21\pm 0.01$. Moreover, we calculate the energy level for the LLs based on the distribution of plateau-plateau transition points, further validating the assignment of the LL index of the QH plateau-plateau transition.

We study the interband transport in a superconducting device composed of graphene with EuO-induced exchange interaction. We show that pure crossed Andreev reflection can be generated exclusively without the parasitic local Andreev reflection and elastic cotunnelling over a wide range of bias and Fermi levels in an EuO-graphene/superconductor/EuO-graphene device. The pure nonlocal conductance exhibits rapid on-off switching and oscillatory behavior when the Fermi levels in the normal and the superconducting leads are varied. The oscillation reflects the quasiparticle propagation in the superconducting lead and can be used as a tool to probe the subgap quasiparticle mode in superconducting graphene, which is inaccessible from the current-voltage characteristics. Our results suggest that the device can be used as a highly tunable transistor that operates purely in the nonlocal and spin-polarized transport regime.

We report the energetics and properties of the $\beta , \alpha$, R, $\gamma , \lambda$, and $\delta$ polymorphs of $\mathrm{MnO}{}_2$ within density functional theory, comparing the performance of the recently introduced SCAN functional with that of conventional exchange-correlation functionals and experiment. We find that SCAN uniquely yields accurate formation energies and properties across all $\mathrm{MnO}{}_2$ polymorphs. We explain the superior performance of SCAN based on its satisfaction of all known constraints appropriate to a semilocal exchange-correlation functional and its accurate representation of all types of orbital overlap.

Previous work on the physical content of exchange-correlation functionals that depend on both charge and spin densities is extended to elemental transition metals and a wider range of perovskite transition metal oxides. A comparison of spectra and magnetic moments calculated using charge-only and spin-dependent exchange-correlation functionals as well as their $+U$ and $+J$ extensions confirms previous conclusions that the spin-dependent part of the exchange-correlation functional provides an effective Hund's interaction acting on the transition metal $d$ orbitals. For the local spin density approximation and spin-dependent generalized gradient approximation in the Perdew-Burke-Ernzerhof parametrization, the effective Hund's exchange implied by the spin dependence of the exchange correlation functional is found to be larger than 1 eV. The results indicate that at least as far as applications to transition metals and their oxides are concerned, $+U$, $+J$, and +dynamical-mean-field-theory extensions of density functional theory should be based on charge-only exchange-correlation functionals.

Electron spin resonance (ESR) is usually viewed as a single-particle phenomenon protected from the effect of many-body correlations. We show that this is not the case in a two-dimensional Fermi liquid (FL) with spin-orbit coupling (SOC). Depending on whether the in-plane magnetic field is below or above some critical value, ESR in such a system probes up to three chiral-spin collective modes, augmented by the spin mode in the presence of the field, or the Silin-Leggett mode. All the modes are affected by both SOC and FL renormalizations. We argue that ESR can be used as a probe not only for SOC but also for many-body physics.

We have investigated the atomic structure of graphene/Ir(111) supported platinum clusters with on average fewer than 40 atoms by means of surface x-ray diffraction (SXRD), grazing incidence small angle x-ray scattering (GISAXS), and normal incidence x-ray standing waves (NIXSW) measurements, in comparison with density functional theory calculations (DFT). GISAXS revealed that the clusters with 1.3 nm diameter form a regular array with domain sizes of 90 nm. SXRD shows that the 1–2 monolayer high, (111) oriented Pt nanoparticles grow epitaxially on the graphene support. From the combined analysis of the SXRD and NIXSW data, a three-dimensional (3D) structural model of the clusters and the graphene support can be deduced which is in line with the DFT results. For the clusters grown in ultrahigh vacuum the lattice parameter is reduced by $(4.6\pm 0.1)\%$ compared to bulk platinum. The graphene layer undergoes a strong Pt adsorption induced buckling, caused by a rehybridization of the carbon atoms below the cluster. In situ observation of the Pt clusters in CO and ${\mathrm{O}}_2$ environments revealed a reversible change of the clusters' strain state while successively dosing CO at room temperature and ${\mathrm{O}}_2$ at 575 K, pointing to a CO oxidation activity of the Pt clusters.

The mechanical loss ($Q^{-1}$) intrinsic to amorphous oxides is the limiting factor for sensitive, high-precision gravitational wave detectors and optical devices. Recent experimental work suggests that doping amorphous tantala with titania reduces $Q^{-1}$, however, the physical processes underlying this reduction are unknown. Here, we calculate $Q^{-1}$ for pure and titania-doped tantala using numerical methods combined with molecular dynamics simulations that have atomic levels of resolution. Our results match experimental trends that titania doping decreases the magnitude of the low-temperature loss peak characteristic of these materials, with 62% titanium cation doping minimizing $Q^{-1}$ at low temperature. We provide a microscopic explanation for this reduced loss by examining how doping affects the potential energy landscape, strain coupling constant, relaxation time, and other properties of the amorphous materials within the framework of the double-well potential model. Analyzing configurational changes provides an atomic description of the transitions driving mechanical loss at various temperatures in these oxides. These results identify the important parameters contributing to $Q^{-1}$ that are most affected by doping and provide guidance for how to screen for optimal doping combinations to minimize loss in other materials.

We study the nonequilibrium dynamics of the linear-to-zigzag structural phase transition exhibited by an ion chain confined in a trap with periodic boundary conditions. The transition is driven by reducing the transverse confinement at a finite quench rate, which can be accurately controlled. This results in the formation of zigzag domains oriented along different transverse planes. The twists between different domains can be stabilized by the topology of the trap, and under laser cooling the system has a chance to relax to a helical chain with nonzero winding number. Molecular dynamics simulations are used to obtain a large sample of possible trajectories for different quench rates. The scaling of the average winding number with different quench rates is compared to the prediction of the Kibble-Zurek theory, and a good quantitative agreement is found.

We introduce a phenomenological theory of dislocation motion appropriate for two-dimensional lattices. A coarse grained description is proposed that involves as primitive variables local lattice rotation and Burgers vector densities along distinguished slip systems of the lattice. We then use symmetry considerations to propose phenomenological equations for both defect energies and their dissipative motion. As a consequence, the model includes explicit dependencies on the local state of lattice orientation, and allows for differential defect mobilities along distinguished directions. Defect densities and lattice rotation need to be determined self-consistently and we show specific results for both square and hexagonal lattices. Within linear response, dissipative equations of motion for the defect densities are derived which contain defect mobilities that depend nonlocally on defect distribution.

We propose a scheme for an in-line spin-torque nano-oscillator (iSTNO) composed of a single nanowire. The oscillating element is an in-plane magnetized region of an otherwise out-of-plane magnetized nanowire, which supports a spin-wave mode. Analytical exploration reveals that the nonadiabatic spin-transfer torque can cancel out the damping and thus induce sustained precession in response to a direct current. Moreover, it predicts that the frequency scales linearly with current and that the wave vector depends only on geometry. Simulations with single iSTNO cells confirm that oscillations occur, and simulations with two iSTNO cells show that they frequency lock to each other roughly in antiphase, even with mismatches in geometry. iSTNO devices can be easily fabricated by irradiating regions of a perpendicular magnetic anisotropy nanowire with ions, do not require an external field, inherently support multiple iSTNOs, and thus form an attractive alternative STNO.

We have investigated the longitudinal and transverse thermoelectric phenomena (Seebeck and Nernst effects) in a single crystal of intermetallic compound MnSi. The Seebeck coefficient is largely decreased ($\approx 50\%$) by magnetic field around the helimagnetic transition temperature, which implies the large entropy release at the magnetic transition. We have discerned the anomalous and normal components of Nernst signal dependent on the magnetization and the magnetic field, respectively. For the anomalous Nernst effect, the intrinsic mechanism induced by the Berry phase of electrons seems dominant over the extrinsic mechanisms induced by scattering at least in the high temperature ($T\ge 15$ K) and resistive ($\rho >4 \mu \mathrm{\Omega }\mathrm{cm}$) region. The Nernst and Seebeck signals show little change in the course of the transition to skyrmion crystal.

The ferrimagnetic spinel ${\mathrm{CoV}}_2{\mathrm{O}}_4$ has been a topic of intense recent interest, both as a frustrated insulator with unquenched orbital degeneracy and as a near-itinerant magnet which can be driven metallic with moderate applied pressure. Here, we report on our recent neutron diffraction and inelastic scattering measurements on powders with minimal cation site disorder. Our main new result is the identification of a weak $\left(\frac{\mathrm{\Delta }a}{a}\sim {10}^{-4}\right)$, first order structural phase transition at $T^{*}=90$ K, the same temperature where spin canting was seen in recent single crystal measurements. This transition is characterized by a short-range distortion of oxygen octahedral positions, and inelastic data further establish a weak $\mathrm{\Delta }\sim 1.25\text{meV}$ spin gap at low temperature. Together, these findings provide strong support for the local orbital picture and the existence of an orbital glass state at temperatures below $T^{*}$.

We present results of the electronic and magnetic structure of Mn nanowires adsorbed on Ag(111) and Au(111) surfaces. For finite Mn nanowires on Ag(111) and Au(111) surfaces, our ab initio results show that the large difference between the spin-orbit splitting of these two surfaces leads to completely different magnetic configurations. The magnetic ordering for Mn nanowires adsorbed on Ag(111) is governed by the strong exchange interaction between Mn adatoms. For Mn nano-chains on Au(111), the competition between Heisenberg and Dzyaloshinskii-Moriya interactions leads to a complex magnetic structure of the clusters considered here. Among the more conspicuous results we note a spin-spiral helical type for the nanowire with seven atoms, and a complex magnetic configuration incommensurate with the substrate lattice for a double-sized Mn wire. The effect of the structural relaxation is also investigated, showing sensitivity of the exchange interactions to the bond distance to the substrate. We also demonstrate that small changes in the band filling of these Mn chains results in drastically different changes of the interatomic exchange. Finally, we show that dispersion of the electronic energy spectrum is possible even in nanostructures with bounded spatial extension.