In this Rapid Communication we will show that, in the presence of a properly modulated Dzyaloshinskii-Moriya (DM) interaction, a $U\left(1\right)$ vortex-antivortex lattice appears at low temperatures for a wide range of the DM interaction. Even more, in the region dominated by the exchange interaction, a standard Berezinskii-Kosterlitz-Thouless (BKT) transition occurs. In the opposite regime, the one dominated by the DM interaction, a kind of inverse BKT transition takes place. As temperature rises, the vortex-antivortex lattice starts melting by the annihilation of vortex-antivortex pairs, in a sort of “inverse” BKT transition.

We have probed the switching dynamics of the Josephson critical current of a superconducting weak link by measuring its voltage/current characteristics while applying an ac current bias in the range 1–200 MHz. The weak link between two Nb reservoirs is formed by a mesoscopic Al wire above its critical temperature. We observe a dynamical phase transition as a function of the frequency and amplitude of the ac current. While at low frequency the transition driven by increasing the current bias is well described by the standard Kramers theory, at high frequency the switching histograms become hysteretic and much narrower than expected by thermal fluctuations. The crossover frequency between the two regimes is set by the electron-phonon interaction rate in the normal metal.

We performed quartz crystal microbalance experiments using a 5-MHz AT-cut crystal for superfluid ${}^4\mathrm{He}$ films on exfoliated graphite (Grafoil) containing up to 0.40 atom/${\mathrm{nm}}^2 {}^3\mathrm{He}$. We found that the mass decoupling of ${}^4\mathrm{He}$ solid layers from the oscillating substrate is considerably sensitive, even with small amounts of ${}^3\mathrm{He}$. For a ${}^4\mathrm{He}$ film of 29.3 atoms/${\mathrm{nm}}^2$, we observed a small drop in resonant frequency at a $T_3$ of $\sim 0.4$ K for a low oscillation amplitude, which is attributed to the sticking of ${}^3\mathrm{He}$ atoms at the ${}^4\mathrm{He}$ solid layer. For higher amplitudes, the ${}^4\mathrm{He}$ solid layer shows a reentrant mass decoupling at $T_R$ close to $T_3$. This decoupling can be explained by the suppression of the superfluid counterflow owing to the adsorption of the ${}^3\mathrm{He}$ atoms on edge dislocations. As the ${}^4\mathrm{He}$ areal density increases, $T_R$ shifts to a lower temperature and disappears around the ${}^4\mathrm{He}$ film of 39.0 atoms/${\mathrm{nm}}^2$.

We report the direct measurement of the topological skyrmion energy barrier through a hysteresis of the skyrmion lattice in the chiral magnet MnSi. Measurements were made using small-angle neutron scattering with a custom-built resistive coil to allow for high-precision minor hysteresis loops. The experimental data were analyzed using an adapted Preisach model to quantify the energy barrier for skyrmion formation and corroborated by the minimum-energy path analysis based on atomistic spin simulations. We reveal that the skyrmion lattice in MnSi forms from the conical phase progressively in small domains, each of which consisting of hundreds of skyrmions, and with an activation barrier of several eV.

Rare-earth monopnictides display rich physical behaviors, featuring most notably spin and orbital orders in their ground state. Here, we grow ErBi single crystal and study its magnetic, thermal, and electrical properties. An analysis of the magnetic entropy and magnetization indicates that the weak magnetic anisotropy in ErBi possibly derives from the mixing effect, namely the anisotropic ground state of $\mathrm{E}{\mathrm{r}}^{3+}\left(4f^{11}\right)$ mingles with the isotropic excited state through exchange interaction. At low temperature, an extremely large magnetoresistance $(\sim {10}^4\%)$ with a parabolic magnetic-field dependence is observed, which can be ascribed to the nearly perfect electron-hole compensation and ultrahigh carrier mobility. When the magnetic field is rotated in the $ab$ $\left(ac\right)$ plane and the current flows in the $b$ axis, the angular magnetoresistance in ErBi shows a twofold (fourfold) symmetry. Similar case has been observed in LaBi where the anisotropic Fermi surface dominates the low-temperature transport. Our theoretical calculation suggests that near the Fermi level ErBi shares similarity with LaBi in the electronic band structures. These findings indicate that the angular magnetoresistance of ErBi could be mainly determined by its anisotropic Fermi surface topology. Besides, contributions from several other possibilities, including the spin-dependent scattering, spin-orbit scattering, and demagnetization correlation to the angular magnetoresistance of ErBi are also discussed.

The discovery of multiple coexisting magnetic phases in the crystallographically homogeneous compound ${\mathrm{Ca}}_3{\mathrm{Co}}_2{\mathrm{O}}_6$ has stimulated ongoing research activity. In recent years the main focus has been on the zero-field-state properties, where exceedingly long timescales have been established. In this study we report a detailed investigation of static and dynamic properties of ${\mathrm{Ca}}_3{\mathrm{Co}}_2{\mathrm{O}}_6$ across the magnetic field induced transition around 3.5 T. This region has so far been practically neglected, while we argue that in some aspects it represents a simpler version of the transition across the $B=0$ state. Investigating the frequency dependence of the ac susceptibility, we reveal that on the high-field side ($B>3.5$ T) the response corresponds to a relatively narrow distribution of magnetic clusters. The distribution appears very weakly dependent on magnetic field, with an associated energy barrier of around 200 K. Below 3.5 T a second contribution arises, with much smaller characteristic frequencies and a strong temperature and magnetic field dependence. We discuss these findings in the context of intrachain and interchain clustering of magnetic moments.

We study magnetic properties in the half-filled Hubbard model on the Ammann-Beenker tiling. First, we focus on the domain structure with locally eightfold rotational symmetry to examine the strictly localized confined states for the tight-binding model. We count the number of vertices and confined states in the larger domains generated by the deflation operations systematically. Then, the fraction of the confined states, which plays an important role for magnetic properties in the weak coupling limit, is obtained as $p=1/2{\tau }^2$, where $\tau (=1+\sqrt{2})$ is the silver ratio. It is also found that the wave functions for confined states are densely distributed in the system and thereby the introduction of the Coulomb interactions immediately induces the finite staggered magnetizations. Increasing the Coulomb interactions, the spatial distribution of the magnetizations continuously changes to those of the Heisenberg model. We discuss crossover behavior in the perpendicular space representation and reveal the superlattice structure in the spatial distribution of the staggered magnetizations.

Using a first-principles fully relativistic scheme based on ultrasoft pseudopotentials and density functional perturbation theory, we study the magnetocrystalline anisotropy free energy of the ferromagnetic binary compound MnBi. We find that differences in the phonon dispersions due to the different orientations of the magnetization (in plane and perpendicular to the plane) give a difference between the vibrational free energies of the high-temperature and low-temperature phases. This vibrational contribution to the magnetocrystalline anisotropy energy (MAE) constant, $K_u$, is non-negligible. When the energy contribution to the MAE is calculated by the PBEsol exchange-correlation functional, the addition of the phonon contribution allows one to get a $T=0\mathrm{K}{K}_u$ and a spin-reorientation transition temperature in reasonable agreement with experiments.

Recently, Chern insulators with Chern numbers $C=1$ and 2 in zero (or very small) magnetic field have been observed in two moire graphene systems: twisted bilayer graphene and ABC trilayer graphene, both aligned with a hexagonal boron-nitride (h-BN) substrate. These Chern insulator states arise due to many-body effects in the Chern bands of these systems when they are partially filled to a total integer filling ${\nu }_T=1,3$ (including spin and valley degrees of freedom). A simple possible explanation is from Hartree-Fock mean-field theory which predicts valley and spin polarization in the zero bandwidth limit, similar to the “quantum Hall ferromagnetism” in Landau levels. Though valley polarization is implied by the existing experiments, the fate of the spin degree of freedom is not presently clear. In this paper, we propose alternative valley polarized—but not spin polarized—candidates for the observed QAH effect. For a valley polarized spinful Chern band at filling ${\nu }_T=1$, we describe a class of exotic Chern insulator phases through spin-charge separation: charge is in a conventional Chern insulator phase with quantized Hall conductivity, while the spin forms disordered spin liquid phase with fractionalization, which we dub quantum Hall spin liquids. We construct a simple class of $Z_2$ quantum Hall spin liquid as analogs of the familiar $Z_2$ spin liquid through slave fermion-Schwinger boson parton theory. Condensation of the spinon from the $Z_2$ quantum Hall spin liquid can lead to a quantum Hall antiferromagnet which is yet another, less exotic, candidate for the experimentally observed Chern insulator. We offer several experimental proposals to probe the quantum Hall spin liquid and the quantum Hall antiferromagnetic phases. We briefly comment on the generalization to the filling ${\nu }_T=2$ and propose possible quantum valley Hall spin liquid without full spin polarization. Finally, we also propose another class of QHSL using fermionic spinons, including phases supporting non-Abelian anyons.

We investigate the magnetic transport properties and electronic structures of single crystal CoSi via angle-dependent quantum oscillation measurements. Prominent de Haas–van Alphen oscillations have been observed with the magnetic field tilting from $B\parallel \left[001\right]$ to $B\parallel \left[110\right]$, from which three fundamental frequencies are extracted. The low frequency $F_{\alpha }$, first observed in quantum oscillations, presents the contribution from the hole pocket at the Brillouin zone (BZ) center $\mathrm{\Gamma }$ point. Two high frequencies $F_{\beta }$ and $F_{\gamma }$, accompanied with asymmetrical peaks in the fast Fourier-transform spectra, are confirmed to stem from the electron pockets with spin-orbit coupling (SOC) at the BZ corner $R$ point. Combined with the analysis of first-principles calculations, the asymmetry of peaks originate from the SOC-induced band splitting when $B\parallel \left[110\right]$ and the irregular Fermi pockets when $B\parallel \left[001\right]$, respectively.

We study multielectron bubble phases in the $N=2$ and $N=3$ Landau levels in a high mobility GaAs/AlGaAs sample. We found that the longitudinal magnetoresistance versus temperature curves in the multielectron bubble region exhibit sharp peaks, irrespective of the Landau-level index. We associate these peaks with an enhanced scattering caused by thermally fluctuating domains of a bubble phase and a uniform uncorrelated electron liquid at the onset of the bubble phases. Within the $N=3$ Landau level, onset temperatures of three-electron and two- electron bubbles exhibit linear trends with respect to the filling factor; the onset temperatures of three-electron bubbles are systematically higher than those of two-electron bubbles. Furthermore, onset temperatures of the two-electron bubble phases across $N=2$ and $N=3$ Landau levels are similar, but exhibit an offset. This offset and the dominant nature of the three-electron bubbles in the $N=3$ Landau level reveals the role of the short-range part of the electron-electron interaction in the formation of the bubbles.

We report a quantitative analysis of the wave-vector diagram emitted by nonlinear photoluminescence generated by a tightly focused pulsed laser beam and distributed along Au nanowire via the mediation of surface plasmon polaritons. The nonlinear photoluminescence is locally excited at key locations along the nanowire in order to understand the different contributions constituting the emission pattern measured in a conjugate Fourier plane of the microscope. Polarization-resolved measurements reveal that the nanowire preferentially emits nonlinear photoluminescence polarized transverse to the long axis at close to the detection limit wave vectors with a small azimuthal spread in comparison to the signal polarized along the long axis. We utilize the finite-element method to simulate the observed directional scattering by using localized incoherent sources placed on the nanowire. Simulation results faithfully mimic the directional emission of the nonlinear signal emitted by the different portions of the nanowire.

${\mathrm{MoS}}_2$ is a two-dimensional material with a band gap depending on the number of layers and tunable by an external electric field. The experimentally observed intralayer band-to-band tunneling and interlayer band-to-band tunneling in this material present an opportunity for new electronic applications in tunnel field-effect transistors. However, such a widely accepted concept has yet to be been supported by theoretical investigations based on first principles. In this paper, using density functional theory, in conjunction with nonequilibrium Green's function techniques and our electric field gating method, enabled by a large-scale computational approach, we study the relation between band alignment and transmission in planar and side-stack ${\mathrm{MoS}}_{2}p\text{–}i\text{–}n$ junction configurations. We demonstrate the presence of negative differential resistance for both in-plane and interlayer current, a staple characteristic of tunnel diode junctions, and analyze the physical origin of such an effect. Electrostatic potentials, the van der Waals barrier, and a complex band analysis are also examined for a thorough understanding of Esaki diodes.

A double-tip scanning tunneling microscope with nanometer-scale tip separation has the ability to access the single-electron Green's function in real and momentum spaces based on second-order tunneling processes. Experimental realization of such measurements has been limited to quasi-one-dimensional systems due to the extremely small signal size. Here we propose an alternative approach to obtain such information by exploiting the current-current correlations from the individual tips and present a theoretical formalism to describe it. To assess the feasibility of our approach we make a numerical estimate for an $\sim 25$-nm Pb nanoisland and show that the wave function in fact extends from tip to tip and the signal depends less strongly on increased tip separation in the diffusive regime than the one in alternative approaches relying on tip-to-tip conductance.

Nodal-line semimetals in two-dimensional (2D) materials have attracted intense attention recently. From fundamental physics and spintronic application points of view, high Curie temperature ferromagnetic (FM) ones with nodal lines robust against spin-orbit coupling (SOC) are extremely desirable. Here, we propose that the FM ${\mathrm{K}}_2\mathrm{N}$ monolayer is such a Weyl nodal-line semimetal. We show that the ${\mathrm{K}}_2\mathrm{N}$ monolayer is dynamically stable and has a FM ground magnetic state with out-of-plane [001] magnetization. It shows two nodal lines in the low-energy band structures. Both nodal lines are robust against SOC, under the protection of mirror symmetry. We construct an effective Hamiltonian, which can well characterize the nodal lines in the system. Remarkably, the nodal-line semimetal proposed here is distinct from the previously studied ones in that the ${\mathrm{K}}_2\mathrm{N}$ monolayer is a 2D $d^0$-type ferromagnet with the magnetism arising from the partially filled the $p$ orbitals of the N atom, which can bring special advantages in spintronic applications. In addition, the Curie temperature in the ${\mathrm{K}}_2\mathrm{N}$ monolayer is estimated to be 942 K, which is significantly higher than previous FM nodal-line materials. We also find that specific tensile strains can transform the nodal line from type-I to type-II, making its nodal-line characteristics even more interesting.

We study possible magnetic instabilities in Dirac semimetals. We find that Dirac electrons naturally host antiferromagnetic or spin-density wave ground states, though their specific configurations may vary depending on a specific model, as well as chemical potential and temperature. We also discuss paramagnetic susceptibility of Dirac semimetals. In the cases when Dirac electrons do not have orbital momentum, the magnetic properties may be $\mu$ and $T$ independent.

We analyze density wave and Pomeranchuk orders in twisted bilayer graphene. This complements our earlier analysis of the pairing instabilities. We assume that near half filling of either conduction or valence band, the Fermi level is close to Van Hove points, where the density of states diverges, and study potential instabilities in the particle-hole channel within a patch model with two valley degrees of freedom. The hexagonal symmetry of twisted bilayer graphene allows for either six or twelve Van Hove points. We consider both cases and find the same two leading candidates for particle-hole order. One is an SU(2)-breaking spin state with ferromagnetism within a valley. A subleading intervalley hopping induces antiferromagnetism between the valleys. The same state has also been obtained in strong-coupling approaches, indicating that this order is robust. The other is a mixed state with ${120}^{\circ }$ complex spin order and orthogonal complex charge order. In addition, we find a weaker but still attractive interaction in nematic channels, and discuss the type of a nematic order.

We study the effects of quenched bond disorder in frustrated easy-plane antiferromagnets in two space dimensions, using a combination of analytical and numerical techniques. We consider local-moment systems which display noncollinear long-range order at zero temperature, with the antiferromagnetic triangular-lattice $XY$ model as a prime example. We show that (i) weak bond disorder transforms the clean ground state into a state with quasi-long-range order, and (ii) strong bond disorder results in a short-range-ordered glassy ground state. Using extensive Monte Carlo simulations, we also track the fate of the quasi-long-range-ordered phase at finite temperature and the associated Kosterlitz-Thouless-like transition. Making contact with previous studies of disordered $XY$ models, we discuss similarities and differences concerning the interplay of disorder and frustration. Our results are also of relevance to noncollinear Heisenberg magnets in an external magnetic field.

The antiferromagnetic-ferromagnetic competition in a diluted magnetic semiconductor is discussed. In a virtual-crystal approximation, the active magnetic ions in the system are assumed to distribute homogeneously, and the quantum magnetic correlations are described in the Kondo lattice model involving local disorder. In the framework of the dynamical mean-field theory, we deliver the signatures of the static magnetic susceptibility function and the $B_{1g}$ channel Raman response adapting to the model. At low temperatures, one finds the antiferromagnetic instability against the ferromagnetic state when the impurity band is nearly filled. For a completely filled impurity band system, the antiferromagnetic state is stabilized in the case of sufficiently large magnetic exchanges. The effect of the thermal fluctuations on the phase competition is also discussed. Analyzing the $B_{1g}$ channel Raman spectrum attributes a competition between the short-range magnetic order and the short-wavelength excitation in the filled impurity band paramagnetic state.

Theoretical studies have shown that new physical properties such as tunable gap openings or quantum spin Hall effects could be expected from group-IV graphene analogs (silicene, germanene, stanene). While there have been numerous studies of growth of such Si, Ge, Sn monolayers, the demonstration of their hexagonal organization has been often based on postgrowth characterization, and their analogy to graphene has remained controversial. Our real-time scanning tunneling microscopy (STM) observation during Ge deposition on Ag(111) in the 380–430 K temperature range reveals that Ag atoms are involved in all the structures observed before the formation of a second layer, rejecting the possible formation of germanene on this substrate within these experimental conditions. The observation by STM of Ge atomic diffusion shows that easy exchange between Ag and Ge atoms is responsible for the Ge-Ag surface alloying at such temperatures.