Skyrmions are topologically protected nanoscale magnetic structures with a wide range of potential applications. Here we determine the life cycle of skyrmions from their creation to their intrinsic dynamics and thermal stability to their eventual thermodynamic demise. Using atomistic simulations of Ir/Co/Pt, parameterized from ab initio calculations, we demonstrate the thermal phase transition to a skyrmion state under application of a perpendicular magnetic field. The created skyrmions exhibit Brownian particlelike dynamics driven by the underlying thermal spin fluctuations. At an elevated temperature window well below the Curie temperature, the skyrmions are metastable and can collapse to a uniform magnetic state. With application of an external magnetic field, the intrinsic local thermal spin fluctuations at this elevated temperature window are sufficiently large to allow the spontaneous formation of new skyrmions in thermodynamic equilibrium analogous to a spontaneous skyrmion gas. Such a system could be used to implement a skyrmion-based true random number generator.

Intrinsic antiferromagnetic ${\mathrm{Mn}}_2{\mathrm{Bi}}_2{\mathrm{Te}}_5$ crystals, a candidate of dynamical axion insulator proposed theoretically recently, were successfully grown by the self-flux method. The crystal structure and chemical composition of ${\mathrm{Mn}}_2{\mathrm{Bi}}_2{\mathrm{Te}}_5$ crystals were experimentally substantiated. Temperature-dependent resistivity of ${\mathrm{Mn}}_2{\mathrm{Bi}}_2{\mathrm{Te}}_5$ shows a metallic behavior $\left(d\rho /dT>0\right)$ when temperature $T>25\mathrm{K}$ and a subsequently semiconductorlike feature $\left(d\rho /dT<0\right)$. Magnetic measurement verifies that ${\mathrm{Mn}}_2{\mathrm{Bi}}_2{\mathrm{Te}}_5$ is ferromagnetic at $ab$ plane but antiferromagnetic along the $c$ axis, whose Néel temperature is around 20 K. Analysis of magnetic hysteresis loops measured at different temperatures substantiates that critical indices of magnetic-paramagnetic phase transition in ${\mathrm{Mn}}_2{\mathrm{Bi}}_2{\mathrm{Te}}_5$ are satisfied to the prediction of Landau mean-field theory. When $T<20\mathrm{K}$, there are unconventional Hall effects, including both anomalous Hall effect and topological Hall effect, when magnetic field $B<3.4\mathrm{T}$. Based on theoretical electronic-band structure of ${\mathrm{Mn}}_2{\mathrm{Bi}}_2{\mathrm{Te}}_5$, the anomalous Hall effect of ${\mathrm{Mn}}_2{\mathrm{Bi}}_2{\mathrm{Te}}_5$ can be described by the Karplus-Luttinger (Berry curvature) mechanism, while the topological Hall effect of ${\mathrm{Mn}}_2{\mathrm{Bi}}_2{\mathrm{Te}}_5$, under $B<3.4\mathrm{T}$, is attributed to noncollinear spin structure. Our results strongly suggest that ${\mathrm{Mn}}_2{\mathrm{Bi}}_2{\mathrm{Te}}_5$ is an axion insulator.

Recently, theoretical work predicted that the cobalt mono-antimonide (CoSb) with a NiAs-type structure could be stabilized in the form of a monolayer PbO-type FeSe structure on a $\mathrm{SrTi}{\mathrm{O}}_3$ substrate [Ding et al., Phys. Rev. Lett. 124, 027002 (2020)], which may host high-temperature superconductivity comparable with monolayer FeSe [Wang et al., Chin. Phys. Lett. 29, 037402 (2012)]. Motivated to explore the possible pressure-induced superconductivity in bulk CoSb associated with structural instability, utilizing the diamond anvil cell technique, we herein report a comprehensive study of the electrical transport, crystalline structure, and electronic band structure of the bulk CoSb with electrical conductivity, synchrotron x-ray diffraction, and first-principles calculations. No pressure-induced superconductivity is detected down to 2 K with pressure up to 42.0 GPa, but a structural phase transition occurs between 11.7 and 16.2 GPa. Instead, an electronic transition, featured by a carrier-type switch from an electron $<10\mathrm{GPa}$ to a hole ($n\to p$) dominant one at higher pressure, is observed. The $n\text{−}p$ switch at room temperature is ascribed to a pressure-induced structural phase transition from hexagonal ($P{6}_3/mmc$, No. 194) to orthorhombic ($Pnma$, No. 62), which is confirmed by a combination of first-principles calculations and structural refinement. Interestingly, the temperature-driven $p\text{−}n$ switch also occurs in the orthorhombic phase at ∼90 K, emphasizing the tunable multiband structure (with charge neutrality point) of the semimetal CoSb by pressure or temperature. Moreover, the electronic transition is assigned as a semimetal-semimetal transition. The resistivity shows a robust $T^3$ dependence trend $<30\mathrm{K}$ for both semimetal phases. At high temperature, all the $\rho \left(T\right)$ curves become saturated, as described by the parallel-resistor model. The results support that the $s\text{−}d$ interband scattering dominates the conductivity of both the hexagonal and orthorhombic phases under pressure. The superconductivity absence implications are discussed, assuming the electron-phonon interaction in CoSb drives the superconductivity.

We have studied the compression behaviors of Ne, Ar, and Kr in comparison with the isoelectronic counterparts of NaF, KCl, and RbBr to 120 GPa, using in situ angle-resolved x-ray diffraction. Interestingly, the pressure-volume compression data indicate the emergence between the isoelectronic pairs of NaF/Ne $>60\mathrm{GPa}$, KCl/Ar $>30\mathrm{GPa}$, and RbBr/Kr $>25\mathrm{GPa}$—analogous to that previously observed in CsI/Xe $>80\mathrm{GPa}$. The interatomic distances of the isoelectronic pairs also become similar as pressure increases, within the difference of $\sim 3\%$, underscoring the pressure-induced electron polarization in novel gas solids, stronger in Xe and getting weaker in Kr, Ar, and Ne. In contrast, the compression curves and interatomic distances of He and LiF remain distinctive to 200 GPa.

Optimizing materials' properties and functions by controlling defects in the crystalline phase has been a cornerstone of materials science and condensed matter physics. However, this paradigm has yet to be established in the broadly defined amorphous materials, which implies the identification of very subtle structural features in an otherwise uniformly disordered medium. Here we propose and define a new integrated glassy defect (IGD), based on machine learning strategy informed by atomistic physics, and also by an extremely wide configurational, thermodynamic, and dynamic variables space of the disordered state. The IGD simultaneously includes positional topology and vibrational features, as well as the local morphology of the potential energy landscape. This unprecedented combination gives rise to a much more comprehensive and more effective definition of the “glassy defect,” much beyond the conventional, purely structural input. IGD can be used not only as an efficient predictor of athermal plasticity but is also transferable to detect both short-time vibrational anomalies (the boson peak), and long-time relaxation and diffusion dynamics in glasses. The integrated strategy is instrumental to build the long-sought structure-property relationship in complex media.

We formulate a fracton-elasticity duality for twisted moiré superlattices, taking into account that they are incommensurate crystals with dissipative phason dynamics. From a dual tensor-gauge formulation, as compared to standard crystals, we identify twice the number of conserved charges that describe topological lattice defects, namely, disclinations and another type of defects that we dub discompressions. The key implication of these conservation laws is that both glide and climb motions of lattice dislocations are suppressed, indicating that dislocation networks may become exceptionally stable. We also generalize our results to other planar incommensurate crystals and quasicrystals.

The origin of the anomalous Hall effect (AHE) in ferromagnetic metallic glasses (MGs) is far from well understood due to the disordered atomic structure. Here, we report that the AHE is found in ${\mathrm{Fe}}_{78}{\mathrm{Si}}_9{\mathrm{B}}_{13}$ MGs, and the anomalous Hall conductivity $\left({\sigma }_{\mathrm{AH}}\right)$ has a linear dependence with magnetization $\left(M_z\right)$, which is a feature of intrinsic mechanism contribution rather than extrinsic mechanism contribution. Moreover, the ${\sigma }_{\mathrm{AH}}$ normalized by $M_z$ is independent of longitudinal conductivity (${\sigma }_{xx}$), which also indicates the characteristic of the nondissipative intrinsic mechanism. This intrinsic contribution could be understood from the density of Berry curvature integrated over occupied energies proposed recently for aperiodic materials, and the linear magnetization dependence can be understood from the fluctuations of spin orientation at the finite temperature. In such case, Berry curvature is proportional to spin magnetization in different orientations. Our results indicate experimental signatures to distinguish the intrinsic mechanism in ferromagnetic MGs and confirm that the intrinsic ${\sigma }_{\mathrm{AH}}$ has a large value $(505–616{\mathrm{Scm}}^{-1})$ in the ${\mathrm{Fe}}_{78}{\mathrm{Si}}_9{\mathrm{B}}_{13}$ MGs.

We report neutron scattering, magnetic susceptibility and Monte Carlo theoretical analysis to verify the short-range nature of the magnetic structure and spin-spin correlations in a ${\mathrm{Yb}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ single crystal. The quantum spin state of ${\mathrm{Yb}}^{3+}$ in ${\mathrm{Yb}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ is verified. The quantum spins organize into a short-ranged emergent director state for $T<0.6$ K derived from anisotropy and near-neighbor exchange. We derive the magnitude of the near-neighbor exchange interactions $0.6<J_1<0.7\mathrm{K},J_2=0.12$ K and the magnitude of the dipolar exchange interaction, $D$, in the range $0.18<D<0.21$ K. Certain aspects of the broad experimental dataset can be modeled using a $J_{1}D$ model with ferromagnetic near-neighbor spin-spin correlations while other aspects of the data can be accurately reproduced using a $J_1{J}_{2}D$ model with antiferromagnetic near-neighbor spin-spin correlation. As such, although we do not quantify all the relevant exchange interactions, we nevertheless provide a strong basis for the understanding of the complex Hamiltonian required to fully describe the magnetic state of ${\mathrm{Yb}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$.

With $T_c\sim 9.6\mathrm{K}, {\mathrm{Be}}_{22}\mathrm{Re}$ exhibits one of the highest critical temperatures among Be-rich compounds. We have carried out a series of high-pressure electrical resistivity measurements on this compound to 30 GPa. The data show that the critical temperature $T_c$ is suppressed gradually at a rate of $d{T}_c/dP=-0.05\mathrm{K}/\mathrm{GPa}$. Using density functional theory (DFT) calculations of the electronic and phonon density of states (DOS) and the measured critical temperature, we estimate that the rapid increase in lattice stiffening in ${\mathrm{Be}}_{22}\mathrm{Re}$ overwhelms a moderate increase in the electron-ion interaction with pressure, resulting in the decrease in $T_c$. High-pressure x-ray diffraction measurements show that the ambient pressure crystal structure of ${\mathrm{Be}}_{22}\mathrm{Re}$ persists to at least 154 GPa.

We present an exhaustive study of Andreev crystals (ACs)—quasi-one-dimensional superconducting wires with a periodic distribution of magnetic regions. The exchange field in these regions is assumed to be much smaller than the Fermi energy. Hence, the transport through the magnetic region can be described within the quasiclassical approximation. In the first part of the paper, by assuming that the separation between the magnetic regions is larger than the coherence length, we derive the effective nearest-neighbor tight-binding equations for ACs with a helical magnetic configuration. The spectrum within the gap of the host superconductor shows a pair of energy-symmetric bands. By increasing the strength of the magnetic impurities in ferromagnetic ACs, these bands cross without interacting. However, in any other helical configuration, there is a value of the magnetic strength at which the bands touch each other, forming a Dirac point. Further increase of the magnetic strength leads to a system with an inverted gap. We study junctions between ACs with inverted spectrum and show that junctions between (anti)ferromagnetic ACs (always) never exhibit bound states at the interface. In the second part, we extend our analysis beyond the nearest-neighbor approximation by solving the Eilenberger equation for infinite ACs and junctions between semi-infinite ACs with collinear magnetization. From the obtained quasiclassical Green functions, we compute the local density of states and the local spin polarization in anti- and ferromagnetic ACs. We show that these junctions may exhibit bound states at the interface and fractionalization of the surface spin polarization.

The description of interactions in strongly correlated topological phases of matter remains a challenge. Here, we develop a stochastic functional approach for interacting topological insulators including both charge and spin channels. We find that the Mott transition of the Kane-Mele-Hubbard model may be described by the variational principle with one equation. We present different views of this equation from the electron Green's function, the free-energy, and the Hellmann-Feynman theorem. In particular, we show the stability of the transition line towards fluctuations, in good agreement with numerical results. The band gap remains finite at the transition and the Mott phase is characterized by antiferromagnetism in the $x\text{−}y$ plane. The interacting topological phase is described through a ${\mathbb{Z}}_2$ number related to helical edge modes. Our results then show that improving stochastic approaches can give further insight on the understanding of interacting phases of matter.

We study magnetic excitations and thermal Hall effect on the Kitaev-Heisenberg model under magnetic fields. By employing the spin-wave theory for the magnetic orders realized in this model, we examine the topological nature of the spin-wave dispersions and calculate the thermal Hall conductivity. The comprehensive investigations on the field-angle dependence clarify that the thermal Hall conductivity is sensitive to the spin ordered pattern and excitation spectra of magnons; this quantity is enhanced by the noncoplanar spin configurations and small magnon gap in the excitation spectrum. On the other hand, we also find a common feature in the field-angle dependence of the thermal Hall conductivity. It vanishes when the magnetic field is on the planes spanned by the spin axes. We reveal that the behavior is intrinsic to the Kitaev-Heisenberg model in an applied field and demonstrate that the introduction of the off-diagonal spin interaction causes the disappearance of the feature in the thermal Hall conductivity.

We present a machine-learning approach that allows to characterize the disorder potential of a two-dimensional electronic system from its quantum transport properties. Numerically simulated transport data for a large number of disorder configurations are used for the training of artificial neural networks. We show that the trained networks are able to recognize details of the disorder potential of an unknown sample from its transport properties, and that they can even reconstruct the complete potential landscape seen by the electrons.

Two-dimensional systems with Rashba spin-orbit coupling are not Galilean invariant and therefore electron–electron collisions in them may affect the current. However, when taken alone, they cannot ensure a nonzero dc resistivity, so their effects are masked by impurity scattering. Here we calculate the related finite-frequency response and show that the electron–electron scattering in clean Rashba conductors decreases the Drude weight while resulting in a finite dissipative component of the response outside of the Drude peak.

We develop a theory of circular photogalvanic effect in nongyrotropic two-dimensional transition metal dichalcogenide monolayers under interband optical transitions. Oblique incidence of circularly polarized electromagnetic field or normal incidence of elliptically polarized electromagnetic field is assumed. In contrast to the linear-in-intensity conventional photogalvanic effect, the effect considered here arises in the second intensity order. The effect is conditioned by (i) the predominant population of the valleys by the circular in-plane electromagnetic field component and (ii) the direct drift of the photoexcited carriers by the linear-polarized in-plane electromagnetic field component in the presence of trigonal valley asymmetry.

We numerically study the three-dimensional (3D) quantum Hall effect (QHE) and magnetothermoelectric transport of Weyl semimetals in the presence of disorder. We obtain a bulk picture that the exotic 3D QHE emerges in a finite range of Fermi energy around the Weyl points determined by the gap between the $n=-1$ and $n=1$ Landau levels (LLs). The quantized Hall conductivity is attributable to the chiral zeroth LLs traversing the gap, and is robust against disorder scattering for an intermediate number of layers in the direction of the magnetic field. Moreover, we predict several interesting characteristic features of the thermoelectric transport coefficients in the 3D QHE regime, which can be probed experimentally. This may open an avenue for exploring Weyl physics in topological materials.

We define, compute, and analyze the nonequilibrium differential optical conductivity of the one-dimensional extended Hubbard model at half-filling after applying a pump pulse, using the time-dependent density matrix renormalization group method. The melting of the Mott insulator is accompanied by a suppression of the local magnetic moment and ensuing photogeneration of doublon-holon pairs. The differential optical conductivity reveals (i) mid-gap states related to parity-forbidden optical states and (ii) strong renormalization and hybridization of the excitonic resonance and the absorption band, yielding a Fano resonance. We offer evidence and interpret such a resonance as a signature of nonequilibrium optical excitations resembling excitonic strings, (bi)excitons, and unbound doublon-holon pairs, depending on the magnitude of the intersite Coulomb repulsion. We discuss our results in the context of pump and probe spectroscopy experiments on organic Mott insulators.

We theoretically study the real-time dynamics of the photoinduced topological phase transition to a nonequilibrium Floquet Chern insulator in an organic conductor $\alpha \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{I}}_3$, which was recently predicted using the Floquet theory. By using a tight-binding model of $\alpha \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{I}}_3$ that hosts a pair of tilted Dirac-cone bands at the Fermi level, we solve the time-dependent Schrödinger equation and obtained time evolutions of physical quantities for continuous-wave and pulse excitations with circularly polarized light. We demonstrate that, for the continuous-wave excitations, time profiles of the Chern number and the Hall conductivity show indications of the Floquet topological insulator. We argue that the Hall conductivity exhibits a slow oscillation with its frequency corresponding to a photoinduced direct gap determined by the Floquet band structure. With pulse excitations, transient excitation spectra are obtained, from which we infer the formation of Floquet bands and the gap opening at the Dirac point during the pulse irradiation. This dynamical gap formation is also manifested by the slow oscillation component of the Hall conductivity; that is, its frequency increases with time toward the pulse peak at which it nearly coincides with the photoinduced direct gap. The relevance of the results to experiments is also discussed.

We study the nonlinear steady-state transport of spinless fermions through a quantum dot with a local two-particle interaction. The dot degree of freedom is in addition coupled to a phonon mode. This setup combines the non-equilibrium physics of the interacting resonant level model and that of the Anderson-Holstein model. The fermion-fermion interaction defies a perturbative treatment. We mainly focus on the antiadiabatic limit, with the phonon frequency being larger than the lead-dot tunneling rate. In this regime also the fermion-boson coupling cannot be treated perturbatively. Our goal is two fold. We investigate the competing roles of the fermion-fermion and fermion-boson interactions on the emergent low-energy scale $T_{\mathrm{K}}$ and show how $T_{\mathrm{K}}$ manifests in the transport coefficients as well as the current-voltage characteristics. For small to intermediate interactions, the latter is in addition directly affected by both interactions independently. With increasing fermion-boson interaction the Franck-Condon blockade suppresses the current at small voltages and the emission of phonons leads to shoulders or steps at multiples of the phonon frequency, while the local fermion-fermion interaction implies a negative differential conductance at voltages larger than $T_{\mathrm{K}}$. We, in addition, use the model to investigate the limitations of our low-order truncated functional renormalization group approach on the Keldysh contour. In particular, we quantify the role of the broken current conservation.

Spin-momentum locking of evanescent waves describes the relationship between the propagation constant of an evanescent mode and the polarization of its electromagnetic field, giving rise to applications in light nanorouting and polarimetry among many others. The use of complex numbers in physics is a powerful representation in areas, such as quantum mechanics or electromagnetism; it is well known that a lossy waveguide can be modeled with the addition of an imaginary part to the propagation constant. Here we explore how these losses are entangled with the polarization of the associated evanescent tails for the waveguide, revealing a well-defined mapping between waveguide losses and the Poincaré sphere of polarizations in what could be understood as a “polarization-loss locking” of evanescent waves. We analyze the implications for near-field directional coupling of sources to waveguides as optimized dipoles must take into account the losses for a perfectly unidirectional excitation. We also reveal the potential advantage of calculating the angular spectrum of a source defined in a complex rather than the traditionally purely real transverse wave-vector space formalism.