Skyrmions are topologically nontrivial, magnetic quasiparticles that are characterized by a topological charge. A regular array of skyrmions, a skyrmion crystal (SkX), features the topological Hall effect (THE) of electrons, which, in turn, gives rise to the Hall effect of the skyrmions themselves. It is commonly believed that antiferromagnetic skyrmion crystals (AFM-SkXs) lack both effects. In this Rapid Communication, we present a generally applicable method to create stable AFM-SkXs by growing a two-sublattice SkX onto a collinear antiferromagnet. As an example we show that both types of skyrmion crystals, conventional and antiferromagnetic, exist in honeycomb lattices. While AFM-SkXs with equivalent lattice sites do not show a THE, they exhibit a topological spin Hall effect. On top of this, AFM-SkXs on inequivalent sublattices exhibit a nonzero THE, which may be utilized in spintronics devices. Our theoretical findings call for experimental realization.

We demonstrate that the differential conductance, $dI/dV$, measured via spectroscopic imaging scanning tunneling microscopy in the doped iron chalcogenide ${\mathrm{FeSe}}_{0.45}{\mathrm{Te}}_{0.55}$, possesses a series of characteristic features that allow one to extract the orbital structure of the superconducting gaps. This yields nearly isotropic superconducting gaps on the two holelike Fermi surfaces, and a strongly anisotropic gap on the electronlike Fermi surface. Moreover, we show that the pinning of nematic fluctuations by defects can give rise to a dumbbell-like spatial structure of the induced impurity bound states, and explains the related $C_2$ symmetry in the Fourier transformed differential conductance.

The quantum anomalous Hall effect, an exotic topological state first theoretically predicted by Haldane and recently experimentally observed, has attracted enormous interest for low-power-consumption electronics. In this work, we derived a Schrödinger-like equation of phonons, where topology-related quantities, time-reversal symmetry, and its breaking can be naturally introduced similar to the process for electrons. Furthermore, we proposed a phononic analog of the Haldane model, which makes the novel quantum (anomalous) Hall-like phonon states characterized by one-way gapless edge modes immune to scattering. The topologically nontrivial phonon states are useful not only for conducting phonons without dissipation but also for designing highly efficient phononic devices, like an ideal phonon diode, which could find important applications in future phononics.

The functionality of many magnetic materials critically depends on first manipulating and then taking advantage of highly nonlinear changes of properties that occur during phase transformations. Unique to lanthanides, property-defining $4f$ electrons are highly localized and, as commonly accepted, play little to no role in chemical bonding. Yet here we demonstrate that the competition between $4f$-electron energy landscapes of Dy $\left(4f^9\right)$ and Er $\left(4f^{11}\right)$ is the key element of the puzzle required to explain complex interplay of magnetic and structural features observed in $\mathrm{E}{\mathrm{r}}_{1-x}\mathrm{D}{\mathrm{y}}_x\mathrm{C}{\mathrm{o}}_2$, and likely many other mixed lanthanide systems. Unlike the parent binaries—$\mathrm{DyC}{\mathrm{o}}_2$ and $\mathrm{ErC}{\mathrm{o}}_2$—$\mathrm{E}{\mathrm{r}}_{1-x}\mathrm{D}{\mathrm{y}}_x\mathrm{C}{\mathrm{o}}_2$ exhibits two successive magnetostructural transitions: a first order at $T_{\mathrm{C}}$, followed by a second order in the ferrimagnetically ordered state. Supported by first-principles calculations, our results offer new opportunities for targeted design of magnetic materials with multiple functionalities, and also provide a critical insight into the role of $4f$ electrons in controlling the magnetism and structure of lanthanide intermetallics.

We consider the Josephson effect through a thin spin-orbit coupled layer in the presence of an exchange field $\mathit{h}$ and discover a set of supercurrent vortices appearing in the system which can be controllably moved around in the system by varying either the direction of $\mathit{h}$, the strength $\left|\mathit{h}\right|$, the spin-orbit coupling magnitude $\alpha$ via a gate voltage, or the phase difference. We refer to this phenomenon as a supercurrent vortex pinball effect and show that its origin is the spin polarization of the triplet Cooper pairs induced in the system. The supercurrent vortices are shown to arise from what resembles a Cooper pair-induced inverse Edelstein effect.

We calculate the effect of a static electric field on the critical temperature of a $s$-wave one band superconductor in the framework of proximity effect Eliashberg theory. In the weak electrostatic field limit the theory has no free parameters while, in general, the only free parameter is the thickness of the surface layer where the electric field acts. We conclude that the best situation for increasing the critical temperature is to have a very thin film of a superconducting material with a strong increase of electron-phonon (boson) constant upon charging.

The Kondo lattice model of spin-1/2 local moments coupled to the conduction electrons at half filling is studied for its orbital response to magnetic field on bipartite lattices. Through an effective charge dynamics, in a canonical representation of electrons that appropriately describes the Kondo insulating ground state, the magnetization is found to show de Haas-van Alphen oscillations from intermediate to weak Kondo coupling. These oscillations are ascribed to the inversion of a dispersion of the gapped charge quasiparticles, whose chemical potential surface is measured by the oscillation frequency. Such oscillations are also predicted to occur in spin-density wave insulators.

We study the spin-$1/2$ Heisenberg model on the triangular lattice with the nearest-neighbor $J_1>0$, the next-nearest-neighobr $J_2>0$ Heisenberg interactions, and the additional scalar chiral interaction $J_{\chi }({\vec{S}}_i\times {\vec{S}}_j)·{\vec{S}}_k$ for the three spins in all the triangles using large-scale density matrix renormalization group calculation on cylinder geometry. With increasing $J_2$ ($J_2/J_1\le 0.3$) and $J_{\chi }$ ($J_{\chi }/J_1\le 1.0$) interactions, we establish a quantum phase diagram with the magnetically ordered ${120}^{\circ }$, stripe, and noncoplanar tetrahedral phase. In between these magnetic order phases, we find a chiral spin liquid (CSL) phase, which is identified as a $\nu =1/2$ bosonic fractional quantum Hall state with possible spontaneous rotational symmetry breaking. By switching on the chiral interaction, we find that the previously identified spin liquid in the $J_1-J_2$ triangular model ($0.08\lesssim J_2/J_1\lesssim 0.15$) shows a phase transition to the CSL phase at very small $J_{\chi }$. We also compute the spin triplet gap in both spin liquid phases, and our finite-size results suggest a large gap in the odd topological sector but a small or vanishing gap in the even sector. We discuss the implications of our results on the nature of the spin liquid phases.

We present new numerical tools to analyze symmetry-broken phases in the context of $\mathrm{SU}\left(2\right)$-symmetric translation-invariant matrix product states (MPS) and density-matrix renormalization-group (DMRG) methods for infinite cylinders, and determine the phase diagram of the geometrically frustrated triangular Heisenberg model with nearest- and next-nearest-neighbor (NN and NNN) interactions. The appearance of Nambu-Goldstone modes in the excitation spectrum is characterized by “tower of states” levels in the momentum-resolved entanglement spectrum. Symmetry-breaking phase transitions are detected by a combination of the correlation lengths and second and fourth cumulants of the magnetic order parameters (which we call the Binder ratio), even though symmetry implies that the order parameter itself is strictly zero. Using this approach, we have identified a ${120}^{\circ }$ order, a columnar order, and an algebraic spin liquid (specific to width-6 systems), alongside the previously studied topological spin liquid phase. For the latter, we also demonstrate robustness against chiral perturbations.

We investigate anisotropic electronic structure and thermal transport properties of bulk black phosphorus (BP). Using density functional dynamical mean-field theory we first derive a correlation-induced electronic reconstruction, showing band-selective Kondoesque physics in this elemental $p$-band material. The resulting correlated picture is expected to shed light onto the temperature and doping dependent evolution of resistivity, Seebeck coefficient, and thermal conductivity, as seen in experiments on bulk single crystal BP. Therein, large anisotropic particle-hole excitations are key to consistently understand thermoelectric transport responses of pure and doped BP.

By employing the nonequilibrium Green's function (NEGF) method, the transient current and the transient behavior of the spin transfer torque (STT) of the magnetic layered system are investigated within the framework of density functional theory (DFT). To reduce the huge computational cost of the transient calculation, especially when the dense mesh of $k$ sampling is present for layered systems, the complex absorbing potential (CAP) and the Padé spectrum decomposition are used so that the energy integrals in calculating transient current and STT can be performed analytically using residue theorem, which dramatically reduces the computational complexity of the first-principles calculation of transient behavior. As an application of the NEGF-DFT-CAP formalism, the transient current and current-induced STT of the Co/Cu/Co trilayer system are studied under an upward bias pulse for different angles of magnetization direction between two leads. The transient current shows a damped oscillatory behavior with the oscillation frequency proportional to the applied bias, leading to a relaxation time of hundreds of femtoseconds. The time-dependent STTs show roughly the same profile for systems with different rotating angles. The oscillation behavior is also observed as the transient STT approaches the steady state value. Such oscillations can be attributed to the interface resonant states.

We investigate the low-energy scaling behavior of an interacting three-dimensional (3D) Weyl semimetal in the presence of disorder. In order to achieve a renormalization group analysis of the theory, we first focus on the effects of a short-ranged-correlated disorder potential, checking nevertheless that this choice is not essential to locate the different phases of the Weyl semimetal. We show that there is a line of fixed points in the renormalization group flow of the interacting theory, corresponding to the disorder-driven transition to a diffusive metal phase. Along that boundary, the critical disorder strength undergoes a strong increase with respect to the noninteracting theory, as a consequence of the unconventional screening of the Coulomb and disorder-induced interactions. The complementary resolution of the Schwinger-Dyson equations allows us to determine the full phase diagram of the system, showing the prevalence of a renormalized semimetallic phase in the regime of intermediate interaction strength, and adjacent to the non-Fermi liquid instability of 3D Weyl semimetals in the strong interaction regime.

We consider a bipartite quantum conductor and discuss the joint probability distribution of a particle number in a subsystem and the self-information associated with the reduced density matrix of the subsystem. By extending the multicontour Keldysh Green-function technique, we calculate the Rényi entropy of a positive integer order $M$ subjected to the particle number constraint, from which we derive the joint probability distribution. For energy-independent transmission, we derive the time dependence of the accessible entanglement entropy, or the conditional entropy. We analyze the joint probability distribution for energy-dependent transmission probability at the steady state under the coherent resonant tunneling and the incoherent sequential tunneling conditions. We also discuss the probability distribution of the efficiency, which measures the information content transferred by a single electron.

Using vicinal surfaces as a template for heteroepitaxial growth offers a unique possibility to control the orientation and the crystallinity of the grown layer. The study presented here focuses on the growth of Ag on a carefully chosen Ni(11 9 9) substrate. Ag adopts a Stranski-Krastanov growth mode with a slightly constrained wetting layer limited to two monolayers on which well crystallized islands grow. The specific orientation of the Ni substrate leads to Ag(7 9 9) orientation for the Ag thin film and it prevents the formation of twin and stacking fault.

We describe a first-principles method to calculate scanning tunneling microscopy (STM) images, and compare the results to well-characterized experiments combining STM with atomic force microscopy (AFM). The theory is based on density functional theory with a localized basis set, where the wave functions in the vacuum gap are computed by propagating the localized-basis wave functions into the gap using a real-space grid. Constant-height STM images are computed using Bardeen's approximation method, including averaging over the reciprocal space. We consider copper adatoms and single CO molecules adsorbed on Cu(111), scanned with a single-atom copper tip with and without CO functionalization. The calculated images agree with state-of-the-art experiments, where the atomic structure of the tip apex is determined by AFM. The comparison further allows for detailed interpretation of the STM images.

Landau theory describes phase transitions as the competition between energy and entropy: The ordered phase has lower energy, while the disordered phase has larger entropy. When heating the system, ordering is reduced entropically until it vanishes at the critical temperature. This picture implicitly assumes that the energy difference between the ordered and disordered phases does not change with temperature. We show that for orbital ordering in the Mott insulator ${\mathrm{KCuF}}_3$, this assumption fails qualitatively: entropy plays a negligible role, while thermal expansion energetically stabilizes the orbitally ordered phase to such an extent that no phase transition is observed. To understand this strong dependence on the lattice constant, we need to take into account the Born-Mayer repulsion between the ions. It is the latter, and not the Jahn-Teller elastic energy, which determines the magnitude of the distortion. This effect will be seen in all materials where the distortion expected from the Jahn-Teller mechanism is so large that the ions would touch. Our mechanism explains not only the absence of a phase transition in ${\mathrm{KCuF}}_3$, but even suggests the possibility of an inverted transition in closed-shell systems, where the ordered phase emerges only at high temperatures.

We study the real-time evolution of the correlation functions in a globally quenched interacting one-dimensional lattice system by means of time-adaptive density matrix renormalization group. We find a clear light-cone behavior quenching the repulsive interaction from the gapped density wave regime. The spreading velocity increases with the final values of the interaction and then saturates at a certain finite value. In the case of a Luttinger liquid phase as the initial state, for strong repulsive interaction quenches, a more complex dynamics occurs as a result of bound state formations. From the other side in the attractive regime, depending on where connected correlation functions are measured, one can observe a delay in the starting time evolution and a coexistence of ballistic and localized signals.

Strain can be used as an effective tool to tune the crystal structure of materials and hence to modify their electronic structures, including topological properties. Here, taking ${\mathrm{Na}}_3\mathrm{Bi}$ as a paradigmatic example, we demonstrated with first-principles calculations and $\mathbf{k}·\mathbf{p}$ models that the topological phase transitions can be induced by various types of strains. For instance, the Dirac semimetal phase of ambient ${\mathrm{Na}}_3\mathrm{Bi}$ can be tuned into a topological insulator (TI) phase by uniaxial strain along the $\langle 100\rangle$ axis. Hydrostatic pressure can let the ambient structure transfer into a new thermodynamically stable phase with $Fm\overline{3}m$ symmetry, coming with a perfect parabolic semimetal having a single contact point between the conduction and valence bands, exactly at the $\mathrm{\Gamma }$ point on the Fermi surface, similar to $\alpha$-Sn. Furthermore, uniaxial strain in the $\langle 100\rangle$ direction can tune the new parabolic semimetal phase into a Dirac semimetal, while shear strains in both the $\langle 100\rangle$ and $\langle 111\rangle$ directions can take the new parabolic semimetal phase into a TI. $\mathbf{k}·\mathbf{p}$ models are constructed to gain more insights into these quantum topological phase transitions. Last, we calculated surface states of $Fm\overline{3}m{\mathrm{Na}}_3\mathrm{Bi}$ without and with strains to verify these topological transitions.

The onset or demise of Kondo effect in a magnetic impurity on a metal surface can be triggered, as sometimes observed, by the simple mechanical nudging of a tip. Such a mechanically driven quantum phase transition must reflect in a corresponding mechanical dissipation peak; yet, this kind of signature has not been focused upon so far. Aiming at the simplest theoretical modeling, we treat the impurity as an Anderson impurity model, the tip action as a hybridization switching, and solve the problem by numerical renormalization group. Studying this model as function of temperature and magnetic field we are able to isolate the Kondo contribution to dissipation. While that is, reasonably, of the order of the Kondo energy, its temperature evolution shows a surprisingly large tail even above the Kondo temperature. The detectability of Kondo mechanical dissipation in atomic force microscopy is also discussed.