We show that thermal stability of skyrmions due to entropic effects can be strongly affected by external control parameters such as magnetic field and interface composition. The lifetimes of isolated skyrmions in atomic Pd/Fe bilayers on Ir(111) and on Rh(111) are calculated in the framework of harmonic transition state theory based on an atomistic spin model parametrized from density functional theory. Depending on the system the attempt frequency for skyrmion collapse can change by up to nine orders of magnitude with the strength of the applied magnetic field. We demonstrate that this effect is due to a drastic change of entropy with skyrmion radius which opens a route toward stabilizing sub-10-nm skyrmions at room temperature.

The reversible metal-insulator transition in ${\mathrm{VO}}_2$ at $T_{\text{c}}\approx 340$ K has been closely scrutinized yet its thermodynamic origin remains ambiguous. We discuss the origin of the transition entropy by calculating the electron and phonon contributions at $T_{\text{c}}$ using density functional theory. The vibration frequencies are obtained from harmonic phonon calculations, with the soft modes that are imaginary at zero temperature renormalized to real values at $T_{\text{c}}$ using experimental information from diffuse x-ray scattering at high-symmetry wave vectors. Gaussian process regression is used to infer the transformed frequencies for wave vectors across the whole Brillouin zone, and in turn compute the finite temperature phonon partition function to predict transition thermodynamics. Using this method, we predict the phase transition in ${\mathrm{VO}}_2$ is driven 5 to 1 by phonon entropy over electronic entropy, and predict a total transition entropy that agrees (within $5\%$) with the calorimetric value.

We study the melting of skyrmions in a two-dimensional Heisenberg chiral magnet with biaxial Dzyaloshinskii-Moriya interactions. At zero temperature, these topological excitations may form a triangular crystal with long-range positional order. We show using large-scale Monte Carlo simulations that at small finite temperature, however, the skyrmions rather form a typical two-dimensional solid, in which the positional correlations decay with distance as power laws while the orientational correlations remain finite. At higher temperature, we observe a direct transition from this two-dimensional solid to a liquid with short-range correlations. This differs from generic two-dimensional homogeneous particle systems, where a hexatic phase is sandwiched between the solid and the liquid.

Two different superstructures form when atomic H is incorporated in the Fe monolayer on Ir(111). Depending on the amount of H provided, either a highly ordered $p(2\times 2)$ hexagonal superstructure or an irregular roughly square structure is created. We present here spin-polarized scanning tunneling microscopy measurements which reveal that in both cases the magnetic nanoskyrmion lattice state of the pristine Fe monolayer is modified. Our measurements of the magnetic states in these hydrogenated films are in agreement with superpositions of cycloidal spin spirals which follow the pattern and the symmetry dictated by the H superstructures. We thus demonstrate here the possibility to vary the symmetry of a noncollinear magnetic state in an ultrathin film without changing its substrate.

Many-body diffusion Monte Carlo is used to obtain the first-principles momentum distribution and Compton profile of vanadium dioxide. Our results for the Compton profile are in good agreement with the experimental values, and we show that good qualitative agreement in the scaled Compton profile difference across the monoclinic to rutile phase transition depends on an accurate description of electron correlation. The electron momentum distribution enables new insights into the metal-insulator phase transition. For example, the probability for electron scattering in the proximity of the Fermi surface (forward scattering) is suppressed in the vanadium chain direction (rutile $c$ axis) but enhanced in perpendicular directions. However, along the $c$ axis we observe an increase at $\sim 2k_{\mathrm{F}}$ in the momentum distribution, which is characteristic for Friedel oscillations (backscattering). Our analysis of the momentum distribution supports experimentally observed anisotropies and provides an explanation for the anomalously low electronic thermal conductivity observed recently in the metallic phase [S. Lee et al., Science 355, 371 (2017)]. Moreover, our results indicate non-Fermi liquid behavior as well as quasi-one-dimensional Friedel oscillations in the metallic rutile phase, which is reminiscent of a Tomanaga-Luttinger liquid with impurities.

The rare-earth chiral compound ${\mathrm{DyNi}}_3{\mathrm{Ga}}_9$ with trigonal structure shows successive phase transitions at 10, 9, 5.5, and 2.5 K. We previously reported that the transverse elastic modulus $C_{66}$ exhibits significant softening toward $T_{\mathrm{Q}}=10$ K and the phase transition at $T_{\mathrm{Q}}$ is ferroquadrupolar ordering due to the quasidegenerate quartet under a crystal electric field (CEF). To investigate the order parameter and magnetic field-temperature phase diagrams of the ferroquadrupolar ordering, we performed ultrasonic measurements under magnetic fields. $T_{\mathrm{Q}}$ decreases drastically at low fields in the magnetic field in the $c$ plane. In contrast, $T_{\mathrm{Q}}$ remains even at high fields in the field along the $c$ axis. We found anisotropic and complex phase diagrams, and this anisotropy of the diagrams originates from the field dependences of the CEF energies and a quadrupole interaction. From our calculated results, we propose that the order parameter of the ferroquadrupolar ordering will be the electric quadrupole $O_{xy}$. There are first-order successive phase transitions in the field sweep in the $c$ plane, and a magnetic-field-induced phase appears in the phase diagram for the $a$ axis.

We explore two approaches to characterize the quantum geometry of the ground state of correlated fermions in terms of the distance matrix in the spectral parameter space. (a) An intrinsic geometry approach, in which we study the intrinsic curvature defined in terms of the distance matrix, and (b) an extrinsic geometry approach, in which we investigate how the distance matrix can be approximately embedded in finite dimensional Euclidean spaces. We implement these approaches for the ground state of a system of one-dimensional fermions on a 18-site lattice with nearest neighbor repulsion. The intrinsic curvature sharply changes around the Fermi points in the metallic regime but is more or less uniform in the insulating regime. In the metallic regime, the embedded points clump into two well-separated sets, one corresponding to modes in the Fermi sea and the other to the modes outside it. In the insulating regime, the two sets tend to merge.

In this work, using the finite-U slave boson mean-field approximation to solve the single-impurity Anderson model, the authors apply two different strategies to study the Kondo cloud, by analyzing quantities that are dependent on the distance to the magnetic impurity and then finding a universal distance scale ${\xi }_K$ through the collapse of the results into an universal function. The first method is based on the analysis of the local density of states of the conduction electrons (denoted as ${\xi }_K^L)$, while the second relies on the analysis of spin correlations $\left({\xi }_K^{\mathrm{\Sigma }}\right)$. Our calculations show that there is exact quantitative agreement, in the way ${\xi }_K$ depends on $U/\mathrm{\Gamma }$, between ${\xi }_K^{\mathrm{\Sigma }}$ and the results obtained through the heuristic expression ${\xi }_K\propto v_F/T_K$, while there is very close quantitative agreement between ${\xi }_K^{\mathrm{\Sigma }}$ and ${\xi }_K^L$. The use of the slave boson technique to calculate the spin correlations, which eliminates finite size effects, allowed us to study very large Kondo clouds, something that is very difficult using other techniques, like the density matrix renormalization Group method, for example. In addition, the very smooth curves obtained for the spin correlations allowed us to qualitatively identify a region in the Kondo cloud, adjacent to the impurity, that had been connected to the free orbital fixed point in previous numerical renormalization group calculations [Phys. Rev. B 84, 115120 (2011)].

The effective mass approximation (EMA) models the response to an external perturbation of an electron in a periodic potential as the response of a free electron with a renormalized mass. For semiconductors used in photovoltaic devices, the EMA allows calculation of important material properties from first-principles calculations, including optical properties (e.g., exciton binding energies), defect properties (e.g., donor and acceptor levels), and transport properties (e.g., polaron radii and carrier mobilities). The conduction and valence bands of semiconductors are commonly approximated as parabolic around their extrema, which gives a simple theoretical description but ignores the complexity of real materials. In this work, we use density functional theory to assess the impact of band nonparabolicity on four common thin-film photovoltaic materials—GaAs, CdTe, ${\mathrm{Cu}}_2{\mathrm{ZnSnS}}_4$ and ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$—at temperatures and carrier densities relevant for real-world applications. First, we calculate the effective mass at the band edges. We compare finite-difference, unweighted least-squares and thermally weighted least-squares approaches. We find that the thermally weighted least-squares method reduces sensitivity to the choice of sampling density. Second, we employ a Kane quasilinear dispersion to quantify the extent of nonparabolicity and compare results from different electronic structure theories to consider the effect of spin-orbit coupling and electron exchange. Finally, we focus on the halide perovskite ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ as a model system to assess the impact of nonparabolicity on calculated electron transport and optical properties at high carrier concentrations. We find that at a concentration of ${10}^{20}{\mathrm{cm}}^{-3}$ the optical effective mass increases by a factor of two relative to the low carrier-concentration value, and the polaron mobility decreases by a factor of three. Our work suggests that similar adjustments should be made to the predicted optical and transport properties of other semiconductors with significant band nonparabolicity.

The spin Hall effect for polaritons (SHEP) in a transition metal dichalcogenides (TMDC) monolayer embedded in a microcavity is predicted. We demonstrate that two counterpropagating laser beams incident on a TMDC monolayer can deflect a superfluid polariton flow due to the generation the effective gauge vector and scalar potentials. The components of polariton conductivity tensor for both noninteracting polaritons without Bose-Einstein condensation (BEC) and for weakly interacting Bose gas of polaritons in the presence of BEC and superfluidity are obtained. It is shown that the polariton flows in the same valley are splitting: the superfluid components of the $A$ and $B$ polariton flows propagate in opposite directions along the counterpropagating beams, while the normal components of the flows slightly deflect in opposite directions and propagate almost perpendicularly to the beams. The possible experimental observation of SHEP in a microcavity is proposed.

We report the results of our detailed magnetotransport studies on single crystals of the antiferromagnetic heavy-fermion superconductor ${\mathrm{Ce}}_3{\mathrm{PdIn}}_{11}$. Electrical resistivity measurements, carried out in different magnetic field orientations with respect to the crystallographic axes, were mainly aimed to amass further insight about the magnetically ordered state. The results manifest a clear metamagnetic transitions in the ordered phase when the applied field is parallel to the tetragonal $c$ axis, while no similar features are seen for the transverse direction, i.e., with the field confined within the $ab$ plane. This finding elucidates the fact that the $c$ axis is the easy magnetic direction in this system. Based on the electrical transport and heat capacity data obtained for ${\mathrm{Ce}}_3{\mathrm{PdIn}}_{11}$, magnetic field-temperature phase diagrams were constructed, which elucidate fairly enigmatic behaviors in this material featuring the existence of both second- and first-order magnetic phase transitions.

The layered honeycomb iridate $\alpha \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$ displays an incommensurate magnetic structure with counter-rotating moments on nearest-neighbor sites, proposed to be stabilized by strongly frustrated anisotropic Kitaev interactions between spin-orbit entangled ${\mathrm{Ir}}^{4+}$ magnetic moments. Here we report powder inelastic neutron scattering measurements that observe sharply dispersive low-energy magnetic excitations centered at the magnetic ordering wave vector, attributed to Goldstone excitations of the incommensurate order, as well as an additional intense mode above a gap $\mathrm{\Delta }\simeq 2.3$ meV. Zero-field muon-spin relaxation measurements show clear oscillations in the muon polarization below the Néel temperature $T_{\mathrm{N}}\simeq 15$ K with a time-dependent profile consistent with bulk incommensurate long-range magnetism. Pulsed-field magnetization measurements observe that only about half the saturation magnetization value is reached at the maximum field of 64 T. A clear anomaly near 25 T indicates a transition to a phase with reduced susceptibility. The transition field has a Zeeman energy comparable to the zero-field gapped mode, suggesting gap suppression as a possible mechanism for the field-induced transition.

Small-angle neutron scattering is used to measure the spin-wave stiffness in the field-polarized state of the Dzyaloshinskii-Moriya helimagnet ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$, which is ordered below $T_C=58$ K. The Dzyaloshinskii-Moriya interaction reflects itself in the anisotropic form of the spin-wave dispersion, ${\epsilon }_{\mathbf{q}}=A{\left({\mathbf{q}-\mathbf{k}}_s\right)}^2+g{\mu }_B(H-H_{C2})$, which is seen as two round spots of the scattering intensity, though overlapping, on the left and right sides of the incident neutron beam. The spin-wave stiffness is equal to $76\pm 1$ meV ${\AA }^2$ at low temperatures and decreases slowly with temperature, showing softening up to $39\pm 3$ meV ${\AA }^2$ close to $T_C$. The temperature behavior of $A$ resembles that of both the archetypical helimagnet MnSi and the archetypical ferrimagnet ${\mathrm{Fe}}_3{\mathrm{O}}_4$, supporting the concept of strong spin interactions within ${\mathrm{Cu}}_4$ tetrahedra, representing essential magnetic building blocks of ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ and thus forming its magnetic energy landscape.

Polarization-dependent photon spectroscopy (dichroism) using signal-enhancing superchiral beams is shown to be sensitive to magnetic properties of the sample, whereas previous investigations explored chargelike electronic properties of chiral samples. In the process of unveiling the potential to observe magnetic circular dichroism (MCD), we underline an affinity between spectroscopies using the Borrmann effect, twisted beams, and superchiral beams. Use of an effective wave vector in a quantum mechanical theory unites the aforementioned spectroscopies and vastly improves our understanding of their advantages. Exploiting an effective wave vector for superchiral beams, natural circular dichroism (NCD) is derived from electric dipole–magnetic dipole (E1-M1) and electric dipole–electric quadrupole (E1-E2) absorption events, and MCD is derived from electric quadrupole–electric quadrupole (E2-E2) absorption. Signal enhancement by superchiral beams is a straightforward gain for the user because NCD and MCD are otherwise precisely the same as for a circularly polarized beam, according to our calculations. An analysis shows that enhancement of E2-E2 is superior to that available for parity-odd events under consideration. Electronic degrees of freedom in all dichroic signals are encapsulated in atomic multipoles that are frequently used in theoretical interpretations of several established experimental techniques.

The magnetic-field-tuned quantum superconductor-insulator transitions of disordered amorphous indium oxide films are a paradigm in the study of quantum phase transitions and exhibit power-law scaling behavior. For superconducting indium oxide films with low disorder, such as the ones reported on here, the high-field state appears to be a quantum-corrected metal. Resistance data across the superconductor-metal transition in these films are shown here to obey an activated scaling form appropriate to a quantum phase transition controlled by an infinite-randomness fixed point in the universality class of the random transverse-field Ising model. Collapse of the field-dependent resistance vs temperature data is obtained using an activated scaling form appropriate to this universality class, using values determined through a modified form of power-law scaling analysis. This exotic behavior of films exhibiting a superconductor-metal transition is caused by the dissipative dynamics of superconducting rare regions immersed in a metallic matrix, as predicted by a recent renormalization group theory. The smeared crossing points of isotherms observed are due to corrections to scaling which are expected near an infinite-randomness critical point, where the inverse disorder strength acts as an irrelevant scaling variable.

Temperature dependence of resistivity under high pressures with magnetic fields up to 16 T is measured in FeSe single crystals with field directions parallel and perpendicular to the FeSe planes. It is found that the tetragonal to orthorhombic structural transition (nematic) temperature is suppressed by pressure and ends at around $P=1.18\mathrm{GPa}$. Below around 0.85 GPa, the superconducting transition shows a narrow width with no indication of antiferromagnetic order. Above this pressure, the superconducting transition temperature drops slightly, forming a small dome of superconducting region with the maximum $T_c$ at around 0.825 GPa. Furthermore, just above this pressure, the superconducting transition exhibits an unusual large transition width which reaches about 6–8 K. This large transition width is an intrinsic feature and does not change with magnetic field. In the high-pressure region above 1.18 GPa, just accompanying the onset of superconducting transition, an upturn of resistivity immediately occurs, which is attributed to the formation of an antiferromagnetic order. This closely attached behavior of superconductivity and antiferromagnetic order indicates that these two orders have a synergy feature. The wide transition width in high-pressure region is interpreted as the concurrence but spatially phase-separated regions of the superconductivity and antiferromagnetic order. Near the critical pressure 0.825 GPa and below, our data illustrate that an antiferromagnetic order emerges when superconductivity is suppressed. From the weak influence of magnetic field to the antiferromagnetic order, we conclude that it may exist already below the superconducting transition temperature in the low-pressure region. This shows a competing feature between superconductivity and antiferromagnetic order. Our results show duality features, namely, synergy and competition between superconductivity and antiferromagnetic order under pressure in FeSe.

A magnetic bimeron is a pair of two merons and can be understood as the in-plane magnetized version of a skyrmion. Here we theoretically predict the existence of single magnetic bimerons as well as bimeron crystals, and compare the emergent electrodynamics of bimerons with their skyrmion analogues. We show that bimeron crystals can be stabilized in frustrated magnets and analyze what crystal structure can stabilize bimerons or bimeron crystals via the Dzyaloshinskii-Moriya interaction. We point out that bimeron crystals, in contrast to skyrmion crystals, allow for the detection of a pure topological Hall effect. By means of micromagnetic simulations, we show that bimerons can be used as bits of information in in-plane magnetized racetrack devices, where they allow for current-driven motion for torque orientations that leave skyrmions in out-of-plane magnets stationary.