Analytic and numerical studies on curved magnetic nano-objects predict numerous exciting effects that can be referred to as magneto-chiral effects, which do not originate from intrinsic Dzyaloshinskii-Moriya interaction or interface-induced anisotropies. In contrast, these chiral effects stem from isotropic exchange or dipole-dipole interaction, present in all magnetic materials, which acquire asymmetric contributions in case of curved geometry of the specimen. As a result, for example, the spin-wave dispersion in round magnetic nanotubes becomes asymmetric; namely, spin waves of the same frequency propagating in opposite directions along the nanotube exhibit different wavelenghts. Here, using time-resolved scanning transmission x-ray microscopy experiments, standard micromagnetic simulations, and a dynamic-matrix approach, we show that the spin-wave spectrum undergoes additional drastic changes when transitioning from a continuous to a discrete rotational symmetry, i.e., from round to hexagonal nanotubes, which are much easier to fabricate. The polygonal shape introduces localization of the modes to both the sharp, highly curved corners and flat edges. Moreover, due to the discrete rotational symmetry, the degenerate nature of the modes with azimuthal wave vectors known from round tubes is partly lifted, resulting in singlet and duplet modes. For comparison with our experiments, we calculate the microwave absorption from the numerically obtained mode profiles, which shows that a dedicated antenna design is paramount for magnonic applications in 3D nanostructures. To our knowledge these are the first experiments directly showing real space spin-wave propagation in 3D nano-objects.

Mn-doped chalcopyrites exhibit rich and unique physical properties due to the presence of magnetic clusters, both at ambient and under high external pressures. Despite recent efforts and rich literature on this topic, it is still necessary to explain some of the phenomena occurring in these systems, such as the origin of magnetic interactions at ambient pressure. In this context, we report the investigations of physical mechanisms responsible for the magnetic order of a ${\mathrm{Cd}}_{1–x}{\mathrm{Mn}}_x\mathrm{Ge}{\mathrm{P}}_2+\mathrm{MnP}$ composite system with changeable Mn contents $x$, ranging from 0.008 to 0.037. The presence of MnP clusters, confirmed by energy dispersive x-ray spectrometry and scanning electron microscopy methods, is responsible for the ferromagnetic phase transition with the Curie temperature $T_{\mathrm{C}}$ around 295 K. Moreover, the transition from ferromagnetic to helical order is observed, with the critical temperature $T_{\mathrm{S}}$ changing from 78 K for the sample with $x=0.008$ down to 54 K for $x=0.037$. Besides the common explanation based on the structural disorder at the cluster surface and finite-size effects, we associate the observed $T_{\mathrm{S}}\left(x\right)$ dependence with the local pressure effect due to Mn doping, where the influence of the external pressure $P\sim 0.25\mathrm{GPa}$ corresponds to the influence of the Mn content $x=0.035$.

We theoretically study a ferrimagnetic domain-wall motion driven by a rotating magnetic field. We find that, depending on the magnitude and the frequency of the rotating field, the dynamics of a ferrimagnetic domain wall can be classified into two regimes. First, when the frequency is lower than a certain critical frequency set by the field magnitude, there is a stationary solution for the domain-wall dynamics, where a domain-wall in-plane magnetization rotates in-phase with the external field. The field-induced precession of the domain wall gives rise to the translational motion of the domain wall via the gyrotropic coupling between the domain-wall angle and position. In this so-called phase-locking regime, a domain-wall velocity increases as the frequency increases. Second, when the frequency exceeds the critical frequency, a domain-wall angle precession is not synchronous with the applied field. In this phase-unlocking regime, a domain-wall velocity decreases as the frequency increases. Moreover, the direction of the domain-wall motion is found to be reversed across the angular compensation point where the net spin density of the ferrimagnet changes its sign. Our work suggests that the dynamics of magnetic solitons under time-varying biases may serve as platform to study critical phenomena.

We study the RKKY Heisenberg model on a three-dimensional stacked-triangular lattice under magnetic fields by extensive Monte Carlo simulations to get insight into the chiral-degenerate symmetric skyrmion crystal (SkX) in centrosymmetric metallic magnets. The triple-$q$ SkX and the double-$q$ states are realized, together with the single-$q$ state. We find an unexpected phenomenon of the replica-symmetry breaking (RSB) well known in glassy systems, although the Hamiltonian and the ordered state are entirely regular. In the RSB SkX phase, the triple-$q$ SkX state macroscopically coexists with the single-$q$ state, even though these ordered states cannot be transformed via any Hamiltonian-symmetry operation. In the thermodynamic limit, the free energies of these states are degenerate whereas the free-energy barrier between the states diverges, breaking the ergodicity. A similar RSB is also observed in the RSB double-$q$ phase where the double-$q$ state macroscopically coexists with the single-$q$ state. Experimental implications are discussed.

A series of Ruddlesden-Popper nickelates ${\mathrm{Nd}}_{n+1}{\mathrm{Ni}}_n{\mathrm{O}}_{3n+1}$ ($n=1–5$) have been stabilized in thin film form using reactive molecular-beam epitaxy. X-ray diffraction and scanning transmission electron microscopy measurements suggest high crystalline quality of these films. The average Ni valence states in these compounds are in accordance with the nominal values, as verified by x-ray photoelectron spectroscopy. The metal-insulator transition temperature ($T_{\mathrm{MI}}$) shows a clear $n$ dependence for $n=3–5$ members. At low temperature, the resistivity for $n=3–5$ members exhibits a log $T$ dependence, which is like that reported in parent compounds of superconducting infinite-layer nickelates.

A strong terahertz pulse is effective for controlling the macroscopic polarization in ferroelectrics. In the present study, we investigated the response of an organic molecular compound, namely ${\left(\mathrm{TMTTF}\right)}_2\mathrm{P}{\mathrm{F}}_6$ (TMTTF: tetramethyltetrathiafulvalene), to a strong electric field using terahertz pulse-pump optical-reflectivity probe spectroscopy. This compound undergoes a transition from Mott insulator to charge-order insulator with lowering temperature, and exhibits electronic ferroelectricity in the charge-order phase. When the terahertz pulse is applied in the Mott-insulator phase, an ultrafast reflectivity change proportional to the square of the electric field waveform of the terahertz pulse emerges, which is attributed to the generation of charge disproportionation in each dimer and the resultant creation of macroscopic polarization. When the terahertz pulse is applied in the charge-order phase, an ultrafast reflectivity change proportional to the electric field waveform of the terahertz pulse is observed, which originates from the modulation of the original charge disproportionation and polarization. These ultrafast reflectivity changes can be ascribed to purely electronic responses. In the midinfrared region, where totally symmetric $\left(a_{\mathrm{g}}\right)$ modes of intramolecular vibrations coupled with intermolecular charge transfers exist, a large reflectivity change is commonly observed in the Mott-insulator and charge-order phases. To interpret this feature, we constructed a model that incorporates a charge-transfer transition and $a_g$-mode intramolecular vibrations. The analyses of the results with this model revealed that the change in the reflectivity spectrum by the terahertz electric field can be explained by the energy shift of the charge-transfer transition caused by the electric field-induced change of charge disproportionation in each dimer, and the transfer of the spectral weight from the intradimer charge-transfer (CT) transition to the interdimer CT transition resulting from the weakening of the dimerization. Our model can be used to analyze the optical responses to electric fields in various organic molecular compounds with electron-intramolecular vibration couplings.

A recent experiment has reported oscillations of the thermal conductivity of $\alpha \text{−}{\mathrm{RuCl}}_3$ driven by an in-plane magnetic field that are reminiscent of the quantum oscillations in metals. At first glance, these observations are consistent with the presence of the long-sought-after spinon Fermi surface state. Strikingly, however, the experiment also reported vanishing thermal Hall conductivity coexisting with the oscillations of the longitudinal one. Such absence of the thermal Hall effect must originate from crystalline symmetries of $\alpha \text{−}{\mathrm{RuCl}}_3$. But if the system was a traditional spinon Fermi surface state, these symmetries would also necessarily prohibit the emergence of a magnetic field acting on the spinons, in stark contradiction with the presence of quantum oscillations in experiments. To reconcile these observations, we introduce a new class of symmetry enriched “pseudoscalar” U(1) spin liquids in which certain crystalline symmetries act as a particle-hole conjugation on the spinons. The associated pseudoscalar spinon Fermi surface states allow for the coexistence of an emergent Landau quantizing magnetic field while having an exactly zero thermal Hall conductivity. We develop a general theory of these states by constructing Gutzwiller-projected wave functions and describing how they naturally appear as U(1) spin liquids with a distinctive projective symmetry group implementation of crystalline symmetries in the fermionic parton representation of spins. We propose that the field induced quantum disordered state in $\alpha \text{−}{\mathrm{RuCl}}_3$ descends from a pseudoscalar spinon Fermi surface state that features compensated spinon-particle and spinon-hole pockets possibly located around the $M$ points of its honeycomb Brillouin zone. These points are connected via a wave vector associated with the emergence of the competing zig-zag antiferromagnetic state.

Exceptional points, which are topological non-Hermitian degeneracies, show up in the collective mode spectrum of Fermi liquids with high angular momentum interactions. In this paper, we look for signatures of these nontrivial singularities by computing the AC conductivity of Fermi liquids with dipolar and quadrupolar interactions in a narrow slab. We show that the finite size of the slab imprints clear signatures of the structures of the collective mode spectrum in the conductivity as well as in the dephasing between the electric field and the current density in a wide range of coupling constants. In particular, we show the fingerprints of exceptional points, observed in the weak attractive dipolar and/or quadrupolar regime. The main result does not depend on the specific model of quasiparticle interactions. We also discuss some actual compounds where these phenomena could be experimentally observed.

Charge ordering is prone to occur in crystalline materials with mixed-valence ions. It is presumably accompanied by a structural phase transition, with possible exceptions in compounds that already present more than one inequivalent site for the mixed-valence ions in the charge-disordered phase. In this work, we investigate the representative case of the homometallic Co ludwigite ${\mathrm{Co}}_2^{2+}{\mathrm{Co}}^{3+}{\mathrm{O}}_2{\mathrm{BO}}_3$ ($Pbam$ space group) with four distinct Co crystallographic sites [$M1$–$M4$] surrounded by oxygen octahedra. The mixed-valent character of the Co ions up to at least $T=873$ K is verified through x-ray absorption near-edge structure (XANES) experiments. Single crystal x-ray diffraction (XRD) and neutron powder diffraction (NPD) confirm that the Co ions at the $M4$ site are much smaller than the others at low temperatures, consistent with a ${\mathrm{Co}}^{3+}$ oxidation state at $M4$ and ${\mathrm{Co}}^{2+}$ at the remaining sites. The size difference between the Co ions in the $M4$ and $M2$ sites is continuously reduced upon warming above $\approx 370$ K, indicating a gradual charge redistribution within the $M4-M2-M4$ (424) ladder in the average structure. Minor structural anomalies with no space group modification are observed near 475 and 495 K, where sharp phase transitions were previously revealed by calorimetry and electrical resistivity data. An increasing structural disorder, beyond a conventional thermal effect, is noted above $\approx 370$ K, manifested by an anomalous increment of XRD Debye-Waller factors and broadened vibrational modes observed by Raman scattering. The local Co-O distance distribution, revealed by Co $K$-edge extended x-ray absorption fine structure (EXAFS) data and analyzed with an evolutionary algorithm method, is similar to that inferred from the XRD crystal structure below $\approx 370$ K. At higher temperatures, the local Co-O distance distribution remains similar to that found at low temperatures, at variance with the average crystal structure obtained with XRD. We conclude that the oxidation states ${\mathrm{Co}}^{2+}$ and ${\mathrm{Co}}^{3+}$ are instantaneously well defined in a local atomic level at all temperatures, however the thermal energy promotes local defects in the charge-ordered configuration of the 424 ladders upon warming. These defects coalesce into a phase-segregated state within a narrow temperature interval ($475<T<495$ K). Finally, a transition at $\approx 500$ K revealed by differential scanning calorimetry (DSC) in the iron ludwigite ${\mathrm{Fe}}_3{\mathrm{O}}_2{\mathrm{BO}}_3$ is discussed.

We present a method of extracting information about topological order from the ground state of a strongly correlated two-dimensional system represented by an infinite projected entangled pair state (iPEPS). As in previous works [A. Francuz et al., Phys. Rev. B 101, 041108(R) (2020) and A. Francuz and J. Dziarmaga, Phys. Rev. B 102, 235112 (2020)] we begin by determining symmetries of the iPEPS represented by infinite matrix product operators (iMPO) that map between the different iPEPS transfer matrix fixed points, to which we apply the fundamental theorem of matrix product states to find zipper tensors between products of iMPO's that encode fusion properties of the anyons. The zippers can be combined to extract topological $F$ symbols of the underlying fusion category, which unequivocally identifies the topological order of the ground state. We bring the $F$ symbols to the canonical gauge, and also compute the Drinfeld center of this unitary fusion category to extract the topological $S$ and $T$ matrices encoding mutual statistics and self-statistics of the emergent anyons. The algorithm is applied to Abelian toric code, Kitaev model, double semion, and twisted quantum double of $Z_3$, as well as to non-Abelian double Fibonacci, double Ising, and quantum double of $S_3$ and $\mathrm{Rep}\left(S_3\right)$ string-net models.

The electronic structures of ${\mathrm{ErTe}}_3$, having double charge density wave (CDW) transitions, have been investigated by performing polarization- and temperature ($T$)-dependent angle-resolved photoemission spectroscopy (ARPES) and Er $4d\to 4f$ resonant PES measurements for high-quality single crystals. It is found that Er $4f$ electrons do not contribute directly to CDW formation. The shadow Fermi surfaces (FSs) are clearly observed, arising from band folding due to the interlayer interaction. Twofold symmetric FS, but with weak FS reconstruction, is observed in the CDW phase of ${\mathrm{ErTe}}_3$, which is similar to that of ${\mathrm{PrTe}}_3$ having the CDW modulation vector $Q\approx 5/7c^{*}\approx 5/7a^{*}$. Very good agreement is found between the constant energy maps and ARPES measured in the CDW phase and the density functional theory band calculation for the non-CDW phase, implying very weak and only partial CDW-induced FS reconstruction in ${\mathrm{ErTe}}_3$. Very weak and similar circular dichroism is observed along different symmetry lines in both FS and ARPES, suggesting weak chiral orbital and spin polarizations in the Te $5p$ orbitals. $T$-dependent FS and ARPES measurements reveal closing of the CDW gap on heating but no clear changes across $T_{\mathrm{CDW}2}$ and $T_{\mathrm{CDW}1}$, supporting the very weak CDW distortion in ${\mathrm{ErTe}}_3$. The origin of the very weak CDW reconstruction in ${\mathrm{ErTe}}_3$ is likely to be the small ionic sizes of Er ions and the concomitant small volume that give rise to the imperfect FS nesting and the reduced density of states at the Fermi level. The very weak FS reconstruction at the first CDW transition along $c^{*}$ is considered to be a reason why there exists a second CDW transition further along $a^{*}$, which elucidates the observed double CDW transitions in ${\mathrm{ErTe}}_3$.

The inclusion of sophisticated density-dependent electronic stopping and electron-phonon coupling calculated with first-principles methods into molecular dynamics simulations of collision cascades has recently become possible thanks to the development of the so-called electron-phonon (EPH) model. This paper aims at employing the EPH model in molecular dynamics simulations of collision cascades in Si. In this context, the electronic stopping power is investigated in Si at low energies with Ehrenfest dynamics calculations. Also, the parametrization of the EPH model for Si, from first-principles Ehrenfest dynamics simulations to actual molecular dynamics simulations of collision cascades is performed and detailed. We demonstrate that the EPH model is able to reproduce very closely the density-dependent features of the energy lost to electrons obtained with ab initio calculations. Molecular dynamics collision cascade simulations results obtained in Si using the EPH model and the simpler but widely employed two-temperature model are compared, showing important discrepancies in the collision cascades results obtained depending on the model employed.

Exploiting methods of quantum field theory we compute the bulk polarization tensor and bulk dielectric functions for Dirac materials in the presence of a mass gap, chemical potential, and finite temperature. Using these results (and neglecting eventual boundary effects), we study the Casimir interaction of Dirac materials. We describe in detail the characteristic features of the dielectric functions and their influence on the Casimir pressure.

We investigated metal-insulator transitions for double-layer two-dimensional electron-hole systems in transition metal dichalcogenides stacked on opposite sides of thin layers of boron nitride. The interparticle interaction is calculated by including the screening due to the polarization charges at different interfaces, including that at the encapsulation and at the substrate of experimental structures. We compute and compare the energies of the metallic electron-hole plasma and the proposed insulating exciton solid with fixed-node diffusion Monte Carlo simulation including the high valley degeneracy of the electron bands. We found that for some examples of current experimental structures, the transition electron/hole density is in an experimentally accessible range between $4.1\times {10}^{12}{\mathrm{cm}}^{-2}$ and $14.5\times {10}^{12}{\mathrm{cm}}^{-2}$ for spacer thicknesses between 2.5 and 7.5 nm. Our result raises the possibility of exploiting this effect for logic device applications.

We investigate the impact of electrostatics on the proximity effect between a magnetic insulator and a semiconductor wire in semiconductor–superconductor–magnetic-insulator hybrid structures. By performing self-consistent Schrödinger-Poisson calculations using an effective model of the hybrid system, we find that large effective Zeeman fields consistent with the emergence of topological superconductivity emerge within a large parameter window in wires with overlapping layers of magnetic insulator and superconductor, but not in nonoverlapping structures. We show that this behavior is essentially the result of electrostatic effects controlling the amplitude of the low-energy wave functions near the semiconductor–magnetic-insulator interface.

We argue and numerically substantiate that the real-space entanglement spectrum (RSES) of chiral Abelian quantum Hall states is given by the spectrum of a local boundary perturbation of a $(1+1)$-dimensional conformal field theory, which describes an effective edge dynamics along the real-space cut. The cut-and-glue approach suggests that the low-lying RSES is equivalent to the low-lying modes of some effective edge action. The general structure of this action is deduced by mapping to a boundary critical problem, generalizing the work of Dubail, Read, and Rezayi [Phys. Rev. B 85, 115321 (2012)]. Using trial wave functions, we numerically test our model of the RSES for the $\nu =2/3$ bosonic composite fermion state.

Topological stability is an important property for topological materials. However, the non-Hermitian effects may change this situation. Here, we investigate the robustness of edge states in the non-Hermitian Kitaev chain with an imbalanced tunneling term and a superconducting pairing term. By defining the similarity of Majorana zero modes (MZMs) and magnetic factor, the coalescing phase diagram of the MZMs and corresponding spin polarization phase diagram are provided. Because of the non-Hermitian coalescence effect and non-Hermitian suppression effect induced by the breakdown of sublattice symmetry and particle-hole symmetry, the system emerges with very interesting phenomena, such as defective MZMs, number-anomalous bulk-boundary correspondence, coalescence of many-body ground states, and the magnetic phase crossover without gap closing. Those non-Hermitian effects offer fresh insights into MZMs and topological physics.

Cerium-based intermetallic ${\mathrm{CeNiGe}}_3$ has been generally believed to be a heavy-fermion material with Kondo behavior at low temperatures. Using femtosecond-resolved coherent phonon spectroscopy, we present a temperature-dependent dynamic investigation of the bosonic quasiparticles in ${\mathrm{CeNiGe}}_3$. Our data do not agree with the heavy-fermion expectation and instead show that the phonon stiffening from room temperature down to 5 K can be well explained by the anharmonic effect in the absence of a Kondo mechanism. Furthermore, the coherent lattice vibration located at ∼7.9 THz exhibits a mode splitting at $T^{+}\approx 105\mathrm{K}$, which is close to the energy scale of the first excited crystal field splitting of the Ce $4f$ level. We argue that an orbital crossover may account for this intriguing observation.

Cerium diantimonide ($\mathrm{Ce}{\mathrm{Sb}}_2$) is one of a family of rare-earth-based magnetic materials that exhibit metamagnetism, enabling control of the magnetic ground state through an applied magnetic field. At low temperatures, $\mathrm{Ce}{\mathrm{Sb}}_2$ hosts a rich phase diagram with multiple magnetically ordered phases for many of which the order parameter is only poorly understood. In this paper, we report a study of its metamagnetic properties by scanning tunneling microscopy (STM) and magnetization measurements. We use STM measurements to characterize the sample magnetostriction with subpicometer resolution from magnetic field and temperature sweeps. This allows us to directly assess the bulk phase diagram as a function of field and temperature and relate spectroscopic features from tunneling spectroscopy to bulk phases. Our magnetostriction and magnetization measurements indicate that the low-temperature ground state at zero field is ferrimagnetic. Quasiparticle interference mapping shows evidence for a reconstruction of the electronic structure close to the Fermi energy upon entering the magnetically ordered phase.