Intrinsic magnetic topological insulators provide an ideal platform to achieve various exciting physical phenomena. However, this kind of materials and related research are still very rare. In this work, we reported the electronic and structural phase transitions in the intrinsic magnetic topological insulator $\mathrm{Mn}{\mathrm{Sb}}_2{\mathrm{Te}}_4$ driven by hydrostatic pressure. Electric transport results revealed that temperature dependent resistance showed a minimum value near short-range antiferromagnetic (AFM) ordering temperature ${T_N}^{\prime }$, which declines with pressure. The short-range AFM ordering was sensitive to pressure and fully suppressed above 11.5 GPa. The intensity of three Raman vibration modes in $\mathrm{Mn}{\mathrm{Sb}}_2{\mathrm{Te}}_4$ declined quickly starting from 7.5 GPa and these modes become undetectable above 9 GPa, suggesting possible insulator-metal transition, which is further confirmed by theoretical calculation. In situ x-ray diffraction demonstrated that an extra diffraction peak appears near 9.1 GPa and $\mathrm{Mn}{\mathrm{Sb}}_2{\mathrm{Te}}_4$ started to enter an amorphouslike state above 16.6 GPa, suggesting the structural origin of suppressed AFM ordering and metallization. This work has demonstrated the correlation among interlayer interaction, magnetic ordering, and electric behavior, which could be benefit the understanding of the fundamental properties of this kind of materials and devices.

Although we know much about Floquet dynamics of pseudospin-$1/2$ systems, namely graphene, we here address the stroboscopic properties of a periodically kicked three-band fermionic system such as the $\alpha \text{−}{\mathrm{T}}_3$ lattice. This particular model provides an interpolation between graphene and dice lattice via the continuous tuning of the parameter $\alpha$ from 0 to 1. In the case of dice lattice ($\alpha =1$), we reveal that one can, in principle, engineer various types of low-energy dispersions around some specific points in the Brillouin zone by tuning the kicking parameter in the Hamiltonian along a particular direction. Our analytical analysis shows that one can experience different quasienergy dispersions, for example, the Dirac type, semi-Dirac type, gapless line, and/or absolute flat quasienergy bands, depending on the specific values of the kicking parameter. Moreover, we numerically study the dynamics of a wave packet in dice lattice. The quasienergy dispersion allows us to understand the instantaneous structure of wave packets at stroboscopic times. We find a situation where absolute flat quasienergy bands lead to a complete dynamical localization of the wave packet. Additionally, we calculate the quasienergy spectrum numerically for the $\alpha \text{−}{\mathrm{T}}_3$ lattice. A periodic kick in a perpendicular (planar) direction breaks (preserves) the particle-hole symmetry for $0<\alpha <1$. Furthermore, it is also revealed that the dynamical localization of a wave packet does not occur at any intermediate $\alpha \ne 0,1$.

While obtaining theoretical predictions for dissipation during sliding motion is a difficult task, one regime that allows for analytical results is the so-called noncontact regime, where a probe is weakly interacting with the surface over which it moves. Studying this regime for a model crystal, we extend previously obtained analytical results and confirm them quantitatively via particle based computer simulations. Accessing the subtle regime of weak coupling in simulations is possible via use of Green-Kubo relations. The analysis allows us to extract and compare the two paradigmatic mechanisms that have been found to lead to dissipation: phonon radiation, prevailing even in a purely elastic solid, and phonon damping, e.g., caused by viscous motion of crystal atoms. While phonon radiation is dominant at large probe-surface distances, phonon damping dominates at small distances. Phonon radiation is furthermore a pairwise additive phenomenon so that the dissipation due to interaction with different parts (areas) of the surface adds up. This additive scaling results from a general one-to-one mapping between the mean probe-surface force and the friction due to phonon radiation, irrespective of the nature of the underlying pairwise interaction. In contrast, phonon damping is strongly nonadditive, and no such general relation exists. We show that for certain cases, the dissipation can even decrease with increasing surface area the probe interacts with. The above properties, which are rooted in the spatial correlations of surface fluctuations, are expected to have important consequences when interpreting experimental measurements, as well as scaling with system size.

We examine isotropic near-zero thermal expansion in the cubic materials ${\mathrm{Zn}}_4{\mathrm{B}}_6{\mathrm{O}}_{12}X(X=\mathrm{O},\mathrm{S},\mathrm{Se})$ using density functional theory calculations of the lattice dynamics within the Grüneisen approximation. The calculated thermal expansion is in excellent agreement with experiment. The near-zero thermal expansion is shown to arise from a balance between two specific groups of vibrations that favor positive and negative thermal expansion, respectively. This gives a clear general indicator of how crystal structures can be engineered to give near-zero thermal expansion.

We investigate the magnetic and crystal electric field (CEF) states of the Kondo lattice system ${\mathrm{CeCuGa}}_3$ by muon spin relaxation ($\mu \mathrm{SR}$), neutron diffraction, and inelastic neutron scattering (INS) measurements. A noncentrosymmetric ${\mathrm{BaNiSn}}_3$-type tetragonal crystal structure (space group $I4mm$) is inferred from x-ray as well as from neutron powder diffraction. The low-temperature magnetic susceptibility and heat capacity data show an anomaly near 2.3–2.5 K, associated with long-range magnetic ordering, which is further confirmed by $\mu \mathrm{SR}$ and neutron diffraction data. The neutron powder diffraction collected at 1.7 K shows the presence of magnetic Bragg peaks indexed by an incommensurate magnetic propagation vector k $=(0.148,0.148,0)$ and the magnetic structure is best described by a longitudinal spin density wave with ordered moments lying in the $ab$-plane. An analysis of the INS data based on a CEF model reveals the presence of two magnetic excitations near 4.5 meV and 6.9 meV. The magnetic heat capacity data suggest an overall CEF splitting of 20.7 meV, however, the excitation between 20 and 30 meV is very broad and weak in our INS data, but could provide an evidence of CEF level in this energy range in agreement with the magnetic entropy. Our analysis of INS data based on the CEF-phonon model indicates that the two excitations at 4.5 meV and 6.9 meV have their origin in CEF-phonon coupling (i.e., splitting of one CEF peak into two peaks, called vibron), with an overall splitting of 28.16 meV, similar to the case of ${\mathrm{CeCuAl}}_3$ and ${\mathrm{CeAuAl}}_3$.

Spin-1/2 maple-leaf lattice antiferromagnets are expected to show interesting phenomena originating from frustration effects and quantum fluctuations. We report the hydrothermal synthesis of a powder sample of bluebellite ${\mathrm{Cu}}_6\mathrm{I}{\mathrm{O}}_3{\left(\mathrm{OH}\right)}_{10}\mathrm{Cl}$ as a first potential candidate. Magnetization and heat capacity measurements reveal moderate frustration with a Curie-Weiss temperature of $–35\mathrm{K}$, and a magnetic transition at $T_N=17\mathrm{K}$. Surprisingly, the magnetic susceptibility and heat capacity above $T_N$ are well reproduced by the Bonner-Fisher model, which suggests that a one-dimensional spin correlation with a magnetic interaction of 25 K occurs in the apparently two-dimensional lattice. This emergent one-dimensionality cannot be explained by orbital ordering or dimensional reduction due to geometrical frustration. We believe that there is an unknown mechanism to cause one-dimensionality in the spin-1/2 maple-leaf lattice antiferromagnet.

The 1-kink skyrmion is a topological spin texture composed of a skyrmion having a chiral kink inside its Néel domain wall, this chiral kink being also called domain wall skyrmion. By micromagnetic simulations we have studied the stability of the 1-kink skyrmion as well as its spin-driven dynamics in Pt/Co/MgO. Such trilayer system allows simultaneously the stabilization of various spin textures including skyrmion (Sk, topological charge $Q=1$), 1-kink skyrmion (1kSk, $Q=2$), and domain wall skyrmion (DWSk, $Q=1$). The 1-kink skyrmion is found to be robust against both thermal fluctuations and normally distributed anisotropy for different grainlike region sizes. Such magnetization pattern has a certain resilience to the variation of the Dzyaloshinskii-Moriya interaction and applied magnetic field. The static analysis is complemented with the investigation of the dynamic regime of motion under electrical current. In the limit of moderate injected current the skyrmion and the 1-kink skyrmion behave similarly. However, depending on the geometry of the injected current, the 1-kink skyrmion can be transformed into a single skyrmion or splits in two skyrmions, opening the path to conceive high density logical devices, enabling novel functionalities for logic operations.

We present a comprehensive experimental and theoretical study of the structural, electronic, magnetic, and thermodynamic properties of a ${\mathrm{Pb}}_6{\mathrm{Co}}_9{\left({\mathrm{TeO}}_6\right)}_5$ single crystal. The ${\mathrm{Pb}}_6{\mathrm{Co}}_9{\left({\mathrm{TeO}}_6\right)}_5$ crystal has shown a unique type of magnetic spin-lattice coupling, in which the lattice structure consists of four different Co ions sites with distorted octahedral coordinations. The x-ray photoelectron spectroscopy (XPS) results confirmed the oxidation states of Pb, Co, Te, and O elements in the sample. Moreover, XPS spectra revealed the adsorbed oxygen in the defect/vacancy sites of the lattice structure. The dc magnetization measurements exhibited a complex magnetic behavior with ferrimagnetic (FIM) transition with Curie temperature $T_C$ at $\sim 21$ K. At lower magnetic fields $H$, the zero-field-cooled and field-cooled curves showed a broad hump at $\sim 10.8$ K and a shoulder peak at $\sim 6.2$ K, which are suppressed at higher magnetic fields. The ac susceptibility data indicated spin-glass-like features. The heat capacity $C_P$ measurements confirmed the FIM transition at $T_C$ at $\sim 21$ K, but without any trace of additional peaks at lower temperatures. The estimated Curie-Weiss constant ${\theta }_{\mathrm{CW}}$ showed a peculiar field-dependent behavior along the $H\parallel c$ direction of the single crystal, where ${\theta }_{\mathrm{CW}}$ is less field dependent for the $H\perp c$ direction. A large coercivity (13 kOe) is observed at 2 K for $H\parallel c$, whereas the magnetization curve of the single crystal is dominated by an antiferromagnetic feature for $H\perp c$. The behaviors indicate the anisotropy nature of the exchange interactions in the compound. The local spin density approximation $+ U$ total energy calculations were performed for various collinear spin configurations of a classical Heisenberg model in order to obtain the magnetic exchange interactions $J_i$ at different distances for different neighbors.

In recent years, multigap superconductors have attracted much attention since the discovery of novel two-gap superconductivity with transition temperature $T_c\sim 39\text{K}$ in bulk ${\mathrm{MgB}}_2$. Based on the first-principles calculation and anisotropic Migdal-Eliashberg theory, we conduct a study on a series of two-dimensional (2D) boron-based materials with doped metal atoms to search for multigap superconductors. We find that ${\mathrm{InB}}_2$ monolayer films and ${\mathrm{InB}}_4$ trilayer films are dynamically stable but not synthesized experimentally yet. An evident three-gap superconductor with high $T_c\sim$ 41.5 K is obtained in a ${\mathrm{InB}}_2$ monolayer film. Similarly, ${\mathrm{InB}}_4$ trilayer film is a novel superconductor with three distinct superconducting gaps and a high $T_c\sim$ 53 K. The superconductivity in both 2D films originates mainly from the covalent-state-driven metallization. In addition, the effect of biaxial strain on the superconducting behavior of $\mathrm{In}{\text{B}}_4$ trilayer films is also involved. The ${\mathrm{InB}}_4$ trilayer films stay dynamically stable under biaxial tensile strain of $-3\%–6\%$, and the highest $T_c$ boosts to 64 K under the biaxial tensile strain of $\sim 4\%$. Meanwhile, we also find that the ${\mathrm{GeB}}_4$ and ${\mathrm{ZnB}}_4$ trilayer films are two-gap superconductors with high $T_c\sim$ 48.5 and 32 K, respectively.

Hexagonal manganites, such as $h\text{−}{\mathrm{LuMnO}}_3$, are ferroelectric and have a narrow electronic band gap of $\approx 1.5\mathrm{eV}$. Here we report on the photoresponse of $h\text{−}{\mathrm{LuMnO}}_3$ single crystals. It is found that the short circuit photocurrent density ($J_{\mathrm{sc}}$) and the open circuit voltage ($V_{\mathrm{oc}}$) are dependent on the direction of the polarization plane of a linearly polarized impinging light. Its angular dependence indicates the contribution of bulk photovoltaic effect to the short circuit photocurrent. It is also observed that a switchable drift photocurrent, originating from the depoling field of the ferroelectric and thus tunable $\left(<10\%\right)$ by its polarization direction, also contributes to $J_{\mathrm{sc}}$. Although its presence precludes accurate determination of the bulk photovoltaic tensor elements and Glass coefficients, some bounds can be established. The Glass coefficients are found to be significantly larger than those obtained in ${\mathrm{BiFeO}}_3$. We argue that the smaller band gap of $h\text{−}{\mathrm{LuMnO}}_3$, its distinctive bipyramidal crystal field, and electronic configuration ($3d^4$ vs $3d^5$), account for the difference and suggest a path towards ferroelectrics of higher photoconversion efficiency.

To date, temperature and conductivity have many outstanding implications in extreme environments but are yet to be fully understood under high pressure and temperature dynamic conditions. Here, we introduce an approach to provide high quality electrical conductivity results under dynamic loading conditions. Emphasis is given to address the skin depth effect's influence in a dynamic loading experiment by using thin films. The thin film samples in this study were at least 100 times thinner than previous samples in dynamic electrical conductivity experiments, increasing the current density to its full potential across the sample's entire cross section. Consideration of the skin depth accounts for at minimum a $4\times$ scaling factor to the final electrical conductivity result that has been neglected in previous dynamic electrical conductivity studies. We also obtained improved signal-to-noise ratio with custom diagnostics optimized for better electrical impedance matching. These considerations were applied to Sn to assess electrical conductivity at elevated pressure and temperature. The high signal-to-noise ratio with reduced skin depth influence results in Sn allowing observation of the conductivity changes related to solid-to-solid and solid-to-liquid phase transitions. Additionally, we calculate the Sn thermal conductivity using the Wiedemann-Franz law for our experiments and compare against thermal transport dependent temperature measurements from previous work.

We investigate the spatial decay and temporal localization properties of quasimodes (i.e., scattering resonances) of two-dimensional Vogel spirals, composed of deterministic, aperiodic arrays of electric dipoles. By determining the structural entropy and localization maps of Vogel spirals using the Green's matrix method, we show that three distinctive decay types of quasimodes coexist in Vogel spirals: exponential, power law, and Gaussian. While the exponential and the power-law decays typically occur in disordered media and multifractal systems, respectively, the Gaussian decay is demonstrated to characterize, on average, the most localized quasimodes of Vogel spirals, both spatially (smallest participation ratios) and temporally (longest lifetimes). These decay forms are demonstrated by a no-fitting analysis of the localization maps, independently corroborated by calculating the electric field in real space, which also provides a direct evidence of the algebraic spatial decay of critical quasimodes. Altogether our findings unveil a rich spectrum of both long-lived and spatially localized quasimodes that coexist in Vogel spirals and can be of direct relevance to novel optical functionalities for applications to light sources and sensing devices.

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.