Motivated by the rapid experimental progress on rare-earth Shastry-Sutherland lattice magnets, we propose a generic effective spin model that describes interacting non-Kramers local moments on the Shastry-Sutherland lattice. We point out that the local moments consist of both magnetic dipole and quadrupole components and the effective model turns out to be an extended XYZ model with an intrinsic field that accounts for the crystal field splitting. We then study the ground-state phase diagram of the model and find that pure quadrupole orders, which are invisible to conventional experimental probes, can be stabilized over a broad regime. In particular, we show that a hidden 1/3 magnetization plateau with quadrupole orders generally exists and discuss its experimental signatures. Finally, we discuss the relevance of our results to the rare-earth Shastry-Sutherland lattice magnets ${\mathrm{Pr}}_2{\mathrm{Ga}}_2{\mathrm{BeO}}_7$ and ${\mathrm{Pr}}_2{\mathrm{Be}}_2{\mathrm{GeO}}_7$.

${\mathrm{NbGe}}_2$ is a superconductor with a chiral crystalline structure such that nontrivial topology may play an important role. Here, we carry out magneto-transport study on microstructured ${\mathrm{NbGe}}_2$. We develop a special recipe that substantially minimizes the influence of the microstructuring-induced surface layer. The fabricated ${\mathrm{NbGe}}_2$ samples show a negative magneto-resistance component at temperatures below 30 K when the magnetic field is parallel to the applied current, akin to the chiral anomaly observed in Weyl semimetals. Furthermore, we realize additional two-dimensional superconductivity by bombarding the ${\mathrm{NbGe}}_2$ surface with gallium ion beam. This surface layer exhibits the supercurrent diode effect under an in-plane magnetic field. Our results demonstrate that microstructuring may pave the way toward exploring exotic properties of such a chiral superconductor.

Thermal transport properties of materials at low temperatures have been a longstanding research topic. However, experimental thermal conductivity data in this regime are scarce and often limited to specific materials and geometries. The recently developed first-principles anharmonic lattice dynamics methods, which can accurately predict the modal phonon transport properties and thermal conductivity, suffer from significant computational challenges, particularly into the Kelvin temperature range. In this paper, we propose a parameter-free, accelerated first-principles approach for obtaining converged thermal conductivity at low temperatures by leveraging on the maximum likelihood estimation and optimizing phonon frequency cutoffs under relaxation time approximation. Specifically, converged thermal conductivities of Si with different sample sizes are obtained at temperatures as low as 5 K. Due to the dominance of boundary and isotope scattering, the difference in the converged simulation results of Si is within 10% with the experimental measurements at temperatures <20 K. The observed temperature dependence for samples with a 4 mm sample size is proportional to $T^{1.4}$ at low temperatures and changes to $T^2$ when accounting for the finite dimension in the direction of heat flux. When the sample sizes are reduced to the micrometer scale, the temperature dependence is restored to $T^3$, as predicted in the Casimir limit. Our analysis underscores the dominant influence of boundary and isotope scattering at low temperatures, while the size effect remains significant for samples with centimeter lengths. The proposed computational framework provides reliable tools to investigate thermal transport properties at low temperatures, and our comprehensive analysis yields insights into the temperature and size-dependent behavior of thermal conductivity under cryogenic conditions.

Magnetic skyrmions emerge as promising quasiparticles for encoding information in next-generation spintronic devices. Their innate flexibility in shape is essential for the applications although they were often ideally treated as rigid particles. In this work, we investigated the voltage-controlled uniform strain mediated dynamics of deformed skyrmions in heterostructures with a flower-shaped magnetic nanostructure, using micromagnetic simulations. The simulated results revealed the possible states of isolated skyrmion nucleated in the nanostructure, which can be mutually switched by applying suitable in-plane strain pulses. In addition, it was found that the skyrmion motions are driven by the emerging internal forces and strain force, which originate from the asymmetric deformation of skyrmion structures. Furthermore, an analytical model of deformed skyrmions was proposed to interpret the dependences of internal forces and strain force on the asymmetric deformation of skyrmion, with some formulas derived for these forces in a semianalytical approach. Further calculations based on these formulas verified the forces appearing in the skyrmion motion, with the resulting forces showing consistence with the simulated data. This suggested that our semianalytical model successfully captures the main physics responsible for the motion of deformed skyrmion in the nanostructure. Our work extends the understanding of the mechanics emerging in deformed skyrmion and provides an effective approach for deterministic manipulation of deformed skyrmion motion via strain forces and internal forces, which may be instructive to the design of skyrmion-based spintronic devices.

Nonlinear photomagnetization is a process by which an oscillating electric field induces a static magnetization. We show that all 32 crystallographic point groups admit such spin polarization using circularly polarized electric fields to second order (as in the usual spin orientation or inverse Faraday effect) but only 29 points groups admit spin polarization using linearly polarized electric fields to second order. The excluded point groups are the highly symmetric $m\overline{3}m, \overline{4}3m$, and 432. Using density functional theory we compute the spectrum of the second-order electric spin susceptibility of prototypical semiconductors Te, Se, ${\mathrm{SnS}}_2$, GaAs, InSb, and Si which corresponds to nonmagnetic materials with and without inversion symmetry. We show that such nonlinear photomagnetization can be comparable to those of naturally occurring ferromagnets.

The origin of superconductivity in the FeSe monolayer on ${\mathrm{SrTiO}}_3$ remains one of the unresolved mysteries in condensed-matter physics. Here, by investigation of the temperature evolution of the dynamic charge response of $\mathrm{FeSe}/{\mathrm{SrTiO}}_3$, we infer that the response of the monolayer itself is nearly temperature independent. This indicates a constant Fermi surface over a wide range of temperature, in stark contrast to that of the bulk FeSe and other Fe-based superconductors. Our results, which manifest the peculiarity of the electronic structure of the FeSe monolayer, may help for a microscopic understanding of the superconductivity in Fe-chalcogenide monolayers on oxide surfaces in general.

Defying the traditional classification into metals and insulators, several materials simultaneously display metallic thermal properties and insulating electric behavior, as if they hosted quasiparticles carrying entropy but not charge. Among them, some materials also possess quantum oscillations in magnetic fields as if they had well-defined Fermi surfaces despite the insulating gap. This remarkable dichotomy has been observed in the topological Kondo insulators ${\mathrm{SmB}}_6$ and ${\mathrm{YbB}}_{12}$. Prompted by the peculiar mixed-valence nature of these compounds, involving $f$ and $d$ electrons of the lanthanide, we propose an explanation of their intriguing properties drawing inspiration from the physics of the pseudogap phase in underdoped cuprates. We argue that the $f$ and $d$ subsystems, when considered separately, act, respectively, as electron- and hole-doped Mott insulators, featuring Fermi pockets coexisting with Luttinger surfaces responsible for the pseudogap. When the two are coupled to each other a hybridization gap opens up, and the whole turns into a topological insulator endowed with genuine chiral edge states. However, the Luttinger surfaces persist and support neutral quasiparticles. This scenario, supported by numerical simulations within the dynamical cluster approximation, effectively resolves the paradoxical phenomenology of ${\mathrm{SmB}}_6$ and ${\mathrm{YbB}}_{12}$.

The quantum criticality of an XXZ-type spin chain antiferromagnet ${\mathrm{Ca}}_2{\mathrm{CoH}}_2{\left({\mathrm{SeO}}_3\right)}_4$, subjected to an external magnetic field, was investigated through ultralow-temperature specific heat, magnetocaloric effect, and magnetic susceptibility measurements. In the absence of an external magnetic field, the system undergoes an antiferromagnetic phase transition at $T_N=1.42\mathrm{K}$. The application of an external field gradually suppresses the antiferromagnetic order. Furthermore, an additional phase emerges in the intermediate field range when the zero-field antiferromagnetic order is suppressed, resulting in a quantum critical point at $B_s\approx 2.55\mathrm{T}$. Near this field-induced quantum critical point, universal scaling behaviors are observed, characterized by the universality class parameters $d=1, \nu =1/2$, and $z=2$.

In the past decade, there has been a systematic investigation of symmetry-protected topological (SPT) phases in interacting fermion systems. Specifically, by utilizing the concept of equivalence classes of finite-depth fermionic symmetric local unitary (FSLU) transformations and the (fluctuating) decorated symmetry domain wall picture, a large class of fixed-point wave functions have been constructed for fermionic SPT (FSPT) phases. Remarkably, this construction coincides with the Atiyah-Hirzebruch spectral sequence, enabling a complete classification of FSPT phases. However, unlike bosonic SPT phases, the stacking group structure in fermion systems proves to be much more intricate. The construction of fixed-point wave functions does not explicitly provide this information. In this paper, we employ FSLU transformations to investigate the stacking group structure of FSPT phases. Specifically, we demonstrate how to compute stacking FSPT data from the input FSPT data in each layer, considering both unitary and antiunitary symmetry, up to 2+1 dimensions. As concrete examples, we explicitly compute the stacking group structure for crystalline FSPT phases in all 17 wallpaper groups and the mixture of wallpaper groups with onsite time-reversal symmetry using the fermionic crystalline equivalence principle. We offer an open-source package sptset for the automatic calculation of classification and stacking group under any discrete symmetries (Y. Qi, sptset package, https://github.com/yangqi137/SptSet). Importantly, our approach can be readily extended to higher dimensions, offering a versatile method for exploring the stacking group structure of FSPT phases.

Photoemission from a solid surface provides a wealth of information about its electronic structure and dynamic evolution. Ultrafast pump-probe experiments offer real-time access to photon-surface interactions and the resulting electron dynamics. Here, we present a femtosecond time-resolved photoelectron spectroscopy study of laser-assisted photoemission (LAPE) from two different metal surfaces, tungsten and platinum. Utilizing synchronized IR laser and x-ray pulses, photoelectron sideband generation up to the sixth order is observed. A significant material-dependent variation of the LAPE response has not been predicted by previous theoretical models, to the best of our knowledge. The observed phenomena are semiquantitatively reproduced by considering distinct dynamic dielectric responses of the two materials. These findings provide a deeper understanding of the LAPE process and insights into the dynamic interplay between optical laser fields and metal surfaces.

In the field of fractional Chern insulators, a great deal of effort has been devoted to characterizi ng Chern bands that exhibit properties similar to the Landau levels. Among them, the concept of the vortex function, which generalizes the complex coordinate used for the symmetric-gauge Landau-level basis, allows for a concise description. In this paper, we develop a theory of constructing the vortex function and basis states of Chern insulators in the tight-binding formalism. In the first half, we consider the optimization process of the vortex function, which minimizes an indicator that measures the difference from the ideal Chern insulators. In particular, we focus on the sublattice position dependence of the vortex function or the quantum geometric tensor. This degree of freedom serves as a discrete analog of the nonuniformity in the spatial metric and magnetic field in a continuous model. In the second half, we construct two types of basis sets for a given vortex function: radially localized basis set and coherent-like basis set. The former basis set is defined as the eigenstates of an analogy of the angular momentum operator. Remarkably, one can always find exact zero mode(s) for this operator, which is explained by the celebrated Atiyah-Singer index theorem. As a byproduct, we propose an inequality rooted in the band topology. We also discuss the subtle differences between our formalism and the previous works about the momentum-space Landau level. The latter basis set generalizes the concept of coherent states on von Neumann lattice. While this basis set is not orthogonal, it is useful to compare the LLL and the given Chern insulator directly in the Brillouin zone. These basis sets are expected to be useful for many-body calculations of fractional Chern insulators.

We investigate the nature of the linear-response tensors in planar Hall and planar thermal Hall setups, where we subject a Rarita-Schwinger-Weyl (RSW) semimetal to the combined influence of an electric field $\mathbf{E}$ (and/or temperature gradient ${\nabla }_{\mathbf{r}}T$) and a weak (i.e., nonquantizing) magnetic field $\mathbf{B}$. For computing the in-plane transport components, we have added an elastic deformation which gives rise to a chirality-dependent effective magnetic field ${\mathbf{B}}^{\text{tot}}=\mathbf{B}+\chi {\mathbf{B}}_5$, where $\chi$ is the chirality of an RSW node. We have included the effects of orbital magnetic moment (OMM), in conjunction with the Berry curvature (BC), both of which appear as a consequence of nontrivial topology of the band structure. Due to the presence of four bands, RSW semimetals provide a richer structure for obtaining the linear-response coefficients, compared to the Weyl semimetals. In particular, we have found that the OMM-contributed terms may oppose or add up to the BC-only parts, depending on which band we are considering. Last, but not the least, we have determined the out-of-plane response comprising the intrinsic anomalous-Hall and the Lorentz-force-contributed currents, whose nature corroborates the findings of some recent experimental results.

We investigate the topological phases of the Haldane model on the Sierpiński gasket. As a consequence of the fractal geometry, multiple fractal gaps arise. Additionally, a flat band appears, and due to a complex next-nearest-neighbor hopping, this band splits and multiple topological flux-induced gaps emerge. Owing to the fractal nature of the model, conventional momentum-space topological invariants cannot be used. Therefore, we characterize the system's topology in terms of a real-space Chern number. In addition, we verify the robustness of the topological states to disorder. Finally, we present phase diagrams for both a fractal gap and a flux-induced gap. Previous work on a similar system claims that fractality squeezes the well-known Haldane phase diagram. However, this result arises because a doubled system was considered with two Sierpiński gaskets glued together. We consider only a single copy of the Sierpiński gasket, keeping global self-similarity. In contrast with these previous results, we find intricate and complex patterns in the phase diagram of this single fractal. Our work shows that the fractality of the model greatly influences the phase space of these structures, and can drive topological phases in the multitude of fractal and flux-induced gaps, providing a richer platform than a conventional integer dimensional geometry.

It has been known that the semiclassical and the response theories can equivalently give the Drude and the intrinsic anomalous Hall conductivities in the linear order of electric field. However, recent theoretical advances implied that the second-order nonlinear conductivities calculated with both approaches are no longer equivalent, which leads to various experimental explanations even in a similar experimental setup conducted in [Science 381, 181 (2023) and Nature (London) 621, 487 (2023), respectively]. Herein, by extending the ac semiclassical theory up to the second order of electric field, we show that the semiclassical theory is still equivalent to the response theory in the second order of electric field when the relaxation is taken into account on the same footing. In particular, we show that the familiar second-order nonlinear current responses, including the nonlinear Drude current and the Berry curvature (quantum metric) dipole driven extrinsic (intrinsic) nonlinear Hall current, can be derived by both approaches. Further, we show that the quantum-corrected intrinsic nonlinear longitudinal current, as recently proposed by the response theory or in a similar manner, can also be reproduced by the semiclassical theory. Beyond those known second-order current responses, with both approaches, we uncover two previously overlooked nonlinear displacement currents unique to the ac electric field. As a consequence of this equivalence, (i) we suggest that the energy of the equilibrium Fermi distribution particularly in the semiclassical theory should be the unperturbed one by assuming that both approaches give the same intrinsic responses; (ii) we unify the intrinsic second-order nonlinear longitudinal current responses calculated with both approaches; and (iii) we argue that the scheme of introducing relaxation in the response theory by modifying the quantum Liouville equation needs to be reconsidered. Our work explicitly shows the equivalence of the ac semiclassical theory and the response theory when calculating the second-order nonlinear conductivities under the electric field and highlights the influence of the ac electric field.

In superconductors that lack inversion symmetry, a supercurrent flow can lead to nondissipative magnetoelectric effects. We offer a straightforward formalism to obtain a supercurrent-induced magnetization in superconductors with broken inversion symmetry, which may have orbital, layer, sublattice, or valley degrees of freedom—multiband noncentrosymmetric superconductors. The nondissipative magnetoelectric effect may find applications in fabricating quantum computation platforms or efficient superconducting spintronic devices. We explore how the current-induced magnetization can be employed to create and manipulate Majorana zero modes in a simple hybrid structure.

Several studies have emphasized the impact of long-range Coulomb interactions in lattice fermions, yet conventional auxiliary field quantum Monte Carlo (QMC) methods face limitations due to their reliance on positive-definite interaction matrices. We address this by decomposing the interaction matrix into positive- and negative-definite components, allowing for QMC calculations with manageable sign properties. This technique enables effective simulations on large lattices. Applying it to a honeycomb lattice with an indefinite interaction matrix, we identify a semimetal to the charge density wave phase transition within the Gross-Neveu criticality class. Notably, the phase transition boundary aligns with regions where the average sign sharply decreases, providing evidence to the increasingly compelling research on the relationship between phase transitions and the sign problem.