We investigate the magnetic order in single crystalline ${\mathrm{EuCd}}_2{\mathrm{Sb}}_2$ thin films using a combined theoretical and experimental approach. Resonant elastic x-ray scattering experiments reveal a sharp magnetic peak at $\mathbf{q}=(0,0,\frac{1}{2})$ below $T_{\mathrm{N}}=7.2$ K, indicative of interlayer antiferromagnetic ordering. Additionally, we observe a weak diffuse magnetic signal centered at $\mathbf{q}=(0,0,1)$ that persists above $T_{\mathrm{N}}$, up to $T_{\mathrm{C}}\sim 11$ K. Our Monte Carlo simulations of a classical spin model approximation of the Eu magnetic sublattice demonstrate that the diffuse signal can arise from ferromagnetic coupling in the top few layers due to surface oxidation. On the other hand, the bulk of the sample exhibits antiferromagnetic coupling between layers. Finally, our fit of the model parameters to the magnetic ordering temperatures shed light on the exchange couplings that are key in stabilizing the observed composite magnetic order.

The frustrated antiferromagnet ${\text{CuFeO}}_2$ exhibits pressure-induced complex magnetic phase transitions from the commensurate collinear (CM1) phase to several incommensurate noncollinear phases. To study the effect of high pressure on magnetic interactions, we performed neutron diffraction and inelastic neutron scattering experiments under high-pressure conditions. With increasing pressure, the CM1 ground state becomes less stable against application of a magnetic field even below the critical pressure ($P\le 3$ GPa), as proved by the significant reduction in the critical magnetic field from $H_{c1}=7.5$ T to 4.5 T at 2.1 GPa. Additionally, the energy gap in the spin-wave dispersion relation is reduced from 1.0 to 0.88 meV by the application of a pressure of $P=2.1$ GPa. Comparing the experimental results with spin-wave calculations revealed that the change in the spin-wave excitation can be explained by the reduction in either the uniaxial anisotropy term or the degree of separation in the nearest-neighbor exchange interactions.

In search of spin-orbit coupling driven nonmagnetic $J=0$ ground state and excitonic magnetism, various pentavalent iridates have been studied in recent years. However, a finite moment was observed in most of the cases due to solid state effects. In this work, we investigate the electronic and magnetic structure of $6H$ hexagonal compound ${\mathrm{Ba}}_3{\mathrm{NiRuIrO}}_9$, where ${\mathrm{Ir}}^{5+}$ is present along with magnetic ${\mathrm{Ni}}^{2+}$ and ${\mathrm{Ru}}^{5+}$ ions. Magnetic susceptibility measurements and neutron powder diffraction (NPD) experiments demonstrate the appearance of short-range magnetic ordering below 170 K and a long-range antiferromagnetic ordering below 80 K. The refinement of the NPD pattern further shows that the Ru and Ir moments interact antiferromagnetically within the dimer and interact ferromagnetically with the Ni sublattice. These experimental findings have been complemented by first-principles density functional theory calculations incorporating spin-orbit coupling effects and electronic correlations for the transitional metal $d$ states. The computed magnetocrystalline anisotropy is also found to be significant and the crystallographic $c$ axis comes out to be the easy axis of magnetization, consistent with the spin alignment direction found from NPD. This study shows that the mixed ruthenate iridate triple perovskite series is a promising family to study the interplay among spin-orbit coupling, electron correlation, and electron filling as a variety of ${\mathrm{Ba}}_{3}M{\mathrm{RuIrO}}_9$ with $M$ as a transition metal ion, rare-earth ion, and alkali metal ions can be synthesized.

The elegant spin physics of Dirac electrons in topological insulators (TIs) have considerably endowed fertile tunability of magnetic/TI heterojunction performance with modified spin-orbit effect engineering. Signatures of proximate hybridization between magnetic states and topological surface states have been reported. However, the nature of the spin relaxation process in these systems remains elusive. Here, we unambiguously demonstrate anisotropic spin relaxation in a spin-orbit-hybridized $\mathrm{Fe}\text{/}{\mathrm{Bi}}_2{\mathrm{Se}}_3$ system. We find a sixfold anisotropy of the Gilbert damping parameter with modulation of up to 33% in $\mathrm{Fe}\text{/}{\mathrm{Bi}}_2{\mathrm{Se}}_3$ in the presence of a topological surface state, together with a sixfold magnetic anisotropy. We anticipate the presence of a spin interplay between the topological spin-orbit texture and magnetic orbital states would manifest an anisotropic Gilbert damping, which corroborates with the density functional theory calculations. It is further demonstrated by the spin Hanle effect indicative of anisotropic spin relaxation time ${\tau }_s$ in the adjacent topological layer, inversely scaling with the Gilbert damping factor ${\alpha }_G$. Our findings present an alternative scenario of the anisotropic spin transport process and offer insights into spin manipulation in spin-logic/memory devices utilizing proximity-hybridized Dirac electrons.

We investigate the quantum spin-1/2 zigzag chain with frustrated $J_1-J_2$ Heisenberg interactions, incorporating additional off-diagonal exchange interactions known as the $\mathrm{\Gamma }$ term, both with and without an applied magnetic field. Based on the density-matrix renormalization group calculation, we map out the ground-state phase diagram that shows a variety of magnetic and nonmagnetic phases including multicritical points and several exactly solvable points. Upon introducing a finite $\mathrm{\Gamma }$ term, we observe the persistent dimer-singlet state of the $J_1-J_2$ Heisenberg model, sustaining a nonzero spin gap, while also hosting a gapless nonmagnetic excitation that manifests in the substantial zero-energy peak in the nematic dynamical structure factor. This gapless peak-mode, remaining almost as a fluctuation to the ground state, induces a dilute but robust concentration of nematicity on top of singlets on dimers, which we call the nematic singlet-dimer phase. When the whole nematic excited mode condenses and replaces the singlet, the nematic-dimer phase transforms into the Ising-type ferromagnetic or antiferromagnetic long-range orders. The $\mathrm{\Gamma }$ term spontaneously selects magnetic easy axes, and their orientations dictate the type of magnetic order under geometric frustration effects as predicted by Landau's mean-field theory. These theoretical findings provide insights into the exotic low-temperature phase observed in ${\mathrm{YbCuS}}_2$, characterized by gapless excitations and seemingly nonmagnetic behavior.

We present a methodology based on the calculation of the inelastic scattering from magnons via the spin-scattering function in confined geometries such as thin films using a second quantization formalism, for both ferromagnetic and antiferromagnetic materials. The case studies are chosen with an aim to demonstrate the effects of film thickness and crystal orientation on magnon modes, using bcc Fe(100) and NiO with (100) and (111) crystallographic orientations as prototypical systems. Due to the quantization of the quasimomentum, we observe a granularity in the inelastic spectra in the reciprocal space path reflecting the orientation of the thin film. This approach also allows for the capture of softer modes that appear due to the partial interaction of magnetic moments close to the surface in a thin film geometry, in addition to bulk modes. The softer modes are also affected by crystallographic orientation, as illustrated by the different surface-related peaks of the NiO magnon density of states at approximately $\sim 65\mathrm{meV}$ for (100) and $\sim 42\mathrm{meV}$ for a (111)-oriented film. Additionally, we explore the role of anisotropy, revealing that anisotropy increases the overall hardness of the magnon modes. The introduction of a surface anisotropy produces a shift of the surface-related magnon DOS peak to higher energies with increased surface anisotropy, and in some cases leads to a surface-confined mode.

We study the electronic band structures and topological properties of nonmagnetic kagome metal $\mathrm{N}{\mathrm{i}}_3\mathrm{Sn}$ when applying in-plane biaxial strains through first-principles calculations. From these calculations, we confirm that the flat band of $\mathrm{N}{\mathrm{i}}_3\mathrm{Sn}$ is located at 250 meV below the Fermi level for pristine $\mathrm{N}{\mathrm{i}}_3\mathrm{Sn}$, and its position can be raised to the Fermi level by applying a finite in-plane biaxial strain. In this deformation process, we also find that there are two distinct topological phase transitions between trivial and nontrivial topological states at 1.5% and 2.9% strains. Finally, through the calculations of electron-phonon coupling, we examine the possibility of superconductivity mediated by electron-phonon coupling for pristine and strained $\mathrm{N}{\mathrm{i}}_3\mathrm{Sn}$ structures and confirm theoretically that the 5% biaxial strained $\mathrm{N}{\mathrm{i}}_3\mathrm{Sn}$ shows superconductivity. Therefore, biaxial strain in kagome metal $\mathrm{N}{\mathrm{i}}_3\mathrm{Sn}$ triggers the flat band to move toward the Fermi level, thereby inducing nontrivial topology and superconductivity simultaneously.

Recent advances in all-dielectric topological photonics offer unprecedented opportunities for achieving robust waveguides. However, most topological waveguides are localized at small areas around the interfaces and usually operate in a single frequency window, leading to the low-throughput energy transmission over a narrow bandwidth. Here we report on the implementation of a dual-band topological large-area waveguide in heterostructures based on valley photonic crystals, in which the bandwidths are effectively widen by exploiting the dual-band topology. Inherited from the valley topology, the topological large-area waveguides exhibit the valley-locking effect at two separate frequency windows and are robust against sharp corners and defects. We also observe intriguing topological refractions of the topological large-area waveguides into the ambient space at zigzag and armchair terminations. Our work highlights both the tunable width and the large bandwidth of topological waveguides, paving the way for topological photonic integrated circuits with high performance and multifunctionality.

Fermi-arc surface states of Weyl semimetals exhibit a unique combination of localization to a surface and the connection to the bulk Weyl fermions. We predict characteristic quantum-oscillation signatures of Fermi arcs in the tunnel mangetoconductance across an interface between two Weyl semimetals. These oscillations stem from a momentum-space analog of Aharonov-Bohm interference of electrons moving along the interface Fermi arcs, driven by an external magnetic field normal to the interface. The Fermi arcs' connection to the bulk enables their characterization via transport normal to the interface, while their localization along the transport direction manifests in a strong field-angle anisotropy of the oscillations. This combination distinguishes these oscillations from conventional Shubnikov-de Haas oscillations and makes them identifiable even in the complex oscillation spectra of real materials.

$(3+1)$-dimensional topological phases can support loop-like excitations in addition to point-like ones, allowing for nontrivial loop-loop and point-loop braiding statistics not permitted to point-like excitations alone. Furthermore, these loop-like excitations can be linked together, changing their properties. In particular, this can lead to distinct three-loop braiding, involving two loops undergoing an exchange process while linked to a third loop. In this work, we investigate the loop-like excitations in a $(3+1)$-dimensional Hamiltonian realization of Dijkgraaf-Witten theory through direct construction of their membrane operators, for a general finite Abelian group and 4-cocycle twist. Using these membrane operators, we find the braiding relations and fusion rules for the loop-like excitations, including those linked to another loop-like excitation. Furthermore, we use these membrane operators to construct projection operators that measure the topological charge and show that the number of distinct topological charges measured by the 2-torus matches the ground-state degeneracy of the model on the 3-torus, explicitly confirming a general expectation for topological phases. This direct construction of the membrane operators sheds significant light on the key properties of the loop-like excitations in $(3+1)$-dimensional topological phases.

Kagome metals with the Fermi energy tuned near the van Hove singularities (vHss) have shown to host exotic phases including unconventional superconductivity and a chiral flux phase arising from a charge density wave. However, most quantum oscillations studies of the electronic structure of kagome metals focus on compounds which electronically or magnetically order, obscuring the unperturbed vHs. Here we present quantum oscillation measurements of ${\mathrm{YV}}_6{\mathrm{Sn}}_6$ which contains a pristine kagome lattice free from long-range order. We discovered quantum oscillations corresponding to a large orbit ($\approx 70\%$ of the Brillouin Zone area) with the heaviest mass ever observed in vanadium-based kagome metals ($\approx 3.3m_e$), consistent with a Fermi pocket whose Fermi level is near the vHs. Comparing with first-principle calculations suggests that the effective mass of this pocket is highly sensitive to the position of Fermi level. Our study establishes the enhanced density of states associated with a vHs in a kagome metal, allowing further insight into a potential driving mechanism for the unconventional electronic orderings in this class of materials.

Strain tuning is a powerful experimental method in probing correlated electron systems. Here we study the strain response of the lattice dynamics and electronic structure in the semiconductor ${\mathrm{Ta}}_2{\mathrm{NiS}}_5$ by polarization-resolved Raman spectroscopy. We observe an increase of the size of the direct semiconducting band gap. Although the majority of the optical phonons show only marginal dependence to applied strain, the frequency of the two ${\mathrm{B}}_{2g}$ phonon modes, which have quadrupolar symmetry and already anomalously soften on cooling under zero strain, increases significantly with tensile strain along the $a$ axis. The corresponding Grüneisen parameters are unusually large in magnitude and negative in sign. These effects are well captured by first-principles density functional theory calculations and indicate close proximity of ${\mathrm{Ta}}_2{\mathrm{NiS}}_5$ to a structural instability, similar to that encountered in excitonic insulator candidate ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$.

The higher-spin Kitaev magnets, in which the Kitaev interaction and off-diagonal exchange couplings are overwhelmingly large, have emerged as a fertile avenue to explore exotic phases and unusual excitations. In this paper, we study the quantum phase diagram of the spin-1 Kitaev-$\mathrm{\Gamma }$ model on the honeycomb lattice using density-matrix renormalization group. It harbors six distinct phases and the intriguing findings are three magnetically ordered phases in which both time-reversal symmetry and lattice symmetry, albeit of different sorts, are broken spontaneously. The chiral spin state originates from the order-by-disorder effect and exhibits an almost saturated scalar spin chirality at the quantum level. Depending on the relative strength of the two interactions, it also features a columnarlike or plaquettelike dimer pattern as a consequence of the translational symmetry breaking. In parallel, the nematic ferromagnets are situated at the ferromagnetic Kitaev side and possess small but finite ferromagnetic ordering. The lattice-rotational symmetry breaking enforces nonequivalent bond energy along one of the three bonds. Although the intrinsic difference between the two nematic ferromagnets remains elusive, the discontinuities in the von Neumann entropy, hexagonal plaquette operator, and Wilson loop operator convincingly suggest that they are separated via a first-order phase transition.

Electronic modes emerge within the band gap at nonzero temperature in strongly correlated insulators such as Mott and Kondo insulators, exhibiting momentum-shifted magnetic dispersion relations from the band edges. As the temperature increases, the emergent modes gain considerable spectral weights and form robust bands that differ from the zero-temperature bands. Here, the origin of the emergent modes, their relation to doping-induced modes, and how their spectral weights increase with temperature are clarified in the ladder and bilayer Hubbard models and one- and two-dimensional Kondo lattice models using effective theory for weak inter-unit-cell hopping and numerical calculations. The results indicate that the temperature-driven change in the band structure reflecting spin excitation, including the change in the number of bands, can be observed in various strongly correlated insulators even with a spin gap, provided that the temperature increases up to about spin-excitation energies. The controlled analyses developed in this study significantly contribute to the fundamental understanding of the band structure of strongly correlated insulators at nonzero temperature.

We present a general-purpose algorithm to extrapolate a low-rank function of two variables from a small domain to a larger one. It is based on the cross-interpolation formula. We apply it to reconstruct physical quantities in some quantum many-body perturbative expansions in the real-time Keldysh formalism, considered as a function of time $t$ and interaction $U$. These functions are of remarkably low rank. This property, combined with the convergence of the perturbative expansion in $U$ both at finite $t$ (for any $U$), and small $U$ (for any $t$), is sufficient for our algorithm to reconstruct the physical quantity at long time, strong coupling regime. Our method constitutes an alternative to standard resummation techniques in perturbative methods, such as diagrammatic quantum Monte Carlo. We benchmark it on the single impurity Anderson model and show that it is successful even in some regime where standard conformal mapping resummation techniques fail.

Solitons formed through the one-dimensional mass-kink mechanism on the edges of two-dimensional systems with nontrivial topology play an important role in the emergence of higher-order (HO) topological phases. In this connection, the existing work in time-reversal symmetric systems has focused on gapping the edge Dirac cones in the presence of particle-hole symmetry, which is not suited to the common spin Chern insulators. Here, we address the emergence of edge solitons in spin Chern number of two insulators, in which the edge Dirac cones are gapped by perturbations preserving time-reversal symmetry but breaking spin-$U\left(1\right)$ symmetry. Through the mass-kink mechanism, we thus explain the appearance of pairwise corner modes and predict the emergence of extra charges around the corners. By tracing the evolution of the mass term along the edge, we demonstrate that the in-gap corner modes and the associated extra charges can be generated through the anisotropic $S_z$-mixing spin-orbit coupling via the mass-kink mechanism. We thus provide strong evidence that an anisotropic even spin Chern number insulator is an HO topological insulator with protected corner charges.

We address the equilibrium and out-of-equilibrium behavior of the particle density in many-body systems undergoing quantum transitions driven by the chemical potential $\mu$. They originate from a nontrivial interplay between noncritical short-range and critical long-range quantum fluctuations. As a paradigmatic model, we consider the one-dimensional fermionic Kitaev model, for which very accurate numerical studies can be performed, up to $O\left({10}^4\right)$ chain sites. The search for dynamic scaling behaviors of the particle density is complicated by the fact that its equilibrium (ground-state) behavior is dominated by short-range fluctuations, giving rise to regular background terms and peculiar logarithmic terms from resonances between renormalization-group perturbations associated with the energy and identity operator families within the conformal field theory. To study these issues, we focus on two dynamic protocols, either instantaneous quenches or quasiadiabatic changes of $\mu$, to the critical value ${\mu }_c$, unveiling out-of-equilibrium scaling behaviors of the particle density, which arise from the critical modes, within a dynamic finite-size scaling framework.