To enable suppression of two-magnon scattering (TMS) in nanometer-thick Co (ultrathin Co) layers and realize low-magnon damping in such layers, the magnon damping in ultrathin Co layers grown on various nonmagnetic seed layers with different surface magnetic anisotropy (SMA) energies are investigated. We verify the significantly weak magnon damping realized by varying the seeding layer species used. Although TMS is enhanced in ultrathin Co on Cu and Al seeding layers, the insertion of a Ti seeding layer below the ultrathin Co greatly suppresses the TMS, which is attributed to suppression of the SMA at the interface between Co and Ti. The Gilbert damping constant of the ultrathin Co layer on Ti (3 nm), 0.020, is comparable to the value for bulk Co, although the Co layer thickness here is only 2 nm. Realization of such weak magnon damping can open the door to tunable magnon excitation, thus enabling coupling of magnons with other quanta such as photons, given that the magnetization of ultrathin ferromagnets can be tuned using an external electric field.

The in-plane magnetoresistance and magnetostriction of ${\mathrm{Ca}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ single crystals were investigated under pulsed magnetic field. Field-induced resistance steps and structural change were observed for specific field directions and far below the metal-to-insulator transition temperature $T_{\mathrm{MI}}=48$ K, which were not observed under static magnetic field. Especially, the resistance steps were only observed in the field-descending branch, indicating the relaxation properties, and no magnetization steps were observed. These results unveil the existence of instantaneous interactions among different degrees of freedom in ${\mathrm{Ca}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$, which are observable under pulsed magnetic field thanks to its fast field sweep rate. Based on the orbital states of ${\mathrm{Ca}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ and the Kugel-Khomskii model, we propose that the resistance steps and the structural change are related to a dynamic spin-orbital interaction, and this interaction can be strongly suppressed by the magnetocrystalline anisotropy.

The Kitaev model, whose ground state is a quantum spin liquid (QSL), was originally conceived for spin $S=1/2$ moments on a honeycomb lattice. In recent years, the model has been extended to higher $S$ from both theoretical and experimental interests, but the stability of the QSL ground state has not been systematically clarified for general $S$, especially in the presence of other additional interactions, which inevitably exist in candidate materials. Here we study the spin-$S$ Kitaev-Heisenberg models by using an extension of the pseudofermion functional renormalization group method to general $S$. We show that, similar to the $S=1/2$ case, the phase diagram for higher $S$ contains the QSL phases in the vicinities of the pristine ferromagnetic and antiferromagnetic Kitaev models, in addition to four magnetically ordered phases. We find, however, that the QSL phases shrink rapidly with increasing $S$, becoming vanishingly narrow for $S\ge 2$, whereas the phase boundaries between the ordered phases remain almost intact. Our results provide a reference for the search for higher-$S$ Kitaev materials.

We investigate the nonequilibrium states of an interacting multicomponent quantum system when only an extensive subsystem is quantum quenched or driven from the ground state. As a concrete example, we consider a system where two XXZ spin chains are coupled to a transverse field Ising (TFI) chain, and only the transverse field in the TFI chain is quantum quenched or periodically driven in time, starting from an initially ordered state. This system is studied using density matrix renormalization group simulations and various entanglement entropy diagnostics. In the case of quantum quenching, when the transverse field is suddenly switched on to become the largest energy scale, the resulting internal dynamics leads to a prethermal steady state with persistent oscillating magnetization (“self driving”) and emergent conservation laws. Upon applying the time-dependent drive to the TFI chain (“external driving”), sufficiently fast drive gives rise to a prethermal steady state with finite magnetization, whereas a slow drive generates a high-temperature disordered state. We briefly discuss the experimental implementation of our protocol in organic materials with quantum-tunneling hydrogen atoms.

Structured optical fields (SOFs) with spatially inhomogeneous phase, amplitude, or polarization can excite substantially different optical responses in nanoscale scattering particles. Inhibition of low-order multipolar resonances, excitation of dark modes and anapoles, and spin and orbital angular momenta dichroism have been demonstrated, broadening the scope of nanoscale manipulations of optical resonances by using these engineered external SOFs. However, studying scattering effects illuminated by SOFs is more complicated than conventional plane wave illuminations, and the difficulty increases for scattering particles on a substrate. In this paper, we present explicit expressions of SOFs propagating through stratified media, or multilayered systems, and provide their beam-shape coefficients. Specifically, simple analytic expressions are provided for circularly symmetric Bessel beams passing through an interface. Then, scattering calculations for a dielectric sphere on a substrate illuminated by an evanescent optical vortex are performed via the $T$-matrix method, which yields accurate and efficient results as compared with finite element analysis. This paper will find practical applications in modeling SOFs for realistic configurations of scattering of particles on a substrate at nonplanar nonparaxial external light source illuminations.

Quantum geometry characterizes the geometric properties of Bloch electrons in the wave space, represented by the quantum metric and the Berry curvature. Recent studies have revealed that the quantum geometry plays a major role in various physical phenomena, from multipole to non-Hermitian physics. For superconductors, the quantum geometry is clarified to appear in the superfluid weight, an essential quantity of superconductivity. Although the superfluid weight was considered to be determined by the Fermi-liquid contribution for a long time, the geometric contribution is not negligible in some superconductors such as artificial flat-band systems and monolayer FeSe. While the superfluid weight is essential for many superconducting phenomena related to the center of mass momenta of Cooper pairs (CMMCP), the full scope of the quantum geometric effect on superconductivity remains unresolved. In this paper, we study the quantum geometric effect on the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state acquiring a finite CMMCP in equilibrium. As a benchmark, the phase diagrams of effective models for monolayer FeSe in an in-plane magnetic field are calculated. In the case of the isotropic $s$-wave pairing, the quantum geometry stabilizes the BCS state, and a metastable BCS state appears in the high-magnetic-field region. In addition, the quantum geometry induces the phase transition from the FFLO state to the BCS state with increasing temperature. However, for the intersublattice pairing, the quantum geometry gives a negative contribution to the superfluid weight; this can induce the FFLO superconductivity in particular parameter sets.

We analyze the real-space paired state with the $\mathbf{k}$-dependent superconducting gap in the presence of Rashba-type spin-orbit coupling and external magnetic field. We show that the extended $s$-wave pairing symmetry is the most probable scenario to appear in the low electron concentration regime. According to our study, the van Hove singularity induced by the spin-orbit coupling may lead to a significant enhancement of the superconducting gap, critical temperature, and critical magnetic field. Moreover, the combined effect of the spin-orbit coupling and the external magnetic field results in a nonzero total momentum of the Cooper pairs, which is a characteristic feature of the so-called helical state. In such a situation, due to the ${\mathrm{C}}_4$ symmetry breaking, a small $d$-wave and $p$-wave contributions to the pairing appear, which significantly change the character of the helical state. The obtained results are discussed in the context of the experimental data related with the unconventional superconducting features of the transition metal oxide interfaces as well as the recently reported supercurrent diode effect.

The $\alpha \text{−}{\mathrm{BiB}}_3{\mathrm{O}}_6$ (BIBO) is a widely used nonlinear optical (NLO) crystal in ultraviolet (UV) energy region. Although the spin-orbit coupling (SOC) effect on the band structure of BIBO has been investigated, the role of SOC effect on the second-order NLO properties is still unclear. In this article, using the first-principles calculations, we discover the unusual polar lone-pair electron can induce large Rashba SOC effect in the electronic structure of $\alpha \text{−}{\mathrm{BiB}}_3{\mathrm{O}}_6$ (BIBO), which can fundamentally change its NLO responses around the band edge. For the second harmonic generation (SHG), it is found that the inclusion of SOC effect can significantly improve the accuracy of SHG coefficients calculations in BIBO, closer to the experimentally measured values. Importantly, we also find the large Rashba SOC effect can strongly modulate the photodriven high-order charge/spin currents in BIBO, giving rise to large bulk photovoltaic effect, large circular photogalvanic effect, and an unusual spin photocurrent effect. Interestingly, the roles of SOC in tuning the charge and spin currents are quite different. Accordingly, we propose the spin current switching around the band edge induced by the large Rashba SOC in a BIBO crystal. Our study not only provides a deep and complete understanding on the unique roles of Rashba SOC effect in the second-order NLO properties of BIBO, but also suggests the BIBO crystal may be used for UV-range spintronics and optoelectronics.

We study a general class of easy-axis spin models on a lattice of corner sharing even-sided polygons with all-to-all interactions within a plaquette. The low-energy description corresponds to a quantum dimer model on a dual lattice of even coordination number with a multidimer constraint. At an appropriately constructed frustration-free Rokhsar-Kivelson (RK) point, the ground-state wavefunction can be exactly mapped onto a classical vertex model on the dual lattice. When the dual lattice is bipartite, the vertex models are bonded and are self-dual under Wegner's duality, with the self-dual point corresponding to the RK point of the original multidimer model. We argue that the self-dual point is a critical point based on known exact solutions to some of the vertex models. When the dual lattice is nonbipartite, the vertex model is arrowed, and we use numerical methods to argue that there is no phase transition as a function of the vertex weights. Motivated by these wavefunction dualities, we construct two other distinct families of frustration-free Hamiltonians whose ground states can be mapped onto these vertex models. Many of these RK Hamiltonians provably host ${\mathbb{Z}}_2$ topologically ordered phases.

The Mott transition plays a key role in strongly correlated physics, but its nature is not yet fully understood. Motivated by the recent development of the Schwinger boson approach for the Kondo lattice, we propose in this paper a slave fermion algorithm to study the Mott transition. Upon local approximation, our method yields a phase diagram with a zero-temperature continuous (Mott) metal-insulator transition at finite Coulomb interaction $U$ for the half-filled one-band Hubbard model on a square lattice, and the resistivity exhibits a critical scaling around the quantum Widom line. We argue that the Mott transition may be associated with a dynamic charge Kondo effect of local degenerate doublon and holon states, causing sharp resonances on the doublon, holon, and electron spectra. The transition is pushed to $U=0$ once intersite antiferromagnetic correlations are included, in agreement with exact numerical calculations. Our approach captures some essential features of the Mott transition and offers an alternative angle to view this important problem. It can be extended to study other correlated electron models with more complicated local interactions.

Topological semimetal states having the minimal number, i.e., only a single pair, of Weyl points are desirable for the study of effects associated with chiral topological charges. So far, the search for such states is focused on magnetic spinful systems. Here, we find that nonmagnetic spinless systems can host a class of single-pair-Weyl-point (SP-WP) states, where the two Weyl points are located at two high-symmetry time-reversal-invariant momenta. We identify 38 candidate space groups that host such states, and we show that the chiral charge of each Weyl point must be an even integer, representing a multi-Weyl point. Besides achieving the minimal number, the two points in SP-WP states are far separated in momentum space, making the physics of each individual point better exposed. The large separation combined with the even topological charge lead to extended surface Fermi loops with a noncontractible winding topology on the surface Brillouin zone torus, distinct from conventional Weyl semimetals. We confirm our proposal in the phonon spectra of two concrete materials ${\mathrm{TlBO}}_2$ and ${\mathrm{KNiIO}}_6$. Our finding applies to a wide range of systems, including electronic, phononic, and various artificial systems. It offers a new direction for the search of ideal platforms to study chiral particles.

We have investigated the interlayer RKKY interaction in ferromagnet/Weyl semimetal/ferromagnet (FM/WSM/FM) trilayer system in the presence of the tilting effect of Weyl cones. Our investigation focuses on the influence of the unique features of WSMs: anisotropy dispersion, tilt effect, intervalley transitions, and Fermi arc states. Due to the separation of Weyl nodes, the RKKY behaviors, such as spin model, decaying rate, and oscillation period, heavily depend on the stacked direction of FMs, exhibiting strong magnetic anisotropy. Tilt of Weyl cones can increase the RKKY amplitude, change spin model, switch the antiferromagnetism and ferromagnetism, and more importantly generate the noncollinear Dzyaloshinskii-Moriya or spin-frustrated terms. Intervalley transition generates extra oscillation with period determined by separated distance between Weyl nodes, which together with Fermi-energy-induced oscillation leads to an interesting battering pattern. These signatures are significantly different from the interlayer RKKY interaction with single-node topological insulators or impurity RKKY case reported in previous literatures. More importantly, we find that the interlayer RKKY interaction decays more slowly with the spacer thickness than one reported extensively in WSMs doped with magnetic impurities, which is explained by providing a general formula. In addition, we discuss the effect from Weyl Fermi arcs. Compared to the bulk contributions, a more slowly decaying rate is found at the edges of ferromagnetic layer if the edges are parallel to the line connecting two Weyl nodes. Our research suggests that trilayer FM/WSM/FM is a more powerful platform to understand the WSM magnetic property and to extract the parameters of WSM materials by measuring the RKKY interaction.

We extend the definition of the Born effective charge to the dynamical regime. This is equal to the Fourier transform of the total electronic current divided by the Fourier transform of the velocity of a particular nucleus. The usual static Born effective charges are recovered in the zero-frequency limit. We calculate these charges for a selection of materials using time-dependent density functional theory in an all-electron code where the nuclei move along a chosen trajectory. A rich response function emerges with prominent resonance peaks. The finite value for the Born effective charge of metals is also reproduced. The dynamical Born effective charges are thus a natural choice of observable for probing the fundamental nonadiabatic coupling of electrons and nuclei.

We demonstrate the formation of robust zero-energy modes close to magnetic impurities in the iron-based superconductor ${\mathrm{FeSe}}_{1-x}{\mathrm{Te}}_x$. We find that the Zeeman field generated by the impurity favors a spin-triplet interorbital pairing as opposed to the spin-singlet intraorbital pairing prevalent in the bulk. The preferred spin-triplet pairing preserves time-reversal symmetry and is topological, as robust, topologically protected zero modes emerge at the boundary between regions with different pairing states. Moreover, the zero modes form Kramers doublets that are insensitive to the direction of the spin polarization or to the separation between impurities. We argue that our theoretical results are consistent with recent experimental measurements on ${\mathrm{FeSe}}_{1-x}{\mathrm{Te}}_x$.

Layered tellurides have been extensively studied as a platform for investigating the Fermi surface (FS) nesting-driven charge density wave (CDW) states. ${\mathrm{EuTe}}_4$, one of the quasi-two-dimensional binary rare-earth tetratelluride CDW compounds with unconventional hysteretic transition, is currently receiving much attention. Here, the CDW modulation vector, momentum, and temperature dependence of CDW gaps in ${\mathrm{EuTe}}_4$ are investigated using angle-resolved photoemission spectroscopy. Our results reveal that (i) a FS nesting vector $\mathit{q}\approx 0.67{\mathit{b}}^{*}$ drives the formation of the CDW state, (ii) a large anisotropic CDW gap is fully open in the whole FS and maintains a considerable size even at 300 K, leading to the appearance of semiconductor properties, (iii) an abnormal behavior of CDW gap in magnitude as a function of temperature, and (iv) an extra, larger gap opens at higher binding energy due to the interaction between the different orbits of the main bands.

We show that an accurate description of the interplay between the anharmonic ionic interaction and the zero-point fluctuation of ions in crystalline aluminum (Al) can be achieved using the path-integral molecular dynamics (PIMD) method in conjunction with improved empirical modified embedded-atom method (MEAM) potentials. Our results show that the zero-point fluctuation of ions is noticeable at a temperature between 200 K and 250 K, roughly half of the Al Debye temperature of 428 K, with a selective influence on mechanical properties. The effect provides appreciable corrections to the lattice constant $a$, bulk modulus $B$, and elastic constant $C_{11}$ at low temperature, but without much influence on elastic constants $C_{12}$ and $C_{44}$. With a revised MEAM potential which takes into consideration the influence of zero-point fluctuations, the PIMD method has a much improved accuracy in $a, B$, and the $C$'s up to 700 K, when compared with precision experimental measurements. The largest errors of $a$ and $B$ can even be reduced by about an order in percentage below 300 K.

In cluster-based quasicrystals, tetrahedra located in conventional Tsai clusters may be replaced by single rare-earth $\left(R\right)$ ions at the cluster centers (pseudo-Tsai clusters). In this study, we investigate the effect of the pseudo-Tsai cluster incorporation on the magnetic structures of two approximants, the Tsai-type Tb-Au-Si [denoted TAS(0)] and Ho-Au-Si [denoted HAS(52)] with partial replacement of conventional Tsai clusters by pseudo-Tsai clusters, up to 52%. The mixture of Tsai and pseudo-Tsai clusters can be considered a different source of randomness/disorder other than the conventional chemical mix sites (Au/Si). The effect of the latter has been previously discussed regarding the origin/cause of spin-glass-like ordering and Anderson localization of electronic states in quasicrystals and approximant crystals. Single crystal neutron diffraction experiments at 2 K were performed and bulk physical properties (magnetization and specific heat) were investigated. In addition, earlier collected powder neutron diffraction data of TAS(14) with 14% replacement was reanalyzed in light of the results on TAS(0) and HAS(52). We find that the arrangement of ordered magnetic spins in the icosahedral shells of these phases is similar, while the cluster-center $R$ magnetic states are different. In the case of TAS(14), the cluster-center Tb magnetic moments seem to affect the arrangement of surrounding icosahedral magnetic moments, and the magnetic structure of the icosahedral shell deviates from that of TAS(0). In the case of HAS(52), however, the icosahedral $R$ magnetic moments are less affected by the cluster-center $R$, while the averaged cluster-center $R$ magnetic moments are significantly diminished. We discuss these results considering the magnetic ordering effect on the bulk physical properties.

We perform the finite-temperature determinant quantum Monte Carlo simulation for the attractive Hubbard model on the half-filled bilayer square lattice. Recent progress on optical lattice experiments lead us to investigate various single-particle properties such as momentum distribution and double occupancies which should be easily measured in cold-atom experiments. The pair-pair and the density-density correlations have been studied in detail and, through finite-size scaling, we show that there is no competing charge density wave order in the bilayer band-insulator model and that the superfluid phase is the stable phase for the interaction range $\left|U\right|/t=5–10$. We show the existence of two energy scales in the system as we increase the attractive interaction, one governing the phase coherence and the other one corresponding to the molecule formation. In the end, we map out the full $T-U$ phase diagram and compare the $T_c$ obtained through the mean-field analysis. We observe that the maximum $T_c/t(=0.27)$ occurs for $\left|U\right|/t=6$, which is roughly twice the reported $T_c$ of the single-layer attractive Hubbard model.

The charge transport properties of zero-temperature multiorbital quantum dot systems with one dot coupled to leads and the other dots coupled only capacitatively are studied within density functional theory. It is shown that the setup is equivalent to an effective single impurity Anderson model. This allows to understand the level occupation switching effect as transitions between ground states of different integer occupations in the uncoupled dots. Level occupation switching is very sensitive to small energy differences and therefore also to the details of the parametrized exchange-correlation functionals. An existing functional already captures the effect on a qualitative level, but we also provide an improved parametrization which is very accurate when compared to reference numerical renormalization group results.

In this work, we find that the complexity of quantum many-body states, defined as a spread in the Krylov basis, may serve as a probe that distinguishes topological phases of matter. We illustrate this analytically in one of the representative examples, the Su-Schrieffer-Heeger model, finding that spread complexity becomes constant in the topological phase. Moreover, in the same setup, we analyze exactly solvable quench protocols where the evolution of the spread complexity shows distinct dynamical features depending on the topological vs nontopological phase of the initial state as well as the quench Hamiltonian.