We report a comprehensive investigation of the lattice and magnetic structure in van der Waals antiferromagnet ${\mathrm{VBr}}_3$, characterized by a ${\mathrm{BiI}}_3$-type structure at room temperature. Neutron diffraction experiments were performed on both polycrystalline and single-crystalline ${\mathrm{VBr}}_3$ samples, revealing clear magnetic Bragg peaks emerging below the Néel temperature of $T_N=26.5$ K. These magnetic Bragg peaks can be indexed by $\mathit{k}=(0, 0.5, 1)$ in hexagonal notation. Our refinement analysis suggests that the antiferromagnetic order in ${\mathrm{VBr}}_3$ manifests as a zigzag structure. Moreover, we observed peak splitting for nuclear Bragg peaks in the HK plane below the structure transition temperature of $T_S=90.4$ K, indicating the breaking of threefold symmetry within the $ab$ plane.

Magnetic skyrmions are nanoscale magnetic whirls that can be driven by currents via spin torques. They are promising candidates for spintronic devices such as the racetrack memory, where a motion along the uniform current is typically desired. However, for spin torque nano-oscillators in Corbino disks, the goal is to achieve a circular motion, perpendicular to the radially applied current. As we show, based on analytical calculations and micromagnetic simulations, Bloch skyrmions engage in a circular motion with frequencies in the MHz range when driven by spin-orbit torques. In contrast, Néel skyrmions get stuck at the edges of the disk. Our analysis reveals that the antagonistic dynamics between Bloch- and Néel-type magnetic textures arise from their helicity. Furthermore, we find that skyrmioniums, which are topologically trivial variations of skyrmions, move even faster and allow an increase in the current density without being pushed toward the edges of the disk. When driven by spin-transfer torques instead, Bloch and Néel skyrmions no longer exhibit different dynamics. Instead, they move along a circular trajectory due to the skyrmion Hall effect caused by their topological charge. Consequently, the topologically trivial skyrmioniums inevitably become trapped at the disk edge in this scenario. To provide a comprehensive understanding, our paper also examines currents applied tangentially, further enriching our insights into skyrmion dynamics and appropriate current injection methods for skyrmion-based devices.

Spin-frustrated systems are well known for exhibiting fascinating and exotic magnetic properties and are often intertwined with other intriguing phenomena. Magnetization and specific-heat studies suggest that the spin-frustrated chiral triangular lattice compound ${\mathrm{MnGeTeO}}_6$ undergoes a long-range antiferromagnetic transition below the Néel temperature $\left(T_{\mathrm{N}}\right)$ ∼ 9 K, which is further validated in the neutron powder-diffraction (NPD) study. In the absence of an external magnetic field $\left(H\right)$, analysis of NPD data reveals a complex incommensurate spin structure, characterized by a mixture of helical and cycloidal spin orderings with a propagation vector of $\mathit{k}=(1/3, 1/3, 0.184)$. Upon the application of $H$, this spin ordering transforms into a conical spin structure, which is manifested as a metamagnetic transition in the isothermal magnetization $\left(M\right)$ curve when $H$ exceeds ∼ 0.9 T. Total energy calculations and estimation of various exchange-interaction energies $\left(J\right)$ by density-functional theory calculations also support the formation of such a complex spin ordering. Moreover, a pronounced dielectric $\left({\varepsilon }^{\prime }\right)$ anomaly is observed near $T_{\mathrm{N}}$, which is suppressed considerably under H. The variation of ${\varepsilon }^{\prime }$ with increasing H closely reflects the $\mathrm{\Delta }{\varepsilon }^{\prime }\propto$ $M^2$ scaling behavior in the low-H regime, thus indicating a higher-order magnetoelectric coupling. Moreover, synchrotron x-ray diffraction reveals an isostructural distortion below $T_{\mathrm{N}}$, suggesting magnetoelastic coupling as a trigger for the dielectric anomaly.

We have performed magnetization, specific heat, and muon spin relaxation $\left(\mu \mathrm{SR}\right)$ measurements on single crystals of the noncentrosymmetric superconductor ${\mathrm{Ir}}_2{\mathrm{Ga}}_9$. The isothermal magnetization measurements show that there is a crossover from type-I to type-II superconductivity with decreasing temperature. Potential multiband superconductivity of ${\mathrm{Ir}}_2{\mathrm{Ga}}_9$ is observed in the specific heat data. $\mu \mathrm{SR}$ measurement is performed to map the phase diagram of ${\mathrm{Ir}}_2{\mathrm{Ga}}_9$, and both type-I and type-II superconductivity characteristics are obtained. Most importantly, a more unique region with the coexistence of type-I and type-II $\mu \mathrm{SR}$ signals is observed. In addition, time reversal symmetry is found to be preserved in ${\mathrm{Ir}}_2{\mathrm{Ga}}_9$ by zero field $\mu \mathrm{SR}$ measurement.

The nature of the insulating phases of the $\mathrm{SU}(N$)-generalization of the one-dimensional Kondo lattice model is investigated by means of nonperturbative approaches. By extending the Lieb-Schultz-Mattis (LSM) argument to multicomponent fermion systems with translation and global $\mathrm{SU}(N$) symmetries, we derive two indices which depend on the filling and the “$\mathrm{SU}(N$)-spin” (representation) of the local moments. These indices strongly constrain possible insulating phases; for instance, when the local moments transform in the $N$-dimensional (defining) representation of $\mathrm{SU}(N$), a featureless Kondo insulator is possible only at filling $f=1-1/N$. To obtain further insight into the insulating phases suggested by the LSM argument, we derive low-energy effective theories by adding an antiferromagnetic Heisenberg exchange interaction among the local moments [the $\mathrm{SU}(N$) Kondo-Heisenberg model]. A conjectured global phase diagram of the $\mathrm{SU}(N$) Kondo lattice model as a function of the filling and the Kondo coupling is then obtained by a combination of different analytical approaches.

We show here two models where changing topology does not necessarily close the bulk insulating charge gap as demanded in the standard noninteracting picture. From extensive determinantal and dynamical cluster quantum Monte Carlo simulations of the half-filled and quarter-filled Kane-Mele-Hubbard model, we show that, for sufficiently strong interactions at either half- or quarter-filling, a transition between topological and trivial insulators occurs without the closing of a charge gap. To shed light on this behavior, we illustrate that an exactly solvable model reveals that while the single-particle gap remains, the many-body gap does, in fact, close. These two gaps are the same in the noninteracting system but depart from each other as the interaction turns on. We purport that for interacting systems, the proper probe of topological phase transitions is the closing of the many-body rather than the single-particle gap.

In this paper, we present a theoretical framework for understanding the extremely correlated Fermi liquid (ECFL) phenomenon within the $U=\infty$ Hubbard model. Our approach involves deriving equations of motion for the single-particle Green's function $G$ and its associated self-energy $\mathrm{\Sigma }$, which involves the product of the bosonic correlation function comprising both density $\left(D_N\right)$ and spin $\left(D_S\right)$ correlations with $G$. By solving these equations self-consistently, we explore the behavior of $G$, $D_N$, and $D_S$ as functions of frequency, temperature, and hole concentration. Our results reveal distinct coherent and incoherent Fermi liquid regimes characterized by the presence or absence of quasiparticle excitations. Additionally, we analyze the intrinsic dc resistivity $\rho \left(T\right)$, observing a crossover from $T^2$ to linear behavior with increasing temperature. Our findings delineate Fermi liquid, quantum incoherent, and “classical” regimes in strongly correlated systems, emphasizing the importance of quantum diffusive local charge and spin fluctuations.

Including the effect of lattice anharmonicity on electron-phonon interactions has recently garnered attention due to its role as a necessary and significant component in explaining various phenomena, including superconductivity, optical response, and temperature dependence of mobility. This study focuses on analytically treating the effects of anharmonic electron-phonon coupling on the polaron self-energy, combined with numerical Diagrammatic Monte Carlo (DiagMC) data. Specifically, we incorporate a quadratic interaction into the method of squeezed phonon states, which has proven effective for analytically calculating the polaron parameters. Additionally, we extend this method to nonparabolic finite-width conduction bands while maintaining the periodic translation symmetry of the system. Our results are compared with those obtained from Diagrammatic Monte Carlo, partially reported in a recent study [S. Ragni et al., Phys. Rev. B 107, L121109 (2023)], covering a wide range of coupling strengths for the nonlinear interaction. Remarkably, our analytic method predicts the same features as the Diagrammatic Monte Carlo simulation.

Experimentally feasible methods to determine the Berry phase, a fundamental quantity characterizing a quantum material, are often needed in applications. We develop an approach to detecting the Berry phase by using a class of two-dimensional (2D) Dirac materials with a flat band, the $\alpha \text{−}{\mathcal{T}}_3$ lattices. The properties of this class of quantum materials are controlled by a single parameter $0\le \alpha \le 1$, where the left and right end points correspond to graphene with pseudospin-$\frac{1}{2}$ and the dice lattice with pseudospin-1 Dirac-Weyl quasiparticles, respectively, and each specific value of $\alpha$ represents a material with a unique Berry phase. Applying a constant electric field to the $\alpha \text{−}{\mathcal{T}}_3$ lattice, we calculate the resulting electric current and find a one-to-one correspondence between the current and the Berry phase in both the linear and nonlinear response regimes. In the linear (Kubo) regime, the main physics is the Zitterbewegung effect. In the nonlinear regime, the Schwinger mechanism dominates. Beyond the nonlinear regime, Bloch-Zener oscillations can arise. Measuring the current thus provides an effective and experimentally feasible way to determine the Berry phase for this spectrum of 2D quantum materials.

Motivated by the prospect of a two-dimensional square-lattice geometry for semiconductor spin qubits, we explore the realization of the parity architecture with quantum dots. We present sequences of spin shuttling and quantum gates that implement the parity quantum approximate optimization algorithm (QAOA) on a lattice constructed of identical unit cells, such that the circuit depth is always constant. We further develop a detailed error model for a hardware-specific analysis of the parity architecture, and we estimate the errors during one round of parity QAOA. The model includes a general description of the shuttling errors as a function of the probability distribution function of the valley splitting, which is the main limitation for the performance. We compare our approach to a superconducting transmon qubit chip, and we find that with high-fidelity spin shuttling the performance of the spin qubits is competitive or even exceeds the results of the transmons. Finally, we discuss the possibility of decoding the logical quantum state and of quantum error mitigation. We find that already with near-term spin qubit devices, a sufficiently low physical error probability can be expected to reliably perform parity QAOA with a short depth in a regime where the success probability compares favorably to standard QAOA.

Spiral spin liquids possess a subextensively degenerate ground-state manifold, represented by a continuum of energy minima in reciprocal space. Since a small change of the spiral state wave vector requires a global change of the spin configuration in real space, it is a priori unclear how such systems can fluctuate within the degenerate ground-state manifold. Only recently it was proposed that momentum vortices are responsible for the liquidity of the spiral phase and that these systems are closely related to an emergent rank-2 U(1) gauge theory H. Yan et al. [Phys. Rev. Res. 4, 023175 (2022)]. As a consequence of this gauge structure, fourfold pinch-point singularities were found in a generalized spin correlator. In this paper, we use classical Monte Carlo and molecular dynamics calculations to embed the previously studied spiral spin liquid into a broader phase diagram of the square lattice XY model. We find a multitude of unusual phases and phase transitions surrounding the spiral spin liquid such as an effective four-state Potts transition into a collinear double-striped phase resulting from the spontaneous breaking of two coupled ${\mathbb{Z}}_2$ symmetries. Since this phase is stabilized by entropic effects selecting the momenta away from the spiral manifold, it undergoes a second phase transition at low temperatures into a nematic spiral phase which only breaks one ${\mathbb{Z}}_2$ symmetry. We also observe a region of parameters where the phase transition into the spiral spin liquid does not break any symmetries and where the critical exponents do not match those of standard universality classes. We study the importance of momentum vortices in driving this phase transition and discuss the possibility of a Kosterlitz-Thouless transition of momentum vortices. Finally, we explore the regime where the rank-2 U(1) gauge theory is valid by investigating the fourfold pinch-point singularities across the phase diagram.

Single crystals of the quasi-one-dimensional ${\mathrm{PrAu}}_2{\mathrm{In}}_4$ have been successfully grown and studied down to very low temperatures. Consistent with the singlet spin-orbit ground state of ${\mathrm{Pr}}^{3+}$, no magnetic ordering is observed in this compound. Nevertheless, ${\mathrm{PrAu}}_2{\mathrm{In}}_4$ reveals a strong magnetic instability arising from the nearly degenerate ground and low-lying singlets that are spaced by only 9 K (termed the pseudodoublet), laying a ground for the competition among crystal field energy, magnetic exchange interaction, and Kondo physics. Moderate Kondo effect comparable to the pseudodoublet spacing is evidenced by a comparative investigation of specific heat and thermopower between ${\mathrm{PrAu}}_2{\mathrm{In}}_4$ and ${\mathrm{LaAu}}_2{\mathrm{In}}_4$. Notably, at $T>2\mathrm{K}$, ${\mathrm{PrAu}}_2{\mathrm{In}}_4$ can be smoothly tuned by field to a strange-metal state near 1 T that is reminiscent of quantum critical fluctuations. At the lowest temperatures of $T<1\mathrm{K}$, however, such quantum fluctuations are quenched and the compound behaves consistently as a Fermi liquid. The strange-metal behavior visible solely at elevated temperatures is considered generic to Kondo lattices with pseudodoublet ground state of non-Kramers ions, characterized by closely spaced singlets.

Ferroelectric (FE) Rashba semiconductors, a class of multifunctional materials with potential applications in spintronic devices, have attracted increasing interests recently. Herein, we employ first-principles calculations and the k·p Hamiltonian method to comprehensively investigate the Rashba effect in $MX/\alpha \text{-I}{\mathrm{n}}_2\mathrm{S}{\mathrm{e}}_3$ ($MX=\mathrm{GaTe}$ and InSe) van der Waals (vdW) heterobilayers and reveal the mechanism underlying the FE manipulation of Rashba spin splitting. Remarkably, the strength of spin splitting in the $\mathrm{GaTe}/\alpha \text{-I}{\mathrm{n}}_2\mathrm{S}{\mathrm{e}}_3$ heterobilayer has been significantly enhanced several times with respect to the intrinsic $\alpha \text{-I}{\mathrm{n}}_2\mathrm{S}{\mathrm{e}}_3$ monolayer. This enhancement is attributed to the effective interfacial electric field contributed from the strong interfacial charge transfer and mirror symmetry breaking. Furthermore, the symmetric and asymmetric $\alpha \text{-I}{\mathrm{n}}_2\mathrm{S}{\mathrm{e}}_3/\mathrm{GaTe}/\alpha \text{-I}{\mathrm{n}}_2\mathrm{S}{\mathrm{e}}_3$ sandwiched structures with four switchable states verify that Rashba spin splitting can be effectively tuned by the FE switch, and its enhancement is achievable if the mirror symmetry is not preserved. Interestingly, spin Hall conductivity can also be manipulated by the spin-orbit coupling associated with the intensified interfacial charge transfer. Our findings highlight the appealing potential of vdW heterostructures as an ideal platform for expanding the family of FE Rashba semiconductors and further promoting their applications in spintronics.

van der Waals (vdW) stacking of two-dimensional (2D) metals and 2D semiconductors has attracted significant interest in metal-semiconductor junctions (MSJs). Unfortunately, the vdW gap always leads to large tunneling barriers even in ohmic contacts. Herein, by constructing 2D electrides possessing sufficient electron gas at the surface, the formation of quasibonds at MSJ interface is expected to overcome the challenge of contact resistance induced by vdW gap. Specifically, 2D $\mathrm{C}{\mathrm{a}}_{2}X{Y}_2$ ($X=\mathrm{Ti},\mathrm{Zr},\mathrm{Hf}$; $Y=\mathrm{N},\mathrm{P}$) electrides possess ultralow work functions ranging from 3.28 to 3.90 eV, accompanied by nearly free electrons on the surface, rendering them efficient electron donors. Taking typical 2D semiconductor $\mathrm{Mo}{\mathrm{S}}_2$ to contact $\mathrm{C}{\mathrm{a}}_{2}X{Y}_2$, the ohmic contact and complete tunneling effect can be achieved. Application of a modest bias voltage yields a noticeable current density of about $0.6\mu \mathrm{A}/{\mathrm{\AA }}^2$. Moreover, these MSJs exhibit superior environmental stability with bromine terminated. Our work not only offers a series of promising 2D electrides, but also paves the way for advancing the progress of 2D electronic and optoelectronic devices.

Photonic nanoparticle arrays that exhibit topologically nontrivial phases are often established by arranging nanoparticles in a lattice configuration in analog to the Su-Schrieffer-Heeger model. This design strategy is based on the principle that the optical interactions among the nanoparticles are predominantly dictated by the distances between them. However, these interactions are also significantly shaped by the near-field profiles of the nanoparticles. As an exemplary case, we examine a zigzag array of metallic ellipsoidal nanoparticles (ENPs) and illustrate how the topological phase of coupled plasmonic waves in such an array can be manipulated by controlling the orientation of the ENPs. More intriguingly, the near-field characteristics of the ENPs can be fine-tuned by the presence of a substrate, suggesting an alternative avenue for modulating the inter-particle interactions. We further show that topological phase transitions in both one-dimensional (1D) and 2D ENP arrays can be triggered by introducing a nearby dielectric substrate, while maintaining the original lattice structure unchanged. Our findings reveal a different mechanism for modulating the topological phases of nanoparticle arrays and provide a strategy that could be extrapolated to other photonic and classical wave systems, suggesting possibilities for the design and control of topological states in carefully engineered photonic systems.

We propose a family of exactly solvable quasiperiodic lattice models with analytical complex mobility edges, which can incorporate mosaic modulations as a straightforward generalization. By sweeping a potential tuning parameter $\delta$, we demonstrate a kind of interesting butterflylike spectra in a complex energy plane, which depicts energy-dependent extended-localized transitions sharing common exact non-Hermitian mobility edges. Applying Avila's global theory, we are able to analytically calculate the Lyapunov exponents and determine the mobility edges exactly. For the minimal model without mosaic modulation, we obtain a compactly analytic formula for the complex mobility edges, which indicates clearly mobility edges having a loop structure in the complex energy plane. Together with an analytical estimation of the range of the complex energy spectrum, we can obtain the true mobility edge. The non-Hermitian mobility edges are further verified by numerical calculations of the fractal dimension and spatial distribution of the wave functions. Tuning the parameters of non-Hermitian potentials, we also investigate the variations of the non-Hermitian mobility edges and the corresponding butterfly spectra, which exhibit a richness of spectrum structures.

The layered triangular lattice owing to $1:2$ order of $B$ and $B^{\prime }$ sites in the triple perovskite $A_{3}B{B}_2^{\prime }{\mathrm{O}}_9$ family provides an enticing domain for exploring the complex phenomena of quantum spin liquids (QSLs). We report a comprehensive investigation of the ground-state properties of ${\mathrm{Sr}}_3{\mathrm{CuTa}}_2{\mathrm{O}}_9$ that belongs to the above family by employing magnetization, specific heat, and muon spin relaxation $\left(\mu \mathrm{SR}\right)$ experiments down to the lowest temperature of 0.1 K. Analysis of the magnetic susceptibility indicates that the spin lattice is a nearly isotropic $S=1/2$ triangular lattice. We illustrate the observation of a gapless QSL in which conventional spin ordering or freezing effects are absent, even at temperatures more than two orders of magnitude smaller than the exchange energy $(J_{\mathrm{CW}}/k_{\mathrm{B}}\simeq -5.04 \mathrm{K})$. Magnetic specific heat in zero field follows a power law, $C_{\mathrm{m}}\sim T^{\eta }$, below 1.2 K with $\eta \approx 2/3$, which is consistent with a theoretical proposal of the presence of a spinon Fermi surface. Below 1.2 K, the $\mu \mathrm{SR}$ relaxation rate shows no temperature dependence, suggesting persistent spin dynamics, as expected for a QSL state. Delving deeper, we also analyze longitudinal field $\mu \mathrm{SR}$ spectra, revealing strong dynamical correlations in the spin-disordered ground state. All of these highlight the characteristics of spin entanglement in the QSL state.

We show how the ${\mathbb{Z}}_2$ topological index of a one-dimensional topological $p$-wave superconductor can be revealed when driving with a classical vector potential, i.e., an electromagnetic wave, through the light-induced transition probabilities and the profile of the induced quasiparticle population. As a function of driving frequency $\omega$, it is possible to obtain a measure of this topological invariant from the resonance envelope classifying the two distinct topological phases of the short-range Kitaev wire. We propose to probe the topological phase transition in the model through the responses of the global capacitance in the presence of the light field and through the Josephson current between the wire and the proximity coupled bulk superconductor. The system may also be implemented on the Bloch sphere allowing alternative ways to measure the $\mathbb{Z}$ and ${\mathbb{Z}}_2$ topological invariants through circuit or cavity quantum electrodynamics.