The structural and magnetic properties of sputter-deposited ${\mathrm{Fe}}_{100-x}{\mathrm{Ru}}_x$ films were studied for $x\le 50$. The crystal structure of ${\mathrm{Fe}}_{100-x}{\mathrm{Ru}}_x$ is shown to be predominantly body-centered cubic for $x<13$ and to undergo a gradual transition to hexagonal close-packed in the Ru concentration range $13\lesssim x\lesssim 20$. Magnetic measurements indicate that the addition of Ru to Fe gives rise to a noncollinear magnetic alignment between Fe atoms in the body-centered cubic FeRu alloys, while the hexagonal close-packed FeRu alloys exhibit paramagnetic behavior. A simple atomistic model was used to show that the competition between ferromagnetic coupling of neighboring Fe atoms and antiferromagnetic coupling of Fe atoms across Ru atoms in cubic FeRu structures can induce noncollinear magnetic order. Magnetic multilayer structures used in thin-film magnetic devices make extensive use of both Fe and Ru layers. Our results reveal that the presence of even a small amount of Ru in Fe influences the magnetic order of Fe, which could impact the performance of these devices.

Understanding the stability of strongly correlated phases of matter when coupled to environmental degrees of freedom is crucial for identifying the conditions under which these states may be observed. Here, we focus on the paradigmatic one-dimensional Bose-Hubbard model, and study the stability of the Luttinger-liquid and Mott insulating phases in the presence of local particle exchange with site-independent baths of noninteracting bosons. We perform a numerically exact analysis of this model by adapting the recently developed wormhole quantum Monte Carlo method for retarded interactions to a continuous-time formulation with worm updates; we show how the wormhole updates can be easily implemented in this scheme. For an Ohmic bath, our numerical findings confirm the scaling prediction that the Luttinger-liquid phase becomes unstable at infinitesimal bath coupling. We show that the ensuing phase is a long-range-ordered superfluid with spontaneously broken U(1) symmetry. While the Mott insulator remains a distinct phase for small bath coupling, it exhibits diverging compressibility and noninteger local boson occupation in the thermodynamic limit. Upon increasing the bath coupling, this phase undergoes a transition to a long-range-ordered superfluid. Finally, we discuss the effects of super-Ohmic dissipation on the Luttinger-liquid phase. Our results are compatible with a stable dissipative Luttinger-liquid phase that transitions to a long-range-ordered superfluid at a finite system-bath coupling.

The fractional quantum anomalous Hall (FQAH) effect in rhombohedral pentalayer graphene (PLG) has attracted significant attention due to its potential for observing exotic quantum states. In this work, we present a self-consistent Hartree-Fock theory for the FQAH effect in rhombohedral PLG. In particular, we focus on the convergence of the Hartree-Fock calculation with various reference fields and discuss the stability of the FQAH states in PLG. We show that the so-called charge neutrality scheme provides an unambiguous result for the Hartree-Fock calculation, as it ensures a convergence with respect to the momentum cutoff. Based on the Hartree-Fock band structure, we further carry out exact diagonalization calculations to explore the stability of the FQAH states in PLG. Our work provides an improved and unified (minimal) theoretical framework to understand the FQAH effect in rhombohedral PLG and paves the way for future studies.

The cooperation of electronic correlation and spin-orbit coupling can stabilize magnetic topological insulators which host novel quantum phenomena such as the quantum anomalous Hall state also known as Chern insulator (CI). Here, we investigate the existence of magnetic topological insulators with antiferromagnetic (AF) order in heavy-fermion materials. Our analysis relies on the half-filled Kane-Mele-Kondo (KMK) model with the AF Kondo interaction $J_K$ coupling the spin of itinerant electrons with a $S=1/2$ localized spin at each lattice site. We consider the Néel AF ordering with the local magnetization not only perpendicular ($z\text{−}\mathrm{AF}$ ordering) but also parallel ($xy\text{−}\mathrm{AF}$ ordering) to the honeycomb plane. We show that in the absence of an energy offset between the two sublattices of the honeycomb structure, the system is always topologically trivial. There is a transition from the trivial $xy\text{−}\mathrm{AF}$ insulator ($xy\text{−}\mathrm{AFI}$) to the trivial Kondo insulator (KI) upon increasing $J_K$. We unveil that an alternating sublattice potential can lead to the stabilization of the $z\text{−}\mathrm{AFCI}$ and the $z\text{−}\mathrm{AF}$ quantum spin Hall insulator ($z\text{−}\mathrm{AFQSHI}$). We address the charge excitations in the bulk as well as at the edges of the KMK model. We provide a systematic comparison between the size of the charge gap in the AFCI in heavy-fermion materials and the size of the charge gap in the AFCI in transition-metal compounds. Our findings can guide future experimental studies searching for AF topological insulators in a novel class of systems which can survive up to higher temperatures.

We present a theory of interacting valence holes in a gate-defined one-dimensional quantum channel in a single layer of a transition metal dichalcogenide material ${\mathrm{WSe}}_2$. Based on a microscopic atomistic tight-binding model and Hartree-Fock and exact configuration-interaction tools, we demonstrate the possibility of symmetry-broken valley polarized states for strongly interacting holes. The interplay between interactions, perpendicular magnetic field, and the lateral confinement asymmetry together with the strong Rashba spin-orbit coupling present in ${\mathrm{WSe}}_2$ material is analyzed, and its impact on valley polarization is discussed. For weaker interactions, an investigation of the pair correlation function reveals a valley-antiferromagnetic phase. For low hole densities, a formation of a zigzag Wigner crystal phase is predicted. The impact of various hole liquid phases on transport in a high mobility quasi-one-dimensional channel is discussed.

We have investigated in-plane magnetotransport properties of trigonal layered ${\mathrm{PtBi}}_2$, which was proposed to be a triply degenerate semimetal. A remarkable planar Hall effect was observed in both ${\mathrm{PtBi}}_2$ bulk and nanoflake samples. We attribute the planar Hall effect in the bulk sample mainly to a chiral anomaly, manifested by the transport signatures of large negative longitudinal magnetoresistance and the quadratic field dependence of magnetoresistance for the field is parallel to and perpendicular to the current case, respectively. For ${\mathrm{PtBi}}_2$ nanoflake, however, further analysis suggests that the in-plane anisotropic orbital resistance rather than the chiral anomaly may play a dominant role on the planar Hall effect. Our results not only give insight into the electronic properties of chiral fermions in triply degenerate semimetal ${\mathrm{PtBi}}_2$, but also provide a promising platform for understanding the mechanism of the planar Hall effect.

${\mathrm{EuAl}}_4$ is proposed to host a topological Hall state. This material also undergoes four consecutive antiferromagnetic (AFM) transitions upon cooling below ${\mathrm{T}}_{N1}=15.4\mathrm{K}$ in the presence of charge density wave (CDW) order that sets in below ${\mathrm{T}}_{\mathrm{CDW}}=140\mathrm{K}$. We use angle-resolved photoemission spectroscopy and density-functional-theory calculations to study how magnetic ordering affects the electronic properties in ${\mathrm{EuAl}}_4$. We found changes in the band structure upon each of the four consecutive AFM transitions including band splitting, renormalizations, and appearance of new bands forming additional Fermi sheets. In addition we also found significant enhancement of the quasiparticles' lifetime due to suppression of spin flip scattering, similar to what was previously reported for ferromagnetic ${\mathrm{EuCd}}_2{\mathrm{As}}_2$. Surprisingly, we observe that most significant changes in electronic properties occur not at ${\mathrm{T}}_{N1}$, but instead at the AFM3 to AFM4 transition, which coincides with the largest drop in resistivity.

Bismuthene is a promising candidate for ultrafast optics and topological applications due to its exceptional properties. In this paper, the optical, electronic, and mechanical properties of monolayer bismuthene modulated by uniaxial and biaxial strain were theoretically investigated using a first-principles approach. The results reveal that the effects of spin-orbit coupling, energy bands, transition dipole moments, and joint density of states combine to cause shifts of optical absorption peaks under strain. Bismuthene can remain stable under a wide range of strains, with the Young's modulus exhibiting consistent variations to the Poisson's ratio. Additionally, a negative Poisson's ratio of bismuthene was displayed under significant tensile strains. These findings provide insights into modulating the optoelectronic and mechanical properties of bismuthene nanomaterials to meet the demands of advanced devices and applications.

Synthetic antiferromagnetic (SAF) multilayers offer a highly versatile platform for skyrmionic devices, providing skyrmions with exceptional mobility and a negligible skyrmion Hall effect (SkHE). However, detecting antiferromagnetic skyrmions is challenging due to their antiparallel spin alignment, and their behavior within asymmetrically parametrized ferromagnetic layers remains unclear. Here, we theoretically investigate the stability and transport properties of antiferromagnetically coupled skyrmions in multilayer systems comprising two ferromagnetic layers with distinct interfacial Dzyaloshinskii-Moriya interactions (DMIs). We show that coupled skyrmion pairs of opposite charge are stable over broad parameter ranges, even when the DMI vectors differ in both magnitude and sense. Our investigation into the transport properties reveals that the SkHE cancellation due to the topological neutrality of the skyrmion pair is robust against imbalances in several material parameters, including DMI and Gilbert damping, but is highly sensitive to imbalance in the current densities and saturation magnetizations. This suggests that synthetic ferrimagnets are generally ineffective in eliminating the SkHE. In contrast, we show that magnetically compensated SAF stacks with antiparallel DMI vectors not only eliminate the SkHE but also facilitate skyrmion detection.

We investigate the effect of a magnetic field on the extended Kitaev spin-liquid state through phonon dynamics. Using a constrained fermionic self-consistent mean field method, we analyze the quantum spin liquid (QSL) ground state for the extended Kitaev model with both the Zeeman term and the perturbative three-spin interaction term $\kappa$. Our results demonstrate the dependence of the stability of the Kitaev QSL state on the field direction, consistent with findings in the literature. Additionally, we calculate the phonon dynamics for acoustic phonons coupled to the Majorana fermion excitations of the Kitaev spin-liquid state, discussing the temperature and field evolution of these quantities.

The observation of large intrinsic anomalous Hall conductivity (AHC) in the noncollinear antiferromagnetic phase of the Weyl semimetal ${\mathrm{Mn}}_3\mathrm{Sn}$ generates enormous interest in uncovering the entanglement between the real-space magnetic ordering and the momentum-space band structure. Previous studies show that changes in the magnetic structure induced by the application of hydrostatic and chemical pressure can significantly affect the AHC of the ${\mathrm{Mn}}_{3+x}{\mathrm{Sn}}_{1-x}$ system. Here, we employ the muon spin relaxation/rotation (${\mu }^{+}\mathrm{SR}$) technique to systematically investigate the evolution of different magnetic states in the ${\mathrm{Mn}}_{3+x}{\mathrm{Sn}}_{1-x}$ as a function of hydrostatic and chemical pressure. We find two muon sites experimentally, which is also supported by our ab initio calculations. Our ${\mu }^{+}\mathrm{SR}$ experiments affirm that the $x=0.05$ compound exhibits a commensurate magnetic state throughout the magnetically ordered phase below the Neel temperature $T_N\approx 420$ K in ambient pressure. In contrast, we observe an incommensurate magnetic state below $T_{IC}\sim 175$ K when a hydrostatic pressure of 1.5 GPa is applied. A similar transition from the commensurate to incommensurate state is also found with chemical pressure for $x=0.04$ and $x=0.03$, using ${\mu }^{+}\mathrm{SR}$ and elastic neutron scattering experiments. Using band structure calculations, we have shown the emergence of Fermi nesting in ${\mathrm{Mn}}_3\mathrm{Sn}$ and the subsequent development of incommensurate magnetic ordering under hydrostatic/chemical pressure.

Telegraph noise caused by frequent switching of the magnetization in small magnetic devices has become a useful resource for probabilistic computing. Conventional theories have been based on a linearization of the fluctuations at the extrema of the magnetic free energy. We show theoretically that the nonlinearities, specifically four-magnon scatterings, reduce the equilibrium fluctuation amplitude of the magnetization as well as the switching frequencies between local minima via the decay of the homogeneous Kittel mode into two spin waves with opposite momenta. Selectively suppressing the effective temperature of the finite-$k$ spin waves or reducing the radius of a thin magnetic disk enhance the switching frequency and improve performance of magnetic tunnel junctions in probabilistic computing applications.

An indispensable ingredient for pair density wave (PDW) superconductivity is the presence of an attractive pairing interaction at finite momentum. Here, we show how this condition can be met with straightforward electron-density interactions in multiband systems. The electron-density interaction, when projected to the band basis, acquires form factors with nontrivial momentum dependence and thereby exhibits a potential tendency to a finite-momentum pairing instability. By applying a mean field analysis to two simple multiband models, the checkerboard lattice and three-band Hubbard model, we find that PDW order can indeed become the leading instability if a strong nearest-neighbor attraction is present. Moreover, the condition for the transition from a uniform superconductor to a PDW superconductor is shown via a simple quantum geometric argument.

Photonic bound states in the continuum (BICs) have been identified as vortex polarization singularities within the momentum space, revealing their captivating topological traits. In contrast, the topological characteristics of acoustic BICs remain largely unexplored. In this paper, we delve into the behavior of BICs within an acoustic Gires-Tournois interferometer (GTI) setting. The underlying physics of BICs formation is illuminated through rigorous analyses of the structure's eigenfields, combining theoretical insights with numerical simulations. Leveraging our eigenfield analyses, we establish that BICs are associated with phase singularities in the out-of-plane radiation, represented by phase winding numbers that act as topological indices, thereby conferring robustness against perturbations in the system's structural parameters. Additionally, we introduce a temporal coupled-wave theory to dissect the phase interplay between total reflection fields and resonant radiation fields under the influence of incident waves. Harnessing this relationship enables the extraction of resonant radiation-field phase information from measured total reflection fields, offering experimental validation that acoustic BICs constitute topological phase singularities. Ultimately, both numerical and experimental results indicate that the acoustic GTI is capable of sustaining phase resonances with high-quality factors and group delays, suggesting significant potential for applications in dispersion engineering.

Polytypism of incommensurately modulated structures was hitherto unobserved. Here, it was found in solid bromine and iodine upon molecular dissociation under pressure up to 112 GPa. Single-crystal synchrotron x-ray diffraction in laser-heated diamond anvil cells revealed a number of allotropes of bromine and iodine, including polytypes of Br-IIIγ $(Fmmm\left(00\gamma \right)s00)$ with γ varying within 0.18 to 0.3, coexisting at a given pressure. This finding opens a research area in the field of solid-solid phase transitions, disordered states of solid matter, and high-pressure structural studies.

The Kitaev honeycomb model provides a paradigmatic example of an exactly solvable quantum spin liquid (QSL), where spins fractionalize into itinerant Majorana fermions coupled to a static background of ${\mathbb{Z}}_2$ gauge fluxes. This model has attracted significant interest due to its potential experimental realization in spin-orbit Mott insulators like $\alpha \text{−}{\mathrm{RuCl}}_3$. Among various experimental techniques, ultrasound measurements of sound attenuation have emerged as a promising approach to detect spin fractionalization in these materials. However, deviations from the ideal Kitaev model, often due to disorder, introduce localized modes that dominate the low-energy physics. To investigate these effects, we calculate the sound attenuation coefficient in the site-disordered Kitaev honeycomb model under an applied magnetic field that breaks time-reversal symmetry. Our analysis reveals that quasilocalized modes from quasivacancies impact the phonon self-energy, yet the sound attenuation coefficient maintains its sixfold symmetry and linear temperature dependence at low temperatures, even with disorder. This robustness underscores sound attenuation's reliability as a probe for spin fractionalization. Furthermore, we show that this linear behavior persists under an external field and in the random-flux sector, highlighting the persistent influence of fractionalized excitations, despite disorder.

Compass models provide insights into the properties of Mott-insulating materials that host bond-dependent anisotropic interactions between their pseudospin degrees of freedom. In this article, we explore the classical and quantum ground-state properties of one such model relevant to certain layered perovskite materials akin to ${\mathrm{Ba}}_2{\mathrm{IrO}}_4$—namely, the Heisenberg-compass model on the square lattice. We first investigate the ground-state phase diagram of this model using classical Monte Carlo simulations. These reveal that the low-temperature classical phase diagram is divided into six different classes of long-range ordered phases, including four phases that exhibit an order by disorder selection and two phases that are stabilized energetically. This model admits a special duality transformation, known as the Klein duality, conveniently allowing to map one region of coupling parameters onto another and constraining the phase diagram, and which we exploit in our study. From the analysis of the zero-point energy and the free energy of the spin waves, we find that order by quantum disorder at zero temperature and order by thermal disorder select the same orderings as those found from classical Monte Carlo simulations. We further investigate the quantum ground states of this model using numerical exact diagonalization on small clusters by exploiting the translational symmetry of the square lattice. We obtain a ground-state phase diagram bearing close resemblance to that found from the classical analysis.

Magnetic potential energy surface is crucial for understanding magnetic materials. This study introduces a time-reversal $E\left(3\right)$-equivariant neural network and physics-informed SpinGNN++ framework for constructing interatomic potentials for magnetic systems, encompassing spin-orbit coupling and noncollinear magnetic moments. SpinGNN++ integrates multitask spin equivariant neural network with explicit spin-lattice terms and time-reversal equivariant neural network to learn high-order spin-lattice interactions using time-reversal $E\left(3\right)$-equivariant convolutions. A complex magnetic model data set is introduced as a benchmark and employed to demonstrate its capabilities. SpinGNN++ provides accurate descriptions of the complex spin-lattice coupling in monolayer ${\mathrm{CrI}}_3$ and ${\mathrm{CrTe}}_2$, achieving sub-meV errors and facilitates large-scale parallel spin-lattice dynamics, thereby enabling the exploration of associated properties, including magnetic ground state and phase transition. Remarkably, SpinGNN++ identifies a differentferrimagnetic state as the ground state for monolayer ${\mathrm{CrTe}}_2$, thereby enriching its phase diagram and providing deeper insights into the distinct magnetic signals observed in various experiments.