The presence of magnetism in potentially altermagnetic ${\mathrm{RuO}}_2$ has been a subject of intense debate. Using broadband infrared spectroscopy combined with density-functional band-structure calculations, we show that the optical conductivity of ${\mathrm{RuO}}_2$, the bulk probe of its electronic structure, is best described by a nonmagnetic model. The sharp Pauli edge demonstrates the presence of a Dirac nodal line lying 45 meV below the Fermi level. An excellent match between the experimental and ab initio plasma frequencies underpins the weakness of electronic correlations. The intraband part of the optical conductivity indicates Fermi-liquid behavior with two distinct scattering rates below 150 K. Fermi-liquid theory also accounts for the temperature-dependent magnetic susceptibility of ${\mathrm{RuO}}_2$ and allows a consistent description of this material as a paramagnetic metal.

Weyl nodal loop semimetals are gapless topological phases that, unlike their insulator counterparts, may be unstable to small perturbations that respect their topology-protecting symmetries. Here, we analyze a clean system perturbed by chiral off-diagonal disorder using numerically exact methods. We establish that the ballistic semimetallic phase is unstable towards the formation of an unconventional topological diffusive metal hosting topological multifractal surface states. Although, as in the clean case, surface states are exponentially localized along the direction perpendicular to the nodal loop, disorder induces a multifractal structure in the remaining directions. Surprisingly, the number of these states also increases with a small amount of disorder. Eventually, as disorder is further increased, the number of surface states starts decreasing.

Electrical spin orientation is the generation of electron spin proportional to the electric current. This phenomenon is allowed by symmetry in gyrotropic systems, e.g., in inversion-asymmetric structures with Rashba spin-orbit splitting. Here we develop a theory of electrical spin orientation for magnetic two-dimensional heterostructures. Spin-orbit coupled graphene and semiconductor heterostructures proximitized by ferromagnets are considered. The analytical theory is based on the Boltzmann kinetic equation for a spin-dependent distribution function and collision integral. We show that the induced spin demonstrates the Hanle effect: a direction of the spin depends on the out-of-plane magnetization. Importantly, the Hanle effect is extremely sensitive to the details of electron elastic scattering. In semiconductor heterostructures, the effect of magnetization is present for scattering by long-range disorder and absent for short-range scattering. In spin-orbit-coupled graphene, the Hanle effect occurs at any scattering potential, but the direction of the spin strongly changes with variation of the disorder type. The theory also describes the effect of valley-Zeeman splitting on the electrical spin orientation in graphene, where the spin experiences opposite Hanle effects in two valleys.

Anisotropy endows topological aspects in optical systems and furnishes a platform to explore non-Hermitian physics, which can be harnessed for the polarization-selective amplification of light. Here, we show a zero-threshold Raman laser can be achieved in an anisotropic optical microcavity via polarization-controlled optical pumping. A loss-gain mechanism between two polarized Stokes modes arises naturally via polarization-dependent stimulated scattering and anisotropic Raman gain of the active layered material inside the microcavity. A parity-time symmetric Hamiltonian has been proposed to explain the emergence of a single polarization mode, essential for achieving a zero-threshold lasing condition. Additionally, intensity correlation measurements of the Stokes modes validate the coherence properties of the emitted light. Our realization of the zero-threshold Raman laser in anisotropic microcavity can open up a new research direction exploring non-Hermitian and topological aspects of light in anisotropic two-dimensional materials.

Ferroic materials with multiple order parameters hold promise for enhanced electrocaloric effect (ECE), but the existence of some order parameters can be detrimental, such as octahedral tiltings in perovskites that are coupled competitively with polarization. In multiferroic ${\mathrm{BiFeO}}_3$, the magnitude of the antiphase octahedral tilting can be tuned by the application of a biaxial strain, in particular that the tilting can be completely annihilated in the super-tetragonal phase ($T$ phase). Here, we study the ECE of strained $R3c$ and $T$ phases of ${\mathrm{BiFeO}}_3$ using an effective Hamiltonian approach. ECE in the $T$ phase is indeed found to be improved compared with that of the $R3c$ phase. The effects of strain on the adiabatic temperature change in both phases under bipolar fields in [001] and $\left[00\overline{1}\right]$ directions, as well as the ECE associated with polarization switching, are reported in details.

As a distinctive scheme towards topological phases with unconventional properties, square-root topological insulators have garnered great interest in recent years. Here, we investigate the topological phases of square-root manipulations on an extended Su-Schrieffer-Heeger (SSH) model with long-range hoppings. By calculating the energy spectra, topological boundary states, and winding number, we demonstrate that the square-root operation allows for the symmetric distribution of quadruple degenerate boundary states in the doubled bulk gaps. These states are inherited from the original extended SSH model and can be characterized by a high winding number $w=2$. Finally, we propose a feasible scheme for realizing our model in an electric circuit system. Our work provides a scheme for the discovery and systematic construction of new topological states.

Utilizing the phenomenological theory based on crystal symmetry operation, we have established the complete angular dependencies of the galvanomagnetic effects, encompassing both anisotropic magnetoresistance (AMR) and the planar Hall effect (PHE), for the ferromagnetic films with $C_{4v}$ symmetry. These dependencies were experimentally confirmed via comprehensive angular mapping of AMR and PHE in single-crystal Fe(001) films at room temperature. We demonstrated that the intrinsic magnetization-induced effects are independent of the field strength by carefully separating the field-induced and magnetization-induced galvanomagnetic effects. Our theoretical and experimental findings highlight the absence of in-plane fourfold angular dependence in PHE, a feature prohibited by the Onsager relation in systems with $C_4$ symmetry. This study affirms that the universal angular dependencies of AMR and PHE in single crystals can be accurately predicted by the conventional phenomenological theory.

We present the thermal Stoner-Wohlfarth (tSW) model and apply it in the context of molecular dynamics simulations. The model is validated against an ensemble of immobilized, randomly oriented uniaxial particles (solid superparamagnet) and a classical dilute ferrofluid for different combinations of anisotropy strength and magnetic field/moment coupling, at a fixed temperature. We compare analytical and simulation results to quantify the viability of the tSW model in reproducing the equilibrium properties (with and without dipole-dipole interactions) and dynamic properties (without dipole-dipole interactions) of magnetic soft matter systems. We show that if the anisotropy of a particle is more than five times higher than the thermal fluctuations, the tSW model is applicable and efficient. This approach allows one to consider the interplay between Néel and Brownian relaxation, often neglected in the fixed point-dipole representation-based magnetic soft matter theoretical investigations.

Investigating the scrambling of quantum information (also known as quantum chaos) in many-body quantum systems provides valuable insights into the postquench evolution of a system. While various methods are available to understand the chaotic dynamics of these quantum systems, scrambling of mutual information following a quench has recently also emerged as a diagnostic of chaotic dynamics. In this work, we consider a chain of spin-$1/2$ particles of a finite length $L$ evolving with the mixed-field Ising Hamiltonian and impose open boundary conditions. We simulate the time evolution of entanglement entropy and mutual information following a quench from the Néel state in this system using tensor networks and find that the entanglement entropy for nonintegrable systems saturates to a constant value at late times, while it continues to oscillate for integrable systems. We also find that mutual information peaks as a function of distance between intervals decay faster for nonintegrable systems compared to integrable systems, in agreement with the results in literature for XXZ chains. We compare the oscillations in entanglement entropy evolution obtained from simulations in the integrable case with analytic results from the quasiparticle picture and find agreement.

Quantum spin Hall insulators (QSHIs) leverage strong spin-orbit coupling (SOC) for efficient spin manipulation, making them promising for spintronics. In this study, we investigate the noncollinear Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between two magnetic impurities in a perturbed Kane-Mele model with strong SOC, relevant to monolayer jacutingaite ${\mathrm{Pt}}_2{\mathrm{HgSe}}_3$ as a prominent QSHI. Following the previous studies that mainly focused on the model and its general applications, we provide a systematic examination of the effects of various perturbations and strong spin-orbit hybridizations, which drive phase transitions that have not been extensively explored before. By incorporating these perturbations into the model and accurately accounting for spin-orbit hybridizations through spin-space Green's functions and the RKKY interactions, we uncover distinct, relative (rather than absolute) signatures of different phase transitions. These phase transitions are induced by both static and dynamic perturbations on the magnetic impurities. Notably, we identify additional phases emerging from the interplay with the magnetic substrate. All these influence the switching between ferromagnetic and antiferromagnetic, as well as clockwise and counterclockwise magnetic interactions. Our results provide a practical way to track topological phases through magnetic properties, offering new insights into phase control and spin manipulation in QSHIs.

We attain a renormalized and iterative expression of the Andreev level in a quantum-dot Josephson junction, which is universal and has advantages as follows. The renormalized form of the Andreev level not only allows us to extend beyond the limitations of small tunnel coupling, quantum-dot energy level and magnetic field, as well as mean-field Coulomb interaction but also enables the capturing of subgap levels that leak out of the superconducting gap into the continuous spectrum. These leaked subgap levels are highly tunable by gate, phase, and field parameters and play a significant role in the novel phenomena and remarkable properties of the superconductor. Furthermore, the iterative form of the Andreev level provides an intuitive understanding of the spin-split and superconducting proximity effects of the superconducting leads. We find a singlet-doublet quantum phase transition (QPT) in the ground state due to the intricate competition between the superconducting and spin-split proximity effects, that differs from the typical QPT arising from the competition between the superconducting proximity effect (favoring singlet phase) and the quantum dot Coulomb interaction (favoring doublet phase). This QPT has a diverse phase diagram owing to the spin-split proximity effects which favors the doublet phase akin to the quantum-dot Coulomb interaction but can be also enhanced by the tunneling coupling like the superconducting proximity effect. Unlike the typical QPT, where tunnel coupling prefers a singlet ground state, this novel QPT enables strong tunnel coupling to suppress the singlet ground state via the spin-split proximity effect, thus allowing a singlet-doublet-singlet transition with increasing tunnel coupling. Our renormalized and iterative formalism of the Andreev level is crucial for the electrostatic gate, external flux, and magnetic field modulations of the Andreev qubits.

Graphyne and its family members are two-dimensional (2D) carbon allotropes with both $sp$ and $s{p}^2$ hybridizations. Authors of recent investigations have highlighted the potential of graphyne as a promising material for exploring both higher-order topological insulators (HOTIs) and semimetals. In this paper, we report the experimental observation of the corner states in 2D graphyne circuit network, which are protected by spatiotemporal inversion symmetry and characterized by the bulk polarization. Furthermore, we demonstrate that an intriguing Weyl semimetal phase emerges during the transition from topological insulating phase to trivial phases, where the Weyl point moves along the $\mathrm{\Gamma }\text{−}M$ line before the band gap reopens. In this paper, we provide both theoretical and experimental evidence to underpin the graphyne structure for realizing both HOTIs and semimetals.

Here we report the hydrostatic pressure effect on low-temperature (80 K) photoluminescence of high-pressure high-temperature diamonds with SiV, GeV, and SnV color centers. A linear blueshift in the zero-phonon line (ZPL) energy with increasing pressure was observed for all centers. The pressure coefficients of SiV, GeV, and SnV were found to increase in the order of increasing dopant mass with a noticeable jump at the transition between SiV and GeV and just a slight increase at the transition between GeV and SnV. The low-temperature conditions of our measurements and the high quality of the studied samples also allowed us to investigate the pressure effect on the quasilocal vibrational modes (QLVMs) of SiV and GeV. Both QLVM's energies were found to increase with pressure, and a method for estimating internal strain based on this phenomenon was proposed. At the same time, the pressure coefficient of the SiV QLVM energy appeared to be almost 3 times as much as for the GeV, while the situation for the pressure coefficients of ZPLs is reversed. The obtained experimental data were summarized on a configurational diagram within the Franck-Condon approximation, and the pressure effect on adiabatic potentials was shown. In addition, we studied the pressure effect on the PL of some co-occurring color centers [720 nm and nitrogen-vacancy centers ($\mathrm{N}{\mathrm{V}}^{-}$ and $\mathrm{N}{\mathrm{V}}^0$)], also presented in the studied diamond samples. For the 720 nm center the noticeable change of the ZPL shape was demonstrated already at $P=4.64\mathrm{GPa}$, which may signal the outstanding (in comparison to other color centers) pressure sensitivity of its electronic structure. Finally, the pressure coefficients of the studied color centers were used to compare the possible impact of the local stress distribution within the diamond sample on the inhomogeneous broadening of the ZPL.

The analytical study of confinement in lattice gauge theories (LGTs) remains a difficult task to this day. Taking a geometric perspective on confinement, we develop a real-space renormalization group (RG) formalism for ${\mathbb{Z}}_2$ LGTs using percolation probability as a confinement order parameter. The RG flow we analyze is constituted by both the percolation probability and the coupling parameters. We consider a classical ${\mathbb{Z}}_2$ LGT in two dimensions, with matter and thermal fluctuations, and analytically derive the confinement phase diagram. We find good agreement with numerical and exact benchmark results and confirm that a finite matter density enforces confinement at $T<\infty$ in the model we consider. Our RG scheme enables future analytical studies of ${\mathbb{Z}}_2$ LGTs with matter and quantum fluctuations and beyond.

We study the emergence of the prethermal Floquet time crystal (pFTC) in disordered chiral multiferroic chains. The model is an extension of the usual periodically driven nearest-neighbor disordered Heisenberg chain, with additional next-nearest-neighbor Heisenberg couplings and Dzyaloshinskii-Moriya interactions due to external magnetic and electric couplings. We derive the phase diagram of the model, characterizing the magnetization, entanglement, and coherence dynamics of the system along the extended interactions. In addition, we explore the application of the pFTC as quantum sensors of AC fields. The sensor performance to estimate small AC fields is quantified through the quantum Fisher information (QFI) measure. The sensor offers several advantages as compared to those composed of noninteracting spins due to its intrinsic robustness, long coherent interrogation time, and many-body correlations. Specifically, the sensor can overcome the standard quantum limit ($\mathrm{SQL}\sim {\mathrm{Nt}}^2$) during the prethermal regime, reaching an optimum performance at the pFTC lifetime $t^{*}$, where the $\mathrm{QFI}/{\mathrm{Nt}}^{{*}^2}\sim {\mathrm{N}}^{\alpha }$ with $\alpha >0$, scaling superlinarly with the number of spins. Different from full FTCs, the prethermal lifetime does not diverge in the thermodynamic limit, nevertheless it can be increasingly long with tuning system parameters.

Surface plasmons are collective electron excitations in metallic systems, and the associated electromagnetic wave usually has the transverse-magnetic polarization. On the other hand, spin waves are spin excitations perpendicular to the equilibrium magnetization and are usually circularly polarized in a ferromagnet. The direct coupling of these two modes is challenging due to the difficulty of matching electromagnetic boundary conditions at the interface of magnetic and nonmagnetic materials. Here, we overcome this challenge by utilizing the linearly polarized spin waves in antiferromagnets (AFM) and show that a strong coupling between AFM magnons and surface plasmons can be realized in a hybrid two-dimensional (2D) material/AFM structure, featuring a clear anticrossing spectrum at resonance. The coupling strength, characterized by the gap of anticrossing at resonance, can be tuned by electric gating on 2D materials and probed by measuring the two reflection minima in the reflection spectrum. Further, as a potential application, we show that plasmonic modes can mediate the coupling of two well-separated AFMs over several micrometers, featuring symmetric and antisymmetric hybrid modes. Our results may open a platform to study antiferromagnetic spintronics and its interplay with plasmonic photonics.

We report on terahertz spectroscopic measurements of quantum spin dynamics on single crystals of a spin-1/2 frustrated spin-ladder antiferromagnet ${\mathrm{BiCu}}_2{\mathrm{PO}}_6$ as a function of temperature, polarization, and applied external magnetic fields. Spin triplon excitations are observed at zero field and split in applied magnetic fields. For magnetic fields applied along the crystallographic $a$ axis, a quantum phase transition at $B_{c1}=21.4$ T is featured by a low-energy excitation mode emerging above $B_{c1}$, which indicates a gap reopening. For fields along the $b$ axis and the $c$ axis, different field dependencies are observed for the spin triplon excitations, whereas no low-lying modes could be resolved at field-induced phase transitions. We perform a theoretical analysis of the magnetic field dependence of the spin triplon modes by using continuous unitary transformations to determine an effective low-energy Hamiltonian. Through an exhaustive parameter search we find numerically optimized parameters to very well describe the experimentally observed modes, which corroborate the importance of significant magnetic anisotropy in the system.

By single crystal diffraction we characterize the chemostructural disorder introduced by Zn-Cu site mixing in the kagome spin $S=\frac{1}{2}$ systems herbertsmithite ${\mathrm{ZnCu}}_3{\left(\mathrm{OH}\right)}_6{\mathrm{Cl}}_2$ and ${\mathrm{YCu}}_3{\left(\mathrm{OH}\right)}_6{\mathrm{Br}}_2{[\mathrm{Br}}_x{\left(\mathrm{OH}\right)}_{1-x}]$. For an untwinned single crystal of herbertsmithite of composition ${\mathrm{Zn}}_{0.95\left(1\right)}{\mathrm{Cu}}_{2.99\left(3\right)}{\mathrm{O}}_{5.9\left(1\right)}{\mathrm{H}}_{5.8\left(1\right)}{\mathrm{Cl}}_2$ we find substitution by Cu of the Zn atoms in the layers separating the kagome layers as well as substantial Zn substitution for Cu in the kagome layers. In ${\mathrm{YCu}}_3{\left(\mathrm{OH}\right)}_6{\mathrm{Br}}_2{[\mathrm{Br}}_x{\left(\mathrm{OH}\right)}_{1-x}]$ site mixing disorder is present for intermediate $x$. Analogous to the Cl homologous system in crystals with $x=1/3$ disorder is absent and a low-temperature structural transition emerges driven by strong magnetophonon coupling as a release of frustration. Apart from this structural anomaly we find the physical properties of these crystals unchanged compared to intermediate $x$ and closely resembling the Cl homologue where long-range magnetic order was observed.

In recent years, twisted bilayer systems such as bilayer graphene attracted a great deal of attention as the twist angle introduced a degree of freedom which can be used to nontrivially modify system properties. This idea has been picked up in the cold atom community, first with a theoretical proposal to simulate twisted bilayers in state-dependent optical lattices, and, more recently, with an experimental realization of twisted bilayers with bosonic atoms in two different spin states. In this article, we theoretically investigate dipolar bosons in a twisted bilayer geometry. The interplay between dipolar interaction and the twist between the layers results in the emergence of quantum states not observed in the absence of twist. We study how system properties vary as we change the twist angle at fixed distance between the layers and fixed dipolar interaction. We find that at a twist angle $\theta =0.1^{\circ }$, the observed quantum phases are consistent with those seen in the absence of twist angle, i.e., paired superfluid, paired supersolid, and paired solid phases. However, a slight increase in the twist angle to $\theta =0.2^{\circ }$ disrupts these paired phases in favor of a phase separation between checkerboard solid and superfluid regions. Notably, at a twist angle of $\theta =5.{21}^{\circ }$, the local occupation number follows the moiré pattern of the underlying moiré bilayers so that a periodic structure of insulating islands is formed. These insulating islands are surrounded by a superfluid.