Single crystals of the van der Waals layered $5d$ transition-metal compound ${\mathrm{Os}}_{0.55}{\mathrm{Cl}}_2$ were grown and characterized by x-ray diffraction, magnetization and heat-capacity measurements, and atomic resolution electron microscopy. The crystals are stable in air and easily cleaved. The structure is derived from the ${\mathrm{CdCl}}_2$ structure type, with triangular layers of transition metal sites coordinated by edge-sharing octahedra of Cl and separated by a van der Waals gap. On average, only 55% of the metal sites are occupied by Os, and evidence for short- and long-ranged vacancy orders is observed by diffraction and real-space imaging. Magnetization data indicate magnetocrystalline anisotropy due to spin-orbit coupling, antiferromagnetic correlations, and no sign of magnetic order or spin freezing down to 0.4 K. Heat-capacity measurements in applied magnetic fields show only a broad, field-dependent anomaly. The magnetic susceptibility and heat capacity obey power laws at low temperature and low field with exponents close to 0.5. The power law behaviors of the low-temperature heat capacity and magnetic susceptibility suggest gapless magnetic fluctuations prevent spin freezing or ordering in ${\mathrm{Os}}_{0.55}{\mathrm{Cl}}_2$. Divergence of the magnetic Gruneisen parameter indicates nearness to a magnetic quantum critical point. Similarities to behaviors of spin-liquid materials are noted, and in total the results suggest ${\mathrm{Os}}_{0.55}{\mathrm{Cl}}_2$ may be an example of a quantum spin liquid in the limit of strong chemical disorder.

We measured NMR and magnetic susceptibility for the quasi-two-dimensional, weakly $XY$-like, spin-$1/2$ square-lattice Heisenberg antiferromagnet $\left[\mathrm{CuCl}{\left(\mathrm{pyz}\right)}_2\right]{\mathrm{BF}}_4$ (pyz = pyrazine $={\mathrm{N}}_2{\mathrm{C}}_4{\mathrm{H}}_4$) near the critical temperature. The Néel temperature $T_N$ and the order-parameter critical exponent $\beta$ were obtained from the NMR line broadening as a function of temperature. As the applied field strength ($H\parallel c$) was increased, $T_N$ increased and $\beta$ decreased. This behavior indicates that the field effectively enhanced $XY$ anisotropy. The susceptibility as a function of temperature did not show a clear feature for $T_N$, but showed field-dependent minima below $T_N$ for both $H\parallel c$ and $H\parallel ab$, where minimum features disappeared for ${\mu }_{0}H>2\mathrm{T}$.

We present a magnetic domain-imaging study by x-ray magnetic circular dichroism photoelectron emission microscopy on a $\mathrm{Co}/{\mathrm{Fe}}_{75}{\mathrm{Gd}}_{25}$ bilayer under exposure to single focused ultrashort (100 fs) infrared laser pulses. Magnetic domain walls experience a force in the intensity gradient of the laser pulses away from the center of the pulse, which can be used to steer domain walls to move in a certain direction. Maximum domain-wall displacements after individual laser pulses close to $1\mu \mathrm{m}$ in zero external field are observed. Quantitative estimates show that electronic spin currents from the spin-dependent Seebeck effect are not strong enough to explain the effect, which we thus attribute to the torque exerted by magnons from the spin Seebeck effect that are reflected at the domain wall.

Frustrated magnets are known to support two-dimensional topological solitons, called skyrmions. A continuum model for frustrated magnets has recently been shown to support both two-dimensional skyrmions and three-dimensional knotted solitons (hopfions). In this article we derive lower bounds for the energies of these solitons expressed in terms of their topological invariants. The bounds are linear in the degree in the case of skyrmions and scale as the Hopf degree to the power 3/4 in the case of hopfions.

A time-dependent model of two interacting spin qutrits is analyzed is analyzed and solved. The two interacting qutrits are subjected to a longitudinal field linearly varying over time as in the Landau-Majorana-Stückelberg- Zener (LMSZ) scenario. Although a transverse field is absent, we show the occurrence of LMSZ transitions assisted by the coupling between the two spin-qutrits. Such a physical effect permits us to estimate experimentally the coupling strength between the spins and allows the generation of entangled states of the two qutrits by appropriately setting the slope of the ramp. Furthermore, the possibility of local and nonlocal control as well as the existence of dark states of the two qutrits have been brought to light. Effects stemming from a noisy surrounding environment are also taken into account by introducing a random fluctuating field component as well as non-Hermitian terms in the Hamiltonian model.

Willis materials are complex media characterized by four macroscopic material parameters, the conventional mass density, the bulk modulus, and two additional Willis coupling terms, which have been shown to enable unsurpassed control over the propagation of mechanical waves. However, virtually all previous studies on Willis materials involved passive structures which have been shown to have limitations in terms of achievable Willis coupling terms. In this Rapid Communication, we show experimentally that linear active Willis metamaterials breaking these constraints enable highly nonreciprocal sound transport in very subwavelength structures. Furthermore, we present an experimental procedure to extract the effective material parameters expressed in terms of acoustic polarizabilities for media in which the Willis coupling terms are not constrained by the relationship holding in unbiased passive materials. The approach presented here will enable a generation of ultracompact Willis materials for enhanced sound control and improved acoustic imaging and signal processing.

Time-dependent density functional theory (TDDFT) has developed into an efficient and versatile description of realistic extended many-electron systems driven, e.g., by strong laser fields. It accounts for the fully coherent evolution of the $N$-electron wave packet representing an isolated system. Decoherence as encountered in open systems by coupling to external degrees of freedom of a bath of, e.g., phonons or defects, is, by construction, absent. In this work we present an open-quantum system (OQS) extension of TDDFT accounting for dephasing and decoherence due to electron-phonon or defect scattering. We test the OQS-TDDFT for high-harmonic generation and irreversible changes of dielectric properties in solids driven by strong ir and mid-ir laser pulses. We present applications to diamond as a prototypical wide band-gap dielectric. For weak pulses we demonstrate the equivalence of OQS-TDDFT with the solution of the Bloch equations for the reduced one-particle density matrix while differences appear at high excitation densities. Our study highlights the importance of the accurate representation of the band structure in simulations of the harmonic spectrum. Narrow avoided crossings within the Brillouin zone can give rise to Bloch-type oscillations.

The fascinating multiple magnetic states observed in the Ni-Mn-Sn based metamagnetic shape memory alloy are addressed through a combined muon spin relaxation ($\mu \mathrm{SR}$) and neutron powder diffraction study. The material used in the present investigation is an off-stoichiometric alloy of nominal composition, ${\mathrm{Ni}}_{2.04}{\mathrm{Mn}}_{1.4}{\mathrm{Sn}}_{0.56}$. This prototypical alloy, similar to other members in the Ni-Mn-Sn series, orders ferromagnetically below $T_{CA}(=320 \mathrm{K})$ and undergoes a martensitic-type structural transition at $T_{MS}(=290 \mathrm{K})$, which is associated with the sudden loss of magnetization. The sample regains its magnetization below another magnetic transition at $T_{CM}=260\mathrm{K}$. Eventually, the composition shows a steplike anomaly at $T_B=120\mathrm{K}$, which is found to coincide with the blocking temperature of the exchange bias effect observed in the alloy. In our study, the initial asymmetry $A_{10}$ of the $\mu \mathrm{SR}$ data falls rapidly below $T_{CA}$, indicating the onset of bulk magnetic order. $A_{10}$ regains its full asymmetry value below $T_{MS}$, suggesting the collapse of the ferromagnetic order into a fully disordered paramagnetic state. Below the second magnetic transition at $T_{CM}$, asymmetry drops again, confirming the reentrance of a long-range ordered state. Interestingly, $A_{10}$ increases sluggishly below $T_B$, indicating that the system attains a disordered/glassy magnetic phase below $T_B$, which is responsible for the exchange bias and frequency dispersion in the ac susceptibility data as previously reported. The neutron powder diffraction data do not show any magnetic superlattice reflections, ruling out the possibility of a long-range antiferromagnetic state at low temperatures. The ground state is likely to be comprised of a concentrated metallic spin glass in the backdrop of an ordered ferromagnetic state.

We use the recently proposed four-spin bond-operator technique (BOT) to discuss the spectral properties of a frustrated spin-$\frac{1}{2}$ $J_1$–$J_2$ Heisenberg antiferromagnet on a square lattice at $J_2<0.4J_1$ (i.e., in the Néel ordered phase). This formalism is convenient for the consideration of low-lying excitations which appear in conventional approaches as multimagnon bound states (e.g., the Higgs excitation) because separate bosons describe them in the BOT. At $J_2=0$, the obtained magnon spectrum describes accurately available experimental data. However, calculated one-magnon spectral weights and the transverse dynamical structure factor (DSF) do not reproduce experimental findings quantitatively around the momentum $\mathbf{k}=(\pi ,0)$. Thus, we do not support the conjecture that the continuum of excitations observed experimentally and numerically near $\mathbf{k}=(\pi ,0)$ is of the Higgs-magnon origin. Upon $J_2$ increasing, one-magnon spectral weights decrease, and spectra of high-energy spin-0 and spin-1 excitations move down. One of the spin-0 quasiparticles becomes long-lived, and its spectrum merges with the magnon spectrum in the majority of the Brillouin zone at $J_2\approx 0.3J_1$. We predict also that the Higgs excitation and another spin-0 quasiparticle become long-lived around $\mathbf{k}=(\pi /2,\pi /2)$ at $J_2\gtrsim 0.3J_1$ and produce sharp anomalies in the longitudinal DSF.

We explore the thermodynamics in two-dimensional arrays consisting of Ising-type nanomagnets lithographically arranged onto random sites and angular orientations. Introducing these basic spin-glass ingredients, we study the characteristic features of the low-energy states achieved, following thermal-annealing protocols. From direct visualization of real-time dynamics, we record relaxation timescales together with magnetic susceptibility variations over temperature, revealing trends towards short-range order as randomness is increased, but falling short of pure spin-glass behavior. Our work provides a route towards the realization of artificial Ising spin-glass systems.

High-resolution time-of-flight powder neutron diffraction and high-field magnetization were measured to investigate the magnetic structure and existence of a field-induced magnetic phase transition in the distorted kagome antiferromagnet ${\mathrm{Cs}}_2{\mathrm{Cu}}_3{\mathrm{SnF}}_{12}$. Upon cooling from room temperature, the compound undergoes a structural phase transition at $T_{\mathrm{t}}=185\mathrm{K}$ from the rhombohedral space-group $R\overline{3}m$ with the perfect kagome spin network to the monoclinic space-group $P{2}_1/n$ with the distorted kagome planes. The distortion results in three inequivalent exchange interactions among the $S=1/2{\mathrm{Cu}}^{2+}$ spins that magnetically order below $T_{\mathrm{N}}=20.2\mathrm{K}$. Magnetization measured with a magnetic field applied within the kagome plane reveals small in-plane ferromagnetism resulting from spin canting. On the other hand, the out-of-plane magnetization does not show a clear hysteresis loop of the ferromagnetic component nor a prominent anomaly up to 170 T with the exception of the subtle kneelike bend around 90 T, which could indicate the 1/3 magnetization plateau. The combined analysis using the irreducible representations of the magnetic space groups and magnetic structure refinement on the neutron powder-diffraction data suggests that the magnetic moments order in the magnetic space-group $P{2}_1^{\prime }/n^{\prime }$ with the all-in–all-out spin structure, which by symmetry allows for the in-plane canting, consistent with the in-plane ferromagnetism observed in the magnetization.

Transition-metal oxides with an ${\text{ABO}}_3$ perovskite structure exhibit strongly entangled structural and electronic degrees of freedom and thus one expects to unveil exotic phases and properties by acting on the lattice through various external stimuli. Using the Jahn-Teller active praseodymium vanadate ${\mathrm{Pr}}^{3+}{\mathrm{V}}^{3+}{\mathrm{O}}_3$ compound as a model system, we show that ${\mathrm{PrVO}}_3$ Néel temperature $T_N$ can be raised by 40 K with respect to the bulk when grown as thin films. Using advanced experimental techniques, this enhancement is unambiguously ascribed to a tetragonality resulting from the epitaxial compressive strain experienced by the films. First-principles simulations not only confirm experimental results, but they also reveal that the strain promotes an unprecedented orbital ordering of the ${\mathrm{V}}^{3+}d$ electrons, strongly favoring antiferromagnetic interactions. These results show that an accurate control of structural aspects of oxides is the key for unveiling unexpected phases in oxides.

We numerically study the spatial spreading of quantized vortex lines in low-temperature liquid helium. The vortex lines, initially concentrated in a small region, diffuse into the surrounding vortex-free helium, a situation which is typical of many experiments. We find that this spreading, which occurs in the absence of viscosity, emerges from the interactions between the vortex lines and can be interpreted as a diffusion process with effective coefficient equal to approximately $0.5\kappa$ where $\kappa$ is the quantum of circulation.

Time-reversal symmetry and magnetoelastic correlations are probed by means of high-resolution volume dilatometry in ${\mathrm{URu}}_2{\mathrm{Si}}_2$ at cryogenic temperatures, and magnetic fields sufficient to suppress the hidden order state at $H_{\mathrm{HO}}(T=0.66\mathrm{K})\simeq 35$ T. We report a significant magnetoelastic volume expansion at and above $H_{\mathrm{HO}}\left(T\right)$, and even above $T_{\mathrm{HO}}$, possibly a consequence of field-induced $f$-electron localization. We investigate in detail the magnetostriction and magnetization as the temperature is reduced across two decades in temperature from 30 K where the system is paramagnetic, to 0.5 K in the realm of the hidden order state. We find a dominant quadratic-in-field dependence $\mathrm{\Delta }L/L\propto H^2$, a result consistent with a state that is symmetric under time reversal. The data shows, however, an incipient yet unmistakable asymptotic approach to linear ($\mathrm{\Delta }L/L\propto 1-H/H_0$) for $15\text{T}<H<H_{\mathrm{HO}}\left(0.66\text{K}\right)\sim 40$ T at the lowest temperatures. We discuss these results in the framework of a Ginzburg-Landau formalism that proposes a complex order parameter for the HO phase to model the $(H,T,p)$ phase diagram.

Symmetry-protected and symmetry-enriched topological (SPT/SET) phases in three dimensions are quantum systems that support nontrivial two-dimensional (2D) surface states. These surface states develop finite excitation energy gaps when the relevant symmetries are broken. On the other hand, one-dimensional (1D) gapless modes can populate along interfaces that separate adjacent gapped surface domains with distinct symmetry-breaking orders. A surface strip pattern in general reduces the low-energy SPT/SET surface degrees of freedom onto a 2D array of gapless 1D channels. These channels can be coupled to one another by quasiparticle tunneling, and these interwire interactions collectively provide an effective description of the surface state. In this paper, we study a general class of symmetry-preserving or -breaking SPT/SET surface states that admit finite excitation energy gaps and Abelian topological orders via the coupled-wire construction. In particular, we focus on the prototype Abelian surface topological orders that fall under the $ADE$ classification of simply laced Lie algebras. We also elaborate on the emergent symmetry and duality properties of the coupled-wire models.

We extend the Vanderbilt ultrasoft pseudopotential scheme by adding kinetic energy density terms, in order to use meta-GGA exchange potentials, such as the Becke-Johnson or Tran-Blaha potentials, in the plane-wave–pseudopotential implementation of density functional theory. Having implemented kinetic energy augmentation and nonlinear core correction terms in the castep density functional package, we evaluate the validity of our approach by comparing the calculated electronic structure of isolated atoms and semiconductor crystals to all-electron benchmark calculations. Based on our results, we provide recommendations for the practical use of the Tran-Blaha exchange in plane-wave–pseudopotential codes.

We employ polarization-resolved Raman scattering spectroscopy to study ferroquadrupolar (FQ) fluctuations and crystal-field (CF) excitations in the ${\mathrm{YbRu}}_2{\mathrm{Ge}}_2$ heavy-fermion metal with FQ transition at $T_Q=10\mathrm{K}$. We demonstrate that the electronic static Raman susceptibilities in quadrupolar symmetry channels exhibit nearly Curie law behavior and that the electron-lattice coupling is essential for the FQ transition at $T_Q$. We establish the CF level scheme of the ${\mathrm{Yb}}^{3+}$ ground state ${}^{2}F_{7/2}$ multiplet. We study the lattice dynamics and demonstrate coupling between CF transitions and phonon modes.

We propose an efficient method to numerically evaluate ${\mathbb{Z}}_4$ indices of Möbius and/or hourglass topological phases with glide symmetry. Our method directly provides ${\mathbb{Z}}_4$ indices in the lattice Brillouin zone while the existing method requires careful observations of momentum-dependent Wannier charge centers. As applications, we perform systematic computation of ${\mathbb{Z}}_4$ indices for Möbius materials $\mathrm{CeNiSn}$ and $\mathrm{UCoGe}$. In particular, our analysis elucidates that $\mathrm{UCoGe}$ shows strong Möbius superconductivity for the $A_u$ or $B_{3u}$ representation whose topology has not been fully characterized. Furthermore, obtained phase diagrams reveal topological gapless excitations in the bulk which are protected by nonsymmorphic glide symmetry. We observe these gapless excitations with glide symmetry by doping holes into the superconducting phase of the $B_{1u}$ or $B_{3u}$ representation.

The dynamical susceptibility of strongly correlated electronic systems can be calculated within the framework of dynamical mean-field theory (DMFT). The required measurement of the four-point vertex of the auxiliary impurity model is however costly and restricted to a finite grid of Matsubara frequencies, leading to a cutoff error. It is shown that the propagation of this error to the lattice response function can be minimized by virtue of an exact decomposition of the DMFT polarization function into local and nonlocal parts. The former is measured directly by the impurity solver, while the latter is given in terms of a ladder equation for the Hedin vertex that features an unprecedentedly fast decay of frequency summations compared to previous calculation schemes, such as the one of the dual boson approach. At strong coupling, the local approximation of the TRILEX approach is viable, but vertex corrections to the polarization should be dropped on equal footing to recover the correct prefactor of the effective exchange. In finite dimensions, the DMFT susceptibility exhibits spurious mean-field criticality, therefore, a two-particle self-consistent and frequency-dependent correction term is introduced, similar to the Moriya $\lambda$ correction of the dynamical vertex approximation. Applications to the two- and three-dimensional Hubbard models on the square and cubic lattices show that the expected critical behavior near an antiferromagnetic instability is recovered.

We study a ferromagnetic instability in a doped single-band Hubbard model by means of dynamical mean-field theory with the continuous-time quantum Monte Carlo simulations. Examining the effect of the strong correlations in the system on the hypercubic and Bethe lattice, we find that the ferromagnetically ordered state appears in the former, while it does not in the latter. We also reveal that the ferromagnetic order is more stable in the case that the noninteracting density of states (DOS) exhibits a slower decay in the high-energy region. The present results suggest that, in the strong-coupling regime, the high-energy part of DOS plays an essential role for the emergence of the ferromagnetically ordered state, in contrast to the Stoner criterion justified in the weak-interaction limit.