Spin waves, or magnons, are fundamental excitations in magnetic materials that provide insights into their dynamic properties and interactions. Magnons are the building blocks of magnonics, which offer promising perspectives for data storage, quantum computing, and communication technologies. These excitations are typically measured through inelastic neutron or x-ray scattering techniques, which involve heavy and time-consuming measurements, data processing, and analysis based on various theoretical models. Here, we introduce a machine-learning algorithm that integrates adaptive noise reduction and active-learning sampling, which enables the restoration from minimal inelastic neutron-scattering point data of spin-wave information and the accurate extraction of magnetic parameters, including hidden interactions. Our findings, benchmarked against the magnon spectra of CrSBr, significantly enhance the efficiency and accuracy in addressing complex and noisy experimental measurements. This advancement offers a powerful machine-learning tool for research in magnonics and spintronics, which can also be extended to other characterization techniques at large facilities.

The spin direction in atomically flat ultrathin Co layers with well-defined thickness is studied experimentally with spin-polarized low-energy electron microscopy and theoretically with first-principles calculations. Spin-orbit coupling and atomic positions at the interface affect the spin direction over a wide thickness range. Spin-dependent quantum size effects produce a layer stacking-dependent oscillatory evolution of the spin direction with thickness not seen in other ultrathin ferromagnetic films. This difference is explained by the theoretical model, which gives a broader understanding of the spin direction in ultrathin ferromagnetic films in general.

We report on the thermodynamic and transport properties of the rare-earth Zintl compound ${\mathrm{Eu}}_5{\mathrm{Sn}}_2{\mathrm{As}}_6$, which orders as a canted antiferromagnetic semiconductor at 10.3 K. The system also displays a complex cascade of magnetic phases arising from geometric and magnetic exchange frustration, with high sensitivity to the application and direction of small magnetic fields. At low temperature, ${\mathrm{Eu}}_5{\mathrm{Sn}}_2{\mathrm{As}}_6$ exhibits negative colossal magnetoresistance of up to a factor of 6000. This represents a lower bound as the conductivity appears to be shunted by an unknown conduction channel, causing the resistivity to saturate. Mechanisms for the low-temperature saturation of resistivity are discussed.

Altermagnets are an exotic class of magnetic materials in which the Fermi surface exhibits a momentum-dependent spin-splitting while maintaining net-zero magnetization. Previous studies showed that this distinctive spin splitting can induce chiral $p$-wave superconductors or Fulde-Ferrell (FF) superconducting states carrying finite momentum. However, the underlying mechanisms of such unconventional superconductivities remain elusive. Here, we propose that the formation of the Bogoliubov Fermi surface (BFS) through the exchange field can play a significant role in such phenomena. Through a systematic self-consistent mean-field analysis of the extended attractive Hubbard model combined with the $d$-wave spin splitting induced by the exchange field, as observed in ${\mathrm{RuO}}_2$, we demonstrate that the formation of the BFS suppresses conventional spin-singlet superconducting states with $s$-wave characteristics. In contrast, the chiral $p$-wave state maintains a fully gapped spectrum without the Fermi surface, thereby becoming the ground state in the strong-field regime. In the intermediate regime, we find that the FF state becomes the predominant state through the optimization of available channels for Cooper pairing. Moreover, we illustrate how the prevalence of the chiral $p$-wave and FF states over the $s$-wave state changes under the variation of the field strength or chemical potential. Our findings provide valuable insights into potential pathways for realizing sought-after topological $p$-wave superconductivity and finite-momentum pairing facilitated by altermagnetism.

The Goldstino, which is a fermionic Nambu-Goldstone mode, has been predicted in a Bose-Fermi mixture when the supersymmetry is broken. To detect this excitation mode, we theoretically investigate the shot noise of the supersymmetry-like tunneling current in a weakly interacting ultracold Bose-Fermi mixture. The Fano factor, which is defined by the noise-to-current ratio, reflects the elementary carriers of the tunneling process. The change of the Fano factor microscopically as the density changes evinces a crossover from the quasiparticle transport to multiparticle (Goldstino) transport. The tunneling channel can also be changed by tuning the potential barrier.

Three-dimensional $d$-wave ($j=2$) superconducting state may result from Cooper pairing in channels with different angular momenta and spin, such as $(l=0,s=2)$ or $(l=2,s=0)$. We consider spin-3/2 Luttinger fermions in the limit of small spin-orbit coupling parameter and with weak attractive interactions in both of these channels and demonstrate that the stable $d$-wave superconducting phase of the system below critical temperature depends critically on the ratio of two interactions. When a weak inversion-breaking term in the kinetic energy is included, all three of the real, ferromagnetic, and cyclic phases can be realized within a narrow range around unit ratio. The result shows how models of multicomponent superconductors can be surprisingly sensitive to precise values of some of its parameters and display radically different superconducting ground states with their variation.

Topological properties of a three-dimensional Dirac semimetal ${\mathrm{Cd}}_3{\mathrm{As}}_2$, protected by crystal rotation and time-reversal symmetry, can be tuned with the application of pressure. Ultrafast spectroscopy is a unique tool to investigate the character and time evolution of electronic states, emphasizing the signatures of transition. We designed an experimental setup for in situ pressure-dependent ultrafast optical pump–optical probe spectroscopy of ${\mathrm{Cd}}_3{\mathrm{As}}_2$ using a symmetric diamond anvil cell. The fast relaxation processes show significant changes across pressure-induced phase transitions at $P_{C_1}\sim 3\mathrm{GPa}$ and $P_{C_2}\sim 9\mathrm{GPa}$. The relaxation time decreases beyond $P_{C_1}$, suggesting a stronger electron-phonon coupling, characteristic of a semiconducting state. A new sub-picosecond timescale relaxation dynamics emerges beyond $P_{C_2}$. Theoretical calculations of differential reflectivity for both interband and intraband processes indicate that the negative (positive) differential reflectivity ($\mathrm{\Delta }R/R$) results from the interband (intraband) processes. The pressure-dependent behavior of relaxation dynamics amplitudes beyond $P_{C_1}$ emphasized the necessity of incorporating quadratic band opening in the calculations, explaining the transition of ${\mathrm{Cd}}_3{\mathrm{As}}_2$ from a Dirac semimetal to a semiconducting phase. The time evolution of differential reflectivity is calculated using the electronic temperature as a function of time, as provided by the two-temperature model, which fits the experimental data.

We study shortcuts to adiabaticity (STAs) through counterdiabatic driving in quantum critical systems in the presence of dissipation. We evaluate unitary as well as nonunitary controls, such that the system density matrix follows a prescribed trajectory corresponding to the eigenstates of a time-dependent reference Hamiltonian at any instant of time. The strength of the dissipator control term for the low-energy states shows universal scaling close to criticality. Using the example of free-fermionic systems, in particular, the transverse-field Ising model, we show that in contrast to STAs in closed quantum critical systems, here, STAs may require multibody interactions terms, even away from criticality, owing to the change in entropy of the time-dependent target state. Further, the associated heat current shows an extremum, while power dissipation changes curvature, close to criticality, and analogous to unitary control, no operational cost is associated with implementation of the exact counterdiabatic Hamiltonian. We believe the counterdiabatic protocol studied here could be of fundamental importance for understanding STAs in many-body open quantum systems and could be highly relevant for varied topics involving open many-body quantum systems, such as quantum computation and many-body quantum heat engines.

Quasielastic neutron scattering (QENS) is a powerful experimental technique to investigate the diffusion of atoms in liquids. It relies on the fact that at low $Q$ values essentially only incoherent scattering is observed from the diffusing atoms. In the case of alloys composed of two chemical elements, it is commonly believed that at low $Q$ QENS mainly probes the self-diffusion of the element with a nonzero incoherent scattering cross-section, while the element with a zero incoherent scattering cross-section does not contribute. However, the findings of this study challenge this belief. The study examines alloys of natural gallium (which has a nonzero incoherent scattering cross-section) and isotopes ${}^{58}\mathrm{Ni}$ and ${}^{60}\mathrm{Ni}$ (both of which have a zero incoherent scattering cross-section). A crucial factor to consider here is chemical incoherence, which typically leads to the renormalization of the incoherent cross-sections of different chemical elements within a mixture. Unfortunately, this renormalization is often disregarded in an ad hoc manner. Significantly, the alloy containing ${}^{58}\mathrm{Ni}$ shows a notable renormalization, scattering incoherently to a similar degree as natural nickel in a comparable alloy with equivalent concentrations of chemical elements. Similarly perplexing, natural nickel scatters with great intensity despite being predominantly composed of ${}^{58}\mathrm{Ni}$ and ${}^{60}\mathrm{Ni}$ (around 94% of the total). This behavior can be attributed to the accepted phenomenon of isotope incoherence. We calculate the integral diffusion coefficients of Ga and Ni atoms in various liquid alloys from QENS experiments. These values are then compared to the results obtained from corresponding molecular dynamics simulations conducted at various temperatures, taking into account chemical incoherence. The comparison shows good agreement within the range of errors for the ${\mathrm{Ga}}_{1-x}{{}^{58}\mathrm{Ni}}_x$ alloy at two concentrations ($x=0.02$ and $x=0.1$). The agreement also partially holds for the ${\mathrm{Ga}}_{1-x}{\mathrm{Ni}}_x$ alloy with natural nickel and for the ${\mathrm{Ga}}_{1-x}{{}^{60}\mathrm{Ni}}_x$ alloy, despite certain difficulties discussed.

Motivated by recent experiments realizing long-lived nonequilibrium states in aperiodically driven quantum many-body systems, we investigate the dynamics of a quasiperiodically driven Rydberg atom chain in the strong Rydberg blockage regime. In this regime, the system is kinetically constrained and the “PXP” model describes its dynamics. Even without driving, the PXP model exhibits many-body scarring and resultant persistent oscillations for dynamics originating from the Néel-ordered initial state. We demonstrate that a rich array of dynamical behaviors emerge when the system is subjected to a continuous drive. In the high-frequency regime, the system exhibits revivals and oscillations for the Néel ordered initial state both for periodic and quasiperiodic drives. We trace the origin of this nonergodicity to an effective PXP Hamiltonian for both of these driving protocols in this regime. Furthermore, we demonstrate that the behavior of the fidelity and the entanglement entropy is nonmonotonic at low frequencies in the high-amplitude regime. This leads to several re-entrant scarring transitions both for both the Néel-ordered and the fully polarized initial state. Our results demonstrate that continuous quasiperiodic drive protocols can provide a promising route to realize prethermal phases of matter in kinetically constrained systems.

We simulate the field-driven magnetization reversal in the exchange-coupled antiferromagnetic-ferromagnetic bilayers, adapting the weakly disordered random-field Ising model to mimic experimental conditions with finite temperatures and driving field rates. Our results reveal how the theoretical ideal hysteresis loop shape with a central loop and side branches with multiple small steps corresponding to a given architecture at low temperatures and slow driving becomes systematically deformed under the combined impact of the elevated temperatures and driving speed. Notably, the loop becomes smoother with the number of steps diminishing as temperature and driving rate increase. The coercive field demonstrates a systematic increase with the driving rate, characterized by a power law with a universal exponent of 1/2, known in random ferromagnets; meanwhile, the nonuniversal factor depends on temperature and the antiferromagnetic layer thickness. For moderate driving rates, the coercivity decays exponentially with the increasing temperature, and the central loop eventually collapses above the ferromagnetic transition temperature, with two side branches dominating the loop shape. However, for substantial driving rates, the incomplete spin relaxation before the new field is applied prevents such thermal effects, and the loop remains almost rectangular, with a step on each branch. Predicting a range of parameters where the desired hysteresis shape is stable, these findings can stimulate further experimental research and applications of the field-driven bilayers.

Explicit interactions, e.g., dipolar and exchange couplings, usually govern magnetization dynamics. Some interactions may be hidden from the global crystal symmetry. We report that in a large class of uniaxial antiferromagnets, a hidden Dzyaloshinskii-Moriya interaction with retaining global inversion symmetry quenches the spin of the magnon along the Néel vector $\mathbf{n}$, thus forbidding its angular-momentum flow. Some magnon spins, termed “nodal” and “corner” spins, survive when they distribute singularly at the hot spots, i.e., high-symmetric degeneracy points in the Brillouin zone, and are protected by crystal symmetries. The biased magnetic field along $\mathbf{n}$ broadens such distributions, allowing bulk spin transport with unique signatures in the magnetic field and temperature dependencies. This explains recent experiments and highlights the role of hidden interaction.

We report a new class of algebraic spin liquids, in which the macroscopically degenerate ground state manifold is not Coulombic, like in spin ices, but Ampère-like. The local constraint characterizing an Ampère phase is not a Gauss law, but rather an Ampère law, i.e., a condition on the curl of the magnetization vector field and not on its divergence. As a consequence, the excitations evolving in such a manifold are not magnetically charged scalar quasiparticles, the so-called magnetic monopoles in Coulomb phases, but instead vectorial magnetic loops (or fictional current lines). We demonstrate analytically that in a macroscopically degenerate manifold inheriting the properties of a cooperative paramagnet and subject to a local curl-free constraint, magnetic correlations decay in space with a power law whose exponent is the space dimension $d$: the Ampère phase is a $d$-algebraic spin liquid. Using Monte Carlo simulations with appropriate cluster dynamics, we confirm this physics numerically in two- and three-dimensional examples and illustrate how the Ampère phase compares to its Coulomb counterpart.

Understanding and quantifying entanglement entropy is crucial to characterize the quantum behaviors that drive phenomena in a variety of systems. Rare-earth spin complexes, with their unique magnetic properties, provide fertile ground for exploring these nonlocal correlations. In this work, we study ${\mathrm{Eu}}^{2+}$ ions deposited on a Au(111) substrate, modeling finite clusters of large spin moments using a Heisenberg Hamiltonian parametrized by first-principles calculations. Our analysis reveals a one-to-one correspondence between structures in the differential conductance profiles and changes in the von Neumann entanglement entropy of bipartite subsystems, influenced by probe-ion separation and applied magnetic fields. Distinct braiding patterns in the conductance profiles are shown to correspond to stepwise changes in the entanglement entropy, providing a new avenue for investigating quantum correlations. These results establish a foundation for experimentally probing and controlling entanglement in lanthanide-based systems, with potential applications in quantum technologies.

Using a time-dependent density matrix renormalization group approach, we study the dynamical structure factor of the $J_1-J_2$ spin-1 chain. As $J_2$ increases, the magnon mode develops incommensurability. The system undergoes a first-order transition at $J_2=0.76J_1$, and at that point, domain walls lead to a continuum of fractional quasiparticles or spinons. By studying small variations in $J_2$ around the transition point, we observe the confinement of spinons into bound states in the spectral function and find a smooth evolution of the spectrum into magnon modes away from the phase transition. We employ the single-mode approximation to accurately account for the dispersion of the magnon mode away from the phase transition and describe the associated continua and bound states. We extend the single-mode approximation to describe the dispersion of a spinon at the phase transition point and obtain its dispersion throughout the Brillouin zone. This allows us to relate the incommensurability at and around the transition point to the competition between a negative nearest-neighbor hopping amplitude and a positive next-nearest-neighbor one for the domain wall.

The spin-lattice interaction, a widely observed yet poorly understood phenomenon, has been often studied in the context of the coupling between lattice and magnetic anisotropies. It is also noteworthy that a quasistatic lattice compression is predicted to reduce the magnetic moment of pure ferromagnets such as cobalt and nickel. In this study, we utilize femtosecond laser pump/probe experiments to investigate the ability of a transient strain pulse to change the magnetic moment of a nanometric Co/Ni multilayer film at the picosecond timescale. In good agreement with static density functional theory calculations, our results reveal that a tensile strain acts oppositely to a compressive strain in increasing the magnetic moment, highlighting the efficiency of such magnetoelastic coupling at THz frequencies.

${\mathrm{RuO}}_2$ is one of the most highlighted candidates for altermagnetism. However, the most recent muon spin spectroscopy and neutron studies demonstrated the absence of magnetic order in this system. The electronic structure of ${\mathrm{RuO}}_2$ hints at a possibility of realizing a magnetically ordered state upon hole doping, and such a possibility was explored experimentally in Cr-doped ${\mathrm{RuO}}_2$, where it was suggested that this system exhibits the anomalous Hall effect (AHE) due to altermagnetism. In this paper, based on our density functional calculations, we revise the results obtained for this system and propose a different interpretation of experimental results. Our calculations suggest that extra holes are bound to Cr impurity and do not dope Ru bands, which remain nonmagnetic. Thus, the observed AHE is not due to the altermagnetism but stems entirely from magnetic Cr ions.

We present the properties of an organic $S=1$ antiferromagnetic chain system $m\text{−}{\mathrm{NO}}_2\mathrm{PhBNO}$ (abbreviated BoNO). In this biradical system, two unpaired electrons from aminoxyl groups are strongly ferromagnetically coupled ($|J_{\text{FM}}|/k_B\gtrsim 500$ K), which leads to the formation of an effective $S=1$ state for each molecule. The chains of BoNO biradicals propagate along the crystallographic $a$ axis. Temperature dependencies of the $g$ factor and electron paramagnetic resonance (EPR) linewidth are consistent with a low-dimensional system with antiferromagnetic interactions. The EPR data further suggest that BoNO is a Haldane system with an almost isotropic $g$ factor ($2.0023\pm 2$‰). The magnetization measurements in magnetic fields up to 40 T and low-field susceptibility, together with ${}^1\mathrm{H}$ nuclear magnetic resonance (NMR) spectra, reveal a dominant intrachain antiferromagnetic exchange coupling of $J_{\text{1D}}/k_B=(11.3\pm 0.1)$ K, and attainable critical magnetic fields of ${\mu }_0{H}_{\text{c1}}\approx 2$ T and ${\mu }_0{H}_{\text{c2}}\approx 33$ T. These measurements therefore suggest that BoNO is a rare example of a Haldane system with extremely small magnetic anisotropy. The present results are crucial for a future in-depth NMR study of the low-temperature Tomonaga-Luttinger liquid and magnetic field-induced phases, which can be performed in the entire phase space.

Thin-film multiphase superconductors are attracting much attention, and it is important to propose how to detect them in experiments. In this work, we study the reciprocal and nonreciprocal paraconductivity of a bilayer model with staggered Rashba-type spin-orbit coupling with and without the potential gradient and Zeeman field. This model shows the Bardeen-Cooper-Schrieffer phase, the pair-density-wave phase, and the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase, and we focus on how their properties are encoded to the charge transport. We show that the reciprocal paraconductivity has a peak associated with the phase transitions between different superconducting states due to the degeneracy of the transition temperatures as well as the paramagnetic depairing effect. We also show that the FFLO superconducting state shows a sizable nonreciprocal paraconductivity once the degeneracy of Cooper pairs is lifted by applying the potential gradient. Observation of the peaked reciprocal and nonreciprocal paraconductivity can be used as a probe of multiphase superconductivity.

The triangular lattice antiferromagnet with $S=1/2$ spins and nearest-neighbor interactions is known to have long-range antiferromagnetic order, with nearest-neighbor spins at an angle of $120{}^{\circ }$. Numerical studies of quantum phases proximate to this state have been limited to small systems because of the sign problem in Monte Carlo simulations in imaginary time. We propose an effective lattice model for quantum fluctuations of the antiferromagnetic order and a sign-problem-free Monte Carlo algorithm enabling studies in large systems sizes. The model is a ${\mathbb{Z}}_2$ gauge theory coupled to gauge-charged scalars which have a relativistic dispersion in the continuum limit. Crucially, the gauge theory is odd, i.e., there is a static, background ${\mathbb{Z}}_2$ gauge charge on each site, accounting for the Berry phases of the half-odd-integer spins on each site. We present results of simulations on lattices of sizes up to $36\times 36\times 36$. Along with the antiferromagnetically ordered phase, our phase diagram has a valence bond solid state with a $\sqrt{12}\times \sqrt{12}$ unit cell and a gapped ${\mathbb{Z}}_2$ spin liquid. Deconfined critical points or phases in intermediate regions are not ruled out by our present simulations.