We present the results of an extensive study of the phase diagram and spin-wave excitations for a general spin model on a hexagonal AB-stacked kagome system. The boundaries of the magnetic phases are determined via a combination of numerical (Monte Carlo) and analytical (Luttinger-Tisza) methods. Depending on the strength of the spin-orbit coupling (SOC), some spin and lattice rotations become decoupled, leading to considerably larger symmetry groups than typical magnetic groups. Thus, we provide a detailed symmetry description of the magnetic Hamiltonian with negligible, weak, and intermediate strengths of SOC. The spin symmetry in these three cases has a strong effect on the splittings observed in the spin excitation spectra. We further identify a number of self-duality transformations that map the Hamiltonian onto itself. These transformations describe the symmetry of the parameter space and provide an exact mapping between the properties of different magnetic orders and lead to accidental degeneracies. Finally, we discuss the physical relevance of our findings in the context of ${\mathrm{Mn}}_{3}X$ compounds.

The determination of accurate magnetic structure of magnetoelectric materials is a mandatory key step to understand their properties. In order to couple to the lattice and induce an electric polarization, this structure is unfortunately complex in most cases, and results from magnetic frustration, either geometrical or exchange in nature. The compound $\mathrm{Ba}{\mathrm{Fe}}_2{\mathrm{Se}}_3$ is one of the rare quasi-one-dimensional compounds that exhibits such magnetoelectric properties. In this paper, thanks to a single crystal neutron diffraction study and symmetry analysis, an umbrellalike magnetic structure has been revealed, with tilted moments in the $(a,c)$ and $(b,c)$ planes, confirmed by polarized neutron diffraction. It corresponds to a magnetic order with antiferromagnetic blocks and a propagation vector $k=(\frac{1}{2},\frac{1}{2},\frac{1}{2})$, belonging to the $C_{a}m$ magnetic space group. This work should allow further theoretical studies of the coupling of this magnetic order with the lattice and the ferroelectric order. It also provides key information on the magnetism, of which fluctuations may be at the origin of the pressure-induced superconductivity in this family.

Laser-induced terahertz spin transport (TST) and ultrafast demagnetization (UDM) are central but so far disconnected phenomena in femtomagnetism and terahertz spintronics. Here, we use broadband terahertz emission spectroscopy to reliably measure both processes in one setup. We find that the rate of UDM in a single simple ferromagnetic metal film $F$ such as ${\mathrm{Co}}_{70}{\mathrm{Fe}}_{30}$ or ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ has the same time evolution as TST from $F$ into an adjacent normal-metal layer $N$ such as $\mathrm{Pt}$ or $\mathrm{W}$. As this remarkable agreement refers to two very different samples, an $F$ layer vs an $F|N$ stack, it does not result from the trivial fact that TST out of $F$ reduces the $F$ magnetization at the same rate. Instead, our observation strongly suggests that UDM in $F$ and TST in $F|N$ are driven by the same force, which is fully determined by the state of the ferromagnet. An analytical model quantitatively explains our measurements and reveals that both UDM in the $F$ sample and TST in the associated $F|N$ stack arise from a generalized spin voltage, i.e., an excess of magnetization, which is defined for arbitrary, nonthermal electron distributions. We also conclude that contributions due to a possible temperature difference between $F$ and $N$, i.e., the spin-dependent Seebeck effect, and optical intersite spin transfer are minor in our experiment. Based on these findings, one can apply the vast knowledge of UDM to TST to significantly increase spin-current amplitudes and, thus, open promising pathways toward energy-efficient ultrafast spintronic devices.

Motivated by the recent experimental realizations of hyperbolic lattices in circuit quantum electrodynamics and in classical electric-circuit networks, we study flat bands and band-touching phenomena in such lattices. We analyze noninteracting nearest-neighbor hopping models on hyperbolic analogs of the kagome and dice lattices with heptagonal and octagonal symmetries. We show that two characteristic features of the energy spectrum of those models, namely, the fraction of states in the flat band as well as the number of touching points between the flat band and the dispersive bands, can both be captured exactly by a combination of real-space topology arguments and a reciprocal-space description via the formalism of hyperbolic band theory. Furthermore, using real-space numerical diagonalization on finite lattices with periodic boundary conditions, we obtain insights into higher-dimensional irreducible representations of the non-Euclidean (Fuchsian) translation group of hyperbolic lattices. First, we find that the fraction of states in the flat band is the same for Abelian and non-Abelian hyperbolic Bloch states. Second, we find that only Abelian states participate in the formation of touching points between the flat and dispersive bands.

Models for nonunitary quantum dynamics, such as quantum circuits that include projective measurements, have recently been shown to exhibit rich quantum critical behavior. There are many complementary perspectives on this behavior. For example, there is a known correspondence between $d$-dimensional local nonunitary quantum circuits and tensor networks on a $[D=(d+1\left)\right]$-dimensional lattice. Here, we show that in the case of systems of noninteracting fermions, there is furthermore a full correspondence between nonunitary circuits in $d$ spatial dimensions and unitary noninteracting fermion problems with static Hermitian Hamiltonians in $D=(d+1)$ spatial dimensions. This provides a powerful perspective for understanding entanglement phases and critical behavior exhibited by noninteracting circuits. Classifying the symmetries of the corresponding noninteracting Hamiltonian, we show that a large class of random circuits, including the most generic circuits with randomness in space and time, are in correspondence with Hamiltonians with static spatial disorder in the 10 Altland-Zirnbauer symmetry classes. We find the criticality that is known to occur in all of these classes to be the origin of the critical entanglement properties of the corresponding random nonunitary circuit. To exemplify this, we numerically study the quantum states at the boundary of Haar-random Gaussian fermionic tensor networks of dimension $D=2$ and 3. We show that the most general such tensor network ensemble corresponds to a unitary problem of noninteracting fermions with static disorder in Altland-Zirnbauer symmetry class DIII, which for both $D=2$ and 3 is known to exhibit a stable critical metallic phase. Tensor networks and corresponding random nonunitary circuits in the other nine Altland-Zirnbauer symmetry classes can be obtained from the DIII case by implementing Clifford algebra extensions for classifying spaces.

An electronic/magnetic order often turns into a different phase of matter when the temperature, pressure, or magnetic field varies, and resulting changes in physical quantities form the basis of various switching devices. By contrast, global phase changes triggered by electric current with minimal Joule heating often remain elusive, although it may offer exotic possibilities for phase manipulation in micro/nanoscale materials. Here, we report the experimental and numerical realizations of a nonthermal current-induced magnetic transition for a spatially confined skyrmion-hosting system. In a microfabricated MnSi, the application of a DC current triggers a nonvolatile suppression of the topological Hall effect, comparable in magnitude to that observed for the skyrmion-to-nonskyrmion thermodynamic transition of the whole system. A similar current-induced global transition is also found in numerical simulations with open boundaries. These phenomena are not seen either in the corresponding bulk system or in the simulation with periodic boundaries, thus, indicating a key role of the confined geometry in the nonthermodynamic phase change triggered by current.

Stark many-body localized (SMBL) systems have been shown both numerically and experimentally to have Bloch many-body oscillations, quantum many-body scars, and fragmentation in the large field tilt limit, but these observations have not been fundamentally understood. We explain and analytically prove all these observations by rigorously perturbatively showing the existence of certain algebraic structures that are exponentially stable in time. In particular, we show that many-body Bloch oscillations persist even at infinite temperature for exponentially long times using a type of dynamical algebra which we refer to as dynamical l-bits and provide a bound on the tilt strength for this nonergodic transition. We numerically confirm our results by studying the prototypical Stark MBL model of a tilted XXZ spin chain. Our work explains why thermalization was observed in a recent two-dimensional tilted experiment. As dynamical l-bits represent stable, localized, and quantum coherent excitations, our work opens possibilities for quantum information processing in Stark MBL systems even at high temperature.

In recent years, it has been shown that the phonon dynamics may serve as an indirect probe of fractionalization of spin degrees of freedom. Here we propose that the sound attenuation measurements allows for the characterization and identification of the Kitaev quantum spin liquid on the hyperhoneycomb lattice, which is particularly interesting since the strong Kitaev interaction was observed in the the hyperhoneycomb magnet $\beta \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$. To this end we consider the low-temperature scattering between acoustic phonons and gapless Majorana fermions with nodal-line band structure. We find that the sound attenuation has a characteristic angular dependence, which is explicitly shown for the high-symmetry planes at temperatures below the flux energy gap.

Conductive domain walls (DWs) in insulating ferroelectrics have recently attracted considerable attention due to their unique topological, optical, and electronic properties, and offer potential applications such as in memory devices or rewritable circuitry. The electronic properties of DWs can be tuned by the application of strain, hence controlling the charge carrier density at DWs. In this paper, we study the influence of uniaxial stress on the conductivity of DWs in the bulk single crystal lithium niobate $\left({\mathrm{LiNbO}}_3\right)$. Using conductive atomic force microscopy, we observe a large asymmetry in the conductivity of DWs, where only negatively screened walls, so called head-to-head DWs, are becoming increasingly conductive, while positively screened, tail-to-tails DWs, show a decrease in conductivity. This asymmetry of DW conductivity agrees with our theoretical model based on the piezoelectric effect. In addition, we observed that the current in the DW increases up to an order of magnitude for smaller compressive stresses of 100 MPa. This response of DWs remained intact for multiple stress cycles over two months, opening a path for future applications.

Ultrafast optical pumping of spatially nonuniform magnetic textures is known to induce far-from-equilibrium spin transport effects. Here, we use ultrafast x-ray diffraction with unprecedented dynamic range to study the laser-induced dynamics of labyrinth domain networks in ferromagnetic CoFe/Ni multilayers. We detected azimuthally isotropic, odd order, magnetic diffraction rings up to fifth order. The amplitudes of all three diffraction rings quench to different degrees within 1.6 ps. In addition, all three of the detected diffraction rings both broaden by 15% and radially contract by 6% during the quench process. We are able to rigorously quantify a 31% ultrafast broadening of the domain walls via Fourier analysis of the order-dependent quenching of the three detected diffraction rings. The broadening of the diffraction rings is interpreted as a reduction in the domain coherence length, but the shift in the ring radius, while unambiguous in its occurrence, remains unexplained. In particular, we demonstrate that a radial shift explained by domain-wall broadening can be ruled out. With the unprecedented dynamic range of our data, our results provide convincing evidence that labyrinth domain structures are spatially perturbed at ultrafast speeds under far-from-equilibrium conditions, albeit the mechanism inducing the perturbations remains yet to be clarified.

The crystal electric field (CEF) plays an essential role in defining the magnetic properties of $4f$ materials. It forces the charge density of $4f$ electrons and the related magnetic moment to be oriented along a certain direction in the crystal. The CEF and related magnetic properties were widely studied in the past with focus on bulk of $4f$ materials, while their surfaces have not received much attention. By the example of the antiferromagnetic material ${\mathrm{TbRh}}_2{\mathrm{Si}}_2$ and using first-principles calculations and classical $4f$ angle-resolved photoemission (PE) measurements, we show how the CEF and related magnetic properties, linked with the orientation of $4f$ moments, are modified at the surface region. Precisely, we studied the CEF characteristics in individual Tb layers for Tb- and Si-terminated surfaces of ${\mathrm{TbRh}}_2{\mathrm{Si}}_2$. We show how strongly the CEF changes near the surface and how dramatically it influences the orientation of the $4f$ moments relative to the bulk. The instructive message of our study is that a rather valuable information about the CEF-related phenomena can be derived from the temperature dependence of $4f$ PE spectra. The presented methodology including the theoretical approach can be further applied to many other layered and quasi-2D rare-earth-based materials for unveiling their surface magnetic properties.

A central property of Chern insulators is the robustness of the topological phase and edge states to impurities in the system. Despite this, the Chern number cannot be straightforwardly calculated in the presence of disorder. Recently, work has been done to propose several local analogs of the Chern number, called local markers, that can be used to characterize disordered systems. However, it was unclear whether the proposed markers represented a physically measurable property of the system. Here we propose a local marker starting from a physical argument, as a local cross conductivity measured in the bulk of the system. We find the explicit form of the marker for a noninteracting system of electrons on the lattice and show that it corresponds to existing expressions for the Chern number. Examples are calculated for a variety of disordered and amorphous systems, showing that it is precisely quantized to the Chern number and robust against disorder.

We investigated the spin excitations of magnetoelectric ${\mathrm{LiFePO}}_4$ by THz absorption spectroscopy in magnetic fields up to 33 T. By studying their selection rules, we found not only magnetic-dipole, but also electric-dipole active (electromagnons) and magnetoelectric resonances. The magnetic field dependence of four strong low-energy modes is reproduced well by a four-spin mean-field model for fields applied along the three orthorhombic axes. From the fit of magnetization and magnon frequencies, we refined the exchange couplings, single-ion anisotropies, and the Dzyaloshinskii-Moriya interaction parameters. Additional spin excitations not described by the mean-field model are observed at higher frequencies. Some of them show a strong shift with the magnetic field, up to 4 ${\mathrm{cm}}^{-1}$ ${\mathrm{T}}^{-1}$, when the field is applied along the easy axis. Based on this field dependence, we attribute these high frequency resonances to the excitation of higher spin multipoles and of two magnons, which become THz-active due to the low symmetry of the magnetically ordered state.

We report magnetic optical second-harmonic generation (SHG) polarimetry and imaging on $\mathrm{Sr}{}_2\mathrm{Cu}{}_3\mathrm{O}{}_4\mathrm{Cl}{}_2$, which allows direct visualization of the mesoscopic antiferromagnetic (AFM) structure of a parent cuprate. Temperature- and magnetic-field-dependent SHG reveals large domains with ${90}^{\circ }$ relative orientations that are stabilized by a combination of uniaxial magnetic anisotropy and the Earth's magnetic field. Below a temperature $T_{\mathrm{R}}$ $\sim$ 97 K, we observe an unusual ${90}^{\circ }$ spin-reorientation transition, possibly driven by competing magnetic anisotropies of the two copper sublattices, which swaps the AFM domain states while preserving the domain structure. This allows deterministic switching of the AFM states by thermal or laser heating. Near $T_{\mathrm{R}}$, the domain walls become exceptionally responsive to an applied magnetic field, with the Earth's field sufficient to completely expel them from the crystal. Our findings unlock opportunities to study the mesoscopic AFM behavior of parent cuprates and explore their potential for AFM technologies.

We present a joint experimental and theoretical study of parametric resonance of spin-wave eigenmodes in ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}/\mathrm{Pt}$ bilayer nanowires. Using electrically detected magnetic resonance, we measure the spectrum of spin-wave eigenmodes in transversely magnetized nanowires and study parametric excitation of these eigenmodes by a microwave magnetic field. We also develop an analytical theory of spin-wave eigenmodes and their parametric excitation in the nanowire geometry that takes into account magnetic dilution at the nanowire edges. We measure tuning of the parametric resonance threshold by antidamping spin Hall torque from a direct current for the edge and bulk eigenmodes, which allows us to independently evaluate frequency, damping, and ellipticity of the modes. We find good agreement between theory and experiment for parametric resonance of the bulk eigenmodes but significant discrepancies arise for the edge modes. The data reveal that ellipticity of the edge modes is significantly lower than expected, which can be attributed to strong modification of magnetism at the nanowire edges. Our work demonstrates that parametric resonance of spin-wave eigenmodes is a sensitive probe of magnetic properties at edges of thin-film nanomagnets.

The flip-flop qubit, encoded in the states with antiparallel donor-bound electron and donor nuclear spins in silicon, showcases long coherence times, good controllability, and, in contrast to other donor-spin-based schemes, long-distance coupling. Electron spin control near the interface, however, is likely to shorten the relaxation time by many orders of magnitude, reducing the overall qubit quality factor. Here, we theoretically study the multilevel system that is formed by the interacting electron and nuclear spins and derive analytical effective two-level Hamiltonians with and without periodic driving. We then propose an optimal control scheme that produces fast and robust single-qubit gates in the presence of low-frequency noise without relying on parametrically restrictive sweet spots. This scheme increases considerably both the relaxation time and the qubit quality factor.

The low-energy physics of interacting quantum systems is typically understood through the identification of the relevant quasiparticles or low-energy excitations and their quantum numbers. We present a quantum information framework that goes beyond this to examine the nature of the entanglement in the corresponding quantum states. We argue that the salient features of the quasiparticles, including their quantum numbers, locality, and fractionalization, are reflected in the entanglement spectrum and in the mutual information. We illustrate these ideas in the specific context of the $d=1$ transverse field Ising model with an integrability breaking perturbation.

Noncentrosymmetric ${\mathrm{Ce}}_3{\mathrm{Bi}}_4{\mathrm{Pd}}_3$ has attracted a lot of attention as a candidate for strongly correlated topological material, yet its experimental ground state remains a matter of contention. Two conflicting scenarios have emerged from a comparison to the prototypical Kondo insulator ${\mathrm{Ce}}_3{\mathrm{Bi}}_4{\mathrm{Pt}}_3$: Either ${\mathrm{Ce}}_3{\mathrm{Bi}}_4{\mathrm{Pd}}_3$ is a spin-orbit-driven topological semimetal or a Kondo insulator with smaller Kondo coupling than its Pt counterpart. Here, we determine the ground state of ${\mathrm{Ce}}_3{\mathrm{Bi}}_4{\mathrm{Pd}}_3$ via electrical resistivity measurements under hydrostatic pressure, which is a clean symmetry-preserving tuning parameter that increases hybridization but virtually preserves spin-orbit coupling. ${\mathrm{Ce}}_3{\mathrm{Bi}}_4{\mathrm{Pd}}_3$ becomes more insulating under pressures up to 2.3 GPa, which is a signature of Ce-based Kondo insulating materials in the considered pressure range. Its small zero-pressure gap increases quadratically with pressure, similar to the behavior observed in the series ${\mathrm{Ce}}_3{\mathrm{Bi}}_4{\left({\mathrm{Pt}}_{1-x}{\mathrm{Pd}}_x\right)}_3$, which indicates that Pt substitution and applied pressure have a similar effect. Our result not only demonstrates that Kondo coupling, rather than spin-orbit coupling, is the main tuning parameter in this class of materials, but it also establishes that ${\mathrm{Ce}}_3{\mathrm{Bi}}_4{\mathrm{Pd}}_3$ has a narrow-gap Kondo insulating ground state.

We use the projective symmetry group analysis to classify the quantum spin liquids on the $S=1/2$ pyrochlore magnet with a breathing anisotropy. We find 40 ${\mathbb{Z}}_2$ spin liquids and 16 $U\left(1\right)$ spin liquids that respect the $F\overline{4}3m$ space group and the time reversal symmetry. As an application, we consider the antiferromagnetic Heisenberg model, which is proposed to be the dominant interaction in the candidate material ${\mathrm{Ba}}_3{\mathrm{Yb}}_2{\mathrm{Zn}}_5{\mathrm{O}}_{11}$. Focusing on the $U\left(1\right)$ spin liquid Ansätze, we find that only two of them are physical when restricted to this model. We present an analytical solution to the parton mean field theory for each of these two $U\left(1\right)$ spin liquids. It is revealed that one of them has gapless, while the other one has gapped, spinon excitations. The two $U\left(1\right)$ spin liquids are equal in energy regardless of the degree of breathing anisotropy, and they can be differentiated by the low-temperature heat capacity contribution from the quadratically dispersing gapless spinons. We further show that the latter is unaffected by fluctuations of the $U\left(1\right)$ gauge field within the random phase approximation. Finally, we demonstrate that a small Dzyaloshinskii-Moriya interaction lifts the degeneracy between the two $U\left(1\right)$ spin liquids, and it eventually causes the lattice to decouple into independent tetrahedra at strong coupling. While current model parameters for ${\mathrm{Ba}}_3{\mathrm{Yb}}_2{\mathrm{Zn}}_5{\mathrm{O}}_{11}$ place it indeed in the decoupled regime, other candidate materials may be synthesized in the near future that realize the spin liquid states discussed in our work.

The nature of an electric ferroaxial moment characterizing a time-reversal-even axial-vector quantity is theoretically investigated under magnetic orderings. We clarify that a vortex spin texture results in the emergence of the ferroaxial moment depending on the helicity. By introducing a multipole description, we show that the ferroaxial nature appears when two types of odd-parity magnetic multipoles become active under the vortex spin texture: One is the magnetic monopole and the other is the magnetic toroidal dipole. We present all the magnetic point groups to possess the ferroaxial moment in the presence of the magnetic orderings with the magnetic monopole and magnetic toroidal dipole. Moreover, we specifically consider a multiorbital model in a four-sublattice tetragonal system to demonstrate the ferroaxial moment induced by the vortex spin texture. We show that the ferroaxial moment is microscopically characterized by both the cluster-scale and atomic-scale axial-vector quantities: The former is described by the vortex of the local electric dipole and the latter is represented by the outer product of the local orbital and spin angular momenta. Moreover, we find that the atomic spin-orbit coupling and the hybridization between orbitals with different parity are key ingredients to induce the ferroaxial moment. We also apply the result to a magnetic skyrmion with a topologically nontrivial spin texture and discuss candidate materials.