The efficiency of the ultrafast excitation of spins in antiferromagnetic $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ using a nearly single-cycle THz pulse is studied as a function of the polarization of the THz pulse and the sample temperature. Above the Morin point the most efficient excitation is achieved when the magnetic field of the THz pulse is perpendicular to the antiferromagnetically coupled spins. Using the experimental results and equations of motion for spins, we show that the mechanism of the spin excitation above and below the Morin point relies on a magnetic-dipole interaction of the THz magnetic field with spins, and the mechanism implies that the efficiency of the coupling is proportional to the time derivative of the magnetic field.

We present a systematic study of higher-order exchange interactions beyond the pairwise Heisenberg exchange in transition metal trilayers based on density functional theory calculations. We show that these terms can play an important role in magnetic trilayers composed of a single hexagonal Fe or Co atomic layer sandwiched between $4d$ and $5d$ transition metal layers. We study the dependence of the biquadratic and the three-site and four-site four spin interaction on the band filling of the $4d$ and $5d$ layers as well as the stacking sequence, i.e., fcc vs. hcp stacking. Our calculations reveal relatively small higher-order interactions for Co based trilayers. For Fe based trilayers with a Rh or Ir layer the higher-order terms can be on the same order of magnitude as pairwise Heisenberg exchange. The trends obtained for freestanding trilayers are used to understand the higher-order interactions in ultrathin film systems on surfaces that are experimentally accessible. It is shown that hcp-Rh/Fe/Ir(111) and hcp-Rh/Fe/Rh(111) exhibit the largest values for the biquadratic and the three-site four spin interaction of all systems under study. We further demonstrate that the three-site four spin interaction is responsible for the experimentally observed change of the magnetic ground state of Rh/Fe/Ir(111) from a spin spiral (single-$Q$) for fcc-Rh to a $2Q$ state for hcp-Rh. We find similar trends for Rh/Fe/Rh(111), i.e., replacing the Ir surface by the isoelectronic Rh surface. For Rh/Co/Ir(111), we obtain a negative value for the four-site four spin interaction which will lead to a reduced stability of magnetic skyrmions which are metastable in this film at zero magnetic field. In contrast, for Pd/Fe/Ir(111), the four-site four spin interaction is positive which leads to an enhanced stability of skyrmions.

The existence and stability of spin-liquid phases represent a central topic in the field of frustrated magnetism. While a few examples of spin-liquid ground states are well established in specific models (e.g., the Kitaev model on the honeycomb lattice), recent investigations have suggested the possibility of their appearance in several Heisenberg-like models on frustrated lattices. An important related question concerns the stability of spin liquids in the presence of small perturbations in the Hamiltonian. In this respect, the magnetoelastic interaction between spins and phonons represents a relevant and physically motivated perturbation, which has been scarcely investigated so far. In this work, we study the effect of the spin-phonon coupling on prototypical models of frustrated magnetism. We adopt a variational framework based upon Gutzwiller-projected wave functions implemented with a spin-phonon Jastrow factor, providing a full quantum treatment of both spin and phonon degrees of freedom. The results on the frustrated $J_1\text{−}{J}_2$ Heisenberg model on one- and two-dimensional (square) lattices show that, while a valence-bond crystal is prone to lattice distortions, a gapless spin liquid is stable for small spin-phonon couplings. In view of the ubiquitous presence of lattice vibrations, our results are particularly important to demonstrate the possibility that gapless spin liquids may be realized in real materials.

The electron transfer dynamics between self-assembled quantum dots and their environment are measured under nonequilibrium conditions by time-dependent capacitance spectroscopy. The quantum dots are embedded in a wide spacer, which inhibits elastic tunneling to or from the reservoirs. At certain bias voltages, electron capture and emission are both significant. A rate equation model is used to determine the corresponding transfer rates and the average occupation numbers of the dots as a function of the bias voltage.

We investigate in atomic scale the electronic phase separation occurring in the well known quasi-one-dimensional (quasi-1D) charge-density wave (CDW) phase of an In atomic wire array on a Si(111) surface. The characteristic atomic scale defects, originating from excess In atoms, are found to be actively involved in the formation of the phase boundary between the metallic and the CDW phases by extensive analysis of scanning tunneling microscopy images at various temperatures. These particular defects flip the phase of the quasi-1D CDW to impose strong local constraints in the CDW correlation. We show that such local constraints and the substantial interwire CDW interaction induce local condensates of CDW and the phase separation between the metallic and the CDW phases. This work unveils the atomistic origin of the electronic phase separation, highlighting the importance of atomic scale structures of defects and their collective interaction in electronically inhomogeneous materials.

Optimization of quantum states using the variational principle has recently seen an upsurge due to developments of increasingly expressive wave functions. In order to improve on the accuracy of the Ansätze, it is a time-honored strategy to impose the systems' symmetries. We present an Ansatz where global non-Abelian symmetries are inherently embedded in its structure. We extend the model to incorporate lattice symmetries as well. We consider the prototypical example of the frustrated two-dimensional $J_1\text{−}{J}_2$ model on a square lattice, for which eigenstates have been hard to model variationally. Our novel approach guarantees that the obtained ground state will have total spin zero. Benchmarks on the 2D $J_1\text{−}{J}_2$ model demonstrate its state-of-the-art performance in representing the ground state. Furthermore, our methodology permits to find the wave functions of excited states with definite quantum numbers associated to the considered symmetries (including the non-Abelian ones), without modifying the architecture of the network.

We not only reproduce a burst of short-wavelength spin waves (SWs) observed in a recent experiment [S. Woo et al., Nat. Phys. 13, 448 (2017)] on magnetic-field-driven annihilation of two magnetic domain walls (DWs) but, furthermore, we predict that this setup additionally generates highly unusual pumping of electronic spin currents in the absence of any bias voltage. Prior to the instant of annihilation, their power spectrum is ultrabroadband, so they can be converted into rapidly changing in time charge currents, via the inverse spin Hall effect, as a source of THz radiation of bandwidth $\simeq 27$ THz where the lowest frequency is controlled by the applied magnetic field. The spin pumping stems from time-dependent fields introduced into the quantum Hamiltonian of electrons by the classical dynamics of localized magnetic moments (LMMs) comprising the domains. The pumped currents carry spin-polarized electrons which, in turn, exert backaction on LMMs in the form of nonlocal damping which is more than twice as large as conventional local Gilbert damping. The nonlocal damping can substantially modify the spectrum of emitted SWs when compared to widely used micromagnetic simulations where conduction electrons are completely absent. Since we use a fully microscopic (i.e., Hamiltonian-based) framework, self-consistently combining time-dependent electronic nonequilibrium Green functions with the Landau-Lifshitz-Gilbert equation, we also demonstrate that previously derived phenomenological formulas miss ultrabroadband spin pumping while underestimating the magnitude of nonlocal damping due to nonequilibrium electrons.

While the Berezinskii-Kosterlitz-Thouless transition (BKT) has been under intense scrutiny for decades, unambiguous experimental signatures in magnetic systems remain elusive. Here, we investigate the interplay between electronic and magnetic degrees of freedom near the BKT transition. Focusing on a metal with easy-plane ferromagnetic order, we establish a framework that accounts both for the coupling between the charge current and the flow of topological magnetic defects and for electron scattering on their inhomogeneous spin texture. We show that electron scattering is responsible for a temperature-dependent magnetoresistance effect scaling as the density of the topological defects, which is expected to increase dramatically above the BKT transition temperature. Our findings call for further experimental investigations.

We report a type of synchronization, termed dancing synchronization, between two spin-torque nano-oscillators (STNOs) coupled through spin waves. Different from the known synchronizations in which two STNOs are locked with various fixed relative phases, in this synchronized state two STNOs have the same frequency, but their relative phase varies periodically within the common period, resulting in a dynamic waving pattern. The amplitude of the oscillating relative phase depends on the coupling strength of two STNOs, as well as the driven currents. The dancing synchronization turns out to be universal and can exist in two nonlinear Van der Pol oscillators coupled both reactively and dissipatively. Our findings open the door for new functional STNO-based devices.

Composite quantum materials are the ideal examples of multifunctional systems, which simultaneously host more than one novel quantum phenomenon in physics. Here, we present a combined theoretical and experimental study to demonstrate the presence of an extremely large exchange bias in the range 0.8–2.7 T and a fully compensated magnetic state (FCF) in a special type of Pt and Ni-doped ${\mathrm{Mn}}_3\mathrm{In}$ cubic alloy. Here, oppositely aligned uncompensated moments in two different atomic clusters sum up to zero, which are responsible for the FCF state. Our density functional theory (DFT) calculations show the existence of several possible ferrimagnetic configurations with the FCF as the energetically most stable one. The microscopic origin of the large exchange bias can be interpreted in terms of the exchange interaction between the FCF background and the uncompensated ferrimagnetic clusters stabilized due to its negligible energy difference with respect to the FCF phase. We utilize pulsed magnetic field up to 60 T and 30 T static-field magnetization measurements to confirm the intrinsic nature of exchange bias in our system. Finally, our Hall effect measurements demonstrate the importance of uncompensated noncoplanar interfacial moments for the realization of large EB. The present finding of gigantic exchange bias in a unique compensated ferrimagnetic system opens up a direction for the design of novel quantum phenomena for the technological applications.

The magnetic properties of the two-dimensional ${\mathrm{VI}}_3$ bilayer are the focus of our first-principles analysis, highlighting the role of $t_{2g}$ orbital splitting and carried out in comparison with the ${\mathrm{CrI}}_3$ prototypical case, where the splitting is negligible. In ${\mathrm{VI}}_3$ bilayers, the empty $a_{1g}$ state is found to play a crucial role in both stabilizing the insulating state and in determining the interlayer magnetic interaction. Indeed, an analysis based on maximally localized Wannier functions allows one to evaluate the interlayer exchange interactions in two different ${\mathrm{VI}}_3$ stackings (labeled AB and ${\mathrm{AB}}^{\prime }$), to interpret the results in terms of the virtual-hopping mechanism, and to highlight the strongest hopping channels underlying the magnetic interlayer coupling. Upon application of electric fields perpendicular to the slab, we find that the magnetic ground state in the ${\mathrm{AB}}^{\prime }$ stacking can be switched from antiferromagnetic to ferromagnetic, suggesting the ${\mathrm{VI}}_3$ bilayer as an appealing candidate for electric-field-driven miniaturized spintronic devices.

A long-standing issue in multiferroic ${\mathrm{MnWO}}_4$ is the electric polarization reversal when magnetic field is applied along the magnetic easy axis $\left(H\left|\right|u\right)$. In this work, we performed comprehensive ferroelectric polarization measurements on ${\mathrm{MnWO}}_4$ in fields up to 52 T and with rotating field direction in the $ac$ plane. When the field is rotated from $H\left|\right|c$ to $H\left|\right|u$, the induced high-field phase $(-P|\left|b\right)$ favors a sign reversal relative to the low-field $+P\left|\right|b$ phase, which is attributed to the vector spin chirality changing from one side of the spin connecting vector to the other. Particularly, further slight deviation of the field from the magnetic easy axis towards $H\left|\right|a$ causes an abrupt reversal of the high-field phase from $-P\left|\right|b$ to $+P\left|\right|b$. We assume that this unusual angular dependence of polarization behavior can be understood by magnetic control of the helicity of the chiral plane with opposite conical spin structures. In addition, we also propose electric-field control of the chiral plane close to the ferroelectric phase transition at high fields. This study results in an overall understanding of this important multiferroic material and sheds light on field-induced polarization switching in related compounds.

We measured the complex conductivity, $\sigma$, of $\mathrm{Fe}{\mathrm{Se}}_{1-x}{\mathrm{Te}}_x$ $(x=0-0.5)$ films in the superconducting state which show a drastic increase of the superconducting transition temperature $T_{\text{c}}$ when the nematic order disappears. Since the magnetic penetration depth $\lambda (>400$ nm) of Fe(Se,Te) is longer than the typical thickness of the film $(\sim 100 \mathrm{nm})$, we combined the coplanar waveguide resonator and cavity perturbation techniques to evaluate both the real and imaginary parts of $\sigma$. Films with a nematic order showed a qualitatively different temperature dependence in penetration depth and quasiparticle scattering time when compared with those without nematic order, suggesting that nematic order influences the superconducting gap structure. Conversely, the proportionality between superfluid density $n_{\text{s}}$ $(\propto {\lambda }^{-2})$ and $T_{\text{c}}$ was observed irrespective of the presence or absence of nematic order. This result indicates that the amount of superfluid has a stronger impact on the $T_{\text{c}}$ of Fe(Se,Te) than the presence or absence of nematic order. Combining these results with band dispersions calculated using density functional theory, we propose that the change of the Fermi surface associated with nematicity is the primary factor influencing the change of $T_{\text{c}}$ and the superconducting gap structure in Fe(Se,Te).

The conditions for the excitation of transverse and longitudinal polar modes in the processes of Raman scattering of light in mono- and polydomain lithium tantalate single crystals are established using a backscattering geometry. The relative intensity of the overtone Raman spectrum on the longitudinal and transverse fundamental polar excitations is much higher in the polydomain sample. Unfortunately, the exact reason for the difference in the Raman spectra of the mono- and polydomain lithium tantalate samples is unknown. The frequencies of the two overtone bands exceeded the exact value of the overtone frequency corresponding to the fundamental longitudinal polar mode $4A_1\left(\text{LO}\right)$, which clearly indicates the existence of bound phonon pairs (biphonons). This is consistent with theory and has been shown for both mono- and polydomain samples. On the basis of the general theory of bound states of quasiparticles, the possibility of the existence of bound states of quadrupole excitations of the antipolar type in the vibrational spectrum of a polydomain crystal of lithium tantalate is predicted.

There is growing interest in the photoinduced generation of rectified current, namely, photocurrent phenomenon. While the response was attributed to noncentrosymmetric structures of crystals, the parity violation accompanied by the magnetic ordering, that is, magnetic parity violation, is recently attracting attention as a platform for a photocurrent generator. In this paper, we investigate the photocurrent response in the current-ordered phase, realizing the magnetic parity violation without the spin degree of freedom, although prior studies focused on the parity-violating spin structure. The loop-current order breaks the inversion symmetry while preserving the parity-time-reversal symmetry. With a model of ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$, we demonstrate the linearly and circularly polarized light-induced photocurrent responses in the current-ordered state. Each photocurrent has a distinct tolerance of the scattering rate according to the mechanism for the photocurrent creation. The predicted photocurrent response is comparable to that in prototypical semiconductors. We propose a probe to detect the hidden-ordered phase in ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$ by the photocurrent response.

Contrasting ground states of quantum magnets with the integer- and half-integer-spin moments are the manifestation of many-body quantum interference effects. In this work, we investigate the distinct nature of the integer- and half-integer-spin quantum spin liquids in the framework of the Kitaev's model on the honeycomb lattice. The models with arbitrary spin quantum numbers are not exactly solvable in contrast to the well-known quantum spin liquid solution of the spin-1/2 system. We use the tensor-network wave functions for the integer-and half-integer-spin quantum spin liquid states to unveil the important difference between these states. We find that the distinct sign structures of the tensor-network wave function for the integer- and half-integer-spin quantum spin liquids are responsible for completely different ground states in the spatially anisotropic limit. Hence the spatial anisotropy would be a useful diagnostic test for distinguishing these quantum spin liquid states, both in the numerical computations and experiments on real materials. We support this discovery via extensive numerics including the tensor-network, DMRG, and exact diagonalization computations.

Despite their simple formulation, short-range classical antiferromagnetic Ising models on frustrated lattices give rise to exotic phases of matter, in particular, due to their macroscopic ground-state degeneracy. Recent experiments on artificial spin systems comprising arrays of chirally coupled nanomagnets provide a significant strengthening of the nearest-neighbor couplings compared to systems with dipolar-coupled nanomagnets. This opens the way to design artificial spin systems emulating Ising models with nearest-neighbor couplings. In this paper, we compare the results of an extensive investigation with tensor network and Monte Carlo simulations of the nearest- and farther-neighbor $(J_1-J_2-J_{3\left|\right|})$ kagome Ising antiferromagnet with the experimental spin-spin correlations of a kagome lattice of chirally coupled nanomagnets. Even though the ratios between the farther-neighbor couplings and the nearest-neighbor coupling estimated from micromagnetic simulations are much smaller than for dipolar-coupled nanomagnets, we show that they still play an essential role in the selection of the correlations.

We propose a multiphysical computational approach that allows for efficient coupling of full-vector Maxwell-based propagation codes with kinetic Boltzmann equations to investigate the spatial dynamics of non-equilibrium processes in plasmonic nanostructures upon intense laser excitation. Accessing the energy-resolved electron distribution provides a direct path towards multidimensional modeling of transient optical, electron emission, and electron transport processes. Simulations are performed for a gold nanoparticle upon infrared ultrashort-pulse excitation close to the melting threshold, evidencing the interplay between strong intrinsic and (non)thermal nonlinearities and accessing simultaneously the non-equilibrium thermal and propagation dynamics. While delivering the results within a reasonable simulation time and while being open to further extensions, the proposed approach can serve as a reliable compromise between point quantum and space-dimensional classical models.

Studying the tight-binding model in an applied rational magnetic field $\left(H\right)$ we show that in graphene there are very unusual Landau levels situated in the immediate vicinity of the saddle point ($M$ point) energy ${\epsilon }_M$. Landau levels around ${\epsilon }_M$ are broadened into minibands (even in relatively weak magnetic fields, $\sim 40–53$ T), with the maximal width reaching 0.4–0.5 of the energy separation between two neighboring Landau levels, though at all other energies the width of Landau levels is practically zero. In terms of the semiclassical approach a broad Landau level or magnetic miniband at ${\epsilon }_M$ is a manifestation of the so-called self-intersecting orbit signifying an abrupt transition from the semiclassical trajectories enclosing the $\mathrm{\Gamma }$ point to the trajectories enclosing the $K$ point in the momentum space. The structure of broad magnetic minibands is clearly fractal, demonstrating fast oscillations of the band energy $E\left(k\right)$ with the magnetic wave vector $k$. Remarkably, the saddle point virtually does not affect the diamagnetic response of graphene, which is caused mostly by electron states in the vicinity of the Fermi energy ${\epsilon }_F$. Experimentally, the effect of the broadening of Landau levels can possibly be observed in twisted graphene where two saddle point singularities can be brought close to the Fermi energy. At zero temperature the longitudinal conductivity is expected to be proportional to the calculated magnetic density of states at the Fermi level.