The nodal line semimetals have attracted much attention due to their unique topological electronic structure and exotic physical properties. A genuine nodal-line semimetal is qualified by the presence of Dirac nodes along a line in the momentum space that are protected against the spin-orbit coupling. In addition, it requires that the Dirac points lie close to the Fermi level, allowing one to dictate the macroscopic physical properties. Although the material realization of nodal-line semimetals have been theoretically predicted in numerous compounds, only a few of them have been experimentally verified and the realization of a genuine nodal-line semimetal is particularly rare. Here we report the realization of a genuine nodal-line semimetal in LaSbTe. We investigated the electronic structure of LaSbTe by band structure calculations and angle-resolved photoemission (ARPES) measurements. Taking spin-orbit coupling into account, our band structure calculations predict that a nodal line is formed in the boundary surface of the Brillouin zone, which is robust and lies close to the Fermi level. The Dirac nodes along the X-R line in momentum space are directly observed in our ARPES measurements, and the energies of these Dirac nodes are all close to the Fermi level. These results constitute clear evidence that LaSbTe is a genuine nodal-line semimetal, providing a platform to explore for novel phenomena and possible applications associated with the nodal-line semimetals.

We report a successful application of the ab initio quantum Monte Carlo (QMC) framework to a phonon dispersion calculation. A full phonon dispersion of diamond is successfully calculated at the variational Monte Carlo (VMC) level, based on the frozen-phonon technique. The VMC-phonon dispersion is in good agreement with the experimental results, giving renormalized harmonic optical frequencies very close to the experimental values, and improving upon previous density functional theory estimates. Key to success for the QMC approach is the statistical error reduction in the atomic force evaluation. We show that this can be achieved by using well conditioned atomic basis sets and by explicitly removing the basis-set redundancy, which reduces the statistical error of forces by up to two orders of magnitude by combining it with the so-called space-warp transformation algorithm. This leads to affordable and accurate QMC-phonons calculations, which are up to ${10}^4$ times more efficient than a bare force treatment, and paves the way to new applications, particularly in correlated materials, where phonons have been poorly reproduced so far.

The usual concepts of topological physics, such as the Berry curvature, are not always relevant for non-Hermitian systems. We show that another object, the quantum metric, which often plays a secondary role in Hermitian systems, becomes a crucial quantity near exceptional points in non-Hermitian systems, where it diverges in a way that fully controls the description of wave-packet trajectories. The quantum metric behavior is responsible for a constant acceleration with a fixed direction, and for a nonvanishing constant velocity with a controllable direction. Both contributions are independent of the wave-packet size.

In Dirac semimetals, interband mixing has been known theoretically to give rise to a giant orbital diamagnetism when the Fermi level is close to the Dirac point. In ${\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x$ and other Dirac semimetals, an enhanced diamagnetism in the magnetic susceptibility χ has been observed and interpreted as a manifestation of such giant orbital diamagnetism. Experimentally proving their orbital origin, however, has remained challenging. The cubic antiperovskite ${\mathrm{Sr}}_3\mathrm{PbO}$ is a three-dimensional Dirac electron system and shows the giant diamagnetism in χ as in the other Dirac semimetals. ${}^{207}\mathrm{Pb}$ NMR measurements are conducted in this study to explore the microscopic origin of diamagnetism. From the analysis of the Knight shift $K$ as a function of χ  and the relaxation rate ${T_1}^{–1}$ for samples with different hole densities, the spin and the orbital components in $K$ are successfully separated. The results establish that the enhanced diamagnetism in ${\mathrm{Sr}}_3\mathrm{PbO}$ originates from the orbital contribution of Dirac electrons, which is fully consistent with the theory of giant orbital diamagnetism.

We report the direct observation of switching of the Néel vector of antiferromagnetic (AFM) domains in response to electrical pulses in micron-scale $\mathrm{Pt}/\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ Hall bars using photoemission electron microscopy. Current pulses lead to reversible and repeatable switching with the current direction determining the final state, consistent with Hall effect experiments that probe only the spatially averaged response. Current pulses also produce irreversible changes in domain structure, in and even outside the current path. In both cases only a fraction of the domains switch in response to pulses. Furthermore, the analysis of images taken with different x-ray polarizations shows that the AFM Néel order has an out-of-plane component in equilibrium that is important to consider in analyzing the switching data. These results show that—in addition to effects associated with spin-orbit torques from the Pt layer, which can produce reversible switching—changes in AFM order can be induced by purely thermal effects.

Photoinduced phase transitions have been intensively studied owing to the potential capability to control a material of interest in the ultrafast manner, which can induce exotic phases unable to be attained at equilibrium. However, the key mechanisms are still under debate, and it is currently a central issue as to how the couplings between the electron, lattice, and spin degrees of freedom are evolving during photoinduced phase transitions. Here, we use a recently developed analysis method, which we call frequency-domain angle-resolved photoemission spectroscopy (FDARPES), and reveal mode- and band-selective electron-phonon couplings during the photoinduced insulator-to-metal transition for ${\mathrm{Ta}}_2\mathrm{Ni}{\mathrm{Se}}_5$. We find that the lattice modulation corresponding to the 2 THz phonon mode, where the Ta lattice is sheared along the $a$ axis, is the most relevant for the emergence of photoinduced semimetallic state. Furthermore, we find that the semimetallic and semiconducting bands coexist in the transient state, and demonstrate that FDARPES spectra can selectively detect the phonon-specific couplings to the two coexistent band structures during the photoinduced phase transition by resolving them in the frequency domain.

Since the discovery of ferromagnetic two-dimensional (2D) van der Waals (vdW) crystals, significant interest on such 2D magnets has emerged, inspired by their appealing physical properties and integration with other 2D family for unique heterostructures. In known 2D magnets, spin-orbit coupling (SOC) stabilizes perpendicular magnetic anisotropy down to one or a few monolayers. Such a strong SOC could also lift the chiral degeneracy, leading to the formation of topological magnetic textures such as skyrmions through the Dzyaloshinskii-Moriya interaction (DMI). Here, we report the experimental observation of Néel-type chiral magnetic skyrmions and their lattice (SkX) formation in a vdW ferromagnet ${\mathrm{Fe}}_3\mathrm{Ge}{\mathrm{Te}}_2$ (FGT). We demonstrate the ability to drive an individual skyrmion by short current pulses along a vdW heterostructure, $\mathrm{FGT}/h\text{−}\mathrm{BN}$, as highly required for any skyrmion-based spintronic device. Using first principle calculations supported by experiments, we unveil the origin of DMI being the interfaces with oxides, which then allows us to engineer vdW heterostructures for desired chiral states. Our finding opens the door to topological spin textures in the 2D vdW magnet and their potential device application.

We study the dynamics of an interacting quantum spin chain under the application of a linearly increasing field. This model exhibits a type of localization known as Stark many-body localization. The dynamics shows a strong dependence on the initial conditions, indicating that the system violates the conventional (“strong”) eigenstate thermalization hypothesis at any finite gradient of the field. This is contrary to reports of a numerically observed ergodic phase. Therefore, the localization is crucially distinct from disorder-driven many-body localization, in agreement with recent predictions on the basis of localization via Hilbert-space shattering.

The emergent induction of spiral magnets that was proposed [N. Nagaosa, Jpn. J. Appl. Phys. 58, 120909 (2019)] and recently demonstrated [T. Yokouchi et al., Nature (London) 586, 232 (2020)] is shown to be further extended by a comprehensive treatment of the Rashba spin-orbit coupling and the electron spin relaxation that affect the underlying processes of spin-transfer torque and spinmotive force. Within adiabatic approximation, we show that the output voltages are widely altered intrinsically via the Rashba effect whereas extrinsically via the nonadiabatic correction due to the spin relaxation and sample disorder. The findings respectively clarify the origins for the amplitude modulation and sign change of the emergent inductance with tunability by electrical gating and careful sample preparation.

We perform an extensive exact diagonalization study of interaction-driven insulators in spin- and valley-polarized moiré flat bands of twisted bilayer graphene aligned with its hexagonal boron nitride substrate. In addition to previously reported fractional Chern insulator phases, we provide compelling evidence for competing charge-density-wave phases at multiple fractional fillings of a realistic single-band model. A thorough analysis at different interlayer hopping parameters, motivated by experimental variability, and the role of kinetic energy at various Coulomb interaction strengths highlight the competition between these phases. The interplay of the single-particle and the interaction-induced hole dispersion with the inherent Berry curvature of the Chern bands is intuitively understood to be the driving mechanism for the ground-state selection. The resulting phase diagram features remarkable agreement with experimental findings in a related moiré heterostructure and affirms the relevance of our results beyond the scope of graphene-based materials.

A micromagnetic study is carried out on the role of using topology to stabilize different magnetic textures, such as a vortex or an antivortex state, in a magnetic heterostructure consisting of a permalloy disk coupled to a set of nanomagnetic bars. The topological boundary condition is set by the stray field contributions of the nanomagnet bars and thus by their magnetization configuration, and can be described by a discretized winding number that will be matched by the winding number of the topological state set in the disk. The lowest number of nanomagnets that defines a suitable boundary is 4, and we identify a critical internanomagnet angle of 225 ° between two nanomagnets, at which the boundary fails because the winding number of the nanomagnet configuration no longer controls that of the disk magnetization. The boundary also fails when the disk-nanomagnets separation is >50 nm and for disk diameters >480 nm. Finally, we provide preliminary experimental evidence from magnetic force microscopy studies in which we demonstrate that an energetically unstable, antivortex-like structure can indeed be stabilized in a permalloy disk, provided that the appropriate topological conditions are set.

We develop a general kinetic theory framework to describe the hydrodynamics of strongly interacting, nonequilibrium quantum systems in which integrability is weakly broken, leaving a few residual conserved quantities. This framework is based on a generalized relaxation-time approximation; it gives a simple, but surprisingly accurate, prescription for computing nonequilibrium transport even in strongly interacting systems. We validate the predictions of this approximation against matrix product operator calculations on chaotic quantum spin chains, finding surprisingly good agreement. We show that despite its simplicity, our framework can capture phenomena distinctive to strongly interacting systems, such as widely separated charge and energy diffusion constants.

We introduce a family of Gross-Neveu-Yukawa models with a large number of fermion and boson flavors as higher dimensional generalizations of the Sachdev-Ye-Kitaev model. The models may be derived from local lattice couplings and give rise to Lorentz invariant critical solutions in $1+1$ and $2+1$ dimensions. These solutions imply anomalous dimensions of both bosons and fermions tuned by the number ratio of boson to fermion flavors. In $1+1$ dimension the solution represents a stable critical phase, while in $2+1$ dimension it governs a quantum phase transition. We compute the out of time order correlators in the $1+1$ dimensional model, showing that it exhibits growth with the maximal Lyapunov exponent ${\lambda }_L=2\pi T$ in the low temperature limit.

Transition metal (TM) defects in silicon carbide have favorable spin coherence properties and are suitable as quantum memory for quantum communication. To characterize TM defects as quantum spin-photon interfaces, we model defects that have one active electron with spin 1/2 in the atomic $D$ shell. The spin structure, as well as the magnetic and optical resonance properties of the active electron, emerge from the interplay of the crystal potential and spin-orbit coupling, and they are described by a general model derived using group theory. We find that the spin-orbit coupling leads to additional allowed transitions and a modification of the $g$-tensor. To describe the dependence of the Rabi frequency on the magnitude and direction of the static and driving fields, we derive an effective Hamiltonian. This theoretical description can also be instrumental in performing and optimizing spin control in TM defects.

We have investigated the THz range magneto-optical response of ultralow carrier density films of ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ using time-domain THz polarimetry. Undoped ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ has a chemical potential that lies inside the bulk valence band. Thus, its topological response is masked by bulk carriers. However, with appropriate buffer-layer engineering and chemical doping, ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ thin films can be grown with extremely low electron or hole densities. The ultralow carrier density samples show unusual optical properties and quantized response in the presence of magnetic fields. Consistent with the expectations for Dirac fermions, a quantized Hall response is seen even in samples where the zero field conductivity falls below detectable levels. The discontinuity in the Faraday angle with small changes in the filling fraction across zero is a manifestation of the parity anomaly in 2D Dirac systems with broken time reversal symmetry (TRS).

The transverse-field Ising model on the triangular lattice is expected to host an intermediate finite-temperature Kosterlitz-Thouless (KT) phase through a mapping of the spins on each triangular unit to a complex order parameter. ${\mathrm{TmMgGaO}}_4$ is a candidate material to realize such physics due to the non-Kramers nature of the ${\mathrm{Tm}}^{3+}$ ion and the resulting two-singlet single-ion ground state. Using inelastic neutron scattering, we confirm this picture by determining the leading parameters of the low-energy effective Hamiltonian of ${\mathrm{TmMgGaO}}_4$. Subsequently, we track the predicted KT phase and related transitions by inspecting the field and temperature dependence of the ac susceptibility. We further probe the spin correlations in both reciprocal space and real space via single-crystal neutron diffraction and magnetic total scattering techniques, respectively. Magnetic pair distribution function analysis provides evidence for the formation of vortex-antivortex pairs that characterize the proposed KT phase around 5 K. Although structural disorder influences the field-induced behavior of ${\mathrm{TmMgGaO}}_4$, the magnetism in zero field appears relatively free from these effects. These results position ${\mathrm{TmMgGaO}}_4$ as a strong candidate for a solid-state realization of KT physics in a dense spin system.

The electron gas hosted in a two-dimensional solid-state matrix, such as a quantum well or a two-dimensional van der Waals heterostructure, supports the propagation of plasma waves. Nonlinear interactions between plasma waves, due to charge conservation and current convection, generate a constant density gradient which can be detected as a DC potential signal at the boundaries of the system. This phenomenon is at the heart of a plasma-wave photodetection scheme, which was first introduced by Dyakonov and Shur for electronic systems with a parabolic dispersion and then extended to the massless Dirac fermions in graphene. In this work, we develop the theory of plasma-wave photodetection in bilayer graphene, which has the peculiarity that the dispersion relation depends locally and dynamically on the intensity of the plasma wave. In our analysis, we show how quantum capacitance effects, arising from the local fluctuations of the electronic dispersion, modify the intensity of the photodetection signal. An external electrical bias, e.g., induced by top and bottom gates, can be used to control the strength of the quantum capacitance corrections, and thus the photoresponse.

The Jaynes-Cummings model (JCM), one of the paradigms of quantum electrodynamics, was introduced to describe the interaction between light and a fictitious two-level atom. Recently it was suggested that the JCM Hamiltonian can be invoked to describe the motional states of electrons trapped on the surface of liquid helium and subjected to a constant uniform magnetic field tilted with respect to the surface [Yunusova et al., Phys. Rev. Lett. 122, 176802 (2019)]. In this case, the surface-bound (Rydberg) states of an electron are coupled to the electron cyclotron motion by the in-plane component of the tilted field. Here we investigate, both theoretically and experimentally, the spectroscopic properties of surface electrons in a tilted magnetic field and demonstrate that such a system exhibits a variety of phenomena common to the light dressed states of atomic and molecular systems. This shows that electrons on helium realize a prototypical atomic system where interaction between components can be engineered and controlled by simple means and with high accuracy, and which therefore can be potentially used as a new flexible platform for quantum experiments. Our work introduces a pure condensed-matter system of electrons on helium into the context of atomic, molecular and optical physics.

Optical spin control is the basis for ultrafast spintronics: circularly polarized light in combination with spin-orbit coupling enables spin manipulation of electronic states in condensed matter. However, the conventional approach is limited to longitudinal spin initialization along one particular axis that is dictated by the direction of light propagation. Here, plasmonics opens new possibilities, allowing one to tailor light polarization at the nanoscale. We demonstrate ultrafast optical excitation of electron spin on femtosecond timescales via plasmon-to-exciton spin conversion. By time resolving the THz spin dynamics in a hybrid (Cd,Mn)Te quantum-well structure covered with a metallic grating, we unambiguously determine the orientation of the photoexcited electron spins which is locked to the propagation direction of the optically excited surface plasmon polaritons. Using the spin of the incident photons as an additional degree of freedom, one can adjust not only the longitudinal, but also the transverse electron spin components normal to the light propagation at will.

We explore correlations of eigenstates around the many-body localization (MBL) transition in their dependence on the energy difference (frequency) $\omega$ and disorder $W$. In addition to the genuine many-body problem, XXZ spin chain in random field, we consider localization on random regular graphs that serves as a toy model of the MBL transition. Both models show a very similar behavior. On the localized side of the transition, the eigenstate correlation function $\beta \left(\omega \right)$ shows a power-law enhancement of correlations with lowering $\omega$; the corresponding exponent depends on $W$. The correlation between adjacent-in-energy eigenstates exhibits a maximum at the transition point $W_c$, visualizing the drift of $W_c$ with increasing system size towards its thermodynamic-limit value. The correlation function $\beta \left(\omega \right)$ is related, via Fourier transformation, to the Hilbert-space return probability. We discuss measurement of such (and related) eigenstate correlation functions on state-of-the-art quantum computers and simulators.