The standard spin transfer torque (STT)—where spin-polarized current drives the dynamics of magnetization viewed as a classical vector—requires noncollinearity between electron spins carried by the current and magnetization of a ferromagnetic layer. However, recent experiments [A. Zholud et al., Phys. Rev. Lett. 119, 257201 (2017)] observing magnetization dynamics in spin valves at cryogenic temperatures, even when electron spin is collinear to magnetization, point at overlooked quantum effects in STT that can lead to highly nonclassical magnetization states. Using quantum many-body treatment, where an electron injected as a spin-polarized wave packet interacts with local spins comprising the anisotropic quantum Heisenberg ferromagnetic chain, we define quantum STT as any time evolution of local spins due to an initial many-body quantum state not being an eigenstate of an electron+local-spins composite system. For time evolution caused by injected spin-$\downarrow$ electron scattering off local $\uparrow$-spins, entanglement between electron and local spin subsystems takes place leading to decoherence and, therefore, shrinking of the total magnetization but without rotation from its initial orientation, which is compatible with the experiments. Furthermore, the same processes—entanglement and thereby induced true decoherence—are present even in the standard noncollinear geometry, intertwined with the usual magnetization rotation. This is because STT in a quantum many-body picture is always caused by an electron spin-$\downarrow$ factor state, and the only difference between collinear and noncollinear geometries is in the relative size of the contribution of the initial separable state containing such a factor to the superposition of separable many-body quantum states generated by time evolution.

In the spin Seebeck effect (SSE), the spin currents generated by thermal gradients applied to a magnetic film are detected by an electric voltage along an adjacent metallic layer due to the spin to charge current conversion through the inverse spin Hall effect. The SSE has been widely studied in bilayers made of insulating ferro- or ferrimagnet insulators and metals with large spin-orbit coupling such as platinum. Few studies of the SSE have been reported in antiferromagnetic insulators despite the increasing attention these materials have been receiving in the field of antiferromagnetic spintronics. Here we report an investigation of the SSE in bilayers made of well-textured polycrystalline films of antiferromagnetic NiO with platinum in a wide range of temperatures and magnetic fields. We demonstrate that the SSE is detected only when the NiO layer is fabricated with a Py underlayer to induce a macroscopic antiferromagnetic arrangement. The results are in good qualitative agreement with a drift-diffusion magnonic theory developed for single-crystal NiO.

We work out a microscopic theory describing complete statistics of voltage fluctuations generated by quantum phase slips (QPS) in superconducting nanowires. We evaluate the cumulant generating function and demonstrate that shot noise of the voltage as well as the thir d and all higher voltage cumulants differ from zero only due to the presence of QPS. In the zero-frequency limit voltage fluctuations in superconducting nanowires are described by Poisson statistics just as in a number of other tunnelinglike problems. However, at nonzero frequencies quantum voltage fluctuations in superconducting nanowires become much more complicated and are not anymore accounted for by Poisson statistics. In the case of short superconducting nanowires we explicitly evaluate all finite-frequency voltage cumulants and establish a nontrivial relation between these cumulants and the current-voltage characteristics of our system.

We propose an interferometer for chiral Majorana modes where the interference effect is caused and controlled by a Josephson junction of proximity-induced topological superconductors, hence, a Majorana-Josephson interferometer. This interferometer is based on a two-terminal quantum anomalous Hall bar, and as such its transport observables exhibit interference patterns depending on both the Josephson phase and the junction length. Observing these interference patterns will establish quantum coherent Majorana transport and further provide a powerful characterization tool for the relevant system.

This paper focuses on spin superfluid transport, observation of which was recently reported in antiferromagnet ${\mathrm{Cr}}_2{\mathrm{O}}_3$ [Yuan et al., Sci. Adv. 4, eaat1098 (2018).]. This paper analyzes the role of dissipation in transformation of spin current injected with incoherent magnons to a superfluid spin current near the interface where spin is injected. The Gilbert damping parameter in the Landau–Lifshitz–Gilbert theory does not describe dissipation properly, and the dissipation parameters are calculated from the Boltzmann equation for magnons scattered by defects. The two-fluid theory is developed similar to the two-fluid theory for superfluids. This theory shows that the influence of temperature variation in bulk on the superfluid spin transport (bulk Seebeck effect) is weak at low temperatures. The scenario that the results of Yuan et al. are connected with the Seebeck effect at the interface between the spin detector and the sample is also discussed. The Landau criterion for an antiferromagnet put in a magnetic field is derived from the spectrum of collective spin modes. The Landau instability starts in the gapped mode earlier than in the Goldstone gapless mode, in contrast to easy-plane ferromagnets where the Goldstone mode becomes unstable. The structure of the magnetic vortex in the geometry of the experiment is determined. The vortex core has the skyrmion structure with finite magnetization component normal to the magnetic field. This magnetization creates stray magnetic fields around the exit point of the vortex line from the sample, which can be used for experimental detection of vortices.

We present the magnetic structure of ${\mathrm{TmMn}}_3{\mathrm{O}}_6$, solved via neutron powder diffraction. We demonstrate that long-range magnetic order develops below 74 K, and at 28 K a spin-flop transition occurs driven by $f-d$ exchange and rare-earth single-ion anisotropy. In both magnetic phases, the magnetic structure may be described as a collinear ferrimagnet, contrary to conventional theories of magnetic order in the manganite perovskites. Instead, we show that these magnetic structures can be understood to arise due to ferro-orbital order, the $A, A^{\prime }$, and $A^{\prime \prime }$ site point symmetry, $mm2$, and the dominance of $A-B$ exchange over both $A-A$ and $B-B$ exchange, which together are unique to the $R{\mathrm{Mn}}_3{\mathrm{O}}_6$ perovskites.

All topological insulators observed so far are the lattice analogs of the integer quantum Hall states with time-reversal symmetry, composed of two decoupled copies of the Chern insulator with opposite chiralities for different spins. The fractional topological insulator (FTI) has been similarly envisioned as being composed of two decoupled copies of the fractional Chern insulator (FCI), which is in turn the lattice analog of the fractional quantum Hall state (FQHS). An important question is if such a vision can be realized for the Coulomb interaction, whose strength is irrespective of spin. To address this question, we investigate the effects of the interspin correlation in the spin-holomorphic Landau levels, where electrons with one spin reside in the usual holomorphic lowest Landau level, while those with the other in the antiholomorphic counterpart. By performing exact diagonalization of the Coulomb interaction Hamiltonian in the spin-holomorphic Landau levels, here, we show that no fractionally filled states in the spin-holomorphic Landau levels can occur as two decoupled copies of the FQHS, suggesting that no FTIs can occur as those of the FCI in the lattice either. Fractionally filled states in this system are generally compressible except at half filling, where a transport gap develops with spontaneous breaking of the space rotational symmetry in the thermodynamic limit, leading to the spatial separation of different spins, i.e., spin separation. It is predicted that there is a novel bulk-edge correspondence at half filling, representing the hallmark of the half-filled spin-separated FTI.

The Sachdev-Ye-Kitaev (SYK) model describes a strange metal that shows peculiar non-Fermi-liquid properties without quasiparticles. It exhibits a maximally chaotic behavior characterized by out-of-time-ordered correlators (OTOCs), and is expected to be a holographic dual to black holes. While a faithful realization of the SYK model in condensed-matter systems may be involved, a striking similarity between the SYK model and the Hund-coupling-induced spin-freezing crossover in multiorbital Hubbard models has recently been pointed out. To further explore this connection, we study OTOCs for fermionic single-orbital and multiorbital Hubbard models, which are prototypical models for strongly correlated electrons in solids. We introduce an imaginary-time four-point correlation function with an appropriate time ordering, which by means of the spectral representation and the out-of-time-order fluctuation-dissipation theorem can be analytically continued to real-time OTOCs. Based on this approach, we numerically evaluate real-time OTOCs for Hubbard models in the thermodynamic limit, using the dynamical mean-field theory in combination with a numerically exact continuous-time Monte Carlo impurity solver. The results for the single-orbital model show that a certain spin-related OTOC captures local moment formation in the vicinity of the metal-insulator transition, while the self-energy does not show SYK-type non-Fermi-liquid behavior. On the other hand, for the two- and three-orbital models with nonzero Hund coupling we find that the OTOC exhibits a rapid damping at short times and an approximate power-law decay at longer times in the spin-freezing crossover regime characterized by fluctuating local moments and a non-Fermi-liquid self-energy $\mathrm{\Sigma }\left(\omega \right)\sim \sqrt{\omega }$. These results are in a good agreement with the behavior of the SYK model, providing firm evidence for the close relation between the spin-freezing crossover physics of multiorbital Hubbard models and the SYK strange metal.

We present detailed studies of the high-field magnetoresistance of the layered organic metal $\kappa \text{−}{\left(\mathrm{BETS}\right)}_2\mathrm{Mn}{\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]}_3$ under a pressure slightly above the insulator-metal transition. The experimental data are analyzed in terms of the Fermi surface properties and compared with the results of first-principles band structure calculations. The calculated size and shape of the in-plane Fermi surface are in very good agreement with those derived from Shubnikov-de Haas oscillations as well as the classical angle-dependent magnetoresistance oscillations. A comparison of the experimentally obtained effective cyclotron masses with the calculated band masses reveals electron correlations significantly dependent on the electron momentum. The momentum- or band-dependent mobility is also reflected in the behavior of the classical magnetoresistance anisotropy in a magnetic field parallel to layers. Other characteristics of the conducting system related to interlayer charge transfer and scattering mechanisms are discussed based on the experimental data. Besides the known high-field effects associated with the Fermi surface geometry, new pronounced features have been found in the angle-dependent magnetoresistance, which might be caused by coupling of the metallic charge transport to a magnetic instability in proximity to the metal-insulator phase boundary.

We calculate the finite temperature x-ray photoemission spectroscopy for the Falicov-Kimball model using a Wiener-Hopf sum equation approach. In the metallic state, the core-hole spectral function shows two side peaks corresponding to the creation of a core hole on an empty site (or a doubly occupied site) and also has two nearly degenerate central peaks (because of our choice of the model parameters) corresponding to the creation of a core hole on a singly occupied site. The nearly doubly degenerate central peaks merge into a single peak at higher temperatures. In the insulating state, we obtain two peaks and a strongly temperature dependent low-energy peak corresponding to the creation of a core hole on a thermally excited empty site. These results for the insulating state should be similar to those of the more general Hubbard model. Also, the strong correlations suggest that even without any additional broadening due to Auger-like processes, the core-hole lifetime will be short.

We study transport properties and topological phase transition in two-dimensional interacting disordered systems. We derive the Hall conductance within real-space dynamical mean-field theory, which is quantized and serves as a topological invariant for insulators, even when the energy gap is closed by localized states. In the spinful Harper-Hofstadter-Hatsugai model, in the trivial insulator regime, we find that the repulsive on-site interaction can assist weak disorder to induce the integer quantum Hall effect, while in the topologically nontrivial regime, it impedes Anderson localization. Generally, the interaction broadens the regime of the topological phase in the disordered system.

An insightful mechanics-based bottom-up framework is developed for probing the frequency dependence of lattice material microstructures. Under a vibrating condition, effective elastic moduli of such microstructured materials can become negative for certain frequency values, leading to an unusual mechanical behavior with a multitude of potential applications. We have derived the fundamental theoretical limits for the minimum frequency, beyond which the negative effective moduli of the materials could be obtained. An efficient dynamic stiffness matrix based approach is developed to obtain the closed-form limits, which can exactly capture the subwavelength scale dynamics. The limits turn out to be a fundamental property of the lattice materials and depend on certain material and geometric parameters of the lattice in a unique manner. An explicit characterization of the theoretical limits of negative elastic moduli along with adequate physical insights would accelerate the process of its potential exploitation in various engineered materials and structural systems under dynamic regime across the length scales.

The series of intermetallic compounds $R{\mathrm{NiSi}}_3 (R=\text{rare} \text{earth})$ shows interesting magnetic properties evolving with $R$ and metamagnetic transitions under applied magnetic field for some of the compounds. The microscopic magnetic structures must be determined to rationalize such rich behavior. Here, resonant x-ray magnetic diffraction experiments are performed on single crystals of ${\mathrm{GdNiSi}}_3$ and ${\mathrm{TbNiSi}}_3$ at zero field. The primitive magnetic unit cell matches the chemical cell below the Néel temperatures $T_N=22.2$ and 33.2 K, respectively. The magnetic structure is determined to be the same for both compounds (magnetic space group $Cmm{m}^{\prime }$). It features ferromagnetic $ac$ planes that are stacked in an antiferromagnetic $+-+-$ pattern, with the rare-earth magnetic moments pointing along the $\vec{a}$ direction, which contrasts with the $+--+$ stacking and moment direction along the $\vec{b}$ axis previously reported for ${\mathrm{YbNiSi}}_3$. This indicates a sign reversal of the coupling constant between second-neighbor $R$ planes as $R$ is varied from Gd and Tb to Yb. The long $b$ lattice parameter of ${\mathrm{GdNiSi}}_3$ and ${\mathrm{TbNiSi}}_3$ shows a magnetoelastic expansion upon cooling below $T_N$, pointing to the conclusion that the $+-+-$ stacking is stabilized under lattice expansion. A competition between distinct magnetic stacking patterns with similar exchange energies tuned by the size of $R$ sets the stage for the magnetic ground state instability observed along this series.

The construction of the vibration-transit theory of liquid dynamics is being presented in three sequential research reports. The first is on the entire condensed-matter collection of $N$-atom potential energy valleys and identification of the random valleys as the liquid domain. The present (second) report defines the vibrational Hamiltonian and describes its application to statistical mechanics. The following is a brief list of the major topics treated here. The vibrational Hamiltonian is universal, in that its potential energy is a single $3N$-dimensional harmonic valley. The anharmonic contribution is also treated. The Hamiltonian is calibrated from first-principles calculations of the structural potential and the vibrational frequencies and eigenvectors. Exact quantum-statistical-mechanical functions are expressed in universal equations and are evaluated exactly from vibrational data. Exact classical-statistical-mechanical functions are also expressed in universal equations and are evaluated exactly from a few moments of the vibrational frequency distribution. The complete condensed-matter distributions of these moments are graphically displayed, and their use in statistical mechanics is clarified. The third report will present transit theory, which treats the motion of atoms between the $N$-atom potential energy valleys.

Magnetic structures are investigated by means of neutron diffraction to shine a light on the intricate details that are believed to be key to understanding the magnetoelectric effect in ${\mathrm{LiCoPO}}_4$. At zero field, a spontaneous spin canting of $\phi =7{\left(1\right)}^{\circ }$ is found. The spins tilt away from the easy $b$-axis toward $c$. Symmetry considerations lead to the magnetic point group $m_z^{\prime }$, which is consistent with the previously observed magnetoelectric tensor form and weak ferromagnetic moment along $b$. For magnetic fields applied along $a$, the induced ferromagnetic moment couples via the Dzyaloshinskii-Moriya interaction to yield an additional field-induced spin canting. An upper limit to the size of the interaction is estimated from the canting angle.

Two-dimensional materials such as graphene or hexagonal boron nitride are indispensable in industry. The recently discovered 2D ferromagnetic materials also promise to be vital for applications. In this work, we develop a phenomenological description of noncentrosymmetric 2D ferromagnets with trigonal prismatic crystal structure. We chose to study this special symmetry group since these materials do break inversion symmetry and therefore, in principle, allow for chiral spin structures such as magnetic helices and skyrmions. However, unlike all noncentrosymmetric magnets known so far, we show that the symmetry of magnetic trigonal prismatic monolayers neither allow for an internal relativistic Dzyaloshinskii-Moriya interaction (DMI) nor a reactive spin-orbit torque. We demonstrate that the DMI only becomes important at the boundaries, where it modifies the boundary conditions of the magnetization and leads to a helical equilibrium state with a helical wave vector that is inherently linked to the internal spin orientation. Furthermore, we find that the helical wave vector can be electrically manipulated via dissipative spin-torque mechanisms. Our results reveal that 2D magnets offer a large potential for unexplored magnetic effects.

We present detailed benchmark ground-state calculations of the one- and two-dimensional Hubbard model utilizing the cluster extensions of the rotationally invariant slave-boson mean-field theory and the density matrix embedding theory. Our analysis shows that the overall accuracy and the performance of these two methods are very similar. Furthermore, we propose a unified computational framework that allows us to implement both of these techniques on the same footing. This provides us with a different line of interpretation and paves the ways for developing systematically distinct generalizations of these complementary approaches.

We investigate the superradiance effects in three-dimensional topological insulator HgTe with conducting surface states. We demonstrate that the superradiance can be explained using the classical electrodynamic approach. Experiments using the continuous-wave spectroscopy allowed us to separate the energy losses in the system into intrinsic and radiation losses, respectively. These results demonstrate that the superradiance effects are not sensitive to the details of the band structure of the material.