Chiral spin pumping is the generation of a unidirectional spin current in half of ferromagnetic films or conductors by dynamic dipolar stray fields from close-by nanomagnets. We formulate a general theory of long-range chiral interactions between magnets mediated by unidirectional traveling waves, e.g., spin waves in a magnetic film or microwaves in a waveguide. The traveling waves emitted by an excited magnet can be perfectly trapped by a second, initially passive magnet by a dynamical interference effect. When both magnets are excited by a uniform microwave, the chiral interaction between them creates a large imbalance in their magnon numbers.

Quantum degeneracy pressure (QDP) underscores the stability of matter and is arguably the most ubiquitous many-body effect. The associated Fermi surface (FS) has broad implications for physical phenomena, ranging from electromagnetic responses to entanglement entropy (EE) area law violations. Given recent fruitful studies in condensed-matter physics under effectively non-Hermitian descriptions, it becomes urgent to study how QDP and many-body interactions interplay with non-Hermitian effects. Through a prototypical critical one-dimensional fermionic lattice with asymmetric gain/loss, a real space FS is shown to naturally emerge, in addition to any existing momentum space FS. We carefully characterize such real space FS with the EE, by a renormalized temperature that encapsulates the interplay of thermal excitations and non-Hermiticity. Nearest-neighbor repulsion is also found to induce a competing charge density wave (CDW) that may erode the real space FS. The underlying physics surrounding criticality and localization is further analyzed with complex flux spectral flows. Our findings can be experimentally demonstrated with ultracold fermions in a suitably designed optical lattice.

Recent work has focused on exploring many-body localization (MBL) in systems without quenched disorder: one such proposal is Stark MBL in which small perturbations to a strong linear potential yield localization. However, as with conventional MBL, it is challenging to experimentally distinguish between noninteracting localization and true MBL. In this paper, we show that several existing experimental probes, designed specifically to differentiate between these scenarios, work similarly in the Stark MBL setting. In particular, we show that a modified spin-echo response shows clear signs of a power-law decay for Stark MBL while quickly saturating for disorder-free Wannier-Stark localization. Furthermore, we observe the characteristic logarithmic-in-time spreading of quantum mutual information in the Stark MBL regime, and an absence of spreading in a noninteracting Stark-localized system. We also show that there are no significant differences in several existing MBL measures for a system consisting of soft-core bosons with repulsive on-site interactions. Lastly, we discuss why curvature or small disorder are needed for an accurate reproduction of MBL phenomenology and how this may be illustrated in experiment. This also connects with recent progress on Hilbert space fragmentation in “fractonic” models with a conserved dipole moment, and we suggest this as an auspicious platform for experimental investigations of these phenomena.

We show that the Mott metal-insulator transition in the standard one-band Hubbard model can be understood as a topological phase transition. Our approach is inspired by the observation that the midgap pole in the self-energy of a Mott insulator resembles the spectral pole of the localized surface state in a topological insulator. We use numerical renormalization-group–dynamical mean-field theory to solve the infinite-dimensional Hubbard model, and represent the resulting local self-energy in terms of the boundary Green's function of an auxiliary tight-binding chain without interactions. The auxiliary system is of generalized Su-Schrieffer-Heeger model type; the Mott transition corresponds to a dissociation of domain walls.

The multiferroic perovskite rare earth manganites $R\mathrm{Mn}{\mathrm{O}}_3$ ($R=\mathrm{Dy}$, Tb, Gd) are known as multiferroics exhibiting pressure-induced gigantic ferroelectric polarization. In this study, we have investigated the magnetic orderings in the pressure-induced phases for $\mathrm{Dy}\mathrm{Mn}{\mathrm{O}}_3$, by neutron diffraction and spherical neutron polarimetry (SNP) experiments up to 8.0 GPa. The magnetic ordering for Mn spins changes from the incommensurate $bc$-cycloid to the commensurate collinear $E$-type structure with ${\mathit{k}}_{\mathrm{Mn}}=\left(0,\frac{1}{2},0\right)$ above 4.0 GPa, which is concomitant with the appearance of a giant ferroelectric polarization. The magnetic ordering for the Dy spins has been determined to be a noncollinear spin structure with $a$ and $b$ spin components and ${\mathit{k}}_{\mathrm{Dy}}=(0,\frac{1}{2},0)$ for the low- and high-pressure phases. The magnetic field along the $a$ axis, $H_a$, affects the Dy ordering, which is seen in the changes in the $\mathit{k}$ vector from ${\mathit{k}}_{\mathrm{Dy}}=(0,\frac{1}{2},0)$ in $H_a\le 3\mathrm{T}$ to ${\mathit{k}}_{\mathrm{Dy}}=(0,0,0)$ in $H_a\ge 3\mathrm{T}$. Considering the lattice distortion generated by the determined magnetic orderings through the exchange striction mechanism, we conclude that the exchange striction for rare earth and Mn bonds, which is added to the uniform polarization generated by the $E$-type Mn ordering, is strongly related to the significant magnetic field enhancement of ferroelectric polarization in the high-pressure phase of the rare earth manganites.

Density matrix embedding theory (DMET) is a powerful quantum embedding method for solving strongly correlated quantum systems. Theoretically, the performance of a quantum embedding method should be limited by the computational cost of the impurity solver. However, the practical performance of DMET is often hindered by the numerical stability and the computational time of the correlation potential fitting procedure, which is defined on a single-particle level. Of particular difficulty are cases in which the effective single-particle system is gapless or nearly gapless. To alleviate these issues, we develop a semidefinite programming (SDP) based approach that can significantly enhance the robustness of the correlation potential fitting procedure compared to the traditional least squares fitting approach. We also develop a local correlation potential fitting approach, which allows one to identify the correlation potential from each fragment independently in each self-consistent field iteration, avoiding any optimization at the global level. We prove that the self-consistent solutions of DMET using this local correlation potential fitting procedure are equivalent to those of the original DMET with global fitting. We find that our combined approach, called L-DMET, in which we solve local fitting problems via semidefinite programming, can significantly improve both the robustness and the efficiency of DMET calculations. We demonstrate the performance of L-DMET on the 2D Hubbard model and the hydrogen chain. We also demonstrate with theoretical and numerical evidence that the use of a large fragment size can be a fundamental source of numerical instability in the DMET procedure.

Magnetic skyrmions are vortexlike topological spin textures often observed in structurally chiral magnets with Dzyaloshinskii-Moriya interaction. Among them, Co-Zn-Mn alloys with a $\beta$-Mn-type chiral structure host skyrmions above room temperature. In this system, it has recently been found that skyrmions persist over a wide temperature and magnetic field region as a long-lived metastable state, and that the skyrmion lattice transforms from a triangular lattice to a square one. To obtain perspective on chiral magnetism in Co-Zn-Mn alloys and clarify how various properties related to the skyrmion vary with the composition, we performed systematic studies on ${\mathrm{Co}}_{10}{\mathrm{Zn}}_{10}$, ${\mathrm{Co}}_9{\mathrm{Zn}}_9{\mathrm{Mn}}_2$, ${\mathrm{Co}}_8{\mathrm{Zn}}_8{\mathrm{Mn}}_4$, and ${\mathrm{Co}}_7{\mathrm{Zn}}_7{\mathrm{Mn}}_6$ in terms of magnetic susceptibility and small-angle neutron scattering measurements. Robust metastable skyrmions with extremely long lifetime are commonly observed in all the compounds. On the other hand, the preferred orientation of the helimagnetic propagation vector and its temperature dependence dramatically change upon varying the Mn concentration. The robustness of the metastable skyrmions in these materials is attributed to topological nature of the skyrmions as affected by structural and magnetic disorder. Magnetocrystalline anisotropy as well as magnetic disorder due to frustrated Mn spins play crucial roles in giving rise to the observed change in helical states and corresponding skyrmion lattice form.

Motivated by the existence of exact many-body quantum scars in the Affleck-Kennedy-Lieb-Tasaki (AKLT) chain, we explore the connection between matrix product state (MPS) wave functions and many-body quantum scarred Hamiltonians. We provide a method to systematically search for and construct parent Hamiltonians with towers of exact eigenstates composed of quasiparticles on top of an MPS wave function. These exact eigenstates have low entanglement in spite of being in the middle of the spectrum, thus violating the strong eigenstate thermalization hypothesis. Using our approach, we recover the AKLT chain starting from the MPS of its ground state, and we derive the most general nearest-neighbor Hamiltonian that shares the AKLT quasiparticle tower of exact eigenstates. We further apply this formalism to other simple MPS wave functions, and derive families of Hamiltonians that exhibit AKLT-like quantum scars. As a consequence, we also construct a scar-preserving deformation that connects the AKLT chain to the integrable spin-1 pure biquadratic model. Finally, we also derive other families of Hamiltonians that exhibit types of exact quantum scars, including a $U\left(1\right)$-invariant perturbed Potts model.

The Einstein-de Haas (EdH) effect and its reciprocal, the Barnett effect, are fundamental to magnetism and uniquely yield measures of the ratio of magnetic moment to total angular momentum. These effects, small and generally difficult to observe, are enjoying a resurgence of interest as contemporary techniques enable new approaches to their study. The high mechanical resonance frequencies in nanomechanical systems offer a tremendous advantage for the observation of EdH torques in particular. At radio frequencies, the EdH effect can become comparable to or even exceed in magnitude conventional cross-product magnetic torques. In addition, the rf-EdH torque is expected to be phase shifted by 90 degrees relative to cross-product torques, provided the magnetic system remains in quasistatic equilibrium, enabling separation in quadratures when both sources of torque are operative. Radio-frequency EdH measurements are demonstrated through the full hysteresis range of micrometer-scale, monocrystalline yttrium iron garnet disks. Equilibrium behavior is observed in the vortex state at low bias field. Barkhausen-like features emerge in the in-plane EdH torque at higher fields in the vortex state, revealing magnetic disorder too weak to be visible through the in-plane cross-product torque. Beyond vortex annihilation, peaks arise in the EdH torque versus bias field, and these together with their phase signatures indicate additional utility of the Einstein-de Haas effect for the study of rf-driven spin dynamics.

Entropy effects substantially modify the growth of self-assembled Ge nanostructures on vicinal Si(001) surfaces, on which one-dimensional nanowire-like structures are formed. As shown by variable temperature scanning tunneling microscopy, these nanostructures are not only tunable in size and shape, but can be fully reversibly erased and reformed without changes in sizes and composition. This unprecedented behavior is caused by the strong free surface energy renormalization due to the large step entropy of vicinal surfaces that strongly increases with increasing temperature. This favors a planar two-dimensional surface at higher temperatures in thermodynamic equilibrium, whereas the nanostructured surface is the preferred low-temperature configuration. Taking the step entropy into account, the critical transition temperature between these surface states derived by free-energy minimization is shown to scale nearly linearly with the Ge coverage—in excellent agreement with the experiments. Most importantly, the nanowire sizes are found to be deterministically controlled by the Ge thickness and vicinal angle, independently of the growth or annealing conditions. Thus, highly reproducible structures with tunable nanogeometries and -dimensions are obtained, which opens promising avenues for device applications.

Here we report on Landau level spectroscopy in magnetic fields up to 34 T performed on a thin film of the topological insulator ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ epitaxially grown on a ${\mathrm{BaF}}_2$ substrate. The observed response is consistent with the picture of a direct-gap semiconductor in which charge carriers closely resemble massive Dirac particles. The fundamental band gap reaches $E_g=(175\pm 5)$ meV at low temperatures and it is not located on the trigonal axis, thus displaying either sixfold or twelvefold valley degeneracy. Interestingly, our magneto-optical data do not indicate any band inversion at the direct gap. This suggests that the fundamental band gap is relatively distant from the $\mathrm{\Gamma }$ point where profound inversion exists and gives rise to the relativisticlike surface states of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$.

The electric-field effect on magnetism was examined in the multiferroic conical magnet $\mathrm{M}{\mathrm{n}}_2\mathrm{Ge}{\mathrm{O}}_4$, which shows a strong coupling between ferromagnetic and ferroelectric order parameters. The systematic evaluation of the electric polarization in the multiferroic phase below 5.5 K under various field cooling conditions reveals that small magnetic fields of 0.1 T significantly reduce the required electric fields needed to reach saturation. By applying electric fields during magnetic field dependent hysteresis measurements of magnetization M and polarization P an electrically controllable exchange bias was observed, a phenomenon exceedingly rare in single phase multiferroics. Furthermore, nonreversible electric switching of P and M domains was achieved under specific magnetic field conditions.

We show that a small conducting object, such as a nanosphere or a nanoring, embedded into or placed in the vicinity of a two-dimensional electron liquid (2DEL) and subjected to a circularly polarized electromagnetic radiation induces “twisted” plasmonic oscillations in the adjacent 2DEL. The oscillations are rectified due to the hydrodynamic nonlinearities leading to the helicity sensitive circular DC current and to a magnetic moment. This hydrodynamic inverse Faraday effect (HIFE) can be observed at room temperature in different materials. The HIFE is dramatically enhanced in a periodic array of the nanospheres forming a resonant plasmonic coupler. Such a coupler exposed to a circularly polarized wave converts the entire 2DEL into a vortex state. Hence, the twisted plasmonic modes support resonant plasmonic-enhanced gate-tunable optical magnetization. Due to the interference of the plasmonic and Drude contributions, the resonances have an asymmetric Fano-like shape. These resonances present a signature of the 2DEL properties not affected by contacts and interconnects and, therefore, providing the most accurate information about the 2DEL properties. In particular, the widths of the resonances encode direct information about the momentum relaxation time and viscosity of the 2DEL.

We report the magnetic structure and spin excitations of ${\mathrm{Mn}}_3\mathrm{Ge}$, a breathing kagome antiferromagnet with transport anomalies attributed to Weyl nodes. Using polarized neutron diffraction, we show the magnetic order is a $\mathbf{k}=\mathbf{0}$ coplanar state belonging to a ${\mathrm{\Gamma }}_9$ irreducible representation, which can be described as a perfect ${120}^{\circ }$ antichiral structure with a moment of 2.2(1) ${\mu }_B$/Mn, superimposed with weak collinear ferromagnetism. Inelastic neutron scattering shows three collective $\mathbf{Q}=0$ excitations at ${\mathrm{\Delta }}_1=2.9\left(6\right)$ meV, ${\mathrm{\Delta }}_2=14.6\left(3\right)$ meV, and ${\mathrm{\Delta }}_3=17.5\left(3\right)$ meV. A field theory of $Q\approx 0$ spin waves in triangular antiferromagnets with a ${120}^{\circ }$ spin structure was used to classify these modes. The in-plane mode ($\alpha$) is gapless, ${\mathrm{\Delta }}_1$ is the gap to a doublet of out-of-plane spin excitations (${\beta }_x,{\beta }_y$), and ${\mathrm{\Delta }}_2$, ${\mathrm{\Delta }}_3$ result from hybridization of optical phonons with magnetic excitations. While a phenomenological spin Hamiltonian including exchange interactions, Dzyaloshinskii-Moriya interactions, and single-ion crystal field terms can describe aspects of the Mn-based magnetism, spin-wave damping [$\mathrm{\Gamma }=25\left(8\right)$ meV] and the extended range of magnetic interactions indicate itinerant magnetism consistent with the transport anomalies.

Recent experiments have observed possible spin- and valley-polarized insulators and spin-triplet superconductivity in twisted double bilayer graphene, a moiré structure consisting of a pair of Bernal-stacked bilayer graphene. Besides the continuously tunable bandwidths controlled by an applied displacement field and twist angle, these moiré bands also possess Van Hove singularities near the Fermi surface and a field-dependent nesting which is far from perfect. Here we carry out a perturbative renormalization group analysis to unbiasedly study the competition among all possible instabilities in twisted double bilayer graphene and related systems with a similar Van Hove fermiology in the presence of weak but finite repulsive interactions. Our key finding is that there are several competing magnetic, valley, charge, and superconducting instabilities arising from interactions in twisted double bilayer graphene, which can be tuned by controlling the displacement field and the twist angle. In particular, we show that spin- or valley-polarized uniform instabilities generically dominate under moderate interactions smaller than the bandwidth, whereas $p$-wave spin-triplet topological superconductivity and exotic spin-singlet modulated paired state become important as the interactions decrease. Realization of our findings in general moiré systems with a similar Van Hove fermiology should open up new opportunities for manipulating topological superconductivity and spin- or valley-polarized states in highly tunable platforms.

We demonstrate that chiral hinge modes naturally emerge in insulating crystals undergoing a slow cyclic evolution that changes the Chern-Simons axion angle $\theta$ by $2\pi$. This happens when the surface (not just the bulk) returns to its initial state at the end of the cycle, in which case it must pass through a metallic state to dispose of the excess quantum of surface anomalous Hall conductivity pumped from the bulk. If two adjacent surfaces become metallic at different points along the cycle, there is an interval in which they are in topologically distinct insulating states, with chiral modes propagating along the connecting hinge. We illustrate these ideas for a tight-binding model consisting of coupled layers of the Haldane model with alternating parameters. The surface topology is determined in a slab geometry using two different markers, surface anomalous Hall conductivity and surface-localized charge pumping (flow of surface-localized Wannier bands), and we find that both correctly predict the appearance of gapless hinge modes in a rod geometry. When viewing the axion pump as a four-dimensional crystal with one synthetic dimension, the hinge modes trace Fermi arcs in the Brillouin zone of the two-dimensional hinge connecting a pair of three-dimensional surfaces of the four-dimensional crystal.

The reduced density matrix of many-body systems possessing an additive conserved quantity can be decomposed in orthogonal sectors which can be independently analyzed. Recently, these have been proven to equally contribute to entanglement entropy for one-dimensional conformal and integrable systems. In this paper, we extend this equipartition theorem to the disordered critical systems by studying the random singlet phase. We analytically compute the disorder averaged symmetry resolved Rényi entropies and show the leading orders are independent of the symmetry sector. Our findings are cross-checked with simulations within the numerical strong disorder renormalization group. We also identify the first subleading term breaking equipartition which is of the form $s^2/ln\ell$ where $s$ is the magnetization of a subsystem of length $\ell$.

Time-reversal symmetric charge and spin transport through a molecule comprising two-orbital channels and connected to two leads is analyzed. It is demonstrated that spin-resolved currents are generated when spin-flip processes are accompanied by a flip of the orbital channels. This surprising finding does not contradict Bardarson's theorem [J. H. Bardarson, J. Phys. A: Math. Theor. 41, 405203 (2008)] for two-terminal junctions: the transmission does possess two pairs of doubly degenerate eigenvalues as required by the theorem. The spin-filtering effect is explicitly demonstrated for a two-terminal chiral molecular junction, modeled by a two-orbital tight-binding chain with intra-atomic spin-orbit interactions (SOI). In the context of transport through organic molecules like DNA, this effect is termed “chirality-induced spin selectivity” (CISS). The model exhibits spin splitting without breaking time-reversal symmetry: the intra-atomic SOI induces concomitant spin and orbital flips. Examining these transitions from the point of view of the Bloch states in an infinite molecule, it is shown that they cause shifts in the Bloch wave numbers, of the size of the reciprocal single turn, whose directions depend on the left and right handedness of the helix. As a result, spin-up and -down states propagate in the opposite directions, leading to the CISS effect. To further substantiate our picture, we present an analytically tractable expression for the $8\times 8$ scattering matrix of such a (single) molecule.

The rare-earth tritellurides ($R{\mathrm{Te}}_3$, where $R=\mathrm{La}$, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Y) form a charge density wave state consisting of a single unidirectional charge density wave for lighter $R$, with a second unidirectional charge density wave, perpendicular and in addition to the first, also present at low temperatures for heavier $R$. We present a quantum oscillation study in magnetic fields up to 65 T that compares the single charge density wave state with the double charge density wave state both above and below the magnetic breakdown field of the second charge density wave. In the double charge density wave state it is observed that there remain several small, light pockets, with the largest occupying around 0.5% of the Brillouin zone. By applying magnetic fields above the independently determined magnetic breakown field, the quantum oscillation frequencies of the single charge density wave state are recovered, as expected in a magnetic breakdown scenario. Measurements of the electronic effective mass do not show any divergence or significant increase on the pockets of Fermi surface observed here as the putative quantum phase transition between the single and the double charge density wave states is approached.

The electronic structure and dynamics of saddle-point excitons formed under $E_1$-resonance excitation around 3.0 eV in GaAs have been studied using time- and angle-resolved photoemission spectroscopy. The momentum- and energy-resolved spectra of the exciton photoionization reveal that the exciton wave function is composed mainly of the electron-hole pair states at the wave number of $0.32\pm 0.04{\AA }^{-1}$ along the Γ-$L$ direction of the bulk Brillouin zone. Time-resolved characteristics of the photoemission is featured by a bimodal decay in the femtosecond (fs) time frame with two different time constants of $11\pm 0.5\mathrm{fs}$ and $120\pm 10\mathrm{fs}$. The short lifetime reflects the autoionization processes of coherently generated excitonic polarization to unbound electron-hole pair states, and the long time constant represents incoherent exciton formation by combination between unbound electron-hole pairs during relaxation. The momentum- and energy-resolved features of photoemission from the exciton are governed by recoil effects of the conjugated holes that restore the valence-band states.