We develop a hydrodynamic theory for an electron system exhibiting the anomalous Hall effect, and show that an additional anomalous Hall effect is induced by a vorticity generated near boundaries. We calculate the momentum flux and force proportional to the electric field using linear response theory. The hydrodynamic equation is obtained by replacing the local electric field with the electric current, focusing on a scale that is sufficiently larger than the mean-free path. It is demonstrated that there is a coupling between a vorticity of an electric current and a magnetization which generates a pressure from nonuniform vorticity. Taking into account Hall viscosity and relaxation forces, a nonuniform flow near a boundary and an additional Hall force are calculated. The additional anomalous Hall force is opposite to a conventional anomalous Hall force, resulting in a sign reversal in thin systems. An antisymmetric viscosity turns out to arise from the side-jump process due to the anomalous velocity.

We propose a streamlined combination of the transcorrelation (TC) and coupled cluster (CC) theory, which not only increases the convergence rate with respect to the basis set but also extends the applicability of the lowest-order CC approximations to strongly correlated systems. Using the three-dimensional uniform electron gas as a model for real periodic solids and highly accurate ground-state energies from the state-of-the-art quantum Monte Carlo methods as benchmark, we show that, with the correct physical insights incorporated, our TC-CC methods gain drastic improvements in accuracy compared with canonical CC methods, and the errors of our methods remain $\pm 0.001$ Hartree per electron (Ha/electron) across a wide range of densities. With the greatly improved efficiencies and the balanced and accurate performances at both low and high densities, our methods present great promise for applications on real solids with weak and strong correlations.

The availability of efficient photon sources with specific properties is important for quantum-technological applications. However, the realization of such photon sources is often challenging and hence alternative perspectives that suggest different means to enhance desired properties while suppressing detrimental processes are valuable. In this work we highlight that ab initio simulations of coupled light-matter systems can provide such alternative avenues. We show for a simple model of a quantum ring that by treating light and matter on equal footing, we can create and enhance pathways for down-conversion processes. By changing the matter subsystem as well as the photonic environment in experimentally feasible ways, we can engineer hybrid light-matter states that enhance at the same time the efficiency of the down-conversion process and the nonclassicality of the created photons. Furthermore, we show that this also leads to a faster down-conversion, potentially avoiding detrimental decoherence effects.

We discuss DC electric-field controls of superexchange interactions. We first present generic results about antiferromagnetic and ferromagnetic superexchange interactions valid in a broad class of Mott insulators, where we also estimate typical field strength to observe DC electric-field effects: $\sim 1\mathrm{MV}/\mathrm{cm}$ for inorganic Mott insulators such as transition-metal oxides and $\sim 0.1\mathrm{MV}/\mathrm{cm}$ for organic ones. Next, we apply these results to geometrically frustrated quantum spin systems. Our theory widely applies to (quasi-)two-dimensional and thin-film systems and one-dimensional quantum spin systems on various lattices such as square, honeycomb, triangular, and kagome ones. In this paper, we give our attention to those on the square lattice and on the chain. For the square lattice, we show that DC electric fields can control a ratio of the nearest-neighbor and next-nearest-neighbor exchange interactions. In some realistic cases, DC electric fields make the two next-nearest-neighbor interactions nonequivalent and eventually turns the square-lattice quantum spin system into a deformed triangular-lattice one. For the chain, DC electric fields can induce singlet-dimer and Haldane-dimer orders. We show that the DC electric-field-induced spin gap $\propto {\left|\mathit{E}\right|}^{2/3}$ in the Heisenberg antiferromagnetic chain will reach $\sim 10\%$ of the dominant superexchange interaction in the case of a spin-chain compound ${\mathrm{KCuMoO}}_4\left(\mathrm{OH}\right)$ when the DC electric field of $\sim 1\mathrm{MV}/\mathrm{cm}$ is applied.

We experimentally investigated magnetization relaxation dynamics in the largely unexplored time window extending from few picoseconds up to two nanoseconds following femtosecond laser pulse excitation. We triggered magnetization dynamics in ${\left[\mathrm{Co}\left(0.4\mathrm{nm}\right)/\mathrm{Pt}\left(0.7\mathrm{nm}\right)\right]}_3$ multilayers and measured the resulting magneto-optic response by recording both transient hysteresis loops as well as transients of magnetization dynamics. We observe that the coercive field of the sample is still strongly suppressed even ∼1 ms after the laser excitation, which is three orders of magnitude longer than the recovery time of the magnetization amplitude. In addition, we succeeded to fit the magnetization relaxation data in the entire experimentally observed time window by considering two phenomenological time constants ${\tau }_f^{*}$ and ${\tau }_s^{*}$ describing fast (ps) and slow (ns) magnetization relaxation processes, respectively. The fits of the data suggest a magnetic field dependent relaxation slowdown beyond 100 ps after excitation. We observe an explosion of the ${\tau }_f^{*}$ and ${\tau }_s^{*}$ values when the magnetization is completely quenched and relaxes intrinsically in the absence of an external magnetic field. We interpret the phenomenological time constants ${\tau }_f^{*}$ and ${\tau }_s^{*}$ using an intuitive physical picture based on the Landau-Lifshitz-Bloch model and numerical solutions of the extended three-temperature model [Shim et al., Sci. Rep. 10, 6355 (2020)].

We consider the quantum theory of paraxial nonrelativistic electron beams in nonuniform magnetic fields, such as the Glaser field. We find the wave function of an electron from such a beam and show that it is a joint eigenstate of two ($z$-dependent) commuting gauge-independent operators. This generalized Laguerre-Gaussian vortex beam has a phase that is shown to consist of two parts, each being proportional to the eigenvalue of one of the two conserved operators and each having different symmetries. We also describe the dynamics of the angular momentum and cross-sectional area of any mode and how a varying magnetic field can split a mode into a superposition of modes. By a suitable change in the frame of reference, all of our analysis also applies to an electron in a quantum Hall system with a time-dependent magnetic field.

We perform an analytical and numerical study of a superconducting instability in quasi-one-dimensional (quasi-1D) disordered systems. Modeling them as an array of Luttinger liquids with Josephson-type interchain coupling, we employed renormalization-group analysis with an extensive search for parameters that support superconductivity enhancement. We have found that this phenomenon is possible in the parameters range that support a latent disorder-driven phase transition between charge- and spin-density-wave phases. Our results may explain the experimental observation of disorder-enhanced superconductivity.

Magnetic-field-induced phase transitions are investigated in the frustrated gapped quantum paramagnet ${\mathrm{Rb}}_2{\mathrm{Cu}}_2{\mathrm{Mo}}_3{\mathrm{O}}_{12}$ through dielectric and calorimetric measurements on single-crystal samples. It is clarified that the previously reported dielectric anomaly at 8 K in powder samples is not due to a chiral spin liquid state as has been suggested, but rather to a tiny amount of a ferroelectric impurity phase. Two field-induced quantum phase transitions between paraelectric and paramagnetic and ferroelectric and magnetically ordered states are clearly observed. It is shown that the electric polarization is a secondary order parameter at the lower-field (gap closure) quantum critical point but a primary one at the saturation transition. Having clearly identified the magnetic Bose-Einstein condensation (BEC) nature of the latter, we use the dielectric channel to directly measure the critical divergence of BEC susceptibility. The observed power-law behavior is in very good agreement with theoretical expectations for three-dimensional BEC. Finally, dielectric data reveal magnetic presaturation phases in this compound that may feature exotic order with unconventional broken symmetries.

The quantum spin liquid (QSL) state is an exotic state of matter featuring a high degree of entanglement and lack of long-range magnetic order in the zero-temperature limit. The triangular antiferromagnet ${\mathrm{YbMgGaO}}_4$ is a candidate QSL host, and precise determination of the Hamiltonian parameters is critical to understanding the nature of the possible ground states. However, the presence of chemical disorder has made directly measuring these parameters challenging. Here we report neutron scattering and magnetic susceptibility measurements covering a broad range of applied magnetic field at low temperature. Our data shows a field-induced crossover in ${\mathrm{YbMgGaO}}_4$, which we reproduce with complementary classical Monte Carlo and density matrix renormalization group simulations. Neutron scattering data above and below the crossover reveal a shift in scattering intensity from $M$ to $K$ points and, collectively, our measurements provide essential characteristics of the phase crossover that we employ to strictly constrain proposed magnetic Hamiltonian parameters despite the chemical disorder. Constrained exchange parameters further suggest the material's proximity to the QSL state in the clean limit. More broadly, our approach demonstrates a means of pursuing QSL candidates where Hamiltonian parameters might otherwise be obscured by disorder.

Chains of magnetic atoms, placed on the surface of $s$-wave superconductors, have been established as a laboratory for the study of Majorana bound states. In such systems, the breaking of time reversal due to magnetic moments gives rise to the formation of in-gap states, which hybridize to form one-dimensional topological superconductors. However, in unconventional superconductors even nonmagnetic impurities induce in-gap states since scattering of Cooper pairs change their momentum but not their phase. Here we propose a path for creating topological superconductivity, which is based on an unconventional superconductor with a chain of nonmagnetic adatoms on its surface. The topological phase can be reached by tuning the magnitude and direction of a Zeeman field, such that Majorana zero modes at its boundary can be generated, moved, and fused. To demonstrate the feasibility of this platform, we develop a general mapping of films with adatom chains to one-dimensional lattice Hamiltonians. This allows us to study unconventional superconductors such as ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ exhibiting multiple bands and an anisotropic order parameter.

One-dimensional gapped phases that avoid any symmetry breaking have drawn enduring attention. In this paper, we study such phases in a bond-alternating spin-1 $K-\mathrm{\Gamma }$ chain built of a Kitaev ($K$) interaction and an off-diagonal $\mathrm{\Gamma }$ term. In the case of isotropic bond strength, a Haldane phase, which resembles the ground state of a spin-1 Heisenberg chain, is identified in a wide region. A gapped Kitaev phase situated at dominant ferromagnetic and antiferromagnetic Kitaev limits is also found. The Kitaev phase has extremely short range spin correlations and is characterized by finite ${\mathbb{Z}}_2$-valued quantities on bonds. Its lowest entanglement spectrum is unique, in contrast to the Haldane phase, whose entanglement spectrum is doubly degenerate. In addition, the Kitaev phase shows a double-peak structure in the specific heat at two different temperatures. In the pure Kitaev limit, the two peaks are representative of the development of short-range spin correlation at $T_h\simeq 0.5680$ and the freezing of ${\mathbb{Z}}_2$ quantities at $T_l\simeq 0.0562$, respectively. By considering bond anisotropy, regions of Haldane phase and Kitaev phase are enlarged, accompanied by the emergence of dimerized phases and three distinct magnetically ordered states.

We present a general theory of classical metastability in open quantum systems. Metastability is a consequence of a large separation in timescales in the dynamics, leading to the existence of a regime when states of the system appear stationary, before eventual relaxation toward a true stationary state at much larger times. In this work, we focus on the emergence of classical metastability, i.e., when metastable states of an open quantum system with separation of timescales can be approximated as probabilistic mixtures of a finite number of states. We find that a number of classical features follow from this approximation, for the manifold of metastable states, long-time dynamics between them, and symmetries of the dynamics. Namely, those states are approximately disjoint and thus play the role of metastable phases, the relaxation toward the stationary state is approximated by a classical stochastic dynamics between them, and weak symmetries correspond to their permutations. Importantly, the classical dynamics is observed not only on average but also at the level of individual quantum trajectories: We show that time coarse-grained continuous measurement records can be viewed as noisy classical trajectories, while their statistics can be approximated by that of the classical dynamics. Among others, this explains how first-order dynamical phase transitions arise from metastability. Finally, to verify the presence of classical metastability in a given open quantum system, we develop an efficient numerical approach that delivers the set of metastable phases together with the effective classical dynamics. Since the proximity to a first-order dissipative phase transition manifests as metastability, the theory and tools introduced in this work can be used to investigate such transitions—which occur in the large size limit—through the metastable behavior of many-body systems of moderate sizes accessible to numerics.

In ${\mathrm{BaV}}_{10}{\mathrm{O}}_{15}$ with quasitriangular V lattices, we found unique superstructures characterized by $\left(qq0\right)$-type peaks in the momentum space, where $q$ varies continuously with temperature between $q=0$ and $1/3$ and locks at $1/3$. Such a threefold superstructure is likely caused by the dense arrangement of the V trimers associated with the orbital ordering of V. The results indicate that the superstructure of the V trimers, each of which is also a superstructure, gives rise to the ordered phases with peculiar characteristics.

We study the generalization of the three-dimensional two-band Hopf insulator to the case of many bands, where all the bands are separated from each other by band gaps. The obtained $\mathbb{Z}$ classification of such an $N$-band Hopf insulator is related to the quantized isotropic magnetoelectric coefficient of its bulk. The boundary of an $N$-band Hopf insulator can be fully gapped, and we find that there is no unique way of dividing a finite system into bulk and boundary. Despite this nonuniqueness, we find that the magnetoelectric coefficient of the bulk and the anomalous Hall conductivity of the boundary are quantized to the same integer value. We propose an experiment where the quantized boundary effect can be measured in a nonequilibrium state.

Adiabatically exchanging anyons gives rise to topologically protected operations on the quantum state of the system, but the desired result is only achieved if the anyons are well separated, which requires a sufficiently large area. Being able to reduce the area needed for the exchange, however, would have several advantages such as enabling a larger number of operations per area and allowing anyon exchange to be studied in smaller systems that are easier to handle. Here, we use optimization techniques to squeeze the charge distribution of Abelian anyons in lattice fractional quantum Hall models, and we show that the squeezed anyons can be exchanged within a smaller area with a close to ideal outcome. We first use a toy model consisting of a modified Laughlin trial state to show that one can shape the anyons without altering the exchange statistics under certain conditions. We then squeeze and braid anyons in the Kapit-Mueller model and an interacting Hofstadter model by adding suitable potentials. We consider a fixed system size, for which the charge distributions of the normal anyons overlap, and we find that the outcome of the exchange process is closer to the ideal value for the squeezed anyons. The time needed for the exchange is also important, and for a particular example we find that the duration needed for the process to be close to the adiabatic limit is about five times longer for the squeezed anyons when the path length is the same. Finally, we show that the exchange outcome is robust with respect to small modifications of the potential away from the optimized value.

As strength of disorder enhances beyond a threshold value in many-body systems, a fundamental transformation happens through which the entire spectrum localizes, a phenomenon known as many-body localization. This has profound implications as it breaks down fundamental principles of statistical mechanics, such as thermalization and ergodicity. Due to the complexity of the problem, the investigation of the many-body localization transition has remained a big challenge. The experimental exploration of the transition point is even more challenging as most of the proposed quantities for studying such an effect are practically infeasible. Here, we experimentally implement a scalable protocol for detecting the many-body localization transition point, using the dynamics of an $N=12$ superconducting qubit array. We show that the sensitivity of the dynamics to random samples becomes maximum at the transition point, which leaves its fingerprints in all spatial scales. By exploiting three quantities, each with a different spatial resolution, we identify the transition point with an excellent match between simulation and experiment. In addition, one can detect the evidence of a mobility edge through slight variation of the transition point as the initial state varies. The protocol is easily scalable and can be performed across various physical platforms.

We measure the colloidal interaction between two silica microspheres in an aqueous solution in the distance range from 0.2 to $0.5\mu \mathrm{m}$ with the help of optical tweezers. When employing a sample with a low salt concentration, the resulting interaction is dominated by the repulsive double-layer interaction which is fully characterized. The double-layer interaction is suppressed when adding $0.22\mathrm{M}$ of salt to our sample, thus leading to a purely attractive Casimir signal. When analyzing the experimental data for the potential energy and force, we find good agreement with theoretical results based on the scattering approach. At the distance range probed experimentally, the interaction arises mainly from the unscreened transverse magnetic contribution in the zero-frequency limit, with nonzero Matsubara frequencies providing a negligible contribution. In contrast, such unscreened contribution is not included by the standard theoretical model of the Casimir interaction in electrolyte solutions, in which the zero-frequency term is treated separately as an electrostatic fluctuational effect. As a consequence, the resulting attraction is too weak in this standard model, by approximately one order of magnitude, to explain the experimental data. Overall, our experimental results shed light on the nature of the thermal zero-frequency contribution and indicate that the Casimir attraction across polar liquids has a longer range than previously predicted.

Understanding magnonic properties of nonperiodic magnetic nanostructures requires real-space imaging of ferromagnetic resonance modes with spatial resolution well below the optical diffraction limit and sampling rates in the 5–100 GHz range. Here, we demonstrate element-specific scanning transmission x-ray microscopy-detected ferromagnetic resonance (STXM-FMR) applied to a chain of dipolarly coupled ${\mathrm{Fe}}_3{\mathrm{O}}_4$ nano-particles (40–50 nm particle size) inside a single cell of a magnetotactic bacterium Magnetospirillum magnetotacticum. The ferromagnetic resonance mode of the nano-particle chain driven at 6.748 GHz and probed with 50 nm x-ray focus size was found to have a uniform phase response but non-uniform amplitude response along the chain segments due to the superposition of dipolar coupled modes of chain segments and individual particles, in agreement with micromagnetic simulations.