We present a high-rate projective measurement-based approach for controlling nonunitary evolution of a quantum chain of interacting spins. In this approach, we demonstrate that local measurement of a single external spin coupled to the chain can produce a spin polaron, which remains stable after the end of the measurement. This stability results from the fact that the Hilbert space of the chain contains a subspace of nondecaying states, stable during the nonunitary evolution. These states determine the resulting final state of the chain and long-term shape of the polaron. In addition to formation of the spin polarons, the presented measurement protocol can be used for distillation of nondecaying states from an initial superposition or mixture.

Tensor network methods have progressed from variational techniques based on matrix-product states able to compute properties of one-dimensional condensed-matter lattice models into methods rooted in more elaborate states, such as projected entangled pair states aimed at simulating the physics of two-dimensional models. In this work, we advocate the paradigm that for two-dimensional fermionic models, matrix-product states are still applicable to significantly higher accuracy levels than direct embeddings into one-dimensional systems allow for. To do so, we exploit schemes of fermionic mode transformations and overcome the prejudice that one-dimensional embeddings need to be local. This approach takes the insight seriously that the suitable exploitation of both the manifold of matrix-product states and the unitary manifold of mode transformations can more accurately capture the natural correlation structure. By demonstrating the residual low levels of entanglement in emerging modes, we show that matrix-product states can describe ground states strikingly well. The power of the approach is exemplified by investigating a phase transition of spinless fermions for lattice sizes up to $10\times 10$.

Many ferroelectric (FE) organic-inorganic hybrid perovskites (OIHPs) show great promise in the fields of photovoltaic cells and information memory devices. We systematically investigated the FE, optical, and electric properties of quasi-one-dimensional OIHPs $\left(\mathrm{MV}\right)A{\mathrm{I}}_3{\mathrm{Cl}}_2$ ($\mathrm{MV}=\mathrm{methylviologen}, A=\mathrm{Bi},\mathrm{Sb}$) using density functional theory calculations. The FE polarization mechanism of $\left(\mathrm{MV}\right)A{\mathrm{I}}_3{\mathrm{Cl}}_2$ mainly originates from octahedral distortion along the direction of octahedral chains with the $p\text{−}p$ coupling of $A^{3+}$ cations and ${\mathrm{I}}^{-}$ anions. Due to the loosely coupled FE chains, $\left(\mathrm{MV}\right)\mathrm{Bi}{\mathrm{I}}_3{\mathrm{Cl}}_2$ and $\left(\mathrm{MV}\right)\mathrm{Sb}{\mathrm{I}}_3{\mathrm{Cl}}_2$ possess a relatively low energy barrier of polarization switching at the 180° reversal of the FE polarization. The estimated upper limit memory densities of 14.9 and $15.0\mathrm{Tb}/\mathrm{c}{\mathrm{m}}^2$ for $\left(\mathrm{MV}\right)\mathrm{Bi}{\mathrm{I}}_3{\mathrm{Cl}}_2$ and $\left(\mathrm{MV}\right)\mathrm{Sb}{\mathrm{I}}_3{\mathrm{Cl}}_2$, respectively, hold promise to be applied to high-density FE memory devices. The strong anisotropic optical absorption of $\left(\mathrm{MV}\right)A{\mathrm{I}}_3{\mathrm{Cl}}_2$ is in the visible light region, and its estimated maximum power conversion efficiencies are >23%. The high anisotropic carrier mobility in $\left(\mathrm{MV}\right)A{\mathrm{I}}_3{\mathrm{Cl}}_2$ enhances the separation of electron-hole pairs. Both $\left(\mathrm{MV}\right)\mathrm{Bi}{\mathrm{I}}_3{\mathrm{Cl}}_2$ and $\left(\mathrm{MV}\right)\mathrm{Sb}{\mathrm{I}}_3{\mathrm{Cl}}_2$ possess positive piezoelectric effect; therefore, strain design is an effective approach to enhance power conversion efficiency and carrier mobility of $\left(\mathrm{MV}\right)A{\mathrm{I}}_3{\mathrm{Cl}}_2$, leading to experimental design of FE photovoltaic cell.

A systematic microscopic approach combining ab-initio density functional theory with the Dirac-Bloch equations is applied to investigate the intra-excitonic transitions of magneto-excitons in transition metal dichalcogenide monolayers. For the example of hBN-encapsulated $\mathrm{Mo}{\mathrm{S}}_2$, the linear optical response and mid-infrared spectra of the pre-excited system are numerically evaluated. It is shown that the transition probability between a subset of the magneto-excitons can be inverted under suitable conditions to display negative absorption, i.e., gain. With the help of an applied magnetic field, the absorption and gain spectra can be tuned over a wide spectral range. Evaluating the Zeeman shift of the excitonic states, effective $g$ factors are deduced that depend on the dielectric environment of the sample under consideration.

Long-distance transfer of quantum information in architectures based on quantum dot spin qubits will be necessary for their scalability. One way of achieving this goal is to simply move the electron between two quantum registers. Precise control over the process of shuttling such a single electron through a chain of tunnel-coupled quantum dots is possible when interdot energy detunings are changed adiabatically. The deterministic character of shuttling is, however, endangered by the open nature of the system, as the transferred electron is coupled to thermal reservoirs: sources of fluctuations of electric fields and lattice vibrations. We present a comprehensive analysis showing how the electron transfer between two voltage-controlled quantum dots is affected by electron-phonon scattering and interaction with sources of $1/f$ and Johnson charge noise in both detuning and tunnel coupling. The electron-phonon scattering turns out to be irrelevant in Si quantum dots, with charge noise dominating the dynamics of the system at slow detuning sweep, when the electron spends more time delocalized between the dots (i.e., near the anticrossing of the tunnel-coupled states). Competition between the effects of charge noise and the Landau-Zener effect leads to an existence of an optimal detuning sweep rate, leading to minimal probability of leaving the electron behind. In GaAs quantum dots, on the other hand, piezoelectric coupling to phonons is strong enough to make the processes of interdot transfer assisted by phonon emission and absorption dominate over transitions caused by charge noise. The probability of leaving the electron behind then depends monotonically on detuning the sweep rate over a broad range of rates, and values much smaller than in silicon can be obtained for slow sweeps. However, after taking into account limitations on transfer time imposed by the need for preservation of a shuttled electron's spin coherence, the minimal probabilities of leaving the electron behind in both GaAs- and Si-based double quantum dots turn out to be of the same order of magnitude. Bringing them down below ${10}^{-3}$ requires temperatures $\le 100$ mK and tunnel couplings above $20\mu \mathrm{eV}$.

Even though the $k·p$ effective Hamiltonian has been widely exploited to predict a large variety of phenomena in condensed matter systems, the general method to construct a $k·p$ Hamiltonian is case-by-case and can even be complicated. Moreover, deriving a $k·p$ model in magnetic materials usually requires representation matrices of a magnetic space group (MSG), however, those for type III/IV MSGs are not constructed well compared with 230 type-I/II MSGs. In this work, we explicitly and exhaustively tabulate the representation matrices for all single-valued/double-valued irreducible representations (irreps) and co-irreps for the little groups of all special $k$ points in 1651 MSGs. Then through analysis by group theory, we obtain a large database composed of 4 857 832 elementary $k·p$ matrix blocks. Directly using these matrix blocks, one can obtain any $k·p$ Hamiltonian for any periodic system, for both nonmagnetic and magnetic materials. In light of the huge successes achieved by $k·p$ models, our work could aid studies in various fields, such as semiconductors, topological physics, spintronics, valleytronics, twistronics, etc. The exhaustive effective models, each attached to an MSG, would facilitate identification and realization of all possible emergent massless/massive excitations. More importantly, through comparing the theoretical model with novel property and those in our database, one can find the MSGs of materials realizations of the model. We also expect our exhaustive results on $k·p$ models to be utilized to connect other fields and condensed matter physics.

Kitaev's toric code is an exactly solvable model with ${\mathbb{Z}}_2$-topological order, which has potential applications in quantum computation and error correction. However, a direct experimental realization remains an open challenge. Here, we propose a building block for ${\mathbb{Z}}_2$ lattice gauge theories coupled to dynamical matter and demonstrate how it allows for an implementation of the toric-code ground state and its topological excitations. This is achieved by introducing separate matter excitations on individual plaquettes, whose motion induce the required plaquette terms. The proposed building block is realized in the second-order coupling regime and is well suited for implementations with superconducting qubits. Furthermore, we propose a pathway to prepare topologically nontrivial initial states during which a large gap on the order of the underlying coupling strength is present. This is verified by both analytical arguments and numerical studies. Moreover, we outline experimental signatures of the ground-state wave function and introduce a minimal braiding protocol. Detecting a $\pi$-phase shift between Ramsey fringes in this protocol reveals the anyonic excitations of the toric-code Hamiltonian in a system with only three triangular plaquettes. Our work paves the way for realizing non-Abelian anyons in analog quantum simulators.

We study the quantum chaos in the Bose-Fermi Kondo model in which the impurity spin interacts with conduction electrons and a bosonic bath at the intermediate temperature in the large-$N$ limit. The out-of-time-ordered correlator is calculated based on the Bethe-Salpeter equation and the Lyapunov exponent ${\lambda }_L$ is extracted. Our calculation shows that the Lyapunov exponent monotonically increases as the Kondo coupling $J_K$ increases, and it can reach an order of ${\lambda }_L\approx T$ as $J_K$ approaches the multichannel Kondo fixed point ($MCK$). Furthermore, we also demonstrate that ${\lambda }_L$ decreases monotonically as the impurity and bosonic bath coupling $g$ increases, which is contrary to the general expectation that the most chaotic property occurs at the quantum critical point with the non-Fermi-liquid nature.

Bias-induced light emission and thermal radiation from conducting channels of metallic armchair carbon nanotubes (CNTs) with defects are studied theoretically within the framework of nonequilibrium Green's function method based on a tight-binding model. Localized states induced by the single vacancy defect and single Stone-Wales defect in the low-energy range enhance electroluminescence significantly while they reduce thermal radiation under zero bias. The influence of the diameters of the CNTs with defects on the radiation is discussed. Radiations from metallic CNTs in thermal equilibrium show black-body-like spectrum. For perfect nanotubes with small diameter, their thermal radiations are nearly independent on the tube diameter due to the confinement of the thermal excitations in the tube's circular direction. Our study is important for optoelectronic applications of CNTs with defects.

Acoustic spin has been recently explored for many applications. In particular, transverse acoustic spin was demonstrated for inhomogeneous acoustic fields. In this Letter, we show the emergence of acoustic spin and torque in rotating acoustic objects of the same physical properties as the surrounding, to single out the effects purely due to rotation. The spinning of a cylindrical column of air or water in the same medium possesses intrinsic spin angular momentum, and we study the torque and force it experiences in evanescent acoustic fields. The resulting discontinuity can thus scatter sound in unusual ways, including a negative radiation force, although it has no imaginary part in its parameters.

We present experimental results on the observed flux screening in proximity coupled superconductor-ferromagnet thin film structures using Nb and Co as the superconductor and ferromagnet respectively. Using the low-energy muon-spin rotation technique to locally probe the magnetic flux density, we find that the addition of the ferromagnet (F) increases the total flux screening inside the superconductor. Two contributions can be distinguished. One is consistent with the predicted spin-polarization (or magnetic proximity) effect, while the other is in line with the recently emerged electromagnetic (EM) proximity models. Furthermore, we show that the addition of a few nanometers of a normal metallic layer between the Nb and the Co fully destroys the contribution due to electromagnetic proximity. This is unanticipated by the current theory models in which the magnetization in the F layer is assumed to be the only driving force for the EM effect and suggests the role of additional factors. Further experiments to explore the influence of the direction of the F magnetization also reveal deviations from theory. These findings are an important step forward in improving the theoretical description and understanding of proximity coupled systems.

Quantum spin Hall edges are envisaged as next-generation transistors, yet they exhibit dissipationless transport only over short distances. Here, we show that in a diffusive sample, where charge puddles with odd spin cause backscattering, a magnetic field drastically increases the mean free path and drives the system into the ballistic regime with a Landauer-Buttiker conductance. A strong nonlinear nonreciprocal current emerges in the diffusive regime with opposite signs on each edge, and vanishes in the ballistic limit. We discuss its detection in state-of-the-art experiments.

Pancharatnam-Berry (PB) metasurfaces have been considered as innovative optical devices for efficient manipulation of both the phase and the polarization of the electromagnetic field. Here, we present the simultaneous generation of circularly, elliptically, and linearly polarized states by the simplest PB metasurface with one-dimensional phase gradient unit cells. Furthermore, we propose that the reconstruction of a multidimensional nonlinear polarization response of a nanomaterial can be achieved in a single heterodyne measurement by active manipulation of the polarization states of incident light. Using multidimensional spectroscopy, we demonstrate the possibility to track both stationary and transient delocalized charge distributions via detecting plasmonic populations and coherences.

Superparamagnetic tunnel junctions (SMTJs) are promising sources for the randomness required by some compact and energy-efficient computing schemes. Coupling SMTJs gives rise to collective behavior that could be useful for cognitive computing. We use a simple linear electrical circuit to mutually couple two SMTJs through their stochastic electrical transitions. When one SMTJ makes a thermally induced transition, the voltage across both SMTJs changes, modifying the transition rates of both. This coupling leads to significant correlation between the states of the two devices. Using fits to a generalized Néel-Brown model for the individual thermally bistable magnetic devices, we can accurately reproduce the behavior of the coupled devices with a Markov model.

The ladder-chain compound ${(\mathrm{Sr},\hspace{0.17em}\mathrm{Ca})}_{14}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$ is a semiconductor at ambient pressure, but becomes a bulk superconductor above the pressure of about 3 GPa. Since the compound is at the verge of the metal-insulator transition, it is reasonable to make tunnel measurements to probe the electronic density of states in its subtleties. We present the results of such measurements carried out at ambient pressure. The break junction (BJ) tunneling gives evidence for the apparent typical gap $2\mathrm{\Sigma }$ of about 140 meV at temperature $T$, equal to 4 K. The gap is smeared out at $T^{*}\approx 90–100$ K. We interpret the gap as that induced by the charge density wave (CDW) formation, although its $T$-dependence differs from the usually observed CDW-like mean-field behavior. Quite unexpectedly, BJ spectra also exhibit distinct zero-bias peak accompanied by the low-energy gaps of $2\mathrm{\Delta }\approx 4–8$ meV immediately after BJ are formed at 4 K. The thermally driven disappearance of this apparently superconducting structure at $T_c\approx 7–13$ K is consistent with the conventional properties of superconducting tunnel junctions. The resultant ratio $\mathrm{\Delta }/T_c$ is consistent with similar observed values for a high-$T_c$ cuprate superconductor. Therefore, we attribute this feature as well as the Josephson-like zero-bias conductance peak to the superconductivity induced at the freshly created BJ surface of the ${(\mathrm{Sr},\hspace{0.17em}\mathrm{Ca})}_{14}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$, which is semiconducting in the sample bulk. Our additional scanning tunneling microscopy data testify that the cracked atomic surface of this compound is substantially modified, which might be the reason for the superconductivity appearance.

We introduce a one-dimensional lattice model whose hopping amplitudes are modulated for equally spaced sites. Such mosaic lattice exhibits many interesting topological and localization phenomena that do not exist in the regular off-diagonal lattices. When the mosaic modulation is commensurate with the underlying lattice, topologically nontrivial phases with zero- and nonzero-energy edge modes are observed as we tune the modulation, where the nontrivial regimes are characterized by quantized Berry phases. If the mosaic lattice becomes incommensurate, Anderson localization will be induced purely by the quasiperiodic off-diagonal modulations. The localized eigenstate is found to be centered on two neighboring sites connected by the quasiperiodic hopping terms. Furthermore, both the commensurate and incommensurate off-diagonal mosaic lattices can host Chern insulators in their two-dimensional generalizations. Our work provides a platform for exploring topological phases and Anderson localization in low-dimensional systems.

Recent experiments demonstrated emerging alternating insulator and metal phases in Mott insulators actuated by a direct bias voltage, leading to oscillating voltage outputs with characteristic frequencies. Here, we develop a physics-based nonequilibrium model to describe the dynamics of oscillating insulator-metal phase transitions and experimentally validate it using a ${\text{VO}}_{\text{2}}$ device as a prototype. The oscillation frequency is shown to scale monotonically with the bias voltage and series resistance and terminate abruptly at lower and upper device-dependent limits, which are dictated by the nonequilibrium carrier dynamics. We derive an approximate analytical expression for the dependence of the frequency on the device operating parameters, which yields a fundamental limit to the frequency and may be utilized to provide guidance to potential applications of insulator-metal transition materials as building blocks of brain-inspired non-von Neumann computers.

Established forms of electromagnetic coupling are usually conservative (in insulators) or dissipative (in metals and semiconductors). Here, we point out the possibility of nondissipative electric driving of magnetization dynamics, if the valence electronic states have nontrivial topology in the combined space of crystal momentum and magnetization configuration. We provide a hybrid insulator system to demonstrate that the topology-based nonconservative electrical generalized force is capable of supporting sustained magnetization motion in the presence of Gilbert damping, with quantized and steady energy pumping into magnetization motion from the electric field. We also generalize our results to magnetic textures, and discuss an electric-field-induced Dzyaloshinskii-Moriya interaction which can be nonconservative.

Magnetic domain walls (DWs) function as a waveguide for low-energy magnons. In this paper we develop the theory for the nonlocal transport of these bound magnons through a ferromagnetic insulator (FMI) that are injected and detected electrically in adjacent normal metal leads by spin-flip scatting processes and the (inverse) spin-Hall effect. Our setup requires a twofold degeneracy of the magnetic ground state, which we realize by an easy axis and hard axis anisotropy, in the FMI. This is readily provided by a broad range of materials. The DW is a topologically protected feature of the system and we obtain the nonlocal spin transport through it, and thereby we provide a framework for reconfigureable magnonic devices.

We develop a self-consistently renormalized spin-wave theory, within a mean-field approximation, for the two-dimensional Heisenberg ferromagnet with perpendicular easy-axis anisotropy on the honeycomb lattice, as well as its few-layer and bulk extensions. In this method, the magnetization dependence on temperature is found as the solution of the self-consistency equation. Furthermore, we account for the physical difference of surface and bulk layers by treating the layers as separate sublattices. Thus the method can be readily generalized to study various magnetic phenomena in a broad range of systems, including those comprising magnetically inequivalent sublattices. Using our theory, we calculate the temperature-dependent magnetization for two chromium-based layered van der Waals insulating magnets, ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ and ${\mathrm{CrI}}_3$, employing various sets of Heisenberg exchange and single-ion anisotropy values reported for these materials in the existing literature. As expected, we observe a strong dimensionality effect where the ordering temperature is reduced and its sensitivity on the anisotropy is enhanced with the decrease of dimensionality.