Topological vortex textures in magnetic disks have garnered great attention due to their interesting physics and diverse applications. However, prior studies have primarily focused on microscale ferromagnetic disks with oscillation frequencies limited to the GHz range. To address this critical bottleneck, we propose a novel mechanism to generate vortex states in antiferromagnetic nanodots surrounded by heavy metals, which enables ultrasmall sizes—hundreds to thousands of times smaller in area—and ultrafast frequencies—hundreds to thousands of times higher. We demonstrate that, interestingly, the interfacial Dzyaloshinskii-Moriya interaction (iDMI) induced by the heavy metal at the dot boundary acts as an effective chemical potential for vortices. Analogous to the formation of superfluid vortices through rotation, we show that the iDMI can stabilize a magnetic vortex state. Moreover, applying an electric current induces vortex oscillations via spin-transfer torque, reaching THz frequencies that can be further tuned with external magnetic fields. These theoretical predictions are validated via micromagnetic simulations. Furthermore, we show that coherent coupling between vortices can be achieved via an antiferromagnetic link, which depends on the vortex polarity and topological charge and is also tunable via the vortex resonance frequency. Our proposal overcomes the critical bottleneck and opens a new vortex-based path toward next-generation information technologies and high-density data storage.

Superconductor–semiconductor hybrid systems play a crucial role in realizing nanoscale quantum devices, including hybrid qubits, Majorana bound states, and Kitaev chains. For such hybrid devices, subgap states play a prominent role in their operation. In this paper, we study these subgap states via Coulomb and tunneling spectroscopy through a superconducting island defined in a semiconductor nanowire fully coated by a superconductor. We systematically explore regimes ranging from an almost decoupled island to the open configuration. In the weak-coupling regime, the experimental observations are very similar in the absence of a magnetic field and when one flux quantum pierces the superconducting shell. Conversely, in the strong-coupling regime, significant distinctions emerge between the two cases. We attribute this distinct behavior to the existence of subgap states at one flux quantum, which become observable only for sufficiently strong coupling to the leads. We support our interpretation using a simple model to describe transport through the island. Our study highlights the importance of studying a broad range of tunnel couplings for understanding the rich physics of hybrid devices.

Interparticle interactions with multiple length scales play a pivotal role in the formation and stability of quasicrystals. Choosing a minimal set of length scales to stabilize a given quasicrystal is a challenging problem. To address this challenge, we propose a symmetry-preserving screening method (SPSM) to design a Landau theory with a minimal number of length scales—referred to as the minimal Landau theory—that includes only the essential length scales necessary to stabilize quasicrystals. Based on a generalized multiple-length-scale Landau theory, SPSM first evaluates various spectral configurations of candidate structures under a hard constraint. It then identifies the configuration with the lowest free energy. Using this optimal configuration, SPSM calculates phase diagrams to explore the thermodynamic stability of desired quasicrystals. SPSM can design a minimal Landau theory capable of stabilizing the desired quasicrystals by incrementally increasing the number of length scales. Our application of SPSM has not only confirmed known behaviors in 10- and 12-fold quasicrystals but also led to a significant prediction that quasicrystals with 8-, 14-, 16-, and 18-fold symmetry could be stable within three-length-scale Landau models.

Numerical methods based on matrix product states (MPSs) are currently the de facto standard for calculating the ground-state properties of (quasi-)one-dimensional quantum many-body systems. While the properties of the low-lying excitations in such systems are often studied in this MPS framework through dynamics by means of time-evolution simulations, we can also look at their statics by directly calculating eigenstates corresponding to these excitations. The so-called MPS excitation ansatz is a powerful method for finding such eigenstates with a single-particle character in the thermodynamic limit. Although this excitation ansatz has been used quite extensively, a general method for calculating expectation values for these states is lacking in the literature: We aim to fill this gap by presenting a recursive algorithm to calculate arbitrary observables expressed as matrix product operators. This method concisely encapsulates existing methods for—as well as extensions to—the excitation ansatz, such as excitations with a larger spatial support and multiparticle excitations, and is robust enough to handle further innovations. We demonstrate the versatility of our method by studying the low-lying excitations in the spin-1 Heisenberg chain and the one-dimensional Hubbard model, looking at how the excitations converge in the former, while in the latter, we present a refined method of targeting single-particle excitations inside a continuum by minimizing the energy variance rather than the energy itself. We hope that this technique will foster further advancements with the excitation ansatz.

Oxygen-deficient titanium dioxide (${\mathrm{TiO}}_{2-x}$) is a very attractive material for several applications ranging from photocatalysis to resistive switching. Oxygen vacancies turn insulating anatase titanium dioxide into a polaronic conductor, while creating a defect-state band below the ultraviolet semiconducting gap. Here we employ a combination of broadband infrared (IR) reflectivity and THz-pump/IR-probe measurements to investigate the relationship between localized defect states and delocalized conducting polaronic states. We show that the THz pump allows to convert deeply localized electrons into metastable polarons with a lifetime in the ns range. These long-lived metastable states may find application in novel optoelectronic applications exploiting the interplay of dc resistivity, with terahertz and infrared signals.

Layered van der Waals tellurides reveal topologically nontrivial properties that give rise to unconventional magneto-transport phenomena. Additionally, their semimetallic character with high mobility makes them promising candidates for large magneto-thermoelectric effects. Remarkable studies on the very large and unconventional Nernst effect in ${\mathrm{WTe}}_2$ have been reported, raising questions about whether this property is shared across the entire family of van der Waals tellurides. In this study, systematic measurements of the Nernst effect in telluride van der Waals Weyl semimetals are presented. Large linear Nernst coefficients in ${\mathrm{WTe}}_2$ and ${\mathrm{MoTe}}_2$ are identified, and moderate Nernst coefficients with nonlinear behavior in magnetic fields are observed in ${\mathrm{W}}_{0.65}{\mathrm{Mo}}_{0.35}{\mathrm{Te}}_2, {\mathrm{TaIrTe}}_4$, and ${\mathrm{TaRhTe}}_4$. Within this sample set, a correlation between the dominant linear-in-magnetic-field component of the Nernst coefficient and mobility is established, aligning with the established Nernst scaling framework, although with a different scaling factor compared with existing literature. This enhancement might be caused by the shared favorable electronic band structure of this family of materials. Conversely, the nonlinear component of the Nernst effect in a magnetic field could not be correlated with mobility. This nonlinear term is almost absent in the binary compounds, suggesting a multiband origin and strong compensation between electron-like and hole-like carriers. This comprehensive study highlights the potential of van der Waals tellurides for thermoelectric conversion.

The metal-organic-framework compound ${\mathrm{Cu}}_3{\left(\mathrm{HOTP}\right)}_2$, also known as ${\mathrm{Cu}}_3{\left(\mathrm{HHTP}\right)}_2$, is a small-gap semiconductor containing a kagome lattice of antiferromagnetically coupled $S=1/2{\mathrm{Cu}}^{\mathrm{II}}$ spins with intralayer nearest-neighbor exchange coupling $J\sim$ 2 K. The intralayer $J$ value obtained from calculations using density functional theory is shown to match with the experimental value for reasonable values of the Hubbard U parameter. Muon spin relaxation confirms no magnetic ordering down to 50 mK and sees spin fluctuations diffusing on a two-dimensional lattice. These properties are consistent with the hypothesis of a quantum spin-liquid ground state being present within highly decoupled kagome layers. Reduction of the spin-diffusion rate on cooling from the paramagnetic region to the low-temperature region reflects quantum entanglement, one of the key properties of a quantum spin liquid. It is also found that the layers become more strongly decoupled in the low-temperature region. Comparison of results for the spin diffusion, magnetic susceptibility, and specific heat in the low-temperature region suggests close proximity to a quantum critical point and a large density of low-energy spinless electronic excitations. A ${\mathrm{Z}}_2$-linear Dirac model for the low-energy spin excitations of the putative quantum spin-liquid ground state is found to provide the best match with experiment.

The decoherence of a central electron spin due to the dynamics of a coupled electron-spin bath is a core problem in solid-state spin physics. Ensemble experiments have studied the central spin coherence in detail, but such experiments average out the underlying quantum dynamics of the bath. Here, we show the coherent back-action of an individual NV center on an electron-spin bath and use it to detect, prepare, and control the dynamics of a pair of bath spins. We image the NV-pair system with subnanometer resolution and reveal a long dephasing time [$T_2^{*}=44\left(9\right)$ ms] for a qubit encoded in the electron-spin pair. Our experiment reveals the microscopic quantum dynamics that underlie the central spin decoherence and provides new opportunities for controlling and sensing interacting spin systems.

Despite acute interest in the dynamics of non-Hermitian systems, there is a lack of consensus in the mathematical formulation of non-Hermitian quantum mechanics in the community. Different methodologies are used in the literature to study non-Hermitian dynamics. This ranges from consistent frameworks like biorthogonal quantum mechanics and metric approach characterized by modified inner products, to normalization by time-dependent norms inspired by open quantum systems. In this work, we systematically explore the similarities and differences among these various methods. Utilizing illustrative models with exact solutions, we demonstrate that these methods produce not only quantitatively different results but also distinct physical interpretations. For dissipative systems where non-Hermiticity arises as an approximation, we find that simply dividing by the norm in the $\mathcal{PT}$-broken regime closely aligns with the full master equation solutions. In contrast, for quantum systems where non-Hermiticity can be engineered exactly, incorporating metric dynamics is crucial for the probabilistic interpretation of quantum mechanics, necessitating the generalizations of similarity transformations and unitarity to non-Hermitian systems. This study lays the groundwork for further exploration of non-Hermitian Hamiltonians, potentially leveraging generalized transformations for novel physical phenomena.

Gaining a deeper understanding of the interplay between charge density wave (CDW) order and superconductivity in transition metal dichalcogenides (TMDs), particularly within the (4H/2H)-${\mathrm{Nb}X}_2 (X=\mathrm{Se}$ and S) family, remains an open and intriguing challenge. A systematic microscopic study across various compounds in this family is therefore required to unravel this complex interplay. Here, we report on muon spin rotation (µSR) and magnetotransport experiments investigating the effects of hydrostatic pressure on the superconducting transition temperature ($T_{\mathrm{c}}$), the temperature-dependent magnetic penetration depth (${\lambda }_{\mathrm{eff}}$), and the charge density wave order (CDW) in two layered chalcogenide superconductors: 4H-${\mathrm{NbSe}}_2$, which exhibits CDW order, and 2H-${\mathrm{NbS}}_2$, which lacks such order. Our observations reveal a substantial $75\%$ enhancement of the superfluid density ($n_s/m^{*}$) in 4H-${\mathrm{NbSe}}_2$ upon the maximum applied pressure of $\sim 2$ GPa, surpassing that of 2H-${\mathrm{NbSe}}_2$. Despite the absence of CDW order, a sizable $20\%$ growth in superfluid density is also observed for 2H-${\mathrm{NbS}}_2$ under an applied pressure of 1.8 GPa. Notably, the evaluated superconducting gaps in all these TMDs remain largely unaffected by changes in applied pressure, irrespective of pressure-induced partial suppression of CDW order in (4H/2H)-${\mathrm{NbSe}}_2$ or its general absence in 2H-${\mathrm{NbS}}_2$. These results underscore the complex nature of pressure-induced behaviors in these TMDs, challenging a simplistic view of competition solely between CDW order and superconductivity. Remarkably, the relationship between $n_s/m^{*}$ and $T_{\mathrm{c}}$ exhibits an unconventional correlation, indicating a noteworthy similarity with the behavior observed in cuprate, kagome, and iron-based superconductors.

Motivated by recent studies of the projected dice lattice Hamiltonian [K. Swaminathan et al., Phys. Rev. Res. 5, 043215 (2023)], we introduce the on-site/bond singlet (OBS) model, a one-dimensional model of a flat band superconductor, in order to better understand the quasiparticle localization and interesting coexistence of ergodic and nonergodic behavior present in the former model. The OBS model is the sum of terms that have direct counterparts in the projected dice lattice Hamiltonian, each of which is parameterized by a coupling constant. Exact diagonalization reveals that the energy spectrum and nonequilibrium dynamics of the OBS model are essentially the same as that of the dice lattice for some values of the coupling constants. The quasiparticle localization and breaking of ergodicity manifest in a striking manner in the level spacing distribution. Its near Poissonian form provides evidence for the existence of local integrals of motion and establishes the OBS model as a nontrivial integrable generalization of the projected Creutz ladder Hamiltonian. These results show that level spacing statistics is a promising tool to study quasiparticle excitations in flat band superconductors.

The remarkable sensitivity of non-Hermitian systems has been extensively studied and stimulated ideas about developing new types of sensors. In this paper, we examine a chain of parametrically driven coupled resonators governed by the squeezed Su-Schrieffer-Heeger model. We emphasize the qualitative difference in sensor performance between configurations depending on bulk topology and boundary modes, specifically for detecting both on-site and non-Hermitian skin effect perturbations. Our analysis goes beyond the scenario of infinitesimal perturbations, extending to arbitrary perturbation strengths beyond the linear response regime. We stress the importance of optimizing the system's parameters to achieve quantum enhancement while avoiding fine-tuned regimes that could limit the practical applicability of this system for real-world quantum sensing.

Semiconductor double quantum dots (DQD) coupled to superconducting microwave resonators offer a promising platform for the detection of single microwave photons. In previous works, the photodetection was studied for a monochromatic source of microwave photons. Here, we theoretically analyze the photodetection of single microwave pulses. The photodetection in this case can be seen as a nonlinear filtering process of an incoming signal, the pulse, to an outgoing one, the photocurrent. This analogy to signal processing motivated the derivation of a Wigner-function formalism which provides a compelling visualization of the time and frequency properties of the photodetector for low intensities. We find a trade-off between detecting the time and the frequency of the incoming photons, in agreement with the time-energy uncertainty relation. As the intensity of the source increases, the photodetection is influenced by coherent Rabi oscillations of the DQD. Our findings give insight into the time-dependent properties of microwave photons interacting with electrons in a DQD-resonator hybrid system and provide guidance for experiments on single microwave pulse detection.

Layered Cu/Fe perovskites with a ${\mathrm{YBaCuFeO}}_5$ structure have attracted significant attention owing to the high thermal stability ($T_S$) of their incommensurate spiral magnetic phase. The intrinsic coupling between spin and ferroelectric orders in these cycloidally ordered multiferroics makes them among the most promising spin-driven magnetoelectric multiferroic candidates. In this system, strong in-plane antiferromagnetic couplings lead to collinearly ordered magnetic planes with spin directions alternating with periodicity ($k_x, k_y$) = (0.5, 0.5), while spin propagation along the perpendicular direction is governed by competing frustrated magnetic couplings of varying signs and strengths. In compounds of this family studied to date, the incommensurate spiral order emerges below $T_S$ due to correlated canting of ferromagnetically coupled spins in bipyramidal layers ($k_z=0.5\to 0.5\pm q_S$). Here, we demonstrate that a spiral magnetic ground state can also arise from a distinct precursor collinear spin configuration, where, above $T_S$, spins in the bipyramids are initially antiferromagnetically coupled ($k_z=0\to \pm q_S$). This newly observed spiral magnetic transition, identified and investigated in ${\mathrm{YBaCu}}_{1-x}{\mathrm{M}}_x{\mathrm{FeO}}_5$ compounds (M: Co, Zn), exists beyond the triple magnetic point, previously considered the critical limit for the ordering of the magnetic spiral in ${\mathrm{YBaCuFeO}}_5$. In the compounds investigated, the new spiral state persists in this region, stabilizing at temperatures up to 365 K.

Superconductivity in few-layer semiconducting transition-metal dichalcogenides (TMDs) can be induced by field-effect doping through ionic-liquid gating. While several experimental observations have been collected over the years, a fully consistent theoretical picture is still missing. Here we develop a realistic framework that combines the predictive power of first-principles simulations with the versatility and insight of Bardeen-Cooper-Schrieffer gap equations to rationalize such experiments. The multivalley nature of semiconducting TMDs is taken into account, together with the doping- and momentum-dependent electron-phonon and Coulomb interactions. Consistently with experiments, we find that superconductivity occurs when the electron density is large enough that the $Q$ valleys get occupied, as a result of a large enhancement of electron-phonon interactions. Despite being phonon driven, the superconducting state is predicted to be sensitive to Coulomb interactions, which can lead to the appearance of a relative sign difference between valleys and thus to a $s_{+-}$ character. We discuss qualitatively how such scenario may account for many of the observed physical phenomena for which no microscopic explanation has been found so far, including in particular the presence of a large subgap density of states, and the sample-dependent dome-shaped dependence of $T_c$ on accumulated electron density. Our results provide a comprehensive analysis of gate-induced superconductivity in semiconducting TMDs, and introduce an approach that will likely be valuable for other multivalley electronic systems, in which superconductivity occurs at relatively low electron density.