Real-time estimation protocols for the frequency shift of optically detected magnetic resonance (ODMR) of nitrogen-vacancy (NV) centers in nanodiamonds (NDs) are the key to the recent demonstrations of diamond quantum thermometry inside living animals. Here we analyze the estimation process in multipoint ODMR measurement techniques (3-, 4-, and 6-point methods) and quantify the amount of measurement artifact derived from the optical power-dependent ODMR spectral shape and instrumental errors of experimental hardware. We propose a practical approach to minimize the effect of these factors, which allows for measuring accurate temperatures of single ND during dynamic thermal events. Further, we discuss integration of noise filters, data estimation protocols, and possible artifacts for further developments in real-time temperature estimation. This study provides technical details regarding quantum diamond thermometry and analyzes the factors that may affect the temperature estimation in biological applications.

Majorana fermions are often proposed to be realized by singling out one Fermi surface without spin degeneracy via spin-orbit coupling, and then imposing boundaries or defects. In this work, we take a different route starting with two degenerate Fermi surfaces without spin-orbit coupling, and show that by the method of “kink on boundary,” the dispersive chiral Majorana fermions can be realized in superconducting systems with $p\pm is$ pairings. The surfaces of these systems develop spontaneous magnetizations whose directions are determined by the boundary orientations and the phase difference between the $p$- and $s$-component gap functions. Along the magnetic domain walls on the surface, there exist chiral Majorana fermions propagating unidirectionally, which can be conveniently dragged and controlled by external magnetic fields. Furthermore, the surface magnetization is shown to be a magnetoelectric effect based on a Ginzburg-Landau free-energy analysis. We also discuss how to use the proximity effects to realize chiral Majorana fermions by performing the kink on boundary method.

We report giant bulk rectification in the bulk trigonal superconductor $\mathrm{PbTa}{\mathrm{Se}}_2$. Nonreciprocal magnetotransport originating from a vortex ratchet motion was successfully observed, showing a significant enhancement during the superconducting transition, which is considered a model behavior for three-dimensional noncentrosymmetric superconductors with strong vortex pinning. Moreover, an unprecedented sign change of nonreciprocal transport was observed with increasing current, which indicates that the rectification effect can be tuned by the current value as well as the magnetic field. The present results represent the rich vortex dynamics and offer a principle for current control of rectification in noncentrosymmetric bulk superconductors.

The geometric properties of a lattice can have profound consequences on its band spectrum. For example, symmetry constraints and geometric frustration can give rise to topologicially nontrivial and dispersionless bands, respectively. Line-graph lattices are a perfect example of both of these features: Their lowest energy bands are perfectly flat, and here we develop a formalism to connect some of their geometric properties with the presence or absence of fragile topology in their flat bands. This theoretical work will enable experimental studies of fragile topology in several types of line-graph lattices, most naturally suited to superconducting circuits.

An analytical formula is derived for particle and energy densities of fermions and bosons, and their ballistic momentum and energy currents for anisotropic energy dispersions in generalized dimensions. The formulation considerably simplifies the comparison of the statistical properties and ballistic particle and energy transport currents of electrons, acoustic phonons, and photons in various dimensions in a unified manner. Assorted examples of its utility are discussed, ranging from blackbody radiation to Schottky diodes and ballistic transistors, quantized electrical and thermal conductance, generalized ballistic Seebeck and Peltier coefficients, their Onsager relations, the generalized Wiedemann-Franz law and the robustness of the Lorenz number, and ballistic thermoelectric power factors, all of which are obtained from the single formula. The new formulation predicts a thermoelectric power factor behavior of three-dimensional Dirac bands which has not been observed yet.

We numerically study coherent errors in surface codes on planar graphs, focusing on noise of the form of $Z$ or $X$ rotations of individual qubits. We find that, similar to the case of incoherent bit and phase flips, a trade-off between resilience against coherent $X$ and $Z$ rotations can be made via the connectivity of the graph. However, our results indicate that, unlike in the incoherent case, the error-correction thresholds for the various graphs do not approach a universal bound. We also study the distribution of final states after error correction. We show that graphs fall into three distinct classes, each resulting in qualitatively distinct final-state distributions. In particular, we show that a graph class exists where the logical-level noise exhibits a decoherence threshold slightly above the error-correction threshold. In these classes, therefore, the logical level noise above the error-correction threshold can retain a significant amount of coherence even for large-distance codes. To perform our analysis, we develop a Majorana-fermion representation of planar-graph surface codes and describe the characterization of logical-state storage using fermion-linear-optics-based simulations. We thereby generalize the approach introduced for the square lattice by Bravyi et al. [npj Quantum Inf. 4, 55 (2018)] to surface codes on general planar graphs.

We study a periodically driven qubit coupled to a quantized cavity mode. Despite its apparent simplicity, this system supports a rich variety of exotic phenomena, such as topological frequency conversion as recently discovered in Martin et al. [Phys. Rev. X 7, 041008 (2017)]. Here we report on a qualitatively different phenomenon that occurs in this platform, where the cavity mode's oscillations lock their frequency to a rational fraction $r/q$ of the driving frequency $\mathrm{\Omega }$. This phenomenon, which we term quantum frequency locking, is characterized by the emergence of $q$-tuplets of stationary (Floquet) states whose quasienergies are separated by $\mathrm{\Omega }/q$, up to exponentially small corrections. The Wigner functions of these states are nearly identical, and exhibit highly regular and symmetric structure in phase space. Similarly to Floquet time crystals, these states underlie discrete time-translation symmetry breaking in the model. We develop a semiclassical approach for analyzing and predicting quantum frequency locking in the model, and use it to identify the conditions under which it occurs.

For massive database-driven materials research, there are increasing demands for both fast and accurate quantum mechanical computational tools. Contemporary density functional theory (DFT) methods can be fast, sacrificing their accuracy, or be precise, consuming a significant amount of resources. Here, to overcome such a problem, we present a DFT method that exploits self-consistent determinations of the on-site and intersite Hubbard interactions ($U$ and $V$) simultaneously and obtain band gaps of diverse materials in the accuracy of the $GW$ method at a standard DFT computational cost. To achieve the self-consistent evaluation of $U$ and $V$, we adapt a recently proposed Agapito–Curtarolo–Buongiorno Nardelli pseudohybrid functional for $U$ to implement a density functional of $V$. This method is found to be appropriate for considering various interactions such as local Coulomb repulsion, covalent hybridization, and their coexistence. We also obtain good agreements between computed and measured band gaps of low-dimensional systems, thus meriting this approach for large-scale as well as high-throughput calculations for various bulk and nanoscale materials with a higher accuracy.

We consider complex three-dimensional polarizations in the interference of several vector wave fields with different commensurable frequencies and polarizations. We show that the resulting polarizations can form knots, and interfering three waves is sufficient to generate a variety of Lissajous, torus, and other knot types. We describe the spin angular momentum, generalized Stokes parameters, and degree of polarization for such knotted polarizations, which can be regarded as partially polarized. Our results are generic for any vector wave fields, including, e.g., optical and acoustic waves. As a directly observable example, we consider knotted trajectories of water particles in the interference of surface water (gravity) waves with three different frequencies.

We study scattering phenomena such as the Kerker effect, superscattering, and scattering dark states in a subwavelength atomic antenna consisting of atoms with only electric dipole transitions. We show that an atomic antenna can exhibit arbitrarily large or small scattering cross sections depending on the geometry of the structure and the direction of the impinging light. We also demonstrate that atoms with only an electric dipole transition can exhibit a directional radiation pattern with zero backscattering when placed in a certain configuration. This is a special case of a phenomenon known as the Kerker effect, which typically occurs in the presence of both electric and magnetic transitions. Our findings open a pathway to design highly directional emitters, nonradiating sources, and highly scattering objects based on individually controlled atoms.

We demonstrate how the properties of light-induced electronic Floquet states in solids impact natural physical observables, such as transport properties, by capturing the environmental influence on the electrons. We include the environment as dissipative processes, such as interband decay and dephasing, often ignored in Floquet predictions. These dissipative processes determine the Floquet band occupations of the emergent steady state, by balancing out the optical driving force. In order to benchmark and illustrate our framework for Floquet physics in a realistic solid, we consider the light-induced Hall conductivity in graphene recently reported by McIver et al. [Nat. Phys. 16, 38 (2020)]. We show that the Hall conductivity is estimated by the Berry flux of the occupied states of the light-induced Floquet bands, in addition to the kinetic contribution given by the average band velocity. Hence, Floquet theory provides an interpretation of this Hall conductivity as a geometric-dissipative effect. We demonstrate this mechanism within a master equation formalism, and obtain good quantitative agreement with the experimentally measured Hall conductivity, underscoring the validity of this approach which establishes a broadly applicable framework for the understanding of ultrafast nonequilibrium dynamics in solids.

We propose an energy-scale correspondence between the Mott physics and the Kondo lattice physics and construct a tentative phase diagram of their correlated electrons with two characteristic energy scales ${\omega }^{*}$ and $\mathrm{\Omega }$ marking the upper boundary of a low-energy regime with well-developed long-range coherence and the lower boundary of localized spectral weight in the energy space, respectively. In between, there exists a crossover region with emergent but damped quasiparticle excitations. We argue that the presence of two separate energy scales is a generic property of correlated electrons on a lattice and reflects an intrinsic two-stage process of the quasiparticle dynamics to build up the lattice coherence. For the Hubbard model, they correspond to the kink and waterfall structures on the dispersion, while for the periodic Anderson model, they are associated with the indirect and direct hybridization gaps. Our work reveals a deep connection between the Mott and Kondo lattice physics and provides a basic ingredient for the study of many-body systems.

Single-molecule magnets (SMMs) possessing bistable states have been considered as promising candidates to realize zero-dimensional (0D) ferroelectrics (FE) and multiferroics (MF) with high storage density. However, how to read or manipulate the FE states with low-energy-dissipation strategy is still a hard challenge. Here, we intercalated a magnetic metal porphyrin molecule, such as TiP with switchable vertical electric polarization, within the ${\mathrm{MoS}}_2$ bilayer to realize 0D FE states. First-principles calculations show that the ${\mathrm{MoS}}_2$ monolayer contacted by SMMs is spin polarized to provide two spin-dependent transport tunnels. As a temperature gradient is applied along the transport channels, a well-defined spin-Seebeck effect (SSE) occurs in the spin-polarized ${\mathrm{MoS}}_2$ layer, helping to generate a pure thermal spin current. More importantly, the spin-Seebeck signal is associated tightly with the FE state in the same layer, and both of them can be switched simultaneously to another layer by an external electric field. The theoretical results not only put forwards a low-energy-dissipation strategy to read the SMM-based FE states, but also develop a new research field of spin-ferroelecto-caloritronics, which focuses on the interplay of electrons' spin and FE states in the presence of a temperature gradient.

We present a comprehensive investigation of lattice dynamics in the double-helix antiferromagnet FeP by means of high-resolution time-of-flight neutron spectroscopy and ab initio calculations. Phonons can hybridize with the magnetic excitations in noncollinear magnets to significantly influence their properties. We observed a rich spectrum of phonon excitations, which extends up to $\sim 50$ meV. We performed detailed analysis of the observed and calculated spectra for all high-symmetry points and high-symmetry directions of the Brillouin zone. We show that the DFT calculations quantitatively capture the essential features of the observed phonons, including both dispersions and scattering intensities. By making use of the detailed intensity comparison between the theory and the data, we were able to identify displacement vectors for the majority of the observed modes. The overall excellent agreement between the DFT predictions and the experimental results breaks down for the lowest mode at the $Y$ point, whose energy is lower than calculated by $\sim 13\%$. The present study provides vital information on the lattice dynamics in FeP and demonstrates applicability of the DFT to novel pressure-induced phenomena in related materials, such as MnP and CrAs.

The study of entanglement spectra is a powerful tool to detect or elucidate universal behavior in quantum many-body systems. We investigate the scaling of the entanglement (or Schmidt) gap $\delta \xi$, i.e., the lowest-laying gap of the entanglement spectrum, at a two-dimensional quantum critical point. We focus on the paradigmatic quantum spherical model, which exhibits a second-order transition and is mappable to free bosons with an additional external constraint. We analytically show that the Schmidt gap vanishes at the critical point, although only logarithmically. For a system on a torus and the half-system bipartition, the entanglement gap vanishes as ${\pi }^2/ln\left(L\right)$, with $L$ the linear system size. The entanglement gap is nonzero in the paramagnetic phase and exhibits a faster decay in the ordered phase. The rescaled gap $\delta \xi ln\left(L\right)$ exhibits a crossing for different system sizes at the transition, although logarithmic corrections prevent a precise verification of the finite-size scaling. Interestingly, the change of the entanglement gap across the phase diagram is reflected in the zero-mode eigenvector of the spin-spin correlator. At the transition quantum fluctuations give rise to a nontrivial structure of the eigenvector, whereas in the ordered phase it is flat. We also show that the vanishing of the entanglement gap at criticality can be qualitatively but not quantitatively captured by neglecting the structure of the zero-mode eigenvector.

We compute the full probability distribution of the spectral form factor in the self-dual kicked Ising model by providing an exact lower bound for each moment and verifying numerically that the latter is saturated. We show that at long enough times the probability distribution agrees exactly with the prediction of random-matrix theory if one identifies the appropriate ensemble of random matrices. We find that this ensemble is not the circular orthogonal one—composed of symmetric random unitary matrices and associated with time-reversal-invariant evolution operators—but is an ensemble of random matrices on a more restricted symmetric space [depending on the parity of the number of sites this space is either $Sp\left(N\right)/U\left(N\right)$ or $O\left(2N\right)/O\left(N\right)\times O\left(N\right)$]. Even if the latter ensembles yield the same averaged spectral form factor as the circular orthogonal ensemble, they show substantially enhanced fluctuations. This behavior is due to a recently identified additional antiunitary symmetry of the self-dual kicked Ising model.

Systematic discovery of reduced-order closure models for multiscale processes remains an important open problem in complex dynamical systems. Even when an effective lower-dimensional representation exists, reduced models are difficult to obtain using solely analytical methods. Rigorous methodologies for finding such coarse-grained representations of multiscale phenomena would enable accelerated computational simulations and provide fundamental insights into the complex dynamics of interest. We focus on a heterogeneous population of oscillators of Kuramoto type as a canonical model of complex dynamics and develop a data-driven approach for inferring its coarse-grained description. Our method is based on a numerical optimization of the coefficients in a general equation of motion informed by analytical derivations in the thermodynamic limit. We show that certain assumptions are required to obtain an autonomous coarse-grained equation of motion. However, optimizing coefficient values enables coarse-grained models with conceptually disparate functional forms, yet comparable quality of representation, to provide accurate reduced-order descriptions of the underlying system.

Heavy metal/ferromagnet interfaces feature emergent spin-orbit effects absent in the bulk materials. Because of their inherent strong coupling between spin, charge, and orbital degrees of freedom, such systems provide a platform for technologically sought-after spin-orbit torques (SOTs). However, the microscopic origin of purely interfacial antidamping SOT, especially in the ultimate atomically thin limit, has proven elusive. Here, using two-dimensional (2D) van der Waals materials as a test bed for interfacial phenomena, we address this problem by means of a microscopic framework accounting for band structure effects and impurity scattering on equal footing and nonperturbatively. A number of unconventional and measurable effects are predicted, the most remarkable of which is a giant enhancement of antidamping SOT in the dilute disorder limit induced by a robust skew scattering mechanism, which is operative in realistic interfaces and does not require magnetic impurities. The newly unveiled skew scattering mechanism activates rich semiclassical spin-charge conversion effects that have gone unnoticed in the literature, including a collinear Edelstein effect with nonequilibrium spin polarization aligned with the direction of the applied current.

Atomistic van der Waals heterostacks are ideal systems for high-temperature exciton condensation because of large exciton binding energies and long lifetimes. Charge transport and electron energy-loss spectroscopy showed first evidence of excitonic many-body states in such two-dimensional materials. Pure optical studies, the most obvious way to access the phase diagram of photogenerated excitons, have been elusive. We observe several criticalities in photogenerated exciton ensembles hosted in $\mathrm{Mo}{\mathrm{Se}}_2\text{−}\mathrm{W}{\mathrm{Se}}_2$ heterostacks with respect to photoluminescence intensity, linewidth, and temporal coherence pointing towards the transition to a coherent many-body quantum state, consistent with the predicted critical degeneracy temperature. For this state, the estimated occupation is approximately 100% and the phenomena survive above 10 K.

When applying a voltage bias across a thin nanopore, localized Joule heating can lead to single-bubble nucleation, offering a unique platform for studying nanoscale bubble behavior, which is still poorly understood. Accordingly, we investigate bubble nucleation and collapse inside solid-state nanopores filled with electrolyte solutions and find that there exists a clear correlation between homo/heterogeneous bubble nucleation and the pore diameter. As the pore diameter is increased from 280 to 525 nm, the nucleation regime transitions from predominantly periodic homogeneous nucleation to a nonperiodic mixture of homogeneous and heterogeneous nucleation. A transition barrier between the homogeneous and heterogeneous nucleation regimes is defined by considering the relative free-energy costs of cluster formation. A thermodynamic model considering the transition barrier and contact-line pinning on curved surfaces is constructed, which determines the possibility of heterogeneous nucleation. It is shown that the experimental bubble generation behavior is closely captured by our thermodynamic analysis, providing important information for controlling the periodic homogeneous nucleation of bubbles in nanopores.