Vanadium dioxide is a strongly correlated material with an ultrafast first-order phase transition between monoclinic/insulator and rutile/metallic close to room temperature. The unusual and complex properties of this transition make $\mathrm{V}{\mathrm{O}}_2$ one of the most heavily investigated materials in modern condensed matter physics. Consequently, high-quality single crystals are in large demand. Here we report the growth of mm-sized $\mathrm{V}{\mathrm{O}}_2$ crystals by thermal decomposition of liquid ${\mathrm{V}}_2{\mathrm{O}}_5$ at $\sim {1000}^{\circ }\mathrm{C}$. Time-resolved zirconia sensor measurements of the oxygen release reveal that the crystal growth rate is limited by liquid-phase diffusion; the properties of the gaseous environment, which were previously assumed to be decisive, are almost insignificant. Consequently, large and stoichiometric single crystals of $\mathrm{V}{\mathrm{O}}_2$ can be obtained at lower temperatures and gas purities than usually applied. These results signify the role of gas-liquid diffusion in crystal growth and will simplify future research on $\mathrm{V}{\mathrm{O}}_2$ and related materials for applications in ultrafast electronics and thermal energy management.

Simulation of the interaction of light with matter, including at the few-photon level, is important for understanding the optical and optoelectronic properties of materials and for modeling next-generation nonlinear spectroscopies that use entangled light. At the few-photon level the quantum properties of the electromagnetic field must be accounted for with a quantized treatment of the field, and then such simulations quickly become intractable, especially if the matter subsystem must be modeled with a large number of degrees of freedom, as can be required to accurately capture many-body effects and quantum noise sources. Motivated by this we develop a quantum simulation framework for simulating such light-matter interactions on platforms with controllable bosonic degrees of freedom, such as vibrational modes in the trapped ion platform. The key innovation in our work is a scheme for simulating interactions with a continuum field using only a few discrete bosonic modes, which is enabled by a Green's function (response function) formalism. We develop the simulation approach, sketch how the simulation can be performed using trapped ions, and then illustrate the method with numerical examples. Our work expands the reach of quantum simulation to important light-matter interaction models and illustrates the advantages of extracting dynamical quantities such as response functions from quantum simulations.

Machine learning has recently emerged as a promising approach for studying complex phenomena characterized by rich datasets. In particular, data-centric approaches lead to the possibility of automatically discovering structures in experimental datasets that manual inspection may miss. Here, we introduce an interpretable unsupervised-supervised hybrid machine learning approach, the hybrid-correlation convolutional neural network (hybrid-CCNN), and apply it to experimental data generated using a programmable quantum simulator based on Rydberg atom arrays. Specifically, we apply hybrid-CCNN to discover and identify new quantum phases on square lattices with programmable interactions. The initial unsupervised dimensionality reduction and clustering stage first reveals five distinct quantum phase regions. In a second supervised stage, we refine these phase boundaries and seek insights into the phases by training multiple CCNN classifiers. A learned spatial weighting, introduced to the CCNNs in this work, enables discovery of spatial structure at scales beyond the filter size. The characteristic spatial weightings and snippets of correlations specifically recognized in each phase capture quantum fluctuations in the striated phase and identify a previously undetected boundary-ordered phase as well as motifs of more exotic ordered phases. These observations demonstrate that a combination of programmable quantum simulators with machine learning can be used as a powerful tool for detailed exploration of correlated quantum states of matter.

O-information is an information-theoretic metric that captures the overall balance between redundant and synergistic information shared by groups of three or more variables. To complement the global assessment provided by this metric, here we propose the gradients of the O-information as low-order descriptors that can characterize how high-order effects are localized across a system of interest. We illustrate the capabilities of the proposed framework by revealing the role of specific spins in Ising models with frustration, in Ising models with three-spin interactions, and in a linear vectorial autoregressive process. We also provide an example of practical data analysis on U.S. macroeconomic data. Our theoretical and empirical analyses demonstrate the potential of these gradients to highlight the contribution of variables in forming high-order informational circuits.

In this work, we investigated polarization-resolved photoinduced carrier relaxation dynamics associated with charge ordering (CO) in geometrically frustrated organic conductors $\theta \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_{2}M\mathrm{Zn}{\left(\mathrm{SCN}\right)}_4$ (M = Rb, Cs) as a function of temperature. In $\theta \text{−}\mathrm{Rb}$ (M = Rb), a polarization-dependent (anisotropic) and independent (isotropic) dynamics show similar temperature dependence. Moreover, discontinuous changes in transient reflectivity amplitude and an enhancement of a relaxation time are observed only upon heating near $T_{\mathrm{co}}=195\mathrm{K}$. This is associated with the first order phase transition due to long-range CO. However, in $\theta \text{−}\mathrm{Cs}$ (M = Cs) where the frustration is stronger than that in $\theta \text{−}\mathrm{Rb}$, as temperature decreases below $T_{\mathrm{nco}}\approx$ 150 K, the anisotropic component only slightly decreases while the isotropic one increases gradually without hysteresis. The behaviors indicate that CO is formed nonuniformly, that is, multiple short-range CO domains appear and are distributed with different orientations. Moreover, we found that the anisotropic dynamics with long-relaxation times suppress below $T_{\mathrm{s}}\approx$ 50 K, which is lower than the charge glass transition temperature of $T_{\mathrm{g}}\sim$ 100 K, whereas the isotropic dynamics grows monotonically. The distinct difference between them suggests the occurrence of a structural transition in which the lattice deforms randomly to match a glassy charge distribution due to the charge-lattice coupling.

The cohesion of a social group is the group's tendency to remain united. It has important implications for the stability and survival of social organizations, such as political parties, research teams, or online groups. Empirical studies suggest that cohesion is affected by both the admission process of new members and the group size. Yet, a theoretical understanding of their interplay is still lacking. To this end, we propose a model where a group grows by a noisy admission process of new members who can be of two different types. Cohesion is defined in this framework as the fraction of members of the same type and the noise in the admission process represents the level of randomness in the evaluation of new candidates. The model can reproduce the empirically reported decrease of cohesion with the group size. When the admission of new candidates involves the decision of only one group member, the group growth causes a loss of cohesion even for infinitesimal levels of noise. However, when admissions require a consensus of several group members, there is a critical noise level below which the growing group remains cohesive. The nature of the transition between the cohesive and noncohesive phases depends on the model parameters and forms a rich structure reminiscent of critical phenomena in ferromagnetic materials.

Despite the exciting implications of the Kitaev spin Hamiltonian, finding and confirming the quantum spin-liquid state have proven incredibly difficult. Recently, the applicability of the model has been expanded through the development of a microscopic description of a spin-1 Kitaev interaction. Here we explore a candidate spin-1 honeycomb system, ${\mathrm{KNiAsO}}_4$, which meets many of the proposed criteria to generate such an interaction. Bulk measurements reveal an antiferromagnetic transition at $\sim 19$ K which is generally robust to applied magnetic fields. Neutron diffraction measurements show magnetic order with a $\mathbf{k}=(\frac{3}{2},0,0)$ ordering vector which results in the well-known “zigzag” magnetic structure thought to be adjacent to the spin-liquid ground state. Field-dependent diffraction shows that while the structure is robust, the field can tune the direction of the ordered moment. Inelastic neutron scattering experiments show a well-defined gapped spin-wave spectrum with no evidence of the continuum expected for fractionalized excitations. Modeling of the spin waves shows that the extended Kitaev spin Hamiltonian is are generally necessary to model the spectra and reproduce the observed magnetic order. First-principles calculations suggest that the substitution of Pd on the Ni sublattice may strengthen the Kitaev interactions while simultaneously weakening the exchange interactions thus pushing ${\mathrm{KNiAsO}}_4$ closer to the spin-liquid ground state.

The differential photoelectron circular dichroism (PECD) of O $1s$ photoelectrons of $R$-trifluoromethyloxirane enantiomers as a function of the photoelectron emission direction in the molecular frame of reference and the direction from which circularly polarized light hits the molecule, is studied experimentally and theoretically for different photoelectron kinetic energies. A coincident detection of the photoelectrons and two ionic molecular fragments, performed with cold target recoil ion momentum spectroscopy, allows us to determine the orientation of the molecule in the laboratory frame and to obtain in addition the molecular-frame photoelectron diffraction patterns. From these we deduce the differential PECD. For given molecular orientations and photoelectron emission directions, we observe a normalized PECD strength clearly beyond 50%. These observations are in agreement with respective relaxed-core Hartree-Fock calculations, performed by employing the single center method. The present results support our recent observation of a huge differential PECD in O $1s$ photoemission of the methyloxirane molecule.

Thouless pumping, not only achieving quantized transport but also immune to moderate disorder, has attracted growing attention in both experiments and theories. Here, we explore how particle-particle interactions affect topological transport in a periodically modulated and tilted optical lattice. Three characterized interaction effects are revealed: topological pumping of bound states, interaction blockade of scattering states, and topologically resonant tunnelings. Attributed to the tilting, we have found a linear scanning in the first Brillouin zone at each Bloch-oscillation period of multiparticle center-of-mass momentum. Not limited to multiparticle Wannier states, our scheme ensures a dispersionless quantized transport even for initial Gaussian-like wave packets of interacting bosons which do not uniformly occupy a given band. Our study deepens the understanding of correlation effects on topological states, and provides a feasible way for detecting topological properties in interacting systems.

X-ray flares are generally believed to be produced by the reactivation of the central engine, and may have the same energy dissipation mechanism as the prompt emission of gamma-ray bursts (GRBs). X-ray flares can therefore provide important clues to understanding the nature of the central engines of GRBs. In this work, we study the physical connection between differential size and return distributions ofx-ray flares of GRBs with known redshifts. We find that the differential distributions of duration, energy, and waiting time can be well fitted by a power-law function. In particular, the distributions for the differences of durations, energies, and waiting times at different times (i.e., the return distributions) well follow a $q$-Gaussian form. The $q$ values in the $q$-Gaussian distributions remain nearly steady for different temporal interval scales, implying a scale-invariant structure of GRB x-ray flares. Moreover, we verify that the $q$ parameters are related to the power-law indices $\alpha$ of the differential size distributions, characterized as $q=(\alpha +2)/\alpha$. These statistical features can be well explained within the physical framework of a self-organizing criticality system.

We theoretically implement a strategy from quantum computation architectures to simulate Stuart-Landau oscillator dynamics in all-to-all connected networks, also referred to as complete graphs. The technique builds upon the triad structure minor embedding which expands dense graphs of interconnected elements into sparse ones which can potentially be realized in future on-chip solid-state technologies with tunable edge weights. As a case study, we reveal that the minor embedding procedure allows simulating the XY model on complete graphs, thus bypassing a severe geometric constraint.

We propose a housekeeping/excess decomposition of entropy production for general nonlinear dynamics in a discrete space, including chemical reaction networks and discrete stochastic systems. We exploit the geometric structure of thermodynamic forces to define the decomposition; this does not rely on the notion of a steady state, and it even applies to systems that exhibit multistability, limit cycles, and chaos. In the decomposition, distinct aspects of the dynamics contribute separately to entropy production: the housekeeping part stems from a cyclic mode that arises from external driving, generalizing Schnakenberg's cyclic decomposition to nonsteady states, while the excess part stems from an instantaneous relaxation mode that arises from conservative forces. Our decomposition refines previously known thermodynamic uncertainty relations and speed limits. In particular, it not only improves an optimal-transport-theoretic speed limit, but it also extends the optimal transport theory of discrete systems to nonlinear and nonconservative settings.

The theoretical and computational description of materials properties is a task of utmost scientific and technological importance. A first-principles description of electron-electron interactions poses an immense challenge that is usually approached by converting the many-electron problem to an effective one-electron problem. There are different ways to obtain an exact one-electron theory for a many-electron system. An emergent method is the exact electron factorization (EEF) – one of the branches of the exact factorization approach to many-body systems. In the EEF, the Schrödinger equation for one electron, in the environment of all other electrons, is formulated. The influence of the environment is reflected in the potential $v^{\mathrm{H}}$, which represents the energy of the environment, and in a potential $v^{\mathrm{G}}$, which has a geometrical meaning. In this paper, we focus on $v^{\mathrm{G}}$ and study its properties in detail. We investigate the geometric origin of $v^{\mathrm{G}}$ as a metric measuring the change of the environment, exemplify how translation and scaling of the state of the environment are reflected in $v^{\mathrm{G}}$, and explain its shape for homo- and heteronuclear diatomic model systems. Based on the close connection between the EEF and density functional theory, we also use $v^{\mathrm{G}}$ to provide an alternative interpretation to the Pauli potential in orbital-free density functional theory.

We study magnetism in the $\kappa$-(${\mathrm{ET})}_{2}X$ family of charge-transfer salts using implanted muon spectroscopy in conjunction with detailed ab initio electronic structure calculations using density functional theory (DFT). ET stands for the electron donor molecule bis(ethylendithio)tetrathiafulvalene and $X$ is an anion. The DFT calculations are used to establish molecular spin distributions, muon stopping sites, and dipolar field parameters, that allow us to make a quantitative interpretation of the experimental results. Materials in the $\kappa$-(${\mathrm{ET})}_{2}X$ family with $X$ = ${\mathrm{Ag}}_2{\left(\mathrm{CN}\right)}_3$ and $X$ = ${\mathrm{Cu}}_2{\left(\mathrm{CN}\right)}_3$ have attracted particular interest, as they have the attributes of quantum spin liquids, showing no magnetic ordering down to 30 mK in zero field $\mu \mathrm{SR}$ and in NMR, despite having exchange couplings of order 200–250 K. In contrast, the material with $X$ = $\mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]\mathrm{Cl}$ has an antiferromagnetic (AF) ordering transition with $T_{\mathrm{N}}$ in the region of 23–30 K. In order to better understand the muon spectroscopy signature of magnetism in this whole family of compounds at both low and high magnetic fields, we look in detail at the case $X$ = $\mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]\mathrm{Cl}$. As the first step in our study, the spin density distribution for the ET dimer is calculated using DFT and used to simulate the 3.7 T ${}^1\mathrm{H}$-NMR spectrum of this salt, with the spectrum showing good agreement with that measured previously [K. Miyagawa, A. Kawamoto, Y. Nakazawa, and K. Kanoda, Phys. Rev. Lett. 75, 1174 (1995)]. Best match to the data is found for antiferromagnetic interlayer ordering and an ordered moment per dimer of $0.25{\mu }_{\mathrm{B}}$. DFT is also used to explore muon stopping sites for this salt, finding one set of sites resulting from muonium addition to C=C double bonds in the ET layer, with muons stopping in the anion layer forming another group of sites. The dipolar fields associated with each of the stopping sites is computed and these are compared with the precession frequencies observed in the ZF-$\mu \mathrm{SR}$ spectrum [M. Ito, T. Uehara, H. Taniguchi, K. Satoh, Y. Ishii, and I. Watanabe, J. Phys. Soc. Jpn. 84, 053703 (2015)]. Best match to the ZF-$\mu \mathrm{SR}$ spectrum is obtained with the mode of interlayer ordering having FM character and an ordered moment per dimer of $0.31{\mu }_{\mathrm{B}}$ for muon sites in the anion layer and $0.36{\mu }_{\mathrm{B}}$ for muonium sites in the ET layer. New measurements of TF-$\mu \mathrm{SR}$ spectra for fields up to 8 T are reported and analyzed to obtain the best estimate of the magnetic order parameter under different measurement conditions, allowing us to observe the variation of $T_{\mathrm{N}}$ with applied field and the field-induced transverse canting of the moments.

We propose and experimentally demonstrate a method to strongly increase the sensitivity of spin measurements on nitrogen vacancy (NV) centers in diamond, which can be readily implemented in existing quantum sensing experiments. While charge state transitions of this defect are generally considered a parasitic effect to be avoided, we show here that these can be used to significantly increase the NV center's spin contrast, a key quantity for high-sensitivity magnetometry and high-fidelity state readout. The protocol consists of a two-step procedure, in which the charge state of the defect is first purified by a strong laser pulse, followed by weak illumination to obtain high spin polarization. We observe a relative improvement of the readout contrast by $17\%$ and infer a reduction of the initialization error of more than 50%. The contrast enhancement is accompanied by a beneficial increase of the readout signal. For long sequence durations, typically encountered in high-resolution magnetometry, a measurement speedup by a factor of $>1.5$ is extracted, and we find that the technique is beneficial for sequences of any duration. Additionally, our findings give detailed insight into the charge and spin polarization dynamics of the NV center and provide actionable insights for direct optical, spin-to-charge, and electrical readout of solid-state spin centers.