Molecular orbital systems with clusters of heavy transition metal (TM) ions are one of the most important classes of model materials for studying the interplay between local physics and effects of itinerancy. Despite a large number of candidates identified in the family of $4d\mathrm{TM}$ materials, an understanding of their physics from competing microscopic energy scales is still missing. We bridge this gap by reporting the first resonant inelastic x-ray scattering (RIXS) measurement on a well-known series of Ru cluster magnets with a $6H$-perovskite structure ${\text{Ba}}_{3}M{\text{Ru}}_2{\mathrm{O}}_9$ ($M^{3+}={\mathrm{In}}^{3+},{\mathrm{Y}}^{3+},{\mathrm{La}}^{3+}$) comprised of Ru dimers. In addition to providing a microscopic explanation for their anomalous magnetic properties, our RIXS measurements combined with theoretical modeling uncover a new regime of molecular orbital physics where the combined effect of large hopping and small spin-orbit coupling results in highly fragile electronic states in the Ru-dimer compounds directly manifested as an abrupt change in the RIXS spectrum accompanying a tiny change in the local structure tuned by the $M$-site ion. This unique combination of energy scales found only in the $4d$ but not the $5d$ cluster magnets highlights the value of these materials as a new platform for studying quantum phase transition involving molecular orbitals.

Local moments with a spin $S>1/2$ can exhibit a rich variety of elementary quasiparticle excitations, such as quadrupolar excitations, that go beyond the dipolar magnons of conventional spin-$1/2$ systems. However, the experimental observation of such quadrupolar excitations is often challenging due to the dipolar selection rules of many linear response probes, rendering them invisible. Here we show that nonlinear spectroscopy, in the form of two-dimensional coherent spectroscopy (2DCS), can be used to reveal quadrupolar excitations. Considering a family of spin-1 Heisenberg ferromagnets with single-ion easy-axis anisotropy as an example, we explicitly calculate their 2DCS signature by combining exact diagonalization and generalized spin wave theory. We further demonstrate that 2DCS can provide access to the quadrupolar weight of an excitation, analogous to how linear response provides access to the dipolar weight. Our work highlights the potential of nonlinear spectroscopy as a powerful tool to diagnose multipolar excitations in quantum magnets.

Despite decades of research, demonstration of all-optical detection and control of free electron spins in silicon remains elusive. Here, we directly probe the electron spin properties in bulk silicon by measuring the polarization of luminescence following circularly polarized light excitation. The all-optical experiments performed for both direct and indirect gap excitation allow not only an experimental determination of the optical selection rules in silicon for the different phonon-assisted transitions but they also lead to the measurement of the spin relaxation of electrons in conditions that are not accessible using transport techniques. We also measure the spin properties of free excitons in bulk silicon, a very little explored field.

Pure mechanical frequency combs attracted a lot of attention recently due to the intriguing nonlinear dynamics and potential of miniatured precision timekeeping but are currently limited by narrow spectra and missing frequency locking physics. In this Letter, we report the design and experiment of a self-injection locked micromechanical frequency comb that is further intrinsically immune to phase offset. We show that by tuning the pump frequency of a pair of nonlinearly coupled flexural and torsional oscillations, self-injection locking could be achieved via aligning the comb teeth from neighboring harmonic clusters, leading to decade-wide cascading of hundreds of equidistant teeth. Inside the injection locking region, the stability of the comb spacing and the phase noise are significantly improved. Moreover, the phase offset in the temporal signal also disappears because all comb lines are locked as integer multiples of the comb spacing and the first tooth is locked to the d.c. frequency due to the comb origination from harmonics. The observation of self-injection locking and zero phase offset in mechanical frequency combs greatly promotes their value for precision applications.

Readout of superconducting qubits faces a trade-off between measurement speed and unwanted backaction on the qubit caused by the readout drive, such as $T_1$ degradation and leakage out of the computational subspace. The readout is typically benchmarked by integrating the readout signal and choosing a binary threshold to extract the “readout fidelity.” We show that readout fidelity may significantly overlook readout-induced leakage errors. Such errors are detrimental for applications that rely on continuously repeated measurements, e.g., quantum error correction. We introduce a method to measure the readout-induced leakage rate by repeatedly executing a composite operation—a readout preceded by a randomized qubit flip. We apply this technique to characterize the readout of a superconducting qubit, optimized for fidelity across four different readout durations. Our technique highlights the importance of an independent leakage characterization by showing that the leakage rates vary from 0.12% to 7.76% across these readouts even though the fidelity exceeds 99.5% in all four cases.

We propose a methodology to measure the cosmological spatial curvature by employing the deviation from statistical isotropy due to the Alcock-Paczyński effect of large scale galaxy clustering. This approach has a higher degree of model independence than most other proposed methods, being independent of the calibration of standard candles, rulers, or clocks, of the power spectrum shape (and thus also of the prerecombination physics), of the galaxy bias, of the theory of gravity, of the dark energy model, and of the background cosmology in general. We find that a combined Dark Energy Spectroscopic Instrument-Euclid galaxy survey can achieve $\mathrm{\Delta }{\mathrm{\Omega }}_{k0}=0.057$ at $1\sigma \mathrm{C}.\mathrm{L}.$ in the redshift range $z<2$ by combining power-spectrum and bispectrum measurements.

We investigate the production of dark matter from metric perturbations produced during inflation or in standard cosmology. Perturbations break Weyl flatness of the Friedmann-Lemaître-Robertson-Walker metric, allowing conformally coupled fields—in particular, fermions studied here—to be produced even when their mass is negligible. Particle production can be computed by studying the Bogoliubov transformation induced by the stochastic background. For perturbations generated during inflation, we present a closed formula for the resulting abundance of particles that depends solely on the power spectrum of curvature perturbations at the end of inflation. This production mechanism can be dominant especially if the amplitude of curvature perturbations is enhanced for modes that exit the horizon toward the end of inflation. In the simplest scenario, the critical dark matter abundance is reproduced for $M\gtrsim {10}^6\mathrm{GeV}$.

Distinct selectivity to the spin angular momenta of photons has garnered significant attention in recent years, for its relevance in basic science and for imaging and sensing applications. While nonlocal metasurfaces with strong chiral responses to the incident light have been reported, these responses are typically limited to a narrow range of incident angles. In this study, we demonstrate a nonlocal metasurface that showcases strong chirality, characterized by circular dichroism ($\sim 0.6$), over a wide range of incident angles $\pm 5°$. Its quality factor, circular dichroism and resonant frequency can be optimized by design. These findings pave the way to further advance the development of valley-selective optical cavities and augmented reality applications.

The quantum metric is known to influence several observables, such as the dielectric constant, superfluid stiffness, and optical mass. However, a direct measurement of the metric has remained elusive. In linear response functions such as electric conductivity, only the matrix elements of the metric appear convoluted with energy prefactors, but to this date no direct observable has been identified that is directly proportional to the integrated metric over the Brillouin zone. In this Letter, we propose relaxation from constrained equilibrium to directly measure the symmetric part of the quantum geometric tensor. This response implements the integral over frequencies appearing in the Souza-Wilkens-Martin sum rule by choosing the electric field appropriately. Our work shines light on other geometric properties of insulators that are absent in the frequency expansions of conductivity in insulators but can, in principle, be revealed in step response.

The destruction of the Kondo effect in a local-moment metal can lead to a topological non-Fermi-liquid phase, dubbed fractionalized Fermi liquid, with spinon-type excitations and an emergent gauge field. We demonstrate that, if the latter displays an internal $\pi$-flux structure, a chiral heavy-fermion metal emerges near the Kondo-breakdown transition. Utilizing a parton mean-field theory describing the transition between a conventional heavy Fermi liquid and a U(1) fractionalized Fermi liquid, we find a novel intermediate phase near the transition whose emergent flux pattern spontaneously breaks both translation and time-reversal symmetries. This phase is an orbital antiferromagnet, and we derive a Landau-type theory which shows that such a phase generically emerges from a $\pi$-flux spin liquid. We discuss the relevance to pertinent experiments.

In driven nonlinear systems, phase locking is an intriguing effect leading to robust stationary states that are stable over extended ranges of control parameters. Recent experiments allow for exploring microscopic mechanisms underlying such phenomena in collective dynamics of micro- and nanoparticles. Here, we show that phase-locked dynamics of hardcore-interacting microparticles in a densely populated periodic potential under time-periodic driving arises from running solitary cluster waves. We explain how values of phase-locked currents are related to soliton velocities and why collective particle dynamics synchronize with the driving for certain particle diameters only. Our analysis is based on an effective potential for the solitary wave propagation and a unit displacement law, which states that the total average shift of all particle positions per soliton period equals one wavelength of the periodic potential.

We examine the navigation behavior of the photosensitive alga Euglena gracilis in confined environments. Under uniform lighting conditions, E. gracilis exhibits stochastic movements with nearly straight runs interrupted by abrupt directional changes. The lengths of these runs follow a long-tailed distribution typical of a Lévy walk, with scaling exponents that vary with light intensity. In gradient lighting conditions, the cells modulate their run durations—extending them upon detecting an increase in light intensity and shortening them when a decrease is detected. This adjustment effectively biases the Lévy walk, enabling the cells to ascend the spatial light gradient. This behavior mirrors well-known prokaryotic stochastic navigation strategies, such as bacterial chemotaxis, offering a eukaryotic parallel. The experimental observations under varied lighting conditions are consistently replicated through an agent-based model.

We introduce a new method for measuring the Hubble parameter from low-redshift large-scale observations that is independent of the comoving sound horizon. The method uses the baryon-to-photon ratio determined by the primordial deuterium abundance, together with big bang nucleosynthesis calculations and the present-day cosmic microwave background (CMB) temperature, to determine the physical baryon density ${\mathrm{\Omega }}_b{h}^2$. The baryon fraction ${\mathrm{\Omega }}_b/{\mathrm{\Omega }}_m$ is measured using the relative amplitude of the baryonic signature in galaxy clustering measured by the Baryon Oscillation Spectroscopic Survey, scaling the physical baryon density to the physical matter density. The physical density ${\mathrm{\Omega }}_m{h}^2$ is then compared with the geometrical density ${\mathrm{\Omega }}_m$ from Alcock-Paczynski measurements from baryon acoustic oscillations (BAO) and voids to give $H_0$. Including type Ia supernovae and uncalibrated BAO, we measure $H_0={67.1}_{-5.3}^{+6.3}\mathrm{km}{\mathrm{s}}^{-1}{\mathrm{Mpc}}^{-1}$. We find similar results when varying analysis choices, such as measuring the baryon signature from the reconstructed correlation function or excluding supernovae or voids. This measurement is currently consistent with both the distance-ladder and CMB $H_0$ determinations, but near-future large-scale structure surveys will obtain $3\times$ to $4\times$ tighter constraints.

Understanding how new physics influences the dynamics of cosmic neutrinos during their decoupling is crucial in light of upcoming precise cosmological observations and the need to reconcile cosmological and laboratory probes. Existing approaches to solving the neutrino Boltzmann equation are often model dependent, computationally inefficient, and yield contradictory results. To solve this problem, we introduce a novel method to comprehensively study neutrino dynamics. We apply this method to several case studies, resolving the discrepancy in the literature about the impact of nonthermal neutrinos on $N_{\mathrm{eff}}$ and providing important insights about the role of decaying new physics particles on MeV plasma.