Physical review letters最新文献

We show that the Luttinger-Ward functional can be formulated as an operator insertion in the path integral and hence can be thought of as a generalized symmetry. The key result is that the associated charge, always quantized, defines the homotopy, not the physical charge. The disconnect between the two arises from divergences in the functional or, equivalently, zeros of the single-particle Green's function. Such divergences produce an anomaly of the triangle-diagram type. As a result of this anomaly, we are able to account for the various deviations of the Luttinger count from the particle density. As a consequence, non-Fermi liquids can be classified generally by the well-known anomaly structures in particle physics. Charges descending from generalized symmetries, as in the divergence of the Luttinger-Ward functional, are inherently nonlocal, their key experimental signature.

Higher-order interactions provide a nuanced understanding of the relational structure of complex systems beyond traditional pairwise interactions. However, higher-order network analyses also incur more cumbersome interpretations and greater computational demands than their pairwise counterparts. Here, we present an information-theoretic framework for determining the extent to which a hypergraph representation of a networked system is structurally redundant and for identifying its most critical higher orders of interaction that allow us to remove these redundancies while preserving essential higher-order structure.

Quantum reservoir computing (QRC) offers a promising paradigm for harnessing quantum systems for machine learning tasks, especially in the era of noisy intermediate-scale quantum devices. While information-theoretical benchmarks like short-term memory capacity (STMC) are widely used to evaluate QRC performance, they fail to provide insights into the physical mechanisms underlying these quantum neural networks. We establish a quantitative connection between the optical absorption spectrum of a quantum reservoir and its memory performance, revealing that optimal STMC aligns directly with maximal absorption, providing a physical explanation for the previously reported "sweet-spot" behavior in QRC performance as a function of dissipation. This connection bridges quantum information theory with experimentally accessible physical properties, opening pathways for targeted engineering of quantum reservoir computers with optimized performance for specific tasks.

We report an unconventional twofold-symmetric magnetoelastic coupling in Ni films, mediated by Rayleigh surface acoustic waves (SAWs). This unique magnetoelastic symmetry originates from a dominant vertical shear strain ϵ_{yz}, which becomes prominent due to the low effective elastic modulus of Ni film. As the film thickness increases, ϵ_{yz} surpasses the conventional Rayleigh SAW strain ϵ_{xx}, a consequence originated from the elastic modulus mismatch at the film-substrate interface. This finding highlights the dominance of ϵ_{yz} in soft thin film under SAW excitation, offering a new platform to excite strong nonreciprocal and topological magnon-phonon hybridization.

The presence of dark matter (DM) stands as one of the most compelling indications of new physics in particle physics. Typically, the detection of wavelike DM involves quantum sensors, such as qubits or cavities. The phase of the sensors is usually discarded as the value of the phase itself is not physically meaningful. However, the difference of the phase between the sensors contains the information of the velocity and direction of the DM wind. We propose a measurement protocol to extract this information from the sensors using quantum states. Our method does not require specific experimental setups and can be applied to any type of DM detector as long as the data from the detectors can be taken quantum mechanically. We also show that our method does not spoil the sensitivity of the DM detectors and is superior to the classical method based on the correlations of the DM signals between the detectors.

The fractional quantum Hall state (FQHS) observed in the lowest Landau level at filling factor ν=1/2 in wide quantum wells has been enigmatic for decades because the two-dimensional electron system (2DES) has a bilayer charge distribution but with significant interlayer tunneling. Of particular interest is whether the 1/2 FQHS in this system has a one-component (1C) or two-component (2C) origin; these are typically identified as the Pfaffian (non-Abelian) or the Ψ_{331} (Abelian) FQHSs, respectively. We report here our experimental study of the evolution of the correlated states of an ultrahigh-quality 2DES confined to a 72.5-nm-wide GaAs quantum well. At the lowest densities, the 2DES displays only odd-denominator FQHSs, and the ground state at ν=1/2 is a composite fermion Fermi sea. As the density is increased, an FQHS emerges at ν=1/2, and becomes very strong. In a finite density range where the 1/2 FQHS is strongest, we also observe its daughter FQHSs at ν=8/17 and 7/13, consistent with the theoretically expected daughter states of a Pfaffian 1/2 FQHS. At the highest densities, the 2DES becomes 2C, signaled by the emergence of a bilayer Wigner crystal state and the transitions of FQHSs flanking ν=1/2. The 1/2 FQHS remains robust near this transition and, notably, its charge transport energy gap exhibits an upward cusp with a maximum value of about 6 K on the 1C side of the transition; this is the largest gap reported for any even-denominator FQHS. Our observation of the transition of the 2DES ground states near ν=1/2 to 2C states at high densities, and our measurements of the robustness of the 1/2 FQHS against charge distribution asymmetry, suggest that the 1/2 FQHS also makes a transition from 1C to 2C. Such a transition from a non-Abelian to Abelian state can open avenues for topological quantum information and quantum criticality.

Gravity can give rise to (pseudo)scalar fields-for instance due to torsion. In particular, axions of gravitational origin have been proposed as a minimal and compelling solution to the strong CP problem. In this Letter, we critically examine the feasibility of this proposal. We demonstrate that models in which the scalar field couples to fermionic currents only through derivatives do not yield a satisfactory axion. Moreover, we identify the necessary conditions for generating a gravitational axion through quantum effects, highlighting Weyl-invariant Einstein-Cartan gravity as a promising theoretical setting.

Magneto-optical trapping of molecules has thus far been restricted to molecules with ^{2}Σ electronic ground states. These species are chemically reactive and only support a simple laser cooling scheme from their first excited rotational level. Here, we demonstrate a magneto-optical trap (MOT) of aluminum monofluoride (AlF), a deeply bound and intrinsically stable diatomic molecule with a ^{1}Σ^{+} electronic ground state. The MOT operates on the strong A^{1}Π←X^{1}Σ^{+} transition near 227.5 nm, whose Q(J) lines are all rotationally closed. We demonstrate a MOT of about 6×10^{4} molecules for the J=1 level of AlF, more than 10^{4} molecules for J=2 and 3, and with no fundamental limit in going to higher rotational levels. Laser cooling and trapping of AlF is conceptually similar to the introduction of alkaline-earth atoms into cold atom physics, and is key to leveraging its spin-forbidden a^{3}Π←X^{1}Σ^{+} transition for precision spectroscopy and narrow-line cooling.

While field-driven electron emission is theoretically understood down to the subcycle regime, its direct experimental temporal characterization using long-wavelength terahertz (THz) fields remains elusive. Here, by driving a graphite tip with phase-stable quasi-single-cycle THz pulses, we reveal distinct subcycle electron emission dynamics including: (i) At a carrier-envelope phase (CEP) zero, spectral peaks scale linearly with THz field strength, characteristic of subcycle emission; (ii) at nearly opposite CEP, dominant deceleration fields generate stationary low-energy peaks. Crucially, we develop a pump-probe-free, direct reconstruction method extracting electron pulse profiles solely from measured energy spectra, obtaining durations from 73.0 to 81.0 fs as the field increases (191-290 kV/cm). Phase-resolved simulations further reveal a 72.8% modulation in the cutoff energy and a near-total (99.7%) suppression of the emission current. This Letter not only validates the field-emisssion theory under THz excitation but also establishes a general framework for the direct temporal characterization of subcycle electron emission, opening pathways for precise electron control in ultrafast electron sources and lightwave nanoelectronics.

We study the gyrotropic magnetic effect (GME), the low-frequency limit of optical gyrotropy, in metals and semimetals coupled to chiral spin textures. In these systems, the chiral spin texture which lacks inversion symmetry can imprint itself upon the electronic structure through Hund's coupling, leading to novel low-frequency optical activity. Using perturbation theory and numerical diagonalization of both relativistic and nonrelativistic models of conduction electrons coupled to spin textures, we analyze how the GME manifests in both single-q and multi-q textures. Analytical expressions for the rotatory power are derived in terms of universal scaling functions. Estimates based on realistic material parameters reveal an experimentally viable range of values for the rotatory power. The GME arises from the orbital and spin magnetic moments of conduction electrons, with the orbital part closely tied to Berry curvature and playing a significant role in relativistic metals but not so in nonrelativistic metals where there is no inherent Berry curvature. The spin contribution to the GME can be significant in nonrelativistic metals with a large Fermi energy. Our Letter shows that the GME can be a sensitive probe of magnetic chirality and symmetry breaking in metallic chiral magnets.