Physical review letters最新文献

The SNO+ Collaboration reports the first evidence of ^{8}B solar neutrinos interacting on ^{13}C nuclei. The charged current interaction proceeds through ^{13}C+ν_{e}→^{13}N+e^{-} which is followed, with a 10 minute half life, by ^{13}N→^{13}C+e^{+}+ν_{e}. The detection strategy is based on the delayed coincidence between the electron and the positron. Evidence for the charged current signal is presented with a significance of 4.2σ. Using the natural abundance of ^{13}C present in the scintillator, 5.7 metric tons of ^{13}C over 231 days of data were used in this analysis. The 5.6_{-2.3}^{+3.0} observed events in the data set are consistent with the expectation of 4.7_{-1.3}^{+0.6} events. This result is the second real-time measurement of CC interactions of ^{8}B neutrinos with nuclei and constitutes the lowest energy observation of neutrino interactions on ^{13}C generally. This enables the first direct measurement of the CC ν_{e} reaction to the ground state of ^{13}N, yielding an average cross section of (16.1_{-6.7}^{+8.5}(stat.)_{-2.7}^{+1.6}(syst.))×10^{-43}  cm^{2} over the relevant ^{8}B solar neutrino energies.

The Landau-Lifshitz-Gilbert (LLG) equation has been the cornerstone of modeling the dynamics of localized spins, viewed as classical vectors of fixed length, within nonequilibrium magnets. When light is employed as the nonequilibrium drive, the LLG equation must be supplemented with additional terms that are usually conjectured using phenomenological arguments for direct optomagnetic coupling between localized spins and (real or effective) magnetic field of light. However, direct coupling of magnetic field to spins is 1/c smaller than coupling of light and electrons; or both magnetic and electric fields are too fast for slow classical spins to be able to follow them. Here, we displace the need for phenomenological arguments by rigorously deriving an extended LLG equation via Schwinger-Keldysh field theory (SKFT). Within such a theory, light interacts with itinerant electrons, and then spin current carried by them exerts spin-transfer torque onto localized spins, so that when photoexcited electrons are integrated out we arrive at a spin-only equation. Unlike the standard phenomenological LLG equation with local-in-time Gilbert damping, our extended one contains a non-Markovian memory kernel whose plot within the plane of its two time arguments exhibits fractal properties. By applying the SKFT-derived extended LLG equation, as our central result, to a light-driven ferromagnet as an example, we predict an optically induced magnetic inertia term. Its magnitude is governed by a spatially nonlocal and time-dependent prefactor, leading to the excitation of coherent magnons at sharp frequencies in and outside of the band of incoherent (or thermal) magnons.

Motivated by the recent observations of superconductivity in twisted bilayer WSe_{2} (tWSe_{2}), we theoretically investigate the superconductivity driven by an electronic mechanism. We first demonstrate that the multiband screened Coulomb interaction within the random phase approximation is insufficient to induce observable pairing instability. Nevertheless, by further including the intervalley antiferromagnetic fluctuations, the pairing interaction is substantially enhanced, yielding superconductivity with critical temperature T_{c} of hundreds of millikelvin at Van Hove singularities. The predicted T_{c} increases with increasing the displacement field and corresponds to a doubly degenerate d-wavelike pairing, which evolves into a topological chiral d±id superconductor below T_{c}. The interplay between superconductivity and intervalley antiferromagnetism results in a phase diagram consistent with experimental observations. These findings provide strong support for intervalley fluctuations as the primary pairing glue in tWSe_{2}.

The intricate competition between coexisting charge density waves (CDWs) can lead to rich phenomena, offering unique opportunities for phase manipulation through electromagnetic stimuli. Leveraging time-resolved x-ray diffraction, we demonstrate ultrafast control of a CDW in EuTe_{4} upon optical excitation. At low excitation intensities, the amplitude of one of the coexisting CDW orders increases at the expense of the competing CDW, whereas at high intensities, it exhibits a nonmonotonic temporal evolution characterized by both enhancement and reduction. This transient bidirectional controllability, tunable by adjusting photoexcitation intensity, arises from the interplay between optical quenching and phase-competition-induced enhancement. Our findings, supported by phenomenological time-dependent Ginzburg-Landau theory simulations, not only clarify the relationship between the two CDWs in EuTe_{4}, but also highlight the versatility of optical control over order parameters enabled by phase competition.

Communication in quantum networks suffers notoriously from photon loss. Resulting errors can be mitigated with a suitable measurement herald at the receiving node. However, waiting for a herald and communicating the measurement result back to the sender in a repeat-until-success strategy makes the protocol slow and prone to errors from false heralds such as detector dark counts. Here, we implement an entanglement herald at the sending node by employing a cascaded two-photon emission of a single atom into two optical fiber cavities: the polarization of one photon is entangled with the spin of the atom, and the second photon heralds entanglement generation. We show that heralding improves the atom-photon entanglement in-fiber efficiency and fidelity to 68(3)% and 87(2)%, respectively. We highlight the potential of our source for noise-limited long-distance quantum communication by extending the range for constant fidelity or, alternatively, increasing the fidelity for a given distance.

The recent realization of Hofstadter spectra and fractional Chern insulators in moiré materials has introduced a new ingredient, a periodic lattice potential, to the study of quantum Hall phases. While the fractionalized states in moiré systems are expected to be in the same universality class as their counterparts in Landau levels, the periodic potential can have qualitative and quantitative effects on physical observables. Here, we examine how the magnetoroton collective modes of fractional quantum Hall (FQH) states are altered by external periodic potentials. Employing a single-mode approximation, we derive an effective Hamiltonian for the low-energy neutral excitations expressed in terms of three-point density correlation functions, which are computed using Monte Carlo. Our analysis is applicable to FQH states in graphene with a hexagonal boron nitride (hBN) substrate and also to fractional Chern insulator (FCI) states in twisted MoTe_{2} bilayers. We predict experimentally testable trends in the THz absorption characteristics of FCI and FQH states and estimate the external potential strength at which a soft-mode phase transition occurs between FQH and charge density wave states.

Euler buckling epitomizes mechanical instabilities: an inextensible straight elastic line in the plane buckles under compression when the compressive force F reaches a critical value F_{*}>0. But how does an elastic line buckle within a general curved surface? Here, we reveal that the classical instability changes fundamentally: by weakly nonlinear analysis of the buckling of an asymptotically short elastic line, we show that the critical force for the lowest buckling mode is F_{*}=0 and discover a new bifurcation structure in which the modes of classical Euler buckling split into pairs. For long elastic lines, we numerically find an additional bifurcation by which the second of these new modes becomes the lowest mode and show that, at sufficiently large F, they snap discontinuously to higher end-to-end compression. Our results constitute the foundations for a class of buckling instabilities that arise within curved surfaces, for example when biological shape emerges in development.

The acoustic spin Hall effect (ASHE) enables the generation of spin current via lattice vibrations driven by surface acoustic waves (SAWs) in heavy metals. Here, we report its reciprocal counterpart-the inverse ASHE-in which an alternating (ac) spin current induces coherent lattice vibrations that propagate SAWs. By injecting ac spin currents into a heavy metal via interfacial spin backflow in a heavy metal-ferromagnet bilayer, we successfully detect such spin-current-induced nonlocal SAWs over distances up to 400  μm in an LiNbO_{3} substrate. As the previously unobserved reciprocal element in spin-lattice interactions, the inverse ASHE completes the framework of spin-phonon interconversion and uncovers a phonon-mediated pathway for long-range spin transport, even through nonmagnetic insulators.

The formation of quantized vortices is a unifying feature of quantum mechanical systems, making it a premier means for fundamental and comparative studies of different quantum fluids. Being excited states of motion, vortices are normally unstable towards relaxation into lower energy states. However, here we exploit the driven-dissipative nature of polaritonic fluids of light to create stationary, multiply charged vortices. We measure the spectrum of collective excitations and observe negative energy modes at the core and positive energy modes at large radii. Their coexistence at the same frequency normally causes the dynamical instability, but here intrinsic losses stabilize the system, allowing for phase pinning by the pump on macroscopic scales. We observe generic features of quantized vortices in quantum fluids and other rotating geometries like astrophysical compact objects, opening the way to the study of generic amplification phenomena.

Understanding the nonequilibrium dynamics of isolated quantum many-body systems is a central goal of modern physics. Measuring genuine many-body correlations in quantum systems is central to this aim, while it remains a fundamental challenge for systems lacking individual addressability. We introduce a multidimensional correlation imprinting technique that encodes the structure of a many-body state into the temporal fluctuation spectrum of a local quantum probe. This allows direct measurement of arbitrary-order correlations in nonequilibrium quantum states. Using a nitrogen-vacancy center in diamond coupled to a ^{13}C nuclear spin bath, we experimentally reconstruct many-body correlation landscape to third order. Furthermore, by analyzing the spectral structure of correlation cumulants, we directly visualize the dynamics of quantum entanglement. Our Letter establishes a general approach to probing complex nonequilibrium phenomena and entanglement in quantum many-body systems at the nanoscale.