Nature Physics最新文献

Controlling the spin current lies at the heart of spintronics and its applications. In ferromagnets, the sign of spin currents is fixed once the current direction is determined. However, spin currents in antiferromagnets can possess opposite polarizations, but this requires enormous magnetic fields to lift the degeneracy between the two modes. Therefore, controlling spin currents with opposite polarization is still a challenge. Here we demonstrate the control of spin currents at room temperature by magnon interference in a canted antiferromagnet, namely, haematite that has recently been classified as an altermagnet. Magneto-optical characterization by Brillouin light scattering reveals that the spatial periodicity of the beating patterns is tunable via the microwave frequency. We further observe that the inverse spin Hall voltage changes sign as the frequency is tuned, evincing a frequency-controlled switching of polarization of pure spin currents. Our work highlights the use of antiferromagnetic magnon interference to control spin currents, which substantially extends the horizon for the emerging field of coherent antiferromagnetic spintronics.

Microwave-to-optical transduction of single photons will play an essential role in interconnecting future superconducting quantum devices. Various transducers have been developed that couple microwave and optical modes by utilizing nonlinear phenomena such as the Pockels effect and a combination of electromechanical, piezoelectric and optomechanical couplings. However, the limited strength of these nonlinearities necessitates the use of high-quality-factor resonators that can require sophisticated nanofabrication methods. Rare-earth-ion-doped crystals have high-quality atomic resonances that result in effective second-order nonlinearities that are many orders of magnitude stronger than those in conventional materials. Here we use ytterbium-171 ions doped in an YVO₄ crystal to implement an on-chip microwave-to-optical transducer. Without an engineered optical cavity, we achieve per-cent-level efficiencies with an added noise referred to the input as low as 1.24(9) photons. We demonstrate the interference of photons originating from two simultaneously operated transducers, enabled by the inherently matching frequencies of the atomic transitions. Our results establish rare-earth-ion-based devices as a competitive platform for transduction and pave the way towards the remote transducer-assisted entanglement of superconducting quantum machines.

The study of van der Waals heterostructures with an interlayer twist, known as twistronics, has been instrumental in advancing the understanding of many strongly correlated phases, many of which derive from the topology of the physical system. Here we explore the application of the twistronics paradigm in plasmonic systems with a non-trivial topology by creating a moiré skyrmion superlattice using two superimposed plasmonic skyrmion lattices with a relative twist. We measure the complex electric field distribution of the moiré skyrmion superlattice using time-resolved polarimetric photoemission electron microscopy. Our results show that each supercell has very large topological invariants and harbours a skyrmion bag, the size of which is controllable by the twist angle and centre of rotation. Our work indicates how twistronics can enable the creation of various topological features in optical fields and provides a route for locally manipulating electromagnetic field distributions.

Blood vessels expand and contract actively as they continuously experience dynamic external stresses from blood flow. The mechanical response of the vessel wall is that of a composite material: its mechanical properties depend on its cellular components, which change dynamically as the cells respond to external stress. Mapping the relationship between these underlying cellular processes and emergent tissue mechanics is an ongoing challenge, particularly in endothelial cells. Here we assess the mechanics and cellular dynamics of an endothelial tube using a microstretcher that mimics the native environment of blood vessels. The characterization of the instantaneous monolayer elasticity reveals a strain-stiffening, actin-dependent and substrate-responsive behaviour. After a physiological pressure increase, the tissue displays a fluid-like expansion, with the reorientation of cell shape and actin fibres. We introduce a mechanical model that considers the actin fibres as a network in the nematic phase and couples their dynamics with active and elastic fibre tension. The model accurately describes the response to the pressure of endothelial tubes.

Quantum simulations offer opportunities both for studying many-body physics and for generating useful entangled states. However, existing platforms are usually restricted to specific types of interaction, fundamentally limiting the models they can mimic. Here we realize an all-to-all interacting model with an arbitrary quadratic Hamiltonian, thus demonstrating an infinite-range tunable Heisenberg XYZ model. This was accomplished by engineering cavity-mediated four-photon interactions between an ensemble of 700 rubidium atoms with a pair of momentum states serving as the effective qubit degree of freedom. As one example of the versatility of this approach, we implemented the so-called two-axis counter-twisting model, a collective spin model that can generate spin-squeezed states that saturate the Heisenberg limit on quantum phase estimation. Furthermore, our platform allows for including more than two relevant momentum states by simply adding additional dressing laser tones. This approach opens opportunities for quantum simulation and quantum sensing with matter–wave interferometers and other quantum sensors, such as optical clocks and magnetometers.

Impurities critically influence crystallization, a process fundamental to both physical sciences and industrial engineering. However, understanding how impurity transport affects crystallization presents substantial experimental challenges. Here we visualized crystallization at the single-particle level for a relatively high concentration of impurities. We observed a bifurcation in growth modes—continuous growth or melting and recrystallization—governed by the ability of the system to remove impurity particles from the growth front. The initial nucleation configuration determines the crystal grain size and growth-front morphology, which in turn influence impurity transport. Small grains promote lateral impurity transport to grain boundaries, thus reducing impurity concentration and favouring continuous growth, whereas larger grains accumulate impurities, leading to melting and recrystallization. We reveal that the latter arises from the competition between crystallization and vitrification, which is a form of devitrification. This study provides insights into the relation between impurity concentration and crystallization pathways and highlights how the initial configuration shapes the final crystal morphology.

Some semiconductors have more than one degenerate minimum of the conduction band in their band structure. These minima—known as valleys—can be used for storing and processing information, if it is possible to generate a difference in their electron populations. However, to compete with conventional electronics, it is necessary to develop universal and fast methods for controlling and reading the valley quantum number of the electrons. Even though selective optical manipulation of electron populations in inequivalent valleys has been demonstrated in two-dimensional crystals with broken time-reversal symmetry, such control is highly desired in many technologically important semiconductor materials, including silicon and diamond. We demonstrate an ultrafast technique for the generation and read-out of a valley-polarized population of electrons in bulk semiconductors on subpicosecond timescales. The principle is based on the unidirectional intervalley scattering of electrons accelerated by an oscillating electric field of linearly polarized infrared femtosecond pulses. Our results are an advance in the development of potential room-temperature valleytronic devices operating at terahertz frequencies and compatible with contemporary silicon-based technology.

Frictional motion is initiated by interface failure that is mediated by ruptures—akin to earthquakes—that typically accelerate to near-sonic velocities. However, slow ruptures may occur in both laboratory and natural fault settings, but the mechanisms that drive them are not fully understood. Although fracture mechanics describes fast frictional ruptures well, its relevance to slow ruptures is uncertain. Here we experimentally show that both extremely slow and fast ruptures—on scales of cm s^–1 and km s^–1, respectively—can repeatably propagate within the same frictional interface. We demonstrate that a dynamic equilibrium between the loading rates and velocity dependencies of both interface resistance and fracture energy enables slow ruptures to nucleate and propagate at very low applied shear stresses. In the same interfaces, fast ruptures also occur, but only when their nucleation becomes possible under higher stress conditions. We find that the dynamics and structure of both rupture classes are well described by fracture mechanics. Their existence results from a close interplay between the interface properties and rupture velocity. These results provide key insights into fault dynamics and related frictional motion.

Quasicrystals are long-range-ordered materials with atypical rotational symmetries, such as 5-fold, 10-fold or 12-fold symmetries, which are incompatible with crystallographic periodicity. Although spin-glass-like freezing phenomena have been observed in quasicrystals, antiferromagnetic order has not. Here we report experimental evidence for antiferromagnetic order in the icosahedral quasicrystal Au₅₆In_28.5Eu_15.5. Its magnetization curve shows a sharp cusp at a Néel temperature of 6.5 K, and both metamagnetic anomaly below and specific heat anomaly at this temperature are consistent with an antiferromagnetic transition. The appearance of magnetic Bragg reflections in the neutron diffraction data below the Néel temperature further confirms the long-range antiferromagnetic order in this icosahedral quasicrystal. Our discovery resolves the long-standing issue of whether antiferromagnetic order is possible in real quasicrystals, inviting further studies particularly on antiferromagnetic icosahedral quasicrystals and quasiperiodic magnetic order, as opposed to periodic magnetic order generally in condensed-matter physics.