Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title><jats:p>Electric polarization is well defined only in insulators not metals, and there is no general scheme to induce and control bulk polarity in metals. Here we circumvent this limitation by utilizing a pseudo-electric field generated by inhomogeneous lattice strain, namely a flexoelectric field, as a means of polarizing and controlling a metal. Using heteroepitaxy and atomic-scale imaging, we show that flexoelectric fields polarize the bulk of an otherwise centrosymmetric metal SrRuO<jats:sub>3</jats:sub>, with off-centre displacements of Ru ions. This further impacts the electronic bands and lattice anisotropy of the flexo-polar SrRuO<jats:sub>3</jats:sub>, potentially leading to an enhancement of electron correlation, ferromagnetism and its anisotropy. Beyond conventional electric fields, flexoelectric fields may be used to create and control electronic states through pure atomic displacements.</jats:p>

<jats:title>Abstract</jats:title><jats:p>Quantum transport can distinguish between dynamical phases of matter. For instance, ballistic propagation characterizes the absence of disorder, whereas in many-body localized phases, particles do not propagate for exponentially long times. Additional possibilities include states of matter exhibiting anomalous transport in which particles propagate with a non-trivial exponent. Here we report the experimental observation of anomalous transport across a broad range of the phase diagram of a kicked quasicrystal. The Hamiltonian of our system has been predicted to exhibit a rich phase diagram, including not only fully localized and fully delocalized phases but also an extended region comprising a nested pattern of localized, delocalized and multifractal states, which gives rise to anomalous transport. Our cold-atom realization is enabled by new Floquet engineering techniques, which expand the accessible phase diagram by five orders of magnitude. Mapping transport properties throughout the phase diagram, we observe disorder-driven re-entrant delocalization and sub-ballistic transport, and we present a theoretical explanation of these phenomena based on eigenstate multifractality.</jats:p>

<jats:title>Abstract</jats:title><jats:p>Ultracold polar molecules combine a rich structure of long-lived internal states with access to controllable long-range anisotropic dipole–dipole interactions. In particular, the rotational states of polar molecules confined in optical tweezers or optical lattices may be used to encode interacting qubits for quantum computation or pseudo-spins for simulating quantum magnetism. As with all quantum platforms, the engineering of robust coherent superpositions of states is vital. However, for optically trapped molecules, the coherence time between rotational states is typically limited by inhomogeneous differential light shifts. Here we demonstrate a rotationally magic optical trap for <jats:sup>87</jats:sup>Rb<jats:sup>133</jats:sup>Cs molecules that supports a Ramsey coherence time of 0.78(4) s in the absence of dipole–dipole interactions. This is estimated to extend to &gt;1.4 s at the 95% confidence level using a single spin-echo pulse. In our trap, dipolar interactions become the dominant mechanism by which Ramsey contrast is lost for superpositions that generate oscillating dipoles. By changing the states forming the superposition, we tune the effective dipole moment and show that the coherence time is inversely proportional to the strength of the dipolar interaction. Our work unlocks the full potential of the rotational degree of freedom in molecules for quantum computation and quantum simulation.</jats:p>

<jats:title>Abstract</jats:title><jats:p>Bilayer graphene is a promising platform for electrically controllable qubits in a two-dimensional material. Of particular interest is the ability to encode quantum information in the valley degree of freedom, a two-fold orbital degeneracy that arises from the symmetry of the hexagonal crystal structure. The use of valleys could be advantageous, as known spin- and orbital-mixing mechanisms are unlikely to be at work for valleys, promising more robust qubits. The Berry curvature associated with valley states allows for electrical control of their energies, suggesting routes for coherent qubit manipulation. However, the relaxation time of valley states—which ultimately limits these qubits’ coherence properties and therefore their suitability as practical qubits—is not yet known. Here we measure the characteristic relaxation times of these spin and valley states in gate-defined bilayer graphene quantum dot devices. Different valley states can be distinguished from each other with a fidelity of over 99%. The relaxation time between valley triplets and singlets exceeds 500 ms and is more than one order of magnitude longer than for spin states. This work facilitates future measurements on valley-qubit coherence, demonstrating bilayer graphene as a practical platform hosting electrically controlled, long-lived valley qubits.</jats:p>

<jats:title>Abstract</jats:title><jats:p>Active processes drive biological dynamics across various scales and include subcellular cytoskeletal remodelling, tissue development in embryogenesis and the population-level expansion of bacterial colonies. In each of these, biological functionality requires collective flows to occur while self-organised structures are protected. However, the mechanisms by which active flows can spontaneously constrain their dynamics to preserve structure are not known. Here, by studying collective flows and defect dynamics in active nematic films, we demonstrate the existence of a self-constraint, namely a two-way, spontaneously arising relationship between activity-driven isosurfaces of flow boundaries and mesoscale nematic structures. We show that self-motile defects are tightly constrained to viscometric surfaces, which are contours along which the vorticity and the strain rate are balanced. This in turn reveals that self-motile defects break mirror symmetry when they move along a single viscometric surface. This is explained by an interdependence between viscometric surfaces and bend walls, which are elongated narrow kinks in the orientation field. These findings indicate that defects cannot be treated as solitary points. Instead, their associated mesoscale deformations are key to the steady-state coupling to hydrodynamic flows. This mesoscale cross-field self-constraint offers a framework for tackling complex three-dimensional active turbulence, designing dynamic control into biomimetic materials and understanding how biological systems can employ active stress for dynamic self-organisation.</jats:p>

<jats:title>Abstract</jats:title><jats:p>Motile cells inside living tissues often encounter junctions, where their path branches into several alternative directions of migration. We present a theoretical model of cellular polarization for a cell migrating along a one-dimensional line, arriving at a symmetric Y junction and extending protrusions along the different paths that originate at the junction. The model predicts the spontaneous emergence of deterministic oscillations of growth and cellular polarization between competing protrusions during the directional decision-making process. The oscillations are modified by cellular noise but remain a dominant feature that affects the time it takes the cell to migrate across the junction. These predictions are confirmed experimentally for two different cell types (non-cancerous endothelial and cancerous glioma cells) migrating on a patterned network of thin adhesive lanes with junctions.</jats:p>

<jats:title>Abstract</jats:title><jats:p>Quantum devices characterized by non-Hermitian topology are predicted to show highly robust and potentially useful properties for precision sensing and signal amplification. However, realizing them has remained a daunting experimental task, as non-Hermiticity is often associated with gain and loss, which would require precise tailoring to produce the signatures of non-trivial topology. Here, instead of gain and loss, we use the non-reciprocity of quantum Hall edge states to directly observe non-Hermitian topology in a multi-terminal quantum Hall ring. Our transport measurements evidence a robust, non-Hermitian skin effect, characterized by currents and voltages showing an exponential profile that persists across Hall plateau transitions away from the regime of maximum non-reciprocity. Our observation of non-Hermitian topology in a quantum device introduces a scalable experimental approach to construct and investigate generic non-Hermitian systems.</jats:p>