Actuation at micro– and nanoscale often requires large displacements and applied forces. The high work energy density that lies inside many phase transitions is an appealing feature for developing new actuating schemes, especially if the transition is reversible and scalable into small actuating domains. We have shown the fabrication of a planar nanomechanical actuator having chevron–type geometry and based on the phase transition of VO2. This device is thermally activated through heating just above room temperature to trigger the VO2 crystalline symmetry change associated with the metal–insulator transition. The large lattice expansion of VO2 phase transition, compared to standard materials, is further amplified by the chevron–type geometry. DC and AC operation of the device are discussed.
Contact person: Nicola Manca, SPIN Genova
Miniaturized frequency comb sources across hard–to–access spectral regions, i.e. mid– and far–infrared, have long been sought. Four–wave–mixing based Quantum Cascade Laser combs (QCL–combs) are ideal candidates, in this respect, due to the unique possibility to tailor their spectral emission by proper nanoscale design of the quantum wells. We have demonstrated full–phase–stabilization of a QCL–comb against the primary frequency standard, proving independent and simultaneous control of the two comb degrees of freedom (modes spacing and frequency offset) at a metrological level. Each emitted mode exhibits a sub–Hz relative frequency stability, while a correlation analysis on the modal phases confirms the high degree of coherence in the device emission, over different power-cycles and over different days. The achievement of fully controlled, phase-stabilized QCL–comb emitters proves that this technology is mature for metrological-grade uses, as well as for an increasing number of scientific and technological applications.
The Berezinskii–Kosterlitz–Thouless phase transition from a disordered to a quasi-ordered state, mediated by the proliferation of topological defects in two dimensions, governs seemingly remote physical systems ranging from liquid helium, ultracold atoms and superconducting thin films to ensembles of spins. We have observed such a transition in a short–lived gas of exciton–polaritons, bosonic light–matter particles in semiconductor microcavities. The observed quasi–ordered phase, characteristic for an equilibrium two–dimensional bosonic gas, with a decay of coherence in both spatial and temporal domains with the same algebraic exponent, is reproduced with numerical solutions of stochastic dynamics, proving that the mechanism of pairing of the topological defects (vortices) is responsible for the transition to the algebraic order. This is made possible thanks to long polariton lifetimes in high–quality samples and in a reservoir–free region. Our results show that the joint measurement of coherence both in space and time is required to characterize driven–dissipative phase transitions and enable the investigation of topological ordering in open systems.
Fifty years ago Walter Kohn speculated that a zero-gap semiconductor might be unstable against the spontaneous generation of excitons–electron–hole pairs bound together by Coulomb attraction. The reconstructed ground state would then open a gap breaking the symmetry of the underlying lattice, a genuine consequence of electronic correlations. We have shown that this excitonic insulator is realized in zero–gap carbon nanotubes by performing first–principles calculations through many–body perturbation theory as well as quantum Monte Carlo. The excitonic order modulates the charge between the two carbon sublattices opening an experimentally observable gap, which scales as the inverse of the tube radius and weakly depends on the axial magnetic field. Our findings call into question the Luttinger liquid paradigm for nanotubes and provide tests to experimentally discriminate between excitonic and Mott insulators.
We have compared the electronic structure of (4×4), (√13×√13) R 13.9° and (2√3×2√3) R 30° silicene monolayers on Ag(111) by means of angle–resolved photoemission spectroscopy and first–principles calculations. We have found that all phases display similar Ag–derived interface states and s bands weakly perturbed by the substrate interaction. Intense spectral features, including those previously attributed to s band emission from the (2√3×2√3) R 30° structure, are found to originate from umklapp replicas of the Ag interface state and Ag sp–bulk bands. All the examined silicene allotropes do not display the characteristic Dirac cones of free–standing silicene, proving that the p bands are strongly modified by the interaction with the substrate bands independently of the structural detail of the allotrope.
Self–propelled particles, both biological and synthetic, are stably trapped by walls and develop high concentration peaks over bounding surfaces. In swimming bacteria, like E. coli, the physical mechanism behind wall entrapment is an intricate mixture of hydrodynamic and steric interactions with a strongly anisotropic character. We have demonstrated that, by using a combination of three-axis holographic microscopy and optical tweezers, it is possible to obtain volumetric reconstructions of individual E. coli cells that are sequentially released at a controlled distance and angle from a flat solid wall. We have found that hydrodynamic couplings can slow down the cell before collision, but reorientation only occurs while the cell is in constant contact with the wall. In the trapped state, all cells swim with the average body axis pointing into the surface. The amplitude of this pitch angle is anticorrelated to the amplitude of wobbling, thus indicating that entrapment is dominated by near–field couplings between the cell body and the wall.
High–quality thin insulating films on graphene (Gr) are essential for field-effect transistors (FETs) and other electronics applications of this material. We have reported a detailed morphological, structural, and electrical investigation of Al2O3 thin films grown by a two–steps ALD process on a large area Gr membrane residing on an Al2O3–Si substrate. This process consists of the H2O–activated deposition of a Al2O3 seed layer a few nanometers in thickness, performed in situ at 100 °C, followed by ALD thermal growth of Al2O3 at 250 °C. Nanoscale–resolution mapping of the current through the dielectric by conductive atomic force microscopy (CAFM) demonstrated an excellent laterally uniformity of the film. Analysis of the transfer characteristics of Gr field–effect transistors (GFETs) allowed us to evaluate the relative dielectric permittivity (ε = 7.45) and the breakdown electric field (EBD = 7.4 MV/cm) of the Al2O3 film as well as the transconductance and the holes field-effect mobility (about 1200 cm2 V–1 s–1).
We have investigated how one can realize a photonic device that combines synthetic dimensions and synthetic magnetic fields with spatially local interactions. Using an array of ring cavities, the angular coordinate around each cavity spans the synthetic dimension. The synthetic magnetic field arises from the circumstance that intercavity photon hopping is associated with a change of angular momentum. Photon-photon interactions are local in the periodic angular coordinate around each cavity. We have also pointed out experimentally observable consequences of the synthetic magnetic field and of the local interactions.
Combining localized surface plasmons (LSPs) and diffractive surface waves (DSWs) in metallic nanoparticle gratings leads to the emergence of collective hybrid plasmonic–photonic modes known as surface lattice resonances (SLRs). These show reduced losses and therefore a higher Q factor with respect to pure LSPs, at the price of larger volumes V. By using aluminum nanoparticle square gratings with unit cells consisting of narrow–gap disk dimers (a geometry featuring a very small modal volume) we have demonstrated that an enhancement of the Q/V ratio with respect to the pure LSP and DSW is obtained for SLRs with a well–defined degree of plasmon hybridization. Simultaneously, we have reported a 5 times increase of the Q/V ratio for the gap–coupled LSP with respect to that of the single nanoparticle. The results of this work open the way toward more efficient applications for the exploitation of excitonic nonlinearities in hybrid plasmonic platforms.
Hole transport in MoS2 field–effect transistors (FETs) is typically hampered by the high Schottky barrier height (SBH) for holes at source/drain contacts, due to the Fermi level pinning close to the conduction band. We have shown that the SBH of multilayer MoS2 surface can be tailored at nanoscale using soft O2 plasma treatments. Nanoscale current–voltage mapping by Conductive Atomic Force Microscopy showed that the SBH maps can be conveniently tuned starting from a narrow SBH distribution (from 0.2 to 0.3 eV) in the case of pristine MoS2 to a broader distribution (from 0.2 to 0.8 eV) after 600 s O2 plasma treatment, which allows both electron and hole injection. Back-gated multilayer MoS2 FETs, fabricated by self–aligned deposition of source/drain contacts in the O2 plasma functionalized areas, exhibit ambipolar current transport with on/off current ratio Ion/Ioff ≈ 103 and field–effect mobilities of 11.5 and 7.2 cm2 V–1s–1 for electrons and holes, respectively.
