arXiv subjects

A. Tagliacozzo

Publications and source records attributed to A. Tagliacozzo.

At least 19 recordsLinked to original sources

arXiv2022

Casimir energy for N superconducting cavities: a model for the YBCO (BSCCO)

In this paper we study the Casimir energy of a sample made by N cavities, with N >> 1, across the transition from the metallic to the superconducting phase of the constituting plates. After having characterized the energy for the configuration in which the layers constituting the cavities are made by dielectric and for the configuration in which the layers are made by plasma sheets, we concentrate our analysis on the latter. It represents the final step towards the macroscopic characterization of a "multi cavity" (with N large) necessary to fully understand the behaviour of the Casimir energy of a YBCO (or a BSCCO) sample across the transition. Our analysis is especially useful to the Archimedes experiment, aimed at measuring the interaction of the electromagnetic vacuum energy with a gravitational field. To this purpose, we aim at modulating the Casimir energy of a layered structure, the multi cavity, by inducing a transition from the metallic to the superconducting phase. After having characterized the Casimir energy of such a structure for both the metallic and the superconducting phase, we give an estimate of the modulation of the energy across the transition.

quant-ph↗

arXiv2021

The extended diffusive Sachdev-Ye-Kitaev model as a sort of "strange metal"

The 0+1-d Sachdev-Ye-Kitaev (SYK) fermionic model attracts nowadays a wide spread interest of the Condensed Matter community, as a benchmark toy model for strong electron correlation and non Fermi Liquid behavior. It is exactly solvable in the infrared limit and reproduces the linear dependence of the resistivity on temperature T, in linear response, typical of the strange metal phase of High Temperature Superconducting (HTS) materials. The breaking of its conformal symmetry requires ultraviolet corrections for a faithful description of its pseudo Goldstone Modes. Extension of the model to higher space dimension includes a local U(1) phase generating collective bosonic excitations driven by the additional ultraviolet contribution to the action. These excitations are studied here, in a temperature window of incoherent dynamics in which the expected chaotic regime has not yet taken over. We identify them as neutral diffusive energy excitations with temperature dependent lifetime h / k_B T. They provide thermalization of the system and contribute to the T dependence of the transport coefficients. The linear unbound T increase of particle current is confirmed by our hydrodynamic modelization. A quantum liquid in interaction with this system would become a Marginal Fermi Liquid.

cond-mat.str-el↗

arXiv2021

Superconducting critical temperature in the extended diffusive SYK model

Models for strongly interacting fermions in disordered clusters forming an array, with electron hopping between sites, reproduce the linear dependence on temperature of the resistivity, typical of the strange metal phase of High Temperature Superconducting materials (Extended Sachdev-Ye-Kitaev (SYK) models). Our hydrodynamical approach to the marginal Fermi liquid emerging out of the interaction, identifies the low energy collective excitations of the system in its coherent phase. These neutral excitations diffuse in the lattice, but the diffusion is heavily hindered by coupling to the pseudo Goldstone modes of the conformal broken symmetry SYK phase, which are local in space. A critical temperature for superconductivity arises in the electron liquid,in case these excitations are assumed to mediate an attractive Cooper-pairing, in the electron liquid, which is not BCS-like.

cond-mat.str-el↗

arXiv2020

Use of a spoof plasmon to optimize the coupling of infrared radiation to Josephson Junction fluxon oscillations

We show that Infrared radiation impinging onto a 1-d array of grooves drilled in the superconductor electrode of a long overlap junction can improve matching between fluxon oscillations at THz frequencies and a spoof plasmon of comparable wavelength. This example proves that metamaterials can be very helpful in integrating superconductive and subwavelength optical circuits with optimized matching bridging the gap between infrared and microwave radiation.

cond-mat.supr-con↗

arXiv2019

Thermal transport driven by charge imbalance in graphene in magnetic field, close to the charge neutrality point at low temperature: Non local resistance

Graphene grown epitaxially on SiC, close to the charge neutrality point (CNP), in an orthogonal magnetic field shows an ambipolar behavior of the transverse resistance accompanied by a puzzling longitudinal magnetoresistance. When injecting a transverse current at one end of the Hall bar, a sizeable non local transverse magnetoresistance is measured at low temperature. While Zeeman spin effect seems not to be able to justify these phenomena, some dissipation involving edge states at the boundaries could explain the order of magnitude of the non local transverse magnetoresistance, but not the asymmetry when the orientation of the orthogonal magnetic field is reversed. As a possible contribution to the explanation of the measured non local magnetoresistance which is odd in the magnetic field, we derive a hydrodynamic approach to transport in this system, which involves particle and hole Dirac carriers, in the form of charge and energy currents. We find that thermal diffusion can take place on a large distance scale, thanks to long recombination times, provided a non insulating bulk of the Hall bar is assumed, as recent models seem to suggest in order to explain the appearance of the longitudinal resistance. In presence of the local source, some leakage of carriers from the edges generates an imbalance of carriers of opposite sign, which are separated in space by the magnetic field and diffuse along the Hall bar generating a non local transverse voltage.

cond-mat.mes-hall↗

arXiv2019

Assessing the quantumness of the annealing dynamics via Leggett Gargs inequalities: a weak measurement approach

Adiabatic quantum computation (AQC) is a promising counterpart of universal quantum computation, based on the key concept of quantum annealing (QA). QA is claimed to be at the basis of commercial quantum computers and benefits from the fact that the detrimental role of decoherence and dephasing seems to have poor impact on the annealing towards the ground state. While many papers show interesting optimization results with a sizable number of qubits, a clear evidence of a full quantum coherent behavior during the whole annealing procedure is still lacking. In this paper we show that quantum non-demolition (weak) measurements of Leggett Garg inequalities can be used to efficiently assess the quantumness of the QA procedure. Numerical simulations based on a weak coupling Lindblad approach are compared with classical Langevin simulations to support our statements.

quant-ph↗

arXiv2018

Anomalous Josephson Effect in S/SO/F/S heterostructures

We study the anomalous Josephson effect, as well as the dependence on the direction of the critical Josephson current, in an S/N/S junction, where the normal part is realized by alternating spin-orbit coupled and ferromagnetic layers. We show that to observe these effects it is sufficient to break spin rotation and time reversal symmetry in spatially separated regions of the junction. Moreover, we discuss how to further improve these effects by engineering multilayers structures with more that one couple of alternating layers.

cond-mat.mes-hall↗

arXiv2016

Incipient Berezinskii-Kosterlitz-Thouless transition in two-dimensional coplanar Josephson junctions

Superconducting hybrid junctions are revealing a variety of novel effects. Some of them are due to the special layout of these devices, which often use a coplanar configuration with relatively large barrier channels and the possibility of hosting Pearl vortices. A Josephson junction with a quasi ideal two-dimensional barrier has been realized by growing graphene on SiC with Al electrodes. Chemical Vapor Deposition offers centimeter size monolayer areas where it is possible to realize a comparative analysis of different devices with nominally the same barrier. In samples with a graphene gap below 400 nm, we have found evidence of Josephson coherence in presence of an incipient Berezinskii-Kosterlitz-Thouless transition. When the magnetic field is cycled, a remarkable hysteretic collapse and revival of the Josephson supercurrent occurs. Similar hysteresis are found in granular systems and are usually justified within the Bean Critical State model (CSM). We show that the CSM, with appropriate account for the low dimensional geometry, can partly explain the odd features measured in these junctions.

cond-mat.supr-con↗

arXiv2016

Josephson coupling in junctions made of monolayer graphene on SiC

Graphene on silicon carbide (SiC) has proved to be highly successful in Hall conductance quantization for its homogeneity at the centimetre scale. Robust Josephson coupling has been measured in co-planar diffusive Al/monololayer graphene/Al junctions. Graphene on SiC substrates is a concrete candidate to provide scalability of hybrid Josephson graphene/superconductor devices, giving also promise of ballistic propagation.

cond-mat.supr-con↗

arXiv2015

Breakdown of the escape dynamics in Josephson junctions

We have identified anomalous behavior of the escape rate out of the zero-voltage state in Josephson junctions with a high critical current density Jc. For this study we have employed YBa2Cu3O7-x grain boundary junctions, which span a wide range of Jc and have appropriate electrodynamical parameters. Such high Jc junctions, when hysteretic, do not switch from the superconducting to the normal state following the expected stochastic Josephson distribution, despite having standard Josephson properties such as a Fraunhofer magnetic field pattern. The switching current distributions (SCDs) are consistent with nonequilibrium dynamics taking place on a local rather than a global scale. This means that macroscopic quantum phenomena seem to be practically unattainable for high Jc junctions. We argue that SCDs are an accurate means to measure nonequilibrium effects. This transition from global to local dynamics is of relevance for all kinds of weak links, including the emergent family of nanohybrid Josephson junctions. Therefore caution should be applied in the use of such junctions in, for instance, the search for Majorana fermions.

cond-mat.supr-con↗

arXiv2015

Spin Orbit coupling and Anomalous Josephson effect in Nanowires

A superconductor-semiconducting nanowire-superconductor heterostructure in the presence of spin orbit coupling and magnetic field can support a supercurrent even in the absence of phase difference between the superconducting electrodes. We investigate this phenomenon, the anomalous Josephson effect, employing a model capable of describing many bands in the normal region. We discuss geometrical and symmetry conditions required to have finite anomalous supercurrent and in particular we show that this phenomenon is enhanced when the Fermi level is located close to a band opening in the normal region.

cond-mat.mes-hall↗

arXiv2014

Influence of Topological Edge States on the Properties of Al/Bi2Se3/Al Hybrid Josephson Devices

In superconductor-topological insulator-superconductor hybrid junctions, the barrier edge states are expected to be protected against backscattering, to generate unconventional proximity effects, and, possibly, to signal the presence of Majorana fermions. The standards of proximity modes for these types of structures have to be settled for a neat identification of possible new entities. Through a systematic and complete set of measurements of the Josephson properties we find evidence of ballistic transport in coplanar Al-Bi2Se3-Al junctions that we attribute to a coherent transport through the topological edge state. The shunting effect of the bulk only influences the normal transport. This behavior, which can be considered to some extent universal, is fairly independent of the specific features of superconducting electrodes. A comparative study of Shubnikov - de Haas oscillations and Scanning Tunneling Spectroscopy gave an experimental signature compatible with a two dimensional electron transport channel with a Dirac dispersion relation. A reduction of the size of the Bi2Se3 flakes to the nanoscale is an unavoidable step to drive Josephson junctions in the proper regime to detect possible distinctive features of Majorana fermions.

cond-mat.supr-con↗

arXiv2013

Topological rf-SQUID with a frustrating pi-junction for probing the Majorana Bound State

Majorana Bound States are predicted to appear as boundary states of the Kitaev model. Here we show that a pi-Josephson Junction, inserted in a topologically non trivial model ring, sustains a Majorana Bound State, which is robust with respect to local and non local perturbations. The realistic structure could be based on a High Tc Superconductor tricrystal structure, similar to the one used to spot the d-wave order parameter. The presence of the Majorana Bound State changes the ground state of the topologically non trivial ring in a measurable way, with respect to that of a conventional one.

cond-mat.mes-hall↗

arXiv2012

Advantages of using YBCO-Nanowire-YBCO heterostructures in the search for Majorana Fermions

We propose an alternative platform to observe Majorana bound states in solid state systems. High critical temperature cuprate superconductors can induce superconductivity, by proximity effect, in quasi one dimensional nanowires with strong spin orbit coupling. They favor a wider and more robust range of conditions to stabilize Majorana fermions due to the large gap values, and offer novel functionalities in the design of the experiments determined by different dispersion for Andreev bound states as a function of the phase difference.

cond-mat.mes-hall↗

arXiv2012

Superconductive proximity in a Topological Insulator slab and excitations bound to an axial vortex

We consider the proximity effect in a Topological Insulator sandwiched between two conventional superconductors, by comparing s-wave spin singlet superconducting pairing correlations and odd-parity triplet pairing correlations with zero spin component orthogonal to the slab ("polar " phase). A superconducting gap opens in the Dirac dispersion of the surface states existing at the interfaces. An axial vortex is included, piercing the slab along the normal to the interfaces with the superconductors. It is known that, when proximity is s-wave, quasiparticles in the gap are Majorana Bound States, localized at opposite interfaces. We report the full expression for the quantum field associated to the midgap neutral fermions, as derived in the two-orbital band model for the TI. When proximity involves odd-parity pairing, midgap modes are charged Surface Andreev Bound States, and they originate from interfacial circular states of definite chirality, centered at the vortex singularity and decaying in the TI film with oscillations. When the chemical potential is moved away from midgap, extended states along the vortex axis are also allowed. Their orbital structure depends on the symmetry of the bulk band from where the quasiparticle level splits off.

cond-mat.mes-hall↗

arXiv2010

Spin connection and boundary states in a topological insulator

We study the surface resistivity of a three-dimensional topological insulator when the boundaries exhibit a non trivial curvature. We obtain an analytical solution for a spherical topological insulator, and we show that a non trivial quantum spin connection emerges from the three dimensional band structure. We analyze the effect of the spin connection on the scattering by a bump on a flat surface. Quantum effects induced by the geometry lead to resonances when the electron wavelength is comparable to the size of the bump.

cond-mat.mes-hall↗

arXiv2009

Mesoscopic conductance fluctuations in YBa$_2$Cu$_3$O$_{7-δ}$ grain boundary Junction at low temperature

The magneto-conductance in YBCO grain boundary Josephson junctions, displays fluctuations at low temperatures of mesoscopic origin. The morphology of the junction suggests that transport occurs in narrow channels across the grain boundary line, with a large Thouless energy. Nevertheless the measured fluctuation amplitude decreases quite slowly when increasing the voltage up to values about twenty times the Thouless energy, of the order of the nominal superconducting gap. Our findings show the coexistence of supercurrent and quasiparticle current in the junction conduction even at high nonequilibrium conditions. Model calculations confirm the reduced role of quasiparticle relaxation at temperatures up to 3 Kelvin.

cond-mat.supr-con↗

arXiv2008

Spin Hall effect in a two-dimensional electron gas in the presence of a magnetic field

We study the spin Hall effect of a two-dimensional electron gas in the presence of a magnetic field and both the Rashba and Dresselhaus spin-orbit interactions. We show that the value of the spin Hall conductivity, which is finite only if the Zeeman spin splitting is taken into account, may be tuned by varying the ratio of the in-plane and out-of-plane components of the applied magnetic field. We identify the origin of this behavior with the different role played by the interplay of spin-orbit and Zeeman couplings for in-plane and out-of-plane magnetic field components.

cond-mat.mes-hall↗