arXiv subjects

Andrea Perali

Publications and source records attributed to Andrea Perali.

At least 37 records · Page 2Linked to original sources

arXiv2023

The payload of the Lunar Gravitational-wave Antenna

The toolbox to study the Universe grew on 14 September 2015 when the LIGO-Virgo collaboration heard a signal from two colliding black holes between 30-250 Hz. Since then, many more gravitational waves have been detected as detectors increased sensitivity. However, the current detector design sensitivity curves still have a lower cut-off of 10 Hz. To detect even lower-frequency gravitational-wave signals, the Lunar Gravitational-wave Antenna will use an array of seismic stations in a permanently shadowed crater. It aims to detect the differential between the elastic response of the Moon and the suspended inertial sensor proof mass motion induced by gravitational waves. A cryogenic superconducting inertial sensor is under development that aims for fm/rtHz sensitivity or better down to 1 Hz and is planned to be deployed in seismic stations. Here, we describe the current state of research towards the inertial sensor, its applications and additional auxiliary technologies in the payload of the lunar gravitational-wave detection mission.

gr-qc↗

arXiv2023

Tunable BCS-BEC crossover, reentrant, and hidden quantum phase transitions in two-band superconductors with tunable valence and conduction bands

Two-band electronic structures with a valence and a conduction band separated by a tunable energy gap and with pairing of electrons in different channels can be relevant to investigate the properties of two-dimensional multiband superconductors and electron-hole superfluids, as monolayer FeSe, recently discovered superconducting bilayer graphene, and double-bilayer graphene electron-hole systems. This electronic configuration allows also to study the coexistence of superconductivity and charge density waves in connection with underdoped cuprates and transition metal dichalcogenides. By using a mean-field approach to study the system above mentioned, we have obtained numerical results for superconducting gaps, chemical potential, condensate fractions, coherence lengths, and superconducting mean-field critical temperature, considering a tunable band gap and different filling of the conduction band, for parametric choice of the pairing interactions. By tuning these quantities, the electrons redistribute among valence and conduction band in a complex way, leading to a new physics with respect to single-band superconductors, such as density induced and band-selective BCS-BEC crossover, quantum phase transitions, and hidden criticalities. At finite temperature, this phenomenon is also responsible for the non-monotonic behavior of the superconducting gaps resulting in a superconducting-normal state reentrant transition, without the need of disorder or magnetic effects.

cond-mat.supr-con↗

arXiv2022

Correlation of strontium anharmonicity with charge-lattice dynamics of the apical oxygens and their coupling to cuprate superconductivity

Cu K edge X-ray absorption spectra of overdoped superconducting $YSr_2Cu_{2.75}Mo_{0.25}O_{7.54}$ and $Sr_2CuO_{3.3}$ show a remarkably strong correlation of their superconductivity with the local dynamics of their Cu-Sr and Cu-apical-O pairs. This finding that the entire alkaline earth cation-apical O "dielectric" layer has an active role in the unusual electronic properties of cuprates has not been previously considered and has far reaching implications. We develop this idea of a possible role for the apical oxygen charge dynamics via a soft mode of the Sr by applying Kuramoto's synchronization technique to exact diagonalization calculations of two neighboring Cu-apical O pairs bridged by Sr and a planar O atom. These calculations show a first order phase transition to a synchronized state of the Internal Quantum Tunneling Polarons (IQTPs) in which a fraction of the hole originally confined to the apical O atoms of the cluster is transferred onto the planar O. This combination of experimental results and theory demonstrates that the Sr-O dielectric layer of cuprates most likely plays an important role in high temperature superconductivity via its collective charge dynamics that extends into the $CuO_2$ conducting planes.

cond-mat.supr-con↗

arXiv2022

Complex phase-fluctuation effects correlated with granularity in superconducting NbN nanofilms

Superconducting nanofilms are tunable systems that can host a 3D-2D dimensional crossover, leading to the Berezinskii-Kosterlitz-Thouless (BKT) superconducting transition approaching the 2D regime. Reducing further the dimensionality, from 2D to quasi-1D, superconducting nanostructures with disorder can generate quantum and thermal phase slips (PS) of the order parameter. Both BKT and PS are complex phase fluctuation phenomena of difficult experimental detection. Here, we have characterized superconducting NbN nanofilms thinner than 15 nm, on different substrates, by temperature dependent resistivity and current-voltage (I-V) characteristics. Our measurements have evidenced clear features related to the emergence of BKT transition and PS events. The contemporary observation in the same system of BKT transition and PS events and their tunable evolution in temperature and thickness, has been explained as due to the nano-conducting paths forming in a granular NbN system. In one of the investigated samples we have been able to trace and characterize the continuous evolution in temperature from quantum to thermal PS. Our analysis has established that the detected complex phase phenomena are strongly related to the interplay between the typical size of the nano-conductive paths and the superconducting coherence length.

cond-mat.supr-con↗

arXiv2022

Causes and consequences of ordering and dynamic phases of confined vortex rows in superconducting nanostripes

Understanding the behaviour of vortices under nanoscale confinement in superconducting circuits is of importance for development of superconducting electronics and quantum technologies. Using numerical simulations based on the Ginzburg-Landau theory for non-homogeneous superconductivity in the presence of magnetic fields, we detail how lateral confinement organises vortices in a long superconducting nanostripe, and present a phase diagram of vortex configurations as a function of the stripe width and magnetic field. We discuss why average vortex density is reduced and reveal that confinement also has profound influence on vortex dynamics in the dissipative regime under sourced electrical current, mapping out transitions between asynchronous and synchronous vortex rows crossing the nanostripe as the current is varied. Synchronous crossings are of particular interest, since they cause single-mode modulations in the voltage drop along the stripe in a high (typically GHz-to-THz) frequency range.

cond-mat.supr-con↗

arXiv2022

Chester supersolid of spatially indirect excitons in double-layer semiconductor heterostructures

A supersolid, a counter-intuitive quantum state in which a rigid lattice of particles flows without resistance, has to date not been unambiguously realised. Here we reveal a supersolid ground state of excitons in a double-layer semiconductor heterostructure over a wide range of layer separations outside the focus of recent experiments. This supersolid conforms to the original Chester supersolid with one exciton per supersolid site, as distinct from the alternative version reported in cold-atom systems of a periodic modulation of the superfluid density. We provide the phase diagram augmented by the supersolid. This new phase appears at layer separations much smaller than the predicted exciton normal solid, and it persists up to a solid--solid transition where the quantum phase coherence collapses. The ranges of layer separations and exciton densities in our phase diagram are well within reach of the current experimental capabilities.

cond-mat.mes-hall↗

arXiv2022

Josephson effect as signature of electron-hole superfluidity in bilayers of van der Waals heterostructures

We investigate a Josephson junction in an electron-hole superfluid in a double layer TMD heterostructure. Observation of a critical tunneling current is a clear signature of superfluidity. In addition, we find the BCS-BEC crossover physics in the narrow barrier region controls the critical current across the entire system. The corresponding critical velocity, which is measurable in this system, has a maximum when the excitations pass from bosonic to fermionic. Remarkably, this occurs for the density at the boundary of the BEC to BCS-BEC crossover regime determined from the condensate fraction. This provides, for the first time in a semiconductor system, an experimental way to determine the position of this boundary.

cond-mat.supr-con↗

arXiv2022

A cryogenic and superconducting inertial sensor for the Lunar Gravitational--Wave Antenna, the Einstein Telescope and Selene-physics

The Lunar Gravitational--Wave Antenna is a proposed low-frequency gravitational-wave detector on the Moon surface. It will be composed of an array of high-end cryogenic superconducting inertial sensors (CSISs). A cryogenic environment will be used in combination with superconducting materials to open up pathways to low-loss actuators and sensor mechanics. CSIS revolutionizes the (cryogenic) inertial sensor field with a modelled displacement sensitivity at 0.5 Hz of 3 orders of magnitude better than the current state-of-the-art. It will allow the Lunar Gravitational-Wave Antenna to be sensitive below 1 Hz, down to 1 mHz and it will also be employed in the forthcoming Einstein Telescope --a third-generation gravitational-wave detector which will make use of cryogenic technologies and that will have an enhanced sensitivity below 10 Hz. Moreover, CSIS seismic data could also be employed to obtain new insights about the Moon interior and what we can call the Selene-physics.

astro-ph.IM↗

arXiv2021

Metastable states in plateaus and multi-wave epidemic dynamics of Covid-19 spreading in Italy

The control of Covid 19 epidemics by public health policy in Italy during the first and the second epidemic waves has been driven by using reproductive number Rt(t) to identify the supercritical (percolative), the subcritical (arrested), separated by the critical regime. Here we show that to quantify the Covid-19 spreading rate with containment measures (CSRwCM) there is a need of a 3D expanded parameter space phase diagram built by the combination of Rt(t) and doubling time Td(t). In this space we identify the dynamics of the Covid-19 dynamics Italy and its administrative Regions. The supercritical regime is mathematically characterized by i) the power law of Td vs. [Rt(t)-1] and ii) the exponential behaviour of Td vs. time, either in the first and in the second wave. The novel 3D phase diagram shows clearly metastable states appearing before and after the second wave critical regime. for loosening quarantine and tracing of actives cases. The metastable states are precursors of the abrupt onset of a next nascent wave supercritical regime. This dynamic description allows epidemics predictions needed by policymakers to activate non-pharmaceutical interventions (NPIs), a key issue for avoiding economical losses, reduce fatalities and avoid new virus variant during vaccination campaign

q-bio.PE↗

arXiv2020

Hidden Pseudogap and Excitation Spectra in a Strongly Coupled Two-Band Superfluid/Superconductor

We investigate single-particle excitation properties in the normal state of a two-band superconductor or superfluid throughout the Bardeen-Cooper-Schrieffer (BCS) to Bose-Einstein-condensation (BEC) crossover, within the many-body T-matrix approximation for multi-channel pairing fluctuations. We address the single-particle density of states and the spectral functions consisting of two contributions associated with a waekly interacting deep band and a strongly interacting shallow band, relevant for iron-based multiband superconductors and multicomponent fermionic superfluids. We show how the pseudogap state in the shallow band is hidden by the deep band contribution throughout the two-band BCS-BEC crossover. Our results could explain the missing pseudogap in recent scanning tunneling microscopy experiments in FeSe superconductors.

cond-mat.supr-con↗

arXiv2020

Epidemic spreading in an expanded parameter space: the supercritical scaling laws and subcritical metastable phases

So far most of the analysis of coronavirus 2020 epidemic data has been focusing on a short-time window and consequently a quantitative test of statistical physical laws of Coronavirus Epidemics with Containment Measures (CEwCM) is currently lacking. Here we report a quantitative analysis of CEwCM over 230 days, covering the full-time lapse of the first epidemic wave. We use a 3D phase diagram tracking the simultaneous evolution of the doubling time Td(t) and reproductive number Rt(t) showing that this expanded parameter space is needed for biological physics of CEwCP. We have verified that in the supercritical [Rt(t)>1, Td(t)<40 days] regime i) the curve Z(t) of total infected cases follows the growth rate called Ostwald law; ii) the doubling time follows the exponential law Td(t)=A exp((t-t0)/s) as a function of time and iii) the power law Td(t)=C(Rt(t)-1)^-n is verified with the exponent n depending on the definition of Rt(t). The log-log plots Td(t) versus (Rt-1) of the second 2020 epidemic wave unveil in the subcritical regime [Td(t)>100 days] arrested metastable phases with Rt>1 where Td(t) was kept constant followed by its explosion and its containment following the same power law as in the first wave

q-bio.PE↗

arXiv2020

Mechanism of screening or enhancing the pseudogap throughout the two-band Bardeen-Cooper-Schrieffer to Bose-Einstein condensate crossover

We demonstrate the rise-and-fall of multiple pseudogaps in the Bardeen-Cooper-Schrieffer-Bose-Einstein-condensation (BCS-BEC) crossover in two-band fermionic systems having different pairing strengths in the deep band and in the shallow band. The striking features of this phenomenon are an unusual many-body screening of pseudogap state and the importance of pair-exchange couplings, which induces multiple pseudogap formation in the two bands. The multi-band configuration suppresses pairing fluctuations and the pseudogap opening in the strongly-interacting shallow band at small pair-exchange couplings by screening effects, with possible connection to the pseudogap phenomenology in iron based superconductors. On the other hand, the multiple pseudogap mechanism accompanies with the emergence of binary preformed Cooper pairs originating from interplay between intra-band and pair-exchange couplings.

cond-mat.supr-con↗

arXiv2020

Electron-hole superfluidity in strained Si/Ge type II heterojunctions

Excitons are promising candidates for generating superfluidity and Bose-Einstein Condensation (BEC) in solid state devices, but an enabling material platform with in-built bandstructure advantages and scaling compatibility with industrial semiconductor technology is lacking. Here we predict that spatially indirect excitons in a lattice-matched strained Si/Ge bilayer embedded into a germanium-rich SiGe crystal, would lead to observable mass-imbalanced electron-hole superfluidity and BEC. Holes would be confined in a compressively strained Ge quantum well and electrons in a lattice-matched tensile strained Si quantum well. We envision a device architecture that does not require an insulating barrier at the Si/Ge interface, since this interface offers a type II band alignment. Thus the electrons and holes can be kept very close but strictly separate, strengthening the electron-hole pairing attraction while preventing fast electron-hole recombination. The band alignment also allows a one-step procedure for making independent contacts to the electron and hole layers, overcoming a significant obstacle to device fabrication. We predict superfluidity at experimentally accessible temperatures of a few Kelvin and carrier densities up to $\sim 6\times 10^{10}$ cm$^{-2}$, while the large imbalance of the electron and hole effective masses can lead to exotic superfluid phases.

cond-mat.mes-hall↗

arXiv2020

Efficiency of Covid-19 Mobile Contact Tracing Containment by Measuring Time Dependent Doubling Time

The Covid-19 epidemic of the novel coronavirus (severe acute respiratory syndrome SARS - CoV-2) has been spreading around the world. While different containment policies using non-pharmaceutical interventions have been applied, their efficiency are not known quantitatively. We show that the doubling time Td(t) with the success s factor, the characteristic time of the exponential growth of Td(t) in the arrested regime, is a reliable tool for early predictions of epidemic spread time evolution and it provides a quantitative measure of the success of different containment measures. The efficiency of the containment policy Lockdown case Finding mobile Tracing (LFT) using mandatory mobile contact tracing is much higher than the Lockdown Stop and Go (LSG) policy proposed by the Imperial College team in London. A very low s factor was reached by LFT policy giving the shortest time width of the dome of positive case curve and the lowest number of fatalities. The LFT policy has been able to reduce by a factor 100 the number of fatalities in the first 100 days of the Covid-19 epidemic, to reduce the time width of the Covid-19 pandemic dome by a factor 2.5 and to rapidly stop new outbreaks avoiding the second wave

q-bio.PE↗

arXiv2020

Ostwald growth rate in controlled Covid-19 epidemic spreading as in arrested growth in quantum complex matter

Here, we focus on the data analysis of the growth of epidemic spread of Covid-19 in countries where different policies of containment were activated. It is known that the growth of pandemic spread at its threshold is exponential, but it is not known how to quantify the success of different containment policies. We identify that a successful approach gives an arrested phase regime following the Ostwald growth, where, over the course of time, one phase transforms into another metastable phase with a similar free energy as observed in oxygen interstitial diffusion in quantum complex matter and in crystallization of proteins. We introduce the s factor which provides a quantitative measure of the efficiency and speed of the adopted containment policy, which is very helpful not only to monitor the Covid-19 pandemic spread but also for other countries to choose the best containment policy. The results show that a policy based on joint confinement, targeted tests, and tracking positive cases is the most rapid pandemic containment policy; in fact, we found values of 9, 5, and 31 for the success s factor for China, South Korea, and Italy, respectively, where the lowest s factor indicates the best containment policy

physics.soc-ph↗

arXiv2020

Experimental conditions for observation of electron-hole superfluidity in GaAs heterostructures

The experimental parameter ranges needed to generate superfluidity in optical and drag experiments in GaAs double quantum wells are determined, using a formalism that includes self-consistent screening of the Coulomb pairing interaction in the presence of the superfluid. The very different electron and hole masses in GaAs make this a particularly interesting system for superfluidity, with exotic superfluid phases predicted in the BCS-BEC crossover regime. We find that the density and temperature ranges for superfluidity cover the range for which optical experiments have observed indications of superfluidity, but that existing drag experiments lie outside the superfluid range. However we also show that for samples with low mobility with no macroscopically connected superfluidity, if the superfluidity survived in randomly distributed localized pockets, standard quantum capacitance measurements could detect these pockets.

cond-mat.supr-con↗

arXiv2020

BCS-BEC Crossover and Pairing Fluctuations in a Two Band Superfluid/Superconductor: A T Matrix Approach

We investigate pairing fluctuation effects in a two band fermionic system, where a shallow band in the Bardeen--Cooper--Schrieffer--Bose--Einstein condensation (BCS-BEC) crossover regime is coupled with a weakly interacting deep band. Within a diagrammatic $T$ matrix approach, we~report how thermodynamic quantities such as the critical temperature, chemical potential, {and~momentum distributions} undergo the crossover from the BCS to BEC regime by tuning the intraband coupling in the shallow band. We also generalize the definition of Tan's contact to a two band system and report the two contacts for different pair-exchange couplings. The present results are compared with those obtained by the simpler Nozières--Schmitt--Rink approximation. We confirm a pronounced enhancement of the critical temperature due to the multiband configuration, as well as to the pair-exchange coupling.

cond-mat.supr-con↗

arXiv2019

A doping-dependent switch from one- to two-component superfluidity at temperature above 100K in coupled electron-hole Van der Waals heterostructures

The hunt for high temperature superfluidity has received new impetus from the discovery of atomically thin stable materials. Electron-hole superfluidity in coupled MoSe2-WSe2 monolayers is investigated using a mean-field multiband model that includes the band splitting caused by the strong spin-orbit coupling. The splitting leads to a large energy misalignment of the electron and hole bands which can be markedly changed by interchanging the doping of the monolayers. The choice of doping determines if the superfluidity is tuneable from one- to two-components. The electron-hole pairing is strong, with high transition temperatures in excess of 100 K.

cond-mat.supr-con↗