arXiv subjects

M. Grilli

Publications and source records attributed to M. Grilli.

At least 37 records · Page 2Linked to original sources

arXiv2016

Dynamical charge density waves rule the phase diagram of cuprates

In the last few years charge density waves (CDWs) have been ubiquitously observed in high-temperature superconducting cuprates and are now the most investigated among the competing orders in the still hot debate on these systems. A wealth of new experimental data raise several fundamental issues that challenge the various theoretical proposals. Here, we account for the complex experimental temperature vs. doping phase diagram and we provide a coherent scenario explaining why different CDW onset curves are observed by different experimental probes and seem to extrapolate at zero temperature into seemingly different quantum critical points (QCPs) in the intermediate and overdoped region. We also account for the pseudogap and its onset temperature T*(p) on the basis of dynamically fluctuating CDWs. The nearly singular anisotropic scattering mediated by these fluctuations also account for the rapid changes of the Hall number seen in experiments and provides the first necessary step for a possible Fermi surface reconstruction fully establishing at lower doping. Finally we show that phase fluctuations of the CDWs, which are enhanced in the presence of strong correlations near the Mott insulating phase, naturally account for the disappearance of the CDWs at low doping with yet another QCP.

cond-mat.supr-con↗

arXiv2015

Archimedes: a feasibility study of an experiment to weigh the electromagnetic vacuum

Archimedes is a feasibility study of a future experiment to ascertain the interaction of vacuum fluctuations with gravity. The experiment should measure the force that the earth's gravitational field exerts on a Casimir cavity by using a small force detector. Here we analyse the main parameters of the experiment and we present its conceptual scheme, which overcomes in principle the most critical problems.

gr-qc↗

arXiv2015

The Archimedes experiment

Archimedes is an INFN-funded pathfinder experiment aimed at verifying the feasibility of measuring the interaction of vacuum fluctuations with gravity. The final experiment will measure the force exerted by the gravitational field on a Casimir cavity whose vacuum energy is modulated with a superconductive transition, by using a balance as a small force detector. Archimedes is a two-year project devoted to test the most critical experimental aspects, in particular the balance resonance frequency and quality factor, the thermal modulation efficiency and the superconductive sample realization.

gr-qc↗

arXiv2015

Confinement of superconducting fluctuations due to emergent electronic inhomogeneities

The microscopic nature of an insulating state in the vicinity of a superconducting state, in the presence of disorder, is a hotly debated question. While the simplest scenario proposes that Coulomb interactions destroy the Cooper pairs at the transition, leading to localization of single electrons, an alternate possibility supported by experimental observations suggests that Cooper pairs instead directly localize. The question of the homogeneity, granularity, or possibly glassiness of the material on the verge of this transition is intimately related to this fundamental issue. Here, by combining macroscopic and nano-scale studies of superconducting ultrathin NbN films, we reveal nanoscopic electronic inhomogeneities that emerge when the film thickness is reduced. In addition, while thicker films display a purely two-dimensional behaviour in the superconducting fluctuations, we demonstrate a zero-dimensional regime for the thinner samples precisely on the scale of the inhomogeneities. Such behavior is somehow intermediate between the Fermi and Bose insulator paradigms and calls for further investigation to understand the way Cooper pairs continuously evolve from a bound state of fermionic objects into localized bosonic entities.

cond-mat.supr-con↗

arXiv2015

Density driven fluctuations in a two-dimensional superconductor

In the vicinity of a phase transition, the order parameter starts fluctuating before vanishing at the critical point. The fluctuation regime, i.e. the way the ordered phase disappears, is a characteristics of a transition, and determines the universality class it belongs to. This is valid for thermal transitions, but also for zero temperature Quantum Phase Transitions (QPT). In the case of superconductivity, the order parameter has an amplitude and a phase, which can both fluctuate according to well identified scenarios. The Ginzburg-Landau theory and its extensions describe the fluctuating regime of regular metallic superconductors, and the associated dynamics of the pair amplitude and the phase. When the system is two-dimensional and/or very disordered, phase fluctuations dominate. Here, we address the possibility that a new type of fluctuations occurs in superconductors with an anomalous dynamics. In particular we show that the superconducting to metal QPT that occurs upon changing the gate voltage in two-dimensional electron gases at LAO/STO and LTO/STO interfaces displays anomalous scaling properties, which can be explained by density driven superconducting critical fluctuations. A Finite Size Scaling (FSS) analysis reveals that the product z.nu (nu is the correlation length exponent and z the dynamical critical one) is z.nu = 3/2. We argue that critical superconducting fluctuations acquire an anomalous dynamics with z=3, since they couple to density ones in the vicinity of a spontaneous electronic phase separation, and that nu=1/2 corresponds to the mean-field value. This approach strongly departs from the conventional z=1 scenario in disordered 2D systems based on long-range Coulomb interactions with dominant phase fluctuations. This scenario can explain recent data in LSCO ultra-thin films, and apply to a whole class of two-dimensional superconductors.

cond-mat.supr-con↗

arXiv2015

Phase separation from electron confinement at oxide interfaces

Oxide heterostructures are of great interest both for fundamental and applicative reasons. In particular the two-dimensional electron gas at the LaAlO$_3$/SrTiO$_3$ or LaTiO$_3$/SrTiO$_3$ interfaces displays many different physical properties and functionalities. However there are clear indications that the interface electronic state is strongly inhomogeneous and therefore it is crucially relevant to investigate possible intrinsic electronic mechanisms underlying this inhomogeneity. Here the electrostatic potential confining the electron gas at the interface is calculated self-consistently, finding that the electron confinement at the interface may induce phase separation, to avoid a thermodynamically unstable state with a negative compressibility. This provides a generic robust and intrinsic mechanism for the experimentally observed inhomogeneous character of these interfaces.

cond-mat.mes-hall↗

arXiv2015

Electronic polymers and soft-matter-like broken symmetries in underdoped cuprates

Empirical evidence in heavy fermion, pnictide, and other systems suggests that unconventional superconductivity appears associated to some form of real-space electronic order. For the cuprates, despite several proposals, the emergence of order in the phase diagram between the commensurate antiferromagnetic state and the superconducting state is not well understood. Here we show that in this regime doped holes assemble in "electronic polymers." Within a Monte Carlo study we find, that in clean systems by lowering the temperature the polymer melt condenses first in a smectic state and then in a Wigner crystal both with the addition of inversion symmetry breaking. Disorder blurs the positional order leaving a robust inversion symmetry breaking and a nematic order, accompanied by vector chiral spin order and with the persistence of a thermodynamic transition. Such electronic phases, whose properties are reminiscent of soft-matter physics, produce charge and spin responses in good accord with experiments.

cond-mat.supr-con↗

arXiv2015

Signatures of nematic quantum critical fluctuations in the Raman spectra of lightly doped cuprates

We consider the lightly doped cuprates Y$_{0.97}$Ca$_{0.03}$BaCuO$_{6.05}$ and La$_{2-x}$Sr$_x$CuO$_4$ (with $x=0.02$,0.04), where the presence of a fluctuating nematic state has often been proposed as a precursor of the stripe (or, more generically, charge-density wave) phase, which sets in at higher doping. We phenomenologically assume a quantum critical character for the longitudinal and transverse nematic, and for the charge-ordering fluctuations, and investigate the effects of these fluctuations in Raman spectra. We find that the longitudinal nematic fluctuations peaked at zero transferred momentum account well for the anomalous Raman absorption observed in these systems in the $B_{2g}$ channel, while the absence of such effect in the $B_{1g}$ channel may be due to the overall suppression of Raman response at low frequencies, associated with the pseudogap. While in Y$_{0.97}$Ca$_{0.03}$BaCuO$_{6.05}$ the low-frequency lineshape is fully accounted by longitudinal nematic collective modes alone, in La$_{2-x}$Sr$_x$CuO$_4$ also charge-ordering modes with finite characteristic wavevector are needed to reproduce the shoulders observed in the Raman response. This different involvement of the nearly critical modes in the two materials suggests a different evolution of the nematic state at very low doping into the nearly charge-ordered state at higher doping.

cond-mat.supr-con↗

arXiv2015

Interplay between density and superconducting quantum critical fluctuations

We consider the case of a density-driven metal-superconductor transition in the proximity of an electronic phase separation. In particular we investigate the interplay between superconducting fluctuations and density fluctuations, which become quantum critical when the electronic phase separation vanishes at zero temperature into a quantum critical point. In this situation the critical dynamical density fluctuations strongly affect the dynamics of the Cooper pair fluctuations, which acquire a more singular character with a z=3 dynamical critical index. This gives rise to a scenario that possibly rules the disappearance of superconductivity when the electron density is reduced by elecrostatic gating at the LaAlO3/SrTiO3 interface.

cond-mat.supr-con↗

arXiv2015

Field-effect control of superconductivity and Rashba spin-orbit coupling in top-gated LaAlO3/SrTiO3 devices

The recent development in the fabrication of artificial oxide heterostructures opens new avenues in the field of quantum materials by enabling the manipulation of the charge, spin and orbital degrees of freedom. In this context, the discovery of two-dimensional electron gases (2-DEGs) at LAlO3/SrTiO3 interfaces, which exhibit both superconductivity and strong Rashba spin-orbit coupling (SOC), represents a major breakthrough. Here, we report on the realisation of a field-effect LaAlO3/SrTiO3 device, whose physical properties, including superconductivity and SOC, can be tuned over a wide range by a top-gate voltage. We derive a phase diagram, which emphasises a field-effect-induced superconductor-to-insulator quantum phase transition. Magneto-transport measurements indicate that the Rashba coupling constant increases linearly with electrostatic doping. Our results pave the way for the realisation of mesoscopic devices, where these two properties can be manipulated on a local scale by means of top-gates.

cond-mat.supr-con↗

arXiv2015

Possible mechanisms of electronic phase separation in oxide interfaces

LaAlO3/SrTiO3 ad LaTiO3/SrTiO3 interfaces are known to host a strongly inhomogeneous (nearly) two-dimensional electron gas (2DEG). In this work we present three unconventional electronic mechanisms of electronic phase separation (EPS) in a 2DEG as a possible source of inhomogeneity in oxide interfaces. Common to all three mechanisms is the dependence of some (interaction) potential on the 2DEG's density. We first consider a mechanism resulting from a sizable density-dependent Rashba spin-orbit coupling. Next, we point out that an EPS may also occur in the case of a density-dependent superconducting pairing interaction. Finally, we show that the confinement of the 2DEG to the interface by a density-dependent, self-consistent electrostatic potential can by itself cause an EPS.

cond-mat.mes-hall↗

arXiv2015

Inhomogeneous multi-carrier superconductivity at LaXO3/SrTiO3 (X=Al or Ti) oxide interfaces

Several experiments reveal the inhomogeneous character of the superconducting state that occurs when the carrier density of the two-dimensional electron gas formed at the LaXO3/SrTiO3 (X=Al or Ti) interface is tuned above a threshold value by means of gating. Re-analyzing previous measurements, that highlight the presence of two kinds of carriers, with low and high mobility, we shall provide a description of multi-carrier magneto-transport in an inhomogeneous two-dimensional electron gas, gaining insight into the properties of the physics of the systems under investigation. We shall then show that the measured resistance, superfluid density, and tunneling spectra result from the percolative connection of superconducting "puddles" with randomly distributed critical temperatures, embedded in a weakly localizing metallic matrix. We shall also show that this scenario is consistent with the characteristics of the superconductor-to-metal transition driven by a magnetic field. A multi-carrier description of the superconducting state, within a weak-coupling BCS-like model, will be finally discussed.

cond-mat.mes-hall↗

arXiv2014

Phase separation and long wave-length charge instabilities in spin-orbit coupled systems

We investigate a two-dimensional electron model with Rashba spin-orbit interaction where the coupling constant $g=g(n)$ depends on the electronic density. It is shown that this dependence may drive the system unstable towards a long-wave length charge density wave (CDW) where the associated second order instability occurs in close vicinity to global phase separation. For very low electron densities the CDW instability is nesting-induced and the modulation follows the Fermi momentum $k_F$. At higher density the instability criterion becomes independent of $k_F$ and the system may become unstable in a broad momentum range. Finally, upon filling the upper spin-orbit split band, finite momentum instabilities disappear in favor of phase separation alone. We discuss our results with regard to the inhomogeneous phases observed at the LaAlO$_3$/SrTiO$_3$ or LaTiO$_3$/SrTiO$_3$ interfaces.

cond-mat.mes-hall↗

arXiv2014

Phase diagrams of voltage-gated oxide interfaces with strong Rashba coupling

We propose a model for the two-dimensional electron gas formed at the interface of oxide heterostructures that includes a Rashba spin-orbit coupling proportional to an electric field oriented perpendicularly to the interface. Taking into account the electron density dependence of this electric field confining the electron gas at the interface, we report the occurrence of a phase separation instability (signaled by a negative compressibility) for realistic values of the spin-orbit coupling and of the electronic band-structure parameters at zero temperature. We extend the analysis to finite temperatures and in the presence of an in-plane magnetic field, thereby obtaining two phase diagrams which exhibit a phase separation dome. By varying the gating potential the phase separation dome may shrink and vanish at zero temperature into a quantum critical point where the charge fluctuates dynamically. Similarly the phase separation may be spoiled by a planar magnetic field even at zero temperature leading to a line of quantum critical points.

cond-mat.mes-hall↗

arXiv2014

Pseudo-gap in tunneling spectra as a signature of inhomogeneous superconductivity in oxide interfaces

We present a theory for the pseudo-gap state recently observed at the LaAlO3/SrTiO3 interface, based on superconducting islands embedded in a metallic background. Superconductivity within each island is BCS-like, and the local critical temperatures are randomly distributed, some of them necessarily exceeding the critical temperature for global percolation to the zero resistance state. Consequently, tunneling spectra display a suppression of the density of states and coherence peaks already well above the percolative transition. This model of inhomogeneous superconductivity accounts well for the experimental tunneling spectra. The temperature dependence of the spectra suggests that a sizable fraction of the metallic background becomes superconducting by proximity effect when the temperature is lowered.

cond-mat.supr-con↗

arXiv2013

Multi-band superconductivity and nanoscale inhomogeneity at oxide interfaces

The two-dimensional electron gas at the LaTiO3/SrTiO3 or LaAlO3/SrTiO3 oxide interfaces becomes superconducting when the carrier density is tuned by gating. The measured resistance and superfluid density reveal an inhomogeneous superconductivity resulting from percolation of filamentary structures of superconducting "puddles" with randomly distributed critical temperatures, embedded in a non-superconducting matrix. Following the evidence that superconductivity is related to the appearance of high-mobility carriers, we model intra-puddle superconductivity by a multi-band system within a weak coupling BCS scheme. The microscopic parameters, extracted by fitting the transport data with a percolative model, yield a consistent description of the dependence of the average intra-puddle critical temperature and superfluid density on the carrier density.

cond-mat.supr-con↗

arXiv2012

Intrinsic instability of electronic interfaces with strong Rashba coupling

We consider a model for the two-dimensional electron gas formed at the interface of oxide heterostructures, which includes a Rashba spin-orbit coupling proportional to the electric field perpendicular to the interface. Based on the standard mechanism of polarity catastrophe, we assume that the electric field is proportional to the electron density. Under these simple and general assumptions, we show that a phase separation instability occurs for realistic values of the spin-orbit coupling and of the band parameters. This could provide an intrinsic mechanism for the recently observed inhomogeneous phases at the LaAlO_3/SrTiO_3 or LaTiO_3/SrTiO_3 interfaces.

cond-mat.supr-con↗

arXiv2012

Multiple Quantum Phase Transitions at the superconducting LaTiO3/SrTiO3 interface

We study the magnetic field driven Quantum Phase Transition (QPT) in electrostatically gated superconducting LaTiO3/SrTiO3 interfaces. Through finite size scaling analysis, we show that it belongs to the (2+1)D XY model universality class. The system can be described as a disordered array of superconducting islands coupled by a two dimensional electron gas (2DEG). Depending on the 2DEG conductance tuned by the gate voltage, the QPT is single (corresponding to the long range phase coherence in the whole array) or double (one related to local phase coherence, the other one to the array). By retrieving the coherence length critical exponent ν, we show that the QPT can be "clean" or "dirty" according to the Harris criteria, depending on whether the phase coherence length is smaller or larger than the island size. The overall behaviour is well described by a theoretical approach of Spivak et al., in the framework of the fermionic scenario of 2D superconducting QPT.

cond-mat.supr-con↗