arXiv subjects

L. Benfatto

Publications and source records attributed to L. Benfatto.

At least 37 records · Page 2Linked to original sources

arXiv2015

Amplitude, density and current correlations of strongly disordered superconductors

We investigate the disorder dependence of the static density, amplitude and current correlations within the attractive Hubbard model supplemented with on-site disorder. It is found that strong disorder favors a decoupling of density and amplitude correlations due to the formation of superconducting islands. This emergent granularity also induces an enhancement of the density correlations on the SC islands whereas amplitude fluctuations are most pronounced in the 'insulating' regions. While density and amplitude correlations are short-ranged at strong disorder we show that current correlations have a long-range tail due to the formation of percolative current paths in agreement with the constant behavior expected from the analysis of one-dimensional models.

cond-mat.supr-con↗

arXiv2015

Massless Dirac cones in graphene: experiments and theory

The opening of a gap in single-layer graphene is often ascribed to the breaking of the equivalence between the two carbon sublattices. We show by angle-resolved photoemission spectroscopy that Ir- and Na-modified graphene grown on the Ir(111) surface presents a very large unconventional gap that can be described in terms of a phenomenological "massless" Dirac model. We discuss the consequences and differences of this model in comparison of the standard massive gap model, and we investigate the conditions under which such anomalous gap can arise from a spontaneous symmetry breaking.

cond-mat.mes-hall↗

arXiv2015

Nature and Raman signatures of the Higgs amplitude mode in the coexisting superconducting and charge-density-wave state

We investigate the behavior of the Higgs (amplitude) mode when superconductivity emerges on a preexisting charge-density-wave state. We show that the weak overdamped square-root singularity of the amplitude fluctuations in a standard BCS superconductor is converted in a sharp, undamped power-law divergence in the coexisting state, reminiscent of the Higgs behavior in Lorentz-invariant theories. This effect reflects in a strong superconducting resonance in the Raman spectra, both for an electronic and a phononic mechanism leading to the Raman visibility of the Higgs. In the latter case, our results are relevant to the interpretation of the Raman spectra measured experimentally in NbSe2.

cond-mat.supr-con↗

arXiv2014

Anisotropy of the superconducting fluctuations in multiband superconductors: the case of LiFeAs

Between the different families of pnictide multiband superconductors, LiFeAs is probably one of the less understood. Indeed, despite the large amount of experiments performed in the last few years on this material, no consensus has been reached yet on the possible pairing mechanism at play in this system. Here we focus on the precursor effects of superconductivity visible in the transport experiments performed above Tc. By analyzing the superconducting fluctuations in a layered multiband model appropriate for this material, we argue that the strong two-dimensional character of the paraconductivity above Tc points towards a significant modulation of the pairing interactions along the z direction. We also discuss the peculiar differences between single-band and multi-band superconductors for what concerns the anisotropy of the superconducting-fluctuations effects above and below Tc.

cond-mat.supr-con↗

arXiv2014

Optical excitation of phase modes in strongly disordered superconductors

According to the Goldstone theorem the breaking of a continuous U(1) symmetry comes along with the existence of low-energy collective modes. In the context of superconductivity these excitations are related to the phase of the superconducting (SC) order parameter and for clean systems are optically inactive. Here we show that for strongly disordered superconductors phase modes acquire a dipole moment and appear as a subgap spectral feature in the optical conductivity. This finding is obtained with both a gauge-invariant random-phase approximation scheme based on a fermionic Bogoliubov-de Gennes state as well as with a prototypical bosonic model for disordered superconductors. In the strongly disordered regime, where the system displays an effective granularity of the SC properties, the optically active dipoles are linked to the isolated SC islands, offering a new perspective for realizing microwave optical devices.

cond-mat.supr-con↗

arXiv2014

Leggett modes in iron-based superconductors as a probe of Time Reversal Symmetry Breaking

Since their discovery, it has been suggested that pairing in pnictides can be mediated by spin fluctuations between hole and electron bands. In this view, multiband superconductivity would substantially differ from other systems like MgB2, where pairing is predominantly intraband. Indeed, interband-dominated pairing leads to the coexistence of bonding and antibonding superconducting channels. Here we show that this has profound consequences on the nature of the low-energy superconducting collective modes. In particular, the so-called Leggett mode for phase fluctuations is absent in the usual two-band description of pnictides. On the other hand, when also the repulsion between the hole bands is taken into account, a more general three-band description should be used, and a Leggett mode is then allowed. Such a model, that has been proposed for strongly hole-doped 122 compounds, can also admit a low-temperature s+is phase which breaks the time reversal symmetry. We show that the (quantum and thermal) transition from the ordinary superconductor to the s+is state is accompanied by the vanishing of the mass of Leggett-like phase fluctuations, regardless the specific values of the interaction parameters. This general result can be obtained by means of a generalized construction of the effective action for the collective degrees of freedom that allows us also to deal with the non-trivial case of dominant interband pairing.

cond-mat.supr-con↗

arXiv2013

Universal scaling of the order-parameter distribution in strongly disordered superconductors

We investigate theoretically and experimentally the statistical properties of the inhomogeneous order-parameter distribution (OPD) at the verge of the superconductor-insulator transition (SIT). We find within two prototype fermionic and bosonic models for disordered superconductors that one can identify a universal rescaling of the OPD. By performing scanning-tunneling microscopy experiments in three samples of NbN with increasing disorder we show that such a rescaling describes also with an excellent accuracy the experimental data. These results can provide a breakthrough in our understanding of the SIT.

cond-mat.supr-con↗

arXiv2013

Robustness of the Berezinskii-Kosterlitz-Thouless Transition in Ultrathin NbN Films near the Superconductor-Insulator Transition

Occurrence of the Berezinskii-Kosterlitz-Thouless (BKT) transition is investigated by superfluid density measurements for two-dimensional (2D) disordered NbN films with disorder level very close to a superconductor-insulator transition (SIT). Our data show a robust BKT transition even near this 2D disorder-tuned quantum critical point (QCP). This observation is in direct contrast with previous data on deeply underdoped quasi-2D cuprates near the SIT. As our NbN films approach the QCP, the vortex-core energy, an important energy scale in the BKT transition, scales with the superconducting gap, not with the superfluid density, as expected within the standard 2D-XY model description of BKT physics.

cond-mat.supr-con↗

arXiv2012

Charged-phonon theory and Fano effect in the optical spectroscopy of bilayer graphene

Since their discovery, graphene-based systems represent an exceptional playground to explore the emergence of peculiar quantum effects. The present paper focuses on the anomalous appearence of strong infrared phonon resonances in the optical spectroscopy of bilayer graphene and on their pronounced Fano-like asymmetry, both tunable in gated devices. By developing a full microscopic many-body approach for the optical phonon response we explain how both effects can be quantitatively accounted for by the quantum interference of electronic and phononic excitations. We show that the phonon modes borrow a large dipole intensity from the electronic background, the so-called charged-phonon effect, and at the same time interfer with it, leading to a typical Fano response. Our approach allows one to disentangle the correct selection rules that control the relative importance of the two (symmetric and antisymmetric) relevant phonon modes for different values of the doping and/or of the gap in bilayer graphene. Finally, we discuss the extension of the same theoretical scheme to the Raman spectroscopy, to explain the lack of the same features on the Raman phononic spectra. Besides its remarkable success in explaining the existing experimental data in graphene-based systems, the present theoretical approach offers a general scheme for the microscopic understanding of Fano-like features in a wide variety of other systems.

cond-mat.mes-hall↗

arXiv2012

Unconventional Hall effect in pnictides from interband interactions

We calculate the Hall transport in a multiband systems with a dominant interband interaction between carriers having electron and hole character. We show that this situation gives rise to an unconventional scenario, beyond the Boltzmann theory, where the quasiparticle currents dressed by vertex corrections acquire the character of the majority carriers. This leads to a larger (positive or negative) Hall coefficient than what expected on the basis of the carrier balance, with a marked temperature dependence. Our results explain the puzzling measurements in pnictides and they provide a more general framework for transport properties in multiband materials.

cond-mat.str-el↗

arXiv2012

Beresinskii-Kosterlitz-Thouless transition within the sine-Gordon approach: the role of the vortex-core energy

One of the most relevant manifestations of the Beresinskii-Kosterlitz-Thouless transition occurs in quasi-two-dimensional superconducting systems. The experimental advances made in the last decade in the investigation of superconducting phenomena in low-dimensional correlated electronic systems raised new questions on the nature of the BKT transitions in real materials. A general issue concerns the possible limitations of theoretical predictions based on the XY model, that was studied as a paradigmatic example in the original formulation. Here we review the work we have done in revisiting the nature of the BKT transition within the general framework provided by the mapping into the sine-Gordon model. While this mapping was already known since long, we recently emphasized the advantages on such an approach to account for new variables in the BKT physics. One such variable is the energy needed to create the core of the vortex, that is fixed within the XY model, while it attains substantially different values in real materials. This has interesting observable consequences, especially in the case when additional relevant perturbations are present, as a coupling between stacked two-dimensional superconducting layers or a finite magnetic field.

cond-mat.supr-con↗

arXiv2012

Current-current Fermi-liquid corrections to the superconducting fluctuations on conductivity and diamagnetism

We analyze the behavior of the superconducting-fluctuations contribution to diamagnetism and conductivity in a model system having current-current interactions. We show that in proximity to a Mott-insulating phase one recovers an overall suppression of the fluctuating contribution to the conductivity with respect to diamagnetism, in close analogy with recent experiments on the underdoped phase of cuprate superconductors.

cond-mat.supr-con↗

arXiv2011

Role of the vortex-core energy on the Beresinkii-Kosterlitz-Thouless transition in thin films of NbN

We analyze the occurrence of the Beresinkii-Kosterlitz-Thouless transition in thin films of NbN at various film thickness, by probing the effect of vortex fluctuations on the temperature dependence of the superfluid density below $T_{BKT}$ and of the resistivity above $T_{BKT}$. By direct comparison between the experimental data and the theory we show the crucial role played by the vortex-core energy in determining the characteristic signatures of the BKT physics, and we estimate its dependence on the disorder level. Our work provides a paradigmatic example of BKT physics in a quasi-two-dimensional superconductor.

cond-mat.supr-con↗

arXiv2011

Superfluid density and phase relaxation in superconductors with strong disorder

As a prototype of a disordered superconductor we consider the attractive Hubbard model with on-site disorder. We solve the Bogoljubov-de-Gennes equations on two-dimensional finite clusters at zero temperature and evaluate the electromagnetic response to a vector potential. We find that the standard decoupling between transverse and longitudinal response does not apply in the presence of disorder. Moreover the superfluid density is strongly reduced by the relaxation of the phase of the order parameter already at mean-field level when disorder is large. We also find that the anharmonicity of the phase fluctuations is strongly enhanced by disorder. Beyond mean-field, this provides an enhancement of quantum fluctuations inducing a zero-temperature transition to a non-superconducting phase of disordered preformed pairs. Finally, the connection of our findings with the glassy physics for extreme dirty superconductors is discussed.

cond-mat.supr-con↗

arXiv2011

Skyrmions in a Doped Antiferromagnet

Magnetization and magnetoresistance have been measured in insulating antiferromagnetic La_{2}Cu_{0.97}Li_{0.03}O_{4} over a wide range of temperatures, magnetic fields, and field orientations. The magnetoresistance step associated with a weak ferromagnetic transition exhibits a striking nonmonotonic temperature dependence, consistent with the presence of skyrmions.

cond-mat.str-el↗

arXiv2011

Effective medium theory for superconducting layers: A systematic analysis including space correlation effects

We investigate the effects of mesoscopic inhomogeneities on the metal-superconductor transition occurring in several two-dimensional electron systems. Specifically, as a model of systems with mesoscopic inhomogeneities, we consider a random-resistor network, which we solve both with an exact numerical approach and by the effective medium theory. We find that the width of the transition in these two-dimensional superconductors is mainly ruled by disorder rather than by fluctuations. We also find that "tail" features in resistivity curves of interfaces between LaAlO3 or LaTiO3 and SrTiO3 can arise from a bimodal distribution of mesoscopic local Tc's and/or substantial space correlations between the mesoscopic domains.

cond-mat.supr-con↗

arXiv2011

Effects of Fermi-surface shrinking on the optical sum rule in pnictides

In this paper we investigate the effects of the band shifts induced by the interband spin-fluctuation coupling on the optical sum rule in pnictides. We show that, despite the shrinking of the Fermi surfaces with respect to first-principle calculations, the charge-carrier concentration in each band is almost unchanged, with a substantial conservation of the total optical sum rule. However, a significant transfer of spectral weight occurs from low-energy coherent processes to incoherent ones, with practical consequences on the experimental estimate of the sum rule, that is carried out integrating the data up to a finite cut-off. This has profound consequences both on the absolute value of the sum rule and on its temperature dependence, that must be taken into account while discussing optical experiments in these systems.

cond-mat.supr-con↗

arXiv2011

Extended Drude model and role of interband transitions in the mid-infrared spectra of pnictides

We analyze the outcomes of an extended-Drude-model approach to the optical spectra of pnictides, where the multiband nature of the electronic excitations requires a careful analysis of the role of interband processes in the optical conductivity.Through a direct comparison between model calculations of the intraband optical spectra and experimental data, we show that interband transitions,whose relevance is shown by first-principle calculations,give a non negligible contribution already in the infrared region. This leads to a substantial failure of the extended-Drude-model analysis on the measured optical data without subtraction of interband contributions.

cond-mat.supr-con↗