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S. Caprara

Publications and source records attributed to S. Caprara.

At least 73 records · Page 4Linked to original sources
arXiv2006

Spectroscopic evidences of quantum critical charge fluctuations in cuprates

We calculate the optical conductivity in a clean system of quasiparticles coupled to charge-ordering collective modes. The absorption induced by these modes may produce an anomalous frequency and temperature dependence of low-energy optical absorption in some cuprates. However, the coupling with lattice degrees of freedom introduces a non-universal energy scale leading to scaling violation in low-temperature optical conductivity.

cond-mat.str-el↗
arXiv2005

Extended paraconductivity regime in underdoped cuprates

We reconsider transport experiments in strongly anisotropic superconducting cuprates and we find that universal Aslamazov-Larkin (AL) paraconductivity in two dimensions is surprisingly robust even in the underdoped regime below the pseudogap crossover temperature T^*. We also establish that the underlying normal state resistivity in the pseudogap phase is (almost) linear in temperature, with all the deviations being quantitatively accounted by AL paraconductivity. The disappearence of paraconductivity is governed by the disappearence of gaussian pair fluctuations at an energy scale related to T^*.

cond-mat.supr-con↗
arXiv2005

Charge-fluctuation contribution to the Raman response in superconducting cuprates

We calculate the Raman response contribution due to collective modes, finding a strong dependence on the photon polarizations and on the characteristic wavevectors of the modes. We compare our results with recent Raman spectroscopy experiments in underdoped cuprates, $La_{2-x}Sr_xCuO_4$ and $(Y_{1.97}Ca_{0.3})Ba_2CuO_{6.05}$, where anomalous low-energy peaks are observed, which soften upon lowering the temperature. We show that the specific dependence on doping and on photon polarizations of these peaks is only compatible with charge collective excitations at finite wavelength.

cond-mat.supr-con↗
arXiv2004

Charge critical fluctuations in cuprates: Isotope effect, pseudogap, conductivity, and Raman spectroscopy

Within the stripe quantum critical point theory for high T_c superconductors, we point out that there is a direct contribution of charge collective fluctuations to the optical absorption and to the Raman spectra. In this latter case, we find that the critical charge collective modes can or can not be excited depending on the direction of their wavevector and on the polarization of the incoming and outgoing photons. This indicates a marked distinction between quasiparticles which are strongly and weakly coupled to critical collective modes and provides a direct confirmation that the order associated to the quantum critical point near optimal doping of cuprates occurs at finite wavevectors.

cond-mat.supr-con↗
arXiv2004

The low-energy phase-only action in a superconductor: a comparison with the XY model

The derivation of the effective theory for the phase degrees of freedom in a superconductor is still, to some extent, an open issue. It is commonly assumed that the classical XY model and its quantum generalizations can be exploited as effective phase-only models. In the quantum regime, however, this assumption leads to spurious results, such as the violation of the Galilean invariance in the continuum model. Starting from a general microscopic model, in this paper we explicitly derive the effective low-energy theory for the phase, up to fourth-order terms. This expansion allows us to properly take into account dynamic effects beyond the Gaussian level, both in the continuum and in the lattice model. After evaluating the one-loop correction to the superfluid density we critically discuss the qualitative and quantitative differences between the results obtained within the quantum XY model and within the correct low-energy theory, both in the case of s-wave and d-wave symmetry of the superconducting order parameter. Specifically, we find dynamic anharmonic vertices, which are absent in the quantum XY model, and are crucial to restore Galilean invariance in the continuum model. As far as the more realistic lattice model is concerned, in the weak-to-intermediate-coupling regime we find that the phase-fluctuation effects are quantitatively reduced with respect to the XY model. On the other hand, in the strong-coupling regime we show that the correspondence between the microscopically derived action and the quantum XY model is recovered, except for the low-density regime.

cond-mat.supr-con↗
arXiv2004

Doping-driven transition to a time-reversal breaking state in the phase diagram of the cuprates

Motivated by recent tunnelling and Andreev-reflection experiments, we study the conditions for a quantum transition within the superconducting phase of the cuprates,in which a bulk imaginary (time-reversal breaking) $id_{xy}$component appears in addition to the $d_{x^2 - y^2}$ order parameter. We examine in detail the role of some important physical features of the cuprates.In particular we show that a closed Fermi surface,a bilayer splitting, an orthorhombic distortion,and the proximity to a quantum critical point around optimal doping favor the appearance of the imaginary component. These findings could explain why the mixed $d_{x^2 - y^2}+ id_{xy}$ order parameter is observed in YBCO and LSCO, and suggest that it could appear also in Bi2212. We also predict that, in all cuprates, the mixed state should be stable only in a limited doping region all contained beneath the $d_{x^2 - y^2}$ dome. The behavior of the specific heat at the secondary transition is discussed.

cond-mat.supr-con↗
arXiv2003

Renormalization group and Ward identities in quantum liquid phases and in unconventional critical phenomena

By reviewing the application of the renormalization group to different theoretical problems, we emphasize the role played by the general symmetry properties in identifying the relevant running variables describing the behavior of a given physical system. In particular, we show how the constraints due to the Ward identities, which implement the conservation laws associated with the various symmetries, help to minimize the number of independent running variables. This use of the Ward identities is examined both in the case of a stable phase and of a critical phenomenon. In the first case we consider the problems of interacting fermions and bosons. In one dimension general and specific Ward identities are sufficient to show the non-Fermi-liquid character of the interacting fermion system, and also allow to describe the crossover to a Fermi liquid above one dimension. This crossover is examined both in the absence and presence of singular interaction. On the other hand, in the case of interacting bosons in the superfluid phase, the implementation of the Ward identities provides the asymptotically exact description of the acoustic low-energy excitation spectrum, and clarifies the subtle mechanism of how this is realized below and above three dimensions. As a critical phenomenon, we discuss the disorder-driven metal-insulator transition in a disordered interacting Fermi system. In this case, through the use of Ward identities, one is able to associate all the disorder effects to renormalizations of the Landau parameters. As a consequence, the occurrence of a metal-insulator transition is described as a critical breakdown of a Fermi liquid.

cond-mat.stat-mech↗
arXiv2003

Quantum phase transition in easy-axis antiferromagnetic integer-spin chains

Antiferromagnetic Heisenberg integer-spin chains are characterized by a spin-liquid ground state with no long-range order, due to the relevance of quantum fluctuations. Spin anisotropy, however, freezes quantum fluctuations, and the system is magnetized in the presence of a sufficiently large easy-axis anisotropy. We numerically investigate the case S=1, by means of the density-matrix renormalization group, and find that the freezing of the spin liquid into a Néel spin solid, with increasing easy-axis anisotropy, is a continuous quantum phase transition. Numerical evidence indicates that the transition is not in the two-dimensional Ising universality class.

cond-mat.str-el↗
arXiv2002

Time Reversal Breaking Superconducting State in the Phase Diagram of the Cuprates

We review and extend a previous study on the symmetry of the superconducting state, stimulated by recent tunneling and Andreev reflection measurements giving robust evidences for the existence of a $d_{x^2-y^2}+id_{xy}$ order parameter in the overdoped regime of two different cuprates. Looking for a possible second-order phase transition from a standard $d_{x^2-y^2}$ to a mixed and time reversal breaking state, we confirm the results of our previous analysis on La$_{2-x}$Sr$_{x}$CuO$_{4}$. In the case of Y$_{1-y}$Ca$_y$Ba$_2$Cu$_3$O$_{7-x}$ as well, among all the allowed symmetries, the $d_{x^2-y^2}+id_{xy}$ is the most favored one and the unconventional state is likely to occur in a small dome of the phase diagram located in the optimal-overdoped region and at very low temperatures.

cond-mat.supr-con↗
arXiv2001

Coherence length in superconductors from weak to strong coupling

We study the evolution of the superconducting coherence length $ξ_0$ from weak to strong coupling, both within a s-wave and a d-wave lattice model. We show that the identification of $ξ_0$ with the Cooper-pair size $ξ_{pair}$ in the weak-coupling regime is meaningful only for a fully-gapped (e.g., s-wave) superconductor. Instead in the d-wave superconductor, where $ξ_{pair}$ diverges, we show that $ξ_0$ is properly defined as the characteristic length scale for the correlation function of the modulus of the superconducting order parameter. The strong-coupling regime is quite intriguing, since the interplay between particle-particle and particle-hole channel is no more negligible. In the case of s-wave pairing, which allows for an analytical treatment, we show that $ξ_0$ is of order of the lattice spacing at finite densities. In the diluted regime $ξ_0$ diverges, recovering the behavior of the coherence length of a weakly interacting effective bosonic system. Similar results are expected to hold for d-wave superconductors.

cond-mat.supr-con↗
arXiv2001

Strong correlation, electron-phonon interaction and critical fluctuations: Isotope effect, pseudogap formation, and phase diagram of the cuprates

Within the Hubbard-Holstein model with long-range Coulomb forces, we revisit the charge-ordering scenario for the superconducting cuprates and account for the presence or the absence of a formed stripe phase in different classes of cuprates. We also evaluate the mean-field and the fluctuation-corrected critical lines for charge ordering and we relate them with the various pseudogap crossover lines occurring in the cuprates and we discuss a mechanism for their peculiar isotopic dependence. Considering the dynamical nature of the charge-ordering transition, we explain the spread of T* and of its isotopic shift, obtained with experimental probes with different characteristic time scales.

cond-mat.supr-con↗
arXiv2001

Single-particle spectra near a stripe instability

We analyze the single-particle spectra of a bi-layered electron system near a stripe instability and compare the results with ARPES experiments on the Bi2212 cuprate superconductor near optimum doping, addressing also the issue of the puzzling absence of bonding-antibonding splitting.

cond-mat.supr-con↗
arXiv2001

Anomalous optical absorption in overdoped cuprates near the charge-ordering instability

We propose an interpretation for the hump observed in the optical conductivity at or below a few hundreds of cm$^{-1}$, in overdoped cuprates like the electron-doped Nd_{2-x}Ce_xCuO_{4-y} at x\gtrsim 0.15 and the hole-doped Bi_2Sr_2CuO_6 and La_{2-x}Sr_xCuO_4. This interpretation is based on the direct excitation of charge collective modes, which become nearly critical in the proximity to a charge-ordering instability. The nearly critical character of these excitations entails a peculiar temperature dependence and a pseudo-scaling form of the lineshapes, which are in agreement with the experimental data.

cond-mat.supr-con↗
arXiv2001

Antiferromagnetic integer-spin chains in a staggered magnetic field: approaching the thermodynamic limit through the infinite-size DMRG

We investigate the behavior of antiferromagnetic integer-spin chains in a staggered magnetic field, by means of the density-matrix renormalization group, carefully addressing the role of finite-size effects within the Haldane phase at small fields. In the case of spin S=2, we determine the dependence of the groundstate energy and magnetization on the external field, in the thermodynamic limit, and show how the peculiar finite-size behavior can be connected with the crossover in the groundstate from a spin liquid to a polarized Néel state.

cond-mat.str-el↗
arXiv2001

Phase fluctuations in superconductors: from Galilean invariant to quantum XY models

We analyze the corrections to the superfluid density due to phase fluctuations within both a continuum and a lattice model for $s$- and d-wave superconductors. We expand the phase-only action beyond the Gaussian level and compare our results with the quantum XY model both in the quantum and in the classical regime. We find new dynamic anharmonic vertices, absent in the quantum XY model, which are responsible for the vanishing of the correction to the superfluid density at zero temperature in a continuum (Galilean invariant) model. Moreover the phase-fluctuation effects are reduced with respect to the XY model by a factor at least of order $1/(k_Fξ_0)^2$.

cond-mat.supr-con↗
arXiv2001

Anomalous isotopic effect near the charge-ordering quantum criticality

Within the Hubbard-Holstein model, we evaluate the various crossover lines marking the opening of pseudogaps in the cuprates, which, in our scenario, are ruled by the proximity to a charge-ordering quantum criticality (stripe formation). We provide also an analysis of their isotopic dependencies, as produced by critical fluctuations. We find no isotopic shift of the temperature $T^0$ marked as a reduction of the quasiparticle density of states in various experiments, and a substantial positive shift of the pseudogap-formation temperature $T^*$. We infer that the superconducting critical temperature $T_c$ has almost no shift in the optimally- and overdoped regimes while it has a small negative isotopic shift in the underdoped, which increses upon underdoping. We account also for the possible dynamical nature of the charge-ordering transition, and explain in this way the spread of the values of $T^*$ and its of isotopic shift, obtained with experimental probes having different characteristic timescales.

cond-mat.supr-con↗
arXiv2001

Charge-ordering quantum criticality in the phase diagram of the cuprates

We discuss how the quantum-critical-point scenario for high-T_c superconductors with density-driven charge order or stripe formation finds support from recent EXAFS experiments. Our phase diagram has the interesting feature, which is also suggested by the EXAFS experiments, that some cuprates may not show well-formed stripe phase. We also consider the extensions of the scenario to include the interesting possibility of first-order transitions which call for further experimental and theoretical investigation.

cond-mat.supr-con↗