S. Caprara

Resonant and crossover phenomena in a multiband superconductor: Tuning the chemical potential near a band edge

Resonances in the superconducting properties, in a regime of crossover from BCS to mixed Bose-Fermi superconductivity, are investigated in a two-band superconductor where the chemical potential is tuned near the band edge of the second miniband generated by quantum confinement effects. The shape resonances at T=0 in the superconducting gaps belonging to the class of Feshbach-like resonances is manifested by interference effects in the superconducting gap at the first large Fermi surface when the chemical potential is in the proximity of the band edge of the second miniband. The case of a superlattice of quantum wells is considered and the amplification of the superconducting gaps at the Lifshitz transition of the type neck-collapsing of Fermi surface topology is clearly shown. The results are found to be in good agreement with available experimental data on a superlattice of honeycomb boron layers intercalated by Al and Mg spacer layers.

Multiple quantum criticality in a two-dimensional superconductor.

The diverse phenomena associated with the two-dimensional electron gas (2DEG) that occurs at oxide interfaces include, among others, exceptional carrier mobilities, magnetism and superconductivity. Although these have mostly been the focus of interest for potential future applications, they also offer an opportunity for studying more fundamental quantum many-body effects. Here, we examine the magnetic-field-driven quantum phase transition that occurs in electrostatically gated superconducting LaTiO3/SrTiO3 interfaces. Through a 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 puddles coupled by a 2DEG and, depending on its conductance, the observed critical behaviour 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). A phase diagram illustrating the dependence of the critical field on the 2DEG conductance is constructed, and shown to agree with theoretical proposals. Moreover, by retrieving the coherence-length critical exponent ν, we show that the quantum critical behaviour can be clean or dirty according to the Harris criterion, depending on whether the phase-coherence length is smaller or larger than the size of the puddles.

Gap and pseudogap evolution within the charge-ordering scenario for superconducting cuprates

Abstract:We describe the spectral properties of underdoped cuprates as resulting from a momentum-dependent pseudogap in the normal-state spectrum. Such a model accounts, within a BCS approach, for the doping dependence of the critical temperature and for the two-parameter leading-edge shift observed in the cuprates. By introducing a phenomenological temperature dependence of the pseudogap, which finds a natural interpretation within the stripe quantum-critical-point scenario for high- Tc superconductors, we reproduce also the Tc – T* bifurcation near optimum doping. Finally, we briefly discuss the different role of the gap and the pseudogap in determining the spectral and thermodynamical properties of the model at low temperatures.

The stripe critical point for cuprates

The experimental determination of the quantum critical point (QCP) that triggers the self-organization of charged striped domains in cuprate perovskites is reported. The phase diagram of doped cuprate superconductors is determined by a first variable, the hole doping ?, and a second variable, the micro-strain ? of the Cu-O bond length, obtained from the Cu K-edge extended x-ray absorption fine structure. For a fixed optimum doping, ?c = 0.16, we show the presence of the QCP for the onset of local lattice distortions and stripe formation at the critical micro-strain ?c. The critical temperature Tc(?,?) reaches its maximum at the quantum critical point (?c,?c) for the formation of bubbles of superconducting stripes. The critical charge, orbital and spin fluctuations near this strain QCP provide the interaction for the pairing.

Anomalous Isotopic Effect Near the Charge-Ordering Quantum Criticality

Within the Hubbard-Holstein model, we evaluate the 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 find that their isotopic dependence, due to critical fluctuations, implies a substantial positive shift of the pseudogap-formation temperature T(*). We infer that the isotopic shift of the superconducting T(c) is nearly absent in the optimally and overdoped regimes and is negative and increasing upon underdoping. The dynamical nature of the charge-ordering transition may explain the spread of the experimental values of T(*).

Shape resonance for the anisotropic superconducting gaps near a Lifshitz transition: the effect of electron hopping between layers

Multigap superconductivity modulated by quantum confinement effects in a superlattice of quantum wells is presented. Our theoretical BCS approach captures the low-energy physics of a shape resonance in the superconducting gaps when the chemical potential is tuned near a Lifshitz transition. We focus on the case of weak Cooper pairing coupling channels and strong pair exchange interaction driven by repulsive Coulomb interaction that allows us to use the BCS theory in the weak-coupling regime neglecting retardation effects, like in quantum condensates of ultracold gases. The calculated matrix element effects in the pairing interaction are shown to yield a complex physics near the particular quantum critical points due to Lifshitz transitions in multigap superconductivity. Strong deviations of the ratio 2Δ/Tc from the standard BCS value as a function of the position of the chemical potential relative to the Lifshitz transition point measured by the Lifshitz parameter are found. The response of the condensate phase to the tuning of the Lifshitz parameter is compared with the response of ultracold gases in the BCS–BEC crossover tuned by an external magnetic field. The results provide the description of the condensates in this regime where matrix element effects play a key role.

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 has a contribution proportional to the electron density. Under these simple and general assumptions, we show that a phase separation instability (signaled by a negative compressibility) occurs for realistic values of the spin-orbit coupling and of the electronic band-structure parameters. This provides an intrinsic mechanism for the inhomogeneous phases observed at the LaAlO(3)/SrTiO(3) or LaTiO(3)/SrTiO(3) interfaces.

Do we have a consistent non-adiabatic quantum-classical mechanics?

Quantum-classical equations of motion satisfying Jacobi identity were recently proposed by V. V. Kisil (Europhys. Lett., 72 (2005) 873), generalizing the Heisenberg group formulation of quantum mechanics. In the attempt to provide a physical interpretation for the resulting quantum-classical dynamics, we have reconstructed and analyzed the derivation, finding that there is a suitable representation from which the purely classical nature of the supposedly quantum-classical equations of motion clearly emerges.

Charge-fluctuation contribution to the Raman response in superconducting cuprates.

We calculate the Raman response contribution due to soft collective modes, finding a strong dependence on the photon polarizations and on the characteristic wave vectors of the modes. We compare our results with recent Raman spectroscopy experiments in underdoped cuprates, La2-xSrxCuO4 and (Y1.97Ca0.3)Ba2CuO6.05, 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 can naturally arise from charge collective excitations at finite wavelength.

SINGLE-PARTICLE PROPERTIES OF A MODEL FOR COEXISTING CHARGE AND SPIN QUASICRITICAL FLUCTUATIONS COUPLED TO ELECTRONS

We study the single-particle spectral properties of a model for coexisting AFM and ICDW critical fluctuations coupled to electrons, which naturally arises in the context of the stripe-quantum-critical-point scenario for high-Tc superconducting materials. Within a perturbative approach, we show that the on-shell inverse scattering time deviates from the normal Fermi-liquid behavior near the points of the Fermi surface connected by the characteristic wave-vectors of the critical fluctuations (hot spots). The anomalous behavior is stronger when the hot spots are located near singular points of the electronic spectrum. The violations to the normal Fermi-liquid behavior are associated with the transfer of spectral weight from the quasi-particle peak to incoherent shadow peaks, which produces an enhancement of incoherent spectral weight near the Fermi level. We use our results to discuss recent ARPES experiments on Bi2212 near optimal doping.