Antonio Valletta

Superconductivity of a striped phase at the atomic limit

Abstract The resonant amplification of the superconducting critical temperature, the isotope effect, the change of the chemical potential in a particular 2D striped phase formed by superconducting stripes of width L alternated by separating stripes of width W with a period λ p at the atomic limit is studied. The critical temperature shows a `shape resonance by tuning the charge density where the chemical potential μ is in the range E n μ E n + ℏ ω 0 , where E n is the bottom of the n th superlattice subband for n >2, and ℏ ω 0 is the energy cutoff for the pairing interaction. The maximum critical superconducting temperature is reached at the cross-over from 2D to 1D behavior. The particular properties of this electronic phase and its similarities with the normal and superconducting properties of doped cuprate perovskites are discussed.

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.

High Tc superconductivity in a superlattice of quantum stripes

Abstract The superconducting gap for 2D electron gas in a superlattice of quantum stripes with a finite 1D periodic potential barrier is calculated. The resonant enhancement of the superconducting gap is found when the Fermi level is tuned near the bottom of the nth superlattice subband with n ≥ 2. The critical charge density ϱc for maximum Tc at the n = 2 resonance is mainly determined by the stripe width L and vice versa, while it is nearly independent on the amplitude of the potential barrier. The width of the resonance is determined by the interaction cutoff h ω 0 and its shape by the ratio ΔE n⊥ h ω 0 , where ΔEn⊥ is the transversal energy dispersion of the n subband due to the transverse hopping between stripes.

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.

Anomalous isotope effect near a 2.5 Lifshitz transition in a multi-band multi-condensate superconductor made of a superlattice of stripes

The doping dependent isotope effect on the critical temperature (Tc) is calculated for multi-band multi-condensate superconductivity near a 2.5 Lifshitz transition. We consider a superlattice of quantum stripes with finite hopping between stripes near a 2.5 Lifshitz transition for the appearance of a new sub-band making a circular electron-like Fermi surface pocket. We describe a particular type of BEC (Bose?Einstein Condensate) to BCS (Bardeen?Cooper?Schrieffer condensate) crossover in multi-band/multi-condensate superconductivity at a metal-to-metal transition that is quite different from the standard BEC?BCS crossover at an insulator-to-metal transition. The results show that the isotope coefficient strongly deviates from the standard BCS value 0.5, when the chemical potential is tuned at the 2.5 Lifshitz transition for the metal-to-metal transition. The critical temperature Tc shows a minimum due to the Fano antiresonance in the superconducting gaps and the isotope coefficient diverges at the point where a BEC coexists with a BCS condensate. In contrast Tc reaches its maximum and the isotope coefficient vanishes at the crossover from a polaronic condensate to a BCS condensate in the newly appearing sub-band.

Evidence for onset of charge density wave in the La-based Perovskite superconductors

Recent studies have shown that structural instabilities play a vital role in high-Tc superconductivity. We have used a local structural probe, extended X-ray absorption fine structure (EXAFS), to investigate the instability of the CuO2 plane in La-based high-Tc perovskite systems. Temperature-dependent CuK-edge EXAFS spectra measured on single crystals of La1.85Sr0.15CuO4 (LSCO) and La2CuO4.1 (LCO) show a characteristic temperature (Tls) where the local structure of the system diverges from the average one. This temperature coincides with a crossover temperature where the system shows anomalous metal-insulator transition in transport measurements. We have discussed the EXAFS results as an evidence for a particular phase transition of the system with segregation of localized and itinerant charge carriers in a short coherence charge density wave (CDW). A phase diagram for this CDW has been proposed.

The isotope effect in a superlattice of quantum stripes

We have calculated the isotope effect coefficient and the critical temperature in a particular superconducting heterostructure formed by a bidimensional (2D) superlattice of metallic quantum stripes. We model the superlattice with a 2D free electron gas confined in a ID periodic potential barrier. Within the BCS theory we calculate the shape resonance for the critical temperature and the isotope coefficient as a function of the chemical potential. The results provide a good description of the anomalous behavior of the isotope coefficient in La{2-x}SrxCuO4 as a function of doping with a sharp peak α ~0.8 in the underdoped regime and a rapid drop to α ~0.2 at 0.125 holes per copper site.

Shape Resonance at a Lifshitz Transition for High Temperature Superconductivity in Multiband Superconductors

We discuss the shape resonance in the superconducting gaps of a two-band superconductor by tuning the chemical potential at a Lifshitz transition for Fermi surface neck collapsing and for spot appearing. The high temperature superconducting scenario for complex matter shows the coexistence of a first BCS condensate made of Cooper pairs in the first band and a second boson-like condensate made of bosons like bipolarons, in the second band where the chemical potential is tuned near a Lifshitz transition. The interband coupling controls the shape resonance in the pair exchange between the two condensates. We discuss the particular BCS–Bose crossover that occurs at the shape resonance tuning the Lifshitz parameter (the energy difference between the chemical potential and the Lifshitz topological transition) like tuning the external magnetic field for the Feshbach resonances in ultracold gases. This superconducting phase provides a particular case of topological superconductivity with multiple condensates of different winding numbers.

Possible Fano resonance for high-Tc multi-gap superconductivity in p-Terphenyl doped by K at the Lifshitz transition

Recent experiments have reported the emergence of high temperature superconductivity with critical temperature

Shape Resonances in superconducting gaps in a 2DEG at oxide- oxide interface

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