Relationships between the superconducting gap, pseudogap and transition temperature in high-Tc superconductors (original) (raw)
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Universal correlations, pseudo‐gaps and condensation mechanisms in high‐Tc superconductors
1997
We combine universal correlations between Tc and ns/m * (superconducting carrier density/effective mass) found by µSR with the pseudo gap behavior found in the underdoped region by various techniques to develop a picture for explaining doping dependence of high-Tc superconductivity in terms of a crossover from Bose-Einstein to BCS condensation. µSR results in overdoped and Zn-substituted systems suggest that behavior on the high-density side is more complicated than expected in a simple BCS model.
Superconducting gap and pseudogap
Low Temperature Physics, 2006
The discovery of the pseudogap has been a fundamental advance in uncovering the new physics of the high-Tc cuprates, yet its meaning is still far from being clear. In particular, its relation to the superconducting gap remains an object of controversy. While many authors consider that it is a high-temperature precursor of superconductivity, which turns into the superconducting gap at low temperatures, others contend that it is a normal-state property related only indirectly to superconductivity. We review a number of experiments such as single-particle tunneling, Andreev–Saint-James reflections, and others, and conclude that in the underdoped regime there exists considerable evidence for the existence of two distinct energy scales, the superconducting gap and the pseudogap, which appear to merge into one another in overdoped samples.
Fermi-condensate quantum phase transition in high-T c superconductors
Journal of Experimental and Theoretical Physics Letters, 2001
The effect of a quantum phase transition associated with the appearance of fermionic condensation in an electron liquid on the properties of superconductors is considered. It is shown that the electron system in both superconducting and normal states exhibits characteristic features of a quantum protectorate after the point of this Fermi-condensate quantum phase transition. The single-particle spectrum of a superconductor can be represented by two straight lines corresponding to two effective masses and . The mass characterizes the spectrum up to the binding energy E 0 , which is of the order of the superconducting gap in magnitude, and determines the spectrum at higher binding energies. Both effective masses are retained in the normal state; however, E 0 Ӎ 4 T. These results are used to explain some remarkable properties of high-T c superconductors and are in good agreement with recent experimental data. © 2001 MAIK "Nauka/Interperiodica".
Temperature dependence of superconducting gap of heavy fermion system
Physica C: Superconductivity, 2000
We present the first detailed data of the momentum-resolved, temperature dependence of the superconducting gap of Bi 2 Sr 2 Ca 1 Cu 2 O 8+x , complemented by similar data on the intensity of the photoemission superconducting condensate spectral area. The gap anisotropy between the Γ−M and Γ−X directions increases markedly with increasing temperature, contrary to what happens for conventional anisotropic-gap superconductors such as lead. Specifically, the size of the superconducting gap along the Γ − X direction decreases to values indistinguishable from zero at temperatures for which the gap retains virtually full value along the Γ −M direction.
Crossover temperatures in the normal-state phase diagram of high-T_ {c} superconductors
Physical Review B, 1998
The widespread reference to phase diagrams for the high-T c superconductors that involve two crossover temperatures T L and T U extending across the underdoped normal-state region is critically examined in the light of NMR and heat capacity data. We conclude that, in both the qϭ(0,0) and qϭ(,) response, there is only one crossover temperature, best described as the pseudogap energy scale rather than a well-defined onset temperature. This has a strong, linearly decreasing dependence on hole concentration. The lower crossover temperature, T L , commonly determined from 63 T 1 T, is shown to be an artefact of convoluting the pseudogap energy scale with an intrinsic linear T dependence arising, in the nearly antiferromagnetic Fermi-liquid model, from the characteristic spin-fluctuation frequency, sf. We conclude that models constructed on the twocrossover temperatures scenario need to be reexamined. ͓S0163-1829͑98͒00338-5͔
Physical Review B, 1998
The existence of a normal state spectral gap in underdoped cuprates raises important questions about the associated superconducting phase. For example, how does this pseudogap evolve into its below Tc counterpart? In this paper we characterize this unusual superconductor by investigating the nature of the "residual" pseudogap below Tc and, find that it leads to an important distinction between the superconducting excitation gap and order parameter. Our approach is based on a conserving diagrammatic BCS Bose-Einstein crossover theory which yields the precise BCS result in weak coupling at any T < Tc and reproduces Leggett's results in the T = 0 limit. We explore the resulting experimental implications.
Journal of Physics and Chemistry of Solids, 2002
In the last few years evidence has been accumulating that there are a multiplicity of energy scales which characterize superconductivity in the underdoped cuprates. In contrast to the situation in BCS superconductors, the phase coherence temperature Tc is different from the energy gap onset temperature T * . In addition, thermodynamic and tunneling spectroscopies have led to the inference that the order parameter ∆sc is to be distinguished from the excitation gap ∆; in this way, pseudogap effects persist below Tc. It has been argued by many in the community that the presence of these distinct energy scales demonstrates that the pseudogap is unrelated to superconductivity. In this paper we show that this inference is incorrect. We demonstrate that the difference between the order parameter and excitation gap and the contrasting dependences of T * and Tc on hole concentration x and magnetic field H follow from a natural generalization of BCS theory. This simple generalized form is based on a BCS-like ground state, but with self consistently determined chemical potential in the presence of arbitrary attractive coupling g. We have applied this mean field theory with some success to tunneling, transport, thermodynamics and magnetic field effects. We contrast the present approach with the phase fluctuation scenario and discuss key features which might distinguish our precursor superconductivity picture from that involving a competing order parameter.
A potential justifying superconductivity and pseudogap formation in high-Tc superconductors
AIP Advances, 2019
Copper and Iron based high temperature superconductors exhibit d-wave type superconducting gap and order parameter and posses a universal phase diagram. Here a potential is introduced that accounts for high temperature superconductivity, justifies d-wave symmetric behavior, and successfully explains phase diagram's salient features. This potential is stipulated by principles of special relativity and arises from the difference between the electric potential of moving electrons and the potential of stationary nuclei. In quasi-two-dimensional materials this difference results in an uncompensated angular dependent attraction force in preferred directions of motion and a repulsion force in the perpendicular directions. The attraction force causes d-wave angular dependent superconducting gap and order parameter at high temperatures for d or p orbitals, which are the orbitals involved in Copper and Iron based superconductors. The repulsion force justifies the existence of angular dependent pseudogap and since the attraction and repulsions forces confine electrons to two directions of motion the number of allowed momentum states are reduced resulting in anti-ferromagnetic Mott-insulator behavior. The combination of the attraction and repulsion forces is shown to create charge density waves in these quasi-two-dimensional materials. This potential is able to justify the main features of the universal phase diagram self-consistently.
Critical Doping in Overdoped High-Tc Superconductors: a Quantum Critical Point?
physica status solidi (b), 1999
Evidence is presented from the scaling of the Knight shift, entropy and transport properties together with the sharp peaking of condensation energy, critical currents, superfluid density and a variety of other physical properties for the occurrence of a common critical doping point in lightly overdoped high-T c superconductors (HTS). This critical doping lies at the point where the doping-dependent normal-state pseudogap energy, E g , falls to zero and bears a strong, though incomplete, resemblance to a quantum critical point (QCP). A QCP scenario could lead directly to an explanation of the non-Fermi liquid behaviour of the normal-state metallic phase and the overall generic behaviour of the HTSC.