Unusual Superconducting State of Underdoped Cuprates (original) (raw)
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Nature Physics, 2006
The superconducting temperature T c of hole-doped hightemperature superconductors has a dome-like shape as a function of hole concentration, with a maximum T c at 'optimal' doping. On the underdoped side, the superconducting state is often described in terms of one energy scale, associated with the maximum of the d-wave gap (at the antinodes), which increases as the doping decreases. Here, we report electronic Raman scattering experiments that show a second energy scale in the gap function: the slope of the gap at the nodes, which decreases with decreasing doping. Our measurements also reveal two distinct quasiparticle dynamics; electronic coherence persists down to low doping levels at the nodes, whereas antinodal quasiparticles become incoherent. Using a sum-rule, we find that the low-frequency Raman response and the temperature dependence of the superfluid density, both controlled by nodal excitations, behave in a qualitatively similar manner with doping variation.
Low-energy quasiparticles in cuprate superconductors: A quantitative analysis
Physical Review B, 2000
A residual linear term is observed in the thermal conductivity of optimally-doped Bi2Sr2CaCu2O8 at very low temperatures whose magnitude is in excellent agreement with the value expected from Fermi-liquid theory and the d-wave energy spectrum measured by photoemission spectroscopy, with no adjustable parameters. This solid basis allows us to make a quantitative analysis of thermodynamic properties at low temperature and establish that thermally-excited quasiparticles are a significant, perhaps even the dominant mechanism in suppressing the superfluid density in cuprate superconductors Bi2Sr2CaCu2O8 and YBa2Cu3O7.
Unconventional superconducting gap in underdoped cuprates
2010
A generic theory of the quasi-particle superconducting gap in underdoped cuprates is derived in the strong coupling limit, and found to describe extremely well the experimental "second gap" in absolute scale. In drastic contrast to the standard theories of Bogoliubov quasi-particle excitations, the quasi-particle gap is shown to originate from anomalous kinetic process, completely unrelated to the pairing strength. Furthermore, the k-dependence of the gap deviates significantly from the pure d x 2 −y 2-wave of the order parameter. Our study reveals a new paradigm for the nature of superconducting gap, and is expected to reconcile numerous apparent contradictions among existing experiments toward a more coherent understanding of high-temperature superconductivity.
Two-gap model for underdoped cuprate superconductors
Physical Review B, 2000
Various properties of underdoped superconducting cuprates, including the momentum-dependent pseudogap opening, indicate a behavior which is neither BCS nor Bose-Einstein condensation (BEC) like. To explain this issue we introduce a two-gap model. This model assumes an anisotropic pairing interaction among two kinds of fermions with small and large Fermi velocities representing the quasiparticles near the M and the nodal points of the Fermi surface respectively. We find that a gap forms near the M points resulting into incoherent pairing due to strong fluctuations. Instead the pairing near the nodal points sets in with phase coherence at lower temperature. By tuning the momentum-dependent interaction, the model allows for a continuous evolution from a pure BCS pairing (in the overdoped and optimally doped regime) to a mixed boson-fermion picture (in the strongly underdoped regime).
Theory of Quasiparticles in the Underdoped High-T_ {c} Superconducting State
1998
The microscopic theory of superconducting (SC) state in the SU (2) slave-boson model is developed. We show how the pseudogap and Fermi surface (FS) segments in the normal state develop into a d-wave gap in the superconducting state. Even though the superfluid density is of order x (the doping concentration), the physical properties of the low lying quasiparticles are found to resemble those in BCS theory. Thus the microscopic theory lay the foundation for our earlier phenomenological discussion of the unusual SC properties in the underdoped cuprates.
High Temperature Superconductivity in Cuprates: a model
A model is proposed such that quasi-particles (electrons or holes) residing in the CuO 2 planes of cuprates may interact leading to metallic or superconducting behaviors. The metallic phase is obtained when the quasi-particles are treating as having classical kinetic energies and the superconducting phase occurs when the quasi-particles are taken as extremely relativistic objects. The interaction between both kinds of particles is provided by a force dependent-on-velocity. In the case of the superconducting behavior, the motion of apical oxygen ions provides the glue to establish the "Cooper pair". The model furnishes explicit relations for the Fermi velocity, the perpendicular and the in-plane coherence lengths, the zero-temperature energy gap, the critical current density, the critical parallel and perpendicular magnetic fields. All these mentioned quantities are expressed in terms of fundamental physical constants as: charge and mass of the electron, light velocity in vacuum, Planck constant, electric permittivity of the vacuum. Numerical evaluation of these quantities show that their values are close those found for the superconducting YBaCuO, leading to think the model as being a possible scenario to explain superconductivity in cuprates. 1-INTRODUCTION Since the discovery of the high temperature superconductivity in copper oxides (cuprates) by Bednorz and Müller [1], a great amount of theoretical work has been dedicated to understand the mechanism behind this phenomenon. One of the first trying to elucidate this puzzle was proposed by Anderson through the resonant-valence-bond model [2]. Another model [3], due to Emery, assumes that the charge carriers are holes in the O(2p) states and the pairing is mediated by strong coupling to local spin configurations in Cu sites. Emery [3] used an extended Hubbard model in order to describe the main features of this mechanism. Meanwhile Plakida et al [4] explained the high-temperature transition in perovskite-type oxides within the framework of the non-harmonic model for superconductors with structurally unstable lattices. In the model of Plakida and collaborators [4] the highly non-harmonic motion is written in terms of a pseudo-spin representation through a Transverse Ising Model and the interaction of the electrons with the non-harmonic ions vibrations is also described in terms of this pseudo-spin representation. Two opposite views of the superconductivity in cuprates have been disputed by Anderson and Schrieffer. Anderson [2] attributes the novel phenomenology present on cuprates materials to a second kind of metallic state, namely, the Luttinger liquid. Schrieffer [5] has pursued the interplay between anti-ferromagnetism and superconductivity, extending the BCS pairing theory beyond the Fermi-liquid regime in terms of spin polarons or "bags". According to Cox and Maple [6] superconductivity in heavy-fermion materials and high-Tc cuprates may involve electron pairing with unconventional symmetries and mechanisms.
PHYSICAL REVIEW B, 2003
Heat transport in the cuprate superconductors YBa2Cu3Oy and La2−xSrxCuO4 was measured at low temperatures as a function of doping. A residual linear term κ0/T is observed throughout the superconducting region and it decreases steadily as the Mott insulator is approached from the overdoped regime. The low-energy quasiparticle gap extracted from κ0/T is seen to scale closely with the pseudogap. The ubiquitous presence of nodes and the tracking of the pseudogap shows that the overall gap remains of the pure d-wave form throughout the phase diagram, which excludes the possibility of a complex component (ix) appearing at a putative quantum phase transition and argues against a non-superconducting origin to the pseudogap. A comparison with superfluid density measurements reveals that the quasiparticle effective charge is weakly dependent on doping and close to unity.
Physical Review B, 1999
We derive in detail a novel solution of the spin fermion model which is valid in the quasi-static limit pi T<<omega_sf, found in the intermediate (pseudoscaling) regime of the magnetic phase diagram of cuprate superconductors, and use it to obtain results for the temperature and doping dependence of the single particle spectral density, the electron-spin fluctuation vertex function, and the low frequency dynamical spin susceptibility. The resulting strong anisotropy of the spectral density and the vertex function lead to the qualitatively different behavior of_hot_ (around k=(pi,0)) and_cold_ (around k=(pi/2,pi/2)) quasiparticles seen in ARPES experiments. We find that the broad high energy features found in ARPES measurements of the spectral density of the underdoped cuprate superconductors are determined by strong antiferromagnetic (AF) correlations and incoherent precursor effects of an SDW state, with reduced renormalized effective coupling constant. The electron spin-fluctuation vertex function, i.e. the effective interaction of low energy quasiparticles and spin degrees of freedom, is found to be strongly anisotropic and enhanced for hot quasiparticles; the corresponding charge-fluctuation vertex is considerably diminished. We thus demonstrate that, once established, strong AF correlations act to reduce substantially the effective electron-phonon coupling constant in cuprate superconductors.
Doping dependence of thermodynamic properties in cuprate superconductors
Physica C: Superconductivity, 2012
The doping and temperature dependence of the thermodynamic properties in cuprate superconductors is studied based on the kinetic energy driven superconducting mechanism. By considering the interplay between the superconducting gap and normal-state pseudogap, the some main features of the doping and temperature dependence of the specific-heat, the condensation energy, and the upper critical field are well reproduced. In particular, it is shown that in analogy to the domelike shape of the doping dependence of the superconducting transition temperature, the maximal upper critical field occurs around the optimal doping, and then decreases in both underdoped and overdoped regimes. Our results also show that the humplike anomaly of the specific-heat near superconducting transition temperature in the underdoped regime can be attributed to the emergence of the normal-state pseudogap in cuprate superconductors.
Pseudogap phenomena in the superconducting phase of the cuprates
1999
The presence of a normal state spectral (pseudo) gap at the superconducting transition temperature in the underdoped cuprates has important implications for the associated superconducting phase. We argue that this normal state pseudogap derives from pairing fluctuations, which necessarily persist below ¢ ¡ and which have important implications on the superconducting state. Our Greens function approach, based on the equation of motion method, can be viewed as a a natural extension of BCS theory for sufficiently strong pairing interaction, suggested by the short coherence length of the cuprates. In addition to the usual fermionic excitations, there are also incoherent (but not pre-formed) pairs of finite center of mass momentum which must be self consistently incorporated in computing ¢ ¡ and other superconducting properties, such as the superfluid density and the Josephson critical current. Finally, we discuss some of the experimental implications of our theory for the cuprates.