ORAL SESSION 2A2: HTS Theory II

Tuesday, Feb. 22, 10:15 a.m. – 12:30 p.m., Room 303A (GRB)

Chairs: S. Maekawa (IMR/Tohoku U), C. Di Castro (U Rome)

2A2.1 Quantum Phase Fluctuation in High T_c Superconductors

B.K. Chakraverty, LEPES/C.N.R.S., B.P. 166, Grenoble, 38042 France

Presenting Author: B.K. Chakraverty

Superconducting order parameter has an amplitude and a phase. In the ground state the phase has to be blocked in all space. It has been suggested that the supeconducting transition temperature T_c in the oxide materials is related to the loss of long range phase coherence. Beyond T_c the system lives, unlike classical superconductors, in an ill understood mixture of Cooper pairs that have a nonzero ampliude and a highly spatially fluctuating phase. As T goes to zero the time variation or the quantum phase fluctuatin becomes important. It is of great interest to understand the role that dynamics of the phase fluctuation plays in bringing about depletion of the superconducting condensate. By calculating the phase correlation function we have shown that in two dimension a linear depletion of the superfluid density as T goes to zero is expected from quantum fluctuation alone [a].

[a] B.K. Chakraverty, cond-mat/9907035.

2A2.2 Coupling to Spin Excitations Determined from Optical Data

J.P. Carbotte and E. Schachinger, Technische Universitaet Graz, Institut fur Theoretical Physics, Graz, Austria

Presenting Author: E. Schachinger

We analyse optical conductivity data as a function of energy to obtain from it information on the coupling of the charge carriers to spin excitations. For optimally doped YBCO, in the superconducting state, a signature of the so called 41 meV spin polarized neutron resonance is seen as is the opening of the superconducting energy gap which has the effect of shifting the derived spin fluctuation structure to higher energy. Also, the coupling to the spin excitations the absolute value of which is determined directly in our approach, is found to be large enough to account for the superconductivity and for the observed value of the critical temperature T_c. In the underdoped case, the spin resonance is found even above T_c and is also shifted by a pseudogap. Charge carrier coupling to the recently observed spin resonance in Bi2212 close to optimum doping is also seen. For optimally doped Tl2201 with T_c = 90K and for YBCO124 the corresponding resonance is found to be relatively broader and for YBCO124 extends over 50 meV. As yet no neutron data exist in these two cases. By way of contrast, in Tl2201 with a T_c value reduced to 23K, no superconducting state resonance is observed and consequently the optical scattering rates look much more like those predicted for an ordinary Fermi liquid with underlying temperature independent spin fluctuation spectral density.

2A2.3 Theoretical Study of Underdoped Cuprates: Cooper-Pair Phase Fluctuations

K.H. Bennemann and Dirk Manske, Institute for Theoretical Physics, Freie Universität Berlin, D-14195 Berlin, Germany

Presenting Author: K.H. Bennemann

We study Cooper-pair phase fluctuations in cuprate superconductors for a spin fluctuation pairing interaction. Using an electronic theory we calculate in particular for the underdoped cuprate superconductors the superfluid density n_s (T), the superconducting transition temperature T_c(x) µ n_s below which phase coherent Cooper-pairs occur, and T_c*(x) where the phase incoherent Cooper-pairs disappear. Also we present results for the penetration depth l (x,T) and for T*(x) at which a gap structure occurs in the spectral density. A Meißner effect is obtained only for T < T_c. We find that phase fluctuations become increasingly important in the underdoped regime and lead to characteris-tic changes in the spectral density.

2A2.4 D-XY-Critical Behavior of Cuprate Superconductors

T. Schneider, Physik-Institut Universität Zürich, Winterthurerstr. 190, CH 8057 Zürich, Switzerland

Presenting Author: T. Schneider

We outline the universal and finite temperature critical properties of the 3D-XY - model, extended to anisotropic extreme type II superconductors, as well as the universal quantum critical properties in 2D. On this basis we review: (1) the mounting evidence for 3D-XY - behavior in optimally doped cuprate superconductors and the 3D to 2D crossover in the underdoped regime; (2) the finite size limitations imposed by inhomogeneities; (3) the experimental evidence for a 2D-XY - quantum critical point in the underdoped limit, where the superconductor to insulator transition occurs; (4) the emerging implications and constraints for microscopic models.

2A2.5 The Sign of D-wave Pairing: Current Anomalies in High T_c Superconductors

J.A. Sauls, Department of Physics and Astronomy, Northwestern University, 2145 Sheridan Rd., Evanston, IL 60208

Presenting Author: J.A. Sauls

Quasiparticle 'jets' are predicted to develop at low temperatures, k_B T << v_f p_s < ½ D _max½ , in superconductors with d-wave symmetry. These jets form in response to a magnetic field or injected current, are carried by quasiparticle states in the vicinity of the nodes of an unconventional order parameter, and give rise to unique signatures of the order parameter symmetry. Low-temperature anomalies in the current response also originate from surface-induced Andreev bound states. These states are observable in both the transverse (Meissner) and longitudinal(Tunneling) current response. The theory of these current anomalies predicts new effects in the surface impedance and tunneling characteristics.

Collaborators: M. Fogelstrom, D. Rainer, S. Yip.