1PO1-20 Quasiparticle thermal conductivity in the vortex state of unconventional superconductors
I. Vekhter, Department of Physics, University of Guelph, Guelph, Ontario, Canada N1G 2W1, and A. Houghton, Department of Physics, Brown University, Providence, RI 02912-1843, USA
Presenting Author: I. Vekhter
We present the results of a microscopic calculation of the quasiparticle thermal conductivity of a d-wave superconductor in the mixed state.
We consider a two-dimensional d-wave superconductor with a cylindrical Fermi surface in a magnetic field applied perpendicular to the basal plane. In the framework of a quasiclassical approximation we compute the field dependence of the thermal conductivity at different temperatures for the thermal gradient in the plane. We find that, in addition to the standard impurity scattering contribution, there exists a contribution to the thermal transport lifetime due to scattering off the vortices, which has the same symmetry at the Fermi surface as the gap function. Consequently, at low temperatures when most of the quasiparticles are near the gap nodes it plays a minor role and the primary effect of the field is to increase the density of states (Volovik effect) and the thermal conductivity. In contrast, at higher temperatures the main effect of the vortices is to introduce a new scattering mechanism, and the thermal conductivity decreases sharply to a plateau-like feature at higher fields (a broad minimum). This behavior is in a qualitative agreement with experimental results on the high-Tc superconducting materials. We also discuss the predictions of the model for the thermal Hall coefficient.
1PO1-21 Depression of Tc due to fluctuations and Uemura's plot
M. Cyrot, Laboratoire Louis Neel, C.N.R.S., B.P. 166, 38042 Grenoble Cedex, France
Presenting Author: M.R. Cyrot
In short coherence length superconductors, fluctuations depress the critical temperature compared to mean field one. To evaluate this effect, we use the standard Hartree approximation on the fourth order term of the Ginzburg-Landau free energy functional. The following results are obtained. Strong reduction of Tc is possible, much larger than the usual estimate. Secondly, Tc becomes proportional to the inverse squared of the penetration depth for large one, which is the relation proposed by Uemura. In cuprates, we explain the large penetration depth, not by a small number of carriers, but by the electronic structure and an anisotropic pairing potential.
1PO1-22 Pseudo-gap and electron-electron interactions in underdoped HTSC
Jacqueline Bouvier and Julien Bok, Thermodynamique Statistique, ESPCI. 10, rue Vauquelin. 75231 Paris Cedex 05, France
Presenting Author: J. Bouvier
Electron-electron interactions are important in disordered metals, where the diffusion length L is small compared to the electron wavelength at the Fermi level1 (FL). This is the case in most of underdoped HTSC cuprates. Following the method of [ref 1] and by numerical computation, we show that particle-particle repulsion produces a dip and a gap in the density of states at the FL. This may explain the ‘pseudogap’ observed in many experiments. From [ref 1], the pseudogap has a width proportional to (1/D)3/2 in 3 dimension, and to (1/D) in 2d. D is the diffusion length given by (1/3) vFl , where vF is the Fermi velocity and l the mean free path. In anisotropic conductors like the cuprates, the Fermi velocity is small is the directions of the saddle points (SP) ((1,0,0) and equivalents). This explains why the pseudogap first appears in these directions. We present detailed calculations of this Coulomb gap using the true band structure of the cuprates. We use a self-consistent procedure to resolve the self-energy equation, including the Hartree-Fock term and the Coulomb interaction term. This leads to the exact position of the energy levels. We obtain a good agreement between our calculated variation of the ‘pseudogap’ with doping and the one observed in ARPES experiments.
[1] B.L. Altshuler and A.G. Aronov. "Electron-electron interactions in disordered systems.", ed. A.L. Efros and M. Pollak. Elsevier Science Publishers B.V. 1985.
1PO1-23 The Spin-Polaron Theory in the Finite-Temperature Green's Function Method as a Mechanism for High-Temperature Superconductivity
Danilo M. Yanga, National Inst. of Physics, College of Science, University of the Philippines-Diliman Campus, Quezon City, Philippines 1101 and Augusto A. Morales, Jr., Math Dept., College of Science, University of the Philippines-Diliman Campus, Quezon City, Philippines 1101
Presenting Author: D.M. Yanga
A finite-temperature Green's function method for the spin-polaron is constructed based on spin-charge separation. Certain physical quantities such as the thermodynamic potential, magnetic spin susceptibility, electrical conductivity, specific heat, and moment of the spectral function are subsequently calculated. This theory is envisioned to describe certain characteristics and structures of high-temperature superconductivity.
1PO1-24 Charged Vortex in the Charged Bose-liquid
Alexandre S. Alexandrov, Department of Physics, Loughborough University, Leics. LE11 3TU, United Kingdom
Presenting Author: A.S. Alexandrov
Equations describing a single vortex in the charged Bose liquid at zero temperature are derived. The zero-temperature coherence length, magnetic field penetration depth, vortex structure and energy, and lower critical field are calculated. The vortex differs from that in type II BCS superconductors or in neutral superfluids. Its core is charged, and there is an electric field inside the core. While the magnetic field profile is the same as in the BCS case, the electric field associated with the vortex could provide unique evidence for charged bosons (bipolarons) in the high-temperature superconductors.
1PO1-25 Nodal Quasiparticles versus Phase Fluctuations in High Tc Superconductors: An Intermediate Scenario*
Qijin Chen, Ioan Kosztin, and K. Levin, The James Franck Institute, The University of Chicago, Chicago, IL 60637
Presenting Author: Q. Chen
A highly debated question in high Tc superconductivity is whether the rich array of pseudogap phenomena can be attributed to fermionic quasi-particles (along the nodal directions) or to bosonic phase fluctuations, both of which are argued to persist above Tc. Here we argue that there exists a third scenario, in which a combination of both fermionic and bosonic excitations are associated with the pseudogap state (above and below Tc). This third scenario is based on the BCS -- Bose Einstein crossover picture, where we have shown [Phys. Rev. Lett. {81}, 4708 (1998)] that bosonic pair excitations play an increasingly important role, as the coupling strength increases. To lend support to this third scenario, we compute the superfluid density and specific heat as a function of low T and of doping concentration x, and show that the results are quantitatively consistent with experiment. We argue that the evidence for x dependent Landau parameters, recently invoked to explain these data, is less compelling than that for combined bosonic and fermionic excitations.
*Work supported by the National Science Foundation (DMR 91-20000) through the Science and Technology Center for Superconductivity.
1PO1-26 Theory of Impurity Induced Resonances in High-Tc
A.V. Balatsky , T-Div, LANL, Los Alamos, NM 87545, USA
Presenting Author: A.V. Balatsky
Scanning tunneling microscopy can provide a probe for the detailed study of quasiparticle states in high-Tc superconductors. It can also be used to acquire specific information about impurity-induced quasiparticle states and the order-parameter structure. In particular, the local density of states is found to be sensitive to impurity-induced resonances and to the symmetry of the superconducting order parameter. The four-fold spatial shape of impurity states and its energy dependence, previously predicted, will be compared with the recent STM results on high-Tc superconductors of the UC Berkeley group. Work done in collaboration with M.Salkola, D. Scalapino and J.R. Schrieffer.
*1PO1-27 Cooper Pair Dispersion Relation
Sadhan K. Adhikari 1, M. Casas 2, A. Puente 2, A. Rigo 2, M. Fortes 3, M.A. Solis 3, M. de Llano 4, Ariel A. Valladares 4, and O. Rojo 5. 1 Instituto de Física Teórica, Universidade Estadual Paulista, 01405-900 Sao Paulo, SP, Brazil. 2 Departament de Física, Universitat de les Illes Balears, 07071 Palma de Mallorca, Spain. 3 Instituto de Física, Universidad Nacional Autónoma de México, 01000 México, DF, México. 5 PESTyC, Secretaría Académica and CINVESTAV, IPN, 04430 México, DF, México.
Presenting Author: M. de Llano
Of crucial importance in a microscopic model of superconductivity based on Bose-Einstein condensation (BEC) of Cooper pairs is their correct energy vs. center-of-mass-momentum (CMM) relation. The BEC transition temperature in two dimensions (2D) is expected to be abnormally large for a linear relation and is well-known to vanish for the usual quadratic one. Cooper pairing in 2D and 3D is analyzed with a set of renormalized equations to determine its binding energy for any (charge-carrier) fermion number density and all coupling assuming a general short-range (even singular) pairwise residual interfermion interaction. Cooper pairs with non-zero CMM are also considered and their binding energy is readily expanded analytically in 2D in powers of the CMM up to the quadratic term. For either 2D or 3D in the weak-coupling (BCS) regime a term linear in the CMM dominates the "pair excitation energy" while in the strong-coupling (Bose) regime the strongly-bound composite-boson pair excitation energy is quadratic in the CMM, as expected. The linear to quadratic crossover of this CMM dependence is studied numerically as a function of coupling/density.
1PO1-28 Magnetisation and upper critical field in YBCO - a unified boson model
Chris J. Dent 1, Alexandre S. Alexandrov 1, and Viktor V. Kabanov 2. 1 Theory Group, Department of Physics, Loughborough University, Loughborough, Leicestershire LE11 3TU, United Kingdom. 2 Department of Solid State Physics, "Jozef Stefan" Institut, Jamova 39, 1001 Ljubljana, Slovenia.
Presenting Author: C.J. Dent
We have extended the charged Bose gas model for the upper critical field to also explain the crossing point in the curves of induced magnetisation divided by the square root of field against temperature in the less anisotropic cuprates. This model has already been shown to provide a parameter-free expression for Tc in a wide range of cuprates. We compare our results with experiment in YBa2Cu3O7.
1PO1-29 Relevance of the pair-pair interaction in the crossover from BCSsuperconductivity to Bose-Einstein condensation
P. Pieri and G.C. Strinati, Dipartimento di Matematica e Fisica, Sezione INFM, Universite di Camerino, I-62032, Camerino (MC), Italy
Presenting Author: P. Strinati
The evolution from BCS superconductivity to Bose-Einstein condensation has become recently of interest owing especially to photoemission data, which show the presence of a pseudo gap above the superconducting critical temperature in the cuprate materials.
In this context, the relevant energy scales can be associated with pair dissociation, pair center-of-mass motion, and pair-pair interaction. While the first two, which correspond to mean-field and Gaussian fluctuations, in the order, have been extensively treated in the literature, the pair-pair interaction has tentatively been treated only within the self-consistent fermionic T-matrix approximation for a "dilute" gas.
We have shown that, to treat the pair-pair interaction correctly for a "dilute" gas, the self-consistency of the fermionic T-matrix has actually to be disregarded and that an appropriate set of diagrams corresponding to a "dilute" Bose gas (made up of fermion pairs) has instead to be included.
To this end, we have developed a systematic mapping between the fermionic and bosonic diagrammatic structures and identified the appropriate fermionic self-energy which takes into account the pair-pair interaction in the "dilute" limit. The resulting effects on the pair-pair scattering length, the superconducting critical temperature, and the pseudo gap are evaluated and discussed.