ORAL SESSION 5DL7: Pseudo Gap II

Friday, Feb. 25, 9:00 a.m. – 11:00 a.m., Arboretum 3-4 (Hyatt)

5DL7.1 Spin-charge separation: From one hole to finite doping

Z.Y. Weng, D.N. Sheng, and C.S. Ting, TCSUH, Univ. of Houston, Houston, TX 77204-5932

Presenting Author: Z.Y. Weng

Quasiparticle properties are discussed in an effective theory of the t-J model which includes two important components: spin-charge separation and unrenormalizable phase shift. We show that the phase shift effect indeed causes the system to be a non-Fermi liquid as conjectured by Anderson on a general ground. But this phase shift also drastically changes a conventional perception of quasiparticles in a spin-charge separation state: an injected hole will remain stable but incoherent due to the confinement of spinon and holon by the phase shift field, despite that the background remains a spinon-holon sea. Only in the zero-doping limit a bare hole will lose its integrity and decay into holon and spinon elementary excitations. The Fermi surface structure is completely different in these two cases, from a large band-structure-like one to four Fermi points in one-hole case, and we show that the so-called underdoped regime actually corresponds to a situation in between, where the ``gap-like'' effect is further amplified by a microscopic phase separation at low temperature.

5DL7.2 Normal and superconducting state anomalies of electron properties in the high-T_c cuprates. Comparison with ARPES and tunneling spectroscopy

Flora Onufrieva and Pierre Pfeuty, LLB, CEA-Saclay, 91191 Gif-sur-Yvette, France

Presenting Author: F. Onufrieva

Based on a microscopical theory developed in [1] we study electronic properties of the hole doped high-T_c cuprates in the normal and superconducting state. An agreement with experiments (angle resolved photoemission (ARPES), tunneling spectroscopy) is remarkable. We explain the normal state pseudogap phenomena including such details observed by ARPES as the anomalous form of the spectral function as a function of energy, the flat form of the "spectrum", the destruction of Fermi surface in a proximity of (0,p ) wavevector, the temperature independence of the pseudogap and its increasing with decreasing doping etc. [2]. We also explain the peak-dip-hump structure of the spectral function in the superconducting state seen by ARPES and tunneling as well as the doping dependences of the peak, dip and hump energies.

[1] F. Onufrieva et al, PRL 82 2370 (1999).

[2] F. Onufrieva and P. Pfeuty PRL, 82 3136 (1999).

5DL7.3 High Energy Secondary Peak Structure (Hump) in Tunneling Spectra as Possible Magnetic Pseudogap

John F. Zasadzinski 1, L. Ozyuzer 2, D. Hinks 2, K.E. Gray 2, N. Miyakawa 3, and C. Kendziora 4. 1 Physics Dept. Illinois Institute of Technology, Chicago, IL 60616. 2 Argonne National Laboratory, Argonne, IL 60439. 3 Science University of Tokyo, Tokyo, Japan. 4 Naval Research Laboratory, Washington, D.C. 20375, USA.

Presenting Author: J.F. Zasadzinski

Tunneling spectra out to high bias voltages are reported for several cuprate superconductors including Bi₂Sr₂CaCu2O₈ (Bi2212) and La_2-xSr_xCuO₄ (La214). In addition to the principal conductance peaks at the superconducting gap it is shown that a reproducible secondary peak structure (hump) is found in all the tunneling spectra that scales approximately as 3 times the gap. We attribute this hump feature to a pseudogap of magnetic origin. The hump feature in Bi2212 has the same magnitude and doping dependence as found in ARPES measurements where the dispersion of the hump was found to be consistent with an incipient SDW gap. Also, the tunneling hump in La214 is consistent with the so-called high energy pseudogap found in angle integrated photoemission. The unusual T dependence of the energy gap near T_c found in a number of tunneling experiments is explained as a consequence of two distinct contributions to the pseudogap.

5DL7.4 Electron-electron interactions in underdoped HTSC

Julien Bok and Jacqueline Bouvier, Thermodynamique Statistique, ESPCI. 10, rue Vauquelin. 75231 Paris Cedex 05, France

Presenting Author: J. Bok

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 reference 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 reference 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)*v_`*l , where v_F 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 have also estimated the electron relaxation time which varies like (1/D) in 2d. It becomes very short in the directions of the SP, this explains why the quasiparticle excitations at the Fermi surface disappear first in ARPES experiments along these directions.

[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.