Session P2: The Electronic Properties of Overdoped Cuprates: The Clean Gateway to High-Tc Superconductivity

The origin of anomalous transport in a high temperature superconductor

The metallic state of high-temperature superconductors is anomalous in that the Hall coefficient is strongly temperature dependent while the resistivity varies linearly in temperature over a wide temperature range. Although this $T$-linear resistivity gradually weakens with doping, crucially it survives until superconductivity is destroyed. Both the superconducting pairing interaction and the origin of this anomalous transport have yet to be determined, though most theoretical approaches consider them to be intrinsically linked. Through novel analysis of polar angular magnetoresistance oscillations, we have succeeded to determine the full temperature and momentum dependence of the mean free path of the charge carriers in highly doped Tl$_{2}$Ba$_{2}$CuO$_{6+\delta }$ ($T_{c}$ = 15K) up to 60K. From this, we have been able to identify the origin of the $T$-linear resistivity \textit{and }the temperature dependence of the Hall coefficient for this particular compound. Given the correlation between the appearance of the $T$-linear resistivity and the onset of superconductivity, this additional scattering is also a prime candidate for the pairing mechanism for high temperature superconductivity itself\textbf{. }   

Nodal-antinodal quasiparticle anisotropy reversal in the overdoped high-T$_{c}$ cuprates.

The cuprate superconductors can be tuned through a remarkable progression of states of matter by doping charge carriers into CuO$_{2}$ planes. The most generic feature of this tuning is a sequence from a Mott antiferromagnetic insulator, to the d-wave superconductor at intermediate doping, and eventually to an overdoped metal which is widely believed to be described by Fermi liquid theory. Of these three, the testing of Fermi liquid theory in the overdoped regime has been particularly hampered by a lack of compounds suitable for a wide range of experimental techniques. Important breakthroughs could come from the study of Tl$_{2}$Ba$_{2}$CuO$_{6+\delta }$ (Tl2201), a clean and structurally simple system with a very high T$_{c}$, whose natural doping range extends from optimal to extreme overdoping as one varies the oxygen content. Recent success in high-purity single crystal growth [1] gave us the opportunity of performing the first extensive ARPES study of the low-energy electronic structure of heavily overdoped Tl2201, which reveals a novel phenomenology: contrary to the case of under and optimally-doped cuprates, quasiparticles are sharp near ($\pi $,0), i.e. the antinodal region where the gap is maximum, and broad at ($\pi $/2, $\pi $/2), i.e. the nodal region where the gap vanishes [1,2]. This reversal of the nodal-antinodal quasiparticle anisotropy across optimal doping and its relevance to scattering, many-body, and quantum-critical phenomena in the high-Tc cuprate superconductors, is discussed. [1] D.C. Peets \textit{et al}., cond-mat/0211028 (2002); [2] M. Plat\'{e} \textit{et al}., PRL \textbf{95}, 077001 (2005).   

Scanning tunneling spectroscopy studies of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ from the strongly underdoped to strongly overdoped regime

Using atomically resolved scanning tunneling microscopy (STS), we investigate the electronic structure Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ across a range of doping levels from x $\sim $ 0.1 up to as high as $\sim $0.23, with significant changes in electronic structure observed above p$\sim $0.21. New sample preparation processes [1] were used to produce heavily overdoped crystals suitable for the imaging of various forms of electronic heterogeneity. The evolution of the gap map $\Delta $(r), coherence peak height map A(r), the inelastic tunneling signatures $\omega $(r), and the quasiparticle interference LDOS modulations, as well as their interrelations across this range of doping levels, will be presented. \newline \newline Additional authors: J. Lee, M. Wang, Laboratory of Atomic and Solid State Physics, Department of Physics, Cornell University, Ithaca, NY 14853, U.S.A; K. Fujita, Department of Advanced Materials Science, University of Tokyo, Tokyo 113-0033, Japan; H. Eisaki, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Central 2, Umezono, Tsukuba, Ibaraki 305-8568; S. Uchida, Department of Physics, University of Tokyo, Tokyo 113-0033; and J. C. Davis, Laboratory of Atomic and Solid State Physics, Department of Physics, Cornell University. \newline \newline [1] J. Slezak, K. Fujita, J. C. Davis, in preparation (2005)   

Evolution of superconducting gap and metallic ground state in cuprates from transport

We report on fundamental characteristics of the ground state of cuprates in the limit of T=0, for both normal and superconducting states, obtained from transport measurements on high-quality single crystals of YBCO and Tl-2201, as a function of hole concentration. The superconducting gap is extracted from thermal conductivity; it is found to scale with the superconducting transition temperature throughout the overdoped regime, with a gap-to-Tc ratio of 5 [1]. The normal state is accessed by suppressing superconductivity with magnetic fields up to 60 T and is characterized by the limiting behavior of its electrical resistivity; while carrier localization is observed in YBCO at low temperature for carrier concentrations p below 0.1 hole/planar Cu, at p=0.1 and above the material remains highly metallic down to T=0 [2]. This shows that the non-superconducting state of underdoped cuprates, deep in the pseudogap phase, is remarkably similar to that of strongly overdoped cuprates, e.g. at p=0.3. We compare these results with similar measurements on other cuprates and discuss their implication for our understanding of the cuprate phase diagram. [1] In collaboration with: D.G. Hawthorn, S.Y. Li, M. Sutherland, E. Boaknin, R.W. Hill, C. Proust, F. Ronning, M. Tanatar, J. Paglione, D. Peets, R. Liang, D.A. Bonn, W.N. Hardy, and N.N. Kolesnikov. [2] In collaboration with: C. Proust, M. Sutherland, N. Doiron- Leyraud, S.Y. Li, R. Liang, D.A. Bonn, W.N. Hardy, N.E. Hussey, S. Adachi, S. Tajima, J. Levallois, and M. Narbone.   

Optical properties of Cuprates in the Normal and superconducting state.

For superconducting materials it is interesting and important to determine the kinetic energy of the conduction electrons, $<<$H$>>_{T}$, because its behavior as a function of temperature dependence, in particular at the superconducting phase transition, provides a direct and profound insight in the mechanisms by which the superconducting phase is stabilized. The intra-band optical spectral weight, W(T), is, apart from a minus sign, closely related to the kinetic energy[1]. With modern optical techniques it is possible to measure W(T) very accurately as a function of temperature. Over the past few years several teams have reported that by the superconducting phase transition affects the optical conductivity over an energy range of several electron Volts[2-8]. Some of these results were accurate enough to determine the effect of superconductivity on W(T). Here we present new optical data for a large number of underdoped and optimally doped samples of various compositions. In order to clearly distinguish the effect of the superconducting phase transition from other temperature dependencies, we use a dense sampling of temperatures (1 spectrum every Kelvin) over a broad range of temperatures and frequencies. All our data support that the change at Tc of W(T) parallel to the CuO2-planes is opposite to the trend expected from the BCS prediction. For strongly overdoped samples the observed behavior of W(T) in the normal state and in the superconducting state is qualitatively different compared to underdoped and optimally doped superconductors. \newline \newline [1] P- F. Maldague, Phys. Rev. 16, 2437 (1977). \newline [2] M. J. Holcomb, et al., Phys. Rev. Lett. 73, 2360 (1994). \newline [3] D. N. Basov et al., Science 283, 49 (1999). \newline [4] H. J. A. Molegraaf et al., Science 295 (2002) 2239 \newline [5] A. F. Santander-Syro et al., Europhys. Lett 62 (2003) 568 \newline [6] V. Boris et al., Science 304, 708 (2004). \newline [7] B. Kuzmenko et al., Phys. Rev. B 72 (2005) 144503 \newline [8] M. Ortolani et al., Phys. Rev. Lett. 94 (2005) 067002.