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Register a new userDoping dependence of superconducting gap in YBa_2Cu_3O_y from universal heat transport
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Johnpierre Paglione
Publication date
2003
fields
Physics
and research's language is
English
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Thermal transport in the T -> 0 limit was measured as a function of doping in high-quality single crystals of the cuprate superconductor YBa_2Cu_3O_y. The residual linear term kappa_0/T is found to decrease as one moves from the overdoped regime towards the Mott insulator region of the phase diagram. The doping dependence of the low-energy quasiparticle gap extracted from kappa_0/T is seen to scale closely with that of the pseudogap, arguing against a non-superconducting origin for the pseudogap. The presence of a linear term for all dopings is evidence against the existence of a quantum phase transition to an order parameter with a complex (ix) component.
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We present low-temperature thermal conductivity measurements on the cuprate Tl_2Ba_2CuO_{6+delta} throughout the overdoped regime. In the T -> 0 limit, the thermal conductivity due to d-wave nodal quasiparticles provides a bulk measurement of the superconducting gap, Delta. We find Delta to decrease with increasing doping, with a magnitude consistent with spectroscopic measurements (photoemission and tunneling). This argues for a pure and simple d-wave superconducting state in the overdoped region of the phase diagram, which appears to extend into the underdoped regime down to a hole concentration of 0.1 hole/Cu. As hole concentration is decreased, the gap-to-Tc ratio increases, showing that the suppression of the superconducting transition temperature Tc (relative to the gap) begins in the overdoped regime.
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The temperature and magnetic-field (H) dependences of thermal conductivity (kappa) of Bi_2Sr_2CaCu_2O_{8+delta} (Bi2212) are systematically measured for a broad doping range by using both pure Bi2212 single crystals with tuned oxygen contents and Bi_2Sr_2Ca_{1-x}Dy_xCu_2O_{8+delta} (Dy-Bi2212) single crystals with different Dy contents x. In the underdoped samples, the quasiparticle (QP) peak below T_c is strongly suppressed, indicating strong QP scattering by impurities or oxygen defects, whereas the phonon conductivity is enhanced in moderately Dy-doped samples and a phonon peak at 10 K is observed for the first time in Bi2212 system, which means Dy^{3+} ions not only introduce the impurities or point defects but also stabilize the crystal lattice. The subkelvin data show that the QP heat conductivity gradually decreases upon lowering the hole doping level. The magnetic-field dependence of kappa at temperature above 5 K is mainly due to the QP scattering off vortices. While the underdoped pure Bi2212 show very weak field dependence of kappa, the Dy-doped samples present an additional dip-like term of kappa(H) at low field, which is discussed to be related to the phonon scattering by free spins of Dy^{3+} ions. For non-superconducting Dy-Bi2212 samples with x simeq 0.50, an interesting plateau feature shows up in the low-T kappa(H) isotherms with characteristic field at 1 -- 2 T, for which we discuss the possible revlevance of magnon excitations.
In order to identify the gap structure of CeIrIn5, we measured field-angle-resolved specific heat C(phi) by conically rotating the magnetic field H around the c axis at low temperatures down to 80 mK. We revealed that C(phi) exhibits a fourfold angular oscillation, whose amplitude decreases monotonically by tilting H out of the ab plane. Detailed microscopic calculations based on the quasiclassical Eilenberger equation confirm that the observed features are uniquely explained by assuming the dx2-y2-wave gap. These results strongly indicate that CeIrIn5 is a dx2-y2-wave superconductor and suggest the universal pairing mechanism in CeMIn5 (M = Co, Rh, and Ir).
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