697 research outputs found
Metal-to-Insulator Crossover in YBa_{2}Cu_{3}O_{y} Probed by Low-Temperature Quasiparticle Heat Transport
It was recently demonstrated that in La_{2-x}Sr_{x}CuO_{4} the magnetic-field
(H) dependence of the low-temperature thermal conductivity \kappa up to 16 T
reflects whether the normal state is a metal or an insulator. We measure the H
dependence of \kappa in YBa_{2}Cu_{3}O_{y} (YBCO) at subkelvin temperatures for
a wide doping range, and find that at low doping the \kappa(H) behavior
signifies the change in the ground state in this system as well. Surprisingly,
the critical doping is found to be located deeply inside the underdoped region,
about the hole doping of 0.07 hole/Cu; this critical doping is apparently
related to the stripe correlations as revealed by the in-plane resistivity
anisotropy.Comment: 4 pages, 3 figures; minor revision, accepted for publication in Phys.
Rev. Let
Low-temperature nodal-quasiparticle transport in lightly doped YBa_{2}Cu_{3}O_{y} near the edge of the superconducting doping regime
In-plane transport properties of nonsuperconducting YBa_{2}Cu_{3}O_{y} (y =
6.35) are measured using high-quality untwinned single crystals. We find that
both the a- and b-axis resistivities show log(1/T) divergence down to 80 mK,
and accordingly the thermal conductivity data indicate that the nodal
quasiparticles are progressively localized with lowering temperature. Hence,
both the charge and heat transport data do not support the existence of a
"thermal metal" in nonsuperconducting YBa_{2}Cu_{3}O_{y}, as opposed to a
recent report by Sutherland {\it et al.} [Phys. Rev. Lett. {\bf 94}, 147004
(2005)]. Besides, the present data demonstrate that the peculiar log(1/T)
resistivity divergence of cuprate is {\it not} a property associated with
high-magnetic fields.Comment: 4 pages, 3 figures. Our previous main claim that the pseudogap state
of cuprates is inherently insulating was found to be erroneous and has been
retracted; the paper now focuses on the log(1/T) resistivity divergence and
its implication
Electron-Hole Asymmetry in GdBaCo_{2}O_{5+x}: Evidence for Spin Blockade of Electron Transport in a Correlated Electron System
In RBaCo_{2}O_{5+x} compounds (R is rare earth) variability of the oxygen
content allows precise doping of CoO_2 planes with both types of charge
carriers. We study transport properties of doped GdBaCo_{2}O_{5+x} single
crystals and find a remarkable asymmetry in the behavior of holes and electrons
doped into a parent insulator GdBaCo_{2}O_{5.5}. Doping dependences of
resistivity, Hall response, and thermoelectric power reveal that the doped
holes greatly improve the conductivity, while the electron-doped samples always
remain poorly conducting. This doping asymmetry provides strong evidence for a
spin blockade of the electron transport in RBaCo_{2}O_{5+x}.Comment: 4 pages, 5 figures, accepted for publication in PR
Origin of the large thermoelectric power in oxygen-variable RBaCo_{2}O_{5+x} (R=Gd, Nd)
Thermoelectric properties of GdBaCo_{2}O_{5+x} and NdBaCo_{2}O_{5+x} single
crystals have been studied upon continuous doping of CoO_2 planes with either
electrons or holes. The thermoelectric response and the resistivity behavior
reveal a hopping character of the transport in both compounds, providing the
basis for understanding the recently found remarkable divergence of the Seebeck
coefficient at x=0.5. The doping dependence of the thermoelectric power evinces
that the configurational entropy of charge carriers, enhanced by their spin and
orbital degeneracy, plays a key role in the origin of the large thermoelectric
response in these correlated oxides.Comment: 5 pages, 4 figures, accepted for publication in PR
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