113 research outputs found
Doping n-type carriers by La-substitution for Ba in YBa_2Cu_3O_y system
Thus far, there is no cuprate system where both n-type and p-type charge
carriers can be doped without changing the crystallographic structure. For
studying the electron-hole symmetry in an identical structure, we try to dope
n-type carriers to YBa2Cu3Oy system by reducing oxygen content and substituting
La3+ ions for Ba2+. Single crystals of La-doped YBa2Cu3Oy are grown by a flux
method with Y2O3 crucibles and it is confirmed that La actually substitutes
\~13% of Ba. The oxygen content y can be varied between 6.21 and 6.95 by
annealing the crystals in an atmosphere with controlled oxygen partial
pressure. The in-plane resistivity rho_ab at room temperature was found to
increase with decreasing oxygen content y down to 6.32, but interestingly
further decrease in y results in a decrease in rho_ab. The most reduced samples
with y = 6.21 show rho_ab of ~30 mOhm cm at room temperature, which is as much
as seven orders-of-magnitude smaller than the maximum value at y = 6.32.
Furthermore, both the Hall coefficient and the Seebeck coefficient of the y =
6.21 samples are found to be negative at room temperatures. The present results
demonstrate that the non-doped Mott-insulating state has been crossed upon
reducing y and n-type carriers are successfully doped in this material.Comment: 4 pages, 4 figures, 1 table, accepted for publication in Phys. Rev.
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
Bulk superconducting phase with a full energy gap in the doped topological insulator Cu_xBi_2Se_3
The superconductivity recently found in the doped topological insulator
Cu_xBi_2Se_3 offers a great opportunity to search for a topological
superconductor. We have successfully prepared a single-crystal sample with a
large shielding fraction and measured the specific-heat anomaly associated with
the superconductivity. The temperature dependence of the specific heat suggests
a fully-gapped, strong-coupling superconducting state, but the BCS theory is
not in full agreement with the data, which hints at a possible unconventional
pairing in Cu_xBi_2Se_3. Also, the evaluated effective mass of 2.6m_e (m_e is
the free electron mass) points to a large mass enhancement in this material.Comment: 4 pages, 3 figure
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
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