407 research outputs found
Doping dependence of superconducting gap in YBa_2Cu_3O_y from universal heat transport
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.Comment: 2 pages, 2 figures, submitted to M2S-Rio 2003 Proceeding
Nonvanishing Energy Scales at the Quantum Critical Point of CeCoIn5
Heat and charge transport were used to probe the magnetic field-tuned quantum
critical point in the heavy-fermion metal CeCoIn. A comparison of
electrical and thermal resistivities reveals three characteristic energy
scales. A Fermi-liquid regime is observed below , with both transport
coefficients diverging in parallel and as , the
critical field. The characteristic temperature of antiferromagnetic spin
fluctuations, , is tuned to a minimum but {\it finite} value at ,
which coincides with the end of the -linear regime in the electrical
resistivity. A third temperature scale, , signals the formation of
quasiparticles, as fermions of charge obeying the Wiedemann-Franz law.
Unlike , it remains finite at , so that the integrity of
quasiparticles is preserved, even though the standard signature of Fermi-liquid
theory fails.Comment: 4 pages, 4 figures (published version
Heat Transport as a Probe of Electron Scattering by Spin Fluctuations: the Case of Antiferromagnetic CeRhIn5
Heat and charge conduction were measured in the heavy-fermion metal CeRhIn5,
an antiferromagnet with T_N=3.8 K. The thermal resistivity is found to be
proportional to the magnetic entropy, revealing that spin fluctuations are as
effective in scattering electrons as they are in disordering local moments. The
electrical resistivity, governed by a q^2 weighting of fluctuations, increases
monotonically with temperature. In contrast, the difference between thermal and
electrical resistivities, characterized by an omega^2 weighting, peaks sharply
at T_N and eventually goes to zero at a temperature T^* ~ 8 K. T^* thus emerges
as a measure of the characteristic energy of magnetic fluctuations.Comment: 4 pages, 4 figure
Influence of a magnetic field on the antiferromagnetic order in UPt_3
A neutron diffraction experiment was performed to investigate the effect of a
magnetic field on the antiferromagnetic order in the heavy fermion
superconductor UPt_3. Our results show that a field in the basal plane of up to
3.2 Tesla, higher than H_c2(0), has no effect: it can neither select a domain
nor rotate the moment. This has a direct impact on current theories for the
superconducting phase diagram based on a coupling to the magnetic order.Comment: 7 pages, RevTeX, 3 postscript figures, submitted to Phys. Rev.
Field-Induced Quantum Critical Point in CeCoIn5
The resistivity of the heavy-fermion superconductor CeCoIn5 was measured as a
function of temperature, down to 25 mK and in magnetic fields of up to 16 T
applied perpendicular to the basal plane. With increasing field, we observe a
suppression of the non-Fermi liquid behavior, rho ~ T, and the development of a
Fermi liquid state, with its characteristic rho = rho_0 + AT^2 dependence. The
field dependence of the T^2 coefficient shows critical behavior with an
exponent of 1.37. This is evidence for a field-induced quantum critical point
(QCP), occuring at a critical field which coincides, within experimental
accuracy, with the superconducting critical field H_c2. We discuss the relation
of this field-tuned QCP to a change in the magnetic state, seen as a change in
magnetoresistance from positive to negative, at a crossover line that has a
common border with the superconducting region below ~ 1 K.Comment: 4 pages, 3 figures (published version
Field-induced thermal metal-to-insulator transition in underdoped LSCO
The transport of heat and charge in cuprates was measured in undoped and
heavily-underdoped single crystal La_{2-x}Sr_xCuO_{4+delta} (LSCO). In
underdoped LSCO, the thermal conductivity is found to decrease with increasing
magnetic field in the T --> 0 limit, in striking contrast to the increase
observed in all superconductors, including cuprates at higher doping. The
suppression of superconductivity with magnetic field shows that a novel thermal
metal-to-insulator transition occurs upon going from the superconducting state
to the field-induced normal state.Comment: 2 pages, 2 figures, submitted to M2S-Rio 2003 Proceeding
The Origin of Anomalous Low-Temperature Downturns in the Thermal Conductivity of Cuprates
We show that the anomalous decrease in the thermal conductivity of cuprates
below 300 mK, as has been observed recently in several cuprate materials
including PrCeCuO in the field-induced normal state,
is due to the thermal decoupling of phonons and electrons in the sample. Upon
lowering the temperature, the phonon-electron heat transfer rate decreases and,
as a result, a heat current bottleneck develops between the phonons, which can
in some cases be primarily responsible for heating the sample, and the
electrons. The contribution that the electrons make to the total low- heat
current is thus limited by the phonon-electron heat transfer rate, and falls
rapidly with decreasing temperature, resulting in the apparent low- downturn
of the thermal conductivity. We obtain the temperature and magnetic field
dependence of the low- thermal conductivity in the presence of
phonon-electron thermal decoupling and find good agreement with the data in
both the normal and superconducting states.Comment: 8 pages, 5 figure
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