626 research outputs found
Wiedemann-Franz law and non-vanishing temperature scale across the field-tuned quantum critical point of YbRh2Si2
The in-plane thermal conductivity kappa(T) and electrical resistivity rho(T)
of the heavy-fermion metal YbRh2Si2 were measured down to 50 mK for magnetic
fields H parallel and perpendicular to the tetragonal c axis, through the
field-tuned quantum critical point, Hc, at which antiferromagnetic order ends.
The thermal and electrical resistivities, w(T) and rho(T), show a linear
temperature dependence below 1 K, typical of the non-Fermi liquid behavior
found near antiferromagnetic quantum critical points, but this dependence does
not persist down to T = 0. Below a characteristic temperature T* ~ 0.35 K,
which depends weakly on H, w(T) and rho(T) both deviate downward and converge
in the T = 0 limit. We propose that T* marks the onset of short-range magnetic
correlations, persisting beyond Hc. By comparing samples of different purity,
we conclude that the Wiedemann-Franz law holds in YbRh2Si2, even at Hc,
implying that no fundamental breakdown of quasiparticle behavior occurs in this
material. The overall phenomenology of heat and charge transport in YbRh2Si2 is
similar to that observed in the heavy-fermion metal CeCoIn5, near its own
field-tuned quantum critical point.Comment: 8 figures, 8 page
Heat Transport in a Strongly Overdoped Cuprate: Fermi Liquid and Pure d-wave BCS Superconductor
The transport of heat and charge in the overdoped cuprate superconductor
Tl_2Ba_2CuO_(6+delta) was measured down to low temperature. In the normal
state, obtained by applying a magnetic field greater than the upper critical
field, the Wiedemann-Franz law is verified to hold perfectly. In the
superconducting state, a large residual linear term is observed in the thermal
conductivity, in quantitative agreement with BCS theory for a d-wave
superconductor. This is compelling evidence that the electrons in overdoped
cuprates form a Fermi liquid, with no indication of spin-charge separation.Comment: 4 pages, 2 figures, published version, title changed, Phys. Rev.
Lett. 89, 147003 (2002
Renormalization of thermal conductivity of disordered d-wave superconductors by impurity-induced local moments
The low-temperature thermal conductivity \kappa_0/T of d-wave superconductors
is generally thought to attain a "universal" value independent of disorder at
sufficiently low temperatures, providing an important measure of the magnitude
of the gap slope near its nodes. We discuss situations in which this inference
can break down because of competing order, and quasiparticle localization.
Specifically, we study an inhomogeneous BCS mean field model with electronic
correlations included via a Hartree approximation for the Hubbard interaction,
and show that the suppression of \kappa_0/T by localization effects can be
strongly enhanced by magnetic moment formation around potential scatterers.Comment: 2 pages, 1 figure, submitted to M2S-HTSC VIII, Dresden 200
Ballistic magnon transport and phonon scattering in the antiferromagnet NdCuO
The thermal conductivity of the antiferromagnet NdCuO was measured
down to 50 mK. Using the spin-flop transition to switch on and off the acoustic
Nd magnons, we can reliably separate the magnon and phonon contributions to
heat transport. We find that magnons travel ballistically below 0.5 K, with a
thermal conductivity growing as , from which we extract their velocity. We
show that the rate of scattering of acoustic magnons by phonons grows as ,
and the scattering of phonons by magnons peaks at twice the average Nd magnon
frequency.Comment: 4 pages, 3 figures, one figure modifie
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
Dynamic charge inhomogenity in cuprate superconductors
The inelastic x-ray scattering spectrum for phonons of -symmetry
including the CuO bond-stretching phonon dispersion is analyzed by a Lorentz
fit in HgBaCuO and BiSrCuO, respectively, using
recently calculated phonon frequencies as input parameters. The resulting mode
frequencies of the fit are almost all in good agreement with the calculated
data. An exception is the second highest -branch compromising the
bond-stretching modes which disagrees in both compounds with the calculations.
This branch unlike the calculations shows an anomalous softening with a minimum
around the wavevector \vc{q}=\frac{2\pi}{a}(0.25, 0, 0). Such a disparity
with the calculated results, that are based on the assumption of an undisturbed
translation- and point group invariant electronic structure of the CuO plane,
indicates some {\it static} charge inhomogenities in the measured probes. Most
likely these will be charge stripes along the CuO bonds which have the
strongest coupling to certain longitudinal bond-stretching modes that in turn
selfconsistently induce corresponding {\it dynamic} charge inhomogenities. The
symmetry breaking by the mix of dynamic and static charge inhomogenities can
lead to a reconstruction of the Fermi surface into small pockets.Comment: 7 pages, 4 figure
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