55 research outputs found
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
Doping dependent Irreversible Magnetic Properties of Ba(Fe1-xCox)2As2 Single Crystals
We discuss the irreversible magnetic properties of self-flux grown
Ba(Fe1-xCox)2As2 single crystals for a wide range of concentrations covering
the whole phase diagram from the underdoped to the overdoped regime, x=0.038,
0.047, 0.058, 0.071, 0.074, 0.10, 0.106 and 0.118. Samples were characterized
by a magneto-optical method and show excellent spatial uniformity of the
superconducting state. The overall behavior closely follows classical Bean
model of the critical state. The field-dependent magnetization exhibits second
peak at a temperature and doping - dependent magnetic field, Hp. The evolution
of this fishtail feature with doping is discussed. Magnetic relaxation is
time-logarithmic and unusually fast. Similar to cuprates, there is an apparent
crossover from collective elastic to plastic flux creep above Hp. At high
fields, the field dependence of the relaxation rate becomes doping independent.
We discuss our results in the framework of the weak collective pinning and show
that vortex physics in iron-based pnictide crystals is much closer to high-Tc
cuprates than to conventional s-wave (including MgB2) superconductors.Comment: for the special issue of Physica C on iron-based pnictide
superconductor
Edge states and determination of pairing symmetry in superconducting Sr2RuO4
We calculate the energy dispersion of the surface Andreev states and their
contribution to tunneling conductance for the order parameters with horizontal
and vertical lines of nodes proposed for superconducting Sr2RuO4. For vertical
lines, we find double peaks in tunneling spectra reflecting the van Hove
singularities in the density of surface states originating from the turning
points in their energy dispersion. For horizontal lines, we find a single
cusp-like peak at zero bias, which agrees very well with the experimental data
on tunneling in Sr2RuO4.Comment: 6 pages, 6 figures. V.2: comparison with experiment added and
discussion of horizontal nodes expanded. v.3: significant expansion: 1 figure
and 2 pages added. v.4: acknowledgements added. Additional viewgraphs with
experimental and theoretical curves superimposed are available at
http://www2.physics.umd.edu/~yakovenk/talks/Sr2RuO4
Detailed study of the ac susceptibility of Sr2RuO4 in oriented magnetic fields
We have investigated the ac susceptibility of the spin triplet superconductor
SrRuO as a function of magnetic field in various directions at
temperatures down to 60 mK. We have focused on the in-plane field configuration
(polar angle ), which is a prerequisite for inducing
multiple superconducting phases in SrRuO. We have found that the
previous attribution of a pronounced feature in the ac susceptibility to the
second superconducting transition itself is not in accord with recent
measurements of the thermal conductivity or of the specific heat. We propose
that the pronounced feature is a consequence of additional involvement of
vortex pinning originating from the second superconducting transition.Comment: Accepted for publication in Phys. Rev.
Multiband superconductivity in NbSe_2 from heat transport
The thermal conductivity of the layered s-wave superconductor NbSe_2 was
measured down to T_c/100 throughout the vortex state. With increasing field, we
identify two regimes: one with localized states at fields very near H_c1 and
one with highly delocalized quasiparticle excitations at higher fields. The two
associated length scales are most naturally explained as multi-band
superconductivity, with distinct small and large superconducting gaps on
different sheets of the Fermi surface.Comment: 2 pages, 2 figures, submitted to M2S-Rio 2003 Proceeding
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
Magnetic field - temperature phase diagram of quasi-two-dimensional organic superconductor lambda-(BETS)_2 GaCl_4 studied via thermal conductivity
The thermal conductivity kappa of the quasi-two-dimensional (Q2D) organic
superconductor lambda-(BETS)_2 GaCl_4 was studied in the magnetic field H
applied parallel to the Q2D plane. The phase diagram determined from this bulk
measurement shows notable dependence on the sample quality. In dirty samples
the upper critical field H_{c2} is consistent with the Pauli paramagnetic
limiting, and a sharp change is observed in kappa(H) at H_{c2 parallel}. In
contrast in clean samples H_{c2}(T) shows no saturation towards low
temperatures and the feature in kappa(H) is replaced by two slope changes
reminiscent of second-order transitions. The peculiarity was observed below ~
0.33T_c and disappeared on field inclination to the plane when the orbital
suppression of superconductivity became dominant. This behavior is consistent
with the formation of a superconducting state with spatially modulated order
parameter in clean samples.Comment: 10 pages, 8 figures, new figure (Fig.5) and references added, title
change
Magnetothemopower study of quasi two-dimensional organic conductor -(BEDT-TTF)KHg(SCN)
We have used a low-frequency magneto-thermopower (MTEP) method to probe the
high magnetic field ground state behavior of
-(BEDT-TTF)KHg(SCN) along all three principal crystallographic
axes at low temperatures. The thermopower tensor coefficients (
and ) have been measured to 30 T, beyond the anomalous low temperature,
field-induced transition at 22.5 T. We find a significant anisotropy in the
MTEP signal, and also observe large quantum oscillations associated with the de
Haas - van Alphen effect. The anisotropy indicates that the ground state
properties are clearly driven by mechanisms that occur along specific
directions for the in-plane electronic structure. Both transverse and
longitudinal magnetothermopower show asymptotic behavior in field, which can be
explained in terms of magnetic breakdown of compensated closed orbits.Comment: 9 pages, 10 figure
Thermal Conductivity near H_c2 for spin-triplet superconducting States with line nodes in Sr_2RuO_4
We calculate the thermal conductivity kappa in magnetic fields near H_c2 for
spin-triplet superconducting states with line nodes vertical and horizontal
relative to the RuO_2-planes. The method for calculating the Green's functions
takes into account the spatial variation of the order parameter and
superconducting flow for the Abrikosov vortex lattice. For in-plane magnetic
field we obtain variations of the in-plane kappa with two-fold symmetry as a
function of rotation angle where the minima and maxima occur for field
directions parallel and perpendicular to the heat flow. The amplitude of the
variation decreases with increasing impurity scattering and temperature. At
higher temperatures the minima and maxima of the variation are interchanged.
Since the results for vertical and horizontal line nodes are almost the same we
cannot say which of the two pairing models is more compatible with recent
measurements of kappa in Sr_2RuO_4. The observed four-fold modulation of kappa
in YBa_2Cu_3O_(7-\delta) is obtained for d-wave pairing by taking into account
the particular shape of the Fermi surface and the finite temperature effect.
The results for kappa for the f-wave pairing state with horizontal line nodes
disagree in some respects with the measurements on UPt_3.Comment: 8 pages, 6 figures. To be published in Phys. Rev.
Field-induced quantum critical point in CeCoIn_5
The resistivity of CeCoIn_5 was measured down to 20 mK in magnetic fields of
up to 16 T. 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 ~4/3. This is evidence
for a new field-induced quantum critical point, occuring in this case at a
critical field which coincides with the superconducting upper critical field
H_c2.Comment: 2 pages, 2 figures, submitted to M2S-Rio 2003 Proceeding
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