50 research outputs found
Magnetic Penetration Depth Measurements of PrCeCuO Films on Buffered Substrates: Evidence for a Nodeless Gap
We report measurements of the inverse squared magnetic penetration depth,
, in PrCeCuO () superconducting films grown on SrTiO (001) substrates coated with a
buffer layer of insulating PrCuO. , and
normal-state resistivities of these films indicate that they are clean and
homogeneous. Over a wide range of Ce doping, ,
at low is flat: it changes by less than 0.15% over a
factor of 3 change in , indicating a gap in the superconducting density of
states. Fits to the first 5% decrease in produce values of
the minimum superconducting gap in the range of .Comment: 4 pages 5 figure
Evidence for a Bulk Complex Order-Parameter in Y0.9Ca0.1Ba2Cu3O7-delta Thin Films
We have measured the penetration depth of overdoped Y0.9Ca0.1Ba2Cu3O7-delta
(Ca-YBCO) thin films using two different methods. The change of the penetration
depth as a function of temperature has been measured using the parallel plate
resonator (PPR), while its absolute value was obtained from a quasi-optical
transmission measurements. Both sets of measurements are compatible with an
order parameter of the form: Delta*dx2-y2+i*delta*dxy, with Delta=14.5 +- 1.5
meV and delta=1.8 meV, indicating a finite gap at low temperature. Below 15 K
the drop of the scattering rate of uncondensed carriers becomes steeper in
contrast to a flattening observed for optimally doped YBCO films. This decrease
supports our results on the penetration depth temperature dependence. The
findings are in agreement with tunneling measurements on similar Ca-YBCO thin
films.Comment: 11 pages, 4 figure
Renormalization of the elementary excitations in hole- and electron-doped cuprates due to spin fluctuations
Extending our previous studies we present results for the doping-, momentum-,
frequency-, and temperature- dependence of the kink-like change of the
quasiparticle velocity resulting from the coupling to spin fluctuations. In the
nodal direction a kink is found in both the normal and superconducting state
while in the antinodal direction a kink occurs only below due to the
opening of the superconducting gap. A pronounced kink is obtained only for
hole-doped, but not for electron-doped cuprates and is characteristically
different from what is expected due to electron-phonon interaction. We further
demonstrate that the kink structure is intimately connected to the resonance
peak seen in inelastic neutron scattering. Our results suggest similar effects
in other unconventional superconductors like .Comment: revised version, 12 pages, 19 figures. accepted for publication in
PR
Hot Spots and Transition from d-Wave to Another Pairing Symmetry in the Electron-Doped Cuprate Superconductors
We present a simple theoretical explanation for a transition from d-wave to
another superconducting pairing observed in the electron-doped cuprates. The
d_{x^2-y^2} pairing potential Delta, which has the maximal magnitude and
opposite signs at the hot spots on the Fermi surface, becomes suppressed with
the increase of electron doping, because the hot spots approach the Brillouin
zone diagonals, where Delta vanishes. Then, the d_{x^2-y^2} pairing is replaced
by either singlet s-wave or triplet p-wave pairing. We argue in favor of the
latter and discuss experiments to uncover it.Comment: 6 pages, 4 figures, RevTeX 4. V.2: Extra figure and many references
added. V.3: Minor update of references for the proof
Evidence for a Nodeless Gap from the Superfluid Density of Optimally Doped Pr_{1.855}Ce_{0.145}CuO_{4-y} Films
We present measurements of the ab-plane magnetic penetration depth,
\lambda(T), in five optimally doped Pr_{1.855}Ce_{0.145}CuO_{4-y} films for 1.6
K \leq T \leq T_c \sim 24 K. Low resistivities, high superfluid densities
n_s(T)\propto \lambda^{-2}(T), high T_c's, and small transition widths are
reproducible and indicative of excellent film quality. For all five films,
\lambda^{-2}(T)/\lambda^{-2}(0) at low T is well fitted by an exponential
temperature dependence with a gap, \Delta_{min}, of 0.85 k_B T_c. This behavior
is consistent with a nodeless gap and is incompatible with d-wave
superconductivity.Comment: 5 pages, 4 figures, reorganized for clarit
Magnetic Field Dependence of Electronic Specific Heat in Pr_{1.85} Ce_{0.15} CuO_4
The specific heat of electron-doped Pr_{1.85} Ce_{0.15} CuO_4 single crystals
is reported for the temperature range 2 - 10 K and magnetic field range 0 - 10
T. A non-linear magnetic field dependence is observed for the field range 0 - 2
T. Our data supports a model with lines of nodes in the gap function of these
superconductors. Theoretical calculations of the electronic specific heat for
dirty d-wave, clean d-wave, and s-wave symmetries are compared to our data.Comment: 10 pages Latex and 4 eps figures, submitted to Phys. Rev.
Strongly correlated s-wave pairing in the n-type infinite-layer cuprate
Quasiparticle tunneling spectra of the electron-doped (n-type) infinite-layer
cuprate Sr_{0.9}La_{0.1}CuO_2 reveal characteristics that counter a number of
common phenomena in the hole-doped (p-type) cuprates. The optimally doped
Sr_{0.9}La_{0.1}CuO_2 with T_c = 43 K exhibits a momentum-independent
superconducting gap \Delta = 13.0 +- 1.0 meV that substantially exceeds the BCS
value, and the spectral characteristics indicate insignificant quasiparticle
damping by spin fluctuations and the absence of pseudogap. The response to
quantum impurities in the Cu-sites also differs fundamentally from that of the
p-type cuprates with d_{x^2-y^2}-wave pairing symmetry.Comment: 4 pages, 3 figures. Published in Physical Review Letter.
Corresponding author: Nai-Chang Yeh (e-mail: [email protected]
Possible Z2 phase and spin-charge separation in electron doped cuprate superconductors
The SU(2) slave-boson mean-field theory for the tt'J model is analyzed. The
role of next-nearest-neighbor hopping t' on the phase-diagram is studied. We
find a pseudogap phase in hole-doped materials (where t'<0). The pseudo-gap
phase is a U(1) spin liquid (the staggered-flux phase) with a U(1) gauge
interaction and no fractionalization. This agrees with experiments on hole
doped samples. The same calculation also indicates that a positive t' favors a
Z2 state with true spin-charge separation. The Z2 state that exists when t' >
0.5J can be a candidate for the pseudo-gap phase of electron-doped cuprates (if
such a phase exists). The experimental situation in electron-doped materials is
also addressed.Comment: 6 pages, 2 figures, RevTeX4. Homepage http://dao.mit.edu/~wen
Point contact spectroscopy of the electron-doped cuprate superconductor Pr{2-x}Ce{x}CuO4: The dependence of conductance-voltage spectra on cerium doping, barrier strength and magnetic field
We present conductance-voltage (G-V) data for point contact junctions between
a normal metal and the electron doped cuprate superconductor Pr{2-x}Ce{x}CuO4
(PCCO). We observe a zero bias conductance peak (ZBCP) for the under-doped
composition of this cuprate (x=0.13) which is consistent with d-wave pairing
symmetry. For optimally-doped (x=0.15) and over-doped (x=0.17) PCCO, we find
that the G-V characteristics indicate the presence of an order parameter
without nodes. We investigate this further by obtaining point contact
spectroscopy data for different barrier strengths and as a function of magnetic
field.Comment: 13 pages, 9 figure
Doping dependence of the gap anisotropy in LCCO studied by millimeter-wave spectroscopy
We measure the penetration depth of optimally doped and underdoped
La2-xCexCuO4 in the millimeter frequency domain (4 - 7 cm-1) and for
temperatures 2 K < T < 300 K. The penetration depth as function of temperature
reveals significant changes on electron doping. It shows quadratic temperature
dependence in underdoped samples, but increases almost exponentially at optimal
doping. Significant changes in the gap anisotropy (or even in the gap symmetry)
may account for this transition.Comment: 4 pages, 4 figure