14 research outputs found
Nernst Effect in Electron-Doped PrCeCuO
The Nernst effect of PrCeCuO (x=0.13, 0.15, and 0.17) has
been measured on thin film samples between 5-120 K and 0-14 T. In comparison to
recent measurements on hole-doped cuprates that showed an anomalously large
Nernst effect above the resistive T and H
\cite{xu,wang1,wang2,capan}, we find a normal Nernst effect above T and
H for all dopings. The lack of an anomalous Nernst effect in the
electron-doped compounds supports the models that explain this effect in terms
of amplitude and phase fluctuations in the hole-doped cuprates. In addition,
the H(T) determined from the Nernst effect shows a conventional behavior
for all dopings. The energy gap determined from H(0) decreases as the
system goes from under-doping to over-dopingin agreement with the recent
tunnelling experiments
Possible pseudogap behavior of electron doped high-temperature superconductors
We have measured the low-energy quasiparticle excitation spectrum of the
electron doped high-temperature superconductors (HTS) Nd(1.85)Ce(0.15)CuO(4-y)
and Pr(1.85)Ce(0.15)CuO(4-y) as a function of temperature and applied magnetic
field using tunneling spectroscopy. At zero magnetic field, for these optimum
doped samples no excitation gap is observed in the tunneling spectra above the
transition temperature Tc. In contrast, below Tc for applied magnetic fields
well above the resistively determined upper critical field, a clear excitation
gap at the Fermi level is found which is comparable to the superconducting
energy gap below Tc. Possible interpretations of this observation are the
existence of a normal state pseudogap in the electron doped HTS or the
existence of a spatially non-uniform superconducting state.Comment: 4 pages, 4 ps-figures included, to be published in Phys. Rev. B,
Rapid Com
An angle-resolved photoemission spectral function analysis of the electron doped cuprate Nd_1.85Ce_0.15CuO_4
Using methods made possible by recent advances in photoemission technology,
we perform an indepth line-shape analysis of the angle-resolved photoemission
spectra of the electron doped (n-type) cuprate superconductor
Nd_1.85Ce_0.15CuO_4. Unlike for the p-type materials, we only observe weak mass
renormalizations near 50-70 meV. This may be indicative of smaller
electron-phonon coupling or due to the masking effects of other interactions
that make the electron-phonon coupling harder to detect. This latter scenario
may suggest limitations of the spectral function analysis in extracting
electronic self-energies when some of the interactions are highly momentum
dependent.Comment: 8 pages, 5 figure
Evidence of a d to s-wave pairing symmetry transition in the electron-doped cuprate superconductor PrCeCuO
We present point contact spectroscopy (PCS) data for junctions between a
normal metal and the electron doped cuprate superconductor
PrCeCuO (PCCO). For the underdoped compositions of this cuprate
() we observe a peak in the conductance-voltage characteristics of
the point contact junctions. The shape and magnitude of this peak suggests the
presence of Andreev bound states at the surface of underdoped PCCO which is
evidence for a d-wave pairing symmetry. For overdoped PCCO () the
PCS data does not show any evidence of Andreev bound states at the surface
suggesting an s-wave pairing symmetry.Comment: 4 pages Latex, 4 eps figures included. Submitted to Phys. Rev. Let
Electron Dynamics in NdCeCuO: Evidence for the Pseudogap State and Unconventional c-axis Response
Infrared reflectance measurements were made with light polarized along the a-
and c-axis of both superconducting and antiferromagnetic phases of electron
doped NdCeCuO. The results are compared to
characteristic features of the electromagnetic response in hole doped cuprates.
Within the CuO planes the frequency dependent scattering rate,
1/, is depressed below 650 cm; this behavior is a
hallmark of the pseudogap state. While in several hole doped compounds the
energy scales associated with the pseudogap and superconducting states are
quite close, we are able to show that in NdCeCuO
the two scales differ by more than one order of magnitude. Another feature of
the in-plane charge response is a peak in the real part of the conductivity,
, at 50-110 cm which is in sharp contrast with the
Drude-like response where is centered at . This
latter effect is similar to what is found in disordered hole doped cuprates and
is discussed in the context of carrier localization. Examination of the c-axis
conductivity gives evidence for an anomalously broad frequency range from which
the interlayer superfluid is accumulated. Compelling evidence for the pseudogap
state as well as other characteristics of the charge dynamics in
NdCeCuO signal global similarities of the cuprate
phase diagram with respect to electron and hole doping.Comment: Submitted to PR
Anomalous dissipation in the mixed state of underdoped cuprates close to the superconductor-insulator boundary
We present a comparative study of Nernst effect and resistivity in underdoped
samples of BiSrCuO and LaSrCuO. The
Nernst effect presents a peak in a region of the H-T diagram where resistivity
shows a non-metallic temperature dependence. Our results illustrate that the
mechanism of dissipation in the mixed state of underdoped cuprates is poorly
understood. Large quantum superconducting fluctuations and vanishing vortex
viscosity are among suggested explanations for an enhanced Nernst signal close
to the superconductor-insulator boundary.Comment: 5 pages including 3 figure
Comparative study of the magnetoresistivity of YBa2Cu3O7−δ and Bi2Sr2Ca Cu2O8+x epitaxial thin films in the fluktuation regime
We measured the magnetoresistivity of high quality, c-axis oriented epitaxial YBa2Cu3O7?? and Bi2Sr2Ca Cu2O8 x (BSCCO) thin films in the fluctuation regime above the critical temperature. The measurements have been performed in magnetic fields up to 12 Tesla applied parallel and perpendicular to the CuO2-planes. The experimental data was analysed in the framework of the Lawrence-Doniach model with the appropriate clean limit expressions for the Aslamazov-Larkin- and Maki-Thompson- fluctuation conductivities. Orbital as well as Zeeman contributions to the magnetoconductivity have been taken into account. With the thermodynamic critical temperature Tc, the coherence lengths parallel and perpendicular to the ab-plane ?ab and ?c, respectively, and the product of the mean free path l and the phase breaking time ?? as fitting parameters, good agreement between experiment and theory was found for YBCO, whereas for BSCCO no convincing fit could be obtained. Fitting the data for YBCO we obtained Tc = 90.2K, ?ab = 13.8Å, ?c = 3.1 Å, and (l?gf)100K 1.5 × 10?22sm. These values are close to those derived from measurements of the transport energy of magnetic flux lines via the Nernst effect at temperatures below Tc. For BSCCO we obtained Tc 90 K and ?ab 24 to 30 Å. The very small coherence length in c-direction ?c ? 0.4 Å reflects the quasi 2-dimensional nature of this material
Nernst, Seebeck, and Hall effects in the mixed state of YBa2Cu3O7-δ and Bi2Sr2CaCu2O8+x thin films : a comparative study
The anisotropy of the high-temperature superconductors has strong impact on their transport properties in the mixed state. We have performed a comparative study of the Nernst, Seebeck, and Hall effects of the anisotropic and extremely anisotropic high-temperature superconductors YBa2Cu3O7-? and Bi2Sr2CaCu2O8 x, respectively. High-quality, c-axis-oriented epitaxial thin films have been used for our study. The temperature and the magnetic-field dependence of the Nernst and Seebeck electric fields were measured both in the mixed state and in the fluctuation regime above the thermodynamic critical temperature. The Seebeck-effect data can be explained well by an extended two-fluid counterflow picture in analogy to the fountain effect in superfluids. From the Nernst-effect data the temperature and magnetic-field dependence of the transport entropy of magnetic-flux lines has been derived. For the magnetic field applied parallel to the c axis of the films, approximately the same value of the transport entropy was found for YBa2Cu3O7-? and Bi2Sr2CaCu2O8 x, showing that the different anisotropy of these materials has little influence on this quantity.
From our experimental data in the mixed state and the fluctuation regime well above the mean-field critical temperature, the upper-critical-field slope was derived. For both regimes, dHc2/dT=-2.4±0.2 and -2.5±0.2 T/K for YBa2Cu3O7-? and Bi2Sr2CaCu2O8 x, corresponding to a Ginzburg-Landau coherence length ?ab=14.5 and 14.2 Å, respectively. The Hall resistivity ?xy of both materials showed a sign anomaly in the mixed state and scaled with the longitudinal resistivity ?xx as ?xy(T)=K-1?xx?(T) with ??2 and a magnetic-field-independent coefficient K