Verlag des Forschungszentrums Jülich
JUEL-3773
YBa2Cu307-x bulk single crystals and epitaxial thin films exhibit a steep drop of
the surface resistance Rs just below the transition temperature Tc to values at 77 K
which are about 300µ[Omega] at 10 GHz. The observed dependence on the frequency f
can be described by fA with sample-dependent A values between 1.5 and 2. At lower
temperatures Rs(T) is substantially different both from that predicted by BCS
theory and observed experimentally for conventional superconductors. Typically, Rs(T)
exhibits a plateau between 30 and 70 K corresponding to a frequency-dependent
maximum in the real part of the conductivity. The observed correlation of the height of
this maximum to the defect density indicates that above about 30 K the
electrodynamic response is dominated by a strong decrease of the quasiparticle lifetime just
below Tc. Below Tc/2 the situation is still quite unclear. The observed temperature
dependences both for Rs and the London penetration depth [lambda]L range from weak
exponential corresponding to energy gaps more than two times smaller than expected
from weak-coupling BCS theory over quadratic to linear dependences. In any case,
a high residual surface resistance Rres = Rs(T -+ 0) remains. In particular, the low
temperature regime was found to be strongly affected by the amount and ordering of
oxygen vacancies in the copper-oxygen chains. The active role of the copper-oxygen
chains as a conducting subsystem was worked out theoretically within a
strongcoupling theory based on phonon-mediated superconductivity and can quantitatively
explain some of the observed results. In particular, gapless superconductivity can
be easily achieved from magnetic pair breaking in the copper-oxygen chains. d-wave
superconductivity, as expected for superconductivity mediated by spin fluctuations
instead of phonons, leads to a gapless behavior as well. The observed temperature
dependences of [lambda]L and Rs can be explained to some extent within d-wave models.
In contradiction to the hole doped cuprates (as e.g. YBa2Cu307-x), the electron
doped cuprates exhibit an electromagnetic response which is similar to predictions
for an isotropic s-wave order parameter.
The dependence of the surface resistance on the amplitude of the high frequency
magnetic field is of particular importance as it defines the limitation of the
applicability of high temperature superconductor films for passive microwave devices
operating at elevated levels of microwave power. The observed dependences were
found to be strongly sampIe dependent. For the best films of YBa2Cu307-x Rs is
independent of the field amplitude up to about 200 Oe at a temperature of 10 K,
limited in most cases by a field breakdown at a certain field level ( quench). The field
amplitude at which the quench occurs decreases with increasing temperature. Such
breakdown is most likely to originate from local defects. However, even films with
low RB values at low field levels sometimes exhibit strong field dependences at field
levels of a few Oerstedt. Generally, the observed field dependences for thin films of
the TI-Ba-Ca-Cu-O compound are stronger than for YBa2Cu307-x.
In the vortex state the electrodynamic response was found to be determined by
the viscous motion of vortices within their pinning potentials. The analysis of the
observed field dependences of the surface impedance at different temperatures allows
to determine the temperature dependence of the average pinning force constant and
the vortex viscosity (at T << Tc). In spite of strongly sample dependent results the
temperature dependence of the pinning force constant can be understood assuming
pinning potentials which are smeared due to thermal fluctuations. For the vortex
viscosity the Bardeen-Stephen formula is in agreement with the observed temperature
dependences.
YBa2Cu307-x bulk single crystals and epitaxial thin films exhibit a steep drop of
the surface resistance Rs just below the transition temperature Tc to values at 77 K
which are about 300µ[Omega] at 10 GHz. The observed dependence on the frequency f
can be described by fA with sample-dependent A values between 1.5 and 2. At lower
temperatures Rs(T) is substantially different both from that predicted by BCS
theory and observed experimentally for conventional superconductors. Typically, Rs(T)
exhibits a plateau between 30 and 70 K corresponding to a frequency-dependent
maximum in the real part of the conductivity. The observed correlation of the height of
this maximum to the defect density indicates that above about 30 K the
electrodynamic response is dominated by a strong decrease of the quasiparticle lifetime just
below Tc. Below Tc/2 the situation is still quite unclear. The observed temperature
dependences both for Rs and the London penetration depth [lambda]L range from weak
exponential corresponding to energy gaps more than two times smaller than expected
from weak-coupling BCS theory over quadratic to linear dependences. In any case,
a high residual surface resistance Rres = Rs(T -+ 0) remains. In particular, the low
temperature regime was found to be strongly affected by the amount and ordering of
oxygen vacancies in the copper-oxygen chains. The active role of the copper-oxygen
chains as a conducting subsystem was worked out theoretically within a
strongcoupling theory based on phonon-mediated superconductivity and can quantitatively
explain some of the observed results. In particular, gapless superconductivity can
be easily achieved from magnetic pair breaking in the copper-oxygen chains. d-wave
superconductivity, as expected for superconductivity mediated by spin fluctuations
instead of phonons, leads to a gapless behavior as well. The observed temperature
dependences of [lambda]L and Rs can be explained to some extent within d-wave models.
In contradiction to the hole doped cuprates (as e.g. YBa2Cu307-x), the electron
doped cuprates exhibit an electromagnetic response which is similar to predictions
for an isotropic s-wave order parameter.
The dependence of the surface resistance on the amplitude of the high frequency
magnetic field is of particular importance as it defines the limitation of the
applicability of high temperature superconductor films for passive microwave devices
operating at elevated levels of microwave power. The observed dependences were
found to be strongly sampIe dependent. For the best films of YBa2Cu307-x Rs is
independent of the field amplitude up to about 200 Oe at a temperature of 10 K,
limited in most cases by a field breakdown at a certain field level ( quench). The field
amplitude at which the quench occurs decreases with increasing temperature. Such
breakdown is most likely to originate from local defects. However, even films with
low RB values at low field levels sometimes exhibit strong field dependences at field
levels of a few Oerstedt. Generally, the observed field dependences for thin films of
the TI-Ba-Ca-Cu-O compound are stronger than for YBa2Cu307-x.
In the vortex state the electrodynamic response was found to be determined by
the viscous motion of vortices within their pinning potentials. The analysis of the
observed field dependences of the surface impedance at different temperatures allows
to determine the temperature dependence of the average pinning force constant and
the vortex viscosity (at T << Tc). In spite of strongly sample dependent results the
temperature dependence of the pinning force constant can be understood assuming
pinning potentials which are smeared due to thermal fluctuations. For the vortex
viscosity the Bardeen-Stephen formula is in agreement with the observed temperature
dependences.
Klein, Norbert
Electrodynamic properties of oxide superconductors
101 S., 2000
The electrodynamic properties of oxide high temperature superconductors are
reviewed. Most of the experimental data are from thin films and bulk single crystals of
YBa2Cu307-x , which - together with thin films of thallium-based oxide
superconductors - provide the lowest microwave losses making epitaxially grown thin films
attractive for applications in microwave technology. Among the various techniques
for the determination of the electrodynamic response of superconducting thin films
cavity, dielectric, and planar resonators are the most successful ones for the
frequency range from 1 to 100 GHz. For the millimeter and submillimeter wave range
nonresonant transmission techniques in frequency and time domain have been used
successfully. Bulk single crystals have been studied by various types of cavity
perturbation techniques.
The electrodynamic properties of oxide high temperature superconductors are
reviewed. Most of the experimental data are from thin films and bulk single crystals of
YBa2Cu307-x , which - together with thin films of thallium-based oxide
superconductors - provide the lowest microwave losses making epitaxially grown thin films
attractive for applications in microwave technology. Among the various techniques
for the determination of the electrodynamic response of superconducting thin films
cavity, dielectric, and planar resonators are the most successful ones for the
frequency range from 1 to 100 GHz. For the millimeter and submillimeter wave range
nonresonant transmission techniques in frequency and time domain have been used
successfully. Bulk single crystals have been studied by various types of cavity
perturbation techniques.
Neuerscheinungen
Schriften des Forschungszentrums Jülich
Ihre Ansprechperson
Heike Lexis
+49 2461 61-5367
zb-publikation@fz-juelich.de