62 research outputs found
DC-assisted microwave quenching of YBa2Cu3O7-{\delta} coplanar waveguide to a highly dissipative state
The paper reports on finding the effect of a strong change in the microwave
losses in an HTS-based coplanar waveguide (CPW) at certain values of the input
power Pin and direct current Idc. CPW on the basis of 150 nm thick
YBa2Cu3O7-{\delta} epitaxial film on a single crystal MgO substrate was studied
experimentally. A sharp and reversible transition of the CPW into a strongly
dissipative state at the certain meanings of Pin and Idc depending on
temperature was observed. Apparently the effect can be explained by
self-heating of HTS structure caused by magnetic flux flow under the joint
influence of MW and DC.Comment: 9 pages, 5 figures, 17 reference
Millimeter-wave surface impedance of optimally-doped Ba(Fe1-xCox)2As2 single crystals
Precision measurements of active and reactive components of in-plane
microwave surface impedance were performed in single crystals of
optimally-doped Fe-based superconductor Ba(Fe1-xCox)2As2 (x = 0.074, Tc = 22.8
K). Measurements in a millimeter wavelength range (Ka band, 35-40 GHz) were
performed using whispering gallery mode excitations in the ultrahigh quality
factor quasioptical sapphire disk resonator with YBa2Cu2O7 superconducting (Tc
= 90 K) end plates. The temperature variation of the London penetration depth
is best described by a power-law function, delta {\lambda}(T) is proportional
to T with the exponent n, n = 2.8, in reasonable agreement with radio-frequency
measurements on crystals of the same batch. This power-law dependence is
characteristic of a nodeless superconducting gap in the extended s-wave pairing
scenario with a strong pair-breaking scattering. The quasiparticle conductivity
of the samples, {\sigma}1(T), gradually increases with the decrease of
temperature, showing no peak below or at Tc, in notable contrast with the
behavior found in the cuprates. The temperature-dependent quasiparticle
scattering rate was analyzed in a two-fluid model, assuming the validity of the
Drude description of conductivity and generalized expression for the scattering
rate. This analysis allows us to estimate the range of the values of a residual
surface resistance from 3 to 6 mOhm.Comment: 7 pages, 7 figure
On the determination of the quasiparticle scattering rate in unconventional superconductors by microwave surface impedance
In the approximation of validity of the Drude expression for quasiparticle
conductivity and the assumption that all electrons at T = 0 transform into a
superfluide condensate, the expression for the quasiparticle scattering rate
tau -1 in terms of the real and imaginary parts of the microwave surface
impedance has been obtained. The resulting expression is a generalization of
the well-known expression for tau^-1, valid for omega x tau is much less 1, to
an arbitrary value of omega x tau, where omega is the frequency of the
microwave field. From experimental Ka-band impedance measurements, temperature
dependence of tau^-1 in superconducting single crystal pnictides
Ba(Fe1-xCox)2As2 is obtained using the generalized expression. It is shown that
under condition of the given work, the approximation omega x tau is much less 1
gives the considerable error in determination of tau^-1.Comment: 3 pages, 1 figur
Millimeter-wave study of London penetration depth temperature dependence in Ba(Fe0.926Co0.074)2As2 single crystal
In-plane surface Ka-band microwave impedance of optimally doped single
crystals of the Fe-based superconductor Ba(Fe0.926Co0.074)2As2 (Tc= 22.8K) was
measured. Sensitive sapphire disk quasi-optical resonator with high-Tc cuprate
conducting endplates was developed specially for Fe-pnictide superconductors.
It allowed finding temperature variation of London penetration depth in a form
of power law, namely \Delta \lambda (T)~ Tn with n = 2.8 from low temperatures
up to at least 0.6Tc consisted with radio-frequency measurements. This exponent
points towards nodeless state with pairbreaking scattering, which can support
one of the extended s-pairing symmetries. The dependence \lambda(T) at low
temperatures is well described by one superconducting small-gap (\Delta \cong
0.75 in kTc units, where k is Boltzman coefficient) exponential dependence.Comment: 6 pages, 2 figures, to be published in Low Temperature
Physics,vol.37, August 201
Unusual microwave response and bulk conductivity of very thin fese0.3te0.7 films as a function of temperature
Results of X-band microwave surface impedance measurements of FeSe1-xTex very
thin film are reported. The effective surface resistance shows appearance of
peak at T less and near Tc when plotted as function of temperature. The authors
suggests that the most well-reasoned explanation can be based on the idea of
the changing orientation of the microwave magnetic field at a SN phase
transition near the surface of a very thin film. The magnetic penetration depth
exhibits a power-law behavior of delta lambda proportional to T with an
exponent n = 2.4 at low temperatures, which is noticeably higher than in the
published results on FeSe1-xTexsingle crystal. However the temperature
dependence of the superfluid conductivity remains very different from the
behavior described by the BCS theory. Experimental results are fitted very well
by a two-gap model with delta1/kTc=0.43 and delta2/kTc=1.22,thus supporting
s(+-)- wave symmetry. The rapid increase of the quasiparticle scattering time
is obtained from the microwave impedance measurements.Comment: 13 pages, 13 figure
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