42 research outputs found
General solution for the complex frequency shift in microwave measurements of thin films
Perturbation of a microwave cavity by a small sample with variable dielectric, magnetic, or conducting properties is considered. The complex frequency shift is derived in terms of a volume integral, or equivalently, in terms of a surface integral. These are used to obtain a general formula for thin films in the microwave electric field maximum. The complex frequency shift depends on the depolarization factor of the film and on its thickness in a nontrivial way. The previously known expressions for the complex frequency shift are shown to be good approximations of the present solution in the low and high conductivity limits. Our formula is applied to calculate the signal shapes in superconducting films of various geometric parameters and conductivities. It is shown that a diversity of signal shapes can result, and experimental support of those shapes is provided. The role of the dielectric substrate on which the thin film is grown is simply reduced to an asymmetry effect
Microwave response of thin niobium films under perpendicular static magnetic fields
The microwave response of high quality niobium films in a perpendicular
static magnetic field has been investigated. The complex frequency shift was
measured up to the upper critical fields. The data have been analyzed by the
effective conductivity model for the type-II superconductors in the mixed
state. This model is found to yield consistent results for the coherence
lengths in high-kappa superconducting samples, and can be used with HTSC even
at temperatures much below T_c. It is shown that for samples with high values
of depinning frequency, one should measure both components of the complex
frequency shift in order to determine the flow resistivity. The thick Nb film
(160 nm) has low resistivity at 10 K, comparable to the best single crystals,
and low kappa value. In contrast, the thinnest (10 nm) film has kappa ~ 9.5 and
exhibits a high depinning frequency (~20 GHz). The upper critical field
determined from microwave measurements is related to the radius of
nonoverlaping vortices, and appears to be larger than the one determined by the
transition to the normal state.Comment: 8 pages, 7 figures; submitted to PRB; measured rho_n; changes due to
the referees' comments (abstract, conclusions, extended introduction
A matrix product solution for a nonequilibrium steady state of an XX chain
A one dimensional XX spin chain of finite length coupled to reservoirs at
both ends is solved exactly in terms of a matrix product state ansatz. An
explicit representation of matrices of fixed dimension 4 independent of the
chain length is found. Expectations of all observables are evaluated, showing
that all connected correlations, apart from nearest neighbor z-z, are zero.Comment: 11 page
Pseudogap in the Optical Spectra of UPd_2Al_3
The in-plane optical conductivity of UPd_2Al_3 was measured at temperatures
K in the spectral range from 1 cm^{-1} to 40 cm^{-1} (0.14
meV to 5 meV). As the temperature decreases below 25 K a well pronounced
pseudogap of 0.2 meV develops in the optical response. In addition we observe a
narrow conductivity peak at zero frequency which at 2 K is less than 1 cm^{-1}
wide but which contains only a fraction of the delocalized carriers. The gap in
the electronic excitations might be an inherent feature of the heavy fermioin
ground state.Comment: 4 pages, 4 figures (submitted to Phys. Rev. Lett.
A Transport and Microwave Study of Superconducting and Magnetic RuSr2EuCu2O8
We have performed susceptibility, thermopower, dc resistance and microwave
measurements on RuSr2EuCu2O8. This compound has recently been shown to display
the coexistence of both superconducting and magnetic order. We find clear
evidence of changes in the dc and microwave resistance near the magnetic
ordering temperature (132 K). The intergranular effects were separated from the
intragranular effects by performing microwave measurements on a sintered
ceramic sample as well as on a powder sample dispersed in an epoxy resin. We
show that the data can be interpreted in terms of the normal-state resistivity
being dominated by the CuO2 layers with exchange coupling to the Ru moments in
the RuO2 layers. Furthermore, most of the normal-state semiconductor-like
upturn in the microwave resistance is found to arise from intergranular
transport. The data in the superconducting state can be consistently
interpreted in terms of intergranular weak-links and an intragranular
spontaneous vortex phase due to the ferromagnetic component of the
magnetization arising from the RuO2 planes.Comment: 20 pages including 6 figures in pdf format. To be published in Phys.
Rev.