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A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenTue, 28 Nov 2023 18:51:37 +0000Current Dependence of the Energy Gap in Superconductors: A Direct Measurement
https://resolver.caltech.edu/CaltechTHESIS:10052015-153159106
Authors: Mitescu, Catalin Dan
Year: 1966
DOI: 10.7907/7c8f-ya60
<p>From the tunneling characteristics of a <i>tin-tin oxide-lead</i> junction, a direct measurement has been made of the energy-gap variation for a superconductor carrying a current in a compensated geometry. Throughout the region investigated – several temperatures near T<sub>c</sub> and down to a reduced temperature t = 0.8 –the observed <i>current</i> dependence agrees quite well with predictions based on the Ginzburg-Landau-Gor’kov theory. Near T<sub>c</sub> the predicted <i>temperature</i> dependence is also well verified, though deviations are observed at lower temperatures; even for the latter, the data are internally consistent with the temperature dependence of the experimental critical current. At the lowest temperature investigated, t = 0.8, a small “Josephson” tunneling current allowed further a direct measurement of the electron drift velocity at low current densities. From this, a preliminary experimental value of the critical velocity, believed to be the first reported, can be inferred in the basis of Ginzburg-Landau theory. For tin at t = 0.8, we find v<sub>c</sub> = 87 m/sec. This value does not appear fully consistent with those predicted by recent theories for superconductors with short electronic mean-free-paths. </p>https://thesis.library.caltech.edu/id/eprint/9201Superfluid Drag in Helium II
https://resolver.caltech.edu/CaltechETD:etd-06022004-140958
Authors: Andelin, John Philip
Year: 1967
DOI: 10.7907/61B5-QS42
The drag on a 1. 21 millimeter diameter sphere due to flowing superfluid helium II was measured at 1.6°K, 1.8°K, 2.0°K and 2.13°K in a "superfluid wind tunnel" (test region 3 cm long and 1 cm diameter) in which 500Å Millipore was used as the barrier to normal component flow for the first experiments, and 100 Å Millipore for the later ones. The superfluid flow through the tunnel was much steadier than it was in similar tunnels used in previous experiments. The velocity of the superfluid was calculated from a measurement of the loss rate of liquid helium in a standpipe at the entrance to the tunnel. (This design eliminated uncertainty in the corrections for evaporation.) The test sphere was mounted 1.3 cm from the axis of a quartz torsion fiber assembly, on which was also mounted a magnetic dipole. The drag was then measured using a null technique in which the torque due to the drag on the sphere in the flowing superfluid was balanced by the torque on the dipole due to an externally applied magnetic field. After corrections were made for a backflow of normal component which leaked through the Millipore, the results were as follows: Within the experimental error, zero drag was often observed for superfluid velocities up to 0.21 cm/ sec at 1.6°K, 0.48 cm/sec at 1.8°K, 1.1 cm/sec at 2.0°K and 4.4 cm/sec at 2.13°K. (These velocities do not necessarily represent superfluid critical velocities, but they are probably limiting velocities imposed by the measurement technique.) At 2.13°K, only zero drag was observed, but at the other three temperatures, drag was also observed with steady values between zero and a drag comparable to that which would be produced by an ordinary, low viscosity liquid having the density and velocity of the superfluid. These results are qualitatively consistent with the Onsager-Feynman model of quantized vortices, but the velocities at which zero drag was observed are much larger than quantitative predictions based on this model.https://thesis.library.caltech.edu/id/eprint/2383Verification of long range quantum phase coherence in superconducting tin utilizing electromagnetically stabilized Josephson junctions
https://resolver.caltech.edu/CaltechETD:etd-09232002-105509
Authors: Vant-Hull, Lorin Lee
Year: 1967
DOI: 10.7907/PHBD-K565
<p>de Broglie wave interferometers have been constructed, successfully utilizing extended superconducting links of tin to couple coherently the quantum phases of two Josephson junctions. The current transmitted by these devices was periodic in the enclosed flux (periodicity (.9 ± .3) h/2e) demonstrating unambiguously coherence of the superconducting order parameter over 1.33 meters.</p>
<p>We may define a "normal" Josephson junction as one for which the quantum mechanical coupling energy is insufficient to overcome the disruptive effects of noise. Consequently, the relative phase of the coupled superconductors is not stabilized and the "zero voltage" Josephson current fluctuates, (bandwidth ~ 2eV<sub>noise</sub>/h) averaging to zero. Two such "normal" junctions have been coupled to an electromagnetic cavity formed by the superconducting arms of a quantum interferometer (junction separation 1. 33 meters). With an applied steady voltage such that the Josephson current excites a normal mode of the cavity, a coherent radiation field is built up. This coherent radiation feeds back to the junctions forcing time coherence on the phase precession, resulting in a dynamic stabilization of the junctions.</p>
<p>Under these conditions a series of "constant voltage steps" has been observed in the I-V characteristic of one interferometer. The Josephson frequency associated with these steps is shown to be characteristic of the electromagnetic resonant modes of the cavity. Frequency modulation analysis, combined with a detailed analysis of the cavity-junction combination, predicts such maxima in the tunneling of pairs when a "selection rule", (m + n) even, is obeyed. Here m/2 is the number of flux quanta (h/2e) linking the interferometer and n is the order of the cavity resonance excited. Experimental confirmation of the detailed predictions of the analysis is presented.</p>
https://thesis.library.caltech.edu/id/eprint/3705Experimental studies of weak superconductivity
https://resolver.caltech.edu/CaltechTHESIS:05022016-090342960
Authors: Kirschman, Randall Keenan
Year: 1972
DOI: 10.7907/y9wt-9r69
<p>Experimental investigations were made of the nature of weak superconductivity in a structure having well-defined, controllable characteristics and geometry. Controlled experiments were made possible by using a thin-film structure which was entirely metallic and consisted of a superconducting film with a localized section that was weak in the sense that its transition temperature was depressed relative to the rest of the film. The depression of transition temperature was brought about by underlaying the superconductor with a normal metal.</p>
<p>The DC and AC electrical characteristics of this structure were studied. It was found that this structure exhibited a non-zero, time-average supercurrent at finite
voltage to at least .2 mV, and generated an oscillating electric potential at a frequency given by the Josephson relation. The DC V-I characteristic and the amplitude of the AC oscillation were found to be consistent with a two-
fluid (normal current-supercurrent) model of weak super-conductivity based on e thermodynamically irreversible process of repetitive phase-slip, and featuring a periodic time dependence in the amplitude of the superconducting order parameter.</p>
<p>The observed linewidth of the AC oscillation could be accounted for by incorporating Johnson noise in the two-fluid model.</p>
<p>Experimentally it was found that the behavior of a short (length on the order of the coherence distance) weak superconductor could be characterized by its critical current and normal-state resistance, and an empirical expression was obtained for the time dependence of the super-current and voltage.</p>
<p>It was found that the results could not be explained on the basis of the theory of the Josephson junction.</p>
https://thesis.library.caltech.edu/id/eprint/9693An experimental study of nonequilibrium superconductivity
https://resolver.caltech.edu/CaltechTHESIS:09172010-102557442
Authors: Yu, Ming Lun
Year: 1974
DOI: 10.7907/2ZWA-Y056
<p>Experimental investigations were made on the transport properties of several superconducting systems in which the superconducting state was out of thermodynamic equilibrium. In such systems, the dynamics of the pairing processes governs the restoration of equilibrium. Systematic investigations into the nonequilibrium dynamics were made possible by creating a steady state depairing region at a supernormal interface with a transport current. Information Was obtained by direct probing of the potential profiles in the depairing region. Measurements were made with superconducting microcircuits with high spatial resolution extending to the scale of the wavelength of light which limits our fabrication system.</p>
<p>It was found that at the depairing region, the experimental results can be described by the existence of two distinct potentials, one for the single electron excitations and one for the electron pairs. A theory for the dynamics of the nonequilibrium process was proposed and succeeded in accounting for most of the experimental observations. The relaxation time associated with this dynamics is found to be longer than the usual equilibrium relaxation time for the condensed electron pairs.</p>
<p>Similar measurements were extended to nonequilibrium regions close to current carrying interfaces between two different superconductors. In this case it was found that the two-potential measurements performed within the nonequilibrium region can be described in terms of a time dependent boundary condition at the interface.</p>
<p>To study the nonequilibrium superconductivity associated with this time dependent boundary condition in a controlled way, two such superconducting boundaries were brought very close to each other, creating a well defined boundary region less than one micron in length. It was found that the boundary region showed Josephson-like effects as well as a unique phenomenon in which electromagnetic radiation increases the apparent superconducting transition temperature. A model for this situation was proposed in which two complex wavefunctions with independent phase angles interfere quantum mechanically inside the boundary region. This model successfully explains most of the observed phenomena associated with the boundary region in the voltage sustaining nonequilibrium state.</p>
https://thesis.library.caltech.edu/id/eprint/6034Investigations of noise and of quantum interference in proximity effect bridges
https://resolver.caltech.edu/CaltechTHESIS:07182014-085322954
Authors: Decker, Stephen K.
Year: 1975
DOI: 10.7907/HZMZ-3C32
<p>This work reports investigations upon weakly superconducting proximity
effect bridges. These bridges, which exhibit the Josephson
effects, are produced by bisecting a superconductor with a short
(<1µ) region of material whose superconducting transition temperature
is below that of the adjacent superconductors. These bridges are
fabricated from layered refractory metal thin films whose transition
temperature will depend upon the thickness ratio of the materials
involved. The thickness ratio is changed in the area of the bridge
to lower its transition temperature. This is done through novel
photolithographic techniques described in the text, Chapter 2.</p>
<p>If two such proximity effect bridges are connected in parallel,
they form a quantum interferometer. The maximum zero voltage current
through this circuit is periodically modulated by the magnetic flux
through the circuit. At a constant bias current, the modulation of
the critical current produces a modulation in the dc voltage across
the bridge. This change in dc voltage has been found to be the result
of a change in the internal dissipation in the device. A simple model
using lumped circuit theory and treating the bridges as quantum
oscillators of frequency ω = 2eV/h, where V is the time average
voltage across the device, has been found to adequately describe the
observed voltage modulation.</p>
<p>The quantum interferometers have been converted to a galvanometer
through the inclusion of an integral thin film current path which
couples magnetic flux through the interferometer. Thus a change in
signal current produces a change in the voltage across the interferometer
at a constant bias current. This work is described in Chapter
3 of the text.</p>
<p>The sensitivity of any device incorporating proximity effect
bridges will ultimately be determined by the fluctuations in their
electrical parameters. He have measured the spectral power density of
the voltage fluctuations in proximity effect bridges using a room
temperature electronics and a liquid helium temperature transformer
to match the very low (~ 0.1 Ω) impedances characteristic of these
devices.</p>
<p>We find the voltage noise to agree quite well with that predicted
by phonon noise in the normal conduction through the bridge plus a
contribution from the superconducting pair current through the bridge
which is proportional to the ratios of this current to the time average
voltage across the bridge. The total voltage fluctuations are given
by <V^2(f ) > = 4kTR^2_d I/V where R_d is the dynamic resistance, I the
total current, and V the voltage across the bridge . An additional
noise source appears with a strong 1/f^(n) dependence , 1.5 < n < 2, if the
bridges are fabricated upon a glass substrate. This excess noise,
attributed to thermodynamic temperature fluctuations in the volume of
the bridge, increases dramatically on a glass substrate due to the
greatly diminished thermal diffusivity of the glass as compared to sapphire.</p>
https://thesis.library.caltech.edu/id/eprint/8559I. Equilibrium and Non-Equilibrium Superconducting Quantum Interference Phenomena. II. Applications of Superconducting Quantum Magnetometer
https://resolver.caltech.edu/CaltechTHESIS:05262023-012237053
Authors: Wang, Run-Han
Year: 1975
DOI: 10.7907/wkq0-rh72
<p>Experimental investigations were made on the equilibrium and nonequilibrium quantum interference phenomena in superconductors. A new instrument was developed for measuring magnetic susceptibility and moment with an improvement of sensitivity of nearly two orders of magnitude over the conventional magnetometers.</p>
<p>A quantitative determination was made on the relaxation rate of a perturbed superconducting phase to relax back to its equilibrium state. The temperature-dependent relaxation time is found to be consistent with the known quasiparticle relaxation time in nonequilibrium superconductors. The effect of relaxation processes on macroscopic quantum interference phenomena was observed for the first time through the temperature modulation of quantum interference in multiply connected superconducting circuits, and implies a modification of the Josephson frequency-voltage relationship. A relaxation model was developed and found adequate to account for nearly all the experimental results. </p>
<p>The development and performance of an ultrasensitive superconducting magnetometer instrument based on various unique properties of superconductors were discussed. Applications of this instrument over a temperature range of 200°K in the fields of magnetochemistry, paleomagnetism and the study of fluctuation effects in superconductivity were illustrated with actual data.</p>https://thesis.library.caltech.edu/id/eprint/15209Nonequilibrium Properties of Superconducting-Normal Metal Boundaries
https://resolver.caltech.edu/CaltechTHESIS:06052023-222013347
Authors: Palmer, David William
Year: 1975
DOI: 10.7907/9zt7-fh22
<p>Three different nonequilibrium states of superconductivity were
experimentally investigated. These experiments included studies of:
the electron transport properties across the interface of a single
superconductor and a normal metal, electron transport between two
weakly coupled superconductors, and electron transport through periodic
arrays of hundreds of weakly coupled superconductors. In all of these
investigations most emphasis was given to quantum interference effects.
All three nonequilibrium states produced time dependent voltages which
were investigated both by inducing step structure on the I-V characteristics
with external microwave radiation, and by directly measuring
the internal oscillating potentials. Josephson's relationship between
frequency and voltage, hν = 2eV , was found to be significantly
modified in certain well defined circumstances leading to a better
understanding of quantum rules for nonequilibrium weak superconductivity,</p>
<p>To produce the structures necessary for these studies, thin
films of soft, hard, and compound superconductors were used. These
thin films were defined into strongly superconducting, weakly superconducting,
and normal regions using highly refined photolithographic
and etching techniques to provide boundary definition of less than
250Å, depth determination to within 5Å, and lateral detail resolution
of 2000Å.</p>
<p>All results can be described using concepts of macroscopic
quantum mechanics and simple equivalent circuit representations for
the nonequilibrium electron state. Macroscopic wave function boundary
conditions and coupling effects were examined with particular emphasis
on characteristic coupling energy, decay lengths, healing time, and
thermal constants. The geometric dimensions and equilibrium electronic
properties were used to predict the critical current-temperature
dependence and the rf responsivity. In some circumstances a single
superconductor-normal metal interface was found to show quantum interference
effects much as though the superconductor were interfering
with itself. Thermal effects, which can dominate at high voltages,
were calculated, measured, and designed around so that the time response
of the macroscopic quantum state could be investigated down to 10<sup>-12</sup>sec.</p>https://thesis.library.caltech.edu/id/eprint/16086I. The phase-current relation at zero voltage in proximity effect bridges. II. The interaction of proximity effect bridges with superconducting microstrip resonators
https://resolver.caltech.edu/CaltechTHESIS:12062011-081306612
Authors: Ganz, Tomas
Year: 1976
DOI: 10.7907/N97W-BW02
<p>Experimental investigations on the proximity effect bridge (a Josephson device) at zero voltage and at finite voltages in the µV range are reported.</p>
<p>The phase-super current relation at zero voltage was measured using an asymmetric superconducting quantum
interferormeter circuit. The data are in agreement with the Josephson supercurrent-phase relation I_S=I_C sinδ with deviation less than 5% of the critical current I_c. The supercurrent density in the measured bridges reached as high as 50-100 µA/µm^2.</p>
<p>Using microcircuitry techniques, proximity effect bridges
were strongly coupled to superconducting microstrip resonators. Selfinduced steps in the I-V characteristics of bridges coupled to resonators were observed in the GHz region at voltages (frequencies) corresponding to the expected modes of the resonators. Two types of steps were
seen depending on whether the resonator impedance on resonance was much higher or much smaller than the bridge resistance. A simple two fluid model of the bridge-resonator circuit was developed and the size and shape of self-induced steps were calculated for a generalized
Josephson oscillator relation I_S = I_c(l-q + q sin∫ 2e/hV dt) where q = 1 corresponds to the original Josephson relation and q = 1/2 represents the phase slip regime. At low critical currents (I_c < 10 µA) and low voltages (V < 3µV) the size and shape of experimentally observed
self-induced steps agree with the q = 1 model. At higher voltages and/or critical currents the step size increasingly deviates from the q = 1 model towards q = 1/2. These observations are interpreted to indicate a progressive reduction of the amplitude of the oscillating
Josephson supercurrent in proximity effect bridges from I_c towards I_c /2 as the critical current and/or voltage are increased.</p>
https://thesis.library.caltech.edu/id/eprint/6748Radio frequency studies of surface resistance and critical magnetic field of type I and type II superconductors
https://resolver.caltech.edu/CaltechTHESIS:07312014-142921170
Authors: Yogi, Tadashi
Year: 1977
DOI: 10.7907/p14g-5751
<p>The surface resistance and the critical magnetic field of lead electroplated on copper were studied at 205 MHz in a half-wave coaxial resonator. The observed surface resistance at a low field level below 4.2°K could be well described by the BCS surface resistance with the addition of a temperature independent residual resistance. The available experimental data suggest that the major fraction of the residual resistance in the present experiment was due to the presence of an oxide layer on the surface. At higher magnetic field levels the surface resistance was found to be enhanced due to surface imperfections.</p>
<p>The attainable rf critical magnetic field between 2.2°K and T_c of lead was found to be limited not by the thermodynamic critical field but rather by the superheating field predicted by the one-dimensional Ginzburg-Landau theory. The observed rf critical field was very close
to the expected superheating field, particularly in the higher reduced temperature range, but showed somewhat stronger temperature dependence than the expected superheating field in the lower reduced temperature
range.</p>
<p>The rf critical magnetic field was also studied at 90 MHz for pure tin and indium, and for a series of SnIn and InBi alloys spanning both type I and type II superconductivity. The samples were spherical with typical diameters of 1-2 mm and a helical resonator was used to generate the rf magnetic field in the measurement. The results of pure samples of tin and indium showed that a vortex-like nucleation of the normal phase was responsible for the superconducting-to-normal phase transition in the rf field at temperatures up to about 0.98-0.99 T_c' where the ideal superheating limit was being reached. The results of the alloy samples showed that the attainable rf critical fields near T_c were well described by the superheating field predicted by the one-dimensional GL theory in both the type I and type II regimes. The measurement was also made at 300 MHz resulting in no significant change in the rf critical field. Thus it was inferred that the nucleation time of the normal phase, once the critical field was reached, was small compared with the rf period in this frequency range.</p>
https://thesis.library.caltech.edu/id/eprint/8624A study of a new electromechanical energy conversion process using superconducting frequency modulated resonators
https://resolver.caltech.edu/CaltechTHESIS:07212014-085741427
Authors: Yen, Huan-chun
Year: 1977
DOI: 10.7907/VY1S-9971
<p>Experimental demonstrations and theoretical analyses of a new
electromechanical energy conversion process which is made feasible only
by the unique properties of superconductors are presented in this
dissertation. This energy conversion process is characterized by a
highly efficient direct energy transformation from microwave energy
into mechanical energy or vice versa and can be achieved at high
power level. It is an application of a well established physical
principle known as the adiabatic theorem (Boltzmann-Ehrenfest theorem)
and in this case time dependent superconducting boundaries provide
the necessary interface between the microwave energy on one hand and
the mechanical work on the other. The mechanism which brings about
the conversion is another known phenomenon - the Doppler effect. The
resonant frequency of a superconducting resonator undergoes continuous
infinitesimal shifts when the resonator boundaries are adiabatically
changed in time by an external mechanical mechanism. These small
frequency shifts can accumulate coherently over an extended period of
time to produce a macroscopic shift when the resonator remains
resonantly excited throughout this process. In addition, the electromagnetic
energy in s ide the resonator which is proportional to the
oscillation frequency is al so accordingly changed so that a direct
conversion between electromagnetic and mechanical energies takes place.
The intrinsically high efficiency of this process is due to the electromechanical
interactions involved in the conversion rather than a
process of thermodynamic nature and therefore is not limited by the
thermodynamic value. </p>
<p>A highly reentrant superconducting resonator resonating in the
range of 90 to 160 MHz was used for demonstrating this new conversion
technique. The resonant frequency was mechanically modulated at a
rate of two kilohertz. Experimental results showed that the time
evolution of the electromagnetic energy inside this frequency modulated
(FM) superconducting resonator indeed behaved as predicted and thus
demonstrated the unique features of this process. A proposed usage
of FM superconducting resonators as electromechanical energy conversion
devices is given along with some practical design considerations.
This device seems to be very promising in producing high power (~10W/cm^3)
microwave energy at 10 - 30 GHz. </p>
<p>Weakly coupled FM resonator system is also analytically studied
for its potential applications. This system shows an interesting
switching characteristic with which the spatial distribution of microwave
energies can be manipulated by external means. It was found that
if the modulation was properly applied, a high degree (>95%) of unidirectional
energy transfer from one resonator to the other could be
accomplished. Applications of this characteristic to fabricate high
efficiency energy switching devices and high power microwave
pulse generators are also found feasible with present superconducting
technology. </p>
https://thesis.library.caltech.edu/id/eprint/8571Phase and amplitude stabilization of superconducting resonators
https://resolver.caltech.edu/CaltechETD:etd-03042008-083340
Authors: Delayen, Jean Roger
Year: 1978
DOI: 10.7907/ZS1J-1E35
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.
This thesis, which stemmed from the superconducting heavy-ion accelerator project at Caltech, deals with the problem of phase and amplitude stabilization of the fields in superconducting resonators. The problem arises from the fast ([...]50 Hz) resonator eigenfrequency modulation of magnitude ([...]100 Hz) which is much larger than the resonator bandwidth ([...]10 Hz). The problem is compounded by the fact that the coupling between the electrical and mechanical modes of the resonator can lead to instabilities (ponderomotive instabilities). The solution suggested involves operating the resonators in self-excited loops, and electronically modifying the loop parameters in order to lock the loop oscillations to an external phase and amplitude reference without attempt to modify the instantaneous resonator eigenfrequency. It is found that this method of phase stabilization is well suited to resonators with small energy contents and small eigenfrequency deviations since the power required is equal to their product; this occurs when the loaded bandwidth of the resonator is twice the maximum eigenfrequency deviation to be compensated for. It is also found that when the loop is free-running, the field amplitude is stable and no ponderomotive instabilities are present as long as the non-ideal effects are limited. When the loop is locked to an external phase and amplitude reference, ponderomotive instabilities can occur; however, the loop can be made stable by adjustment of the loop phase shift, and the stable range can be increased by using high amplitude and phase feedback gains. It is also found that under certain feedback conditions, the error on the particle energy gain can be made to vanish, although residual phase and amplitude errors are still present. A microprocessor-controlled feedback system based on this analysis is then described and results of experiments performed in conjunction with a 150 MHz lead (Pb) plated superconducting split-ring resonator are presented. The experiments show excellent agreement with the analysis.
https://thesis.library.caltech.edu/id/eprint/867Microwave Dynamics of Quasiparticles and Critical Fields in Superconducting Films
https://resolver.caltech.edu/CaltechTHESIS:03032016-130102032
Authors: Sridhar, Srinivas
Year: 1983
DOI: 10.7907/xyc7-qy52
<p>The microwave response of the superconducting state in equilibrium and non-equilibrium configurations was examined experimentally and analytically. Thin film superconductors were mostly studied in order to explore spatial effects. The response parameter measured was the surface impedance.</p>
<p>For small microwave intensity the surface impedance at 10 GHz was measured for a variety of samples (mostly Sn) over a wide range of sample thickness and temperature. A detailed analysis based on the BCS theory was developed for calculating the surface impedance for general thickness and other experimental parameters. Experiment and theory agreed with each other to within the experimental accuracy. Thus it was established that the samples, thin films as well as bulk, were well characterised at low microwave powers (near equilibrium).</p>
<p>Thin films were perturbed by a small dc supercurrent and the effect on the superconducting order parameter and the quasiparticle response determined by measuring changes in the surface resistance (still at low microwave intensity and independent of it) due to the induced current. The use of fully superconducting resonators enabled the measurement of very small changes in the surface resistance (< 10<sup>-9</sup> Ω/sq.). These experiments yield information regarding the dynamics of the order parameter and quasiparticle systems. For all the films studied the results could be described at temperatures near T<sub>c</sub> by the thermodynamic depression of the order parameter due to the static current leading to a quadratic increase of the surface resistance with current.</p>
<p>For the thinnest films the low temperature results were surprising in that the surface resistance <i>decreased</i> with increasing current. An explanation is proposed according to which this decrease occurs due to an additional high frequency quasiparticle current caused by the combined presence of both static and high frequency fields. For frequencies larger than the inverse of the quasiparticle relaxation time this additional current is out of phase (by π) with the microwave electric field and is observed as a decrease of surface resistance. Calculations agree quantitatively with experimental results. This is the first observation and explanation of this non-equilibrium quasiparticle effect.</p>
<p>For thicker films of Sn, the low temperature surface resistance was found to <i>increase</i> with applied static current. It is proposed that due to the spatial non-uniformity of the induced current distribution across the thicker films, the above purely temporal analysis of the local quasiparticle response needs to be generalised to include space and time non-equilibrium effects.</p>
<p>The nonlinear interaction of microwaves arid superconducting films was also examined in a third set of experiments. The surface impedance of thin films was measured as a function of the incident microwave magnetic field. The experiments exploit the ability to measure the absorbed microwave power and applied microwave magnetic field absolutely. It was found that the applied surface microwave field could not be raised above a certain threshold level at which the absorption increased abruptly. This critical field level represents a dynamic critical field and was found to be associated with the penetration of the app1ied field into the film at values well below the thermodynamic critical field for the configuration of a field applied to one side of the film. The penetration occurs despite the thermal stability of the film which was unequivocally demonstrated by experiment. A new mechanism for such penetration via the formation of a vortex-antivortex pair is proposed. The experimental results for the thinnest films agreed with the calculated values of this pair generation field. The observations of increased transmission at the critical field level and suppression of the process by a metallic ground plane further support the proposed model.</p>
https://thesis.library.caltech.edu/id/eprint/9602