- Email: [email protected]
Triggering of phase slip in superconducting microbridges by impact of low-energy atoms
Volume 78A, number 2
TRIGGERING
PHYSICS LETTERS
21 July 1980
OF PHASE SLIP IN SUPERCONDUCTING
MICROBRIDGES
BY IMPACT OF LOW-ENERGY ATOMS
J. HOYLE Department
of Mechanical Engineering,
Auburn
University, Auburn, AL 36830,
USA
and R. HUMPHRIS Department of Nuclear Engineering and Engineering Physics, University of Virginia, Charlottesville, VA 22903, USA
Received 10 March 1980 Revised manuscript received 16 May 1980
Voltage pulses have been observed across a long superconducting microbridge biased below its critical current when ions or atoms of argon or helium with energies from 150 to 800 eV are incident on the bridge. These pulses are attributed to the triggering of phase slip at spatially discrete localized centers along the superconducting film by the impact of single incident particles.
An investigation has been made on the use of superconducting thin-film bridges as detectors of lowenergy atomic particles. This was done by biasing the bridge with a current slightly less than the critical current and observing the voltage pulses produced when ions and atoms of helium and argon with energies 150 to 800 eV were incident on the bridge [l-3] . In the course of these experiments, it was found that the magnitude and duration of the voltage pulses were independent of the energy and type of incident particle. The purpose of this note is to relate these initial observations to intrinsic phase-slip phenomena in these film bridges and to point out some important experimental implications of these results. The measurements reported here were made on tin-indiurn (about 14% indium) films approximately 500 A thick, evaporated onto glass substrates and mechanically scribed using a micromanipulator to form bridges approximately 1 pm wide by 2 mm in length. A close examination of the voltage-current curve and of the voltage-as-a-function-of-temperature curves at constant bias current (fig. 1) revealed a step-like structure. Voltage steps were first reported by Webb
rI-
--l
)--
I i I
I-
l-
3-
Fig. 1. Voltage across current-biased film as a function of temperature. The temperature is set at a particular value and the vertical dashes represent the recorder output. Curve b is an expanded scale of the initial portion.
181
Volume
78A, number
2
PHYSICS
and Warburton [4] , who observed them on the V-Z and V-T curves of superconducting tin whiskers. The voltage steps of thin wires and whiskers have been extensively studied by Meyer and Minnigerode [S] Skocpol et al. [6] studied voltage steps on the I/-Z curves of long tin microbridges configured similar to those of this investigation. Their studies showed that the voltage steps were due to spatially localized voltage units distributed along the bridge. Various explanations of these steps have been presented, based primarily on the idea of localized phase-slip centers, which occur at places along the bridge with locally lower critical current. For a bias current slightly larger than the local critical current at one of these centers, the magnitude of the Landau-Ginsburg order parameter executes a relaxation oscillation and the phase slips by 2n each time the magnitude drops to zero. The voltage across the phase-slip center is then related to the frequency of this relaxation oscillation by the Josephson relation, and the magnitude of the voltage is determined by local material parameters (e.g., critical current and relaxation times for elastic and inelastic scattering). More recently, Weitz et al. [7,8] investigated Josephson steps induced on the dc I- V curves of niobium contacts by far-infrared laser radiation. In comparing I- I’ curves of typical high-quality junctions with the available theoretical predictions of different’models (Werthamer’s FDSC, the RSJ model, and the TDGL theory), they found each to explain some features of the observed I- V curves and concluded that some synthesis of the present models might be necessary to closely account for their observation. A characteristic feature shown in b of fig. 1 is the approximate 100 /.LVheight of the first step, which is the approximate size of all the pulse heights produced by the incident beam for particle energies, either neutrals or ions, from 150 to 800 eV. It was determined that random shifts and slight changes in the pulse height spectra, sometimes producing multiple peaks, were due to beam alignment. This further indicated that specific sections of the film were sensitive to incident particles. Fig. 2 shows a PHA for argon ions striking the film. Note that the average of the peak is about 100 pV, which closely corresponds to the first step in the voltage-temperature curve for the same film shown in b of fig. 1. The duration of the pulses was typically 1 ms, 182
LETTERS
21 July 1980
25
50
75
100
125
PULSE HEIGHT (!A)
Fig. 2. Pulse height analysis for argon ions striking film which is biased slightly below critical current (44.5 /IA). It is believed that the double pulse occurring with energy change is due to beam alignment variations, resulting in the triggering of another phase-slip channel.
with no apparent dependence on incident particle energy. When phase slip occurs at an area of lower critical current, the region goes normal, and this localized hot spot spreads from the narrow-neck origin to some wider area of the microbridge where the heat conduction is such that further spreading of the normal region is halted. Then, after a period during which heat is dissipated through the substrate and the superconducting film, the normal area reverts to the superconducting state. A I-D transient thermal analysis considering the parameters of the film/substrate configuration revealed dissipation times on the order of milliseconds. Even with no ions striking the film, spontaneous pulses were observed when the bias current was great enough. As noted in fig. 3, for a film at a particular temperature, the bias current is set at 49 PA and a certain pulse height spectrum is recorded. As the bias current is increased, a new step is encountered on the I- V curve, and corresponding to the onset of one of these steps is a new feature or peak added to the pulse height distribution, caused by a new phaseslip channel that opens.
Volume 78A, number 2
PHYSICS LETTERS
21 July 1980
1
98
PULSE HEIGHT ($')
40
42
44
46
40
50
Fig. 3. Pulse height analysis for intrinsic pulses. Note the pulse height distribution change with increasing bias current.
Fig. 4. Count data as a function of superconducting film current. A 300 eV A: beam striking the film, which is at 4.328 K.
The count rate as a function of bias current is shown in fig. 4 for a particular film when a beam of constant energy particles is striking the film surface. For a bias current of less than 38 PA, there are no phase slips and, hence, no counts. At slightly above 38 PA, or near the critical current for a region of the film, each particle collision triggers a phase-slip event and the count rate is proportional to the particle flux striking that particular channel. At a bias current of 42 PA, another channel becomes sensitive and hence, the count rate again begins to increase as phase-slip events are now occurring in at least two channels. This process continues as the current is increased until the point is reached at which the current flow itself triggers the events without the energy from the particle collisions and “run-away” in counts is observed. Intrinsic pulses or counts result-
ing from current flow were observed for currents greater than 48 PA with no incident beam particles. From the film dimensions, its normal resistance, and the magnitude of the voltage pulses, an estimate of the size of the sensitive area can be made. A comparison of the measured count rate with a given flux density of incident particles on this sensitive area indicates that it is essentially 100% efficient in detecting argon ions. In summary, it is believed (1) that strong supportive evidence has been shown that ions or neutral particles of sufficient energy trigger phase-slip channels along discrete portions of the film, (2) that this may be an extremely useful technique for studying phase-slip phenomenon, and (3) that this may be developed into a practical low energy particle detector by using state-of-the-art photolithographic tech183
Volume
78A, number
2
PHYSICS
niques to greatly increase the sensitive area over that of the films used in this investigation. References [l]
J.A. Hoyle, Ph.D. Dissertation, Univ. of Virginia (December 1973). [2] J.A. Hoyle, R.R. Humphris and J.W. Boring, IEEE Trans. Magn. MAG-11 (March 1975) No. 2. [3] R. Connolly, Masters Thesis, Univ. of Virginia (August 1976).
184
LETTERS
21 July 1980
[4] W.W. Webb and R.J. Warburton, Phys. Rev. Lett. 20 (1968) 461. [5] J. Meyer and G.V. Minnigerode, Phys. Lett. 38A (1972) 529. [6] W.J. Skocpol, M.R. Beasley and M. Tinkham, J. Low Temp. Phys. 16 (1974) 145. [7] D.A. Weitz, W.J. Skocpol and M. Tinkham, Phys. Rev. Lett. 40 (1978) 253. [S] D.A. Weitz, W.J. Skocpol and M. Tinkham, Phys. Rev. B18 (1978) 3282.
TRIGGERING
PHYSICS LETTERS
21 July 1980
OF PHASE SLIP IN SUPERCONDUCTING
MICROBRIDGES
BY IMPACT OF LOW-ENERGY ATOMS
J. HOYLE Department
of Mechanical Engineering,
Auburn
University, Auburn, AL 36830,
USA
and R. HUMPHRIS Department of Nuclear Engineering and Engineering Physics, University of Virginia, Charlottesville, VA 22903, USA
Received 10 March 1980 Revised manuscript received 16 May 1980
Voltage pulses have been observed across a long superconducting microbridge biased below its critical current when ions or atoms of argon or helium with energies from 150 to 800 eV are incident on the bridge. These pulses are attributed to the triggering of phase slip at spatially discrete localized centers along the superconducting film by the impact of single incident particles.
An investigation has been made on the use of superconducting thin-film bridges as detectors of lowenergy atomic particles. This was done by biasing the bridge with a current slightly less than the critical current and observing the voltage pulses produced when ions and atoms of helium and argon with energies 150 to 800 eV were incident on the bridge [l-3] . In the course of these experiments, it was found that the magnitude and duration of the voltage pulses were independent of the energy and type of incident particle. The purpose of this note is to relate these initial observations to intrinsic phase-slip phenomena in these film bridges and to point out some important experimental implications of these results. The measurements reported here were made on tin-indiurn (about 14% indium) films approximately 500 A thick, evaporated onto glass substrates and mechanically scribed using a micromanipulator to form bridges approximately 1 pm wide by 2 mm in length. A close examination of the voltage-current curve and of the voltage-as-a-function-of-temperature curves at constant bias current (fig. 1) revealed a step-like structure. Voltage steps were first reported by Webb
rI-
--l
)--
I i I
I-
l-
3-
Fig. 1. Voltage across current-biased film as a function of temperature. The temperature is set at a particular value and the vertical dashes represent the recorder output. Curve b is an expanded scale of the initial portion.
181
Volume
78A, number
2
PHYSICS
and Warburton [4] , who observed them on the V-Z and V-T curves of superconducting tin whiskers. The voltage steps of thin wires and whiskers have been extensively studied by Meyer and Minnigerode [S] Skocpol et al. [6] studied voltage steps on the I/-Z curves of long tin microbridges configured similar to those of this investigation. Their studies showed that the voltage steps were due to spatially localized voltage units distributed along the bridge. Various explanations of these steps have been presented, based primarily on the idea of localized phase-slip centers, which occur at places along the bridge with locally lower critical current. For a bias current slightly larger than the local critical current at one of these centers, the magnitude of the Landau-Ginsburg order parameter executes a relaxation oscillation and the phase slips by 2n each time the magnitude drops to zero. The voltage across the phase-slip center is then related to the frequency of this relaxation oscillation by the Josephson relation, and the magnitude of the voltage is determined by local material parameters (e.g., critical current and relaxation times for elastic and inelastic scattering). More recently, Weitz et al. [7,8] investigated Josephson steps induced on the dc I- V curves of niobium contacts by far-infrared laser radiation. In comparing I- I’ curves of typical high-quality junctions with the available theoretical predictions of different’models (Werthamer’s FDSC, the RSJ model, and the TDGL theory), they found each to explain some features of the observed I- V curves and concluded that some synthesis of the present models might be necessary to closely account for their observation. A characteristic feature shown in b of fig. 1 is the approximate 100 /.LVheight of the first step, which is the approximate size of all the pulse heights produced by the incident beam for particle energies, either neutrals or ions, from 150 to 800 eV. It was determined that random shifts and slight changes in the pulse height spectra, sometimes producing multiple peaks, were due to beam alignment. This further indicated that specific sections of the film were sensitive to incident particles. Fig. 2 shows a PHA for argon ions striking the film. Note that the average of the peak is about 100 pV, which closely corresponds to the first step in the voltage-temperature curve for the same film shown in b of fig. 1. The duration of the pulses was typically 1 ms, 182
LETTERS
21 July 1980
25
50
75
100
125
PULSE HEIGHT (!A)
Fig. 2. Pulse height analysis for argon ions striking film which is biased slightly below critical current (44.5 /IA). It is believed that the double pulse occurring with energy change is due to beam alignment variations, resulting in the triggering of another phase-slip channel.
with no apparent dependence on incident particle energy. When phase slip occurs at an area of lower critical current, the region goes normal, and this localized hot spot spreads from the narrow-neck origin to some wider area of the microbridge where the heat conduction is such that further spreading of the normal region is halted. Then, after a period during which heat is dissipated through the substrate and the superconducting film, the normal area reverts to the superconducting state. A I-D transient thermal analysis considering the parameters of the film/substrate configuration revealed dissipation times on the order of milliseconds. Even with no ions striking the film, spontaneous pulses were observed when the bias current was great enough. As noted in fig. 3, for a film at a particular temperature, the bias current is set at 49 PA and a certain pulse height spectrum is recorded. As the bias current is increased, a new step is encountered on the I- V curve, and corresponding to the onset of one of these steps is a new feature or peak added to the pulse height distribution, caused by a new phaseslip channel that opens.
Volume 78A, number 2
PHYSICS LETTERS
21 July 1980
1
98
PULSE HEIGHT ($')
40
42
44
46
40
50
Fig. 3. Pulse height analysis for intrinsic pulses. Note the pulse height distribution change with increasing bias current.
Fig. 4. Count data as a function of superconducting film current. A 300 eV A: beam striking the film, which is at 4.328 K.
The count rate as a function of bias current is shown in fig. 4 for a particular film when a beam of constant energy particles is striking the film surface. For a bias current of less than 38 PA, there are no phase slips and, hence, no counts. At slightly above 38 PA, or near the critical current for a region of the film, each particle collision triggers a phase-slip event and the count rate is proportional to the particle flux striking that particular channel. At a bias current of 42 PA, another channel becomes sensitive and hence, the count rate again begins to increase as phase-slip events are now occurring in at least two channels. This process continues as the current is increased until the point is reached at which the current flow itself triggers the events without the energy from the particle collisions and “run-away” in counts is observed. Intrinsic pulses or counts result-
ing from current flow were observed for currents greater than 48 PA with no incident beam particles. From the film dimensions, its normal resistance, and the magnitude of the voltage pulses, an estimate of the size of the sensitive area can be made. A comparison of the measured count rate with a given flux density of incident particles on this sensitive area indicates that it is essentially 100% efficient in detecting argon ions. In summary, it is believed (1) that strong supportive evidence has been shown that ions or neutral particles of sufficient energy trigger phase-slip channels along discrete portions of the film, (2) that this may be an extremely useful technique for studying phase-slip phenomenon, and (3) that this may be developed into a practical low energy particle detector by using state-of-the-art photolithographic tech183
Volume
78A, number
2
PHYSICS
niques to greatly increase the sensitive area over that of the films used in this investigation. References [l]
J.A. Hoyle, Ph.D. Dissertation, Univ. of Virginia (December 1973). [2] J.A. Hoyle, R.R. Humphris and J.W. Boring, IEEE Trans. Magn. MAG-11 (March 1975) No. 2. [3] R. Connolly, Masters Thesis, Univ. of Virginia (August 1976).
184
LETTERS
21 July 1980
[4] W.W. Webb and R.J. Warburton, Phys. Rev. Lett. 20 (1968) 461. [5] J. Meyer and G.V. Minnigerode, Phys. Lett. 38A (1972) 529. [6] W.J. Skocpol, M.R. Beasley and M. Tinkham, J. Low Temp. Phys. 16 (1974) 145. [7] D.A. Weitz, W.J. Skocpol and M. Tinkham, Phys. Rev. Lett. 40 (1978) 253. [S] D.A. Weitz, W.J. Skocpol and M. Tinkham, Phys. Rev. B18 (1978) 3282.