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Low-frequency voltage noise in mesa-shaped stacks of intrinsic high-Tc Josephson junctions
Physica C 403 (2004) 37–44 www.elsevier.com/locate/physc
Low-frequency voltage noise in mesa-shaped stacks of intrinsic high-Tc Josephson junctions J. Scherbel a
a,*
, M. Mans a, F. Schmidl a, H. Schneidewind b, P. Seidel
a
Institut f€ ur Festk€orperphysik, Friedrich-Schiller-Universit€at Jena, Helmholtzweg 5, D-07743 Jena, Germany b Institut f€ur Physikalische Hochtechnologie e.V., Albert-Einstein-Str. 9, D-07745 Jena, Germany Received 10 September 2003; received in revised form 28 October 2003; accepted 20 November 2003
Abstract We investigated the low-frequency noise behaviour in high temperature superconductor (HTS) Josephson junction stacks made of thin film TBCCO-2212 layers on sapphire substrates. The measured stacks possess a top gold electrode and they are 3 lm · 3 lm in size as well as about 70 nm in height. They are prepared by means of photolithography and Ar ion milling. The voltage noise density showed clearly visible Lorentz-like bumps. We observe random telegraph noise in the sub-mV range as well as random switching in the mV-range. Furthermore the telegraph noise shows a clearly discernible two level fluctuation behaviour with one level dominating in time. With increasing bias current the absolute noise amplitude slightly grows and the lifetimes of both levels decrease considerably. However, the lifetimes of the upper and lower level do not show a simple Poisson distribution. We assume that the observed low-frequency behaviour is due to the quasiparticle injection through the normal conducting top electrode into the stacks. 2003 Elsevier B.V. All rights reserved. PACS: 74.40.+k; 74.72.Jt; 74.81; 85.25; 73.40.)c Keywords: Josephson junction; Low-frequency noise; Random telegraph noise; Quasiparticle injection
1. Introduction The strong anisotropy in some layered high temperature superconductors (HTS), e.g. such as Bi2 Sr2 CaCu2 Ox (BSCCO-2212) or Tl2 Ba2 CaCu2 O8d (TBCCO-2212), leads to the intrinsic Josephson effects for c-axis transport currents. In these materials the weak coupling layer between
*
Corresponding author. Tel.: +49-36-41947427; fax: +49-3641947412. E-mail address: [email protected] (J. Scherbel).
two CuO2 -planes on an atomic scale forms a single Josephson junction. Thus a finite layer thickness of some nanometers offers the possibility to create a serial array of many Josephson junctions with a high package density. Within the last years for Josephson junction stacks using BSCCO and TBCCO layers or single crystals different preparation techniques were developed. The used materials and their belonging techniques depend on the possibilities and the requirements of the investigated devices. A survey of the published results suggests that the preparation technology used may influence some device properties. The
0921-4534/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2003.11.006
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current transport mechanism in the stacks and the interaction of the junctions are very important for the understanding of the electrodynamics and thus for the application of the HTS Josephson junction stacks. The investigation of the low-frequency noise spectrum reveals very important indications concerning the transport mechanisms in electronic devices. Furthermore, for a high efficiency in the application of stacked Josephson junctions a synchronisation of the junctions is necessary. Noise and random switches in Josephson junction arrays substantially reduce the potential of synchronisation. Some measurements of the c-axis low-frequency noise for large mesa-shaped stacks of single crystal BSCCO-2212 are performed recently [1–3]. The noise characteristics of these samples often show clearly visible Lorentz bumps. In the low-frequency behaviour shown in [1] the Lorentz bumps are remarkably noticeable and the authors proved that they are caused by a large randomtelegraph voltage noise (RTVN). This RTVN is due to the switching between the different resistive branches. The origins of the rather inconspicuous Lorentz bumps reported in the latter both publications are not clarified yet. In this work we investigate the low-frequency noise of small mesashaped stacks of TBCCO-2212 layers and explain the origin of the low-frequency noise behaviour of HTS Josephson junction stacks considering the suggestions and measurements published in [2,3].
2. Sample preparation and experimental setup The TBCCO-2212 films were grown epitaxially on buffered (1 )1 0 2) oriented sapphire substrates in a two step process. At first a Tl-free precursor of amorphous Ba–Ca–Cu–O is produced by dcmagnetron rf sputtering and ensuing this precursor is oxythallinized in a thallium oxide atmosphere at high temperatures. The details of this process are described in the publication of Schneidewind et al. [4]. These films are approximately 270 nm thick, subsequently a 200 nm thick gold layer is magnetron dc sputtered ex situ. To pattern the ground electrodes and the mesa-shaped stacks of a 3 lm · 3 lm area and a height of about 70 nm we use photolithographical techniques and Arþ -ion beam
milling. The sidewall is filled up with magnetron rf-sputtered CeOx and the top electrode contact pads are made by a photolithographic reversal process, sputtering and lift off technique. The caxis current–voltage characteristics (IVC) and the low-frequency voltage noise are measured by a three-probe method. The ground electrode includes separate pads for the current bias and for the voltage measurement wire. In contrast, the top electrode does not provide separated pads for current and voltage. Both current wires are separately electro-magnetically shielded by grounded covers as well as both voltage wires are. Additionally the whole cable harness is shielded with another grounded cover. This considerably reduces the influence of extrinsic noise sources. The IVCs were obtained by a very low noise source measurement unit (SMU) from Keithley. The lowfrequency voltage noise measurements are performed by a HP 35665A spectrum analyzer in combination with the SMU in order to set the constant bias points, respectively. The dc voltage offset is compensated.
3. Measurement results At an operating temperature of 4.2 K the IVCs of the TBCCO mesa-shaped stacks show lowest critical currents Ic between 50 and 150 lA. The normal resistances for the single Josephson junctions RN are estimated to be about 13–22 X by passing through the different branches. The Ic RN products vary approximately from 3 to 7 mV. Fig. 1a shows the IVC of a TBCCO Josephson junction stack. Obviously the first transition at about 80 lA is caused by the simultaneous switching of two intrinsic Josephson junctions. Furthermore a clear nonlinear voltage background can be seen. The origin of this background can be either a diode behavior due to a degradation of the superconductor to a semiconductor at the interface to the gold top electrode or due to the tunneling behavior of the quasiparticles [5] injected by the top electrode or a superposition of both effects. We assume that the last mentioned superposition whereas the quasiparticle tunneling is responsible to the main contribution of this nonlinear voltage background
J. Scherbel et al. / Physica C 403 (2004) 37–44
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explains the phenomenon best. The low-frequency noise characteristics for different current bias points, which are marked in Fig. 1a, are shown in Fig. 1b for increasing bias current. The low frequency noise density shows a 1=f background superposed with Lorentz-like bumps which emerge with higher bias currents. Concurrent the bumps get broader and shift to higher frequencies with increasing bias. To achieve more clarity the noise characteristics are plotted starting with 100 Hz for increasing bias in Fig. 1c and for decreasing in Fig. 1d, respectively. This behavior was observed for large BSCCO mesa-shaped stacks, too [2,3]. The Lorentz-like bumps suggest: the response of the investigated device contains two level fluctuations such as RTVN [6]. Indeed we find a RTVN by investigating the time domain of the noise signal. Fig. 2 shows the time domain measurement for different constant bias currents. Apparently there are only two certain voltage levels whereas the
upper level dominates. The evaluated voltage difference of both levels amounts between 0.15 and 0.3 mV and it increases with the bias current of the Josephson junction (Fig. 3). Thus the voltage steps are considerably smaller than the lowest voltage steps between the branches in the IV characteristics of the stacks. Furthermore, Fig. 2 reveals that the time constants of the upper level sup and the lower level sdown get smaller with higher constant bias currents. The statistics of the respective time constants are shown in Fig. 4. We extracted them in 10 measurements in the time domain for each constant bias current. Fig. 4 shows clearly that both time constants strongly decrease with higher bias currents. Additionally the statistics do not show a plain Poisson distribution which would be expected for two level processes with one dominant level. Instead they show the shape of a superposition of different Poisson distributions, which is quite distinct in the statistics for the lower constant bias
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some mV. Generally, these fluctuations do not reveal the RTVN shape described in [1]. Their behaviour coincides more with the results of quasiparticle injection experiments on small BSCCO single crystal double mesas which are reported in [7]. These discrete steps seem to be switches of single Josephson junctions into the resistive state and back.
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currents. With rising constant c-axis bias currents the peaks of the different Poisson distributions increasingly come closer. The increase of the bias current above the critical current leads to additional observable discrete long term voltage steps. These steps have time constants of several seconds and amplitudes of
4. Discussion 4.1. Origin of the RTVN It is well known that magnetic vortices may exist within the superconducting planes of the stacks. Separated by the normal conducting or insulating barrier these vortices form out a socalled pancake structure (see e.g. [8]). It is also well known that there are many defects in the thin film stack acting as pinning centres for the several pancakes. So a small disturbance within the stack due to thermal fluctuation or bias
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J. Scherbel et al. / Physica C 403 (2004) 37–44
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fluctuations may bring the pancake structure into imbalance. This may lead to pancakes stepping out of their pinning centre into another one. Because the moving magnetic vortices generate dissipation within the stack this leads to a telegraph-like response in the voltage. It can be assumed that the many statistical distributed defects in the thin film stacks usually lead to more levels in the telegraph voltage noise. We assume that moving pancake structures cannot cause the observed telegraph noise because in our RTVN only two distinct levels are seen. For
example Zybtsev et al. [9] measured RTVN responses in thin-BSCCO-2212 whisker-based superconducting sub-lm bridges, too. They could show that this response was due to spontaneous addition and removal of vortex trains. Differing from our experiments this response also includes multi levels. Furthermore, even after several warming up and cooling down cycles the features of the random telegraph voltage noise we measure are always the same. It is assumed that the pancake structure within the stack should be different for each cycle because there are different
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investigations on BSCCO-2212 double mesa geometries suggest that Leff should be in the range of some few intrinsic junction thicknesses [7]. Due to the quasiparticle injection the superconducting gap is reduced especially within Leff [13]. Hence the critical currents of the upper Josephson junctions are strongly reduced as well as their characteristic voltages Ic RN are. Basing on this scenario and starting from the upper voltage level in the RTVN the physical procedure can be described as follows: When Leff rises slightly the Cooper pair density around Leff decreases. Consequently the superconducting current density JS decreases below the critical current density Jc of the last Josephson junction in the normal conducting state on the top region of the mesa. Hence this junction switches back to the superconducting state. That switch leads to a slight displacement of the superconducting background within the stack towards the top electrode. At that time Leff gets slightly smaller by DLeff (Fig. 5a). This increases the Cooper pair density in the respective location of the involved junction. Since the Copper pair density is higher now the Jc of the just switched junction is exceeded and the junction switches to the normal conducting state again. Hence the superconducting background within the stack is shifting slightly away from the top electrode and Leff rises slightly again, which continues the just described process. In that case the switches show the transition of the respective involved degraded Josephson junction.
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distributed defects in the superconducting planes of the stack and the initial conditions differ every time. Although therefore the RTVN should slightly change its features, e.g. in the amplitude or in the statistics of the switching frequencies but this is not the case in our observations. Apart from that, the observed RTVN in the time domain of the HTS mesa-shaped Josephson junction stack responses is only reported for devices with normal conducting top electrodes yet. Furthermore, investigations of BSCCO and TBCCO single crystal mesas made by technologies which enable superconducting bias wiring [10,11] have not shown this conspicuous lowfrequency noise behaviour. Therefore we assume that this RTVN is caused by the quasiparticle injection through the normal conducting top electrode into the stack of Josephson junctions. In this case the injected quasiparticles trough the upper Josephson junction barriers relax towards the gap edge by emission of phonons and form Cooper pairs at the gap edge by emission of 2D phonons. Here D is the respective superconducting gap. The resultant phonons with energies above 2D once more contribute to pair breaking to excite some quasiparticles. These processes continue until some steady non-equilibrium state is established [12]. The effective way from the top electrode into the superconducting stack where the density of quasiparticles NQP is larger than the density of Cooper pairs NCP , is called effective conversion length Leff . Charge-imbalance effect
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J. Scherbel et al. / Physica C 403 (2004) 37–44
4.2. Effects of bias current increment The increase of the injected quasiparticle current exceeds the critical current densities of the next Josephson junctions. Hence a larger number of degraded Josephson junctions switch into the normal state and the superconducting background is shifted away from the top electrode towards the superconducting ground electrode. This increases the effective conversion length from Leff;1 to Leff;2 (Fig. 5b). Additionally, due to the higher density of the quasiparticles the slope of both charge densities in the region around Leff is respectively larger. Due to this a slight random increment of Leff causes that JS falls deeper below Jc compared with the case of lower quasiparticle injection. This better enables a switch into the superconducting state which leads to earlier switches. Vice versa, if JS rises again due to the processes just described the transgression of the respective Jc is also higher. This leads to an earlier switch into the normal conducting state again. So the effective time constants of the upper and lower level reduce with increasing bias currents. Fig. 3 shows the enlargement of the voltage level with increased constant bias currents. The fact that the junctions switching at higher bias currents are situated deeper in the stack towards to the superconducting electrode show higher characteristic voltages may offer an explanation for this phenomenon. Furthermore the distributions of sup and sdown reveal not a single Poisson distribution but it seems that they represent a bundle of Poisson distributions with different mean time constants, which merge more and more at higher bias currents. Therefore we assume that over long terms the respectively switching junction can differ from the previous switched one in the next neighbourhood. This hypothesis is reinforced by the long term fluctuation of the offset in the sub-mV range. 4.3. Discrete long term jumps in the mV-range Random discrete long term jumps in the range of several mV of HTS Josephson junction mesastacks are only found for geometries with normal conducting top electrodes yet. Pepe et al. [14] car-
43
ried out experiments on a stacked double tunnel Nb-based Josephson system, where the bottom Josephson junction was used as a quasiparticle pulse injector to the top junction. These measurements have shown that quasiparticle pulses into the top junction are able to induce a switch between the normal conducting and superconducting state if the top junction is biased with a current lower than its critical current. Therefore we assume that the reported long term steps in the mV-ranges are pulse-induced switches of the non-degraded Josephson junctions in the stack due to the pulses generated by the RTVN in the sub-mV range. The non-RTVN behaviour we observed are in contrast to the observation of Saito et al. in [1] which might be explained by the substantial smaller area of our mesa-shaped stacks. Due to this smaller area the large voltage step generates a higher quasiparticle density pulse and an additional non-negligible electrical field, which could induce a switch of another junction in the next neighbourhood and so on, which may lead to a chain reaction.
5. Summary and conclusion The investigated TBCCO thin film mesa-shaped stacks with a normal conducting top electrode showed random telegraph voltage noise in the lowfrequency range. The lifetimes of the upper and lower level decreased and the voltage difference between both levels increased with rising bias current. Furthermore, we observed discrete long term steps of the voltage in the range of some mV. We explained the origin and the behaviour of the random telegraph noise as well as the discrete long term steps by the dynamics due to the quasiparticle injection through the normal conducting top electrode. Note that the telegraph voltage noise and the long term random switches reduce the potential of a stable synchronisation of the Josephson junctions within the stack substantially. However, for a high efficiency in the application of stacked Josephson junctions a stable synchronisation of the junctions is necessary. Therefore intrinsic Josephson junction stacks with superconducting leads are advantageous for the application. In [10,11], it is reported on preparation
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technologies which enable superconducting leads for TBCCO or BSCCO single crystal mesas or whiskers. Up to now the focused ion beam technology described in [11] seems to be the only possibility for thin films. However, that kind of preparation of thin film mesa-shaped stacks is quite difficult. To pattern micro-bridges into misaligned thin films of TBCCO, the new approach described in [15], appears to be more promising concerning thin film preparations. Acknowledgements The authors thank Professor Dr. M. Yu. Kupriyanov for the helpful discussions about the interpretation of our measurement results. This work was partially supported by the German DFG within the contract no. Se 664/10-1.
References [1] A. Saito, K. Hamasaki, A. Irie, G. Oya, IEEE Trans. Appl. Supercond. 11 (2001) 304. [2] A. Saito, K. Hamasaki, A. Irie, G. Oya, J. Appl. Phys. 90 (2001) 2911.
[3] A. Saito, H. Ishida, A. Irie, A. Kawakami, Z. Wang, G. Oya, K. Hamasaki, IEEE Trans. Appl. Supercond. 13 (2003) 904. [4] H. Schneidewind, M. Manzel, G. Bruchlos, K. Kirsch, Supercond. Sci. Technol. 14 (2001) 200. [5] P. Seidel, A.N. Grib, Yu.M. Shukrinov, J. Scherbel, U. H€ ubner, F. Schmidl, Physica C 326 (2001) 102. [6] F. Herbstritt, T. Kemen, L. Alff, A. Marx, R. Gross, Appl. Phys. Lett. 78 (2001) 955. [7] S. Rother, Y. Koval, P. M€ uller, R. Kleiner, D.A. Ryndyk, J. Keller, Phys. Rev. B 67 (2003) 024510. [8] J.R. Clem, T. Pe, M. Benkraouda, Physica C 282–287 (1997) 311. [9] S.G. Zybtsev, V.Ya. Prokovskii, I.G. Gorlova, Yu.I. Latyshev, Dynamics of random trains and random telegraph signal in submicron BSCCO bridges, presented at the 6th European Conference of Applied Superconductivity, 14–18 Septemper 2003, Sorrento/Napoli. [10] H.B. Wang, P.H. Wu, T. Yamashita, Appl. Phys. Lett. 78 (2001) 4010. [11] S.-J. Kim, Yu.I. Latyshev, T. Yamashita, S. Kishida, Physica C 362 (2001) 150. [12] I. Iguchi, K. Nukui, K. Lee, Phys. Rev. B 50 (1994) 457. [13] I. Iguchi, J. Low Temp. Phys. 33 (1978) 439. [14] G.P. Pepe, G. Peluso, M. Valentino, A. Barone, L. Parlato, E. Esposito, C. Granata, M. Russo, C. De Leo, G. Rotoli, Appl. Phys. Lett. 79 (2001) 2770. [15] O.S. Chana, A.R. Kuzhakhmetov, P.A. Warburton, D.M.C. Hyland, D. Dew-Hughes, C.R.M. Grovenor, R.J. Kinsey, G. Burnell, W.E. Booij, M.G. Blamire, R. Kleiner, P. M€ uller, Appl. Phys. Lett. 76 (2000) 3603.
Low-frequency voltage noise in mesa-shaped stacks of intrinsic high-Tc Josephson junctions J. Scherbel a
a,*
, M. Mans a, F. Schmidl a, H. Schneidewind b, P. Seidel
a
Institut f€ ur Festk€orperphysik, Friedrich-Schiller-Universit€at Jena, Helmholtzweg 5, D-07743 Jena, Germany b Institut f€ur Physikalische Hochtechnologie e.V., Albert-Einstein-Str. 9, D-07745 Jena, Germany Received 10 September 2003; received in revised form 28 October 2003; accepted 20 November 2003
Abstract We investigated the low-frequency noise behaviour in high temperature superconductor (HTS) Josephson junction stacks made of thin film TBCCO-2212 layers on sapphire substrates. The measured stacks possess a top gold electrode and they are 3 lm · 3 lm in size as well as about 70 nm in height. They are prepared by means of photolithography and Ar ion milling. The voltage noise density showed clearly visible Lorentz-like bumps. We observe random telegraph noise in the sub-mV range as well as random switching in the mV-range. Furthermore the telegraph noise shows a clearly discernible two level fluctuation behaviour with one level dominating in time. With increasing bias current the absolute noise amplitude slightly grows and the lifetimes of both levels decrease considerably. However, the lifetimes of the upper and lower level do not show a simple Poisson distribution. We assume that the observed low-frequency behaviour is due to the quasiparticle injection through the normal conducting top electrode into the stacks. 2003 Elsevier B.V. All rights reserved. PACS: 74.40.+k; 74.72.Jt; 74.81; 85.25; 73.40.)c Keywords: Josephson junction; Low-frequency noise; Random telegraph noise; Quasiparticle injection
1. Introduction The strong anisotropy in some layered high temperature superconductors (HTS), e.g. such as Bi2 Sr2 CaCu2 Ox (BSCCO-2212) or Tl2 Ba2 CaCu2 O8d (TBCCO-2212), leads to the intrinsic Josephson effects for c-axis transport currents. In these materials the weak coupling layer between
*
Corresponding author. Tel.: +49-36-41947427; fax: +49-3641947412. E-mail address: [email protected] (J. Scherbel).
two CuO2 -planes on an atomic scale forms a single Josephson junction. Thus a finite layer thickness of some nanometers offers the possibility to create a serial array of many Josephson junctions with a high package density. Within the last years for Josephson junction stacks using BSCCO and TBCCO layers or single crystals different preparation techniques were developed. The used materials and their belonging techniques depend on the possibilities and the requirements of the investigated devices. A survey of the published results suggests that the preparation technology used may influence some device properties. The
0921-4534/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2003.11.006
38
J. Scherbel et al. / Physica C 403 (2004) 37–44
current transport mechanism in the stacks and the interaction of the junctions are very important for the understanding of the electrodynamics and thus for the application of the HTS Josephson junction stacks. The investigation of the low-frequency noise spectrum reveals very important indications concerning the transport mechanisms in electronic devices. Furthermore, for a high efficiency in the application of stacked Josephson junctions a synchronisation of the junctions is necessary. Noise and random switches in Josephson junction arrays substantially reduce the potential of synchronisation. Some measurements of the c-axis low-frequency noise for large mesa-shaped stacks of single crystal BSCCO-2212 are performed recently [1–3]. The noise characteristics of these samples often show clearly visible Lorentz bumps. In the low-frequency behaviour shown in [1] the Lorentz bumps are remarkably noticeable and the authors proved that they are caused by a large randomtelegraph voltage noise (RTVN). This RTVN is due to the switching between the different resistive branches. The origins of the rather inconspicuous Lorentz bumps reported in the latter both publications are not clarified yet. In this work we investigate the low-frequency noise of small mesashaped stacks of TBCCO-2212 layers and explain the origin of the low-frequency noise behaviour of HTS Josephson junction stacks considering the suggestions and measurements published in [2,3].
2. Sample preparation and experimental setup The TBCCO-2212 films were grown epitaxially on buffered (1 )1 0 2) oriented sapphire substrates in a two step process. At first a Tl-free precursor of amorphous Ba–Ca–Cu–O is produced by dcmagnetron rf sputtering and ensuing this precursor is oxythallinized in a thallium oxide atmosphere at high temperatures. The details of this process are described in the publication of Schneidewind et al. [4]. These films are approximately 270 nm thick, subsequently a 200 nm thick gold layer is magnetron dc sputtered ex situ. To pattern the ground electrodes and the mesa-shaped stacks of a 3 lm · 3 lm area and a height of about 70 nm we use photolithographical techniques and Arþ -ion beam
milling. The sidewall is filled up with magnetron rf-sputtered CeOx and the top electrode contact pads are made by a photolithographic reversal process, sputtering and lift off technique. The caxis current–voltage characteristics (IVC) and the low-frequency voltage noise are measured by a three-probe method. The ground electrode includes separate pads for the current bias and for the voltage measurement wire. In contrast, the top electrode does not provide separated pads for current and voltage. Both current wires are separately electro-magnetically shielded by grounded covers as well as both voltage wires are. Additionally the whole cable harness is shielded with another grounded cover. This considerably reduces the influence of extrinsic noise sources. The IVCs were obtained by a very low noise source measurement unit (SMU) from Keithley. The lowfrequency voltage noise measurements are performed by a HP 35665A spectrum analyzer in combination with the SMU in order to set the constant bias points, respectively. The dc voltage offset is compensated.
3. Measurement results At an operating temperature of 4.2 K the IVCs of the TBCCO mesa-shaped stacks show lowest critical currents Ic between 50 and 150 lA. The normal resistances for the single Josephson junctions RN are estimated to be about 13–22 X by passing through the different branches. The Ic RN products vary approximately from 3 to 7 mV. Fig. 1a shows the IVC of a TBCCO Josephson junction stack. Obviously the first transition at about 80 lA is caused by the simultaneous switching of two intrinsic Josephson junctions. Furthermore a clear nonlinear voltage background can be seen. The origin of this background can be either a diode behavior due to a degradation of the superconductor to a semiconductor at the interface to the gold top electrode or due to the tunneling behavior of the quasiparticles [5] injected by the top electrode or a superposition of both effects. We assume that the last mentioned superposition whereas the quasiparticle tunneling is responsible to the main contribution of this nonlinear voltage background
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explains the phenomenon best. The low-frequency noise characteristics for different current bias points, which are marked in Fig. 1a, are shown in Fig. 1b for increasing bias current. The low frequency noise density shows a 1=f background superposed with Lorentz-like bumps which emerge with higher bias currents. Concurrent the bumps get broader and shift to higher frequencies with increasing bias. To achieve more clarity the noise characteristics are plotted starting with 100 Hz for increasing bias in Fig. 1c and for decreasing in Fig. 1d, respectively. This behavior was observed for large BSCCO mesa-shaped stacks, too [2,3]. The Lorentz-like bumps suggest: the response of the investigated device contains two level fluctuations such as RTVN [6]. Indeed we find a RTVN by investigating the time domain of the noise signal. Fig. 2 shows the time domain measurement for different constant bias currents. Apparently there are only two certain voltage levels whereas the
upper level dominates. The evaluated voltage difference of both levels amounts between 0.15 and 0.3 mV and it increases with the bias current of the Josephson junction (Fig. 3). Thus the voltage steps are considerably smaller than the lowest voltage steps between the branches in the IV characteristics of the stacks. Furthermore, Fig. 2 reveals that the time constants of the upper level sup and the lower level sdown get smaller with higher constant bias currents. The statistics of the respective time constants are shown in Fig. 4. We extracted them in 10 measurements in the time domain for each constant bias current. Fig. 4 shows clearly that both time constants strongly decrease with higher bias currents. Additionally the statistics do not show a plain Poisson distribution which would be expected for two level processes with one dominant level. Instead they show the shape of a superposition of different Poisson distributions, which is quite distinct in the statistics for the lower constant bias
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some mV. Generally, these fluctuations do not reveal the RTVN shape described in [1]. Their behaviour coincides more with the results of quasiparticle injection experiments on small BSCCO single crystal double mesas which are reported in [7]. These discrete steps seem to be switches of single Josephson junctions into the resistive state and back.
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currents. With rising constant c-axis bias currents the peaks of the different Poisson distributions increasingly come closer. The increase of the bias current above the critical current leads to additional observable discrete long term voltage steps. These steps have time constants of several seconds and amplitudes of
4. Discussion 4.1. Origin of the RTVN It is well known that magnetic vortices may exist within the superconducting planes of the stacks. Separated by the normal conducting or insulating barrier these vortices form out a socalled pancake structure (see e.g. [8]). It is also well known that there are many defects in the thin film stack acting as pinning centres for the several pancakes. So a small disturbance within the stack due to thermal fluctuation or bias
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fluctuations may bring the pancake structure into imbalance. This may lead to pancakes stepping out of their pinning centre into another one. Because the moving magnetic vortices generate dissipation within the stack this leads to a telegraph-like response in the voltage. It can be assumed that the many statistical distributed defects in the thin film stacks usually lead to more levels in the telegraph voltage noise. We assume that moving pancake structures cannot cause the observed telegraph noise because in our RTVN only two distinct levels are seen. For
example Zybtsev et al. [9] measured RTVN responses in thin-BSCCO-2212 whisker-based superconducting sub-lm bridges, too. They could show that this response was due to spontaneous addition and removal of vortex trains. Differing from our experiments this response also includes multi levels. Furthermore, even after several warming up and cooling down cycles the features of the random telegraph voltage noise we measure are always the same. It is assumed that the pancake structure within the stack should be different for each cycle because there are different
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investigations on BSCCO-2212 double mesa geometries suggest that Leff should be in the range of some few intrinsic junction thicknesses [7]. Due to the quasiparticle injection the superconducting gap is reduced especially within Leff [13]. Hence the critical currents of the upper Josephson junctions are strongly reduced as well as their characteristic voltages Ic RN are. Basing on this scenario and starting from the upper voltage level in the RTVN the physical procedure can be described as follows: When Leff rises slightly the Cooper pair density around Leff decreases. Consequently the superconducting current density JS decreases below the critical current density Jc of the last Josephson junction in the normal conducting state on the top region of the mesa. Hence this junction switches back to the superconducting state. That switch leads to a slight displacement of the superconducting background within the stack towards the top electrode. At that time Leff gets slightly smaller by DLeff (Fig. 5a). This increases the Cooper pair density in the respective location of the involved junction. Since the Copper pair density is higher now the Jc of the just switched junction is exceeded and the junction switches to the normal conducting state again. Hence the superconducting background within the stack is shifting slightly away from the top electrode and Leff rises slightly again, which continues the just described process. In that case the switches show the transition of the respective involved degraded Josephson junction.
NQP(z)/NQP(0); NCP(z)/NCP, max
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distributed defects in the superconducting planes of the stack and the initial conditions differ every time. Although therefore the RTVN should slightly change its features, e.g. in the amplitude or in the statistics of the switching frequencies but this is not the case in our observations. Apart from that, the observed RTVN in the time domain of the HTS mesa-shaped Josephson junction stack responses is only reported for devices with normal conducting top electrodes yet. Furthermore, investigations of BSCCO and TBCCO single crystal mesas made by technologies which enable superconducting bias wiring [10,11] have not shown this conspicuous lowfrequency noise behaviour. Therefore we assume that this RTVN is caused by the quasiparticle injection through the normal conducting top electrode into the stack of Josephson junctions. In this case the injected quasiparticles trough the upper Josephson junction barriers relax towards the gap edge by emission of phonons and form Cooper pairs at the gap edge by emission of 2D phonons. Here D is the respective superconducting gap. The resultant phonons with energies above 2D once more contribute to pair breaking to excite some quasiparticles. These processes continue until some steady non-equilibrium state is established [12]. The effective way from the top electrode into the superconducting stack where the density of quasiparticles NQP is larger than the density of Cooper pairs NCP , is called effective conversion length Leff . Charge-imbalance effect
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4.2. Effects of bias current increment The increase of the injected quasiparticle current exceeds the critical current densities of the next Josephson junctions. Hence a larger number of degraded Josephson junctions switch into the normal state and the superconducting background is shifted away from the top electrode towards the superconducting ground electrode. This increases the effective conversion length from Leff;1 to Leff;2 (Fig. 5b). Additionally, due to the higher density of the quasiparticles the slope of both charge densities in the region around Leff is respectively larger. Due to this a slight random increment of Leff causes that JS falls deeper below Jc compared with the case of lower quasiparticle injection. This better enables a switch into the superconducting state which leads to earlier switches. Vice versa, if JS rises again due to the processes just described the transgression of the respective Jc is also higher. This leads to an earlier switch into the normal conducting state again. So the effective time constants of the upper and lower level reduce with increasing bias currents. Fig. 3 shows the enlargement of the voltage level with increased constant bias currents. The fact that the junctions switching at higher bias currents are situated deeper in the stack towards to the superconducting electrode show higher characteristic voltages may offer an explanation for this phenomenon. Furthermore the distributions of sup and sdown reveal not a single Poisson distribution but it seems that they represent a bundle of Poisson distributions with different mean time constants, which merge more and more at higher bias currents. Therefore we assume that over long terms the respectively switching junction can differ from the previous switched one in the next neighbourhood. This hypothesis is reinforced by the long term fluctuation of the offset in the sub-mV range. 4.3. Discrete long term jumps in the mV-range Random discrete long term jumps in the range of several mV of HTS Josephson junction mesastacks are only found for geometries with normal conducting top electrodes yet. Pepe et al. [14] car-
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ried out experiments on a stacked double tunnel Nb-based Josephson system, where the bottom Josephson junction was used as a quasiparticle pulse injector to the top junction. These measurements have shown that quasiparticle pulses into the top junction are able to induce a switch between the normal conducting and superconducting state if the top junction is biased with a current lower than its critical current. Therefore we assume that the reported long term steps in the mV-ranges are pulse-induced switches of the non-degraded Josephson junctions in the stack due to the pulses generated by the RTVN in the sub-mV range. The non-RTVN behaviour we observed are in contrast to the observation of Saito et al. in [1] which might be explained by the substantial smaller area of our mesa-shaped stacks. Due to this smaller area the large voltage step generates a higher quasiparticle density pulse and an additional non-negligible electrical field, which could induce a switch of another junction in the next neighbourhood and so on, which may lead to a chain reaction.
5. Summary and conclusion The investigated TBCCO thin film mesa-shaped stacks with a normal conducting top electrode showed random telegraph voltage noise in the lowfrequency range. The lifetimes of the upper and lower level decreased and the voltage difference between both levels increased with rising bias current. Furthermore, we observed discrete long term steps of the voltage in the range of some mV. We explained the origin and the behaviour of the random telegraph noise as well as the discrete long term steps by the dynamics due to the quasiparticle injection through the normal conducting top electrode. Note that the telegraph voltage noise and the long term random switches reduce the potential of a stable synchronisation of the Josephson junctions within the stack substantially. However, for a high efficiency in the application of stacked Josephson junctions a stable synchronisation of the junctions is necessary. Therefore intrinsic Josephson junction stacks with superconducting leads are advantageous for the application. In [10,11], it is reported on preparation
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technologies which enable superconducting leads for TBCCO or BSCCO single crystal mesas or whiskers. Up to now the focused ion beam technology described in [11] seems to be the only possibility for thin films. However, that kind of preparation of thin film mesa-shaped stacks is quite difficult. To pattern micro-bridges into misaligned thin films of TBCCO, the new approach described in [15], appears to be more promising concerning thin film preparations. Acknowledgements The authors thank Professor Dr. M. Yu. Kupriyanov for the helpful discussions about the interpretation of our measurement results. This work was partially supported by the German DFG within the contract no. Se 664/10-1.
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[3] A. Saito, H. Ishida, A. Irie, A. Kawakami, Z. Wang, G. Oya, K. Hamasaki, IEEE Trans. Appl. Supercond. 13 (2003) 904. [4] H. Schneidewind, M. Manzel, G. Bruchlos, K. Kirsch, Supercond. Sci. Technol. 14 (2001) 200. [5] P. Seidel, A.N. Grib, Yu.M. Shukrinov, J. Scherbel, U. H€ ubner, F. Schmidl, Physica C 326 (2001) 102. [6] F. Herbstritt, T. Kemen, L. Alff, A. Marx, R. Gross, Appl. Phys. Lett. 78 (2001) 955. [7] S. Rother, Y. Koval, P. M€ uller, R. Kleiner, D.A. Ryndyk, J. Keller, Phys. Rev. B 67 (2003) 024510. [8] J.R. Clem, T. Pe, M. Benkraouda, Physica C 282–287 (1997) 311. [9] S.G. Zybtsev, V.Ya. Prokovskii, I.G. Gorlova, Yu.I. Latyshev, Dynamics of random trains and random telegraph signal in submicron BSCCO bridges, presented at the 6th European Conference of Applied Superconductivity, 14–18 Septemper 2003, Sorrento/Napoli. [10] H.B. Wang, P.H. Wu, T. Yamashita, Appl. Phys. Lett. 78 (2001) 4010. [11] S.-J. Kim, Yu.I. Latyshev, T. Yamashita, S. Kishida, Physica C 362 (2001) 150. [12] I. Iguchi, K. Nukui, K. Lee, Phys. Rev. B 50 (1994) 457. [13] I. Iguchi, J. Low Temp. Phys. 33 (1978) 439. [14] G.P. Pepe, G. Peluso, M. Valentino, A. Barone, L. Parlato, E. Esposito, C. Granata, M. Russo, C. De Leo, G. Rotoli, Appl. Phys. Lett. 79 (2001) 2770. [15] O.S. Chana, A.R. Kuzhakhmetov, P.A. Warburton, D.M.C. Hyland, D. Dew-Hughes, C.R.M. Grovenor, R.J. Kinsey, G. Burnell, W.E. Booij, M.G. Blamire, R. Kleiner, P. M€ uller, Appl. Phys. Lett. 76 (2000) 3603.