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One-dimensional arrays of ultrasmall tunnel junctions with alternating superconducting and normal metal islands
PHYSICA
Physica B 194-196 (1994) 1337-1338 Noah-Holland
One-dimensional arrays of ultrasmall tunnel junctions with alternating superconducting and normal metal islands M. Matters and J.E. Mooij Department of Applied Physics, Delft University of Technology P.O. Box 5046, 2600 GA Delft, The Netherlands We have fabricated and performed measurements on one-dimensional arrays of ultra.small tunnel junctions. These arrays consist of alternating superconducting and normal metal islands. With two gate electrodes, capacitively coupled to the aluminum islands, the array is irradiated with microwaves of frequency f with an adjustable phase difference between the two microwave signals. In the normal state the I-V characteristics show clear plateaus at currents I = nef up to n = 3. 1. I n t r o d u c t i o n One-dimensional arrays of ultrasmall metal tunnel junctions have been studied extensively by Delsing et al.[1]. In the normal state SEToscillations can be observed in these arrays when microwaves with frequency f are inductively coupled to the leads. These oscillations manifest themselves as peaks in the differential resistance at currents I = nef. Also, the microwaves can be capacitively coupled to the array by means of a gate. The one-dimensional array can then be operated as a turnstile [2, 3] and a clear plateau at I = e f is observed in the I-V characteristic. Recently, transport measurements were performed on a normal- superconductor-normal (NSN) Coulomb blockade transistor [4]. At low bias, a 2e-periodic modulation of the current was observed when changing the gate voltage. This is due to the fact that an isolated superconductor with an odd number of electrons has a larger ground state energy compared to the situation were the electron number is even and all electrons can be paired. In this NSN device, when the charging energy is smaller then the superconducting gap, the conductance at low bias is due to electrons passing in pairs through the island. It is interesting to study the transport in onedimensional arrays consisting of alternating superconducting and normal metal islands. These arrays can also be irradiated by microwaves. The current due to co-tunneling in these arrays can be much smaller then in arrays with only normal metal islands [5]. Thus the array can be used as a lead to single charge traps to study (the absence of) detailed balance.[6]
2. D e s c r i p t i o n o f t h e d e v i c e The device layout is shown in fig.1. The two gates are capacitively coupled to the superconducting islands . Two rf signals with an adjustable phase difference can be applied to the gates. A magnetic field of 2 T can be used to perform measurements on the array in the normal state. The device was not fabricated by the
I I I
I I
I I
I I
T I Figure 1. Layout of a device with 10 junctions. The dashed lines indicate the normal metal (gold) islands. standard shadow evaporation technique. Rather, crossing wires, making an angle of 90 ° , are first defined in a double-layer resist. Then aluminum is evaporated at an angle of 45 ° in such a way that
0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved SSDI 0921-4526(93)E1249-L
1338 material is only deposited on the substrate in parallel wires (in the wires making a 900 angle the material is deposited against the wall of the resist). After oxidizing, the substrate is rotated 90 ° in the plane and gold is evaporated at 45 o . Tunnel junctions are thus formed at the intersections of the resulting aluminum and gold wires. We employ this particular fabrication method to avoid the formation of extra islands (as is unavoidable during shadow evaporation) and the mixing of the gold and aluminum.
islands. This work was supported by the Dutch Foundation for Fundamental Research on Matter (FOM).
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8
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3. R e s u l t s Up to now, only a 22-junction array was measured. The capacitance of the junctions was estimated to be C = 0.3 fF, the resistance R = 90 k~. From measurements on double junctions, the self-capacitance of the islands is estimated to be Co = 0.04 fF. This leads to a threshold voltage for electrons to enter the array of about 0.7 mV and a soliton length of about 3. In fig.2 we show the I - V characteristic of this array in the normal state. Two rf-signals (f=10 MHz) were applied to the gates with a phase difference of ~r/2 (this phase difference applied outside the cryostat can be different from the one at the device). For other phase differences, the observed plateaus became less clear. The plateaus show up at values I = nef, except for the last plateau which occurs at somewhat larger value. For higher values of the bias voltage the current rises without exhibiting any plateaus. An rf-amplitude of 5 mV (10dB attenuated at 1 K) was used . No extra dc-voltage was applied to the gates. The I-V characteristic in the superconducting state showed conduction below the superconducting gap, however no evidence of the formation of plateaus was yet found. It should be noted that after switching back to the nomal state, the steps did not reappear. In conclusion we have found that the fabrication method used produces junctions with a reasonably small capacitance. In the normal state sharp plateaus at I = n e f up to n=3 were found in the I - V characteristic of the array under rf modulation. Although in the superconducting state no plateaus were found, it is still interesting to study the transport in a turnstile consisting of alternating superconducting and normal metal
H
4
2
I
0 0
0.I"
V
I
I
0.2
0.3
(mV)
Figure 2. 1-V characteristic of the 22-junction array in the normal state with 2 rf-signals applied to the gates with a frequency f -- 10 MHz, an amplitude A ~- 5 mV and a phase difference ¢ ----~'/2. References 1. For a review see P. Delsing in Single Charge lunneling edited H. Grabert and M.H. Devoret (Plenum Press, New York 1992). 2. L.J. Geerligs, V.F. Anderegg, P.A.M. Holweg, J.E. Mooij, H. Pothier, D. Esteve, C. Urbina and M.H. Devoret Phys. Rev. Lett. 64 2691 (1990). 3. P. Delsing, D.B. Haviland, T. Claeson, K.K. Likharev and A.N. Korotkov preprint. 4. T.M. Eiles, J.M. Martinis and M.H. Devoret preprint to be published in Phys]ca B. 5. D.V. Averin and Yu.V. Nazarov, preprint. 6. R. Bauernschmitt and Yu.V. Nazarov,
preprint.
Physica B 194-196 (1994) 1337-1338 Noah-Holland
One-dimensional arrays of ultrasmall tunnel junctions with alternating superconducting and normal metal islands M. Matters and J.E. Mooij Department of Applied Physics, Delft University of Technology P.O. Box 5046, 2600 GA Delft, The Netherlands We have fabricated and performed measurements on one-dimensional arrays of ultra.small tunnel junctions. These arrays consist of alternating superconducting and normal metal islands. With two gate electrodes, capacitively coupled to the aluminum islands, the array is irradiated with microwaves of frequency f with an adjustable phase difference between the two microwave signals. In the normal state the I-V characteristics show clear plateaus at currents I = nef up to n = 3. 1. I n t r o d u c t i o n One-dimensional arrays of ultrasmall metal tunnel junctions have been studied extensively by Delsing et al.[1]. In the normal state SEToscillations can be observed in these arrays when microwaves with frequency f are inductively coupled to the leads. These oscillations manifest themselves as peaks in the differential resistance at currents I = nef. Also, the microwaves can be capacitively coupled to the array by means of a gate. The one-dimensional array can then be operated as a turnstile [2, 3] and a clear plateau at I = e f is observed in the I-V characteristic. Recently, transport measurements were performed on a normal- superconductor-normal (NSN) Coulomb blockade transistor [4]. At low bias, a 2e-periodic modulation of the current was observed when changing the gate voltage. This is due to the fact that an isolated superconductor with an odd number of electrons has a larger ground state energy compared to the situation were the electron number is even and all electrons can be paired. In this NSN device, when the charging energy is smaller then the superconducting gap, the conductance at low bias is due to electrons passing in pairs through the island. It is interesting to study the transport in onedimensional arrays consisting of alternating superconducting and normal metal islands. These arrays can also be irradiated by microwaves. The current due to co-tunneling in these arrays can be much smaller then in arrays with only normal metal islands [5]. Thus the array can be used as a lead to single charge traps to study (the absence of) detailed balance.[6]
2. D e s c r i p t i o n o f t h e d e v i c e The device layout is shown in fig.1. The two gates are capacitively coupled to the superconducting islands . Two rf signals with an adjustable phase difference can be applied to the gates. A magnetic field of 2 T can be used to perform measurements on the array in the normal state. The device was not fabricated by the
I I I
I I
I I
I I
T I Figure 1. Layout of a device with 10 junctions. The dashed lines indicate the normal metal (gold) islands. standard shadow evaporation technique. Rather, crossing wires, making an angle of 90 ° , are first defined in a double-layer resist. Then aluminum is evaporated at an angle of 45 ° in such a way that
0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved SSDI 0921-4526(93)E1249-L
1338 material is only deposited on the substrate in parallel wires (in the wires making a 900 angle the material is deposited against the wall of the resist). After oxidizing, the substrate is rotated 90 ° in the plane and gold is evaporated at 45 o . Tunnel junctions are thus formed at the intersections of the resulting aluminum and gold wires. We employ this particular fabrication method to avoid the formation of extra islands (as is unavoidable during shadow evaporation) and the mixing of the gold and aluminum.
islands. This work was supported by the Dutch Foundation for Fundamental Research on Matter (FOM).
i0
8
o~ v
3. R e s u l t s Up to now, only a 22-junction array was measured. The capacitance of the junctions was estimated to be C = 0.3 fF, the resistance R = 90 k~. From measurements on double junctions, the self-capacitance of the islands is estimated to be Co = 0.04 fF. This leads to a threshold voltage for electrons to enter the array of about 0.7 mV and a soliton length of about 3. In fig.2 we show the I - V characteristic of this array in the normal state. Two rf-signals (f=10 MHz) were applied to the gates with a phase difference of ~r/2 (this phase difference applied outside the cryostat can be different from the one at the device). For other phase differences, the observed plateaus became less clear. The plateaus show up at values I = nef, except for the last plateau which occurs at somewhat larger value. For higher values of the bias voltage the current rises without exhibiting any plateaus. An rf-amplitude of 5 mV (10dB attenuated at 1 K) was used . No extra dc-voltage was applied to the gates. The I-V characteristic in the superconducting state showed conduction below the superconducting gap, however no evidence of the formation of plateaus was yet found. It should be noted that after switching back to the nomal state, the steps did not reappear. In conclusion we have found that the fabrication method used produces junctions with a reasonably small capacitance. In the normal state sharp plateaus at I = n e f up to n=3 were found in the I - V characteristic of the array under rf modulation. Although in the superconducting state no plateaus were found, it is still interesting to study the transport in a turnstile consisting of alternating superconducting and normal metal
H
4
2
I
0 0
0.I"
V
I
I
0.2
0.3
(mV)
Figure 2. 1-V characteristic of the 22-junction array in the normal state with 2 rf-signals applied to the gates with a frequency f -- 10 MHz, an amplitude A ~- 5 mV and a phase difference ¢ ----~'/2. References 1. For a review see P. Delsing in Single Charge lunneling edited H. Grabert and M.H. Devoret (Plenum Press, New York 1992). 2. L.J. Geerligs, V.F. Anderegg, P.A.M. Holweg, J.E. Mooij, H. Pothier, D. Esteve, C. Urbina and M.H. Devoret Phys. Rev. Lett. 64 2691 (1990). 3. P. Delsing, D.B. Haviland, T. Claeson, K.K. Likharev and A.N. Korotkov preprint. 4. T.M. Eiles, J.M. Martinis and M.H. Devoret preprint to be published in Phys]ca B. 5. D.V. Averin and Yu.V. Nazarov, preprint. 6. R. Bauernschmitt and Yu.V. Nazarov,
preprint.