Coulomb blockade of Andreev reflection in the NSN single-electron transistor

PHYSICA

Physica B 194-196 (1994) 1111-1112 North-Holland

Coulomb Blockade of Andreev Reflection in the NSN Single-Electron Transistor Travis M. Eiles, a,b Michel H. Devoret,c and John M. Martinis a aNational Institute of Standards and Technology, Boulder, Colorado, USA bDeparlment of Physics, University of Colorado, Boulder, Colorado, USA eService de Physique de l'Etat Condens6, CEA-Saclay, Gif-sur-Yvette, France We have measured at low temperatures the current through a submicrometer superconducting island connected to normal metal leads by ultrasmall tunnel junctions. At low bias voltages, the current changes from being e-periodic in the applied gate charge to 2e-periodic. We interpret this 2e-periodic current as a manifestation of a sequence of Andreev reflection events which transports two electrons at a time across the island. This behavior is clear evidence that there is a difference in total energy between ground states of differing electron number parity. 1. INTRODUCTION It is well known that the behavior of systems with a small number of particles, like atomic nuclei, depends on the even or odd parity of the total number of particles. The pairing of electrons in a superconductor may lead to an observation of this effect in an isolated sample with a macroscopic (-109 ) number of such particles. At very low temperatures, a simple view is that all the electrons should condense into the BCS ground state if their number is even. However, an odd number of electrons will require one electron to remain in an excited quasiparticle state of energy d. The superconducting island can be conveniently probed by connecting it to metal leads by two ultrasmall tunnel junctions, which maintain the isolation of the island. We present currentvoltage characteristics for a sample with normal leads (NSN) 1. In this case, the mechanism of tunneling is understood more readily than in the all-superconducting case (SSS) where Josephson tunneling and its interaction with the electromagnetic environment play a major role. For a normal metal island at zero temperature, the zero-bias conductance vanishes unless the gate charge removes the Coulomb energy barrier at charges Q/e = n + 1/2. The associated current will be e-periodic with the gate charge. H o w e v e r , if the island is superconducting, the gap energy significantly alters the situation (see Fig. 1). As long as A exceeds the charging energy, zero-bias conduction can occur only at gate charges Q/e = 2n+1. However, the mechanism for the

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Figure 1: (a) Energy and Co) zero-bias conductance vs. gate charge for several values of the island electron number. current must provide for the simultaneous Iransfer of two electrons. In the absence of Josephson coupling, we envision a process where two electrons enter the island through one junction and form a Cooper pair, followed by the disappearance of another Cooper pair as two electrons exit the island. This process is similar to Andreev reflection at each junction. 2. DATA The device we tested had circuit parameters Rr]--Rr2=65 k~, EJe=126 ~tV, and A/e=235 IxV. In Fig. 2 we show experimental I -V8 data taken at 35 mK for two bias voltages: (a)V=478 ~tV (--2Ale) and (b)V=63 I.tV (-A/4e). The higher voltage data are e-periodic as expected at voltage scales exceeding the energy gap. The low voltage data have twice the period of the higher voltage

0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved S S D I 0921-4526(93)El136-A

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Figure 2. I -Vs curves for two values of the bias voltage. data, and the peaks are positioned at minima of the high voltage data. This behavior is exactly predicted by the simple Coulomb blockade model incorporating an energy asymmetry between even and odd states. The maximum current decreased linearly with increasing temperature (Fig. 3) and disappeared completely at To=130 mK. At zero bias voltage and gate charges Q/e = 2n + 1, the energy difference between even and odd levels is A - Ec. Thus the odd level becomes significantly populated when the temperature reaches To = (AEc)/lnNe// where Ne#is the total number of available quasiparticles, 2 in good agreement with our measured parameters. Figure 4 shows I-V characteristics for three 0.8 I ,( ~0.6 "~ 0.4 '~ 0.2

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Figure 4. l-VCurves for three values of the gate charge (mod 2e). values of charge bias. The 2e-periodicity is evident in the different characteristics between the Q=O and Q=e data. The curve for Q=l.06e shows a small Coulomb gap opening up at the origin. This behavior is markedly different from what is seen in the SSS device where the charge transfer is coherent. The gap at the origin points to an incoherent process where voltage thresholds must be exceeded in order that sequential tunneling occurs. The two-electron mechanism we suggest does not require voltages greater than A, since no quasi-particles are excited in the island. The associated current increases linearly with voltage, and is 2e-periodic with gate charge due to the two-electron transfer. We calculate the zero-bias conductance to be on the order of RK/(4Rr~Meff), where Me# corresponds to the number of effective channels contributing to the conduction: REFERENCES 1. T.M. Eiles, John M. Martinis, and Michel H. Devoret, Phys. Rev. Lett., 70, 1862 (1993). 2. M.T. Tuominen, J.M. Hergenrother, T.S. Tighe, and M. Tinldaam, Phys. Rev. Lett. 69, 1997 (1992). 3. Travis M. Eiles, John M. Martinis, and Michel H. Devoret, to appear in Physica B; F.W.J. Hekking, L.I. Glazman, K.A. Matveev, and R.I. Shekhter, submitted to Phys. Rev. Left.