Physica B 189 (1993) 210-217 North-Holland SDI: 0921-4526(93)E0025-C PHYSICA Even-odd symmetry breaking in the NSN Coulomb blockade electrometer • ab Travis M. Elles ' , John M. Martinis" and Michel H. Devoret .... ~'National Institute of Standards and Technology, Boulder, CO, USA bphysics Department, University of Colorado, Boulder, CO, USA CService de Physique de l'Etat Condense, CEA-Saclay, Gif-sur-Yvette, France We have measured at low temperature the current through a submicrometer superconducting island connected to two normal metal leads by ultra-small tunnel junctions. As the bias voltage is lowered below the superconducting gap of this Coulomb blockade electrometer, the current changes from being e-periodic with gate charge to 2e-periodic. We interpret the 2e-periodic current at low voltages as a manifestation of a sequence of Andreev reflections which shuttles two electrons at a time through the island. This process can only exist if the island favors a state with a definite parity of the number of conduction electrons. 1. Introduction It is well known that systems with a small number of particles, like atomic nuclei, behave in a very different way if the number of particles is even or odd [1]. Nevertheless, one is inclined to believe that the macroscopic properties of a large isolated metallic electrode or "island" should not depend on the even or odd character of the number of its conduction electrons. There is one case, however, where the oddness or evenness of electron number would in principle show up at the macroscopic level. According to the BCS theory, electrons in a superconductor tend to form Cooper pairs. But an isolated superconductor with an odd number of electrons cannot have all its electrons paired. In the ground state, one electron would remain as a quasi-particle excitation, which has an energy corresponding to the superconducting energy gap. On the contrary, if the number of electrons were even, the ground state would be free of such excitation. This simple picture may not be applicable to a real superconducting electrode: Correspondence to: M.H. Devoret, Service de Physique de l*Etat Condensd, CEA-Saclay, 91191 Gif-sur-Yvette, France. the presence of only one quasiparticle state located within k~T of the Fermi level would restore the even-odd symmetry at an arbitrary low temperature T. A very small but finite density of states below the superconducting gap would be undetectable in usual tunneling experi- ments. In this paper, we report novel experimen- tal results supporting the simple picture of even- odd symmetry breaking in an isolated supercon- ductor, at least when time scales of the order of an hour are considered. 2. The NSN Coulomb blockade electrometer Our experiment uses the property, well estab- lished experimentally [2] and theoretically [3], that a tunneling current passing through an island in the Coulomb blockade regime (i.e. kBT~E c =eZ/2C,=, Cz being the total island capacitance) depends on the total energy differ- ence between the ground states of the island differing by one charge carrier. We connect the island to two normal metal leads via tunnel junctions (insets of fig. 1). A voltage difference V is applied to the leads. The potential of the island and hence, its number of electrons, can be 0921-4526/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved