We present measurements on a novel electronic microrefrigerator that can cool conduction electrons significantly below the lattice temperature. A normal-insulator-superconductor tunnel junction is used to extract electrons from the normal metal electrode whose energy is higher than the Fermi energy. Electrons with an average energy equal to the Fermi energy are returned to the metal by a superconducting contact. Consequently, the high-energy thermal excitations are removed from the normal metal, thus cooling the electrons. For lattice temperatures higher than 100 mK the data can be explained by a simple theory incorporating the BCS density of states in the superconducting electrode and the coupling between electrons and phonons. At lower temperatures our measurement suggests that the electron energies in the normal electrode depart strongly from an equilibrium distribution.
We have measured the noise of a Coulomb blockade electrometer. Below 100 Hz, the noise referred to the input charge has a 1/f power spectrum with a charge noise of 3×10−4 e/√Hz and an energy sensitivity EN of 3×104 ℏ at 10 Hz. The 1/f noise probably results from the stochastic occupation of charge traps which could in principle be eliminated. The theoretical noise floor is set by shot noise, and indirect measurements show that this contribution to EN can be as small as 1.5 ℏ, suggesting that the electrometer will be a quantum limited amplifier if the 1/f noise can be eliminated.
We have measured at low temperatures the current through a submicrometer superconducting island connected to two normal metal leads by ultrasmall tunnel junctions. As the bias voltage is lowered well below twice the superconducting energy gap, the current changes from being e periodic with gate charge to 2e periodic. This behavior is clear evidence that there is a difference in the total energy between the ground states of an even and odd number of electrons on the island. The 2e-periodic current peaks are the first observation of the Coulomb blockade of Andreev reflection.PACS numbers: 73.40. Gk, 73.40.Rw, 74.50.+r Systems with a small number of particles like nuclei behave differently if the number of particles is even or odd [1]. Does this symmetry breaking occur for a large metallic particle or "island" where the number of conduction electrons is macroscopic, say, 10^? In a superconductor, according to the BCS theory, the electronic state consists of pairs of electrons. At very low temperatures, a simple view is that all the electrons should condense into the ground state if their number is even. However, if the number is odd, one electron should remain in an excited quasiparticle state with energy A, the superconducting gap. This simple picture might not be applicable to a real experimental system because, for example, there might be quasiparticle states located within ksT of the Fermi level. Even one of these states, normally undetectable in usual tunneling experiments, could then capture and release single electrons and restore the even-odd symmetry.The isolated superconducting island can be conveniently probed by connecting it to metal leads via two tunnel junctions. If the tunnel junction resistances are much greater than the resistance quantum, then the charge on the island is quantized, and this Coulomb blockade electrometer can be used to probe the energy of the island [2]. In experiments with a device having both superconducting island and leads, the SSS electrometer [3], a current 2e periodic with a gate charge applied to the island has been measured for bias voltages in a certain range [4,5]. These 2e-periodic data, which could be observed only on some samples, display a complicated structure which is diflftcult to interpret since the basic model should at least include, in addition to the charging energy of the system, both quasiparticle and Josephson tunneling. Nevertheless, Tuominen et al. [5] have observed that the amplitude of the 2e-periodic component vanished at a temperature of about 300 mK, which they explain as the temperature at which a single quasiparticle of energy A can be thermally excited and thus restore the even-odd symmetry.In this Letter, we present the first measurements on a superconducting island with normal metal leads, the NSN electrometer. This system can be more readily understood because the two SN junctions do not allow Josephson tunneling [6]. The only allowed conduction process at low bias voltages is the Andreev reflection [7] of an electron into a hole on the N s...
We have used a scanning YBa2Cu3O7 superconducting quantum interference device (SQUID) at 77 K to image currents in room-temperature integrated circuits. We acquired magnetic field data and used an inversion technique to convert the field data to a two-dimensional current density distribution, allowing us to locate current paths. With an applied current of 1 mA at 3 kHz, and a 150 μm separation between the sample and the SQUID, we found a spatial resolution of 50 μm in the converted current density images. This was about three times smaller than the SQUID–sample separation, i.e., three times better than the standard near-field microscopy limit, and about 10 times sharper than the raw magnetic field images.
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