Matthew Brenner

Distribution of Supercurrent Switching in Graphene under the Proximity Effect

We study the stochastic nature of switching current in hysteretic current-voltage characteristics of superconductor-graphene-superconductor junctions. We find that the dispersion of the switching current distribution scales with temperature as σ(I) proportional to T(α(G)) with α(G) as low as 1/3. This observation is in sharp contrast to the known Josephson junction behavior where σ(I) proportional to T(α(J)) with α(J)=2/3. We propose an explanation using a generalized version of Kurkijärvis theory for the flux stability in rf-SQUID and attribute this anomalous effect to the temperature dependence of the critical current which persists down to low temperatures.

Current-phase relationship, thermal and quantum phase slips in superconducting nanowires made on a scaffold created using adhesive tape.

Quantum phase slippage (QPS) in a superconducting nanowire is a new candidate for developing a quantum bit [Mooij et al. New J. Phys. 2005, 7, 219; Mooij et al. Nat. Phys. 2006, 2, 169; Khlebnikov http://arxiv.org/abs/quant-ph/0210019 2007]. It has also been theoretically predicted that the occurrence of QPS significantly changes the current-phase relationship (CPR) of the wire due to the tunneling between topologically different metastable states [Khlebnikov Phys. Rev. B 2008, 78, 014512]. We present studies on the microwave response of the superconducting nanowires to reveal their CPRs. First, we demonstrate a simple nanowire fabrication technique, based on commercially available adhesive tapes, which allows making thin superconducting wire from different metals. We compare the resistance vs temperature curves of Mo(76)Ge(24) and Al nanowires to the classical and quantum models of phase slips. In order to describe the experimentally observed microwave responses of these nanowires, we use the McCumber-Stewart model [McCumber J. Appl. Phys. 1968, 39, 3113; Stewart Appl. Phys. Lett. 1968, 12, 277], which is generalized to include either classical or quantum CPR.

Dynamics of superconducting nanowires shunted with an external resistor

We present a study of superconducting nanowires shunted with an external resistor, geared towards understanding and controlling coherence and dissipation in nanowires. The dynamics is probed by measuring the evolution of the V -I characteristics and the distributions of switching and retrapping currents upon varying the shunt resistor and temperature. Theoretical analysis of the experiments indicates that as the value of the shunt resistance is decreased, the dynamics turns more coherent, presumably due to stabilization of phase-slip centers in the wire, and furthermore the switching current approaches the Bardeen’s prediction for equilibrium depairing current. By a detailed comparison between theory and experiment, we make headway into identifying regimes in which the quasi-one-dimensional wire can effectively be described by a zero-dimensional circuit model analogous to the resistively and capacitively shunted Josephson junction model of Stewart and McCumber. Aside from its fundamental significance, our study has implications for a range of promising technological applications.

Delayed-choice quantum cryptography

Delayed-choice quantum cryptography is a novel protocol, incorporating an optical storage system, which achieves substantially higher effective data rates for quantum key distribution.

Little-Parks oscillations at low temperatures: Gigahertz resonator method

A thin-film Fabry–Perot superconducting resonator is used to reveal the Little and Parks (LP) effect [Phys. Rev. Lett. 9, 9 (1962)], even at temperatures much lower than the critical temperature. A pair of parallel nanowires is incorporated into the resonator at the point of the supercurrent antinode. As the magnetic field is ramped, Meissner currents develop, changing the resonance frequency of the resonator. The LP oscillation is revealed as a periodic set of distorted parabolas observed in the transmission of the resonator and corresponds to the states of the wire loop having different vorticities. We also report a direct observation of single and double phase slip events.

Nanoslits in silicon chips

Potassium hydroxide (KOH) etching of a patterned [100] oriented silicon wafer produces V-shaped etch pits. We demonstrate that the remaining thickness of silicon at the tip of the etch pit can be reduced to approximately 5 microm using an appropriately sized etch mask and optical feedback. Starting from such an etched chip, we have developed two different routes for fabricating 100 nm scale slits that penetrate through the macroscopic silicon chip (the slits are approximately 850 microm wide at one face of the chip and gradually narrow to approximately 100-200 nm wide at the opposite face of the chip). In the first process, the etched chips are sonicated to break the thin silicon at the tip of the etch pit and then further KOH etched to form a narrow slit. In the second process, focused ion beam milling is used to etch through the thin silicon at the tip of the etch pit. The first method has the advantage that it uses only low-resolution technology while the second method offers more control over the length and width of the slit. Our slits can be used for preparing mechanically stable, transmission electron microscopy samples compatible with electrical transport measurements or as nanostencils for depositing nanowires seamlessly connected to their contact pads.