Sukumar Rajauria

Andreev Current-Induced Dissipation in a Hybrid Superconducting Tunnel Junction

We have studied hybrid superconducting microcoolers made of a double superconductor-insulator-normal metal tunnel junction. Under subgap conditions, the Andreev current is found to dominate the single-particle tunnel current. We show that the Andreev current introduces additional dissipation in the normal metal equivalent to Joule heating. By analyzing quantitatively the heat balance in the system, we provide a full description of the evolution of the electronic temperature with the voltage. The dissipation induced by the Andreev current is found to dominate the quasiparticle tunneling-based cooling over a large bias range.

Electron and phonon cooling in a superconductor-normal-metal-superconductor tunnel junction.

We present evidence for the cooling of normal-metal phonons, in addition to the well-known electron cooling, by electron tunneling in a superconductor-normal-metal-superconductor tunnel junction. The normal-metal electron temperature is extracted by comparing the device current-voltage characteristics to the theoretical prediction. We use a quantitative model for the heat transfer that includes the electron-phonon coupling in the normal metal and the Kapitza resistance between the substrate and the metal. It gives a very good fit to the data and enables us to extract an effective phonon temperature in the normal metal.

Efficiency of quasiparticle evacuation in superconducting devices

In a normal metal - insulator - superconductor (N-I-S) junction, charge transport is mainly governed by quasi- particles (1). The presence of the superconducting en- ergy gapinduces an energy selectivity of quasiparticles tunneling out of the normal metal (2, 3). The quasiparti- cle tunnel current is thus accompanied by a heat trans- fer from the normal metal to the superconductor that is maximum at a voltage bias just below the superconduct- ing gap (V ≤ �/e). For a double junction geometry (S-

Inherent Thermometry in a Hybrid Superconducting Tunnel Junction

We discuss inherent thermometry in a Superconductor-Normal metal-Superconductor tunnel junction. In this configuration, the energy selectivity of single-particle tunneling can provide a significant electron cooling, depending on the bias voltage. The usual approach for measuring the electron temperature consists in using an additional pair of superconducting tunnel junctions as probes. In this paper, we discuss our experiment performed on a different design with no such thermometer. The quasi-equilibrium in the central metallic island is discussed in terms of a kinetic equation including injection and relaxation terms. We determine the electron temperature by comparing the micro-cooler experimental current-voltage characteristic with isothermal theoretical predictions. The limits of validity of this approach, due to the junctions asymmetry, the Andreev reflection or the presence of sub-gap states are discussed.

Porous Superhydrophobic Membranes: Hydrodynamic Anomaly in Oscillating Flows

We have fabricated and characterized a novel superhydrophobic system, a meshlike porous superhydrophobic membrane with solid area fraction Φ(s), which can maintain intimate contact with outside air and water reservoirs simultaneously. Oscillatory hydrodynamic measurements on porous superhydrophobic membranes as a function of Φ(s) reveal surprising effects. The hydrodynamic mass oscillating in phase with the membranes stays constant for 0.9≲Φ(s)≤1, but drops precipitously for Φ(s)<0.9. The viscous friction shows a similar drop after a slow initial decrease proportional to Φ(s). We attribute these effects to the percolation of a stable Knudsen layer of air at the interface.

Competition between Electronic Cooling and Andreev Dissipation in a Superconducting Micro-Cooler

We discuss very low temperature experiments on superconducting micro-coolers made of a double Normal metal–Insulator–Superconductor junction. We investigate with a high resolution the differential conductance of the micro-cooler as well as of additional probe junctions. There is an explicit crossover between the single quasi-particle current and the phase-coherent Andreev current. We establish a thermal model by considering the thermal contribution due to the Andreev current. The related increase of the electron temperature is discussed, including the influence of several parameters like the phase-coherence length or the tunnel junction transparency.

Electrostatically Tunable Adhesion in a High Speed Sliding Interface

Contact hysteresis between sliding interfaces is a widely observed phenomenon from macro- to nanoscale sliding interfaces. Most such studies are done using an atomic force microscope (AFM) where the sliding speed is a few μm/s. Here, we present a unique study on stiction between the head-disk interface of commercially available hard disk drives, wherein the vertical clearance between the head and the disk is of the same order as in various AFM-based fundamental studies but with a sliding speed that is nearly 6 orders of magnitude higher. We demonstrate that, although the electrostatic force (dc or ac voltage) is an attractive force, the ac-voltage-induced out-of-plane oscillation of the head with respect to the disk is able to completely suppress the contact hysteresis.

Dynamic Friction Study on Voltage Assisted Overcoat Wearing Head-Disk Interface

Voltage assisted wear at the head and the disk interface is investigated with the motive of understanding the head overcoat wear processes. In this work, we report the quantitative analysis of voltage assisted wear on head carbon overcoat at high sliding speed interfaces. We found that voltage assisted TFC head wear acts asymmetrically at the interface with positive voltage leading to high wear. We quantitatively analyzed the interface using a strain gauge based friction measurement.Copyright

Overcoat Wear and Dynamic Friction Study at the Head-Disk Interface

The dynamic friction and wear at the head and the disk interface is investigated with the motive of understanding the head overcoat wear process associated with physical contact between the head and the disk. In this work, the results from systematic experiments under overpush conditions are presented. Various regimes of head wear are identified based on the individual wear rate of the participating overcoat layers. A strain gauge based friction measurement is used to extract the friction coefficient and the adhering shear strength between the head and the disk.Copyright