Fred Sharifi

Experimental evidence for s-wave pairing symmetry in superconducting Cu(x)Bi2Se3 single crystals using a scanning tunneling microscope.

Topological superconductors represent a newly predicted phase of matter that is topologically distinct from conventional superconducting condensates of Cooper pairs. As a manifestation of their topological character, topological superconductors support solid-state realizations of Majorana fermions at their boundaries. The recently discovered superconductor CuxBi2Se3 has been theoretically proposed as an odd-parity superconductor in the time-reversal-invariant topological superconductor class and point-contact spectroscopy measurements have reported the observation of zero-bias conductance peaks corresponding to Majorana states in this material. Here we report scanning tunneling spectroscopy (STS) measurements of the superconducting energy gap in CuxBi2Se3 as a function of spatial position and applied magnetic field. The tunneling spectrum shows that the density of states at the Fermi level is fully gapped without any in-gap states. The spectrum is well described by the Bardeen-Cooper-Schrieffer (BCS) theory with a momentum independent order parameter, which suggests that Cu0.2Bi2Se3 is a classical s-wave superconductor contrary to previous expectations and measurements.

Stable field emission from nanoporous silicon carbide.

We report on a new type of cold cathode field emission electron source capable of extremely high emission, producing current densities at levels comparable to thermal sources. Record stable emission in excess of 6 A/cm2 at 7.5 V/μm and a maximum current density of 11 A/cm2 at 9.0 V/μm are demonstrated. The emitter is a unique structure, comprised of a monolithic and rigid nanostructured foam with homogenously distributed emission sites. The fabrication process is simple and scalable, as it consists of wet chemistry and ion etching and does not require high temperature gas phase synthesis. Electric field enhancement is tunable and is set by a two-level hierarchy consisting of local nanostructure morphology and the macroscopic shape.

Field emission from nanoporous silicon carbide

A new type of cold cathode field emission electron source capable of emission at levels comparable to thermal sources is described. The emitter is a unique structure, comprised of a monolithic and rigid porous semiconductor nanostructure with homogenously distributed emission sites, and is fabricated through a room temperature process which allows for control of emission properties. These electron sources potentially enable breakthroughs in several critical technologies, including microwave electronics and x-ray imaging.

Fabrication and characterization of nanostructured III-V thermoelectric materials

Approximately two thirds of all fossil fuel used is lost as heat. Thermoelectric materials, which convert heat into electrical energy, may provide a solution to partially recover some of this lost energy. To date, most commercial thermoelectric materials are too inefficient to be a viable option for most waste heat applications. This research proposes to investigate the fabrication and characterization of nanostructured III-V semiconductor thermoelectric materials with the goal of increasing the performance of existing technology. In order to improve thermoelectric material efficiency, either the lattice thermal conductivity must be lowered or the thermoelectric power factor must be increased. This research will focus on the latter by modifying the density of states of the semiconductor material and studying the effect of quantum confinement on the material’s thermoelectric properties. Using focused ion beam milling, nanostructured cantilevers are fabricated from single crystal wafers. An all around gate dielectric and electrode are deposited to create a depletion region along the outer core of the cantilever, thus creating an inner conductive core. The Seebeck coefficient can then be measured as a function of confinement by varying the gate voltage. This technique can be applied to various material systems to investigate the effects of confinement on their thermoelectric properties.

Stable field emission from nanoporous silicon carbide

We report on a new type of cold cathode field emission electron source capable of extremely high emission, producing current densities at levels comparable to thermal sources. Record stable emission in excess of 6 A/cm2 at 7.5 V/μm and a maximum current density of 11 A/cm2 at 9.0 V/μm are demonstrated. The emitter is a unique structure, comprised of a monolithic and rigid nanostructured foam with homogenously distributed emission sites. The fabrication process is simple and scalable, as it consists of wet chemistry and ion etching and does not require high temperature gas phase synthesis. Electric field enhancement is tunable and is set by a two-level hierarchy consisting of local nanostructure morphology and the macroscopic shape.