Non-equilibrium Tunneling Spectroscopy in Carbon Nanotubes

We report measurements of the non-equilibrium electron energy distribution in carbon nanotubes. Using tunneling spectroscopy via a superconducting probe, we study the shape of the local electron distribution functions, and hence energy relaxation rates, in nanotubes that have bias voltages applied between their ends. At low temperatures, electrons interact weakly in nanotubes of a few microns channel length, independent of end-to-end conductance values. Surprisingly, the energy relaxation rate can increase substantially when the temperature is raised to only 1.5 K

Transport Through Andreev Bound States in a Graphene Quantum Dot

Andreev reflection-where an electron in a normal metal backscatters off a superconductor into a hole-forms the basis of low energy transport through superconducting junctions. Andreev reflection in confined regions gives rise to discrete Andreev bound states (ABS), which can carry a supercurrent and have recently been proposed as the basis of qubits [1-3]. Although signatures of Andreev reflection and bound states in conductance have been widely reported [4], it has been difficult to directly probe individual ABS. Here, we report transport measurements of sharp, gate-tunable ABS formed in a superconductor-quantum dot (QD)-normal system, which incorporates graphene. The QD exists in the graphene under the superconducting contact, due to a work-function mismatch [5, 6]. The ABS form when the discrete QD levels are proximity coupled to the superconducting contact. Due to the low density of states of graphene and the sensitivity of the QD levels to an applied gate voltage, the ABS spectra are narrow, can be tuned to zero energy via gate voltage, and show a striking pattern in transport measurements.Comment: 25 Pages, included SO

Magnetoresistance in Single Layer Graphene: Weak Localization and Universal Conductance Fluctuation Studies

We report measurements of magnetoresistance in single-layer graphene as a function of gate voltage (carrier density) at 250 mK. By examining signatures of weak localization (WL) and universal conductance fluctuations (UCF), we find a consistent picture of phase coherence loss due to electron-electron interactions. The gate-dependence of the elastic scattering terms suggests that the effect of trigonal warping, i.e., the non-linearity of the dispersion curves, may be strong at high carrier densities, while intra-valley scattering may dominate close to the Dirac point. In addition, a decrease in UCF amplitude with decreasing carrier density can be explained by a corresponding loss of phase coherence

Visualizing landscapes of the superconducting gap in heterogeneous superconductor thin films: geometric influences on proximity effects

The proximity effect is a central feature of superconducting junctions as it underlies many important applications in devices and can be exploited in the design of new systems with novel quantum functionality. Recently, exotic proximity effects have been observed in various systems, such as superconductor-metallic nanowires and graphene-superconductor structures. However, it is still not clear how superconducting order propagates spatially in a heterogeneous superconductor system. Here we report intriguing influences of junction geometry on the proximity effect for a 2D heterogeneous superconductor system comprised of 2D superconducting islands on top of a surface metal. Depending on the local geometry, the superconducting gap induced in the surface metal region can either be confined to the boundary of the superconductor, in which the gap decays within a short distance (~ 15 nm), or can be observed nearly uniformly over a distance of many coherence lengths due to non-local proximity effects.Comment: 17 pages, 4 figure

Professional Exposure to Goats Increases the Risk of Pneumonic-Type Lung Adenocarcinoma: Results of the IFCT-0504-Epidemio Study

Pneumonic-type lung adenocarcinoma (P-ADC) represents a distinct subset of lung cancer with specific clinical, radiological, and pathological features. Given the weak association with tobacco-smoking and the striking similarities with jaagsiekte sheep retrovirus (JSRV)-induced ovine pulmonary adenocarcinoma, it has been suggested that a zoonotic viral agent infecting pulmonary cells may predispose to P-ADC in humans. Our objective was to explore whether exposure to domestic small ruminants may represent a risk factor for P-ADC. We performed a multicenter case-control study recruiting patients with P-ADC as cases and patients with non-P-ADC non-small cell lung cancer as controls. A dedicated 356-item questionnaire was built to evaluate exposure to livestock. A total of 44 cases and 132 controls were included. At multivariate analysis, P-ADC was significantly more associated with female gender (Odds-ratio (OR) = 3.23, 95% confidence interval (CI): 1.32–7.87, p = 0.010), never- smoker status (OR = 3.57, 95% CI: 1.27–10.00, p = 0.015), personal history of extra-thoracic cancer before P-ADC diagnosis (OR = 3.43, 95% CI: 1.10–10.72, p = 0.034), and professional exposure to goats (OR = 5.09, 95% CI: 1.05–24.69, p = 0.043), as compared to other subtypes of lung cancer. This case-control suggests a link between professional exposure to goats and P-ADC, and prompts for further epidemiological evaluation of potential environmental risk factors for P-ADC

Tunneling spectroscopy of carbon nanostructures: a romance in many dimensions

In this dissertation we present results from various methods of tunneling spectroscopy in carbon nanotubes, which shed light on electron ??? electron interaction in carbon nanotubes and low dimensional systems in general. We also apply those methods to two dimensional graphene sheets. We first review the fabrication techniques used to make the devices studied here. Some of the techniques are standard in nanofabrication, and some were developed in-house to make the particular device geometries studied here possible. In particular, we developed recipes for the growth and contact of clean, ultra-long carbon nanotubes as well as for the fabrication of non-invasive top tunnel probes. We then present results on normal metal tunneling spectroscopy of carbon nanotube devices of varying length. We measure the exponent of the conductance power law in the density of states as a function of device length over two orders of magnitude and find unexpected evidence of finite size effects in long devices. Next, we present results from the first measurement of the non-equilibrium electron energy distribution function in carbon nanotubes measured via non-equilibrium superconducting tunneling spectroscopy and find little evidence of scattering at low temperatures, which is consistent with a clean, strongly interacting Luttinger liquid. In addition, we discuss two ways we are working to extend this powerful technique. We also present results of superconducting tunneling spectroscopy of a clean carbon nanotube quantum dot. We are able to characterize the energy spectrum of the quantum dot and distinguish between spin singlet and spin triplet shell filling. We observe elastic and inelastic co-tunneling features which are not visible when the probe is made normal by a magnetic field. These co-tunneling rates have important technological implications for carbon nanotubes as single electron transistors. We also observe an energetically forbidden conductance inside the superconducting gap that could be related to inelastic scattering in the carbon nanotube quantum dot. Finally, we present results from the first application of the superconducting tunneling spectroscopy technique to graphene, a two dimensional system. We observe conductance oscillations consistent with Fabry-Perot interference. We also observe a gate dependant pair of subgap peaks, symmetric about bias voltage. We hypothesize that these peaks are due to conductance through bound Andreev states confined to a graphene quantum dot below the superconducting tunnel probe