2D materials based heterostructure for quantum tunneling: a lithography free technique.

Two-dimensional electron gas (2DEG) systems have played a vital role in the development of superior electronic devices including tunnel junctions consisting of two such 2DEG systems. With the advent of the new 2D electronic material systems, it has opened a new route for 2D–2D tunneling in such extended systems. In this study, we have utilized a plasma enhanced chemical vapor deposition (PECVD) technique to directly deposit graphene (nanowalls) and h-BN on Si/SiO2 substrates to construct two-dimensional material based, vertically stacked electron tunneling devices free of expensive and cumbersome microfabrication steps. In the first study, we fabricated direct quantum tunneling devices by depositing atomically thin tunnel barriers of h-BN as the tunneling barrier with equally doped (p-doped under ambient conditions) graphene nanowalls as the active electrode layers (top and bottom) on Si/SiO2 substrates. Current-voltage (I-V) measurements for varying h-BN thicknesses of these single barrier tunneling devices showed linear I-V characteristics at low bias but an exponential dependence at higher bias. Our measurements of the electron tunnel current through the barrier demonstrated that the h-BN films act as a good tunnel barrier. The barrier thickness dependent tunneling current was in good agreement with the tunnelling currents computed using the Bardeen transfer Hamiltonian approach with equally doped top and bottom graphene electrodes. Presence of negative differential resistance (NDR) is characteristic of the current–voltage relationship of a resonant tunneling device, enabling many unique applications. NDR arises at a voltage bias corresponding to aligned band structures of the 2D systems, causing a sharp peak in the tunnelling current. The existence of devices with NDR has been reported since the late 1950\u27s in devices that contained degenerately doped p-n junctions with thin oxide barriers (tunnel diodes) and double barrier heterojunction devices where quantum tunneling effects are utilized. The NDR in the I-V characteristics of these devices has been used in many applications involving microwave/millimeter wave oscillators, high speed logic devices and switches. We investigated NDR phenomenon in our graphene/h-BN systems in two different routes. In the first case, graphene/h-BN/graphene single barrier device, the bottom and top graphene layers were unequally doped. One of the graphene layers was n-type doped using ammonia or hydrazine. Nitrogen doping using ammonia was accomplished during the growth by incorporating ammonia in the PECVD system. Hydrazine doping was accomplished by exposing the graphene to hydrazine vapor in vacuum. The unequal doping of graphene causes alignment of the band structures of graphene systems giving rise to NDR. The tunnelling devices consisting of unequally doped graphene with a single barrier shows resonant quantum tunneling with the presence of a pronounced peak in the current corresponding to NDC whose peak current value and the voltage value depend on the doping levels. The results are explained according to the modified Bardeen tunneling model. Next, resonant tunneling behavior was demonstrated in Graphene/h-BN/Graphene/h-BN/Graphene double barrier (DB) devices by directly depositing graphene and h-BN successive layers on Si/SiO2 substrates using PECVD. DB Tunneling junctions with various barrier widths were investigated (by varying the thickness of the second graphene layer). The I-V parameters of tunneling current at room temperature demonstrated resonant tunneling with negative differential conductance. A quantum mechanical double barrier tunneling model was used to explain the phenomenon, by solving the Schrödinger\u27s equation in either side of the system. A systematic behavior of the current peak values and the corresponding voltage values in I-V curves were seen to be in good agreement with the transmission coefficient calculated using a quantum mechanical model. Josephson tunneling is a different kind of tunneling phenomenon in superconductors, in which superconducting cooper pairs tunnel across a thin insulating barrier. A supercurrent can flow between two superconductors that are separated by a narrow insulating barrier. The current is influenced by the phase difference between the two superconductors. We fabricated Josephson junctions with atomically thin tunnel barriers by combining h-BN with magnesium diboride (MgB2) active electrode layers on a Si/SiO2 substrate using a PECVD (for h-BN) and a Hybrid Physical-Chemical Vapor Deposition (HPCVD) (for Mg ). The I-V characteristics were measured above and below the transition temperature Tc (37 K). A measurable supercurrent was detected below Tc

Superconductor

This book contains a collection of works intended to study theoretical and experimental aspects of superconductivity. Here you will find interesting reports on low-Tc superconductors (materials with Tc 30 K). Certainly this book will be useful to encourage further experimental and theoretical researches in superconducting materials

Topological Insulator-Superconductor Heterostructures and Devices

A 3D topological insulator has fully gapped insulating bulk state but a conducting surface. Such conducting “surface” states are formed with helical Dirac fermions, with spin-momentum strictly locked by spin-orbital coupling. When coupled to a conventional s-wave superconductor, the surface state behaves just like the desired p-wave superconductor. It has been predicted that Majorana zero-modes obeying non-Abelian statistics can appear in such a system. Braiding operations on the Majorana zero-modes can realize topological quantum computing that is innately error-tolerant. Therefore, it has boosted extensive interest in the topological insulator-superconductor (TI-S) heterostructures. High quality TI-S heterostructures on wafer size scale is demanded for both fundamental studies as well as applications in the future. The study in this thesis explored molecular beam epitaxy (MBE) growth of TI-S bilayer heterostructures by depositing 3D topological insulator (Bi1-xSbx)2Te3 (BST) on superconductors including Nb and MgB2. The BST-Nb heterostructure has been demonstrated to have high crystalline quality and clear interface. BST-MgB2 heterostructure suffers from chemical reaction happening at its interface, which can be inhibited by lowering growth temperature and prevented by inserting a thin layer of Nb in between to form BST-Nb/MgB2 structure. In addition, by depositing a top layer of Nb on the bilayer heterostructures and proceeding to fabrication, we achieved vertical S-TI-S junctions that have seldomly been studied. Josephson coupling between the two superconducting electrodes and between proximity induced superconducting TI surfaces was observed on the Nb-BST-Nb and Nb-BST-Nb/MgB2 junctions, giving two different critical current transitions on the I-V characteristics. In the Nb-BST-MgB2 junction, tunneling effect through normal electrons was dominating without Josephson current being observed, which is due to an insulating layer formed at the BST-MgB2 interface. Despite the difference in overall conductance features, a zero bias conductance peak (ZBCP) was observed on the dI/dV-V curves from all three kinds of junction. The origin of this ZBCP was discussed. The thin film heterostructures obtained in this project can be good platforms to search for Majorana zero-modes through various approaches. We have also demonstrated the heterostructures are compatible with fabrication processes by patterning vertical junctions. Transport measurement results on these junctions could provide some insight into understanding the physical processes happening at the TI-S interfaces

The structure of the superconducting gap in MgB2 from point-contact spectroscopy

We have studied the structure of the superconducting gap in MgB2 thin films by means of point-contact spectroscopy using a gold tip. The films were produced by depositing pure boron on a sapphire substrate, using e-beam evaporation, followed by reaction with magnesium vapour. The films have a Tc of 38.6 +- 0.3 K and resistivity of about 20 microOhm cm at 40 K. The point-contact spectra prove directly the existence of a multi-valued order parameter in MgB2, with two distinct values of the gap, DELTA1=2.3+-0.3 meV and DELTA2=6.2+-0.7 meV at 4.2 K. Analysis of the spectra in terms of the Blonder-Tinkham-Klapwijk model reveals that both gaps close simultaneously at the Tc of the film. Possible mechanisms that can explain the intrinsic co-existence of two values of the gap are discussed.Comment: 9 pages, 9 figure

Superconductivity in doped semiconductors

International audienceA historical survey of the main normal and superconducting state properties of several semiconductors doped into superconductivity is proposed. This class of materials includes selenides, tellurides, oxides and column-IV semiconductors. Most of the experimental data point to a weak coupling pairing mechanism, probably phonon-mediated in the case of diamond, but probably not in the case of strontium titanate, these being the most intensively studied materials over the last decade. Despite promising theoretical predictions based on a conventional mechanism, the occurrence of critical temperatures significantly higher than 10 K has not been yet verified. However, the class provides an enticing playground for testing theories and devices alike

Evidence for two distinct scales of current flow in polycrystalline Sm and Nd iron oxypnictides

Early studies have found quasi-reversible magnetization curves in polycrystalline bulk rare-earth iron oxypnictides that suggest either wide-spread obstacles to intergranular current or very weak vortex pinning. In the present study of polycrystalline samarium and neodymium rare-earth iron oxypnictide samples made by high pressure synthesis, the hysteretic magnetization is significantly enhanced. Magneto optical imaging and study of the field dependence of the remanent magnetization as a function of particle size both show that global currents over the whole sample do exist but that the intergranular and intragranular current densities have distinctively different temperature dependences and differ in magnitude by about 1000. Assuming that the highest current density loops are restricted to circulation only within grains leads to values of ~5 MA/cm2 at 5 K and self field, while whole-sample current densities, though two orders of magnitude lower are 1000-10000 A/cm2, some two orders of magnitude higher than in random polycrystalline cuprates. We cannot yet be certain whether this large difference in global and intragrain current density is intrinsic to the oxypnictides or due to extrinsic barriers to current flow, because the samples contain significant second phase, some of which wets the grain boundaries and produces evidences of SNS proximity effect in the whole sample critical current.Comment: 28 pages, 14 figure

Experiments in thin film deposition : plasma-based fabrication of carbon nanotubes and magnesium diboride thin films.

Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2004.A simple, low-cost plasma reactor was developed for the purpose of carrying out thin film deposition experiments. The reactor is based largely on the Atmospheric Pressure Nonequilibrium Plasma (APNEP) design with a simple modification. It was used in an attempt to fabricate magnesium diboride thin films via a novel, but unsuccessful CVD process where plasma etching provides a precursor boron flux. Carbon nanotubes were successfully synthesised with the apparatus using a plasma-based variation of the floating catalyst or vapour phase growth method. The affect of various parameters and chemicals on the quality of nanotube production was assessed