Andreev Reflection without Fermi surface alignment in High Tc_{c}-Topological heterostructures

We address the controversy over the proximity effect between topological materials and high T$_{c}$ superconductors. Junctions are produced between Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ and materials with different Fermi surfaces (Bi$_{2}$Te$_{3}$ \& graphite). Both cases reveal tunneling spectra consistent with Andreev reflection. This is confirmed by magnetic field that shifts features via the Doppler effect. This is modeled with a single parameter that accounts for tunneling into a screening supercurrent. Thus the tunneling involves Cooper pairs crossing the heterostructure, showing the Fermi surface mis-match does not hinder the ability to form transparent interfaces, which is accounted for by the extended Brillouin zone and different lattice symmetries

Evidence for a New Excitation at the Interface Between a High-Tc Superconductor and a Topological Insulator

High-temperature superconductors exhibit a wide variety of novel excitations. If contacted with a topological insulator, the lifting of spin rotation symmetry in the surface states can lead to the emergence of unconventional superconductivity and novel particles. In pursuit of this possibility, we fabricated high critical-temperature (Tc ~ 85 K) superconductor/topological insulator (Bi2Sr2CaCu2O8+delta/Bi2Te2Se) junctions. Below 75 K, a zero-bias conductance peak (ZBCP) emerges in the differential conductance spectra of this junction. The magnitude of the ZBCP is suppressed at the same rate for magnetic fields applied parallel or perpendicular to the junction. Furthermore, it can still be observed and does not split up to at least 8.5 T. The temperature and magnetic field dependence of the excitation we observe appears to fall outside the known paradigms for a ZBCP

Proximity Effect and Tunneling Spectroscopy of High-temperature Superconductor-semiconductor Hybrid Structures

This thesis presents Andreev scattering studies of a variety of high-temperature superconductor Bi2Sr2CaCu2O8+x-normal material (SN) junctions. The SN devices were fabricated with our newly-developed technique, called the mechanical-bonding method. High-temperature superconductivity was observed in topological insulators Bi2Se3 and Bi2Te3 (as well as graphite) by proximity to the high-temperature superconductor Bi2Sr2CaCu2O8+x. This proximity effect was further confirmed by the application of a magnetic field and observation of a shifting in the energy of the Andreev-reflected quasiparticles as a result of diamagnetic screening supercurrents. However, the ability to probe the topological surface states in Bi2Se3 and Bi2Te3 was limited, due to the Fermi energy being deep in the bulk conduction band. Thus we turned to Bi2Te2Se, which is quite insulating in the bulk. The differential conductance measurements on Bi2Sr2CaCu2O8+x/Bi2Te2Se devices demonstrated a zero-bias conductance peak, suggesting evidences of a new excitation emerging at the interface between the two materials. Furthermore, the mechanical bonding method was employed as a technique to probe the density of states of Bi2Sr2CaCu2O8+x at various doping levels, observing the superconducting gap and the pseudogap of Bi2Sr2CaCu2O8+x at various temperatures.Ph.D

Hybrid High-Temperature-Superconductor–Semiconductor Tunnel Diode

We report the demonstration of hybrid high-Tc-superconductor-semiconductor tunnel junctions, enabling new interdisciplinary directions in condensed matter research. The devices were fabricated by our newly-developed mechanical bonding technique, resulting in high-Tc-semiconductor planar junctions acting as superconducting tunnel diodes. Tunneling-spectra characterization of the hybrid junctions of Bi2Sr2CaCu2O8+{\delta} combined with bulk GaAs, or a GaAs/AlGaAs quantum well, exhibits excess voltage and nonlinearity - in good agreement with theoretical predictions for a d-wave superconductor-normal material junction, and similar to spectra obtained in scanning tunneling microscopy. Additional junctions are demonstrated using Bi2Sr2CaCu2O8+{\delta} combined with graphite or Bi2Te3. Our results pave the way for new methods in unconventional superconductivity studies, novel materials and quantum technology applications

Hybrid High-Temperature-Superconductor–Semiconductor Tunnel Diode

Physical Review X. Volume 2, Issue 4, 2012, Article number 041019.We report the demonstration of hybrid high-Tc-superconductor-semiconductor tunnel junctions, enabling new interdisciplinary directions in condensed matter research. The devices are fabricated by our newly developed mechanical-bonding technique, resulting in high-Tc-superconductor-semiconductor tunnel diodes. Tunneling-spectra characterization of the hybrid junctions of Bi2Sr2CaCu2O8+δ combined with bulk GaAs, or a GaAs/AlGaAs quantum well, exhibits excess voltage and nonlinearity, similarly to spectra obtained in scanning-tunneling microscopy, and is in good agreement with theoretical predictions for a d-wave-superconductor-normal-material junction. Additional junctions are demonstrated using Bi2Sr2CaCu2O8+δ combined with graphite or Bi2Te3. Our results pave the way for new methods in unconventional superconductivity studies, novel materials, and quantum technology applications