Induced Topological Superconductivity in Two Dimensional Systems

Harvard University, Cambridge MA

Investigators

Abstract

Non-Technical Abstract: Nearly a hundred years after its discovery, superconductivity remains one of the most intriguing phases of matter. In 1957 Bardeen, Cooper and Schrieffer (BCS) presented their theory of superconductivity describing this state in terms of pairs of electrons arranged in a spatially isotropic wave function with no net momentum and a spin singlet configuration. Immediately thereafter, a search began to find materials with unconventional superconductivity where pairing deviates from conventional BCS theory. One particular class of unconventional superconductors involves pairs arranged in triplet rather than singlet configurations. Such superconductors may enable dissipationless transport of spin and may also give rise to elementary excitations that do not obey the conventional Fermi or Bose statistics but rather have non-Abelian statistics where the exchange of two particles transforms the state of the system into a new quantum mechanical state. This project explores unconventional superconductivity that arises due to the proximity effect between a conventional superconductor and a semiconductor with strong spin-orbit interaction. The research team looks to identify signatures of non-Abelian excitations and explore their properties using a variety of transport techniques. The research addresses three of the broader impact criteria set forth by the NSF: Benefits to Society through the exploration of dissipationless storage and transfer of information; Promoting teaching, training and education through the development of undergraduate courses; and broader participation of underrepresented groups through the active hiring of promising female scientists into the program. Technical Abstract: A research project is proposed to investigate topological superconductivity in two dimensional systems. The work follows recent theoretical work suggesting three distinct experimental platforms where topological superconductivity in two dimensions emerges. The first consists of topological HgCdTe quantum wells. The robust helical nature of the quantum spin Hall Effect seen in this system and its predicted dependence on an external magnetic field is particularly promising for exploring the underlying Andreev bound states spectrum as direct means for observing signatures of Majorana modes. The second platform consists of two dimensional Josephson junctions where the normal medium consists of non-topological 2D semiconductors with strong spin-orbit interaction (such as InAs quantum wells and narrow wells of HgCdTe). Such a platform was recently proposed as a promising controllable system where topological superconductivity can be tuned using an external in-plane magnetic field and the phase difference across the two superconducting contacts. The third approach consists of using quantum Hall edges in HgTe and quantum Anomalous Hall edges in vanadium doped (Bi,Sb)2Te3 for exploring topological superconductivity in chiral edges. The strong spin orbit coupling in HgTe quantum wells and the absence of an external magnetic field required for exploring the anomalous quantum Hall effect opens up rich and unexplored possibilities for exploring chiral Majorana modes. The experimental methods proposed include tunneling spectroscopy of Andreev bound states and noise magnetometry using nitrogen vacancy centers in diamond for exploring the fractional current phase relations in these systems.