Measurement Platform for Assessment of Semiconductor-Superconductor Hybrid Systems

A major obstacle in the advancement of quantum computers is the susceptibility of quantum bits (qubits) to decoherence. Decoherence occurs when a system of qubits encounters local noise like gamma radiation and heat due to incomplete isolation from its surroundings. The noise causes the qubits to change their states, thereby losing information. A new type of quantum computer, called a topological quantum computer, will be built with qubits that use inherent properties to protect against decoherence. Excitations in two-dimensional electron systems can act as this type of qubit. Realizing such a system requires confining electrons to two-dimensional planes inside structures of semiconductor and superconductor layers. A variety of low temperature measurements can be taken in order to evaluate the quality and characteristics of structure samples. These measurements can take many hours at the temperatures necessary to evaluate the sample’s properties. To streamline this process, a cryogenic measurement platform was designed that will allow for rapid assessment of new structures before they are measured at lower temperatures. A probe and a 32-pin sample mount were designed and constructed for the system. A 48 switch BNC panel was machined and wired, and magnet cables were made to charge the 5 Tesla magnet inside the cryostat. A 40-pin sample mount will also be constructed, and the system will be cooled to 4K to take measurements. This cryostat is expected to speed up the sample assessment process greatly

Hydrodynamic electron pumping in two-dimensional electron systems as a signature of viscous transport

Hydrodynamic effects arising from electron-electron interactions can have a significant influence on transport dynamics in ultra-clean two-dimensional electron systems in the solid state. A growing interest in electron hydrodynamics in the solid state has been noted due to the development of new materials systems. Hence signatures of this hydrodynamic regime, where the rate of momentum conserving collisions exceed that of momentum relaxing collisions, are increasingly being explored. Here, we experimentally study a hydrodynamic pumping phenomenon using a transverse magnetic focusing geometry, whereby a ballistic electron jet sweeping past a lithographic aperture can extract (pump) electrons from this aperture. This phenomenon highlights the importance of electron-electron interactions and concomitant hydrodynamic phenomena in mesoscopic ballistic transport, delivers an experimentally supported explanation of nonlocal negative resistances observed in transverse magnetic focusing as signatures of the hydrodynamic regime, and indicates that the Coulombic repulsive interaction can result in a net attractive force.Comment: 13 page

Stability of High-Density Two-Dimensional Excitons against a Mott Transition in High Magnetic Fields Probed by Coherent Terahertz Spectroscopy

We have performed time-resolved terahertz absorption measurements on photoexcited electron-hole pairs in undoped GaAs quantum wells in magnetic fields. We probed both unbound- and bound-carrier responses via cyclotron resonance and intraexciton resonance, respectively. The stability of excitons, monitored as the pair density was systematically increased, was found to dramatically increase with increasing magnetic field. Specifically, the 1$s$-2$p_-$ intraexciton transition at 9 T persisted up to the highest density, whereas the 1$s$-2$p$ feature at 0 T was quickly replaced by a free-carrier Drude response. Interestingly, at 9 T, the 1$s$-2$p_-$ peak was replaced by free-hole cyclotron resonance at high temperatures, indicating that 2D magnetoexcitons do dissociate under thermal excitation, even though they are stable against a density-driven Mott transition.Comment: 5 pages, 4 figure