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

Dynamic Response of Wigner Crystals

The Wigner crystal, an ordered array of electrons, is one of the very first proposed many-body phases stabilized by the electron-electron interaction. This electron solid phase has been reported in ultra-clean two-dimensional electron systems at extremely low temperatures, where the Coulomb interaction dominants over the kinetic energy, disorder potential and thermal fluctuation. We closely examine this quantum phase with capacitance measurements where the device length-scale is comparable with the crystal's correlation length. The extraordinarily high performance of our technique makes it possible to quantitatively study the dynamic response of the Wigner crystal within the single crystal regime. Our result will greatly boost the study of this inscrutable electron solid

Metastable Charge Distribution Between Degenerate Landau Levels

We study two dimensional electron systems confined in wide quantum wells whose subband separation is comparable with the Zeeman energy. Two N = 0 Landau levels from different subbands and with opposite spins are pinned in energy when they cross each other and electrons can freely transfer between them. When the disorder is strong, we observe clear hysteresis in our data corresponding to instability of the electron distribution in the two crossing levels. When the intra-layer interaction dominates, multiple minima appear when a Landau level is 1/3 or 2/3 filled and fractional quantum hall effect can be stabilized

Suppression of Spin Pumping at Metal Interfaces

An electrically conductive metal typically transmits or absorbs a spin current. Here, we report on evidence that interfacing two metal thin films can suppress spin transmission and absorption. We examine spin pumping in ferromagnet/spacer/ferromagnet heterostructures, in which the spacer -- consisting of metallic Cu and Cr thin films -- separates the ferromagnetic spin-source and spin-sink layers. The Cu/Cr spacer largely suppresses spin pumping -- i.e., neither transmitting nor absorbing a significant amount of spin current -- even though Cu or Cr alone transmits a sizable spin current. The antiferromagnetism of Cr is not essential for the suppression of spin pumping, as we observe similar suppression with Cu/V spacers where V is a nonmagnetic analogue of Cr. We speculate that diverse combinations of spin-transparent metals may form interfaces that suppress spin pumping, although the underlying mechanism remains unclear. Our work may stimulate a new perspective on understanding and engineering spin transport in metallic multilayers