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uperfluid 3He, a TwoFluid System, with the NormalFluid Dynamics Dominated by Andreev Reflection1 G. R. Pickett Department of Physics, Lancaster University, Lancaster, LA1 4YB UK email: [email protected] Received May 26, 2014

Abstract—As a specific offering towards his festschrift, we present a review the various properties of the excita tion gas in superfluid 3He, which depend on Andreev reflection. This phenomenon dominates many of the properties of the normal fluid, especially at the lowest temperatures. We outline the ideas behind this dominance and describe a sample of the many experiments in this system which the operation of Andreev reflection has made possible, from temperature measurement, particle detection, vortex imaging to cosmological analogues. Contribution for the JETP special issue in honor of A.F. Andreev’s 75th birthday DOI: 10.1134/S1063776114120140 1

1. INTRODUCTION

This article provides a brief review of the influence of Andreev reflection on the behavior of superfluid 3He. Since, as we see below, the dispersion curve of the qua siparticle/quasihole excitations depends on the relative motions of the fluid, any mechanical disturbance of the liquid gives rise to a disordered effective gap for the exci tations which are no longer free to move through the liquid unconstrained. In this landscape of varying gaps, excitations are constantly being subjected to Andreev processes to the extent that the whole mechanical behavior of the normal fluid of excitations is completely dominated by such processes. This is the system where the Andreev process indeed comes into its own. How ever, this behavior is not a drawback. On the contrary, we are able to exploit these processes which allow us to undertake a range of experimental investigations which are otherwise experimentally inaccessible. 2. THE ANALOGY BETWEEN ANDREEV REFLECTION AT A BOUNDARY SEPARATING DIFFERENT SUPERCONDUCTORS, AND IN A REGION SEPARATING STATIONARY AND MOVING SUPERFLUID 3He The original Andreev reflection process was pro posed by Aleksandr Fedorovich for describing the behavior of quasiparticles in superfluids in regions where the energy gap is changing spatially, as for example at a superconductingnormal interface or at an interface between two dissimilar superconductors 1 The article is published in the original.

with different energy gaps [1]. Of course, the unique aspect of this process is the “flavor” change made by excitations as they approach a region where the gap rises above the total energy of the excitation. In this situation an approaching excitation finds itself arriving at a minimum in the dispersion curve at which its group velocity falls to zero and then it retraces its tra jectory with almost no change in its momentum but with the opposite group velocity. Thus an incoming quasiparticle is retroreflected as a quasihole and an incoming quasihole will be reflected as a quasiparticle. From this new understanding of the process, a whole spectrum of unique phenomena can be recog nized, quasiparticlequas