Persistent Coherent Spin Precession in SuperfluidH3e−BDriven by Off-Resonant Excitation

Abstract

In superfluid 3 He-B under conditions of negligible normal fluid density we are able to trap a domain of coherent spin precession away from the destructive influence of boundaries. After the domain is generated by an NMR pulse, the free spin precession may continue for several minutes. Remarkably, we find that these precessing domains may also be generated by cw excitation but only at a frequency considerably higher than that of the precession. We believe that this behavior may indicate that at our temperatures the precessing spin is able to stimulate the orbital moment into sympathetic precession.[S0031-9007 (99)09224-8] PACS numbers: 67.57.Lm, 67.57.Fg, 67.57.JjSuperfluid 3 He has a complex condensate with coherent ordering of the angular momenta and nuclear spins of the Cooper pairs. The coupled orbital and spin motions are both strongly damped by interaction with the normal fluid. Indeed, the orbital viscosity is so large that we can usually take the orbital moment as clamped and regard the condensate nuclear spin system as constituting an independent spin superfluid, the dynamics of which display a very complex range of behavior, readily probed by NMR. However, at the lowest temperatures where the orbital viscosity becomes negligible, there must also appear a similar range of behaviors associated with the orbital fluid. While spin precession is a general feature of magnetic materials, coupled coherent spin precession is a unique feature of the magnetic superfluids. The equivalent orbital motion has no analog in normal materials and is certain to yield as spectacular properties as has the spin superfluid. The search for such new phenomena provides a motive for cooling the superfluid to temperatures where we may be able to see the unfettered behavior of the condensate in all its aspects.With our present cell, we are able to cool the superfluid to temperatures low enough that domains of free coherent spin precession can persist for periods of many minutes [1]. These are the persistent induction signals (PIS) previously observed in 3 He-B at low temperatures [2][3][4][5]. Such domains form after a short NMR excitation pulse. After the pulse scrambles the spin and texture transiently in the experimental region, the long-lived precessional mode then appears as the superfluid attempts to relax to its previous state. In this paper we report that we can also excite these domains continuously by cw excitation. However, the behavior observed is entirely unique in NMR terms; the system responds with a domain precessing at a completely different frequency from that of the excitation. Since something more than just a simple spin fluid is responding here, we believe that these observations may provide the first evidence of orbital precession.The experiments are made in the double cell shown in Fig. 1. The liquid in the inner 5 mm diameter experimental volume can be cooled to 130 mK at zero bar. A steady magnetic field is applied vertically and a field gradient arranged such that the closed base of the cell is normally in l...

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“…5. The relaxation of lowest energy Q-balls can be compensated by additional RF pumping of magnons at any frequency above the frequency of Q-ball, as was found in 27 and explained in 28 .…”

Magnon Condensation into aQBall inHe3−B

“…It can be independently generated in different parts of the sample, with the signal being dependent on the position in the container. Such a state lives extremely long, up to a few minutes, without external pumping [9,10]. That is why the name Persistent Signal (PS).…”

Decay of persistent precessing domains in3He-Bat very low temperatures

“…Instead, the PPD required a new technique, named off-resonant cw NMR. 19 Here the continuous excitation is supplied at a different frequency to the signal. The PPD was only excited if the excitation frequency was higher than the signal frequency.…”