Positive Ion Mobility in Superfluid 3He in Magnetic Fields

1
Positive Ion Mobility in Superfluid 3He in
Magnetic Fields
V.Efimov Lancaster University, Lancaster, UK
Institute of Solid State Physics RAS,
Chernogolovka, Russia Hidehiko Ishimoto, Ken
OBara, Daigo Ueno Akira Yamaguchi Institute for
Solid State Physics, University of Tokyo,
Kashiwanoha, Japan

• Influence of Superfluid transition on positive
  ion mobility in 3He
• Critical velocity in superfluid state.
• Mobility in B, A and A1 phases.

2
Purpose of experiments

• Investigation of superfluid 3He by using probe
  particles positive ion complex.
• Study structure of the complex positive ions
  surrounded with solid helium (snowball).
• Investigation of the interaction between heat
  excitation of Fermi liquid in normal and in
  superfluid state and spin system of ions
  surrounding.
• Experimental study of the change in the number of
  quasiparticles at transitions between N and SF
  states NàA1 NàA (A2), AàB, A1àA2.

3
P-T diagram of 3He and our experimental
conditions

• Point P, bar T, mK
• A 34.4 2.79
• B 34.4 2.16
• PCP 21.5 2.56
• Z 00.0 1.04
•  (polycritical point)

The measurements were made at pressures 3, 28.5
and 32.3 bar, in temperature range down to 1 mK,
in magnetic fields up to 15 T
4
Experimental technique and ions emission.

• Measurements were made in dilution fridge with
  copper nuclear demagnetization stage.
• Thermal contact through a Pt-Ag sintered powder
  heat exchanger (180 m2 surface area)
• Thermometers - Pt NMR and melting curve.
• Ion emission by short ( 20 ms) high voltage
  pulses from W tip.
• The ionization current10-12A, which corresponds
  to 105 ions.
• The measurement in the cell used four operating
  grids.
• The concentration of impurities (0.05 4He) was
  found by mass spectrometry.

Charge motion
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Critical ion velocity in superfluid state.

• At low electric field we observed the the linear
  v-E region of drift velocity. All results of the
  temperature and magnetic field dependence of ion
  mobility in superfluid state were taken in the
  linear region.
• In the superfluid phase, the positive ion
  mobility is quite high. Hence the electric field
  should be as low as possible, so as to avoid
  breaking the Cooper pair.
• At high electric fields, the velocity is
  sufficient for breaking Cooper pair (velocity gt 5
  cm/s on diagram)

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The 3He phase diagram in magnetic field

• SF state in 3He with Spair 1, hence L1 and S
  may have components Sz1, 0
• A1 2 gt Anisotropy
• A gt gt Anisotropy
• B gt gt Isotropy

A1 polarisation along H SF, Another
polarisation - normal
H
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Temperature dependence of positive ions mobility
Superfluid 3He, A1 phase
The temperature dependence of mobility in A1
phase may be described as linear (or very weak
exp). The normalised mobility of polluted
ions has a stronger dependence ?(T)/?(Tc). The
concentration of normal phase (pink line)
increases very slowly.
The measurements at pressures of 32.34 bar and
in a high magnetic field showed that transitions
in superfluid states are accompanied by change
of slope µ(T) curve. We estimated the
temperature of transitions into A1 and A2. It
was found the temperatures of transitions Tc
with accuracy 0.02 mK corresponded to the data
from literature.
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3He in superfluid B and A2 (A) phases
Transition between A and B phases
A2 phase
9
3He in superfluid phase, the literature data.
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The effect of magnetic field on positive ion
mobility
P29 bar

• Temperature of 2.38 mK corresponds to the case,
  when increasing the magnetic field expands the
  width of A1 phase. For example, at pressure 29
  bar with magnetic field 3 T the TA1 2.49 mK and
  TA2 2.34 mK at B10 T TA1 2.8 mK and TA2 2.2
  mK. The experiments at T2.38 mK with torsion
  oscillator showed a reduction of normal fluid
  fraction only by 4 (from 100 to 96) when
  magnetic field rises up to 10 T. This change is
  indicated on the graph as a dotted line.
• The same torsion measurements shown in A2 phase
  the normal fluid fraction is not depended by
  magnitude of magnetic field. (At T2.1 mK ?N/?
  92)

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P32.3 bar
P28.5 bar
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Conclusions

• Superfluid Fermi Liquid.
• The positive ion mobility in superfluid 3He has
  been measured at 28.8 and 32.3 bars in magnetic
  fields up to 15 T.
• The temperature dependence shows a sharp change
  of ?(T) at the transition from the normal to A1
  and also the A1 to A2 phase. It reflects the
  step-like rises of the rate of the decreasing
  number of normal excitations.
• The magnetic field dependence in the A1 and A2
  phase exhibits a small broad peak followed by a
  drastic decrease of mobility up on increasing the
  field. This behaviour is similar to that in the
  normal phase at temperatures close to the
  superfluid transition. In the A1 phase we must be
  take into account the effect of suppression of
  the normal density by high magnetic fields.
• The temperature dependence of positive ion
  mobility observed in A1 phase is stronger than
  that from a simple model transition of one spin
  polarisations into superfluid state.
• The measured temperature and field dependences of
  positive ion mobility suggests the existence of
  an exchange interaction between the 3He
  quasi-particles and the 3He spins on the
  "snowball".