Spin Dynamics of Superfluid 3He in Aerogel

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Spin Dynamics of Superfluid 3He in Aerogel

• Osamu Ishikawa
• Osaka City University

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Collaborator
H. Nakagawa, R. Kado, K. Obara, H. Yano, T.
Hata H. Yokogawa, M. Yokoyama
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Outline of talk

• Aerogel properties and temporary phase diagram of
  superfluid 3He in 97.5 porosity
• coexistence phenomena of A-like and B-like phases
• Spin Dynamics in A-like phase and B-like phase by
  pulsed NMR experiments
• Suppresion of Tc

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Aerogel properties
Aerogel in cylindrical glass for NMR cell
aerogel is directly grown inside glass tube to
avoid any space between glass and aerogel
NMR coils are wound on outside surface of glass
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• Surface to volume ratio

about 50 m2/cc estimated by the B.E.T surface
measurements and 1000 m2/g with aerogel mass
6 m2 surface area for aerogel in sample

• Pre-coating of 4He film

2.5 layers of 4He film on silica strand in
aerogel are controlled with large surface area
in a heat exchanger in the sample cell of 660
m2.
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• Characteristic distance between strands
• by neutron scattering experiment

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small-angle X-ray scattering

• silica beads with diameter of 8.4 nm
• smooth surface of silica beads
• (DS 2.0)
• Fractal dimension is 1.8

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Temporary phase diagram of superfluid 3He in 97.5
porosity
B-like phase
A-like phase
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NMR experiments showed two phases in superfluid
3He in aerogel A-like phase and B-like phase.
(1) in A-like phase an absorption spectrum
being a narrow peak, shifted as a whole
from the Larmor freq., showing a positive
or negative shift depending on aerogels
the magnetization is the same as in normal state
(2) in B-like phase an absorption spectrum
widely spread due to a texture
depending on aerogels the magnetization
decreased as decreasing temperatures
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Coexisting signals on cooling from normal state
3.2MPa
4He film
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Warming up from coexisting temperature (from
Region A)
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Turn around experiment (1)
3.2 MPa with 4He film
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Turn around experiment (2)
2.35 MPa with 4He film
Different spectra at 1.3 mK with cooling
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coexistence phenomena without 4He film
Rapid spin exchange mechanism between liquid and
solid covers coexisting phenomena
3He at 3.2MPa
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3He at 2.1MPa
Frequencies changed continuously with cooling
liquid
This is an evidence of coexisting phases
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Spin dynamics by pulsed NMR (A-like phase)
2.4 MPa with 4He film
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3.2 MPa with 4He film
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Magnetization measurement
At 3.2 MPa, surface solid 3He shows Curie-Weiss
magnetism on 2.5 layers of 4He film
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At 2.4 MPa, Curie-Weiss behavior almost disappears
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Tipping angle dependent frequency shift is
attributed to superfluidity
But they are different from those in bulk liquid
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A rubust phase
A rubust phase proposed by I.A. Fomin explains
FID frequencies
M.Miura and K.Nagai (Hiroshima univ.)
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Spin dynamics by pulsed NMR (B-like phase)
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Different from flare out texture
Similar to absorption of bulk liquid confined
in slab geometry where magnetic field parallel to
surface
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An evidence showing that B-like phase is B phase
V.V.Dmitriev et al., JETP Letters 76 312-317
(2002)
5.3 mm diameter 5.6 mm height 98 aerogel
Confirmation of the order parameter of B-like
phase as that of B phase in bulk liquid
n-vector
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Suppression of Tc

• Inhomogeneous Impurity Scattering Model includes
  a length L as a parameter.
• L16.0 mm for pure 3He,
• L16.5 mm for 3He 4He
• with Radius R4.6L.

Thuneberg et al.
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Summary

• Aerogel has average periodicity
• Adding a small amount 4He helped us to observe
  coexisting phenomena by reducing solid
  contribution to NMR signal
• Boundaries between phases are probably pinned at
  inhomogeneous parts
• Tipping angle dependent frequency shifts were
  observed in A-like phase and in B-like phase.
• A-like phase seems to be a robust phase by Fomin
• B-lile phase seems to be BW state as in bulk
  liquid