Spin Dynamics of Superfluid 3 He in Aerogel Osamu Ishikawa Osaka City University

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

Outline of talk Aerogel properties and temporary phase diagram of superfluid 3 He 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

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 aerogel empty

about 50 [m 2 /cc] estimated by the B.E.T surface measurements and ~1000 [m 2 /g] with aerogel mass Surface to volume ratio ~ 6 [m 2 ] surface area for aerogel in sample Pre-coating of 4 He film 2.5 layers of 4 He film on silica strand in aerogel are controlled with large surface area in a heat exchanger in the sample cell of 660 [m 2 ].

concentration of silica [%] density [g/cc] 97.5% porosity

Characteristic distance between strands by neutron scattering experiment Evidence of weak periodicity in aerogel

silica beads with diameter of 8.4 nm smooth surface of silica beads （ D S = 2.0 ） Fractal dimension is 1.8 small-angle X-ray scattering

Temporary phase diagram of superfluid 3 He in 97.5 % porosity A-like phase B-like phase

NMR experiments showed two phases in superfluid 3 He 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

Coexisting signals on cooling from normal state Region A Continuous phase conversion from A-like to B-like phase 3.2MPa + 4 He film

Warming up from coexisting temperature (from Region A)

One peak signal in on cooling from normal state and on warming from below region A Turn around experiment (1) Region B 3.2 MPa with 4 He film

Turn around experiment (2) Different spectra at 1.3 mK with cooling 2.35 MPa with 4 He film

coexistence phenomena without 4 He film 3 He at 3.2MPa Rapid spin exchange mechanism between liquid and solid covers coexisting phenomena

3 He at 2.1MPa Frequencies changed continuously with cooling liquid This is an evidence of coexisting phases

Spin dynamics by pulsed NMR (A-like phase) on cooling on warming Temperature [mK]  f [Hz] 2.4 MPa with 4 He film on cooling on warming Temperature [mK]  f [Hz] Tipping angle [deg]

Temperature [mK]  f [Hz] 3.2 MPa with 4 He film 1.55 mK Tipping angle [deg]  f [Hz] Temperature [mK]  f [Hz]

Magnetization measurement Temperature [mK] Magnetization [a.u.] pure 3 He Temperature [mK] Magnetization [a.u.] 2.5 layers 4 He film Temperature [mK] Magnetization [a.u.] pure 3 He At 3.2 MPa, surface solid 3 He shows Curie-Weiss magnetism on 2.5 layers of 4 He film

Magnetization [a.u.] Temperature [mK] pure 3 He Magnetization [a.u.] Temperature [mK] with 2.5 layers 4 He film 2.5 layers 4 He films At 2.4 MPa, Curie-Weiss behavior almost disappears

 f [Hz] Tipping angle [deg] Tipping angle dependent frequency shift is attributed to superfluidity But they are different from those in bulk liquid

A rubust phase A rubust phase proposed by I.A. Fomin explains FID frequencies M.Miura and K.Nagai (Hiroshima univ.)

Temperature [mK] Full Width at Half Maximum [Hz]

Spin dynamics by pulsed NMR (B-like phase) Flare out texture in cylindrical cell Large peak at Larmor freq. And having a tail to higher freq. by cw NMR.

 f [Hz]  [ a.u.] 2.35 MPa Different from flare out texture Similar to absorption of bulk liquid confined in slab geometry where magnetic field parallel to surface

V.V.Dmitriev et al., JETP Letters (2002) 5.3 mm diameter 5.6 mm height 98 % aerogel 104˚ An evidence showing that B-like phase is B phase Confirmation of the order parameter of B-like phase as that of B phase in bulk liquid “n-vector”

Tipping angle [deg]  f [kHz] BS-mode

Suppression of Tc T c aero / T c pure  Inhomogeneous Impurity Scattering Model includes a length L as a parameter. L=16.0  m for pure 3 He, L=16.5  m for 3 He+ 4 He with Radius R=4.6L. Thuneberg et al.

Summary Aerogel has average periodicity Adding a small amount 4 He 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