Pressure dependence of elementary excitations in normal liquid helium-3

Abstract

The neutron scattering function for liquid 3 He at 120 mK and pressures of 0, 0.5, 1, and 2 MPa has been determined for wave vectors in the range 3 nm -1 < Q < 20 nm _1 . This represents the first experimental information on the density dependence of the zero-sound frequency and damping at finite wave vectors, and should serve as a useful test of theories on excitations in neutral Fermi liquids. The results are in qualitative agreement with extensions of the Landau Fermi-liquid theory to finite wave vectors.PACS numbers: 67.50.Dg Neutron inelastic scattering offers a unique method to examine directly the elementary excitations in liquid 3 He on a microscopic scale, i.e., for wave vectors of order 10 nm -1 . Through their spin-dependent interaction with the 3 He nuclei, neutrons probe the nuclear spin fluctuations, in addition to the density fluctuations which are probed by the spin-averaged interaction. In the neutron scattering function, one distinguishes three components: the excitation of single particle-hole (p-h) pairs, a collective density mode (zero sound), and a broad distribution corresponding to multiple p-h excitations. The current theoretical and experimental situation in this area has recently been reviewed by Glyde. l Previous neutron-scattering experiments 2 " 5 have shown that the single p-h spectrum resembles the one expected for a noninteracting Fermi gas but with an effective mass m* ~ 3mo which lowers the energy of the p-h band compared with that of the noninteracting system. For Q < 10 nm -1 , the single-pair spin fluctuations are enhanced and are well described by the paramagnon model. 6 The zero-sound mode is well resolved for wave vectors less than 10 nm -1 and shows anomalous dispersion, i.e., the energies are above those corresponding to linear dispersion co^coQ, where Co is the ultrasonic zero-sound velocity. It broadens considerably with increasing wave vector even before the mode overlaps with the p-h band. All the results published so far at saturated-vapor pressure (SVP) are in good general agreement with the predictions of the polarization-potential theory developed by Pines and co-workers. 7 Here the potential parameters, obtained by a fitting to the neutron-scattering results and to the Landau parameters, have been used to calculate transport properties for both normal and superfluid 3 He which are in excellent agreement with measured values. 8 The effective interactions in the polarization-potential theory are expected to be sensitive to the density, as discussed by Aldrich and Pines 7 and as shown by the Landau parameters, which are the low-(? limits of the effective interactions.While in an earlier experiment 9 the scattering function for pressurized liquid 3 He could only be measured for wave vectors larger than 12 nm -1 , the present experiment explores the wave-vector range 3 nm ~l < Q < 20 nm" 1 . This therefore represents the first measurement of the zero-sound mode at elevated pressures.The measurements were made at the time-of-flight spectrometer 10 IN6 at the high-flux...

Microscopic collective dynamics in liquidpara−H2

“…Also, the relative deviation from hydrodynamic sound is here the largest ͑close to 60%͒ found for a simple liquid, which comes some 20 times stronger than that found for superfluid liquid 4 He, 17 about four times above that found for normal-phase liquid 4 He, 22 and about three times stronger than that observed for 3 He. 20 The presence of such a large deviation from hydrodynamic sound enables the decay of phonons into two excitations, one of them having at least a lower velocity than the original phonon. In other words, an excitation with velocity well above c T ͑i.e.…”

“…The departure from hydrodynamics, usually referred to as anomalous dispersion, is a known experimental fact since work on superfluid 4 He, 19 liquid 3 He, 20 or liquid o-D 2 . 10 In fact, the phenomenon is also exhibited by a weakly interacting Bose gas.…”

Effective mass, spin fluctuations, and zero sound in liquid3He

“…In our preliminary study, 31 we selected the values m 0 ϭ1.7 and f ϭ0.35 to best reproduce the Q and dependence of S c (Q, ) and S I (Q, ) observed in earlier neutron-scattering measurements at Tϭ0.1 K. 27,28 The present study focuses on the temperature dependence. It shows that the same model with identical parameter values predicts the temperature dependence of both S c (Q, ) and S I (Q, ) well, especially S I (Q, ).…”

“…This is similar to the temperature measured in our previous experiment. 27,28 While we can only make an estimate of the lowest temperature in the present experiment, it is clear from the detailed-balance condition of the neutron scattering cross section that the temperature cannot be higher than 0.3 K.…”

“…Similarly, in neutron scattering experiments [25][26][27][28][29][30] quasiparticles are excited from states k (kϽk F ) to states kϩQ (͉k ϩQ͉Ͼk F ), where Q is the wave vector transfer from the neutron to the fluid. At low Q, only states immediately below k F and immediately above k F will be sampled and the quasiparticles involved will have a mass m*(k F ).…”