Collective Modes and f-Wave Pairing Interactions in Superfluid He3 (original) (raw)
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High Frequency Sound in Superfluid 3He-B
Journal of Low Temperature Physics, 2008
We present measurements of the absolute phase velocity of transverse and longitudinal sound in superfluid 3 He-B at low temperature, extending from the imaginary squashing mode to near pair-breaking. Changes in the transverse phase velocity near pair-breaking have been explained in terms of an order parameter collective mode that arises from f -wave pairing interactions, the so-called J = 4 − mode. Using these measurements, we establish lower bounds on the energy gap in the B-phase. Measurement of attenuation of longitudinal sound at low temperature and energies far above the pair-breaking threshold, are in agreement with the lower bounds set on pair-breaking. Finally, we discuss our estimations for the strength of the fwave pairing interactions and the Fermi liquid parameter, F s 4 .
Journal of Low Temperature Physics, 2009
The whole collective mode spectrum in axial and planar phases of superfluid 3He with dispersion corrections is calculated for the first time. In axial A-phase the degeneracy of clapping modes depends on the direction of the collective mode momentum k with respect to the vector l (mutual orbital moment of Cooper pairs), namely: the mode degeneracy remains the same as in case of zero momentum k for k‖l only. For any other directions there is a threefold splitting of these modes, which reaches maximum for k ⊥ l. In planar 2D-phase, which exists in the magnetic field (at H>H C ) we find that for clapping modes the degeneracy depends on the direction of the collective mode momentum k with respect to the external magnetic field H, namely: the mode degeneracy remains the same as in case of zero momentum k for k‖H only. For any other directions different from this one (for example, for k ⊥ H) there is twofold splitting of these modes. The obtained results imply that new interesting features can be observed in ultrasound experiments in axial and planar phases: the change of the number of peaks in ultrasound absorption into clapping mode. One peak, observed for these modes by Ling et al. (J. Low Temp. Phys. 78:187, 1990), will split into two peaks in a planar phase and into three peaks in an axial phase under the change of ultrasound direction with respect to the external magnetic field H in a planar phase and with respect to the vector l in an axial phase. In planar phase, some Goldstone modes in the magnetic field become massive (quasi-Goldstone) and have a similar twofold splitting under the change of ultrasound direction with respect to the external magnetic field H. The obtained results as well will be useful under interpretation of the ultrasound experiments in axial and planar phases of superfluid 3He.
Discovery of an excited pair state in superfluid 3He
Nature Physics, 2008
Order parameter collective modes are the fingerprint of a condensed phase. The spectroscopy of these modes in superfluid 3 He and unconventional superconductors can provide key information on the symmetry of the condensate as well as the microscopic pairing mechanism responsible for the ground state and excitation energies. We report the discovery of a new collective mode in superfluid 3 He-B which we identify as an excited bound state of Cooper pairs. We use interferometry within an acoustic cavity that is very sensitive to changes in the velocity of transverse sound. Our measurements of sound velocity and mode frequency, together with the observation of acoustic birefringence indicate that this new mode is weakly bound with an excitation energy within 1% of the pair-breaking edge of 2∆. Based on the selection rules for coupling of transverse sound to a collective mode in 3 He-B, combined with the observation of acoustic birefringence near the collective mode frequency, we infer that the new mode is most likely a spin-triplet (S = 1), f -wave pair exciton (L = 3) with total angular momentum, J = 4. The existence of a pair exciton with J = 4 suggests an attractive, sub-dominant, f -wave pairing interaction in liquid 3 He.
Dispersion induced splitting of the collective mode spectrum in A-phase of superfluid 3He
Physics Letters A, 2009
The whole collective mode spectrum in axial and planar phases of superfluid 3 He with dispersion corrections is calculated for the first time. In axial A-phase the degeneracy of clapping modes depends on the direction of the collective mode momentum k with respect to the vector l (mutual orbital moment of Cooper pairs), namely: the mode degeneracy remains the same as in case of zero momentum k for k l only. For any other directions there is a threefold splitting of these modes, which reaches maximum for k⊥l.
Nonlinear acoustic effects in superfluid 3He-B
Physica B: Condensed Matter, 1992
We consider the nonlinear interaction of zero sound with the collective modes of the order-parameter in superfluid 3He-B. The approximate particle-hole symmetry of the 3He-Fermi liquid determines selection rules for the linear and nonlinear coupling of zero sound to the collective modes. Starting from the quasiclassical theory of superfluid 3He, we have shown that the coupling strengths have a simple representation in terms of Feynman diagrams. We predict measurable two-phonon absorption and nonlinear-Raman scattering by the J = 2 + (real squashing) modes at low pressures. Recent observations of two-phonon absorption by a group in Helsinki are compared to the theoretical predictions. Two-phonon absorption can be used to determine the dispersion of the J = 2 + modes. 0921-4526/92/$05.00
Collisionless collective modes in superfluid 3He
1977
A systematic and complete discussion of the collective modes of the BW and ABM states of 3He is given for the colfisionless regime. In addition to the gapless modes associated with the spontaneous breakdown of symmetries there exist a number of pair vibration modes with nonzero energy gap in the limit of wave vector tending to zero. The temperature dependence and dispersion of these modes as well as the possibility of exciting them is discussed.
Collective Modes in a Unitary Fermi Gas across the Superfluid Phase Transition
Physical Review Letters, 2013
We provide a joint theoretical and experimental investigation of the temperature dependence of the collective oscillations of first sound nature exhibited by a highly elongated harmonically trapped Fermi gas at unitarity, including the region below the critical temperature for superfluidity. Differently from the lowest axial breathing mode, the hydrodynamic frequencies of the higher-nodal excitations show a temperature dependence, which is calculated starting from Landau two-fluid theory and using the available experimental knowledge of the equation of state. The experimental results agree with high accuracy with the predictions of theory and provide the first evidence for the temperature dependence of the collective frequencies near the superfluid phase transition.
Physical Review B, 2004
The signature of superfluidity in bosonic systems is a sound wave-like spectrum of the single particle excitations which in the case of strong interactions is roughly temperature independent. In fermionic systems, where fermion pairing arises as a resonance phenomenon between free fermions and paired fermionic states (examples are: the atomic gases of 6 Li or 40 K controlled by a Feshbach resonance, polaronic systems in the intermediary coupling regime, d-wave hole pairing in the strongly correlated Hubbard system), remnants of such superfluid characteristics are expected to be visible in the normal state. The single particle excitations maintain there a sound wave like structure for wave vectors above a certain qmin(T) where they practically coincide there with the spectrum of the superfluid phase for T < Tc. Upon approaching the transition from above this region in q-space extends down to small momenta, except for a narrow region around q = 0 where such modes change into damped free particle like excitations.