Yu. M. Bunkov

Orientation effect on superfluid 3He in anisotropic aerogel

The orientation of the order parameter in the A-like and B-like phases of superfluid 3He immersed in uniaxially compressed aerogel is reported. With the use of NMR methods, it is found that the orbital momentum of the A-and B-like phases is oriented along the deformation. In the A-like phase, a relatively narrow NMR line with an anomalously large negative frequency shift is observed. The Leggett frequency in the A-like phase, which shows the same energy gap suppression as in the B-like phase, is measured.

Catastrophic Relaxation in 3He-B at 0.4Tc

We have observed a rapid decrease in the lifetime of a homogeneously precessing domain in 3He-B at a temperature of 0.4Tc. This phenomenon does not depend on pressure, magnetic field, or on the presence of 4He covering the surface of the chamber. We suppose this effect is the result of a magnetic relaxation through some unknown channel. Catastrophic relaxation may possibly be connected to some new phase transition in 3He.

Self-trapping of magnon Bose-Einstein Condensates in the ground state and on excited levels: from harmonic to box cofinement

Long-lived coherent spin precession of (3)He-B at low temperatures around 0.2T(c) is a manifestation of Bose-Einstein condensation of spin-wave excitations or magnons in a magnetic trap which is formed by the order-parameter texture and can be manipulated experimentally. When the number of magnons increases, the orbital texture reorients under the influence of the spin-orbit interaction and the profile of the trap gradually changes from harmonic to a square well, with walls almost impenetrable to magnons. This is the first experimental example of Bose condensation in a box. By selective rf pumping the trap can be populated with a ground-state condensate or one at any of the excited energy levels. In the latter case the ground state is simultaneously populated by relaxation from the exited level, forming a system of two coexisting condensates.

Coherent precession of magnetization in the superfluid 3He A-phase.

We report the first observation of coherent precession of magnetization in superfluid 3He A-like phase (CP-A) in aerogel. The coherent precession in bulk 3He A-phase is unstable due to the positive feedback of spin supercurrent to the gradient of phase of precession. It was predicted that the homogeneous precession will be stable if the orbital momentum of the 3He A-phase can be oriented along the magnetic field. We have succeeded to prepare this configuration by emerging 3He in uniaxially deformed anisotropic aerogel. The dissipation rate of coherent precession states in aerogel is much larger than that in bulk 3He B-phase. We propose a mechanism of this dissipation.

SINGLE-VORTEX NUCLEATION IN ROTATING SUPERFLUID 3HE-B

Nucleation of vortices in units of one quantum has been observed with c.w. NMR in a rotating cylinder filled with 3He-B. During acceleration a new vortex is created each time the counterflow velocity at the perimeter reaches a critical value vc(T, p). The measured vc resembles the calculated velocity vcb(T, p) of the bulk superflow instability, but is smaller by a factor with power law dependence on the superfluid coherence length. This indicates that a nucleation event takes place whenever the flow exceeds vcb locally at the nucleation site and the nucleation barrier vanishes.

On the Possibility of the Homogeneously Precessing Domain in Bulk 3He-A

The spatially coherent Larmor precession, which was discovered in 1984 in superfluid 3He-B, can be stabilized also in bulk superfluid 3He-A. If the tipping angle of precessing magnetization is within the range 75° < β < 123°, the orbital anisotropy vector is spontaneously reoriented and the stable homogeneously precessing domain (HPD) is formed. The HPD is characterized by a single precession frequency for the whole sample even under inhomogeneous external field and by spatial correlation of precession phases. Two-domain precession is also possible with the domain boundary separating the HPD and the stationary domain.

Ultra-Low Temperature Magnetic Properties of Liquid 3He Films

We report measurements of the nuclear magnetization of submonolayer liquid3He films adsorbed on a graphite substrate (Papyex) preplated by a monolayer of4He. In the submilliKelvin temperature range we observe a substantial enhancement of the nuclear magnetization with respect to the degenerate Fermi Liquid value. The unusual temperature dependence of this new contribution to the liquid3He film magnetization agrees well with that expected from the theory of weak disorder in two-dimensional (2D) correlated Fermion systems. The effects of disorder and reduced dimensionality suppress the superfluid transition at least to below 180 μK.

Search for supersymmetric Dark Matter with superfluid 3He (MACHe3)

Abstract MACHe3 (MAtrix of Cells of superfluid 3 He) is a project of a new detector for direct Dark Matter search, using superfluid 3 He as a sensitive medium. This Letter presents a phenomenological study done with the DarkSUSY code, in order to investigate the discovery potential of this project of detector, as well as its complementarity with existing and planned devices.

Magnon condensation into a Q ball in 3He-B.

The theoretical prediction of Q balls in relativistic quantum fields is realized here experimentally in superfluid 3He-B. The condensed-matter analogs of relativistic Q balls are responsible for an extremely long-lived signal of magnetic induction observed in NMR at the lowest temperatures. This Q ball is another representative of a state with phase coherent precession of nuclear spins in 3He-B, similar to the well-known homogeneously precessing domain, which we interpret as Bose-Einstein condensation of spin waves--magnons. At large charge Q, the effect of self-localization is observed. In the language of relativistic quantum fields it is caused by interaction between the charged and neutral fields, where the neutral field provides the potential for the charged one. In the process of self-localization the charged field modifies locally the neutral field so that the potential well is formed in which the charge Q is condensed.

Superconducting aluminium heat switch prepared by diffusion welding

Abstract The heat switch for a nuclear demagnetization refrigerator has been produced by diffusion welding of copper and aluminium foils. The design of the heat switch and the procedure of its fabrication are described.