Phase Diagram of Superfluid 3 He in a Nematic Aerogel in a Strong Magnetic Field
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DISORDER, AND PHASE TRANSITION IN CONDENSED SYSTEM
Phase Diagram of Superfluid 3He in a Nematic Aerogel in a Strong Magnetic Field E. V. Surovtsev Kapitza Institute for Physical Problems, Russian Academy of Sciences, Moscow, 119334 Russia e-mail: [email protected] Received September 27, 2018; revised October 23, 2018; accepted October 23, 2018
Abstract—The simultaneous effect of a nematic aerogel and a strong magnetic field on the phase diagram of superfluid 3He is considered. Using the Ginzburg–Landau theory, the domains of existence of new phases, viz., β phase (P1), distorted β phase (P2), distorted A phase ( A4(I) ), and distorted planar phase ( A4(II)), are determined. It is shown that order parameter components of the β phase have the maximal transition temperature. The conditions for the existence of the second-order transition from the distorted β phase to the distorted planar phase is formulated. DOI: 10.1134/S1063776119020250
1. The use of aerogels in experiments with superfluid 3He revealed the possibility of introducing global anisotropy (aerogel) into the initially isotropic system (superfluid 3He), which substantially changes the properties of the considered system. In contrast to restricted-geometry systems (3He in the slabs or in nanochannels), in which the order parameter changes significantly in the vicinity of restricting surfaces, 3He in an aerogel can be treated in the first approximation as a system with a uniform order parameter. This is possible due to the smallness of aerogel strand radius as compared to the coherence length of superfluid 3He and in the absence of correlations in the arrangement of aerogel elements on scales of the order of the coherence length. If these conditions are satisfied, the effect of aerogel is not reduced to only local suppression of the order parameter, but leads to a global change in the system symmetry in the entire volume. Different types of aerogels can have different types of global symmetry. For example, silica aerogel can be made absolutely isotropic [1], which means that the direction of aerogel strands changes in space at random. In the case of small uniaxial stretching or compression of such aerogel, uniaxial anisotropy appears in the system [2, 3]. This can be visualized as the emergence of a preferred direction in the orientation of strands. The spatial direction of each strand can be characterized by a certain director; aerogel will be treated as stretched if directors of its strands have predominantly the same direction and as compressed if the directors lie predominantly in the same plane. In most experiments, aerogel strands are coated with a few 4He layers; therefore, we can assume that aerogel is a system of non-
magnetic impurities. In this case, the presence of axially anisotropic aerogel imposes a limitation on the system symmetry associated with the rotation of the orbital part of the order parameter of superfluid 4He about a preferred direction, which reduces the corresponding symmetry to the axial symmetry. Depending on anisotropy
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