Superfluid ß phase of

It is known that in low magnetic fields the superfluid transition of ^{3}He in nematic aerogel occurs into the polar phase. Using a vibrating aerogel resonator, we observe that in high magnetic fields this transition splits into two discrete transitions, occurring at different temperatures. According to theoretical models, a new superfluid phase-the ß phase-should be realized between these two transitions. The temperature range of existence of the new phase is measured as a function of magnetic field. The results are well consistent with theoretical expectations for the ß phase.

Half-quantum vortices and walls bounded by strings in the polar-distorted phases of topological superfluid 3He.

Symmetries of the physical world have guided formulation of fundamental laws, including relativistic quantum field theory and understanding of possible states of matter. Topological defects (TDs) often control the universal behavior of macroscopic quantum systems, while topology and broken symmetries determine allowed TDs. Taking advantage of the symmetry-breaking patterns in the phase diagram of nanoconfined superfluid 3He, we show that half-quantum vortices (HQVs)-linear topological defects carrying half quantum of circulation-survive transitions from the polar phase to other superfluid phases with polar distortion. In the polar-distorted A phase, HQV cores in 2D systems should harbor non-Abelian Majorana modes. In the polar-distorted B phase, HQVs form composite defects-walls bounded by strings hypothesized decades ago in cosmology. Our experiments establish the superfluid phases of 3He in nanostructured confinement as a promising topological media for further investigations ranging from topological quantum computing to cosmology and grand unification scenarios.

Bose-Einstein Condensation of Magnons and Spin Superfluidity in the Polar Phase of

The polar phase of ^{3}He, which is topological spin-triplet superfluid with the Dirac nodal line in the spectrum of Bogoliubov quasiparticles, has been recently stabilized in a nanoconfined geometry. We pump magnetic excitations (magnons) into the sample of polar phase and observe how they form a Bose-Einstein condensate, revealed by coherent precession of the magnetization of the sample. Spin superfluidity, which supports this coherence, is associated with the spontaneous breaking of U(1) symmetry by the phase of precession. We observe the corresponding Nambu-Goldstone boson and measure its mass emerging when applied rf field violates the U(1) symmetry explicitly. We suggest that the magnon BEC in the polar phase is a powerful probe for topological objects such as vortices and solitons and topological nodes in the fermionic spectrum.

Effect of Magnetic Boundary Conditions on Superfluid

We report results of experiments with superfluid ^{3}He confined in aerogels with parallel strands which lead to anisotropic scattering of ^{3}He quasiparticles. We vary boundary conditions for the scattering by covering the strands with different numbers of atomic ^{4}He layers and observe that the superfluid phase diagram and the nature of superfluid phases strongly depend on the coverage. We assume the main reason for these phenomena is a magnetic channel of the scattering which becomes important at low coverages and can be essential in other Fermi systems with triplet pairing.

Observation of Half-Quantum Vortices in Topological Superfluid

One of the most sought-after objects in topological quantum-matter systems is a vortex carrying half a quantum of circulation. They were originally predicted to exist in superfluid ^{3}He-A but have never been resolved there. Here we report an observation of half-quantum vortices (HQVs) in the polar phase of superfluid ^{3}He. The vortices are created with rotation or by the Kibble-Zurek mechanism and identified based on their nuclear magnetic resonance signature. This discovery provides a pathway for studies of unpaired Majorana modes bound to the HQV cores in the polar-distorted A phase.

Polar Phase of Superfluid (3)He in Anisotropic Aerogel.

We report the first observation of the polar phase of superfluid (3)He. This phase appears in (3)He confined in a new type of aerogel with a nearly parallel arrangement of strands which play the role of ordered impurities. Our experiments qualitatively agree with theoretical predictions and suggest that in other systems with unconventional Cooper pairing (e.g., in unconventional superconductors) similar phenomena may be found in the presence of anisotropic impurities.

Stochastic properties of one-parameter discrete model of genetic diversity in the unique random parameter case.

Stochastic properties of previously introduced one parameter discrete model of genetic diversity (M. Yu. Shchelkanov et al, J. Biomol. Struct. Dyn. 15, 887-894 (1998)) are investigated. Two approaches are compared: (A) when the on-step substitution number and/or the number of substitution steps are random variables; (B) referred parameters are replaced by mathematical expectations of the respective variables. It has been demonstrated, that estimations of sequence measure based on the number of replication steps are more under the assumption of case (A) as compared with (B). Thus, real biological situation relating to the case (A) could additionally promote the increasing of distinctions between different taxons (e.g. HIV, etc.). Peculiarities of one-parameter discrete model of genetic diversity during calculation of the distinctions between symbol (e.g. nucleotide) sequence sets are also discussed.

Variability analysis of HIV-1 gp120 V3 region: IV. Distribution functions for intra- and inter-subtype amino acid hamming distances.

Distribution functions for intra- and inter- HIV-1 V3-loop subtypes amino acid Hamming distances were calculated (850 V3-loop sequences from the Los Alamos HIV-1 Database (1996) were used). These functions have pronounced bell-like shape. Such shapes of the histograms for HIV-1 V3 intra- and inter-subtype distriutions are discussed to confirm the applicability of different hierarchical cluster procedures for HIV-1 V3 classification. Two-mode distribution for the subtype E could sertificate that this subtype includes two thinner taxons.

One-parameter discrete model of the genetic diversity.

One-parameter discrete model estimating genetic distance between precursor and descendant nucleotide sequences after several steps of substitution acts is developed. This model based on the previously introduced symbol sequences enumeration procedure (M.Y. Shchelkanov, A.N. Yudin, A.V. Antonov, N.S. Starikov, A.A. Vedenov, E.V. Karamov, J. Biomol. Struct. Dyn. 15, 231-241 (1997)) differs from Jukes-Cantor and Kimura models by the absence of the assumptions usual for continuous Markov's processes. Formula obtained with the help of our model are more preferable since they take into account multiple repetition substitution ability and they are correct in the entire admissible range of parameters.

Variability analysis of HIV-1 gp120 V3 region: I. Point estimators for the amino acid distribution characteristics.

Enumerating procedure for symbol sequences is proposed. Relationship between Hamming distance for symbol sequences and Euclidean distance for corresponding enumerations is established, and more universal Hamming-transformed Euclidean measure is constructed. A distribution function of amino acid substitutions and some of its point estimators (consensus, subconsensus, sample mean, sample central moments and asymmetry coefficient) are introduced. Hamming-transformed Euclidean measures between consensus, subconsensus and sample means for ten HIV-1 taxons of gp120 V3 regions are calculated. It is demonstrated that these taxons have a complicated pattern which is significant for their classification.

Variability analysis of HIV-1 gp120 V3 region: II. Hierarchy of taxons.

In the previous work (M. Yu. Shchelkanov, A. N. Yudin, A. V. Antonov, N. S. Starikov, A. A. Vedenov, E. V. Karamov, J. Biomol. Struct. Dyn. 15, 217-229 (1997)) we have introduced the amino acid distribution function within HIV-1 taxons and Hamming-transformed Euclidean measures between their characteristics: consensus, subconsensus and sample mean. In this work the referred characteristics are used for hierarchical classification of amino acid sequences of gp120 V3 region belonging to different HIV-1 taxons. A comparative analysis of the results produced by various classification methods is carried out. Multidimensional scaling of distance matrix for the specified characteristics is used to visualize the pattern of HIV-1 variability.

Variability analysis of HIV-1 gp120 V3 region: III. Distinctions between various sets of peptide fragments derived from the sequences belonging to different HIV-1 taxons.

Distinction criterion for various sets of fixed length peptide fragments and integral distinction measure for various sets of peptide fragments with different length and start position ranges have been introduced on the base of an enumeration procedure and a point estimators for the amino acid distribution characteristics introduced previously (M. Yu. Shchelkanov, A. N. Yudin, A. V. Antonov, N. S. Starikov, A. A. Vedenov, E. V. Karamov, J. Biomol. Struct. Dyn. 15, 231-241 (1997)). Differences between 6-10-mer peptides derived from the majority of HIV-1 taxon pairs are demonstrated to be located generally in the vicinity of the V3-loop top. This validates the suitability of V3 top mimicking synthetic peptides for HIV-1 serotyping. A significant difference between E subtype V3 C-terminus peptides and the corresponding peptides derived from the other subtypes has been demonstrated. Taking into account the Langerhans' cells tropism of E subtype virus variants we have hypothesized the influence of mutations in the V3 C-terminus on HIV-1 cell tropism.