Bose-Einstein condensation (BEC) has been observed in a dilute gas of sodium atoms. A Bose-Einstein condensate consists of a macroscopic population of the ground state of the system, and is a coherent state of matter. In an ideal gas, this phase transition is purely quantum-statistical. The study of BEC in weakly interacting systems which can be controlled and observed with precision holds the promise of revealing new macroscopic quantum phenomena that can be understood from first principles.

Bose-Einstein condensation in a gas of sodium atoms

2. GRAVITY 7. MICROSCOPIC PHYSICS 13. TOPOLOGY OF DEFECTS 18. ANOMALOUS NON-CONSERVATION OF FERMIONIC CHARGE 22. EDGE STATES AND FERMION ZERO MODES ON SOLITON 26. LANDAU CRITICAL VELOCITY 29. CASIMIR EFFECT AND VACUUM ENERGY

The Universe in a Helium Droplet

日本物理学会誌及びJournal of the Physical Society of Japanの月刊について

In the present review, aerogels designate dried gels with a very high relative pore volume. These are versatile materials that are synthesized in a first step by low-temperature traditional sol-gel chemistry. However, while in the final step most wet gels are often dried by evaporation to produce so-called xerogels, aerogels are dried by other techniques, essentially supercritical drying. As a result, the dry samples keep the very unusual porous texture which they had in the wet stage. In general these dry solids have very low apparent densities, large specific surface areas, and in most cases they exhibit amorphous structures when examined by X-ray diffraction (XRD) methods. In addition, they are metastable from the point of view of their thermodynamic properties. Consequently, they often undertake a structural evolution by chemical transformation, when aged in a liquid medium and/or heat treated. As aerogels combine the properties of being highly divided solids with their metastable character, they can develop very attractive physical and chemical properties not achievable by other means of low temperature soft chemical synthesis. In other words, they form a new class of solids showing sophisticated potentialities for a range of applications. These applications as well as chemical and physical aspects of these materials were regularly detailed and discussed in a series of symposia on aerogels,1-5 the last of them being held in Albuquerque in 2000.6 Reviews were also regularly published, either on both xerogels and aerogels7 or more focused on the applications of aerogels.8-13 The particularly interesting properties of aerogels arise from the extraordinary flexibility of the solgel processing, coupled with original drying techniques. The wet chemistry is not basically different for making xerogels and aerogels. As this common basis has been extensively detailed in recent books,14 it does not need to be reviewed. Compared to traditional xerogels, the originality of aerogels comes from * To whom all correspondence should be addressed. † Institut de Recherches sur la Catalyse. ‡ Laboratoire d’Application de la Chimie à l’Environnement. 4243 Chem. Rev. 2002, 102, 4243−4265

https://is.muni.cz/el/1431/podzim2006/C7780/um/Read/2711172/aerogel_Pajonk_ChRev02_4243.pdf

Chemistry of Aerogels and Their Applications

Since the 1950s, Heisenberg and others have addressed the problem of how to explain the appearance of countable particles in continuous fields. Stable localized field configurations were searched for an ingredient for a general field theory of elementary particles, but the majority of nonlinear field models were unable to predict them. As an exception, Skyrme succeeded in describing nuclear particles as localized states, so-called 'skyrmions'. Skyrmions are a characteristic of nonlinear continuum models ranging from microscopic to cosmological scales. Skyrmionic states have been found under non-equilibrium conditions, or when stabilized by external fields or the proliferation of topological defects. Examples are Turing patterns in classical liquids, spin textures in quantum Hall magnets, or the blue phases in liquid crystals. However, it has generally been assumed that skyrmions cannot form spontaneous ground states, such as ferromagnetic or antiferromagnetic order, in magnetic materials. Here, we show theoretically that this assumption is wrong and that skyrmion textures may form spontaneously in condensed-matter systems with chiral interactions without the assistance of external fields or the proliferation of defects. We show this within a phenomenological continuum model based on a few material-specific parameters that can be determined experimentally. Our model has a condition not considered before: we allow for softened amplitude variations of the magnetization, characteristic of, for instance, metallic magnets. Our model implies that spontaneous skyrmion lattice ground states may exist generally in a large number of materials, notably at surfaces and in thin films, as well as in bulk compounds, where a lack of space inversion symmetry leads to chiral interactions.

Spontaneous skyrmion ground states in magnetic metals

TOPOLOGICAL defects in the geometry of space–time (such as cosmic strings) may have played an important role in the evolution of the early Universe, by supplying initial density fluctuations which seeded the clusters of galaxies that we see today1. The formation of cosmic strings during a symmetry-breaking phase transition shortly after the Big Bang is analogous to vortex creation in liquid helium following a rapid transition into the superfluid state; the underlying physics of this cosmological defect-forming process (known as the Kibble mechanism1) should therefore be accessible to experimental study. Superfluid vortices have been observed in 4He following rapid quenching to the superfluid state2, lending qualitative support to Kibble's contention that topological defects are generated by such phase transitions. Here we quantify this process by using an exothermic neutron-induced nuclear reaction to heat small volumes of super-fluid 3He above the superfluid transition temperature, and then measuring the deficit in energy released as these regions of normal liquid pass back into the superfluid state. By ascribing this deficit to the formation of a tangle of vortices, we are able to infer the resulting vortex density; we find that this agrees very well with the predictions of Zurek's modification3 of the original Kibble mechanism1.

Laboratory simulation of cosmic string formation in the early Universe using superfluid 3He

We apply exact high-temperature series expansions of the multiple-spin-exchange Hamiltonian for a triangular lattice to describe high precision NMR measurements of the nuclear magnetic susceptibility of 3He films adsorbed on graphite, reported here, as well as all available specific heat data. A consistent quantitative description of the unusual thermodynamic properties of the second layer solid, which provides canonical examples of two-dimensional magnets, is obtained as a function of temperature and areal density. We prove that cyclic multiple-spin exchange processes are responsible for the large degree of frustration found in the antiferromagnetic phase and that they remain significant in the ferromagnetic phase. [S0031-9007(97)05249-6]

https://iramis.cea.fr/spec/Docspec/articles/s97/124/public/aps.pdf

Multiple-Spin Exchange on a Triangular Lattice: A Quantitative Interpretation of Thermodynamic Properties of Two-Dimensional Solid 3 He

The frequency of the ''long-lived'' induction signal in /sup 3/He-B depends on the time, and the rate of change of this frequency increases with increasing field gradient. Experiments confirm Fomin's theoretical explanation of this phenomenon, according to which a /sup 3/He-B sample in which a uniform magnetization precession is excited breaks up into two phases. In one, the magnetization is parallel to the field and does not precess; in the other, the magnetization precesses with a deflection angle of approximately 104/sup 0/.

Long-lived induction signal in superfluid /sup 3/He-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.

/pdf/magnon-condensation-into-a-q-ball-in-3he-b-sysdk8rdkq.pdf

Magnon condensation into a Q ball in 3He-B

Magnetic supercurrents are observed in superfluid $^{3}\mathit{B}$. These supercurrents transport the longitudinal magnetization from one experimental cell to another through a narrow channel, driven by gradients of the order parameter. In both cells NMR is induced by separate signal generators at the same frequency. The supercurrent arises if a phase difference of the magnetization precession is established across the narrow channel. A critical spin supercurrent is measured at which precession phase slippage takes place. Experimental values of the critical spin supercurrent are in agreement with theoretical predictions.

Yu. M. Bunkov

Papers

Laboratory simulation of cosmic string formation in the early Universe using superfluid 3He

Multiple-Spin Exchange on a Triangular Lattice: A Quantitative Interpretation of Thermodynamic Properties of Two-Dimensional Solid 3 He

Long-lived induction signal in superfluid /sup 3/He-B

Magnon condensation into a Q ball in 3He-B

Investigation of spin supercurrents in 3He-B.