China 
Africa 
Explore chapters and articles related to this topic
Boson Condensation and Liquid Helium
Published in Teunis C. Dorlas, Statistical Mechanics, 2021
One may now ask: is the boson gas a model for a realistic physical system? And in particular, is there a system where Bose-Einstein condensation actually occurs? In fact the above analysis may give the impression that Bose-Einstein condensation is a peculiarity of the free gas. This is not the case as we demonstrate below. It was suggested in 1938 by Fritz London that the transition to superfluidity in helium might be an example of Bose-Einstein condensation. Helium is indeed a very remarkable substance. It has two isotopes: 3He and 4He. Both remain fluid down to the absolute zero of temperature, but otherwise they behave quite differently. Liquid 4He has another phase transition, the so-called λ-transition, from a phase called He I to a phase He II. The latter is a superfluid phase, which means that the fluid flows with zero viscosity. The transition takes place at a temperature Tλ=2.18 K along the vapour pressure curve and at specific volume vλ=46.2×10−24 cm3 atom-1. Figure 34.2 shows the phase diagram of 4He (see also figure 12.6). At the superfluid transition the specific heat diverges (figure 34-3). The name λ-transition is derived from the shape of this curve.
Novel Nanoscience in Superfluid Helium
Published in Klaus D. Sattler, 21st Century Nanoscience – A Handbook, 2020
Arin Mizouri, Charlotte Pughe, Berlian Sitorus, Andrew M. Ellis, Shengfu Yang, Berlian Sitorus
The quantization of circulation in superfluids and superconductors is one of the most remarkable macroscopic illustrations of quantum mechanics. In superfluids, the circulation is carried by the continuant atoms, each possessing an angular momentum equal to an integer multiple of ħ. The investigation of the rotational behavior of superfluid 4He can be traced back to the 1950s when Osborne,39 Andronikashvili, and Kaverkin40 found that the liquid rotates uniformly as a whole in rotating containers, and the shape of the meniscus generated in the containers is independent of the temperature. This was puzzling until Onsager proposed quantized vortices and modeled rotation in a bucket of superfluid helium as an array of concentric vortices rotating around a fixed point with quantized circulation41: Quantized circulation=nhm4
A Comparison of the Properties of Superconductors and Superfluid Helium
Published in R. D. Parks, Superconductivity, 2018
It appears that at sufficiently low velocities no superfluid vortices are created, and superfluid flow remains irrotational and frictionless. (This is to be expected, since insufficient energy will be available to create vortices.) Thus at low velocities a mass flow through a channel should be possible without any driving force, and this is observed to be the case. We note from Eq. (5) that the driving force on the superfluid is the gradient in the chemical potential. At low velocities this chemical potential is independent of velocity, and we have simply () ∇μ=−S∇T+(1/ρ)∇p
Les vertus des défauts: The scientific works of the late Mr Maurice Kleman analysed, discussed and placed in historical context, with particular stress on dislocation, disclination and other manner of local material disbehaviour
Published in Liquid Crystals Reviews, 2022
Only a few months after appearance of the Toulouse-Kléman paper, and independently of it, the Monastyrsky work was picked up by Vladimir Mineev and Grigory Volovik at the Landau Institute for Theoretical Physics at the USSR Academy of Sciences in Moscow [200–202]. They were studying defects in low-temperature phases of superfluid He. These phases (He-A and He-B) are analogous to complex superconductors. At the time there was intense interest in superfluid helium phases; the superfluid He phase has been known since the late 1930s, and it has a low-temperature phase whose order parameter has the symmetry of the XY model. He is fermionic and hence expected at low temperatures to behave differently from the bosonic He. The defects in He are well-studied superfluid vortices, and this is consistent with the predictions of homotopy theory. But the He phases are more complex – indeed they are often described as complex liquid crystalline phases – and their defects require much more detailed study.
Superconductivity in the twisted bilayer graphene: emergent mystery in the magic angle, the topological bosons and the Bardeen Cooper Schrieffer – Bose Einstein unconventional crossover
Published in Philosophical Magazine, 2021
When , no excitation can be created, and the topological boson moves through the tBLG medium without dissipation, as if the viscosity is zero, characterising a superfluid of the Dirac fermions’ liquid in tBLG. This is the Landau criterion for superfluidity [134,135]. Consequently, the following criterion is established for the superconductivity in tBLG at the magic angle θ (as follows from (80) and inequality ) Under the condition (75), superconductivity emerges in the tBLG, and the BCS-BEUC arises. We place in Table 1 the values of the magic angles θ and the effective masses meff of the Dirac fermions which are pairing into the topological boson with mass m* = 2meff, zero spin, and charge q* = 2e (e is the electron charge). The effective masses in the Table 1 are computed by relations (74) and (75). We refer to the BCS–BEUC as the unconventional one because it occurs not directly due to virtual phonons which are responsible for te creation of the Cooper pairs [56–58]. The tBLG phonon system participates indirectly [136,137] in BCS–BEUC via the sound velocity because of the Landau criterion for superfluidity.
Bose–Einstein condensation in a mixture of interacting Bose and Fermi particles
Published in Phase Transitions, 2020
Yu. M. Poluektov, A. A. Soroka
The phenomenon of Bose–Einstein condensation [1,2] was used by London [3] and Tisza [4] to explain the phenomenon of superfluidity of liquid helium discovered by Kapitsa [5] and Allen [6]. Yet the model of an ideal gas is too simple to explain properties of dense systems in which the interparticle interaction plays a substantial role, the fact that was pointed out by Landau [7]. But, as was demonstrated in experiments on neutron scattering in the superfluid He [8,9], Bose–Einstein condensate exists also in the presence of the interparticle interaction. A new splash of interest to the phenomenon of superfluidity and its relationship to condensation is associated with the discovery about 20 years ago of Bose–Einstein condensation in atomic gases of alkali metals confined in magnetic [10,11] and laser traps [12].