Condensed-Matter Physics (1986) / Chapter Skim
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8 Low-Temperature Physics
8 Low-Temperature Physics
Pages 164-189
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From page 164... ...
QUANTUM FLUIDS The term quantum fluid is used as the generic expression to cover any material in which the particles of interest do not solidify when in the ground state. Quantum fluids remain as a liquid or gas at T = 0 K because the particles are sufficiently light that the ground-state kinetic energy is larger than the interparticle potential energy that normally causes crystallization.
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From page 165... ...
Rapid progress in the understanding of superconductivity followed the development of the microscopic theory by Bardeen, Cooper, and Schrieffer in 1957. The BCS theory attributes the phenomenon to an attractive interaction between pairs of electrons in a spin singlet (antiparallel spins)
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From page 166... ...
In the former case the orbital part of the pair wave function is required to be antisymmetric in the interchange of the coordinates of the two atoms; in the latter it must be symmetric. The members of a pair having parallel spins must be in states of odd angular momentum, and in the case of 3He they are in a state with L = 1.
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From page 167... ...
An important consequence of the odd angular momentum of the orbital wave function is that superfluid 3He has an anisotropic spatial character. The fluid in all the superfluid phases has an intrinsic bending energy that favors the persistence of a particular orientation of the pair wave functions over rather long ranges in the liquid.
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From page 168... ...
168 o Ct .= .~ Ct so o by Cat o an rid ._ ._ o Cal Cal Cal ._ ma: V)
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From page 169... ...
169 o ~ 0 0, ~ ~O ~ 3 ~° ~ 3 ct 3 o ;> ~ _ v: Z L ~V)
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From page 170... ...
When the same flow experiments were repeated in the B phase, no dependence of the flow on the direction of the magnetic field was observed. NUCLEAR MAGNETIC RESONANCE IN SUPERFLUID SHE The macroscopic quantum nature of 3He has profound effects on its NMR.
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From page 171... ...
The 5~ = + 1 polarization of the spin triplet in the Al phase was determined through comparing a mechanically induced second-sound pulse with the magnetization change measured in an NMR coil. Fourth sound, a compression wave in a superleak, was used to make the first convincing demonstration of superfluid flow in 3He.
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From page 172... ...
The critical velocities limiting the superfluid flow in both the A and B phases appear to be much smaller than originally expected. The viscosity measured in the fluid at low temperatures is also much smaller than expected.
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From page 173... ...
The newest materials in this class that have come under investigation are the gases of atomic hydrogen and deuterium, which can be stabilized against the formation of H2 and D' through polarization of the atomic electrons in large magnetic fields at low temperatures. MIXTURES OF SHE IN SHE At low temperatures 3He is soluble in 4He in concentrations up to 6 percent for fluid with no external pressure, and up to 10 percent for fluid under pressures greater than 10 bar.
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From page 174... ...
LIQUID 4HE IN UNUSUAL GEOMETRIES During the past decade, the superfluid transition in liquid 4He has been extensively studied in thin films on a variety of substrates. Films deposited on smooth and flat surfaces provided the first model for measuring the way in which the order of a two-dimensional system is disrupted by thermally activated defects.
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From page 175... ...
In this system, TC decreases as the amount of fluid deposited on the substrate decreases. For thick films with a high transition temperature, the character of the superfluid mass change below TC appears much like that in the bulk fluid where critical fluctuations dominate the behavior.
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From page 176... ...
Small-scale electronic-type applications include the fastest-switching and least-powerconsuming electronic devices, the most sensitive and lowest-noise magnetic and electromagnetic sensors, and the most accurate voltage standard, to name but a few. At present the worldwide economic impact of superconductivity is estimated to be a few hundred million dollars annually.
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From page 177... ...
Theoretical understanding of many of the remarkable features of type 11 materials was soon achieved in terms of extreme parametric regimes of existing phenomenological theories of Ginzburg, Landau, and Abrikosov, which had been linked to BCS theory by Gor'kov. The second highly significant scientific advance was the prediction and subsequent experimental confirmation of the Josephson effects, viz., that supercurrents (dissipationless currents)
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From page 178... ...
(4) Superconducting systems have been used in examinations of fundamental questions in statistical physics and quantum mechanics, including quantum noise, special types of two-dimensional phase transitions, and, most recently, macroscopic quantum tunneling and chaotic behavior.
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From page 179... ...
Although much progress in this area has been made, many research opportunities remain, particularly in the area of extreme nonequilibrium conditions. Novel Superconducting Materials Searches for superconductivity in a variety of novel or exotic materials have been richly rewarded in the past decade.
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From page 180... ...
As it is cooled further, the rare-earth ions begin to order magnetically until the superconductivity is destroyed near the Curie temperature just below 1 K Recent small-angle neutron-scattering experiments suggest that some of these compounds exhibit a new phase of matter in which superconductivity coexists with magnetic order in periodic structures with a wavelength of about 200 A with superconductivity .
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From page 181... ...
FIGURE 8.3 High-performance multifilament superconducting conductor used in the Mirror Fusion Test Facility superconducting magnets at Lawrence Livermore National Laboratory. Nb-Ti alloy superconducting filaments (dark regions seen end on in center square)
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From page 182... ...
Central to rapid progress in this area during the past decade has been the highly successful development of techniques for fabricating micrometer- and submicrometer-size junction structures of high quality, uniformity, and reliability. This has made possible the application of nearly ideal Josephson structures to a number of exciting scientific and technological endeavors.
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From page 183... ...
A major consequence of the quantum motion in solid 3He appears in its magnetic properties. The overlap between the wave functions of neighboring atoms leads to an
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From page 184... ...
There have been recent investigations into the nature of the interface between liquid and solid helium, to determine whether the surface has facets at low temperatures. In all other crystals there is a roughening transition at which the low-temperature state, with flat faces related to the crystal structure, is disrupted by thermally activated defects so that the facets disappear.
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From page 185... ...
. 900 1 9so 2000 YEAR FIGURE 8.5 Graph representing progress in cryogenic technology.
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From page 186... ...
The lowest temperatures have been obtained by using two cascaded stages of nuclear magnetic cooling. Typically PrNis has been used as the first magnetic stage, to remove the heat of magnetization from copper.
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From page 187... ...
The Fermi temperature of pure liquid 3He is roughly 1 K It is not practical to produce a highly polarized specimen through the bruteforce application of a large magnetic field (at any temperature)
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From page 188... ...
There are no reliable estimates for the pairing transition temperature in the dilute mixtures; nor is it known whether the pairing will be a triplet state like that in pure liquid 3He or a singlet state similar to that in superconductors. In pure 3He and in superconductors Tc is ~ 10-3 Tf, where Tf is the Fermi temperature.
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From page 189... ...
Even without the unpredictable discovery of a material with a much higher Tc, of an alternative pairing mechanism other than the electron-phonon interaction, or of new phenomena with the impact of the Josephson effects, the field is likely to continue to prosper along the lines of the recent past. It is reasonable to predict that new and unusual superconducting materials will continue to be discovered and avidly studied.
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