High-Magnetic-Field Research and Facilities [Final Report] (1979) / Chapter Skim
Currently Skimming:
SCIENTIFIC OPPORTUNITIES
SCIENTIFIC OPPORTUNITIES
Pages 8-37
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 8... ...
Significant scientific opportunities exist over the entire field range greater than 30 T As the available field strengths are increased, the number of opportunities increases.
|
From page 9... ...
Considerable preparatory work has already been accomplished, and we conclude that the time is ripe for exploitation of available methods that explore an unknown and potentially scientifically rich field regime. CONCLUSIONS AND HIGHLIGHTS In this section we summarize our principal scientific conclusions and give an indication of the excitement and richness of the research that could be ac
|
From page 10... ...
In summary, we conclude that significant new scientific opportunities exist in the utilization of steady-state, highly homogeneous magnetic fields up to 75 T and that a design program to attain such fields should be undertaken. The design and construction of facilities for the production of quasi-static magnetic fields approaching 100 T is a step toward this goal, and sufficient scientific opportunities exist to warrant construction of such facilities in their own right.
|
From page 11... ...
Transition to Lower Dimensionality and Wigner Crystallization Magnetic fields reduce the dimensionality of motion of an electron gas. Confinement to orbits along the magnetic-field lines results in a lowering of the electronic kinetic energy, allowing the potential energy greater influence over electronic motion.
|
From page 12... ...
High-field investigations are absolutely vital for the last two of these properties. Metallurgical Phase Transitions At present, pulsed high magnetic fields are not used extensively in metallurgical research.
|
From page 13... ...
Study of collision dynamics under high magnetic fields involves the time of collision directly and promises new insights into the collision potential. On a strictly spectroscopic level, study of atomic and molecular spectra in very high magnetic fields and at high temperatures can be used to simulate conditions at the surface of white dwarf stars (which exhibit fields of the order of 100 to 1000 T)
|
From page 14... ...
Magnetic fields reduce the dimensionality of the electron gas. For a three-dimensional system, the electron orbits are cylindrical for sufficiently large fields (o>cr > 1)
|
From page 15... ...
Wigner crystallization is predicted for low electron densities (too low for these structures in the absence of magnetic fields at reasonable temperatures) , because the electron kinetic energy decreases more rapidly than the correlation energy as the density is reduced.
|
From page 16... ...
This allows one to make the transition from slow to fast electronic motion by merely increasing the cyclotron frequency, that is, by increasing the magnetic field. Some experiments that exhibit the beginning of this "undressing" of the electron have been performed by choosing systems for which m*
|
From page 17... ...
When this ratio is not large, the electron experiences rapid changes in its potential, leading to a breakdown of the effective mass approximation. Large magnetic fields force the electrons into orbits whose radii are ~(fic/eH)
|
From page 18... ...
In these materials, cyclotron resonance measurements require high magnetic fields for two reasons: (a) fields greater than the upper critical field Hc2 are required so that the field penetrates the superconductor uniformly (fields of the order of 20-30 T are required for many A15 compounds, for example Nb3Sn)
|
From page 19... ...
This unequivocal test of Overhauser's prediction is an important one that high magnetic fields can make of our understanding of the behavior of the so-called simple metals. Electronic Structure of Exotic Metals.
|
From page 20... ...
CATEGORY 3. The availability of very high magnetic fields would allow measurements of electronic scattering rates and of dimensional changes in Fermi surfaces in binary alloys to be extended beyond the dilute limit.
|
From page 21... ...
In the presence of a magnetic field, the A -phase transition line splits into two, with the parallel nuclear spins predominantly along the magnetic-field direction for the AI phase. Leggett has argued that a ferromagnetic moment exists in 3He-A because of chemical and pairing effects.
|
From page 22... ...
A second important field of research on critical phenomena concerns Ising systems in transverse magnetic fields. It has been shown for the one-dimensional case that the Ising model ground state in a transverse field can be mapped onto the Ising model in the absence of a field in two dimensions at arbitrary temperature.
|
From page 23... ...
High magnetic fields can compete with the crystal field splitting, in some cases allowing magnetic ordering where none existed before. It would be of great interest to follow the development of the induced moment as the applied field reduced the splitting of the ground and excited state.
|
From page 24... ...
There is little doubt that the availability of research facilities for higher dc magnetic fields would be a boon for investigators. Work at high fields should yield a better understanding of the basic material parameters that control both the critical temperature, TC, and the upper critical field He2.
|
From page 25... ...
CATEGORY 3. When there is a difference in the magnetic susceptibility or the magnetization between the phases involved, an applied magnetic field can induce or suppress the phase transformation.
|
From page 26... ...
understanding the electronic structures of highly excited states. The discussion that follows deals with ion-cyclotron resonance, spectroscopy and structure determination, chemical-biological reactivity, and chemical dynamics.
|
From page 27... ...
CATEGORY 3. Magnetically related research avenues that will lead to improved understanding of chemical reactivity are studies of reaction mechanisms by variations of relative reaction rates in complex systems; studies of new chemical properties brought about by changing molecular wavefunctions in the magnetic field; studies of the effects of magnetically induced anisotropy on otherwise isotropic chemically reacting systems; and studies of photochemical reaction pathways involving magnetically sensitive singlet and triplet states of molecules in high fields.
|
From page 28... ...
The magnetic field in this case causes both orientation and spin selectivity. The rates of chemical reactions in solutions are described in terms of rate constants that are ensemble and orientation averages of microscopic rates.
|
From page 29... ...
Other important applications in dynamics are the study of magnetic effects on collisions at high fields, on the effects of energy excess on energy transfer, and on orientational selection rules in moderately large molecules. The nonradiative relaxation and autoionization of molecules will also be influenced by 50-100 T magnetic fields that will couple Rydberg levels significantly to the ionization continuum.
|
From page 30... ...
These effects influence the angular distribution of the photoelectron spectrum. Photochemical reaction paths and rates can be modified because sufficiently high magnetic fields will influence nonradiative relaxation processes in molecules.
|
From page 31... ...
The number for a variety of proteins is given in Table 1. At currently available field strengths, we can resolve a small fraction of the resonances in proteins of low molecular weight, such as ribonuclease, while those of higher molecular weight are not yet open to study.
|
From page 32... ...
NMR Imaging: The use of higher applied fields for NMR imaging will result in greater sensitivity, which means that smaller volumes of tissue will give
|
From page 33... ...
Each of these measurements would be done on a "pure" electrodynamic system-electrons in an external magnetic field. The first of these experiments would involve tests of radiative corrections to quantum electrodynamics in strong fields.
|
From page 34... ...
The magnetic fields at the surfaces of white dwarfs are of the order of 100-1000 T Current observations are fitted to theoretical spectra of atoms appropriate to this field range, which may not be a sound procedure.
|
From page 35... ...
CATEGORY 2. When an atom is subjected to a large magnetic field, its emission lineshape will be dominated by the motional Stark effect if its velocity (temperature)
|
From page 36... ...
CATEGORY 2. The presence of an intense magnetic field generates a precession of the ground and excited moments of atoms or molecules that can change the collision cross section in a gas.
|
From page 37... ...
The future potential of such studies lies in the ability to alter a particular zero-field excitation transfer process by application of a field (in the Na-Na* resonant transfer case, shutting off the role of the long-range dipole-dipole interaction)
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.