Biographical Memoirs Volume 64 (1994) / Chapter Skim
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Felix Bloch
Felix Bloch
Pages 34-71
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From page 35... ...
that magnetic moments of nuclei in general could be measured by resonance methods. This idea led to the discovery of nuclear magnetic resonance, which Bloch originally caller!
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From page 36... ...
He entered public elementary school when he was six years oIcI. Experiences in school at that tender age were difficult for Felix, who spoke Swiss German with a somewhat different accent than most members of the class.
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From page 37... ...
He tract a preference for Bach's harmonies. At twelve, Felix f~nishect elementary school and began secondary school.
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From page 38... ...
Coming across Sommerfeld's famous book, Atomic Structure and Spectral Lines, Felix found that he neecled to know what was meant by an "electromagnetic field." He did his own reading about that subject and many others in classical physics and made a brief foray into experimental physics that he never completecI. On the other hanct, he was absorbed by the lectures in the small colloquia held alternately in the departments of the University of Zurich anct the ETH.
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From page 39... ...
Debye suggested that Felix shouIct stucly Schrodinger's new wave mechanics. Many years later, Felix returned to the Compto Effect and wrote a paper on his original idea, this time · .
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From page 40... ...
This work resulted in Bloch's first paper and, as he later remarked, it was a forerunner of the paper by Weisskopf and Wigner on radiation damping and the natural line widths of spectral lines. Heisenberg took Felix as his first graduate student and suggested that for his thesis Felix should study the conductivity of metals by applying the new quantum mechanical theory.
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From page 41... ...
By straight Fourier analysis I found to my delight that the wave differed from a plane wave of free electron only by a periodic modulation. This was so simple that I didn't think it could be much of a discovery, but when I showed it to Heisenberg he said right away, "That's it." Well, that wasn't quite it yet, and my calculations were only completed in the summer when I wrote my thesis on "The Quantum Mechanics of Electrons in Crystal Lattices."
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From page 42... ...
The waves that Felix discovered have been called "Bloch Waves" or "Bloch States," and the concept of these waves turns up everywhere in the theory of condensed matter. Tncicientally, the wave solution that Felix discoverect was a version of what was known in mathematics as Floquet's Theorem anct had been user!
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From page 43... ...
we know about it mainly from references by others, such as Bethe Lonclon, Brillouin, anct PauTi. Bloch and London pointed out that it was necessary, on thermodynamic grounds, that the superconducting state required a minimum of the energy below the critical temperature but that at temperatures above that point a zero current state is more probable.
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From page 44... ...
~ magnetoresistance of metals and shortly afterwards attacked the funcIamental problems of ferromagnetism. Ferromagnetism tract already been treated by Heisenberg, who showed that the basic explanation depencled on the exchange interaction of electrons.
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From page 45... ...
There he wrote a short qualitative paper on the interactions betwen metallic electrons, summarizing his icleas on spin waves developed in Utrecht. Felix greatly enjoyed!
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From page 46... ...
A quotation from a historical paper on the solid state by Lillian HocIdeson, Gordon Baym, anct Michael Eckert follows: Nominally devoted to exchange-ir~teraction problems and residual magnetization in ferromagnets, the paper presents an exceptional wealth of formalism which has become part of the fabric of the modern theory of condensed matter physics and collective phenomena. Beyond its contribution to the theory of domain walls, this work serves as a bridge between the quantum theory of ferromagnetism in the 1930's and present theories of many-particle systems.
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From page 47... ...
While there he completecI the paper on stopping power that he submitted to the Annalen der Physik in the summer of 1932. At the end of 1932 he made an elegant mollification of the stopping power calculation by employing the Thomas-Fermi atomic mode} in a particularly successful and practical way.
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From page 48... ...
In the fall of 1933, actually while visiting Bohr in Copenhagen, Bloch received a telegram from David Webster at Stanford offering him a position in the Physics Department. ~ quote from Bloch's interview with Charles Weiner in 1968: There's a rather amusing story there.
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From page 49... ...
shortly afterwards Fermi wrote his famous paper on beta clecay. At age twenty-eight Felix left Europe to go to Stanford.
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From page 50... ...
It was known that the neutron tract a magnetic moment, and Felix's earlier knowlecige of ferromagnetism macle him think about polarizing neutrons in a ferromagnetic material. In July 1936 Bloch submitted a "Letter to the Editor" of the Physical Review in which he first described the theory of magnetic scattering of neutrons.
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From page 51... ...
so Bloch applied his idea to a different sort of experiment. He thought of using a polarizect beam that would pass through a region of constant magnetic field!
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From page 52... ...
Between the two plates a constant strong magnetic field was placed, and, in acIdition, a weak oscillating magnetic field was introclucec! normal to the constant field anct of variable frequency.
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From page 53... ...
But a first step in getting accuracy was made by using two flip coils, one just as used previously, with the second inserted in the magnetic field of the cyclotron. The resonant orbital cyclotron frequency for protons, or a harmonic of it, could then be used to determine the proton moment.
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From page 54... ...
Hamermesh and H Staub, a figure of 8 percent was established at saturation magnetic field!
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From page 55... ...
"nuclear induction." The idea is the following: If atomic nuclei are placed in a constant, say vertical (z direction magnetic field, an alignment of their magnetic moments would take place that wouic! be limited by thermal agitation.
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From page 56... ...
It is clear that nuclear induction had a close filial relation with the Bloch-Alvarez experiment. The nuclear induction idea was first applied experimentally to water, and the proton moment was measured to be in agreement with the value previously determined in the Rabi experiments with molecular beams.
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From page 57... ...
After a period of working with the novel nuclear induction measurements and obtaining interesting new results, Felix returned to a precise measurement of the magnetic moment of the neutron by the Bloch-Alvarez method. The small Stanford cyclotron was used as a neutron source, and the measurement was carried out by Bloch, Nicodemus, and Staub.
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From page 58... ...
Even so, when he went to Geneva, he took along Stanford equipment, and he and two colleagues, Jim Arnoict anct Wes Anderson, continued nuclear induction experiments. As Felix had predictecI, he clidn't care for administrative work, and after a trial period of one year, he returned to Stanforcl, but not before leaving a great ant!
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From page 59... ...
K Wangsness, worked out a microscopic justification for the use of the phenomenological relation parameters T~ anct T2 in nuclear induction.
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From page 60... ...
These remarks were really an attempt, once more, to develop a simplified physical theory of superconductivity. Although he made considerable progress, as outlined in his talk, he stated modestly: It seems to become of the many cherished traditions of the American Physical Society that every third retiring presidential address somehow refers to the theory of superconductivity.
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From page 61... ...
In choosing physics there could harcIly have been a better time for him to enter the fielct, for during the years 192~27 modern quantum theory emerged in great splendor and he was a witness to it. He rocle the crest of the waves of this great new science, contributed to it, anct shower!
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From page 62... ...
Felix helped in mending fences and bringing in firewood, and his hearty appetite macle shared mealtimes a double pleasure. It is very hard right now to think of those times, but
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From page 63... ...
Schiff had suggested me as a cancticiate for a position in the Stanford Physics Department. Felix traveler!
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From page 64... ...
graciously. During the fifteen years or so T spent on the electron scattering program, ~ would often come to see Felix after ~ had worked through the night on a particularly successful run.
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From page 65... ...
Lore Bloch; the Stanford University Archives; and the Niels Bohr Library, Center for History of Physics, at the American Institute of Physics, for access to documentary source material. He wishes also to thank Ms.
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From page 66... ...
32:881. 1932 Zur Theorie der Austauschproblems und der Remanenzerscheinung der Feromagnetika.
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Rabi. Atoms in variable magnetic fields.
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41:48. 1956 Dynamical theory of nuclear induction II.
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From page 69... ...
A 7:2187. 1976 Reminiscences of Heisenberg and the early days of quantum mechanics.
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From page 70... ...
A 25:102. 1987 Past, present and future of nuclear magnetic resonance.
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