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Felix Bloch and Twentieth-Century Physics

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Felix Bloch and Twentieth-Century Physics Felix Bloch RICE UNIVERSITY STUDIES FELIX BLOCH AND TWENTIETH-CENTURY PHYSICS Dedicated to Felix Bloch on the Occasion of his Seventy-Fifth Birthday M. CHODOROW, R. HOFSTADTER, H. E. RORSCHACH, and A. L. SCHAWLOW, Editors H. GUTFREUND W. A. LITTLE E. L. HAHN JOHN D. ROBERTS WALTER A. HARRISON MICHAEL SCHICK 0LEG JARDETZKY HANS H. STAUB CARSON D. JEFFRIES H. SUHL WALTER KOHN NORMA W ADE-JARDETZKY L. KOWARSKI J.D. WALECKA WILLIS LAMB CHEN NING YANG AYISHA Y YANIY PUBLISHED BY WILLIAM MARSH RICE UNIVERSITY HOUSTON, TEXAS Vol. 66, No. 3 Summer 1980 Copyright I 980 by Rice University US-ISSN-0035-4996 US-ISBN-0-89263-246-1 This work relates to Department of the Navy Research Grant N00014-80-G-0063 issued by the Office of Naval Research. The United States Government has a royalty-free license throughout the world in all copyrightable material contained herein. VJ ~~ C. Q I> FELIX BLOCH This volume is dedicated to Felix Bloch on the occasion of his seventy­ fifth birthday. The editors hope that these papers by former students and colleagues will entertain and enlighten and will convey a sense of the high respect and regard in which Felix is held by all who have been privileged to be his associates. We also hope that the broad range of these papers will be indicative of the impact of his work on twentieth-century physics. One expects that book-editing will be a frustrating chore. This book was quite otherwise. The authors were eager to write, quick to submit their manuscripts, and anxious to be a part of this heartfelt expression of love and respect. We editors want to express our . own gratitude to Felix Bloch. As friends, associates, and colleagues for many years, we can presume to speak for all who came in contact with Felix. We have treasured his friendship, been inspired by his rigorous insights, and been brought to a greater love for and devotion to the beauty of physics by his example. It is with profound gratitude and love that we present this volume to him . "I always felt Bohr was somewhat disappointed that I was not more interested in epistemology, but being of a pragmatic nature, I always want­ ed to deal with concrete problems." Felix Bloch spent six months with Niels Bohr in the winter of 1931-1932. Even then, at the age of 26, he was a well­ respected theorist and had come into contact with many of the prominent European physicists who were associated with the development of quantum mechanics . Bloch had first met Bohr in 1929, when he had gone with H. A. Kramers to one of Bohr's meetings held annually in Copenhagen. "I learned 'qualitative' physics from him," recalls Bloch. Those who have heard Bohr speak will know that he was not easy to understand. But a part of his inspiration was due to this very characteristic. Bloch found conversa­ tions with Bohr to be often "confusing," but they left him with an urge to clarify the physics by a detailed calculation. Many of the European physicists of that period owe their initiation in modern physics to Arnold Sommerfeld, whose monumental work v vi RICE UNIVERSITY STUDIES Atombau und Spectralinien can be compared in its impact only with P. A. M. Dirac's "The Principles of Quantum Mechanics." Bloch was no exception. He learned modern physics from Sommerfeld's book, but that was not his only tie to Sommerfeld. He was the first student of Werner Heisenberg, who had in turn been Sommerfeld's most brilliant student. Bloch had begun his study of physics at the Federal Institute of Technology (ETH) in his home town of ZUrich. He was strongly influenced by Peter Debye, having taken Debye's introductory physics course, in which he was greatly impressed by Debye's ability to catch the simple and essential point of a problem. When Debye left ZUrich for Leipzig in 1927, he advised Bloch to come to Leipzig and to work there under Heisenberg. Bloch accepted this advice and after being introduced to Heisenberg told him about calculations on the radiation damping of wave packets that he had begun in ZUrich. Heisenberg encouraged Bloch to complete this work, later published as his first paper.' He then suggested that the time was ripe for the fruitful appli­ cation of quantum mechanics to the solid state and that the electric conduc-· tion of metals might be a good topic to consider. This work, which Bloch began immediately, was not only his thesis, but, when published in 1928, 2 established the theory of metals in its modern form and brought to Bloch a reputation as a brilliant and promising young theorist. In 1928, Bloch returned to ZUrich to be Wolfgang Pauli's assistant. Pauli proposed that he work on the theory of superconductivity, since in his opinion this was the only remaining problem of importance in solid-state physics. Bloch tried to explain persistent currents by the fact that they corre­ spond to states at a relative minimum of the energy but under seemingly quite general conditions found that no such state carried a current. This result has often been referred to as "Bloch's Theorem" and was jocularly summarized by the statement that "any theory of superconductivity can be disproven." The statement is an oversimplification, but the result was an important contribution, indicating that the phenomenon of superconduc­ tivity was more profound than had been revealed by the existence of persist­ ent currents. Bloch also published work' on the ferromagnetic behavior of free electrons, but it was not a very happy year for Felix, particularly since he had hurt a knee while skiing that winter. The year that followed was a much happier one. In 1929 Bloch traveled to Holland as a Lorentz Fellow, where he spent six months with Kramers in Utrecht and a few months with A. D. Fokker in Haarlem. This was a fruit­ ful time, during which he developed the T 5 law for the temperature depend­ ence of the electrical resistance, • thus proving that the profound analysis of the electron-lattice interaction first introduced in his thesis was capable of explaining in detail the experimental results on the low-temperature resist­ ance of metals. He also published his paper containing the introduction of spin waves, 5 which became the forerunner of all modern quasiparticle FELIX BLOCH vii theories. (Bloch has often referred to this work as an "embellishment" on Heisenberg's theory of ferromagnetism!) Kramers was a delightful host, and he and Bloch became good friends. L. D. Landau, whom Bloch hc;td met earlier at Copenhagen, also visited him in Utrecht, and their close friendship led Bloch to visit Landau later (in 1931) in Russia. In 1930, Heisenberg asked Bloch to return to Leipzig as his assistant. It was at this time that Bloch's work on ferromagnetism was extended, culmi­ nating in his discovery of "Bloch Walls" which were to prove of great importance in understanding the magnetic properties of real materials, especially the phenomena of remanence and hysteresis. This work was included in his "Habilitationsschrift" on the exchange problem in mag­ netism. 6 (This is a kind of second thesis, required of those who wish to pur­ sue an academic career at a German university.) After one year with Heisenberg, Bloch was invited to Copenhagen for six months as an Oersted fellow in Bohr'·s institute. He established a close friendship with Bohr during this period-a friendship that was to last through the trying years of the war and for .the rest of Bohr's life, and one that would be renewed by frequent visits. (Bloch has remarked that he was in almost continual contact with Bohr through the years, and that the same deep problems never ceased to occupy Bohr's mind.) As a result of con­ versations with him, Bloch became interested in the stopping power of matter for charged particles, 7 a subject in which Bohr had had a long in­ terest. In 1932, Bloch returned to Leipzig, where he had become a lecturer and continued as Heisenberg's asistant. In January 1933, Hitler was appointed Chancellor of Germany. This event was to have the same profound effect on Bloch's career as it did on such other great physicists as Albert Einstein, James Franck, and Hans Bethe. Bloch realized that he would have to leave Germany, and he spent the spring and summer of 1933 in Switzerland and in Paris (where he stayed at the home of Paul Langevin). He had applied for and was awarded a Rockefeller Fellowship for a year, starting in the fall of 1933, which he intended to use partly in Rome with Enrico Fermi and partly at the Cavendish Laboratory in Cambridge. On his way to Rome, he visited Bohr at Copenhagen, where he received a letter from the Chairman of the Stanford Physics Department, David L. Webster, offering him a position at Stanford University. He knew nothing of Stanford, and he turned for advice to Heisenberg, who was also visiting Bohr. Heisenberg remarked, "0 yes, those are the people who steal one another's axes" (a reference to the Stanford-Berkeley football rivalry). Bohr was more helpful, since he had met Webster and some of the other faculty at Stanford and knew that it was a fine university. Bloch continued on to Rome, but Bohr's advice per­ suaded him to accept Webster's offer and to abandon his planned visit to Cambridge. viii RICE UNIVERSITY STUDIES Bloch's time in Rome was short, but he learned something of Fermi's style.
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