|Bloch is considered one of the founders of solid-state physics. He made particularly significant contributions to the quantum theory of metals and solids, he worked on the magnetic scattering of neutrons and, together with Luis Alvarez, he experimentally measured the magnetic moment of the neutron. His discovery of nuclear magnetic resonance won him the Nobel Prize in Physics for 1952, which he shared with Edward Mills Purcell.
|Research on Theory of Metals and Collective Phenomena. Felix Bloch proposed a satisfactory electron theory of conduction on the basis of quantum mechanics in his doctoral thesis, “Über die Quantenmechanik der Elektronen in Kristallgittern’’ (The quantum mechanics of electrons in crystal lattices), which was published in the Zeitschrift für Physik (1928). The electrons in a metal were considered to be uncoupled, though the field in which any one electron moved was found by an averaging process over the other electrons. If the metal was at absolute zero, its lattice determined a periodic potential field for the electronic motions, and the electrical resistance by the immobile lattice was zero. An electron could move freely through a perfect crystal and a finite free path could only be due to the imperfections in the lattice. In general the imperfections were caused predominantly by the thermal motion of the atoms and were strongly temperature dependent, increasing with increasing temperature. Impurities, however, also scattered the electrons, but in this case the free path would not vary appreciably with temperature. The resistance, therefore, consisted of the “impurity resistance’’ and the resistance due to the thermal motion of the atoms. According to Bloch’s analysis of the motion of an electron in a perfect lattice, all the electrons in a metal could be considered to be “free,” but it did not necessarily follow that they were all conduction electrons. This theory accounted for metals, semiconductors, and insulators but not for superconductors. Physics of the Neutron. In March 1933, with the Nazis already in power, Bloch left Germany with a Rockefeller Fellowship. He was planning to start working in the fall with Fermi’s group in Rome. In the meantime he traveled to Paris, Utrecht, and Copenhagen, and a short while before going to Rome, he was contacted by the Physics Department of Stanford University to be offered a position there. He took the position as acting associate professor in April 1934. While in Stanford, he had the opportunity to organize seminars in theoretical physics, jointly with Robert Oppenheimer, who was at Berkeley. In the summer of 1935, he combined a trip he took to Switzerland with a trip to Copenhagen. Bohr thought that Bloch’s experience with problems of ferromagnetism would be useful for thinking about the physics of the newly discovered neutron. Since the magnetic moment of neutron had already been discovered, Bloch started considering the possibilities of polarized neutrons in ferromagnetic materials. In a letter to the Physical Review Bloch submitted in 1936, he outlined his theory of magnetic scattering of neutrons. It was also shown that the scattering could lead to a beam of polarized neutrons and how temperature variations of the ferromagnet could be used to separate the atomic scattering from the nuclear scattering. Nuclear Magnetic Resonance and the Nobel Prize. After the war, Bloch devised a method for measuring atomic magnetic moments. This method he called nuclear induction. When the atomic nuclei were placed in a constant magnetic field, then their magnetic moments would be aligned. If a weak oscillating magnetic field is superposed on the constant field in a direction which is perpendicular to the constant magnetic field, then, as the Larmor frequency is approached, the original rotating polarization vector will be forced nearer the plane perpendicular to the constant magnetic field. The rotating horizontal component of the polarization vector will induce a signal in a pickup coil whose axis is perpendicular to the weak oscillating field. The exact value of the frequency that gives the maximum signal can then be used, as in the Larmor resonance formula, to calculate the magnetic moment. Using this method, the proton moment was measured and found to be in close agreement with the value that had been already determined by Rabi in his experiments with molecular beams. Bloch’s collaborators in the experiments were William. W. Hansen and a graduate student, Martin Packard.
|1905 - 1983