NATURE|Vol 455|30 October 2008 NEWS & VIEWS

OBITUARY the effective electrons. In 1955, during a sabbatical year at the University of Chicago, Pippard measured the surface resistances of Brian Pippard (1920–2008) single-crystal samples of copper in various Low-temperature physicist who excelled in subtle intuitive concepts. orientations, and on his return to Cambridge used his data to map out the Fermi surface of copper, a widely recognized tour de force. He Brian Pippard, who died on 21 September, was elected to the Royal Society in 1956. had many achievements, but will be In 1962, Pippard’s student Brian Josephson remembered particularly as the first independently proposed a theory of quantum experimenter to map the Fermi surface tunnelling between superconductors, which of a metal, for his non-local theories subsequently earned him a Nobel prize (in CAMBRIDGE UNIV. of the response of normal metals and 1973). Pippard was not directly involved, but superconductors to electric fields, and as later enthusiastically took up the study of the dynamic head of physics at the University of physics of SNS Josephson junctions, and of Cambridge from 1971 to 1982. the peculiar phenomena that occur when Pippard grew up in Bristol, UK, and the superconducting current is converted to at school proved exceptional in music as normal current at an SN interface. During well as science. He entered Clare College, the same period, his book Dynamics of Cambridge, in 1938, and, doubting his Conduction Electrons established him as mathematical ability, chose first to study an expert on all aspects of Fermi-surface chemistry, before switching to physics to phenomena, especially magnetoresistance, make himself useful during the Second helicon waves, and magnetic breakdown in World War. He worked at the Great Malvern which electrons tunnel through momentum research establishment on a radar device space from one part of the surface to another. for tracking mortar projectiles, returning to In 1966, Pippard became the first president Cambridge for his PhD in 1945. complete theory of the effect, published in of Clare Hall, informal and happy home of He was already familiar with the skin 1948 by Harry Reuter and Ernst Sondheimer, graduates and distinguished visitors, and of effect, the restriction of microwave fields and Pippard’s simple ineffectiveness his wife Charlotte and their three daughters. to a thin surface layer in metals, and his concept was later applied in understanding In 1971, he was elected to the Cavendish supervisor David Shoenberg encouraged many other phenomena, such as cyclotron chair in the department of physics, but even him to study it in superconductors — resonance and ultrasonic attenuation in before that had been the moving spirit behind materials that mysteriously lose all electrical metals. the much needed transfer of the Cavendish resistance below a sharply defined transition After his doctorate, Pippard became Laboratory to new buildings. temperature. In 1935, Fritz and Heinz London interested in the stability of normal metal– As head of department from 1971, he had hinted that a superconductor might be superconductor (NS) boundaries, and encouraged innovative research and tried a quantum fluid, a system of many electrons introduced the idea that their energy was persistently — though not always successfully governed by a wavefunction similar to that determined by a characteristic coherence — to close down areas he judged to be less of a single electron, as though they were all length. This length was later identified with worthy. He was proactive in the reform of in the same quantum state. In such a system, that of the celebrated Ginzburg–Landau undergraduate teaching, and insisted on the inertia of the superelectrons should lead theory, and proved crucial in understanding lecturing ability in staff appointments. He to a purely inductive conductivity at high the physics of type II superconductors, used was himself always a stimulating teacher, frequencies. By measuring the frequency and in making superconducting magnets. and his perceptive Elements of Classical bandwidth of microwave resonators made Pippard’s work on the anomalous skin Thermodynamics and challenging Cavendish of tin and mercury, Pippard successfully effect encouraged him to wonder whether, Problems in Classical Physics excited and observed this effect when he cooled the just as in normal metals the current at a tormented generations of students. He was resonators through their superconducting given point depends on the electric field president of the Institute of Physics during phase transitions. at neighbouring points through which 1974–76. Pippard’s experiments were models of the electrons had travelled since last being Brian could be idiosyncratic. He was simplicity. For instance, the frequency of his scattered, the superconducting current suspicious of mathematical formalism, and klystron microwave source had to be stable might likewise depend on the electric field at once, when invited to deliver a keynote speech to one part in a million, yet he achieved this distant points — the range being limited by to an international audience, selected as its using no more than draught protection, a an electromagnetic coherence length. In 1953 title ‘The cat and the cream’, and startled his micrometer tuner with a weight and pulley he proposed a model of this non-local effect. listeners by announcing the imminent demise to eliminate backlash, and a stack of carefully It proved entirely successful; and in 1957, of solid-state physics. He was capable of great aged high-voltage batteries as power supply. when John Bardeen, Leon Cooper and Robert kindness, but also relished being boyishly In 1940, Heinz London had noticed that Schrieffer published their revolutionary clever: his inaugural lecture as Cavendish the microwave surface resistance of normal theory of superconductivity, they were at professor was planned around an intriguing metals, instead of falling steadily with falling pains to demonstrate that it confirmed series of bench experiments whose outcomes temperature as the resistivity did, reached a Pippard’s model. the assembled practitioners, young and old, limit. Pippard confirmed this ‘anomalous skin In 1954, Pippard pointed out that in the were invited to predict, by show of hands. We effect’ in his own samples and realized that it anomalous skin effect, the surface impedance duly got most of our predictions wrong, as he occurred when the electron free path became of a metal is related to its Fermi surface — the intended. larger than the skin depth, and that only theoretical boundary in momentum-space John Waldram those electrons moving parallel to the sample within which the conduction electrons John Waldram was formerly at the Cavendish surface remained effective in responding to of a metal reside — and in fact provides a Laboratory, Cambridge CB3 OHE, UK. the electric field. This idea led quickly to a measure of the curvature of this surface near e-mail: [email protected]

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