Yury Bunkov#

Short laudatio by Tom W.B. Kibble#

Professor Yury Bunkov is the discoverer of "Coherent Quantum States of Spin Precession and Spin Superfluidity" in superfluid 3 He. For this he was awarded the International London Memorial Prize in Low Temperature Physics in 2008 (together with I.A. Fomin and V.V. Dmitriev). Superfluid 3 He is the first man-made ultra-pure material which supports rnulti-dimensional order parameter fields (of orbital p-wave and spin-triplet nature). Today it is used as a laboratory model system of superfluidity in a magnetically ordered quantum liquid crystal environment. It has many properties in common with relativistic quantum fields and provides a testing ground for new ideas in particle physics and cosmology.

Yury Bunkov started his career as researcher in the renowned Kapitza Institute for Physical Problems in Moscow. Here he constructed the first nuclear demagnetization refrigerator in Soviet Union and cooled liquid 3 He to sub-millikelvin temperatures. This led in 1984 to his discovery of the unusual spin dynamics and NMR properties of superfluid 3He, observed as a dynamic coherent state of matter, with spontaneous phase-coherent macroscopic precession of magnetization. In this state inhomogeneity in the spin precession frequency creates spin supercurrents which redistribute the magnetization in such a way that precession remains uniform. This coherence of the phase in the Larmor precession throughout the system, which persists even in the presence of inhomogeneous external magnetic fields, is the first example of a magnetic analogue of off-diagonal long-range order observed in phase-coherent Bose condensates, in superfluids, and superconductors, and was hailed as the first realization of spin superfluidity. It can also be interpreted as Bose-Einstein condensation of spin-wave excitations or magnons. Indeed, it is now recognized that historically these coherently precessing states were the first examples of experimentally stabilized Bose condensates.

After the discovery of spin superfluidity Prof Bunkov demonstrated many other consequences of his work, such as the spin supercurrent, spin-current Josephson effect, spin-current vortices, and non-topological solitons (known as Q-balls in high energy physics). He also developed new measuring techniques based on coherent spin precession, which made it possible to observe other novel effects in 3 He superfluids, such as a quantized vortex terminating on a soliton sheet, observation of the Goldstone mode of the vortex with a non-axisymmetric core, identification of the order parameter states of 3 He in aerogel via resonance measurements and the observation of coherently precessing states in the highly disordered superfluid 3 He-aerogel systems.

Since l995 Prof Bunkov’s home institute is the low temperature physics laboratory in the Institute Louis Nécl of CNRS in Grenoble. Here he is known as the foremost expert on magnetic resonance and relaxation in 3 He superfluids. He was one of the originators of the first model experiments with superfluid 3 He on the cosmological Kibble-Zurek mechanism, which was devised by T.W.B. Kibble to explain cosmic string formation in cosmological phase transitions. At present one of Yury Bunkov’s efforts is to construct a superfluid 3 He detector to observe and identify cosmological dark matter.

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