Nikolai Kopnin, Low-temperature Physicist

January 2, 2014 (PO25). Our friend and colleague Professor Nikolai Borisovich Kopnin passed away on October 20, 2013, during a lecturing trip to Rome. He was 67 years old.

Nikolai Kopnin had been a frequent visitor at the Aalto Low Temperature Laboratory¹ for several years until in 2000 he became a member of the staff. His early collaboration with the Laboratory was on the dynamics of quantized vortices in helium-3 superfluids. Lately, his studies contributed extensively to the development of superconducting nanoelectronics, the new backbone of the Laboratory's research.

Nikolai received his physics education in Moscow. In 1973 he defended his PhD thesis on vortices in Type II superconductors under the supervision of Academician Lev P. Gor’kov in the Landau Institute of Theoretical Physics. He remained a researcher of the Landau Institute, and in 1984 he received his higher doctoral degree.  In 2011, he was awarded the International Francis Simon Prize (together with his Landau colleague, S.V. Iordanskii). The Prize was for his work on forces acting on quantum vortices in superfluids and superconductors. One of these forces is now known as the "Kopnin force".

The main area of Nikolai Kopnin's research was superconductivity, primarily its non-equilibrium and non-stationary phenomena. His research work has been highly recognized and he was one of the leading experts in this field worldwide, as shown by the citations of his monograph “Nonequilibrium superconductivity" (Oxford University Press, 2001). He has contributed to the studies of anisotropic and layered superconductors, developed the microscopic theories for dissipative and non-stationary flow in Fermi superfluids, in particular in superfluid 3He, worked on new mechanisms for the formation of topological defects during rapid quench-cooled phase transitions, which has applications in cosmology, and he constructed theories of superfluid quantum turbulence. During more recent years he investigated the physics of mesoscopic structures and devices, including graphene. He published more than 150 highly cited articles in leading international journals.

Already in 1991, by extending his theory of the "Kopnin force" to chiral superfluids Nikolai predicted the existence of fermionic bound states, which have exactly zero energy. Today these quasiparticles are known as Majorana fermions - objects, which are still elusive in particle physics but may be observable in topological superfluids and superconductors. The bound states in chiral superfluids found by him have in addition the remarkable property that their spectrum is dispersionless. Now such flat bands are intensively searched for in solid-state materials. According to Nikolai’s recent work, the singular density of states in materials with a flat band may open the route to superconductivity at room temperature.

Nikolai Kopnin was a modest person who was always fair and considerate to his colleagues and friends, and they considered him a noble man with a strong passion for science. He was a person whom one could always trust and who dedicated and committed himself fully to his research activity. His free time Nikolai devoted to his family and enjoyed interesting hikes and journeys to wilderness. Those who were fortunate to be friends with Nikolai know that there is nothing better to look for.

Nikolai was always caring and loyal, with warm human spirits and intelligent sense of humor.

Matti Krusius, Jukka Pekola and Grigori Volovik

¹Aalto University, School of Science, FI-00076 Aalto, Finland.