Alexander Sergeyevich Davydov (born 26.12.1912) is a Soviet theoretical physicist and an academician of the Academy of Sciences of the Ukrainian SSR (1964). He was born in Yevpatoria. He graduated from Moscow University (1939). In 1945-53 he worked at the Institute of Physics of the Academy of Sciences of the Ukrainian SSR, in 1953 — 56 he was head of a department at the Institute of Physics and Power Engineering in Obninsk, and in 1956—64 professor and head of a chair at Moscow University. In 1964—66 he was head of a department at the Institute of Physics of the Academy of Sciences of the Ukrainian SSR, from 1966 head of a department at the Institute of Theoretical Physics of the Academy of Sciences of the Ukrainian SSR and from 1973 its director.
His work is in the field of the theory of solids, the theory of the nucleus and theoretical biophysics. He was the first to extend (1948) the concept of the exciton to molecular crystals of complex structure and to point out the decisive part that excitons play in the processes of light absorption, luminescence and photoconductivity. He proved that even a slight molecular interaction in most cases substantially alters the energy spectrum of a real crystal, leading to the splitting of non-degenerate molecular terms in crystals (“Davydov splitting”). He was the first to introduce, in 1951, the concept of deforming excitons. He studied the impurity absorption of light by crystals and in 1952 developed the theory of this phenomenon. In 1966, for his theoretical studies of excitons in crystals, he was awarded the Lenin Prize (together with others).
In 1958, together with G. F. Filippov, he formulated and developed the main propositions of the model of a rigid non-axial rotator, which made it possible to explain many regularities in the spectra of the low excitations of a large group of non-spherical nuclei, and in 1960, together with O. O. Chaban, he developed a model of collective excitations that takes into account the deformability of the shape of the nucleus as it rotates. As a result Davydov developed (1958 — 60) the theory of the collective excited states of non-spherical atomic nuclei, which allows for departures from the axial symmetry of nuclei (Davydov's theory of non-axial nuclei).
(State Prize of the Ukrainian SSR, 1969). In 1973—76 he developed a theory of solitons, on the basis of which he constructed a model of muscle contraction. Hero of Socialist Labour (1982).