Yakov Il'ich Frenkel (Russian: Яков Ильич Френкель; 10 February 1894 – 23 January 1952) was a Soviet physicist renowned for his works in the field of condensed-matter physics.

In 1912, while studying in the Karl May Gymnasium in St. Petersburg, he completed his first physics work on the Earth's magnetic field and atmospheric electricity.

He considered moving to the USA (which he visited in the summer of 1913, supported by money hard-earned by tutoring) but was nevertheless admitted to St. Petersburg University in the winter semester of 1913, at which point any emigration plans ended.

[1] Frenkel graduated from the university in three years and remained there to prepare for a professorship (his oral exam for the master's degree was delayed due to the events of the October revolution).

[1] Early works of Yakov Frenkel focused on electrodynamics, statistical mechanics and relativity, though he soon switched to the quantum theory.

Frenkel enthusiastically entered the field through discussions (he reportedly discovered what is now called the Klein–Gordon equation simultaneously with Oskar Klein) but his first scientific paper on the matter (considering electrodynamics in metals) was published in 1927.

In 1930 to 1931, Frenkel showed that neutral excitation of a crystal by light is possible, with an electron remaining bound to a hole created at a lattice site identified as a quasiparticle, the exciton.

Mention should be made of Frenkel's works on the theory of metals, nuclear physics (the liquid drop model of the nucleus, in 1936), and semiconductors.

During the 1930s, Frenkel and Ioffe opposed dangerous tendencies in Soviet physics, tying science to the materialist ideology, with remarkable courage.

[1] Yakov Frenkel was involved in the studies of the liquid phase, too, since the mid-1930s (he undertook some research in colloids) and during the World War II, when the institute was evacuated to Kazan.