Effective mass (solid-state physics) from both sides gives where is the th element of , is the th element of , and are the th and th elements of , respectively, and is the total energy of the particle according to the Planck–Einstein relation. The index is contracted by the use of Einstein notation (there is an implicit summation over ). Since Newton's second law uses the inertial mass (not the gravitational mass), we can identify the inverse of this mass in the equation above as the tensor This tensor expresses the change in group velocity due to a change in crystal momentum. Its
C. N. Yang Institute for Theoretical Physics C. N. Yang Institute for Theoretical Physics The C. N. Yang Institute of Theoretical Physics (YITP) is a research center at Stony Brook University. In 1965, it was the vision of then University President J.S. Toll and Physics Department chair T.A. Pond to create an institute for theoretical physics and invite the famous physicist Chen Ning Yang from Institute for Advanced Study to serve as its director with the Albert Einstein Professorship of Physics. While the center is often referred to as "YITP", this can be confusing as YITP also stands for the Yukawa Institute for Theoretical Physics in Japan.