Strong-coupling theory of the universal linear temperature dependence of the nodal Fermi velocity in layered cuprates

We explain the recently observed linear temperature dependence of the nodal Fermi velocity upsilon(F)(T) in nearly optimally doped cuprates. We argue that it originates from electron-electron interaction and it is a fundamental property of an arbitrary two-dimensional (2D) Fermi liquid. We consider a spin-fermion model with input parameters extracted from the data, and we show that the T term is about 30% at 300 K, in agreement with the data. We show that the sub-leading term in upsilon(F)(T) is a regular (and small) T(2) correction. We also show that at a 2k(F) quantum-critical point, temperature corrections to the dispersion are singular.