Magnetic-field-induced criticality in superconducting two-leg ladders

We study critical singularities in the d-wave-like superconducting phase of the hole-doped Hubbard model of repulsively interacting electrons, defined on a two-leg ladder, induced by a magnetic field applied parallel to the ladder plane. We argue that, provided the lowest energy spin excitations in doped ladders carry as well charge quantum numbers, the low temperature thermodynamic quantities, such as specific heat coefficient and magnetic susceptibility will show logarithmic singularities in quantum critical regime. This behavior is in drastic contrast with the magnetic field induced criticality in undoped Mott insulator ladders, which is governed by the zero scle-factor universality with its hallmark square root singularities.