Spin-split states in metals

A new type of Fermi-surface instability of metals is described: As the temperature is lowered below a critical temperature, the Fermi surfaces of up and down electrons shift relative to each other. The low-temperature phase has broken parity but is time-reversal invariant. It is shown that such a state can be driven by a certain off-diagonal matrix element of the Coulomb interaction. A one-dimensional model is solved within the mean-field approximation, and the anomalies associated with the transition in the susceptibility and specific heat are examined. It is suggested that chromium exhibits a realization of this state.