Quantum critical behavior of clean itinerant ferromagnets

We consider the quantum ferromagnetic transition at zero temperature in clean itinerant electron systems, We find that the Landau-Ginzburg-Wilson order parameter field theory breaks down since the electron-electron interaction leads to singular coupling constants in the Landau-Ginzburg-Wilson functional, These couplings generate an effective long-range interaction between the spin or order parameter fluctuations of the form 1/r(2d-1), With d the spa tial dimension. This leads to unusual scaling behavior at the quantum critical point in 1 < d less than or equal to 3, which we determine exactly, We also discuss the quantum-to-classical crossover at small but finite temperatures, which is characterized by the appearance of multiple temperature scales, A comparison with recent results on disordered itinerant ferromagnets is given.