The single-Q state of chromium

It is shown that in an idealized but realistic model for the band structure of chromium, the single-Q sinusoidal antiferromagnet state must be lower in free energy than other antiferromagnet states including a cubic one. Calculations beyond the idealized model show that the only requirement for this to occur is that on each face of the electron or hole quasi-octahedrons important for the condensate, the root mean square Fermi-velocity average (v f perpendicular to 2 ) 1/2 perpendicular to the (1,1,1) direction in that octant must be much smaller than (v f //). Even with the perfect-octahedron model, and with no adjustable parameters, calculated values for the condensate amplitude and the temperature dependence of the vector Q(T) are within 20% of experimental results. The order of the phase transition at T N and the character of the macroscopic quantum condensate are discussed.