why Rb85 is difficult to bose-condense?

wdlang

some guy told me Rb87 is easy to bose-condense, while Rb85 is very difficult.

i do not why.

i guess these two isotopes share almost the same internal levels and atom-atom interactions.

genneth

Recall that the usual trick is to play games with hyperfine levels --- these depend on the nuclear moments and thus the nucleon number.

Cthugha

The scattering length for Rb87 is positive while it is negative for Rb85. This means that at low temperature Rb87 atoms will on average repel each other, while Rb85 atoms will on average attract each other. This attractive interaction severely limits the possible number of atoms, which can condense, because the condensate will contract and finally collapse as more atoms are added. However, one can apply a magnetic field near a Feshbach resonance to tune the magnitude and sign of the scattering length to some extent.

wdlang

Thanks a lot!

I do not even know this fact before.

wdlang

Thanks for directing the way for me.

tirrel

ok... but:

- why does the scattering length have this behaviour? I guess it is connected to what genneth said but precisely?;

- what do you mean by "contracting"? is it a technical expression?

- why, given an attractive interaction, a gas should condensate at lower temperature then in absence of interaction? (here I think it is because it can host more particles at lower energy without involving the lowest energy state)

Cthugha

To be honest, I do not know. I am a "solid-stater". Maybe someone with some experience in atom optics can answer that.

The condensate is literally contracting. On increasing the number of atoms, the attractive interaction will increase and they move closer to each other.

Might be. Again I am not sure.

Additionally most bosons used to produce BEC are only composite bosons. At large interparticle spacings the substructure will not play a role. However it does as the average distance between particles gets smaller.