Why Rb85 is difficult to bose-condense?

  • Thread starter wdlang
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In summary, Rb87 is easier to bose-condense than Rb85 due to its positive scattering length, which causes atoms to repel each other at low temperatures. On the other hand, Rb85 has a negative scattering length, leading to an attractive interaction that limits the number of atoms that can condense. However, this can be manipulated by applying a magnetic field near a Feshbach resonance. The scattering length is dependent on the nuclear moments and nucleon number, and the condensate will contract as more atoms are added. The exact reason for this behavior is not known, but it may be related to the ability to host more particles at lower energy levels.
  • #1
wdlang
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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.
 
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  • #2
Recall that the usual trick is to play games with hyperfine levels --- these depend on the nuclear moments and thus the nucleon number.
 
  • #3
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.
 
  • #4
Cthugha said:
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.

Thanks a lot!

I do not even know this fact before.
 
  • #5
genneth said:
Recall that the usual trick is to play games with hyperfine levels --- these depend on the nuclear moments and thus the nucleon number.

Thanks for directing the way for me.
 
  • #6
Cthugha said:
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.

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)
 
  • #7
tirrel said:
- why does the scattering length have this behaviour? I guess it is connected to what genneth said but precisely?;

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

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

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

tirrel said:
- 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)

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.
 

1. Why is Rb85 difficult to bose-condense?

Rb85 is difficult to bose-condense because it has an odd number of protons and neutrons, which results in a half-integer spin. This type of spin does not allow for the formation of a Bose-Einstein condensate, which requires particles with integer spin.

2. Can Rb85 be bose-condensed at all?

No, Rb85 cannot be bose-condensed due to its half-integer spin. Only particles with integer spin, such as Rb87, can form a Bose-Einstein condensate.

3. Are there any ways to overcome the difficulty of bose-condensing Rb85?

There are currently no known methods to overcome the difficulty of bose-condensing Rb85. Scientists have been exploring different techniques, such as using optical lattices, to try and manipulate the spin of Rb85, but so far, none have been successful.

4. What are the potential applications of bose-condensing Rb85?

Bose-condensing Rb85 could have potential applications in quantum computing and precision measurements. It could also provide insights into understanding the behavior of other half-integer spin systems.

5. Is there ongoing research on Rb85 and bose-condensation?

Yes, there is ongoing research on Rb85 and bose-condensation. Scientists are continually exploring new techniques and approaches to try and overcome the difficulty of bose-condensing Rb85. This research could lead to new breakthroughs and a better understanding of the fundamental properties of matter.

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