Magneticfield control of ultracold molecular interactions

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Magnetic-field control of ultracold molecular
interactions
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Introduction

• The Group
• Computational Quantum Chemists/Physicists
• Durham University
• Jeremy Hutsons Group

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Introduction

• Two main activities
• Calculating Potential Energy Surfaces (PESs)
• Scattering calculations

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Structure of Presentation

• Motivation
• Why ultracold matter?
• Background
• Terminology and Basic ideas
• Some recent results
• Calculations on 3/4He16O2

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How cold is ultracold?

• Electronic excitation
• Rotational excitation
• Quantum and ultracold regime
• Bose-Einstein condensation

- 1000K -1K -1mK -1µK -1nK
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Why ultracold matter?

• Novel quantum mechanical effects (Condensation)
• Fundamental constants (standard model)
• Dipole moment of electron
• Fine-structure constants
• Ultracold chemistry (control of reaction)
• High-resolution spectroscopy

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Why Ultracold matter?

• Molecules versus Atoms
• Richer structure than atoms ( e.g. dipole moment)
• Interesting many-body physics
• Fundamental constants
• But
• Richer structure ? more difficult to cool and
  control

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Quantum Regime

• HF Bond length 0.92 Angs
• de Broglie wavelength ? h/v(3mkT)?

T ?/Å
300K 1.03 1
K 18 1mK 565 1µk 18
e4 1nK 5.6 5 e10
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Fields

• Effects of Fields
• Introduce coupling between states (tuneable)
• Break symmetries Changing manifold of energy
  levels
• Change the energy (reaction thresholds etc.)

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Some experimental challenges
Trap loss ? sel/sinel
Association of ultracold atoms ? Feshbach
Resonance
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Scattering Channels

• Channels

A1, A2, . . . . An
A
B1, B2, . . . . Am
B
Channel A1B1, A1B2,....,AiBj,.AnBm
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Scattering Channels
Before Collision AkBi
After Collision AlBj
Bj
Bi
Al
Ak
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Scattering Cross sections

• Collisions cause Transition AkBi?AlBj
• For inelastic collisions skilj(E) a
  P(AkBi?AlBj)
• We calculate cross sections of atom-molecule
  systems

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Scattering Cross sections and Resonances
Area
Energy
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Scattering What are Resonance?
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Resonances Bound states in continuum
P.E.C of a single channel z
Z
Shape Resonance
Feshbach Resonance
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Feshbach Resonance Tuning
Magnetic Field
Collision / K.E Eres ? Resonance
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Scattering Tuning Feshbach Resonance
Resonance
Y
X
?E
E(B)
Bres
K.E ?E ? Resonance
?E (Bres) 0 ? Zero energy Res
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Scattering Tuning Feshbach Resonance
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Scattering Resonances

• Important points to note
• Cross sections vary sharply across a resonance
• We can tune states including resonances
• So we can tune sel and sinel

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Recent calculations

• 3He 16O2

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Bound states 3He 16O2
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Energy / cm-1
Magnetic Field /Gauss
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Recent calculations

• 4He 16O2

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Conclusion

• Crucial results
• It is possible to tune the elastic to inelastic
  cross section
• For 4He16O2 we have tuned the ratio from 1 to
  200, and this occurs over 1000 Gauss

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Acknowledgements

• Professor Jeremy Hutson
• Group members

END
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Understanding the results
Differences in the two systems
Quantum numbers of resonance states and threshold
Coupling strengths between resonances and
thresholds
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Understanding More general
Determining factors of
The background elastic/inelastic scattering
The dependence of cross sections on magnetic
field
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Scattering Shape Resonances