martin zwierlein
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TOPS, MIT, Cambridge, June 24 th , 2009. Pairs and Loners in Ultracold Fermi Gases. Martin Zwierlein. Massachusetts Institute of Technology Center for Ultracold Atoms at MIT and Harvard $$$: NSF, AFOSR- MURI, Sloan Foundation. Bosons vs Fermions. E F. Bosons. Fermions. - PowerPoint PPT PresentationTRANSCRIPT
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Martin Zwierlein
TOPS, MIT, Cambridge, June 24th, 2009
Pairs and Loners inUltracold Fermi Gases
Massachusetts Institute of Technology
Center for Ultracold Atoms at MIT and Harvard
$$$: NSF, AFOSR- MURI, Sloan Foundation
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EF
Bosons vs Fermions
e.g.: 1H, 23Na, 6Li2 e.g.: e-, 3He, 6Li, 40K
CT T
CT T
0T
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Degenerate gases
Want lifetime > 1s Ultradilute
Ultracold
de Broglie wavelength ~ Interparticle spacing
Good news: Bosons condense at
1/3n1/3
dB n
15 310 cmn 2
2/3 1 KFB
T nk m
C FT T
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How to measure temperature?
Gas Effusive atomic beam
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How to measure temperature?
Gas Effusive atomic beam
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Atom cloudLens
CCDCamera
Laser beam
Observation of the atom cloud
Shadow imageof the cloud
TrappedExpanded
1 mm
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BEC @ MIT, 1995 (Sodium)
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BEC @ JILA, Juni ‘95(Rubidium)
BEC @ MIT, Sept. ‘95 (Sodium)
Superfluidity in Bosonic Gases
• BEC 1995All atoms occupy same
macroscopic wavefunction
MIT
• Phase coherence 1997
JILA
ENS
MIT• Superfluidity 1999/2000Frictionless flow,
quantized vorticity
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Fermions – The Building Blocks of Matter
Harvard-Smithsonian Center for Astrophysics
Lithium-6
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Can we have superfluidityin a Fermi gas?
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1911: Discovery of Superconductors
HeikeKamerlingh-Onnes
• Discovery of Superconductivity in Metals
Res
ista
nce
TemperatureNobel prize 1913
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• No energy loss• persistent flow• Doesn’t want to rotate
• No energy loss• persistent currents• expels magnetic fields
Flow without friction Current without resistance
Onnes 1908,Kapitza, Allen & Misener 1938
Onnes 1911Müller & Bednorz 1987
SuperconductorsSuperfluids
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What are superconductors?
• Apparently the electrical current flows without friction
• But: Carrier of electrical current are Electrons
Electrons are Fermions
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What are superconductors?
• Apparently the electrical current flows without friction
• But: Carrier of electrical current are Electrons
Electrons are Fermions
L. Cooper (1956) (45 years after Onnes):
Pairing of electronsPairs are Bosons
Superconductivity: Condensation of Electron Pairs
J. Bardeen, L. Cooper, R. Schrieffer (BCS), 1957, Nobel prize 1972
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Fermionic Superfluidity
Superconductors: Charged superfluids of electron pairsFrictionless flow Resistance-less current
Condensation of Fermion Pairs
John Bardeen Leon N. Cooper John R. Schrieffer
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High-temperature Superconductors
J. Georg Bednorz K. Alex Müller
Nobel prize 1987
Critical temperature:35 K above Absolute Zero (-238 °C)
Record today:138 K (-135 °C)
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Room temperature superconductors?
Today:• ~5-10% energy loss only due to transport of energy
The problem:High-temperature superconductivity not really understood
Electrons interact so strongly that it’s hard to model
The hope:• Superconducting cables• No resistance No energy loss during transport
We need:A model system for superconductors
Ultracold atomic gases
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Can we do this with atoms?YES! The ultracold Fermi gas at MIT:• Lithium-6 (3p, 3n, 3e-) is a fermion
• The atoms form pairs likeelectrons in a superconductor
• Size of pairs isfreely controllable
• The gas becomes superfluid
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How can you distinguish a superfluid from a normal one?
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Rotating bucket
Normal Super
Fluid Fluid
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Rotating superfluid
Superfluids are described by matter wave
The wave has to close in itself(Example: Vibrating rubber band)
Superfluid does not want to rotate Only possibility:
Vortices, “Mini-Tornados”, “Quantum whirlpools”
Only full wavelengths are allowed
Circulation is only possible in certainunits (“Quanta”), carried by the Vortices
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Look from top into the bucket
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Abrikosov lattice (honeycomb lattice)
Look from top into the bucket
Aleksei A.Abrikosov
Nobel prize2003
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Vortex latticesVortex latticesin bosonic gases/fluidsin bosonic gases/fluids
ENSENS(J. Dalibard, 2000)(J. Dalibard, 2000)
Rubidium BECRubidium BEC
BerkeleyBerkeley(R.E. Packard, 1979)(R.E. Packard, 1979)
Helium-4Helium-4
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U. Essmann and H. Träuble,Physics Letters A, 24, 526 (1967)
Rotation of a neutral Fluid
Coriolis Force
Superconductor in a magnetic field
Lorentz Force
2F mv
F qv B
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Demonstration of superfluidity in a Fermi gas
Ultracold gas
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Vortex lattices
M.W. Zwierlein, J.R. Abo-Shaeer, A. Schirotzek, C.H. Schunck, W. Ketterle,Nature 435, 1047-1051 (2005)
- 0.7
B
• Demonstration of superfluidity in a gas of atom pairs• A high-temperature superfluid
Pair size
Scaled to the density of electrons in a metal, the gaswould become superfluid far above room temperature
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What if there are too many singles?
Fermionic Superfluidity withImbalanced Spin Populations
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94%90%56%30%22%12%6%
Fermionic Superfluidity with Imbalanced Spin Populations
|2>
0%
|1>
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What is the Nature of theImbalanced State?
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Cooling Down
Direct observation of the density difference
Y. Shin, M.W. Zwierlein, C.H. Schunck, A. Schirotzek, W. Ketterle,PRL 97, 030401 (2006)
SuperfluidNormal
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Reconstruction of 3D density profile
Only assumption: cylindrical symmetry
Phase Separation !
= 0.6
Fermionic Superfluidity does not tolerate loners
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Atomic Bose-EinsteinCondensates (Sodium)
Molecular Bose-EinsteinCondensates (6Li2)
Pairs of fermionic atoms(6Li)
Gallery of superfluid Gases
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Ultracold Atoms
As Model systems:• How does matter work?
new quantum states, development of new materialsQuantum computer, Quantum simulators (Bose and Fermi gases)
As measuring device:• Development of highly sensitive sensors
gravitational gradient sensors (important for mining, geophysics),sensors for navigation
• New highly accurate atomic clocks as time standardbasis of all GPS-systems, more accurate positioning, faster telecommunication requires accurate clocks
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The teamBEC 1:
Andre SchirotzekAriel Sommer
Fermi 1:Cheng-Hsun Wu
Ibon SantiagoDr. Peyman Ahmadi
Undergraduates:Caroline FiggattJacob SharpeSara CampbellKevin Fischer
39K 40K 6Li
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