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 $$$: NSF, AFOSR- MURI, Sloan Foundation. Bosons vs Fermions. E F. Bosons. Fermions. - PowerPoint PPT PresentationTRANSCRIPT
Martin Zwierlein
TOPS, MIT, Cambridge, June 24th, 2009
Pairs and Loners inUltracold Fermi Gases
Massachusetts Institute of Technology
Center for Ultracold Atoms at MIT and Harvard
$$$: NSF, AFOSR- MURI, Sloan Foundation
EF
Bosons vs Fermions
e.g.: 1H, 23Na, 6Li2 e.g.: e-, 3He, 6Li, 40K
CT T
CT T
0T
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
How to measure temperature?
Gas Effusive atomic beam
How to measure temperature?
Gas Effusive atomic beam
Atom cloudLens
CCDCamera
Laser beam
Observation of the atom cloud
Shadow imageof the cloud
TrappedExpanded
1 mm
BEC @ MIT, 1995 (Sodium)
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
Fermions – The Building Blocks of Matter
Harvard-Smithsonian Center for Astrophysics
Lithium-6
Can we have superfluidityin a Fermi gas?
1911: Discovery of Superconductors
HeikeKamerlingh-Onnes
• Discovery of Superconductivity in Metals
Res
ista
nce
TemperatureNobel prize 1913
• 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
What are superconductors?
• Apparently the electrical current flows without friction
• But: Carrier of electrical current are Electrons
Electrons are Fermions
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
Fermionic Superfluidity
Superconductors: Charged superfluids of electron pairsFrictionless flow Resistance-less current
Condensation of Fermion Pairs
John Bardeen Leon N. Cooper John R. Schrieffer
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)
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
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
How can you distinguish a superfluid from a normal one?
Rotating bucket
Normal Super
Fluid Fluid
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
Look from top into the bucket
Abrikosov lattice (honeycomb lattice)
Look from top into the bucket
Aleksei A.Abrikosov
Nobel prize2003
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
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
Demonstration of superfluidity in a Fermi gas
Ultracold gas
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
What if there are too many singles?
Fermionic Superfluidity withImbalanced Spin Populations
94%90%56%30%22%12%6%
Fermionic Superfluidity with Imbalanced Spin Populations
|2>
0%
|1>
What is the Nature of theImbalanced State?
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
Reconstruction of 3D density profile
Only assumption: cylindrical symmetry
Phase Separation !
= 0.6
Fermionic Superfluidity does not tolerate loners
Atomic Bose-EinsteinCondensates (Sodium)
Molecular Bose-EinsteinCondensates (6Li2)
Pairs of fermionic atoms(6Li)
Gallery of superfluid Gases
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
The teamBEC 1:
Andre SchirotzekAriel Sommer
Fermi 1:Cheng-Hsun Wu
Ibon SantiagoDr. Peyman Ahmadi
Undergraduates:Caroline FiggattJacob SharpeSara CampbellKevin Fischer
39K 40K 6Li