tommaso macrì sissa...ultracold diluted fermi systems some numbers: number of particles: 1.000...
TRANSCRIPT
Cold Atom Fermi Systems
Tommaso MacrìTommaso MacrìSissaSissa
February 13February 13thth 2009 2009
Plan of the talk
Cold Atom Fermi Systems
Fermion scattering problem
BCSBEC crossover
Dynamics and Quenching
Tunneling in Cold Atoms
Conclusions
Ultracold Bosons vs Fermions
Quantum statistics effectsQuantum statistics effects
Bosons: condensation
Fermions: quantum degeneracy
Different temperature scalesDifferent temperature scales
Bosons: condensation temperatureBosons: condensation temperature
Fermions: Fermi temperatureFermions: Fermi temperature
Evidence for quantum degeneracyEvidence for quantum degeneracy(De Marco, Papp and Jin, 2001)
Ultracold Diluted Fermi Systems
Some numbers:Some numbers:
Number of particles: 1.000 100.000Number of particles: 1.000 100.000
Temperature: 100 nK – 1Temperature: 100 nK – 1 K K
Density of particles: 10Density of particles: 101111 particles / cm particles / cm3 3 normal metal: 10normal metal: 102323 particles / cm particles / cm33
Dilution parameter: n |aDilution parameter: n |a33| | ~~ 10 1033
What do these numbers tell us?What do these numbers tell us?
Diluted particles interactingDiluted particles interacting relevance of two body physics!relevance of two body physics!
Moreover: Moreover: possibility of tuning the interaction!possibility of tuning the interaction!
Scattering in QM
Quantum scattering problemQuantum scattering problem
Long away from the potentialLong away from the potential
Partial wave expansionPartial wave expansion
Free interactingFree interacting
Partial wave scattering amplitudePartial wave scattering amplitude
Fermion scattering
Identical particle scatteringIdentical particle scattering
Spin wave function: sym.
No swave scattering for polarized fermions!
Low energy scatteringLow energy scattering
Swave dominant
Define a scattering length and an effective radius
3d Square well3d Square wellRepulsive: Repulsive: a > 0a > 0Attractive : barrier height!Attractive : barrier height!
a < 0 a < 0 a > 0 a > 0 : : presence of a bound state
Example
Take the potential:Take the potential:
Explicit solution is available for the sExplicit solution is available for the swavewave
From this result we get:
Where CWhere C00 is a constant depending on is a constant depending on
Feshbach resonances
Tuning the scattering lenghtTuning the scattering lenght
Two channels (e.g. hyperfine states)
Bound state in the closed one
Coupling between the two
Resonance in cold atomsResonance in cold atoms
Tuning parameter: magnetic field
Broad vs Narrow resonances: |R* | b |R* | >> b
Fermionic atoms: 40K and 6Li
Scattering lenght in Scattering lenght in 66LiLi with B = 834 G with B = 834 G (Bourdel et al. 2003)
Interacting manybody problem
Ideal Gas model for Ideal Gas model for spinpolarized fermionsspinpolarized fermions works well. works well.
When different spin states are occupied When different spin states are occupied solution of the interacting solution of the interacting many body problem!many body problem!
Ingredients for cold atomsIngredients for cold atoms
Dilute systemDilute systemWeakly interacting to strongly interacting regime by tuning the interactions!Weakly interacting to strongly interacting regime by tuning the interactions!Interplay among different regimes:Interplay among different regimes:
BCS regime as in superconductivityBCS regime as in superconductivityBoseEinstein condensation as in cold atom bosonsBoseEinstein condensation as in cold atom bosonsUnitary limitUnitary limit
BCS – BEC crossover
BCS regime (a < 0, kF|a|<<1)
Basic features:Basic features:
Instability of Fermi surface in presence of a weak Instability of Fermi surface in presence of a weak attraction leading to attraction leading to CCooper pairsooper pairs
Exact solution of the reduced BCS hamiltonian:Exact solution of the reduced BCS hamiltonian:
Energy spectrumEnergy spectrum
Critical temperatureCritical temperature
(Gorkov 1961)(Gorkov 1961)
Gap at T = 0 Gap at T = 0
Experimental difficulty in reaching critical Experimental difficulty in reaching critical temperature: too low to observe superfluidity for temperature: too low to observe superfluidity for cold atoms!cold atoms!
Example:Example:
Spin singlet pairing
BEC (a > 0, kF a<<1)
Scattering lenght a > 0 and smallScattering lenght a > 0 and small
Naively repulsive gas Naively repulsive gas (Huang and Yang, (Huang and Yang, 1957)1957)
Gas of composite bosons! Gas of composite bosons!
e.g. Presence of a bound state in e.g. Presence of a bound state in square wellsquare well
Dimension of dimers > size of Dimension of dimers > size of molecules (deeply bound)molecules (deeply bound)
a >> a >> ||R*R*||
BEC for condensed gases applicable:BEC for condensed gases applicable:
for a uniform gas
Jila 1995
BEC emerging from Fermi sea
Ketterle (2004)Ketterle (2004)
Unitary regime (kF |a|
KKFF |a| >> 1 with: |a| >> 1 with:
Dilute Fermi gasDilute Fermi gas
Strongly correlatedStrongly correlated
Basic features:Basic features:
Universal behaviorUniversal behavior of the scattering amplitude of the scattering amplitude
Only relevant quantities:Only relevant quantities: kkFF and and
Thermodynamic quantities depend only on T/TThermodynamic quantities depend only on T/TFF
Quantity of interest:Quantity of interest:
Chemical potential at T = 0Chemical potential at T = 0
Pameter Pameter attractive interactionattractive interaction!!
(quantum MC, Carlson et al., 2003)
BCS – BEC crossover
Define the microscopic potential Define the microscopic potential
Minimize Free energy for the Minimize Free energy for the hamiltonian:hamiltonian:
Solve 2 coupled integral equations:Solve 2 coupled integral equations:
For a single parameter potential For a single parameter potential (removing the divergence) (removing the divergence) ((Leggett,1980Leggett,1980))
Determine Determine and and as functions of a as functions of a00
Numerical study for the solution of Numerical study for the solution of the coupled equationsthe coupled equations
BCS – BEC crossover
Energy per particleEnergy per particle
Critical temperatureCritical temperature
Astrakharchik et al. (2004)Astrakharchik et al. (2004) High–Temperature superfluidity High–Temperature superfluidity Sa de Melo, Randeria et al. (1993)Sa de Melo, Randeria et al. (1993)
Up to now: equilibrium properties important for ongoing experimentsUp to now: equilibrium properties important for ongoing experiments But what about dynamics?...But what about dynamics?...
Quenched Fermi Systems
1 particle in harmonic potential1 particle in harmonic potential
Quench the potentialQuench the potential
Compute the dispersion of the packetCompute the dispersion of the packet
N free particles in harmonic potentialN free particles in harmonic potential
Where is N? Not in the frequencyWhere is N? Not in the frequency
What if bosons?What if bosons?
What about interactions? ...What about interactions? ...
Tunneling effects
Tunneling phenomena:Tunneling phenomena: quantum quantum effecteffect!!
Probability of passing through the Probability of passing through the barrier:barrier:
Single particle tunnelingSingle particle tunneling Cooper pair tunnelingCooper pair tunneling
Josephson effect in superconductors
Single particle tunnelingSingle particle tunneling
nn: Ohm's Lawnn: Ohm's Law
ns: j starts from V = ns: j starts from V =
ss: j starts from V = 2ss: j starts from V = 2
Quantum effect involving Quantum effect involving coherencecoherence of the superconductors!of the superconductors!
Where do we see coherence?Where do we see coherence?
Phase dependence of the tunneling Phase dependence of the tunneling currentcurrent
Cooper pairs tunnelingCooper pairs tunneling
Exact formula for the current is Exact formula for the current is availableavailable
Example with Example with = 1= 1
Josephson effect along the crossover
Supercurrent as a function of crossover parametersSupercurrent as a function of crossover parameters
Integrate now over all energies!Integrate now over all energies!
Interaction is not limited among particles around Fermi surfaceInteraction is not limited among particles around Fermi surface
See also recent papers by Links et al. on the Bethe Ansatz solution for a finite number of fermions in the BCS limit
see e.g. Abrikosov (1988)see e.g. Abrikosov (1988)
Conclusions
Manybody physics in cold atom fermions leads to BECBCS crossover
.BECBCS crossover and hightemperature superfluidityBECBCS crossover and hightemperature superfluidity
On going experiments on dynamics of fermions:On going experiments on dynamics of fermions:
Collective oscillationsCollective oscillations
Dynamics of Bose Fermi mixturesDynamics of Bose Fermi mixtures
Dynamics close to unitarityDynamics close to unitarity
Work in progress:Work in progress:
Quench of fermions for polarized / nonpolarized fermionsQuench of fermions for polarized / nonpolarized fermions
Dynamics in double well potentialsDynamics in double well potentials Josephson beyond BCSJosephson beyond BCS