coherent quantum gases - from cold atoms to condensed ... · -1 0 1 650 834.15-20-10 0 10 20 x10....
TRANSCRIPT
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Center for Ultracold
Atoms at MIT and Harvard
Radio Frequency Spectroscopy of Ultracold
Fermi Gases (1)
Andre Schirotzek
Coherent Quantum Gases - From Cold Atoms to Condensed MatterCopenhagen, Jan. 15th/16th 2009
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OutlinePart 1: 1.) Introduction
-
strongly interacting Fermions-
effect of density imbalance
-
rf
spectroscopy
2.) Experiments using rf
spectroscopy:Problems and solutions
Part 2: 3.) Quantitative studies with rf
spectroscopy
a)
Quasiparticle
spectroscopy and determination of the superfluid
gap
b) The ‘N+1’
body problem: Observation of polarons
in a highly imbalanced Fermi gas
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Pairing and Superfluidity of Fermions
Formation of Pairs
Fermi mixtureInteracting
Condensation Superfluidity
L. Cooper (1956):
Bound State
Condensed Matter PhysicsSuperconductivitySuperfluidity in 3He Nuclear Physics
Superfluidity inside nuclei(protons & neutrons)
AstrophysicsDense quark matter
Quark (Color) SuperfluidityNeutron star
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Ultracold Atom System: Interacting Fermi Mixture
An Ideal Model System: A New Sample
No impurities
Temperature
Dimensionality
Interaction strength
Density
Highly controllable:
Ultradilute
Ultracold
TF = 0.1~10 μK
T/TF < 0.05
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-1
0
1
834.15650-20
-10
0
10
20x103
-1
0
1
834.15650 [G]Magnetic Field
Ener
gy[M
Hz]
Tuning Interactions: Feshbach ResonancesBetween the two lowest hf states of 6Li
a0
]Scattering Length [
Atoms formstable molecules
Atoms formstable molecules
Isolated atom pairsare unstable
Isolated atom pairsare unstable
bound state
Scattering length a>0Bound state in
interatomic potentialBEC
limit: Condensation of
tightly
bound fermion pairs
Scattering length a<0:Presence of Fermi sea
leads to Cooper pairingBCS-limit: Condensation oflong-range Cooper pairs
UnitarityEF
only relevant energy scale
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The BEC-BCS
Crossover
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The BEC-BCS
Crossover
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The experiment
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Optical trapping @ 1064 nm
νaxial
= 22 Hz
νradial
= 160 HzEtrap
= 0.5 -
5 μKStates |1> and |3> correspond to
Pseudospin
|↑> and |↓>
Preparation of an interacting Fermi system in 6Li6Li -
Atom:
6 hyperfine states
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Cool fermionic lithium-6using sodium as a refrigerator
107
atoms in BEC (w/o Li) 5 x 107
Li atoms at
Z. Hadzibabic
et.al., PRL
91, 160401 (2003)
Source of ultracold fermions
Bosons Fermions
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From Fermion Pair Condensation
to High Temperature Superfluidity
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M.W. Zwierlein
et al, PRL
91, 250401 (2003)
BEC of Fermion Pairs (“Molecules”)
Boulder
Nov ‘03 Innsbruck Nov ‘03, Jan ’04
MIT
Nov ’03 Paris March ’04
Rice, Duke
These days: Up to 10 million condensed molecules
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C.A. Regal et al., PRL
92, 040403 (2004) M.W. Zwierlein
et al.,
PRL
92, 120403 (2004)
T/TF
= 0.05T/TF
= 0.1T/TF
= 0.2
At
900 G
Observation of Pair Condensates
Condensate Fraction
BEC BCS
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Vortex lattices in the BEC-BCS
crossover
M.W. Zwierlein
et al, Nature 435, 1047-1051 (2005)
Establishes superfluidity and
phase coherencein gases of fermionic
atom pairs
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Interaction Parameter 1/kF
a0 -11
TC
/TF
-
∞∞
TBEC
/TFSuperfluid 3He,normal and High-TC superconductors
High-TC
Superfluid
High Temperature Superfluid at 100 nK?
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What if there are too many singles?
Fermionic
Superfluidity
with Imbalanced 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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Increase population imbalance
The Clogston-Chandrasekhar limit
↑⟩⎜
↓⟩⎜
Majority component
Minority component
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Phase Diagram for Unequal Mixtures
Breakdown: Critical polarization Pc
∝
Gap ΔBCSBEC
P
M.W. Zwierlein
et al, Science 311, 492 (2006)
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Radiofrequency spectroscopyNo interactions
↑⟩⎜
↓⟩⎜
f ⟩⎜
0ωh
0 2B kEω ω ε= + +h h
Photon energy = Zeeman + Binding + Kinetic energy
Molecular Pairing
↑⟩⎜
↓⟩⎜0 BEω +h
f ⟩⎜
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C.Chin
et al. Science, 305, 1128 (2004)
Loss in |2>
[kHz]ω
BE
0 2B kEω ω ε= + +h h
Photon energy = Zeeman + Binding + Kinetic energy
Radiofrequency spectroscopyMolecular Pairing
↑⟩⎜
↓⟩⎜0 BEω +h
f ⟩⎜
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Radiofrequency spectroscopy
0 BEω +h
Photon energy = Zeeman + Quasiparticle
+ Kinetic energy
↑⟩⎜
↓⟩⎜
3⟩⎜
0 BEω +h
0 k kEω ω μ ε= + − +h h
Onset at 2
0 2 FEω ω Δ
= +h h
Binding energy per particle:
F
2
B EE Δ
∝
(in the BCS
limit)
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RF Spectroscopy vs
Tunneling
freeparticledispersion
quasiparticledispersionF
2
E2Δ
1.0
0.5
0.0
-0.51.00.50.0
Δ
−+ω=ω k0 Ehh )( kε−μ
Fkk
F
k
EE
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EF
2
0 2 FEω ω Δ
− =h h
Ek
BCS
limit, onset at:
RF
Photon
Final state empty:Connection to tunneling experiments
1.0
0.5
0.0
-0.51.00.50.0
Δ
RF
spectroscopy directly measures the binding energy
of fermion
pairs (not the gap)
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Connection to tunneling experiments
Superconductor Normal metalInsulator
EF
Ek
Δ
eV = ΔOnset at: Source: Tinkham,
Introduction to Superconductivity
Transfer at Fermi surface k=kF
Final state (metal):
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OutlinePart 1: 1.) Introduction
-
strongly interacting Fermions-
effect of density imbalance
-
rf
spectroscopy
2.) Experiments using rf
spectroscopy:Problems and solutions
Part 2: 3.) Quantitative studies with rf
spectroscopy
a)
Quasiparticle
spectroscopy and determination of the superfluid
gap
b) The ‘N+1’
body problem: Observation of polarons
in a highly imbalanced Fermi gas
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Previous Experiments on RF
Spectroscopy
First evidence a pairing gap, Grimm group:
Equal Mixture
C. Chin et al., Science 305, 1128 (2004)
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Previous Experiments on RF
Spectroscopy
Spectral gap in the normal phase, MIT group:
1 2
T
Super
Normal
1
2
P
C.H. Schunck
et al, Science 316, 815 (2007)
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Problems of Previous RF
experiments I
RF spectroscopy with equal mixtures
Num
ber d
iffer
ence
(a.u
.)
f-f0
(kHz)
• Ultracold atom sample confined in a harmonic trap• Inhomogeneous density profiles
Density broadening of the spectroscopic signalDifficult to compare with theoretical predictions
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In situ phase contrast images measure the depletion region
RF Spectroscopy with in situ Images
Sig
nal
(a.u
.)
Radio-frequency offset (kHz)
Y. Shin et al, PRL
99, 090403 (2007)
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3d Density Reconstruction
Cylindrical symmetry, Inverse Abel transformation:
Tomographic
RF spectroscopy:
3D reconstruction
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Spatially resolved RF Spectrum
Inhomogeneous spectrumHomogeneous spectrum
at trap center
BUT: Still not the shape of a dissociation spectrum!
Y. Shin et al, PRL
99, 090403 (2007)
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Problems of Previous RF
experiments II
BEC BCS
C.H. Schunck
et al, Nature 454, 739 (2008)
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RF Spectroscopy: Final State Interactions
|1>
|3>
|2>
Final state interactions can strongly modify the spectrum
e.g. transition from 1-2 molecule to 1-3 molecule
How to change the final state interactions without changing the initial state?
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Figure: M. Bartenstein
et al., PRL 94 103201 (2005)
3 candidates for high Tc
Superfluids in 6Li
Mixture Resonance
(1,2) 834 G
(2,3) 811 G
(1,3) 690 G
Lifetime of all mixtures exceeds 10 s
We observe fermion pair condensation in all mixtures: Two new high Tc superfluids
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3 high Tc
Superfluids for Rf Spectroscopy
Mixture Final state scattering length at unitarity
(1,2) a13
= -
3290 a0
(2,3) a13
= -
3560 a0
(1,3) a23
= +1140 a0
a12
= +1450 a0
afinal
: large and negative
|1>
|3>
|2>
|1>
|3>
|2>
afinal
: smaller and positive
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Dramatic effect of final state interactions
(1,3) mixture at unitarity (691 G)
a23
= + 1140 a0
200 kHz “tail”
to higher frequencies
(1,2) mixture at unitarity (833 G)
a13
= -
3290 a0
< 20 kHz linewidth
εF ~ h*30 kHz, T/TF ~ 0.1C.H. Schunck
et al, Nature 454, 739 (2008)
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Rf
spectroscopy in the (1,3) mixture
BEC BCSResonance
C.H. Schunck
et al, Nature 454, 739 (2008)
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Experiments on RF
Spectroscopy
Photoemission Spectroscopy, extraction of Δ
and μ, Jin group:
0
10
20
-10
-20
-30Sin
gle-
parti
cle
ener
gy (k
Hz)
0 105 15k (μm-1)
J. Stewart et al, Nature 454, 744 (2008)
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Center for Ultracold
Atoms at MIT and Harvard
Radio Frequency Spectroscopy of Ultracold
Fermi Gases (2)
Andre Schirotzek
Coherent Quantum Gases - From Cold Atoms to Condensed MatterCopenhagen, Jan. 15th/16th 2009
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OutlinePart 1: 1.) Introduction
-
strongly interacting Fermions-
effect of density imbalance
-
rf
spectroscopy
2.) Experiments using rf
spectroscopy:Problems and solutions
Part 2: 3.) Quantitative studies with rf
spectroscopy
a)
Quasiparticle
spectroscopy and determination of the superfluid
gap
b) The ‘N+1’
body problem: Observation of polarons
in a highly imbalanced Fermi gas
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Quantitative Studies
Address open questions, Superfluid
state:
-
Magnitude of Δ
- Local double peak structure? evidence of quasiparticles
Presence of important Hartree energy
- “Pairing”
in the normal phase?Need majority spectra
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Motivation I: Quasiparticles?
Shin et al, PRL
97, 030401 (2006)
Polarized Superfluid=
Finite Quasiparticle
Occupation
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Motivation II: The Superfluid
Gap
Direct measure of the gap Δ
Note: Only difference to tunneling:Stokes vs
Anti-Stokes
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Motivation III: Minority Spectra
T
Super
Normal
1
2
P
1 2
Minority Spectra do NOT reveal the onset of superfluidity
Full pairing of minority cloud?Bosons which do not condense even at T=0 ?
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Ingredient: Imbalanced Fermi Mixtures
94%90%56%30%22%12%6%
|2>
0%
|1>
Increasing imbalance
-
System remains superfluid
up to a critical imbalance
-Two (of several) ways to destroy Superfluidity:- Imbalance- Temperature
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Phase Diagram of a Polarized Fermi Gas
Spin polarization:
↓↑
↓↑
+−
nnnn
Y. Shin et al,Nature, 451,689 (2008).
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Quasiparticles
in equal mixtures?
We were not able to resolve a local double peak structure between 0.1 < T / TF
< 0.55
How can we create quasiparticles?
420420420
spec
tral r
espo
nse
/ a.u
.
420RF offset / εF↑
A DCB
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Solution: Low Temperature QuasiparticlesGenerate quasiparticles
at the minimum energy
of the dispersion curve:
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RF spectra for varying local imbalances
Elliptic Average
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RF spectra for varying local imbalances
Equal density superfluid
Polarized transition region
Highly polarized normal phase
Polarized superfluid
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Polarized SuperfluidBCS Theory: Excess Fermions = Quasiparticles
→
local double peaks in RF spectrum-U
BUT:Note that the quasiparticlepeak is at ωRF
> 0 and
not at the expected position -
Δ
Hartree
terms U introducean overall shift of theRF spectrum
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Two Peaks = Two Equations
UΔ −−
μ)U,,(ω34
th Δ
0.15=
0.75=
finalE+
finalE+
(I)
(I)
(II)
(II)
Δ
= 0.44(3) EF
U = -
0.43(3) EF
(μ ≈ 0.42 from QMCor experiment)
A. Schirotzek et al, PRL
101, 140403 (2008)
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Hartree
Terms: Where do they come from?
Hamiltonian in general difficult: 4321 ccccH ∝ † †
Hartree-Fock, replace 4-fermion-operator by best 2-fermion-operator you can find:
∝H 32414132 cccccccc +† † † † Hartree(direct)
31424231 cccccccc −− † † † † Fock(exchange)
214343
*
21 cccccccc321321
Δ
+
Δ
+ †† ††Non-zero only in BCS
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Hartree
Terms: Where do they come from?
∑ Δ+Δ− ↑↓−↓−↑k
kk*
kk )cccc( † †
BCS
Hamiltonian:↑−↓↓−↑−= 'k'kkk0BCS ccccVV † †
σσσ
∑ −μ−ε=μ− kkk
k cc))U((NHFree Energy:
†
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Hartree
Terms: Where do they go?
Quasiparticle
excitation spectrum:
22kk ))U((E Δ+−μ−ε±=
Quasiparticle
operator (Bogoliubov):
↓−↑↑ γ−γ= k*kkkk vuc† †
excess Fermion = Quasiparticle
BCSBCSc kk ↑↑ γ∝† †
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Hartree
Terms: Where do they go?
RF
spectrum (from FGR):
(...))( 2th ×
ωω−ω
∝ωΓ
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Hartree
Terms: Where do they go?
RF
spectrum, including Hartree
energy U:
(...)'
'')( 2th ×
ωω−ω
∝ωΓ
with:
Same functional form. Main effect:Shifting the x-axis
U'U'
−μ=μ+ω=ω
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The importance of Hartree
Terms
(I)Δ
= 0.76 EF
(again: μ ≈ 0.42)
(I) 75.0E),(34
finalth =+Δμω
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Peak Positions in the Polarized Superfluid
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Conclusions, a)
• Comparison of Majority and Minority RF spectrareveal a change in pairing character
• Polarized Superfluid: Observation of Quasiparticles
• Position of Quasiparticle
peak highlights importanceof Hartree
terms U in RF spectroscopy
• Double peak spectrum allows determinationof Δ
and U
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OutlinePart 1: 1.) Introduction
-
strongly interacting Fermions-
effect of density imbalance
-
rf
spectroscopy
2.) Experiments using rf
spectroscopy:Problems and solutions
Part 2: 3.) Quantitative studies with rf
spectroscopy
a)
Quasiparticle
spectroscopy and determination of the superfluid
gap
b) The ‘N+1’
body problem: Observation of polarons
in a highly imbalanced Fermi gas
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Quantitative Studies
Address open questions, Normal
state:
-
Magnitude of Fermi liquid parameters (μ, Z, m*, F)
-
Critical interaction strength Fermi liquid Bose liquid ?
BCSBEC
P
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Swimming in the Fermi seaWhat is the fate of a single
impurity in a Fermi sea?
Crucial question for• electron transport in lattices•
Kondo problem
(single magnetic impurity)•
mobility of 4He in 3He
…
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Polarons•
Historically: e-
interacting with ion lattice
L. D. Landau, Phys. Z. Sowjetunion 3 664 (1933).•
Quasi-particle:
particle dressed by surrounding•
Long lifetime•
“Dressed”
energy•
Quasiparticle
residue or Weight Z•
Effective mass m*•
Same question asked across all of physics for different types of
particles, environments, and coupling between them (colossal magnetoresistance, the pseudo-gap phase of High-TC
superconductors, fullerenes, polymers, etc…)
Polarons: The “N+1”-body problem
Theory: Landau, Froehlich, Feynman, Anderson
Every man, wherever he goes, is encompassed by a cloud of comforting convictions, which move with him like flies
on a summer day.(Bertrand Russell, 1919)
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Molecule Mean FieldPolaron
strong attraction weak attraction
Swimming in the Fermi Sea
24 a nm
π↑
h
Theory: Chevy, Lobo, Giorgini, Stringari, Prokof’ev, Svistunov, Sachdev, Sheehy,
Radzihovsky, Lamacraft, Combescot, Sa de Melo
2
2mah
Energy?
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Swimming in the Fermi sea
A single atom immersed in a cloud with unitarity
limited interactions
Binding energy must be universal
F. Chevy PRA 74, 063628 (2006), Variational
Cooper pair Ansatz
C. Lobo, A. Recati, S. Giorgini, S. Stringari, PRL
97, 200403 (2006), Monte-Carlo
↑⟩⎜↓⟩⎜
μ↓
= γ EF↑ γ = −
0.6
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Swimming in the Fermi sea
F. Chevy PRA 74, 063628 (2006), Variational
Cooper pair Ansatz
C. Lobo, A. Recati, S. Giorgini, S. Stringari, PRL
97, 200403 (2006), Monte-Carlo
Polaron:
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Swimming in the Fermi sea
F. Chevy PRA 74, 063628 (2006), Variational
Cooper pair Ansatz
C. Lobo, A. Recati, S. Giorgini, S. Stringari, PRL
97, 200403 (2006), Monte-Carlo
Polaron:
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2
1 11 1 1 1 1
4 2 2F
F F
q k
k k q k
Em
a Eπ ε ε ε ε ε<
> <−
=Ω ⎛ ⎞
+ − −⎜ ⎟⎜ ⎟Ω − + − Ω⎝ ⎠
∑∑ ∑
k q q k k qh
Polaron
Energy
Polaronbare
Molecule
‐1/kF
a
E/EFMolecular limit Mean-field limit
Meanfield
Diagrammatic
Monte‐Carlo
Diagrammatic Monte‐Carlo: N. V. Prokof'ev and B. V. Svistunov, PRB 77, 125101 (2008)
Variational
approach/T‐Matrix: F. Chevy PRA 74, 063628 (2006), R. Combescot
and S. Giraud, PRL
101, 050404 (2008), R. Combescot
et al.,PRL
98, 180402 (2007), T‐matrix/ladder approximation: P. Massignan, G. Bruun
and H. Stoof, PRA 78, 031602 (2008)
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‐1/kF
a
Polaron
Quasi‐particle weight2
0
2
11 11
1 1 1 1 14 2 2
F
F F
q k
k k q k
Z
E ma E
ϕ
π ε ε ε ε ε<
> <−
= =∂
−∂ Ω ⎛ ⎞
+ − −⎜ ⎟⎜ ⎟Ω − + − Ω⎝ ⎠
∑∑ ∑
k q q k k qh
20
1
1 ( )Z
EE
ϕ= =∂
− Σ∂
F. Chevy PRA 74, 063628 (2006), Variational
Cooper pair Ansatz
C. Lobo, A. Recati, S. Giorgini, S. Stringari, PRL
97, 200403 (2006), Monte-Carlo
or
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Experimental Realization6Li -
Atom:
6 hyperfine states
| >
| >
|free>
• Spatially resolved• 3D reconstructed
RF
spectroscopy
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RF
Spectra with high density imbalance
BEC
limit towards Unitarity
A. Schirotzek et al., in preparation
76543210 6543210
rf energy / εF
atom
tran
sfer
/ a.
u.
43210
a) c)b) 1/kFa = 0.43(1)1/kFa = 0.76(1) 1/kFa = 0
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High energy
High momentum
Short range physicsScattered states
(particle-hole excitations)
Polaron
weight: Ζ
Spectrum @ 680 G
Interpretation of the spectra
Narrow peaklong lifetime
Narrow peaklong lifetime
Polaron
= Free particle + scattered statesZ 1 Z−
Radio frequency
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Z for various interaction strengths
Interaction Strength 1/kF
a
Polaron
weight 40% on resonance
•
Polarons
become
molecules for a
critical interaction strength: 0.76
•
Breakdown of
Fermi liquid•
Coincides with
critical strength for
always having a
BEC
|1> majority|3> majority
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Diagrammatic Monte‐Carlo: N. V. Prokof'ev and B. V. Svistunov, PRB 77, 125101 (2008)
Variational
approach/T‐Matrix: F. Chevy PRA 74, 063628 (2006), R. Combescot
and S. Giraud, PRL
101, 050404 (2008), R. Combescot
et al.,PRL
98, 180402 (2007), T‐matrix/ladder approximation: P. Massignan, G. Bruun
and H. Stoof, PRA 78, 031602 (2008)
Molecule limit Meanfield
limit
No final state
interactions
Including final state
interactions
Polaron
Energy vs
Interaction Strength
Peak position|1> majority|3> majority
Bare molecule
Mean-field
Pol
aron
ener
gy /
ε F
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Conclusion
Molecular limit towards Unitarity
• Observation of Spin-Polarons
in a new Fermi liquid with tunable interactions• Benchmark test for many-body theories•
Very good agreement in the binding energy
• Determines the low-temperature phase diagram of imbalanced Fermi gases• Future: Dynamic properties. Measure effective mass m*.
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Martin Zwierlein,
Andre Schirotzek,
ChristianSchunck
Wolfgang Ketterle,
Yong-Il Shin
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The team
Potassium-40
BEC
1:Andre SchirotzekAriel Sommer
Fermi 1:Cheng-Hsun
WuIbon
SantiagoDr. Peyman
Ahmadi
Caroline FiggattJacob Sharpe
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Center for Ultracold
Atoms at MIT and Harvard
Radio Frequency Spectroscopy of Ultracold
Fermi Gases
Andre Schirotzek
Coherent Quantum Gases - From Cold Atoms to Condensed MatterCopenhagen, Jan. 15th/16th 2009
Thanks for your attention!
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Realization in ultracold
Fermi gases
Atomic Fermi Gases allow to freely choose• Spin imbalance• Interaction strength between spin up and spin down
Situation maps onto tunable square well (kF
reff
<<1):
Vr
E
r r
V
V
E E
strong attraction deep bound state
weak attraction no bound state
Resonance bound state appears
scattering length0a > a → ±∞ 0a <
R
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Full Phase Diagram
S. Pilati
and S. Giorgini, PRL
101, 030408 (2008)
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Phase Transition from Density Distribution
Yong-il
Shin et al., PRL
070404 (2008)