Magnetism in ultracold Fermi gases and New physics with ultracold ions: many-body systems with non-equilibrium noise $$ NSF, AFOSR MURI, DARPA Harvard-MIT David Pekker (Harvard), Rajdeep Sensarma (Harvard/JQI Maryland), Mehrtash Babadi (Harvard), Nikolaj Zinner (Harvard/Niels Bohr Institute), Antoine Georges (Ecole Polytechnique), Ehud Altman (Weizmann), Emanuele Dalla Torre (Weizmann), Thierry Giamarchi (Geneva), Eugene Demler (Harvard)

Outline Stoner instability in ultracold atoms Motivated by experiments of G.-B. Jo et al., Science (2009) Introduction to Stoner instability. Possible observation of Stoner instability in MIT experiments. Domain formation. Competition of molecule formation and Stoner instability Ref: M. Babadi et al., arXiv: and unpublished New physics with ultracold ions Quantum many-body systems in the presence of non-equilibrium noise Ref: Dalla Torre et al., arXiv:

Stoner model of ferromagnetism Spontaneous spin polarization decreases interaction energy but increases kinetic energy of electrons Mean-field criterion U N(0) = 1 U – interaction strength N(0) – density of states at Fermi level Theoretical proposals for observing Stoner instability with ultracold Fermi gases: Salasnich et. al. (2000); Sogo, Yabu (2002); Duine, MacDonald (2005); Conduit, Simons (2009); LeBlanck et al. (2009); … Kanamori’s counter-argument: renormalization of U then

Magnetic domains could not be resolved. Why? T.L. Ho (2009)

Stoner Instability New feature of cold atoms systems: non-adiabatic crossing of U c Screening of U (Kanamori) occurs on times 1/E F Two timescales in the system: screening and magnetic domain formation Magnetic domain formation takes place on much longer time scales: critical slowing down

Quench dynamics across Stoner instability Find collective modes Unstable modes determine characteristic lengthscale of magnetic domains For U>U c unstable collective modes

For MIT experiments domain sizes of the order of a few l F Quench dynamics in D=3 Dynamics of magnetic domain formation near Stoner transition Moving across transition at a finite rate 0u u*u* domains coarsen slow growth domains freeze Growth rate of magnetic domains Domain size Domains freeze when Domain size at “freezing” point M. Babadi et al. (2009)

Is it sufficient to consider effective model with repulsive interactions when analyzing experiments? Feshbach physics beyond effective repulsive interaction

Feshbach resonance Review: Duine and Stoof, 2004 Chin et al., 2009 Two particle bound state formed in vacuum BCS instability Stoner instability Molecule formation and condensation This talk: Prepare Fermi state of weakly interacting atoms. Quench to the BEC side of Feshbach resonance. System unstable to both molecule formation and Stoner ferromagnetism. Which instability dominates ?

Many-body instabilities Imaginary frequencies of collective modes Magnetic Stoner instability Pairing instability

Change from bare interaction to the scattering length Instability to pairing even on the BEC side Pairing instability

Intuition: two body collisions do not lead to molecule formation on the BEC side of Feshbach resonance. Energy and momentum conservation laws can not be satisfied. This argument applies in vacuum. Fermi sea prevents formation of real Feshbach molecules by Pauli blocking. Molecule Fermi sea

Pairing instability Time dependent variational wavefunction Time dependence of u k (t) and v k (t) due to D BCS (t) For small D BCS (t):

Pairing instability From wide to narrow resonances

Stoner vs pairing Does Stoner instability really exceed molecule formation rate?

Stoner instability = Divergence in the scattering amplitude arises from bound state formation. Bound state is strongly affected by the Fermi sea. Stoner instability is determined by two particle scattering amplitude

Stoner instability Spin susceptibility

Growth rate of pairing instability Growth rate of magnetic Stoner instability RPA with bare scattering length RPA with Cooperon Changing from scattering length to Cooperon gives strong suppression of the Stoner instability Stoner instability suppressed when using a Cooperon. Strong suppression due to Pauli blocking Stoner instability ?

Stoner vs pairing G.B. Jo et al., Science (2009)

Stoner vs pairing Increase in the kinetic energy: consistent with pairing. In the BCS state kinetic energy goes up and the interaction energy goes down

Conclusions for part I Competition of pairing and Stoner instabilities New features due to dynamical character of experiments Simple model with contact repulsive interactions may not be sufficient to understand experiments Strong suppression of Stoner instability by Fechbach resonance physics + Pauli blocking Possible ways to recover Stoner instability: many-body correlations, e.g. effective mass renormalization. Interesting questions beyond linear instability analysis.

QUANTUM MANY-BODY SYSTEMS IN THE PRESENSE OF NONEQUILIBRIUM NOISE NEW PHYSICS WITH ULTRACOLD IONS

Question: What happens to low dimensional quantum systems when they are subjected to external non- equilibrium noise? Ultracold polar molecules Trapped ions E One dimensional Luttinger state can evolve into a new critical state. This new state has intriguing interplay of quantum critical and external noise driven fluctuations

A brief review: Universal long-wavelength theory of 1D systems Displacement field: Long wavelength density fluctuations (phonons): Haldane (81) Weak interactions : K >>1 Hard core bosons: K = 1 Strong long range interactions: K < 1

1D review cont’d: Wigner crystal correlations No crystalline order ! Scale invariant critical state (Luttinger liquid) Wigner crystal order parameter:

1D review cont’d: Effect of a weak commensurate lattice potential How does the lattice potential change under rescaling ? Quantum phase transition: K 2 – Critical phase (Luttinger liquid)

New systems more prone to external disturbance E + - Ultracold polar molecules Trapped ions R. Blatt’s talk at this conference (from NIST group )

Linear ion trap Linear coupling to the noise:

Measured noise spectrum in ion trap f From dependence of heating rate on trap frequency. - Direct evidence that noise spectrum is 1/f - Short range spatial correlations (~ distance from electrodes) Monroe group, PRL (06), Chuang group, PRL (08)

Ultra cold polar molecules E Polarizing electric field: + - System is subject to electric field noise from the electrodes ! Molecule polarizability

Long wavelength description of noisy low D systems

Effective coupling to external noise Long wavelength component of noise Component of noise at wavelengths near the inter-particle spacing The “backscattering”  can be neglected if the distance to the noisy electrode is much larger than the inter-particle spacing. >>

Effective harmonic theory of the noisy system Dissipative coupling to bath needed to ensure steady state (removes the energy pumped in by the external noise) Implementation of bath: continuous cooling (Quantum) Langevin dynamics: Thermal bath External noise

Wigner crystal correlations 1/f noise is a marginal perturbation ! Critical steady state Case of local 1/f noise: - Decay of crystal correlations remains power-law. - Decay exponent tuned by the 1/f noise power.

Effect of a weak commensurate lattice potential How does the lattice change under a scale transformation? Without lattice: Scale invariant steady state. Phase transition tuned by noise power (Supported also by a full RG analysis within the Keldysh formalism) KcKc F 0 /  Localized Critical state 2

1D-2D transition of coupled tubes Scaling of the inter-tube hopping: KcKc F 0 /  1/4 2D superfluid 1D critical

Global phase diagram KcKc F 0 /  2D crystal Critical state 1 KcKc F 0 /  1/4 2D superfluid 1D critical Inter-tube interactions Inter-tube tunneling Both perturbations 2 KcKc F 0 /  2D superfluid 2D crystal 1D critical

Conclusions for part II New perspectives on many-body physics from chains of ions and polar molecules Effects of external noise on quantum critical states -new critical state -new phases and phase transitions tuned by noise

Summary Stoner instability in ultracold atoms Motivated by experiments of G.-B. Jo et al., Science (2009) Introduction to Stoner instability. Possible observation of Stoner instability in MIT experiments. Domain formation. Competition of molecule formation and Stoner instability Ref: M. Babadi et al., arXiv: and unpublished New physics with ultracold ions Quantum many-body systems in the presence of non-equilibrium noise Ref: Dalla Torre et al., arXiv: Harvard-MIT