Observation of Anomalous Spin-State Segregation in a Trapped Ultracold Vapor

Lewandowski, H. J.; Harber, D.; Whitaker, Dwight; Cornell, Eric A.

doi:10.1103/physrevlett.88.070403

Cited by 134 publications

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“…In 2002, a group at JILA [6] showed that, in an trapped ultracold atomic gas of bosons, the ISRE can result in a spontaneous spatial "segregation" of two atomic internal states |1 and |2 (equivalent to a pseudo-spin 1/2). Several groups then proposed a theoretical explanation of these observations, using either one-dimensional spin 1/2 hydrodynamic [7] or kinetic [8,9] equations.…”

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confidence: 99%

Cumulative Identical Spin Rotation Effects in Collisionless Trapped Atomic Gases

2009

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We discuss the strong spin segregation in a dilute trapped Fermi gas recently observed by Du et al. with "anomalous" large time scale and amplitude. In a collisionless regime, the atoms oscillate rapidly in the trap and average the inhomogeneous external field in an energy dependent way, which controls their transverse spin precession frequency. During interactions between atoms with different spin directions, the identical spin rotation effect (ISRE) transfers atoms to the up or down spin state, depending on their motional energy. Since low energy atoms are closer to the center of the trap than high energy atoms, the final outcome is a strong correlation between spins and positions. PACS numbers:Spin waves in dilute gases were predicted at the beginning of the eighties [1, 2] and confirmed experimentally soon after [3,4,5]. They can be understood in two equivalent ways, either as a consequence of spin mean field [1], or in more microscopic terms as the cumulative result of the identical spin rotation effect (ISRE) -an effect taking place during binary collisions between identical atoms [2]. Similarly, the Faraday effect is a rotation of the spin of photons that can be seen, either as a consequence of a macroscopic index of refraction, or as resulting from the accumulation of microscopic forward scattering events between photons and atoms.Experiments with ultracold atomic gases have renewed the interest in spin waves. In 2002, a group at JILA [6] showed that, in an trapped ultracold atomic gas of bosons, the ISRE can result in a spontaneous spatial "segregation" of two atomic internal states |1 and |2 (equivalent to a pseudo-spin 1/2). Several groups then proposed a theoretical explanation of these observations, using either one-dimensional spin 1/2 hydrodynamic [7] or kinetic [8,9] equations. More recently, Du et al. from Duke University [10] did an experiment that explores the properties of spin waves in quantum gases of fermions ( 6 Li) in the collisionless Knudsen regime, while most previous experiments were performed in the hydrodynamic regime (see nevertheless [11]). The spin segregation they observe is a hundred times larger and a hundred times slower than would be predicted by hydrodynamic theory. They call this spectacular effect "anomalous spin segregation" and suggest that its explanation may require "a modification of spin wave theory or possibly a new mechanism" for fermions.The purpose of this letter is twofold. First we argue that no modification of spin wave theory is necessary to understand the experiment; the physical mechanism behind the observations is the usual ISRE. The difference between bosons and fermions is not essential; what is important is the collisionless regime. Second, we discuss why this collisionless regime, combined with the presence of a trap, gives access to unexpected and interesting new physics. In fact, one can observe the ISRE almost as in an ideal experiment, where a single spin polarized atom is sent through a target of a gas polarized in another direction, and where t...

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Section: Pacs Numbersmentioning

confidence: 99%

Cumulative Identical Spin Rotation Effects in Collisionless Trapped Atomic Gases

2009

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“…Quantum collisional effects can also produce spatiotemporal spin oscillations known as spin waves. In a recent experiment, we observed in an ultra-cold atomic gas a startling spin migration effect which we called "anomalous spin-state segregation" [3]. Several theory groups have shown that this effect can be thought of as a halfcycle of a large-amplitude, over-damped spin wave [4].…”

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“…The cumulative effect of many such spin-rotating collisions is such that inhomogeneities of spin propagate like waves rather than diffusively. The characteristic frequency scale for the spin rotation effect is the exchange collision rate, ω exch = 4πhan/m, where m, n, and a are the mass of the atoms, the number density, and the s-wave scattering length respectively.The experimental apparatus and spectroscopy method used to prepare and probe the ultra-cold gas are described in [3] and briefly will be summarized here. 87 Rb atoms are precooled and trapped in a magneto-optical trap, transferred to a Ioffe-Pritchard magnetic trap via a servo-controlled linear track, and cooled further using forced radio-frequency evaporation.…”

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Spatial Resolution of Spin Waves in an Ultracold Gas

et al. 2002

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We present the first spatially resolved images of spin waves in a gas. The complete longitudinal and transverse spin field as a function of time and space is reconstructed. Frequencies and damping rates for a standing-wave mode are extracted and compared with theory.PACS numbers: 05.30. Jp, 32.80.Pj, 75.30.Ds, 51.10.+y, 51.60.+a Macroscopic collective behavior can arise in a quantum gas even above the onset of degeneracy. When indistinguishable atoms collide, the scattering can be significantly altered by the need to symmetrize the scattered waves. Examples of quantum scattering effects are seen in the strong polarization dependence of heat conduction and viscosity coefficients in spin polarized 3 He [1] and of thermalization rates in nondegenerate Fermi gases [2]. Quantum collisional effects can also produce spatiotemporal spin oscillations known as spin waves. In a recent experiment, we observed in an ultra-cold atomic gas a startling spin migration effect which we called "anomalous spin-state segregation" [3]. Several theory groups have shown that this effect can be thought of as a halfcycle of a large-amplitude, over-damped spin wave [4]. In this Letter we verify experimentally their viewpoint and present spatially resolved images of coupled transverse and longitudinal spin perturbances propagating through a magnetically trapped atomic cloud. Frequencies and damping rates for the standing-wave excitations are studied as a function of density and temperature.Although the concept of spin waves in ferromagnets dates back to Bloch's predictions in 1930 [5], the theory of spin waves in liquids and in dilute gases was not formulated until the much later [6]. The first evidence for spin waves (in the form of extra resonances in nuclear magnetic resonance spectra) occurred in spin polarized hydrogen and soon after in polarized 3 He and dilute mixtures of 3 He in 4 He [7]. On a microscopic level, spin waves arise from interference effects in lowest partial-wave collisions. When identical particles collide, the 180 • backscattering event causes a scattered atom to propagate along a trajectory indistinguishable from that of either a forward scattered atom or an unscattered atom. When the two atoms are spin aligned, the backward scattering contribution is indistinguishable from the forward scattering contribution, which adds a factor of two to the mean-field collisional energy shift. When the spins are antiparallel, the backward scattering event is distinguishable and the mean-field shift arises from the forward scattering event only. In the case of intermediate spin alignment, the backward scattering event is only partially distinguishable from the unscattered event, and one needs to add coherently two unparallel spin amplitudes, leading to a rotation of the spins of the scattered atoms. The cumulative effect of many such spin-rotating collisions is such that inhomogeneities of spin propagate like waves rather than diffusively. The characteristic frequency scale for the spin rotation effect is the exchange collision...

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“…In recent years, successes at loading atoms directly onto fabricated structures [2,3], evaporation in purely optical traps [4,5,6], and transport of atoms from one vacuum chamber to another [7,8,9] have created Bose-Einstein condensates (and in some cases degenerate gases of new species) in environments where enhanced optical access and proximity to surfaces allows for the discovery of new phenomena.…”

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Optically plugged quadrupole trap for Bose-Einstein condensates

Naik¹,

Raman²

2005

Phys. Rev. A

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We created sodium Bose-Einstein condensates in an optically plugged quadrupole magnetic trap (OPT). A focused, 532nm laser beam repelled atoms from the coil center where Majorana loss is significant. We produced condensates of up to 3 × 10 7 atoms, a factor of 60 improvement over previous work [1], a number comparable to the best all-magnetic traps, and transferred up to 9 × 10 6 atoms into a purely optical trap. Due to the tight axial confinement and azimuthal symmetry of the quadrupole coils, the OPT shows promise for creating Bose-Einstein condensates in a ring geometry.

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Observation of Anomalous Spin-State Segregation in a Trapped Ultracold Vapor

Cited by 134 publications

References 17 publications

Cumulative Identical Spin Rotation Effects in Collisionless Trapped Atomic Gases

Cumulative Identical Spin Rotation Effects in Collisionless Trapped Atomic Gases

Spatial Resolution of Spin Waves in an Ultracold Gas

Optically plugged quadrupole trap for Bose-Einstein condensates

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