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arXiv:1007.4339 · New Journal of Physics

Optical excitation of nonlinear spin waves

J M McGuirk, Lydia Neary‐Zajiczek

Published Oct 13, 2010• 6 citations
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Abstract

We demonstrate a technique for exciting spin waves in an ultracold gas of 87 Rb atoms on the basis of tunable ac Stark potentials. This technique allows us to excite normal modes of spin waves with arbitrary amplitudes in the trapped gas, including dipole, quadrupole, octupole and hexadecapole modes. These modes exhibit strong nonlinearities, which manifest as amplitude dependence of the excitation frequencies and departure from sinusoidal behavior. Our results are in good agreement with a full treatment of a quantum Boltzmann transport equation.

DOI
10.1088/1367-2630/12/10/103020

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We demonstrate a technique for exciting spin waves in an ultracold gas of 87 Rb atoms on the basis of tunable ac Stark potentials. This technique allows us to excite normal modes of spin waves with arbitrary amplitudes in the trapped gas, including dipole, quadrupole, octupole and hexadecapole modes. These modes exhibit strong nonlinearities, which manifest as amplitude dependence of the excitation frequencies and departure from sinusoidal behavior. Our results are in good agreement with a full treatment of a quantum Boltzmann transport equation.
We demonstrate a technique for exciting spin waves in an ultracold gas of Rb-87 atoms based on tunable AC Stark potentials. This technique allows us to excite normal modes of spin waves with arbitrary amplitudes in the trapped gas, including dipole, quadrupole, octupole, and hexadecapole modes. These modes exhibit strong nonlinearities, which manifest as amplitude dependence of the excitation frequencies and departure from sinusoidal behavior. Our results are in good agreement with a full treatment of a quantum Boltzmann transport equation.
An ultracold Bose gas of two-level atoms can be thought of as a spin-1/2 Bose gas. It supports spin-wave collective modes due to the exchange mean field. Such collective spin oscillations have been observed in recent experiments at JILA with 87Rb{}^{87}\mathrm{Rb}87Rb atoms confined in a harmonic trap. We present a theory of the spin-wave collective modes based on the moment method for trapped gases. In the collisionless and hydrodynamic limits, we derive analytic expressions for the frequencies and damping rates of modes with dipole and quadrupole symmetry. We find that the frequency for a given mode is given by a temperature-independent function of the peak density n, and falls off as 1/n.1/n.1/n. We also find that, to a very good approximation, excitations in the radial and axial directions are decoupled. We compare our model to the numerical integration of a one-dimensional version of the kinetic equation and find very good qualitative agreement. The damping rates, however, show the largest deviation for intermediate densities, where one expects Landau damping---which is unaccounted for in our moment approach---to play a significant role.
Ultracold trapped atomic systems are ideal model systems to investigate physical phenomena in the quantum regime.In this work we studied instabilities in spin dynamics of a sample of nondegenerate trapped 87 Rb gas.This is the first experimental investigation of the so-called Castaing instability in such systems.The Castaing instability is an instability in the spin dynamics of spin-polarized systems as a result of introducing sharp spin gradients in their spin profile.We used an optical technique via the ac Stark effect to initialize arbitrary spin profiles in a sample of evaporatively cooled nondegenerate 87 Rb gas.The experimental results manifest evidence for the presence of the Castaing instability in both transverse and longitudinal spin channels.The results agree reasonably well with theoretical studies and results of a numerical solution to the spin transport equation.Some imperfection in the longitudinal spin gradient due to the spin-state preparation technique makes the signature of the instability less clear in the transverse channel.We propose another preparation technique to overcome this shortcoming in future experiments.First, I would like to thank my supervisor, Dr. Jeffrey McGuirk.This thesis would not be possible without his help and guidance.I have been fortunate to work in his group and benefit from his incredible knowledge, enthusiasm and patience.He is always supportive and willing to give advice, yet knows when to let you experience research and learn science on your own.I am grateful for having two great physicists
We study the collapse and revival of coherence induced by dipolar spin waves in a trapped gas of 87Rb^{87}\mathrm{Rb}87Rb atoms. In particular, we observe spatially localized collapse and revival of Ramsey fringe contrast and show how the pattern of coherence depends on the strength of the spin-wave excitation. We find that the spatial character of the coherence dynamics is incompatible with a simple model based only on position-space overlap of wave functions. We show that this phenomenon requires a full phase-space description of the atomic spin using a quantum Boltzmann transport equation, which highlights spin-wave-induced coherent spin currents and the ensuing dynamics they drive.
A semiclassical method is used to study Landau damping of transverse pseudo-spin waves in harmonically trapped ultracold gases in the collisionless Boltzmann limit. In this approach, the time evolution of a spin is calculated numerically as it travels in a classical orbit through a spatially dependent mean field. This method reproduces the Landau damping results for spin-waves in unbounded systems obtained with a dielectric formalism. In trapped systems, the simulations indicate that Landan damping occurs for a given spin-wave mode because of resonant phase space trajectories in which spins are "kicked out" of the mode (in spin space). A perturbative analysis of the resonant and nearly resonant trajectories gives the Landau damping rate, which is calculated for the dipole and quadrupole modes as a function of the interaction strength. The results are compared to a numerical solution of the kinetic equation by Nikuni et al.
We present a theory of spin waves in a noncondensed gas of spin-1 bosons and provide both analytic calculations of the linear theory and full numerical simulations of the nonlinear response. We highlight the role of spin-dependent contact interactions in the dynamics of a thermal gas. Although these interactions are small compared to the thermal energy, they set the scale for low-energy, long-wavelength spin waves. In particular, we find that the polar state of {sup 87}Rb is unstable to collisional mixing of magnetic sublevels even in the normal state. We augment our analytic calculations by providing full numerical simulations of a trapped gas, explicitly demonstrating this instability. Further, we show that for strong antiferromagnetic interactions, the polar gas is unstable. Finally, we explore coherent population dynamics in a collisionless transversely polarized gas.