arXiv subjects

Stefan S. Natu

Publications and source records attributed to Stefan S. Natu.

27 records · Page 2Linked to original sources

arXiv2013

Spin dynamics in a spin-orbit coupled Fermi gas

We study the dynamics of a non-degenerate, harmonically trapped Fermi gas following a sudden ramp of the spin-orbit coupling strength. In the non-interacting limit, we solve the Boltzmann equation in the presence of spin orbit coupling analytically, and derive expressions for the dynamics of an arbitrary initial spin state. In particular we show that for a fully spin polarized initial state, the total magnetization exhibits collapse and revival dynamics in time with a period set by the trapping potential. In real space, this corresponds to oscillations between a fully polarized state and a spin helix. We numerically study the effect of interactions on the dynamics using a collisionless Boltzmann equation.

cond-mat.quant-gas↗

arXiv2013

Dynamics of correlations in shallow optical lattices

We explore how correlations evolve in a gas of lattice bosons when the lattice depth is rapidly reduced. We find a simple closed form expression for the static structure factor in the limit of vanishing interactions. The corresponding real-space density correlation function shows multiple spatial oscillations which linearly disperse as a function of time. By perturbatively including the effects of the interactions we calculate how the boson quasi-momentum evolves following the quench.

cond-mat.quant-gas↗

arXiv2012

Evolution of condensate fraction during rapid lattice ramps

Combining experiments and numerical simulations, we investigate the redistribution of quasi-momentum in a gas of atoms trapped in an optical lattice when the lattice depth is rapidly reduced. We find that interactions lead to significant momentum redistribution on millisecond timescales, thereby invalidating previous assumptions regarding adiabaticity. We show that this phenomenon is driven by the presence of low-momentum particle-hole excitations in an interacting system. Our results invalidate bandmapping as an equilibrium probe in interacting gases.

cond-mat.quant-gas↗

arXiv2011

Pairing, Ferromagnetism, and Condensation of a normal spin-1 Bose gas

We theoretically study the stability of a normal, spin disordered, homogenous spin-1 Bose gas against ferromagnetism, pairing, and condensation through a Random Phase Approximation which includes exchange (RPA-X). Repulsive spin-independent interactions stabilize the normal state against both ferromagnetism and pairing, and for typical interaction strengths leads to a direct transition from an unordered normal state to a fully ordered single particle condensate. Atoms with much larger spin-dependent interaction may experience a transition to a ferromagnetic normal state or a paired superfluid, but, within the RPA-X, there is no instability towards a normal state with spontaneous nematic order. We analyze the role of the quadratic Zeeman effect and finite system size.

cond-mat.quant-gas↗

arXiv2010

Local versus global equilibration near the bosonic Mott-superfluid transition

We study the response of trapped two dimensional cold bosons to time dependent lattices. We find that in lattice ramps from 11 (superfluid, $\hbar/U_{\text{i}} = 3$ms, $\hbar/J_{\text{i}} = 45$ms) to 16 recoils (Mott, $\hbar/U_{\text{f}} = 2$ms, $\hbar/J_{\text{f}} = 130$ms) the local number fluctuations remains at their equilibrium values if ramps are slower than 3 ms. Global transport, however, is much slower (1s), especially in the presence of Mott shells. This separation of timescales has practical implications for cold atom experiments and cooling protocols.

cond-mat.quant-gas↗

arXiv2010

Domain wall dynamics in a two-component Bose-Mott insulator

We model the dynamics of two species of bosonic atoms trapped in an optical lattice within the Mott regime by mapping the system onto a spin model. A field gradient breaks the cloud into two domains. We study how the domain wall evolves under adiabatic and diabatic changes of this gradient. We determine the timescales for adiabaticity, and study how temperature evolves for slow ramps. We show that after large, sudden changes of the field gradient, the system does not equilibrate on typical experimental timescales. We find interesting spin dynamics even when the initial temperature is large compared to the super-exchange energy. We discuss the implication of our results for experiments wishing to use such a two-component system for thermometry, or as part of a cooling scheme.

cond-mat.quant-gas↗

arXiv2009

Spin waves in a spin-1 Bose gas

We present a theory of spin waves in a non-condensed gas of spin-1 bosons: providing 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 Rb-87 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 enough anti-ferromagnetic interactions, the polar gas is unstable. Finally we explore coherent population dynamics in a collisionless transversely polarized gas.

cond-mat.quant-gas↗

arXiv2009

Anomalous Spin segregation in a weakly interacting two-component Fermi gas

We explain the spin segregation seen at Duke in a two-component gas of 6Li [Du, Luo, Clancy and Thomas, Phys. Rev. Lett. 101,150401 (2008)] as a mean-field effect describable via a collisionless Boltzmann equation. As seen in experiments, we find that slight differences in the trapping potentials in the two spin states drive small spin currents. Hartree-Fock type interactions convert these currents into a redistribution of populations in energy space, and consequently a long lived spin texture develops. We explore the interaction strength dependence of these dynamics, finding close agreement with experiment.

cond-mat.other↗

arXiv2008

Elastic theory of Normal-Superfluid Boundary in trapped Fermi Gases

By modeling the normal-superfluid boundary in a trapped polarized Fermi gas as an elastic membrane, we calculate the atomic density profile. For small trapping anisotropy, we find that the superfluid-normal boundary remains approximately elliptical, and has an aspect ratio different from that of the trap. For very prolate clouds the boundary becomes distorted into a capsule-like shape. We present an analytic explanation of this shape.

cond-mat.other↗