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Vortices and Persistent Currents in Rotating Bose Gases - a Diagonalization Approach

Bargi, Sara LU (2010)
Abstract (Swedish)
Popular Abstract in Swedish

Bose-Einsteinkondensat är de kallaste system människan känner till, och de styrs helt av kvantmekaniska lagar. Därför har de många speciella egenskaper, som bland annat visar sig under rotation. De kan till exempel inte rotera med vilken hastighet som helst, utan bara vissa specifika hastigheter är tillåtna. Vidare kan man - under vissa omständigheter - få ett Bose-Einsteinkondensat att rotera "för evigt" utan märkbar minskning av rotationshastighet.

Jag har gjort noggranna datorsimuleringar för att bättre förstå hur och när dessa fenomen uppkommer. I dagsläget går det inte att dra slutsatser rakt av från mina beräkningar och applicera dem på experimenten, eftersom jag räknar på några... (More)
Popular Abstract in Swedish

Bose-Einsteinkondensat är de kallaste system människan känner till, och de styrs helt av kvantmekaniska lagar. Därför har de många speciella egenskaper, som bland annat visar sig under rotation. De kan till exempel inte rotera med vilken hastighet som helst, utan bara vissa specifika hastigheter är tillåtna. Vidare kan man - under vissa omständigheter - få ett Bose-Einsteinkondensat att rotera "för evigt" utan märkbar minskning av rotationshastighet.

Jag har gjort noggranna datorsimuleringar för att bättre förstå hur och när dessa fenomen uppkommer. I dagsläget går det inte att dra slutsatser rakt av från mina beräkningar och applicera dem på experimenten, eftersom jag räknar på några tiotal atomer och experimenten hanterar miljontals atomer. Men experimentalister hoppas inom rimlig tid kunna göra experiment där man kan studera några få atomer åt gången. Då hade vi fått något som är ganska ovanligt i fysiken: en modell som utan approximationer beskriver ett verkligt system som saknar orenheter. (Less)
Abstract
In this thesis, I explore the behavior of rotating ultra-cold Bose gases, by diagonalizing the Hamiltonian. This method has the advantage of being exact in the limit of weak interactions, and thus is a useful complement to the more commonly used Gross-Pitaevskii (GP) equation, which relies on a mean-field approximation. It is generally known that an ultra-cold Bose gas shows some remarkable properties under rotation, such as quantized vortices and persistent currents, and these are one main reason why we choose to study rotation.



Abstract A repulsively interacting Bose-Einstein condensate (BEC) in a harmonic trapping potential will form singly quantized vortices when rotated. Papers I-V are examples of different kinds... (More)
In this thesis, I explore the behavior of rotating ultra-cold Bose gases, by diagonalizing the Hamiltonian. This method has the advantage of being exact in the limit of weak interactions, and thus is a useful complement to the more commonly used Gross-Pitaevskii (GP) equation, which relies on a mean-field approximation. It is generally known that an ultra-cold Bose gas shows some remarkable properties under rotation, such as quantized vortices and persistent currents, and these are one main reason why we choose to study rotation.



Abstract A repulsively interacting Bose-Einstein condensate (BEC) in a harmonic trapping potential will form singly quantized vortices when rotated. Papers I-V are examples of different kinds of generalizations to this basic system, each showing its particular non-trivial new features.



Abstract In Paper I we make the trapping potential weakly anharmonic, and confirm the existence of multiply quantized vortices in both repulsively and attractively interacting condensates, which were previously predicted in Gross-Pitaevskii calculations.



Abstract In Papers II and III, we generalize to a two-component system in a harmonic trap. The vortices that form under rotation will in general be coreless, i.e. one component will form a vortex and rotate around the other component. In Paper II we give some analytical expressions for the dispersion relation and occupation numbers at low angular momenta, and in Paper III we show how multiply quantized vortices can be stable in these systems.



Abstract Finally we investigate bosons in potentials which can support persistent currents, i.e. rotating states that do not decay in a finite amount of time. These are possible if there is an energy barrier separating the rotating state from the non-rotating ground state. In Paper IV, we have a two-component condensate in a one-dimensional potential and we calculate among other things the minimum interaction strength where persistent currents can occur. In Paper V, we study an experimentally relevant two-dimensional annulus. Again we find that persistent currents are possible above a certain interaction strength, and we find this strength as a function of the width of the annulus, as well as how it depends on the relative population of the two species. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Professor Papenbrock, Thomas, University of Tennessee, Knoxville, USA
organization
publishing date
type
Thesis
publication status
published
subject
keywords
Bose-Einstein condensation, vortices, superfluidity, Fysicumarkivet A:2010:Bargi
pages
138 pages
defense location
Lecture hall B, Fysicum, Sölvegatan 14A, Lund University Faculty of Engineering
defense date
2010-10-01 14:15
ISBN
978-91-7473-005-0
language
English
LU publication?
yes
id
2858ae23-62f5-4770-9b93-fc3e562ce14d (old id 1667356)
date added to LUP
2010-09-07 16:24:46
date last changed
2016-09-19 08:45:16
@phdthesis{2858ae23-62f5-4770-9b93-fc3e562ce14d,
  abstract     = {In this thesis, I explore the behavior of rotating ultra-cold Bose gases, by diagonalizing the Hamiltonian. This method has the advantage of being exact in the limit of weak interactions, and thus is a useful complement to the more commonly used Gross-Pitaevskii (GP) equation, which relies on a mean-field approximation. It is generally known that an ultra-cold Bose gas shows some remarkable properties under rotation, such as quantized vortices and persistent currents, and these are one main reason why we choose to study rotation. <br/><br>
<br/><br>
Abstract A repulsively interacting Bose-Einstein condensate (BEC) in a harmonic trapping potential will form singly quantized vortices when rotated. Papers I-V are examples of different kinds of generalizations to this basic system, each showing its particular non-trivial new features.<br/><br>
<br/><br>
Abstract In Paper I we make the trapping potential weakly anharmonic, and confirm the existence of multiply quantized vortices in both repulsively and attractively interacting condensates, which were previously predicted in Gross-Pitaevskii calculations.<br/><br>
<br/><br>
Abstract In Papers II and III, we generalize to a two-component system in a harmonic trap. The vortices that form under rotation will in general be coreless, i.e. one component will form a vortex and rotate around the other component. In Paper II we give some analytical expressions for the dispersion relation and occupation numbers at low angular momenta, and in Paper III we show how multiply quantized vortices can be stable in these systems.<br/><br>
<br/><br>
Abstract Finally we investigate bosons in potentials which can support persistent currents, i.e. rotating states that do not decay in a finite amount of time. These are possible if there is an energy barrier separating the rotating state from the non-rotating ground state. In Paper IV, we have a two-component condensate in a one-dimensional potential and we calculate among other things the minimum interaction strength where persistent currents can occur. In Paper V, we study an experimentally relevant two-dimensional annulus. Again we find that persistent currents are possible above a certain interaction strength, and we find this strength as a function of the width of the annulus, as well as how it depends on the relative population of the two species.},
  author       = {Bargi, Sara},
  isbn         = {978-91-7473-005-0},
  keyword      = {Bose-Einstein condensation,vortices,superfluidity,Fysicumarkivet A:2010:Bargi},
  language     = {eng},
  pages        = {138},
  school       = {Lund University},
  title        = {Vortices and Persistent Currents in Rotating Bose Gases - a Diagonalization Approach},
  year         = {2010},
}