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Sökning: WFRF:(Jonsell Svante) > Doktorsavhandling

  • Resultat 1-4 av 4
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1.
  • Saers, Robert, 1977- (författare)
  • Ultracold rubidium atoms in periodic potentials
  • 2008
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • This thesis includes both experimental and theoretical investigations, presented in a series of eight papers. The experimental part ranges from the construction procedures of an apparatus for Bose-Einstein condensates, to full scale experiments using three different set-ups for ultracold atoms in optical lattices. As one of the main themes of the thesis, an experimental apparatus for production of Bose-Einstein Condensates is under construction. A magneto-optically trapped sample, hosting more than 200 million 87Rb atoms, have successfully been loaded into a magnetic trap with high transfer rate. The lifetime of the sample in the magnetic trap is in the range of 9 s, and the atoms have been shown to respond to evaporative cooling. The experiment is ready for optimization of the magnetic trap loading, and evaporative cooling parameters, which are the final steps for reaching Bose-Einstein condensation. The set-up is designed to host experiments including variable geometry optical lattices, and includes the possibility to align laser beams with high angular precision for this purpose. The breakdown of Bloch waves in a Bose-Einstein condensate is studied, attributed to the effect of energetic and dynamical instability. This experimental study is performed using a Bose-Einstein condensate in a moving one-dimensional optical lattice at LENS, Florence Italy. The optical lattice parameters, and the thermal distribution of the atomic sample required to trigger the instabilities, are detected, and compared with a theoretical model developed in parallel with the experiments. In close connection with these one-dimensional lattice studies, an experimental survey to characterize regimes of superradiant Rayleigh scattering and Bragg scattering is presented. Tunneling properties of repulsively bound atom pairs in double well potentials are characterized in an experiment at Johannes Gutenberg University, Mainz Germany. A three-dimensional optical lattice, producing an array of double wells with tunable properties is let to interact with a Bose-Einstein condensate. Pairs of ultracold atoms are produced on one side in the double wells, and their tunneling behavior, dependent on potential barrier and repulsion properties, is studied. A theoretical study of the crossover between one- and two-dimensional systems has been performed. The simulations were made for a two-dimensional array of atoms, where the behavior for different tunneling probabilities and atom-atom repulsion strengths was studied. Scaling relations for systems of variable sizes have been examined in detail, and numerical values for the involved variables have been found.
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2.
  • Sjölund, Peder, 1976- (författare)
  • Laser cooling mechanisms and Brownian motors in optical lattices
  • 2007
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Denna avhandling innefattar såväl experimentella som numeriska studier av laserkylda atomer i optiska kristallgitter. Bland annat har laserkylningsprocesser studerats, där atomers rörelser i optiska kristallgitter har uppvisat andra typer av bakomliggande mekanismer än de som tidigare förutsågs genom “Sisyfoskylningsmodellen”. Sedan atomer kylda till några mikrokelvin först realiserades (sent 60-tal) så har Sisyfoskylningsmodellen varit hörnstenen för förståelsen av laserkylda och lokaliserade atomer i dissipativa optiska kristallgitter. I dissipativa optiska kristallgitter finns det en balans mellan den uppvärmande diffusionen och den kylande friktionen. Studier i denna avhandling visar att laserkylningsprocesser är mer komplexa än vad denna modell innefattar. Både experimentella och numeriska resultat visar att atomer i optiska kristallgitter har två hastighetsfördelningar där en “kallare” och en “varmare” mod av atomer omfördelas mellan moderna. Speciellt så visar det sig att varma atomer dels värms och diffunderar ut ur gittret, men samtidigt populeras den kalla moden med en tidsutveckling som inte förändrar dess temperatur nämnvärt. I detta arbete presenteras också resultat från den första realiserade tredimensionella Brownska motorn baserad på ljus-atom-växelverkan. Det unika med denna Brownska motor är att den är kontrollerbar både vad gäller dess hastighet som dess riktning. Den underliggande principen för denna Brownska motor är tämligen generell och den kan därför vara applicerbar inom andra vetenskapliga discipliner såsom nanoteknik, biologi, kemi och elektronik. Generellt så är förståelsen av Brownska motorer viktigt eftersom de återfinns i vår omgivning, från exempelvis härkomsten av muskelsammandragningar och materialtransporter i levande celler till rörelsen hos bakterier och mindre organismer. Det flesta av de experimentella resultaten presenterade i denna avhandling har varit möjliga genom utveckling och förbättringar av den experimentella uppställningen. Framförallt så har kvaliten och reproducerbarheten vid de olika mätningar som gjorts blivit avsevärt förbättrade jämfört med tidigare vilket utgör en bra grund för framtida studier av ultrakalla atomer.
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3.
  • Tempest, Kajsa-My, 1986- (författare)
  • Three-body Universality Controlled by a Feshbach Resonance
  • 2023
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • The quantum Efimov effect manifests itself in the limit of resonant two-body interactions, where the scattering length diverges. In this scenario, an infinite number of shallow bound trimer states are formed with binding energies that obey a discrete geometric scaling law. The effect is universal in the sense that it can, in principle, appear in any system of bosons with resonant interactions. These states have been studied experimentally in dilute gases of ultracold atoms, where the magnetic-field-dependent coupling to a different molecular state, i.e., where a Feshbach resonance, can be used to tune the scattering length to the resonant regime. A limited number of Efimov states can also be formed for scattering lengths of finite magnitude. As experimental observations of these states accumulated, an unexpected regularity was found in the value of the scattering length where the first Efimov state emerges from the three-body continuum. This regularity is commonly referred to as van der Waals universality. It is believed to exclusively appear when broad Feshbach resonances are used to tune the scattering length. Theoretical predictions suggest a different kind of regularity for narrow Feshbach resonances, but the few experimental observations that exist are inconclusive in regard to these predictions.This thesis summarises the methodology I have used for developing a multichannel model that includes a magnetically tunable Feshbach resonance used for varying the scattering length. With this model, I am able to study effects related to the resonance width, which the commonly used single-channel models are unable to capture. By solving the Schrödinger equation in the adiabatic hyperspherical approximation, I obtain three-body hyperradial potentials, which I subsequently use to obtain the discrete Efimov energy spectrum. The origin of the van der Waals universality is believed to be caused by a universally positioned repulsive wall in the three-body effective hyperradial potential responsible for the Efimov effect. I observe a slight shift in the position of this wall as the system approaches the narrow resonance limit. More notably, the attractive well becomes exceedingly shallow. In the Efimov energy spectra, I find a clear dependence on the width of the resonance and the position of the first appearing Efimov state. The results agree qualitatively with the predictions of others but for the narrow resonance, there is a large quantitative difference. The observed scaling between consecutive Efimov states agrees well with Efimov's prediction.
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4.
  • Umair, Muhammad, 1984- (författare)
  • Resonances in Three-Body Systems
  • 2016
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Three particles interacting via Coulomb forces represent a fundamental problem in quantum mechanics with no known exact solution. We have investigated resonance states composed of three particles interacting via Coulombic and more general potentials in non-relativistic quantum mechanics, using the complex scaling method. Our calculations have been applied to three different systems.(i) An investigation of resonances in the positron-alkali (Li, Na, K) systems has been conducted. Some calculations have previously been reported on the resonances in positron-alkali systems; however, most of the work was limited to the lower partial wave, such as S-wave resonances. In this thesis, we have extended the calculations to higher partial waves and extracted the resonance positions and widths using the more accurate complex scaling method. A dipole series of resonances has been found under positronium n = 2 threshold, for natural parity and n = 3 threshold for unnatural parity states. Furthermore, these resonances were found to agree well with an analytically derived scaling law. This series in the positron-alkali system are caused by the attractive potential formed by the dipole moment of positronium (the bound state of an electron and a positron). This dipole moment is a hydrogen-like system, and hence its energy levels are degenerate with respect to orbital angular momentum. We have also predicted several new resonances.(ii) A calculation of resonances in positron-hydrogen scattering, which shows that we can represent this system with the accuracy needed for future scattering calculations. Such cross sections are of interest since this is a way to form anti-hydrogen.(iii) A search for possible resonances in the pµe system, which has been suggested as a possible reason for unexpected results from a recent measurement of the proton radius in muonic hydrogen. We have ruled out the possibility of such resonances.In all calculations we used the Couple Rearrangement Channel Method, where the wave function is represented by Gaussians expressed in Jacobi coordinates. Thus effects due to mass polarization are automatically.
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