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Coeficiente de expansão térmica de um gás de Bose durante a sua transição de fase

E. D. Mercado-Gutiérrez

Published Jul 18, 2016• 0 citations
OpenAlex

Abstract

Ultra cold quantum gas is a convenient system to study fundamental questions of modern physics, such as phase transitions and critical phenomena. This master thesis is devoted to experimental investigation of the thermodynamics susceptibilities, such as the isothermal compressibility and the thermal expansion coefficient of a trapped Bose-Einstein condensate (BEC) of 87Rb atoms. The critical phenomena and the critical exponents across the transition can explain the behavior of the isothermal compressibility and the thermal expansion coefficient near the critical temperature TC. By employing the developed formalism of global thermodynamics variables, we carry out a statistical treatment of Bose gas in a 3D harmonic potential. After that, comparison of obtained results reveals the most appropriate state variables describing the system, namely volume and pressure parameter V and Π respectively. The both are related with the confining frequencies and BEC density distribution. We apply this approach to define the set of new thermodynamic variables of BEC, and also to construct the isobaric phase diagram V T. Its allows us to extract the compressibility κT and the thermal expansion coefficient βΠ. The behavior of the isothermal compressibility corresponds to the second-order phase transition, while the thermal expansion coefficient around the critical point behaves as β ∼ tr-α, where tr is reduced temperature of the system and α is the critical exponent on the basic of these. Results we have obtained the critical exponent α = 0.15±0.09, which allows us to determine the system dimensionality by means of the scaling theory, relating the critical exponents with the dimensionality. As a result, we found out that the dimensionality of the system to be d ∼ 3, one is in agreement with the real dimension of the system.

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Ultra cold quantum gas is a convenient system to study fundamental questions of modern physics, such as phase transitions and critical phenomena. This master thesis is devoted to experimental investigation of the thermodynamics susceptibilities, such as the isothermal compressibility and the thermal expansion coefficient of a trapped Bose-Einstein condensate (BEC) of 87Rb atoms. The critical phenomena and the critical exponents across the transition can explain the behavior of the isothermal compressibility and the thermal expansion coefficient near the critical temperature TC. By employing the developed formalism of global thermodynamics variables, we carry out a statistical treatment of Bose gas in a 3D harmonic potential. After that, comparison of obtained results reveals the most appropriate state variables describing the system, namely volume and pressure parameter V and Π respectively. The both are related with the confining frequencies and BEC density distribution. We apply this approach to define the set of new thermodynamic variables of BEC, and also to construct the isobaric phase diagram V T. Its allows us to extract the compressibility κT and the thermal expansion coefficient βΠ. The behavior of the isothermal compressibility corresponds to the second-order phase transition, while the thermal expansion coefficient around the critical point behaves as β ∼ tr-α, where tr is reduced temperature of the system and α is the critical exponent on the basic of these. Results we have obtained the critical exponent α = 0.15±0.09, which allows us to determine the system dimensionality by means of the scaling theory, relating the critical exponents with the dimensionality. As a result, we found out that the dimensionality of the system to be d ∼ 3, one is in agreement with the real dimension of the system.
We apply the global thermodynamic variables approach to experimentally determine the isothermal compressibility parameter ${\ensuremath{\kappa}}_{T}$ of a trapped Bose gas across the phase transition. We demonstrate the behavior of ${\ensuremath{\kappa}}_{T}$ around the critical pressure, revealing the second-order nature of the phase transition. Compressibility is the most important susceptibility to characterize the system. The use of global variables shows advantages with respect to the usual local density approximation method and can be applied to a broad range of situations.
We report the measurement of the global thermal expansion coefficient of a confined Bose gas of 87Rb^{87}\rm Rb87Rb in a harmonic potential around the Bose-Einstein condensation transition temperature. We use the concept of global thermodynamic variable, previously introduced and appropriated for a non homogeneous system. The data show divergence on the thermal expansion coefficient near the transition temperature and analyses of the universality behavior shows a critical exponent α=0.15±0.09\alpha = 0.15\pm 0.09α=0.15±0.09. With this obtained value and the critical exponent for the correlation length recently presented in the literature we analyse the scaling law combining dimension and critical exponents.
Thermodynamics of non-uniform systems has always been a challenging topic in the physical sciences. In the field of quantum gases, that subject is mainly studied with ultracold samples trapped in a harmonic potential, the first kind of confinement ever made to atomic fluids.1 Nascimbène et al.2 have already described a method to measure locally the equation of state and heat capacity of a harmonically trapped quantum gas. Nonetheless, local measurements cannot provide the volume-dependent susceptibilities of an inhomogeneous system, namely the thermal expansion and compressibility, meaning that the thermodynamic description of harmonically trapped gases is still incomplete. To solve that issue, Romero and Bagnato3, 4 proposed canonical variables of work for a gas in a harmonic trap, which work like pressure and volume for a gas in a vessel and allow global measurements on the system. Therefore, we describe here our methodology to measure the thermodynamic susceptibilities of a harmonically trapped gas around the Bose-Einstein transition using the Romero-Bagnato formalism. Our experiments were divided in different phases, each having the harmonic trap with unique frequencies, related to the formalism´s extensive variable. In each phase, we prepared individual samples of rubidium-87 gas in the trap and imaged them using the time-of-flight technique. Next, we fitted our images with the bimodal model and found the in situ density profiles of the samples with the standard regression procedures. Then, we determined the phase´s equation of state as a function of temperature and number of atoms. For the thermodynamic analysis, we used an empirical model that we developed to fit the equation-of-state curves, allowing us to represent our data in terms of mathematical coefficients. In that way, we found the curves of internal energy, heat capacity, thermal expansion and compressibility, thus achieving a full thermodynamic description of a harmonically trapped gas. Our method and its results are unprecedented in the literature and might contribute to the further understanding of non-uniform systems, as well as the future development of quantum thermal engines.
In this thesis we present some studies that were done in a trapped 87Rb Bose-Einstein condensate using the thermodynamic global variables. We have measured the global sound velocity by studying the variation of the total number of trapped atoms as a function of temperature. This method allowed us to determine the contribution of thermal and BEC components at each temperature. In order to study the sound velocity in each component, we treated both fluids as they were completely independent and we found great similarity with a published work. In addition, we analyze theoretically the validity of the global variables by comparing this approach with other methods. The specific heat for an ideal gas was evaluated using the global variables and by using the usual statistical approach found in textbooks. After finding the same result for both methods, we used the simplicity to implement the interaction in the global approach, to study the variation in the specific heat when the interactions are taken into account. The last comparison was done between global variables and the local density approximation. We have obtained that, for the isothermal compressibility and a equation of state, both methods provide equivalent results.
We present theoretical aspects concerning the thermodynamics of an ideal bosonic gas trapped by a harmonic potential. Working in the Grand Canonical ensemble we are able to properly identify the extensive thermodynamic variable equivalent to the volume and the intensive thermodynamic variable equivalent to the pressure. These are called the "harmonic volume" and the "harmonic pressure" and their physical meaning is discussed. With these variables, the problem of Bose-Einstein condensation is studied in terms of the behavior of the corresponding equation of state and in terms of measurable susceptibilities such as the heat capacities, the isothermal compressibility and the coefficient of thermal expansion. From the analysis, an interesting analogy with Black-Body radiation emerges, showing that at and below the critical temperature, the non-condensate fraction of atoms behaves thermodynamically like a gas of massless particles.