The interfacial profiles and interfacial tensions of phase-separated binary mixtures of Bose-Einstein condensates are studied theoretically. The two condensates are characterized by their respective healing lengths and £2 and by the interspecies repulsive interaction K. An exact solution to the Gross-Pitaevskii (GP) equations is obtained for the special case I 2/ I 1 = 1/2 and K = 3/2. Furthermore, applying a double-parabola approximation (DPA) to the energy density featured in GP theory allows us to define a DPA model, which is much simpler to handle than GP theory but nevertheless still captures the main physics. In particular, a compact analytic expression for the interfacial tension is derived that is useful for all f ,, | 2, and K. An application to wetting phenomena is presented for condensates adsorbed at an optical wall. The wetting phase boundary obtained within the DPA model nearly coincides with the exact one in GP theory.

The phase structure of the linear sigma model at finite isospin chemical potential μ and temperature T is systematically studied in the non-standard case of symmetry breaking by means of the Cornwall-Jackiw-Tomboulis effective potential. The latter quantity is calculated in the improved Hartree-Fock approximation which preserves the Goldstone theorem and the thermodynamic consistency. It results that the charged pions are condensed for μ equal to the pion mass in vacuum and the pion condensation corresponds to a first-order phase transition for 0 T 175.813 MeV, whereas at higher temperature it becomes a second-order phase transition. Moreover, the chiral-symmetry restoration, which is a first-order phase transition, occurs for 138.464 MeV T 147.168 MeV. The phase diagrams for both pion and chiral condensates are obtained.

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By means of Cornwall-Jackiw-Tomboulis effective action we investigate a dilute weakly interacting Bose gas at finite temperature. The shift of critical temperature is obtained in the universal form ∆T C /T (0) C = c.n 1/3 0 a s with constant c. The non-condensate fraction is expressed in sum od three terms, which correspond to the quantum fluctuation, thermal fluctuation and both.

The localized low-energy interfacial excitations, or Nambu-Goldstone modes, of phase-segregated binary mixtures of Bose-Einstein condensates are investigated analytically by means of a doubleparabola approximation (DPA) to the Lagrangian density in Gross-Pitaevskii theory for a system in a uniform potential. Within this model analytic expressions are obtained for the excitations underlying capillary waves or "ripplons" for arbitrary strength K (> 1) of the phase segregation.The dispersion relation ω ∝ k 3/2 is derived directly from the Bogoliubov-de Gennes equations in limit that the wavelength 2π/k is much larger than the healing length ξ. The proportionality constant in the dispersion relation provides the static interfacial tension. A correction term in ω(k) of order k 5/2 is calculated analytically, entailing a finite-wavelength correction factor (1 +). This prediction may be tested experimentally using (quasi-)uniform optical-box traps. Explicit expressions are obtained for the structural deformation of the interface due to the passing of the capillary wave. It is found that the amplitude of the wave is enhanced by an amount that is quadratic in the ratio of the phase velocity ω/k to the sound velocity c. For generic asymmetric mixtures consisting of condensates with unequal healing lengths an additional modulation is predicted of the common value of the condensate densities at the interface.

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By means of Cornwal–Jackiw–Tomboulis effective action approach we considered the finite-size effect on Bose–Einstein condensate mixtures confined between two parallel palates. Keeping upto double-bubble approximation and taking into account the conservation of Goldstone boson, our results are quite different from those in previous works within one-loop approximation. The order parameters strongly depend on the distance between palates and can be expressed via the correction terms. The Casimir force is also considered.

Using field theory we calculate the Casimir energy and Casimir force of two-component BoseEinstein condensates restricted between two parallel plates, in which Dirichlet and periodic boundary conditions applied. Our results show that, in one-loop approximation, the Casimir force equals to summation of the one of each component and it is vanishing in some cases: (i) inter-distance between two plates becomes large enough; (ii) intraspecies interaction is zero; (iii) interspecies interaction is full strong segregation.