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## Seminar details

 Name BEC Seminar Title Excitations and thermodynamics in a one-dimensional Bose gas Details We study the solitonic Lieb II branch of excitations the in one-dimensional Bose gas in homogeneous and trapped geometry. Using Bethe-ansatz Lieb's equations we calculate the effective number of atoms'' and the effective mass'' of the excitation. The equations of motion of the excitation are defined by the ratio of these quantities. The frequency of oscillations of the excitation in a harmonic trap is calculated.We show that the chemical potential exhibits a nonmonotonic temperature dependence which is peculiar of superfluids. The effect is a direct consequence of the phononic nature of the excitation spectrum at large wavelengths exhibited by 1D Bose gases. We demonstrate that the quadratic coefficient in the expansion low-temperature expansion of the chemical potential is entirely defined by the zero-temperature density dependence of the sound velocity, while the quartic coefficient experiences opposite-direction effects originating from (a) non-linearity of the excitation spectrum (b) negative excluded-volume corrections.Finally, the energy of a 1D Bose gas is found in a box with zero boundary condition. It is shown that a "standard" solution dating back to Gauden's work in 1971 in the thermodynamic limit actually describes a dark soliton and not zero boundary condition. Time Friday, 29 May 2020, at 13:30 CEST The seminar was held! Speaker Grigori Astrakharchik  (Universitat Politècnica de Catalunya) Seminar Language English Organisers prof. Kazimierz Rzążewski (CFT); prof. Mariusz Gajda (IF PAN); prof. Mirosław Brewczyk (UB);