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Viktor Cikojević defended his thesis on the study of ultracold atomic mixtures

Viktor Cikojević defended his thesis co-directed by Leandra Vranješ Markić and Jordi Boronat on April 20th 2021 at Campus Diagonal Nord, titled "Ab-initio Quantum Monte Carlo study of ultracold atomic mixtures". The thesis presents a systematic study of self-bound quantum droplets in a Bose-Bose mixture, employing techniques of Quantum Monte Carlo and Density Functional Theory.
Viktor Cikojević defended his thesis on the study of ultracold atomic mixtures
Dependence of the critical atom number of a 39K droplet on the magnetic field. Full circles are predictions using the QMC functional within DFT with the interaction potentials which reproduce both s-wave scattering length and the effective range. Diamond points are data from the experiment. Empty points show the prediction using the QMC functional with the HCSW model potentials

The properties of mixtures of Bose-Einstein condensates at T = 0 have been investigated with the aim of understanding physics beyond the mean-field theory in Bose-Bose mixtures. In particular, quantum liquid droplets with attractive intraspecies and repulsive interspecies attraction were studied, for which we observed significant contributions beyond Lee Huang Yang (LHY) theory that affect the energy, saturation density, and surface tension.

For a homogeneous system, extensive calculations of the equations of state were performed and we report the influence of finite-range effects in beyond-Bogoliubov theory. In systems interacting with a small (large) effective range, we observe repulsive (attractive) beyond-LHY contributions to the energy.

For the droplets in a mixture of 39K atoms, which were observed experimentally for the first time, the calculations of equations of state were performed. Combining QMC-based functionals with DFT, the discrepancy in the estimation of critical atom number between the mean-field theory and experimental results was explained by the proper inclusion of the effective range in inter-particle interaction models. The influence of finite-range effects on breathing and quadrupole modes in 39 K quantum droplets was investigated. We predicted a significant deviation in the excitation frequencies when entering a more correlated regime.

Finally, the phase diagram of repulsive Bose-Bose mixtures in a spherical harmonic trap using Quantum Monte Carlo calculations was studied. Density profiles were obtained and we found the occurrence of three phases: separation of condensates in two blobs, fully mixed and shell-separated phase. A comparison with the Gross-Pitaevskii solutions showed a large deviation in the regime of large mass imbalance and strong interactions. We showed the universality in the density profiles with respect to the s-wave scattering length and found numerical evidence for Gross-Pitaevskii scaling present beyond the regime of applicability of Gross-Pitaevskii equations.


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