Title Ab-initio Quantum Monte Carlo study of ultracold atomic mixtures Title (english) Ultrahladne atomske mješavine istražene ab-initio kvantnom Monte Carlo metodom Author Viktor Cikojević Mentor Leandra Vranješ Markić (mentor)Mentor Jordi Boronat Committee member Hrvoje Buljan (predsjednik povjerenstva)Committee member Pietro Alberto Massignan Committee member Luis Santos Sanchez (član povjerenstva)000000000000000 Committee member Joaquim Casulleras Ambros (član povjerenstva)87484488 Committee member Dario Jukić (član povjerenstva)Granter University of Zagreb Defense date and country 2021, Croatia Scientific / art field, NATURAL SCIENCES Universal decimal classificationUDC ) 53 - Physics Abstract The properties of mixtures of Bose-Einstein condensates at T=0 have been investigated using quantum Monte Carlo(QMC) methods and Density Functional Theory (DFT) 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. The critical atom number in droplets in free space for total number of atoms N between N=30 and N =2000 was obtained. Results of the surface tension for three values of the attractive interspecies interactions are presented. 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 39^K atoms, which were observed experimentally for the first time, the calculations of equations of state were performed. Combining QMC-built 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 reported, 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.
Abstract (english) Svojstva smjesa Bose-Einsteinovih kondenzata pri T=0 istražena su korištenjem metoda kvantnog Monte Carla (QMC) i teorije funkcionala gustoće(DFT) s ciljem proučavanja fizike izvan teorije srednjeg polja u bozonskim mješavinama. Proučili smo kvantne kapljice s jednakim i odbojnim interakcijama između atoma istovrsne komponente te privlačnim interakcijama atoma različitih komponenti u interakciji i opazili smo značajne doprinose povrh Lee Huang Yang (LHY) teorije koji utječu na energiju, saturacijsku gustoću i površinsku napetost. Odredili smo kritični broj atoma za kapljice u slobodnom prostoru za broj atoma u kapljici N između N=30 i N=2000. Izračunali smo površinsku napetost za tri vrijednosti privlačnih međuatomskih interakcija. Izvršili smo opsežne proračune jednadžbi stanja iznimno rijetke tekućine bozonske mješavine i uočili utjecaj efekata konačnog dosega koji nije predviđen Bogoliubovljevom teorijom. U sustavima koji interagiraju s malim (velikim) efektivnim dosegom, opaženi su odbojni (privlačni) doprinosi koje ne predviđa LHY teorija. Izračunali smo jednadžbe stanja za kapljice bozonskih mješavina koje su po prvi put eksperimentalno uočene u smjesi 39^K atoma. Kombinirajući funkcionale gustoće izgrađene pomoću kvantnog Monte Carla s DFT-om, neslaganje u procjeni kritičnog broja atoma između teorije srednjeg polja i eksperimentalnih rezultata je objašnjeno preko pravilnog uključivanja efektivnog dosega u modele međudjelovanja čestica. Istražen je utjecaj efektivnog dosega na pobuđenja kapljice 39^K, i to na mod disanja i kvadrupolni mod. Dobiveni rezultati prikazuju značajno odstupanje frekvencija pobude pri ulasku u korelirani režim. Detaljno smo proučili fazni dijagram odbojnih Bose-Bose mješavina u sfernoj harmonijskoj zamci koristeći kvantne Monte Carlo račune. Dobiveni su profili gustoće koji pokazuju pojavu tri faze: separacija kondenzata u dvije nakupine, potpuno miješanje i separacije u obliku ljuske. Usporedba s rješenjima Gross-Pitaevskii jednadžbi pokazuje veliko odstupanje u režimu velike masene neravnoteže i jakih interakcija. Pokazali smo univerzalnost profila gustoće s obzirom na s-valnu duljinu raspršenja te postojanje Gross-Pitaevskii skaliranja prisutnog izvan dosega primjenjivosti Gross-Pitaevskii jednadžbi.
Keywords
quantum Monte Carflo methods
diffusion Monte Carlo
densfity functional theory
Gross-Pitaevskii equation
Bose-Bose mixtures
quantum liquids
beyond-Bogoliubov calculations
finite-range effects
excitation modes
Keywords (english)
kvantne Monte Carlo metode
difuzijski Monte Carlo
teorija funkcionala gustoće
Gross-Pitaevskii jednadžba
bozonske mješavine
kvantne tekućine
povrh-Bogoliubov računi
efekti konačnog dosega
modovi pobuđenja
Language english URN:NBN urn:nbn:hr:217:633063 Promotion 2021 Study programme Title: Physics Type of resource Text Extent 128 str. File origin Born digital Access conditions Open access Terms of use Created on 2021-06-07 09:51:27
