High Field Induced Bose-Einstein Condensation Studies Of Quantum Magnets ?Sr, Ba?₃Cr_2-xM_xO₈

A Bose-Einstein condensation (BEC) is a state of a large fraction of bosons with the same lowest quantum state. Quantum magnets Sr3CrO8 and Ba3CrO8 have the same trigonal R3m crystal structure and Cr ions with spin s= 1/2. In these materials, a Cr atom and its nearest Cr atoms interact with each other by spin coupling, resulting in an energy gap Jo between the ground state with total spin S= 0 and the first triplet excited state with total spin S=1. With an applied magnetic field, the first triplet excited state is split due to the Zeeman Effect, in which the|SZ= 1> state is gradually suppressed with increasing magnetic field and eventually condenses to ground state at the first critical field. In this case, a BEC occurs. In order to explore the nature of BEC in quantum magnets, and regulate this phenomenon by temperature and magnetic field, we studied the influence of the ferromagnetic, antiferromagnetic and non-magnetic ions dopant on BEC. The main contents are as follows:Firstly, a brief introduction of the development of BEC, researching significance of quantum magnetic materials, and explanation of the BEC mechanism in quantum magnets Sr3Cr2O8 and Ba3Cr2O8 systems were given.Secondly, the single crystal growth conditions of SraCr2O8 and BaaCr2O8 and their doped samples have been explored. The single crystals of (Sr, Ba)3Cr2-xMxO8 (M=Fe?V? Nb) have been successfully grown. Based on the pulsed high magnetic field facility, the temperature and magnetic field dependent magnetization have been measured. Experimental results show that with the temperature increasing, the first critical field increases, but the second critical magnetic field decreases. Furthermore, the dopant and its doping level affect the critical fields. Finally, the phase diagram of BEC of the Sr3Cr2O8 and Ba3Cr2O8 has been obtained.Thirdly, the effects of different doping elements (Fe, V) on BEC have been studied. Experimental results show that dopants enhance the intradimer interaction J0, change the critical temperature, and increase the first critical field. Doped Fe ions make the phase transition process from singlet state to the first excited triplet state more sensitive to the applied magnetic field. However the non-magnetic V ion has little impact on this transition. Especially, the BEC behavior was observed even with temperature up to 10 Kelvin, thus, this systemic materials provide an excellent platform for the study of BEC phenomena.Fourthly, the effects of different Fe doping level on BEC have been studied. Experimental results show that the Fe dopant can enhance the magnetic sensitivity of the intradimer interaction Jo, the interdimer interaction J',and the changing rate of temperature ?x/?T.Finally, exploratory studies of optical absorption with zero magnetic field and Faraday rotation under ultra-high magnetic field of Sr3Cr2O8 single crystal have been done. It has been found that at 4.2 Kelvin and zero magnetic field, an absorption peak was observed at 1731 nm wavelength in a-100 microns thick sample. However, under ultra-high magnetic field and present measuring conditions, no Faraday rotation signal was detected in the Sr3Cr2O8 single crystal sample.