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DTSTAMP:20260410T213922
DTSTART:20161221T140000
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DTEND:20161221T140000
URL:https://murmitoyen.com/events/vanille/udem/detail/737558-soutenance-de-
 these-de-doctorat-de-reza-moosavi-askari
LOCATION:Pavillon Roger-Gaudry \, 2900\, boul. Édouard-Montpetit\, Local M
 -415\, Montréal\, QC\, Canada
SUMMARY:Soutenance de thèse de doctorat de Reza Moosavi-Askari
DESCRIPTION:Experimental and numerical study of a magnetic realization of a
  Bose-Einstein Condensate in a purely organic spin-1/2 quantum magnet (NIT
 2Py)
\nReza Moosavi-Askari supervisor: Andrea Bianchi co-supervisor: Miche
 l Côté président: Sjoerd Roorda member: François Schittekatte examinat
 or: Claude Bourbonnais (Université de Sherbrooke)
\nAbstract:
\nThe resea
 rch presented in this thesis focuses on studying the magnetic properties o
 f a purely organic free radical molecule NIT2Py\, which can condense into 
 highly sta- ble crystals at room temperature. The magnetization measuremen
 ts reveal that each molecule carries a magnetic moment due to the existenc
 e of a spin-1/2 unpaired elec- tron. The magnetic order is found to be ant
 iferromagnetic (AFM). The field dependence of the magnetization isotherm m
 easurement at 0.5 K shows a 1/2 magnetization plateau. Temperature depende
 nce specific heat and magnetocaloric effect are used to produce the (H\,T)
  phase diagram. In the vicinity of a quantum critical point\, the (H\,T) p
 hase boundary moves towards a power law with a critical exponent of 1.47(9
 ) that is com- patible with the universal value of 1.5 for a 3D BEC. The d
 ome-shaped field-induced ordering phases and the intersecting spin levels 
 observed in NIT2Py are signatures of magnetic realisation of BEC\, which i
 s seen for the first time in a purely organic quantum spin-1/2 AFM.
\n 
\
 nWe propose a relevant model spin Hamiltonian based on the Heisenberg exch
 ange interactions. We exploit state-of-the-art electronic structure calcul
 ations through den- sity functional theory combined with the broken symmet
 ry and the energy-mapping ap- proaches to evaluate the exchange coupling c
 onstants (Js)\, which in turn provide us with the sign and strengths of th
 e exchange interactions in the system. The strongest ex- change interactio
 ns are found to be related to three AFM Js. An appropriate magnetic lattic
 e\, which is a repeat pattern of a minimal set of strong Js\, is construct
 ed. We found that the magnetic lattice consisting of spin tetramers\, each
  having four spins with two intratetramer AFM interactions\, explains the 
 magnetic properties of NIT2Py. By resort- ing to mean-field approximation\
 , we establish a magnetization model for a system of interacting tetramers
 \, which can reproduce the magnetic properties of the system such as the t
 wo-step saturation with an intermediate plateau at half saturated magnetiz
 ation. Furthermore\, an exact diagonalization of the Heisenberg Hamiltonia
 n for a system in- cluding four tetramers is carried out\, and yields the 
 magnetization and other required thermodynamic properties of the system. 
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