**Experimental and numerical study of a magnetic realization of a Bose-Einstein Condensate in a purely organic spin-1/2 quantum magnet (NIT2Py)**

Reza Moosavi-Askari

supervisor: Andrea Bianchi

co-supervisor: Michel Côté

président: Sjoerd Roorda

member: François Schittekatte

examinator: Claude Bourbonnais (Université de Sherbrooke)

Abstract:

The research presented in this thesis focuses on studying the magnetic properties of a purely organic free radical molecule NIT2Py, which can condense into highly sta- ble crystals at room temperature. The magnetization measurements reveal that each molecule carries a magnetic moment due to the existence of a spin-1/2 unpaired elec- tron. The magnetic order is found to be antiferromagnetic (AFM). The field dependence of the magnetization isotherm measurement at 0.5 K shows a 1/2 magnetization plateau. Temperature dependence 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*) phase 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 dome-shaped field-induced ordering phases and the intersecting spin levels observed in NIT2Py are signatures of magnetic realisation of BEC, which is seen for the first time in a purely organic quantum spin-1/2 AFM.

We propose a relevant model spin Hamiltonian based on the Heisenberg exchange interactions. We exploit state-of-the-art electronic structure calculations through den- sity functional theory combined with the broken symmetry and the energy-mapping ap- proaches to evaluate the exchange coupling constants (*J*s), which in turn provide us with the sign and strengths of the exchange interactions in the system. The strongest ex- change interactions are found to be related to three AFM *J*s. An appropriate magnetic lattice, which is a repeat pattern of a minimal set of strong *J*s, is constructed. 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 two-step saturation with an intermediate plateau at half saturated magnetization. Furthermore, an exact diagonalization of the Heisenberg Hamiltonian for a system in- cluding four tetramers is carried out, and yields the magnetization and other required thermodynamic properties of the system.