Damping and noise of magnetic dipoles collectively coupled with a resonant circuit

Author(s)

Alencar José de Faria

• Document type: Doctoral Thesis
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Instituto de Física (IF/SBI)
• Defense date: 2008-03-17

Examining board members

Humberto de Menezes Franca; Antonio Vidiella Barranco; Ibere Luiz Caldas; Kaled Dechoum; Said Rahnamaye Rabbani

• Advisor: Humberto de Menezes Franca

Abstract

We study the radiation damping and the spin noise of a magnetic material coupled with a resonant circuit. Radiation damping in magnetic resonance is a dissipation phenomenon, where magnetization prepared after a Rabi pulse decays toward its equilibrium state. The magnetic sample loses its energy by the coupling with resonant circuit, that must be tuned in Larmor frequency of the sample spins. Even though this phenomenon had been studied many years ago, no full quantum description was done. We present a quantum Hamiltonian model, that explains the radiation damping. We use quantum Langevin equation method for this task. Beyond radiation damping, we show the magnetization acquires an unusual intrincate motion, if the circuit initial state is coherent. Using the same Langevin equation, we study the sample influence on the resonant circuit noise. We calculate the current spectral density in the case of thermal equilibrium of whole system. We can verify the method efectiveness, comparing former papers. Moreover we study modifcations in the circuit noise, if an external oscillating tension is applied. In this situation, other two peaks emerge in the central peak sidebands of the sample absorption spectrum. It leads to appear three dips in circuit current spectrum. This efect is due to the splitting of the spin energy states. We comment about the analogy between this phenomenon and the resonance fluorescence in Quantum Optics. (AU)