Buljubasich Lisandro, Sánchez Claudia M, Dente Axel D, Levstein Patricia R, Chattah Ana K, Pastawski Horacio M
Instituto de Física Enrique Gaviola (IFEG-CONICET), Córdoba 5000, Argentina.
Facultad de Matemática, Astronomía y Física, Universidad Nacional de Córdoba, Ciudad Universitaria, Córdoba 5000, Argentina.
J Chem Phys. 2015 Oct 28;143(16):164308. doi: 10.1063/1.4934221.
We performed Loschmidt echo nuclear magnetic resonance experiments to study decoherence under a scaled dipolar Hamiltonian by means of a symmetrical time-reversal pulse sequence denominated Proportionally Refocused Loschmidt (PRL) echo. The many-spin system represented by the protons in polycrystalline adamantane evolves through two steps of evolution characterized by the secular part of the dipolar Hamiltonian, scaled down with a factor |k| and opposite signs. The scaling factor can be varied continuously from 0 to 1/2, giving access to a range of complexity in the dynamics. The experimental results for the Loschmidt echoes showed a spreading of the decay rates that correlate directly to the scaling factors |k|, giving evidence that the decoherence is partially governed by the coherent dynamics. The average Hamiltonian theory was applied to give an insight into the spin dynamics during the pulse sequence. The calculations were performed for every single radio frequency block in contrast to the most widely used form. The first order of the average Hamiltonian numerically computed for an 8-spin system showed decay rates that progressively decrease as the secular dipolar Hamiltonian becomes weaker. Notably, the first order Hamiltonian term neglected by conventional calculations yielded an explanation for the ordering of the experimental decoherence rates. However, there is a strong overall decoherence observed in the experiments which is not reflected by the theoretical results. The fact that the non-inverted terms do not account for this effect is a challenging topic. A number of experiments to further explore the relation of the complete Hamiltonian with this dominant decoherence rate are proposed.
我们进行了洛施密特回波核磁共振实验，以通过一种称为比例重聚焦洛施密特（PRL）回波的对称时间反转脉冲序列，研究在缩放偶极哈密顿量下的退相干。由多晶金刚烷中的质子所代表的多自旋系统通过以偶极哈密顿量的长期部分为特征的两个演化步骤进行演化，该长期部分按因子|k|缩放且符号相反。缩放因子可以从0连续变化到1/2，从而能够探究一系列动力学复杂性。洛施密特回波的实验结果表明，衰减率的展宽与缩放因子|k|直接相关，这表明退相干部分受相干动力学支配。应用平均哈密顿量理论来深入了解脉冲序列期间的自旋动力学。与最广泛使用的形式不同，我们针对每个射频块进行了计算。对一个8自旋系统数值计算得到的平均哈密顿量的一阶项表明，随着长期偶极哈密顿量变弱，衰减率逐渐降低。值得注意的是，传统计算中忽略的一阶哈密顿量项对实验退相干率的排序给出了解释。然而，实验中观察到存在很强的整体退相干，而理论结果并未反映这一点。非反转项无法解释这种效应这一事实是一个具有挑战性的课题。我们提出了一些实验，以进一步探索完整哈密顿量与这种主导退相干率之间的关系。
