Field-theoretical methods in quantum magnetism

Shoibal Chakravarty

Chia-Hao Chang, Cheng‐Liang Huang, Chi‐Kung Ni +6 more

The effects of collisions and magnetic fields on quantum beat are treated by the density matrix method. Some experimental quantum beat results of biacetyl in the presence of collisions are presented and theoretically analysed. It is shown that in this case not only pure dephasing but also pressure dependent inherent decay rate constants can be observed; this implies that the Schrödinger method cannot be employed for this case. The magnetic field effect on quantum beat is examined also and it is shown that for the biacetyl molecule magnetic splitting can be observed even in fields of a few gauss.

Swapan Kumar Pati, S. Ramasesha, Diptiman Sen

Quantum magnetism in low dimensions has been one of the central areas of theoretical research for many decades now. One of the key reasons for the long standing interest in this field has been the existence of simplified models, which serve as paradigms for understanding the role of strong interactions in many-electron systems. Although simple, these models quite often can not be solved exactly. In this review, we discuss a variety of analytical and numerical methods, which treat the system in a systematic and controlled manner. The central method employed in all the studies is the density matrix renormalization group (DMRG) method. This is supported by small scale numerical exact calculations, and by analytical methods using field theoretic techniques. We have considered a number of magnetic systems, from magnetic clusters to extended lattices, and have found some novel quantum ground states and low-energy elementary excitations. In some cases, we have also employed the finite-temperature DMRG method to accurately compute the low-temperature thermodynamic properties such as specific heat and magnetic susceptibility.

Ian B. Spielman

Zhi Liu, Lihong Dong, Zhe Qi +3 more

Abstract This work proposes a novel method for in situ measurement of the non‐orthogonal angles of magnetic field coils based on single‐beam magnetic resonance. The orthogonality of magnetic field coils is crucial for ensuring the accuracy of atomic magnetometers. The theoretical principles are derived to establish a measurement method for the non‐orthogonal angles. The proposed method requires only one single beam of light to perform in situ measurements of non‐orthogonal angles in the YZ and XZ directions. The experimental results demonstrate that the angle between the Y and Z coils is 89.3001 ± 0.0045°, a value that remains stable despite variations in experimental parameters such as temperature and light power, thereby confirming the robustness of the method. Additionally, the measured gyromagnetic ratio is 6.9963 ± 0.0005 Hz nT −1 , which closely matches the theoretical value of 6.9958 Hz nT −1 , further validating the method's accuracy. The proposed method provides a more straightforward approach and has significant potential for widespread application across various atomic magnetometers, especially in miniaturized configurations. Furthermore, this method may be extended to the field of atomic gyroscopes.