Development of multi-extreme terahertz ESR and its application to quantum spin system

Abstract

Multi-extreme terahertz ESR developed at Kobe University will be presented. Multi-extreme terahertz ESR in Kobe covers the frequency region between 0.03 and 7 THz and the temperature region between 1.8 and 300 K. Multi-extreme conditions such as the high field and the high pressure can be combined with our system. High field up to 55 T can be applied using a pulsed magnet and a 300 kJ (10kV) capacitor bank. High pressure up to 1.4 GPa can be also applied together with the high field. Developments of two new detection methods will be also shown. One is a micro-cantilever ESR system using a torque method, which enables the ESR measurement of μm size single crystal. We have succeeded in making the measurement up to 315 GHz and the achieved sensitivity is about 1010 spins/G, which is much higher than the sensitivity of 1015 spins/G using the conventional transmission method. Another method is a longitudinal ESR detection using a SQUID magnetometer recently. The potentials of new methods will be discussed. Secondly, as an application of multi-extreme terahertz ESR, ESR measurement of natural mineral dioptase (Cu₆Si₆O₁₈6H₂O), which has a unique S=1/2 antiferromagnetic spin network called the dioptase lattice, will be presented. In this system Cu²⁺ ions form the helical antiferromagnetic chain along the c-axis and the chains are also antiferromagnetically coupled. The magnetic susceptibility shows a maximum around 50 K suggesting the low dimensional character of the system, and it shows an antiferromagnetic order below TN=15.5 K. EPR cannot be observed by the conventional X-band ESR because of its very broad linewidth, and the terahertz ESR is essential to observe EPR. Moreover, the measurement up to 1 THz is required to obtain the whole profile of AFMR at 4.2 K. The details of our measurement results and analyses will be discussed.