Evidence of unconvensional density wave in low dimensional organic compounds

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In a electron spin resonance (ESR) measurement by using a cavity, it is important that the temperature is well controlled to be constant in order that the frequency should be stable. In the ESR measurement system, a lock-in amplifier to detect output, a constant voltage generator for the precise magnetic field control and the temperature controller are connected to a computer through IEEE-488 interface. The program is made to conduct the measurement automatically by using LabVIEW software.The organic compound λ-(BEDT-TSF)_2Fe_<1-x>Ga_xCl_4(x=0,0.4,1.0) is the typical example of so called n-d system. There is the strong correlation between the Π electrons at the BEDT-TSF molecules and the d electrons at Fe ions. For x=0, the material shows the paramagnetic metal above 9K and it becomes the antiferromagnetic insulator below 9 K. As the Ga concentration x increases, the metal insulator transition temperature decreases. For x>5.0, the superconducting phase appears. For the material with x= … More 0, the anomaly in the π electrons are reported by the dielectric response, the specific heat, the proton NMR and the X-ray diffraction measurements at around 70 K in addition to the metal insulator transition at 9 K.The g-value and the line width along H//a^*, H//b^* and H//c are measured in the temperature range from 1.5 to 300 K, The magnetic field direction dependencies are measured at several temperatures between 20 and 300 K. In the ESR measurements for the crystal with x=0, only one resonance signals observed in the temperature range from 7 to 300 K. The magnetic field direction dependencies at 120 K do not show the symmetry of the two-dimensional conducting plane. Therefore, the response from the π electrons is combined with that from the d electrons of Fe ions. The shift of the g-value and the enhancement of the line width are observed below 70 K. It is a strong evidence of the strong π-d coupling that the ESR anomaly occurs accompanied by the anomaly in the π electrons.For the crystal with x=0.4, similar anomaly in the ESR signal is observed. This suggests the dielectric anomaly also in the compounds except for x=0. In the case of the crystal with x=1.0, both of the temperature dependencies and magnetic field direction dependencies show that the ESR signal originates from the π electrons as expected. Less