Charge-Ordering Transition in a Quasi-Two-Dimensional Organic Superconductor Probed by RF Penetration Depth Measurements in Pulsed Magnetic Fields

We have seen evidence of a charge ordering transition in β′′-(BEDT-TFF)₄​[(H₃​O)Ga(C₂​O₄​)₃​]C₆H₅ as a function of temperature based on a change of the effective electron mass. In addition to this transition, our data also identified a possible new phase transition as a function of magnetic field in the metallic state not far above the superconducting transition. Charge ordering transitions have been a focus of study recently, because they may be associated with the unconventional pairing mechanisms that lead to superconductivity. Charge ordering is often the result of enhanced electron-electron interactions, which in turn can increase the effective mass. We have measured rf penetration depth using a tunnel diode oscillator in pulsed magnetic fields up to 50 tesla and at various temperatures that yields a T_c = 8.5 K and shows clear quantum oscillations. Analyzing the oscillations using the Lifshitz-Kosevich formula shows a change in effective mass consistent with data reported by Imajo’s group [1]. The rf penetration depth also contained a feature at 8 tesla that could be related to a phase transition, further supported by specific heat and magnetization measurements. This feature has not been previously reported in magnetoresistance measurements suggesting it is magnetic in nature. The origin of this feature remains unclear given that there are no magnetic ions in this material.