Antiferromagnetic Insulating Ground State of Molecular π-d System Λ-(BETS)2FeCl4 (BETS = Bis(ethylenedithio)tetraselenafulvalene): A Theoretical and Experimental Review

Yugo Oshima,Heng‐Bo Cui,Reizo Kato

doi:10.3390/magnetochemistry3010010

Abstract

The πd molecular conductor λ‐(BETS)2FeCl4, where BETS is bis(ethylenedithio) tetraselenafulvalene, has attracted considerable interest for the discovery of its field induced superconducting state. A mystery of this system is its antiferromagnetic insulating ground state. The point still under strong debate is whether the d spins in Fe3+ are ordered or not. Here, we review experimental and theoretical studies on the antiferromagnetic insulating phase in λ‐(BETS)2FeCl4 and mention our perspective based on our ESR measurements for λ‐(BETS)2FexGa1−xCl4. Our ESR results indicate that the π‐d interaction in the system is very strong and there is no sign of paramagnetic Fe spins in the antiferromagnetic ground state.

Highlights

Molecular conductors with finite π-d interactions have attracted many interests for the past few decades since interaction between the itinerant π-electron and localized d-electron yields some intriguing physical phenomena
A complementary study was performed by Brossard et al [8]. This compound is metallic in the high temperature phase, and the magnetic susceptibility, where the dominant magnetic contribution originating from the Fe3+ ion, shows the paramagnetic behavior
The authors found that the metal to insulator (MI) transition temperature is suppressed by applying a high magnetic field, and a metallic state is recovered above 10 T

Summary

Introduction

Molecular conductors with finite π-d interactions have attracted many interests for the past few decades since interaction between the itinerant π-electron and localized d-electron yields some intriguing physical phenomena. Λ-(BETS)2FeCl4, where BETS is bis(ethylenedithio)tetraselenafulvalene, is one of the most studied and well-known π-d molecular conductors for the appearance of superconductivity in a high magnetic field [3,4]. In contrast to the superconducting state in a high magnetic field, this system shows cooperative conducting and magnetic properties in the low-field region thanks to the strong interaction between π- and d-electrons, namely, π-d interaction. In the high temperature phase, λ-(BETS)2FeCl4 is a metal where the magnetic property shows the paramagnetic behavior. A complementary study was performed by Brossard et al [8] This compound is metallic in the high temperature phase, and the magnetic susceptibility, where the dominant magnetic contribution originating from the Fe3+ ion, shows the paramagnetic behavior. In the intermediate region (0.3 < x < 0.5), a complicated phase diagram accompanied by small changes of Tc and TN is observed [13,15]

Transport and Magnetic Properties

Specific Heat Measurements and the Paramagnetic Fe Model

Theoretical Studies

Other Experimental Studies

Findings

Previous ESR Studies