Spin-carrier coupling induced ferromagnetism and giant resistivity peak in EuCd2P2

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First glimpses of phase transitions in new materials are often gained through measurement of the temperature-dependent resistivity.  The resistivity of EuCd₂P₂, the subject of this research project, stands out as an extreme example of unconventional temperature dependence, quantitatively and qualitatively different from other systems exhibiting a resistivity peak. The metallic high-temperature resistivity undergoes a hundredfold increase with decreasing temperature and subsequently returns to metallic values. Both the rise and fall of the resistance take place well above the Neel temperature of 11.5 K. The peak value of the resistivity, which is found at 18K, is suppressed by modest magnetic fields, yielding a giant negative magnetoresistance. 

We have discovered that the resistivity peak occurs when time-reversal symmetry is broken by a previously undiscovered magnetic phase transition.  To detect this transition we introduced a new experimental technique enabling unambiguous detection of time-reversal symmetry breaking in complex correlated systems. The theoretical basis for the technique is microscopic time reversibility as encoded by Onsager’s relations: in time-reversal invariant systems the symmetric (with respect to exchange of indices) part of the dielectric tensor is an even function of the magnetic field. This ensures that in time-reversal symmetric systems, birefringence (defined here as the difference in reflectivity of vertically and horizontally polarized light) is also an even function of the magnetic field. Consequently, the detection of birefringence that is linear in applied magnetic field, an effect we refer to as linear magneto-birefringence (LMB), is direct evidence of time-reversal symmetry breaking. In order to measure LMB with high signal-to-noise ratio and free of spurious background signal, we developed a modulation technique in which the sample is placed in a coil that applies an oscillating magnetic field. Detection of birefringence at the fundamental modulation frequency arises exclusively from spontaneous time-reversal symmetry breaking. The sensitivity of this technique was crucial to recognize the onset of time reversal symmetry breaking in EuCd₂P₂, and it has the potential to identify novel phases in a broad range of materials. A variation of the technique can be applied to both metals and insulators, across a broad range of temperatures.