Quantum matter in high magnetic fields

Abstract:

Experiments on three double-layer (n=2) Ruddlesden–Popper (RP) systems are reported. Doping Sr1.8La1.2Mn2O7 (Tc=126 K) with Nd to form Sr1.8La1.2−xNdxMn2O7 leads to a reduction in Curie temperature for low doping levels (x=0.2), and to behavior reminiscent of Sr1.8Nd1.2Mn2O7 for x⩾0.7. This suggests that it may be possible to control the temperature of maximum magnetoresistance chemically in these phases. The application of pressure (0<P/GPa⩽1.8) is shown to modify the magnetotransport properties of Sr2NdMn2O7 to resemble those of Sr1.9Nd1.1Mn2O7. The changes can be explained by considering the relative strength of ferromagnetic and antiferromagnetic interactions within the material. Finally, the need for careful phase analysis of n=2 RP materials is demonstrated by the misleading magnetization data recorded for a sample of Sr1.8Sm1.2Mn2O7 containing ∼2.8% of an n=∞ perovskite phase.

Abstract:

Weyl semimetals exhibit exceptional quantum electronic transport due to the presence of topologically-protected band crossings called Weyl nodes. The nodes come in pairs with opposite chirality, but their number and location in momentum space is otherwise material specific. Following the initial discoveries there is now a need for better material realizations, ideally comprising a single pair of Weyl nodes located at or very close to the Fermi level and in an energy window free from other overlapping bands. Here we propose the layered intermetallic EuCd$_2$As$_2$ to be such a system. We show that Weyl nodes in EuCd$_2$As$_2$ are magnetically-induced via exchange coupling, emerging when the Eu spins are aligned by a small external magnetic field. The identification of EuCd$_2$As$_2$ as a model magnetic Weyl semimetal, evidenced here by ab initio calculations, photoemission spectroscopy, quantum oscillations and anomalous Hall transport measurements, opens the door to fundamental tests of Weyl physics.