Transport properties of space-time asymmetric conductive antiferromagnets

我们在BaMn2Pn2反铁磁绝缘体家族中发现了非常大的磁阻效应，其中Pn代表As， Sb和Bi。在这种效应下，当反铁磁矩被外磁场倾斜时，材料的电导率会得到强烈增强。在BaMn2Bi2的情况下，这种效应足以抑制绝缘基态，并诱导出金属基态。有趣的是，BaMn2Pn2的尼尔温度很高，没有观察到磁转变。这使得我们的发现从其他已知的巨大磁阻效应中脱颖而出，其中磁场诱导从反铁磁绝缘体到铁磁金属的转变。从对称性的角度来看，BaMn2Pn2中的磁电阻可以解释为从反铁磁序继承的奇偶时间对称性被打破的影响。在BaMn2Pn2中，自旋向上和自旋向下的Mn离子可以通过反转中心交换它们的晶体位置；因此，时间和空间反演操作是交织在一起的。空间反转同时反转Mn离子的自旋，这被称为奇偶时间对称。在一定条件下，奇偶-时间对称性允许甚至促进载流子的局域化，类似于更常规材料中的时间反转对称性的效果。当磁场使反铁磁矩倾斜时，奇偶时间对称性就会被破坏，而局域化又会被提升，从而产生较大的负磁阻。

We discovered a very large magnetoresistive effect in the family of BaMn2Pn2 antiferromagnetic insulators, where Pn stands for As, Sb, and Bi. In this effect, the conductivities of the materials are strongly enhanced when the antiferromagnetic moments are tilted by an external magnetic field. In the case of BaMn2Bi2, the effect is strong enough to suppress the insulating ground states and a metallic ground state is induced. Interestingly, the Neel temperatures of the BaMn2Pn2 are high, and no magnetic transition was observed. This makes our findings stand out from other well-known colossal magnetoresistive effects, in which the magnetic field induces a transition from an antiferromagnetic insulator to a ferromagnetic metals.From the viewpoint of symmetry, the magnetoresistance in BaMn2Pn2 can be explained as an effect of the breaking the parity-time symmetry inherited from the antiferromagnetic order. In BaMn2Pn2, the spin up and spin down Mn ions can exchange their crystallographic positions via an inversion center; and hence the time- and space-inversion operations are intertwined. A space inversion simultaneously invert the spins of the Mn ions, and this is called parity-time symmetry. Under certain conditions, the parity-time symmetry allows or even facilitates the localization of carriers, being similar the effect of time reversal symmetry in more conventional materials. The parity-time symmetry is destroyed by a magnetic fields that tilts the antiferromagnetic moments, and in turn the localization can be lifted, producing a large negative magnetoresistance.