Transport properties of space-time asymmetric conductive antiferromagnets

我们发现了一个非常大的磁阻效应在家庭的BaMn 2 Pn 2反铁磁绝缘体，其中Pn代表的As，Sb和Bi。在这种效应中，当反铁磁磁矩被外部磁场倾斜时，材料的电导率被强烈地增强。在BaMn 2Bi 2的情况下，该效应强到足以抑制绝缘基态，并且诱导金属基态。有趣的是，BaMn_2 Pn_2的Neel温度很高，并且没有观察到磁性转变。从对称性的角度来看，BaMn_2 Pn_2中的磁电阻效应可以解释为反铁磁有序的宇称时间对称性被打破的效应.在BaMn 2 Pn 2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;因此时间和空间反转操作交织在一起。空间反转同时反转Mn离子的自旋，这被称为宇称-时间对称。在某些条件下，宇称-时间对称性允许或甚至促进载流子的局部化，类似于更常规材料中的时间反演对称性的效果。宇称-时间对称性被磁场破坏，反铁磁矩倾斜，反过来局域化可以被提升，产生大的负磁阻。

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.