自旋$1/2$磁体Ba $_2$ CuGe $_2$ O $_7$磁电响应的自旋电流物理指纹
Fingerprints of spin-current physics on magnetoelectric response in the spin-$1/2$ magnet Ba$_2$CuGe$_2$O$_7$
摘要：
作为Kramers简并的结果，自旋为1/2的单站点各向异性消失。我们认为，类似的性质适用于磁感应电极化的强度P，这应该只取决于在键中的自旋的相对取向，而不是每个单独的自旋的方向。因此，对于绝缘的多铁性化合物，P可以分解成对的各向同性，反对称和各向异性的贡献，这可以严格推导出超交换（SE）理论的框架，在类似的自旋哈密顿量。SE理论还使我们能够识别每个贡献背后的微观机制。其中最有争议和最有趣的是反对称或自旋电流机制。在这项工作中，我们提出，有争议的磁电（ME）性能的Ba 2CuGe 2 O 7可以单独解释的自旋电流机制，而其他贡献是小的或禁止的对称性。首先，我们明确地表明如何摆线自旋顺序诱导实验观察到的P在垂直于xy平面的方向，这可以很自然地解释的自旋电流机制在平面外的债券。然后，我们揭示了以前被忽视的ME效应，其中垂直于平面的磁场的应用不仅会导致无限公度转变，而且由于自旋电流机制在该平面内的相邻键中起作用，P也会翻转到平面内。在这两种情况下，P的大小和方向可以通过在xy平面中旋转自旋图案来控制。我们的分析是基于一个现实的自旋模型，这是严格来自第一性原理计算，并补充了新的算法的本地化Wannier函数的建设服从晶体对称性的Ba 2CuGe 2 O 7。
英文摘要：
The single-site anisotropy vanishes for the spin-1/2 as a consequence of Kramers degeneracy. We argue that similar property holds for the magnetically induced electric polarization P, which should depend only on the relative orientation of spins in the bonds but not on the direction of each individual spin. Thus, for insulating multiferroic compounds, P can be decomposed in terms of pairwise isotropic, antisymmetric, and anisotropic contributions, which can be rigorously derived in the framework of the superexchange (SE) theory, in an analogy with the spin Hamiltonian. The SE theory also allows us to identify the microscopic mechanism, which stands behind each contribution. The most controversial and intriguing one is antisymmetric or spin-current mechanism. In this work, we propose that the disputed magnetoelectric (ME) properties of Ba2CuGe2O7 can be explained solely by the spin-current mechanism, while other contributions are either small or forbidden by symmetry. First we explicitly show how the cycloidal spin order induces the experimentally observed P in the direction perpendicular to the xy plane, which can be naturally explained by the spin-current mechanism operating in the out-of-plane bonds. Then, we unveil previously overlooked ME effect, where the application of the magnetic field perpendicular to the plane not only causes the incommensurate-commensurate transition, but also flips P into the plane due to the spin-current mechanism operating in the neighboring bonds within this plane. In both cases, the magnitude and direction of P can be controlled by rotating the spin pattern in the xy plane. Our analysis is based on a realistic spin model, which was rigorously derived from the first-principles calculations and supplemented with the new algorithm for the construction of localized Wannier functions obeying the crystallographic symmetry of Ba2CuGe2O7.
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