无质量Dirac费米子中倾斜锥诱导的易平面赝自旋铁磁体和Kosterlitz-Escherless跃迁
Tilted-Cone-induced easy-plane pseudo-spin ferromagnet and Kosterlitz-Thouless transition in massless Dirac fermions
摘要：
本文研究了在压力作用下二维无质量Dirac费米子在填充因子为$ν=0$的情况下的量子Hall铁磁体，其中二维Dirac费米子在压力作用下具有斜锥和时间反演对称性导致的双重谷简并，在$α$ -（BEDT-TTF）$_2$ I $_3$中的零能（N=0）朗道能级.在没有倾斜的狄拉克锥的情况下，N=0朗道能级中的长程库仑相互作用表现出SU（2）谷赝自旋对称性，甚至达到$O(a/l_{\rm H})$级，这与$N \ne 0$朗道能级相反，其中$a$和$l_{\rm H}$分别代表晶格常数和磁长度。这种特性来自于这样一个事实，即在特定谷中的零能态仅限于其中一个旋量分量，而另一个旋量分量必然为零。在$α$ -（BEDT-TTF）$_2$ I $_3$中发现的倾斜狄拉克锥的情况下，人们得到第二分量的非零值，然后谷之间的ack散射过程变得非零。结果表明，这一事实可以导致易平面赝自旋铁磁性（XY型）。在这种情况下，序参量的相位起伏可以用XY模型来描述，导致在较低温度下的Kosterlitz-无相变。根据这些理论结果，讨论了$α$ -（BEDT-TTF）$_2$ I $_3$的电阻率实验结果。
英文摘要：
The possible quantum Hall ferromagnet at a filling factor $ν=0$ is investigated for the zero-energy (N=0) Landau level of the two dimensional massless Dirac fermions in $α$-(BEDT-TTF)$_2$I$_3$ under pressure with tilted cones and a twofold valley degeneracy resulting from time-reversal symmetry. In the case of the Dirac cones without tilting, the long-range Coulomb interaction in the N=0 Landau level exhibits the SU(2) valley-pseudo-spin symmetry even to the order $O(a/l_{\rm H})$, in contrast to $N \ne 0$ Landau levels, where $a$ and $l_{\rm H}$ represent the lattice constant and the magnetic length, respectively. Such a characteristic comes from a fact that zero-energy states in a particular valley are restricted to only one of the spinor components, whereas the other spinor component is necessarily zero. In the case of the tilted Dirac cones as found in $α$-(BEDT-TTF)$_2$I$_3$, one obtains a non-zero value of the second component and then the ackscattering processes between valleys becomes non-zero. It is shown that this fact can lead to easy-plane pseudospin ferromagnetism (XY-type). In this case, the phase fluctuations of the order parameters can be described by the XY model leading to Kosterlitz-Thouless transition at lower temperature. In view of these theoretical results, experimental findings in resistivity of $α$-(BEDT-TTF)$_2$I$_3$ are discussed.
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