Quantum Hall effect in intentionally disordered two‐dimensional electron systems
Quantum Hall effect in intentionally disordered two‐dimensional electron systems
复制标题
故意无序二维电子系统中的量子霍尔效应
作者：
K. Buth;U. Merkt
在这项工作中，故意无序的二维电子系统调制掺杂GaAs/GaAlAs异质结的磁输运实验进行了研究。这种无序是由带负电荷的铍受体的δ掺杂层提供的。在低磁场下，观察到强烈的负磁阻，这可以归因于磁场诱导的离域。在增加的磁场下，量子霍尔效应表现出宽的霍尔平台，其中心被转移到更高的磁场，即更低的填充因子。这种转变可以用不对称的态密度来解释。一致地，在高磁场中量子霍尔液滴到绝缘状态的转变发生在临界填充因子约为Vc =0.4时，即远低于对称无序势所预期的值1/2。绝缘体转变的特征在于纵向电阻以及霍尔电阻的发散。这与在绝缘状态下观察到有限霍尔电阻的其他实验相反，并且以前没有观察到。根据最近的理论研究，霍尔电阻的发散指向通过量子霍尔液滴之间的隧穿的量子相干输运。磁输运实验通过模拟排斥散射体的随机和相关分布的潜在景观来补充，这使得能够确定远红外激发的逾渗阈值，态密度和振子强度。这些模拟表明，霍尔平台的强烈移位和观察到的临界填充因子的绝缘体过渡在高磁场中需要一个非对称的状态密度，只能产生一个强相关的铍分布。在相同的样品上的回旋共振也表明铍受体之间的相关性的可能性。
In this work intentionally disordered two‐dimensional electron systems in modulation doped GaAs/GaAlAs heterostructures are studied by magnetotransport experiments. The disorder is provided by a δ‐doped layer of negatively charged beryllium acceptors. In low magnetic fields a strong negative magnetoresistance is observed that can be ascribed to magnetic‐field‐induced delocalization. At increased magnetic fields the quantum Hall effect exhibits broad Hall plateaus whose centers are shifted to higher magnetic fields, i.e. lower filling factors. This shift can be explained by an asymmetric density of states. Consistently, the transition into the insulating state of quantum Hall droplets in high magnetic fields occurs at critical filling factors around νc=0.4, i.e. well below the value 1/2 that is expected for symmetric disorder potentials. The insulator transition is characterized by the divergence of both the longitudinal resistance as well as the Hall resistance. This is contrary to other experiments which observe a finite Hall resistance in the insulating regime and has not been observed previously. According to recent theoretical studies the divergence of the Hall resistance points to quantum coherent transport via tunneling between quantum Hall droplets. The magnetotransport experiments are supplemented by simulations of potential landscapes for random and correlated distributions of repulsive scatterers, which enable the determination of percolation thresholds, densities of states, and oscillator strengths for far‐infrared excitations. These simulations reveal that the strong shift of the Hall plateaus and the observed critical filling factor for the insulator transition in high magnetic fields require an asymmetric density of states that can only be generated by a strongly correlated beryllium distribution. Cyclotron resonance on the same samples also indicates the possibility of correlations between the beryllium acceptors.