[论文解读] Orbital Multiferroicity in Pentalayer Rhombohedral Graphene

Tonghang Han, Zhengguang Lu+9 more|arXiv (Cornell University)|Aug 17, 2023

Multiferroics and related materials参考文献 40被引用 10

一句话总结

该论文报道了五层菱菱形石墨烯中的轨道多铁性，揭示了一个谷–磁化四重态以及可通过电场耦合于 E·B 而实现调控的 ferro-谷电子序。

ABSTRACT

Ferroic orders describe spontaneous polarization of spin, charge, and lattice degrees of freedom in materials. Materials featuring multiple ferroic orders, known as multiferroics, play important roles in multi-functional electrical and magnetic device applications. 2D materials with honeycomb lattices offer exciting opportunities to engineer unconventional multiferroicity, where the ferroic orders are driven purely by the orbital degrees of freedom but not electron spin. These include ferro-valleytricity corresponding to the electron valley and ferro-orbital-magnetism supported by quantum geometric effects. Such orbital multiferroics could offer strong valley-magnetic couplings and large responses to external fields-enabling device applications such as multiple-state memory elements, and electric control of valley and magnetic states. Here we report orbital multiferroicity in pentalayer rhombohedral graphene using low temperature magneto-transport measurements. We observed anomalous Hall signals Rxy with an exceptionally large Hall angle (tanΘH > 0.6) and orbital magnetic hysteresis at hole doping. There are four such states with different valley polarizations and orbital magnetizations, forming a valley-magnetic quartet. By sweeping the gate electric field E we observed a butterfly-shaped hysteresis of Rxy connecting the quartet. This hysteresis indicates a ferro-valleytronic order that couples to the composite field E\cdot B, but not the individual fields. Tuning E would switch each ferroic order independently, and achieve non-volatile switching of them together. Our observations demonstrate a new type of multiferroics and point to electrically tunable ultra-low power valleytronic and magnetic devices.

研究动机与目标

• Motivate discovery of multiferroicity driven by orbital degrees of freedom in 2D honeycomb materials.
• Demonstrate orbital multiferroicity via low-temperature magneto-transport in pentalayer rhombohedral graphene.
• Show how gate electric fields can control valley and orbital magnetization states.
• Identify coupling of ferroic orders to the composite field E·B rather than to E or B alone.

提出的方法

• Use low-temperature magneto-transport measurements on pentalayer rhombohedral graphene.
• Observe anomalous Hall signals with a large Hall angle (tan ΘH > 0.6) and orbital magnetic hysteresis at hole doping.
• Identify four valley-polarized/orbital-magnetization states forming a valley-magnetic quartet.
• Sweep gate electric field E to reveal butterfly-shaped hysteresis in Rxy connecting the quartet.
• Interpret hysteresis as ferro-valleytronic order coupling to E·B and enabling electric switching of ferroic orders.

实验结果

研究问题

• RQ1 Can orbital degrees of freedom alone drive multiferroicity in 2D honeycomb systems?
• RQ2 Do valley polarization and orbital magnetization form a controllable quartet in pentalayer rhombohedral graphene?
• RQ3 How does an external electric field gate influence ferro-valleytronic order and its coupling to magnetic fields?
• RQ4 Is there a device-relevant way to achieve non-volatile control of valley and orbital states via E-field tuning?

主要发现

• There are four states with different valley polarizations and orbital magnetizations forming a valley-magnetic quartet.
• Anomalous Hall signals with a large Hall angle (tan ΘH > 0.6) and orbital magnetic hysteresis observed at hole doping.
• Butterfly-shaped hysteresis in Rxy appears when sweeping gate electric field E, connecting the quartet.
• The ferroic orders couple to the composite field E·B, not to E or B alone.
• Tuning the electric field E allows independent switching of each ferroic order, enabling potential non-volatile control.