Data Archive of Graphene on two-dimensional hexagonal BN, AlN, and GaN: Electronic, spin-orbit, and spin relaxation properties

We investigate the electronic band structure of graphene on a series of two-dimensional hexagonal nitride insulators hXN, X = B, A1, and Ga, with first-principles calculations. A symmetry-based model Hamiltonian is employed to extract orbital parameters and spin-orbit coupling (SOC) from the low-energy Dirac bands of the proximitized graphene. While commensurate hBN induces a staggered potential of about 10 meV into the Dirac band structure, less lattice-matched hA1N and hGaN disrupt the Dirac point much less, giving a staggered gap below 100 mu eV. Proximitized intrinsic SOC surprisingly does not increase much above the pristine graphene value of 12 mu eV; it stays in the window of 1-16 mu eV, depending strongly on stacking. However, Rashba SOC increases sharply when increasing the atomic number of the boron group, with calculated maximal values of 8, 15, and 65 mu eV for B-, Al-, and Ga-based nitrides, respectively. The individual Rashba couplings also depend strongly on stacking, vanishing in symmetrically sandwiched structures, and can be tuned by a transverse electric field. The extracted spin-orbit parameters were used as input for spin transport simulations based on Chebyshev expansion of the time-evolution of the spin expectation values, yielding interesting predictions for the electron spin relaxation. Spin lifetime magnitudes and anisotropies depend strongly on the specific (hXN)/graphene/hXN system, and they can be efficiently tuned by an applied external electric field as well as the carrier density in the graphene layer. A particularly interesting case for experiments is graphene/hGaN, in which the giant Rashba coupling is predicted to induce spin lifetimes of 1-10 ns, short enough to dominate over other mechanisms, and lead to the same spin relaxation anisotropy as that observed in conventional semiconductor heterostructures: 50%, meaning that out-of-plane spins relax twice as fast as in-plane spins.

本研究通过第一性原理计算，探究了一系列二维六方氮化物绝缘体hXN（X = B, A1, Ga）上石墨烯的电子能带结构。采用基于对称性的模型哈密顿量，从邻近石墨烯的低能狄拉克带中提取轨道参数和自旋-轨道耦合（SOC）。尽管具有相同晶格匹配度的hBN导致约10 meV的错位势能进入狄拉克能带结构，而晶格匹配度较低的hA1N和hGaN对狄拉克点的破坏较小，错位能隙低于100 mu eV。邻近本征SOC出人意料地并未显著高于原始石墨烯的12 mu eV值；它保持在1-16 mu eV的范围内，强烈依赖于层叠方式。然而，当增加硼族元素的原子序数时，Rashba SOC急剧增加，对于基于B、Al和Ga的氮化物，分别计算出的最大值分别为8、15和65 mu eV。个别Rashba耦合也强烈依赖于层叠方式，在对称嵌套结构中消失，并可由横向电场进行调节。提取的自旋-轨道参数被用作基于Chebyshev展开的旋量期望值时间演化的旋量输运模拟的输入，从而对电子自旋弛豫提出了有趣的预测。自旋寿命的幅度和各向异性强烈依赖于特定的(hXN)/石墨烯/hXN系统，并且可以通过施加的外部电场以及石墨烯层中的载流子密度进行有效调节。对于实验来说，一个特别有趣的情况是石墨烯/hGaN，在此情况下，预测的巨量Rashba耦合将诱导1-10 ns的自旋寿命，足够短以至于可以主导其他机制，并导致与常规半导体异质结构中观察到的相同的自旋弛豫各向异性：50%，意味着垂直于平面的自旋弛豫速度是平面内自旋的两倍。