Structural fluctuations cause spin-split states in tetragonal (CH3NH3)PbI3 as evidenced by the circular photogalvanic effect
Structural fluctuations cause spin-split states in tetragonal (CH3NH3)PbI3 as evidenced by the circular photogalvanic effect
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DOI：
10.1073/pnas.1805422115
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发表时间：
2018-09-18
影响因子：
11.1
通讯作者：
Fauster, Thomas
中科院分区：
文献类型：
--
作者：
Niesner, Daniel;Hauck, Martin;Fauster, Thomas
卤化铅钙钛矿用于薄膜太阳能电池，其高效率归功于光载流子的长寿命。各种计算发现，动态拉什巴效应可以显著地促进这些长寿命。这一效应预计会在有限温度下引起反转对称晶体材料电子带的自旋分裂，导致轻微的间接带隙。关于自旋分裂存在或强度的直接实验证据是缺乏的。在这里，我们用圆偏振光共振激发单晶（CH3NH3）PbI3中的光电流，以澄清在能带结构中存在自旋分裂。我们观察到一个圆形的光电效应，即光电流取决于光螺旋度，在正交和四方（CH3NH3）PbI3。在室温下，激发光子能量δ E = 110 meV的效应峰值低于直接光学带隙。温度相关的测量揭示了正交-四方相变效应的符号变化，表明两相的微观起源不同。在四方相内，E和圆形光电效应的振幅随温度的升高而增大。我们的发现支持这一阶段的动态Rashba效应，即由热诱导的结构波动引起的自旋分裂，破坏了反转对称性。
Lead halide perovskites are used in thin-film solar cells, which owe their high efficiency to the long lifetimes of photocarriers. Various calculations find that a dynamical Rashba effect could significantly contribute to these long lifetimes. This effect is predicted to cause a spin splitting of the electronic bands of inversion-symmetric crystalline materials at finite temperatures, resulting in a slightly indirect band gap. Direct experimental evidence of the existence or the strength of the spin splitting is lacking. Here, we resonantly excite photocurrents in single crystalline (CH3NH3)PbI3 with circularly polarized light to clarify the existence of spin splittings in the band structure. We observe a circular photogalvanic effect, i.e., the photocurrent depends on the light helicity, in both orthorhombic and tetragonal (CH3NH3)PbI3. At room temperature, the effect peaks for excitation photon energies Delta E = 110 meV below the direct optical band gap. Temperature-dependent measurements reveal a sign change of the effect at the orthorhombic-tetragonal phase transition, indicating different microscopic origins in the two phases. Within the tetragonal phase, both Delta E and the amplitude of the circular photogalvanic effect increase with temperature. Our findings support a dynamical Rashba effect in this phase, i.e., a spin splitting caused by thermally induced structural fluctuations which break inversion symmetry.