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Frontiers of Physics

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Front. Phys.    2023, Vol. 18 Issue (6) : 62304    https://doi.org/10.1007/s11467-023-1320-4
RESEARCH ARTICLE
Generation and modulation of multiple 2D bulk photovoltaic effects in space-time reversal asymmetric 2H-FeCl2
Liang Liu1,2, Xiaolin Li1, Luping Du3, Xi Zhang1,4()
1. Institute of Nanosurface Science and Engineering, Guangdong Provincial Key Laboratory of Micro/Nano Optomechatronics Engineering, Shenzhen University, Shenzhen 518060, China
2. School of Physics, State Key Laboratory for Crystal Materials, Shandong University, Jinan 250100, China
3. Nanophotonics Research Centre, Institute of Microscale Optoelectronics, Shenzhen University, Shenzhen 518060, China
4. Research Center of Plasma Medical Technology, Shenzhen University, Shenzhen 518060, China
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Abstract

The two-dimensional (2D) bulk photovoltaic effect (BPVE) is a cornerstone for future highly efficient 2D solar cells and optoelectronics. The ferromagnetic semiconductor 2H-FeCl2 is shown to realize a new type of BPVE in which spatial inversion (P), time reversal (T), and space−time reversal (PT) symmetries are broken (PT-broken). Using density functional theory and perturbation theory, we show that 2H-FeCl2 exhibits giant photocurrents, photo-spin-currents, and photo-orbital-currents under illumination by linearly polarized light. The injection-like and shift-like photocurrents coexist and propagate in different directions. The material also demonstrates substantial photoconductance, photo-spin-conductance, and photo-orbital-conductance, with magnitudes up to 4650 (nm·μA/V2), 4620 [nm·μA/V2 ℏ/(2e)], and 6450 (nm·μA/V2 ℏ/e), respectively. Furthermore, the injection-currents, shift-spin-currents, and shift-orbital-currents can be readily switched via rotating the magnetizations of 2H-FeCl2. These results demonstrate the superior performance and intriguing control of a new type of BPVE in 2H-FeCl2.
Keywords 2D ferromagnetism      bulk photovoltaic effects      photo-spin-currents      photo-orbital-currents      nonlinear optoelectronics     
Corresponding Author(s): Xi Zhang   
Issue Date: 25 July 2023
 Cite this article:   
Liang Liu,Xiaolin Li,Luping Du, et al. Generation and modulation of multiple 2D bulk photovoltaic effects in space-time reversal asymmetric 2H-FeCl2[J]. Front. Phys. , 2023, 18(6): 62304.
 URL:  
https://academic.hep.com.cn/fop/EN/10.1007/s11467-023-1320-4
https://academic.hep.com.cn/fop/EN/Y2023/V18/I6/62304
ClassSymmetryBPVE mechanismConcrete 2D crystal
1P-brokenT-conservedLinear light: shiftCircular light: injectionMoS2 [16, 17, 36]MoSe2, CuInP2S6 [8]
2P-brokenT-brokenPT-conservedLinear light: injectionCircular light: shift + injectionBilayer CrI3 [7], bilayerMnBi2Te4 [9, 34], 1T-FeCl2 [37]Monolayer MnPSe3 [35]
3P-brokenT-brokenPT-brokenLinear light: shift + injectionCircular light: shift + injectionMonolayer 2H-FeCl2 (this work)
Tab.1  The symmetry classification, mechanism, and typical platforms for the nonlinear photocurrents.
Fig.1  Structure of class 3 BPVEs in 2H-FeCl2. (a) Geometry structure of 2H-FeCl2. Green and brown balls denote Cl and Fe atoms. Dashed lines outline the periodic cell. (b) Local structures of 2H-FeCl2. Blue dots denote the magnetic moments of Fe atoms along +z-direction. Blue crosses denote the magnetic moments along ?z-direction. Left part is the original structure, right part involves transformed structures through spatial inversion or time reversal. (c) The typical valley (K and K' = ?K) electronic structures in systems with both P and T symmetries, with only T symmetry, and without P or T symmetries. Black, red, and blue curves denote the spin compensated, spin up and spin down states. Arrows denote the electron-hole generation processes via absorbing photons. (d) The prototype of 2D BPVE device, in which steady currents, spin-currents and orbital-currents are generated by applying light.
Fig.2  Electronic structures of 2H-FeCl2. (a) Band structure without SOC and (b) with SOC. Red and blue lines represent spin down and up states. The irreducible representations of several states in high symmetric k-points were marked. Energy gaps at K and K' were defined as ΔK and ΔK'. (c) The atomic wave functions of irreducible representations of states in FeCl2. Color denotes the phases of wavefunctions. (d) Irreducible Brillouin zone of 2H-FeCl2. a1, a2 are lattice vectors. g1, g2 are reciprocal lattice vectors.
Fig.3  Calculated 2nd order photoconductance (a, b), photo-spin-conductance (c, d), and photo-orbital-conductance (e, f). The magnetizations of 2H-FeCl2 in (a), (c), and (e) are in +z-direction. The magnetizations in (b), (d), and (f) are in ?z-direction.
Fig.4  The distribution of photocurrents on BZ of 2H-FeCl2 under illumination of x-polarized light with energy 1.05 eV. (a, b) The photocurrents along x-direction. (c, d) The photocurrents along y-direction. Insets depict the magnetization orientation of FeCl2 in each case.
Fig.5  Detections of photocurrents, photo-spin-currents, and photo-orbital-currents in 2H-FeCl2. (a) Detecting the photocurrents along x-axis via measuring the currents under zero-bias. The Green and purple arrows along x-axis denote the flowing of electrons and holes induced by the illuminations of linearly polarized lights. The arrows along y-axis denote the unfavored propagation directions of photo-carriers. (b) Detecting the photo-spin-currents via measuring the Hall voltage produced by the ISHE. Red and blue arrows denote the flowing of spin-currents with spin-polarizations along +z and ?z directions. (c) Detecting the orbital magnetic moments on edges via MOKE. The colored tori represent the orbital angular moments and the colors are the phases. The in-plane blue and red arrows denote the flowing of orbital angular moments. The out-of-plane arrows on edges denote the orbital magnetic moments caused by the accumulations of orbital moments on edges.
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