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1. Effect of finite spin-orbit splitting on the electron-hole exchange interaction in excitons confined in semiconductor nanocrystals

   https://doi.org/10.1103/PhysRevB.108.235419  

   Goupalov, S. V. (December 2023, Physical Review B)

   We derive an effective spin-Hamiltonian accounting for the exciton fine structure in quasi-spherical zinc-blende semiconductor nanocrystals within the k · p formalism explicitly taking into account the spin-orbit split-off valence band. It is shown that, for excitons in nanocrystals made of III-V and II-VI semiconductors with fairly small spin-orbit splitting, the scaling of the electron-hole exchange interaction with the nanocrystal size insignificantly differs from the inverse nanocrystal volume law predicted within the model neglecting the spin-orbit split-off band. Numerical calculations are performed for InP nanocrystals. 

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2. Revealing the Band-Edge Exciton Fine Structure of Single InP Nanocrystals

   https://doi.org/10.1021/acs.nanolett.3c01419  

   Prin, Elise; Xia, Chenghui; Won, Yu-Ho; Jang, Eunjoo; Goupalov, Serguei V.; Tamarat, Philippe; Lounis, Brahim (July 2023, Nano Letters)

   We investigate the fundamental optical properties of single zinc-blende InP/ZnSe/ZnS nanocrystals (NCs) using frequency- and time-resolved magneto-photoluminescence spectroscopy. At liquid helium temperature, highly resolved spectral fingerprints are obtained and identified as the recombination lines of the three lowest states of the band-edge exciton fine structure. The evolutions of the photoluminescence spectra and decays under magnetic fields show evidence for a ground dark exciton level 0L with zero angular momentum projection along the NC main elongation axis. It lies 300 to 600 μeV below the ±1L bright exciton doublet, which is finely split by the NC shape anisotropy. These spectroscopic findings are well reproduced with a model of exciton fine structure accounting for shape anisotropy of the InP core. Our spectral fingerprints are extremely sensitive to the NC morphologies and unveil highly uniform shapes with prolate deviations of less than 3% from perfect sphericity. 

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3. Chiral photocurrent in a Quasi-1D TiS ₃ (001) phototransistor

   https://doi.org/10.1088/1361-648x/acb581  

   Gilbert, Simeon J; Li, Mingxing; Chen, Jia-Shiang; Yi, Hemian; Lipatov, Alexey; Avila, Jose; Sinitskii, Alexander; Asensio, Maria C; Dowben, Peter A; Yost, Andrew J (February 2023, Journal of Physics: Condensed Matter)

   Abstract The presence of in-plane chiral effects, hence spin–orbit coupling, is evident in the changes in the photocurrent produced in a TiS 3 (001) field-effect phototransistor with left versus right circularly polarized light. The direction of the photocurrent is protected by the presence of strong spin–orbit coupling and the anisotropy of the band structure as indicated in NanoARPES measurements. Dark electronic transport measurements indicate that TiS 3 is n-type and has an electron mobility in the range of 1–6 cm 2 V −1 s −1 . I – V measurements under laser illumination indicate the photocurrent exhibits a bias directionality dependence, reminiscent of bipolar spin diode behavior. Because the TiS 3 contains no heavy elements, the presence of spin–orbit coupling must be attributed to the observed loss of inversion symmetry at the TiS 3 (001) surface. 

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4. Circular photogalvanic spectroscopy of Rashba splitting in 2D hybrid organic–inorganic perovskite multiple quantum wells

   https://doi.org/10.1038/s41467-019-14073-6  

   Liu, Xiaojie; Chanana, Ashish; Huynh, Uyen; Xue, Fei; Haney, Paul; Blair, Steve; Jiang, Xiaomei; Vardeny, Z. V. (January 2020, Nature Communications)

   Abstract The two-dimensional (2D) Ruddlesden−Popper organic-inorganic halide perovskites such as (2D)-phenethylammonium lead iodide (2D-PEPI) have layered structure that resembles multiple quantum wells (MQW). The heavy atoms in 2D-PEPI contribute a large spin-orbit coupling that influences the electronic band structure. Upon breaking the inversion symmetry, a spin splitting (‘Rashba splitting’) occurs in the electronic bands. We have studied the spin splitting in 2D-PEPI single crystals using the circular photogalvanic effect (CPGE). We confirm the existence of Rashba splitting at the electronic band extrema of 35±10 meV, and identify the main inversion symmetry breaking direction perpendicular to the MQW planes. The CPGE action spectrum above the bandgap reveals spin-polarized photocurrent generated by ultrafast relaxation of excited photocarriers separated in momentum space. Whereas the helicity dependent photocurrent with below-gap excitation is due to spin-galvanic effect of the ionized spin-polarized excitons, where spin polarization occurs in the spin-split bands due to asymmetric spin-flip. 

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5. Pure spin current injection of single-layer monochalcogenides

   https://doi.org/10.1088/2053-1591/acbf99  

   Mendoza, Bernardo S; Grillo, Simone; Juárez-Reyes, Lucila; Fregoso, Benjamin M (March 2023, Materials Research Express)

   Cao, Yi; Wu, Judy (Ed.)

   Abstract We compute the spectrum of pure spin current injection in ferroelectric single-layer SnS, SnSe, GeS, and GeSe. The formalism takes into account the coherent spin dynamics of optically excited conduction states split in energy by spin–orbit coupling. The velocity of the electron’s spins is calculated as a function of incoming photon energy and angle of linearly polarized light within a full electronic band structure scheme using density functional theory. We find peak speeds of 520, 360, 270 and 370 Km s⁻¹for SnS, SnSe, GeS and GeSe, respectively which are an order of magnitude larger than those found in bulk semiconductors, e.g., GaAs and CdSe. Interestingly, the spin velocity is almost independent of the direction of polarization of light in a range of photon energies. Our results demonstrate that single-layer SnS, SnSe, GeS and GeSe are candidates to produce on demand spin-current in spintronics applications. 

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