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Non-Equilibrium Green's Function based Circuit Models for Coherent Spin Devices
摘要： With recent developments in spintronics, it is now possible to envision 'spin-driven' devices with magnets and interconnects that require a new class of transport models using generalized Fermi functions and currents, each with four components: one for charge and three for spin. The corresponding impedance elements are not pure numbers but 4 × 4 matrices. Starting from the Non-Equilibrium Green's Function (NEGF) formalism in the elastic, phase-coherent transport regime, we develop spin generalized Landauer-Büttiker formulas involving such 4 × 4 conductances, for multi-terminal devices in the presence of Normal-Metal (NM) leads. In addition to usual 'terminal' conductances describing currents at the contacts, we provide 'spin-transfer torque' conductances describing the spin currents absorbed by ferromagnetic (FM) regions inside the conductor, specifying both of these currents in terms of Fermi functions at the terminals. We derive universal sum rules and reciprocity relations that would be obeyed by such matrix conductances. Finally, we apply our formulation to two example Hamiltonians describing the Rashba and the Hanle effect in 2D. Our results allows the use of pure quantum transport models as building blocks in constructing circuit models for complex spintronic and nano-magnetic structures and devices for simulation in SPICE-like simulators.
关键词： Rashba effect,spintronics,Landauer-Büttiker formula,spin-transfer torque,Non-Equilibrium Green's Function,Hanle effect,SPICE simulation
更新于2025-09-23 15:23:52
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Ferromagnet structural tuning of interfacial symmetry breaking and spin Hall angle in ferromagnet/heavy metal bilayers
摘要： We investigate the influence of ferromagnet crystalline structures on spin Hall angle (SHA) in ferromagnet (FM)/heavy metal (HM) bilayers with reversed stacking order. X-ray diffraction and transmission electron microscopy show that a pronounced permalloy (Py) (111) texture emerges in substrate/Ta/Py compared to that in substrate/Py/Ta. Spin torque ferromagnetic resonance measurements show that the out-of-plane SHA (h?, associated with Rashba-like effective field) is (cid:2)2 times larger in substrate/Ta/Py than that in substrate/Py/Ta, while in-plane SHA (hjj, associated with spin Hall-like effective field) shows no distinct difference. We ascribe the FM structure dependent SHA to the interfacial symmetry breaking between the FM and HM, which is tuned by the work functions due to different crystalline structures of the FM. The emergence of the Py (111) structure enlarges the interfacial symmetry breaking and consequently enhances the Rashba effect at the FM/HM interface.
关键词： crystalline structure,heavy metal,ferromagnet,Rashba effect,spin Hall angle
更新于2025-09-23 15:21:21
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Vectorial control of the spin-orbit interaction in suspended InAs nanowires
摘要： Semiconductor nanowires featuring strong spin-orbit interactions (SOI), represent a promising platform for a broad range of novel technologies, such as spintronic applications or topological quantum computation. However, experimental studies into the nature and the orientation of the SOI vector in these wires remain limited despite being of upmost importance. Typical devices feature the nanowires placed on top of a substrate which modifies the SOI vector and spoils the intrinsic symmetries of the system. In this work, we report experimental results on suspended InAs nanowires, in which the wire symmetries are fully preserved and clearly visible in transport measurements. Using a vectorial magnet, the non-trivial evolution of weak anti-localization (WAL) is tracked through all 3D space, and both the spin-orbit length lSO and coherence length l? are determined as a function of the magnetic field magnitude and direction. Studying the angular maps of the WAL signal, we demonstrate that the average SOI within the nanowire is isotropic and that our findings are consistent with a semiclassical quasi-1D model of WAL adapted to include the geometrical constraints of the nanostructure. Moreover, by acting on properly designed side gates, we apply an external electric field introducing an additional vectorial Rashba spin-orbit component whose strength can be controlled by external means. These results give important hints on the intrinsic nature of suspended nanowire and can be interesting for the field of spintronics as well as for the manipulation of Majorana bound states in devices based on hybrid semiconductors.
关键词： weak anti-localization,spin-orbit interaction,Rashba effect,indium arsenide,nanowire
更新于2025-09-23 15:21:01
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Estimation of the Rashba Strength from Second Harmonic Generation in 2D and 3D Hybrid Organic-Inorganic Perovskites
摘要： The Rashba e?ect (RE), caused by concurrent spin-orbit coupling and structural inversion asymmetry (SIA), has been invoked to explain outstanding optoelectronic properties of hybrid organic inorganic perovskites (HOIPs). The SIA in HOIPs also manifests itself in second harmonic generation (SHG). Here we establish a quantitative relation between the RE strength and the low-frequency SHG value in 2D and 3D HOIPs using a newly developed e?ective-mass model. The calculated SHG, which exhibits one- and two-photon resonances between the valence and conduction bands, approaches a ?nite value at low frequencies. Using the measured SHG values in literature, the estimated RE strength is 10?3 eV?A for 3D CH3NH3PbI3 and 4 × 10?2 eV?A for 2D (C4H9NH3)2PbI4. Our results facilitate a simple and reliable determination of the RE strength of 2D and 3D HOIPs and help quantify the impact of RE on various properties in HOIPs.
关键词： second harmonic generation,Rashba effect,hybrid organic-inorganic perovskites,structural inversion asymmetry,spin-orbit coupling
更新于2025-09-23 15:21:01
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Indirect tail states formation by thermal-induced polar fluctuations in halide perovskites
摘要： Halide perovskites possess enormous potential for various optoelectronic applications. Presently, a clear understanding of the interplay between the lattice and electronic effects is still elusive. Specifically, the weakly absorbing tail states and dual emission from perovskites are not satisfactorily described by existing theories based on the Urbach tail and reabsorption effect. Herein, through temperature-dependent and time-resolved spectroscopy on metal halide perovskite single crystals with organic or inorganic A-site cations, we confirm the existence of indirect tail states below the direct transition edge to arise from a dynamical Rashba splitting effect, caused by the PbBr6 octahedral thermal polar distortions at elevated temperatures. This dynamic effect is distinct from the static Rashba splitting effect, caused by non-spherical A-site cations or surface induced lattice distortions. Our findings shed fresh perspectives on the electronic-lattice relations paramount for the design and optimization of emergent perovskites, revealing broad implications for light harvesting/photo-detection and light emission/lasing applications.
关键词： thermal fluctuations,halide perovskites,Rashba effect,optoelectronics,tail states
更新于2025-09-19 17:15:36
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Ultrafast acoustic phonon scattering in CH <sub/>3</sub> NH <sub/>3</sub> PbI <sub/>3</sub> revealed by femtosecond four-wave mixing
摘要： Carrier scattering processes are studied in CH3NH3PbI3 using temperature-dependent four-wave mixing experiments. Our results indicate that scattering by ionized impurities limits the interband dephasing time (T2) below 30 K, with strong electron-phonon scattering dominating at higher temperatures (with a time scale of 125 fs at 100 K). Our theoretical simulations provide quantitative agreement with the measured carrier scattering rate and show that the rate of acoustic phonon scattering is enhanced by strong spin-orbit coupling, which modifies the band-edge density of states. The Rashba coefficient extracted from fitting the experimental results (γc = 2 eV ?) is in agreement with calculations of the surface Rashba effect and recent experiments using the photogalvanic effect on thin films.
关键词： electron-phonon scattering,carrier scattering,CH3NH3PbI3,four-wave mixing,Rashba effect
更新于2025-09-19 17:13:59
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Modulation of the spin-orbit interaction and the transition probability of polaron in disk quantum dot under electromagnetic field
摘要： This work presents the important role played by the spin-orbit interaction in nanostructures such as disk quantum dot when subjected to an external field. Here by using the LLP variational method, the Rashba effect on polaron in this nanostructure with Gaussian confinement potential under electromagnetic field is investigated. The polaron ground, first excited state energy and the electron transition probability are derived. Results show that the external electromagnetic field highly modulates the spin-orbit coupling strength αr, the polaron energies and transition probability. The polaron transition probability splits in two branches due to the splitting of the eigenstates energies. Our study also reveals that DQD properties can be adjusted by the external field. Therefore, the transport and the optical properties of the quantum dot can be controlled by turning the spin-orbit coupling.
关键词： quantum dot,transition probability,spin-orbit interaction,Rashba effect,polaron
更新于2025-09-16 10:30:52
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Exploring Orbit–Orbit Interaction in Relationship to Photoluminescence Quantum Efficiency in Perovskite Quantum Dots through Rashba Effect
摘要： This study demonstrates the influence of orbit-orbit interaction on photoluminescence quantum efficiency (PLQE) of metal halide perovskite quantum dots (QDs) through Rashba effect. The orbit-orbit interaction between excitons was characterized by using the minimal excitation intensity required to generate photoluminescence difference (?PL) between linearly and circularly polarized photoexcitations. It was observed that changing the surface functionalization from PFOA-OA to PFSH-OAm and OA can largely increase the minimal excitation intensity for generating ?PL. This indicates that orbit-orbit interaction is essentially decreased in CsPbBr1I2 QDs with surface functionalization. Simultaneously, the PLQE is increased from 39 % to 59 % and 72 % in CsPbBr1I2 QDs upon surface functionalization. Furthermore, the PL lifetime is decreased with increasing the PLQE in CsPbBr1I2 QDs upon surface functionalization. This phenomenon implies that decreasing the orbit-orbit interaction can essentially weaken Rashba effect and consequently reduces the disallowed transitions, leading to an enhancement on PLQE in perovskite QDs.
关键词： Rashba effect,orbit-orbit interaction,Perovskite quantum dots,photoluminescence quantum efficiency
更新于2025-09-12 10:27:22
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Influence of Disorder and Anharmonic Fluctuations on the Dynamical Rashba Effect in Purely Inorganic Lead-Halide Perovskites
摘要： Doping organic metal-halide perovskites with cesium could be the best solution to stabilize highly-efficient perovskite solar cells. The understanding of the respective roles of the organic molecule, on one hand, and the inorganic lattice, on the other, is thus crucial in order to be able to optimize the physical properties of the mixed-cation structures. In particular, the study of the recombination mechanisms is thought to be one of the key challenges towards full comprehension of their working principles. Using molecular dynamics and frozen phonons, we evidence sub-picosecond anharmonic fluctuations in the fully inorganic CsPbI3 perovskite. We reveal the effect of these fluctuations, combined with spin-orbit coupling, on the electronic band structure, evidencing a dynamical Rashba effect. Our study shows that under certain conditions space disorder can quench the Rashba effect. As for time disorder, we evidence a dynamical Rashba effect which is similar to what was found for MAPbI3 and which is still sizable despite temperature disorder, the large investigated supercell, and the absence of the organic cations’ motion. We show that the spin texture associated to the Rashba splitting cannot be deemed responsible for a consistent reduction of recombination rates, although the spin mismatch between valence and conduction band increases with the ferroelectric distortion causing the Rashba splitting.
关键词： spin-orbit coupling,perovskite solar cells,Rashba effect,molecular dynamics,CsPbI3
更新于2025-09-04 15:30:14