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Microscopic theory of spin relaxation anisotropy in graphene with proximity-induced spin-orbit coupling
摘要： We present a microscopic theory of spin dynamics in weakly disordered graphene with uniform proximity-induced spin-orbit coupling (SOC). A time-dependent perturbative treatment is employed to derive the spin Bloch equations governing the spin dynamics at high electronic density for arbitrary ratio λSOC/η, where η is the disorder-induced quasiparticle broadening and λSOC is the largest spin-orbit energy scale. Rich scenarios are predicted, depending on a delicate competition between interface-induced Bychkov-Rashba and spin-valley interaction. In the motional narrowing regime of weak SOC (λSOC (cid:2) η), the anisotropy ratio of out-of-plane to in-plane spin lifetimes ζ = τ ⊥ s agrees qualitatively with a toy model of spins in a fluctuating SOC field proposed recently by Cummings and co-workers Phys. Rev. Lett. 119, 206601 (2017). For well-resolved SOC (λSOC (cid:2) η), the spin dynamics is characterized by fast damped oscillations with spins relaxing on the timescale of a single scattering event. In this regime, qualitatively different formulas for ζ are obtained, which can be useful to model spin transport in ultraclean van der Waals heterostructures.
关键词： spin-orbit coupling,van der Waals heterostructures,spin dynamics,Bychkov-Rashba,graphene,spin-valley interaction
更新于2025-09-04 15:30:14
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Metamorphic InAs(Sb)/InGaAs/InAlAs nanoheterostructures grown on GaAs for efficient mid-IR emitters
摘要： High-efficiency semiconductor lasers and light-emitting diodes operating in the 3–5 μm mid-infrared (mid-IR) spectral range are currently of great demand for a wide variety of applications, in particular, gas sensing, noninvasive medical tests, IR spectroscopy etc. III-V compounds with a lattice constant of about 6.1 ? are traditionally used for this spectral range. The attractive idea to fabricate such emitters on GaAs substrates by using In(Ga,Al)As compounds is restricted by either the minimum operating wavelength of ～8 μm in case of pseudomorphic AlGaAs-based quantum cascade lasers or requires utilization of thick metamorphic InxAl1-xAs buffer layers (MBLs) playing a key role in reducing the density of threading dislocations (TDs) in an active region, which otherwise result in a strong decay of the quantum efficiency of such mid-IR emitters. In this review we present the results of careful investigations of employing the convex-graded InxAl1-xAs MBLs for fabrication by molecular beam epitaxy on GaAs (001) substrates of In(Ga,Al)As heterostructures with a combined type-II/type-I InSb/InAs/InGaAs quantum well (QW) for efficient mid-IR emitters (3–3.6 μm). The issues of strain relaxation, elastic stress balance, efficiency of radiative and non-radiative recombination at T = 10–300 K are discussed in relation to molecular beam epitaxy (MBE) growth conditions and designs of the structures. A wide complex of techniques including in-situ reflection high-energy electron diffraction, atomic force microscopy (AFM), scanning and transmission electron microscopies, X-ray diffractometry, reciprocal space mapping, selective area electron diffraction, as well as photoluminescence (PL) and Fourier-transformed infrared spectroscopy was used to study in detail structural and optical properties of the metamorphic QW structures. Optimization of the growth conditions (the substrate temperature, the As4/III ratio) and elastic strain profiles governed by variation of an inverse step in the In content profile between the MBL and the InAlAs virtual substrate results in decrease in the TD density (down to 3 × 107 cm?2), increase of the thickness of the low-TD-density near-surface MBL region to 250–300 nm, the extremely low surface roughness with the RMS value of 1.6–2.4 nm, measured by AFM, as well as rather high 3.5 μm-PL intensity at temperatures up to 300 K in such structures. The obtained results indicate that the metamorphic InSb/In(Ga,Al)As QW heterostructures of proper design, grown under the optimum MBE conditions, are very promising for fabricating the efficient mid-IR emitters on a GaAs platform.
关键词： Buffer layer,Non-radiative recombination,Mid-infrared emitters,Photoluminescence,Metamorphic heterostructures,InSb,InAs,Structural properties,Molecular beam epitaxy,Nanostructures,In(Ga,Al)As ternary alloys,Threading dislocations,Quantum well
更新于2025-09-04 15:30:14
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MBE Growth and Optical Properties of Isotopically Purified ZnSe Heterostructures
摘要： Wide-gap II/VI heterostructures (HS) and quantum wells (QW) composed of ZnSe, CdSe, MgSe and their ternary and quaternary compounds are attractive candidates for modern quantum optical devices such as single photon sources and optically controlled spin qubits in the visible spectral range. In contrast to similar III/V semiconductor based devices, generally, most of the II/VI compounds allow for isotope purification towards zero nuclear spin species in the semiconductor environment. Using the same molecular beam epitaxy (MBE) system for natural and isotopically purified materials opposes the challenge to achieve superior isotope purity for example of Zn and Se species on the background of operation of the other effusion cells filled with natural isotopes. Here we report on the crystallographic and optical properties of ZnMgSe/ZnSe heterostructures and quantum wells grown by using 64Zn and 80Se isotopes and Mg with natural isotope distribution. We present a detailed quantitative secondary ion mass spectrometry (SIMS) analysis, which confirms that an extremely high grade of isotope purification of the ZnSe can be maintained although naturally and enriched Zn and Se elements are used in the same MBE system. This pioneering growth study forms a solid base to generate a spin vacuum ZnSe host crystal that is particularly suited for future studies on the dynamics of localized spins in II/VI heterostructures on a strongly extended coherence time scale.
关键词： Heterostructures,Molecular beam epitaxy,Optical properties,Crystal structure,X-ray diffraction,Isotope purification,Elementary analysis
更新于2025-09-04 15:30:14
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films
摘要： Noncollinear magnetism can play an important role in multiferroic materials but is relatively understudied in oxide heterostructures compared to their bulk counterparts. Using variable temperature magnetometry and neutron diffraction, we demonstrate the presence of helical magnetic ordering in CaMn7O12 and Ca1?xSrxMn7O12 (for x up to 0.51) thin ?lms. Consistent with bulk Ca1?xSrxMn7O12, the net magnetization increases with Sr doping. Neutron diffraction con?rms that the helical magnetic structure remains incommensurate at all values of x, while the fundamental magnetic wavevector increases upon Sr substitution. This result demonstrates a chemical-based approach for tuning helical magnetism in quadruple perovskite ?lms and enables future studies of strain and interfacial effects on helimagnetism in oxide heterostructures.
关键词： helical magnetism,oxide heterostructures,magnetometry,Sr-doped CaMn7O12,neutron diffraction
更新于2025-09-04 15:30:14
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Performance of resonant tunneling diodes based on the nonpolar-oriented AlGaN/GaN heterostructures
摘要： The performance of resonant tunneling diodes (RTDs) based on nonpolar-oriented AlGaN/GaN heterostructures has been theoretically investigated by numerical simulation. Simulation results show that the RTDs grown along the nonpolar orientation can yield the current–voltage (I–V ) characteristics with the peak current, the peak-to-valley ratio (PVCR), and the peak voltage of about 3.5 times and 1.5 times higher and 1.1 times lower than those of RTDs grown along the conventional polar orientation, respectively. The results also show excellent symmetry, high reproducibility of I–V characteristics, and high intrinsic response frequency.
关键词： current–voltage characteristics,resonant tunneling diodes,numerical simulation,AlGaN/GaN heterostructures,nonpolar orientation
更新于2025-09-04 15:30:14
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Phase and Defect Engineering of MoS <sub/>2</sub> Stabilized in Periodic TiO <sub/>2</sub> Nanoporous Film for Enhanced Solar Water Splitting
摘要： Phase and defect engineering of the heterostructured MoS2@TiO2 nanoporous film is investigated to achieve a broad solar spectrum light absorption and high solar water splitting efficiency. The phase transition from the semiconducting 2H-MoS2 to the metallic 1T-MoS2 is achieved by a hydrothermal exfoliation treatment. Experimental studies elucidate that the solar water splitting activity is greatly improved by forming 1T-MoS2 along with increasing S-vacancies because of the significantly enhanced surface plasmon resonance. The mixed-phase MoS2@TiO2 film shows a high H2 yield rate of 308 μmol h?1 cm?2 and long-term durability for 30 h, which is superior to the state-of-the-art catalysts for solar water splitting. This study offers a universal and efficient avenue to rationalize the plasmonic catalysts for solar water splitting and other energy and environmental applications.
关键词： solar water splitting,defect engineering,MoS2,heterostructures,phase transitions
更新于2025-09-04 15:30:14
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Site‐Selective and van der Waals Epitaxial Growth of Rhenium Disulfide on Graphene
摘要： The surface property of growth substrate imposes significant influence in the growth behaviors of 2D materials. Rhenium disulfide (ReS2) is a new family of 2D transition metal dichalcogenides with unique distorted 1T crystal structure and thickness-independent direct bandgap. The role of growth substrate is more critical for ReS2 owing to its weak interlayer coupling property, which leads to preferred growth along the out-of-plane direction while suppressing the uniform in-plane growth. Herein, graphene is introduced as the growth substrate for ReS2 and the synthesis of graphene/ReS2 vertical heterostructure is demonstrated via chemical vapor deposition. Compared with the rough surface of SiO2/Si substrate with dangling bonds which hinders the uniform growth of ReS2, the inert and smooth surface nature of graphene sheet provides a lower energy barrier for migration of the adatoms, thereby promoting the growth of ReS2 on the graphene surface along the in-plane direction. Furthermore, patterning of the graphene/ReS2 heterostructure is achieved by the selective growth of ReS2, which is attributed to the strong binding energy between sulfur atoms and graphene surface. The fundamental studies in the role of graphene as the growth template in the formation of van der Waals heterostructures provide better insights into the synthesis of 2D heterostructures.
关键词： rhenium disulfide,chemical vapor deposition,2D heterostructures,graphene,site-selective growth
更新于2025-09-04 15:30:14
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Band structure tuning and charge separation of MNX monolayers and MNX/GaS van der Waals heterostructures
摘要： Efficient charge separation and band structure tuning are very important for application to photoelectric device. Here, based on first-principles calculations, we propose two-dimensional (2D) MNX (M=Zr, Hf; X=Cl, Br, I) monolayers which can be exfoliated from the corresponding layered bulk phase due to the low cleavage energy. The phonon band structure and the mechanical analysis indicate that 2D MNX monolayers can form free-standing membranes. The calculated results suggest that 2D MNX monolayers are indirect band gap semiconductors with band gaps in the range of 1.55-3.37 eV. Among them, MNI (M=Zr, Hf) monolayers with effective charge separation and moderate band gaps would have potential application to photocatalytic water splitting. In order to realize the effective charge separation for the other MNX monolayers, MNX/GaS (X=Cl, Br) heterostructures are investigated. Our calculations reveal that MNX/GaS heterostructures are typical type-II band alignment, facilitating the separation of photogenerated carriers where electrons and holes are localized in MNX and GaS monolayers, respectively. Furthermore, the band gaps of the MNX/GaS heterostructures are obviously narrowed compared to those of the isolated constituent monolayers. In addition, the band gaps and band edge positions of MNX/GaS heterostructures can further be tuned by biaxial strain to match better with the redox potentials of water. These findings in this study not only enrich the family of 2D materials, but also demonstrate that MNI (M=Zr, Hf) monolayers and MNX/GaS heterostructures are promising candidates for photocatalytic materials.
关键词： van der Waals heterostructures,photocatalytic water splitting,band structure tuning,MNX monolayers,charge separation
更新于2025-09-04 15:30:14
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Selective Ferroelectric BiOI/Bi4Ti3O12 Heterostructures For Visible-light-driven Photocatalysis
摘要： Ferroelectric-photocatalyst/photocatalyst heterojunctions have very attractive photocatalytic activities. Beside enhanced charge carrier separation due to their internal electric fields, charge transfer could be even further enhanced by designing the heterojunction interface. In this work, the polarization-adsorption interaction that exists in ferroelectric materials was employed for successful deposition of BiOI on specific surfaces of Bi4Ti3O12 plates in the dark at room temperature, where the positively polarized region was found. The crystal structure, morphology, and composition of samples were confirmed by X-ray diffraction, field emission scanning electron microscopy, and X-ray photoelectron spectroscopy, respectively. Higher photocatalytic activity was achieved by the use of heterojunctions, with the reason behind the enhancement of activity confirmed to be the modified band structure, which contributed to the transfer of photoelectrons from Bi4Ti3O12 to BiOI, the increased visible light absorption, the increased active site area of positively polarized Bi4Ti3O12, and the elimination of the screening layer, which contributes impedance in charge transfer.
关键词： Visible-light-driven,Photocatalysis,Heterostructures,BiOI/Bi4Ti3O12,Ferroelectric
更新于2025-09-04 15:30:14