Theoretical study of the zero-gap organic conductor α-(BEDT-TTF)2I3

Abstract

The quasi-two-dimensional molecular conductor α-(BEDT-TTF)₂I₃ exhibits anomalous transport phenomena where the temperature dependence of resistivity is weak but the ratio of the Hall coefficient at 10 K to that at room temperature is of the order of 10⁴. These puzzling phenomena were solved by predicting massless Dirac fermions, whose motions are described using the tilted Weyl equation with anisotropic velocity. α-(BEDT-TTF)₂I₃ is a unique material among several materials with Dirac fermions, i.e. graphene, bismuth, and quantum wells such as HgTe, from the view-points of both the structure and electronic states described as follows. α-(BEDT-TTF)₂I₃ has the layered structure with highly two-dimensional massless Dirac fermions. The anisotropic velocity and incommensurate momenta of the contact points, ±k₀, originate from the inequivalency of the BEDT-TTF sites in the unit cell, where ±k₀ moves in the first Brillouin zone with increasing pressure. The massless Dirac fermions exist in the presence of the charge disproportionation and are robust against the increase in pressure. The electron densities on those inequivalent BEDT-TTF sites exhibit anomalous momentum distributions, reflecting the angular dependences of the wave functions around the contact points. Those unique electronic properties affect the spatial oscillations of the electron densities in the vicinity of an impurity. A marked behavior of the Hall coefficient, where the sign of the Hall coefficient reverses sharply but continuously at low temperatures around 5 K, is investigated by treating the interband effects of the magnetic field exactly. It is shown that such behavior is possible by assuming the existence of the extremely small amount of electron doping. The enhancement of the orbital diamagnetism is also expected. The results of the present research shed light on a new aspect of Dirac fermion physics, i.e. the emergence of unique electronic properties owing to the structure of the material.     

夹层对层状岩体稳定性的影响分析

在对层状岩体结构及力学特性分析的基础上,将含夹层岩体视为一个完整的力学系统,建立了含夹岩体组合系统力学模型,揭示了含夹层岩体的破坏及失稳机理,并研究确定出含夹层岩体稳定性的判别指标,为层状岩体稳定性的控制设计提供了依据。最后,通过一工程实例分析了确定含夹层极限厚度的方法以及提高系统稳定性的措施。     

Interband Cascade Lasers with Wavelengths Spanning 2.9 μm to 5.2 μm

We report an experimental investigation of four interband cascade lasers with wavelengths spanning the mid-infrared spectral range, i.e., 2.9 μm to 5.2 μm, near room temperature in pulsed mode. One broad-area device had a pulsed threshold current density of only 3.8 A/cm² at 78 K (λ = 3.6 μm) and 590  A/cm² at 300 K (λ = 4.1 μm). The room-temperature threshold for the shortest-wavelength device (λ = 2.6 μm to 2.9 μm) was even lower, 450 A/cm². A␣cavity-length study of the lasers emitting at 3.6 μm to 4.1 μm yielded an internal loss varying from 7.8 cm⁻¹ at 78 K to 24 cm⁻¹ at 300 K, accompanied by a decrease of the internal efficiency from 77% to 45%.     

Modeling Spin-Dependent Transport in InAs/GaSb/AlSb Resonant Tunneling Structures

The Rashba effect resonant tunneling diode is a candidate for achieving spin polarizing under zero magnetic field using only conventional non-magnetic III–V semiconductor heterostructures. We point out the challenges involved based on simple arguments, and offer strategies for overcoming these difficulties. We present modeling results that demonstrate the benefits of the InAs/GaSb/AlSb-based asymmetric resonant interband tunneling diode (a-RITD) for spin filtering applications.     

丙丁烷TDLAS测量系统的吸收峰自动检测

基于丙丁烷在3.37μm处的基频吸收峰,使用相应中心波长的连续带间级联激光器和长光程吸收池,研制了基于波长调制技术的丙丁烷气体检测系统。为自动确定波长调制过程中的锯齿波中心值参数,基于电流与输出光频率的线性关系,通过均匀增加激光器驱动电流获得直接吸收光谱。使用基于洛伦兹线型的拟合算法确定吸收峰值对应电流,使锯齿波输出光以其为中心覆盖吸收峰。实验证明,以拟合结果31.94mA为中心值的锯齿波,在叠加频率为20 000Hz、幅值为0.03V的高频正弦调制波后,系统二次谐波峰值超过100mV。算法全过程不依赖标气和F-P标准具等精密光学器件,可适应实际生产条件下的系统自动定参。     

Direct and Indirect Exciton Dynamics in Few‐Layered ReS2 Revealed by Photoluminescence and Pump‐Probe Spectroscopy

Atomically thin‐layered ReS₂ with a distorted 1T structure has attracted attention because of its intriguing optical and electronic properties. Here, the direct and indirect exciton dynamics of a three‐layered ReS₂ is investigated by polarization‐resolved transient photoluminescence (PL) and ultrafast pump‐probe spectroscopy. The various time scales of the decay signals of the time‐resolved PL (<10 ps), with monitoring of the populations of electron–hole pairs (exciton), and the transient differential reflectance (≈1 and 100 ps), with monitoring of the populations of electrons and/or holes in the excited states, are observed. These results reveal the characteristic exciton dynamics: rapid relaxation of direct excitons (electron–hole pairs) and slow relaxation of the momentum‐mismatched indirect excitons accompanied by a one‐phonon emission process. These findings provide important information regarding the indirect bandgap nature of few‐layered ReS₂ and its characteristic exciton dynamics, boosting the understanding of the novel electronic and optical properties of atomically thin‐layered ReS₂.     

InAs/GaAsSb带间级联中波红外焦平面研究

针对高工作温度红外探测器的迫切需求,设计并利用分子束外延技术制备了高晶格质量的2级带间级联中波红外探测材料,带间级联单元器件在最高323 K下可以测试到清晰的响应光谱,140 K下暗电流密度达到4×10~(-5)A/cm~(-2).并在此基础上利用干法刻蚀技术实现了320×256规模的台面型带间级联红外焦平面原型器件.焦平面测试结果表明其在80-120K范围内量子效率达到30%,127 K下噪声等效温差为55.1 mK,盲元率为2.3%.采用该焦平面器件在127 K下获得了较为清晰的演示性室温目标红外热成像.     

用变n次抛物线叠加敷设缓和曲线

按汽车转向时的实际特性分析了公路缓和曲线应满足的理论关系，并采用了变ｎ次抛物线在切线上和圆弧上分别叠加，从而使所敷设的缓和曲线更简化，更附合实际。     

金属／溶液界面氧化还原反应的量化处理初探

对金属腐蚀体系中界面反应进行综合量子化学处理还是一个探索性的工作,本文用简化势垒模型的方法,把问题适当简化后进行量化计算,给出了微观参数与界面反应速率的半定量关系式,并同电化学测量结果相比较,证明了该计算所给出的半定量判断与实验事实相等。     

非线性传输对光节点引入的串扰的影响

利用软件首次模拟了光纤的非线性传输对串扰的影响,发现光纤的非线性传输在很大程度上加大了串扰对系统的影响;由于自相位调制引起的调制不稳定性,主信号和串扰完全同频时的误码并不是最坏情况;相反地,主信号和串扰有一定频差时误码比较大。模拟结果表明,在设计长距离光传送网时,应尽量保持光节点引入的串扰量小于40dB。