Pure spin current generation in monolayer graphene by quantum pumping

We study a method to generate pure spin current in monolayer graphene over a wide range of Fermi energy by adiabatic quantum pumping. The device consists of three gate electrodes and two ferromagnetic strips, which induce a spin-splitting in the graphene through the proximity effect. A pure spin current is generated by applying two periodic oscillating gate voltages. We find that the pumped pure spin current is a sensitive oscillatory function of the Fermi energy. Large spin currents can be found at Fermi energies where there are Fabry-Perot resonances in the barriers. Furthermore, we analyze the effects of the parameters of the system on the pumped currents. Our predicted pumped spin current can be of the order of 100 nA which is measurable using the current technology. The proposed method is useful in the realization of graphene spintronic devices.     

Thermoelectric transport and spin density of graphene nanoribbons with Rashba spin–orbit interaction

In the present paper, we have theoretically investigated thermoelectric transport properties of armchair and zigzag graphene nanoribbons with Rashba spin–orbit interaction, as well as dephasing scattering processes by applying the nonequilibrium Green function method. Behaviors of electronic and thermal currents, as well as thermoelectric coefficients are studied. It is found that both electronic and thermal currents decrease, and thermoelectric properties been suppressed, with increasing strength of Rashba spin–orbit interaction. We have also studied spin split and spin density induced by Rashba spin–orbit interaction in the graphene nanoribbons.     

Coherence control of Hall charge and spin currents

Using two-color optical coherence control techniques in intrinsic GaAs at 80 K with orthogonally polarized 70 fs, 1430 and 715 nm pulses, we generate a pure spin source current that yields a transverse Hall pure charge current; or alternatively, with parallel polarized pulses, we generate a pure charge source current that yields a pure spin current. By varying the relative phase or polarization of the incident pulses, one can effectively tune the type, magnitude and direction of both the source and transverse currents without application of electric or magnetic fields.     

L型石墨纳米结的热输运

利用非平衡格林函数方法研究了由半无限长扶手椅型和锯齿型边界石墨纳米带连接而成的L型石墨纳米结的热输运性质.结果表明,L型石墨纳米结的热导依赖于L型石墨纳米结的夹角和石墨纳米带的宽度.在L型石墨纳米结的夹角从30°增加到90°再增加到150°过程中,其热导显著增大.夹角为90°的L型石墨纳米结的热导随着扶手椅型纳米带宽度增加时,在低温区热导随着宽度的增大而降低,在高温区热导随宽度的增大而升高.对于夹角为150°的L型石墨纳米结,其热导无论是在低温区还是在高温区都随着锯齿型纳米带宽度的增加而降低.利用声子透射谱对这些热输运现象进行了合理的解释.研究结果阐明了不同L型石墨纳米结中的热输运机理,为设计基于石墨纳米结的热输运器件提供了重要的物理模型和理论依据. 关键词： 石墨纳米结 热输运 热导     

Scattering of Circularly Polarized Terahertz Waves on a Graphene Nanoantenna

We present a surface current method to model the graphene rectangular nanoantenna scattering in the terahertz band with Comsol.Compared with the equivalent thin slab method,the results obtained by the surface current method are more accurate and efficient.Then the electromagnetic scattering of circularly polarized terahertz waves on graphene nanoantennas is numerically analyzed by utilizing the surface current method.The dependences of the antenna resonant frequency with the circularly polarized wave on width and length are consistent with those for the linear polarized waves.These results are proved to be useful to design efficient nanoantennas in terahertz wireless communications.     

Thermally driven spin transport through a transverse-biased zigzag-edge graphene nanoribbon

In the framework of the Landauer-Büttiker formalism, we investigate coherent spin transport through a transverse-biased magnetic zigzag-edge graphene nanoribbon, with a temperature difference applied between the source and the drain. It is shown that a critical source temperature is needed to generate a spin-polarized current due to the presence of a forbidden transport gap. The magnitude of the obtained spin polarization exceeds 90% in a wide range of source temperatures, and its polarization direction could be changed by reversing the transverse electric field. We also find that, at fixed temperature difference, the spin-polarized current undergoes a transition from increasing to decreasing as the source temperature rises, which is attributed to the competition between the excited energy of electrons and the relative temperature difference. Moreover, by modulating the transverse electric field, the source temperature and the width of the ribbon, we can control the device to work well for generating a highly spin-polarized current.     

Thermoelectric-transport in metal/graphene/metal hetero-structure

We investigate the thermoelectric-transport properties of metal/graphene/metal hetero-structure. We use a single band tight-binding model to present the two-dimensional electronic band structure of graphene. Using the Landauer--Butticker formula and taking the coupling between graphene and the two electrodes into account, we can calculate the thermoelectric potential and current versus temperature. It is found that in spite of metal electrodes, the carrier type of graphene determines the electron motion direction driven by the difference in temperature between the two electrodes, while for n type graphene, the electrons move along the thermal gradient, and for p type graphene, the electrons move against the thermal gradient.     

Spin and spin-valley Hall effects in a honeycomb lattice with antiferromagnetism and spin-orbit couplings

We study linear response to a longitudinal electric field on an antiferromagnetic honeycomb lattice with intrinsic and Rashba spin-orbit couplings (SOCs). It is found that the spin-valley Hall effect could emerge alone or coexist with the spin Hall effect. The spin and spin-valley Hall conductivities exhibit some peculiarities that depend on the distinct topological states of the graphene lattice. Furthermore, the spin and spin-valley Hall conductivities could be remarkably modulated by changing the Fermi level. Our findings suggest that the antiferromagnetic honeycomb lattice with SOCs is an excellent platform for potential applications of spintronics and valleytronics.     

Thermoelectric spin diffusion in a ferromagnetic metal

We present a semiclassical theory of spin diffusion in a ferromagnetic metal subject to a temperature gradient. Spin-flip scattering can generate pure thermal spin currents by short-circuiting spin channels while suppressing spin accumulations. A thermally induced spin density is locally generated when the energy dependence of the density of states is spin polarized.     

Valley Hall effect and Nernst effect in strain engineered graphene

We theoretically predict the existence of tunneling valley Hall effect and Nernst effect in the normal/strain/normal graphene junctions, where a strained graphene is sandwiched by two normal graphene electrodes. By applying an electric bias a pure transverse valley Hall current with longitudinal charge current is generated. If the system is driven by a temperature bias, a valley Nernst effect is observed, where a pure transverse valley current without charge current propagates. Furthermore, the transverse valley current can be modulated by the Fermi energy and crystallographic orientation. When the magnetic field is further considered, we obtain a fully valley-polarized current. It is expected these features may be helpful in the design of the controllable valleytronic devices.     

Spin polarized quantum pump effect in zigzag graphene nanoribbons

The spin polarized adiabatic quantum pump effect in zigzag graphene nanoribbons has been numerically analyzed. Since the ground state of such a ribbon is antiferromagnetic (the opposite spin electrons are located on the opposite edges of the ribbon), the spin currents can be generated in this system with the help of the quantum pump effect when symmetry between the opposite spin states is broken. Two methods of this breaking by means of defects at the ribbon edge and the transverse electric field have been proposed. It has been shown that the generation of not only the electron and spin currents, but also the purely spin current is possible in both cases.     

Spin Caloritronic Transport of Tree-Saw Graphene Nanoribbons

Using density functional theory combined with non-equilibrium Green's function method, we investigate the spin caloritronic transport properties of tree-saw graphene nanoribbons. These systems have stable ferromagnetic ground states with a high Curie temperature that is far above room temperature and exhibit obvious spin-Seebeck effect. Moreover, thermal colossal magnetoresistance up to 1020% can be achieved by the external magnetic field modulation. The underlying mechanism is analyzed by spin-resolved transmission spectra, current spectra and band structures.     

纳米石墨烯片-正十八烷复合相变材料制备及热物性研究

本文分别制备了纳米石墨烯片质量分数为0%, 0.5%, 1%, 1.5%, 2%的纳米石墨烯片-正十八烷复合相变材料,并通过扫描电镜测试、红外光谱分析、差示扫描量热实验及导热分析等实验对其形貌结构及热物性进行表征和研究.实验表明本文制备的纳米石墨烯-正十八烷复合相变材料具有很好的相变稳定性;当纳米石墨烯片的质量分数达到2%时,复合相变材料的导热系数相对于纯十八烷高出了89.4%.     

The inelastic electron tunneling spectroscopy of edge-modified graphene nanoribbon-based molecular devices

The inelastic electron tunneling spectroscopy(IETS) of four edge-modified finite-size grapheme nanoribbon(GNR)-based molecular devices has been studied by using the density functional theory and Green's function method. The effects of atomic structures and connection types on inelastic transport properties of the junctions have been studied. The IETS is sensitive to the electrode connection types and modification types. Comparing with the pure hydrogen edge passivation systems, we conclude that the IETS for the lower energy region increases obviously when using donor–acceptor functional groups as the edge modification types of the central scattering area. When using donor–acceptor as the electrode connection groups, the intensity of IETS increases several orders of magnitude than that of the pure ones. The effects of temperature on the inelastic electron tunneling spectroscopy also have been discussed. The IETS curves show significant fine structures at lower temperatures. With the increasing of temperature, peak broadening covers many fine structures of the IETS curves.The changes of IETS in the low-frequency region are caused by the introduction of the donor–acceptor groups and the population distribution of thermal particles. The effect of Fermi distribution on the tunneling current is persistent.     

Investigation of the mode splitting induced by electro-optic birefringence in a vertical-cavity surface-emitting laser by polarized electroluminescence

The mode splitting induced by electro-optic birefringence in a P-I-N InGaAs/GaAs/A1GaAs vertical-cavity surface- emitting laser （VCSEL） has been studied by polarized electroluminescence （EL） at room temperature. The polarized EL spectra with E｜｜[110] and E ｜｜ [150] directions, are extracted for different injected currents. The mode splitting of the two orthogonal polarized modes for a VCSEL device is determined, and its value increases linearly with the increasing injected current due to electro-optic birefringence; This article demonstrates that the polarized EL is a powerful tool to study the mode splitting and polarization anisotropy of a VCSEL device.     

Temperature-Dependent Debye Temperature and Specific Capacity of Graphene

The shot-range interaction and the atomic anharmonic vibration are both considered,and then the analytic functions of the Debye temperature,the specific capacity and the thermal conductivity of graphene with the temperature are obtained.The influence of anharmonic vibration on these thermal physical properties is also investigated.Some theoretical results are given.If only the harmonic approximation is considered,the Debye temperature of the graphene is unrelated to the temperature.If the anharmonic terms are considered,it increases slowly with the increasing temperature.The molar heat capacity of the graphene increases nonlinearly with the increasing temperature.The mean free path of phonons and the thermal conductivity of the graphene decrease nonlinearly with the increasing temperature.The relative changes of the Debye temperature,the specific heat capacity and the thermal conductivity caused by the anharmonic terms increase with the increasing temperature.The anharmonic effect of atomic vibration becomes more significant under higher temperature.     

石墨烯气凝胶复合相变材料的热物性研究

相变材料利用其相变潜热能力可吸收储存和释放利用热量,同时在相变过程中其温度浮动小,能够实现温度控制从而用于热管理.但是其低热导率和易泄露问题严重制约了其性能.石墨烯气凝胶因其丰富的多孔结构而具有较大的比表面积,可吸附相变材料解决其泄露问题,同时石墨烯的高导热系数可提高相变材料的热导率.这里选取正十八烷为相变材料,制备了不同质量分数的石墨烯气凝胶复合相变材料.测得石墨烯气凝胶含量为13.99 wt%的样品,其导热系数比纯正十八烷高出306.2%,熔化潜热和凝固潜热分别下降了13.8%和10.8%.分子动力学模拟结果表明,石墨烯气凝胶的引入会在一定程度上增强正十八烷分子的有序性和一致性,即在同一温度下复合相变材料中的正十八烷分子比纯正十八烷分子拥有更集中分布的末端距和扭转角,径向分布函数和自扩散系数都相对较低,说明石墨烯材料的引入可以提升正十八烷的导热系数.     

Dimensional crossover of thermal conductance in graphene nanoribbons: a first-principles approach

First-principles density-functional calculations are performed to investigate the thermal transport properties in graphene nanoribbons (GNRs). The dimensional crossover of thermal conductance from one to two dimensions (2D) is clearly demonstrated with increasing ribbon width. The thermal conductance of GNRs of a few nanometers width already exhibits an approximate low-temperature dependence of T(1.5), like that of 2D graphene sheets which is attributed to the quadratic nature of the dispersion relation for the out-of-plane acoustic phonon modes. Using a zone-folding method, we heuristically derive the dimensional crossover of thermal conductance with the increase of ribbon width. Combining our calculations with the experimental phonon mean-free path, some typical values of thermal conductivity at room temperature are estimated for GNRs and for 2D graphene sheet. Our findings clarify the issue of the low-temperature dependence of thermal transport in GNRs and suggest a calibration range of thermal conductivity for experimental measurements in graphene-based materials.     

Shot noises of spin and charge currents in a ferromagnet-quantum-dot-ferromagnet system

We have investigated the shot noises of charge and spin current by considering the spin polarized electron tunneling through a ferromagnet-quantum-dot-ferromagnet system. We have derived the spin polarized current noise matrix, from which we can derive general expressions of shot noises associated with charge and spin currents. The spin and charge currents are intimately related to the polarization angles, and they behave quite differently from each other. The shot noise of charge current is symmetric about the gate voltage whose structure is modified by the Zeeman field considerably. There exists oscillations in spin current shot noise in the absence of source-drain bias at zero temperature, and it is asymmetric in the positive and negative regimes of sourcedrain voltage. The shot noise of spin current behaves quite differently from the shot noise of charge current, since the spin current components I_x^s, I_y^s oscillate sinusoidally with the frequency ?_? in the ?th lead, while the I_z^s component of spin current is independent of time.     

Unexpected large thermal rectification in asymmetric grain boundary of graphene

We have investigated the lattice thermal transport across the asymmetric tilt grain boundary between armchair and zigzag graphene by nonequilibrium molecular dynamics (NEMD). We have observed significant temperature drop and ultra-low temperature-dependent thermal boundary resistance. More importantly, we find an unexpected thermal rectification phenomenon. The thermal conductivity and Kapitza conductance is direction-dependent. The effect of thermal rectification could be amplified by increasing the difference of temperature imposed on two sides. Our results propose a promising kind of thermal rectifier and phonon diodes based on polycrystalline graphene without delicate manipulation of the atomic structure.