Measurement of the quantum of thermal conductance

The physics of mesoscopic electronic systems has been explored for more than 15 years. Mesoscopic phenomena in transport processes occur when the wavelength or the coherence length of the carriers becomes comparable to, or larger than, the sample dimensions. One striking result in this domain is the quantization of electrical conduction, observed in a quasi-one-dimensional constriction formed between reservoirs of two-dimensional electron gas. The conductance of this system is determined by the number of participating quantum states or 'channels' within the constriction; in the ideal case, each spin-degenerate channel contributes a quantized unit of 2e(2)/h to the electrical conductance. It has been speculated that similar behaviour should be observable for thermal transport in mesoscopic phonon systems. But experiments attempted in this regime have so far yielded inconclusive results. Here we report the observation of a quantized limiting value for the thermal conductance, Gth, in suspended insulating nanostructures at very low temperatures. The behaviour we observe is consistent with predictions for phonon transport in a ballistic, one-dimensional channel: at low temperatures, Gth approaches a maximum value of g0 = pi2kB2T/3h, the universal quantum of thermal conductance.     

禁带区域内负的群速度

研究超声波经过两个烧结样品的行为.两个烧结样品为有序的面心立方结构,它们的结构几乎相同.研究表明:在两个有序的准均匀的介质中分别有一个禁带区域.就禁带区域而言,在两个样品中产生禁带的区域非常接近.同时在禁带出现的区域群速度为负值.我们的结果表明群速度的负值的确存在于有序介质中的禁带区域内.     

Negative intrinsic resistivity of an individual domain wall in epitaxial (Ga,Mn)As microdevices

Magnetic domains, and the boundaries that separate them (domain walls, DWs), play a central role in the science of magnetism. Understanding and controlling domains is important for many technological applications in spintronics, and may lead to new devices. Although theoretical efforts have elucidated several mechanisms underlying the resistance of a single DW, various experiments report conflicting results, even for the overall sign of the DW resistance. The question of whether an individual DW gives rise to an increase or decrease of the resistance therefore remains open. Here we report an approach to DW studies in a class of ferromagnetic semiconductors (as opposed to metals) that offer promise for spintronics. These experiments involve microdevices patterned from monocrystalline (Ga,Mn)As epitaxial layers. The giant planar Hall effect that we previously observed in this material enables direct, real-time observation of the propagation of an individual magnetic DW along multiprobe devices. We apply steady and pulsed magnetic fields, to trap and carefully position an individual DW within each separate device studied. This protocol reproducibly enables high-resolution magnetoresistance measurements across an individual wall. We consistently observe negative intrinsic DW resistance that scales with channel width. This appears to originate from sizeable quantum corrections to the magnetoresistance.     

传统农业生态系统的保护

世界的生物多样性很大程度上受到传统农业耕作方式的保护。这些生态上复杂的农业系统与作物遗传多样性中心相联系,有传统的栽培品种或“地方品种”,作为世界作物遗传资源的重要组成,还有野生的植物和动物种群,作为生物资源服务于人类。