Zero field spin splitting in AlSb/InAs/AlSb quantum wells induced by surface proximity effects

InAs quantum well heterostructures are of considerable interest for mesoscopic device applications such as scanning probe and magnetic recording sensors, which require the channel to be close to the surface. Here we report on magnetotransport measurements of AlSb/InAs/AlSb Hall bars at a shallow depth of 20 nm. Analysis of the observed Shubnikov-de Haas oscillations and modeling show that spin splitting energies in excess of 2.3 meV occur at zero magnetic field. We conclude that the spin-splitting results from the Rashba effect due to the band bending in the quantum well. This is caused by substantial electron transfer from the surface to the quantum well and becomes significant when the quantum well is located near the surface.Comment: 14 pages, 2 figures. (To be published in APL

Spin and charge optical conductivities in spin-orbit coupled systems

We study the frequency dependent spin- and charge- conductivity tensors of a two-dimensional electron gas (2DEG) with Rashba and Dresselhaus spin-orbit interaction. We show that the angular anisotropy of the spin-splitting energy induced by the interplay between the Rashba and Dresselhaus couplings gives rise to a characteristic spectral behavior of the spin and charge response which is significantly different from that of pure Rashba or Dresselhaus case. Such new spectral structures open the possibility for control of the optical response by applying an external bias and/or by adjusting the light frequency. In addition, it is shown that the relative strength of the spin-orbit coupling parameters can be obtained through optical probing.Comment: 13 pages, 4 figures. Revised versio

Spin torque contribution to the a.c. spin Hall conductivity

Using the recently proposed definition of a conserved spin-current operator [J. Shi et al., Phys. Rev. Lett. 96, 076604 (2006)] we explore the frequency dependent spin Hall conductivity for a two-dimensional electron gas with Rashba and Dresselhaus spin-orbit interaction in response to an oscillating electric field. We show that the optical spectrum of the spin Hall conductivity exhibit remarkable changes when the new definition of spin current is applied. Such behavior is mainly due to a significant contribution of the spin torque term which is absent in the conventional form of the spin current. In addition, it is observed that the magnitude and direction of the dynamic spin Hall current strongly depends on the electric field frequency as with the interplay of the spin-orbit coupling strengths.Comment: 8 pages, 4 figures, pape

Tuning Fano-type resonances in coupled quantum point contacts by applying asymmetric voltages

We study the ballistic magnetotransport in a double quantum point contact (QPC) device consisting of a quasi-one-dimensional quantum wire with an embedded island-like impurity - etched nano-hole as in a recently published experiment [J. C. Chen, Y. Lin, K.-T. Lin, T. Ueda and S. Komiyama, Appl. Phys. Lett. 94, 012105 (2009)]. We reproduce the zero field quantized conductance, the interference phenomenon induced by the coupled QPCs, as well as the Ramsauer-like resonances observed in the experiments. At finite magnetic fields Fano-type resonances arises in the conductance due to the formation of localized states at the impurity periphery and to an inter-edge state resonant coupling effect. It is predicted that the Fano-type resonances can be controlled by an asymmetric confinement of the QPCs.Comment: published in AP

Development Status of a CVD System to Deposit Tungsten onto UO2 Powder via the WCI6 Process

Nuclear Thermal Propulsion (NTP) is under development for deep space exploration. NTP's high specific impulse (> 850 second) enables a large range of destinations, shorter trip durations, and improved reliability. W-60vol%UO2 CERMET fuel development efforts emphasize fabrication, performance testing and process optimization to meet service life requirements. Fuel elements must be able to survive operation in excess of 2850 K, exposure to flowing hydrogen (H2), vibration, acoustic, and radiation conditions. CTE mismatch between W and UO2 result in high thermal stresses and lead to mechanical failure as a result UO2 reduction by hot hydrogen (H2) [1]. Improved powder metallurgy fabrication process control and mitigated fuel loss can be attained by coating UO2 starting powders within a layer of high density tungsten [2]. This paper discusses the advances of a fluidized bed chemical vapor deposition (CVD) system that utilizes the H2-WCl6 reduction process

Optimization of a Wcl6 CVD System to Coat UO2 Powder with Tungsten

In order to achieve deep space exploration via Nuclear Thermal Propulsion (NTP), Marshall Space Flight Center (MSFC) is developing W-UO2 CERMET fuel elements, with focus on fabrication, testing, and process optimization. A risk of fuel loss is present due to the CTE mismatch between tungsten and UO2 in the W-60vol%UO2 fuel element, leading to high thermal stresses. This fuel loss can be reduced by coating the spherical UO2 particles with tungsten via H2/WCl6 reduction in a fluidized bed CVD system. Since the latest incarnation of the inverted reactor was completed, various minor modifications to the system design were completed, including an inverted frit sublimer. In order to optimize the parameters to achieve the desired tungsten coating thickness, a number of trials using surrogate HfO2 powder were performed. The furnace temperature was varied between 930 C and 1000degC, and the sublimer temperature was varied between 140 C and 200 C. Each trial lasted 73-82 minutes, with one lasting 205 minutes. A total of 13 trials were performed over the course of three months, two of which were re-coatings of previous trials. The powder samples were weighed before and after coating to roughly determine mass gain, and Scanning Electron Microscope (SEM) data was also obtained. Initial mass results indicated that the rate of layer deposition was lower than desired in all of the trials. SEM confirmed that while a uniform coating was obtained, the average coating thickness was 9.1% of the goal. The two re-coating trials did increase the thickness of the tungsten layer, but only to an average 14.3% of the goal. Therefore, the number of CVD runs required to fully coat one batch of material with the current configuration is not feasible for high production rates. Therefore, the system will be modified to operate with a negative pressure environment. This will allow for better gas mixing and more efficient heating of the substrate material, yielding greater tungsten coating per trial

Spin rotation for ballistic electron transmission induced by spin-orbit interaction

We study spin dependent electron transmission through one- and two-dimensional curved waveguides and quantum dots with account of spin-orbit interaction. We prove that for a transmission through arbitrary structure there is no spin polarization provided that electron transmits in isolated energy subband and only two leads are attached to the structure. In particular there is no spin polarization in the one-dimensional wire for which spin dependent solution is found analytically. The solution demonstrates spin evolution as dependent on a length of wire. Numerical solution for transmission of electrons through the two-dimensional curved waveguides coincides with the solution for the one-dimensional wire if the energy of electron is within the first energy subband. In the vicinity of edges of the energy subbands there are sharp anomalies of spin flipping.Comment: 9 oages, 7 figure