Quantum oscillations in graphene in the presence of disorder and interactions

Quantum oscillations in graphene is discussed. The effect of interactions are addressed by Kohn's theorem regarding de Haas-van Alphen oscillations, which states that electron-electron interactions cannot affect the oscillation frequencies as long as disorder is neglected and the system is sufficiently screened, which should be valid for chemical potentials not very close to the Dirac point. We determine the positions of Landau levels in the presence of potential disorder from exact transfer matrix and finite size diagonalization calculations. The positions are shown to be unshifted even for moderate disorder; stronger disorder, can, however, lead to shifts, but this also appears minimal even for disorder width as large as one-half of the bare hopping matrix element on the graphene lattice. Shubnikov-de Haas oscillations of the conductivity are calculated analytically within a self-consistent Born approximation of impurity scattering. The oscillatory part of the conductivity follows the widely invoked Lifshitz-Kosevich form when certain mass and frequency parameters are properly interpreted.Comment: Appendix A was removed, as the content of it is already contained in Ref. 17. Thanks to M. A. H. Vozmedian

Characteristics of a tunneling quantum-dot infrared photodetector operating at room temperature

We report high-temperature (240–300 K)(240–300K) operation of a tunneling quantum-dot infrared photodetector. The device displays two-color characteristics with photoresponse peaks at ∼ 6 μm∼6μm and 17 μm17μm. The extremely low dark current density of 1.55 A/cm21.55A∕cm2 at 300 K300K for 1 V1V bias is made possible by the tunnel filter. For the 17 μm17μm absorption, the measured peak responsivity is 0.16 A/W0.16A∕W (300 K)(300K) for a bias of 2 V2V and the specific detectivity D*D* is 1.5×107 cm Hz1/2/W1.5×107cmHz1∕2∕W (280 K)(280K) for a bias of 1 V1V. Excellent performance characteristics are also measured for the 6 μm6μm photoresponse.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87841/2/191106_1.pd

Terahertz detection with tunneling quantum dot intersublevel photodetector

The characteristics of a tunnel quantum dot intersublevel photodetector, designed for the absorption of terahertz radiation, are described. The absorption region consists of self-organized In0.6Al0.4As/GaAsIn0.6Al0.4As∕GaAs quantum dots with tailored electronic properties. Devices exhibit spectral response from 20 to 75 μm20to75μm ( ∼ 4 THz)(∼4THz) with peak at ∼ 50 μm∼50μm. The peak responsivity and specific detectivity of the device are 0.45 A/W0.45A∕W and 108 cm Hz1/2/W108cmHz1∕2∕W, respectively, at 4.6 K4.6K for an applied bias of 1 V1V. Response to terahertz radiation is observed up to 150 K150K.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87819/2/031117_1.pd

Modulated Rashba interaction in a quantum wire: Spin and charge dynamics

It was recently shown that a spatially modulated Rashba spin-orbit coupling in a quantum wire drives a transition from a metallic to an insulating state when the wave number of the modulation becomes commensurate with the Fermi wave length of the electrons in the wire. It was suggested that the effect may be put to practical use in a future spin transistor design. In the present article we revisit the problem and present a detailed analysis of the underlying physics. First, we explore how the build-up of charge density wave correlations in the quantum wire due to the periodic gate configuration that produces the Rashba modulation influences the transition to the insulating state. The interplay between the modulations of the charge density and that of the spin-orbit coupling turns out to be quite subtle: Depending on the relative phase between the two modulations, the joint action of the Rashba interaction and charge density wave correlations may either enhance or reduce the Rashba current blockade effect. Secondly, we inquire about the role of the Dresselhaus spin-orbit coupling that is generically present in a quantum wire embedded in semiconductor heterostructure. While the Dresselhaus coupling is found to work against the current blockade of the insulating state, the effect is small in most materials. Using an effective field theory approach, we also carry out an analysis of effects from electron- electron interactions, and show how the single-particle gap in the insulating state can be extracted from the more easily accessible collective charge and spin excitation thresholds. The smallness of the single-particle gap together with the anti-phase relation between the Rashba and chemical potential modulations pose serious difficulties for realizing a Rashba-controlled current switch in an InAs-based device. Some alternative designs are discussed.Comment: 20 pages, 6 figure

Demonstration of all‐optical modulation in a vertical guided‐wave nonlinear coupler

The performance characteristics of an AlGaAs dual waveguide vertical coupler with a nonlinear GaAs/AlGaAs multiquantum well coupling medium are demonstrated. The structure was grown by molecular beam epitaxy and fabricated by optical lithography and ion milling. The nonlinear coupling and modulation behavior is identical to that predicted theoretically. The nonlinear index of refraction and critical input power are estimated to be n2=1.67×10−5 cm2/W and Pc=170 W/cm2, respectively. This device also allows reliable measurement of the nonlinear refractive index for varying quantum well and optical excitation parameters.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69681/2/APPLAB-52-14-1125-1.pd

An electrically injected InAs/GaAs quantum-dot photonic crystal microcavity light-emitting diode

An electrically injected InAs/GaAs self-organized quantum-dot photonic crystal microcavity light-emitting diode operating at 1.04 μm is demonstrated. Light–current characteristics are obtained for devices with two- and five-defect period cavities with maximum light output of 0.17 μW measured in the surface-normal direction. Near-field images were also obtained for an injection current of 8.35 mA, showing light confinement within a few periods of the photonic crystal defect microcavity. © 2002 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70407/2/APPLAB-81-20-3876-1.pd

Climate Resilient Agriculture Experiences from NICRA Implementation in North Bank Plains Zone of Assam

Not AvailableClimate change impacts on agriculture are being witnessed all over the world, but countries like India are more vulnerable in view of the huge population dependent on agriculture, excessive pressure on natural resources and poor coping mechanisms. The warming trend in India over the past 100 years has indicated an increase of 0.60°C. The projected impacts are likely to further aggravate field fluctuations of many crops thus impacting food security. There are already evidences of negative impacts on yield of wheat and paddy in parts of India due to increased temperature, water stress and reduction in number of rainy days. Increased abnormalities in amount and distribution of rainfall has also been observed in North Bank Plains Zone (NBPZ) of Assam along with increase in numbers of wet spell driven flash floods and seasonal droughts in recent years. Planned adaption is essential to increase the resilience of agricultural production to climate change. Several improved agricultural practices evolved over time for diverse agroecological regions in India have potential to enhance climate change adaptation, if deployed prudently. Management practices that increase agricultural production under adverse climatic conditions also tend to support climate change adaptation because they increase resilience and reduce yield variability under variable climate and extreme events. Keeping this in view, National Innovations on Climate Resilient Agriculture was implemented under All India Coordinated Research Project for Dryland Agriculture, Biswanath Chariali centre on 2011in two selected villages of North Lakhimpur, Chamua and Ganakdoloni. The main aim of the project is to enhance resilience of Indian agriculture to climate change and climate variability through strategic research and technology demonstration. In this bulletin the authors have penned down the experiences gained from implementation of NICRA in NBPZ of Assam. It is hoped that the bulletin will be able to serve the purposes towards achieving its objectives and act as a benchmark study for the entire area. It is expected that the information contained in this bulletin will be exploited by the scientist of different disciplines and extension officials for making rational recommendation. The authors will remain ever grateful to Project Coordinator, (AICRPDA), CRIDA, Hyderabad and Director of Research (Agri), AAU for their guidance, encouragement and sincere support. The authors extend thankfulness to the fellow Scientists, Research Associates and other technical staff for their cooperation and help. The authors express their thankfulness to Sharada Offset, Biswanath Chariali for printing the report.Not Availabl

Devolatilization of coals of North-Eastern India under fluidized bed conditions in oxygen-enriched air

Oxygen-enriched air can increase the combustion efficiency, boiler efficiency, and sulfur absorption efficiency of atmospheric fluidized bed combustion (AFBC) boilers which use high-sulfur coal, and other combustion systems that use coal. Devolatilization is the first step in the gasification or combustion of coal. In this work, devolatilization characteristics of five run-of-mine (ROM) coals of North-Eastern India having particle-size between 4mmand 9 mm are reported. The experiments were performed under fluidized bed conditions at 1123 K in enriched air containing 30% oxygen. The devolatilization time was correlated with the particle diameter by a power law correlation. The variation of mass with time was correlated by an exponential correlation. It was observed that the average ratio of yield of volatile matter to the proximate volatile matter decreased with the increase in volatilecontent of the coals. A shrinking-core model was used to determine the role of filmdiffusion, ash-diffusion and chemical reaction. The experimental results indicate the likelihood of film-diffusion to be the rate-controlling mechanism in presence of oxygenenriched air. A cost-analysis was carried out to study the economy of the proces

Competing order, Fermi surface reconstruction, and quantum oscillations in underdoped high temperature superconductors

We consider incommensurate $d$-density wave order in underdoped high temperature superconductors. We find that Fermi surface reconstruction can correctly capture the phenomenology of the recent quantum oscillation experiments that suggest incommensurate order. The predicted frequencies are a frequency around 530 T arising from the electron pocket, a hole frequency at around 1650 T, and a new low frequency from a smaller hole pocket at 250 T for which there are some indications that require further investigation. The oscillation corresponding to the electron pocket will be further split due to bilayer coupling but the splitting is sufficiently small to require more refined measurements. The truly incommensurate $d$-density wave breaks both time reversal and inversion but the product of these two symmetry operations is preserved. There is some similarity of our results with the spiral spin density wave order, which, as pointed out by Overhauser, also breaks time reversal and inversion. Calculations corresponding to higher order commensuration produces results similar to anti-phase spin stripes, but appear to us to be an unlikely explanation of the experiments. The analysis of the Gorkov equation in the mixed state shows that the oscillation frequencies are unshifted from the putative normal state and the additional Dingle factor arising from the presence of the mixed state can provide a subtle distinction between the spiral spin density wave and the $d$-density wave.Comment: 16 pages and 8 figure