Universal Properties of Linear Magnetoresistance in Strongly Disordered Semiconductors

Linear magnetoresistance occurs in semiconductors as a consequence of strong electrical disorder and is characterized by nonsaturating magnetoresistance that is proportional to the applied magnetic field. By investigating a disordered MnAs-GaAs composite material, it is found that the magnitude of the linear magnetoresistance (LMR) is numerically equal to the carrier mobility over a wide range and is independent of carrier density. This behavior is complementary to the Hall effect that is independent of the mobility and dependent on the carrier density. Moreover, the LMR appears to be insensitive to the details of the disorder and points to a universal explanation of classical LMR that can be applied to other material systems.Comment: Accepted by Phys. Rev. B (2010

Effect of Fluid Suction on an Oscillatory MHD Channel Flow with Heat Transfer

Magnetohydrodynamics (MHD) is generally concerned with the study of the magnetic properties (behaviour) of electrically conducting fluids (plasmas, liquid metals etc.) moving in an electromagnetic field. The importance of the concept of MHD in various fields such as astrophysics, bio-medical research, missile technology and geophysics motivates the modelling and investigation of MHD flow and transport problems. The role of fluid suction is paramount in laminar flow control and has wide applications in fields such as aeronautical engineering, automobile engineering and rocket science. This fact inspires the study of the effects of fluid suction in flow and transport models. Time dependent flows are widely encountered in engineering applications such as turbines and in physiological studies such as flow of bio-fluid (blood etc.). In the present paper, an attempt has been made to investigate analytically the problem of a time dependent channel flow with heat transfer, where the channel is bounded by two infinite parallel porous walls. The pressure gradient is assumed to be oscillatory in nature. A magnetic field of uniform strength is assumed to be applied normal to the walls. After necessary idealization of the momentum and energy equations, the governing equations of our problem are solved by adopting the regular perturbation technique. The effects of magnetic field, suction velocity, viscous dissipation, Reynolds number, Prandtl number etc. on the flow and heat transfer are studied and demonstrated graphically. It is seen that magnetic field, fluid suction, viscous dissipation, Reynolds number, Prandtl number have a significant effect on the flow and heat transfer characteristic. For instance, the imposition of the magnetic field enhances the rates of heat transfer at the walls and the fluid suction decreases the temperature and aids in laminar flow control

Experimental demonstration of single-mode large mode area multi-trench fiber for UV-VIS light transmission

Step-index optical fibers are widely used waveguides for light transmission. However, non-linear effects are always a severe challenge for optical fibers with increasing power level. This challenge is more severe at shorter wavelengths, where core size has to be much smaller in order to maintain a single mode operation. For an example, for 0.005 core refractive index with respect to cladding, a 10µm core diameter can ensure single mode operation at 1550nm. On other hand, core diameter has to be ~4µm and ~2µm at lambda=632nm and 300nm respectively for single mode operation. At these shorter wavelengths, photonic crystal fiber (PCF) and hollow core photonic bandgap fiber (HC-PBGF) have been proposed to address non-linear effects by offering large core diameter and air-core respectively [1-2]. However, such fibers are relatively difficult to fabricate. Moreover, presence of air-holes causes considerable difficulties in cleaving and splicing.Recently, we proposed an all-solid fiber design known as multi-trench fiber (MTF) as shown in Fig. 1(a) [3]. In this paper, we demonstrated their mode area scaling capability for UV-VIS wavelengths. Numerical simulations show the potential of achieving an effective single mode for 10µm and 20µm core diameter MTF by ensuring high loss to the higher order modes (HOMs) at ~300nm and ~632nm respectively. Fig. 1(b) shows numerically computed bending loss of a 20µm core MTF with trench thickness (t)=1.34µm, refractive index difference between core and cladding (Delta.n)=0.005, and resonant ring thickness (d)=6µm at 632nm. It is important to note that MTF ensures similar level of loss (as shown in Fig. 1(b)) to the HOMs even in an unbent case thanks to the resonant coupling between modes of core and resonant ring, which ensures suitability for beam delivery applications. Fig. 1(c) shows the RIP and microscope image of a 20µm core MTF fabricated by MCVD process in conjunction with rod-in-tube technique. Fig. 1(d) shows the measured bending loss of fiber. The loss remains lower than 0.2dB/m and 0.5dB/m at ~30cm and ~15cm bend radius respectively at ~632nm. This loss can even be reduced down with further refinement in fabrication process. We investigated the output profile of a 2m long MTF using the experimental set-up shown in Fig. 1(e) with respect to the multi-mode input beam as shown in Fig. 1(g). Fig. 1(f) shows the output for different offset launching, while fiber is coiled at ~20cm bend radius. On the other hand, Fig. 1(h) shows the output at different coil radii for optimum launching. In this paper, for the first time, to the best of our knowledge, an effective-single-mode behaviour of an all-solid and cylindrical symmetrical fiber of a large core (~20µm) fiber at ~632nm has been demonstrated. All-solid design and cylindrical symmetry ensure suitability for mass-scale production and easy cleaving and splicing. Further, with these advantages, a large core (~10µm) MTF at ~300nm as confirmed by our simulations can also be achieved and details will be presented at conference

Proximity-induced ferromagnetism and chemical reactivity in few-layer VSe2 heterostructures

Among transition-metal dichalcogenides, mono and few-layers thick VSe2 has gained much recent attention following claims of intrinsic room-temperature ferromagnetism in this system, which have nonetheless proved controversial. Here, we address the magnetic and chemical properties of Fe/VSe2 heterostructure by combining element sensitive x-ray absorption spectroscopy and photoemission spectroscopy. Our x-ray magnetic circular dichroism results confirm recent findings that both native mono/few-layer and bulk VSe2 do not show intrinsic ferromagnetic ordering. Nonetheless, we find that ferromagnetism can be induced, even at room temperature, after coupling with a Fe thin film layer, with antiparallel alignment of the moment on the V with respect to Fe. We further consider the chemical reactivity at the Fe/VSe2 interface and its relation with interfacial magnetic coupling

Unsteady MHD free convective flow past a vertical porous plate immersed in a porous medium with Hall current, thermal diffusion and heat source

Abstract An attempt has been made to study the unsteady MHD free convective flow past a vertical porous plate immersed in a porous medium with Hall current, thermal diffusion and heat source. Analytical solution has been found depending on the physical parameters including the Hartmann number M, the Prandtl number Pr, the Grashof number for heat transfer Gr , the Grashof number for mass transfer Gc , the Schmidt number Sc , the Hall parameter m, the Soret number 0 S , heat source S , frequency parameter Ω . The influence of these parameters on velocity, temperature, species concentration, and shearing stress at the plate are demonstrated graphically and the results obtained are discussed. It is found that the concentration at the plate-surface increases under Soret effect. Further, it is observed that the Soret effect causes the main-flow shear stress to rise and the crossflow shear stress to fall. It is also found that a decrease in the Soret effect leads to an increase in both the main flow and crossflow velocities. 2000 Mathematics subject classification: 76 W 0

Role of serum CA 19-9 as a tumor marker in TCC bladder

Background: There is a dearth of reliable blood and urine markers for transitional cell carcinoma of urinary bladder. CA 19-9 is a well-known marker for gastrointestinal malignancies and is being investigated for other malignancies including carcinoma bladder. In this prospective study, we evaluated the role of serum CA 19-9 as a tumor marker and correlated its level with tumor grade and stage.Methods: One hundred and fifteen patients with transitional cell carcinoma of urinary bladder and 69 healthy volunteers, as controls were included in the study. Preoperative blood sample was analysed for level of CA 19-9 using ELISA kit (normal - 0 U/ml to 37U/ml) and were correlated with grade and TNM stage of tumor.Results: The range of the control group is 2-38U/ml (mean: 17.67±9.68U/ml); TCC group is 1-94U/ml (mean: 37.12±31.52U/ml) (p=0.304). When CA 19-9 level >37IU/ml was taken as cut-off for a positive test, sensitivity of detecting T3 disease, T4 disease, MIBC, presence of node and high grade tumour were 80%, 75%, 70.3%, 78% and 57.8% respectively. However, there was a statistically significant increase in levels of CA19-9 in relation to higher grade (<0.001), presence of muscle invasion (<0.001), T stage (<0.001) and N stage (<0.001).Conclusions: Serum CA19-9 is almost invariably raised in patients with high grade and invasive disease. Thus, it has a place as a prognostic marker rather than as a diagnostic tool due to its low sensitivity for TCC bladder

Astrophysical S_{17}(0) factor from a measurement of d(7Be,8B)n reaction at E_{c.m.} = 4.5 MeV

Angular distribution measurements of $^2$H($^7$Be,$^7$Be)$^2$H and $^2$H($^7$Be,$^8$B)$n$ reactions at $E_{c.m.}\sim$~4.5 MeV were performed to extract the astrophysical $S_{17}(0)$ factor using the asymptotic normalization coefficient (ANC) method. For this purpose a pure, low emittance $^7$Be beam was separated from the primary $^7$Li beam by a recoil mass spectrometer operated in a novel mode. A beam stopper at 0$^{\circ}$ allowed the use of a higher $^7$Be beam intensity. Measurement of the elastic scattering in the entrance channel using kinematic coincidence, facilitated the determination of the optical model parameters needed for the analysis of the transfer data. The present measurement significantly reduces errors in the extracted $^7$Be(p,$\gamma$) cross section using the ANC method. We get $S_{17}$~(0)~=~20.7~$\pm$~2.4 eV~b.Comment: 15 pages including 3 eps figures, one figure removed and discussions updated. Version to appear in Physical Review

The relationship between web enjoyment and student perceptions and learning using a web-based tutorial

Web enjoyment has been regarded as a component of system experience. However, there has been little targeted research considering the role of web enjoyment alone in student learning using web-based systems. To address this gap, this study aims to examine the influence of web enjoyment on learning performance and perceptions by controlling system experience as a variable in the study. 74 students participated in the study, using a web-based tutorial covering subject matter in the area of 'Computation and algorithms'. Their learning performance was assessed with a pre-test and a post-test and their learning perceptions were evaluated with a questionnaire. The results indicated that there are positive relationships between the levels of web enjoyment and perceived usefulness and non-linear navigation for users with similar, significant levels of system experience. The implications of these findings in relation to web-based learning are explored and ways in which the needs of students who report different levels of web enjoyment might be met are discussed

Periodic harmonic functions on lattices and points count in positive characteristic

This survey addresses pluri-periodic harmonic functions on lattices with values in a positive characteristic field. We mention, as a motivation, the game "Lights Out" following the work of Sutner, Goldwasser-Klostermeyer-Ware, Barua-Ramakrishnan-Sarkar, Hunzikel-Machiavello-Park e.a.; see also 2 previous author's preprints for a more detailed account. Our approach explores harmonic analysis and algebraic geometry over a positive characteristic field. The Fourier transform allows us to interpret pluri-periods of harmonic functions on lattices as torsion multi-orders of points on the corresponding affine algebraic variety.Comment: These are notes on 13p. based on a talk presented during the meeting "Analysis on Graphs and Fractals", the Cardiff University, 29 May-2 June 2007 (a sattelite meeting of the programme "Analysis on Graphs and its Applications" at the Isaac Newton Institute from 8 January to 29 June 2007