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| 2. |
- Van Kampen, M., et al.
(författare)
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On the realization of artificial XY spin chains
- 2005
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Ingår i: Journal of Physics. - 0953-8984 .- 1361-648X. ; 17:2, s. L27-L33
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Tidskriftsartikel (refereegranskat)abstract
- Spin chains are the most elementary entities in the study of magnetic ordering behavior. The authors describe a route toward the realization of artificial XY spin chains. Using thin-film deposition and e-beam lithog., 300 and 550 nm diam. pillars consisting of alternating disks of permalloy and Al2O3 were made. Shape anisotropy forces the magnetization in plane, creating a chain of two-dimensional magnetic moments →i. The disks couple to each other by their dipolar stray fields, resulting in an interaction Jij→i·→j, with Jij the coupling between disks i and j. This form closely resembles a chain of xy spins, coupled by effective interactions Jij. The authors present a magnetic characterization of the pillars, and compare the results with model calcns. Also, spin dynamics and ordering phenomena are addressed.
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| 3. |
- Bloom, F. L., et al.
(författare)
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Correspondence of the sign change in organic magnetoresistance with the onset of bipolar charge transport
- 2008
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Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 93:26
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Tidskriftsartikel (refereegranskat)abstract
- In this work we examine the transition between positive and negative organic magnetoresistance in poly[2-methoxy-5-(3(), 7()-dimethyloctyloxy)-p-phenylenevinylene] in order to understand how different regimes of charge transport affect the organic magnetoresistance effect. To characterize the charge transport in these devices we measured the current, low frequency differential capacitance, and electroluminescence efficiency as a function of voltage. These measurements show that the sign change of the magnetoresistance corresponds with a change from a unipolar diffusive transport below the built in voltage (V(bi)) to a regime of bipolar drift transport above V(bi).
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| 4. |
- Bloom, F. L., et al.
(författare)
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Separating positive and negative magnetoresistance in organic semiconductor devices
- 2007
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 99:25
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Tidskriftsartikel (refereegranskat)abstract
- We study the transition between positive and negative organic magnetoresistance (OMAR) in tris-(8 hydroxyquinoline) aluminium (Alq(3)), in order to identify the elementary mechanisms governing this phenomenon. We show how the sign of OMAR changes as function of the applied voltage and temperature. The transition from negative to positive magnetoresistance (MR) is found to be accompanied by an increase in slope of log(I) versus log(V). ac admittance measurements show this transition coincides with the onset of minority charge (hole) injection in the device. All these observations are consistent with two simultaneous contributions with opposite sign of MR, which may be assigned to holes and electrons having different magnetic field responses.
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| 5. |
- Bloom, F. L., et al.
(författare)
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Sign Inversion of Magnetoresistance in Space-Charge Limited Organic Devices
- 2009
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 103:6
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Tidskriftsartikel (refereegranskat)abstract
- In this Letter, we explain the puzzling sign change of organic magnetoresistance in space-charge limited devices by device physics. We prove analytically and numerically that in the case of bipolar conduction with an Ohmic majority carrier and an injection limited minority carrier contact, a decrease in minority carrier mobility may give rise to an increase in the device current. It is shown that when the magnetic field acts to decrease the mobility of both carriers, a sign change in the magnetoconductivity as a function of applied bias may result. This behavior is in agreement with experimental observations.
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| 6. |
- Boulle, Olivier, et al.
(författare)
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Non-adiabatic spin transfer torque in high anisotropic magnetic nanowires with narrow domain walls
- 2008
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 101:21, s. 1-4
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Tidskriftsartikel (refereegranskat)abstract
- Current induced domain wall (DW) depinning of a narrow DW in out-of-plane magnetized (Pt/Co)3/Pt multilayer elements is studied by magnetotransport. We find that for conventional measurements Joule heating effects conceal the real spin torque efficiency and so we use a measurement scheme at a constant sample temperature to unambiguously extract the spin torque contribution. From the variation of the depinning magnetic field with the current pulse amplitude we directly deduce the large nonadiabaticity factor in this material and we find that its amplitude is consistent with a momentum transfer mechanism.
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| 7. |
- Harker, Geraint J. A., et al.
(författare)
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Detection and extraction of signals from the epoch of reionization using higher-order one-point statistics
- 2009
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 393, s. 1449-1458
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Tidskriftsartikel (refereegranskat)abstract
- Detecting redshifted 21-cm emission from neutral hydrogen in the early Universe promises to give direct constraints on the epoch of reionization (EoR). It will, though, be very challenging to extract the cosmological signal (CS) from foregrounds and noise which are orders of magnitude larger. Fortunately, the signal has some characteristics which differentiate it from the foregrounds and noise, and we suggest that using the correct statistics may tease out signatures of reionization. We generate mock data cubes simulating the output of the Low Frequency Array (LOFAR) EoR experiment. These cubes combine realistic models for Galactic and extragalactic foregrounds and the noise with three different simulations of the CS. We fit out the foregrounds, which are smooth in the frequency direction, to produce residual images in each frequency band. We denoise these images and study the skewness of the one-point distribution in the images as a function of frequency. We find that, under sufficiently optimistic assumptions, we can recover the main features of the redshift evolution of the skewness in the 21-cm signal. We argue that some of these features - such as a dip at the onset of reionization, followed by a rise towards its later stages - may be generic, and give us a promising route to a statistical detection of reionization.
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| 8. |
- Harker, Geraint, et al.
(författare)
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Non-parametric foreground subtraction for 21-cm epoch of reionization experiments
- 2009
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 397, s. 1138-1152
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Tidskriftsartikel (refereegranskat)abstract
- One of the problems facing experiments designed to detect redshifted 21-cm emission from the epoch of reionization (EoR) is the presence of foregrounds which exceed the cosmological signal in intensity by orders of magnitude. While fitting them so that they can be removed, we must be careful to minimize `overfitting', in which we fit away some of the cosmological signal, and `underfitting', in which real features of the foregrounds cannot be captured by the fit, polluting the signal reconstruction. We argue that in principle it would be better to fit the foregrounds non-parametrically - allowing the data to determine their shape - rather than selecting some functional form in advance and then fitting its parameters. Non-parametric fits often suffer from other problems, however. We discuss these before suggesting a non-parametric method, Wp smoothing, which seems to avoid some of them. After outlining the principles of Wp smoothing, we describe an algorithm used to implement it. Some useful results for implementing an alternative algorithm are given in an appendix. We apply Wp smoothing to a synthetic data cube for the Low Frequency Array (LOFAR) EoR experiment. This cube includes realistic models for the signal, foregrounds, instrumental response and noise. The performance of Wp smoothing, measured by the extent to which it is able to recover the variance of the cosmological signal and to which it avoids the fitting residuals being polluted by leakage of power from the foregrounds, is compared to that of a parametric fit, and to another non-parametric method (smoothing splines). We find that Wp smoothing is superior to smoothing splines for our application, and is competitive with parametric methods even though in the latter case we may choose the functional form of the fit with advance knowledge of the simulated foregrounds. Finally, we discuss how the quality of the fit is affected by the frequency resolution and range, by the characteristics of the cosmological signal and by edge effects.
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| 9. |
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| 10. |
- Thomas, Rajat M., et al.
(författare)
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Fast large-scale reionization simulations
- 2009
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 393, s. 32-48
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Tidskriftsartikel (refereegranskat)abstract
- We present an efficient method to generate large simulations of the epoch of reionization without the need for a full three-dimensional radiative transfer code. Large dark-matter-only simulations are post-processed to produce maps of the redshifted 21-cm emission from neutral hydrogen. Dark matter haloes are embedded with sources of radiation whose properties are either based on semi-analytical prescriptions or derived from hydrodynamical simulations. These sources could either be stars or power-law sources with varying spectral indices. Assuming spherical symmetry, ionized bubbles are created around these sources, whose radial ionized fraction and temperature profiles are derived from a catalogue of one-dimensional radiative transfer experiments. In case of overlap of these spheres, photons are conserved by redistributing them around the connected ionized regions corresponding to the spheres. The efficiency with which these maps are created allows us to span the large parameter space typically encountered in reionization simulations. We compare our results with other, more accurate, three-dimensional radiative transfer simulations and find excellent agreement for the redshifts and the spatial scales of interest to upcoming 21-cm experiments. We generate a contiguous observational cube spanning redshift 6 to 12 and use these simulations to study the differences in the reionization histories between stars and quasars. Finally, the signal is convolved with the Low Frequency Array (LOFAR) beam response and its effects are analysed and quantified. Statistics performed on this mock data set shed light on possible observational strategies for LOFAR.
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