140 research outputs found
The amylose - free potato mutant as a model plant to study gene expression and gene silencing
In this thesis, gene-expression and gene silencing were examined for Granule Bound Starch Synthase (GBSS) which catalyses the formation of amylose and Branching Enzyme (BE) which catalyses the formation of amylopectin. The (GBSS) deficient, with iodine, red staining amylose-free (amf) potato mutant was used in order to facilitate the experiments; GBSS-gene expression and BE-gene silencing resulted in blue staining starch.A dosage effect of the wild-type GBSS-allele on GBSS-activity and amylose content was found in a tetraploid dosage population. The presence of amylose had a distinct influence on the physico-chemical properties of the starch.Insertion of the wild-type GBSS-gene in the amf -mutant resulted in fully and partially complemented plants. The minimum number of independently segregating active GBSS-inserts was estimated by genetic analysis after microspore staining. The complemented phenotype was normally transmitted to the F1 for the fully complemented, but not for the partially complemented plants. For one plant the partial complementation was correlated with the presence of a block of five inserts. This block was also capable of inhibiting the endogenous GBSS-gene in a wild- type. This indicates that partial complementation can be caused by co-suppression, besides low transgene expression. Co-suppression was also obtained when the full size (GBSS) sequence or (GBSS) cDNA were introduced in a wild-type potato.Introduction of the distal 1.5 kb cDNA coding for BE in both sense and antisense orientation resulted in several transgenic plants with a small blue core in these starch granules. This could indicate the presence of loosely branched amylopectin. in the core of the starch granules. The expression of the endogenous BE-gene was largely or fully inhibited as judged by the absence of BE mRNA and protein. This did not result in a measurable effect on the branching degree, but resulted in altered physico-chemical properties of the starch compared to amf -starch.It is concluded that this study shows that the amf -mutant is successfully used as a model plant to examine different aspects of gene expression
Spin-dependent Seebeck coefficients of Ni_{80}Fe_{20} and Co in nanopillar spin valves
We have experimentally determined the spin-dependent Seebeck coefficient of
permalloy (Ni_{80}Fe_{20}) and cobalt (Co) using nanopillar spin valve devices.
The devices were specifically designed to completely separate heat related
effects from charge related effects. A pure heat current through the nanopillar
spin valve, a stack of two ferromagnetic layers (F) separated by a non-magnetic
layer (N), leads to a thermovoltage proportional to the spin-dependent Seebeck
coefficient S_{S}=S_{\uparrow}-S_{\downarrow} of the ferromagnet, where
S_{\uparrow} and S_{\downarrow} are the Seebeck coefficient for spin-up and
spin-down electrons. By using a three-dimensional finite-element model (3D-FEM)
based on spin-dependent thermoelectric theory, whose input material parameters
were measured in separate devices, we were able to accurately determine a
spin-dependent Seebeck coefficient of -1.8 microvolt/Kelvin and -4.5
microvolt/Kelvin for cobalt and permalloy, respectively corresponding to a
Seebeck coefficient polarization P_{S}=S_{S}/S_{F} of 0.08 and 0.25, where
S_{F} is the Seebeck coefficient of the ferromagnet. The results are in
agreement with earlier theoretical work in Co/Cu multilayers and spin-dependent
Seebeck and spin-dependent Peltier measurements in Ni_{80}Fe_{20}/Cu spin valve
structures
Observation of the spin Peltier effect
We report the observation of the spin Peltier effect (SPE) in the
ferrimagnetic insulator Yttrium Iron Garnet (YIG), i.e. a heat current
generated by a spin current flowing through a Platinum (Pt)|YIG interface. The
effect can be explained by the spin torque that transforms the spin current in
the Pt into a magnon current in the YIG. Via magnon-phonon interactions the
magnetic fluctuations modulate the phonon temperature that is detected by a
thermopile close to the interface. By finite-element modelling we verify the
reciprocity between the spin Peltier and spin Seebeck effect. The observed
strong coupling between thermal magnons and phonons in YIG is attractive for
nanoscale cooling techniques.Comment: 5 pages, 3 figures, 4 pages supplementary information, 4
supplementary figure
Sign of inverse spin Hall voltages generated by ferromagnetic resonance and temperature gradients in yttrium iron garnet|platinum bilayers
We carried out a concerted effort to determine the absolute sign of the
inverse spin Hall effect voltage generated by spin currents injected into a
normal metal. We focus on yttrium iron garnet (YIG)|platinum bilayers at room
temperature, generating spin currents by microwaves and temperature gradients.
We find consistent results for different samples and measurement setups that
agree with theory. We suggest a right-hand-rule to define a positive spin Hall
angle corresponding to with the voltage expected for the simple case of
scattering of free electrons from repulsive Coulomb charges.Comment: incorporated additions from the published versio
Simultaneous ultrafast probing of intramolecular vibrations and photoinduced charge carriers in rubrene using broadband time-domain THz spectroscopy
The ultrafast frequency- and time-resolved complex dielec. responses of photoexcited, single-crystal rubrene at n = 10-30 THz were detd. using ultrafast broadband far-IR spectra. In this frequency range, the responses of both photogenerated mobile charges and intramol. vibrational modes were obsd. simultaneously, both of which vary with time after excitation. The data in conjunction with a theor. model indicate a dynamic blueshift of the 15.5 THz phonon. [on SciFinder (R)
Charge transport modulation by a redox supramolecular spin-filtering chiral crystal
The chirality induced spin selectivity (CISS) effect is a fascinating
phenomena correlating molecular structure with electron spin-polarisation in
excited state measurements. Experimental procedures to quantify the
spin-filtering magnitude relies generally on averaging data sets, especially
those from magnetic field dependent conductive-AFM. We investigate the
underlying observed disorder in the IV spectra and the origin of spikes
superimposed. We demonstrate and explain that a dynamic, voltage sweep rate
dependent, phenomena can give rise to complex IV curves for chiral crystals of
coronene bisimide. The redox group, able to capture localized charge states,
acts as an impurity state interfering with a continuum, giving rise to Fano
resonances. We introduce a novel mechanism for the dynamic transport which
might also provide insight into the role of spin-polarization. Crucially,
interference between charge localisation and delocalisation during transport
may be important properties into understanding the CISS phenomena
Diamond deposition on modified silicon substrates: Making diamond atomic force microscopy tips for nanofriction experiments
Fine-crystalline diamond particles are grown on standard Si atomic force microscopy tips, using hot filament-assisted chemical vapor deposition. To optimize the conditions for diamond deposition, first a series of experiments is carried out using silicon substrates covered by point-topped pyramids as obtained by wet chemical etching. The apexes and the edges of the silicon pyramids provide favorable sites for diamond nucleation and growth. The investigation of the deposited polycrystallites is done by means of optical microscopy, scanning electron microscopy and micro-Raman spectroscopy. The resulting diamond-terminated tips are tested in ultra high vacuum using contact-mode atomic force microscope on a stepped surface of sapphire showing high stability, sharpness, and hardnes
Development of a Near-Field Magneto-Optical Microscopy for Studying Ultrafast Magnetization Dynamics
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