Photo-induced persistent inversion of germanium in a 200-nm-deep surface region.

The controlled manipulation of the charge carrier concentration in nanometer thin layers is the basis of current semiconductor technology and of fundamental importance for device applications. Here we show that it is possible to induce a persistent inversion from n- to p-type in a 200-nm-thick surface layer of a germanium wafer by illumination with white and blue light. We induce the inversion with a half-life of ~12 hours at a temperature of 220 K which disappears above 280 K. The photo-induced inversion is absent for a sample with a 20-nm-thick gold capping layer providing a Schottky barrier at the interface. This indicates that charge accumulation at the surface is essential to explain the observed inversion. The contactless change of carrier concentration is potentially interesting for device applications in opto-electronics where the gate electrode and gate oxide could be replaced by the semiconductor surface.

Engineering spin propagation across a hybrid organic/inorganic interface using a polar layer.

Spintronics has shown a remarkable and rapid development, for example from the initial discovery of giant magnetoresistance in spin valves to their ubiquity in hard-disk read heads in a relatively short time. However, the ability to fully harness electron spin as another degree of freedom in semiconductor devices has been slower to take off. One future avenue that may expand the spintronic technology base is to take advantage of the flexibility intrinsic to organic semiconductors (OSCs), where it is possible to engineer and control their electronic properties and tailor them to obtain new device concepts. Here we show that we can control the spin polarization of extracted charge carriers from an OSC by the inclusion of a thin interfacial layer of polar material. The electric dipole moment brought about by this layer shifts the OSC highest occupied molecular orbital with respect to the Fermi energy of the ferromagnetic contact. This approach allows us full control of the spin band appropriate for charge-carrier extraction, opening up new spintronic device concepts for future exploitation.

Direct measurement of the electronic spin diffusion length in a fully functional organic spin valve by low-energy muon spin rotation.

Electronic devices that use the spin degree of freedom hold unique prospects for future technology. The performance of these 'spintronic' devices relies heavily on the efficient transfer of spin polarization across different layers and interfaces. This complex transfer process depends on individual material properties and also, most importantly, on the structural and electronic properties of the interfaces between the different materials and defects that are common to real devices. Knowledge of these factors is especially important for the relatively new field of organic spintronics, where there is a severe lack of suitable experimental techniques that can yield depth-resolved information about the spin polarization of charge carriers within buried layers of real devices. Here, we present a new depth-resolved technique for measuring the spin polarization of current-injected electrons in an organic spin valve and find the temperature dependence of the measured spin diffusion length is correlated with the device magnetoresistance.

Size effect in the spin glass magnetization of thin AuFe films as studied by polarized neutron reflectometry.

We used polarized neutron reflectometry to determine the temperature dependence of the magnetization of thin AuFe films with 3% Fe concentration. We performed the measurements in a large magnetic field of 6 T in a temperature range from 295 to 2 K. For the films in the thickness range from 500 to 20 nm we observed a Brillouin-type behavior from 295 K down to 50 K and a constant magnetization of about 0.9 micro(B) per Fe atom below 30 K. However, for the 10 nm thick film we observed a Brillouin-type behavior down to 20 K and a constant magnetization of about 1.3 micro(B) per Fe atom below 20 K. These experiments are the first to show a finite-size effect in the magnetization of single spin-glass films in large magnetic fields. Furthermore, the ability to measure the deviation from the paramagnetic behavior enables us to prove the existence of the spin-glass state where other methods relying on a cusp-type behavior fail.

Muon spin relaxation studies of superconductivity in a crystalline array of weakly coupled metal nanoparticles.

We report muon-spin-relaxation studies in weak transverse fields of the superconductivity in the metal cluster compound, Ga84[N(SiMe3)2]20-Li6Br2(thf)20.2 toluene. The temperature and field dependence of the muon-spin-relaxation rate and Knight shift clearly evidence type II bulk superconductivity below Tc approximately 7.8 K, with Bc1 approximately 0.06 T, Bc2 approximately 0.26 T, kappa approximately 2, and weak flux pinning. The data are well described by the s-wave BCS model with weak electron-phonon coupling in the clean limit. A qualitative explanation for the conduction mechanism in this novel type of narrow-band superconductor is presented.

Evidence for electronic phase separation between orbital orderings in SmVO3.

We report evidence for phase coexistence of orbital orderings of different symmetry in SmVO3 by high resolution x-ray powder diffraction. The phase coexistence is triggered by an antiferromagnetic ordering of the vanadium spins near 130 K, below an initial orbital ordering near 200 K. The phase coexistence is the result of the intermediate ionic size of samarium coupled to exchange striction at the vanadium spin ordering.

Glassy spin dynamics in non-fermi-liquid UCu5-xPdx, x = 1.0 and 1.5.

Local f-electron spin dynamics in the non-Fermi-liquid heavy-fermion alloys UCu5-xPdx, x = 1.0 and 1.5, have been studied using muon spin-lattice relaxation. The sample-averaged asymmetry function G(t) indicates strongly inhomogeneous spin fluctuations and exhibits the scaling G(t,H) = G(t/H(gamma)) expected from glassy dynamics. At 0.05 K gamma(x = 1.0) = 0.35+/-0.1, but gamma(x = 1.5) = 0.7+/-0.1. This is in contrast to inelastic neutron scattering results, which yield gamma = 0.33 for both concentrations. There is no sign of static magnetism approximately greater than 10(-3)(B)/U ion in either material above 0.05 K. Our results strongly suggest that both alloys are quantum spin glasses.

Observation of two time scales in the ferromagnetic manganite La1-xCaxMnO3, x approximately 0.3.

We report new zero-field muon spin relaxation and neutron spin echo measurements in ferromagnetic (FM) (La,Ca)MnO3 which suggest at least two spatially separated regions possessing very different Mn-ion spin dynamics. One region displays diffusive relaxation, "critical slowing down" near T(C) and an increasing volume fraction below T(C), suggesting overdamped FM spin waves below T(C). The second region possesses more slowly fluctuating spins, a linewidth independent of q, and a decreasing volume fraction below T(C). The estimated length scale for the inhomogeneity is

Dose uniformity of ferromagnetic seed implants in tissue with discrete vasculature: a numerical study on the impact of seed characteristics and implantation techniques.

The results from simulations with a new three-dimensional treatment planning system for interstitial hyperthermia with ferromagnetic seeds are presented in this study. The thermal model incorporates discrete vessel structures as well as a heat sink and enhanced thermal conductivity. Both the discrete vessels and the ferroseeds are described parametrically in separate calculation spaces. This parametric description has the advantage of an arbitrary orientation of the structures within the tissue grid, easy manipulation of the structures and independence from the resolution of the tissue voxels (tissue calculation space). The power absorption of the self-regulating seeds is according to empirical data. The thermal effects of an unlimited number of thin layers surrounding the seed (coatings, catheters) can be modelled. The initial calculations have been performed for an array of 12 identical ferromagnetic seeds in a tissue volume with a computer generated artificial vessel network spanning four vessel generations in both the arterial and venous tree. The heterogeneously distributed large isolated vessels impair the temperature distribution significantly, indicating the limited accuracy of continuum models. Simulations with different types of ferromagnetic seeds have confirmed that the efforts of previous studies to optimize the self-regulating temperature control and the implantation techniques of the ferroseeds will improve the homogeneity of the temperature distribution in the target volume. Multifilament seeds implanted in brachytherapy needles and tubular seeds appear to be the most favourable configurations. The division of long seeds into shorter segments with the appropriate Curie temperature will further improve the homogeneity of the temperature distribution without increasing the average temperature in the volume of interest. Given the proper thermal tissue data, the model presented in this study will prove to be a useful tool in making choices for the implant geometry, seed spacing and Curie temperature.

Three-dimensional temperature control of palladium-nickel thermoseeds: a computer aided and experimental evaluation.

The capability of self-regulating thermoseeds to compensate for nonuniform cooling along their longitudinal axis has been investigated in this study. For this purpose a quasi three-dimensional computer model has been developed. Calculations of the temperature profile in tissue with nonuniform heat loss demonstrated a clear improvement in the longitudinal temperature control of PdNi seeds compared to constant power seeds. Further, two strategies for improved control of nonuniform cooling along the longitudinal axis of ferromagnetic seeds have been investigated: (1) application of a 'normal' undivided seed; and (2) division of a long seed in smaller segments of which each segment is able to respond more directly to local variations in the temperature distribution. Calculations with the quasi three-dimensional model showed that the loose segments do respond more directly to their close proximity. However, the equilibrium temperature of a segment in an area with high local blood flow will be relatively low due the limited heat production of PdNi thermoseeds. In the undivided seeds the high thermal conductivity of PdNi causes some levelling of the longitudinal temperature gradient in the seed. In addition, calorimetric experiments have shown that the heat production of a segmented seed is less effective because of a demagnetizing field. Also, the absence of PdNi between the segments reduces the heat production of the seed.

The effect of catheters and coatings on the performance of palladium-nickel thermoseeds: evaluation and design of implantation techniques.

In the development of materials for self-regulating thermoseeds much effort is put in improvement of the self-regulating temperature control mechanism of the seeds. The catheters and coatings which are needed to implant the seeds or to guarantee biocompatibility, generally impair the optimized performance of the ferromagnetic seeds. The influence of various coatings on the performance of PdNi seeds has been investigated by means of one-dimensional modelling and calorimetric experiments. Implantation using thin walled catheters is acceptable provided that the catheters are filled with water to assure good thermal coupling. Air layers inside catheters should be avoided as they reduce the sharp gradient of the heat production at the Curie temperature significantly. An alternative for the application of catheters is to insert the seeds into metallic needles. The effect of shielding by the metal needle can be minimized by driving the seed into its saturated state using a high magnetic field strength. The thermal interaction between the seed and surrounding tissue can also be enhanced by placing PdNi, e.g. tubular, on the outside of the catheter or brachytherapy needle. An additional advantage of this new design is an increase in the heat production and the quality of temperature control due to an increase in the amount of PdNi. For permanent implantation seeds can be coated with an inert metal, ceramics or plastic. The performance of the seeds is not affected by any of the coatings if certain conditions are met. For plastic coatings the thickness of the coating has to be very thin, preferably < or = 20 microns, to avoid thermal isolation.

Power absorption and temperature control of multi-filament palladium-nickel thermoseeds for interstitial hyperthermia.

In interstitial hyperthermia using ferromagnetic seeds, multi-filament seeds have gained interest because of a more effective power absorption than solid seeds. Palladium-nickel (PdNi) seeds composed of filaments with diameters in the range from 0.1 to 1.0 mm (maximally 90 filaments) have been investigated to find the conditions for optimal power absorption and temperature control. Magnetic and calorimetric experiments have shown that a decreasing filament radius results in a more effective power absorption. The power absorption approaches a common asymptote for high field intensities at all filament diameters. This asymptotic behaviour can be understood as a consequence of the approach of saturation magnetization of PdNi. The sharpness of the transition at the Curie temperature, which is a measure for the quality of temperature control, improves as the magnetic field strength increases, but it is limited by the asymptote of the power absorption. When the asymptote has been reached the quality of temperature regulation of a seed can only be improved by increasing the amount of PdNi, e.g. by increasing the number of filaments. Calculations of the power absorption, using the generally applied theory based on a linear relation between the magnetization of PdNi and the magnetic field strength, do not correspond quantitatively with experimental results for seeds having an induction number smaller than the 'optimal value' of 2.5. For these seeds the measured heat production is larger than the calculated one.

The development of PdNi thermoseeds for interstitial hyperthermia.

Magnetic induction heating of thermoseed implants can be used to produce highly localized hyperthermia in deep-seated tumors. Automatic temperature control throughout the tumor can be achieved by the self-regulating character of ferromagnetic seeds, which corrects for local variations in heat loss due to blood perfusion. An increased sharpness of the ferromagnetic transition at the Curie temperature, Tc, improves the performance of self-regulating control. This was realized for palladium-nickel alloys by a "cold working" procedure preceded and followed by annealing. Palladium-nickel seeds with a predetermined Tc were produced, showing a sharp decrease at Tc of the magnetic susceptibility and the heat production.