Separating spin and charge transport in single wall carbon nanotubes

We demonstrate spin injection and detection in single wall carbon nanotubes using a 4-terminal, non-local geometry. This measurement geometry completely separates the charge and spin circuits. Hence all spurious magnetoresistance effects are eliminated and the measured signal is due to spin accumulation only. Combining our results with a theoretical model, we deduce a spin polarization at the contacts of approximately 25 %. We show that the magnetoresistance changes measured in the conventional two-terminal geometry are dominated by effects not related to spin accumulation.Comment: Number of pages: 11 Number of figures:

The Magneto-coulomb effect in spin valve devices

We discuss the influence of the magneto-coulomb effect (MCE) on the magnetoconductance of spin valve devices. We show that MCE can induce magnetoconductances of several per cents or more, dependent on the strength of the coulomb blockade. Furthermore, the MCE-induced magnetoconductance changes sign as a function of gate voltage. We emphasize the importance of separating conductance changes induced by MCE from those due to spin accumulation in spin valve devices.Comment: This paper includes 3 figure

Bistable hysteresis and resistance switching in hydrogen gold junctions

Current-voltage characteristics of H2-Au molecular junctions exhibit intriguing steps around a characteristic voltage of 40 mV. Surprisingly, we find that a hysteresis is connected to these steps with a typical time scale > 10 ms. This time constant scales linearly with the power dissipated in the junction beyond an ofset power P_s = IV_s. We propose that the hysteresis is related to vibrational heating of both the molecule in the junction and a set of surrounding hydrogen molecules. Remarkably, we can engineer our junctions such that the hysteresis' characteristic time becomes >days. We demonstrate that reliable switchable devices can be built from such junctions.Comment: Submitted to Phys. Rev. Let

The role of Joule heating in the formation of nanogaps by electromigration

We investigate the formation of nanogaps in gold wires due to electromigration. We show that the breaking process will not start until a local temperature of typically 400 K is reached by Joule heating. This value is rather independent of the temperature of the sample environment (4.2-295 K). Furthermore, we demonstrate that the breaking dynamics can be controlled by minimizing the total series resistance of the system. In this way, the local temperature rise just before break down is limited and melting effects are prevented. Hence, electrodes with gaps < 2 nm are easily made, without the need of active feedback. For optimized samples, we observe quantized conductance steps prior the gap formation.Comment: including 7 figure

Effects of Oestrogens and FSH on LH Stimulation of Steroid Production by Testis Leydig Cells from Immature Rats

Hypophysectomy of immature male rats results after 5 days in a decreased production of testosterone by isolated testis Leydig cells in response to LH. The LH responsiveness of the Leydig cells can be partly restored by treatment of the hypophysectomized rats with FSH. In continuation of previous reports on this subject (Steroids 28 (1976) 847; and 30 (1978) the following conclusions were derived from the results in the present paper: 1. After hypophysectomy of immature male rats the production of testosterone (T) as well as of 5‐pregnenolone (Δ5P) by isolated Leydig cells in response to LH is reduced. 2. Daily administration of FSH after hypophysectomy restores the Δ5P production in response to LH almost completely, but has a much smaller effect on the restoration of T production. 3. Administration of oestradiol benzoate (E2B) together with FSH has no effect on the restoration of LH‐stimulated Δ5P production, but causes a reduction of T production, when compared with Leydig cells from animals treated with FSH only. 4. Treatment of intact immature rats with E2B results in a decreased production of T and an increased production of Δ5P in isolated Leydig cells. 5. From experiments with labelled pregnenolone it appears that E2B and diethylstilboestrol (DES) inhibit the 17α‐hydroxylase activity of Leydig cells from intact as well as from hypophysectomized rats. This results in a reduced conversion of pregnenolone to C1:)‐steroids and in increased production of 3α‐hydroxy‐5α‐pregnan‐20‐one from δ5P. 6. The observed effects of FSH and E, were similar within a dose range of 100–10000 ng LH per 106 Leydig cells. Copyrigh

The mechanical response of lithographically defined break junctions

We present an experimental study on the mechanical response of lithographically defined break junctions by measuring atomic chain formation, tunneling traces and Gundlach oscillations. The calibration factor, i.e., the ratio between the electrode movement and the bending of the substrate, is found to be 2.5 times larger than expected from a simple mechanical model. This result is consistent with previous finite-element calculations. Comparing different samples, the mechanical response is found to be similar for electrode separations >4 angstrom. However, for smaller electrode separations significant sample-to-sample variations appear. These variations are ascribed to differences in the shape of the two electrodes on the atomic scale which cannot be controlled by the fabrication process