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University of Twente Publications (54,724 recursos)
University of Twente Repository provides publications of UT scientific staff, in many cases full text and publicly available.

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Mostrando recursos 1 - 20 de 23,957

1. Continuous wave near‐infrared atomic Xe laser excited by a radio frequency discharge in a slab geometry - Tskhai, S.N.; Udalov, Yu.B.; Peters, P.J.M.; Witteman, W.J.; Ochkin, V.N.
Near‐infrared atomic Xe laser lines have been generated from an Ar:He:Xe laser gas mixture excited by a radio frequency (rf) discharge in a slab geometry. A maximum continuous wave (cw) output power of 1.5 W (270 W/l) was obtained at an rf frequency of 125 MHz from a gas mixture containing Ar:He:Xe (50:49:1) at a total gas pressure of 90 Torr.

2. Noise properties of direct current SQUIDs with quasiplanar YBa2Cu3O7 Josephson junctions - Faley, M.I.; Poppe, U.; Urban, K.; Hilgenkamp, H.; Hemmes, H.; Aarnink, W.; Flokstra, J.; Rogalla, H.
We describe the noise performance of dc SQUIDs fabricated with quasiplanar ramp‐type Josephson junctions on the basis of c‐axis‐oriented YBa2Cu3O7/PrBa2Cu3O7 thin‐film heterostructures. The noise spectrum of the dc SQUIDs was measured with dc‐ and ac‐bias schemes at different temperatures and showed values below 10−5 Φ0/Hz1/2 down to frequencies of about 1 Hz at 70 K. Up to now for the magnetic fluxnoise and the energy resolution obtained at 1 kHz and 77 K the best values were 2.5×10−6, Φ0/Hz1/2 and 3×10−31 J/Hz, respectively. A study of the white and 1/fnoises of the SQUIDs was performed. The influence of magnetic flux,...

3. Subharmonic Shapiro steps in high‐Tc Josephson junctions - Terpstra, D.; IJsselsteijn, R.P.J.; Rogalla, H.
We studied the response of high‐Tc biepitaxial grain boundaryjunctions to 100 GHz radiation in the presence of a magnetic field. Integer as well as subharmonic constant voltage steps are observed, even at one‐fifth of the voltage separation between integer (Shapiro) steps. Our results indicate that this behavior is due to the synchronized motion of Josephsonvortices along the junction. We show that this effect is directly related to the width of the junction, and not to an array of weaker regions in the barrier.

4. Distribution of the critical current density and flux trapping in YBa2Cu3O7−δ ramp‐edge Josephson junctions - Marx, A.; Husemann, K.-D.; Mayer, B.; Nissel, T.; Gross, R.; Verhoeven, M.A.J.; Gerritsma, G.J.
We have studied the spatial distribution of the critical current density in YBa2Cu3O7−δ ramp edge Josephson junctions using low‐temperature scanning electron microscopy. Applying this technique allows the imaging of the critical current density distribution with a spatial resolution of about 1 μm. Our measurements show that the geometry of the ramp‐edge junction eases the trapping of magnetic flux quanta in the YBa2Cu3O7−δ layer covering the ramp edge. These trapped flux quanta result in a spatially inhomogeneousmagnetic field parallel to the barrier layer, which in turn results in a spatially modulated supercurrent density and an unusual magnetic field dependence of the...

5. Enhanced gain and output power of a sealed‐off rf‐excited CO2 waveguide laser with gold‐plated electrodes - Heeman-Ilieva, M.B.; Udalov, Yu.B.; Hoen, K.; Witteman, W.J.
The small‐signal gain and the laser output power have been measured in a cw sealed‐off rf‐excited CO2waveguide laser for two different electrodematerials, gold‐plated copper and aluminum, at several excitation frequencies, gas pressures and mixture compositions. In the case of the gold‐plated electrodes an enhancement of the gain up to a factor of 2 and the output power up to a factor of 1.4 with time at a frequency of 190 MHz and 60 Torr of 1:1:5+5% (CO2:N2:He+Xe) mixture is observed. This is believed to be the result of the goldcatalytic activities which are favored by increased electrode temperatures and helium...

6. Formation and quenching mechanisms of the electron beam pumped (XeRb)+ ionic excimer in different buffer gases - Lankhorst, F.T.J.L.; Bastiaens, H.M.J.; Peters, P.J.M.; Witteman, W.J.
The ionic excimer molecule XeRb+ is formed in an electron beam excited gas mixture of Xe, Rb, and a buffer gas. The formation and quenching mechanisms of ionic excimers are investigated by measuring the XeRb+fluorescence as a function of the gas composition and gas pressure. The formation of XeRb+ is achieved by a three‐body association reaction between Xe+, Rb, and a buffer gas atom. For the buffer gases He, Ne, or Ar the values of the important formationrate constants are determined from the observed fluorescence signal decay.

7. Four layer monolithic integrated high Tc dc SQUID magnetometer - Hilgenkamp, J.W.M.; Brons, G.C.S.; Soldevilla, J.G.; IJsselsteijn, R.P.J.; Flokstra, J.; Rogalla, H.
YBa2Cu3O7−x based monolithic integrated dc SQUID magnetometers, consisting of a dc SQUID integrated with a flux transformer on a single bicrystalline substrate, have been fabricated and characterized. The devices consist of four layers, including two superconducting layers, and first realizations operate up to 73 K. A maximum voltage modulation of 32 μV is observed at 40 K. A field sensitivity of 0.17 pT/√Hz is obtained above 200 Hz at 45 K and 0.49 pT/√Hz at 1 Hz and 65 K.

8. High sensitivity double relaxation oscillation superconducting quantum interference devices - Adelerhof, Derk Jan; Kawai, Jun; Uehara, Gen; Kado, Hisashi
Double relaxation oscillationsuperconducting quantum interference devices(SQUIDs) (DROSs) have been fabricated with estimated relaxation frequencies up to 14 GHz. Both the intrinsic flux noise and the performance in a flux locked loop with direct voltage readout have been studied. In flux locked loop, a noise level of 0.55 μφ0/√Hz corresponding to an energy sensitivity of 34 h has been obtained for a DROS with a SQUIDinductance of 29 pH. The intrinsic sensitivity improves with increasing relaxation frequency, leveling off to a value of 13 h at relaxation frequencies higher than about 3 GHz.

9. Noise study of a high‐Tc Josephson junction under near‐millimeter‐wave irradiation - Gupta, R.; Hu, Qing; Terpstra, D.; Gerritsma, G.J.; Rogalla, H.
Noise studies of both the dc and ac Josephson effects have been performed on a high‐Tc ramp‐type Josephson junction irradiated at 176 GHz. Well‐established analytical results for noise in overdamped RSJs are used to model the measured I‐V characteristics, and their agreement is excellent. Noise‐rounded I‐V curves at the critical current and the first and second Shapiro steps under coherent 176 GHz radiation have been studied in detail at several temperatures and rf power levels. The noise temperatures inferred from these simulations are close to the physical temperatures. An increase of noise temperatures at high radiation power levels is a...

10. Phase‐matched second‐harmonic generation by periodic poling of fused silica - Kashyap, Raman; Veldhuis, Gerrit J.; Rogers, David C.; Mckee, F.
90° phase matching in periodically poled fused silica is achieved over approximately 2 mm at a wavelength of 1064 nm. Electrical poling was done at 5 kV for 120 min using photolithographed periodic electrodes on fused silica glass at 250 °C. A nonlinearity equivalent to d 11/200 of quartz is estimated from measurements. Several phase‐matched interactions are reported and the implications for device applications discussed.

11. Anisotropic excitation spectra of GaAs/AlGaAs quantum wells grown on vicinal plane substrates - Wentink, D.J.; Dawson, P.; Foxon, G.T.
We report measurements of the photoluminescence excitation spectra of a series of GaAs/AlGaAs quantum well samples grown on vicinal plane substrates with differing off‐cut angles. When the plane of polarization of the exciting light is changed we have observed a clear variation in the ratio of the strength of the n=1 light and heavy hole exciton transitions in samples grown on vicinal plane substrates. This behavior is attributed to anisotropic scattering at steps in the heterointerface.

12. High specific laser output energy at 157 nm from an electron beam pumped He/Ne/F2 gas mixture - Lankhorst, F.T.J.L.; Bastiaens, H.M.J.; Peters, P.J.M.; Witteman, W.J.
The output energy and the temporal behavior of a molecular F*2 laser pumped by a coaxial electron beam have been measured in gas mixtures of He/F2 and He/Ne/F2. The highest output energy of 172 mJ has been obtained in a mixture of He/Ne/F2 (19.9%/80%/0.1%) at a pressure of 12 bar, corresponding to a specific output energy of 10.8 J/l and an intrinsic efficiency of 2.6%.

13. Near‐millimeter‐wave response of high Tc ramp‐type Josephson junctions - Gupta, R.; Hu, Qing; Terpstra, D.; Gerritsma, G.J.; Rogalla, H.
We have studied the response of a YBCO/PBCO/YBCO ramp‐type junction to coherent radiation at 176 and 270 GHz. The I‐V characteristic of the junction closely resembles the prediction of the RSJ model. The I cR n product of the junction is 0.25 mV at 5 K. The millimeter‐wave radiation is coupled to the junction via a quasioptical structure that focuses the radiation onto the junction through a yttrium‐stabilized ZrO2 substrate. At 176 GHz, we have observed as many as six Shapiro steps at the maximum power level of our Gunn oscillator‐pumped frequency doubler. Shapiro steps are still clearly seen up...

14. New continuous wave infrared Ar‐Xe laser at intermediate gas pressures pumped by a transverse radio frequency discharge - Udalov, Y.B.; Peters, P.J.M.; Heeman-Ilieva, M.B.; Ernst, F.H.J.; Ochkin, V.N.; Witteman, W.J.
An atomic Xe laser with a transverse rf excitation has been operated in a cw mode in the intermediate pressure regime. The laser output spectrum consisted of 5 Xe lines with wavelengths of 2.03, 2.63, 2.65, 3.37, and 3.51 μm. The unoptimized total output power of 330 mW was obtained for a gas mixture Ar:He:Xe=59:40:1 at a pressure of 85 Torr and a rf input power of 150 W and excitation frequency of 121 MHz.

15. Small‐signal gain measurements in an electron beam pumped F2 laser - Bastiaens, H.M.J.; Dam, B.M.C. van; Peters, P.J.M.; Witteman, W.J.
The net‐small‐signal gain of a molecular fluorine F2 * laser pumped by a coaxial electron beam has been measured in gas mixtures of He/F2 and He/Ne/F2. A peak net‐small‐signal gain of 0.63 cm−1 has been measured in a mixture of He/Ne/F2 at a pressure of 8 bar and a pumping power density of 13 MW/cm3.

16. Vacuum ultraviolet fluorescence of (XeRb)+ produced in an electron‐beam‐pumped gas mixture - Bastiaens, H.M.J.; Lankhorst, F.T.J.L.; Peters, P.J.M.; Witteman, W.J.
With a pulsed electron beam a gas mixture of Ar, Xe, and Rb was excited producing (XeRb)+ ionic excimer molecules. To study the formation kinetics the (XeRb)+fluorescence pulse was measured as a function of the gas composition and the pumping density. From the observed fluorescence signal decay a value of 6±1×10−30 cm6/s for the formation rate constant of (XeRb)+ from Xe+, Ar, and Rb was determined.

17. Van kaal naar integral : het opstellen van een integraal kostenafwegingsmodel voor varianten van kunstwerken in de studiefase - Cremer, Frits; Wind, H.G.; Crijns, R.J.H.A.; Reij, A.W.F.; Schouwstra, L.P.

18. The effects of water on the morphology and the swelling behavior of sulfonated poly(ether ether ketone) films - Koziara, B.T.; Akkilic, N.; Nijmeijer, K.; Benes, N.E.
Thin sulfonated poly(ether ether ketone) films swell excessively in water. The extent of water-induced swelling is shown to be correlated with the optical anisotropy of the films, due to two distinct phenomena. Firstly, the optical anisotropy is directly related to the amount of water taken up from the surrounding ambient atmosphere, and thus to amount of water present in the material just prior to swelling. Secondly, the optical anisotropy corresponds to internal stresses in the film that affect the free energy of the film, and thus the potential of the film to swell. The anisotropy vanishes upon sorption of liquid...

19. Acyl Ghrelin Improves Synapse Recovery in an In Vitro Model of Postanoxic Encephalopathy - Stoyanova, I.I.; Hofmeijer, J.; Putten, M.J.A.M. van; Feber, J. le
Comatose patients after cardiac arrest have a poor prognosis. Approximately half never awakes as a result of severe diffuse postanoxic encephalopathy. Several neuroprotective agents have been tested, however without significant effect. In the present study, we used cultured neuronal networks as a model system to study the general synaptic damage caused by temporary severe hypoxia and the possibility to restrict it by ghrelin treatment. Briefly, we applied hypoxia (pO2 lowered from 150 to 20 mmHg) during 6 h in 55 cultures. Three hours after restoration of normoxia, half of the cultures were treated with ghrelin for 24 h, while the...

20. Avoiding preamorphization damage in MeV heavy ion‐implanted silicon - Schreutelkamp, R.J.; Custer, J.S.; Liefting, J.R.; Saris, F.W.
Implantation of 1.0 MeV 115In in Si results in secondary‐defect formation during subsequent 900 °C annealing if the total number of displaced Si atoms is greater than 1.6×1017/cm2, achieved with a dose near 1.5×1013/cm2. We demonstrate, though, that higher total In doses can be introduced without forming secondary defects by repetitive subthreshold implants each followed by an anneal to remove the implant damage. While a single 6×1013 In/cm2implant results in a high density of dislocation loops after annealing, instead using four separate 1.5×1013 In/cm2 implants each followed by an anneal leads to the formation of only a few partial dislocations.

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