H-T phase diagram and the nature of Vortex-glass phase in a quasi two-dimensional superconductor: Sn metal layer sandwiched between graphene sheets

The magnetic properties of a quasi two-dimensional superconductor, Sn-metal graphite (MG), are studied using DC and AC magnetic susceptibility. Sn-MG has a unique layered structure where Sn metal layer is sandwiched between adjacent graphene sheets. This compound undergoes a superconducting transition at $T_{c}$ = 3.75 K at $H$ = 0. The $H$-$T$ diagram of Sn-MG is similar to that of a quasi two-dimensional superconductors. The phase boundaries of vortex liquid, vortex glass, and vortex lattice phase merge into a multicritical point located at $T^{*}$ = 3.4 K and $H^{*}$ = 40 Oe. There are two irreversibility lines denoted by $H_{gl}$ (de Almeida-Thouless type) and $H_{gl^{\prime}}$ (Gabay-Toulouse type), intersecting at $T_{0}^{\prime}$= 2.5 K and $H_{0}^{\prime}$ = 160 Oe. The nature of slow dynamic and nonlinearity of the vortex glass phase is studied.Comment: 24 pages, 13 figures; Physica C (2003), in pres

An investigation of the electronic and magnetic properties taking account of the film morphology

Titel Inhaltsverzeichnis 5 1. Einleitung 15 2. Experimentelles 19 2.1. Probenpräparation und -charakterisierung 19 2.2. Versuchsaufbau 27 2.2.1. Apparatur zur DC-Leitfähigkeitsmessung 27 2.2.2. Apparatur zur AC-Suszeptibilitätsmessung und AC-Leitfähigkeitsmessung 29 3. Theoretische Grundlagen 33 3.1. Leitfähigkeit 33 3.1.1. Leitfähigkeit in einem Metall 33 3.1.2. Leitfähigkeit in einem Halbleiter 35 3.2. Magnetowiderstand 39 3.3. Suszeptibilität 41 3.4. Supraleitung 42 3.5. Supraleitung in dünnen Schichten 51 3.5.1. Größeneffekt 51 3.5.2. Granularität 54 3.5.3. Kopplungseffekt 55 3.5.4. Andreev-Reflexion 58 3.5.5. Erhöhung des kritischen Magnetfeldes 59 3.5.6. Winkelabhängigkeit des kritischen Magnetfeldes 61 4. Ergebnisse 65 4.1. Leitfähigkeitsuntersuchungen 65 4.1.1. kritische Temperatur Tc 68 4.1.2. kritisches Magnetfeld Bc 70 4.1.3. Magnetowiderstand 72 4.1.4. Strom-Spannungs-Kennlinien 79 4.2. Suszeptibilitätsmessungen 80 5. Diskussion 83 5.1. Temperaturabhängigkeit der Leitfähigkeit 83 5.2. Magnetowiderstand 85 5.3. Übergangstemperatur Tc zur Supraleitung 86 5.4. kritisches Magnetfeld Bc 93 5.5. Winkelabhängigkeit kritischen Magnetfeld Bc 95 5.6. Strom-Spannungs-Kennlinien 98 5.7. Probenstabilität 101 6. Zusammenfassung 103 Literatuverzeichnis 113 Danksagung 115 Publikationsliste 117 Lebenslauf 119Aus LEED-, AES- und Ramanspektroskopie-Untersuchungen an den Zinn-Schichten auf InSb direkt nach der Präparation konnte festgestellt werden, daß die Schichten nicht, wie aus der Literatur erwartet, nur aus dem halbmetallischen a-Zinn bestehen, sondern aus einer Mischung von halbmetallischem a\- und metallischem supraleitenden b-Zinn. AFM-Untersuchungen an den Schichten zeigten ein deutliches Insel-Wachstum mit kleiner werdenen Inseln bei abnehmender Schichtdicke. Durch diese Struktur- und Morphologieuntersuchungen konnte bestimmt werden, daß die Schichten aus b -Zinn-Inseln eingebettet in einer a-Zinn-Matrix bestehen, so daß eine zweidimensionale Schicht mit einem Netzwerk statistisch verteilter Normalleiter-Supraleiter-Normalleiter-Kontakte existiert. Die gefundene Probenmorphologie hat einen großen Einfluß auf die supraleitenden Eigenschaften der Zinn-Schichten, wie es sich aus Widerstands- und Suszeptibilitätsmessungen ergab. Anhand der Widerstandsmessungen konnte gezeigt werden, daß die supraleitenden kritischen Temperaturen bei allen hier untersuchten Zinn-Schichten deutlich unterhalb der Übergangstemperatur des Volumenmaterials (Tc=3,7 K) liegen. Das Verhalten des kleiner werdenden Tc mit abnehmender Schichtdicke kann durch das Auftreten des "Proximity"- und des "Size"-Effekts erklärt werden. Das kritische Magnetfeld Bc der Schichten zeigte ebenfalls ein abweichendes Verhalten gegenüber dem des Volumenmaterials . Das kritische Magnetfeld liegt bei allen Schichten oberhalb des kritischen Magnetfeldes des Volumenmaterials (Bc=30mT) und wurde mit abnehmender Schichtdicke größer. Diese Erhöhung des kritischen Magnetfeldes entspricht der Erwartung der London-Theorie, die diese Erhöhung mit dem geringer werdenden Feldprofil im Inneren einer supraleitenden Schicht bei abnehmender Dicke erklärt. Bei senkrecht zum Magnetfeld orientierten Schichten wurde ebenfalls eine Erhöhung des kritischen Magnetfeldes gefunden. Dies kann durch die Ausbildung eines supraleitenden Mischzustandes in der Schicht senkrecht zum Magnetfeld erklärt werden. Messungen der Winkelabhängigkeit des kritischen Magnetfeldes unterstützen dies. Durch die oberflächenphysikalischen Untersuchungen (LEED, AES, AFM, Raman) an den Zinn-Schichten und die Bestimmung der supraleitenden Eigenschaften an denselben Zinn-Schichten, konnte erstmals die Abhängigkeit der Supraleitung in zweidimensionalen Schichten von der Morphologie der Schicht direkt bestimmt werden. Da zum einen aus der Untersuchung der kritischen Magnetfelder der Schichten in parallelen und senkrechten Orientierungen zum Magnetfeld klar geht hervor, daß es sich bei den untersuchten Zinn-Schichten um ein quasizweidimensionale Systeme handelt. Zum anderen zeigen die Messungen der Temperaturabhängigkeit des Widerstands, daß eine globale Supraleitung in den granularen Schichten auftritt. Diese globale Supraleitung manifestiert sich in einem scharfen supraleitenden Übergang im Widerstand und einem Widerstandsabfall auf Null und wird unterstützt durch das Verhalten der Strom- Spannungs-Kennlinien, die keine Hinweise auf lokale Supraleitung zeigten. Insgesamt läßt sich daraus schließen, daß in den zweidimensionalen Schichten eine Kopplung der b-Zinn-Inseln in der a-Zinn-Matrix vorliegt.From the LEED, AES and Raman spectroscopy it is known that the films consist of a-islands in an b-matrix differently from the outcome in the literature. The spectroscopy investigations were taken in the ultrahigh vacuum chamber. The films are grown in islands and the island became smaller with decreasing film thickness which is observed in AFM investigations. From these structural and morphology studies it is well known that the films are two dimensional and it exists a random network of normal-superconductor-normal-contacts. The resistance and susceptibility measurements have shown that the sample morphology had a strong influence on superconducting properties of the tin films. The superconducting critical temperatures of the tin films with different film thickness are clearly smaller as in the bulk material (Tc=3.7 K) this was found in the resistance measurements. The decreasing superconducting critical temperature with decreasing film thickness could be explained with the proximity and size effect. The critical magnetic field Bc of the films had another behavior as in the bulk material too. It was always higher than in the bulk (Bc=30mT) and the critical magnetic field increase with decreasing film thickness. The Explanation of this behavior came from the London theory. The magnetic field variation gets smaller with decreasing thickness in a film with a magnetic field parallel to the film plane. The critical magnetic fields were also higher when the film plane parallel to the external magnetic field. The superconducting tin films are composing a Shubnikov phase in this orientation which was supported from the angular dependence of the critical magnetic field. The dependence on the superconductivity in two dimensional films from the film morphology could be shown directly for the first time because we were taking the surface investigation (LEED, AES, AFM, Raman), resistance and susceptibility measurements on the same sample. On the one hand it is known from the angular dependence of Bc the films are two dimensional, on the other hand the temperature dependence of the resistance is showing the global superconductivity in the granular films. The global superconductivity was observed in the resistance by a sharp drop and falling to zero when the tin film became superconducting. There was also no indication of local superconductivity in the current-voltage-curves. In conclusion, it could say that the superconducting behavior of these tin films is deciding the coupling between the a-islands and the b-matrix in the two-dimensional film

Thermal conductance measurements of bolted copper to copper joints at sub-Kelvin temperatures

We have measured the thermal contact conductance of several demountable copper joints below 1 K. Joints were made by bolting together either two flat surfaces or a clamp around a rod. Surfaces were gold plated, and no intermediate materials were used. A linear dependence on temperature was seen. Most of the measured conductance values fell into a narrow range: 0.1–0.2 W K−1 at 1 K. Results in the literature for similar joints consist of predictions based on electrical resistance measurements using the Wiedemann–Franz law. There is little evidence of the validity of this law in the case of joints. Nevertheless, our results are in good agreement with the literature predictions, suggesting that such predictions are a reasonable approximation

In-flight calibration sources for Herschel-SPIRE

SPIRE, the Spectral and Photometric Imaging Receiver, will be a bolometer instrument for ESA's Herschel satellite. The instrument comprises a three-band imaging photometer covering the 250-500 μm range, and an imaging Fourier Transform Spectrometer (FTS) covering 200-670 μm. This paper presents the requirements for and design of the photometer and spectrometer calibration/illumination sources, and the results of laboratory tests on prototypes. The photometer calibrator is an electrically heated thermal source of submillimetre radiation, the purpose of which is to provide a repeatable signal for in-flight monitoring of health and responsivity of the SPIRE photometer detectors. It is not required to provide absolute calibration or uniform illumination of the arrays, but it may be used as part of the overall calibration scheme. The spectrometer calibrator is located at a pupil at the second input port of the FTS. It is designed to enable matching of the telescope emission for a range of telescope temperature (60-90 K) and emissivity (2% - 10%). By matching the telescope emission at this port, the high background from the Herschel telescope emission can be nulled to a high degree, resulting in an interferogram in which the contribution from the astronomical source is not overwhelmed by the telescope offset. The flexibility for telescope matching inherent in the design is important, as the exact telescope parameters will be unknown until the satellite is in operation. The FTS calibrator will also be used to assist in the absolute calibration scheme for SPIRE FTS observations

Electron beam crystallised thin film silicon solar cell on floatglass

Sufficient optical thickness and a minimum of recombination losses are the requirements for efficient silicon thin film solar cells with efficiencies above 15 . A capable solution would be a 5 10 m thick polycrystalline Si absorber [1] grown on a low cost substrate and completed to a solar cell by an amorphous a Si H heteroemitter [2]. This article describes a polycrystalline thin film solar cell with grain sizes of 5 10 m at a layer thickness of 5 7 m prepared on an easily available low cost float glass substrate. The grain growth is achieved by zone melting crystallisation ZMC of amorphous silicon with a line shaped electron beam [3]. An intermediate layer is necessary to protect the glass softening point 700 C from the melt zone and to prevent the silicon from being contaminated by elements from the glass. Presently, a combination of silicon carbide SiC and silicon oxide SixOy is used as diffusion barrier and structural fitment. The silicon carbide is deposited by an RF magnetron sputtering process at room temperature with a typical layer thickness of 200 nm, silicon oxide by a PECVD process with a thickness of 300nm. For the purpose of absorber doping by diffusion during the crystallization, a thin layer of spin on glass, containing either phosphorus or boron was applied on top of the SiC SiO sandwich before the absorber layer was deposited. Three alternative technologies have been used for absorber deposition PECVD at 600 C, LPCVD at 600 C and high rate vacuum e beam evaporation with growth rates up to 2 m min with typical absorber thicknesses from 5 to 15 m. The succeeding crystallisation is carried out after preheating the substrate to a temperature near 600 C. Scan velocities are varying from 5 up to 30 mm s through the focus of the line shaped electron source. Typical energy densities are around 450 mJ mm2. The silicon morphology before crystallization does not affect the crystallisation process. After crystallization the poly Si layers exhibit crystalline grain sizes comparable to the layer thickness, i. e. up to 10 m as shown by Scanning Electron Microscopy SEM investigations figure 1 . The surface morphology is smooth shingle type modulated due to the grains. XRD measurements indicate lt;111 gt;, lt;110 gt; and lt;311 gt; crystal orientations figure 2 . Raman spectroscopy analysis indicates that the structure of the poly Si films is crystalline over the whole area figure 3 ; no amorphous phase was detected. Furthermore, the line shapes are symmetrical no Fano effect . Therefore, a defect concentration below 1019cm 3 can be expected. These results are supported by photoluminescence spectra which display a detectable maximum at about 1150nm figure 4 . This indicates that the fast non radiative recombination does not completly annihilate the excess charge carriers. The minority carrier lifetime was directly investigated by photoconductance decay. These first measurements are resulting an effective carrier lifetime of about 40 s figure 5 . This result can be improved for example by passivating the grain boundaries with hydrogen. The surface photovoltage SPV of the poly Si absorber and its decay was measured. Surface band bending modulation via field dependent SPV was analysed. Such an modulation is an indispensable precondition for the formation of a built in voltage. Finally, heterojunction solar cells were prepared by PECVD deposition of 10 30nm thin a Si H n,p emitter on top of the crystallised c Si p,n absorber. The solar cell was then characterized by taking current voltage curves under AM1.5 conditions, measuring internal quantum efficiency and open circuit voltages by Suns Voc. [1] R. B. Bergmann. J.H. Werner, Thin Solid Films 403 404, p. 162 2002 [2] J. Poortmans et. al., Proceedings IEEE 4th WCPEC, p.1449 2006 [3] F. Gromball et. al., Rev. Sci. Instrum. 76, p. 063901 200

Ultrathin SiO2 layers on Si 111 preparation, interface gap states and the influence of passivation

Essential prerequisite for successful application of Si SiO2 nanostructures in photovoltaics is the realization of well defined and abrupt interfaces with low densities of interface gap states. Here, a complete in situ process from preparation and hydrogen passivation to interface gap state analysis by near UV photoelectron spectroscopy without breaking ultrahigh vacuum UHV conditions is introduced. It is demonstrated, that by RF plasma oxidation of Si 111 substrates with thermalized neutral oxygen atoms ultrathin SiO2 layers with compositionally and structurally abrupt Si SiO2 interfaces with minimal amount of intermediate oxidation states bridging the transition from Si to SiO2 can be realized. Plasma oxidized samples have significantly lower interface gap states than samples oxidized by thermal oxidation at 850 C. Interface gap state densities were further reduced by in situ hydrogen plasma passivation with nearly thermalized H atoms. The resulting reduction of interface recombination velocity and the increase of effective majority and minority carrier lifetimes are revealed by constant photocurrent measurements and quasi steady state photoconductance, respectivel

A novel high efficiency buried grid rear contact amorphous crystalline silicon heterojunction solar cell concept

A novel high efficiency Silicon wafer based solar cell concept, combining the amorphous crystalline silicon heterojunction solar cell technology with a specifically adopted rear contact scheme is presented The Buried Grid RECASH Rear Contact Amorphous Crystalline Silicon Heterojunction Solar Cell. This newly developed solar cell design patent pending [1] consists of an electrically insulated contact grid buried within a rear side a Si H emitter layer and needs no structuring processes of the solar cell absorber or emitter. The efficiency potential of the concept is discussed by means of numerical computer simulation, and first experimental results of the RECASH technology are presente

Temperature behaviour of irreversibility fields of various Hg- based superconducting ceramics

A variety of Hg-1223 high-T-c superconducting: systems, partly substituted by lead or fluorine, are investigated by means of magnetic susceptibility measurements. As already observed earlier, the double peak structure in the imaginary parts of the ac signals below about 1 T is confirmed indicating different pinning mechanisms. Irreversibility lines are determined from the upper peak values and analysed applying the theory of Matsushita which is based on a depinning mechanism caused by thermally activated flux creep. It leads to a suitable description in the whole temperature range investigated and allows a discussion of differences of the various samples. (C) 2002 Elsevier Science B.V. All rights reserved

In-flight calibration sources for Herschel-SPIRE

SPIRE, the Spectral and Photometric Imaging Receiver, will be a bolometer instrument for ESA's Herschel satellite. The instrument comprises a three-band imaging photometer covering the 250-500 μm range, and an imaging Fourier Transform Spectrometer (FTS) covering 200-670 μm. This paper presents the requirements for and design of the photometer and spectrometer calibration/illumination sources, and the results of laboratory tests on prototypes. The photometer calibrator is an electrically heated thermal source of submillimetre radiation, the purpose of which is to provide a repeatable signal for in-flight monitoring of health and responsivity of the SPIRE photometer detectors. It is not required to provide absolute calibration or uniform illumination of the arrays, but it may be used as part of the overall calibration scheme. The spectrometer calibrator is located at a pupil at the second input port of the FTS. It is designed to enable matching of the telescope emission for a range of telescope temperature (60-90 K) and emissivity (2% - 10%). By matching the telescope emission at this port, the high background from the Herschel telescope emission can be nulled to a high degree, resulting in an interferogram in which the contribution from the astronomical source is not overwhelmed by the telescope offset. The flexibility for telescope matching inherent in the design is important, as the exact telescope parameters will be unknown until the satellite is in operation. The FTS calibrator will also be used to assist in the absolute calibration scheme for SPIRE FTS observations

Temperature behaviour of irreversibility fields of various Hg- based superconducting ceramics

A variety of Hg-1223 high-T-c superconducting: systems, partly substituted by lead or fluorine, are investigated by means of magnetic susceptibility measurements. As already observed earlier, the double peak structure in the imaginary parts of the ac signals below about 1 T is confirmed indicating different pinning mechanisms. Irreversibility lines are determined from the upper peak values and analysed applying the theory of Matsushita which is based on a depinning mechanism caused by thermally activated flux creep. It leads to a suitable description in the whole temperature range investigated and allows a discussion of differences of the various samples. (C) 2002 Elsevier Science B.V. All rights reserved