Low field extension for magnetometers (TinyBee) used for investigations on low-dimensional superconductors with Bc1 < 5G

In this article a simple and easy to install low magnetic field extension of the SQUID magnetometer Quantum Design MPMS-7 is described. This has been accomplished by complementing the MPMS-7 magnet control system with a laboratory current supply for the low magnetic field region (B \leq 200G). This hard- and software upgrade provides a significant gain in the magnetic field accuracy up to an order of magnitude compared with the standard instrument's setup and is improving the resolution to better than 0.01G below 40G. The field control has been integrated into the Quantum Design MultiVu software for a transparent and user-friendly operation of this extension. The improvements achieved are especially useful, when low magnetic field strengths (B < 1G) are required at high precision. The specific advantages of this application are illustrated by sophisticated magnetic characterisation of lowdimensional superconductors like Sc3CoC4 and SnSe2{Co({\eta}-C5H5)2}x.Comment: 16 pages, 7 figure

Unilateral Favre-Racouchot Disease: Evidence for the Etiological Role of Chronic Solar Damage

Favre-Racouchot disease commonly presents as comedones, cysts andelastosis in the periocular region of older men. Its etiology has beenlinked to several exogenous factors. Here we present 2 patients withstrictly unilateral manifestation of the disease and a correspondinghistory of predominantly one-sided chronic occupational sun exposure andsmoking, making the case for the causative role of these two factors

Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit

postprin

Interkalationsverbindungen von NbSe2 und SnSe2: Modellsysteme für niederdimensionale Supraleiter

Quasi zweidimensionale (2D) Metalldichalkogenide erfahren beträchtliches Interesse, seitdem ihre komplexen anisotropen elektronischen Eigenschaften mittels Interkalation mit Donormolekülen kontrolliert werden können. Obwohl schichtförmig aufgebaute Dichalcogenide im Hinblick auf zahlreiche chemische und physikalische Eigenschaften untersucht wurden, wird ihr zweidimensionaler Charakter nur wenig verstanden. Die vorliegende Arbeit befasst sich mit den schichtförmig aufgebauten Dichalkogeniden SnSe2 und NbSe2. Das Wirtsgitter SnSe2 wurde durch chemischen Transport mit Hilfe von Jod als Transportmittel in evakuierten Quarzampullen synthetisiert. Die Interkalation der halbleitenden Einkristalle SnSe2 mit dem metallorganischen Donor Cobaltocen (CoCp2) führen in Kombination zu einem Supraleitenden Hybridmaterial mit einer Sprungtemperatur von etwa 8 Kelvin. Ex-situ Interkalationsstudien zeigen einen Interkalationsmechanismus ausgehend vom Wirtsgitter 2H-SnSe2 über in eine Stage-2 Phase, die bei weiterer Interkalation in einer Stage-1 Phase endet. Darüber hinaus zeigt SnSe2{CoCp2}x bemerkenswerte Tieftemperatureigenschaften wie die Koexistenz von Supraleitung und Magnetismus in Abhängigkeit des Interkalationsgrades. Für Interkalationsgrade von mehr als 17% Cobaltocen, konnte in 18R-SnSe2{CoCp2}x eine langreichweitige magnetische Ordnung beobachtet werden. Außerdem zeigt SnSe2{CoCp2}x ein äußerst empfindliches Pinning-Verhalten in sehr kleinen magnetischen Feldern von unter B < 0,1 mT. Für die Untersuchung dieser Felder wurde eine neue Geräteoption für das SQUID-Magnetometer MPMS-7 von QUANTUM DESIGN namens "TinyBee" in Betrieb genommen. Mit dieser Geräteerweiterung wird eine bedeutende Verbesserung der Einstellbarkeit von magnetischen Feldern unterhalb von B = 1 mT erzielt. Temperaturabhängige elektrische Transportmessungen in unterschiedlichen äußeren Magnetfeldern deuten auf ein hoch anisotropes supraleitendes Verhalten hin. Die Anisotropie nimmt mit steigendem Cobaltocen-Gehalt ab. Ein erstes Phasendiagramm über den gesamten Interkalationsgrad zwischen 0% und 33% wurde erstellt. Zum Vergleich der Tieftemperatureigenschaften von SnSe2{CoCp2}x wurde ein weiterer schichtförmig aufgebauter Supraleiter NbSe2 mit CoCp2 interkaliert. Er gehört zu den bekanntesten schichtförmigen Supraleitern mit Multiband-Charakter. Um die Anisotropie im Kristall zu erhöhen, wurde 2H-NbSe2 ebenfalls mit CoCp2 interkaliert, was zu einer Aufweitung des Gitter-Parameters in der kristallographischen c-Richtung von 12,53 Å auf 23,81 Å führt. Die supraleitende Sprungtemperatur verändert sich durch die Interkalation mit dem Donormolekül unwesentlich und man erhält eine Sprungtemperatur von Tc = 7,35 K. Außerdem deutet die starke Zunahme des oberen kritischen Magnetfeldes Bc2|| ≈ 18,5 T im Vergleich zum Wirtsgitter NbSe2 (Bc2|| ≈ 14,5 T) auf eine Zunahme der Anisotropie hin. Elektrische Transportmessungen und spezifische Wärmemessungen parallel und senkrecht zu den NbSe2-Schichten von 2H-NbSe2{CoCp2}0,26 zeigen sowohl eine magnetfeldabhängige Reentrant-Supraleitung als auch ein reversibles Verhalten in M(B) oberhalb von B > 3,5 T, wie es auch vom Wirtsgitter NbSe2 her bekannt ist.Quasi-two-dimensional (2D) metal dichalcogenides have received considerable research interest since their complex anisotropic electronic properties can be controlled by the intercalation of donor species. Although layered dichalcogenides have been studied by many aspects of chemical and physical properties, their two-dimensional character is only poorly understood. The present work deals with the layer-shaped dichalcogenides SnSe2 and NbSe2. The host-material SnSe2 was synthesized by chemical transport with Iodine as transport agent in sealed quartz ampoules. The intercalation of the semiconducting layered single crystals SnSe2 with the organometallic compound cobaltocene (CoCp2) leads to superconductivity up to T = 8 K. Ex-situ intercalation studies show an intercalation-mechanism outgoing from the host material 2H-SnSe2 in a stage-2 phase which goes over in a stage-1 phase for higher intercalation degrees. In addition, SnSe2{CoCp2}x show remarkable low-temperature properties e.g. the coexistence of superconductivity and magnetism in dependence of the staging and cobaltocene-content of the material. Starting from an intercalation degree of 17% CoCp2 long range ordered magnetism (with increasing saturation magnetization) was observed in 18R-SnSe2{CoCp2}x. Furthermore SnSe2{CoCp2}x show an extremely sensitive superconducting pinning behavior in very small magnetic fields partially below B < 0.1 mT. For investigation of these fields, a new low magnetic field extension for SQUID magnetometers ("TinyBee") was taken into use. With the upgrade of the SQUID magnetometer, a significant improvement of the magnetic field accuracy better than 1 µT will be achieved. Temperature dependent resistivity studies in various magnetic fields exhibit a highly anisotropic superconducting behaviour. This anisotropy decreases with increasing CoCp2-content. A phase diagram was developed in dependence of the degree of intercalation over the whole range of intercalation between 0 % and 33 %. For comparison of the low-temperature character of SnSe2{CoCp2}x, another layer-shaped superconductor NbSe2 was intercalated with CoCp2. The layered high-k s-wave superconductor 2H-NbSe2 belongs to the most prominent low-dimensional materials studied during the past fifty years. After the discovery of the high temperature superconductor MgB2, a benchmark system for multi-band superconductivity, NbSe2 experienced a renascence of research activities. Especially, since it represents a well-suited candidate for probing the multi-band model in a quasi-two-dimensional superconductor, due to the negligible vortex pinning in NbSe2 single crystals. In order to enhance the anisotropic character we intercalated high quality 2H-NbSe2 single crystals with the organometallic donor molecule cobaltocene, leading to an expansion of the lattice parameter in c direction from 12.53 Å to 23.81 Å. While the intercalation of organic compounds (which usually act as electron donors) reduces the superconducting transition temperature Tc from 7.1 K in 2H-NbSe2 to temperatures below Tc < 3 K. An opposite behavior is observed in 2H-NbSe2{CoCp2}0.26 with Tc = 7.35 K. Furthermore, the strong increase of the upper critical magnetic field Bc2 ≈ 18.5 T in comparison to the native parent compound (Bc2(NbSe2) ≈ 14,5 T) indicates a more pronounced anisotropic behavior. Resistivity, susceptibility and specific heat studies parallel and perpendicular to the NbSe2-layers of 2H-NbSe2{CoCp2}0.26 reveal both, a field-dependent reentrant superconductivity and a reversibility of the magnetization M(B) over a wide range above 3.5 T, also observed in the native parent NbSe2. Both intercalated materials NbSe2{CoCp2}x and SnSe2{CoCp2}x are good candidates for further theoretical investigation of the low dimensional superconductivity. The experimental results of the layered materials presented in this thesis will contribute to a better understanding of the low dimensional superconducting behavior

Refractive indices and band-gap properties of rocksalt Mg \u3csub\u3e\u3ci\u3ex\u3c/i\u3e\u3c/sub\u3eZn sub\u3e1-\u3ci\u3ex\u3c/i\u3e\u3c/sub\u3eO (0.68 ≤x≤1)

The room-temperature optical pseudo-dielectric-functions of single-phase, single-crystalline rocksalt-structure MgxZn1-xO with Mg-content x between 0.68 and 1 were determined in the photon energy range from 0.75 to 9.10 eV using spectroscopic ellipsometry. The refractive index determined in the spectral region below the fundamental absorption edge decreases with increasing Mg content. The pseudo-dielectric-functions reveal structures caused in critical points due to electronic band-to-band transitions and free exciton formation at the fundamental band-gap transition. Standard model dielectric function approaches were applied for line shape analysis. Upon increase of the bond ionicity with increasing Mg content the energies of the band-to-band transitions as well as the fundamental-band-gap exciton binding energy parameters increase, while the Γ-point spin-orbit-splitting energy parameter decreases. We compare our results with the band-gap properties of wurtzite-structure MgxZn1-xO with Mg-content x between 0 and 0.5 obtained previously, and we discuss discontinuities across the phase transition. We provide estimates for the band gap bowing parameter of rocksalt-structure MgxZn1-xO and the band-gap value of rocksalt-structure ZnO

Unilateral Favre-Racouchot Disease: Evidence for the Etiological Role of Chronic Solar Damage

Favre-Racouchot disease commonly presents as comedones, cysts andelastosis in the periocular region of older men. Its etiology has beenlinked to several exogenous factors. Here we present 2 patients withstrictly unilateral manifestation of the disease and a correspondinghistory of predominantly one-sided chronic occupational sun exposure andsmoking, making the case for the causative role of these two factors