Two-channel Kondo physics due to As vacancies in the layered compound ZrAs1.58Se0.39

We address the origin of the magnetic-field independent -|A| T^{1/2} term observed in the low-temperature resistivity of several As-based metallic systems of the PbFCl structure type. For the layered compound ZrAs_{1.58}Se_{0.39}, we show that vacancies in the square nets of As give rise to the low-temperature transport anomaly over a wide temperature regime of almost two decades in temperature. This low-temperature behavior is in line with the non-magnetic version of the two-channel Kondo effect, whose origin we ascribe to a dynamic Jahn-Teller effect operating at the vacancy-carrying As layer with a C_4 symmetry. The pair-breaking nature of the dynamical defects in the square nets of As explains the low superconducting transition temperature T_{\rm{c}}\approx 0.14 K of ZrAs_{1.58}Se_{0.39}, as compared to the free-of-vacancies homologue ZrP_{1.54}S_{0.46} (T_{\rm{c}}\approx 3.7 K). Our findings should be relevant to a wide class of metals with disordered pnictogen layers.Comment: 17 pages, 8 figures; submitte

Type-II Superconductivity in Ternary Zirconium Pnictide Chalcogenide Single Crystals

Layered Pnictides are proven to be a great reservoir for superconductors in the past and ternary zirconium pnictide chalcogenides of ZrXY-type (X = P, As; Y = S, Se) might be a platform for new superconductors. The superconducting properties of carefully grown (chemical transport reaction) single crystals of ZrP1.54S0.46 with a transition temperature of Tc = 3.5 K are investigated. This compound (PbFCl structure type) contains square planar nets: One of the nets is completely occupied (no vacancies) by P, the other one characterized by a random distribution of P and S (full occupation: no vacancies). Besides zero-field-cooling (zfc), field-cooling (fc), and remanent moment (rem) measurements, especially magnetization and ac susceptibility measurements are performed. A nearly ideal type-II behavior with a Ginzburg-Landau parameter κ = 24 is found. The magnetization curves between Bc1 and Bc2 for increasing field are in excellent agreement with theoretical calculations performed by E. H. Brandt based on the Ginzburg-Landau theory. The decreasing branches of the magnetization curves are asymmetric about the field axis indicating weak pinning and also large diamagnetic behavior

Towards democratic intelligence oversight: Limits, practices, struggles

Despite its common usage, The meaning of 'democratic' in democratic intelligence oversight has rarely been spelled out. In this paper, we situate questions regarding intelligence oversight within broader debates about the meanings and practices of democracy. We argue that the literature on intelligence oversight has tended to implicitly or explicitly follow liberal and technocratic ideas of democracy, which have limited the understanding of oversight both in academia and in practice. Thus, oversight is mostly understood as an expert, institutional and partially exclusive arrangement that is supposed to strike a balance between individual freedom and collective security with the goal of establishing the legitimacy of, and trust in intelligence work in a national setting. ‘Healthy’ or ‘efficient’ democratic oversight then becomes a matter of technical expertise, non-partisanship, and the ability to guard secrets. By analysing three moments of struggle around what counts as intelligence oversight across Germany, the UK, and the USA, this paper elucidates their democratic stakes. Through a practice-based approach, we argue that oversight takes much more agonistic, contentious, transnational, and public forms. However, these democratic practices reconfiguring oversight remain contested or contained by dominant views on what constitutes legitimate and effective intelligence oversight

Monoclonal antibodies against the human lymphocyte differentiation antigen CD 76 bind to gangliosides

AbstractTwo monoclonal antibodies, HD 66 and CRIS-4, by which the new CD 76 B-cell-associated cluster was defined, bound to several gangliosides (sialic acid containing glycolipids) of different polarity. One of the gangliosides recognized by HD 66 could be identified as NeuAcα2-6Galβl-4GlcNAcβl-3Galβl-4Glc-βl-l'Cer. This antigen was enzymatically synthesized. Sialidase treatment of the ganglioside antigens abolished binding of HD 66 and CRIS-4

Crystallographic disorder and electron scattering on structural two-level systems in ZrAs1.4Se0.5

Single crystals of ZrAs1.4Se0.5 (PbFCl type structure) were grown by chemical vapour transport. While their thermodynamic and transport properties are typical for ordinary metals, the electrical resistivity exhibits a shallow minimum at low temperatures. Application of strong magnetic fields does not influence this anomaly. The minimum of the resistivity in ZrAs1.4Se0.5 apparently originates from interaction between the conduction electrons and structural two-level systems. Significant disorder in the As-Se substructure is inferred from X-ray diffraction and electron microprobe studies