A final-state wavefunction, involving three appropriate confluent hypergeometric functions, which satisfies the correct asymptotic condition is used to study positron-impact ionization of atomic hydrogen Results are reported for the double as well as triple difterenlial cross sections and compared with lhe available theoretical findings. The double difterential cross section curves for diKerent ejection angles show the presence of a cusp-like structure at the region of equal energy sharing by the outgoing positron and lhe ejected electron. This is in conformity with the findings of Mandal et a1 at zero ejection angle but unlike the structure exhibited by the curves of Schdtz and Reinhold.

Spin-orbit coupling (SOC) plays a crucial role in magnetic and electronic properties of 5d iridates.In this paper we have experimentally investigated the structural and physical properties of a series of Ir-based double perovskite compounds Pr 2−x Sr x MgIrO 6 (x = 0, 0.5, 1; abbreviated as PMIO, PSMIO1505 and PSMIO from now on). Interestingly, these compounds have recently been proposed to undergo a transition from the spin-orbit-coupled Mott insulating phase at x = 0 to the elusive half-metallic antiferromagnetic (HMAFM) state with Sr-doping at x = 1. However, our detailed magnetic and electrical measurements refute any kind of HMAFM possibility in either of the doped samples. In addition, we establish that within these Pr 2−x Sr x MgIrO 6 double perovskites, changes in Ir-oxidation states (4+ for PMIO to 5+ for PSMIO via mixed 4+/5+ for PSMIO1505) lead to markedly different magnetic behaviors. While SOC on Ir is at the root of the observed insulating behaviors for all the three samples, the correlated magnetic properties of these three compounds develop entirely due to the contribution from local Ir-moments. Additionally, the magnetic Pr 3+ (4f 2 ) ions, instead of showing any kind of ordering, only contributes to the total paramagnetic moment. It is seen that the PrSrMgIrO 6 sample does not order down to 2 K despite antiferromagnetic interactions. But, the d 5 iridate Pr 2 MgIrO 6 shows a sharp AFM transition at around 14 K, and in mixed valent Pr 1.5 Sr 0.5 MgIrO 6 sample the AFM transition is shifted to much lower temperature (∼ 6 K) due to weakening of AFM exchange.

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Colossal electroresistance (CER) in manganites, i.e., a large change in electrical resistance under the influence of either an applied electric field or an applied electric current, has often been described as complimentary to the colossal magnetoresistance (CMR) effect. Mixed valent vanadates with active t 2g and empty e g orbitals, unlike manganites, have not naturally been discussed in this context, as double exchange based CMR is not realizable in them. However, presence of coupled spin and orbital degrees of freedom, metal-insulator transition (MIT) accompanied by orbital order-disorder transition, etc., anyway make the vanadates an exciting group of materials. Here we probe a Fe-doped hollandite lead vanadate PbFe 1.75 V 4.25 O 11 (PFVO), which exhibits a clear MIT as a function of temperature. Most importantly, a giant fall in the resistivity, indicative of a CER, as well as a systematic shift in the MIT towards higher temperature are observed as a function of applied electric current. Detailed structural, magnetic, thermodynamic and transport studies point towards a complex interplay between orbital order/disorder effect, MIT and double exchange in this system.

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