Superconductivity and phase relationships were explored in the Na-Fe-As system. The PbFCl-type 111 phase is stable only within a Na stoichiometry range of 1.00 to ϳ0.85, and exhibits bulk superconductivity within an even narrower range around 0.90 in Na 0.9 FeAs. In particular, stoichiometric NaFeAs is not a bulk superconductor. The onset of the superconducting transition varies in a totally different way and the highest T c occurs in multiphase samples with a nominal composition of Na: Fe: As= 0.5:1:1, where the superconductive volume-fraction is almost zero. Such doping dependency is rather surprising and in disagreement with most expectations. DOI: 10.1103/PhysRevB.79.184516 PACS number͑s͒: 74.70.Dd, 74.62.Dh, 74.62.Bf The recent discovery of superconductivity in layered transition-metal oxypnictides, La͑O , F͒FeAs, 1 has attracted intense interest in the FeAs-based compounds. Superconductivity up to 55 K has been observed in three classes of FeAsbased compounds, i.e., ͑R ,Ae͒͑O , F͒FeAs, ͑Ae, A͒Fe 2 As 2 and AFeAs, where R, Ae, and A are rare earth, alkaline earth, and alkali elements, respectively.2-7 The FeAs-based superconducting compounds have often been compared with the well-investigated cuprate superconductors. The doping dependency of the superconductivity, however, appears to be rather different in the FeAs-family as it varies significantly from one member to another. 7,8 The main doping effects reported so far in the FeAs family, however, appear still to be a smooth, bell-like T c vs. carrier filling x 0 , where T c is the transition temperature. Competitions with magnetic ordering are often suggested in interpreting the data.9,10 Significant change in the superconducting volume-fraction V S , on the other hand, occurs only near the normal conductorsuperconductor boundary. The V S , it should be pointed out, is actually a convolution of the T c ͑x 0 ͒ and the local x 0 -distribution ͑composition inhomogeneity͒ if x 0 is a sole parameter. A constant V S , therefore, is expected if the superconductive range, ⌬, is much broader than the x 0 -spread, e.g., the full width at half height ͑FWHH͒ of a normal distribution. The effect on T c , in such cases, will be the main focus. At the opposite extreme of ⌬Ӷ, however, the spread would lead to the same T c distribution but a drastic V S change with x 0 , though this is rarely observed or discussed. Herein we report our observations in the superconducting system, Na y FeAs, which possesses a PbFCl-type structure isotypic to that of LiFeAs. This PbFCl-type structure as well as ͑trace͒ superconductivity exist over the whole nominalcomposition range investigated, i.e., with the nominal composition of Na y FeAs, with 0.5Յ y Յ 1.0. The samples are single phase, however, only for y Ն 0.9, and the impurity phase FeAs appears at lower y. A rather unusual doping effect is also observed. On one hand, the samples become bulk superconducting, e.g., with V S Ͼ 10%, only around y = 0.9 with an estimated spread Ӷ0.1. The apparent T c , on the other hand, monotonically...

No abstract

The hysteretic and reversible polarity-dependent resistive switch driven by electric pulses is studied in both Ag/Pr0.7Ca0.3MnO3/YBa2Cu3O7 sandwiches and single-layer Pr0.7Ca0.3MnO3 strips. The data demonstrate that the switch takes place at the Ag-Pr0.7Ca0.3MnO3 interface. A model, which describes the data well, is proposed. We further suggest that electrochemical diffusion is the cause for the switch.Pr 0.7 Ca 0.3 MnO 3 (PCMO) has attracted extensive interest recently. Below 150 K, its free energies corresponding to the paramagnetic, the charge-ordered, and the ferromagnetic states differ only slightly. Therefore, a slight external disturbance, e.g. magnetic field, light, isotope mass, pressure, or electric field, may lead to a large resistivity (ρ) change, but only at low temperatures.1 Therefore, it is interesting to note the report of Liu et al.2 that the two-lead resistance, R, of a PCMO layer sandwiched between an Ag top-electrode and a YBa 2 Cu 3 O 7 (YBCO) or a Pt bottom-electrode can be drastically and repeatably alternated at room temperature by applying electric pulses with different polarities.3 This R-switch may thus offer potential device applications, e.g. nonvolatile memory. Similar R-changes in single-layer PCMO films with the four-lead configuration were also reported. The R-switch has therefore been attributed to bulk properties of PCMO, in terms of the alignment of the presumed ferromagnetic clusters by the electric field.2 The interpretation, if confirmed, presents a major challenge to the physics of manganites and, possibly, to the basic law of parity conservation. The reported R-change of ∆R ≥ 3000 Ω across a 600 nm thick PCMO film represents a ρ-increase of ∆ρ ≈ 105 Ω cm, and suggests a novel state with a ρ(297 K) far greater than the ρ(297 K) << 10 1 Ω cm ever reported in PCMO. According to the commonly accepted polaron model, ρ(297 K) of manganites is controlled by the polaron mobility and should be ultimately restricted by the hopping barrier (10 −1 eV ≈ k B T at 297 K) associated with the Jahn-Teller distortion, which is only a few eV. 4 The experimental ρ(297 K) is only 10 −2 to 10 0 Ω cm in (La y Pr 1−y ) 1−x Ca x MnO 3 for 0.2 ≤ x ≤ 0.5 and 0 ≤ y ≤ 0.7, 5 and ≤ 10 4 Ω cm even in extreme cases, such as Nd 0.7 Ba 0.3 MnO 3 and LaMnO 3 .6 A ρ(297 K) of 10 5 Ω cm or higher would suggest a new insulating state never observed before and challenge the polaron model commonly accepted. In a more general sense, this polarity-dependent ρ in a uniform material reported, if proven, represents a violation of the law of parity conservation in the electromagnetic field. It may occur without parity violation only if the sample is asymmetric due to either an inhomogeneity in the thickness direction or poling by electric pulses ("training"); neither bears any obvious relation to the alignment model proposed.2 The present study is motivated by our attempt to elucidate the mechanism responsible for, and the nature of, the R-switch. Our data demonstrate that the switch occurs at the Ag-PCMO interface, ...

No abstract

We investigate the polarity-dependent field-induced resistive switching phenomenon driven by electric pulses in perovskite oxides. Our data show that the switching is a common occurrence restricted to an interfacial layer between a deposited metal electrode and the oxide. We determine through impedance spectroscopy that the interfacial layer is no thicker than 10 nm and that the switch is accompanied by a small capacitance increase associated with charge accumulation. Based on interfacial I − V characterization and measurement of the temperature dependence of the resistance, we propose that a field-created crystalline defect mechanism, which is controllable for devices, drives the switch.Recent observation of room-temperature resistive switching driven by electric fields in various perovskite oxides has garnered attention due to the potential for nonvolatile memory applications. This fieldinduced resistive switch has been reported in several compounds. 1,2,3,4,5,6 The switching observed shares several features: a moderate switch speed; an altered resistance inconsistent with the well-accepted bulk resistivity; and a sensitivity to surface treatments. Various models have been put forth, but inconsistencies remain. For instance, bulk charge ordering in Pr 0.7 Ca 0.3 MnO 3 (PCMO) has been suggested, but this creates an apparent conflict with both the spatial symmetry and the reported bulk properties.1 Although interface models have been proposed involving either lattice defects or carrier concentration, 2,5,7 the exact nature of this interface remains vague. We therefore investigate the interface properties associated with the resistive switch and have observed: a) the switching is a common phenomenon to the metal-oxide interfacial layer; b) the interfacial transport properties are rather different from that of the bulk; c) the interfacial resistance is dominated by the carrier trapping with no indication of Schottky barriers; and d) the capacitance of the interfacial layer, on the order of 1000 nF/cm 2 , changes with switching, which is indicative of a change in the space-charge. These observations can be self-consistently understood using a carrier-trapping model and suggest that the field-induced switch is not restricted to a small class of materials. Therefore, the potential for controlling this phenomenon for device applications is considerable.Ceramic samples synthesized by standard solid-state reactions were determined to be single-phase based on X-ray powder diffraction patterns taken on a Rigaku DMAX-IIIB diffractometer. PCMO thin films were ac sputtered on LaAlO 3 substrates at 760• C in an Ar:O 2 = 2 : 3 mixed atmosphere at 140 mTorr. The films were found to be highly epitaxial. Pt leads were attached to the ceramic samples using Ted Pella Leitsilber 200 Ag paint. The thin film samples received sputtered Ag electrodes to which Pt leads were also attached using Ag paint.Ceramic LaCoO 3 , La 0.7 Ca 0.3 MnO 3 , Pr 0.7 Ca 0.3 MnO 3 , SrFeO 2.7 , RuSr 2 GdCu 2 O 3 , and YBa 2 Cu 3 O 7 were tested. We adopted ...