Low-temperature behavior of specific heat curves in ruthenocuprate RuSr<Subscript>2</Subscript>Gd<Subscript>1.5</Subscript>Ge<Subscript>0.5</Subscript>Cu<Subscript>2</Subscript>O<Subscript>10 − δ</Subscript>: Superconducting,magnetic and crossing-point effects

Magnetic and superconducting properties of polycrystalline samples of RuSr₂Gd_1.5Ce_0.5Cu₂O_{(10 − δ)}, asprepared (by solid-state reaction) and annealed in pure oxygen at different pressure are presented. Specific heat and magnetization were investigated in the temperature range 1.8–300 K with a magnetic field up to 8 T. Specific heat, C (T), shows a jump at the superconducting transition (with onset at T ≈ 37.5 K) and a Schottky-type anomaly below 20 K. It is found that curves C(T) taken for different values of magnetic field have the same crossing point (at T _* ≈ 2.7 K) for all samples studied. At the same time, C(H) curves taken for different temperatures have a crossing point at a characteristic field H _* ≈ 3.7 T. These effects are manifestations of the crossing-point phenomenon, which is supposed to be inherent for strongly correlated electron systems.     

Thermal transport in the oxygenated magnetic superconductor, Eu1.5Ce0.5RuSr2Cu2O10+δ

The thermal conductivity and thermopower are reported for a hole doped Eu_1.5Ce_0.5RuSr₂Cu₂O_10+δ sample that has been annealed at 1100 K under an oxygen pressure of 54 atm. At T_c=45 K superconductivity and weak ferromagnetism coexist (T_m=180 K). Weak features in the thermopower, S(T), and thermal conductivity, κ(T), are observed both at T_m and at T^*=140 K. The thermopower begins to decrease sharply toward zero at T_c, and there is an extremely sharp increase of about 30% in the thermal conductivity at T_c. This “first order” transition may be related to the sudden appearance of a spontaneous vortex phase at T_c. A small shoulder is observed in κ(T) in the temperature range T=5–13 K.     

Metamagnetic states in single crystal HoNi<Subscript>2</Subscript>B<Subscript>2</Subscript>C at <Emphasis Type="Italic">T</Emphasis> ≈ 1.9 K: Torque magnetometry study

Metamagnetic transitions in single-crystal rare-earth nickel borocarbide HoNi₂B₂C have been studied at T ≈ 1.9 K with a Quantum Design torque magnetometer. With increasing field, transitions to antiferromagnetic, ferrimagnetic, non-collinear, and saturated paramagnetic states take place. The critical fields of the transitions depend crucially on the angle θ between applied field and the easy axis [110]. Measurements of torque along the c axis have been made while changing the angular direction of the magnetic field (parallel to basal tetragonal ab planes) and with changing field at fixed angle over a wide angular range. Two new phase boundaries in the region of the non-collinear phase have been observed, and the direction of the magnetization in this phase has been precisely determined. At low field the antiferromagnetic phase is observed to be multidomain. In the angular range around the hard axis (?6° ≤ ? ≤ 6°, where ? is the angle between the field and hard axis [100]) the magnetic behavior is found to be “frustrated” with a mixture of phases with different directions of the magnetization.     

N2O Reductase Activity of a [Cu4S] Cluster in the 4CuI Redox State Modulated by Hydrogen Bond Donors and Proton Relays in the Secondary Coordination Sphere

The model complex [Cu₄(μ₄‐S)(dppa)₄]²⁺ ( 1 , dppa=μ₂‐(Ph₂P)₂NH) has N₂O reductase activity in methanol solvent, mediating 2 H⁺/2 e^? reduction of N₂O to N₂+H₂O in the presence of an exogenous electron donor (CoCp₂). A stoichiometric product with two deprotonated dppa ligands was characterized, indicating a key role of second‐sphere N?H residues as proton donors during N₂O reduction. The activity of 1 towards N₂O was suppressed in solvents that are unable to provide hydrogen bonding to the second‐sphere N?H groups. Structural and computational data indicate that second‐sphere hydrogen bonding induces structural distortion of the [Cu₄S] active site, accessing a strained geometry with enhanced reactivity due to localization of electron density along a dicopper edge site. The behavior of 1 mimics aspects of the Cu_Z catalytic site of nitrous oxide reductase: activity in the 4Cu^I:1S redox state, use of a second‐sphere proton donor, and reactivity dependence on both primary and secondary sphere effects.     

Open-tubular nanoelectrochromatography (OT-NEC): gel-free separation of single stranded DNAs (ssDNAs) in thermoplastic nanochannels

Electrophoresis or electrochromatography carried out in nanometer columns (width and depth) offers some attractive benefits compared to microscale columns. These advantages include unique separation mechanisms that are scale dependent, fast separation times, and simpler workflow due to the lack of a need for column packing and/or wall coatings to create a stationary phase. We report the use of thermoplastics, in this case PMMA, as the substrate for separating single-stranded DNAs (ssDNAs). Electrophoresis nanochannels were created in PMMA using nanoimprint lithography (NIL), which can produce devices at lower cost and in a higher production mode compared to the fabrication techniques required for glass devices. The nanochannel column in PMMA was successful in separating ssDNAs in free solution that was not possible using microchip electrophoresis in PMMA. The separation could be performed in <1 s with resolution >1.5 when carried out using at an electric field strength of 280 V/cm and an effective column length of 60 μm (100 nm × 100 nm, depth and width). The ssDNAs transport through the PMMA column was driven electrokinetically under the influence of an EOF. The results indicated that the separation was dominated by chromatographic effects using an open tubular nano-electrochromatography (OT-NEC) mode of separation. Interesting to these separations was that no column packing was required nor a wall coating to create the stationary phase; the separation was affected using the native polymer that was UV/O₃ activated and an aqueous buffer mobile phase.     

Probing the electronic and mechanistic roles of the μ4-sulfur atom in a synthetic CuZ model system

Nitrous oxide (N₂O) contributes significantly to ozone layer depletion and is a potent greenhouse agent, motivating interest in the chemical details of biological N₂O fixation by nitrous oxide reductase (N₂OR) during bacterial denitrification. In this study, we report a combined experimental/computational study of a synthetic [4Cu:1S] cluster supported by N-donor ligands that can be considered the closest structural and functional mimic of the Cu_Z catalytic site in N₂OR reported to date. Quantitative N₂ measurements during synthetic N₂O reduction were used to determine reaction stoichiometry, which in turn was used as the basis for density functional theory (DFT) modeling of hypothetical reaction intermediates. The mechanism for N₂O reduction emerging from this computational modeling involves cooperative activation of N₂O across a Cu/S cluster edge. Direct interaction of the μ₄-S ligand with the N₂O substrate during coordination and N–O bond cleavage represents an unconventional mechanistic paradigm to be considered for the chemistry of Cu_Z and related metal–sulfur clusters. Consistent with hypothetical participation of the μ₄-S unit in two-electron reduction of N₂O, Cu K-edge and S K-edge X-ray absorption spectroscopy (XAS) reveal a high degree of participation by the μ₄-S in redox changes, with approximately 21% S 3p contribution to the redox-active molecular orbital in the highly covalent [4Cu:1S] core, compared to approximately 14% Cu 3d contribution per copper. The XAS data included in this study represent the first spectroscopic interrogation of multiple redox levels of a [4Cu:1S] cluster and show high fidelity to the biological Cu_Z site.

Experimental data and computational modeling indicates an active role for the bridging sulfide ligand in a synthetic Cu_Z model.     

Influence of high-energy electron irradiation on the transport properties of La1-xCaxMnO3 films ($$(x \approx 1/3)$$ )

The effect of crystal lattice disorder on the conductivity and colossal magnetoresistance in La_1-xCa_xMnO₃ ( ) films has been examined. The lattice defects are introduced by irradiating the film with high-energy ( MeV) electrons with a maximal fluence of about cm^-2. This comparatively low dose of irradiation produces rather small radiation damage in the films. The number of displacements per atom (dpa) in the irradiated sample is about 10^-5. Nevertheless, this results in an appreciable increase in the film resistivity. The percentage of the resistivity increase in the ferromagnetic metallic state (below the Curie temperature ) was much greater than that observed in the insulating state (above ). At the same time irradiation has much less effect on or on the magnitude of the colossal magnetoresistance. A possible explanation of such behavior is proposed. Received 21 July 1999 and Received in final form 27 December 1999