P. Klavins

Giant magnetoresistance at 300 K in single crystals of La0.65(PbCa)0.35MnO3

Giant magnetoresistance (GMR) at 300 K and 5.5 T with ΔR/R(0)=74% was observed in single crystals of La0.65(PbCa)0.35MnO3. The maximum GMR occurs exactly at the ferromagnetic–paramagnetic phase transition temperature (Tc). For T<Tc, the R(T) curve is closely related to the M(T) curve. A sign change in the curvatures of R(H) curves below and above Tc has been observed, indicating that two different mechanisms are responsible for the magnetic scatterings of the carriers in two separate temperature regions.

Vapor transport growth of C60 crystals

A vapor transport technique has been used to produce large, high quality C60 single crystals. This technique, which uses an open tube method with a flowing helium gas carrier, eliminates the crystal size limitation found in the vacuum sublimation growth method.

Reactions of M{N(SiMe3)2}2 (M=Mn, Fe or Co) with pyridine and 4,4′-bipyridyl: structural and magnetic studies

Abstract The reactions of the aromatic nitrogen donor ligands pyridine (py) and 4,4′-bipyridyl (4,4′-bipy) with the transition metal silylamides M{N(SiMe3)2}2 (M=Mn, Fe or Co) were investigated. Treatment of the metal amides with pyridine afforded the bispyridine complexes M{N(SiMe3)2}(py)2 (M=Mn, 1a; Fe, 2a; Co, 3a). Distillation or sublimation of 2a or 3a afforded the monopyridine complexes M{N(SiMe3)2}2(py) (M=Fe, 2b; Co, 3b). The addition of pyrazine (prz) to Mn{N(SiMe3)2}2 also yielded the bispyrazine adduct Mn{N(SiMe3)2}2(prz)2, 1b. However, the reaction of 4,4′-bipyridyl with Fe{N(SiMe3)2}2 or Co{N(SiMe3)2}2 afforded the polymeric chain-like complexes {M{N(SiMe3)2}2(4,4′-bipy)}∞ (M=Fe, 4; Co, 5). With the exception of 2b, all complexes were characterized by X-ray crystallography. The complexes 1a, 1b, 2a and 3a displayed monomeric structures and metal geometries that are based on a tetrahedron with interligand angles that deviate markedly from idealized values. Complex 3b was found to have a distorted trigonal planar geometry. In contrast, complexes 4 and 5 displayed polymeric zig-zag chain structures which have four coordinate metal centers connected by 4,4′-bipyridyl ligands. Magnetic studies indicated that complexes 1–3b have high-spin electron configurations and that the paramagnetic centers in 4 and 5 did not interact with each other through the 4,4′-bipyridyl ligands.

Synthesis, structure, magnetism, and high temperature thermoelectric properties of Ge doped Yb14MnSb11

The Zintl phase Yb(14)MnSb(11) was successfully doped with Ge utilizing a tin flux technique. The stoichiometry was determined by microprobe analysis to be Yb(13.99(14))Mn(1.05(5))Sb(10.89(16))Ge(0.06(3)). This was the maximum amount of Ge that could be incorporated into the structure via flux synthesis regardless of the amount included in the reaction. Single crystal X-ray diffraction could not unambiguously determine the site occupancy for Ge. Bond lengths varied by about 1% or less, compared with the undoped structure, suggesting that the small amount of Ge dopant does not significantly perturb the structure. Differential scanning calorimetry/thermogravimetry (DSC/TG) show that the doped compounds melting point is greater than 1200 K. The electrical resistivity and magnetism are virtually unchanged from the parent material, suggesting that Yb is present as Yb(2+) and that the Ge dopant has little effect on the magnetic structure. At 900 K the resistivity and Seebeck coefficient decrease resulting in a zT of 0.45 at 1100 K, significantly lower than the undoped compound.

Magnetic properties and negative colossal magnetoresistance of the rare earth Zintl phase EuIn2As2.

Large, high quality single crystals of a new Zintl phase, EuIn(2)As(2), have been synthesized from a reactive indium flux. EuIn(2)As(2) is isostructural to the recently reported phase EuIn(2)P(2), and it is only the second reported member of the group of compounds with formula AM(2)X(2) (A = alkali, alkaline earth, or rare earth cation; M = transition or post-transition metal; and X = Group 14 or 15 element) that crystallizes in the hexagonal space group P6(3)/mmc (a = 4.2067(3) A, c = 17.889(2) A and Z = 2). The structure type contains layers of A(2+) cations separated by [M(2)X(2)](2-) layers along the crystallographic c-axis. Crystals of the title compound were mounted for magnetic measurements, with the crystallographic c-axis oriented either parallel or perpendicular to the direction of the applied field. The collective magnetization versus temperature and field data indicate two magnetic exchange interactions near 16 K, one involving Eu(2+)...Eu(2+) intralayer coupling and the other involving Eu(2+)...Eu(2+) coupling between layers. EuIn(2)As(2) is metallic and magnetoresistive, as is the isostructural phosphide, and both compounds have coincident resistivity and magnetic ordering transitions, consistent with the observation of colossal magnetoresistance. Negative colossal magnetoresistance (MR = {[rho(H) - rho(0)]/rho(H)} x 100%) of up to -143% (at T = 17.5 K, H = 5 T) is observed for EuIn(2)As(2), approximately half of that observed for the more resistive phosphide, which has a higher magnetic ordering temperature and local moment coupling strength.

Meissner effect and magnetic critical current density in detwinned YBa2Cu3O7−x single crystals

Abstract A simple uniaxial compression technique was used to detwin YBa 2 Cu 3 O 7− x single crystals. The Meissner effect and magnetic critical current density were measured on the same crystals before and after the detwinning treatment. Our results show that at low temperature the twin boundaries are not the primary pinning centers responsible for the high critical current density observed in the copper-oxygen plane.

Electrical resistivity of single crystal Ba1-xKxBiO3

Abstract Resistive measurements and characterization of superconductivity have been performed for Ba 1- x K x BiO 3 single crystals grown by an electrochemical method. Depending on the synthesis conditions, the crystals showed semiconductive and metallic behavior in the normal state with transition temperatures as high as 30.5 K. The upper critical field H c2 (O) and Ginzburg-Landau coherence length ξ(O) of the crystal are 16.1 T and 45 A, respectively.

Long range order and two-fluid behavior in heavy electron materials

The heavy electron Kondo liquid is an emergent state of condensed matter that displays universal behavior independent of material details. Properties of the heavy electron liquid are best probed by NMR Knight shift measurements, which provide a direct measure of the behavior of the heavy electron liquid that emerges below the Kondo lattice coherence temperature as the lattice of local moments hybridizes with the background conduction electrons. Because the transfer of spectral weight between the localized and itinerant electronic degrees of freedom is gradual, the Kondo liquid typically coexists with the local moment component until the material orders at low temperatures. The two-fluid formula captures this behavior in a broad range of materials in the paramagnetic state. In order to investigate two-fluid behavior and the onset and physical origin of different long range ordered ground states in heavy electron materials, we have extended Knight shift measurements to URu2Si2, CeIrIn5, and CeRhIn5. In CeRhIn5 we find that the antiferromagnetic order is preceded by a relocalization of the Kondo liquid, providing independent evidence for a local moment origin of antiferromagnetism. In URu2Si2 the hidden order is shown to emerge directly from the Kondo liquid and so is not associated with local moment physics. Our results imply that the nature of the ground state is strongly coupled with the hybridization in the Kondo lattice in agreement with phase diagram proposed by Yang and Pines.

Preparation and superconductivity of Ba1−ϰKϰBiO3 single crystals

Electrochemical synthesis of millimeter-sized single crystals of Ba 1−ϰ K ϰ Bio 3 with superconducting transition temperatures of 31.0 K is detailed. The resulting crystals are cubic, well-faceted and exhibit remarkably high quality surfaces which aid in important studies of the transport properties of the material. In addition, an oxygenation treatment which increases and sharpens the transition temperature of the as-grown crystals is reported.

Synthesis and properties of hole-doped Li1−xBC

Stoichiometric LiBC has been synthesized using both arc melting and sealed tantalum ampoule methods. Hole-doped Li1−xBC (0<x⩽0.37) compounds have been realized through vacuum deintercalation from LiBC. The hexagonal crystal lattice has remained largely intact, with only slight decreases in lattice parameters upon hole-doping. The samples are intrinsically diamagnetic and are semiconducting in the 2–300 K range studied. Increased hole-doping leads to a color darkening as well as a decrease in resistivity.