[en] In order to determine the T_c-dependence of the colossal magnetoresistance (MR) exhibited by the ferromagnetic La_0.7M_0.3MnO_3+σ (M = Ba, Ca, Sr) system, the authors examine the magnetic-field and temperature dependent resistivity and magnetization of a series of thin films that were grown via pulsed-laser deposition. The films had magnetic ordering temperatures (T_C) ranging from 150 to 350 K; all samples displayed a large negative MR that is largest near T_c. The magnitude of a given sample's MR at T_c inversely correlates with T_c; samples with a low T_c display significantly larger MR values than do samples with large T_c's. The quantity ρ(T_c)/ρ(4 K), the amount by which the resistivity is reduced by full ferromagnetic order, is an activated function of T_c with an activation energy E_a = 0.1 eV. These results indicate that the magnitude of the CMR effect in a given specimen is controlled not by ρ(T_c), but by T_c via the ratio ρ(T_c)/ρ(4 K). Phenomenological scaling relationships are also reported that link ρ(H,T) to both H and M(H, T)

[en] Doping studies are presented on three materials exhibiting hybridization gaps: Ce₃Bi₄Pt₃, U₃Sb₄Pt₃ and CeRhSb. In the case of trivalent La, Y, or Lu substituting for Ce or U, there is a suppression of the low temperature gap and an increase in the electronic specific heat, γ. In the case of tetravalent Th substitutions for U there is no change in γ and in the case of tetravalent Zr substitution for Ce in CeRhSb, there is an enhanced semiconductor-like behavior in the electrical resistance. These results are discussed in the light of a simple model of hybridization gapped systems. (orig.)

[en] CeRhIn₅ is known to show an unusual transition at a critical pressure of ∼15 kbar. Specific-heat data show a gradual change in the zero-field 'magnetic' specific-heat anomaly from one typical of antiferromagnetic (AF) ordering at ambient pressure to one more characteristic of a Kondo singlet ground state at 21 kbar. However, at 15 kbar there is a discontinuous change from an AF ground state to a superconducting ground state, and evidence of a weak thermodynamic first-order transition. Above the critical pressure, the low-energy excitations are characteristic of superconductivity with line nodes in the energy gap, and, at intermediate pressures, of extended gaplessness

[en] Measurements of magnetic susceptibility χ(T), specific heat C(T), Hall coefficient R_H(T), and Yb valence ν = 2 + n_f [f-occupation number n_f (T) determined from Yb L₃ x-ray absorption measurements] were carried out on single crystals of Yb_1-xLu_xAl₃. The low temperature anomalies observed in χ(T) and C(T) corresponding to an energy scale T_coh ∼ 40 K in the intermediate valence, Kondo lattice compound YbAl₃ are suppressed by Lu concentrations as small as 5% suggesting these low-T anomalies are extremely sensitive to disorder and, therefore, are a true coherence effect. By comparing the temperature dependence of various physical quantities to the predictions of the Anderson Impurity Model, the slow crossover behavior observed in YbAl₃, in which the data evolve from a low-temperature coherent, Fermi-liquid regime to a high temperature local moment regime more gradually than predicted by the Anderson Impurity Model, appears to evolve to fast crossover behavior at x ∼ 0.7 where the evolution is more rapid than predicted. These two phenomena found in Yb_1-xLu_xAl₃, i.e., the low-T anomalies and the slow/fast crossover behavior are discussed in relation to recent theories of the Anderson lattice

[en] The anisotropic electrical resistivity rho and thermoelectric power S have been determined on single crystals of La₂CuO/sub 4+//sub δ/ in which the hole doping δ has been varied by controlled anneals. An enormous decrease and hysteresis in both rho and S occur at the Neel temperature T/sub N/ for small δ, suggesting strong coupling between magnetic order and transport. Possible origins of these transport anomalies are discussed

[en] In this paper we examine the physical properties of the f-electron system Ce₃Bi₄Pt₃. In the majority of cases f-moment compounds are metallic, and, depending upon the relative strength of competing magnetic interactions, have a superconducting, paramagnetic, or antiferromagnetic ground state. In rare instances f-electron compounds have a narrow-gapped insulating ground state instead. This gap is thought to result from hybridization between conduction and f electrons. We have measured the magnetic susceptibility, low-temperature specific heat, resistivity, Hall effect, and thermoelectric power of Ce₃Bi₄Pt₃ and find that this material is non-metallic at all temperatures. Isostructural lanthanum substitution for cerium suppresses the gap and produces transport and thermodynamic behavior that is characteristic of a relatively heavy Kondo impurity system. Further, the energy scale at which coherence develops is roughly equal to the energy gap in this compound. This suggests that the gap in Ce₃Bi₄Pt₃ appears to stem from the same requirements of lattice periodicity necessary for coherence to manifest itself in metallic systems. Hence, there is strong evidence that the insulating behavior displayed by Ce₃Bi₄Pt₃ results form the presence of a Kondo coherence-induced gap. 15 refs., 4 figs

[en] Short communication

[en] The electronic transport properties of YbBiPt are examined in the temperature range 4 - 325 K to gain insight into the electronic structure of this compound. The magnitude of the strongly magnetic-field-dependent Hall effect is consistent with a semimetallic electronic structure. The data place an upper limit on the carrier concentration of 6x10²⁰holes/cm³. The thermoelectric power (TEP) is dominated by a large phonon-drag peak below 100 K, while above 200 K the TEP is dominated by a large linear-in-temperature contribution. The temperature-dependent resistivity manifests very little indication of a magnetic scattering contribution. These characteristics indicate the YbBiPt is a low-carrier concentration semimetal. It appears that this low-carrier conducting state is closely tied to the massively renormalized electronic state evident in this compound below 1 K. copyright 1997 The American Physical Society

[en] Growth of LaMnO₃ films that exhibit colossal magnetoresistance (CMR) has concentrated heavily on Ca doped materials. However, since the 33% Sr doped films are ferromagnetic at room temperature, they are ideal candidates for dual growth-magnetic structure studies using scanned probe techniques. In this study, interest was focused on the relations between growth/processing parameters, film morphology, and electronic/magnetic properties. In addition, films were grown on both LaAlO₃ (LAO) and SrTiO₃ (STO) to examine the results of stress induced by different substrate mismatches. La_0.67Sr_0.33MnO₃ (LSMO) was grown using pulsed laser deposition (PLD) at temperatures between 500 C and 800 C. The film microstructure, crystallinity, and magnetic and electrical properties were characterized by room temperature scanning tunneling microscopy (STM), atomic force microscopy (AFM), magnetic force microscopy (MFM), x-ray diffraction, and temperature dependent transport and magnetization measurements. The growth trends follow those previously reported for Ca doped films. Grains increase in size with increasing temperature and coalesce into extended layers after annealing. Although topographic contributions complicate interpretation of some MFM data, local magnetic structure observed here is generally associated with film defects

[en] We report electrical-resistivity, Hall-effect, and thermoelectric-power (TEP) measurements on the superconducting oxide La/sub 1.85/Sr/sub 0.15/CuO₄. The transport properties indicate that in the normal state the material conducts via holes with a carrier concentration of 6.0 x 10/sup 21/ cm/sup -3/. Evidence is observed for unusual phonon-drag effects above the superconducting transition temperature T/sub c/ and granular or inhomogeneous superconductivity in the vicinity of T/sub c/. Below T/sub c/ the resistivity, Hall constant, and TEP all drop to zero, as expected for a superconducting ground state