We present results of temperature and magnetic field dependent resistivity ρ(H,T) and bulk magnetization M(H,T) measurements on post-annealed La0.7Ca0.3MnO3 thin films that were grown via pulsed-laser deposition. Both the resistivity and the anomalously large negative magnetoresistance peak near the ferromagnetic ordering temperature (Tc=250 K), with Δρ/ρ0=−85% at 50 kOe. A clear correlation is found between ρ and M that is described by the phenomenological expression ρ(H,T)∝exp[−M(H,T)/M0]. This correlation reflects the important interplay between transport and magnetism in this system, and suggests that the transport below Tc involves polaron hopping.

Using neutron pair distribution function analysis over the temperature range from 1000 to 15 K, we demonstrate the existence of local polarization and the formation of medium-range, polar nanoregions (PNRs) with local rhombohedral order in a prototypical relaxor ferroelectric Pb(Mg(1/3)Nb(2/3))O3. We estimate the volume fraction of the PNRs as a function of temperature and show that this fraction steadily increases from 0% to a maximum of approximately 30% as the temperature decreases from 650 to 15 K. Below T approximately 200 K the volume fraction of the PNRs becomes significant, and PNRs freeze into the spin-glass-like state.

Zero-field muon-spin-resonance and lower-critical-field data are presented for a wide range of Th concentrations in Uix Th v Bei3, spanning the region where both a superconducting and a second, lowertemperature phase transition are observed. Overall T-x phase boundaries are assigned and discussed according to the nature of the lower phase transition. Arguments for associating the lower phase with a possible magnetic (time-reversal-violating) superconducting state are given.

The temperature and pressure dependence of the thermal displacements and lattice parameters were obtained across the γ → α phase transition of Ce using high-pressure, high-resolution neutron and synchrotron x-ray powder diffraction. The estimated vibrational entropy change per atom in the γ → α phase transition, ∆S γ−α vib ≈ (0.75±0.15)kB, is about half of the total entropy change. The bulk modulus follows a power-law pressure dependence which is well described using the framework of electron-phonon coupling. These results clearly demonstrate the importance of lattice vibrations, in addition to the spin and charge degrees of freedom, for a complete description of the γ → α phase transition in elemental Ce.PACS numbers: 64.70. Kb, 71.27.+a, 61.12.Ld Materials with electrons near the boundary between itinerant and localized behavior continue to present a major theoretical challenge to a complete description of their properties, including multiple phases and anomalous thermodynamics. This is particularly true in the 4f and 5f systems, where this boundary appears to occur in or near the elements Ce and Pu, respectively [1]. In Pu, which possesses five allotropic phases at ambient pressure, a partial localization of some of the five 5f electrons appears necessary to understand the higher temperature phases [2]. Partial localization may also be present in U compounds [3]. Ce metal is in principle simpler, possessing only a single 4f electron, but still displays four different phases at ambient pressure. One of the most interesting and still not completely understood phenomena in Ce is the isostructural (fcc) γ → α phase transition, which involves about 17% volume collapse at room temperature and pressure of roughly 0.8 GPa [4].In the majority of theoretical models [4,5,6,7,8,9, 10] the γ → α transition has been attributed to an instability of the single 4f 1 electron. The earliest models focused on charge instability, while later models dealt with spin instability. The promotional model postulates a transition from 4f 1 5d 1 6s 2 (γ-phase) to 4f 0 5d 2 6s 2 (α-phase), but is inconsistent with the 4f binding energy and the cohesive energies of other 5d 2 6s 2 materials [5]. In the Mott transition (MT) model [5,6] the 4f electron in the γ phase is localized and non-binding, but is itinerant and binding in the lower volume α phase. The energy for the phase transition is provided by the kinetic energy of the itinerant f electron. In the Kondo-volumecollapse (KVC) model [9, 10] the 4f electron is assumed to be localized in both the γ and α phases, and the phase transition is driven by the Kondo spin fluctuation energy and entropy within the context of the single-impurity Anderson model. These early models ignored altogether an explicit treatment of the lattice degrees of freedom; even the lattice entropy is not considered. More recent treatments [8,11,12] include both the lattice and spin entropies, but still do not deal explicitly with the consequences of electron-lattice coupling despite the large volume collapse at t...

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The anisotropy and temperature dependence of the magnetic-field penetration in superconducting UPt3 have been measured by muon spin relaxation. The extrapolated zero-temperature values for the penetration depths parallel and perpendicular to the c axis are Xw =7070 ± 30 A and X± =7820 ± 30 A, respectively. The temperature dependences of Xw and X± are different and can both be accounted for by a superconducting gap function with a line of nodes in the basal plane and axial point nodes.PACS numbers: 74.70.Tx, 74.60.Ec, 76.75.+i There are many experiments which give indirect evidence for anisotropic superconductivity in heavy-fermion systems. These fall into two categories, namely, measurements of transport coefficients or specific heat at low temperatures (70, 12 and measurements showing transitions between different superconducting states. 3,4 Experiments of the first type have shown that the temperature dependence of bulk properties of heavy-fermion superconductors follow power laws at low temperatures as opposed to the exponentially activated behavior characteristic of conventional superconductors. The absence of activated T dependence implies nodes in the gap functions, which can be due to either unconventional pairing in a clean sample or conventional pairing in the presence of magnetic impurities. 5 In spite of the great activity 5 on heavy-fermion superconductivity, the only experiment which by itself resolves this ambiguity in favor of unconventional pairing is a transverse ultrasound measurement on UPt3, 2 where the attenuation was found to rise (from 7=0) by Aa~~T and Aa~~T 3 for sound propagating in the basal plane with polarization parallel and perpendicular to the basal plane, respectively. For conventional gapless superconductors, Aa~ T 2 , independent of polarization direction. Ultrasound attenuation is a nonequilibrium property, and a prerequisite for its interpretation is a detailed understanding of the mean free path for thermally excited quasiparticles. In the present paper, we describe the first demonstration that a static property, namely, the magnetic penetration depth X, of a heavyfermion superconductor has an anisotropic T dependence. Previous measurements of X for several heavyfermion systems showed that X increased from its 7=0 value in proportion to T 2 , but did not address the issue of anisotropy. 6,7 The technique used here, muon spin re-laxation (n + SR), is unique in that it both gives absolute values for X and allows anisotropy to be measured in a single experimental run for a single face of the sample, with no mechanical disturbance of the apparatus. Furthermore, the 4.2-MeV muons used in this experiment penetrate ~ 70 jim into UPt3 and thus probe bulk superconducting material. The important results of our experiment are (i) that X(T=0) is consistent with the London formula where the effective mass is the renormalized (heavy) mass, and (ii) that the T dependence of X is anisotropic in a manner which implies that the superconducting gap function has a line of nodes in the basal p...

We report new zero-field muon spin relaxation and neutron spin echo measurements in ferromagnetic (FM) (La,Ca)MnO3 which suggest at least two spatially separated regions possessing very different Mn-ion spin dynamics. One region displays diffusive relaxation, "critical slowing down" near T(C) and an increasing volume fraction below T(C), suggesting overdamped FM spin waves below T(C). The second region possesses more slowly fluctuating spins, a linewidth independent of q, and a decreasing volume fraction below T(C). The estimated length scale for the inhomogeneity is

to be published in Phys. Rev. Lett., LA-UR-98-3125) X-ray-absorption fine-structure measurements of the local structure in UCu4Pd are described which indicate a probable lattice-disorder origin for non-Fermi-liquid behavior in this material. Short Pd-Cu distances are observed, consistent with 24 ± 3% of the Pd atoms occupying nominally Cu sites. A "Kondo disorder" model, based on the effect on the local Kondo temperature TK of this interchange and some additional bond-length disorder, agrees quantitatively with previous experimental susceptibility data, and therefore also with specific heat and magnetic resonance experiments.