R. P. Guertin

UFe4P12 and CeFe4P12: Nonmetallic isotypes of superconducting LaFe4P12

The new compound UFe4P12, which was found to be isostructural to superconducting LaFe4P12 and have a lattice constant of 7.7729 A, is a semiconductor and shows ferromagnetic order below 3.15 K. CeFe4P12 is also a semiconductor, and its magnetic susceptibility is unusually small in comparison to LaFe4P12. The semiconducting behaviors of both UFe4P12 and CeFe4P12 seem anomalous and may arise from strong f‐electron hybridization.

Crystalline electric field and structural effects in f-electron systems

Section 1: Crystal Field and Structural Effects.- Symmetry and Experiment in Magnetism (Invited).- Structural Phase Transitions and Magnetostriction in Metallic Rare-Earth Compounds (Invited).- Pressure Effects and Lattice Parameter Correlations in CEF Split Systems (Invited).- Random Crystal Field Effects in Amorphous Rare-Earth Alloys (Invited).- Section 2: Lattice Effects I.- Low Frequency Dynamics in Concentrated 4f CEF Systems (Invited).- Interactions of Rare-Earth Ions with Phonons (Invited).- New Experimental Evidence for Quadrupolar Interactions in Cubic Rare-Earth Compounds (Invited).- Magnetic Anisotropy and Spin Reorientations in HoxTb1-xFe2, DyxTb1-xFe2 and HoxTbyDy1-x-y Fe2.- Thermal Expansion and Magnetostriction of Mg Single Crystals with Heavy Rare Earth Impurities.- Elastic Moduli of Amorphous Rare-Earth Alloys.- Magnetostriction of Dilute Tb Ions in Several Metallic Matrices.- First Order Transitions and the Magnetic Phase Diagram of CeSb.- Origin of Crystal Field and Magnetoelasticity in Rare Earth Intermetallic Compounds.- Terbium3+ Coupling Parameters and the Phase Diagram of the Singlet Ground State Terbium Monopnictides.- Section 3: Lattice Effects II.- Exchange and Crystal Field Excitations in Rare-Earth Iron and Rare Earth Cobalt Laves-Phase Compounds (Invited).- Soft Modes and Mode Splitting in Paramagnetic TbP (Invited).- Theory of Static and Dynamic Properties of Crystal Field Systems Including Correlation Effects (Invited).- Pressure Effects on the Crystal Field in Rare Earth Chalcogenides and Pnictides.- Excitation Spectrum of Van Vleck Magnets.- H3+ - A Basis for Clustering in Metallic Hydrides.- Determination of Crystal Fields Combining Paramagnetic Anisotropy and Neutron Scattering.- Rare Earth-Iron Exchange Interaction and Crystal Field-Like Terms in Garnets.- Investigation of Crystal Field in PrNi5.- Section 4: Actinides.- Actinide 5f Systems: Experimental Determination of the Magnetic Response Function (Invited).- Neutron Scattering Studies of Uranium Compounds (Invited).- Sources of Anisotropy and Extraordinary Magnetic Ordering in Cerium and Actinide Intermetallics (Invited).- Dispersion of Collective Magnetic Excitons in a Single Crystal of the Actinide Intermetallic UPd3 (Invited).- The Magnetic Susceptibility of 249Bk Metal.- Ordered Quadrupolar State with a Nearly Zero Magnetic Moment in NpO2.- Is the Intermediate Valence State Possible in the Uranium Tellurides?.- Crystal Field Effects in Some Ionic Uranium Compounds.- Transport Properties, Lattice Constants, Susceptibility and Specific Heat of UPd3.- Section 5: Kondo and Intermediate Valence Properties.- Energy Parameters and Crystal Field Effects in the Mixed Valence Problem: Chemically Collapsed Cerium (Invited).- Valence Instabilities as a Source of Actinide System Inconsistencies.- Thermal Properties of Exotic Cerium Compounds.- A Novel Low Temperature Phase Transition in Mixed Valent Sm4Bi3.- Mixed Valence, Crystalline Electric Fields and the Kondo Effect in Ce (Pd,Ag)3.- Low Temperature Properties of (La1-xSmx) Sn3 and CeSn3.- Ground State and Magnetic Responses of Model Valence-Fluctuation Systems.- Section 6: Transport and Thermodynamic Properties.- Multipolar f Electron-Conduction Band Interactions in Rare Earth Compounds (Invited).- Electrical Resistivity Investigations on Metallic Rare Earths (Invited).- Magneto-Transport in F-Electron Systems-Quadrupolar and Orbital Exchange Effects (Invited).- Electrical Resistivity in Pr3T1-an Induced Moment Ferromagnet Near Threshold.- Antiferromagnetic Ordering in ErCu: Resistivity and Specific Heat.- Electrical and Thermal Properties of Magnetically Ordered Metallic Compounds TbZn, TbGd, GdCd.- Kondo Effect in the Presence of Crystalline Electric Fields - Ce (In, Sn)3.- Section 7: Singlet Ground State and Other Properties.- Series Expansion for CEF Split Systems (Invited).- Theory of Singlet-Ground-State Ferromagnetism in Pr3Se4: A Cubic System with Low Local Symmetry.- Van Vleck Orbit-Lattice Interactions in LaZ (Z=P, As, Sb, Bi).- Theory of g-shift and Linewidth in CeP Excited State EPR.- REA12s: A Real and Yet Unresolved Problem.- Electric Field Gradient (Crystal Field) in Amorphous f-Electron Systems.- Magnetic Excitations in Holmium Phosphide.- A General Method for Determining Crystal Field Effects on Magnetic Ions.- Determination of Crystal Field Parameters in ErBe13 by Inelastic Neutron Scattering.- Section 8: Superconductivity.- The Influence of Crystalline Electric Fields on the Superconducting Properties of Rare Earth Alloys and Compounds (Invited).- Superconductivity, Long-Range Magnetic Order, and Crystal Field Effects in RERh4B4 Compounds (Invited).- Crystal Field Effects in Magnetic Superconductors (Invited).- Roles of Crystal Fields in Magnetic Superconducting Rare-Earth Rhodium Borides (Invited).- Crystalline Electric Field Manifestations in Tunneling into Superconductors.- Crystal Field Effects in the Magnetic Susceptibility of Dilute Alloys of LaSn3 Containing Nd Impurities.- Section 9: Lifetime Effects.- Neutron Studies of CEF Excitations and Linewidths of Dilute Systems (Invited).- Linewidth of Quasi-Elastic and Inelastic Crystal Field Excitations in CeAg and CeMg Compounds (Invited).- List of Participants.- Material Index.

Crystalline electric field effects in f-electron magnetism

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RBa2Cu3O7−δ (R = rare earth) high-Tc magnetic superconductors

Electrical resistivity p, magnetization M, and specific heat C measurements have been made on polycrystalline RBa2Cu3O7–δ compounds (R = Y or a rare earth element) which, except for R = Ce, Pr and Tb, exhibit superconductivity with TC ≈ 90K. Measurements of C(T) below Tc reveal (1) a contribution linear in T and Debye and Einstein lattice contributions for R = Y, and (2) electronic Schottky anomalies due to crystalline electric field effects and magnetic ordering for many R ions with partially-filled 4f electron shells. Measurements of ρ(T, H) and M(T, H) yield upper critical field slopes near Tc of ≥ 3T/K and critical current densities ≥3.5 × 104 A/cm2 at 4.2 K in zero field.

DESTRUCTION OF THE MOTT INSULATING GROUND STATE OF CA2RUO4 BY A STRUCTURALTRANSITION

We report a first-order phase transition at T_M=357 K in single crystal Ca_2RuO_4, an isomorph to the superconductor Sr_2RuO_4. The discontinuous decrease in electrical resistivity signals the near destruction of the Mott insulating phase and is triggered by a structural transition from the low temperature orthorhombic to a high temperature tetragonal phase. The magnetic susceptibility, which is temperature dependent but not Curie-like decreases abruptly at TM and becomes less temperature dependent. Unlike most insulator to metal transitions, the system is not magnetically ordered in either phase, though the Mott insulator phase is antiferromagnetic below T_N=110 K.

Magnetic field alignment of high-Tc superconductorsRBa2Cu3O7–δ (R=rare earth)

Finely ground powders ofRBa2Cu3O7 − δ (R=Y, Nd, Sm, Eu, Dy, Ho, Er, Tm, Yb) have been mixed at dilute 3%-by-volume concentrations into epoxy matrices which were then allowed to harden in applied magnetic fieldsHA=18 kOe. X-ray diffractometry studies and 4.3 K measurements of supercurrent-induced magnetization hysteresisΔM are interpreted as indicating at least partial alignment of single-crystal-grain c-axes (1) parallel toHA forR=Y, Nd, Sm, Dy, Ho (as earlier found by Farrell et al. forR=Y), and (2) perpendicular toHA forR=Eu, Er, Tm, Yb. With a few exceptions (Y, Sm, Eu) the alignment direction correlates with the sign of the second-order Stevens factorαJ of the crystalline electric field Hamiltonian in the manner suggested by Livingston et al. For the best aligned specimens (Ho, Dy) critical current densitiesJc (4.3 K, 5 kOe) for individual grains are estimated fromΔM and the Bean model to be of order 107 A/cm2 for the measuring fieldH parallel to the original alignment fieldHA, and of order 106 A/cm2 forH perpendicular toHA.

d-Hole localization and the suppression of superconductivity in YBa2(Cu1-xZnx)3O7−y

The temperature and Zn concentration dependence of the electrical resistivity, specific heat, magnetic susceptibility, and electron paramagnetic resonance (EPR) spectra of YBa2(Cu1−xZnx)3O7−y withy∼0.1 has been measured forx≤0.16. In addition, the temperature and field dependence of the magnetization has been measured for 2<T<300K and 0<H<9.0T, along with the temperature and quasihydrostatic pressure dependence of the electrical resistivity for selected samples for 0<P<13 GPa. The substitution of Zn for Cu in YBa2Cu3O7−y causes a rapid and nearly linear depression of the superconducting transition temperature,Tc, withTc going to 0 K forx≥ 0.10. YBa2(Cu1−xZnx)3O7−y retains the YBa2Cu3O7-y orthorhombic structure forx≤0.16 for both the superconducting and nonsuperconducting samples. Initially, the unit cell volume increases nearly linearly with Zn content; however, an abrupt change occurs in the vicinityx=0.8–0.10. Forx<0.10, the temperature dependence of the electrical resistivity,ρ(T), is metallic-like (dρ/dT>0) andρ increases gradually with increasing Zn content. However, forx≥ 0.10,ρ(T) becomes semiconductor-like, with a very rapid increase of the resistivity with increasingx. The electrical resistivity, magnetic susceptibility, EPR spectra, and specific heat all indicate that thed-holes associated with the Cu ions become localized in the nonsuperconducting phase,x>-0.10.

Low temperature specific heat and magnetoresistance of the ferromagnetic semiconductor UFe4P12

Abstract Low temperature (T) specific heat and magnetoresistance measurements have been performed on the ferromagnetic semiconducting compound UFe4P12 in the vicinity of its Curie temperature Tc=3.15 K. The specific heat data reveal a pronounced peak near Tc. The magnetoresistance above Tc is large and negative, whereas near Tc, it is initially positive, passes through a maximum, then decreases and becomes large and negative.

Normal and superconducting state magnetic properties of RBa2Cu3O7−δ compounds

Abstract Magnetic properties have been investigated for polycrystalline samples of RBa 2 Cu 2 O 7−δ with R = Y and all the rare earth elements except Pm. The normal-state magnetic susceptibilities of these compounds can be described as χ ( T )= χ 0 + Nμ 2 eff /[3 k b ( T − ϑ )], with μ eff comparable to the values of free R 3+ ions. The superconducting-state magnetizations for all measured superconducting compounds are very different for different R while the field and temperature dependence of critical current densities J c ( H , T ) inferred from the hysteresis of M ( H ) isotherms at different temperatures are similar. J c decreases with H , rapidly in low fields and slowly in high fields, and decreases by two orders of magnitude as the temperatures increase from 4.2 to 65 K. At 4.2 K and 180 kOe, the critical current densities of Gd, Yb, and Tm compounds range from 1.6 to 17 kA/cm 2 .

Extraordinary behaviour of the Y1−xPrxBa2Cu3O7−δ system

Investigations of Y1−xMxBa2Cu3O7−δ (M=Ce, Th)c-axis oriented thin film specimens show that the rate of depression ofTc withx is larger for M=Th, than for M=Ce and Pr, and suggest that Ce, like Th, is tetravalent in this compound. Hall effect measurements on Y1−xPrxBa2Cu3O7−δ single crystals reveal aT2 dependence of the cotangent of the Hall angle in the normal state and a “negative Hall anomaly” belowTc in the superconducting state, in agreement with recent reports. Our research shows that the depth, Δ, of the negative Hall signal scales withT/Tc and that the maximum value of Δ decreases linearly withx and vanishes atx≈0.24. Magnetoresistance measurements on Y1−xPrxBa2Cu3O7−δ single crystals indicate that the irreversibility lineH(T*) obeys a universal scaling relation characterized by anm=3/2 power law nearTc, with a crossover to a more rapid temperature dependence of belowT/Tc≈0.6, similar to that observed for polycrystalline specimens.