A. P. Ramirez

Defects in carbon nanostructures

Previous high-resolution electron microscopy (HREM) observations of the carbon nanotubes have led to a Russian doll structural model that is based on hollow concentric cylinders capped at both ends. The structures of the carbon nanotubes and particles were characterized here by bulk physical and chemical property measurements. The individual nanostructure is as compressible as graphite in the c axis, and such nanostructures can be intercalated with potassium and rubidium, leading to a saturation composition of MC8. These results are counter to expectations that are based on a Russian doll structure. HREM after intercalation with potassium and deintercalation indicates that individual nanoparticles are a paper-mache of smaller graphite layers. Direct current magnetization and electron spin resonance measurements indicate that the electronic properties of the nanostructures are distinctly different from those of graphite. Although the nanostructures have distinct morphologies and electronic properties, they are highly defective and have a local structure similar to turbostratic graphite.

Synthesis and characterization of alkali metal fullerides: AxC60

Abstract Alkali metal fullerides (A x C 60 ) are a subject of considerable current interest because of the occurrence of superconductivity for A x C 60 at temperatures surpassed only by the high T c copper oxides. The preparation and characterization of A x C 60 (A = alkali metal, x = 2,3,4,6) by powder X-ray diffraction, NMR ( 13 C, 23 Na and 87 Rb), and d.c. magnetization are reported. The structures are described as intercalation compounds of the FCC structure of pristine c 60 or of hypothetical BCC or BCT structures. The structures and phase diagrams can be rationalized on the basis of ion size and electrostatic considerations. Only the A 3 C 60 compounds are metallic (and superconducting). The superconducting T c increases nearly linearly with unit cell size. EHT (Extended Huckel Theory) calculations and 13 C NMR relaxation measurements indicate higher densities of states for the higher T c compositions.

13C NMR Spectroscopy of KxC60: Phase Separation, Molecular Dynamics, and Metallic Properties

The results of 13C nuclear magnetic resonance (NMR) measurements on alkali fullerides KxC60 are reported. The NMR spectra demonstrate that material with 0 < x < 3 is in fact a two-phase system at equilibrium, with x = 0 and x = 3. NMR lineshapes indicate that C3–60 ions rotate rapidly in the K3C60 phase at 300 K, while C6–60 ions in the insulating K6C60 phase are static on the time scale of the lineshape measurement. The temperature dependence of the 13C spin-lattice relaxation rate in the normal state of K3C60 is found to be characteristic of a metal, indicating the important role of the C3–60 ions in the conductivity. From the relaxation measurements, an estimate of the density of electronic states at the Fermi level is derived.

Superconductivity and heavy fermions

B BATLOGG, D J BISHOP, E BUCHER, B GOLDING, Jr, A P RAMIREZ AT& T Bell Laboratorles, Murray Hill, NJ 07974, USA Z FISK, J L SMITH Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos NM 87545 USA and H R OTT Laboratonum fur FestkorperphysIk, ETH-Honggerberg 8093, Zurich, Switzerland Experimental studies are discussed which shed hght on the nature of the heavy fermlon normal state, the occurrence of heavy fermion superconductivity and anomalous properties of the superconducting state l) Magnetization and mag- netoreslstance of the Kondo lattice UBel~ demonstrate the breakdown of a description m terms of a single energy scale TK Here T K becomes temperature dependent below ~ 10 K, and the canomcal relationship between R(H) and M(H) does not hold n) Superconductivity in UPta occurs at the borderline to magnetism spin-density wave-hke instabihtJes are induced by a few percent of Th or Au, whereas mass enhancement and susceptlbihty are rapidly reduced by Ir subsntution This suggests that spin fluctuations, rather than phonons, are ~mportant in medmtmg superconducting pairing of heavy fermions in UPt~ lU) The posstbillhes of noncubic strain induced by amsotropic superconductivity in UBe~ ~ is investigated by ultrasound experiments The suggestion of tetragonal strata in (U Th)Be~ is found to be not applicable, and observed frequency and amplitude dependent absorption anomalies at T,~ T, might reflect domain wall motion

Doping a semiconductor to create an unconventional metal.

Landau–Fermi liquid theory, with its pivotal assertion that electrons in metals can be simply understood as independent particles with effective masses replacing the free electron mass, has been astonishingly successful. This is true despite the Coulomb interactions an electron experiences from the host crystal lattice, lattice defects and the other ∼1022u2009cm-3 electrons. An important extension to the theory accounts for the behaviour of doped semiconductors. Because little in the vast literature on materials contradicts Fermi liquid theory and its extensions, exceptions have attracted great attention, and they include the high-temperature superconductors, silicon-based field-effect transistors that host two-dimensional metals, and certain rare-earth compounds at the threshold of magnetism. The origin of the non-Fermi liquid behaviour in all of these systems remains controversial. Here we report that an entirely different and exceedingly simple class of materials—doped small-bandgap semiconductors near a metal–insulator transition—can also display a non-Fermi liquid state. Remarkably, a modest magnetic field functions as a switch which restores the ordinary disordered Fermi liquid. Our data suggest that we have found a physical realization of the only mathematically rigorous route to a non-Fermi liquid, namely the ‘undercompensated Kondo effect’, where there are too few mobile electrons to compensate for the spins of unpaired electrons localized on impurity atoms.

Magnetic fluctuations and the superconducting transition in the heavy-fermion material UPd2Al3

Abstract Inelastic neutron scattering has been performed on single crystals of the heavy-fermion superconductor UPd 2 Al 3 . The antiferromagnetically ordered state is characterized by an acoustic spin wave mode with no gap. The low-frequency magnitude excitations are unaffected by the transition to a superconducting state despite coupling to the conduction electrons as evidenced by the significant damping.

Low‐temperature specific heat and thermal expansion in the frustrated garnet Gd3Ga5O12

Gd3Ga5O12 is strongly frustrated, possessing a Curie–Weiss temperature of −2.3 K, while not ordering down to 25 mK, in zero field. Specific‐heat and thermal‐expansion measurements were made as a function of temperature (0–2 K) and magnetic field (0–2 T). The unusual phase diagram of this material, reported in the susceptibility measurements of Hov, Bratsberg, and Skjeltorp [J. Magn. Magn. Mater. 15–18, 455 (1980)], is verified by the present thermal behavior.

Broken spin rotation symmetry without magnetic Bragg peaks in Kagomé antiferromagnets

We review experimental results for SrCr 8− x Ga4 4+x O 19 (SCGO(x)), a system which includes Kagome layers of antiferromagnetically (AFM)-coupIed Cr 3+ ions. Even though the thermodynamic and inelastic neutron scattering measurements suggest a broken continuous symmetry, the AFM correlation length remains very short. Consideration of the possible ground states for xy AFMs on Kagome lattices provides a framework for understanding the observations on SCGO( x ).

Magnetothermal properties of UIrGe and systematic trends in magnetic heavy fermion compounds

We have measured the low‐temperature susceptibility and specific heat of the compound UIrGe and find both an antiferromagnetic phase transition at 16 K and a high‐temperature electronic specific heat coefficient of 145 mJ/moleu2009K2. Comparison of this behavior is made with other heavy fermion magnetic systems and a monotonic relationship is found between TN and the difference in the electronic specific heat coefficient γ above and below the transition.We have measured the low‐temperature susceptibility and specific heat of the compound UIrGe and find both an antiferromagnetic phase transition at 16 K and a high‐temperature electronic specific heat coefficient of 145 mJ/moleu2009K2. Comparison of this behavior is made with other heavy fermion magnetic systems and a monotonic relationship is found between TN and the difference in the electronic specific heat coefficient γ above and below the transition.

Soft Manifold Dynamics behind Negative Thermal Expansion

Minimal models are developed to examine the origin of large negative thermal expansion in underconstrained systems. The dynamics of these models reveals how underconstraint can organize a thermodynamically extensive manifold of low-energy modes which not only drives negative thermal expansion but extends across the Brillioun zone. Mixing of twist and translation in the eigenvectors of these modes, for which in ZrW2O8 there is evidence from infrared and neutron scattering measurements, emerges naturally in our model as a signature of the dynamics of underconstraint.