K. Holczer

Organic molecular soft ferromagnetism in a fullerene C60

The properties of an organic molecular ferromagnet [C60TDAE0.86; TDAE is tetrakis(dimethylamino)ethylene] with a Curie temperature ;Tc = 16.1 kelvin are described. The ferromagnetic state shows no remanence, and the temperature dependence of the magnetization below ;Tc does not follow the behavior expected of a conventional ferromagnet. These results are interpreted as a reflection of a three-dimensional system leading to a soft ferromagnet.

Alkali-Fulleride Superconductors: Synthesis, Composition, and Diamagnetic Shielding

The recent report of a superconductivity onset near the critical temperature {Tc} = 18 K in potassium-doped C{sub 60} raises questions concerning the composition and stability of the superconducting phase. The effects of mixing and heat treatment of K{sub chi}C{sub 60} samples prepared over a wide range of initial compositions on the superconducting transition was determined from shielding diamagnetism measurements. A single superconducting phase ({Tc} = 19.3 K) occurs for which the composition is K{sub 3}C{sub 60}. The shielding reaches a maximum of greater than 40% of the perfect diamagnetism, a high value for a powder sample, in samples prepared from 3:1 mixtures. A Rb{sub chi}C{sub 60} sample prepared and analyzed in an analogous way exhibited evidence for superconductivity with {Tc} {equals} 30 K and a diamagnetic shielding of 7% could be obtained.

Pressure Dependence of Superconductivity in Single-Phase K3C60

The superconducting compound K3C60 (with transition temperature Tc = 19.3 kelvin at ambient pressure), formed as a single phase by reaction of alkali vapor with solids of the icosahedral C60 molecule (buckminsterfullerene), shows a very large decrease of Tc with increasing pressure. Susceptibility measurements on sintered pellets showing bulk superconductivity are reported up to 21 kilobars of pressure, where Tc is already less than 8 kelvin. The results are consistent with a piling up of the density of states at the Fermi level.

Superconducting and normal state properties of the A3C60 compounds

This report is a critical review of the measurements and their interpretations of the normal and superconducting state of the A3C60 compounds, where A = alkali atom. These compounds are highly ionic [A+]3 · [C60]3− and form fcc lattices (cryolite structure) which locate the C60 icosahedra in sites of local cubic symmetry, thereby preserving the degeneracy of the tlu orbitals, allowing for the formation of a narrow half-filled band of a width comparable to or smaller than the different molecular excitation energies. The Tc -s of the more than a dozen compounds synthesized so far span the range 2–33 K; the variation of Tc with pressure and from material to material is assessed as an empirical Tc lattice parameter relation, suggestive that the attraction responsible for the Cooper pair formation is a local property of the C60 molecules and variations of the density of state ρ(ϵf) at the Fermi level (i.e., band width) determine Tc. The superconducting parameters, λL and ξ0 determined from critical field and μSR measurements, favoring a local pairing image, are marginally supportive of the expected density of state variations. The so far available 13C nuclear relaxation, susceptibility and ESR measurements in the normal state manifest several features more related to the complex correlated nature of the C60 molecules than free electron band effects of the simple lattice they are arranged in. The paper emphasizes these unusual characteristics.

Microwave cavity perturbation technique: Part II: Experimental scheme

In this paper, the second in a three part series, we describe an experimental scheme used to measure the electrodynamical response of a material in the millimeter wave range of frequency. In particular, with this technique we can directly evaluate the complex conductivity from a measurement ofboth the bandwidth and characteristic frequency of a resonator containing the specimen. We will describe in detail all the technical improvements achieved which provide the required accuracy.

Collisional probes and possible structures of La2C80

Abstract The soluble molecule La2C80 recently reported by Alvarez et al. exhibits a robustness similar to other fullerenes. It survives intact for ≈ 10−6s following impact against solid surfaces Si(100) at up to 200 eV, yielding an estimate Ef>4 eV for fragmentation processes. The high threshold for electron emission suggests that the electron affinity in La2C−80 is significantly larger than in C60. These facts seem to rule out an external site for either La atom and disfavor a network-site interpretation, and thus favor endohedral (encapsulation) sites. Among the possible cages, the Ih-C80 fullerene appears to be highly favorable for the oxidation state (La3+)2C6−80.

Pressure dependence of magnetism in C60TDAE

Abstract We have determined the pressure response of magnetism in C60TDAE, where TDAE is tetrakis (dimethylamino) ethylene, and its temperature dependent susceptibility above and below the magnetic ordering temperature TC=16.1K. The magnetism is depressed very rapidly with applied pressure. We consider several possible interpretations for the magnetic transition and its pressure dependence and conclude that an itinerant-ferromagnet description is most likely.

Defect dependence of the dielectric permeability of Qn(TCNQ)2

Abstract The conductivity and 9.1 GHz dielectric constant of neutron irradiated Qn(TCNQ) 2 single crystals are strongly affected by defect concentrations less than 1 %. This indicates long range electronic coherence of the order of 100 lattice constants in the non-irradiated material for which the coherence length may still be impurity limited. It is suggested that a collective excitation of electrons is responsible for the temperature dependence of the large dielectric constant.

Non-Korringa 13C Nuclear Relaxation in the Normal State of the K3C60 Superconductor

We present 13C NMR results on K3C60 showing that nuclear relaxation is exponential only above 250 K where molecular motion narrows the NMR line. The non-exponential nuclear relaxation at low temperatures is an intrinsic property of the frozen state and is associated with intramolecular variations of the electron spin density distribution. Dipolar coupling to the electron spins at the Fermi level of the carbon pπ-t1u band is identified as the dominant source of 13C relaxation. The measured mean value of (T1T)-1 is determined not only by the total density of states at the Fermi level, but also by the intramolecular electron spin density distribution.

Ohmic and radiation losses in superconducting films

The losses associated with leakage through thin‐film superconductors with film thickness comparable to the skin depth in the normal state, and to the penetration depth in the superconducting state, are described. The method is used to describe the surface resistance of YBa2Cu3O7 films deposited on various substrates.