The density of states N(0) of A_3C_60 at the Fermi level can be estimated
from various types of experiments. The value deduced from the specific heat and
NMR measurements is 5-6 and 7 states/(eV spin), respectively, whereas the
estimate obtained form the measured susceptibility -- without taking into
account many-body effects -- is 10-16 states/(eV spin). To understand this
discrepancy, we have calculated the Pauli susceptibility using a fixed-node
diffusion Monte-Carlo method. To describe A_3C_60 we use a multi-band
Hubbard model, including the three-fold degenerate t_1u derived band and
the on-site Coulomb interaction. We find that the many-body enhancement-factor
of the Pauli susceptibility is of the order of three. This reconciles the
different experimental estimates of N(0). The large enhancement suggests that
A_3C_60 is in a strongly correlated metallic state, which is in line with
the recent finding that these systems are close to a Mott transition.(
O.~Gunnarsson, E.~Koch, and R.M.~Martin, Phys.~Rev.~B 54), R11026 (1996)
and \tt cond-mat/9608107
[K25.02] Rotational Excitations of Interstitial Hydrogen in C_60
Stephen FitzGerald, Lou Santodonato, John Copley, Taner Yildirim, Dan Neumann, John Rush (NIST Gaithersburg, MD)
The fcc lattice of solid C_60 has large interstitial sites which can accommodate various atomic and molecular species. Analyzing the structure and dynamics of small molecules placed in these sites provides information about the trapped molecules and the intermolecular potential of C_60. We report a neutron diffraction study which demonstrates that hydrogen molecules adsorbed into the C_60 lattice sit exclusively at the octahedral sites. Modest loading pressures on the order of 100 bar are sufficient to fill roughly 40% of the available sites. Inelastic scattering results taken on a neutron time-of-flight spectrometer clearly show the ortho to para conversion of the interstitial hydrogen, which occurs via a transition from the J=1 to J= 0 rotational levels. The transition occurs at 14.2 meV slightly shifted in energy from that of free hydrogen at 14.7 meV. As expected measurements performed with interstitial D_2 show the same transition reduced by a fact! or of two in energy but the transi
[K25.03] Diffusion kinetics of Ne in C_60 measured by neutron powder diffraction
Z. Hu, J.D. Jorgensen, S. Short (Materials Science Division, Argonne National Laboratory), B. Morosin, J.E. Schirber (Sandia National Laboratories), G.H. Kwei (Lawrence Livermore National Laboratory)
In situ high-pressure neutron powder diffraction has been used to probe the Ne content and diffusion kinetics in C_60 by monitoring the C_60 lattice parameter vs. time following sudden changes of the applied Ne pressure. In the face-centered phase where the C_60 balls are rotating (below 2.4 kbar at room temperature), diffusion is slower for higher Ne pressures. In the primitive phase where the C_60 balls do not rotate (above 3.4 kbar at room temperature), the diffusion rate is slower, but shows little pressure dependence. These observations can be understood in terms of three competing effects: Increasing the external Ne pressure increases the driving force for diffusion; (2) Compression of the C_60 lattice slows diffusion; and, (3) C_60 ball rotation enhances diffusion in the face-centered phase by a "paddle wheel" effect, but pressure slows the rotation rate and, thus, slows diffusion.
[K25.04] Variable Valence and Superconductivity in Fullerides
Taner Yildirim (University of Maryland, College Park, MD 20742, and National Institute of Standards and Technology, Gaithersburg, MD 20899)
Superconductivity in the fullerides has been exciting and controversial problem. The initial rigid band arguments predict metallic behavior for alkali fullerides M_xC_60 with any valence x between 0 and 6. However it was quickly realized that definite phases exist only for special integral values of x and only one of these (x=3, M_3C_60) is metallic and superconducting. There followed a number of theories based on strong correlation effects, lattice symmetry, Jahn-Teller distortions, etc to explain the insulating ground states for x\ne 3--phases, e.g. M_4C_60. Some of these theories also predicted M_3C_60 to be insulating, leading to the argument that M_3C_60 is metallic only because it is accidentally underdoped. It is now believed in some quarters that fulleride superconductors fall into the same category as the cuprates-- low electron density, highly correlated metals. To resolve these issues there are two distinct but related questions to be answered;
Does superconductivity occur at x different from 3 in the fcc fullerides? and
(2) Is non--integer electron concentration essential for superconductivity?
In this talk we will try to answer these questions by x-ray, Raman, ESR, magnetization, and NMR studies of two new families of fullerides; Na_2Cs_xC_60 (T. Yildirim et al), Phys. Rev. Lett. 77, 167 (1996). and Ba_2M_xC_60, BaMCsC_60 (T. Yildirim et al), Phys. Rev. B 54, 11981 (1996). which at last allow us to address the correlation between superconductivity and molecular valence while maintaining the crystal structure of M_3C_60 superconductors.
\bigskip This work was done in close collaborations with L. Barbedette and J. E. Fischer (Univ. of Pennsylvania), C. Goze and P.W. Stephens (BNL and SUNY Stony Brook), C. L. Lin (Temple University), J. Robert and P. Petit (CNRS-ULP, France), and T.T.M. Palstra (Univ. of Groningen, The Netherlands).
[K25.05] Analysis of Superconducting Parameters from Tunneling Measurements on K_3C_60/Al_2O_3/Al and Rb_3C_60/Al_2O_3/Al Junctions
J. R. Ostrick, L. M. Merchant, F. Hellman, R. C. Dynes (Department of Physics, University of California at San Diego, La Jolla, Ca. 92093)
Tunneling and transport measurements on K_3C_60 and Rb_3C_60 have been performed. The tunnel junctions are made by sequentially depositing C_60 and K or Rb onto a pre-fabricated Al_2O_3/Al structure in a UHV deposition system. In both K_3C_60 and Rb_3C_60, the tunneling measurements show evidence for strong electron-phonon coupling. From the measured conductance, we have numerically inverted the zero-temperature Eliashberg equations in order to extract the gap function, \Delta(ømega), the spectral function, \alpha^2F(ømega), and the Coulomb pseudo-potential, \mu^*. The results and implications of these calculations will be discussed. Research is supported by ATamp;T Ph.D Scholarship Program and NSF Grants DMR-9113631 and DMR-9208599.
[K25.06] Superconductivity in ternary fullerides A_3Ba_3C_60 (A=K and Rb)
Y. Iwasa, M. Kawaguchi, T. Mitani (JAIST, Ishikawa 923-12, Japan), N. Wada, T. Hasegawa, K. Ishii, H. Suematsu (University of Tokyo, Tokyo, Japan), Y. Murakami (KEK PF, Tsukuba 305, Japan)
We report superconductivity of ternary fullerides A_3Ba_3C_60 synthesized by intercalation of alkali metals A to Ba_3C_60. A naive electron counting indicates that C_60 molecules are reduced to 9- state, which corresponds to the half-filling of the LUMO+1 conduction band. Both K_3Ba_3C_60 and Rb_3Ba_3C_60 have bcc unit cells with the latice parameters of 11.24 Åand 11.34 Årespectively. T_c was 5.6K and 2.0K for K and Rb compounds, respectively. Decrease of T_c with increasing the lattice parameter in the A_3Ba_3C_60 superconductors differs from the well-known trend in the conventional A_3C_60-type superconductors.
[K25.07] Divalent Yb Cations and Distorted, Inequivalently-Charged C_60 Anions in Yb_2.75C_60
P. H. Citrin, S. Schuppler, E. Ozdas, A. R. Kortan, K. B. Lyons (Bell Laboratories, Murray Hill, NJ)
Near-edge x-ray absorption measurements from Yb_2.75C_60 establish that the Yb cations are exclusively divalent, meaning that the average negative formal charge of the C_60 anions is 5.5. Extended x-ray absorption fine structure and Raman measurements also reveal that the C_60 anions are distorted in shape, despite their well ordered local structure around Yb. Together, these findings are used to show that the charge transferred from Yb to C_60 is both anisotropically distributed on the anions and inequivalently partitioned among them within the unit cell. The inhomogeneous charge distribution stems from the partially covalent interactions between Yb and C_60, a situation similar to (albeit weaker than) that between Pt and C_60 in [(C_6H_5)_3P]_2Pt-C_60.The inequivalent anionic charge states stem from the three crystallographically distinct types of C_60 anions surrounded by 8, 10, or 12 Yb cations, a situation similar to (and more pronounced than) the inequivalent O anions in YBa_2Cu_3O_7. Comparisons are made with analogous metal-doped fullerides and organometallic compounds, which provide new insight into the distinctive structure of Yb_2.75C_60 and its unique distribution of anion charge.
More generally, our results are shown to have implications to all other alkaline- and
rare-earth (i.e., divalent-metal) fullerides.
[K25.08] Structural Energetics of Yb_2.75C_60: Charge Transfer and Local Distortions
Karin M. Rabe (Yale University, New Haven, CT 06520-8284), P. H. Citrin (Bell Laboratories, Murray Hill, NJ 07974)
The crystal structure of Yb_2.75C_60 is closely related to that of the well-known superconducting K_3C_60, but with an ordered cationic vacancy sublattice and large displacements of the Yb cations from their ideal sites.(E. Ozdas et al., Nature 375), 126 (1995). This structure is unique among known fullerene compounds in that there are crystallographically inequivalent C_60 molecules with distinct nearest-neighbor cation environments. Recent EXAFS measurements(P. H. Citrin et al., unpublished.) indicate that the C_60 molecules are also significantly distorted. Using a simple electrostatic model of Yb^2+ point cations and C_60 anions as classical charged spheres with an average charge of -5.5, we find that the structural energy is minimized by charge transfer among the inequivalent C_60 anions. The result is three different charge states of -6.2, -5.5, and -4.8. Refinements of this simple model, including charge-state- and local-field-induced distortions of the C_60 molecules and covalent bonding to the nearest-neighbor Yb cations, will be discussed. Within the model, the experimental structural data and the reasons for the stability of this structure in the Yb-C_60 system will be analyzed.
[K25.09] Carbon Isotope Effect in Rb_3C_60
M. S. Fuhrer, K. Cherrey, A. Zettl (Department of Physics, University of California at Berkeley, and Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.)
We have synthesized single crystals of isotopically enriched Rb_3C_60. The very sharp superconducting transitions in single crystal superconducting fullerides allow us to determine the isotope effect on T_c very accurately. We find that the carbon isotope shift for 99% ^13C substitution is significantly smaller than other values reported in the literature. This, coupled with the near-zero rubidium isotope effect reported by B. Burk et. al.(B. Burk, V. H. Crespi, A. Zettl and M. L. Cohen, PRL 72, 3706 (1994).) , should place considerable restraints on any theoretical model of superconductivity in the alkali fullerides. Further experiments to investigate the anomalous isotope shift reported by C.-C. Chen et. al.(Chia-Chun Chen and C. M. Lieber, Science 259, 655 (1993).) for isotopically disordered systems will be discussed.
[K25.10] Density of States at Fermi Level in SC and FCC A_3C_60
Katsumi Tanigaki (Fundamental Research Laboratories, NEC Corporation, 34 Miyukigaoka Tsukuba 305, Japan), Ichiro Hirosawa (NEC Fundamental Research Lab, Tsukuba 305, Japan), Kosmas Prassides (University of Sussex, Falmer, Brighton, U. K)
The rotational order and disorder of C_60 molecules may give an influence on the density of states at the Fermi level (N_Ef) in A_3C_60 (here A denotes alkali-metals and their mixtures) fullerdies. This has been one of the important special issues of the C_60 related compounds and debated in recent years. A series of Na_2A'_1-xC_60 (A'=K, Rb and Cs; x=0-1) fullerides show such a phase transition from disordered high-temperature fcc to low-temperature ordered sc among the ^1A_3C_60's so far synthesized, and can be used for this research purpose. We have attempted to study this C_60-correlation effect on N_Ef from the magnetic susceptibility point of view by SQUID using Na_2CsC_60, since this compound is the most stable phase in the series. The measured spin susceptibility, estimated by substracting the value at themagnetic field of 2T from that at 4T in order to cancell out existing ferro-magnetic contributions, shows that N_Ef is slightly larger (by around 5%) in fcc than that in sc taking into account the correction of lattice jump occurring at this transition. This is in agreement with previous theoretical expectations.
[K25.11] Mott-Hubbard transition in integer-doped Fullerides
Erik Koch (Department of Physics, University of Illinois at Urbana-Champaign), Olle Gunnarsson (Max-Planck-Institut für Festkörperforschung, Stuttgart), Richard M. Martin (Department of Physics, University of Illinois at Urbana-Champaign)
In a tight-binding representation, the doped Fullerides can be described by a multiband Hubbard model. Considering only the partly occupied three-fold degenerate t_1u band, the Hamiltonian comprises three degenerate orbitals per site. This orbital degeneracy shifts the critical ratio U_c/W at which the Mott-Hubbard transition occurs towards larger values.( O.~Gunnarsson, E.~Koch, and R.M.~Martin, Phys.~Rev.~B 54), R11026 (1996) and \tt cond-mat/9608107 Prompted by the synthesis of isostructural families of integer doped Fullerides with fcc structure, we investigate the Mott transition in these systems at integer fillings not equal to three (half-filling). The calculations are done by fixed-node Diffusion Monte-Carlo, using a trial function which permits to systematically vary the magnetic character of the system.