Jahn-Teller Spectral Fingerprint in Molecular Photoemission: C60

The h_u hole spectral intensity for C60 -> C60+ molecular photoemission is calculated at finite temperature by a parameter-free Lanczos diagonalization of the electron-vibration Hamiltonian, including the full 8 H_g, 6 G_g, and 2 A_g mode couplings. The computed spectrum at 800 K is in striking agreement with gas-phase data. The energy separation of the first main shoulder from the main photoemission peak, 230 meV in C60, is shown to measure directly and rather generally the strength of the final-state Jahn-Teller coupling.Comment: 5 pages, 3 figure

Low-energy excitations of a linearly Jahn-Teller coupled orbital quintet

The low-energy spectra of the single-mode h x (G+H) linear Jahn-Teller model is studied by means of exact diagonalization. Both eigenenergies and photoemission spectral intensities are computed. These spectra are useful to understand the vibronic dynamics of icosahedral clusters with partly filled orbital quintet molecular shells, for example C60 positive ions.Comment: 14 pages revte

The Phase Diagram of Correlated Electrons in a Lattice of Berry Molecules

A model for correlated electrons in a lattice with local additional spin--1 degrees of freedom inducing constrained hopping, is studied both in the low density limit and at quarter filling. We show that in both 1D and 2D two particles form a bound state even in presence of a repulsive U<U_c. A picture of a dilute Bose gas, leading to off-diagonal long range order (LRO) in 2D (quasi-LRO in 1D), is supported by quantitative calculations in 1D which allow for a determination of the phase diagram.Comment: 7 pages + 2 ps figures, published versio

Simulation and Comparison of Various Gamma-Ray Imaging Detector Configurations for IPRL Devices

Simulations are performed for seven different geometrical configurations of CdZnTe (CZT) detector arrays for Intelligent Personal Radiation Locator (IPRL) devices. IPRL devices are portable radiation detectors that have gamma-ray imaging capability. The detector performance is analyzed for each type of IPRL configuration, and the intrinsic photopeak efficiency, intrinsic photopeak count rate, detector image resolution, imaging efficiency, and imaging count rate are determined

Simulated Performance of a Second-Generation Compact Compton Imaging Detector

Simulations are performed using GEANT4 of a second-generation compact Compton imaging detector called CCI2 which uses silicon and germanium detector crystals. Realistic simulated detector geometry and realistic detector parameters are used. Results are obtained for the CCI2 detector for the intrinsic photopeak efficiency, imaging efficiency, and angular resolution, and simulated images are created for point sources with various energies and source angles

Dynamical Jahn-Teller Effect and Berry Phase in Positively Charged Fullerene I. Basic Considerations

We study the Jahn-Teller effect of positive fullerene ions $^2$C$_{60}^{+}$ and $^1$C$_{60}^{2+}$. The aim is to discover if this case, in analogy with the negative ion, possesses a Berry phase or not, and what are the consequences on dynamical Jahn-Teller quantization. Working in the linear and spherical approximation, we find no Berry phase in $^1$C$_{60}^{2+}$, and presence/absence of Berry phase for coupling of one $L=2$ hole to an $L=4$/$L=2$ vibration. We study in particular the special equal-coupling case ($g_2=g_4$), which is reduced to the motion of a particle on a 5-dimensional sphere. In the icosahedral molecule, the final outcome assesses the presence/absence of a Berry phase of $\pi$ for the $h_u$ hole coupled to $G_g$/$H_h$ vibrations. Some qualitative consequences on ground-state symmetry, low-lying excitations, and electron emission from C$_{60}$ are spelled out.Comment: 31 pages (RevTeX), 3 Postscript figures (uuencoded

Enhanced Electron Pairing in a Lattice of Berry Phase Molecules

We show that electron hopping in a lattice of molecules possessing a Berry phase naturally leads to pairing. Our building block is a simple molecular site model inspired by C$_{60}$, but realized in closer similarity with Na$_3$. In the resulting model electron hopping must be accompanied by orbital operators, whose function is to switch on and off the Berry phase as the electron number changes. The effective hamiltonians (electron-rotor and electron-pseudospin) obtained in this way are then shown to exhibit a strong pairing phenomenon, by means of 1D linear chain case studies. This emerges naturally from numerical studies of small $N$-site rings, as well as from a BCS-like mean-field theory formulation. The pairing may be explained as resulting from the exchange of singlet pairs of orbital excitations, and is intimately connected with the extra degeneracy implied by the Berry phase when the electron number is odd. The relevance of this model to fullerides, to other molecular superconductors, as well as to present and future experiments, is discussed.Comment: 30 pages, RevTe

The role of the Berry Phase in Dynamical Jahn-Teller Systems

The presence/absence of a Berry phase depends on the topology of the manifold of dynamical Jahn-Teller potential minima. We describe in detail the relation between these topological properties and the way the lowest two adiabatic potential surfaces get locally degenerate. We illustrate our arguments through spherical generalizations of the linear T x h and H x h cases, relevant for the physics of fullerene ions. Our analysis allows us to classify all the spherical Jahn-Teller systems with respect to the Berry phase. Its absence can, but does not necessarily, lead to a nondegenerate ground state.Comment: revtex 7 pages, 2 eps figures include