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Nicola Manini

Publications and source records attributed to Nicola Manini.

81 records · Page 5Linked to original sources
arXiv1998

Exact zero-point energy shift in the $e\otimes (n~E)$, $t\otimes (n~H)$ many modes dynamic Jahn-Teller systems at strong coupling

We find the exact semiclassical (strong coupling) zero-point energy shifts applicable to the $e\otimes (n E)$ and $t\otimes (n H)$ dynamic Jahn-Teller problems, for an arbitrary number $n$ of discrete vibrational modes simultaneously coupled to one single electronic level. We also obtain an analytical formula for the frequency of the resulting normal modes, which has an attractive and apparently general Slater-Koster form. The limits of validity of this approach are assessed by comparison with O'Brien's previous effective-mode approach, and with accurate numerical diagonalizations. Numerical values obtained for $t\otimes (n H)$ with $n =8$ and coupling constants appropriate to C$_{60}^-$ are used for this purpose, and are discussed in the context of fullerene.

cond-mat↗
arXiv1997

Berry-Phase and Symmetry of the Ground State in Dynamical Jahn-Teller Systems

We show through selected examples, relevant to the physics of fullerene ions, that the presence of a Berry phase in dynamical Jahn-Teller systems does not guarantee the degeneracy of the ground state, contrary to what previously believed. Moreover, we introduce a new class of Jahn-Teller systems with no Berry phase, as a generalization of the basic icosahedral $H \otimes h$ case.

cond-mat↗
arXiv1996

Surprises in the Orbital Magnetic Moment and g-Factor of the Dynamic Jahn-Teller Ion C_{60}^-

We calculate the magnetic susceptibility and g-factor of the isolated C_{60}^- ion at zero temperature, with a proper treatment of the dynamical Jahn-Teller effect, and of the associated orbital angular momentum, Ham-reduced gyromagnetic ratio, and molecular spin-orbit coupling. A number of surprises emerge. First, the predicted molecular spin-orbit splitting is two orders of magnitude smaller than in the bare carbon atom, due to the large radius of curvature of the molecule. Second, this reduced spin-orbit splitting is comparable to Zeeman energies, for instance, in X-band EPR at 3.39KGauss, and a field dependence of the g-factor is predicted. Third, the orbital gyromagnetic factor is strongly reduced by vibron coupling, and so therefore are the effective weak-field g-factors of all low-lying states. In particular, the ground-state doublet of C_{60}^- is predicted to show a negative g-factor of \sim -0.1.

cond-mat↗
arXiv1995

Phase Diagram of a Model of Correlated Hopping of Electrons in a Lattice of Berry Molecules

The $1D$ phase diagram of a model for correlated hopping of electrons in a lattice of Berry phase molecules is presented. Electrons hop in presence of an extra orbital degree of freedom at each site. This is mimicked as a spin-1 variable whose allowed states depend on the electron occupancy so as to take into account the orbital degeneracies of different molecular occupancies. In the $1D$ case we find that at low electron densities $n<<1$ there is a region with dominant superconducting correlations surviving an additional repulsive on-site interaction $U$ as strong as the bandwidth, $W=4t$. The critical value $U_c$ of $U$ below which superconductivity is found to be dominant decreases with increasing density $n$. For $n=1/2$ we find $U_c/t\approx 1$, whereas at $n=1$ (half-filling) our (less accurate) results are compatible with $U_c/t\approx 0$. For $U>U_c(n)$ and away from half-filling ($n\neq 1$) the system is metallic with dominant $2k_F$ charge density wave (CDW) correlations. At half-filling a charge gap opens for $U>U_c$ and the system becomes an insulator. A spin-gap characterizes the phase-diagram for all densities and for all values of $U$, even in the metallic regime $U>U_c$.

cond-mat↗
arXiv1995

Low lying excitations of the Dynamical Jahn-Teller ions C_60^- and C_60^2-

We compute the vibronic spectrum of gas phase fullerene negative ions C_{60}^{-} and C_{60}^{2-}. We treat accurately the linear dynamical Jahn-Teller of these ions in their ground state. In particular, coupling of the t_{1u} orbital to the eight H_g vibrational modes is handled by exact diagonalizations including up to six vibrons. The resulting spectrum is characterized by large splittings, which should be readily observable spectroscopically. The lowest excitation symmetry is argued to be related to the Berry phase present in this problem.

cond-mat↗
arXiv1994

Anomalous low-energy electron attachment in C_{60}

We propose that thermal electron attachment to C$_{60}$ should occur preferentially in the p-wave channel, following an analysis of the vibron excitation spectrum of C$_{60}^-$. A very simple model based on this idea is shown to account very well for recent attachment data. The unexplained activation energy of $\approx$ 0.26 eV found experimentally is attributed to the p-wave centrifugal barrier.

cond-mat↗
arXiv1994

Electron--Vibron Interactions and Berry Phases in Charged Buckminsterfullerene: Part I

A simple model for electron-vibron interactions on charged buckminsterfullerene C$_{60}^{n-}$, $n=1,\ldots 5$, is solved both at weak and strong couplings. We consider a single $H_g$ vibrational multiplet interacting with $t_{1u}$ electrons. At strong coupling the semiclassical dynamical Jahn-Teller theory is valid. The Jahn-Teller distortions are unimodal for $n$=1,2,4,5 electrons, and bimodal for 3 electrons. The distortions are quantized as rigid body pseudo--rotators which are subject to geometrical Berry phases. These impose ground state degeneracies and dramatically change zero point energies. Exact diagonalization shows that the semiclassical level degeneracies and ordering survive well into the weak coupling regime. At weak coupling, we discover an enhancement factor of $5/2$ for the pair binding energies over their classical values. This has potentially important implications for superconductivity in fullerides, and demonstrates the shortcoming of Migdal--Eliashberg theory for molecular crystals.

cond-mat↗
arXiv1994

Electron--Vibron Interactions in Charged Buckminsterfullerene: Pair Energies and Spectra (Part II)

The ground state energy shifts and excitation spectra of charged buckminsterfullerene C$_{60}^{n-}$, $n=1,\ldots 5$ are calculated. The electron--vibron Hamiltonian of Part I is extended to include all $A_g$ and $H_g$ modes with experimentally determined frequencies and theoretically estimated coupling constants. Complex splitting patterns of $H_g$ vibrational levels are found. Our results are relevant to EPR measurements of spin splittings in C$_{60}^{2-}$ and C$_{60}^{3-}$ in solution. Spectroscopic gas-phase experiments will be of interest for further testing of this theory. As found in Part I, degeneracies in the electron and vibron Hamiltonians give rise to a dynamical Jahn Teller effect, and to a considerable enhancement of the electronic pairing interaction. This helps to overcome repulsive Coulomb interactions and has important implications for superconductivity in K$_3$C$_{60}$ and the insulating state in K$_4$C$_{60}$.

cond-mat↗