arXiv subjects

Nicola Manini

Publications and source records attributed to Nicola Manini.

At least 73 records · Page 4Linked to original sources

arXiv2006

Theoretical aspects of highly correlated fullerides: metal-insulator transition

We review some theoretical aspects connected with the interplay of strong electron correlations and vibron phenomena in negative fullerene ions and in solid fulleride conductors, superconductors and insulators. The first part covers molecular ions, their intra-molecular Jahn-Teller effect, Coulomb (Hund's rule) exchange interactions, molecular vibrons and multiplet splittings. The second part addresses electron propagation in molecular fullerides, with special emphasis given to trivalent cases such as K3C60 and NH3 K3C60, where metallic, superconducting and Mott insulating phases are at play. Dynamical mean field theory approaches to a simplified Hamiltonian for this system are discussed in the light of some of the observed phenomenology. It is argued in particular that NH3 K3C60 is a Mott-Jahn-Teller insulator, which under pressure turns into a strongly correlated superconductor, thus sharing some important elements with the high-Tc cuprates.

cond-mat.str-el↗

arXiv2005

Enumeration of many-body skeleton diagrams

The many-body dynamics of interacting electrons in condensed matter and quantum chemistry is often studied at the quasiparticle level, where the perturbative diagrammatic series is partially resummed. Based on Hedin's equations for self-energy, polarization, propagator, effective potential, and vertex function in zero dimension of space-time, dressed Feynman (skeleton) diagrams are enumerated. Such diagram counts provide useful basic checks for extensions of the theory for future realistic simulations.

cond-mat.str-el↗

arXiv2005

Condensate Fraction of a Fermi Gas in the BCS-BEC Crossover

We investigate the Bose-Einstein condensation of Fermionic pairs in a uniform two-component Fermi gas obtaining an explicit formula for the condensate density as a function of the chemical potential and the energy gap. We analyze the condensate fraction in the crossover from the Bardeen-Cooper-Schrieffer (BCS) state of weakly-interacting Cooper pairs to the Bose-Einstein Condensate (BEC) of molecular dimers. By using the local density approximation we study confined Fermi vapors of alkali-metal atoms for which there is experimental evidence of condensation also on the BCS side of the Feshbach resonance. Our theoretical results are in agreement with these experimental data and give the behavior of the condensate on both sides of the Feshbach resonance at zero temperature.

cond-mat.stat-mech↗

arXiv2004

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.

physics.chem-ph↗

arXiv2004

Thermal Effects in Photoemission and Electron-Phonon Couplings of Fullerene

We show that thermal effects play a relevant role in the determination of the electron-phonon couplings based on the intensity of the phonon-shakeup peaks in photoemission spectra. In particular, we re-consider the determination of the electron-phonon couplings of fullerene based on a fit of the photoemission spectrum of C60- in gas phase [Gunnarsson PRB 1995]. We show that taking thermal effects into account reduces the obtained couplings by approximately 10%.

cond-mat.mtrl-sci↗

arXiv2003

Jahn-Teller distortions and excitation energies in C60(n+)

Based on previously computed parameters for the electron-phonon couplings and the Coulomb exchange, we compute and classify the static Jahn-Teller distortions, i.e. the minima of the lowest adiabatic potential energy surface, of C60(n+), for all values of charge 1 <= n <= 9 and spin. We compute the intra-band electronic excitation energies in the different optimal geometries in the sudden approximation, and find a spread of the electronic states of roughly 1 eV. We also obtain the leading vibronic quantum corrections to the ground-state energy, equal to zero-point energy lowering due to the softening of the phonons at the adiabatic Jahn-Teller minima: these non-adiabatic corrections are so large that for 4 <= n <= 6 states of different spin symmetry turn lower than the high-spin adiabatic ground state.

cond-mat.str-el↗

arXiv2003

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.

cond-mat.mtrl-sci↗

arXiv2003

Comment to "Experimental Evidence of a Dynamic Jahn-Teller Effect in C60+"

A recent Letter reports photoemission data for the free C60 molecule, showing an interesting three-peak structure, presented as evidence of dynamic Jahn-Teller (DJT) effect in the C60+ ion. Those data constitute, along with earlier spectra by the Uppsala group, the best available piece of experimental evidence about the spectrum of a hole in fullerene. DJT must indeed affect the fivefold-degenerate h_u hole molecular orbital, but we contend that the energy separation of these three peaks is far too large for the proposed tunneling interpretation to be correct.

cond-mat.mtrl-sci↗

arXiv2003

Hund's rule Magnetism in C60 ions?

We investigate the occurrence of Hund's rule magnetism in C60(n+-) molecular ions, by computing the ground-state spin for all charge states n from -3 to +5. The two competing interactions, electron-vibration (e-v, including Jahn Teller, favoring low spin) and electron-electron (e-e, including Hund-rule exchange, favoring high spin), are accounted for based on previously computed ab-initio coupling parameters. Treating the ion coordinates as classical, we first calculate and classify the static Jahn-Teller distorted states for all n, inclusive of both e-v and e-e effects. We then correct the adiabatic result by including the zero-point energy lowering associated with softening of vibrations at the adiabatic Jahn-Teller minima. Our overall result is that while, like in previous investigations, low-spin states prevail in negative ions, Hund's rule high spin dominates all positive C60(n+) ions. This suggests also that Hund-rule magnetism could arise in fullerene cation-based solid state compounds, particularly those involving C60(2+).

cond-mat.str-el↗

arXiv2002

Coulomb couplings in positively charged fullerene

We compute, based on density-functional electronic-structure calculations, the Coulomb couplings in the h_u highest occupied orbital of molecular C60. We obtain a multiplet-averaged Hubbard U ~ 3 eV, and four Hund-rule-like intra-molecular multiplet-splitting terms, each of the order of few hundreds of meVs. According to these couplings, all C60^n+ ions should possess a high-spin ground state if kept in their rigid, undistorted form. Even after molecular distortions are allowed, however, the Coulomb terms still appear to be somewhat stronger than the previously calculated Jahn-Teller couplings, the latter favoring low-spin states. Thus for example in C60^2+, unlike C60^2-, the balance between Hund rule and Jahn Teller yields, even if marginally, a high-spin ground state. That seems surprising in view of reports of superconductivity in field-doped C60^n+ systems.

cond-mat.str-el↗

arXiv2002

Sensitivity of the Mott Transition to Non-cubic Splitting of the Orbital Degeneracy: Application to NH3 K3C60

Within dynamical mean-field theory, we study the metal-insulator transition of a twofold orbitally degenerate Hubbard model as a function of a splitting Δof the degeneracy. The phase diagram in the U-Δplane exhibits two-band and one-band metals, as well as the Mott insulator. The correlated two-band metal is easily driven to the insulator state by a strikingly weak splitting Δ<< W of the order of the Kondo-peak width zW, where z << 1 is the metal quasiparticle weight. The possible relevance of this result to the insulator-metal transition in the orthorhombic expanded fulleride NH3 K3C60 is discussed.

cond-mat.str-el↗

arXiv2001

Electron-vibration coupling constants in positively charged fullerene

Recent experiments have shown that C60 can be positively field-doped. In that state, fullerene exhibits a higher resistivity and a higher superconducting temperature than the corresponding negatively doped state. A strong intramolecular hole-phonon coupling, connected with the Jahn-Teller effect of the isolated positive ion, is expected to be important for both properties, but the actual coupling strengths are so far unknown. Based on density functional calculations, we determine the linear couplings of the two a_g, six g_g, and eight h_g vibrational modes to the H_u HOMO level of the C60 molecule. The couplings predict a D_5 distortion, and an H_u vibronic ground state for C60^+. They are also used to generate the dimensionless coupling constant which controls the superconductivity and the phonon contribution to the electrical resistivity in the crystalline phase. We find that is 1.4 times larger in positively-charged C60 than in the negatively-doped case. These results are discussed in the context of the available transport data and superconducting temperatures. The role of higher orbital degeneracy in superconductivity is also addressed.

cond-mat.supr-con↗

arXiv2000

Geometric Phases and Multiple Degeneracies in Harmonic Resonators

In a recent experiment Lauber et al. have deformed cyclically a microwave resonator and have measured the adiabatic normal-mode wavefunctions for each shape along the path of deformation. The nontrivial observed cyclic phases around a 3-fold degeneracy were accounted for by Manolopoulos and Child within an approximate theory. However, open-path geometrical phases disagree with experiment. By solving exactly the problem, we find unsuspected extra degeneracies around the multiple one that account for the measured phase changes throughout the path. It turns out that proliferation of additional degeneracies around a multiple one is a common feature of quantum mechanics.

cond-mat↗

arXiv2000

Off-Diagonal Geometric Phases

We investigate the adiabatic evolution of a set of non-degenerate eigenstates of a parameterized Hamiltonian. Their relative phase change can be related to geometric measurable quantities that extend the familiar concept of Berry phase to the evolution of more than one state. We present several physical systems where these concepts can be applied, including an experiment on microwave cavities for which off-diagonal phases can be determined from published data.

quant-ph↗

arXiv2000

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.

cond-mat↗

arXiv1999

Berry Phase and Ground State Symmetry in H x h Dynamical Jahn-Teller Systems

Due to the ubiquitous presence of a Berry phase, in most cases of dynamical Jahn-Teller systems the symmetry of the vibronic ground state is the same as that of the original degenerate electronic state. As a single exception, the linear H x h icosahedral model, relevant to the physics of C60 cations, is determined by an additional free parameter, which can be continuously tuned to eliminate the Berry phase from the low-energy closed paths: accordingly, the ground state changes to a totally-symmetric nondegenerate state.

cond-mat.mtrl-sci↗

arXiv1999

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

Due to the frequent presence of a Berry phase, in most cases of dynamical Jahn-Teller systems the symmetry of the ground state is the same as that of the electronic state. However, the H x h icosahedral case, relevant for the physics of fullerene ions, provides a first example of linear coupling leading, at strong coupling, to a change in symmetry of the ground state to a totally symmetric nondegenerate state. We generalize this observation and show through detailed examples that the absence of a Berry phase can, but does not necessarily, lead to a nondegenerate ground state.

cond-mat↗

arXiv1998

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.

cond-mat.mtrl-sci↗