Molecular fingerprints in the electronic properties of crystalline organic semiconductors: from experiment to theory.

By comparing photoemission spectroscopy with a nonperturbative dynamical mean field theory extension to many-body ab initio calculations, we show in the prominent case of pentacene crystals that an excellent agreement with experiment for the bandwidth, dispersion, and lifetime of the hole carrier bands can be achieved in organic semiconductors, provided that one properly accounts for the coupling to molecular vibrational modes and the presence of disorder. Our findings rationalize the growing experimental evidence that even the best band structure theories based on a many-body treatment of electronic interactions cannot reproduce the experimental photoemission data in this important class of materials.

Quantum critical scaling in the single-particle spectrum of a novel anisotropic metal.

We report an angle resolved photoemission spectroscopy study of quantum critical scaling in the single-particle spectral function of a novel anisotropic metal Li_{0.9}Mo_{6}O_{17}. We find a temperature (T) scaling exponent value and also low-T angle resolved photoemission spectroscopy line shapes that are very challenging for current one-dimensional theory frameworks. These results add a new spectroscopic component to a growing collection of puzzling low-T transport behaviors of this material.

New Luttinger-liquid physics from photoemission on Li0.9M06O17.

Temperature dependent high resolution photoemission spectra of quasi-one-dimensional Li(0.9)Mo(6)O(17)evince a strong renormalization of its Luttinger-liquid density-of-states anomalous exponent. We trace this new effect to interacting charge neutral critical modes that emerge naturally from the two-band nature of the material. Li(0.9)Mo(6)O(17) is shown thereby to be a paradigm material that is capable of revealing new Luttinger physics.

Spectroscopic signatures of a bandwidth-controlled Mott transition at the surface of 1T-TaSe2.

High-resolution angle-resolved photoemission data show that a metal-insulator Mott transition occurs at the surface of the quasi-two-dimensional compound 1T-TaSe2. The transition is driven by the narrowing of the Ta 5d band induced by a temperature-dependent modulation of the atomic positions. A dynamical mean-field theory calculation of the spectral function of the half-filled Hubbard model captures the main qualitative feature of the data, namely, the rapid transfer of spectral weight from the observed quasiparticle peak at the Fermi surface to the Hubbard bands, as the correlation gap opens up.

Spontaneous breaking of time-reversal symmetry in the pseudogap state of a high-Tc superconductor.

A change in 'symmetry' is often observed when matter undergoes a phase transition-the symmetry is said to be spontaneously broken. The transition made by underdoped high-transition-temperature (high-Tc) superconductors is unusual, in that it is not a mean-field transition as seen in other superconductors. Rather, there is a region in the phase diagram above the superconducting transition temperature Tc (where phase coherence and superconductivity begin) but below a characteristic temperature T* where a 'pseudogap' appears in the spectrum of electronic excitations. It is therefore important to establish if T* is just a cross-over temperature arising from fluctuations in the order parameter that will establish superconductivity at Tc (refs 3, 4), or if it marks a phase transition where symmetry is spontaneously broken. Here we report that, for a material in the pseudogap state, left-circularly polarized photons give a different photocurrent from right-circularly polarized photons. This shows that time-reversal symmetry is spontaneously broken below T*, which therefore corresponds to a phase transition.

Spectroscopic indications of polaronic carriers in the quasi-one-dimensional conductor (TaSe4)2I.

Angle-resolved photoemission (ARPES) on the quasi-one-dimensional conductor (TaSe4)2I shows a hidden Fermi-surface crossing in its metallic state and the opening of a Peierls gap at low temperatures. The underlying quasiparticles have vanishing spectral weight and extremely short coherence lengths. They are interpreted as polarons in the strong-coupling adiabatic limit, and almost all their ARPES weight is incoherent. These observations suggest a scenario where the long-standing contradictions between ARPES and other experiments on Peierls materials could be resolved.

Fermi surface of Bi(111) measured by photoemission spectroscopy.

Synchrotron radiation angle-resolved photoemission spectroscopy of Bi(111) shows that the Fermi surface consists of six elongated hole pockets along the gamma M right macro directions surrounding a ring-shaped electron pocket centered at gamma, all of which have two-dimensional character. The associated hole and electron sheet densities are p(s) = 1.1 x 10(13) cm(-2) and n(s) = 5.5 x 10(12) cm(-2), respectively. A weak emission feature associated with the bulk hole pocket in the Fermi surface was identified. The Fermi momentum of the bulk hole band near the T point is k(F) = 0.013+/-0.003 A(-1).

Electronic structure of solids with competing periodic potentials.

When electrons are subject to a potential with two incommensurate periods, translational invariance is lost, and no periodic band structure is expected. However, model calculations based on nearly free one-dimensional electrons and experimental results from high-resolution photoemission spectroscopy on a quasi-one-dimensional material do show dispersing band states with signatures of both periodicities. Apparent band structures are generated by the nonuniform distribution of electronic spectral weight over the complex eigenvalue spectrum.