Gap ratio in anharmonic charge-density-wave systems

Many experimental systems exist that possess charge-density-wave order in their ground state. While this order should be able to be described with models similar to those used for superconductivity, nearly all systems have a ratio of the charge-density-wave order parameter to the transition temperature that is too high for conventional theories. Recent work explained how this can happen in harmonic systems, but when the lattice distortion gets large, anharmonic effects must play an increasingly important role. Here we study the gap ratio for anharmonic charge-density wave systems to see whether the low-temperature properties possess universality as was seen previously in the transition temperature and to see whether the explanation for the large gap ratios survives for anharmonic systems as well.Comment: (5 pages, 3 figures, ReVTeX

Segregation and charge-density-wave order in the spinless Falicov-Kimball model

The spinless Falicov-Kimball model is solved exactly in the limit of infinite-dimensions on both the hypercubic and Bethe lattices. The competition between segregation, which is present for large U, and charge-density-wave order, which is prevalent at moderate U, is examined in detail. We find a rich phase diagram which displays both of these phases. The model also shows nonanalytic behavior in the charge-density-wave transition temperature when U is large enough to generate a correlation-induced gap in the single-particle density of states.Comment: 10 pages, 10 figure

Holstein model in infinite dimensions at half-filling

The normal state of the Holstein model is studied at half-filling in infinite dimensions and in the adiabatic regime. The dynamical mean-field equations are solved using perturbation expansions around the extremal paths of the effective action for the atoms. We find that the Migdal-Eliashberg expansion breaks down in the metallic state if the electron-phonon coupling $\lambda$ exceeds a value of about 1.3 in spite of the fact that the formal expansion parameter $\lambda \omega_0/E_F$ ($\omega_0$ is the phonon frequency, $E_F$ the Fermi energy) is much smaller than 1. The breakdown is due to the appearance of more than one extremal path of the action. We present numerical results which illustrate in detail the evolution of the local Green's function, the self-energy and the effective atomic potential as a function of $\lambda$.Comment: Revtex + 17 postscript figures include

Resonant Enhancement of Electronic Raman Scattering

We present an exact solution for electronic Raman scattering in a single-band, strongly correlated material, including nonresonant, resonant and mixed contributions. Results are derived for the spinless Falicov-Kimball model, employing dynamical mean field theory; this system can be tuned through a Mott metal-insulator transition.Comment: 4 pages, 3 figures, contribution to the SNS'2004 conferenc

Resonant electronic Raman scattering near a quantum critical point

We calculate the resonant electronic Raman scattering for the Falicov-Kimball model near the Mott transition on a hypercubic lattice. The solution is exact, and employs dynamical mean field theory.Comment: 2 pages, 2 figures, contribution to the SCES04 conferenc

Semiclassical action based on dynamical mean-field theory describing electrons interacting with local lattice fluctuations

We extend a recently introduced semiclassical approach to calculating the influence of local lattice fluctuations on electronic properties of metals and metallic molecular crystals. The effective action of electrons in degenerate orbital states coupling to Jahn-Teller distortions is derived, employing dynamical mean-field theory and adiabatic expansions. We improve on previous numerical treatments of the semiclassical action and present for the simplifying Holstein model results for the finite temperature optical conductivity at electron-phonon coupling strengths from weak to strong. Significant transfer of spectral weight from high to low frequencies is obtained on isotope substitution in the Fermi-liquid to polaron crossover regime.Comment: 10 pages, 7 figure

Exact solution of a variety of X-ray probes in the Falicov-Kimball model with dynamical mean-field theory

We examine the core-level X-ray photoemission spectroscopy (XPS), X-ray absorption near-edge spectroscopy (XANES) and X-ray emission spectroscopy (XES) in the Falicov-Kimball model by using the exact solution from dynamical mean-field theory. XPS measures the core-hole propagator, XANES measures the absorption of X-rays when the core electron is excited to an unoccupied electronic state of the solid and is not emitted, and XES measures the spectra of light emitted as electrons fill the core-hole state created via some form of X-ray excitation. These three spectra are closely related to one another and display orthogonality catastrophe behavior at T=0. We show an efficient way of evaluating these spectra at finite temperature, with a primary focus on the details of XANES.Дослiджуються рентґенiвськi спектри фотоелектронної емiсiї (XPS), передкрайового поглинання (XANES) та фотоемiсiї (XES) для моделi Фалiкова-Кiмбала, використовуючи точнi розв’язки теорiї динамiчного середнього поля. XPS вимiрює пропагатор дiрки йонного залишку, XANES — поглинання X-променiв, коли електрон йонного залишку збуджується в незаповнену електронну зону твердого тiла, i XES — спектр випромiненого свiтла при заповненнi електроном наперед створеної X-променями дiрки йонного залишку. Всi цi три типи спектрiв тiсно пов’язанi мiж собою i проявляють особливостi типу катастрофи ортогональ-ностi при T = 0. Показано, як ефективно розраховувати такi спектри для скiнчених температур, зокрема з детальнiшим наголосом на спектри XANES

Non-resonant Raman scattering through a metal-insulator transition: an exact analysis of the Falicov-Kimball model

For years, theories for Raman scattering have been confined to either the insulating or fully metallic state. While much can be learned by focusing attention on the metal or insulator, recent experimental work on the cuprate systems points to the desirability of formulating a theory for Raman response which takes one through a quantum critical point – the metalinsulator transition. Using the Falicov-Kimball model as a canonical model of a MIT, we employ dynamical mean-field theory to construct an exact theory for non-resonant Raman scattering. In particular we examine the formation of charge transfer peaks and pseudogaps as well as the low-energy dynamics. The results are qualitatively compared to the experimental B₁g Raman spectra in the cuprates, which probes the hot quasiparticles along the Brillouin zone axes. The results shed important information on normal state electronic transport and the pseudo-gap in the cuprates.Упродовж років теорія комбінаційного розсіяння (КР) обмежувалася розглядом або ізоляторів або суто металічного стану. Хоча можна багато довідатися, зосередивши увагу тільки на металах чи ізоляторах, останні експериментальні роботи з купратних систем вказують на бажаність формулювання теорії раманівського відгуку, яке придатне при проходженні через квантову критичну точку - перехід метал-ізолятор (ПМІ). Використовуючи модель Фалікова-Кімбала як канонічну модель ПМІ, ми застосовуємо теорію динамічного середнього поля для побудови точної теорії нерезонансного КР. Зокрема, ми розглядаємо утворення піків, зумовлених переносом заряду, та псевдощілин, а також низькоенергетичну динаміку. Результати якісно зіставимі з експериментальними B₁g спектрами КР у купратах, в яких фіксують “гарячі” квазічастинки вздовж осей зони Брілюена. Результати дають важливу інформацію про електронний транспорт у нормальному стані та псевдощілину в купратах

Nonresonant inelastic light scattering in the Hubbard model

Inelastic light scattering from electrons is a symmetry-selective probe of the charge dynamics within correlated materials. Many measurements have been made on correlated insulators, and recent exact solutions in large dimensions explain a number of anomalous features found in experiments. Here we focus on the correlated metal, as described by the Hubbard model away from half filling. We can determine the B1g Raman response and the inelastic X-ray scattering along the Brillouin zone diagonal exactly in the large dimensional limit. We find a number of interesting features in the light scattering response which should be able to be seen in correlated metals such as the heavy fermions.Comment: 9 pages, 7 figures, typeset with ReVTe

Revealing Superfluid--Mott-Insulator Transition in an Optical Lattice

We study (by an exact numerical scheme) the single-particle density matrix of $\sim 10^3$ ultracold atoms in an optical lattice with a parabolic confining potential. Our simulation is directly relevant to the interpretation and further development of the recent pioneering experiment by Greiner et al. In particular, we show that restructuring of the spatial distribution of the superfluid component when a domain of Mott-insulator phase appears in the system, results in a fine structure of the particle momentum distribution. This feature may be used to locate the point of the superfluid--Mott-insulator transition.Comment: 4 pages (12 figures), Latex. (A Latex macro is corrected