Charge-density-wave formation in the Edwards fermion-boson model at one-third band filling

We examine the ground-state properties of the one-dimensional Edwards spinless fermion transport model by means of large-scale density-matrix renormalization-group calculations. Determining the single-particle gap and the Tomonaga-Luttinger liquid parameter ($K_\rho$) at zero temperature, we prove the existence of a metal-to-insulator quantum phase transition at one-third band filling. The insulator---established by strong correlation in the background medium---typifies a charge density wave (CDW) that is commensurate with the band filling. $K_\rho=2/9$ is very small at the quantum critical point, and becomes $K_\rho^{\rm CDW}=1/9$ in the infinitesimally doped three-period CDW, as predicted by the bosonization approach.Comment: 6 pages, 3 figures, contributions to SCES 201

Signatures of polaronic charge ordering in optical and dc conductivity using dynamical mean field theory

We apply dynamical mean field theory to study a prototypical model that describes charge ordering in the presence of both electron-lattice interactions and intersite electrostatic repulsion between electrons. We calculate the optical and d.c. conductivity, and derive approximate formulas valid in the limiting electron-lattice coupling regimes. In the weak coupling regime, we recover the usual behavior of charge density waves, characterized by a transfer of spectral weight due to the opening of a gap in the excitation spectrum. In the opposite limit of very strong electron-lattice coupling, instead, the charge ordering transition is signaled by a global enhancement of the optical absorption, with no appreciable spectral weight transfer. Such behavior is related to the progressive suppression of thermally activated charge defects taking place below the critical temperature. At intermediate values of the coupling within the polaronic regime, a complex behavior is obtained where both mechanisms of transfer and enhancement of spectral weight coexist.Comment: 1 figure added, illustrating the optical sum rul

The pseudogap phase in (TaSe_4)_2I

We have developed the mean-field theory of coexisting charge-density waves (CDW) and unconventional charge-density waves (UCDW). The double phase transition manifests itself in the thermodynamic quantities and in the magnetic response, such as spin susceptibility and spin-lattice relaxation rate. Our theory applies to quasi-one dimensional (TaSe_4)_2I, where above the CDW transition, thermal fluctuations die out rapidly, but robust pseudogap behaviour is still detected. We argue, that the fluctuations are suppressed due to UCDW, which partially gaps the Fermi surface, and causes non-Fermi-liquid (pseudogap) behaviour.Comment: 7 pages, 6 figure

Elementary Excitations in One-Dimensional Electromechanical Systems; Transport with Back-Reaction

Using an exactly solvable model, we study low-energy properties of a one-dimensional spinless electron fluid contained in a quantum-mechanically moving wire located in a static magnetic field. The phonon and electric current are coupled via Lorentz force and the eigenmodes are described by two independent boson fluids. At low energies, the two boson modes are charged while one of them has excitation gap due to back-reaction of the Lorentz force. The theory is illustrated by evaluating optical absorption spectra. Our results are exact and show a non-perturbative regime of electron transport

Mean-field quantum phase transition in graphene and in general gapless systems

We study the quantum critical properties of antiferromagnetism in graphene at T=0 within mean-field (MF) theory. The resulting exponents differ from the conventional MF exponents, describing finite temperature transitions. Motivated by this, we have developed the MF theory of general gapless phases with density of states rho(E) |E|^r, r>-1, with the interaction as control parameter. For r>2, the conventional MF exponents \'a la Landau are recovered, while for -1<r<2, the exponents vary significantly with r. The critical interaction is finite for r>0, therefore no weak-coupling solution exists in this range. This generalizes the results on quantum criticality of the gapless Kondo systems to bulk correlated phases.Comment: 5 pages, 1 figure, 2 table

Boundary effect on CDW: Friedel oscillations, STM image

We study the effect of open boundary condition on charge density waves (CDW). The electron density oscillates rapidly close to the boundary, and additional non-oscillating terms (~ln(r)) appear. The Friedel oscillations survive beyond the CDW coherence length (v_F/Delta), but their amplitude gets heavily suppressed. The scanning tunneling microscopy image (STM) of CDW shows clear features of the boundary. The local tunneling conductance becomes asymmetric with respect to the Fermi energy, and considerable amount of spectral weight is transferred to the lower gap edge. Also it exhibits additional zeros reflecting the influence of the boundary.Comment: 7 pages, 6 figure

Downward shift of infrared conductivity spectral weight at the DDW transition: role of anisotropy

We consider the motion of conductivity spectral weight at a finite-temperature phase transition at which $d_{x^2-y^2}$ density-wave (DDW) order develops. We show that there is a shift of spectral weight to higher frequencies if the quasiparticle lifetime is assumed to be isotropic, but a shift to lower frequencies if the quasiparticle lifetime is assumed to be anisotropic. We suggest that this is consistent with recent experiments on the pseudogap phase of the cuprate superconductors and, therefore, conclude that the observation of a downward shift in the spectral weight at the pseudogap temperature does not militate against the DDW theory of the pseudogap.Comment: 8 pages, 7 figures. Added reference

Self Organization and a Dynamical Transition in Traffic Flow Models

A simple model that describes traffic flow in two dimensions is studied. A sharp {\it jamming transition } is found that separates between the low density dynamical phase in which all cars move at maximal speed and the high density jammed phase in which they are all stuck. Self organization effects in both phases are studied and discussed.Comment: 6 pages, 4 figure

Competing charge density waves and temperature-dependent nesting in 2H-TaSe2

Multiple charge density wave (CDW) phases in 2H-TaSe2 are investigated by high-resolution synchrotron x-ray diffraction. In a narrow temperature range immediately above the commensurate CDW transition, we observe a multi-q superstructure with coexisting commensurate and incommensurate order parameters, clearly distinct from the fully incommensurate state at higher temperatures. This multi-q ordered phase, characterized by a temperature hysteresis, is found both during warming and cooling, in contrast to previous reports. In the normal state, the incommensurate superstructure reflection gives way to a broad diffuse peak that persists nearly up to room temperature. Its position provides a direct and accurate estimate of the Fermi surface nesting vector, which evolves non-monotonically and approaches the commensurate position as the temperature is increased. This behavior agrees with our recent observations of the temperature-dependent Fermi surface in the same compound [Phys. Rev. B 79, 125112 (2009)]