Interplay between the edge-state magnetism and long-range Coulomb interaction in zigzag graphene nanoribbons: quantum Monte Carlo study

We perform projective quantum Monte Carlo simulations of zigzag graphene nanoribbons within a realistic model with long-range Coulomb interactions. Increasing the relative strength of nonlocal interactions with respect to the on-site repulsion does not generate a phase transition but has a number of nontrivial effects. At the single-particle level we observe a marked enhancement of the Fermi velocity at the Dirac points. At the two-particle level, spin- and charge-density-wave fluctuations compete. As a consequence, the edge magnetic moment is reduced but the edge dispersion relation increases in the sense that the single-particle gap at momentum $q=\pi/|{\pmb a}_1|$ grows. We attribute this to nonlocal charge fluctuations which assist the spin fluctuations to generate the aforementioned gap. In contrast, the net result of the interaction-induced renormalization of different energy scales is a constant spin-wave velocity of the edge modes. However, since the particle-hole continuum is shifted to higher energies---due to the renormalization of the Fermi velocity---Landau damping is reduced. As a result, a roughly linear spin-wave-like mode at the edge spreads out through a larger part of the Brillouin zone.Comment: 11 pages, 11 figures, comment about doped nanoribbon

Melting of stripe phases and its signature in the single-particle spectral function

Motivated by the recent experimental data [Phys. Rev. B 79, 100502 (2009)] indicating the existence of a pure stripe charge order over unprecedently wide temperature range in La_{1.8-x}Eu_{0.2}Sr_xCuO_4, we investigate the temperature-induced melting of the metallic stripe phase. In spite of taking into account local dynamic correlations within a real-space dynamical mean-field theory of the Hubbard model, we observe a mean-field like melting of the stripe order irrespective of the choice of the next-nearest neighbor hopping. The temperature dependence of the single-particle spectral function shows the stripe induced formation of a flat band around the antinodal points accompanied by the opening a gap in the nodal direction.Comment: 4 pages, 5 figures, minor changes, added Ref. 1

Emergent Coherent Lattice Behavior in Kondo Nanosystems

How many magnetic moments periodically arranged on a metallic surface are needed to generate a coherent Kondo lattice behavior? We investigate this fundamental issue within the particle-hole symmetric Kondo lattice model using quantum Monte Carlo simulations. Extra magnetic atoms forming closed shells around the initial impurity induce a fast splitting of the Kondo resonance at the inner shells which signals the formation of composite heavy-fermion bands. The onset of the hybridization gap matches well the enhancement of antiferromagnetic spin correlations in the plane perpendicular to the applied magnetic field, a genuine feature of the coherent Kondo lattice. In contrast, the outermost shell remains dominated by a local Kondo physics with spectral features resembling the single-impurity behavior.Comment: 4+ pages plus supplemental material; published versio

d-wave Superconductivity, Orbital Magnetism, and Unidirectional Charge Order in the t-J Model

Recent scanning tunneling microscopy in the superconducting regime of two different cuprate families has revealed unidirectional bond-centered modulation in the local electronic density of states. Motivated by this result we investigate the emergence of modulated d-wave superconductivity coexisting with charge domains that form along one of the crystal axes. While detailed stripe profiles depend on the used form of the Gutzwiller factors, the tendency towards a valence bond crystal remain robust. We also find closely related stripe phase originating from the staggered flux phase, a candidate for the pseudogap phase of lightly doped cuprates.Comment: 8 pages, 4 figure

Mechanism of the reorientation of stripes in the cuprates

Using the mean field theory in the slave-boson approach we analyzed the electron correlation effects in the stripe phases. One finds that a finite next-nearest neighbor hopping $t'$ plays an important role in the low doping regime, where it controls the crossover from the filled diagonal to half-filled vertical/horizontal stripes at doping $x\simeq 1/16$.Comment: 2 pages, 1 figur

Stability of RVB hole stripes in high-temperature superconductors

Indications of density-wave states in underdoped cuprates, coming from recent STM (scanning tunneling microscopy) and Hall-resistance measurements, have raised new concerns whether stripes could be stabilized in the superconducting phase of cuprate materials, even in the absence of antiferromagnetism. Here, we investigate this issue using state-of-the-art quantum Monte Carlo calculations of a $t-J$ model. In particular we consider the stability of unidirectional hole domains in a modulated superconducting background, by taking into account the effect of tetragonal-lattice distortions, next-nearest neighbor hopping and long-range Coulomb repulsion.Comment: 8 pages, 9 figures. Enlarged versio

Spin-rotationally symmetric domain flux phases in underdoped cuprates

We propose a new form of inhomogeneous phases consisting of out-of-phase staggered flux domains separated by diagonal charged domain walls centered on bonds or on sites. Remarkably, such domain flux phases are spin-rotationally symmetric and exhibit cone-like quasiparticle dispersion as well as incommensurate order of orbital currents. Such features are consistent with the pseudogap behavior and the diagonal stripes observed experimentally in lightly doped cuprates. A renormalized mean field theory shows that such solutions are competitive candidates within the $t$--$J$ model.Comment: 6 pages, 3 figure