Cedric Weber - Academia.edu (original) (raw)

Papers by Cedric Weber

Physical review letters, Jan 21, 2014

We study the phase diagram of an effective three-orbital model of the cuprates using variational ... more We study the phase diagram of an effective three-orbital model of the cuprates using variational Monte Carlo calculations on asymptotically large lattices and exact diagonalization on a 24-site cluster. States with ordered orbital current loops (LC), itinerant antiferromagnetism, d-wave superconductivity, and the Fermi liquid are investigated using appropriate Slater determinants refined by Jastrow functions for on-site and intersite correlations. We find an LC state stable in the thermodynamic limit for a range of parameters compatible with the Fermi surface of a typical hole doped superconductor provided the transfer integrals between the oxygen atoms have signs determined by the effects of indirect transfer through the Cu-4s orbitals as suggested by Andersen. The results of the calculations are that the LC phase gives way at lower dopings to an antiferromagnetism phase, and at larger dopings to superconductivity and Fermi liquid phases.

Scientific reports, 2013

We address the general question of the extent to which the hydrodynamic behaviour of microscopic ... more We address the general question of the extent to which the hydrodynamic behaviour of microscopic freely fluctuating objects can be reproduced by macrosopic rigid objects. In particular, we compare the sedimentation speeds of knotted DNA molecules undergoing gel electrophoresis to the sedimentation speeds of rigid stereolithographic models of ideal knots in both water and silicon oil. We find that the sedimentation speeds grow roughly linearly with the average crossing number of the ideal knot configurations, and that the correlation is stronger within classes of knots. This is consistent with previous observations with DNA knots in gel electrophoresis.

Physical review letters, Jan 24, 2003

We study the thermal properties of the classical antiferromagnetic Heisenberg model with both nea... more We study the thermal properties of the classical antiferromagnetic Heisenberg model with both nearest (J1) and next-nearest (J2) exchange couplings on the square lattice by extensive Monte Carlo simulations. We show that, for J2/J1>1/2, thermal fluctuations give rise to an effective Z2 symmetry leading to a finite-temperature phase transition. We provide strong numerical evidence that this transition is in the 2D Ising universality class, and that T(c)-->0 with an infinite slope when J2/J1-->1/2.

We report on the formation of tetrahydrofuran clathrate hydrate studied by x-ray Raman scattering... more We report on the formation of tetrahydrofuran clathrate hydrate studied by x-ray Raman scattering measurements at the oxygen K-edge. A comparison of x-ray Raman spectra measured from water/tetrahydrofuran mixtures and tetrahydrofuran hydrate at different ...

We use the Local Density Approximation in combination with the Dynamical Mean Field Theory to inv... more We use the Local Density Approximation in combination with the Dynamical Mean Field Theory to investigate intermediate energy properties of the copper oxides. We identify coherent and incoherent spectral features that results from doping a charge transfer insulator, namely quasiparticles, Zhang-Rice singlet band, and the upper and lower Hubbard bands. Angle resolving these features, we identify a waterfall like feature, between the quasiparticle part and the incoherent part of the Zhang-Rice band. We investigate the assymetry between particle and hole doping. On the hole doped side, there is a very rapid transfer of spectral weight upon doping in the one particle spectra. The optical spectral weight increases superlinearly on the hole doped side in agreement with experiments.

We use the Local Density Approximation in combination with the Dynamical Mean Field Theory to car... more We use the Local Density Approximation in combination with the Dynamical Mean Field Theory to carry out a comparative investigation of a typical electron doped and a typical hole doped copper oxide, NCCO and LSCO respectively. The parent compounds of both materials are strongly correlated electron systems in the vicinity of the metal to charge transfer insulator transition. In NCCO the magnetic long range order is essential to open a charge transfer gap, while Mott physics is responsible for the gap in LSCO. We highlights the role of the apical oxygens in determining the strength of the correlations and obtaining overall good agreement between theory and several experimentally determined quantities. Results for optical conductivity, polarized X-ray absorption and angle resolved photoemission are presented and compared with experiments. arXiv:1005.3100v1 [cond-mat.str-el]

High temperature superconductivity was achieved by introducing holes in a parent compound consist... more High temperature superconductivity was achieved by introducing holes in a parent compound consisting of copper oxide layers separated by spacer layers. Realizations of this phenomena has been achieved in multiple crystal structures and has been the subject of numerous investigations and extensive controversy. In a small number of copper oxide based materials, it is possible to dope the parent compound with electrons [1-3] , and their physical properties are bearing some similarities but also significant differences from the hole doped counterparts. For example, in the electron doped materials the antiferromagnetic phase is much more robust than the superconducting phase, while the normal state has a resistivity with a quadratic temperature dependence which is expected in normal metals rather than the linear temperature dependence observed in the hole doped systems. Here, we use a recently developed first principles method, to study the electron doped cuprates and elucidate the deep physical reasons why their behavior is so different than the hole doped materials. The crystal structure of the electron doped materials, characterized by a lack of oxygen in the apical position, results in a parent compound which is a Slater insulator. Namely, a material where the insulating behavior is the result of the presence of magnetic long range order. This is in sharp contrast with the hole doped materials, where the parent compound is a Mott charge transfer insulator, namely a material which is insulating due to the strong electronic correlations but not the magnetic order. We study the evolution of the angle resolved photoemission spectra and the optical properties of the normal state of the electron doped cuprates as a function of doping, clarifying how their unique position close to, but below the metal to charge transfer insulator transition, accounts for their surprising differences from the hole doped cuprates.

Physical Review Letters, 2004

Low temperature (3.9K) scanning-tunneling spectroscopy on a hexagonal superlattice of Ce adatoms ... more Low temperature (3.9K) scanning-tunneling spectroscopy on a hexagonal superlattice of Ce adatoms on Ag(111) reveals site-dependent characteristic features in differential conductance spectra and in spectroscopic images at atomic-scale spatial resolution. Using a tight-binding model, we relate the overall spectral structures to the scattering of Ag(111) surface-state electrons by the Ce adatoms, the site dependence to the disorder induced by imperfections

Proceedings of the National Academy of Sciences, 2014

Physical Review Letters, 2013

We propose a mechanism for binding of diatomic ligands to heme based on a dynamical orbital selec... more We propose a mechanism for binding of diatomic ligands to heme based on a dynamical orbital selection process. This scenario may be described as bonding determined by local valence fluctuations. We support this model using linear-scaling first-principles calculations, in combination with dynamical mean-field theory, applied to heme, the kernel of the hemoglobin metalloprotein central to human respiration. We find that variations in Hund's exchange coupling induce a reduction of the iron 3d density, with a concomitant increase of valence fluctuations. We discuss the comparison between our computed optical absorption spectra and experimental data, our picture accounting for the observation of optical transitions in the infrared regime, and how the Hund's coupling reduces, by a factor of five, the strong imbalance in the binding energies of heme with CO and O2 ligands. arXiv:1206.0412v2 [cond-mat.str-el]

Physical Review Letters, 2009

Motivated by the recent report of broken time-reversal symmetry and zero momentum magnetic scatte... more Motivated by the recent report of broken time-reversal symmetry and zero momentum magnetic scattering in underdoped cuprates, we investigate under which circumstances orbital currents circulating inside a unit cell might be stabilized in extended Hubbard models that explicitly include oxygen orbitals. Using Gutzwiller projected variational wave functions that treat on an equal footing all instabilities, we show that orbital currents indeed develop on finite clusters, and that they are stabilized in the thermodynamic limit if additional interactions, e.g. strong hybridization with apical oxygens, are included in the model. Despite intensive efforts in the last twenty years, the physics of high Tc superconductors remains largely mysterious [1]. This is especially true of the pseudogap phase of underdoped cuprates, for which various explanations have been put forward ranging from preformed superconducting pairs [5] to the existence of orbital currents with or without broken translational symmetry. The latter case has received considerable attention due on one hand to recent neutron experiments indicating the presence of magnetic moments, compatible with the translation invariant pattern of currents predicted by Varma , and on the other hand to Kerr effect measurements showing evidence of time-reversal symmetry breaking.

Physical Review Letters, 2012

We study the steady-state dynamics of the Hubbard model driven out-of-equilibrium by a constant e... more We study the steady-state dynamics of the Hubbard model driven out-of-equilibrium by a constant electric field and coupled to a dissipative heat bath. For very strong field, we find a dimensional reduction: the system behaves as an equilibrium Hubbard model in lower dimensions. We derive steady-state equations for the dynamical mean-field theory in the presence of dissipation. We discuss how the electric field induced dimensional crossover affects the momentum resolved and integrated spectral functions, the energy distribution function, as well as the steady current in the non-linear regime.

Physical Review Letters, 2012

Vanadium dioxide undergoes a first order metal-insulator transition at 340 K. In this work, we de... more Vanadium dioxide undergoes a first order metal-insulator transition at 340 K. In this work, we develop and carry out state of the art linear scaling DFT calculations refined with non-local dynamical mean-field theory. We identify a complex mechanism, a Peierls-assisted orbital selection Mott instability, which is responsible for the insulating M1 phase, and furthermore survives a moderate degree of disorder.

Physical Review Letters, 2003

We study the thermal properties of the classical antiferromagnetic Heisenberg model with both nea... more We study the thermal properties of the classical antiferromagnetic Heisenberg model with both nearest (J 1 ) and next-nearest (J 2 ) exchange couplings on the square lattice by extensive Monte Carlo simulations. We show that, for J 2 =J 1 > 1=2, thermal fluctuations give rise to an effective Z 2 symmetry leading to a finite-temperature phase transition. We provide strong numerical evidence that this transition is in the 2D Ising universality class, and that T c ! 0 with an infinite slope when J 2 =J 1 ! 1=2.

Physical Review Letters, 2004

Low temperature (3.9 K) scanning-tunneling spectroscopy on a hexagonal superlattice of Ce adatoms... more Low temperature (3.9 K) scanning-tunneling spectroscopy on a hexagonal superlattice of Ce adatoms on Ag(111) reveals site-dependent characteristic features in differential conductance spectra and in spectroscopic images at atomic-scale spatial resolution. Using a tight-binding model, we relate the overall spectral structures to the scattering of Ag(111) surface-state electrons by the Ce adatoms, the site dependence to the disorder induced by imperfections of the superlattice, and the opening of a gap in the local density of states to the observed stabilization of superlattices with adatom distances in the range of 2.3-3.5 nm.

Physical Review B, 2006

Motivated by the observation of inhomogeneous patterns in some high-T$_c$ cuprate compounds, seve... more Motivated by the observation of inhomogeneous patterns in some high-T$_c$ cuprate compounds, several variational Gutzwiller-projected wave-functions with built-in charge and bond order parameters are proposed for the extended t−J−Vt-J-Vt−J−V model on the square lattice at low doping. First, following a recent Gutzwiller-projected mean-field approach by one of us (Phys. Rev. B. {\bf 72}, 060508(R) (2005)), we investigate, as a function of doping and Coulomb repulsion, the stability of the staggered flux phase with respect to small spontaneous modulations of squared unit cells ranging from 2times22\times 22times2 to sqrt32timessqrt32\sqrt{32}\times\sqrt{32}sqrt32timessqrt32. It is found that a 4times44\times 44times4 bond-order (BO) modulation appears spontaneously on top of the staggered flux pattern for hole doping around 1/8. A related wave-function is then constructed and optimized accurately and its properties studied extensively using an approximation-free variational Monte Carlo scheme. Finally, the competition of the BO-modulated staggered flux wave-function w.r.t. the d-wave RVB wave-function or the commensurate flux state is investigated. It is found that a short range Coulomb repulsion penalizes the d-wave superconductor and that a moderate Coulomb repulsion brings them very close in energy. Our results are discussed in connection to the STM observations in the under-doped regime of some cuprates.

Physical Review B, 2012

We present Monte Carlo simulations for a classical antiferromagnetic Heisenberg model with both n... more We present Monte Carlo simulations for a classical antiferromagnetic Heisenberg model with both nearest (J1) and next-nearest (J2) exchange couplings on the square lattice in the presence of nonmagnetic impurities. We show that the order-by-disorder entropy selection, associated with the Ising-like phase transition that appears for J2/J1 > 1/2 in the pure spin model, is quenched at low temperature due to the presence of non-magnetic impurities. Evidences that a new competing order is stabilized around the impurities, and in turn induces a re-entrance phase transition are reported. Implications for local magnetic measurement of the parent compound of iron pnictides are briefly discussed.

Physical Review B, 2011

We derive a general formalism for evaluating the high-frequency limit of the thermoelectric power... more We derive a general formalism for evaluating the high-frequency limit of the thermoelectric power of strongly correlated materials, which can be straightforwardly implemented in available first principles LDA+DMFT programs. We explore this formalism using model Hamiltonians and we investigate the validity of approximating the static thermoelectric power S 0 , by its hightemperature limit, S * . We point out that the behaviors of S * and S 0 are qualitatively different for a correlated Fermi liquid near the Mott transition, when the temperature is in the coherent regime.

Journal of Physics: Condensed Matter, 2006

ABSTRACT Gel electrophoresis can be used to separate nicked circular DNA molecules of equal lengt... more ABSTRACT Gel electrophoresis can be used to separate nicked circular DNA molecules of equal length but forming different knot types. At low electric fields, complex knots drift faster than simpler knots. However, at high electric field the opposite is the case and simpler knots migrate faster than more complex knots. Using Monte Carlo simulations we investigate the reasons of this reversal of relative order of electrophoretic mobility of DNA molecules forming different knot types. We observe that at high electric fields the simulated knotted molecules tend to hang over the gel fibres and require passing over a substantial energy barrier to slip over the impeding gel fibre. At low electric field the interactions of drifting molecules with the gel fibres are weak and there are no significant energy barriers that oppose the detachment of knotted molecules from transverse gel fibres. © 2006 IOP Publishing Ltd.

Physical review letters, Jan 21, 2014

We study the phase diagram of an effective three-orbital model of the cuprates using variational ... more We study the phase diagram of an effective three-orbital model of the cuprates using variational Monte Carlo calculations on asymptotically large lattices and exact diagonalization on a 24-site cluster. States with ordered orbital current loops (LC), itinerant antiferromagnetism, d-wave superconductivity, and the Fermi liquid are investigated using appropriate Slater determinants refined by Jastrow functions for on-site and intersite correlations. We find an LC state stable in the thermodynamic limit for a range of parameters compatible with the Fermi surface of a typical hole doped superconductor provided the transfer integrals between the oxygen atoms have signs determined by the effects of indirect transfer through the Cu-4s orbitals as suggested by Andersen. The results of the calculations are that the LC phase gives way at lower dopings to an antiferromagnetism phase, and at larger dopings to superconductivity and Fermi liquid phases.

Scientific reports, 2013

We address the general question of the extent to which the hydrodynamic behaviour of microscopic ... more We address the general question of the extent to which the hydrodynamic behaviour of microscopic freely fluctuating objects can be reproduced by macrosopic rigid objects. In particular, we compare the sedimentation speeds of knotted DNA molecules undergoing gel electrophoresis to the sedimentation speeds of rigid stereolithographic models of ideal knots in both water and silicon oil. We find that the sedimentation speeds grow roughly linearly with the average crossing number of the ideal knot configurations, and that the correlation is stronger within classes of knots. This is consistent with previous observations with DNA knots in gel electrophoresis.

Physical review letters, Jan 24, 2003

We study the thermal properties of the classical antiferromagnetic Heisenberg model with both nea... more We study the thermal properties of the classical antiferromagnetic Heisenberg model with both nearest (J1) and next-nearest (J2) exchange couplings on the square lattice by extensive Monte Carlo simulations. We show that, for J2/J1>1/2, thermal fluctuations give rise to an effective Z2 symmetry leading to a finite-temperature phase transition. We provide strong numerical evidence that this transition is in the 2D Ising universality class, and that T(c)-->0 with an infinite slope when J2/J1-->1/2.

We report on the formation of tetrahydrofuran clathrate hydrate studied by x-ray Raman scattering... more We report on the formation of tetrahydrofuran clathrate hydrate studied by x-ray Raman scattering measurements at the oxygen K-edge. A comparison of x-ray Raman spectra measured from water/tetrahydrofuran mixtures and tetrahydrofuran hydrate at different ...

We use the Local Density Approximation in combination with the Dynamical Mean Field Theory to inv... more We use the Local Density Approximation in combination with the Dynamical Mean Field Theory to investigate intermediate energy properties of the copper oxides. We identify coherent and incoherent spectral features that results from doping a charge transfer insulator, namely quasiparticles, Zhang-Rice singlet band, and the upper and lower Hubbard bands. Angle resolving these features, we identify a waterfall like feature, between the quasiparticle part and the incoherent part of the Zhang-Rice band. We investigate the assymetry between particle and hole doping. On the hole doped side, there is a very rapid transfer of spectral weight upon doping in the one particle spectra. The optical spectral weight increases superlinearly on the hole doped side in agreement with experiments.

We use the Local Density Approximation in combination with the Dynamical Mean Field Theory to car... more We use the Local Density Approximation in combination with the Dynamical Mean Field Theory to carry out a comparative investigation of a typical electron doped and a typical hole doped copper oxide, NCCO and LSCO respectively. The parent compounds of both materials are strongly correlated electron systems in the vicinity of the metal to charge transfer insulator transition. In NCCO the magnetic long range order is essential to open a charge transfer gap, while Mott physics is responsible for the gap in LSCO. We highlights the role of the apical oxygens in determining the strength of the correlations and obtaining overall good agreement between theory and several experimentally determined quantities. Results for optical conductivity, polarized X-ray absorption and angle resolved photoemission are presented and compared with experiments. arXiv:1005.3100v1 [cond-mat.str-el]

High temperature superconductivity was achieved by introducing holes in a parent compound consist... more High temperature superconductivity was achieved by introducing holes in a parent compound consisting of copper oxide layers separated by spacer layers. Realizations of this phenomena has been achieved in multiple crystal structures and has been the subject of numerous investigations and extensive controversy. In a small number of copper oxide based materials, it is possible to dope the parent compound with electrons [1-3] , and their physical properties are bearing some similarities but also significant differences from the hole doped counterparts. For example, in the electron doped materials the antiferromagnetic phase is much more robust than the superconducting phase, while the normal state has a resistivity with a quadratic temperature dependence which is expected in normal metals rather than the linear temperature dependence observed in the hole doped systems. Here, we use a recently developed first principles method, to study the electron doped cuprates and elucidate the deep physical reasons why their behavior is so different than the hole doped materials. The crystal structure of the electron doped materials, characterized by a lack of oxygen in the apical position, results in a parent compound which is a Slater insulator. Namely, a material where the insulating behavior is the result of the presence of magnetic long range order. This is in sharp contrast with the hole doped materials, where the parent compound is a Mott charge transfer insulator, namely a material which is insulating due to the strong electronic correlations but not the magnetic order. We study the evolution of the angle resolved photoemission spectra and the optical properties of the normal state of the electron doped cuprates as a function of doping, clarifying how their unique position close to, but below the metal to charge transfer insulator transition, accounts for their surprising differences from the hole doped cuprates.

Physical Review Letters, 2004

Low temperature (3.9K) scanning-tunneling spectroscopy on a hexagonal superlattice of Ce adatoms ... more Low temperature (3.9K) scanning-tunneling spectroscopy on a hexagonal superlattice of Ce adatoms on Ag(111) reveals site-dependent characteristic features in differential conductance spectra and in spectroscopic images at atomic-scale spatial resolution. Using a tight-binding model, we relate the overall spectral structures to the scattering of Ag(111) surface-state electrons by the Ce adatoms, the site dependence to the disorder induced by imperfections

Proceedings of the National Academy of Sciences, 2014

Physical Review Letters, 2013

We propose a mechanism for binding of diatomic ligands to heme based on a dynamical orbital selec... more We propose a mechanism for binding of diatomic ligands to heme based on a dynamical orbital selection process. This scenario may be described as bonding determined by local valence fluctuations. We support this model using linear-scaling first-principles calculations, in combination with dynamical mean-field theory, applied to heme, the kernel of the hemoglobin metalloprotein central to human respiration. We find that variations in Hund's exchange coupling induce a reduction of the iron 3d density, with a concomitant increase of valence fluctuations. We discuss the comparison between our computed optical absorption spectra and experimental data, our picture accounting for the observation of optical transitions in the infrared regime, and how the Hund's coupling reduces, by a factor of five, the strong imbalance in the binding energies of heme with CO and O2 ligands. arXiv:1206.0412v2 [cond-mat.str-el]

Physical Review Letters, 2009

Motivated by the recent report of broken time-reversal symmetry and zero momentum magnetic scatte... more Motivated by the recent report of broken time-reversal symmetry and zero momentum magnetic scattering in underdoped cuprates, we investigate under which circumstances orbital currents circulating inside a unit cell might be stabilized in extended Hubbard models that explicitly include oxygen orbitals. Using Gutzwiller projected variational wave functions that treat on an equal footing all instabilities, we show that orbital currents indeed develop on finite clusters, and that they are stabilized in the thermodynamic limit if additional interactions, e.g. strong hybridization with apical oxygens, are included in the model. Despite intensive efforts in the last twenty years, the physics of high Tc superconductors remains largely mysterious [1]. This is especially true of the pseudogap phase of underdoped cuprates, for which various explanations have been put forward ranging from preformed superconducting pairs [5] to the existence of orbital currents with or without broken translational symmetry. The latter case has received considerable attention due on one hand to recent neutron experiments indicating the presence of magnetic moments, compatible with the translation invariant pattern of currents predicted by Varma , and on the other hand to Kerr effect measurements showing evidence of time-reversal symmetry breaking.

Physical Review Letters, 2012

We study the steady-state dynamics of the Hubbard model driven out-of-equilibrium by a constant e... more We study the steady-state dynamics of the Hubbard model driven out-of-equilibrium by a constant electric field and coupled to a dissipative heat bath. For very strong field, we find a dimensional reduction: the system behaves as an equilibrium Hubbard model in lower dimensions. We derive steady-state equations for the dynamical mean-field theory in the presence of dissipation. We discuss how the electric field induced dimensional crossover affects the momentum resolved and integrated spectral functions, the energy distribution function, as well as the steady current in the non-linear regime.

Physical Review Letters, 2012

Vanadium dioxide undergoes a first order metal-insulator transition at 340 K. In this work, we de... more Vanadium dioxide undergoes a first order metal-insulator transition at 340 K. In this work, we develop and carry out state of the art linear scaling DFT calculations refined with non-local dynamical mean-field theory. We identify a complex mechanism, a Peierls-assisted orbital selection Mott instability, which is responsible for the insulating M1 phase, and furthermore survives a moderate degree of disorder.

Physical Review Letters, 2003

We study the thermal properties of the classical antiferromagnetic Heisenberg model with both nea... more We study the thermal properties of the classical antiferromagnetic Heisenberg model with both nearest (J 1 ) and next-nearest (J 2 ) exchange couplings on the square lattice by extensive Monte Carlo simulations. We show that, for J 2 =J 1 > 1=2, thermal fluctuations give rise to an effective Z 2 symmetry leading to a finite-temperature phase transition. We provide strong numerical evidence that this transition is in the 2D Ising universality class, and that T c ! 0 with an infinite slope when J 2 =J 1 ! 1=2.

Physical Review Letters, 2004

Low temperature (3.9 K) scanning-tunneling spectroscopy on a hexagonal superlattice of Ce adatoms... more Low temperature (3.9 K) scanning-tunneling spectroscopy on a hexagonal superlattice of Ce adatoms on Ag(111) reveals site-dependent characteristic features in differential conductance spectra and in spectroscopic images at atomic-scale spatial resolution. Using a tight-binding model, we relate the overall spectral structures to the scattering of Ag(111) surface-state electrons by the Ce adatoms, the site dependence to the disorder induced by imperfections of the superlattice, and the opening of a gap in the local density of states to the observed stabilization of superlattices with adatom distances in the range of 2.3-3.5 nm.

Physical Review B, 2006

Motivated by the observation of inhomogeneous patterns in some high-T$_c$ cuprate compounds, seve... more Motivated by the observation of inhomogeneous patterns in some high-T$_c$ cuprate compounds, several variational Gutzwiller-projected wave-functions with built-in charge and bond order parameters are proposed for the extended t−J−Vt-J-Vt−J−V model on the square lattice at low doping. First, following a recent Gutzwiller-projected mean-field approach by one of us (Phys. Rev. B. {\bf 72}, 060508(R) (2005)), we investigate, as a function of doping and Coulomb repulsion, the stability of the staggered flux phase with respect to small spontaneous modulations of squared unit cells ranging from 2times22\times 22times2 to sqrt32timessqrt32\sqrt{32}\times\sqrt{32}sqrt32timessqrt32. It is found that a 4times44\times 44times4 bond-order (BO) modulation appears spontaneously on top of the staggered flux pattern for hole doping around 1/8. A related wave-function is then constructed and optimized accurately and its properties studied extensively using an approximation-free variational Monte Carlo scheme. Finally, the competition of the BO-modulated staggered flux wave-function w.r.t. the d-wave RVB wave-function or the commensurate flux state is investigated. It is found that a short range Coulomb repulsion penalizes the d-wave superconductor and that a moderate Coulomb repulsion brings them very close in energy. Our results are discussed in connection to the STM observations in the under-doped regime of some cuprates.

Physical Review B, 2012

We present Monte Carlo simulations for a classical antiferromagnetic Heisenberg model with both n... more We present Monte Carlo simulations for a classical antiferromagnetic Heisenberg model with both nearest (J1) and next-nearest (J2) exchange couplings on the square lattice in the presence of nonmagnetic impurities. We show that the order-by-disorder entropy selection, associated with the Ising-like phase transition that appears for J2/J1 > 1/2 in the pure spin model, is quenched at low temperature due to the presence of non-magnetic impurities. Evidences that a new competing order is stabilized around the impurities, and in turn induces a re-entrance phase transition are reported. Implications for local magnetic measurement of the parent compound of iron pnictides are briefly discussed.

Physical Review B, 2011

We derive a general formalism for evaluating the high-frequency limit of the thermoelectric power... more We derive a general formalism for evaluating the high-frequency limit of the thermoelectric power of strongly correlated materials, which can be straightforwardly implemented in available first principles LDA+DMFT programs. We explore this formalism using model Hamiltonians and we investigate the validity of approximating the static thermoelectric power S 0 , by its hightemperature limit, S * . We point out that the behaviors of S * and S 0 are qualitatively different for a correlated Fermi liquid near the Mott transition, when the temperature is in the coherent regime.

Journal of Physics: Condensed Matter, 2006

ABSTRACT Gel electrophoresis can be used to separate nicked circular DNA molecules of equal lengt... more ABSTRACT Gel electrophoresis can be used to separate nicked circular DNA molecules of equal length but forming different knot types. At low electric fields, complex knots drift faster than simpler knots. However, at high electric field the opposite is the case and simpler knots migrate faster than more complex knots. Using Monte Carlo simulations we investigate the reasons of this reversal of relative order of electrophoretic mobility of DNA molecules forming different knot types. We observe that at high electric fields the simulated knotted molecules tend to hang over the gel fibres and require passing over a substantial energy barrier to slip over the impeding gel fibre. At low electric field the interactions of drifting molecules with the gel fibres are weak and there are no significant energy barriers that oppose the detachment of knotted molecules from transverse gel fibres. © 2006 IOP Publishing Ltd.