Interaction induced collapse of a section of the Fermi sea in in the zig-zag Hubbard ladder

Using the next-nearest neighbor (zig-zag) Hubbard chain as an one dimemensional model, we investigate the influence of interactions on the position of the Fermi wavevectors with the density-matrix renormalization-group technique (DMRG). For suitable choices of the hopping parameters we observe that electron-electron correlations induce very different renormalizations for the two different Fermi wavevectors, which ultimately lead to a complete destruction of one section of the Fermi sea in a quantum critical point

Jahn–Teller instability in cationic boron and carbon buckyballs B₈₀⁺ and C₆₀⁺: a comparative study

This paper investigates the Jahn–Teller effect in the icosahedral cation B80+ and compares the descent in symmetry with that in C60+. For both cations the icosahedral ground state is a 2Hu state, which exhibits a H [multiply sign in circle] (g ⊕ 2h) Jahn–Teller instability. A detailed construction of the potential energy surface of B80+ using different DFT methods including B3LYP/6-31G(d), VWN/6-31G(d), PBE/TZP and PBE/6-31G(d) shows that, contrary to C60+, which prefers D5d symmetry, the ground state of B80+ adopts S6 point group symmetry. A D3d structure is identified as a saddle point among the S6 minima of B80+. The distortion of D3d to S6 in B80+ is attributed to a superposition of Jahn–Teller and pseudo-Jahn–Teller effects. Imaginary modes, transforming as the gg representation, which are present in neutral icosahedral B80, form the dominant symmetry breaking active modes. The pronounced difference between the JT effects in the boron and carbon buckyball cations is due to the plasticity of the boron caps. The calculated Jahn–Teller stabilization of B80+ is nearly 1549 cm−1 (PBE/TZP), which exceeds the stabilization of 596 cm−1 computed for C60+ at the same level

Relationship between spiral and ferromagnetic states in the Hubbard model in the thermodynamic limit

We explore how the spiral spin(SP) state, a spin singlet known to accompany fully-polarized ferromagnetic (F) states in the Hubbard model, is related with the F state in the thermodynamic limit using the density matrix renormalization group and exact diagonalization. We first obtain an indication that when the F state is the ground state the SP state is also eligible as the ground state in that limit. We then follow the general argument by Koma and Tasaki [J. Stat. Phys. {\bf 76}, 745 (1994)] to find that: (i) The SP state possesses a kind of order parameter. (ii) Although the SP state does not break the SU(2) symmetry in finite systems, it does so in the thermodynamic limit by making a linear combination with other states that are degenerate in that limit. We also calculate the one-particle spectral function and dynamical spin and charge susceptibilities for various 1D finite-size lattices. We find that the excitation spectrum of the SP state and the F state is almost identical. Our present results suggest that the SP and the F states are equivalent in the thermodynamic limit. These properties may be exploited to determine the magnetic phase diagram from finite-size studies.Comment: 17 figures, to be published in Phys. Rev.

An electron correlation originated negative magnetoresistance in a system having a partly flat band

Inspired from an experimentally examined organic conductor, a novel mechanism for negative magnetoresistance is proposed for repulsively interacting electrons on a lattice whose band dispersion contains a flat portion (a flat bottom below a dispersive part here). When the Fermi level lies in the flat part, the electron correlation should cause ferromagnetic spin fluctuations to develop with an enhanced susceptibility. A relatively small magnetic field will then shift the majority-spin Fermi level to the dispersive part, resulting in a negative magnetoresistance. We have actually confirmed the idea by calculating the conductivity in magnetic fields, with the fluctuation exchange approximation, for the repulsive Hubbard model on a square lattice having a large second nearest-neighbor hopping.Comment: RevTex, 5 figures in Postscript, to be published in Phys. Rev.

Dielectric catastrophe at the Mott transition

We study the Mott transition as a function of interaction strength in the half-filled Hubbard chain with next-nearest-neighbor hopping t' by calculating the response to an external electric field using the Density Matrix Renormalization Group. The electric susceptibility chi diverges when approaching the critical point from the insulating side. We show that the correlation length xi characterizing this transition is directly proportional to fluctuations of the polarization and that chi ~ xi^2. The critical behavior shows that the transition is infinite-order for all t', whether or not a spin gap is present, and that hyperscaling holds.Comment: 4 pages, 4 eps figures, REVTe

High magnetic anisotropy of Fe+ ions in KTaO3 and SrCl2

The zero-field splitting constant, D, and the gyromagnetic tensor of the off-center system KTaO3:Fe+ and SrCl2:Fe+ have been explored by means of calculations based on the density-functional theory at the C4v local equilibrium geometry. The calculated D values for KTaO3:Fe+ (9?cm?1) and SrCl2:Fe+ (53 cm?1) are found to be much higher than typical figures measured for insulating compounds containing common 3d Kramers ions with a spin S>1/2 in the ground state. This result together with the calculated g? and g? values concur with available experimental information. The high magnetic anisotropy derived for Fe+ in KTaO3 and SrCl2 is shown to be strongly related to the existence of a 4E excited state lying only at about 3000 and 600?cm?1, respectively, above the ground state. Implications of present findings in the search of new molecular magnets with high values of the magnetic anisotropy are discussed in some detail.The support by the Spanish Ministerio de Ciencia y Tecnología under Projects No. FIS2006-02261 and No. FIS2009-07083 is acknowledged.The support by the Spanish Ministerio de Ciencia y Tecnología under Projects No. FIS2006-02261 and No. FIS2009-07083 is acknowledged

Investigation of Quantum Phase Transitions using Multi-target DMRG Methods

In this paper we examine how the predictions of conformal invariance can be widely exploited to overcome the difficulties of the density-matrix renormalization group near quantum critical points. The main idea is to match the set of low-lying energy levels of the lattice Hamiltonian, as a function of the system's size, with the spectrum expected for a given conformal field theory in two dimensions. As in previous studies this procedure requires an accurate targeting of various excited states. Here we discuss how this can be achieved within the DMRG algorithm by means of the recently proposed Thick-restart Lanczos method. As a nontrivial benchmark we use an anisotropic spin-1 Hamiltonian with special attention to the transitions from the Haldane phase. Nonetheless, we think that this procedure could be generally valid in the study of quantum critical phenomena.Comment: 14 pages, LaTeX2e (svjour class), 8 EPS figures. Same version as the published one, with new references and English corrections of the proofreade

Flat-band ferromagnetism induced by off-site repulsions

Density matrix renormalization group method is used to analyze how the nearest-neighbor repulsion V added to the Hubbard model on 1D triangular lattice and a railway trestle (t-t') model will affect the electron-correlation dominated ferromagnetism arising from the interference (frustration). Obtained phase diagram shows that there is a region in smaller-t' side where the critical on-site repulsion above which the system becomes ferromagnetic is reduced when the off-site repulsion is introduced.Comment: 4 pages, RevTex, 6 figures in Postscript, to be published in Phys. Rev.

Existence of Saturated Ferromagnetic and Spiral States in 1D Lieb-Ferrimagnetic Models away from Half-Filling

In order to study conditions for the appearance of ferromagnetism in a wide filling region, we investigate numerically three types of one-dimensional Lieb-ferrimagnetic Hubbard models: a periodic diamond (PD) chain, a periodic alternately-attached leg (PAAL) chain and an open diamond (OD) chain. All of these models have a flat band (or equivalently, degenerate single-electron eigenvalues). The PD and OD chains commonly have a local-loop structure. Nagaoka's theorem holds only in the PD chain. At half-filling, it have been rigorously proven that all of these models are ferrimagnet. Away from half-filling, however, quite different magnetic properties are found. In the fillings 1/3< rho_e <1/2, the ground state of the PD chain for a infinitely-large U is the extended ferromagnetic state, that is, the saturated ferromagnetic state or the spiral state for odd or even number of electrons, respectively. In the PAAL chain, on the other hand, there is no magnetic order. Thus, the flat band is found to be not a sufficient condition of the extended ferromagnetic state. We find, moreover, that the saturated ferromagnetism appears in the OD chain, although the Nagaoka theorem does not hold on this chain. This indicates that the local-loop structure plays an important role on the appearance of the extended ferromagnetic state.Comment: 4 pages, 4 figures, 2 tables. to be published in J. Phys. Soc. Jpn. Vol. 68 No.

Quasiparticle States at a d-Wave Vortex Core in High-Tc Superconductors: Induction of Local Spin Density Wave Order

The local density of states (LDOS) at one of the vortex lattice cores in a high Tc superconductor is studied by using a self-consistent mean field theory including interactions for both antiferromagnetism (AF) and d-wave superconductivity (DSC). The parameters are chosen in such a way that in an optimally doped sample the AF order is completely suppressed while DSC prevails. In the mixed state, we show that the local AF-like SDW order appears near the vortex core and acts as an effective local magnetic field on the quasiparticles. As a result, the LDOS at the core exhibits a double-peak structure near the Fermi level that is in good agreement with the STM observations on YBCO and BSCCO. The presence of local AF order near the votex core is also consistent with the recent neutron scattering experiment on LSCO.Comment: 4 pages, 2 ps figure