arXiv subjects

S. Yunoki

Publications and source records attributed to S. Yunoki.

28 records · Page 2Linked to original sources

arXiv2002

Optical Conductivity $σ(ω)$ and Resistivity $ρ_{dc}$ of a Hole Doped Spin-Fermion Model for Cuprates

The optical conductivity and Drude weight of a Spin-Fermion model for cuprates are studied as a function of electronic density and temperature. This model develops stripes and robust D-wave pairing correlation upon hole doping, and it has the advantage that it can be numerically simulated without sign problems. Both static and dynamical information can be obtained. In this work it was possible to analyze up to 12x12 site clusters at low temperatures ranging between 0.01t and 0.1t (between 50K and 500K for a hopping amplitude $t \sim 0.5 eV$). As the temperature is reduced, spectral weight is transferred from high to low frequencies in agreement with the behavior observed experimentally. Varying the hole density, the Drude weight has a maximum at the optimal doping for the model, i.e., at the density where the pairing correlations are stronger. It was also observed that the inverse of the Drude weight, roughly proportional to the resistivity, decreases linearly with the temperature at optimal doping, and it is abruptly reduced when robust pairing correlations develop upon further reducing the temperature. The behavior and general form of the optical conductivity are found to be in good agreement with experimental results for the cuprates. Our results also establish the Spin-Fermion model for cuprates as a reasonable alternative to the t-J model, which is much more difficult to study accurately.

cond-mat.supr-con↗

arXiv2001

Robust D-wave Pairing Correlations in a Hole-Doped Spin-Fermion Model for Cuprates

Pairing Correlations are studied numerically in the hole-doped spin-fermion model for cuprates. Simulations performed on up to 12x12 clusters provide robust indications of D-wave superconductivity away from half-filling. The pairing correlations are the strongest in the direction perpendicular to the dynamic stripe-like inhomogeneities that appear in the ground state at some densities. An optimal doping, where the correlations reach a maximum value, was observed at about 25% doping, in qualitative agreement with hight T_{c} cuprates' experiments. On the other hand, pairing correlations are suppressed by static stripe inhomogeneities.

cond-mat.supr-con↗

arXiv2000

Fermi Surface and Spectral Functions of a Hole Doped Spin-Fermion Model for Cuprates

Using numerical techniques we study the spectral function $A(k,ω)$ of a spin-fermion model for cuprates in the regime where magnetic and charge domains (stripes) are developed upon hole-doping. From $A(k,ω)$ we study the electronic dynamics and determine the Fermi Surface (FS), which is compared with angular resolved photoemission results for $La_{2-x}Sr_xCuO_2$. A pseudogap is observed in the density of states at the chemical potential for all finite dopings. The striped ground state appears to be metallic in this model since there is finite spectral weight at the chemical potential, but the electronic hopping seems to be stronger perpendicular to the stripes rather than along them. The band structure is not rigid, contrary to the behavior found in mean-field studies, and changes with doping. Both mid-gap (stripe induced) and valence band states determine the FS. For vertical (horizontal) stripes, a clear FS appears close to $(π,0)$ $((0,π))$, while no FS is observed close to $(0,π)$ $((π,0))$. Along the diagonal direction the spectral function shows a clear quasi-particle peak close to (0,0), but its weight is reduced as the chemical potential is approached. A weak FS develops along this direction as the system is doped.

cond-mat.supr-con↗

arXiv2000

Spin-flux phase in the Kondo lattice model with classical localized spins

We provide numerical evidence that a spin-flux phase exists as a ground state of the Kondo lattice model with classical local spins on a square lattice. This state manifests itself as a double-Q magnetic order in the classical spins with spin density at both $(0,π)$ and $(π,0)$ and further exhibits fermionic spin currents around an elementary plaquette of the square lattice. We examine the spin-wave spectrum of this phase. We further discus an extension to a face centered cubic (FCC) lattice where a spin-flux phase may also exist. On the FCC lattice the spin-flux phase manifests itself as a triple-Q magnetically ordered state and may exist in $γ$-Mn alloys.

cond-mat.str-el↗

arXiv1998

Phase Separation Induced by Orbital Degrees of Freedom in Models for Manganites with Jahn-Teller Phonons

The two-orbital Kondo model with classical Jahn-Teller phonons is studied using Monte Carlo techniques. The observed phase diagram is rich, and includes a novel regime of phase separation induced by the orbital degrees of freedom. Experimental consequences of our results are discussed. In addition, the optical conductivity $σ(ω)$ of the model is presented. It is shown to have several similarities with experimental measurements for manganites.

cond-mat.str-el↗

arXiv1998

Phase Separation in Models for Manganites: Theoretical Aspects and Comparison with Experiments

This paper is an informal review of the present status of the computational studies of models for manganites that recently reported the existence of phase separation at low temperatures (S. Yunoki et al., Phys. Rev. Lett. 80, 845 (1998) and cond-mat/9807149). Results for both the one-orbital and two-orbital models, the latter with Jahn-Teller phonons, are included. A large list of experimental results discussed in the paper are compatible with the existence of charge-inhomogeneities in the manganites, which may be an important ingredient for the explanation of the CMR effect in these compounds.

cond-mat.str-el↗

arXiv1998

Influence of Nearest-Neighbor Coulomb Interactions on the Phase Diagram of the Ferromagnetic Kondo Model

The influence of a nearest-neighbor Coulomb repulsion of strength V on the properties of the Ferromagnetic Kondo model is analyzed using computational techniques. The Hamiltonian studied here is defined on a chain using localized S=1/2 spins, and one orbital per site. Special emphasis is given to the influence of the Coulomb repulsion on the regions of phase separation recently discovered in this family of models, as well as on the double-exchange-induced ferromagnetic ground state. When phase separation dominates at V=0, the Coulomb interaction breaks the large domains of the two competing phases into small ``islands'' of one phase embedded into the other. This is in agreement with several experimental results, as discussed in the text. Vestiges of the original phase separation regime are found in the spin structure-factor as incommensurate peaks, even at large values of V. In the ferromagnetic regime close to density $n=0.5$, the Coulomb interaction induces tendencies to charge ordering without altering the fully polarized character of the state. This regime of ``charge- ordered ferromagnetism'' may be related with experimental observations of a similar phase by C. H. Chen and S-W. Cheong (Phys. Rev. Lett. 76, 4042 (1996)). Our results reinforce the recently introduction notion (see e.g., S. Yunoki et al., Phys. Rev. Lett. 80, 845 (1998)) that in realistic models for manganites analyzed with unbiased many-body techniques, the ground state properties arise from a competition between ferromagnetism and phase-separation/charge-ordering tendencies.

cond-mat.str-el↗

arXiv1997

Phase Separation in Electronic Models for Manganites

The Kondo lattice Hamiltonian with ferromagnetic Hund's coupling as a model for manganites is investigated. The classical limit for the spin of the (localized) $t_{2g}$ electrons is analyzed on lattices of dimension 1,2,3 and $\infty$ using several numerical methods. The phase diagram at low temperature is presented. A regime is identified where phase separation occurs between hole undoped antiferromagnetic and hole-rich ferromagnetic regions. Experimental consequences of this novel regime are discussed. Regions of incommensurate spin correlations have also been found. Estimations of the critical temperature in 3D are compatible with experiments.

cond-mat.str-el↗

arXiv1997

The Ferromagnetic Kondo Model for Manganites: Phase Diagram and Charge Segregation Effects

The phase diagram of the ferromagnetic Kondo model for manganites is investigated using computational techniques. In clusters of dimensions 1 and 2, Monte Carlo simulations in the limit where the localized spins are classical show a rich low temperature phase diagram with three dominant regions: (i) a ferromagnetic phase, (ii) phase separation between hole-poor antiferromagnetic and hole-rich ferromagnetic domains, and (iii) a phase with incommensurate spin correlations. Possible experimental consequences of the regime of phase separation are discussed. Studies using the Lanczos algorithm and the Density Matrix Renormalization Group method applied to chains with localized spin 1/2 (with and without Coulombic repulsion for the mobile electrons) and spin 3/2 degrees of freedom give results in excellent agreement with those in the spin localized classical limit. The Dynamical Mean Field ($D=\infty$) approximation was also applied to the same model. At large Hund coupling phase separation and ferromagnetism were identified, again in good agreement with results in low dimensions. In addition, a Monte Carlo study of spin correlations allowed us to estimate the critical temperature for ferromagnetism $T_c^{FM}$ in 3 dimensional clusters. It is concluded that $T_c^{FM}$ is compatible with current experimental results.

cond-mat.str-el↗

arXiv1997

Insulator-Metal Transition in One Dimension Induced by Long-Range Electronic Interactions

The effects of a long range electronic potential on a one dimensional commensurate Charge Density Wave (CDW) state are investigated. Using numerical techniques it is shown that a transition to a metallic ground state is reached as the range of the electron-electron repulsion increases. In this metallic state, the optical conductivity exhibits a large Drude weight. Possible interpretations of our results are discussed.

cond-mat.str-el↗