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arXiv:0903.3168 · Physical Review B

Incommensurate antiferromagnetic fluctuations in the two-dimensional Hubbard model

H. C. Krahl, Simon Friederich, CHRISTOF WETTERICH

Published Jul 31, 2009• 13 citations
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Abstract

Commensurate and incommensurate antiferromagnetic fluctuations in the two-dimensional repulsive $t\text{\ensuremath{-}}{t}^{\ensuremath{'}}$-Hubbard model are investigated using functional renormalization group equations. For a sufficient deviation from half-filling we establish the existence of local incommensurate order below a pseudocritical temperature ${T}_{pc}$. Fluctuations not accounted for in the mean-field approximation are important---they lower ${T}_{pc}$ by a factor $\ensuremath{\approx}2.5$.

DOI
10.1103/physrevb.80.014436

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Commensurate and incommensurate antiferromagnetic fluctuations in the two-dimensional repulsive t\ensuremath-t\ensuremath′t\text{\ensuremath{-}}{t}^{\ensuremath{'}}t\ensuremath-t\ensuremath′-Hubbard model are investigated using functional renormalization group equations. For a sufficient deviation from half-filling we establish the existence of local incommensurate order below a pseudocritical temperature Tpc{T}_{pc}Tpc​. Fluctuations not accounted for in the mean-field approximation are important---they lower Tpc{T}_{pc}Tpc​ by a factor \ensuremath≈2.5\ensuremath{\approx}2.5\ensuremath≈2.5.
The magnetic states occurring in the two-dimensional Hubbard model in the vicinity of half-filling of the conduction band are studied in the case of weak correlations, using Hartree-Fock theory. It is shown that at zero temperature the commensurate antiferromagnetic state is unstable against domain-wall formation for arbitrarily small deviations from half-filling. The resulting incommensurate antiferromagnet remains initially insulating. The predictions of the model for the polarization and modulation wave vector of the incommensurate antiferromagnet are discussed.
We analyze the interplay of antiferromagnetism and pairing in the two-dimensional Hubbard model with a moderate repulsive interaction. Coupled charge, magnetic, and pairing fluctuations above the energy scale of spontaneous symmetry breaking are treated by a functional renormalization group flow, while the formation of gaps and order below that scale is treated in mean-field theory. The full frequency dependences of the interaction vertices and gap functions are taken into account. We compute the magnetic and pairing gap functions as a function of doping ppp and compare with results from a static approximation. In spite of the strong frequency dependences of the effective interactions and of the pairing gap, important physical results from previous static functional renormalization group calculations are confirmed. In particular, there is a sizable doping regime with robust pairing coexisting with N\'eel or incommensurate antiferromagnetism. The critical temperature for magnetic order is interpreted as the pseudogap crossover temperature. Computing the Kosterlitz-Thouless temperature from the superfluid phase stiffness, we obtain a superconducting dome in the (p,T)(p,T)(p,T) phase diagram centered around 15% hole doping.
A scheme of symmetry breaking is proposed for the two-dimensional Hubbard model at less than half filling. The self-induced mean field is a random spin-dependent potential. The energy of the spin-disordered state is shown to be lower than that of commensurate or incommensurate antiferromagnetic states. Long-wavelength charge and spin fluctuations obey the diffusion equation, leading to deviations from Fermi-liquid behavior. At wave vector q=(\ensuremath{\pi},\ensuremath{\pi}) the transverse spin-excitation spectrum has a gap.
We study the low-temperature critical behavior of the one-dimensional Hubbard model near half filling caused by enhanced antiferromagnetic fluctuations. We use a mean-field-type approximation with a two-particle self-consistency renormalizing the bare interaction. It allows us to control a transition from high to low temperatures as well as from weak to strong-coupling. We show that there is a crossover temperature T0 = t exp{−1/Uρ(0)} for arbitrary interaction U > 0 and the bare density of states at the Fermi energy ρ(0) > 0. The solution at lower temperatures goes over to strong coupling and approaches a quantum critical point with the diverging staggered susceptibility and a gap in the excitation spectrum at zero temperature.
The interplay between antiferromagnetism and superconductivity is studied by using the three-dimensional nearly half-filled Hubbard model with anisotropic transfer matrices tz{t}_{z}tz​ and ${t}_{\ensuremath{\perp}}.$ The phase diagrams are calculated for varying values of the ratio ${r}_{z}{=t}_{z}{/t}_{\ensuremath{\perp}}$ using the spin fluctuation theory within the fluctuation-exchange approximation. The antiferromagnetic phase around the half-filled electron density expands while the neighboring phase of the anisotropic ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave superconductivity shrinks with increasing rz.{r}_{z}.rz​. For small rz{r}_{z}rz​ Tc{T}_{c}Tc​ decreases slowly with increasing rz.{r}_{z}.rz​. For moderate values of rz{r}_{z}rz​ we find a second order transition, with lowering temperature, from the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave superconducting phase to a phase where an incommensurate spin density wave (SDW) coexists with ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave superconductivity. Resonance peaks discussed previously for two-dimensional (2D) superconductors are shown to survive in the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave superconducting phase of 3D systems. Soft components of the incommensurate SDW spin fluctuation mode grow as the coexistent phase is approached.
Salmhofer [Commun. Math. Phys. 194, 249 (1998)] recently developed a new renormalization-group method for interacting Fermi systems, where the complete flow from the bare action of a microscopic model to the effective low-energy action, as a function of a continuously decreasing infrared cutoff, is given by a differential flow equation which is local in the flow parameter. We apply this approach to the repulsive two-dimensional Hubbard model with nearest- and next-nearest-neighbor hopping amplitudes. The flow equation for the effective interaction is evaluated numerically on a one-loop level. The effective interactions diverge at a finite-energy scale which is exponentially small for small bare interactions. To analyze the nature of the instabilities signaled by the diverging interactions we extend Salmhofer's renormalization group for the calculation of susceptibilities. We compute the singlet superconducting susceptibilities for various pairing symmetries, and also charge- and spin-density susceptibilities. Depending on the choice of the model parameters (hopping amplitudes, interaction strength, and band filling) we find commensurate and incommensurate antiferromagnetic instabilities or d-wave superconductivity as leading instability. We present the resulting phase diagram in the vicinity of half-filling, and also results for the density dependence of the critical energy scale.