spin-density wave and superconductivity in an extended two-dimensional hubbard model with...
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PHYSICAL REVIEW B, VOLUME 64, 172507
Spin-density wave and superconductivity in an extended two-dimensional Hubbard modelwith nearest-neighbor attraction
W. P. Su and Yan ChenDepartment of Physics and Texas Center for Superconductivity, University of Houston, Houston, Texas 77204
~Received 1 March 2001; published 4 October 2001!
We study the interplay of antiferromagnetic and superconducting orders in a two-dimensional Hubbardmodel with on-site repulsion and nearest-neighbor attractive interaction. This type of interaction has beenwidely used to modeld-wave superconductivity. Our mean-field calculations indicate that such an attractiveinteraction enhances the staggered magnetization produced by the on-site repulsion. As a result, superconduc-tivity is strongly suppressed. Stripe formation is also affected.
DOI: 10.1103/PhysRevB.64.172507 PACS number~s!: 74.72.2h, 74.20.2z, 74.80.2g
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I. INTRODUCTION
A salient feature of the high-temperature superconducity in the cuprates is the association with antiferromagnetiBesides being an interesting subject by itself, the interacbetween those two orders may provide important informaton the mechanism of superconductivity.
The current theoretical understanding of the antiferromnetic order is more complete than that of the supercondtivity. A simple on-site Hubbard model seems capableexplaining some features of the antiferromagnetism. In pticular, the antiferromagnetic domain-wall-like structur~the stripes1,2!, which have been receiving increasing expemental support,3,4 emerge naturally from mean-field studythe Hubbard model.
Most of our knowledge about superconductivity comfrom experiment. A completely microscopic understandof the mechanism is not available yet. There has nonethebeen theoretical activity of modeling superconductivityemploying some effective Hamiltonian. Just like the cawhere one uses the on-site attractive Hubbard model to ss-wave pairing, one is interested in studying models whwould generated-wave superconductivity. Indeed, this hbeen done by adding a nearest-neighbor attracinteraction5 to the on-site repulsive Hubbard model. Manphenomenological studies ofd-wave superconductivity6–12
have been based on this model. In those studies, howevespin-density wave~SDW! produced by the repulsive interaction has been ignored. A more complete treatment involvboth the SDW and superconductivity has been done orecently by Martinet al.13 They showed that superconducing stripes are inhomogeneous solutions of the lineari~mean-field! Hamiltonian.
In deriving the mean-field Hamiltonian, Martinet al.havedropped a Hartree term coming from the attractive intertion. They have assumed that such a term is not goingoverwhelm the charge and spin order determined by thesite repulsion. The purpose of this paper is to assesseffect of this term. As we will see, this term does signicantly affect the spin and charge orders and thereby strosuppresses the superconductivity as well.
The extended Hubbard Hamiltonian runs as
H52t (^ i j &s
cis† cj s1U(
ini↑ni↓1V(̂
i j &ni↓nj↑ , ~1!
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the linearization of which leads to the following mean-fieHamiltonian:
Hm f52t (^ i j &s
cis† cj s1U(
i~ni↑^ni↓&1^ni↑&ni↓!
1V(̂i j &
~ni↓^nj↑&1^ni↓&nj↑!1(̂i j &
ci↓cj↑D i j* 1H.c.,
~2!
where D i j 5V^ci↓cj↑& is the superconducting order parameter andnis is the number operator~the HamiltonianHm f isthe subject of the present study! and
V(̂i j &
~ni↓^nj↑&1^ni↓&nj↑! ~3!
is the Hartree term that has been ignored by Martinet al.
II. METHODOLOGY
To solveHm f we follow the standard iterative procedurFor a given initial set of̂ nis& andD i j , the Bogoliubov–deGennes equations are solved numerically and the electrwave functions obtained are used to calculate the new^nis&andD i j for the next iteration step. The calculation is repeauntil the solution converges. By varying the chemical potetial, one obtains solutions corresponding to various dopconcentrations. Periodic boundary conditions are adoptedboth the x and y directions. We refer to earliepublications6,13 for details of this numerical procedure.
III. RESULTS
Hereafter we taket51, U54 and measure energiesunits of t. For V50, the ground-state configuration of thhalf-filled case is well known to be a uniform spin-denswave. The magnitude of the staggered magnetizationumi u5u^ni↑&2^ni↓&u is about 0.69. A negativeV enhances themagnetization. It also widens the single-particle energy gas shown in Fig. 1. This enhancement can be understoonoting that at half-filling, apart from a constant, the expection value^nj↓& in Eq. ~3! can be replaced by2^ni↑& assum-ing a uniform SDW in the ground state. The Hartree term
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BRIEF REPORTS PHYSICAL REVIEW B 64 172507
then equivalent to a linearized on-site Hubbard term wU524V.
As we have alluded to in the Introduction, the originintention of introducing a negativeV is to produced-wavesuperconductivity. In the absence of the Hartree termSDW ground state produced byU can be made superconducting by makingV sufficiently negative. For examplefor U54 the d-wave superconducting order parameterDdis essentially zero foruVu less than 3.6 but it increases to 0for V524.4, as seen in Fig. 2. It turns out that the ordparameters are not uniform even for the half-filled caThe range of the order parameters is indicated byerror bars. The magnitude of the SDW is also displayedFig. 2.
As soon as the Hartree term~3! is reinstated, however,Ddis always negligible for half-filling and for any doping concentration. The result can be interpreted as the enhanceof antiferromagnetism by the Hartree term. In other wordsnegativeV will generate a SDW which is strong enoughsuppress the superconductivity.
An enhanced SDW also affects the formation of stripTo get some idea, we examine a few solutions on a 12312
FIG. 1. Staggered magnetization (h) and single-particle energygap (d) as a function of2V for a 12312 lattice at half-filling.
FIG. 2. Staggered magnetization (s) andd-wave superconducting order parameter (l) as a function of2V for an 838 lattice athalf-filling calculated without the Hartree term~3!.
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lattice for U54. Figure 3~a! depicts the staggered magnezation profile for 20 holes withV520.8. In theV50 case,the valley of the surface would reach20.69 with a looplikedomain wall ~line of zero magnetization!. In contrast, herethe staggered magnetization stays far above20.85. As oneproceeds to a slightly higher doping~24 holes!, one expectsto see two straight domain walls~stripes!. That indeedhappens, but the separation between the two stripes naras V becomes more negative. AtV520.8 they almostannihilate each other as shown in Fig. 3~b!. For an even morenegativeV521.5 the depression in SDW no longer appeastraight; instead it becomes square like and the botis shallower than before. It should be emphasized againfor all the solutions in Fig. 3, we do not detect any trace
FIG. 3. Distribution of staggered magnetization on a 12312lattice for ~a! 20 holes withV520.8, ~b! 24 holes withV520.8,and ~c! 24 holes withV521.5.
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BRIEF REPORTS PHYSICAL REVIEW B 64 172507
superconductivity.Martin et al. have argued that the addition of a diagon
hopping term to the Hamiltonian~2! would reduce thesingle-particle gap in the SDW energy spectrum, theremaking pairing more likely. We have added such a termmagnitude2t8 and calculated the SDW gap for 16 holesan 838 lattice. As shown in Fig. 4, the gap actually increases witht8. In any case, we fail to find any supercondutivity again.
IV. DISCUSSION
From our study so far it seems that nearest-neigh
FIG. 4. Single-particle energy gap as a function of the diagohopping strengtht8.
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attractive interaction leads to stronger antiferromagnetinstead ofd-wave superconductivity, in line with the obsevation made by Emery, Kivelson, and Zachar14 regardingthe difficulty of achieving high-temperature superconductity through strong effective attractions. Previous employmof this interaction to generated-wave superconductivityis not self-consistent at the mean-field level. Whethconsideration of a more long-range or more complicatedteraction might change the conclusion remains to be invegated.
We end by emphasizing that our study is a mean-fieldand has its limitations. It is well known that mean-fietheory tends to overestimate the stability of the SDW phaIn particular Hellberg and Manousakis15 have investigatedthe t-J model and have shown that the striped phasesexcited states rather than the ground-state in the perigeometry. They have attributed16 the ground state stripephase found by White and Scalapino17 in another approach toa cylindrical boundary condition. While we have used fuperiodic geometry in all our calculations, the possibility rmains open that correlation effect may favor superconducity over SDW.
This work was partially supported by the Texas CenterSuperconductivity, the Robert A. Welch Foundation, tTexas Advanced Technology Program under Grant N003652-0222-1999, and the Texas Advanced Researchgram under Grant Nos. 003652-707-1997. We thank CTing, Jian-Xin Zhu, and Z. Y. Weng for stimulating discusions.
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