Development of a two-particle self-consistent method for multi-orbital systems and its application to unconventional superconductors

0.68 MB·English

by  Hideyuki Miyahara

Development of a two-particle self-consistent method for multi-orbital systems and its
application to unconventional superconductors
Hideyuki Miyahara1, Ryotaro Arita1,2, and Hiroaki Ikeda3
1Department of Applied Physics, University of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan
2JST PRESTO, Kawaguchi, Saitama, 332-0012, Japan and
3Department of Physics, Kyoto University, Kyoto 606-8502, Japan
(Dated: January 16, 2013)
3
1 We extend the two-particle self-consistent method proposed by Vilk and Tremblay (J. Phys. I
0 France7, 1309-1368 (1997)) to studysuperconductivityin multi-orbital systems. Startingwith the
2 sumrulesforthespinandchargesusceptibilities,wederiveself-consistentequationstodeterminethe
n renormalized effectiveinteractions. We apply this method to the two-orbital dx2−y2-d3z2−r2 model
for La CuO and the five-orbital d-model for LaFeAsO. Comparing the results with those of the
a 2 4
J randomphaseapproximationorthefluctuationexchangeapproximationinwhichvertexcorrections
areignored,wediscusshowthevertexcorrectionsaffectthepairinginstabilityofLa CuO andthe
5 2 4
dominant pairing symmetry of LaFeAsO.
1

]
n I. INTRODUCTION In this method, vertex corrections in the charge and
o spin channel are assumed to be momentum and fre-
c SincetheseminalstudiesbySuhl1 andKondo,2 super- quency independent, and they are determined in such a
-
r conductivityinmulti-orbitalsystemshasbeenoneofthe way that the correlation functions meet their sum rules.
p major topics in condensed matter physics. So far, many Withthisnumericallyinexpensivetreatment,ithasbeen
u
s kinds of multi-orbital superconductors such as MgB2 demonstrated for the single-band Hubbard model that
. (Ref. 3), alkali-doped C (Ref. 4), Na CoO · yH O TPSCshowsgoodagreementwithquantumMonteCarlo
t 60 x 2 2
a (Ref.5),Sr RuO (Ref.6),iron-basedsuperconductors,7 (QMC) calculations.
2 4
m and heavy fermion superconductors8 have been discov- Inthispaper,weformulateTPSCforthemulti-orbital
- ered and studied extensively. Theoretically, a variety Hubbard model. First, we derive a series of equa-
d
of exotic unconventional pairing mechanisms going be- tions to determine the vertex corrections in the spin
n
yondtheMigdal-Eliashbergtheory9 havebeenproposed. and charge channel, and then apply this method to a
o
c Forexample,ithas beenconsideredfor the cobaltatesu- two-orbital model for La2−x(Sr/Ba)xCuO4 and a five-
[ perconductor that the Hund’s coupling (which of course orbital model for F-doped LaFeAsO. Recently, the two-

1 does not exist for single-orbital systems) induces triplet orbital model (which we call the dx2−y2-d3z2−r2 model)
superconductivity,10 and it has recently become an is- was studied by FLEX18 to give an insight into the ma-
v
4 sue of hot debates whether orbital fluctuations mediate terial dependence of superconducting transition temper-
5 superconductivity in the iron-based superconductors.11 ature (Tc). While FLEX successfully describes the dif-
2 To investigate these fascinating possibilities, accurate ference between La2−x(Sr/Ba)xCuO4 (Tc ∼ 40 K) and
3 calculations of superconductivity in correlated multi- HgBa2CuO4+δ (Tc ∼ 90 K), it underestimates the pair-
1. orbitalmodels areindispensable. Amongmany available ing instability for La2−x(Sr/Ba)xCuO4 and Tc is much
0 approaches,fromtheweakcouplingside,oneoftenstarts lowerthanthe experimentalvalue. We show that, in the
3 with the random phase approximation (RPA). Since the present multi-orbitalTPSC calculation, the inter-orbital
1 pioneering work for the single-band Hubbard model by scattering enhances the d-wave instability and reason-
v: Scalapino et al.,12 RPA has been successfully applied to ablevalueofTc isobtainedfortheintermediatecoupling
i various multi-orbital systems. The fluctuation exchange regime. Forthefive-orbitalmodel,ithasbeenextensively
X
approximation (FLEX) developed by Bickers et al.,13 studied by RPA19,20,22 and FLEX.21 There, strong spin
ar whichincludestheself-energycorrectionself-consistently, fluctuation has been shown to mediate the s-wave su-
has beenalsowidely used. Here, the self-energyis calcu- perconductivitywithsignchanges(theso-calleds±-wave
latedinthemannerofBaymandKadanoff,14andconser- pairing). Ontheotherhand,recently,ithasbeenpointed
vation laws for one-particle quantities such as the total out that vertex corrections can enhance orbital fluctua-
energy and momentum are satisfied. However, due to tions, which mediate s-wave superconductivity without
the absence of vertex corrections,FLEX violates conser- sign changes(the s++-wave pairing).16 In this paper, we
vation laws for two-particle quantities. show that orbital fluctuations are enhanced in TPSC,
Recently, several diagrammatic methods, which take while the dominant pairing symmetry is still s± when
into account some vertex corrections, have been the system resides in the weak coupling regime.
proposed.15,16 Among them, the two-particle self- This paper is organized as follows. In Sec. II, we for-
consistent method (TPSC) proposed by Vilk and mulate multi-orbital TPSC for the Hubbard model. We
Tremblay17 is a promising approach in that it is com- discuss how we calculate the charge (orbital) and spin
patible with conservation laws in the two-particle level. correlationfunctions. InSec. III,weshowtheresultsfor
2
the two- and five-orbital Hubbard model and the sum- Note that the RPA violates the Pauli principles, and
mary of the present study is given in Sec. IV. it does not fulfill the following two sum rules,
T
χsp(q)=h(n (r)−n (r))(n (r)−n (r))i
↑ ↓ ↑ ↓
N
II. METHOD Xq
=n−2hn n i, (2a)
↑ ↓
A. Model T
χch(q)=h(n (r)+n (r))(n (r)+n (r))i−n2
↑ ↓ ↑ ↓
N
Xq
The Hamiltonian of the multi-orbital Hubbard model
=n+2hn n i−n2, (2b)
is given by ↑ ↓
which are exact relations derived via the Pauli princi-
H = ǫµnµσ(r)+ tµrrν′c†µσ(r)cνσ(r′) ples, hnσ(r)2i = hnσ(r)i (see Appendix A). Here, n is
Xrµσ rrX′µνσ the particle number per site and for nonmagnetic states,
+ U nµ↑(r)nµ↓(r)+U′ nµσ(r)nνσ′(r) hn↑(r)i = hn↓(r)i = n/2. Note that the double occu-
Xr h Xµ µX>νXσσ′ pancy,hn↑(r)n↓(r)i≡hn↑n↓iisalsotranslationinvariant
and does not depend on site r.
−J Sµ(r)·Sν(r)+J′ c†µ↑(r)c†µ↓(r)cν↓(r)cν↑(r) , In TPSC, to meet the above conditions [Eqs. (2)], we
µX=6 ν µX6=ν i introduce two independent effective interactions, Usp for
the spin channel and Uch for the charge channel. Then
where c† (r) is a creation operator of an electron the full susceptibilities of Eq. (1) are replaced with
µσ
with spin σ and orbital µ at site r, and n (r) =
µσ
c†µσ(r)cµσ(r), Sµ(r) = (c†µ↑(r),c†µ↓(r))σ(cµ↑(r),cµ↓(r))T χsp(q)= 2χ0(q) , χch(q)= 2χ0(q) . (3)
with the Pauli matrices σ. The on-site Coulomb inter- 1−Uspχ0(q) 1+Uchχ0(q)
actions, U,U′,J,J′ denote the intra-orbital,inter-orbital
Finally, we put the following ansatz:
Coulomb repulsions, the Hund’s exchange, and the pair-
hopping term, respectively. hn n i
Usp = ↑ ↓ U, (4)
hn ihn i
↑ ↓
which is compatible with the equations of motion (see
B. Two-particle self-consistent method for the
single-orbital Hubbard model Appendix B). Equations (2), (3), and (4) provide a set
of self-consistent equations in TPSC. Namely, Uch, Usp,
and hn n i are self-consistently determined for given n
Let us start with a review of TPSC for the ↑ ↓
andU. Theone-particleGreen’sfunctionandself-energy
single-orbital Hubbard model formulated by Vilk and
are calculated by
Tremblay.17 The central quantities in this method are
the spin and charge correlation functions. In the non- G(k)=G0(k)+G0(k)Σ(k)G(k),
magnetic state, the system holds SU(2) symmetry, and
1 T
the spin-spin correlationfunctions do not depend on the Σ(k)= Uspχsp(q)U +Uchχch(q)U G(k−q).
4N
spin directions. Thus we consider the z component of Xq h i
the spin operator Sz(r) = n (r)−n (r) and the charge
↑ ↓
operator n(r) = n (r)+n (r). In RPA, the spin and For the single-band Hubbard model, it has been demon-
↑ ↓
charge correlationfunctions are evaluated as follows: strated that TPSC agrees well with QMC.17
2χ0(q) 2χ0(q)
χsp (q)= , χch (q)= , (1) C. Extension to multi-orbital systems
RPA 1−Uχ0(q) RPA 1+Uχ0(q)
LetushereformulateTPSCforthemulti-orbitalHub-
with the irreducible susceptibility,
bard model. Hereafter, we follow the matrix form em-
T ployed in Refs. 10 and 23. The irreducible susceptibility
χ0(q)=− G0(k)G0(k+q), is defined as
N
Xk
T
χ0 (q)=− G0 (k)G0 (k+q), (5)
where T and N are temperature and number of sites in λµνξ N νλ µξ
the system, and G0(k) = 1/(iǫ +µ−ǫ(k)) is the bare Xk
n
Green’s function with chemical potential µ and energy which can be considered as a matrix element with a row
dispersion ǫ(k). Here, we have introduced the abbrevi- λµ and a column νξ of a matrix χ0(q). In the non-
ations k = (k,iǫ ) and q = (q,iν ) being the fermionic magnetic state, the system is invariant for spin rota-
n n
and bosonic Matsubara frequencies, respectively. tion, and then 2χspz(q)=χsp±(q) holds, where χsp±(q)
3
is the in-plane correlation function between S± (r) = ceptibilities and the double occupancy:
λµ
Sx (r) ± iSy (r) = c† (r)c (r) with σ =↑ or ↓. In
λµ λµ λσ µσ¯ T
TPSC, similar to the single-orbital case, the spin and χch (q)=h(n +n )(n +n )i−hn ihn i,
N µµµµ µ↑ µ↓ µ↑ µ↓ µ µ
charge susceptibilities are given by Xq
=n +2hn n i−n2 (9a)
µ µ↑ µ↓ µ
χsp(q)=(1−χ0(q)Usp)−12χ0(q), (6a) T
χch (q)=h(c† c +c† c )(c† c +c† c )i
χch(q)=(1+χ0(q)Uch)−12χ0(q), (6b) N Xq µνµν µ↑ ν↑ µ↓ ν↓ ν↑ µ↑ ν↓ µ↓
=hn (1−n )i+hn (1−n )i
µ↑ ν↑ µ↓ ν↓
where Usp(ch) is the renormalized effective interaction +hc† c c† c i+hc† c c† c i
µ↓ ν↓ ν↑ µ↑ µ↑ ν↑ ν↓ µ↓
matrixforthespin(charge)channel.10,23 Fortwo-orbital
T
systems, for instance, these are represented as = χspz (q)+2χspz (q) . (9b)
N(cid:18) µνµν µµνν (cid:19)
Xq
Usp Jsp 0 0 Finally, as in the single-band case, we introduce the
1111
Usp = Jsp U2s2p22 0 0 , (7a) following ansatz between the two-particle quantities and
0 0 Usp Jsp the interaction parameters (see Appendix C);
1212
 0 0 Jsp Usp 
 2121
hn n i
Uch 2Uch −Jch 0 0 Usp = σµ σ¯µ U, (10a)
2Uch11−11Jch 11U22ch 0 0  µµµµ hnσµihnσ¯µi
Uch = 2211 2222 ,
 0 0 −U1c2h12+2Jch 0  Usp = hnσµnσ¯νi U′, (10b)
 0 0 0 −Uch +2Jch µνµν hn ihn i
 2121  σµ σ¯ν
(7b) hn n i
Usp −Jsp= σµ σν (U′−J). (10c)
µνµν hn ihn i
σµ σν
where Usp (Uch ) is the intra-orbital Coulomb in-
µµµµ µµµµ Equations. (6)-(10) are a set of self-consistent equations
teraction; Usp (Uch ) with µ 6= ν, the inter-orbital
µνµν µνµν in the multi-orbital case.
Coulombinteraction;Jsp(Jch),theHund’scoupling.10,23
Inthe presentstudy,forsimplicity,weignoretheHund’s
coupling in the charge channel, namely, Jch = 0.24 In D. Eliashberg equation
RPA, one employs the unperturbed bare vertex as fol-
lows, Uµspµ(µcµh) = U, Uµspµ(νcνh) =Uµspν(µcνh) =U′, and Jsp(ch) = Superconductivity has been studied by the following
J. linearized Eliashberg equation,
Nextletus considerthe sumruleformulti-orbitalsys-
tems(seeAppendixA). Inthen-orbitalHubbardmodel, λ∆ll′(k)= Vlm1m4l′(k,k′)Gm1m2(k′)
there are n4 sum rules for χsp(q) and χch(q). Among kX′,mi (11)
them, we use the following equations to determine Usp: ×∆ (k′)G (−k′).
m2m3 m4m3
T Eigenstate ∆ll′(k) with the largest eigenvalue λ was nu-
χspz (q)=2hn i−2hn n i, (8a) mericallyevaluatedbythepowermethod. Thesupercon-
N µµµµ µ↑ µ↑ µ↓
Xq ducting transitionoccursatthe temperatureforwhichλ
T becomesunity. Here,Gll′(k)isthe dressedGreen’sfunc-
N χsµpν±µν(q)=2hc†µ↑cν↓c†ν↓cµ↑i tion,
Xq
=2hnµ↑i−2hnµ↑nν↓i, (8b) Gll′(k)=G0ll′(k)+G0lm(k)Σmm′(k)Gm′l′(k), (12)
T
N χzµ±µνν(q)=2hnµ↑nν↑i−2hnµ↑nµ↓i. (8c) and the self-energy Σll′(k) is given by
Xq
1 T
Σll′(k)= 4N Uspχsp(q)Us0p
Note that the intra-orbital component of the sum rule Xq h (13)
hmaosdtehl.eFsoarmtehefoirnmtera-sorthbaittalocfotmheposinnegnltes-,orwbeituasleHχusbpb±a(rqd) +Uchχch(q)Uc0hilml′m′Gmm′(k−q).
ratherthanχspz(q),sincetheycanbeexpressedinterms
In the present study, we omit the Hartree-Fock term,
of the density operators.
since a part of its contribution is already considered in
For Uch, we use the followingsum rules for the charge the one-body part of the Hamiltonian, which is derived
susceptibilitieswhichcanbe representedbythespinsus- from density functional calculation.
4
The effective interaction Vll′mm′(k,k′) for the spin- La2CuO4
singlet pairing can be expressed in a matrix form as fol-
2
lows:
]
V
V(k)=−3Uspχsp(k) Usp+ 1Uchχch(k)Uch [e 1
2 0 2 0 y
(14) g
− 1Usp− 1Uch, ner 0
2 0 2 0 E
where Usp and Uch are the bare vertex in the spin and -1
0 0
charge channel, respectively.10,23
-2
Γ N X Γ
III. RESULTS
FIG. 1: (Color online) Band structure of the two-orbital
Let us move on to the application of the multi-orbital
model for La2CuO4. The model consists of the dx2−y2 or-
TPSC method to the effective models for La2CuO4 and bital and the d3z2−r2 orbital. The Fermi level is set at 0 eV.
LaFeAsO. Using the technique of the maximally local-
ized Wannier functions,25 these models are derived from
1
first-principles calculations. In the density-functional TPSC
calculations,weemployedthe exchangecorrelationfunc- eq. FLEX
tional proposed by Perdew et al.,26 and the augmented erg 0.8
b
planewaveandlocalorbital(APW+lo)methodasimple- ash 0.6
mentedintheWIEN2Kprogram.27Wethenconstructed Eli
the Wannier functions for the d bands around the Fermi e of 0.4
level, using the WIEN2Wannier (Ref. 28) and the wan- alu
nier90 (Ref. 29) codes. nv 0.2
e
g
Ei
0
A. La CuO 0 0.02 0.04 0.06
2 4
Temperature [eV]
Recently, the two-orbital dx2−y2-d3z2−r2 Hubbard
model for the cuprates were studied to understand the
FIG. 2: (Color online) Temperature dependence of the max-
materialdependenceofTc byFLEX.18 There,theenergy imum eigenvalue of the linearized Eliashberg equation ob-
difference betweenthe dx2−y2 orbitalandthe d3z2−r2 or- tainedbyTPSC(redsolidline)andFLEX(bluedottedline).
bital was found to be a key parameter to characterize U and J are 2.0 eV and 0.2 eV, respectively, and n is set to
La CuO andHgBa CuO . Namely,inthe FLEXcalcu- be2.85.
2 4 2 4
lationforthetwo-orbitalmodel,thepairinginstabilityis
stronger in the latter. On the other hand, in the former,
the eigenvalue of the Eliashberg equation within FLEX orbital and strong commensurate inter-orbital fluctua-
does not reach unity down to T ∼ 40 K. The purpose of tions. We here stress that there is no large peak in the
this subsection is to examine how the vertex corrections charge susceptibilities in the RPA and FLEX calcula-
in TPSC affect the superconductivity in La CuO . tions, so that these enhanced inter-orbital charge fluc-
2 4
The band structure of the effective two-orbital model tuations are purely due to the effects of vertex correc-
for La CuO is shown in Fig. 1. We set U = 2.0 eV, tions. It should be noted that χch (Q) has a negative
2 4 1221
U′ = 1.6 eV, J = 0.2 eV, and n = 2.85. We employ peak around Q=(π,π), which works as attractive force
64×64k-pointmeshesand2048Matsubarafrequencies. betweenthetwoorbitalsford-wavepairingjustlikeanti-
Hereafter,orbitals1and2denotethedx2−y2 andd3z2−r2 ferromagnetic spin fluctuations. [Note that the spin and
orbitals, respectively.30 charge sectors in Eq. (14) have opposite signs.] As we
In Fig. 2, we plot temperature dependence of λ, the will see below, there is a close correlation between the
maximum eigenvalue of the Eliashberg equation. While characteristic enhancement of λ and the charge fluctua-
λ does not show appreciable temperature dependence tions.
in FLEX, λ is drastically enhanced at low temperature Figure 4 depicts temperature dependence of the max-
<0.02eVinTPSC.Thecharacteristicenhancementofλ imum value in the spin susceptibility, the orbital suscep-
in TPSC is attributed to the low-temperature behaviors tibility, and its inverse. We can see that the enhance-
of the spin and charge susceptibility. In Fig. 3, we plot ment of the peaks in χch and −χch dominate over
1212 1221
χsp (q,ω = 0), χch (q,ω = 0), and −χch (q,ω = 0) that of χsp for T < 0.02 eV. This behavior comes
1111 1212 1221 1111
at T = 0.020 eV, which indicate that the system has from the fact that while Usp is renormalized substan-
a strong incommensurate spin correlation in the dx2−y2 tially (U1s1p11 ∼ 1.16 eV at T = 0.020 eV), Uc3h3 (Uc4h4)
5
(a)χsp (b)χch (c) -χch
1111 1212 1221
9
12 12
6
6 6
3
0 (π, π) 0 (π, π) 0 (π, π)
q q q
(0, 0) q y (0, 0) q y (0, 0) q y
x (π, 0) x (π, 0) x (π, 0)
FIG.3: (Coloronline)(a)χsp (q,ω=0),(b)χch (q,ω=0),and(c)−χch (q,ω=0)atT =0.020eV,whereorbitals1and
1111 1212 1221
2 denote thedx2−y2 and d3z2−r2 orbitals, respectively.
(a) (b) (c)
20
10 1.6
π-δ) 68 |π,π) 1126 -χχc1c1h2h21221 |-1π,π) 1.2 -χχc1c1h2h21221
spπ,χ(1111 24 χ(1212(1221) 48 chχ(1212(1221) 00..48
0 | 0 | 0
0 0.02 0.04 0.06 0 0.02 0.04 0.06 0 0.02 0.04 0.06
Temperature [eV] Temperature [eV] Temperature [eV]
FIG. 4: (Color online) Temperature dependenceof themaximum value in (a) χsp , (b) |χch |, and (c) |χch −1|.
1111 1212(1221) 1212(1221)
becomes ∼ 1.55 eV, which is even larger than the bare ture T = 0.018 eV. We see that characteristic structure
value 1.2eVusedinRPAandFLEX.Infact, similaren- emerges at (π,0) and (0,π) in the d3z2−r2 gap function,
hancement of charge channelis also observedin the case due to the inter-orbital effective interaction, dominantly
of the single-orbitalmodel.17 mediated by the orbital fluctuation χch (q), which con-
1221
Figure5showsthegapfunctionsforthedx2−y2 orbital nects ∆11 and ∆22 [Eq. (11)]. Since χc1h221(q) takes a
and the d3z2−r2 orbital (∆11 and ∆22) at T =0.022 eV. largenegativevaluearound(π,π),itcooperateswithan-
They have the d-wave symmetry, which is mediated by tiferromagnetic spin fluctuation to enhance the d-wave
the dominant spin fluctuation, denoted by the black ar- pairing instability.
row. This is the conventionalsituationwhere the orbital
fluctuations remains small.
(a) dx2-y2 (b) dz2
(a) dx2-y2 (b) dz2 π π
π π
q 0 q 0
y y
q 0 q 0
y y
-π -π
-π 0 π -π 0 π
-π -π
q q
-π 0 π -π 0 π x x
q q
x x -0.003 0 0.003 -0.0015 0 0.0015
-0.006 0 0.006 -0.0008 0 0.0008
FIG.6: (Coloronline)PlotssimilartoFig.5forT =0.018eV.
Thedashedblackarrowdenotesthepairscatteringmediated
FIG. 5: (Color online) Gap function for the (a) dx2−y2 or- byorbital fluctuations.
bital and the (b) d3z2−r2 orbital at T = 0.022 eV. The gap
functionshaved-wavesymmetryandtheblackarrowdenotes
the pair scattering mediated by antiferromagnetic spin fluc- This situationchangesinthe strongercouplingregime
tuations. (U ∼ 2.5 eV), where the system goes away from a su-
perconducting instability. This is because the dominant
In Fig. 6, we plot the gap functions at lower tempera- spin/orbital fluctuations make the quasi-particle damp-
6
(a) (b) (c)
100 50 q. 1
π-δ) 6800 |π,π() 3400 -χχc1c1h2h21221 Eliashberg e 00..68
spχπ,(1111 2400 chχ1212(1221) 1200 nvalue of 00..24
0 | 0 ge 0
0 0.02 0.04 0.06 0 0.02 0.04 0.06 Ei 0 0.02 0.04 0.06
Temperature [eV] Temperature [eV] Temperature [eV]
FIG. 7: (Color online) Temperature dependence of the maximum value in (a) χsp , (b) |χch |, and (c) the maximum
1111 1212(1221)
eigenvalue of the linearized Eliashberg equation obtained by TPSC for U =2.5 eV, U′ =2.0 eV,and J =0.25 eV.
ing around (0,π) and (π,0) significant, as is observed in LaFeAsO
the previous FLEX calculation.18 Temperature depen-
2
dence of the maximum value in the spin, orbital suscep-
tibility and the maximum eigenvalue of the Eliashberg ]
V 1
equationλfor U =2.5eV, U′ =2.0 eVand J =0.25eV e
[
areshowninFigs. 7(a),7(b),and7(c),respectively. The y
g
maximum value in χsp always dominates over that of er 0
1111 n
χch and−χch ,andλdoesnotshowanyenhancement. E
1212 1212
-1
-2
B. Iron-based superconductor: LaFeAsO
(0, 0) (π, 0) (π, π) (0, 0)
Letusnowapplymulti-orbitalTPSCtotheiron-based
superconductor, LaFeAsO. The recent discovery of high FIG. 8: (Color online) Band structure of the five-orbital d-
Tc superconductivity in F-doped LaFeAsO32 has stimu- model for LaFeAsO.The Fermi level is set at 0 eV.
latedarenewedinterestinmulti-orbitalsuperconductors.
Asforthepairingmechanismoftheiron-basedsupercon-
ductors, several scenarios have been proposed. Among than that of χsp (q,ω = 0) for T > 0.01 eV. While
2222
them, the possibility of the sign-reversing s -wave su- χch (q,ω = 0) has a broad maximum peak around
± 2424
perconductivity mediated by spin fluctuations22,33 have T = 0.01 eV, χsp (q,ω = 0) grows monotonously as
2222
been extensively studied. While the s -wave solution temperature lowers.
±
has been obtained in the RPA19,20,22 or FLEX21 calcu- The enhancement in the orbital susceptibility comes
lations for the five-orbital d-model, recently, it has been from the vertex correction in the charge susceptibility.
proposedthatvertexcorrectionscanenhanceorbitalfluc- Tomakethispointclear,inFig. 11,weplottemperature
tuations, and the s -pairing without sign reversingbe- dependenceofthemaximumvalueinχch (q,ω =0)and
++ 2424
comes dominant.11 In this subsection, we discuss how χsp (q,ω = 0) obtained by RPA, for which the bare
2222
vertex corrections in TPSC affects superconductivity in coupling constants are set to be U = 1.2 eV, U′ = 0.96
the five-orbital d-model for LaFeAsO. eV, and J = 0.12 eV. We see that while χsp (q,ω = 0)
2222
The band structure of the d-model is shown in Fig. 8. diverges around T = 0.01 eV, χch (q,ω = 0) has no
2424
The bare coupling constants are set to be U = 1.5 eV, significant temperature dependence.
U′ =1.2eV,andJ =0.15eV.34 Thenumberofelectrons In Fig. 12, we show temperature dependence of the
n is 6.1. We employ 64 × 64 k-point meshes and 2048 maximum eigenvalue of the Eliashberg equation. We see
Matsubara frequencies. Hereafter orbitals 1, 2, 3, 4, and thatthesystemhasasuperconductingtransitionaround
5 denote the d3z2−r2, dxz, dyz, dx2−y2, and dxy orbitals, T ∼0.005 eV.
respectively. The associated eigenfunctions of the Eliashberg equa-
To see that TPSC can give enhanced orbital fluctua- tion at T = 0.015 eV (the gap functions) are shown
tions,weplotχsp (q,ω =0)andχch (q,ω =0)atT = in Fig. 13 for the three bands crossing the Fermi level.
2222 2424
0.015 eV in Figs. 9(a) and 9(b), respectively. Clearly, We see that these gap functions have the s symme-
±
both susceptibilities havepeaks around(π,0) and (0,π), try, indicating that the spin fluctuation is the primary
and the peak in the orbital susceptibility is higher than glue of superconductivity. However, there is a notable
that of the spin susceptibility. Temperature dependence difference between TPSC and RPA results in the am-
of these peaks are shown in Fig. 10. We see that the plitudes of the gap functions on the Fermi surface. As
peak of χch (q,ω = 0) is more drastically enhanced we can see in Fig. 14, the gap amplitude is larger for
2424
7
(a) χsp (b) χch
2222 2424
1.6 4.5
3
0.8
1.5
(π, π) (π, π)
0 0
q q
(0, 0) y (0, 0) y
q q
x (π, 0) x (π, 0)
FIG.9: (Color online) (a) χs2p222(q,ω=0) and (b)χc2h424(q,ω=0) at T =0.015 eV,whereorbitals 2and 4denotethedxz and
dx2−y2 orbitals, respectively.
) 1
δ, 0 5 χs2p222 Eigenvalue of
χspπ-(2222 34 χc2h424 hberg Eq. 00..68 Eliashberg Eq.
d as
an Eli
π-δ, 0) 12 alue of 0.4
( v 0.2
χch2424 0 0 0.01 0.02 0.03 0.04 0.05 Eigen 0
Temperature [eV] 0 0.01 0.02 0.03 0.04 0.05
Temperature [eV]
FIG.10: (Coloronline)Temperaturedependenceofthemax-
imum value in χsp (red solid line) and χch (blue dotted FIG.12: (Coloronline)Temperaturedependenceofthemax-
2222 2424
line) obtained byTPSC. imum eigenvalue of the linearized Eliashberg equation. The
bluedotted line is a guide tothe eye.
)
0 10
χsp-RPAπ-δ, (2222 68 χχs2c2p2h4-22-RR24PPAA Vtthh(eeknF)ew=rem−fiins32udUrftsahpcaχetsapts(hkien)gUtahps0epRaimnPAptlhirteeusdEuellitab[seshecbeoemFrgeigse.lq1au4rag(dtei)o]on.n,
d
n
a 4
)
0
δ, IV. SUMMARY
- 2
π
(A
χch-RP2424 0 0 0.01 0.02 0.03 0.04 0.05 conTsoisstuemntmmaertihzeo,dw(eThPaSvCe)dfeovretlohpeemdutlhtei-otwrboi-tpaalrHtiucblebaserldf
Temperature [eV]
model. Wederivedself-consistentequationstodetermine
vertexcorrectionsinthespinandcharge(orbital)suscep-
FIG.11: (Coloronline)Temperaturedependenceofthemax- tibilities. We applied this method to the effective mod-
imum value in χs2p222 (red solid line) and χc2h424 (blue dotted els for La2CuO4 and LaFeAsO. We solved the linearized
line) obtained by RPA. The bare coupling constants are set Eliashberg equation and found that vertex corrections
to beU =1.2 eV, U′ =0.96 eV,and J =0.12 eV. play a crucial role in the multi-orbital superconductors.
In the two-orbital dx2−y2-d3z2−r2 model for La2CuO4,
while FLEX shows much lower T than its experimental
c
RPA than TPSC. This indicates that there is a frus- value ∼ 40 K, the present TPSC can increase Tc dra-
tration between the orbital-fluctuation-mediated pair- maticallyduetoenhancedorbitalfluctuationsviavertex
ing and the spin-fluctuation-mediated pairing. Indeed, corrections for intermediate U ∼ 2.0 eV.
in TPSC, if we drop the contribution of the charge In the iron-based superconductor LaFeAsO, we have
channel in the pairing interaction, namely consider only studiedwhetherorbitalfluctuationscanbeenhancedand
8
(a) Band 2 (b) Band 3 (c) Band 4
π π π
q q q
0 0 0
y y y
-π -π -π
-π 0 π -π 0 π -π 0 π
q q q
x x x
0 0.001 0.002 -0.001 0 0.001 0.002 -0.0012 -0.0008 -0.0004 0
FIG. 13: (Color online) Gap functions obtained by TPSC for the bands with the (a) second, (b) third, and (c) fourth Kohn-
Sham energy at T = 0.015 eV. The black line and dotted green line represent the Fermi surface and nodes of gap functions,
respectively.
(b) (c) (d)
(a)
]
π -30 6 FS1 6 6
FS4 1 FS2
[ FS3
FS1 FS3 s ∆ 4 FS4 4 4
q 0 n
y FS2 ctio 2 2 2
n
u
-π p f 0 0 0
-π 0 π Ga
q -2 -2 -2
x
0 π/2 0 π/2 0 π/2
Angle θ
FIG.14: (Coloronline)Gapfunctiononthe(a)Fermisurfacesby(b)TPSC,(c)RPA,and(d)TPSCwithoutchargefluctuation,
where θ is therotation angle from theky axis.
induce the s -wave pairing within TPSC. Indeed we corrections in multi-orbital systems for cooperative and
++
have found that some kinds of orbital fluctuations are competitivephenomenabetweenspinandorbitaldegrees
enhanced by considering vertex correction and become of freedoms.
even stronger than spin fluctuations. However, their or-
bitalfluctuationsarenotstrongenoughtocausethes -
++
wave pairing, and the obtained gap function has the s
±
symmetry,althoughthe gapmagnitude is relativelysup-
pressedduetoafrustrationbetweentwokindsofpairing
interactions mediated by spin and orbital fluctuations.
Acknowledgments
It is an interesting problem in future research whether
the pairing symmetry changes for larger interaction pa-
rameters. Anotherimportantfutureissueisasystematic
WethankH.KontaniandS.Onariforstimulatingdis-
comparisonbetweenthepresentmulti-orbitalTPSCand
cussions. This work was supported by Grants-in-Aid for
other(diagrammatic)methodswhichconsiderthevertex
ScientificResearch(No.23340095)fromMEXTandJST-
corrections. Finally, we stress the importance of vertex
PRESTO, Japan.
9
Appendix A: Definition of correlation functions (δ >0).
Inthe multi-orbitalcase,we considercorrelationfunc-
Inthesingle-orbitalcase,correlationfunctionsforspin tions for S (1) = (c† (1),c† (1))σ(c (1),c (1))T,
µν µ↑ ν↓ µ↑ ν↓
Sz(r) and charge n(r) are defined as and n (1) = c† (1)c (1)+c† (1)c (1). These corre-
µν µ↑ ν↑ µ↓ ν↓
lation functions for spin and charge channels are defined
χsp(1,2)=hT Sz(1)Sz(2)i, (A1a)
τ as
χch(1,2)=hT n(1)n(2)i−hn(1)ihn(2)i, (A1b)
τ
χspz (1,2)=hT Sz (1)Sz (2)i, (A2a)
where an abbreviation 1=(r ,τ ) denotes a position r λµνξ τ λµ ξν
1 1 1
and an imaginary time τ1, and Tτ is the time ordering χsλpµ±νξ(1,2)=hTτS+λµ(1)S−ξν(2)i, (A2b)
operator. Time dependence of a generic operator Q(r)
χch (1,2)=hT n (1)n (2)i−hn (1)ihn (2)i.
is defined as Q(r,τ) = e−τHQ(r)eτH. χsp(ch)(q) in the λµνξ τ λµ ξν λµ ξν
(A2c)
maintextistheFouriertransformoftheabovereal-space
representation. Thetwosumrules(Eqs.(2)),whichplay
a central role in TPSC, originate from the definition at The sum rules of Eqs. (8) and (9) come from the fol-
equal time, that is, χsp(ch)(1,1+) with 1+ = (r ,τ +δ) lowing definitions at equal time,
1 1
χspz (1,1+)=h(c† (1)c (1)−c† (1)c (1))(c† (1+)c (1+)−c† (1+)c (1+))i
λµνξ λ↑ µ↑ λ↓ µ↓ ξ↑ ν↑ ξ↓ ν↓
(A3a)
−h(c† (1)c (1)−c† (1)c (1))ih(c† (1+)c (1+)−c† (1+)c (1+))i
λ↑ µ↑ λ↓ µ↓ ξ↑ ν↑ ξ↓ ν↓
χsp± (1,1+)=2hc† (1)c (1)c† (1+)c (1+)i (A3b)
λµνξ λ↑ µ↓ ξ↓ ν↑
χch (1,1+)=h(c† (1)c (1)+c† (1)c )(c† (1+)c (1+)+c† (1+)c (1+))i
λµνξ λ↑ µ↑ λ↓ µ↓ ξ↑ ν↑ ξ↓ ν↓
(A3c)
−h(c† (1)c (1)+c† (1)c )(c† (1+)c (1+)+c† (1+)c (1+))i.
λ↑ µ↑ λ↓ µ↓ ξ↑ ν↑ ξ↓ ν↓
Appendix B: Ansatz for effective interactions in with σ¯ = −σ. Here a bar over a number means the in-
single-orbital case tegral over position and imaginary time. The four-point
correlation function can be approximated by the local
Following Vilk and Tremblay,17 let us derive the correlationfunction and the Green’s function as follows,
ansatz, Eq. (4), used in TPSC calculations. The four-
point vertex function, Γσσ′, between electrons with spin
σ and σ′ is given by
Γσσ′δ(1−3)δ(2−4)δ(2−1+)= δΣσ(1,2), (B1) −UhTτ[cσ†¯(1++)cσ¯(1+)cσ(1)c†σ(2)]i
δGσ′(3,4) hn (1)n (1)i (B3)
∼U ↑ ↓ G (1,1+)G (1,2).
σ¯ σ
hn (1)ihn (1)i
where G (1,2) and Σ (1,2) is the dressed Green’s func- ↑ ↓
σ σ
tionandselfenergywithspinσ. Theequationofmotion
and the Dyson equation leads to the relation,
Σ (1,¯1)G (¯1,2)
σ σ
(B2) By substituting Eq. (B3) into Eq. (B1), we can obtain
=−UhT [c†(1++)c (1+)c (1)c†(2)]i,
τ σ¯ σ¯ σ σ
δ U hn↑n↓i G (1,1+)δ(1−2)
Γσσ′δ(1−3)δ(2−4)δ(2−1+)= δΣσ(1,2) = h hn↑ihn↓i σ¯ i
δGσ′(3,4) δGσ′(3,4)
(B4)
= δhUhnhn↑↑inhn↓↓iiiG (1,1+)δ(1−2)+U hn↑n↓i δGσ¯(1,1+)δ(1−2).
σ¯
δGσ′(3,4) hn↑ihn↓i δGσ′(3,4)
The lastterm ofthis equationis proportionalto δσ¯σ′ via and then contributes to the spin channel, Usp = Γσσ¯ −
δG (1,1+) Γσσ. This just provides Eq. (4) for Usp.
σ¯ =δσ¯σ′δ(1−3)δ(4−1+), (B5)
δGσ′(3,4)
10
Appendix C: Ansatz in multi-orbital systems pointvertexfunctionhasorbitalindices,λ,µ,ν,ξbesides
spin index, σ. We here consider Γ , which can
(µµσ)(ννσ′)
In this section, let us extend the above-mentioned be written by only orbital-diagonalcomponents.
ansatz into the multi-orbital case. In this case, the four-
Γ δ(1−3)δ(2−4)δ(2−1+)= δΣµµσ(1,2) = δ Σµµσ(1,¯5)[G(¯5,¯6)G−1(¯6,2)]µµσ
(µµσ)(ννσ′) δGννσ′(3,4) (cid:2) δGννσ′(3,4) (cid:3)
δ hn n i hn n i hn n i
∼ −U µσ µσ¯ Gµµσ¯(1,1+)− U′ µσ ξσ¯ Gξξσ¯(1,1+)− (U′−J) µσ ξσ Gξξσ(1,1+)
δGννσ′(3,4) hnµσihnµσ¯i ξX6=µ hnµσihnξσ¯i ξX6=µ hnµσihnξσi
 
hn n i δG (1,1+) hn n i δG (1,1+) hn n i δG (1,1+)
∼−U µσ µσ¯ µµσ¯ − U′ µσ ξσ¯ ξξσ¯ − (U′−J) µσ ξσ ξξσ . (C1)
hnµσihnµσ¯i δGννσ′(3,4) ξX6=µ hnµσihnξσ¯i δGννσ′(3,4) ξX6=µ hnµσihnξσi δGννσ′(3,4)
Here, following the single-orbital case, we have intro- Eq. (10a) for Usp can be obtained from the first term
µµµµ
duced the following approximations, of Eq. (C1), since the intra-orbital Coulomb interaction
for the orbital µ is proportional to δ δ . In the same
µµ σσ¯
Σ G G−1 ∼−U hnµσnµσ¯i G (1,1+), way, Eq. (10b) for Uµspνµν and Eq. (10c) for Uµspνµν −Jsp
µµσ µµσ µµσ hn ihn i µµσ¯ with µ 6= ν can be obtained from the second and the
µσ µσ¯
hn n i third terms in Eq. (C1), respectively.
Σ G G−1 ∼−U′ µσ ξσ¯ G (1,1+),
µµσ ξξσ ξξσ hn ihn i ξξσ¯
µσ ξσ¯
hn n i
Σ G G−1 ∼−U′ µσ ξσ¯ G (1,1+).
µµσ ξξσ¯ ξξσ¯ hn ihn i ξξσ
µσ ξσ¯
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