Origin of Model Fractional Chern Insulators in All Topological Ideal Flatbands Explicit Color-entangled Wavefunction and Exact Density Algebra Jie Wang1 2 3Semyon Klevtsov4and Zhao Liu5y

2025-04-29 0 0 3.94MB 22 页 10玖币
侵权投诉
Origin of Model Fractional Chern Insulators in All Topological Ideal Flatbands:
Explicit Color-entangled Wavefunction and Exact Density Algebra
Jie Wang,1, 2, 3, Semyon Klevtsov,4and Zhao Liu5,
1Center for Mathematical Sciences and Applications,
Harvard University, Cambridge, MA 02138, USA
2Department of Physics, Harvard University, Cambridge, MA 02138, USA
3Center for Computational Quantum Physics, Flatiron Institute, 162 5th Avenue, New York, NY 10010, USA
4IRMA, Universit´e de Strasbourg, UMR 7501, 7 rue Ren´e Descartes, 67084 Strasbourg, France
5Zhejiang Institute of Modern Physics, Zhejiang University, Hangzhou 310027, China
It is commonly believed that nonuniform Berry curvature destroys the Girvin-MacDonald-
Platzman algebra and as a consequence destabilizes fractional Chern insulators. In this work we
disprove this common lore by presenting a theory for all topological ideal flatbands with nonzero
Chern number C. The smooth single-particle Bloch wavefunction is proved to admit an exact color-
entangled form as a superposition of Clowest Landau level type wavefunctions distinguished by
boundary conditions. Including repulsive interactions, Abelian and non-Abelian model fractional
Chern insulators of Halperin type are stabilized as exact zero-energy ground states no matter how
nonuniform Berry curvature is, as long as the quantum geometry is ideal and the repulsion is
short-ranged. The key reason behind is the existence of an emergent Hilbert space in which Berry
curvature can be exactly flattened by adjusting wavefunction’s normalization. In such space, the
flatband-projected density operator obeys a closed Girvin-MacDonald-Platzman type algebra, mak-
ing exact mapping to Clayered Landau levels possible. In the end we discuss applications of the
theory to moir´e flatband systems with a particular focus on the fractionalized phase and spontaneous
symmetry breaking phase recently observed in graphene based twisted materials.
I. INTRODUCTION
Flatbands are ideal platforms for realizing exotic quan-
tum phases of matter. Of particular interests are topo-
logical flatbands where fractionalized topological phases
are possible [17]. During recent years, there has been
numerous progress in creating and engineering flatband
systems such as by stacking and twisting [811]. Under-
standing the stability of fractionalized phases with re-
spect to tuning parameters such as energy dispersion,
wavefunction geometry, interaction range is crucial for
material engineering, experimental realization and the-
oretical characterization of these exotic phases. Along
this path, exact statements are particularly important.
In this work, we present an exact condition for the stabil-
ity of fractional Chern insulators (FCIs), i.e. the lattice
version of fractional quantum Hall (FQH) states [37].
Our results also provide a framework to systematically
explore the interplay between wavefunction’s geometry
and interactions, and are useful in guiding material engi-
neering toward realizing exotic quantum phases of mat-
ter.
Landau levels (LLs) are the simplest topological Chern
flatbands, with unit Chern number C= 1 and exactly
zero bandwidth. They are well known for their extreme
uniformness that is crucial for the stability of FQH states:
LLs are covariant under any smooth area preserving de-
formations [12,13] which is encoded in the LL projected
jiewang.phy@gmail.com
zhaol@zju.edu.cn
density algebra initially noticed by Girvin, MacDonald,
and Platzman (GMP) for lowest Landau level (LLL)
states [14,15]:
[ˆρq1,ˆρq2] = eq
1q2l2
Beq1q
2l2
Bˆρq1+q2,(1)
where ˆρqis the LLL projected density operator, qis
the complex coordinate of momentum which is q=
(qx+iqy)/2 in an isotropic LL, and lBis the mag-
netic length. The density operator deforms the shape of
LLs while preserving their LL index [1215]. The GMP
algebra is important for FQH physics in many aspects
including the conformal field theory mapping [16,17],
constructing exact pseudopotential projectors that pro-
tect the stability of FQH states [18,19] and others. The
GMP algebra was initially derived based on the holomor-
phicity of LLL wavefunctions [20] which arises from the
non-commutativity of guiding centers and in fact applies
to any LL [21].
Topological flatbands realized in lattice systems are
more complicated than LLs. They can carry arbitrary
integer Chern number, and typically do not exhibit uni-
form Berry curvature. Due to this complication, nei-
ther the holomorphicity of wavefunction nor the exact
projected density algebra exist, thereby Eq. (1) becomes
approximate. As a consequence FCIs are no longer ex-
pected to be exact and their stability is supposed to be
reduced [22]. There have been important attempts to-
wards restoring the GMP algebra in flatband systems.
For instance, Ref. [23] highlighted the importance of the
Fubini-Study metric gab
kwhich is the intrinsic wavefunc-
tion distance measure. Given constant Berry curvature
k= Ω and the so-called trace condition Trgk= Ωk,
arXiv:2210.13487v1 [cond-mat.mes-hall] 24 Oct 2022
2
Ref. [23] derived a GMP algebra for all topological flat-
bands of C 6= 0. However, the assumption of constant
Berry curvature oversimplified the flatband problem.
In this work, we relax the above assumption by fo-
cusing on ideal flatbands satisfying the following condi-
tions [24]:
gab
k=1
2ωabk,k>0 for k.(2)
The ωab is a constant uni-modular matrix. The Berry
curvature is assumed to be positive definite but not nec-
essarily uniform. Recently, it is known that Eq. (2) is
necessarily [25] and sufficiently [2628] equivalent to the
momentum-space holomorphicity, which is a key geo-
metric property of the momentum-space (= boundary-
condition space). For all C= 1 ideal flatbands such
as those realized in the chiral model of twisted bilayer
graphene [29,30] and others [3133], it has been shown
that momentum-space holomorphicity highly constrains
the wavefunction to admit a universal form descend-
ing from the LLL wavefunction [24]. Such simplicity
enables exact construction of many-body model wave-
functions [30] as well as exact projective pseudopotential
Hamiltonians [24], hence the ideal condition plays an im-
portance role in experimental realization and identifica-
tion of FCIs [34,35]. Recently the ideal condition Eq. (2)
has been generalized to a larger family called “vortexabil-
ity” [36] by allowing nonlinear real-space embedding [37].
While much progress are made for C= 1, less is known
for generic cases of C>1. Recently, solvable models
based on twisted multilayer graphene sheets were pro-
posed which realize ideal flatbands of arbitrary Chern
number [38,39]. Remarkably, exact Halperin type FCIs
were found by numerical diagonalization [38]. Moti-
vated by the C= 1 case, it was conjectured that the
momentum-space holomorphicity is also the fundamen-
tal reason for their emergence [38], however, the nature
of the Bloch wavefunction and why FCIs are stable are
still far from thorough understanding.
In this work, we prove that the ideal quantum geo-
metric condition, without assuming the flatness of Ωk
or gkthemselves, is sufficient to guarantee exact GMP
algebra and model FCI states occurring as exact zero-
energy ground states of proper short-ranged interactions
in all C 1 ideal flatbands. We achieve these results
by pointing out the importance of an emergent Hilbert
space in which the wavefunctions’ normalization factors
are adjusted to flatten Berry curvature in an exact man-
ner. Importantly, this leads to a simple algebra for
the projected coordinates identical to the guiding cen-
ter algebra in LLs, and enables an exact derivation for
the single-particle wavefunction in ideal flatbands. The
general form Bloch wavefunction is found to be a non-
linear superposition of LLL wavefunctions of Cdistinct
boundary conditions, generalizing the previously pro-
posed color-entangled wavefunctions with uniform Berry
curvature [4042] to the case of fluctuating Berry cur-
vature. The density algebra is proved to be closed, ex-
tending the GMP algebra initially derived for C= 1 LLs
to generic C 1 ideal flatbands. The closeness of the
density algebra directly leads to the exact FCIs stabi-
lized by generic Mbody repulsive interactions which
are Halperin type when C>1 and typically non-Abelian
when M > 2. Thus the ideal condition Eq. (2) provides a
general and exact statement for stabilizing FCIs regard-
less of the nonuniformness of Berry curvature.
The paper is structured as follows. In Sec. II, we pose
the problem by showing unusual numerical observations.
In Sec. III, we review key analytical results for ideal
flatbands, emphasizing their real and momentum-space
boundary conditions and define the emergent Hilbert
space. We show in Sec. IV a well defined guiding center in
the ideal flatband problems which leads to the derivation
of single-particle wavefunctions in Sec. V. In Sec. VI, the
density algebra is derived and the origin of model FCIs is
explained. We discuss application of the theory to FCIs
and symmetry breaking phases [34,43] in moir´e flatband
systems in Sec. VII.
II. MOTIVATION FROM UNUSUAL
NUMERICAL OBSERVATIONS
The ideal flatbands are not abstract concepts, rather
they can be realized in concrete models including Dirac
fermion based models [29,33,38,39] and Kapit Mueller
models [31,32,44,45]. In particular, in the chiral twisted
multilayer graphene models [38,39] ideal flatbands of ar-
bitrary C 6= 0 are realized exactly. In this section, we use
this model as a concrete platform to numerically study
the interacting spectra, which point out a couple of im-
portant and unusual aspects of the projected density op-
erator that motivates the theory to be discussed in the
following sections.
For self-consistency, we first briefly review the chiral
twisted multilayer graphene models [38,39], although the
model details are not crucial for the current discussion.
Details about the chiral twisted graphene model and be-
yond can be found in the literature [29,30,4651]. The
chiral twisted multilayer model consists of two sheets of
graphene multilayers each of which has nlayers and is
A/B stacked without twist [5254]. The top sheets are
twisted relative to the bottom sheets as a whole by a twist
angle θ. See Fig. 1(a) for illustration of the setup. At
magic twist angles, the model exhibits two exactly dis-
persionless degenerate bands showing in Fig. 1(b), which
have Chern number C=±n. Such degeneracy can be
easily lifted by sublattice contrasting potential, and we
focus on the C=nband. The flatband wavefunction is
derived in Refs. [38,39], and its components on the out-
most layers are found to be responsible for the high Chern
number and zero-mode FCIs. We hence project the wave-
function to the outmost layers to get an effective single-
component wavefunction, denoted as φBloch
k(r). It can
be numerically verified that φBloch
k(r) is smooth in both
kand r. The Berry curvature associated to φBloch
k(r),
3
defined through the standard definition,
k=aba
kAb
k,Aa
k≡ −ihuBloch
k|a
kuBloch
ki,(3)
is plotted in Fig. 1(c), which integrates to the Chern
number C=n. Here uBloch
k(r) = eik·rφBloch
k(r) is the
cell-periodic part of the Bloch wavefunction. Its ideal
geometry condition can be either analytically proved or
numerically verified to be satisfied [38,39]. See Fig. 1(d)
for the plot of the trace condition for the n= 2 model
where errors are finite-size artifact that will vanish in the
infinite-size limit.
We then consider interacting phenomena when the flat-
band is partially filled. We focus on finite systems on the
torus geometry at band filling ν=N/(N1N2), where N
is the number of particles and N1,2is the number of unit
cells in each primitive direction of the lattice. The two-
body density-density interaction takes the form of
H=X
q
v(q) : ˆρqˆρq:,(4)
where v(q) is the Fourier transform of the interaction po-
tential, ˆρqis the band-projected density operator, and : :
is the normal ordering. Because of the translation invari-
ance by lattice vectors, the energy spectrum of Hcan
be resolved by the center of mass momentum (K1, K2).
We are particularly interested in the repulsive interac-
tion vm(r1r2) = Pqvm(q)eiq·(r1r2), whose Fourier
transform vm(q) has series expansion of orders no higher
than m:
vm(q) = c0+c1|q|2+c2|q|4+... +cm|q|2m.(5)
The above series expansion is equivalent as interaction
range expansion, as the real space form of the |q|2nterm
is given by the 2nth derivative of the contact interaction
δ(r1r2). In particular, the v0and v1are the shortest
interactions for bosons and fermions, respectively [55].
We first consider two particles interacting with vm,
where mis even for bosons and odd for fermions. While
it is unrealistic for bosons to occupy the flatband near
charge neutrality of twisted double bilayer graphene, we
can still use that setup to examine the band property. Re-
markably, for any choice of mand Chern number C, the
two-particle spectrum always shows a dispersive “band”
with a fixed number of nonzero eigenvalues at each mo-
mentum k, no matter what the lattice size is. The dimen-
sion of this finite-energy “band” is precisely (m+ 1)Cfor
fermions and (m+1)C+1 for bosons [56]. The “band” dis-
persion reflects the fact that Berry curvature is nonuni-
form. Apart from these finite-energy levels, there are also
massive zero modes whose dimension increases with the
system size. The typical example with m= 1,C= 2 is
demonstrated in Fig. 1(e) for two fermions. Such two-
particle spectra are different from those in either LLs or
generic flatbands. For generic flatbands without ideal
geometry, the zero modes would not appear and the di-
mension of finite-energy levels is hence lattice-size de-
pendent [5,57]. In a LL, as the vminteraction only picks
FIG. 1. (a) Illustration of the chiral twisted multilayered
graphene model, where solid and empty dots represent the A
and B sublattice, respectively, and the interlayer tunneling is
assumed to be chiral and represented by the dashed line. The
φ1,...,n and φ0
1,...,n label the layer component. (b) The band
structure where a tiny sublattice bias potential is used to split
the two-fold degeneracy of the flatbands. The resulting two
bands are entirely sublattice polarized. (c) and (d) The Berry
curvature and the trace condition of uBloch
kdefined in the
main text. The Berry curvature is positive definite. (e) The
energy spectrum of the two-body vm=1 interaction for two
fermions on a N1×N2= 5 ×5 lattice. At each momentum
k, there are precisely (m+ 1)C= 4 non-zero eigenvalues.
(f) The energy spectrum of the three-body onsite interaction
for N= 10 bosons on a N1×N2= 5 ×3 lattice. The exact
six-fold degenerate zero-energy ground states are non-Abelian
model FCIs which are analogous to the FQH non-Abelian spin
singlet state (NASS). In (a)-(f) we use n= 2, namely the
twisted double bilayer graphene model, as an example.
out two-particle coherent states of relative angular mo-
mentum no greater than m, both a finite-energy band
whose dimension is independent of the system size and
massive zero modes appear, however, the “band” is non-
dispersive due to the continuous translation symmetry.
Therefore, Fig. 1(e) clearly shows there is an emergent
projector property associated to the density operator of
ideal flatbands which however, unlike in LLs, is influ-
enced by the nonuniform Berry curvature.
We can further generalize Eq. (4) to multibody interac-
4
tions. In Fig. 1(f), the spectrum of the three-body onsite
interaction is plotted for bosons at ν= 2/3 in C= 2
band. In this case, a six-fold exact ground-state degen-
eracy at zero energy is obtained. These zero modes are
expected to be lattice analogs of the non-Abelian spin
singlet states, which are generalizations of the Abelian
Halperin topological order in bilayer LLL [58]. This re-
sult indicates that suitable short-range M-body repul-
sions can stabilize model FCIs of both Abelian and non-
Abelian types in high Chern number ideal flatbands.
Moreover it points out that two-body interaction is not
special: there should be a general property that the pro-
jected density operator obey in order to give rise to FCIs
stabilized by generic M-body interactions [59].
To summarize, numerical studies on interacting two-
particle spectra and three-body interactions point out
the importance of the flatband projected density oper-
ator which exhibits emergent projector properties and
internal Cdegrees of freedom playing the role of layers,
although the underlying Berry curvature can be highly
nonuniform. It motivates the following two questions:
What internal degrees of freedom in the scalar-
valued wavefunction uk(r) play the role of layers?
Why the nonuniform Berry curvature does not
destabilize the Abelian and non-Abelian FCIs?
Thorough this work, we will show the answers to these
two questions by deriving the general form of C 6= 0 ideal
flatband wavefunctions, illustrating their physical mean-
ings and studying their density algebra which we find
generalizes the GMP algebra Eq. (1) in a nontrivial way.
III. ANALYTICAL PROPERTIES OF IDEAL
FLATBANDS
As we have shown, numerical experiments in ideal flat-
bands point out the unusual effects of ideal quantum
geometry on interacting problems. To address the two
questions at the end of the last section, we will focus on
analytical derivations in the remaining parts of this arti-
cle. We start with reviewing the ideal quantum geomet-
ric condition and emphasizing its relation to momentum-
space K¨ahler condition in this section. The momentum-
space K¨ahler condition gives novel properties of the Bloch
wavefunction in many aspects, such as the unique bound-
ary condition and the relation between Berry curvature
and normalization factor. These general results were de-
rived for ideal flatbands in Ref. [24]. In the next section
Sec. IV, we will discuss new results about their impor-
tant implication for the well defined guiding center which
establishes connection to LL physics in the presence of
nonuniform Berry curvature.
A. Ideal quantum geometry and K¨ahler geometry
We first of all set up conventions in more detail. Our
exact results applies to arbitrary system size N1,2. The
system is spanned by primitive lattice vectors a1,2con-
taining an area |a1×a2|= 2πS and throughout this work
we set S= 1 as the unit area scale. The reciprocal/ mo-
mentum space is spanned by reciprocal lattice vectors bi
such that ai·bj= 2πδij is satisfied.
We consider a single component complex valued Bloch
wavefunction defined on this torus. Its cell periodic part
uBloch
k(r) is lattice translational invariant following the
Bloch theorem uBloch
k(r) = uBloch
k(r+a) for arbitrary
lattice vectors a=m1a1+m2a2and m1,2Z. The
uBloch
k(r) is assumed to be smooth in both kand r. Mo-
mentum quantization restricts k= (n1/N1+φ1/2π)b1+
(n2/N2+φ2/2π)b2where n1,2Zand φ1,2are ficti-
tious boundary condition fluxes. Our result applies to
arbitrary φ1,2too so without loss of generality we set
φ1,2= 0. Because the flux space (torus formed by con-
tinuous variable φ1,2) is equivalent to the momentum
space in the presence of translational symmetry, we use
these two spaces interchangeably. The discussions in this
work can be generalized to flux space for systems without
translation symmetry where the notion of momentum is
not defined.
The uBloch
k(r) satisfies the ideal quantum geometric
condition while not necessarily carrying flat Berry curva-
ture. We assume uBloch
khas positive Chern number; neg-
ative Chern number is a simple generalization. As being
a positive integer, the Chern number can be factorized
into two positive integers C=C1C2and the factorization,
as we will discuss, is a gauge choice.
Systematic studies of flatbands with momentum-space
holomorphicity start from Refs. [25,60] where the au-
thors found that the ideal condition is automatically
satisfied if the cell-periodic part of Bloch wavefunction
uBloch
k(r) is given by Eq. (6). Very recently, Refs. [2628]
pointed out the inverse is also true: the ideal condition
generally implies uBloch
kcan be written as Eq. (6) under a
gauge choice up to a positive-definite normalization fac-
tor Nk:
uBloch
k(r) = Nkuholo
k(r),¯
kuholo
k= 0,(6)
where the holomorphic coordinate k(unbold letter) is de-
fined as kωakaand the associated anti-holomorphic
derivative is defined as ¯
kωaa
k. Technically, the no-
tion of holomorphicity ωais defined from factorizing the
constant uni-modular matrix ωab and the anti-symmetric
tensor ab as follows [21]:
ωab =ωaωb+ωaωb,
iab =ωaωbωaωb.(7)
The vectors satisfy ωaωa= 1, ωaωa= 0 where and
throughout the paper Einstein summation is implicitly
assumed, and their indices are raised or lowered by ωab.
Note that in the isotropic case (ωx, ωy) = (1, i)/2 the
5
complex coordinate reduces to the conventional form k=
(kx+iky)/2. We will work in the general case without
assuming isotropy.
It is worth to mention that ωahas an equivalent canon-
ical definition as being the constant null vector of the
quantum geometric tensor [24]:
Qab
kωb= 0 for k,(8)
where the quantum geometric tensor is defined by us-
ing the covariant derivative of wavefunctions Da
k=a
k
iAa
k, whose real symmetric and imaginary anti-symmetric
parts are the Fubini-Study metric and the Berry curva-
ture, respectively:
Qab
k≡ hDa
kuk|Db
kuki=gab
k+iab
2k.(9)
B. Implications from K¨ahler geometry
We have reviewed the relation between ideal quantum
geometric condition and momentum-space holomorphic-
ity [2528]. In this section we discuss two implications
from K¨ahler geometry: the uniqueness of the boundary
condition and the relation between normalization factor
and Berry curvature. They are important in deriving
ideal flatband wavefunction and their density algebra.
1. Uniqueness of the boundary condition
Following Ref. [24], we can generally denote the
momentum-space boundary boundary condition of uholo
k
as:
uholo
k+bi(r) = uholo
k(r)·exp (ibi·r+k,bi),(10)
where the complex phase φk,b is not only required to be
holomorphic in kbut also is constrained to satisfy a co-
cycle relation [24],
2πC=φb1,b2φ0,b2+φ0,b1φb2,b1.(11)
Such a constraint Eq. (11) is required from the simplified
Chern number formula derived for ideal flatbands [24]:
C=1
2πi Idk ∂kln uholo
k(r),(12)
which means that the winding of the wavefunction in the
momentum space around the Brillouin zone at any fixed
rmust reflect the topology of the wavefunction. The co-
cycle relation Eq. (11) implies the boundary condition
factor φk,b must be a linear function of holomorphic co-
ordinate k[24]. The phase φk,b is the so-called factor
of automorphy which appears in classifying holomorphic
line bundles [61]. Without loss of generality, we choose
the symmetric gauge,
φk,bj=Cb
j(ik ibj/2) + Cjπ, j = 1,2,(13)
where C1,2are the two positive integers dividing the
Chern number C=C1C2, and the ambiguity in their
choice will prove to be unimportant.
FIG. 2. The normalized flatband wavefunction defines a
Hilbert space Hwhere the Berry curvature is nonuniform.
For ideal flatbands, the wavefunction’s normalization can be
tuned such that the resulting wavefunction uk(r) define a
modified Hilbert space ˜
Hin which Berry curvature can be
exactly flattened. As a consequence uk˜
Hperceives iden-
tical k-dependence as LLL wavefunctions. Such property
greatly simplifies the problem, enabling a thorough charac-
terization of flatband wavefunction from a momentum-space
formulation. As an illustration, we used kto represent the
2D momentum-space. The normalization and Berry curvature
are represented by the dashed and solid lines, respectively.
2. ahler potential
The anti-holomorphic component of the Berry connec-
tion ¯
AkωaAa
kcan be computed from the standard
definition:
¯
Ak≡ −ihuBloch
k|¯
kuBloch
ki,
=iZd2rNkuholo
k(r)¯
kNkuholo
k(r),
=iN1
k¯
kNkZd2rN2
kuholo
k(r)uholo
k(r),
=i¯
klog Nk.(14)
Then from Ωk=ik¯
Ak¯
kAkwhere Akis the
complex-conjugate of ¯
Ak, we obtain the expression for
the Berry curvature [24,62,63]:
k=2k¯
kln Nk.(15)
Eq. (15) has an important implication: for ideal flat-
bands, the Berry curvature can be effectively flattened by
adjusted by the normalization of the wavefunction. More
precisely, by factorizing Nkinto,
Nk=eC
4|k|2˜
Nk,(16)
the Berry curvature is accordingly split into Ωk=C+˜
k,
i.e. the uniform and fluctuating part. The fluctuating
part ˜
k=2k¯
kln ˜
Nkaverages to zero when being
integrated over the Brillouin zone which does not con-
tributes to the quantization.
摘要:

OriginofModelFractionalChernInsulatorsinAllTopologicalIdealFlatbands:ExplicitColor-entangledWavefunctionandExactDensityAlgebraJieWang,1,2,3,SemyonKlevtsov,4andZhaoLiu5,y1CenterforMathematicalSciencesandApplications,HarvardUniversity,Cambridge,MA02138,USA2DepartmentofPhysics,HarvardUniversity,Cambri...

展开>> 收起<<
Origin of Model Fractional Chern Insulators in All Topological Ideal Flatbands Explicit Color-entangled Wavefunction and Exact Density Algebra Jie Wang1 2 3Semyon Klevtsov4and Zhao Liu5y.pdf

共22页,预览5页

还剩页未读, 继续阅读

声明:本站为文档C2C交易模式,即用户上传的文档直接被用户下载,本站只是中间服务平台,本站所有文档下载所得的收益归上传人(含作者)所有。玖贝云文库仅提供信息存储空间,仅对用户上传内容的表现方式做保护处理,对上载内容本身不做任何修改或编辑。若文档所含内容侵犯了您的版权或隐私,请立即通知玖贝云文库,我们立即给予删除!
分类:图书资源 价格:10玖币 属性:22 页 大小:3.94MB 格式:PDF 时间:2025-04-29

开通VIP享超值会员特权

  • 多端同步记录
  • 高速下载文档
  • 免费文档工具
  • 分享文档赚钱
  • 每日登录抽奖
  • 优质衍生服务
/ 22
客服
关注