Exact Many-Body Ground States from Decomposition of Ideal Higher Chern Bands Applications to Chirally Twisted Graphene Multilayers Junkai Dong1Patrick J. Ledwith1Eslam Khalaf2Jong Yeon Lee3and Ashvin Vishwanath1

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Exact Many-Body Ground States from Decomposition of Ideal Higher Chern Bands:
Applications to Chirally Twisted Graphene Multilayers
Junkai Dong,1, Patrick J. Ledwith,1Eslam Khalaf,2Jong Yeon Lee,3and Ashvin Vishwanath1
1Department of Physics, Harvard University, Cambridge, MA 02138, USA
2Department of Physics, The University of Texas at Austin, TX 78712, USA
3Kavli Institute for Theoretical Physics, University of California, Santa Barbara, CA 93106, USA
(Dated: October 26, 2022)
Motivated by the higher Chern bands of twisted graphene multilayers, we consider flat bands
with arbitrary Chern number Cwith ideal quantum geometry. While C > 1 bands differ from
Landau levels, we show that these bands host exact fractional Chern insulator (FCI) ground states
for short range interactions. We show how to decompose ideal higher Chern bands into separate
ideal bands with Chern number 1 that are intertwined through translation and rotation symmetry.
The decomposed bands admit an SU(C) action that combines real space and momentum space
translations. Remarkably, they also allow for analytic construction of exact many-body ground
states, such as generalized quantum Hall ferromagnets and FCIs, including flavor-singlet Halperin
states and Laughlin ferromagnets in the limit of short-range interactions. In this limit, the SU(C)
action is promoted to a symmetry on the ground state subspace. While flavor singlet states are
translation symmetric, the flavor ferromagnets correspond to translation broken states and admit
charged skyrmion excitations corresponding to a spatially varying density wave pattern. We confirm
our analytic predictions with numerical simulations of ideal bands of twisted chiral multilayers
of graphene, and discuss consequences for experimentally accessible systems such as monolayer
graphene twisted relative to a Bernal bilayer.
I. INTRODUCTION
The discovery of correlated states in twisted bilayer
graphene (TBG) has inspired interest in the study of
topological flat bands [1–6]. The interplay of band
topology and strong interactions in TBG makes it an
ideal candidate to realize strongly correlated topological
physics. A simplified “chiral” model[7], where intrasub-
lattice moir´e tunneling is ignored, yields exactly flat topo-
logical bands at the magic angle, thereby capturing the
most essential features of the system. The chiral model
hosts analytically-attainable wavefunctions [7] that are
equivalent to those of the LLL in an inhomogeneous mag-
netic field [8]. Its mathematical properties and mysteries
have turned the chiral model into a sub-field of study of
its own right [7–19].
At the same time, it was shown [8] that the chi-
ral model satisfies a quantum-geometric[20–29] iden-
tity known as the “trace condition”. It was recently
shown that the trace condition enables intra-band vor-
tex attachment[30, 31], which has strong implications
for interacting physics. For example, it directly enables
the construction of FQHE-like trial states which are ex-
act ground states of the system under short-range re-
pulsive interactions. For TBG, this led to the analytic
prediction[8] of fractional Chern insulators (FCI)[32–41],
states that host the fractional quantum Hall effect at
fractional filling of a Chern band first observed[42] in
Hofstadter bands [43–47]. FCIs in TBG were simultane-
ously predicted in numerical works [48–50] and were re-
cently experimentally observed [51] in a small magnetic
junkaidong@g.harvard.edu
field that effectively restored the ideal chiral limit [52].
We will refer to trace condition satisfying bands as ideal
bands; they have also been referred to as “vortexable”
[31]. The trace condition is also related to momentum
space holomorphicity [21, 23, 24, 29, 31, 53], a powerful
analytic tool.
Multilayer twisted graphene systems consisting of
nchirally stacked[54–56] graphene layers, i.e. AB,
ABC, etc., stacked with a single twist on top of
mchirally stacked layers are a natural extension of
TBG. Experimentally, twisted double bilayer graphene,
(n, m) = (2,2) [57–64], and twisted mono-bilayer
graphene (n, m) = (1,2) [65–72], have been fabricated
and a nearly quantized anomalous Hall effect has repeat-
edly been observed with C > 1 [57, 63, 65, 66]. This
has generated extensive theoretical interest in study-
ing topological physics in these systems at different fill-
ings [30, 73–85]. Furthermore, symmetry broken Chern
insulators have been found experimentally at half fill-
ing [70]. A variety of these states was recently identi-
fied numerically together with signs of an approximate
SU(2) symmetry [86], although its origin has not been
understood. FCIs have also been studied in these sys-
tems [29, 79], and other systems with higher Chern
bands [40, 44, 46, 87–92], though analytic progress has
largely been limited to toy models. These advances mo-
tivate the need for a systematic understanding of corre-
lation phenomena in higher Chern bands, including both
symmetry-broken and fractional Chern insulators.
Chiral models have been constructed for the twisted
chiral multilayers in Refs. [30, 80] through neglecting cer-
tain interlayer tunneling terms. Chiral twisted multilayer
graphene yields flat and ideal higher Chern bands that
are analytically tractable when tuned to the same magic
arXiv:2210.13477v1 [cond-mat.mes-hall] 24 Oct 2022
2
angle as chiral TBG. By combining ideality, which allows
the construction of trial wavefunctions for topologically
ordered states, with higher Chern number in an exper-
imentally feasible system, these models provide a novel
and unexplored platform to study and realize interacting
topological phases. Ref. [80] numerically found model
FCI states with particle entanglement suggestive of par-
ticular “color-singlet” [40] Halperin states, though a gen-
eral analytic understanding of the range of possible states
in such a system remains lacking.
One approach to understanding correlated states in a
C > 1 band is to employ the hybrid Wannier functions
to decompose the band into CChern 1 bands by enlarg-
ing the unit cell [93]. However, this approach does not
respect the ideality condition: an ideal Chern C > 1
band generally decomposes into a set of C= 1 non-ideal
bands such that analytic techniques cannot be applied
to the decomposed basis. Furthermore, this decomposi-
tion assumes broken translation symmetry in a specific
direction from the outset, making it difficult to under-
stand translation-unbroken states like the FCIs observed
in Ref. [80]. This naturally leads to the question: is it
possible to decompose an ideal C > 1 band into ideal
C= 1 bands and in a way that captures both transla-
tionally symmetric and translation-breaking states? A
partial answer to this question was provided in Ref. [30]
which showed that it is impossible to decompose a generic
ideal C > 1 bands in terms of orthogonal and ideal C= 1
bands.
Here, we show that by lifting the orthogonality con-
straint, it is possible to decompose a generic ideal C > 1
into Cideal Chern 1 bands. From this decomposition,
we reveal a hidden non-unitary SU(C) action among the
decomposed bands combining real space and momentum
space translations. We note that although this SU(C)
action is generally non-unitary and is not a symmetry of
the Hamiltonian, it is often a symmetry of the ground
state manifold for short-range repulsive interactions, en-
abling us to make sharp predictions regarding the many-
body ground state at partial filling. Furthermore, the
SU(C) structure is reminiscent of the structure of multi-
component Landau levels [40], allowing us to interpret
these ground states in terms of more familiar correlated
states in multi-component quantum Hall systems. It
should be emphasized that the states we obtain are phys-
ically distinct and have non-trivial translation symmetry
breaking patterns visible through real-space charge den-
sity, compared to multi-component Landau levels which
have uniform charge density.
By employing this decomposition, we analytically con-
struct ground states at a variety of fillings of ideal Chern
C > 1 bands and make concrete predictions on their re-
alization. First, we identify a ground state manifold of
charge density waves (CDWs) at filling 1/C with emer-
gent SU(C) symmetry. Such states can be interpreted
as generalized quantum Hall ferromagnets in the decom-
posed basis. An immediate consequence of this iden-
tification is the existence of charged skyrmion textures
which correspond to a characteristic winding pattern of
the CDW order parameter (cf. Fig. 1). Such a wind-
ing pattern can be readily observed with local charge
probes such as STM, which has already been employed
in these systems [71, 72, 94–98]. Second, we characterize
the structure of translation symmetric fractional Chern
insulator states at fillings 1/(2Cs + 1), for each positive
integer s, by establishing a direct analogy with flavor-
singlet Halperin states. Finally, we discuss a manifold of
translation-breaking Laughlin states that appear at low
fillings 1/C(2s+ 1) where fractionalization and topolog-
ical order coexists with CDW order. All our results are
verified by numerical exact diagonalization (ED) on the
chiral model for twisted mono-bilayer graphene. These
results enable us to make experimental predictions for
graphene multilayers.
The paper is organized as follows. We begin with
an overview of the central theoretical results as well as
some consequences for experiment in Sec. II. Next, we re-
view the physical model for chiral graphene multilayers
in Sec. III and quantum geometry techniques to under-
stand the single particle physics in Sec. IV. We present
the central technical result, the decomposition of an ideal
higher Chern band, in Sec. V for C= 2. We study quan-
tum ferromagnets, which manifest as topological CDWs,
and their associated skyrmion description at half filling
for a spinless C= 2 band in Sec. VI and a spinful C= 2
band in Sec. VII. We discuss the possible fractional Chern
insulators, both translation symmetric and translation
broken, in Sec. VIII. We generalize the physical implica-
tions to ideal bands with C > 1 in Sec. IX. We conclude
with some future questions in Sec. X.
II. SUMMARY OF RESULTS
We begin by briefly summarizing our results and dis-
cussing their main implications. By now, a full under-
standing of ideal C= 1 bands has been achieved: they
have been related to the LLL of a Dirac particle in a mag-
netic field [8, 29], which enables the construction of exact
many body Laughlin-like ground states for short range
interactions [31]. Our main technical achievement in this
work is the generalization of such understanding for any
ideal higher Chern band. In particular, we provide a gen-
eral explicit construction to decompose any ideal higher
Chern band into ideal but non-orthogonal Chern 1 bands
with a non-unitary SU(C) action. Compared to the hy-
brid Wannier decomposition [93], our procedure has the
distinct advantages of preserving the ideal band geome-
try and allowing access to translation broken states in all
possible directions in the same basis. The preservation
of ideal band geometry means we leverage the analytic
knowledge of ideal C= 1 bands to analytically study in-
teracting phases – in particular, FCIs – in C > 1 bands.
The decomposition provides analytic many-body
states that are the exact ground states at certain frac-
3
FIG. 1. Main results of our paper. (a) Plots of the density profile of a CDW-skyrmion that arises out of a manifold of topological
charge density waves from the ideal C= 2 band in chiral twisted monolayer-bilayer graphene. Windings of the order parameter
corresponds to windings of the translation breaking pattern. We zoom into different regions to highlight different regions that
correspond to particular topological charge density waves that repeat in a 2 ×2 unit cell. (b) shows many-body spectrum from
ED at different fillings for the ideal C= 2 band in chiral twisted monolayer-bilayer graphene: boxed energy levels correspond to
the ground state manifold of model FCIs. The first panel is the translation invariant (332) Halperin state at ν= 1/5 with the
interaction V2δ00 (ˆ
riˆ
rj) and the expected ground state degeneracy of 5. The second panel is the translation broken Laughlin
state at ν= 1/6 under a screened Coulomb interaction with a ground state degeneracy of 21 that combines the Laughlin
degeneracy of 3 with the generalized-ferromagnetic degeneracy of Ne+ 1 = 7. Both of these collections of states originate from
the depicted analytic vortex attachment construction summarized in Sec. IV.
tional fillings for short range interactions. For example,
for a spinless band with Chern number Cthese ground
states include topological CDWs at 1/C filling. At lower
fillings, we construct Halperin-like translationally sym-
metric FCIs at ν= 1/(2Cs + 1) and Laughlin-like trans-
lation broken FCIs at ν= 1/C(2s+ 1) for any positive
integer s.
The topological CDWs are best understood as a man-
ifold of generalized quantum Hall ferromagnets (FM),
where translations act as pseudospin rotations on the fer-
romagnets. The pseudospin ferromagnets are generically
translation symmetric only in a C×Cunit cell. Crys-
talline symmetries, both translations and discrete rota-
tions, emerge as rotations in the order parameter mani-
fold. For C= 2, this manifold is a sphere and different
topological CDW states may be understood as different
pseudospin directions, pictorially represented in Fig. 2(a)
and summarized in Tab. 2(b). The ±X, ±Z, ±Yaxes
of the pseudospin sphere correspond to states that pre-
serve the translations along a1,2,3, where a1,2are prim-
itive lattice vectors and a3=a1a2. For rotational
symmetry, we focus on the three-fold C3zcase which
is relevant for graphene moir`e systems, and other rota-
tional symmetries may be understood similarly. We find
that C3zacts as a 120rotation around a particular n0
axis of the pseudospin sphere. There are also rotations
C(i)
3z=TaiC3zTaiaround lattice vectors aiand corre-
sponding invariant axes ni; these are distinct operations
because we have broken translations.
We construct exact model FCIs with the vortex at-
tachment procedure [30, 31] reviewed in Sec. IV. By at-
taching vortices to the fully filled Chern Cband, we ar-
rive at an explicit first quantized wavefunction for the
translationally-symmetric Halperin states. If we instead
choose a C= 1 topological charge density wave as a
parent state we obtain the translation broken Laughlin
states. We numerically verify these predictions in ex-
act diagonalization for the C= 2 band of chiral twisted
monolayer-bilayer graphene, see Fig. 1(b).
Finally, based on our analysis in the ideal limit, we
make concrete predictions for the ground states and ex-
citations of twisted mono-bilayer graphene at several fill-
ings as follows. Note, here we allow for valley and spin
degeneracy, hence the filling νranges from 0 ν4.
We will focus on states doped on top of a ν= 3 spin and
valley polarized state such that there is a single empty
C= 2 band left over.
1. At ν= 3+1/2, we expect that the ground state will
be a continuous manifold of spin polarized topo-
logical CDWs, or generalized quantum Hall fer-
romagnets. The observed topological CDWs in
Ref. [70] are special points in a continuous manifold
of generalized quantum Hall ferromagnets. Our
main prediction is that there will be associated
charged skyrmion textures with a particular wind-
ing of translation breaking patterns illustrated in
Fig. 1(a). These textures can be directly probed
by STM.
2. At ν= 3+1/5, we expect that the ground state will
be a translationally invariant Halperin (332) state:
see first panel of Fig. 1(b) for numerical spectrum.
4
FIG. 2. Sphere of flavor ferromagnets (FMs) at half filling
for a C3zsymmetric ideal band with C= 2. Each point on
the sphere correspond to a topological CDW with unit cell
2a1×2a2. (a) We show the order parameter sphere of fer-
romagnets: red arrows correspond to C3zsymmetric CDWs
(second and third insets of Fig. 1(a)) and yellow dots corre-
spond to CDWs that preserve certain translation symmetries
Tai(first inset of Fig. 1(a)). C3zrotations in real space cor-
respond to rotations around red arrows in order parameter
space. Table (b) explains the representation of special CDWs
as special ferromagnets on the sphere and crystalline symme-
tries as rotations. (c) We show the real space unit cell and
the lattice vectors.
In particular, the Hall conductivity (or the slope
of the gapped feature on the Landau fan diagram)
will be 2/5.
3. At ν= 3 + 1/6, we expect that the ground state
will be translation breaking ferromagnetic Laughlin
states: see second panel of Fig. 1(b) for numerical
spectrum. These states will in general be transla-
tion breaking striped phases with 1/3 Hall conduc-
tivity.
We end our summary of results with some cautionary
comments on application to experiments. First, realis-
tic systems will not be in the chiral limit, which means
that the higher Chern band will not have exactly ideal
quantum geometry, and may be dispersive. However, the
band geometry and flatness could be improved by exter-
nal perturbations such as out of plane electric and mag-
netic fields as believed to occur in magic angle graphene
[52], bringing the system closer to the ideal limit. Sec-
ond, in most of our discussions, we have assumed a spin-
less band for simplicity; exotic magnetic orders at half
filling such as the tetrahedral antiferromagnetic (TAF)
order exist in spinful bands [86, 99–104]. Remarkably,
after adding spin we find that the TAF states lie in the
same CP3manifold as the previously mentioned charge
density waves. Furthermore, a magnetic field may be
used to favor the spin-polarized submanifold that hosts
charge-density skyrmions. A final caveat is that typi-
cal twisted graphene systems contain twist angle disorder
which leads to large inhomogeneous heterostrain. Strain
greatly increases single particle dispersion and pushes the
band away from ideality. We leave a detailed quantita-
tive study of the realistic system that addresses the above
issues to future work.
III. CHIRAL TWISTED GRAPHENE
MULTILAYERS
In this section, we review the chiral twisted graphene
multilayers: they give rise to ideal higher Chern bands
and closely describe experimentally relevant systems. We
use them as our primary numerical example. Experimen-
tally, these graphene multilayers have been fabricated
both as twisted monolayer-bilayer (mono-bi) [65–68] and
twisted bilayer-bilayer (bi-bi) [57–61] graphene systems.
The following discussion will closely follow recent works
which introduced these models [30, 80].
We consider n+mgraphene layers. The first nlayers
are untwisted and chirally stacked on each other: chiral
stacking means that the successive layers have Bernal
stacking AB or BA. These nchirally stacked layers are
then twisted by the magic angle θof chiral TBG, and put
on top of another mlayers of chirally stacked graphene.
The mono-bilayer system corresponds to n= 1, m = 2 and
the bilayer-bilayer system corresponds to n= 2, m = 2.
For concision, consider electrons of a specific valley and
spin flavor. We make the approximation that the electron
can only tunnel between neighboring layers, and that the
interlayer tunneling is on-site (this neglects e.g. trigonal
warping terms [105]). The Hamiltonian can be written
as
H=hm,σ TM
T
Mhn,σ0(1)
where hn,σ describes the nuntwisted chirally stacked
graphene layers with chirality σ=±depending on
whether the layers have AB (+1) or BA (1) stack-
ing. TMonly tunnels between layers nand n+ 1 and
is identical to the moir´e tunneling of chiral TBG, where
intra-sublattice tunneling is switched off. Its explicit di-
mensionless form is given by
TM=α0U(r)
U(r) 0 (2)
where U1(r) = P2
n=0 e2πin
3iqn·r,qn=Cn
3(0,1)T, and
C3is a 120rotation. The parameter α=w1/vkθis
a dimensionless tunneling strength, where w1110
meV and the angle-dependent kinetic energy scale is
vkθ=v|Ktop Kbot|, where vis the Fermi velocity of
the Dirac cone and Ktop,bot are the graphene K-points
for the top and bottom stacks respectively. The chiral
5
FIG. 3. Band structure of twisted monolayer-bilayer
graphene. The higher Chern band is highlighted in red. We
use the chiral model (1) with α= 0.586 and β= 2; the two
Chern bands are degenerate and both of them have zero en-
ergy.
TBG magic angle corresponds to α0.586 [7].
The form of hn,σ is tri-diagonal:
hn,±=
· ∇ T±0. . .
T
±· ∇ T±. . .
0T
±· ∇ . . .
. . . . . . . . . ...
(3)
The diagonal terms describe the Dirac cones of each layer,
and the off diagonal terms describe the tunneling due to
two kinds of Bernal stacking: T±=βσx±y
2.
Most importantly, the model hosts zero energy flat
bands at particular values of αidentical to the magic an-
gles for chiral TBG. These zero energy bands are closely
related to the chiral symmetry of the model: {H, σz}= 0.
The chiral symmetry arises because the model is purely
off diagonal in the sublattice basis:
H=0D
D0AB
,(4)
where the subscript AB denotes that the matrix is block
diagonalized in its sublattice. Thus, we may choose zero
energy eigenstates of Hto be eigenstates of σz. At the
magic angle α= 0.586, two zero energy bands labeled by
the sublattice index Aand Bemerge independent of the
value of β[30, 80]. These bands carry Chern numbers,
whose signs depend on the stacking chiralities (σ, σ0) and
whose values depend on (n,m).
For example, when both upper nlayers and lower m
layers are AB-stacked with σ=σ0= 1, the Chern num-
ber for the A-sublattice polarized flatband (zero-energy)
is CA=n, while for the B-sublattice polarized flatband
CB=m. In particular, both mono-bilayer and bilayer-
bilayer systems give rise to Chern bands with |C|= 2
for this stacking orientation. For an analytic derivation
of the flat band wavefunctions ψk(r) for any stacking
(σ, σ0), in terms of the n=m= 1 chiral TBG wave-
functions, see Ref. [30]. We numerically plot the band
structure of mono-bi with the chiral model in Fig. 3; the
higher Chern band is highlighted in red.
In the next section we will review ideal band geometry
and show that the zero modes (flat bands) of magic-angle
chiral graphene multilayers in Eq. (4) attain the ideal ge-
ometry. Crucially, the zero mode operator Donly depend
on coordinates x, y and the antiholomorphic derivative
=1
2(x+i∂y): D(r, ∂x, ∂y) = D(r, ∂). This means
that
[D, z]=0,where z=x+iy. (5)
This property of Dwill be equivalent to the “vortex-
attachment” description of ideal quantum geometry.
IV. QUANTUM GEOMETRY OF IDEAL
CHERN BANDS
In this section we review our characterization of ideal
band geometry via the equivalent perspectives of the
“trace condition,” momentum-space holomorphicity, and
real-space “vortexability.” We will review the utility of
each of these perspectives. Finally, we will conclude by
developing certain technical tools for working with ideal
bands that we will use in the subsequent decomposition
section.
Consider a set of bands with total Chern number C
0 and periodic wavefunctions uka(r) = eik·rψka(r),
where ais a band index. The set of bands is ideal if
they satisfy the trace condition defined as one of the fol-
lowing three equivalent conditions [31]:
(i) The inequality tr g(k)Ω(k) is saturated, where
gµν (k) = Re ηµν (k) is the Fubini Study metric and
Ω(k) = µν Im ηµν is the Berry curvature. [20, 23,
24, 33]. Here, ηis the quantum metric defined by
ηµν (k) = X
ahkνuka|Q(k)kµuka(6)
and Q(k) = IPka|ukaihuka|.
(ii) The periodic wavefunctions |ukai=eik·r|ψkai
may be chosen to be holomorphic functions of k=
kx+ikyin a suitable choice of gauge. Non-unitary
gauge transformations are generically required to
reach this gauge, and the wavefunctions |ukaiare
generically not orthonormal.
(iii) The Bloch wavefunctions ψsatisfy zψ =Pzψ where
z=x+iy and P=Pka|ψkaihψka|is the projector
onto the bands of interest, written here using an
orthonormal basis |ψkai.
See Ref. [24] for the equivalence of (i) and (ii) and de-
tailed description of the associated K¨ahler geometry on
the Brillouin Zone (BZ). The condition (iii) and its gen-
eralizations, together with its relationship to conditions
(i) and (ii), is discussed in Ref. [31]. Pictorially, these
relations are shown in Fig. 4. See also Ref. [106] which
摘要:

ExactMany-BodyGroundStatesfromDecompositionofIdealHigherChernBands:ApplicationstoChirallyTwistedGrapheneMultilayersJunkaiDong,1,PatrickJ.Ledwith,1EslamKhalaf,2JongYeonLee,3andAshvinVishwanath11DepartmentofPhysics,HarvardUniversity,Cambridge,MA02138,USA2DepartmentofPhysics,TheUniversityofTexasatAust...

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