Line-Graph Approach to Spiral Spin Liquids Shang Gao1 2yGanesh Pokharel2 3Andrew F. May2Joseph A. M. Paddison2Chris Pasco2Yaohua Liu1Keith M. Taddei1Stuart Calder1David G. Mandrus2 3 4Matthew B. Stone1and Andrew D. Christianson2

2025-05-03 0 0 3.06MB 20 页 10玖币
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Line-Graph Approach to Spiral Spin Liquids
Shang Gao,1, 2, Ganesh Pokharel,2, 3 Andrew F. May,2Joseph A. M. Paddison,2Chris Pasco,2Yaohua Liu,1Keith
M. Taddei,1Stuart Calder,1David G. Mandrus,2, 3, 4 Matthew B. Stone,1and Andrew D. Christianson2
1Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
2Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
3Department of Physics & Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA
4Department of Material Science & Engineering,
University of Tennessee, Knoxville, Tennessee 37996, USA
(Dated: October 24, 2022)
Competition among exchange interactions is able to induce novel spin correlations on a bipar-
tite lattice without geometrical frustration. A prototype example is the spiral spin liquid, which
is a correlated paramagnetic state characterized by sub-dimensional degenerate propagation vec-
tors. Here, using spectral graph theory, we show that spiral spin liquids on a bipartite lattice can
be approximated by a further-neighbor model on the corresponding line-graph lattice that is non-
bipartite, thus broadening the space of candidate materials that may support the spiral spin liquid
phases. As illustrations, we examine neutron scattering experiments performed on two spinel com-
pounds, ZnCr2Se4and CuInCr4Se8, to demonstrate the feasibility of this new approach and expose
its possible limitations in experimental realizations.
Introduction.— A spiral spin liquid (SSL) is an ex-
otic correlated paramagnetic state of sub-dimensional
degeneracy, meaning that the propagation vectors qof
the ground states form a continuous manifold, or spiral
surface, in a dimension that is reduced from the orig-
inal system [1–14]. Similar to geometrically frustrated
magnets [15, 16], a SSL may host topological spin tex-
tures [17–19] and quantum spin liquid states [3–5, 20–22].
What differentiates a SSL from a conventional frustrated
magnet is the sub-dimensional degeneracy, which induces
highly distinctive dynamics since the spins are confined
to fluctuate collectively as nonlocal spirals [23]. Recent
calculations on a square lattice reveal that the low-energy
fluctuations in a SSL may behave as topological vortices
in momentum space [23], leading to an effective tensor
gauge theory with unconventional fracton quadrupole ex-
citations that are deeply connected to theories of quan-
tum information, elasticity, and gravity [24–29].
To date, bipartite lattices have been the primary av-
enue through which SSLs are studied. This is because
the ground state degeneracy on a bipartite lattice can
be exact, so that all spin spirals with qover the spi-
ral surface have exactly the same energy [5]. Although
this degeneracy stabilizes the SSL down to very low tem-
This manuscript has been authored by UT-Battelle, LLC un-
der Contract No. DE-AC05-00OR22725 with the U.S. Depart-
ment of Energy. The United States Government retains and the
publisher, by accepting the article for publication, acknowledges
that the United States Government retains a non-exclusive, paid-
up, irrevocable, world-wide license to publish or reproduce the
published form of this manuscript, or allow others to do so, for
United States Government purposes. The Department of En-
ergy will provide public access to these results of federally spon-
sored research in accordance with the DOE Public Access Plan
(http://energy.gov/downloads/doe-public-access-plan).
sgao.physics@gmail.com
peratures [1, 3], it also imposes a strong constraint on
real materials because most of the known bipartite-lattice
compounds are dominated by the nearest-neighbor inter-
actions J1[30–44]. Even for the established model com-
pounds where the second-neighbor interactions J2are rel-
atively strong [17, 45, 46], the degeneracy over the spiral
surface is only approximate due to the existence of fur-
ther perturbations [14]. This degeneracy lifting results
in an approximate SSL state at elevated temperatures
where thermal fluctuations overcome the slight energy
difference among the spirals.
Inspired by recent density functional theory (DFT)
calculations for the breathing pyrochlore lattice com-
pounds [47], here we seek the realization of an approx-
imate SSL, i.e. a SSL with an approximate degener-
acy, beyond the bipartite lattices. According to the
Luttinger-Tisza theory [48, 49], the degeneracy of a SSL
model is encoded in the minimum manifold of the in-
teraction matrix. Using graph theory [50, 51], we show
that the J1-J2model on a bipartite lattice shares the
same minimum manifold with a J1-J3model on the cor-
responding line-graph lattice, where J3denotes the third-
neighbor interaction. Thus an approximate SSL state is
achieved in the latter case when J3is sufficiently strong,
which greatly expands the range of materials that may
support a SSL state. This line-graph approach to SSL
is vetted through neutron scattering experiments per-
formed on two Cr-based chalcogenide spinels ZnCr2Se4
and CuInCr4Se8.
Line-graph approach.— Our starting point is a Heisen-
berg model on a l-regular lattice, where lcounts the num-
ber of the nearest-neighbor (NN) sites. In the presence of
a uniform NN exchange interaction J1, the coupled spins
form a undirected graph G= (V, D), with Vdenoting the
set of vertices (i.e. the spin sites) and Ddenoting the set
of edges (i.e. the NN bonds). Two vertices iand jare
arXiv:2210.11781v1 [cond-mat.str-el] 21 Oct 2022
2
called adjacent if the graph contains an edge e={i, j},
and the adjacency matrix is defined as A(G)ij = 1 for
{i, j} ∈ Dand 0 otherwise. Following this definition, the
spin Hamiltonian, H=J1PhijiSi·Sjwith hijidenoting
the NN bonds, can be expressed through the adjacency
matrix as H=1
2J1A(G)ij Si·Sj. Therefore, according
to the Luttinger-Tisza theory [1, 5], the classical ground
state of Hcan be determined from the eigensolution of
A(G), of which the eigenvalues are defined as the spec-
trum of the graph, denoted as σ(G).
Algebraic graph theory defines that the related graphs
should have related spectra [50, 51]. Of interest here is
the line graph LG[52, 53], whose vertices correspond to
the edges of the root graph Gand are adjacent if the
original edges share a vertex: LG= (D, {{e, e0} | ee06=
, e 6=e0}). For a regular bipartite graph G, a conve-
nient way to define its line graph is to select the vertices
at the midpoints of the original edges. As illustrated in
Fig. 1, for the honeycomb (l= 3) and diamond (l= 4)
lattices that are the prototype hosts of the SSL, their line
graphs form the kagome and pyrochlore lattices, respec-
tively. According to graph theory [50, 51], the spectra of
Gand LGare related by
σ(LG) = (2)mnσ(G) + l2 , (1)
where m(n) is the total number of edges (vertices) of the
root graph G. In reciprocal space, Eq. (1) indicates the
existence of flat eigenbands on LGwith a degeneracy of
l2, which has been the focus of many recent studies [51,
54–56]. More importantly, it reveals that the non-flat
eigenbands of σ(LG) and σ(G) share the same dispersion
up to a constant of l2, and their eigenvectors are related
by the incident matrix as discussed in Ref. [57].
Such a spectrum correspondence can be immediately
verified for Heisenberg models. On a regular lattice of g
sublattices and Nprimitive cells, the interaction matrix
of a J1-only model
Jαβ
1(q) = J1
NX
iα,jβ
ij∈hiji
exp [iq·(rirj)] (2)
is a g×ghollow matrix with zero diagonal elements.
The eigenbands ν(q) for J1<0 are shown as solid lines
in Fig. 1(b) for the honeycomb and kagome lattices, and
in Fig. 1(d) for the diamond and pyrochlore lattices. The
minimum of the eigenbands νmin has been subtracted for
comparison. Analytical expressions for the eigenbands
are presented in the Supplemental Material [58], which
includes additional Refs. [59–69]. Aside from the top flat
bands in blue color, the two dispersive bands ν±(q)νmin
on the kagome (pyrochlore) lattice overlap exactly with
those on the honeycomb (diamond) lattice, which is a
direct consequence of spectral graph theory.
This eigenband correspondence is maintained un-
der the addition of certain further-neighbor interac-
tions. For the J1-J2model on a bipartite lattice,
LΓX W K Γ
0
2
4
6
8
10
E/|J|
1
J2
J3b
J3a
× 2
× 1
(a)
honeycomb - kagome
(b)
(d)
honeycomb - kagome
(c)
J3b
J3a
J2
c
b
a
KΓM K
0
1
2
3
4
5
6
7
E/|J|
1
diamond - pyrochlore
a
b
diamond - pyrochlore
FIG. 1. (a) The line graph of a bipartite honeycomb lat-
tice (large spheres linked by orange bonds) is a kagome lat-
tice (small spheres linked by gray bonds). J1is the nearest-
neighbor exchange coupling. Black dashed (red solid) arrows
indicate the second-neighbor coupling J2(third-neighbor cou-
plings J3aand J3b) over the honeycomb (kagome) lattice. (b)
Eigenbands of the interaction matrix of Heisenberg models on
the honeycomb and kagome lattices. Orange solid (dashed)
lines are the two eigenbands for the J1model (J1-J2model
with J2/J1=0.25) on the honeycomb lattice, which overlap
with the two lower eigenbands of the J1model (J1-J3model
with J3/J1=0.25) on the kagome lattice shown in blue
solid (dashed) lines. (c-d) Similar correspondence exists be-
tween the diamond and pyrochlore lattices. The interaction
matrices for the J1model (J1-J2model with J2/J1=0.3)
on the diamond lattice and the J1model (J1-J3model with
J3/J1=0.3) on the pyrochlore lattice share two same
eigenbands shown by the overlapping orange and blue solid
(dashed) lines.
J2couples the spins of the same sublattices. There-
fore, its contribution to J(q) is a diagonal matrix
J2(q) that commutes with J1(q), leading to a q-
dependent shift γG(q) of the eigenbands with γG(q) =
(J2/Ng)Pij∈hhijii exp [iq·(rirj)], where hhijii are
the second-neighbor bonds [58]. Similar conclusions can
be drawn for the J1-J3model on the line-graph lattices,
as J3(including both J3aand J3b) also couples spins of
the same sublattices [58]. Since J2(G) on the root-graph
lattice and J3(LG) on the line-graph lattice share the
same exchange paths as compared in Fig. 1(a,c), we ex-
pect, in the case of J2(G) = J3(LG), the same dispersive
eigenbands up to a constant on the root- and line-graph
lattices. This correspondence is illustrated by the dashed
lines in Fig. 1(b,d).
Approximate SSL.— An approximate SSL can be re-
alized on the line-graph lattices through the eigenband
correspondence. Following the results of the J1-J2model
on the bipartite lattices [1, 5], it is clear that for the J1-
3
J3model on a line-graph lattice the eigenband minima of
J(q) will form a degenerate manifold in reciprocal space
at |J3/J1|>1/(2l), where lis the number of the NN sites
on the root-graph lattice. Here J1and J3should be FM
and AF, respectively, as the additional flat bands on line
graphs remove the FM-AF duality of a bipartite lattice.
Since the equal moment constraint over the eigenvectors
of J(q) is not always satisfied for qover the minimum
manifold [58], the SSL realized through the line-graph ap-
proach is approximate and needs to be stabilized by ther-
mal fluctuations. As discussed in the Supplemental Ma-
terial [58], the degeneracy breaking over the spiral surface
is relatively weak, which contrasts the strong modulation
in the previously studied half-moon patterns [57, 70–72].
This weak degeneracy breaking on the line-graph lattices
leads to a stable SSL state in a wide temperature regime
that is comparable to that on the bipartite lattices.
Before making comparisons to the experimental data,
we further generalize the SSL model by incorporating
a breathing distortion on the line-graph lattice so that
the NN interactions are modulated alternately as J1and
J0
1[47, 73, 74]. For the breathing pyrochlore lattice shown
in Fig. 2(a), the eigenbands of J(q) can be solved as [58]:
ν1,2=J3κ(q)±q4(J1J0
1)2+J1J0
1|η(q)|2+ (J1+J0
1),
ν3,4=J3κ(q)(J1+J0
1) ,
where η(q) = P4
n=1 exp(iq·dn) with dndenoting
the 4 bonding vectors around each spin site [58] and
κ(q) = |η(q)|24. Assuming J1< J0
1<0 and J3>0,
an approximate SSL state is realized for 1
4
J1J0
1
|J1+J0
1|< J3<
1
4
J1J0
1
|J1J0
1|. The corresponding phase diagram is presented
in Fig. 2(b). Representative spiral surfaces in the approx-
imate SSL phase at |J3/J1|= 0.2 are shown in Fig. 2(c-e)
for J0
1/J1= 0.8, 0.6, and 0.5, respectively. The surfaces
are identical to those on the diamond lattice [1]. Similar
conclusions on the breathing kagome lattice are presented
in the Supplemental Material [58].
ZnCr2Se4with a regular pyrochlore lattice.— The Cr-
based chalcogenide spinels present nearly ideal model
compounds to demonstrate the proposed line-graph ap-
proach to SSLs. In these systems, J1is FM due to the
90superexchange path, while J20 due to negligible
orbital overlap [75, 76]. As the first example, we study
the short-range spin correlations in ZnCr2Se4, where the
Cr3+ (S= 3/2) ions form a regular pyrochlore lattice.
Single crystals of ZnCr2Se4were grown using the chem-
ical vapor transport method [58]. Figure 3 summa-
rizes the diffuse neutron scattering results measured on
CORELLI at the Spallation Neutron Source (SNS), Oak
Ridge National Laboratory (ORNL) [58]. With the sta-
tistical chopper, the elastic channel of our CORELLI
data has an average energy resolution of about 0.8 meV
for an incident neutron energy range of 13 to 33 meV. At
20 K, below the N´eel transition temperature TN22 K,
(c) (d) (e)
0 0.2 0.4 0.6 0.8 1
0
0.1
0.2
0.3
0.4
0.5
0.6
FM
1D degeneracy
= 0
(a)
c
J1/J1
|J3 /J1|
J1
J1
J4
'
J2
J3a
J3b
(b)
Approx. SSL
e d
'
X
L
W
ZCS
FIG. 2. (a) The breathing pyrochlore lattice is composed of
corner-sharing tetrahedra of two different sizes, which results
in alternating J1and J0
1couplings over the gray and orange
tetrahedra, respectively. The exchange paths of the J2,J3a,
J3b, and J4interactions are also indicated. (b) Phase dia-
gram on the breathing pyrochlore lattice with ferromagnetic
J1and J0
1interactions together with antiferromagnetic J3in-
teractions assuming J3a=J3b. The shaded area indicates
the region where an approximate SSL phase can be stabilized
by thermal fluctuations. Contour lines for constant |η(q)|are
shown in the approximate SSL phase. Along these lines, the
spiral surface stay the same. Location of ZnCr2Se4(ZCS)
with J1=J0
1and |J3/J1|= 0.15 is indicated by the yel-
low dot. Inset shows the degenerate manifold (blue lines)
for J3>1
4
J1J0
1
|J1J0
1|out of the SSL regime. (c-e) Characteris-
tic spiral surfaces with quasi-degenerate energies in the first
Brillouin zone at |J3/J1|= 0.2 and J0
1/J1= 0.8 (c), 0.6 (d),
and 0.5 (e). These points are indicated on the phase diagram
in (b) using the panel labels.
magnetic Bragg peaks indexed by q= (0,0,0.47) are ob-
served (see Fig. 3(a)). This is consistent with the helical
ground state reported previously [77–79], with the weak
ring-like scattering mainly arising from the low-energy
magnon excitations [76, 80]. At 30 K, above TN, mag-
netic Bragg peaks are replaced by broad diffuse scattering
with a spherical shape (see Fig. 3(b-d)), evidencing the
emergence of an approximate SSL state where gapped
excitations are replaced by quasielastic fluctuations [58].
Assuming a Heisenberg model with exchange interac-
tions up to the fourth neighbors (J4), we fit the diffuse
scattering data using the self consistent Gaussian approx-
imation (SCGA) method [58]. The calculated slices in
Fig. 3(b-d) reproduce the experimental data. The cou-
pling strengths are fitted as J1=2.86(8), J2= 0.00(1),
J3= 0.48(1), and J4=0.057(1) meV, which is very
close to the values determined by inelastic neutron scat-
tering (INS) [76, 80] and indicates marginal changes in
the coupling strengths across the phase transition. The
weak strengths of the J2and J4interactions allow a direct
verification of the line-graph approach that is based on
4
2.4
1.6 2
(h,0,2)
2.4
1.6 22.4
1.6 2
(h,0,2) (h,0,2)
(a) (hk2), 20 K (hhl), 30 K
(111), 30 K
calc.
(hk2), 30 K
(b)
(c)
(d)
4
0 8 12
Int. (a.u.)
2.4
1.6 2
(h,h,0)
2.4
1.6 2
(h,h,0)
2.4
1.6 2
(h,h,0)
2.4
1.6 2
(h,h,0)
2.4
1.6
2
(0,k,2)
2.4
1.6
2
(0,k,2)
2.4
1.6
2
(0,0,l )
0.2
0.1
-0.1
-0.2
0
(l,-l,2l+2)
calc.
calc.
FIG. 3. Quasistatic spin correlations in ZnCr2Se4measured
on CORELLI at (a) 20 K and (b-d) 30 K. Slices are along
the (hk2) plane in panels (a,b), (hhl) plane in panel (c), and
(111) plane in panel (d). The right sub-panel in (a) shows the
complete spiral surface for the J1-J3model on a pyrochlore
lattice with J3/J1=0.15. The flat planes correspond to
the slice directions as indicated by the colored plaquette at
the top left corner in each panel. The right sub-panels in (b-
d) are the calculated diffuse scattering patterns for the fitted
J1-J2-J3-J4model. Solid line is the spiral surface predicted
by the J1-J3model with J3/J1=0.15. The same linear
intensity scale is employed in all panels.
aJ1-J3model. The solid circles on top of the calculated
patterns in Fig. 3(b-d) are the contours of the spiral sur-
face predicted by the J1-J3model with J3/J1=0.15.
The contours capture the shapes of the diffuse scatter-
ing patterns. As compared in the Supplemental Mate-
rial [58], the existence of the FM J4interaction slightly
reduces the radius of the spiral surface while strongly
modulates the scattering intensities.
CuInCr4Se8with a breathing pyrochlore lattice.— Until
now, we have assumed a uniform J3interaction. In real
materials, the J3exchange paths on a line-graph lattice
can be different as indicated in Fig. 1. This may lead
to different coupling strengths and destabilize the SSL
state. As the second example, we study the spin cor-
relations in the breathing pyrochlore lattice compound
CuInCr4Se8[81], which has been proposed as a SSL can-
didate from DFT calculations [47].
A polycrystalline sample of CuInCr4Se8was synthe-
sized through the solid state reaction method [58]. INS
experiments were performed on SEQUOIA at the SNS,
ORNL. Neutron diffraction experiments were performed
on HB-2A at the high flux isotope reactor (HFIR),
ORNL. As is consistent with the previous report [81],
magnetic susceptibility shown in Fig. 4(a) suggests a spin
glass-like transition at Tf15 K with a clear frequency
dependence. This is confirmed in the neutron diffraction
results presented in Fig. 4(b), where only broad mag-
netic features are observed down to 0.25 K. The weak
features at Q0.41 and 0.72 ˚
A1can be indexed by
q= (0.48,0.48,0), while their intensities exhibit a his-
0.2 0.3 0.4 0.5 0.6 0.7 0.8
Q-1)
0
50
100
150
Int. (a.u.)
0 100 200 300
T (K)
0
0.2
0.4
0.6
0.8
1
1.2
(emu/mol-Cr Oe)
10 Hz
10 kHz
(a)
10 15 20
T(K)
0
0.5
1
'(emu/mol-Cr Oe)
0.5 1 1.5 2 2.5
0
1
2
3
Int. (a.u.)
0.5 1 1.5 2 2.5
-1 0 1
(h,0,0)
-1 0 1
(h,0,0)
-1
0
1
(0,k,0)
-1
0
1
(0,k,0)
0.5 1 1.5 2 2.5
-1 0 1
(h,0,0)
-1
0
1
(0,k,0)
-1 0 1
(h,0,0)
-1
0
1
(0,k,0)
(0.48,0.48,0) (0.48, 0.48,1)(0.3, 0.3, 0)
0
10
20
T = 20 K0.25 K 25 K
(b)
T = 100 KT = 40 K T = 200 K
× 1.5 × 2.5 × 3.5
(d)(c) (e)
Q-1)
Q-1)Q-1)
FIG. 4. (a) DC magnetic susceptibility of CuInCr4Se8mea-
sured in a 1 ×104Oe field (circles). Red line shows the
Curie Weiss fit at temperatures between 200 and 380 K.
The fitted magnetic moment and Weiss temperature are
µeff = 4.6µB/Cr and 97(2) K, respectively. The inset shows
the frequency dependence of the real part of the ac suscep-
tibility measured in a 10 Oe field. Imaginary part of the ac
susceptibility is presented in the Supplemental Material [58].
(b) Difference between the neutron diffraction intensity at
0.25 and 25 K measured on HB-2A. The dashed line is a
guide to the eyes. Positions of the characteristic wave vec-
tors are indicated by arrows. The full width at half maxi-
mum (FHWM) of the closest nuclear reflection is indicated
by the horizontal bar. Inset shows the calculated neutron
diffuse scattering pattern in the (h, k, 0) plane at T= 20 K
for J3a= 0.07 meV (see text). (c-e) Circles show the Q-
dependence of the equal-time spin correlations at T= 40 (c),
100 (d), and 200 K (e). Results of the SCGA fits using the
J1-J0
1-J3a-J3bmodel are shown by the red solid lines. Insets
are the calculated neutron diffuse scattering patterns in the
(h, k, 0) plane for J3a= 0.07 meV at the corresponding tem-
peratures (see text).
tory dependence as expected for a spin glass state.
Figures 4(c-e) present the equal-time spin correla-
tions obtained by integrating the INS spectra from [20,
20] meV [58]. Assuming a J1-J0
1-J3a-J3bHeisenberg
spin model, we fit the diffuse scattering data by the
SCGA method [58, 82]. The fitted results are shown in
Fig. 4(c-d) as solid lines. The fitted coupling strengths
are J1=1.6(2), J0
1=5.4(3), J3a= 0.1(1), and
J3b= 0.6(1) meV, where the different strengths for J3a
and J3bare necessary for a satisfactory fit. As shown
in the inset of Fig. 4(b-e), the spin correlations in the
(hk0) plane does not follow a circular shape, implying
the absence of a SSL state due to the unequal J3aand
J3binteractions.
The fitted parameter set provides an explanation for
the glassy ground state in CuInCr4Se8.J3a, being the
weakest interaction in the J1-J0
1-J3a-J3bmodel, is how-
ever the most crucial parameter in determining the exact
length of the LRO q= (q, q, 0). Within the standard de-
viation of J3a,qvaries from 0 at J3a= 0 up to 0.4 at
J3a= 0.2 meV. Therefore, the broad diffuse scattering at
摘要:

Line-GraphApproachtoSpiralSpinLiquidsShangGao,1,2,yGaneshPokharel,2,3AndrewF.May,2JosephA.M.Paddison,2ChrisPasco,2YaohuaLiu,1KeithM.Taddei,1StuartCalder,1DavidG.Mandrus,2,3,4MatthewB.Stone,1andAndrewD.Christianson21NeutronScatteringDivision,OakRidgeNationalLaboratory,OakRidge,Tennessee37831,USA2Mat...

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Line-Graph Approach to Spiral Spin Liquids Shang Gao1 2yGanesh Pokharel2 3Andrew F. May2Joseph A. M. Paddison2Chris Pasco2Yaohua Liu1Keith M. Taddei1Stuart Calder1David G. Mandrus2 3 4Matthew B. Stone1and Andrew D. Christianson2.pdf

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