The observation of quantum fluctuations in a kagome Heisenberg antiferromagnet Fangjun Lu1Long Yuan1Jian Zhang1Boqiang Li1Yongkang Luo1yand Yuesheng Li1z 1Wuhan National High Magnetic Field Center and School of Physics

2025-05-06 0 0 2.58MB 21 页 10玖币
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The observation of quantum fluctuations in a kagome Heisenberg antiferromagnet
Fangjun Lu,1, Long Yuan,1, Jian Zhang,1Boqiang Li,1Yongkang Luo,1, and Yuesheng Li1,
1Wuhan National High Magnetic Field Center and School of Physics,
Huazhong University of Science and Technology, 430074 Wuhan, China
(Dated: October 25, 2022)
Abstract
The search for the experimental evidence of quantum
spin liquid (QSL) states is critical but extremely challeng-
ing, as the quenched interaction randomness introduced
by structural imperfection is usually inevitable in real ma-
terials. YCu3(OH)6.5Br2.5(YCOB) is a spin-1/2 kagome
Heisenberg antiferromagnet (KHA) with strong coupling
of hJ1i 51 K but without conventional magnetic freez-
ing down to 50 mK 0.001hJ1i. Here, we report a Br
nuclear magnetic resonance (NMR) study of the local spin
susceptibility and dynamics on the single crystal of YCOB.
The temperature dependence of NMR main-line shifts and
broadening can be well understood within the frame of
the KHA model with randomly distributed hexagons of
alternate exchanges, compatible with the formation of a
randomness-induced QSL state at low temperatures. The
in-plane spin fluctuations as measured by the spin-lattice
relaxation rates (1/T1) exhibit a weak temperature depen-
dence down to T0.03hJ1i. Our results demonstrate that
the majority of spins remain highly fluctuating at low tem-
peratures despite the quenched disorder in YCOB.
Introduction
Quantum spin liquid (QSL) is a state of matter that ex-
hibits exotic fractional excitations and long-range en-
tanglement without symmetry breaking [1–4]. Since Ander-
son’s proposal of the prototype, i.e., resonating-valence-bond
(RVB) state, in 1973 [5], QSL has been attracting researchers
for decades, due to its key role in understanding high-
temperature superconductivity [6] and the possible realization
of the topological quantum computation [7]. Experimentally,
many prominent two-dimensional QSL candidate compounds
have been extensively studied (the one-dimensional scenario
of QSL is qualitatively different [2]), including the kagome-
lattice ZnCu3(OH)6Cl2(herbertsmithite) [8–17], triangular-
lattice κ-(ET)2Cu2(CN)3[18, 19], EtMe3Sb[Pd(dmit)2]2[20,
21], YbMgGaO4[22, 23], etc., all of which generally exhibit
gapless QSL behaviors [9, 11, 14, 16, 18–20, 22, 24–27], but
without evident magnetic thermal conductivity [28–32].
Despite the progress, the existing experimental evidence for
QSL remains circumstantial and strongly depends on theoreti-
cal interpretation. The root cause lies in the quenched interac-
tion randomness introduced by structural imperfection that is
inevitable in all real materials [2, 17, 33]. Therefore, great
efforts are being devoted to exploring for ultrahigh-quality
candidate materials, which is extremely challenging [2]. On
the other hand, disorder-free QSL, even if successfully pre-
pared, is usually very fragile. For instance, the most frus-
trated kagome Heisenberg antiferromagnet (KHA) falls back
to conventional long-range magnetic ordering in the presence
of a weak next-nearest-neighbor coupling |J2| ≥ 0.03J1[34]
or Dzyaloshinsky-Moriya interaction |D| ≥ 0.012J1[35].
These constrictions further compress the “living space” of
disorder-free perfect QSL compounds.
Alternatively but more realistically, one could first find
out whether the inherent randomness is fatal or vital to the
QSL physics [4]. In fact, this same question can also be
raised for high-temperature superconductivity, as it is gener-
ally believed that Cooper pairs naturally form once the RVB
states are charged upon chemical doping [6, 36]. The pres-
ence of quenched vacancies in the KHA can lead to a va-
lence bond glass ground state (GS) [37]. Further, Kawamura
et al. found that randomness-induced QSL GSs instead of
spin glasses form in both KHA and triangular Heisenberg an-
tiferromagnet with strong bond randomness, J/J10.4
and 0.6 [38, 39], respectively, which may explain the gapless
behaviors observed in ZnCu3(OH)6Cl2,κ-(ET)2Cu2(CN)3,
EtMe3Sb[Pd(dmit)2]2[33], etc. Later similar scenarios have
been generally applied to the gapless QSL behaviors ob-
served in the strongly-spin-orbital-coupled triangular-lattice
YbMgGaO4[40, 41] with the mixing of Mg2+/Ga3+ [26, 42],
as well as in other relevant materials [43]. Despite the growing
interest in theory, the key issue is whether the paramagnetic
phase conspired by frustration and randomness in real materi-
als is relevant to the exotic QSL/RVB state with strong quan-
tum fluctuations, or simply a trivial product state of quenched
random singlets. To address this issue, local and dynamic
measurements on QSL candidates with quantifiable random-
ness are particularly needed.
Recently, a S= 1/2 KHA YCu3(OH)6.5Br2.5(YCOB) has
been proposed, without any global symmetry reduction of the
kagome lattice (space group P¯
3m1, see Fig. 1d) [44–46].
Neither long-range magnetic ordering nor spin-glass freezing
was observed down to 50 mK 0.001hJ1i, as evidenced by
specific heat [46], thermal conductivity [47], and ac suscep-
tibility [44, 45] measurements. The observed power-law T
dependence of low-Tspecific heat suggests the appearance of
gapless spin excitations [44, 46]. Unlike other known QSL
materials (e.g. ZnCu3(OH)6Cl2[14–16, 48, 49]), the mixing
between Cu2+ and other nonmagnetic ions is prohibited due
to the significant ionic difference, thus defect orphan spins
are essentially negligible [44]. Further, the antisite mixing of
the polar OHand nonpolar Brcauses 70(2)% of randomly
distributed hexagons of alternate exchanges (e.g. Fig. 1b) on
the kagome lattice (Fig. 1d), which accounts for the measured
arXiv:2210.12627v1 [cond-mat.str-el] 23 Oct 2022
2
Br1-
Br2-
Br2-
Cu2+
O22-
H2+
Y23+
a b c
-37.8 eV/FU
108.2o
CuOCu
~ 116.6oCuOCu
~ 115.5o
zy
x
H1+
O12- Y13+
-35.7 eV/FU
J1a
J1c J1b
Vzz
Vxx Vyy
Vzz
Vxx Vyy
Y2 (~70%)
Y1 (~30%)
Br2 (~50%)
OH2 (~50%)
Br1
d
FIG. 1. Crystal structure of YCu3(OH)6.5Br2.5around the kagome layer of Cu2+.The Br1 nuclear (81 Br or 79Br ) spin detects three
equidistant Cu2+ electronic spins of each triangle on the kagome lattice (a), and the inset defines the coordinate system for the spin components.
Whereas, the Br2 nuclear spin mainly probes the nonsymmetric hexagon of spins with alternate exchanges (b), instead of the symmetric
hexagon with almost uniform exchange (c). The formation energies of band cstacking sequences are listed, and the different exchange paths
of Cu-O-Cu (J1a,J1b, and J1c, depending on the bond angles) are marked. The principal axes of the electric field gradient calculated by density
functional theory at the Br1 (a) and Br2 (b) sites are displayed by arrows scaled by the modulus of the tensor element, Vzz ∼ −2Vxx ∼ −2Vyy
(see Supplementary Note 6). In our measurements, the external magnetic field is always applied along the zaxis, and thus the second-order
quadrupole shifts of the main NMR lines are negligibly small. (d) The crystal structure determined by the single-crystal x-ray diffraction [44].
The thin lines mark the unit cell.
thermodynamic properties above T0.1hJ1i[44]. There-
fore, YCOB provides an excellent platform for a quantita-
tive study of the aforementioned randomness-induced QSL
physics.
Herein, the effect of inherent randomness on the QSL prop-
erties was investigated locally by 81Br and 79Br nuclear mag-
netic resonance (NMR) measurements on high-quality YCOB
single crystals. We successfully identify the two NMR sig-
nals originating from the intrinsic kagome spins (Br1, see
Fig. 1a) and spins of hexagons with alternate exchanges on the
kagome lattice (Br2, see Fig. 1b). Simulations of the random
exchange model show good agreement with the measured T
dependence of NMR line shifts and broadening, which sug-
gests the formation of the randomness-induced QSL phase at
low T. The measured in-plane spin fluctuations of the kagome
spin system exhibit a weak power-law Tdependence down
to T0.03hJ1idespite the quenched exchange randomness,
thus supporting the survival of strong quantum fluctuations in
YCOB.
Results
NMR spectra. Figure 2a shows the 81Br NMR spectra mea-
sured on the crystal S1(Supplementary Fig. 1) at µ0Hk
10.75 T (0.14hJ1i). Two well-separated peaks are ob-
served, originating respectively from two different Wyckoff
positions of 2d(Br1) and 1a(Br2). Above 15 K, the
ratio between the integrated intensities of these two peaks
IBr2/IBr1 0.2 is well consistent with the stoichiometric ra-
tio of Br2 and Br1 determined by single-crystal x-ray diffrac-
tion (XRD) fBr2/fBr1 = 0.22(1) [44] (see inset of Fig. 2b).
Below 15 K, the weight of Br1 NMR line decreases drastically
and IBr2/IBr1 increases sharply, due to the reduced spin-spin
relaxation times (Supplementary Note 4 and Supplementary
Fig. 5) [50]. Neither Br1 nor Br2 peak splits down to 1.7
K, suggesting the absence of conventional magnetic ordering
within the ability of our resolution.
The NMR shift of Br1 (K1) detecting three (z1= 3) equidis-
tant spins of each triangle on the kagome lattice (Fig. 1a), fol-
lows the bulk susceptibility χkmeasured at the same magnetic
field strength in the full temperature range (Fig. 2b), com-
patible with the absence of defect orphan spins in YCOB. In
contrast, the NMR shift of Br2 (K2) shows an obvious devi-
ation from the bulk susceptibility below 100 K 2hJ1i.
Generically, the NMR shift consists of a T-dependent term
proportional to the local susceptibility and a T-independent
term (K0). Above 100 K, the NMR line broadening is in-
3
- 1 . 4 - 0 . 7 0 . 0 0 . 7
0 . 0 0
0 . 0 3
0 . 0 6
- 0 . 8
- 0 . 4
0 . 0
0 . 0 0
0 . 0 3
0 . 0 6
0 100 200 300
0 . 0
0 . 3
0 . 6
0 100 200 300
0 . 0
0 . 2
0 . 4
0 . 6
122 123 124
- 0 . 4
- 0 . 2
0 . 0
0 . 0 0 . 2 0 . 4 0 . 6
0 . 0
0 . 1
0 . 2
0 . 3
c
b
300 K
200 K
230 K
200 K
165 K
100 K
130 K
8 0 K
6 0 K
5 0 K
4 0 K
3 0 K
2 0 K
1 5 K
1 0 K
6 K
S p i n e c h o i n t e n s i t y ( a r b . u n i t s )
f/f0- 1 ( % )
1 . 7 K
a
Sz
K1 ( % )
Sz
f0
T ( K )
K2 ( % )
IB r 2 /IB r 1
T ( K )
X R D , fB r 2 /fB r 1
f ( M H z )
K1 ( % )
100 K
K2 ( % )
M||/H|| ( 0 . 0 1 B/ T )
300 K
FIG. 2. Nuclear magnetic resonance spectra of YCu3(OH)6.5Br2.5.(a) Frequency-sweep spectra measured on the sample S1at a field
µ0Hk10.75 T (the reference frequency f0=81γnµ0Hk123.64 MHz). The shifts of two lines, K1and K2, are marked by solid and
hollow triangles, respectively. Temperature dependence of K1(b) and K2(c), with the bulk magnetization (hSzi) measured at µ0Hk= 10.75
T for comparison. The inset of bshows the ratio between the integrated intensities of 81Br2 and 81 Br1 lines (IBr2/IBr1 ), as well as the
stoichiometric ratio from x-ray diffraction [44]. Inset of cdisplays K1and K2shifts vs bulk susceptibility χk(i.e. Mk/Hk). The red bars
display the normalized frequency regions where the intensity is larger than half of the maximum value in band c, and error bars on IBr2/IBr1
show a standard error from the fit.
significant (see Fig. 2) and the calculated local magnetization
is nearly spatially homogeneous (see Fig. 3a), and thus one ex-
pects a scaling law K=Ahf χk+K0, where Ahf presents the
hyperfine coupling between Br (Br1 or Br2) nuclear and Cu2+
electronic spins. By fitting the experimental data (see inset of
Fig. 2c), we obtain Ahf1 =0.68(2) T/µB,K01 =0.015(7)%
and Ahf2 = 0.55(3) T/µB,K02 = 0.02(1)%. The presence of
both negative and positive hyperfine couplings of the same
nuclear species is surprising, and the underlying mechanism
must be complex, including both the positive and negative
contributions.
The NMR shift of Br2 probes spins of hexagons on the
kagome lattice (Fig. 1b,c), but is obviously smaller than
Ahf2χk(K02 K2) below 100 K (Fig. 2c). In fact, the
formation energy of the optimized nonsymmetric Br2-OH2
stacking sequence (Fig. 1b) (37.8 eV/FU), is 2.5×104
K/FU lower than that of the symmetric Br2-Br2 configuration
(Fig. 1c) (35.7 eV/FU). Therefore, Br2 ions actually prefer
the nonsymmetric local environments, and Br2 nuclear spins
mainly probe the nonsymmetric hexagons with alternate ex-
changes [44] (J1a > J1c) as illustrated in Fig. 1b. Intuitively,
the nonsymmetric hexagon tends to locally release the frustra-
tion and form three nonmagnetic singlets along the stronger
couplings J1a (see Fig. 3e), which accounts for the relatively
smaller Br2 shifts observed below 100 K (Fig. 3a). More-
over, the Br1 line detects the site susceptibility/magnetization
of all the Cu2+ spins with exchange couplings J1a,J1b, and
J1c, whereas the Br2 line mainly probes the hexagons of spins
only with J1a and J1c. The Br2 line probes less kinds of Cu2+
spins, and thus is narrower than the Br1 line.
Quantitatively, the temperature dependence of both Br1
and Br2 shifts can be reproduced by the average magnetiza-
tion of all the triangles and nonsymmetric hexagons on the
kagome lattice, viz. K1Ahf1gkhSz
ti/(µ0Hk)and K2
Ahf2gkhSz
nhi/(µ0Hk), respectively (see Fig. 3a), with J1a =
89 K, J1b = 48 K, and J1c =16K(hJ1i 51 K) experi-
mentally determined by bulk susceptibilities (Supplementary
Note 1 and Supplementary Fig. 2). Taking all the triangles
and nonsymmetric hexagons into account, we are able to sim-
ulate the broadening of both Br1 and Br2 lines by introduc-
ing the distributed density dnS(hSzi)/dhSzi(see Fig. 3b,c),
where dnS(hSzi)is the number of triangles or nonsymmetric
hexagons with the local magnetization (per site) ranging from
hSzito hSzi+dhSzi.hSziis thermally averaged, so the dis-
tributed density is a function of T.
The simulations also enable us to revisit the correlation
functions hSi·Sjiin this KHA system with randomness. Com-
pared to the ideal case (Fig. 3f), a small fraction (3/54) of
well-defined singlets with hSi·Sji∼−0.7 → −0.75 are frozen
4
a b
a b
- 0 . 8
- 0 . 4
0 . 0 0 50 100 150 200 250 300
0 . 0
0 . 3
0 . 6
V S M Snhz
Stz Ss h z
0 . 0 0
0 . 0 3
0 . 0 6
- 1 . 0
- 0 . 5
0 . 0 0 . 0
0 . 4
0 . 8
0 . 1 1
- 0 . 5
0 . 0
012345
f
e
b 3
b 5
b 4
b 2
J1
d
b
J1 a
J1 b
J1 c
- 0 . 5
0 . 0
0 . 5
Szi
- 0 . 7 5
- 0 . 2 5
0 . 2 5
SiSj
a
b 1
K1 ( % )
B r 1
T ( K )
K2 ( % )
B r 2
Sz
0 100 200 3000 . 0 0
0 . 0 3
0 . 0 6
Stz
T ( K )
0 . 0 0 . 1 0 . 2
d e n s i t y
K1 ( % )
0 100 200 3000 . 0 0
0 . 0 3
0 . 0 6
Snhz
T ( K )
0 . 0 0 . 1 0 . 2
d e n s i t y
T ( K )
K2 ( % )
b 1
b 2
b 3
b 4
b 5
c
SiSj
T/J1
T/J1
FIG. 3. Simulations of the spatially inhomogeneous susceptibility probed by nuclear magnetic resonance (NMR) spectra. (a) Temper-
ature dependence of NMR shifts K1and K2, as well as bulk hSzimeasured in a vibrating sample magnetometer at a field µ0Hk= 10.75 T.
The colored lines show the calculated average hSziover all the triangles (hSz
ti), nonsymmetric hexagons (hSz
nhi), and symmetric hexagons
(hSz
shi). (b,c) The finite-temperature Lanczos diagonalization results of local magnetization of triangles (hSz
ti) and nonsymmetric hexagons
(hSz
nhi), along with the measured K1and K2, respectively. (d) Calculated local correlations of the selected spin pairs (see e,f) as function of
normalized temperature T/hJ1i. (e) Calculations of a sample within the random kagome Heisenberg antiferromagnet (KHA) model at T=
0.1hJ1iand µ0Hk= 10.75 T. The solid circles and squares stand for local magnetization hSz
iiat the kagome site iand correlation function
hSi·Sjiof the nearest-neighbor spin pair hiji, respectively. (f) The same calculations as in e, but within the ideal KHA model. The dashed
lines mark the clusters with periodic boundary conditions, and J1a,J1b ,J1c, and J1present the exchange couplings. The color scale in band
cquantifies the distributed density, whereas the ones in eand fquantify the local magnetization (circles) and correlation function (squares),
respectively. The bars in a-cshow the regions where the intensities are larger than half of the maximum value.
at low Tin YCOB (Fig. 3e), which is a signature of releas-
ing frustration due to the quenched randomness. These local
singlets might confine the mobile spinons [27, 37, 51, 52],
which might be responsible for the absence of large mag-
netic thermal conductivity observed in nearly all of the ex-
isting gapless QSL candidates, including the well-known
ZnCu3(OH)6Cl2[28, 29], EtMe3Sb[Pd(dmit)2]2[30, 31], etc.
However, the majority of antiferromagnetic interactions re-
main not fully satisfied at low T(see Fig. 3d,e), and the GS
wavefunction should be represented by a superposition of var-
ious pairings of spins. It is worth to mention that the weights
of different pairings should be different due to the quenched
randomness. The survival of strong frustration in the S= 1/2
random KHA speaks against the product GS wavefunction of
randomly distributed singlets, and may still give rise to strong
quantum fluctuations. To testify this, we turn to the spin dy-
namics of YCOB mainly probed by the spin-lattice relaxation
rates as follow.
Spin dynamics. The representative spin-lattice relaxation
data measured on YCOB are displayed in Figs 4a and 4b,
which can be well fitted to the single-exponential function for
the central transition of I= 3/2 nuclear spins, i.e.,
M(t) = M02M0F(1
10et
T1+9
10e6t
T1),(1)
where M0and Fare scale parameters for intensity. Alter-
natively, the relaxation data may be fitted with the stretched-
exponential one,
M(t) = M02M0F[1
10e(t
T1)β+9
10e(6t
T1)β].(2)
5
c d
ab
FIG. 4. Nuclear spin-lattice relaxation of YCu3(OH)6.5Br2.5.(a) A representative T= 2.8 K spin-lattice relaxation of Br1 measured by
an inversion recovery method, fitted to the stretched-exponential function with fixing β= 1 (red) and tuning β(blue). The pulse sequence for
T1measurements is depicted in the inset. (b) The same fits to the relaxation data measured at other selected temperatures. The inset shows
the fitted stretching exponent β. (c) Temperature dependence of 81Br1 nuclear spin-lattice relaxation rate 1/T1measured on the sample S1
at a field µ0Hk= 10.75 T, as well as 81Br1 and 79 Br1 1/T1measured on S2at µ0Hk= 10.75 and 11.59 T, respectively. The colored lines
present the power-law fits to the experimental data below 10 K 0.2hJ1i, the dashed black and blue lines are the 1/T1and energy (per
site), respectively, calculated by using the random kagome Heisenberg antiferromagnet model of YCu3(OH)6.5Br2.5. (d) The Curie (T1,
black lines) and critical ((TTc)α01, the critical temperature Tc=0.1±0.4 K, green lines) fits to 1/(T1T)at 1.7 T300 K. The
blue line is the low-Tpower-law dependence as shown in c. The inset shows the T1Tvs Tplot, where the dashed violet line displays the
antiferromagnetic Curie-Weiss behavior (T1TT+hJ1i, with coupling hJ1i ∼ 50 K). The T1data presented in cand dare obtained from the
single-exponential fits (i.e., β= 1, see aand b), and the stretched-exponential fits are made only for comparison in aand b. Error bars on the
experimental data points show a standard error from fit, and the error bars in aand bare small.
Here, the stretching exponent βslightly decreases at low tem-
peratures (see inset of Fig. 4b), but remains large down to the
lowest temperature of 1.7 K, β > 0.8. Moreover, the fit to
the single T1function (i.e. β= 1) is still good even at 1.7 K
(Fig. 4b), with the adj. R2= 0.997 [53]. The inclusion of
the additional fitting parameter βonly slightly improves the
fit (the adj. R2increases to 0.998), and even makes the stan-
dard error on T1larger. Therefore, all the following T1data
are obtained with the single-exponential fits.
Figure 4c shows the results of Br1 nuclear spin-lattice re-
laxation rates (1/T1). Br1 1/T1is highly sensitive to the elec-
tronic spin fluctuations perpendicular to the applied magnetic
field on the kagome layer via the hyperfine coupling [39, 54–
56],
1
T1
T
hf0X
q
χ00(q, f0)πγ2
nA2
hf1g2
ZX
q,m,m0
eEm
kBT
× |hm0|S
q|mi|2δ(f0Em0Em
h),(3)
where the qdependence of Ahf1 is neglected, f0=γnµ0Hk
(kBT/h)0 presents the NMR frequency, S
qis
the Fourier transform of S
iover all triangles, and Z=
Pmexp(Em/kB/T )the partition function. In our simu-
lation of 1/T1, the delta function is replaced by a Gaussian
distribution with narrow width 104hJ1i/h [39, 56].
At high temperatures (T hJ1i), the Moriya para-
magnetic limit yields the T-independent 1/T
1=
摘要:

TheobservationofquantumuctuationsinakagomeHeisenbergantiferromagnetFangjunLu,1,LongYuan,1,JianZhang,1BoqiangLi,1YongkangLuo,1,yandYueshengLi1,z1WuhanNationalHighMagneticFieldCenterandSchoolofPhysics,HuazhongUniversityofScienceandTechnology,430074Wuhan,China(Dated:October25,2022)AbstractThesearchf...

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The observation of quantum fluctuations in a kagome Heisenberg antiferromagnet Fangjun Lu1Long Yuan1Jian Zhang1Boqiang Li1Yongkang Luo1yand Yuesheng Li1z 1Wuhan National High Magnetic Field Center and School of Physics.pdf

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