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 T∼0.03hJ1i. 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/J1≥0.4
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 OH−and nonpolar Br−causes 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