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Register a new userMagnetic phase diagram of Cu$_{4-x}$Zn$_x$(OH)$_6$FBr studied by neutron-diffraction and $mu$SR techniques
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We have systematically studied the magnetic properties of Cu$_{4-x}$Zn$_x$(OH)$_6$FBr by the neutron diffraction and muon spin rotation and relaxation ($mu$SR) techniques. Neutron-diffraction measurements suggest that the long-range magnetic order and the orthorhombic nuclear structure in the $x$ = 0 sample can persist up to $x$ = 0.23 and 0.43, respectively. The temperature dependence of the zero-field (ZF) $mu$SR spectra provide two characteristic temperatures, $T_{A0}$ and $T_{lambda}$. Comparison between $T_{A0}$ and $T_M$ from previously reported magnetic-susceptibility measurements suggest that the former comes from the short-range interlayer-spin clusters that persist up to $x$ = 0.82. On the other hand, the doping level where $T_{lambda}$ becomes zero is about 0.66, which is much higher than threshold of the long-range order, i.e., $sim$ 0.4. Our results suggest that the change in the nuclear structure may alter the spin dynamics of the kagome layers and a gapped quantum-spin-liquid state may exist above $x$ = 0.66 with the perfect kagome planes.
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Barlowite Cu$_4$(OH)$_6$FBr shows three-dimensional (3D) long-range antiferromagnetism, which is fully suppressed in Cu$_3$Zn(OH)$_6$FBr with a kagome quantum spin liquid ground state. Here we report systematic studies on the evolution of magnetism in the Cu$_{4-x}$Zn$_x$(OH)$_{6}$FBr system as a function of $x$ to bridge the two limits of Cu$_4$(OH)$_6$FBr ($x$=0) and Cu$_3$Zn(OH)$_6$FBr ($x$=1). Neutron-diffraction measurements reveal a hexagonal-to-orthorhombic structural change with decreasing temperature in the $x$ = 0 sample. While confirming the 3D antiferromagnetic nature of low-temperature magnetism, the magnetic moments on some Cu$^{2+}$ sites on the kagome planes are found to be vanishingly small, suggesting strong frustration already exists in barlowite. Substitution of interlayer Cu$^{2+}$ with Zn$^{2+}$ with gradually increasing $x$ completely suppresses the bulk magnetic order at around $x$ = 0.4, but leaves a local secondary magnetic order up to $xsim 0.8$ with a slight decrease in its transition temperature. The high-temperature magnetic susceptibility and specific heat measurements further suggest that the intrinsic magnetic properties of kagome spin liquid planes may already appear from $x>0.3$ samples. Our results reveal that the Cu$_{4-x}$Zn$_x$(OH)$_6$FBr may be the long-thought experimental playground for the systematic investigations of the quantum phase transition from a long-range antiferromagnet to a topologically ordered quantum spin liquid.
Quantum spin liquid (QSL) represents a new class of condensed matter states characterized by the long-range many-body entanglement of topological orders. The most prominent feature of the elusive QSL state is the existence of fractionalized spin excitations. Subject to the strong quantum fluctuations, the spin-1/2 antiferromagnetic system on a kagome lattice is the promising candidate for hosting a QSL ground state, but the structurally ideal realization is rare. Here, we report Raman scattering on the single crystalline Cu$_3$Zn(OH)$_6$FBr, and confirm that the ideal kagome structure remains down to low temperatures without any lattice distortion by the angle-resolved polarized Raman responses and second-harmonic-generation measurements. Furthermore, at low temperatures the Raman scattering reveals a continuum of the spin excitations in Cu$_3$Zn(OH)$_6$FBr, in contrast to the sharp magnon peak in the ordered kagome antiferromagnet EuCu$_3$(OH)$_6$Cl$_3$. Such magnetic Raman continuum, in particular, the substantial low-energy one-pair spinon excitation serves as strong evidence for fractionalized spin excitations in Cu$_3$Zn(OH)$_6$FBr.
We report the results of ac and dc magnetization (M) and heat-capacity (C) measurements on the solid solution, Sr$_3$Cu$_{1-x}$Zn$_x$IrO$_6$. While the Zn end member is known to form in a rhombohedral pseudo one-dimensional K$_4$CdCl$_6$ structure with an antiferromagnetic ordering temperature of (T$_N$ =) 19 K, the Cu end member has been reported to form in a monoclinically distorted form with a Curie temperature of (T$_C$ =) 19 K. The magnetism of the Zn compound is found to be robust to synthetic conditions and is broadly consistent with the behavior known in the literature. However, we find a lower magnetic ordering temperature (T$_o$) for our Cu compound (~ 13 K), thereby suggesting that T$_o$ is sensitive to synthetic conditions. The Cu sample appears to be in a spin-glass-like state at low temperatures, judged by a frequency dependence of ac magnetic susceptibility and a broadening of the C anomaly at the onset of magnetic ordering, in sharp contrast to earlier proposals. Small applications of magnetic field, however, drive this system to ferromagnetism as inferred from the M data. Small substitutions for Cu/Zn (x = 0.75 or 0.25) significantly depress magnetic ordering; in other words, T$_o$ varies non-monotonically with x (T$_o$ ~ 6, 3 and 4 K for x = 0.25, 0.5, and 0.67 respectively). The plot of inverse susceptibility versus temperature is non-linear in the paramagnetic state as if correlations within (or among) the magnetic chains continuously vary with temperature. The results establish
We systematically study the low-temperature specific heats for the two-dimensional kagome antiferromagnet, Cu$_{3}$Zn(OH)$_6$FBr. The specific heat exhibits a $T^{1.7}$ dependence at low temperatures and a shoulder-like feature above it. We construct a microscopic lattice model of $Z_2$ quantum spin liquid and perform large-scale quantum Monte Carlo simulations to show that the above behaviors come from the contributions from gapped anyons and magnetic impurities. Surprisingly, we find the entropy associated with the shoulder decreases quickly with grain size $d$, although the system is paramagnetic to the lowest temperature. While this can be simply explained by a core-shell picture in that the contribution from the interior state disappears near the surface, the 5.9-nm shell width precludes any trivial explanations. Such a large length scale signifies the coherence length of the nonlocality of the quantum entangled excitations in quantum spin liquid candidate, similar to Pippards coherence length in superconductors. Our approach therefore offers a new experimental probe of the intangible quantum state of matter with topological order.
Barlowite, Cu$_{4}$(OH)$_{6}$FBr, has attracted much attention as the parent compound of a new series of quantum spin liquid candidates, Zn$_{x}$Cu$_{4-x}$(OH)$_{6}$FBr. While it is known to undergo a magnetic phase transition to a long-range ordered state at $T_{N} = 15$ K, there is still no consensus over either its nuclear or magnetic structures. Here, we use comprehensive powder neutron diffraction studies on deuterated samples of barlowite to demonstrate that the only space group consistent with the observed nuclear and magnetic diffraction at low-temperatures is the orthorhombic $Pnma$ space group. We furthermore conclude that the magnetic intensity at $T < T_{N}$ is correctly described by the $Pn^prime m^prime a$ magnetic space group, which crucially allows the ferromagnetic component observed in previous single-crystal and powder magnetisation measurements. As such, the magnetic structure of barlowite resembles that of the related material clinoatacamite, Cu$_{4}$(OH)$_{6}$Cl$_{2}$, the parent compound of the well-known quantum spin liquid candidate hebertsmithite, ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$.
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