Abstract
The BerezinskiiKosterlitzThouless (BKT) mechanism, building upon proliferation of topological defects in 2D systems, is the first example of phase transition beyond the LandauGinzburg paradigm of symmetry breaking. Such a topological phase transition has long been sought yet undiscovered directly in magnetic materials. Here, we pin down two transitions that bound a BKT phase in an ideal 2D frustrated magnet TmMgGaO_{4}, via nuclear magnetic resonance under inplane magnetic fields, which do not disturb the lowenergy electronic states and allow BKT fluctuations to be detected sensitively. Moreover, by applying outofplane fields, we find a critical scaling behavior of the magnetic susceptibility expected for the BKT transition. The experimental findings can be explained by quantum Monte Carlo simulations applied on an accurate triangularlattice Ising model of the compound which hosts a BKT phase. These results provide a concrete example for the BKT phase and offer an ideal platform for future investigations on the BKT physics in magnetic materials.
Introduction
Topology plays an increasingly important role in understanding different phases and phase transitions in correlated quantum matters and materials. One prominent example is the Berezinskii–Kosterlitz–Thouless (BKT) mechanism in twodimensional (2D) systems^{1,2,3,4,5}, which is associated with the binding and unbinding of topological defects. The BKT transition cannot be characterized by conventional order parameters and constitutes the earliest example of phase transition beyond the Landau–Ginzburg paradigm of spontaneous symmetry breaking. Historically, the BKT mechanism was introduced in the XY spin model and long predicted to occur in magnetic thin films^{1,4}. In experiments, signatures of the BKT transition have been observed in superfluid helium films^{6}, as well as in 2D superconducting films^{7,8} and arrays^{9}. Regarding the original proposal in layered XYtype magnets, despite intensive efforts^{10,11,12,13,14,15}, direct and unambiguous observation of the BKT transition is still lacking. One major obstacle is the threedimensional (3D) couplings in the magnets, although weak, will inevitably enhance the confining potential of vortices^{15}, leading to 3D ordering that masks the BKT transition. Therefore, it is of fundamental interest to find and identify BKT materials that could overcome the obstacle and study the topologyrelated lowenergy dynamics.
Recently, a layered frustrated rareearth antiferromagnet TmMgGaO_{4}^{16,17,18} was reported to ideally realize the triangularlattice quantum Ising (TLI) model^{19}. The relatively large interlayer distance of 8.3774 Å along the c axis gives rise to excellent two dimensionality^{17} and no sign of conventional 3D phase transition was observed in either specific heat or magnetic susceptibility measurements. Nevertheless, it was reported from neutron scattering that TmMgGaO_{4} ordered below ~1 K into an antiferromagnetic phase with a sixfold symmetry breaking^{16,18}, which closely resembles the ground state of the TLI model originated from an orderbydisorder mechanism^{20,21}. At higher temperatures, the effective XY degrees of freedom emerge and the BKT mechanism is expected to come into play^{21}.
In TmMgGaO_{4}, each Tm^{3+}, with a 4f^{12} electron configuration and a spin–orbit moment J = 6, forms a nonKramers doublet due to the crystalelectricfield splitting. The doublet is well separated from the rest levels by about 400 K^{16} and can thus be regarded as an effective spin1/2. There is further a fine energy splitting within the doublet, induced by the local trigonal crystal field^{17}, acting as an intrinsic “transverse field” applied on the effective spin. From the magnetization measurements^{16,17,18}, one observes that Tm^{3+} ions contribute highly anisotropic Isingtype moments with J_{z} = ±6 along the c axis, resulting in an effective outofplane gfactor ~ 13.2^{16,19}. On the contrary, the effective inplane gfactor and dipolar moment in the ab plane are negligible.
Facilitated by this feature in TmMgGaO_{4}, in this work we employed nuclear magnetic resonance (NMR), a sensitive lowenergy probe, to detect the BKT phase. We applied a moderate inplane field of 3 T, which is adequate to collect the ^{69}Ga NMR signals and, at the same time, hardly disturbs the lowenergy electronic states of the material. This is important, as in the TLI model that is believed to accurately model TmMgGaO_{4}^{19}, the BKT phase can be fragile against outofplane fields^{22,23,24}, thus posing a challenge to NMR measurements. Taking advantage of the fact that inplane moment in TmMgGaO_{4} is mostly multipolar^{16,19}, our NMR experiments with inplane fields, which merely couple to the nuclear spins, can clearly identify the BKT phase in the material.
As shown in Fig. 1, from our NMR measurements of the spinlattice relaxation rate 1/T_{1}, we identify T_{U} ≃ 1.9 K and T_{L} ≃ 0.9 K, which represent the upper and lowerBKT transitions, where a critical BKT phase resides at zero magnetic field in between the highT paramagnetic and lowT antiferromagnetic phases. This finding is further substantiated by our scaling analysis of the measured susceptibility data near T_{L}, as well as the simulated NMR and susceptibility data using largescale quantum Monte Carlo (QMC) calculations.
Results
NMR probe of the BKT phase
The obtained NMR spectra with an inplane magnetic field μ_{0}H = 3 T are shown in Fig. 2a at representative temperatures. To better resolve the magnetic transition, the hyperfine shifts ^{69}K_{n} of the spectra were analyzed and plotted in Fig. 2b as a function of temperature. Upon cooling, ^{69}K_{n} peaks at about 0.8–0.95 K and then starts to drop at lower temperatures. Therefore, the ordering temperature is determined to be T_{L} ≃ 0.9 K, consistent with neutron scattering experiments^{16,18}. In addition, both the second moments (width of the NMR spectra) and the third moments (asymmetry of the spectra) of the spectra change dramatically below ~ 2 K, suggesting the onset of local hyperfine fields enhanced by the static or quasistatic magnetic ordering (Supplementary Fig. 4). These two characteristic temperatures signal the twostep melting of magnetic order through two BKT transitions, suggesting an intermediate floating BKT phase in the system. We suspect that there is some inhomogeneity of the local hyperfine fields, which is very likely caused by the quenched disorder from Mg/Ga site mixing^{16}, although no significant influence on the electronic and more importantly the magnetic properties is seen (see more detailed discussions in Supplementary Note 4).
The spinlattice relaxation rate 1/T_{1} provides a highly sensitive detection of lowenergy spin fluctuations^{25,26,27,28,29}, and thus the BKT transition. In Fig. 2c, we show the 1/^{69}T_{1} obtained under inplane fields μ_{0}H = 3 T and 1 T, which reflects intrinsic spin fluctuations with zero outofplane field. At 3 T, 1/^{69}T_{1} first decreases upon cooling from 10 K then suddenly increases below T_{U} ≈ 1.9 K, indicating the onset of strong lowenergy spin fluctuations. The data at 1 T show similar behaviors. Below T_{L} ≃ 0.9 K, 1/^{69}T_{1} drops sharply, consistent with the onset of the magnetic ordering as also inferred from the hyperfine shift. Here, 1/^{69}T_{1} is dominated by the gapped long wavelength excitations in the ordered state. At the magnetic phase transition, a peaked feature in 1/T_{1} develops, caused by the gapless lowenergy spin fluctuations with diverging correlation length. Remarkably, at temperatures between T_{U} ≃ 1.9 K and T_{L} ≃ 0.9 K, 1/^{69}T_{1} exhibits a plateaulike structure, indicating the emergence of a highly fluctuating phase with diverging spin correlations yet no true longrange order, which is the hallmark of a BKT phase^{1,2,3,4,5}. Therefore, it is for the first time that such a phase is unambiguously observed in a magnetic crystalline material.
Our unbiased QMC simulations on the TLI model of the material (see “Methods”), with accurate coupling parameters determined in ref. ^{19}, quantitatively justifies the existence of the BKT phase between T_{L} and T_{U}. We computed 1/T_{1} and compared with the experiment below. Figure 2d shows the calculated 1/T_{1} by including fluctuations from all momentum points in the Brillouin zone (cf. Supplementary Fig. 2) and compare to that from K′ (around the K point at the corner of Brillouin zone). The former shows a decrease upon cooling below 4 K and then an upturn above \({T}_{{\rm{U}}}^{* }\simeq 2\) K, followed by a rapid decrease below \({T}_{{\rm{L}}}^{* }\simeq 1\) K. These behaviors are in excellent agreement with the measured 1/^{69}T_{1}. The latter reflects gapless excitations of the XY degrees of freedom emergent in the BKT phase, where the calculated 1/T_{1} from K′ exhibits an anomalous increase down to \({T}_{{\rm{U}}}^{* }\), below which the increment slows down. The contribution to 1/T_{1} near the K point reaches a maximum at \({T}_{{\rm{L}}}^{* }\) and drops rapidly below it. The absence of critical spin fluctuations at momentum away from the K point suggests that the measured 1/^{69}T_{1} below 2 K is mainly contributed by excitations around the K point (see Supplementary Note 3).
Overall, the quasiplateau behaviors in the QMC results and the two characteristic temperature scales are in full consistency with the NMR measurements. This constitutes both strong support for the accurate quantum manybody modeling of the material TmMgGaO_{4} and also solid proof of the BKT phase therein detected by NMR. Nevertheless, we note that there are still subtle differences between the experimental and numerical data. Needless to say, the real material is always more complicated than our theoretical minimal model. For example, influences from higher crystalelectricfield levels above the nonKramers doublet, the interlayer couplings not included in our model calculations, and the lack of knowledge on the precise local hyperfine coupling constant, etc., may explain the difference remaining between Fig. 2c, d.
Universal magnetic susceptibility scaling
Magnetic susceptibility χ measurements were also performed to strengthen the finding of the BKT phase. In Fig. 3a, we show the overall temperature dependence of χ at different outofplane fields. For T ≳ 2 K, χ increases monotonically upon cooling and barely changes with fields. However, for T ≲ 2 K, approximately the upper BKT transition T_{U} as obtained from the 1/^{69}T_{1} measurements, χ increases as the field decreases, suggesting the onset of peculiar magnetic correlations. With further cooling, the susceptibility gets flattened with temperature. The magnetization M(H) was further measured at selected temperatures (data shown in Supplementary Fig. 6), and for the sake of clarity, the differential susceptibility dM/dH is plotted in Fig. 3b. At around 2.5 T, a pronounced peak can be observed at low temperature, indicating the existence of a quantum phase transition, and the phase at lower fields should be a magnetically ordered phase, although its precise nature remains to be uncovered. Besides the highfield feature, for temperatures at 0.8 K and above, a kinked feature is clearly resolved on each dM/dH curve at low fields, whereas at 0.4 K, the lowfield kink disappears, which posts a question of whether there is a quantum transition or merely a crossover from the zerofield AFM phase to the finitefield ordered phase under the dome in Fig. 1. The temperature and field values indicated by the down arrows in Fig. 3 are denoted as \(T^{\prime}\) and T″ in the phase diagram (Fig. 1).
Fieldtheoretical analysis of the TLI model^{23,24} has predicted that upon applying a small outofplane field, the differential susceptibility dM/dH shall exhibit a divergent powerlaw behavior as dM/dH ~ H^{−α} in proximity to the BKT phase. At T_{L}, α has the value of 2/3, which corresponds to a critical exponent η = 1/9 at the lowerBKT transition and is originated from the sixfold symmetry breaking^{23}. The exponent α gradually decreases as temperature increases, and above an intermediate temperature between T_{L} and T_{U}, α = 0 due to nonuniversal contributions. This is exactly what we observe in Fig. 3c. We fit the dM/dH with the powerlaw function at different temperatures, with the fitting regime chosen between 0.6–0.9 T. At 0.8 K, α is very close to the expected value of 2/3 (and thus η = 1/9) at T_{L}, which constitutes a remarkable fingerprint evidence for the BKT transition. At lower temperatures such as 0.4 K, the exponent is also close to 2/3, because the susceptibility saturates with temperature, as shown in Fig. 3a. At high temperatures, α drops rapidly to a small value 0.12 at 2.1 K and becomes effectively zero at 3.0 K.
Therefore, the susceptibility scaling gives the lowerBKT transition at about 0.8 K and upper transition probably between 2.1 and 3 K, in good agreement with the T_{L} and T_{U} estimated from NMR. These results are also fully consistent with our QMC calculations on the susceptibility shown in Fig. 3d. At T_{L} or lower, α is 2/3, then decreases to a very small exponent 0.086 at 2.67 K, and above 3 K, becomes zero within numerical uncertainty. Such a powerlaw behavior in dM/dH again signifies the finitetemperature window of the BKT phase with diverging magnetic correlations, which gives rise to the universal powerlaw scaling of magnetic susceptibility.
Discussion
We believe the findings in this work are of various fundamental values. Since the original proposal of a BKT phase in magnetic films^{3,4,5}, which also triggered the currently thriving research field of topology in quantum materials, tremendous efforts have been devoted to finding the BKT phase in magnetic crystalline materials, yet hindered by the obstacle outlined in the Introduction. Here, benefiting from NMR as a sensitive lowenergy probe, and the nearly zero planar gyromagnetic factor in a TLI magnet TmMgGaO_{4}, we are able to reveal two BKT transitions and a critical BKT phase with an emergent XY symmetry. Together with the powerlaw behavior of the susceptibility and excellent agreement between our QMC simulation and experiment data, we unambiguously identify the longsought BKT phase in a magnetic crystalline material.
Many intriguing questions are stimulated, based on the phase diagram in Fig. 1 obtained here. First, what is the nature of the ordered phase under finite fields, are there further exotic phases and transitions in the phase diagram, and will there be a fieldinduced quantum phase transition at the highfield side of the dome—these are all of great interests to be addressed in future studies. Second, it should be noted that the dynamical properties obtained by QMC calculations in Fig. 2 are computed on a large, while finitesize, 36 × 36 lattice, which already produces 1/T_{1} data in very good agreement with the experimental measurements. Such a great agreement is surprising, given the possible existence of randomness from Ma/Ga site mixing in the material TmMgGaO_{4}^{16}, and also the lattice disorder revealed by the large hightemperature second moment of the NMR spectra (Supplementary Note 4). Although the random distribution in intrinsic transverse fields and spin couplings does not seem to alter the lowtemperature spinordered phase and the sharp spin excitation line shapes^{18,19}, its intriguing effects on the finitetemperature phase diagram of TLI and also the compound TmMgGaO_{4} call for further studies.
Third, in the study of BKT transition in superfluid systems, it has been observed experimentally and understood theoretically that additional dissipations also appear above the transition temperature due to fluctuations of vortices^{6}. Hence, the plateau of 1/T_{1} we observe may also cover regions slightly above the upper BKT transition temperature. We leave this subtlety to future numerical and experimental efforts. Lastly, in general terms, whether there are other rareearth magnetic materials in the same family of TmMgGaO_{4} that, acquire similar 2D competing magnetic interactions from highly anisotropic gyromagnetic factor and unique triangularlattice structures, and also exhibit the BKT physics, is quite intriguing and calls for future investigations. All these directions are ready to be explored from here.
Methods
Crystal growth and susceptibility measurements
Single crystals were grown by the opticalfloatingzone method with an image furnace (IR Image Furnace G2, Quantum Design). The natural cleavage surface of the crystals is the ab plane, which allows us to align the field orientation within an error of 2^{∘}. The dc susceptibility was measured in a PPMS VSM (Quantum Design) for temperatures above 2 K and in a He3 MPMS (Quantum Design) for temperatures ranging from 0.4 to 2 K.
NMR measurements
The ^{69}Ga (I = 3/2, γ = 10.219 MHz/T) NMR spectra were collected with the standard spinecho sequence, with frequency sweep by a 50 kHz step using a topping tuning circuit. The NMR hyperfine shift was obtained by calculating the change of the first moment of the spectra. The spinlattice relaxation rate 1/^{69}T_{1} was measured by the inversionrecovery method, where a π/2 pulse was used as the inversion pulse. The NMR data from 1.8 K and above were measured in a variable temperature insert, and the data from 1.8 K and below were measured in a dilution refrigerator. The weak NMR signal at low fields and the rapid decrease of ^{69}T_{2} upon cooling (Supplementary Note 5) prevented us to measure the 1/^{69}T_{1} for inplane fields <3 T, with temperature below 1.8 K. Whereas for inplane fields of 4 T and higher, the sample could not be held in position because of the large anisotropy in the gfactor and unavoidable sample misalignment (≲2^{∘}). At T = 1.2 K, we did not find any change of 1/^{69}T_{1} with two different radio frequency excitation levels (14 mT and 24 mT), and with different frequencies across the NMR line, within the error bar.
Triangularlattice Ising model
At zero field, the intralayer couplings in TmMgGaO_{4} can be described by the TLI Hamiltonian,
where J_{1} and J_{2} are the superexchange interactions among Tm^{3+}, 〈i, j〉 and 〈〈i, j〉〉 refer to the nearest neighbors and the nextnearest neighbors, respectively, and Δ is the energy splitting within the nonKramers doublet imposed by the crystal field. We have shown that the parameter set J_{1} = 0.99 meV, J_{2}/J_{1} ≈ 0.05 and Δ/J_{1} ≈ 0.54 reproduces the experimental results of the transition temperatures and the inelastic neutron scattering spectra^{19}.
In the TLI model [Eq. (1)], we can define a complex field ψ as a combination of the Ising (Z) components \({m}_{{\rm{A}},{\rm{B}},{\rm{C}}}^{z}\) on three sublattices, i.e.,
Notably, ψ = ∣ψ∣e^{iθ} is a complex order parameter that represents the emergent XY degree of freedom relevant to the BKT physics in the TLI model.
QMC calculations
QMC simulations were performed in the path integral in the \({S}_{i,\tau }^{z}\) basis with discretization in space and time. The lattice of L × L × L_{τ}, where L = 36 and L_{τ} = β/Δτ with Δτ = 0.05 and β = 1/T, were simulated with both local and Wolffcluster update schemes^{30,31}. The 1/T_{1} results were obtained by first computing the dynamical spin–spin correlation function \(\langle {S}_{i}^{z}(\tau ){S}_{j}^{z}(0)\rangle\) and then acquiring its realfrequency dependence S(q, ω) from the stochastic analytic continuation^{32}. We then determined the 1/T_{1} either by summing the contributions close to momentum K or over the entire Brillouin zone, as discussed in the Fig. 2d of the main text,
where A_{hf}(q) is the hyperfine coupling form factor (see Supplementary Note 2). Similar analyses have been successfully applied to the QMC computation of NMR 1/T_{1} for the spin1/2 and spin1 chains^{33,34}.
Data availability
The data that support the findings of this study are available from the corresponding authors upon reasonable request.
Code availability
All numerical codes in this paper are available upon request to the corresponding authors.
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Acknowledgements
We thank Changle Liu, Rong Yu, Nvsen Ma, and Anders Sandvik for stimulating discussions. We acknowledge the supports from the National Key Projects for Research and Development of China through Grant numbers 2016YFA0300502 and 2016YFA0300504, the National Natural Science Foundation of China through Grant numbers 11574359, 11674370, 11822405, 11674157, 11974036, 11834014, 11874115, and 51872328, RGC of Hong Kong SAR China through Grant number 17303019, Natural Science Foundation of Jiangsu Province with Grant number BK20180006, Fundamental Research Funds for the Central Universities with Grant number 020414380117, and the Research Funds of Renmin University of China. We thank the Center for Quantum Simulation Sciences in the Institute of Physics, Chinese Academy of Sciences, the Computational Initiative at the Faculty of Science and the Information Technology Services at the University of Hong Kong, the Platform for DataDriven Computational Materials Discovery at the Songshan Lake Materials Laboratory, Guangdong, China, and the TianheI, TianheII, and TianheIII prototype platforms at the National Supercomputer Centers in Tianjin and Guangzhou for their technical support and generous allocation of CPU time.
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W.Q.Y. and J.S.W. designed the experiments, with proposals from Y.Q., W.L., and Z.Y.M. Z.M. grew and characterized the single crystals and performed susceptibility measurements and analysis, with help from Y.Y.S.G., Z.T.H., Z.H., and H.L. Z.H., C.S.M., and Y.C. performed NMR measurements and analysis. Y.D.L. and H.L. carried out the largescale quantum manybody calculations, with the guidance from Y.Q., W.L., and Z.Y.M. W.Q.Y., J.S.W., W.L., Z.Y.M., and Y.Q. wrote the manuscript with comments from all coauthors.
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Hu, Z., Ma, Z., Liao, YD. et al. Evidence of the BerezinskiiKosterlitzThouless phase in a frustrated magnet. Nat Commun 11, 5631 (2020). https://doi.org/10.1038/s4146702019380x
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DOI: https://doi.org/10.1038/s4146702019380x
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