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arXiv:1903.00246 · Physical review. B./Physical review. B

Quantum thermal Hall effect of chiral spinons on a kagome strip

Pavel Tikhonov, Efrat Shimshoni

Published May 28, 2019• 4 citations
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Abstract

We develop a theory for the thermal Hall coefficient in a spin-$\frac{1}{2}$ system on a strip of kagome lattice, where a chiral spin-interaction term is present. To this end, we model the kagome strip as a three-leg $XXZ$ spin-ladder, and use bosonization to derive a low-energy theory for the spinons in this system. Introducing further a Dzyaloshinskii-Moriya interaction ($D$) and a tunable magnetic field ($B$), we identify three distinct $B$-dependent quantum phases: a valence-bond crystal (VBC), a ``metallic'' spin liquid (MSL), and a chiral spin liquid (CSL). In the presence of a temperature difference $\mathrm{\ensuremath{\Delta}}T$ between the top and the bottom edges of the strip, we evaluate the net heat current ${J}_{h}$ along the strip, and consequently the thermal Hall conductivity ${\ensuremath{\kappa}}_{xy}$. We find that the VBC-MSL-CSL transitions are accompanied by a pronounced qualitative change in the behavior of ${\ensuremath{\kappa}}_{xy}$ as a function of $B$. In particular, analogously to the quantum Hall effect, ${\ensuremath{\kappa}}_{xy}$ in the CSL phase exhibits a quantized plateau centered around a commensurate value of the spinon filling factor ${\ensuremath{\nu}}_{s}\ensuremath{\propto}B/D$.

DOI
10.1103/physrevb.99.174429

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We investigate the spin-12\frac{1}{2}21​ antiferromagnetic Heisenberg model with a Dzyaloshinskii-Moriya interaction on kagome lattice, making use of the variational Monte Carlo technique. An exotic quantum spin state is found to arise from a melting of the $\mathbit{Q}=0$ long-range magnetic order by a topological transition, when a small anisotropic third-nearest-neighbor antiferromagnetic Heisenberg interaction is turned on. This state is a gapped quantum spin liquid, characterized by a topological order with ground-state degeneracy ng=4{n}_{g}=4ng​=4 and topological entanglement entropy $\ensuremath{\gamma}=ln2$, suggesting it is an Abelian topological phase. Furthermore, the Chern numbers of the spin-up (-down) spinon occupied bands of this state are ${C}_{\ensuremath{\uparrow}\ensuremath{\downarrow}}=\ifmmode\pm\else\textpm\fi{}1$, respectively. From this perspective, this state is also a time-reversal symmetric (total Chern number Ctotal=0{C}_{\text{total}}=0Ctotal​=0) topological insulator with spinons as the chiral edge states, which carry opposite spin and move in the opposite direction. It is analogous to the quantum spin Hall state but the spin current is carried by deconfined spinons in a quantum spin liquid, and so is dubbed as the spinon quantum spin Hall state.
We perform an exact-diagonalization study of the spin-12\frac{1}{2}21​ XXZ Heisenberg antiferromagnet on the kagome lattice at finite magnetization m=23m=\frac{2}{3}m=32​ with an emphasis on the XYXYXY point (Jz=0)({J}_{z}=0)(Jz​=0) and in the presence of a small chiral term. Recent analytic work by Kumar et al. [K. Kumar, K. Sun, and E. Fradkin, Phys. Rev. B 90, 174409 (2014)] on the same model, using a newly developed flux attachment transformation, predicts a plateau at this value of the magnetization described by a chiral spin liquid (CSL) with a spin Hall conductance of ${\ensuremath{\sigma}}_{xy}=\frac{1}{2}$. Such a state is topological in nature, has a ground-state degeneracy, and exhibits fractional excitations. We analyze the degeneracy structure in the low-energy manifold, identify the candidate topological states, and use them to compute the modular matrices and Chern numbers, all of which strongly agree with expected theoretical behavior for the ${\ensuremath{\sigma}}_{xy}=\frac{1}{2}$ CSL. In the limit of zero chirality, we find on most (not all) clusters that the topological invariants are still those of a CSL.
Motivated by recent transport measurements in high-Tc{T}_{c}Tc​ cuprate superconductors in a magnetic field, we study the thermal Hall conductivity in materials with topological order, focusing on the contribution from neutral spinons. Specifically, different Schwinger boson mean-field Ans\"atze for the Heisenberg antiferromagnet on the square lattice are analyzed. We allow for both Dzyaloshinskii-Moriya interactions, and additional terms associated with scalar spin chiralities that break time-reversal and reflection symmetries, but preserve their product. It is shown that these scalar spin chiralities, which can either arise spontaneously or are induced by the orbital coupling of the magnetic field, can lead to spinon bands with nontrivial Chern numbers and significantly enhanced thermal Hall conductivity. Associated states with zero-temperature magnetic order, which is thermally fluctuating at any T>0T>0T>0, also show a similarly enhanced thermal Hall conductivity.
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We calculate the thermal Hall conductivity ${\ensuremath{\kappa}}_{xy}$ for the columnar valence bond solid phase of a two-dimensional frustrated antiferromagnet. In particular, we consider the square lattice spin-$1/2\phantom{\rule{4pt}{0ex}}{J}_{1}\text{\ensuremath{-}}{J}_{2}$ antiferromagnetic Heisenberg model with an additional Dzyaloshinskii-Moriya interaction between the spins and in the presence of an external magnetic field. We concentrate on the intermediate parameter region of the ${J}_{1}\text{\ensuremath{-}}{J}_{2}$ model, where a quantum paramagnetic phase is stable, and consider a Dzyaloshinskii-Moriya vector pattern associated with the couplings between the spins in the CuO2{\mathrm{CuO}}_{2}CuO2​ planes of the YBa2Cu3O6{\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6}YBa2​Cu3​O6​ compound. We describe the columnar valence bond solid phase within the bond-operator formalism, which allows us to map the Heisenberg model into an effective interacting boson model written in terms of triplet operators. The effective boson model is studied within the harmonic approximation, and the triplon excitation bands of the columnar valence bond solid phase are determined. We then calculate the Berry curvature and the Chern numbers of the triplon excitation bands and, finally, determine the thermal Hall conductivity due to triplons as a function of the temperature. We find that the Dzyaloshinskii-Moriya interaction yields a finite Berry curvature for the triplon bands, but the corresponding Chern numbers vanish. Although the triplon excitations are topologically trivial, the thermal Hall conductivity of the columnar valence bond solid phase in the square lattice antiferromagnet is finite at low temperatures. Our results complement a previous study by Samajdar et al., Phys. Rev. B 99, 165126 (2019) concerning the thermal Hall effect due to spinons of a spin-liquid phase on a square lattice. We also comment on the relations of our results with a no-go condition for a thermal Hall effect previously derived for ordered magnets by Katsura et al., Phys. Rev. Lett. 104, 066403 (2010).
In recent work, we argued that a particular algebraic spin liquid (ASL) may be the ground state of the S=1/2S=1/2S=1/2 kagome lattice Heisenberg antiferromagnet. Furthermore, this state, which lacks a spin gap, is appealing in light of recent experiments on herbertsmithite [ZnCu3(OH)6Cl2][{\text{ZnCu}}_{3}{(\text{OH})}_{6}{\text{Cl}}_{2}][ZnCu3​(OH)6​Cl2​]. Here, we study the properties of this ASL in more detail using both the low-energy effective field theory and Gutzwiller-projected wave functions of fermionic spinons. We identify the competing orders of the ASL, which are observables having slowly decaying power-law correlations---among them we find a set of magnetic orders lying at the MMM points of the Brillouin zone, the familiar $\mathbit{q}=0$ magnetic ordered state, the ``Hastings'' valence-bond solid (VBS) state, and a pattern of vector spin-chirality ordering corresponding to one of the Dzyaloshinskii--Moriya (DM) interaction terms present in herbertsmithite. Identification of some of these orders requires an understanding of the quantum numbers of magnetic monopole operators in the ASL. We discuss the detection of the magnetic and VBS competing orders in experiments. While we primarily focus on a clean system without DM interaction, we consider the effects of small DM interaction and argue that, surprisingly, it leads to spontaneously broken time-reversal symmetry (for DM interaction that preserves XYXYXY spin rotation symmetry, there is also XYXYXY magnetic order). Our analysis of the projected wave function provides an estimate of the ``Fermi velocity'' vF{v}_{F}vF​ that characterizes all low-energy excitations of the ASL---this allows us to estimate the specific heat, which compares favorably with experiments. We also study the spin and bond correlations of the projected wave function and compare these results with those of the effective field theory. While the spin correlations in the effective field theory and wave function seem to match rather well (although not completely), the bond correlations are more puzzling. We conclude with a discussion of experiments in herbertsmithite and make several predictions.