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Register a new userFractionalization on the surface: Is type-II terminated 1T-TaS$_2$ surface an anomalously realized spin liquid?
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Gang Chen Professor
Publication date
2021
fields
Physics
and research's language is
English
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The type-II terminated 1T-TaS$_2$ surface of a three-dimensional 1T-TaS$_2$ bulk material realizes the effective spin-1/2 degree of freedom on each David-star cluster with ${T^2=-1}$ such that the time reversal symmetry is realized anomalously, despite the bulk three-dimensional 1T-TaS$_2$ material has an even number of electrons per unit cell with ${T^2=+1}$. This surface is effectively viewed as a spin-1/2 triangular lattice magnet, except with a symmetry-protected topological bulk. We further propose this surface termination realizes a spinon Fermi surface spin liquid with the surface fractionalization but with a non-exotic three-dimensional bulk. We analyze possible experimental consequences of the type-II terminated surface spin liquid.
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A quantum spin liquid (QSL) is an exotic state of matter characterized by quantum entanglement and the absence of any broken symmetry. A long-standing open problem, which is a key for fundamental understanding the mysterious QSL states, is how the quantum fluctuations respond to randomness due to quenched disorder. Transition metal dichalcogenide 1T-TaS$_2$ is a candidate material that hosts a QSL ground state with spin-1/2 on the two-dimensional perfect triangular lattice. Here, we performed systematic studies of low-temperature heat capacity and thermal conductivity on pure, Se-substituted and electron irradiated crystals of 1T-TaS$_2$. In pure 1T-TaS$_2$, the linear temperature term of the heat capacity $gamma T$ and the finite residual linear term of the thermal conductivity in the zero-temperature limit $kappa_{0}/Tequivkappa/T(Trightarrow0)$ are clearly resolved, consistent with the presence of gapless spinons with a Fermi surface. Moreover, while the strong magnetic field slightly enhances $kappa_0/T$, it strongly suppresses $gamma$. These unusual contrasting responses to magnetic field imply the coexistence of two types of gapless excitations with itinerant and localized characters. Introduction of additional weak random exchange disorder in 1T-Ta(S$_{1-x}$Se$_x$)$_2$ leads to vanishing of $kappa_0/T$, indicating that the itinerant gapless excitations are sensitive to the disorder. On the other hand, in both pure and Se-substituted systems, the magnetic contribution of the heat capacity obeys a universal scaling relation, which is consistent with a theory that assumes the presence of localized orphan spins forming random singlets. Electron irradiation in pure 1T-TaS$_2$ largely enhances $gamma$ and changes the scaling function dramatically, suggesting a possible new state of spin liquid.
To reveal the nature of elementary excitations in a quantum spin liquid (QSL), we measured low temperature thermal conductivity and specific heat of 1T-TaS$_2$, a QSL candidate material with frustrated triangular lattice of spin-1/2. The nonzero temperature linear specific heat coefficient $gamma$ and the finite residual linear term of the thermal conductivity in the zero temperature limit $kappa_0/T=kappa/T(Trightarrow 0)$ are clearly resolved. This demonstrates the presence of highly mobile gapless excitations, which is consistent with fractionalized spinon excitations that form a Fermi surface. Remarkably, an external magnetic field strongly suppresses $gamma$, whereas it enhances $kappa_0/T$. This unusual contrasting behavior in the field dependence of specific heat and thermal conductivity can be accounted for by the presence of two types of gapless excitations with itinerant and localized characters, as recently predicted theoretically (I. Kimchi et al., arXiv:1803.00013 (2018)). This unique feature of 1T-TaS$_2$ provides new insights into the influence of quenched disorder on the QSL.
We study the electronic structure of the Pd-terminated surface of the non-magnetic delafossite oxide metal PdCoO$_2$. Combining angle-resolved photoemission spectroscopy and density-functional theory, we show how an electronic reconstruction driven by surface polarity mediates a Stoner-like magnetic instability towards itinerant surface ferromagnetism. Our results reveal how this leads to a rich multi-band surface electronic structure, and provide spectroscopic evidence for an intriguing sample-dependent coupling of the surface electrons to a bosonic mode which we attribute to electron-magnon interactions. Moreover, we find similar surface state dispersions in PdCrO$_2$, suggesting surface ferromagnetism persists in this sister compound despite its bulk antiferromagnetic order.
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