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1. Quantum liquid from strange frustration in the trimer magnet Ba4Ir3O10

   https://doi.org/10.1038/s41535-020-0232-6  

   Cao, Gang ; Zheng, Hao ; Zhao, Hengdi ; Ni, Yifei ; Pocs, Christopher A. ; Zhang, Yu ; Ye, Feng ; Hoffmann, Christina ; Wang, Xiaoping ; Lee, Minhyea ; et al ( May 2020 , npj Quantum Materials)

   Quantum spin systems such as magnetic insulators usually show magnetic order, but such classical states can give way toquantum liquids with exotic entanglementthrough two known mechanisms of frustration: geometric frustration in lattices with triangle motifs, and spin-orbit-coupling frustration in the exactly solvable quantum liquid of Kitaev’s honeycomb lattice. Here we present the experimental observation of a new kind of frustrated quantum liquid arising in an unlikely place: the magnetic insulator Ba₄Ir₃O₁₀where Ir₃O₁₂trimers form an unfrustrated square lattice. The crystal structure shows no apparent spin chains. Experimentally we find a quantum liquid state persisting down to 0.2 K that is stabilizedmore »by strong antiferromagnetic interaction with Curie–Weiss temperature ranging from −766 to −169 K due to magnetic anisotropy. The anisotropy-averaged frustration parameter is 2000, seldom seen in iridates. Heat capacity and thermal conductivity are both linear at low temperatures, a familiar feature in metals but here in an insulator pointing to an exotic quantum liquid state; a mere 2% Sr substitution for Ba produces long-range order at 130 K and destroys the linear-T features. Although the Ir⁴⁺(5d⁵) ions in Ba₄Ir₃O₁₀appear to form Ir₃O₁₂trimers of face-sharing IrO₆octahedra, we propose that intra-trimer exchange is reduced and the lattice recombines into an array of coupled 1D chains with additional spins. An extreme limit of decoupled 1D chains can explain most but not all of the striking experimental observations, indicating that the inter-chain coupling plays an important role in the frustration mechanism leading to this quantum liquid.

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2. Probing topological spin liquids on a programmable quantum simulator

   https://doi.org/10.1126/science.abi8794  

   Semeghini, G. ; Levine, H. ; Keesling, A. ; Ebadi, S. ; Wang, T. T. ; Bluvstein, D. ; Verresen, R. ; Pichler, H. ; Kalinowski, M. ; Samajdar, R. ; et al ( December 2021 , Science)

   Quantum spin liquids, exotic phases of matter with topological order, have been a major focus in physics for the past several decades. Such phases feature long-range quantum entanglement that can potentially be exploited to realize robust quantum computation. We used a 219-atom programmable quantum simulator to probe quantum spin liquid states. In our approach, arrays of atoms were placed on the links of a kagome lattice, and evolution under Rydberg blockade created frustrated quantum states with no local order. The onset of a quantum spin liquid phase of the paradigmatic toric code type was detected by using topological string operatorsmore »that provide direct signatures of topological order and quantum correlations. Our observations enable the controlled experimental exploration of topological matter and protected quantum information processing.« less
3. Engineering parafermions in helical Luttinger liquids

   https://doi.org/10.1117/12.2595632  

   Lyanda-Geller, Yuli ; Ponomarenko, Vadim ; Wang, Ying ; Rokhinson, Leonid ( August 2021 , SPIE Nanoscience + Engineering)

   Drouhin, Henri-Jean M. ; Wegrowe, Jean-Eric ; Razeghi, Manijeh (Ed.)

   Parafermions or Fibonacci anyons leading to universal quantum computing, require strongly interacting systems. A leading contender is the fractional quantum Hall effect, where helical channels can arise from counterpropagating chiral modes. These modes have been considered weakly interacting. However, experiments on transport in helical channels in the fractional quantum Hall effect at a 2/3 filling shows current passing through helical channels on the boundary between polarized and unpolarized quantum Hall liquids nine-fold smaller than expected. This current can increase three-fold when nuclei near the boundary are spin polarized. We develop a microscopic theory of strongly interacting helical states and showmore »that emerging helical Luttinger liquid manifests itself as unequally populated charge, spin and neutral modes in polarized and unpolarized fractional quantum Hall liquids. We show that at strong coupling counter-propagating modes of opposite spin polarization emerge at the sample edges, providing a viable path for generating proximity topological superconductivity and parafermions. Current, calculated in strongly interacting picture is in agreement with the experimental data.« less
4. Fractionalized conductivity and emergent self-duality near topological phase transitions

   https://doi.org/10.1038/s41467-021-25707-z  

   Wang, Yan-Cheng ; Cheng, Meng ; Witczak-Krempa, William ; Meng, Zi Yang ( December 2021 , Nature Communications)

   Abstract The experimental discovery of the fractional Hall conductivity in two-dimensional electron gases revealed new types of quantum particles, called anyons, which are beyond bosons and fermions as they possess fractionalized exchange statistics. These anyons are usually studied deep inside an insulating topological phase. It is natural to ask whether such fractionalization can be detected more broadly, say near a phase transition from a conventional to a topological phase. To answer this question, we study a strongly correlated quantum phase transition between a topological state, called a $${{\mathbb{Z}}}_{2}$$ Z 2 quantum spin liquid, and a conventional superfluid using large-scale quantummore »Monte Carlo simulations. Our results show that the universal conductivity at the quantum critical point becomes a simple fraction of its value at the conventional insulator-to-superfluid transition. Moreover, a dynamically self-dual optical conductivity emerges at low temperatures above the transition point, indicating the presence of the elusive vison particles. Our study opens the door for the experimental detection of anyons in a broader regime, and has ramifications in the study of quantum materials, programmable quantum simulators, and ultra-cold atomic gases. In the latter case, we discuss the feasibility of measurements in optical lattices using current techniques.« less
5. Anderson–Kitaev spin liquid

   https://doi.org/10.1038/s41535-020-00285-3  

   Yamada, Masahiko G. ( November 2020 , npj Quantum Materials)

   The bond-disordered Kitaev model attracts much attention due to the experimental relevance inα-RuCl₃andA₃LiIr₂O₆(A= H, D, Ag, etc.). Applying a magnetic field to break the time-reversal symmetry leads to a strong modulation in mass terms for Dirac cones. Because of the smallness of the flux gap of the Kitaev model, a small bond disorder can have large influence on itinerant Majorana fermions. The quantization of the thermal Hall conductivityκ^xy/Tdisappears by a quantum Hall transition induced by a small disorder, andκ^xy/Tshows a rapid crossover into a state with a negligible Hall current. We call this immobile liquid state Anderson–Kitaev spin liquidmore »(AKSL). Especially, the critical disorder strengthδJ_c1~ 0.05 in the unit of the Kitaev interaction would have many implications for the stability of Kitaev spin liquids.

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