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1. Spectrum of quantum KdV hierarchy in the semiclassical limit

   https://doi.org/10.1007/JHEP09(2022)169  

   Dymarsky, Anatoly; Kakkar, Ashish; Pavlenko, Kirill; Sugishita, Sotaro (September 2022, Journal of High Energy Physics)

   A<sc>bstract</sc> We employ semiclassical quantization to calculate spectrum of quantum KdV charges in the limit of large central chargec. Classically, KdV chargesQ_2n−1generate completely integrable dynamics on the co-adjoint orbit of the Virasoro algebra. They can be expressed in terms of action variablesI_k, e.g. as a power series expansion. Quantum-mechanically this series becomes the expansion in 1/c, while action variables become integer-valued quantum numbersn_i. Crucially, classical expression, which is homogeneous inI_k, acquires quantum corrections that include terms of subleading powers inn_k. At first two non-trivial orders in 1/cexpansion these “quantum” terms can be fixed from the analytic form ofQ_2n−1acting on the primary states. In this way we find explicit expression for the spectrum ofQ_2n−1up to first three orders in 1/cexpansion. We apply this result to study thermal expectation values ofQ_2n−1and free energy of the KdV Generalized Gibbs Ensemble. 

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2. Quantum oscillations of electrical resistivity in an insulator

   https://doi.org/10.1126/science.aap9607  

   Xiang, Z.; Kasahara, Y.; Asaba, T.; Lawson, B.; Tinsman, C.; Chen, Lu; Sugimoto, K.; Kawaguchi, S.; Sato, Y.; Li, G.; et al (October 2018, Science)

   In metals, orbital motions of conduction electrons on the Fermi surface are quantized in magnetic fields, which is manifested by quantum oscillations in electrical resistivity. This Landau quantization is generally absent in insulators. Here, we report a notable exception in an insulator—ytterbium dodecaboride (YbB₁₂). The resistivity of YbB₁₂, which is of a much larger magnitude than the resistivity in metals, exhibits distinct quantum oscillations. These unconventional oscillations arise from the insulating bulk, even though the temperature dependence of the oscillation amplitude follows the conventional Fermi liquid theory of metals with a large effective mass. Quantum oscillations in the magnetic torque are also observed, albeit with a lighter effective mass. 

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3. Probing the mesoscopic size limit of quantum anomalous Hall insulators

   https://doi.org/10.1038/s41467-022-31105-w  

   Deng, Peng; Eckberg, Christopher; Zhang, Peng; Qiu, Gang; Emmanouilidou, Eve; Yin, Gen; Chong, Su Kong; Tai, Lixuan; Ni, Ni; Wang, Kang L. (July 2022, Nature Communications)

   Abstract The inelastic scattering length (L_s) is a length scale of fundamental importance in condensed matters due to the relationship between inelastic scattering and quantum dephasing. In quantum anomalous Hall (QAH) materials, the mesoscopic length scaleL_splays an instrumental role in determining transport properties. Here we examineL_sin three regimes of the QAH system with distinct transport behaviors: the QAH, quantum critical, and insulating regimes. Although the resistance changes by five orders of magnitude when tuning between these distinct electronic phases, scaling analyses indicate a universalL_samong all regimes. Finally, mesoscopic scaled devices with sizes on the order ofL_swere fabricated, enabling the direct detection of the value ofL_sin QAH samples. Our results unveil the fundamental length scale that governs the transport behavior of QAH materials. 

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4. Uniaxial stress effect on the electronic structure of quantum materials

   https://doi.org/10.3389/femat.2024.1392760  

   Jo, Na Hyun; Gati, Elena; Pfau, Heike (May 2024, Frontiers in Electronic Materials)

   Uniaxial stress has proven to be a powerful experimental tuning parameter for effectively controlling lattice, charge, orbital, and spin degrees of freedom in quantum materials. In addition, its ability to manipulate the symmetry of materials has garnered significant attention. Recent technical progress to combine uniaxial stress cells with quantum oscillation and angle-resolved photoemission techniques allowed to study the electronic structure as function of uniaxial stress. This review provides an overview on experimental advancements in methods and examines studies on diverse quantum materials, encompassing the semimetal WTe₂, the unconventional superconductor Sr₂RuO₄, Fe-based superconductors, and topological materials. 

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5. Exchange‐Biased Quantum Anomalous Hall Effect

   https://doi.org/10.1002/adma.202300391  

   Zhang, Peng; Balakrishnan, Purnima_P; Eckberg, Christopher; Deng, Peng; Nozaki, Tomohiro; Chong, Su_Kong; Quarterman, Patrick; Holtz, Megan_E; Maranville, Brian_B; Qiu, Gang; et al (June 2023, Advanced Materials)

   Abstract The quantum anomalous Hall (QAH) effect is characterized by a dissipationless chiral edge state with a quantized Hall resistance at zero magnetic field. Manipulating the QAH state is of great importance in both the understanding of topological quantum physics and the implementation of dissipationless electronics. Here, the QAH effect is realized in the magnetic topological insulator Cr‐doped (Bi,Sb)₂Te₃(CBST) grown on an uncompensated antiferromagnetic insulator Al‐doped Cr₂O₃. Through polarized neutron reflectometry (PNR), a strong exchange coupling is found between CBST and Al‐Cr₂O₃surface spins fixing interfacial magnetic moments perpendicular to the film plane. The interfacial coupling results in an exchange‐biased QAH effect. This study further demonstrates that the magnitude and sign of the exchange bias can be effectively controlled using a field training process to set the magnetization of the Al‐Cr₂O₃layer. It demonstrates the use of the exchange bias effect to effectively manipulate the QAH state, opening new possibilities in QAH‐based spintronics. 

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