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1. Disorder-dominated quantum criticality in moiré bilayers

   https://doi.org/10.1038/s41467-022-35103-w  

   Tan, Yuting ; Tsang, Pak Ki Henry ; Dobrosavljević, Vladimir ( December 2022 , Nature Communications)

   Moiré bilayer materials have recently attracted much attention following the discovery of various correlated insulating states at specific band fillings. Here we discuss the metal-insulator transitions (MITs) that have been observed in the same devices, but at fillings far from the strongly correlated regime dominated by Mott-like physics, displaying many similarities to other examples of disorder-dominated MITs. We propose a minimal theoretical model describing the interplay of interactions and disorder, which is able to capture all the universal aspects of quantum criticality, as observed in experiments performed on several devices.

    

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2. Signature of a randomness-driven spin-liquid state in a frustrated magnet

   https://doi.org/10.1038/s42005-022-00879-2  

   Khatua, J. ; Gomilšek, M. ; Orain, J. C. ; Strydom, A. M. ; Jagličić, Z. ; Colin, C. V. ; Petit, S. ; Ozarowski, A. ; Mangin-Thro, L. ; Sethupathi, K. ; et al ( April 2022 , Communications Physics)

   Collective behaviour of electrons, frustration induced quantum fluctuations and entanglement in quantum materials underlie some of the emergent quantum phenomena with exotic quasi-particle excitations that are highly relevant for technological applications. Herein, we present our thermodynamic and muon spin relaxation measurements, complemented by ab initio density functional theory and exact diagonalization results, on the recently synthesized frustrated antiferromagnet Li₄CuTeO₆, in which Cu²⁺ions (S= 1/2) constitute disordered spin chains and ladders along the crystallographic [101] direction with weak random inter-chain couplings. Our thermodynamic experiments detect neither long-range magnetic ordering nor spin freezing down to 45 mK despite the presence of strong antiferromagnetic interaction between Cu²⁺moments leading to a large effective Curie-Weiss temperature of − 154 K. Muon spin relaxation results are consistent with thermodynamic results. The temperature and magnetic field scaling of magnetization and specific heat reveal a data collapse pointing towards the presence of random-singlets within a disorder-driven correlated and dynamic ground-state in this frustrated antiferromagnet.

    

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3. Momentum-independent magnetic excitation continuum in the honeycomb iridate H3LiIr2O6

   https://doi.org/10.1038/s41467-023-40769-x  

   de la Torre, A. ; Zager, B. ; Bahrami, F. ; Upton, M. H. ; Kim, J. ; Fabbris, G. ; Lee, G. -H. ; Yang, W. ; Haskel, D. ; Tafti, F. ; et al ( August 2023 , Nature Communications)

   Understanding the interplay between the inherent disorder and the correlated fluctuating-spin ground state is a key element in the search for quantum spin liquids. H₃LiIr₂O₆is considered to be a spin liquid that is proximate to the Kitaev-limit quantum spin liquid. Its ground state shows no magnetic order or spin freezing as expected for the spin liquid state. However, hydrogen zero-point motion and stacking faults are known to be present. The resulting bond disorder has been invoked to explain the existence of unexpected low-energy spin excitations, although data interpretation remains challenging. Here, we use resonant X-ray spectroscopies to map the collective excitations in H₃LiIr₂O₆and characterize its magnetic state. In the low-temperature correlated state, we reveal a broad bandwidth of magnetic excitations. The central energy and the high-energy tail of the continuum are consistent with expectations for dominant ferromagnetic Kitaev interactions between dynamically fluctuating spins. Furthermore, the absence of a momentum dependence to these excitations are consistent with disorder-induced broken translational invariance. Our low-energy data and the energy and width of the crystal field excitations support an interpretation of H₃LiIr₂O₆as a disordered topological spin liquid in close proximity to bond-disordered versions of the Kitaev quantum spin liquid.

    

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4. Time-varying quantum channel models for superconducting qubits

   https://doi.org/10.1038/s41534-021-00448-5  

   Etxezarreta Martinez, Josu ; Fuentes, Patricio ; Crespo, Pedro ; Garcia-Frias, Javier ( July 2021 , npj Quantum Information)

   The decoherence effects experienced by the qubits of a quantum processor are generally characterized using the amplitude damping time (T₁) and the dephasing time (T₂). Quantum channel models that exist at the time of writing assume that these parameters are fixed and invariant. However, recent experimental studies have shown that they exhibit a time-varying (TV) behaviour. These time-dependant fluctuations ofT₁andT₂, which become even more pronounced in the case of superconducting qubits, imply that conventional static quantum channel models do not capture the noise dynamics experienced by realistic qubits with sufficient precision. In this article, we study how the fluctuations ofT₁andT₂can be included in quantum channel models. We propose the idea of time-varying quantum channel (TVQC) models, and we show how they provide a more realistic portrayal of decoherence effects than static models in some instances. We also discuss the divergence that exists between TVQCs and their static counterparts by means of a metric known as the diamond norm. In many circumstances this divergence can be significant, which indicates that the time-dependent nature of decoherence must be considered, in order to construct models that capture the real nature of quantum devices.

    

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5. Evidence of a coupled electron-phonon liquid in NbGe2

   https://doi.org/10.1038/s41467-021-25547-x  

   Yang, Hung-Yu ; Yao, Xiaohan ; Plisson, Vincent ; Mozaffari, Shirin ; Scheifers, Jan P. ; Savvidou, Aikaterini Flessa ; Choi, Eun Sang ; McCandless, Gregory T. ; Padlewski, Mathieu F. ; Putzke, Carsten ; et al ( September 2021 , Nature Communications)

   Whereas electron-phonon scattering relaxes the electron’s momentum in metals, a perpetual exchange of momentum between phonons and electrons may conserve total momentum and lead to a coupled electron-phonon liquid. Such a phase of matter could be a platform for observing electron hydrodynamics. Here we present evidence of an electron-phonon liquid in the transition metal ditetrelide, NbGe₂, from three different experiments. First, quantum oscillations reveal an enhanced quasiparticle mass, which is unexpected in NbGe₂with weak electron-electron correlations, hence pointing at electron-phonon interactions. Second, resistivity measurements exhibit a discrepancy between the experimental data and standard Fermi liquid calculations. Third, Raman scattering shows anomalous temperature dependences of the phonon linewidths that fit an empirical model based on phonon-electron coupling. We discuss structural factors, such as chiral symmetry, short metallic bonds, and a low-symmetry coordination environment as potential design principles for materials with coupled electron-phonon liquid.

    

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