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1. Gapped magnetic ground state in quantum spin liquid candidate κ-(BEDT-TTF) 2 Cu 2 (CN) 3

   https://doi.org/10.1126/science.abc6363  

   Miksch, Björn ; Pustogow, Andrej ; Rahim, Mojtaba Javaheri ; Bardin, Andrey A. ; Kanoda, Kazushi ; Schlueter, John A. ; Hübner, Ralph ; Scheffler, Marc ; Dressel, Martin ( April 2021 , Science)

   Geometrical frustration, quantum entanglement, and disorder may prevent long-range ordering of localized spins with strong exchange interactions, resulting in an exotic state of matter. κ-(BEDT-TTF)₂Cu₂(CN)₃is considered the prime candidate for this elusive quantum spin liquid state, but its ground-state properties remain puzzling. We present a multifrequency electron spin resonance (ESR) study down to millikelvin temperatures, revealing a rapid drop of the spin susceptibility at 6 kelvin. This opening of a spin gap, accompanied by structural modifications, is consistent with the formation of a valence bond solid ground state. We identify an impurity contribution to the ESR response that becomes dominant when the intrinsic spins form singlets. Probing the electrons directly manifests the pivotal role of defects for the low-energy properties of quantum spin systems without magnetic order.

    

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2. Hyperfine interactions and coherent spin dynamics of isotopically purified 167 Er 3+ in polycrystalline Y 2 O 3

   https://doi.org/10.1088/2633-4356/ac9e86  

   Rajh, Tijana ; Sun, Lei ; Gupta, Shobhit ; Yang, Jun ; Zhang, Haitao ; Zhong, Tian ( November 2022 , Materials for Quantum Technology)

   Abstract 167 Er 3+ doped solids are a promising platform for quantum technology due to erbium’s telecom C-band optical transition and its long hyperfine coherence times. We experimentally study the spin Hamiltonian and dynamics of 167 Er 3+ spins in Y 2 O 3 using electron paramagnetic resonance (EPR) spectroscopy. The anisotropic electron Zeeman, hyperfine and nuclear quadrupole matrices are fitted using data obtained by X-band (9.5 GHz) EPR spectroscopy. We perform pulsed EPR spectroscopy to measure spin relaxation time T 1 and coherence time T 2 for the 3 principal axes of an anisotropic g tensor. Long electronic spin coherence time up to 24.4 μ s is measured for lowest g transition at 4 K, exceeding previously reported values at much lower temperatures. Measurements of decoherence mechanism indicates T 2 limited by spectral diffusion and instantaneous diffusion. Long spin coherence times, along with a strong anisotropic hyperfine interaction makes 167 Er 3+ :Y 2 O 3 a rich system and an excellent candidate for spin-based quantum technologies. 

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3. Monolayer Superconductivity and Tunable Topological Electronic Structure at the Fe(Te,Se)/Bi 2 Te 3 Interface

   https://doi.org/10.1002/adma.202210940  

   Moore, Robert G. ; Lu, Qiangsheng ; Jeon, Hoyeon ; Yao, Xiong ; Smith, Tyler ; Pai, Yun‐Yi ; Chilcote, Michael ; Miao, Hu ; Okamoto, Satoshi ; Li, An‐Ping ; et al ( April 2023 , Advanced Materials)

   The interface between 2D topological Dirac states and ans‐wave superconductor is expected to support Majorana‐bound states (MBS) that can be used for quantum computing applications. Realizing these novel states of matter and their applications requires control over superconductivity and spin‐orbit coupling to achieve spin‐momentum‐locked topological interface states (TIS) which are simultaneously superconducting. While signatures of MBS have been observed in the magnetic vortex cores of bulk FeTe_0.55Se_0.45, inhomogeneity and disorder from doping make these signatures unclear and inconsistent between vortices. Here superconductivity is reported in monolayer (ML) FeTe_1–ySe_y(Fe(Te,Se)) grown on Bi₂Te₃by molecular beam epitaxy (MBE). Spin and angle‐resolved photoemission spectroscopy (SARPES) directly resolve the interfacial spin and electronic structure of Fe(Te,Se)/Bi₂Te₃heterostructures. Fory = 0.25, the Fe(Te,Se) electronic structure is found to overlap with the Bi₂Te₃TIS and the desired spin‐momentum locking is not observed. In contrast, fory = 0.1, reduced inhomogeneity measured by scanning tunneling microscopy (STM) and a smaller Fe(Te,Se) Fermi surface with clear spin‐momentum locking in the topological states are found. Hence, it is demonstrated that the Fe(Te,Se)/Bi₂Te₃system is a highly tunable platform for realizing MBS where reduced doping can improve characteristics important for Majorana interrogation and potential applications.

    

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4. Electron and Spin Delocalization in [Co 6 Se 8 (PEt 3 ) 6 ] 0/+1 Superatoms

   https://doi.org/10.1002/cphc.202300064  

   Xu, Yunyao ; Chen, Jia ; Aydt, Alexander P. ; Zhang, Lichirui ; Sergeyev, Ivan ; Keeler, Eric G. ; Choi, Bonnie ; He, Shoushou ; Reichman, David R. ; Friesner, Richard A. ; et al ( January 2024 , ChemPhysChem)

   ChemPhysChem (Ed.)

   Molecular clusters can function as nanoscale atoms/superatoms, assembling into superatomic solids, a new class of solid‐state materials with designable properties through modifications on superatoms. To explore possibilities on diversifying building blocks, here we thoroughly studied one representative superatom, Co₆Se₈(PEt₃)₆. We probed its structural, electronic, and magnetic properties and revealed its detailed electronic structure as valence electrons delocalize over inorganic [Co₆Se₈] core while ligands function as an insulated shell.⁵⁹Co SSNMR measurements on the core and³¹P,¹³C on the ligands show that the neutral Co₆Se₈(PEt₃)₆is diamagnetic and symmetric, with all ligands magnetically equivalent. Quantum computations cross‐validate NMR results and reveal degenerate delocalized HOMO orbitals, indicating aromaticity. Ligand substitution keeps the inorganic core nearly intact. After losing one electron, the unpaired electron in [Co₆Se₈(PEt₃)₆]⁺¹is delocalized, causing paramagnetism and a delocalized electron spin. Notably, this feature of electron/spin delocalization over a large cluster is attractive for special single‐electron devices.

    

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5. Ionic Modulation at the LaAlO 3 /KTaO 3 Interface for Extreme High‐Mobility Two‐Dimensional Electron Gas

   https://doi.org/10.1002/admi.202201633  

   Yan, Hong ; Zeng, Shengwei ; Rubi, Km ; Omar, Ganesh Ji ; Zhang, Zhaoting ; Goiran, Michel ; Escoffier, Walter ; Ariando, Ariando ( October 2022 , Advanced Materials Interfaces)

   Due to the coexistence of many emergent phenomena, including 2D superconductivity and a large Rashba spin‐orbit coupling, 5d transition metal oxides‐based two‐dimensional electron systems (2DESs) have been prospected as one of the potential intrants for modern electronics. However, despite the lighter electron mass, the mobility of carriers, a key requisite for high‐performance devices, in 5d‐oxides devices remains far behind their 3d‐oxides analogs. The carriers’ mobility in these oxides is significantly hampered by the inevitable presence of defects. Here, very high mobility (≈22 650 cm²V⁻¹s⁻¹) of 5d‐2DES confined at the LaAlO₃/KTaO₃interface is reported. The high mobility, which is beyond the values observed in SrTiO₃2DESs in the same carrier‐density range, is achieved using the ionic‐liquid gating at room temperature. The authors postulate that the ionic‐liquid gating affects the oxygen vacancies and efficiently reduces any disorder at the interface. Investigating density and mobility in a broad range of back‐gate voltage, the authors reveal that the mobility follows the power‐law µ ∝ n^1.2, indicating the very high quality of ionic‐liquid‐gated LaAlO₃/KTaO₃devices, consistent with the postulate. Furthermore, the analysis of the quantum oscillations confirms that the high‐mobility electrons occupy the electronic sub‐bands emerging from the Ta:5d orbitals of KTaO₃.

    

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