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1. Comparison of temperature and doping dependence of elastoresistivity near a putative nematic quantum critical point

   https://doi.org/10.1038/s41467-022-28583-3  

   Palmstrom, J. C. ; Walmsley, P. ; Straquadine, J. A. W. ; Sorensen, M. E. ; Hannahs, S. T. ; Burns, D. H. ; Fisher, I. R. ( February 2022 , Nature Communications)

   Strong electronic nematic fluctuations have been discovered near optimal doping for several families of Fe-based superconductors, motivating the search for a possible link between these fluctuations, nematic quantum criticality, and high temperature superconductivity. Here we probe a key prediction of quantum criticality, namely power-law dependence of the associated nematic susceptibility as a function of composition and temperature approaching the compositionally tuned putative quantum critical point. To probe the ‘bare’ quantum critical point requires suppression of the superconducting state, which we achieve by using large magnetic fields, up to 45 T, while performing elastoresistivity measurements to follow the nematic susceptibility. We performed these measurements for the prototypical electron-doped pnictide, Ba(Fe_1−xCo_x)₂As₂, over a dense comb of dopings. We find that close to the putative quantum critical point, the elastoresistivity appears to obey power-law behavior as a function of composition over almost a decade of variation in composition. Paradoxically, however, we also find that the temperature dependence for compositions close to the critical value cannot be described by a single power law.

    

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2. Quantum-critical scale invariance in a transition metal alloy

   https://doi.org/10.1038/s42005-020-00448-5  

   Nakajima, Yasuyuki ; Metz, Tristin ; Eckberg, Christopher ; Kirshenbaum, Kevin ; Hughes, Alex ; Wang, Renxiong ; Wang, Limin ; Saha, Shanta R. ; Liu, I-Lin ; Butch, Nicholas P. ; et al ( October 2020 , Communications Physics)

   Quantum-mechanical fluctuations between competing phases induce exotic collective excitations that exhibit anomalous behavior in transport and thermodynamic properties, and are often intimately linked to the appearance of unconventional Cooper pairing. High-temperature superconductivity, however, makes it difficult to assess the role of quantum-critical fluctuations in shaping anomalous finite-temperature physical properties. Here we report temperature-field scale invariance of non-Fermi liquid thermodynamic, transport, and Hall quantities in a non-superconducting iron-pnictide, Ba(Fe_1/3Co_1/3Ni_1/3)₂As₂, indicative of quantum criticality at zero temperature and applied magnetic field. Beyond a linear-in-temperature resistivity, the hallmark signature of strong quasiparticle scattering, we find a scattering rate that obeys a universal scaling relation between temperature and applied magnetic fields down to the lowest energy scales. Together with the dominance of hole-like carriers close to the zero-temperature and zero-field limits, the scale invariance, isotropic field response, and lack of applied pressure sensitivity suggests a unique quantum critical system unhindered by a pairing instability.

    

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3. Two superconducting states with broken time-reversal symmetry in FeSe 1−x S x

   https://doi.org/10.1073/pnas.2208276120  

   Matsuura, Kohei ; Roppongi, Masaki ; Qiu, Mingwei ; Sheng, Qi ; Cai, Yipeng ; Yamakawa, Kohtaro ; Guguchia, Zurab ; Day, Ryan P. ; Kojima, Kenji M. ; Damascelli, Andrea ; et al ( May 2023 , Proceedings of the National Academy of Sciences)

   Iron-chalcogenide superconductors FeSe_1−xS_xpossess unique electronic properties such as nonmagnetic nematic order and its quantum critical point. The nature of superconductivity with such nematicity is important for understanding the mechanism of unconventional superconductivity. A recent theory suggested the possible emergence of a fundamentally new class of superconductivity with the so-called Bogoliubov Fermi surfaces (BFSs) in this system. However, such an ultranodal pair state requires broken time-reversal symmetry (TRS) in the superconducting state, which has not been observed experimentally. Here, we report muon spin relaxation (μSR) measurements in FeSe_1−xS_xsuperconductors for0≤x≤0.22covering both orthorhombic (nematic) and tetragonal phases. We find that the zero-field muon relaxation rate is enhanced below the superconducting transition temperatureT_cfor all compositions, indicating that the superconducting state breaks TRS both in the nematic and tetragonal phases. Moreover, the transverse-fieldμSR measurements reveal that the superfluid density shows an unexpected and substantial reduction in the tetragonal phase (x>0.17). This implies that a significant fraction of electrons remain unpaired in the zero-temperature limit, which cannot be explained by the known unconventional superconducting states with point or line nodes. The TRS breaking and the suppressed superfluid density in the tetragonal phase, together with the reported enhanced zero-energy excitations, are consistent with the ultranodal pair state with BFSs. The present results reveal two different superconducting states with broken TRS separated by the nematic critical point in FeSe_1−xS_x, which calls for the theory of microscopic origins that account for the relation between nematicity and superconductivity.

    

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4. (Digital Presentation) Multimodal Encapsulation of p-SnOx to Engineer the Carrier Density for Thin Film Transistor Applications

   https://doi.org/10.1149/MA2022-0215821mtgabs  

   Lee, Dong Hun ; Zhang, Yuxuan ; No, Kwangsoo ; Song, Han Wook ; Lee, Sunghwan ( October 2022 , ECS Meeting Abstracts)

   It has been challenging to synthesize p-type SnO_x(1≤x<2) and engineer the electrical properties such as carrier density and mobility due to the narrow processing window and the localized oxygen 2p orbitals near the valence band.

   We recently reported on the processing of p-type SnOx and an oxide-based p-n heterostructures, demonstrating high on/off rectification ratio (>10³), small turn-on voltage (<0.5 V), and low saturation current (~1×10^-10A)¹. In order to further understand the p-type oxide and engineer the properties for various electronic device applications, it is important to identify (or establish) the dominating doping and transport mechanisms. The low dopability in p-type SnOx, of which the causation is also closely related to the narrow processing window, needs to be mitigated so that the electrical properties of the material are to be adequately engineered^{2, 3}.

   Herein, we report on the multifunctional encapsulation of p-SnO_xto limit the surface adsorption of oxygen and selectively permeate hydrogen into the p-SnO_xchannel for thin film transistor (TFT) applications. Time-of-flight secondary ion mass spectrometry measurements identified that ultra-thin SiO₂as a multifunctional encapsulation layer effectively suppressed the oxygen adsorption on the back channel surface of p-SnO_xand augmented hydrogen density across the entire thickness of the channel. Encapsulated p-SnO_x-based TFTs demonstrated much-enhanced channel conductance modulation in response to the gate bias applied, featuring higher on-state current and lower off-state current. The relevance between the TFT performance and the effects of oxygen suppression and hydrogen permeation is discussed in regard to the intrinsic and extrinsic doping mechanisms. These results are supported by density-functional-theory calculations.

   Acknowledgement

   This work was supported by the U.S. National Science Foundation (NSF) Award No. ECCS-1931088. S.L. and H.W.S. acknowledge the support from the Improvement of Measurement Standards and Technology for Mechanical Metrology (Grant No. 20011028) by KRISS. K.N. was supported by Basic Science Research Program (NRF-2021R11A1A01051246) through the NRF Korea funded by the Ministry of Education.

   References

   Lee, D. H.; Park, H.; Clevenger, M.; Kim, H.; Kim, C. S.; Liu, M.; Kim, G.; Song, H. W.; No, K.; Kim, S. Y.; Ko, D.-K.; Lucietto, A.; Park, H.; Lee, S., High-Performance Oxide-Based p–n Heterojunctions Integrating p-SnOx and n-InGaZnO.ACS Applied Materials & Interfaces2021,13(46), 55676-55686.

   Hautier, G.; Miglio, A.; Ceder, G.; Rignanese, G.-M.; Gonze, X., Identification and design principles of low hole effective mass p-type transparent conducting oxides.Nat Commun2013,4.

   Yim, K.; Youn, Y.; Lee, M.; Yoo, D.; Lee, J.; Cho, S. H.; Han, S., Computational discovery of p-type transparent oxide semiconductors using hydrogen descriptor.npj Computational Materials2018,4(1), 17.

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5. Dramatically Enhanced Valley‐Polarized Emission by Alloying and Electrical Tuning of Monolayer WTe 2 x S 2(1‐ x ) Alloys at Room Temperature with 1T′‐WTe 2 ‐Contact

   https://doi.org/10.1002/advs.202304890  

   Lin, Wei‐Hsiang ; Li, Chia‐Shuo ; Wu, Chih‐I ; Rossman, George R. ; Atwater, Harry A. ; Yeh, Nai‐Chang ( November 2023 , Advanced Science)

   Monolayer ternary tellurides based on alloying different transition metal dichalcogenides (TMDs) can result in new two‐dimensional (2D) materials ranging from semiconductors to metals and superconductors with tunable optical and electrical properties. Semiconducting WTe_2xS_2(1‐x)monolayer possesses two inequivalent valleys in the Brillouin zone, each valley coupling selectively with circularly polarized light (CPL). The degree of valley polarization (DVP) under the excitation of CPL represents the purity of valley polarized photoluminescence (PL), a critical parameter for opto‐valleytronic applications. Here, new strategies to efficiently tailor the valley‐polarized PL from semiconducting monolayer WTe_2xS_2(1‐x)at room temperature (RT) through alloying and back‐gating are presented. The DVP at RT is found to increase drastically from < 5% in WS₂to 40% in WTe_0.12S_1.88by Te‐alloying to enhance the spin‐orbit coupling. Further enhancement and control of the DVP from 40% up to 75% is demonstrated by electrostatically doping the monolayer WTe_0.12S_1.88via metallic 1T′‐WTe₂electrodes, where the use of 1T′‐WTe₂substantially lowers the Schottky barrier height (SBH) and weakens the Fermi‐level pinning of the electrical contacts. The demonstration of drastically enhanced DVP and electrical tunability in the valley‐polarized emission from 1T′‐WTe₂/WTe_0.12S_1.88heterostructures paves new pathways towards harnessing valley excitons in ultrathin valleytronic devices for RT applications.

    

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