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1. Bulk evidence of anisotropic s-wave pairing with no sign change in the kagome superconductor CsV3Sb5

   https://doi.org/10.1038/s41467-023-36273-x  

   Roppongi, M. ; Ishihara, K. ; Tanaka, Y. ; Ogawa, K. ; Okada, K. ; Liu, S. ; Mukasa, K. ; Mizukami, Y. ; Uwatoko, Y. ; Grasset, R. ; et al ( February 2023 , Nature Communications)

   The recently discovered kagome superconductorsAV₃Sb₅(A= K, Rb, Cs) exhibit unusual charge-density-wave (CDW) orders with time-reversal and rotational symmetry breaking. One of the most crucial unresolved issues is identifying the symmetry of the superconductivity that develops inside the CDW phase. Theory predicts a variety of unconventional superconducting symmetries with sign-changing and chiral order parameters. Experimentally, however, superconducting phase information inAV₃Sb₅is still lacking. Here we report the impurity effects in CsV₃Sb₅using electron irradiation as a phase-sensitive probe of superconductivity. Our magnetic penetration depth measurements reveal that with increasing impurities, an anisotropic fully-gapped state changes to an isotropic full-gap state without passing through a nodal state. Furthermore, transport measurements under pressure show that the double superconducting dome in the pressure-temperature phase diagram survives against sufficient impurities. These results support that CsV₃Sb₅is a non-chiral, anisotropics-wave superconductor with no sign change both at ambient and under pressure.

    

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2. Fermi surface nesting and the Lindhard response function in the kagome superconductor CsV 3 Sb 5

   https://doi.org/10.1063/5.0081081  

   Kaboudvand, Farnaz ; Teicher, Samuel M. L. ; Wilson, Stephen D. ; Seshadri, Ram ; Johannes, Michelle D. ( March 2022 , Applied Physics Letters)

   The recently discovered kagome net compounds AV₃Sb₅( A =  K, Rb, and Cs) become superconducting on cooling, in addition to displaying interesting topological features in the electronic structure. They also exhibit charge density wave ordering, which manifests as a breathing-mode distortion in the kagome layers. It has been suggested that such ordering derives from nesting between saddle points on the Fermi surface. In aid of the evolving understanding of this intriguing materials class, we present calculations of Fermi surface nesting and Lindhard susceptibility of CsV₃Sb₅. The breathing mode distortions appear to not display a simple link with Fermi surface nesting (FSN) and do not display the signatures of a Peierls-like transition. The FSN is agnostic to changes along k_zand is only mildly impacted by small shifts of the Fermi level. The results suggest that FSN is largely independent of specific features in the saddle point.

    

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3. Two types of charge order with distinct interplay with superconductivity in the kagome material CsV3Sb5

   https://doi.org/10.1038/s42005-022-01011-0  

   Gupta, Ritu ; Das, Debarchan ; Mielke, Charles ; Ritz, Ethan T. ; Hotz, Fabian ; Yin, Qiangwei ; Tu, Zhijun ; Gong, Chunsheng ; Lei, Hechang ; Birol, Turan ; et al ( December 2022 , Communications Physics)

   Abstract The kagome metals of the family A V 3 Sb 5 , featuring a unique structural motif, harbor an array of intriguing phenomena such as chiral charge order and superconductivity. CsV 3 Sb 5 is of particular interest because it displays a double superconducting dome in the region of the temperature-pressure phase diagram where charge order is still present. However, the microscopic origin of such an unusual behavior remains an unsolved issue. Here, to address it, we combine high-pressure, low-temperature muon spin relaxation/rotation with first-principles calculations. We observe a pressure-induced threefold enhancement of the superfluid density, which also displays a double-peak feature, similar to the superconducting critical temperature. This leads to three distinct regions in the phase diagram, each of which features distinct slopes of the linear relation between superfluid density and the critical temperature. These results are attributed to a possible evolution of the charge order pattern from the superimposed tri-hexagonal Star-of-David phase at low pressures (within the first dome) to the staggered tri-hexagonal phase at intermediate pressures (between the first and second domes). Our findings suggest a change in the nature of the charge-ordered state across the phase diagram of CsV 3 Sb 5 , with varying degrees of competition with superconductivity. 

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4. Phonon mixing in the charge density wave state of ScV6Sn6

   https://doi.org/10.1038/s41535-023-00590-7  

   Gu, Yanhong ; Ritz, Ethan T. ; Meier, William R. ; Blockmon, Avery ; Smith, Kevin ; Madhogaria, Richa Pokharel ; Mozaffari, Shirin ; Mandrus, David ; Birol, Turan ; Musfeldt, Janice L. ( December 2023 , npj Quantum Materials)

   Kagomé metals are widely recognized, versatile platforms for exploring topological properties, unconventional electronic correlations, magnetic frustration, and superconductivity. In theRV₆Sn₆family of materials (R= Sc, Y, Lu), ScV₆Sn₆hosts an unusual charge density wave ground state as well as structural similarities with theAV₃Sb₅system (A= K, Cs, Rb). In this work, we combine Raman scattering spectroscopy with first-principles lattice dynamics calculations to reveal phonon mixing processes in the charge density wave state of ScV₆Sn₆. In the low temperature phase, we find at least four new peaks in the vicinity of the V-containing totally symmetric mode near 240 cm⁻¹suggesting that the density wave acts to mix modes ofP6/mmmand$$R\bar{3}m$$$R\overline{3}m$symmetry - a result that we quantify by projecting phonons of the high symmetry state onto those of the lower symmetry structure. We also test the stability of the short-range ordered density wave state under compression and propose that both physical and chemical pressure quench the effect. We discuss these findings in terms of symmetry and the structure-property trends that can be unraveled in this system.

    

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5. (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.

   Figure 1

    

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