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1. Intermediate valence state in YbB 4 revealed by resonant x-ray emission spectroscopy

   https://doi.org/10.1088/1361-648X/ac7629  

   Frontini, Felix ; Lebert, Blair W. ; Cho, K. K. ; Song, M. S. ; Cho, B. K. ; Pollock, Christopher J. ; Kim, Young-June ( June 2022 , Journal of Physics: Condensed Matter)

   We report the temperature dependence of the Yb valence in the geometrically frustrated compound${\mathrm{Y}\mathrm{b}\mathrm{B}}_4$from 12 to 300 K using resonant x-ray emission spectroscopy at the Yb$L_{{\alpha }_1}$transition. We find that the Yb valence,v, is hybridized between thev = 2 andv = 3 valence states, increasing from$v=2.61\pm 0.01$at 12 K to$v=2.67\pm 0.01$at 300 K, confirming that${\mathrm{Y}\mathrm{b}\mathrm{B}}_4$is a Kondo system in the intermediate valence regime. This result indicates that the Kondo interaction in${\mathrm{Y}\mathrm{b}\mathrm{B}}_4$is substantial, and is likely to be the reason why${\mathrm{Y}\mathrm{b}\mathrm{B}}_4$does not order magnetically at low temperature, rather than this being an effect of geometric frustration. Furthermore, the zero-point valence of the system is extracted from our data and compared with other Kondo lattice systems. The zero-point valence seems to be weakly dependent on the Kondo temperature scale, but not on the valence change temperature scaleT_v.

    

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2. Tuning metal/superconductor to insulator/superconductor coupling via control of proximity enhancement between NbSe 2 monolayers

   https://doi.org/10.1088/1361-648X/acbf92  

   Chiatti, Olivio ; Mihov, Klara ; Griffin, Theodor U. ; Grosse, Corinna ; Alemayehu, Matti B. ; Hite, Kyle ; Hamann, Danielle ; Mogilatenko, Anna ; Johnson, David C. ; Fischer, Saskia F. ( March 2023 , Journal of Physics: Condensed Matter)

   The interplay between charge transfer and electronic disorder in transition-metal dichalcogenide multilayers gives rise to superconductive coupling driven by proximity enhancement, tunneling and superconducting fluctuations, of a yet unwieldy variety. Artificial spacer layers introduced with atomic precision change the density of states by charge transfer. Here, we tune the superconductive coupling between$\text{NbS}{\text{e}}_{\text{2}}$monolayers from proximity-enhanced to tunneling-dominated. We correlate normal and superconducting properties in${\left[{\left(\text{SnSe}\right)}_{1+\delta }\right]}_m{\left[\text{NbS}{\text{e}}_{\text{2}}\right]}_1$tailored multilayers with varying SnSe layer thickness ($m=1-15$). From high-field magnetotransport the critical fields yield Ginzburg–Landau coherence lengths with an increase of$140\mathrm{\%}$cross-plane ($m=1-9$), trending towards two-dimensional superconductivity for$m>9$. We show cross-overs between three regimes: metallic with proximity-enhanced coupling ($m=1-4$), disordered-metallic with intermediate coupling ($m=5-9$) and insulating with Josephson tunneling ($m>9$). Our results demonstrate that stacking metal mono- and dichalcogenides allows to convert a metal/superconductor into an insulator/superconductor system, prospecting the control of two-dimensional superconductivity in embedded layers.

    

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3. Deciphering the sensitivity of urban canopy air temperature to anthropogenic heat flux with a forcing-feedback framework

   https://doi.org/10.1088/1748-9326/ace7e0  

   Wang, Linying ; Sun, Ting ; Zhou, Wenyu ; Liu, Maofeng ; Li, Dan ( August 2023 , Environmental Research Letters)

   The sensitivity of urban canopy air temperature ($T_a$) to anthropogenic heat flux ($Q_{AH}$) is known to vary with space and time, but the key factors controlling such spatiotemporal variabilities remain elusive. To quantify the contributions of different physical processes to the magnitude and variability of$\mathrm{\Delta }{T}_a/\mathrm{\Delta }{Q}_{AH}$(where$\mathrm{\Delta }$represents a change), we develop a forcing-feedback framework based on the energy budget of air within the urban canopy layer and apply it to diagnosing$\mathrm{\Delta }{T}_a/\mathrm{\Delta }{Q}_{AH}$simulated by the Community Land Model Urban over the contiguous United States (CONUS). In summer, the median$\mathrm{\Delta }{T}_a/\mathrm{\Delta }{Q}_{AH}$is around 0.01$\text{K\hspace{0.17em}}{\left(\text{W\hspace{0.17em}}{\text{m}}^{-\text{2}}\right)}^{-1}$over the CONUS. Besides the direct effect of$Q_{AH}$on$T_a$, there are important feedbacks through changes in the surface temperature, the atmosphere–canopy air heat conductance ($c_a$), and the surface–canopy air heat conductance. The positive and negative feedbacks nearly cancel each other out and$\mathrm{\Delta }{T}_a/\mathrm{\Delta }{Q}_{AH}$is mostly controlled by the direct effect in summer. In winter,$\mathrm{\Delta }{T}_a/\mathrm{\Delta }{Q}_{AH}$becomes stronger, with the median value increased by about 20% due to weakened negative feedback associated with$c_a$. The spatial and temporal (both seasonal and diurnal) variability of$\mathrm{\Delta }{T}_a/\mathrm{\Delta }{Q}_{AH}$as well as the nonlinear response of$\mathrm{\Delta }{T}_a$to$\mathrm{\Delta }{Q}_{AH}$are strongly related to the variability of$c_a$, highlighting the importance of correctly parameterizing convective heat transfer in urban canopy models.

    

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4. Characterizing the fundamental bending vibration of a linear polyatomic molecule for symmetry violation searches

   https://doi.org/10.1088/1367-2630/ace471  

   Jadbabaie, Arian ; Takahashi, Yuiki ; Pilgram, Nickolas H ; Conn, Chandler J ; Zeng, Yi ; Zhang, Chi ; Hutzler, Nicholas R ( July 2023 , New Journal of Physics)

   Polyatomic molecules have been identified as sensitive probes of charge-parity violating and parity violating physics beyond the Standard Model (BSM). For example, many linear triatomic molecules are both laser-coolable and have parity doublets in the ground electronic$\tilde{X}{}^2{\mathrm{\Sigma }}^{+}(010)$state arising from the bending vibration, both features that can greatly aid BSM searches. Understanding the$\tilde{X}{}^2{\mathrm{\Sigma }}^{+}(010)$state is a crucial prerequisite to precision measurements with linear polyatomic molecules. Here, we characterize the fundamental bending vibration of${}^{174}$YbOH using high-resolution optical spectroscopy on the nominally forbidden$\tilde{X}{}^2{\mathrm{\Sigma }}^{+}(010)$$\to \tilde{A}{}^2{\mathrm{\Pi }}_{1/2}(000)$transition at 588 nm. We assign 39 transitions originating from the lowest rotational levels of the$\tilde{X}{}^2{\mathrm{\Sigma }}^{+}(010)$state, and accurately model the state’s structure with an effective Hamiltonian using best-fit parameters. Additionally, we perform Stark and Zeeman spectroscopy on the$\tilde{X}{}^2{\mathrm{\Sigma }}^{+}(010)$state and fit the molecule-frame dipole moment to$D_{\mathrm{m}\mathrm{o}\mathrm{l}}=2.16(1)$Dand the effective electrong-factor to$g_S=2.07(2)$. Further, we use an empirical model to explain observed anomalous line intensities in terms of interference from spin–orbit and vibronic perturbations in the excited$\tilde{A}{}^2{\mathrm{\Pi }}_{1/2}(000)$state. Our work is an essential step toward searches for BSM physics in YbOH and other linear polyatomic molecules.

    

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5. Electronic structure in a transition metal dipnictide TaAs 2

   https://doi.org/10.1088/1361-648X/ad04fc  

   Regmi, Sabin ; Huang, Cheng-Yi ; Khan, Mojammel A. ; Wang, Baokai ; Pradhan Sakhya, Anup ; Hosen, M. Mofazzel ; Thompson, Jesse ; Singh, Bahadur ; Denlinger, Jonathan D. ; Ishigami, Masahiro ; et al ( November 2023 , Journal of Physics: Condensed Matter)

   The family of transition-metal dipnictides has been of theoretical and experimental interest because this family hosts topological states and extremely large magnetoresistance (MR). Recently,${\mathrm{T}\mathrm{a}\mathrm{A}\mathrm{s}}_2$, a member of this family, has been predicted to support a topological crystalline insulating state. Here, by using high-resolution angle-resolved photoemission spectroscopy (ARPES), we reveal both closed and open pockets in the metallic Fermi surface (FS) and linearly dispersive bands on the ($\bar{2}01$) surface, along with the presence of extreme MR observed from magneto-transport measurements. A comparison of the ARPES results with first-principles computations shows that the linearly dispersive bands on the measured surface of${\mathrm{T}\mathrm{a}\mathrm{A}\mathrm{s}}_2$are trivial bulk bands. The absence of symmetry-protected surface state on the ($\bar{2}01$) surface indicates its topologically dark nature. The presence of open FS features suggests that the open-orbit fermiology could contribute to the extremely large MR of${\mathrm{T}\mathrm{a}\mathrm{A}\mathrm{s}}_2$.

    

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