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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. 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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3. Ground-state phase diagram of the t-t ′ -J model

   https://doi.org/10.1073/pnas.2109978118  

   Jiang, Shengtao ; Scalapino, Douglas J. ; White, Steven R. ( October 2021 , Proceedings of the National Academy of Sciences)

   We report results of large-scale ground-state density matrix renormalization group (DMRG) calculations on t-$t^{\prime }$-J cylinders with circumferences 6 and 8. We determine a rough phase diagram that appears to approximate the two-dimensional (2D) system. While for many properties, positive and negative$t^{\prime }$values ($t^{\prime }/t=\pm 0.2$) appear to correspond to electron- and hole-doped cuprate systems, respectively, the behavior of superconductivity itself shows an inconsistency between the model and the materials. The$t^{\prime }<0$(hole-doped) region shows antiferromagnetism limited to very low doping, stripes more generally, and the familiar Fermi surface of the hole-doped cuprates. However, we find$t^{\prime }<0$strongly suppresses superconductivity. The$t^{\prime }>0$(electron-doped) region shows the expected circular Fermi pocket of holes around the$\left(\pi ,\pi \right)$point and a broad low-doped region of coexisting antiferromagnetism and d-wave pairing with a triplet p component at wavevector$\left(\pi ,\pi \right)$induced by the antiferromagnetism and d-wave pairing. The pairing for the electron low-doped system with$t^{\prime }>0$is strong and unambiguous in the DMRG simulations. At larger doping another broad region with stripes in addition to weaker d-wave pairing and striped p-wave pairing appears. In a small doping region near$x=0.08$for$t^{\prime }\sim -0.2$, we find an unconventional type of stripe involving unpaired holes located predominantly on chains spaced three lattice spacings apart. The undoped two-leg ladder regions in between mimic the short-ranged spin correlations seen in two-leg Heisenberg ladders.

    

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4. Plasmonically enhanced mid-IR light source based on tunable spectrally and directionally selective thermal emission from nanopatterned graphene

   https://doi.org/10.1038/s41598-020-73582-3  

   Shabbir, Muhammad Waqas ; Leuenberger, Michael N. ( October 2020 , Scientific Reports)

   We present a proof of concept for a spectrally selective thermal mid-IR source based on nanopatterned graphene (NPG) with a typical mobility of CVD-grown graphene (up to 3000$$\hbox {cm}^2\,\hbox {V}^{-1}\,\hbox {s}^{-1}$$${\text{cm}}^2{\text{V}}^{-1}{\text{s}}^{-1}$), ensuring scalability to large areas. For that, we solve the electrostatic problem of a conducting hyperboloid with an elliptical wormhole in the presence of anin-planeelectric field. The localized surface plasmons (LSPs) on the NPG sheet, partially hybridized with graphene phonons and surface phonons of the neighboring materials, allow for the control and tuning of the thermal emission spectrum in the wavelength regime from$$\lambda =3$$$\lambda =3$to 12$$\upmu$$$\mu$m by adjusting the size of and distance between the circular holes in a hexagonal or square lattice structure. Most importantly, the LSPs along with an optical cavity increase the emittance of graphene from about 2.3% for pristine graphene to 80% for NPG, thereby outperforming state-of-the-art pristine graphene light sources operating in the near-infrared by at least a factor of 100. According to our COMSOL calculations, a maximum emission power per area of$$11\times 10^3$$$11\times {10}^3$W/$$\hbox {m}^2$$${\text{m}}^2$at$$T=2000$$$T=2000$K for a bias voltage of$$V=23$$$V=23$V is achieved by controlling the temperature of the hot electrons through the Joule heating. By generalizing Planck’s theory to any grey body and deriving the completely general nonlocal fluctuation-dissipation theorem with nonlocal response of surface plasmons in the random phase approximation, we show that the coherence length of the graphene plasmons and the thermally emitted photons can be as large as 13$$\upmu$$$\mu$m and 150$$\upmu$$$\mu$m, respectively, providing the opportunity to create phased arrays made of nanoantennas represented by the holes in NPG. The spatial phase variation of the coherence allows for beamsteering of the thermal emission in the range between$$12^\circ$$${12}^{\circ }$and$$80^\circ$$${80}^{\circ }$by tuning the Fermi energy between$$E_F=1.0$$$E_F=1.0$eV and$$E_F=0.25$$$E_F=0.25$eV through the gate voltage. Our analysis of the nonlocal hydrodynamic response leads to the conjecture that the diffusion length and viscosity in graphene are frequency-dependent. Using finite-difference time domain calculations, coupled mode theory, and RPA, we develop the model of a mid-IR light source based on NPG, which will pave the way to graphene-based optical mid-IR communication, mid-IR color displays, mid-IR spectroscopy, and virus detection.

    

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5. Proximity-enhanced valley Zeeman splitting at the WS 2 /graphene interface

   https://doi.org/10.1088/2053-1583/acd5df  

   Faria Junior, Paulo E. ; Naimer, Thomas ; McCreary, Kathleen M. ; Jonker, Berend T. ; Finley, Jonathan J. ; Crooker, Scott A. ; Fabian, Jaroslav ; Stier, Andreas V. ( May 2023 , 2D Materials)

   The valley Zeeman physics of excitons in monolayer transition metal dichalcogenides provides valuable insight into the spin and orbital degrees of freedom inherent to these materials. Being atomically-thin materials, these degrees of freedom can be influenced by the presence of adjacent layers, due to proximity interactions that arise from wave function overlap across the 2D interface. Here, we report 60 T magnetoreflection spectroscopy of the A- and B- excitons in monolayer WS₂, systematically encapsulated in monolayer graphene. While the observed variations of the valley Zeeman effect for the A- exciton are qualitatively in accord with expectations from the bandgap reduction and modification of the exciton binding energy due to the graphene-induced dielectric screening, the valley Zeeman effect for the B- exciton behaves markedly different. We investigate prototypical WS₂/graphene stacks employing first-principles calculations and find that the lower conduction band of WS₂at the$K/K^{\mathrm{\prime }}$valleys (the${\mathrm{C}\mathrm{B}}^{-}$band) is strongly influenced by the graphene layer on the orbital level. Specifically, our detailed microscopic analysis reveals that the conduction band at theQpoint of WS₂mediates the coupling between${\mathrm{C}\mathrm{B}}^{-}$and graphene due to resonant energy conditions and strong coupling to the Dirac cone. This leads to variations in the valley Zeeman physics of the B- exciton, consistent with the experimental observations. Our results therefore expand the consequences of proximity effects in multilayer semiconductor stacks, showing that wave function hybridization can be a multi-step energetically resonant process, with different bands mediating the interlayer interactions. Such effects can be further exploited to resonantly engineer the spin-valley degrees of freedom in van der Waals and moiré heterostructures.

    

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