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1. Inhomogeneous Kondo-lattice in geometrically frustrated Pr2Ir2O7

   https://doi.org/10.1038/s41467-021-21698-z  

   Kavai, Mariam ; Friedman, Joel ; Sherman, Kyle ; Gong, Mingda ; Giannakis, Ioannis ; Hajinazar, Samad ; Hu, Haoyu ; Grefe, Sarah E. ; Leshen, Justin ; Yang, Qiu ; et al ( March 2021 , Nature Communications)

   Magnetic fluctuations induced by geometric frustration of local Ir-spins disturb the formation of long-range magnetic order in the family of pyrochlore iridates. As a consequence, Pr₂Ir₂O₇lies at a tuning-free antiferromagnetic-to-paramagnetic quantum critical point and exhibits an array of complex phenomena including the Kondo effect, biquadratic band structure, and metallic spin liquid. Using spectroscopic imaging with the scanning tunneling microscope, complemented with machine learning, density functional theory and theoretical modeling, we probe the local electronic states in Pr₂Ir₂O₇and find an electronic phase separation. Nanoscale regions with a well-defined Kondo resonance are interweaved with a non-magnetic metallic phase with Kondo-destruction. These spatial nanoscale patterns display a fractal geometry with power-law behavior extended over two decades, consistent with being in proximity to a critical point. Our discovery reveals a nanoscale tuning route, viz. using a spatial variation of the electronic potential as a means of adjusting the balance between Kondo entanglement and geometric frustration.

    

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2. 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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3. Nanolaser-based emulators of spin Hamiltonians

   https://doi.org/10.1515/nanoph-2020-0230  

   Parto, Midya ; Hayenga, William E. ; Marandi, Alireza ; Christodoulides, Demetrios N. ; Khajavikhan, Mercedeh ( July 2020 , Nanophotonics)

   Abstract Finding the solution to a large category of optimization problems, known as the NP-hard class, requires an exponentially increasing solution time using conventional computers. Lately, there has been intense efforts to develop alternative computational methods capable of addressing such tasks. In this regard, spin Hamiltonians, which originally arose in describing exchange interactions in magnetic materials, have recently been pursued as a powerful computational tool. Along these lines, it has been shown that solving NP-hard problems can be effectively mapped into finding the ground state of certain types of classical spin models. Here, we show that arrays of metallic nanolasers provide an ultra-compact, on-chip platform capable of implementing spin models, including the classical Ising and XY Hamiltonians. Various regimes of behavior including ferromagnetic, antiferromagnetic, as well as geometric frustration are observed in these structures. Our work paves the way towards nanoscale spin-emulators that enable efficient modeling of large-scale complex networks. 

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4. Magnetic field-tuned Fermi liquid in a Kondo insulator

   https://doi.org/10.1038/s41467-019-13421-w  

   Kushwaha, Satya K. ; Chan, Mun K. ; Park, Joonbum ; Thomas, S. M. ; Bauer, Eric D. ; Thompson, J. D. ; Ronning, F. ; Rosa, Priscila F. S. ; Harrison, Neil ( December 2019 , Nature Communications)

   Kondo insulators are expected to transform into metals under a sufficiently strong magnetic field. The closure of the insulating gap stems from the coupling of a magnetic field to the electron spin, yet the required strength of the magnetic field–typically of order 100 T–means that very little is known about this insulator-metal transition. Here we show that Ce$${}_{3}$$${}_3$Bi$${}_{4}$$${}_4$Pd$${}_{3}$$${}_3$, owing to its fortuitously small gap, provides an ideal Kondo insulator for this investigation. A metallic Fermi liquid state is established above a critical magnetic field of only$${B}_{{\rm{c}}}\approx$$$B_c\approx$11 T. A peak in the strength of electronic correlations near$${B}_{{\rm{c}}}$$$B_c$, which is evident in transport and susceptibility measurements, suggests that Ce$${}_{3}$$${}_3$Bi$${}_{4}$$${}_4$Pd$${}_{3}$$${}_3$may exhibit quantum criticality analogous to that reported in Kondo insulators under pressure. Metamagnetism and the breakdown of the Kondo coupling are also discussed.

    

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5. Compositional Influence of Local and Long-Range Polarity in the Frustrated Pyrochlore System Bi 2−x RE xTi 2 O 7 ( RE = Y 3+ , Ho 3+ )

   https://doi.org/10.1039/D2TC01328B  

   Bailey, Owen ; Husremovic, Samra ; Murphy, Madison ; Ross, Jason ; Gong, Joyce ; Olds, Daniel ; Laurita, Geneva ( July 2022 , Journal of Materials Chemistry C)

   Structural distortions such as cation off-centering are frustrated in the pyrochlore structure due to the triangular arrangement of cations on the pyrochlore lattice. This geometric constraint inhibits a transition from a paraelectric to ferroelectric phase in majority of pyrochlore oxide materials. Few pyrochlore materials can overcome this frustration and exhibit polar crystal structures, and unraveling the origin of such leads to the understanding of polarity in complex materials. Herein we hypothesize that frustration on the pyrochlore lattice can be relieved through A -site doping with rare earth cations that do not possess stereochemically active lone pairs. To assess if frustration is relieved, we have analyzed cation off-centering in various Bi 2−x RE xTi 2 O 7 ( RE = Y 3+ , Ho 3+ ) pyrochlores through neutron and X-ray total scattering. Motivated by known distortions from the pyrochlore literature, we present our findings that most samples show local distortions similar to the β-cristobalite structure. We additionally comment on the complexity of factors that play a role in the structural behavior, including cation size, bond valence, electronic structure, and magnetoelectronic interactions. We posit that the addition of magnetic cations on the pyrochlore lattice may play a role in an extension of the real-space correlation length of electric dipoles in the Bi-Ho series, and offer considerations for driving long-range polarity on the pyrochlore lattice. 

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