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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)

   Abstract 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. Pristine quantum criticality in a Kondo semimetal

   https://doi.org/10.1126/sciadv.abf9134  

   Fuhrman, Wesley T.; Sidorenko, Andrey; Hänel, Jonathan; Winkler, Hannes; Prokofiev, Andrey; Rodriguez-Rivera, Jose A.; Qiu, Yiming; Blaha, Peter; Si, Qimiao; Broholm, Collin L.; et al (May 2021, Science Advances)

   The observation of quantum criticality in diverse classes of strongly correlated electron systems has been instrumental in establishing ordering principles, discovering new phases, and identifying the relevant degrees of freedom and interactions. At focus so far have been insulators and metals. Semimetals, which are of great current interest as candidate phases with nontrivial topology, are much less explored in experiments. Here, we study the Kondo semimetal CeRu 4 Sn 6 by magnetic susceptibility, specific heat, and inelastic neutron scattering experiments. The power-law divergence of the magnetic Grünesien ratio reveals that, unexpectedly, this compound is quantum critical without tuning. The dynamical energy over temperature scaling in the neutron response throughout the Brillouin zone and the temperature dependence of the static uniform susceptibility, indicate that temperature is the only energy scale in the criticality. Such behavior, which has been associated with Kondo destruction quantum criticality in metallic systems, could be generic in the semimetal setting. 

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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. Observation of an antiferromagnetic quantum critical point in high-purity LaNiO3

   https://doi.org/10.1038/s41467-020-15143-w  

   Liu, Changjiang; Humbert, Vincent F.; Bretz-Sullivan, Terence M.; Wang, Gensheng; Hong, Deshun; Wrobel, Friederike; Zhang, Jianjie; Hoffman, Jason D.; Pearson, John E.; Jiang, J. Samuel; et al (December 2020, Nature Communications)

   null (Ed.)

   Abstract Amongst the rare-earth perovskite nickelates, LaNiO 3 (LNO) is an exception. While the former have insulating and antiferromagnetic ground states, LNO remains metallic and non-magnetic down to the lowest temperatures. It is believed that LNO is a strange metal, on the verge of an antiferromagnetic instability. Our work suggests that LNO is a quantum critical metal, close to an antiferromagnetic quantum critical point (QCP). The QCP behavior in LNO is manifested in epitaxial thin films with unprecedented high purities. We find that the temperature and magnetic field dependences of the resistivity of LNO at low temperatures are consistent with scatterings of charge carriers from weak disorder and quantum fluctuations of an antiferromagnetic nature. Furthermore, we find that the introduction of a small concentration of magnetic impurities qualitatively changes the magnetotransport properties of LNO, resembling that found in some heavy-fermion Kondo lattice systems in the vicinity of an antiferromagnetic QCP. 

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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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