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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 (December 2021, Nature Communications)

   null (Ed.)

   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, Pr 2 Ir 2 O 7 lies 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 Pr 2 Ir 2 O 7 and 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. Strain-dependent insulating state and Kondo effect in epitaxial ${\mathrm{SrIrO}}_3$ films

   https://doi.org/10.1103/PhysRevMaterials.8.L071201  

   Rimal, Gaurab; Tasnim, Tanzila; Calderon_Ortiz, Gabriel; Sterbinsky, George E; Hwang, Jinwoo; Comes, Ryan B (July 2024, Physical Review Materials)

   The large spin-orbit coupling in iridium oxides plays a significant role in driving novel physical behaviors, including emergent phenomena in the films and heterostructures of perovskite and Ruddlesden-Popper iridates. In this Letter, we study the role of epitaxial strain on the electronic behavior of thin SrIrO3 films. We find that compressive epitaxial strain leads to metallic transport behavior, but a slight tensile strain shows gapped behavior. Temperature-dependent resistivity measurements are used to examine different behaviors in films as a function of strain. We find Kondo contributions to the resistivity, with stronger effects in films that are thinner and under less compressive epitaxial strain. These results show the potential to tune SrIrO3 into Kondo insulating states and open possibilities for a quantum critical point that can be controlled with strain in epitaxial films. 

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4. 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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5. Amplified multipartite entanglement witnessed in a quantum critical metal

   https://doi.org/10.1038/s41467-025-57778-7  

   Fang, Yuan; Mahankali, Mounica; Wang, Yiming; Chen, Lei; Hu, Haoyu; Paschen, Silke; Si, Qimiao (March 2025, Nature Communications)

   Strong correlations in matter promote a landscape of quantum phases and associated quantum critical points. For metallic systems, there is increasing recognition that the quantum criticality goes beyond the Landau framework and, thus, further means are needed to characterize the quantum critical fluid. Here we do so by studying an entanglement quantity, the quantum Fisher information, in a strange metal system, focusing on the exemplary case of an Anderson/Kondo lattice model near its Kondo destruction quantum critical point. The spin quantum Fisher information peaks at the quantum critical point and indicates a strongly entangled ground state. Our results are supported by the quantum Fisher information extracted from inelastic neutron scattering measurements in heavy fermion metals. Our work elucidates the loss of quasiparticles in strange metals, opens a quantum information avenue to advance the understanding of metallic quantum criticality in a broad range of strongly correlated systems, and points to a regime of quantum matter to realize amplified entanglement. 

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