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The non-symmorphic rutile space group enforces the existence of a network of Dirac nodal lines in the Brillouin zones of transition metal rutiles like RuO₂ and IrO₂ A recent report published in Nature Communications demonstrates that oxygen vacancies in IrO₂ and RuO₂ nanowires drive a fragile many-body state known as the orbital Kondo effect.

Interestingly, the symmetries that enforce the existence of DNLs also promote the formation of nonmagnetic Kondo correlations. The demonstration that the non-symmorphic rutile space group supports oxygen-driven orbital Kondo effects in transition metal rutiles holds promise for the realization of novel states of matter and may have ramifications in other materials featuring oxygen vacancies. 

An ever growing number of complex materials have been identified that defy the standard theory of metals. Among the simplest ways of generating an unconventional metal is through the two-channel Kondo effect, which leads to the formation of a fragile many-body state. This article reviews challenges, attemps and successes of realizing this two-channel Kondo effect in engineered structures and real quantum materials.

Two‐Channel Kondo Physics: From Engineered Structures to Quantum Materials Realizations

A recent publication analyzes the fate of the g-theorem in the pseudogap Bose-Fermi Kondo model. This model contains various quantum impurity models as special cases. Contrary to earlier investigations, we reach the conclusion that the g-theorem is not valid for the SU(N) symmetric version of this model. As a result, the boundary entropy may increase along the RG flow. The origin of the inapplicability of the g-theorem is traced back to anomalous contributions of the large-N scaling functions.

Quantum criticality in the spin-isotropic pseudogap Bose-Fermi Kondo model: Entropy, scaling, and the g-theorem

Angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM) have become indispensable tools in the study of correlated quantum materials. A new review article, published in the Colloquiua section of Reviews of Modern Physics, explicates the potential of these techniques in the context of heavy-electron quantum criticality.

In “Colloquium: Heavy-electron quantum criticality and single-particle spectroscopy“, the authors survey recent progress and possible challenges in the experimental investigations. The STM and ARPES spectra for several quantum-critical heavy-electron compounds are compared, and the prospects for further advances are outlined.