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.

Congratulations!

S. Kirchner has been named an ‘Excellent Individual of the Department‘ of the year 2019.

A collaboration between physicists from Zhejiang University and Rice University in Houston establishes a new means of probing quantum entanglement of strange metals at a quantum critical point.

Just like water vaporizes or freezes upon the variation of thermal fluctuations, matter can undergo transformation from one quantum phase to another through the control of quantum fluctuations. A quantum critical point develops at a continuous quantum phase transition, and is expected to possess amplified quantum entanglement. This is the spooky action at a distance as Albert Einstein called it, which may drive such technologies as quantum computing.

In this work, which was published in Physical Review Letters, Cai et al. focus on an idealized but realistic model for quantum materials, featuring strongly correlated electrons at the border of itinerancy and localization. Only the spin degrees of freedom of the model interact with each other, and conventional expectation is that only physical quantities made up of the spins will be critical. Surprisingly, they show that quantities from the charge sector are also critical. These results provide a theoretical basis to understand the striking puzzles raised by recent experiments in “heavy fermion metals” and indicate new means to probe the quantum entanglement of electrons in a broad range of quantum materials.

Transition metal rutiles – a quantum simulation platform of non-Fermi liquid behavior

Our new preprint reports the discovery of orbital two-channel Kondo impurity physics in transition metal oxides of composition MO₂ possessing the rutile structure. The two-channel Kondo effect was proposed by Noziere in the early 1980s but despite its conceptual simplicity and a few claims no convincing observation in quantum matter was available so far. The study is based onwell-characterized high-quality nanowires.

Oxygen vacancies in IrO₂ drive a non-Fermi liquid ground state which leads to a characteristic magnetic-field independent √T increase of the resistivity at low temperatures T. This anomalous behavior has been observed over two decades in T.

A big challenge in irrefutably establishing two-channel behavior is that weakly disordered metals generically display a √T correction in their transport behavior. We therefore demonstrate that the same mechanics in antiferromagnetic RuO₂ drive an orbital one-channel Kondo effect.

The data for RuO₂ display a scaling collapse for a wide range of Kondo temperatures and covers almost three decades in temperature.

Heavy electron quantum criticality: Perspectives and challenges for single-electron spectroscopy

Our new preprint reviews existing scanning tunneling and angle-resolved photoemission spectrocopy studies in the context of Kondo-destroying quantum criticality in heavy-electron compounds. A discussion of current challenges, prospects and future research for single-electron spectroscopy near quantum criticality is provided.

Illustrations of basic concepts of Kondo-destruction quantum criticality: Local quantum criticality with Kondo destruction, under the variation of the control parameter δ. Here, T₀ is a high-energy scale that describes the initial onset of dynamical Kondo correlations and that smoothly evolves across the QCP. This high-energy scale is reflected in the onset of hybridization-gap formation. The low-energy physics is described in terms of T_N and T_FL which are respectively the temperatures for the Néel transition and the crossover into the paramagnetic Fermi-liquid state.
This phase diagram also involves the Kondo-destruction energy scale E^*_loc, which characterizes the Kondo destruction. The E^*_loc-line divides the phase diagram in terms of the flow of the system towards either the Kondo-screened or the Kondo-destruction ground state.

Mixed-Order Quantum Phase Transition far from Equilibrium

In an article published this week in Physical Review Letters, a team of scientists from China and Portugal establishes that quantum phase transitions out of equilibrium can be of mixed order: first order and continuous. Their findings exemplify that far from equilibrium, novel critical phenomena exist which are not possible in equilibrium.

New quantum and classical liquid states in an old model

In research published in Physical Review Letters, a team of scientists from Zhejiang University and University of Lisbon identify classical and quantum liquid states as classical order gets suppressed by quantum fluctuations.

These results promise a new vantage point for understanding how quantum liquids can emerge from their classical counterparts.