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Register a new userQuantum Criticality and Novel Phases: Summary and Outlook
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A. J. Schofield
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This conference summary and outlook provides a personal overview of the topics and themes of the August 2009 Dresden meeting on quantum criticality and novel phases. The dichotomy between the local moment and the itinerant views of magnetism is revisited and refreshed in new materials, new probes and new theoretical ideas. New universality and apparent zero temperature phases of matter move us beyond the old ideas of quantum criticality. This is accompanied by alternative pairing interactions and as yet unidentified phases developing in the vicinity of quantum critical points. In discussing novel order, the magnetic analogues of superconductivity are considered as candidate states for the hidden order that sometimes develops in the vicinity of quantum critical points in metallic systems. These analogues can be thought of as pairing in the particle-hole channel and are tabulated. This analogy is used to outline a framework to study the relation between ferromagnetic fluctuations and the propensity of a metal to nematic type phases which at weak coupling correspond to Pomeranchuk instabilities. This question can be related to the fundamental relations of Fermi liquid theory.
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Strain tuning is increasingly being recognized as a clean tuning parameter to induce novel behavior in quantum matter. Motivated by the possibility of straining graphene up to $20$ percent, we investigate novel quantum criticality due to interplay between strain-induced anisotropic band structure and critical antiferromagnetic spin fluctuations (AFSF) in this setting. We detail how this interplay drives $(i)$ a quantum phase transition (QPT) between the Dirac-semimetal-incoherent pseudogapped metal-correlated insulator as a function of strain ($epsilon$), and $(ii)$ critical AFSF-driven divergent nematic susceptibility near critical strain ($epsilon_{c}$) manifesting as critical singularities in magneto-thermal expansion and Gruneisen co-efficients. The correlated band insulator at large strain affords realization of a two-dimensional dimerized spin-singlet state due to this interplay, and we argue how doping such an insulator can lead to a spin-charge separated metal, leading to anomalous metallicity and possible unconventional superconductivity. On a wider front, our work serves to illustrate the range of novel states realizable by strain-tuning quantum materials.
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New electronic phases have been identified and placed in the (T,H) phase diagram of metallic NaxCoO2. At low Na-content (x = 0.36), the magnetic susceptibility diverges with a power law T^(-n), n<1, and shows (T,H) scaling, indicating the proximity to a magnetic quantum phase transition. At high Na contents (x = 0.6) the mass of the quasiparticles does never diverge, but renormalizes and becomes strongly field dependent at low temperatures, forming a heavy Fermi-Liquid. Our results make superconducting NaxCoO2 a clear candidate for magnetically mediated pairing.
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