Article

# Quantum critical spin liquids and superconductivity in the cuprates

• ##### André LeClair
11/2006;
Source: arXiv

ABSTRACT We describe a new kind of quantum critical point in the context of quantum anti-ferromagnetism in 2d that can be understood as a quantum critical spin liquid. Based on the comparison of exponents with previous numerical work, we argue it describes a transition from an anti-ferromagnetic N\'eel ordered state to a VBS-like state. We argue further that the symplectic fermions capture the proper degrees of freedom in the zero temperature phase that is the parent to the superconducting phase in the cuprates. We then show that our model reproduces some features found recently in experiments and also in the Hubbard model.

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##### Article: A model of a 2d non-Fermi liquid with SO(5) symmetry, AF order, and a d-wave SC gap
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ABSTRACT: Demanding a consistent quantum field theory description of spin 1/2 particles near a circular Fermi surface in 2d leads to a unique fermionic theory with relevant quartic interactions which has an emergent Lorentz symmetry and automatically has an Sp(4) = SO(5) internal symmetry. The free theory has resistivity linear in temperature. The interacting theory has a low-energy interacting fixed point and is thus a non-Landau/Fermi liquid. Anti-ferromagnetic (AF) and superconducting (SC) order parameters are bilinears in the fields and form the 5-dimensional vector representation of SO(5). An AF phase occurs at low doping which terminates in a first order transition. We incorporate momentum dependent scattering of Cooper pairs near the Fermi surface to 1-loop and derive a new kind of SC gap equation beyond mean field with a d-wave gap solution. Taking into account the renormalization group (RG) scaling properties near the low energy fixed point, we calculate the complete phase diagram as a function of doping, which shows some universal geometric features. The d-wave SC dome terminates on the over-doped side at the fixed point of the RG,which is a quantum critical point. Optimal doping is estimated to occur just below 3/2\pi^2. The critical temperature for SC at optimal doping is set mainly by the universal nodal Fermi velocity and lattice spacing, and is estimated to average around 140K for LSCO. The pseudogap energy scale is identified as the RG scale of the coupling. Comment: published version
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##### Article: Quantum critical spin liquids and conformal field theory in 2+1 dimensions
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ABSTRACT: We describe new conformal field theories based on symplectic fermions that can be extrapolated between 2 and 4 dimensions. The critical exponents depend continuously on the number of components N of the fermions and the dimension D. In the context of anti-ferromagnetism, the N=2 theory is proposed to describe a deconfined quantum critical spin liquid corresponding to a transition between a Neel ordered phase and a VBS-like phase.
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##### Article: 3D Ising and other models from symplectic fermions
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ABSTRACT: We study a model of N component symplectic fermions in D spacetime dimensions. It has an infra-red stable fixed point in 2<D<4 dimensions referred to as Sp{2N}{D}. Based on the comparison of exponents, we conjecture that the critical exponents for the 3D Wilson-Fisher fixed point for an O(N) invariant N-component bosonic field can be computed in the Sp{-2N}{3} theory. The 3D Ising model corresponds to Sp{-2}{3}. The exponent beta agrees with known results to 1 part in 1000 and is within current error bars. The nu exponent agrees to 1% and we suggest this because we only went to 1-loop for this exponent.
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