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Mean field theories of quantum spin glasses Antoine Georges Olivier Parcollet Nick Read Subir Sachdev Jinwu Ye Talk online: Sachdev Classical Sherrington-Kirkpatrick model H J ij Si S j i j Jij : a Gaussian random variable with zero mean Si : a unit length n component vector Two routes to quantization A. Quantum rotor model 1 dSi Action = d H 2 g i d 2 n=1: Ising model is a transverse field g Spectrum at Jij=0 j 1 j 2 j 1 j 2 g n=3: randomly coupled spin dimers j j 1 , j j 2 Spectrum at Jij=0 g 1 j 2 j j , j Two routes to quantization B. Heisenberg spins dS j Action = d iSA S j H d j First term is kinematic Berry phase which ensures S j , Sk i jk S j and S 2j S ( S 1) Spectrum at Jij=0 , j j (2S+1)-fold degeneracy Generalize model to SU(N) spins and explore phase diagram in N, S plane Outline A. Insulating quantum rotors. B. Insulating Heisenberg spins C. DMFT of a random t-J model D. Metallic spin glasses: DMFT of a random Kondo lattice A. Insulating quantum rotors A. Quantum rotor model 1 dSi Action = d J ij Si S j 2 g i d i j 2 Jij : a Gaussian random variable with zero mean T=0 phases Local dynamic spin susceptibility n 1/ T 0 d S j S j 0 ein Spin glass Paramagnet '' ~ qEA '' gapped Specific heat C ~ T (?) '' ~ g D.A. Huse and J. Miller, Phys. Rev. Lett. 70, 3147 (1993). J. Ye, S. Sachdev, and N. Read, Phys. Rev. Lett. 70, 4011 (1993). N. Read, S. Sachdev, and J. Ye, Phys. Rev. B 52, 384 (1995). A. Georges, O. Parcollet, and S. Sachdev, Phys. Rev. B 63, 134406 (2001). T > 0 phase diagram Quantum critical '' ~ 1/ 2 T / ln 1/ T g gc J. Ye, S. Sachdev, and N. Read, Phys. Rev. Lett. 70, 4011 (1993). N. Read, S. Sachdev, and J. Ye, Phys. Rev. B 52, 384 (1995). B. Insulating Heisenberg spins B. Heisenberg spin glass dS j Action = d iSA S j J ij Si S j d i j j Jij : a Gaussian random variable with zero mean S j : a SU(N ) spin with N 2 1 components and "length" S S. Sachdev and J. Ye, Phys. Rev. Lett. 70, 3339 (1993). T=0 phase diagram S Spin glass order '' ~ qEA Specific heat C ~ T (C ~ T2 ?) Quantum critical "spin slush" phase with "marginal Fermi liquid" spectrum: sgn '' ~ J N S. Sachdev and J. Ye, Phys. Rev. Lett. 70, 3339 (1993). A. Georges, O. Parcollet, and S. Sachdev, Phys. Rev. Lett. 85, 840 (2000). A. Camjayi and M. J. Rozenberg, Phys. Rev. Lett. 90, 217202 (2003). Quantum critical phase is described by fractionalized S=1/2 neutral spinon excitations S ~ f f † Spinon spectral density kBT 1 S. Sachdev and J. Ye, Phys. Rev. Lett. 70, 3339 (1993). T > 0 phase diagram kBT '' ~ sgn S. Sachdev and J. Ye, Phys. Rev. Lett. 70, 3339 (1993). A. Georges, O. Parcollet, and S. Sachdev, Phys. Rev. Lett. 85, 840 (2000). A. Camjayi and M. J. Rozenberg, Phys. Rev. Lett. 90, 217202 (2003). C. Doping the quantum critical spin liquid C. DMFT of a random t-J model Hamiltonian = tij Pc c j P J ij Si S j † i ij ij 1 † Si ci ci 2 Jij : a Gaussian random variable with zero mean O. Parcollet and A. Georges, Phys. Rev. B 59, 5341 (1999). = carrier density Quantum critical "incoherent" physics with universal /k BT scaling above a coherence scale F* ~ k BT * ~ t 2 J O. Parcollet and A. Georges, Phys. Rev. B 59, 5341 (1999). Physical consequences of quantum criticality 1. Electron spectral function (photoemission) 1 Momentum integrated electron spectral density at T 0 : * t F 1 1 as 0 and as Momentum resolved spectral density 2 Quasiparticle peak with residue Z ~ * 1 O. Parcollet and A. Georges, Phys. Rev. B 59, 5341 (1999). Physical consequences of quantum criticality 2. d.c Resistivity h T dc T 2 * e F T T * F * F ! 2 O. Parcollet and A. Georges, Phys. Rev. B 59, 5341 (1999). Physical consequences of quantum criticality 3. NMR 1/T1 relaxation rate 1 1 T * T1 J F constant (MFL) T Korringa * F O. Parcollet and A. Georges, Phys. Rev. B 59, 5341 (1999). Physical consequences of quantum criticality 4. Optical conductivity F* In quantum critical regime, with T J , Re k BT * F F* for T J with Re * F for T T O. Parcollet and A. Georges, Phys. Rev. B 59, 5341 (1999). Phenomenological phase diagram for cuprates O. Parcollet and A. Georges, Phys. Rev. B 59, 5341 (1999). D. Metallic spin glasses C. DMFT of a random Kondo lattice model Hamiltonian = t c † ij i ij c j J ij Si S j JK 2 ij S c i † i ci i Jij : a Gaussian random variable with zero mean S. Sachdev, N. Read, and R. Oppermann, Phys. Rev. B 52, 10286 (1995). A. M. Sengupta and A. Georges, Phys. Rev. B 52, 10295 (1995). Quantum critical '' ~ sgn '' ~ qEA sgn 1/ 2 1/ 2 3/ 2 T JK S. Sachdev, N. Read, and R. Oppermann, Phys. Rev. B 52, 10286 (1995). A. M. Sengupta and A. Georges, Phys. Rev. B 52, 10295 (1995). Outlook • Spin glass order is an attractive candidate for a quantum critical point in the cuprates, on both theoretical and experimental grounds. (Impurities break the translational symmetry associated with chargeordered states, and the Imry-Ma argument then prohibits a quantum critical point associated with charge order in the presence of randomness in two dimensions) • A simple mean-field theory of a doped Heisenberg spin glass naturally reproduces all the “marginal” phenomenology. • Needed: better theory of fluctuations in low dimensions