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Theory of the pairbreaking superconductor-metal transition in nanowires Talk online: sachdev.physics.harvard.edu Talk online: sachdev.physics.harvard.edu

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Theory of the pairbreaking superconductor-metal transition in nanowires Adrian Del Maestro, Bernd Rosenow, Nayana Shah, and Subir Sachdev, Physical Review B 77, (2008). Adrian Del Maestro, Bernd Rosenow, Markus Mueller, and Subir Sachdev, Physical Review Letters 101, (2008). Adrian Del Maestro, Bernd Rosenow, and Subir Sachdev, Annals of Physics 324, 523 (2009).

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Adrian Del Maestro University of British Columbia

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T Superconductor Metal TcTc cc

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Pair-breaking in nanowires Evidence for magnetic moments on the wire’s surface A. Rogachev et al., PRL (2006) B. Spivak et. al, PRB (2001) Inhomogeneity in the pairing interaction

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T Superconductor Metal TcTc cc

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Field theory for pair-breaking superconductor-metal transition Field theory for pair-breaking superconductor-metal transition Outline Dirty wires and the strong disorder renormalization group Dirty wires and the strong disorder renormalization group Field theory for pair-breaking superconductor-metal transition Field theory for pair-breaking superconductor-metal transition Evidence for an infinite randomness fixed point Finite temperature transport (weak disorder)

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Computation of fluctuation conductivity in metal at low temperatures T Superconductor Metal TcTc cc

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Computation of fluctuation conductivity in metal at low temperatures T Superconductor Metal TcTc cc

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Computation of fluctuation conductivity in metal at low temperatures T Superconductor Metal TcTc cc

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Computation of fluctuation conductivity in metal at low temperatures T Superconductor Metal TcTc cc

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Theory for quantum-critical region, and beyond T Superconductor Metal TcTc cc Quantum critical

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Theory for quantum-critical region, and beyond T Superconductor Metal TcTc cc Quantum critical In one dimension, theory reduces to the Langer-Ambegaokar- McCumber-Halperin theory (Model A dynamics), near mean-field T c

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Role of charge conservation in quantum critical theory Dynamics of quantum theory (and model A) does not conserve total charge. Analogous to the Fermi-liquid/spin-density-wave transition (Hertz theory), where dynamics of critical theory does not conserve total spin.

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Role of charge conservation in quantum critical theory Dynamics of quantum theory (and model A) does not conserve total charge. Analogous to the Fermi-liquid/spin-density-wave transition (Hertz theory), where dynamics of critical theory does not conserve total spin. Cooper pairs (SDW) fluctuations decay into fermionic excitations at a finite rate, before any appreciable phase precession due to changes in chemical potential (magnetic field).

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Field theory for pair-breaking superconductor-metal transition Field theory for pair-breaking superconductor-metal transition Outline Dirty wires and the strong disorder renormalization group Dirty wires and the strong disorder renormalization group Evidence for an infinite randomness fixed point Finite temperature transport (weak disorder)

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T Superconductor Metal TcTc cc Quantum critical Theory for quantum-critical region, and beyond in d=1

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T Superconductor Metal TcTc cc Quantum critical Theory for quantum-critical region, and beyond in d=1

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Accuracy of large-N expansion In the metallic phase, exact agreement is found between the large-N and microscopic calculations for the electrical and thermal conductivity Lopatin, Shah and Vinokur (2005,2007)

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Non-monotonic temp. dependance A crossover is observed as the temperature is reduced for Bollinger, Rogachev and Bezryadin (2005)

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The Wiedemann-Franz law The large-N results can be used to evaluate both the electrical and thermal conductivity QCM

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Corrections to the WF law All couplings between bosons and fermions scale to universal values and we are left with a universal correction to the WF law

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Field theory for pair-breaking superconductor-metal transition Field theory for pair-breaking superconductor-metal transition Outline Dirty wires and the strong disorder renormalization group Dirty wires and the strong disorder renormalization group Evidence for an infinite randomness fixed point Finite temperature transport (weak disorder) Dirty wires and the strong disorder renormalization group Dirty wires and the strong disorder renormalization group

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Spatially dependent random couplings Hoyos, Kotabage and Vojta, PRL 2007 Real space RG predicts the flow to a strong randomness fixed point for z = 2 RTFIM Fisher’s T = 0

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Strong disorder renormalization group D. Fisher, PRL, (1992); PRB (1995) First developed by Ma, Dasgupta and Hu, applied to the RTFIM by D.S. Fisher Clusters are created or decimated as the energy is reduced

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Manifestations of strong disorder Under renormalization, probability distributions for observables become extremely broad Dynamics are highly anisotropic in space and time: activated scaling Averages become dominated by rare events (spurious disorder configurations)

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Exact predictions from the RTFIM For a finite size system D. Fisher, PRL (1992); PRB (1995) In the disordered phase critical exponents

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Discretize to a chain of L sites Measure all quantities with respect to and set Measure all quantities with respect to and set

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Take the large-N limit Enforce a large-N constraint by solving the self- consistent saddle point equations numerically Y. Tu and P. Weichman, PRL (1994); J. Hartman and Weichman, PRL (1995)

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Numerical investigation of observables Measure observables for L = 16, 32, 64, 128 averaged over 3000 realizations of disorder Tune the transition by shifting the mean of the distribution, Tune the transition by shifting the mean of the distribution, Equal time correlators are easily determined from the self-consistency condition Numerical solution by solve-join-patch method

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Equal time correlation functions Fisher’s asymptotic scaling form L = 64

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Energy gap statistics Juhász, Lin and Iglói (2006) The minimum excitation energy is due to a rare event, an extremal value

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Activated scaling Divergence cannot be fit with any power law!

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Dynamic susceptibilites We have direct access to real dynamical quantities real frequency

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Local order parameter suceptibility

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Average order parameter suceptibility

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Susceptibility scaling forms The scaling forms for the disorder averaged order parameter susceptibilities follow from an analysis of a single cluster

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Susceptibility scaling collapse average susceptibility average susceptibility local susceptibility local susceptibility Observe data collapse consistent with expectations from scaling

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Susceptibility ratio Too much parameter freedom in data collapse to confidently extract the tunneling exponent The ratio of the average to local susceptibility is related to the average cluster moment

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Activated dynamical scaling dynamic confirmation of infinite randomness

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Putting it all together νψφ RTFIM21/2(1+√5)/2 SMT1.9(2)0.53(6)1.6(2) Numerical confirmation of the strong disorder RG calculations of Hoyos, Kotabage and Vojta

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Conclusions Field theory for quantum critical transport near the superconductor-metal transition. Crosses over to phase fluctuation theory (in the superconductor) and conventional pairing fluctuation theory (in the metal) at low temperatures.

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Conclusions Field theory for quantum critical transport near the superconductor-metal transition. Crosses over to phase fluctuation theory (in the superconductor) and conventional pairing fluctuation theory (in the metal) at low temperatures.

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Conclusions Field theory for quantum critical transport near the superconductor-metal transition. Crosses over to phase fluctuation theory (in the superconductor) and conventional pairing fluctuation theory (in the metal) at low temperatures. New Wiedemann-Franz ratio in the quantum critical region.

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Conclusions First dynamical confirmation of activated scaling from numerical simulations in disordered systems. Field theory for quantum critical transport near the superconductor-metal transition. Crosses over to phase fluctuation theory (in the superconductor) and conventional pairing fluctuation theory (in the metal) at low temperatures. New Wiedemann-Franz ratio in the quantum critical region.

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Conclusions SMT has an infinite randomness fixed point in the RTFIM universality class First dynamical confirmation of activated scaling from numerical simulations in disordered systems. Field theory for quantum critical transport near the superconductor-metal transition. Crosses over to phase fluctuation theory (in the superconductor) and conventional pairing fluctuation theory (in the metal) at low temperatures. New Wiedemann-Franz ratio in the quantum critical region.

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Conclusions SMT has an infinite randomness fixed point in the RTFIM universality class First dynamical confirmation of activated scaling from numerical simulations in disordered systems. Scratching the surface of transport calculations near a strong disorder fixed point Field theory for quantum critical transport near the superconductor-metal transition. Crosses over to phase fluctuation theory (in the superconductor) and conventional pairing fluctuation theory (in the metal) at low temperatures. New Wiedemann-Franz ratio in the quantum critical region.

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