Hong Kong Forum of Condensed Matter Physics: Past, Present, and Future December 20, 2006 Adam Durst Department of Physics and Astronomy Stony Brook University Dirac Quasiparticles in Condensed Matter Physics

Hong Kong Forum of Condensed Matter Physics: Past, Present, and Future December 20, 2006 Adam Durst Department of Physics and Astronomy Stony Brook University Dirac Quasiparticles in Condensed Matter Physics (mostly d-wave superconductors)

Outline I.Background II.d-Wave Superconductivity III.Universal Limit Thermal Conductivity (w/ aside on Graphene) IV.Quasiparticle Transport Amidst Coexisting Charge Order V.Quasiparticle Scattering from Vortices VI.Summary

Dirac Fermions Relativistic Fermions (electrons) Massless Relativistic Fermions (neutrinos)

We need only non-relativistic quantum mechanics and electromagnetism But in many important cases, the low energy effective theory is described by Dirac Hamiltonian and Dirac energy spectrum Examples include: Quasiparticles in Cuprate (d-wave) Superconductors Electrons in Graphene etc… Low energy excitations are two-dimentional massless Dirac fermions What does this have to do with Condensed Matter Physics?

High-T c Cuprate Superconductors T x AF dSC

s-Wave Superconductor Fully gapped quasiparticle excitations

d-Wave Superconductor Quasiparticle gap vanishes at four nodal points Quasiparticles behave more like massless relativistic particles than normal electrons

Two Characteristic Velocities Quasiparticle Excitation Spectrum Anisotropic Dirac Cone d-Wave Superconductivity

Disorder-Induced Quasiparticles N(  )  Disorder-Induced Quasiparticles Density of States L. P. Gorkov and P. A. Kalugin, JETP Lett. 41, 253 (1985)

Universal Limit Transport Coefficients Disorder generates quasiparticles Disorder scatters quasiparticles Disorder-independent conductivities P. A. Lee, Phys. Rev. Lett. 71, 1887 (1993) M. J. Graf, S.-K. Yip, J. A. Sauls, and D. Rainer, Phys. Rev. B 53, (1996) A. C. Durst and P. A. Lee, Phys. Rev. B 62, 1270 (2000) Disorder-dependentDisorder-independent

Low Temperature Thermal Conductivity Measurements YBCO: BSCCO: Taillefer and co-workers, Phys. Rev. B 62, 3554 (2000)

Graphene Single-Layer Graphite

Universal Conductivity? Bare Bubble: Novosolov et al, Nature, 438, 197 (2005)  max (h/4e 2 ) 1 0  (cm 2 /Vs) 0 8, , devices Missing Factor of  !!! Can vertex corrections explain this? Shouldn’t crossed (localization) diagrams be important here?

Low Temperature Quasiparticle Transport in a d-Wave Superconductor with Coexisting Charge Density Wave Order (with S. Sachdev (Harvard) and P. Schiff (Stony Brook)) STM from Davis Group, Nature 430, 1001 (2004) Checkerboard Charge Order in Underdoped Cuprates x T underdoped

Hamiltonian for dSC + CDW Current Project: Doubles unit cell Future:

CDW-Induced Nodal Transition K. Park and S. Sachdev, Phys. Rev. B 64, (2001) Nodes survive but approach reduced Brillouin zone boundary Nodes collide with their “ghosts” from 2nd reduced Brillouin zone Nodes are gone and energy spectrum is gapped

Thermal Conductivity Calculation Green’s Function Disorder Heat Current Kubo Formula 4×4 matrix

Analytical Results in the Clean Limit

Beyond Simplifying Approximations Realistic Disorder Vertex Corrections 4×4 matrix Self-energy calculated in presence of dSC+CDW 32 real components in all (at least two seem to be important) Not clear that these can be neglected in presence of charge order Work in Progress with Graduate Student, Philip Schiff

Scattering of Dirac Quasiparticles from Vortices H x 2R Two Length Scales Scattering from Superflow + Aharonov-Bohm Scattering (Berry phase effect) (with A. Vishwanath (UC Berkeley), P. A. Lee (MIT), and M. Kulkarni (Stony Brook))

Model and Approximations Account for neighboring vortices by cutting off superflow at r = R Neglect Berry phase acquired upon circling a vortex - Quasiparticles acquire phase factor of (-1) upon circling a vortex - Only affects trajectories within thermal deBroglie wavelength of core Neglect velocity anisotropy v f = v 2 R R r PsPs

Single Vortex Scattering Momentum Space Coordinate Space

Cross Section Calculation Start with Bogoliubov-deGennes (BdG) equation Extract Berry phase effect from Hamiltonian via gauge choice Shift origin to node center Separate in polar coordinates to obtain coupled radial equations Build incident plane wave and outgoing radial wave Solve inside vortex (r R) to all orders in linearized hamiltonian and first order in curvature terms Match solutions at vortex edge (r = R) to obtain differential cross section Small by k/p F

Contributions to Differential Cross Section from Each of the Nodes

Calculated Thermal Conductivity YBa 2 Cu 3 O Experiment (Ong and co-workers (2001))Calculated

What about the Berry Phase? Should be important for high field (low temperature) regime where deBroglie wavelength is comparable to distance between vortices Over-estimated in single vortex approximation Branch Cut Better to consider double vortex problem Elliptical Coordinates Work in Progress with Graduate Student, Manas Kulkarni

Summary The low energy excitations of the superconducting phase of the cuprate superconductors are interesting beasts – Dirac Quasiparticles Cuprates provide a physical system in which the behavior of these objects can be observed In turn, the study of Dirac quasiparticles provides many insights into the nature of the cuprates (as well as many other condensed matter systems)