News and Events

Program:

Abstracts of the Talks:

Infinite Randomness and "Quantum" Griffiths Effects in a Classical System: the Randomly Layered Magnet

Rajesh NARAYANAN (Indian Institute of Technology, Madras)

The phase transition in a three dimensional Heisenberg magnet with planar defects is studied. By laying recourse to a strong disorder renormalization group approach we show the critical behavior in these models is controlled by an infinite disorder type fixed point. The infinite disorder fixed point is accompanied with power-law Griffiths type singularities. We also compute various thermodynamic observables, laying particular emphasis on finite size effects which are crucial for the experimental verification of our theory.

Quantum Hall Effects in a Weyl Semimetal: Possible Application in Pyrochlore Iridates

Kaiyu YANG (Boston College)

There has been much interest in pyrochlore iridates A₂Ir₂O₇ where both strong spin-orbital coupling and strong correlation are present. This system is likely in a three-dimensional topological semimetallic phase: a Weyl semimetal. In this talk, I will discuss two quantum effects of this system in a magnetic feld: a pressure-induced anomalous Hall effect and a magnetic-feld-induced charge density wave at the pinned wave vector connecting Weyl nodes with opposite chiralities. The existence of 12 pairs of Dirac points enhanced the proposed effects substantially. Those effects can be used to detect the Weyl semimetal phase in A₂Ir₂O₇.

Numerical studies of novel quantum phase and transport phenomena in 2D interacting and disordered systems

Hao WANG (HKU), in collaboration with D. N. Sheng (California State University Northridge) and F. D. M. Haldane (Princeton University)

Using exact diagonalization method with torus geometry, we numerically study quantum phases in different 2D electron systems. For the Dirac fermions in Graphene system with partially filled N=3 Landau level (LL), our results show that at half-filling, the equal-time density-density correlation function displays sharp peaks at nonzero wavevectors q*. Finite-size scaling reveals that the peak value grows with electron number and diverges in thermodynamic limit, suggesting an instability towards a unidirectional charge density wave. This symmetry-broken stripe phase is found robust against perturbation from disorder scattering, indicating experimentally observable through transport measurements. Associated with the special wavefunctions of the Dirac LL in Graphene, both stripe and bubble phases become possible candidates for the ground state of the Dirac fermions with lower filling factors in the N=3 LL. We also study the conventional 2D electron system with 5/2 fractional quantum hall effect. A model Hamiltonian with the additional three-body (3b) interaction has been investigated. The 3b interaction plays a role in breaking the particle-hole (PH) symmetry of the system and induces a phase transition of the ground state (GS) towards a Pfaffian (Pf) state or its PH conjugate (APf) state depending on the sign of the three-body interaction. The results of the low energy spectrum, the wave function overlap, and the PH parity evolution, have shown strong evidence of the existence of a first order phase transition between the Pf and the APf, with the pure Coulomb system sitting at the critical point of the transition. Modulated by an extra short-range pseudopotential, the above induced Pf or APf system can transfer to the nearby compressible phases with the stripe order or the composite-fermion-liquid (CFL) state.

Effects of the Vortices and Impurities on the Nuclear Spin Relaxation Rate in Iron-based Superconductors

Hong-min JIANG (HKU)

The effects of magnetic vortices and nonmagnetic impurities on the low energy quasiparticle excitations and the spin-lattice relaxation rate are examined in the iron-based superconductors for the s_±-, s- and d -wave pairing symmetries, respectively. The main effect of the vortices is to enhance the quasiparticle excitations and the spin-lattice relaxation rate for all symmetries, and leads to a T³ dependence of the relaxation rate followed by a nearly T-linearity at lower temperatures. This enhancement can only be seen for the s_±- and d-wave symmetries in the presence of nonmagnetic impurities. These results suggest that the s_±-wave and d-wave pairing states behave similarly in response to the magnetic field and nonmagnetic impurities, therefore it may be impossible to distinguish them on the basis of the measurements of spin-lattice relaxation rates when a magnetic field and/or impurity scatterings are present.