V. Ravi Chandra

Classical dipoles on the kagome lattice

Motivated by recent developments in magnetic materials, frustrated nanoarrays and cold atomic systems, we investigate the behaviour of dipolar spins on the frustrated two-dimensional kagome lattice. By combining the Luttinger-Tisza approach, numerical energy minimization, spin-wave analysis and parallel tempering Monte-Carlo, we study long-range ordering and finite-temperature phase transitions for a Hamiltonian containing both dipolar and nearest-neighbor interactions. For both weak and moderate dipolar interactions, the system enters a three-sublattice long-range ordered state, with each triangle having vanishing dipole and quadrupole moments; while for dominating dipolar interactions we uncover ferrimagnetic three-sublattice order. These are also the ground states for XY spins. We discuss excitations of, as well as phase transitions into, these states. We find behaviour consistent with Ising criticality for the 120-degree state, while the ferrimagnetic state appears to be associated with drifting exponents. The celebrated flat band of zero-energy excitations of the kagome nearest-neighbour Heisenberg model is lifted to finite energies but acquires only minimal dispersion as dipolar interactions are added.

p6 - Chiral Resonating Valence Bonds in the Kagome Antiferromagnet

The Kagome Heisenberg antiferromagnet is mapped onto an effective Hamiltonian on the star superlattice by Contractor Renormalization. Comparison of ground state energies on large lattices to Density Matrix Renormalization Group justifies truncation of effective interactions at range 3. Within our accuracy, magnetic and translational symmetries are not broken (i.e. a spin liquid ground state). However, we discover doublet spectral degeneracies which signal the onset of p6 - chirality symmetry breaking. This is understood by simple mean field analysis. Experimentally, the p6 chiral order parameter should split the optical phonons degeneracy near the zone center. Addition of weak next to nearest neighbor coupling is discussed.

Renormalization group study of the Kondo problem at a junction of several Luttinger wires

We study a system consisting of a junction of N wires, where the junction is characterized by a scalar S-matrix, and an impurity spin is coupled to the electrons near the junction. The wires are modeled as weakly interacting Tomonaga-Luttinger liquids. We derive the renormalization group RG equations for the Kondo couplings of the spin to the electrons on different wires. We analyze the RG flows and fixed points for different values of the initial Kondo couplings and of the junction S-matrix, such as the decoupled S-matrix and the Griffiths connected S-matrix. We find that the Kondo couplings flow either towards a ferromagnetic FM fixed point or towards large and antiferromagnetic AFM values in one of two ways. For the Griffiths S-matrix, one of the strong coupling flows is towards a FM fixed point with decoupled wires; this is seen by a perturbative analysis. Thus if we start with a system of connected wires with an AFM coupling to the spin impurity, the flow at large distances is towards a system of disconnected wires at the FM fixed point. For the decoupled S-matrix, the flow is either to a FM fixed point or to one of two strong coupling fixed points in which all the channels are strongly coupled to each other through the impurity spin. Strong interactions between the electrons with

Magnetic properties of a helical spin chain with alternating isotropic and anisotropic spins: Magnetization plateaus and finite entropy

K_\rho< N/(N+2)

Exact magnetization plateaus and phase transitions in spin- S Heisenberg antiferromagnets in arbitrary dimensions

can stabilize a multichannel fixed point in which the coupling between different channels goes to zero. We also study the temperature dependence of the conductance at the decoupled and Griffiths S-matrices.

A multi-channel fixed point for a Kondo spin coupled to a junction of Luttinger liquids

We propose a model which could explain some of the unusual magnetic properties observed for the onedimensional helical spin system

Frustrated spin ladder with alternating spin-1 and spin-1/2 rungs

Co(hfac)_2NITPhOMe.

Entanglement and level crossings in frustrated ferromagnetic rings

One of these properties is that the magnetization shows some plateaus if a magnetic field is applied along the helical axis. The system consists of cobalt ions (which have easy axes which are tilted at an angle

Gapless points of dimerized quantum spin chains: Analytical and numerical studies

\theta_i

Antiferromagnetic sawtooth chain with spin-1/2 and spin-1 sites

with respect to the helical axis) and organic radicals alternating with each other. We consider a model in which the tilt angles