Brijesh Kumar

Substantial magnetoelectric coupling near room temperature in Bi2Fe4O9

We report remarkable multiferroic effects in polycrystalline Bi2Fe4O9. High-resolution X-ray diffraction shows that this compound has orthorhombic structure. Magnetic measurements confirm an antiferromagnetic transition around 260 K. A pronounced inverse S-shape anomaly in the loss tangent of dielectric measurement is observed near the Neel temperature. This feature shifts with the application of magnetic field. These anomalies are indicative of substantial coupling between the electric and magnetic orders in this compound.

Superconductivity in CoO 2 layers and the resonating valence bond mean-field theory of the triangular lattice t − J model

Motivated by the recent discovery of superconductivity in two-dimensional

SrCu2(BO3)2: A Unique Mott Hubbard Insulator

CoO_2

Quantum spin models with exact dimer ground states

layers, we present some possibly useful results of the resonating valence bond mean-field theory applied to the triangular lattice. An interesting time reversal breaking superconducting state arises from strongly frustrated interactions. Away from half filling, the order parameter is found to be complex, and yields a fully gapped quasiparticle spectrum. The sign of the hopping plays a crucial role in the analysis, and we find that superconductivity is as fragile for one sign as it is robust for the other.

Bond-operators and triplon analysis for spin-S dimer antiferromagnets

Na_xCoO_2 \cdot yH_2O

Theory of quantum oscillations of magnetization in Kondo insulators

is argued to belong to the robust case, by comparing the local-density approximation fermi surface with an effective tight-binding model. The high-frequency Hall constant in this system is potentially interesting, since it is pointed out to increase linearly with temperature without saturation for T>Tdegeneracy.

Intertwined nematic orders in a frustrated ferromagnet

We discuss the recently discovered system SrCu2(BO3)(2), a realization of an exactly solvable model proposed two decades earlier. We propose its interpretation as a Mott Hubbard insulator. The possible superconducting phase arising from doping is explored, and its nature as well as its importance for testing the RVB theory of superconductivity are discussed.

Quantum Ising dynamics and Majorana-like edge modes in the Rabi lattice model

Inspired by the exact solution of the Majumdar-Ghosh model, a family of one-dimensional, translationally invariant spin Hamiltonians is constructed. The exchange coupling in these models is antiferromagnetic, and decreases linearly with the separation between the spins. The coupling becomes identically zero beyond a certain distance. It is rigorously proved that the dimer configuration is an exact, superstable ground-state configuration of all the members of the family on a periodic chain. The ground state is twofold degenerate, and there exists an energy gap above the ground state. The Majumdar-Ghosh Hamiltonian with a twofold degenerate dimer ground state is just the first member of the family. The scheme of construction is generalized to two and three dimensions, and illustrated with the help of some concrete examples. The first member in two dimensions is the Shastry-Sutherland model. Many of these models have exponentially degenerate, exact dimer ground states.

Level crossings induced by a longitudinal coupling in the transverse field Ising chain

The mean-field triplon analysis is developed for spin-

Exact spin-orbital separation in a solvable model in one dimension

S