Kingshuk Majumdar

Magnetoresistance of the double-tunnel-junction Coulomb blockade with magnetic metals

We have studied the junction magnetoresistance (JMR) and the differential junction magnetoresistance (DJMR) for double tunnel junctions with magnetic metals in the Coulomb blockade regime. Spikes are seen in both the JMR and the DJMR vs voltage curves. They occur at those places where the current increases by a step. In all cases the large bias limit can be obtained by adding the resistances of each of the junctions in series. The JMR is positive in all the cases we studied, whereas the DJMR can be positive or negative as a function of the voltage. Moreover, the relative variation of the DJMR as a function of the voltage is larger than the variation of the JMR with the voltage.

Magnetic phase diagram of a spatially anisotropic, frustrated spin-\frac {1}{2} Heisenberg antiferromagnet on a stacked square lattice

The magnetic phase diagram of a spatially anisotropic, frustrated spin-[Formula: see text] Heisenberg antiferromagnet on a stacked square lattice is investigated using a second-order spin-wave expansion. The effects of interlayer coupling and the spatial anisotropy on the magnetic ordering of two ordered ground states are explicitly studied. It is shown that with increase in next nearest neighbor frustration the second-order corrections play a significant role in stabilizing the magnetization. We obtain two ordered magnetic phases (Néel and stripe) separated by a paramagnetic disordered phase. Within the second-order spin-wave expansion we find that the width of the disordered phase diminishes with increase in the interlayer coupling or with decrease in spatial anisotropy but it does not disappear. Our obtained phase diagram differs significantly from the phase diagram obtained using linear spin-wave theory.

Non-linear spin wave theory results for the frustrated S=\frac {1}{2} Heisenberg antiferromagnet on a body-centered cubic lattice

At zero temperature the sublattice magnetization of the quantum spin- 1/2 Heisenberg antiferromagnet on a body-centered cubic lattice with competing first and second neighbor exchange (J(1) and J(2)) is investigated using the non-linear spin wave theory. The zero temperature phases of the model consist of a two sublattice Néel phase for small J(2) (AF(1)) and a collinear phase at large J(2) (AF(2)). We show that quartic corrections due to spin wave interactions enhance the sublattice magnetization in both the AF(1) and the AF(2) phase. The magnetization corrections are prominent near the classical transition point of the model and in the J(2)>J(1) regime. The ground state energy with quartic interactions is also calculated. It is found that up to quartic corrections the first order phase transition (previously observed in this model) between the AF(1) and the AF(2) phase survives.

Spin-wave energy dispersion of a frustrated spin-\frac {1}{2} Heisenberg antiferromagnet on a stacked square lattice

The effects of interlayer coupling and spatial anisotropy on the spin-wave excitation spectra of a three-dimensional spatially anisotropic, frustrated spin-½ Heisenberg antiferromagnet (HAFM) are investigated for the two ordered phases using second-order spin-wave expansion. We show that the second-order corrections to the spin-wave energies are significant and find that the energy spectra of the three-dimensional HAFM have similar qualitative features to the energy spectra of the two-dimensional HAFM on a square lattice. We also discuss the features that can provide experimental measures for the strength of the interlayer coupling, spatial anisotropy parameter, and magnetic frustration.

Zero Temperature Phases of the Frustrated J1–J2 Antiferromagnetic Spin-1/2 Heisenberg Model on a Simple Cubic Lattice

At zero temperature magnetic phases of the quantum spin-1/2 Heisenberg antiferromagnet on a simple cubic lattice with competing first and second neighbor exchanges (J1 and J2) is investigated using the non-linear spin wave theory. We find existence of two phases: a two sublattice Néel phase for small J2 (AF), and a collinear antiferromagnetic phase at large J2 (CAF). We obtain the sublattice magnetizations and ground state energies for the two phases and find that there exists a first order phase transition from the AF-phase to the CAF-phase at the critical transition point, pc=0.56 or J2/J1=0.28. We also show that the quartic 1/S corrections due spin-wave interactions enhance the sublattice magnetization in both the phases which causes the intermediate paramagnetic phase predicted from linear spin wave theory to disappear.

Logarithmic corrections to finite-size spectrum of SU(N) symmetric quantum chains

We consider SU(N) symmetric one-dimensional quantum chains at finite temperature. For such systems the correlation lengths, ground state energy and excited state energies are investigated in the framework of conformal field theory. The possibility of different types of excited states is discussed. Logarithmic corrections to the ground state energy and different types of excited states in the presence of a marginal operator are calculated. The known results for SU(2) and SU(4) symmetric systems follow from our general formula.We consider SU(N) symmetric one-dimensional quantum chains at finite temperature. For such systems the correlation lengths, ground state energy and excited state energies are investigated in the framework of conformal field theory. The possibility of different types of excited states is discussed. Logarithmic corrections to the ground state energy and different types of excited states in the presence of a marginal operator are calculated. The known results for SU(2) and SU(4) symmetric systems follow from our general formula.

Quantum critical point in a periodic Anderson model

We investigate the symmetric periodic Anderson model (PAM) on a three-dimensional cubic lattice with nearest-neighbor hopping and hybridization matrix elements. Using Gutzwillers variational method and the Hubbard-III approximation (which corresponds to an exact solution of the appropriate Falicov-Kimball model in infinite dimensions) we demonstrate the existence of a quantum critical point at zero temperature. Below a critical value

Von Neumann entropy and on-site localization for perpetually coupled qubits

{V}_{c}

Vortex Deconfinement in the XY Model with a Magnetic Field

of the hybridization (or above a critical interaction

Deconfinement and Phase Diagram of Bosons in a Linear Optical Lattice with a Particle Reservoir

{U}_{c})