Abstract:
We study spin-flip and spin-wave excitations for arbitrarily polarized quantum Hall states by employing a fermionic Chern-Simons gauge theory in the low Zeeman energy limit. We show that the spin-flip correlation functions do not get renormalized by the fluctuations of Chern-Simons gauge field. As a consequence, the excitations for a given integer quantum Hall state are identical to fractional quantum Hall states in the lowest Landau level having the same numerator equal to the integer quantum Hall state. Fully and partially polarized states possess only spin-wave excitations while spin-flip excitations are possible for all states, irrespective of their polarizations.

Abstract:
I propose quasiparticle and quasihole operators which operating on the Laughlin wave functions describing the Laughlin condensates (LCs) at the filling factors $\nu =1/m$ in a specific Hilbert-subspace, {\em generate} composite fermions (CFs) by expelling electrons from the condensate to a different Hilbert-subspace. The condensation of these expelled electrons into $\nu =1/m$ together with the original LC, form a new condensate at $\nu = 2/[2(m- 1) + 1]$. In general, hierarchically constructed states are coupled LCs formed at different Hilbert-subspaces and the corresponding wave functions are {\em identical} with those proposed in the CF theory. This theory further predicts that the half and the quarter filled lowest Landau level are quantum critical points for topological phase transitions.

Abstract:
We show that neutron scattering and Raman scattering experiments can unambiguously determine a composite fermion parameter, viz., the effective number of Landau Levels filled by the composite fermions. For this purpose, one needs partially polarized or more preferably unpolarized quantum Hall states. We further find that spin correlation function acts as an order parameter in the spin transition.

Abstract:
We determine the wave functions for arbitrarily polarized quantum Hall states by employing the doublet model which has been proposed recently to describe arbitrarily polarized quantum Hall states. Our findings recover the well known fully polarized Laughlin wave functions and unpolarized Halperin wave function for the filling fraction $\nu =2/5$. We have also confirmed by an explicit One-loop computation that the Hall conductivity does indeed get quantized at those filling fractions that follow from the model. Finally, we have given a physical picture for the non-analytic nature of the wave functions, and shown that quantum fluctuations restore the Kohn mode.

Abstract:
We propose a global model which accounts for all the observed quantum Hall states in terms of an abelian doublet of Chern-Simons gauge fields, with the strength of the Chern-Simons term given by a coupling matrix. The model is employed within the composite fermion picture.

Abstract:
We determine wave number $q$ and frequency $\omega$ dependent spin Hall conductivity $\sigma_{yx}^s(q, \omega)$ for a disordered two dimensional electron system with Rashba spin orbit interaction when $\q$ is {\it transverse} to the electric field. Both the conventional definition of spin current and its new definition which takes care of the conservation of spins, have been considered. The spin Hall conductivitivities for both of these definitions are qualitatively similar. $\sigma_{yx}^s(q, \omega)$ is zero at $q=0, \omega =0$ and is maximum at $q=0$ and at small but finite $\omega$ whose value depends on different parameters of the system. Interestingly for $\omega \to 0$, $\sigma_{yx}^s(q)$ resonates when $\Lambda \simeq L_{so}$ which are the wavelength $(\Lambda = 2\pi/q)$ of the electric field's spatial variation and the length for one cycle of spin precession respectively. The sign of the out-of-plane component of the electrons' spin flips when the sign of electric field changes due to its spatial variation along transverse direction. It changes the mode of spin precession from clockwise to anti-clockwise or {\it vice versa} and consequently a finite spin Hall current flows in the bulk of the system.

Abstract:
We have determined the off-diagonal and diagonal conductivities for a quantum Hall effect system at exactly integer filling at finite temperatures and in the presence of weak short ranged disorder potential within the self consistent Born approximation. We find that there is a finite temperature contribution to off-diagonal conductivity $\sigma_{xy}$ which is `anomalous' in nature as it survives even in the zero impurity limit. The diagonal conductivity $\sigma_{xx}$ survives only when both temperature and disorder is non zero. At low temperatures, $\sigma_{xx}$ activates with a temperature dependent prefactor. Inverting the conductivity matrix, we determine the resistivities. The deviation of the off-diagonal resistivity $\rho_{xy}$ from its zero temperature value and the diagonal resistivity $\rho_{xx}$ activate with a temperature dependent prefactor at low temperatures, in agreement with experiments. Further, we find two physical regimes both of which are at low temperatures and low broadening, which provide the experimentally observed linear relationship between the deviation of $\rho_{xy}$ and the $\rho_{xx}$ with different signs. We have also estimated the effective masses from the experimental data of $\rho_{xy}$ and find them to be reasonable. Finally, our result on compressibility as a function of temperature shows that there is no phase transition involved in the system as far as the temperature is concerned.

Abstract:
We derive an effective non-linear sigma model for quantum hall systems with arbitrary polarizations, by employing the recently proposed doublet model. We study the topological excitations, in particular, the skyrmions, as a function of the filling fraction as well as the polarization. We determine the relationship between the topological charge density and the electronic charge density, and the statistics of skyrmions. We also estimate the value of spin stiffness by using the dispersion relations that we have obtained recently by employing the time dependent Hartree-Fock approximation for the doublet model. Finally, we point out how the skyrmionic excitations reveal information directly on the number of flux tubes that get attached to the electrons in order to form composite fermions.

Abstract:
We have studied the integer quantum Hall effect at finite temperatures by diagonalizing a single body tight binding model Hamiltonian including Aharonov-Bohm phase. We have studied the energy dependence of the specific heat and the Hall conductivity at a given temperature. The specific heat shows a sharp peak between two consecutive Hall plateaus. At very low temperatures, the numerical results of the temperature variations of specific heat (in the plateau region) are in good agreement with the analytical results.

Abstract:
It is known that the anomalous Hall conductivity (AHC) in a disordered two dimensional electron system with Rashba spin-orbit interaction and finite ferromagnetic spin-exchange energy is zero in the metallic weak-scattering regime because of the exact cancellation of the bare-bubble contribution by the vertex correction. We study the effect of inhomogeneous longitudinal electric field on the AHC in such a system. We predict that AHC increases from zero (at zero wavenumber), forms a peak, and then decreases as the wavenumber for the variation of electric field increases. The peak-value of AHC is as high as the bare-buble contribution. We find that the wave number, q, at which the peaks occur is the inverse of the geometric mean of the mean free path of an electron and the spin-exchange length scale. Although the Rashba energy is responsible for the peak-value of AHC, the peak position is independent of it.