Composite Fermions in Negative Effective Magnetic Field: A Monte-Carlo Study

The method of Jain and Kamilla [PRB {\bf 55}, R4895 (1997)] allows numerical generation of composite fermion trial wavefunctions for large numbers of electrons in high magnetic fields at filling fractions of the form nu=p/(2mp+1) with m and p positive integers. In the current paper we generalize this method to the case where the composite fermions are in an effective (mean) field with opposite sign from the actual physical field, i.e. when p is negative. We examine both the ground state energies and the low energy neutral excitation spectra of these states. Using particle-hole symmetry we can confirm the correctness of our method by comparing results for the series m=1 with p>0 (previously calculated by others) to our results for the conjugate series m=1 with p <0. Finally, we present similar results for ground state energies and low energy neutral excitations for the states with m=2 and p <0 which were not previously addressable, comparing our results to the m=1 case and the p > 0, m=2 cases.Comment: 11 page

High-speed direct-modulation of InP microdisk lasers

We demonstrate for the first time high-speed direct-modulation of InP microdisk lasers by exploiting longitudinal mode competition. High-speed operation is demonstrated by means of S21 and PRBS modulation. We show open eye diagrams and bit-error rates up to 10 Gb/s

Excitation gaps in fractional quantum Hall states: An exact diagonalization study

We compute energy gaps for spin-polarized fractional quantum Hall states in the lowest Landau level at filling fractions nu=1/3, 2/5,3/7 and 4/9 using exact diagonalization of systems with up to 16 particles and extrapolation to the infinite system-size limit. The gaps calculated for a pure Coulomb interaction and ignoring finite width effects, disorder and LL mixing agree with predictions of composite fermion theory provided the logarithmic corrections to the effective mass are included. This is in contrast with previous estimates, which, as we show, overestimated the gaps at nu=2/5 and 3/7 by around 15%. We also study the reduction of the gaps as a result of the non-zero width of the 2D layer. We show that these effects are accurately accounted for using either Gaussian or z*Gaussian' (zG) trial wavefunctions, which we show are significantly better variational wavefunctions than the Fang-Howard wavefunction. For quantum well parameters typical of heterostructure samples, we find gap reductions of around 20%. The experimental gaps, after accounting heuristically for disorder,are still around 40% smaller than the computed gaps. However, for the case of tetracene layers inmetal-insulator-semiconductor (MIS) devices we find that the measured activation gaps are close to those we compute. We discuss possible reasons why the difference between computed and measured activation gaps is larger in GaAs heterostructures than in MIS devices. Finally, we present new calculations using systems with up to 18 electrons of the gap at nu=5/2 including width corrections.Comment: 18 pages, 17 figure

Interaction dependence of composite fermion effective masses

We estimate the composite fermion effective mass for a general two particle potential r^{-\alpha} using exact diagonalization for polarized electrons in the lowest Landau level on a sphere. Our data for the ground state energy at filling fraction \nu=1/2 as well as estimates of the excitation gap at \nu=1/3, 2/5 and 3/7 show that m_eff \sim \alpha^{-1}.Comment: 4 pages, RevTeX, 5 figure

Microfabricated Chemical Analysis Systems for Environmental Applications

Recent contributions to the design, development, and fabrication of microtechnological devices for chemical analysis are summarized. The discussion includes microdisk-electrode arrays for voltammetric analysis of trace metals, and micro total-analysis systems for coulometric nanotitrations of different analytes

Dynamical Correlations in a Half-Filled Landau Level

We formulate a self-consistent field theory for the Chern-Simons fermions to study the dynamical response function of the quantum Hall system at $\nu=1/2$. Our scheme includes the effect of correlations beyond the random-phase approximation (RPA) employed to this date for this system. The resulting zero-frequency density response function vanishes as the square of the wave vector in the long-wavelength limit. The longitudinal conductivity calculated in this scheme shows linear dependence on the wave vector, like the experimentals results and the RPA, but the absolute values are higher than the experimental results.Comment: 4 pages, revtex, 3 figures included. Corrected typo

Possible composite-fermion liquid as a crossover from Wigner crystal to bubble phase in higher Landau level

The ground state cohesive energies per electron of the composite fermion (CF) Fermi sea, the Laughlin state and the charge density wave (CDW) at higher Landau levels (LLs) are computed. It is shown that whereas for $n\geq 2$ LL, the CDW state is generally more energetically preferable than those of the CF liquid and the Laughlin liquid, the $\nu =4+1/6$ CF liquid state unexpectedly has lower ground state energy than that of the CDW state. We suggest this CF liquid between the Wigner crystal and the bubble phase may lead to the crossover from the normal integer quantum Hall liquid to the novel re-entrant integer quantum Hall state observed in the recent magneto-transport experiments

A Theory of Ferroelectric Phase Transition in SrTiO3_3 induced by Isotope Replacement

A theory to describe the dielectric anomalies and the ferroelectric phase transition induced by oxygen isotope replacement in SrTiO$_3$ is developed. The proposed model gives consistent explanation between apparently contradictory experimental results on macroscopic dielectric measurements versus microscopic lattice dynamical measurements by neutron scattering studies. The essential feature is described by a 3-state quantum order-disorder system characterizing the degenerated excited states in addition to the ground state of TiO$_6$ cluster. The effect of isotope replacement is taken into account through the tunneling frequency between the excited states. The dielectric properties are analyzed by the mean field approximation (MFA), which gives qualitative agreements with experimental results throughout full range of the isotope concentration.The phase diagram in the temperature-tunneling frequencycoordinate is studied by a QMC method to confirm the qualitative validity of the MFA analysis.Comment: 26 pages, 8 figure

Fermi-sea-like correlations in a partially filled Landau level

The pair distribution function and the static structure factor are computed for composite fermions. Clear and robust evidence for a $2k_F$ structure is seen in a range of filling factors in the vicinity of the half-filled Landau level. Surprisingly, it is found that filled Landau levels of composite fermions, i.e. incompressible FQHE states, bear a stronger resemblance to a Fermi sea than do filled Landau levels of electrons.Comment: 23 pages, revte