arXiv subjects

Ming-Che Chang

Publications and source records attributed to Ming-Che Chang.

22 records · Page 2Linked to original sources

arXiv1999

Finite-temperature phase transitions in $ν=2$ bilayer quantum Hall systems

In this paper, the influence of an in-plane magnetic field B_\parallel on the finite-temperature phase transitions in nu=2 bilayer quantum Hall systems are examined. It is found that there can exist two types of finite-temperature phase transitions. The first is the Kosterlitz-Thouless (KT) transitions, which can have an unusual non-monotonic dependence on B_\parallel; the second type originates from the crossing of energy levels and always increases with B_\parallel. Based on these results, we point out that the threshold temperature observed in the inelastic light scattering experiments cannot be the KT transition temperature, because the latter shows a totally different B_\parallel-dependence as compared with the experimental observation. Instead, it should be the level-crossing temperature, which we found agrees with the B_\parallel-dependence observed. Moreover, combining the knowledge of these two transition temperatures, a complete finite-temperature phase diagram is presented.

cond-mat.mes-hall↗

arXiv1999

Effect of in-plane magnetic field on magnetic phase transitions in nu=2 bilayer quantum Hall systems

By using the effective bosonic spin theory, which is recently proposed by Demler and Das Sarma [ Phys. Rev. Lett. 82, 3895 (1999) ], we analyze the effect of an external in-plane magnetic field on the magnetic phase transitions of the bilayer quantum Hall system at filling factor nu=2. It is found that the quantum phase diagram is modified by the in-plane magnetic field. Therefore, quantum phase transitions can be induced simply by tilting the magnetic field. The general behavior of the critical tilted angle for different layer separations and interlayer tunneling amplitudes is shown. We find that the critical tilted angles being calculated agree very well with the reported values. Moreover, a universal critical exponent for the transition from the canted antiferromagnetic phase to the ferromagnetic phase is found to be equal to 1/2 within the present effective theory.

cond-mat.mes-hall↗

arXiv1997

Multi-band energy spectra of spin-1/2 electrons with two-dimensional magnetic modulations

The energy spectra of spin-1/2 electrons under two-dimensional magnetic field modulations are calculated beyond the one-band approximation. Our formulation is generally applicable to a modulation field with a rectangular lattice symmetry. The field distribution within a plaquette is otherwise arbitrary. The spectra being obtained are qualitatively different from their electric modulated counterparts. Peculiar features of the spectra are that, for an electron with a g factor precisely being equal to two, no matter how strong the modulation is, the zero-energy level seems to be unaffected by the modulation and is separated from higher energy levels with a nonzero energy gap. Moreover, there is a two-fold degenerancy for all states with positive energies with respect to spin flip. These features agree with earlier analytical studies of the periodically magnetic modulated systems.

cond-mat.mes-hall↗

arXiv1997

Resonant Optical Nonlinearity of Conjugated Polymers

When the energy of a pump wave is in resonance with the exciton creation energy, the electric susceptibility of a conjugated polymer in response to the probe wave is altered by the exciton gas. In this paper, we calculate the dependence of this change on the the exciton populations by the equation of motion (EOM) method. The magnitude of optical nonlinearity is also influenced by ambient temperature, by the extent of exciton wave functions, and by the strength of electron-electron interaction. All of these factors can be easily incorporated in the EOM approach systematically. Using the material parameters for polydiacetylene (PDA), the optical Kerr coefficient $n_2$ obtained is about $10^{-8} cm^2/W$, which is close to experimental value, and is four orders of magnitude larger than the value in nonresonant pump experiments.

cond-mat↗