Compressible Quantum Research Articles

Stability of the Compressible Quantum Hall State around the Half-Filled Landau Level

We study the compressible states in the quantum Hall system using a mean field theory on the von Neumann lattice. In the lowest Landau level, a kinetic energy is generated dynamically from Coulomb interaction. The compressibility of the state is calculated as a function of the filling factor $\nu$ and the width $d$ of the spacer between the charge carrier layer and dopants. The compressibility becomes negative below a critical value of $d$ and the state becomes unstable at $\nu=1/2$. Within a finite range around $\nu=1/2$, the stable compressible state exists above the critical value of $d$.

Bulk and edge correlations in the compressible half-filled quantum Hall state

We study bulk and edge correlations in the compressible half-filled state, using a modified version of the plasma analogy. The corresponding plasma has anomalously weak screening properties, and as a consequence we find that the correlations along the edge do not decay algebraically as in the Laughlin (incompressible) case, while the bulk correlations decay in the same way. The results suggest that due to the strong coupling between charged modes on the edge and the neutral Fermions in the bulk, reflected by the weak screening in the plasma analogue, the (attractive) correlation hole is not well defined on the edge. Hence, the system there can be modeled as a free Fermi gas of {\em electrons} (with an appropriate boundary condition). We finally comment on a possible scenario, in which the Laughlin-like dynamical edge correlations may nevertheless be realized.

Optical intersubband transitions in strained quantum wells utilizing In1−x GaxAs/InP solid solutions

Infrared absorption in strained p-type In1−xGaxAs/InP quantum wells is investigated for both possible types of strain (tensile and compressive). It is observed that the normalincidence absorption increases considerably under compressive strain (when the ground state is a heavy-hole state) and decreases under tensile strain (when the ground state is a light-hole state). The peak absorption in the compressed quantum well can attain very large values, on the order of 5000 cm−1 at a hole density ∼ 1012 cm−2; this attribute makes “compressed” p-type quantum wells attractive for IR detection applications.

Tunneling into the Edge of a Compressible Quantum Hall State

We present a composite-fermion theory of tunneling into the edge of a compressible quantum Hall system. The tunneling conductance is non-Ohmic, due to slow relaxation of electromagnetic and Chern-Simons field disturbances caused by the tunneling electron. Universal results are obtained in the limit of a large number of channels involved in the relaxation. The tunneling exponent is found to be a continuous function of the Hall resistivity ${\ensuremath{\rho}}_{\mathrm{xy}}$, with a slope that is discontinuous at filling factor $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1/2$, in the limit of vanishing bulk resistivity ${\ensuremath{\rho}}_{\mathrm{xx}}$. When $\ensuremath{\nu}$ corresponds to a principal fractional quantized Hall state, our results agree with the chiral Luttinger liquid theories of Wen and Kane, Fisher, and Polchinski.

Study of 1.3-μm tapered waveguide spotsize transformers

Laterally tapered waveguide structures have been fabricated using a novel, though simple fabrication technique to achieve spot-size transformation. The effect of mesa shape on the far-field pattern and the coupling efficiency to a single-mode fiber (SMF) is reported. A tapered active region having both tensile and compressive quantum wells has been used and dual polarization characteristics are observed for the first time. An experimental analysis of the waveguide losses due to the taper has also been shown.

Diffusion thermopower at even-denominator fractions.

We compute the electron diffusion thermopower at compressible Quantum Hall states corresponding to even denominator fractions in the framework of the composite fermion approach. It is shown that the deviation from the linear low temperature behavior of the termopower is dominated by the logarithmic temperature corrections to the conductivity and not to the thermoelectric coefficient, although such terms are present in both quantities. The enhanced magnitude of this effect compared to the zero field case may allow its observation with the existing experimental techniques.

Wavelength conversion in a quantum well polarization insensitive amplifier

Polarization insensitivity of the gain saturation properties of a 1.3 μm semiconductor amplifier with tensile and compressive strain quantum well active region are used to demonstrate a WDM wavelength conversion configuration in which the outputs at the two wavelengths are separated on the basis of polarization. Use of orthogonal polarizations allows operation of a wavelength converter without an optical frequency filter at the output. Alternatively, inherent polarization insensitivity of the device can be used for wavelength conversion of an arbitrarily polarized signal.

Gain saturation properties of a polarization insensitive semiconductor amplifier implemented with tensile and compressive strain quantum wells

The gain saturation properties of a 1.3 /spl mu/m polarization insensitive semiconductor amplifier implemented with tensile and compressive strain quantum well active region are experimentally investigated in order to determine how well the amplifier maintains its polarization insensitivity in the saturation regime. The amplifier has unsaturated gain of 12 dB and in the saturation regime the maximum observed gain imbalance between TE and TM gains is 0.9 dB. The measured 3 dB saturation output power is 5 mW. >

Gain saturation properties of a semiconductor gain medium with tensile and compressive strain quantum wells

Gain saturation properties of a multiple-quantum-well structure with both tensile and compressively strained quantum wells are investigated analytically. This type of structure has recently been experimentally demonstrated to serve as a basis for the implementation of a two-polarization/two-frequency laser and polarization insensitive travelling wave(TW) amplifier. The performance of these devices strongly depends on the interaction between the TE and TM gains of the structure. The gain medium model appropriate for this type of structure is developed and the rate equation approach is used to describe the saturation properties of TE/TM gains and the coupling between the TE and TM gains due to gain saturation. The minimum amount of coupling between the two is governed by the basic symmetry of the light-hole wavefunction which interacts with photons of both polarization: photon cross-coupling. The finite rate of carrier escape from the quantum wells provides for carrier induced coupling between the populations of the two well types and therefore also couples TE and TM gains: carrier cross-coupling. The performance of a polarization insensitive amplifier, laser, and polarization control element is evaluated as a function of the amount of carrier cross-coupling, which is a structure dependent parameter. A structure with high degree of cross-coupling is desirable for polarization insensitive TW amplifier, while two-polarization lasers and polarization control elements require minimum cross-coupling.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Lateral thickness modulations in alternate tensile—compressive strained GaInAsP multilayers grown by gas source molecular beam epitaxy

Compressive GaInAsP multiple quantum wells (MQWs) grown on (001)InP have demonstrated high potential for the production of lasers emitting at 1.5 μm. A GaInAsP tensile barrier is now used instead of a lattice matched barrier to compensate for the compressive strain. However, zero-net strained MWQs grown at 500 °C by gas source molecular beam epitaxy show a decrease in structural and optical quality. A structural investigation of two MQWs using a −0.5% mismatched barrier was performed using transmission electron microscopy to understand why using a tensile barrier induces such degradation. All the samples studied show a lateral quasi-periodic thickness modulation which is strongly anisotropic with a periodicity of about 50 nm along the [110] direction, while no modulation occurs along the [110] direction. Since the growth is easier on relaxed areas than on strained areas, we deduce that the thicker parts of the layer are more relaxed than the thinner parts. Moreover, the average period of 50 nm is found to be independent of both the barrier thickness and the mismatch of the compressive well, while the amplitude of the modulation is drastically enhanced when the barrier thickness increases. This confirms that the phenomenon is controlled by the partial elastic relaxation of tensile layers.

Lateral modulations in zero-net-strained GaInAsP multilayers grown by gas source molecular-beam epitaxy

Compressive GaInAsP multiple quantum wells (MQW) grown by gas source molecular-beam epitaxy present altered structural and optical characteristics when tensile GaInAsP barriers are used instead of lattice-matched ones. An alternate tensile/compressive GaInAsP MQW has been examined by transmission electron microscopy. A strong lateral modulation of thickness, strain, and probably chemical composition was shown. This modulation exhibits pronounced anisotropy, with a periodicity of about 50 nm along the [110] direction. Although its origin is not fully accounted for yet, it seems to allow partial elastic relaxation of tensile layers. Based on this analysis, a schematic description of distortion modulation is proposed.

Very low threshold current density 1.5 μm GaInAs/AlGaInAs graded-index separate-confinement-heterostructure strained quantum well laser diodes grown by organometallic chemical vapor deposition

Very low threshold current densities of 200 and 400 A/cm2 were obtained in 1.5 μm GaInAs/AlGaInAs tensile and compressive strained-layer quantum well laser diodes (SL-QW LDs), grown by organometallic chemical vapor deposition, with continuously graded-index separate-confinement-heterostructure. The differential quantum efficiency of SL-QW LDs showed less sensitive to temperature in contrast to that of a lattice matched QW LD. This is attributed to the decrease of intervalence band absorption due to the strain-induced reduction in the valence band density of state. The polarization of output power for a tensile SL-QW LD showed transverse magnetic (TM) mode, while that for a lattice matched and a compressive SL-QW LDs showed transverse electric (TE) mode.

Very low threshold 1.5 μm GaInAs/AlGaInAs BH GRINSCH strained-layer quantum well laser diodes grown by MOCVD

A very low threshold current density of 400 A/cm2 was obtained in a 1.5 μm GaInAs/AlGaInAs compressive strained-layer quantum well laser diode, grown by metal organic chemical vapour deposition (MOCVD), with continuously graded-index separate-confinement-heterostructure. A very low threshold current of 3.6 mA was achieved in a high reflective coated 130μm long buried heterostructure laser diode with MOCVD grown semi-insulating current blocking layer. The lasing wavelength was 1.48 μm.

Compressibility and superfluidity in the fractional-statistics liquid.

We study the low-temperature properties of a fractional-statistics liquid. Using a hydrodynamic approach based on an extended mean-field approximation, we show that the excitation is gapless, in agreement with a random-phase calculation of Fetter, Hanna, and Laughlin [Phys. Rev. B 39, 9679 (1989)]. A compressible quantum fluid with only a phonon as low-lying excitation is shown to be a superfluid with a ``Meissner effect.'' We develop a two-fluid model and calculate some properties of the superfluid phase at finite temperatures. These properties are consistent with high-temperature superconductivity. We also discuss experimental signals of time reversal and parity breaking in the superfluid state.