D. C. Tsui

Fractional quantum Hall effect of composite fermions.

In a GaAs/AlGaAs quantum well of density 1 x 10(11) cm(-2) we observed a fractional quantum Hall effect (FQHE) at nu = 4/11 and 5/13, and weaker states at nu = 6/17, 4/13, 5/17, and 7/11. These sequences of fractions do not fit into the standard series of integral quantum Hall effects of composite fermions (CF) at nu = p/(2mp +/- 1). They rather can be regarded as the FQHE of CFs attesting to residual interactions between these composite particles. In tilted magnetic fields the nu = 4/11 state remains unchanged, strongly suggesting it to be spin polarized. The weak nu = 7/11 state vanishes quickly with tilt.

Far‐infrared emission spectroscopy of hot two‐dimensional plasmons in Al0.3Ga0.7As/GaAs heterojunctions

We have investigated the radiative decay of hot two‐dimensional (2D) plasmons in Al0.3Ga0.7As/ GaAs heterostructures by far‐infrared emission spectroscopy and determined the spectral line shape of the radiation. Narrowband plasmon emission lines are obtained in the terahertz regime. The experimentally observed energies of plasmon emission are in good agreement with the results of a recently developed full grating theory. The plasmon emission intensity is found to increase with increasing input electrical power to the electron system and follows the Bose–Einstein distribution law characterized by the electron temperature of the hot 2D electron system. This fact indicates that 2D plasmons are thermally excited in the present experimental regime.

Low-frequency noise in transport through quantum pOint contacts

We report the noise characteristics of quantum point contacts between 100 Hz and 100 kHz at 4.2 K. The noise consists of a 1/f component on top of a white background. The 1/f noise increases as the contact width decreases and shows peaks between the quantized resistance plateaus. The white noise background increases with current but is much lower than the full shot noise level, suggesting that shot noise is not generated in an ideal quantum point contact, where the electrons do not suffer backscattering as they enter and traverse the contact.

Magneto‐optics of a quasi‐zero‐dimensional electron gas

We have investigated theoretically and experimentally the energy levels and allowed optical transitions with energies ΔE of a quasi‐zero‐dimensional (Q0D) electron gas in a magnetic field B. Using a two‐dimensional harmonic confining potential with oscillator frequency ω0, the theory predicts two values for ΔE. The resonance position in the magnetotransmission spectra from the Q0D system realized on a grid‐gate GaAs/AlGaAs heterostructure, depends strongly on the 2D confining potential induced by the gate voltage Vg and, when Vg=−0.5 and −1.5 V, agrees with ΔE calculated with ℏω0=1.5 and 2.8 meV, respectively.

A different view of the quantum Hall plateau-to-plateau transitions

We demonstrate experimentally that the transitions between adjacent integer quantum Hall (QH) states are equivalent to a QH-to-insulator transition occurring in the top Landau level, in the presence of an inert background of the other completely filled Landau levels, each contributing a single unit of quantum conductance, e 2 /h, to the total Hall conductance of the system.

Experimental evidence for a two-dimensional quantized Hall insulator

Quite generally, an insulator is theoretically defined by a vanishing conductivity tensor at the absolute zero of temperature. In classical insulators, such as band insulators, vanishing conductivities lead to diverging resistivities. In other insulators, in particular when a high magnetic field (B) is added, it is possible that while the magneto-resistance diverges, the Hall resistance remains finite, which is known as a Hall insulator. In this letter we demonstrate experimentally the existence of another, more exotic, insulator. This insulator, which terminates the quantum Hall effect series in a two-dimensional electron system, is characterized by a Hall resistance which is approximately quantized in the quantum unit of resistance h/e^2. This insulator is termed a quantized Hall insulator. In addition we show that for the same sample, the insulating state preceding the QHE series, at low-B, is of the HI kind.The general theoretical definition of an insulator is a material in which the conductivity vanishes at the absolute zero of temperature. In classical insulators, such as materials with a band gap, vanishing conductivities lead to diverging resistivities. But other insulators can show more complex behaviour, particularly in the presence of a high magnetic field, where different components of the resistivity tensor can display different behaviours: the magnetoresistance diverges as the temperature approaches absolute zero, but the transverse (Hall) resistance remains finite. Such a system is known as a Hall insulator. Here we report experimental evidence for a quantized Hall insulator in a two-dimensional electron system—confined in a semiconductor quantum well. The Hall resistance is quantized in the quantum unit of resistance h/e2, where h is Plancks constant and e the electronic charge. At low fields, the sample reverts to being a normal Hall insulator.

Evidence for Charge-Flux Duality near the Quantum Hall Liquid-to-Insulator Transition

A remarkable symmetry has been observed between the diagonal, nonlinear, current-voltage (I-Vxx) characteristics taken in the fractional quantum Hall effect (FQHE) liquid state of the two-dimensional electron system and those taken in the bordering insulating phase. When properly selected, the I-Vxx traces in the FQHE regime are identical, within experimental errors, to Vxx-I traces in the insulator, that is, with the roles of the currents and voltages exchanged. These results can be interpreted as evidence for the existence of charge-flux duality symmetry in the system.

The Quantized Hall Effect

Quantization of the Hall effect is one of the most surprising discoveries in recent experimental solid-state research. At low temperatures and high magnetic fields the ratio of the Hall voltage to the electric current in a two-dimensional system is quantized in units of h/e2, where h is Plancks constant and e is the electronic charge. Concomitantly, the electrical resistance of the specimen drops to values far below the resistances of the best normal metals.

Semicircle: An exact relation in the integer and fractional quantum Hall effect

We present experimental results on the quantized Hall insulator in two dimensions. This insulator, with vanishing conductivities, is characterized by the quantization (within experimental accuracy) of the Hall resistance in units of the quantum unit of resistance, h/e2. The measurements were performed in a two-dimensional hole system, confined in a Ge/SiGe quantum well, when the magnetic field is increased above the ν = 1 quantum Hall state. This quantization leads to a nearly perfect semicircle relation for the diagonal and Hall conductivities. Similar results are obtained with a higher-mobility n-type modulation-doped GaAs/AlGaAs sample, when the magnetic field is increased above the ν = 1/3 fractional quantum Hall state.

Experimental observation of a striking similarity between quantum hall transport coefficients

Abstract The derivative of the Quantum Hall resistance, ρ xy , with respect to the carrier density, n , has been measured for a two-dimensional electron gas in a GaAs-Al x Ga 1− x As heterostructure, as a function of magnetic field. dρ xy /d n exhibits a remarkable similarity to the diagonal resistivity, ρ xy , to the extent that one is almost directly proportional to the other. Our result suggests the possibility of a fundamental connection between the two quantities.