1D Constriction Research Articles

The in-plane anisotropy of the effective g factors in Al0.25Ga0.75N/GaN based quantum point contacts with narrow channels

The effective g factors (g*) of the lowest three subbands of Al0.25Ga0.75N/GaN-based quantum point contacts (QPCs) are obtained in various magnetic field orientations. The g factors show an anisotropic enhancement compared to those in the two-dimensional electron gas, among which an in-plane anisotropy becomes manifested, when the QPC is depleted from the third to the first subband and the channel width shrinks from the order of 10−7 m–10−8 m. In the one-dimensional (1D) regime with only one subband in the QPC, the in-plane g* perpendicular to the current is evidently larger than the parallel one. The in-plane anisotropy can be attributed to the Rashba spin–orbit coupling (SOC) in the narrow confinement. The reason that the anisotropy can be observed in this work, but not in conventional III–V semiconductors studied previously, is due to the fact that the Al0.25Ga0.75N/GaN-based QPCs have larger polarization-field-induced Rashba SOC and much stronger 1D constriction.

Non-Kondo zero-bias anomaly in quantum wires

It has been suggested that a zero-bias conductance peak in quantum wires signifies the presence of Kondo spin-correlations, which might also relate to an intriguing one-dimensional (1D) spin effect known as the 0.7 structure. These zero-bias anomalies (ZBA) are strongly temperature dependent, and have been observed to split into two peaks in magnetic field, both signatures of Kondo correlations in quantum dots. We present data in which ZBAs in general do not split as magnetic field is increased up to 10 T. A few of our ZBAs split in magnetic field but by significantly less than the Kondo splitting value, and evolve back to a single peak upon moving the 1D constriction laterally. The ZBA therefore does not appear to have a Kondo origin, and instead we propose a simple phenomenological model to reproduce the ZBA which is in agreement mostly with observed characteristics.

Thermal measurements of a one-dimensional wire in the quantum limit

The single-particle properties of a ballistic one-dimensional (1D) conductorare reviewed; the breakdown in these properties is used to characterize the0.7 structure, an anomalous but reproducible feature in the conductanceG that ismeasured at G = 0.7 × 2e2/h.We show how alternating current heating creates a temperature differenceΔT across a 1D wire, allowing thermopower measurements. The thermopower characteristicsS(Vg) show deviations from Cutler–Mott predictions (a single-particle theory) close tothe 0.7 structure. The magnitude of a thermovoltage peak is used to measurethe electron temperature, and we have incorporated a mesoscopic thermometerinto a simple thermal circuit that allows us to measure the thermal conductanceof a 1D wire. For the first four 1D subbands the heat carried by electronspassing through the 1D constriction is proportional to the electrical conductanceG(Vg); this is the first demonstration that the thermal conductance due to electronsis quantized. In the vicinity of the 0.7 structure the Wiedemann–Franzratio breaks down, and a plateau at the quantum of thermal conductanceπ2kB2T/3h is observed.

Distribution of an Ohmic current in the close vicinity of a quantum point contact

We present the essential findings of the screening theory of the integer quantum Hall effect (IQHE) considering a quantum point contact (QPC). Our approach is to solve the Poisson and the Schrödinger equations self-consistently, taking into account electron interactions, within a Hartree type approximation for a two dimensional electron gas (2DEG) subject to high perpendicular magnetic fields. The Coulomb interaction between the electrons separates 2DEG into two co-existing regions, namely quasi-metallic compressible and quasi-insulating incompressible regions, which exhibit peculiar screening and transport properties. In the presence of an external current, we show that this current is confined into the incompressible regions where the drift velocity is finite. In particular, we investigate the distribution of these incompressible strips and their relation with the quantum Hall plateaus considering a quasi 1D constriction, i.e. a QPC.

Energy-loss rate of a two-dimensional electron gas measured using a mesoscopic thermometer

Using a simple setup, the linear thermopower is measured in etched one-dimensional (1D) constrictions with large subband energy spacings, allowing the temperature of the two-dimensional electron gas on one side of the constriction to be determined as a function of the applied heating power. As tested by the Cutler-Mott relation, the 1D constriction acts as a reliable electron thermometer for lattice temperatures in the range ${T}_{L}=4.2--13\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The power dissipated per electron, measured as a function of electron temperature, reveals that at ${T}_{L}=4.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ the acoustic phonon scattering is dominated by ${T}^{5}$ behavior due to an unscreened deformation potential. At higher lattice temperatures the scattering can be described by an interpolation between the Gr\"uneisen-Bloch and equipartition regimes.

Quantum Thermal Conductance of Electrons in a One-Dimensional Wire

We use an electron thermometer to measure the temperature rise of approximately 2 x 10(5) electrons in a two-dimensional box, due to heat flow into the box through a ballistic one-dimensional (1D) constriction. Using a simple model we deduce the thermal conductance kappa(Vg) of the 1D constriction, which we compare to its electrical conductance characteristics; for the first four 1D subbands the heat carried by the electrons passing through the wire is proportional to its electrical conductance G(Vg). In the vicinity of the 0.7 structure this proportionality breaks down, and a plateau at the quantum of thermal conductance pi(2)k(2/B)T/3h is observed.

Gradual decrease of conductance of an adiabatic ballistic constriction below2e2∕h

We have performed four-terminal conductance measurements of a one-dimensional (1D) channel in which it is possible to modulate the potential profile using three overlaying finger gates. In such a 1D ballistic structure we have observed, {\em for the first time,} that the conductance steps show a gradual decrease from $2e^2/h$ to $0.97 \times 2e^2/h$ with increasing negative finger gate voltage in a short, clean 1D constriction. We suggest this phenomenon is due to differing shifts of 1D subbands with changing spilt-gate voltage. Both a simple analytical estimate for an adiabatic constriction and, realistic modeling of the device, give the same magnitude of the conductance decrease as observed in our experiments.

Can the conductance step of a single-mode ballistic constriction be lower than 2 e2/ h?

We have performed four-terminal measurements on a split-gate device with three overlaying finger gates. In our system, a one-dimensional (1D) ballistic constriction can be formed by applying a negative voltage on the split-gate. By changing the finger gate voltage, we are able to modulate the potential profile within the 1D constriction. In such a 1D ballistic structure we have observed that the conductance steps show a gradual decrease from 2 e 2/ h to 0.97×2 e 2/ h with increasing negative finger gate voltage. We suggest this phenomenon is due to different shifts of 1D subbands with changing spilt-gate voltage. Both simple analytical estimate for an adiabatic constriction and realistic modeling of the device give the same magnitude of the conductance decrease as observed in our experiments.

Magnetoconductance of independently tunable tunnel-coupled double quantum wires

We report on our recent experimental studies of vertically coupled quantum point contacts subject to in-plane magnetic fields. Using a novel flip–chip technique, mutually aligned split gates on both sides of sub-micron thick double quantum well heterostructure define a closely coupled pair of ballistic one-dimensional (1D) constrictions. We observe quantized conductance steps due to each quantum well and demonstrate independent control of each 1D constriction width. In addition, a novel magnetoconductance feature at ∼6 T is observed when a magnetic field is applied perpendicular to both the current and growth directions. This conductance dip is observed only when 1D subbands are populated in both the top and bottom constrictions. This data is consistent with a counting model whereby the number of subbands crossing the Fermi level changes with field due to the formation of an anticrossing in each pair of 1D subbands.

Threshold Features in Transport through a 1D Constriction

Suppression of electron current $ \Delta I$ through a 1D channel of length $L$ connecting two Fermi liquid reservoirs is studied taking into account the Umklapp electron-electron interaction induced by a periodic potential. This interaction causes Hubbard gaps $E_H$ for $L \to \infty$. In the perturbative regime where $E_H \ll v_c/L$ ($v_c:$ charge velocity), and for small deviations $\delta n$ of the electron density from its commensurate values $- \Delta I/V$ can diverge with some exponent as voltage or temperature $V,T$ decreases above $E_c=max(v_c/L,v_c \delta n)$, while it goes to zero below $E_c$. This results in a nonmonotonous behavior of the conductance.

Conductance of a quantum channel containing a small number of impurities

The crossover properties of the quantum conductance in a 1D constriction from the ballistic to the diffusive regime are investigated analytically. In the ballistic regime, the well-known plateau structures are found. As more impurities are added to the system, we are able to show that interesting “dip” structures should be found when new quantum channels become open. The higher the new quantum channel index, the deeper the dip will be. When the degree of disorder increases, the ensemble-averaged conductance is shown to be reduced, but the dip structure can still be seen as long as the diffusive regime is not reached. Our analytical results are in agreement with recent experiments as well as numerical simulations.