Ballistic Quantum Point Contact Research Articles

Fabry\u2013P\xe9rot resonances and a crossover to the quantum Hall regime in ballistic graphene quantum point contacts

We report on the observation of quantum transport and interference in a graphene device that is attached with a pair of split gates to form an electrostatically-defined quantum point contact (QPC). In the low magnetic field regime, the resistance exhibited Fabry–Pérot (FP) resonances due to np’n(pn’p) cavities formed by the top gate. In the quantum Hall (QH) regime with a high magnetic field, the edge states governed the phenomena, presenting a unique condition where the edge channels of electrons and holes along a p–n junction acted as a solid-state analogue of a monochromatic light beam. We observed a crossover from the FP to QH regimes in ballistic graphene QPC under a magnetic field with varying temperatures. In particular, the collapse of the QH effect was elucidated as the magnetic field was decreased. Our high-mobility graphene device enabled observation of such quantum coherence effects up to several tens of kelvins. The presented device could serve as one of the key elements in future electronic quantum optic devices.

Dynamical transport measurements of a quantum point contact in GHz

We report the time-dependent measurements in a ballistic quantum point contact at gigahertz frequencies. The Nyquist curves (Im(G) versus Re(G)) of the admittance show a general quadratic function, which extend the linear behaviour reported by Gabelli et al. in the previous experiment. We find that the deviations from the linear behaviour in Nyquist curves are increased with the conductance and the strength of the applied magnetic filed. The Nqyquist curves give direct access to the relaxation time in a quantum RL circuit, which governs the dynamic response of the circuit. The oringin of the deviations is that the geometric capacitance is much smaller than the quantum capacitance of a single quantum point contact device. Hence, our results demonstrate that the dynamic transport strongly depends on the geometric structure of the mesoscopic structures.

Boundary scattering effects on the dynamic magnetotransport of a ballistic quantum point contact

We report dynamic magnetotransport measurements in a ballistic quantum point contact (QPC) at gigahertz frequencies. We observed a gate-controlled crossover of the magnetoadmittance from the partially diffusive boundary scattering effect to weak localization. Moreover, the charge relaxation time of this mesoscopic circuit shows similar behaviors, indicating that the density of states of QPC channels is also sensitively adjusted by the boundary scattering in addition to their transmission. Our experiment demonstrates that boundary scattering plays a dominant role in the dynamic transport in a ballistic QPC and could influence the coherent dynamics in other QPC-based phase-coherent systems such as the quantum dots and qubits.

Split-Channel Ballistic Transport in an InSb Nanowire.

We report an experimental study of one-dimensional (1D) electronic transport in an InSb semiconducting nanowire. A total of three bottom gates are used to locally deplete the nanowire, creating a ballistic quantum point contact with only a few conducting channels. In a magnetic field, the Zeeman splitting of the corresponding 1D sub-bands is revealed by the emergence of conductance plateaus at multiples of e2/h, yet we find a quantized conductance pattern largely dependent on the configuration of voltages applied to the bottom gates. In particular, we can make the first plateau disappear, leaving a first conductance step of 2 e2/ h, which is indicative of a remarkable 2-fold sub-band degeneracy that can persist up to several tesla. For certain gate voltage settings, we also observe the presence of discrete resonant states producing conductance features that can resemble those expected from the opening of a helical gap in the sub-band structure. We explain our experimental findings through the formation of two spatially separated 1D conduction channels.

Connecting strongly correlated superfluids by a quantum point contact.

Point contacts provide simple connections between macroscopic particle reservoirs. In electric circuits, strong links between metals, semiconductors, or superconductors have applications for fundamental condensed-matter physics as well as quantum information processing. However, for complex, strongly correlated materials, links have been largely restricted to weak tunnel junctions. We studied resonantly interacting Fermi gases connected by a tunable, ballistic quantum point contact, finding a nonlinear current-bias relation. At low temperature, our observations agree quantitatively with a theoretical model in which the current originates from multiple Andreev reflections. In a wide contact geometry, the competition between superfluidity and thermally activated transport leads to a conductance minimum. Our system offers a controllable platform for the study of mesoscopic devices based on strongly interacting matter.

Evidence for the formation of quasibound states in an asymmetrical quantum point contact

Features below the first conductance plateau in ballistic quantum point contacts (QPCs) are often ascribed to electron interaction and spin effects within the single mode limit. In QPCs with a highly asymmetric geometry, we observe sharp resonance peaks when the point contacts are gated to the single mode regime, and surprisingly, under certain gating conditions, a complete destruction of the $2{e}^{2}/h$ first quantum plateau. The temperature evolution of the resonances suggest non-Fermi-liquid behavior, while the overall nonlinear characterizations reveal features reminiscent of the 0.7 effect. We attribute these unusual behaviors to the formation of a quasibound state, which is stabilized by a momentum mismatch accentuated by asymmetry.

Very High Thermopower of Bi Nanowires with Embedded Quantum Point Contacts

Quantum confinement effects in bismuth (Bi) nanowires (NWs) are predicted to impart them with high thermopower values and hence make them efficient thermoelectric materials. Yet, boundary scattering of charge carriers in these NWs operating in the diffusion transport regime mask any quantum effects and impede their use for nanoscale thermoelectric applications. Here we demonstrate quantum confinement effects in Bi NWs by forming in their structure ballistic quantum point contacts (QPCs) leading to exceptionally high thermopower values (S > 2 mV/K). The power factor, S(2)G, of the QPCs is maximized at G ~ 0.25G(0) (where G(0) is the quantum of conductance) within agreement with a one-band model with step edge characteristics.

Current noise in a quantum point contact

The current correlation is evaluated for a ballistic quantum point contact interacting with a classical time-dependent field. The formula of low-frequency current noise spectra is obtained and is evaluated analytically for a very short defect and numerically for a smooth potential. In contract to the previous study, there is a parameter region where the current noise with a weak random field is smaller than the Johnson–Nyquist/shot noise expected for a static potential.

Effects of exchange and electron correlation on conductance and nanomagnetism in ballistic semiconductor quantum point contacts

The spontaneous magnetization of a quantum point contact (QPC) formed between two large quantum dots by a lateral confinement of a high-mobility two-dimensional electron gas is studied for a realistic GaAs/AlxGa1-xAs heterostructure. The model of the device incorporates the contributions from a patterned gate, doping, surface states, and mirror charges. To explore the magnetic properties, the Kohn-Sham local spin-density formalism is used with exchange and correlation potentials that allows for local spin polarization. Exchange is the dominant mechanism behind local magnetization within the QPC, while the correlation part is less prominent. However, the correlation potential gives rise to an important correction in the QPC potential. Below the first conduction plateau we thus find a magnetized regime corresponding approximately to a single electron spin. Using an approximate separable saddle potential we compute the conductance and recover the so-called similar to0.7 (2e(2)/h) conduction anomaly plus an additional anomaly at similar to0.4 (2e(2)/h) below which the magnetization collapses.

Influence of processing parameters on the transport properties of quantum point contacts fabricated with an atomic force microscope

We describe a reliable technique for fabricating ballistic quantum point contacts (QPCs) with large energy separation between one-dimensional subbands. The technique is based on lithography with an atomic force microscope and wet chemical etching of a GaAs/AlGaAs heterostructure. The high-mobility two-dimensional electron gas located 55 nm below the surface is laterally confined by 20 nm or 50 nm deep grooves with separations ranging between 65–105 nm or 100–185 nm, respectively. The conductance characteristics at T = 4.2 K exhibit clear quasi-plateaux at multiples of 2e2/h. Both the conductance threshold voltage and the plateau widths are directly related to the QPC geometry. The energy separation ΔE1,2 of the lowest subbands is determined from the conductance under nonzero dc drain voltage. Upon reducing the QPC width, ΔE1,2 varies from 6 meV to 15 meV.

Acoustoelectric current and pumping in a ballistic quantum point contact.

The acoustoelectric current induced by a surface acoustic wave (SAW) in a ballistic quantum point contact is considered using a quantum approach. We find that the current is of the "pumping" type and is not related to drag, i.e., to the momentum transfer from the wave to the electron gas. At gate voltages corresponding to the plateaus of the quantized conductance the current is small. It is peaked at the conductance step voltages. The peak current oscillates and decays with increasing SAW wave number for short wavelengths. These results contradict previous calculations, based on the classical Boltzmann equation.

Current Noise in an Irradiated Point Contact

We propose a new approach to calculate current and current correlations in a ballistic quantum point contact interacting with a classical field. The approach is based on the concept of scattering states for a time dependent Hamiltonian neglecting electron-electron interaction. Using this approach we calculated the spectra of the current noise in a biased point contact irradiated by a weak random field. For typical radiation frequencies \nu less than the temperature T and the bias voltage V we find a narrow peak of width \nu on top of a broad background of width \max (T,eV).

Resistively shunted superconducting quantum point contacts

We have studied the Josephson dynamics of resistively-shunted ballistic superconducting quantum point contacts at finite temperatures and arbitrary number of conducting modes. Compared to the classical Josephson dynamics of tunnel junctions, dynamics of quantum point contacts exhibits several new features associated with temporal fluctuations of the Josephson potential caused by fluctuations in the occupation of the current-carrying Andreev levels.

Coulomb effects in a ballistic one-channel S-S-S device

We develop a theory of Coulomb oscillations in superconducting devices in the limit of small charging energy E C ≪Δ. We consider a small superconducting grain with finite capacitance connected to two superconducting leads by nearly ballistic single-channel quantum point contacts. The temperature is assumed to be very low, so there are no single-particle excitations on the grain. Then the behavior of the system can be described in terms of the quantum mechanics of the superconducting phase on the island. The Josephson energy as a function of this phase has two minima that become degenerate when the phase difference on the leads equals to π, the tunneling amplitude between them being controlled by the gate voltage on the grain. We find the Josephson current and its low-frequency fluctuations, and predict their periodic dependence with period 2e on the induced charge Q x =CV g .

Giant Andreev backscattering and reentrant resistance in a 2-dimensional electron gas coupled to superconductors

We have investigated the superconducting-phase modulated reduction in the resistance of a ballistic quantum point contact (QPC) connected via a disordered 2-dimensional electron gas (2DEG) to superconductors. We show that this reduction is caused by coherent Andreev backscattering of holes through the QPC, which increases monotonically by reducing the bias voltage to zero. In contrast, the magnitude of the phase-dependent resistance of the disordered 2DEG displays a non-monotonic reentrant behavior versus bias voltage.

Reentrant resistance and giant Andreev backscattering in a two-dimensional electron gas coupled to superconductors

We first present the bias-voltage dependence of the superconducting phase-dependent reduction in the differential resistance of a disordered T-shaped two-dimensional electron gas (2DEG) coupled to two superconductors. This reduction exhibits a reentrant behavior, since it first increases upon lowering the bias voltage, exhibits a maximum for a bias voltage of about the Thouless energy and disappears almost completely at zero bias voltage. Second, we have investigated the superconducting-phase-modulated reduction in the resistance of a ballistic quantum point contact (QPC) connected via a disordered 2DEG to superconductors. We show that this reduction is caused by coherent Andreev backscattering of holes through the QPC, which increases monotonically by reducing the bias voltage to zero.

Where is the potential drop in a quantum point contact?

A scattering theory calculation is presented for the potential distribution and the associated current–voltage characteristic in a ballistic quantum point contact. Following Landauer's idea, the electric field is calculated as the response to a given incoming current. The field is finite only near the entrance to or exit from the constriction, while no potential drop occurs in the device itself, where the motion is adiabatic. As a result dissipation is restricted to the reservoirs, whose detailed description, however, requires inelastic scattering.

Coulomb effects in a ballistic one-channel S-S-S device

We develop a theory of Coulomb oscillations in superconducting devices in the limit of small charging energy $E_C \ll \Delta$. We consider a small superconducting grain of finite capacity connected to two superconducting leads by nearly ballistic single-channel quantum point contacts. The temperature is supposed to be very low, so there are no single-particle excitations on the grain. Then the behavior of the system may be described as quantum mechanics of the superconducting phase on the island. The Josephson energy as a function of this phase has two minima which become degenerate at the phase difference on the leads equal to $\pi$, the tunneling amplitude between them being controlled by the gate voltage at the grain. We find the Josephson current and its low-frequency fluctuations and predict their periodic dependence on the induced charge $Q_x=C V_g$ with period $2e$.

Local spin polarization in ballistic quantum point contacts

Self-consistent calculations of the electronic structure and conductance of a ballistic quantum point contact (QPC) in the one-subband limit are reported. The spin-polarized density-functional theory of Kohn-Sham is used. The self-consistent results show that spontaneous spin polarization occurs locally in the region of the saddle point as the electron density is lowered. As a consequence, the effective potential barriers become different for spin up and spin down electrons. Transport associated with spin up electrons, let us say, then suddenly takes place via tunneling, while spin down electrons still carry the current via propagating states in a normal way. The onset of spontaneous spin polarization induces anomalies in the conductance. Our results support the recent interpretations of experimentally observed conductance anomalies in QPCs near pinch off. The agreement with measurements is qualitative, however, rather than quantitative. Reasons for the discrepancy are proposed.

Giant Andreev Backscattering through a Quantum Point Contact Coupled via a Disordered Two-Dimensional Electron Gas to Superconductors

We have investigated the superconducting-phase-modulated reduction in the resistance of a ballistic quantum point contact (QPC) connected via a disordered two-dimensional electron gas (2DEG) to superconductors. We show that this reduction is caused by coherent Andreev backscattering of holes through the QPC, which increases monotonically by reducing the bias voltage to zero. In contrast, the magnitude of the phase-dependent resistance of the disordered 2DEG displays a nonmonotonic reentrant behavior versus bias voltage.