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Register a new userMulti-Valued Logic Gates based on Ballistic Transport in Quantum Point Contacts
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Multi-valued logic gates, which can handle quaternary numbers as inputs, are developed by exploiting the ballistic transport properties of quantum point contacts in series. The principle of a logic gate that finds the minimum of two quaternary number inputs is demonstrated. The device is scalable to allow multiple inputs, which makes it possible to find the minimum of multiple inputs in a single gate operation. Also, the principle of a half-adder for quaternary number inputs is demonstrated. First, an adder that adds up two quaternary numbers and outputs the sum of inputs is demonstrated. Second, a device to express the sum of the adder into two quaternary digits [Carry (first digit) and Sum (second digit)] is demonstrated. All the logic gates presented in this paper can in principle be extended to allow decimal number inputs with high quality QPCs.
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Linear and non-linear transport properties through an atomic-size point contact based on oxides two-dimensional electron gas is examined using the tight-binding method and the $mathbf{kcdot p}$ approach. The ballistic transport is analyzed in contacts realized at the (001) interface between band insulators $LaAlO_3$ and $SrTiO_3$ by using the Landauer-Buttiker method for many sub-bands derived from three Ti 3d orbitals ($d_{yz}$, $d_{zx}$ and $d_{xy}$) in the presence of an out-of-plane magnetic field. We focus especially on the role played by the atomic spin-orbit coupling and the inversion symmetry breaking term pointing out three transport regimes: the first, at low energies, involving the first $d_{xy}$-like sub-bands, where the conductance quantization is robust; a second one, at intermediate energies, entailing further $d_{xy}$-like sub-bands, where the sub-band splitting induced by the magnetic field is quenched; the third one, where the mixing between light $d_{xy}$-like, heavy $d_{yz}$-like and $d_{zx}$-like sub-bands is so strong that the conductance plateaus turn out to be very narrow. Very good agreement is found with recent experiments exploring the transport properties at low energies.
We study nonlinear transport and non-equilibrium current noise in quasi-classical point contacts (PCs) defined in a low-density high-quality two-dimensional electron system in GaAs. At not too high bias voltages $V$ across the PC the noise temperature is determined by a Joule heat power and almost independent on the PC resistance that can be associated with a self-heating of the electronic system. This commonly accepted scenario breaks down at increasing $V$, where we observe extra noise accompanied by a strong decrease of the PCs differential resistance. The spectral density of the extra noise is roughly proportional to the nonlinear current contribution in the PC $delta Sapprox2F^*|edelta I|sim V^2$ with the effective Fano factor $F^*<1$, indicating that a random scattering process is involved. A small perpendicular magnetic field is found to suppress both $delta I$ and $delta S$. Our observations are consistent with a concept of a drag-like mechanism of the nonlinear transport mediated by electron-electron scattering in the leads of quasi-classical PCs.
In the ballistic regime, the transport across a normal metal (N)/superconductor (S) point-contact is dominated by a quantum process called Andreev reflection. Andreev reflection causes an enhancement of the conductance below the superconducting energy gap, and the ratio of the low-bias and the high-bias conductance cannot be greater than 2 when the superconductor is conventional in nature. In this regime, the features associated with Andreev reflection also provide energy and momentum-resolved spectroscopic information about the superconducting phase. Here we theoretically consider various types of N/S point contacts, away from the ballistic regime, and show that even when the superconductor under investigation is simple conventional in nature, depending on the shape, size and anatomy of the point contacts, a wide variety of spectral features may appear in the conductance spectra. Such features may misleadingly mimic theoretically expected signatures of exotic physical phenomena like Klein tunneling in topological superconductors, Andreev bound states in unconventional superconductors, multiband superconductivity and Majorana zero modes.
The Josephson effect in ballistic point contacts between single-band and multi-band superconductors was investigated. It was found that in the case of Josephson junctions formed by a single-band and an $s_pm$-wave two-band superconductor as well as by a single-band and a three-band superconductor the junctions become frustrated, demonstrating the $phi$-contact properties. Depending on the ground state of a three-band superconductor with broken time-reversal symmetry (BTRS), the Josephson junction can have from one to three energy minima, some of which can be locally stable. We also study the behavior of a dc SQUID based on the Josephson junctions between single-band and multi-band superconductors. Some features on the dependences of the critical current and the total magnetic flux on the applied flux of a dc SQUID based on the Josephson point contacts between a single-band superconductor and an $s_pm$-wave superconductor, three-band superconductor with BTRS and three-band superconductor without BTRS as compared to the conventional dc SQUIDs based on single-band superconductors were found. The results can be used as an experimental tool to detect the existence of multi-band structure and BTRS.
We study a superconducting quantum point contact made of a narrow In$_{0.75}% $Ga$_{0.25}$As channel with Nb proximity electrodes. The narrow channel is formed in a gate-fitted constriction of InGaAs/InAlAs/InP heterostructure hosting a two-dimensional electron gas. When the channel opening is varied with the gate, the Josephson critical current exhibits a discretized variation that arises from the quantization of the transverse momentum in the channel. The quantization of Josephson critical current persists down to the single-channel regime, providing an unambiguous demonstration of a semiconductor--superconductor hybrid Josephson junction involving only a single ballistic channel.
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