Promising electroplating solution for facile fabrication of Cu quantum point contacts

In this article, we report on the fabrication and transport measurements of Cu quantum point contacts prepared by a novel, electrochemically assisted mechanically controllable break junction （EC-MCBJ） method. By employing photolithography and wet-etching processes, suspended electrode pairs were patterned and fabricated successfully on Si microchips. Rather than adopting an acid Cu electroplating solution, a novel alkaline electroplating solution was developed and utilized to establish Cu nanocontacts between electrode pairs. Typically, the widths of the as-fabricated Cu nanocontacts were found to be smaller than 18 nm. A large number of Cu quantum point contacts were then produced and characterized by a home-built MCBJ setup. In addition to the conventional histogram, where peaks tend to decrease in amplitude with increasing conductance, an anomalous type of conductance histogram, exhibiting different peak amplitudes, was observed. Through statistical analysis of the maximum allowable bending of the Si microchips, and theoretical calculations, we demonstrated that our alkaline Cu electroplating solution affords Cu nanocontacts that are compatible with subsequent MCBJ operations, which is essential for the fabrication of Cu quantum point contacts. As sophisticated e-beam lithography is not required, the EC-MCBJ method is fast, simple, and cost-effective. Moreover, it is likely to be suitable for the fabrication and characterization of quantum point contacts of various metals from their respective electroplating solutions.

Shot Noise Suppression in a Quantum Point Contact with Short Channel Length

An experimental study on the current shot noise of a quantum point contact with short channel length is reported. The experimentally measured maximum energy level spacing between the ground and the first excited state of the device reached up to 7.5meV, probably due to the hard wall confinement by using shallow electron gas and sharp point contact geometry. The two-dimensionM non-equilibrium shot noise contour map shows noise suppression characteristics in a wide range of bias voltage. Fano factor analysis indicates spin-polarized transport through a short quantum point contact.

The 0.7-Anomaly in Quantum Point Contact;Many-Body or Single-Electron Effect?

Apart from usual quantization steps on the ballistic conductance of quasi-one-dimensional conductor, an additional plateau-like feature appears at a fraction of about 0.7 below the first conductance step in GaAs-based quantum point contacts (QPCs). Despite a tremendous amount of research on this anomalous feature, its origin remains still unclear. Here, a unique model of this anomaly is proposed relying on fundamental principles of quantum mechanics. It is noticed that just after opening a quasi-1D conducting channel in the QPC a single electron travels the channel at a time, and such electron can be—in principle—observed. The act of observation destroys superposition of spin states, in which the electron otherwise exists, and this suppresses their quantum interference. It is shown that then the QPC-conductance is reduced by a factor of 0.74. “Visibility” of electron is enhanced if the electron spends some time in the channel due to resonant transmission. Electron’s resonance can also explain an unusual temperature behavior of the anomaly as well as its recently discovered feature: oscillatory modulation as a function of the channel length and electrostatic potential. A recipe for experimental verification of the model is given.

Statistical evaluation of 571 GaAs quantum point contact transistors showing the 0.7 anomaly in quantized conductance using cryogenic on-chip multiplexing

Themass production and the practical number of cryogenic quantum devices producible in a single chip are limited to the number of electrical contact pads and wiring of the cryostat or dilution refrigerator.It is,therefore,beneficial to contrast themeasurements of hundreds of devices fabricated in a single chip in one cooldown process to promote the scalability,integrability,reliability,and reproducibility of quantum devices and to save evaluation time,cost and energy.Here,we used a cryogenic on-chip multiplexer architecture and investigated the statistics of the 0.7 anomaly observed on the first three plateaus of the quantized conductance of semiconductor quantum point contact(QPC)transistors.Our single chips contain 256 split gate field-effect QPC transistors(QFET)each,with two 16-branch multiplexed source-drain and gate pads,allowing individual transistors to be selected,addressed and controlled through an electrostatic gate voltage process.A total of 1280 quantum transistors with nano-scale dimensions are patterned in 5 different chips of GaAs heterostructures.From the measurements of 571 functioning QFETs taken at temperatures T[1.4 K and T[40 mK,it is found that the spontaneous polarisation model and Kondo effect do not fit our results.Furthermore,some of the features in our data largely agreed with van Hove model with short-range interactions.Our approach provides further insight into the quantum mechanical properties and microscopic origin of the 0.7 anomaly in QFETs,paving the way for the development of semiconducting quantum circuits and integrated cryogenic electronics,for scalable quantum logic control,readout,synthesis,and processing applications.