Mesoscopic Point Contacts Research Articles

Back Cover: Erasable Superconductivity in Topological Insulator Bi2Se3 Induced by Voltage Pulse (Adv. Quantum Technol. 9/2021)

Emergent superconductivity is observed in the local mesoscopic point-contacts on the topological insulator Bi2Se3 by applying a voltage pulse through the Ag contacts. The superconductivity can be erased with thermal cycles by warming up to high temperatures (300 K) and repeatedly induced by the voltage pulse at base temperature (1.9 K), suggesting a significance for designing new types of quantum devices. For further details, see article number 2100067 by Tian Le, Xiaozhi Wang, Xin Lu and co-workers.

In Situ Growth Characterization of Electrodeposited Metal Contacts

We have developed a new electrochemical method to study the morphology of electrodeposited metal in-situ at the nano-scale , without the need of any additional surface characterization techniques. A metal-insulator-metal (MIM) pillar structure with exposed edges is fabricated and subsequently contacted in a customize 4-electrode electrochemical cell. The current is measured during electrodeposition while a voltage bias is applied between the two working electrodes, separated by a 50 nm thin dielectric. For appropriate choices of the voltage bias and deposition potential, the current between the two working electrodes can easily be distinguished from the electrodeposition current. This current represents the tunnelling current and eventually the mesoscopic contact current. The change in conductance of the MIM structure during electrodeposition is highly indicative of the growth mode of the electrodeposited metal. A rough film growth leads to step-like, quantized current transients typical of mesoscopic point contacts. Some steps correspond to the conductance quantum but smaller steps in the tunnelling current are also observed [1]. In contrast, a uniformly growing film leads to smooth, exponential current transients due to the significant contribution of a tunnelling current before physical contact is made.We use this method to investigate the growth of Au thin films from a thiosulfate-sulfite bath, and the growth of Cu from a sulfate bath. We also show the effect of common levelling additives for Cu electrodeposition, such as polyethylene glycol (PEG) and benzotriazole (BTA). Additionally, we have observed a significant dependence of the film growth on the voltage bias applied between the top and bottom metals of the MIM that is independent of the deposition potential.This technique provides a novel platform to study the effect of additives and other electrodeposition variables in a wide range of electrolytes. Furthermore, it provides an efficient platform to fabricate nanogap structures.[1] Li, C. Z., H. X. He, and N. J. Tao. "Quantized tunneling current in the metallic nanogaps formed by electrodeposition and etching." Applied Physics Letters 77.24 (2000): 3995-3997.

Mesoscopic superconductivity above 10 K in silicon point contacts

Silicon, perhaps the most ubiquitously used material in the digital age of today, has also been a material of choice for testing the fundamental differences between various electronic ground states, e.g., metals and insulators. This is mainly because ultimate control has been achieved in growing extremely pure silicon crystals and doping them with varying concentrations of charge carriers and their mobility. Here, we show that by forming mesoscopic point contacts with non-superconducting metals on insulating (doped) silicon, it is possible to obtain a superconducting phase with a remarkably high critical temperature above 10 K and an average superconducting energy gap of 2 meV. Apart from its importance in advancing the understanding of nanoscale superconductivity, this discovery is also expected to boost the efforts to realize silicon based superconducting devices with far reaching application potential.

Large enhancement of superconductivity in Zr point contacts

For certain complex superconducting systems, the superconducting properties get enhanced under mesoscopic point contacts made of elemental non-superconducting metals. However, understanding of the mechanism through which such contact induced local enhancement of superconductivity happens has been limited due to the complex nature of such compounds. In this paper we present a large enhancement of superconducting transition temperature Tc and superconducting energy gap Δ in a simple elemental superconductor Zr. While bulk Zr shows a critical temperature around 0.6 K, superconductivity survives at Ag/Zr and Pt/Zr point contacts up to 3 K with a corresponding five-fold enhancement of Δ. Further, the first-principles calculations on a model system provide useful insights. We show that the enhancement in superconducting properties can be attributed to a modification in the electron–phonon coupling accompanied by an enhancement of the density of states which involves the appearance of a new electron band at the Ag/Zr interfaces.

Enhanced zero-bias conductance peak and splitting at mesoscopic interfaces between an s-wave superconductor and a 3D Dirac semimetal

Mesoscopic point contacts between elemental metals and the topological 3D Dirac semimetal Cd3As2 have been recently shown to be superconducting with unconventional pairing while Cd3As2 itself does not superconduct. Here we show that the same superconducting phase at mesoscopic interfaces on Cd3As2 can be induced with a known conventional superconductor Nb where a pronounced zero-bias conductance peak is observed which undergoes splitting in energy under certain conditions. The observations are consistent with the theory of the emergence of Andreev bound states due to the presence of a pair potential with broken time reversal symmetry. The data also indicate the possibility of Majorana bound states as expected at the interfaces between s-wave superconductors and topologically non-trivial materials with a high degree of spin-orbit coupling.

Unconventional superconductivity at mesoscopic point contacts on the 3D Dirac semimetal Cd3As2.

Three-dimensional (3D) Dirac semimetals exist close to topological phase boundaries which, in principle, should make it possible to drive them into exotic new phases, such as topological superconductivity, by breaking certain symmetries. A practical realization of this idea has, however, hitherto been lacking. Here we show that the mesoscopic point contacts between pure silver (Ag) and the 3D Dirac semimetal Cd3As2 (ref.) exhibit unconventional superconductivity with a critical temperature (onset) greater than 6 K whereas neither Cd3As2 nor Ag are superconductors. A gap amplitude of 6.5 meV is measured spectroscopically in this phase that varies weakly with temperature and survives up to a remarkably high temperature of 13 K, indicating the presence of a robust normal-state pseudogap. The observations indicate the emergence of a new unconventional superconducting phase that exists in a quantum mechanically confined region under a point contact between a Dirac semimetal and a normal metal.