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Register a new userEfficient thermionic operation and phonon isolation by a semiconductor-superconductor junction
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Control of heat flux at small length scales is crucial for numerous solid-state devices and systems. In addition to the thermal management of information and communication devices the mastering of heat transfer channels down to the nanoscale also enable, e.g., new memory concepts, high sensitivity detectors and sensors, energy harvesters and compact solid-state refrigerators. Electronic coolers and thermal detectors for electromagnetic radiation, especially, rely on the maximization of electro-thermal response and blockade of phonon transport. In this work, we propose and demonstrate that efficient electro-thermal operation and phonon transfer blocking can be achieved in a single solid-state thermionic junction. Our experimental demonstration relies on suspended semiconductor-superconductor junctions where the electro-thermal response arises from the superconducting energy gap, and the phonon blocking naturally results from the transmission bottleneck at the junction. We suspend different size degenerately doped silicon chips (up to macroscopic scale) directly from the junctions and cool these by biasing the junctions. The electronic cooling operation characteristics are accompanied by measurement and analysis of the thermal resistance components in the structures indicating the operation principle of phonon blocking in the junctions.
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Braiding operations are challenging to create topological quantum computers. It is unclear whether braiding operations can be executed with any materials. Although various calculations based on Majorana fermions show braiding possibilities, a braiding operation with a Majorana fermion has not yet been experimentally proven. Herein, braiding operations are demonstrated using a molecular topological superconductor (MTSC) that utilizes the topological properties intrinsic in molecules. The braiding operations were implemented by controlling two MTSC modules made by pelletizing crystals of 4,5,9,10-tetrakis(dodecyloxy)pyrene, which is proposed as the first MTSC material through n-MOSFETs. It shows the elements of topological quantum computers that can be demonstrated without an external magnetic field at room temperature.
We present evidence for the cooling of normal metal phonons by electron tunneling in a Superconductor - Normal metal - Superconductor tunnel junction. The normal metal electron temperature is extracted by comparing the device current-voltage characteristics to the theoretical prediction. We use a quantitative model for the phonon cooling that includes the electron-phonon coupling in the normal metal and the Kapitza resistance between the substrate and the metal. It gives an excellent fit to the data and enables us to extract an effective phonon temperature in the normal metal.
The superconducting proximity effect has played an important role in recent work searching for Majorana modes in thin semiconductor devices. Using transport measurements to quantify the changes in the semiconductor caused by the proximity effect provides a measure of dynamical processes such as screening and scattering. However, in a two terminal measurement the resistance due to the interface conductance is in series with resistance of transport in the semiconductor. Both of these change, and it is impossible to separate them without more information. We have devised a new three terminal device that provides two resistance measurements that are sufficient to extract both the junction conductance and the two dimensional sheet resistance under the superconducting contact. We have compared junctions between Nb and InAs and Nb and 30% InGaAs all grown before being removed from the ultra high vacuum molecular beam epitaxy growth system. The most transparent junctions are to InAs, where the transmission coefficient per Landauer mode is greater than 0.6. Contacts made with ex-situ deposition are substantially more opaque. We find that for the most transparent junctions, the largest fractional change as the temperature is lowered is to the resistance of the semiconductor.
Many present and future applications of superconductivity would benefit from electrostatic control of carrier density and tunneling rates, the hallmark of semiconductor devices. One particularly exciting application is the realization of topological superconductivity as a basis for quantum information processing. Proposals in this direction based on proximity effect in semiconductor nanowires are appealing because the key ingredients are currently in hand. However, previous instances of proximitized semiconductors show significant tunneling conductance below the superconducting gap, suggesting a continuum of subgap states---a situation that nullifies topological protection. Here, we report a hard superconducting gap induced by proximity effect in a semiconductor, using epitaxial Al-InAs superconductor-semiconductor nanowires. The hard gap, along with favorable material properties and gate-tunability, makes this new hybrid system attractive for a number of applications, as well as fundamental studies of mesoscopic superconductivity.
We report on Andreev reflections at clean NbSe2-bilayer graphene junctions. The high transparency of the junction, which manifests as a large conductance enhancement of up to 1.8, enables us to see clear evidence of a proximity-induced superconducting gap in bilayer graphene and two Andreev reflections through a vertical NbSe2-graphene and a lateral graphene-graphene junction respectively. Quantum transport simulations capture the complexity of the experimental data and illuminate the impact of various microscopic parameters on the transmission of the junction. Our work establishes the practice and understanding of an all-van-der-Waals, high-performance superconducting junction. The realization of a highly transparent proximized graphene-graphene junction opens up possibilities to engineer emergent quantum phenomena.
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