Alexander Savin

Opportunities for mesoscopics in thermometry and refrigeration: Physics and applications

This review presents an overview of the thermal properties of mesoscopic structures. The discussion is based on the concept of electron energy distribution, and, in particular, on controlling and probing it. The temperature of an electron gas is determined by this distribution: refrigeration is equivalent to narrowing it, and thermometry is probing its convolution with a function characterizing the measuring device. Temperature exists, strictly speaking, only in quasiequilibrium in which the distribution follows the Fermi-Dirac form. Interesting nonequilibrium deviations can occur due to slow relaxation rates of the electrons, e.g., among themselves or with lattice phonons. Observation and applications of nonequilibrium phenomena are also discussed. The focus in this paper is at low temperatures, primarily below

Heat transistor: Demonstration of gate-controlled electronic refrigeration

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Efficient electronic cooling in heavily doped silicon by quasiparticle tunneling

, where physical phenomena on mesoscopic scales and hybrid combinations of various types of materials, e.g., superconductors, normal metals, insulators, and doped semiconductors, open up a rich variety of device concepts. This review starts with an introduction to theoretical concepts and experimental results on thermal properties of mesoscopic structures. Then thermometry and refrigeration are examined with an emphasis on experiments. An immediate application of solid-state refrigeration and thermometry is in ultrasensitive radiation detection, which is discussed in depth. This review concludes with a summary of pertinent fabrication methods of presented devices.

Thermal budget of superconducting digital circuits at subkelvin temperatures

We present experiments on a superconductor-normal-metal electron refrigerator in a regime where single-electron charging effects are significant. The system functions as a heat transistor; i.e., the heat flux out from the normal-metal island can be controlled with a gate voltage. A theoretical model developed within the framework of single-electron tunneling provides a full quantitative agreement with the experiment. This work serves as the first experimental observation of Coulombic control of heat transfer and, in particular, of refrigeration in a mesoscopic system.

High-resolution superconducting single-flux quantum comparator for sub-Kelvin temperatures

Cooling of electrons in a heavily doped silicon by quasiparticle tunneling using a superconductor–semiconductor–superconductor double-Schottky-junction structure is demonstrated at low temperatures. In this work, we use Al as the superconductor and thin silicon-on-insulator (SOI) film as the semiconductor. The electron–phonon coupling is measured for the SOI film and the low value of the coupling is shown to be the origin of the observed significant cooling effect.

Ferromagnetic (Ga,Mn)As nanowires grown by Mn-assisted molecular beam epitaxy

Superconducting single-flux-quantum (SFQ) circuits have so far been developed and optimized for operation at or above helium temperatures. The SFQ approach, however, should also provide potentially viable and scalable control and readout circuits for Josephson-junction qubits and other applications with much lower, millikelvin, operating temperatures. This paper analyzes the overheating problem which becomes important in this temperature range. We suggest a thermal model of the SFQ circuits at subkelvin temperatures and present experimental results on overheating of electrons and silicon substrate which support this model. The model establishes quantitative limitations on the dissipated power both for “local” electron overheating in resistors and “global” overheating due to ballistic phonon propagation along the substrate. Possible changes in the thermal design of SFQ circuits in view of the overheating problem are also discussed.

Optimization of electron cooling by SIN tunnel junctions

A design of sub-Kelvin single-flux quantum (SFQ) circuits with reduced power dissipation and additional cooling of shunt resistors has been developed and characterized. The authors demonstrate operation of SFQ comparators with current resolution of 40nA at 2GHz sampling rate. Due to improved cooling the electron temperature in shunt resistors of a SFQ comparator is below 50mK when the bath temperature is about 30mK.

Rapid single-flux-quantum circuits for low noise mK operation

(Ga,Mn)As nanowires were grown by molecular beam epitaxy using Mn as a growth catalyst on GaAs(001) substrates at 485u2009°C, i.e., at intermediate temperatures higher than ones used for the growth of (Ga,Mn)As thin films, but lower than the ordinary temperatures of Au-assisted growth of GaAs nanowires. (Ga,Mn)As nanowires obtained with typical lengths between 0.8 and 4u2009μm and diameters 50–90 nm do not have defects, such as dislocations or precipitates, except for the stacking faults lying parallel to the growth direction. The investigation of magnetic and optical properties has been carried out not only for as-grown samples with nanowires but also for peeled off nanowires from the host substrate. The results obtained demonstrate that (Ga,Mn)As nanowires exhibit ferromagnetic ordering around 70 K.

Cold electron Josephson transistor

We report on the optimization of electron cooling by SIN tunnel junctions due to the advanced geometry of superconducting electrodes and very effective normal metal traps for more efficien tr emoval of quasiparticles at temperatures from 25 to 500 mK. The maximum decrease in electron temperature of about 200 mK has been observed at bath temperatures 300–350 mK. We used four-junction geometry with Al–AlOx –Cr/Cu tunnel junctions and Au traps. Efficient electron cooling was realized due to the improved geometry of the cooling tunnel junctions (quadrant shape of the superconducting electrode) and optimized Au traps just near the junctions (≈0.5 µm) to reduce reabsorption of quasiparticles after removing them from normal metal. The maximum cooling effect wa si ncreased from a temperature drop of dT =− 56 mK (ordinary cross geometry) to −130 mK (improve dg eometry of superconducting electrodes) and to dT =− 200 mK (improve dg eometry of superconducting electrodes and effective Au traps). The heating peak (instead of cooling) near the zero voltage across cooling junctions has been observed in practice for all samples at temperatures below 150 mK. For higher cooling voltages close to the superconducting gap, the heating was converted to cooling with decreased amplitude. The leakage resistance of the tunnel junctions gives a reasonable explanation of the heating peak. The phonon reabsorption due to the recombination of quasiparticles in superconducting electrodes gives an additional improvement in the theoretical fitting but could not explain the heating peak. An anomalous zero-bias resistance peak has been observed for all tested structures. The peak is explained by Coulomb blockade of tunnelling in transistor-type structures with relatively small tunnel junctions. The work on electron cooling is devoted to the development o fac old-e lectron bolometer (CEB) with capacitive coupling by SIN tunnel junctions to the antenna for sensitive detection in the terahertz region. Direct electron cooling of an absorber plays a crucial role in supersensitive detection in the presence of a realistic background power load. (Some figures in this article are in colour only in the electronic version)

Application of superconductor–semiconductor Schottky barrier for electron cooling

Rapid single-flux-quantum (RSFQ) technology has been proposed as control electronics for superconducting quantum bits because of the material and working temperature compatibility. In this work, we consider practical aspects of RSFQ circuit design for low noise low power operation. At the working temperature of 20 mK and operational frequency of 2 GHz, dissipated power per junction is reduced to 25 pW by using 6 νA critical current junctions available at the Hypres and VTT low Jc fabrication process. To limit phonon temperature to 30 mK, a maximum of 40 junctions can be placed on a 5 mm × 5 mm chip. Electron temperature in resistive shunts of Josephson junctions is minimized by use of cooling fins, giving minimum electron temperatures of about 150 mK for the Hypres process and 70 mK for the VTT process.