Electron cooling with graphene-insulator-superconductor tunnel junctions and applications to fast bolometry

Abstract

Electronic cooling in hybrid normal metal-insulator-superconductor junctions is a promising technology for the manipulation of thermal loads in solid state nanosystems. One of the main bottlenecks for efficient electronic cooling is the electron-phonon coupling, as it represents a thermal leakage channel to the phonon bath. Graphene is a two-dimensional material that exhibits a weaker electronphonon coupling with respect to standard metals. For this reason, we study the electron cooling in graphene-based systems consisting of a graphene sheet contacted by two insulator/superconductor junctions. We show that, by properly biasing the graphene, its electronic temperature can reach base values lower than those achieved in similar systems based on metallic ultra thin films. Moreover, the lower electron-phonon coupling is mirrored in a lower heat power pumped into the superconducting leads, thus avoiding their overheating and preserving the cooling mechanisms. Finally, we analyze the possible application of cooled graphene as a bolometric radiation sensor. We study its main figures of merit, i.e. responsivity, noise equivalent power and response time. In particular, we show that the built-in electron refrigeration allows to reach a responsivity of the order of 50 nA/pW and a noise equivalent power of order of 10 −18 W Hz −1/2 while the response speed is about 10 ns corresponding to a thermal bandwidth in the order of 100 MHz.

Thermodynamics of a Phase-Driven Proximity Josephson Junction

“…This can be obtained in a qualitatively by considering that when ϕ = 0, the minigap is closed and the DoS is modified in a region surrounding the weak link in such a way to return expression (23), that is linear like a normal metal. The volume Ṽ of the proximized region can be estimated by comparing expression (24) with the entropy of a normal metal S N = 2π 2 N S ṼT/3:…”

“…It is possible to find an analytic expression for Q cs valid for T R , T L ∆ 0 . Considering the scheme in Figure 9b, it can be noticed that at low temperature the heat absorbed by the CS is ruled by the purple area defined by the linear expression of entropy in Equations ( 23) and (24). Approximating the T 2 temperature to 0, due to the strong isentropic cooling at low temperatures, the Q cs is given at the leading order by the CS temperature…”

“…Superconducting hybrid systems, i.e., constituted of superconducting parts in electric contact with normal (non-superconducting) parts, are in practice coherent electron systems with striking thermodynamic equilibrium/transport properties, resulting in a wide variety of applicative devices: low-temperature sensitive thermometers [15][16][17][18][19], sensitive detectors [20][21][22][22][23][24][25][26][27][28][29][30][31][32], heat valves [33][34][35][36][37][38][39][40][41][42][43], caloritronics (heat computing) [11,37,[44][45][46][47][48], solid-state micro-refrigerators [18,[49][50][51][52][53][54][55], solid-state quantum machines [56][57][58]…”

“…(b) Magnification of panel (a) around T = 0.2T c , highlighting the passage from a exponential suppressed behavior at ϕ = 0 to a linear behavior at ϕ = π. The dashed curve is the analytical low-temperature in expression (23),(24).…”