Search

The allegedly unconventional superconducting phase of magic-angle twisted bilayer graphene (MATBG)1 has been predicted to possess extraordinary thermal properties, as it is formed from a highly diluted electron ensemble with both a record-low carrier density n ~ 10^11 cm-2 and electronic heat capacity Ce < 100 kB. While these attributes position MATBG as a ground-breaking material platform for revolutionary calorimetric applications2, these properties have so far not been experimentally shown. Here we reveal the ultra-sensitive calorimetric properties of a superconducting MATBG device, by monitoring its temperature dependent critical current Ic under continuous laser heating with a wavelength of 1550nm. From the bolometric effect, we are able to extract the temperature dependence of the electronic thermal conductance Gth, which remarkably has a non-zero value Gth = 0.19 pW/K at 35mK and in the low temperature limit is consistent with a power law dependence, as expected for nodal superconductors. Photo-voltage measurements on this non-optimized device reveal a peak responsivity of S = 5.8 x 10^7 V/W when the device is biased close to Ic, with a noise-equivalent power of NEP = 5.5 x 10^-16 WHz^-1/2. Analysis of the intrinsic perfor-mance shows that a theoretically achievable limit is defined by thermal fluctuations and can be as low as NEPTEF < 10^-20 WHz-1/2, with operation speeds as fast as ~ 500 ns. This establishes superconducting MATBG as a revolutionizing active material for ultra-sensitive photon-detection applications, which could enable currently unavailable technologies such as THz photon-number-resolving single-photon-detectors.

Electrons incident from a normal metal onto a superconductor are reflected back as holes - a process called Andreev reflection. In a normal metal where the Fermi energy is much larger than a typical superconducting gap, the reflected hole retraces the path taken by the incident electron. In graphene with ultra low disorder, however, the Fermi energy can be tuned to be smaller than the superconducting gap. In this unusual limit, the holes are expected to be reflected specularly at the superconductor-graphene interface due to the onset of interband Andreev processes, where the effective mass of the reflected holes change sign. Here we present measurements of gate modulated Andreev reflections across the low disorder van der Waals interface formed between graphene and the superconducting NbSe2. We find that the conductance across the graphene-superconductor interface exhibits a characteristic suppression when the Fermi energy is tuned to values smaller than the superconducting gap, a hallmark for the transition between intraband retro- and interband specular- Andreev reflections.