Fluctuation or noise may feel undesirable, but can contain intricate detail about a system not accessible by the average of the underlying parameter. In the context of quantum transport in mesoscopic systems, we are interested in the fluctuation of current. Among the several kinds of noise encountered in condensed matter systems shot noise is unique, as it is related to the quantized nature of the particles those carry the current. Interestingly, the particle responsible for current flow, i.e. electrons, do not always behave as free electrons. When they interact, either among themselves or with different kind of fields, a new concept of current transport appears, where the current is carried by new quasiparticles, whose charge can be different from free electron charge. Shot noise has emerged as the most valuable tool, using which we can probe the transport properties of these quasiparticles or measure charge of the quasiparticles. In fact, the charge of Laughlin quasiparticles was measured for the first-time using shot noise. In our lab, we use shot noise to study the quasiparticle dynamics in graphene – superconductor junctions.
Enhanced shot noise at bilayer graphene–superconductor junction: Graphene, if realized in ultraclean form, shows signatures of ballistic transport at higher carrier densities. Close to neutrality point, where the density of states becomes vanishingly small, current transport happen in a pseudo diffusive manner. The pseudo diffusive transport manifests as 1/3 Fano factor of shot noise in short and wide graphene, which reduces away from the neutrality point due to ballistic transport. When connected to a superconductor, twice Fano factor is expected as an indicator of 2e quasiparticle charge of superconductor. In this work, we measured shot noise in a bilayer graphene – Niobium superconductor junction revealing the higher charge transport when the excitation energy is less than the superconducting gap. When the excitation energy is more than superconducting gap, normal quasiparticle transport happens, and we observe 1/3 Fano factor at neutrality point, which reduces at higher carrier densities as expected. Within superconducting gap, we observed ~1.5 times enhanced noise. The reduction of shot noise from expected doubling can be understood as due to heat leak through the superconductor, which in turn is due to presence of residual density of states at the interface.
(Top-left) The edge contacted bilayer graphene device was fabricated using standard dry transfer technique. All the measurements were performed between contact-1 (Au) and contact 2 (Nb). Inset shows gate response of the device. (Bottom-left) The schematic of the device and the shot noise measurement set-up. (Right) Fano factor is plotted as a function of gate voltage for both bias energies higher and lower than superconducting gap. A global enhancement of Fano factor is clearly visible within the superconducting gap and the ratio (FS=FN) is shown in the inset.
Probing chiral Andreev edge states: Chiral quantum Hall (QH) edge states in proximity with a superconductor (SC) can give rise to exotic excitations. A notable example is Majorana fermion, whose evidence in condensed matter systems is still inconclusive. Graphene with moderate perpendicular magnetic field hosts clean QH edge states with insulating bulk, thus emerging as an ideal system to study superconducting proximity effect in QH regime. There are several promising theoretical proposals of realizing chiral Majorana fermion at QH-SC interfaces. Realization of the electron-hole hybrid states called Andreev edge states (AESs) at the QH-SC interface is an important step in this quest. The recent developments of several superconductors with large critical magnetic field (Bc) and transparent interfaces with high quality graphene have paved the way for a number of interesting experimental discoveries and demonstrations, such as crossed Andreev conversion (CAC), supercurrent in QH regime, inter-Landau-level Andreev reflection and interference of chiral AESs in graphene QH-SC junctions. Despite these progresses, the identification of AESs remains scarce, and its dynamics have remained unexplored in the presence of disorders and dissipations. AESs result from the successive Andreev reflections at a QH-SC interface, where an incident electron successively turns into a hole and back into an electron. Quantum mechanically, AESs are the fermionic modes, which are linear combinations of electron and hole. In this context, the shot noise, which arises due to the discrete nature of the current carriers can provide intriguing information about the AESs interference.
(Top-left) Schematic of the Andreev edge states (wavy lines), the electron-hole hybrid states formed at the QH-SC interface. A quantum mechanical model predicts half Fano factor for QH-SC junction in contrast to the vanishing Fano factor for the QH-NM case. (Right) The schematic of the MoRe-SLG-MoRe device along with the shot noise measurement setup. The MoRe lead is characterized separately in a He3 Refrigerator by standard four probe. I vs V measurement using a DC source meter and a DC multi-meter. The IV curve of MoRe showing existing superconductivity at 8 T at 240 mK temperature. (c) The conductance of the MoRe – graphene – MoRe device at filling factor 2 QH plateau at B = 2 T plotted as a function of back gate voltage VBG. The conductance at filling 2 plateau remain independent of excitation energy as can be seen from the G vs VSD plot in the inset. (Bottom-left) Differential Fano factor as a function of applied current (ISD) showing maxima close to zero current.