Quantum Phase Transition in Few-Layer NbSe2 Probed through Quantized Conductance Fluctuations

We present the first observation of dynamically modulated quantum phase transition between two distinct charge density wave (CDW) phases in two-dimensional $2H\text{\ensuremath{-}}{\mathrm{NbSe}}_{2}$. There is recent spectroscopic evidence for the presence of these two quantum phases, but its evidence in bulk measurements remained elusive. We studied suspended, ultrathin $2H\text{\ensuremath{-}}{\mathrm{NbSe}}_{2}$ devices fabricated on piezoelectric substrates---with tunable flakes thickness, disorder level, and strain. We find a surprising evolution of the conductance fluctuation spectra across the CDW temperature: the conductance fluctuates between two precise values, separated by a quantum of conductance. These quantized fluctuations disappear for disordered and on-substrate devices. With the help of mean-field calculations, these observations can be explained as to arise from dynamical phase transition between the two CDW states. To affirm this idea, we vary the lateral strain across the device via piezoelectric medium and map out the phase diagram near the quantum critical point. The results resolve a long-standing mystery of the anomalously large spectroscopic gap in ${\mathrm{NbSe}}_{2}$.