Quantum phase transition is one of most important topics in condensed matter physics. For the first time, we observe the divergence of dynamical critical exponent for the superconductor-metal transion in ultrathin cyrtalline Ga films grown on GaN substrate, which is a the major signature of quantum Griffiths singularity and manifests a new quantum phase transition in 2D superconductors [1,2]. This discovery is further revealed in LAO/STO(110) interface superconductors [3] and monolayer NbSe2 films[4].

It is well known that so far there are two classes of quantum oscillations. One is B periodic oscillations, such as AB and AAS effects for mesoscopic system and Little Parks oscillations for superconducting systems. The other one, i.e. 1/B periodic SdH oscillations from quantized Landau levels, might be more universal. However, we discover a new class of quantum oscillations beyond quantum limit in high quality topological materials, showing exotic log B period. Further theoretical investigation reveals that the Efimovian bound states can explain the log-periodic quantum oscillations (i.e. discrete scale invariance) well. [5]
 
References
[1] Science 350, 542 (2015);
[2] Science 350, 509 (2015);
[3] Phys. Rev. B 94, 144517 (2016);
[4] Nano Letters (2017), DOI: 10.1021/acs.nanolett.7b03026;
[5] arXiv:1704.00995