Two-dimensional (2D) crystalline superconductor [1] has attracted much attention due to its novel properties such as anomalous metallic state [2], quantum Griffiths singularities [3] and Ising superconductivity [4]. Although the metallic ground state is theoretically not expected in 2D bosonic systems, in recent years many experimental groups have reported the possible signature of anomalous metallic state (i.e. quantum metallic state) at low temperatures in different 2D superconducting systems. However, the high frequency noise from the external environments usually affects the measurement results in the ultralow temperature region, especially for the nanosystems showing fragile 2D superconductivity [5]. This raises doubts about previous experimental observations on the anomalous metallic state without appropriate filters. Therefore, the experimental study of the quantum ground state in ultralow temperature region is quite challenging and further investigations on the intrinsic ground states in crystalline 2D superconductors, especially for nanodevices, is still necessary.
Recently, Professor Jian Wang from International Center for Quantum Materials, School of Physics, Peking University, in collaboration with Prof. Xi Lin at Peking University, Prof. Ying Xing at China University of Petroleum (Beijing), Prof. Yi Liu at Renmin University of China and others, observed the external radiation induced resistance saturation (named as the extrinsic anomalous metallic state) at low temperatures in 4Ha-TaSe2 nanodevices. Interestingly, by sufficiently filtering the external perturbations, the lower temperature and higher magnetic field measurements manifest that the intrinsic anomalous metallic ground state still exists when approaching zero temperature. This work demonstrates the presence of an intrinsic anomalous metallic state even in the nanosystems with relatively fragile 2D superconductivity and confirms the necessity of effective filtering for low-temperature measurements in the research of quantum ground states.
Besides, Ising superconductivity revealed by ultrahigh in-plane critical field going beyond 6 times of the Pauli paramagnetic limit is detected in the one-unit-cell (around 2.8 nm) thick 4Ha-TaSe2 device with in-plane inversion symmetry breaking. Different with previous studies on Ising superconductivity, the in-plane critical field beyond the Pauli paramagnetic limit can even be detected in 4Ha-TaSe2 bulk crystals, which might be due to the weak coupling between the TaSe2 submonolayers.
The paper was published in Nano Letters on August 30, 2021 (Nano Letters 2021, DOI: 10.1021/acs.nanolett.1c01426). Prof. Jian Wang, Prof. Xi Lin at Peking University and Prof. Yi Liu at Renmin University of China are corresponding authors of this paper. Prof. Ying Xing, Pu Yang at China University of Petroleum (Beijing), Jun Ge and Jiaojie Yan at Peking University contributed equally to this work. This work was financially supported by the National Natural Science Foundation of China, the National Key Research and Development Program of China, Beijing Natural Science Foundation, and Strategic Priority Research Program of Chinese Academy of Sciences.
Paper link：https://doi.org/10.1021/acs.nanolett.1c01426
Figure: Extrinsic (a) and intrinsic (b) experimental evidences of anomalous metallic states in superconducting TaSe2 nanodevices. (c) Phase diagram of 2D TaSe2 showing zero resistance and intrinsic anomalous metallic (AM) state as quantum ground states of 2D crystalline superconductors.
【References】
[1] Saito, Y.; Nojima, T.; Iwasa, Y. Highly crystalline 2D superconductors. Nat. Rev. Mater. 2016, 2(1), 16094.
[2] Yang, C.; Liu, Y.; Wang, Y.; Feng, L.; He, Q.; Sun, J.; Tang, Y.;Wu, C.; Xiong, J.; Zhang, W.; Lin, X.; Yao, H.; Liu, H.; Fernandes, G.;Xu, J.; Valles, J. M.; Wang, J.; Li, Y. Intermediate bosonic metallic state in the superconductor-insulator transition. Science 2019, 366,505.
[3] Xing, Y.; Zhang, H. M.; Fu, H. L.; Liu, H. W.; Sun, Y.; Peng, J.P.; Wang, F.; Lin, X.; Ma, X. C.; Xue, Q. K.; Wang, J.; Xie, X. C. Science 2015, 350, 542.
[4] Xing, Y.; Zhao, K.; Shan, P.; Zheng, F.; Zhang, Y.; Fu, H.; Liu,Y.; Tian, M.; Xi, C.; Liu, H.; Feng, J.; Lin, X.; Ji, S.; Chen, X.; Xue, Q.-K.; Wang, J. Ising Superconductivity and Quantum Phase Transitionin Macro-Size Monolayer NbSe2. Nano Lett. 2017, 17, 6802−6807.
[5]Tamir, I.; Benyamini, A.; Telford, E. J.; Gorniaczyk, F.; Doron, A.; Levinson, T.; Wang, D.; Gay, F.; Sacépé, B.; Hone, J.; Watanabe, K.; Taniguchi, T.; Dean, C. R.; Pasupathy, A. N.; Shahar, D. Sensitivity of the superconducting state in thin films. Sci. Adv. 2019, 5, eaau3826.
