For Japanese publications, please see Japanese page.
(*: equal contribution  : corresponding author)

Selected Papers

Quantum phase transitions in highly crystalline two-dimensional superconductors
Y. Saito†, T. Nojima and Y. Iwasa
Nature Communications 9, 778 (2018).
According to the magneto-transport studies in 2D superconducting ZrNCl and MoS2, we found that the quantum metallic state commonly observed at low magnetic fields is converted via the quantum Griffiths state to the weakly localized metal at high magnetic fields. The scaling behavior, characterized by the diverging dynamical critical exponent (Griffiths singularity), indicates that the quantum fluctuation manifests itself as superconducting puddles. The present results suggest that an evolution from the quantum metallic to the quantum Griffiths state is generic nature in highly crystalline 2D superconductors with weak pinning potentials.

Nonreciprocal charge transport in noncentrosymmetric superconductors
R. Wakatsuki*, Y. Saito*(co-first), S. Hoshino, Y. M. Itahashi, T. Ideue, M. Ezawa, Y. Iwasa and N. Nagaosa
Science Advances 3, e1602390 (2017).
We demonstrated experimentally and theoretically that the nonreciprocal transport, which means that the electrical resistance depends on the current direction, in general, can be observed in the resistive superconducting fluctuation in noncentrosymmetric superconductors. Especially, with the example of gate–induced superconductivity in MoSsingle crystals, which possess threefold symmetry, the signal of the nonreciprocal transport in the superconducting state was found to be much enhanced compared to that in the normal state. This experimental result indicates that intrinsic nature of noncentrosymmetric systems can be accessed by the coherence of Cooper pairs.

RIKEN Research “An uneven resistance” (Link)
UTokyo Research “Seeking out new functions for superconducting nanoelectronics” (Link)

Highly crystalline 2D superconductors
Y. Saito, T. Nojima, and Y. Iwasa
Nature Reviews Materials 2, 16094 (2016).
Recent advances in materials fabrication enabled the manufacturing of ordered 2D electron systems, which are highly crystalline, despite their atomic layer thickness. In this review, we review recent developments in the field of newly emerged highly crystalline 2D superconductors and highlight the unprecedented physical properties of these systems. In particular, we explore the novel quantum phases, and the anomalous superconducting state maintained in large external magnetic fields.

Superconductivity protected by spin-valley locking in ion-gated MoS2
Y. Saito, Y. Nakamura, M. S. Bahramy, Y. Kohama, J. T. Ye, Y. Kasahara, Y. Nakagawa, M. Onga, M. Tokunaga, T. Nojima, Y. Yanase and Y. Iwasa
Nature Physics 12, 144ー149 (2016).   (arXiv:1506.04146)
By performing pulsed high-magnetic-field measurements up to 55 Tesla, we found that the gate-induced superconductivity in MoS2 single crystals exhibits an anomalously large upper critical field of approximately 52 T, which indicates an enhancement of the Pauli limit by a factor of four compared to the conventional one. Combined with first-principles calculation and associated realistic tight-binding calculations based numerical estimation of the upper critical field, we reveal that this unusual behavior is due to an inter-valley Cooper pairing that is protected by spin-valley locking originating from inplane mirror symmetry breaking in MoS2 single layer and strong spin-orbit coupling (Ising superconductivity).

Perspective “Opening the gate on superconductivity” Science 350, 13161317 (2015).
News and Views “Two-dimensional superconductivity: The Ising on the monolayer” Nature Physics 12, 112113 (2016).
UTokyo Research “2D superconductor surviving in high magnetic fields over 50 Tesla” (Link)

Metallic ground state in an ion-gated two-dimensional superconductor
Y. Saito, Y. Kasahara, J. T. Ye, Y. Iwasa and T. Nojima
Science 350, 409413 (2015).   (arXiv:1511.03266)
We found that an ion-gated ZrNCl single crystal, exhibiting a dome-shaped phase diagram (Tc vs carrier density), becomes a truly 2D crystalline superconductor with the effective superconducting thickness of 1.8 nanometers, which is thinner than one-unit-cell. In particular, we discovered that the zero resistance state is immediately destroyed by the application of out-of-plane magnetic fields, and consequently, a quantum metallic state is stabilized in a wide range of magnetic fields. We concluded that this quantum metallic is caused by the quantum tunneling of vortices due to the extremely weak pinning and disorder.

UTokyo Research “The ultimate 2D superconductor” (Link)
Highlighted in Asian Scientist as “A Minimally Disordered 2D Superconductor” (Link)

Ambipolar insulator-to-metal transition in black phosphorus by ionic-liquid gating
Y. Saito and Y. Iwasa
ACS Nano 9, 31923198 (2015).   (arXiv:1505.04859)
We investigated ambipolar transport properties in black phosphorus single crystals using an electric-double-layer transistor (EDLT) configuration. By inducing an ultra-high carrier density of ~ 1014 cm-2, an electric-field-induced transition from the insulating state to the metallic state was realized, due to electron and hole doping.