Investigation of Superconducting Phase Diagram of an Organic Mott Insulator
Using Simultaneous Control of Electric Field and Strain
Yoshitaka Kawasugi1, Kazuhiro Seki2, Masashi Tajima3, Jiang Pu4, Taishi Takenobu4, Seiji
Yunoki5, Hiroshi M. Yamamoto1,6, and Reizo Kato1
1

Condensed Molecular Materials Laboratory, RIKEN , SISSA2, Toho University3, Nagoya University4,
Computational Condensed Matter Physics Laboratory, RIKEN5, Institute for Molecular Science6

In materials with a half-filled band, strong on-site Coulomb repulsion localizes the conduction electron ateach site. This type of insulator is called Mott insulator. The phase transition between the Mott insulating andmetallic states (the Mott transition) has attracted much attention for these decades because unconventional
superconductivity emerges in the proximity of the Mott transition in some cuprates and molecular conductors(organic conductors).
There are two routes for the Mott transition. One is to weaken the electron correlation effectively byincreasing the electron transfer energy. This is experimentally achieved through pressure control. The otheris to reduce the commensurability between the electron density and lattice potential. Carrier doping enables
this operation. If we are able to tune the lattice spacing and doping concentration in a single Mott insulator,
we can thoroughly investigate the relationship between the Mott transition and the unconventional
superconductivity. Nevertheless, such an experiment had not been done yet because of lacking suitablesamples. Inorganic Mott insulators can be precisely doped with chemical substitution but are mostly notcompressible enough for the Mott transition. On the other hand, organic Mott insulators can be easily
compressed, but have been difficult to dope.
In RIKEN Condensed Molecular Materials Lab, we have been developing the field-effect doping tomolecularconductors.

Recently,

weachieved

dopingintoanorganic

Mottinsulator

κ-(BEDT-TTF)2Cu[N(CN)2]Cl (hereafter κ-Cl) up to approximately 20%, using the principle ofelectric-double-layer transistor (EDLT) [2]. In this study, we attempt exploring the superconducting phase
diagram in the proximity of the Mott insulating state by combining the EDLT doping and pressure control.
Without using a pressure cell, the effective pressure can be varied by bending the substrate [3].

Fig. 1. Schematic side view and optical top view of the organic Mott EDLT based on κ-Cl. A gate voltage varies
doping and a strain modifies the effective pressure in the same sample.

Figure 2 shows the temperature dependence of the resistivity in our organic Mott EDLT based on κ-Cl [4].
Without gating (Fig. 2a), the sample changes from a superconductor to an insulator with varying the tensile
strain. Non-monotonic temperature dependences are observed in the intermediate state, indicating that theinsulating state is antiferromagnetically ordered at low temperature. This reentrant behavior is consistentwith the pressure dependence of the resistivity in thebulk κ-Cl crystals [5].
Namely, the effective pressure for κ-Cl was
successfully tuned by bending of the substrateenough for the pressure-control phase transition.
Next,

ata

fixed

electric-field-induced

strain,wesuperconductivity

observedintheFig. 2. Temperature dependences of the resistivity (a)insulating state of the same sample (Fig. 2b). Thisat various tensile strains with no gate voltage and (b) atalso indicates that the EDLT doping is enough forvarious gate voltages at a fixed tensile strain.

the doping-control phase transition. Thus, byprecisely varying the strain and gate voltage, we caninvestigate the outlines of the superconducting andinsulating phases.
Figure 3 shows color plots of the resistivity
against gate voltage and tensile strain, whichindicatetheoutlinesofsuperconducting

phase

doping-asymmetricandthosephases.

appears
surroundingThehighlytheMottinsulating phase. On the hole-doped side, theresistivity monotonically decreases against gate
voltage and the superconducting state appears in aFig. 3. Gate voltage and tensile strain dependence ofthe resistivity at 5.5 K. The dark blue regions indicate

region where the hole doping level is above ca. 10%.

the superconducting phases. AFI, h-SC, e-SC denoteOn the other hand, the superconducting state

theemerges more drastically on the electron-doped sidesuperconducting, and electron-doped superconducting

antiferromagnetic

insulating,hole-dopedphases, respectively.

and it disappears by further doping.
The above doping asymmetry (except for thedisappearance of the superconducting state under high electron doping) has been qualitatively reproduced byour variational cluster approximation (VCA) calculations for the Hubbard model on an anisotropic triangularlattice. Seemingly it originates from the difference of the density of states between the bottom of the upper
Hubbard band and the top of the lower Hubbard band. For future works, we will investigate the origin of thedisappearance of the electron-doped superconductivity and apply the method for quantum spin liquid
candidates (paramagnetic Mott insulators) to gain insights into the relationship between thesuperconductivity and magnetism.
References[1] M. Imada et al., Rev. Mod. Phys. 70, 1039 (1998).
Suda et al., Adv. Mater. 26, 3490 (2014).
B 69, 064511 (2004).

[2] Y. Kawasugi et al., Nat. Commun. 7, 12356 (2016).

[3] M.

[4] Y. Kawasugi et al., Sci. Adv. accepted. [5] F. Kagawa et al., Phys. Rev.