Non-Fermi-liquid Behavior and their Doping Asymmetry
in an Organic Mott Electric-double-layer Transistor
Yoshitaka Kawasugi1, Kazuhiro Seki2, Jiang Pu3, Taishi Takenobu3, and Seiji Yunoki4, Hiroshi M.
Yamamoto5, and Reizo Kato1
1

Condensed Molecular Materials Laboratory, RIKEN, 2SISSA,
3
Nagoya University, 4Computational Condensed Matter Physics Laboratory, RIKEN, 5Institute for MolecularScience

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. Electron or hole doping into the Mott insulator
reduces the commensurability between the electron density and the lattice potential, and the electrons aredelocalized (band-filling-controlled Mott transition). The Mott transition has been intensively studied inrelation to the high-temperature (high-TC) superconductivity because the high-TC cuprates have been found to
be doped Mott insulators.
In the cuprates, high-TC superconductivity emerges between the Mott insulating and metallic (Fermiliquid) states as we increase the doping concentration. Indeed, the normal state of the high-TC
superconductors has been found to be a strange metal. The resistivity increases with temperature far above
the Mott–Ioffe–Regel limit at sufficiently high temperatures (the bad metal behavior). The Hall coefficient isstrongly temperature-dependent despite the metallic-like transport. On the other hand, the cotangent of theHall angle is proportional to T2 as in a Fermi liquid. These anomalies have often been viewed as key
unresolved signatures of strong correlation, which is probably relevant to the superconductivity. To discuss
their origins, it is important to investigate these behaviors in different systems which can be described withsimilar models, to speculate what behavior is material-dependent and what behavior universally appears in
the proximity of the Mott transition.
Recently, we have developed electric-double-layer doping into an organic Mott insulator
κ-(BEDT-TTF)2Cu[N(CN)2]Cl (hereafter κ-Cl), resulting in observation of ambipolar superconductivity
surrounding the Mott insulating state in one and the same sample. Therefore, our organic Mott electricdouble-layer transistor (EDLT) would likely show the non-Fermi-liquid behavior in the normal state. In thisstudy, we investigated the temperature dependence of the transport properties in an organic Mott EDLT, withvarying doping across half filling in a single sample.

Fig. 1. (a) Top view of the conducting BEDT-TTF layer in κ-Cl. Each BEDT-TTF dimer has one hole carrier. t and t' denotenearest- and next-nearest-neighbor hopping energy. (b), (c) Band structures of single-band models for κ-Cl and the cuprates.

Figure2showsthetemperaturedependence of the surface resistivity underelectrondoping.

metallic-like

Above

ca.

conduction

100

above

K,
theMott–Ioffe–Regel limit is observed in a wide
range of doping level. This is the bad metal
behavior in the sense that the mean-free pathof carriers is shorter than the site distance.
On the other hand, at low temperatures, theFermi-liquidbehavior(quadratic-in-temperatureresistivity)

andsemiconducting

appearsunderbehavior

electron and hole doping, respectively.
Figure3showsthetemperature

Fig. 2. Temperature dependence of the resistivity in a κ-Cl EDLT
under (a) electron and (b) hole doping. Solid black line denotes theMott–Ioffe–Regel limit in two dimensions. The resistivity undersufficient electron doping (VG ≥ 0.5 V) is quadratic in temperature(the Fermi liquid behavior).

dependence of the Hall coefficient and thecotangent of the Hall angle. Under electron doping, the Hall coefficient does not depend significantly on
temperature and corresponds to the carrier density in the non-interacting case. On the other hand, under holedoping, it largely increases with cooling despite the metallic-like conduction, indicating a reduction of thecarriers down to approximately one-third of the non-interacting carrier density. Nevertheless, the cotangentof the Hall angle is quadratic in temperature in a wide temperature range.
To summarize these results, (1) the bad metal behavior widely appears except at half filling regardless ofthe doping polarity at high temperatures. (2) At lower temperatures, the pseudo gap behavior is observed
only under hole doping, while (3) the Fermi-liquid-like behavior is observed under electron doping, within
the doping region studied here (up to ~±20%).
Although we need further studies to conclude, the bad metal behavior seems a universal high-energy scale
phenomenon in doped Mott insulators, while the pseudo gap behavior is based on lower energy scale physics
that can be influenced by details of the band structure, according to these results and the theoretically
predicted effect for a quantum spin liquid [3].

Fig. 3. (a) Temperature dependenceof the Hall coefficient in a κ-ClEDLT. (b) Hall angle vs T2 plots.
They appear more linear under holedoping.

References[1] P. A. Lee et al., Rev. Mod. Phys. 78, 17 (2006). [2] Y. Kawasugi et al., Nat. Commun. 7, 12356 (2016). [3] T.
Senthil, Phys. Rev. B 78, 045109 (2008).