Introduction

Control of electronic phases in a field-effect transistor (FET) configuration has been an emergent concept both for fundamental research and the development of future electronic devices1,2,3,4,5. Recent demonstrations of electric-field-induced superconductivity6,7,8 and ferromagnetism9,10 exemplify the broad applicability of an electric field of sufficient strength for phase control. In this context, the electric double-layer transistor (EDLT) plays a central role due to the extremely high electric fields attainable at the solid/liquid interface, enabling charge accumulation in the channel surface at densities as high as 1015/cm2 (ref. 11). This is one to two orders of magnitude larger than that in conventional FETs, offering an easy access to electrostatic modulation of electronic phases. The benefits of facile and quasi-continuous control of the doping level have been clearly demonstrated in high-temperature superconductors recently12,13. The merit of the FET is exceptionally valuable in strongly correlated electron systems, in which the electrostatic modulation of electronic phases has attracted considerable interest because of its potential to achieve greater functionality and to realize a material-independent scaling limit14,15,16,17,18,19,20. The competition between the charge, spin and orbital degrees of freedom, gives rise to rich electronic phases, as exemplified in Mott insulators, various charge, spin and orbital orderings, multiferroics and superconductivity. At the boundary between these phases, the system is expected to be extremely sensitive to external stimuli21,22,23,24. In order to approach the phase boundary, one needs to control relevant parameters such as the doping level x and band-width W23. The tuning of x is ordinarily achieved by chemical doping whereas W is modified through chemical substitution or high pressure. However, these techniques are cumbersome and do not allow fine-tuning.

Here, we report on the search for a phase boundary in the ground-state phase diagram of the perovskite manganites R1−xAxMnO3 (R = rare earth elements and A = alkaline-earth elements), where, x stands for nominal hole doping. Manganites are unique in that W can be effectively controlled by application of an external magnetic field through the double-exchange mechanism25,26. Among a variety of perovskite manganites, we selected Pr1−xSrxMnO3 (PSMO) in the vicinity of x ~ 0.5 corresponding to the 1/4-filled state27,28. The phase diagram of bulk materials at B = 0 T is known to be very rich as shown in Fig. 1a. Upon hole doping, the ground-state evolves from a ferromagnetic metal (FM) to an A-type antiferromagnetic insulator (A-AFI) near x = 0.5, where the phase boundary is very sensitive to the hole doping. Quasi-continuous scans of both x and W, by means of EDLT and magnetic field, respectively, enabled us to uncover an insulating state in an extremely narrow range of carrier density (vide infra). Utilizing this critical phase, an ambipolar phase control by gate voltage (VG) was realized in the strongly correlated electron system, demonstrating the importance of field-effect phase control in the search for new states and functions of matter.

Figure 1
figure 1

The electric-field effect on the transport properties of a 5 nm-thick Pr1−xSrxMnO3 (PSMO) thin film.

(a), The electronic phase diagram of bulk PSMO with varying doping level 0.0 < x < 0.6. PI, A-AFI, FI and FM, denote paramagnetic insulator, A-type antiferromagnetic insulator, ferromagnetic insulator and ferromagnetic metal, respectively. Open red circles and blue triangles indicate Curie temperature (TC) and the metal-insulator transition temperature (TMI), respectively. (b), Schematic diagram of the electric double layer transistor with an ionic liquid electrolyte. (c), The temperature (T) dependence of the sheet resistance (Rs) at gate voltage (VG) of 0 and ±3 V. The positive and negative VG correspond to electron and hole doping, respectively. The inset shows gate voltage dependence of both TC and TMI defined as the temperature where d(log Rs)/dT = 0 on cooling. (d), Rs-T curves of the device at the magnetic field (B) of 5 T for VG = 0 and ±3 V.

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Results

The sample was a PSMO epitaxial thin film prepared by a pulsed-laser deposition method. It was patterned into a Hall-bar structure and both the channel and the gate electrode were covered by the ionic liquid for the application of electric fields (Fig. 1b). We examined the VG dependence of sheet resistance (Rs) vs. temperature (T) curves for a 5 nm-thick PSMO film, as shown in Fig. 1c. At VG = 0 V, the Rs showed an appreciable decrease below the Curie temperature (TC) of 193 K, followed by a sharp metal-insulator (MI) transition at the transition temperature (TMI) of 117 K in a cooling process. The temperature hysteresis was rather reproducible, indicating that it is originated from quenched disorder such as defects existing in the channel. With a positive bias of VG = 3 V, corresponding to electron doping, TMI decreased to 96 K, whereas TC slightly increased to 195 K. For hole doping with a negative bias of VG = −3 V, TMI increased to 130 K and TC decreased to 187 K. Thus, TMI shifted by 30 K through gating of VG = −3 V to 3 V as summarized in the inset of Fig. 1c, which quantitatively agrees with an expanded phase diagram of chemically doped PSMO around x ~ 0.5. Note that the device performance was highly reversible against repeated application of VG and was also reproducible in different devices as shown in Supplementary Information.

To investigate the effect of W modulation, we measured the gate effects under a magnetic field of B = 5 T as shown in Fig. 1d. Owing to the enhanced W by the magnetic field, VG = 3 V was strong enough to completely suppress the insulating state observed below TMI of ~60 K at VG = 0 V. In contrast, the negative gate voltage of VG = −3 V increased TMI to ~80 K and increased Rs below TMI concomitantly. Application of magnetic field made the Rs-T curve much more sensitive to VG. The colossal electroresistance was as large as eight orders of magnitude at 20 K. A further increase of W (B = 7.5 T) revealed an unexpected VG dependence of Rs as displayed in Fig. 2a. The insulating behavior at T = 20 K was already suppressed even at VG = 0 V and the FM state was realized by applying only VG = 1 V. As VG was swept to negative, Rs at low temperature started to increase, reached a maximum at VG = −1.4 V and then decreased, showing ‘ambipolar’ device operation. This indicates that hole doping also stabilized a metallic state although Rs below TMI was not as low as that in the electron doped state. This behavior is clearly illustrated in a three-dimensional plot of Rs in the VG-T plane as shown in Fig. 2b. There is a peculiar singular point, where an insulating state is stabilized in a very narrow VG region near −1.4 V. A VG sweep of ~±50 mV results in a four orders of magnitude change in Rs. This gigantic and steep ambipolar behavior suggests that VG = −1.4 V corresponds to the exactly 1/4-filled state (x = 0.5), as will be discussed below. Naturally, it is highly unlikely that the exactly 1/4-filled state will be realized to this level of accuracy by chemical means and thus it must have escaped our observation thus far. Note that this critical feature reproduces even in other devices (see Supplementary Information).

Figure 2
figure 2

A robust insulating state confined in a very narrow gate voltage region.

(a), The temperature dependence of Rs at different VG ranging from −3 V to 1 V under B = 7.5 T. (b), A three-dimensional plot of Rs measured in a warming process under B = 7.5 T as functions of temperature and VG. (c), Contour plots of Rs on the T-VG plane at various magnetic fields from B = 5 T to 8 T. These plots correspond to electronic phase diagrams under magnetic fields in the vicinity of x = 0.5. Note that the direction of abscissa is taken to facilitate the comparison of VG with x in the phase diagram, Fig. 1a. See also Fig. 3a below.

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Discussion

To understand the singular behavior at VG = −1.4 V, we explored the VG dependence of the Rs-T curve while varying the magnetic field strength. Figure 2c displays pseudo-color plots of Rs in the VG-T plane for B = 5–8 T. At B = 5 T, the AFI state (the red-to-green colored region) rises from VG = 1.5 V toward the negative VG region, showing no anomaly at VG = −1.4 V, i.e., at the exactly 1/4-filled state. With increasing W, the FM state (the blue colored region) becomes dominant not only at the electron doping side (VG > −1.4 V) but also at the hole doping side (VG < −1.4 V) with the appearance of a robust insulating phase only at VG = −1.4 V, which clearly stands out above 7 T. Note that Rs at the hole side is higher than that at the electron side. Figure 3a highlights this anomaly at VG = −1.4 V, where we plot Rs at 20 K in the VG-B plane, equivalent to a phase diagram in x-W coordinates. Although the overall trend in Fig. 3a is metallization on increasing W, the insulating state at the 1/4-filling robustly remains on the narrow region of ~±50 mV at B = 7.5 T. By passing over this sharp insulating phase, one should expect the gate-induced conversion from the insulating state to conducting states only by infinitesimal voltages. This would lead to the new concept of low-power-consumption and steep switching devices rather exceeding conventional FETs.

Figure 3
figure 3

The ground-state phase diagram of PSMO.

(a), Contour plot of Rs on the VG-B plane at T = 20 K showing the ground state phase diagram in the doping level x - band width W plane. A change in VG by ±1.5 V corresponds to a change in x of ±0.01 as estimated by the electrostatic carrier accumulation with a capacitance of 10 μF/cm2 (ref. 11). (b), A schematic of a metal-insulator phase diagram of perovskite manganites in the plane of x and W. A rectangle with dashed line located around x ~ 0.5 indicates the scan area shown in Fig. 3a. COO denotes charge/orbital ordered insulating phase.

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The singular nature of the insulating phase at VG ~ −1.4 V, which penetrates deep into the wider W region (shown in Fig. 3a), indicates that its origin differs from that for the broader insulating phase seen in smaller W. Based on the x-W phase diagram (shown in Fig. 3b), we attribute the stable insulating phase only at the 1/4-filling to the charge/orbital ordered (COO) state, whereas the broader insulating state away from the 1/4-filled state is ascribed to the A-AFI. Figure 3b shows a schematic phase diagram in terms of x and W in the vicinity of x ~ 0.5, proposed for bulk perovskite manganite compounds at zero magnetic field29. The chemical composition corresponding to the particular value of W is shown on the right side. As prototypical materials, La1−xSrxMnO3 and Pr1−xCaxMnO3 are well known for having large and small W, respectively30,31. While the latter exhibits very wide COO phase, it is much reduced in Nd1−xSrxMnO3 with a larger W, as shown in Fig. 3b. With a yet larger W, PSMO has been considered to be located just around the critical point, at which three phases compete29. However, by comparing Figs. 3a and 3b, we are compelled to ascribe that the COO state is anomalously stabilized extending deep into the larger W at the exact 1/4 filling. Therefore, the scanned region of our experiment shown in Fig. 3a lies within the rectangle delineated by dashed lines in Fig. 3b. The robustness of the COO phase arises from the commensurability of the 1/4-filling. i.e., one eg electron per two Mn sites. This phenomenon has escaped our detection in the past because Rs in the A-AFI state under ordinary circumstances is too high to distinguish it from COO in simple transport measurements and because the exact filling condition is too severe to be achieved by chance. Indeed, the width of COO state in the doping level, i.e. the doping modulation (Δx) in the ±50 mV sweeping of VG, is roughly estimated as Δx ~ ±0.0004 by assuming the penetration of accumulated carriers into the whole 5 nm-thick film (see Supplementary Information). The narrow range of Δx ~ ±0.0004 in PSMO together with the fact that it stands out only near a particular W would cause serious difficulties in chemical approach. This is the reason why the existence of the COO in bulk PSMO single crystals is still in debate29,32,33. The nature of the robust insulating state at the 1/4-filling is left to be clarified by in-situ neutron and x-ray scattering studies.

In summary, we demonstrate the gate-induced ambipolar phase switching associated with the colossal electroresistance in transistor devices of 1/4-filled PSMO by using a combination of external electric and magnetic fields. Quasi-continuous parameter control allowed us to map out the ground-state phase diagram of PSMO near a critical point, where we found a stable insulating state existing in a very narrow region of Δx = ±0.0004, which could be collapsed by a small voltage of ±50 mV generating a gigantic responses in Rs. More generally, our results revealed that a relatively small carrier modulation of ±1019/cm3, corresponding to Δx = ±0.0004, has a tremendous impact on a correlated electron system containing more than 1022/cm3 order of carriers. Such a gigantic response and a buried state at the critical point should not be peculiar to perovskite manganites alone, but should be common to strongly correlated electron systems, offering a novel opportunity to construct phase transition FET devices that operate with quite a low voltage.

Methods

The epitaxial thin film of PSMO was grown on a (110)-oriented (LaAlO3)0.3 – (SrAl0.5Ta0.5O3)0.7 substrate by a pulsed-laser deposition method (+0.7% mismatch). The film is coherently grown, i.e., the in-plane lattice is clamped to the substrate. Here the use of the (110)-oriented substrate allowed the Jahn-Teller distortion in the film required for the first-order phase transition and enabled us to observe a sharp metal-insulator transition even in ultra-thin films28,34. The film was patterned into a Hall-bar structure with a side gate located in the vicinity of channel by photo-lithography and Ar-ion milling. The channel was 520 μm long and 30 μm wide. All Au electrodes were deposited by electron-beam evaporation. Prior to this evaporation, we treated the film surface with O2 plasma for enhancing the Au adhesion. A hard-baked photoresist was used for electrical isolation between the side gate and channel. We completed the device by putting a small amount of ionic liquid N,N-diethyl-N-(2-methoxyethyl)-N-methylammonium bis-trifluoromethylsulfonyl)-imide on the channel and side gate. We measured Rs with a standard four-probe configuration by applying VG using a semiconductor parameter analyzer. Temperature was swept from 220 K to 15 K at a cooling/heating rate of 2.5 K/min. We applied VG at 220 K and kept it until the gate current was saturated before starting temperature sweep.