A strained organic field-effect transistor with a gate-tunable superconducting channel

Abstract

In state-of-the-art silicon devices, mobility of the carrier is enhanced by the lattice strain from the back substrate. Such an extra control of device performance is significant in realizing high-performance computing and should be valid for electric-field-induced superconducting (SC) devices, too. However, so far, the carrier density is the sole parameter for field-induced SC interfaces. Here we show an active organic SC field-effect transistor whose lattice is modulated by the strain from the substrate. The soft organic lattice allows tuning of the strain by a choice of the back substrate to make an induced SC state accessible at low temperature with a paraelectric solid gate. An active three-terminal Josephson junction device thus realized is useful both in advanced computing and in elucidating a direct connection between filling-controlled and bandwidth-controlled SC phases in correlated materials.

ARTICLE

Received 19 May 2013 | Accepted 1 Aug 2013 | Published 23 Aug 2013

In state-of-the-art silicon devices, mobility of the carrier is enhanced by the lattice strain from the back substrate. Such an extra control of device performance is signicant in realizing high-performance computing and should be valid for electric-eld-induced superconducting (SC) devices, too. However, so far, the carrier density is the sole parameter for eld-induced SC interfaces. Here we show an active organic SC eld-effect transistor whose lattice is modulated by the strain from the substrate. The soft organic lattice allows tuning of the strain by a choice of the back substrate to make an induced SC state accessible at low temperature with a paraelectric solid gate. An active three-terminal Josephson junction device thus realized is useful both in advanced computing and in elucidating a direct connection between lling-controlled and bandwidth-controlled SC phases in correlated materials.

DOI: 10.1038/ncomms3379

A strained organic eld-effect transistor with a gate-tunable superconducting channel

Hiroshi M. Yamamoto1,2,3, Masaki Nakano4,5, Masayuki Suda1,2, Yoshihiro Iwasa5,6, Masashi Kawasaki5,6

& Reizo Kato2

1 Division of Functional Molecular Systems, Research Center of Integrative Molecular Systems (CIMoS), Institute for Molecular Science, 38 Nishigounaka, Myodaiji, Okazaki 444-8585, Japan. 2 RIKEN , 2-1 Hirosawa, Wako 351-0198, Japan. 3 JST, PRESTO (Precursory Research for Embryonic Science and Technology), 4-1-8 Honcho, Kawaguchi 332-0012, Japan. 4 Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Sendai 980-8577,Japan. 5 RIKEN Center for Emergent Matter Science (CEMS), 2-1 Hirosawa,, Wako 351-0198, Japan. 6 Quantum Phase Electronics Center and Department of Applied Physics, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Tokyo 113-8656, Japan. Correspondence and requests for materials should be addressed to H.M.Y. (email: mailto:[email protected]

Web End [email protected] ).

NATURE COMMUNICATIONS | 4:2379 | DOI: 10.1038/ncomms3379 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications 1

& 2013 Macmillan Publishers Limited. All rights reserved.

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3379

Strained silicon shows enhanced mobility because of the band-structure tuning and is widely used in modern high-spec circuits1. This technique should be also applicable to

the innovation of wider range of electronic devices including a eld-induced superconducting (SC) interface26. If a strained SC eld-effect transistor (FET) is realized, it will contribute in nding new materials for, in guring out unknown phase diagrams of and in utilizing quantum devices based on, superconductivities. In general, organic materials enjoy soft lattice and therefore the strain effect on its properties should be signicant because of the alteration in the non-covalent intermolecular interaction. Indeed, the organic superconductivity is known to be sensitive to the physical and chemical pressure effects, providing a well-investigated platform to check the inuence of strain on the SC transition. An organic superconductor that neighbours Mott-insulating (MI) phase has several advantages in realizing SC-FET with a strained interface with following reasons. First, the relationship between the strain and the electron correlation U/W, where U is the effective Coulomb interaction and W is the bandwidth, is well established7,8, leading to precise analysis of the obtained result. Second, the relationship between U/W and SC state in the phase diagram with half-lled (non-doped) band is already well investigated both experimentally and theoretically912. Third, the carrier number required for the induction of SC state is expected to be relatively low because of the large molecular size along with the dimerized crystal structure. For example, the half-lled carrier density for the k-type BEDT-TTF (bis(ethyelenedithio)tetrathiafulvalene)

material (ca. 2 1014cm 2)13 is less than one-third of that for

high-Tc cuprates (ca. 7 1014 cm 2), which will make a eld-

induced superconductivity within the reach of carrier density modulation with solid paraelectric gate insulator, and thus an active control of the device in a real-time low-temperature experiment is possible. Finally, there should be no dangling bond and associated carrier trap on the surface because molecular crystal is constructed only by weak intermolecular interactions, which is good for transistor operation.

The above four reasons justify a strained organic SC-FET as an ideal test ground for the strain effect in gate-controlled SC devices. At the same time, SC-FET with organic material merits, when it is realized, also in understanding the phase diagram of correlated materials superconductivity in simultaneous control of bandwidth and bandlling, which has been impossible to be studied by other methods. For example, bandwidth-controlled SC phase with half-lled condition has not yet been realized in cuprates because of its hard lattice, although it is important in verifying the superconductivity mechanism to know whether lling-controlled and bandwidth-controlled SC phases are connected to one another or not.

Here we demonstrate an active eld-effect control of super-conductivity in a strained FET with an organic Mott insulator, k-(BEDT-TTF)2Cu[N(CN)2]Br (k-Br), whose ground state is tuned in the vicinity of a strain-induced Mott transition. This device provides a novel three-terminal Josephson junction (JJ) whose transport characteristics reveals a phase diagram of an organic Mott insulator, where a direct connection between lling-controlled and bandwidth-controlled SC phases is suggested.

ResultsStrain effect from the substrate. We fabricated FETs by laminating thin single crystals of k-Br on top of metallic Nb-doped

SrTiO3 (STO) substrates covered with 30 nm of Al2O3 dielectric layer grown by atomic layer deposition (Fig. 1a). k-Br is a highly correlated organic superconductor (Tc 11.6 K)14 whose

bandwidth-controlled SC phase neighbours an antiferromagnetic

MI phase11 (Fig. 1b). The ground state of k-Br can be nely tuned by physical and/or chemical pressure. In our previous studies15, by laminating it onto SiO2/Si substrates, we applied tensile strain to thin crystals of k-Br and guided them into a MI state at low temperature. As thermal expansion coefcients are different between a Si substrate and a k-Br crystal (ca. 2 and 60 p.p.m. K 1 at room temperature, respectively), the k-Br crystal is expanded at low temperature as shown in Fig. 1b, blue arrow. In the present study, STO was chosen as a substrate because of its relatively large thermal expansion coefcient (ca. 10 p.p.m. K 1 at room temperature) to adjust the ground state of k-Br very close to a partially SC region by a weaker tensile strain. In Fig. 1c, we show normalized temperature dependence of the resistance (RT plot) for two devices (1 (orange) and 2 (red)) as well as those for k-Br bulk (black) and k-Br on a SiO2/Si substrate (blue). Because of the weak tensile strain effect from STO substrate, the resistances for these two samples remain in between a complete SC state and a highly resistive MI state, as indicated in Fig. 1b by orange and red arrows. Device 1 showed insulating behaviour at low temperature, whereas device 2 showed a small resistance drop at 12 K, followed by a re-entrant percolation transition around 9 K. This behaviour of device 2 indicates that the system is in the partially SC phase where separation between SC and MI phases occurs and the SC fraction is maximized around 9 K. Such a situation has already been investigated in detail for a bulk material by infrared spectroscopy mapping16, nuclear magnetic resonance17, noise measurement18 and so on. In our devices, magnetization measurement for another sample (device 3) also showed partial superconductivity and maximization of SC fraction at medium temperature range, which will be described later. The above sample dependence between devices 1 and 2 indicates that the conductivity of k-Br is very sensitive to the small difference in the strain that is produced in the lamination and cooling processes.

n-type eld-effect and superconductivity in device 1. By applying gate voltage (VG), the resistance of the device 1 drastically changed with n-type polarity as shown in Fig. 2a. With negative VG, the resistance increases rapidly, whereas the application of positive VG squeezed out the insulating phase into low-temperature region and the device became weakly metallic at VG 2 V. By further increasing VG, it showed superconductivity

at VG48 V. The current-voltage characteristic in the SC region (Fig. 2a, inset) showed typical bistable switching of a resistance-shunted JJ with a McCumber parameter much larger than unity19, which implied an inhomogeneous SC transition at the interface. The contour map of the resistance in logarithmic scale is shown in Fig. 2b, suggesting a SC dome very similar to that for cuprate superconductors (white-dashed line). This provides evidences that the cuprates and the k-type organic system share the common phase diagram in the lling-controlled regime.

p-type eld-effect and superconductivity in device 2. In Fig. 3a, we show an RT plot below 13 K for device 2. The device entered partially SC state at 12 K even at VG 0 V, followed by a reen

trant percolation transition around 9 K (OFF-ON transition at9.2 K and ON-OFF transition at 8.1 K). This re-entrant transition is already known in a k-type bulk crystal in the vicinity of bandwidth-controlled Mott transition20, and can be explained by the recurrent decrease of SC volume fraction at low temperature. A schematic image of a percolation transition of a JJ network (JJN) in our device is shown in Fig. 3b. The ON switching of JJN designates the minimum of the free energy for the SC state with respect to that for the MI state because the fraction ratio of these two competing phases reects the relative difference in their free energies. The JJN in device 2 was switchable in situ by changing

2 NATURE COMMUNICATIONS | 4:2379 | DOI: 10.1038/ncomms3379 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications

& 2013 Macmillan Publishers Limited. All rights reserved.

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3379 ARTICLE

a

BEDT-TTF+0.5

BEDT-TTF+0.5

BEDT-TTF+0.5

Cu[N(CN)2]Br1

Cu[N(CN)2]Br1

V+ V

I

Al2O3

I+ -Br

V+ V

I

I+

-Br

Al2O3 (28 nm)

VG

Nb-SrTiO3

b c

-Cl

-Br

100

-Br

103

PI

-Br

PM

-Br

-Br

Si

ST O

SC (percolative)

AFI (Mott-insulator)

SC (complete)

RN

Si

Tensile

102

R(T )/R (300 K)

101

-Br

T/K

STO

10

100

101

-Br/SiO2/Si

-Br/Al2O3/SrTiO3 2

-Br/Al2O3/SrTiO3 1

-Br (bulk)

-Br

1

102

5 10 50 100

Tensile

Compressive

Strain

T/K

Strong Weak Electron-correlation (U/W)

Figure 1 | Device conguration and the basic conduction properties. (a) Microscope image (upper right; scale bar, 100 mm) and schematic section (lower right) of the device, along with crystal structure of k-Br, where cationic BEDT-TTF layers and anionic Cu[N(CN)2]Br layers alternate each other (left). (b) Phase diagram of k-type BEDT-TTF system with bandlling 0.5, after refs 17,20,31,32. The horizontal axis represents electron correlation

that can be tuned by physical or chemical pressures as well as strain from the substrates. These pressure effects control bandwidth of the half-lled conduction band. PI, PM, AFI and RN denote paramagnetic insulator, paramagnetic metal, antiferromagnetic insulator and recurrent non-metallic state, respectively. Between AFI and complete SC, there is a percolative SC region where MI and SC phases coexist. Blue, orange, red and black arrows indicate trajectories that k-Br crystals on SiO2/Si, on Al2O3/SrTiO3(1 and 2) and as bulk experience upon cooling, respectively. (c) Temperature dependencies of relative resistance for k-Br on SiO2/Si (blue) or Al2O3/SrTiO3 substrates (1 orange, 2 red), as well as that of bulk crystal (black).

either magnetic or electric elds too. When the magnetic eld was swept, the device exhibited switching with large hysteresis (ON-OFF at 1.2 T, OFF-ON at 0.8 T; Fig. 3a, inset). As for the gate-electric eld, the device exhibited re-entrant JJN switching with p-type polarity upon VG sweeping below 8 K (Fig. 3c). The VG required for the switching was shifted to larger negative value as the temperature decreased. The whole resistance mapping as a function of T and VG is shown in Fig. 3d. In a mid-gap VG area ( 4 VoVGo11 V), where essentially no mobile carriers were

introduced because of mid-gap traps (that is, the essential bandlling is just 0.5), the JJN was switched to the ON state around 89 K (blue area). At VGo 4 V, on the other hand, the

switching temperature started shifting downwards as the hole injection proceeded. The blue area continued to reach to a fully hole-doped SC state around (T, VG) (3 K, 13 V). As these JJN

switchings at around (T, VG) (8.5 K, 0 V) and (T, VG) (3 K, 13 V) reect the free-energy minima for the bandwidth-controlled and the lling-controlled SC states, respectively, Fig. 3d

clearly shows a seamless connection between these two different SC states.

Magnetic measurement for n-type device 3. We also conrmed by magnetic susceptibility measurement (for device 3, Fig. 4) that the devices exhibited diamagnetic shielding effect whose volume fraction could be modulated by applying VG. Although there was strong sample dependence, device 3 exhibited the largest Meissner and shielding effect among several samples we have measured, with the magnetic eld applied parallel to the BEDT-TTF layers. At VG 0 V, the OFF-state shielding effect was observed

below 7 K, whereas the shielding effect was enhanced by applying a positive VG, implying the device switching to the ON state with the n-type characteristics. This enhancement was reproducible at VG sweep. With an assumption that the shielding effect is proportional to the volume fraction of the superconductivity and that the volume fraction of the (unstrained) bulk crystal is 100%, one

NATURE COMMUNICATIONS | 4:2379 | DOI: 10.1038/ncomms3379 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications 3

& 2013 Macmillan Publishers Limited. All rights reserved.

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3379

Change in electron number (/ BEDT-TTF dimer)

Device 1

VG = 9 V T = 4 K

0.1

0.05

0

0.05

0.1

6

Log R

VG = 3 V

2 V

1 V

0 V

0.5 V

1 V

2 V

7.8

106

7.0

R()

6.0

5

T(K)

105

5.0

0.1

I(A)

9 V

3.8

0

0.1

104

0 V (V)

4

4 5 6

10

5

0

5

10

T (K)

VG (V)

Figure 2 | Characteristics for device 1. (a) Temperature dependencies of the resistance of the device 1 under various VG in logarithmic scale. The inset shows current (I)-voltage(V) characteristics at VG 9 V and T 4 K. The results can be repeated more than three times without any signicant

difference. (b) Contour plot of resistance as a function of temperature and VG. The white broken line indicates the n-doped SC area. The upper horizontal scale shows the eld-induced change in the electron number per BEDT-TTF dimer at the rst monolayer interface.

a b

Free energy

Free energy

Device 2

20 VG = 0 V

B B = 1 T

= 3 T

OFF

ON

MI SC

R (k)

SC MI

JJ

B = 0 T

10

B, T or VG

I+

V+ IV

T=8.5 K

10 VG = 0 V

R (k)

0 1.0 2.0

0 B (T)

0 5 10 13

MI SC

T (K)

Change in electron number (/BEDT-TTF dimer)

c d

0.1

0 0.05

0.05 0.1

13

13

13 K

5 K

10

13.0 (k)

7.8

R (k)

5

T (K)

8

8

3.9

Device 2

0

0

Device 2

10 0 10

3 10

5

0

VG (V)

5 10 VG (V)

Figure 3 | Characteristics and switching mechanism for device 2. (a) Temperature dependencies of device 2 resistance under various magnetic elds without VG. A partial SC transition occurs at 12 K, after which percolation OFF/ON and ON/OFF transitions of JJN is observed. The inset shows resistance change under magnetic eld sweep at T 8.5 K, where percolation transition with large hysteresis is observed. (b) Schematic images

representing OFF and ON states of the JJN, along with free-energy diagrams for these two states. When the temperature, magnetic eld or VG is changed, the ratio of the SC fraction changes to evoke the JJN switching. (c) Resistance change under VG sweep at 13 and 5 K. The arrows indicate the sweep directions. (d) Contour plot of resistance as a function of temperature and VG with rising VG sweep direction. These results (ad) can be repeated more than three times without any signicant difference.

can estimate the SC fraction of both OFF and ON states of the device by comparing the diamagnetic values of device 3 with that of bulk k-Br. As a result, the temperature dependency of the

volume fraction of superconductivity in device 3 at various VG is

estimated as shown in Fig. 4b. Taking into account that the thickness of the charge accumulation layer is one or two layers15

4 NATURE COMMUNICATIONS | 4:2379 | DOI: 10.1038/ncomms3379 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications

& 2013 Macmillan Publishers Limited. All rights reserved.

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3379 ARTICLE

Device 3

0.0

FC

VG = 10 V

VG = 5 V

VG = 0 V

VG = 5 V

VG = 10 V

Bulk

FC

Magnetization/e.m.u. g1

0.1

ZFC

Relative shielding fraction

(M device 3/M bulk)

1.0

0.8

0.6

0.4

0.2

0.0

ON state VG = 10 V

OFF state VG = 0 V

0.2

0.23

H

0.3

0.24

0.4

ZFC

-Br

Al2O3/STO

0.25

2

3

4

Device 3

0.5

0

2

4

6

8

10

12

14

0

2

4

6

8

10

T / K

T / K

Figure 4 | Magnetic measurements for device 3. (a) Magnetic susceptibility of device 3 under various VG (red, yellow, black, green and blue) along with that for bulk k-Br (grey). FC and ZFC designate the measurements under eld-cooled and zero-eld-cooled conditions, respectively. The inset shows a magnied plot at low-temperature region. Note that the inter-layer coupling in a bulk k-Br crystal is a naturally formed Josephson coupling that allows penetration of magnetic uxes to some extent. (b) Relative shielding fraction of device 3 in OFF (VG 0 V) and ON (VG 10 V) states

calculated by the data in panel a. The shielding diamagnetism for device 3 is divided by that for the bulk k-Br at the same temperature to estimate the volume fraction. H denotes the direction of applied magnetic eld.

and the thickness of k-Br crystal on the device 3 (550 nm) corresponds that of about 350 BEDT-TTF layers, the SC transition seems not only to take place in the charge-injected layers but also to propagate into the neighbouring layers in the thickness direction because ca. 5% of the change of the volume fraction between OFF (VG 0 V) and ON (VG 10 V) states well exceeds the expected

value for monolayer transition (ca. 0.3%). Such a bulk phase transition seems to originate from an interlayer dielectric screening of the Coulomb interaction U21. The maximization of the SC fraction (or recurrent decrease of SC fraction) around 5 K of device 3 also designates a characteristic feature of k-Br system in this percolative region, which is commonly seen in the re-entrant percolation transition of device 2.

DiscussionSo far, the hole- or electron-doped cuprates superconductivity and k-type organic superconductivity have been separately discussed as in Fig. 5a, although scientists have pointed out many common physical properties22. These similarities are a consequence of a Mott transition, where doping controls the bandlling of cuprates while a pressure/strain controls bandwidth of organic Mott insulators. However, the simultaneous control of the strain (bandwidth) and the carrier density (bandlling) in a single Mott device is necessary for a deeper discussion of SCMI phase competition in the unied diagram. For example, the order parameter symmetry is pointed out to be dx2 y2 for cuprate and dxy for k-BEDT-TTF23,24. To discuss the origin of this discrepancy, it is important to realize both lling-controlled and bandwidth-controlled superconductivity in the same material and map out the connection between these two phases, which has been impossible to be checked with cuprates that exhibits a hard lattice. In the present experiments, it turned out that these SC phases are directly connected to one another as drawn in Fig. 5b. Inside the yellow SC arc, the U/W is too high to evoke superconductivity, whereas it is too weak outside the arc. The present organic FET devices having controllable bandwidth and bandlling therefore provide an indispensable opportunity for exploring a phase diagram of a channel material in a wide parameter space.

Judging from the RT plots for devices 1 and 2, the difference in the strain for these devices corresponds to ca. 10 MPa

difference in hydrostatic pressure for bulk material20. From crystallographic experiment, the lattice of k-type BEDT-TTF material is known to shrink at a rate of about 0.025% per 10 MPa, and the transfer integrals between molecules increases at a rate of about 0.2% per 10 MPa (ref. 25). This means that the difference in U/W for devices 1 and 2 (in Fig. 5b) is only 0.2% with respect to the absolute value of U/W (an order of unity), when one takes into account the fact that W is proportional to the transfer integral (W 4|t| where t is the inter-dimer transfer integral), whereas

effective U is not sensitive to the change in the transfer integral7 (because UE2|tdimer| 4t

dimer

2/Ubare, where tdimer is intra-dimer

transfer integral whose value is about 0.25 eV and Ubare is on-site

Coulomb repulsion for a single BEDT-TTF molecule whose value is about 1.0 eV, small changes in tdimer cancels out.). As such a small strain of 0.025% in the lattice evokes a drastic change in the conductivity behaviour and associated FET characteristics, the present results have proved that the control of the strain is a signicant tool for the quest for organic SC-FETs.

As the gate insulator in the present device is made of a paraelectric solid, an active switching of JJ has become possible. Such an electrostatically switchable three-terminal JJ device may nd applications in SC computation methods such as rapid single-ux quantum26 and quantum computers27 because these beyond Complementary Metal-Oxide Semiconductor methods utilizes SC quantum interference device. In addition, an in situ gate-sweep measurement at low temperature allows a detection of rst-order (hysteric) transitions in the bandlling-controlled regime. One example of such a transition is already visible in Fig. 3c, where the resistance showed a bistable behaviour depending on the sweep direction just inside the SC region. This rst-order phase transition is presumably relevant to the discrepancy in one-particle excitation spectrum of a Mott insulator that is dependent on the sweep direction of the correlation strength as anticipated by theories2830.

In summary, we have realized a strained organic FET with gate-tunable SC channel. The phase diagram obtained by the device operation shows a direct connection between bandwidth-controlled and lling-controlled SC phases around a MI phase, which has been impossible to realize in inorganic cuprates. The gate-tunable JJ may be useful in beyond Complementary Metal-Oxide Semiconductor quantum devices and detecting a hidden rst-order phase transition in a lling-controlled regime.

NATURE COMMUNICATIONS | 4:2379 | DOI: 10.1038/ncomms3379 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications 5

& 2013 Macmillan Publishers Limited. All rights reserved.

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3379

Anderson-insulator

Anderson-insulator

Anderson-insulator

Anderson-insulator

SC (hole-doped cuprate)

Mott-insulator

SC (electron-doped cuprate)

SC (p-doped)

Mott-insulator

SC (n-doped)

Electron-correlation (U/W)

Electron-correlation (U/W)

Device 1

Device 2

Tensile

SC (organic)

SC (intrinsic)

Tensile

Doping

Metal

Doping

Metal

Compressive

Compressive

0 0.5

0.5 1

1

Bandfilling

0

Bandfilling

Figure 5 | Phase diagrams around a Mott insulator. (a) Phase diagram that has been experimentally determined before this experiment. (b) Phase diagram that has been conrmed by this experiment. SC (intrinsic) denotes bandwidth-controlled SC phase, whereas SC (n-dope) and SC (p-dope) denote bandlling-controlled SC phases. Devices 1 and 2 can be mapped as red arrows.

Methods

Substrate preparation. A 30-nm-thick amorphous Al2O3 gate insulator was grown at 150 C on a 0.05 wt% Nb-doped SrTiO3 (001) single-crystal substrate (Shinkosha Co., Ltd.), which was used as a bottom gate. A thickness of Al2O3 was

evaluated by low-angle X-ray reection measurement. A surface roughness was o0.2 nm, which is comparable to that of a substrate. Typical electrical properties of a Al2O3 dielectric layer were characterized by current-voltage and capacitance-voltage characteristics of a capacitor made of Au/Ti/Al2O3/Nb:SrTiO3, showing large breakdown eld (B67 MV cm 1) and high dielectric constant (B910).

Crystal growth and lamination process. All the chemical compounds were purchased from commercial sources and used without further purication unless noted. 1,1,2-trichloroethane was puried by a basic aluminium column. TTP[N(CN)2] (TTP, tetraphenylphosphonium) was precipitated from an equi-molar mixture of Na[N(CN)2] and TTP-Br aqueous solutions and recrystallized from ethanol-ethyl acetate. A thin (100300 nm) single crystal of k-Br was grown electrochemically by oxidizing BEDT-TTF (50 mg) dissolved in 100 ml of 1,1,2-trichloroethane (10% v/v ethanol) in the presence of TTP[N(CN)2] (200 mg),

CuBr (50 mg) and TTP-Br (20 mg). After applying galvanostatic current of 5.0 mA for 15 h, thin crystal was picked up under microscopic observation. Then, it was transferred into an ethanol (10 ml) by pipette. A 3-mm square-STO substrate was immersed in the same ethanol and the crystal was guided on top of the substrate using the tip of a strand of hair. The substrate was then removed from the alcohol to be dried.

Transport measurements. The k-Br crystal was cut by laser (V-technology; VL-C30-GB) to form terminals. Gold wires (15 mmf) were attached to these terminals with silver and/or carbon paste. Standard four-probe measurement was done under He atmosphere in an automatic cryochamber (Niki-glass) equipped with a SC magnet.

Magnetic measurements. The magnetization of the device was measured using SC quantum interference device (Quantum Design, MPMS) equipped with reciprocating sample option. The weight of the thin k-Br crystal on device 3 was calculated by using its thickness, area and specic gravity. Two phosphorous bronze wires were attached on the k-Br and the gate substrate to modulate the VG during the magnetic measurement. For eld-cooled measurement, the device was cooled down from 20 to 2 K, during which time a magnetic eld of 8 T was applied, before the measurement. The magnetization was measured at a magnetic eld of 100 Oe that is parallel to the BEDT-TTF layers in all the measurements.

References

1. Ghani, T. et al. A 90 nm high volume manufacturing logic technology featuring novel 45 nm gate length strained silicon CMOS transistors. IEDM Tech. Dig. 2003, 11611163 (2003).

2. Bollinger, A. T. et al. Superconductor-insulator transition in La2-xSrxCuO4 at

the pair quantum resistance. Nature 472, 458460 (2011).3. Ye, J. T. et al. Superconducting dome in a gate-tuned band insulator. Science 338, 11931196 (2012).

4. Leng, X., Garcia-Barriocanal, J., Bose, S., Lee, Y. & Goldman, A. M. Electrostatic control of the evolution from a superconducting phase to an insulating phase in ultrathin YBa2Cu3O7-x lms. Phys. Rev. Lett. 107, 027001 (2011).

5. Ueno, K. et al. Electric-eld-induced superconductivity in an insulator. Nat. Mater. 7, 855858 (2008).

6. Takahashi, K. S. et al. Local switching of two-dimensional superconductivity using the ferroelectric eld effect. Nature 441, 195198 (2006).

7. Tamura, M. & Kato, R. Effective on-site repulsion in molecular conductors with dimeric structure: is the transfer integral a good measure of correlation? J. Phys. Soc. Jpn 73, 31083110 (2004).

8. Mori, T. et al. The intermolecular interaction of tetrathiafulvalene and bis(ethyelendithio)tetrathiafulvalene in organic metals. calculation of orbital overlaps and models of energy-band structures. Bull. Chem. Soc. Jpn 57, 627633 (1984).

9. Toyota, N., Lang, M. & Muller, J. in Low-Dimensional Molecular Metals (Springer-Verlag, 2007).

10. Ishiguro, T., Yamaji, K. & Saito, G. in Organic Superconductors (Springer-Verlag, 1998).

11. Kanoda, K. Metal-insulator transition in k-(ET)2X and (DCNQI)2M: two contrasting manifestation of electron correlation. J. Phys. Soc. Jpn 75, 051007 (2006).

12. Kino, H. & Fukuyama, H. Electronic states of conducting organic k-(BEDTTTF)2X. J. Phys. Soc. Jpn 64, 27262729 (1995).

13. Kato, R. et al. A new ambient-pressure superconductor, k-(BEDT-TTF)2I3.

Chem. Lett. 1987, 507510 (1987).14. Kini, A. M. et al. A new ambient-pressure organic superconductor, k-(ET)2Cu[N(CN)2]Br, with the highest transition temperature yet observed (inductive onset Tc 11.6 K, resistive onset 12.5 K). Inorg. Chem. 29,

25552557 (1990).15. Kawasugi, Y. et al. Electric-eld-induced Mott transition in an organic molecular crystal. Phys. Rev. B 84, 125129 (2011).

16. Sasaki, T. & Yoneyama, N. Spatial mapping of electronic states in k-(BEDT-TTF)2X using infrared reectivity. Sci. Technol. Adv. Mat. 10, 024306 (2009).

17. Miyagawa, K., Kanoda, K. & Kawamoto, A. NMR studies on two-dimensional molecular conductors and superconductors: Mott transition in k-(BEDT

TTF)2X. Chem. Rev. 104, 56355653 (2004).18. Muller, J. Fluctuation spectroscopy: a new approach for studying low-dimensional molecular metals. Chemphyschem. 12, 12221245 (2011).

19. McCumber, D. E. Effect of Ac impedance on dc voltage-current characteristics of superconductor weak-link junctions. J. Appl. Phys. 39, 31133118 (1968).

20. Ito, H., Ishiguro, T., Kubota, M. & Saito, G. Metal-nonmetal transition and superconductivity localization in the two-dimensional conductor k-(BEDT

TTF)2Cu[N(CN)2]Cl under pressure. J. Phys. Soc. Jpn 65, 29872993 (1996).21. Shinaoka, H., Misawa, T., Nakamura, K. & Imada, M. Mott transition and phase diagram of k-(BEDT-TTF)2Cu(NCS)2 studied by two-dimensional model derived from ab initio method. J. Phys. Soc. Jpn 81, 034701 (2012).

22. McKenzie, R. H. Similarities between organic and cuprate superconductors. Science 278, 820821 (1997).

23. Izawa, K., Yamaguchi, H., Sasaki, T. & Matsuda, Y. Superconducting gap structure of k-(BEDT-TTF)2Cu(NCS)2 probed by thermal conductivity tensor.

Phys. Rev. Lett. 88, 027002 (2002).24. Arai, T. et al. Tunneling spectroscopy on the organic superconductor k-(BEDT-TTF)2Cu(NCS)2 using STM. Phys. Rev. B 63, 104518 (2001).

25. Rahal, M. et al. Isothermal compressibility and pressure dependence of the crystal structures of the superconducting charge-transfer salt

6 NATURE COMMUNICATIONS | 4:2379 | DOI: 10.1038/ncomms3379 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications

& 2013 Macmillan Publishers Limited. All rights reserved.

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3379 ARTICLE

k-(BEDT-TTF)2Cu(NCS)2 [BEDT-TTF Bis(ethyelenedithio)tetra

thiafulvalene]. Acta Cryst. B53, 159167 (1997).26. Likharev, K. K. & Semenov, V. K. RSFQ logic/memory family: a new Josephson-junction technology for sub-terahertz-clock-frequency digital systems. IEEE Trans. Appl. Supercon. 1, 328 (1991).

27. Niskanen, A. O. et al. Quantum coherent tunable coupling of superconducting qubits. Science 316, 723726 (2007).

28. Kotliar, G., Lange, E. & Rozenberg, M. J. Landau theory of the nite temperature Mott transition. Phys. Rev. Lett. 84, 51805183 (2000).

29. Furukawa, Y., Ohashi, T., Koyama, Y. & Kawakami, N. Mott transition in the Hubbard model on the anisotropic kagome lattice. Phys. Rev. B 82, 161101 (R) (2010).

30. Sordi, G., Haule, K. & Tremblay, A. M. S. Mott physics and rst-order transition between two metals in the normal-state phase diagram of the two-dimensional Hubbard model. Phys. Rev. B 84, 075161 (2011).

31. Zhang, S. C. A unied theory based on SO(5) symmetry of superconductivity and antiferromagnetism. Science 275, 10891096 (1997).

32. Murakami, S. & Nagaosa, N. Multicritical phenomena of superconductivity and antiferromagnetism in organic conductor k-(BEDT-TTF)2X. J. Phys. Soc. Jpn 69, 23952398 (2000).

Acknowledgements

We thank Y. Tokura, I. Kawayama and A. Fujimaki for discussions. This work was partly supported by Grant-in-Aid for Scientic Research (No. 22224006, 21224009, 25708040, 24226002 and 25000003) and the FIRST Program from JSPS and SICORP from JST.

Author contributions

H.M.Y., M.N. and Y.I. designed the project. M.N. provided the substrates to H.M.Y. who grew the crystal, made the devices and measured the transport properties. M.S. measured the magnetic susceptibility. All authors discussed the results and commented on the manuscript.

Additional information

Competing nancial interests: The authors declare no competing nancial interests.

Reprints and permission information is available online at http://npg.nature.com/reprintsandpermissions/

Web End =http://npg.nature.com/ http://npg.nature.com/reprintsandpermissions/

Web End =reprintsandpermissions/

How to cite this article: Yamamoto, H. M. et al. A strained organic eld-effect transistor with a gate-tunable superconducting channel. Nat. Commun. 4:2379 doi: 10.1038/ ncomms3379 (2013).

NATURE COMMUNICATIONS | 4:2379 | DOI: 10.1038/ncomms3379 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications 7

& 2013 Macmillan Publishers Limited. All rights reserved.