Yoshihiro Iwasa (Tohoku U): Electric field induced superconductivity with electric double layer transistors

Outline:

• electric field induced SC
• double layer Transistor
• oxides
• increase of carrier density
• layered ZrNCl
• summary

Intro:

charge accumulation in capacitor lead to MOSFET by replacement of one electrode structure, however, difficult to make in the past

at the same time (Glover 1960) electric field control of SC was investigated (In)

field effect on high-Tc cuprates (YBCO): by applying gate voltage increase of Tc

dream: gate control of localization-SC transition (e.g. a-Bi, Parendo PRL 2005)

best would be SC in an insulator by gating

conventional FET with weak electric field ~1 MV/cm, 10^13 cm^-2 which is too little, therefore attempt to use liquid gate (ionic), this leads to charge accumulation devices like Li battery or electric double layer capacitor

comparison of capacitance: double layer capacitor b/w electrolytic capacitor and battery, in double layer capacitor voltage drop right at the electrode

application of EDLT to organic conductors: sharp raise of conductance --> low operation voltage, but conductance in order of 4 microS therefore switch to inorganics


made of ZnO and patterned, ZnO shows large increase of sheet conductance at about 1.7 V, carrier densities are several 10^13 cm^-2, the capacitance is 7.8 microF/cm^2, e. i. half that as on Au

low T measurements;

crossover from insulator to metallic behavior at 0.7 V
now STO/PEO(KClO4) is being used, current two order of magnitude larger, insulator to metal transition at around a few volts

electric field induced SC was then found at V_g = 3 V, first field induced SC without doping, critical magnetic field about 30 mT, H_c2 is smaller than in the bulk due to interface which is not working as a pinning center

increase of carrier density:

uniqueness of STO:
STO is lowest carrier density SC, also atomically flat surfaces can readily be made

higher carrier density in ionic liquids: polymer electrolyte (solvent + salt) but large solvent molecules are bad, therefore solvent is to be removed --> melting at RT (organic material)

now accumulated carrier density is in range of 10^14 cm^-2, gate voltages of 0.5 V needed at room temperature, gate V goes up with T --> enhanced electric charging at low temperature, that should be advantageous for SC



Device:


ZrNCl:

discovered about ten years ago, layered material, cleaveable and interesting phase diagram

very small specific heat capacity if compared to other SC but relatively high Tc of ~15 K (goes down with Li-doping, pairing interaction increases which is strange, magnetic fluctuation are present but it is not clear where they come from, crossover from isotropic gap to an anisotropic one

try to explain these strange properties by a fluctuation exchange approx. theory, afm fluctuation develops causing d pairing even in doped band insulator







device fabrication:

exfoliating single crystals, electrodes by e-beam lithography, TiAu electrodes, single crystal size some 10 micrometer,

sigma kicks in at around 1,5 V at 220 K, goes up to 1.5 mS, by increasing the gate voltage remarkable decrease of resistivity, at about 4 V SC is found below ~14 K, however it is not clear how much volume of the sample turns SC

doping study: Tc goes down to below 12 K but there seems to be a maximum at ~0.04 as shown by recent low-doping studies

novel materials:

also, KTaO_3 turns SCbelow 0.04 K

• new tunable 2D systems
• challenges: increase Tc, discover new SC
• new states at interface which are inaccessible by conventional chemistry