Electrostatic Modification of Cuprates. Allen M. Goldman School of Physics and Astronomy University of Minnesota

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1 Electrostatic Modification of Cuprates Allen M. Goldman School of Physics and Astronomy University of Minnesota

2 Principal Collaborators Xiang Leng (BNL) Javier Garcia-Barriocanal (Complutense) Joseph Kinney Boyi Yang Yeonbae Lee (LBL/UCB) JJ Nelson Xiang Leng et al., Phys. Rev. Lett. 107, (2011). Xiang Leng et al., Phys. Rev. Lett (2012). Garcia-Barriocanal et al., Phys. Rev. B 87, (2013)

3 How can one modify the electronic properties of a material? Chemical doping (changes composition, structure, ) Electrostatic charging (reversible?, continuous, fast, ) Field Effect Transistor Induced carriers : A Q = CVG = ε VG d ε : dielectric constant d : thickness of the gate dielectric

4 This approach is compatible with Complex oxides: cuprate superconductors, Mott insulators, and colossal magnetoresistive compounds Two-dimensional and interfacial superconductors Organic and inorganic semiconductors, novel carbon structures, single molecules, graphene, topological insulators. Can provide a tool for studying quantum critical behavior. It may be possible to dope into regimes not accessible by chemical doping.

5 FET Structure: Combined Substrate and Gate Insulator Back of a micro-machined substrate. Height profile is superimposed on the picture. Thickness in middle can range from 10µm to 100µm Surface roughness of approximately 1µm. Diameter of the thinned region is typically 4mm. Cartoon of insulating substrate separating a Bi film from the gate Thickness of the film is about 10 Å, and that of the source and drain about 100 Å. Separation between the gate and the film is approximately 50 µm. A. Bhattacharya, et al., APL 85, 997 (2004)

6 Electrostatically Tuned S-I Transition Δn = 0 d =10.22Å Δn =3.4 x /cm 2 Δn c = 1.4 x /cm 2 n 0 = /cm 2 (W. Buckel) K. Parendo et al., PRL 95, (2005)

7 Electric Double Layer Transistor Pt Gate EDLT Ionic Liquid Drain Source V g Sample Substrate

8 1853. ANNALEN ZO. 7. DER PHYSTK UND CHEMIE. BAND LXXXIX. 1. Ueber einige Gesetze dcr Vertheilung elektrischer Striime in kiirperlichen Leitern, mit Anwendung au f die thierisch -elektrischen Versuche; con H. Helmholtz. ( s c I, I I1 Ts. ) IV. Tlieareni von der gleieheu gegeoseitigen Wirkung elektromotorisehen Flichenelemente. zn-eier,

9 Electric Double-Layer FET- SrTiO 3 Electrolyte: polyethylene oxide, containing KClO 4 K. Ueno et al., Nature Materials 7, 855 (2008)

10 Search for Superconductivity of KTaO 3 a b Ionic liquid (DEME-BF 4 ) Source Gate Gate Ionic liquid Source Drain Channel KTaO mm Drain Channel V G I G + Separator Gate V D I D Ionic liquid Source Drain c BF 4 O DEME + KTaO 3 + H O C N Ta K [001] KTaO 3 Channel [100] [010] K. Ueno et al, Nature Nanotechnology (2011)

11 NATURE NANOTECHNOLOGY DOI: /NNANO a KTaO 3 EDLT sample D V G = 5 V I = 50 na b R S (Ω) T = 20 mk R S (Ω) T c = 47 mk T (mk) c I (μa) μ 0 H (Oe) T = 20 mk V (μv) Figure 4 Superconducting properties. a, SheetresistanceR S versus K. Ueno et al, Nature Nanotechnology (2011) temperature T at gate voltage V G ¼ 5 V in an EDL transistor in which the channel is a layer of KTaO 3.Thesolidlinedenotesthemid-pointofthe superconducting transition. b, R S versus magnetic field m 0 H at 20 mk. c, CurrentI versus differential voltage V at 20 mk, measured in a

12 Properties of materials as a function of 2D charge density SrTiO 3 : ε = at Room T SiO 2 : ε = 3.9 at Room T SiO 2 FET SrTiO 3 FET EDLTs From: C.H. Ahn, J.-M. Triscone, J. Mannhart, Nature (2003) Also see: Ahn et al. Rev. Mod. Phys. (2006).

13 Generic phase diagram for HTS C. Varma. Nature 468, 184 (2010) Superconductivity starts at x ~ 0.03,corresponding to 3.5 x carriers per plane

14 Ionic liquid-gated cuprates l 10 nm may be too thick; l Thomas-Fermi screening length < 1 nm (1 UC~1.1nm); Affected by Ionic Liquid YBCO 1 UC Dead Layers Large ΔTc in YBCO Need ultrathin YBCO films! A.S. Dhoot et al., Adv. Mater. 22, 2529 (2010)

15 Film deposition and characterization l High pressure oxygen sputtering system; l Deposition rate ~1UC/minute (1 UC ~1.1 nm) l Thicknesses determined from X-ray reflectivity data; l Roughness ~0.5 nm; l Control thickness of nm scale. 28 YBCO 15 u.c. 5.5nm 8.0nm 24 R s (Ω) T(K) Intensity (arb. units) 11.0nm θ 13.5nm θ

16 EDLT fabrication Pt DEME Pt TFSI Pt a- a- Al 2 O YBCO 3 Al 2 O 3 STO SOURC E DRAIN YBCO very sensitive, reacts with most chemicals Pre-pattern: Deposition of a-al 2 O 3 STO surface treatment: TiO 2 termination YBCO deposition Electrode evaporation: Pt Glue glass cylinder DEME TFSI Top gating: Pt coil

17 Gate voltage changed at 240 K H. Yuan, Adv. Funct. Mater. 19, 1046 (2009)

18 7 UC thick YBCO film T C (K) Effective doping level 0.165/R sheet (T=200K) Carriers/Cu Good match in the SI transition regime Not good in the optimal doping regime X. Leng et al., PRL 107, (2011)

19 7 UC thick YBCO film Phase diagram derived from R(T) curves Quantum critical point? X. Leng et al., PRL 107, (2011)

20 SI transition: Quantum phase transitions T c QPT can only be accessed by varying a tuning parameter (g) in the limit of zero temperature. The tuning parameter may be thickness, magnetic field, disorder, doping, etc. Quantum critical region exhibits universal power-law behaviors. Spatial correlation length: ξ ~ δ -ν, δ=g-g c Characteristic frequency: Ω ~ ξ - z 0 At finite temperature, Ω is cut off by k B T, leading to a scaling relation: g R S = R C f(δ/t 1/ ν z ) The critical resistance R C and the critical exponent product νz determine the universality class of the QPT g M. P. A. Fisher, PRL (1990)

21 7 UC thick YBCO film Finite size scaling analysis 10k R sheet (Ω) 1k T=6.5K-22K νz~2.2 1E x-x C /T 1/νz X. Leng et al., PRL 107, (2011) Breakdown at low temperature

22 SI transition in La 2-x Sr x CuO 4 film l Scaling analysis from 4.5 K to 20 K; l R C =6.45kΩ, νz =1.5 A. T. Bollinger et al., Nature 472, 458 (2011)

23 7 UC thick YBCO film In the presence of magnetic field R s (Ω) 120k 12k 1.2k V G =1.52 V V G =0 V m T(K) l A cleaner SI transition. l Inhomogeneity has been suppressed. l Measurements down to 2K only ZFC B=9T

24 Finite size scaling analysis R s (Ω) 120k 12k 1.2k 120 2K x c =0.055 R c =4.5kΩ 22K B=9T x=0.165/r(200k) l x c changed to higher doping level l νz remains the same; l Critical resistance is not universal 10 1 R s /R c E-3 1E-4 νz=2.2 1E x-x C /T 1/νz

25 Underdoped to Overdoped Transition V 1.0 R s (kω) a) b) -2.38V -2.38V R s (kω) a) b) V +1.46V 3.0 R s (kω) V c) T(K) 1.5 d) +0.7V T(K) a) Holes injected, R norm drops and T C increases then saturates R s (kω) d) c) b) Further injection of holes, T C drops and R norm increases c) and d) The process is reversible

26 R xy (Ω) R xy (Ω) (a) (b) T=180 K 0 Hall effect measurement -2.32V -2.52V -2.32V B(T) n H = 1/( RH e) A peak around the optimal doping point X. Leng et al., PRL 108, (2012)

27 V V R s (kω) R s (kω) -2.65V -2.65V T (K) T (K) The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. Tc (K) R 180K (kω) p (holes/cu) A 5.5 Unit Cell Thick Film of YBCO

28 Hall effect in cuprates F. Balakirev et al., Nature 424,912(2003) F. Balakirev et al., PRL 102, (2009)

29 Electron Doping of YBCO

30 dv/di (Ω) 1M 100k 10k 1k m 10m dv/di at the lowest current (<0.01 na) V G (V) T(K)

31 dv/di(ω) 1 10 T(K) k 10k V G (V) 100k

32 R H (10-10 m 3 /C) Positive holes 1.70 V R H (10-10 m 3 /C) Negative, electrons 1.95 V 2.0 V B(T)

33 La 2 CuO 4+δ (δ-lco) vs La 2-X M X CuO 4 (LMCO) LCO Lee et al. PRB (1999) δ-lco δ-lco is a HTS derived from the antiferromagnetic - Mott insulator La 2 CuO 4 (LCO) Oxygen interstitials (i-o) are located in between La 2 O 2+δ layers, far away from the CuO 2 superconducting planes i-o are introduced in specific positions of the crystal structure Chailout et al. Physica C (1989) Disorder is weak and T C is the highest of the La 214 family of compounds 33

34 Epitaxial growth of δ-lco thin films Ozone Assisted Molecular Beam Epitaxy - UHV chamber: Torr Torr - Shuttered growth technique - T GROWTH = 750 C - On SLAO (0 0 1) substrates - P[O 3 ] = Torr - Reflection High Energy Electron Cu Sr Diffraction accuracy I (arb. units) (RHEED): ½ unit cell Number of Repititions

35 Structural characterization Atomic Force Microscopy and Scanning Transmission Electron STEM Microscopy SAMPLE ADF Z-Contrast (011) cross-section SUBSTRATE C O u La Maria ORNL 35

36 Transport and Magneto Transport Experiment Results R S (Ω) 10 7 RS Rs R S1 9 T T (K) R S2 Rs 2 R S2 9 T 1.2V 1.6V 2.1V 2.4V 2.45V 2.5V 2.55V 2.6V 2.65V 2.75V 2.8V 2.85V 2.9V 2.95V 3V 75 levels of charge doping p (holes/cu) R S (Ω) Rs 91 T T (K) 9 T T (K) Rs 2 33 levels of charge doping 36

37 Superconductor Insulator Transition Hall resistance The presence of a maximum in the R XY could again be revealing an electronic phase transition R XY 9 T Is the QPT at zero temperature related to the high temperature changes in R XY?? T (K) p (holes/cu) p C 37

38 Minimum in n H near the edge of the SC dome in δ-lco 1x10 15 Tc mid K n H (cm -2 ) 8x x T C (K) holes/cu 0

39 Summary and Future Prospects Electronic Double Layer Transistors employing Ionic Liquids can be used to tune the properties of novel materials such as the Cuprates. We have emphasized the study of YBCO and are now investigating LCO. One can tune through the phase diagram with some surprises. Currently the measurement of the Hall effect is the major complement to the study of resistance and magnetoresistance. There is evidence from this work of electronic transitions in YBCO near the maximum of the dome of the transition temperature vs. doping, and in LCO near the SI transition. We plan X-ray spectroscopy and kinetic inductance measurements. An open question is whether this approach can be efficiently used to search for new superconductors. Caveats: don t know the actual depth of charge pentratiion failures interaction of the ionic liquid with the sample chemistry or physics, movement of oxygen