OXIDE THIN FILM HETEROSTRUCTURES A ROUTE TO SUPERCONDUCTIVITY AT AMBIENT TEMPERATURES?? A VIEW FROM TECHNOLOGICAL PERSPECTIVES

Transcription

1 OXIDE THIN FILM HETEROSTRUCTURES A ROUTE TO SUPERCONDUCTIVITY AT AMBIENT TEMPERATURES?? A VIEW FROM TECHNOLOGICAL PERSPECTIVES H.-U. Habermeier MPI-FKF Stuttgart Germany

2 The quest for hightemperature superconductivity ½

3 NOT THE EASY HIGHWAY HERE COMPILATION OF THOUGHTS AND CON- CEPTS ARISING FROM OUR WORK ON FM- SC SUPERLATTICES ENRICHED BY THE WORK OF OTHERS TO BE APPLIED TO THE SEARCH OF SUPERCONDUCTIVITY BEYOND HTC CUPRATES

4 NOT FOCUS ON YOUR OWN NARROW FIELD AND IGNORE EVERYTHING ELSE BUT FOCUS ON THE INTERPLAY OF THEORY MODELS AND DREAMS EXPERIMENT PROPERTIES AT NANOSCALE CHEMISTRY TAILOR BANDS AND BONDS TECHNOLOGY FABRICATE REALISTIC SAMPLES

5 OUTLINE 1. INTRODUCTION - PHENOMENOLOGY SOME COMMENTS ON COMPLEX OXIDE HETEROSTRUCTURES 2. SAMPLES AND THEIR BASIC CHARACTERIZATION 3. OXIDE FERROMAGNET- SUPERCONDUCTOR INTERACTIONS STRUCTURAL ELECTRONIC MAGNETIC 4. OXIDE HETEROSTRUCTURES AS A LABORATORY FOR MANY BODY PHYSICS - NICKELATE PROJECT 5. CONCEPTS TO STABILIZE PREFORMED PAIRS IN THE PSEUDOGAP REGION 6. CONCLUSIONS

6 COOPERATION WITH Ch. Bernhard 2, J. Chakhalian 1,3, V. Hinkov 1,B. Keimer 1, G. Khaliullin 1, G. Christiani 1,G. Ravikumar 4,S.Soltan 1,6, Zheng RK 5 1 MPI-FKF Heisenbergstr. 1 D Stuttgart, Germany 2 University Fribourg, Switzerland 3 University of Arkansas, Fayetteville, Arkansas, U.S.A. 4 Bhaba Atomic Research Institute, Mumbai, India 5 Hong Kong Polytechnic University, Hong Kong 6 Helwan University Cairo, Egypt

7 1. INTRODUCTION PHENOMENOLOGY COMMENTS ON OXIDE HETEROSTRUCTURES CONCEPT: WHAT and WHY PHYSICS AT THE NANOSCALE STRONG CHARGE CARRIER CORRELATION BREAKDOWN OF QUASI - CONTINUUM APPROXIMATION BREAKDOWN OF SINGLE PARTICLE APPROXIMATION INTERFACES

8 INTERFACES OF INSULATING COMPLEX OXIDES Nature 427 (2004) 423

9 SrTiO 3 -LaAlO 3 field effect transistor Thiel et al., Science 2006 Much work done by H. Hwang et al, J. Mannhart et al, D. Blank et al..

10 James Eckstein: Nature Mat. July 2007 SrTiO 3 and oxygen

11 CHEMISTRY ( real picture ) DEFECT INTERFACE Modification of defect concentration c j at distance x from interface Dissimilar materials at both sides :HETEROGENEOUS DOPING Fe- doped SrTiO 3 ( R. Merkle ) Σ : charge of the core

12 COMPLEX OXIDES WITH LONG RANGE ORDER LRO-1 LRO-2 LRO-1 LRO-2 LRO-1 SUBSTRATE QUESTIONS: a.) who wins or peaceful together or mutual ignorance? b.) crosstalk?? c.) interfaces?? d.) construct new materials?? ANALOGY TO METALS: Giant Magnetoresistance Tunneling Magnetoresistance 2007 Peter Grünberg / Albert Fert

13 CUPRATE SUPERCONDUCTORS Perovskite-type structure Antiferromagnetic ground state d- wave symmetry of order parameter T c and conductivity sensitively depend on CuO 2 plane doping

14 1. INTRODUCTION WHAT REMAINS IN HTS RESEARCH AFTER 20 YEARS?? MAIN CHALLENGES IN HTS RESEARCH MECHANISM(S) CAUSING HIGH T c ROLE OF PSEUDOGAP EXPLORATION OF HTS APPLICATION POTENTIAL

15 FERROMAGNETIC MANGANITES Perovskite-type structure Doping dependent magnetic ground state 100% spin-polarization of e g electrons Spin-Charge-Orbital-Lattice coupling

16 FERROMAGNET SUPERCONDUCTOR OXIDE INTERFACE INTERACTION AT INTERFACE SC FM New groundstate New properties New material? INTERACTION ACROSS FM (SC) LAYER SC FM SC FM SC FM SC FM

17 GOAL USE CUPRATE MANGANITE HETEROSTRUCTURES AND SUPERLATTICES AS PROTOTYPE MATERIALS TO STUDY THE INTERACTION OF OXIDES WITH DIFFERENT FUNCTIONALITIES FOCUS ON ROLE OF THE INTERFACE ACTIVITIES INITIATED BY G. Jakob et al. Appl. Phys. Lett 66 (1995) 2564 P. Przyslupski et al. IEEE Trans. Appl. Superc. 7, 2192 (1997) P. Prieto, P. Vivas, G. Campillo, E. Baca, L. F. Castro, M. Varela, C. Ballesteros, J. E. Villegas, D. Arias, C. Leon, and J. Santamaria, J. Appl. Phys. 89, (2001) 8026 H.-U. Habermeier et al. Physica C 354, (2001) 298

18 2. SAMPLES AND BASIC CHARACTERIZATION PULSED LASER DEPOSITION KrF - Excimerlaser 248 nm Oxygen:.5 mbar Computer - controlled target exchange Pyrometric temperature control 180nm YBCO dep. rate ~15 nm/min Total high T exposure < 20 min

19 UHV PLD SYSTEM WITH RHEED and TRANSFER- CHAMBER

20 STRUCTURAL CHARACTERIZATION X-ray diffraction Y. Kuru 2008

21 HIGH RESOLUTION TEM BaO Y CuO CuO2 YBCO O Cu Ba A BaO MIRROR INTERFACES LCMO MnO A - B vs B A B LaCaO CuO2 ARE NOT IDENTICAL YBCO Mn Y La(Ca) Z. Zhang, U. Kaiser, Uni Ulm (7 uc LCMO in between of 50 nm YBCO)

22 bilayers depression of T Curie and T c magnetism matters superconductivity matters - regime of pseudogap?? - H.-U. H., G. Christiani et al. Physica C 364 ( 2001 ) 298 H.-U. H. and G. Christiani J. of Supercond. 15 (2002) 425

23 YBCO- LCMO 5nmx5nm superlattice YBCO-SRO 20nmx20nm superlattice magnetic coupling across the SC spacer

24 SPECTROSCOPIC ELLIPSOMETRY T c =85 K; T mag =245 K T c =73 K; T mag =215 K T c =60 K; T mag =120 K strange metal (SC) + strange metal (FM) strange insulator T. Holden, C. Bernhard, H.-U. H. et al., Phys. Rev. B 69, (2004)

25 3. OXIDE FERROMAGNET- SUPERCONDUCTOR INTERACTIONS STRUCTURAL ELECTRONIC MAGNETIC LCMO YBCO LCMO YBCO LCMO YBCO Charge Transfer Santamaria, Varela et al. Diffusion of spinpolarized quasiparticles Soltan et al. Proximity effect coupling via YBCO Bulaevski, Efetov. Sa de Melo Magnetic proximity effect Bergeret, Efetov Coupling via FM spacer STO

26 CHARGE REDISTRIBUTION AT INTERFACE Distance for movement of charges is given by Thomas Fermi screening length λ TF = 1 2 a 0 n 1/3 Bohr radius Charge carrier density Typically cm -3 in complex oxides λ TF = 2 6 Å (1 2 unit cells)

27 EXPERIMENTALLY: STEM- EELS STUDIES M. Varela, S. Pennycock, J. Santamaria et al.. cond-mat Amount of excess (depleted) electrons per formula within the YBCO (LCMO) layers as a function of distance along the growth direction, as measured from EELS.

28 PHASE DIAGRAMS Charge Transfer across the interface 1/3 YBaCu 2 O 7-x YBCO LCMO IS THAT THE WHOLE STORY???

29 HRTEM ( 50nm YBCO / 3nm LCMO / 50 nm YBCO Τ C = 83 Κ Τ Curie = 210 K!!!!

30 POLARIZED RESONANT X-RAY ABSORPTION Recent results Chakhalian, Freeland, HUH,Keimer, Khaliullin Science 318 (2007) 1114

31 MAGNETIC CORRELATIONS AT INTERFACE AS REVEALED BY NEUTRON REFLECTOMETRY

32 specular neutron reflectivity Bragg reflections due to structural and magnetic periodicity J. Stahn et al. Phys. Rev. B 71 (05) R second Bragg peak forbidden if structural and magnetic denisty profiles are equal

33 J. Chakhalian,J.W. Freeland, H.-U. H., G. Christiani,G. Khaliullin, Ch. Bernhard, and B. Keimer Nature Physics 2 (2006) 244 ELEMENT SPECIFIC PROBE OF MAGNETIC MOMENT La2/3Ca1/3MnO3 YBaCuO c X-ray beam Resonant soft X-ray absorption (XAS) reflected light H N absorbed light S SrTiO 3 Soft x-ray range -100 ev -1.5 KeV (3 d generation facilities - ESRF, APS, ALS, SLS and CLS) A Total electron yield

34 X-ray X-ray magnetic dichroism in YBCO/LCMO SL e - H µ µ s µ l LCMO YBCO 3.89A 3.82A Ba YBCO Y Ba Cu O Mn Mn-O-Cu interface La LCMO 3.87A Magnetic moment on Cu below T sc! Cu and Mn magnetic moments are anti-aligned. aligned.

35 MODEL vs. REALITY U. Kaiser+ Z Zhang Uni Ulm Okt 2007

36 J.Chakhalian, J. Freeland, H.-U. H. et al. Nature Physics 2(2006) 244 assume bulk orbital occupancy is maintained at interface x 2 -y 2 3z 2 -r 2 metallic LCMO: fluctuating orbital occupancy x 2 -y 2 orbital reconstruction at interface LCMO: 3z 2 -r 2 orbital depleted metallic YBCO: x 2 -y 2 orbital occupied ferromagnetic exchange coupling across interface inconsistent with experiment YBCO: antiferromagnetic coupling, as observed Superexchange interaction across interface

37 SUMMARY YBCO LCMO SUPERLATTICES 1. High quality YBCO - LCMO ( and derivates ) can be fabricated so far exclusively c-axis- epitaxial 2. Complex interplay of magnetism and superconductivity Magnetic domains affected by SC transition 3. Formation of the molecular orbital across the interface: strong AF binding of Cu and Mn spins 4. Orbital repopulation within the Cu e g doublet: hole transfer from a planar x 2 -y 2 orbital to the apical z 2 5. Charge transfer across the interface: hole depletion on Cu AF enhanced, SC suppressed

38 4. OXIDE HETEROSTRUCTURES AS A LABORATORY FOR MANY BODY PHYSICS - NICKELATE PROJECT Chaloupka & Khaliullin, PRL 100, (2008) CAN WE GENERATE THE CUPRATE SITUATION IN OTHER TRANSITION METAL OXIDES? Key elements in cuprate physics: no orbital degeneracy spin 1/2 strong AF coupling two-dimensionality Candidates: RTiO 3, Sr 2 VO 4, Sr 2 CoO 4, NaNiO 2, RNiO 3

39 Intermediate character between an itinerant metal and a localized Mott insulator J.-S. Zhou, J.B. Goodenough et al., PRL 84, 526 (2000) Ni 3+ 3d 7 t 6 2g e1 g e g S=1/2 t 2g W ~ E G ~ J H bandwidth driven M-I transition: continuous and monotonic upon R size

40 LaNiO 3 -LaMO 3 Chaloupka & Khaliullin, PRL D electronic structure x 2 -y 2 orbital favored cuprate Hamiltonian?

41 LaNiO 3 -LaMO 3 superconductivity? mean-field superconducting transition temperature cuprates LaNiO 3 -LaMO 3 Chaloupka & Khaliullin, PRL 2008

42 Nickelate-based superlattices LaNiO 3 (paramagnetic metal) Possible 2D- and interface- induced - metal- insulator transition with unusual magnetic and charge ordering - orbital reconstruction - superconductivity!? LaAlO 3 wide-band-gap (~ 5eV) insulator

43 La 2 CuO 4 spin-1/2 2D bond network orbitally non-degenerate Mott insulator LaNiO 3 O 2- (2p 6 ) O 2- (2p 6 ) spin-1/2 3D bond network Ni 3+ (3d 7 ) Cu 2+ (3d 9 ) orbitally degenerate metal x 2 -y 2 x 2 -y 2 3z 2 -r 2 3z 2 -r 2 yz xy xz yz xz xy TASKS Cut 3D network to a 2D one prevent c-axis conduction Lift orbital degeneracy orbital engineering

44 STRAIN-INDUCED ORBITAL POLARIZATION X-ray linear dichroism experiments z 2 LSMO z-in e g x 2 -y 2 STO t 2g yz xz xy x 2 -y 2 LSMO z-out e g z 2 xy t 2g LAO yz xz C. Aruta, G. Ghiringhelli, A Tebano, NG Boggio, NB Brookes, G Balestrino PRB 73 ( 2006 )

45 LaNiO 3 LaAlO 3 SUPERLATTICES LaAlO 3 LaNiO 3 5 nm [(LaNiO 3 ) 3 /(LaAlO 3 ) 3 ] 42

46 ε 1 at 0.85 ev vs. temperature SrTiO 3 tensile strain LaSrAlO 4 compressive strain A. Boris 2008/2009: stabilization of in-plane charge ordering

47 from itinerant to localized electrons Effective number of electrons localized: bulk NdNiO 3 ΔN eff 2m 2 π e N ω 0 ( ω) = Δσ ( ω ) Ni dω 0 σ 1 (100Κ, ω) σ 1 (10Κ, ω) 800 LNO-LAO-SB2 on STO (3u.c. 3u.c.)*42 ΔN eff =0.058 Δσ 1, (Ω 1 cm -1 ) SW T MI ~100K ΔN eff =0.04 T MI =200K Photon energy (ev) T.Katsufuji, Y.Tokura et al., PRB 51, 4830 (1995)

48 LaNiO 3 SLs charge dynamics Temperature-driven itinerant to localized electronic transition has been observed in heterostructured LaNiO 3 ultrathin ( < 3 u.c.) films. Effective number of localized electrons and characteristic energy scales are consistent with those observed in the charge ordering phase of bulk nickelates NdNiO 3 and PrNiO 3. Is that the end of the nickelate story???

49 Khaliullin s idea ( 2 ): Make Ni 3+ (d 7 )-based HTSCs by sandwiching hole doped LaO-NiO 2 layers between insulating layers through heterostructuring (orbital engineering) The confinement together with the electronic correlations should make it possible to localize or empty the 3z 2 -r band thus leaving the conduction electron in the x 2 -y 2 band If the 3z 2 -r orbital can be manipulated to lie above x 2 -y 2, it might play the role of the axial orbital in the cuprate d 9 HTSCs J. Chaloupka and G. Khaliullin, PRL 100, (2008), P. Hansmann, X.P. Yang et al., arxiv: (2008); X.P. Yang and O.K. Andersen, in preparation

50 d 7 nickelates Simplified e g conduction-band structure in 2D 3D square cubic lattice: 3 2 Energy (ev) z 2-1 x 2 -y 2-2 Γ Z R A Z Γ(000) X(001) M(101) R(111) X(001) (0,0,0) (0,0,1) (1,0,1) (1,1,1) (0,0,1) Z (0,0,½) taken from OKA

51 Band structure Γ Z x 2 -y z 2-1 R LDA A LaAl 3 (ev) Z -2 LDA+DMFT U -2J = 6.7 ev = 5.3 ev (For U -2J > 6 ev, DMFT yields an AFI) The Coulomb correlations enhance the crystal-field splitting and simplifies the Fermi surface to one sheet when ε 3z 2-1 (Γ) > ε F, i.e. with a shape (r ~ ½), like that in the cuprates with the highest T cmax Fermi surface taken from OKA

52 Control by lattice constant of the substrate Control by the chemistry (Al, Ti, Sc) of the insulating layers

53 Bandstructure calculations using the local spin-polarized density approximation (LSDA) in combination with LDA+U method. single sheet Fermi surface like in cuprates

54 5. CONCEPTS TO STABILIZE PREFORMED PAIRS IN THE PSEUDOGAP REGION BASIC REQUIREMENTS FOR SUPERCONDUCTIVITY BINDING OF e s INTO PAIRS DESTROYED BY AMPLITUDE FLUCTUATIONS PAIR BREAKING LONG RANGE PHASE COHERENCE OF PAIR WAVE FUNCTION DESTROYED BY PHASE FLUCTUATIONS

55 CAN WE FABRICATE HYBRID SYSTEMS COMBINING HIGH ELECTRON BINDING ENERGY WITH HIGH PHASE STIFFNESS?? T PAIRING > T PHASEORDER HIGH PHASE STIFFNESS Canwemakethem talk to each other HIGH T PAIRING SUBSTRATE

56 CONCEPT OF STABILIZATION OF PREFORMED PAIRS PROXIMITY INDUCED SUPERCONDUCTIVITY SC requires Cooper-Pair formation ( at T CP ) and phase coherence between pairs ( usually T c ) T CP > T c claimed in cuprates Combine metal with strong phase stiffness with cuprate with high T CP INDUCE phase CP at interface layer with thickness of ξ

57 MODEL SYSTEM UNDOPED (110) ORIENTED YBCO FILM COVERED WIT AN APPROPRIATE METAL ξ METAL WITH HIGH PHASE STIFFNESS REGION OF PROXIMITY-INDUCED SUPERCONDUCTIVITY UNDERDOPED CUPRATE SUPERCONDUCTOR T C < T < T P

58 Experimental Hints High Temperature Interface Superconductivity in La 2 CuO 4 Based Bilayers ( Theory: Kivelson, Orgad et al. Experiments: Bozovic Group, G.Koren, Yuli Group )

59 La 2-x Sr x CuO 4 BILAYER EXPERIMENTS Yuli, Asulin, Millo, Orgad; Iomin and Gad Koren PRL 101, , (2008) (a) RT of a bare LSCO(0.35) film. (b) (d) RT curves of the LSCO0:35-LSCOx bilayers with x 0, 0.10, 0.18, and 0.12, and of the corresponding bare films.

60 (a) Tc vs x of the bilayers (open symbols) and bare films (solid symbols) (b) Tc of LSCO films grown on LaSrAlO4 (open symbols), and on STO (solid symbols), as compiled from Refs. [13,19]. The dotted line depicts the Tc of bulk LSCO.

61 INTERFACE SUPERCONDUCTIVITY IN CUPRATES R(T) for single-phase layers of I and M. R(T) for various bilayers. I-M and M-I bilayers show superconductivity even though neither of the two constituents does. In single-phase S or S films grown under the same conditions T c 40 K. [ Gozar et al, Nature (2008) ]

62 INTERFACE-ENHANCED ENHANCED SUPERCONDUCTIVITY The temperature dependence of the imaginary part of the mutual inductance of the single phase, S, and bilayer, M-S, samples. The onset of the Meissner response occurs at T c of the sample. The sharp drop in the imaginary component indicates a very narrow superconducting transition.

63 5. CONCLUSIONS 1. Interfaces composed of complex oxides are an intriguing new playground for many-body physics. 2. The properties of interfaces of ferromagnetic manganites and superconducting cuprates are determined by orbital reconstruction and charge transfer 3. The manipulation of the electronic structure of nickelates sandwiched in between insulators can mimick the cuprate situation and can pave the way for higher T c s 4. Bilayer systems of underdoped cuprates and metals with high phase stiffness can be a route to high T c s

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