Scanning Tunneling Microscopy Local probes at high magnetic fields?

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1 Scanning Tunneling Microscopy Local probes at high magnetic fields? Hermann Suderow Laboratorio de Bajas Temperaturas Departamento de Física de la Materia Condensada Instituto de Ciencia de Materiales Nicolás Cabrera Universidad Autónoma de Madrid (UAM)

2 Microscopy optical C. Zeiss, Jena Antoni van Leeuwenhoek ( ) Delft Franz Griendel von Ach Always fascinating to obtain real space images

3 Microscopy Optical, SEM, SPM SPM at cryogenic temperatures Cryogenic operation of a scanning probe microscope eliminates Brownian motion altogether, provides for resolution in energy and enables macroscopic quantum behavior

4 Atomic scale manipulation with cryogenic scanning probe microscopy T=4.2 K D. Eigler, IBM Cryogenic scanning probe microscopy eliminates Brownian motion altogether and enables macroscopic quantum behavior 4

5 Status of cryogenic scanning probe microscopy

6 Spectroscopy at high magnetic fields pa What? Tunneling spectroscopy Tunneling microscopy

7 STM: Binnig and Rohrer (1982) I ( V, z ) VN ( E ) F e Φz I= 1 na ~ z= 1Å Piezoelectrics x, y 20 A V 1Å ~ 250 mv

8 Tunneling current between a metallic tip and a sample: Overlap between tip and sample s wavefunctions. Bardeen s formalism ( ) ( ) ( ) ( ) ( ) ( ) = + = + = * * ; ; 1 1 ) ( 1 2 µ ψ µ ψ ψ ψ ψ ψ δ π ν ν µν ν ν µ µ ν µ µν ν µν µ ds m M E E e E f E E M ev E f E f e I T k E E B F h h STM: The tunneling current

9 STM: The tunneling current Ideal tip, low bias voltage : d 0 2 ( d ) δ ( E ) ρ( ) I ψν 0 ν E F = d 0, E F ν STM: Contour map of the local density of states

10 Low temperature spectroscopy di dv N( E) 87 µev = 1 K f ( E ev ) de V E N( E) = E 2 2 Normalized tunnelling conductance =1mV T=0.3 K T=1 K Bias voltage (ev/ 0 ) αβ ψ r k, α ψ r k, β ψ = ψ e iϕ E v k = ε 2 v k + α, β r (k ) 2 N( E; x, y)

11 Tunneling spectroscopy with STM: vacuum Giaever tunneling

12 The superconducting gap through local tunneling spectroscopy at very low temperatures High T c superconductors Geneva, Cornell, Paris, Tokyo, 1.5 Two band superconductors Magnetic superconductors normalized conductance Normalized conductance TmNi 2 B 2 C T c = 10.5 K T = 0.8 K Bias voltage (mv) Normalized conductance ErNi 2 B 2 C T c =11K T = 0.15 K Bias voltage (mv) MgB Normalized bias voltage Sr 2 RuO T = 150 mk 200 G = 0.28 mev BULK superconducting density of states Averagedoverpartof the Fermi surface Normalized conductance P ro s 4 S b 1 2 T c = K B ia s vo lta ge / 0 ( = k B T c ) Normalized conductance Bias voltage (mv)

13 NbSe 2 and NbS 2 : two two-gap superconductors 2H-NbSe 2 2H-NbS Normalized conductance Normalized conductance nm B ia s v o lt a g e ( m V ) Bias voltage (mv)

14 1.25 nm 0.35 nm 0.29 nm Charge order in 2H-NbSe 2 at 100 mk c a Se Nb b 1/λ CDW 1/a 2H-NbSe 2 1.7nm λ CDW = 3 a( ±1% ) 2H-NbS 2

15 Two gap superconductor NbS 2 2H-NbS 2 Electronic density of states vs. temperature 5 5 Normalized conductance Normalized density of States di dv Bias voltage (mv) f ( E ev ) N S E) de V Energy (mev) ( ( E) N S

16 Two gap superconductor NbS 2 2H-NbS 2 Maxima of the gap distribution (two gap) vs. temperature =1.73 k B T c Normalized density of states (mev) Energy (mev) Temperature (K)

17 Tunneling spectroscopy URu 2 Si Phase diagrams Q = 0.59 Q = 0.58 H (Tesla) Q=0.57 Kondo systems 0.25 Q = Temperatura (K) Strong coupling features Andreev features

18 Spectroscopy at high magnetic fields pa What? Tunneling spectroscopy Tunneling microscopy

19 STM: Binnig and Rohrer (1982) I ( V, z ) VN ( E ) F e Φz I= 1 na ~ z= 1Å Piezoelectrics x, y 20 A V 1Å ~ 250 mv

20 Microscopy of the superconducting gap in type II materials H H c2 Mixed state Normal phase Mixed state H Meissner state H c H c1 2 Ψ = n s Meissner state T c T H d(nm) 50 / H(T) H c 2 = φ0 2 2πξ d H J ξ λ r ψ iϕ = ψ e αβ ψ r k, αψ k r, β J E N( E) = E 2 2

21 The vortex lattice through STM Measure far below T c (here 7 K)

22 Atomic size tunneling spectroscopy inside vortices in 2H-NbSe 2 T=100 mk di/dv 2.23 µs 0 mv 0 mv di/dv 0.85 µs 0 µs 0.4 µs 0.6 nm 21nm Conductance (µs) Se atom Hole Bias voltage (mv)

23 Local tunneling spectroscopy in the mixed state of superconductors High T c materials: Geneva, Paris, Tokyo, Cornell, Princeton, FeAs Beijing, Harvard, Tokyo V 3 Si and Chevrel phases Maryland, Geneva MgB 2 Geneva, Paris, Argonne, Madrid Nickel borocarbides Tokyo, Argonne, Madrid NbSe 2 and related compounds Bell labs, Paris, Madrid, Leiden

24 Temperature between (0.1K-2.1K) 144 images 8mineachone SeveralimagesateachT Direct observation of thermally induced vortex depinning Physics Today, see youtube channel physics update

25 Spectroscopy at high magnetic fields pa How?

26 Principles of design of STMs : From first large devices First cryogenic STM in Madrid 1988

27 Facilities in Madrid STM/S in a dilution refrigerator Compact design to guarantee mechanical stability (piezoelectric drive) Home made electronics, cryogenics and mechanics 100 mk 100 mk + 10 T Superinsulator Vortex physics Nanostructures 10 mk + 9 T Current Drive STS Heavy Fermions (Kondo lattice) 100 mk, 3D VECTOR MAGNET Layered materials 1. Spectroscopy 2. Macroscopic movement over the sample 3. Tip and sample preparation methods 2 µm 300 mk, 13 T -> 17 T Superconducting tips

28 Principle of operation of a scanning tunneling microscope Fine positioning piezoelectrics + Coarse motor x, y 20 nm V Scanning window in the µm range with sub atomic resolution Position without heating with nm resolution z Z y x Y X

29 Designing a cryogenic scanning tunneling microscope Stiffness f vibration isolation Soft and heavy Stiff and light f microscope f 0 = 1 2π k m eff Tip-sample motion shall be in phase

30 STM holder Mezclas for dilution de 3 He refrigeration y 4 He STM head is posed on fiber glass loom Specific precision motion system using a series of ropes with a room temperature actuator without measurable heating at 100 mk

31 STM Mezclas electronics de at 3 He LBTUAM y 4 He Switchable filter PA243 Motor Z RF filters I-V converter Tunnel current Bias voltage RF filters DAC ADC Industrial PC RF filters 6x PA243 OP27 RF filters Power supply +/- 15 V RF filters 6 PIEZO SCANNING DRIVES Power supply +/- 15 V +/- 140 V Power supply Resolution in spectroscopy of <15 µev (<150 mk)

32 Spectroscopy at high magnetic fields Compact system Compact system just for tunneling Z maximally simplified X pa Madrid: 17 T superconducting magnet Microscope for high field superconducting solenoid z Z y x Y X

33 Web page of LBTUAM

34 Workshop on high magnetic field science and technology Miraflores de la Sierra, 6-9 November Organizers : H. Suderow and I. Guillamón

35 Scanning probe microscopy A. Maldonado, J.A. Galvis Echeverry, P. Kulkarni, A. Buendía, I. Guillamón, J.G. Rodrigo, S. Vieira Laboratorio de Bajas Temperaturas Dpto. Física de la Materia Condensada Instituto de Ciencia de Materiales Nicolás Cabrera Universidad Autónoma de Madrid (UAM) J. Sesé, R. Córdoba, A. Fernández Pacheco, J.M. de Teresa, R. Ibarra ICMA, Unizar, Instituto de Nanociencia de Aragón F. Guinea Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas, Madrid S. Bud ko, P.C. Canfield Ames Laboratory, Ames USA S. Bannerjee IIT Kanpur India V. Tissen, Chernogolovka, Russia T. Baturina, Novosibirsk, Russia, V. Vinokur, Argonne, USA L. Cario, Nantes; P. Rodiere, P. Lejay, J.P. Brison, Dai Aoki, J. Flouquet Institut Néel and SPSMS/DRFMC E. Navarro Moratalla, C. Martí Gastaldo, E. Coronado, ICMol Valencia

36 The SEGAINVEX team (headed by M. Pazos)

37 Increasing the field in steps No time variation of flux distribution Pinning produces spatial variation of B Critical state F L =-F P Vortex bundleswithweakpinningat 100 mk Campo Crítico H c2 (T) Temperatura (K) 33 STS images 0.04 T steps