1. Interlayer tunneling spectroscopy of NbSe 3 and graphite at high magnetic fields Yu.I. Latyshev Institute of Raduio-Engineering and Electronics RAS, Mokhovaya 11-7, Moscow 125009 In collaboration with А.P. Оrlov, A.Yu. Latyshev IREE RAS, Moscow A.A. Sinchenko Moscow Eng. Physical Institute А.V. Irzhak Moscow Inst. of Steel and Alloys P. Monceau, Th. Fournier Neel Institute, Grenoble, France J.Marcus D. Vignolles LNCMP, Toulouse, France

2. OUTLINE 1.Introduction to interlayer tunneling in layered superconductors and charge density wave materials. 2.CDW gap spectroscopy at high magnetic field in NbSe 3. 3.Graphite. Nanostractures fabrication with focused ion beam. 4.Pseudogap. 5. Interlayer tunneling spectroscopy of Landau levels. 6.Behaviour in high magnetic fields. 7.Conclusions.

3. Interlayer tunneling in layered HTS and CDW materials

4. Layered crystalline structure s LL L NbSe 3 Sample configuration σ ║ /σ ┴ =10 3 -10 4

5. Bi-2212. Gap/pseudogap spectroscopy Yu.I. Latyshev et al.ISS Conf. 1999, Physica C, 2001; V.M. Krasnov et al. PRL, 2000, 2001

6. Spectroscopy of CDW gap and intragap states. NbSe 3 4.2К  170К Yu.I. Latyshev, P. Monceau, S. Brazovskii, A.P. Orlov, Th. Fournier, PRL 2005, 2006

7. 1. CDW gap spectroscopy in high magnetic fields

8. Anomalously high magnetoresistance in NbSe 3. Orbital effect on partly gapped CDW state. Q Q 2K F F perfect imperfect H=0 H R.V. Coleman et al. PRL 1985 A. Bjelis, D. Zanchi, G. Montambeaux PR B 1996, cond-mat /1999 also have shown the possibility to increase T p by magnetic field. L.P. Gor’kov and A.G. Lebed 1984 C.A. Balseiro and L.M. Falicov 1984, 1985 Magnetic field destroys ungapped pockets Magnetic field improves nesting condition and thus can increase CDW gap

9. Zeeman splitting effect on CDW ordering In a zero field the CDW state is degenerated with respect to spin up  and spin down  configurations. Magneticfield releasess degeneration due to Zeeman splitting. As a result Q  CDW vector increaseswith fieldwhile Q  decreases Q  > Q 0  Therefore a CDW state with a fixed Q 0 tends to be destroyed with field. One can expect the interplay between orbital and Pauli effects at high fields of the scale 2  B H ~ kT p.. For NbSe 3 with T p =60K that requires experiments at fields ~50T

10. Experiments at pulsed magnetic field (see also p. 29 at poster session) LNCMP, Toulouse High speed acquisition system

11. Field-induced gap. 3D picture. T=65 K

12. Induced CDW gap above Peierls transition temperature H=0 H=35T

13. Phase diagram. Interplay of orbital and Pauli effects. Non-monotonic behaviour of T p (H) is defined by interplay between orbital and Pauli effects on CDW pairing. Orbital effect is realized in improving of nesting condition and, thus, in increase of  and T p, while Zeeman splitting tends to destroy CDW pairing. Experimental crossover field corresponds to H  30T, 2  B H 0  kT p That is consistent with calculatons of Zanchi, Bjelis, Montambeau PRB 1996 for the case of moderate imperfection parameter (valid for NbSe 3 )  B H 0 / (2  T p )  0.1 or For T p = 61 K that corresponds to H 0  30T Field induceed CDW state

14. 2. Interlayer tunneling spectroscopy of graphite

15. Questions: 1.Is there interlayer correlation? 2.Is that possible to observe Dirac fermion features by interlayer tunneling technique? 3.Which is the inter-graphene behaviour in high magnetic fields?

16. Fabrication of nanostructures

17. FIB microetching method Yu.I. Latyshev, T. Yamashita, et al. Phys. Rev. Lett., 82 (1999) 5345. S.-J. Kim, Yu.I.Latyshev, T. Yamashita, Supercond. Sci. Technol. 12 (1999) 729. FIB machine Seiko Instruments Corp. SMI-9000(SP) Ga+ ions 15-30 kV Beam current : 8pA – 50 nA Minimal beam diameter: 10nm

18. Stacked structures fabricated from layered materials by FIB methods Figure 2. (a-c) Stages of the double sided FIB processing technique for fabrication of the stacked structure; (d) SEM image of the structure. The structure sizes are 1  x 1  x 0.02 - 0.3  Yu. I. Latyshev et al. Supercond.Sci.Techn. 2007 NbSe 3 single crystals are thin whiskers with a thickness of 1-3  m, a width of 20  m and a length of about 1 mm

19. Pseudogap in graphite

20. Interlayer tunneling in graphite mesas We found an evidence of pseudogap formation in graphite below T 0 =30K. Vpg  10-15 mV Vpg  3.5 kT 0 !? Yu.I.Latyshev, A.P.Orlov, A.Yu. Latyshev,Th. Fournier, J. Marcus and P. Monceau 2007 At 300K    0.2  cm,  //  50  cm,   /  // ~ 4000 At 4.2K   /  // ~ 30 000 Mesa sizes; 1  m x 1  m x 0.02-0.03 

21. Observation of Dirac fermions in graphite previous experiments

22. ARPES on graphite S.Y. Zhou et al. Nature Physics, 2006

23. Landau quantization in graphite from STM G.Li and E.Andrei, Nature Phys. 07 Graphene spectrum E(k) =  v F (h/2  ) k Landau quantization E(n)= sgn n [2e (h/2  ) V F 2 |n|B] 1/2 E(n)  (nB) 1/2 Bilayer graphene E(n)= sgn n h  c [|n|(|n|+1)] 2  c = eB/m* Fit: v F = 1.07 10 8 cm/s as for graphene and for graphite data from ARPES For linear E(H) dependence m* = 0.028 m 0

24. Landau quantization in graphite from magneto-transmission experiment M. Orlita et al. Phys. Rev. Lett. 2008 selection rule:  n =  1,

25. Interlayer tunneling our experiments

26. Landau quantization in graphite (Interlayer tunneling Yu.I.L, A.P. Orlov, D. Viqnolles 07 66 0.41 0.47 0.54 0.61 0.69 0.77 0.87 0.97 1.08 1.20 1.34 1.49 1.65 1.82 2.02 2.23 2.46 2.70 2.92 G #1 N  30G #3 N  20 We found Landau quantization from interlayer tunneling transitions -1 1, -2 2 consistent with STM and magneto-transmission data Spectra are well reproducible, peak position does not dependent on N аnother selection rule: |  n| = 0 valid for coherent tunnеling

27. Comparison of the 1 st level energy for two samples V  H 1/2 typical for Dirac fermions

28. Comparison with STM and magneto transmission data Transitions -1 1, -2 2, -3 3 observed are consistent with STM and magneto-transmission data. V F = 10 8 cm/s, E n  (nH) 1/2

29. Effects in strong magnetic fields

30. Graphite at strong fields Yu.I.L., A.P.Orlov, D. Vignolles, P. Monceau 07 Observation H. Ochimizu et al., Phys. Rev. B46, 1986 (1992). Explanation was related with the CDW formation along the field axis D. Yoshioka and H. Fukuyama, J. Phys. Soc. Jpn. 50, 725 (1981). We attempted to find CDW gap above 30 T Effect nearly disappeared for 20 graphene layers

31. Pseudogap at graphite at high fields Yu.L., A.P. Orlov, D. Vignolles, P. Monceau 06-07 Pseudogap appears above 20T, Vpg  150 mV Remarkable features: (1) increase of tunnel conductivity with field (2) field induced PG ???

32. Field dependence of pseudogap value No essential field dependence above 25 T We consider that the big value of the field induced pseudogap is an indication of some collective excitations in graphene at high fields

33. CONCLUSIONS 1.FIB technique has been adapted for fabrication mesa type structures on various nanomaterials as HTS materials, CDW layered materials and graphite. 2.We found the effect of CDW gap induction by high magnetic field above Peierls transition temperature. We also found non-monotonic dependence of Tp(H) which is interpreted as the interplay between orbital and Pauli effects on CDW ordering. 3.We found interlayer correllative gap in graphite below 25K with energy of 10- 15 mV. 4. Using interlayer tunneling we identified in graphite Landau levels typical for Dirac fermions in graphene. 5.We found field induced pseudogap in graphite. The high value of the pseudogap, 150 mV, points out to its possible origin related with collective excitations in graphene.