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Interlayer tunneling spectroscopy in layered CDW materials. Yu.I. Latyshev Institute of Radio-Engineering and Electronics, Russian Acad. of Sci., Mokhovaya 11-7, Moscow 101999, Russia In collaboration with P. Monceau, Th. Fournier CRTBT-CNRS, Grenoble, France

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slide1

Interlayer tunneling spectroscopy in layered CDW materials

Yu.I. Latyshev

Institute of Radio-Engineering and Electronics, Russian Acad. of Sci., Mokhovaya 11-7, Moscow 101999, Russia

In collaboration with

P. Monceau, Th. Fournier CRTBT-CNRS, Grenoble, France

S. Brazovskii LPTMS-CNRS, Orsay, France

A.P. Orlov IREE RAS, Moscow, Russia

A.A. Sinchenko MEPI, Moscow, Russia

E. Mossang LCMI-CNRS, Grenoble, France

At early stages

L.N. Bulaevskii LANL, Los Alamos, USA

T. Yamashita, T. Hatano, T. Kawae NIMS, Tsukuba, Japan

slide2

OUTLINE

  • Introduction to interlayer tunneling.
  • Mesa fabrication and experimental technique.
  • CDW gap spectroscopy.
  • CDW amplitude soliton spectroscopy.
  • Threshold for interlayer tunneling.
  • CDW spectroscopy at high magnetic fields.
  • Conclusions
slide3

Introduction to intrinsic Josephson effects (IJE) and Josephson flux-flow (JFF) in layered HTS materials

The amplitude of the OP is vertically modulated, while phases are coupled. One can expect both phase interference effect and quasiparticle tunneling.

IJE - Josephson effects on naturally layered crystalline structure of layered superconductors

I. Phase effects

Early ideas in 70-s: W.E.Lawrence, S.Doniach 1971L.N. Bulaevskii 1973

Further development in 90-s, after discovery of HTS

L.Bulaevskii, J.Clem, L.Glazman 1992

stationary IJE for short stacks L< 2J,

J, = s c /ab~ 1m in Bi-2212

L

L

L

s

slide4

First experimental evidence of IJE: R.Kleiner, P.Mueller et al. PRL 1992

R. Kleiner and P.Mueller PR 1994

They had junctions of big lateral size ~ 30 m and Fraunhoffer oscillations of critical current were not so clear

Yu.I.Latyshev, N. Pavlenko, S-J.Kim, T.Yamashita ISS-99, Morioka

Bi-2212 stack: L=1.4 m, s=1.5 nm, H=1.01 T

slide5

Josephson vortices

Long stack, L>>J

Josephson vortex (phase topological defect). No normal core.

J. Clem and M.Coffey, PR 1990

Phase coherency is locally broken with appearance of JV at H>Hc1

Josephson-vortex lattice (JVL)

L. Bulaevskii and J. Clem, PR 1991

Dense Josephson-vortex lattice

~103

Bcr~0.3T

L. Bulaevskii, D.Dominguez et al. PR 1996

Dense lattice can move as a whole being driven by DC current across the layers

slide6

Josephson flux-flow regime

Swihart velocity, velocity

of electromagnetic wave

propagation

when

the slowest mode

(triangular lattice)

~0.1% c

resonance occurs => flux-flow step

Experiments

Linear ff, low fields J.U Lee et al. 1995

------------------------------------------------------------------

ff-step B1TYu. Latyshev, P. Monceau et al. Physica C 1997

------------------------------------------------------------------

ff-step B<0.3T G. Hechtfischer et al. PR 1997

------------------------------------------------------------------

ff-step 0.5<B<3.5TG. Hechtfischer et al. PRL 1997

slide7

The experimental ways to identify JVL

(1) Shapiro step response in JFF regime to subterahertz external radiation. Coherent response of 60 elementary junctions

Yu.Latyshev, M.Gaifullin et al. PRL 2001

with N=57

slide8

(2) Commensurate oscillations of Josephson flux-flow resistance in narrow mesas

Commensurate boundary pinning of JVL

w

triangular lattice

rectangular lattice

H s w = 0, w= 1.8 m

S. Ooi, T. Mochiku, K.Hirata, PRL, 2002

H s w = ½ 0, w=7.3; 18; 31m

I. Kakeya, K. Kadowaki et al. 2004

T. Hatano et al. 2004

slide9

II. Quasiparticle tunneling over a gap: multibranched IVs, gap/pseudogap spectroscopy

Yu.I. Latyshev et al.ISS Conf. 1999, Physica C, 2001; V.M. Krasnov et al. PRL, 2000, 2001

slide11

CDW interlayer tunneling spectroscopy: NbSe3

The elementary prisms are assembled in elementary conducting layers with higher density of conducting chains (shaded layers in a figure) separated by a double barrier of insulating prism bases. That results in a very high interlayer conductivity anisotropy a*/b ~ 10-3 at low temperatures compared with intralayer anisotropy c/b ~ 10-1.

That provides the ground for interlayer tunneling spectroscopy of CDW layered materials..

slide12

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

slide13

stack

Stacked structures fabricated from CDW materials by FIB methods

NbSe3 mesa fabricated by double-sided FIB method (CRTBT-CNRS, Grenoble)

TaS3 mesa fabricated by lateral FIB etching method (RIEC, Tohoku Univ.,Sendai)

KMo6O17 mesa fabricated by lateral FIB etching method (Crystals were kindly provided by C. Schlenker and J. Markus, LEPES)

slide14

NbSe3: low temperature interlayer tunneling spectra

Yu.I. Latyshev, P. Monceau, A.A.Sinchenko, L.N. Bulaevskii, S.A. Brazovskii, T. Kawae, T Yamashita,. J.Phys. A, 2003

Coexisting of both CDW gaps, zero bias conductance peak (ZBCP)

22  50-60 mV,

21  130-150 mV

Consistent with STM, optics and low temperature ARPES data

slide15

22

(a)

21

Temperature evolution of the spectra

Point contact spectra NbSe3-NbSe3 along the a*-axis A.A. Sinchenko et al., 2003

Stacked junction behaves as a single junction. We consider that as the weakest junction in the stack.

slide17

Zero bias conductance peak

Bulaevskii theory of coherent tunneling of the ungapped carriers L.M Bulaevskii, JETP Lett. 2002.

Coherent tunneling implies the conservation of particle in-plane momentum in the process of tunneling. This is necessary because the pockets represent some localized small parts of the Fermi surface and electron momrntum should not be scattered beyond the pockets by tunneling from one layer to another. The width of ZBCP characterizes the energy uncertainty for the state characterized by momentum p.

Fitting parameters: eff = 0.25 mV, sc= 0.13 mV, N=30, m*=0.24 me, (T) was taken from the paper of N.P.Ong PRB 1978

for dI/dV

slide18

Intragap states: NbSe3

There are two new features inside the CDW gap with characteristic energies Vs ~2/3 and Vt ~ 2/10

slide19

Amplitude solitons in the incommensurate CDW (ICDW)

The order parameter in the ground state is 0 =A cos (Qx + ) with Q the CDW wave vector Q = 2kF and  the arbitrary phase in the ICDW state and A=const. That means that ground state is degenerated with respect to A  -A.

That leads to the possibility of configuration with accepting of one electron and formation of new ground state with A=tanh (x/0) called amplitude soliton (AS).

AS is a self-localized state with an energy Es= 20 /. S.A. Brazovskii, Sov. Phys.-JETP, 1980

This state is preferable since its energy is smaller than the lowest energy 0 of the free band electron by 0/3.

The existence of ASs has been well documented for in dimeric CDW materials (polyacetilene or CuGeO3). However , for ICDW materials of higher order incommensurability as MX3 existence of ASs has not been reliably demonstrated yet.

slide20

Intragap CDW spectroscopy in NbSe3. (I) ZBCP is suppressed by temperature

Temperature dependence at high temperatures T > Tp/2

CDW 1

Peak at Vs 2/3 for both CDWs,

Scaling Vs and 2 is temperature independent!!!

CDW 2

slide21

(II) ZBCP narrowed by magnetic field

Soliton peak at low temperatures

At high magnetic fields parallel to the layers, H//c

T=4.2K

Parallel magnetic field narrows ZBCP and soliton peak becomes clear

Perpendicular magnetic field to the contrast broadens ZBCP

slide22

CDW gap spectroscopy: o-TaS3

Parabolic background substraction at high temperatures

ICDW-CCDW transition at T  130 K

slide23

Schematic view of tunneling spectra

NbSe3

o-TaS3

at CCDW amplitude solitons with accepting of one electron are forbidden

slide24

Threshold for interlayer tunneling

Specially designed mesa oriented across the chain direction to avoid contribution of CDW sliding in connecting electrodes

Set up

Threshold behaviour is very clear

slide25

Threshold voltage scaling with CDW gap

CDW 1

o-TaS3

CDW 2

Scaling with 

eVt 0.2 

NbSe3

slide27

CDW dislocation lines (DLs)

The energy ~ Tp is known as an energy of 3D CDW ordering. As known from structural measurements, above Tp transversal phase coherence of the CDW becomes lost.

Therefore, Vt may be interpreted as phase decoupling between neighbour elementary layers.

S. Brazovskii suggested that this decoupling occurs via successive entering in the “weakest” junction the set of dislocation lines.

DLs appear as a result of share stress induced by electric field across the layers. Each DL is oriented across the chains in elementary junction and corresponds to the charge 2e per chain or entering of one unit of CDW period. DL can be considered as phase CDW vortex. There is also some similarity between Vtand Hc1 in superconductors.

Electric field concentrates within dislocation core: dz~ 10Å i.e. that drops within one junction,

L= p/Tp ~ 20. L~ 200 Å. For 1 m size junction one needs 5-10 DLs to overlap all the junction area and to have complete decoupling of neigbour layers.

So one can expect multiple threshold for successive entering of a of DLs.

slide30

When dislocation cores start to overlap at high voltage bias all the voltage drops on a single elementary junction. That can explain puzzling equivalence of the behaviour of the stacked junction and point contact containing one junction.

slide31

Metal - insulator transition for interlayer conductivity in NbSe3 induced by high magnetic field

Yu.I. Latyshev, P. Monceau, A.P. Orlov, A.A.Sinchenko, T. Fournier, E. Mossang , 2004

Colossal magnetoresistance at eV< 2, that can achieve 2 orders at H=25 T,

some enhancement of the lower CDW gap by magnetic field

Questions

1. Field induced CDW? One can expect then enhancement of lower Tp.

2. The ZBCP still survives and acquires step-like form. We will study this state in future experiments.

slide32

SUMMARY

  • The FIB technique has been adapted for fabrication of micron sized stacked junctions for interlayer tunneling spectroscopy of layered CDW`materials.
  • Using interlayer tunneling spectroscopy we identified the CDW gap and zero bias conductance peak (NbSe3) attributed to coherent tunneling of uncondensed carriers.
  • We found some evidence of an existence of amplitude soliton states inside the CDW gap for both materials, NbSe3 and TaS3, and for both CDW states in NbSe3.
  • We found a threshold and staircase structure for interlayer dynamic conductivity as a function of bias voltage. We associate a threshold with the nucleation of the first CDW dislocation line in the weakest intrinsic junction. The staircase structure is explained as a successive nucleation of a row of CDW dislocation lines.
  • We found that magnetic field perpendicular to conducting planes essentially suppresses tunneling density of states at eV<2 leading to a colossal magnetoresistance. That is a sort of metal-insulator transition mediated by magnetic field.
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