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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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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
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< 2J,
J, = s c /ab~ 1m in Bi-2212
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
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
L. Bulaevskii, D.Dominguez et al. PR 1996
Dense lattice can move as a whole being driven by DC current across the layers
Swihart velocity, velocity
of electromagnetic wave
the slowest mode
resonance occurs => flux-flow step
Linear ff, low fields J.U Lee et al. 1995
ff-step B1TYu. 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
(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
(2) Commensurate oscillations of Josephson flux-flow resistance in narrow mesas
Commensurate boundary pinning of JVL
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; 31m
I. Kakeya, K. Kadowaki et al. 2004
T. Hatano et al. 2004
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
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..
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.
Seiko Instruments Corp. SMI-9000(SP)
Ga+ ions 15-30 kV
Beam current : 8pA – 50 nA
Minimal beam diameter: 10nm
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)
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)
22 50-60 mV,
21 130-150 mV
Consistent with STM, optics and low temperature ARPES data
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.
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
There are two new features inside the CDW gap with characteristic energies Vs ~2/3 and Vt ~ 2/10
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= 20 /. 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.
Intragap CDW spectroscopy in NbSe3. (I) ZBCP is suppressed by temperature
Temperature dependence at high temperatures T > Tp/2
Peak at Vs 2/3 for both CDWs,
Scaling Vs and 2 is temperature independent!!!
Soliton peak at low temperatures
At high magnetic fields parallel to the layers, H//c
Parallel magnetic field narrows ZBCP and soliton peak becomes clear
Perpendicular magnetic field to the contrast broadens ZBCP
Parabolic background substraction at high temperatures
ICDW-CCDW transition at T 130 K
at CCDW amplitude solitons with accepting of one electron are forbidden
Specially designed mesa oriented across the chain direction to avoid contribution of CDW sliding in connecting electrodes
Threshold behaviour is very clear
eVth 1.3 kTp
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.
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.
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
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.