Download K. Morikawa

A new relaxation state of negative resistance in
nano-fabricated CDW
K.Morikawa*, A.Goto*, N.Shinjo*, Y.Nishi*, A.Nakada*, H.Kubota*
Kumamoto University Japan
Outline
1.Introduction
2.Procedure of Nano-fabricated CDW sample
3. Experimental Method of the pulsed
photoconductive measurement & Result
4. Conclusion
Introduction
As for Charge density wave (CDW) in one-dimensional , various experiments
have been performed.
And in the case of sliding motion of CDW, the quantum tunneling effects has
been expected.(J.Bardeen Phys. Rev. B 5, 1989)
However, it is difficult to observe the quantum tunneling phenomenon in the
actual experimental results, because the quasi one-dimensional effect
･ Coulomb interaction
･ phonon scattering
･ normal-carrier screening
Aim of this study
We controlled the generation and disappearance of CDW spatially by using
two kinds of ion beam irradiation, and the dopants isolated nano-fabricated
CDW in the bulk K0.3MoO3
The phenomenon of nano-fabricated CDW original is observed
Procedure of nano-fabricated of CDW crystal
Dimension
1.
The single crystal of bulk K0.3MoO3 was
prepared by an electrochemical growth
technique(3D).
In this time the sample size was
5x3x0.3mm3
2.
A hydrogen ions irradiation with 4.5keV
in the direction perpendicular to the one
dimensional axis(3D→2D). K0.3MoO3
dopes about 10% of H ion to the K, CDW
will disappear in the pouring domain. In
the result generates a ultra-then film
CDW layer on the sample surface.
3.
The line & space patterns are exposed to
the Si ion irradiation by using Focused
Ion Beam（60keV） dimensional axis and
etching by the use of alkaline solution
(2D→1D)
K0.3MoO3
Single crystal
3D
H
+
Ultra Thin CDW
layer
2D
No CDW layer
(due to doped H ion)
Si2+
1D
Nano-fabricated CDW
thickness and wide
Nano-fabricated
CDW line
<< x^ : ~ 500nm at 50K
We created nano-fabricated CDW was
40nm thickness and 100nm line & space
all over the area on the surface of
K0.3MoO3
In the case of nano-fabricated CDW phenomenon
with the electrode on the sample surface
1. Narrow Band Noise (NBN) and changing Broad
Band Noise (BBN) that could not be observed with
the balk sample in K0.3MoO3 was observed
(Journal de phys. IV Vol. 12 No.9 2002. )
2. Negative resistance appeared in the current-voltage
characteristic (ICSM 1998)
In this experimental
We are reporting on the relaxation state of the CDW by
using the pulsed photoconductive signal with blocking
electrode at 4.2K.
Experimental method of the pulsed
photoconductive measurement
Pulse
generator
Amplifier
X100
A sample is placed between two
blocking electrode isolated by the
Mylar sheet
Cu
Mylar (10mm)
K0.3MoO3
Xenon flash lamp
Digital
Oscilloscope
RELAY
R2=
1kW
R1=1~10
MW
Measurement circuit of pulsed
photoconductivity method
Impressed voltage and light pulses
delay time was controlled pulse
generator
Relay switch is controlled charge
time of sample
1.At t=0, the relay switch is in R2, and
applied pulse voltage V, charge up the
condenser in a few microseconds
2.When the charge collects in the
condenser, the relay is changed into
R1 before td
3.At t=td, the sample is illuminated by
pulsed light to induce probe photo
carriers, which immediately move
under the internal field Ein(t) inside
the sample.
4.Read the probe signal DQ(t) by Digital
Oscilloscope
Conductivity s Relation of the internal field
Ein(t) and relaxation time t is
DQ  F ( E in )
E in (t d )  ( VD ) exp[ tt ]
Equivalent circuit and timing of pulse signal
t  RC  sk
Experiment result
Relaxation state of the balk CDW
initial voltage
100V
80V
50V
balk-sample @4.2K
Inσ[a.u.]
Internal field Ein [a.u.]
T=4.2K
0
0.002
0.004
Time, t [sec]
Time dependence of internal field in sample
Ein [a.u.]
Internal field dependency of CDW conductivity
1.Relaxation characteristic was decreased at a same rate as the all
impressed voltage ranges
2.Conductivity of the bulk samples was not changed
Relaxation state of the nano-fabricated CDW
Inσ[a.u.]
1D-sample @4.2K 50V init.
1D-sample @4.2K 100Vinit.
Time dependence of internal field in sample
Ein [a.u.]
Internal field dependency of CDW conductivity
1.Relaxation characteristic decrease a fixed rate at the bulk CDW
when the impressed below the threshold voltage (50V)
2.Relaxation characteristic had changed nonlinearly when the
impressed more than the threshold voltage (100V)
3.Relaxation characteristic after CDW stops is different from the state that
CDW doesn't move originally
New relaxation state is caused
New relaxation state was expressed negative resistance
Image of the Current-Internal field Ein
characteristic of nano-fabricated CDW
E
σ２:sliding CDW state
Coherent CDW
Relaxation time τ2 is fast
electrode
New relaxation state is caused
σ３: State after
CDW stops
Relaxation time τ3
is the slowest
σ3 ＜ σ1 ＜σ２
I
Negative resistance
Balk CDW
Nano- fabricated
CDW
σ１: non-sliding CDW state
incoherent CDW
The electric field hangs only to
coherent CDW
The electric field is saturated with
a current increase.
Relaxation time τ1 is slow
Coherence length of
CDW(1-dimensionality )
Appearance of
negative resistance
quantum tunneling has occurred
CONCLUSION
We observed the internal field relaxation characteristic of
nano-fabricated K0.3MoO3 at 4.2K, the state that showed
nano-fabricated CDW state.
New conduction state existence that shows
negative resistance is confirmed
The state of nano-fabricated CDW was
expressed by using Current-Internal field
Ein characteristic that considers onedimensionality
Condition of no CDW formation
E(k)
E(k)
E'
F
gapIon irradiation
E
F
EF
E'
F
k
k
-k
F
0
k
F
0
k
defect
H
-k
electron density
F
F
atoms
No CDW layer
H
No CDW condition at 10% dopant of H to potassium
[scenario]
Fermi level should be influenced by H induced defects
Density of states at Fermi level changes
CDW will disappear
The defect induced region will play a role to separate adjacent CDWs
defect