A new relaxation state of negative resistance in nano-fabricated CDW

1. A new relaxation state of negative resistance in nano-fabricated CDW K.Morikawa*, A.Goto*, N.Shinjo*, Y.Nishi*, A.Nakada*, H.Kubota* Kumamoto UniversityJapan

2. Outline 1.Introduction 2.Procedure of Nano-fabricated CDW sample 3. Experimental Method of the pulsed photoconductive measurement & Result 4. Conclusion

3. 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

4. Procedure of nano-fabricated of CDW crystal Dimension • The single crystal of bulk K0.3MoO3 was prepared by an electrochemical growth technique(3D). • In this time the sample size was 5x3x0.3mm3 • 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. • The line & space patterns are exposedto 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+ We created nano-fabricated CDW was 40nm thickness and 100nm line & space all over the area on the surface of K0.3MoO3 1D Nano-fabricated CDW line Nano-fabricated CDW thickness and wide x^ < < : ~ 500nm at 50K

5. 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.

6. Pulse generator Cu Mylar (10mm) Xenon flash lamp K0.3MoO3 Digital Oscilloscope RELAY R1=1~10MW R2=1kW Amplifier X100 Experimental method of the pulsed photoconductive measurement A sample is placed between two blocking electrode isolated by the Mylar sheet Impressed voltage and light pulses delay time was controlled pulse generator Relay switch is controlled charge time of sample Measurement circuit of pulsed photoconductivity method

7. 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 Equivalent circuit and timing of pulse signal

8. initial voltage T=4.2K balk-sample @4.2K 100V 80V 50V Inσ[a.u.] Internal field Ein [a.u.] 0.002 0.004 0 Time, t [sec] Ein [a.u.] Experiment result Relaxation state of the balk CDW Time dependence of internal field in sample 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

9. 1D-sample @4.2K 50V init. 1D-sample @4.2K 100Vinit. Inσ[a.u.] Ein [a.u.] Relaxation state of the nano-fabricated CDW Time dependence of internal field in sample 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 relaxationstate is caused New relaxation state was expressed negative resistance

10. Image of the Current-Internal field Ein characteristic of nano-fabricated CDW E σ２:sliding CDW state electrode Coherent CDW Relaxation time τ2is fast New relaxation state is caused σ３: State after CDW stops σ3　＜　σ1　＜σ２ incoherent CDW Relaxation time τ3 is the slowest The electric field hangs only to coherent CDW I Negative resistance The electric field is saturated with a current increase. Balk CDW σ１: non-sliding CDW state Relaxation time τ1 is slow Nano- fabricated CDW Appearance of negative resistance Coherence length of CDW(1-dimensionality ) quantum tunneling has occurred

11. 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 one-dimensionality CONCLUSION We observed the internal field relaxation characteristic of nano-fabricated K0.3MoO3 at 4.2K, the state that showed nano-fabricated CDW state.

12. E(k) E(k) ' E F gap Ion irradiation E E F F ' E F k k k - k - k 0 0 k F F F F electron density atoms No CDW layer defect H defect H Condition of no CDW formation 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