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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