Charge-density wave conduction with extremely low differential resistance in K0.3MoO3: Current oscillations

Solid State Communications, Printed in Great Britain.

Vo1.66,No.2,

WAVE CONDUCTION

CHARGE-DENSITY

pp.149-152,

0038-1098/88 $3.00 + .OO Pergamon Press plc

1988.

WITH EXTRJZHELY LOW DIFFERENTIAL

RESISTANCE

IN Ko_3kfo03:

CURRENTOSCILLATIONS

G. BihAly*,

associated

P. Beauchf!ne

and J.

Warcus

Centre National de la Recherche Scientifique Laboratoire d’Etudes des Proprietes Electroniques with Universite Scientifique, Technique et Medicale B.P. 166, 38042 Grenoble Cedex, France (Received

25 January

de Grenoble

1988 by A. Zawadowski)

We have studied the real time current oscillations in the low temperature zero differential resistance state of the blue bronze (pdiff< 10 -4 Gem), where the current increases more than seven orders of magnitude without observable change in the voltage. The linear relation between the oscillation frequency and the current as well as the pulse duration memory effect observed demonstrate that the electronic transport is due to the charge-density waves (CDW). The drift velocity of the condensate reaches values as large as a few hundred cm/set, leading to a metallic conductivity in an “insulating” medium at T=4.2 K. The dramatic drop of the CDW damping reflects the qualitative change in the conduction mechanism as normal carriers freeze out.

chain Ko.3BoD3 is a well known2 linear compound, where the development of incommensurate charge-density waves below Tp= 180 K opens a Peierls gap, E ~0.15 eV. At T= 4.2 K, where experiments of &he present study were performed, the normal carrier density is negligible. In the low voltage range, the system is an insulator, with resistivity higher than 1Oi2 Dem. At higher above a sample dependent threshold, voltages, however, sharp current rise was reported at the pA levels4-6. Fig. Idemonstrates that the current can be increased much further, up to - one Ampere, without observable change in the voltage. High current regions of the curves shown in Fig. lwere measured by pulsed technique (HP 214B pulse generator, TK 2230 digital storage oscilloscope 1. Pulse shape studies in the high current range1 and long time (5~10~ set) observation of stable continuous current at mA levels confirmed that the measured values correspond to steady state currents. We found that the current response at high current densities is quicker than 20 nanoseconds. Such a short rise time excludes the possibility of thermal run-away; the thermal time constant is about four orders of magnitude longer at this temperatureT.

recentlyi that at low We have shown temperatures the blue bronze2 KO_3Mo03, exhibits practically zero differentlal resistance over an extremely wide range of current above a contact dependent threshold voltage. The narrow band noise observed in the BHz frequency range indicated that the current is due to sliding charge-density waves (CDWs). Abrupt changes in associated to between noise spectra jumps distinct current levels revealed that at low currents the conducting cross section of the The possibility that the sample may vary. sharpness of I-V curves Is related to a breakdown phenomenon due to filamentary instability with strongly inhomogeneous current densities could not be complately excluded’. In this letter we present results of real time current oscillation experiments showing that the steep increase of the current is a bulk phenomenon and it arises from the highly coherent motion of CDWs. Differential resistivity as low as 10s4 Dcm characterises this sliding state at liquid helium temperature - a value never reached at higher temperatures. We argue that freezing out of normal carriers changes qualitatively the conduction mechanism of the collective mode, leading to a “free sliding” state. This could correspond to an anomalously low damping which goes to zero as the temperature is lowered. It is also tempting to speculate about the Frohlich mechanism3 - sliding without scattering even at finite temperature.

*Permanent Physics,

address:

Central

1525 Budapest

Research

P.O.B.49,

Institute

The threshold voltage VT depends on the quality of the sample. For crystals A, B and C with geometry of 3.85x0.37x0.066 mm3, SxSxO.9 mm3 and 3x 0.85x x 0.26 mm3 (length x width x thickness) VT = 2, 6 and 13 V was found respectively. The voltage on the crystal is stable withln -10% during the sharp rise of the current. Segments with negative characteristics and slight shifts of the curves in Fig. 1 have instrumental origin (e.g. due to the change of

for

Hungary 149

EXTREMELYLOW DIFFERENTIAL RESISTANCE IN K 0. 3M003

150

sample

10-3

B

sample

c

1

-

f i :

a

-6 10

0.1

1 Voltage

(VI

Figure 1. Current vs voltage curves for three blue bronze samples measured at T = 4.2 K by pulsed technique in two probes configuration. (Continuous lines are d.c. results) _ Further increase of the current instrumentally was limited. serial resistance, which was applied to regulate the current of the voltage pulse generator). current increase of the was Further instrumentally limited. Current density in the order of lo4 A/cm2 and differential resistivity less than 10m4 ncm were found in sample A. The chordal conductivity is larger than 800 n-1cm-1, which is close to the metallic value measured Note that the above the Peirls transition. resistance of the crystals is higher than 10 GD when measured at voltage levels below VT, and it drops more than nine orders of magnitude above the threshold voltage. A unique manifestation of charge-density wave conduction8-11 is the narrow band noise phenomenon12, the appearance of an alternating voltage is component when constant current direct (or voltage modulation at applied current). The fundamental frequency of the oscillations (uf) is proportional to the CDW current density (jCDW) with a ratio of uf/jCDW close to 2enl, where nI is the number of conducting chains, per unit cross section. In spite of large experimental efforts to find the source of noise generation there is no unambiguous evidence to decide whether it is a bulk or surface phenomenon. It is clear however

Vol.

66,

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that noise arises from CDW domainsi and can be generated inside the crystal, not only at contact Different theoretical descriptions surfacesl$. modulation were suggested for noise generation: of drift velocityIS, phase slips at7CDW current conversionih, backward scatte~~~~~~i~~d s?EEs l~tti~~~?-‘% the following we consider the narrow band noise phenomenon as an evidence for charge-density wave current. Fig. 2shows the current response of samp1e.C for single voltage pulses. The length of the pulses and instrumental time constant determine a frequency window of about loSO0 KHz. For currents in the 3-10 mA interval high amplitude oscillations were observed in this frequency range. (In this experiment the circuit was close to a voltage driven arrangement. Ideal voltage source cannot be applied because of the nearly zero differential resistance of the sample). The phase of the oscillations is Independent of the averaging several leading edge of the pulse, randomly applied pulses results in a smooth In some cases we observed current pulse shape. change in the oscillation period within a single with harmonic oscillation periods. pulse, In Fig. 3we have plotted the frequency of the real time oscillations recorded in single pulse experiment as a function of the current. (Some oscillations with higher frequencies were also identified as observed, most of them were harmonics of the fundamental frequency shown in Fig. 3). The linear u - I relation suggests CDW the slope u/I (4.2 K) = 14 HHz/A conduction, is close to the temperature independent value measured in the conventional CDW conduction range, v/ICDW (78 K) = 20 MHz/A. With the value of- the current density calculated from the cross section, one finds u/j = 19 kHz cr stal cm3 /A at 4.2 K, which is to be compared to the theoretical value of v/jCDW = 12.5 kHz cm2/A (calculated for band degeneracy 2). Although the deviation indicates that some part of the crystal in the section does not participate cross conduction, it is clear that current flow through most of the contact area. Consequently not only in this range, but also at higher currents I.e. from the mA to the 1A region the increase of the

I

E 04

r Figure 2. measured on T-4.2 K.

Real sample

t.ime current oscillations C with single pulses

at

2

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66,

No.

2

151

EXTREMELY LOW DIFFERENTIAL RESISTANCE IN K o.3Moo3

Surprisingly the phase is locked both to the leading and trailing edges; the frequency of oscillation changes with the pulse length in such a way that the number of oscillations remains (Fig. 4(b)). The frequency can be constant shifted by about 507. until a new oscillation appears. The phenomenon described above is the pulse duration memory effectzO characteristic of the It indicates, charge-density wave conduction. in the simplest interpretation, that CDW tends to slide by integral multiples of wavelength. In some sense the pulse duration memory effect mode locking of the iS the manifestatlon phenomenonz2 in a real time experiment. The observation of pulse duration memory in the oscillations current at T4.2 K strongly supports that the current rise shown in Fig. 1 arises from sliding CDWs. Summarizing the results we found that the charge-density wave current can be increased without observable voltage change. The metallic conductivity reached at high current densities indicates that the damping of CDW motion is extremely low. The limit for the dynamic resistivity is p< 10s4 Ocm, considerably lower

4.2 K

I( mA) Oscillation Figure 3. single pulse experiment total current.

frequency measured as a function of

in the

current at constant voltage (Fig 1, sample C) reflects a real increase of the current density. Real time oscillations shown In Fig. 2 are different from low clearly the nA l~:~~~~?y reported oscillations In the Oscillations with line shape characteristic to relaxation oscillator& and switching between different distinct current levels5 arise from instabilities between parallel conductin domains. We showed by narrow band noise studies H that at the beginning of the current rise not only the current density but also the conducting fraction of the crystal changes. Oscillations due to volume instabilities as well as the broad noise at low band E:;::ZZ:~~ are not related to the single domain properties. The current range covered by the present experiment, however, is above these instabilities. The most important conclusion of Fig. 31s that the steep increase of the current at high current levels is not due to an avalanche breakdown associated to inhomogeneous filamentary conduction, but it reflects the increase of the CDW drift velocity. the current oscillations As mentioned above, disappear when randomly applied pulses are averaged. For certain repetition rates however the oscillations are locked at the pulse edges. Fig. 4(a) shows three current traces averaged over 64 current pulses with different repetition rates. The oscillations are phase locked for repetition time of about IOms and well shaped sinusoidal modulations can be recorded.

2m trep.

-

_ -

I---

12.5ms 125 ms 1.25 ms

Figure 4. a) Average of 64 repetitive current pulses with Phase is locked three different rates t’crep). (The curves are shifted for for Trep=lOms. clarity.) b) Pulse duration memory: phase is locked both at the leading and trailing edges. Average of 64 12.5 ms) for three repetitive pulses (Trep= different pulse lengths.

152

EXTREMELYLOW DIFFERENTIAL RESISTANCE IN KU 3MoOY

than ever observed at higher temperatures, in the conventional sliding state. The highest drift vd= jCDW X/Zen p 400 velocity reached is : cm/set (as calculated from the total current). The unusually sharp current rise indicates that absence of normal carriers changes the qualitatively the CDW scattering mechanism. We believe that in a large gap Peierls system at low temperatures rigid CDW could develop23 and the increased phase coherence is reflected in the A complete translational electronic transport. allow superinvariance would FrGhlich conductivity3, a finite phase velocity without With the scattering on the underlaying lattice. present experimental limits it is also possible that in the new sliding state a low, but nonzero which might disappear only at damping exist, zero tempereture. We have considered the narrow band noise phenomenon as a proof for charge density wave conduction. However, if damping is negligible in the bulk, noise must appear at the contacts. The role of the contacts is also to be investigated as a possible origin of the threshold voltage. VT could be a barrier voltage at the contact area due to current injection from a metal into

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a medium where no free carriers are present. It could also reflect a real threshold field which must be exceeded, or a phase slip energy the conversion of normal corresponding to electrons into CDW condensate. We assumed that at low temperatures the internal deformations of the charge-density waves freeze out as the number of the free carriers decrease. It is to be noted, however, that previous low temperature experiments on blue bronze were interpreted in terms of deformable charge-density waves5*24. “Phase organization”2S, a recent description of pulse duration memory effect, also assumes deformations, and the model was developed for a dissipative many-body system. In conclusion we have shown that in the blue bronze at low temperatures a new form of collective electron transport exists, chargedensity wave conduction with extremely low These results suggest the possibility damping. of Friihlich mechanism3 In large gap Peierls systems. Acknowledgement - We wish to thank L. Mihaly, A. Janossy, J. Dumas and C. Schlenker for discussions and valuable help.

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24.

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