1. GROWTH, CRYSTAL STRUCRURE AND TRANSPORT PROPERTIES OF ONE-DIMENSIONAL CONDUCTORS NbS3.
S. G. Zybtsev, V. Ya. Pokrovskii,
S. V. Zaitsev-Zotov, and V. F. Nasretdinova.
Kotel’nikov Institute of Radioengineering and Electronics of Russian Academy of Sciences,
11-7 Mokhovaya Moscow, Russia.

2. Main conclusions of Vukovar-2010
Whiskers of quasi one-dimensional conductor NbS3, phase II, have been synthesized. The samples show two charge-density wave (CDW) states: below 360 and 150 K. Both CDWs show sharp threshold field and coherent transport revealed by the AC-DC coupling – the Shapiro steps [1], centered at the so-called fundamental frequency, fo=ICDW MeN (ICDW is the CDW current, e - the elementary charge, M2 – the number of electrons per CDW wavelength, N – the total number of conducting chains).
The results indicate that for each of the CDW states only 1 Nb chain per unit cell participates in the CDW transport.
The thinnest samples (cross-section below 104 nm2) show the Shapiro steps under at least 16 GHz irradiation at room temperature.
1. S. G. Zybtsev, V. Ya. Pokrovskii,a V. F. Nasretdinova, and S. V. Zaitsev-Zotov, Appl. Phys. Lett. 94, (2009).

3. Synthesis and microstructure of NbS3 whiskers
. The samples were grown from the vapor phase by direct reaction of Nb and S in mole ratio 1:3 with a 10% excess of sulfur. The growth continued for two weeks in a quartz tube. The temperature gradient over the tube length (20 cm) was C. .
TEM pictures of NbS3 whiskers. .

4. Microstructures based on NbS3 whiskers
Microbrodges are fabricated by vacuum laser deposition of gold through thin micron-sized masks (we used NbSe3 and BSCCO whiskers as the masks). We controlled dimensions of bridges and their location on the substrate by the AFM technique. Using the laser micro-etching we isolated electrically the selected microbridge from other bridges.
AFM pictures of 3m-long microbridges Au Au Au Au Au
NS102208
NS102808
NS100208

5. Arrhenius plot of typical R(T) curves. R is normalized by L.7 5 4 6

6. dI /dV versus V curves under rf radiation for the upper and lower CDWs.

7. Differential I-V curves of NS060110b bridge at room temperature

8. The dependences of the CDW current corresponding to the 1-st Shapiro step on f.
T=300 K

9. High resolution TEM pictures of NbS3 whiskers synthesized at different temperatures
Ts = oC
Ts = oC q1 q2
atom interlayer (along b) distance is 0.99 nm
the doubling of the parameter a of the unit cell from 9.9 Å to 19.8 Å
one can see two superstructures q1=(1/2a*, b*,0.297) and q2=(1/2a*,b*, 0.353)) . Phase II is monoclinic with 8 Nb chains per unit cell and shows approximate trimerisation below TP = 365 K

10. Temperature dependencies R(T) of synthesized samples
=150 K
=365 K
One samples group (Ts = oC) has two Pierles transitions (red line) TP1=365 K and TP2 =150 K
Another group (Ts = oC) has one Pierles transition (blue line) TP1 =365 K (more high ohmic)

11. Non-linear properties
Low ohmic samples
High ohmic samples

12. PRL 93, 106404-1, (2004) A. Ayari, R. Danneau, H. Requardt, L
PRL 93, , (2004) A. Ayari, R. Danneau, H. Requardt, L. Ortega, J. E. Lorenzo, P. Monceau, R. Currat, S. Brazovskii, and G. Grubel
temperature variation of the depinning threshold fields of NbSe3 for the Q1 (Ec1 : from dV/dI, and from BBN) and Q2 (Ec2: from dV/dI) CDWs.
In the regime EC1 <E<EC2, when the Q2 CDW is in the free sliding state, the Q 1 CDW experiences a timedependent periodic perturbation from the moving Q2 CDW. Its action resembles that of an external ac field, which is known to anneal the frozen pinning, most probably by relaxing metastable states. This effect together with the ‘‘averaging out’’ of the phase coupling term is the most natural reason for the observed drop of Ec1 after the sliding onset of the Q2 CDW.

13. dI /dV versus V curves under rf radiation for low ohmic samples (upper CDW).

14. dI /dV versus V curves under rf radiation for low ohmic samples (lower CDW).

15. Temperature dependencies of dI /dV versus V curves under rf radiation for low ohmic samples (lower CDWs).

16. In the paper PRB, 46, 7413 (1992) F. Ya. Nad‘, P
In the paper PRB, 46, 7413 (1992) F. Ya. Nad‘, P. Monceau authors explained decreasing of Jc/f (more than 1000 times) by phase-slip process on contacts. At some temperature, the value of the reciprocal relaxation time becomes smaller than the NBN frequency, CDW will not have time enough to be sufficiently deformed and therefore the amplitude of oscillations of energy gap will begin to decrease. Simultaneously, the flow of screening electrons in the region of phase slip begins to retard and, consequently, the number of charges involved in the phase-slip process decreases. This mechanism may yield the decrease of the current oscillation amplitude and, consequently, the decrease of the Ic/f ratio.
However, in our case despite the small Jc/f, the synchronization (amplitude of Shapiro steps) is high, and Jc/f doesn't depend on temperature, though resistance changes by more than an order. Hence, another explanation for this effect is required.

17. Study of Peierls state at high temperatures (T>300 K)
In the work PRB, 40, (1989) Z.Z. Wang, P. Monceau et al. authors observed the evolution of electronic diffraction spots q1 and q2 on temperature. They found that q1 spots strongly decrease above 77 oC. But q2 spots have shown only very weak variation. They concluded that for T>300 K NbS3 exhibits two CDW’s, like NbSe3 and monoclinic TaS3: TP1 = K and TP0>450 K (the temperature of irreversible deformation of the sample in electron microscope).
This work stimulated us to investigate NbS3 at high temperatures (T>360 K).
Our TEM and Shapiro steps studies support the idea of existing of CDW0 (TP0>360 K). We found that at T<360 K only 1-2 Nb chains per unit cell participate in the CDW transport. Other six chains could be in CDW0 state above 360 K. Indeed our diffraction patterns have shown that intensity of q2 spots is much higher intensity of q1 spots.

18. Study of Peierls state at high temperatures (T>300 K)
We designed the low-inertia furnace with argon blow to prevent the degradation of samples
Slow measurements using lock-in amplifier Stanford 830

19. Fast measurements using the digital oscilloscope
Samples were supplied by a sawtooth voltage (voltage is linearly proportional to time). The response (current) were measured by digital oscilloscope. Then data were differentiated by mathematically.
T=300K

20. Recording of 500 IV curves during heating from room to 400 oC temperatures

21. Fast I-V measurements
Complete variant of temperature dependencies of resistance of NbS3 whiskers
Temperature dependencies of resistance for two samples

22. Comparison of two transitions: TP1 TP2 and TP0 TP1
The common in two plots is visible. Approaching to TP2 from high temperatures, the threshold field of CDW1 at first increases then begin to decrease. The threshold field of CDW0 is very high (not detected), but approaching to TP1 from high temperatures threshold field of CDW0 begin to be detected simultaneously we can see a onset of sliding of CDW1.
inflection points of curves are connected by dash red line

23. Conclusions
1. Whiskers of quasi one-dimensional conductor NbS3, phase II, have been synthesized.
2. Grown crystals NbS3 have three CDW’s with transitions: TP0=620K, TP1=365 K and TP2=150 K.
3. In the range of K CDW0 is strongly pinned. A sharp threshold ~200 V/cm is seen at 360 K<T< 470 K.
4. CDW1 is stable and not so sensitive to growth conditions. It can move and be synchronized by the external microwave irradiation up to 16 GHz.
5. CDW2 strongly depends on growth conditions. However, it can also move and can be synchronized by the external microwave irradiation.

24. The common behavior in two plots is visible
The common behavior in two plots is visible. Approaching to TP2 from high temperatures, the threshold field of CDW1 at first increases then begin to decrease. the Threshold field of CDW0 is very high (not detected), but approaching to TP1 from high temperatures threshold field of CDW0 begin to be detected simultaneously we can see a onset of sliding of CDW1.

25. For lower CDW (CDW1) Jc/f =18A/Mhz/cm2 that corresponds to 1 CDW chain per cell. For upper CDW (CDW2) Jc/f =0.13 A/Mhz/cm2 that is less more 100 times than for CDW2.
In the paper PRB, 46, 7413 (1992) F. Ya. Nad‘, P. Monceau authors explained decreasing of Jc/f (more than 1000 times) by phase-slip process on contacts. At some temperature, the value of the reciprocal relaxation time becomes smaller than the NBN frequency, CDW will not have time enough to be sufficiently deformed and therefore the amplitude of oscillations of energy gap will begin to decrease. Simultaneously, the flow of screening electrons in the region of phase slip begins to retard and, consequently, the number of charges involved in the phase-slip process decreases. This mechanism may yield the decrease of the current oscillation amplitude and, consequently, the decrease of the Ic/f ratio.
However, in our case despite a small Jc/f, the synchronization (amplitude of Shapiro steps) is high and Jc/f doesn't depend on temperature though resistance changes more than on an order.
Hence, other explanation for this effect is required.

26. To slide 11 (high ohmic sample) f=400 MHz

27. To slide 13

28. High resolution TEM picture (atom interlayer distance along b direction is 0.95 nm ) and the microdiffraction with the beam of [100]B (one can see two superstructures q1=(1/2a*, b*,0.297) and q2=(1/2a*,b*, 0.353)) . Phase II is monoclinic with 8 Nb chains per unit cell and shows approximate trimerisation below TP =  K

29. The differential resistance versus voltage dV/ dI versus V at different T for the NbS3 samples No. 3a
The curves suggest formationof two CDWs: below 360 and 150 K. This is the first observation of such abrupt depinning of the CDWs in NbS3.

30. dV/dI vs I for a NbS3 nanowhisker (sample NS102208) under 2
dV/dI vs I for a NbS3 nanowhisker (sample NS102208) under 2.56 GHz irradiation.

31. AFM pictures of 1m-long microbridges
Differential I-V curve
Sample NS031710f
Profile of NbS3 whisker
1.1 m
S=14 nm*44 nm = 2 (AFM measurement)
S = 180 Å  I/(2efo) =180* /(2* * ) = 2 (From Shapiro steps)

32. Differential I-V curves of NS052010 bridge at room temperature

33. Rd vs I dependencies at utmost currents
Jmax ~ A/cm2, corresponding to CDW velocities 200 m/s or fundamental frequencies 200 GHz.

34. The table of samples

35. The table of samples

36. PRB, 28, 1646 (1983)

37. The origin of such high frequency Shapiro steps
Low density of energy dissipation due to low density of conducting chains.
The highly anisotropic structure of NbS3 allows to synthesize the whiskers down to nanometer dimensions retaining their CDW properties that make possible passing extremely high current densities through the samples without heating.
Apart from high currents high characteristic frequencies are required. Characteristic frequencies fc~Ete/(m*) [1] can be high due to high threshold fields (Et ≥ 100 V/cm for nanometer samples) and low friction of CDW (1/).
1. Rigid overdamped oscillator model. G. Gruner et al. Phys.Rev.Lett 46, 511 (1981); P. Monceau et al. Phys.Rev.B 25, 931 (1982).