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 125009 Moscow, Russia.e-mail: zybt@mail.cplire.ru

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, M2 – 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, 152112 (2009).

TEM pictures of NbS3 whiskers..

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 665-715 C. .

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 3m-long microbridges

NS102208 NS102808 NS100208

Au

Au Au

Au

Au

Arrhenius plot of typical R(T) curves. R is normalized by L.

4 6 8 10 12 14 4 6 8 10 1210

-2

100

102

104

106

108

1

2

3a3b

103/T (K-1)

R/L

(

/m

)

100 200 3000

1000

2000

3000

4000

5000

dln(R)/d(1/T) vs.T

45

6

6

54

7

dI /dV versus V curves under rf radiation forthe upper and lower CDWs.

2

3

T=330 K

V,Volts

(a)

-0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.40

1

2

T=101 K

Voltage (V)

dI/dV

(M

-1

)

(b)

0 0.1 0.2 0.30

0.05

0.1

0.15

0.2

f0 (GHz)

I CD

W ( A

)

f=60 MHz

f=160 MHz

f=300 MHz

f=256 MHz

f=175 MHz

T=101 K

T=330 K

Differential I-V curves of NS060110b bridge at room temperature

-1.5 -1 -0.5 0 0.5 1 1.5

x 10-5

4

6

8

10

12

14x 10

4

I, A

Rd, O

hm

fex

=13.85 GHz

9.34 GHz

6.28 GHz

0.8 GHz

The dependences of the CDW current correspondingto the 1-st Shapiro step on f.

0 2 4 6 8 10 12 14 160

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6x 10

-5

fex

, GHz

I cdw

NS051310

NS051410

NS052010b

NS052010

NS052010e

NS060110

NS060110a

NS060110b

T=300 K

High resolution TEM pictures of NbS3 whiskers synthesized at different temperatures

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

Ts =670 -700 oC Ts =715 -740 oC

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

q1

q2

Temperature dependencies R(T) of synthesized samples

2 4 6 8 10 12 14

x 10-3

105

106

107

108

109

1010

1011

1012

T, K

R, O

hm

TP2

TP1

One samples group (Ts =670 -700 oC) has two Pierles transitions (red line) TP1=365 K and TP2 =150 K

Another group (Ts =715 -740 oC) has one Pierles transition (blue line) TP1 =365 K (more high ohmic)

=365 K

=150 K

Non-linear properties

-1.5 -1 -0.5 0 0.5 1 1.5

10-9

10-8

10-7

10-6

10-5

V, V

dI/d

V, O

hm-1

T=297K260242218199188174154

120

105

78

-10 -5 0 510

-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

V, V

dI/d

V, O

hm-1

T= 304 K

280

262

249

223

190

159

139

122

97

77.5

Low ohmic samples High ohmic samples

-1 0 10

2

4

6

8

10

12

14

16

x 10-7

V, V

dI/d

V, O

hm-1

T=300 K

295

285

256

211

187

172

159

152

146

143

132

127

114

96

PRL 93, 106404-1, (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.

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

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

x 10-5

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8x 10

-5

I, A

dI/dV

, Ohm

-1

T=295 Kf=400 MHz

150 200 250 300 35014.5

15

15.5

16

16.5

17

17.5

18

T, K

J CD

W/f,

A/M

Hz/

cm2

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

-6 -4 -2 0 2 4 6

x 10-7

0

5

10

15x 10

-7

I, A

dI/dV

, Ohm

-1

T=117 Kf=400 MHz

0 100 200 300 400 500 600 700 8000

0.5

1

1.5

2

2.5

3

3.5

4

4.5x 10

-7

f, MHz

J CDW

, A

Jc/f~0.7 A/Mhz/cm2

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

-2 -1 0 1 2

10-8

10-7

10-6

10-5

V, V

dI/d

V, O

hm

-1

Jc/f0.13 A/cm2/MHz

Jc/f18 A/cm2/MHz

f=400 MHz

T=300 K 295 285 256 211

187 172 159 152 146 143 132 127 114 96

•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.

Study of Peierls state at high temperatures (T>300 K)

• In the work PRB, 40, 11589 (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 = 340-360 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.

Study of Peierls state at high temperatures (T>300 K)

-2 -1 0 1 2 30

1

2

3

4

5

6

7

x 10-5

V, V

dI/d

V, O

hm-1

T=200 oC 140 120

25 oC

1.5 2 2.5 3 3.5

x 10-3

105

1/T, K-1

R, O

hm

TP1

=365 K

TP0

=620 K

We designed the low-inertia furnace with argon blow to prevent the degradation of samples

Slow measurements using lock-in amplifier Stanford 830

Fast measurements using the digital oscilloscope

80 90 100 110 120 130

1

1.5

2x 10

-5

I, A

80 90 100 110 120 130-4

-2

0

2

4x 10

-5

t, sec

dI/d

V, O

hm

-1

-0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.20

0.5

1

1.5

2

2.5

3

x 10-5

V, V

dI/d

V, O

hm-1

f=400 MHz

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

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

0 50 100 150 200 250 300 350 400

-1.5

-1

-0.5

0

0.5

1

1.5

x 10-4

T, oC

dI/d

V, O

hm

-1

TP1

TP0

Fast I-V measurements

0 0.002 0.004 0.006 0.008 0.01 0.012 0.01410

4

105

106

107

108

109

1/T, K-1

R, O

hm

TP1

TP2

TP0

1 1.5 2 2.5 3 3.5

x 10-3

103

104

105

1/T, K-1

R, O

hm

TP0

TP1

Temperature dependencies of resistance for two samples

Complete variant of temperature dependencies of resistance of NbS3 whiskers

Comparison of two transitions: TP1 TP2 and TP0 TP1

-1 0 10

2

4

6

8

10

12

14

16

x 10-7

V, V

dI/d

V, O

hm

-1

T=300 K

295

285

256

211

187

172

159

152

146

143

132

127

114

96

-4 -2 0 2 4

0.5

1

1.5

2

2.5

x 10-4

V, V

dI/d

V, O

hm-1

T=390 oC

345

265

215

175

145

120 90 65 50 30

inflection points of curves are connected by dash red line

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.

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 620-360 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.

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.

• 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.

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

-8 -6 -4 -2 0 2 4 610

-8

10-7

10-6

10-5

10-4

V, V

dI/d

V, O

hm

-1

T= 304 K

280

262

249

223

To slide 13

-3 -2 -1 0 1 210

-7

10-6

10-5

10-4

V, V

dId

V, O

hm

-1

T=347 K, f=400 MHz 329 293

282 247 210 180

Jc/f =18 A/cm2/MHz

T= 102 K, f=800MHz

Jc/f =0.086 A/cm2/MHz

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 = 340-355 K

-0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4

106

78 K89

101133

140

145

151

153

157

164

196273299

308

347

360

368

Voltage (V)

dV/dI (

)

1063

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.

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

-6 -4 -2 0 2 4 6

70

80

90

100

110

120

130

I (A)

dV/dI (

k)

T=287 K

f=2.56 GHz

Au

Au

Sapphire

1.1 m 0 500 1000 1500100

150

200

250

300

x, Angstroms

z, A

ngst

rom

s

-1 -0.5 0 0.5 1

x 10-6

2

2.2

2.4

2.6

2.8

3x 10

5

I, AR

d,

f=800 MHz

Sample NS031710f

AFM pictures of 1m-long microbridges

Profile of NbS3 whisker

Differential I-V curve

S=14 nm*44 nm = 6 10-4 2 (AFM measurement)

S = 180 Å I/(2efo) =180*7.8 10-8 /(2*1.6 10-19*800 106) = 5.5 10-4 2 (From Shapiro steps)

Differential I-V curves of NS052010 bridge at room temperature

Rd vs I dependencies at utmost currents

-4 -2 0 2 4 6 8 10 12

x 10-5

0

0.5

1

1.5

2

2.5x 105

I, A

Rd,

Oh

m

Shapiropeak 12 GHz

Shapiropeak15.3GHz

Samlesburned out

Jmax ~ 6 106 A/cm2, corresponding to CDW velocities 200 m/s or fundamental frequencies 200 GHz.

№ Fig R(T)

Sample T, K h*w*l (nm2*mcm)

ICDW/f/(2e) S per chain A2

N per cell

NS102208 295 140*50*3.3 2.1623 103 (<1 GHz)

322 0.56

2 NS100208(dual) 326 120*50*3.7 7.6762e+003 78 2.3 NS121108 295 150*20*3.2 1.95 103

1.2806 103

(4 GHz)

154 234

1.2 0.78

NS102708 295 750*23*3.5 104 171 1.06 NS102808 295 160*40*5.5 1875 341 0.54 1 NS052608 650*280*

(40-57-44)

330 470*25* (33-42)

2250 522 0.35 3 NS052808

101 1844 637 0.29 4 NS042408 730*40*

(50-95-60)

NS060408 337 5000*1200 2 106 300 0.61 5 NS072108 337

131* 1000*400*500 1.64 105

1.05 104 244 3.8e3

0.75 0.0478

6 NS052008 17000*1000*2400

7 NS031510 1.7 103nm2* 3.5 937

8 NS031710e 295 90*28*2.7 1367 184 1 9 NS031710f 295 44*14*0.86 304 202 0.9

The table of samples

The table of samples

PRB, 28, 1646 (1983)

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).