Impurity effects in photoconduction of orthorhombic

TaS3

S.V. Zaitsev-Zotov, V. F. Nasretdinova, V.E. Minakova,

IRE RAS, Moscow, Russia

K. Biljaković, D. Staresinić,

Institute of Physics, Zagreb, Croatia

ECRYS'2008, Cargèse, August 28, 2008

Outline● Motivation● Temperature variation of conduction and I-V

curves of doped crystals● Temperature variation of photoconduction of

pure and doped crystals● Spectral study

● Peiers gap value● Impurity states

● Conclusions

Motivation

● Decrease of transverse sizes of NbSe

3 leads to qualitative

change in character of R(T) and I-V curves.

● Power dependences for R(T) and I-V curves expected for 1D [Kane, Fisher (1992)].

[ZZ, Pokrovskii, Monceau, JETPL (2001); ZZ, Microel. Eng. (2003); Slot et al., PRL (2004)]

Motivation

o-TaS3

● Variable-range hopping for R(T), R exp([T

0/T]1/2)

(expected for 3D [Fogler et al. (2003)], and1D [Artemenko (2005)]).

● Sample sizes are still too big to suggest 1D effects (like Luttinger liquid).

[ZZ, Microel. Eng. (2003)]

Motivation

Possible explanation: this behavior may be a manifistation of a new state – impurity-stabilized Luttinger liquid (Artemenko, JETP Letters (2004)], when impurities does not allow to established 3D order of the CDW and a quasi-1D sample consists of 1D chains of bounded Luttiger liquid. Then G exp(-[T

0/T]1/2) [Artemenko (2005)]).

Motivation

Experimental search for impurity stabilized

Luttinger liquid:● Is it possible to observe 1D behavior in

doped samples: any qualitative changes in G(T) and I-Vs?

● What happens with the Peierls gap, any changes in density of states?

● What about impurity levels?

Temperature variation of conduction of doped crystals of

o-TaS3

0.2-0.5 at. % Nb

Temperature variation of conduction of doped crystals of

o-TaS3

● Power dependence G (Kane, Fisher, LL?)

Current-voltage characteristics of doped samples

● Power dependence I V LL?) and(space-charge limited current?=6

=2

Temperature variation of conduction of doped crystals of

o-TaS3

Nb 0.2 – 0.5 at. % Variable-range

hopping for G(T), G exp(-[T

0/T]1/2)

(expected for 3D [Fogler et al. (2003)], and1D [Artemenko (2005)]). E

T >103 V/cm

Temperature variation of conduction of doped crystals of

o-TaS3

Variable-range hopping for G(T), G exp(-[T

0/T]3/4)

expected for 3D [Fogler et al.

(2003)]

Methods

Al2O

3

TaS3

IR LED

Monochromator

Cryostat

Sample

Mirror

Temperature-dependent photoconduction

Spectral study

Photoconduction as a method of analysis of single-particle

conduction● Linear-to-quadratic

recombination temperature crossover

● Linear condution of electrons and holes is shunted by collective conduction

Photoconduction at different light intensities (pure o-TaS

3)

[ZZ, Minakova, PRL (2006)]

Temperature variation of photoconduction of doped crystals● Power

dependencies● VRH-like

dependencies

Temperature variation of photoconduction of pure and

impure crystals● Nb impurities do

not change the low-temperature single-particle conduction of o-TaS

3 (no doping

as in semicon-ductors)

Spectral study

● Optical gap edge at 180 meV

● Sharp gap edge / = 0.2

● Impurity levels?

Photoconduction of pure crystals of o-TaS

3 (poster P35)

● Photoconduction● Bolometric response● Nb and

Ta doped● Minton

Brill (1988)

pure● Nad'

Itkis (1996)

Photoconduction of pure crystals of o-TaS

3 (poster P35)

Photoconduction of pure crystals of o-TaS

3 (poster P35)

Impurity states

Brazovskii JETP (1981)

Chord solitons, ingap states

two levels: Ei = Δ cos ( θ )

Tutto, Zawadovskii, PRB (1985)

two levels inside the gap

CDW-phase dependent impurity states (=> dependence on elecrtric field)

2θ

CDW order parameter

Impurity states

h

kT

● Two levels => two lines● Extra energy due to

thermal excitation● => temperature

dependent intensitykT

Photoconduction of pure crystals of o-TaS

3 (poster P35)

Conclusions● Nb impurities in amount 0.2-0.5 at. % does not

change the Peierls gap shape => No impurity-stabilized Luttinger liquid (yet)

● G T , G exp( - [ T0 / T ] ) and I V are

intrincic properties of the CDW conductors

● There are impurity states in CDW conductors

● The low-temperature gap value is much bigger the high-temperature one

● We need careful measurements of impurity states

● We need theories of impurity states, doping, and single-particle life-time in CDW conductors