Impurity effects in photoconduction of orthorhombic TaSImpurity states Brazovskii JETP (1981) Chord...
Transcript of Impurity effects in photoconduction of orthorhombic TaSImpurity states Brazovskii JETP (1981) Chord...
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