Tuning charge density wave glass transition by introducing lattice disorder ECRYS 2011 D. Dominko, K. Biljaković, D. Starešinić Institute of Physics, Zagreb, Croatia M. Jakšić Institute Ruđer Bošković, Zagreb, Croatia P. Lunkenheimer, A. Loidl University of Augsburg, Germany

Scope introduction glass transition sample preparation transport properties dielectric response discussion conclusion

Introduction- pinning Friedel oscillations around impurity site pin CDW phase  domain structure CDW phase bends to minimise pinning energy on two neighbour impurity sites  change of wave vector q~p-n  two degenerate polarisations around impurity site simplified scheme:

Introduction- response to external field overdamped response two (three) processes   - high T ~ freezes at 40 K determines glass transition temperature   - below 60 K  both relaxation times follow resistivity polarisation change governed by free carriers?  ~  ~viscosity?

Introduction- freezing criterion Free carrier number follows activation law  increase of viscosity  process slows down and freeze when  reaches 100 s (T G determined by fit)  when exactly does it happen? domain density: cm -3 at T g 1 e per domain freezing criterion? n e (T g )~10 15 e/cm 3 Q TSD ~10 15 electrons/cm 3 if so - polarisation is governed by free carriers screening tested by reducing domain size

Sample preparation irradiation by H + ions  sample thickness <10  m  2 MeV, Van der Graaf at IRB  mainly just vacancies and interstitials  uniform up to 10  m (range: 30  m)  recombination (annealing)? assumption: number of long living defects is proportional to the number right at the beginning

Results - transport transition smearing  el unchanged minimum in E T is disappearing limiting curve for high dosage

Results – dielectric response  ’’  (f,T) pure

Results – dielectric response  ’’  (f,T) 2.2 ppm

Results – dielectric response  ’’  (f,T) 67 ppm

Results – dielectric response  ’’  (f,T) 220 ppm

1/n Results – dielectric response higher domain density reduction of  proces amplitude hi T:   ~ 1/n  process unchanged  proces  proces 100 kHz

Results – dielectric response at high T- minimum in   pure ones – flattening activation energy increases

00 TgTg E act increases above  el Discussion – fit parameters 1/n  0 reaches phonon frequencies  =  0 (n)e -  (n)/T,  (T G )=100 s

T g increases electron density increases 2 orders of magnitude on interval T g (n i ) consistent with radiation level! Discussion – fit parameters- T G n-1/Tg has activational behavior with  =  el !

Discussion - threshold low T part of E T “spoons” detaches from the most irradiated sample (~e -T/50 K ) higher dosage -> higher the crossover temperature T cross exponential E T cross ~ e -Tcross/T0 with T 0 =18 K same T 0 as found for E T (T) bellow 100 K in pure samples* *Itkis, Nad and Monceau, JPCM 1990

 process insensitive to doping level  is not governed by the carrier number alone any more – domain cooperativity slows it down even more? T G tuned by defect number having one electron per domain  activation temperature for n i (T G ) is 700 K- follows electron count n(T)! high T minimum in  appears after irradiation pinning resonance increases? at high T- E T insensitive to doping level, at low T it increases Conclusions – glass

Attention! Thank you for your attention!

Low temperature state - Coulomb stiffening prefers constant phase - only local distortions allowed - CDW chain cut with topological defects (solitons) - TLS - like - bound e - states at impurity Bound states Tutto, Zawadowski PRL ‘85

Results - transport E T -n power low, but weaker than ~n