Detection of charge density wave ground state in granular thin films of blue bronze K0.3MoO3 by femtosecond spectroscopy

Abstract

During the last years, femtosecond time-resolved spectroscopy (fsTRS) has become an important new tool to investigate low energy excitations in strongly correlated systems. By studying energy relaxation pathways linking various degrees of freedom (e.g., electrons, spin, or lattice), the interaction strengths between different subsystems can be deduced. Here we report on yet another application of fsTRS, where the technique is used to unambiguously determine the nature of the ground state in granular thin films of a prototype charge density wave system blue bronze, K0.3MoO3. These, potassium blue bronze, films, obtained for the first time ever, have been prepared by pulsed laser deposition and investigated by various standard characterization methods. While the results of all used methods indicate that the thin films consist of nanometer grains of K0.3MoO3, it is only the non-destructive fsTRS that demonstrates the charge density wave nature of the ground state. Furthermore, the comparison of the fsTRS data obtained in thin films and in single crystals shows the reduction of the charge density wave transition temperature and of the photoinduced signal strength in granular thin films in respect to single crystals, which is attributed to the granularity and crystal growth morphology.

Variable range hopping conductivity in nanocrystalline films of K0.3MoO3

“…However, a smeared feature appeared in the logarithmic derivative of R(T) in respect to 1/T around 150 K, similarly as in highly doped BB samples and patterned BB wires [1]. At the same temperature, the Raman-active CDW amplitude mode appears in fs-TRS [3,5]. The films with lower resistance and a more pronounced anomaly at 150 K also had better texture, the grains preferably ordered in orthogonal directions, as observed by atomic force microscopy (AFM) [4,5].…”

“…We have previously reported on the production of the BB thin films by pulsed laser deposition (PLD) [3]. Standard characterization techniques showed that our films are composed of nanocrystalline grains [3][4][5], while the presence of the CDW in grains below 150 K was unambiguously demonstrated by the observation of CDW amplitude mode in femto-second time resolved spectroscopy (fs-TRS) [3].…”

“…The angle between the incident beam and the surface normal vector of a film was 20°. The scattered and recoiled ions were detected at 37.5° [3].…”

“…On the other hand, granular films of CDW materials might enable investigation of their physical properties at mesoand micro-scales when the grain size becomes comparable to or smaller than the phase and the amplitude correlation lengths of the CDW order parameter [1,3].…”

“…Standard characterization techniques showed that our films are composed of nanocrystalline grains [3][4][5], while the presence of the CDW in grains below 150 K was unambiguously demonstrated by the observation of CDW amplitude mode in femto-second time resolved spectroscopy (fs-TRS) [3]. In contrast to this, initial transport measurements showed no evidence of the sharp Peierls transition (characteristics of the nominally pure bulk samples) in the temperature dependence of the resistance R(T) of the films, and the data could be fit to a one-dimensional (1d) variable range hopping (VRH) dependence [3]. However, a smeared feature appeared in the logarithmic derivative of R(T) in respect to 1/T around 150 K, similarly as in highly doped BB samples and patterned BB wires [1].…”