Single particle and collective excitations in the one-dimensional charge density wave solid K 0.3 MoO 3 probed in real time by femtosecond spectroscopy.

We present the first systematic studies of the photoinduced phase transition from the ground charge density wave (CDW) state to the normal metallic state in the prototype quasi-1D CDW system K0.3MoO3. Ultrafast nonthermal CDW melting is achieved at the absorbed energy density that corresponds to the electronic energy difference between the metallic and CDW states. The results imply that on the subpicosecond time scale when melting and subsequent initial recovery of the electronic order takes place the lattice remains unperturbed.

A comprehensive study of selected properties of four (TiZrNbCu)1-xNix (x ≤ 0.25) amorphous high entropy alloys (a-HEA) has been performed. The samples were ribbons about 20 µm thick and their fully amorphous state was verified by X-ray diffraction and thermal analysis. The surface morphology, precise composition and the distribution of components were studied with a Scanning electron microscope (SEM) with an energy dispersive spectroscopy (EDS) attachment. The properties selected were the melting temperature (Tm), the low temperature specific heat (LTSH), the magnetic susceptibility χexp and the Young´s modulus (E). Whereas LTSH and χexp were measured for the as-cast samples, E was measured both for as-cast samples and relaxed samples (after a short anneal close to the glass transition temperature). The LTSH showed that the electronic density of states at the Fermi level, N0(EF), decreases with increasing x, whereas the Debye temperature (θD) increases with x. This is similar to what is observed in binary and ternary amorphous alloys of early transition metals (TE) with late transition metals (TL) and indicates that N0(EF) is dominated by the d-electrons of the TE.The LTSH also showed the absence of superconductivity down to 1.8K and indicated the emergence of the Boson peak above 4K in all alloys.The free-electron like paramagnetic contribution to χexp also decreases with x, whereas E, like θD, increases with x, indicating enhanced interatomic bonding on addition of Ni. The applicability of the rule of mixtures to these and other similar HEAs is briefly discussed.

We report on the high resolution studies of the temperature (T) dependence of the q = 0 phonon spectrum in the quasi one-dimensional charge density wave (CDW) compound K0.3MoO3 utilizing time-resolved optical spectroscopy. Numerous modes that appear below T c show pronounced Tdependences of their amplitudes, frequencies and dampings. Utilizing the time-dependent GinzburgLandau theory we show that these modes result from linear coupling of the electronic part of the order parameter to the 2kF phonons, while the (electronic) CDW amplitude mode is overdamped.Femtosecond optical spectroscopy is becoming an important tool for investigation of the so called strongly correlated systems due to its intrinsic ability to determine the interaction strengths between various degrees of freedom which lead to fascinating phenomena like superconductivity or giant magnetoresistance. Low dimensional charge density wave (CDW) systems, with their inherently multi-component order parameter (modulation of carrier density is accompanied by the modulation of the underlying lattice) present no exception. In the past decade or so various one and two dimensional CDWs have been studied by time-resolved optical [1][2][3][4][5][6] as well as photoemission [7, 8] techniques. The initial focus of research was in identifying various components in the observed photoinduced transients with the corresponding ones obtained by standard time-averaging spectroscopic techniques, as well as in coherent control of the collective modes [9]. Recently it was shown that photoexcitation with an intense optical pulse can nonthermally drive the phase transition from the low temperature CDW state to a metastable state, characterized by a suppressed carrier modulation with the lattice remaining nearly frozen [6]. This observation has an important implication for the understanding of ultrafast relaxation processes in this class of materials, and, as we will show, for the general understanding of the cooperative phenomena leading to the appearance of the CDW state and the nature of their collective excitations.In this Letter we present high resolution studies of Tdependent time-resolved reflectivity dynamics in a prototype quasi-1D CDW material K 0.3 MoO 3 . The high sensitivity achieved in this experiment enabled us to measure the T-evolution of the low frequency phonons with unprecedented resolution. We were able to show that not only the 1.68 THz (57 cm −1 ) mode, that is commonly assigned to the collective amplitude mode (AM) of the CDW, shows softening upon increasing T towards T c = 183 K, the phase transition to the normal metallic state. Qualitatively the same softening is observed also for a number of phonon modes that appear below T c . The frequencies of these modes correspond well to the phonon frequencies at the 2k F modulation vector as observed by neutron experiments [10,11], and are shown to result from the linear coupling of the electronic part of the order parameter (EOP), ∆, to 2k F phonons. Utilizing the time-dependent Ginzburg-Landau (TDGL) mod...

No abstract

We present measurements of the dielectric response of quasi one-dimensional (TMTTF) 2 AsF 6 in a wide temperature and frequency range. We provide a thorough characterization of the relaxational dynamics observed close to the ferroelectric-like transition at T c = 100 K. Our measurements, extending up to 100 MHz, reveal a continuous slowing down of the mean relaxation time when approaching T c from high as well as from low temperatures. The simultaneous critical rise of dielectric constant and relaxation time point to an explanation of the transition in terms of a classic ferroelectric scenario.

We present a detailed investigation of the low-frequency dielectric response of the charge-density-wave system ͑CDW͒ o-TaS 3 in wide temperature ͑5-300 K͒ and frequency ͑10 mHz to 100 MHz͒ ranges. Although our measurements agree relatively well with data performed in some restricted frequency and temperature ranges and previously published by several groups, we show that they do not correspond to a single lowfrequency process. Instead, three distinctive processes are found to contribute to the dielectric function below the CDW transition temperature. The temperature evolution of the characteristic relaxation time of the three processes bears a close resemblance to the phenomenology of the dielectric response of glasses. The freezing of some of these processes at finite temperatures leads to changes in the CDW properties, as is thoroughly documented in the literature. Based on these results, we propose a consistent model of the temperature evolution of the CDW ground state.

No abstract