Katica Biljaković

SINGLE PARTICLE AND COLLECTIVE EXCITATIONS IN THE ONE-DIMENSIONAL CHARGE DENSITY WAVE SOLID K0.3MOO3 PROBED IN REAL TIME BY FEMTOSECOND SPECTROSCOPY

Ultrafast transient reflectivity changes caused by collective and single particle excitations in the quasi one-dimensional charge-density wave (CDW) semiconductor K0.3MoO3 are investigated with optical pump-probe spectroscopy. The temperature-dependence of non-equilibrium single particle excitations across the CDW gap and their recombination dynamics are reported for the first time. In addition, amplitude mode reflectivity oscillations are observed in real time. A T-dependent overdamped response is also observed which is attributed to relaxation of the phason mode.

Dynamics of photoinduced Charge-Density-Wave to metal phase transition in K0.3MoO3

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.

Evidence of 1D behavior of He4 confined within carbon-nanotube bundles.

We present the first low-temperature thermodynamic investigation of the controlled physisorption of He4 gas in carbon single-wall nanotube (SWNT) samples. The vibrational specific heat measured between 100 mK and 6 K demonstrates an extreme sensitivity to outgassing conditions. For bundles with a few number of nanotubes the extra contribution to the specific heat, C(ads), originating from adsorbed He4 at very low density displays 1D behavior, typical for He atoms localized within linear channels as grooves and interstitials, for the first time evidenced. For larger bundles, C(ads) recovers the 2D behavior akin to the case of He4 films on planar substrates (grafoil).

Disentanglement of the Electronic and Lattice Parts of the Order Parameter in a 1D Charge Density Wave System Probed by Femtosecond Spectroscopy

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 K(0.3)MoO(3) utilizing time-resolved optical spectroscopy. Numerous modes that appear below T(c) show pronounced T dependences of their amplitudes, frequencies, and dampings. Utilizing the time-dependent Ginzburg-Landau theory we show that these modes result from linear coupling of the electronic part of the order parameter to the 2k(F) phonons, while the (electronic) CDW amplitude mode is overdamped.

Giant dielectric response in the one-dimensional charge-ordered semiconductor (NbSe4)3I.

We report on broadband dielectric spectroscopy on the one-dimensional semiconductor (NbSe4)3I. Below the structural phase transition close to 270 K, which is accompanied by complex charge-order processes, we observe colossal dielectric constants with a frequency and temperature dependence very similar to what is observed in charge-density wave systems.

Fractional power law susceptibility and specific heat in low-temperature insulating state of o-TaS3

Measurements of the magnetic susceptibility and its anisotropy in the quasi–one-dimensional system o-TaS3 in its low-T charge-density-wave (CDW) ground state are reported. Both sets of data reveal below 40 K an extra paramagnetic contribution obeying a power law temperature dependence χ(T) = AT−0.7. The fact that the extra term measured previously in specific heat in zero field, ascribed to low-energy CDW excitations, also follows a power law CLEE(0,T) = CT0.3, strongly revives the case of random exchange spin chains. Introduced impurities (0.5% Nb) only increase the amplitude C, but do not change essentially the exponent. Within the two-level system (TLS) model, we estimate from the amplitudes A and C that there is one TLS with a spin s = 1/2 localized on the chain at the lattice site on average every 900 Ta atoms. We discuss the possibility that it is the charge frozen within a soliton-network below the glass transition Tg ~ 40 K determined recently in this system.

Low-Energy Excitations of Charge Density Wave Metastable States in TaS3

We have measured the specific heat of orthorhombic TaS3 between 0.08 K and 8 K. An extra-phononic contribution is shown to follow a Cp ∝ T0.32 law below 2 K. We ascribed this excess contribution to low-energy excitations between charge density wave metastable states.

Interplay between phase defects and spin polarization in the specific heat of the spin-density-wave compound (TMTTF)2Br in a magnetic field.

Equilibrium heat relaxation experiments provide evidence that the ground state of the commensurate spin-density-wave compound (TMTTF)2Br after the application of a sufficient magnetic field is different from the conventional ground state. The experiments are interpreted on the basis of the local model of strong pinning as the deconfinement of soliton-antisoliton pairs triggered by the Zeeman coupling to spin degrees of freedom, resulting in a magnetic-field-induced density-wave glass for the spin carrying the phase configuration.

Low-temperature thermodynamic investigation of the sulphur organic salts (TMTTF)2PF6 and (TMTTF)2Br (TMTTF?tetramethyltetrathiafulvalene): I. General aspects

A detailed thermodynamical investigation of the quasi-one-dimensional sulphur-based organic salts (TMTTF)2PF6 and (TMTTF)2Br is presented in the temperature range from 30 mK to 7 K. In this part (part I), we consider the general aspects of the low-temperature specific heat of these materials in relationship with their ground states and we compare them with those previously measured for selenium members of the same family. All these compounds exhibit very similar thermodynamical behaviour, despite a variety of electronic ground states: spin-Peierls for (TMTTF)2PF6, commensurate antiferromagnetic spin modulation for (TMTTF)2Br, incommensurate spin-density wave for (TMTSF)2PF6 (TMTSF≡tetramethyltetraselenafulvalene). We show that the low-energy excitations which are predominant below 1 K have a similar character to that observed in glassy systems, at least on short timescales of measurements. The dynamical aspects of the non-equilibrium thermodynamics will be presented separately in the following part (part II).

Glass transition from a slush to a real glass in charge-density-wave compound TaS3

Abstract We have found by broad-band dielectric spectroscopy of charge density wave (CDW) compound TaS 3 that the main relaxation time representing CDW viscosity shows a critical slowing down in the range 55-50 K. At lower temperatures two additional processes appear that we assign to the defects in CDW superlattice. Based on very strong similarity with the glass transition in supercooled liquids, we propose a consistent scenario of the glass transition from a CDW slush to a real CDW glass. The length/time scales involved imply the existence of a new family of glasses in systems with the long-range order such as CDW.