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K. Biljakovic

Publications and source records attributed to K. Biljakovic.

14 recordsLinked to original sources
arXiv2010

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 K0.3MoO3 utilizing time-resolved optical spectroscopy. Numerous modes that appear below Tc 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 2kF phonons, while the (electronic) CDW amplitude mode is overdamped.

cond-mat.str-el↗
arXiv2009

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

We present first systematic studies of the photoinduced phase transition from the ground charge density wave (CDW) state to the normal metallic (M) state in the prototype quasi-1D CDW system K0.3MoO3. Ultrafast non-thermal 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 sub-picosecond timescale when melting and subsequent initial recovery of the electronic order takes place the lattice remains unperturbed.

cond-mat.str-el↗
arXiv2006

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 (SDW) compound (TMTTF)$_2$Br 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 phase configuration.

cond-mat.dis-nn↗
arXiv2005

Slowing down of the relaxational dynamics at the ferroelectric phase transition in one-dimensional (TMTTF)2AsF6

We present measurements of the dielectric response of quasi one-dimensional (TMTTF)2AsF6 in a wide temperature and frequency range. We provide a thorough characterization of the relaxational dynamics observed close to the ferroelectric-like transition at Tc = 100 K. Our measurements, extending up to 100 MHz, reveal a continuous slowing down of the mean relaxation time when approaching Tc 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.

cond-mat.str-el↗
arXiv2005

Bimodal energy relaxations in quasi-one-dimensional systems

We show that the low temperature ($T<0.5$ K) time dependent non-exponential energy relaxation of quasi-one-dimensional (quasi-1D) compounds strongly differ according to the nature of their modulated ground state. For incommensurate ground states, such as in (TMTSF)$_2$PF$_6$ the relaxation time distribution is homogeneously shifted to larger time when the duration of the heat input is increased, and exhibits in addition a scaling between the width and the position of the peak in the relaxation time distribution, $w^{2}\sim\ln{(τ_{m})}$. For a commensurate ground state, as in (TMTTF)$_2$PF$_6$, the relaxation time spectra show a bimodal character with a weight transfer between well separated slow and fast entities. Our interpretation is based on the dynamics of defects in the modulated structure, which depend crucially on the degree of commensurability.

cond-mat.dis-nn↗
arXiv2005

Charge-Density-Wave like Behavior 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 we observe colossal dielectric constants with a frequency and temperature dependence very similar to what is observed in canonical charge-density wave systems. Guided by structural details we interpret this structural phase transition as driven by complex charge-order processes.

cond-mat.str-el↗
arXiv2004

Energy relaxation in disordered charge and spin density waves

We investigate collective effects in the strong pinning model of disordered charge and spin density waves (CDWs and SDWs) in connection with heat relaxation experiments. We discuss the classical and quantum limits that contribute to two distinct contribution to the specific heat (a $C_v \sim T^{-2}$ contribution and a $C_v \sim T^α$ contribution respectively), with two different types of disorder (strong pinning versus substitutional impurities). From the calculation of the two level system energy splitting distribution in the classical limit we find no slow relaxation in the commensurate case and a broad spectrum of relaxation times in the incommensurate case. In the commensurate case quantum effects restore a non vanishing energy relaxation, and generate stronger disorder effects in incommensurate systems. For substitutional disorder we obtain Friedel oscillations of bound states close to the Fermi energy. With negligible interchain couplings this explains the power-law specific heat $C_v \sim T^α$ observed in experiments on CDWs and SDWs combined to the power-law susceptibility $χ(T)\sim T^{-1+α}$ observed in the CDW o-TaS$_3$.

cond-mat.dis-nn↗
arXiv2004

Ultrafast real-time spectroscopy of low dimensional charge density wave compounds

We present a femtosecond time-resolved optical spectroscopy (TRS) as an experimental tool to probe the changes in the low energy electronic density of states as a result of short and long range charge density wave order. In these experiments, a femtosecond laser pump pulse excites electron-hole pairs via an interband transition in the material. These hot carriers rapidly release their energy via electron-electron and electron-phonon collisions reaching states near the Fermi energy within 10-100 fs. The presence of an energy gap in the quasiparticle excitation spectrum inhibits the final relaxation step and photoexcited carriers accumulate above the gap. The relaxation and recombination processes of photoexcited quasiparticles are monitored by measuring the time evolution of the resulting photoinduced absorption. This way, the studies of carrier relaxation dynamics give direct information of the temperature-dependent changes in the low energy density of states. Here we present the application of the femtosecond time-resolved optical spectroscopy for studying changes in the low energy electronic density of states in low dimensional charge density wave systems associated with various charge density wave (CDW) transitions and review some recent experiments on quasi 1D and 2D CDW compounds.

cond-mat.str-el↗
arXiv2003

Evidence of 1D behaviour of He$^4$ confined within carbon-nanotube bundles

We present the first low-temperature thermodynamic investigation of the controlled physisorption of He$^{4}$ 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 NTs the extra contribution to the specific heat, C$_{ads}$, originating from adsorbed He$^{4}$ 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 behaviour akin to the case of He$^{4}$ films on planar substrates (grafoil).

cond-mat.mtrl-sci↗
arXiv2002

Fractional power-law susceptibility and specific heat in low temperature insulating state of o-TaS_{3}

Measurements of the magnetic susceptibility and its anisotropy in the quasi-one-dimensional system o-TaS_{3} 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 C_{LEE}(0,T)=CT^{0.3}, 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 per cca 900 Ta atoms. We discuss the possibility that it is the charge frozen within a soliton-network below the glass transition T_{g}~40 K determined recently in this system.

cond-mat↗
arXiv2002

Slow relaxation experiments in disordered charge and spin density waves: collective dynamics of randomly distributed solitons

We show that the dynamics of disordered charge density waves (CDWs) and spin density waves (SDWs) is a collective phenomenon. The very low temperature specific heat relaxation experiments are characterized by: (i) ``interrupted'' ageing (meaning that there is a maximal relaxation time); and (ii) a broad power-law spectrum of relaxation times which is the signature of a collective phenomenon. We propose a random energy model that can reproduce these two observations and from which it is possible to obtain an estimate of the glass cross-over temperature (typically $T_g \simeq 100 - 200$ mK). The broad relaxation time spectrum can also be obtained from the solutions of two microscopic models involving randomly distributed solitons. The collective behavior is similar to domain growth dynamics in the presence of disorder and can be described by the dynamical renormalization group that was proposed recently for the one dimensional random field Ising model [D.S. Fisher, P. Le Doussal and C. Monthus, Phys. Rev. Lett. {\bf 80}, 3539 (1998)]. The typical relaxation time scales like $τ^{\rm typ} \sim τ_0 \exp{(T_g/T)}$. The glass cross-over temperature $T_g$ related to correlations among solitons is equal to the average energy barrier and scales like $T_g \sim 2 x ξ_0 Δ$. $x$ is the concentration of defects, $ξ_0$ the correlation length of the CDW or SDW and $Δ$ the charge or spin gap.

cond-mat.stat-mech↗
arXiv1999

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.

cond-mat.str-el↗