Tomislav Vuletić - Academia.edu (original) (raw)

Papers by Tomislav Vuletić

Research paper thumbnail of Structure modulation and charge ordering in self-doped M x CuO 2 crystals

Acta Crystallographica Section A Foundations of Crystallography, 2012

Research paper thumbnail of Influence of quantum Hall effect on linear and nonlinear conductivity in the FISDW states of the organic conductor (TMTSF) 2PF 6

Zeitschrift für Physik B Condensed Matter

We report a detailed characterization of quantum Hall effect (QHE) influence on the linear and no... more We report a detailed characterization of quantum Hall effect (QHE) influence on the linear and non-linear resistivity tensor in FISDW phases of the organic conductor (TMTSF)2PF6. We show that the behavior at low electric fields, observed for nominally pure single-crystals with different values of the resistivity ratio, is fully consistent with a theoretical model, which takes QHE nature of FISDW and residual quasi-particle density associated with different crystal imperfection levels into account. The non-linearity in longitudinal and diagonal resistivity tensor components observed at large electric fields reconciles preceding contradictory results. Our theoretical model offers a qualitatively good explanation of the observed features if a sliding of the density wave with the concomitant destruction of QHE, switched on above a finite electric field, is taken into account.

Research paper thumbnail of Single-particle and spin-density wave charge dynamics in (TMTSF)2PF6 and (TMTSF)2AsF6 : A comparative overview

We present the results of DC and AC (100 mHz - 1MHz) electrical transport measurements in low and... more We present the results of DC and AC (100 mHz - 1MHz) electrical transport measurements in low and high electric fields performed in the spin-density wave (SDW) state of the Bechagaards salts (TMTSF)2PF6 and (TMTSF)2AsF6. We argue that a degree of complex structure of the SDW ground state which is unfolded in a particular experiment depends strongly on the chosen experimental probe and the crystal measured.

Research paper thumbnail of Charge-density wave formation in Sr_ {14-x} Ca_ {x} Cu_ {24} O_ {41}

The electrodynamic response of the spin-ladder compound Sr14-xCaxCu24O41(x=0,3,9) has been studie... more The electrodynamic response of the spin-ladder compound Sr14-xCaxCu24O41(x=0,3,9) has been studied from radio frequencies up to the infrared. At temperatures below 250 K a pronounced absorption peak appears around 12cm-1 in Sr14Cu24O41 for the radiation polarized along the chains/ladders (E‖c). In addition a strongly temperature-dependent dielectric relaxation is observed in the kHz – MHz range. We explain this behavior by a charge-density wave which develops in the ladders and produces a phason mode pinned at 12cm-1. With increasing x the mode shifts up in frequency and eventually disappears for x=9 because the dimensionality of the system crosses over from one to two dimensions, giving way to the superconducting ground state under pressure.

Research paper thumbnail of Variable-range hopping conductivity in the copper-oxygen chains of La_3Sr_3Ca_8Cu_24O_41

We show that the spin chain/ladder compound La3Sr3Ca8Cu24O41 is an insulator with hopping transpo... more We show that the spin chain/ladder compound La3Sr3Ca8Cu24O41 is an insulator with hopping transport along the chains. In the temperature range 35 -280 K, DC conductivity σDC(T ) follows Mott's law of variable-range hopping conduction; the frequency dependence has the form σ(ν, T ) = σDC(T )+A(T )·ν s , where s ≈ 1. The conduction mechanism changes from variable-range hopping to nearest-neighbor hopping around Tc = 300 K. The chain array thus behaves like a one-dimensional disordered system. Disorder is due to random structural distortions of chains induced by irregular coordination of the La/Sr/Ca ions.

Research paper thumbnail of PROPERTIES OF MOTT-PEIERLS INSULATING PHASE IN DEUTERATED COPPER-DCNQI SYSTEMS

We overview low frequency dielectric spectroscopy and non-linear dc electrical transport in singl... more We overview low frequency dielectric spectroscopy and non-linear dc electrical transport in single crystals of a member of copper-DCNQI charge-transfer salt with N = 3 commensurate charge density wave. Broad relaxation mode of moderate strength weakens and becomes narrower with decreasing temperature. Threshold electrical field diverges, concomitant with the slight increase of the non-ohmic conductivity effect. These features suggest that the entities responsible for the dielectric response are pairs of charged domain walls between metallic and insulating phases.

Research paper thumbnail of Polyelectrolyte Composite: Hyaluronic Acid Mixture with DNA

Macromolecules, 2015

We studied salt-free, highly concentrated (5−200 g/L) mixtures of unfragmented (μm contour length... more We studied salt-free, highly concentrated (5−200 g/L) mixtures of unfragmented (μm contour length) DNA and hyaluronic acid (HA) as a borderline example of rigid-rod/flexible-chain composite, across a broad range of concentration ratios c HA /c DNA = 0.05−50. By polarizing microscopy, we established that the DNA and HA form clearly separated thread-like domains defined and oriented by solution shear. Within its domains, DNA shows birefringent banded patterns, routinely observed for long chain mesogens. We applied small-angle X-ray scattering (SAXS) to the mixtures and observed a polyelectrolyte (PE) correlation peak at q ★ wave vector. This peak was ascribed to DNA subphase and was used as a measure of the effective DNA concentration in the subphase c DNA ★ , according to deGennes scaling relationship between the DNA mesh size ξ =2 π/q ★ ∝ c −1/2 and monomer concentration c. From c DNA ★ (and initial c HA and c DNA ), we inferred the effective c HA ★ of HA subphase. We find a concentration-independent ratio Γ = c HA ★ / c DNA ★ ≈ 0.85 across a broad range of 0.02−0.4 M. As there is the osmotic pressure (Π) equilibrium between DNA and HA subphases, the constant Γ indicates that Π ∝ c 9/8 scaling common for DNA and other highly charged PEs is valid also for HA (a weak PEdoes not feature counterion condensation). Therefore, we propose that this deviation from the conventional osmotic pressure scaling Π ∝ c cannot originate from the concentration dependence of counterion condensation, which is an implicit but common interpretation in the literature. Further, as HA releases all its counterions into the solution, the HA osmotic coefficient ϕ HA we took as a measure of the DNA osmotic coefficient which we found to be ϕ DNA ≈ 0.28. This, double the Manning value, corroborates a result by Raspaud et al. 1

Research paper thumbnail of Low-frequency dielectric spectroscopy of commensurate density waves

Synthetic Metals, 2001

ABSTRACT We overview complex conductivity measurements (20Hz - 1MHz) in low-dimensional organic s... more ABSTRACT We overview complex conductivity measurements (20Hz - 1MHz) in low-dimensional organic systems kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Cl, and deuterated Cu-DCNQI and its alloys, with commensurate density wave ground states. We discuss the obtained results in comparison with those for the incommensurate density waves, like in (TMTSF)(2)PF6, in which the phason relaxation in a disordered background dominates. We show that in the commensurate systems with a random domain structure charged domain walls provide a more appropriate description of the observed dielectric relaxation.

Research paper thumbnail of Variable-range hopping conductivity in the copper-oxygen chains of La3Sr3Ca8Cu24O41

Physical Review B, 2003

We show that the spin chain/ladder compound La3Sr3Ca8Cu24O41 is an insulator with hopping transpo... more We show that the spin chain/ladder compound La3Sr3Ca8Cu24O41 is an insulator with hopping transport along the chains. In the temperature range 35 -280 K, DC conductivity σDC(T ) follows Mott's law of variable-range hopping conduction; the frequency dependence has the form σ(ν, T ) = σDC(T )+A(T )·ν s , where s ≈ 1. The conduction mechanism changes from variable-range hopping to nearest-neighbor hopping around Tc = 300 K. The chain array thus behaves like a one-dimensional disordered system. Disorder is due to random structural distortions of chains induced by irregular coordination of the La/Sr/Ca ions.

Research paper thumbnail of Diffraction analysis of incommensurate modulation in “chain-ladder” composite crystal (Sr/Ca)14Cu24O41

EMC 2008 14th European Microscopy Congress 1–5 September 2008, Aachen, Germany, 2008

ABSTRACT The “chain-ladder” compounds with formula (Sr/La/Ca)14Cu24O41 exhibit a rather complex c... more ABSTRACT The “chain-ladder” compounds with formula (Sr/La/Ca)14Cu24O41 exhibit a rather complex crystallographic structure due to their composite character. These materials consist of an alternating stacking of two distinct types of layers forming interpenetrated subsystems [1]. One subsystem is formed of (Sr/La/Ca)2Cu2O3 layers with a ladder-like structure and orthorhombic unit cell (Fmmm), while the other contains layers of CuO2 chains with orthorhombic unit cell (Amma) [2]. All lattice parameters slightly vary with La/Ca for Sr substitution, but the one dimensional incommensurate modulation results from the misfit between the c lattice parameters of these layer elements. In pure Sr compound: c Ch = 0.273nm, c Ld = 0.393 nm ≈ √2·c Ch; the a parameter (≈ 1.147 nm) is the same in both lattices, as well as b (≈ 1.341 nm) pointing along stacking direction. For high Ca for Sr substitution in Sr3Ca11Cu24O41: c Ch = 0.276 nm, c Ld = 0.391 nm, so that the ratio of these parameters is always close to √2. The c Ld/c Ch ratio also varies from 1.416 for pure undoped Sr14Cu24O41, to 1.445 for highly doped SrxCa14−xCu24O41[3].

Research paper thumbnail of Charge-density wave formation in Sr14-xCaxCu24O41

PHYSICAL REVIEW B, 2002

The electrodynamic response of the spin-ladder compound Sr14−xCaxCu24O41 (x = 0, 3, 9) has been s... more The electrodynamic response of the spin-ladder compound Sr14−xCaxCu24O41 (x = 0, 3, 9) has been studied from radiofrequencies up to the infrared. At temperatures below 250 K a pronounced absorption peak appears around 12 cm −1 in Sr14Cu24O41 for the radiation polarized along the chains/ladders (E c). In addition a strongly temperature dependent dielectric relaxation is observed in the kHz -MHz range. We explain this behavior by a charge density wave which develops in the ladders sub-system and produces a mode pinned at 12 cm −1 . With increasing Ca doping the mode shifts up in frequency and eventually disappears for x = 9 because the dimensionality of the system crosses over from one to two dimensions, giving way to the superconducting ground state under pressure.

Research paper thumbnail of Modalities of self-organized charge response in low dimensional systems

We present modalities of self-organized charge response in low dimensional systems, like diverse ... more We present modalities of self-organized charge response in low dimensional systems, like diverse organic and quantum spin systems, studied by the low-frequency (10 mHz - 1 MHz) dielectric spectroscopy. Density wave structures with the order of commensurability N cz 4 can be recognized as phasons in a random impurity potential, whereas those with N = 3 can be viewed as topological defects like charge domain wall pairs in the background domain structure.

Research paper thumbnail of Non-ohmic electrical transport in the Peierls-Mott state of deuterated copper-DCNQI systems

Synthetic Metals, 2001

ABSTRACT Electric-field-dependent measurements are reported in the low-temperature state of the o... more ABSTRACT Electric-field-dependent measurements are reported in the low-temperature state of the organic conductors Cu[2,5(CD3)(2)-DCNQI](2) and Cu[(2,5(CH3)(2)-DCNQI)(0.70)(2,5(CD3)(2)-DCNQI)(0.30)](2) With N=3 commensurate CDW. We correlate observed features of the electric conduction below and above threshold field with the temperature evolution of the N=3 CDW order as detected by the low-frequency dielectric measurements.

Research paper thumbnail of Low frequency dielectric response in spin density wave phase of bechgaard salts

Synthetic Metals, 1999

We report the dielectric response (100mHz-1MHz) of the spin density wave (SDW) in (TMTSF)2PF6 and... more We report the dielectric response (100mHz-1MHz) of the spin density wave (SDW) in (TMTSF)2PF6 and (TMTSF)2AsF6 between 1.4K–4.2K. The relaxation gradually slows down in PF6, while in AsF6 it shows a tendency of divergence hinting the appearance of a glassy transition. The latter correlates with the important increase of the electrical threshold field for the SDW sliding found in AsF6, but not observed in PF6 compound.

Research paper thumbnail of Suppression of the Charge-Density-Wave State in Sr14Cu24O41 by Calcium Doping

Physical Review Letters, 2003

The charge response in the spin chain/ladder compound Sr14−xCaxCu24O41 is characterized by DC res... more The charge response in the spin chain/ladder compound Sr14−xCaxCu24O41 is characterized by DC resistivity, low-frequency dielectric spectroscopy and optical spectroscopy. We identify a phase transition below which a charge-density wave (CDW) develops in the ladder arrays. Calcium doping suppresses this phase with the transition temperature decreasing from 210 K for x = 0 to 10 K for x = 9, and the CDW gap from 130 meV down to 3 meV, respectively. This suppression is due to the worsened nesting originating from the increase of the inter-ladder tight-binding hopping integrals, as well as from disorder introduced at the Sr sites. These results altogether speak in favor of two-dimensional superconductivity under pressure.

Research paper thumbnail of Screening and Fundamental Length Scales in Semidilute Na-DNA Aqueous Solutions

Physical Review Letters, 2006

The fundamental length scales in semidilute Na-DNA aqueous solutions have been investigated by di... more The fundamental length scales in semidilute Na-DNA aqueous solutions have been investigated by dielectric spectroscopy. The low-and the high-frequency relaxation modes are studied in detail. The length scale of the high-frequency relaxation mode at high DNA concentrations can be identified with the de Gennes-Pfeuty-Dobrynin correlation length of polyelectrolytes in semidilute solution, whereas at low DNA concentrations and in the low added salt limit the length scale shows an unusual exponent reminiscent of semidilute polyelectrolyte chains with hydrophobic backbone. The length scale of the low-frequency relaxation mode corresponds to a Gaussian chain composed of correlation blobs in the low added salt limit, and to the Odijk-Skolnick-Fixman value of the single chain persistence length in the high added salt limit.

Research paper thumbnail of Manning free counterion fraction for a rodlike polyion: Aqueous solutions of short DNA fragments in presence of very low added salt

Physical Review E, 2011

We quantified the Manning free (uncondensed) counterions fraction θ for dilute aqueous solutions ... more We quantified the Manning free (uncondensed) counterions fraction θ for dilute aqueous solutions of rodlike polyions: 150 bp DNA fragments, in the presence of a very low concentration of monovalent salt c salt < 0.05 mM. Conductivity measurements of these solutions for DNA base pair concentration range 0.015 c 8 mM were complemented by fluorescence correlation spectroscopy (FCS) measurements of the DNA polyion diffusion coefficient D p (c). We observed a crossover in the normalized conductivity σ (c)/c that nearly halved across the c = 0.05-1 mM range, while D p (c) remained rather constant, as we established by FCS. Analyzing these data we extracted θ (c) = 0.30-0.45, and taking the Manning asymmetry field effect on polyelectrolyte conductivity into account we got θ (c) = 0.40-0.60. We relate the θ(c) variation to gradual DNA denaturation occurring, in the very low salt environment, with the decrease in DNA concentration itself. The extremes of the experimental θ(c) range occur toward the highest, above 1 mM, and the lowest, below 0.05 mM, DNA concentrations and correspond to the theoretical θ values for dsDNA and ssDNA, respectively. Therefore, we confirmed Manning condensation and conductivity models to be valuable in description of dilute solutions of rodlike polyions.

Research paper thumbnail of Structure and dynamics of hyaluronic acid semidilute solutions: A dielectric spectroscopy study

Physical Review E, 2010

Dielectric spectroscopy is used to investigate fundamental length scales describing the structure... more Dielectric spectroscopy is used to investigate fundamental length scales describing the structure of hyaluronic acid sodium salt (Na-HA) semidilute aqueous solutions. In salt-free regime, the length scale of the relaxation mode detected in MHz range scales with HA concentration as c −0.5 HA and corresponds to the de Gennes-Pfeuty-Dobrynin correlation length of polyelectrolytes in semidilute solution. The same scaling was observed for the case of long, genomic DNA. Conversely, the length scale of the mode detected in kHz range also varies with HA concentration as c −0.5 HA which differs from the case of DNA (c −0.25 DNA ). The observed behavior suggests that the relaxation in the kHz range reveals the de Gennes-Dobrynin renormalized Debye screening length, and not the average size of the chain, as the pertinent length scale. Similarly, with increasing added salt the electrostatic contribution to the HA persistence length is observed to scale as the Debye length, contrary to scaling pertinent to the Odijk-Skolnick-Fixman electrostatic persistence length observed in the case of DNA. We argue that the observed features of the kHz range relaxation are due to much weaker electrostatic interactions that lead to the absence of Manning condensation as well as a rather high flexibility of HA as compared to DNA.

Research paper thumbnail of Anisotropic charge modulation in ladder planes of Sr14−xCaxCu24O41

Physical Review B, 2005

The charge response of the ladders in Sr14−xCaxCu24O41 is characterized by dc resistivity, low fr... more The charge response of the ladders in Sr14−xCaxCu24O41 is characterized by dc resistivity, low frequency dielectric and optical spectroscopy in all three crystallographic directions. The collective charge-density wave screened mode is observed in the direction of the rungs for x=0, 3 and 6, in addition to the mode along the legs. For x=8 and 9, the charge-density-wave response along the rungs fully vanishes, while the one along the legs persists. The transport perpendicular to the planes is always dominated by hopping.

Research paper thumbnail of Crossover in charge transport from one-dimensional copper-oxygen chains to two-dimensional ladders in (La,Y)y(Sr,Ca)14−yCu24O41

Physical Review B, 2008

The charge transport in the copper-oxygen chain/ladder layers of ͑La, Y͒ y ͑Sr, Ca͒ 14−y Cu 24 O ... more The charge transport in the copper-oxygen chain/ladder layers of ͑La, Y͒ y ͑Sr, Ca͒ 14−y Cu 24 O 41 is investigated along two crystallographic directions in the temperature range from 50 to 700 K and for doping levels from y Ϸ 6 ͑number of holes n h Ͻ 1͒ to y =0 ͑number of holes n h =6͒. A crossover from a one-dimensional hopping transport along the chains for y Ն 3 to a quasi-two-dimensional charge conduction in the ladder planes for y Շ 2 is observed. This is attributed to a partial hole transfer from chains to ladders when the hole doping exceeds n h Ϸ 4 and approaches fully doped value n h = 6. For y Շ 2 a weak dielectric relaxation at radio frequencies and a microwave mode are detected, which might be recognized as signatures of a charge-density wave phase developed at short length scales in the ladder planes.

Research paper thumbnail of Structure modulation and charge ordering in self-doped M x CuO 2 crystals

Acta Crystallographica Section A Foundations of Crystallography, 2012

Research paper thumbnail of Influence of quantum Hall effect on linear and nonlinear conductivity in the FISDW states of the organic conductor (TMTSF) 2PF 6

Zeitschrift für Physik B Condensed Matter

We report a detailed characterization of quantum Hall effect (QHE) influence on the linear and no... more We report a detailed characterization of quantum Hall effect (QHE) influence on the linear and non-linear resistivity tensor in FISDW phases of the organic conductor (TMTSF)2PF6. We show that the behavior at low electric fields, observed for nominally pure single-crystals with different values of the resistivity ratio, is fully consistent with a theoretical model, which takes QHE nature of FISDW and residual quasi-particle density associated with different crystal imperfection levels into account. The non-linearity in longitudinal and diagonal resistivity tensor components observed at large electric fields reconciles preceding contradictory results. Our theoretical model offers a qualitatively good explanation of the observed features if a sliding of the density wave with the concomitant destruction of QHE, switched on above a finite electric field, is taken into account.

Research paper thumbnail of Single-particle and spin-density wave charge dynamics in (TMTSF)2PF6 and (TMTSF)2AsF6 : A comparative overview

We present the results of DC and AC (100 mHz - 1MHz) electrical transport measurements in low and... more We present the results of DC and AC (100 mHz - 1MHz) electrical transport measurements in low and high electric fields performed in the spin-density wave (SDW) state of the Bechagaards salts (TMTSF)2PF6 and (TMTSF)2AsF6. We argue that a degree of complex structure of the SDW ground state which is unfolded in a particular experiment depends strongly on the chosen experimental probe and the crystal measured.

Research paper thumbnail of Charge-density wave formation in Sr_ {14-x} Ca_ {x} Cu_ {24} O_ {41}

The electrodynamic response of the spin-ladder compound Sr14-xCaxCu24O41(x=0,3,9) has been studie... more The electrodynamic response of the spin-ladder compound Sr14-xCaxCu24O41(x=0,3,9) has been studied from radio frequencies up to the infrared. At temperatures below 250 K a pronounced absorption peak appears around 12cm-1 in Sr14Cu24O41 for the radiation polarized along the chains/ladders (E‖c). In addition a strongly temperature-dependent dielectric relaxation is observed in the kHz – MHz range. We explain this behavior by a charge-density wave which develops in the ladders and produces a phason mode pinned at 12cm-1. With increasing x the mode shifts up in frequency and eventually disappears for x=9 because the dimensionality of the system crosses over from one to two dimensions, giving way to the superconducting ground state under pressure.

Research paper thumbnail of Variable-range hopping conductivity in the copper-oxygen chains of La_3Sr_3Ca_8Cu_24O_41

We show that the spin chain/ladder compound La3Sr3Ca8Cu24O41 is an insulator with hopping transpo... more We show that the spin chain/ladder compound La3Sr3Ca8Cu24O41 is an insulator with hopping transport along the chains. In the temperature range 35 -280 K, DC conductivity σDC(T ) follows Mott's law of variable-range hopping conduction; the frequency dependence has the form σ(ν, T ) = σDC(T )+A(T )·ν s , where s ≈ 1. The conduction mechanism changes from variable-range hopping to nearest-neighbor hopping around Tc = 300 K. The chain array thus behaves like a one-dimensional disordered system. Disorder is due to random structural distortions of chains induced by irregular coordination of the La/Sr/Ca ions.

Research paper thumbnail of PROPERTIES OF MOTT-PEIERLS INSULATING PHASE IN DEUTERATED COPPER-DCNQI SYSTEMS

We overview low frequency dielectric spectroscopy and non-linear dc electrical transport in singl... more We overview low frequency dielectric spectroscopy and non-linear dc electrical transport in single crystals of a member of copper-DCNQI charge-transfer salt with N = 3 commensurate charge density wave. Broad relaxation mode of moderate strength weakens and becomes narrower with decreasing temperature. Threshold electrical field diverges, concomitant with the slight increase of the non-ohmic conductivity effect. These features suggest that the entities responsible for the dielectric response are pairs of charged domain walls between metallic and insulating phases.

Research paper thumbnail of Polyelectrolyte Composite: Hyaluronic Acid Mixture with DNA

Macromolecules, 2015

We studied salt-free, highly concentrated (5−200 g/L) mixtures of unfragmented (μm contour length... more We studied salt-free, highly concentrated (5−200 g/L) mixtures of unfragmented (μm contour length) DNA and hyaluronic acid (HA) as a borderline example of rigid-rod/flexible-chain composite, across a broad range of concentration ratios c HA /c DNA = 0.05−50. By polarizing microscopy, we established that the DNA and HA form clearly separated thread-like domains defined and oriented by solution shear. Within its domains, DNA shows birefringent banded patterns, routinely observed for long chain mesogens. We applied small-angle X-ray scattering (SAXS) to the mixtures and observed a polyelectrolyte (PE) correlation peak at q ★ wave vector. This peak was ascribed to DNA subphase and was used as a measure of the effective DNA concentration in the subphase c DNA ★ , according to deGennes scaling relationship between the DNA mesh size ξ =2 π/q ★ ∝ c −1/2 and monomer concentration c. From c DNA ★ (and initial c HA and c DNA ), we inferred the effective c HA ★ of HA subphase. We find a concentration-independent ratio Γ = c HA ★ / c DNA ★ ≈ 0.85 across a broad range of 0.02−0.4 M. As there is the osmotic pressure (Π) equilibrium between DNA and HA subphases, the constant Γ indicates that Π ∝ c 9/8 scaling common for DNA and other highly charged PEs is valid also for HA (a weak PEdoes not feature counterion condensation). Therefore, we propose that this deviation from the conventional osmotic pressure scaling Π ∝ c cannot originate from the concentration dependence of counterion condensation, which is an implicit but common interpretation in the literature. Further, as HA releases all its counterions into the solution, the HA osmotic coefficient ϕ HA we took as a measure of the DNA osmotic coefficient which we found to be ϕ DNA ≈ 0.28. This, double the Manning value, corroborates a result by Raspaud et al. 1

Research paper thumbnail of Low-frequency dielectric spectroscopy of commensurate density waves

Synthetic Metals, 2001

ABSTRACT We overview complex conductivity measurements (20Hz - 1MHz) in low-dimensional organic s... more ABSTRACT We overview complex conductivity measurements (20Hz - 1MHz) in low-dimensional organic systems kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Cl, and deuterated Cu-DCNQI and its alloys, with commensurate density wave ground states. We discuss the obtained results in comparison with those for the incommensurate density waves, like in (TMTSF)(2)PF6, in which the phason relaxation in a disordered background dominates. We show that in the commensurate systems with a random domain structure charged domain walls provide a more appropriate description of the observed dielectric relaxation.

Research paper thumbnail of Variable-range hopping conductivity in the copper-oxygen chains of La3Sr3Ca8Cu24O41

Physical Review B, 2003

We show that the spin chain/ladder compound La3Sr3Ca8Cu24O41 is an insulator with hopping transpo... more We show that the spin chain/ladder compound La3Sr3Ca8Cu24O41 is an insulator with hopping transport along the chains. In the temperature range 35 -280 K, DC conductivity σDC(T ) follows Mott's law of variable-range hopping conduction; the frequency dependence has the form σ(ν, T ) = σDC(T )+A(T )·ν s , where s ≈ 1. The conduction mechanism changes from variable-range hopping to nearest-neighbor hopping around Tc = 300 K. The chain array thus behaves like a one-dimensional disordered system. Disorder is due to random structural distortions of chains induced by irregular coordination of the La/Sr/Ca ions.

Research paper thumbnail of Diffraction analysis of incommensurate modulation in “chain-ladder” composite crystal (Sr/Ca)14Cu24O41

EMC 2008 14th European Microscopy Congress 1–5 September 2008, Aachen, Germany, 2008

ABSTRACT The “chain-ladder” compounds with formula (Sr/La/Ca)14Cu24O41 exhibit a rather complex c... more ABSTRACT The “chain-ladder” compounds with formula (Sr/La/Ca)14Cu24O41 exhibit a rather complex crystallographic structure due to their composite character. These materials consist of an alternating stacking of two distinct types of layers forming interpenetrated subsystems [1]. One subsystem is formed of (Sr/La/Ca)2Cu2O3 layers with a ladder-like structure and orthorhombic unit cell (Fmmm), while the other contains layers of CuO2 chains with orthorhombic unit cell (Amma) [2]. All lattice parameters slightly vary with La/Ca for Sr substitution, but the one dimensional incommensurate modulation results from the misfit between the c lattice parameters of these layer elements. In pure Sr compound: c Ch = 0.273nm, c Ld = 0.393 nm ≈ √2·c Ch; the a parameter (≈ 1.147 nm) is the same in both lattices, as well as b (≈ 1.341 nm) pointing along stacking direction. For high Ca for Sr substitution in Sr3Ca11Cu24O41: c Ch = 0.276 nm, c Ld = 0.391 nm, so that the ratio of these parameters is always close to √2. The c Ld/c Ch ratio also varies from 1.416 for pure undoped Sr14Cu24O41, to 1.445 for highly doped SrxCa14−xCu24O41[3].

Research paper thumbnail of Charge-density wave formation in Sr14-xCaxCu24O41

PHYSICAL REVIEW B, 2002

The electrodynamic response of the spin-ladder compound Sr14−xCaxCu24O41 (x = 0, 3, 9) has been s... more The electrodynamic response of the spin-ladder compound Sr14−xCaxCu24O41 (x = 0, 3, 9) has been studied from radiofrequencies up to the infrared. At temperatures below 250 K a pronounced absorption peak appears around 12 cm −1 in Sr14Cu24O41 for the radiation polarized along the chains/ladders (E c). In addition a strongly temperature dependent dielectric relaxation is observed in the kHz -MHz range. We explain this behavior by a charge density wave which develops in the ladders sub-system and produces a mode pinned at 12 cm −1 . With increasing Ca doping the mode shifts up in frequency and eventually disappears for x = 9 because the dimensionality of the system crosses over from one to two dimensions, giving way to the superconducting ground state under pressure.

Research paper thumbnail of Modalities of self-organized charge response in low dimensional systems

We present modalities of self-organized charge response in low dimensional systems, like diverse ... more We present modalities of self-organized charge response in low dimensional systems, like diverse organic and quantum spin systems, studied by the low-frequency (10 mHz - 1 MHz) dielectric spectroscopy. Density wave structures with the order of commensurability N cz 4 can be recognized as phasons in a random impurity potential, whereas those with N = 3 can be viewed as topological defects like charge domain wall pairs in the background domain structure.

Research paper thumbnail of Non-ohmic electrical transport in the Peierls-Mott state of deuterated copper-DCNQI systems

Synthetic Metals, 2001

ABSTRACT Electric-field-dependent measurements are reported in the low-temperature state of the o... more ABSTRACT Electric-field-dependent measurements are reported in the low-temperature state of the organic conductors Cu[2,5(CD3)(2)-DCNQI](2) and Cu[(2,5(CH3)(2)-DCNQI)(0.70)(2,5(CD3)(2)-DCNQI)(0.30)](2) With N=3 commensurate CDW. We correlate observed features of the electric conduction below and above threshold field with the temperature evolution of the N=3 CDW order as detected by the low-frequency dielectric measurements.

Research paper thumbnail of Low frequency dielectric response in spin density wave phase of bechgaard salts

Synthetic Metals, 1999

We report the dielectric response (100mHz-1MHz) of the spin density wave (SDW) in (TMTSF)2PF6 and... more We report the dielectric response (100mHz-1MHz) of the spin density wave (SDW) in (TMTSF)2PF6 and (TMTSF)2AsF6 between 1.4K–4.2K. The relaxation gradually slows down in PF6, while in AsF6 it shows a tendency of divergence hinting the appearance of a glassy transition. The latter correlates with the important increase of the electrical threshold field for the SDW sliding found in AsF6, but not observed in PF6 compound.

Research paper thumbnail of Suppression of the Charge-Density-Wave State in Sr14Cu24O41 by Calcium Doping

Physical Review Letters, 2003

The charge response in the spin chain/ladder compound Sr14−xCaxCu24O41 is characterized by DC res... more The charge response in the spin chain/ladder compound Sr14−xCaxCu24O41 is characterized by DC resistivity, low-frequency dielectric spectroscopy and optical spectroscopy. We identify a phase transition below which a charge-density wave (CDW) develops in the ladder arrays. Calcium doping suppresses this phase with the transition temperature decreasing from 210 K for x = 0 to 10 K for x = 9, and the CDW gap from 130 meV down to 3 meV, respectively. This suppression is due to the worsened nesting originating from the increase of the inter-ladder tight-binding hopping integrals, as well as from disorder introduced at the Sr sites. These results altogether speak in favor of two-dimensional superconductivity under pressure.

Research paper thumbnail of Screening and Fundamental Length Scales in Semidilute Na-DNA Aqueous Solutions

Physical Review Letters, 2006

The fundamental length scales in semidilute Na-DNA aqueous solutions have been investigated by di... more The fundamental length scales in semidilute Na-DNA aqueous solutions have been investigated by dielectric spectroscopy. The low-and the high-frequency relaxation modes are studied in detail. The length scale of the high-frequency relaxation mode at high DNA concentrations can be identified with the de Gennes-Pfeuty-Dobrynin correlation length of polyelectrolytes in semidilute solution, whereas at low DNA concentrations and in the low added salt limit the length scale shows an unusual exponent reminiscent of semidilute polyelectrolyte chains with hydrophobic backbone. The length scale of the low-frequency relaxation mode corresponds to a Gaussian chain composed of correlation blobs in the low added salt limit, and to the Odijk-Skolnick-Fixman value of the single chain persistence length in the high added salt limit.

Research paper thumbnail of Manning free counterion fraction for a rodlike polyion: Aqueous solutions of short DNA fragments in presence of very low added salt

Physical Review E, 2011

We quantified the Manning free (uncondensed) counterions fraction θ for dilute aqueous solutions ... more We quantified the Manning free (uncondensed) counterions fraction θ for dilute aqueous solutions of rodlike polyions: 150 bp DNA fragments, in the presence of a very low concentration of monovalent salt c salt < 0.05 mM. Conductivity measurements of these solutions for DNA base pair concentration range 0.015 c 8 mM were complemented by fluorescence correlation spectroscopy (FCS) measurements of the DNA polyion diffusion coefficient D p (c). We observed a crossover in the normalized conductivity σ (c)/c that nearly halved across the c = 0.05-1 mM range, while D p (c) remained rather constant, as we established by FCS. Analyzing these data we extracted θ (c) = 0.30-0.45, and taking the Manning asymmetry field effect on polyelectrolyte conductivity into account we got θ (c) = 0.40-0.60. We relate the θ(c) variation to gradual DNA denaturation occurring, in the very low salt environment, with the decrease in DNA concentration itself. The extremes of the experimental θ(c) range occur toward the highest, above 1 mM, and the lowest, below 0.05 mM, DNA concentrations and correspond to the theoretical θ values for dsDNA and ssDNA, respectively. Therefore, we confirmed Manning condensation and conductivity models to be valuable in description of dilute solutions of rodlike polyions.

Research paper thumbnail of Structure and dynamics of hyaluronic acid semidilute solutions: A dielectric spectroscopy study

Physical Review E, 2010

Dielectric spectroscopy is used to investigate fundamental length scales describing the structure... more Dielectric spectroscopy is used to investigate fundamental length scales describing the structure of hyaluronic acid sodium salt (Na-HA) semidilute aqueous solutions. In salt-free regime, the length scale of the relaxation mode detected in MHz range scales with HA concentration as c −0.5 HA and corresponds to the de Gennes-Pfeuty-Dobrynin correlation length of polyelectrolytes in semidilute solution. The same scaling was observed for the case of long, genomic DNA. Conversely, the length scale of the mode detected in kHz range also varies with HA concentration as c −0.5 HA which differs from the case of DNA (c −0.25 DNA ). The observed behavior suggests that the relaxation in the kHz range reveals the de Gennes-Dobrynin renormalized Debye screening length, and not the average size of the chain, as the pertinent length scale. Similarly, with increasing added salt the electrostatic contribution to the HA persistence length is observed to scale as the Debye length, contrary to scaling pertinent to the Odijk-Skolnick-Fixman electrostatic persistence length observed in the case of DNA. We argue that the observed features of the kHz range relaxation are due to much weaker electrostatic interactions that lead to the absence of Manning condensation as well as a rather high flexibility of HA as compared to DNA.

Research paper thumbnail of Anisotropic charge modulation in ladder planes of Sr14−xCaxCu24O41

Physical Review B, 2005

The charge response of the ladders in Sr14−xCaxCu24O41 is characterized by dc resistivity, low fr... more The charge response of the ladders in Sr14−xCaxCu24O41 is characterized by dc resistivity, low frequency dielectric and optical spectroscopy in all three crystallographic directions. The collective charge-density wave screened mode is observed in the direction of the rungs for x=0, 3 and 6, in addition to the mode along the legs. For x=8 and 9, the charge-density-wave response along the rungs fully vanishes, while the one along the legs persists. The transport perpendicular to the planes is always dominated by hopping.

Research paper thumbnail of Crossover in charge transport from one-dimensional copper-oxygen chains to two-dimensional ladders in (La,Y)y(Sr,Ca)14−yCu24O41

Physical Review B, 2008

The charge transport in the copper-oxygen chain/ladder layers of ͑La, Y͒ y ͑Sr, Ca͒ 14−y Cu 24 O ... more The charge transport in the copper-oxygen chain/ladder layers of ͑La, Y͒ y ͑Sr, Ca͒ 14−y Cu 24 O 41 is investigated along two crystallographic directions in the temperature range from 50 to 700 K and for doping levels from y Ϸ 6 ͑number of holes n h Ͻ 1͒ to y =0 ͑number of holes n h =6͒. A crossover from a one-dimensional hopping transport along the chains for y Ն 3 to a quasi-two-dimensional charge conduction in the ladder planes for y Շ 2 is observed. This is attributed to a partial hole transfer from chains to ladders when the hole doping exceeds n h Ϸ 4 and approaches fully doped value n h = 6. For y Շ 2 a weak dielectric relaxation at radio frequencies and a microwave mode are detected, which might be recognized as signatures of a charge-density wave phase developed at short length scales in the ladder planes.