Publications (13)22.17 Total impact
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Article: Cooperative dynamics in charge-ordered state of alpha-(BEDT-TTF)2I3
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ABSTRACT: Electric-field-dependent pulse measurements are reported in the charge-ordered state of alpha-(BEDT-TTF)2I3. At low electric fields up to about 50 V/cm only negligible deviations from Ohmic behavior can be identified with no threshold field. At larger electric fields and up to about 100 V/cm a reproducible negative differential resistance is observed with a significant change in shape of the measured resistivity in time. These changes critically depend whether constant voltage or constant current is applied to the single crystal. At high enough electric fields the resistance displays a dramatic drop down to metallic values and relaxes subsequently in a single-exponential manner to its low-field steady-state value. We argue that such an electric-field induced negative differential resistance and switching to transient states are fingerprints of cooperative domain-wall dynamics inherent to two-dimensional bond-charge density wave with ferroelectric-like nature.08/2012; -
Article: The Hall effect in the organic conductor TTF-TCNQ: choice of geometry for accurate measurements of a highly anisotropic system.
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ABSTRACT: We have measured the Hall effect on recently synthesized single crystals of the quasi-one-dimensional organic conductor TTF-TCNQ (tetrathiafulvalene-tetracyanoquinodimethane), a well known charge transfer complex that has two kinds of conductive stacks: the donor (TTF) and the acceptor (TCNQ) chains. The measurements were performed in the temperature interval 30 K < T < 300 K and for several different magnetic field and current directions through the crystal. By applying the equivalent isotropic sample approach, we have demonstrated the importance of the choice of optimal geometry for accurate Hall effect measurements. Our results show, contrary to past belief, that the Hall coefficient does not depend on the geometry of measurements and that the Hall coefficient value is approximately zero in the high temperature region (T > 150 K), implying that there is no dominance of either the TTF or the TCNQ chain. At lower temperatures our measurements clearly prove that all three phase transitions of TTF-TCNQ could be identified from Hall effect measurements.Journal of Physics Condensed Matter 02/2012; 24(4):045602. · 2.55 Impact Factor -
Article: Electrodynamic Response of the Charge Ordering Phase: Dielectric and Optical Studies of alpha-(BEDT-TTF)2I3
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ABSTRACT: We report on the anisotropic response, the charge and lattice dynamics of normal and charge-ordered phases with horizontal stripes in single crystals of the organic conductor alpha-(BEDT-TTF)2I3 determined by dc resistivity, dielectric and optical spectroscopy. An overdamped Drude response and a small conductivity anisotropy observed in optics is consistent with a weakly temperature dependent dc conductivity and anisotropy at high temperatures. The splitting of the molecular vibrations nu27(Bu) evidences the abrupt onset of static charge order below TCO=136 K. The drop of optical conductivity measured within the ab plane of the crystal is characterized by an isotropic gap that opens of approximately 75 meV with several phonons becoming pronounced below. Conversely, the dc conductivity anisotropy rises steeply, attaining at 50 K a value 25 times larger than at high temperatures. The dielectric response within this plane reveals two broad relaxation modes of strength Deltaepsilon_LD ~= 5000 and Deltaepsilon_SD ~= 400, centered at 1 kHz < f_LD < 100 MHz and f_SD ~= 1 MHz. The anisotropy of the large-mode (LD) mean relaxation time closely follows the temperature behavior of the respective dc conductivity ratio. We argue that this phason-like excitation is best described as a long-wavelength excitation of a 2kF bond-charge density wave expected theoretically for layered quarter-filled electronic systems with horizontal stripes. Conversely, based on the theoretically expected ferroelectric-like nature of the charge-ordered phase, we associate the small-mode (SD) relaxation with the motion of domain-wall pairs, created at the interface between two types of domains, along the a and b axes. We also consider other possible theoretical interpretations and discuss their limitations.11/2010; -
Article: Collective excitations in the charge-ordered phase of α-(BEDT-TTF)2I3.
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ABSTRACT: The charge response of a charge-ordered state in the organic conductor α-(BEDT-TTF)2I3 is characterized by dc resistivity, dielectric and optical spectroscopy in different crystallographic directions within the two-dimensional conduction layer. Two dielectric modes are detected. The large mode is related to the phasonlike excitation of the 2k(F) bond-charge density wave which forms in the ab plane. The small dielectric mode is associated with the motion of domain-wall pairs along the a and b axes between two types of domains which are created due to inversion symmetry breaking.Physical Review Letters 05/2010; 104(20):206406. · 7.37 Impact Factor -
Article: Magnetoresistance in spin-density-wave phase of (TMTSF)2PF6: Single-particle picture
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ABSTRACT: We present the b'-axis transverse magnetoresistance study of the spin-density-wave (SDW) ground state in the quasi–one-dimensional conductor (TMTSF)2PF6 for temperatures down to 2.2 K and magnetic fields up to 5 T. The magnetoresistance is very large and highly anisotropic. We present the clear change in the activation energy for T > 4.2 K and T < 4.2 K, which depends on magnetic field. The results can be well described by the quasi-particles with the energy gap given in the model with imperfect nesting.EPL (Europhysics Letters) 01/2007; 43(4):450. · 2.17 Impact Factor -
Article: Suppression of the charge-density-wave state in Sr14Cu24O41 by calcium doping.
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ABSTRACT: The charge response in the spin chain and/or 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 interladder 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.Physical Review Letters 07/2003; 90(25 Pt 1):257002. · 7.37 Impact Factor -
Article: Negative Magnetoresistance in (TMTTF)$\mathsf {_2}$Br
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ABSTRACT: We report the transverse magnetoresistance measurements at ambient pressure in the organic conductor (TMTTF)$_2$Br in the temperature range between 4.2 K and 40 K and in magnetic fields up to 8.5 T. We have found isotropic, negative and temperature dependent magnetoresistance which becomes negligible, close to 40 K. We interpret the observed behaviour within the picture of strongly correlated quasi-one dimensional systems.http://dx.doi.org/10.1051/jp1:1996193. -
Article: Phase diagrams of (La,Y,Sr,Ca)$_{14}$Cu$_{24}$O$_{41}$: Switching between the ladders and the chains
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ABSTRACT: The most comprehensive charge response study of the intrinsically hole doped, spin-chain and spin-ladder composites is overviewed. Results of dc and electric-field-dependent resistivity, low frequency dielectric, and optical spectroscopy in all crystallographic directions are used to build phase diagrams of the underdoped materials (hole count$=6-y$ per formula unit, f.u.) and of the fully doped , 6 holes per f.u. The underdoped materials are insulators with hopping transport along the chains, which behave as a one-dimensional disordered system. For the fully doped materials the charge transport switches to the ladders due to transfer of holes from the chains. Two-dimensional (2D) charge-density wave (CDW) ground state is formed, i.e. besides CDW phason response along the ladders for $0\leq x \leq 9$, we also found it along the ladder rungs for $x \leq 6$. However, CDW sliding conductivity, as observed in the standard CDW materials, is not observed in either of directions. Normal to the ladder planes no CDW response was found, and transport is presumably hopping-like. For the first time in any system, CDW response away from the principal direction was identified. For $x > 9$ both CDW in ladders and CO in chains are supressed.http://dx.doi.org/10.1051/jp4:2005131076. -
Article: Hall effect in the normal phase of the organic conductors: (TMTSF)$_{2}$ReO$_{4}$ vs. (TMTTF)$_{2}$AsF$_{6}$
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ABSTRACT: We report Hall effect measurements performed in the normal phase and at ambient pressure of (TMTSF)$_{2}$ReO$_{4}$ and (more anisotropic) (TMTTF)$_{2}$AsF$_{6}$ quasi-1D organic conductors. For $T>200$ K, the Hall coefficient of (TMTSF)$_{2}$ReO$_{4}$ is smoothly temperature-dependent, and its value corresponds to a carrier concentration of one hole/unit cell. On the other hand, the Hall coefficient of (TMTTF)$_{2}$AsF$_{6}$ is enhanced above 220 K, and the corresponding carrier concentration is almost 100 times lower than the expected one for one hole/unit cell. Our results give evidence for the existence of a high temperature regime above 200 K in (TMTTF)$_{2}$X salts, where the 1D Luttinger liquid features appear in the transport properties. Key words. Organic conductors, transport properties, magnetotransport.http://dx.doi.org/10.1051/jp4:2004114013. -
Article: Hall effect in Sr14-xCaxCu24O41
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ABSTRACT: We present Hall effect measurements of two-leg ladder Sr14-xCaxCu24O41 (0⩽x⩽11.5). In these composite materials charge and spin ordering occur within chain and ladder subsystems. They have been investigated intensively during the last years because of their fascinating physical properties, especially after the discovery of superconductivity under pressure (for x>10). They are the first superconducting copper oxides (superconductivity occurs under pressure for x>10) without the CuO2 planes, characteristic for high-temperature superconductors. The question of charge dynamics and hole distribution between the ladders and chains is still open. Our Hall effect results show typical semiconducting behavior for x<10. The x=11.5 compound shows metallic behavior and our Hall effect results match qualitatively the behavior found in high-Tc cuprates. We have determined the effective number of carriers and its dependence on composition (x) and compared it to the number of holes in the ladders obtained by other experimental techniques at room temperature.Physica B Condensed Matter 404:385-388. · 1.06 Impact Factor -
Article: Hall effect in the organic conductor (TMTSF)2NO3
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ABSTRACT: Hall effect measurements of the organic conductor (TMTSF)2NO3 at low temperatures (1.8 ≤ T ≤7 K) and high magnetic fields (H ≤ 8.5 T), are reported. The large Hall coefficient RH indicates the semimetallic nature of the spin density wave (SDW) ground state, whereas its positive sign shows the dominance of the hole carriers. Both RH and resistivity increase with decreasing temperature, which suggests the possibility that the carrier concentration is reduced as the temperature is lowered. The field variation of Hall resistance Rxy deviates from the linear dependence for T ≤ 4 K and H ≥ 3 T. This could be interpreted not only by the temperature induced change in the number of carriers or anisotropy of their mobilities, but also by the possibility that the SDW state might change with the applied magnetic field.Solid State Communications 97(5):333-337. · 1.65 Impact Factor -
Article: Influence of doping on the Hall coefficient in Sr_ {14− x} Ca_ {x} Cu_ {24} O_ {41}
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ABSTRACT: We present Hall-effect measurements of two-leg ladder compounds Sr14−xCaxCu24O41 (0≤x≤11.5) with the aim to determine the number of carriers participating in dc transport. Distribution of holes between the ladder and chain subsystems is one of the crucial questions important for understanding the physics of these compounds. Our Hall-effect and resistivity measurements show typical semiconducting behavior for x<11.5. However, for x=11.5, the results are completely different, and the Hall coefficient and resistivity behavior are qualitatively similar to that of high-temperature copper-oxide superconductors. We have determined the effective number of carriers at room temperature and compared it to the number of holes in the ladders obtained by other experimental techniques. We propose that going from x=0 to x=11.5 less than 1 hole per f.u. is added to the ladders and is responsible for a pronounced change in resistivity with Ca doping.Phys. Rev. B. 78(15). -
Article: Charge-density wave formation in Sr_ {14-x} Ca_ {x} Cu_ {24} O_ {41}
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ABSTRACT: 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.Phys. Rev. B. 66(6).
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Institutions
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2007–2010
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Institute of Physics, Zagreb
Zagreb, Grad Zagreb, Croatia
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