Size effects on the charge-density-wave pinning in the quasi-one-dimensional conductor K0.30MoO3

We report size effects on the charge-density-wave pinning in the quasi-one-dimensional conductor K_0.30MoO₃ in the temperature range 77 K-160 K. The threshold field is approximately one order of magnitude larger in needle-like samples than in bulk crystals, temperature independent and strongly dependent on the width of the sample. In addition, the nonlinear conductivity shows a maximum near 130 K. The results are discussed in relation with various pinning models and CDW dislocations.     

Specific heat and critical behavior of Tl0.3MoO3 near the Peierls phase transition

The specific heat of the quasi-one-dimensional charge-density-wave (CDW) compound Tl_0.3MoO₃ has been measured using an adiabatic continuous heating method from 100 to 220 K. A specific heat jump associated with the Peierls phase transition occurs at 172.3 K. A good scaling relation between the excess specific heat and the susceptibility is found between 140 K and 190 K. Further analysis indicates that the width of the critical region of Tl_0.3MoO₃ is about 10 K and the specific-heat critical behavior can be well described by the three-dimensional XY model.     

EPR study of the low temperature charge density wave state of o-TaS3

We report an EPR study of the chain conductor o-TaS₃ in the low temperature charge density wave (CDW) state. The EPR spectrum is attributed to Fe³⁺ (S=5/2) impurities. A power law for the temperature dependence of the EPR intensity, (T^−α with an exponent α∼0.8) found below ∼30 K is very close to that previously found in magnetic susceptibility measurements. The possible role of these impurities in the susceptibility data are discussed.     

Specific heat anomalies in the quasi two-dimensional monophosphate tungsten bronzes KxP4W8O32

The monophosphate tungsten bronzes K_xP₄W₈O₃₂ (0.75<x<2) are quasi-two-dimensional conductors which show electronic transitions at a critical temperature T_c depending on the concentration of the alkali metal. The phase diagram shows a maximum at for x=1.30. We report specific heat measurements in the range 120-190 K. The thermal anomalies found at the transition temperature are larger than in conventional charge density wave materials. This corroborates that the transition is not a ‘pure’ charge density wave transition and that a structural transition dominates the instability.     

The thermoelectric power of charge-density-wave conductors Rb0.3MoO3 and Rb0.15K0.15MoO3

The thermoelectric power (TEP) of the quasi-one-dimensional charge-density-wave (CDW) conductors rubidium blue bronze Rb_0.3MoO₃ and its alloy Rb_0.15K_0.15MoO₃ were measured in the temperature range 80-280 K. The result showed a sign change from a small positive value to a great negative value where the Peierls transition temperatures (T_p) are 183 and 180 K for Rb_0.3MoO₃ and Rb_0.15K_0.15MoO₃, respectively. Above T_p, the TEP for both samples can be described with the empirical relation S=AT+B; while below T_p, the TEP fits well the relation S=AT+B/T based on the experimental data. The Fermi energies ε_F for Rb_0.3MoO₃ and Rb_0.15K_0.15MoO₃ are estimated to be 1.55 and 0.53 eV, respectively.     

Photo-induced enhancement of charge density wave current in the quasi-one-dimensional conductor TaS3

In the near vicinity of Peierls transition temperature T_P, we have measured the V-I characteristics of the quasi-one-dimensional conductor TaS₃ under dark and photo-irradiation conditions. It is found that a significant enhancement of CDW current occurs only around the threshold voltage V_t under photo-irradiation. This effect can be interpreted as a result of screening of pinning potential for CDW condensate by photo-excited quasi-particles (QP's). Further the distribution of pinning potential intensity is reflected in the behavior of V-I characteristics near V_t. Our finding suggests that the strength of pinning potential can be controlled by the photo-excited QP's in quasi-1D conductors.     

Modification of the charge ordering transition in the quasi-one-dimensional conductor (TMTTF)2SbF6 under pressure

We have measured the temperature dependences of the conductance G and the dielectric permittivity ε′ of the (TMTTF)₂SbF₆ compound under a moderate pressure. The maximum of G(T) associated with the Mott-Hubbard localization disappears under pressure. With increasing pressure the peak in ε′(T), corresponding to the charge ordering (CO) phase transition, shifts to lower temperatures and broadens. At pressures above 0.24 GPa, ε′(T) becomes strongly frequency dependent. These modifications are explained in the frame of the extended Hubbard model and a slowing down behavior.     

Temperature dependence of the second threshold field for rubidium blue bronze Rb0.3MoO3

We have investigated the threshold properties of Rubidium blue bronze Rb_0.3MoO₃ under high dc electric field in a large temperature range 20-150 K. The second threshold fields have been observed at temperature up to 102.4 K, and have quasi-linear relationships with temperatures 20-45 K and 55-100 K, respectively. A novel crossover platform has been found firstly in the temperature dependence of the second threshold field E_T2 at about 45-55 K. The results indicate that the dynamical behavior of the second threshold effects may originate from different mechanisms. We suggest that the highly conducting state at 20-45 K and 55-100 K result from the undamped sliding motion of rigid CDW and current inhomogeneity, respectively.     

Transient electrical response of K0.30MoO3: Effects of isoelectronic dopants

In this paper, we examine the variation of threshold fields and the transient response of tungsten- and rubidium- substituted K_0.30MoO₃. We find that the dc threshold electric field for the onset of nonlinear behavior scales linearly with tungsten concentration, but varies as the square of the rubidium concentration, indicating strong and weak pinning effects, respectively. In tungsten- substituted samples, the threshold field becomes a strong function of frequency in the range 0.01– 1000 Hz. Above a critical frequency which depends on the doping level, the threshold field is proportional to -log (frequency). We interpret these results as a further example of the spin glass- like response of the CDW. Nonlinear conductivity is observed only after the voltage has exceeded the threshold for a finite time interval.     

Thermal Expansion Anomaly of Tb2Fe14Cr3 Compound

We investigate the thermal expansion property of the Tb₂Fe₁₄Cr₃ compound by means of x-ray diffraction. The result shows that the Tb₂Fe₁₄Cr₃ compound has a hexagonal Th₂Ni₁₇-type structure. Negative thermalexpansion is found in the Tb₂Fe₁₄Cr₃ compound from 296 to 493K by x-ray dilatometry. The coefficient of the average thermal expansion is α=-2.82 × 10^-5K^-1. In the temperature range 493--692K, the coefficient of the average thermal expansion is α=1.59 × 10^-5K^-1. The physical mechanism of thermal expansion anomaly of the Tb₂Fe₁₄Cr₃ compound is discussed according to the temperature dependence of magnetization measured by a superconducting quantum interference device.     

The blue bronze K0.3MoO3: A new one-dimensional conductor

The optical reflectivity of the blue bronze K_0.3MoO₃ has been measured on single crystals for photon energies between 0.03 and 12 eV at temperatures from 10 to 300 K using polarized light. The data are interpreted that this compound is a one-dimensional conductor for temperatures above 180 K and that the metal-semiconductor transition at 180 K is due to a Peierls type transition, leading to a gap of 0.15 eV in the density of states.     

X-ray study of field induced deformation of sliding CDW in K0.30MoO3

A deformation of the CDW is investigated under the electric field whose magnitude is intensionally made inhomogeneous in the one-dimensional conductor K_0.30MoO₃. It is verified that the CDW's deformation, which occurs in the transverse (2a¹-c¹)-direction, is caused by the field gradient in the sample. In the sample where the CDW deformation is found without the intensional inhomogeneity of the electroc field, the deformation is not uniform from point to point. Possible relations are discussed between the structural change and the electrical polarization observed in pulse measurements of the conductivity.     

X-ray evidence for a deformation of CDW during the sliding motion in K0.30MoO3

The wave vector and the correlation length of the CDW ordering in K_0.30MoO₃ are studied by x-ray diffraction as functions of electric fields applied along the one-dimensional axis. Several samples change their CDW-ordering above a threshold field. The change of the wave vector occurs mainly in the (2a¹-c¹) - direction, which is perpendicular to the one-dimensional b¹ - axis. It depends also on the field direction and magnitude. Time resolved x-ray experiments show that characteristic time of this change is of the order of 1 ms. The correlation length becomes longer along the (2a¹-c¹)-direction in the transient period.     

Quasiperiodic noise voltage and metastability in the charge density wave conductivity of K0.30MoO3

Slow relaxation phenomena as well as quasiperiodic noise have been studied in the non linear regime of conductivity which takes place above a well defined threshold electric field in the semiconducting incommensurate phase of the quasi one-dimensional blue bronze K_0.30MoO₃. The noise frequencies are found to be proportional to the excess current attributed to the charge density wave (CDW). In some temperature range, the CDW current is found to decrease logarithmically vs time. These results indicate the presence of metastable states related to domains and domain walls.     

Thermal and optical properties of the ferroelectric (C3N2H5)5Bi2Cl11 crystal

Thermal (specific heat) and optical (linear birefringence) studies were performed for a new ferroelectric crystal (C₃N₂H₅)₅Bi₂Cl₁₁. Two phase transitions were confirmed and described. The first-order paraelastic-ferroelastic phase transition at 360 K was studied with a polarizing microscope. The continuous second-order phase transition at 165 K to the ferroelectric phase is described by the Landau model using specific heat and linear birefringence data. The Landau expansion coefficients B and C are of an order of magnitude higher than the closely related ferroelectric crystal; MAPCB — (CH₃NH₃)₅Bi₂Cl₁₁. Thermal parameters (such as the excess enthalpy and the excess entropy ) of the continuous transition were estimated and discussed. The ‘two-site’ model describing the motion of three of the five imidazolium cations, which is proposed from the structural studies, is fully confirmed by the data from the ac-calorimetric measurements.     

Quantum self-consistent approach to the charge gap of the quasi-one-dimensional organic conductors

We apply an extended quantum self-consistent method, in which quantum fluctuations are taken into account, to the bosonization Hamiltonian to investigate analytically the charge gap in the quasi-one-dimensional (1D) organic conductors at quarter-filling described by the 1D Hubbard model with dimerization and alternate potential on site. It is shown that either dimerization or alternate potential gives rise to the enhancement of the charge gap. Our results are compared with those of the numerical and the other analytical theories. Our results are also consistent with the experimental data of the actual organic materials when the effect of nearest-neighbor Coulomb interaction is taken into account.     

Thermal expansion behaviors of hcp and fcc Co nanowire arrays

The lattice parameters of as-prepared and annealed Co nanowires with hcp and fcc structures have been measured using the in situ high-temperature x-ray diffraction method. The hcp and fcc Co nanowires have been fabricated within the porous anodic alumina membranes by a direct-current electrodeposition technique. The results indicate that the variational quantity of the interplanar spacing for hcp Co nanowire arrays is bigger than that for fcc Co nanowire arrays in spite of as-prepared and annealed samples. The structural difference between hcp and fcc Co nanowires results in the different thermal expansion behaviors.     

Polarized infrared reflectivity study in charge density wave conductor Tl0.3MoO3

Polarized infrared reflectivity measurements between 300 and 10 K have been carried out on charge density waves (CDW) conductor blue bronze Tl_0.3MoO₃. Three important features are observed: (i) A bump at 1155 cm⁻¹ in the reflectivity spectra of Tl_0.3MoO₃ at 300 K is a precursor of the Peierls gap due to optical excitations across a pseudogap, and this kind of Peierls-like gap opens gradually with decreasing temperature from 180 to 160 K. (ii) The three sharp modes as “triplet” of infrared reflectivity between 800 and 1000 cm⁻¹ of Tl_0.3MoO₃ along [1 0 2] axis show red shift compared to K_0.3MoO₃ and Rb_0.3MoO₃, which is assigned to the increase of the distance of Mo-O bond with the substitution of thallium ions. (iii) Two peaks at about 514 and 644 cm⁻¹ in the far-infrared reflectivity spectra of Tl_0.3MoO₃ along [1 0 2] direction are suggested to be the electronic transitions from the valence band to the midgap state and from occupied midgap state to the conduction band, respectively.     

Optical properties of the red bronze K0.33MoO3

The optical reflectivity of the red bronze K_0.33MoO₃ has been measured on single crystals in the spectral energy range between 0.03 and 12 eV at temperatures from 4 K to 300 K using polarized light. The optical constants have been determined by means of a Kramers-Kronig analysis; the data are interpreted that this compound is a 0.5 eV energy gap semiconductor with very strong anisotropy in the infrared and visible energy range.