Blue Bronze Research Articles

X-ray and ultraviolet photoemission spectra study of Tl 0.3MoO 3

X-ray and ultraviolet photoemission spectra of thallium blue bronze Tl 0.3MoO 3 are presented. The results show that the conduction band extends to 3.2eV below the Fermi level, the valence band lies between3.2 and 10 eV and the gap between conduction and valence band is insignificant. The O ls and Tl 4f core lines exhibit the distinctive feature of weak shoulders, understood as excitation of small energy plasmon ( hω ∼ 2 eV). Asymmetric line shape of O ls is due to contribution from inequivalent oxygen sites. The Mo 3d core-electron spectrum is dominated by two final-state effect, representing alternate screening channels.

Electromodulated infrared transmission in blue bronze.

The infrared transmittance of the quasi-one-dimensional charge-density-wave (CDW) material blue bronze (${\mathrm{K}}_{0.3}$${\mathrm{MoO}}_{3}$) is observed to be affected by an applied electric field. The effect is interpreted in terms of changes in free carrier absorption resulting from surprisingly large changes in the density of quasiparticles which screen the CDW polarization at electrical contacts. The magnitude of the screening charge has a maximum at 110 K, and is thermally activated below this temperature.

Investigations of the breakdown transition, hysteresis, and periodic-pulse oscillation in Tl/W-doped blue bronzes.

Nonlinear transport behavior, which shows a sudden breakdown transition to zero- or negative-differential resistance in the high field, is observed in the quenched thallium- or tungsten-doped blue bronzes ${\mathrm{K}}_{0.15}$${\mathrm{Tl}}_{0.15}$${\mathrm{MoO}}_{3}$ and ${\mathrm{Tl}}_{0.3}$${\mathrm{Mo}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{W}}_{\mathit{x}}$${\mathrm{O}}_{3}$ at 77 K. It is found that the critical breakdown transition threshold is unstable for the induced field and decreases with increasing cycling times of the conducting current and, finally, reaches a definite value. When the current increases from 30 to 100 mA, the voltage between the two ends of the sample does not show a clear increase, i.e., a similar phenomenon with zero-differential resistance. On the downward cycle of the current, the voltage-current curve exhibits a large electronic hysteresis over the nonlinear region. On the other hand, in the critical region, the breakdown transition is usually accompanied by a voltage pulse with a large amplitude and exhibits a negative-differential resistance. These effects of an electric field on a pinned charge-density wave are phenomenologically interpreted in analogy with the mechanical deformations of the crystal solid.

LOW FREQUENCY VOLTAGE NOISE IN IMPURITY- DOPED BLUE BRONZES

In this paper, low frequency voltage noise property in impurity-doped blue bronzes is investigated. It is found that these low frequency voltage noise can be strongly enhanced by impurities or defects, for which the amplitude and shape of the oscillations are closely related to the initial conducting current of sample during cooling. The results indicate that the origin of the broad-band noise in charge-density-wave system may result from the macroscopic defects or impurities.

CHARGE DENSITY WAVE PROPERTIES OF MOLYBDENUM BRONZES

The physical properties of the low dimensional molybdenum bronzes showing charge density wave (CDW) instabilities are reviewed. The mechanisms of the Peierls transition, CDW and spin density wave (SDW) instabilities are first introduced. The properties of the quasi-one-dimensional (1D) blue bronze, A 0.30 MoO 3, including non linear transport due to the depinning of the incommensurate CDW, are described. The transport properties and the effect of pressure on the superconducting properties of the quasi-1D Li purple bronze Li 0.9 Mo 6 O 17 are reported. Finally, one presents the properties of the commensurate CDW state of the quasi-2D purple bronze KMo 6 O 17, especially under high magnetic field and hydrostatic pressure.

Effect of impurities on thermoelectric power in thallium blue bronze Tl 0.3MoO 3

Temperature dependence of thermoelectric power of thallium blue bronze Tl 0.3MoO 3 doped with K and W atoms impurities are presented. The experimental results display that thermoelectric power data for all the doped samples can be fitted to an expression of the type, S = At + B/ T, below and above T p . With the addition of impurities, the absolute values of the thermoelectric power decrease and the temperature of the sign change of thermoelectric power T s increases. The mechanisms of drift diffusion and phonon drag of the thermoelectric power are involved. The donor effect of the impurities and the influence of weak inter-chain coupling are proposed.

Synthesis of a reduced niobium “blue” with a layered perovskite structure

We have prepared a member with n=2 of the series of layered perovskites of general formula AA' n−1 Nb n O 3 n+1 (KLaNb 2O 7) and its proton-exchanged derivative HLaNb 2O 7. We have found that a reductive intercalation of additional protons is possible for the latter under reducing condition, to yield a “blue” derivative H 1+ x LaNb 2O 7, where the Nb ion is partially reduc ed to Nb(IV). The reduced species is readily oxidized in air to form back its parent compound. Alternatively, a more stable form of the reduced oxide can be obtained by treatment under Ar/H 2 at 650°C. The mixed valence state is confirmed by the observation of an Inter Valence Charge Transfer band at 1100 nm. This species can be considered as a blue hydrogen bronze of niobium.

The incommensurately modulated structures of the blue bronzes K0.3MoO3 and Rb0.3MoO3

The incommensurately modulated structures of the isostructural blue bronzes of K and Rb with modulation wavevector q = a* + 0.748 (1)b* + 1/2c* at 100 K have been determined by X-ray diffraction. The lattice parameters of the C-centred monoclinic cell for K0.3MoO3 are: a = 18.162 (2), b = 7.554 (1), c = 9.816 (1) angstrom, beta = 17.393(6)degrees, V = 1195.7 angstrom3, Z = 20, mu = 55.7 cm-1, lambda = 0.7107 angstrom, M(r) = 156.9. For Rb0.3MoO3: a = 18.536 (2), b = 7.556 (1), c = 10.035 (5) angstrom, beta = 118.52 (1)degrees , V = 1234.9 angstrom3, Z = 20, mu = 110.0 cm-1, lambda = 0.7107 angstrom, M(r) = 172.4. The symmetry of the structure can be described as consisting of a one-dimensionally modulated system with the four-dimensional superspace group C(s 1BAR)C2/m (0beta1/2). Th final R(F) = 0.033 for 7985 reflections for the K bronze and 0.032 for 4458 reflections for the Rb bronze. In the modulated structure, valence calculations show that the phase transition to the semiconductor state is accompanied by ordering of Mo5+ along the infinite-chain direction. The metallic conductivity, with delocalization of 4d electrons between clusters by overlapping Mo-O-Mo orbitals along the infinite-chain direction, tums into semiconductor properties by localization of 4d electrons on individual Mo(2) and Mo(3) octahedra (not on Mo10O30 clusters as a whole), modulated with wavevector q. By a comparison of the structures of the blue and red bronzes, their physical properties can be interpreted.

Collective modes in charge-density waves and long-range Coulomb interactions.

We study theoretically the collective modes in charge-density waves in the presence of long-range Coulomb interaction. We find that earlier works by Takada and his collaborators are inadequate since they introduced inconsistent approximations in evaluating a variety of correlation functions. The amplitude mode is unaffected by the Coulomb interaction, while the phase mode splits into the phason with linear dispersion (i.e., acoustic mode) and the optical mode with an energy gap in the presence of the Coulomb interaction. In particular, we establish the temperature dependence of the phason velocity ${\mathit{v}}_{\mathrm{\ensuremath{\varphi}}}$. A comparison with recent neutron-scattering data on the phason velocity in the charge-density wave of a single crystal of blue bronze ${\mathrm{K}}_{0.3}$${\mathrm{MoO}}_{3}$ indicates that mean-field theory which includes the long-range Coulomb interaction gives an excellent description of the observed phason velocity.

On the dimensionality of quasi-one-dimensional conductors

The anisotropic Ginzburg-Landau model is applied to the Bechgaard salts and to the blue bronze. The former is found to be well described by mean-field theory, whereas the latter shows strong fluctuations in the vicinity of T c

Transient charge density wave dynamics in pure, non-stoichiometric and V-doped blue bronzes A0.30MoO3 (A= K, Rb)

We report the observation of the transient voltage response to current pulses in pure, non-stoichiometric and V-doped blue bronzes A 0.30 MoO 3 (A=K, Rb) near threshold for depinning of the charge density wave. The transient response waveforms involve overshoot and sluggish behaviors at the leading edge of the pulse in pure samples, weak sluggish response in non-stoichiometric samples and strong sluggish behavior in V-doped blue bronzes. The time decays of the transient responses obey stretched exponential laws. The characteristic time for the overshoot obeys a power law as a function of the electric field and is divergent as threshold is approached from above, suggesting that depinning can be viewed as a dynamical critical phenomenon. The transient response has also been studied under inhomogeneous conditions when a current pulse is injected in a segment of the sample while a dc current is flowing in a neighboring segment the different regimes found in the transient response are analyzed in analogy with strain transient and plastic yielding of a crystal and in terms of combined motion of edge and screw charge density wave phase dislocations interacting with impurities

Low temperature preparation of the blue potassium bronze from a hydrated potassium molybdenum bronze by heat treatment in a nitrogen atmosphere

The blue potassium bronze was obtained from the hydrated potassium molybdenum bronze in a nitrogen atmosphere at lower temperature by about 200 K than the usual fused method. The blue bronze is obtained directly in a solid phase. This is a new preparation method, compared with the usual method.

Transient charge density wave dynamics in A 0.30MoO 3 (A = K, Rb)

We have investigated the transient voltage response to current pulses in the blue bronzes A 0.30MoO 3 (A = K, Rb) near threshold for depinning of the charge density wave in the temperature range 40K–110K. The time dependence of the overshoot at the leading edge of the voltage response and of the tail at the trailing edge obey stretched exponential laws. The effect of a dc current superimposed on the current pulses on threshold values and voltage response is reported. The different regimes found for the transient response are discussed in terms of charge density wave phase dislocations and in analogy with strain transient and plastic yielding of a crystal.

CDW depinning under inhomogeneous conditions in the blue bronzes

We have studied the non-linear voltage response to current pulses in a segment of a blue bronze sample where the CDW is depinned when a dc current is flowing in a neighboring segment. As the dc current is increased, various voltage response waveforms are found: overshoot, sluggish, undershoot. These non-local properties are discussed in terms of nucleation of edge and screw phase dislocations between the two neighboring segments.

Photoconduction in the blue bronze

We have investigated the photoconductivity in the linear chain compound K 0.3MoO 3. The experiments were performed at T = 4.2 K using pulsed infrared laserbeam with photon energy corresponding to the single particle gap. The electric field dependence of the observed photocurrent suggests that the optically excited electrons initiate the sliding of the charge density wave condensate, and the current of the collective mode dominates over any single particle contribution. We discuss the role of the electrostatic screening in this mechanism and give an estimation of the efficiency for a photon to excite an electron across the gap.

Long range coulomb interaction and phason propagator in charge density wave

Within mean field theory, we reexamine the collective modes associated with the phase fluctuation in the presence of long range Coulomb interaction. We find that the phason propagator is dominated by an acoustic mode and an optical mode. These modes have already been described by Nakane and Takada and Wong and Takada. However, due to their inconsistent approximations, their results became unreliable; particularly in the vicinity of the transition temperature. Relevance of these modes to those found in the Raman and neutron scattering from blue bronze is briefly discussed.

AC calorimetry at CDW and SDW transitions

We have measured the specific heat at the SDW transition in κ-(BEDT-TTF) 2Cu[N(CN) 2]Cl and the CDW transitions in ZrTe 3, blue bronze, and (TMTSF) 2ReO 4. No anomaly is observed at the SDW transition. For ZrTe 3 s small peak, Δc p ∼0.065R, is observed. For blue bronze and (TMTSF) 2ReO 4, the changes in c p are 3–4 times their calculated mean-field values, as expected for 1-D systems. For (TMTSF) 2ReO 4, there is also sample dependent latent heat associated with anion ordering.

Microwave conductivity of sliding charge-density-waves

The results of measurements of microwave impedance at f=2GHz as a function of a bias voltage for orthorombic TaS 3 and blue bronze K 0.3MoO 3 are presented. The results have been discussed in a correlation with dc non-linear characteristics and with direct mixing experiment data. Both dc conductivity and microwave one of the blue bronze are linear below a threshold field and become non-linear as the steady field exceeds the threshold. The non-linear microwave conductivity in TaS 3 is clearly defined below the dc threshold field.

New approach for preparing blue potassium molybdenum bronze thin films

Molybdenum bronze with a low concentration of K can be synthesized by electrolytic techniques. We report that blue potassium molybdenum bronze has been prepared by the simultaneous vacuum evaporation of potassium molybdate and molybdenum trioxide. The compound prepared under certain conditions is described. The material was analyzed with a PCXA analyzer program to acquire an energy-dispersive spectrum from the detector using a scanning microscope. The DC I-V characteristics and conductivity at different temperatures have been measured on the M/film/M sandwich structure. The results of these measurements reveal a transition region at the temperature of 165–180 K.

Hexagonal tungsten bronze synthesized from potassium peroxo-polytungstate and its electrical properties

Peroxo-polytungstic acid, formed by the direct reaction of metallic tungsten with hydrogen peroxide, precipitated an amorphous salt with potassium bromide. Decomposing the precipitate at 400–800°C in air yielded white hexagonal potassium tungstate, K x WO 3+ x 2 (x= 0.3) , with lattice parameters of a=7.31 and c=7.65 Å. On the other hand, thermal decomposition in a reducing atmosphere (H 2/H 2O) at 500°C gave dark blue hexagonal tungsten bronze (HTB), K x WO 3, with a=7.38 and c=7.52 Å, which were almost identical to those reported for HTB. It was found that the redox reaction between tungstate and HTB was completely reversible. A porous pellet of the present HTB showed metallic conductivity (≈10 S/cm, at 25°C) under dry conditions. In a wet atmosphere with only slight humidity, however, its conductivity was dramatically reduced. The conductivity was recovered by treating it again in a dry condition.