The resistivity measurements of bulk super-conducting oxides La2−xSrxCuO4−x/2+ 〈0.15〈x〈0.2) irradiated by 2.5 Mev-electrons and YBa2Cu3O7-δ 〈0〈δ〈0.15) irradiated by 2.9 GeU-krypton ions have shown a regular and constant decrease of the critical temperature Tc as a function of the fluence. On the contrary, an increase of Tc has been observed durino irradiation of super-conduct ino La2CuO4 compound by 2.9 GeV-krypton ions, which saturates at Tc/Tco = 0.1 for fluences hioher than 3.1012 Kr.cm−. The evolution of the R(T) curves during irradiation is very similar to the one observed under Dressure. A comparison of the variations of Tc as a function of the number of di solacements oer atom for various particles (n. e−. He+. O. Kr) suggests that the hijjh electronic stoppino power of the heavy ions could be the main factor of damage.

The monophosphate tungsten bronzes (PO2)(4)(WO3)(2m) form family of two-dimensional metals which exhibit charge density wave (CDW) instabilities. These materials are generally built by the regular stacking of (a,b) layers in which chains made of segments of m WO6 octahedra directed along the a and a+/-b directions are delimited. Their electronic structure thus originates from quasi-one-dimensional (1D) bands located on these chains. As a consequence their Fermi surface (FS) exhibits large flat portions whose nesting gives rise to successive CDW instabilities. Here we present a structural study of the CDW instability of the (PO2)(4)(WO3)(10) member formed by the alternate stacking of layers built with segments of m=4 and m=6 WO6 octahedra. Its ab initio electronic structure calculation shows that the FS of this member exhibits large flat portions which can be extremely well nested. Its best nesting wave vector accounts for the modulation wave vector stabilized by the CDW transition which occurs at 156 K. Because of the regular stacking of layers of different m values the FS is slightly split. The unusual thermal dependence of the x-ray satellite intensity provides evidence that the two types of layers become modulated at different temperature. This also leads to a slight thermal sliding of the CDW-nesting modulation wave vector, which can be accounted for within the framework of a Landau-Ginzburg theory. In addition, the observation of a global hysteresis in the thermal cycling of the satellite intensity, as well as the degradation of the interlayer order upon cooling, suggest the formation of a disordered lattice of dilute solitons. Such solitons allow to accomodate the charge transferred between the two types of layer. Finally the relevance of local charge transfers, at intergrowth defects, for example, to create pinned discommensurations that break the CDW coherence is emphasized in this whole family of bronzes.

The potassium doped monophosphate tungsten bronzes KxP4W8O32 are two-dimensional metals which show a metal-to-metal transition at a critical temperature which depends on the doping level. The metal-to-metal transition is accompanied by the formation of a commensurate charge density wave with wave vector (π/b,0) which is independent of the doping level. Undoped P4W8O32, on the other hand, has two metal-to-metal transitions which are connected to the formation of incommensurate charge density waves. We measured the infrared reflectivity of the series KxP4W8O32 (x = 0 - 1.57) in the spectral range from 100 to 10 000 cm-1 for room temperature and well below the critical temperature. Polarization-dependent infrared spectra find a two-dimensional behavior in the normal and the charge density wave state and show signatures of hybridization between one- and two-dimensional conduction bands. In undoped P4W8O32 the essentials of the charge density wave state can be understood from the nesting vectors of the calculated Fermi surface and two gaps are observed in the infrared spectra. The gap sizes are a factor of about 2.5 bigger than the predictions from mean-field theory in the weak-coupling limit which suggests medium- or strong electron-phonon coupling. For potassium doped KxP4W8O32 one gap is observed in the charge density wave state. The energetics of the charge density formation may be dominated by the energy required for the lattice modulation.

We report a comparative study of the electrical resistivity under hydrostatic pressure up to 18 kbar of the quasi-two-dimensional conductors (PO2)4(WO3)2m (m = 4, 5, 6) and η-Mo4O11, which show Peierls transitions towards a charge density wave state. The pressure dependences of the transition temperatures are discussed in relation with the hidden nesting properties of these oxides. The results for η-Mo4O11 are compared with previous works.

: The monophosphate tungsten bronzes (PO 2 ) 4 (WO 3 ) 2m are quasi-two-dimensional conductors which show charge density wave type electronic instabilities. We report electrical resistivity and magnetoresistance measurements down to 0.30 K and in magnetic fields up to 16 T for the m = 7, 8 and 9 members of this family. We show that these compounds exhibit at low temperature an upturn of resistivity and field dependences of the magnetoresistance characteristic of localization effects. We discuss the dimensionality of the regime of localization as m is varied. We show that for m =7 , the regime is quasi-two-dimensional and three-dimensional for m = 8, 9 .

The charge-density-wave (CDW) structure of the monophosphate tungsten bronze P4W20O68, the m = 10 member of the (PO2)(4)(WO3)(2m) series has been solved at room temperature from single-crystal x-ray-diffraction data. In agreement with previous x-ray diffuse scattering experiments, intense first-order satellite reflections at +/- 3/7a* reduced wave vectors as well as second-order satellite reflections at +/- 1/7a* were observed. The CDW structure was refined in a four-dimensional (4D) formalism and found to have the superspace group P2(1) (alpha 00) 0. The modulation, of the displacive type, involves mainly the tungsten atoms inside the different WO6 octahedra of the WO3-type slab. The W displacements, which are mostly oriented in the direction of the segment of 10 WO6 octahedra building the slab, tends to be oriented in opposite directions between neighboring segments. It is suggested that for the large in members these features represent a good compromise between the CDW instability of the metallic bronzes, related to their quasi-1D electronic structure, and the incipient antiferroelectric lattice distortion of the insulating oxide WO3, which corresponds to the limit m-->infinity of this series. These structural features play a crucial role in setting the coupling between the differently oriented 1D portions of the Fermi surface required for the hidden nesting mechanism at the basis of the stabilization of a CDW ground state in the ReO3-type metallic W and Mo bronzes and oxides.

The monophosphate tungsten bronzes KxP4W8O32 are quasi-two-dimensional conductors. The parent compound P4W8O32 shows two charge density wave instabilities (CDW). The layered structure of the doped compounds contains pseudo hexagonal tunnels stabilised by the insertion of potassium for 0.75<x<2. In order to study the role of the band filling in the CDW instabilities, we have performed resistivity, magnetoresistance, Hall effect, thermopower and specific heat measurements on the doped compounds with 0.8<x<1.94. Anomalies at a temperature \(\) depending on x appear on all the transport properties. From transport data, we have obtained a phase diagram \(\) which shows surprisingly a maximum at 170 K for x=1.3. In the whole range of temperature studied (4.2 K-300 K), the transport properties of KxP4W8O32 show a change for \(\). The diagram \(\) is discussed in term of non-monotonous behaviour for the density of states versus the energy and in relation to previous X-ray studies. The transitions in KxP4W8O32 do not seem to be conventional Peierls instabilities in contrast with those observed in pure P4W8O32.

Resistivity, thermoelectric power and magnetotransport measurements have been performed on single crystals of the quasi two-dimensional monophosphate tungsten bronzes (PO2)4(WO3)2m for m=5 with alternate structure, between 0.4 K and 500 K, in magnetic fields of up to 36 T. These compounds show one charge density instability (CDW) at \(\) 160 K and a possible second one at \(\) 30 K. Large positive magnetoresistance in the CDW state is observed. The anisotropic Shubnikov-de Haas and de Haas-van Alphen oscillations detected at low temperatures are attributed to the existence of small electron and hole pockets left by the CDW gap openings. Angular dependent magnetoresistance oscillations (AMRO) have been found at temperatures below \(\) 30 K. The results are discussed in terms of a weakly corrugated cylindrical Fermi surface. They are shown to be consistent with a change of the Fermi surface below \(\) 30 K.

Besides the standard configuration with all the columnar defects parallel to the c→-axis, different splayed configurations with all the defects inclined at the same angle θi from the c→-axis were obtained by irradiation of single crystals of the most anisotropic high-Tc superconductor Bi2Sr2CaCu2O8 with 6 GeV Pb ions. The pinning abilities of these configurations were studied by magnetic measurements performed with H→ parallel to c→. We found that, in the case of this very anisotropic compound, it is not necessary to invoke a splay effect to account for the pinning properties of the splayed configurations of defects. The major role of two parameters has been seen. First, the role of the effective size of the projection of the defects in the (a→,b→) planes has been studied by introducing columnar defects of different diameters or by tilting the ion beam direction from the c→-axis. Second, the symmetry of the defect distribution with respect to the applied magnetic field has been shown to allow a better accommodation of the vortices to columnar defects. These two parameters can account for the observed crossover between a low-T regime and a high-T regime. This crossover is not predicted as a splay effect in the theoretical models.

The monophosphate tungsten bronzes (PO2)4(WO3)2m with pentagonal tunnels are quasi-two-dimensional conductors that show charge density wave type electronic instabilities. These series of compounds provide a model system where the low-dimensional character and the average electron concentration are functions of the m parameter. The low m compounds (m=4, 6) show conventional charge density wave instabilities. The m=5 compound exists with two different crystal structures and shows instabilities with slightly different properties. We report measurements of transport properties for the compounds m=5, 7, 8, 9. We show that, for m>7, these compounds exhibit an upturn of resistivity and field dependence of the magnetoresistance characteristic of quantum interference effects. We also report transport properties of the compounds KxP4W8O32 with pseudo-hexagonal tunnels that show electronic instabilities with critical temperatures depending on x.

Phase relations in the system Na-K-P-W-O have been studied for two values m = 4 and m = 6 of the relevant parameter m which characterizes the thickness of the perovskite WO3-type slabs within the series of the low dimensional tungsten phosphate bronzes (K-x-Na-y)(PO2)(4)(WO3)(2m). New NayP4W12O44 bronzes (0 less than or equal to y less than or equal to 1) belonging to the monophosphate tungsten bronzes with pentagonal tunnels family have been isolated and studied by single-crystal X-ray diffraction and resistivity measurements. The crystal structure of Na0.96P4W12O44 has been solved and refined to conventional R = 0.0289 (wR = 0.0414) with 2924 reflections with I greater than or equal to 3 sigma(I). Like that of P4W12O44, the unit cell is orthorhombic (space group P2(1)2(1)2(1)) with a = 5.3083(3) Angstrom, b = 6.5790(7) Angstrom, and c = 23.6380(10) Angstrom. The location of Na+ cations in the O-18 cages with pentagonal windows is compared to that of Na+ cations in the O-18 cages with hexagonal windows of the monophosphate tungsten bronzes with hexagonal tunnels Na1.7P4W12O44. Electron transport properties of single crystals of both monophosphate tungsten bronzes with pentagonal and hexagonal tunnels are described and discussed in terms of possible charge density wave (CDW) instabilities.

The conductors (PO2)4(WO3)2m show CDW type electronic instabilities. We report electrical properties of the m = 5 crystal variety made of a regular 5/5/5 stacking of WO3 layers. An anomaly at T~60K is observed in the electrical resistivity, Hall coefficient and thermoelectric power. The origin of the anomaly is discussed.

Resistivity and magnetoresistance have been measured on single crystals of the monophosphate tungsten bronzes (PO 1 ) 4 (WO 3 ) 2m for m=5. The m = 5 compound exists in two structural varieties which lead to different resistivity and magnetoresistance properties. Shubnikov de Haas oscillations with large amplitude are observed in one of the variety. The origin of the observed anomalies in the resistivity vs temperature curves is discussed.

A new member, m = 6, Of the series K-x(PO2)(4)(WO3)(2m) has been isolated and studied by single crystal X-ray diffraction and electrical transport measurements. The structure corresponds to that of the MonoPhosphates Tungsten Bronzes with hexagonal tunnels (MPTBh's). It has been solved and refined to conventional R = 0.0268 with 2342 independent reflections with I> 3 sigma(I). The unit cell is monoclinic (space group P2(1)/m) with a = 6.6736(2), b=5.3543(3), c=11.9005(5) Angstrom, beta = 92.615(3)degrees. The oxygen surroundings of K; P and W atoms are described and compared with those of other members of the series including Pb-, Na- and K-based analogues. Electrical properties and correlations with structural data are discussed. Large magnetoresistance effects have been observed at low temperature and assigned to possible CDW instabilities.

The quasi-two-dimensional conductors (PO2)4(WO3)2m show charge density wave type electronic instabilities. The average number of conduction electrons per W atom is 2/m, decreasing with m. We report resistivity and magnetoresistance measurements for the m=7 compound. The upturn of resistivity at low temperature and the field dependence of the magnetoresistance are discussed in terms of weak localization effects.

A new variety of P 4 W 10 O 38 , m = 5 member of the monophosphate tungsten-bronze family with pentagonal tunnels (MPTBp), \(\), has been isolated and studied by X-ray diffraction measurements. The structure has been solved by direct methods from single crystal X-ray data. The monoclinic unit cell corresponds to a regular stacking of WO 3 -type slabs which are all five-WO 6 -octahedra thick and connected through PO 4 tetrahedra. This structure is comparable to that previously described for the MPTBp'series with m = 4 (P 4 W 8 O 32 ) and m = 6 (P 4 W 12 O 44 ). An X-ray diffuse scattering investigation has revealed that this member is subjected to charge density wave instabilities (CDW) located on chains running along the \(\) directions. Two CDW transitions have been observed at \(\) and \(\), bearing some resemblance with those exhibited by the m = 4 member. The corresponding modulation wave vectors can be accounted for by a hidden nesting mechanism which connects the crossing points of differently oriented quasi-planar Fermi surfaces, as found for the low m (4 and 6) members of the MPTB's.

We present a comparative study of the structural instabilities of the monophosphate tungsten bronzes (PO2)(4)(WO3)(2m) With m=5 which exists in two crystal varieties corresponding to different stacking sequences of the conducting layers made of m WO6 octahedra. The variety built with a regular stacking of m=5 (m=5/5) exhibits two charge density wave (CDW) transitions similar to those already found in the regular m=4 and 6 bronzes. On the contrary the variety built with an alternate stacking of m=4 and 6 layers (m=4/6) exhibits an unusual high temperature CDW transition with a non monotonous thermal variation of the satellite intensity and a large hysteresis between the cooling and heating intensity curves. These surprising features could be related to the presence of non equivalent CDW instabilities located in the m=4 and m=6 layers.

The monophosphate tungsten bronzes KxP4W8O32 with K in the pseudo-hexagonal tunnels show instabilities with critical temperatures Tc depending on x and satellite wave vectors independent of x. Tc is found to have a maximum for x~1.30. Possible origins for the alkaline concentration dependence of Tc are discussed.