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Two decades after the discovery of superconductivity in a Bechgaard salt, the pairing mechanism is not yet identified. The first step towards such an identification is to determine the symmetry of the superconducting order parameter. Results on heat transport in the superconducting state of (TMTSF) 2 ClO 4 indicate a finite gap all over the Fermi surface. The magnitude of this gap is confirmed by the first experiments done on break junctions. A nodeless gap imposes a number of constraints on the symmetry of the order parameter

With the advent of PSA dosing, an increasing number of prostate cancers are being detected at a local stage. Since 1989, our group has been developing a research project with the aim of establishing treatment of localised prostate cancer by means of HIFU. The treatment is performed transrectally, using ultrasound imaging guidance only. The quality of HIFU treatment depends on four factors: the intensity of the transmitted pulse, the exposure time, the signal frequency, and the time between two firing bursts. The lesions are created by a thermal effect. Their slightly conical form is due to the absorption of ultrasound by tissue, enhanced by cavitation bubbles. Results obtained since 1993 demonstrate that transrectally administered HIFU treatment achieves local control of localised prostate cancer in 80% of cases, with 70% complete success and 30% partial response. The use of an annular array probe with variable focus and frequency should significantly improve results in the future. Finally, real time visual display of the damaged tissue via differential imaging of the attenuation coefficient should give the surgeon an instant appreciation of the result of the sequence. It would thus be possible to repeat treatment of insufficiently covered zones in the same session.

High intensity focused ultrasound (HIFU) is an effective technique for creating coagulative necrotic lesions in biological tissue, with a view to treating localized tumors. Although good results have already been obtained, notably in urology, current systems lack a real time monitoring system to check the efficacy of the treatment procedures. This study describes the development and assessment of a noninvasive system for making local measurements of attenuation variations during HIFU treatment procedures. An apparatus (Ablatherm, Edap-Technomed, France), combining a 2.5 MHz therapeutic transducer and a 5.5 MHz twin plane imaging probe (connected to an ultrasound scanner), was used to produce lesions. The rf signals needed to calculate the attenuation were recorded as outputs from the ultrasound scanner, before and after the high intensity firing sequences, which were performed on ten pieces of porcine liver. Each firing sequence involved producing a lesion volume comprising 42 individual lesions. A number of recordings were also made without producing lesions, in order to test the reproducibility of the measurements. The attenuation function was evaluated locally using the centroid and the multinarrowband methods. Initially, changes in the integrated attenuation αbar; (mean attenuation in the 4–7 MHz range) and the attenuation slope β were examined for the lesion volume. β values did not vary significantly within this range, whereas α values varied significantly (in the region of 86% of the initial level) in comparison to measurements performed without forming lesions. The differential attenuation Δα (representing local variations in α) was subsequently used to generate images revealing the lesion areas. There was a strong similarity between these ‘Δα images’ and the lesion volumes defined by the operator. ‘Δα images’ offer several advantages over existing attenuation imaging techniques. Any problems related to the heterogeneity of the medium are eliminated, since only the change in attenuation is taken into account. Furthermore, there is no need to compensate for diffraction when estimating Δα, as the rf signals are captured in exactly the same positions before and after treatment. This technique can be used during in vivo treatment procedures. It can be implemented in real time, since the computational algorithms (based primarily on FFT calculations) are very fast. The technique should provide clinical practitioners with valuable qualitative and quantitative information for use in HIFU ultrasound surgery.

The thermal conductivity of a YBCO single crystal has been studied as a function of the relative orientation of the crystal axes and a magnetic field rotating in the Cu-O planes. Measurements were carried out at several temperatures below T_c and at a fixed field of 30 kOe. A four-fold symmetry characteristic of a superconducting gap with nodes at odd multiples of 45 degrees in k-space was resolved. Experiments were performed to exclude a possible macroscopic origin for such a four-fold symmetry such as sample shape or anisotropic pinning. Our results impose an upper limit of 10% on the weight of the s-wave component of the essentially d-wave superconducting order parameter of YBCO., Comment: 10 pages, 4 figures

The metallic state of a high-quality (TMTTF)2 Br sample was stabilized under a pressure of 26 kbars. The magnetoresistance of the system was then studied up to 20 teslas for a field rotating in the least-conducting plane. The anisotropy of the magnetoresistance was found to be different from what had been observed in the TMTSF family. For a field along the c-axis, a small kink in the magnetoresistance at H = 13 T is detected which may be the first signature of the appearance of the FISDW states.

The study of the magnetoresistance in the organic quasi-one-dimensional conductor (TMTSF)2NO3 at a pressure (8 kbar) exceeding the critical pressure required for the stabilization of a conducting state at low temperature has revealed the existence of quantum oscillations with a frequency of 190 T in good agreement with an extended Huckel-type calculation of the band structure. Unlike other members of the (TMTSF)2X series the ambient pressure SDW ground state in (TMTSF)2NO3 can be understood in terms of an instability involving the nesting of 2D semi-metallic Fermi surfaces.

We present a study of the angular dependence of the magnetoresistance of the quasi-one-dimensional organic conductor (TMTSF)2PF6. For some special orientations of magnetic field in the least-conducting plane, a commensurability effect in the electronic open-orbit motion is expected. At these special “magic angles”, dips in the resistance along the least-conducting axis in both metallic and field-induced-spin-density-wave (FISDW) states are detected. In the latter states, the dips gradually fade away with the increasing field. Moreover, the commenusurability effect intereferes directly with FISDW transitions: For magic angles, the threshold field goes through a weak minimum and the amplitude of the FISDW magnetoresistance oscillations is diminished.

We report magnetotransport measurements in the quasi one dimensional (Q-1-D) organic conductor (TMTTF) 2 Br at pressures up to 26 kbar, down to 0.45 K in magnetic fields up to 19 T along the c * direction. It is found that a superconducting ground state is stabilized under 26 kbar at T c =0.8 K. No magnetic field induced spin density wave (FISDW) transitions are observed below 19T unlike other Q-1-D superconductors pertaining to the selenium series. The computed amplitude of the interchain coupling along transverse directions is unable to explain the missing FISDW instability

In very anisotropic materials the transport properties of a single crystal may be equivalent to those of a long thin bar. In this geometry we show that an extension of the Montgomery method allows a detailed discussion of the experimental results. It is then possible to establish the correct value of the anisotropy of the resistivity tensor. The method is used to show that in La 2 CuO 4+0.018 , the conductivity is activated in the Cu-O planes while it results from variable range hopping process perpendicular to the planes as previously established

The angular dependence of the magnetoresistance along the least conducting axis has been investigated in both metallic and field-induced spin-density-wave (FISDW) states of the quasi-one-dimensional organic conductor (TMTSF)2PF6. Strong dips in the resistance occur for Lebed's magic field orientations associated to resonant effects in the open-orbit electron motion. The dips exist in both the metallic and the FISDW states but gradually fade away with increasing field in the latter state. The study of the FISDW phase transitions as a function of field orientation confirms the existence of a minimum for the threshold field in the vicinity of a magic angle. Such an anomaly is not observed in the angular dependence of successive transition fields within the FISDW state.

We report on transport properties of (EDT-TTF) 2 [Pd(dmit) 2 ] 2 . The occurence of a resistivity anomaly and of a metallic ground state, which is stabilized under pressure, is discussed within tight-binding band structure calculations.

We have measured the resistivity perpendicular to the CuO 2 planes, in the magnetically ordered phases of a single crystal of La 2 CuO 4+y as a function of temperature. Within a limited temperature range a one-dimensional variable range hopping conduction mechanism is identified, in zero magnetic field. The analysis of both the temperature range and the magnetic field variation shows that this mechanism is strongly dependent on the intraplane and on the interplane couplings.

This letter reports upon transport properties of the (EDT-TTF)2 [Pd(dmit)2]2 compound. A nonlinear effect is observed below 50 K at ambient pressure. A resistivity bump, observed at ca. 40 K at ambient pressure, is shifted towards lower temperature under pressure and a metallic ground state is stabilized up to 10 kbar. In order to understand lack of superconductivity above 500 mK, low-dimensional electronic correlations are stressed on.

The pressure-temperature phase diagram of the molecular superconductor TTF(Ni(dmit) 2 ) 2 was determined by a.c. resistivity measurements up to 14 kbar. Increasing pressures induce electronic phase transitions between a high temperature metal and successive, semimetallic, insulating and reentrant superconducting ground states. This phase diagram is discussed in connection with ambient pressure charge density wave (CDW) instabilities. Their wave vector can be well accounted for by an original conduction band structure. This structure involves both the partially filled HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) bands of the acceptor slabs. The superconductivity coexists with a high temperature CDW instability and is in weak competition with low temperature CDW fluctuations. This explains the pressure increase of the superconducting temperature T c .

The pressure-temperature phase diagram of the quasi-one-dimensional molecular superconductor TTF [Ni(dmit${)}_{2}$${]}_{2}$ has been carefully determined by ac resistivity measurements up to 14 kbar. Increasing pressures induce electronic phase transitions between a high-temperature metal and successive, metallic or semimetallic, semiconducting and reentrant superconducting ground states. This unusual phase diagram is compared with that of TTF [Pd(dmit${)}_{2}$${]}_{2}$. It is discussed in connection with ambient-pressure charge-density-wave (CDW) instabilities, the wave vector of which can be well accounted for by an original conduction-band structure involving both the highest occupied molecular orbitals and the lowest unoccupied molecular orbitals of the acceptor slabs. It is confirmed that the superconducting temperature increases slowly with increasing pressure. It is suggested that the superconductivity coexists with a high-temperature CDW instability and is in weak competition with low-temperature CDW fluctuations; these CDW's affect different parts of the Fermi surface.

We have carried out a comparative study of the thermal conductivity of two rectangular single crystals: a niobium sample and a detwinned YBCO. Thermal conductivity was measured as a function of the relative orientation of the crystaline axes and a magnetic field applied parallel to the sample plane. Different angular variations, two-fold for Nb and four-fold for YBCO, were observed in the two case. We discuss the microscopic effects related to the gap structure which are at the origin of this qualitative difference in the behavior of thermal conductivity in these two superconductors.

We have carried out a comparative study of the thermal conductivity of two rectangular single crystals: a niobium sample and a detwinned YBCO. Thermal conductivity was measured as a function of the relative orientation of the crystaline axes and a magnetic field applied parallel to the sample plane. Different angular variations, two-fold for Nb and four-fold for YBCO, were observed in the two case. We discuss the microscopic effects related to the gap structure which are at the origin of this qualitative difference in the behavior of thermal conductivity in these two superconductors.

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Summary form only given. We studied the Fermi surface of the organic conductor (TMTSF)/sub 2/NO/sub 3/ through the measurements of magnetic quantum oscillations. Two different families of oscillations are observed. Well defined Shuvnikov-de Haas(SdH) oscillations with frequency of ca. 60T can be observed below 8K and above 6T at the ambient pressure. The higher frequency oscillations reported elsewhere is not visible be low 12T. The angular magnetoresistance does not show any structure neither at Yamaji's magic angles nor at Lebed's resonance condition. Only the SdH oscillations for the field scaled by Cos /spl theta/ are added and easily separated. Under a pressure of 10kbar, different oscillations with frequency of ca. 200T is observed above 10T at 450mK. Even though it is the same frequency as so called rapid oscillations observed in other Bechgaard salts, angular dependence of oscillation frequency does not follow the simple cosine law contrary to that of (TMTSF)/sub 2/CIO/sub 4/. The amplitude of oscillations depend very oddly with sample orientation, too. Both suggest that the electron structure at low temperature is strongly modified upon application of pressure which may explain the absence of superconductivity and the field-induced spin-density waves. The angular magnetoresistance shows actually three maxima at angles which are not determined from the known theories.

Superconductivity has been observed by resistive measurements in the quasi-one dimensional organic conductor di-(tetramethyltetraselenafulvalene)-hexafluorophosphate, (TMTSF)2PF6 under a hydrostatic pressure of 12 kbar with a transition temperature of 0.9 K. The effect of magnetic fields perpendicular to the chain axis is also reported.

We present the first study of thermal conductivity of a quasi two dimensional organic superconductor. κ-(BEDT-TTF) 2 Cu(NCS) 2 presents features which have already been detected in the high T c cuprates. Notably we observed an increase of the thermal conductivity at the superconducting transition which is suppressed by a moderate magnetic field. At low temperatures we clearly resolved a residual electronic contribution close to the universal limit value which is consistent with an anisotropic pairings with lines of nodes in the superconducting gap function.

We present a study of the thermal conductivity of a detwinned YBa 2 Cu 3 O 6.9 single crystal as a function of the angle between the crystal axes and a rotating magnetic field. Measurements were done at several different temperatures below T c and at a fixed field of 30 kOe. When the field rotates in a plane parallel to the CuO planes, thermal conductivity shows a four-fold angular variation with maxima at 45 degrees off the principal symmetry axes. This is caracteristic of an anisotropic superconducting gap associated with a d x 2 − y 2 order parameter. An upper limit on the weight of the possible s-wave component can be deduced from the angular positions of the extrema.

The thermal conductivity of high-quality untwinned single crystals of YBa2Cu3O6.9 was studied as a function of temperature (0.1 K < T < 1 K) and with heat current injected along two different orientations (a and b crystalline axes). We detect the presence of a linear term with comparable magnitude for the two orientations. This is in contrast with the high-temperature data, where an anisotropic behaviour is reported. The possible electronic origin of the linear term and its implication for the gap symmetry are discussed.

We studied the Fermi surface of the organic conductor (TMTSF) 2 NO 3 through the measurements of magnetic quantum oscillations. Well defined Shuvnikov-de Haas(SdH) oscillations with frequency of ca. 60T can be observed below 8K and above 6T at the ambient pressure. Under a pressure of 8kbar, different oscillations with frequency of ca. 180T is observed above 10T at 450mK. Electron structure at low temperature seems strongly modified upon application of pressure which may explain the absence of superconductivity and the field-induced spin-density waves. The angular magnetoresistance could not be fit neither by Yamaji's model nor by Lebed's.

Specific heat measurements on CeAl3 are described and compared to thermal‐expansion results. At very low temperatures, the Gruneisen parameter reaches giant negative values. Comparisons are made with the properties of 3He and the unstable‐valent 4f compounds (YbCuAl, CeBe13, CeSn3, and CePd3). It is suggested that the magnetic anisotropy may play a major role.

Observation of a large pseudo-gap at the Fermi level by Schottky tunnelling of electrons from N doped-GaSb into the organic superconductor (TMTSF) 2PF6 under a pressure of 11 kbar (Tc = 1 K) establishes, on an unambiguous experimental basis the one-dimensional character of the low temperature superconducting divergence and so justifies the presence of 1-D superconducting fluctuations to high temperatures. The intrachain pairing energy 2 Δ(0) = 3.6 meV derived from tunnelling characteristics emphasizes the strength of the superconducting channel in the family of organic superconductors (TMTSF)2X.

Datas for magnetoresistance of a single crystal of La 2 CuO 4−y are presented; this effect has been studied as a function of temperature and applied field. A discussion of the various models of magnetism in this crystal is added as far as experimental datas are involved.

The comparison between the disordered metallic state created by irradiation and the metallic regime stabilised by high pressure in TMTSF-DMTCNQ ands TMTSF2-PF6 establishes the existence of a contribution from superconducting fluctuations to the conductivity below 50K. A calculation using Ginzburg-Landau theory is in very good agreement with the experimental temperature dependence of the paraconductivity. The authors show that the metal-insulator transition in TMTSF2-PF6 is shifted by the disorder to a lower temperature. However, the effect is less for the spin-density-wave transition in TMTSF-PF6 than for the charge-density-wave ordering in TMTSF-DMTCNQ. Finally they show that the superconductivity transition is strongly depressed by irradiation-induced defects.

The (Y3+1−xCa2+x)(Ba2+2−xLa3+x)Cu3O7 system is investigated by X-ray diffraction on powders and single crystals. The homogeneity range is limited to x≈0.5, after which the La substitution occurs on both yttrium and barium sites. The pseudo “Ca(BaLa)Cu3O7” (x=1) corresponds to a mixture of (Ca0.5La0.5)(Ba1.25La0.75)Cu3O7.125 and CaCuO2. All the materials exhibit a superconductive transition, broadened by disorder, with 90 K>Tc≥75 K. The Meissner effect is large, ≈60%, for the ideal materials (xCa=xLa) and decreases for the compounds with xCa

In nuclear orientation experiments performed on 3d radioactive ions of 54Mn, 51Cr and 60Co dissolved in FCC ytterbium, the occurrence of the 3d magnetism is exactly that found for divalent matrices. At 4.2K the magnetisation of pure ytterbium varies linearly up to 130 kG. These measurements are in agreement with the existence, in a divalent matrix, of localised Yb3+ condensed at low temperature in a Kondo state. The authors discuss the possible origin of these Yb3+.

Below 1 K the thermal expansion coefficient α of Al3Ce is negative and linearly approaches zero at 0 K. The pressure variation of the specific heat yT deduced from the initial variation of α is very strong : ∂γ/∂P = 0.62 J/K2 mole kbar . These properties cannot be interpreted with a single ion Kondo effect model but can be interpreted in terms of Fermi liquid (F.L.) behaviour. The magnetostriction experiments clearly show the differences between an ordinary F.L. such as 3He and the F.L. built with a lattice of magnetic ions coupled with and by itinerant electrons.

Quasihydrostatic pressure resistivity experiments on a single crystal of SmS are reported up to 27.5 kbar. At low temperature, a regime change occurs at P ∼ 20 kbar, between a “quasiinsulating” behavior (P

A detailed investigation of the magnetic and transport properties of La/sub 1.85/Sr/sub 0.15/CuO/sub 4/ in the superconducting state is presented. It is concluded that the magnetic and transport behavior is that of a granular type-II superconductor in which strongly superconducting grains are coupled via weak superconducting links. Five characteristic fields are identified. H/sub c//sub 1//sup w/ (approx. =1 Oe at 4 K) is the field at which the first vortex line penetrates the weak links. At H = H/sup w/ (approx. =11 Oe at 4.2 K) the London penetration depth in the weak links is very large compared to their dimensions. H/sub c//sub 1//sup g/ (approx. =300 Oe at 4.2 K) defines the first flux thread entering the grains themselves. H/sup g/ is the value above which the magnetic behavior of the grain is thought to be governed by Abrikosov's negative surface energy model. The critical current density measured by the resistivity technique is about 8 A cm/sup 2/ and is imposed by the sample size and compositional inhomogeneities and by H/sub c//sub 1//sup w/, whereas the bulk (or intragrain) critical density could attain 10/sup 5/--10/sup 6/ A/cm/sup 2/.