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The reduction of the remanent magnetization, Mr, induced by low-energy excitations has been studied at very low temperatures ((0.06 ÷ 5) K) in spin glasses for which Tg ranges from 35 to 100 K. A large nuclear Curie contribution to the measured magnetization is evidenced in CuMn below 1 K. The variation of Mr is found to scale as T2 down to the lowest T (10-3 Tg) in CuMn and AuCr. Similar data taken on the Ising spin glass (Fe0.8Ga0.2)2TiO5 yield a quite different behaviour for Mr with a T3-dependence between 0.05 and 0.02Tg and a slower one below 0.02Tg. The origin of the differences between these two cases is discussed.

The salient properties of SmS are reported: the re-entrant shape of the transition line between the divalent black phase and the intermediate valent gold phase, the softening of the lattice in the gold phase, the probable occurence of an insulator-metal transition under pressure in the gold phase. By means of comparison with TmSe, it is suggested that this may be due to a pressure driven crossing of the Sm 2+ and Sm 3+ configurations.

To examine the role of anion ordering for superconductivity we studied the low temperature properties of rapidly cooled (Q-state) and slowly cooled (R-state) states of (TMTSF)2ClO4 through resistivity measurements under pressure. In the R-state the anion ordering remained at 25 K up to 8 kb (maximum studied pressure), while the superconductivity disappeared above 3 kb. In the Q-state, below 4.2 K, we observed resistance upturn below 2 kb and metallic behaviour without superconductivity above 3 kb. The superconductivity was suppressed either by anion disorder or by pressures higher than 3 kb.

The authors report measurements of the Hall effect in (TMTSF)2PF6 under a pressure of 10.5 kbar performed at temperatures from 0.15 to 77K and in magnetic fields up to 9 T parallel to the c* axis. The low-temperature data suggest a series of transitions between semimetallic (SDW) sub-phases above a threshold field. The giant oscillations of the Hall voltage observed above the threshold field suggest for (TMTSF)2PF6 that, unlike in (TMTSF)2ClO4, electrons and holes play important roles. A particularly sharp anomaly of the Hall voltage detected around 7 T may be the signature of a first-order transition between SDW states with transverse and longitudinal nesting, respectively, below and above the transition field.

Nuclear orientation experiments show the different behaviour between the Al 2 Ce and Al 3 Ce compounds. Despite the occurrence of a Kondo coupling, the first compound is well ordered down to 5 mK. No long range ordering seems to exist in Al 3 Ce but crystal field effects can partly mask the ordering.

The superconducting properties of (TMTSF)2ClO4 were found to be highly dependent on the origin of the TMTSF obtained by different synthetic procedures. These differences were assigned to the presence of low amounts of sulfur (up to 0.3%), introduced, in place of selenium, in the course of the synthesis avoiding the use of CSe2. The latter method was improved so the superconducting transition of (TMTSF)2 ClO4 therefrom was found to be the same as for samples prepared with TMTSF synthesized from CSe2.

We report pressure dependent studies of the a-axis resistivity as a function of temperature for several members of the isostructural families of organic charge transfer salts, (TMTSF)2X and (TMTTF) 2X. For a typical (TMTSF)2X material the low temperature metal-insulator transition seen at 1 bar is suppressed above some critical pressure, Pc, where a superconducting transition is observed near 1 K. We find a correlation between Pc and the ambient pressure c lattice parameter which reflects the anion size. The (TMTTF) cX salts exhibit very different ambient pressure behaviour but we find that with the application of sufficiently high pressures (∼30 kbar) their behaviour resembles that seen in the (TMTSF)2X family but at lower pressures. In particular we find evidence of a possible superconducting transition near 4 K in (TMTTF)2Br at 25 kbar. At this pressure the conductivity near 4 K is extremely high with a value approaching 106 (Ωcm)−1 and the resistivity ratio is about 400.

The resistance of single crystals of NbSe3 under pressure, between 1.2 and 70K has been measured in magnetic fields up to 180 kG. The CDW which appears at ambient pressures at 59K, is totally suppressed for pressures greater than 5.5 kbar. At this pressure NbSe3 is fully superconducting as shown by a total Meissner effect. Fermi surface studies by Shubnikov-de Haas oscillations show that the Fermi surface is strongly affected by the CDW formation. At ambient pressure, when H is parallel to the plane (b, c) one frequency (0.3 MG) is detected, but when the pressure has suppressed the CDW, at least eight frequencies (up to 7 MG) were detected. The experiments are consistent with the model of different types of chains in NbSe3 on which the two CDW form.

After a short summary of the nuclear orientation results obtained on the cerium compounds, we discuss their low temperature properties mainly the phase diagram magnetic field-critical temperature.

At low temperature, in (TMTSF)2ClO4, first order phase transitions are detected in the (40 – 90) kOe magnetic field range. In a fixed magnetic field, the transition from the high temperature metallic state to the low temperature semi-metallic state is a second order phase transition. The general features of the Hall voltage in (TMTSF)2PF6 are similar to those observed in (TMTSF)2ClO4 but in the former material a large number of subphases appears. The small number of fully developed phases in (TMTSF)2ClO4 results from defects induced by anion ordering.

The authors report resistivity measurements along the high-conductivity axis of (TMTSF)2ClO4, (TMTSF)2TaF6 and (TMTSF)2SbF6 for various hydrostatic pressures up to 12.5 kbar, and for transverse magnetic fields of up to 50 kG, in the temperature range 100 mK-4K. These measurements show that all three compounds have a superconducting phase transition near 1K when sufficient pressure is applied (TMTSF)2SbF6 and (TMTSF)2TaF6 become superconducting for pressures above 10 kbar and 11 kbar respectively. At lower pressures a metal-insulator phase transition is observed which is probably associated with a spin density wave (SDW) instability. They observe a superconducting phase transition in (TMTSF)2ClO4 at much lower pressures (

We report the study of the Hall effect, at low temperature, in (TMTSF) 2 ClO 4 (slow-cooled state), up to 80 kOe, with the field perpendicular to the (a-b) plane. We show that the linear field dependence of the weak Hall resistance, up to 40 kOe, supports the interpretation of the conducting phase in terms of a quasi planar Fermi surface with a density of carriers corresponding to the stoechiometry of the salt. At higher field, a strong and sharp increase of the Hall constant, is the signature of a phase transition to a semi-metallic state. The transition magnetic field is very temperature dependent. Below 0.7 K, in the semi-metallic state, remarkable steps and plateaux, of the Hall resistance are observed, suggesting either the quantization of the Hall constant of carriers in weakly coupled (a-b) planes or the existence of a sequence of electron instabilities (excitonic phases) Etude de l'effet Hall a basse temperature apres refroidissement lent, jusqu'a un champ de 80 kOe perpendiculaire au plan a-b. Variation lineaire avec le champ de la tres faible resistance de Hall jusqu'a 40 kOe, ce qui indique l'existence d'un conducteur a surface de Fermi quasi plane et une densite de porteurs correspondant a la stoechiometrie. Croissance tres brutale de la constante de Hall en champ eleve, qui correspond a une transition vers un etat semi metallique, avec marches et plateaux de la resistance de Hall au-dessous de 0,7 K. Interpretation soit par la quantification de la constante de Hall des porteurs bidimensionnels a-b, soit par l'existence d'une serie de transitions du type electron-trou (phases excitoniques)

Systematic thermogravimetric measurements under variable oxygen pressure followed by magnetic determination of the transition temperature of the superconducting compound YBa 2 Cu 3 O x yield the phase diagram of T c versus the oxygen content for 6 Etude par des mesures thermogravimetriques sous pression d'oxygene variable suivies de mesures de l'effet Meissner, pour x compris entre 6 et 7,0; mise en evidence d'un systeme biphase a faible quantite de phase supraconductrice pour x 6,6. Accord qualitatif avec la theorie BCS dans l'hypothese d'une bande rigide 3D; temperature de Debye, parametre de couplage NV

Magnetization and nuclear orientation experiments on dilute alloys of $\mathrm{Pd}\mathrm{Mn}$ allow the first observation of the giant moment of the manganese impurity down to 10 mK without Kondo-like coupling. Below 10 mK, spin-glass magnetic interactions occur due to iron parasitic impurities even for an alloy containing no more than 3 ppm of iron. The nuclear orientation results are compared with those obtained for a $\mathrm{Au}\mathrm{Mn}$ alloy which has almost the same hyperfine constant.

We report measurements of the resistive superconducting transitions of NbSe3 as a function of magnetic field for sufficient pressures to destroy the lower CDW. These indicate that NbSe3 is a bulk anisotropic superconductor with a total Meissner effect. The critical field anisotropies are well described by an effective mass model and the values obtained are in good agreement with those obtained by conductivity or Shubnikov-de Haas measurements. At ambient pressure we have measured zero-resistivity transitions for compacted samples, but do not find a total Meissner effect. Hence this superconductivity is not a bulk effect but is probably associated with barriers between domains in the sample.

Apres avoir rappele ce qu'est l'effet de Haas-van Alphen nous discutons ses conditions de mise en evidence sous pression. Les realisations experimentales presentees correspondent a toutes les etapes de ce type de mesure : preparation de monocristaux par distillation d'ytterbium sous ultravide, utilisation d'enceintes de pression comprenant des passages de courant etanches jusqu'a 12 kbar, realisation de sondes de mesure a faible bruit.Nous presentons les premieres mesures jamais realisees d'effet de Haas-van Alphen dans l'ytterbium cubique face centree. Dans une direction voisine de l'axe nous avons mesure une frequence d'oscillation variant lineairement en fonction de la pression depuis 5,7 MG a 4 kbar a 2,3 MG a 8,6 kbar. La masse effective cyclotron des porteurs varie peu sous pression de (0,97 ± 0,03) m0 a 4 kbar a (1,11 ± 0,05) m0 a 7,4 kbar. La temperature de Dingle ne peut etre tiree des mesures realisees qu'apres interpretation, nous l'estimons a (3 ± 0,8) K.Nous rapprochons ensuite l'ytterbium des metaux alcalino-terreux et interpretons nos resultats dans le cadre des calculs de bandes de S. L. Altmann, A. R. Harford et R. G. Blake. Nous trouvons un bon accord entre notre interpretation et les mesures relatives a la densite d'etats : chaleur specifique electronique et susceptibilite, le desaccord constate dans le cas des proprietes de transport est justifie simplement.

The observation of Shubnikov-de Haas (SdH) oscillations in the metallic, high pressure phase (Metal-II) of TMTSF-DMTCNQ (P > 10 kbar) demonstrates the 3-dimensional, closed nature of the Fermi surface at low temperature (T < 2 K). We observe very small Fermi surface cross-sections of 1.16 x 1017 m-2 and 2.6 x 1017 m-2 in the ab and ac planes respectively. From the field and temperature dependence of the SdH oscillations we extract a cyclotron mass mc/m0 ≃ 0.56 and a Dingle temperature TD = 0.8 K. Finally we discuss the discrepancy between these results and the very high dc conductivity (σ 1.2K ≃ 3 x 105 (Ωcm)-1).

Electron tunnelling data in (TMTSF)2 X-GaSb Schottky barriers support the idea that strong superconducting fluctuations exist in quasi-one-dimensional (TMTSF)2X up to temperatures which are about one order or magnitude higher than the 3-D ordering critical temperature. Other tunnelling data tend to indicate that stabilization of superconductivity is possible at high temperature via chain cross-linking.

High pressure resistivity measurements reveal a very large decrease of the superconducting transition temperature of (TMTSF)2PF 6 with pressure. It is difficult to explain this phenomenon with current theories of narrow band superconductors. An explanation in terms of the soft mode of the low pressure metal to insulator transition is proposed.

Superconductivity has been observed in a second quasi-one-dimensional organic conductor : (TMTSF)2AsF6, at a temperature of 1.1 K under 12 kbar. The establishment of the superconducting state is apparently not related to the existence of a soft external phonon mode at 2 k F, since under atmospheric pressure X-ray diffuse scattering fails to detect any Kohn anomaly at low temperature.

Temperature dependence of resistivity (up to 24 Kbar) and of E.S.R. (at 1 bar) measurements suggest that centered monoclinic TTF[Pd(Dmit) 2 ] 2 could be an anisotropic and weak antiferromagnetic fluctuating metal. At low pressure, gradual charge and spin localizations at different temperatures are the signature of a Mott-Hubbard insulator. Above 20 Kbar, “high” superconducting onset temperatures (6.5 K) are observed.

We have measured the current-voltage characteristic of a point contact on a sample of Sr-doped La2CuO4. Josephson junction characteristics are observed even when the metal tip is nonsuperconducting, a result that indicates that the junction is inside the material under the tip. Under microwave irradiation, we observe well-defined Shapiro steps which yield the usual value h/2e for the coupling constant between the quantum phase and the electromagnetic field. Preliminary measurements on samples of Y1Ba2Cu3O9-x also yield h/2e. We conclude that in these newly discovered materials, the superconducting state is formed of paired electrons.

The diamagnetic shielding and the Meissner effect measured on a single crystal of (TMTSF)2ClO4 are strongly anisotropic increasing from the a to b* and c* directions. In very low magnetic fields the diamagnetic shielding is complete for all directions. In the a direction the Meissner effect never stands higher than 1% of its expected value whereas it reaches 80 % in the c* direction. The aimantation curves versus magnetic field can be explained by pinning effects, but overestimated values of H c1 are obtained. The Meissner effect and the Ginzburg-Landau parameter for the a direction show that this direction is distinct.

Reports resistivity against temperature measurements along the high-conductivity a axis of TMTSF2CIO4, TMTSF2SbF6 and TMTSF2TaF6, under pressure and as a function of applied magnetic field, that show that all three compounds exhibit superconducting phase transitions near 1K, when sufficient pressure is applied. TMTSF2SbF6 and TMTSF2TaF6 become superconducting above critical pressures of approximately 10 and 11 kbar respectively whereas the authors observe a superconducting phase transition in TMTSF2ClO4 at much lower pressures (

We have measured, via electrical transport measurements, the effect of pressure on the anion ordering (AO) transition in (TMTSF)2BF4. The effect of low pressures (

We present an experimental study of the organic alloy (TMTSF) 2 (ClO 4 ) x , 0 ⩽ x ⩽ 1. The results show an important influence of nonmagnetic impurities on the low temperature behaviour of the electron gas and, in particular, on the stabilization of the superconducting ground state.

The (0 1 2 1 2 ) anion ordering's kink observed at 205 K in the resistivity curve of (TMTSF)2ReO4 under a pressure of 14 Kbar is related to the rapid Shubnikov de Haas oscillations observed just below 7.5 Tesla by magnetoresistance and Hall effect measurements.

Superconductivity is observed at 1.62 K by resistive measurements on the three dimensional molecular conductor tetrathiofulvalene bis-[bis-(4,5 dimercapto- 1,3- dithiole -2- thione) Nickel (II)] under an hydrostatic pressure of 7 kbar. Resistivity p decreases linearly between 300 and 30 K and then follows a T 2 law down to 5 K where a metallic state is stabilized with p; = 15 μωcm. The width of the superconductive transition is rather large (.57 K) and sample seem to be inhomogeneous. At 400 mK, the critical current density is greater than 12 A/cm 2 . Transverse magnetic fields parallel to the c axis of 1.3 and 2.8 T restore the normal state at 1.08 K and 400 mK respectively. Specific features of this molecular superconductor are compared with quasi Id organic conductors behaviours.

Above a temperature dependent threshold field Hth, a transverse magnetic field H parallel to c∗ axis induces giant oscillations in the Hall voltage VH of (TMTSF)2PF6 under an hydrostatic pressure of 10.5 Kbar.Hysteresis are observed only when the Hall voltage goes from condensed holes (VH O) : this is the signature of first order transitions between various SDW sub-phases, as for (TMTSF)2ClO4 in the relaxed state. But each SDW sub-phase is itself subdivided by a weakly first order transition driving the transverse nesting vector of the Fermi surface from an electron pocket to an hole one, as for (TMTSF)2ClO4 in the hyper-relaxed state. At a given H, the amplitude of VH increases and then saturates with decreasing temperature. Although this suggests a semi-metallic behaviour, the compound is never compensated due to the alternate condensation of electrons and holes with H. These features are compared with the Hall voltage results in (TMTSF)2ClO4.

We discuss the transverse magnetoresistance of nickel at low temperature and its dependence on residual impurity. It appears from our results on some nickel dilute alloys that the difference between the relaxation times of spin ↑ and spin ↓ conduction electrons is a dominant mechanism for the magnetoresistance.

The nuclear orientation results obtained under pressure for 3d impurities dissolved in the f.c.c. Ytterbium are compared with magnetization and de Haas Van Alphen experiments performed on the matrix. Open questions are the description of the 3d impurities at the metal semiconductor transition and the origin of the magnetic carriers of the matrix.

2014 Using a very crude model, we calculate at 0 K the magnetic ground state of the cerium intermetallic compounds and derive the corresponding value of the linear temperature coefficient of the specific heat. In the ordered systems, an abnormal point appears due to the variation of the symmetry. This point seems to reflect the same property as the Kondo point of the local magnetism problem. LE JOURNAL DE PHYSIQUE LETTRES TOME 39, 1 er AVRIL 1978, Classification Physics Abstracts 75.30

We have developed high pressure techniques connected with dilution refrigerators down to 8 mK. We present here two complementary aspects of this work : i/ pressure cells able to maintain, on solids, pressures up to 20 kbar in an experimental diameter of 3 mm, for an over all diameter down to 10 mm ; ii/ a hydraulic press used down to 10 mK developing loads up to 1.2 x 103 kg and having an external diameter of 50 mm.

Analysis of the temperature-pressure phase diagram of the centered monoclinic phase of TTF(Pd(dmit) 2 ) 2 reveals the absence of 1D instabilities such as Spin Peierls and Spin Density Wave ground states. At low pressures and between 70 and 250 K, a gradual localization of the carriers leads to a small gap semi-conductor or to a semi-metal. At high pressures, a dirty superconductivity sets in below 6 K and seems compatible with Charge Density Wave instabilities observed at 1 bar Les instabilites 1D de type Spin Peierls et Onde de Densite de Spin sont absentes du diagramme de phase (temperature, pression) de la phase monoclinique centree de TTF(Pd(dmit) 2 ) 2 . A basse pression, une localisation graduelle des porteurs apparait a 70 K

Nuclear orientation experiments on In3Ce, Pb3Ce, Sn3Ce show that the first compound is well ordered, that Sn 3Ce is a Kondo lattice and that further experiments must be performed in order to confirm the Kondo state of Pb3Ce. A simple interpretation is proposed to explain the behaviour of these abnormal compounds.

We report a resistive investigation of superconductivity in the system La2 - xSrxCuO4 - y on samples which have been analysed by thermogravimetric techniques for oxygen deficiency determination. A maximum superconductive transition temperature is observed in La2 - xSr xCuO4 - y near the tetragonal-orthorhombic phase transition, together with a resurgence of high temperature superconductivity in non-substituted La2CuO4 - y. Drastic localization effects are observed in the vicinity of x = 0.04 with no sign of superconductivity at low temperature. These observations are coherent with a simple BCS scheme with a band filling dominated by Sr-doping and localization effects dominated by an interplay between 0 vacancies and Sr-doping. This situation leads to half-filling of the band near x = 0.04 with enhanced electron localization via Umklapp scattering through a near nested Fermi surface. Moreover, our data suggest some contribution of antiferromagnetic fluctuations to the superconducting pairing mechanism.

Centre de Recherches sur les Tres Basses Temperatures, C.N.R.S., BP 166 X, 38042 Grenoble-Cedex, France. (Laboratoire associe a l'Universite Scientifique et Medicale de Grenoble). + Permanent address : IBM Watson Center, Yorktown Heights, NJ 10598, USA. * Permanent address : Laboratoire de Physique des Solides, Orsay, France. Resume.- Nous decrivons des mesures de resistivite d'un echantillon presque stoechiometrique sous des pressions allant jusqu'a 6 kbars, jusqu'a une temperature de 25 mK. A 6 kbars, la resistivite atteint 120 mfl.cra a 30 mK alors que, au-dessus de T„, elle est de 1,65 mfl.om a 4,2 K. La resistivite croit exponentiellement avec la pression a tres basse temperature et lineairement au voisinage de T . Nous comparons ces resultats a la variation sous pression de l'aimantation a'un sous reseau mesuree par diffraction neutronique et a la resistivite d'autres composes de valence intermediaire. Abstract.- Resistivity measurements of a nearly stoichiometric sample up to 6 kbar and down to 25 mK are reported. For 6 kbar , the resistivity p rises up to 120 mSJ.cm at 30 mK whereas just above the ordering temperature T at 4.2 K its value is 1.65 mfl.cra. The increase of p is exponential with the pressure at very low temperature and linear in the vicinity of T . Comparisons are made with the pressure dependence of the sublattice magnetization measured by neutron diffraction and with resistivity of other intermediate valence compounds. The low temperature properties of the intermediate valent (IV) compound TmSe have been studied extensively in the last few years /1-7/. The occurence of a magneti­cally ordered ground state and the main features of the magnetic phase diagram have been well documented by various experimental methods ; in particular, the magnetic structure in zero magnetic field was shown by neutron diffraction to be simple type I antiferromagnetic (AF) /2/. As in many IV systems, the electrical transport properties are very anomalous due to the presence of a narrow 4f band at the Fermi level. The resistivity of nearly stoichiometric TmSe in the paramagnetic state increases on cooling from % 200 u°,.cm at room temperature to about 1500 u£2.cm at 4.2 K /4-5/ with a n almost logarithmic term between 6 and 40 K. This has been argued on qualitative grounds to originate from the Kondo effect. In zero applied field, the resistivity p jumps at T and increases very rapidly down to 10 mK where it may attain 14 inn.cm /6/. The intrinsic amplitude of this anomaly is unknown since it is dramatically reduced by small deviations from stoichiometry. This effect is destroyed by the magnetic field ; above the critical field of the metamagnetic transition, the resistivity in the FM phase lies below the RT value /7/. There is presently no definitive explanation for this rather sharp transition to a much higher resistivity below T at H = 0. The few pictures which have been proposed previously fall in two categories : i) those which consider the AF structur e to be responsible for the poor conductivity below T ; the situation would be closed to that described by Adler /8/ for Ti