Low-temperature properties of pseudoternary R(Rh1−x Ir x )4B4 (R=Ho and Dy) systems

Pseudoternary Ho(Rh_1–x Ir_x)₄B₄ and Dy(Rh_1–x Ir_x)₄B₄ systems were investigated with respect to the interplay between superconductivity (SC) and long-range magnetic ordering by means of specific heat (C) and upper critical magnetic field (H _c2) measurements. For Ho(Rh_1–x Ir_x)₄B₄ withx0.2 and Dy(Rh_1–x Ir_x)₄B₄ withx0.15, ferromagnetic (FM) ordering, reminiscent of spin-1/2 mean-field theory, was observed. In particular, FM ordering in Ho(Rh_1–x Ir_x)₄B₄ with 0.07x0.2 was found to destroy SC at a second critical temperatureT _c2 below the superconducting transition temperatureT _c, and was accompanied by a spike-shaped feature inC atT _c2. Coexistence of antiferromagnetic (AFM) ordering with SC was observed in both systems for 0.2x0.7. The compounds withx0.5 exhibited Néel temperaturesT _N that were smaller thanT _c and enhanced values ofH _c2 belowT _N. TheT _c andT _N undergo an abrupt decrease and increase, respectively, nearx0.5, and then cross atx0.6, above whichT _N>T _c. For Ho(Rh_1–x Ir_x)₄B₄ with 0.275x0.4, two different AFM transitions were detected in a double-peak feature inC.     

Direct evidence for the first-order phase transition at the lower critical temperature in Er1−x Ho x Rh4B4

Using a heat-pulse and a relaxation technique, we have performed an extensive calorimetric investigation on the reentrant superconductors Er_1–xHo_xRh₄B₄ (x=0.4 and 0.6) around their lower critical temperature. These experiments revealed a supercooling effect and thermal hysteresis, thus establishing the first-order nature of the transition. Based on the measurement of the latent heat, our thermodynamic analysis suggests that the onset of ferromagnetism is lowered by only 55 mK due to the competing superconducting phase. Results of magnetization and ac susceptibility experiments on these compounds are also included.This research was supported by the Fonds National Suisse de la Recherche Scientifique.     

Hydrostatic pressure effects on the superconductivity of La1.2−x Eu x Mo6S8 pseudoternary compounds

The effect of hydrostatic pressure on the superconductivity of La_1.2–x Eu _x Mo₆S₈ pseudoternary compounds was studied by means of low-frequency ac and dc magnetic susceptibility measurements up to 20 kbar. The superconducting transition temperatureT _c at ambient pressure is a weak function of Eu concentrationx for 0x0.2, and then decreases smoothly withx forx>0.2 until superconductivity is completely suppressed forx0.95. Pressure was found to decreaseT _c in the La-rich compounds and to increaseT _c in the Eu-rich compounds. Pressure also induces superconductivity in compounds withx>0.95 that are not superconducting at ambient pressure. A La_0.2Eu_1.0Mo₆S₈ compound revealed a sizeable Meissner effect for pressures larger than 7 kbar.Research supported by the U.S. Department of Energy under contract DE-AT03-76ER70227.On leave from Instituto de Fisica Gleb Wataghin, UNICAMP, Brazil.Research supported by the National Science Foundation under grant DMR81-09950.Supported by the National Science Foundation.     

Two distinct structural phases in Sr3−x Ca x Ru2O y

Non-cuprate layered perovskite Sr₂RuO₄, which is an analogue of (La,Sr)₂CuO₄, exhibits superconductivity below T_c1K. We have synthesized a new system Sr_3–xCa_xRu₂O_y and found the solubility of Ca to be at least as much as x = 2. These samples are not superconductive down to 2K but magnetically ordered. The x dependence of the lattice parameters and that of T_mag clearly indicate that there are at least two distinct structural phases in Sr_3–xCa_xRu₂O_y.On leave from Hiroshima University.     

Superconducting Density of States from the Magnetic Penetration Depth of Electron-Doped Cuprates La2−x Ce x CuO4−y and Pr2−x Ce x CuO4−y

From measurements of the magnetic penetration depth, (T), from 1.6 K to T _c in films of electron-doped cuprates La_2–x Ce _x CuO_4–y and Pr_2–x Ce _x CuO_4–y we obtain the normalized density of states, N _s(E) at T=0 by using a simple model. In this framework, the flat behavior of ^–2(T) at low T implies N _s(E) is small, possibly gapped, at low energies. The upward curvature in ^–2(T) near T _c seen in overdoped films implies that superfluid comes from an anomalously small energy band within about 3k _B T _c of the Fermi surface.     

The synthesis of the Nb3(Al-Ge) system by sputtering

The variation of the superconducting transition temperatureT _chas been studied in the Nb₃Al _1–x Ge _x system for0x1 for samples produced by rf sputtering onto heated substrates. The effect that different sputtering gas pressures have upon the rate of energy loss of the sputtered atoms due to collisions with neutral sputtering gas atoms is considered. Also considered is how thermalization can be achieved in the fewest number of collisions by matching the mass of the sputtering gas atoms to that of the sputtered atoms. For the case of Nb ₃ (Al-Ge) we show that it is advantageous to use a mixture of sputtering gases so that the light Al atoms can be thermalized as well as the heavier Nb and Ge atoms. It is also thus shown that the same sputtering conditions that are optimal for forming high-T _cNb₃Ge onto heated substrates are not optimal for forming high-T _cNb ₃ Al.Supported by CUNY FRAP and National Science Foundation DMR 74-18138.     

Transition from antiferromagnetism to ferromagnetism in the superconducting mixed ternary system (Sm1−x Er x )Rh4B4

The mixed ternary system (Sm_1–x Er _x )Rh₄B₄ has been investigated by means of measurements of ac electrical resistance in various applied magnetic fields, ac magnetic susceptibility, and specific heat. The low-temperature superconducting and magnetic transition temperature versus Er concentrationx phase diagram reveals the occurrence of a single transition to long-range magnetic order for all concentrations, changing from antiferromagnetism for SmRh₄B₄ to ferromagnetism for ErRh₄B₄ atx 0.2. The upper critical magnetic field versus temperature curves for various Er concentrations exhibit combined features characteristic of both SmRh₄B₄ and ErRh₄B₄, rather than a gradual evolution from one type to the other. A model for such a mixed magnetic system is discussed, and comparisons with other mixed ternary systems are made.Research supported by the U.S. Department of Energy under Contract No. DE-AT03-76ER70227.     

Superconducting Properties in Amorphous MgxB1−x Films

We report the observation of superconductivity in a thick amorphous (a-) Mg_xB_1–x film with x0.3, which was prepared by coevaporation of Mg and B. The resistivity in the normal state is by approximately two order of magnitude higher than that for clean MgB₂ crystals. The transition temperature T _c at which the resistivity vanishes is found to be 6 K, that is even lower than T _c=39 K for clean MgB₂ crystals. The upper critical field extrapolated to zero temperature is about 5 T. Based on the data of resistivity measured down to 0.4 K for various fields B, we construct the possible B–T phase diagram over the broad temperature/field region.     

Investigation on the Structure and Superconducting Properties of Mg1 ? xMxB2 (M = deficiency or Ca)

We report the preparation of Mg_{1 – x} M _x B₂ (M = deficiency or Ca) compounds and their structure and superconducting properties. For Mg_{1 – x} B₂, although nearly single-phase samples can be obtained for x = 0, MgB₄ coexists with the MgB₂ phase and some minor impurity phases, and the amount of MgB₄ increases with x for 0 < x 0.5. The lattice parameters a and c of MgB₂ decreases and increases, respectively, with the increase of x, and T _c also decreases. While for Mg_{1 – x} Ca _x B₂, the superconducting transition temperature remains unchanged for x 0.3 and loss of superconductivity occurs for x > 0.3. X-ray diffraction patterns for x 0.3 samples show that MgB₂ phase coexists with CaB₆, Mg, and MgO. With increasing x, the amount of CaB₆, Mg, and MgO increases, while the amount of MgB₂ decreases. The lattice parameters of MgB₂ phase do not show any obvious change in contrast to Mg_{1 – x} B₂. The results were discussed by considering some possible contributions.     

Investigation of the temperature dependence of electron and phonon Raman scattering in single crystal YBa2Cu3O6.952

Detailed Raman-scattering measurements have been performed on high-quality YBa₂Cu₃O_6.952 single crystal (T _c =93 K, T _c =0.3 K). A sharp (FWHM 7.2 cm^–1 at 70 K and 10.0 cm^–1 at 110 K) 340 cm^–1phonon mode has been observed inB _1g polarization. An electronic scattering peak at 500 cm^–1 in theB _1g polarization extends down to 250 cm^–1. These FWHM values determine the upper limit of the homogeneous linewidth of the phonon and electronic excitations. The start of the electronic spectral function renormalization and of the 340 cm^–1 mode anomalies (frequency softening, linewidth sharpening, and intensity increase) have been observed to occur approximately 40 K aboveT _c . The 340 cm^–1 mode Fano shape analysis has been performed and the temperature dependences of the Fano shape parameters have been estimated. All 340 cm^–1 mode anomalies have been explained by the electronic spectral function renormalization.This work was supported by Swedish Natural Sciences Research Council (G.B. and L.B.) and by the National Science Foundation (DMR 91-20000) through the Science and Technology Center for Superconductivity (G.B. and M.V.K.).     

Pressure Effect on the Coexistence of Magnetism and Superconductivity in Ho1−x Dy x Ni2B2C

We have measured the transport properties of Ho_(1–x)Dy _x Ni₂B₂C (x=0.1, 0.4) single crystals to study the effects of high pressure conditions on the superconductivity and magnetism. The pressure affects the pair-breaking interaction by changing the magnetic exchange integral J _sf. In the case of x=0.1, the superconducting transition temperature T _c is higher than the Néel temperature T _N and decreases as the pressure increases. On the other hand, for x=0.4, when have lower T _N than T _c, T _c does not change with increasing pressure. These results are due to the effect of the pressure on J _sf.     

Diffusion in3He near its critical point

Pulsed NMR techniques were used to measure the spin diffusion coefficient in ³ He along eight isotherms between 3.0 and 4.2 K. Densities were also obtained simultaneously from the amplitude of the signal, assuming the magnetization obeys the Curie law. Absolute values of the density were obtained by normalizing to existing data. Although values of|T–T _c |/T _c and|– _c |/ _c of less than 10^–3 were achieved, no anomaly was observed. This is consistent with similar measurements on other substances. Our low-density results agree well with previous measurements and theory.Work supported in part by a National Science Foundation Department Science Development Grant, No. GU-2603, and the Ohio University Research Committee.     

Magnetization curves of (La1−x Sr x )2CuO4−δ single crystals as functions ofx and δ

The second peak effect in magnetization curves for overdoped (La_1–xSr_x)CuO_4– single crystals was examined by means of systematic variations of Sr content x and oxygen deficiency . Oxygen defect concentration was found to sensitively affect the critical temperature T_c and the macroscopic pinning force F_p, resulting in significant changes in magnetization hysteresis loops. Observations of dependence of M and F_p at the same reduced temperature T/T_c yielded an insight into the role of oxygen defects: increasing results in increasing the pinning center density N. The second peak field B_2pk seemed to be determined by the development of a percolating network of magnetically reversible regions from the observation that the temperature dependence of B_2pk showed similar behavior to the irreversibility field B_irr.     

Effect of Al Substitution on the Mg Sites in Mg11B2 Prepared Under High-Pressure Conditions

The effect of aluminum substitution on superconductivity in Mg¹¹B₂ prepared under a high pressure (HP) of P3 GPa was compared with that in (Mg_1–x Al _x )B₂ prepared at ambient pressure (AP). In both the HP and the AP samples, the superconducting transition temperature (T _c) tended to decrease in the same way due to the substitution, but the width of the superconducting transition (90%–10%), T _c, decreased more drastically in the HP sample (HPS) than it did in the AP sample (APS). Moreover, bulk superconductivity lasted up to x=0.3 for the HPS, which is not the case for the APS. A systematic change in the lattice constants due to the substitution of Al for Mg was observed by X-ray diffraction. A scanning electron microscopic image showed that the grains were compactly connected in the HPS, thus explaining the reduced normal-state resistivity.     

Magnetic scattering of conduction carriers in 3d transition-metal intercalates of M x TiS2 (M=Mn,Fe, Co,and Ni)

Galvanomagnetic measurements on layer-structured 3d transition-metal intercalates of M_xTiS₂ (M = Mn, Fe, Co, and Ni;x0.33) have been made over the temperature range 0.34–20 K. Resistivity minima and negative magneto-resistances are observed for paramagnetic Mn, Fe, and Ni intercalates with low guest concentrations (x0.1), as found in well-known dilute alloys. However, in the case of the weak-ferromagnetic phase of Co_xTiS₂ (0.10x0.33) with the Curie temperatureT _c =120–140 K, such anomalous behaviors are also observed, which is not commonly found in the magnetically ordered phases of various magnetic materials. With these data we have given qualitative discussions on the magnetic scattering process of conduction carriers. Further, some of the experimental results are discussed in connection with the band calculations for M_1/3TiS₂.     

Crystallization and properties of CaO-P2O2-B2O3 glasses

Glasses were prepared with compositions (50–0.5 x) CaO.(50–0.5 x) P₂O₅ · x B₂O₃ with B₂O₃ contents (x) from 0 to 45 mol%. The glass transformation temperature (T _g), dilatational softening temperature (T _D) and Vickers hardness (H _V) initially increased with x, but showed maxima at about x=20 for T _g and T _D and at about x=35 for H _V. The thermal expansion coefficient decreased with x, levelling off at about 35 mol% B₂O₃. The maximum tendency to crystallize occurred at around 25 mol% B₂O₃. Volume nucleation (and hence glass-ceramic formation) and surface nucleation were obtained for x between 15 and 25 mol%. The first phase to appear was BPO₄, which was probably homogeneously nucleated. Subsequently the 4CaO · P₂O₅ phase was heterogeneously nucleated on the BPO₄. For 10 x 35 only surface nucleation was observed. The kinetics of nucleation were investigated in the 20 mol% B₂O₃ glass. The changes in properties and crystallisation behaviour with B₂O₃ content were related to short-range structural information. Infrared spectra and literature data indicated a threedimensional network of B-O-B and B-O-P linkages in the glasses.     

Mixing entropy of 3He-4He monolayers adsorbed on graphite

Calorimetric measurements on ³He-⁴He monolayers by Hickernell, McLean, and Vilches indicate that isotopic ordering takes place in both the low-density and registered lattice gas regimes. The low-density film heat capacity corresponds to imperfect 2D gas mixtures above T – 1 K. At lower temperatures the ⁴He fraction condenses to a 2D liquid, and the concurrent decrease in mixing entropy indicates concurrent isotopic separation. In the registered lattice gas mixtures the mixing entropy changes by a much smaller amount over the experimental region. It is argued that the observed changes are due to shortrange isotopic ordering tending towards segregation. From a detailed comparison with an Ising model, we estimate a transition temperature T _c 30 mK.Research supported by The National Science Foundation.     

Effects of Non-Magnetic Impurities on Cu-Spin Dynamics and Superconductivity in La2−x Sr x Cu1−y Zn y O4 Around x = 0.115

Muon-spin-relaxation (SR) and magnetic susceptibility measurements have been carried out in La_2–x Sr _x Cu_1–y Zn _y O₄ with x=0.13 changing y finely up to 0.10, with the aim at clarifying effects of the non-magnetic impurity Zn on the Cu-spin dynamics and superconductivity. It has been found that the y dependence of the volume fraction of the superconducting region estimated from susceptibility measurements highly correlates with that of the magnetically ordered region estimated from the SR results. The rapid decrease in the superconducting region and the rapid increase in the magnetically ordered region by the doping of a small amount of Zn can be interpreted as follows; Zn pins the dynamical spin correlation or the dynamical stripe correlations and hence the superconductivity is destroyed around itself.     

A review of the size of the energy gap in superconducting oxides

An attempt has been made to summarize a large body of data dealing with measurements of the energy gap in superconducting oxides like La_2–x Sr _x CuO_4–y , YBa₂Cu₃O_7–y , and related compounds. A plot of available data of the energy gap 2 as a function of the superconducting transition temperatureT _c revealed a large dispersion of some data from 2/k _B T _c =3.53 for the weak coupling BCS prediction. Nevertheless, we could conclude that the size of the energy gap in these exotic materials is within the weak coupling BCS prediction, in view of some controversial issues that could be responsible for the dispersion, such as the gap anisotropy as well as identification of the energy gap.