Effect of Disorder Induced by Heavy-Ion Irradiation on CeCoIn5 Superconductivity

We present a study of the effect of heavy-ion irradiation on a thinned single crystal of the unconventional heavy fermion superconductor CeCoIn₅. Magnetic susceptibility and low-temperature specific heat results show that the superconducting transition temperature (T _c) changed only 4% with irradiation of 1×10¹² ions/cm², Energy=1 GeV, while the specific heat jump at T _c, ΔC, divided by C _n, where C _n is the normal state specific heat just above T _c, was reduced to 3.6 from ΔC/C _n=4.5 for unirradiated CeCoIn₅. The increase of low-temperature magnetic susceptibility and the saturation in magnetization suggest that the defects induced by heavy-ion irradiation are magnetic in nature, as was seen in the case of neutron irradiation on the heavy fermion superconductors, UBe₁₃ and UPt₃. The non-Fermi liquid behavior of the irradiated sample, based on the temperature dependence of the low temperature magnetic susceptibility, is significantly altered.     

The superconductivity and magnetic susceptibility of some zirconium-transition-metal compounds; evidence for an anticorrelation

The properties of the superconducting transition temperatureT _cof the CuAl ₂ -type compounds of Zr ₂ Co, Zr ₂ Ni, Zr ₂ Rh, Zr ₂ Fe, and Zr ₂ Ir are discussed with respect to effects due to alloying and heat treatment. Dilute pseudobinary alloys of transition-metal elements with Zr ₂ Rh (T _c=11.3 K) produced lower transition temperatures suggesting that the valence-electron concentration of 5.67 electrons/atom gives a maximum inT _c for this crystal type. Results for Zr ₂ Co (T _c=5.0 K) and Zr ₂ Ir (T _c=7.3 K) show that their transition temperatures are raised somewhat when the electron concentration is increased by alloying. A peak is seen near 5.72 electrons/atom. A peak in the room-temperature magnetic susceptibility in the Zr ₂ Co-Zr ₂ Ni system occurs near the same electron concentration as the peak inT _c. Similar susceptibility behavior takes place in the Zr ₂ Rh-Zr ₂ Ni system, however, with no peak inT _c. Results of susceptibility measurements on Zr ₂ Ir and its isomorphic alloys indicate correlation betweenT _c and susceptibility. An anticorrelation occurs for Zr ₂ Co alloys suggesting the presence of Coulomb interactions. A sharp symmetric drop inT _c near the stoichiometric composition as well as a marked decrease of the transition temperature with a low-temperature (600 C) anneal indicate that the coulomb interactions are sensitive to crystalline order. The lattice parameters of Zr ₂ Ir are reported as a=6.508 Å andc=5.721 Å. The superconductivity of Zr ₃ Co (T _c=3.9 K) and Zr ₃ Ir (T _c=2.13 K) is reported.Research sponsored by the Air Force Office of Scientific Research, Office of Aerospace Research, United States Air Force, under AFOSR grant number AF-AFOSR-631-67-A.     

Scaling Behavior and Estimation of Activation Energy of Polycrystalline RuSr2GdCu2O8 Superconductor from AC Susceptibility Measurements

In this study, the measured curves of AC susceptibility (ACS) components, χ′(T) and χ″(T), of polycrystalline RuSr₂GdCu₂O₈ (Ru-1212) superconductor were scaled onto a single curve using the peak temperature of its imaginary part (T _p) as the scaling parameter for various AC field amplitudes from 0.5 to 24 G. The dependence of the AC magnetic field amplitude on T _p is scaled as: H _ac ～ (1?T _p/T _c)^2.25. Similarly, the current density J _c, extracted from the AC field amplitude is also scaled as: J _c ～ (1 ? T _p/T _c)^2.25. The dependencies of T _p on frequency and AC field amplitude are also investigated and the time parameter t ₀ of the order of 10^?8 s is estimated from the dependence of T _p on frequency. The dependencies of activation energy on temperature, T, and the field amplitude, H _ac, are obtained from the Arrhenius-like semilog plot of frequency (ν) and T _p. Such dependencies on temperature and field amplitude can be described by a scaling law of the form: U(H _ac,T) = U ₀[1 ? T/T _p]H _ac ^?0.17.     

Pair breaking, “intrinsic”T c,and the upper limit ofT c in the cuprates

The presence of magnetic impurities leads to a drastic decrease inT _c in the overdoped region, gaplessness, and the usual temperature dependence ofH _c2. The magnetic moments are localized on the apical oxygen site, and this allows us to explain the increase inT _c with the increase in the number of Cu-O planes in the unit cell. Applied pressure can raiseT _c above the usual value at optimum doping, toward the “intrinsic”T _c. The cuprates as a class of compounds have an upper limit ofT _c in the rangeT _c,upp=160–170 K.     

Influence de divers types de substitution sur les températures de transition des oxyfluorures ferroélectriques de type chiolite

A great number of oxyfluorides with a chiolite related structure have been prepared. They show all a ferroelastic, ferroelectric-prototype, paraelectric transition. The highest Curie temperature (T_c = 800 K) has been obtained for the tungsten compound Na₅W₃O₉F₅. The value of T_c is found to vary as a function of the substitutions from Na₅W₃O₉F₅. The largest decrease is observed for the composition Na₄Lu (WNb₂)O₉F₅ with T_c = 88K. Pyroelectric and piezoelectric applications are expected. The influence of the composition on other transitions observed in these materials has been determined.     

Superconductivity in MgB2: Phonon modes and influence of carbon doping

Following a brief overview, results of our investigations on phonon modes in MgB₂, and superconducting transition in carbon doped MgB₂ are presented. The superconducting transition temperature in MgB_{2 x}C_x as obtained from susceptibility and resistivity measurements is observed to decrease systematically from 39-4 K forx = 0 to 26 K forx = 0.5. It is shown the changes in lattice volume, as obtained from x-ray diffraction measurements, can account only partially for the observed decrease inT _c . The observed variation ofT _c with carbon content is seen to correlate with the Debye temperatures, obtained from an analysis of the resistivity data. Investigation of the phonon modes in MgB₂, through infrared absorption measurements indicate three modes at 410,475 and 560 cm^-1. The former two are associated with the infrared active modes, and the third component is associated with the Raman mode, that gets activated due to disorder. A study of the temperature dependence of these modes indicates no changes across the superconducting transition. The mode at 560 cm^-1 shows a significant hardening and a corresponding decrease in linewidth, with the lowering of temperature, that can been accounted in terms of anharmonicity.     

Superconductivity and Ferromagnetism of the Ru-1232 Compounds in the (Ru1−x Nb x )Sr2(GdCe1.8Sr0.2)Cu2O z System

The Ru-1232 compounds have been synthesized in the (Ru_1?xNb _x )Sr₂(GdCe_1.8Sr_0.2)Cu₂O _z system, and effects of Nb substitution for Ru on superconductivity and ferromagnetism of the Ru-1232 compounds have been investigated. First, X-ray powder diffraction study shows that nearly the single 1232 phase samples can be obtained in the x composition range from 0.0 to 0.3. Then, from the electrical resistivity study, it is found that each of the samples shows resistivity dropping phenomenon at two temperatures of T _c ^l and T _c ^h, which originates from superconductivity of the Ru-1232 phase and the Ru-1222 one, respectively. Both of the starting temperatures are lowering with increasing Nb content x. Lastly, from the magnetic susceptibility study, it is found that superconducting transition temperature T _c is 20 K for the Ru-1232 sample with x = 0.0 and the ferromagnetic transition temperature T _m is about 90 K. This study also shows that both of the values of T _c and T _m become low with increasing x from 0.0 to 0.3.     

RF susceptibility of magnetron sputtered YBa[]Cu[]O[]−x films

We measured the complex susceptibility of two different quality magnetron sputtered YBa₂Cu₃O_7−x films in RF range. Temperature and static magnetic field dependence of complex magnetic susceptibility is a useful indicator of appearance of phase coherence in the film including fluctuation region above T_c. The characteristics showed a low frequency type behaviour at loss peak and diamagnetic drop at near transition temperature and normal state non-zero susceptibility. One of our samples displayed an enhanced diamagnetism in the normal state close to T_c, which was probably attributed to the combination of the normal skin effect and excess conductivity due to superconductive fluctuations. The low temperature susceptibility was also frequency dependent and qualitatively agreed with the vortex-glass scaling model.     

Superconductivity and Ferromagnetism of the Ru-1232 Compounds in the (Ru1?xNbx)Sr2(GdCe1.8Sr0.2)Cu2Oz System

The Ru-1232 compounds have been synthesized in the (Ru_1–xNb _x )Sr₂(GdCe_1.8Sr_0.2)Cu₂O _z system, and effects of Nb substitution for Ru on superconductivity and ferromagnetism of the Ru-1232 compounds have been investigated. First, X-ray powder diffraction study shows that nearly the single 1232 phase samples can be obtained in the x composition range from 0.0 to 0.3. Then, from the electrical resistivity study, it is found that each of the samples shows resistivity dropping phenomenon at two temperatures of T _c ^l and T _c ^h, which originates from superconductivity of the Ru-1232 phase and the Ru-1222 one, respectively. Both of the starting temperatures are lowering with increasing Nb content x. Lastly, from the magnetic susceptibility study, it is found that superconducting transition temperature T _c is 20 K for the Ru-1232 sample with x = 0.0 and the ferromagnetic transition temperature T _m is about 90 K. This study also shows that both of the values of T _c and T _m become low with increasing x from 0.0 to 0.3.     

Fluctuations of the Order Parameter's Phase in Layered Superconductors

The influence of the quantum fluctuations of the order parameter's phase on the critical temperature T _c is studied for a Josephson coupled layered superconductor. Two characteristic critical temperatures exist for a system, namely the superconducting critical temperature T ⁽²⁾ _c for a single layer estimated by the mean-field theory and the transition temperature for the outset or the superconducting phase coherence T* _c . The true critical temperature T _c is shown to vary inside the intervals T* _c ≤ T _c ≤ T ⁽²⁾ _c . For a strong quantum phase fluctuation limit, the superconducting layers become decoupled.     

Intergranular critical currents and excess conductivity in the Fe-substituted Bi-2223 high Tc superconductor

In this paper me present the effect of iron substitution in the (Bi_1.6Pb_0,4)(Sr_1.88Ba_0.2)Ca₂(Cu_1–xFe_x)_3.1O_y system. From the resistivity and AC susceptibility data we observed that a monotonie decrease of T_c occurs with increasing Fe concentration. A strong reduction of J_c (calculated from AC susceptibility data performed at low magnetic fields in the range 0.4A/m-800A/m) is observed in the doped samples. The crossover temperature from the Anderson-Kim thermally activated flux creep to the thermally activated phase slippage behaviour is depending on the doping concentration. The excess conductivity and 2D-3D crossover is well described by considering /₀ as a product of a Gaussian centered at T_c with a Student distribution centered at 2T_c. The alteration of 2D dimensionality above T_c by doping results in the decrease of the number of freedom degrees of the Student distribution.     

Formation of the high-Tc phase in lead-doped Bi-Sr-Ca-Cu-O superconductors

The characteristics of Bi-Sr-Ca-Cu-O superconductors have been investigated as a function of the amount of Pb addition. The volume fraction of the high-T _c phase increases with increasing Pb addition up to x=0.3 in (Bi_1–xPb_x)₂Sr₂Ca₂Cu₃O_y and above that it decreases. The grain size is increased and density is decreased with increasing Pb addition, due to two-dimensional grain growth which results from formation of the high-T _c phase. The specimen with x=0.3 has a transition temperature of 104 K and high magnetic susceptibility due to the fact that most of the volume fraction is of the high-T _c phase. Pb addition cause formation of Ca₂PbO₄ as a secondary phase and it produces a partially melted liquid phase below the sintering temperature, which acts as a flux and promotes formation of the high-T _c phase.     

Textures of Superfluid 3He-B in Applied Flow and?Comparison with Hydrostatic Theory

Measurements of the order parameter texture of rotating superfluid ³He-B have been performed as a function of the applied azimuthal counterflow velocity down to temperatures of 0.2 T _c. The results are compared to the hydrostatic theory of ³He-B. Good agreement is found at all measured temperatures and rotation velocities when the flow anisotropy contribution to the textural free energy is adjusted. This gives a superfluid energy gap ??(T) which agrees with that measured by Todoshchenko et al., with ??(0)=1.97 k _B T _c at 29.0 bar. The B-phase susceptibility, longitudinal resonance frequency, and textural phase transition have been extracted from the measurements as a function of temperature and azimuthal counterflow velocity. Owing to decreasing absorption intensities the present measuring method, based on the line shape analysis of the NMR spectrum, loses its sensitivity with decreasing temperature. However, we find that in practice the measurement of vortex numbers and counterflow velocities is still feasible down to 0.2 T _c.     

Superconductivity in Ca<Subscript>0.5</Subscript>La<Subscript>0.5</Subscript>FBiSe<Subscript>2</Subscript>

Through the measurement of resistivity, magnetic susceptibility, and Hall effect, we discovered a novel BiSe₂-based superconductor Ca_0.5La_0.5FBiSe₂ with T _c of 3.9 K. A strong diamagnetic signal below T _c in susceptibility χ(T) is observed indicating the bulk superconductivity. The negative Hall coefficient throughout the whole temperature regime implies the dominant electron-type carriers in the sample. Different to most of BiS₂-based compounds where superconductivity develops from a semiconducting-like normal state, its resistivity in the present compound exhibits a metallic behavior down to T _c . Together with the enhanced T _c , the metallic character of the normal state implies that the electronic structure of Ca_0.5La_0.5FBiSe₂ may be different to those in the other BiS₂-based compounds.     

AC susceptibility study of YBCO

The temperature dependence of ac susceptibility of YBCO bulk samples was measured as a function of ac field amplitude and frequency. Analysis of the temperature dependence of the ac susceptibility near the transition temperature (T _c ) has been done employing the simplified Kim model. We have obtained an empirical function for the penetration field H _p = H _α (1?t) ^β , t = T=T _c . Best fitting to data was obtained with parameters H _α ≈ 6:2 × 10³ A/m and β ≈ 1.50. The experimental value agrees well with the model calculations. In addition, as the frequency increases, the peak temperature (T _p ) shifts to higher temperature. This effect can be interpreted in terms of flux creep. The field dependence of activation energy obtained from the Arrhenius plots for the frequency (f) and (T _p ) can be described as U ∝ (H _ac )^?β′ with β′ ≈ 0:38 for YBCO.     

Resistivity anomalies for ferromagnetic metals at curie points. II. Short-range magnetization fluctuations

Using the itinerant model of magnetic moment in ferromagnetic metals, we have investigated the resistivity anomalies near the Curie temperatureT _c . In this paper, the short-range magnetization fluctuations have been taken into account in calculating the spin correlation function in the paramagnetic region. The mean free path is considered to be comparable with the correlation length and to be temperature dependent nearT _c . A critical exponent is introduced for the mean free path and its value is established to be not smaller than 1/2. The resistivity is found to be continuous throughT _c . The temperature derivative of the resistivity is found to be divergent linearly and positively near and aboveT _c . These results confirm the phase transition nature of the resistivity anomalies of the ferromagnetic metals nearT _c and show that the short-range order must also exist in the itinerant model of the magnetic electrons.     

The effect of pressure on the superconducting and crystallographic transitions of La3S4, La3Se4 and La3Te4

The superconducting transition temperatures (T_c's) for La₃S₄ and La₃Se₄ vary in a non-linear fashion under hydrostatic compression to 22 kbar. Both T_c and the crystallographic transformation temperature rise with pressure for La₃S₄ and show a maximum near 10 kbar for La₃Se₄. Non-transforming isostructural La₃Te₄ exhibits a linear pressure dependence of T_c. However, the telluride undergoes a pressure induced phase transformation at ～70 kbar. In this case the sign of the pressure dependence is different from the sulfide and selenide. Variation of T_c with the crystallographic transformation temperature shows a new and unexpected behavior.     

The spin susceptibility of singlet correlated oxygen band in La2−x Sr x CuO4

We have shown that the unconventional temperature dependence of the static susceptibility?(T) of the perovskite high-T _c superconductors above the superconducting transition temperatureT _c can be explained in terms of two relevant band models containing the singlet-correlated oxygen band and the copper character band. The usual copper-oxygen Hamiltonian containing hopping and Coulomb repulsion terms has been reduced to an effective Hubbard-liket-t′-t″-U _eff model to describe the low-energy properties. The unusual behaviour of the susceptibility is due to thermally activated oxygen holes coming into the hybridization singularity peak in the density of states. A possible physical origin ofT _max in the temperature dependence of the susceptibility is discussed.     

The composition dependence of the grey tin → white tin transition in dilute tin-germanium alloys

It is shown that the temperature, T_c, at which the grey modification will convert to the white phase in dilute SnGe alloys increases with the amount of germanium, according as T_c = 32.0 + 51.8C^0.33. It is concluded that this increase in transition temperature is a hysteresis effect and is probably due to the reduced mobility of the $\text{β}\text{α}$ martensitic interface.     

The effects of oxygen doping on copper Chevrel compounds

The Chevrel compounds Cu_1.8Mo₆S_8–x O _x have been studied, wherex=0.00, 0.05, and 0.10. In order to establish the effects of incorporating a small amount of oxygen into Cu_1.8Mo₆S₈, we have measured lattice parameters, superconducting transition temperatureT _c, and the temperature dependence of both the upper critical fieldH _c2 and the magnetic susceptibility x. We show that the usual method of making this compound incorporates oxygen, and that this impurity may be removed by gettering the material with Y₂S₃. We observe that the substitution of oxygen for sulfur in Cu_1.8Mo₆S₈ increasesT _c and decreases the ratioc/a of the hexagonal lattice constants and x. A breakdown in the rigid-band approximation is indicated by a comparison of the relative signs of the effect onT _c and that on x. In addition to these observations, we report on a correlation between hardness andT _c of Cu_1.8Mo₆S₈ and on the temperature dependence of the magnetic susceptibility of Cu_3.2Mo₆S₈.