The specific heat jump of superconducting La1-xTbxAl2 related to crystal electric field splitting of the Tb3+ levels

Measurements of the specific heat jump Δc(Tinc) were performed on superconducting La_1-xTb_xAl₂ compounds in order to investigate the pairbreaking effect of Tb³⁺ ions in the LaAl₂ host. The results deviate from the Abrikosov-Gorkov theory for superconductors with magnetic impurities. Taking into account the crystal field level structure of Tb³⁺ in LaAl₂ the results can be qualitatively understood within the model of Fulde, Keller and Peschel. According to calculations published by Müller-Hartmann and Zittartz an influence of the Kondo effect can be ruled out.     

Hall effect in CeAl3

We have studied temperature dependences of the Hall coefficient R_H(T) and resistivity ?(T) in substitutional solid solutions Ce_xLa_1?xAl₃ (0 ≤ x ≤ 1) which realize the transition from Kondo impurity regime (X?1) to Kondo lattice (X = 1). Lowering temperature results in the anomalous increase of R_H(T). The ratio r = R_H(3 K)/R_H(100 K) has been found to increase with cerium content up to the r ? 15 in the marginal compound CeAl₃. The specific features of R_H(T) behaviour in Ce_xLa_1?xAl₃ alloys seem to be related to the existence at low temperatures of heavy fermions due to the formation of the narrow Abrikosov-Suhl resonance at the Fermi level.     

Evidence for intermediate temperature superconductivity as a bulk effect

The specific heat was measured in a highly homogenized sample of (La_0.9936Ce_0.0064) Al₂ between 0.35 and 1.5 K, both in the superconducting and in the normal state. The difference of the specific heats ΔC = C_S ? C_N was observed to change its sign twice. Hence the existence of an ‘intermediate temperature superconductor’ is suggested by means of a true volume effect. Reasonable agreement is found between the conclusions from the specific heat experiment and recent results of Winzer obtained for the upper critical magnetic field.     

Magnetic properties of (Ce1−xLax)PdIn2

We report on results of X-ray powder diffraction, magnetization and specific heat measurements of the pseudo-ternary (Ce_1−xLa_x)PdIn₂ system with x=0; 0.2; 0.4 and 0.6. The results show a linear increase of the unit cell volume and a reduction of the ferromagnetic transition as La content increases. The Debye temperature, Sommerfeld coefficient and crystal field parameters were estimated from specific heat data, and are found to be weakly dependent of the Ce concentration. Also, the variation of magnetic entropy at T_C is only weakly dependent on x (ΔS≅0.92Rln2) indicating that T_K/T_C is approximately constant along the series. The T_C and T_K behaviors are explained by the variation of the exchange parameter due to the volume change when Ce is replaced by La. Our results indicate that the chemical pressure is the dominant effect rather than the chemical disorder for determining the physical proprieties of the (Ce_1−xLa_x)PdIn₂ system.     

The effect of pressure on the crystalline electric field levels of superconducting La1−xTbxAl2

The pressure dependence of the superconducting transition temperature (dT_c/dP) is smaller for La_1?xTb_xAl₂ than for LaAl₂. The difference is attributed to an increase with pressure of the crystalline electric field energies experienced by the Tb ion.     

Electrical transport of (1−x)La0.7Ca0.3MnO3+xAl2O3 composites

We report the resistivity (ρ)-temperature (T) patterns in (1-x)La_0,7Ca_0,3MnO₃+xAl₂O₃ composites (0≤x≤0.05) over a temperature regime of 50-300 K. Al₂O₃ addition has increased the resistivity of these composites. The Curie temperature (T_C) is almost independent on the Al₂O₃ content and is about 250 K for all the samples, while the metal-insulator transition temperature (T_MI) decreases with increasing Al₂O₃ content. Based on the phenomenological equation for conductivity under a percolation approach, which is dependent on the phase segregation of ferromagnetic metallic clusters and paramagnetic insulating regions, we fitted the experimental data (ρ−T) from 50 to 300 K and find that the activation barrier increases as Al₂O₃ content increases.     

Kondo behaviour of the solid solution (Ce1−xLax)PtGa

The solid solution (Ce_1−xLa_x)PtGa has been studied through X-ray diffraction, magnetization (σ(B)), magnetic susceptibility (χ(T)), electrical resistivity (ρ(T)), magnetoresistivity (MR) and heat capacity (C_P(T)) measurements. The Néel temperature (T_N=3.3 K) for CePtGa is lowered upon La substitution as observed from χ(T) and ρ(T) measurements. The Kondo temperature T_K as calculated from MR measurements is comparable to T_N and also decreases with La substitution. The volume dependence of T_K is in accordance with the compressible Kondo lattice model and a Doniach diagram of the results is presented. C_P(T) measurements are presented for CePtGa, Ce_0.2La_0.8PtGa and LaPtGa and the results are discussed in terms of the electronic and magnetic properties. Other features of interest are anomalies in ρ(T) and C_P(T) due to crystalline electric field effects and metamagnetism as observed in σ(B) studies for samples with 0≤x≤ 0.3.     

The influence of crystal field split impurities (Tb) on the superconducting properties of LaAl2

The superconducting transition temperatures of the system La_1-xTbxAl₂ have been measured. By making use of the theory of Keller and Fulde the energy splitting between the ground state and the first excited state of Tb³⁺ is found to be 5 ± 1 K. Furthermore, we have determined the upper critical field H_c2(T) of these alloys and compared the results with those found for La_1-xGd_xAl₂. In this way the influence of the crystal field splitting of Tb³⁺ on the critical field data is clearly demonstrated.     

Superconductivity and calorimetric studies of Pr1.85Ce0.15CuO4+δ under different annealing conditions

Polycrystalline samples of electron-doped Pr_1.85Ce_0.15CuO_4+δ have been prepared under different annealing conditions and investigated by means of X-ray-diffraction, oxygen content analysis, electrical resistivity, magnetic susceptibility and low temperature specific heat measurements. X-ray-diffraction patterns show that samples contain a single T′ phase. The superconducting transition temperatures T_cm taken with the onset of diamagnetism in magnetic-susceptibility measurements are 20 and 19.5 K for sample annealed in flowing Ar gas and in vacuum (∼10⁻³ torr), respectively. The data of the samples, which are annealed in flowing Ar gas, show clear evidence for an αT² term at zero magnetic field in superconducting electronic specific heat, and are consistent with d-wave superconductivity. However, this behavior is not observed in the other sample, which is annealed in vacuum. These results indicate that different heat treatments affect the oxygen content, homogeneity, superconducting transition temperature T_c, superconducting volume fraction, and the superconducting pairing symmetry of Pr_1.85Ce_0.15CuO_4+δ.     

Non-Fermi liquid behavior in polycrystalline Ce2PdIn8

We report on the formation of a novel ternary compound Ce₂PdIn₈ that is isostructural with the heavy-fermion superconductors Ce₂CoIn₈ and Ce₂RhIn₈. Its magnetic, electrical transport and thermodynamic properties were studied on polycrystalline samples in wide ranges of temperature and magnetic field strength. The results revealed Ce₂PdIn₈ to be a paramagnetic Kondo lattice with a coherence temperature of about 12 K. The C/T ratio of the specific heat reaches at 350 mK a strongly enhanced magnitude of about per Ce-atom, thus clearly indicating a heavy-fermion nature of this material. Moreover, a logarithmic divergence of C/T vs. T, observed below 3 K, which is accompanied by a linear temperature dependence of the electrical resistivity below 6 K, hint at a non-Fermi liquid character of the electronic ground state in the new compound reported.     

Calorimetric investigation under high pressure of the heavy fermion superconductor CeCu2Si2

The heavy fermion compound CeCu₂Si₂ is commonly regarded as a Kondo lattice system. Though it has been shown that the heavy mass quasiparticles participate in its superconductivity below ～ 0.7 K, a detailed understanding of the interdependence of the superconducting and the Kondo lattice parameters is still to be developed. The application of pressure is one useful approach to study this problem. In this paper we present results of specific heat measurements between 0.3 K and 2 K under pressures up to 5.9 kbar. While in our sample T_c hardly changes, the normal state specific heat, which is exclusively of electronic origin in the present temperature range, is rapidly decreased in a monotonous way, qualitatively corresponding to the expected rise of the Kondo temperature with pressure. In contrast to this behaviour, a strong nonlinear change of the jump Δc(T_c) passing through a maximum near 3 kbar is observed. We suggest that this reflects changes of the Kondo lattice coherence structure in the quasiparticle density of states near E_F.     

Simultaneous enhancements of Curie temperature and magnetocaloric effects in the La1−xCexFe11.5Si1.5Cy compounds

The effects of introducing Ce and C atoms on the Curie temperature (T_C), the magnetic entropy change (ΔS_M) and the hysteresis loss have been investigated in the NaZn₁₃-type LaFe_11.5Si_1.5 compound. Partial replacement of La with Ce leads to a decrease in T_C and an increase in ΔS_M; however, the introduction of interstitial C atoms can adjust T_C to high temperature. The itinerant-electron metamagnetic transition is weakened after carbonization, which results in a reduction of both the hysteresis loss and magnetocaloric effect (MCE). The maximum value of ΔS_M for La_0.8Ce_0.2Fe_11.5Si_1.5C_0.2 is found to be −28 J/kg K at T_C=207 K with an effective refrigeration capacity of 420 J/kg for a field change from 0 to 5 T. Our study reveals that the enhancements of both T_C and MCEs can be achieved simultaneously in the La_1−xCe_xFe_11.5Si_1.5C_y compounds by adjusting the concentrations of Ce and C atoms appropriately.     

On the valence regime in the (CexLa1−x)Be13 system

High-field magnetization, susceptibility and heat capacity data for the (Ce_xLa_1?x)Be₁₃ system are presented. A mixed valence behaviour is followed through the whole system with a n_f occupation number increasing from 0.65 toward 1 with increasing Ce dilution. For x ? 0.1 a Kondo regime is possible.     

Specific heat of Ce x La1 − x B6 in the low cerium concentration limit (x ≤ 0.03)

The specific heat of high-quality Ce _x La_{1 ? x} B₆ (x = 0, 0.01, 0.03) single crystals is studied in the temperature range 0.4–300 K. LaB₆ samples with various boron isotope compositions (¹⁰B, ¹¹B, ^nat B) are analyzed to estimate the effect of boron vacancies. The experimental data are used to take into account the electron component correctly under the renormalization of the density of states at T < 8 K, the contribution of the quasi-local vibrational mode of a rare-earth ion with the Einstein temperature Θ_E ≈ 152 K, the Debye contribution from the rigid cage of boron atoms with the Debye temperature Θ_D ≈ 1160 K, and the low-temperature Schottky contribution related to the presence of 1.5?2.3% boron vacancies in the rare-earth hexaborides. The detected low-temperature anomalies in the specific heat are shown to be interpreted in terms of the formation of two-level systems with an energy ΔE = 92–98 K caused by the displacement of rare-earth ions from their centrosymmetric positions. A scenario of heavy fermion formation that is alternative to the Kondo mechanism is proposed for the systems with a magnetic impurity.     

Magnetic properties of RPd5Al2 (R=Y, Ce, Pr, Nd, Sm, Gd)

Polycrystalline samples of a new rare-earth series RPd₅Al₂ crystallizing in the tetragonal ZrNi₂Al₅-type structure have been prepared. Their physical properties by electrical resistivity ρ, magnetic susceptibility χ, magnetization M and specific heat C_p measurements are reported. The ingots are composed of elongated grains preferentially aligned in the c direction; therefore, measurements were conducted parallel and perpendicular to the grains. Antiferromagnetic ordering appears in R=Ce, Nd, Gd, and Sm at low temperatures. CePd₅Al₂ has two AFM transitions at 4.1 and 2.9 K and ρ(T) indicates a Kondo metal behavior with large anisotropy. In PrPd₅Al₂ no magnetic transition was observed down to 0.4 K. The C_p(T) shows a broad peak around 13 K due to the CEF effect, suggesting a non-magnetic singlet ground state. In NdPd₅Al₂, χ(T) shows anisotropy and the C_p(T) shows a sharp peak at 1.2 K. The magnetic entropy at 3 K is very close to Rln2, indicating a Kramers doublet ground state. In SmPd₅Al₂, C_p(T) shows a magnetic transition at 1.7 K. C_p(T) for GdPd₅Al₂ shows a peak at 6 K, followed by a broad anomaly around 3 K. Within this series, T_N's for CePd₅Al₂ and NdPd₅Al₂ clearly deviate from the relation predicted by de Gennes scaling, which is ascribed to the CEF effect.     

Complex magnetic behavior in Ce5Rh4Sn10 doped with La

Ce₅Rh₄Sn₁₀ is known as a heavy-fermion antiferromagnet with magnetic-ordering temperature T _N2?=?4.3?K. We report a new antiferromagnetic phase transition at T _N1?=?1.4?K. In the series of Ce_1?x La _x Rh₄Sn₁₀ compounds both magnetic phase transitions at T _N1 and T _N2 shift towards lower temperatures. For Ce₃La₂Rh₄Sn₁₀ and Ce_3.5La_1.5Rh₄Sn₁₀ we found only one transition. In the temperature region T _N1<T<T _N2, the magnetization M measured as a function of magnetic field H exhibits a large jump at very low fields of ～0.03?T with a very small hysteresis loop. This abnormal behavior could be due to spin-flip transitions. Thermodynamical properties of the series of Ce_1?x La _x Rh₄Sn₁₀ compounds are discussed in terms of the antiferromagnetic Kondo lattice.     

Anomalous ultrasonic attenuation and velocity change in LaAl2

The attenuation and velocity of longitudinal waves has been measured in superconducting and normal LaAl₂ in the frequency range from 15 to 225 MHz. Both in the superconducting and in the normal state an unusually large damping has been found superimposed on the relatively small attenuation due to the well known electron-phonon interaction. On cooling into the superconducting state the attenuation starts to drop only at temperatures well below T_c and — as in insulating glasses — it rises again below 0.8 K. In our view this indicates that two-level systems similar to the ones found in amorphous superconductors may be present in our crystalline LaAl₂ sample and are responsible for the observed acoustic anomalies.     

Magnetocaloric properties of the La0.7Pr0.3Fe13−xSix compounds

Magnetic entropy change ΔS_m, hysteresis loss and refrigerant capacity of NaZn₁₃-type La_0.7Pr_0.3Fe_13−xSi_x (1.5?x?2.0) compounds have been investigated. The Curie temperature T_C increases linearly with the increase of silicon concentration. Although the maximum value of ΔS_m under a field change of 0−5 T decreases from 30.5 to 11.4 J/kg K as x increases from 1.5 to 2.0, the hysteresis loss at T_C reduces remarkably from 89.2 J/kg for x=1.5 to zero for x=2.0 because the increase of Si content can weaken the itinerant electron metamagnetic transition. The effective refrigerant capacity RC_eff is maintained at high values of 362−439 J/kg for a field change of 0−5 T. This implies that a large ΔS_m and a high RC_eff can be achieved simultaneously in the La_0.7Pr_0.3Fe_13−xSi_x compounds.     

Enhancement of superconductivity by an external magnetic field in magnetic alloys

An infinite-volume limit solution of the thermodynamics of a BCS superconductor containing spin 1/2 and 7/2 magnetic impurities, obtained recently in [D. Borycki, J. Ma?kowiak, Supercond. Sci. Technol. 24, 035007 (2011)] is exploited to derive the expressions for critical magnetic field $\mathcal{H}_c$ (T). The credibility of the resulting thermodynamically limited theoretical equations, which depend on the magnetic coupling constant g and impurity concentration c, is verified on the experimental data for the following superconducting alloys: LaCe, ThGd and SmRh₄B₄. Good quantitative agreement with experimental data is found for sufficiently small values of c. The discrepancies between theoretical and experimental values of $\mathcal{H}_c$ (T) for larger values of c in case of LaCe and ThGd are reduced by introducing the concept of the effective temperature $\tilde T$ , which accounts for the Coulomb interactions between the electron gas and impurity ions. At low temperatures, the critical magnetic field is found to increase with decreasing temperature T. This enhancement of the critical magnetic field provides evidence of the Jaccarino-Peter effect, which was experimentally observed in the Kondo systems like LaCe, (La_{1 ? x} Ce _x )Al₂ and also in the pseudoternary compounds, including Sn_{1 ? x} Eu _x Mo₆S₈, Pb_{1 ? x} Eu _x Mo₆S₈ and La_{1.2 ? x} Eu _x Mo₆S₈. The effect of an external magnetic field $\mathcal{H}$ on a BCS superconductor perturbed by magnetic impurities was also studied. On these grounds, by analyzing the dependence of superconducting transition temperature T _c on $\mathcal{H}$ of (La_{1 ? x} Ce _x )Al₂, we have shown, that for certain parameter values, external magnetic field compensates the destructive effect of magnetic impurities.