R. Lackner

Structure and physical properties of type-I clathrate solid-solution Ba 8 Pt x Ge 46 − x − y ◻ y ( ◻ = vacancy )

Formation, crystal chemistry, and physical properties were investigated for the solid-solution Ba{sub 8}Pt{sub x}Ge{sub 46-x-y}{open_square}{sub y} ({open_square} is a vacancy) deriving from binary clathrate Ba{sub 8}Ge{sub 43}{open_square}{sub 3} with a solubility limit of {approx}3.5 Pt atoms/f.u. at T=800 deg. C. Structural investigations throughout the homogeneity region confirm isotypism with the cubic primitive clathrate type-I structure (space group type Pm3n) and lattice parameters ranging from a=1.0657(2) nm for Ba{sub 8}Ge{sub 43}{open_square}{sub 3} to a=1.0752(2) nm for Ba{sub 8}Pt{sub 3.5}Ge{sub 41.5}{open_square}{sub 1.0}. Phase relations for the region concerning the clathrate solution were derived at subsolidus temperatures as well as at 800 deg. C. Transport properties evidence electrons as the majority charge carriers in the system with a slight dependency on the Pt content. The system is located close to a semiconducting regime with a gap in the electronic density of states of a few thousand K. No low temperature maximum is obvious from thermal conductivity which is dominated by the lattice contribution. Thermal conductivity furthermore documents a high efficiency of phonon scattering on vacancies.

New orthorhombic modification of equiatomic CePdAl

The crystal structure of a new low-temperature modification of CePdAl was determined from single-crystal x-ray data: CePdAl-type; space group Pmmn (No. 59), Z = 14, oP42, a = 0.426 07 nm, b = 2.887 58 nm, c = 0.721 90 nm; RF = 0.048. Physical properties of orthorhombic CePdAl are governed by a mutual balance of the RKKY interaction, the Kondo effect and crystalline electric field splitting, resulting in antiferromagnetic ordering below ≈2.5 K. Electronic transport is reminiscent of a textbook-like Kondo lattice, which comes along with a significant negative magnetoresistance of more than 50% at low temperatures. Although geometrical frustration is absent when compared to the hexagonal modification of CePdAl, the ordering temperature TN is even smaller. A possible cause is enhanced Kondo interactions.

REPt3Si (RE = La, Pr, Nd, Sm and Gd): isotypes of the heavy fermion superconductor CePt3Si

Novel representatives REPt3Si (RE = La, Pr, Nd, Sm and Gd) of the CePt3B type have been synthesized and characterized by means of Rietveld x-ray powder diffraction. Measurements of the magnetic susceptibility, isothermal magnetization, temperature dependent specific heat as well as temperature and field dependent resistivity were employed to derive basic information on the low temperature behaviour. Long range antiferromagnetic order from 2.2 K (CePt3Si) to 15.1 K (GdPt3Si) was observed. PrPt3Si, however, is non-magnetic, at least down to 400 mK, as a consequence of crystalline electric field splitting of the non-Kramers ion Pr3+ in tetragonal symmetry. Whilst the ordering temperature of SmPt3Si appears to be almost unaffected in external fields up to 12 T, magnetic order in GdPt3Si, although twice as high as for the Sm homologue, is easily suppressed by external magnetic fields due to the absence of anisotropy.

Pair breaking by nonmagnetic impurities in the noncentrosymmetric superconductor CePt3Si

We have studied the effect of Ge substitution and pressure on the heavy-fermion superconductor CePt3Si. Ge substitution on the Si site acts as negative chemical pressure leading to an increase in the unit-cell volume but also introduces chemical disorder. We carried out electrical resistivity and ac heat-capacity experiments under hydrostatic pressure on CePt3Si1-xGex (x=0, 0.06). Our experiments show that the suppression of superconductivity in CePt3Si1-xGex is mainly caused by the scattering potential, rather than volume expansion, introduced by the Ge dopants. The antiferromagnetic order is essentially not affected by the chemical disorder.

Low temperature properties of the ternary compounds CePt2B and CePt3B

Ground state properties of ternary CePt2B and CePt3B have been characterized from studies of various bulk properties. Both compounds order antiferromagnetically below 2.1 and 7.5 K, respectively, and the latter exhibits an additional phase transition at T1 = 4.5 K. The spin wave dispersion relation of these systems is gapped, with and , for CePt2B and CePt3B, respectively. Kondo-type interaction and crystal field splitting are present, reducing the absolute values of the ordered moments. The characteristic temperatures are TK = 3.5–5 K for the former and K for the latter.

The influence of substitution and doping on the thermoelectric properties of CePd 3

Intermediate valent CePd₃ exhibits one of the highest Seebeck coefficients (up to 120 muV/K at a broad maximum around 150 K) among Ce based intermetallics. Such strongly enhanced S(T) values are referred to an intense Kondo interaction responsible for a characteristic temperature T_K of about 240 K. Substitutions of Pd in CePd₃ by Rh increase the Ce valence towards the non-magnetic 4+ state, while a substitution with Ag shifts the valence towards 3+. Doping interstitial lattice-sites with B also reduces the valency. We have synthesized samples with following compositions Ce(Pd_1-xRh_x)₃B_0.05 and Ce(Pd_0.94-xRh_xAg_0.06)₃ to examine the possibility to compensate the loss of thermopower due to Ag substitution or B doping by Rh substitution and whether ZT can be improved by a further increase of the phonon scattering part of the thermal conductivity. The electrical resistivity rho(T), thermal conductivity lambda(T) and S(T) were measured allowing to calculate ZT. A further investigation of the measured data in the scope of empirical and theoretical models is presented.

CePt3Si: Heavy Fermion Superconductivity and Magnetic Order without Inversion Symmetry

Ternary CePt3Si crystallizes in the tetragonal P4mm structure which lacks a center of inversion. Antiferromagnetic order sets in at TN ≈ 2.2 K followed by superconductivity (SC) below Tc ≈ 0.75 K. Large values of H′c2 ≈ −8.5 T/K and Hc2(0) ≈ 4 to 5 T were derived, referring to Cooper pairs formed out of heavy quasiparticles. The mass enhancement originates from Kondo interactions with a characteristic temperature TK of roughly 10 K. NMR and μSR results show that both magnetic order and SC coexist on a microscopic scale without having spatial segregation of both phenomena. The absence of an inversion symmetry gives rise to a lifting of the degeneracy of electronic bands by spin‐orbit coupling. As a consequence, the SC order parameter may be composed of spin‐singlet and spin‐triplet components as indicated from a very unique NMR relaxation rate 1/T1 and a linear temperature dependence of the penetration depth λ. A helical modification of the order parameter would explain the absence of significant anisotrop...