Competition between phonon superconductivity and Kondo screening in mixed valence and heavy fermion compounds

We consider competition of Kondo effect and s-wave superconductivity in heavy fermion and mixed valence superconductors, using the phenomenological approach for the periodic Anderson model. Similar to the well known results for single-impurity Kondo effect in superconductors, we have found principal possibility of a re-entrant regime of the superconducting transition temperature, $T_c$, in heavy fermion superconductors in a narrow range of model parameters and concentration of f-electrons. Suppression of $T_c$ in mixed valence superconductors is much weaker. Our theory has most validity in the low-temperature Fermi liquid regime, without re-entrant behavior of $T_c$. To check its applicability, we performed the fit for the $x$-dependence of $T_c$ in Ce$_{1-x}$La$_x$Ru$_3$Si$_2$ and obtained an excellent agreement with the experimental data, although no re-entrance was found in this case. Other experimental data are discussed in the light of our theoretical analysis. In particular, we compare temperatures of the superconducting transition for some known homologs, i.e., the analog periodic lattice compounds with and without f-elements. For a few pairs of homologs superconductivity exists only in the heavy fermion materials, thus confirming uniqueness of superconductivity mechanisms for the latter. We suggest that for some other compounds the value of $T_c$ may remain of the same order in the two homologs, if superconductivity originates mainly on some light Fermi surface, but induces sizable superconducting gap on another Fermi surface,for which hybridization or other heavy fermion effects are more significant.Comment: 11 pages, 4 figures, pd

Effects of La substitution on superconducting state of CeCoIn5

We report effects of La substitution on superconducting state of heavy fermion superconductor CeCoIn5, as seen in transport and magnetization measurements. As opposed to the case of conventional superconductors, pair breaking by nonmagnetic La results in depression of Tc and indicates strong gap anisotropy. Upper critical field Hc2 values decrease with increased La concentration, but the critical field anisotropy, gamma=Hc2(a)/Hc2(c), does not change in the Ce_{1-x}La_xCoIn5 (x=0-0.15). The electronic system is in the clean limit for all values of x.Comment: Submitted to Phys. Rev.

Heavy fermions and superconductivity: “Superconducting spectroscopy” of non-magnetic impurities in CeCu₂Si₂

We report results on the normal-state (n-state) and the superconducting properties for CeCu2.2Si2 and the quasi-binary alloys Ce1-xMxCu2.2Si2 with M=La (x⩽11 at%) and M=Y(x⩽3 at%). Upon increasing the dopant concentration, the specific heat jump height, ΔC, is depressed much more strongly than Tc. This proves a pair-breaking effect of the La- and Y-induced “Kondo holes” — characterized by temperatures Th, La and Th, Y, that show an anticorrelation to the characteristic temperatures (T∗La, T∗Y) of the underlying Ce1-xMxCu2.2Si2 systems. Compared with La, Y impurities are more efficient in pair breaking and in destroying coherence in the normal state, presumably owing to a less anisotropic scattering potential

Investigation of new lanthanum-, cerium- and uranium-based ternary intermetallics

While searching for new Ce- and U-based “heavy fermion” systems, we have investigated the following intermetallic compounds: CeCu2Ge2, CeCu2Sn2, CeAg2Ge2, CeZn2Al2 (tetragonal ThCr2Si2-structure and modifications thereof); CeRu3Si2, URu3Si2 (hexagonal LaRu3Si2-structure); Ce2Cu7Al10, U2Cu7Al10 (trigonal Th2Mn17-structure); CeCu4Al8, CeAg4Al8, UCu4Al8 (tetragonal CeMn4Al8-structure); LaCu13−xAlx with x = 3, 5.5, and 6.5, as well as CeCu6.5Al6.5 (cubic NaZn13-structure). Resistive and inductive measurements were undertaken between T = 30 mK and room temperature, and two new superconductors were found: LaCu6.5Al6.5 (Tc = 0.65 K) and CeRu3Si2 (Tc = 1.2 K). The specific heat and thermopower of two of these systems have also been measured. CeCu4Al8 shows interesting features that are presumably related to fine structure in the “heavy-fermion” density of states near the Fermi level. CeCu6.5A16.5 shows a magnetic transition at T = 0.5 K which resembles a “spin-glass freezing” rather than a formation of long-range magnetic order

Critical magnetic fields and specific heats of heavy fermion superconductors

The present status concerning lower and upper critical magnetic fields, Bc1(T) and Bc2(T), and specific heats, Cs(T), of heavy-fermion superconductors is reviewed. For CeCu2Si2, the data near Tc are found to be in full accord with GL theory. Yet, a surprising enhancement of the elastic mean free path over that in the normal state is inferred. Structure in both Bc1(T) and Bc2(T) of UBe13 appears related to a bump in the normal-state specific heat (B = 0) around T = 0.5 This may be a precursor to the lower of the two calorimetric transitions inU0.97Th0.03Be13 which are confirmed by new Bc1(T)… results

Upper critical magnetic fields of the heavy fermion superconductors CeCu₂Si₂, UPt₃, and UBe₁₃: Comparison between experiment and theory

We report measurements of the upper critical magnetic field,B c2 (T), on single crystal and polycrystalline samples of the heavy-fermion superconductors CeCu2Si2, UPt3 and UBe13. Comparison is made with experimental results for a “conventional” superconductor (CeRu3Si2) and with results of the weak-coupling theory applied to both ans-wave superconductor in the “dirty-limit” and ap-wave superconductor. New anomalies are discovered to add to the unusualB c2 (T) behavior

Impurities in heavy fermion superconductors

Stimulated by the unusual phase diagram of the superconducting states in U]_,.Th,.Bei3 which furnished a renewed debate about the nature of the heavy-fermion superconduct¬ ing order parameter (conventionally anisotropic vs. unconventional), we have initiated a study of the effect of impurities on the superconducting state of CeCu2Si2. We conclude a 'diamagnetic' pair-breaking effect by the non-magnetic impurities La and Y and an additional "paramagnetic" one by Gd. Comparison with low-temperature resistivity results in the normal state leads to the assumption that the Tc — depression for vanishing dopant concentration is determined by a highly anisotropic scattering of the heavy quasiparticles off these impurities

CeCu₂Si₂ and UPt₃: Two different cases of heavy fermion superconductivity

Specific heat and thermal conductivity in the superconducting state of the Kondo-lattice system CeCu2Si2 and the paramagnon system UPt3 point to substantial mass anisotropies at the Fermi surface (FS): while in CeCu2Si2 bare band electrons at certain portions of the FS seem to remain normal, in UPt3 heavier fermions seem to stay normal and Cooper pairs are formed by somewhat less heavy ones. The upper critical field of UPt3 shows no influence of Pauli limiting and can reasonably well be explained with S = 0 as well as S = 1 pairing. CeCu2Si2 is strongly Pauli limited, independent of orientation. This substantiates recent Josephson experiments which show that CeCu2Si2 is a singlet superconductor

Specific heat measurements of Yb₄As₃: A heavy fermion system with low carrier concentration?

The specific heat of Yb4As3, a mixed-valence system, has been measured down to 100 mK in magnetic fields up to 4 T. At low temperature a nuclear contribution in the specific heat is seen. The Sommerfeld coefficient gamma is approximately 150 mj/K2 mole at zero field. The magnetic field opens up a gap at low temperatures

Coexistence of superconductivity and intermediate valence in CeRu<sub>3</suB>Si₂

Thermopower and superconductivity measurements are reported on CeRu 3Si 2 and Ce 1-xLa xRu 3Si 2. For the compound, a mixed valence derived contribution to the thermopower is found. The transition temperatures T c(x) of the alloys interpolate smoothly between T c(CeRu 3Si 2)=1.0K and T c(LaRu 3Si 2)=7.2K