New Superconducting and Magnetic Phases Emerge on the Verge of Antiferromagnetism in CeIn3_3

We report the discovery of new superconducting and novel magnetic phases in CeIn$_3$ on the verge of antiferromagnetism (AFM) under pressure ($P$) through the In-nuclear quadrupole resonance (NQR) measurements. We have found a $P$-induced phase separation of AFM and paramagnetism (PM) without any trace for a quantum phase transition in CeIn$_3$. A new type of superconductivity (SC) was found in $P=2.28-2.5$ GPa to coexist with AFM that is magnetically separated from PM where the heavy fermion SC takes place. We propose that the magnetic excitations such as spin-density fluctuations induced by the first-order magnetic phase transition might mediate attractive interaction to form Cooper pairs.Comment: 4 pages, 4 EPS figures, submitted to J. Phys. Soc. Jp

Meta-orbital Transition in Heavy-fermion Systems: Analysis by Dynamical Mean Field Theory and Self-consistent Renormalization Theory of Orbital Fluctuations

We investigate a two-orbital Anderson lattice model with Ising orbital intersite exchange interactions by means of dynamical mean field theory combined with the static mean field approximation of the intersite orbital interactions. Focusing on Ce-based heavy-fermion compounds, we examine the orbital crossover between the two orbital states, when the total f-electron number per site n_f is n_f ~ 1. We show that a "meta-orbital" transition, at which the occupancy of the two orbitals changes steeply, occurs when the hybridization between the ground-state f-electron orbital and conduction electrons are smaller than that between the excited f-electron orbital and conduction electrons. Near the meta-orbital critical end point, the orbital fluctuations are enhanced, and couple with the charge fluctuations. A critical theory of the meta-orbital fluctuations is also developed by applying the self-consistent renormalization theory of itinerant electron magnetism to the orbital fluctuations. The critical end point, first-order transition and crossover are described within Gaussian approximations of orbital fluctuations. We discuss the relevance of our results to CeAl2, CeCu2Si2, CeCu2Ge2 and the related compounds, which all have low-lying crystalline-electric-field excited states.Comment: 11 pages, 6 figures, J. Phys. Soc. Jpn. 79, (2010) 11471

Calorimetric and transport investigations of CePd_{2+x}Ge_{2-x} (x=0 and 0.02) up to 22 GPa

The influence of pressure on the magnetically ordered CePd_{2.02}Ge_{1.98} has been investigated by a combined measurement of electrical resistivity, $\rho(T)$, and ac-calorimetry, C(T), for temperatures in the range 0.3 K<T<10 K and pressures, p, up to 22 GPa. Simultaneously CePd_2Ge_2 has been examined by $\rho(T)$ down to 40 mK. In CePd_{2.02}Ge_{1.98} and CePd_2Ge_2 the magnetic order is suppressed at a critical pressure p_c=11.0 GPa and p_c=13.8 GPa, respectively. In the case of CePd_{2.02}Ge_{1.98} not only the temperature coefficient of $\rho(T)$, A, indicates the loss of magnetic order but also the ac-signal $1/V_{ac}\propto C/T$ recorded at low temperature. The residual resistivity is extremely pressure sensitive and passes through a maximum and then a minimum in the vicinity of p_c. The (T,p) phase diagram and the A(p)-dependence of both compounds can be qualitatively understood in terms of a pressure-tuned competition between magnetic order and the Kondo effect according to the Doniach picture. The temperature-volume (T,V) phase diagram of CePd_2Ge_2 combined with that of CePd_2Si_2 shows that in stoichiometric compounds mainly the change of interatomic distances influences the exchange interaction. It will be argued that in contrast to this the much lower p_c-value of CePd_{2.02}Ge_{1.98} is caused by an enhanced hybridization between 4f and conduction electrons.Comment: 9 pages, 7 figure

Signatures of valence fluctuations in CeCu2Si2 under high pressure

Simultaneous resistivity and a.c.-specific heat measurements have been performed under pressure on single crystalline CeCu2Si2 to over 6 GPa in a hydrostatic helium pressure medium. A series of anomalies were observed around the pressure coinciding with a maximum in the superconducting critical temperature, $T_c^{max}$. These anomalies can be linked with an abrupt change of the Ce valence, and suggest a second quantum critical point at a pressure $P_v \simeq 4.5$ GPa, where critical valence fluctuations provide the superconducting pairing mechanism, as opposed to spin fluctuations at ambient pressure. Such a valence instability, and associated superconductivity, is predicted by an extended Anderson lattice model with Coulomb repulsion between the conduction and f-electrons. We explain the T-linear resistivity found at $P_v$ in this picture, while other anomalies found around $P_v$ can be qualitatively understood using the same model.Comment: Submitted to Phys. Rev.

Mass Enhancement in an Intermediate-Valent Regime of Heavy-Fermion Systems

We study the mechanism of the mass enhancement in an intermediate-valent regime of heavy-fermion materials. We find that the crossovers between the Kondo, intermediate valent, and almost empty f-electron regimes become sharp with the Coulomb interaction between the conduction and f electrons. In the intermediate-valent regime, we find a substantial mass enhancement, which is not expected in previous theories. Our theory may be relevant to the observed nonmonotonic variation in the effective mass under pressure in CeCu2Si2 and the mass enhancement in the intermediate-valent compounds alpha-YbAlB4 and beta-YbAlB4.Comment: 4 pages, 4 figure

Unconventional Superconductivity in Heavy Fermion Systems

We review the studies on the emergent phases of superconductvity and magnetism in the $f$-electron derived heavy-fermion (HF) systems by means of the nuclear-quadrupole-resonance (NQR) under pressure. These studies have unraveled a rich variety of the phenomena in the ground state of HF systems. In this article, we highlight the novel phase diagrams of magnetism and unconventional superconductivity (SC) in CeCu$_2$Si$_2$, HF antiferromagnets CeRhIn$_5$, and CeIn$_3$. A new light is shed on the difference and common features on the interplay between magnetism and SC on the magnetic criticality.Comment: 15 pages, 13 figures, to appear in J. Phys. Soc. JPN, 74, No.1 (2005), special issue "Kondo Effect- 40 Years after the Discovery

Gutzwiller Method for an Extended Periodic Anderson Model with the c-f Coulomb Interaction

We study an extended periodic Anderson model with the Coulomb interaction Ucf between conduction and f electrons by the Gutzwiller method. The crossovers between the Kondo, intermediate-valence, and almost empty f-electron regimes become sharper with Ucf, and for a sufficiently large Ucf, become first-order phase transitions. In the Kondo regime, a large enhancement in the effective mass occurs as in the ordinary periodic Anderson model without Ucf. In addition, we find that a large mass enhancement also occurs in the intermediate-valence regime by the effect of Ucf.Comment: 9 pages, 7 figure

Multiband Superconductivity in Heavy Fermion Compound CePt3Si without Inversion Symmetry: An NMR Study on a High-Quality Single Crystal

We report on novel superconducting characteristics of the heavy fermion (HF) superconductor CePt3Si without inversion symmetry through 195Pt-NMR study on a single crystal with T_c= 0.46 K that is lower than T_c= 0.75 K for polycrystals. We show that the intrinsic superconducting characteristics inherent to CePt3Si can be understood in terms of the unconventional strong-coupling state with a line-node gap below T_c= 0.46 K. The mystery about the sample dependence of T_c is explained by the fact that more or less polycrystals and single crystals inevitably contain some disordered domains, which exhibit a conventional BCS s-wave superconductivity (SC) below 0.8 K. In contrast, the Neel temperature T_N= 2.2 K is present regardless of the quality of samples, revealing that the Fermi surface responsible for SC differ from that for the antiferromagnetic order. These unusual characteristics of CePt3Si can be also described by a multiband model; in the homogeneous domains, the coherent HF bands are responsible for the unconventional SC, whereas in the disordered domains the conduction bands existing commonly in LaPt3Si may be responsible for the conventional s-wave SC. We remark that some impurity scatterings in the disordered domains break up the 4f-electrons-derived coherent bands but not others. In this context, the small peak in 1/T_1 just below T_c reported in the previous paper (Yogi et al, 2004) is not due to a two-component order parameter composed of spin-singlet and spin-triplet Cooper pairing states, but due to the contamination of the disorder domains which are in the s-wave SC state.Comment: 10 pages, 9 figures, Accepted for publication in J. Phys. Soc. Jpn., vol.78, No.1 (2009