Session B8: Focus Session: Novel Superconductors II: CeCoIn5 and NaxCoO2

Influence of Al doping on the superconducting properties of magnesium diboride single crystals

We present a systematic study of the evolution of the lower ($H_{c1}$) and upper ($H_{c2}$) critical fields, gap values and Sommerfeld coefficient of Mg$_{1-x}$Al$_{x}$B$_2$ single crystals (for $x = 0$, $0.1$ and $\geq 0.2$). We show that $H_{c1}$ and $H_{c2}$ deduced from Hall probe magnetization and specific heat measurements, respectively are both decreasing with increasing doping content. The corresponding anisotropy parameter $\Gamma_{H_{c2}}(0) = H^{ab}_{c2}(0)/H^c_{c2}(0)$ value also decreases from $\sim 5$ in pure MgB$_2$ samples down to $\sim 1.5$ for $x \geq 0.2$ whereas $\Gamma_{H_{c1}}(0)=H^c_{c1}(0)/H^{ab}_{c1}(0)$ remains on the order of 1 in all samples. The magnetic field dependence of the anisotropy parameter $\Gamma_{H_{c1}} \leq \Gamma(H) \leq \Gamma_{H_{c2}}$ has then been deduced from a detailled analysis of the angular dependence of the Sommerfeld coefficient for different values of the applied fields. The small and large gap values have been obtained both by fitting the temperature dependence of the zero field electronic contribution to the specific heat to the two gap model and by point contact spectroscopy measurements. Both measurements led to very similar values and the evolution of those gaps with Al concentration suggests that both band filling and interband scattering effects are present.\\ This work has been performed in collaboration with : L.Lyard, C.Marcenat, J.Marcus, Z.Holanova, P.Szabo, P.Samuely, B.W.Kang, H-J.Kim, H-S.Lee, H-K.Lee,S-I.Lee, S.Tajima and S.Lee.   

De Haas-van Alphen Effect across the CeRh$_{1-x}$Co$_x$In$_5$ Phase Diagram

We present de Haas-van Alphen (dHvA) data across the phase diagram of CeRh$_{1-x}$Co$_x$In$_5$ down to ultra-low temperatures. The chemical substitution of Co for Rh, which changes the electronic structure of CeRhIn$_5$ from a localized 4\textit{f} electron configuration at $x=0$ to itinerant behaviour at $x=1$, is analogous to the application of external pressure to the antiferromagnet CeRhIn$_5$, which was shown to exhibit a discontinuous evolution of its Fermi surfaces near 2.35GPa [Shishido \textit{et al.} J Phys. Soc. Jpn. \textbf{74}, 1103 (2005)]. Exploiting the rotational degree of freedom afforded by ambient pressure measurements of single-crystal specimens, we analyze both the Fermi surface geometry and quasiparticle effective masses of CeRh$_{1-x}$Co$_x$In$_5$ as a function of chemical substitution in order to investigate the evolution of electronic structure through the antiferromagnetic quantum critical point of this system.   

Synthesis and Characterization of High Quality Single Crystal CeCoIn$_{5}$

Novel electronic materials play an ever-increasing role in technological applications. They cover a rather broad spectrum of materials that exhibit exotic transport and magnetic properties. These include superconductors, narrow band gap semiconductors, electronically active polymers, colossal magneto resistive systems, nanoscale electronic materials, and Fullerenes, to name a few. Understanding the interplay between strong electronic correlations and structural properties of these materials is important. In this communication, we report the~growth and preliminary characterization of single crystal~Cerium based~intermetallic compounds that are known to show superconductivity at Tc = 2.3K.   

NMR study of the Cd-doped CeCoIn$_{5}$ heavy-fermion superconductor

We have investigated the local environment of In and Co sites of the heavy fermion compound CeCo(In$_{1-x}$Cd$_{x}$)$_{5}$ ($x$ = 0.0, 0.10 and 0.15) using Nuclear Magnetic Resonance (NMR) and Nuclear Quadrupole Resonance (NQR) measurements. Recently, it was found that Cd-doping acts as an electronic tuning agent in CeCoIn$_{5}$ and that superconductivity (SC) and antiferromagnetism (AFM) coexist at ambient-pressure for $0.05 < x < 0.15$. The NMR/NQR spectra of In and Co indicate the presence of electronic inhomogeneity, and the spin-lattice relaxation rate $1/T_{1}$ shows dramatic change in the spin dynamics only below $T \simeq 2T_{c}$.   

Impurities induced configuration space phase transition in the Fulde-Ferrell-Larkin-Ovchinnikov state

We study the effects of impurities on the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state at low temperature in d-wave superconductors. At low impurity concentration, the order parameter remains two-dimensional lattice like. When the impurity concentration reaches certain level, nodal lines of the order parameter no-longer cross each other and the variation of the order parameter becomes essentially one dimensional. We will also discuss the effects of impurities on the quasi-particle density of states.   

Fermi-liquid effects in the Fulde-Ferrell-Larkin-Ovchinnikov state of two-dimensional d-wave superconductors

We study the effects of Fermi-liquid interactions on quasi-two-dimensional d-wave superconductors in a magnetic field. The phase diagram of the superconducting state, including the periodic Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state in high magnetic fields, is discussed for different strengths of quasiparticle many-body interactions within Landau's theory of Fermi liquids. Decreasing the Fermi-liquid parameter $F_0^a$ causes the magnetic spin susceptibility to increase, which in turn leads to a reduction of the FFLO phase. It is shown that a negative $F_0^a$ results in a first-order phase transition from the normal to the uniform superconducting state in a finite temperature interval. Finally, we discuss the thermodynamic implications of a first-order phase transition for the heavy-fermion superconductor CeCoIn$_5$.   

Evolution of FFLO state of CeCo(In$_{1-x}$Cd$_{x})_{5}$

Unconventional superconductor CeCoIn$_{5}$ at high magnetic field displays first order superconducting transition and an additional high field-low temperature superconducting phase (previously proposed to be an inhomogeneous superconducting FFLO state). Both phenomena were attributed to strong Pauli limiting effects. Our specific heat measurements on low Cd-doping (for In) samples, at fields close to the superconducting critical field $H_{c2}$, show that superconducting transition remains first order for samples with $H_{c2}$ up to 5.7 T (from 4.95 T in a pure compound), for field out of plane orientation ($H \quad \vert \vert $ c). We discuss systematic evolution of the proposed FFLO state with Cd content.   

Specific Heat of Na$_{0.35}$CoO$_{2}\cdot $1.3H$_{2}$O: Effects of Sample Age; Two Energy Gaps; Non-Magnetic Pair Breaking

Specific-heat measurements on three samples of Na$_{0.35}$CoO$_{2}\cdot $1.3H$_{2}$O show an evolution of the superconductivity and its eventual disappearance with increasing sample age. The results, in combination with other recent work [1], provide a basis for understanding the extreme ``sample dependence'' of the properties of this material. Samples of different age are in effect samples of slightly different materials: A non-magnetic pair-breaking action produces a residual density of electron states that increases with sample age. It occurs preferentially in the electron band (one of two with different energy gaps) with the smaller gap, producing a change in the nature of the superconducting condensation. It also weakens the overall electron pairing of the superconducting state until it gives way to a competing ordering, possibly a CDW. The same combinations of features in the specific heat have been seen in measurements on other individual samples, showing that they are ``intrinsic''. The changes in the specific heat are evidently related to structural and electronic changes that occur on a similar time scale [1], and include an increasing concentration of O vacancies, which could be the pair-breaking scattering centers. \newline [1] P. W. Barnes et al., Phys. Rev. B \textbf{72}, 134515 (2005).   

Systematic ARPES study on high Na-level Na$_{x}$CoO$_{2}$ (x$>$0.75)

The phase diagram of the cobaltae Na$_{x}$CoO$_{2}$, with varying Na concentration x, is very rich and complicated. At the high-doping regime (x$>$0.75), the system was found to be more correlated, with a spin-density-wave state emerging at low temperatures. A previous ARPES study has observed a usually large Fermi surface which apparently violates the Luttinger theorem at x = 0.8. We will report our recent ARPES results of the high Na-level Na$_{x}$CoO$_{2}$ (x$>$0.75) samples.   

Complete d-Band Dispersion Relation in Sodium Cobaltates

We utilize fine-tuned polarization selection coupled with excitation-energy variation of photoelectron signal to image the complete d-band dispersion relation in sodium cobaltates. A hybridization gap anticrossing is observed along the Brillouin zone corner and the full quasiparticle band is found to emerge as a many-body entity lacking a pure orbital polarization. At low dopings, the quasiparticle bandwidth (Fermion scale, many- body EF $\sim$ 0.25 eV) is found to be smaller than most known oxide metals. The low-lying density of states is found to be in agreement with bulk-sensitive thermodynamic measurements for nonmagnetic dopings where the 2D Luttinger theorem is also observed to be satisfied.   

Angle-resolved photoemission study of superconducting cobalt oxide Na$_{x}$CoO$_{2}$ yH$_{2}$O

Superconducting cobalt oxide Na$_{x}$CoO$_{2}$ yH$_{2}$O is studied by angle-resolved photoemission spectroscopy. We report the Fermi surface topology and electronic structure near the Fermi level in the normal state of Na$_{x}$CoO$_{2}$ yH$_{2}$O. Our result indicates the presence of the hexagonal Fermi surfce centered at $\Gamma $ point, while the small pocket Fermi surfaces along $\Gamma $-K direction are absent, similar to Na$_{x}$CoO$_{2}$. The top of the e$_{g}$' band, which is expected in band calculations to form the small pocket FSs, extends to within $\sim $30 meV below Fermi level, more closer to Fermi level than in Na$_{x}$CoO$_{2}$. Its possible role in superconductivity will be discussed, comparing with other experimental and theoretical results.   

Phase Diagrams, Thermodynamic Quantities and Possible Two Different Superconducting States of Multiorbital Superconductor NaxCoO2.yH2O

Motivated by recently reported experimental phase diagrams, we theoretically study the effects of CoO$_6$ distortion on the electronic structure in Na$_x$CoO$_2 \cdot y$H$_2$O by constructing the multiorbital model. By analyzing the model, we show the deformation of band dispersions and Fermi-surface (FS) topology caused by the variation of CoO$_2$-layer thickness. We propose that two different pairing states, an extended $s$-wave and a $p$-wave pairings, are possible depending on the layer thickness or the FS topology. Furthermore, microscopic calculations of thermodynamic quantities show that two different specific-heat data and two distinct superfluid- density data can be explained fairly well for these two pairing states. We also discuss that inconsistent and scattered experimental results on the magnetic properties, which have been reported from NMR/NQR, $\mu$SR and neutron-scattering measurements, can also be clarified well if we consider the strong layer-thickness dependence.   

Unified origin for superconductivity and 3D magnetism in Na$_x$CoO$_2$

The pairing mechanism of a hydrated cobaltate superconductor Na$_x$CoO$_2\cdot y$H$_2$O has been of great interest recently. Some experiments point toward unconventional pairing, while others suggest s-wave-like pairing. Recently, based on a fluctuation exchange study for an extended Hubbard model, we have proposed a possibility of unconventional s-wave pairing, where the nesting between the outer and the inner Fermi surfaces that arise due to the local minimum structure of the band at the $\Gamma$ point plays an important role. The superconducting gap changes sign between the inner and outer Fermi surfaces due to the repulsive interaction originating from the spin fluctuations at the nesting vector, while the gap does not change sign within each Fermi surface. We have further found that this nesting becomes three dimensional when a small hopping integral in the z-direction is considered, which gives rise to an in-plane ferromagnetic, out-of-plane antiferromagnetic spin correlation, consistent with the experiments for the non-hydrated Na-rich systems. The calculated magnetic ordering temperature and the spin wave dispersion explain well the experimental results.