Session J12: Experimental Studies of the Heavy Fermion 115 Compounds

3D Fermi Surface Mapping of Ce$_2$RhIn$_8$

Photon-dependent multi-Brillouin zone angle-resolved photoemission (ARPES) measurements are presented for the antiferromagnet heavy fermion bilayer compound Ce$_2$RhIn$_8$. Highly automated Fermi Surface (FS) mapping for excitation energies of 90-120 eV allow visualization of k$_z$-variations of the electronic structure and permit a k$_z$-tomographic determination of the three-dimensional (3D) FS topology with sufficient detail for quantitative comparison to dHvA orbit areas and LDA-predicted topological shapes. In addition to confirming the quasi-2D circular and square topologies centered on the Brilluoin zone corner, highly 3D k$_z$-variations are observed along the (100) directions $\Gamma$-X (Z-R). Results are compared to a similar ARPES determination of the 3D electronic structure and FS of the single layer compound CeCoIn$_5$.   

Anisotropic quantum criticality in heavy-fermion metal CeCoIn5

We previously reported a violation of the Wiedemann-Franz law in the heavy-fermion metal CeCoIn5 when tuned to its quantum critical point, depending on the direction of electron motion relative to the crystal lattice, which points to an anisotropic destruction of the Fermi surface [1]. Here we present new measurements of electric, thermal and thermo-electric transport coefficients which reveal different anisotropic responses. [1] M.A. Tanatar et al., Science 316, 1320 (2007).   

Nature of the superconducting state of CeCoIn$_5$ as revealed by NMR.

We report low temperature nuclear magnetic resonance (NMR) measurements of the heavy-fermion superconductor CeCoIn$_5$ in high magnetic fields. The effect of the RF penetration on the NMR spectrum for the different parts of the phase diagram is studied. The implications of this study for the nature of a possible inhomogeneous superconducting state, the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, are discussed.   

Thermal transport in the vortex state of heavy-fermion superconductor CeCoIn$_5$

The thermal conductivity of heavy-fermion superconductor CeCoIn$_5$ was measured as a function of temperature and magnetic field throughout the vortex state. An anomalous decrease is found in the field dependence at low fields and low temperature. We discuss the origin of this behavior and its possible relation to the presence of uncondensed electrons, found in a previous doping study of this material [1]. \newline \newline [1] M.A. Tanatar et al., Phys. Rev. Lett. 95, 067002 (2005).   

NMR investigation of a hole doped CeCoIn5

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 $0.05 < x < 0.15$. It has also been observed on Cd doped compounds that pressure $P$ recovers the SC ground-state observed for the undoped compound suggesting it as a reversible tuning parameter. In this work we report data indicating that these phases indeed coexist microscopically. The NMR/NQR spectra of In and Co indicate the presence of electronic inhomogeneity, and the spin-lattice relaxation measurements $T_{1}^{-1}$ suggest that Cd doping induces changes to the low frequency spin dynamics only below $T \simeq 5 K$. Furthermore, $T_{1}^{-1}$ measurements for $x = 0.10$ under pressure show a different spin dynamics response in the paramagnetic state, in contrast with the effect of the chemical pressure. We show that chemical doping and pressure are not equivalent.   

Thermal transport at a field-tuned quantum critical point in CeCoIn5

The heavy-fermion metal CeCoIn5 exhibits a field-tuned quantum critical point which coincides with the upper critical field for superconductivity for directions of magnetic field both parallel [1] and perpendicular to the tetragonal c-axis of the material [2]. Here we report a study of this field-tuned critical point using electrical resistivity and thermal conductivity measurements performed in magnetic fields parallel to the conducting plane. [1] J. Paglione et al., Phys. Rev. Lett. 91, 246405 (2003). [2] F. Ronning et al., Phys. Rev. B. 71, 104528 (2005).   

Evolution of the superconducting properties of CeCoIn$_5$ with Yb substitution

We report on the evolution of the physical properties of the Yb substitution series starting from the unconventional superconductor (SC) CeCoIn$_5$ to the isostructural normal metal YbCoIn$_5$. This study was motivated by the recent results of Cd or Hg substitution at the percentage level on the In site in CeCoIn$_5$ which for low concentrations first was shown to lead to the coexistence of antiferromagnetism with SC and to a complete suppression of SC at higher concentrations. At the same time, the lattice constant of YbCoIn$_5$ indicates that Yb enters this compound in a partially divalent configuration suggesting that Yb could also be suitable for doping holes into CeCoIn$_5$. In our substitution series we find that that the unit cell volume stays roughly constant up to an Yb concentration of about 40~\%, after which the cell volume begins to decrease gradually to the value of YbCoIn$_5$. At the same time we observe a gradual suppression of the transition temperature $T_c$ to zero at an Yb concentration of 60~\%. Interestingly, the shape of $H$-$T$-phase diagram remains the same when the axis is scaled with the respective $T_c$ and upper critical field $H_{c2}$, suggesting that the ratio between Pauli $H_p$ and the orbital critical field $H_{c20}$ remains constant.   

Quantum oscillations in heavy fermion CeRhIn5

Construction of Fermi surface is the key to understand the physical properties of variant materials. In correlated electron systems, the heavy carrier mass strongly attenuates the amplitude of quantum oscillations, and, therefore, the Fermi surface sheets from the heavy electrons are usually not observable at low fields. High magnet fields are an indispensable tool to uncover these important features of electronic structure. Furthermore, strong magnetic field above the Neel critical field may lead to the reconstruction or a volume change of Fermi surface in f-electron antiferromagnets (N. Harrison et al, PRL 99, 056401, 2007). Using the unique facilities in the national high magnetic field lab at LANL, in this presentation we will study the quantum oscillations in the heavy fermion compound CeRhIn$_5$ with a field up to 65T.   

Pressure effect of single ion Kondo temperature in Ce$_{.02}$La$_{.98}$RhIn$_{5}$

Near a critical pressure P$_{c} \quad \sim $ 25 kbar, CeRhIn$_{5}$ assumes characteristics of CeCoIn$_{5}$ at atmospheric pressure: they have comparable T$_{C}$, similar dHvA frequencies, and display quantum-critical behaviors. Many properties of CeCoIn$_{5}$ can be interpreted within a two-fluid phenomenology$^{1}$ in which there are interpenetrating fluids, a localized f-electron Kondo gas (energy scale T$_{K})$ and an interacting Kondo liquid (energy scale T*). We have measured transport properties of Ce$_{.02}$La$_{.98}$RhIn$_{5}$ under pressures to 50 kbar to determine T$_{K}$(P), which at P=0 is estimated to be $\sim $ 0.03K from specific heat measurements. T$_{K}$(P) increases rapidly, reaching $\sim $1.35K at 25 kbar, where it becomes comparable to T$_{K}$ ($\sim $1.8K) of CeCoIn$_{5}$ at P=0. A comparison of T$_{K}$(P) with T*(P), determined from the pressure studies of CeRhIn$_{5}$, reveals the same correlation between T$_{K}$ and T* inferred from a two-fluid analysis of CeCoIn$_{5}$, further supporting the similarity of these two compounds and the two-fluid phenomenology. [1] S. Nakatsuji et al., Phys. Rev. Lett. 92, 016401 (2004).   

Uniaxial Pressure and the Superconducting Transition of CeIrIn$_5$

We measure the superconducting transition temperature of CeIrIn$_5$ with applied uniaxial pressure. Heat capacity shows almost no shift in T$_c$ as pressure is applied along the {\em c}-axis, contrary to the large decrease expected from earlier thermal expansion measurements. With {\em a}-axis pressure, however, T$_c$ increases about 20 mK per kbar. These results indicate that another factor besides the {\em c}/{\em a} ratio has a strong effect on T$_c$. Furthermore, applied pressure along either crystal axis strongly reduces the size of the heat capacity transition. We will also discuss the effect of uniaxial pressure on the resistive transtition, which at ambient pressure occurs at a temperature well above the heat capacity transition. Finally, we present x-ray diffraction measurements correlating our applied pressures with changes in the crystal lattice constants.   

Evolution of the superconducting state through quantum criticality in CeRh$_{1-x}$Co$_x$In$_5$

The Ce-based 115 materials exhibit a host of novel ground states separated by experimentally tunable quantum instabilities. In the single-crystal alloy series CeRh$_{1-x}$Co$_x$In$_5$, long range antiferromagnetic order is gradually suppressed upon chemical substitution of Co for Rh and followed by a robust superconducting state extending to the 2.3~K transition of the infamous heavy-fermion superconductor CeCoIn$_5$. Here we present a thorough study of heat transport measurements of high- quality single crystals of CeRh$_{1-x}$Co$_x$In$_5$ for several different superconducting samples spanning both the coexistent magnetic and non-magnetic regions of the $x$-$T$ phase diagram. By extracting the residual ($T\to 0$ limit) electronic thermal conductivity of samples at several $x$ values, we analyze the nature of the superconducting state on either side of the incipient quantum critical point near $x\simeq 0.65$ and study the influence of coexistent magnetism on the pairing state of these materials.   

Electronic Duality in the Pressure-tuned Quantum Critical Metal CeRhIn$_{5}$

The heavy fermion compound CeRhIn$_{5}$ is a prototypical strongly correlated antiferromagnet where the localized 4f electron of Ce hybridizes weakly with ligand electrons. Applying pressure to this material increases hybridization and induces bulk unconventional superconductivity that arises from pressure-enhanced itinerancy of 4f electrons and that simultaneously coexists with large-moment antiferromagnetic order among localized 4f electrons. This microscopic coexistence of local-moment magnetic order and superconductivity in CeRhIn$_{5}$ is distinctly different from conventional models that attribute coexisting spin-density wave magnetism and superconductivity to a Fermi-surface instability. Electronic duality, which is unambiguously revealed in the single 4f electron of cerium in CeRhIn$_{5}$, is a new framework emerging from strongly correlated electron matter, ranging from the high-T$_{c}$ cuprates and heavy fermion superconductors to plutonium.   

Interplay of Magnetism and Superconductivity in CeM(In$_{1-x}$Hg$_x$)$_5$

The CeMIn$_5$ (M=Co, Rh, Ir) heavy fermion superconductors have attracted interest in recent years due to their high superconducting transition temperatures (e.g., $T_c$=2.3 K in CeCoIn$_5$), unconventional superconductivity, and magnetic- field induced exotic ground states. In particular, field- induced quantum criticality at the upper critical field H$_{c2} $=5 T in CeCoIn$_5$ and a possible field-induced magnetic state within the superconducting state suggests close proximity to antiferromagnetism.$^1$ The exciting discovery of slight changes in the electronic structure of CeMIn$_5$ with Cd or Hg substitution$^2$ at the percent level in CeCoIn$_5$ appears to have ``uncovered'' the hidden magnetism in this material. Therefore, substitution of Hg in CeMIn$_5$ offers yet another way to probe the proximity to magnetism in CeCoIn$_5$, the field-induced magnetic state under pressure in CeRhIn$_5$, and the coexistence of magnetism and superconductivity. The physical properties of CeM(In$_{1-x}$Hg$_x$)$_5$ system will be discussed and contrasted with those found with isoelectronic substitutions. $^1$ {J. Paglione {\textit{et al.}} Phys. Rev. Lett. {\textbf {91}} 246405 (2003); A. Binachi {\textit{et al.}} Phys. Rev. Lett. {\textbf{91}} 257001 (2003)} $^2$ {L.D. Pham {\textit{et al.}} Phys. Rev. Lett. {\textbf {97}} 056404 (2006)}   

$d_{x^{2}-y^{2}}$ paring symmetry of heavy fermion CeIrIn$_{5}$ remote from antiferromagnetic quantum critical point

Quasi-two dimensional heavy Fermion CeIrIn$_{5}$ involves two distinct superconducting domes in the phase diagram, which appear as a function of pressure or Rh substitution of Ir. In the analogy to CeCu$_{2}$Si$_{2}$, two distinct superconducting domes with different symmetry has been invoked. We report on the results of low-temperature thermal transport of CeIrIn$_{5}$ in the second dome, which locates away from an antiferromagnetic quantum critical point. The thermal conductivity is measured under a magnetic field rotated with respect to the crystal axes, which give direct evidence for superconducting gap structure. Clear fourfold oscillation with minima at [110] and [1-10] directions is observed as rotating magnetic field within the basal $ab$-plane, while no oscillation is observed within the $bc$-plane. In sharp contrast to previous reports that suggested $E_{g}$ symmetry with horizontal line node within the $ab$-plane [1], our results are most consistent with $d_{x^{2}- y^{2}}$ symmetry with vertical line nodes along the $c$-axis. These results imply that two superconducting domes have the same gap symmetry which appears to be mediated by antiferromagnetic spin fluctuations. \newline [1] H.~Shakeripour et al., Phys. Rev. Lett. 99, 187004 (2007).