The easily tuned balance among competing interactions in Kondo-lattice metals allows access to a zero-temperature, continuous transition between magnetically ordered and disordered phases, a quantum-critical point (QCP). Indeed, these highly correlat
ed electron materials are prototypes for discovering and exploring quantum-critical states. Theoretical models proposed to account for the strange thermodynamic and electrical transport properties that emerge around the QCP of a Kondo lattice assume the presence of an indefinitely large number of itinerant charge carriers. Here, we report a systematic transport and thermodynamic investigation of the Kondo-lattice system CeNi$_{2-delta}$As$_2$ ($delta$$thickapprox$0.28) as its antiferromagnetic order is tuned by pressure and magnetic field to zero-temperature boundaries. These experiments show that the very small but finite carrier density of $sim$0.032 $e^-$/f.u. in CeNi$_{2-delta}$As$_2$ leads to unexpected transport signatures of quantum criticality and the delayed development of a fully coherent Kondo lattice state with decreasing temperature. The small carrier density and associated semi-metallicity of this Kondo-lattice material favor an unconventional, local-moment type of quantum criticality and raise the specter of Nozi`{e}res exhaustion idea that an insufficient number of conduction-electron spins to separately screen local moments requires collective Kondo screening.
The synthesis, crystal structure, and physical properties studied by means of x-ray diffraction, magnetic, thermal and transport measurements of CeMAl$_{4}$Si$_{2}$ (M = Rh, Ir, Pt) are reported, along with the electronic structure calculations for L
aMAl$_{4}$Si$_{2}$ (M = Rh, Ir, Pt). These materials adopt a tetragonal crystal structure (space group P4/mmm) comprised of BaAl$_4$ blocks, separated by MAl$_2$ units, stacked along the $c$-axis. Both CeRhAl$_{4}$Si$_{2}$ and CeIrAl$_{4}$Si$_{2}$ order antiferromagnetically below $T_{N1}$=14 and 16 K, respectively, and undergo a second antiferromagnetic transitition at lower temperature ($T_{N2}$=9 and 14 K, respectively). CePtAl$_{4}$Si$_{2}$ orders ferromagnetically below $T_C$ =3 K with an ordered moment of $mu_{sat}$=0.8 $mu_{B}$ for a magnetic field applied perpendicular to the $c$-axis. Electronic structure calculations reveal quasi-2D character of the Fermi surface.
Density functional theory calculations of the electronic structure of Ce- and Pu-based heavy fermion superconductors in the so-called 115 family are performed. The gap equation is used to consider which superconducting order parameters are most favor
able assuming a pairing interaction that is peaked at (pi,pi,q_z) - the wavevector for the antiferromagnetic ordering found in close proximity. In addition to the commonly accepted $d_{x^2-y^2}$ order parameter, there is evidence that an extended s-wave order parameter with nodes is also plausible. We discuss whether these results are consistent with current observations and possible measurements that could help distinguish between these scenarios.
The nature of the second order phase transition that occurs in URu2Si2 at 17.5 K remains puzzling despite intensive research over the past two and half decades. A key question emerging in the field is whether a hybridization gap between the renormali
zed bands can be identified as the long-sought hidden order parameter. We report on the measurement of a hybridization gap in URu2Si2 employing a spectroscopic technique based on quasiparticle scattering across a ballistic metallic junction. The differential conductance exhibits an asymmetric double-peak structure, a clear signature for a Fano resonance in a Kondo lattice. The extracted hybridization gap opens well above the transition temperature, indicating that it is not the hidden order parameter. Our results put stringent constraints on the origin of the hidden order transition in URu2Si2 and demonstrate that quasiparticle scattering spectroscopy can probe the band renormalizations in a Kondo lattice via detection of a novel type of Fano resonance.
We have performed low-temperature specific heat $C$ and thermal conductivity $kappa$ measurements on the Ni-pnictide superconductors BaNi$_2$As$_2$ ($T_mathrm{c}$=0.7 K and SrNi$_2$P$_2$ ($T_mathrm{c}$=1.4 K). The temperature dependences $C(T)$ and $
kappa(T)$ of the two compounds are similar to the results of a number of s-wave superconductors. Furthermore, the concave field responses of the residual $kappa$ for BaNi$_2$As$_2$ rules out the presence of nodes on the Fermi surfaces. We postulate that fully gapped superconductivity could be universal for Ni-pnictide superconductors. Specific heat data on Ba$_{0.6}$La$_{0.4}$Ni$_2$As$_2$ shows a mild suppression of $T_mathrm{c}$ and $H_mathrm{c2}$ relative to BaNi$_2$As$_2$.
We investigated the effect of electron and hole doping on the high-field low-temperature superconducting state in CeCoIn$_5$ by measuring specific heat of CeCo(In$_{rm 1-x}$M$_{rm x}$)$_5$ with M=Sn, Cd and Hg and $x$ up to 0.33% at temperatures down
to 0.1,K and fields up to 14,T. Although both Cd- and Hg-doping (hole-doping) suppresses the zero-field $T_c$ monotonically, $H_{c2}$ increases with small amounts of doping and has a maximum around $x$=0.2% (M=Cd). On the other hand, with Sn-doping (electron-doping) both zero-field $T_c$ and $H_{c2}$ decrease monotonically. The critical temperature for the high-field low-temperature superconducting state (so called {it Q}-state) correlates with $H_{c2}$ and $T_c$, which we interpret in support of the superconducting origin of this state.
The magnetic susceptibilities and specific heats of the crystalline garnet and glass forms of Mn3Al2Si3O12 are reported. This allows a direct comparison of the degree of magnetic frustration of the triangle-based garnet lattice and the structurally d
isordered solid at the same composition for isotropic spin 5/2 Mn^2+ (3d^5). The results show that the glass phase shows more pronounced signs of magnetic frustration than the crystalline phase. Through comparison of the specific heats of Ca3Al2Si3O12 (grossular) and Mn3Al2Si3O12 (spessartine) garnets, information is provided concerning the anomalous extra specific heat in the latter material.
We review the properties of Ni-based superconductors which contain Ni2X2 (X=As, P, Bi, Si, Ge, B) planes, a common structural element found also in the recently discovered FeAs superconductors. Strong evidence for the fully gapped nature of the super
conducting state has come from field dependent thermal conductivity results on BaNi2As2. Coupled with the lack of magnetism, the majority of evidence suggests that the Ni-based compounds are conventional electron-phonon mediated superconductors. However, the increase in Tc in LaNiAsO with doping is anomalous, and mimics the behavior in LaFeAsO. Furthermore, comparisons of the properties of Ni- and Fe-based systems show many similarities, particularly with regards to structure-property relationships. This suggests a deeper connection between the physics of the FeAs superconductors and the related Ni-based systems which deserves further investigation.
Heat capacity, magnetic susceptibility, NMR, and resistivity of SrNi2P2 single crystals are presented, illustrating a purely structural transition at 325 K with no magnetism. Bulk superconductivity is found at 1.4 K. The magnitude of the transition t
emperature T_c, fits to the heat capacity data, the small upper critical field $H_{c2}$ = 390 Oe, and Ginzburg-Landau parameter $kappa$ = 2.1 suggests a conventional fully gapped superconductor. With applied pressure a second structural phase transition occurs which results in an 8% reduction in the c/a ratio of lattice parameters. We find that superconductivity persists into this high pressure phase, although the transition temperature is monotonically suppressed with increasing pressure. Comparison of these Ni-P data as well as layered Fe-As and Ni-As superconductor indicates that reduced dimensionality can be a mechanism for increasing the transition temperature.
Superconductivity without phonons has been proposed for strongly correlated electron materials that are tuned close to a zero-temperature magnetic instability of itinerant charge carriers. Near this boundary, quantum fluctuations of magnetic degrees
of freedom assume the role of phonons in conventional superconductors, creating an attractive interaction that glues electrons into superconducting pairs. Here we show that superconductivity can arise from a very different spectrum of fluctuations associated with a local or Kondo-breakdown quantum-critical point that is revealed in isotropic scattering of charge carriers and a sub-linear temperature-dependent electrical resistivity. At this critical point, accessed by applying pressure to the strongly correlated, local-moment antiferromagnet CeRhIn5, magnetic and charge fluctuations coexist and produce electronic scattering that is maximal at the optimal pressure for superconductivity. This previously unanticipated source of pairing glue opens possibilities for understanding and discovering new unconventional forms of superconductivity.
