Signatures of quantum criticality in the thermopower of Ba(Fe(1-x)Co(x))2As2

We demonstrate that the thermopower (S) can be used to probe the spin fluctuations (SFs) in proximity to the quantum critical point (QCP) in Fe-based superconductors. The sensitivity of S to the entropy of charge carriers allows us to observe an increase of S/T in Ba(Fe(1-x)Co(x))2As2 close to the spin-density-wave (SDW) QCP. This behavior is due to the coupling of low-energy conduction electrons to two-dimensional SFs, similar to heavy-fermion systems. The low-temperature enhancement of S/T in the Co substitution range 0.02 < x < 0.1 is bordered by two Lifshitz transitions, and it corresponds to the superconducting region, where a similarity between the electron and non-reconstructed hole pockets exists. The maximal S/T is observed in proximity to the commensurate-to-incommensurate SDW transition, for critical x_c ~ 0.05, close to the highest superconducting T_c. This analysis indicates that low-T thermopower is influenced by critical spin fluctuations which are important for the superconducting mechanism

Crystal growth and annealing study of fragile, non-bulk superconductivity in YFe2_2Ge2_2

We investigated the occurrence and nature of superconductivity in single crystals of YFe$_2$Ge$_2$ grown out of Sn flux by employing x-ray diffraction, electrical resistivity, and specific heat measurements. We found that the residual resistivity ratio (RRR) of single crystals can be greatly improved, reaching as high as $\sim$60, by decanting the crystals from the molten Sn at $\sim$350$^\circ$C and/or by annealing at temperatures between 550$^\circ$C and 600$^\circ$C. We found that samples with RRR $\gtrsim$ 34 showed resistive signatures of superconductivity with the onset of the superconducting transition $T_c\approx1.4$ K. RRR values vary between 35 and 65 with, on average, no systematic change in $T_c$ value, indicating that systematic changes in RRR do not lead to comparable changes in $T_c$. Specific heat measurements on samples that showed clear resistive signatures of a superconducting transition did not show any signature of a superconducting phase transition, which suggests that the superconductivity observed in this compound is either some sort of filamentary, strain stabilized superconductivity associated with small amounts of stressed YFe$_2$Ge$_2$ (perhaps at twin boundaries or dislocations) or is a second crystallographic phase present at levels below detection capability of conventional powder x-ray techniques.Comment: 8 pages, 11 figure

Remarkably robust and correlated coherence and antiferromagnetism in (Ce1−x_{1-x}Lax_x)Cu2_2Ge2_2

We present magnetic susceptibility, resistivity, specific heat, and thermoelectric power measurements on (Ce$_{1-x}$La$_x$)Cu$_2$Ge$_2$ single crystals (0 $\leq x\leq$ 1). With La substitution, the antiferromagnetic temperature $T_N$ is suppressed in an almost linear fashion and moves below 0.36 K, the base temperature of our measurements for $x>$ 0.8. Surprisingly, in addition to robust antiferromagnetism, the system also shows low temperature coherent scattering below $T_{coh}$ up to $\sim$ 0.9 of La, indicating a small percolation limit $\sim$ 9$\%$ of Ce that separates a coherent regime from a single-ion Kondo impurity regime. $T_{coh}$ as a function of magnetic field was found to have different behavior for $x$ 0.9. Remarkably, $(T_{coh})^2$ at $H$ = 0 was found to be linearly proportional to $T_N$. The jump in the magnetic specific heat $\delta C_{m}$ at $T_N$ as a function of $T_K/T_N$ for (Ce$_{1-x}$La$_x$)Cu$_2$Ge$_2$ follows the theoretical prediction based on the molecular field calculation for the $S$ = 1/2 resonant level model

Thermoelectric power of Ba(Fe1-xRux)2As2 and Ba(Fe1-xCox)2As2: possible changes of Fermi surface with and without changes in electron count

Temperature-dependent, in-plane, thermoelectric power (TEP) data are presented for Ba(Fe1-xRux)2As2 (0 < x < 0.36) single crystals. The previously outlined x - T phase diagram for this system is confirmed. The analysis of TEP evolution with Ru-doping suggests significant changes in the electronic structure, correlations and/or scattering occurring near ~7% and ~30% of Ru-doping levels. These results are compared with an extended set of TEP data for the electron-doped Ba(Fe1-xCox)2As2 series

Anisotropic transport and magnetic properties, and magnetic-field tuned states of CeZn11 single crystals

We present detailed temperature and field dependent data obtained from magnetization, resistivity, heat capacity, Hall resistivity and thermoelectric power measurements performed on single crystals of CeZn11. The compounds orders antiferromagnetically at $\sim$ 2 K. The zero-field resistivity and TEP data show features characteristic of a Ce-based intermetallic with crystal electric field splitting and possible correlated, Kondo lattice effects. We constructed the T-H phase diagram for the magnetic field applied along the easy, [110], direction which shows that the magnetic field required to suppress T_N below 0.4 K is in the range of 45-47.5 kOe. A linear behavior of the rho(T) data, H||[110], was observed only for H=45 kOe for 0.46 K<T<1.96 K followed by the Landau-Fermi-liquid regime for a limited range of fields, 47.5 kOe< H<60 kOe. From the analysis of our data, it appears that CeZn11 is a weakly to moderately correlated local moment compound with rather small Kondo temperature. The thermoelectric and transport properties of CeZn11 are mostly governed by the CEF effects. Given the very high quality of our single crystals, quantum oscillations are found for both CeZn11 and its non-magnetic analogue, LaZn11

Single crystal investigation of proposed type-II Weyl semimetal CeAlGe

We present details of materials synthesis, crystal structure, and anisotropic magnetic properties of single crystals of CeAlGe, a proposed type-II Weyl semimetal. Single-crystal x-ray diffraction confirms that CeAlGe forms in noncentrosymmetric I4$_1$md space group, in line with predictions of non-trivial topology. Magnetization, specific heat and electrical transport measurements were used to confirm antiferromagnetic order below 5 K, with an estimated magnon excitation gap of $\Delta$ = 9.11 K from heat capacity and hole-like carrier density of 1.44 $\times$ 10$^{20}$ cm$^{-3}$ from Hall effect measurements. The easy magnetic axis is along the [100] crystallographic direction, indicating that the moment lies in the tetragonal $\it{ab}$-plane below 7 K. A spin-flop transition to less than 1 $\mu_B$/Ce is observed to occur below 30 kOe at 1.8 K in the $M(H)$ ($\bf{H}\|\bf{a}$) data. Small magnetic fields of 3 kOe and 30 kOe are sufficient to suppress magnetic order when applied along the $\it{a}$- and $\it{c}$-axes, respectively, resulting in a complex $\it{T-H}$ phase diagram for $\bf{H}\|\bf{a}$ and a simpler one for $\bf{H}\|\bf{c}$