Tuning of 4f- and Fe-based correlated electron systems by magnetic field and chemical substitution

Compounds with 3d- and 4f -electrons can often be tuned to manifest new physics and evolve into new ground states with multiple parameters: pressure, magnetic field, and chemical substitution. In this work chemical substitution and magnetic field were used to tune correlated states coming from 3d- and 4f-electrons. The first part of this thesis summarizes the study of Lifshitz transitions in K- and TM- (TM=Co, Rh, Ru, and Mn) substituted BaFe2As2 single crystals by thermoelectric power (TEP) measurements. - TM=Co (0 \u3c=x\u3c= 0.42): the TEP is negative for all Co concentrations studied. x ~ 0.02, 0.11, and 0.22 are the concentrations where Lifshitz transitions occur. - TM=Rh (0 \u3c=x\u3c= 0.171): the temperature dependence of the TEP is very similar (sign and absolute value) to that of Co-substitution, x ~ 0.015 and 0.1 are the concentrations where Lifshitz transitions may possibly occur. - TM=Ru (0 \u3c=x\u3c= 0.36): very complex temperature dependent TEP behavior. x ~ 0.07, 0.2, and 0.3 are the concentrations where either Lifshitz transitions or other significant changes of the electronic structure or correlations might occur. - TM=Mn (0 \u3c=x\u3c=0.147): for x \u3c= 0.042, the TEP exhibits a minimum at low temperatures and is negative over the whole temperature range studied. With further increase of Mn content, S(T) at low temperatures evolves into a maximum and changes sign once or twice at higher temperatures. The ranges of Mn concentrations 0.012 \u3c= x \u3c= 0.017 and 0.092 \u3c= x \u3c= 0.102 were identified to be regions where either Lifshitz transitions or other significant changes of the electronic structure or correlation might occur. The latter region corresponds to a region where the structural transition abruptly disappears and different magnetic order is observed. - K (0.44 \u3c= x \u3c= 1): for the K-substitutions studied, the TEP is positive over the whole temperature range measured. The functional behavior of the TEP, S(T), is somewhat similar, except for the sign, to that of the heavier Co-substituted samples. x ~ 0.55 and x ~ 0.8-0.9 were delineated as the K concentrations where Lifshitz transitions may occur. The second part of this thesis presents two studies of tuning the low-temperature states of Ce-based materials. The first of these is a comprehensive study of transport and thermodynamic properties of CeZn11 and LaZn11 single crystals as well as the search for a possible field-induced quantum critical point in CeZn11. CeZn11 orders antiferromagnetically below ~ 2 K. The zero-field resistivity and thermoelectric power data show features characteristic of a Ce-based intermetallic with crystal-electric-field splitting and possible Kondo-lattice effects. The constructed T - H phase diagram for the magnetic field applied along the easy [110] direction shows that the magnetic field required to suppress TN 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 \u3c= T \u3c= 1.96 K followed by the Landau-Fermi-liquid regime for a limited range of fields, 47.5 kOe \u3c= H\u3c= 60 kOe. From the analysis of the data, it appears that CeZn11 is a local moment compound with little or no electronic correlations arising from the Ce 4f-shell. Given the very high quality of the single crystals, quantum oscillations are found for both CeZn11 and LaZn11. In order to study a system with clearer Kondo-like features, the effects of La dilution of the Kondo lattice CeCu2Ge2 were studied as well. CeCu2Ge2 orders antiferromagnetically below TN 4 K with the Kondo temperature TK in the range of 4-6 K. The study of (Ce{1-x}La{x})Cu2Ge2 system indicated that with La-substitution TN is suppressed in an almost linear fashion and moves below 0.36 K, the base temperature of the measurements, for x \u3e 0.8. Remarkably, in addition to robust antiferromagnetism, the system also shows low temperature coherent scattering below Tcoh up to ~ 0.9 of La, indicating a small percolation limit ~ 9% of Ce that separates a coherent state from a single-ion Kondo impurity state. Tcoh as a function of magnetic field was found to have different functional dependencies in coherent and single-ion regimes. Remarkably, (Tcoh)^2 was found to be linearly proportional to TN. The Kondo temperature was found to slowly change in a non-linear fashion from ~ 4 K to ~ 1 K upon La substitution. For Ce concentrations, y = 1 - x, in the range of 0.01 \u3c= y \u3c= 0.08, Tmin in the resistivity data is proportional to y^{1/5} as expected for the single-ion Kondo impurity. The jump in the magnetic specific heat deltaCm at TN as a function of TK/TN for (Ce{1-x}La{x})Cu2Ge2 system follows the theoretical prediction based on the molecular field calculation for the S =1/2 resonant level model

Physical properties of CeGe2-x (x = 0.24) single crystals

We present data on the anisotropic magnetic properties, heat capacity and transport properties of CeGe2-x (x = 0.24) single crystals. The electronic coefficient of the heat capacity, gamma ~ 110 mJ/mol K^2, is enhanced; three magnetic transitions, with critical temperatures of ~ 7 K, ~ 5 K, and ~ 4 K are observed in thermodynamic and transport measurements. The ground state has a small ferromagnetic component along the c - axis. Small applied field, below 10 kOe, is enough to bring the material to an apparent saturated paramagnetic state (with no further metamagnetic transitions up to 55 kOe) with a reduced, below 1 mu_B, saturated moment

Boron isotope effect in single crystals of ErNi2_2B2_2C superconductor

The influence of local moment magnetism on the boron isotope effect of T$_c$ was studied on single crystals of ErNi$_2$B$_2$C. Values of the partial isotope effect exponent of $\alpha_B$=0.10$\pm$0.02 and $\alpha_B$=0.10$\pm$0.04 were obtained based on two different criteria applied to extract $T_c$. No significant change in the partial isotope effect exponent compared to the ones obtained for LuNi$_2$B$_2$C was observed. Based on this result we conclude that pair-breaking due to the Er local magnetic moment appears to have no detectable influence on boron isotope effect of T$_c$.Comment: 7 pages, 3 figure

Evolution of Structure and Superconductivity in Ba(Ni1−x_{1-x}Cox_x)2_2As2_2

The effects of Co-substitution on Ba(Ni$_{1-x}$Co$_x$)$_2$As$_2$ ($0\leq x\leq 0.251$) single crystals grown out of Pb flux are investigated via transport, magnetic, and thermodynamic measurements. BaNi$_2$As$_2$ exhibits a first order tetragonal to triclinic structural phase transition at $T_s=137 K$ upon cooling, and enters a superconducting phase below $T_c=0.7 K$. The structural phase transition is sensitive to cobalt content and is suppressed completely by $x\geq0.133$. The superconducting critical temperature, $T_c$, increases continuously with $x$, reaching a maximum of $T_c=2.3 K$ at the structural critical point $x=0.083$ and then decreases monotonically until superconductivity is no longer observable well into the tetragonal phase. In contrast to similar BaNi$_2$As$_2$ substitutional studies, which show an abrupt change in $T_c$ at the triclinic-tetragonal boundary that extends far into the tetragonal phase, Ba(Ni$_{1-x}$Co$_x$)$_2$As$_2$ exhibits a dome-like phase diagram centered around the first-order critical point. Together with an anomalously large heat capacity jump $\Delta C_e/\gamma T\sim 2.2$ at optimal doping, the smooth evolution of $T_c$ in the Ba(Ni$_{1-x}$Co$_x$)$_2$As$_2$ system suggests a mechanism for pairing enhancement other than phonon softening.Comment: 7 pages, 8 figure

Electrical resistivity study of CeZn11: Magnetic field and pressure phase diagram up to 5 GPa

Thorough resistivity measurements on single crystals of CeZn11 under pressure p and magnetic field H are presented. At ambient pressure, CeZn11 orders antiferromagnetically at TN=2 K. The pressure dependence of the resistivity reveals an increase of the Kondo effect. We determine the pressure evolution of the magnetic exchange interaction between conduction and localized 4f electrons. It qualitatively reproduces the pressure evolution of the magnetic ordering temperature TO1 (with TO1=TN at ambient pressure). In addition to TO1, a new anomaly TO2appears under pressure. Both anomalies are found to increase with applied pressure up to 4.9 GPa, indicating that CeZn11 is far from a pressure induced quantum critical point. Complex T-H phase diagrams are obtained under pressure which reveal the instability of the ground state in this compound

Fermi surface reconstruction in (Ba1−x_{1-x}Kx_x)Fe2_2As2_2 (0.44 ≤x≤\leq x \leq 1) probed by thermoelectric power measurements

We report in-plane thermoelectric power measurements on single crystals of (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ (0.44 $\leq x \leq$ 1). We observe a minimum in the S$|_{T=const}$ versus x at x ~ 0.55 that can be associated with the change in the topology of the Fermi surface, a Lifshitz transition, related to the electron pockets at the center of M point crossing the Fermi level. This feature is clearly observable below ~ 75 K. Thermoelectric power also shows a change in the x ~ 0.8 - 0.9 range, where maximum in the thermoelectric power collapses into a plateau. This Lifshitz transition is most likely related to the reconstruction of the Fermi surface associated with the transformation of the hole pockets at the M point into four blades as observed by ARPES measurements.Comment: Accepted for publication in Phys. Rev.

CoAs: The Line of 3d Demarcation

Transition metal-pnictide compounds have received attention for their tendency to combine magnetism and unconventional superconductivity. Binary CoAs lies on the border of paramagnetism and the more complex behavior seen in isostructural CrAs, MnP, FeAs, and FeP. Here we report the properties of CoAs single crystals grown with two distinct techniques along with density functional theory calculations of its electronic structure and magnetic ground state. While all indications are that CoAs is paramagnetic, both experiment and theory suggest proximity to a ferromagnetic instability. Quantum oscillations are seen in torque measurements up to 31.5 T and support the calculated paramagnetic Fermiology

Quantum Oscillations of the J=3/2 Fermi Surface in the Topological Semimetal Yptbi

The bismuth-based half-Heusler materials host a nontrivial topological band structure, unconventional superconductivity, and large spin-orbit coupling in a system with very low electron density. In particular, the inversion of p-orbital-derived bands with an effective angular momentum j of up to 3/2 is thought to play a central role in anomalous Cooper pairing in the cubic half-Heusler semimetal YPtBi, which is thought to be the first high-spin superconductor. Here, we report an extensive study of the angular dependence of quantum oscillations (QOs) in the electrical conductivity of YPtBi, revealing an anomalous Shubnikov-de Haas effect consistent with the presence of a coherent j=3/2 Fermi surface. The QO signal in YPtBi manifests an extreme anisotropy upon rotation of the magnetic field from the [100] to [110] crystallographic direction, where the QO amplitude vanishes. This radical anisotropy for such a highly isotropic system cannot be explained by trivial scenarios involving changes in effective mass or impurity scattering, but rather is naturally explained by the warping feature of the j=3/2 Fermi surface of YPtBi, providing direct proof of active high angular momentum quasiparticles in the half-Heusler compounds

CoAs: The line of 3d demarcation

Transition metal-pnictide compounds have received attention for their tendency to combine magnetism and unconventional superconductivity. Binary CoAs lies on the border of paramagnetism and the more complex behavior seen in isostructural CrAs, MnP, FeAs, and FeP. Here we report the properties of CoAs single crystals grown with two distinct techniques along with density functional theory calculations of its electronic structure and magnetic ground state. While all indications are that CoAs is paramagnetic, both experiment and theory suggest proximity to a ferromagnetic instability. Quantum oscillations are seen in torque measurements up to 31.5~T, and support the calculated paramagnetic Fermiology.Comment: 10 pages, 6 figure

Campbell penetration depth in low carrier density superconductor YPtBi

Magnetic penetration depth, $\lambda_{m}$, was measured as a function of temperature and magnetic field in single crystals of low carrier density superconductor YPtBi by using a tunnel-diode oscillator technique. Measurements in zero DC magnetic field yield London penetration depth, $\lambda_{L}\left(T\right)$, but in the applied field the signal includes the Campbell penetration depth, $\lambda_{C}\left(T\right)$, which is the characteristic length of the attenuation of small excitation field, $H_{ac}$, into the Abrikosov vortex lattice due to its elasticity. Whereas the magnetic field dependent $\lambda_C$ exhibit $\lambda_{C}\sim B^{p}$ with $p=1/2$ in most of the conventional and unconventional superconductors, we found that $p\approx 0.23\ll1/2$ in YPtBi due to rapid suppression of the pinning strength. From the measured $\lambda_{C}(T,H)$, the critical current density is $j_{c}\approx40\,\mathrm{A}/\mathrm{cm^{2}}$ at 75 mK. This is orders of magnitude lower than that of conventional superconductors of comparable $T_{c}$. Since the pinning centers (lattice defects) and vortex structure are not expected to be much different in YPtBi, this observation is direct evidence of the low density of the Cooper pairs because $j_{c}\propto n_s$