#### Title

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

#### Degree Type

Dissertation

#### Date of Award

2014

#### Degree Name

Doctor of Philosophy

#### Department

Physics and Astronomy

#### First Advisor

Paul C. Canfield

#### Second Advisor

Sergey L. Bud'ko

#### Abstract

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 <=x<= 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 <=x<= 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 <=x<= 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 <=x<=0.147): for x <= 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 <= x <= 0.017 and 0.092 <= x <= 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 <= x <= 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 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 <= 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 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 T_{N} 4 K with the Kondo temperature T_{K} in the range of 4-6 K. The study of (Ce{1-x}La{x})Cu2Ge2 system indicated that with La-substitution T_{N} is suppressed in an almost linear fashion and moves below 0.36 K, the base temperature of the measurements, for x > 0.8. Remarkably, in addition to robust antiferromagnetism, the system also shows low temperature coherent scattering below T_{coh} 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. T_{coh} as a function of magnetic field was found to have different functional dependencies in coherent and single-ion regimes. Remarkably, (T_{coh})^2 was found to be linearly proportional to T_{N}. 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 <= y <= 0.08, T_{min} 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 deltaC_{m} at T_{N} as a function of T_{K}/T_{N} 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.

#### DOI

https://doi.org/10.31274/etd-180810-3716

#### Copyright Owner

Halyna Hodovanets

#### Copyright Date

2014

#### Language

en

#### File Format

application/pdf

#### File Size

218 pages

#### Recommended Citation

Hodovanets, Halyna, "Tuning of 4f- and Fe-based correlated electron systems by magnetic field and chemical substitution" (2014). *Graduate Theses and Dissertations*. 14166.

https://lib.dr.iastate.edu/etd/14166