Veronika Fritsch

Entropy Evolution in the Magnetic Phases of Partially Frustrated CePdAl

In the heavy-fermion metal CePdAl, long-range antiferromagnetic order coexists with geometric frustration of one-third of the Ce moments. At low temperatures, the Kondo effect tends to screen the frustrated moments. We use magnetic fields B to suppress the Kondo screening and study the magnetic phase diagram and the evolution of the entropy with B employing thermodynamic probes. We estimate the frustration by introducing a definition of the frustration parameter based on the enhanced entropy, a fundamental feature of frustrated systems. In the field range where the Kondo screening is suppressed, the liberated moments tend to maximize the magnetic entropy and strongly enhance the frustration. Based on our experiments, this field range may be a promising candidate to search for a quantum spin liquid.

Synthesis and structural characterization of novel clathrate-II compounds of silicon

The search for new and better thermoelectric (TE) materials has recently brought to light a somewhat forgotten class of compounds, the clathrates of group 14. The rich phenomenology of these and related cage-like structures has driven the most recent research on their electronic, magnetic, spectral, and conducting properties [Rowe, ed., 1995]. The present studies try to address the issue whether the clathrates are closed-shell compounds (i.e. semiconductor-like) by nature or whether the metallic behavior in some clathrates is simply a failure of the Zintl concept. The underlying principle in our work is based upon the formal oxidation states - a defect-free clathrate framework is electronically balanced since each group 14 element carries 4 valence electrons, which are shared with the 4 neighbors to form 4 covalent bonds. Each guest atom (alkali, alkaline-earth or rare-earth element) is considered merely as an electron donor that transfers its valence electron(s) to the network. Metallic behavior is expected even if the cages are not fully occupied, provided the framework retains the ideal tetrahedral topology. There are two possible ways to balance the extra electrons: 1) to create a vacancy in the framework, or 2) to substitute network atoms with electron-poorer elements, from groups 13 or 12 for example. Such considerations, however, are not always supported by experiments, suggesting that such a clear-cut approach is perhaps overly simplistic

Structure and properties of the tetragonal phase of MnCuAs

The tetragonal modification (space group P4/nmm, no. 129) of the MnCuAs ternary intermetallic compound has been prepared for the first time. The crystallographic data related to the structure and the atom positions were determined using the X-ray analysis method for MnCuAs single crystals of the specified modification. The magnetic properties of the tetragonal phase obtained were studied in the temperature range of 5–300 K.

CePdAl - A Kondo lattice with partial frustration

Magnetic frustration, which is well-defined in insulating systems with localized magnetic moments, yields exotic ground states like spin ices, spin glasses, or spin liquids. In metals magnetic frustration is less well defined because of the incipient delocalization of magnetic moments by the interaction with conduction electrons, viz., the Kondo effect. Hence, the Kondo effect and magnetic frustration are antithetic phenomena. Here we present experimental data of electrical resistivity, magnetization, specific heat and neutron diffraction on CePdAl, which is one of the rare examples of a geometrically frustrated Kondo lattice, demonstrating that the combination of Kondo effect and magnetic frustration leads to an unusual ground state.

Evolution of the partially frustrated magnetic order in CePd 1 − x Ni x Al6212

We report on a single-crystal neutron diffraction study of the evolution of the antiferromagnetic order in the heavy-fermion compound CePd

Approaching quantum criticality in a partially geometrically frustrated heavy-fermion metal

_{1-x}

Magnetization and specific heat of CePd1 − xNixAl

Ni

Signature of frustrated moments in quantum critical CePd1-xNixAl

_x

Evolution of the Magnetic Structure in CeCu5.5Au0.5 under Pressure towards Quantum Criticality

Al which exhibits partial geometric frustration due to its distorted Kagome structure. The magnetic structure is found to be unchanged with a propagation vector

Low-temperature properties of CeAu2Ge2single crystals grown from Au-Ge and Sn flux

Q_\mathrm{AF} \approx (0.5~0~0.35)