arXiv subjects

G. Giester

Publications and source records attributed to G. Giester.

7 recordsLinked to original sources

arXiv2024

Ce$_3$Bi$_4$Ni$_3$ $-$ A large hybridization-gap variant of Ce$_3$Bi$_4$Pt$_3$

The family of cubic noncentrosymmetric 3-4-3 compounds has become a fertile ground for the discovery of novel correlated metallic and insulating phases. Here, we report the synthesis of a new heavy fermion compound, Ce$_3$Bi$_4$Ni$_3$. It is an isoelectronic analog of the prototypical Kondo insulator Ce$_3$Bi$_4$Pt$_3$ and of the recently discovered Weyl-Kondo semimetal Ce$_3$Bi$_4$Pd$_3$. In contrast to the volume-preserving Pt-Pd substitution, structural and chemical analyses reveal a positive chemical pressure effect in Ce$_3$Bi$_4$Ni$_3$ relative to its heavier counterparts. Based on the results of electrical resistivity, Hall effect, magnetic susceptibility, and specific heat measurements, we identify an energy gap of 65-70 meV, about eight times larger than that in Ce$_3$Bi$_4$Pt$_3$ and about 45 times larger than that of the Kondo-insulating background hosting the Weyl nodes in Ce$_3$Bi$_4$Pd$_3$. We show that this gap as well as other physical properties do not evolve monotonically with increasing atomic number, i.e., in the sequence Ce$_3$Bi$_4$Ni$_3$-Ce$_3$Bi$_4$Pd$_3$-Ce$_3$Bi$_4$Pt$_3$, but instead with increasing partial electronic density of states of the $d$ orbitals at the Fermi energy. To understand under which condition topological states form in these materials is a topic for future studies.

cond-mat.str-el↗

arXiv2017

Clathrate formation in the systems Sr-Cu-Ge and {Ba,Sr}-Cu-Ge

In the ternary system Sr-Cu-Ge, a clathrate type-I phase, Sr8Cu5.3Ge40.7 (a = 1.06311(3), exists close to the Zintl limit in a small temperature interval. Sr8Cu5.3Ge40.7 decomposes eutectoidally on cooling at 730°C into (Ge), SrGe2 and tau1-SrCu2-xGe2+x. Phase equilibria at 700°C have been established for the Ge rich part and are characterized by the appearance of only one ternary compound, tau1-SrCu2-xGe2+x, which crystallizes with the ThCr2Si2 structure type and forms a homogeneity range up to x=0.4 (a = 0.42850(4), c = 1.0370(1) nm). Additionally, the extent of the clathrate type-I solid solution Ba8-xSrxCuyGe46-y (5.2 < y < 5.4) has been studied at various temperatures. The clathrate type-I crystal structure (space group ) has been proven by X-ray single crystal diffraction on two single crystals with composition Sr8Cu5.3Ge40.7 (a = 1.06368(2) nm) and Ba4.9Sr3.1Cu5.3Ge40.7 (a = 1.06748(2) nm) measured at 300, 200 and 100 K. From the temperature dependency of the lattice parameters and the atomic displacement parameters, the thermal expansion coefficients, the Debye- and Einstein-temperatures and the speed of sound have been determined. From heat capacity measurements of Sr8Cu5.3Ge40.7 at low temperatures, the Sommerfeld coefficient and the Debye temperature have been extracted, whereas from a detailed analysis of these data at higher temperatures, Einstein branches of the phonon dispersion relation have been derived and compared with those obtained from the atomic displacement parameters. Electrical resistivity measurements of Sr8Cu5.3Ge40.7 reveal a rather metallic behaviour in the low temperature range (< 300 K).

cond-mat.mtrl-sci↗

arXiv2017

On Ni-Sb-Sn based skutterudites

Novel filled skutterudites EpyNi4Sb12-xSnx (Ep = Ba and La) have been prepared by arc melting followed by annealing at 250C, 350C and 450C up to 30 days in sealed quartz vials. A maximum filling level of y = 0.93 and y = 0.65 was achieved for the Ba and La filled skutterudite, respectively. Single-phase samples with the composition Ni4Sb8.2Sn3.8, Ba0.42Ni4Sb8.2Sn3.8 and Ba0.92Ni4Sb6.7Sn5.3 were employed for measurements of the physical properties i.e. temperature dependent electrical resistivity, Seebeck coefficient and thermal conductivity. Resistivity data showed a crossover from metallic to semiconducting behaviour. The corresponding gap width was extracted from maxima in the Seebeck coefficient data as a function of temperature. Temperature dependent single crystal X-ray structure analyses (at 100 K, 200 K and 300 K) revealed the thermal expansion coefficients, Einstein and Debye temperatures for two selected samples Ba0.73Ni4Sb8.1Sn3.9 and Ba0.95Ni4Sb6.1Sn5.9. These data compare well with Debye temperatures from measurements of specific heat (4.4 K < T < 200 K). Several mechanical properties were measured and evaluated. Thermal expansion coefficients are 11.8.10-6 K-1 for Ni4Sb8.2Sn3.8 to 13.8.10-6 K-1 for Ba0.92Ni4Sb6.7Sn5.3. Room temperature Vicker's hardness values (up to a load of 24.5 mN) vary within the range of 2.6 GPa to 4.7 GPa. Severe plastic deformation (SPD) via high-pressure torsion (HPT) was used to introduce nanostructuring. Physical properties before and after HPT were compared, showing no significant effect on the material's thermoelectric behaviour.

cond-mat.mtrl-sci↗

arXiv2007

Superconductivity in novel Ge-based skutterudites: {Sr,Ba}Pt_4Ge_{12}

Combining experiments and ab initio models we report on $\rm SrPt_4Ge_{12}$ and $\rm BaPt_4Ge_{12}$ as members of a novel class of superconducting skutterudites, where Sr or Ba atoms stabilize a framework entirely formed by Ge-atoms. Below $T_c=5.35$ K, and 5.10 K for $\rm BaPt_4Ge_{12}$ and $\rm SrPt_4Ge_{12}$, respectively, electron-phonon coupled superconductivity emerges, ascribed to intrinsic features of the Pt-Ge framework, where Ge-$p$ states dominate the electronic structure at the Fermi energy.

cond-mat.supr-con↗

arXiv2005

Ferromagnetism and possible heavy fermion behavior in single crystals of NdOs$_4$Sb$_{12}$

Single crystals of the filled-skutterudite compound NdOs$_4$Sb$_{12}$ have been investigated by means of electrical resistivity, magnetization, and specific heat measurements. The NdOs$_4$Sb$_{12}$ crystals have the LaFe$_4$P$_{12}$-type cubic structure with a lattice parameter of 9.3 Å. Possible heavy-fermion behavior is inferred from specific heat measurements, which reveal a large electronic specific heat coefficient $γ\approx 520$ mJ/mol-K$^2$, corresponding to an effective mass $m^* \sim$ 98 $m_e$. Features related to a ferromagnetic transition at {$\sim$ 0.9 K} can be observed in electrical resistivity, magnetization and specific heat. Conventional Arrott-plot analysis indicates that NdOs$_4$Sb$_{12}$ conforms to mean-field ferromagnetism.

cond-mat.str-el↗

arXiv2004

Intermediate valence behavior in CeCo9Si4

The novel ternary compound CeCo$_9$Si$_4$ has been studied by means of specific heat, magnetisation, and transport measurements. Single crystal X-ray Rietveld refinements reveal a fully ordered distribution of Ce, Co and Si atoms with the tetragonal space group I4/mcm isostructural with other RCo9Si4. The smaller lattice constants of CeCo9Si4 in comparison with the trend established by other RCo9Si4 is indicative for intermediate valence of cerium. While RCo9Si4 with R= Pr, .. Tb, and Y show ferromagnetism and LaCo9Si4 is nearly ferromagnetic, CeCo9Si4 remains paramagnetic even in external fields as large as 40 T, though its electronic specific heat coefficient (g~190 mJ/molK^2) is of similar magnitude as that of metamagnetic LaCo9Si4 and weakly ferromagnetic YCo9Si4.

cond-mat.str-el↗

arXiv2003

Itinerant electron metamagnetism in LaCo$_9$Si$_4$

The strongly exchange enhanced Pauli paramagnet LaCo$_9$Si$_4$ is found to exhibit an itinerant metamagnetic phase transition with indications for metamagnetic quantum criticality. Our investigation comprises magnetic, specific heat, and NMR measurements as well as ab-initio electronic structure calculations. The critical field is about 3.5 T for $H||c$ and 6 T for $H\bot c$, which is the lowest value ever found for rare earth intermetallic compounds. In the ferromagnetic state there appears a moment of about 0.2 $μ_B$/Co at the $16k$ Co-sites, but sigificantly smaller moments at the 4d and $16l$ Co-sites.

cond-mat.str-el↗