arXiv subjects

Ulrich Häussermann

Publications and source records attributed to Ulrich Häussermann.

3 recordsLinked to original sources

arXiv2025

Split-gasket approach to the integration of electrical leads into diamond anvil cells

Transport and heat capacity measurements under pressure must reconcile the limited available space and complicated geometry of a high-pressure cell with the need for multiple electrical connections. One solution for diamond anvil cells is to use customized diamonds with deposited electrical leads. Here, we instead address the problem through a split-gasket approach, intended for diamond anvil cells at moderate pressures and low temperature. A key component is the use of a substrate with lithographically defined leads, which enables connections to components such as thermometer, heater, and/or sample within the confined sample volume of the cell. The design includes an elaborate BeCu gasket sandwich with a preparation method that ensures electrical contact integrity. Using this configuration, we bring 12 leads to within 100 $μ$m of the center of the diamond anvil at a pressure of about 2 GPa, comparable to the pressure reached with a regular gasket, demonstrating the setup's capability for high-pressure experiments. The split-gasket approach may come at the cost of reduced maximum pressure, but brings versatility and reproducibility, and alleviates the experimental efforts of maintaining multiple electrical leads both intact and electrically isolated.

physics.ins-det↗

arXiv2023

Ab initio Determination of Phase Stabilities of Dynamically Disordered Solids: rotational C2 disorder in Li2C2

The temperature-induced orthorhombic to cubic phase transition in Li2C2 is a prototypical example of a solid to solid phase transformation between an ordered phase, which is well described within the phonon theory, and a dynamically disordered phase with rotating molecules, for which the standard phonon theory is not applicable. The transformation in Li2C2 happens from a phase with directionally ordered C2 dimers to a structure, where they are dynamically disorderd. We provide a description of this transition by employing ab initio molecular dynamics (AIMD) based stress-strain thermodynamic integration on a deformation path that connects the ordered and dynamically disordered phases. The free energy difference between the two phases is obtained. The entropy that stabilizes the dynamically disordered cubic phase is captured by the behavior of the stress on the deformation path. We also show that a combination of the stress-strain thermodynamic integration and machine learning force field methodologies appears as a promising path in studies of dynamically disordered materials.

cond-mat.mtrl-sci↗

arXiv2015

Thermal and Vibrational Properties of Thermoelectric ZnSb - Exploring the Origin of Low Thermal Conductivity

The intermetallic compound ZnSb is an interesting thermoelectric material, largely due to its low lattice thermal conductivity. The origin of the low thermal conductivity has so far been speculative. Using multi-temperature single crystal X-ray diffraction (9 - 400 K) and powder X-ray diffraction (300 - 725 K) measurements we characterized the volume expansion and the evolution of structural properties with temperature and identify an increasingly anharmonic behavior of the Zn atoms. From a combination of Raman spectroscopy and first principles calculations of phonons we consolidate the presence of low-energy optic modes with wavenumbers below 60 cm-1. Heat capacity measurements between 2 and 400 K can be well described by a Debye-Einstein model containing one Debye and two Einstein contributions with temperatures ΘD = 195K, ΘE1 = 78 K and ΘE2 = 277 K as well as a significant contribution due to anharmonicity above 150 K. The presence of a multitude of weakly dispersed low-energy optical modes (which couple with the acoustic, heat carrying phonons) combined with anharmonic thermal behavior provides an effective mechanism for low lattice thermal conductivity. The peculiar vibrational properties of ZnSb are attributed to its chemical bonding properties which are characterized by multicenter bonded structural entities. We argue that the proposed mechanism to explain the low lattice thermal conductivity of ZnSb might also control the thermoelectric properties of electron poor semiconductors, such as Zn4Sb3, CdSb, Cd4Sb3, Cd13-xInyZn10, and Zn5Sb4In2-x.

cond-mat.mtrl-sci↗