arXiv subjects

Tim Bartsch

Publications and source records attributed to Tim Bartsch.

4 recordsLinked to original sources

arXiv2026

Bonding Signatures of Incipient Electron Localization in Topological Chiral Semimetals Near the Metal-Insulator Transition

How do electronic localization and delocalization compete in solids beyond the traditional limiting cases of metals and iono-covalent insulators? Topological chiral semimetals (TCSMs), characterized by their unique crystal symmetry, offer an intriguing platform to explore this question. Here, we systematically compare TCSMs with covalent compounds, ordinary metals, and metavalent solids (incipient metals), and show that TCSMs occupy a distinct region in a multidimensional property fingerprint. Atom probe tomography reveals an unusual bond-rupture signature, consistent with a bonding regime intermediate between electron localization and delocalization. This interpretation is supported by measurements of optical properties showing a transfer of spectral weight from interband to intraband transitions. For highly conductive TCSMs, this transition is accompanied by the disappearance of the Born effective charge, a measure of chemical bond polarizability, while less conductive TCSMs retain a nonzero value. Together, these results identify a property based bonding perspective on TCSMs that distinguishes them from metals, covalent solids, and metavalent compounds. Although metavalent solids and TCSMs both lie near the metal-insulator transition and exhibit distorted crystal structures, ultrafast coherent phonon spectroscopy reveals fundamentally different lattice-dynamical responses: a phonon-driven Peierls-like instability in metavalent solids versus a robust chiral B20 bonding motif in TCSMs.

cond-mat.mtrl-sci↗

arXiv2026

Classification of Metal - Insulator Transitions: Relating characteristic Properties to Quantum Chemical Bonding Descriptors

Pressure induced metal insulator transitions (MIT) are classified by the evolution of characteristic optoelectronic and vibrational properties calculated with density functional theory. Three classes emerge: ionic solids metallize continuously at band-gap closure with hardening phonons; covalent solids show discontinuous changes in atomic arrangement and optical phonon frequencies; a third class exhibits complete lattice softening and drastically enhanced electron phonon coupling. A one dimensional hydrogen chain reproduces this behavior and serves as a toy model of the underlying bonding mechanism, termed metavalent. Two quantum-chemical descriptors capture the distinct bonding changes behind the three classes. In metavalent solids, competing electron localization and delocalization yield soft optical modes and Peierls distortions on the insulating side, superconductivity on the metallic side, and low lattice thermal conductivity near the MIT.

cond-mat.mtrl-sci↗

arXiv2026

Divergent Coherent Phonon Responses Across the Metal-Insulator Crossover

Ultrafast laser control of material properties hinges on understanding light-matter interactions. We use two experimentally accessible response functions, laser fluence induced phonon softening and the amplitude of coherent reflectance oscillations, to compare how strongly different materials respond to ultrafast photoexcitation. Comparing a diverse set of materials, we find that only a narrow class, including Sb, GeTe, and Bi2Te3, shows exceptional responses such as pronounced phonon softening and a giant increase of reflectance oscillations with increasing fluence. These response functions peak in an intermediate conductivity regime of about 102 - 104 S/cm, at the crossover between localized and delocalized electronic states. The corresponding class of solids also shows other unconventional properties including high dielectric constants, enhanced Born effective charges, coordination numbers exceeding the 8-N rule and uncommon bond rupture. This suggests that these materials employ a unique bonding mechanism, coined metavalent bonding. Frozen-phonon DFT calculations show that the strong fluence dependence arises from Peierls-like instabilities, leading to large deformation potentials and anharmonic double-well potentials. These findings identify metavalent bonding as a design principle for enhanced coherent phonon control and provide a quantitative framework for identifying materials with exceptional ultrafast responses.

cond-mat.mtrl-sci↗

arXiv2022

Yttrium incorporation in Cr2AlC: On the metastable phase formation and decomposition of (Cr,Y)2AlC MAX phase thin films

Herein we report on the synthesis of a metastable (Cr,Y)2AlC MAX phase solid solution by co-sputtering from a composite Cr-Al-C and elemental Y target, at room temperature, followed by annealing. While direct high-temperature synthesis resulted in multiphase films, as evidenced by X-ray diffraction analyses, room temperature depositions, followed by annealing to 760 {\deg}C led to the formation of phase pure (Cr,Y)2AlC by diffusion. Higher annealing temperatures caused decomposition of the metastable phase into Cr2AlC, Y5Al3 , and Cr-carbides. In contrast to pure Cr2AlC, the Y-containing phase crystallizes directly in the MAX phase structure instead of first forming a disordered solid solution. Furthermore, the crystallization temperature was shown to be Y-content dependent and was increased by ~200 {\deg}C for 5 at.% Y compared to Cr2AlC. Calculations predicting the metastable phase formation of (Cr,Y)2AlC and its decomposition are in excellent agreement with the experimental findings.

cond-mat.mtrl-sci↗