Kornelius Nielsch

Electronic phases in quantum materials are often governed by nanoscale inhomogeneity, where local order develops within spatially confined regions, or puddles. A prominent example is an incommensurate charge-density-wave (I-CDW) comprising locally commensurate domains. In 2H-NbSe2, such an I-CDW state persists alongside lattice anharmonicity and superconductivity, raising fundamental questions about the dynamical stabilization of CDW order in puddles. Here, using Raman scattering, we reveal a strong Fano-coupling between interlayer shear phonons and the CDW amplitude mode. Our time-resolved reflectivity measurement shows a low-frequency ∼0.2 THz coherent overdamped oscillation onsetting within the CDW regime at ∼17K, indicating a previously unexplored transition. This we identify as a Fano-coupled phonon-CDW hybrid emerging from the collective dynamics of CDW puddles. We establish an overdamped-oscillator framework for capturing such intertwined glassy dynamics. These results highlight how lattice pinning and electronic correlations shape CDW order in layered materials, insight crucial for designing novel van der Waals devices.

Bi12Rh3Ag6I9 extends the Bi14Rh3I9 family of layered bismuth-rich iodides in which intermetallic topological-insulator sheets alternate with salt-like spacer layers. High-temperature synthesis yields phase-pure powders and millimeter-sized platelets. Single-crystal X-ray diffraction reveals a rhombohedral structure composed of [Bi12Rh3I]2+ layers and [Ag6I8]2– spacers. In this spacer, Ag+ cations occupy 75% of the tetrahedral voids of a double iodide layer, giving rise to intrinsic two-dimensional cation disorder and mobility. Electrochemical impedance spectroscopy reveals a room-temperature ionic conductivity of about 10–6 S cm–1. Density-functional theory including spin–orbit coupling predicts band inversion and weak-topological-insulator invariants, while angle-resolved photoemission spectroscopy corroborates the calculated bulk electronic structure, yielding an experimental bulk band gap of 286 meV. Transport measurements show strongly anisotropic, initially n-type conduction. During the initial measurement cycle, applied bias reduces the electron doping through slow electrochemical deintercalation of silver. After conditioning, bulk conduction is strongly reduced; transport below about 130 K becomes nonactivated, the bias-induced drift ceases below about 50 K as Ag+ motion freezes, and a pronounced resistivity drop perpendicular to the layers below 3.2 K is suggestive of a boundary-sensitive low-dimensional transport regime. Bi12Rh3Ag6I9 demonstrates that spacer chemistry provides a chemical lever for carrier-density control in layered topological insulators.

We report a novel atomic layer deposition (ALD) process for SnO2 that does not require water or other strong oxidizing agents, such as H2O, O2 plasma, O3, or H2O2. The development of alternative oxidant-free ALD processes is highly attractive because it enables a gentler and more controllable chemical environment, which is crucial for next-generation nanoscale devices and complex material stacks. In this approach, Sn-based complementary metal precursors are employed: Sn(OtBu)4 serves as both the tin and oxygen precursor, while SnCl4 acts as an additional tin source. For this precursor combination, an optimal ALD temperature window of 70–90 °C was established, within which amorphous SnO2 films are deposited. Post-deposition annealing is subsequently required to develop the desired crystallinity and to adjust the oxygen-vacancy concentration and impurity levels, which ultimately determine the electrical and optical properties of the resulting SnO2 films and their potential for various applications. To gain mechanistic insight into this oxidant-free SnO2 growth process, we employ density functional theory (DFT) calculations to investigate the surface reactions during sequential exposure of Sn(OtBu)4 and SnCl4 on a SiO2 substrate. The results reveal key atomic-level processes, including ligand-exchange pathways, oxygen transfer, and surface regeneration, which govern film growth. This combined experimental and theoretical approach provides fundamental understanding of the SnO2 ALD mechanism and offers guidance for post-deposition optimization of the material.

LaSb is a compensated semimetal that exhibits non-saturating extreme magnetoresistance (XMR), yet the microscopic origin of its transport behavior remains debated. We report a combined magnetotransport and quantum-oscillation study on high-quality LaSb single crystals to disentangle the roles of carrier compensation, mobility, and Fermi-surface topology. The data revealacompensatedmultibandstatewithhighmobility,afield-inducedresistivityupturn,alow- temperature plateau, and pronounced angular magnetoresistance anisotropy. Shubnikov–de Haas anddeHaas–vanAlphenoscillationsresolvethreedominantFermipockets.Landau-fan-diagram analysis indicates mixed pocket topology: the electron pockets are consistent with a trivial Berry phase, whereas the Γ-centered hole pocket shows a nontrivial phase offset. Multicarrier conductivity analysis further demonstrates that the field-dependent transport cannot be fully captured by a minimal two-carrier model, despite near overall electron–hole compensation at low temperature. These results show that XMR in LaSb arises from the cooperative interplay of compensation, high mobility, and complex multiband Fermi-surface topology, with signatures of a temperature-dependent redistribution of transport-active pockets.

Multilayer thin film structuring offers a versatile strategy to tailor nanostructures through interface engineering. Atomic layer deposition (ALD) provides precise control of composition and structure via supercycle approaches that enable the multiple metal species introduction. However, the resulting growth behavior is often interpreted without fully accounting for interfacial effects that govern nucleation and nanostructure evolution. In this work, we demonstrate that introducing SnSe2 layers into Sb2Te3/SnSe2 multilayer films using ALD supercycle significantly enhances Sb2Te3 nucleation and accelerates out-of-plane growth, leading to an increase in growth per cycle (GPC) from 0.18 to 0.69 Å/cycle. First-principles atomistic simulations of precursor deposition chemistry and extremely surface-sensitive low-energy ion scattering (LEIS) analysis confirm the promoted growth of Sb2Te3 in Sb2Te3/SnSe2 multilayer structures, which is also accompanied by elemental exchange reactions between Te and Se atoms. The resulting Sb2Te3/SnSe2 multilayer thin films exhibit increased charge carrier and phonon scattering, leading to a significant reduction in lattice thermal conductivity from 1.09 to 0.25 W/mK at room temperature due to the modulated nanostructure. This study establishes a robust analytical framework for understanding and engineering interfacial reactions and nucleation phenomena in ALD-based multilayer systems.

Grain boundary engineering has been extensively applied to improve thermoelectric performance, but its potential to enhance chemical stability remains underexplored. Here, we demonstrate that modifying grain boundary chemistry can effectively suppress the chemical degradation of Y ₂ Te ₃ under ambient conditions. Scanning transmission electron microscopy and atom probe tomography reveal that H ₂ O preferentially infiltrates along grain boundaries, initiating oxidation of Y ₂ Te ₃ into Y–O–H phases and causing chemo-mechanical breakdown of the matrix. This process, remarkably, can be retarded by just 1 at.% of Bi incorporation due to its segregation along grain boundaries. Density functional theory calculations reveal the thermodynamic and kinetic origins of Bi segregation, and show how segregated Bi modifies the local electronic and chemical environment of grain boundaries, thereby linking GB chemistry to both chemical stability and thermoelectric performance. These findings establish multifunctional grain boundary engineering as a generalizable strategy for the design of next-generation thermoelectric materials.

Electron–phonon (e–ph) interactions govern photoinduced nonequilibrium dynamics of MXenes, determining hot-carrier relaxation and parasitic heat accumulation. However, strategies to deliberately modulate these interactions through chemical control, together with mechanistic understanding, remain underexplored. Here, we demonstrate the effective modulation of nonequilibrium e–ph interactions in Ti ₃ C ₂ T _x MXene via surface-anchored Mo ₃ S ₇ nanoclusters, which introduce a rapid energy harvesting pathway competing with intrinsic e–ph relaxation. Using mild ligand substitution, Mo ₃ S ₇ nanoclusters are densely and homogenously anchored onto Ti ₃ C ₂ T _x via coordination bonding between Mo centers and O-terminations. Femtosecond transient absorption and optical-pump terahertz-probe spectroscopy reveal an ultrafast, sub-100 fs nonthermal electron and/or energy extraction, with efficiency increasing from ~28.6 % at 1.55 eV to ~38.2 % at 3.88 eV. This excitation-energy-dependent enhancement is enabled by improved energetic alignment between hot electrons in Ti ₃ C ₂ T _x and the conduction-band manifold of Mo ₃ S ₇ . The competitive depletion of nonthermal electrons suppresses coherent A _1g phonon excitation, reducing effective e–ph interactions. Our study offers a viable strategy for modulating e–ph interactions in MXenes, advancing hot carrier relaxation and thermal management in next-generation optoelectronic devices.

LaSb is a compensated semimetal that exhibits non-saturating extreme magnetoresistance (XMR), yet the microscopic origin of its transport behavior remains debated. We report a combined magnetotransport and quantum-oscillation study on high-quality LaSb single crystals to disentangle the roles of carrier compensation, mobility, and Fermi-surface topology. The data revealacompensatedmultibandstatewithhighmobility,afield-inducedresistivityupturn,alow- temperature plateau, and pronounced angular magnetoresistance anisotropy. Shubnikov–de Haas anddeHaas–vanAlphenoscillationsresolvethreedominantFermipockets.Landau-fan-diagram analysis indicates mixed pocket topology: the electron pockets are consistent with a trivial Berry phase, whereas the Γ-centered hole pocket shows a nontrivial phase offset. Multicarrier conductivity analysis further demonstrates that the field-dependent transport cannot be fully captured by a minimal two-carrier model, despite near overall electron–hole compensation at low temperature. These results show that XMR in LaSb arises from the cooperative interplay of compensation, high mobility, and complex multiband Fermi-surface topology, with signatures of a temperature-dependent redistribution of transport-active pockets.

Metal ion migration under operational gradients triggers irreversible decomposition and performance collapse in thermoelectric (TE) materials. β-Zn ₄ Sb ₃ has high TE performance but suffers from severe zinc (Zn) ion migration under an external field. This work uses powder atomic layer deposition (pALD) to engineer atomic-scale zinc oxide (ZnO) interfaces that simultaneously suppress Zn ion migration and enhance phonon scattering. Through precise ZnO coatings (50 to 200 cycles), we create continuous barriers that immobilize interstitial Zn ions, eliminating Zn motion and inhibiting phase decomposition. Optimized 100 ALD cycle coatings reduce lattice thermal conductivity by >20% through intensified boundary-phonon scattering, yielding a stabilized, nondegrading figure of merit compared to uncoated performance. Crucially, the thermal stability of 100-ALD-cycle-coated sample persists through 39,260 thermal cycles under gradients of 220 kelvin, and Seebeck coefficient mapping exhibits a uniform distribution along temperature difference. Our approach establishes pALD as a promising atomic-level interface design in migration-prone TE materials, bridging high performance with long-term operational reliability.

Quantum confinement in artificial superlattices enables the engineering of electronic and optical properties that exceed bulk limitations. Despite the unrivaled precision in thickness control and scalability of atomic layer deposition (ALD), the experimental demonstration of confinement effects in the superlattice geometry has remained elusive, especially for nonoxide systems. In this study, we report the experimental demonstration of distinct quantum confinement in the chalcogenide-based heteromorphic superlattices via an ALD supercycle approach. Polycrystalline PbS and amorphous SnS₂ are alternately deposited with subnanometer thickness control, resulting in the formation of a well-defined heteromorphic superlattice structure with sharp, strain-relieved, and defect-passivated interfaces. Raman spectroscopy also revealed the activation of low-frequency vibrational modes, indicating strong interlayer coupling of strain-free layers within the superlattice. A systematic reduction in the PbS sublayer thickness below its excitonic Bohr radius enables a substantial and controllable widening of the bandgap, from 1.74 eV for PbS (14 nm)/SnS₂ (5 nm) to 2.51 eV for PbS (3 nm)/SnS₂ (5 nm), compared to 1.54 eV for individually grown PbS films. This marked bandgap modulation unambiguously demonstrates the strong quantum confinement of charge carriers within the strain-relaxed PbS layers. Density functional theory (DFT) calculations confirm the experimental observations, revealing the emergence of both lateral and vertical quantum confinement and elucidating the role of the superlattice architecture in shaping the electronic structure. Together, these results establish ALD as an effective platform for quantum superlattice engineering, enabling precise control of confinement effects in complex chalcogenide heterostructures and their integration into next-generation optoelectronics and quantum devices.