Topological Singularity Induced Chiral Kohn Anomaly in a Weyl Semimetal
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-06-11 |
| Journal | Physical Review Letters |
| Authors | Thanh Nguyen, Fei Han, Nina Andrejevic, Ricardo PabloâPedro, Anuj Apte |
| Institutions | Argonne National Laboratory, University of Maryland, College Park |
| Citations | 42 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive Summaryâ- Novel Phenomenon Observed: The first experimental observation of a topological, chiral Kohn anomaly in a Weyl Semimetal (WSM), specifically Tantalum Phosphide (TaP).
- Origin and Mechanism: This anomaly is induced by the electron-phonon interaction (EPI) arising from inter-node scattering between topologically protected chiral Weyl nodes (W1 and W2).
- Distinct Divergence: Unlike the logarithmic divergence typical of conventional Fermi liquids, this WSM anomaly exhibits a stronger power-law divergence, a counterintuitive result stemming from the 3D linear dispersion of Weyl fermions.
- Dynamical Robustness: The anomaly is dynamically significant, causing phonon softening over a finite regime in the Brillouin zone, rather than being confined to a single wavevector (q) point.
- Chirality Selection: The anomaly is highly sensitive to chirality conservation, manifesting strongly at the W2 node (which allows for chirality-conserved nesting) but being imperceptible at the W1 node.
- Methodology: The findings are supported by field-theoretical calculations and confirmed experimentally using Inelastic X-ray Scattering (IXS) and Inelastic Neutron Scattering (INS) on high-quality TaP single crystals across a wide temperature range (18K to 300K).
- Tool for EPI: This discovery establishes the Kohn anomaly as a new, ubiquitous tool for extracting and elucidating EPI strength and mechanisms in emergent topological materials.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Material System | TaP | N/A | Type-I Weyl Semimetal |
| Crystal Structure | Body-centered tetragonal | N/A | Space group I41md (109) |
| Lattice Parameter (a=b) | 3.32 | Angstrom | Room Temperature (RT) |
| Lattice Parameter (c) | 11.34 | Angstrom | Room Temperature (RT) |
| W1 Weyl Node Energy | ~60 | meV | Below Fermi level (EF) |
| W2 Weyl Node Energy | Few | meV | Above Fermi level (EF) |
| Phonon Energy Range Studied | <25 | meV | Low-energy optical and acoustic modes |
| Fermi Velocity (vF) | ~1.5 x 105 | m/s | Estimated for TaP |
| Initial Momentum Mismatch | ~4 | % | q mismatch for W1-W2 nesting |
| Dynamical Compensation | ~1 | % | Reduced mismatch due to dynamical effect |
| IXS Incident Beam Energy | 21.657 | keV | Advanced Photon Source (APS) |
| IXS Energy Resolution | 2.1 | meV | Overall instrument resolution |
| IXS Sample Thickness | ~20 | ”m | Optimal thickness for 0.5725 Angstrom X-ray |
| IXS Measurement Temperatures | 18, 60, 100, 300 | K | Temperature range for phonon dispersion |
| INS Fixed Final Energy (Ef) | 14.7 | meV | Triple-axis spectrometer setup |
Key Methodologies
Section titled âKey MethodologiesâTaP Single Crystal Synthesis (Chemical Vapor Transport)
Section titled âTaP Single Crystal Synthesis (Chemical Vapor Transport)â- Precursor Preparation: Tantalum (Ta, 99.95%) and Phosphorus (P, 99.999%) powders were weighed, mixed, and flame-sealed inside a quartz tube within a glovebox.
- Pre-reaction: The sealed tube was heated to 70°C and held for 20 hours.
- Transport Setup: The resulting TaP powder was sealed in a second quartz tube with 0.4g of Iodine (I2) as the transport agent.
- Growth Conditions (Two-Zone Furnace): The tube was placed horizontally in a two-zone furnace with optimized temperatures: 900°C (cold end, source) and 950°C (hot end, deposition).
- Duration: Single crystals were condensed into single-crystalline form within 14 days.
Sample Preparation and Characterization
Section titled âSample Preparation and Characterizationâ- Orientation: Crystal orientation was determined using a back-scattering Laue diffractometer.
- Thinning: For IXS experiments, the TaP crystal was thinned down to an optimal thickness of approximately 20 ”m via polishing to ensure adequate photon transmission (0.33 at 0.5725 Angstrom).
- Structural Confirmation: X-ray and neutron diffraction confirmed the lattice parameters (a=b=3.32 A, c=11.34 A at RT), agreeing with established values.
Spectroscopic Measurements and Analysis
Section titled âSpectroscopic Measurements and Analysisâ- Inelastic X-ray Scattering (IXS): Performed at the APS HERIX instrument (21.657 keV). Measurements were conducted at 18K, 60K, 100K, and 300K.
- Inelastic Neutron Scattering (INS): Performed at ORNL and NIST using triple-axis spectrometers with a fixed final energy of 14.7 meV.
- Data Fitting: Intensity spectra were fitted using a core function of damped harmonic oscillators, which was convoluted with the instrumentâs resolution function (pseudo-Voigt).
- Computational Modeling: Ab initio calculations (VASP, PAW pseudopotentials, PBE functional) were used to model the bare phonon dispersion, providing a baseline for comparison against the experimentally observed softening.
- Theoretical Framework: Field-theoretical calculations derived the dynamical polarization operator Π(Μ, q) for 3D linear dispersive Weyl fermions to predict the power-law divergence and dynamical effects of the Kohn anomaly.
Commercial Applications
Section titled âCommercial ApplicationsâThe fundamental understanding of electron-phonon interaction (EPI) in topological materials like TaP is critical for several advanced technology sectors:
- High-Performance Computing & Electronics: WSMs exhibit ultra-high carrier mobility due to their linear dispersion. Controlling EPI is essential for minimizing scattering and dissipation, leading to faster, more energy-efficient transistors and interconnects.
- Quantum Information Processing (QIP): The study of EPI mechanisms directly impacts spin relaxation times. Materials where EPI can be precisely controlled are candidates for robust quantum bits (qubits) and quantum sensors.
- Thermoelectrics and Thermal Management: The unique chiral anomaly in phonon spectra (mentioned as an exotic property of WSMs) suggests potential for engineering thermal transport, enabling highly efficient thermoelectric devices or advanced heat dissipation solutions in microelectronics.
- Topological Sensing: The topologically protected nature of Weyl fermions makes them robust against local disorder. Devices based on WSMs could be used for highly stable magnetic or electrical sensing applications.
- Unconventional Superconductivity: Since the Kohn anomaly is a key signature of strong EPI, this research provides foundational knowledge for designing and synthesizing new materials exhibiting unconventional or high-temperature superconductivity.
View Original Abstract
The electron-phonon interaction (EPI) is instrumental in a wide variety of phenomena in solid-state physics, such as electrical resistivity in metals, carrier mobility, optical transition, and polaron effects in semiconductors, lifetime of hot carriers, transition temperature in BCS superconductors, and even spin relaxation in diamond nitrogen-vacancy centers for quantum information processing. However, due to the weak EPI strength, most phenomena have focused on electronic properties rather than on phonon properties. One prominent exception is the Kohn anomaly, where phonon softening can emerge when the phonon wave vector nests the Fermi surface of metals. Here we report a new class of Kohn anomaly in a topological Weyl semimetal (WSM), predicted by field-theoretical calculations, and experimentally observed through inelastic x-ray and neutron scattering on WSM tantalum phosphide. Compared to the conventional Kohn anomaly, the Fermi surface in a WSM exhibits multiple topological singularities of Weyl nodes, leading to a distinct nesting condition with chiral selection, a power-law divergence, and non-negligible dynamical effects. Our work brings the concept of the Kohn anomaly into WSMs and sheds light on elucidating the EPI mechanism in emergent topological materials.