Phonon Coupling
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2022 ◽  
Vol 12 (1) ◽  
M. B. Shoker ◽  
T. Alhaddad ◽  
O. Pagès ◽  
V. J. B. Torres ◽  
A. V. Postnikov ◽  

AbstractRaman scattering and ab initio Raman/phonon calculations, supported by X-ray diffraction, are combined to study the vibrational properties of Zn1−xBexTe under pressure. The dependence of the Be–Te (distinct) and Zn–Te (compact) Raman doublets that distinguish between Be- and Zn-like environments is examined within the percolation model with special attention to x ~ (0,1). The Be-like environment hardens faster than the Zn-like one under pressure, resulting in the two sub-modes per doublet getting closer and mechanically coupled. When a bond is so dominant that it forms a matrix-like continuum, its two submodes freely couple on crossing at the resonance, with an effective transfer of oscillator strength. Post resonance the two submodes stabilize into an inverted doublet shifted in block under pressure. When a bond achieves lower content and merely self-connects via (finite/infinite) treelike chains, the coupling is undermined by overdamping of the in-chain stretching until a «phonon exceptional point» is reached at the resonance. Only the out-of-chain vibrations «survive» the resonance, the in-chain ones are «killed». This picture is not bond-related, and hence presumably generic to mixed crystals of the closing-type under pressure (dominant over the opening-type), indicating a key role of the mesostructure in the pressure dependence of phonons in mixed crystals.

2022 ◽  
Vol 13 (1) ◽  
Hailan Luo ◽  
Qiang Gao ◽  
Hongxiong Liu ◽  
Yuhao Gu ◽  
Dingsong Wu ◽  

AbstractThe Kagome superconductors AV3Sb5 (A = K, Rb, Cs) have received enormous attention due to their nontrivial topological electronic structure, anomalous physical properties and superconductivity. Unconventional charge density wave (CDW) has been detected in AV3Sb5. High-precision electronic structure determination is essential to understand its origin. Here we unveil electronic nature of the CDW phase in our high-resolution angle-resolved photoemission measurements on KV3Sb5. We have observed CDW-induced Fermi surface reconstruction and the associated band folding. The CDW-induced band splitting and the associated gap opening have been revealed at the boundary of the pristine and reconstructed Brillouin zones. The Fermi surface- and momentum-dependent CDW gap is measured and the strongly anisotropic CDW gap is observed for all the V-derived Fermi surface. In particular, we have observed signatures of the electron-phonon coupling in KV3Sb5. These results provide key insights in understanding the nature of the CDW state and its interplay with superconductivity in AV3Sb5 superconductors.

2022 ◽  
Vol 13 (1) ◽  
Philipp Kurzhals ◽  
Geoffroy Kremer ◽  
Thomas Jaouen ◽  
Christopher W. Nicholson ◽  
Rolf Heid ◽  

AbstractElectron-phonon coupling, i.e., the scattering of lattice vibrations by electrons and vice versa, is ubiquitous in solids and can lead to emergent ground states such as superconductivity and charge-density wave order. A broad spectral phonon line shape is often interpreted as a marker of strong electron-phonon coupling associated with Fermi surface nesting, i.e., parallel sections of the Fermi surface connected by the phonon momentum. Alternatively broad phonons are known to arise from strong atomic lattice anharmonicity. Here, we show that strong phonon broadening can occur in the absence of both Fermi surface nesting and lattice anharmonicity, if electron-phonon coupling is strongly enhanced for specific values of electron-momentum, k. We use inelastic neutron scattering, soft x-ray angle-resolved photoemission spectroscopy measurements and ab-initio lattice dynamical and electronic band structure calculations to demonstrate this scenario in the highly anisotropic tetragonal electron-phonon superconductor YNi2B2C. This new scenario likely applies to a wide range of compounds.

2022 ◽  
Vol 13 (1) ◽  
Emre Ergeçen ◽  
Batyr Ilyas ◽  
Dan Mao ◽  
Hoi Chun Po ◽  
Mehmet Burak Yilmaz ◽  

AbstractIn van der Waals (vdW) materials, strong coupling between different degrees of freedom can hybridize elementary excitations into bound states with mixed character1–3. Correctly identifying the nature and composition of these bound states is key to understanding their ground state properties and excitation spectra4,5. Here, we use ultrafast spectroscopy to reveal bound states of d-orbitals and phonons in 2D vdW antiferromagnet NiPS3. These bound states manifest themselves through equally spaced phonon replicas in frequency domain. These states are optically dark above the Néel temperature and become accessible with magnetic order. By launching this phonon and spectrally tracking its amplitude, we establish the electronic origin of bound states as localized d–d excitations. Our data directly yield electron-phonon coupling strength which exceeds the highest known value in 2D systems6. These results demonstrate NiPS3 as a platform to study strong interactions between spins, orbitals and lattice, and open pathways to coherent control of 2D magnets.

2022 ◽  
Vol 119 (3) ◽  
pp. e2113967119
Laurent P. René de Cotret ◽  
Martin R. Otto ◽  
Jan-Hendrik Pöhls ◽  
Zhongzhen Luo ◽  
Mercouri G. Kanatzidis ◽  

SnSe is a layered material that currently holds the record for bulk thermoelectric efficiency. The primary determinant of this high efficiency is thought to be the anomalously low thermal conductivity resulting from strong anharmonic coupling within the phonon system. Here we show that the nature of the carrier system in SnSe is also determined by strong coupling to phonons by directly visualizing polaron formation in the material. We employ ultrafast electron diffraction and diffuse scattering to track the response of phonons in both momentum and time to the photodoping of free carriers across the bandgap, observing the bimodal and anisotropic lattice distortions that drive carrier localization. Relatively large (18.7 Å), quasi-one-dimensional (1D) polarons are formed on the 300-fs timescale with smaller (4.2 Å) 3D polarons taking an order of magnitude longer (4 ps) to form. This difference appears to be a consequence of the profoundly anisotropic electron–phonon coupling in SnSe, with strong Fröhlich coupling only to zone-center polar optical phonons. These results demonstrate a high density of polarons in SnSe at optimal doping levels. Strong electron-phonon coupling is critical to the thermoelectric performance of this benchmark material and, potentially, high performance thermoelectrics more generally.

2022 ◽  
Vol 128 (1) ◽  
Meng-Xue Guan ◽  
Xin-Bao Liu ◽  
Da-Qiang Chen ◽  
Xuan-Yi Li ◽  
Ying-Peng Qi ◽  

Zhong-zhen Luo ◽  
Songting Cai ◽  
Shiqiang Hao ◽  
Trevor Bailey ◽  
Yubo Luo ◽  

Although Ga doping can weaken the electron phonon coupling of n-type PbTe, Ga-doped PbTe has a relatively low carrier concentration (n) and high lattice thermal conductivity (κlat), resulting in a...

Nanoscale ◽  
2022 ◽  
Ruowei Wu ◽  
Mei Qi ◽  
Qiyi Zhao ◽  
Yuanyuan Huang ◽  
Yixuan Zhou ◽  

Low-symmetry of ReS2 has not only in-plane but also out-of-plane anisotropic light scattering, which is complicated, yet interesting with intrinsic strong electron-phonon coupling. In such case, the Raman tensor also...

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