Phonon Lifetime Research Articles

Discovery of superconductivity and electron-phonon drag in the non-centrosymmetric Weyl semimetal LaRhGe3

We present an exploration of the effect of electron-phonon coupling and broken inversion symmetry on the electronic and thermal properties of the semimetal LaRhGe3. Our transport measurements reveal evidence for electron-hole compensation at low temperatures, resulting in a large magnetoresistance of 3000% at 1.8 K and 14 T. The carrier concentration is on the order of 1021/cm3 with high carrier mobilities of 2000 cm2/Vs. When coupled to our theoretical demonstration of symmetry-protected almost movable Weyl nodal lines, we conclude that LaRhGe3 supports a Weyl semimetallic state. We discover superconductivity in this compound with a Tc of 0.39(1) K and Bc(0) of 2.2(1) mT, with evidence from specific heat and transverse-field muon spin relaxation. We find an exponential dependence in the normal state electrical resistivity below ~50 K, while Seebeck coefficient and thermal conductivity measurements each reveal a prominent peak at low temperatures, indicative of strong electron-phonon interactions. To this end, we examine the temperature-dependent Raman spectra of LaRhGe3 and find that the lifetime of the lowest energy A1 phonon is dominated by phonon-electron scattering instead of anharmonic decay. We conclude that LaRhGe3 has strong electron-phonon coupling in the normal state, while the superconductivity emerges from weak electron-phonon coupling. These results open up the investigation of electron-phonon interactions in the normal state of superconducting non-centrosymmetric Weyl semimetals.

Ultralow lattice thermal conductivity in K2AuBi and its thermoelectric properties

Thermoelectric materials are best known for their prowess to transform the environment’s waste heat into electricity. In an endeavor to explore new thermoelectric prospects, in the present study, we investigate K2AuBi using density functional theory-based first-principles simulations. From our simulations, we find an intrinsically low lattice thermal conductivity of 0.43 W m−1 K−1 at 300 K in K2AuBi. Based on our detailed analysis, we find the reasons for such a low value of lattice thermal conductivity as, low phonon group velocities, short phonon lifetimes, anharmonicity in the lattice vibrations, and significant mean square displacements of K and Au atoms. The large mean square displacements hint at weak bonding and anharmonicity in the lattice vibrations, favoring more phonons scattering. We also find that the vibrations of K-atoms can be related to rattlers, conducive to low lattice thermal conductivity. Our simulations predict a high value, ∼784 μV K−1, of Seebeck coefficient at 700 K on account of the large density of states in the vicinity of Fermi level. Combining our computed lattice thermal conductivity with electrical transport properties, we obtain a high figure of merit, ZT∼ 1.04, at 700 K in K2AuBi.

Influence of Mo dopants on the vibrational, emission, dielectric, electrical, and magnetic properties of combustion synthesized ZnFe2O4 nanostructures for optoelectronic and spintronic applications

This report investigates the dielectric and magnetic behavior of Molybdenum (Mo)-incorporated ZnFe2O4 prepared via combustion route with different dopant concentrations (0.0, 0.1, 0.25, 0.5, 0.75, and 1.0 wt.%). XRD patterns reveal the cubic spinel structures with a slight increase in lattice constant while replacing Mo at Fe sites. Mo doped induced lattice constant increase from 8.444 to 8.469 Å coupled with a significant increase in density. Raman spectroscopy reveals a decrement in the peak broadening of the A1g mode at higher Mo concentrations, indicating longer phonon lifetimes. Scanning electron microscopy (SEM) and EDX analysis confirm the agglomerated pseudo-spherical structures with uniform elemental distribution over the surface. Further, the dielectric constant values exhibit a slightly decreasing trend with increasing frequency, and the mechanisms were discussed based on the intrinsic polarization due to the charge imbalance between Fe3+ and Fe2+ states. Further, the magnetic measurements confirm the soft magnetic behavior with saturation magnetization ranging from 13.72 to 14.61 emu/g and coercivity between 07 (Oe) to 44 (Oe). The overall findings demonstrate that Mo doping in ZnFe₂O₄ significantly modifies the dielectric and magnetic properties, making it a promising material for various technological applications.

Electronic structure investigation and anisotropic phonon anharmonicity in ternary ZrGeTe4 single crystals

Ternary two-dimensional (2D) transition metal chalcogenides have gained immense attention because of their ability to overcome the intrinsic limitations of their binary counterparts. Layered 2D materials are important for future electronic and photonic devices owing to their low structural symmetry and in-plane anisotropy with tunable bandgap. Herein, the electronic structure and detailed vibrational properties of bulk ZrGeTe4 layered single crystals were investigated using angle-resolved photoemission spectroscopy (ARPES) and Raman scattering (RS). The ARPES results revealed an anisotropic Fermi surface of different momentum along kx and ky from the zone center and an anisotropic band structure with varying band curvatures along the high-symmetry directions. Furthermore, the RS of ZrGeTe4 was investigated under different polarizations and varying temperatures. The polarized RS exhibited twofold and fourfold symmetry orientations in different configurations, revealing the anisotropic phonon dispersions for bulk ZrGeTe4. The observed softening of Raman modes was corroborated with the anharmonic phonon dispersion, which was further supported by our third-order force constant calculations of thermal transport using density functional theory. Low lattice thermal conductivity with increasing temperature is linked with enhanced phonon–phonon scattering, which is evident from the decreased phonon lifetime and peak linewidth. In addition to these fundamental aspects, the anisotropic nature and unique layered structure of such materials reveal their bright future for next-generation nanoelectronic applications.

Research on Multi-Core Curvature Sensing Measurement Based on PPP-BOTDA.

To address the issue of spatial resolution limitations in traditional Brillouin optical time-domain analysis systems due to phonon lifetime constraints, we employed pre-pumped pulse technology. Additionally, to mitigate the double-peak phenomenon observed in pre-pumped Brillouin optical time-domain analysis systems, we implemented a two-sided band interference method to reduce the linewidth of the double-peak fitting. We conducted bending measurements on three eccentric cores and intermediate cores spaced 120° apart. Our results demonstrate that the system described in this paper can achieve a spatial resolution of 30 cm, with bimodal linewidths of 23.1 MHz and 16.0 MHz. Using the parallel transmission frame algorithm, we determined the curvature of a seven-core fiber with a curvature diameter of approximately 10 cm to be 20.67 m-1, with an error margin of 3.2%.

Electronic structure and phonon transport properties of HfSe2 under in-plane strain and finite temperature

The electronic structure and phonon transport properties of HfSe2 under different in-plane strains at finite temperatures are systematically investigated by combining first-principles calculations with machine learning force field molecular dynamics simulations. Within a strain range of -3% to 3%, the electronic band gap value of HfSe2 varies between 0.39 and 0.87 eV. Under compressive strain, the conduction band minimum moves towards the Fermi level, with the distribution of electrons near the valence band maximum becoming more delocalized. This will reduce the scattering of electrons during the transport process, helping to improve the carrier mobility. Under tensile strain, the localization of the density of states near the valence band maximum is strengthened, accompanied by enhanced metallic properties of the Hf-Se bonds, which facilitates the enhancement of the thermoelectric power factor. Both compressive and tensile strains intensify the coupling of phonon normal modes with phonon scattering, and elevating the temperature amplifies this impact. The anharmonicity-induced reduction in phonon frequencies is especially pronounced for modes in the vicinity of the Debye frequency. This not only curtails the phonon lifetimes but also diminishes the lattice thermal conductivity through the enhancement of vibrational coupling among optical branches and the reduction of the group velocity in acoustic branches. These results demonstrate the synergistic effects of strain and temperature on the electronic structure and phonon transport of HfSe2.

Temperature dependence of phonon energies and lifetimes in single- and few-layered graphene

Temperature dependence of phonon energies and lifetimes in single- and few-layered graphene

Ultrafast Polarization-Resolved Phonon Dynamics in Monolayer Semiconductors.

Monolayer transition metal dichalcogenide semiconductors exhibit unique valleytronic properties interacting strongly with chiral phonons that break time-reversal symmetry. Here, we observed the ultrafast dynamics of linearly and circularly polarized E'(Γ) phonons at the Brillouin zone center in single-crystalline monolayer WS2, excited by intense, resonant, and polarization-tunable terahertz pulses and probed by time-resolved anti-Stokes Raman spectroscopy. We separated the coherent phonons producing directional sum-frequency generation from the incoherent phonon population emitting scattered photons. The longer incoherent population lifetime than what was expected from coherence lifetime indicates that inhomogeneous broadening and momentum scattering play important roles in phonon decoherence at room temperature. Meanwhile, the faster depolarization rate in circular bases than in linear bases suggests that the eigenstates are linearly polarized due to lattice anisotropy. Our results provide crucial information for improving the lifetime of chiral phonons in two-dimensional materials and potentially facilitate dynamic control of spin-orbital polarizations in quantum materials.

Identifying the effect of photo-generated carriers on the phonons in rutile TiO2 through Raman spectroscopy

Abstract Investigating lattice vibrations through Raman spectroscopy is a crucial method for studying crystalline materials. Carriers can interact with lattices and influence lattice vibrations; thus, it is feasible to study the effect of photo-generated carriers on phonons by analyzing changes in the Raman spectra of semiconductors. Rutile is one of the predominant crystalline phases of TiO2, which is a widely utilized metal oxide semiconductor. In this work, rutile TiO2 is coated on a thinned optical fiber to concentrate ultraviolet light energy within the material, thereby enhancing the generation of carriers and amplifying the changes in the Raman spectra. A Raman detection laser with a wavelength of 532 nm is utilized to collect the Raman spectra of rutile TiO2 during irradiation. Using this setup, the impact of photo-generated carriers on the phonons corresponding to Raman vibrational modes is researched. The localization and non-radiative recombination of photo-generated carriers contribute to a reduction in both the frequencies and lifetimes of phonons. This work provides a novel approach to researching the effect of carriers on phonons.

Active control of thermal conductivity of low-dimensional α -PbS by strain-induced ferroelectric

Active control of heat transfer in nanostructured materials is crucial for the development of microelectronic devices. Thermal switch is a typical heat management device, which has attracted widespread attention. In this work, based on first-principles calculations, we propose a two-dimensional thermal switch based on the strain-induced ferroelectric phase transition in α-PbS. It is found that thermal conductivity can be significantly reduced by external strain and a room temperature two-dimensional thermal switch with a switch ratio of 3.7 can be constructed. The calculated phonon lifetime and scattering rate reveal that phonons around 2 THz frequency range predominantly contribute to the modulation in thermal conductivity when the strain is smaller than 2.0%. A detailed analysis on phonon dispersion indicates that these phonons are LO2 and TO3 branches. When the strain is larger than 2.0%, the decrease in phonon group velocity leads to the reduction in thermal conductivity. Our work elucidates the mechanisms for changes in the thermal conductivity of α-PbS under strain and provides a low-dimensional thermal switch, which is promising for future applications in microelectronic devices.

First-principles study on thermal transport properties of GaN under different cross-plane strain

With the development of power devices towards high integration and power, the electrical and thermal stresses per unit area become more concentrated and intensified. The impact of cross-plane strain resulting from inverse piezoelectric effect and thermal expansion on power devices cannot be ignored. Cross-plane strain has a substantial influence on the thermal properties of GaN. However, the research on the influence of cross-plane strain on the thermal conductivity of GaN has not been reported in the literature. Based on the first-principles calculation method and the phonon Boltzmann transport equation, the influence of cross-plane strain on the thermal conductivity and phonon characteristics of the GaN lattice is systematically studied in this study. The thermal conductivity of GaN has anisotropy and increases significantly with the decrease in temperature. The thermal conductivity of GaN at room temperature under the free state is calculated to be 257 and 275 W/(m·K) for in-plane (k⊥) and cross-plane (k∥) directions, respectively. Compared with previous theoretical reports, our calculation results are more consistent with the existing experiment values. Under the state of cross-plane strain, the lattice thermal conductivity changes remarkably. In detail, the average thermal conductivity at room temperature decreased by 35 % under a 5 % cross-plane tensile strain state, while it increased by 11.5 % under a 5 % cross-plane compressive strain state. According to the calculation results, the influence of cross-plane on lattice thermal conductivity is mainly due to the change in phonon lifetime. The analysis of two mechanisms of phonon lifetime suppression indicates that the cross-plane strain will significantly change the frequency of the high-frequency optical acoustic branch. This result leads to a change in the phonon scattering process and thus affects the phonon lifetime. Besides, the anisotropy of thermal conductivity changes under different strain values, which may be due to the weakened piezoelectric polarization effect induced by strain.

Triaxial strain enhanced thermoelectric performance and conversion efficiency in Tl3TaSe4

Thermoelectric (TE) materials have garnered significant attention due to their potential in converting waste heat into useful electrical energy. Strain engineering has emerged as a promising strategy for tailoring the properties of materials, offering opportunities to optimize TE performance. In this study, we employ first-principles calculations to investigate the electronic and thermal transport properties of cubic Tl3TaSe4 under triaxial strain. Our results demonstrate that tensile strain can reduce the effective mass of carriers and contribute to high carrier mobility. The phonon calculation and AIMD simulation reveal excellent dynamic stability and thermal stability of Tl3TaSe4 both with and without strain. The red shift phenomenon of phonon vibration modes and the higher portion of phonon DOS in the low frequency region indicate that tensile strain enhances the anharmonicity of Tl3TaSe4. In addition, triaxial tensile strain can effectively reduce the heat capacity, group velocity, and phonon lifetime, while enhancing the anharmonicity of Tl3TaSe4, which contributes to the lower κl. Compared to the condition without strain, the room temperature κl of Tl3TaSe4 under 1 % and 2 % tensile strain is reduced by about 43 % and 79 %, respectively. Furthermore, the tensile strain exhibits a positive enhancement effect on the ZT value of Tl3TaSe4. At 700 K, the optimal ZT values of p-type and n-type Tl3TaSe4 under 2 % tensile strain are 1.31 and 0.94, respectively, which are three times higher than that of 0.4 and 0.33 without strain. Finally, the TE conversion efficiency of Tl3TaSe4 under 2 % tensile strain is 6.8 %, which is higher than BiCuSeO and α-Cu2+xSe, and comparable to SnSe. Our results demonstrate the effectiveness of triaxial tensile strain in enhancing the phonon scattering, reducing the κl, and improving the TE properties of Tl3TaSe4.

Theoretical investigation into potential thermoelectric material Monolayer InI with direct bandgap and ultralow lattice thermal conductivity

Monolayer metal iodides are garnering widespread attention due to their ultralow lattice thermal conductivity and their potential applications in the field of thermoelectrics. Here, we utilized first-principles calculations and Boltzmann transport theory to thoroughly investigate the thermoelectric behavior of monolayer InI. Results indicate that this material functions as a direct bandgap semiconductor and features an exceptionally low lattice thermal conductivity of 0.27 W m−1K−1 at 300 K, a result of its low group velocities and short phonon lifetimes. Furthermore, transport coefficients were calculated with enhanced precision through the Wannier interpolation method, incorporating the HSE06 hybrid functional and spin-orbit coupling (SOC) effects. Remarkably, the maximum thermoelectric figure of merit (ZT) values for monolayer InI demonstrate substantial growth with increased temperatures, surging from 0.31 at 300 K to 1.20 at 450 K for p-type, and escalating from 0.26 at 300 K to 0.72 at 450 K for n-type. These promising low-temperature thermoelectric properties exhibited by monolayer InI suggest its suitability for integration into the production of thermoelectric devices.

Performance evaluation of SILAR deposited Rb-Doped ZnO thin films for photodetector applications

ZnO-based photodetectors (PDs) compose a remarkable optoelectronic device field due to their high optical transmittance, electrical conductivity, wide band gap, and high binding energy. This study examined the visible light photodetector performance of the pristine and Rubidium (Rb)-doped ZnO thin films. The influence of Rb doping amount (2, 4, and 6 wt% in solution) on the electrical, optical, and structural properties of the ZnO-based thin films produced by the Successive Ion Layer Adsorption and Reaction (SILAR) technique was analyzed. Structural analyses showed that all peaks correspond to hexagonal wurtzite structure with no other peak from Rb-based phases, suggesting the high quality of the crystalline pristine and Rb-doped ZnO thin films. The morphology of the thin films shows homogenous layers formed of nanoparticles where particle size was first decreased and then increased with the increasing Rb doping according to Scanning Electron Microscope (SEM) morphology analysis. Besides that, Raman spectroscopy analyses indicate that the phonon lifetimes of the ZnO-based thin films slightly increased due to the improvement of the crystal quality with the increasing amount of Rb in the SILAR solution. Photosensor measurements of the nanostructured pristine and Rb-doped ZnO thin films were measured at different light power intensities under the visible light environment. Photosensor properties were examined depending on the doping amount and light power density. In light of the literature review, our study is the first to produce Rb-doped ZnO thin films via the SILAR method, which has a promising potential for photosensor applications.Graphical

Quantification of electron–phonon interaction in bismuth telluride under hydrostatic pressure via ultrafast spectroscopy

Here, we demonstrate a strategy for the quantification of electron–phonon interaction (EPI) of bismuth telluride (Bi2Te3) under hydrostatic pressure through systematic femtosecond pump-probe spectroscopy. Two optical phonon modes, namely A1g and Eg with frequencies of 1.87 and 3.71 THz at ambient pressure, are detected using time-resolved transient reflection (TR) measurement. Frequencies of both coherent phonon oscillations increase monotonically by around 33% and 17%, respectively, with the rising pressure up to 5.67 GPa, indicating pressure-induced phonon-hardening effect. The mode-specific electron–phonon coupling constant λ of Bi2Te3 under different pressures are calculated with the frequency of the A1g mode. It turns out that the variation of phonon lifetime and the corresponding phonon dephasing rate of the A1g mode may result from the pressure modification of λ. Our findings reveal the significant role of EPI in phonon transport and shed light on further manipulation on thermoelectric efficiency of Bi2Te3 with external strain.

Investigation of pressure-driven superconductivity in TlInTe2: an ab initio study

The Zintl compound TlInTe2 is an intriguing material because of its outstanding thermoelectric properties at ambient pressure. Interestingly, it has recently been found that TlInTe2 exhibits a V-shape dependence of the superconducting critical temperature (T c) under increasing pressure, which has been linked to the reversed behavior of the Raman active Ag phonon mode and anharmonic effects. In this study, we have performed first-principles calculations of the electron-phonon interactions and the superconducting properties of TlInTe2 in order to understand this unusual pressure-induced response. In contrast to experiment, we find a dome-shaped pressure-induced dependence of T c with a maximum value of 0.23 K at 18 GPa, significantly lower than the experimental results. Electron doping has the potential to adjust the T c to fall within the experimental range, but it necessitates considerably high levels of doping. Furthermore, our analysis of the phonon spectra and phonon lifetimes, including anharmonic effects, show that anharmonicity is unlikely to influence the superconducting properties of TlInTe2. It remains an open question whether there is indeed an unusual V-shape T c dependence with pressure or whether the phonon-mediated theory of superconductivity used here breaks down in this system.

Large magnetocaloric refrigeration performance near room temperature in monolayer transition metal dihalides

Magnetocaloric effect (MCE) exhibits highly efficient and ecological cooling abilities for solid-state refrigeration in contrast to traditional vapor-compression refrigeration. Successive emerging two-dimensional (2D) magnetic materials provide a fertile platform for exploring low-dimensional MCE systems. Here, we focus on a series of 2D transition metal dihalides MX2 (M = Fe, Ru, Os; X = Cl, Br) to explore the maximum isothermal magnetic entropy change (−ΔSmagmax) and adiabatic temperature change (ΔTadmax) under external magnetic field. It is found that FeCl2, FeBr2, and RuCl2 have intrinsically sizable −ΔSmagmax, ΔTadmax, and high thermal conductivity near room temperature, demonstrating superior comprehensive refrigeration performance in comparison with other 2D magnets. It is revealed that strong nearest-neighbor ferromagnetic exchange interaction plays a decisive role in −ΔSmagmax, and the high lattice thermal conductivities of FeCl2 and RuCl2 are attributed to the longer phonon lifetime and larger group velocity of low-frequency acoustic branch. Moreover, moderate strain and carriers doping are able to effectively regulate Curie temperature and magnetocrystalline anisotropy energy and correspondingly enhance −ΔSmagmax. The present work provides important insights for the exploration of 2D magnets for magnetocaloric refrigeration near room temperature.

Raman spectrum and phonon thermal transport in van der Waals semiconductor GaPS4

The emergent van der Waals semiconductor GaPS4 is heralding frontiers for gallium-based semiconductors. Despite its potential, the intricacies of its Raman spectrum and phonon heat transport remain elusive. In this research, experimental and theoretical methods are employed to give a comprehensive portrayal. The Raman spectra and phonon calculations obtained were cross-validated, affirming the study's credibility. A total of 28 Raman peaks were identified, with all phonon irreducible representations delineated. Advanced calculations unveiled notable shifts in the transition of GaPS4 from bulk to monolayer. During this process, phonons undergo a red shift, and the vibration contributions of different atoms change. The lifetime and group velocity of low wavenumber phonons are markedly reduced, suppressing the thermal conductivity in the monolayer. The thermal conductivity of GaPS4 bulk at 300 K is 0.5 W/m K, and 0.13 W/m K for monolayer, while the thermal conductivity in the cleavage direction is lower. These findings offer a detailed account of the complex Raman spectra and phonon thermal transport properties of GaPS4, setting the stage for its subsequent exploration and prospective applications in electronic and thermal devices, and contributing to enriching condensed matter theory of phonon thermal transport in van der Waals materials.

Approaching the low thermal conductivity in layered oxychalcogenide Bi2-xPrxO2Se (0 ≤ x ≤ 0.15) via mass and strain field fluctuatiocn for thermoelectric application

Bi2O2Se, a layered oxychalcogenide has gained much attention among the conventional oxides and chalcogenides based thermoelectric materials because of its high stability and less toxicity. Herein, Bi2-xPrxO2Se (0 ≤ x ≤ 0.15) is synthesized by ball milling followed by hot press densification technique, the x represents the replacement of Bi with Pr in atomic%. The influence of isovalent Pr substitution on the structural and thermal transport properties of Bi2O2Se were studied. The formation of tetragonal crystal structure with space group I4/mmm is confirmed by X ray diffraction (XRD) Pattern. HRTEM micrographs reveals the crystalline nature and lattice defects in the synthesized composition. The influence of 10 at % Pr substitution at Bi site reduces the thermal conductivity to 0.276 Wm−1K−1 at 653 K, originated from short phonon lifetime due to the effect of mass and strain field fluctuation. This result exhibits that the incorporation of Pr substitution in Bi2O2Se can dramatically enhance the thermoelectric performance of Bi2O2Se ceramic by reducing thermal conductivity.

Electronic Transport and Interaction of Lattice Dynamics in Topological Nodalline Semimetal HfAs2 Single Crystals

AbstractTopological semimetals represent a novel class of quantum materials displaying non‐trivial topological states that host Dirac/Weyl fermions. The intersection of Dirac/Weyl points gives rise to essential properties in a wide range of innovative transport phenomena, including extreme magnetoresistance, high mobilities, weak antilocalization, electron hydrodynamics, and various electro‐optical phenomena. In this study, the electronic, transport, phonon scattering, and interrelationships are explored in single crystals of the topological semimetal HfAs2. It reveals a weak antilocalization effect at low temperatures with high carrier density, which is attributed to perfectly compensated topological bulk and surface states. The angle‐resolved photoemission spectroscopy (ARPES) results show anisotropic Fermi surfaces and surface states indicative of the topological semimetal, further confirmed by first‐principle density functional theory (DFT) calculations. Moreover, the lattice dynamics in HfAs2 are investigated both with the Raman scattering and density functional theory. The phonon dispersion, density of states, lattice thermal conductivity, and the phonon lifetimes are computed to support the experimental findings. The softening of phonons, the broadening of Raman modes, and the reduction of phonon lifetimes with temperature suggest the enhancement of phonon anharmonicity in this new topological material, which is crucial for boosting the thermoelectric performance of topological semimetals.