Changes In Phonon Frequencies Research Articles

Control of coherent phonon dynamics in CoFeB/heavy metal heterojunction structures for future hybrid phononic and spintronic devices

Abstract The heterojunction structure of CoFeB/heavy metal has shown significant potential for spintronics, where both electrons and magnons potentially can serve as information carriers. However, another promising information carrier, coherent phonons, has not been fully explored for hybrid phononic and spintronic devices. In this study, we used time-resolved pump–probe spectroscopy to investigate the dynamic behaviour of coherent phonons. We observed variations in reflectivity spectra, corresponding to changes in phonon frequency and relaxation times, with different thicknesses of the heavy metal and CoFeB layers. The experimental results demonstrated a decrease in coherent phonon oscillation frequency as the thickness of the CoFeB and heavy metal layers increased. These findings were further supported by first-principles calculations, which showed that the frequency of the optical modes is suppressed due to interface relaxation between the magnetic and heavy metal layers.

Studies on structural and optical characterization of Na:ZnS nanocomposites

In this work, we report the structural, morphological and optical characterization of pure and sodium (Na) mixed ZnS nanocomposites. The Na:ZnS nanocomposites were synthesized by the co-precipitation method using the Na2SO4. The powder XRD pattern reveals a mixed phase of cubic and hexagonal structure. The crystalline size was found to be ∼6 nm based on the full width at half maximum (FWHM) by using the Debye Scherrer formula. Moreover, the diffraction peaks were shifted towards a higher angle compared to pure ZnS. This can be due to the difference in the ionic radii of Na+ and Zn2+ ions. The SEM images show the spherical shape of nanocomposites. The UV-visible absorption spectra of pure and Na:ZnS show an absorption cut-off wavelength of 278 nm (4.46 eV) compared to bulk band gap of ZnS (3.78 eV). The blue shift can be attributed to the quantum confinement effect. In addition to that, the optical absorption increases with increasing Na concentration. The functional groups are identified by FTIR analysis. The photoluminescence (PL) study reveals the near band edge emission at 468 nm (2.64 eV). The broad and high relative intensity of defect level emission is observed in the lower energy side of the PL spectra. The intensity of luminescence increases with increasing Na concentration due to the radiative recombination process. The Raman spectra show the formation of nanocomposite through change in phonon frequency, structural disorder and width for LO phonon mode. A change of 1.7 cm−1 phonon width suggests the formation of disorder.

Stress-induced changes in phonon frequencies of ZrSiO4: Infrared spectroscopy-based pressure sensor

Functional materials that can serve as high-pressure transducers are limited, making such sensor material sought after. It has been reported that hydrostatic pressures highly influence Raman shifts of ZrSiO4. Therefore, zirconium silicate has been suggested as a Raman spectroscopic pressure sensor. However, mass applications of a Raman-based sensor technology poses a wide range of challenges. We demonstrate that ZrSiO4 also exhibits pressure-dependent infrared (IR) spectra. Furthermore, the IR peaks of ZrSiO4 are sensitive to shear stresses and non-hydrostatic pressures, making this material a unique sensor for determining a variety of mechanical stresses through IR spectroscopy.

(Invited) Micromechanical Aspects of GaN Hemt Performance and Reliability

Performance and reliability of microelectronic devices are governed by the mechanical strain. Typically, strain engineering implies uniformly distributed strain. However, we offer a different perspective by hypothesizing that very small but localized strain (or stress) may have significant impact on the overall behavior of AlGaN/GaN high electron mobility transistors (HEMTs). Micro to nanoscale confined mechanical stress fields may develop unavoidably and are ignored because their spatial average is insignificant. We exploit high resolution techniques such as transmission electron microscopy (TEM) and micro-Raman spectroscopy to spatially resolve the stress field in GaN HEMTs to demonstrate the stress localization and then radiation effects.To study stress localization effects on the global electrical characteristics, we introduced a highly localized strain relief by milling a 20×30 μm2 micro trench about 70 μm deep on the backside of an 800×840 μm2 size HEMT die. The resulting local relaxation of in-plane residual strain was mapped using micro-Raman technique. Our results show that a decrease of only 0.02% strain can decrease the overall output saturation current up to ~20 %. The drop of output current is attributed to reduced two-dimensional electron gas (2DEG) sheet carrier density and electron mobility due to the strain relief in the device layers. However, the mechanistic process of strain relief also causes defect generation at the interfaces, which increases leakage current. Our technique for localized strain re-distribution could be an effective tool to surrogate the influence of inherent localized strain build-up across the channel of electronic devices.To study radiation effects, we exposed the HEMTs to gamma rays (up to 10 megaRads dose at 180 kiloRads per hour). We report the thermal and mechanical responses in terms of the changes in lattice strain and temperature, which were simultaneously characterized by changes in phonon frequency of E2 (high) and A1 (LO) from on- state and unpowered/pinched off reference states. Lower doses of radiation improved the electrical properties, however degradation initiated at about 1 megaRads. We observed about 16% decrease in saturation current and 6% decrease in transconductance at the highest dose. However, leakage current increase by 3 orders of magnitude was the most notable radiation effect. We observed temperature increase by 40% and mechanical stress increase by a factor of three at dose of 10 megaRads compared to the pristine devices. Spatial mapping of mechanical stress along the channel identifies the gate region as mechanically affected area, whereas the thermal degradation was mostly uniform. Transmission electron microscopy showed contrast changes reflecting high vacancy concentration in the gate region. These findings suggest that localized stress (mechanical hotspots) may increases vulnerability to radiation damage by accommodating higher concentration of defects that promote leakage current.Finally, we present experimental results on a novel concept of electron wind force driven room temperature annealing of GaN and SiC devices. This technique was applied on Gamma irradiated HEMT and SiC diodes. We demonstrate full recovery of the HEMTs irradiated to 10 mega-Rads gamma radiation. About 200% performance improvement was recorded for the SiC diodes. Our hypothesis is that mechanical stress waves are instrumental in eliminating interfacial defects.

Activation energy of homogeneous nucleation of Zr hydride: Density functional theory calculation

Considering the nucleation process of Zr hydrides as phase transformation from hexagonal closed-packed (HCP) to face-centered tetragonal (FCT) structure, we calculated the activation energy of the homogeneous nucleation process of Zr hydrides and atomic rearrangement during nucleation for Zr4H, Zr2H, ZrH and ZrH2 using density functional theory calculations and minimum energy path detection. At 0 K limit, although ZrH and ZrH2 have lower chemical potentials and are more energetically stable than Zr4H and Zr2H, the latter have lower activation energies for nucleation. At finite temperatures, the crossover of activation energies occurs around 300 K, where ZrH becomes the most possible candidate with the lowest activation energy. This was explained by the difference in the atomic rearrangement and change in phonon frequency during phase transformation.

High Pressure Raman Study of Layered Semiconductor Tlgase2

AbstractRaman spectroscopy measurements of a monoclinic layered semiconductor TlGaSe2were performed in a pressure range up to 10.24 GPa. The pressure-induced first-order phase transition accompanied by reconstruction of the layer structure was observed at the pressure P ~ 0.9 GPa. The mode-Grüneisen parameters of intralayer bonds were calculated for TlGaSe2. The contribution of thermal expansion to temperature changes of phonon frequencies was defined. The type of intralayer bonds and their pressure transformation were analyzed in layered TlGaSe2. It was shown that the nature of intramolecular forces in molecular crystals and intralayer forces in layered GaS, GaSe and TlGaSe2is similar

Nuclear quantum effect with pure anharmonicity and the anomalous thermal expansion of silicon

Despite the widespread use of silicon in modern technology, its peculiar thermal expansion is not well understood. Adapting harmonic phonons to the specific volume at temperature, the quasiharmonic approximation, has become accepted for simulating the thermal expansion, but has given ambiguous interpretations for microscopic mechanisms. To test atomistic mechanisms, we performed inelastic neutron scattering experiments from 100 K to 1,500 K on a single crystal of silicon to measure the changes in phonon frequencies. Our state-of-the-art ab initio calculations, which fully account for phonon anharmonicity and nuclear quantum effects, reproduced the measured shifts of individual phonons with temperature, whereas quasiharmonic shifts were mostly of the wrong sign. Surprisingly, the accepted quasiharmonic model was found to predict the thermal expansion owing to a large cancellation of contributions from individual phonons.

Probing the interface strain in a 3D-2D van der Waals heterostructure

The interaction at a van der Waals (vdW) epitaxial interface is perceived to be too weak to induce substantial elastic strain in films due to the weak nature of dipole-dipole interactions. Recent research on discovering giant strain in vdW heterostructures in 2D-2D configurations has reformed such an understanding. In this letter, the study on strain was extended to the regime of 3D-2D vdW heterostructures. Using temperature dependent Raman spectroscopy, the strain behaviors were revealed, through the change of phonon frequency, at a vdW epitaxial interface of CdS-mica. Although the interface was free of interactions for relatively thick CdS films, it was found that at least 0.4% strain can be borne at the interface when the film was only 6 nm thick and the strain was estimated to be 1.2% if a two-monolayer can be grown. The finding suggests that non-trivial strain could be enabled in 3D-2D vdW heterostructures if the thickness can be well-controlled, providing an avenue for strain engineering. This work also indicated a different changing tendency in the linewidth of phonon scattering as a function of temperature, which may be related to defects and serve as a reference for studying the linewidth of ultrathin films.

Ultrafast structural phenomena: theory of phonon frequency changes and simulations with code for highly excited valence electron systems

When a femtosecond-laser pulse excites a solid it may, among other ultrafast processes, induce coherent phonons, phonon frequency changes, thermal phonon squeezing, and nonthermal melting. Using our in-house code for highly excited valence electron systems, where laser-induced interatomic forces are computed “on the fly” from ab initio theory, we performed molecular dynamics simulations of supercells with up to 800 atoms. For Si we found that thermal phonon squeezing precurses nonthermal melting as a function of fluence. Furthermore, our molecular dynamics trajectories showed that nonthermal melting includes a stage during which the atoms move fractionally diffusive. We also simulated femtosecond-laser-excited Ge. In addition, we explain the electronic origin of laser-induced phonon frequency redshifts and blueshifts in Mg.

Electronic origin of bond softening and hardening in femtosecond-laser-excited magnesium

Many ultrafast structural phenomena in solids at high fluences are related to the hardening or softening of particular lattice vibrations at lower fluences. In this paper we relate femtosecond-laser-induced phonon frequency changes to changes in the electronic density of states, which need to be evaluated only in the electronic ground state, following phonon displacement patterns. We illustrate this relationship for a particular lattice vibration of magnesium, for which we—surprisingly—find that there is both softening and hardening as a function of the femtosecond-laser fluence. Using our theory, we explain these behaviours as arising from Van Hove singularities: We show that at low excitation densities Van Hove singularities near the Fermi level dominate the change of the phonon frequency while at higher excitations Van Hove singularities that are further away in energy also become important. We expect that our theory can as well shed light on the effects of laser excitation of other materials.

Far infrared reflection spectra of InAsxSb1−x (x = 0-0.4) thin films

A series of InAsxSb1−x ternary thin films (x = 0-0.4) has been studied in a far-infrared reflection experiment over the range of 50-4000 cm−1 at room temperature. The obtained spectra are fitted using a multi-oscillator model. Two types of lattice vibration modes, InSb-like and InAs-like, plus one plasmon mode have been identified in x > 0 samples. The lattice vibration in these ternary alloy films shows a typical two-mode behavior. Within the studied fraction range, the InSb transverse-optical (TO) phonon frequency decreases with x, while the InAs TO frequency increases. A random-element-isodisplacement model has been employed to describe the phonon frequency changes. The fitted plasmon parameters have been used to extract the carrier concentrations and mobility. The carrier concentration increases monotonously with the increase of As fraction and is attributed to the bandgap narrowing effect. The mobility decreases with x, indicating an increased scattering.

Anomalous surface lattice dynamics in the low-temperature phase of Ba(Fe 1− x Co x ) 2 As 2

In complex materials, how correlation between charge, spin, and lattice affects the emergent phenomena remains unclear. The newly discovered iron-based high-temperature superconductors and related compounds present to the community a prototype family of materials, where interplay between charge, spin, and lattice degrees of freedom can be explored. With the occurrence of structural, magnetic, and superconducting transitions in the bulk of these materials, creating a surface will change the delicate balance between these phases, resulting in new behavior. A surface lattice dynamics study on (001) Ba(Fe(1-x)Co(x))(2)As(2), through electron energy loss spectroscopy measurements, reveals unusual temperature dependence of both the phonon frequency and line width in the low-temperature orthorhombic phase. The rate of change of phonon frequency with temperature is gigantic, two orders of magnitude larger than in the bulk. This behavior cannot be explained using conventional models of anharmonicity or electron-phonon coupling; instead, it requires that a large surface-spin-charge-lattice coupling be included. Furthermore, the higher surface-phase-transition temperature driven by surface stabilization of the low-temperature orthorhombic phase seems to turn the first-order transition (bulk) into the second-order type, equivalent to what is observed in the bulk by applying a uniaxial pressure. Such equivalence indicates that the surface mirrors the bulk under extreme conditions.

Spin-lattice coupling and phonon dispersion of CdCr2O4from first principles

First-principles calculations are used to investigate the effects of magnetic ordering on the minimum-energy structure and on the full phonon dispersion relation of CdCr${}_{2}$O${}_{4}$, focusing on the changes through the coupled magnetic/structural transition which shows relief of the geometric frustration of the antiferromagnetic ordering on the pyrochlore lattice. We computed the full phonon dispersion relations for the ferromagnetic and antiferromagnetic orderings in cubic and tetragonal structures of CdCr${}_{2}$O${}_{4}$. We extracted the phonon dispersion for the cubic paramagnetic phase and found that it compares well with the experimental results. The AFM ordering is seen to lower the symmetry and induce a lattice distortion comparable in magnitude to that observed in the transition. While the spin-phonon couplings are large for modes which involve displacement of the Cr atoms, there are no unstable modes at any point in the Brillouin zone for either of the magnetic orderings considered, and thus we conclude that the phase transition is driven not by spin-phonon coupling, but by the atomic forces and stresses induced by the magnetic order. Finally, by comparison of the phonon frequencies for structures with different magnetic orderings and structural distortions, we find that the spin-phonon coupling, rather than the coupling of the phonons to the structural change, is the dominant factor in the observed changes of phonon frequencies through the phase transition.

Nondestructive Measurement of Carrier Density in GaAs Using Relative Reflectivity of Two Terahertz Waves

A new nondestructive method using terahertz waves for determining the carrier density of GaAs is proposed. The reflectance around the longitudinal optical (LO) phonon frequency changes with carrier density, whereas the reflectance around the transverse optical (TO) phonon frequency is constant. The relative reflectance, which is evaluated from the reflectance at the two frequencies related to the TO and LO phonon, as a function of the carrier density of GaAs was calculated from the dielectric function. A broadband tunable terahertz source is convenient for producing two waves around these phonon frequencies. Imaging of n-type GaAs wafer using this new terahertz method is described.

Unexpected isotopic effect of a deuteriumed countercation on the spin-Peierls-type transitions in quasi-one-dimensional bis(maleonitriledithiolato)nickelate monoanion spin systems

Two deuteriumed quasi-one-dimensional (quasi-1D) spin-Peierls-type compounds, 4-X-benzylpyridinium-d(5) bis(maleo-nitriledithiolato)nickelate (the substituent X = Br or Cl), were structurally characterized. Compared with the corresponding non-deuteration compounds, the transition temperature T(C) shifts to higher temperature. The isotopic effect of countercations on T(C) is probably related to the change of phonon frequency ω(0) and 'chemical pressure' resulted from the substitution of pyridine by pyridine-d(5).

Low-temperature infrared spectroscopic study of OH-stretching modes in kaolinite and dickite

The powder-transmission infrared spectra of OH-stretching modes in well-ordered samples of kaolinite and dickite are recorded from 270 to 10 K. Effective medium modeling of the kaolinite spectrum confirms the coupled motion of non-equivalent OH groups in the inter-layer OH-stretching modes. A full assignment of OH-stretching bands in dickite is proposed. In particular, the band observed at 3685 cm−1 in room-temperature spectra is attributed to a kaolinite-like defect, whereas that at 3706 cm−1 is related to OH3 groups of the dickite structure. The changes in phonon frequency and line shape are analyzed as a function of temperature. The results evidence the quasi-harmonic nature of the OH-stretching modes in kaolinite-group minerals. Coupling of OH-stretching modes to each other can be related to the macroscopic electrostatic properties of the composite medium, made of mineral particles and KBr matrix, and not to microscopic anharmonic mechanisms. The low saturation temperatures of vibrational frequencies and linewidths indicate that the OH-stretching modes probe the average thermal bath of the crystal, without coupling to a specific decay channel.

Pressure dependence of phonon modes across the tetragonal to collapsed-tetragonal phase transition inCaFe2As2

The pressure dependence of a large number of phonon modes in CaFe2As2 with energies covering the full range of the phonon spectrum has been studied using inelastic x-ray and neutron scattering. The observed phonon frequency changes are in general rather small despite the sizable changes of the lattice parameters at the phase transition. This indicates that the bonding properties are not profoundly altered by the phase transition. The transverse acoustic phonons propagating along the c-direction are an exception because they stiffen very significantly in response to the large contraction of the c-axis. The lattice parameters are found to change significantly as a function of pressure before, during and after the first-order phase transition. However, the frequencies change nearly uniformly with the change in the lattice parameters due to pressure, with no regard specifically to the first-order phase transition. Density functional theory describes the frequencies in both the zero pressure and in the collapsed phase in a satisfactory way if based on the respective crystal structures.

Cooper-pair formation by anharmonic rattling modes in the β-pyrochlore superconductor KOs2O6

We study the influence of anharmonic rattling phonons in the β-pyrochlore superconductor KOs2O6 using the strong-coupling Eliashberg approach. In particular, by analyzing the specific heat data, we find that the rattling phonon frequency changes discontinuously at the critical temperature of the first-order phase transition. Solving the strong-coupling Eliashberg equations with effective temperature-dependent α2F(ω), we investigate the consequence of this first-order phase transition for the anomalous temperature dependence of the superconducting gap. We discuss our results in the context of the recent experimental data.

Low-temperature and high-pressure Raman and x-ray studies of pyrochloreTb2Ti2O7: Phonon anomalies and possible phase transition

We have carried out temperature- and pressure-dependent Raman and x-ray measurements on single crystals of Tb2Ti2O7. We attribute the observed anomalous temperature dependence of phonons to phonon-phonon anharmonic interactions. The quasiharmonic and anharmonic contributions to the temperature-dependent changes in phonon frequencies are estimated quantitatively using mode Gruneisen parameters derived from pressure-dependent Raman experiments and bulk modulus from high-pressure x-ray measurements. Further, our Raman and x-ray data suggest a subtle structural deformation of the pyrochlore lattice at ~9 GPa. We discuss possible implications of our results on the spin-liquid behavior of Tb2Ti2O7.

Electron-phonon interactions and high-temperature thermodynamics of vanadium and its alloys

Inelastic neutron scattering was used to measure the phonon densities of states (DOSs) for pure V and solid solutions of V with 6 to 7at% of Co, Nb, and Pt, at temperatures from 10 K to 1323 K. Ancillary measurements of heat capacity and thermal expansion are reported on V and V-7at%Co and used to help identify the different sources of entropy. Pure V exhibits an anomalous anharmonic stiffening of phonons with increasing temperature. This anharmonicity is suppressed by Co and Pt, but not by isoelectronic Nb solutes. The changes in phonon frequency with alloying and with temperature both correlate to the decrease in electron density of states (DOS) at the Fermi level as calculated using density functional theory. The effects of both temperature and alloying can be understood in terms of an adiabatic electron-phonon interaction (EPI), which broadens sharp features in the electron DOS. These results show that the adiabatic EPI can influence the phonon thermodynamics at temperatures exceeding 1000 K, and that thermal trends of phonons may help assess the strength of the EPI.