Polar Phonon Modes Research Articles

Dispersive response of polar optical phonon modes on the dielectric and size effects in a nitride nanostructure

Within the framework of the macroscopic dielectric continuum model, the dispersive spectra of polar optical phonon modes of III-nitride based nanostructures with different inner and outer dielectric media are investigated in this paper. The dielectric and quantum size effects on the dispersive frequencies of phonon modes are focused and emphasized. In the case of GaN quantum rings (QRs), we found that the dielectric constant of the inner/outer medium could significantly influence the dispersive spectra of the surface optical (SO) phonon mode as the high/low-frequency SO phonon modes are mainly localized at the inner/outer surface of QRs, respectively. Moreover, such effects associated with different crystallographic structures of GaN, i.e., the wurtzite (WZ) and zinc blende (ZB) crystal structures, are also studied in detail, showing the different degenerating behavior and electron–phonon coupling strength. The results obtained in this study provide a beneficial opportunity to independently adjust dispersive spectra of the two branches of SO modes in GaN nanostructures; thus, it could be used to design and develop phonon-based detectors and sensors.

Terahertz pulse induced second harmonic generation and Kerr effect in the quantum paraelectric KTaO3

We present a joint experimental and computational study of second harmonic generation and Kerr effect induced by intense terahertz electric fields in the quantum paraelectric material ${\mathrm{KTaO}}_{3}$. The single-cycle terahertz (THz) electric pulses induce the free induction decay of the polar soft phonon mode and a ferroelectric polarization response that lasts for several picoseconds in both experiment and computation. We observe a much longer-lasting (43-ps) transient Kerr effect that follows the THz excitation. We explain the disparity in the relaxation times between the THz-induced second harmonic generation and the Kerr effect by the presence of the polar nanoregions intrinsic to ${\mathrm{KTaO}}_{3}$. The origin of the slow THz Kerr effect dynamics is not presently understood. We speculate that it may be associated with the rotational motion of polar nanoregions due to the material's proximity to the ferroelectric phase transition.

Polaronic Effects on the Electron Raman Scattering in a Spherical Anisotropic Quantum Dot

We study the polaronic effect on the differential cross section (DCS) for an electron Raman scattering process in a spherical anisotropic semiconductor quantum dot in the presence of a homogeneous external electric field. The Fröhlich coupling of electrons with confined and surface polar optical phonon modes for resonance Raman scattering is considered. The influence of electron-phonon coupling on the energy spectrum is taken into account in the framework of perturbation theory. The emission spectra are discussed for different anisotropy cases. Resonant peaks in the spectra are found and interpreted.

Terahertz Magnetic and Lattice Excitations in van der Waals Ferromagnet VI3.

We use the synergy of infrared, terahertz, and Raman spectroscopies with DFT calculations to shed light on the magnetic and lattice properties of VI3. The structural transition at TS1 = 79 K is accompanied by a large splitting of polar phonon modes. Below TS1, strong ferromagnetic fluctuations are observed. The variations of phonon frequencies at 55 K induced by magnetoelastic coupling enhanced by spin-orbit interaction indicate the proximity of long-range ferromagnetic order. Below TC = 50 K, two Raman modes simultaneously appear and show dramatic softening in the narrow interval around the temperature TS2 of the second structural transition associated with the order-order magnetic phase transition. Below TS2, a magnon in the THz range appears in Raman spectra. The THz magnon observed in VI3 indicates the application potential of 2D van der Waals ferromagnets in ultrafast THz spintronics, which has previously been considered the exclusive domain of antiferromagnets.

Role of phonon mode in the enhancement of ferroelectric polarization in a perovskite-based eco-friendly functional material

We report here the role of component freezing of three-dimensional polar () phonon mode corresponding to the center of cubic Brillouin zone in tuning the structure-property correlations of a scientifically enriched and technologically important barium-titanate–based eco-friendly functional material (Ba0.92Ca0.08)(Zr0.05Ti0.95−x Sn x )O3; BCZTSnx synthesized via solid-state reaction method. The combined X-ray diffraction, Raman spectroscopic analysis, and temperature-dependent dielectric studies have revealed the presence of several crystallographic phase transitions with coexisting phases, viz., , as a function of Sn(x) content. These crystallographic phases, viz., P4mm, Amm2, and R3m results due to freezing of the component(s) of phonon mode (belonging to space group), with the respective order parameter directions (0,0,a), (a,a,0), and (a,a,a) leading to ferroelectric polarization along , and directions, respectively. The ceramic composition corresponding to exhibits a significant reduction in the coercive field (E c ) and an enhancement in ferroelectric polarization (P r ) in comparison to , inferred from PE loop measurements. The enhancement in ferroelectric polarization at has been attributed to the inter-ferroelectric three-phase coexistence around this composition and significantly enhanced amplitudes of ferroelectric phonon modes corresponding to orthorhombic and rhombohedral phases, calculated using the symmetry mode analysis technique. The existence of a high ferroelectric polarization and low coercive field may lead to composition as an eco-friendly candidate for ferroelectric memory devices.

Band versus Polaron: Charge Transport in Antimony Chalcogenides.

Antimony sulfide (Sb2S3) and selenide (Sb2Se3) are emerging earth-abundant absorbers for photovoltaic applications. Solar cell performance depends strongly on charge-carrier transport properties, but these remain poorly understood in Sb2X3 (X = S, Se). Here we report band-like transport in Sb2X3, determined by investigating the electron–lattice interaction and theoretical limits of carrier mobility using first-principles density functional theory and Boltzmann transport calculations. We demonstrate that transport in Sb2X3 is governed by large polarons with moderate Fröhlich coupling constants (α ≈ 2), large polaron radii (extending over several unit cells), and high carrier mobility (an isotropic average of >10 cm2 V–1 s–1 for both electrons and holes). The room-temperature mobility is intrinsically limited by scattering from polar phonon modes and is further reduced in highly defective samples. Our study confirms that the performance of Sb2X3 solar cells is not limited by intrinsic self-trapping.

Systematic search of the rotation-compatible polar phases in perovskite oxides: A first-principles study

In many perovskite oxides, ferroelectricity is intimately related to octahedral rotation patterns, which can suppress or enhance polar distortions. Using first-principles density functional theory, we investigate the relation between octahedral rotation patterns and polar instability. Based on the rotation patterns commonly observed in perovskite oxides, we present a workflow that allows to systematically and efficiently search for the unstable polar phonon modes and identify metastable polar structures. We apply the workflow to investigate rotation-pattern-dependent polar phases of CaSnO3 and find metastable polar structures by changing rotation patterns. Further investigation of the polar R3c structures shows sizable polarization comparable to the conventional ferroelectrics. We discuss substrate materials having the potential to stabilize the polar structure. Our work provides an efficient way to identify new polar phases by changing the octahedral rotation patterns, which will be useful to design new functional materials in the thin-film/substrate configuration utilizing interfacial coupling.

Fröhlich electron–phonon interaction Hamiltonian and potential distribution of a polar optical phonon mode in wurtzite nitride triangular nanowires

Polar optical phonon modes of wurtzite triangular nanowires (NWs) with three different cross sections, including the hemi-equilateral triangle (HET), the isosceles right triangle (IRT), and the equilateral triangle (ET), are deduced and analyzed using the dielectric continuum model. The exact and analytical phonon states of exactly confined (EC) modes in nitride NWs with HET, IRT, and ET cross sections are derived. The characteristic frequency of EC phonon modes in the triangular nitride NW systems is specified. Fröhlich electron–phonon interaction Hamiltonians in wurtzite NWs with three types of triangular cross sections are obtained. It is found from the numerical results that, among the three types of GaN NWs, the electron–phonon coupling of EC modes in NWs with an HET cross section is the weakest one, that in NWs with an ET cross section is the strongest one, and that in NWs with an IRT cross section is in the middle. The electrostatic potentials of EC modes in HET NWs are neither symmetric nor antisymmetric. The potential functions of EC modes in the ET NW structures have one (three) symmetric axis (axes) as the quantum numbers p and q take fractions (integers). The potential functions of EC modes in IRT NWs behave either symmetrically or anti-symmetrically, which are closely dependent on the parities of the quantum numbers p and q. With the increase of order-number of EC modes, the electron–phonon coupling becomes weaker and weaker. This reveals that cross-sectional morphology of quantum structures has an important influence on the symmetries of phonon modes and electron–phonon coupling strengths in low-dimensional quantum systems.

Strong Electron–Phonon Coupling in β-Ga2O3: A Huge Broadening of Self-Trapped Exciton Emission and a Significant Red Shift of the Direct Bandgap

The temperature dependence of self-trapped exciton (STE) emission and the optical absorption edge of monoclinic gallium oxide (β-Ga2O3) has been carefully studied. According to this research, it is found that as temperature increases (from 10 to 300 K), the STE and the direct bandgap of β-Ga2O3 exhibit a huge broadening (∼120 meV) and a significant red shift (∼250 meV), respectively. Combined with theoretical analysis, these temperature-dependent change trends are found related to the strong electron-phonon coupling (EPC) effect in crystal, which is caused by the high localization (i.e., self-trapping) of carriers in β-Ga2O3. This finding further indicates that in the transition process of carriers' absorbing and releasing photons, the influence of lattice vibration needs to be considered and described by the configuration coordinate model. The strength of EPC can be measured by the Huang-Rhys factor, which is about S ≈ 9 for β-Ga2O3 with the polar longitudinal optical phonon mode of lower energy (∼31 meV) being involved.

Potential distribution of polar optical phonons and their electron–phonon coupling properties of a wurtizite GaN-based trianglar quantum dot

Polar optical phonon modes of a wurtzite nitride trianglar quantum dot (QD) are deduced and analyzed by using the dielectric continuum model. Based on a method of nonseparable variables, the Laplace equation of electristatic potential of the structure is solved, and the exact and analytical phonon states of exact confined (EC) modes are derived. It is found that the frequency of EC modes in the nitride trianglar QD systems can take both the longitudinal optical phonons in [Formula: see text]-direction [Formula: see text] and the longitudinal optical phonons in [Formula: see text]-plane [Formula: see text], which is obviously different from the cases of EC modes in wurtzite GaN quantum wells and quantum wires due to their different confined dimensions. The polarization eigenvectors and their orthogonality relations for the EC phonon modes are derived, which could form a set of orthogonal and completed basis vectors. By using the orthogonality relations of polarization eigenvector and field quantization method, the free phonon field of EC modes and the corresponding Fröhlich electron–phonon interaction Hamiltonian are obtained. Numerical calculations on a wurtztie GaN trianglar QD are performed. The spatial distributions of phonon potentials of EC phonons both in [Formula: see text]-plane and in [Formula: see text]-direction as well as the electron–phonon coupling strength are plotted and discussed. It is observed that the potential of these EC phonon modes is strictly confined within the trianglar QD ranges. The dependence of symmetries and electron–phonon coupling strength on the quantum numbers [Formula: see text] and [Formula: see text] in [Formula: see text]-plane, and the quantum number [Formula: see text] in [Formula: see text]-direction are analyzed in detail. The relationship of the total number of peaks and troughs with these quantum numbers are also discussed and induced.

Revisiting eigen displacements of tetragonal BaTiO3: Combined first principle and experimental investigation

BaTiO3 is a classical ferroelectric studied for last one century for its ferroelectric properties. Lattice dynamics of BaTiO3 is crucial as the utility of devices is governed by phonons. In this work, we show that traditional characterization of the polar phonon modes is ambiguous and often misinterpreted. By combining Raman, Neutron and X-ray diffraction, dielectric spectroscopic observations with first principle calculations, we have re-examined the character of the normal modes of phonons of tetragonal BaTiO3. We obtained Eigen displacements of vibrational modes through DFT calculations and reclassified the polar modes being Slater (Ti–O), Last (Ba–TiO3) and Axe (BO6) vibrations by correlating experimental and theoretical calculations. The study thus provides authentic nomenclature of the polar modes.

Piezoelectric properties of ferroelectric perovskite superlattices with polar discontinuity

The stability of a high-symmetry P4mm polar phase in seventeen short-period ferroelectric perovskite superlattices with polar discontinuity is studied from first principles within the density-functional theory. It is shown that in most superlattices this phase exhibits either the ferroelectric instability or the antiferrodistortive one, or both of them. For each superlattice, the ground-state structure, the structure of possible metastable phases, the spontaneous polarization, and piezoelectric properties are calculated. The properties of superlattices with the polar discontinuity are compared with those of superlattices with broken symmetry and of ordinary superlattices, which have no the polar discontinuity. It is shown that high piezoelectric coefficients (up to 150–270 pC/N) in some superlattices with the polar discontinuity are due to the appearance of strong lattice distortions, whose symmetry follows that of a low-lying polar phonon mode of the ground-state structure, under the influence of external strain.

Temperature-Dependent Cationic Doping-Driven Phonon Dynamics Investigation in CdO Thin Films Using Raman Spectroscopy

In the present work, undoped cadmium oxide (CdO)- and 1% Tin (Sn)-doped CdO thin films were prepared by the sol–gel route. These samples have been analyzed by temperature (T)-dependent (80–500 K) Raman spectroscopy and studied for their lattice dynamics and vibrational density of states. Results indicate that the room T synthesized pure CdO thin film manifests two prime second order features, that is, 300.5 cm–1 transverse optical (TO) and 488 cm–1 longitudinal optical (LO) phonon modes. However, incorporation of cationic-assisted impurity (Sn) with larger ionic radii results in a softening of the high-frequency LO (480.5 cm–1) mode via lattice deformation scattering potential. In fact, selective Sn doping increases the carrier concentration in the host CdO matrix which subsequently mitigates intraionic anharmonicity owing to increased Coulomb screening, leading to disappearance of the low-frequency TO mode (300.5 cm–1). In the case of pure CdO thin films, surface electron-induced electron–LO phonon coupling causes the intensity enhancement in LO modes, while negligible four-phonon anharmonic coupling results in lowering of full width at half maxima below Debye T. On the other hand, Frühlich interaction in the polar LO phonon mode supersedes the impact of anharmonic decay and dominates the overall phonon decay process by impurity incorporation, via appropriate Sn doping. Theoretical phonon dispersion profiles throughout the Brillouin zone with increasing T suggests stronger TO phonon mode softening along with optical branch broadening followed by LO–TO splitting. The acoustic branch barely suffers any shift with changing T which cannot be observed in experimental spectra. However, the flexural phonon modes confer a direct indication of changed rigidity and bond stiffness with varying T. Overall, the present investigation provides experimental evidence regarding the significance of Debye T, below and above which the three- and four-phonon anharmonicity are feasible in phonon scattering in conjunction with theoretical insights.

Giant Thermal Enhancement of the Electric Polarization in Ferrimagnetic BiFe_{1-x}Co_{x}O_{3} Solid Solutions near Room Temperature.

Thermal excitations typically reduce the electric polarization in ferroelectric materials. Here, we show by means of first-principles calculations that multiferroic BiFe_{1-x}Co_{x}O_{3} solid solutions with 0.25≤x≤0.50 (BFCO) represent a noteworthy exception to this behavior. In particular, we find that, at room temperature and for moderate pressures of 0.1-1.0GPa, depending on the composition, the electric polarization of bulk BFCO increases by ∼150%. The origin of such an exceptional behavior is a phase transformation involving a low-T rhombohedral (R) phase and a high-T supertetragonal (T) phase. Both R and T phases are ferrimagnetic near room temperature with an approximate net magnetization of 0.13 μ_{B} per formula unit. Contrary to what occurs in either bulk BiFeO_{3} or BiCoO_{3}, the T phase is stabilized over the R by increasing temperature due to its higher vibrational entropy. This extraordinary T-induced R→T phase transition is originated by polar phonon modes that involve concerted displacements of transition-metal and oxygen ions.

Microwave dielectric properties of Co2P2O7 ceramics

Co2P2O7 ceramics were prepared through the traditional solid-state sintering technique. The phase composition, grain size distribution, and densification were researched via X-ray diffraction and scanning electron microscope. The influence of pores on permittivity was described by various models. The dielectric loss was found highly dependent on porosity. Moreover, the low εr (<10) values of Co2P2O7 ceramics were explained by the covalent feature of P–O bonds. Raman spectroscopy was used for exploring the relationship between polar phonon modes and dielectric properties in terms of intrinsic factors. The optimum dielectric properties (εr = 6.76, Qf = 36,400 GHz and τf = −23.9 ppm/°C) were obtained at 1160 °C for 4 h.

Engineering Weyl Phases and Nonlinear Hall Effects in T_{d}-MoTe_{2}.

MoTe_{2} has recently attracted much attention due to the observation of pressure-induced superconductivity, exotic topological phase transitions, and nonlinear quantum effects. However, there has been debate on the intriguing structural phase transitions among various observed phases of MoTe_{2} and their connection to the underlying topological electronic properties. In this work, by means of density-functional theory calculations, we investigate the structural phase transition between the polar T_{d} and nonpolar 1T^{'} phases of MoTe_{2} in reference to a hypothetical high-symmetry T_{0} phase that exhibits higher-order topological features. In the T_{d} phase we obtain a total of 12 Weyl points, which can be created/annihilated, dynamically manipulated, and switched by tuning a polar phonon mode. We also report the existence of a tunable nonlinear Hall effect in T_{d}-MoTe_{2} and propose the use of this effect as a probe for the detection of polarity orientation in polar (semi)metals. By studying the role of dimensionality, we identify a configuration in which a nonlinear surface response current emerges. The potential technological applications of the tunable Weyl phase and the nonlinear Hall effect are discussed.

Boron phosphide as a p -type transparent conductor: Optical absorption and transport through electron-phonon coupling

Boron phosphide has recently been identified as a potential high hole mobility transparent conducting material. This promise arises from its low hole effective masses. However, BP has a relatively small 2 eV indirect band gap which will affect its transparency. In this work, we computationally study both optical absorption across the indirect gap and phonon-limited electronic transport to quantify the potential of boron phosphide as a \emph{p}-type transparent conductor. We find that phonon-mediated indirect optical absorption is weak in the visible spectrum and that the phonon-limited hole mobility is very high (around 900 cm$^2$/Vs) at room temperature. This exceptional mobility comes from a combination of low hole effective mass and very weak scattering by polar phonon modes. We rationalize the weak scattering by the less ionic bonding in boron phosphide compared to oxides. We suggest this could be a general advantage of non-oxides for \emph{p}-type transparent conducting applications. Using our computed properties, we assess the transparent conductor figure of merit of boron phosphide and shows that it exceeds by one order of magnitude that of established \emph{p}-type transparent conductors, confirming the potential of this material.

Symmetry degeneration and room temperature ferroelectricity in ion-irradiated SrTiO3

Polar phonon modes associated with room temperature ferroelectricity are observed in SrTiO3 single crystals irradiated with Ti ions. Quantitative strain analysis reveals that irradiation-induced out-of-plane strain drives the centrosymmetric cubic SrTiO3 to a tetragonal-like structure in the maximum damaged region. Energy transfer from ions to electrons during ion irradiation yields defects in SrTiO3 that also plays an important role for the room temperature ferroelectricity. Different from thin film techniques, the ferroelectricity in the ion irradiated SrTiO3 can occur for much larger thicknesses, depending on the energy and type of ion.

High-temperature structural phase transition and infrared dielectric features of La2CoMnO6

In this paper, we present the temperature-dependent far-infrared reflectivity spectra of partially ordered magnetodielectric La2CoMnO6 from room temperature to 673 K. We demonstrate a clear first-order structural phase transition (SPT) from a monoclinic structure with P21/n symmetry to a rhombohedral phase with R3¯ symmetry from the behavior of the polar phonon modes at TC ∼ 590 K. The temperature-dependent transversal and longitudinal phonon branches, dielectric strengths, and damping of the strongest dielectric modes support the significant contribution of the phonon modes to the SPT and reveal an important lattice anharmonicity, especially for the low-frequency modes. Also, these investigations showed that structural ordering does not inhibit the SPT and provided valuable information regarding the polar phonons and their effects on the intrinsic dielectric constant in double perovskites and related compounds.

Strain-Dependent Polar Optical Phonon Scattering and Drive Current Optimization in Nanoscale Monolayer MoS2 FETs

Recently, microelectromechanical system has been used to dynamically strain atomically thin materials including MoS2. While strain can significantly modulate the electronic and phonon structures of monolayer MoS2, its impact on electron transport, especially in the dissipative regime, has not been well explored. In this paper, using a three-dimensional particle-based quantum-corrected Monte Carlo device simulator, the effects of uniaxial and biaxial strain on room-temperature electron transport in a model monolayer molybdenum disulphide (MoS2) based field-effect transistor have been investigated. In the beginning, the simulator has been validated against recently published experimental results. Overall, strain in monolayer MoS2 strongly affects the polar optical phonon modes as well as the electronic bandstructure. Uniaxial strain breaks the degeneracy of E′ Raman mode and results in phonon softening. In this case, our results show that, for both E+ and E− Raman modes, ON current first increases for up to 3.7% of applied strain and then decreases as the strain is increased further. As for biaxial strain, we consider the effects of both tensile and compressive stresses. We find that the application of biaxial tensile strain boosts the ON current for up to 4% of strain. Especially, biaxial tensile strain leads to ~ 15.56% increase in the ON current, which is highest for any type of applied stress.