Role Of Phonons Research Articles

RECENT DEVELOPMENTS ON ELECTRON-PHONON INTERACTIONS IN STRUCTURES FOR ELECTRONIC AND OPTOELECTRONIC DEVICES

As device dimensions in electronic and optoelectronic devices are reduced, the characteristics and interactions of dimensionally-confined longitudinal-optical (LO) and acoustic phonons deviate substantially from those of bulk semiconductors. Furthermore, as würtzite materials are applied increasingly in electronic and optoelectronic devices it becomes more important to understand the phonon modes in such systems. This account emphasizes the properties of bulk optical phonons in würtzite structures, the properties of LO-phonon modes and acoustic-phonon modes arising in polar-semiconductor quantum wells, superlattices, quantum wires and quantum dots, with a variety of cross sectional geometries and, lastly, the properties of optical phonons in würtzite materials as predicted by the dielectric continuum model. Emphasis is placed on the dielectric continuum and elastic continuum models of bulk, confined and interface phonons. This article emphasizes device applications of confined phonons in GaAs-based systems and provides a brief discussion of carrier-LO-phonon interactions in bulk würtzite structures. This account also includes discussions on the use of metal-semiconductor heterointerfaces to reduce scattering and on the role of phonons in Fröhlich, deformation and piezoelectric interactions in electronic and optoelectronic structures; specific device applications high-lighted here include quantum cascade lasers, mesoscopic devices, thermoelectric devices and optically-pumped resonant intersubband lasers.

Role of phonons in electronic and spin excitations of high- Tc superconductors

By using large single crystals of YBa 2Cu 3O y , precise temperature and doping dependence of the c-axis infrared active phonons are investigated. The effect of the superconducting gap opening on the phonon self-energy is observed for the optimally doped crystal but not for the overdoped crystal, which suggests that this effect becomes pronounced owing to a lower crystal symmetry due to oxygen deficiencies. The observed phonon anomalies for the in-plane oxygen mode above T c in the underdoped crystals might be associated with the c-axis conductivity suppression, and cannot be an evidence for a direct coupling of phonons to he spin system. An indirect contribution of the spin gap to the phonon system is suggested by a spectral growth around 450 cm −1.

Towards a dynamical theory of magnetic polaron formation

Calculations have been carried out of the magnetic polaron energies associated with a simple donor-type carrier as a function of its mass. Evaluation of the free energy surfaces show the important role of phonons in magnetic polaron formation. A long-range mechanism for the transfer of spin angular momentum out of the polaron complex is proposed, which can account for spin-diffusion associated with polaron formation.

Magnetization and spin-lattice relaxation in semimagnetic semiconductors at high magnetic fields

We review the methods of studying the spin-lattice and magnetization relaxation at high magnetic fields taking as an example the results obtained for semimagnetic semiconductors. The role of phonons which cannot be described by the Debye approximation is shown. The different relaxation paths are discussed. The effect of relaxation in exchange coupled pairs is presented.

A new theory of the Invar behavior considering the role of phonon

Abstract By treating the roles of electrons and phonons on the same footing, we numerically calculate thermal expansion coefficient of an itinerant electron ferromagnet for both T T c and T > T c with a simple model of the electronic density of 1758 0727 states, and find thermal phonons to play a crucial role in the Invar behavior.

Simulation of laser annealing of Hg0.8Cd0.2Te

Computer simulation of the diffusion processes of the atoms in a semiconductor was performed. It was demonstrated that one can cause a great local change of concentration of impurities or intrinsic defects in epitaxial films of Hg0.8Cd0.2Te by long pulses of laser radiation. The inclusion of the action of phonons in the diffusion processes leads to the observation of an unexpected effect of laser annealing. The role of phonons in the diffusion processes depends on the temperature gradient.

The Nuclear Quadrupole Interaction in High Temperature Superconductors

Abstract Since their discovery in 1986 [11], the high temperature superconducting (HTS) copper oxides have presented a continuing challenge to both experiment and theory. The identification of the underlying mechanism (or mechanisms) responsible for their superconductivity remains an unanswered question. Numerous theories have been proposed ranging from phonon-mediated pairing of the charge carriers, similar to the Bardeen–Cooper–Schrieffer (BCS) [2] theory developed for conventional low-temperature superconductors, to novel concepts independent of phonons [3–-l0]. For conventional superconductors the variation of the transition temperature Tc , with isotopic mass M (from BCS theory Tc ∼ M−a ) was an important verification of the contribution of electron-phonon interactions to electron pairing. Measurements of this effect of HTS cuprates resulted in isotope shifts much smaller than predicted by theory [ll-14], raising doubts about the role of phonons. However, Barbee [15] argued that the size of the isoto...

MAGNETIZATION DEPENDENCE OF PHONON EFFECT ON VOLUME OF AN ITINERANT ELECTRON FERROMAGNET

Volume behavior is often anomalous for T<Tc of an itinerant electron ferromagnet. Recently Kim pointed out that phonons also can be responsible for such anomalous behavior. Further to explore such possible role of phonons, in this paper we carry out a numerical calculation on the temperature dependences of phonon Debye temperature and Grüneisen constant of an itinerant electron ferromagnet by using a model electronic density of states. Our numerical results support our earlier conclusion and demonstrate the importance of taking into account the effect of the electron-phonon interaction in understanding the anomalous volume and elasticity behavior of an itinerant electron ferromagnet.

Laser excited luminescence of CaF2:Ho. The role of phonons

The laser excited luminescence of CaF2:Ho has been studied between 17 and 300 K using various Ar+ and He-Ne laser lines. The dependence of the effectiveness of excitation on the laser photon energy finds explanation if one-phonon assisted processes are taken into account. The adiabatic model and the model of phonon relaxation has been applied to the experimental linewidth temperature dependence of the A-sites of Ho3+ (transition 5F4, 5S2 to 5I7) and Er3+ (transition 4S3/2 to 4I15/2) and the results are discussed.

The role of phonons in the temperature dependence of magnetization in an itinerant electron ferromagnet

From a numerical analysis on model itinerant electron ferromagnets, we show the importance of considering the effect of the electron-phonon interaction in accounting for the observed temperature dependence of magnetization of a ferromagnetic metal.

The role of phonons in the volume behavior near the Curie point of ferromagnetic metals

The volume behavior of ferromagnetic metals is quite anomalous particularly below and around the Curie point TC. We often observe a zero or negative thermal expansion coefficient, β, below TC, and a peak or dip in β near TC; the relative size of such a peak or dip is quite large, being of the order of unity. Reccently we pointed out that for T<TC the phonon Grüneisen constant, γD, can be negative and change quite drastically with magnetization. We now show that such behavior of γD enables us to understand the occurrence of the peak or dip in β around TC.

Role of phonons and electron—hole pairs in hydrogen diffusion on a corrugated metal surface

The possibility of correlated jumps in hydrogen self-diffusion on the Ni(001) surface has been studied numerically at T=250 K. It is found that excitations of electron—hole pairs among the conduction electrons are the dominating mechanism for energy dissipation in the present situation. Correlated jumps as well as recrossing events are observed, indicating that transition state theory is erroneous. The hydrogen atom is assumed to move classically and the results are found consistent with recent diffusion experiments using the laser induced desorption technique.

Role of phonons in superconductivity

Though the phonon mediated electron-electron interaction has been widely accepted as the cause of superconductivity, yet some doubts have been raised on the basis of the fact that many properties of superconductors are explained by employing a single Einstein frequency without taking a proper account of phonons in the superconductor. In order to check the role of phonons in superconductivity, we involve the whole of the phonon spectra for obtaining superconducting state parameters of seven superconductors, namely Mg, Zn, Be, Cd, A1, T1 and Pb in anisotropic as well as in isotropic phonon models. It has been observed that the electronphonon coupling strengthλ and the transition temperatureT c exhibit strong directional dependence and are sensitive to the phonons, reaffirming that the phonon mechanism is responsible for superconductivity. The computed values of isotope effect exponentα and the interaction strengthN 0 V also show similar trend, supporting the phonon mechanism.

Phonon Dispersion Curves and Density of States Curves of Bi2Sr2CaCu2O10-xand Bi2Sr2Ca2Cu3O12-y

By making use of an algebraic method in classifying the normal modes, we calculated phonon dispersion curves and density of states curves of the high T c super-conductors, Bi 2 Sr 2 CaCu 2 O 10- x , and Bi 2 Sr 2 Ca 2 Cu 3 O 12- y . Here, the crystal structures are approximated by tetragonal ones with one formula unit, instead of an orthorhombic one with 5 formula units proposed by Kajitani et at . The force constants between atoms are estimated from the results in our previous paper. Similarly to that of YBa 2 Cu 3 O 7 , the density of states curve splits into, roughly speaking, two parts in both Bi 2 Sr 2 CaCu 2 O 10- x and Bi 2 Sr 2 Ca 2 Cu 3 O 12- y , the higher energy part being mainly contributed from Cu–O, O–O, Ca–O and Sr–O bonds. The role of phonons in high T c superconductivity is briefly discussed in relation with YBa 2 Cu 3 O 7 .

Phonon dispersion curves of Bi 2Sr 2CaCu 2O 10−x and Bi 2Sr 2Ca 2Cu 3O 12−y

By making use of an algebraic method in classifying the normal modes, we calculated phonon dispersion curves and density of states curves of the high T c superconductors, Bi 2Sr 2CaCu 2O 10−x. Here, the crystal structures are approximated by tetragonal ones with one formula units, instead of an orthorhombic one with 5 formula units proposed by Kajitani et al. Some of the force constants between atoms are estimated from the results in our previous paper. Similarly to that of YBa 2Cu 3O 7, the density of states curve roughly splits into two parts in both Bi 2Sr 2CaCu 2O 10−x and Bi 2Sr 2Ca 2Cu 3O 12−y, the higher energy part being mianly contributed from CuO, OO, CaO and SrO bonds. The role of phonons in high T c superconductivity is briefly discussed in relation with YBa 2Cu 3O 7.

Second-order Raman scattering and infra-red absorption in SnS2

The second-order Raman scattering and infra-red absorption spectra of 2H−SnS2 are reported. The temperature, excitation wavelength and polarization dependences of Raman spectra are studied; resonance effects are put in evidence. The assignment of the second-order bands is discussed on the basis of the appliable selection rules. The role of phonons of high symmetry points different from Γ in second-order spectra is also taken into account.

Sticking of molecular hydrogens — the role of phonons and rotational excitations

We report measurements, with use of nozzle-beams of n-H 2, p-H 2, n-D 2 and o-D 2 gases, of the initial sticking probability, S o, on a cold Cu(100) surface. In the investigated ranges of incident energies and angles 20–75 meV, 30° – 70°, S o comprises a smooth background and a series of peaks with maxima occurring close to the molecular rotational excitation energies. Both background and structure show striking isotope effects. We argue that S o is due primarily to parallel-momentum non-conserving phonon processes augmented by rotational resonances that enhance the phonon excitation probability.

Acoustic plasmons and superconductivity. I. The role of exchange

Acoustic plasmon excitations have been proposed as a possible mechanism for superconductivity in systems with 'heavy' and 'light' electrons. The authors formulate a theory of superconductivity in which electron excitations play the role of phonons in the conventional theory. The main qualitatively new aspect of such a theory is the role of exchange. The effect of exchange on the electron-acoustic-plasmon coupling parameter lambda is estimated.

On the Role of Phonons for the Pressure Induced Change of Valence

On the Role of Phonons for the Pressure Induced Change of Valence

ROLE OF PHONONS IN ORIENTATIONAL INTRA-PLANE ORDERING IN GRAPHITE INTERCALATION COMPOUNDS

Theoretical study of orientational intra-plane ordering in the second stage graphite intercalation compounds is presented with particular emphasis on the role of phonons. It is shown that the rotation angle of intercalant layers relative to the graphite layers is predominantly determined by intra = plane transverse modes and also depends on the ratio of lattice constants of both intercalant and graphite layens.