Phonon Mechanism Research Articles

Phonon Mechanisms and Transformation Paths in Pu

A long-standing problem in Pu science is the crystallographic mechanism for the delta-->alpha' (fcc-->monoclinic) transformation. Orientation relations between the two structures impose severe restrictions on the possible mechanisms and require the transition to be reconstructive, which we describe as a sequence of three displacive transitions: fcc-->trigonal-->hexagonal-->monoclinic. We predict instabilities along the Lambda and Sigma branches in the phonon dispersion of the delta phase and formulate a free energy to describe the displacement of atoms across the transition. We suggest that the delta-->alpha' transition in Pu lies at the threshold of a change in character of the orientation relationship from lighter to heavier actinides, correlating with changes in electron itinerancy, magnetism, and volume.

Isotope effect in high-Tcsuperconductors

For high-${T}_{c}$ superconductors in which transition temperatures ${T}_{c}$ are reduced by doping, the oxygen isotope effect (OIE) coefficient in ${T}_{c}$ is shown to increase systematically with the pair-breaking rate and with the valence difference between the substituted and native ions. Moreover, the OIE in ${T}_{c}$ tends to zero as one approaches optimum (or ideal) stoichiometry at which the quality of the superconducting condensate is maximized. In materials with isovalent substitutions, e.g., Sr for Ba or Zn for Cu in $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$, the small OIE of the parent (unsubstituted) compound is magnified owing to pair-breaking disorder. In materials with heterovalent substitutions, e.g., La or Pr for Ba, where carrier densities are necessarily changed, pair breaking induces a much larger OIE. A seminal case is Pr-doped $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$, where data were previously misinterpreted owing to the false assumption that Pr substitutes only for Y. It is now clear, however, that the decrease in ${T}_{c}$ observed with Pr doping actually arises from pair breaking caused by Pr-on-Ba-site defects introduced during crystal growth. When $\mathrm{Pr}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ is grown correctly without such defects, ${T}_{c}$ remains unchanged (as in the case of most other rare-earth substitutions for Y). Invariance of ${T}_{c}$ under a 60% rare-earth mass increase provides strong evidence against phononic pairing mechanisms. The fact that ${T}_{c}$ drops when Pr substitutes for Ba but not for Y indicates that the superconducting hole condensate resides in the BaO layers, where pair breaking degrades ${T}_{c}$ and dramatically increases the OIE. Superconductive pairing modeled on Coulomb coupling between the hole (BaO) and the electron $(\mathrm{Cu}{\mathrm{O}}_{2})$ layers is shown to resolve the serious shortcomings inherent in approaches based on electron-phonon interactions and is found to be generally applicable. Moreover, the OIE in the magnetic penetration depth in alloys is shown to be constant, with degradation of ${T}_{c}$ up to 50%, and thus is unrelated to the OIE in ${T}_{c}$.

Possible mechanism of superconductivity inPuCoGa5probed by self-irradiation damage

Measurements of the electrical resistivity of a polycrystalline PuCoGa{sub 5} sample reveal significant modifications of the superconducting properties as a function of time, due to the increase of defects and impurities resulting from self-irradiation damage. More than four years of aging were necessary to detect a deviation from linearity in the time dependence of the critical temperature. The observed behavior is understood in the framework of the Eliashberg theory, confirming the 'dirty' d-wave character which was already suggested by nuclear magnetic resonance. We show that experimental data accumulated so far can be well reproduced by assuming a phononic mechanism for superconductivity, with reasonable values of the electron-phonon coupling and Coulomb pseudopotential. Further experiments are then required to assess the role of spin fluctuations in stabilizing the superconducting state in this compound.

Orientational isotopic effects in the thermal conductivity of CH4∕CD4 solid solutions

The thermal conductivity of (CH4)1−c(CD4)c solid solutions with c=0, 0.03, 0.065, 0.13, 0.22, 0.4, 0.78, and 1.0 is measured in the region of existence of three orientational phases: disordered (phase I), partially ordered (phase II), and completely ordered (phase III). The temperature range is 1.3–30K. It is shown that the thermal conductivity has different temperature dependences κ(T) in these phases. Its value increases with the degree of the orientational order in the phase. In phase I the thermal conductivity is independent of c and weakly dependent on T. The impurity effect in κ(T) is much stronger in the low-temperature part of phase II than in phase III. As the concentration c grows, the κ(T) curve of phase II approaches the dependence κ(T) typical of phase I. There is a hysteresis in the vicinity of the II↔III phase transition. In phase III the impurity effect in κ(T) can be considered as phonon scattering at rotational defects developing due to the difference between the moments of inertia of the CH4 and CD4 molecules. The obtained dependences of thermal conductivity on temperature and concentration can be explained qualitatively assuming that the dominant mechanism of phonon scattering is connected with the interaction of phonons with the rotational motion of the molecules in all of the three orientational phases of the CH4–CD4 system.

Thermal Conductivity of a Molecular Crystal with Rotational Degrees of Freedom: Orientational Defect Scattering

Measurements of thermal conductivity of solid methane-deuteromethane solutions at equilibrium vapor pressure in the temperature range 1.2÷20 K are reported. The obtained dependences of thermal conductivity on temperature and concentration can be explained qualitatively assuming that the dominant mechanism of phonon scattering is connected with the interaction of phonons with the rotational motion of the molecules in all of the three orientational phases of the CH4-CD4 system. The contribution of the orientational defect scattering to the thermal conductivity is discussed in frame of the model of local changes in the moments of inertia of molecules.

Phonon-assisted absorption of hypersound in a rectangular quantum wire

The mechanisms of the Landau-Rumer process and of the process of two-phonon decay of microwave phonons are considered for a rectangular quantum wire. For GaAs wires with a free surface and various transverse dimensions, the coefficients of absorption of confined microwave phonons of the lowest hybrid width mode are calculated. The phononic and electronic mechanisms of hypersound absorption in rectangular wires and unbounded solids are compared. At low temperatures, the inclusion of cubic anharmonicity in a wire leads to an exponential temperature dependence of the hypersound absorption for the Landau-Rumer process and to the dominant absorption due to decay processes; in the latter case, the exponent of the frequency dependence decreases. At sufficiently high temperatures, the phonon-assisted absorption dominates over the electron-assisted absorption in a quantum wire of a nondegenerate material.

Electrical and thermal properties of the manganite La0.8Ag0.15MnO3

A comprehensive investigation of the electro- and thermophysical properties of the manganite La0.8Ag0.15MnO3 is carried out over a wide temperature interval (4.2–350K) and in magnetic fields up to 26kOe. It is shown that the colossal magnetoresistance in a magnetic field of 11kOe amounts to 57%, and the effect is maximum at room temperature. The dominant mechanisms of current carrier scattering in the ferromagnetic and paramagnetic phases are established. An analysis of the data on the low-temperature heat capacity provides estimates of the electronic density of states at the Fermi level, NF=6.82×1024eV−1mol−1, and of the Debye temperature, θD=370K. The results of thermal expansion measurements are used to find the spontaneous magnetostriction. It is found that the phonon mechanism of heat transfer is dominant, and the local Jahn–Teller distortions are considered as the main mechanism of phonon scattering.

EXPLORATION AND SEARCH ON NEW HIGH TC SUPERCONDUCTIVE MATERIALS

On the basis of Fan and Malozovsky theory, which had discussed the non-phonon mechanism of the layered Fermi liquid superconductivity, and of the experimental studies and theoretical analyses obtained by Zhang et al. as well as other scientists, we have analyzed and discussed the extrinsic and intrinsic factors of affecting the transition temperature Tc of cuprate superconductors. We also will present some possible approaches to explore new high Tc superconductive materials, and pay more attention to the non-traditional phonon mechanism, low dimensional or quasi two-dimensional system, and hexagonal structure materials, etc.

On the role of atomic motion soft modes in acoustic phonon broadening below the boson peak in glasses

Abstract This Letter presents a mechanism of acoustic phonon broadening for frequencies lower than the boson peak frequency in glasses exhibiting a high-frequency sound above the boson peak. The mechanism is based on a resonant interaction of an acoustic phonon with harmonic vibrational excitations of soft modes in such glasses. The related width of the phonon is found to be independent of temperature and characterized by a power-law frequency dependence ν ϰ , with the exponent ϰ varying from ϰ ≈ 2 below the boson peak to ϰ ≈ 4 at lower frequencies. The dependencies do not appear to contradict some recent experimental data, for the glasses under discussion.

Plasma oscillation and isotope effect

Superconducting isotropic crystal is presented by two subsystems in terms of anharmonic oscillation model. The superconducting energy gap below the phase transition temperature is deduced by the free electron plasma energy. The anisotropic nature of the materials is appeared by the superconducting gaps in x, y, z directions. The phase transition temperature-square plasma energy relation is established. The observed facts for high temperature superconductors are consistent with the plasma theory. The isotope phase transition temperature shift is connected with the plasma energy. The fixing temperature divided by the phase transition temperature is the function of the molecular mass divided by effective mass density. The phase transition temperature depends on the interaction parameter. The equations for mercury and MgB 2 isotopes are given. The interaction parameters reduce with the phase transition temperature rise. The isotope distinctions in the superconducting lines are explained. The phonon mechanism is concluded to be special case of the plasma mechanism.

Unconventional pairing in bipolaronic theories

Various mechanisms have been put forward for cuprate superconductivity, which fit largely into two camps: spin-fluctuation and electron–phonon (el–ph) mechanisms. However, in spite of a large effort, electron–phonon interactions are not fully understood away from clearly defined limits. To this end, we use a numerically exact algorithm to simulate the binding of bipolarons. We present the results of a continuous-time quantum Monte-Carlo (CTQMC) algorithm on a tight-binding lattice, for bipolarons with arbitrary interaction range in the presence of strong coulomb repulsion. The algorithm is sufficiently efficient that we can discuss properties of bipolarons with various pairing symmetries. We investigate the effective mass and binding energies of singlet and triplet real-space bipolarons for the first time, and discuss the extensions necessary to investigate d-symmetric pairs.

Giant enhancement of noncontact friction between closely spaced bodies by dielectric films and two-dimensional systems

The effect of an external bias voltage and spatial variations of the surface potential on the damping of cantilever vibrations in an atomic force microscope (AFM) is considered. The damping is due to an electrostatic friction that arises due to dissipation of the energy of an electromagnetic field generated in the sample by oscillating static charges induced on the surface of the AFM probe tip by the bias voltage or spatial variations of the surface potential. A similar effect appears when the tip is oscillating in an electrostatic field created by charged defects present in the dielectric sample. The electrostatic friction is compared to the van der Waals (vdW) friction between closely spaced bodies, which is caused by a fluctuating electromagnetic field related to the quantum and thermal fluctuations of current density inside the bodies. It is shown that the electrostatic friction and the vdW friction can be strongly enhanced in the presence of dielectric films or two-dimensional (2D) structures—such as a 2D electron system or an incommensurate layer of adsorbed ions exhibiting acoustic oscillations—on the probe tip and sample surfaces. It is also shown that the damping of cantilever oscillations caused by the electrostatic friction in the presence of such 2D structures can have the same order of magnitude and the same dependence on the distance as observed in experiment by Stipe et al. [Phys. Rev. Lett. 87, 096801 (2001)]. At small distances, the vdW friction can be large enough to be measured in experiment. In interpreting the experimental data that obey a quadratic dependence on the bias voltage, one can reject a phonon mechanism according to which the friction depends on the fourth power of the voltage.

Electron-Phonon Interaction via Electronic and Lattice Wannier Functions: Superconductivity in Boron-Doped Diamond Reexamined

We present a first-principles technique for investigating the electron-phonon interaction with millions of k points in the Brillouin zone, which exploits the spatial localization of electronic and lattice Wannier functions. We demonstrate the effectiveness of our technique by elucidating the phonon mechanism responsible for superconductivity in boron-doped diamond. Our calculated phonon self-energy and Eliashberg spectral function show that superconductivity cannot be explained without taking into account the finite-wave-vector Fourier components of the vibrational modes introduced by boron, as well as the breaking of the diamond crystal periodicity induced by doping.

A possible explanation of the phase diagram of cuprate superconductors

A d–p pairing curve that is consistent with the pseudogap curve observed in experiments is found on a d–p model on phonon mechanism. On the discovery we suggest that there are two pseudogaps associated with the nearly localized d–p pairs and nearly free p–p pairs. The p–p pairs look like bosons and are responsible for superconductivities.

Thermal diffusivity of samarium–gadolinium zirconate solid solutions

We synthesized samarium–gadolinium zirconate solid solutions and determined their thermal diffusivities, Young's moduli and thermal expansion coefficients, which are very important for their application in thermal barrier coatings. Samarium–gadolinium zirconate solid solutions have extremely low thermal diffusivity between 20 and 600 °C. The solid solutions have lower Young's moduli and higher thermal expansion coefficients than those of pure samarium and gadolinium zirconates. This combination of characteristics is promising for the application of samarium and gadolinium zirconates in gas turbines. The mechanism of phonon scattering by point defects is discussed.

Infrared spectroscopy of the intermediate-valence semiconductor YbB12

The dynamic conductivity and permittivity spectra of the intermediate-valence compound YbB12 are measured in the frequency range (6–104) cm−1 (quantum energy 0.75 meV-1.24 eV) at temperatures of 5–300 K. Analysis of the spectral singularities associated with the response of free charge carriers has made it possible for the first time to determine the temperature dependences of their microscopic parameters, viz., concentration, effective mass, relaxation frequency and time, mobility, and plasma frequency. It is shown that the relaxation frequency decreases upon cooling from 300 K to the coherence temperature T * = 70 K for YbB12, which is mainly associated with the phonon mechanism of scattering of charge carriers. For cooling below the coherence temperature T * = 70 K, the temperature dependence of the relaxation frequency for charge carriers of the Fermi-liquid type is found to be γ ∼ γ0 + T 2, while their effective mass and relaxation time increase, respectively, to m *(20 K) = 34m 0 (m 0 is the free electron mass) and τ(20 K) = 4 × 10−13 s, indicating the establishment of coherent scattering of carriers from localized magnetic moments of the f centers. At a temperature of T = 5 K, the conductivity spectrum contains an absorption line at a frequency of 22 cm−1 (2.7 meV); the origin of this line can be associated with the exciton-polaron bound state. Since such a state was observed earlier in other intermediate-valence semiconductors (such as SmB6, TmSe1−x Te, and (Sm, Y)S), it is probably typical of this class of compounds.

The effective Hamiltonian for cuprates at different energy scales

A multiband p−d model describes electronic properties of cuprates in the large energy scale ∼ U∼10 eV. In the strong electron correlation (SEC) limit we project out double occupied intra-atomic states and obtain the singlet–triplet t - t ′ - t ″ - J * model at scale ∼ Ω ≈ 0.5 eV , where Ω is the excitation energy between two-hole singlet and triplet states. At smaller energies ∼ J one gets the usual t - t ′ - t ″ - J * model. At energies ∼ ω D one has to add electron–phonon interaction, and the low-energy effective Hamiltonian with magnetic and phonon mechanisms of pairing is constructed in SEC limit with parameters calculated from the ab initio approach. Magnetic mechanism of pairing results in a very high value of the critical temperature T c while electron–phonon interaction suppresses d-pairing to the T c∼100 K.

The electronic properties of amorphous and crystallized In2O3 films

Polycrystalline In2O3 films with high crystallinity were prepared by annealing the amorphous In2O3 films deposited at room temperature by using dc/rf sputtering method. General electronic behaviors of both these amorphous and polycrystalline films have been investigated by means of Hall effect and resistivity measurements at temperatures between 4.2 and 300K. For the amorphous films, ionized impurity scattering dominates the electronic transport, and Hall mobility agrees well with the theory of degenerate semiconductors with divalent impurity scattering centers. On the other hand, the conduction mechanism of the polycrystalline films is governed dominantly by phonon scattering, since these films exhibit a clearly positive temperature coefficient of resistivity with the maximum Hall mobility of 150cm2∕Vs at 300K and 230cm2∕Vs at 4.2K, with little change in carrier concentration between these temperatures. Conductivity of both the amorphous and polycrystalline films is found to show a linear relationship in a double-logarithmic plot against carrier concentration between 4×1015 and 5×1020cm−3, a typical behavior of the impurity semiconductors with parabolic conduction band. For each set of films with the same thermal history, Hall mobility is found to vary on a convex upward curve with respect to carrier concentration. This behavior suggests a general and strong correlation of the structural defects introduced upon deposition and localization of conduction electrons with electron mobility, which led to a proposition of three categories for temperature coefficient of resistivity with major transport mechanisms of phonon scattering, divalent impurity scattering, and weak localization, respectively.

Power- and Polarization-Dependences of Coherent Phonons of Tetrahedrally Bonded Semiconductors

Generation of coherent optical phonons in GaAs and Ge are systematically investigated by pump-probe reflectivity measureents. While the amplitude of the coherent optical phonon of Ge increases linearly with increasing optical excitation density, that of the coherent LO phonon of GaAs shows a saturation above a certain optical density. The coherent optical phonon of Ge follows the Raman selection rule, while that of GaAs is generated isotropically regardless of pump polarization. These differences are explained by different generation mechanisms of coherent phonons; impulsive stimulated Raman scattering and transient depletion field screening.

Excitation mechanisms of coherent phonons unravelled by femtosecond X‐ray diffraction

AbstractCoherent zone‐folded longitudinal acoustic phonons (ZFLAPs) were measured nondestructively with sub‐picometer spatial and sub‐picosecond time resolution via ultrafast X‐ray diffraction (XRD). The femtosecond excitation of a GaAs/AlGaAs superlattice with 1.5 eV photons results in spatially periodic stress, which induces a coherent ZFLAP with a 3.5 ps period. The motion is directly monitored by femtosecond X‐ray pulses at a kilohertz repetition rate. The intensity change ΔR /R = 0.03 of the −1st order satellite peak of the (002) reflection of bulk AlGaAs is directly proportional to the induced lattice strain of Δa /a 0 = 1.5 × 10−4. The phase and amplitude of the oscillatory XRD signal around a new equilibrium demonstrates the displacive excitation of coherent phonons (DECP) as the dominant mechanism for strong excitation. We discuss different mechanisms which account for impulsive stress, and argue that the electronic stress due to the deformation potential dominates. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)