Effect Of Lattice Disorder Research Articles

Hot Electron Cooling by Acoustic Phonons in Graphene

We have investigated the energy loss of hot electrons in metallic graphene by means of GHz noise thermometry at liquid helium temperature. We observe the electronic temperature T ∝ V at low bias in agreement with the heat diffusion to the leads described by the Wiedemann-Franz law. We report on T ∝ √V behavior at high bias, which corresponds to a T(4) dependence of the cooling power. This is the signature of a 2D acoustic phonon cooling mechanism. From a heat equation analysis of the two regimes we extract accurate values of the electron-acoustic phonon coupling constant Σ in monolayer graphene. Our measurements point to an important effect of lattice disorder in the reduction of Σ, not yet considered by theory. Moreover, our study provides a strong and firm support to the rising field of graphene bolometric detectors.

Spin correlations in the geometrically frustrated pyrochloreTb2Mo2O7

We report neutron-scattering studies of the spin correlations of the geometrically frustrated pyrochlore ${\text{Tb}}_{2}{\text{Mo}}_{2}{\text{O}}_{7}$ using single-crystal samples. This material undergoes a spin-freezing transition below ${T}_{g}\ensuremath{\simeq}24\text{ }\text{K}$, similar to ${\text{Y}}_{2}{\text{Mo}}_{2}{\text{O}}_{7}$, and has little apparent chemical disorder. Diffuse elastic peaks are observed at low temperatures, indicating short-range ordering of the Tb moments in an arrangement where the Tb moments are slightly rotated from the preferred directions of the spin-ice structure. In addition, a $\stackrel{P\vec}{Q}$-independent signal is observed which likely originates from frozen but completely uncorrelated Tb moments. Inelastic measurements show the absence of sharp peaks due to crystal-field excitations. These data show how the physics of the Tb sublattice responds to the glassy behavior of the Mo sublattice with the associated effects of lattice disorder.

Raman study of gap mode and lattice disorder effect in InN films prepared by plasma-assisted molecular beam epitaxy

Raman scattering has been used to study lattice defects induced by non-stoichiometry in indium nitride films grown by plasma-assisted molecular beam epitaxy with different In/N ratios. A gap mode located at about 375 cm −1 is observed in InN films grown at low In/N ratios. This is in good agreement with the recursion method calculation for the In vacancy-induced vibration mode. In addition, a spatial correlation model has been used to estimate the lattice disorder in InN samples. The shortest correlation length is L = 5.9 nm.

Dissolution of nanocrystalline fluorite powders: An investigation by XRD and solution chemistry

The effect of lattice disorder and mineral surface area on the reactivity of finely ground fluorite was studied on ball-milled powders. Structural information was provided by X-ray whole powder pattern modeling (WPPM). The mean size of coherent scattering domains decreases with milling time from 70 nm to ∼20 nm, whereas the density of lattice defects increases with both time and intensity of milling treatment, from 4 × 10 15 m -2 to 24 × 10 15 m -2. High resolution transmission electron microscopy (HRTEM) of ground fluorite grains shows several line defects and a general tendency of nanometric crystalline domains to agglomerate in larger grains. Solution chemistry was investigated using batch reactors with free drift of solution saturation state with respect to fluorite. Total surface area was measured by the Brunaver, Emmet and Teller (BET) method, and dissolution rates were measured at pH = 2 (HCl) and T = 295 K. In far from equilibrium conditions, dissolution rates normalized by BET area do not increase with the dislocation density. In near-equilibrium condition, however, measured stationary ionic product clearly increases with both time and intensity of milling treatment. Thermodynamic predictions of the solubility constant indicate negligible or little effect of total surface area. Consequently, the observed increase in the stationary ionic product can be related to the increasing lattice defect content. This confirms the significant role of dislocation outcrops on mineral dissolution in close to equilibrium conditions.

Dynamic annealing in III-nitrides under ion bombardment

We study the evolution of structural defects in AlxGa1−xN films (with x=0.0–0.6) bombarded with kilo-electron-volt heavy ions at 77 and 300 K. We use a combination of Rutherford backscattering/channeling spectrometry and cross-sectional transmission electron microscopy. Results show that an increase in Al content not only strongly enhances dynamic annealing processes but can also change the main features of the amorphization behavior. In particular, the damage buildup behavior at 300 K is essentially similar for all the AlGaN films studied. Ion-beam-produced disorder at 300 K accumulates preferentially in the crystal bulk region up to a certain saturation level (∼50%–60% relative disorder). Bombardment at 300 K above a critical fluence results in a rapid increase in damage from the saturation level up to complete disordering, with a buried amorphous layer nucleating in the crystal bulk. However, at 77 K, the saturation effect of lattice disorder in the bulk occurs only for x≳0.1. Based on the analysis of these results for AlGaN and previously reported data for InGaN, we discuss physical mechanisms of the susceptibility of group-III nitrides to ion-beam-induced disordering and to the crystalline-to-amorphous phase transition.

Energy band structure of strained Si1−xCx alloys on Si (001) substrate

We report the energy band structures of strained Si1−xCx alloys on Si (001) substrates. All calculations are based on a 20×20 Hamiltonian matrix constructed from the linear combination of atomic orbital approximation with spin–orbit interaction, strain effect, and lattice disorder effect taken into account. The lattice disorder parameter is obtained from fittings with the experimental band gap of strained Si1−xCx alloy with small carbon concentration and reflects the initial reduction of band gap of relaxed Si1−xCx alloy, while simple virtual crystal approximation does not. The effect of strain on band structure is incorporated in terms of the interatomic interaction parameters, which are functions of bond length and bond angle. The strained Si1−xCx alloy becomes metallic when x=28%. All the directional effective masses are affected by the strain. Overall agreements are found between our theoretical calculations and recent experimental results.

Solution-mediated phase transformation of anhydrous to dihydrate carbamazepine and the effect of lattice disorder

This paper describes the kinetics of the solution-mediated phase transformation of the anhydrous monoclinic polymorph of carbamazepine (CBZ(A)) to the dihydrate crystal form (CBZ(D)). Monitoring both solution concentration and solid phase composition identified the steps and mechanisms that control the kinetic processes, and regulate the concentration of drug achieved during dissolution of the metastable solid phase, CBZ(A). The results show that the kinetics and the rate-controlling step for the transformation depend on grinding and storage conditions of CBZ(A). Grinding CBZ(A) shortened the transformation times and changed the rate-controlling step from crystallization of CBZ(D) to dissolution of CBZ(A). Grinding may cause various degrees of disorder in the form of lattice defects and/or amorphous regions. These disordered regions promote the anhydrous to dihydrate transformation by facilitating the surface nucleation of CBZ(D) on freshly ground CBZ(A) and on amorphous CBZ. The concentration-time profiles revealed aging effects on the solution-mediated transformation of ground CBZ(A) that were undetectable by diffraction and thermal analysis. These results have significant consequences on the concentration-time profiles of active pharmaceutical ingredients during dissolution of metastable solid phases, crystalline or amorphous.

Effects of lattice disorder in theUCu5−xPdxsystem

The UCu(5-x)Pd(x) system exhibits non-Fermi liquid (NFL) in the thermodynamic and transport properties at low temperatures for Pd concentrations 0.9 <~ x <~ 1.5. The local structure around the U, Cu, and Pd atoms has been measured for 0 <= x <= 1.5 using the X-ray Absorption Fine Structure (XAFS) technique in order to quantify the effects of lattice disorder on the NFL properties. A model which allows a percentage of the Pd atoms to occupy nominal Cu (16e) sites, s, was used to fit the Pd and Cu K edge and U L(III) edge data. Pd/Cu site interchange was found to occur in all samples (x != 0), reaching a minimum value of s ~0.17 at x = 0.7 and increasing monotonically to s ~ 0.4 at x=1.5. These data also determine the static disorder the nearest neighbor U--Cu pairs. The results indicate that the measured U--Cu static disorder is not sufficient to explain the NFL behavior of the magnetic susceptibility within the single-ion Kondo disorder model and casts doubt on the applicability of this model to UCu(5-x)Pd(x).

The Effect of Lattice Disorder on the Channeling Potential in fcc Single Crystals

The axial and planar potential for α-particles in copper single crystal disordered by dumb-bell configuration (DBC) or body-centered interstitial (BCI) is calculated according to the continuum potential approximation by using Moliere potential and Biersack's universal potential and compared with the channeling potential of the normal lattice. The calculations showed that the axial potential for DBC or BCI are the same, but the planar potential has different estimations for each kind of lattice disorder. The point of minimum potential in the disordered lattice has been determined for both axial and planar channel.

Lattice Disorder Effects on The Vacancy-Oxygen Centre in Ion-Irradiated Silicon

ABSTRACTThe Vacancy-Oxygen (VO) centre is one of the most prominent defects appearing in silicon after irradiation with energetic particles and gamma rays. It is formed when migrating vacancies are trapped by interstitial oxygen atoms. It gives rise to a deep level in the upper part of the silicon bandgap at 0.164 eV below the conduction band edge that can be conveniently studied by deep level transient spectroscopy (DLTS). It is furthermore a very important defect from a technological point of view since it normally controls the charge carrier lifetimes in silicon power devices at high injection levels.Various doped n-type float zone Si samples have been irradiated with MeV ions at low doses. DLTS measurements of the charge carrier capture kinetics reveal lower capture rates if heavier ions, i.e. larger collision cascades, have been used to generate the defects. This effect can be interpreted as a local lowering of the Fermi level following lattice disorder in regions with a high elastic energy deposition. It will be shown that the VO is also very sensitive to lattice disorder in a similar way as previously reported for the divacancy centre.In the DLTS spectra another defect, originating from an interstitial-carbon—substitutional-carbon pair (Cj-Cs), overlaps with the VO-peak. The Ci-Cs pair has been observed to posses bistable properties and in this paper we will further elucidate the various contributions from the two defects to the Ec-0.164 eV peak. In particular, the temperature dependence of the electron capture cross section of the Cj-Cs level will be discussed.

Lattice disorder and texture in diamond coatings deposited by HFCVD on Co-cemented tungsten carbide

Diamond coatings were deposited by hot filament chemical vapour deposition on 5.8 wt.% Co-cemented tungsten carbide (WCCo). Under the same deposition conditions, two sets of samples were produced by varying the CH 4 percentage (0.5 and 1.0%) in the H 2 atmosphere, and changing the substrate temperature in the range 650–950 °C. The effect of different CH 4 percentage and of temperature on diamond microstructure was investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). In particular, pole figure measurement and line profile analysis (LPA) were used to study diamond texture and lattice disorder, respectively. Diamond grains were not preferentially oriented for deposition temperatures below 800 °C. Above this temperature, a [111] fibre texture developed in samples deposited with 0.5% CH 4 in H 2. The XRD profile broadening indicated a marked increase of lattice disorder in the diamond film with increasing deposition temperature. This observation was correlated with the Raman peak broadening, also attributed to lattice disorder effects. A detailed LPA showed a decrease of crystallite mean size and a corresponding microstrain increase with raising temperature, although SEM morphological observations indicated a progressive grain coarsening at higher deposition temperatures.

Effects of lattice disorder on the superconducting properties of YBa 2Cu 3O 6.9 films

We report on striking sensitivity of the superconducting properties of ion-beam sputtered YBa 2Cu 3O 6.9 films to the lattice disorder (induced by varying growth temperatures). T c decreases with increasing disorder, while the width of the resistive transition and the normal state resistivity increase. We give a quantitative significance to this trend by expressing the degree of the disorder in terms of the lattice coherence length r c is extracted from the width of X-ray diffraction rocking curves. We find that T c saturates to the maximum of ∼ 92 K for r c > 10 nm.

Two-dimensional time-resolved X-ray diffraction studies of live isometrically contracting frog sartorius muscle.

Results were obtained from contracting frog muscles by collecting high quality time-resolved, two-dimensional, X-ray diffraction patterns at the British Synchrotron Radiation Source (SERC, Daresbury, Laboratory). The structural transitions associated with isometric tension generation were recorded under conditions in which the three-dimensional order characteristic of the rest state is either present or absent. In both cases, new layer lines appear during tension generation, subsequent to changes from activation events in the thin filaments. Compared with the 'decorated' actin layer lines of the rigor state, the spacings of the new layer lines are similar whereas their intensities differ substantially. We conclude that in contracting muscle an actomyosin complex is formed whose structure is not like that in rigor, although it is possible that the interacting sites are the same. Transition from rest to plateau of tension is accompanied by approximately 1.6% increase in the axial spacing of the myosin layer lines. This is explained as arising from axial disposition of the interacting myosin heads in the actomyosin complex. Model calculations are presented which support this view. We argue that in a situation where an actomyosin complex is formed during contraction, one cannot describe the diffraction features as being either thick or thin filament based. Accordingly, the layer lines seen during tension generation are referred to as actomyosin layer lines. It is shown that these layer lines can be indexed as submultiples of a minimum axial repeat of approximately 218.7 nm. After lattice disorder effects are taken into account, the intensity increases on the 15th and 21st AM layer lines at spacings of approximately 14.58 and 10.4 nm respectively, show the same time course as tension rise. However, the time course of the intensity increase of the other actomyosin layer lines and of the spacing change (which is the same for both phenomena) shows a substantial lead over tension rise. These findings suggest that the actomyosin complex formed prior to tension rise is a non-tension-generating state and that this is followed by a transition of the complex to a tension-generating state. The intensity increase in the 15th actomyosin layer line, which parallels tension rise, can be accounted for assuming that in the tension-generating state the attached heads adopt (axially) a more perpendicular orientation with respect to the muscle axis than is seen at rest or in the non-tension-generating state. This suggests the existence of at least two structurally distinct interacting myosin head conformations.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of Lattice Disorder on the Excitonic States in Superionic Crystals

AbstractThe effect of the lattice disorder on excitonic states in superionic crystals is studied experimentally and theoretically. The spectral density of the excitonic states is measured in AgI‐type superionic crystals within a wide temperature region. The theory based on multiple scattering approach is developed. The Lipatov‐Brezin‐Parisi method is applied for the calculation of the averaged scattering matrix in the region of localized excitonic states. The experimental behaviour of the scattering matrix is recovered from the excitonic spectral density after removal of the exciton–phonon interaction effects. A comparison of the experimental and theoretical scattering matrices is made.

Effect of ion beam mixing/implantation at low temperature on superconductivity in palladium-hydrogen and palladium-copper-hydrogen systems

The highest superconducting transition temperature of 8.42 and 5.16K has been observed in the alloy system PdH and PdCuH, where the atomic ratio n=H/Pd(Cu) are 1 and 0.6, respectively. The effect of lattice disorder induced by 220 keV Ar ion beam bombardment at liquid helium-temperature is detrimental to the superconductivity.

Formation, properties, and ion irradiation effects of hexagonal structure MoN thin films

Thin films (100-120 nm) of hexagonal structure MoN have been fabricated by reaction of Mo films in an NH 3 atmosphere. The as-formed films possessed superconducting transition temperatures T c ≃ 13 K, with resistance ratios r = R(296K)/R(T c ) in the range 5 to 10, low-temperature normal state resistivities ρ o = 4 to 10 μ Ω-cm, and extrapolated upper critical fields H c2 (0) = 4.0 to 5.0 T. Thin film X-ray diffraction patterns revealed no visible second phase, with measured lattice parameters close to literature values. The effects of lattice disorder on the superconducting and electronic properties were investigated by irradiation with nitrogen ions of energy 45 and 340 keV, resulting in a nearly uniform damage profile without the introduction of any new chemical species. The results indicate that ordered hexagonal MoN shows some of the unusual properties characteristic of moderate-to-high T c transition metal compounds, but is relatively insensitive to degradation of the superconducting properties by lattice disorder. For ion fluences Φ up to 2 × 1016N-ions/cm2, T c is found to decrease monotonically and saturate at 9.5 K, almost 3/4 the initial value, while H c2 (0) undergoes a gradual increase to 11T.

Effect of lattice disorder and strain on T C in sputtered “Nb 3Ge” thin films

Disorder and strain introduced into sputtered “Nb 3Ge” films by mechanical polishing slightly lowers the onset temperature and/or very markedly broadens the width of the superconducting transition. The structural damage is reversible and annealing restores the superconducting behavior of the film very nearly to its initial state.

Enhancement of the superconducting transition temperatures in ion-implanted aluminium alloys

Random alloys of C, Si, and Ge with Al, prepared by ion implantation at liquid helium temperatures, have maximum superconducting transition temperatures Tc of 4.2 K (C), 7.35 K (Ge) and 8.35 K (Si). The latter value is the highest yet obtained for an Al-based alloy. All these values are obtained for covalent element concentrations of ∼ 25 %. The effect of lattice disorder is not found sufficient to account for the observed Tc enhancements.

Coupled order parameters, lattice disorder, and magnetic phase transitions

A mean-field theory is developed which describes the effects of lattice disorder on magnetic phase transitions. Depending on the parameter values, three types of renormalization effects on ${T}_{C}$ can occur, i.e., weak, strong, or catastrophic; the last implies a first-order phase transition in both magnetic and lattice order parameter. Phase diagrams are obtained which show both the existence of critical triple points and the effects of phonon entropy in determining ${T}_{C}$. It is also shown that even in the weak-coupling limit, the lattice disordering process can be driven by magnetic entropy alone. The model is applied to MnBi, which undergoes a first-order magnetic phase transition well below its mean-field-value Curie temperature.

Semiclassical theory of atom-solid surface collisions: Elastic scattering

The elastic scattering of atoms from solid surfaces is examined within the semiclassical framework. Explicit expression for diffraction intensities are obtained which utilize classical trajectory information as the computational device. Effect of lattice disorder are examined.The results of Beeby and Weinberg are considered.An alternative method for extracting the surface atom displacements and the attractive well depth of the atom-surface interaction is discussed.