Local entropy and structure in a two-dimensional frustrated system

We calculate the local contributions to the Shannon entropy and excess entropy and use these information theoretic measures as quantitative probes of the order arising from quenched disorder in the diluted Ising antiferromagnet on a triangular lattice. When one sublattice is sufficiently diluted, the system undergoes a temperature-driven phase transition, with the other two sublattices developing magnetizations of equal magnitude and opposite sign as the system is cooled.(1) The diluted sublattice has no net magnetization but exhibits spin glass ordering. The distribution of local entropies shows a dramatic broadening at low temperatures; this indicates that the system's total entropy is not shared equally across the lattice. The entropy contributions from some regions exhibit local reentrance, although the entropy of the system decreases monotonically as expected. The average excess entropy shows a sharp peak at the critical temperature, showing that the excess entropy is sensitive to the structural changes that occur as a result of the spin glass ordering.     

Superparamagnetic iron oxide (SPIO) enhancement in the cirrhotic liver: a comparison of two doses of ferumoxides in patients with advanced disease

The aim of this study was to establish whether enhancement of the liver by the MRI contrast agent ferumoxides could be effectively achieved at a reduced dose of 7.5 micromol/kg in patients with advanced liver cirrhosis. Forty-two liver transplant candidates with end-stage cirrhosis underwent SPIO-enhanced MRI at 1.5T, using either 15 micromol/kg or 7.5 micromol/kg ferumoxides. The lower dose of ferumoxides was also used in 21 non-cirrhotic patients with colorectal liver metastases who acted as a control group. The percentage signal intensity loss (PSIL) after SPIO was measured in all patients, and in those patients with tumors the post-SPIO contrast-to-noise ratio (CNR) was measured. The median PSIL after SPIO in the high dose cirrhotic (HDLC), low dose non-cirrhotic (LDNC) and low dose cirrhotic (LDLC) patients was 86.3%, 74.6%, and 64.2% respectively. These differences were significant using the Mann-Whitney U test. Tumors were found in 8 patients in the high dose cirrhotic group, 9 in the low dose cirrhotic group, and all 21 of the control group. No significant differences were found between the CNR values after SPIO in the 3 groups (median values HDLC 15.1, LDNC 23.7, LDLC 19.5). In patients with late-stage cirrhosis the PSIL after SPIO was significantly less at 7.5 micromol/kg than at 15 micromol/kg, but both doses produced a substantial loss of signal. Lesion to liver CNR was not adversely affected by using the lower dose, so when imaging at 1.5T the authors would recommend using 7.5 micromol/kg in patients with liver cirrhosis.     

Difference in icosahedral short-range order in early and late transition metal liquids

New short-range order data are presented for equilibrium and undercooled liquids of Ti and Ni. These were obtained from in situ synchrotron x-ray diffraction measurements of electrostatically levitated droplets. While the short-range order of liquid Ni is icosahedral, consistent with Frank's hypothesis, significantly distorted icosahedral order is observed in liquid Ti. This is the first experimental observation of distorted icosahedral short-range order in any liquid, although this has been predicted by theoretical studies on atomic clusters.     

Voice Analysis of Postlingually Deaf Adults Pre- and Postcochlear Implantation

Objectives

To ascertain whether cochlear implantation (CI), without specific vocal rehabilitation, is associated with changes in perceptual and acoustic vocal parameters in adults with severe to profound postlingual deafness.

Hypothesis

Merely restoring auditory feedback could allow the individual to make necessary adjustments in vocal pattern.

Study Design

Prospective and longitudinal.

Methods

The experimental group composed of 40 postlingually deaf adults (20 males and 20 females) with no previous laryngeal or voice disorders. Participants’ voices were recorded before CI and 6–9 months after CI. To check for chance modifications between two evaluations, a control group of 12 postlingually deaf adults, six male and six female, without CI was also evaluated. All sessions composed of the recording of read sentences from Consensus Auditory-Perceptual Evaluation of Voice and sustained vowel /a/. Auditory and acoustic analyses were then conducted.

Results

We found a statistically significant reduction in overall severity, strain, loudness, and instability in auditory analysis. In vocal acoustic analysis, we found statistically significant reduction fundamental frequency (F0) values (in male participants) and F0 variability (in both genders). The control group showed no statistically significant changes in most vocal parameters assessed, apart from pitch and F0 (in female participants only). On comparing the interval of variation of results between the experimental and control groups, we found no statistically significant difference in vocal parameters between CI recipients and nonrecipients, with the exception of F0 variability in male participants.

Conclusions

The patients in our sample showed changes in overall severity, strain, loudness, and instability values, and reductions in F0 and its variability. On comparing the variation of results between the groups, we were able to prove in our study that implant recipients postlingually deaf adults (experimental group), without specific vocal rehabilitation, differed from nonrecipients (control group) in loudness and F0 variability sustained vowel /a/ in male participants.     

Local adsorption sites and bondlength changes in Ni(1 0 0)/H/CO and Ni(1 0 0)/CO

The local adsorption geometry of CO adsorbed in different states on Ni(1 0 0) and on Ni(1 0 0) precovered with atomic hydrogen has been determined by C 1s (and O 1s) scanned-energy mode photoelectron diffraction, using the photoelectron binding energy changes to characterise the different states. The results confirm previous spectroscopic assignments of local atop and bridge sites both with and without coadsorbed hydrogen. The measured Ni–C bondlengths for the Ni(1 0 0)/CO states show an increase of 0.16 ± 0.04 Å in going from atop to bridge sites, while comparison with similar results for Ni(1 1 1)/CO for threefold coordinated adsorption sites show a further lengthening of the bond by 0.05 ± 0.04 Å. These changes in the Ni–CO chemisorption bondlength with bond order (for approximately constant adsorption energy) are consistent with the standard Pauling rules. However, comparison of CO adsorbed in the atop geometry with and without coadsorbed hydrogen shows that the coadsorption increases the Ni–C bondlength by only 0.06 ± 0.04 Å, despite the decrease in adsorption energy of a factor of 2 or more. This result is also reproduced by density functional theory slab calculations. The results of both the experiments and the density functional theory calculations show that CO adsorption onto the Ni(1 0 0)/H surface is accompanied by significant structural modification; the low desorption energy may then be attributed to the energy cost of this restructuring rather than weak local bonding.     

Temperature-dependent surface X-ray diffraction on K/Ag(001)-(2 × 1)

Temperature-dependent surface X-ray diffraction experiments have been performed on the K/Ag(001)-(2 × 1) adsorption system. The structure is characterized by a missing-row geometry in which alternate Ag rows along [1 0] are missing. The K atoms reside within the large grooves coordinated by six Ag atoms at a distance of 3.44(5) Å, corresponding to an effective K-radius of 2.00(5) Å. Large anisotropic disorder is observed for both the K-atoms and the top-layer (“ridge”) Ag atoms. The K-atom displacements are largest in the direction along the grooves, whereas for the Ag atoms the vibrations along [110] are significantly larger. The temperature dependence of the Ag vibrations is in accordance with Debye theory for the [110] direction, but deviates from it for the [1 0] vibrations at high temperature. In contrast to the K-atoms, the out-of-plane vibrations of the top-layer Ag atoms are larger than the in-plane vibrations. The inclusion of anharmonic contributions to describe the Ag disorder significantly improves the fits. It is shown that if anharmonicity is neglected the interlayer contraction is overestimated (Δd₁₂/d₁₂ only −3.2%, instead of −12.7% if anharmonicity is neglected). Due to the anharmonicity, different definitions of the atomic position arise (mean, mode and equilibrium position), which are discussed on the basis of the results.     

Application and assessment of laser Doppler velocimetry for underwater acoustic measurements

The majority of traditional methods for making underwater acoustic pressure measurements involve placing all or part of a measurement transducer in the acoustic field. A variety of optical metrology techniques have been developed in an attempt to reduce or remove any perturbing effects. An example of this is the use of laser interferometry which has been developed as the primary method of calibrating hydrophones in the frequency range - at the National Physical Laboratory (NPL). This technique involves suspending a thin Mylar pellicle in the acoustic field and recording the displacement of the pellicle surface using a Michelson Interferometer. This study details a comparison of a Laser Doppler Velocimeter (LDV) with the NPL Laser Interferometer, which gives a good correlation where agreement is within approximately 4% and 7% for two different power levels from a plane piston transducer and within 2.5% and 1% for the same power levels from a plane piston transducer. A novel, non-perturbing method of recording temporally resolved acoustic pressure distributions in water using an LDV is also described. The technique is shown to benefit from the consistent frequency response of the LDV detection system, such that the measured output resembles the drive voltage input to the transducer more closely than a similar hydrophone measurement. For the experimental arrangement described, the LDV system is shown to be sensitive to minimum pressure amplitudes of nominally .     

Field-induced self-assembly: does size matter?

ABSTRACT

Simulated external electric fields are applied to polarisable species containing either a monodisperse of bidisperse distribution of polarisabilities. The magnitude of the external field and the polarisabilities are systematically varied. The application of an external field (of sufficient magnitude) is found to induce chain formation (as expected). The monodisperse systems are found to ‘self-assemble’ with larger induced dipole moments effectively clustering in chains as a result of significant dipole-induced dipole effects. The distribution of the chain lengths is characterised as a function of the applied field and the atom polarisability. For the bidisperse systems, the external field induces chain formation and a partial segregation, in which the more polarisable species preferentially form chains. The chain lengths are again determined as a function of field strength and the atom polarisabilities. Scaling behaviour is analysed.     

Effect of relativistic plasma on extreme-ultraviolet harmonic emission from intense laser-matter interactions

Experiments were performed in which intense laser pulses (up to 9x10(19) W/cm(2)) were used to irradiate very thin (submicron) mass-limited aluminum foil targets. Such interactions generated high-order harmonic radiation (greater than the 25th order) which was detected at the rear of the target and which was significantly broadened, modulated, and depolarized because of passage through the dense relativistic plasma. The spectral modifications are shown to be due to the laser absorption into hot electrons and the subsequent sharply increasing relativistic electron component within the dense plasma.     

Return to equilibrium in theXY model

We prove that the locally perturbedXY model returns to equilibrium under the unperturbed evolution but the unperturbed model does not necessarily approach equilibrium under the perturbed evolution. In fact this latter property is false for perturbation by a local magnetization. The failure is directly attributable to the formation of bound states. If the perturbation is quadratic these problems are reduced to spectral analysis of the one-particle Hamiltonian. We demonstrate that the perturbed Hamiltonian has a finite set of eigenvalues of finite multiplicity together with some absolutely continuous spectrum. Eigenvalues can occur in the continuum if, and only if, the perturbation dislocates the system. Singular continuous spectrum cannot occur.