1,509 research outputs found
Structural Properties of Two-Dimensional Polymers
We present structural properties of two-dimensional polymers as far as they
can be described by percolation theory. The percolation threshold, critical
exponents and fractal dimensions of clusters are determined by computer
simulation and compared to the results of percolation theory. We also describe
the dependence of the typical cluster structures on the reaction rate.Comment: 7 pages, LaTeX with RevTeX and epsf styles and PostScript figures
included (uuencoded shell archive), TVP-93051
The non-centrosymmetric lamellar phase in blends of ABC triblock and ac diblock copolymers
The phase behaviour of blends of ABC triblock and ac diblock copolymers is
examined using self-consistent field theory. Several equilibrium lamellar
structures are observed, depending on the volume fraction of the diblocks,
phi_2, the monomer interactions, and the degrees of polymerization of the
copolymers. For segregations just above the order-disorder transition the
triblocks and diblocks mix together to form centrosymmetric lamellae. As the
segregation is increased the triblocks and diblocks spatially separate either
by macrophase-separating, or by forming a non-centrosymmetric (NCS) phase of
alternating layers of triblock and diblock (...ABCcaABCca...). The NCS phase is
stable over a narrow region near phi_2=0.4. This region is widest near the
critical point on the phase coexistence curve and narrows to terminate at a
triple point at higher segregation. Above the triple point there is two-phase
coexistence between almost pure triblock and diblock phases. The theoretical
phase diagram is consistent with experiments.Comment: 9 pages, 8 figures, submitted to Macromolecule
Absence of non-linear Meissner effect in YBa2Cu3O6.95
We present measurements the field and temperature dependence of the
penetration depth (lambda) in high purity, untwinned single crystals of
YBa2Cu3O6.95 in all three crystallographic directions. The temperature
dependence of lambda is linear down to low temperatures, showing that our
crystals are extremely clean. Both the magnitude and temperature dependence of
the field dependent correction to lambda however, are considerably different
from that predicted from the theory of the non-linear Meissner effect for a
d-wave superconductor (Yip-Sauls theory). Our results suggest that the
Yip-Sauls effect is either absent or is unobservably small in the Meissner
state of YBa2Cu3O6.95.Comment: 4 pages, 4 figures (Latex file + Postscipt figures
Image of the Energy Gap Anisotropy in the Vibrational Spectum of a High Temperature Superconductor
We present a new method of determining the anisotropy of the gap function in
layered high-Tc superconductors. Careful inelastic neutron scattering
measurements at low temperature of the phonon dispersion curves in the (100)
direction in La_(1.85)Sr_(.15)CuO_4 would determine whether the gap is
predominately s-wave or d-wave. We also propose an experiment to determine the
gap at each point on a quasi-two-dimensional Fermi surface.Comment: 12 pages + 2 figures (included
Conductivity of Metallic Si:B near the Metal-Insulator Transition: Comparison between Unstressed and Uniaxially Stressed Samples
The low-temperature dc conductivities of barely metallic samples of p-type
Si:B are compared for a series of samples with different dopant concentrations,
n, in the absence of stress (cubic symmetry), and for a single sample driven
from the metallic into the insulating phase by uniaxial compression, S. For all
values of temperature and stress, the conductivity of the stressed sample
collapses onto a single universal scaling curve. The scaling fit indicates that
the conductivity of si:B is proportional to the square-root of T in the
critical range. Our data yield a critical conductivity exponent of 1.6,
considerably larger than the value reported in earlier experiments where the
transition was crossed by varying the dopant concentration. The larger exponent
is based on data in a narrow range of stress near the critical value within
which scaling holds. We show explicitly that the temperature dependences of the
conductivity of stressed and unstressed Si:B are different, suggesting that a
direct comparison of the critical behavior and critical exponents for stress-
tuned and concentration-tuned transitions may not be warranted
Multicenter trial of neo-adjuvant chemotherapy followed by extrapleural pneumonectomy in malignant pleural mesothelioma
Background: The aim of this multicenter trial was to prospectively evaluate neo-adjuvant chemotherapy followed by extrapleural pneumonectomy (EPP) and radiotherapy, including quality of life as outcome. Patients and methods: Eligible patients had malignant pleural mesothelioma of all histological types, World Health Organization performance status of zero to two and clinical stage T1-T3, N0-2, M0 disease considered completely resectable. Neo-adjuvant chemotherapy consisted of three cycles of cisplatin and gemcitabine followed by EPP. Postoperative radiotherapy was considered for all patients. Results: In all, 58 of 61 patients completed three cycles of neo-adjuvant chemotherapy. Forty-five patients (74%) underwent EPP and in 37 patients (61%) the resection was complete. Postoperative radiotherapy was initiated in 36 patients. The median survival of all patients was 19.8 months [95% confidence interval (CI) 14.6-24.5]. For the 45 patients undergoing EPP, the median survival was 23 months (95% CI 16.6-32.9). Psychological distress showed minor variations over time with distress above the cut-off score indicating no morbidity with 82% (N = 36) at baseline and 76% (N = 26) at 3 months after surgery (P = 0.5). Conclusions: The observed rate of operability is promising. A median survival of 23 months for patients undergoing EPP compares favourably with the survival reported from single center studies of upfront surgery. This approach was not associated with an increase in psychological distres
Scaling of the Conductivity with Temperature and Uniaxial Stress in Si:B at the Metal-Insulator Transition
Using uniaxial stress to tune Si:B through the metal-insulator transition we
find the conductivity at low temperatures shows an excellent fit to scaling
with temperature and stress on both sides of the transition. The scaling
functions yield the conductivity in the metallic and insulating phases, and
allow a reliable determination of the temperature dependence in the critical
regions on both sides of the transition
Genome-wide DNA methylation detection by MethylCap-seq and Infinium HumanMethylation450 BeadChips: an independent large-scale comparison.
Two cost-efficient genome-scale methodologies to assess DNA-methylation are MethylCap-seq and Illumina's Infinium HumanMethylation450 BeadChips (HM450). Objective information regarding the best-suited methodology for a specific research question is scant. Therefore, we performed a large-scale evaluation on a set of 70 brain tissue samples, i.e. 65 glioblastoma and 5 non-tumoral tissues. As MethylCap-seq coverages were limited, we focused on the inherent capacity of the methodology to detect methylated loci rather than a quantitative analysis. MethylCap-seq and HM450 data were dichotomized and performances were compared using a gold standard free Bayesian modelling procedure. While conditional specificity was adequate for both approaches, conditional sensitivity was systematically higher for HM450. In addition, genome-wide characteristics were compared, revealing that HM450 probes identified substantially fewer regions compared to MethylCap-seq. Although results indicated that the latter method can detect more potentially relevant DNA-methylation, this did not translate into the discovery of more differentially methylated loci between tumours and controls compared to HM450. Our results therefore indicate that both methodologies are complementary, with a higher sensitivity for HM450 and a far larger genome-wide coverage for MethylCap-seq, but also that a more comprehensive character does not automatically imply more significant results in biomarker studies
MGMT Promoter Methylation Cutoff with Safety Margin for Selecting Glioblastoma Patients into Trials Omitting Temozolomide: A Pooled Analysis of Four Clinical Trials.
The methylation status of the O <sup>6</sup> -methylguanine DNA methyltransferase (MGMT) gene promoter is predictive for benefit from temozolomide in glioblastoma (GBM). A clinically optimized cutoff was sought allowing patient selection for therapy without temozolomide, while avoiding to withhold it from patients who may potentially benefit.Experimental Design: Quantitative MGMT methylation-specific PCR data were obtained for newly diagnosed patients with GBM screened or treated with standard radiotherapy and temozolomide in four randomized trials. The pooled dataset was randomly split into a training and test dataset. The unsupervised cutoff was obtained at a 50% probability to be (un)methylated. ROC analysis identified an optimal cutoff supervised by overall survival (OS).
For 4,041 patients valid MGMT results were obtained, whereof 1,725 were randomized. The unsupervised cutoff in the training dataset was 1.27 (log <sub>2</sub> [1,000 × (MGMT+1)/ACTB]), separating unmethylated and methylated patients. The optimal supervised cutoff for unmethylated patients was -0.28 (AUC = 0.61), classifying "truly unmethylated" (≤-0.28) and "gray zone" patients (>-0.28, ≤1.27), the latter comprising approximately 10% of cases. In contrast, for patients with MGMT methylation (>1.27) more methylation was not related to better outcome. Both methylated and gray zone patients performed significantly better for OS than truly unmethylated patients [HR = 0.35, 95% confidence interval (CI), 0.27-0.45, P < 0.0001; HR = 0.58, 95% CI, 0.43-0.78, P < 0.001], validated in the test dataset. The MGMT assay was highly reproducible upon retesting of 218 paired samples (R <sup>2</sup> = 0.94).
Low MGMT methylation (gray zone) may confer some sensitivity to temozolomide treatment, hence the lower safety margin should be considered for selecting patients with unmethylated GBM into trials omitting temozolomide
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