arXiv subjects

J. L. Robertson

Publications and source records attributed to J. L. Robertson.

4 recordsLinked to original sources

arXiv2004

Acid-mediated tumor invasion: How does vasculature affect the growth characteristics?

We study the growth of an implanted a-vascular tumor seed in two-dimensions based on a model where the mechanism of invasion is centered on tumor-induced acidification of the micro-environment and consequent death of normal cells. The spatial distribution of the acid density around the tumor is found using mean-field analysis. By assuming that the viability of both normal and tumor cells falls sharply below certain threshold values of the local pH, we determine the conditions for the formation of a necrotic core at the center, as well as its radius as a function of the tumor radius. We show that the mean micro-vessel density (MVD) plays a pivotal role in determining the growth characteristics of the tumor. When the MVD is sufficiently small, accumulation of excess acid inside the tumor leads to the formation of a necrotic core, which occupies a significant fraction of the total area in large tumors. However, necrosis is reduced when the mean MVD inside the tumor is larger than outside because of the more efficient removal of excess acid. At sufficiently high MVD, necrosis is absent in the tumor, or confined to small regions mostly devoid of micro-vessels. Quantitative estimates of MVD for these different phases of growth are obtained, and verified using explicit cellular automaton simulations. Recent experimental studies on the correlation between necrosis and MVD support our main conclusions.

q-bio.CB↗

arXiv2003

Neutron Scattering Study of Relaxor Ferroelectric (1-x)Pb(Zn1/3Nb2/3)O3-xPbTiO3

Neutron elastic diffuse scattering experiments performed on Pb(Zn1/3Nb2/3)O3 (PZN) and on its solid solution with PbTiO3(PT), known as PZN-xPT, with x=4.5% and 9% around many reflections show that diffuse scattering is observed around reflections with mixed indices in the transverse and diagonal directions only. From the width of the diffuse scattering peak a correlation length is extracted. In PZN, we have reported that the diffuse scattering is more extended in the transverse than in the diagonal directions. In the present work, the results show that the addition of PT leads to a broadening of the diffuse scattering along the diagonal, relative to the transverse directions, indicating a change in the orientation of the polar regions. Also, with the addition of PT, the polar nanoregions condense at a higher temperature above the transition than in pure PZN (> 40 K), due to stronger correlations between the polar regions. Neutron inelastic scattering measurements have also been performed on PZN-xPT. The results show the broadening of the transverse acoustic (TA) phonon mode at a small momentum transfer q upon cooling. We attribute this broadening to the appearance of the polar nanoregions.

cond-mat.mtrl-sci↗

arXiv2002

Diffuse Neutron Scattering Study of Relaxor Ferroelectric (1-x)Pb(Zn1/3Nb2/3)O3-xPbTiO3(PZN-xPT)

Diffuse neutron scattering is a valuable tool to obtain information about the size and orientation of the polar nanoregions that are a characteristic feature of relaxor ferroelectrics. In this paper, we present new diffuse scattering results obtained on Pb(Zn1/3Nb2/3)O3 (PZN for short) and (1-x)Pb(Zn1/3Nb2/3)O3-xPbTiO3(PZN-xPT)single crystals (with x=4.5 and 9%), around various Bragg reflections and along three symmetry directions in the [100]-[011] zone. Diffuse scattering is observed around reflections with mixed indices, (100), (011) and (300), and along transverse and diagonal directions only. No diffuse scattering is found in longitudinal scans. The diffuse scattering peaks can be fitted well with a Lorentzian function, from which a correlation length is extracted. The correlation length increases with decreasing temperatures down to the transition at Tc, first following a Curie-Weiss law, then departing from it and becoming flat at very low temperatures. These results are interpreted in terms of three temperature regions: 1) dynamic polarization fluctuations (i.e. with a finite lifetime) at high temperatures, 2) static polarization reorientations (condensation of polar nanoregions) that can still reorient as a unit (relaxor behavior) at intermediate temperatures and 3) orientational freezing of the polar nanoregions with random strain fields in pure PZN or a structural phase transition in PZN-xPT at low temperatures. The addition of PT leads to a broadening of the diffuse scattering along the diagonal ([111]) relative to the transverse ([100]) direction, indicating a change in the orientation of the polar regions. Also, with the addition of PT, the polar nanoregions condense at a higher temperature above Tc.

cond-mat.mtrl-sci↗

arXiv2001

Diffuse Neutron Scattering Study of a Disordered Complex Perovskite Pb(Zn1/3Nb2/3)O3 Crystal

Diffuse scattering around the (110) reciprocal lattice point has been investigated by elastic neutron scattering in the paraelectric and the relaxor phases of the disordered complex perovskite crystal-Pb(Zn1/3Nb2/3)O3(PZN). The appearance of a diffuse intensity peak indicates the formation of polar nanoregions at temperature T*, approximately 40K above Tc=413K. The analysis of this diffuse scattering indicates that these regions are in the shape of ellipsoids, more extended in the <111> direction than in the <001> direction. The quantitative analysis provides an estimate of the correlation length, ξ, or size of the regions and shows that ξ<111>~1.2ξ< 001>, consistent with the primary or dominant displacement of Pb leading to the low temperature rhombohedral phase. Both the appearance of the polar regions at T*and the structural transition at Tc are marked by kinks in the ξ< 111> curve but not in the ξ< 001> one, also indicating that the primary changes take place in a <111> direction at both temperatures.

cond-mat.mtrl-sci↗