Application of selective epitaxy to fabrication of nanometer scale wire and dot structures

The selective growth of nanometer scale GaAs wire and dot structures using metalorganic vapor phase epitaxy is demonstrated. Spectrally resolved cathodoluminescence images as well as spectra from single dots and wires are presented. A blue shifting of the GaAs peak is observed as the size scale of the wires and dots decreases

Facet modulation selective epitaxy–a technique for quantum-well wire doublet fabrication

The technique of facet modulation selective epitaxy and its application to quantum-well wire doublet fabrication are described. Successful fabrication of wire doublets in the AlxGa1–xAs material system is achieved. The smallest wire fabricated has a crescent cross section less than 140 Å thick and less than 1400 Å wide. Backscattered electron images, transmission electron micrographs, cathodoluminescence spectra, and spectrally resolved cathodoluminescence images of the wire doublets are presented

Unconventional Spin Density Waves in Dipolar Fermi Gases

The conventional spin density wave (SDW) phase (Overhauser, 1962), as found in antiferromagnetic metal for example (Fawcett 1988), can be described as a condensate of particle-hole pairs with zero angular momentum, $\ell=0$, analogous to a condensate of particle-particle pairs in conventional superconductors. While many unconventional superconductors with Cooper pairs of finite $\ell$ have been discovered, their counterparts, density waves with non-zero angular momenta, have only been hypothesized in two-dimensional electron systems (Nayak, 2000). Using an unbiased functional renormalization group analysis, we here show that spin-triplet particle-hole condensates with $\ell=1$ emerge generically in dipolar Fermi gases of atoms (Lu, Burdick, and Lev, 2012) or molecules (Ospelkaus et al., 2008; Wu et al.) on optical lattice. The order parameter of these exotic SDWs is a vector quantity in spin space, and, moreover, is defined on lattice bonds rather than on lattice sites. We determine the rich quantum phase diagram of dipolar fermions at half-filling as a function of the dipolar orientation, and discuss how these SDWs arise amidst competition with superfluid and charge density wave phases.Comment: 5 pages, 3 figure

Self-Consistent Hopping Theory of Activated Relaxation and Diffusion of Dilute Penetrants in Dense Crosslinked Polymer Networks

We generalize and apply a microscopic force-level statistical mechanical theory of the activated dynamics of dilute spherical penetrants in glass-forming liquids to study the influence of permanent crosslinking in polymer networks on the penetrant relaxation time and diffusivity over a wide range of temperature and crosslink density. Calculations are performed for model parameters relevant to recent experimental studies of an nm-sized organic molecule diffusing in crosslinked PnBA networks. The theory predicts the penetrant alpha relaxation time increases exponentially with the crosslink fraction ($f_n$) dependent glass transition temperature, $T_g$, which grows roughly linearly with the square root of $f_n$. Moreover, $T_g$ is also found to be proportional to a geometric confinement parameter defined as the ratio of the penetrant diameter to the mean network mesh size. The decoupling ratio of the penetrant to polymer Kuhn segment alpha relaxation times displays a complex non-monotonic dependence on crosslink density and temperature that can be well collapsed based on the variable $T_g(f_n)/T$. The microscopic mechanism for activated penetrant relaxation is elucidated and a model for the penetrant diffusion constant that combines activated segmental dynamics and entropic mesh confinement is proposed which results in a significantly stronger suppression of mass transport with degree of effective supercooling than predicted for the penetrant alpha time. This behavior corresponds to a new polymer network-based type of decoupling of diffusion and relaxation. In contrast to the diffusion of larger nanoparticles in high temperature rubbery networks, our analysis in the deeply supercooled regime suggests that for the penetrants studied the mesh confinement effects are of secondary importance relative to the consequences of crosslink-induced slowing down of glassy activated relaxation.Comment: 42 pages and 14 figure

How Segmental Dynamics and Mesh Confinement Determine the Selective Diffusivity of Molecules in Crosslinked Dense Polymer Networks

The diffusion of molecules (penetrants) of variable size, shape, and chemistry through dense crosslinked polymer networks is a fundamental scientific problem that is broadly relevant in materials, polymer, physical and biological chemistry. Relevant applications include molecular separations in membranes, barrier materials for coatings, drug delivery, and nanofiltration. A major open question is the relationship between molecular transport, thermodynamic state, and chemical structure of the penetrant and polymeric media. Here we address this question by combining experiment, simulation, and theory to unravel the competing effects of penetrant chemistry on its transport in rubbery and supercooled polymer permanent networks over a wide range of crosslink densities, size ratios, and temperatures. The crucial importance of the coupling of local penetrant hopping to the polymer structural relaxation process, and the secondary importance of geometric mesh confinement effects, are established. Network crosslinks induce a large slowing down of nm-scale polymer relaxation which greatly retards the rate of penetrant activated relaxation. The demonstrated good agreement between experiment, simulation, and theory provides strong support for the size ratio variable (effective penetrant diameter to the polymer Kuhn length) as a key variable, and the usefulness of coarse-grained simulation and theoretical models that average over Angstrom scale chemical details. The developed microscopic theory provides a fundamental understanding of the physical processes underlying the behaviors observed in experiment and simulation. Penetrant transport is theoretically predicted to become even more size sensitive in a more deeply supercooled regime not probed in our present experiments or simulations, which suggests new strategies for enhancing selective polymer membrane design.Comment: including a main text and a supporting information. For the main text, 28 pages and 5 figures; For the supporting information, 23 pages and 14 figures. Totally, 51 pages and 19 figure

Bond order solid of two-dimensional dipolar fermions

Recent experimental realization of dipolar Fermi gases near or below quantum degeneracy provides opportunity to engineer Hubbard-like models with long range interactions. Motivated by these experiments, we chart out the theoretical phase diagram of interacting dipolar fermions on the square lattice at zero temperature and half filling. We show that in addition to p-wave superfluid and charge density wave order, two new and exotic types of bond order emerge generically in dipolar fermion systems. These phases feature homogeneous density but periodic modulations of the kinetic hopping energy between nearest or next-nearest neighbors. Similar, but manifestly different, phases of two-dimensional correlated electrons have previously only been hypothesized and termed "density waves of nonzero angular momentum". Our results suggest that these phases can be constructed flexibly with dipolar fermions, using currently available experimental techniques.Comment: 5 pages, 3 figures, supplementary material also included; to appear in Phys. Rev. Lett. (in press

Quantum wire and quantum dot semiconductor lasers

There is currently great interest in fabrication of structures that are two and three dimensional analogs of the conventional quantum well. We review here the physics behind the use of arrays of such lower dimensional structures in semiconductor laser active layers. Methods which are currently under investigation for producing such structures will be discussed

DHODH modulates transcriptional elongation in the neural crest and melanoma

Melanoma is a tumour of transformed melanocytes, which are originally derived from the embryonic neural crest. It is unknown to what extent the programs that regulate neural crest development interact with mutations in the BRAF oncogene, which is the most commonly mutated gene in human melanoma1. We have used zebrafish embryos to identify the initiating transcriptional events that occur on activation of human BRAF(V600E) (which encodes an amino acid substitution mutant of BRAF) in the neural crest lineage. Zebrafish embryos that are transgenic for mitfa:BRAF(V600E) and lack p53 (also known as tp53) have a gene signature that is enriched for markers of multipotent neural crest cells, and neural crest progenitors from these embryos fail to terminally differentiate. To determine whether these early transcriptional events are important for melanoma pathogenesis, we performed a chemical genetic screen to identify small-molecule suppressors of the neural crest lineage, which were then tested for their effects on melanoma. One class of compound, inhibitors of dihydroorotate dehydrogenase (DHODH), for example leflunomide, led to an almost complete abrogation of neural crest development in zebrafish and to a reduction in the self-renewal of mammalian neural crest stem cells. Leflunomide exerts these effects by inhibiting the transcriptional elongation of genes that are required for neural crest development and melanoma growth. When used alone or in combination with a specific inhibitor of the BRAF(V600E) oncogene, DHODH inhibition led to a marked decrease in melanoma growth both in vitro and in mouse xenograft studies. Taken together, these studies highlight developmental pathways in neural crest cells that have a direct bearing on melanoma formation