ABSTRACT
We describe the growth of Zn(1-x)Mn(x)Se nanowires in ultrahigh vacuum seeded by Au nanodroplets. Electron microscopy reveals the formation of single-crystal c-axis wurtzite nanowires (typically 1-3 microm long) with Mn concentrations up to x approximately 0.6, accompanied by a dense horizontal undergrowth of shorter, crooked nanowires. Magnetophotoluminescence measurements show evidence for sp-d exchange effects in a reduced symmetry environment. We find that the optical emission is surprisingly dominated by the undergrowth of crooked nanowires.
Subject(s)
Magnetics , Manganese/chemistry , Nanowires/chemistry , Selenium/chemistry , Zinc/chemistry , Gold/chemistry , Materials Testing , Metal Nanoparticles/chemistry , Nanotechnology , Optics and Photonics , Particle Size , Surface PropertiesABSTRACT
Frustration, defined as a competition between interactions such that not all of them can be satisfied, is important in systems ranging from neural networks to structural glasses. Geometrical frustration, which arises from the topology of a well-ordered structure rather than from disorder, has recently become a topic of considerable interest. In particular, geometrical frustration among spins in magnetic materials can lead to exotic low-temperature states, including 'spin ice', in which the local moments mimic the frustration of hydrogen ion positions in frozen water. Here we report an artificial geometrically frustrated magnet based on an array of lithographically fabricated single-domain ferromagnetic islands. The islands are arranged such that the dipole interactions create a two-dimensional analogue to spin ice. Images of the magnetic moments of individual elements in this correlated system allow us to study the local accommodation of frustration. We see both ice-like short-range correlations and an absence of long-range correlations, behaviour which is strikingly similar to the low-temperature state of spin ice. These results demonstrate that artificial frustrated magnets can provide an uncharted arena in which the physics of frustration can be directly visualized.
ABSTRACT
Recurrent perineal Crohn's disease can be an extremely debilitating complication that may be difficult to treat. We report a patient with progressively worsening perineal and biopsy-proven cutaneous Crohn's disease that had been refractory to surgery and medical treatment (sulfasalazine, steroids, 6-mercaptopurine, metronidazole, antibiotics). As the lesions were reminiscent of problem wounds occurring in other situations, hyperbaric oxygen treatment was instituted while the patient was continued on metronidazole. Response was dramatic with almost immediate relief of symptoms and regression within 2.5 mo of wounds that had previously defied therapy for 8 yr. Clinical remission has not been sustained as four subsequent courses of hyperbaric oxygen have been given over a period of 11 mo. However, the patient has been essentially asymptomatic since her initial course and the extent of her cutaneous disease has been minimal compared with that before hyperbaric oxygen. Hyperbaric oxygen treatment is costly and should not be routinely used in every patient with perineal Crohn's disease. However, this case report may herald an advance in the understanding of the pathogenesis of this complication and ultimately, its therapy.
