Excitonic instability and electric-field-induced phase transition towards a two dimensional exciton condensate

We present an InAs-GaSb-based system in which the electric-field tunability of its 2D energy gap implies a transition towards a thermodynamically stable excitonic condensed phase. Detailed calculations show a 3 meV BCS-like gap appearing in a second-order phase transition with electric field. We find this transition to be very sharp, solely due to exchange interaction, and so, the exciton binding energy is greatly renormalized even at small condensate densities. This density gradually increases with external field, thus enabling the direct probe of the Bose-Einstein to BCS crossover.Comment: LaTex, 11 pages, 3 ps figures, To appear in PR

Unusual development of light-reflecting pigment cells in intact and regenerating tail in the periodic albino mutant of Xenopus laevis

Unusual light-reflecting pigment cells, “white pigment cells”, specifically appear in the periodic albino mutant (ap/ap) of Xenopus laevis and localize in the same place where melanophores normally differentiate in the wild-type. The mechanism responsible for the development of unusual pigment cells is unclear. In this study, white pigment cells in the periodic albino were compared with melanophores in the wild-type, using a cell culture system and a tail-regenerating system. Observations of both intact and cultured cells demonstrate that white pigment cells are unique in (1) showing characteristics of melanophore precursors at various stages of development, (2) accumulating reflecting platelets characteristic of iridophores, and (3) exhibiting pigment dispersion in response to α-melanocyte stimulating hormone (α-MSH) in the same way that melanophores do. When a tadpole tail is amputated, a functionally competent new tail is regenerated. White pigment cells appear in the mutant regenerating tail, whereas melanophores differentiate in the wild-type regenerating tail. White pigment cells in the mutant regenerating tail are essentially similar to melanophores in the wild-type regenerating tail with respect to their localization, number, and response to α-MSH. In addition to white pigment cells, iridophores which are never present in the intact tadpole tail appear specifically in the somites near the amputation level in the mutant regenerating tail. Iridophores are distinct from white pigment cells in size, shape, blue light-induced fluorescence, and response to α-MSH. These findings strongly suggest that white pigment cells in the mutant arise from melanophore precursors and accumulate reflecting platelets characteristic of iridophores

Adenomatoid tumour of the liver

An unusual primary adenomatoid tumour arising in the normal liver is described. Hepatectomy was performed, and the patient is alive and free of disease 1 year postsurgery. Grossly, the tumour showed a haemorrhagic cut surface with numerous microcystic structures. Histological examination revealed cystic or angiomatoid spaces of various sizes lined by cuboidal, low-columnar, or flattened epithelioid cells with vacuolated cytoplasm and round to oval nuclei. The epithelioid cells were entirely supported by proliferated capillaries and arteries together with collagenous stroma. Immunohistochemical studies showed that the epithelioid cells were strongly positive for a broad spectrum of cytokeratins (AE1/AE3, CAM5.2, epithelial membrane antigen and cytokeratin 7) and mesothelial markers (calretinin, Wilms’ tumour 1 and D2-40). These cells were negative for Hep par-1, carcinoembryonic antigen, neural cell adhesion molecule, CD34, CD31 and HMB45. Atypically, abundant capillaries were observed; however, the cystic proliferation of epithelioid cells with vacuoles and immunohistochemical profile of the epithelioid element were consistent with hepatic adenomatoid tumour

Drag in paired electron-hole layers

We investigate transresistance effects in electron-hole double layer systems with an excitonic condensate. Our theory is based on the use of a minimum dissipation premise to fix the current carried by the condensate. We find that the drag resistance jumps discontinuously at the condensation temperature and diverges as the temperature approaches zero.Comment: 12 pages, 1 Figure, .eps file attache

Charged Many-Electron -- Single Hole Complexes in a Double Quantum Well near a Metal Plate

It has been shown that the presence of a metal plate near a double quantum well with spatially separated electron and hole layers may lead to a drastic reconstruction of the system state with the formation of stable charged complexes of several electrons bound to a spatially separated hole. Complexes of both the Fermi and the Bose statistics may coexist in the ground state and their relative densities may be changed with the change of the electron and hole densities. The stability of the charged complexes may be increased by an external magnetic field perpendicular to the layers plane.Comment: to appear in Phys.Rev.Lett. 77, No.7 (1996). 4 pages, RevTeX, 1 figur