Electric field effect modulation of transition temperature, mobile carrier density and in-plane penetration depth in NdBa2Cu3O(7-delta) thin films

We explore the relationship between the critical temperature, T_c, the mobile areal carrier density, n_2D, and the zero temperature magnetic in-plane penetration depth, lambda_ab(0), in very thin underdoped NdBa2Cu3O{7-delta} films near the superconductor to insulator transition using the electric field effect technique. We observe that T_c depends linearly on both, n_2D and lambda_ab(0), the signature of a quantum superconductor to insulator (QSI) transition in two dimensions with znu-bar where z is the dynamic and nu-bar the critical exponent of the in-plane correlation length.Comment: 4 pages, 4 figure

A consistent interpretation of the low temperature magneto-transport in graphite using the Slonczewski--Weiss--McClure 3D band structure calculations

Magnetotransport of natural graphite and highly oriented pyrolytic graphite (HOPG) has been measured at mK temperatures. Quantum oscillations for both electron and hole carriers are observed with orbital angular momentum quantum number up to $N\approx90$. A remarkable agreement is obtained when comparing the data and the predictions of the Slonczewski--Weiss--McClure tight binding model for massive fermions. No evidence for Dirac fermions is observed in the transport data which is dominated by the crossing of the Landau bands at the Fermi level, corresponding to $dE/dk_z=0$, which occurs away from the $H$ point where Dirac fermions are expected.Comment: 3 figure

Massive Infrared-Quiet Dense Cores: Unveiling the Initial Conditions of High-Mass Star Formation

As Pr. Th. Henning said at the conference, cold precursors of high-mass stars are now "hot topics". We here propose some observational criteria to identify massive infrared-quiet dense cores which can host the high-mass analogs of Class 0 protostars and pre-stellar condensations. We also show how far-infrared to millimeter imaging surveys of entire complexes forming OB stars are starting to unveil the initial conditions of high-mass star formation

NLTE spectroscopic analysis of the 3^3He anomaly in subluminous B-type stars

Several B-type main-sequence stars show chemical peculiarities. A particularly striking class are the $^3$He stars, which exhibit a remarkable enrichment of $^3$He with respect to $^4$He. This isotopic anomaly has also been found in blue horizontal branch (BHB) and subdwarf B (sdB) stars, which are helium-core burning stars of the extreme horizontal branch. Using a hybrid local/non-local thermodynamic equilibrium (LTE/NLTE) approach for B-type stars, we analyzed high-quality spectra of two known $^3$He BHBs and nine known $^3$He sdBs to determine their isotopic helium abundances and $^4$He/$^3$He abundance ratios. We redetermined their atmospheric parameters and analyzed selected neutral helium lines, including $\lambda$4922 $\mathring{A}$ and $\lambda$6678 $\mathring{A}$, which are very sensitive to $^4$He/$^3$He. Most of the $^3$He sdBs cluster in a narrow temperature strip between 26000 K and 30000 K and are helium deficient in accordance with previous LTE analyses. BD+48$^\circ$ 2721 is reclassified as a BHB star because of its low temperature ($T_{\mathrm{eff}}=$ 20700 K). Whereas $^4$He is almost absent ($^4$He/$^3$He$<$ 0.25) in most of the known $^3$He stars, other sample stars show abundance ratios up to $^4$He/$^3$He$=$2.51. A search for $^3$He stars in the ESO SPY survey led to the discovery of two new $^3$He sdB stars (HE 0929-0424 and HE 1047-0436). The observed helium line profiles of all BHBs and of three sdBs are not matched by chemically homogeneous atmospheres, but hint at vertical helium stratification. This phenomenon has been seen in other peculiar B-type stars, but is found for the first time for sdBs. We estimate helium to increase from the outer to the inner atmosphere by factors ranging from 1.4 (SB 290) up to 8.0 (BD+48$^\circ$ 2721).Comment: 19 pages, 79 figures submitted to Astronomy&Astrophysic

A Note on Asymptotic Freedom at High Temperatures

This short note considers, within the external field approach outlined in hep-ph/0202026, the role of the lowest lying gluon Landau mode in QCD in the high temperature limit. Its influence on a temperature- and field-dependent running coupling constant is examined. The thermal imaginary part of the mode is temperature-independent in our approach and exactly cancels the well-known zero temperature imaginary part, thus rendering the Savvidy vacuum stable. Combining the real part of the mode with the contributions from the higher lying Landau modes and the vacuum contribution, a field-independent coupling alpha_s(T) is obtained. It can be interpreted as the ordinary zero temperature running coupling constant with average thermal momenta \approx 2pi T for gluons and \approx pi T for quarks.Comment: 4 pages; minor changes, version to appear in Phys. Rev.

CII, CI, and CO in the massive star forming region W3 Main

We have used the KOSMA 3m telescope to map the core 7'x5' of the Galactic massive star forming region W3Main in the two fine structure lines of atomic carbon and four mid-J transitions of CO and 13CO. In combination with a map of singly ionized carbon (Howe et al. 1991), and FIR fine structure line data observed by ISO/LWS at the center position, these data sets allow to study in detail the physical structure of the photon dominated cloud interface regions (PDRs) where the occurance of carbon changes from CII to CI, and to CO.Comment: 4 pages, 4 figures, to appear in "Proceedings of the 4th Cologne-Bonn-Zermatt-Symposium, The dense interstellar medium in galaxies", eds. S. Pfalzner, C. Kramer, C. Straubmeier, and A. Heithausen (Springer Verlag

Graphite from the viewpoint of Landau level spectroscopy: An effective graphene bilayer and monolayer

We describe an infrared transmission study of a thin layer of bulk graphite in magnetic fields up to B = 34 T. Two series of absorption lines whose energy scales as sqrtB and B are present in the spectra and identified as contributions of massless holes at the H point and massive electrons in the vicinity of the K point, respectively. We find that the optical response of the K point electrons corresponds, over a wide range of energy and magnetic field, to a graphene bilayer with an effective inter-layer coupling 2\gamma_1, twice the value for a real graphene bilayer, which reflects the crystal ordering of bulk graphite along the c-axis. The K point electrons thus behave as massive Dirac fermions with a mass enhanced twice in comparison to a true graphene bilayer.Comment: 4 pages, 2 figure

DY determinants, possibly associated with novel class II molecules, stimulate autoreactive CD4+ T cells with suppressive activity

A set of T cell clones (TCC) isolated from HLA-DR-, Dw-, DQ-matched allogeneic MLCs was found to proliferate autonomously when stimulated with cells carrying a wide range of class I or II specificities. This apparently unrestricted proliferation was relatively weak, and only low levels of IL-2 were present in the supernatants of stimulated cells. Autologous as well as allogeneic PBMC and B lymphoblastoid cell lines (B-LCL) were capable of stimulating such clones, which were also restimulated by suppressive, but not by helper, TCC. Moreover, such clones displayed the unusual property of autostimulation. mAb inhibition experiments suggested that class II- or class II-restricted antigens were involved in stimulation. Thus, certain "broad" mAbs (TU39, SG520) reacting with multiple locus products inhibited activation of these reagents, but none of those reacting more specifically with DR (TU34, TU37, L243, Q2/70, SG157), DQ (TU22, SPV- L3, Leu 10), or DP (B7/21), or mixtures of these mAbs, were able to do so. Evidence from sequential immunoprecipitation experiments suggested that mAb TU39 bound class II-like molecules other than DR, DQ, and DP on TCC and B-LCL, and it is therefore proposed that such putative novel class II-like molecules may carry the stimulating determinants for these autoreactive clones. DY-reactive clones lacked helper activity for B cells but mediated potent suppressive activity on T cell proliferative responses that was not restricted by the HLA type of the responding cells. Suppressive activity was induced in normal PBMC by such clones, as well as by independent suppressive clones, which was also inhibited only by mAb TU39. These findings lead to the proposal that DY-reactive autostimulatory cells may constitute a self- maintaining suppressive circuit, the level of activity of which would be regulated primarily by the availability of IL-2 in the microenvironmen