396 research outputs found

    Quantum railroads and directed localization at the juncture of quantum Hall systems

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    The integer quantum Hall effect (QHE) and one-dimensional Anderson localization (AL) are limiting special cases of a more general phenomenon, directed localization (DL), predicted to occur in disordered one-dimensional wave guides called "quantum railroads" (QRR). Here we explain the surprising results of recent measurements by Kang et al. [Nature 403, 59 (2000)] of electron transfer between edges of two-dimensional electron systems and identify experimental evidence of QRR's in the general, but until now entirely theoretical, DL regime that unifies the QHE and AL. We propose direct experimental tests of our theory.Comment: 11 pages revtex + 3 jpeg figures, to appear in Phys. Rev.

    Structure and Stability of a Splat-Cooled Fe-P-C Alloy

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    The structure and the stability of a splat-cooled iron-based alloy (80Fe-13P-7C) were investigated by using electron and X-ray diffraction methods, high voltage electron microscopy and field ion microscopy. No appreciable change occurred in the transmission electron diffraction pattern as well as in the transmission electron microscope image by the irradiation of electrons being accelerated at any voltage in the range of 100 kV-1000 kV up to the total dose of 10^ electrons/cm^2. However, when the specimen was heated in-situ up to 200℃ under irradiation, the splitting of the second halo ring disappeared already at the total dose of 2×10^ electrons/cm^2, whereas such change did not occur in the unirradiated region of the specimen. Furthermore, crystallization was retarded in the irradiated region. Pole figure determination indicates no development of preferred orientation in the crystallization process of the splat-cooled alloy and also the cold rolled sheet of the alloy. The field ion micrographs obtained with the splat-cooled alloy tips were far different from the imaging ring patterns which were observed with the crystallized one and the data which support positively the existence of the microcrystalline phase have not been obtained so far (color superposition method is now being applied). The observations by dark field electron microscopy and lattice fringe electron microscopy made in parallel to FIM observation so far support the FIM observation

    Ballistic spin-polarized transport and Rashba spin precession in semiconductor nanowires

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    We present numerical calculations of the ballistic spin-transport properties of quasi-one-dimensional wires in the presence of the spin-orbit (Rashba) interaction. A tight-binding analog of the Rashba Hamiltonian which models the Rashba effect is used. By varying the robustness of the Rashba coupling and the width of the wire, weak and strong coupling regimes are identified. Perfect electron spin-modulation is found for the former regime, regardless of the incident Fermi energy and mode number. In the latter however, the spin-conductance has a strong energy dependence due to a nontrivial subband intermixing induced by the strong Rashba coupling. This would imply a strong suppression of the spin-modulation at higher temperatures and source-drain voltages. The results may be of relevance for the implementation of quasi-one-dimensional spin transistor devices.Comment: 19 pages (incl. 9 figures). To be published in PR

    Gauged Nambu-Jona-Lasinio model with extra dimensions

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    We investigate phase structure of the D (> 4)-dimensional gauged Nambu-Jona-Lasinio (NJL) model with ή(=D−4)\delta(=D-4) extra dimensions compactified on TeV scale, based on the improved ladder Schwinger-Dyson (SD) equation in the bulk. We assume that the bulk running gauge coupling in the SD equation for the SU(N_c) gauge theory with N_f massless flavors is given by the truncated Kaluza-Klein effective theory and hence has a nontrivial ultraviolet fixed point (UVFP). We find the critical line in the parameter space of two couplings, the gauge coupling and the four-fermion coupling, which is similar to that of the gauged NJL model with fixed (walking) gauge coupling in four dimensions. It is shown that in the presence of such walking gauge interactions the four-fermion interactions become ``nontrivial'' even in higher dimensions, similarly to the four-dimensional gauged NJL model. Such a nontriviality holds only in the restricted region of the critical line (``nontrivial window'') with the gauge coupling larger than a non-vanishing value (``marginal triviality (MT)'' point), in contrast to the four-dimensional case where such a nontriviality holds for all regions of the critical line except for the pure NJL point. In the nontrivial window the renormalized effective potential yields a nontrivial interaction which is conformal invariant. The exisitence of the nontrivial window implies ``cutoff insensitivity'' of the physics prediction in spite of the ultraviolet dominance of the dynamics. In the formal limit D -> 4, the nontrivial window coincides with the known condition of the nontriviality of the four-dimensional gauged NJL model, 9/(2Nc)<Nf−Nc<9/2Nc9/(2N_c) < N_f - N_c < 9/2 N_c.Comment: 34 pages, 6 figures, references added, to appear in Phys.Rev.D. The title is changed in PR

    Dynamical chiral symmetry breaking in gauge theories with extra dimensions

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    We investigate dynamical chiral symmetry breaking in vector-like gauge theories in DD dimensions with (D−4D-4) compactified extra dimensions, based on the gap equation (Schwinger-Dyson equation) and the effective potential for the bulk gauge theories within the improved ladder approximation. The non-local gauge fixing method is adopted so as to keep the ladder approximation consistent with the Ward-Takahashi identities. Using the one-loop MSˉ\bar{\rm MS} gauge coupling of the truncated KK effective theory which has a nontrivial ultraviolet fixed point (UV-FP) g∗g_* for the (dimensionless) bulk gauge coupling g^{\hat g}, we find that there exists a critical number of flavors, NfcritN_f^{\rm crit} (≃4.2,1.8\simeq 4.2, 1.8 for D=6,8D=6, 8 for SU(3) gauge theory): For Nf>NfcritN_f > N_f^{\rm crit}, the dynamical chiral symmetry breaking takes place not only in the ``strong-coupling phase'' (g^>g∗{\hat g} >g_*) but also in the ``weak-coupling phase'' (g^<g∗{\hat g} <g_*) when the cutoff is large enough. For Nf<NfcritN_f < N_f^{\rm crit}, on the other hand, only the strong-coupling phase is a broken phase and we can formally define a continuum (infinite cutoff) limit, so that the physics is insensitive to the cutoff in this case. We also perform a similar analysis using the one-loop ``effective gauge coupling''. We find the NfcritN_f^{\rm crit} turns out to be a value similar to that of the MSˉ\bar{\rm MS} case, notwithstanding the enhancement of the coupling compared with that of the MSˉ\bar{\rm MS}.Comment: REVTEX4, 38 pages, 18 figures. The abstract is shortened; version to be published in Phys. Rev.

    Gauge Dependence of Mass and Condensate in Chirally Asymmetric Phase of Quenched QED3

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    We study three dimensional quenched Quantum Electrodynamics in the bare vertex approximation. We investigate the gauge dependence of the dynamically generated Euclidean mass of the fermion and the chiral condensate for a wide range of values of the covariant gauge parameter Ο\xi. We find that (i) away from Ο=0\xi=0, gauge dependence of the said quantities is considerably reduced without resorting to sophisticated vertex {\em ansatze}, (ii) wavefunction renormalization plays an important role in restoring gauge invariance and (iii) the Ward-Green-Takahashi identity seems to increase the gauge dependence when used in conjunction with some simplifying assumptions. In the Landau gauge, we also verify that our results are in agreement with those based upon dimensional regularization scheme within the numerical accuracy available.Comment: 14 pages, 11 figures, uses revte

    Chiral Symmetry Breaking in Quenched Massive Strong-Coupling QED4_4

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    We present results from a study of subtractive renormalization of the fermion propagator Dyson-Schwinger equation (DSE) in massive strong-coupling quenched QED4_4. Results are compared for three different fermion-photon proper vertex {\it Ans\"{a}tze\/}: bare ÎłÎŒ\gamma^\mu, minimal Ball-Chiu, and Curtis-Pennington. The procedure is straightforward to implement and numerically stable. This is the first study in which this technique is used and it should prove useful in future DSE studies, whenever renormalization is required in numerical work.Comment: REVTEX 3.0, 15 pages plus 7 uuencoded PostScript figure

    Emulsion sheet doublets as interface trackers for the OPERA experiment

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    New methods for efficient and unambiguous interconnection between electronic counters and target units based on nuclear photographic emulsion films have been developed. The application to the OPERA experiment, that aims at detecting oscillations between mu neutrino and tau neutrino in the CNGS neutrino beam, is reported in this paper. In order to reduce background due to latent tracks collected before installation in the detector, on-site large-scale treatments of the emulsions ("refreshing") have been applied. Changeable Sheet (CSd) packages, each made of a doublet of emulsion films, have been designed, assembled and coupled to the OPERA target units ("ECC bricks"). A device has been built to print X-ray spots for accurate interconnection both within the CSd and between the CSd and the related ECC brick. Sample emulsion films have been extensively scanned with state-of-the-art automated optical microscopes. Efficient track-matching and powerful background rejection have been achieved in tests with electronically tagged penetrating muons. Further improvement of in-doublet film alignment was obtained by matching the pattern of low-energy electron tracks. The commissioning of the overall OPERA alignment procedure is in progress.Comment: 19 pages, 19 figure

    Measurement of the atmospheric muon charge ratio with the OPERA detector

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    The OPERA detector at the Gran Sasso underground laboratory (LNGS) was used to measure the atmospheric muon charge ratio in the TeV energy region. We analyzed 403069 atmospheric muons corresponding to 113.4 days of livetime during the 2008 CNGS run. We computed separately the muon charge ratio for single and for multiple muon events in order to select different energy regions of the primary cosmic ray spectrum and to test the charge ratio dependence on the primary composition. The measured charge ratio values were corrected taking into account the charge-misidentification errors. Data have also been grouped in five bins of the "vertical surface energy". A fit to a simplified model of muon production in the atmosphere allowed the determination of the pion and kaon charge ratios weighted by the cosmic ray energy spectrum.Comment: 14 pages, 10 figure

    The detection of neutrino interactions in the emulsion/lead target of the OPERA experiment

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    The OPERA neutrino detector in the underground Gran Sasso Laboratory (LNGS) was designed to perform the first detection of neutrino oscillations in appearance mode through the study of ΜΌ→Μτ\nu_\mu\to\nu_\tau oscillations. The apparatus consists of an emulsion/lead target complemented by electronic detectors and it is placed in the high energy long-baseline CERN to LNGS beam (CNGS) 730 km away from the neutrino source. Runs with CNGS neutrinos were successfully carried out in 2007 and 2008 with the detector fully operational with its related facilities for the emulsion handling and analysis. After a brief description of the beam and of the experimental setup we report on the collection, reconstruction and analysis procedures of first samples of neutrino interaction events
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