251 research outputs found
Nonmetallic thermal transport in low-dimensional proximity structures with partially preserved time-reversal symmetry in a magnetic field
Gapped excitation spectra of Andreev states are studied in one- and
two-dimensional (1D and 2D) normal systems in superconducting contacts subject
to a parallel magnetic field. In the ballistic regime, a specific interplay
between magnetic field spin splitting and the effect of a screening
supercurrent is found to preserve time-reversal symmetry for certain groups of
Andreev states remaining gapped despite the presense of the magnetic field. In
1D wires such states can lead to a fractional thermal magnetoconductance equal
to half of the thermal conductance quantum. In 2D systems the thermal
magnetoconductance is also predicted to remain suppressed well below the
normal-state value in a wide range of magnetic fields.Comment: 21 pages, 7 figure
Ballistic quantum spin Hall state and enhanced edge backscattering in strong magnetic fields
The quantum spin Hall (QSH) state, observed in a zero magnetic field in HgTe
quantum wells, respects the time-reversal symmetry and is distinct from quantum
Hall (QH) states. We show that the QSH state persists in strong quantizing
fields and is identified by counter-propagating (helical) edge channels with
nonlinear dispersion inside the band gap. If the Fermi level is shifted into
the Landau-quantized conduction or valence band, we find a transition between
the QSH and QH regimes. Near the transition the longitudinal conductance of the
helical channels is strongly suppressed due to the combined effect of the
spectrum nonlinearity and enhanced backscattering. It shows a power-law decay
1/B^2N with magnetic field B, determined by the number of backscatterers on the
edge, N. This suggests a rather simple and practical way to probe the quality
of recently realized quasiballistic QSH devices using magnetoresistance
measurements.Comment: 4 pages, 3 figures, minor changes, accepted for publication in PR
Measuring Multijet Structure of Hadronic Energy Flow Or What IS A Jet?
Ambiguities of jet algorithms are reinterpreted as instability wrt small
variations of input. Optimal stability occurs for observables possessing
property of calorimetric continuity (C-continuity) predetermined by kinematical
structure of calorimetric detectors. The so-called C-correlators form a basic
class of such observables and fit naturally into QFT framework, allowing
systematic theoretical studies. A few rules generate other C-continuous
observables. The resulting C-algebra correctly quantifies any feature of
multijet structure such as the "number of jets" and mass spectra of "multijet
substates". The new observables are physically equivalent to traditional ones
but can be computed from final states bypassing jet algorithms which reemerge
as a tool of approximate computation of C-observables from data with all
ambiguities under analytical control and an optimal recombination criterion
minimizing approximation errors.Comment: PostScript, 94 pp (US Letter), 18 PS files, [email protected]
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