619 research outputs found
Quantum Hall Effect of Massless Dirac Fermions in a Vanishing Magnetic Field
The effect of strong long-range disorder on the quantization of the Hall
conductivity in graphene is studied numerically. It is shown that
increasing Landau-level mixing progressively destroys all plateaus in
except the plateaus at (per valley and
per spin). The critical state at the charge-neutral Dirac point is robust to
strong disorder and belongs to the universality class of the conventional
plateau transitions in the integer quantum Hall effect. We propose that the
breaking of time-reversal symmetry by ripples in graphene can realize this
quantum critical point in a vanishing magnetic field.Comment: 5 pages, 4 figure
Electromagnetic and thermal responses in topological matter: topological terms, quantum anomalies and D-branes
We discuss the thermal (or gravitational) responses in topological
superconductors and in topological phases in general. Such thermal responses
(as well as electromagnetic responses for conserved charge) provide a
definition of topological insulators and superconductors beyond the
single-particle picture. In two-dimensional topological phases, the Str\v{e}da
formula for the electric Hall conductivity is generalized to the thermal Hall
conductivity. Applying this formula to the Majorana surface states of
three-dimensional topological superconductors predicts cross-correlated
responses between the angular momentum and thermal polarization (entropy
polarization). We also discuss a use of D-branes in string theory as a
systematic tool to derive all such topological terms and topological responses.
In particular, we relate the index of topological insulators
introduced by Kane and Mele (and its generalization to other symmetry classes
and to arbitrary dimensions) to the K-theory charge of non-BPS D-branes, and
vice versa. We thus establish a link between the stability of non-BPS D-branes
and the topological stability of topological insulators.Comment: 16 pages, 2 figures; Submitted to a topical issue of the Comptes
Rendus de l Academie des Sciences (CRAS
Physical properties of portland cement based concrete exposed at a depth of 3520 m in the Nankai Trough
Concrete is widely used in large-scale construction of submarine infrastructure because of its high strength, durability, and ease of handling. However, knowledge of its durability in deep seawater is lacking. In the deep sea, materials are exposed to high pressures and low temperatures, which may cause early deterioration of concrete over time. Concrete materials may also be affected by the chemical composition of seawater, which induces the leaching of calcium. In situ exposure tests are therefore important for understanding degradation processes in the deep sea. In this study, Portland cement based concrete specimens were placed at a depth of 3520 m on the northern edge of the Nankai Trough in 2018 and retrieved in 2019, in the deepest exposure testing conducted to date. Here we provide an outline of the tests, describe the physical properties of materials exposed to deep seawater, freshwater, and air, and discuss possible concrete degradation mechanisms
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