25,651 research outputs found
The kHz QPOs as a probe of the X-ray color-color diagram and accretion-disk structure for the atoll source 4U 1728-34
We have taken the kHz QPOs as a tool to probe the correlation between the
tracks of X-ray color-color diagram (CCD) and magnetosphere-disk positions for
the atoll source 4U 1728-34, based on the assumptions that the upper kHz QPO is
ascribed to the Keplerian orbital motion and the neutron star (NS)
magnetosphere is defined by the dipole magnetic field. We find that from the
island to the banana state, the inner accretion disk gradually approaches the
NS surface with the radius decreasing from r ~33.0km to ~15.9 km, corresponding
to the magnetic field from B(r) ~4.8*10^6 G to ~4.3*10^7 G. In addition, we
note the characteristics of some particular radii of magnetosphere-disk -r are:
firstly, the whole atoll shape of the CCD links the disk radius range of ~15.9
- 33.0 km, which is just located inside the corotation radius of 4U 1728-34
-r_co ( ~34.4 km), implying that the CCD shape is involved in the NS spin-up
state. Secondly, the island and banana states of CCD correspond to the two
particular boundaries: (I)-near the corotation radius at r ~27.2 - 33.0 km,
where the source lies in the island state; (II)-near the NS surface at r ~15.9
- 22.3 km, where the source lies in both the island and banana states. Thirdly,
the vertex of the atoll shape in CCD, where the radiation transition from the
hard to soft photons occurs, is found to be near the NS surface at r ~16.4 km.
The above results suggest that both the magnetic field and accretion
environment are related to the CCD structure of atoll track, where the
corotation radius and NS hard surface play the significant roles in the
radiation distribution of atoll source.Comment: 6 pages, 3 figures, 1 table, accepted by Astronomy & Astrophysic
Nonlinear Hall Effects in Strained Twisted Bilayer WSe
Recently, it has been pointed out that the twisting of bilayer WSe would
generate topologically non-trivial flat bands near the Fermi energy. In this
work, we show that twisted bilayer WSe (tWSe) with uniaxial strain
exhibits a large nonlinear Hall (NLH) response due to the non-trivial Berry
curvatures of the flat bands. Moreover, the NLH effect is greatly enhanced near
the topological phase transition point which can be tuned by a vertical
displacement field. Importantly, the nonlinear Hall signal changes sign across
the topological phase transition point and provides a way to identify the
topological phase transition and probe the topological properties of the flat
bands. The strong enhancement and high tunability of the NLH effect near the
topological phase transition point renders tWSe and related moire materials
new platforms for rectification and second harmonic generations.Comment: 5 pages, 3 figures. Comments are welcom
- …