74,738 research outputs found
Nonsymmorphic symmetry-required band crossings in topological semimetals
We show that for two-band systems nonsymmorphic symmetries may enforce the
existence of band crossings in the bulk, which realize Fermi surfaces of
reduced dimensionality. We find that these unavoidable crossings originate from
the momentum dependence of the nonsymmorphic symmetry, which puts strong
restrictions on the global structure of the band configurations. Three
different types of nonsymmorphic symmetries are considered: (i) a unitary
nonsymmorphic symmetry, (ii) a nonsymmorphic magnetic symmetry, and (iii) a
nonsymmorphic symmetry combined with inversion. For nonsymmorphic symmetries of
the latter two types, the band crossings are located at high-symmetry points of
the Brillouin zone, with their exact positions being determined by the algebra
of the symmetry operators. To characterize these band degeneracies we introduce
a \emph{global} topological charge and show that it is of type,
which is in contrast to the \emph{local} topological charge of Fermi points in,
say, Weyl semimetals. To illustrate these concepts, we discuss the -flux
state as well as the SSH model at its critical point and show that these two
models fit nicely into our general framework of nonsymmorphic two-band systems.Comment: 6.5 pages, 4 figure
Breakdown of QCD Factorization for P-Wave Quarkonium Production at Low Transverse Momentum
Quarkonium production at low transverse momentum in hadron collisions can be
used to extract Transverse-Momentum-Dependent(TMD) gluon distribution
functions, if TMD factorization holds there. We show that TMD factorization for
the case of P-wave quarkonium with holds at one-loop
level, but is violated beyond one-loop level. TMD factorization for other
P-wave quarkonium is also violated already at one-loop.Comment: Published version in Physics Letters B (2014), pp. 103-10
Transverse Momentum Dependent Factorization for Quarkonium Production at Low Transverse Momentum
Quarkonium production in hadron collisions at low transverse momentum
with as the quarkonium mass can be used for probing
transverse momentum dependent (TMD) gluon distributions. For this purpose, one
needs to establish the TMD factorization for the process. We examine the
factorization at the one-loop level for the production of or .
The perturbative coefficient in the factorization is determined at one-loop
accuracy. Comparing the factorization derived at tree level and that beyond the
tree level, a soft factor is, in general, needed to completely cancel soft
divergences. We have also discussed possible complications of TMD factorization
of p-wave quarkonium production.Comment: Title changed in the journal, published versio
Large magnetothermal conductivity of HoMnO_3 single crystals and its relation to the magnetic-field induced transitions of magnetic structure
We study the low-temperature heat transport of HoMnO_3 single crystals to
probe the magnetic structures and their transitions induced by magnetic field.
It is found that the low-T thermal conductivity (\kappa) shows very strong
magnetic-field dependence, with the strongest suppression of nearly 90% and the
biggest increase of 20 times of \kappa compared to its zero-field value. In
particular, some ``dip"-like features show up in \kappa(H) isotherms for field
along both the ab plane and the c axis. These behaviors are found to shed new
light on the complex H-T phase diagram and the field-induced re-orientations of
Mn^{3+} and Ho^{3+} spin structures. The results also demonstrate a significant
spin-phonon coupling in this multiferroic compound.Comment: 5 pages, 4 figures, accepted for publication in Phys. Rev.
Enhanced spin-orbit torques in MnAl/Ta films with improving chemical ordering
We report the enhancement of spin-orbit torques in MnAl/Ta films with
improving chemical ordering through annealing. The switching current density is
increased due to enhanced saturation magnetization MS and effective anisotropy
field HK after annealing. Both damplinglike effective field HD and fieldlike
effective field HF have been increased in the temperature range of 50 to 300 K.
HD varies inversely with MS in both of the films, while the HF becomes liner
dependent on 1/MS in the annealed film. We infer that the improved chemical
ordering has enhanced the interfacial spin transparency and the transmitting of
the spin current in MnAl layer
Unified theory of PT and CP invariant topological metals and nodal superconductors
As PT and CP symmetries are fundamental in physics, we establish a unified
topological theory of PT and CP invariant metals and nodal superconductors,
based on the mathematically rigorous theory. Representative models are
constructed for all nontrivial topological cases in dimensions , and
, with their exotic physical meanings being elucidated in detail.
Intriguingly, it is found that the topological charges of Fermi surfaces in the
bulk determine an exotic direction-dependent distribution of topological subgap
modes on the boundaries. Furthermore, by constructing an exact bulk-boundary
correspondence, we show that the topological Fermi points of the PT and CP
invariant classes can appear as gapless modes on the boundary of topological
insulators with a certain type of anisotropic crystalline symmetry.Comment: 4 pages, 1 figure and supplemental material, to be published in PR
- …