10,443 research outputs found
Global phase diagram of a spin-orbit-coupled Kondo lattice model on the honeycomb lattice
Motivated by the growing interest in the novel quantum phases in materials
with strong electron correlations and spin-orbit coupling, we study the
interplay between the spin-orbit coupling, Kondo interaction, and magnetic
frustration of a Kondo lattice model on a two-dimensional honeycomb lattice. We
calculate the renormalized electronic structure and correlation functions at
the saddle point based on a fermionic representation of the spin operators. We
find a global phase diagram of the model at half-filling, which contains a
variety of phases due to the competing interactions. In addition to a Kondo
insulator, there is a topological insulator with valence bond solid
correlations in the spin sector, and two antiferromagnetic phases. Due to a
competition between the spin-orbit coupling and Kondo interaction, the
direction of the magnetic moments in the antiferromagnetic phases can be either
within or perpendicular to the lattice plane. The latter antiferromagnetic
state is topologically nontrivial for moderate and strong spin-orbit couplings.Comment: 8 pages, 9 figure
STM Study of Quantum Hall Isospin Ferromagnetic States of Zero Landau Level in Graphene Monolayer
A number of quantum Hall isospin ferromagnetic (QHIFM) states have been
predicted in the relativistic zero Landau level (LL) of graphene monolayer.
These states, especially the states at LL filling factor v = 0 of
charge-neutral graphene, have been extensively explored in experiment. To date,
identification of these high-field broken-symmetry states has mostly relied on
macroscopic transport techniques. Here, we study splitting of the zero LL of
graphene at partial filling and demonstrate a direct approach by imaging the
QHIFM states at atomic scale with a scanning tunneling microscope. At half
filling of the zero LL (v = 0), the system is in a spin unpolarized state and
we observe a linear magnetic-field-scaling of valley splitting. Simultaneously,
the spin degeneracy in the two valleys is also lifted by the magnetic fields.
When the Fermi level lies inside the spin-polarized states (at v = 1 or -1),
the spin splitting is dramatically enhanced because of the strong many-body
effects. At v = 0, we direct image the wavefunctions of the QHIFM states at
atomic scale and observe an interaction-driven density wave featuring a Kekule
distortion, which is responsible for the large gap at charge neutrality point
in high magnetic fields.Comment: 3 figures in main tex
Cosmic Reionization Study : Principle Component Analysis After Planck
The study of reionization history plays an important role in understanding
the evolution of our universe. It is commonly believed that the intergalactic
medium (IGM) in our universe are fully ionized today, however the reionizing
process remains to be mysterious. A simple instantaneous reionization process
is usually adopted in modern cosmology without direct observational evidence.
However, the history of ionization fraction, will influence cosmic
microwave background (CMB) observables and constraints on optical depth .
With the mocked future data sets based on featured reionization model, we find
the bias on introduced by instantaneous model can not be neglected. In
this paper, we study the cosmic reionization history in a model independent
way, the so called principle component analysis (PCA) method, and reconstruct
at different redshift with the data sets of Planck, WMAP 9 years
temperature and polarization power spectra, combining with the baryon acoustic
oscillation (BAO) from galaxy survey and type Ia supernovae (SN) Union 2.1
sample respectively. The results show that reconstructed is consistent
with instantaneous behavior, however, there exists slight deviation from this
behavior at some epoch. With PCA method, after abandoning the noisy modes, we
get stronger constraints, and the hints for featured evolution could
become a little more obvious.Comment: 12 pages, 10 figure
Primordial Gravitational Waves Measurements and Anisotropies of CMB Polarization Rotation
Searching for the signal of primordial gravitational waves in the B-modes
(BB) power spectrum is one of the key scientific aims of the cosmic microwave
background (CMB) polarization experiments. However, this could be easily
contaminated by several foreground issues, such as the thermal dust emission.
In this paper we study another mechanism, the cosmic birefringence, which can
be introduced by a CPT-violating interaction between CMB photons and an
external scalar field. Such kind of interaction could give rise to the rotation
of the linear polarization state of CMB photons, and consequently induce the
CMB BB power spectrum, which could mimic the signal of primordial gravitational
waves at large scales. With the recent polarization data of BICEP2 and the
joint analysis data of BICEP2/Keck Array and Planck, we perform a global
fitting analysis on constraining the tensor-to-scalar ratio by considering
the polarization rotation angle which can be separated into a background
isotropic part and a small anisotropic part. Since the data of BICEP2 and Keck
Array experiments have already been corrected by using the "self-calibration"
method, here we mainly focus on the effects from the anisotropies of CMB
polarization rotation angle. We find that including the anisotropies in the
analysis could slightly weaken the constraints on , when using current CMB
polarization measurements. We also simulate the mock CMB data with the
BICEP3-like sensitivity. Very interestingly, we find that if the effects of the
anisotropic polarization rotation angle can not be taken into account properly
in the analysis, the constraints on will be dramatically biased. This
implies that we need to break the degeneracy between the anisotropies of the
CMB polarization rotation angle and the CMB primordial tensor perturbations, in
order to measure the signal of primordial gravitational waves accurately.Comment: 7 pages, 5 figure
Tibets Window on Primordial Gravitational Waves
As an essential part of China’s Gravitational Waves Program, the Ali CMB
Polarization Telescope (AliCPT) is a ground-based experiment aiming at the
Primordial Gravitational Waves (PGWs) by measuring B-mode polarization of
Cosmic Microwave Background (CMB). First proposed in 2014 and currently in fast
construction phase, AliCPT is China’s first CMB project that plans for
commissioning in 2019. Led by the Institute of High Energy Physics (IHEP) under
the Chinese Academy of Sciences (CAS), the project is a worldwide collaboration
of more than fifteen universities and research institutes. Ali CMB Project is
briefly introduced
Testing CPT Symmetry with Current and Future CMB Measurements
In this paper we use the current and future cosmic microwave background (CMB)
experiments to test the Charge-Parity-Time Reversal (CPT) symmetry. We consider
a CPT-violating interaction in the photon sector which gives rise to a rotation of the
polarization vectors of the propagating CMB photons. By combining the nine-year
WMAP, BOOMERanG 2003 and BICEP1 observations, we obtain the current constraint
on the isotropic rotation angle (),
indicating an about significance of the CPT violation. Here, we
particularly take the systematic errors of CMB measurements into account. Then,
we study the effects of the anisotropies of the rotation angle
[] on the CMB polarization power spectra in
detail. Due to the small effects, the current CMB polarization data can not
constrain the related parameters very well. We obtain the 95\% C.L. upper limit
of the variance of the anisotropies of the rotation angle
from all the CMB datasets. More interestingly, including the anisotropies of
rotation angle could lower the best fit value of and relax the tension on
the constraints of between BICEP2 and Planck. Finally, we investigate the
capabilities of future Planck polarization measurements on and
. Benefited from the high precision of Planck
data, the constraints of the rotation angle can be significantly improved.Comment: 8 pages, 4 figures, 1 table, ApJ in press. arXiv admin note: text
overlap with arXiv:1201.445
Constraints on dark matter interactions from the first results of DarkSide-50
In an extended effective operator framework of isospin violating interactions
with light mediators, we investigate the compatibility of the candidate signal
of the CDMS-II-Si with the latest constraints from DarkSide-50 and XENON-1T,
etc. We show that the constraints from DarkSide-50 which utilizes Argon as the
target is complementary to that from XENON-1T which utilizes Xenon. Combining
the results of the two experiments, we find that for isospin violating
interaction with light mediator there is no parameter space which can be
compatible with the positive signals from CDMS-II-Si. As a concrete example of
this framework, we investigate the dark photon model in detail. We obtain the
combined limits on the dark matter mass , the dark photon mass
, and the kinetic mixing parameter in the dark photon
model. The DarkSide-50 gives more stringent upper limits in the region of
mediator mass from 0.001 to 1 GeV, for GeV in the
(,) plane, and more stringent constraints for
GeV and in the
(,) plane.Comment: 22 pages, 5 figure
Constraints on Dark Energy from New Observations including Pan-STARRS
In this paper, we set the new limits on the equation of state parameter (EoS)
of dark energy with the observations of cosmic microwave background radiation
(CMB) from Planck satellite, the type Ia supernovae from Pan-STARRS and the
baryon acoustic oscillation (BAO). We consider two parametrization forms of
EoS: a constant and time evolving . The results show
that with a constant EoS, (), which is
consistent with CDM at about confidence level. For a time
evolving model, we get (),
(), and in this case CDM can
be comparable with our observational data at confidence level. In
order to do the parametrization independent analysis, additionally we adopt the
so called principal component analysis (PCA) method, in which we divide
redshift range into several bins and assume as a constant in each redshift
bin (bin-w). In such bin-w scenario, we find that for most of the bins
cosmological constant can be comparable with the data, however, there exists
few bins which give deviating from CDM at more than
confidence level, which shows a weak hint for the time evolving behavior of
dark energy. To further confirm this hint, we need more data with higher
precision.Comment: 9 pages, 8 figures, 1 tabl
Type-II topological metals
Topological metals (TMs) are a kind of special metallic materials, which
feature nontrivial band crossings near the Fermi energy, giving rise to
peculiar quasiparticle excitations. TMs can be classified based on the
characteristics of these band crossings. For example, according to the
dimensionality of the crossing, TMs can be classified into nodal-point,
nodal-line, and nodal-surface metals. Another important property is the type of
dispersion. According to degree of the tilt of the local dispersion around the
crossing, we have type-I and type-II dispersions. This leads to significant
distinctions in the physical properties of the materials, owing to their
contrasting Fermi surface topologies. In this article, we briefly review the
recent advances in this research direction, focusing on the concepts, the
physical properties, and the material realizations of the type-II nodal-point
and nodal-line TMs.Comment: 12 pages, 16 figure
Localization Trajectory and Chern-Simons axion coupling for Bilayer Quantum Anomalous Hall Systems
Quantum anomalous Hall (QAH) multilayers provide a platform of topological
materials with high Chern numbers. We investigate the localization routes of
bilayer QAH systems with Chern number C = 2 under strong disorder, by numerical
simulations on their quantum transport properties and the Chern-Simons axion
coupling. Compared to the single layer counterpart with C = 2, the localization
trajectories present much richer behaviors, for example, the existence of the
stable intermediate state with C = 1 can be tuned by model parameters. This
state was always unstable in the single layer case. Furthermore, the two
parameter scaling trajectories also exhibit multiple patterns, some of which
were not captured by the standard Pruisken picture. During the process towards
localization, the Chern-Simons axion coupling shows a surprisingly remarkable
peak which is even higher and sharper in the large size limit. Therefore the
disordered bilayer QAH system can be a good candidate for this nontrivial
magnetoelectric coupling mediated by orbital motions.Comment: 11 pages, 11 figure
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