59,072 research outputs found
Symmetry protected topological orders of 1D spin systems with D2+T symmetry
In [Z.-X. Liu, M. Liu, X.-G. Wen, arXiv:1101.5680], we studied 8 gapped
symmetric quantum phases in S=1 spin chains %/ladders which respect a discrete
spin rotation and time reversal symmetries. In this
paper, using a generalized approach, we study all the 16 possible gapped
symmetric quantum phases of 1D integer spin systems with only symmetry.
Those phases are beyond Landau symmetry breaking theory and cannot be
characterized by local order parameters, since they do not break any symmetry.
They correspond to 16 symmetry protected topological (SPT) orders. We show that
all the 16 SPT orders can be fully characterized by the physical properties of
the symmetry protected degenerate boundary states (end `spins') at the ends of
a chain segment. So we can measure and distinguish all the 16 SPT orders
experimentally. We also show that all these SPT orders can be realized in S=1
spin ladder models. The gapped symmetric phases protected by subgroups of
are also studied. Again, all these phases can be distinguished by
physically measuring their end `spins'.Comment: 10+page
Identification of the CRP regulon using in vitro and in vivo transcriptional profiling
The Escherichia coli cyclic AMP receptor protein (CRP) is a global regulator that controls transcription initiation from more than 100 promoters by binding to a specific DNA sequence within cognate promoters. Many genes in the CRP regulon have been predicted simply based on the presence of DNA-binding sites within gene promoters. In this study, we have exploited a newly developed technique, run-off transcription/microarray analysis (ROMA) to define CRP-regulated promoters. Using ROMA, we identified 176 operons that were activated by CRP in vitro and 16 operons that were repressed. Using positive control mutants in different regions of CRP, we were able to classify the different promoters into class I or class II/III. A total of 104 operons were predicted to contain Class II CRP-binding sites. Sequence analysis of the operons that were repressed by CRP revealed different mechanisms for CRP inhibition. In contrast, the in vivo transcriptional profiles failed to identify most CRP-dependent regulation because of the complexity of the regulatory network. Analysis of these operons supports the hypothesis that CRP is not only a regulator of genes required for catabolism of sugars other than glucose, but also regulates the expression of a large number of other genes in E.coli. ROMA has revealed 152 hitherto unknown CRP regulons
Complete classification of 1D gapped quantum phases in interacting spin systems
Quantum phases with different orders exist with or without breaking the
symmetry of the system. Recently, a classification of gapped quantum phases
which do not break time reversal, parity or on-site unitary symmetry has been
given for 1D spin systems in [X. Chen, Z.-C. Gu, and X.-G. Wen, Phys. Rev. B
\textbf{83}, 035107 (2011); arXiv:1008.3745]. It was found that, such symmetry
protected topological (SPT) phases are labeled by the projective
representations of the symmetry group which can be viewed as a symmetry
fractionalization. In this paper, we extend the classification of 1D gapped
phases by considering SPT phases with combined time reversal, parity, and/or
on-site unitary symmetries and also the possibility of symmetry breaking. We
clarify how symmetry fractionalizes with combined symmetries and also how
symmetry fractionalization coexists with symmetry breaking.
In this way, we obtain a complete classification of gapped quantum phases in
1D spin systems. We find that in general, symmetry fractionalization, symmetry
breaking and long range entanglement(present in 2 or higher dimensions)
represent three main mechanisms to generate a very rich set of gapped quantum
phases. As an application of our classification, we study the possible SPT
phases in 1D fermionic systems, which can be mapped to spin systems by
Jordan-Wigner transformation.Comment: 15 pages, 3 figure
Synthesis of monodispersed Ag-doped bioactive glass nanoparticles via surface modification
© 2016 by the authors.Monodispersed spherical Ag-doped bioactive glass nanoparticles (Ag-BGNs) were synthesized by a modified Stöber method combined with surface modification. The surface modification was carried out at 25, 60, and 80 °C, respectively, to investigate the influence of processing temperature on particle properties. Energy-dispersive X-ray spectroscopy (EDS) results indicated that higher temperatures facilitate the incorporation of Ag. Hydroxyapatite (HA) formation on Ag-BGNs was detected upon immersion of the particles in simulated body fluid for 7 days, which indicated that Ag-BGNs maintained high bioactivity after surface modification. The conducted antibacterial assay confirmed that Ag-BGNs had an antibacterial effect on E. coli. The above results thereby suggest that surface modification is an effective way to incorporate Ag into BGNs and that the modified BGNs can remain monodispersed as well as exhibit bioactivity and antibacterial capability for biomedical applications
From Galaxy-Galaxy Lensing to Cosmological Parameters
Galaxy-galaxy lensing measures the mean excess surface density DS(r) around a
sample of lensing galaxies. We develop a method for combining DS(r) with the
galaxy correlation function xi_gg(r) to constrain Omega_m and sigma_8, going
beyond the linear bias model to reach the level of accuracy demanded by current
and future measurements. We adopt the halo occupation distribution (HOD)
framework, and we test its applicability to this problem by examining the
effects of replacing satellite galaxies in the halos of an SPH simulation with
randomly selected dark matter particles from the same halos. The difference
between dark matter and satellite galaxy radial profiles has a ~10% effect on
DS(r) at r<1 Mpc/h. However, if radial profiles are matched, the remaining
impact of individual subhalos around satellite galaxies and environmental
dependence of the HOD at fixed halo mass is <5% in DS(r) for 0.1<r<15 Mpc/h. We
develop an analytic approximation for DS(r) that incorporates halo exclusion
and scale-dependent halo bias, and we demonstrate its accuracy with tests
against a suite of populated N-body simulations. We use the analytic model to
investigate the dependence of DS(r) and the galaxy-matter correlation function
xi_gm(r) on Omega_m and sigma_8, once HOD parameters for a given cosmological
model are pinned down by matching xi_gg(r). The linear bias prediction is
accurate for r>2 Mpc/h, but it fails at the 30-50% level on smaller scales. The
scaling of DS(r) ~ Omega_m^a(r) sigma_8^b(r) approaches the linear bias
expectation a=b=1 at r>10 Mpc/h, but a(r) and b(r) vary from 0.8 to 1.6 at
smaller r. We calculate a fiducial DS(r) and scaling indices a(r) and b(r) for
two SDSS galaxy samples; galaxy-galaxy lensing measurements for these samples
can be combined with our predictions to constrain Omega_m and sigma_8.Comment: 18 pages, 10 figures, accepted for publication in The Astrophysical
Journa
The J1-J2 model: First order phase transition versus deconfinement of spinons
We revisit the phase transition from the N\'eel ordered to a valence bond
solid (VBS) state in the two-dimensional antiferromagnetic Heisenberg
model. In the first part we address the question whether or not this transition
could be an example of a second order phase transition due to a deconfinement
of spinons. We give arguments based on series expansion and spin-wave theory
that this is not the case and the transition is most likely first order. The
method proposed here to detect first order phase transitions seems to be very
sensitive and might be useful in other models as well. In the second part we
analyze possible VBS patterns in the magnetically disordered phase based on
numerical data for different susceptibilities, obtained in the ordered phase,
which test the breaking of lattice symmetries. We conclude that a columnar
dimerization pattern is the most likely candidate.Comment: 7 pages, 7 figures, small changes, references adde
- …