A general group theoretical method to unfold band structures and its application

We present a general method to unfold energy bands of supercell calculations to primitive Brillouin zone using group theoretical techniques, where an isomorphic factor group is introduced to connect the primitive translation group with the supercell translation group via a direct product. Originating from the translation group symmetry, our method gives an uniform description of unfolding approaches based on various basis sets, and therefore, should be easy to implement in both tight-binding model and existing ab initio code packages using different basis sets. This makes the method applicable to a variety of problems involving the use of supercells, such as defects, disorder, and interfacial reconstructions. As a realistic example, we calculate electronic properties of an monolayer FeSe on SrTiO$_3$ in checkerboard and collinear antiferromagnetic spin configurations, illustrating the potential of our method.

Quantum Unfolding: A program for unfolding electronic energy bands of materials

We present Quantum Unfolding, a Fortran90 program for unfolding first-principles electronic energy bands. It unfolds energy bands accurately by handling the Fourier components of Bloch wavefunctions, which are reconstructed from Wannier functions from Wannier90. Due to the wide application of Wannier90 package and the possibility of focusing only on the most important energy bands, the present code works very conveniently.

A highly magnified candidate for a young galaxy seen when the Universe was 500 Myrs old

The early Universe at redshift zsim6-11 marks the reionization of the intergalactic medium, following the formation of the first generation of stars. However, those young galaxies at a cosmic age of lesssim 500 million years (Myr, at z gtrsim 10) remain largely unexplored as they are at or beyond the sensitivity limits of current large telescopes. Gravitational lensing by galaxy clusters enables the detection of high-redshift galaxies that are fainter than what otherwise could be found in the deepest images of the sky. We report the discovery of an object found in the multi-band observations of the cluster MACS1149+22 that has a high probability of being a gravitationally magnified object from the early universe. The object is firmly detected (12 sigma) in the two reddest bands of HST/WFC3, and not detected below 1.2 {mu}m, matching the characteristics of zsim9 objects. We derive a robust photometric redshift of z = 9.6 pm 0.2, corresponding to a cosmic age of 490 pm 15Myr (i.e., 3.6% of the age of the Universe). The large number of bands used to derive the redshift estimate make it one of the most accurate estimates ever obtained for such a distant object. The significant magnification by cluster lensing (a factor of sim15) allows us to analyze the objects ultra-violet and optical luminosity in its rest-frame, thus enabling us to constrain on its stellar mass, star-formation rate and age. If the galaxy is indeed at such a large redshift, then its age is less than 200 Myr (at the 95% confidence level), implying a formation redshift of zf lesssim 14. The object is the first z>9 candidate that is bright enough for detailed spectroscopic studies with JWST, demonstrating the unique potential of galaxy cluster fields for finding highly magnified, intrinsically faint galaxies at the highest redshifts.

Bright Strongly Lensed Galaxies at Redshift z~ 6-7 behind the Clusters Abell 1703 and CL0024+161

We report on the discovery of three bright, strongly-lensed objects behind Abell 1703 and CL0024+16 from a dropout search over 25 square arcminutes of deep NICMOS data, with deep ACS optical coverage. They are undetected in the deep ACS images below 8500 A and have clear detections in the J and H bands. Fits to the ACS, NICMOS and IRAC data yield robust photometric redshifts in the range z~6-7 and largely rule out the possibility that they are low-redshift interlopers. All three objects are extended, and resolved into a pair of bright knots. The bright i-band dropout in Abell 1703 has an H-band AB magnitude of 23.9, which makes it one of the brightest known galaxy candidates at z>5.5. Our model fits suggest a young, massive galaxy only ~ 60 million years old with a mass of ~ 1E10 solar mass. The dropout galaxy candidates behind CL0024+16 are separated by 2.5 (~ 2 kpc in the source plane), and have H-band AB magnitudes of 25.0 and 25.6. Lensing models of CL0024+16 suggest that the objects have comparable intrinsic magnitudes of AB ~ 27.3, approximately one magnitude fainter than L* at z~6.5. Their similar redshifts, spectral energy distribution, and luminosities, coupled with their very close proximity on the sky, suggest that they are spatially associated, and plausibly are physically bound. Combining this sample with two previously reported, similarly magnified galaxy candidates at z~6-8, we find that complex systems with dual nuclei may be a common feature of high-redshift galaxies.

Prediction of Half Metallicity along the Edge of Boron Nitride Zigzag Nanoribbons

First-principles calculations reveal half metallicity in zigzag boron nitride (BN) nanoribbons (ZBNNRs). When the B edge, but not the N edge, of the ZBNNR is passivated, despite being a pure $sp$-electron system, the ribbon shows a giant spin splitting. The electrons at the Fermi level are 100% spin polarized with a half-metal gap of 0.38 eV and its conductivity is dominated by metallic single-spin states. The two states across at the Dirac point have different molecular origins, which signals a switch of carrier velocity. The ZBNNR should be a good potential candidate for widegap spintronics.