Linear magnetoresistance in a quasi-free two dimensional electron gas in an ultra-high mobility GaAs quantum well

We report a magnetotransport study of an ultra-high mobility ($\bar{\mu}\approx 25\times 10^6$\,cm$^2$\,V$^{-1}$\,s$^{-1}$) $n$-type GaAs quantum well up to 33 T. A strong linear magnetoresistance (LMR) of the order of 10$^5$ % is observed in a wide temperature range between 0.3 K and 60 K. The simplicity of our material system with a single sub-band occupation and free electron dispersion rules out most complicated mechanisms that could give rise to the observed LMR. At low temperature, quantum oscillations are superimposed onto the LMR. Both, the featureless LMR at high $T$ and the quantum oscillations at low $T$ follow the empirical resistance rule which states that the longitudinal conductance is directly related to the derivative of the transversal (Hall) conductance multiplied by the magnetic field and a constant factor $\alpha$ that remains unchanged over the entire temperature range. Only at low temperatures, small deviations from this resistance rule are observed beyond $\nu=1$ that likely originate from a different transport mechanism for the composite fermions

Single-photon threshold photoionization of NO

Single‐photon threshold photoionization spectra for jet‐cooled NO have been measured for the v^+=0 and 1 vibrational levels of the X ^1Σ^+ ground state of NO^+. The NO^+ rotational state distribution for the v^+=0 level is shown to be perturbed by nearby autoionizing levels, whereas the v^+=1 level exhibits a cation rotational distribution which is in near quantitative agreement with calculated spectra near threshold. Only small changes in total angular momentum are observed (‖ΔJ‖=‖J^+ − J‘‖≤5/2) even though a wide range of photoelectron angular momenta (l=0–3) are predicted to contribute to the near‐threshold photoelectron continua. The present results are also discussed in light of recently published two‐photon threshold photoionization spectra of NO which exhibit nearly identical NO^+ rotational state distributions

SNP discovery in swine by reduced representation and high throughput pyrosequencing

<p>Abstract</p> <p>Background</p> <p>Relatively little information is available for sequence variation in the pig. We previously used a combination of short read (25 base pair) high-throughput sequencing and reduced genomic representation to discover > 60,000 single nucleotide polymorphisms (SNP) in cattle, but the current lack of complete genome sequence limits this approach in swine. Longer-read pyrosequencing-based technologies have the potential to overcome this limitation by providing sufficient flanking sequence information for assay design. Swine SNP were discovered in the present study using a reduced representation of 450 base pair (bp) porcine genomic fragments (approximately 4% of the swine genome) prepared from a pool of 26 animals relevant to current pork production, and a GS-FLX instrument producing 240 bp reads.</p> <p>Results</p> <p>Approximately 5 million sequence reads were collected and assembled into contigs having an overall observed depth of 7.65-fold coverage. The approximate minor allele frequency was estimated from the number of observations of the alternate alleles. The average coverage at the SNPs was 12.6-fold. This approach identified 115,572 SNPs in 47,830 contigs. Comparison to partial swine genome draft sequence indicated 49,879 SNP (43%) and 22,045 contigs (46%) mapped to a position on a sequenced pig chromosome and the distribution was essentially random. A sample of 176 putative SNPs was examined and 168 (95.5%) were confirmed to have segregating alleles; the correlation of the observed minor allele frequency (MAF) to that predicted from the sequence data was 0.58.</p> <p>Conclusion</p> <p>The process was an efficient means to identify a large number of porcine SNP having high validation rate to be used in an ongoing international collaboration to produce a highly parallel genotyping assay for swine. By using a conservative approach, a robust group of SNPs were detected with greater confidence and relatively high MAF that should be suitable for genotyping in a wide variety of commercial populations.</p

Coexistence of bulk and surface states probed by Shubnikov-de Haas oscillations in Bi2_2Se3_3 with high charge-carrier density

Topological insulators are ideally represented as having an insulating bulk with topologically protected, spin-textured surface states. However, it is increasingly becoming clear that these surface transport channels can be accompanied by a finite conducting bulk, as well as additional topologically trivial surface states. To investigate these parallel conduction transport channels, we studied Shubnikov-de Haas oscillations in Bi$_2$Se$_3$ thin films, in high magnetic fields up to 30 T so as to access channels with a lower mobility. We identify a clear Zeeman-split bulk contribution to the oscillations from a comparison between the charge-carrier densities extracted from the magnetoresistance and the oscillations. Furthermore, our analyses indicate the presence of a two-dimensional state and signatures of additional states the origin of which cannot be conclusively determined. Our findings underpin the necessity of theoretical studies on the origin of and the interplay between these parallel conduction channels for a careful analysis of the material's performance.Comment: Manuscript including supplemental materia

Novel porcine repetitive elements

BACKGROUND: Repetitive elements comprise ~45% of mammalian genomes and are increasingly known to impact genomic function by contributing to the genomic architecture, by direct regulation of gene expression and by affecting genomic size, diversity and evolution. The ubiquity and increasingly understood importance of repetitive elements contribute to the need to identify and annotate them. We set out to identify previously uncharacterized repetitive DNA in the porcine genome. Once found, we characterized the prevalence of these repeats in other mammals. RESULTS: We discovered 27 repetitive elements in 220 BACs covering 1% of the porcine genome (Comparative Vertebrate Sequencing Initiative; CVSI). These repeats varied in length from 55 to 1059 nucleotides. To estimate copy numbers, we went to an independent source of data, the BAC-end sequences (Wellcome Trust Sanger Institute), covering approximately 15% of the porcine genome. Copy numbers in BAC-ends were less than one hundred for 6 repeat elements, between 100 and 1000 for 16 and between 1,000 and 10,000 for 5. Several of the repeat elements were found in the bovine genome and we have identified two with orthologous sites, indicating that these elements were present in their common ancestor. None of the repeat elements were found in primate, rodent or dog genomes. We were unable to identify any of the replication machinery common to active transposable elements in these newly identified repeats. CONCLUSION: The presence of both orthologous and non-orthologous sites indicates that some sites existed prior to speciation and some were generated later. The identification of low to moderate copy number repetitive DNA that is specific to artiodactyls will be critical in the assembly of livestock genomes and studies of comparative genomics

Optimizing the wireless power transfer over MIMO Channels

In this paper, the optimization of the power transfer over wireless channels having multiple-inputs and multiple-outputs (MIMO) is studied. Therefore, the transmitter, the receiver and the MIMO channel are modeled as multiports. The power transfer efficiency is described by a Rayleigh quotient, which is a function of the channel's scattering parameters and the incident waves from both transmitter and receiver side. This way, the power transfer efficiency can be maximized analytically by solving a generalized eigenvalue problem, which is deduced from the Rayleigh quotient. As a result, the maximum power transfer efficiency achievable over a given MIMO channel is obtained. This maximum can be used as a performance bound in order to benchmark wireless power transfer systems. Furthermore, the optimal operating point which achieves this maximum will be obtained. The optimal operating point will be described by the complex amplitudes of the optimal incident and reflected waves of the MIMO channel. This supports the design of the optimal transmitter and receiver multiports. The proposed method applies for arbitrary MIMO channels, taking transmitter-side and/or receiver-side cross-couplings in both near- and farfield scenarios into consideration. Special cases are briefly discussed in this paper in order to illustrate the method

Rotationally resolved threshold photoelectron spectra of OH and OD

The results of combined experimental and theoretical studies of the rotationally resolved photoelectron spectra of OH and OD following single‐photon ionization are presented. The measured zero‐kinetic‐energy (ZEKE) spectra were obtained using pulsed field ionization in conjunction with a vacuum ultraviolet laser source. The OH^+ and OD^+ (X ^3Σ^−, v^+=0) rotational distributions were studied over the range 95.0–95.4 nm. Agreement between the observed and calculated spectra is very encouraging. Improved values for the ionization potentials of OH and OD (104 989 and 105 085 ± 2 cm^(−1), respectively) are reported and the unusual dynamics favoring ΔN<0 transitions are discussed

Anisotropic and strong negative magneto-resistance in the three-dimensional topological insulator Bi2Se3

We report on high-field angle-dependent magneto-transport measurements on epitaxial thin films of Bi2Se3, a three-dimensional topological insulator. At low temperature, we observe quantum oscillations that demonstrate the simultaneous presence of bulk and surface carriers. The magneto- resistance of Bi2Se3 is found to be highly anisotropic. In the presence of a parallel electric and magnetic field, we observe a strong negative longitudinal magneto-resistance that has been consid- ered as a smoking-gun for the presence of chiral fermions in a certain class of semi-metals due to the so-called axial anomaly. Its observation in a three-dimensional topological insulator implies that the axial anomaly may be in fact a far more generic phenomenon than originally thought.Comment: 6 pages, 4 figure