Biomedical subjects

X-G Zhang

Publications and source records attributed to X-G Zhang.

10 recordsLinked to original sources

PubMed2006

Probing spin-flip scattering in ballistic nanosystems.

Because spin-flip length is longer than the electron mean-free path in a metal, past studies of spin-flip scattering are limited to the diffusive regime. We propose to use a magnetic double barrier tunnel junction to study spin-flip scattering in the nanometer sized spacer layer near the ballistic limit. We extract the voltage and temperature dependence of the spin-flip conductance Gs in the spacer layer from magnetoresistance measurements. In addition to spin scattering information including the mean-free path (70 nm) and the spin-flip length (1.0-2.6 microm) at 4.2 K, this technique also yields information on the density of states and quantum well resonance in the spacer layer.

Journal Article↗

PubMed2006

First-principles theory of quantum well resonance in double barrier magnetic tunnel junctions.

Quantum well (QW) resonances in Fe(001)/MgO/Fe/MgO/Fe double barrier magnetic tunnel junctions are calculated from first principles. By including the Coulomb blockade energy due to the finite size islands of the middle Fe film, we confirm that the oscillatory differential resistance observed in a recent experiment [T. Nozaki, Phys. Rev. Lett. 96, 027208 (2006)10.1103/PhysRevLett.96.027208] originates from the QW resonances from the Delta1 band of the Fe majority-spin channel. The primary source of smearing at low temperatures is shown to be the variation of the Coulomb blockade energy.

Journal Article↗

PubMed2006

Characterization of the tunneling conductance across DNA bases.

Characterization of the electrical properties of the DNA bases (adenine, cytosine, guanine, and thymine), in addition to building the basic knowledge on these fundamental constituents of a DNA, is a crucial step in developing a DNA sequencing technology. We present a first-principles study of the current-voltage characteristics of nucleotidelike molecules of the DNA bases, placed in a 1.5 nm gap formed between gold nanoelectrodes. The quantum transport calculations in the tunneling regime are shown to vary strongly with the electrode-molecule geometry and the choice of the density-functional theory exchange-correlation functionals. Analysis of the results in the zero-bias limit indicates that distinguishable current-voltage characteristics of different DNA bases are dominated by the geometrical conformations of the bases and nanoelectrodes.

Computer Simulation↗

PubMed2006

Spin-dependent transport through a magnetic carbon nanotube-molecule junction.

The electronic structure and spin-dependent conductance of a magnetic junction consisting of two Fe-doped carbon nanotubes and a C60 molecule are investigated using a first-principles approach that combines the density functional theory with the nonequilibrium Greens function technique. The tunneling magnetoresistance ratio is found to be 11%. The density of states and transmission coefficient through the molecular junction are analyzed and compared to layered magnetic tunneling junctions. Our findings suggest new possibilities for experiments and for future technology.

Journal Article↗

PubMed2006

First-principles transversal DNA conductance deconstructed.

First-principles calculation of the transverse conductance across DNA fragments placed between gold nanoelectrodes reveals that such conductance describes electron tunneling that depends critically on geometrical rather than electronic-structure properties. By factoring the first-principles result into two simple and approximately independent tunneling factors, we show that the conductances of the A, C, G, and T fragments differ only because of their sizes: the larger is the DNA base, the smaller its distance to the electrode, and the larger its conductance. Because the geometrical factors are difficult to control in an experiment, the direct-current measurements across DNA with gold contact electrodes may not be a convenient approach to DNA sequencing.

Computer Simulation↗

PubMed2005

First-principles mobility calculations and atomic-scale interface roughness in nanoscale structures.

Calculations of mobilities have so far been carried out using approximate methods that suppress atomic-scale detail. Such approaches break down in nanoscale structures. Here we report the development of a method to calculate mobilities using atomic-scale models of the structures and density functional theory at various levels of sophistication and accuracy. The method is used to calculate the effect of atomic-scale roughness on electron mobilities in ultrathin double-gate silicon-on-insulator structures. The results elucidate the origin of the significant reduction in mobility observed in ultrathin structures at low electron densities.

Journal Article↗

PubMed2005

Spin-dependent resonant tunneling through quantum-well states in magnetic metallic thin films.

Quantum-well (QW) states in nonmagnetic metal films between magnetic layers are known to be important in spin-dependent transport, but QW states in magnetic films remains elusive. Here we identify the conditions for resonant tunneling through QW states in magnetic films and report first principles calculations of Fe/MgO/FeO/Fe/Cr and Co/MgO/Fe/Cr. We show that, at resonance, the current increases by 1 to 2 orders of magnitude. The tunneling magnetoresistance ratio is much larger than in simple spin tunnel junctions and is positive (negative) for majority- (minority-) spin resonances, with a large asymmetry between positive and negative biases. The results can serve as a basis for novel spintronic devices.

Journal Article↗

PubMed2005

Characterization and application of two novel monoclonal antibodies against 2IgB7-H3: expression analysis of 2IgB7-H3 on dendritic cells and tumor cells.

2IgB7-H3 has recently been identified as a new member of the B7 family. Its expression at the protein level remains largely unknown due to the lack of the specific monoclonal antibody (mAb). To characterize the expression of 2IgB7-H3, we newly generated two mouse antihuman 2IgB7-H3 mAbs (4H7 and 21D4). We found the constitutive expression of 2IgB7-H3 on a series of tumor cell lines. Furthermore, the expression was examined on monocyte-derived dendritic cells (Mo-DCs) and DCs from CD34(+) hematopoietic progenitor cells (HPC) by means of mAb staining. The results showed that 2IgB7-H3 was expressed on Mo-DCs at a high and stable level during differentiation in vitro. With the maturation of DCs from CD34(+) HPCs, the expression of the molecule was upregulated. However, the 2IgB7-H3 was not expressed on fresh isolated T and B lymphocytes, monocytes, or CD34(+) HPCs. These results suggested that 2IgB7-H3 may be a valuable surface antigen for the detection of DCs.

Animals↗

PubMed2004

Coherent electron transport through an azobenzene molecule: a light-driven molecular switch.

We apply a first-principles computational approach to study a light-sensitive molecular switch. The molecule that comprises the switch can convert between a trans and a cis configuration upon photoexcitation. We find that the conductance of the two isomers varies dramatically, which suggests that this system has potential application as a molecular device. A detailed analysis of the band structure of the metal leads and the local density of states of the system reveals the mechanism of the switch.

Journal Article↗

PubMed2004

Evidence for a single median fin-fold and tail in the Lower Cambrian vertebrate, Haikouichthys ercaicunensis.

In this study, we illustrate an exceptionally well-preserved Haikouichthys ercaicunensis from the Lower Cambrian Chengjiang fauna that displays complete single dorsal, ventral and caudal fins. This 530-million-year old vertebrate is fish-shaped and characterized by a single median fin-fold, which is an essential trait of the initial vertebrate chordates. The radially orientated ray-like structures in its dorsal fin somewhat resemble but are probably not real radials seen in basal vertebrates, such as hagfishes and lampreys. The unique design of primitive fins and fin structures provides additional insights into the early evolution of vertebrates.

Animal Structures↗