A field theoretic approach to master equations and a variational method beyond the Poisson ansatz

We develop a variational scheme in a field theoretic approach to a stochastic process. While various stochastic processes can be expressed using master equations, in general it is difficult to solve the master equations exactly, and it is also hard to solve the master equations numerically because of the curse of dimensionality. The field theoretic approach has been used in order to study such complicated master equations, and the variational scheme achieves tremendous reduction in the dimensionality of master equations. For the variational method, only the Poisson ansatz has been used, in which one restricts the variational function to a Poisson distribution. Hence, one has dealt with only restricted fluctuation effects. We develop the variational method further, which enables us to treat an arbitrary variational function. It is shown that the variational scheme developed gives a quantitatively good approximation for master equations which describe a stochastic gene regulatory network.Comment: 13 pages, 2 figure

The stochastic pump current and the non-adiabatic geometrical phase

We calculate a pump current in a classical two-state stochastic chemical kinetics by means of the non-adiabatic geometrical phase interpretation. The two-state system is attached to two particle reservoirs, and under a periodic perturbation of the kinetic rates, it gives rise to a pump current between the two-state system and the absorbing states. In order to calculate the pump current, the Floquet theory for the non-adiabatic geometrical phase is extended from a Hermitian case to a non-Hermitian case. The dependence of the pump current on the frequency of the perturbative kinetic rates is explicitly derived, and a stochastic resonance-like behavior is obtained.Comment: 11 page

Population III Gamma Ray Bursts

We discuss a model of Poynting-dominated gamma-ray bursts from the collapse of very massive first generation (pop. III) stars. From redshifts of order 20, the resulting relativistic jets would radiate in the hard X-ray range around 50 keV and above, followed after roughly a day by an external shock component peaking around a few keV. On the same timescales an inverse Compton component around 75 GeV may be expected, as well as a possible infra-red flash. The fluences of these components would be above the threshold for detectors such as Swift and Fermi, providing potentially valuable information on the formation and properties of what may be the first luminous objects and their black holes in the high redshift Universe.Comment: 12 pages; Apj, subm. 12/10/2009; accepted 04/12/201

In-situ photoemission study of Pr_{1-x}Ca_xMnO_3 epitaxial thin films with suppressed charge fluctuations

We have performed an {\it in-situ} photoemission study of Pr_{1-x}Ca_xMnO_3 (PCMO) thin films grown on LaAlO_3 (001) substrates and observed the effect of epitaxial strain on the electronic structure. We found that the chemical potential shifted monotonically with doping, unlike bulk PCMO, implying the disappearance of incommensurate charge fluctuations of bulk PCMO. In the valence-band spectra, we found a doping-induced energy shift toward the Fermi level (E_F) but there was no spectral weight transfer, which was observed in bulk PCMO. The gap at E_F was clearly seen in the experimental band dispersions determined by angle-resolved photoemission spectroscopy and could not be explained by the metallic band structure of the C-type antiferromagnetic state, probably due to localization of electrons along the ferromagnetic chain direction or due to another type of spin-orbital ordering.Comment: 5 pages, 4 figure

Potential Energy Function for Continuous State Models of Globular Proteins

One of the approaches to protein structure prediction is to obtain energy functions which can recognize the native conformation of a given sequence among a zoo of conformations. The discriminations can be done by assigning the lowest energy to the native conformation, with the guarantee that the native is in the zoo. Well-adjusted functions, then, can be used in the search for other (near-) natives. Here the aim is the discrimination at relatively high resolution (RMSD difference between the native and the closest nonnative is around 1 Å) by pairwise energy potentials. The potential is trained using the experimentally determined native conformation of only one protein, instead of the usual large survey over many proteins. The novel feature is that the native structure is compared to a vastly wider and more challenging array of nonnative structures found not only by the usual threading procedure, but by wide-ranging local minimization of the potential. Because of this extremely demanding search, the native is very close to the apparent global minimum of the potential function. The global minimum property holds up for one other protein having 60% sequence identity, but its performance on completely dissimilar proteins is of course much weaker.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/63101/1/106652700750050835.pd

Spin-filter tunnel junction with matched Fermi surfaces

Efficient injection of spin-polarized current into a semiconductor is a basic prerequisite for building semiconductor-based spintronic devices. Here, we use inelastic electron tunneling spectroscopy to show that the efficiency of spin-filter-type spin injectors is limited by spin scattering of the tunneling electrons. By matching the Fermi-surface shapes of the current injection source and target electrode material, spin injection efficiency can be significantly increased in epitaxial ferromagnetic insulator tunnel junctions. Our results demonstrate that not only structural but also Fermi-surface matching is important to suppress scattering processes in spintronic devices.Comment: 5 pages, 4 figure

Roles of proton-neutron interactions in alpha-like four-nucleon correlations

An extended pairing plus QQ force model, which has been shown to successfully explain the nuclear binding energy and related quantities such as the symmetry energy, is applied to study the alpha-like four-nucleon correlations in 1f_{7/2} shell nuclei. The double difference of binding energies, which displays a characteristic behavior at $N \approx Z$, is interpreted in terms of the alpha-like correlations. Important roles of proton-neutron interactions forming the alpha-like correlated structure are discussed.Comment: 10 pages, 2 figures, RevTex, submitted to Phys. Rev.