Biomedical subjects

Y L Loh

Publications and source records attributed to Y L Loh.

5 recordsLinked to original sources

PubMed2006

Efficient algorithm for random-bond ising models in 2D.

We present an efficient algorithm for calculating the properties of Ising models in two dimensions, directly in the spin basis, without the need for mapping to fermion or dimer models. The algorithm computes the partition function and correlation functions at a single temperature on any planar network of N Ising spins in O(N;{3/2}) time or less. The method can handle continuous or discrete bond disorder and is especially efficient in the case of bond or site dilution, where it executes in O(NlnN) time near the percolation threshold. We demonstrate its feasibility on the ferromagnetic Ising model and the +/-J random-bond Ising model and discuss the regime of applicability in cases of full frustration such as the Ising antiferromagnet on a triangular lattice.

Journal Article↗

PubMed2006

Thermal conductivity of a granular metal.

Using the Kubo formula approach, we study the effect of electron interaction on thermal transport in the vicinity of a metal-insulator transition, with a granular metal as our model. For small values of dimensionless intergrain tunneling conductance, g<<1, we find that the thermal conductivity surprisingly shows a phononlike algebraic decrease, kappa(T) approximately g2T3/E2c even though the electrical conductivity obeys an Arrhenius law, sigma(T) approximately ge-Ec/T ; therefore the Wiedemann-Franz (WF) law is seriously violated. We explicitly show that this violation arises from nonmagnetic bosonic excitations of low energy that transport heat but not charge. At large values of intergrain tunneling, we find it plausible that the WF law weakly deviates from the free-electron theory due to potential fluctuations. Implications for experiment are discussed.

Journal Article↗

PubMed2001

Fast Chebyshev-polynomial method for simulating the time evolution of linear dynamical systems.

We present a fast method for simulating the time evolution of any linear dynamical system possessing eigenmodes. This method does not require an explicit calculation of the eigenvectors and eigenfrequencies, and is based on a Chebyshev polynomial expansion of the formal operator matrix solution in the eigenfrequency domain. It does not suffer from the limitations of ordinary time-integration methods, and can be made accurate to almost machine precision. Among its possible applications are harmonic classical mechanical systems, quantum diffusion, and stochastic transport theory. An example of its use is given for the problem of vibrational wave-packet propagation in a disordered lattice.

Journal Article↗

PubMed2001

Origin of the boson peak in systems with lattice disorder.

The origin of the boson peak in models with force-constant disorder has been established by calculations using the coherent potential approximation. The analytical results obtained are supported by precise numerical solutions. The boson peak in the disordered system is associated with the lowest van Hove singularity in the spectrum of the reference crystalline system, pushed down in frequency by disorder-induced level-repelling and hybridization effects.

Journal Article↗

PubMed2001

Control of exuT activity for galacturonate transport by the negative regulator ExuR in Erwinia chrysanthemi EC16.

The negative regulatory protein ExuR in Erwinia chrysanthemi regulates expression of the galacturonate uptake (exuT) and utilization (uxaA, uxaB, uxaC) genes. We cloned and determined the nucleotide sequence of the exuR gene from E. chrysanthemi EC16. Analysis of the deduced amino acid sequence indicates that this protein possesses a helix-turn-helix motif and belongs to the GntR family of transcriptional repressors. Northern blot analysis and studies with transcriptional fusions of exuT in wild-type and exuR mutant backgrounds indicate that exuT transcription is deregulated in the exuR strain in vivo and in planta. [14C]-galacturonic acid uptake was constitutively high under inducing and noninducing conditions in the exuR mutant. Maximal exuT transcription activity was observed within 8 h of bacterial inoculation into potato tubers, well before any visible symptoms of disease were detected. This suggests that ExuT transport activity in E. chrysanthemi is important in the early stages of disease development.

Amino Acid Sequence↗