Spinons and helimagnons in the frustrated Heisenberg chain

We investigate the dynamical spin structure factor S(q,w) for the Heisenberg chain with ferromagnetic nearest (J1<0) and antiferromagnetic next-nearest (J2>0) neighbor exchange using bosonization and a time-dependent density-matrix renormalization group algorithm. For |J1|<< J2 and low energies we analytically find and numerically confirm two spinon branches with different velocities and different spectral weights. Following the evolution of S(q,w) with decreasing J1/J2 we find that helimagnons develop at high energies just before entering the ferromagnetic phase. Furthermore, we show that a recent interpretation of neutron scattering data for LiCuVO4 in terms of two weakly coupled antiferromagnetic chains (|J_1|<< J2) is not viable. We demonstrate that the data are instead fully consistent with a dominant ferromagnetic coupling, J1/J2 ~ -2.Comment: 5 pages, 3 figure

Order independent structural alignment of circularly permuted proteins

Circular permutation connects the N and C termini of a protein and concurrently cleaves elsewhere in the chain, providing an important mechanism for generating novel protein fold and functions. However, their in genomes is unknown because current detection methods can miss many occurances, mistaking random repeats as circular permutation. Here we develop a method for detecting circularly permuted proteins from structural comparison. Sequence order independent alignment of protein structures can be regarded as a special case of the maximum-weight independent set problem, which is known to be computationally hard. We develop an efficient approximation algorithm by repeatedly solving relaxations of an appropriate intermediate integer programming formulation, we show that the approximation ratio is much better then the theoretical worst case ratio of $r = 1/4$. Circularly permuted proteins reported in literature can be identified rapidly with our method, while they escape the detection by publicly available servers for structural alignment.Comment: 5 pages, 3 figures, Accepted by IEEE-EMBS 2004 Conference Proceeding

Expert system training and control based on the fuzzy relation matrix

Fuzzy knowledge, that for which the terms of reference are not crisp but overlapped, seems to characterize human expertise. This can be shown from the fact that an experienced human operator can control some complex plants better than a computer can. Proposed here is fuzzy theory to build a fuzzy expert relation matrix (FERM) from given rules or/and examples, either in linguistic terms or in numerical values to mimic human processes of perception and decision making. The knowledge base is codified in terms of many implicit fuzzy rules. Fuzzy knowledge thus codified may also be compared with explicit rules specified by a human expert. It can also provide a basis for modeling the human operator and allow comparison of what a human operator says to what he does in practice. Two experiments were performed. In the first, control of liquid in a tank, demonstrates how the FERM knowledge base is elicited and trained. The other shows how to use a FERM, build up from linguistic rules, and to control an inverted pendulum without a dynamic model

Majorana Fermion Induced Non-local Current Correlations in Spin-orbit Coupled Superconducting Wires

Recent observation of zero bias conductance peaks in semiconductor wire/superconductor heterostructures has generated great interest, and there is a hot debate on whether the observation is associated with Majorana fermions (MFs). Here we study the local and crossed Andreev reflections in a junction of two normal leads and a sandwiched superconductor-semiconductor wire with two spatially separated but strongly coupled MF end states. The conductance and Fano factors of such a device are sharply different from the topologically trivial case even in the presence of disorder, and can hence be used to identify MFs unambiguously.Comment: 7+ pages, 10 figure

Realizing quantum controlled phase-flip gate through quantum dot in silicon slow-light photonic crystal waveguide

We propose a scheme to realize controlled phase gate between two single photons through a single quantum dot in slow-light silicon photonic crystal waveguide. Enhanced Purcell factor and beta factor lead to high gate fidelity over broadband frequencies compared to cavity-assisted system. The excellent physical integration of this silicon photonic crystal waveguide system provides tremendous potential for large-scale quantum information processing.Comment: 9 pages, 3 figure

Zero-bias peaks in spin-orbit coupled superconducting wires with and without Majorana end-states

One of the simplest proposed experimental probes of a Majorana bound-state is a quantized (2e^2/h) value of zero-bias tunneling conductance. When temperature is somewhat larger than the intrinsic width of the Majorana peak, conductance is no longer quantized, but a zero-bias peak can remain. Such a non-quantized zero-bias peak has been recently reported for semiconducting nanowires with proximity induced superconductivity. In this paper we analyze the relation of the zero-bias peak to the presence of Majorana end-states, by simulating the tunneling conductance for multi-band wires with realistic amounts of disorder. We show that this system generically exhibits a (non-quantized) zero-bias peak even when the wire is topologically trivial and does not possess Majorana end-states. We make comparisons to recent experiments, and discuss the necessary requirements for confirming the existence of a Majorana state.Comment: 5 pages, 4 Figure