Surface enhanced Raman scattering from mildly roughened surfaces: variation of signal with metal grain size

The intensity of surface enhanced Raman scattering from benzoic acid derivatives on mildly roughened, thermally evaporated Ag films shows a remarkably strong dependence on metal grain size. Large grained (slowly deposited) films give a superior response, by up to a factor of 10, to small grained (quickly deposited) films, with films of intermediate grain size yielding intermediate results. The optical field amplification underlying the enhancement mechanism is due to the excitation of surface plasmon polaritons (SPPs). Since surface roughness characteristics, as determined by STM, remain relatively constant as a function of deposition rate, it is argued that the contrast in Raman scattering is due to differences in elastic grain boundary scattering of SPPs (leading to different degrees of internal SPP damping), rather than differences in the interaction of SPPs with surface inhomogeneities.     

Ward's identity in critical dynamics

We consider the relaxation of an order-parameter fluctuation of wave numberk in a system undergoing a second-order phase transition. In general, close to the critical point, wherek ^{–1 –1} (the correlation length) the relaxation rate has a linear dependence on/k of the form (k, ) = (k, 0)x(1–a/k). In analogy with the use of Ward's identity in elementary particle physics, we show that the numerical coefficienta is readily calculated by means of a mass insertion. We demonstrate, furthermore, that this initial linear drop is the main feature of the full/k dependence of the scaling functionR ^–x (k,), wherex is the dynamic critical exponent andR=(k²+ ²)^1/2 is the distance variable.     

Crossover critical correlation function

The anomalous dimension index assumes its critical value for small momenta and vanishes for large momenta. We provide, in the form of a quadrature, a momentum dependent (k) which interpolates between the above limits. High- and low-momentum expansions for the spectral function are obtained, leading to a Padé approximant from which (k) is determined, thereby giving a crossover critical correlation function.Dedicated to B. Mühlschlegel on the occasion of his 60th birthday     

Selective Monofunctionalization Enabled by Reaction‐History‐Dependent Communication in Catalytic Rotaxanes

Selective monofunctionalization of substrates with distant, yet equally reactive functional groups is difficult to achieve, as it requires the second functional group to selectively modulate its reactivity once the first functional group has reacted. We now show that mechanically interlocked catalytic rings can effectively regulate the reactivity of stoppering groups in rotaxanes over a distance of about 2 nm. Our mechanism of communication is enabled by a unique interlocked design, which effectively removes the catalytic rings from the substrates by fast dethreading as soon as the first reaction has taken place. Our method not only led to a rare example of selective monofunctionalization, but also to a “molecular if function”. Overall, the study presents a way to get distant functional groups to communicate with each other in a reaction‐history‐dependent manner by creating linkers that can ultimately perform logical operations at the molecular level.     

Addendum to “field theory of the two-dimensional Ising model: Equivalence to the free particle one-dimensional dirac equation”

Journal of Statistical Physics -     

Bistability in cell signaling: How to make continuous processes discontinuous,and reversible processes irreversible

Xenopus oocyte maturation is an example of an all-or-none, irreversible cell fate induction process. In response to a submaximal concentration of the steroid hormone progesterone, a given oocyte may either mature or not mature, but it can exist in intermediate states only transiently. Moreover, once an oocyte has matured, it will remain arrested in the mature state even after the progesterone is removed. It has been hypothesized that the all-or-none character of oocyte maturation, and some aspects of the irreversibility of maturation, arise out of the bistability of the signal transduction system that triggers maturation. The bistability, in turn, is hypothesized to arise from the way the signal transducers are organized into a signaling circuit that includes positive feedback (which makes it so that the system cannot rest in intermediate states) and ultrasensitivity (which filters small stimuli out of the feedback loop, allowing the system to have a stable off-state). Here we review two simple graphical methods that are commonly used to analyze bistable systems, discuss the experimental evidence for bistability in oocyte maturation, and suggest that bistability may be a common means of producing all-or-none responses and a type of biochemical memory. (c) 2001 American Institute of Physics.     

Anomalous thermopower in the metalliclike phase of a 2D hole system

We report very low temperature ( T) thermopower and resistivity ( rho) measurements on variable-density, two-dimensional hole systems confined to GaAs quantum wells. As the hole density is lowered from 1.49x10(11) cm(-2) to 0.14x10(11) cm(-2), the system crosses from an insulating ( drho / dT less, similar0) to a metallic regime ( drho / dT>0) and finally displays insulating behavior ( drho / dT<0). Diffusion thermopower shows a striking sign reversal in a narrow range of density in the metallic regime, suggesting a qualitative change in the conduction or the scattering mechanism.