When considering new sensory technologies one should look to nature for guidance. Indeed, living organisms have developed the ultimate chemical sensors. Many insects can detect chemical signals with perfect specificity and incredible sensitivity. Mammalian olfaction is based on an array of less discriminating sensors and a memorized response pattern to identify a unique odor. It is important to recognize that the extraordinary sensory performance of biological systems does not originate from a single element. In actuality, their performance is derived from a completely interactive system wherein the receptor is served by analyte delivery and removal mechanisms, selectivity is derived from receptors, and sensitivity is the result of analyte-triggered biochemical cascades. Clearly, optimal artificial sensory sys-

Conjugated polymer-based chemical sensors.

This article reviews the physics of high-temperature superconductors from the point of view of the doping of a Mott insulator. The basic electronic structure of cuprates is reviewed, emphasizing the physics of strong correlation and establishing the model of a doped Mott insulator as a starting point. A variety of experiments are discussed, focusing on the region of the phase diagram close to the Mott insulator (the underdoped region) where the behavior is most anomalous. The normal state in this region exhibits pseudogap phenomenon. In contrast, the quasiparticles in the superconducting state are well defined and behave according to theory. This review introduces Anderson's idea of the resonating valence bond and argues that it gives a qualitative account of the data. The importance of phase fluctuations is discussed, leading to a theory of the transition temperature, which is driven by phase fluctuations and the thermal excitation of quasiparticles. However, an argument is made that phase fluctuations can only explain pseudogap phenomenology over a limited temperature range, and some additional physics is needed to explain the onset of singlet formation at very high temperatures. A description of the numerical method of the projected wave function is presented, which turns out to be a very useful technique for implementing the strong correlation constraint and leads to a number of predictions which are in agreement with experiments. The remainder of the paper deals with an analytic treatment of the $t\text{\ensuremath{-}}J$ model, with the goal of putting the resonating valence bond idea on a more formal footing. The slave boson is introduced to enforce the constraint againt double occupation and it is shown that the implementation of this local constraint leads naturally to gauge theories. This review follows the historical order by first examining the U(1) formulation of the gauge theory. Some inadequacies of this formulation for underdoping are discussed, leading to the SU(2) formulation. Here follows a rather thorough discussion of the role of gauge theory in describing the spin-liquid phase of the undoped Mott insulator. The difference between the high-energy gauge group in the formulation of the problem versus the low-energy gauge group, which is an emergent phenomenon, is emphasized. Several possible routes to deconfinement based on different emergent gauge groups are discussed, which leads to the physics of fractionalization and spin-charge separation. Next the extension of the SU(2) formulation to nonzero doping is described with a focus on a part of the mean-field phase diagram called the staggered flux liquid phase. It will be shown that inclusion of the gauge fluctuation provides a reasonable description of the pseudogap phase. It is emphasized that $d$-wave superconductivity can be considered as evolving from a stable U(1) spin liquid. These ideas are applied to the high-${T}_{c}$ cuprates, and their implications for the vortex structure and the phase diagram are discussed. A possible test of the topological structure of the pseudogap phase is described.

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Doping a Mott insulator: Physics of high-temperature superconductivity

Structure of lipid bilayers

This article reviews the effort to understand the physics of high temperature superconductors from the point of view of doping a Mott insulator. The basic electronic structure of the cuprates is reviewed, emphasizing the physics of strong correlation and establishing the model of a doped Mott insulator as a starting point. A variety of experiments are discussed, focusing on the region of the phase diagram close to the Mott insulator (the underdoped region) where the behavior is most anomalous. We introduce Anderson's idea of the resonating valence bond (RVB) and argue that it gives a qualitative account of the data. The importance of phase fluctuation is discussed, leading to a theory of the transition temperature which is driven by phase fluctuation and thermal excitation of quasiparticles. We then describe the numerical method of projected wavefunction which turns out to be a very useful technique to implement the strong correlation constraint, and leads to a number of predictions which are in agreement with experiments. The remainder of the paper deals with an analytic treatment of the t-J model, with the goal of putting the RVB idea on a more formal footing. The slave-boson is introduced to enforce the constraint of no double occupation. The implementation of the local constraint leads naturally to gauge theories. We give a rather thorough discussion of the role of gauge theory in describing the spin liquid phase of the undoped Mott insulator. We next describe the extension of the SU(2) formulation to nonzero doping. We show that inclusion of gauge fluctuation provides a reasonable description of the pseudogap phase.

Doping a Mott Insulator: Physics of High Temperature Superconductivity

Supramolecular nanotube architectures based on amphiphilic molecules.

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X-Ray and Optical Studies of the Thickness Dependence of the Phase Diagram of Liquid-Crystal Films

X-ray scattering studies were performed to understand the structure and correlations in the lamellar phases of thick, freely suspeded films of (i) the hydrated phospholipid dimyristoylphosphotidylcholine (DMPC) and (ii) the ternar system consisting of the surfactant sodium dodecyl sulfate (SDS), cosurfactant (pentanol), and water. The films were drawn in a temperature- and humidity-controlled environment, where the layers were oriented to within 0.1 degrees . In the DMPC system, this made it possible to directly observe the orientation of the P(beta') modulation and to identify phase L(beta') as three distinct phases distinguished by the direction of chain tilt with respect to the lattice. In the L(alpha) phase of the ternary system, power law behavior of the (0,0,L) structure factor arising from the algebraic decay of layer correlatios was observed in single crystals.

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X-ray Scattering Studies of Aligned, Stacked Surfactant Membranes.

Real surfaces are rough and chemically heterogeneous on many length scales and wetting phenomena on these surfaces are affected by this inhomogeneous nature. We have used x‐ray reflectivity to examine the structure of static precursing films which precede the macroscopic meniscus of water on glass. We have examined the thickness of the film, the roughness of the interfaces bounding the film, and the conformality or correlations between the roughness features of these bounding interfaces. The wetting film consists of three basic components: the bulk meniscus, a thin (≤100 A) precursing wetting film which has a sudden termination, and beyond that, a much thinner film characteristic of the glass surface in contact with water vapor only. The roughness and chemical heterogeneity of the solid manifest themselves in the roughness of the water/vapor interface of the film, in the irregularity of the terminating boundary of the precursor wetting film, and in the hysteretic pinning of that boundary. The roughness of...

The effects of substrate roughness on ultrathin water films

The effects of high pressure gases (P≤400 bar) on the RII, RI, and RV rotator phases of 21, 23, and 25 carbon normal alkanes were studied via x‐ray scattering. We have measured the pressure and temperature dependence of the rotator structures and present these results in terms of the essential structural parameters: layer spacing, area per molecule, lattice distortion, and tilt. The pressure was generated by one of three gases: helium, nitrogen, or argon. In the rotator phases, argon and nitrogen intercalate between the layers while helium acts mostly as a noninteracting pressurizing medium. The thermal expansion and compressibility are anomalously large in the rotator phases, and this implies that the heat capacity in the rotator phases is dominated by anharmonic effects.

Structural effects of high pressure gas on the rotator phases of normal alkanes

A path of polymer crystallization has recently been proposed by Strobl by which lamellae are first formed in a mesophase, which then convert to the crystal form with a nanoscale granular or blocklike structure. The crystallization temperature dependence of the lamellar thickness was argued to imply a transient mesophase. Here we argue that the granular structure of the lamellae is itself a fingerprint of the transient mesophase into which a lamella originally grows and its conversion to the higher-density stable form. The lower interfacial energy of the mesophases, which cause them to be favored as the initial growing phase, is directly connected to their lower density. The reduction in area at the mesophase-stable transition, being frustrated by the tethering of the stems to the amorphous region, results in the granular structure. We also discuss the effect of the relaxation of the stresses in the amorphous region and importance of hysteresis at the mesophase-to-crystal transition.

Eric B. Sirota

Papers

X-Ray and Optical Studies of the Thickness Dependence of the Phase Diagram of Liquid-Crystal Films

X-ray Scattering Studies of Aligned, Stacked Surfactant Membranes.

The effects of substrate roughness on ultrathin water films

Structural effects of high pressure gas on the rotator phases of normal alkanes

Polymer crystallization : Metastable mesophases and morphology