Showing papers by "University of California, Santa Barbara published in 1994"

Review of particle properties.

Abstract: This biennial Review summarizes much of Particle Physics. Using data from previous editions, plus 2205 new measurements from 667 papers, we list, evaluate, and average measured properties of gauge bosons, leptons, quarks, mesons, and baryons. We also summarize searches for hypothetical particles such as Higgs bosons, heavy neutrinos, and supersymmetric particles. All the particle properties and search limits are listed in Summary Tables. We also give numerous tables, figures, formulae, and reviews of topics such as the Standard Model, particle detectors, probability, and statistics. This edition features expanded coverage of CP violation in B mesons and of neutrino oscillations. For the first time we cover searches for evidence of extra dimensions (both in the particle listings and in a new review). Another new review is on Grand Unified Theories. A booklet is available containing the Summary Tables and abbreviated versions of some of the other sections of this full Review. All tables, listings, and reviews (and errata) are also available on the Particle Data Group website: http://pdg.lbl.gov.

Chaos and quantum thermalization

Abstract: We show that a bounded, isolated quantum system of many particles in a specific initial state will approach thermal equilibrium if the energy eigenfunctions which are superposed to form that state obey Berry's conjecture. Berry's conjecture is expected to hold only if the corresponding classical system is chaotic, and essentially states that the energy eigenfunctions behave as if they were Gaussian random variables. We review the existing evidence, and show that previously neglected effects substantially strengthen the case for Berry's conjecture. We study a rarefied hard-sphere gas as an explicit example of a many-body system which is known to be classically chaotic, and show that an energy eigenstate which obeys Berry's conjecture predicts a Maxwell-Boltzmann, Bose-Einstein, or Fermi-Dirac distribution for the momentum of each constituent particle, depending on whether the wave functions are taken to be nonsymmetric, completely symmetric, or completely antisymmetric functions of the positions of the particles. We call this phenomenon eigenstate thermalization. We show that a generic initial state will approach thermal equilibrium at least as fast as O(\ensuremath{\Elzxh}/\ensuremath{\Delta})${\mathit{t}}^{\mathrm{\ensuremath{-}}1}$, where \ensuremath{\Delta} is the uncertainty in the total energy of the gas. This result holds for an individual initial state; in contrast to the classical theory, no averaging over an ensemble of initial states is needed. We argue that these results constitute a sound foundation for quantum statistical mechanics.

Generalized synthesis of periodic surfactant/inorganic composite materials

Abstract: THE recent synthesis of silica-based mesoporous materials1,2 by the cooperative assembly of periodic inorganic and surfactant-based structures has attracted great interest because it extends the range of molecular-sieve materials into the very-large-pore regime. If the synthetic approach can be generalized to transition-metal oxide mesostructures, the resulting nanocomposite materials might find applications in electrochromic or solid-electrolyte devices3,4, as high-surface-area redox catalysts5 and as substrates for biochemical separations. We have proposed recently6 that the matching of charge density at the surfactant/inorganic interfaces governs the assembly process; such co-organization of organic and inorganic phases is thought to be a key aspect of biomineralization7. Here we report a generalized approach to the synthesis of periodic mesophases of metal oxides and cationic or anionic surfactants under a range of pH conditions. We suggest that the assembly process is controlled by electrostatic complementarity between the inorganic ions in solution, the charged surfactant head groups and—when these charges both have the same sign—inorganic counterions. We identify a number of different general strategies for obtaining a variety of ordered composite materials.

Time-dependent transport in interacting and noninteracting resonant-tunneling systems

Abstract: We consider a mesoscopic region coupled to two leads under the influence of external timedependent voltages. The time dependence is coupled to source and drain contacts, the gates controlling the tunnel-barrier heights, or to the gates that define the mesoscopic region. We derive, with the Keldysh nonequilibrium-Green-function technique, a formal expression for the fully nonlinear, time-dependent current through the system. The analysis admits arbitary interactions in the mesoscopic region, but the leads are treated as noninteracting. For proportionate coupling to the leads, the time-averaged current is simply the integral between the chemical potentials of the time-averaged density of states, weighted by the coupling to the leads, in close analogy to the time-independent result of Meir and Wingreen [Phys. Rev. Lett. 68, 2512 (1992)]. Analytical and numerical results for the exactly solvable noninteracting resonant-tunneling system are presented. Due to the coherence between the leads and the resonant site, the current does not follow the driving signal adiabatically: a "ringing" current is found as a response to a voltage pulse, and a complex time dependence results in the case of harmonic driving voltages. We also establish a connection to recent linear-response calculations, and to earlier studies of electron-phonon scattering effects in resonant tunneling.

Lyapunov stability theory of nonsmooth systems

Abstract: This paper develops nonsmooth Lyapunov stability theory and LaSalle's invariance principle for a class of nonsmooth Lipschitz continuous Lyapunov functions and absolutely continuous state trajectories. Computable tests based on Filipov's differential inclusion and Clarke's generalized gradient are derived. The primary use of these results is in analyzing the stability of equilibria of differential equations with discontinuous right-hand side such as in nonsmooth dynamic systems or variable structure control. >

On black hole entropy

Abstract: Two techniques for computing black hole entropy in generally covariant gravity theories including arbitrary higher derivative interactions are studied. The techniques are Wald's Noether charge approach introduced recently, and a field redefinition method developed in this paper. Wald's results are extended by establishing that his local geometric expression for the black hole entropy gives the same result when evaluated on an arbitrary cross section of a Killing horizon (rather than just the bifurcation surface). Further, we show that his expression for the entropy is not affected by ambiguities which arise in the Noether construction. Using the Noether charge expression, the entropy is evaluated explicitly for black holes in a wide class of generally covariant theories. For a Lagrangian of the functional form L\ifmmode \tilde{}\else \~{}\fi{}=L\ifmmode \tilde{}\else \~{}\fi{}(${\mathrm{\ensuremath{\psi}}}_{\mathit{m}}$, ${\mathrm{\ensuremath{ abla}}}_{\mathit{a}}$${\mathrm{\ensuremath{\psi}}}_{\mathit{m}}$,${\mathit{g}}_{\mathit{a}\mathit{b}}$,${\mathit{R}}_{\mathit{a}\mathit{b}\mathit{c}\mathit{d}}$, ${\mathrm{\ensuremath{ abla}}}_{\mathit{e}}$${\mathit{R}}_{\mathit{a}\mathit{b}\mathit{c}\mathit{d}}$), it is found that the entropy is given by S=-2\ensuremath{\pi}\ensuremath{\oint}(${\mathit{Y}}^{\mathit{a}\mathit{b}\mathit{c}\mathit{d}}$-${\mathrm{\ensuremath{ abla}}}_{\mathit{e}}$${\mathit{Z}}^{\mathit{e}:\mathit{a}\mathit{b}\mathit{c}\mathit{d}}$) \ensuremath{\epsilon}${\mathrm{^}}_{\mathit{a}\mathit{b}}$\ensuremath{\epsilon}${\mathrm{^}}_{\mathit{c}\mathit{d}}$\ensuremath{\epsilon}\ifmmode\bar\else\textasciimacron\fi{}, where the integral is over an arbitrary cross section of the Killing horizon, \ensuremath{\epsilon}${\mathrm{^}}_{\mathit{a}\mathit{b}}$ is the binormal to the cross section, ${\mathit{Y}}^{\mathit{a}\mathit{b}\mathit{c}\mathit{d}}$=\ensuremath{\partial}L\ifmmode \tilde{}\else \~{}\fi{}/\ensuremath{\partial}${\mathit{R}}_{\mathit{a}\mathit{b}\mathit{c}\mathit{d}}$, and ${\mathit{Z}}^{\mathit{e}:\mathit{a}\mathit{b}\mathit{c}\mathit{d}}$=\ensuremath{\partial}L\ifmmode \tilde{}\else \~{}\fi{}/\ensuremath{\partial}${\mathrm{\ensuremath{ abla}}}_{\mathit{e}}$${\mathit{R}}_{\mathit{a}\mathit{b}\mathit{c}\mathit{d}}$.Further, it is shown that the Killing horizon and surface gravity of a stationary black hole metric are invariant under field redefinitions of the metric of the form g${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}_{\mathit{a}\mathit{b}}$\ensuremath{\equiv}${\mathit{g}}_{\mathit{a}\mathit{b}}$+${\mathrm{\ensuremath{\Delta}}}_{\mathit{a}\mathit{b}}$, where ${\mathrm{\ensuremath{\Delta}}}_{\mathit{a}\mathit{b}}$ is a stationary tensor field that vanishes at infinity and is regular on the horizon (including the bifurcation surface). Using this result, a technique is developed for evaluating the black hole entropy in a given theory in terms of that of another theory related by field redefinitions. Remarkably, it is established that certain perturbative, first order, results obtained with this method are in fact exact. A particular result established in this fashion is that a scalar matter term of the form ${\mathrm{\ensuremath{ abla}}}^{2\mathit{p}}$\ensuremath{\varphi}${\mathrm{\ensuremath{ abla}}}^{2\mathit{q}}$\ensuremath{\varphi} in the Lagrangian makes no contribution to the black hole entropy. The possible significance of these results for the problem of finding the statistical origin of black hole entropy is discussed.

Critical layer thickness for self-assembled InAs islands on GaAs.

Abstract: Using atomic force microscopy (AFM), we have directly observed the progression of surface morphology of InAs deposited by molecular-beam epitaxy on GaAs(100). InAs self-assembled dots (coherent) or relaxed InAs islands (incoherent) are formed depending on the InAs coverage. The InAs coverage was varied continuously and AFM was used to monitor in detail the nucleation and resulting size and shape transition of the InAs self-assembled dots. Dots of uniform size were observed only at the initial stages of this Stranski-Krastanow growth-mode transition. The self-assembled dot density increased very abruptly with total deposited amount of InAs. Treating this InAs growth-mode transition as a first-order phase transition with InAs total coverage as the critical parameter, we extract a critical thickness for surface elastic relaxation of 1.50 ML.

DSC: scheduling parallel tasks on an unbounded number of processors

Abstract: We present a low-complexity heuristic, named the dominant sequence clustering algorithm (DSC), for scheduling parallel tasks on an unbounded number of completely connected processors. The performance of DSC is on average, comparable to, or even better than, other higher-complexity algorithms. We assume no task duplication and nonzero communication overhead between processors. Finding the optimum solution for arbitrary directed acyclic task graphs (DAG's) is NP-complete. DSC finds optimal schedules for special classes of DAG's, such as fork, join, coarse-grain trees, and some fine-grain trees. It guarantees a performance within a factor of 2 of the optimum for general coarse-grain DAG's. We compare DSC with three higher-complexity general scheduling algorithms: the ETF by J.J. Hwang, Y.C. Chow, F.D. Anger, and C.Y. Lee (1989); V. Sarkar's (1989) clustering algorithm; and the MD by M.Y. Wu and D. Gajski (1990). We also give a sample of important practical applications where DSC has been found useful. >

Langmuir-Blodgett films

Abstract: The controlled transfer of organized monolayers of amphiphilic molecules from the airwater interface to a solid substrate was the first molecular-scale technology for the creation of new materials. However, the potential benefits of the technology envisioned by Langmuir and Blodgett in the 1930s have yet to be fully realized. Problems of reproducibility and defects and the lack of basic understanding of the packing of complex molecules in thin films have continued to thwart practical applications of Langmuir-Blodgett films and devices made from such films. However, modern high-resolution x-ray diffraction and scanning probe microscopy have proven to be ideal tools to resolve many of the basic questions involving thin organic films. Here, studies are presented of molecular order and organization in thin films of fatty acid salts, the prototypical system of Katharine Blodgett. Even these relatively simple systems present liquid, hexatic, and crystalline order; van der Waals and strained layer epitaxy on various substrates; wide variations in crystal symmetry and interfacial area with counterions; modulated superstructures; and coexisting lattice structures. The wide variety of possible structures presents both a challenge and an opportunity for future molecular design of organic thin-film devices.

Spectroscopy of quantum levels in charge-tunable InGaAs quantum dots.

Abstract: Imbedding self-assembled lens-shaped InGaAs quantum dots in a suitably designed field-effect-type GaAs/AlAs heterostructure allows us to charge the lowest discrete quantum levels in the dots with single electrons. Because of their small diameters of about 20 nm the Coulomb charging energy is significantly smaller than the quantization energies. We extract energy spacings of about 41 meV between the $s$-like ground state and the first excited $p$-like state from capacitance as well as infrared transmission spectroscopy at low temperatures and under application of high magnetic fields.

Human luminance pattern-vision mechanisms: masking experiments require a new model.

Abstract: A widely used model of simultaneous luminance pattern masking is based on mechanisms that sum inputs linearly and produce a response that is an S-shaped function of that sum This model makes two predictions about masking: (1) Changing the masker spatial waveform will shift the threshold-versus-masker contrast function horizontally by a multiplicative constant (2) Adding a second fixed-contrast masker will shift this function horizontally by an additive constant Experimental tests do not support these predictions The results can be explained by a new model that incorporates broadband divisive inhibition, consistent with physiology

Direct Visualization of Surfactant Hemimicelles by Force Microscopy of the Electrical Double-Layer

Abstract: The morphology of ionic surfactant molecules adsorbed from aqueous solution onto hydrophobic substrates has been determined by atomic force microscopy. Near the critical micelle concentration (cmc), noncontact imaging using double-layer repulsion between the tip and sample shows parallel, epitaxially oriented stripes spaced apart by about twice the surfactant length. This represents the first direct imaging of «hemimicelles», liquid-crystalline aggregates of amphiphilic molecules (analogous to bulk micelles) which form at interfaces. The striped pattern is indicative of hemicylindrical hemimicelles, which is further corroborated by images of the monolayer adsorbate (in contact mode) below the cmc

The physics and mechanics of fibre-reinforced brittle matrix composites

Abstract: This review compiles knowledge about the mechanical and structural performance of brittle matrix composites. The overall philosophy recognizes the need for models that allow efficient interpolation between experimental results, as the constituents and the fibre architecture are varied. This approach is necessary because empirical methods are prohibitively expensive. Moreover, the field is not yet mature, though evolving rapidly. Consequently, an attempt is made to provide a framework into which models could be inserted, and then validated by means of an efficient experimental matrix. The most comprehensive available models and the status of experimental assessments are reviewed. The phenomena given emphasis include: the stress/strain behaviour in tension and shear, the ultimate tensile strength and notch sensitivity, fatigue, stress corrosion and creep.

Interactions of silica surfaces

Abstract: Adhesion, friction, and colloidal forces in air and aqueous salt solutions have been measured between various silica surfaces prepared by depositing amorphous but highly smooth silica films on mica. The results show four interesting and interrelated phenomena: (i) The adhesion of silica surfaces in air increases slowly with contact time, especially in humid air where the "contacting" surfaces become separated by an ∼20-A-thick layer of hydrated silica or silica gel. (ii) The friction of two silica surfaces exhibits large sticking or "stiction" spikes, whose magnitude increases in the presence of water and when the surfaces are kept in contact longer before sliding. (iii) The non-DLVO repulsion commonly seen at short range (<40 A) between silica surfaces immersed in aqueous solutions is monotonically repulsive, with no oscillatory component, and is quite unlike theoretical expectations and previous measurements of forces due to solvent structure. (iv) Dynamic contact angle measurements reveal time-dependent effects which cannot be due to a fixed surface chemical heterogeneity or roughness. The results indicate that silica surfaces undergo slow structural and chemical changes during interactions with water and with each other. More specifically, we propose that the unusual interfacial and colloidal properties of silica are due, not to hydration effects, but to the presence of an ∼10-Athick gel-like layer of protruding silanol and silicilic acid groups that grow on the surfaces in the presence of water. These protruding groups react chemically (sinter) with similar groups located on an opposing surface and give rise to the unusual time-dependent adhesion, friction, and non-DLVO forces observed. Concerning the effects on colloidal interactions, the surface gel-layer effectively shifts the OHP outward and adds a monotonic short-range polymer-like steric repulsion to the DLVO interaction. The mechanism proposed here is quite different from the commonly accepted one, in which modified water structure at the silica surface is believed to give rise to a repulsive "hydration force." The proposed mechanism in terms of a surface layer of silica gel is consistent with the known surface chemistry of silica and accounts for the results reported here and also for many other unusual surface and colloidal properties of silica.

High abundance of Archaea in Antarctic marine picoplankton

Abstract: Archaea (archaebacteria) constitute one of the three major evolutionary lineages of life on Earth. Previously these prokaryotes were thought to predominate in only a few unusual and disparate niches, characterized by hypersaline, extremely hot, or strictly anoxic conditions. Recently, novel (uncultivated) phylotypes of Archaea have been detected in coastal and subsurface marine waters, but their abundance, distribution, physiology and ecology remain largely undescribed. Here we report exceptionally high archaeal abundance in frigid marine surface waters of Antarctica. Pelagic Archaea constituted up to 34% of the prokaryotic biomass in coastal Antarctic surface waters, and they were also abundant in a variety of other cold, pelagic marine environments. Because they can make up a significant fraction of picoplankton biomass in the vast habitats encompassed by cold and deep marine waters, these pelagic Archaea represent an unexpectedly abundant component of the Earth's biota.

Integer quantum Hall transition: An alternative approach and exact results

Abstract: We introduce and analyze a class of model systems to study transitions in the integer quantum Hall effect (IQHE). Even without disorder our model exhibits an IQHE transition as a control parameter is varied. We find that the transition is in the two-dimensional Ising universality class and compute all associated exponents and critical transport properties. The fixed point has time-reversal, particle-hole, and parity invariance. We then consider the effect of quenched disorder on the IQHE transition and find the following. (i) Randomness in the control parameter (which breaks all the above symmetries) translates into bond randomness in the Ising model and is hence marginally irrelevant. The transition may equally well be viewed as a quantum percolation of edge states localized on equipotentials. The absence of random-phase factors for the edge states is responsible for the nongeneric (Ising) critical properties. (ii) For a random magnetic field (which preserves particle-hole symmetry in every realization) the model exhibits an exactly solvable fixed line, described in terms of a product of a Luttinger liquid and an SU(n) spin chain. While exponents vary continuously along the fixed line, the longitudinal conductivity is constant due to a general conformal sum rule for Kac-Moody algebras (derived here), and is computed exactly. We also obtain a closed expression for the extended zero-energy wave function for every realization of disorder and compute its exact multifractal spectrum f(\ensuremath{\alpha}) and the exponents of all participation ratios. One point on the fixed line corresponds to a recently proposed model by Gade and Wegner. (iii) The model in the presence of a random on-site potential scales to a strong disorder regime, which is argued to be described by a symplectic nonlinear-sigma-model fixed point. (iv) We find a plausible global phase diagram in which all forms of disorder are simultaneously considered. In this generic case, the presence of random-phase factors in the edge-state description indicates that the transition is described by a Chalker-Coddington model, with a so far analytically inaccessible fixed point.

A state-space approach to adaptive RLS filtering

Abstract: Adaptive filtering algorithms fall into four main groups: recursive least squares (RLS) algorithms and the corresponding fast versions; QR- and inverse QR-least squares algorithms; least squares lattice (LSL) and QR decomposition-based least squares lattice (QRD-LSL) algorithms; and gradient-based algorithms such as the least-mean square (LMS) algorithm. Our purpose in this article is to present yet another approach, for the sake of achieving two important goals. The first one is to show how several different variants of the recursive least-squares algorithm can be directly related to the widely studied Kalman filtering problem of estimation and control. Our second important goal is to present all the different versions of the RLS algorithm in computationally convenient square-root forms: a prearray of numbers has to be triangularized by a rotation, or a sequence of elementary rotations, in order to yield a postarray of numbers. The quantities needed to form the next prearray can then be read off from the entries of the postarray, and the procedure can be repeated; the explicit forms of the rotation matrices are not needed in most cases. >

Plane curves associated to character varieties of 3-manifolds.

Abstract: Consider a compact 3-manifold M with boundary consisting of a single torus. The papers [CS1, CS2, CGLS] discuss the variety of characters of SL2(C) representations of zl(M), and some of the ways in which the topological structure of M is reflected in the algebraic geometry of the character variety. We will describe in this paper a certain affine algebraic curve DM which is naturally associated to the character variety of M. A basis B = {~, 9.1/} for the peripheral subgroup of M determines an embedding P8 of DM into C* x C* with coordinates l and m. The closure in C 2 of ps(DM) is a plane algebraic curve and therefore is defined by a polynomial AM,n0, m) that, after certain normalizations, is uniquely determined up to multiplication by constants. The polynomial AM, B is effectively computable and is an invariant of the manifold M together with the choice of basis B. The results in this paper describe how geometric properties of the character variety, and hence topological properties of M, are reflected by the polynomial AM, B. In the case that M is the complement of a knot K in a homology 3-sphere we may take the basis B to consist of the longitude and meridian of K. With the usual orientation conventions this basis is well-defined modulo the involution which inverts both the longitude and meridian. It will follow from the construction of DM that if the basis B' is obtained from B by inverting both generators then the regular maps PB and PB, have the same image. Thus Ar:= AM, s = AM, B, is an invariant of the knot. The polynomial AM, B displays, in a striking way, information about the incompressible surfaces in M. This involves the Newton polygon of AM.B,

Adaptive control of plants with unknown dead-zones

Abstract: Model reference adaptive controllers are designed for plants with unknown dead-zones. Several control strategies are investigated in which two sets of adjustable parameters, one belonging to a dead-zone inverse and the other to a linear controller, are either kept fixed or adaptively updated. The developed adaptive control schemes ensure boundedness of all closed-loop signals and reduce the tracking error. >

Active integrated antennas

Abstract: This paper provide a review of the active integrated antenna (AIA) technologies. After a brief introduction on the definition and some historical remarks, the paper concentrates on the research effort on the past decades or so. The AlAs are reviewed in its various functions. First, an oscillator-type AIA is presented, followed by very interesting aspects of coupled oscillator arrays for phase control. Use of an AIA concept for efficient RF front ends is described with examples on high-power amplifier AlAs. Next, a phase-conjugation-based retrodirective array is reviewed. Finally, AIA systems for receiving, transmitting, and duplexing are reviewed.

Catalysis of dynamical flavor symmetry breaking by a magnetic field in 2 +1 dimensions

Abstract: It is shown that in 2 + 1 dimensions, a constant magnetic field is a strong catalyst of dynamical flavor symmetry breaking, leading to generating a fermion dynamical mass even at the weakest attractive interaction between fermions. The effect is illustrated in the Nambu-Jona-Lasinio model in a magnetic field. The low-energy effective action in this model is derived, and the thermodynamic properties of the model are established.

Terrestrial remote sensing science and algorithms planned for EOS/MODIS

Abstract: The Moderate Resolution Imaging Spectroradiometer (MODIS) will be the primary daily global monitoring sensor on the NASA Earth Observing System (EOS) satellites, scheduled for launch on the EOS-AM platform in June 1998 and the EOS-PM platform in December 2000. MODIS is a 36 channel radiometer covering 0·415-14·235 μm wavelengths, with spatial resolution from 250 m to 1 km at nadir. MODIS will be the primary EOS sensor for providing data on terrestrial biospheric dynamics and process activity. This paper presents the suite of global land products currently planned for EOSDIS implementation, to be developed by the authors of this paper, the MODIS land team (MODLAND). These include spectral albedo, land cover, spectral vegetation indices, snow and ice cover, surface temperature and fire, and a number of biophysical variables that will allow computation of global carbon cycles, hydrologic balances and biogeochemistry of critical greenhouse gases. Additionally, the regular global coverage of these var...

Anderson model out of equilibrium: Noncrossing-approximation approach to transport through a quantum dot.

Abstract: The infinite-U Anderson model is applied to transport through a quantum dot. The current and density of states are obtained via the noncrossing approximation for two spin-degenerate levels weakly coupled to two leads. At low temperatures, the Kondo peak in the equilibrium density of states strongly enhances the linear-response conductance. Application of a finite voltage bias reduces the conductance and splits the peak in the density of states. The split peaks, one at each chemical potential, are suppressed in amplitude by a finite dissipative lifetime. We estimate this lifetime perturbatively as the time to transfer an electron from the higher-chemical-potential lead to the lower-chemical-potential one. At zero magnetic field, the clearest signatures of the Kondo effect in transport through a quantum dot are the broadening, shift, and enhancement of the linear-response conductance peaks at low temperatures, and a peak in the nonlinear differential conductance around zero bias.

Surface morphology and mechanical properties of MDCK monolayers by atomic force microscopy

Abstract: We describe the morphology and mechanical stability of the apical surface of MDCK monolayers by atomic force microscopy (AFM). Living cells could be imaged in physiological solution for several hours without noticeable deterioration. Cell boundaries appear as ridges that clearly demarcate neighboring cells. In some cases the nucleus of individual cells could be seen, though apparently only in very thin areas of the monolayer. Two types of protrusions on the surface could be visualized. Smooth bulges that varied in width from a few hundred nanometers to several micrometers, which appear to represent relatively rigid subapical structures. Another type of protrusion extended well above the membrane and was swept back and forth during the imaging. However, the microvilli that are typically present on the apical surface could not be resolved. For comparison, a transformed MDCK cell line expressing the K-ras oncogene was also examined. When cultured on solid substrata at low density, the R5 cells spread out and are less than 100 nm thick over large areas with both extensive processes and rounded edges. Many intracellular structures such as the nucleus, cytoskeletal elements and vesicles could be visualized. None of the intracellular structures seen in the AFM images could be seen by scanning electron microscopy. Both R5 cells and MDCK monolayers required imaging forces of > 2 nN for good image contrast. Force measurements on the MDCK monolayers show that they are very soft, with an effective spring constant of approximately 0.002 N/m for the apical plasma membrane, over the first micrometer of deformation, resulting in a height deformation of approximately 500 nm per nanoNewton of applied force. The mechanical properties of the cells could be manipulated by addition of glutaraldehyde. These changes were monitored in real time by collecting force curves during the fixation reaction. The curves show a stiffening of the apical plasma membrane that was completed in approximately 1 minute. On the basis of these measurements and the imaging forces required, we conclude that deformation of the plasma membrane is an important component of the contrast mechanism, in effect 'staining' structures based on their relative rigidity.