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The ground state of two-dimensional electron systems (2DESs) at low Landau level filling factors ($\nu\lesssim1/6$) has long been a topic of interest and controversy in condensed matter. Following the recent breakthrough in the quality of ultra-high-mobility GaAs 2DESs, we revisit this problem experimentally and investigate the impact of reduced disorder. In a GaAs 2DES sample with density $n=6.1\times10^{10}$ /cm$^2$ and mobility $\mu=25\times10^6$ cm$^2$/Vs, we find a deep minimum in the longitudinal magnetoresistance ($R_{xx}$) at $\nu=1/7$ when $T\simeq104$ mK. There is also a clear sign of a developing minimum in the $R_{xx}$ at $\nu=2/13$. While insulating phases are still predominant when $\nu\lesssim1/6$, these minima strongly suggest the existence of fractional quantum Hall states at filling factors that comply with the Jain sequence $\nu=p/(2mp\pm1)$ even in the very low Landau level filling limit. The magnetic field dependent activation energies deduced from the relation $R_{xx}\propto e^{E_A/2kT}$ corroborate this view, and imply the presence of pinned Wigner solid states when $\nu\neq p/(2mp\pm1)$. Similar results are seen in another sample with a lower density, further generalizing our observations., Comment: 6 pages, 4 figures

Pinning modes of a Wigner solid show remarkably small screening by a fractional quantum Hall liquid in close proximity., We studied a bilayer system hosting two-dimensional electron systems (2DESs) in close proximity but isolated from one another by a thin barrier. One 2DES has low electron density and forms a Wigner solid (WS) at high magnetic fields. The other has much higher density and, in the same field, exhibits fractional quantum Hall states (FQHSs). The WS spectrum has resonances which are understood as pinning modes, oscillations of the WS within the residual disorder. We found the pinning mode frequencies of the WS are strongly affected by the FQHSs in the nearby layer. Analysis of the spectra indicates that the majority layer screens like a dielectric medium even when its Landau filling is ~1/2, at which the layer is essentially a composite fermion (CF) metal. Although the majority layer is only ~ one WS lattice constant away, a WS site only induces an image charge of ~0.1e in the CF metal.

Microwave frequency conductivity $\mathrm{Re}({\ensuremath{\sigma}}_{\mathrm{xx}})$ of high quality two-dimensional hole systems (2DHS) in a large perpendicular magnetic field (B) is measured with the carrier density ${(n}_{s})$ of the 2DHS controlled by a backgate bias. The high-$B$ insulating phase of the 2DHS exhibits a microwave resonance that remains well defined, but shifts to higher peak frequency ${(f}_{\mathrm{pk}})$ as ${n}_{s}$ is reduced. In different regimes, ${f}_{\mathrm{pk}}$ vs ${n}_{s}$ can be fit to ${f}_{\mathrm{pk}}\ensuremath{\propto}{n}_{s}^{\ensuremath{-}1/2}$ or to ${f}_{\mathrm{pk}}\ensuremath{\propto}{n}_{s}^{\ensuremath{-}3/2}.$ The data clearly indicate that both carrier-carrier interactions and disorder are indispensable in determining the dynamics of the insulator. The ${n}_{s}$ dependence of ${f}_{\mathrm{pk}}$ is consistent with a weakly pinned Wigner crystal in which domain size increases with ${n}_{s},$ due to larger carrier-carrier interaction.

We have measured the real part of the diagonal conductivity, Re(σxx) for a 2D hole system in the low Landau filling, insulating regime. The spectra, Re(σxx) versus frequency f, show a well-defined, single resonance peak, which is surprisingly sharp. Plots of the resonance peak frequency and integrated intensity versus the applied magnetic field B are in conflict with the model of a 2D harmonic oscillator in B.

The experimental results of Faraday rotation (FR) measurements in dilute magnetic semiconductors in high magnetic field (`Dirac Series' – Los Alamos) are presented. The magnetic field is produced by an explosive-driven flux-compression generator (150 T). Measurements have been carried with samples of Cd 1− x Mn x Te with x =0.43 using 633 nm light at liquid helium temperature. The FR increases in such samples when the magnetic field exceeds 60 T. Interband exchange interaction and the direct influence of the external magnetic field on the exchange interaction must be considered to interpret the experimental results.

We report reflectivity measurements at 810 nm wavelength on GaAs/GaAlAs multiple quantum wells and NbSe2 layers at 75 K up to magnetic fields of 800 T. In the GaAs system, we observed in two separate measurements new, reproducible oscillatory phenomena in the reflectivity between 30 and 800 T, and in a third measurement on 2H-NbSe2 we observed a decrease in reflectivity of about 50% above 200 T, with some additional evidence for oscillatory behavior. We discuss these measurements based on the expected behavior in terms of their known physical properties, and consider future prospects for the application of optical methods to study condensed matter physics under these extremes.

We present data on finite frequency (0.2 ≤ f ≤ 8.6 GHz) conductivity, Re(σxx), of the insulator terminating a fractional quantum Hall effect series in the high magnetic field (B) limit. As B is increased within the insulator, at temperatures of 50 and 200 mK Re(σxx) vs f evolves from a decreasing curve into a peak. The height and f of the peak increase with B. Integrals of Re(σxx) vs f become B-independent at large enough B. The results are discussed within the framework of the Fukuyama-Lee model of the dynamics of a pinned Wigner crystal.

We describe purpose-built instrumentation that has proved successful in making transport measurements of materials in ultrahigh magnetic fields generated by explosive-driven flux compression. The experimental arrangement minimizes severe problems of heating and pick-up associated with the microsecond pulsed field, for which dB/dt can be as large as 109 T/s. Electrical connection to multiple samples in the high field region is via long, lithographically-defined, nested coplanar transmission lines. Contactless measurements by capacitive coupling, as well as resistively coupled measurements, are made at frequencies of order 1 GHz so that pick-up from the microsecond pulse can be effectively filtered. We demonstrate our technique with data on semiconductor samples taken to 450 T using Russian MC-1 type flux compression generators combined with U.S. explosives at the Ancho Canyon firing point, Los Alamos National Laboratory. We discuss the obstacles encountered during these experiments, and outline improvement...

We have measured microwave conductivity, Ressxxd, of a high quality two-dimensional hole system (2DHS) in the high magnetic field (B) insulating phase for frequency ( f) between 0.2 and 9 GHz. For n , 0.3, Ressxxd vs f shows a well-defined peak at frequency fpk. As B is increased, the maximal Ressxxd increases. The resonance Q (fpk divided by full line width at half maximum) increases linearly with B. fpk vs B increases, but is nearly flat, at 1.25 GHz, for B $ 10 T. Oscillator strength, S, measured by integrating the spectrum, is likewise flat for B $ 10 T. The simultaneously B-independent S and fpk are inconsistent with the conventional (Fukuyama-Lee) interpretation of the resonance as a Wigner crystal pinning mode. [S0031-9007(97)03876-3]

The conductivity of the high magnetic field insulating phases of two-dimensional electron and hole systems (2DES and 2DHS) of sufficiently low disorder exhibits a resonance at microwave frequency. We present and compare results on this resonance in a moderate disorder 2DES and in a high quality 2DHS. The 2DES exhibits the resonance even though the high- B insulator is not reentrant around the 1 5 fractional quantum Hall effect . The resonance in the 2DHS is much sharper than that in the 2DES, and appears to be inconsistent with the standard model of a two-dimensional harmonic oscillator in a magnetic field.

In the regime of the fractional quantum Hall effect, with Landau filling ν

We investigate the low-temperature magneto-transport properties of monolayer epitaxial graphene films formed on the Si-face of semi-insulating 4H-SiC substrates by a high temperature sublimation process. A high-k top-gate on the epitaxial graphene is realized by inserting a fully oxidized nanometer thin aluminum film as a seeding layer, followed by an atomic layer deposition process. At low temperatures, the devices demonstrate a strong field effect by the top gate with an on/off ratio of ~7 and an electron mobility up to ~3250 cm^2/Vs. After the observation of the half-integer quantum Hall effect for monolayer epitaxial graphene films, detailed magneto-transport measurements have been carried out including varying densities, temperatures, magnetic fields and currents. We study the width of the distinguishable quantum-Hall plateau to plateau transition (Landau level index n=0 to n=1) as temperature (T) and current are varied. For both gate voltage and magnetic field sweeps and T>10 K the transition width goes as T^{-\kappa} with exponent \kappa ~0.42. This universal scaling exponent agrees well with those found in III-V heterojunctions with short range alloy disorders and in exfoliated graphene., Comment: accepted by Journal of Applied Physics

We report the observation of a resonance in the microwave spectra of the real diagonal conductivities of a two-dimensional electron system within a range of ∼ ± 0.015 from filling factor ν = 1/3. The resonance is remarkably similar to resonances previously observed near integer ν, and is interpreted as the collective pinning mode of a disorder-pinned Wigner solid phase of e/3-charged carriers.

We have measured the microwave conductance of mechanically exfoliated graphene at frequencies up to 8.5 GHz. The conductance at 4.2 K exhibits quantum oscillations, and is independent of the frequency.

We report the observation of a resonance in the microwave spectra of the real diagonal conductivities of a two-dimensional electron system within a range of ∼ ± 0.015 from filling factor ν = 1/3. The resonance is remarkably similar to resonances previously observed near integer ν, and is interpreted as the collective pinning mode of a disorder-pinned Wigner solid phase of e/3-charged carriers.

Microwave pinning-mode resonances found around integer quantum Hall effects, are a signature of crystallized quasiparticles or holes. Application of in-plane magnetic field to these crystals, increasing the Zeeman energy, has negligible effect on the resonances just below Landau-level filling $\ensuremath{\nu}=2$, but increases the pinning frequencies near $\ensuremath{\nu}=1$, particularly for smaller quasiparticle or hole densities. The charge dynamics near $\ensuremath{\nu}=1$, characteristic of a crystal order, are affected by spin, in a manner consistent with a Skyrme crystal.

We have measured high frequency magnetotransport of a high quality two-dimensional electron system (2DES) near the reentrant insulating phase (RIP) at Landau fillings ($\nu$) between 1/5 and 2/9. The magneto\textit{conductivity} in the RIP has resonant behavior around 150 MHz, showing a \textit{peak} at $\nu$$\sim$0.21. Our data support the interpretation of the RIP as due to some pinned electron solid. We have also investigated a narrowly confined 2DES recently found to have a RIP at 1/3$, Comment: 4 pages and 1 figure (amsart format), 16th International Conference on High Magnetic Fields in Semiconductor Physics (SemiMag16), August 2-6, 2004, Tallahassee

We present measurements of the real diagonal microwave conductivity of the reentrant insulating quantum Hall phases in the first excited Landau level at temperatures below $50\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$. A resonance is detected around filling factor $\ensuremath{\nu}=2.58$ and weaker frequency dependence is seen at $\ensuremath{\nu}=2.42$ and 2.28. These measurements are consistent with the formation of a bubble phase crystal centered around these $\ensuremath{\nu}$ at very low temperatures.

Ultrashort electrical pulses were used to characterize the magnetoplasmon resonance of a two-dimensional electrongas formed in an AlGaAs/GaAs heterostructure at frequencies up to 400 gigahertz. This was accomplished byincorporating the sample into a guided wave probe operating in a dilution refrigerator. A bath temperatureof 50mK was recorded during measurements, demonstrating the feasibility of this approach in other situationsrequiring high magnetic “eld and mK temperatures.Keywords: Auston Switch, Magnetoplasmon, Cyclotron Resonance, Micr owave 1. INTRODUCTION The far infrared properties of two-dimensional electron gas (2DEG) systems have been studied extensively withcommon examples being observations of cyclotron resonance 1,2 and magnetoplasmon resonances. 3,4 This paperaddresses a novel approach to these measurements that implements Auston switches for picosecond electricalpulse launching and detection in a waveguide in order to allow for time-resolved detection of the magnetoplasmonresonance. Although the incorporation of Auston switches into waveguides has been well characterized, therehave been relatively few low temperature applications.An Auston switch is a relatively simple optoelectronic device comprised of two conductors on a photocon-ductive layer having a short recombination time (

We present the results of an experimental study of the current-voltage characteristics in strong magnetic field ($B$) of disordered, superconducting, thin-films of amorphous Indium-Oxide. As the $B$ strength is increased superconductivity degrades, until a critical field ($B_c$) where the system is forced into an insulating state. We show that the differential conductance measured in the insulating phase vanishes abruptly below a well-defined temperature, resulting in a clear threshold for conduction. Our results indicate that a new collective state emerges in two-dimensional superconductors at high $B$., Comment: TeX file and 4 figures; final accepted version

We present the results of an experimental study of superconducting, disordered, thin-films of amorphous Indium Oxide. These films can be driven from the superconducting phase to a reentrant insulating state by the application of a perpendicular magnetic field ($B$). We find that the high-$B$ insulator exhibits activated transport with a characteristic temperature, $T_I$. $T_I$ has a maximum value ($T_{I}^p$) that is close to the superconducting transition temperature ($T_c$) at $B$ = 0, suggesting a possible relation between the conduction mechanisms in the superconducting and insulating phases. $T_{I}^p$ and $T_c$ display opposite dependences on the disorder strength., Comment: Tex file and 5 figures; Revised version; To appear in Phys. Rev. Lett. (2004)

We have measured the diagonal conductivity, ${\ensuremath{\sigma}}_{xx}$, in the microwave regime of an ultrahigh mobility two dimensional electron system. We find a sharp resonance in $\mathrm{R}\mathrm{e}[{\ensuremath{\sigma}}_{xx}]$ versus frequency when $\ensuremath{\nu}g4$ and the partial filling of the highest Landau level, ${\ensuremath{\nu}}^{*}$, is $\ensuremath{\sim}1/4$ or $3/4$ and temperatures $l0.1\text{ }\text{ }\mathrm{K}$. The resonance appears for a range of ${\ensuremath{\nu}}^{*}$ from 0.20 to 0.38 and again from 0.64 to 0.80. The peak frequency ${f}_{\mathrm{p}\mathrm{k}}$ changes from $\ensuremath{\sim}500$ to $\ensuremath{\sim}150\text{ }\text{ }\mathrm{M}\mathrm{H}\mathrm{z}$ as ${\ensuremath{\nu}}^{*}=1/2$ is approached. This range of ${f}_{\mathrm{p}\mathrm{k}}$ shows no dependence on $\ensuremath{\nu}$ where the resonance is observed. The quality factor, $Q$, of the resonance is maximum at about ${\ensuremath{\nu}}^{*}=0.25$ and 0.74. We interpret the resonance as due to a pinning mode of the bubble phase crystal.

We have measured the diagonal conductivity, sigma(xx), in the microwave regime of an ultrahigh mobility two dimensional electron system. We find a sharp resonance in Re[sigma(xx)] versus frequency when nu4 and the partial filling of the highest Landau level, nu(*), is approximately 1/4 or 3/4 and temperatures0.1 K. The resonance appears for a range of nu(*) from 0.20 to 0.38 and again from 0.64 to 0.80. The peak frequency f(pk) changes from approximately 500 to approximately 150 MHz as nu(*)=1/2 is approached. This range of f(pk) shows no dependence on nu where the resonance is observed. The quality factor, Q, of the resonance is maximum at about nu(*)=0.25 and 0.74. We interpret the resonance as due to a pinning mode of the bubble phase crystal.

We measure microwave frequency (4-40 GHz) photoresistance at low magnetic field B, in high mobility 2D electron gas samples, excited by signals applied to a transmission line fabricated on the sample surface. Oscillatory photoresistance vs B is observed. For excitation at the cyclotron resonance frequency, we find an unprecedented, giant relative photoresistance (\Delta R)/R of up to 250 percent. The photoresistance is apparently proportional to the square root of applied power, and disappears as the temperature is increased., Comment: 4 pages, 3 figures

This article presents an overview of the Human Genome Project. The goals of the program are outlined; the genetic, physical, and DNA sequence maps that will be its products are defined; the brief origins of the project are traced; the management structure of the US Human Genome Project, in which the Department of Energy and the National Institutes of Health participate as partners, is outlined; international dimensions are discussed; the excellent progress to date with respect to each of the eight scientific goals is reported; and the projected impact of the project on biology and medicine is discussed.

Epitaxial graphene films were formed on the Si-face of semi-insulating 4H-SiC substrates by a high temperature sublimation process. A high-k gate stack on epitaxial graphene is realized by inserting a fully oxidized nanometer thin aluminum film as a seeding layer followed by an atomic-layer deposition process. The electrical properties of epitaxial graphene films are sustained after gate stack formation without significant degradation. At low temperatures, the quantum-Hall effect in Hall resistance is observed along with pronounced Shubnikov-de Hass oscillations in diagonal magneto-resistance of gated epitaxial graphene on SiC (0001)., 2 new references added

A rigid formalism has been developed for the calculation of the order parameter S for fluorescence probes embedded in environments that hinder the motions of the probes and for calculation of a "cone angle" of fluorophore rotation from the order parameters. The motions of the fluorescence probes 1,6-diphenylhexa-1,3,5-triene (DPH) and 1-[4-(trimethylamino)phenyl]-6-phenylhexa-1,3,5-triene (TMA-DPH) embedded in lipid bilayers were then analyzed in terms of the order parameter and the cone angle. Order parameters for such fluorescence can only be compared to the average order parameter over a segment of a fatty acyl chain or of membrane thickness. Also, because the bilayer may be perturbed by the fluorophore at regions distant from the immediate location of the probe, these "averaged" order parameters cannot be easily compared to those calculated from nuclear magnetic resonance data but are more readily compared to order parameters of electron spin resonance probes. There is no defined mathematical relation between the order parameter and a dynamical parameter which would afford a calculation of membrane "microviscosity". A Gaussian angular freedom parameter or cone angle of fluorophore motion has been calculated from the order parameters and shows in a geometric sense the limitations imposed on the angular displacements of TMA-DPH as compared to DPH.

Sodium butyrate treatment of cultures of ChaGo (human lung cancer) cells resulted in increased production of human chorionic gonadotropin (hCG) and its alpha subunit (hCG-alpha) and induced a variety of morphologic changes. Elongation and flattening of cells were seen by light microscopy. Immunocytochemistry with antisera against hCG and against hCG-alpha showed an increase in cells containing stainable hCG-alpha. Scanning electron microscopy demonstrated enhanced adhesion of cells to glass cover slips, with elongation, flattening, and decreased cytoplasmic blebs. Ultrastructural changes were examined by transmission electron microscopy and evaluated quantitatively by an unbiased observer. Significant findings included increases in perinuclear tonofilaments, smooth endoplasmic reticulum vesicles, dense mitochondrial inclusions, and lipid granules, as well as decreases in intercellular desmosomes, free polyribosomes, mitochondrial dense granules, and Golgi complexes. The most notable change, a marked decrease in condensed chromatin clumps, may have reflected a butyrate-induced biochemical modification of chromatin leading to enhanced accessibility of certain genes for transcription.

Three continuous lines of mammary tumor cells (ZR-75-1, ZR-75-27, and ZR-75-30) have been established from malignant effusions of two women with breast cancer. Differentiated properties expressed by each cell line include: (a) epithelial morphology (by light and electron microscopy) resembling that of the parental tumors; (b) presence of receptors for estrogen and other steroid hormones; and (c) growth responsiveness to estrogen and/or progesterone. All three cell lines possess human karyotypes that differ from one another in modal chromosome number as well as in characteristic marker chromosomes. Two of the cultures (ZR-75-27 and ZR-75-30), although derived from the same patient, have stable differences in their karyotypes.

A comprehensive listing of putative human breast carcinoma cell lines and the extent to which each has been characterized is presented. Criteria used to certify the human, mammary, and malignant origin of a cell line include: (a) a reliable histopathological diagnosis; (b) interspecies specificity established by human karyotype, isoenzyme profiles, and/or cell surface antigenicity; (c) intraspecies specificity, demonstrated by genetic evidence of a unique, human donor distinct from other cells including HeLa cells; and (d) organ specficity, supported by morphological evidence of epithelial structure and secretory activity, and especially by the expression of differentiated functions; these include presence of receptors for sex steroid hormones, hormone responsiveness, and production of milk proteins, fatty acids, or milk-specific antigens. Of the 47 cell lines for which data are here reported, 22 have been shown to be derived from human non-HeLa donors and to have epithelial morphology as revealed by light or electron microscopy. Differentiated function has been recorded for 19 cell lines. Additional human breast cancer cell lines have been reported, but characterization of some of these has been insufficient to judge the legitimacy of their predigrees. For others mammary origin is questionable. Six purported breast cell lines are in reality HeLa cells, and one is of nonhuman origin.

The experimental results of Faraday rotation (FR) measurements in the sample with Eu3+ ion at 4 K in high magnetic fields up to 700 T (`Dirac Series' – Los Alamos) are presented. The magnetic field is produced by the explosive-driven flux-compression generator MC-1. Measurements have been carried using 850 nm light at liquid helium temperature. The magnetic field-induced quantum reorientation of the spin and orbital magnetic moments in the Sm3+ and Eu3+ ions have been investigated.