Search

Newton’s law of cooling describes an object whose temperature decreases exponentially with time. Because of its many applications, it is a frequent topic of introductory physics labs. In this article, I describe an experiment designed for the freshman year algebra-based physics course that applies this law to answering the question, “Why don’t small mammals live in the ocean?” The experiment is part of a process under way to include more biology-related examples in this first-year sequence to better serve and provide relevance to our biology majors, who predominantly take the course. The experiment focuses on scaling laws and modeling, and demonstrates the utility of physics in answering questions in biology.

We present reflectance and polarization phase curve measurements of highly reflective planetary regolith analogues having physical characteristics expected on atmosphereless solar system bodies (ASSBs) such as a eucritic asteroids or icy satellites. We used a goniometric photopolarimeter (GPP) of novel design to study thirteen well-sorted particle size fractions of aluminum oxide (Al2O3). The sample suite included particle sizes larger than, approximately equal to, and smaller than the wavelength of the incident monochromatic radiation (λ = 635 nm). The observed phase angle, α, was 0.056 o ∼95%). The incident radiation has a very high probability of being multiply scattered before being backscattered toward the incident direction or ultimately absorbed. The five smallest particle sizes exhibited extremely high void space (> ∼95%). The reflectance phase curves for all particle size fractions show a pronounced non-linear reflectance increase with decreasing phase angle at α∼ The polarization phase curves for particle size fractions with size parameter (particle radius/wavelength) r/λ ∼1 we detect no polarization. This polarization behavior is distinct from that observed in low albedo solar system objects such as the Moon and asteroids and for absorbing materials in the laboratory. We suggest this behavior arises because photons that are backscattered have a high probability of having interacted with two or more particles, thus giving rise to the CB process. These results may explain the unusual negative polarization behavior observed near small phase angles reported for several decades on highly reflective ASSBs such as the asteroids 44 Nysa, 64 Angelina and the Galilean satellites Io, Europa and Ganymede. Our results suggest these ASSB regoliths scatter electromagnetic radiation as if they were extremely fine grained with void space > ∼95%, and grain sizes of the order The GPP used in this study was modified from our previous design so that the sample is presented with light that is alternatingly polarized perpendicular to and parallel to the scattering plane. There are no analyzers before the detector. This optical arrangement, following the Helmholtz Reciprocity Principle (HRP), produces a physically identical result to the traditional laboratory reflectance polarization measurements in which the incident light is unpolarized and the analyzers are placed before the detector. The results are identical in samples measured by both methods. We believe that ours is the first experimental demonstration of the HRP for polarized light, first proposed by Helmholtz in 1856.

Bacterial biofilms are more resilient to standard killing methods than free-living bacteria. Pseudomonas aeruginosa PAO1 biofilms grown on borosilicate coupons were treated with gas-discharge plasma for various exposure times. Almost 100% of the cells were inactivated after a 5-min plasma exposure. Atomic force microscopy was used to image the biofilms and study their micromechanical properties. Results show that the adhesiveness to borosilicate and the thickness of the Pseudomonas biofilms are reduced upon plasma treatment.

The geology and physics programs at Western Carolina University were revised to provide investigative, quantitative, and interdisciplinary opportunities at all levels of the curriculum. Central to the revisions was the development of the Cullowhee Creek Environmental Field Station (CCEFS). The field station, a part of the Western Carolina University campus, includes three groundwater wells, a gaging station, a weather station, and areas to investigate the physical, hydrological, and environmental systems on campus. Hydrological measurements are made using continuously monitored loggers and probes, hand-held instruments, and flow meters. Shallow subsurface characteristics are evaluated through surface material observations, shallow soil probes, and geophysical (seismic, resistivity, magnetic and ground-penetrating radar) surveys. Curricular reforms using the CCEFS have emphasized development of investigative projects related to the campus geological environment in several introductory through advanced cour...

Biofilms are microbial communities attached to a surface and embedded in a matrix composed of exopolysaccharides and excreted nucleic acids. Bacterial biofilms are responsible for undesirable effects such as disease, prostheses colonization, biofouling, equipment damage, and pipe plugging. Biofilms are also more resilient than free-living cells to regular sterilization methods and therefore it is indispensable to develop better ways to control and remove them. The use of gas discharge plasmas is a good alternative since plasmas contain a mixture of reactive agents well-known for their decontamination potential against free microorganisms. We have previously reported that Pseudomonas aeruginosa biofilms were inactivated after a 1-min plasma exposure. We determined that the adhesiveness and the thickness of Pseudomonas biofilms grown on borosilicate were reduced. We also reported sequential morphological changes and loss of viability upon plasma treatment. However, the studies were carried out in batch cultures. The use of a continuous culture results in a more homogenous environment ensuring reproducible biofilm growth. The aim of this work was to study plasma-mediated inactivation of P. aeruginosa biofilms grown on borosilicate in a continuous culture system. In this paper we show that biofilms grown on glass under continuous culture can be inactivated by using gas discharge plasma. Both biofilm architecture and cell culturabilty are impacted by the plasma treatment. The inactivation kinetics is similar to previously described ones and cells go through sequential changes ranging from minimal modification without loss of viability at short plasma exposure times, to major structure and viability loss at longer exposure times. We report that changes in biofilm structure leading to the loss of culturability and viability are related to a decrease of the biofilm matrix adhesiveness. To our knowledge, there has been no attempt to evaluate the inactivation/sterilization of biofilms grown in a continuous system., Centro de Investigación y Desarrollo en Fermentaciones Industriales

Compared to pyrolytic graphite (PG) and pyrolytically coated polycrystalline graphite (PCPG), which are commonly employed substrates for graphite furnace atomic absorption spectrometry (GFAAS), highly oriented pyrolytic graphite (HOPG) is characterized by a high degree ofc-axis alignment with relatively few active sites and imperfections present on the surface. The usefulness of HOPG and commercial PG platforms for the determination of lead, copper, and aluminum by GFAAS was compared. Qualitative comparison of the absorption profiles and temperature optimizations suggest that lead, copper with a chemical modifier (palladium), and aluminum interact similarly with the two graphite substrates, while copper without a modifier interacts less strongly with HOPG than with PG. These results are in agreement with previous studies that have demonstrated that copper interacts strongly with PG and PCPG. HOPG and PG were employed for the determination of a pine needles standard reference material (SRM) and Fraser fir (Abies fraseri) samples. The analyses of the SRM gave good agreement with the certified value using both substrates, and comparable values were obtained for the samples. These results suggest that HOPG may be useful as a model for PG or PCPG when techniques such as scanning tunneling microscopy that require a high degree ofc-axis alignment are employed.

Using a scanning tunneling microscope in a large number of approach and retraction experiments, quantized conductance is observed for the first time in gallium. We compare the conductance quantization at temperatures between 300 and 4.2 K, just slightly below and far below the bulk melting point of gallium. We find that the conductance is significantly dependent on temperature and find a moderate conductance peak at 1Go (2e 2 /h) that increases in intensity with decreasing temperature. At 4.2 K, a second conductance peak at 2Go is observed. We attribute our observations to gallium’s unique orthorhombic crystal structure and a significant increase in atomic disorder at temperatures near the melting point. q 1999 Elsevier Science Ltd. All rights reserved.

We describe a diffraction grating experiment for visible and microwave optics. The groove spacing for the visible-optics grating is determined through atomic force microscopy, so that an additional feature, the blaze angle, is revealed. From the blaze angle, students calculate the theoretical shift in the diffraction pattern and are then able to visualize this shift through measurements of a microwave diffraction pattern produced using a macroscopic grating. The experiment demonstrates the utility of an atomic force microscope in an engineering application and allows separation of diffraction pattern effects associated with slit width and slit spacing.

NUE funded work at California State Polytechnic University involved development and implementation of nanotechnology modules for physics courses spanning all levels of the undergraduate curriculum, from freshman service courses to senior level laboratories and independent research projects. These modules demonstrate the application of fundamental physics at the nanoscale that complement macroscopic investigations. The introductory level and some of the advanced level modules have been described previously in journal papers and will be outlined briefly here. The main focus of this article, however, is to describe some newer work involving nanoscale experiments that have been developed for senior level laboratories and independent research. These experiments involve applications as diverse as tunneling diodes, gas discharge plasmas for biofilm inactivation, and quantized conductance in gold nanowires.

Scanning tunneling microscopy (STM) was used to elucidate the submicrometer defect structures on the graphite substrates used in graphite furnace atomic absorption spectrometry (GFAAS). Images were obtained on pristine pyrolytic coated and uncoated polycrystalline graphite tubes and on pure pyrolytic graphite platforms. For comparison, images of highly oriented pyrolytic graphite, not used in GFAAS, were also obtained. Polycrystalline tubes were characterized by disordered surfaces with extensive oxidation. Pyrolytic coated tubes and pure pyrolytic graphite platforms were characterized by scaled structures, island columns, or smoothly varying contours. Scaled structures and island columns presented the greatest abundance of exposed carbon edge sites and seemed probable areas for analyte reactivity and intercalation. In areas displaying smoothly varying contours, atomic imaging revealed basal plane surface layers with moderate curvature but regular spacing between the carbon atoms within the layers. The results imply that the weak ordering between graphite layers along the c-axis does not preclude good atomic ordering within the layers, and this factor may be sufficient for discouraging analyte—substrate interactions.

Proximity effect gaps were observed consistently in Nb/Au, Bi2Sr2CaCu2O8+y/Au, and HgBa2CuO4/Ag bilayers using a low temperature scanning tunneling microscope. The Nb/Au conductance curves [dI(V)/dV vs V] were fit by the Arnold model, using the accepted energy gap value of Nb, 1.55 meV. The application of gold and silver overlayers allowed vacuum tunneling and topographic images of BSCCO single crystals and the HgBa2CuO4 polycrystalline sample. Cleavage steps of varying sizes (up to 1500 A) in the BSCCO/Au sample were observed, and gaplike features were found on such steps. Consistent, reproducible energy gaps were also found in the Hg‐compound sample.

A brief, selective review shows the unconventional nature of the normal state, the loss of spectral weight and appearance of a 500 cm −1 psuedo-gap in the low temperature infrared conductivity, and the evidence for pair formation above T c in YBa 2 Cu 3 O 7-x .

The temperature dependence of the energy gap of bulk polycrystalline Ba1−xKxBiO3 (BKBO) was measured with a low‐temperature scanning tunneling microscope in the point contact mode. At 4.2 K, the tunneling conductance shows nearly ideal BCS behavior, however at higher temperatures, the lifetime broadening term Γ was required to fit the data. Γ(T) displays a surprising T3 temperature dependence and the magnitude is significantly larger than the calculated theoretical recombination times. Comparisons with other conductance data obtained from grain boundary and sandwich‐type BKBO junctions show similar results. The large scattering rates for BKBO may be important for the utility of this material in SIS mixers.

We have studied the structural and superconducting properties of La[sub 2[minus][ital x]]Sr[sub [ital x]]CuO[sub 4] by neutron and x-ray powder diffraction, thermogravimetry, ac susceptibility, and dc magnetization. The kinetics of the synthesis reaction were studied in order to determine the optimum conditions for preparing single-phase samples. The room-temperature and low-temperature structural properties, as determined by neutron powder diffraction, are discussed in detail. By comparing the results of the neutron-diffraction and magnetic measurements, we conclude that superconductivity exists in both the orthorhombic and tetragonal phases. The thermodynamic conditions for the stability of the La[sub 2[minus][ital x]]Sr[sub [ital x]]CuO[sub 4] phase are also investigated.

Most studies dealing with growth and physiology of bacteria have been carried out using free-living cells. However, most bacteria live in communities referred to as biofilms where cooperative interactions among their members make conventional methods of controlling microbial growth often ineffective. The use of gas discharge plasmas represents an alternative to traditional decontamination/sterilization methods. We studied biofilms using two organisms, Chromobacterium violaceum and Pseudomonas aeruginosa. With the first organism we demonstrated almost complete loss of cell culturability after a 5-min plasma treatment. However, additional determinations showed that non-culturable cells were still alive after short exposure times. We have recently reported the effect of plasma on P. aeruginosa biofilms grown on borosilicate coupons. In this paper, we present results for plasma treatments of 1-, 3-, and 7-day old P. aeruginosa biofilms grown on polycarbonate or stainless-steel coupons. Results indicate nearly 100% of ­biofilm inactivation after 5 min of exposure with similar inactivation kinetics for 1-, 3-, and 7-day-old biofilms, and for both materials used. The inactivation kinetics is similar for both organisms, suggesting that the method is useful regardless of the type of biofilm. AFM images show changes in biofilm structure for various plasma exposure times.

We present measurements of the magnetization hysteresis on neutron- and proton-irradiated ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ single crystals. The crystals were irradiated to fluences of 2\ifmmode\times\else\texttimes\fi{}${10}^{17}$ n ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ (${\mathit{E}}_{\mathit{n}}$g0.1 MeV) and 1\ifmmode\times\else\texttimes\fi{}${10}^{16}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ (${\mathit{E}}_{\mathit{p}}$=3.5 MeV), respectively. The magnetization was measured for both field directions. We observed increases of the hysteresis by a factor of 5 at 10 K and 1 T for H\ensuremath{\parallel}c for the neutron-irradiated sample and a factor of 9 for the proton-irradiated sample. For H\ensuremath{\parallel}ab the hysteresis increased by a factor of 8 and 6, respectively. The irradiated crystals were annealed at 100, 200, and 300 \ifmmode^\circ\else\textdegree\fi{}C in air for 8 h each. The irradiation-induced decrease of ${\mathit{T}}_{\mathit{c}}$ is fully recovered after the final annealing step for both types of irradiation. The magnetization hysteresis for H\ensuremath{\parallel}ab decreases almost to its preirradiation level after annealing at 300 \ifmmode^\circ\else\textdegree\fi{}C for both types of irradiation. In contrast, for H\ensuremath{\parallel}c, 50% of the initial neutron-irradiation- and 30% of the proton-irradiation-induced enhancements of the magnetization hysteresis are still maintained after the final annealing. The differences in annealing between neutron- and proton-irradiated samples are attributed to the different amount of point defects created by these types of irradiation.

An in situ x-ray reflectivity study of the dynamic evolution of a growing interface was carried out for gold sputter-deposited onto a polished silicon substrate. X-ray reflectivity data were recorded during growth for thicknesses of the gold film ranging from 50 to 3500 \AA{}. A progressive kinetic roughening of the gold-vacuum interface was observed and the time-dependent interfacial width exhibits a power-law behavior. Aided by scanning-tunneling-microscopy measurements the scaling exponents were determined and compared with theoretical studies.

We have calibrated the displacement/voltage (A/V) response of our piezoelectric scanning tube (PZT‐5A) by imaging graphite at over 40 temperatures between 4 and 300 K. We have also calibrated the (A/V) response as a function of voltage up to 220 V at room temperature, imaging a gold‐plated diffraction grating. We find that the temperature dependence of the (A/V) response is linear to within 10% and is reduced by a factor of 5.5 on decreasing temperature from 300 to 4.2 K. The near linearity with temperature of the (A/V) response makes the PZT‐5A lead zirconate titanate composition a convenient choice for low temperature scanning tunneling microscope piezo tube elements.

Results of dc magnetization on neutron (n) -irradiated ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ single crystals are presented. The crystals were irradiated to a fluence of 2\ifmmode\times\else\texttimes\fi{}${10}^{17}$ n/${\mathrm{cm}}^{2}$ (Eg0.1 MeV). The magnetization has been measured at 10, 40, and 70 K for both field orientations. The magnetization is thereby obtained by cooling the sample in zero field to the desired temperature. Then the field is ramped in steps of 0.25 T up to 5 T and reversed in the same steps. At each field the magnetization is measured. The hysteresis \ensuremath{\Delta}M is defined as the difference in magnetization between descending and ascending branches in the hysteresis loop. At 10 K, we observe an increase of the hysteresis by a factor of 3 for H\ensuremath{\parallel}c and by a factor of 10 for H\ensuremath{\parallel}a,b due to n irradiation. After irradiation, some of the crystals were annealed at 100, 200, and 300 \ifmmode^\circ\else\textdegree\fi{}C for 8 h each. After each annealing step, the critical current densities and the transition temperatures were determined again. We observe that following the 300 \ifmmode^\circ\else\textdegree\fi{}C annealing, the magnetization hysteresis for H\ensuremath{\parallel}a,b is almost reduced to the preirradiation level (a reduction in ${\mathit{J}}_{\mathit{c}}$ of 90%), in contrast to the H\ensuremath{\parallel}c case, in which the decrease in hysteresis due to annealing is much less (a reduction in ${\mathit{J}}_{\mathit{c}}$ of 30%). Crystals irradiated under the same conditions were used for transmission-electron-microscopy (TEM) studies. By comparison between TEM and magnetization measurements, the role of the cascade defects in pinning was determined. It is furthermore shown by TEM that the cascade defects do not anneal for comparable annealing temperatures. We find that the hysteresis for H\ensuremath{\parallel}c is governed by both the cascade defects which do not anneal and point defects or their clusters on the Y layers or ${\mathrm{CuO}}_{2}$ planes which do anneal, whereas for H\ensuremath{\parallel}a,b, pinning is determined by oxygen defects in the CuO chains which anneal due to oxygen diffusion.

Using rotating analyzer ellipsometry we have measured all three components of the dielectric tensor of YBa 2 Cu 3 O 7 from the near-infrared to the vacuum-ultraviolet (0.7–24 eV). In the low-energy region the spectra for a − b polarization can be well modelled by a Drude free-carrier response with a frequency independent scattering rate. Below 10 eV we observe strongly anisotropic interband transitions involving the partially filled valence band and the conduction band. At higher energies the anisotropy becomes smaller. In that region we see two prominent double structures around 16 and 20 eV that we assign to transitions from lower-lying bands with O s and Ba p character to states close to the Fermi level.

Conventional disinfection and sterilization methods are often ineffective with biofilms, which are ubiquitous, hard-to-destroy microbial communities embedded in a matrix mostly composed of exopolysaccharides. The use of gas-discharge plasmas represents an alternative method, since plasmas contain a mixture of charged particles, chemically reactive species and UV radiation, whose decontamination potential for free-living, planktonic micro-organisms is well established. In this study, biofilms were produced using Chromobacterium violaceum, a Gram-negative bacterium present in soil and water and used in this study as a model organism. Biofilms were subjected to an atmospheric pressure plasma jet for different exposure times. Our results show that 99.6 % of culturable cells are inactivated after a 5 min treatment. The survivor curve shows double-slope kinetics with a rapid initial decline in c.f.u. ml−1 followed by a much slower decline with D values that are longer than those for the inactivation of planktonic organisms, suggesting a more complex inactivation mechanism for biofilms. DNA and ATP determinations together with atomic force microscopy and fluorescence microscopy show that non-culturable cells are still alive after short plasma exposure times. These results indicate the potential of plasma for biofilm inactivation and suggest that cells go through a sequential set of physiological and morphological changes before inactivation.

The reduction in flux motion resistivity due to pinning at twin boundaries in single crystal YBa 2 Cu 3 O x is demonstrated. Pinning takes place only for magnetic fields aligned within a critical angle of the twin boundary plane. The critical angle varies with field direction and with field strength for H | c . The onset temperature of twin boundary pinning is established by its strong anisotropy, and shown to correspond with a feature in the temperature dependent resistivity for H | c .

Over the past several years a coherent phenomenology of the high {Tc} cuprate superconductors has begun to emerge. Infrared measurements have contributed several important ingredients to this picture including: (1) the inference of a scattering rate that is linear in frequency for {omega}>T, and of order {omega}, (2) a characteristic energy scale in the superconducting state of 500 cm{sup {minus}1} (60 meV), which can be interpreted as a superconducting pair excitation threshold or energy gap, and (3) evidence for very unusual temperature dependence in the vicinity of {Tc}. An attempt to describe these aspects of the data is presented here.

Magnetization and lattice-constant measurements were done on four single crystals of YBa{sub 2}Cu{sub 3{minus}{ital x}}Fe{sub {ital x}}O{sub 7{minus}{delta}} with Fe concentrations of {ital x}=0.06, 0.09, 0.15, and 0.24 to determine the structural and normal-state properties. Measurements of magnetization as a function of temperature were done in a temperature range between 90 and 300 K and analyzed using a Curie-Weiss law with a temperature-independent background. The effective magnetic moment obtained corresponds to a low spin state of Fe{sup 3+} with a magnitude of 2.18{mu}{sub {ital B}}. Magnetic anisotropy between the {ital ab} plane and the {ital c} direction is only present for the concentration of {ital x}=0.06 and is attributed to the various oxygen environments surrounding the iron. For concentrations larger than {ital x}=0.06, the moment on Fe is isotropic and Curie-Weiss fits predict uniform antiferromagnetic ordering. Structural measurements exhibit an orthorhombic-tetragonal transition at an Fe concentration of {ital x}=0.15, similar to that observed in the polycrystalline material.

In this article we describe a simple SQUID magnetometer system useful for the magnetic characterization of high temperature superconductors. The system employs a commercial rf SQUID probe with associated electronics located in a 4He gas exchange cryostat. The system is equipped with a copper coil dc magnet for fields up to 50 Gauss. The cryostat is surrounded by a μ‐metal shield that keeps the remanent field to less than 20 mGauss. The apparatus has been used extensively to measure both the superconducting transition temperature and width as well as the relative quantities of magnetic flux expulsion and exclusion. It has therefore served as an important first step in the evaluation of sample quality.

We present dc-magnetization measurements in fields up to 5 T on oxygen-deficient ({ital T}{sub {ital c}}{congruent}60 K) YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} single crystals. The upper-critical-field slopes are {minus}2.0 and {minus}8.7 T/K and the lower critical fields at 5 K are 83 and 25 Oe for fields perpendicular and parallel to the Cu-O planes, respectively. The upper-critical-field slope anisotropy is not substantially different from the anisot- ropy found for the 90-K crystals. The {ital H}{sub {ital c}1} values are, however, much smaller for the 60-K crystals. We explain these results through a model in which the chains do not contribute significantly to the coupling between the Cu-O planes but rather serve as a charge reservoir for the superconductivity that occurs in the planes.

We have developed the concept of a characteristic temperature {ital T}{sup *} at which the flux first fully penetrates the sample. Based on this concept, we have interpreted the effects of the temperature on the magnetic relaxation that have been measured in a single crystal of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub {ital x}} in a broad range of temperature (5--35 K) and applied magnetic field (0.1--4 kG). The magnetic relaxation exhibits a clear peak in the {ital dM}/{ital d} ln{ital t}-versus-temperature plot, and the peak position is highly affected by the applied field. We have also interpreted the peak shift of {ital dM}/{ital d} ln{ital t} with different fields in terms of the flux-creep and the critical-state models.

The syntheses, structures, selected physical properties, and band electronic structures of three copper (I) dicyanamide halide salts of bis(ethylenedithio)tetrathiafulvalene [κ-(ET) 2 Cu[N(CN) 2 ]X, where X = Cl, Br, and I] are discussed. X-ray crystallographic studies demonstrate that the three derivatives are isostructural. The bromide salt is an ambient pressure superconductor with an inductive onset at 11.6 K and a resistive onset at 12.5 K. κ-(ET) 2 Cu[N(CN) 2 ]Cl exhibits the highest reported superconducting transition temperature (T c = 12.8 K. 0.3 kbar) for an organic superconductor, once a semiconductor-semiconductor transition (42 K) is suppressed. The application of GE varnish or Apiezon N grease to crystals of κ-(ET) 2 Cu[N(CN) 2 ]Cl provides sufficient stress to induce superconductivity at “ambient pressure”. Crystals of the iodide remain metallic to ∼150 K, where they become weakly semiconductive. No sign of superconductivity was detected at pressures (hydrostatic and shearing) up to 5.2 kbar and at temperatures as low as 1.1 K. The band electronic structures of the three salts are essentially identical. The differences in superconducting properties are explained in terms of differences in lattice softness, which are strongly influenced by short C-H…donor and C-H…anion contacts.

DC magnetization measurements of H c2 are used to establish a 3D Ginzburg-Landau model of the superconductivity near T c Resistive measurements of the characteristic sin 2 θ angular dependence associated with flux motion models show that only a fraction of the dissipation in the superconducting state is due to Lorentz force effects.

We report a determination of the temperature dependence of the upper critical magnetic fields ${\mathit{H}}_{\mathit{c}2}$ for \ensuremath{\kappa}-(BEDT-TTF${)}_{2}$Cu[N(CN${)}_{2}$]Br, with the use of dc-magnetization measurements that show a well-defined diamagnetic onset. We find large slopes of -20 T/K and -2.2 T/K for H\ensuremath{\parallel}ac and H\ensuremath{\parallel}b, respectively, which lead to coherence lengths of 37 and 4 \AA{}. There is unusual structure in the critical fields and a possible dimensional crossover at T/${\mathit{T}}_{\mathit{c}}$\ensuremath{\sim}0.97 for H\ensuremath{\parallel}ac. Neither the slopes nor the structure are observable in the superconducting transition measured by ac magnetoresistance, which exhibits strong magnetic-field broadening similar to that of the high-${\mathit{T}}_{\mathit{c}}$ copper oxide superconductor ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$. We find a strong depression of ${\mathit{T}}_{\mathit{c}}$, as large as 1.2 K, when the sample is rapidly cooled.

We report measurements of the field of first magnetic flux entry and the derived values of the lower critical field {ital H}{sub {ital c}1} for different orientations of the applied field with respect to the twin boundaries. For a field orientation perpendicular to the twin boundaries, increased values of {ital H}{sub {ital c}1} are found. The results are interpreted in terms of an increased vortex creation energy when the vortex intersects a twin boundary. In an untwinned crystal, {ital H}{sub {ital c}1} saturates at low temperatures in agreement with measurements of penetration depths from other experiments. We find no significant {ital a}-{ital b} anisotropy in {ital H}{sub {ital c}1} in the untwinned crystal.

We have carried out angle-resolved photoemission spectroscopy on single crystals of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.9}$. The crystals were cleaved in situ, under ultrahigh vacuum while the sample stage was cooled to 8 K. By observing the dispersion of the electron energy bands as they cross the Fermi energy, we have mapped the Fermi surfaces. There is reasonable agreement between the experimental results and the predictions of band-structure calculations using the local-density approximation, as well as with positron annihilation experiments. Rather than \ensuremath{\delta} holes, these results indicate that the Fermi surface of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.9}$ encloses 1+\ensuremath{\delta} holes, where \ensuremath{\delta} is the doping level.

New data on the upper critical magnetic field of single crystals of URu2Si2 show distinct upward curvature for the field H in the tetragonal basal plane, but not for fields along the tetragonal axis. The data for H

By quenching the tetragonal phase during crystal growth of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} and then oxygenating the crystals without mechanical stress, we obtain some essentially untwinned crystals. Magnetic measurements on such crystals, with fields applied along the {ital a} or {ital b} axis, show anisotropy. A qualitative analysis of our relaxation and hysteresis data indicates that the critical current density is higher along {ital a} than along {ital b} (the chain direction). The anisotropic Meissner percent data show that pinning is higher for fields along {ital b} than along {ital a}.

We have carried out angle-resolved photoemission Spectroscopy on single crystals of YBa 2 Cu 3 O 6.9 . The crystals were cleaved in situ, under ultra high vacuum while the sample stage was cooled to below 10°K. By observing the dispersion of the electron energy bands as they cross the Fermi energy, we have mapped the Fermi surfaces. There is reasonable agreement between the experimental results and the predictions of band structure calculations using the local density approximation, as well as with positron annihilation experiments.

We have performed angle-integrated and angle-resolved photoemission measurements at 20 K on well-characterized single crystals of high Tc cuprates (both 1:2:3-type and 2:2:1:2-type) cleaved in situ, and find a relatively large, resolution limited Fermi edge which shows large amplitude variations with photon energy, indicative of band structure final state effects. The lineshape of the spectra of the 1:2:3 materials as a function of photon energy are well reproduced by band structure predictions, indicating a correct mix of 2p and 3d orbitals in the calculations, while the energy positions of the peaks agree with calculated bands to within ≈0.5 eV. This may yet prove to reflect the effects of Coulomb correlation. We nevertheless conclude that a Fermi liquid approach to conductivity is appropriate. Angle-resolved data, while still incomplete, suggest agreement with the Fermi surface predicted by the LDA calculations. A BCS-like energy gap is observed in the 2:2:1:2 materials, whose magnitude is twice the weak coupling BCS value (i.e., 2Δ ≈ 7kTc).

Summary form only given. The removal and disposal of biofilms is an expensive, time-consuming effort required in many industries (medical, and water purification, to name a few). Biofilms are microbial communities that coalesce in an exopolysaccharide matrix which enables them to adhere to different surfaces and make them more resistant to conventional sterilization techniques than free, mobile bacteria. Gas discharge plasmas are a novel approach to inactivating biofilms. They contain a mixture of charged particles, chemically reactive species, and UV radiation, all of which are well-established sterilization agents against free microorganisms. Four day-old single-species biofilms were grown in 96-well polystyrene microplates using Chromobacterium violaceum, a gram negative bacterium. The gas discharge plasma was produced with an Atomflo 250 plasma reactor (Surfx Technologies) in which discharge occurs in gas flowing through two narrowly spaced, perforated electrodes (one at ground, one at 100 MHz RF). A mixture of helium and a secondary gas, nitrogen, was used. Bacterial biofilms were exposed to it for various time periods. The spectroscopy data were obtained using an Ocean Optics HR-2000 spectrometer with 0.5 nm resolution. The microscopy data were obtained using a Quesant atomic force microscope (AFM) in air in intermittent contact mode. Our data show that after a 10-minute plasma treatment, almost 100% of the viable cells are removed. The kinetics show a rapid initial decline in CFU/mL followed by a much slower subsequent decline. Spectroscopy data show the presence of two different chemically reactive oxygen species: NO and OH. These reactive species interact with biomolecules in the bacterial cell membranes and walls. The microscopy data show that non culturable cells are still intact at short exposure times. The microscopy data also suggest that cells may undergo cell wall damage after longer plasma exposure periods. These results indicate the potential of plasma as an alternative way for biofilm inactivation.

A series of laboratory modules were developed to introduce atomic force microscope (AFM) applications into undergraduate physics courses. The goal is to elucidate fundamental physics concepts at the nanoscale that will complement existing investigations at the macroscale, and to expose students to advanced instrumentation at an early level. The experiments allow students to experience the full range of scanning probe modes available, which include contact, intermittent contact, lateral force, and magnetic force microscopy as well as force distance spectroscopy. The course levels span the range from freshman introductory to advanced senior level and allow students to experience AFM applications at successive levels of complexity.

Meissner- and shielding-effect measurements were made on an oxygen-deficient single crystal of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ as a function of aging time at room temperature following a quench from 225 \ifmmode^\circ\else\textdegree\fi{}C. It is seen that the Meissner fraction systematically increases from 11.1% to 13.2% as ${\mathit{T}}_{\mathit{c}}$ changes from 11.2 to 25.0 K. All aging was done within the helium exchange gas environment of the sample chamber. The results point to oxygen vacancy ordering rather than oxygen absorption as the mechanism for both the ${\mathit{T}}_{\mathit{c}}$ and Meissner-fraction increase.

Bacterial biofilms are microbial communities that are less susceptible to standard killing methods than free-living bacteria. Gas-discharge plasmas were used to treat biofilms for various exposure times. After 5-min plasma exposure, 90% of culturable cells were removed. Atomic-force-microscope images that reveal the sequential changes in cell morphology occurring during plasma treatment are presented.

We have measured ac magnetoresistance in fields up to 8 T in a variety of YBa2Cu3O7−δ single crystals containing (a) twin boundaries only in one direction, (b) multi‐twin domain structure, and (c) growth steps. We find two components of the resistance: an angular dependent Lorentz force induced ‘‘excess resistivity’’ superimposed on an angular independent resistivity. The temperature and magnetic field dependence of the angular‐dependent resistivity exhibit a strong correlation with crystal defects, whereas the angular‐independent resistivity remains virtually unchanged. In addition, we find crystals exhibiting both ohmic and non‐ohmic behavior in their I‐V characteristics depending on crystal imperfections.

Measurements of the magnetization hysteresis of the same crystal in the twinned and untwinned state reveal that pinning due to twinning planes may contribute a substantial, anisotropic fraction of the observed critical currents. A linear scaling between magnetization hysteresis and sample dimension shows that the twinned and untwinned crystal does not exhibit any subgranular effects.

Low‐field dc magnetization measurements in fields between 0.5 and 40 G are reported on a single crystal of nominal composition Tl2Ba2Ca2Cu3O10. It is observed that Hc2, determined from the diamagnetic onset temperature, is quite anisotropic. Extrapolating Hc2 to zero field yields zero field superconductive transition temperatures of Tc=118 K for H parallel to the c axis and Tc=111 K for H perpendicular to the c axis. This apparent inconsistency is thought to be due to the variation in stacking sequence of the Cu‐O sheets.

AC magnetoresistance measurements were performed on twinned single crystals of YBa2Cu3O7−δ to study the effect of dissipative flux flow motion due to Lorentz force. We report the first direct observation of Lorentz force induced flux flow resistivity in high-Tc superconductors which varies as sin2(θ) with respect to transport current-magnetic field orientation. However, the intrinsic broadening of the resistive transition in magnetic field exists even in the absence of Lorentz force and hence cannot be explained by flux flow dissipative resistivity.

The upper critical field of polycrystalline Ba 0.65 K 0.35 BiO 3 has been measured in magnetization and in resistivity experiments. Both methods yield essentially the same results: a clear upward curvature near T c , and a more nearly linear region at lower temperature with a slope of ∼−0.8T/K. The agreement between the magnetic and resistive measurements rules out any significant flux flow resistivity.

We discuss the evolution of our understanding of the normal state infrared conductivity of cuprate superconductors. The discussion begins with results from polycrystalline samples and continues up to the recent work on single domain (untwinned) crystals. The issue of whether the conductivity at low frequency (i.e. ω≲1000 cm−1) should be described with one (intraband) component, or with two (intraband + interband) components is central to the discussion.

A single biconical fiber taper is a simple and low-cost yet powerful sensor. With a distinct strength in refractive index RI sensing, biconical tapered fiber sensors can find their place in handheld sensor platforms, especially as biosensors that are greatly needed in health care, environmental protection, food safety, and biodefense. We report doubling of sensitivity for these sensors with two passes through the tapered region, which becomes possible through the use of sensitive and high-dynamic-range photodetectors. In a proof-of-principle experiment, we measured transmission through the taper when it was immersed in isopropyl alcohol-water mixtures of varying concentrations, in which a thin gold layer at the tip of the fiber acted as a mirror enabling two passes through the tapered region. This improved the sensitivity from 0.43 dB/vol % in the single-pass case to 0.78 dB/vol % with two passes through the taper. The refractive index detection limit was estimated to be 1.210 5 RI units RIU and 0.610 5 RIU in the single- and double-pass schemes, respectively. We predict that further enhance- ment of sensitivity may be achieved with a higher number of passes through the taper. © 2010 Society of Photo-Optical Instrumentation