Penetration Depth Measurements Research Articles

Monitoring Gramicidin Conformations in Membranes: A Fluorescence Approach

We have monitored the membrane-bound channel and nonchannel conformations of gramicidin utilizing red-edge excitation shift (REES), and related fluorescence parameters. In particular, we have used fluorescence lifetime, polarization, quenching, chemical modification, and membrane penetration depth analysis in addition to REES measurements to distinguish these two conformations. Our results show that REES of gramicidin tryptophans can be effectively used to distinguish conformations of membrane-bound gramicidin. The interfacially localized tryptophans in the channel conformation display REES of 7 nm whereas the tryptophans in the nonchannel conformation exhibit REES of 2 nm which highlights the difference in their average environments in terms of localization in the membrane. This is supported by tryptophan penetration depth measurements using the parallax method and fluorescence lifetime and polarization measurements. Further differences in the average tryptophan microenvironments in the two conformations are brought out by fluorescence quenching experiments using acrylamide and chemical modification of the tryptophans by N-bromosuccinimide. In summary, we report novel fluorescence-based approaches to monitor conformations of this important ion channel peptide. Our results offer vital information on the organization and dynamics of the functionally important tryptophan residues in gramicidin.

Probing the superconducting gap symmetry ofPrRu4Sb12:A comparison withPrOs4Sb12

We report measurements of the magnetic penetration depth $\lambda$ in single crystals of PrRu$_{4}$Sb$_{12}$ down to 0.1 K. Both $\lambda$ and superfluid density $\rho_{s}$ exhibit an exponential behavior for $T$ $<$ 0.5$T_{c}$, with parameters $\Delta$(0)/\textit{k}$_{B}$\textit{T}$_{c}$ = 1.9 and $\lambda(0)$ = 2900 \AA. The value of $\Delta$(0) is consistent with the specific-heat jump value of $\Delta C/\gamma T_{c}$ = 1.87 measured elsewhere, while the value of $\lambda(0)$ is consistent with the measured value of the electronic heat-capacity coefficient $\gamma$. Our data are consistent with PrRu$_{4}$Sb$_{12}$ being a moderate-coupling, fully-gapped superconductor. We suggest experiments to study how the nature of the superconducting state evolves with increasing Ru substitution for Os.

Polaronic origin of the isotope effect on the London penetration depth in high-temperature superconducting oxides

Direct measurements of the in-plane London penetration depth λ L have recently been performed on high-temperature superconducting copper oxides by a new low-energy muon spin rotation technique. The results show that λ L is isotope dependent, evidencing unconventional electron–phonon interactions as its source. The data are interpreted here in terms of polaronic effects on the single-particle energies, which leads to level shifts and exponential band narrowing. Good agreement with the experimental data is obtained.

Microstructure and temperature dependence of microwave penetration depth of Ag doped Y 1Ba 2Cu 3O 7− x thin films

We report the measurements of magnetic penetration depth λ( T) of Ag-doped YBa 2Cu 3O 7− δ (YBCO) thin films in the thickness range 1500–4000 A and temperature range 18–88 K. The films are in situ grown by laser ablation on 〈1 0 0〉 LaAlO 3 substrates. The penetration depth measurements are performed by microstrip resonator technique. A correlation of λ( T) with the film microstructure observed with atomic force microscopy has shown that λ( T) depends critically on the film microstructure. Temperature dependence of magnetic penetration depth has also been studied for best quality films. The experimental results are discussed in terms of BCS theory (s-wave pairing) and d-wave Pairing with and without unitary scattering. The results are found to be best fitted to the d-wave model with unitary scattering limit. Near T c, we have also compare the (3D) XY critical regime and the Ginzburg–Landau (GL) behaviour.

Hydrophobic barrier in liposomes studied by FT-ESEEM.

An improvement of the electron spin echo envelope modulation technique (ESEEM) for studying the hydrophobic barrier in lipid membranes was proposed. Water penetration depth into the lipid bilayer composed of egg-phosphatidylcholine and dicetylphosphate was studied. For this purpose the stearic acid spin probes with nitroxide moiety at different positions on the acyl chain were dissolved in the bilayer of liposomes prepared in buffer solution with D2O. The ESEEM method is based on the deuterium effect to the 14N-hyperfine splitting constant of the spin probe. For the analysis of the spectra we suggested the frequency domain approach (Fourier transform-ESEEM) instead of the time domain. In order to test this approach, the influence of cholesterol and octanol on the water penetration depth into liposomes was measured. FT-ESEEM appeared to be a more sensitive method for the quantitative measurements of the water penetration depth than the time domain. This approach can become one of the most effective methods for studying the influence of surfactants and other biologically active molecules on water penetration depth into membranes.

Polymer Treatments for Stone Conservation: Methods for Evaluating Penetration Depth

This study examines the measurement of penetration depths for three polymer treatments for stone conservation—acrylic, partially fluorinated acrylic, and siloxane polymers. The treatments were tested on three types of calcareous stones of increasing porosity, widely present in historic architecture and monuments in Italy. Various approaches were explored, including direct determination with various Fourier transform infrared spectroscopic techniques (micro-attenuated total reflection, diamond cell transmission, and hyphenated with thermogravimetry) as well as indirect methods such as static angle measurements. Micro-attenuated total reflection on cross sections of treated specimens proved to be a particularly powerful technique for detection of polymers, with fairly good spatial resolution and sensitivity. The degree of penetration of polymers inside the porous matrix of stone is an important parameter when evaluating the efficacy and durability of treatments. Therefore, developing reliable testing methods for determining penetration depth is crucial.

Polymer Treatments for Stone Conservation: Methods for Evaluating Penetration Depth

This study examines the measurement of penetration depths for three polymer treatments for stone conservation—acrylic, partially fluorinated acrylic, and siloxane polymers. The treatments were tested on three types of calcareous stones of increasing porosity, widely present in historic architecture and monuments in Italy. Various approaches were explored, including direct determination with various Fourier transform infrared spectroscopic techniques (micro-attenuated total reflection, diamond cell transmission, and hyphenated with thermogravimetry) as well as indirect methods such as static angle measurements. Micro-attenuated total reflection on cross sections of treated specimens proved to be a particularly powerful technique for detection of polymers, with fairly good spatial resolution and sensitivity. The degree of penetration of polymers inside the porous matrix of stone is an important parameter when evaluating the efficacy and durability of treatments. Therefore, developing reliable testing methods for determining penetration depth is crucial.

New method to measure the rapid chloride migration coefficient of chloride-contaminated concrete

The apparent chloride diffusion coefficient, D app, which is obtained by fitting chloride profiles as the result of time-consuming immersion tests can be substituted in a model on chloride ingress by the rapid chloride migration (RCM) coefficient of concrete, D RCM, which is determined under electrically accelerated conditions. Until now, it was not possible to measure D RCM of chloride-contaminated concrete, as already inherent chlorides interfere with the common colorimetric indicator used for penetration depth measurements. Furthermore, carbonation may also interfere with the penetration depth reading. To overcome these problems, the regular test has been modified by using iodide as penetrating ion and iodate–starch acetic acid as indicator. A strong linear relationship between the regular RCM test and the newly developed Rapid Iodide Migration (RIM) Test was found. Carbonated specimens can be tested using the RIM test without additional interference. Thus, the new method enables the quantification of the actual concrete quality sampled from existing structures during a condition assessment.

In situ sputtering growth and characterization of MgB/sub 2/ films for microwave applications

High-quality c-axis oriented MgB/sub 2/ superconducting thin films were grown on MgO and Al/sub 2/O/sub 3/ single crystal substrates by a d.c. planar magnetron sputtering technique. The films were obtained starting by Mg rich Mg-B precursor films, subsequently annealed in situ at 800 /spl deg/C for 10 min in a In sealed Nb box in the presence of saturated Mg vapor. The process is highly reproducible and can be easily scaled to produce large area films. Resistive transition of the 1 /spl mu/m thick resulting films showed a maximum critical temperature T/sub c/ of 35 K and a transition width lower than 0.5 K. The residual resistivity ratio was 1.6 for the best samples. The films were characterized by a variety of structural and electronic techniques including profilometry, XRD, EDS, and STM-AFM analyses, critical current, upper critical field and penetration depth measurements. In view of possible device applications, the microwave response has been studied in detail. The measured dependence of the surface impedance from temperature and r.f. field amplitude at 20 GHz via a dielectric resonator technique is reported and discussed.

Sputter deposition conditions and penetration depth in NbN thin films

NbN films have been reactively sputter deposited from a 15.24 cm Nb target using a variety of deposition conditions. Film penetration depth has been measured using Taber's parallel plate resonator technique. These measurements have been compared with penetration depth measurements obtained from SQUID measurements. Penetration depth results have also been correlated with film superconducting transition temperature, resistivity, resistance ratio, and x-ray diffraction patterns. The films have been deposited over a variety of substrates and buffer layers including oxidized Si, sapphire, and a variety of metal and metal nitride seed layers.

Data processing methods in the measurements of magnetic penetration depth

The data processing methods that are always used in the measurements of superconducting magnetic penetration depth λ are discussed. The strong and weak points of each method are analyzed in detail. It is emphasized that the adjustable parameters in the comparison between theory and experiment always lead to some uncertainties in the conclusions. To eliminate these uncertainties, some effective data processing methods are recommended for different experiments.

Spin Injection Into YBa2Cu3Ox Films

We report a range of experiments designed to test the efficacy of spin polarized injection from colossal magnetoresistive material (CMR) into high temperature superconductors (HTS). These include traditional transport measurements, studies of CMR/YBCO contacts, measurements of penetration depth and studies of the effect of spin-injection on the critical state of square thin films and rings. We have used pulsed currents to minimize heating and have made comparisons with the non-magnetic homologue of CMR LaNiO3. Our experiments provide evidence that there is a measurable effect, but that it is small and mainly confined to 10–20 nm of the YBCO surface. Current gain of about 0.2 has been observed.

Nonlocality and strong coupling in the heavy fermion superconductorCeCoIn5:A penetration depth study

We report measurements of the magnetic penetration depth $\ensuremath{\lambda}$ in single crystals of ${\mathrm{CeCoIn}}_{5}$ down to $\ensuremath{\sim}0.14\mathrm{K}$ using a tunnel-diode-based, self-inductive technique at 28 MHz. While the in-plane penetration depth tends to follow a power law, ${\ensuremath{\lambda}}_{//}\ensuremath{\sim}{T}^{3/2},$ the data are better described as a crossover between linear $(T\ensuremath{\gg}{T}^{*})$ and quadratic $(T\ensuremath{\ll}{T}^{*})$ behavior, with ${T}^{*}$ the crossover temperature in the strong-coupling limit. The c-axis penetration depth ${\ensuremath{\lambda}}_{\ensuremath{\perp}}$ is linear in T. Both the magnitude of ${T}^{*}$ and the different temperature dependencies in the two directions rule out impurity effects, but instead indicate that the penetration depth is governed by nonlocal electrodynamics in a d-wave superconductor with line nodes along the c axis. This is experimental confirmation of directional nonlocality, predicted theoretically by Kosztin and Leggett.

Superconducting density of states from the magnetic penetration depth of electron-doped cuprates La2-xCexCuO4-y and Pr2-xCexCuO4-y

From measurements of the magnetic penetration depth, λ(T), from 1.6 K to T c in films of electron-doped cuprates La 2-x Ge x CuO 4-y and Pr 2-x Ge x CuO 4-y we obtain the normalized density of states, N s (E) at T = 0 by using a simple model. In this framework, the flat behavior of λ -2 (T) at low T implies N s (E) is small, possibly gapped, at low energies. The upward curvature in λ -2 (T) near T c seen in overdoped films implies that superfluid comes from an anomalously small energy band within about 3k B T c of the Fermi surface.

Penetration depth and core radius μSR measurements in the vortex state near the lower critical field

μSR measurements of the magnetic field distribution in the high- T c superconductor YBa 2Cu 3O 6+ x and conventional superconductor NbSe 2 near the lower critical field are presented. The muon spin polarization signal is analyzed in terms of a Ginzburg–Landau (GL) field distribution. Our analysis reveals that the increase in the vortex core radius levels off below μ 0H=0.05 T in underdoped YBa 2Cu 3O 6+ x and the penetration depth stops decreasing. We find that λ=2000 A ̊ in YBa 2Cu 3O 6.50, λ=1850 A ̊ in YBa 2Cu 3O 6.67, and λ=1600 A ̊ in NbSe 2. At still lower external magnetic fields, the GL field distribution no longer fits the μSR signal, likely due to a transition to the Meissner state.

Fermi-liquid interactions and the superfluid density ind-wave superconductors

We construct a phenomenological superfluid Fermi-liquid-theory for a two-dimensional d-wave superconductor on a square lattice, and study the effect of quasiparticle interactions on the superfluid density. Using simple models for the dispersion and the Landau interaction function, we illustrate the deviation of these results from those for the isotropic superfluid. This allows us to reconcile the value and doping dependence of the superfluid density slope at low temperature obtained from penetration depth measurements, with photoemission data on nodal quasiparticles.

RESISTIVITY AND PENETRATION DEPTH MEASUREMENTS OF ORGANIC SUPERCONDUCTORS IN HIGH MAGNETIC FIELDS USING A TUNNEL DIODE OSCILLATOR

We have made measurements of resistivity and penetration depth in dc-and pulsed-magnetic fields of the organic superconductors α- (ET) 2 NH 4 Hg(SCN) 4, and κ- (ET) 2 Cu(NCS) 2 using a resonant rf circuit powered by a tunnel diode oscillator (TDO). We compare the critical fields as measured by resistivity, and the TDO. All the superconductors we have studied have anisotropic critical fields with a ratio H c2 // to H c2 ⊥ of 5-17. In κ- (ET) 2 Cu(NCS) 2 we observed an almost monotonic increase in the parallel critical fields up to 24 T at 390 mK, and a sharp cusp in the critical field as a function of angle when the sample is parallel to the magnetic field. This suggests that the superconducting layers are only Josephson-coupled and the superconductivity is two-dimensional(2D). Another salt, α- (ET) 2 NH 4 Hg(SCN) 4, shows similar cusp behavior, but this material shows evidence of extreme Pauli limiting when the field is turned parallel to the conducting layers.

Reflection of a two-gap nature in penetration-depth measurements ofMgB2film

The magnetic penetration depth, $\lambda^{-2}(T)$, in the basal plane of a magnesium diboride (MgB$_2$) film was measured using a two-coil mutual inductance technique at 50 kHz. This film has $T_c\simeq 38$ K, $\Delta T_c \leq 1$ K, and $\lambda(0)\sim 1500$ \AA. At low temperatures, $\lambda^{-2}(T)$ shows a clear exponential temperature dependence, indicating s-wave superconducting order parameter symmetry. However, the data are not quantitatively well described by theory assuming a single gap. From the data fit by the full BCS calculation assuming a double gap, the values of the two distinct gaps were obtained: $\Delta_S(0)=2.61\pm0.41$ meV and $\Delta_L(0)=6.50\pm0.33$ meV. The contributions of the small and the large gaps to the total superfluid density at T=0 were estimated to be 21% and 79%, respectively. Finally, we consider the effect of gap anisotropy on the penetration depth measurements, and find that the anisotropy does not play a significant role in determining the temperature dependence of the penetration depth.

Microwave penetration depth measurement for high T/sub c/ superconductors by dielectric resonators

The penetration depth /spl lambda/(T) dependence on temperatures for high T/sub c/ superconducting YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin films stored in various environments was measured by a well-designed microwave dielectric resonator. A d-wave T/sup 2/ dependence was observed at low temperatures, while an exponential dependence of the penetration depth /spl lambda/(T) relevant to the s wave was detected as temperature increases due to thermal fluctuation. An abnormal upturn of the penetration depth at temperatures below 10 K attributed to the surface current carried by the defect surface-induced Andreev bound states can be apparently observed without applying heavy-ion bombardment from this relatively higher frequency measurement. Readers who endeavor to start this kind of measurement can use the well-modified dielectric cavity in conjunction with the detailed measuring procedure.

Evidence for a transition in the pairing symmetry of the electron-doped cuprates La(2-x)Ce(x)CuO(4-y) and Pr(2-x)Ce(x)CuO(4-y).

We present measurements of the magnetic penetration depth, lambda(-2)(T), in Pr(2-x)Ce(x)CuO(4-y) and La(2-x)Ce(x)CuO(4-y) films at three Ce doping levels, x, near optimal. Optimal and overdoped films are qualitatively and quantitatively different from underdoped films. For example, lambda(-2)(0) decreases rapidly with underdoping but is roughly constant above optimal doping. Also, lambda(-2)(T) at low T is exponential at optimal and overdoping but is quadratic at underdoping. In light of other studies that suggest both d- and s-wave pairing symmetry in nominal optimally doped samples, our results are evidence for a transition from d- to s-wave pairing near optimal doping.