Radio Frequency Penetration Depth Research Articles

In Vivo Molecular Electron Paramagnetic Resonance-Based Spectroscopy and Imaging of Tumor Microenvironment and Redox Using Functional Paramagnetic Probes.

A key role of the tumor microenvironment (TME) in cancer progression, treatment resistance, and as a target for therapeutic intervention is increasingly appreciated. Among important physiological components of the TME are tissue hypoxia, acidosis, high reducing capacity, elevated concentrations of intracellular glutathione (GSH), and interstitial inorganic phosphate (Pi). Noninvasive in vivo pO2, pH, GSH, Pi, and redox assessment provide unique insights into biological processes in the TME, and may serve as a tool for preclinical screening of anticancer drugs and optimizing TME-targeted therapeutic strategies. Recent Advances: A reasonable radiofrequency penetration depth in living tissues and progress in development of functional paramagnetic probes make low-field electron paramagnetic resonance (EPR)-based spectroscopy and imaging the most appropriate approaches for noninvasive assessment of the TME parameters. Here we overview the current status of EPR approaches used in combination with functional paramagnetic probes that provide quantitative information on chemical TME and redox (pO2, pH, redox status, Pi, and GSH). In particular, an application of a recently developed dual-function pH and redox nitroxide probe and multifunctional trityl probe provides unsurpassed opportunity for in vivo concurrent measurements of several TME parameters in preclinical studies. The measurements of several parameters using a single probe allow for their correlation analyses independent of probe distribution and time of measurements. The recent progress in clinical EPR instrumentation and development of biocompatible paramagnetic probes for in vivo multifunctional TME profiling eventually will make possible translation of these EPR techniques into clinical settings to improve prediction power of early diagnostics for the malignant transition and for future rational design of TME-targeted anticancer therapeutics. Antioxid. Redox Signal. 28, 1365-1377.

Upper critical fields and two-band superconductivity in Sr1−xEux(Fe0.89Co0.11)2As2(x=0.20and0.46)

The upper critical fields ${H}_{c2}(T)$ of single crystals of Sr${}_{1\ensuremath{-}x}$Eu${}_{x}$(Fe${}_{0.89}$Co${}_{0.11}$)${}_{2}$As${}_{2}$ ($x=0.20$ and $0.46$) were determined by radio-frequency penetration depth measurements in pulsed magnetic fields. ${H}_{c2}(T)$ approaches the Pauli limiting field but shows an upward curvature with an enhancement from the orbital limited field, as inferred from the Werthamer-Helfand-Hohenberg theory. We discuss the temperature dependence of the upper critical fields and the decreasing anisotropy using a two-band BCS model.

Radio-frequency measurements of UNiX compounds (X=Al, Ga, Ge) in high magnetic fields

We performed radio-frequency (RF) skin-depth measurements of antiferromagnetic UNiX compounds (X=Al, Ga, Ge) in magnetic fields up to 60 T and at temperatures between 1.4 to ~60 K. Magnetic fields are applied along different crystallographic directions and RF penetration-depth was measured using a tunnel-diode oscillator (TDO) circuit. The sample is coupled to the inductive element of a TDO resonant tank circuit, and the shift in the resonant frequency Δ f of the circuit is measured. The UNiX compounds exhibit field-induced magnetic transitions at low temperatures, and those transitions are accompanied by a drastic change in Δ f. The results of our skin-depth measurements were compared with previously published B– T phase diagrams for these three compounds.

Determination of anisotropicHc2up to 60 T inBa0.55K0.45Fe2As2single crystals

The radio frequency penetration depth was measured in the superconductor $({\text{Ba}}_{0.55}{\text{K}}_{0.45}){\text{Fe}}_{2}{\text{As}}_{2}$ under pulsed magnetic fields extending to 60 T and down to 14 K. Using these data we are able to infer a ${H}_{c2}(T)$, $H\text{\ensuremath{-}}T$ phase diagram, for applied fields parallel and perpendicular to the crystallographic $c$ axis. The upper critical-field curvature is different for the respective orientations, but they each remains positive down to 14 K. The upper critical-field anisotropy is moderate, $\ensuremath{\approx}3.5$ close to ${T}_{c}$, and drops with the decrease in temperature, reaching $\ensuremath{\approx}1.2$ at 14 K. These data and analysis indicate that (i) $({\text{Ba}}_{0.55}{\text{K}}_{0.45}){\text{Fe}}_{2}{\text{As}}_{2}$ anisotropy diminishes with temperature and has an unusual temperature dependence and (ii) ${H}_{c2}(T=0)$ for this compound may easily approach fields of 75 T.

Intergrain Connectivity and Resistive Broadening in Vortex State: A Comparison Between ${\rm MgB}_{2}$, ${\rm NbSe}_{2}$ and ${\rm Bi}_{2}{\rm Sr}_{2}{\rm Ca}_{2}{\rm Cu}_{3}{\rm O}_{10}$ Superconductors

Magnetoresistance and radio frequency penetration depth techniques are used to study grain connectivity and broadening of superconducting transition. We study and compare these issues in clean polycrytalline samples of three different superconducting systems e.g. MgB2, NbSe2 and Bi2Sr2Ca2Cu3O10. From the rf response, the bulk pinning force constant is evaluated. From high field transport measurements, H-T phase diagram is ascertained for the three systems with varying degrees of fluctuation and connectivity.

Dynamics of the superconducting mixed state in YBa2Cu3O7-δ/PrBa2Cu3O7-δ superlattices in radio frequency regime

Epitaxial multilayers of YBa2Cu3O7-δ and P1Ba2Cu3O7-δ have been deposited on (100) cut SrTiO3 substrates using the technique of pulsed laser deposition. Standard ϑ-2ϑ X-ray diffraction measurements on the films showed excellent superlattice reflections. The mixed state of these superlattices has been probed through measurements of radio frequency penetration depth (λ) as a function of temperature (T), magnetic field (H) and it’s orientation (ϑ) with respect to the planes of the superlattices. These data reflect the two-dimensional nature of the mixed state in these systems.

Radio frequency vortex dynamics in heavy ion irradiated (Bi–Pb) 2Sr 2Ca 2Cu 3O 10 superconducting platelets

We report measurements of vortex dynamics in heavy ion irradiated (Bi–Pb) 2Sr 2Ca 2Cu 3O 10 superconducting tapes in the high-temperature ( T⩾60 K) and low-field ( μ 0 H<0.3 T) regimes. The temperature, magnetic field strength and angular dependences of radio-frequency (RF) penetration depth λ(H, T) are measured and compared with the behavior of unirradiated samples. The pinning force constant, k p, and viscosity coefficient η are extracted from the field dependence of λ(H, T) . The angular dependence of the RF penetration depth in samples irradiated at the field equivalent fluence of 0.5 and 3.0 T is minimum when the external field is brought into alignment with the columnar defects (CDs). While this characteristic feature of anisotropic pinning due to CDs disappear in a temperature regime where J c is high, in the vortex liquid state closer to T c, this feature grows with the increasing field strength. The overall changes in η, k p and the angular dependence of λ(H, T) are discussed in the framework of the Bose glass theory for vortex dynamics in highly layered superconductors.

Effects of granularity and strong pinning on high frequency vortex dynamics in (Bi–Pb) 2Sr 2Ca 2Cu 3O 10 superconducting platelets

An anomalous hysteretic behaviour is seen in the radio frequency penetration depth of unirradiated and heavy ion irradiated (Bi–Pb) 2Sr 2Ca 2Cu 3O 10 platelets. Characterized by a large remanence, this anomaly is due to the high mobility of the magnetic flux trapped at the ubiquitous grain boundaries in these otherwise highly c-axis oriented samples. The range of temperature over which the hysteresis is seen depends on the strength of intragranular pinning. The observed behaviour is discussed quantitatively on the basis of the two-level critical state model of Ji, Rzchowski, Anand and Tinkham for microwave field penetration in inhomogeneous superconductors.

Scaling behavior of the rf vortex penetration depth in an organic superconductor.

The radiofrequency penetration depth \ensuremath{\lambda}(H,T) of the organic superconductor (BEDT-TTF${)}_{2}$Cu(NCS${)}_{2}$ was studied in the mixed state at moderate fields. The data can be scaled into a single functional form \ensuremath{\Delta}\ensuremath{\lambda}(H,T)=\ensuremath{\Delta}${\ensuremath{\lambda}}^{\mathrm{*}}$(T)f(H/${\mathit{H}}^{\mathrm{*}}$(T)), where ${\mathit{H}}^{\mathrm{*}}$(T)\ifmmode\times\else\texttimes\fi{}(1-t${)}^{3/2}$/t can be identified with a ``depinning'' or ``irreversibility'' line. We show that the scaling features are contained in a theory of vortex motion in a periodic pootential under the influence of thermal fluctuations, which describes the functional form quantitatively. The data for \ensuremath{\Delta}\ensuremath{\lambda}(H=0, T) are governed by the same (vortex) length scale as the finite-field data, suggesting that the zero-field state is not a conventional Meissner state obeying a London relation.

Radio-frequency surface impedance of type-II superconductors: Dependence upon the magnitude and angle of an applied static magnetic field

The application of a static magnetic field introduces vortices into a type-II superconductor. The dynamic response of these vortices strongly affects the surface impedance (surface resistance and surface reactance) of the superconductor. Using a self-consistent approach for including the influence of the vortices upon the complex-valued radio-frequency penetration depth, we calculate in this paper how the surface impedance depends upon the static magnetic field's magnitude and angle relative to the surface and to the rf magnetic field. The surface resistance is maximized when the induced rf currents are perpendicular to the static field.

Pinning forces and lower critical fields in YBa2Cu3Oy crystals: Temperature dependence and anisotropy.

Measurements of the field-dependent radiofrequency penetration depth {lambda}({ital H},{ital T}) are used to delineate the lower-critical-field {ital H}{sub {ital c}1}-{ital T} phase boundary, and to study flux dynamics in YBa{sub 2}Cu{sub 3}O{sub {ital y}} crystals. For both {bold H}{parallel}{bold {cflx c}} and {bold H}{perpendicular}{bold {cflx c}} {ital H}{sub {ital c}1} obeys a BCS temperature dependence with a temperature-independent anisotropy of 3.4{plus minus}0.3. In the mixed state, the data obey {lambda}{sup 2}({ital H})=({phi}{sub 0}/{mu}{sub 0}{alpha}({ital T})){ital B}({ital H}), and yield both the functional dependence {ital B}({ital H}) and the pinning force constant {alpha}({ital T}). The latter is anisotropic, obeys an approximately (1{minus}{ital t}{sup 2}){sup 2} temperature dependence, and vanishes slightly below the bulk transition temperature.

Rf penetration depth in superconducting UPt3 with double heat capacity anomaly

The radiofrequency penetration depth has been measured in a polycrystalline sample of UPt3 that has been observed to have a double-peak structure in the specific heat. At low temperatures, the penetration depth va ries as T4, consistent with similar measurements on single crystals. No change in the penetration depth, or its variation with temperature, is detected at the onset of the lower transition.

RF penetration depth in superconducting upt 3 with double heat capacity anomaly

The radiofrequency penetration depth has been measured in a polycrystalline sample of UPt 3 that has been observed to have a double-peak structure in the specific heat. At low temperatures, the penetration depth va ries as T 4, consistent with similar measurements on single crystals. No change in the penetration depth, or its variation with temperature, is detected at the onset of the lower transition.

Temperature dependence of electrodynamic properties of YBa2Cu3Oy crystals.

The temperature dependences of the radio-frequency penetration depth {lambda}{sub {ital ab}}, microwave surface resistance {ital R}{sub {ital s}}, and the lower critical field {ital H}{sub {ital c}1} were measured for YBa{sub 2}Cu{sub 3}O{sub {ital y}} crystals. Pair-breaking effects of magnetic fields on {lambda}{sub {ital ab}}, represented by a parameter {ital k}{identical to}{ital d}{lambda}{sub {ital ab}}/d(H{sup 2}), were also measured. The temperature dependences of all these parameters {lambda}{sub {ital ab}}, {ital H}{sub {ital c}1}, {ital k}, and {ital R}{sub {ital s}} are in good agreement with the Bardeen-Cooper-Schrieffer temperature dependences for an {ital s}-wave superconductor, and suggest a mean-field behavior of the gap parameter.

Radio frequency penetration depth in HTSC ceramic samples

We present measurements of the temperature dependence of the RF penetration depth δ(T) in YBa 2Cu 3O 6.9 and BiSrCaCu 2O x ceramics. We found that δ(T) closely follows the Gorter - Casimir two-fluid model at T< T c , but with an unusually high value of δ(0)=(1.5 − 10)·10 5 A ̊ , which is at least two orders of magnitude higher than reported for the London penetration depth in the HTSC. A large absorption peak in RF losses was observed at T c and for ω<10 MHz which disappears at higher frequencies. These effects are most likely caused by weakly coupled superconducting loops within ceramic HTSC.

Field variation of the penetration depth in ceramic Y1Ba2Cu3Oy.

The radio-frequency penetration depth is found to increase quadratically with applied dc magnetic field $B$, i.e., $\ensuremath{\Delta}{\ensuremath{\lambda}}_{\mathrm{eff}}=k(T){B}^{2}$, between 4.2 and 85 K in samples of ceramic ${\mathrm{Y}}_{1}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$. The parameter $k(T)$ rises steeply with increasing temperature $T$, and even at 4.2 K is orders of magnitude larger than a Ginzburg-Landau prediction. Instead, the ${B}^{2}$ dependence and the large values of $k$ can be quantitatively explained in terms of the dc flux dependence of the high-frequency response of a Josephson junction network, with typical junction area about 6 \ensuremath{\mu}${\mathrm{m}}^{2}$. The analysis provides an explanation of the observed correspondence between ${\ensuremath{\lambda}}_{\mathrm{eff}}(T)$, $k(T)$, and the threshold field for irreversible flux entry.