Behavior Of Tc Research Articles

Transition temperature enhancement of nano-structured superconductors: Shape dependence and effects of Cooper pair distribution

Enhancement of superconducting critical temperature Tc for nano-structured square and rectangular plates is investigated, using the Gor’kov equations and the finite element method. Tc increases with decreasing system size and shows sawtooth behavior as the function of system size. We find the enhancement of Tc becomes larger for narrower rectangular plate. We discuss mechanisms of these behaviors of Tc, investigating quasi-particle eigen-energies and spatial order parameter distributions.

Impact of nitrate on the redox chemistry and solubility of Tc(IV) in alkaline, dilute to concentrated aqueous NaCl solutions

The impact of nitrate on the redox behaviour and solubility of 99Tc(IV) was investigated in 0.1–5.0 M NaCl–NaNO3–NaOH solutions with 0.1 M ≤ [NO3−] ≤ 1.0 M and 8.5 ≤ pHm ≤ 14.5 (with pHm = –log [H+]) in the presence or absence of reducing agents (as Fe powder, Na2S2O4, Sn(II)). The evolution of the redox conditions as a function of time (for t ≤ 450 days) was investigated: (i) in the absence of technetium (inactive samples), (ii) in the presence of 99Tc(IV) (added as 1–2 mg of TcO2∙0.6H2O(am)), and (iii) in the presence of 137Cs(I) (and absence of 99Tc). All experiments were conducted in an Ar-glovebox (O2 < 1 ppm) at T = (22 ± 2) °C. An improved understanding of Tc redox processes in nitrate-rich alkaline solutions is relevant to predict Tc behaviour in specific nuclear waste disposal scenarios and a topic of fundamental scientific interest.Nitrate induces a clear increase in Eh over time in the investigated aqueous solutions both in the presence and absence of reducing agents. In the case of Fe powder, this increase is observed almost immediately whereas in the presence of Na2S2O4 or Sn(II) the oxidizing effect is only observed after several months in inactive systems (absence of Tc). In the presence of technetium, the increase in Eh occurs within a few days (5–50, depending upon reducing system, pHm and ionic strength). The evolution of Eh in the presence of 137Cs (with similar β– dose as in solubility experiments with 99Tc) was analogous to that in the inactive systems. These observations indicate that the solid TcO2∙0.6H2O(am) may play a role in catalysing nitrate reduction and radiation induced reactions have no significant impact in our experiments. Liquid-liquid extraction and XANES measurements confirm that technetium in the aqueous phase is mostly present as Tc(VII). Colorimetric nitrate/nitrite tests show the formation of nitrite, confirming that the oxidation of Tc(IV) to Tc(VII) is coupled to the reduction of nitrate. Our experimental observations support that, although kinetically hindered, nitrate can effectively oxidize Tc(IV).

Evolution of a Novel Ribbon Phase in Optimally Doped Bi2Sr2CaCu2O8+δ at High Pressure and Its Implication to High- TC Superconductivity.

One challenge in studying high-temperature superconductivity (HTSC) stems from a lack of direct experimental evidence linking lattice inhomogeneity and superconductivity. Here, we apply synchrotron hard X-ray nanoimaging and small-angle scattering to reveal a novel micron-scaled ribbon phase in optimally doped Bi2Sr2CaCu2O8+δ (Bi-2212, with δ = 0.1). The morphology of the ribbon-like phase evolves simultaneously with the dome-shaped TC behavior under pressure. X-ray absorption studies show that the increasing of TC is associated with oxygen-hole redistribution in the CuO2 plan, while TC starts to decrease with pressure when oxygen holes become immobile. Additional X-ray irradiation experiments reveal that nanoscaled short-range ordering of oxygen vacancies could further lower TC, which indicates that the optimal TC is affected not only by an optimal morphology of the ribbon phase, but also an optimal distribution of oxygen vacancies. Our studies thereby provide for the first time compelling experimental evidence correlating the TCwith micron to nanoscale inhomogeneity.

Chemometric analyses of XANES data collected on 99Tc bearing silicate glasses

A challenge to the incorporation of 99Tc into a glass matrix is that 99Tc is volatile at vitrification temperatures. Understanding how this volatilization occurs requires knowledge of the multiple chemistries which Tc may take during vitrification. This paper presents an overview of how the localized chemistry of 99Tc has been determined in a series of 99Tc-bearing glasses by chemometric analyses of X-ray absorption near edge spectra (XANES). Linear combination fitting and principal component analysis of the glasses’ XANES spectra suggested that the glasses contained 3–4 chemically distinct Tc environments. The identity of the detected chemistries were pertechnetates, (Tc(VII) as isolated oxyanions, or locally coordinated by Na or K), or isolated Tc(IV) species. The linear combination fitting distribution of local Tc sites agrees with reanalyzed Raman spectra, suggesting that as targeted KTcO4 incorporation increases, a significant ion exchange takes place, and speciation in the glass changes to favor Tc(VII) formation, specifically NaTcO4. Based on the statistical suggestion that not all Tc environments are accounted by the available standards, a new mechanism for the behavior of Tc during vitrification is proposed.

Exotic superfluidity and pairing phenomena in atomic Fermi gases in mixed dimensions

Atomic Fermi gases have been an ideal platform for simulating conventional and engineering exotic physical systems owing to their multiple tunable control parameters. Here we investigate the effects of mixed dimensionality on the superfluid and pairing phenomena of a two-component ultracold atomic Fermi gas with a short-range pairing interaction, while one component is confined on a one-dimensional (1D) optical lattice whereas the other is in a homogeneous 3D continuum. We study the phase diagram and the pseudogap phenomena throughout the entire BCS-BEC crossover, using a pairing fluctuation theory. We find that the effective dimensionality of the non-interacting lattice component can evolve from quasi-3D to quasi-1D, leading to strong Fermi surface mismatch. Upon pairing, the system becomes effectively quasi-two dimensional in the BEC regime. The behavior of Tc bears similarity to that of a regular 3D population imbalanced Fermi gas, but with a more drastic departure from the regular 3D balanced case, featuring both intermediate temperature superfluidity and possible pair density wave ground state. Unlike a simple 1D optical lattice case, Tc in the mixed dimensions has a constant BEC asymptote.

Interlayer Bond Formation in Black Phosphorus at High Pressure.

Black phosphorus was compressed at room temperature across the A17, A7 and simple‐cubic phases up to 30 GPa, using a diamond anvil cell and He as pressure transmitting medium. Synchrotron X‐ray diffraction showed the persistence of two previously unreported peaks related to the A7 structure in the pressure range of the simple‐cubic phase. The Rietveld refinement of the data demonstrates the occurrence of a two‐step mechanism for the A7 to simple‐cubic phase transition, indicating the existence of an intermediate pseudo simple‐cubic structure. From a chemical point of view this study represents a deep insight on the mechanism of interlayer bond formation during the transformation from the layered A7 to the non‐layered simple‐cubic phase of phosphorus, opening new perspectives for the design, synthesis and stabilization of phosphorene‐based systems. As superconductivity is concerned, a new experimental evidence to explain the anomalous pressure behavior of Tc in phosphorus below 30 GPa is provided.

Interlayer Bond Formation in Black Phosphorus at High Pressure

AbstractBlack phosphorus was compressed at room temperature across the A17, A7 and simple‐cubic phases up to 30 GPa, using a diamond anvil cell and He as pressure transmitting medium. Synchrotron X‐ray diffraction showed the persistence of two previously unreported peaks related to the A7 structure in the pressure range of the simple‐cubic phase. The Rietveld refinement of the data demonstrates the occurrence of a two‐step mechanism for the A7 to simple‐cubic phase transition, indicating the existence of an intermediate pseudo simple‐cubic structure. From a chemical point of view this study represents a deep insight on the mechanism of interlayer bond formation during the transformation from the layered A7 to the non‐layered simple‐cubic phase of phosphorus, opening new perspectives for the design, synthesis and stabilization of phosphorene‐based systems. As superconductivity is concerned, a new experimental evidence to explain the anomalous pressure behavior of Tc in phosphorus below 30 GPa is provided.

Electrochemical studies of technetium–ruthenium alloys in HNO3: Implications for the behavior of technetium waste forms

The electrochemical behavior of Tc–Ru alloys (Ru content, at. %: 3.2, 5.2, 20.1, 24.7) in 1 M HNO3 was studied. The transpassivation potentials (Etp) of Tc–Ru alloys were determined by linear voltammetry. The results show that the transpassivation potentials of the alloys increase with the Ru content. To understand the dissolution mechanism, electrolysis experiments at 1.2 V vs. Ag/AgCl were performed; the corrosion products of the alloys were characterized in solution by UV-visible spectroscopy and electrospray ionization mass spectrometry (ESI-MS). For Ru, a polymeric Ru(IV) species was detected, while for Tc the speciation was dominated by TcO4–.

Effect of BSO addition on Cu-O bond of GdBa2Cu3O7-xfilms with varying thickness probed by extended x-ray absorption fine structure

We investigated the relation between the Cu-O bond length and the superconducting properties of BaSnO3 (BSO)-added GdBa2Cu3O7-x (GdBCO) thin films by using extended x-ray absorption fine structure (EXAFS) spectroscopy. 4 wt.% BaSnO3 (BSO) added GdBa2Cu3O7-x (GdBCO) thin films with varying thickness from 0.2 μm to 1.0 μm were fabricated by using pulsed laser deposition (PLD) method. The transition temperature (Tc) and the residual resistance ratio (RRR) of the GdBCO films increased with increasing thickness up to 0.8 μm, where the crystalline BSO has the highest peak intensity, and then decreased. This uncommon behaviors of Tc and RRR are likely to be created by the addition of BSO, which may change the ordering of GdBCO atomic bonds. Analysis from the Cu K-edge EXAFS spectroscopy showed an interesting thickness dependence of ordering behavior of BSO-added GdBCO films. It is noticeable that the ordering of Cu-O bond and the transition temperature are found to show opposite behaviors in the thickness dependence. Based on these results, the growth of BSO seemingly have evident effect on the alteration of the local structure of GdBCO film.

Online ultrasonic characterization of La1-xNaxMnO3: Bulk and nanostructured perovskites

ABSTRACTA series of manganites (bulk and nanostructured) with three different compositions (x = 0.16, 0.18, and 0.20) were synthesized and characterized using solid state decomposition and sonication methods. The prepared manganite La1−xNaxMnO3 (LNMO) belongs to rhombohedral structures. On-line ultrasonic related physical parameters measured using unique tool in the temperature range from 300 to 400 K to explore the structural changes and phase transitions. The observed anomalous behavior in LNMO manganites was correlated with occurrence of ferromagnetic–paramagnetic transition temperature (TC). The shift in TC and magnitude and width of the observed anomalies are used to study the behavior of TC.

Research Update: Conductivity and beyond at the LaAlO3/SrTiO3 interface

In this review, we focus on the celebrated interface between two band insulators, LaAlO3 and SrTiO3, that was found to be conducting, superconducting, and to display a strong spin-orbit coupling. We discuss the formation of the 2-dimensional electron liquid at this interface, the particular electronic structure linked to the carrier confinement, the transport properties, and the signatures of magnetism. We then highlight distinctive characteristics of the superconducting regime, such as the electric field effect control of the carrier density, the unique tunability observed in this system, and the role of the electronic subband structure. Finally we compare the behavior of Tc versus 2D doping with the dome-like behavior of the 3D bulk superconductivity observed in doped SrTiO3. This comparison reveals surprising differences when the Tc behavior is analyzed in terms of the 3D carrier density for the interface and the bulk.

Little–Parks effect in proximity superconductor-ferromagnet coaxial cylinders

The linearized Usadel equations in a generalized form containing both the spin singlet and the odd frequency spin triplet pair amplitudes in an s-wave state are used to study the oscillatory behavior of the superconducting critical temperature Tc due to the influence of an external magnetic field in proximity coaxial cylinders of a ferromagnetic core surrounded by a superconducting shell. Such geometry modifies the pairing wavefunction periodically with the vortex number L. The exchange field in the ferromagnetic core is chosen to be of the spiral type which rotates in the plane with a spiral wave vector Q. The external magnetic field assumes a constant value and is directed perpendicularly to the plane. The switching phenomena of the induced superconductivity in the ferromagnet with various vortex numbers are investigated in detail by solving self-consistently the Usadel equations to determine the Little–Parks effect i.e., the behavior of Tc versus a magnetic flux threading the ferromagnetic core.

Computational Investigation of Technetium(IV) Incorporation into Inverse Spinels: Magnetite (Fe3O4) and Trevorite (NiFe2O4).

Iron oxides and oxyhydroxides play an important role in minimizing the mobility of redox-sensitive elements in engineered and natural environments. For the radionuclide technetium-99 (Tc), these phases hold promise as primary hosts for increasing Tc loading into glass waste form matrices, or as secondary sinks during the long-term storage of nuclear materials. Recent experiments show that the inverse spinel, magnetite [Fe(II)Fe(III)2O4], can incorporate Tc(IV) into its octahedral sublattice. In that same class of materials, trevorite [Ni(II)Fe(III)2O4] is also being investigated for its ability to host Tc(IV). However, questions remain regarding the most energetically favorable charge-compensation mechanism for Tc(IV) incorporation in each structure, which will affect Tc behavior under changing waste processing or storage conditions. Here, quantum-mechanical methods were used to evaluate incorporation energies and optimized lattice bonding environments for three different, charge-balanced Tc(IV) incorporation mechanisms in magnetite and trevorite (∼5 wt % Tc). For both phases, the removal of two octahedral Fe(II) or Ni(II) ions upon the addition of Tc(IV) in an octahedral site is the most stable mechanism, relative to the creation of octahedral Fe(III) defects or increasing octahedral Fe(II) content. Following hydration-energy corrections, Tc(IV) incorporation into magnetite is energetically favorable while an energy barrier exists for trevorite.

Paramagnetic Meissner Effect in Electrochemically Doped Indium-Tin Oxide Films

Transparent conductive indium-tin oxide (ITO) thin films, electrochemically intercalated with alkali (Li+, Na+, K+, Rb+, Cs+), alkali earth (Mg+2, Ca+2), or complex NH\(_{4}^{+}\) ions, show tunable superconducting transitions with dome-shaped behavior of Tc versus electron density around the maximum at ∼ 5 K. On field cooling, the transition into the superconducting state is accompanied by a paramagnetic response, i.e., an increase of magnetization, rather than the usual diamagnetic Meissner response. We provide an extensive study of this so-called paramagnetic Meissner effect (PME), using DC SQUID, transport measurements and a variety of sample sizes and growth conditions. We show that the PME in electrochemically doped ITO films results from a higher Tc at the sample edges than at the center of disk-shaped samples, causing flux to be expelled towards the center of the disk, following the flux-compression theory of Koshelev and Larkin. Changing to the opposite spatial Tc profile largely removes the paramagnetic response. The paramagnetic magnetization is strongly influenced by sample geometry and flux pinning conditions. The reduction of pinning defects by thermal annealing removes the paramagnetic response. An alternation of the external magnetic field restores the usual Meissner diamagnetism.

In Vitro/In Vivo Evaluation of Radiolabeled [99mTc(CO)3]+-Hydroxyurea and Fluorescein Isothiocyanate-Hydroxyurea

The aim of current study is to examine hydroxyurea (HU), which is an antineoplastic drug used for the treatment of leukemia, sickle-cell disease, HIV, psoriasis, thrombocythemia, and various neoplastic diseases in two aspects. The active ingredient hydroxyurea was obtained by purification of the capsule form drug, commercially named as HYDREA. Then, [(99m)Tc(CO)3](+)core radiolabeling with HU was performed as first aspect. Quality control studies of (99m)Tc(CO)3-HU complex were performed by thin-layer radiochromatography and high-performance liquid radiochromatography methods. The results demonstrated that the radiolabeling yield was quite high (98.43% ± 2.29%). Also, (99m)Tc(CO)3-HU complex has good stability during the 24-hour period. Biological behavior of (99m)Tc(CO)3-HU complex is evaluated by biodistribution studies on Wistar Albino rats. Fluorescein isothiocyanate (FITC) labeling of HU was performed as second aspect. Fluorometric evaluation of binding efficacy and fluorescence imaging studies on MCF7 and Hela cell lines were carried out. It was thought that the knowledge achieved in this study would contribute to using (99m)Tc(CO)3-HU complex as an imaging agent, which inhibits the DNA synthesis selectively, by inhibiting ribonucleotide reductase enzyme. It was observed that FITC-HU has noteworthy incorporation on both cell lines.

Close relation between Mg–Mg bonds and Tc of SiC buffered-MgB2 tapes

A series of superconducting tapes were prepared by growing MgB2 film on Hastelloy tapes buffered with SiC buffer layers. When the SiC buffer layer increases in thickness, the superconducting transition temperature (Tc) of MgB2 tapes show slight increment with no depression detected. This Tc behavior may have been originated from the changes occurred in the local structure of MgB2. From EXAFS analysis, the ordering of the Mg–Mg bonds was systematically increased when the thicknesses of SiC buffer layers were decreased. We also found that unlike the relatively random bond lengths of Mg–B bond, the Mg–Mg bond lengths are expanding when SiC adds its thickness, inferring the clear effect of SiC to the Mg–Mg bond lengths. On the basis of the results shown in this letter we argue that SiC buffer layers have significant effects on the local structure of MgB2 which contributes to the fundamentals of superconducting properties of MgB2 tapes.

Emergence of Multiple Superconducting Phases in (NH3)yMxFeSe (M: Na and Li).

We previously discovered multiple superconducting phases in the ammoniated Na-doped FeSe material, (NH3)yNaxFeSe. To clarify the origin of the multiple superconducting phases, the variation of Tc was fully investigated as a function of x in (NH3)yNaxFeSe. The 32 K superconducting phase is mainly produced in the low-x region below 0.4, while only a single phase is observed at x = 1.1, with Tc = 45 K, showing that the Tc depends significantly on x, but it changes discontinuously with x. The crystal structure of (NH3)yNaxFeSe does not change as x increases up to 1.1, i.e., the space group of I4/mmm. The lattice constants, a and c, of the low-Tc phase (Tc = 32.5 K) are 3.9120(9) and 14.145(8) Å, respectively, while a = 3.8266(7) Å and c = 17.565(9) Å for the high-Tc phase (~46 K). The c increases in the high Tc phase, implying that the Tc is directly related to c. In (NH3)yLixFeSe material, the Tc varies continuously within the range of 39 to 44 K with changing x. Thus, the behavior of Tc is different from that of (NH3)yNaxFeSe. The difference may be due to the difference in the sites that the Na and Li occupy.

Ferromagnetic–paramagnetic transition temperature in bulk and nanostructured La0.7Sr x Ca0.3−x MnO3 (x = 0.10, 0.15, and 0.20) manganite materials

Series of bulk and nanostructured La0.7Sr x Ca0.3−x MnO3 (x = 0.10, 0.15, and 0.20) manganites were synthesized and characterized using different techniques. In the series, both the bulk and nanostructured La0.7Sr0.10Ca0.20MnO3 and La0.7Sr0.20Ca0.10MnO3 manganites have orthorhombic and rhombohedral structures, whereas La0.7Sr0.15Ca0.15MnO3 manganite has rhombohedral and orthorhombic structures, respectively. Online ultrasonic velocity and attenuation measurements were taken through an indigenously designed ultrasonic setup, and the analysis was done from 300 to 400 K during the aging of the samples to explore the structural/phase transitions. The bulk and nanocrystalline La0.7Sr x Ca0.3−x MnO3 perovskite samples show the particle size distribution in the range of 197–943 and 24–93 nm, respectively. The addition of Sr2+ alters the size of particles, which decrease in size. The observed anomaly in ultrasonic velocities, attenuations, and elastic moduli is correlated with the ferromagnetic–paramagnetic (FM–PM) transition temperature (T C) in both bulk and nanocrystalline perovskites. In addition, the shift in T C and the magnitude and width of observed anomaly are correlated with the value of x to study the behavior of T C.

Towards the equation of state of quark-gluon matter from lattice QCD with Wilson twisted mass fermions

This contribution gives an overview of a long-term study of quark-gluon matter under extreme conditions employing Euclidean lattice discretized Quantum Chromodynamics (QCD) with Wilson twisted mass fermions and improved gauge actions. The analysis relies on methods and results of the European Twisted Mass Collaboration at zero temperature. We will start for two mass-degenerate light flavour degrees of freedom with a thorough investigation of the three-dimensional phase diagram of the bare coupling and the two mass-related parameters. Then for the same case we are going to describe the search for the pseudo-critical behaviour at various quark mass values. We shall discuss the behaviour of the (pseudo-)critical temperature Tc as a function of the (charged) pion mass mπ and the difficulty to extract the chiral limit in order to establish or reject the conjectured O(4)-universality. For the two-flavour case we shall further present very recent results for the thermodynamic Equation of State at non-zero temperature. Finally we will provide a view on an ongoing first exploration of QCD thermodynamics under the inclusion of virtual strange and charm quarks at their realistic mass values.

High-Tc/high-coupling relaxed PZT-based single crystal thin films

Pb(Zr,Ti)O3 (PZT)-based ferroelectric ceramics exhibit high piezoelectricity, however, their Curie temperature (Tc) values are not so high, i.e., Tc &amp;lt; 400 °C. PZT-based piezoelectric thin films with higher Tc would be beneficial for improved micro actuators, sensors, memories, and piezoelectric micro-electro mechanical systems. In-plane biaxial strained PZT thin films in a laminated composite structure are known to exhibit enhanced Tc; however, the thickness of PZT-based thin films is limited to below a critical thickness typically &amp;lt;50 nm. The Tc of relaxed PZT-based thin films with thicknesses greater than the critical thickness is the same as bulk Tc. However, a sort of relaxed PZT-based single-crystal thin films exhibit extraordinary high Tc, Tc = ∼600 °C. In addition, the films show extremely low dielectric constant, ɛ/ɛo ∼ 100 with high coupling factor, kt ∼ 0.7, and large remnant polarization, Pr ∼ 100 μC/cm2. These exotic properties would result from the single-domain/single-crystal structure. The enhanced Tc is possibly caused by the highly stable interface between the PZT-based thin films and substrates. Their ferroelectric performances are beyond those of conventional PZT. The high-Tc/high-coupling performances are demonstrated, and the possible mechanisms of the high Tc behavior in relaxed PZT-based single-crystal thin films are discussed.