Low High Field Research Articles

“Free” oxide ions in silicate melts: Thermodynamic considerations and probable effects of temperature

Reactions of simple metal oxides with silica often describe the melt solution in terms of equilibria among three energetically distinct oxygen ion species: bridging oxygens connecting two network cations, non-bridging oxygens shared by one network cation and multiple modifier cations of lower charge and larger radius, and “free” oxide ion bonded only to modifiers. Interest in the role and concentration of “free” oxide ions has recently been renewed by spectroscopic studies that have directly determined its abundance in a few glassy and amorphous silicates with low silica contents and high cation field strengths, as well as being inferred to be present by less direct measurements of bridging/non-bridging oxygen ratios in more silica-rich compositions. Here we review and evaluate recent evidence on “free” oxide concentrations in silicate glasses and melts, and discuss the important clues about effects of composition that come from comparisons of heats of formation of silicates and measurements of oxide component activities: relative proportions of oxygen species are expected to depend strongly on the charge and size of the modifier cation and its degree of covalent/ionic bonding. We also present simple thermodynamic approximations that allow temperature effects on “free” oxide concentrations to be estimated. In most compositions, this relative high energy species is expected to become more abundant at high temperatures, and may reach quite significant concentrations (>10% of total oxygens?) in hot, ultramafic, Mg- and Fe-rich melts in nature. In contrast, in most common, glass-forming compositions the concentrations of “free” oxide ion are probably much lower, particularly on cooling to glass transition temperatures, but this species has been suggested to potentially have an important role in melt dynamics even at low abundances. In any case, further experimental determinations, and theoretical evaluations, of this somewhat elusive and controversial species are well warranted.

High Magnetic Field Study on Giant Negative Magnetoresistance in the Molecular Conductor TPP[Cr(Pc)(CN)2]2

We investigated the magnetic and transport properties of the phthalocyanine molecular conductor TPP[Cr(Pc)(CN)2]2 in magnetic fields of up to 54 T. We observed giant negative magnetoresistance which hardly depends on the magnetic field direction owing to the isotropic nature confirmed by electron spin resonance measurements. The magnitude of magnetoresistance [|ρ(μ0H)/ρ(0 T) − 1|] is proportional to the square of magnetization as observed in the case of the spin scattering process, while the proportionality coefficient increases with decreasing the temperature. The magnetization does not saturate even at 53 T, indicating the existence of the large antiferromagnetic exchange interactions between the localized spins. In spite of this antiferromagnetic exchange interaction and low dimensionality, a convex magnetization curve was observed in the low temperature and high magnetic field range. To reproduce this magnetization curve, we proposed a model taking into account the antiferromagnetic exchange interaction between the neighboring π-electron spins.

Efficient plasmonic emission by the quantum Čerenkov effect from hot carriers in graphene.

Graphene plasmons have been found to be an exciting plasmonic platform, thanks to their high field confinement and low phase velocity, motivating contemporary research to revisit established concepts in light–matter interaction. In a conceptual breakthrough over 80 years old, Čerenkov showed how charged particles emit shockwaves of light when moving faster than the phase velocity of light in a medium. To modern eyes, the Čerenkov effect offers a direct and ultrafast energy conversion scheme from charge particles to photons. The requirement for relativistic particles, however, makes Čerenkov emission inaccessible to most nanoscale electronic and photonic devices. Here we show that graphene plasmons provide the means to overcome this limitation through their low phase velocity and high field confinement. The interaction between the charge carriers flowing inside graphene and the plasmons enables a highly efficient two-dimensional Čerenkov emission, giving a versatile, tunable and ultrafast conversion mechanism from electrical signal to plasmonic excitation.

Low turn-on field and high field emission current density from Ag/TiO2 nanocomposite

High current density of 1.24mA/cm2 was drawn at an applied field of 4.4V/μm from Ag/TiO2 nanocomposite. Also the turn-on field has been reduced from 3.9V/μm to 2.7V/μm for the emission current density of 10μA/cm2. Ag/TiO2 nanocomposite was synthesized by using UV-switchable reducing agent. TiO2 nanotube wall was decorated by Ag nanoparticles with average diameter of 17nm. To the best of our knowledge this is the first report on the field emission studies of Ag/TiO2 nanocomposite. Simple synthesis route coupled with superior field emission properties indicate the possible use of Ag/TiO2 nanocomposite for micro/nanoelectronic devices.

Overview of Performance Test of Korean CICCs for ITER TF Magnet

The Korean Domestic Agency (KODA) for the International Thermonuclear Experimental Reactor (ITER) project has taken the responsibility of the procurement of cable-in-conduit conductors for the ITER toroidal field magnet. While procuring 27 conductor unit lengths, which are the Korean sharing, 10 conductors have gone through the superconducting performance test, under low temperature and high magnetic field, as one of the final acceptance tests. The sample assembly and instrumentation of various voltage taps and temperature sensors have been done according to the specified procedure, which had been agreed between domestic agencies and the ITER International Organization. The performance test program, which has also been agreed, was carried out. For the assessment of the current sharing temperature (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cs</sub> ), the standard analysis procedure has been adopted. T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cs</sub> of all samples, at 68 kA and a background field of 10.78 T, after 1000 cyclic current loads are well above the acceptance criterion. By analyzing the electric field as a function of temperature, the effective strain (ε <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> ) and transition index (n-value) were reduced, and the behavior of ε <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> will be discussed in terms of the electromagnetic load, i.e., I × B. The effective strain was compared with the physical strain and results with a numerical simulation.

High frequency magnetic fluctuations correlated with the inter-ELM pedestal evolution in ASDEX Upgrade

In order to understand the mechanisms that determine the structure of the high confinement mode (H-mode) pedestal, the evolution of the plasma edge electron density and temperature profiles between edge localised modes (ELMs) is investigated. The onset of radial magnetic fluctuations with frequencies above 200 kHz is found to correlate with the stagnation of the electron temperature pedestal gradient. During the presence of these magnetic fluctuations the gradients of the edge electron density and temperature are clamped and stable against the ELM onset. The detected magnetic fluctuation frequency is analysed for a variety of plasma discharges with different electron pressure pedestals. It is shown that the magnetic fluctuation frequency scales with the neoclassically estimated velocity at the plasma edge. This points to a location of the underlying instability in the gradient region. Furthermore, the magnetic signature of these fluctuations indicates a global mode structure with toroidal mode numbers of approximately 10. The fluctuations are also observed on the high field side with significant amplitude, indicating a mode structure that is symmetric on the low field side and high field side. The associated fluctuations in the current on the high field side might be attributed to either a strong peeling part or the presence of non-adiabatic electron response.

Decoupled sources of the 2.3–2.2 Ga Yuanjiacun banded iron formation: Implications for the Nd cycle in Earth’s early oceans

The recognized worldwide gap in BIF deposition between 2.4 and 2.0 billion years ago has long been considered as an obstacle to fully determining the geochemical composition of seawater at that time. However, the recently dated 2.3–2.2Ga Yuanjiacun banded iron formation (BIF) in the North China Craton (NCC) offers a possibility to redress these uncertainties. Shale-normalized rare earth element-yttrium (REE+Y) patterns of the BIF and interlayered meta-chert samples show features characteristic of other Archean and Paleoproterozoic BIFs, with HREE enrichment relative to LREE, positive La and Eu anomalies, and superchondritic Y/Ho ratios comparable to modern seawater. Very low Al2O3 (<0.5wt%) and high field strength elements (HFSE) concentrations (<10ppm) indicate an essentially detritus-free depositional setting, while positive Eu anomalies are attributed to an imprint of high-temperature hydrothermal fluids. Sm–Nd isotopic features further point to two periodically interacting water masses controlling the deposition of the BIF. The first is seafloor-vented hydrothermal fluids (εNd(t)∼+3.5) derived from interaction with a depleted mantle source and associated with high Fe fluxes. The second is ambient surface seawater (εNd(t)∼−2.4), which obtained its signature through weathering of the nearby landmasses and associated with high Si fluxes. Our findings suggest that the REE budget of the oceans prior to 2.3Ga was generally dominated by hydrothermal circulation of seawater through depleted mantle-derived source rocks. However, where evolved local continental crust and/or an enriched mantle source were present, this positive mantle Nd signal became discernible in the BIFs. By comparing the Nd isotopic features of similar aged BIFs and marine carbonates, it is concluded that similar to modern oceans, the early Precambrian ocean was not well-mixed with respect to its Nd isotopic composition.

Effects of oxygen plasma etching on Sb2Te3 explored by torque detected quantum oscillations

De Haas–van Alphen measurements evidence that oxygen plasma etching strongly affects the properties of the three-dimensional topological insulator Sb2Te3. The quantum oscillations in magnetization down to low temperature (T ≥ 2 K) and high magnetic field (B ≤ 7 T) have been systematically investigated using a high-sensitive cantilever torque magnetometer. The effective mass and the oscillation frequency obtained from de Haas–van Alphen measurements first increase and then decrease as the oxygen plasma etching time increases from 0 to 12 min, corresponding to an up- and down-shift of the Dirac point. We establish the cantilever torque magnetometer as a powerful contactless tool to investigate the oxygen sensitivity of the surface state in topological insulators.

Low-Temperature Intergranular Spin Transport in $${\mathbf {La_{0.5}Ca_{0.4}Li_{0.1}MnO_3}}$$ La 0.5 Ca 0.4 Li 0.1 MnO 3 Manganite Under High Magnetic Field $$\mathbf {(1\,T\le H \le 14\,T)}$$ ( 1 T ≤ H ≤ 14 T )

The quantitative analysis of a low-temperature minimum of the resistivity of $$\mathrm {La_{0.5}Ca_{0.4}Li_{0.1}MnO_3}$$ under high magnetic fields (up to 14 T) was performed. It was shown that the behavior of resistivity and magnetoresistivity at low temperature and high magnetic field can be fitted well by a spin-polarized intergrain tunneling model.

Antiferromagnetic Spin Seebeck Effect.

We report on the observation of the spin Seebeck effect in antiferromagnetic MnF_{2}. A device scale on-chip heater is deposited on a bilayer of MnF_{2} (110) (30 nm)/Pt (4nm) grown by molecular beam epitaxy on a MgF_{2} (110) substrate. Using Pt as a spin detector layer, it is possible to measure the thermally generated spin current from MnF_{2} through the inverse spin Hall effect. The low temperature (2-80K) and high magnetic field (up to 140kOe) regime is explored. A clear spin-flop transition corresponding to the sudden rotation of antiferromagnetic spins out of the easy axis is observed in the spin Seebeck signal when large magnetic fields (>9 T) are applied parallel to the easy axis of the MnF_{2} thin film. When the magnetic field is applied perpendicular to the easy axis, the spin-flop transition is absent, as expected.

Thermally stable yttrium–scandium oxide high-k dielectrics deposited by a solution process

We investigated the thermal stability of electrical properties in ternary alloy (YxSc1−x)2O3 high-k oxides as a function of yttrium fraction, x. The yttrium–scandium oxide dielectric films are deposited using a facile ink-based process. The oxides have a stoichiometry-dependent relative dielectric constant of 26.0 to 7.7 at 100 kHz, low leakage current density of 10−8 A·cm−2, high breakdown field of 4 MV⋅cm−1, and interface trap density of 1012 cm−2·eV−1 with silicon. Compared with binary oxides, ternary alloys exhibit less frequency dispersion of the dielectric constant and a higher crystallization temperature. After crystallization is induced through a 900 °C anneal, ternary (Y0.6Sc0.4)2O3 films maintain their low leakage current and high breakdown field. In contrast, the electrical performance of the binary oxides significantly degrades following the same treatment. The solution-processed ternary oxide dielectrics demonstrated here may be used as high-k gate insulators in complementary metal-oxide semiconductor (CMOS) technologies, in novel electronic material systems and devices, and in printed, flexible thin film electronics, and as passivation layers for high power devices. These oxides may also be used as insulators in fabrication process flows that require a high thermal budget.

Intrinsic plasma rotation and Reynolds stress at the plasma edge in the HSX stellarator

Using multi-tipped Langmuir probes in the edge of the HSX stellarator, the radial electric field and parallel flows are found to deviate from the values calculated by the neoclassical transport code PENTA for the optimized quasi-helically symmetric (QHS) configuration. To understand whether Reynolds stress might explain the discrepancy, fluctuating floating potential measurements are made at two locations in the torus corresponding to the low field and high field sides of the device. The measurements at the two locations show clear evidence of a gradient in the Reynolds stress. However, the resulting flow due to the gradient in the stress is found to be large and in opposite directions for the two locations. This makes an estimation of the flux surface average using a small number of measurement locations impractical from an experimental perspective. These results neither confirm nor rule out whether Reynolds stress plays an important role for the QHS configuration. Measurements made in configurations with the quasi-symmetry degraded show even larger flows and greater deviations from the neoclassically calculated velocity profiles than the QHS configuration while the fluctuation magnitudes are reduced. Therefore, for these configurations in particular, the Reynolds stress is most likely not responsible for the additional momentum.

Geochemistry, geochronology, and tectonic setting of early Permian (~290 Ma) volcanic-hosted massive sulphide deposits of the Tasik Chini district, Peninsular Malaysia

ABSTRACTThe age and nature of Permian volcanism and sulphide mineralization in Malaysia are poorly understood. The Tasik Chini district is located in the Central Belt of Peninsular Malaysia and hosts the Bukit Botol and Bukit Ketaya volcanic-hosted massive sulphide (VHMS) deposits. These deposits are hosted by felsic-dominated Permian felsic volcanics that are part of a mixed volcano-sedimentary rock succession. Four mineralization zones were identified in both deposits: (1) stringer sulphide/zone; (2) massive sulphide zone; (3) barite zone; and (4) Fe-Mn and Fe-Si zones. The stringer and massive sulphides generally define the lower mineralized zones, whereas the barite, Fe-Mn, and Fe-Si layers define the stratigraphically upper mineralized zones. The main sulphide phases are pyrite, chalcopyrite, sphalerite, rare galena, and trace Sn-, Au-, and Ag-bearing minerals, with the latter two confined to the massive sulphide and barite zones. Trace and rare earth element data for the host rhyodacite-rhyolite at both deposits are consistent with formation in a volcanic arc palaeotectonic setting. In comparison, the trace element data for Triassic volcanic and intrusive rocks from the Tasik Chini area have moderate to low high field strength element (HFSE) concentrations along with transitional (Zr/Y = 4–7) to tholeiitic (Zr/Y = 2–4) affinities, but have similar magmatic arc signatures. Laser ablation inductively coupled mass spectrometry (LA-ICP-MS) U–Pb zircon dating of rhyolites from the Bukit Botol deposit yields early Permian (286 ± 4 to 292 ± 3 Ma) ages. Similarly, the zircon U–Pb age results for Bukit Ketaya rhyolites reveal early Permian (286 ± 2 to 288 ± 4 Ma) ages. The differences in geochemical and geochronological results between the early Permian host and the later Triassic volcanic and intrusive rocks are likely due to tectonic progression from a volcanic arc environment to a collisional setting.

Study on temperature and electric field dependence of conductivity in polyimide

The deep dielectric charging (DDC) imposes a potential threat on spacecrafts. On the one hand, this kind of polymer insulator dielectric, represented by polyimide, is significantly dependent on temperature; on the other hand, during the charging process the high electric field (at the level of 107 V/m) will enhance the conductivity of the dielectric. Therefore, in order to make a precise assessment of DDC by computer simulation, the conductivity model should take into account the temperature and electric field dependences. In this field, two conductivity models are usually adopted for DDC simulation. One of them is proposed by Adamec. It puts emphasis on the enhanced conductivity due to high electric field, while its temperature dependence is based on the famous Arrhenius formula. Adamec model can make good performance versus electric field, but it is inappropriate in low temperatures. Another model combines the thermally assistant hopping conductivity and the variable-range hopping conductivity together, so it shows advantage in the temperature dependence, which is named as TAH VRH model. Although this model also can include the influence from electric fields, the effectiveness is not so good as that of Adamec model. In order to combine the advantages of these two models, i.e. the Adamec model and TAH VRH model, a new conductivity model is proposed with fewer parameters than those in TAH VRH. It is derived by replacing the Arrhenius formula in Adamec model with a simplified temperature model referred to as TAH VRH model. This formulation enables the new model to deal with a wider temperature range and keep the good performance versus high electric fields. The proposed model is verified partly by the measured data of a kind of polyimide. Satisfactory agreement is obtained in data fitting by using the new model, where the temperature dependence is better than that of Adamec model. In addition, to overcome the unreasonable increase in conductivity in low temperature and high electric field, a useful technique is proposed. By temperature mapping in the electric field correlated factors namely the carrier concentration and mobility enhancement factor, this technique can extend the feasible temperature range to a lower limit. This is done according to the assumption that the carrier concentration is small at low temperatures, and consequently the electric field influence should not be large. At high temperatures or in low electric fields, the temperature mapping is of little effect. Finally, analysis of the model's sensitivity versus several parameters is provided, demonstrating the advantage of applicability of the new model with fewer parameters.

Multiple phase transitions in Pauli-limited iron-based superconductors

Specific heat measurements have been successfully used to probe unconventional superconducting phases in one-band heavy-fermion and organic superconductors. We extend the method to study successive phase transitions in multi-band materials such as iron-based superconductors. The signatures are multiple peaks in the specific heat, at low temperatures and high magnetic field, which can lead to the experimental verification of unconventional superconducting states with non-zero total momentum.

Test of 60 kA coated conductor cable prototypes for fusion magnets

Coated conductors could be promising materials for the fabrication of the large magnet systems of future fusion devices. Two prototype conductors (flat cables in steel conduits), each about 2 m long, were manufactured using coated conductor tapes (4 mm wide) from Super Power and SuperOx, with a total tape length of 1.6 km. Each flat cable is assembled from 20 strands, each strand consisting of a stack of 16 tapes surrounded by two half circular copper profiles, twisted and soldered. The tapes were measured at 12 T and 4.2 K and the results of the measurements were used for the assessment of the conductor electromagnetic properties at low temperature and high field. The two conductors were assembled together in a sample that was tested in the European Dipole (EDIPO) facility. The current sharing temperatures of the two conductors were measured at background fields from 8 T up to 12 T and for currents from 30 kA up to 70 kA: the measured values are within a few percent of the values expected from the measurements on tapes (short samples). After electromagnetic cycling, Tcs at 12 T and 50 kA decreased from about 12 K to 11 K (about 10%), corresponding to less than 3% of Ic.

Characterization of Leakage and Reliability of SiN&lt;sub&gt;&lt;italic&gt;x&lt;/italic&gt;&lt;/sub&gt; Gate Dielectric by Low-Pressure Chemical Vapor Deposition for GaN-based MIS-HEMTs

In this paper, we systematically investigated the leakage and breakdown mechanisms of the low-pressure chemical vapor deposition (LPCVD) silicon nitride thin film deposited on AlGaN/GaN heterostructures. The LPCVD-SiN x gate dielectric exhibits low leakage and high breakdown electric field. The dominant mechanism of the leakage current through LPCVD-SiN x gate dielectric is identified to be Poole–Frenkel emission at low electric field and Fowler–Nordheim tunneling at high electric field. Both electric-field-accelerated and temperature-accelerated time-dependent dielectric breakdown of the LPCVD-SiN x gate dielectric were also investigated.

A device for the application of uniaxial strain to single crystal samples for use in synchrotron radiation experiments.

We present the design, construction, and testing of a straining device compatible with many different synchrotron radiation techniques, in a wide range of experimental environments (including low temperature, high field and ultra-high vacuum). The device has been tested by X-ray diffraction on single crystal samples of quasi-one-dimensional Cs2Mo6Se6 and K2Mo6Se6, in which microscopic strains up to a Δc/c = 0.12% ± 0.01% change in the c lattice parameters have been achieved. We have also used the device in an inelastic X-ray scattering experiment, to probe the strain-dependent speed of sound ν along the c axis. A reduction Δν/ν of up to -3.8% was obtained at a strain of Δc/c = 0.25% in K2Mo6Se6.

A general one-dimension nonlinear magneto-elastic coupled constitutive model for magnetostrictive materials

For magnetostrictive rods under combined axial pre-stress and magnetic field, a general one-dimension nonlinear magneto-elastic coupled constitutive model was built in this paper. First, the elastic Gibbs free energy was expanded into polynomial, and the relationship between stress and strain and the relationship between magnetization and magnetic field with the polynomial form were obtained with the help of thermodynamic relations. Then according to microscopic magneto-elastic coupling mechanism and some physical facts of magnetostrictive materials, a nonlinear magneto-elastic constitutive with concise form was obtained when the relations of nonlinear strain and magnetization in the polynomial constitutive were instead with transcendental functions. The comparisons between the prediction and the experimental data of different magnetostrictive materials, such as Terfenol-D, Metglas and Ni showed that the predicted magnetostrictive strain and magnetization curves were consistent with experimental results under different pre-stresses whether in the region of low and moderate field or high field. Moreover, the model can fully reflect the nonlinear magneto-mechanical coupling characteristics between magnetic, magnetostriction and elasticity, and it can effectively predict the changes of material parameters with pre-stress and bias field, which is useful in practical applications.

Barkhausen noise in the random field Ising magnetNd2Fe14B

With sintered needles aligned and a magnetic field applied transverse to its easy axis, the rare-earth ferromagnet Nd_2Fe_(14)B becomes a room-temperature realization of the random field Ising model. The transverse field tunes the pinning potential of the magnetic domains in a continuous fashion. We study the magnetic domain reversal and avalanche dynamics between liquid helium and room temperatures at a series of transverse fields using a Barkhausen noise technique. The avalanche size and energy distributions follow power-law behavior with a cutoff dependent on the pinning strength dialed in by the transverse field, consistent with theoretical predictions for Barkhausen avalanches in disordered materials. A scaling analysis reveals two regimes of behavior: one at low temperature and high transverse field, where the dynamics are governed by the randomness, and the second at high temperature and low transverse field, where thermal fluctuations dominate the dynamics.