Low High Field Research Articles

Room temperature ultrahigh magnetoresistance nanostructure (La2/3Sr1/3)MnO3 films growth on SrTiO3 substrate

Nanostructure (La2/3Sr1/3)MnO3 thin films were epitaxial grown on SrTiO3 (001) single crystal substrate by pulsed laser deposition. Both high Curie temperature (Tc=360K) and high magnetoresistance (MR=−38% at 500Oe magnetic fields and 320K) were simultaneously achieved. The high Curie temperature was attributed to the high quality epitaxial LSMO films. The high low field magnetoresistance (LFMR) was attributed to the regular square nanostructure of the deposited LSMO films, where the boundaries serve as the barrier for spin-polarized tunneling and/or spin-dependent scattering. The high Tc and large MR made the LSMO film very useful in room temperature magnetic devices.

Overlapping-Gate Architecture for Silicon Hall Bar MOSFET Devices in the Low Electron Density and High Magnetic Field Regime

A common issue in low temperature measurements of enhancement-mode metal-oxide-semiconductor (MOS) field-effect transistors (FETs) in the low electron density regime is the high contact resistance dominating the device impedance. In that case a voltage bias applied across the source and drain contact of a Hall bar MOSFET will mostly fall across the contacts (and not across the channel) and therefore magneto-transport measurements become challenging. However, from a physical point of view, the study of MOSFET nanostructures in the low electron density regime is very interesting (impurity limited mobility [1], carrier interactions [2,3] and spin-dependent transport [4]) and it is therefore important to come up with solutions [5,6] that work around the problem of a high contact resistance in such devices (c.f. Fig. 1 (a)).

Recent results of the T-10 tokamak

Poloidal asymmetry and radial correlation lengths of turbulence were investigated in T-10 at low field side and high field side by correlation reflectometry. Correlation of plasma confinement with the turbulence type was observed. Improvements in heavy ion beam probe diagnostic enabled us to measure the plasma potential during electron cyclotron resonance heating (ECRH) in a wide range of radial positions and operational regimes. The turbulence appeared to rotate close to E × B velocity. The concept of electron internal transport barrier (e-ITB) formation at low-order rational surfaces under conditions of low density of the rational surfaces was proved by the observation of e-ITB formation near the q = 1.5 surface in discharges with non-central ECRH and current ramp-up. The kinetic phenomena were investigated by means of electron cyclotron emission (ECE) under the strong on-axis ECRH. Lithium gettering of the limiter and the wall allowed us to significantly reduce the impurity level and obtain a recycling coefficient as low as 0.3. The rates of carbon film deposition were measured in the working and cleaning discharges. Second harmonic EC assisted start-up was investigated. ECRH allowed us to control the generation of runaway electrons and the current decay rate after the energy quench at the density limit disruption.

Near-field radiative heat transfer between closely spaced graphene and amorphous SiO2

We study the near-field radiative energy transfer between graphene and an amorphous SiO${}_{2}$ substrate. In comparison with the existing theories of near-field radiative heat transfer our theory takes into account that the free carriers in graphene are moving relative to the substrate with a drift velocity $v$. In this case the heat flux is determined by both thermal and quantum fluctuations. We find that quantum fluctuations give an important contribution to the radiative energy transfer for low temperatures and high electric field (large drift velocities). For nonsuspended graphene the near-field radiative energy transfer gives a significant contribution to the heat transfer in addition to the contribution from phononic coupling. For suspended graphene (large separation) the corresponding radiative energy transfer coefficient at a nanoscale gap is $~$3 orders of magnitude larger than radiative heat transfer coefficient of the blackbody radiation limit.

A Small 1.5 T Persistent Current Operating Test Magnet Using ${\rm MgB}_{2}$ Wire With High $j_{\rm c}$ Joints

Persistent current (PC) operating magnet with bore diameter of about 32 mm was design and developed based on multi- and mono-filament MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> wires. The magnet was in a multi-solenoid structure. One pilot coil was wound and joints were fabricated at both ends, connecting it to a superconducting switch for PC operation tests. The magnet was to demonstrate the high field and high temperature performance at different temperatures. Numerical and experimental results demonstrated a promising future of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> wires in low cost magnetic resonance imaging (MRI) systems and high field scientific magnets.

Study of non-magnetic iron mononitride thin films

Abstract Iron mononitride, FeN, is known to exist in different phases and there is a debate on the exact crystal type and the coexistence of these phases. We prepared single phase iron mononitride thin films by magnetron sputtering with various deposition rates (sputtering power) and investigated them with X-ray diffraction, neutron reflectometry and low temperature and high magnetic field Mössbauer spectroscopy. It was observed that with an increase in the sputtering power a more disordered structure is formed while the local chemical environment of iron remains unaffected. The low temperature and high magnetic field Mössbauer spectroscopy measurements confirmed that the FeN phase is paramagnetic at 5 K and an applied magnetic field of 5 T reflects magnetic splitting caused by the applied field. The obtained results are discussed in this study.

Swirling flow of natural gas in supersonic separators

The supersonic swirling separator is a new apparatus for offshore and subsea natural gas separation, due to its lightweight and the viability of unmanned operation. A new supersonic swirling separator was designed for the numerical calculation, in which a central body was inserted based on the principle of conservation of angular momentum. Axial and radial distribution of the main parameters of natural gas flow was investigated with RNG K-ɛ turbulence model. The effects of the shock waves on the natural gas flow fields were analyzed in the supersonic separator. The results show that water and heavy hydrocarbons can be separated from natural gas due to the low temperature and high centrifugal field. The non-uniformity of radial distribution of the gas dynamic parameters significantly affects the gas/liquid separation. The position of the shock wave determines the distribution of the temperature, which has a great influence on the re-evaporation of liquid droplets.

Simultaneous low extinction and high local field enhancement in Ag nanocubes**Project supported by the National Natural Science Foundation of China (Grant Nos. 60736041 and 10874238), and the National Key Basic Research Special Foundation of China (Grant No. 2007CB613205).

We theoretically investigate surface plasmon resonance properties in Au and Ag cubic nanoparticles and find a novel plasmonic mode that exhibits simultaneous low extinction and high local field enhancement properties. We analyse this mode from different aspects by looking at the distribution patterns of local field intensity, energy flux, absorption and charge density. We find that in the mode the polarized charge is highly densified in a very limited volume around the corner of the nanocube and results in very strong local field enhancement. Perturbations of the incident energy flux and light absorption are also strongly localized in this small volume of the corner region, leading to both low absorption and low scattering cross section. As a result, the extinction is low for the mode. Metal nanoparticles involving such peculiar modes may be useful for constructing nonlinear compound materials with low linear absorption and high nonlinearity.

Effect of metal nanoparticles decoration on electron field emission property of graphene sheets

The electron field emission from metal nanoparticle decorated hydrogen exfoliated graphene (metal/HEG) occurs at low turn on and threshold fields due to its low work function and high field enhancement factor.

High Performance Metal-Insulator-Metal Capacitors with Er2O3 on ALD SiO2 for RF Applications

MIM capacitors for RF applications with sputtered Er2O3 on ALD SiO2 stacked dielectrics with excellent capacitance performance were demonstrated for the first time. An MIM capacitor with 8.8 nm Er2O3 on 3 nm SiO2 stacked dielectrics displayed a high capacitance density of 7 fF/μm2, a low quadratic VCC of less than 100 ppm/V2, a low leakage current at 3.3 V bias of 1 × 10−8 A/cm2, and a high dielectric field strength of 8.7 MV/cm. Having leakage currents of 1 × 10−7 and 4 × 10−8 A/cm2 at 3.3 and 2 V, respectively, the MIM capacitor with 7 nm Er2O3 on 3 nm SiO2 stacked dielectrics demonstrated a capacitance density of 7.4 fF/μm2 and quadratic VCCs of −91 and −255 ppm/V2 at 10 and 100 kHz, respectively. The low quadratic VCC α was obtained by compensating the positive α of Er2O3 with the negative α of SiO2, while the low leakage current and high oxide field strength demonstrated were due to the high quality SiO2 deposited by the ALD method.

Temperature and Doping Dependencies of the Transport Properties within GaN and GaAs

Temperature and doping dependencies of the transport properties have been calculated using an ensemble Monte Carlo simulation. We consider the polar optical phonon, acoustic phonons, piezoelectric, intervalley scatterings and Charged impurity scattering model of Ridley; furthermore, a non nonparabolic three-valley model is used. Our simulation results have shown that the electron velocity in GaN is less sensitive to changes in temperature than that associated with GaAs. Also it is found that GaN exhibits high peak drift velocity at room temperature, 2.8 × 105m/s, at doping concentration of 1 × 1020 m–3and the electron drift velocity relaxes to the saturation value of 1.3 × 105 m/s which is much larger than that of GaAs. The weakening of the phonon emission rate at low temperature explains the extremely high low field mobility. Our results suggest that the transport characteristics of GaN are superior to that of GaAs, over a wide range of temperatures, from 100 K to 700 K, and doping concentrations, up to 1 × 1025 m–3

X-ray absorption and magnetic circular dichroism investigation of bis(phthalocyaninato)terbium single-molecule magnets deposited on graphite

Bisphthalocyaninato terbium complexes show a long magnetization relaxation time at relatively high temperatures---which makes them very interesting as magnets at single-molecule level. Their technological exploitation, however, requires the addressing of the individual molecules, therefore the deposition of single-molecule magnets on surfaces is a topic of great interest as the interaction with the substrate can play a crucial role in the definition of the molecule properties. In this work we investigate the electronic and magnetic properties of anionic and neutral forms of a bis(phthalocyaninato)terbium derivative deposited on graphite by means of x-ray absorption spectroscopy and x-ray magnetic circular dichroism, performed at low temperature and high magnetic field at the ${M}_{4,5}$ edge of Tb. We were able to reproduce the experimental spectra by means of multiplet calculations and to validate the applicability of sum rules to the present case. Sum rules were then used for determining the orbital and spin moments of thick (several monolayers) and of thin films (submonolayer range). Calculations of spectra as a function of the molecule orientation with respect to the impinging x-ray beam, allowed us to ascertain the adsorption geometry of molecules. For both compounds, molecules stay essentially flat when adsorbing as thin film on graphite. This result is also confirmed by scanning probe microscopy, which also finds a very interesting ordered arrangement for the molecules of the neutral form. In the thick film of the neutral compound the molecules keep the same orientational order, arranging almost flat as well. On the contrary, in the thick film of the anionic compound their orientation appears to be random. The origin of this different behavior can be related to the hindrance of the counterion moiety and/or to the different solvent used for each compound. Finally, the comparison of the magnetization values and their dependence on the external magnetic field and temperature suggest that the magnetic properties of molecules are preserved when adsorbed onto the graphite surface.

High magnetic field induced charge density waves and sign reversal of the Hall coefficient ingraphite

We report on the investigation of magnetic field induced charge density waves and Hallcoefficient sign reversal in a quasi-two-dimensional electronic system of highly orientedpyrolytic graphite under very strong magnetic field. The change of Hall sign coefficientfrom negative to positive occurs at low temperature and high magnetic field just after thecharge density wave transition, suggesting the role of hole-like quasi-particles inthis effect. Angular dependent measurements show that the charge density wavetransition and Hall sign reversal fields follow the magnetic field component along thec-axis of graphite.

Comparison of Supersonic Molecular Beam Injection from Both Low Field Side and High Field Side of HL-2A

SMBI (supersonic molecular beam injection), as an effective fueling method for fusion plasmas, has been widely used on the HL-2A tokamak and other fusion devices. Two different types of SMBI system are now installed on HL-2A. One is an electromagnetic valve injector on the low field side (LFS), and the other is a pneumatic valve injector on the high field side (HFS). A new electron density record n̄e = 4.7 × 1019 m−3, which exceeds both the Greenwald density limit and the maximum density obtained by gas puffing (GP), was obtained on HL-2A with single-null-divertor operation. The HFS injection system is still under test, however, its outstanding fueling characteristics have already been observed, e.g. it has higher fueling efficiency compared to the LFS system. This excellent feature is still preserved during the process of ECRH.

(Invited) Germanium on Sapphire Technology

Silicon-on-sapphire (SOS) substrates have been proven to offer significant advantages in the integration of passive and active devices in RF circuits. Germanium on insulator technology is a candidate for future higher performance circuits. Thus the advantages of employing a low loss dielectric substrate other than a silicon-dioxide layer on silicon will be even greater. This paper covers the production of germanium on sapphire (GeOS) substrates by wafer bonding. The quality of the germanium back interface is studied and a tungsten self-aligned gate process MOST process has been developed. High low field mobilities of 450-500 cm2/V-s have been achieved for p-channel MOSTs produced on GeOS substrates. Thick germanium on alumina (GOAL) substrates have also been produced.

Anisotropic magnetic susceptibility record of the kinematics of the Boltaña Anticline (Southern Pyrenees)

AbstractAnisotropic magnetic susceptibility (AMS) analysis of 1454 samples evenly distributed along an 1800 m section in the South Pyrenean Zone is presented. The sampled rocks are pre‐ and syn‐tectonic limestones, marls and sandstones of Ypresian‐Lutetian age. They are weakly deformed rocks sampled from both limbs of the N–S trending Boltaña Anticline (oblique to the WNW–ESE main Pyrenean trend). Rock magnetism analyses, including low temperature and high field magnetic susceptibility measurements confirm a dominant paramagnetic signal and hence allow the interpretation of AMS in terms of preferred rock fabric or finite strain orientation. Therefore, this study was focused on obtaining an independent kinematic information from a vast set of AMS data from particular structural positions. The three directional maxima for the maximum axis of the magnetic ellipsoid (KMAX, magnetic lineation) can be tentatively related to major tectonic events in the Central Pyrenees. A first stage (recording a N–S magnetic lineation) is related to an oblique ramp of the Cotiella Thrust, emplaced during Ypresian times. A pseudo‐progressive variation turns into a second stage (linked to a NW–SE KMAX orientation maximum) that can be related to Pyrenean compression. Both stages witness a far‐field imprint of active structures on the foreland sediments. Once the Boltaña Anticline was nucleated (during the Lutetian), a predominant, more constant NW–SE trend is observed, this near‐field magnetic fabric being the only lineation imprinted in the rocks. A final stage arrived in the course of vertical axis rotation during the Late Lutetian‐Bartonian. This latest event was responsible for the present configuration of the Boltaña Anticline (N–S). Copyright © 2010 John Wiley &amp; Sons, Ltd.

Increased low field magnetoresistance in electron doped system Sr0.4Ba1.6−xLaxFeMoO6

Magnetotransport properties of electron doped polycrystalline system Sr0.4Ba1.6−xLaxFeMoO6 are presented in this paper. We have observed increased low field magnetoresistance values with significant Curie temperature increase in the electron doped system Sr0.4Ba1.6−xLaxFeMoO6. The low field magnetoresistance value (at 1000 Oe) in 20% La3+ doped sample is observed to be 2.17% at 300 K. At 0.64 T and 80 K, the magnetoresistance change measured in this sample is 21.4%. This sample also showed 50 K increase in Curie temperature over the pristine sample. The increased low field magnetoresistance values are associated with modified grain boundary barriers of the system. The results confirm the fact that the modification of grain boundary barriers has enough potential to possess high low field magnetoresistance values even in the systems with lower spin polarization values. The role of grain to grain connectivity is observed to be dominantly determining the low field magnetoresistance values in grain to grain tunnel type magnetoresistance.

Upgrading Design to a 25 T Cryogen-Free Superconducting Magnet Based on Low Temperature and High Magnetic Field Properties of the Practical CVD Processed Coated Conductors

We evaluated critical current density and mechanical properties of the Y123 coated conductor (CC) tapes on buffered Hastelloy substrates prepared by the chemical vapor deposition method in high magnetic fields and low temperatures. The Jc values of the tape are about 1.1 MA/cm at 77.3 K, 0 T and 1.8 MA/cm2 at 4.2 K and 17 T for B//c. In addition, the hoop stress test of the single layer coil shows that the stress limit of the CVD-Y123 CC tapes on Hastelloy is over 1 GPa. On the basis of those experimental data, we designed the innermost high temperature superconducting insert coil for upgrading of the 18 T cryogen-free superconducting magnet (18 T-CSM) under the condition of the stress limit of 600 MPa. The coil, which consists of the 22 double pancake coils can generate 9.4 T with the operation current of 295 A in the backup field of 15.6 T. In this case, the central field of the 18 T-CSM can be improved up to 25 T from 18.1 T.

Rotation dependence of a phase delay between plasma edge electron density and temperature fields due to a fast rotating, resonant magnetic perturbation field

Measurements of the plasma edge electron density ne and temperature Te fields during application of a fast rotating, resonant magnetic perturbation (RMP) field show a characteristic modulation of both, ne and Te coherent to the rotation frequency of the RMP field. A phase delay Φ between the ne(t) and Te(t) waveforms is observed and it is demonstrated that this phase delay Φ is a function of the radius with Φ(r) depending on the relative rotation of the RMP field and the toroidal plasma rotation. This provides for the first time direct experimental evidence for a rotation dependent damping of the external RMP field in the edge layer of a resistive high-temperature plasma which breaks down at low rotation and high resonant field amplitudes.

Effects of La, Nd and Sm substitution of Sr in Sr 2CrReO 6 on the structural, magnetic and transport properties

A detailed study of Strontium substitution (up to 25% at most) with Lanthanide (La, Nd and Sm) in the Sr 2CrReO 6 double perovskite has been carried out by means of neutron powder diffraction (NPD), scanning electron microscopy (SEM), low magnetic field and high pulsed magnetic field magnetometry as well as transport measurements. In the present manuscript we will focus on the consequences of such substitution on structural, electronic and magnetic properties. The presented results show the lack of expected enhancement of the Curie temperature by the Lanthanide addition. This puts doubts on the possibility of electron doping in the Sr 2CrReO 6 system via the standard solid-state reaction growth techniques with rare-earth doping, commonly used for other double perovskite compounds.