Raman spectroscopy as a versatile tool for studying the properties of graphene.

Raman spectroscopy is an integral part of graphene research. It is used to determine the number and orientation of layers, the quality and types of edge, and the effects of perturbations, such as electric and magnetic fields, strain, doping, disorder and functional groups. This, in turn, provides insight into all sp(2)-bonded carbon allotropes, because graphene is their fundamental building block. Here we review the state of the art, future directions and open questions in Raman spectroscopy of graphene. We describe essential physical processes whose importance has only recently been recognized, such as the various types of resonance at play, and the role of quantum interference. We update all basic concepts and notations, and propose a terminology that is able to describe any result in literature. We finally highlight the potential of Raman spectroscopy for layered materials other than graphene.

Numerical study of the shielding properties of macroscopic hybrid ferromagnetic/superconductor hollow cylinders

We study the magnetic shielding properties of hybrid ferromagnetic/ superconductor (F/S) structures consisting of two coaxial cylinders, with one of each material. We use an axisymmetric finite-element model in which the electrical properties of the superconducting tube are modeled by a nonlinear E-J power law with a magnetic-field-dependent critical current density whereas the magnetic properties of the ferromagnetic material take saturation into account. We study and compare the penetration of a uniform axial magnetic field in two cases: 1) a ferromagnetic tube placed inside a larger superconducting tube (Ferro-In configuration) and 2) a ferromagnetic tube placed outside the superconducting one (Ferro-Out configuration). In both cases, we assess how the ferromagnetic tube improves the shielding properties of the sole superconducting tube. The influence of the geometrical parameters of the ferromagnetic tube is also studied: It is shown that, upon an optimal choice of the geometrical parameters, the range of magnetic fields that are efficiently shielded by the high-temperature superconductor tube alone can be increased by a factor of up to 7 (2) in a Ferro-Out (Ferro-In) configuration. The optimal configuration uses a 1020 carbon steel with a thickness of 2 mm and a height that is half that of the superconducting cylinder (80 mm). © 2009 IEEE.

DC and AC Shielding properties of bulk high-tc

We have studied numerically and experimentally the magnetic flux penetration in high-Tc superconducting tube subjected to a uniform magnetic field parallel to its long axis. This study is carried in view of designing low-frequency magnetic shields by exploiting the diamagnetic properties of high-Tc superconducting ceramics. We have measured the field attenuation for applied magnetic fields in the frequency range 5 mHz-0.1 Hz by Hall probe measurements and at audio frequencies using a sensing coil. A simple 1D analysis using the Kim critical state model was found to be able to reproduce the experimental data satisfactorily. We have also determined the phase shift between the internal and the applied field both experimentally and numerically. Finally, we have studied the sweep rate dependence of the magnetic shielding properties, using data recorded either at several constant sweep rates dB /dt or at several AC fields of various amplitudes and frequencies. Both methods agree with each other and lead to a n-value of the E ∼ Jn law equal to ∼40 at 77 K. © 2009 IEEE.

Magnetotransport study of MgB2 superconductor

Precise magnetotransport studies of heat and charge carriers in polycrystalline MgB2 show that magnetic fields up to 8 T remarkably influence electrical resistivity, thermoelectric power and thermal conductivity. The superconducting transition temperature shifts from 39 K to 19 K at 8 T as observed on electric signals. The temperature transition width is weakly broadened. Electron and phonon contributions to the thermal conductivity are separated and discussed. The Debye temperature calculated from a phonon drag thermoelectric power component is inconsistent with values derived through other effects.

Magneto-transport study of Nb-doped Bi/Pb2223 superconductor

The magneto-transport properties of Bi1.5Pb0.4Nb0.1Sr2Ca2Cu 3O10-x polycrystalline, superconducting ceramic are reported. The material was found to be chemically homogeneous and partially textured. The mixed state properties were investigated by measuring the electrical resistivity, longitudinal and transverse (Nernst effect) thermoelectric power, and thermal conductivity. The magnetization and AC susceptibility measurements were also performed. The variation of these characteristics for magnetic fields up to 5 T are discussed and compared to those of the zero field case. The transport entropy and thermal Hall angle are extracted and quantitatively compared to previously reported data of closely related systems. © 2003 Elsevier Science B.V. All rights reserved.

Measurement of filling-factor-dependent magnetophonon resonances in graphene using Raman spectroscopy

We perform polarization-resolved Raman spectroscopy on graphene in magnetic fields up to 45 T. This reveals a filling-factor-dependent, multicomponent anticrossing structure of the Raman G peak, resulting from magnetophonon resonances between magnetoexcitons and E2g phonons. This is explained with a model of Raman scattering taking into account the effects of spatially inhomogeneous carrier densities and strain. Random fluctuations of strain-induced pseudomagnetic fields lead to increased scattering intensity inside the anticrossing gap, consistent with the experiments. © 2013 American Physical Society.

Turbulence in Rotating, Stratified and Electrically Conducting Fluids

This book investigates how turbulence responds to rotation, stratification or magnetic fields, identifying common themes, where they exist, as well as the essential differences which inevitably arise between different classes of flow.

Noise and vibration in brushless doubly fed machine and brushless doubly fed reluctance machine

This study investigates modes of vibration in brushless doubly fed machine and brushless doubly fed reluctance machine due to the interaction of its fundamental magnetic fields, via the bending forces they set up in the back iron. It is shown that the presence of two field components of different pole numbers leads to vibration components in addition to those that would be expected in single field machines such as the induction motor. Formulations for the frequencies and magnitudes of the expected vibration components are given and verified experimentally. It is shown that the strength of the vibration components is highly dependent on the choice of pole numbers in the machine, with some much worse than equivalent induction machines and some very similar. The methodology presented enables designers to determine whether their machines are likely to suffer from vibration problems in advance of construction, and to apply remedies where appropriate. © The Institution of Engineering and Technology 2014.

A trapped field of 17.6 T in melt-processed, bulk Gd-Ba-Cu-O reinforced with shrink-fit steel

The ability of large-grain (RE)Ba2Cu3O7-δ ((RE)BCO; RE = rare earth) bulk superconductors to trap magnetic fields is determined by their critical current. With high trapped fields, however, bulk samples are subject to a relatively large Lorentz force, and their performance is limited primarily by their tensile strength. Consequently, sample reinforcement is the key to performance improvement in these technologically important materials. In this work, we report a trapped field of 17.6 T, the largest reported to date, in a stack of two silver-doped GdBCO superconducting bulk samples, each 25 mm in diameter, fabricated by top-seeded melt growth and reinforced with shrink-fit stainless steel. This sample preparation technique has the advantage of being relatively straightforward and inexpensive to implement, and offers the prospect of easy access to portable, high magnetic fields without any requirement for a sustaining current source. © 2014 IOP Publishing Ltd.

Temperature-dependent transport and spin accumulation in a quantum wire with Rashba spin-orbit interaction

We study the theory of temperature-dependent electron transport, spin polarization, and spin accumulation in a Rashba spin-orbit interaction (RSOI) quantum wire connected nonadiabatically to two normal conductor electrode leads. The influence of both the wire-lead connection and the RSOI on the electron transport is treated analytically by means of a scattering matrix technique and by using an effective free-electron approximation. Through analytical analysis and numerical examples, we demonstrate a simple way to design a sensitive spin-transfer switch that operates without applying any external magnetic fields or attaching ferromagnetic contacts. We also demonstrate that the antisymmetry of the spin accumulation can be destroyed slightly by the coupling between the leads and the wire. Moreover, temperature can weaken the polarization and smear out the oscillations in the spin accumulation.

Positive magnetoresistance in In0.53Ga0.47As/In0.52Al0.48As quantum well

Magnetotransport measurements have been carried out on In0.53Ga0.17As/In0.52Al0.48 As quantum wells in a temperature range between 1.5 and 77 K. We have observed a large positive magnetoresistance in the low magnetic field range, but saturating in high magnetic fields. The magnetoresistance results from two occupied subbands in the two-dimensional electron gas. With the intersubband scattering considered, we obtained the subband mobility by analyzing the positive magnetoresistance. It is found that the second subband mobility is larger than that of the first due to the existence of the intersubband scattering.

Thermodynamic properties of Cu-3(CO3)(2)(OH)(2) and the spin-1/2 distorted diamond Heisenberg chain

The thermodynamic properties of the spin-1/2 diamond quantum Heisenberg chain model have been investigated by means of the transfer matrix renormalization group (TMRG) method. Considering different crystal structures, by changing the interactions among different spins and the external magnetic fields, we first investigate the magnetic susceptibility, magnetization, and specific heat of the distorted diamond chain as a model of ferrimagnetic spin systems. The susceptibility and the specific heat show different features for different ferromagnetic (F) and antiferromagnetic (AF) interactions and different magnetic fields. A 1/3 magnetization plateau is observed at low temperature in a magnetization curve. Then, we discuss the theoretical mechanism of the double-peak structure of the magnetic susceptibility and the three-peak structure of the specific heat of the compound Cu-3(CO3)(2)(OH)(2), on which an elegant measurement was performed by Kikuchi [Phys. Rev. Lett. 94, 227201 (2005)]. Our computed results are consistent with the main characteristics of the experimental data. Meanwhile, we find that the double-peak structure of susceptibility can be found in several different kinds of spin interactions in the diamond chain. Moreover, a three-peak behavior is observed in the TMRG results of magnetic susceptibility. In addition, we perform calculations relevant for some experiments and explain the characteristics of these materials. (c) 2007 American Institute of Physics.

Properties of exchange interaction in Yb3Fe5O12 under extreme conditions

In Yb3Fe5O12, the exchange effective field can be expressed as H-eff = -lambda center dot center dot center dot M-Fe = -lambda chi(eff)center dot center dot center dot H-e = -gamma center dot center dot center dot H-e where gamma is named as the exchange field parameter and H-e is the external magnetic field. Then, in this paper, by the discussions on the characteristics of the exchange field parameter gamma, the properties of exchange interaction in ytterbium iron garnet (Yb3Fe5O12) are analyzed under extreme conditions (high magnetic fields and low temperatures). Our theory suggests that the exchange field parameter gamma is the function of the temperatures under different external magnetic fields, and gamma = a+b center dot center dot center dot T+c center dot center dot center dot T-2, where the coefficients a, b, c are associated with the external magnetic fields and the magnetized directions. Thus, the temperature-dependence, field-dependence and anisotropic characteristics of the exchange interaction in Yb3Fe5O12 are revealed. Also, excellent fits to the available experiments are obtained. (C) 2009 Elsevier B.V. All rights reserved.

Measurement of single electron spin with submicron Hall magnetometer

Submicron Hall magnetometry has been demonstrated as an efficient technique to probe extremely weak magnetic fields. In this letter, we analyze the possibility of employing it to detect single electron spin. Signal strength and readout time are estimated and discussed with respect to a number of practical issues. (C) 2005 American Institute of Physics.

Subband electron properties of InGaAs/InAlAs high-electron-mobility transistors with different channel chickness

Magnetotransport properties of In-0.53 GaAs/In-0.52 AlAs high electron mobility transistor (HEMT) structures with different channel thickness of 10-35 nm have been investigated in magnetic fields up to 13 T at 1.4 K. Fast Fourier transform has been employed to obtain the subband density and mobility of the two-dimensional electron gas in these HEMT structures. We found that the thickness of channel does not significantly enhance the electron density of the two-dimensional electron gas, however, it has strong effect on the proportion of electrons inhabited in different subbands. When the size of channel is 20 nm, the number of electrons occupying the excited subband, which have higher mobility, reaches the maximum. The experimental values obtained in this work are useful for the design and optimization of InGaAs/InAlAs HEMT devices.