Showing papers on "Field (physics) published in 2009"

Observation of a large-gap topological-insulator class with a single Dirac cone on the surface

Abstract: Recent experiments and theories have suggested that strong spin–orbit coupling effects in certain band insulators can give rise to a new phase of quantum matter, the so-called topological insulator, which can show macroscopic quantum-entanglement effects. Such systems feature two-dimensional surface states whose electrodynamic properties are described not by the conventional Maxwell equations but rather by an attached axion field, originally proposed to describe interacting quarks. It has been proposed that a topological insulator with a single Dirac cone interfaced with a superconductor can form the most elementary unit for performing fault-tolerant quantum computation. Here we present an angle-resolved photoemission spectroscopy study that reveals the first observation of such a topological state of matter featuring a single surface Dirac cone realized in the naturally occurring Bi_2Se_3 class of materials. Our results, supported by our theoretical calculations, demonstrate that undoped Bi_2Se_3 can serve as the parent matrix compound for the long-sought topological device where in-plane carrier transport would have a purely quantum topological origin. Our study further suggests that the undoped compound reached via n-to-p doping should show topological transport phenomena even at room temperature.

Magnetic metal-organic frameworks.

Abstract: The purpose of this critical review is to give a representative and comprehensive overview of the arising developments in the field of magnetic metal–organic frameworks, in particular those containing cobalt(II). We examine the diversity of magnetic exchange interactions between nearest-neighbour moment carriers, covering from dimers to oligomers and discuss their implications in infinite chains, layers and networks, having a variety of topologies. We progress to the different forms of short-range magnetic ordering, giving rise to single-molecule-magnets and single-chain-magnets, to long-range ordering of two- and three-dimensional networks (323 references).

Artificial Brownian motors: Controlling transport on the nanoscale

Abstract: In systems possessing spatial or dynamical symmetry breaking, Brownian motion combined with unbiased external input signals, deterministic and random alike, can assist directed motion of particles at submicron scales. In such cases, one speaks of ``Brownian motors.'' In this review the constructive role of Brownian motion is exemplified for various physical and technological setups, which are inspired by the cellular molecular machinery: the working principles and characteristics of stylized devices are discussed to show how fluctuations, either thermal or extrinsic, can be used to control diffusive particle transport. Recent experimental demonstrations of this concept are surveyed with particular attention to transport in artificial, i.e., nonbiological, nanopores, lithographic tracks, and optical traps, where single-particle currents were first measured. Much emphasis is given to two- and three-dimensional devices containing many interacting particles of one or more species; for this class of artificial motors, noise rectification results also from the interplay of particle Brownian motion and geometric constraints. Recently, selective control and optimization of the transport of interacting colloidal particles and magnetic vortices have been successfully achieved, thus leading to the new generation of microfluidic and superconducting devices presented here. The field has recently been enriched with impressive experimental achievements in building artificial Brownian motor devices that even operate within the quantum domain by harvesting quantum Brownian motion. Sundry akin topics include activities aimed at noise-assisted shuttling other degrees of freedom such as charge, spin, or even heat and the assembly of chemical synthetic molecular motors. This review ends with a perspective for future pathways and potential new applications.

Optical tomography: forward and inverse problems

Abstract: This is a review of recent mathematical and computational advances in optical tomography. We discuss the physical foundations of forward models for light propagation on microscopic, mesoscopic and macroscopic scales. We also consider direct and numerical approaches to the inverse problems that arise at each of these scales. Finally, we outline future directions and open problems in the field.

Editorial: Mapping the Field of Mixed Methods Research

Abstract: The intent of this editorial is to advance my list of topics currently being discussed in the field of mixed methods research. As one who has been involved in the mixed methods field since its beginning 20 years ago, I have some sense of the topics that have evolved and that hold center stage in mixed methods discussions. Furthermore, as one of the founding coeditors of the Journal of Mixed Methods Research (JMMR; and outgoing editor after June 2009), I have been privileged to have examined close to 300 manuscripts submitted to the journal in the past 3 years, and I have taken notes on what I have seen as potential contributions of these manuscripts to the field of mixed methods research. My discussion will first address why we, as researchers, need a map of the field at this current time. Then I will advance a list of 30 topics that are being discussed today in the mixed methods literature. To crosscheck the accuracy of my list, I will reflect on the topics that were discussed last summer at the 2008 Mixed Methods Conference at Cambridge University, UK. I will note differences and similarities between the papers presented at the conference and my list. Then I will select four topics from my list, discuss the development of the topics, and note recent insightful contributions that have emerged in the literature. Finally, I will end with some thoughts about the future of mixed methods research. I hope that by reading this editorial you will learn how one individual constructs the field of mixed methods today, obtain a glimpse into what topics current writers presented at the Mixed Methods Conference last July, and assess how your mixed methods manuscript might make a contribution to the field.

Electrical impedance tomography and Calderón's problem

Abstract: We survey mathematical developments in the inverse method of electrical impedance tomography which consists in determining the electrical properties of a medium by making voltage and current measurements at the boundary of the medium. In the mathematical literature, this is also known as Calderon's problem from Calderon's pioneer contribution (Calderon 1980 Seminar on Numerical Analysis and its Applications to Continuum Physics (Rio de Janeiro, 1980) p 65 (Soc. Brasil. Mat.)). We concentrate this review around the topic of complex geometrical optics solutions that have led to many advances in the field. In the last section, we review some counterexamples to Calderon's problems that have attracted a lot of interest because of connections with cloaking and invisibility.

Relativistic theory of tidal Love numbers

Abstract: In Newtonian gravitational theory, a tidal Love number relates the mass multipole moment created by tidal forces on a spherical body to the applied tidal field. The Love number is dimensionless, and it encodes information about the body's internal structure. We present a relativistic theory of Love numbers, which applies to compact bodies with strong internal gravities; the theory extends and completes a recent work by Flanagan and Hinderer, which revealed that the tidal Love number of a neutron star can be measured by Earth-based gravitational-wave detectors. We consider a spherical body deformed by an external tidal field, and provide precise and meaningful definitions for electric-type and magnetic-type Love numbers; and these are computed for polytropic equations of state. The theory applies to black holes as well, and we find that the relativistic Love numbers of a nonrotating black hole are all zero.

Influence of molecular parameters and increasing magnetic field strength on relaxivity of gadolinium- and manganese-based T1 contrast agents.

Abstract: Simulations were performed to understand the relative contributions of molecular parameters to longitudinal (r(1)) and transverse (r(2)) relaxivity as a function of applied field, and to obtain theoretical relaxivity maxima over a range of fields to appreciate what relaxivities can be achieved experimentally. The field-dependent relaxivities of a panel of gadolinium and manganese complexes with different molecular parameters, water exchange rates, rotational correlation times, hydration state, etc. were measured to confirm that measured relaxivities were consistent with theory. The design tenets previously stressed for optimizing r(1) at low fields (very slow rotational motion; chelate immobilized by protein binding; optimized water exchange rate) do not apply at higher fields. At 1.5 T and higher fields, an intermediate rotational correlation time is desired (0.5-4 ns), while water exchange rate is not as critical to achieving a high r(1). For targeted applications it is recommended to tether a multimer of metal chelates to a protein-targeting group via a long flexible linker to decouple the slow motion of the protein from the water(s) bound to the metal ions. Per ion relaxivities of 80, 45, and 18 mM(-1) s(-1) at 1.5, 3 and 9.4 T, respectively, are feasible for Gd(3+) and Mn(2+) complexes.

Characterizing the swimming properties of artificial bacterial flagella.

Abstract: Artificial bacterial flagella (ABFs) consist of helical tails resembling natural flagella fabricated by the self-scrolling of helical nanobelts and soft-magnetic heads composed of Cr/Ni/Au stacked thin films. ABFs are controlled wirelessly using a low-strength rotating magnetic field. Self-propelled devices such as these are of interest for in vitro and in vivo biomedical applications. Swimming tests of ABFs show a linear relationship between the frequency of the applied field and the translational velocity when the frequency is lower than the step-out frequency of the ABF. Moreover, the influences of head size on swimming velocity and the lateral drift of an ABF near a solid boundary are investigated. An experimental method to estimate the propulsion matrix of a helical swimmer under a light microscope is developed. Finally, swarm-like behavior of multiple ABFs controlled as a single entity is demonstrated.

Radiative magnetohydrodynamic simulation of sunspot structure

Abstract: Results of a three-dimensional MHD simulation of a sunspot with a photospheric size of about 20 Mm are presented. The simulation has been carried out with the MURaM code, which includes a realistic equation of state with partial ionization and radiative transfer along many ray directions. The largely relaxed state of the sunspot shows a division in a central dark umbral region with bright dots and a penumbra showing bright filaments of about 2-3 Mm length with central dark lanes. By a process similar to the formation of umbral dots, the penumbral filaments result from magnetoconvection in the form of upflow plumes, which become elongated by the presence of an inclined magnetic field; the upflow is deflected in the outward direction while the magnetic field is weakened and becomes almost horizontal in the upper part of the plume near the level of optical depth unity. A dark lane forms owing to the piling up of matter near the cusp-shaped top of the rising plume that leads to an upward bulging of the surfaces of constant optical depth. The simulated penumbral structure corresponds well to the observationally inferred interlocking-comb structure of the magnetic field with Evershed outflows along dark-laned filaments with nearly horizontal magnetic field and overturning perpendicular ("twisting") motion, which are embedded in a background of stronger and less inclined field. Photospheric spectral lines are formed at the very top and somewhat above the upflow plumes, so that they do not fully sense the strong flow as well as the large field inclination and significant field strength reduction in the upper part of the plume structures.

Gigahertz Dynamics of a Strongly Driven Single Quantum Spin

Abstract: Two-level systems are at the core of numerous real-world technologies such as magnetic resonance imaging and atomic clocks. Coherent control of the state is achieved with an oscillating field that drives dynamics at a rate determined by its amplitude. As the strength of the field is increased, a different regime emerges where linear scaling of the manipulation rate breaks down and complex dynamics are expected. By calibrating the spin rotation with an adiabatic passage, we have measured the room-temperature "strong-driving" dynamics of a single nitrogen vacancy center in diamond. With an adiabatic passage to calibrate the spin rotation, we observed dynamics on sub-nanosecond time scales. Contrary to conventional thinking, this breakdown of the rotating wave approximation provides opportunities for time-optimal quantum control of a single spin.

Energy flux determines magnetic field strength of planets and stars

Abstract: The magnetic fields of Earth and Jupiter, along with those of rapidly rotating, low-mass stars, are generated by convection-driven dynamos that may operate similarly (the slowly rotating Sun generates its field through a different dynamo mechanism). The field strengths of planets and stars vary over three orders of magnitude, but the critical factor causing that variation has hitherto been unclear. Here we report an extension of a scaling law derived from geodynamo models to rapidly rotating stars that have strong density stratification. The unifying principle in the scaling law is that the energy flux available for generating the magnetic field sets the field strength. Our scaling law fits the observed field strengths of Earth, Jupiter, young contracting stars and rapidly rotating low-mass stars, despite vast differences in the physical conditions of the objects. We predict that the field strengths of rapidly rotating brown dwarfs and massive extrasolar planets are high enough to make them observable.

Magnetic fields from inflation

Abstract: We consider the possibility of generation of the primordial magnetic field on inflation and show that the effect of the back reaction of this field can be very important. Assuming that the back rea ...

Axisymmetric magnetic fields in stars: relative strengths of poloidal and toroidal components

Abstract: In this third paper in a series on stable magnetic equilibria in stars, I look at the stability of axisymmetric field configurations and, in particular, the relative strengths of the toroidal and poloidal components. Both toroidal and poloidal fields are unstable on their own, and stability is achieved by adding the two together in some ratio. I use Tayler's stability conditions for toroidal fields and other analytic tools to predict the range of stable ratios and then check these predictions by running numerical simulations. If the energy in the poloidal component as a fraction of the total magnetic energy is written as E p /E, it is found that the stability condition is a(E/U) < Ep/E ≤ 0.8 where E/U is the ratio of magnetic to gravitational energy in the star and a is some dimensionless factor whose value is of order 10 in a main-sequence star and of order 10 3 in a neutron star. In other words, whilst the poloidal component cannot be significantly stronger than the toroidal, the toroidal field can be very much stronger than the poloidal-given that in realistic stars we expect E/U < 10- 6 . The implications of this result are discussed in various contexts such as the emission of gravitational waves by neutron stars, free precession and a 'hidden' energy source for magnetars.

Magneto-thermal evolution of neutron stars

Abstract: Context. The presence of magnetic fields in the crust of neutron stars c auses a non-spherically symmetric temperature distribution. The strong temperature dependence of the magnetic diffusivity and thermal conductivity, together with the heat generated by magnetic dissipation, couple the magnetic and thermal evolution of NSs, that cannot be formulated as separated one‐dimensional problems. Aims. We study the mutual influence of thermal and magnetic evoluti on in a neutron star’s crust in axial symmetry. Taking into account realistic microphysical inputs, we find the heat rel eased by Joule effect consistent with the circulation of currents in the crust , and we incorporate its effects in 2‐dimensional cooling calculations. Methods. We solve the induction equation numerically using a hybrid method (spectral in angles, but a finite‐di fferences scheme in the radial direction), coupled to the thermal diffusion equation. To improve the boundary conditions, we also revisit the envelope stationary solutions updating the well known Tb− Ts‐relations to include the effect of 2‐D heat transfer calculations and new microphysical inputs. Results. We present the first long term 2‐dimensional simulations of t he coupled magneto-thermal evolution of neutron stars. This substantially improves previous works in which a very crude approximation in at least one of the parts (thermal or magnetic diffusion) has been adopted. Our results show that the feedback between Joule heating and magnetic diffusion is strong, resulting in a faster dissipation of the stronger fields during the first 10 5 − 10 6 years of a NS’s life. As a consequence, all neutron stars born with fields larger than a critical value (> 5×10 13 G) reach similar field strengths (≈ 2−3×10 13 G) at late times. Irrespectively of the initial magnetic field strength, after 10 6 years the temperature becomes so low that the magnetic diffusion timescale becomes longer than the typical ages of radio‐pulsars, thus resulting in apparently no diss ipation of the field in old NS. We also confirm the strong correl ation between the magnetic field and the surface temperature of relatively young NSs discussed in preliminary works. The effective temperature of models with strong internal toroidal components are systematically higher than those of models with purely poloidal fie lds, due to the additional energy reservoir stored in the toroidal field tha t is gradually released as the field dissipates.

Quasinormal modes of black holes and black branes

Abstract: Quasinormal modes are eigenmodes of dissipative systems. Perturbations of classical gravitational backgrounds involving black holes or branes naturally lead to quasinormal modes. The analysis and classification of the quasinormal spectra requires solving non-Hermitian eigenvalue problems for the associated linear differential equations. Within the recently developed gauge-gravity duality, these modes serve as an important tool for determining the near-equilibrium properties of strongly coupled quantum field theories, in particular their transport coefficients, such as viscosity, conductivity and diffusion constants. In astrophysics, the detection of quasinormal modes in gravitational wave experiments would allow precise measurements of the mass and spin of black holes as well as new tests of general relativity. This review is meant as an introduction to the subject, with a focus on the recent developments in the field.

The gravity duals of N=2 superconformal field theories

Abstract: We study the gauge/gravity duality for theories with four dimensional ${\cal N}=2$ supersymmetries. We consider the large class of generalized quiver field theories constructed recently by one of us (D.G.). These field theories can also be viewed as the IR limit of M5 branes wrapping a Riemann surface with punctures. We give a prescription for constructing the corresponding geometries and we discuss a few special cases in detail. There is a precise match for various quantities between the field theory and the M-theory description.

Fundamental processes of quantum electrodynamics in laser fields of relativistic power

Abstract: In this review we summarize our progress in the investigation of fundamental processes of quantum electrodynamics in laser fields of relativistic power in view of the more recent experimental progress in the generation of laser field intensities, yielding ponderomotive energy shifts Up of the order of magnitude mc2 and beyond. In particular, the generation of electron–positron pairs during the collision of laser pulses with ions or protons appears to become feasible.

Design and experimental realization of a broadband transformation media field rotator at microwave frequencies.

Abstract: We designed a metamaterial field rotator that can rotate electromagnetic wave fronts. Our starting point was the transformation-media concept. Effective medium theories and full simulations facilitated the actual design process. We created at a very simple structure comprising of an array of identical aluminum metal plates. We made and measured a sample and we experimentally demonstrated the field rotation effect as well as the broadband functionality at microwave frequencies.

Quantum Error Correction for Beginners

Abstract: Quantum error correction (QEC) and fault-tolerant quantum computation represent one of the most vital theoretical aspect of quantum information processing. It was well known from the early developments of this exciting field that the fragility of coherent quantum systems would be a catastrophic obstacle to the development of large scale quantum computers. The introduction of quantum error correction in 1995 showed that active techniques could be employed to mitigate this fatal problem. However, quantum error correction and fault-tolerant computation is now a much larger field and many new codes, techniques, and methodologies have been developed to implement error correction for large scale quantum algorithms. In response, we have attempted to summarize the basic aspects of quantum error correction and fault-tolerance, not as a detailed guide, but rather as a basic introduction. This development in this area has been so pronounced that many in the field of quantum information, specifically researchers who are new to quantum information or people focused on the many other important issues in quantum computation, have found it difficult to keep up with the general formalisms and methodologies employed in this area. Rather than introducing these concepts from a rigorous mathematical and computer science framework, we instead examine error correction and fault-tolerance largely through detailed examples, which are more relevant to experimentalists today and in the near future.

The dusty heart of nearby active galaxies -- II. From clumpy torus models to physical properties of dust around AGN

Abstract: The dusty environments (= "dust tori'') of AGN are now in reach of observations. Following our paper I on ground-based mid-IR spectro-photometry (H\"onig et al. 2010), we present an upgrade to our radiative transfer model of 3-dimensional clumpy dust tori. The upgrade with respect to H\"onig et al. (2006) concerns an improved handling of the diffuse radiation field in the torus which is approximated by a statistical approach. The models are presented as tools to translate classical and interferometric observations into characteristic properties of the dust distribution. We compare model SEDs for different chemical and grain-size compositions of the dust and find that clouds with standard ISM dust and optical depth tau_V~50 appear in overall agreement with observed IR SEDs. By studying parameter dependencies, it is shown that type 1 AGN SEDs, in particular the mid-IR spectral index, can be used to constrain the radial dust cloud distribution power-law index 'a', while other parameters are more difficult to assess using SEDs only. Interferometry adds important additional information for modeling when interpreted simultaneously with the SED. Although type 2 AGN can, in principle, be used to constrain model parameters as well, obscuration effects make the analysis more ambiguous. We propose a simple, interferometry-based method to distinguish between "compact'' and "extended'' radial dust distributions without detailed modeling of the data and introduce a way to easily determine individual or sample average model parameters using the observed optical depth in the silicate feature and the mid-IR spectral index.

Non-Gaussian Statistics and Extreme Waves in a Nonlinear Optical Cavity

Abstract: A unidirectional optical oscillator is built by using a liquid crystal light valve that couples a pump beam with the modes of a nearly spherical cavity. For sufficiently high pump intensity, the cavity field presents complex spatiotemporal dynamics, accompanied by the emission of extreme waves and large deviations from the Gaussian statistics. We identify a mechanism of spatial symmetry breaking, due to a hypercycle-type amplification through the nonlocal coupling of the cavity field.

Probing the Magnetic Field of Light at Optical Frequencies

Abstract: Light is an electromagnetic wave composed of oscillating electric and magnetic fields, the one never occurring without the other. In light-matter interactions at optical frequencies, the magnetic component of light generally plays a negligible role. When we "see" or detect light, only its electric field is perceived; we are practically blind to its magnetic component. We used concepts from the field of metamaterials to probe the magnetic field of light with an engineered near-field aperture probe. We visualized with subwavelength resolution the magnetic- and electric-field distribution of propagating light.

Quantum Computing With an Electron Spin Ensemble

Abstract: We propose to encode a register of quantum bits in different collective electron spin wave excitations in a solid medium. Coupling to spins is enabled by locating them in the vicinity of a superconducting transmission line cavity, and making use of their strong collective coupling to the quantized radiation field. The transformation between different spin waves is achieved by applying gradient magnetic fields across the sample, while a Cooper pair box, resonant with the cavity field, may be used to carry out one- and two-qubit gate operations.

Modeling of Gamma-Ray Pulsar Light Curves with Force-Free Magnetic Field

Abstract: (Abridged) Gamma-ray emission from pulsars has long been modeled using a vacuum dipole field. This approximation ignores changes in the field structure caused by the magnetospheric plasma and strong plasma currents. We present the first results of gamma-ray pulsar light curve modeling using the more realistic field taken from 3D force-free magnetospheric simulations. Having the geometry of the field, we apply several prescriptions for the location of the emission zone, comparing the light curves to observations. We find that the conventional two-pole caustic model fails to produce double-peak pulse profiles, mainly because the size of the polar cap in force-free magnetosphere is larger than the vacuum field polar cap. The conventional outer-gap model is capable of producing only one peak under general conditions, because a large fraction of open field lines does not cross the null charge surface. We propose a novel "separatrix layer" model, where the high-energy emission originates from a thin layer on the open field lines just inside of the separatrix that bounds the open flux tube. The emission from this layer generates two strong caustics on the sky map due to the effect we term "Sky Map Stagnation" (SMS). It is related to the fact that force-free field asymptotically approaches the field of a rotating split monopole, and the photons emitted on such field lines in the outer magnetosphere arrive to the observer in phase. The double-peak light curve is a natural consequence of SMS. We show that most features of the currently available gamma-ray pulsar light curves can be reasonably well reproduced and explained with the sepatratrix model using the force-free field. Association of the emission region with the current sheet will guide more detailed future studies of the magnetospheric acceleration physics.

Field-free orientation of CO molecules by femtosecond two-color laser fields.

Abstract: We report the first experimental observation of nonadiabatic field-free orientation of a heteronuclear diatomic molecule (CO) induced by an intense two-color (800 and 400 nm) femtosecond laser field. We monitor orientation by measuring fragment ion angular distributions after Coulomb explosion with an 800 nm pulse. The orientation of the molecules is controlled by the relative phase of the two-color field. The results are compared to quantum mechanical rigid rotor calculations. The demonstrated method can be applied to study molecular frame dynamics under field-free conditions in conjunction with a variety of spectroscopy methods, such as high-harmonic generation, electron diffraction, and molecular frame photoelectron emission.

Nonlinear study of bell's cosmic ray current-driven instability

Abstract: The cosmic ray current-driven (CRCD) instability, predicted by Bell, consists of nonresonant, growing plasma waves driven by the electric current of cosmic rays (CRs) that stream along the magnetic field ahead of both relativistic and nonrelativistic shocks. Combining an analytic, kinetic model with one-, two-, and three-dimensional particle-in-cell simulations, we confirm the existence of this instability in the kinetic regime and determine its saturation mechanisms. In the linear regime, we show that, if the background plasma is well magnetized, the CRCD waves grow exponentially at the rates and wavelengths predicted by the analytic dispersion relation. The magnetization condition implies that the growth rate of the instability is much smaller than the ion cyclotron frequency. As the instability becomes nonlinear, significant turbulence forms in the plasma. This turbulence reduces the growth rate of the field and damps the shortest wavelength modes, making the dominant wavelength, λ d , grow proportional to the square of the field. At constant CR current, we find that plasma acceleration along the motion of CRs saturates the instability at the magnetic field level such that vA ~ v d,cr, where vA is the Alfven velocity in the amplified field, and v d,cr is the drift velocity of CRs. The instability can also saturate earlier if CRs get strongly deflected by the amplified field, which happens when their Larmor radii get close to λ d . We apply these results to the case of CRs propagating in the upstream medium of the forward shock in supernova remnants. If we consider only the most energetic CRs that escape from the shock, we obtain that the field amplification factor of ~10 can be reached. This confirms the CRCD instability as a potentially important component of magnetic amplification process in astrophysical shock environments.