Showing papers in "Reports on Progress in Physics in 2009"

Fundamentals of zinc oxide as a semiconductor

Abstract: In the past ten years we have witnessed a revival of, and subsequent rapid expansion in, the research on zinc oxide (ZnO) as a semiconductor. Being initially considered as a substrate for GaN and related alloys, the availability of high-quality large bulk single crystals, the strong luminescence demonstrated in optically pumped lasers and the prospects of gaining control over its electrical conductivity have led a large number of groups to turn their research for electronic and photonic devices to ZnO in its own right. The high electron mobility, high thermal conductivity, wide and direct band gap and large exciton binding energy make ZnO suitable for a wide range of devices, including transparent thin-film transistors, photodetectors, light-emitting diodes and laser diodes that operate in the blue and ultraviolet region of the spectrum. In spite of the recent rapid developments, controlling the electrical conductivity of ZnO has remained a major challenge. While a number of research groups have reported achieving p-type ZnO, there are still problems concerning the reproducibility of the results and the stability of the p-type conductivity. Even the cause of the commonly observed unintentional n-type conductivity in as-grown ZnO is still under debate. One approach to address these issues consists of growing high-quality single crystalline bulk and thin films in which the concentrations of impurities and intrinsic defects are controlled. In this review we discuss the status of ZnO as a semiconductor. We first discuss the growth of bulk and epitaxial films, growth conditions and their influence on the incorporation of native defects and impurities. We then present the theory of doping and native defects in ZnO based on density-functional calculations, discussing the stability and electronic structure of native point defects and impurities and their influence on the electrical conductivity and optical properties of ZnO. We pay special attention to the possible causes of the unintentional n-type conductivity, emphasize the role of impurities, critically review the current status of p-type doping and address possible routes to controlling the electrical conductivity in ZnO. Finally, we discuss band-gap engineering using MgZnO and CdZnO alloys.

The hydrodynamics of swimming microorganisms

Abstract: Cell motility in viscous fluids is ubiquitous and affects many biological processes, including reproduction, infection and the marine life ecosystem. Here we review the biophysical and mechanical principles of locomotion at the small scales relevant to cell swimming, tens of micrometers and below. At this scale, inertia is unimportant and the Reynolds number is small. Our emphasis is on the simple physical picture and fundamental flow physics phenomena in this regime. We first give a brief overview of the mechanisms for swimming motility, and of the basic properties of flows at low Reynolds number, paying special attention to aspects most relevant for swimming such as resistance matrices for solid bodies, flow singularities and kinematic requirements for net translation. Then we review classical theoretical work on cell motility, in particular early calculations of swimming kinematics with prescribed stroke and the application of resistive force theory and slender-body theory to flagellar locomotion. After examining the physical means by which flagella are actuated, we outline areas of active research, including hydrodynamic interactions, biological locomotion in complex fluids, the design of small-scale artificial swimmers and the optimization of locomotion strategies. (Some figures in this article are in colour only in the electronic version) This article was invited by Christoph Schmidt.

The physics of dipolar bosonic quantum gases

Abstract: This paper reviews the recent theoretical and experimental advances in the study of ultra-cold gases made of bosonic particles interacting via the long-range, anisotropic dipole–dipole interaction, in addition to the short-range and isotropic contact interaction usually at work in ultra-cold gases. The specific properties emerging from the dipolar interaction are emphasized, from the mean-field regime valid for dilute Bose–Einstein condensates, to the strongly correlated regimes reached for dipolar bosons in optical lattices. (Some figures in this article are in colour only in the electronic version)

X-ray diffraction of III-nitrides

Abstract: The III-nitrides include the semiconductors AlN, GaN and InN, which have band gaps spanning the entire UV and visible ranges. Thin films of III-nitrides are used to make UV, violet, blue and green light-emitting diodes and lasers, as well as solar cells, high-electron mobility transistors (HEMTs) and other devices. However, the film growth process gives rise to unusually high strain and high defect densities, which can affect the device performance. X-ray diffraction is a popular, non-destructive technique used to characterize films and device structures, allowing improvements in device efficiencies to be made. It provides information on crystalline lattice parameters (from which strain and composition are determined), misorientation (from which defect types and densities may be deduced), crystallite size and microstrain, wafer bowing, residual stress, alloy ordering, phase separation (if present) along with film thicknesses and superlattice (quantum well) thicknesses, compositions and non-uniformities. These topics are reviewed, along with the basic principles of x-ray diffraction of thin films and areas of special current interest, such as analysis of non-polar, semipolar and cubic III-nitrides. A summary of useful values needed in calculations, including elastic constants and lattice parameters, is also given. Such topics are also likely to be relevant to other highly lattice-mismatched wurtzite-structure materials such as heteroepitaxial ZnO and ZnSe.

Electron energy-loss spectroscopy in the TEM

Abstract: Electron energy-loss spectroscopy (EELS) is an analytical technique that measures the change in kinetic energy of electrons after they have interacted with a specimen. When carried out in a modern transmission electron microscope, EELS is capable of giving structural and chemical information about a solid, with a spatial resolution down to the atomic level in favourable cases. The energy resolution is typically 1 eV but can approach 0.1 eV if an electron-beam monochromator is used. This review provides an overview of EELS instrumentation and of the physics involved in the scattering of kilovolt electrons in solids. Features of the energy-loss spectrum are discussed, including plasmon peaks, inner-shell ionization edges and fine structure related to the electronic densities of states. Examples are given of the use of EELS for the measurement of local properties, including specimen thickness, mechanical and electronic properties (such as bandgap) and chemical composition. Factors that determine the spatial resolution of the analysis are outlined, including radiation damage to the specimen. Comparisons are made with related techniques, particularly x-ray absorption spectroscopy.

Fundamental questions relating to ion conduction in disordered solids

Abstract: A number of basic scientific questions relating to ion conduction in homogeneously disordered solids are discussed. The questions deal with how to define the mobile ion density, what can be learnt from electrode effects, what the ion transport mechanism is, the role of dimensionality and what the origins of the mixed-alkali effect, the time-temperature superposition, and the nearly constant loss are. Answers are suggested to some of these questions, but the main purpose of the paper is to draw attention to the fact that this field of research still presents several fundamental challenges.

Approaches to Understanding Cosmic Acceleration

Abstract: Theoretical approaches to explaining the observed acceleration of the universe are reviewed. We briefly discuss the evidence for cosmic acceleration, and the implications for standard general relativity coupled to conventional sources of energy–momentum. We then address three broad methods of addressing an accelerating universe: the introduction of a cosmological constant, its problems and origins; the possibility of dark energy and the associated challenges for fundamental physics and the option that an infrared modification of general relativity may be responsible for the large-scale behavior of the universe.

Nearly perfect fluidity: from cold atomic gases to hot quark gluon plasmas

Abstract: Shear viscosity is a measure of the amount of dissipation in a simple fluid. In kinetic theory shear viscosity is related to the rate of momentum transport by quasi-particles, and the uncertainty relation suggests that the ratio of shear viscosity ? to entropy density s in units of /kB is bounded by a constant. Here, is Planck's constant and kB is Boltzmann's constant. A specific bound has been proposed on the basis of string theory where, for a large class of theories, one can show that ?/s ? /(4?kB). We will refer to a fluid that saturates the string theory bound as a perfect fluid. In this review we summarize theoretical and experimental information on the properties of the three main classes of quantum fluids that are known to have values of ?/s that are smaller than /kB. These fluids are strongly coupled Bose fluids, in particular liquid helium, strongly correlated ultracold Fermi gases and the quark gluon plasma. We discuss the main theoretical approaches to transport properties of these fluids: kinetic theory, numerical simulations based on linear response theory and holographic dualities. We also summarize the experimental situation, in particular with regard to the observation of hydrodynamic behavior in ultracold Fermi gases and the quark gluon plasma.

Fröhlich polaron and bipolaron: recent developments

Abstract: It is remarkable how the Frohlich polaron, one of the simplest examples of a Quantum Field Theoretical problem, as it basically consists of a single fermion interacting with a scalar Bose field of ion displacements, has resisted full analytical or numerical solution at all coupling since ~1950, when its Hamiltonian was first written. The field has been a testing ground for analytical, semi-analytical and numerical techniques, such as path integrals, strong-coupling perturbation expansion, advanced variational, exact diagonalization (ED) and quantum Monte Carlo (QMC) techniques. This paper reviews recent developments in the field of continuum and discrete (lattice) Frohlich (bi)polarons starting with the basics and covering a number of active directions of research.

Physics at a future Neutrino Factory and super-beam facility

Abstract: The conclusions of the Physics Working Group of the International Scoping Study of a future Neutrino Factory and super-beam facility (the ISS) are presented. The ISS was carried out by the international community between NuFact05, (the 7th International Workshop on Neutrino Factories and Super-beams, Laboratori Nazionali di Frascati, Rome, 21–26 June 2005) and NuFact06 (Ivine, CA, 24–30 August 2006). The physics case for an extensive experimental programme to understand the properties of the neutrino is presented and the role of high-precision measurements of neutrino oscillations within this programme is discussed in detail. The performance of second-generation super-beam experiments, beta-beam facilities and the Neutrino Factory are evaluated and a quantitative comparison of the discovery potential of the three classes of facility is presented. High-precision studies of the properties of the muon are complementary to the study of neutrino oscillations. The Neutrino Factory has the potential to provide extremely intense muon beams and the physics potential of such beams is discussed in the final section of the report.

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.

Warm Inflation and its Microphysical Basis

Abstract: The microscopic quantum field theory origins of warm inflation dynamics are reviewed. The warm inflation scenario is first described along with its results, predictions and comparison with the standard cold inflation scenario. The basics of thermal field theory required in the study of warm inflation are discussed. Quantum field theory real time calculations at finite temperature are then presented and the derivation of dissipation and stochastic fluctuations are shown from a general perspective. Specific results are given of dissipation coefficients for a variety of quantum field theory interaction structures relevant to warm inflation, in a form that can be readily used by model builders. Different particle physics models realizing warm inflation are presented along with their observational predictions.

Biophysical models of tumour growth

Abstract: Tumour growth is a multifactorial process, which has stimulated in recent decades the development of numerous models trying to figure out the mechanisms controlling solid tumours morphogenesis. While the earliest models were focusing on cell proliferation kinetics, modulated by the availability of supplied nutrients, new modelling approaches emphasize the crucial role of several biophysical processes, including local matrix remodelling, active cell migration and traction, and reshaping of host tissue vasculature. After a brief presentation of this experimental background, this review will outline a number of representative models describing, at different scales, the growth of avascular and vascularized tumours. Special attention will be paid to the formulation of tumour–host tissue interactions that selectively drive changes in tumour size and morphology, and which are notably mediated by the mechanical status and elasticity of the tumour microenvironment. Emergence of invasive behaviour through growth instabilities at the tumour–host interface will be presented considering both reaction–diffusion and mechano-cellular models. In the latter part of the review, patient-oriented implications of tumour growth modelling are outlined in the context of brain tumours. Some conceptual views of the adaptive strategies and selective barriers that govern tumour evolution are presented in conclusion as potential guidelines for the development of future models.

Advanced quantum dot configurations

Abstract: We present an overview on approaches currently employed to fabricate advanced quantum dot configurations by epitaxial growth. Widely investigated self-assembled quantum dots, i.e. In(Ga)As/GaAs and (Si)Ge/Si, are first introduced. Different quantum dot structures can be derived from In(Ga)As quantum dots by combining them with in situ etching, by layer stacking or by using them as stressors. Other fabrication methods include droplet epitaxy and multilayer deposition on hole patterned substrates.The combination of bottom–up and top–down methods results in absolute position control of self-assembled quantum dots. We review these 'seeded quantum dot crystals' in detail. Finally, we discuss a promising approach to realize quantum dot crystals with controlled spatial and optical properties.

Hybrid atomistic simulation methods for materials systems

Abstract: We review recent progress in the methodology of hybrid quantum/classical (QM/MM) atomistic simulations for solid-state systems, from the earliest reports in 1993 up to the latest results. A unified terminology is defined into which the various and disparate schemes fit, based on whether the information from the QM and MM calculations is combined at the level of energies or forces. We discuss the pertinent issues for achieving 'seamless' coupling, the advantages and disadvantages of the proposed schemes and summarize the applications and scientific results that have been obtained to date.

The formation and interactions of cold and ultracold molecules: new challenges for interdisciplinary physics

Abstract: Progress on research in the field of molecules at cold and ultracold temperatures is reported in this review. It covers extensively the experimental methods to produce, detect and characterize cold and ultracold molecules including association of ultracold atoms, deceleration by external fields and kinematic cooling. Confinement of molecules in different kinds of traps is also discussed. The basic theoretical issues related to the knowledge of the molecular structure, the atom–molecule and molecule–molecule mutual interactions, and to their possible manipulation and control with external fields, are reviewed. A short discussion on the broad area of applications completes the review.

LUNA: a laboratory for underground nuclear astrophysics

Abstract: It is in the nature of astrophysics that many of the processes and objects one tries to understand are physically inaccessible. Thus, it is important that those aspects that can be studied in the laboratory are rather well understood. One such aspect is the nuclear fusion reactions, which are at the heart of nuclear astrophysics. They sensitively influence the nucleosynthesis of the elements in the earliest stages of the universe and in all the objects formed thereafter, and control the associated energy generation, neutrino luminosity and evolution of stars. We review a new experimental approach for the study of nuclear fusion reactions based on an underground accelerator laboratory, named LUNA.

How far are we from the quantum theory of gravity

Abstract: I give a pedagogical explanation of what it is about quantization that makes general relativity go from being a nearly perfect classical theory to a very problematic quantum one. I also explain why some quantization of gravity is unavoidable, why quantum field theories have divergences, why the divergences of quantum general relativity are worse than those of the other forces, what physicists think this means and what they might do with a consistent theory of quantum gravity if they had one. Finally, I discuss the quantum gravitational data that have recently become available from cosmology.

The physics of light transmission through subwavelength apertures and aperture arrays

Abstract: The passage of light through apertures much smaller than the wavelength of the light has proved to be a surprisingly subtle phenomenon. This report describes how modern developments in nanofabrication, coherent light sources and numerical vector field simulations have led to the upending of early predictions from scalar diffraction theory and classical electrodynamics. Optical response of real materials to incident coherent radiation at petahertz frequencies leads to unexpected consequences for transmission and extinction of light through subwavelength aperture arrays. This paper is a report on progress in our understanding of this phenomenon over the past decade.

Muonium as a model for interstitial hydrogen in the semiconducting and semimetallic elements

Abstract: Although the interstitial hydrogen atom would seem to be one of the simplest defect centres in any lattice, its solid state chemistry is in fact unknown in many materials, not least amongst the elements. In semiconductors, the realization that hydrogen can profoundly influence electronic properties even as a trace impurity has prompted its study by all available means-but still only in the functionally important or potentially important materials-for the elements, Si, Ge and diamond. Even here, it was not studies of hydrogen itself but of its pseudo-isotope, muonium, that first provided the much needed microscopic pictures of crystallographic site and local electronic structure-now comprehensively confirmed by ab initio computation and such data as exists for monatomic, interstitial hydrogen centres in Si. Muonium can be formed in a variety of neutral paramagnetic states when positive muons are implanted into non-metals. The simple trapped atom is commonly only metastable. It coexists with or reacts to give defect centres with the unpaired electron in somewhat more extended orbitals. Indications of complete delocalization into effective mass states are discussed for B, α-Sn, Bi and even Ge, but otherwise all the muonium centres seen in the elemental semiconductors are deep and relatively compact. These are revealed, distinguished and characterized by μSR spectroscopy-muon spin rotation and resonance informing on sites and spin-density distributions, muon spin relaxation on motional dynamics and charge-state transitions. This Report documents the progress of μSR studies for all the semiconductors and semimetals of the p-block elements, Groups III-VI of the Periodic Table. The striking spectra and originally unanticipated results for Group IV are for the most part well known but deserve summarizing and updating; the sheer diversity of muonium states found is still remarkable, especially in carbon allotropes. The interplay of crystallographic site and charge state in Si and Ge at high temperatures, or under illumination, reflects the capture and loss of charge carriers that should model the electrical activity of monatomic hydrogen but still challenges theoretical descriptions. Spin-flip scattering of conduction electrons by the paramagnetic centres is revealed in heavily doped n-type material, as well as some modification of the local electronic structures. The corresponding spectroscopy for the solid elements of Groups III, V and VI is rather less well known and is reviewed here for the first time; a good deal of previously unpublished data is also included. Theoretical expectations and computational modelling are sparse, here. Recent results for B suggest a relatively shallow centre with molecular character; P and As show deeper quasi-atomic states, but still with substantial overlap of spin density onto surrounding host atoms. Particular attention is paid to the chalcogens. Muonium centres in Te show charge-state transitions already around room temperature; the identification of those in S and Se has been complicated by unusual spin dynamics of a different character, here attributed to spin-orbit coupling and interstitial reorientation. In the metals and semimetals, muonium is not formed as a paramagnetic centre. Here the implanted muons mimic interstital protons and interest shifts to a variety of other topics, including aspects of charge screening (α-Sn, Sb, Bi), site preference and quantum mobility (Al, β-Sn, Pb). The post-transition metals receive only a brief mention, by way of contrast with the nonmetals. Systematic studies of local susceptibility via measurements of muon Knight shifts extends in favourable cases to revealing the elusive high-field Condon domains (Al, Sn, Pb, Bi). Some new information is available on the superconducting phases. Appendices include a derivation of the spin Hamiltonian for paramagnetic muonium centres or molecular radicals having varying admixtures of orbital angular momentum, including the extreme case of orbital degeneracy, and examine the consequences of significant spin-orbit coupling for μSR spectroscopy and muon spin relaxation. This is the framework for the tentative assignments made here for the muonium defect centres formed in sulphur and selenium, namely diatomic species resembling the chalcogen monohydrides. Equally, it provides guidelines for eventual solid-state detection of OMu-the elusive muoniated hydroxyl radical.

Nucleation in the atmosphere

Abstract: Small particles play major roles in modulating radiative and hydrological fluxes in the atmosphere and thus they impact both climate (IPCC 2007) and weather. Most atmospheric particles outside clouds are created in situ through nucleation from gas phase precursors and most ice particles within clouds are formed by nucleation, usually from the liquid. Thus, the nucleation process is of great significance in the Earth's atmosphere.The theoretical examination of nucleation in the atmosphere has been based mostly on classical nucleation theory. While diagnostically very useful, the prognostic skill demonstrated by this approach has been marginal. Microscopic approaches such as molecular dynamics and density functional theory have also proven useful in elucidating various aspects of the process but are not yet sufficiently refined to offer a significant prognostic advantage to the classical approach, due primarily to the heteromolecular nature of atmospheric nucleation. An important aspect of the nucleation process in the atmosphere is that the degree of metastability of the parent phase for the nucleation is modulated by a number of atmospheric processes such as condensation onto pre-existing particles, updraft velocities that are the main driving force for supersaturation of water (a major factor in all atmospheric nucleation), and photochemical production rates of nucleation precursors. Hence, atmospheric nucleation is both temporally and spatially inhomogeneous.

Electrothermal transport coefficients at finite frequencies

Abstract: We review a recently developed formalism for computing thermoelectric coefficients in correlated matter. The usual difficulties of such a calculation are circumvented by a careful generalization of the transport formalism to finite frequencies, from which one can extract the high frequency objects. The technical parallel between the Hall constant and the Seebeck coefficient is explored and used to advantage. For small clusters, exact diagonalization gives the full spectrum for the Hubbard and especially the t–J model, a prototypical model for strong correlations, and this spectrum can be used to compute the exact finite frequency transport coefficients and hence to benchmark various approximations.An application of this formalism to the physically important case of sodium cobaltate NaxCoO2 is made, and interesting predictions for new materials are highlighted.

Nuclear magnetic resonance in the heavy fermion superconductors

Abstract: Nuclear magnetic resonance has emerged as a vital technique for investigating strongly correlated electron systems, and is particularly important for studying superconductivity. In this paper the basic features of NMR as a technique for probing the superconducting state are reviewed. Topics include spin relaxation processes, studies of vortex lattices and phenomena associated with unconventional pairing symmetries. Recent experimental work is reviewed, with a particular emphasis on the heavy fermion superconductors.

Precision mass measurements

Abstract: Mass as a physical quantity and its measurement are described After some historical remarks, a short summary of the concept of mass in classical and modern physics is given Principles and methods of mass measurements, for example as energy measurement or as measurement of weight forces and forces caused by acceleration, are discussed Precision mass measurement by comparing mass standards using balances is described in detail Measurement of atomic masses related to 12C is briefly reviewed as well as experiments and recent discussions for a future new definition of the kilogram, the SI unit of mass

The Kondo effect in coupled-quantum dots

Abstract: We discuss Kondo systems in coupled-quantum dots, with emphasis on the semiconductor quantum dot system. The rich variety of behaviors, such as distinct quantum phases, non-Fermi-liquid behavior and associated quantum phase transitions and cross-over behaviors are reviewed. Experimental evidence for such novel characteristics is summarized. The observed behaviors may provide clues as to the relevance of the 2-impurity Kondo (2IK) effect and the 2-channel Kondo (2CK) effect to the unusual characteristics in strongly correlated systems, such as the heavy fermion system.

Generalized Kramers-Kronig relations in nonlinear optical- and THz-spectroscopy

Abstract: Kramers?Kronig (K?K) relations have constituted one of the principal tools in the optical spectroscopy for the assessment of the optical properties of media from measured spectra. The underlying principle for the existence of the K?K relations is causality. Thanks to the K?K relations we have achieved a better understanding of both macroscopic and microscopic properties of media.Recently, various kinds of modified K?K relations have been presented in the literature. Such relations have been applied, e.g. to the nonlinear optical properties of polymers. A typical advantage of these generalized K?K relations is that the measured data do not need to be manipulated as in the case of the traditional K?K relations. Hence, the accuracy of the inverted data on linear or nonlinear optical properties of media becomes higher.A novel way to utilize generalized K?K relations is related to the measurement and correction of terahertz spectra in the time-domain reflection spectroscopy. Terahertz spectroscopy is nowadays one of the most rapidly developing fields in modern physics with applications being related to, e.g. security at the airports or inspection of pharmaceutical tablets. While recording THz spectra it is also possible to perform a chemical mapping of species. Therefore, correctness of the spectrum is of crucial importance for the identification of different species. This is possible by the generalized K?K relations. In this review paper we consider advances of K?K relations both in nonlinear optical and THz spectroscopy.

Femtosecond superradiant emission in inorganic semiconductors

Abstract: A review of an experimental study of superradiance in semiconductor inorganic structures is presented. It is demonstrated that unique properties of superradiant emission are determined by unusual properties of electrons and holes, namely, the formation of BCS-like state in a system of collectively paired electrons and holes. This can adequately explain all features of superradiance, including its femtosecond pulse duration, record peak power, optical spectrum, spatial and temporal coherency and macroscopically large fluctuations. The effect of non-equilibrium condensation of electrons and holes in the phase domain at room temperature is experimentally demonstrated. The critical temperature of condensation in a strongly degenerate system of electrons and holes is finally theoretically estimated.

Quantum Hall ferromagnets

Abstract: Quantum Hall (QH) systems continuously provide us with fascinating phenomena both physically and mathematically. They have received renewed interest owing to the discovery of quantum coherence associated with the spin and layer degrees of freedom. They have also proved to be ideal systems to play with noncommutative geometry. When an electron is confined within the lowest Landau level, its position is described solely by the guiding center, whose X and Y coordinates do not commute with one another. Hence, the QH system is formulated as a dynamical system in the noncommutative plane. We construct the microscopic theory of the QH system based on noncommutative geometry. Although the microscopic theory is necessary to derive some key formulae, it is intuitively clear to use the composite-boson theory to understand the mechanism how quantum coherence develops spontaneously. In the spontaneously broken phase of the spin SU(2) symmetry, there arises a topological soliton flipping several spins coherently. It is the quasiparticle (charged excitation) called a skyrmion. Skyrmions have been experimentally observed both in integer and fractional QH systems. More remarkable is the bilayer QH system, where the layer degree of freedom acts as the pseudospin. Due to the parallelism between the spin and the pseudospin, in the spontaneously broken phase of the pseudospin SU(2) symmetry, the Goldstone mode is the pseudospin wave, and the quasiparticle is a topological soliton to be identified with the pseudospin skyrmion. A new feature is the phase current, which induces anomalous behavior of the Hall resistance in a counterflow geometry. Another new feature is the tunnelling current, which demonstrates the Josephson-like phenomena. Furthermore, the parallel magnetic field penetrates between the two layers, and forms a soliton lattice beyond the commensurate–incommensurate phase-transition point. There are experimental indications for the phase current, the dc Josephson current, the pseudospin skyrmion and the soliton lattice.

Theory of radio frequency spectroscopy experiments in ultracold Fermi gases and their relation to photoemission in the cuprates

Abstract: In this paper we present an overview of radio frequency (RF) spectroscopy in the atomic Fermi superfluids with the ultimate goal of suggesting new directions in the cold gas research agenda from the condensed matter perspective. We review the experimental and theoretical literature on cold gases and the photoemission spectroscopy of the cuprates particularly as it pertains to areas of overlap. In addition to a comparison with the cuprates, this paper contains a systematic overview of the theory of RF spectroscopy, both momentum integrated and momentum resolved. It should be noted that the integrated and momentum resolved forms of photoemission are equally important in the high Tc cuprate literature. For the cold gases we introduce the reader to such topical issues as the effects of traps, population imbalance, final state interactions and, over the entire range of temperatures, we compare theory and experiment. We show that this broad range of phenomena can be accommodated within the BCS-Leggett description of BCS–BEC crossover. Importantly, this scheme captures some of the central observations in photoemission experiments in the cuprates.

Suspended single-wall carbon nanotubes: synthesis and optical properties

Abstract: A single-wall carbon nanotube (SWNT) is a rolled-up graphene sheet, and all the carbon atoms are in the surface layer. Thus, when lying on the substrate surface, or forming a bundle, SWNTs suffer from strong interaction with the substrate atoms or other nanotubes. However, when a SWNT is suspended between mesa structures, the interactions with the substrate and other nanotubes are minimized, which is important for extracting the intrinsic properties of nanotubes. In this paper, we review the synthesis of suspended SWNTs between mesa structures and their optical properties. The first part focuses on the growth and characterization of suspended SWNTs, the mechanisms of suspended structure formation and control of the structures (individual or bundled). The second part describes photoluminescence and Raman spectroscopy of individual and ensemble SWNTs. Highly enhanced photoluminescence and Raman signals enable us to examine the structure of individual SWNTs. The third part describes device applications of suspended SWNTs.