Showing papers in "Reviews of Modern Physics in 2001"
TL;DR: In this paper, a simple model based on the power-law degree distribution of real networks was proposed, which was able to reproduce the power law degree distribution in real networks and to capture the evolution of networks, not just their static topology.
Abstract: The emergence of order in natural systems is a constant source of inspiration for both physical and biological sciences. While the spatial order characterizing for example the crystals has been the basis of many advances in contemporary physics, most complex systems in nature do not offer such high degree of order. Many of these systems form complex networks whose nodes are the elements of the system and edges represent the interactions between them.
Traditionally complex networks have been described by the random graph theory founded in 1959 by Paul Erdohs and Alfred Renyi. One of the defining features of random graphs is that they are statistically homogeneous, and their degree distribution (characterizing the spread in the number of edges starting from a node) is a Poisson distribution. In contrast, recent empirical studies, including the work of our group, indicate that the topology of real networks is much richer than that of random graphs. In particular, the degree distribution of real networks is a power-law, indicating a heterogeneous topology in which the majority of the nodes have a small degree, but there is a significant fraction of highly connected nodes that play an important role in the connectivity of the network.
The scale-free topology of real networks has very important consequences on their functioning. For example, we have discovered that scale-free networks are extremely resilient to the random disruption of their nodes. On the other hand, the selective removal of the nodes with highest degree induces a rapid breakdown of the network to isolated subparts that cannot communicate with each other.
The non-trivial scaling of the degree distribution of real networks is also an indication of their assembly and evolution. Indeed, our modeling studies have shown us that there are general principles governing the evolution of networks. Most networks start from a small seed and grow by the addition of new nodes which attach to the nodes already in the system. This process obeys preferential attachment: the new nodes are more likely to connect to nodes with already high degree. We have proposed a simple model based on these two principles wich was able to reproduce the power-law degree distribution of real networks. Perhaps even more importantly, this model paved the way to a new paradigm of network modeling, trying to capture the evolution of networks, not just their static topology.
18,415 citations
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TL;DR: The author revealed that quantum teleportation as “Quantum one-time-pad” had changed from a “classical teleportation” to an “optical amplification, privacy amplification and quantum secret growing” situation.
Abstract: Quantum cryptography could well be the first application of quantum mechanics at the individual quanta level. The very fast progress in both theory and experiments over the recent years are reviewed, with emphasis on open questions and technological issues.
6,949 citations
TL;DR: In this paper, the current status of lattice-dynamical calculations in crystals, using density-functional perturbation theory, with emphasis on the plane-wave pseudopotential method, is reviewed.
Abstract: This article reviews the current status of lattice-dynamical calculations in crystals, using density-functional perturbation theory, with emphasis on the plane-wave pseudopotential method. Several specialized topics are treated, including the implementation for metals, the calculation of the response to macroscopic electric fields and their relevance to long-wavelength vibrations in polar materials, the response to strain deformations, and higher-order responses. The success of this methodology is demonstrated with a number of applications existing in the literature.
6,917 citations
TL;DR: This article considers the empirical data and then reviews the main approaches to modeling pedestrian and vehicle traffic, including microscopic (particle-based), mesoscopic (gas-kinetic), and macroscopic (fluid-dynamic) models.
Abstract: Since the subject of traffic dynamics has captured the interest of physicists, many surprising effects have been revealed and explained. Some of the questions now understood are the following: Why are vehicles sometimes stopped by ``phantom traffic jams'' even though drivers all like to drive fast? What are the mechanisms behind stop-and-go traffic? Why are there several different kinds of congestion, and how are they related? Why do most traffic jams occur considerably before the road capacity is reached? Can a temporary reduction in the volume of traffic cause a lasting traffic jam? Under which conditions can speed limits speed up traffic? Why do pedestrians moving in opposite directions normally organize into lanes, while similar systems ``freeze by heating''? All of these questions have been answered by applying and extending methods from statistical physics and nonlinear dynamics to self-driven many-particle systems. This article considers the empirical data and then reviews the main approaches to modeling pedestrian and vehicle traffic. These include microscopic (particle-based), mesoscopic (gas-kinetic), and macroscopic (fluid-dynamic) models. Attention is also paid to the formulation of a micro-macro link, to aspects of universality, and to other unifying concepts, such as a general modeling framework for self-driven many-particle systems, including spin systems. While the primary focus is upon vehicle and pedestrian traffic, applications to biological or socio-economic systems such as bacterial colonies, flocks of birds, panics, and stock market dynamics are touched upon as well.
3,117 citations
TL;DR: The generalization of field theory to space-time with noncommuting coordinates has been studied intensively in the last few years and many qualitatively new phenomena have been discovered, on both the classical and quantum level as discussed by the authors.
Abstract: This article reviews the generalization of field theory to space-time with noncommuting coordinates, starting with the basics and covering most of the active directions of research. Such theories are now known to emerge from limits of M theory and string theory and to describe quantum Hall states. In the last few years they have been studied intensively, and many qualitatively new phenomena have been discovered, on both the classical and the quantum level.
2,306 citations
TL;DR: The concept of entanglement plays an essential role in quantum physics as mentioned in this paper, and it is also essential to understand decoherence, the process accounting for the classical appearance of the macroscopic world.
Abstract: After they have interacted, quantum particles generally behave as a single nonseparable entangled system. The concept of entanglement plays an essential role in quantum physics. We have performed entanglement experiments with Rydberg atoms and microwave photons in a cavity and tested quantum mechanics in situations of increasing complexity. Entanglement resulted either from a resonant exchange of energy between atoms and the cavity field or from dispersive energy shifts affecting atoms and photons when they were not resonant. With two entangled particles (two atoms or one atom and a photon), we have realized new versions of the Einstein-Podolsky-Rosen situation. The detection of one particle projected the other, at a distance, in a correlated state. This process could be viewed as an elementary measurement, one particle being a ``meter'' measuring the other. We have performed a ``quantum nondemolition'' measurement of a single photon, which we detected repeatedly without destroying it. Entanglement is also essential to understand decoherence, the process accounting for the classical appearance of the macroscopic world. A mesoscopic superposition of states (``Schr\"odinger cat'') gets rapidly entangled with its environment, losing its quantum coherence. We have prepared a Schr\"odinger cat made of a few photons and studied the dynamics of its decoherence, in an experiment which constitutes a glimpse at the quantum/classical boundary. We have also investigated entanglement as a resource for the processing of quantum information. By using quantum two-state systems (qubits) instead of classical bits of information, one can perform logical operations exploiting quantum interferences and taking advantage of the properties of entanglement. Manipulating as qubits atoms and photons in a cavity, we have operated a quantum gate and applied it to the generation of a complex three-particle entangled state. We finally discuss the perspectives opened by these experiments for further fundamental studies.
2,303 citations
TL;DR: In this article, the authors review the properties of low-capacitance Josephson tunneling junctions and the practical and fundamental obstacles to their use for quantum information processing and describe how the basic physical manipulations on an ideal device can be combined to perform useful operations.
Abstract: Quantum-state engineering, i.e., active control over the coherent dynamics of suitable quantum-mechanical systems, has become a fascinating prospect of modern physics. With concepts developed in atomic and molecular physics and in the context of NMR, the field has been stimulated further by the perspectives of quantum computation and communication. Low-capacitance Josephson tunneling junctions offer a promising way to realize quantum bits (qubits) for quantum information processing. The article reviews the properties of these devices and the practical and fundamental obstacles to their use. Two kinds of device have been proposed, based on either charge or phase (flux) degrees of freedom. Single- and two-qubit quantum manipulations can be controlled by gate voltages in one case and by magnetic fields in the other case. Both kinds of device can be fabricated with present technology. In flux qubit devices, an important milestone, the observation of superpositions of different flux states in the system eigenstates, has been achieved. The Josephson charge qubit has even demonstrated coherent superpositions of states readable in the time domain. There are two major problems that must be solved before these devices can be used for quantum information processing. One must have a long phase coherence time, which requires that external sources of dephasing be minimized. The review discusses relevant parameters and provides estimates of the decoherence time. Another problem is in the readout of the final state of the system. This issue is illustrated with a possible realization by a single-electron transistor capacitively coupled to the Josephson device, but general properties of measuring devices are also discussed. Finally, the review describes how the basic physical manipulations on an ideal device can be combined to perform useful operations.
2,225 citations
TL;DR: The fundamental physical properties of doped oxides and their underlying physics were known more than 40 years ago as mentioned in this paper, and the concept of double exchange in particular, and points out the missing elements that have led to a massive resurgence of interest in these and related materials.
Abstract: The fundamental physical properties of doped ${\mathrm{LaMnO}}_{3},$ generically termed ``manganites,'' and much of the underlying physics, were known more than 40 years ago. This article first reviews progress made at that time, the concept of double exchange in particular, and points out the missing elements that have led to a massive resurgence of interest in these and related materials. More recent research is then described, treating first the ground states that emerge as divalent atoms are substituted for trivalent La. A wide range of ground states appear, including ferromagnetic metals, orbital- and charge-ordered antiferromagnets, and more complex stripe and spin-glass states. Because of the interest in so-called colossal magnetoresistance that occurs in the ferromagnetic/metallic composition range, a section is devoted to reviewing the atypical properties of that phase. Next the high-temperature phase is examined, in particular, evidence for the formation of self-trapped small polarons and the importance of Jahn-Teller coupling in this process. The transitions between the high-temperature polaronic phase and the ferromagnetic and charge-ordered states are treated in a fourth section. In each section, the authors stress the competition among charge, spin, and lattice coupling and review the current state of theoretical understanding. They conclude with some comments on the impact that research on these materials has on our understanding of doped oxides and other strongly correlated electronic materials.
2,060 citations
TL;DR: In this paper, the authors describe variational and fixed-node diffusion quantum Monte Carlo methods and how they may be used to calculate the properties of many-electron systems and describe a selection of applications to ground and excited states of solids and clusters.
Abstract: This article describes the variational and fixed-node diffusion quantum Monte Carlo methods and how they may be used to calculate the properties of many-electron systems. These stochastic wave-function-based approaches provide a very direct treatment of quantum many-body effects and serve as benchmarks against which other techniques may be compared. They complement the less demanding density-functional approach by providing more accurate results and a deeper understanding of the physics of electronic correlation in real materials. The algorithms are intrinsically parallel, and currently available high-performance computers allow applications to systems containing a thousand or more electrons. With these tools one can study complicated problems such as the properties of surfaces and defects, while including electron correlation effects with high precision. The authors provide a pedagogical overview of the techniques and describe a selection of applications to ground and excited states of solids and clusters.
1,957 citations
TL;DR: In this paper, the authors present a tutorial review of some ideas that are basic to our current understanding of Bose-Einstein condensation in the dilute atomic alkali gases, with special emphasis on the case of two or more coexisting hyperfine species.
Abstract: The author presents a tutorial review of some ideas that are basic to our current understanding of the phenomenon of Bose-Einstein condensation (BEC) in the dilute atomic alkali gases, with special emphasis on the case of two or more coexisting hyperfine species. Topics covered include the definition of and conditions for BEC in an interacting system, the replacement of the true interatomic potential by a zero-range pseudopotential, the time-independent and time-dependent Gross-Pitaevskii equations, superfluidity and rotational properties, the Josephson effect and related phenomena, and the Bogoliubov approximation.
1,695 citations
TL;DR: A relatively new class of materials has been found in which exhibit unusual temperature dependences in their low-temperature properties, including several examples in which the specific heat divided by temperature shows a singular $\mathrm{log}T$ temperature dependence over more than two orders of magnitude as mentioned in this paper.
Abstract: A relatively new class of materials has been found in which the basic assumption of Landau Fermi-liquid theory---that at low energies the electrons in a metal should behave essentially as a collection of weakly interacting particles---is violated. These ``non-Fermi-liquid'' systems exhibit unusual temperature dependences in their low-temperature properties, including several examples in which the specific heat divided by temperature shows a singular $\mathrm{log}T$ temperature dependence over more than two orders of magnitude, from the lowest measured temperatures in the milliKelvin regime to temperatures over 10 K. These anomalous properties, with their often pure power-law or logarithmic temperature dependences over broad temperature ranges and inherent low characteristic energies, have attracted active theoretical interest from the first experimental report in 1991. This article first describes the various theoretical approaches to trying to understand the source of strong temperature- and frequency-dependent electron-electron interactions in non-Fermi-liquid systems. It then discusses the current experimental body of knowledge, including a compilation of data on non-Fermi-liquid behavior in over 50 systems. The disparate data reveal some interesting correlations and trends and serve to point up a number of areas where further theoretical and experimental work is needed.
TL;DR: In this paper, the first analytical description of anomalous scaling laws in turbulent transport has been obtained and the underlying physical mechanism reveals the role of statistical integrals of motion in nonequilibrium systems.
Abstract: The understanding of fluid turbulence has considerably progressed in recent years. The application of the methods of statistical mechanics to the description of the motion of fluid particles, i.e., to the Lagrangian dynamics, has led to a new quantitative theory of intermittency in turbulent transport. The first analytical description of anomalous scaling laws in turbulence has been obtained. The underlying physical mechanism reveals the role of statistical integrals of motion in nonequilibrium systems. For turbulent transport, the statistical conservation laws are hidden in the evolution of groups of fluid particles and arise from the competition between the expansion of a group and the change of its geometry. By breaking the scale-invariance symmetry, the statistically conserved quantities lead to the observed anomalous scaling of transported fields. Lagrangian methods also shed new light on some practical issues, such as mixing and turbulent magnetic dynamo.
TL;DR: In this article, the authors present the current knowledge and understanding of the interstellar medium of our galaxy and discuss the interaction of these interstellar constituents, both with each other and with stars, in the framework of the general galactic ecosystem.
Abstract: This article reviews the current knowledge and understanding of the interstellar medium of our galaxy. The author first presents each of the three basic constituents---ordinary matter, cosmic rays, and magnetic fields---of the interstellar medium, with emphasis on their physical and chemical properties as inferred from a broad range of observations. The interaction of these interstellar constituents, both with each other and with stars, is then discussed in the framework of the general galactic ecosystem.
TL;DR: Chiang et al. as mentioned in this paper discovered conducting polymers and the ability to dope these polymers over the full range from insulator to metal, which was particularly exciting because it created a new field of research on the boundary between chemistry and condensed-matter physics.
Abstract: In 1976, Alan MacDiarmid, Hideki Shirakawa, and I, together with a talented group of graduate students and postdoctoral researchers, discovered conducting polymers and the ability to dope these polymers over the full range from insulator to metal (Chiang et al., 1977; Shirakawa et al., 1977). This was particularly exciting because it created a new field of research on the boundary between chemistry and condensed-matter physics, and because it created a number of opportunities:
TL;DR: In this paper, the essential elements of the theory of brown dwarfs and giant planets are described, including their evolution, atmospheric composition, and spectra, including the new spectroscopic classes L and T. The theory explains the basic systematics of substellar-mass objects over three orders of magnitude in mass and age, and a factor of 30 in temperature.
Abstract: Straddling the traditional realms of the planets and the stars, objects below the edge of the main sequence have such unique properties, and are being discovered in such quantities, that one can rightly claim that a new field at the interface of planetary science and astronomy is being born. This article extends the previous review of Burrows and Liebert (1993) and describes the essential elements of the theory of brown dwarfs and giant planets. It discusses their evolution, atmospheric composition, and spectra, including the new spectroscopic classes L and T. Particular topics which are important for an understanding of the spectral properties include the effects of condensates, clouds, molecular abundances, and atomic opacities. Moreover, it discusses the distinctive features of these extrasolar giant planets that are irradiated by a central primary, in particular, their reflection spectra, albedos, and transits. Overall, the theory explains the basic systematics of substellar-mass objects over three orders of magnitude in mass and age, and a factor of 30 in temperature.
TL;DR: For about 20 years, it has been the prevailing view that there can be no metallic state or metal insulator transition in two dimensions in zero magnetic field as mentioned in this paper, however, unusual behavior suggestive of such a transition has been reported in a variety of dilute two-dimensional electron and hole systems.
Abstract: For about 20 years, it has been the prevailing view that there can be no metallic state or metal-insulator transition in two dimensions in zero magnetic field. In the last several years, however, unusual behavior suggestive of such a transition has been reported in a variety of dilute two-dimensional electron and hole systems. The physics behind these observations is at present not understood. The authors review and discuss the main experimental findings and suggested theoretical models.
TL;DR: In this paper, the Anderson impurity model and cluster model are used to model the effects of intra-atomic multiplet coupling and weak interatomic electron transfer. But the Anderson model is not suitable for the case of transition-metal compounds, where the 4f electrons are localized with strong electron correlation.
Abstract: conservation are discussed. At the opposite extreme are rare-earth systems (metals and oxides), in which the 4f electrons are almost localized with strong electron correlation. The observations are interpreted based on the effects of intra-atomic multiplet coupling and weak interatomic electron transfer, which are well described with an Anderson impurity model or a cluster model. In this context a narrowing of spectral width in the excitation spectrum, polarization dependence, and the magnetic circular dichroism in ferromagnetic materials are discussed. The authors then consider transition-metal compounds, materials with electron correlation strengths intermediate between semiconductors and rare-earth systems. In these interesting cases there is an interplay of intra-atomic and interatomic electronic interactions that leads to limitations of both the band model and the Anderson impurity model. Finally, other topics in resonant x-ray emission studies of solids are described briefly.
KEK1
TL;DR: In this article, the current theoretical and experimental status of the field of muon decay and its potential to search for new physics beyond the standard model is reviewed, together with the recent up-to-date motivations of lepton flavor violation based on supersymmetric models.
Abstract: This article reviews the current theoretical and experimental status of the field of muon decay and its potential to search for new physics beyond the standard model. The importance of rare muon processes with lepton flavor violation is highly stressed, together with precision measurements of normal muon decay. Recent up-to-date motivations of lepton flavor violation based on supersymmetric models, in particular supersymmetric grand unified theories, are described along with other theoretical models. Future prospects of experiments and muon sources of high intensity for further progress in this field are also discussed.
TL;DR: In this paper, the electronic structure of atoms, molecules, and bulk matter, as well as the thermodynamic properties of dense plasma, in strong magnetic fields, were reviewed, and various theoretical approaches and numerical results were also discussed.
Abstract: The properties of matter are drastically modified by strong magnetic fields, $B\ensuremath{\gg}{m}_{e}^{2}{e}^{3}c/{\ensuremath{\Elzxh}}^{3}=2.35\ifmmode\times\else\texttimes\fi{}{10}^{9}\mathrm{G}$ $(1\mathrm{G}{=10}^{\ensuremath{-}4}\mathrm{T}),$ as are typically found on the surfaces of neutron stars. In such strong magnetic fields, the Coulomb force on an electron acts as a small perturbation compared to the magnetic force. The strong-field condition can also be mimicked in laboratory semiconductors. Because of the strong magnetic confinement of electrons perpendicular to the field, atoms attain a much greater binding energy compared to the zero-field case, and various other bound states become possible, including molecular chains and three-dimensional condensed matter. This article reviews the electronic structure of atoms, molecules, and bulk matter, as well as the thermodynamic properties of dense plasma, in strong magnetic fields, ${10}^{9}\mathrm{G}\ensuremath{\ll}B\ensuremath{\lesssim}{10}^{16}\mathrm{G}.$ The focus is on the basic physical pictures and approximate scaling relations, although various theoretical approaches and numerical results are also discussed. For a neutron star surface composed of light elements such as hydrogen or helium, the outermost layer constitutes a nondegenerate, partially ionized Coulomb plasma if $B\ensuremath{\lesssim}{10}^{15}$ G (at temperature $T\ensuremath{\gtrsim}{10}^{6}$ K), and may be in the form of a condensed liquid if the magnetic field is stronger (and T $\ensuremath{\gtrsim}{10}^{6}\mathrm{K}).$ For an iron surface, the outermost layer of the neutron star can be in a gaseous or a condensed phase, depending on the cohesive property of the iron condensate.
TL;DR: In this paper, the authors define a heterogeneous semiconductor structure built from two or more different semiconductors, in such a way that the transition region or interface between the different materials plays an essential role in any device action.
Abstract: Heterostructures, as I use the word here, may be defined as heterogeneous semiconductor structures built from two or more different semiconductors, in such a way that the transition region or interface between the different materials plays an essential role in any device action. Often, it may be said that the interface is the device. The participating semiconductors all involve elements from the central portion of the periodic table of the elements (Table I). In the center is silicon, the backbone of modern electronics. Below Si is germanium. Although Ge is rarely used by itself, Ge-Si alloys with a composition-dependent position play an increasingly important role in today’s heterostructure technology. In fact, historically this was the first heterostructure device system proposed, although it was also the system that took longest to bring to practical maturity, largely because of the 4% mismatch between the lattice constants of Si and Ge. Silicon plays the same central role in electronic metallurgy that steel plays in structural metallurgy. But just as modern structural metallurgy draws on metals other than steel, electronics draws on semiconductors other than silicon, namely, the compound semiconductors. Every element in column III may be combined with every element in column V to form a so-called III-V compound. From the elements shown, twelve different discrete III-V compounds may be formed. The most widely used compound is GaAs—gallium arsenide—but all of them are used in heterostructures, the specific choice depending on the application. In fact, today the III-V compounds are almost always used in heterostructures, rather than in isolation. Two or more discrete compounds may be used to form alloys. A common example is aluminum-gallium arsenide, AlxGa12xAs, where x is the fraction of columnIII sites in the crystal occupied by Al atoms, and 1 2 x is occupied by Ga atoms. Hence we have not just 12 discrete compounds, but a continuous range of materials. As a result, it becomes possible to make compositionally graded heterostructures, in which the composition varies continuously rather than abruptly throughout the device structure.
TL;DR: In this article, the concept of spontaneous symmetry breaking is applied to the rotating mean field of nuclei, which is based on the tilted-axis cranking model, taking into account that the rotational axis can take any orientation with respect to the deformed density distribution.
Abstract: The concept of spontaneous symmetry breaking is applied to the rotating mean field of nuclei. The description is based on the tilted-axis cranking model, which takes into account that the rotational axis can take any orientation with respect to the deformed density distribution. The appearance of rotational bands in nuclei is analyzed, focusing on weakly deformed nuclei at high angular momentum. The quantization of the angular momentum of the valence nucleons leads to new phenomena. Magnetic rotation represents the quantized rotation of the anisotropic current distribution in a near spherical nucleus. The restricted amount of angular momentum of the valence particles causes band termination. The discrete symmetries of the mean-field Hamiltonian provide a classification scheme of rotational bands. New symmetries result from the combination of the spatial symmetries of the density distribution with the vector of the angular momentum. The author discusses in detail which symmetries appear for a reflection-symmetric density distribution and how they show up in the properties of the rotational bands. In particular, the consequences of rotation about a nonprincipal axis and of breaking the chiral symmetry are analyzed. Also discussed are which symmetries and band structures appear for non-reflection-symmetric mean fields. The consequences of breaking the symmetry with respect to gauge and isospin rotations are sketched. Some analogies outside nuclear physics are mentioned. The application of symmetry-restoring methods to states with large angular momentum is reviewed.
TL;DR: The matrix model of M theory as mentioned in this paper is an 11-dimensional quantum theory of gravity that is believed to underlie all superstring theories and is currently the most plausible candidate for a theory of fundamental physics which reconciles gravity and quantum field theory.
Abstract: This article reviews the matrix model of M theory. M theory is an 11-dimensional quantum theory of gravity that is believed to underlie all superstring theories. M theory is currently the most plausible candidate for a theory of fundamental physics which reconciles gravity and quantum field theory in a realistic fashion. Evidence for M theory is still only circumstantial -- no complete background-independent formulation of the theory exists as yet. Matrix theory was first developed as a regularized theory of a supersymmetric quantum membrane. More recently, it has appeared in a different guise as the discrete light-cone quantization of M theory in flat space. These two approaches to matrix theory are described in detail and compared. It is shown that matrix theory is a well-defined quantum theory that reduces to a supersymmetric theory of gravity at low energies. Although its fundamental degrees of freedom are essentially pointlike, higher-dimensional fluctuating objects (branes) arise through the non-Abelian structure of the matrix degrees of freedom. The problem of formulating matrix theory in a general space-time background is discussed, and the connections between matrix theory and other related models are reviewed.
TL;DR: In this article, double-heterostructures and double-wells, including quantum wells, wires, and dots, are the subject of research of two-thirds of the semiconductor physics community, and the ability to control the type of conductivity of a semiconductor material by doping with various impurities and injecting nonequilibrium charge carriers could be said to be the seeds from which semiconductor electronics developed.
Abstract: It is impossible to imagine now modern solid-state physics without semiconductor heterostructures. Semiconductor heterostructures and, particularly, double heterostructures, including quantum wells, wires, and dots, are today the subject of research of two-thirds of the semiconductor physics community. The ability to control the type of conductivity of a semiconductor material by doping with various impurities and the idea of injecting nonequilibrium charge carriers could be said to be the seeds from which semiconductor electronics developed. Heterostructures developed from these beginnings, making it possible to solve the considerably more general problem of controlling the fundamental parameters inside the semiconductor crystals and devices: band gaps, effective masses of the charge carriers and the mobilities, refractive indices, electron energy spectrum, etc. Development of the physics and technology of semiconductor heterostructures has resulted in remarkable changes in our everyday life. Heterostructure electronics are widely used in many areas of human civilization. It is hardly possible to imagine our recent life without double heterostructure (DHS) laser-based telecommunication systems, heterostructure-based light-emitting diodes (LED’s), heterostructure bipolar transistors, or lownoise high-electron-mobility transistors for highfrequency applications including, for example, satellite television. Double-heterostructure lasers now enter practically every house with CD players. Heterostructure solar cells have been widely used for space and terrestrial applications. Our interest in semiconductor heterostructures was not occasional. Systematic studies of semiconductors were started in the early 1930s at the Physico-Technical Institute under the direct leadership of its founder, Abraham Ioffe. V. P. Zhuze and B. V. Kurchatov studied the intrinsic and impurity conductivity of semiconductors in 1932, and the same year Ioffe and Ya. I. Frenkel created a theory of rectification in a metalsemiconductor contact based on the tunneling phenom-
TL;DR: Gauge invariance is the basis of the modern theory of electroweak and strong interactions (the so-called standard model). as discussed by the authors describes the 19th century developments that led to the discovery that different forms of the vector potential are physically equivalent, if accompanied by a change in the scalar potential.
Abstract: Gauge invariance is the basis of the modern theory of electroweak and strong interactions (the so-called standard model). A number of authors have discussed the ideas and history of quantum guage theories, beginning with the 1920s, but the roots of gauge invariance go back to the year 1820 when electromagnetism was discovered and the first electrodynamic theory was proposed. We describe the 19th century developments that led to the discovery that different forms of the vector potential (differing by the gradient of a scalar function) are physically equivalent, if accompanied by a change in the scalar potential: $\mathbf{A}\ensuremath{\rightarrow}{\mathbf{A}}^{\ensuremath{'}}=\mathbf{A}+\ensuremath{
abla}\ensuremath{\chi}, \ensuremath{\Phi}\ensuremath{\rightarrow}{\ensuremath{\Phi}}^{\ensuremath{'}}=\ensuremath{\Phi}\ensuremath{-}\ensuremath{\partial}\ensuremath{\chi}/c\ensuremath{\partial}t.$ L. V. Lorenz proposed the condition ${\ensuremath{\partial}}_{\ensuremath{\mu}}{A}^{\ensuremath{\mu}}=0$ in the mid-1860s, but this constraint is generally misattributed to the better known H. A. Lorentz. In the work in 1926 on the relativistic wave equation for a charged spinless particle in an electromagnetic field by Schr\"odinger, Klein, and Fock, it was Fock who discovered the invariance of the equation under the above changes in A and \ensuremath{\Phi} if the wave function was transformed according to $\ensuremath{\psi}\ensuremath{\rightarrow}{\ensuremath{\psi}}^{\ensuremath{'}}=\ensuremath{\psi}\mathrm{exp}(ie\ensuremath{\chi}/\ensuremath{\Elzxh}c).$ In 1929, H. Weyl proclaimed this invariance as a general principle and called it Eichinvarianz in German and gauge invariance in English. The present era of non-Abelian gauge theories started in 1954 with the paper by Yang and Mills on isospin gauge invariance.
TL;DR: In this article, an overview of recent x-ray diffraction experiments with time resolutions down to 10^-13s is presented, using examples from solid state physics and biology.
Abstract: This article gives an overview of recent x-ray diffraction experiments with time resolutions down to 10^-13s. The scientific motivation behind the development is outlined, using examples from solid state physics and biology. The ultrafast resolution may be provided either by fast detectors or short x-ray pulses, and the limitations of both techniques are discussed on the basis of state of the art experiments. In particular, it is shown that with present designs, high time resolution reduces the structural information attainable with high spatial resolution, thereby limiting feasible experiments on the ultrashort time-scale. The first experiment showing subpicosecond conformation changes was recently achieved with simple solids using an ultrafast laser-produced plasma x-ray source. The principles of this experiment are described in detail.
TL;DR: In this article, the early investigations that preceded and eventually led to the discovery of chemical doping are discussed, and a discussion of the early development of conducting polymers is presented, with a focus on the early work on polyacetylene.
Abstract: The Nobel Prize in Chemistry 2000 was awarded for our discovery and development of conducting polymers, but that discovery only happened after much work on polyacetylene. In this lecture, I would like to talk about the early investigations that preceded and eventually led to the discovery of chemical doping. I do hope my talk will be of use for you, the audience, to deepen your understanding of what had happened before and how we arrived at the idea of chemical doping.
TL;DR: In this article, an overview is presented of the various phases predicted to occur when gases are absorbed within a bundle of carbon nanotubes, which is characterized by an effective dimensionality, which depends on the species and the temperature.
Abstract: An overview is presented of the various phases predicted to occur when gases are absorbed within a bundle of carbon nanotubes. The behavior may be characterized by an effective dimensionality, which depends on the species and the temperature. Small molecules are strongly attracted to the interstitial channels between tubes. There, they undergo transitions between ordered and disordered quasi-one-dimensional (1D) phases. Both small and large molecules display 1D and/or 2D phase behavior when adsorbed within the nanotubes, depending on the species and thermodynamic conditions. Finally, molecules adsorbed on the external surface of the bundle exhibit 1D behavior (striped phases), which crosses over to 2D behavior (monolayer film) and eventually 3D behavior (thick film) as the coverage is increased. The various phases exhibit a wide variety of thermal and other properties.
TL;DR: In this article, the authors describe recent measurements of atmospheric neutrinos by large underground detectors and show that the (v μ + v μ )/(v e + v e ) ratio is significantly smaller than expected.
Abstract: Atmospheric neutrinos arise from the decay of secondaries (π, K and μ) produced by primary cosmic-ray interactions in the atmosphere. In the energy range below 1GeV, where all secondaries decay, we roughly expect that (v μ + v μ )/(v e + v e ) ≃2. This is because a π-decay produces a v μ and a μ; the μ, when it decays, produces another v μ and a v e . These neutrinos enter into the detector from all directions nearly isotropically. The path length of the neutrinos (from the production point to the detector) ranges from a few km to 13000km, the diameter of the earth. In this article we describe recent measurements of atmospheric neutrinos by large underground detectors. Some of the experiments have observed that the (v μ + v μ )/(v e + v e ) ratio is significantly smaller than expected. The interpretation of these interesting results, the future prospect on atmospheric neutrinos and related neutrino physics are also described. In most parts of this article, when mention is made of neutrinos, we imply both neutrinos and antineutrinos.
TL;DR: In this paper, a pedagogical exploration of the nonperturbative issues entwining lattice gauge theory, anomalies, and chiral symmetry is presented, along with a scheme for exploring this physics on the lattice.
Abstract: This article is a pedagogical exploration of the nonperturbative issues entwining lattice gauge theory, anomalies, and chiral symmetry. After briefly reviewing the importance of chiral symmetry in particle physics, the author discusses how anomalies complicate lattice formulations. Considerable information can be deduced from effective chiral Lagrangians, helping interpret the expectations for lattice models and elucidating the role of the CP-violating parameter \ensuremath{\Theta}. One particularly elegant scheme for exploring this physics on the lattice is presented in some detail. This uses an auxiliary extra space-time dimension, with the physical world being a four-dimensional interface.