The charge exchange of slow highly charged ions at surfaces unraveled with freestanding 2D materials

The property of a variable charge state makes ions unique to other types of radiation a material surface can be exposed to. As a consequence of charge exchange between ions and surfaces, energy is transferred to the surface and material damage may be triggered. Furthermore, a changing charge state of the ion alters its slowing down process in solids and has important implications when back-scattered ions are to be measured for material analysis purposes. Over the last decades extensive research was devoted to the understanding of ion charge exchange with solids. Here I review recent progress in this field with special emphasize on slow ions in high charge states. This class of ions allows a detailed analysis of charge exchange in experiments, which employ also ultra-thin solid targets and therefore give experimental access to electronic processes on the femtosecond timescale. In this review I will discuss general properties of charge exchange and present typical experimental techniques. I will also discuss current developments in the modelling and simulation of ion-surface interaction. Recent findings using freestanding 2D materials are discussed as well as results from spectroscopy of emitted secondary particles. The paper concludes with a unified picture of ion charge exchange at surfaces and presents possible applications based on the understanding of the underlying physics.     

Phase transition between three- and two-flavor QCD?

We explore the possibility that QCD may undergo a phase transition as a function of the strange quark mass. This would hint towards models with ”spontaneous color symmetry breaking” in the vacuum. For two light quark flavors we classify possible colored quark-antiquark, diquark and gluon condensates that are compatible with a spectrum of integer charged states and conserved isospin and baryon number. The ”quark mass phase transition” would be linked to an unusual realization of baryon number in QCD₂ and could be tested in lattice simulations. We emphasize, however, that at the present stage the Higgs picture of the vacuum cannot predict a quark mass phase transition - a smooth crossover remains as a realistic alternative. Implications of the Higgs picture for the high-density phase transition in QCD₂ suggest that this transition is characterized by the spontaneous breaking of isospin for nuclear and quark matter. Received: 12 March 2001 / Revised version: 2 April 2003 / Published online: 2 June 2003 RID="a" ID="a" e-mail: C.Wetterich@thphys.uni-heidelberg.de     

Violations of parton-hadron duality in Deep Inelastic Scattering

I present an overview of recent data on Deep Inelastic Scattering at large Bjorken x and low invariant mass, W², where parton-hadron duality was originally observed. I discuss the concept of parton-hadron duality from the perspective of perturbative QCD. Within this framework, I show that parton-hadron duality is broken for low values of W², in the - and S₁₁-resonance production region. A model that accounts for the anomalous scale dependence ensuing from this situation is developed.Received: 1 November 2002, Published online: 15 July 2003PACS: 13.60.-r Photon and charged-lepton interactions with hadrons - 13.60.Hb Total and inclusive cross-sections (including deep-inelastic processes)     

用光学方法实现多灰度级图像的区域分割

利用光学中的阶跃滤波器实现对多灰度级图像的区域分割，并从理论上对用阶跃滤波器处理具有硬边界和软边界的多灰度级图像及最终实现多灰度级图像的区域分割进行了讨论。当多灰度级图像存在灰度不连续变化(硬边界)和有转折变化(尖点)时，使用阶跃滤波器都会在这样的区域内呈现边界增强。     

Hadronic radiation patterns in vector boson fusion higgs production

We consider the hadronic radiation patterns for the generic process of forward jet production at the LHC, where the (centrally produced) originate either from a Higgs, a Z or from standard QCD production processes. A numerical technique for evaluating the radiation patterns for non-trivial final states is introduced and shown to agree with the standard analytic results for more simple processes. Significant differences between the radiation patterns for the Higgs signal and the background processes are observed and quantified. This suggests that hadronic radiation patterns could be used as an additional diagnostic tool in Higgs searches in this channel at the LHC.Received: 25 July 2003, Published online: 26 September 2003     

Recent Notable Approaches to Study Self-Assembly of Nanoparticles with X-Ray Scattering and Electron Microscopy

Self-assembly of nanoparticles (NPs) has evolved into a powerful tool for the synthesis of superstructures with tailored properties. The quality, diversity, and complexity of synthesized structures are continuously improving and fascinating new collective properties are demonstrated. At the same time, the rapid development of electron microscopy and synchrotron sources for X-rays has enabled new exciting experimental approaches to study structure and structure formation in the context of NP self-assembly. In this review, some recent studies and what can be learned from them are highlighted and discussed. It is started with a general introduction covering important concepts, experimental approaches, commonly obtained structures, the ideas of artificial atoms, and emerging properties are discussed. Recent experimental in situ and ex situ approaches with state-of-the-art electron microscopy and X-ray diffraction and scattering that helped to obtain a detailed picture of NP self-assembly processes and resulting structures are then presented.     

Muon level-crossing resonance in magnetically ordered materials

The positive muon is widely used as a microscopic probe of internal fields at interstitial sites in magnetically ordered materials. Recently, we have demonstrated that the hyperfine fields on the neighboring host nuclear spins can be measured using a novel muon level-crossing resonance technique, thus providing a more detailed picture of the electronic and magnetic environment around the muon. In this paper I will describe the fundamentals of muon level-crossing resonance as applied to magnetically ordered materials, and report an example in MnF₂.     

Theoretical aspects of hydrogen in crystalline semiconductors

Hydrogen in semiconductors displays behavior quite different from any other impurity, causing effects which can be either beneficial or detrimental to the electronic properties. Theoretical calculations have been able to elucidate many puzzling aspects of this behavior, by providing a microscopic picture of the interactions of hydrogen with the host lattice and with other impurities. I will critically review the available theoretical techniques, and address the following topics: microscopic structure of isolated interstitial hydrogen, as well as various hydrogen-impurity complexes; electronic structure, including the relative stability of different charge states; diffusion; and H motion around impurities.     

Asymmetric reactions in sonochemistry.

Sonochemical processes involved in asymmetric synthesis are reviewed. The goal of this overview is to provide a comprehensive picture about the fields of both enantioselective and diastereoselective reactions accelerated or initiated by ultrasounds. Since the most, in many cases comparative, data are available in heterogeneous metal catalysis, the emphasis will be placed on these enantioselective hydrogenations, however, other applications such as phase transfer catalysis etc. will also be cited.     

Segmental dynamics of polymers in nanoscopic confinements,as probed by simulations of polymer/layered-silicate nanocomposites

In this paper we review molecular modeling investigations of polymer/layered-silicate intercalates, as model systems to explore polymers in nanoscopically confined spaces. The atomic-scale picture, as revealed by computer simulations, is presented in the context of salient results from a wide range of experimental techniques. This approach provides insights into how polymeric segmental dynamics are affected by severe geometric constraints. Focusing on intercalated systems, i.e. polystyrene (PS) in 2 nm wide slit-pores and polyethylene-oxide (PEO) in 1 nm wide slit-pores, a very rich picture for the segmental dynamics is unveiled, despite the topological constraints imposed by the confining solid surfaces. On a local scale, intercalated polymers exhibit a very wide distribution of segmental relaxation times (ranging from ultra-fast to ultra-slow, over a wide range of temperatures). In both cases (PS and PEO), the segmental relaxations originate from the confinement-induced local density variations. Additionally, where there exist special interactions between the polymer and the confining surfaces (e.g., PEO) more molecular mechanisms are identified.Received: 1 January 2003, Published online: 14 October 2003PACS: 83.10.Rs Computer simulation of molecular and particle dynamics - 81.07.Nb Molecular nanostructures - 81.07.Pr Organic-inorganic hybrid nanostructures     

The Theory of the Rise of Sap in Trees: Some Historical and Conceptual Remarks

The ability of trees to suck water from roots to leaves, sometimes to heights of over a hundred meters, is remarkable given the absence of any mechanical pump. In this study I deal with a number of issues, of both a historical and conceptual nature, in the orthodox Cohesion-Tension (CT) theory of the ascent of sap in trees. The theory relies chiefly on the exceptional cohesive and adhesive properties of water, the structural properties of trees, and the role of evaporation (“transpiration”) from leaves. But it is not the whole story. Plant scientists have been aware since the inception of the theory in the late 19^th century that further processes are at work in order to “prime” the trees, the main such process – growth itself – being so obvious to them that it is often omitted from the story. Other factors depend largely on the type of tree, and are not always fully understood. For physicists, in particular, it may be helpful to see the fuller picture, which is what I attempt to provide in nontechnical terms.     

Phase behavior and collective excitations of the Morse ring chain

Using primarily numerical methods we study clustering processes and collective excitations in a one-dimensional ring chain. The ring chain is constituted by N identical point particles with next neighbors interacting via nonlinear Morse springs. If the system is coupled to a heat bath (Gaussian white noise and viscous friction), then depending on the particle density and the bath temperature different phase-like states can be distinguished. This will be illustrated by means of numerically calculated phase diagrams. In order to identify collective excitations activated by the heat bath we calculate the spectrum of the normalized dynamical structure factor (SDF). Our numerical results show that the transition regions between different phase-like states are typically characterized by a 1/f-type SDF spectrum, reflecting the fact that near critical points correlations on all length and time scales become important. In the last part of the paper we also discuss a non-equilibrium effect, which occurs if an additional nonlinearly velocity-dependent force is included in the equations of motions. In particular it will be shown that such additional dissipative effects may stabilize cluster configurations.Received: 27 June 2003, Published online: 2 October 2003PACS: 05.70.Fh Phase transitions: general studies - 05.70.Ln Non-equilibrium and irreversible processes - 05.40.-a Fluctuation phenomena, random processes, noise and Brownian motion     

A relativistic superalgebra in a generalized Schrödinger picture

We consider a relativistic superalgebra in the picture in which the time and spatial derivative cannot be presented in the operators of the particle. The supersymmetry generators as well as the Hamilton operators for the massive relativistic particles with spin 0 and spin 1/2 are expressed in terms of the principal series of the unitary representations of the Lorentz group. We also consider the massless case. New Hamilton operators are constructed for the massless particles with spin 0 and spin 1/2. Received: 20 November 2002 / Published online: 14 April 2003 RID="a" ID="a" e-mail: rf@thp.uni-koeln.de     

A possible interpretation of the \pi_1(1400) exotic meson

I will discuss the current theoretical and experimental status of exotic mesons and a possible interpretation of the light, exotic meson as a final state interaction effect.Received: 30 September 2002, Published online: 22 October 2003PACS: 12.39.Mk Glueball and nonstandard multi-quark/gluon states - 13.60.Le Meson productionA.P. Szczepaniak: Present address: Department of Physics, Indiana University, Bloomington, IN 47405, USA     

Correlations between multiplicities and average transverse momentum in the percolating color strings approach

Long range multiplicity-multiplicity, p_T²-multiplicity and p²_T- p²_T correlations are studied in the percolating color string picture under different assumptions of the dynamics of the string interaction. It is found that the strength of these correlations is rather insensitive to these assumptions; nor is it sensitive to the geometry of the fused string clusters that formed, the percolation phase transition in particular. Both multiplicity-multiplicity and p_T²-multiplicity correlations are found to scale and depend only on the string density. p_T²-multiplicity correlations, which are absent in the independent string picture, are found to be of the order of 10% for central heavy ion collisions and can serve as a clear signature of string fusion. In contrast p²_T- p²_T correlations turned out to be inversely proportional to the number of strings and therefore to be very small for realistic collisions.Received: 4 July 2003, Revised: 12 September 2003, Published online: 18 December 2003     

The big problems in star formation: The star formation rate,stellar clustering,and the initial mass function

Star formation lies at the center of a web of processes that drive cosmic evolution: generation of radiant energy, synthesis of elements, formation of planets, and development of life. Decades of observations have yielded a variety of empirical rules about how it operates, but at present we have no comprehensive, quantitative theory. In this review I discuss the current state of the field of star formation, focusing on three central questions: What controls the rate at which gas in a galaxy converts to stars? What determines how those stars are clustered, and what fraction of the stellar population ends up in gravitationally-bound structures? What determines the stellar initial mass function, and does it vary with star-forming environment? I use these three questions as a lens to introduce the basics of star formation, beginning with a review of the observational phenomenology and the basic physical processes. I then review the status of current theories that attempt to solve each of the three problems, pointing out links between them and opportunities for theoretical and numerical work that crosses the scale between them. I conclude with a discussion of prospects for theoretical progress in the coming years.     

Progress in neutrino oscillation searches and their implications

Neutrino oscillation, in which a given flavor of neutrino transforms into another is a powerful tool for probing small neutrino masses. The intrinsic neutrino properties involved are neutrino mass squared difference Δm ² and the mixing angle in vacuum θ. In this paper I will summarize the progress that we have achieved in our search for neutrino oscillation with special emphasis on the recent results from the Sudbury Neutrino Observatory (SNO) on the measurement of solar neutrino fluxes. I will outline the current bounds on the neutrino masses and mixing parameters and discuss the major physics goals of future neutrino experiments in the context of the present picture.     

Some phenomenological consequences of the time-ordered perturbation theory of QED on non-commutative spacetime

A framework was recently proposed for doing perturbation theory on non-commutative (NC) spacetime. It preserves the unitarity of the S matrix and differs from the naive, popular approach already at the lowest order in perturbation when time does not commute with space. In this work, we investigate its phenomenological implications at linear colliders, especially the TESLA at DESY, through the processes of . We find that some NC effects computed previously are now modified and that there are new processes which now exhibit NC effects. Indeed, the first two processes get corrected at tree level as opposed to the null result in the naive approach, while the third one coincides with the naive result only in the low energy limit. The impact of the earth's rotation is incorporated. The NC signals are generally significant when the NC scale is comparable to the collider energy. If this is not the case, the non-trivial azimuthal angle distribution and day-night asymmetry of events due to Lorentz violation and the earth's rotation will be useful in identifying signals. We also comment briefly on the high energy behavior of the cross section that grows linearly in the center of mass energy squared and argue that it does not necessarily contradict some statements, e.g., the Froissart-Martin bound, achieved in ordinary theory. Received: 29 November 2002 / Revised version: 11 March 2003 / Published online: 5 May 2003 RID="a" ID="a" e-mail: liaoy@itp.uni-leipzig.de RID="b" ID="b" e-mail: dehne@itp.uni-leipzig.de     

Malignant Mimickers: Chronic Bacterial and Fungal Infections of the Larynx

Chronic infections of the larynx are notorious "copycats" of squamous cell carcinoma. Patients typically present with a historical picture and symptoms identical to those seen in a neoplastic setting: dyspnea, hoarseness, odynophagia, weight loss, and a history of tobacco and alcohol abuse. Historically, these patients were subject to an extensive resection for what was in reality a benign disease. A better understanding and awareness of these conditions has reinforced the need for a direct laryngoscopy, biopsy, and culture in the evaluation of long-lasting laryngeal lesions; this has led to more appropriate and focused treatment. The clinical mimicry of chronic laryngeal infections will be illustrated in two recent case reports, histoplasmosis and botryomycosis of the larynx, which will lead into a discussion on the differential diagnosis of bacterial and fungal laryngeal infections, their evaluation, and treatment options.