Eigenmodes and thermodynamics of a Coulomb chain in a harmonic potential

The density of ions trapped in a harmonic potential in one dimension is not uniform. Consequently the eigenmodes are not phonons. We calculate the long wavelength modes in the continuum limit, and evaluate the density of states in the short wavelength limit for chains of $N\gg 1$ ions. Remarkably, the results that are found analytically in the thermodynamic limit provide a good estimate of the spectrum of excitations of small chains down to few tens of ions. The spectra are used to compute the thermodynamic functions of the chain. Deviations from extensivity of the thermodynamic quantities are found. An analytic expression for the critical transverse frequency determining the stability of a linear chain is derived.Comment: 4 pages, 4 figure

Two-photon and EIT-assisted Doppler cooling in a three-level cascade system

Laser cooling is theoretically investigated in a cascade three-level scheme, where the excited state of a laser-driven transition is coupled by a second laser to a top, more stable level, as for alkali-earth atoms. The second laser action modifies the atomic scattering cross section and produces temperatures lower than those reached by Doppler cooling on the lower transition. When multiphoton processes due to the second laser are relevant, an electromagnetic induced transparency modifies the absorption of the first laser, and the final temperature is controlled by the second laser parameters. When the intermediate state is only virtually excited, the dynamics is dominated by the two-photon process and the final temperature is determined by the spontaneous decay rate of the top state.Comment: 5 pages, 3 figures. Revised version, accepted for publication in Phys. Rev A (Rapid Comm.

Physical model of quantum-well infrared photodetectors

A fully quantum mechanical model for electron transport in quantum well infrared photodetectors is presented, based on a self-consistent solution of the coupled rate equations. The important macroscopic parameters like current density, responsivity and capture probability can be estimated directly from this first principles calculation. The applicability of the model was tested by comparison with experimental measurements from a GaAs/AlGaAs device, and good agreement was found. The model is general and can be applied to any other material system or QWIP design

Nuclear Physics for Cultural Heritage

Nuclear physics applications in medicine and energy are well known and widely reported. Less well known are the many important nuclear and related techniques used for the study, characterization, assessment and preservation of cultural heritage. There has been enormous progress in this field in recent years and the current review aims to provide the public with a popular and accessible account of this work. The Nuclear Physics Division of the EPS represents scientists from all branches of nuclear physics across Europe. One of its aims is the dissemination of knowledge about nuclear physics and its applications. This review is led by Division board member Anna Macková, Head of the Tandetron Laboratory at the Nuclear Physics Institute of the Czech Academy of Sciences, and the review committee includes four other members of the nuclear physics board interested in this area: Faiçal Azaiez, Johan Nyberg, Eli Piasetzky and Douglas MacGregor. To create a truly authoritative account, the Scientific Editors have invited contributions from leading experts across Europe, and this publication is the combined result of their work. The review is extensively illustrated with important discoveries and examples from archaeology, pre-history, history, geography, culture, religion and curation, which underline the breadth and importance of this field. The large number of groups and laboratories working in the study and preservation of cultural heritage across Europe indicate the enormous effort and importance attached by society to this activity

A physical model of quantum cascade lasers: Application to GaAs, GaN and SiGe devices

The philosophy behind this work has been to build a predictive bottom up physical model of quantum cascade lasers (QCLs) for use as a design tool, to interpret experimental results and hence improve understanding of the physical processes occurring inside working devices and as a simulator for developing new material systems. The standard model uses the envelope function and effective mass approximations to solve two complete periods of the QCL under an applied bias. Other models, such as k·p and empirical pseudopotential, have been employed in p-type systems where the more complex band structure requires it. The resulting wave functions are then used to evaluate all relevant carrier-phonon, carrier-carrier and alloy scattering rates from each quantised state to all others within the same and the neighbouring period. This information is then used to construct a rate equation for the equilibrium carrier density in each subband and this set of coupled rate equations are solved self-consistently to obtain the carrier density in each eigenstate. The latter is a fundamental description of the device and can be used to calculate the current density and gain as a function of the applied bias and temperature, which in turn yields the threshold current and expected temperature dependence of the device characteristics. A recent extension which includes a further iteration of an energy balance equation also yields the average electron (or hole) temperature over the subbands. This paper will review the method and describe its application to mid-infrared and terahertz, GaAs, GaN, SiGe cascade laser designs

STABILITY OF Z-STRINGS IN STRONG MAGNETIC FIELDS

We show that the Z-strings of the standard electroweak theory can be stabilized by strong external magnetic fields, provided that $\beta^{1/2} \equiv M_H/M_Z\leq 1$, where $M_H$ and $M_Z$ are the Higgs and Z masses. The magnetic fields needed are larger than $\beta^{1/2} B_c$ and smaller than $B_c$, where $B_c\equiv M^2_W/e$ is the critical magnetic field which causes W-condensation in the usual broken phase vacuum. If such magnetic fields were present after the electroweak transition, they would stabilize strings for a period comparable to the inverse Hubble rate at that time. Pair creation of monopoles and antimonopoles linked by segments of string is briefly considered.Comment: 12 pages, 3 uuencoded figs ,LaTeX (RevTeX). A postcript version can be obtained via anonymous ftp at ftp://ftp.ifae.es/preprint.f

Drift-diffusion model for single layer transition metal dichalcogenide field-effect transistors

A physics-based model for the surface potential and drain current for monolayer transition metal dichalcogenide (TMD) field-effect transistor (FET) is presented. Taking into account the 2D density-of-states of the atomic layer thick TMD and its impact on the quantum capacitance, a model for the surface potential is presented. Next, considering a drift-diffusion mechanism for the carrier transport along the monolayer TMD, an explicit expression for the drain current has been derived. The model has been benchmarked with a measured prototype transistor. Based on the proposed model, the device design window targeting low-power applications is discussed.Comment: 10 pages, 3 figure

Comment on "Do Earthquakes Exhibit Self-Organized Criticality?"

It is shown that earthquakes do not know how large they will become, at least from the information collected at seismic catalogs. In other words, the magnitude is independent on previous magnitudes as well as on the waiting time between previous earthquakes. In contrast, the time to the next event does depend on the magnitude. Also it is argued that SOC systems do not necessarily shown a Poisson-type behavior in time, and SOC does not exclude the possibility of some degree of prediction.Comment: Tentative comment to Yang, Du, Ma, PRL 92, 228501 (2004

Universal Earthquake-Occurrence Jumps, Correlations with Time, and Anomalous Diffusion

Spatiotemporal properties of seismicity are investigated for a worldwide (WW) catalog and for Southern California in the stationary case (SC), showing a nearly universal scaling behavior. Distributions of distances between consecutive earthquakes (jumps) are magnitude independent and show two power-law regimes, separated by jump values about 200 km (WW) and 15 km (SC). Distributions of waiting times conditioned to the value of jumps show that both variables are correlated in general, but turn out to be independent when only short or long jumps are considered. Finally, diffusion profiles reflect the shape of the jump distribution.Comment: Short pape

Suppression of Bragg scattering by collective interference of spatially ordered atoms with a high-Q cavity mode

When N driven atoms emit in phase into a high-Q cavity mode, the intracavity field generated by collective scattering interferes destructively with the pump driving the atoms. Hence atomic fluorescence is suppressed and cavity loss becomes the dominant decay channel for the whole ensemble. Microscopically 3D light-intensity minima are formed in the vicinity of the atoms that prevent atomic excitation and form a regular lattice. The effect gets more pronounced for large atom numbers, when the sum of the atomic decay rates exceeds the rate of cavity losses and one would expect the opposite behaviour. These results provide new insight into recent experiments on collective atomic dynamics in cavities.Comment: 4 pages, 5 figure