Depth of interaction and bias voltage depenence of the spectral response in a pixellated CdTe detector operating in time-over-threshold mode subjected to monochromatic X-rays

High stopping power is one of the most important figures of merit for X-ray detectors. CdTe is a promising material but suffers from: material defects, non-ideal charge transport and long range X-ray fluorescence. Those factors reduce the image quality and deteriorate spectral information. In this project we used a monochromatic pencil beam collimated through a 20μm pinhole to measure the detector spectral response in dependance on the depth of interaction. The sensor was a 1mm thick CdTe detector with a pixel pitch of 110μm, bump bonded to a Timepix readout chip operating in Time-Over-Threshold mode. The measurements were carried out at the Extreme Conditions beamline I15 of the Diamond Light Source. The beam was entering the sensor at an angle of \texttildelow20 degrees to the surface and then passed through \texttildelow25 pixels before leaving through the bottom of the sensor. The photon energy was tuned to 77keV giving a variation in the beam intensity of about three orders of magnitude along the beam path. Spectra in Time-over-Threshold (ToT) mode were recorded showing each individual interaction. The bias voltage was varied between -30V and -300V to investigate how the electric field affected the spectral information. For this setup it is worth noticing the large impact of fluorescence. At -300V the photo peak and escape peak are of similar height. For high bias voltages the spectra remains clear throughout the whole depth but for lower voltages as -50V, only the bottom part of the sensor carries spectral information. This is an effect of the low hole mobility and the longer range the electrons have to travel in a low field

Directed percolation with a single defect site

In a recent study [arXiv:1011.3254] the contact process with a modified creation rate at a single site was shown to exhibit a non-universal scaling behavior with exponents varying with the creation rate at the special site. In the present work we argue that the survival probability decays according to a stretched exponential rather than a power law, explaining previous observations.Comment: 8 pages, 3 figure

Absorbing boundaries in the conserved Manna model

The conserved Manna model with a planar absorbing boundary is studied in various space dimensions. We present a heuristic argument that allows one to compute the surface critical exponent in one dimension analytically. Moreover, we discuss the mean field limit that is expected to be valid in d>4 space dimensions and demonstrate how the corresponding partial differential equations can be solved.Comment: 8 pages, 4 figures; v1 was changed by replacing the co-authors name "L\"ubeck" with "Lubeck" (metadata only

Kondo effect in a Luttinger liquid: nonuniversality of the Wilson ratio

Using a precise coset Ising-Bose representation, we show how backscattering of electrons off a magnetic impurity destabilizes the two-channel Kondo fixed point and drives the system to a new fixed point, in agreement with previous results. In addition, we verify the scaling proposed by Furusaki and Nagaosa and prove that the other possible critical fixed point, namely the local Fermi liquid class, is not completely universal when backscattering is included because the Wilson ratio is not well-defined in the spinon basis.Comment: 4 pages, RevTeX; to appear in Physical Review

Magnetic impurities in the one-dimensional spin-orbital model

Using one-dimensional spin-orbital model as a typical example of quantum spin systems with richer symmetries, we study the effect of an isolated impurity on its low energy dynamics in the gapless phase through bosonization and renormalization group methods. In the case of internal impurities, depending on the symmetry, the boundary fixed points can be either an open chain with a residual spin or (and) orbital triplet left behind, or a periodic chain. However, these two fixed points are indistinguishable in the sense that in both cases, the lead-correction-to-scaling boundary operators (LCBO) only show Fermi-liquid like corrections to thermodynamical quantities. (Except the possible Curie-like contributions from the residual moments in the latter cases.) In the case of external (Kondo) impurities, the boundary fixed points, depending on the sign of orbital couplings, can be either an open chain with an isolated orbital doublet due to Kondo screening or it will flow to an intermediate fixed point with the same LCBO as that of the two-channel Kondo problem. Comparison with the Kondo effect in one-dimensional (1D) Heisenberg spin chain and multi-band Hubbard models is also made.Comment: 7 pages, No figur

Boundary contributions to specific heat and susceptibility in the spin-1/2 XXZ chain

Exact low-temperature asymptotic behavior of boundary contribution to specific heat and susceptibility in the one-dimensional spin-1/2 XXZ model with exchange anisotropy 1/2 < \Delta \le 1 is analytically obtained using the Abelian bosonization method. The boundary spin susceptibility is divergent in the low-temperature limit. This singular behavior is caused by the first-order contribution of a bulk leading irrelevant operator to boundary free energy. The result is confirmed by numerical simulations of finite-size systems. The anomalous boundary contributions in the spin isotropic case are universal.Comment: 6 pages, 3 figures; corrected typo

Ungdomar på marginalen : En fallstudie om ungas utanförskap

Abstract: In the past ten years in Sweden, the numbers of people between the ages of 16-19 who were unemployed or not in upper secondary school education have increased. This was assumed to be caused by three factors. Firstly the upper secondary school education reforms in 1992, secondly a substantially reduced labour market for young people without the upper secondary school education and thirdly an increase in demand for work experience. Our purposes was to describe the effects on the outsiders through a document analysis study, but also to understand which individual, social and structural causes that were thought to increase the risk of marginality. Our second purposes were to understand how the Kristianstad community worked to prevent young people of ending up as outsiders. Through interviews with local authorities, we tried to find out if the Kristianstad community lives up to their responsibility of looking after people less than twenty years of age. Our conclusion was that, although the Kristianstad community seems to have a good system, the community has the highest number of unemployed in the county of Skåne. It was obvious that the local authorities lack means to work effective with inactive youths under the age of 20. Sammanfattning: Ungdomar i åldrarna 16-19 år som stod utanför såväl gymnasieskola som arbetsmarknad har ökat i Sverige de senaste tio åren. Detta tros dels bero på gymnasieskolans förlängda utbildningstid och förändringen av yrkesprogrammen i mer teoretisk riktning i och med den stora gymnasiereform som trädde i kraft i Sverige år 1992, men även på en kraftigt försvagad arbetsmarknad för dem utan någon gymnasieutbildning eller arbetslivserfarenhet. Syftet med uppsatsen har dels varit att via dokumentanalyser studera och beskriva effekterna av utanförskap eller marginalisering. Vi har också velat studera vilka individuella, sociala och strukturella orsaker som tros öka riskerna för en marginaliserad position på arbetsmarknaden. Vårt andra syfte med uppsatsen har varit att titta på hur man i Kristianstads kommun arbetar för att motverka att unga i åldrarna 16-19 år riskerar att hamna utanför. Genom intervjuer med tjänstemän som arbetar med denna grupp har vi försökt kartlägga hur och om man lever upp till det kommunala uppföljningsansvaret som åligger kommunerna och regleras i Skollagen (1985:1100). Uppföljningsansvaret innebär att kommunerna har en skyldighet att hålla sig informerad om och att erbjuda alla en plats vid något gymnasieprogram. Resultatet av vår undersökning var att man i Kristianstads kommun tycktes ha ett väl fungerande system för uppföljning, men att man trots det låg i topp vad det gällde antalet arbetslösa ungdomar hösten 2004 i Skåne län. Dessutom framgick det att kommunerna saknade medel för att arbeta effektivt med inaktiva ungdomar under 20 år som saknade gymnasieutbildning

Low-density series expansions for directed percolation II: The square lattice with a wall

A new algorithm for the derivation of low-density expansions has been used to greatly extend the series for moments of the pair-connectedness on the directed square lattice near an impenetrable wall. Analysis of the series yields very accurate estimates for the critical point and exponents. In particular, the estimate for the exponent characterizing the average cluster length near the wall, $\tau_1=1.00014(2)$, appears to exclude the conjecture $\tau_1=1$. The critical point and the exponents $\nu_{\parallel}$ and $\nu_{\perp}$ have the same values as for the bulk problem.Comment: 8 pages, 1 figur

Directed Fixed Energy Sandpile Model

We numerically study the directed version of the fixed energy sandpile. On a closed square lattice, the dynamical evolution of a fixed density of sand grains is studied. The activity of the system shows a continuous phase transition around a critical density. While the deterministic version has the set of nontrivial exponents, the stochastic model is characterized by mean field like exponents.Comment: 5 pages, 6 figures, to be published in Phys. Rev.

Do quantum dots allow one access to pseudogap Kondo physics?

For the last decade, tunable quantum dot systems have allowed the investigation of Kondo physics wherein the quenching of a single spin on an artificial atom affects the conductance. The pseudogap Kondo model featuring a density of states $\rho(\epsilon)$ = C$|\epsilon|^{r}$, introduced by Withoff and Fradkin in 1990 was predicted to exhibit Kondo-like physics above a critical value of the Kondo coupling, J$_c$, which several groups have shown by numerical renormalization group (RG) is finite for r$< {1/2}$. Gonzalez-Buxton {\it{et al}} showed that the strong coupling limit of the particle-hole symmetric model leads to a non-trivial $\frac{\pi (1-r)}{2}$ phase shift at low temperatures indicating incomplete screening of the local moment, while away from particle-hole symmetry one generically flows towards a ground state with $\delta \sim \pi$. We examine the implications of this model for quantum dots whose leads are Fermi-liquid-like, yet possess a tunneling density of states which is suppressed at the Fermi energy as a power law.Comment: 2 pages, 1 figure, poster to be presented at SCES'04 Karlsruhe July 26-3