arXiv subjects

H. Pettersson

Publications and source records attributed to H. Pettersson.

14 recordsLinked to original sources

arXiv2015

Enhanced sputtering and incorporation of Mn in implanted GaAs and ZnO nanowires

We simulated and experimentally investigated the sputter yield of ZnO and GaAs nanowires, which were implanted with energetic Mn ions at room temperature. The resulting thinning of the nanowires and the dopant concentration with increasing Mn ion fluency were measured by accurate scanning electron microscopy (SEM) and nano-X-Ray Fluorescence (nanoXRF) quantification, respectively. We observed a clear enhanced sputter yield for the irradiated nanowires compared to bulk, which is also corroborated by iradina simulations. These show a maximum if the ion range matches the nanowire diameter. As a consequence of the erosion thinning of the nanowire, the incorporation of the Mn dopants is also enhanced and increases non-linearly with increasing ion fluency.

cond-mat.mtrl-sci↗

arXiv2009

Double-Pionic Fusion of Nuclear Systems and the ABCEffect -- Aproaching a Puzzle by Exclusive and Kinematically Complete Measurements

The ABC effect - a puzzling low-mass enhancement in the $ππ$ invariant mass spectrum - is well-known from inclusive measurements of two-pion production in nuclear fusion reactions. Here we report on first exclusive and kinematically complete measurements of the most basic double pionic fusion reaction $pn \to d π^0π^0$ at 1.03 and 1.35 GeV. The measurements, which have been carried out at CELSIUS-WASA, reveal the ABC effect to be a $(ππ)_{I=L=0}$ channel phenomenon associated with both a resonance-like energy dependence in the integral cross section and the formation of a $ΔΔ$ system in the intermediate state. A corresponding simple s-channel resonance ansatz provides a surprisingly good description of the data.

nucl-ex↗

arXiv2008

Polarisation of the omega meson in the pd-->3He+omega reaction at 1360 and 1450 MeV

The tensor polarisation of omega mesons produced in the pd-->3He+omega reaction has been studied at two energies near threshold. The 3He nuclei were detected in coincidence with the pi0pi+pi- or pi0gamma decay products of the omega. In contrast to the case of phi meson production, the omega mesons are found to be unpolarised. This brings into question the applicability of the Okubo-Zweig-Iizuka rule when comparing the production of vector mesons in low energy hadronic reactions.

nucl-ex↗

arXiv2008

The WASA Detector Facility at CELSIUS

The WASA 4pi multidetector system, aimed at investigating light meson production in light ion collisions and eta meson rare decays at the CELSIUS storage ring in Uppsala is presented. A detailed description of the design, together with the anticipated and achieved performance parameters are given.

nucl-ex↗

arXiv2008

Measurement of eta meson decays into lepton-antilepton pairs

A search for rare lepton decays of the eta meson was performed using the WASA detector at CELSIUS. Two candidates for double Dalitz decay eta->e+e-e+e- events are reported with a background of 1.3+/-0.2 events. This allows to set an upper limit to the branching ratio of 9.7E-5 (90% CL). The branching ratio for the decay eta->e+e-gamma is determined to (7.8+/-0.5 stat+/-0.8 syst)E-3. An upper limit (90% CL) for the branching ratio for the eta->e+e- decay is 2.7E-5 and a limit for the sum of the eta->mu+mu-mu+mu- and eta->pi+pi-mu+mu- decays is 3.6E-4.

hep-ex↗

arXiv2008

Tunneling Anisotropic Magnetoresistance in Co/AlOx/Au Tunnel Junctions

We observe spin-valve-like effects in nano-scaled thermally evaporated Co/AlOx/Au tunnel junctions. The tunneling magnetoresistance is anisotropic and depends on the relative orientation of the magnetization direction of the Co electrode with respect to the current direction. We attribute this effect to a two-step magnetization reversal and an anisotropic density of states resulting from spin-orbit interaction. The results of this study points to future applications of novel spintronics devices involving only one ferromagnetic layer.

physics.pop-ph↗

arXiv2007

Measurement of the Slope Parameter for the eta->3pi0 Decay in the pp->pp eta Reaction

The CELSIUS/WASA setup is used to measure the 3pi0 decay of eta mesons produced in pp interactions with beam kinetic energies of 1.36 and 1.45 GeV. The efficiency-corrected Dalitz plot and density distributions for this decay are shown, together with a fit of the quadratic slope parameter alpha yielding alpha = -0.026 +/- 0.010(stat) +/- 0.010(syst). This value is compared to recent experimental results and theoretical predictions.

nucl-ex↗

arXiv2007

Large Magnetoresistance in Co/Ni/Co Ferromagnetic Single Electron Transistors

We report on magnetotransport investigations of nano-scaled ferromagnetic Co/Ni/Co single electron transistors. As a result of reduced size, the devices exhibit single electron transistor characteristics at 4.2K. Magnetotransport measurements carried out at 1.8K reveal tunneling magnetoresistance (TMR) traces with negative coercive fields, which we interpret in terms of a switching mechanism driven by the shape anisotropy of the central wire-like Ni island. A large TMR of about 18% is observed within a finite source-drain bias regime. The TMR decreases rapidly with increasing bias, which we tentatively attribute to excitation of magnons in the central island.

cond-mat.other↗

arXiv2007

The pp -> pp pi pi pi reaction channels in the threshold region

The cross section for prompt neutral and charged three pion production in pp interactions was measured at excess energies in the range 160 - 217 MeV. That comprises the first measurement of the pp->pp pi0pi0pi0 reaction and the comparison with the pp->pp pi+pi-pi0 reaction, in a very direct way. The experiment was performed above the eta meson production threshold and the cross section normalization was obtained from a concurrent measurement of the reaction pp->pp eta with the eta decaying into 3 pions. Since the same final states are selected, the measurement has a low systematical error. The measured cross section ratio sigma(pp->pp pi+pi-pi0)/sigma(pp->pp pi0\pi0\pi0) is compared to predictions of dominance of different isobars in the intermediate state.

nucl-ex↗

arXiv2006

Probing Spin Accumulation in Ni/Au/Ni Single-Electron Transistors with Efficient Spin Injection and Detection Electrodes

We have investigated spin accumulation in Ni/Au/Ni single-electron transistors assembled by atomic force microscopy. The fabrication technique is unique in that unconventional hybrid devices can be realized with unprecedented control, including real-time tunable tunnel resistances. A grid of Au discs, 30 nm in diameter and 30 nm thick, is prepared on a SiO2 surface by conventional e-beam writing. Subsequently, 30 nm thick ferromagnetic Ni source, drain and side-gate electrodes are formed in similar process steps. The width and length of the source and drain electrodes were different to exhibit different coercive switching fields. Tunnel barriers of NiO are realized by sequential Ar and O2 plasma treatment. Using an atomic force microscope with specially designed software, a single non-magnetic Au nanodisc is positioned into the 25 nm gap between the source and drain electrodes. The resistance of the device is monitored in real-time while the Au disc is manipulated step-by-step with Angstrom-level precision. Transport measurements in magnetic field at 1.7 K reveal no clear spin accumulation in the device, which can be attributed to fast spin relaxation in the Au disc. From numerical simulations using the rate-equation approach of orthodox Coulomb blockade theory, we can put an upper bound of a few ns on the spin-relaxation time for electrons in the Au disc. To confirm the magnetic switching characteristics and spin injection efficiency of the Ni electrodes, we fabricated a test structure consisting of a Ni/NiO/Ni magnetic tunnel junction with asymmetric dimensions of the electrodes similar to those of the SETs. Magnetoresistance measurements on the test device exhibited clear signs of magnetic reversal and a maximum TMR of 10%, from which we deduced a spin-polarization of about 22% in the Ni electrodes.

cond-mat.other↗

arXiv2006

Evidence for a 'Narrow' Roper Resonance - the Breathing Mode of the Nucleon

All the time since its discovery the N$^*$(1440) baryon state, commonly known as Roper resonance, has been a state with many question marks - despite of its 4-star ranking in the particle data book. One reason is that it does not produce any explicit resonance-like structures in the observables of $π$N or $γ$N reactions. Only in partial wave analyses of $π$N scattering data a clear resonance strcuture gets obvious in the $P_{11}$ partial wave. Very recent measurements of the J/$Ψ$ decay by the BES collaboration and of the $pp \to npπ^+$ reaction at 1.3 GeV by the CELSIUS-WASA collaboration show for the first time a clear resonance structure in the invariant $nπ^+$ mass spectrum for the Roper resonance at M $\approx$ 1360 MeV with a width of about 150 MeV. These values agree very favorably with the pole position results of recent $π$N phase shift analyses. In consequence of this very low-lying pole postion, which is roughly 100 MeV below the nominal value, the decay properties have to be reinvestigated. From our two-pion production data we see that the decay mainly proceeds via N$^* \to $N$σ$, i.e. a monopole transition as expected for the breathing mode of the nucleon.

nucl-ex↗

arXiv2006

Two-Pion Production in Nucleon-Nucleon Collisions and the ABC-Effect - Approaching a Puzzleby exclusive and Kinematically complete Measurements

The ABC effect - a puzzling low-mass enhancement in the $ππ$ invariant mass spectrum - is known from inclusive measurements of two-pion production in nuclear collisions, where it always showed up, if the participating nucleons fused to a bound nuclear system in the final state. The first exclusive measurements on the ABC effect have been carried out very recently at CELSIUS-WASA for the fusion reactions leading to d, $^3$He and $^4$He nuclei in the final state. The data analyzed so far for the fusion processes to d and $^3$He reveal this effect to be a $σ$ channel phenomenon associated with the formation of a strongly attractive $ΔΔ$ system. The data for the strictly isospin-selective double-pionic fusion to $^4$He, where we expect the largest effect, are currently still analyzed. All inclusive data on this system are well described by our model, too. This case also constitutes the heaviest nuclear system, where exclusive measurements of double-pionic fusion can be carried out with present-day instruments. Surprisingly, the $pp \to ppπ^0π^0$ reaction in the $ΔΔ$ region is observed to also show a ABC-like low-$ππ$ mass enhancement, a phenomenon, which deserves special attention.

nucl-ex↗

arXiv2006

Measurement of the eta->pi+pi-e+e- decay branching ratio

The reaction pd->3He eta at threshold was used to provide a clean source of eta mesons for decay studies with the WASA detector at CELSIUS. The branching ratio of the decay eta->pi+pi-e+e- is measured to be (4.3+/-1.3+/-0.4)x10^-4.

hep-ex↗

arXiv2006

Anisotropy in the pion angular distribution of the reaction pp -> pp pi0 at 400 MeV

The reaction pp -> pp pi0 was studied with the WASA detector at the CELSIUS storage ring. The center of mass angular distribution of the pi0 was obtained by detection of the gamma decay products together with the two outgoing protons, and found to be anisotropic with a negative second derivative slope, in agreement with the theoretical predictions from a microscopic calculation.

nucl-ex↗