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Search Results: 1 - 10 of 401352 matches for " M. Hakala "
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Calculation of valence electron momentum densities using the projector augmented-wave method
I. Makkonen,M. Hakala,M. J. Puska
Physics , 2004, DOI: 10.1016/j.jpcs.2005.02.009
Abstract: We present valence electron Compton profiles calculated within the density-functional theory using the all-electron full-potential projector augmented-wave method (PAW). Our results for covalent (Si), metallic (Li, Al) and hydrogen-bonded ((H_2O)_2) systems agree well with experiments and computational results obtained with other band-structure and basis set schemes. The PAW basis set describes the high-momentum Fourier components of the valence wave functions accurately when compared with other basis set schemes and previous all-electron calculations.
Modeling the momentum distributions of annihilating electron-positron pairs in solids
I. Makkonen,M. Hakala,M. J. Puska
Physics , 2005, DOI: 10.1103/PhysRevB.73.035103
Abstract: Measuring the Doppler broadening of the positron annihilation radiation or the angular correlation between the two annihilation gamma quanta reflects the momentum distribution of electrons seen by positrons in the material.Vacancy-type defects in solids localize positrons and the measured spectra are sensitive to the detailed chemical and geometric environments of the defects. However, the measured information is indirect and when using it in defect identification comparisons with theoretically predicted spectra is indispensable. In this article we present a computational scheme for calculating momentum distributions of electron-positron pairs annihilating in solids. Valence electron states and their interaction with ion cores are described using the all-electron projector augmented-wave method, and atomic orbitals are used to describe the core states. We apply our numerical scheme to selected systems and compare three different enhancement (electron-positron correlation) schemes previously used in the calculation of momentum distributions of annihilating electron-positron pairs within the density-functional theory. We show that the use of a state-dependent enhancement scheme leads to better results than a position-dependent enhancement factor in the case of ratios of Doppler spectra between different systems. Further, we demonstrate the applicability of our scheme for studying vacancy-type defects in metals and semiconductors. Especially we study the effect of forces due to a positron localized at a vacancy-type defect on the ionic relaxations.
RX J0806+15: the shortest period binary?
G. Ramsay,P. Hakala,M. Cropper
Physics , 2002, DOI: 10.1046/j.1365-8711.2002.05471.x
Abstract: The X-ray source RX J0806+15 was discovered using ROSAT, and shows an X-ray light curve with a prominent modulation on a period of 321.5 sec. We present optical observations in which we report the detection of its optical counterpart. We find an optical period consistent with the X-ray period. We do not find convincing evidence for a second period in the data: this implies the 321.5 sec period is the orbital period. As such it would be the shortest period stellar binary system yet known. We discuss the nature of this system. We conclude that an isolated neutron star and an intermediate polar interpretation is unlikely and that a double degenerate interpretation is the most likely.
First-Principles Calculations of Positron Annihilation in Solids
B. Barbiellini,M. Hakala,R. M. Nieminen,M. J. Puska
Physics , 1999,
Abstract: We present first-principles approaches based on density functional theory for calculating positron states and annihilation characteristics in condensed matter. The treatment of the electron-positron correlation effects (the enhancement of the electron density at the positron with respect to mean-field density) is shown to play a crucial role when calculating the annihilation rates. A generalized gradient approximation (GGA) takes the strong inhomogeneities of the electron density in the ion core region into account and reproduces well the experimental total annihilation rates (inverses of the positron lifetimes) by suppressing the rates given by a local density approximation (LDA). The GGA combined with an electron-state-dependent enhancement scheme gives a good description for the momentum distributions of the annihilating positron-electron pairs reproducing accurately the trends observed in the angular correlation (ACAR) or Doppler broadening measurements of the annihilation radiation. The combination of the present positron lifetime and momentum density calculations with the corresponding measurements yields a unique tool for defect identification. Especially, the investigation of various vacancy-type defects in semiconductors able to trap positrons will be an important field for these methods. We will show that the identification of vacancy-impurity complexes in highly n-Type Si and the study of the SiO$_2$/Si interface are particularly interesting applications.
Comparison of chain versus sheet crystal structures for cyanides $M$CN ($M$ = Cu-Au) and dicarbides $M$C$_2$ ($M$ = Be-Ba; Zn-Hg). Alternatives to graphene ?
P. Zaleski-Ejgierd,P. Pyykk?,M. Hakala
Physics , 2007, DOI: 10.1103/PhysRevB.76.094104
Abstract: The cyanides $M$CN, $M$=Cu, Ag, Au, have experimentally a structure with hexagonally packed, infinite -$M$-CN-$M$-CN- chains. Following our earlier study for AuCN, we now predict that all three $M$CN could have an alternative $M_3$C$_3$N$_3$ sheet structure of comparable energy with the known one. The valence isoelectronic systems $M$C$_2$ versus $M_3$C$_6$, $M$=Be-Ba; Zn-Hg are also studied. Now, the known dicarbides have the CaC$_2$ or MgC$_2$ chain structures. The predicted sheets lie energetically below the chains for $M$ = Zn, Cd, and Hg. All these systems are experimentally unknown. Indeed, they are clearly endothermic, compared to the elements. For some sheet structures the densities of states suggests rather small band gaps and even metallic character. When available, the experimental geometries agree well with the calculated ones for both cyanides and dicarbides.
Elements of Radio Waves
Frank G. Borg,Ismo Hakala,Jukka M??tt?l?
Physics , 2007,
Abstract: We present a summary of the basic properties of the radio wave generation, propagation and reception, with a special attention to the gigahertz bandwidth region which is of interest for wireless sensor networks. We also present some measurement results which use the so-called RSSI indicator in order to track how the field strength varies with position and distance of the transceivers. We hope the paper may be useful to anyone who looks for a quick review of the fundamentals of electromagnetic theory with application to antennas.
Stream eclipse mapping with 'fire-flies'
C. M. Bridge,Pasi Hakala,Mark Cropper,Gavin Ramsay
Physics , 2003,
Abstract: We apply a new method of eclipse mapping to the light curves of eclipsing polars. The technique aims to locate the bright emission associated with the accretion stream, using a technique that makes the fewest prior assumptions about the location of the accretion stream material. We have obtained data of EP Dra and HU Aqr with the S-Cam 2 superconducting tunnel junction camera using the William Herschel Telescope. The location of emission regions in both systems show that previously assumed trajectories are consistent with those found using our technique. Most of the emission is located in a region where we expect material to be confined to magnetic field lines, particularly for HU Aqr, while there appears to be less emission from where we conventionally expect material to follow a ballistic trajectory from the L1-point.
The origin of the tilted disk in the low mass X-ray binary GR Mus (XB 1254-690)
R. Cornelisse,M. M. Kotze,J. Casares,P. A. Charles,P. J. Hakala
Physics , 2013,
Abstract: We present photometric and spectroscopic observations of the low mass X-ray binary GR Mus (XB 1254-690), and find strong evidence for the presence of a negative superhump with a period that is 2.4+/-0.3% shorter than the orbital. This provides further support that GR Mus indeed harbours a precessing accretion disk (with a period of 6.74+/-0.07 day) that has retrograde precession and is completely tilted out of the orbital plane along its line of nodes. This tilt causes a large fraction of the gas in the accretion stream to either over- or underflow the accretion disk instead of hitting the disk rim, and could be a feature of all low mass X-ray binaries with characteristics similar to GR Mus (i.e. the so-called atoll sources). Furthermore, we also find marginal evidence for the presence of a positive superhump, suggesting that the accretion disk in GR Mus is eccentric due to tidal resonances. If true, than the relationship between the positive superhump period excess and the mass ratio (q) provides a constraint of q=M_donor/M_NS=0.33-0.36. Together with the radial velocity semi-amplitude measurements of the compact object, and previous modeling of the inclination we obtain a mass for the neutron star of 1.2
Orbital modulation of X-ray emission lines in Cygnus X-3
O. Vilhu,P. Hakala,D. C. Hannikainen,M. McCollough,K. Koljonen
Physics , 2009, DOI: 10.1051/0004-6361/200811293
Abstract: We address the problem where the X-ray emission lines are formed and investigate orbital dynamics using Chandra HETG observations, photoionizing calculations and numerical wind-particle simulations.The observed Si XIV (6.185 A) and S XVI (4.733 A) line profiles at four orbital phases were fitted with P Cygni-type profiles consisting of an emission and a blue-shifted absorption component. In the models, the emission originates in the photoionized wind of the WR companion illuminated by a hybrid source: the X-ray radiation of the compact star and the photospheric EUV-radiation from the WR star. The emission component exhibits maximum blue-shift at phase 0.5 (when the compact star is in front), while the velocity of the absorption component is constant (around -900 km/s). The simulated FeXXVI Ly alpha line (1.78 A) from the wind is weak compared to the observed one. We suggest that it originates in the vicinity of the compact star, with a maximum blue shift at phase 0.25 (compact star approaching). By combining the mass function derived with that from the infrared HeI absorption (arising from the WR companion), we constrain the masses and inclination of the system. Both a neutron star at large inclination (over 60 degrees) and a black hole at small inclination are possible solutions.
UV Spectroscopy of AB Doradus with the Hubble Space Telescope. Impulsive flares and bimodal profiles of the CIV 1549 line in a young star
O. Vilhu,P. Muhli,J. Huovelin,P. Hakala,S. M. Rucinski,A. Collier Cameron
Physics , 1997, DOI: 10.1086/300268
Abstract: We observed AB Doradus, a young and active late type star (K0 - K2 IV-V, P= 0.514 d) with the Goddard High Resolution Spectrograph of the post-COSTAR Hubble Space Telescope with the time and spectral resolutions of 27 s and 15 km, respectively. The wavelength band (1531 - 1565 A) included the strong CIV doublet (1548.202 and 1550.774, formed in the transition region at 100 000 K). The mean quiescent CIV flux state was close to the saturated value and 100 times the solar one. The line profile (after removing the rotational and instrumental profiles) is bimodal consisting of two Gaussians, narrow (FWHM = 70 km/s) and broad (FWHM =330km/s). This bimodality is probably due to two separate broadening mechanisms and velocity fields at the coronal base. It is possible that TR transient events (random multiple velocities), with a large surface coverage, give rise to the broadening of the narrow component,while true microflaring is responsible for the broad one. The transition region was observed to flare frequently on different time scales and magnitudes. The largest impulsive flare seen in the CIV 1549 emission reached in less than one minute the peak differential emission measure (10**51.2 cm-3) and returned exponentially in 5 minutes to the 7 times lower quiescent level.The 3 min average line profile of the flare was blue-shifted (-190 km/s) and broadened (FWHM = 800 km/s). This impulsive flare could have been due to a chromospheric heating and subsequent evaporation by an electron beam, accelerated (by reconnection) at the apex of a coronal loop.
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