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Sökning: L773:0921 4526 > (2000-2004)

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1.
  • Fang, AP, et al. (författare)
  • Theoretical study of electromagnetically induced transparency in Er3+: YAlO3 crystal
  • 2003
  • Ingår i: Physica B: Condensed Matter. - 0921-4526. ; 328:3-4, s. 204-210
  • Tidskriftsartikel (refereegranskat)abstract
    • By using the density matrix theory of interaction between light and matter, and relevant parameter calculations of the relaxation rate, the dipole matrix elements and the ion density for a three-level ladder model, we have discussed theoretically the possibility to realize electromagnetically induced transparency (EIT) in Er3+:YA1O(3) crystal. (C) 2002 Elsevier Science B.V. All rights reserved.
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2.
  • Koblischka, M. R., et al. (författare)
  • Paramagnetic Meissner response of an artificially granular YBCO thin film
  • 2000
  • Ingår i: Physica B: Condensed Matter. - 0921-4526. ; 284-288, s. 599-600
  • Konferensbidrag (refereegranskat)abstract
    • Field-cooling measurements on an YBa2Cu3Oy thin film, patterned into a hexagonal close-packed lattice of disks with a diameter of 50 m reveal a paramagnetic Meissner response (PME) when cooled in fields below 1 mT. In contrast to previous investigations on Nb and melt-cast Bi-2212, the PME in this artificial granular film is attributed to field trapping in the space between the disks
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3.
  • Lipinska-Kalita, KE, et al. (författare)
  • High-pressure studies of titanium pyrophosphate by Raman scattering and infrared spectroscopy
  • 2003
  • Ingår i: Physica B: Condensed Matter. - 0921-4526. ; 337:1-4, s. 221-229
  • Tidskriftsartikel (refereegranskat)abstract
    • High-pressure investigations of titanium pyrophosphate, TiP2O7, in diamond anvil cell have been performed at room temperature using in situ Raman scattering and Fourier transform infrared spectroscopy (FTIR). The endeavor was to acquire information on pressure-induced structural transformations such as phase transitions and amorphization occurring in the crystal lattice. The pressure-stimulated alterations in the spectral profile, the position, and the intensity of the stretching and bending modes Of PO4 tetrahedral structural units have been investigated up to 42.8 and 49.4 GPa for Raman and infrared-active modes, respectively. The spectral changes pointed mostly to the densification and partial amorphization of the crystal lattice. FTIR spectra confirmed that the investigated compound, TiP2O7, compressed smoothly up to the highest investigated pressures. The spectroscopic studies did not indicate an unambiguous structural transformation matching to a pressure-driven phase transition. The reversibility to ambient pressure structure upon decompression was implied by FTIR but was not confirmed by Raman spectroscopy. The mode Gruneisen parameters were calculated for the various Raman and infrared-active vibrational modes. The results obtained are consistent with our previous high-pressure synchrotron radiation-based X-ray diffraction investigations. (C) 2003 Elsevier B.V. All rights reserved.
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4.
  • Agren, P., et al. (författare)
  • Hysteretic current-voltage characteristics and Coulomb blockade in 1D-arrays of Josephson junctions
  • 2000
  • Ingår i: Physica. B, Condensed matter. - 0921-4526 .- 1873-2135. ; 280:04-jan, s. 414-415
  • Tidskriftsartikel (refereegranskat)abstract
    • The IV characteristics (IVC) of 1D-arrays of small capacitance Josephson junctions with E-C similar to E-J have been measured. The IVC show Coulomb blockade of Cooper pair tunneling and exhibit a pronounced hysteresis which appears to be dual to the well-known resistively shunted junction behavior of ordinary Josephson junctions. A dual serially resistive junction model is used to qualitatively explain the measured data.
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5.
  • Andersen, O., et al. (författare)
  • Piezospectroscopic analysis of the hydrogen-carbon complexes in silicon
  • 2001
  • Ingår i: Physica. B, Condensed matter. - 0921-4526 .- 1873-2135. ; 308, s. 139-142
  • Tidskriftsartikel (refereegranskat)abstract
    • We have observed the donor (Ec-0.22 eV) and acceptor (Ec-0.16 eV) levels related to hydrogen-carbon complexes in silicon. The donor level is only detected at low temperatures after proton implantation. This hydrogen-carbon complex irreversibly reconfigures at temperatures above 225 K to a configuration characterized by the acceptor level, which is stable up to room temperature. The same acceptor level is also observed after atomic hydrogen diffusion. We have used Laplace transform deep level transient spectroscopy (DLTS) to show the influence of uniaxial stress on the electron emission process and the effect of the stress-induced alignment for the acceptor state. The pattern of the Laplace DLTS peak splittings indicate a trigonal symmetry of the defect. First principles calculations were carried out on the hydrogen-carbon defects with a view of determining their electrical levels and stress response for comparison with the experimental results.
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6.
  • Andersson, K., et al. (författare)
  • Synchronous Cooper pair tunneling in a 1D-array of Josephson junctions
  • 2000
  • Ingår i: Physica. B, Condensed matter. - 0921-4526 .- 1873-2135. ; 284, s. 1816-1817
  • Tidskriftsartikel (refereegranskat)abstract
    • We have studied 1D-arrays of Josephson junctions with a gate capacitively coupled to the middle of the array. The SQUID shape of each junction enables tuning of the Josephson coupling energy, El. By adding a signal to the gate with a radio-frequency f we create a step in the I-V characteristics at the value I = 2ef: The step is clearly visible as a peak in the differential resistance dI/dV and can be seen in the whole frequency range studied (10-90 MHz).
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7.
  • Axnas, J., et al. (författare)
  • Magnetoconductivity of polycrystalline Hg,Tl-1223
  • 2000
  • Ingår i: Physica. B, Condensed matter. - 0921-4526 .- 1873-2135. ; 284, s. 1009-1010
  • Tidskriftsartikel (refereegranskat)abstract
    • The magnetoconductivity of polycrystalline Hg1-xTlxBa2Ca2Cu3O8+delta With x = 0 and 0.2 has been measured above T-c and analysed in terms of superconducting fluctuations. Such studies are numerous and successful for Y- and Bi-based materials, but still rare for Hg- and Tl-based materials. Results for the coherence lengths and the role of the Maki-Thompson contribution are briefly discussed.
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8.
  • Bergsten, T., et al. (författare)
  • Hall resistance in two-dimensional arrays of Josephson junctions
  • 2000
  • Ingår i: Physica. B, Condensed matter. - 0921-4526 .- 1873-2135. ; 284, s. 1818-1819
  • Tidskriftsartikel (refereegranskat)abstract
    • We have measured the Hall resistance in two-dimensional arrays of ultrasmall aluminium Josephson junctions. We found that the Hall resistance was periodical with respect to an external magnetic field applied perpendicular to the plane of the array. We also found that the Hall resistance was affected by an applied voltage to a nearby gate electrode, but not by a gate plane silting underneath the array.
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9.
  • Buyanova, Irina A., et al. (författare)
  • Strain relaxation in GaNxP1-x alloy : Effect on optical properties
  • 2001
  • Ingår i: Physica. B, Condensed matter. - : Elsevier. - 0921-4526 .- 1873-2135. ; 308-310, s. 106-109
  • Tidskriftsartikel (refereegranskat)abstract
    • By using scanning electron microscopy and cathodoluminescence (CL), a decrease in radiative efficiency of GaNP alloy with increasing N content is seen due to the formation of structural defects. The defect formation is attributed to relaxation of tensile strain in the GaNP layer, which is lattice mismatched to GaP substrate. Several types of extended defects including dislocations, microcracks and pits are revealed in partly relaxed GaNxP1-x epilayers with x=1.9%, whereas coherently strained layers exhibit high crystalline quality for x up to 4%. According to the CL measurements, all extended defects act as competing, non-radiative channels leading to the observed strong decrease in the radiative efficiency. From CL mapping experiments, non-uniformity of strain distribution around the extended defects is partly responsible for the broadening of the photoluminescence (PL) spectra recorded in the macro-PL experiments. © 2001 Elsevier Science B.V. All rights reserved.
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10.
  • Coutinho, J., et al. (författare)
  • Over-coordinated oxygen in the interstitial carbon-oxygen complex
  • 2001
  • Ingår i: Physica. B, Condensed matter. - 0921-4526 .- 1873-2135. ; 308, s. 305-308
  • Tidskriftsartikel (refereegranskat)abstract
    • The interstitial carbon-oxygen complex is one of the most prominent defects formed in e-irradiated Cz-Si containing carbon. Stress alignment investigations have shown that the oxygen atom only perturbs the carbon interstitial but the lack of a high frequency oxygen mode has been taken to imply that the oxygen atom is over-coordinated. Local vibrational mode spectroscopy and ab initio modeling are used to investigate the defect. We find new modes whose oxygen isotopic shifts, along with the piezoscopic stress-energy tensor support the trivalent model, thus providing evidence for oxygen over-coordination.
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11.
  • Eberlein, T.A.G., et al. (författare)
  • Self-interstitial clusters in silicon
  • 2001
  • Ingår i: Physica. B, Condensed matter. - 0921-4526 .- 1873-2135. ; 308-310, s. 454-457
  • Tidskriftsartikel (refereegranskat)abstract
    • Although there have been made many calculations for structures of the self-interstitial in Si and small aggregates of interstitials, In, there have been relatively few attempts to relate these defects with experimental data. Here, we discuss the assignments of the self-interstitial to the AA12 EPR centre and the di-interstitial to the P6 EPR centre.
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12.
  • Festin, Örjan, et al. (författare)
  • Zero-field vortex dynamics in YBCO thin films
  • 2000
  • Ingår i: Physica. B, Condensed matter. - 0921-4526 .- 1873-2135. ; 284-288:Pt. 1, s. 963-964
  • Tidskriftsartikel (refereegranskat)abstract
    • The dynamics of spontaneously generated vortices in epitaxial YBCO films have been investigated in the superconducting transition region by means of flux noise measurements. The investigated samples differ with respect to film thickness and sample quality and the influence of these parameters on the flux noise spectra is discussed.
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13.
  • Geller, Michael, et al. (författare)
  • Theory of electron–phonon dynamics in insulating nanoparticles
  • 2002
  • Ingår i: Physica. B, Condensed matter. - : Elsevier. - 0921-4526 .- 1873-2135. ; 316-317, s. 430-433
  • Tidskriftsartikel (refereegranskat)abstract
    • We discuss the rich vibrational dynamics of nanometer-scale semiconducting and insulating crystals as probed by localized electronic impurity states, with an emphasis on nanoparticles that are only weakly coupled to their environment. Two principal regimes of electron--phonon dynamics are distinguished, and a brief survey of vibrational-mode broadening mechanisms is presented. Recent work on the effects of mechanical interaction with the environment is discussed.
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14.
  • Ghiringhelli, G., et al. (författare)
  • Probing the singlet character of the two-hole states in cuprate superconductors
  • 2002
  • Ingår i: Physica. B, Condensed matter. - 0921-4526 .- 1873-2135. ; 312, s. 34-35
  • Tidskriftsartikel (refereegranskat)abstract
    • Using spin-resolved resonant photoemission we have probed the singlet vs. triplet character of the two-hole state in the layered cuprates Bi2Sr2CaCu2O8+delta La2-xSrxCuO4 and Sr2CuO2Cl2. The combination of the photon circular polarization with the photoelectron spin detection gives access to the character of the photoemission final states, which correspond to the two-hole configurations localized at a (CuO4) site. In particular, the lowest energy state is found to have a very high singlet character in all the measured compounds. This can be considered as a strong indication of the existence and stability of the so-called Zhang-Rice singlets in the layered cuprates.
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15.
  • Godlewski, M., et al. (författare)
  • In-plane and in-depth nonuniformities in defect distribution in GaN and InGaN epilayers
  • 2001
  • Ingår i: Physica. B, Condensed matter. - 0921-4526 .- 1873-2135. ; 308-310, s. 102-105
  • Tidskriftsartikel (refereegranskat)abstract
    • The in-plane and in-depth characteristics of the GaN and InGaN epilayers grown by the metalorganic chemical vapour deposition (MOCVD) on three different substrates (sapphire, SiC and bulk GaN) are evaluated. Relatively large intensity fluctuations of "edge" GaN and InGaN emissions are observed and are related to the details of the micro-structure of the GaN and InGaN films studied. The experiments indicate a nonuniform defect distribution in all types of the MOCVD films studied. In particular, the decoration of structural defects with impurities, an increased defect accumulation at the interfaces and a surprisingly small influence of the micro-structure on the in-plane homogeneity of the yellow band cathodoluminescence emission are observed. © 2001 Elsevier Science B.V. All rights reserved.
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16.
  • Haviland, David B., et al. (författare)
  • Quantum-phase transition in 1D Josephson junction arrays
  • 2000
  • Ingår i: Physica. B, Condensed matter. - 0921-4526 .- 1873-2135. ; 284, s. 1808-1809
  • Tidskriftsartikel (refereegranskat)abstract
    • One-dimensional arrays of small capacitance SQUIDs have been studied experimentally. The effective Josephson coupling between neighboring electrodes is tunable in situ. The arrays can be tuned from a Josephson-like state, with low resistance and sharp critical current, to a Coulomb blockade state, with infinite resistance and sharp threshold voltage. A quantum phase transition occurs at the crossover between these two types of behavior, which is evident from an analysis of the temperature dependence of the zero bias resistance.
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17.
  • Hermansson, J., et al. (författare)
  • Complexes of the self-interstitial with oxygen in irradiated silicon : A new assignment of the 936 cm-1 band
  • 2001
  • Ingår i: Physica. B, Condensed matter. - 0921-4526 .- 1873-2135. ; 302-303, s. 188-192
  • Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
    • Three vibrational infrared absorption bands at about 936, 944 and 956 cm-1 appear commonly in spectra of Czochralski-grown silicon irradiated at low temperatures. All three bands have earlier been assigned to local vibrational modes related to oxygen in the complex of the silicon and the oxygen interstitials (IOi). However, it is shown that such an assignment of the 936 cm-1 band clearly is inconsistent with many facts and observations and that the band is most likely due to oxygen vibrations in the Si interstitial pair and interstitial oxygen complex, I2Oi. © 2001 Published by Elsevier Science B.V.
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18.
  • Hong, V. V., et al. (författare)
  • Nitrogenation studies of Nd2Fe17 alloys
  • 2003
  • Ingår i: Physica. B, Condensed matter. - 0921-4526 .- 1873-2135. ; 327:04-feb, s. 404-409
  • Tidskriftsartikel (refereegranskat)abstract
    • In the present investigation, the kinetics of nitrogenation of Nd2Fe17 alloys were studied in order to optimize the magnetic properties of the pure Nd2Fe17 alloy by the introduction of nitrogen interstitials. The nitrogenation kinetics was followed in the temperature interval 1173-1473K by the use of the thermogravimetric (TG) technique. The nitrogenated alloys were prepared from a Nd2Fe17 master alloy by heat treating in different partial pressures of nitrogen under well-controlled conditions. The results of the TG experiments showed that there was an initial incubation time before the onset of nitrogenation. The length of the incubation period was found to be a function of temperature and the partial pressure of nitrogen in the gas. The nitrogen used was of high purity (99.9999% N-2). The gas purification system adopted in the present work could effectively remove traces of reducing as well as oxidizing impurity gases in the system. In order to remove the traces of CO, CH4 and H-2, (as well as other hydrocarbons) if present, the gas was passed through a column of Cu turnings kept at 973 K. The lower portion of the Cu column was surface oxidized. The reducing gases got oxidized passing through this column and CO2 and H2O were formed. Further, the oxygen impurity in the gas (ca. 10(-2) Pa) could react with the remaining reducing gases. These products were absorbed by columns of ascarite and dehydrite, respectively. Remaining oxygen impurity in the gas could then be removed by passing the gas through a furnace containing Mg turnings at 773 K. The rate of nitrogen uptake after this period was found to increase with increasing temperature. After an initial rapid nitrogenation period, the reaction rate was found to slow down, possibly due to nitrogen atoms diffusing into the bulk. The reaction was found to be rapid, and the incubation time was conspicuously absent, when nitrogen gas was used without the purification steps mentioned earlier. Oxygen impurity in the gas was found to have a strong influence on the reaction kinetics. Heat capacity measurements, as well as magnetic hysteresis measurements, were also conducted. The heat capacity of the Nd2Fe17 master alloy, as well as that of the nitrogenated alloys, Was determined in the temperature interval 293-773 K by the use of the differential scanning calorimetric technique. From the experimental heat-capacity data the Curie temperatures of the alloys were derived. Compared to the master alloy, significantly increased Curie temperatures were obtained for the nitrogenated alloys. The magnetic measurements were conducted at 300 K by the use of a quantum design magnetometer. The results obtained indicate that the nitrogenated alloys were soft magnets as no large hysteresis loop was found to exist.
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19.
  • Hourahine, B., et al. (författare)
  • Evidence for H2* trapped by carbon impurities in silicon
  • 2001
  • Ingår i: Physica. B, Condensed matter. - 0921-4526 .- 1873-2135. ; 308, s. 197-201
  • Tidskriftsartikel (refereegranskat)abstract
    • Local mode spectroscopy and ab initio modelling are used to investigate two trigonal defects found in carbon-rich Si into which H had been in-diffused. Isotopic shifts with D and 13C are reported along with the effect of uniaxial stress. Ab initio modelling studies suggest that the two defects are two forms of the CH2* complex where one of the two hydrogen atoms lies at an anti-bonding site attached to C or Si, respectively. The two structures are nearly degenerate and possess vibrational modes in good agreement with those observed.
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20.
  • Johannson, J., et al. (författare)
  • Coulomb blockade in anodised titanium nanostructures
  • 2000
  • Ingår i: Physica. B, Condensed matter. - 0921-4526 .- 1873-2135. ; 284, s. 1796-1797
  • Tidskriftsartikel (refereegranskat)abstract
    • We investigate the electrical transport properties of narrow titanium (Ti) wires which are thinned by anodic oxidation. At temperatures below 1 K the current-voltage (IV) characteristics of the resulting Ti/TiOx nanostructures exhibit a zero current state due to the occurrence of Coulomb blockade. An indium gate electrode is placed on top of the anodised region, and the modulation of the conductance as a function of the gate voltage is investigated.
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21.
  • Jones, R., et al. (författare)
  • Thermal double donors in Si and Ge
  • 2001
  • Ingår i: Physica. B, Condensed matter. - 0921-4526 .- 1873-2135. ; 308, s. 8-12
  • Tidskriftsartikel (refereegranskat)abstract
    • Three experiments appear to cast doubt on self-interstitial-free models of the family of thermal donors, based on oxygen defects of increasing size. These are (a) the rapid transformation of TDD(N) into TDD(N+1) with activation energies considerably lower than that of oxygen diffusion, (b) the lack of any appreciable spin-density on oxygen in TDD(N)+, and (c) the observation of only two oxygen related vibrational modes associated with each donor. However, we show that the oxygen-only model of the donors is compatible with experiment for a structure involving an insulating core with normal oxygen coordination, surrounded by over-coordinated oxygen atoms which are responsible for the donor activity. It is also shown that the calculated stress-energy tensors for the early donors are in good agreement with the measurements.
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22.
  • Lindstrom, J.L., et al. (författare)
  • Interaction between self-interstitials and the oxygen dimer in silicon
  • 2001
  • Ingår i: Physica. B, Condensed matter. - 0921-4526 .- 1873-2135. ; 308-310, s. 284-289
  • Tidskriftsartikel (refereegranskat)abstract
    • Interactions between the oxygen dimer (O2i) and silicon self-interstitials (I) and vacancies (V) have been studied in Czochralski-grown silicon (Cz-Si) crystals using infrared absorption and deep level transient spectroscopies. The focus in this report is on reactions of O2i with I. The first step in this interaction is found to be the formation of a self-interstitial-dioxygen centre (IO2i) with oxygen-related local vibrational mode (LVM) bands at 922 and 1037 cm-1. During the second formation step, another centre, I2O2i, with LVM bands at 918 and 1034 cm-1 is suggested to appear. A Si-related band at about 545 cm-1 is also assigned to both the IO2i and I2O2i centres. The IO2i centre is found to be electrically active with an acceptor level at Ec - 0.11 eV. The both defects, IO2i and I2O2i, are stable at room temperature and anneal out at about 400 and 550 K, respectively. © 2001 Elsevier Science B.V. All rights reserved.
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23.
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24.
  • Lundqvist, Björn, et al. (författare)
  • Vortex dynamics for in- and out-of-plane magnetic fields in oxygen deficient single crystals of YBa2Cu3O7-delta
  • 2000
  • Ingår i: Physica. B, Condensed matter. - 0921-4526 .- 1873-2135. ; 284:Part. 1, s. 705-706
  • Tidskriftsartikel (refereegranskat)abstract
    • The resistive transition into a glassy vortex state in oxygen deficient single crystals of YBa2Cu3O7-delta (YBCO) has been studied for magnetic fields 0 less than or equal to B less than or equal to 12 T along the crystallographic c-axis, parallel to the ab plane, and also with a small misalignment of 1 degrees from the ab plane. Angular sweeps of the magnetic field reveal a sharp dip in the resistivity for B \textbackslash\textbackslash ab, which is attributed to intrinsic pinning of vortices between the planes. For Bile the vortex glass line is well described by B-g(T)= B-0[(1 - T/T-c)/(T/T-c)](alpha), where B-0 is anisotropy dependent and alpha approximate to 1.0-1.2. For B \textbackslash\textbackslash ab the behaviour is significantly different, and at high magnetic fields a change to a field-independent vortex solid-to-liquid transition is observed. This behaviour disappears for small misalignments between B and ab.
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25.
  • Marcinkevicius, Saulius, et al. (författare)
  • Changes in carrier dynamics induced by proton irradiation in quantum dots
  • 2002
  • Ingår i: Physica. B, Condensed matter. - 0921-4526 .- 1873-2135. ; 314:04-jan, s. 203-206
  • Tidskriftsartikel (refereegranskat)abstract
    • The effects of proton irradiation on carrier dynamics were investigated by time-resolved photoluminescence on different InGaAs/GaAs quantum-dot (QD) structures varying in QD surface density and substrate orientation, as well as thin InGaAs quantum wells. The carrier lifetimes in the dots are much less affected by proton irradiation than in the wells. Decrease in lifetimes of only 40 percent at the highest proton dose are observed in some of the QDs, whereas an similar to20 to similar to40-fold decrease is observed in the wells. Similar trends were observed for all quantum dot samples.
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26.
  • Murin, L.I., et al. (författare)
  • Early stages of oxygen clustering in hydrogenated Cz-Si : IR absorption studies
  • 2001
  • Ingår i: Physica. B, Condensed matter. - 0921-4526 .- 1873-2135. ; 302-303, s. 180-187
  • Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
    • The formation kinetics of small oxygen clusters in hydrogenated Si has been studied by means of infrared absorption measurements. Hydrogen was introduced into the crystals by in-diffusion from H2 gas at 1200-1300°C. The samples were heated at temperatures in the range of 280-370°C for different durations. At initial stages of heat-treatment, enormous generation rates of the oxygen dimer have been observed in hydrogenated samples. This indicates highly enhanced diffusion of the interstitial oxygen atoms. The maximum achievable concentration of the dimers is found to be limited by their dissociation rate even at temperatures of about 300°C, while in as-grown crystals the capture processes are known to be dominant in this temperature region. An explanation of this phenomenon is presented. © 2001 Published by Elsevier Science B.V.
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27.
  • Murin, L.I., et al. (författare)
  • Isotopic effects on vibrational modes of thermal double donors in Si and Ge
  • 2001
  • Ingår i: Physica. B, Condensed matter. - 0921-4526 .- 1873-2135. ; 308-310, s. 290-293
  • Tidskriftsartikel (refereegranskat)abstract
    • The local vibrational modes of thermal double donors in Si and Ge are investigated by FTIR and ab initio modelling. At most, two oxygen modes are detected from each donor, which exhibit regular shifts with increasing donor number. By using mixtures of 16O and 18O, it is found that in Si the upper band does not yield any new modes suggesting that any oxygen atom is decoupled from any other. However, the lower frequency bands exhibit unique mixed modes proving for the first time that oxygen atoms are coupled together and in close spatial proximity. Ab initio calculations demonstrate that the modes and their isotopic shifts are consistent with a model involving two parallel chains of oxygen atoms linking second neighbour Si atoms, terminated by O-atoms close to the normal position for interstitial O. © 2001 Elsevier Science B.V. All rights reserved.
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28.
  • Narayan, V., et al. (författare)
  • Calculation of the temperature dependence of hot electron scattering in heavily p-doped GaAs using a high-temperature approximation to the dielectric function
  • 2002
  • Ingår i: Physica. B, Condensed matter. - 0921-4526 .- 1873-2135. ; 324:1-4, s. 393-402
  • Tidskriftsartikel (refereegranskat)abstract
    • Using a high-temperature approximation to the dielectric function within the random phase approximation, we calculate hot electron scattering rates, as a function of temperature and doping density, in p-doped GaAs. The dielectric function of the holes contains contributions from intraband excitations and interband excitations. The former reduces to an analytic form within the two pole approximation (which used Boltzmann statistics), whereas the latter was calculated numerically. The collective excitation mode of the holes was defined by intraband excitations, since at very small wavevectors, the interband excitations vanish. However, at low temperature the interband excitations were found to be the dominant Landau damping mechanism, which strongly suppressed the plasmon at moderate doping levels. At high temperature the excitations from the heavy to light band were partially suppressed, and the plasmon was not overwhelmingly Landau damped by either interband or intraband excitations. At room temperature, an analytic dielectric function where the interband excitations have been neglected, may be used to accurately calculate hot electron mean free paths. This approximation was found to become more accurate with lower doping levels, but was not appropriate at low temperature. © 2002 Elsevier Science B.V. All rights reserved.
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29.
  • Nilsson, Hans-Erik, et al. (författare)
  • Monte Carlo simulation of high field hole transport in 4H-SiC including band to band tunneling and optical interband transisitons
  • 2002
  • Ingår i: Physica. B, Condensed matter. - 0921-4526 .- 1873-2135. ; 314:1-4, s. 68-71
  • Tidskriftsartikel (refereegranskat)abstract
    • The high field hole transport in 4H-SiC has been studied using a full band Monte Carlo (MC) simulation model that includes band to band tunneling and allows mixing of the band states during carrier drift. Impact ionization coefficients along the c-axis direction have been extracted and compared with experimental data. It is shown that the band to band tunneling mechanism is crucial in order to explain experimental results. The carrier distribution function obtained from the MC simulations has been used to determine the breakdown luminescence spectra coming from interband transitions. Our results are in good agreement with the available experimental luminescence spectra for SiC polytypes, and the importance of including interband tunneling is clearly demonstrated.
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30.
  • Nordblad, Per, et al. (författare)
  • Fe/V and Fe/Co(001) superlattices: growth, anisotropy, magnetisation and magnetoresistance
  • 2003
  • Ingår i: PHYSICA B-CONDENSED MATTER. - 0921-4526. ; 327:2-4, s. 344-
  • Tidskriftsartikel (refereegranskat)abstract
    • Some physical properties of BCC Fe/V and Fe/Co (0 0 1) superlattices are reviewed. The dependence of the magnetic anisotropy on the in-plane strain introduced by the lattice mismatch between Fe and V is measured and compared to a theoretical derivation. The dependence of the magnetic anisotropy (and saturation magnetisation) on the layer thickness ratio Fe/Co is measured and a value for the anisotropy of BCC Co is derived from extrapolation. The interlayer exchange coupling of Fe/V superlattices is studied as a function of the V layer thickness (constant Fe thickness) and layer thickness of Fe (constant V thickness). A region of antiferromagnetic coupling and GMR is found for V thicknesses 12-14 monolayers. However, surprisingly, a 'cutoff' of the antiferromagnetic coupling and GMR is found when the iron layer thickness exceeds about 10 monolayers.
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31.
  • Pinho, N.M.C., et al. (författare)
  • Mg-H and Be-H complexes in c-BN
  • 2001
  • Ingår i: Physica. B, Condensed matter. - 0921-4526 .- 1873-2135. ; 308-310, s. 1027-1030
  • Tidskriftsartikel (refereegranskat)abstract
    • Ab initio calculations on hydrogen complexes with Mg and Be impurities in c-BN are reported. We find that both impurities are acceptors and bind H to form passive defects. However, their structures are different. For Be-H, the H atom lies at a puckered bond centre configuration similar to C-H in GaAs, while in Mg-H the H atom lies at an anti-bonding site to an N neighbour of the impurity. The hydrogen related vibrational modes of the two complexes are also reported.
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32.
  • Qiu, Min, et al. (författare)
  • Interference of signals in parallel waveguides in a two-dimensional photonic crystal
  • 2001
  • Ingår i: Physica. B, Condensed matter. - 0921-4526 .- 1873-2135. ; 299:02-jan, s. 187-193
  • Tidskriftsartikel (refereegranskat)abstract
    • The interference of signals in parallel waveguides in a two-dimensional photonic crystal is studied and compared for two different types of waveguides, namely, an air waveguide formed by removing a row of dielectric columns, and a dielectric waveguide formed by introducing a dielectric slab inside the photonic crystal. For the case of air waveguides, two such parallel waveguides should be separated by at least five rows of dielectric columns to avoid interference between signals (with frequencies in the band gap) traveling in different waveguides. For the case of dielectric slab waveguides, one can achieve very low energy leakage (with negligible interference) between two such waveguides even if they are separated by only a single row of dielectric columns. A dielectric slab waveguide in a photonic crystal is more efficient for confining the electromagnetic energy inside the waveguide than an air waveguide in the photonic crystal.
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33.
  • Rydh, Andreas, et al. (författare)
  • Magnetic field scaling of the vortex glass resistivity in oxygen deficient YBa2Cu3O7-delta single crystals
  • 2000
  • Ingår i: Physica. B, Condensed matter. - 0921-4526 .- 1873-2135. ; 284, s. 707-708
  • Tidskriftsartikel (refereegranskat)abstract
    • The in-plane resistivity of an oxygen deficient YBa2Cu3O7-delta single crystal has been measured as a function of magnetic field B \\ c-axis in the range 0 T less than or equal to B less than or equal to 12 T. A scaling relation is found for the linear resistivity at p less than or similar to 0.1 rho(n) above the glass line that incorporates all studied temperatures and magnetic fields. Our scaling can be understood within a simple model where the mean pinning energy balances the thermal energy at the glass transition, U-0(B, T-g) = k(B)Th(g), with U-0(B, T) resembling a generalized condensation energy, U-0 similar to (1 - T/T-c)(m)/B-beta.
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34.
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35.
  • Slotte, J., et al. (författare)
  • Vacancy type defects in Al implanted 4H-SiC studied by positron annihilation spectroscopy
  • 2001
  • Ingår i: Physica. B, Condensed matter. - 0921-4526 .- 1873-2135. ; 308, s. 664-667
  • Tidskriftsartikel (refereegranskat)abstract
    • Vacancy-type defects produced by 100 keV Al+ ion implantation in epitaxial 4H-SiC have been studied by positron annihilation spectroscopy. The implantation dose varied from 3 x 10(13) to 1 X 10(15)/cm(2) and the implantation temperature was 25-800degreesC. From the experimental results it is clear that the implantation at elevated temperatures reduces the number of vacancy type defects. Furthermore, the defect-cluster size depends on the implantation temperature, suggesting that the vacancies involved are mobile. The positron annihilation experiments also suggest that the defect profile extends deeper into the samples than expected from the deposited energy distribution and from previous Rutherford backscattering/channeling measurements.
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36.
  • Tjernberg, Oscar, et al. (författare)
  • Resonant spin resolved photoemission on Ce
  • 2000
  • Ingår i: Physica. B, Condensed matter. - 0921-4526 .- 1873-2135. ; 281, s. 723-724
  • Tidskriftsartikel (refereegranskat)abstract
    • Spin resolved resonant photoemission using circularly polarized light has been performed on thick gamma-Ce films and the measured photoelectrons show a high degree of polarization in agreement with a simple model calculation. The two near Fermi edge features are shown to influence the polarization spectrum in opposite directions, as intuitively expected for states with opposite spin-orbit alignment.
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37.
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38.
  • Abdul-Redah, T, et al. (författare)
  • Anomalous neutron Compton scattering in metallic hydrides: new experiments
  • 2000
  • Ingår i: PHYSICA B. - : ELSEVIER SCIENCE BV. - 0921-4526. ; 276, s. 824-825
  • Tidskriftsartikel (refereegranskat)abstract
    • The time-of-flight spectra of the neutron Compton scattering of solid palladium hydrides were measured at room temperature. This experiment, which was motivated by previous measurements on liquid H2O/D2O mixtures as mel as on niobium hydrides, showed anom
  •  
39.
  • Blumenau, A.T., et al. (författare)
  • Basal plane partial dislocations in silicon carbide
  • 2003
  • Ingår i: Physica. B, Condensed matter. - : Elsevier BV. - 0921-4526 .- 1873-2135. ; 340, s. 160-164
  • Tidskriftsartikel (refereegranskat)abstract
    • Under operating conditions (forward bias) bipolar 4H- and 6H-SiC devices are known to degrade rapidly through stacking fault formation and expansion in the basal plane. It has been suggested that a recombination-enhanced dislocation glide (REDG) mechanism allows the bordering Shockley partial dislocations to overcome their barrier to glide motion and thus results in the observed stacking fault growth. In this work, we investigate the structure and properties of the participating Shockley partials by means of density functional-based atomistic calculations. Their glide motion is modelled in a process involving the formation and subsequent migration of kinks. This in combination with an analysis of the electronic structure of the partials allows an identification of those types which will be affected by the REDG mechanism.
  •  
40.
  • Borlado, CR, et al. (författare)
  • Neutron strain scanning in bimetallic tubes: experimental and Monte Carlo simulation results
  • 2000
  • Ingår i: PHYSICA B. - : ELSEVIER SCIENCE BV. - 0921-4526. ; 276, s. 907-908
  • Tidskriftsartikel (refereegranskat)abstract
    • Neutron strain scanning measurements have been performed on straight and curved parts of a component manufactured from bimetallic tubes. Instrumental and sample-related effects contributing to non-strain induced Bragg peak shifts have been corrected for b
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41.
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42.
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43.
  • Broddefalk, A, et al. (författare)
  • Structural and magnetic properties of TlCo2-xCuxSe2, 0 <= x <= 1
  • 2000
  • Ingår i: PHYSICA B. - 0921-4526. ; 284, s. 1317-1318
  • Tidskriftsartikel (refereegranskat)abstract
    • Structural and magnetic properties of single-crystal TlCo2-xCuxSe2, 0 less than or equal to x less than or equal to 1, have been investigated by means of X-ray diffraction experiments and magnetisation measurements. TlCo2Se2 and TlCu2Se2 are isostructural
  •  
44.
  • Castleton, C W M, et al. (författare)
  • Ab initio study of neutral vacancies in InP using supercells and finite size scaling
  • 2003
  • Ingår i: Physica. B, Condensed matter. - : Elsevier BV. - 0921-4526 .- 1873-2135. ; 340, s. 407-411
  • Tidskriftsartikel (refereegranskat)abstract
    • The formation energies and relaxed structures of the neutral vacancies in InP are calculated by planewave density functional theory, using supercells of 8, 64, 216 and 512 atoms together with finite size scaling. The effects of electrostatic defect-defect interactions and the truncation of elastic relaxations by the finite supercell are examined. The unrelaxed formation energies are 4.95+/-0.10 and 3.00+/-0.10eV for V-In(+0) and V-P(+0) respectively, and the relaxed formation energies are 4.20+/-0.05 and 2.35+/-0.15 eV. When relaxed, V-In(+0) remains very nearly symmetric with a 40% volume reduction, V-P(+0) on the other hand develops a double dimerized structure, with an similar to12% Jahn-Teller distortion and a 45% volume reduction. It is shown that finite size scaling sometimes reveals that energies are not converged even in the 512 atom supercell, and can thus be important tool in theoretical studies of defects in semiconductors, if sufficient care is taken with strongly Jahn Teller active defects.
  •  
45.
  • Coutinho, J., et al. (författare)
  • The formation, dissociation and electrical activity of divacancy-oxygen complexes in Si
  • 2003
  • Ingår i: Physica. B, Condensed matter. - : Elsevier BV. - 0921-4526 .- 1873-2135. ; 340, s. 523-527
  • Tidskriftsartikel (refereegranskat)abstract
    • Density functional calculations are carried out on divacancy-oxygen (V2O and V2O2) complexes in silicon, paying particular attention to their formation and dissociation mechanisms as well as their electrical activity. The formation of V2O around 220°C is controlled by the diffusion of V2 to immobile oxygen traps, while it dissociates around 300°C into VO and V. V2O and V2O2 are found to possess deep single and double acceptor levels as well as deep donor levels similar to those of V2.
  •  
46.
  • Edström, Kristina, et al. (författare)
  • The magnetic structure in K+ beta-ferrite
  • 2000
  • Ingår i: PHYSICA B. - 0921-4526. ; 276, s. 746-747
  • Tidskriftsartikel (refereegranskat)abstract
    • The magnetic structures of stoichiometric, KFe11O17, and non-stoichiometric, K1.33Fe11O17, K+ beta-ferrite, have been studied with neutron powder diffraction at 10, 295 and 923 K. Parameters describing the crystal and magnetic structures were refined with
  •  
47.
  • Gali, Adam, et al. (författare)
  • Anti-site pair in SiC : A model of the DI center
  • 2003
  • Ingår i: Physica B. - : Elsevier BV. ; , s. 175-179
  • Konferensbidrag (refereegranskat)abstract
    • The DI low-temperature photoluminescence center is a well-known defect stable up to 1700°C annealing in SiC, still its structure is not known after decades of study. Combining experimental and theoretical studies in this paper we will show that the properties of an anti-site pair can reproduce the measured one-electron level position and local vibration modes of the D I center and the model is consistent with other experimental findings as well. We give theoretical values of the hyperfine constants of the anti-site pair in its paramagnetic state as a means to confirm our model. © 2003 Elsevier B.V. All rights reserved.
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48.
  • Gu, Y. W., et al. (författare)
  • Structural evolution in Ti-Si alloy synthesized by mechanical alloying
  • 2004
  • Ingår i: Physica. B, Condensed matter. - : Elsevier BV. - 0921-4526 .- 1873-2135. ; 352:1-4, s. 299-304
  • Tidskriftsartikel (refereegranskat)abstract
    • Mechanical alloying of Ti-Si powder mixture was performed by high-energy ball milling at ambient temperature (25 °C). The structural and compositional evolutions during the mechanical alloying process were investigated by X-ray diffraction, energy dispersive spectroscopy, scanning electron microscopy and transmission electron microscopy. Results showed, that the crystallite size of Ti and Si decreased with increasing milling time and the steady-state crystallite size was between 5 and 15nm. The mechanically alloyed Ti-Si powder was predominantly nanocrystalline with traces of the remnant amorphous phase. It was found that a significant increase in solid solubility of Si in Ti was achieved by mechanical alloying. Interdiffusion between the two elements occurred during milling and the dissolution of Si in Ti was obtained after 60 h milling, forming Ti(Si) solid solution. The lattice parameter ratio c/a and the unit cell volume of h.c.p. Ti(Si) were found to decrease with increasing milling time, indicating that the shrinkage of Ti lattice was caused by diffusion of Si atoms into Ti. © 2004 Elsevier B.V. All rights reserved.
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49.
  • Heisel, B, et al. (författare)
  • mu SR-Experiments on proton-conducting oxides
  • 2000
  • Ingår i: PHYSICA B. - : ELSEVIER SCIENCE BV. - 0921-4526. ; 289, s. 487-490
  • Tidskriftsartikel (refereegranskat)abstract
    • Positive muons implanted in proton-conducting oxides can be used to gain insight into the diffusion and trapping behaviour of light particles in these materials which are used as solid oxide electrolytes in high-temperature fuel cells. We have studied bot
  •  
50.
  • Henry, Anne, et al. (författare)
  • Boron-related luminescence in SiC
  • 2003
  • Ingår i: Physica B: Condensed Matter. - : Elsevier BV. - 0921-4526.
  • Konferensbidrag (refereegranskat)abstract
    • We report a photoluminescence (PL) study for both 4H and 6H-SiC epilayers on boron-related recombination. The PL is not observed from as-grown epilayers, but after secondary ion mass spectrometry. In 4H the no-phonon (NP) line spectrum is near 3838Å, whereas it is located close to 4182 Å in the 6H. The two spectra have almost the same phonon structure with localized modes. The luminescence is predominantly polarized perpendicular to the c-axis. The temperature dependence shows that the NP lines have at least three excited states higher in energy with energy separation depending on the polytype. The luminescence is quenched at T > 70K with a thermalization energy of about 40meV. The absence of splitting or shift of the lines originating from excited states under applied magnetic field shows that the excited states have singlet character, whereas splitting is observed for the low-temperature NP lines. The luminescence of the NP lines in these samples is shown to increase with excitation time. © 2003 Elsevier B.V. All rights reserved.
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