Showing papers in "Physica B-condensed Matter in 1993"
TL;DR: In this article, the main formulas governing the analysis of the Bragg magnetic scattering are summarized and shortly discussed and the method of profile fitting without a structural model to get precise integrated intensities and refine the propagation vector(s) of the magnetic structure is discussed.
Abstract: In spite of intrinsic limitations, neutron powder diffraction is, and will still be in the future, the primary and most straightforward technique for magnetic structure determination. In this paper some recent improvements in the analysis of magnetic neutron powder diffraction data are discussed. After an introduction to the subject, the main formulas governing the analysis of the Bragg magnetic scattering are summarized and shortly discussed. Next, we discuss the method of profile fitting without a structural model to get precise integrated intensities and refine the propagation vector(s) of the magnetic structure. The simulated annealing approach for magnetic structure determination is briefly discussed and, finally, some features of the program FullProf concerning the magnetic structure refinement are presented and discussed. The different themes are illustrated with simple examples.
11,923 citations
TL;DR: In this article, the authors describe instrumentation and computational methods for acquiring and analyzing the resonant ultrasound spectrum of very small (0.001 cm3) samples as a function of temperature, and provide examples to demonstrate the power of the technique.
Abstract: The mechanical resonant response of a solid depends on its shape, density, elastic moduli and dissipation. We describe here instrumentation and computational methods for acquiring and analyzing the resonant ultrasound spectrum of very small (0.001 cm3) samples as a function of temperature, and provide examples to demonstrate the power of the technique. The information acquired is in some cases comparable to that obtained from other more conventional ultrasonic measurement techniques, but one unique feature of resonant ultrasound spectroscopy (RUS) is that all moduli are determined simultaneously to very high accuracy. Thus in circumstances where high relative or absolute accuracy is required for very small crystalline or other anisotropic samples RUS can provide unique information. RUS is also sensitive to the fundamental symmetry of the object under test so that certain symmetry breaking effects are uniquely observable, and because transducers require neither couplant nor a flat surface, broken fragments of a material can be quickly screened for phase transitions and other temperature-dependent responses.
513 citations
TL;DR: In this article, the current use of electrical and optical methods to study the semiconducting properties of SiC was reviewed. And the authors treated Hall measurements, deep-level transient spectroscopy, infrared absorption and luminescence.
Abstract: We review the currentuse of electrical and optical methods to study the semiconducting properties of SiC. More specifically we treat Hall measurements, deep-level transient spectroscopy, infrared absorption and luminescence. Some very recent results, not yet available in the literature, on donor and acceptor levels in 3C-SiC, 4H-SiC and 6H-SiC are discussed.
268 citations
TL;DR: In this article, a GaN-buffer was grown at low temperature and then the rest of the GaN films were grown at higher temperatures, leading to a relatively small two-dimensional nucleation rate (∼20 nuclei/μm2 h) and high lateral growth rate (100 times faster than the vertical growth rate).
Abstract: High-quality GaN films have been grown on a variety of substrates by electron cyclotron resonance microwave plasma-assisted molecular beam epitaxy (ECR-MBE). The films were grown in two steps. First, a GaN-buffer was grown at low temperature and then the rest of the film was grown at higher temperatures. We found that this method of growth leads to a relatively small two-dimensional nucleation rate (∼20 nuclei/μm2 h) and high lateral growth rate (100 times faster than the vertical growth rate). This type of quasi-layer-by-layer growth results in a smooth surface morphology to within 100 A. Growth on Si(1 0 0) leads to single-crystalline GaN films having the zinc-blende structure. Growth on Si(1 1 1) leads to GaN films having the wurtzitic structure with a large concentration of stacking faults. The crystallographic orientation and the surface morphology of GaN films on sapphire depends on the orientation of sapphire. To this date, the best films were grown on the basal plane of sapphire.
232 citations
TL;DR: There are four primary application areas for 6H-SiC devices: optoelectronics, high-temperature electronics, high power/high frequency devices, and nonvolatile memories as discussed by the authors.
Abstract: A variety of devices with promising characteristics have recently been demonstrated in 6H-SiC. There are four primary application areas for 6H-SiC devices: (1) optoelectronics, (2) high-temperature electronics, (3) high-power/high- frequency devices, and (4) nonvolatile memories. These applications, and current device results in each area, are discussed below.
187 citations
TL;DR: In this paper, the relation between the NMR spin-echo attenuation in the magnetic field gradient and the velocity autocorrelations of molecular random migration has been derived by using the comulant expansion theorem for averaging stochastic processes.
Abstract: The relation between the NMR spin-echo attenuation in the magnetic field gradient and the velocity autocorrelations of molecular random migration has been derived by using the comulant expansion theorem for averaging stochastic processes. It may explain certain cases of time-dependent diffusion observed in many systems. In the limit of long correlation times it is identical to Torrey's well-known formula for spin echo attenuation due to self-diffusion. The method is used to derive the expression for spin-echo attenuation of diffusion in restricted regions.
152 citations
TL;DR: In this article, a review of recent progress made in microscopic understanding of point defects in silicon carbide (SiC) is given, including shallow nitrogen donors, group-III acceptors, vanadium impurities, and radiation induced defects.
Abstract: A review is given on recent progress made in a microscopic understanding of point defects in silicon carbide (SiC). Defect structures to be discussed include shallow nitrogen donors, group-III acceptors, the transition metal impurities vanadium and titanium and radiation induced defects, like the silicon vacancy in SiC.
121 citations
TL;DR: In this paper, the reverse bias breakdown appears to occur via avalanche multiplication processes exhibiting a sharp knee at breakdown, which corresponds to an extrapolated room temperature current density of ∼2 × 10-17 A/cm2 at -1.0 V.
Abstract: Single junction devices in silicon carbide have been developed for use as blue LEDs, UV photodiodes and high- temperature rectifiers. As a light emitter, 6H-SiC junctions can be tailored to emit light across the visible spectrum. The most widely commercialized device is the blue LED. Over the past two years, the quantum efficiency of the Cree blue LED has increased significantly. The devices emit light with a peak wavelength of 470 nm with a spectral halfwidth of ∼70 nm. The optical power output is typically between 12 and 18 μW for a forward current of 20 mA at 3 V. This represents a power efficiency of ∼0.02–0.03%. In addition to blue emission, the energy bandgap of ∼3.0 eV allows for inherently low dark currents and high quantum efficiencies for ultraviolet photodiode detectors made in 6H-SiC, even at high temperatures. These devices typically exhibit a quantum efficiency of 80–100% and peak response of ∼250–280 nm. These characteristics are maintained to at least 623 K. The dark current density at -1.0 V and 473 K is ∼10-11 A/cm2. This corresponds to an extrapolated room temperature current density of ∼2 × 10-17 A/cm2 at -1.0 V. Rectifiers with blocking voltages as high as ∼1400 V and a forward current rating of 400 mA at ∼3.0 V have been fabricated. For a 710 V rectifier, the reverse bias leakage current density at 200 V is shown to increase from ∼10-9 to ∼10-7 A/cm2 from 300 to 673 K, respectively. The reverse bias breakdown appears to occur via avalanche multiplication processes exhibiting a sharp knee at breakdown. For a ∼1400 V rectifier, the reverse bias leakage current at 1375 V is less than 1 μA at room temperature.
120 citations
TL;DR: The extreme properties of diamond, SiC and GaN provide combinations of attributes for high-power, -temperature, -frequency and optoelectronic applications as mentioned in this paper, and the results of chemical, structural, microstructural and electrical characterization and device development are reviewed for thin films of these three materials.
Abstract: The extreme properties of diamond, SiC and GaN provide combinations of attributes for high-power, -temperature, -frequency and optoelectronic applications. The methods of deposition, the results of chemical, structural, microstructural and electrical characterization and device development are reviewed for thin films of these three materials. Problems and areas of future research are also noted.
116 citations
TL;DR: In this article, the sum rules for dipolar linear and quadrupolar circular dichroism in the X-ray region were derived for transition metals and rare earths and applied to transition metals.
Abstract: Sum rules are derived for dipolar linear and quadrupolar circular dichroism in the X-ray region. They relate the intensity of the dichroic signal to the ground state expectation values of orbital and spin dependent effective operators, which act on the valence electrons. The results obtained are applied to transition metals and rare earths.
113 citations
TL;DR: In this article, a series of differently prepared polycrystalline diamond (PCD) films exhibiting considerable differences in the shape of the C-H stretch absorption band has been studied.
Abstract: Hydrogen in polycrystalline diamond (PCD) films has been analysed by infrared spectroscopy. A series of differently prepared PCD films exhibiting considerable differences in the shape of the C-H stretch absorption band has been studied. The spectra are consistently explained by a superposition of lines from sp3 hybridized CH3, CH2 and CH groups. The corresponding vibrational frequencies coincide with those of amorphous hydrogenated carbon (a-C:H). Contributions of sp2 hybridized CHn groups are found to be less than 10%. In addition to the known a-C:H lines a diamond specific C-H vibration at 2822 cm-1 has been observed for the first time in PCD. It is shown that hydrogen is incorporated in noncrystalline and defective regions of the PCD film, i.e. at grain boundaries and dislocations. For decreasing grain size (increasing defect content) the hydrogen concentration increases. In many samples the sp3 hybridized CH2 group is dominant (up to 69%). Within a series, where the hydrocarbon concentration in the feed gas was varied, samples with high abundances of sp3CH3 (49%) and sp3CH (28%) have been prepared.
TL;DR: In this article, an angle-dispersive X-ray diffraction was used to study anatase up to 49 GPa in a diamond anvil cell by angle dispersive diffraction with B 0 values of 59(5) and 98(9) GPa, respectively.
Abstract: TiO 2 (anatase) was studied up to 49 GPa in a diamond anvil cell by angle-dispersive X-ray diffraction. Two phase transitions were observed, the first to the orthorhombic α-PbO 2 -type phase and the second to a monoclinic phase, which appears to be baddeleyite-structured. Birch-Murnaghan equations of state were calculated for both anatase and the α-PbO 2 -type phase yielding B 0 values of 59(5) and 98(9) GPa, respectively. Accordingly, anatase is four times and the α-PbO 2 phase twice as compressible as rutile. The high-pressure, monoclinic phase, in contrast, was found to be much harder than rutile.
TL;DR: In this paper, the authors proposed a way to build a resonant tunneling interferometer by considering discrete levels of finite width on both sides of tunnel barrier, where the tunneling occurs only between levels spaced less than their width.
Abstract: The quantum interference between two semiclassical trajectories was shown to be a great issue in mesoscopic systems with metallic conductivity. The effect persists also in the systems of tunnel junctions. It determines reproducible fluctuations of the I–V curve and Aharonov-Bohm effect in the system of two junctions. Quantum interference leads to quantization and discrete levels. To build a qualitative picture one may consider discrete levels of finite width on both sides of tunnel barrier. The tunneling occurs only between levels spaced less than their width. The picture allows for correct estimations of the effect for different layouts. If the tunnel junctions connect two almost isolated metal islands the perfect level quantization in the islands refines the effect. The electron tunneling transmission peaks sharply when energies of two levels match. If the islands form a loop, the magnetic flux penetrating the loop modulates strongly the peak height. This is a way to build a resonant tunneling interferometer.
TL;DR: In this paper, the first pressure derivatives of the cerium and uranium mono-chalcogenides or -pnictides were calculated from the pressure-volume relationships using Birch's equation of state.
Abstract: The bulk moduli and their first pressure derivatives of the cerium and uranium mono-chalcogenides or -pnictides have been calculated from the pressure-volume relationships using Birch's equation of state. The possible existence of anisotropic stress components has been considered. The bulk moduli of cerium mono-chalcogenides and -pnictides exhibit a linear relationship with 1 V 0 (V0: volume of the unite cell). The presence of the Kondo effect is detected from a small (CeO excepted) decrease of the bulk modulus of a compound but more often from a marked reduction in the first pressure derivative of the bulk modulus. The bulk moduli of uranium mono-chalcogenides and -pnictides exhibit a linear relationship with ( 1 V 0 ) 5 3 which indicates the presence of metallic bonding. For the uranium pnictides a decrease of the pressure derivatives of the bulk moduli results from the mixing of p-f orbitals.
TL;DR: A review of the role of polarized neutron reflectivity in measuring the magnetic profile close to the surface and in thin films of superconductors and magnetic materials is given in this paper.
Abstract: A review is given of the role of polarized neutron reflectivity (PNR) in measuring the magnetic profile close to the surface and in thin films of superconductors and magnetic materials. For type I and type II superconductors PNR provided a new and direct determination of the penetration depth. For very thin ferromagnetic films PNR was able to determine the absolute value of the magnetic moments. In magnetic superlattices, formed by the alternation of ferromagnetic layers and nonmagnetic spacers, PNR was used to confirm the basic magnetic structure as well as to determine the direction of the magnetic moments of the individual layers. In addition to reflectivity, forward magnetic scattering may very well extend the usefulness of PNR to the case of laterally dishomogeneous systems.
TL;DR: In this paper, the structure and properties of nitrogen-containing defects in diamond are reviewed and the present understanding of hydrogen, a major impurity in type II diamond, is reviewed.
Abstract: Recent theoretical investigations into the structure and properties of two major impurities in diamond are reviewed. Particular attention is placed on nitrogen (N), the dominant impurity in natural type I diamond. It is shown how calculations of the structure, electronic and vibrational properties of nitrogen-containing defects have shed light on a variety of experimental information gathered in the last 50 years. Problems considered include nitrogen as an isolated impurity, in the A and B centres (complexes believed to contain two and four nitrogen atoms, respectively) and in the platelet defects which may contain thousands of N atoms. Finally, the present understanding of hydrogen, a major impurity in type II diamond, is reviewed.
TL;DR: The development of the technology for heteroepitaxial growth of high-crystalline quality GaN films on sapphire substrates using AlN buffer layers, the establishment of a technique for control of conductivity for n-type GaN by Si-doping, and the realization of p-type Mg-doped GaN using low-energy electron-beam irradiation treatment have enabled us to fabricate a high performance, short wavelength LED based on GaN.
Abstract: The development of the technology for heteroepitaxial growth of high-crystalline-quality GaN films on sapphire substrates using AlN buffer layers, the establishment of a technique for control of conductivity for n-type GaN by Si-doping, and the realization of p-type Mg-doped GaN using low-energy electron-beam irradiation treatment have enabled us to fabricate a high performance, short wavelength LED based on GaN. In this paper, we review the recent developments of AlGaN/GaN multi-layered structures showing quantum-size effects, and of LEDs with AlGaN/GaN double heterostructures which emit light in the blue to UV region with output power of more than a few milliwatts at room temperature.
TL;DR: In this article, the authors demonstrate that the Coulomb interaction and the binding forces due to the confinement and the magnetic field induces transitions of the ground state of quantum dots with few electrons.
Abstract: The competition between the Coulomb interaction and the binding forces due to the confinement and the magnetic field induces transitions of the ground state of quantum dots with few electrons. We demonstrate that for quantum dot helium these transitions occur between states with differently strong correlations between the electrons. By the response of quantum dot helium to far infrared radiation we exemplify that a proper treatment of the correlation effects may be essential for a correct interpretation of experimental data on quantum dots with few electrons.
TL;DR: In this paper, a study of hexagonal GaN thin films grown by low-pressure chemical vapor deposition on sapphire substrates was presented, where synchrotron radiation photoemission spectroscopy was used to probe the electronic density of states and test the validity of existing band structure calculations.
Abstract: We present a study of hexagonal GaN thin films grown by low-pressure chemical vapor deposition on sapphire substrates. We used synchrotron radiation photoemission spectroscopy to probe the electronic density of states and test the validity of existing band structure calculations. Parallel studies of surface composition and atomic structure by means of Auger spectroscopy and reflection high-energy electron diffraction were performed following sputtering/annealing cycles using argon, xenon and nitrogen ions. We found that sputtering with nitrogen ions followed by annealing in vacuum is an effective method to reduce nitrogen surface depletion and produce quasi-stoichiometric ordered GaN(0 0 0 1)1 × 1 surfaces.
TL;DR: In this paper, the edge absorption, edge emission and the two-and three-phonon absorption in the infrared spectral region are briefly reviewed for intrinsic diamond and the role of nitrogen and boron are then discussed in depth and the current classification scheme for diamond, and its relation to the aggregation model for nitrogen in diamond, are presented.
Abstract: In this paper the edge absorption, edge emission and the two- and three-phonon absorption in the infrared spectral region are briefly reviewed for intrinsic diamond. The role of nitrogen and boron are then discussed in depth and the current classification scheme for diamond, and its relation to the aggregation model for nitrogen in diamond, are presented. Radiation damage, followed by thermal annealing creates a wide range of absorption and luminescence bands in diamond. Some of these bands display localised vibrational modes and current studies of these optical centres, using diamonds grown from mixtures of 12C and 13C, are presented in detail.
TL;DR: In this article, the authors examined the effect of the zero-phonon line (ZPL) in the absorption spectrum of synthetic type-Ib diamonds under high magnetic fields B, up to 150 T at 77 K.
Abstract: To study the electronic states of an NV center in synthetic type-Ib diamonds, the Zeeman effect of the zero-phonon line (ZPL) in the absorption spectrum has been examined under high magnetic fields B, up to 150 T at 77 K. The absorption peak energies for circularly polarized lights σ+ and σ- are shifted oppositely and nonlinearly to the magnetic fields for B‖;[1 0 0] in the Faraday configuration. The Zeeman splitting is 2.2 meV at 150 T, which leads to a g value of 0.22. It has been concluded that the orbital magnetic moment of the doubly degenerate excited state is partially quenched because the orbital degeneracy is lifted by an internal strain.
TL;DR: In this article, a ternary intermetallics with the ThMn12 tetragonal structure and their nitrides YFe12−xMoxNy(y≈1) were prepared.
Abstract: Iron-rich ternary intermetallics YFe12−xMox (0.5⩽x⩽4.0) with the ThMn12 tetragonal structure and their nitrides YFe12−xMoxNy(y≈1) were prepared. The lattice parameters increased linearly with Mo concentration (5.06 A3/Mo) The values a = 8.468 A and c = 4.752 A were extrapolated for the hypothetical YFe12. Upon nitrogenation, the cell volume expanded more than 3%, where the a-axis mainly lengthens. The Fe-Fe exchange interactions were greatly enhanced by interstitial N atoms, resulting in a Curie temperature increase of about 200 K on average. Saturation magnetization was increased by nitrogenation. While saturation magnetization on the host compounds could be explained by the ‘magnetic valence model’, the data on the nitrides did not fit in the same model. Both nitrides with x = 0.5 and 1.0 have fairly high TC (708 K and 664 K) and saturation magnetization at room temperature (167 Am2/kg and 151 Am2/kg), being interesting for applications.
TL;DR: In this paper, an expression for the magic angular momenta in the spin-polarized case is given and electron probability distributions are computed to illustrate the physical difference between magic and non-magnetic states.
Abstract: The electron-electron interaction in quantum dots leads to interesting effects which are highly elusive. For example, the magnetization of dots containing very small numbers of electrons is predicted to oscillate with magnetic field. The reason is that the ground states prefers to be at certain magic values of the total angular momentum which are field-dependent and the oscillations mirror jumps from one magic angular momentum value to another. This behaviour is a direct consequence of the Pauli principle which enables the electrons to reduce their energy optimally only at the magic angular momenta. An expression for the magic angular momenta in the spin-polarized case is given and electron probability distributions are computed to illustrate the physical difference between magic and nonmagic states. In the limit of large angular momentum the ground state appears to be the molecular analogue of a Wigner crystal.
TL;DR: In this article, the authors reported new experimental results on the pressure dependence of the superconducting transition temperature, Tc(p), for the heavy-fermion superconductor CeCu2Si2.
Abstract: We report new experimental results on the pressure dependence of the superconducting transition temperature, Tc(p), for the heavy-fermion superconductor CeCu2Si2. Two characteristic pressures, p1 = 3.0 GPa and p2 = 7.7 GPa, discriminate three regimes. A fairly linear decrease is observed in the intermediate regime. Data suggest that Tc at zero pressure is strongly reduced relatively to this regime due to an unknown mechanism.
TL;DR: In this article, the specific heat of linear temperature dependence is observed in low carrier systems of Ce and Yb monopnictides, Yb 4 As 3 as well as insulating Sm 3 X 4 (X=Se, Te).
Abstract: Typical examples of a heavy fermion state of low carrier concentration systems including extreme cases of insulating substances are shown. Large specific heat of linear temperature dependence is observed in low carrier systems of Ce and Yb monopnictides and Yb 4 As 3 as well as insulating Sm 3 X 4 (X=Se, Te).
NEC1
TL;DR: In this paper, the authors observed the Aharonov-Bohm effect in magnetoresistance of a two-dimensional electron gas with a twodimensional array of depleted circular regions, called antidots.
Abstract: We have observed the Aharonov-Bohm (AB) effect in magnetoresistance of a two-dimensional electron gas with a two-dimensional array of depleted circular regions, called antidots. Fine oscillations with a period of about h/ea2 (a = period of the array) are found in the magnetoresistance peak which originates from the formation of the localized orbits encircling a single antidot. These oscillations are a manifestation of the AB effect accompanied by quantization of the localized orbits.
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TL;DR: In this article, the authors performed optical reflectance and transmission measurements on SmB6 and their temperature dependence, and two peak structures were resolved at 0.1 − 0.5 eV in the optical conductivity spectrum (σ).
Abstract: We performed optical reflectance and transmission measurements on SmB6 and their temperature dependence. At low temperature where the gap was formed, two peak structures were resolved at 0.1–0.5 eV in the optical conductivity spectrum (σ). Such structures are discussed in relation to the gap state.
TL;DR: In this article, an electrostatic model for explaining polytype inheritance in SiC is proposed and step-controlled epitaxial growth of 6H-SiC is discussed in detail and the growth mechanism and effects of off-direction and off-angle are described.
Abstract: Recent progress in epitaxial growth of SiC in the author's group is surveyed. Growth of 3C-SiC on a Si substrate, a typical success in heteroepitaxial growth with large lattice mismatch, is briefly reviewed. Step-controlled epitaxy of 6H-SiC, which the author's group named and which is a key technology for future development, is discussed in detail. The growth mechanism and effects of off-direction and off-angle are described. An electrostatic model for explaining polytype inheritance in SiC is proposed. As an advanced epitaxial growth, atomic layer epitaxy in SiC crystal growth by gas source molecular beam epitaxy is given.
TL;DR: The measured orbital susceptibility of an array of 10 5 isolated Ga-As squares of area L 2 = 4.5 x 4,5 μm 2 has a paramagnetic contribution ≃ k F L = 100 times larger than the Landau susceptibility.
Abstract: The measured orbital susceptibility of an array of 10 5 isolated Ga-As squares of area L 2 = 4.5 x 4,5 μm 2 has a paramagnetic contribution ≃ k F L = 100 times larger than the Landau susceptibility. With increasing magnetic field, this paramagnetic susceptibility decreases on a field scale of order H 0 ≃ φ 0 /L 2 where φ 0 = h/e is the flux quantum. As the temperature is raised, this orbital paramagnetism decreases on the scale of 0.5K ≃ E c /2 where E c = hυ F /L is the correlation energy. These experimental findings can be checked using Coulomb blockade spectroscopy. To this end, the thermodynamic relations between chemical potential and orbital currents are derived