N. E. Christensen

Aarhus University, Aarhus, Central Jutland, Denmark

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Publications (258)684.75 Total impact

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    ABSTRACT: The electronic structures of nonpolar short-period InN/GaN superlattices grown in the wurtzite a -direction, have been calculated and compared to earlier calculations for polar superlattices (grown in the c-direction). For the nonpolar superlattices it is found that the calculated band gaps and their pressure coefficients are quite similar to those of bulk InGaN alloys with an equivalent In/Ga composition ratio. Also, they are much closer than the values calculated for polar superlattices to the photolu-minescence emission energies and their pressure coefficients measured on polar structures. Possible explanations of the observed phenomena are suggested and discussed. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 01/2014;
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    ABSTRACT: Electronic and transport properties of CuGaTe$_2$, a hole-doped ternary copper based chalcopyrite type semiconductor, are studied using calculations within the Density Functional Theory and solving the Boltzmann transport equation within the constant relaxation time approximation. The electronic bandstructures are calculated by means of the full-potential linear augmented plane wave method, using the Tran-Blaha modified Becke-Johnson potential. The calculated band gap of 1.23 eV is in agreement with the experimental value of 1.2 eV. The carrier concentration- and temperature dependent thermoelectric properties of CuGaTe$_2$ are derived, and a figure of merit of $zT= 1.69$ is obtained at 950 K for a hole concentration of $3.7\cdot10^{19}$ cm$^{-3}$, in agreement with a recent experimental finding of $zT= 1.4$, confirming that CuGaTe$_2$ is a promising material for high temperature thermoelectric applications. The good thermoelectric performance of p-type CuGaTe$_2$ is associated with anisotropic transport from a combination of heavy and light bands. Also for CuSbS$_2$ (chalcostibite) a better performance is obtained for p-type than for n-type doping. The variation of the thermopower as a function of temperature and concentration suggests that CuSbS$_2$ will be a good thermoelectric material at low temperatures, similarly to the isostructural CuBiS$_2$ compound.
    01/2014;
  • I. Gorczyca, K. Skrobas, T. Suski, N.E. Christensen, A. Svane
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    ABSTRACT: The electronic structures and internal electric fields of semipolar short-period mInN/nGaN superlattices (SLs) have been calculated for several compositions (m, n). Two types of SL are considered, (112¯2) and (202¯1), corresponding to growth along the wurtzite s2 and s6 directions, respectively. The results are compared to similar calculations for polar SLs (grown in the c-direction) and nonpolar SLs (grown in the a- and m-directions). The calculated band gaps for the semipolar SLs lie between those calculated for the nonpolar and polar SLs: For s2-SLs they fall slightly below the band gaps of a-plane SLs, whereas for s6-SLs they are considerably smaller.
    Applied Physics Letters 01/2014; 104(23):232101-232101-4. · 3.79 Impact Factor
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    ABSTRACT: Quantitative high resolution transmission electron microscopy studies of intentionally grown 1InN/nGaN short-period superlattices (SLs) were performed. The structures were found to consist of an InxGa1−xN monolayer with an Indium content of x = 0.33 instead of the intended x = 1. Self-consistent calculations of the band structures of 1In0.33Ga0.67N/nGaN SLs were carried out, including a semi-empirical correction for the band gaps. The calculated band gap, Eg, as well as its pressure derivative, dEg/dp, are in very good agreement with the measured photoluminescence energy, EPL, and its pressure derivative, dEPL/dp, for a series of 1In0.33Ga0.67N/nGaN samples with n ranging from 2 to 40. This resolves a discrepancy found earlier between measured and calculated optical emission properties, as those calculations were made with the assumption of a 1InN/nGaN SL composition.
    Applied Physics Letters 01/2014; 104(18):182103-182103-4. · 3.79 Impact Factor
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    ABSTRACT: The electronic, structural, vibrational and elastic properties of PaN have been studied at both ambient and high pressures, using first principles methods with several commonly used parameterizations of the exchange-correlation energy. The generalized gradient approximation (GGA) reproduces the ground state properties satisfactorily. The high pressure behavior of the acoustic phonon branch along the [1, 0, 0] and [1, 1, 0] directions and the C44 elastic constant are anomalous, which signals a structural transition. With GGA exchange-correlation, a topological transition in the charge density occurs near the structural transition, which may be regarded as a quantum phase transition, where the order parameter obeys a mean field scaling law. However, here it is found that the topological transition is absent when other exchange-correlation functionals are invoked (local density approximation (LDA) and hybrid functional). This constitutes an example of GGA and LDA leading to qualitatively different predictions, and therefore it is of great interest to examine experimentally whether this topological transition occurs.
    Journal of Physics Condensed Matter 12/2013; 26(3):035403. · 2.22 Impact Factor
  • D. Richard, L. A. Errico, A. Svane, N. E. Christensen
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    ABSTRACT: The electronic, structural, and hyperfine properties of pure and Cd-doped lanthanide (Ln) sesquioxides with the cubic bixbyite structure (Ln_{2}O_{3}, Ln ranging from La to Lu) have been studied using the full-potential augmented plane wave plus local orbital (APW+lo) method within the local spin density approximation (LSDA) and the Coulomb-corrected LSDA+U. In the case of the pure systems, our calculations show that LSDA+U gives a better representation of the band structure compared to LSDA. The predicted equilibrium structures and the electric field gradient (EFG) tensor at Ln sites were calculated and compared with those obtained by means of hyperfine techniques and with theoretical results obtained in In_{2}O_{3}, Sc_{2}O_{3}, and Lu_{2}O_{3} reported in the literature. The origin of the EFG at Ln sites and the role played by the 4f electrons on this quantity are discussed. In the case of the Cd-doped systems, the APW+lo method (also within LSDA and LSDA+U) was applied to treat the electronic structure of the doped system. The role of the Ln 4f electrons on the EFG at Cd impurity sites, and other variables like structural distortions induced by the Cd impurity, were investigated in detail and are discussed and compared with available experimental results. An excellent agreement between the experimental and calculated EFGs was found for all Cd-doped systems.
    Physical Review B 10/2013; 88(16). · 3.66 Impact Factor
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    ABSTRACT: The deformation potentials acz−D1, act−D2, D3, D4, and D5 are determined for random AlGaN and InGaN alloys using electronic band structure calculations based on the density functional theory. A sublinear composition dependence is obtained for acz−D1 and D3 in AlGaN, and D3 in InGaN, whereas superlinear behavior on composition is found for act−D2, D4, and D5 in AlGaN, and act−D2 and D5 in InGaN. The optical polarization properties of nitride quantum wells are very well described by the k·p method when the obtained deformation potentials are included. In m-plane AlGaN/AlN and InGaN/GaN quantum wells, the difference between the interband transition energies for light polarized parallel and orthogonal to the crystalline c axis compares more favorably to experimental data, than when deformation potentials previously reported in literature are used.
    Physical Review B 08/2013; 88(8). · 3.66 Impact Factor
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    ABSTRACT: First-principles calculations have been performed for americium (Am) metal using the generalized gradient approximation + orbital-dependent onsite Coulomb repulsion via Hubbard interaction (GGA+U) and hybrid density functional theory (HYB-DFT) methods to investigate various ground state properties and pressure-induced structural transitions. Both methods yield equilibrium volume and bulk modulus in good agreement with the experimental results. The GGA+spin orbit coupling+U method reproduced all structural transitions under pressure correctly, but the HYB-DFT method failed to reproduce the observed Am-I to Am-II transition. Good agreement was found between calculated and experimental equations of states for all phases, but the first three phases need larger U (α) parameters (where α represents the fraction of Hartree-Fock exchange energy replacing the DFT exchange energy) than the fourth phase in order to match the experimental data. Thus, neither the GGA+U nor the HYB-DFT methods are able to describe the energetics of Am metal properly in the entire pressure range from 0 GPa to 50 GPa with a single choice of their respective U and α parameters. Low binding-energy peaks in the experimental photoemission spectrum at ambient pressure relate, for some parameter choices, well to peak positions in the calculated density of states function of Am-I.
    Physical Review B 07/2013; 88(1). · 3.66 Impact Factor
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    ABSTRACT: The structural and elastic properties of orthorhombic black phosphorus have been investigated using first-principles calculations based on density functional theory. The structural parameters have been calculated using the local density approximation (LDA), the generalized gradient approximation (GGA), and with several dispersion corrections to include van der Waals interactions. It is found that the dispersion corrections improve the lattice parameters over LDA and GGA in comparison with experimental results. The calculations reproduce well the experimental trends under pressure and show that van der Waals interactions are most important for the crystallographic b axis in the sense that they have the largest effect on the bonding between the phosphorus layers. The elastic constants are calculated and are found to be in good agreement with experimental values. The calculated C22 elastic constant is significantly larger than the C11 and C33 parameters, implying that black phosphorus is stiffer against strain along the a axis than along the b and c axes. From the calculated elastic constants, the mechanical properties, such as bulk modulus, shear modulus, Young's modulus, and Poisson's ratio are obtained. The calculated Raman active optical phonon frequencies and their pressure variations are in excellent agreement with available experimental results.
    · 3.77 Impact Factor
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    ABSTRACT: The electronic structures, densities of states, Fermi surfaces and elastic properties of AB3 (A =La, Y; B =Pb, In, Tl) compounds are studied under pressure using the full-potential linear augmented plane wave (FP-LAPW) method within the local density approximation for the exchange-correlation functional and including spin-orbit coupling. Fermi surface topology changes are found for all the isostructural AB3 compounds under compression (at V/V0 = 0.90 for LaPb3 (pressure = 8 GPa), at V/V0 = 0.98 for AIn3 (pressure = 1.5 GPa), at V/V0 = 0.80 for ATl3 (pressure in excess of 18 GPa)) apart from YPb3, although its electronic structure at zero pressure is very similar to that of LaPb3. For LaPb3 a softening of the C44 elastic constant under pressure (equivalent to 8 GPa) may be related to the appearance of a new hole pocket around the X point. From the calculated elastic properties and other mechanical properties, all the compounds investigated are found to be ductile in nature with elastic anisotropy. The states at the Fermi level (EF) are dominated by B p states with significant contributions from the A d states. For the La compounds, small hybridizations of the La f states also occur around EF.
    Journal of Physics Condensed Matter 03/2013; 25(15):155501. · 2.22 Impact Factor
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    ABSTRACT: Measurements of photoluminescence and its dependence on hydrostatic pressure are performed on a set of InN/nGaN superlattices with one InN monolayer and with different numbers of GaN monolayers. The emission energies, EPL, measured at ambient pressure, are close to the value of the band gap, Eg, in bulk GaN, in agreement with other experimental findings. The pressure dependence of the emission energies, dEPL/dp, however, resembles that of the InN energy gap. Further, the magnitudes of both EPL and dEPL/dp are significantly higher than those obtained from ab-initio calculations for 1InN/nGaN superlattices. Some causes of these discrepancies are suggested.
    Journal of Applied Physics 03/2013; 113(12). · 2.21 Impact Factor
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    ABSTRACT: Measurements of photoluminescence and its dependence on hydrostatic pressure are performed on a set of InN/nGaN superlattices with one InN monolayer, and with different numbers of GaN monolayers (n from 1 to 40). The emission energies, EPL, measured at ambient pressure, are close to the value of the band gap, Eg, in bulk GaN, in agreement with other experimental findings. The pressure dependence of the emission energies, dEPL/dp, however, resembles that of the InN energy gap. Further, the magnitudes of both EPL and dEPL/dp are significantly higher than those obtained from abinitio calculations for 1InN/nGaN superlattices. Some causes of these discrepancies are suggested...Detailed analysis of the electronic band structure of 1InN/5GaN superlattice is performed showing that the built-in electric field plays an important role in the mInN/nGaN structures. It strongly influences the valence- and conduction-band profiles and thus determines the effective band gap.
    Proc SPIE 03/2013;
  • I. Gorczyca, K. Skrobas, T. Suski, N.E. Christensen, A. Svane
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    ABSTRACT: The electronic structures of nonpolar short-period InN/GaN superlattices (SLs) grown in the wurtzite a- and m-directions have been calculated and compared to previous calculations for polar superlattices (grown in the c-direction). The variation of the band gaps with the composition (m, n) of the mInN/nGaN unit cells of the superlattices was examined. The band structures were obtained by self-consistent calculations based on the local density approximation to the density functional theory using the Linear-Muffin-Tin-Orbital method with a semi-empirical correction for the band gaps. The calculated band gaps and their pressure coefficients for nonpolar superlattices are similar to those calculated for bulk InGaN alloys with an equivalent In/Ga concentration ratio. This is very different from what has been found in polar superlattices where the band gaps are much smaller and vanish when the number m of InN layers in the unit cell exceeds three. A strong internal electric field is responsible for this behavior of polar structures. Experimental photoluminescence data for polar SLs agree very well with gaps calculated for the nonpolar structures. It is suggested that this is caused by screening of the electric field in the polar structures by carriers originating from unintentional defects.
    Journal of Applied Physics 01/2013; 114(22):223102-223102-7. · 2.21 Impact Factor
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    ABSTRACT: Electronic and transport properties of CuGaTe2, a hole-doped ternary copper based chalcopyrite type semiconductor, are studied using calculations within the Density Functional Theory and solving the Boltzmann transport equation within the constant relaxation time approximation. The electronic band structures are calculated by means of the full-potential linear augmented plane wave method, using the Tran-Blaha modified Becke-Johnson potential. The calculated band gap of 1.23 eV is in agreement with the experimental value of 1.2 eV. The carrier concentration- and temperature dependent thermoelectric properties of CuGaTe2 are derived, and a figure of merit of zT = 1.69 is obtained at 950 K for a hole concentration of 3.7·1019 cm-3, in agreement with a recent experimental finding of zT = 1.4, confirming that CuGaTe2 is a promising material for high temperature thermoelectric applications. The good thermoelectric performance of p-type CuGaTe2 is associated with anisotropic transport from a combination of heavy and light bands. Also for CuSbS2 (chalcostibite), a better performance is obtained for p-type than for n-type doping. The variation of the thermopower as a function of temperature and concentration suggests that CuSbS2 will be a good thermoelectric material at low temperatures, similarly to the isostructural CuBiS2 compound.
    Journal of Applied Physics 01/2013; 114(22):223707-223707-8. · 2.21 Impact Factor
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    ABSTRACT: The structural and elastic properties of orthorhombic black phosphorus have been investigated using first-principles calculations based on density functional theory. The structural parameters have been calculated using the local density approximation (LDA), the generalized gradient approximation (GGA), and with several dispersion corrections to include van der Waals interactions. It is found that the dispersion corrections improve the lattice parameters over LDA and GGA in comparison with experimental results. The calculations reproduce well the experimental trends under pressure and show that van der Waals interactions are most important for the crystallographic b-axis, in the sense that they have the largest effect on the bonding between the phosphorus layers. The elastic constants are calculated and are found to be in good agreement with experimental values. The calculated C$_{22}$ elastic constant is significantly larger than the C$_{11}$ and C$_{33}$ parameters, implying that black phosphorus is stiffer against strain along the a-axis than along the b- and c-axes. From the calculated elastic constants,the mechanical properties such as bulk modulus, shear modulus, Young's modulus and Poisson's ratio are obtained. The calculated Raman active optical phonon frequencies and their pressure variations are in excellent agreement with available experimental results.
    Physical Review B. 11/2012; 86(3).
  • I. Gorczyca, T. Suski, N. E. Christensen, A. Svane
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    ABSTRACT: The electronic structures of short-period pseudomorphically grown superlattices (SLs) of the form mInN/nGaN are calculated and the band gap variation with the well and the barrier thicknesses is discussed including hydrostatic pressure effects. The calculated band gap shows a strong dependence on the superlattice geometry. The superlattice gap vanishes for n = m ≥ 4. These effects are related to the existence of the internal electric fields that strongly influence the valence- and conduction-band profiles and thus determine the effective band gap and emission energies. The electric field strength depends strongly on the strain conditions and SL geometry, but weakly on the applied external hydrostatic pressure.
    Applied Physics Letters 08/2012; 101(9). · 3.79 Impact Factor
  • I. Gorczyca, T. Suski, N. E. Christensen, A. Svane
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    ABSTRACT: InN/GaN superlattices offer an important way of band gap engineering in the blue-green range of the spectrum. This approach represents a more controlled method than the band gap tuning in quantum well systems by application of InGaN alloys. The electronic structures of short-period wurtzite InN/GaN(0001) superlattices are investigated, and the variation of the band gap with the thicknesses of the well and the barrier is discussed. Superlattices of the form mInN/nGaN with n ≥ m are simulated using band structure calculations in the Local Density Approximation with a semiempirical correction for the gap error. The calculated band gap shows a strong decrease with the thickness (m) of the InN well. In superlattices containing a single layer of InN (m = 1) the band gap increases weakly with the GaN barrier thickness n, reaching a saturation value around 2 eV. In superlattices with n = m and n > 5 the band gap closes and the systems become “metallic”. These effects are related to the existence of the built-in electric fields that strongly influence valence- and conduction-band profiles and thus determine effective band gap and emission energies of the superlattices. Varying the widths of the quantum wells and barriers one may tune band gaps over a wide spectral range, which provides flexibility in band gap engineering.
    Crystal Growth & Design 06/2012; 12(7):3521–3525. · 4.69 Impact Factor
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    ABSTRACT: The electronic structure, Fermi surface, and elastic properties of the isostructural and isoelectronic LaSn3 and YSn3 intermetallic compounds are studied under pressure within the framework of density functional theory including spin-orbit coupling. The LaSn3 Fermi surface consists of two sheets, of which the second is very complex. Under pressure a third sheet appears around compression V/V0=0.94, while a small topology change in the second sheet is seen at compression V/V0=0.90. This may be in accordance with the anomalous behavior in the superconducting transition temperature observed in LaSn3, which has been suggested to reflect a Fermi surface topological transition, along with a nonmonotonic pressure dependence of the density of states at the Fermi level. The same behavior is not observed in YSn3, the Fermi surface of which already includes three sheets at ambient conditions, and the topology remains unchanged under pressure. The reason for the difference in behavior between LaSn3 and YSn3 is the role of spin-orbit coupling and the hybridization of La 4f states with the Sn p states in the vicinity of the Fermi level, which is well explained using the band structure calculation. The elastic constants and related mechanical properties are calculated at ambient as well as at elevated pressures. The elastic constants increase with pressure for both compounds and satisfy the conditions for mechanical stability under pressure.
    Physical review. B, Condensed matter 05/2012; 85(17). · 3.77 Impact Factor
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    ABSTRACT: A combination of experiments and ab initio quantum-mechanical calculations has been applied to examine electronic, structural, and hyperfine interactions in pure and Ta-doped zirconium dioxide in its monoclinic phase (m-ZrO2). From the theoretical point of view, the full-potential linear augmented plane wave plus local orbital (APW+lo) method was applied to treat the electronic structure of the doped system including the atomic relaxations introduced by the impurities in the host in a fully self-consistent way using a supercell approach. Different charge states of the Ta impurity were considered in the study and its effects on the electronic, structural, and hyperfine properties are discussed. Our results suggest that two different charge states coexist in Ta-doped m-ZrO2. Further, ab initio calculations predict that depending on the impurity charge state, a sizeable magnetic moment can be induced at the Ta-probe site. This prediction is confirmed by a new analysis of experimental data.
    Physical Review B 04/2012; 85(15). · 3.66 Impact Factor
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    ABSTRACT: An electronic quantity, the correlation strength, is defined as a necessary step for understanding the properties and trends in strongly correlated electronic materials. As a test case, this is applied to the different phases of elemental Pu. Within the GW approximation we have surprisingly found a "universal" scaling relationship, where the f-electron bandwidth reduction due to correlation effects is shown to depend only on the local density approximation bandwidth and is otherwise independent of crystal structure and lattice constant.
    Physical review. B, Condensed matter 01/2012; 87(4). · 3.77 Impact Factor

Publication Stats

4k Citations
684.75 Total Impact Points

Institutions

  • 1990–2014
    • Aarhus University
      • Department of Physics and Astronomy
      Aarhus, Central Jutland, Denmark
  • 2013
    • Indian Institute of Technology Hyderabad
      • Department of Physics
      Hyderābād, State of Andhra Pradesh, India
  • 2005–2010
    • Polish Academy of Sciences
      • Institute of High Pressure Physics
      Warsaw, Masovian Voivodeship, Poland
  • 2009
    • Los Alamos National Laboratory
      • Theoretical Division
      Los Alamos, NM, United States
  • 1987–2007
    • Max Planck Institute for Solid State Research
      Stuttgart, Baden-Württemberg, Germany
  • 2000
    • European Synchrotron Radiation Facility
      Grenoble, Rhône-Alpes, France
  • 1989
    • Centro Universitário Sant’Anna
      San Paulo, São Paulo, Brazil
  • 1983
    • Technical University of Denmark
      København, Capital Region, Denmark