S. Shallcross

Publications (10)7.37 Total impact

• Article: Spectral density and metal-insulator phase transition in Mott insulators within RDMFT

  S. Sharma, J. K. Dewhurst, S. Shallcross, E. K. U. Gross
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  ABSTRACT: We present a method for calculating the spectrum of periodic solids within reduced density matrix functional theory. This method is validated by a detailed comparison of the angular momentum projected spectral density with that of well established many-body techniques, in all cases finding an excellent agreement. The physics behind the pressure induced insulator-metal phase transition in MnO is investigated. The driving mechanism of this transition is identified as increased crystal field splitting with pressure, resulting in a charge redistribution between the Mn $e_g$ and $t_2g$ symmetry projected states.
  06/2012;
• Source

  Available from: ArXiv

  Article: Spectrum of extended systems from Reduced Density Matrix Functional Theory

  S. Sharma, S. Shallcross, J. K. Dewhurst, E. K. U. Gross
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  ABSTRACT: We present a method for calculating the spectrum of extended solids within reduced density matrix functional theory. An application of this method to the strongly correlated transition metal oxide series demonstrates that (i) an insulating state is found in the absence of magnetic order and, in addition, (ii) the interplay between the change transfer and Mott-Hubbard correlation is correctly described. In this respect we find that while NiO has a strong charge transfer character to the electronic gap, with substantial hybridization between $t_{2g}$ and oxygen-$p$ states in the lower Hubbard band, for MnO this is almost entirely absent. As a validation of our method we also calculate the spectra for a variety of weakly correlated materials, finding good agreement with experiment and other techniques.
  12/2009;
• Source

  Available from: ArXiv

  Article: Why magnetism in CeO$_{1-x}$F$_x$FeAs and LaO$_{1-x}$F$_x$FeAs is different

  S. Sharma, S. Shallcross, J. K. Dewhurst, A. Sanna, C. Bersier, H. Glawe, E. K. U. Gross
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  ABSTRACT: Using state-of-the-art first-principles calculations we study the magnetic behaviour of CeOFeAs. We find the Ce layer moments oriented perpendicular to those of the Fe layers. An analysis of incommensurate magnetic structures reveals that the Ce-Ce magnetic coupling is rather weak with, however, a strong Fe-Ce coupling. Comparison of the origin of the tetragonal to orthorhombic structural distortion in CeOFeAs and LaOFeAs show marked differences; in CeOFeAs the distortion is stabilized by a lowering of spectral weight at the Fermi level, while in LaOFeAs by a reduction in magnetic frustration. Finally, we investigate the impact of electron doping upon CeOFeAs and show that while the ground state Fe moment remains largely unchanged by doping, the stability of magnetic order goes to zero at a doping that corresponds well to the vanishing of the N\'eel temperature.
  04/2009;
• Source

  Available from: ArXiv

  Article: Magnetic properties of LaO$_{1-x}$F$_x$FeAs

  S. Sharma, J. K. Dewhurst, S. Shallcross, C. Bersier, F. Cricchio, A. Sanna, S. Massidda, E. K. U. Gross, L. Nordström
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  ABSTRACT: Using state-of-the-art first-principles calculations we have elucidated the complex magnetic and structural dependence of LaOFeAs upon doping. Our key findings are that (i) doping results in an orthorhombic ground state and (ii) there is a commensurate to incommensurate transition in the magnetic structure between $x=0.025$ and $x=0.04$. Our calculations further imply that in this system magnetic order persists up to the onset of superconductivity at the critical doping of $x=0.05$. Finally, our investigations of the undoped parent compound reveal an unusually pronounced dependence of the magnetic moment on details of the exchange-correlation (xc) functional used in the calculation. However, for all choices of xc functional an orthorhombic structure is found.
  11/2008;
• Article: First-principles approach to noncollinear magnetism: towards spin dynamics.

  S Sharma, J K Dewhurst, C Ambrosch-Draxl, S Kurth, N Helbig, S Pittalis, S Shallcross, L Nordström, E K U Gross
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  ABSTRACT: A description of noncollinear magnetism in the framework of spin-density functional theory is presented for the exact exchange energy functional which depends explicitly on two-component spinor orbitals. The equations for the effective Kohn-Sham scalar potential and magnetic field are derived within the optimized effective potential (OEP) framework. With the example of a magnetically frustrated Cr monolayer it is shown that the resulting magnetization density exhibits much more noncollinear structure than standard calculations. Furthermore, a time-dependent generalization of the noncollinear OEP method is well suited for an ab initio description of spin dynamics. We also show that the magnetic moments of solids Fe, Co, and Ni are well reproduced.
  Physical Review Letters 06/2007; 98(19):196405. · 7.37 Impact Factor
• Source

  Available from: S. Kurth

  Article: Comparison of exact-exchange calculations for solids in current-spin-density- and spin-density-functional theory

  S. Sharma, S. Pittalis, S. Kurth, S. Shallcross, J. K. Dewhurst, E. K. U. Gross
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  ABSTRACT: The relative merits of current-spin-density- and spin-density-functional theory are investigated for solids treated within the exact-exchange-only approximation. Spin-orbit splittings and orbital magnetic moments are determined at zero external magnetic field. We find that for magnetic (Fe, Co and Ni) and non-magnetic (Si and Ge) solids, the exact-exchange current-spin-density functional approach does not significantly improve the accuracy of the corresponding spin-density functional results. Comment: 4 pages, 1 fig, 2 tables
  04/2007;
• Source

  Available from: S. Kurth

  Article: First-principles approach to Non-Collinear Magnetism: towards Spin-dynamics

  S. Sharma, J. K. Dewhurst, C. Ambrosch-Draxl, S. Kurth, N. Helbig, S. Pittalis, E. K. U. Gross, S. Shallcross, L. Nordstroem
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  ABSTRACT: A description of non-collinear magnetism in the framework of spin-density functional theory is presented for the exact exchange energy functional which depends explicitly on two-component spinor orbitals. The equations for the effective Kohn-Sham scalar potential and magnetic field are derived within the optimized effective potential (OEP) framework. With the example of a magnetically frustrated Cr monolayer it is shown that the resulting magnetization density exhibits much more non-collinear structure than standard calculations. Furthermore, a time-dependent generalization of the non-collinear OEP method is well suited for an ab-initio description of spin dynamics. We also show that the magnetic moments of solids Fe, Co and Ni are well reproduced.
  11/2005;
• Source

  Available from: arxiv.org

  Article: Magnetism in Exact Exchange Density Functional Theory

  S. Sharma, J. K. Dewhurst, C. Persson, S. Shallcross, L. Nordstroem, C. Ambrosch-Draxl
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  ABSTRACT: The magnetic properties of the intermetallic compound FeAl are investigated using exact exchange density functional theory. This is implemented within a state of the art all-electron full potential method. We find that FeAl is magnetic with a moment of 0.70 $\mu_B$, close to the LSDA result of 0.69 $\mu_B$. A comparison with the non-magnetic density of states with experimental negative binding energy result shows a much better agreement than any previous calculations. We attribute this to the fine details of the exchange field, in particular its asymmetry, which is captured very well with the orbital dependent exchange potential.
  02/2005;
• Source

  Available from: fu-berlin.de

  Article: Magnetism in CeFeAsO1-xFx and LaFeAsO1-xFx from first principles

  Sangeeta Sharma, S. Shallcross, J. K. Dewhurst, A. Sanna, C. Bersier, S. Massidda, E. K. U. Gross
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  ABSTRACT: Using state-of-the-art first-principles calculations we study the magnetic behavior of CeOFeAs. We find the Ce layer moments oriented perpendicular to those of the Fe layers. An analysis of incommensurate magnetic structures reveals that the Ce-Ce magnetic coupling is rather weak with, however, a strong Fe-Fe and Fe-Ce coupling. Comparison of the origin of the tetragonal to orthorhombic structural distortion in CeOFeAs and LaOFeAs shows marked differences; in CeOFeAs the distortion is stabilized by a lowering of spectral weight at the Fermi level, while in LaOFeAs by increase in Fe spin moment. Finally, we investigate the impact of electron doping upon CeOFeAs and LaOFeAs and show that (a) while in CeOFeAs the ground-state Fe moment remains largely unchanged by doping, the stability of magnetic order goes to zero at a doping that corresponds well to the vanishing of the Néel temperature and, (b) in contrast the LaOFeAs system remains magnetic with a slowly vanishing moment as a function of doping.
  Physical Review B, v.80, 184502-1-184502-6 (2009).
• Article: Magnetic phase diagrams from non-collinear canonical band theory

  S. Shallcross, L. Nordström, S. Sharma
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  ABSTRACT: A canonical band theory of non-collinear magnetism is developed and applied to the close packed fcc and bcc crystal structures. This is a parameter-free theory where the crystal and magnetic symmetry and exchange splitting uniquely determine the electronic bands. In this way, we are able to construct phase diagrams of magnetic order for the fcc and bcc lattices. Several examples of non-collinear magnetism are seen to be canonical in origin, in particular, that of γ-Fe. In this approach, the determination of magnetic stability results solely from changes in kinetic energy due to spin hybridization, and on this basis we are able to analyze the microscopic reasons behind the occurrence of non-collinear magnetism in the elemental itinerant magnets.
  Phys. Rev. B. 76(5).