Energy band with Wannier functions of ferromagnetic symmetry as the cause of ferromagnetism in iron

Krüger, Ekkehard

doi:10.1103/physrevb.59.13795

Cited by 20 publications

(30 citation statements)

References 11 publications

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“…V of Ref. [4]. The second assumption is corroborated by the fact that a spin structure alone (together with the related magnetostriction) represents a distortion of the lattice without any spatial distortion.…”

Section: Doped La2−xsrxcuo4supporting

confidence: 49%

“…Observations on the band structures of Fe [4] and Cr [5] suggest that in both Fe and Cr a narrow, roughly half-filled magnetic band is responsible for the stability of the ferromagnetic and the (antiferromagnetic) spin-density wave state, respectively, in these materials. This observation can be interpreted within a nonadiabatic extension of the Heisenberg model, the "nonadiabatic Heisenberg model" (NHM) [6].…”

Section: La2−xsrxcuo4mentioning

confidence: 99%

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Antiferromagnetic, neutral, and superconducting band in La2CuO4

Krüger¹

2005

J Supercond

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The symmetry of the Bloch functions in the conduction band of tetragonal and orthorhombic La2CuO4 is examined for the existence of symmetry-adapted and optimally localizable (usual or spin-dependent) Wannier functions. It turns out that such Wannier functions do not exist in the tetragonal phase. In the orthorhombic phase, on the other hand, the Bloch functions can be unitarily transformed in three different ways into optimally localizable Wannier functions: they can be chosen to be adapted to each of the three phases observed in the pure or doped material, that is, to the antiferromagnetic phase, to the superconducting phase or to the phase evincing neither magnetism nor superconductivity. This group-theoretical result is proposed to be interpreted within a nonadiabatic extension of the Heisenberg model. Within this model, atomiclike states represented by these Wannier functions are responsible for the stability of each of the three phases. However, all the three atomiclike states cannot exist in the tetragonal phase, but are stabilized by the orthorhombic distortion of the crystal. A simple model is proposed which might explain the physical properties of La2−xSrxCuO4 as a function of the Sr concentration x.

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Section: Doped La2−xsrxcuo4supporting

confidence: 49%

Section: La2−xsrxcuo4mentioning

confidence: 99%

Antiferromagnetic, neutral, and superconducting band in La2CuO4

Krüger¹

2005

J Supercond

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“…The nonadiabatic Heisenberg model (NHM) as proposed in this and previous papers 7,8,9,10 extends the original Heisenberg model on the basis of Wannier functions which form an exactly unitary transformation of the Bloch functions of the bands of interest. Within this model, it is not allowed to replace any Bloch function in the calculated band structure by functions with a new symmetry.…”

mentioning

confidence: 99%

“…There is a deep connection between the symmetry of these exact Wannier functions in a metal and the physical properties of the electrons at the Fermi level. 7,8,9,10,11 The original Heisenberg model of magnetism is defined by the assumption that there is exactly one electron on each atom of a metal. The NHM replaces this assumption by introducing three postulates which will be given in the following Sec.…”

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confidence: 99%

Nonadiabatic extension of the Heisenberg model

Krüger

2001

Phys. Rev. B

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The localized states within the Heisenberg model of magnetism should be represented by best localized Wannier functions forming a unitary transformation of the Bloch functions of the narrowest partly filled energy bands in the metals. However, as a consequence of degeneracies between the energy bands near the Fermi level, in any metal these Wannier functions cannot be chosen symmetry-adapted to the complete paramagnetic group M P . Therefore, it is proposed to use Wannier functions with the reduced symmetry of a magnetic subgroup M of M P [case (a)] or spin dependent Wannier functions [case (b)]. The original Heisenberg model is reinterpreted in order to understand the pronounced symmetry of these Wannier functions. While the original model assumes that there is exactly one electron at each atom, the extended model postulates that in narrow bands there are as many as possible atoms occupied by exactly one electron. However, this state with the highest possible atomiclike character cannot be described within the adiabatic (or Born-Oppenheimer) approximation because it requires a more realistic description of the electronic motion. Within the (true) nonadiabatic system the electrons move on localized orbitals that are still symmetric on the average of time, but not at any moment. These nonadiabatic states have the same symmetry as the adiabatic states and determine the commutation properties of the nonadiabatic Hamiltonian H n . The nonadiabatic Heisenberg model is a purely group-theoretical model which interprets the commutation properties of H n that are explicitly given in this paper for the two important cases (a) and (b). There is evidence that the occurrence of these two types of Wannier functions in the band structure of a metal is connected with the occurrence of magnetism and superconductivity, respectively.

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“…In view of our present results on LaFeAsO and former results on the magnetic materials Cr [23], Fe [24], La 2 CuO 4 [22], and YBa 2 Cu 3 O 6 [25], on the high-T c superconductors La 2 CuO 4 [22], YBa 2 Cu 3 O 7 , and MgB 2 [14], and on numerous elemental superconductors [26,27], we propose that the nonadiabatic condensation energy E (see Sect. 2) generally stabilizes magnetism and superconductivity.…”

Section: Discussionmentioning

confidence: 92%

The Reason why Doping Causes Superconductivity in LaFeAsO

Krüger

Strunk

2012

J Supercond Nov Magn

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The experimental observation of superconductivity in LaFeAsO appearing on doping is analyzed with the group-theoretical approach that evidently led in a foregoing paper (Krüger and Strunk in J. Supercond. 24:2103Supercond. 24: , 2011 to an understanding of the cause of both the antiferromagnetic state and the accompanying structural distortion in this material. Doping, like the structural distortions, means also a reduction of the symmetry of the pure perfect crystal. In the present paper we show that this reduction modifies the correlated motion of the electrons in a special narrow halffilled band of LaFeAsO in such a way that these electrons produce a stable superconducting state.

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Energy band with Wannier functions of ferromagnetic symmetry as the cause of ferromagnetism in iron

Cited by 20 publications

References 11 publications

Antiferromagnetic, neutral, and superconducting band in La2CuO4

Antiferromagnetic, neutral, and superconducting band in La2CuO4

Nonadiabatic extension of the Heisenberg model

The Reason why Doping Causes Superconductivity in LaFeAsO

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