Materials and mechanisms of hole superconductivity

Hirsch, J. E.

doi:10.1016/j.physc.2011.10.006

Cited by 16 publications

(17 citation statements)

References 47 publications

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“…To conclude we point out that a strong argument in favor of our point of view is that if it is correct for the cuprates it is likely to also explain the high-temperature superconductivity of several other materials, such as MgB 2 [59], iron pnictides, and dichalcogenides [60] and explain the reason for why all hightemperature superconducting materials appear to have negative ions [61,62], an observation also made by Overhauser [63]. It also suggests an explanation for why elements under high pressure are superconducting [64] for the fact that electron-doped cuprates have hole carriers in the regime where they become superconducting [65] and for the pervasive presence of hole carriers in superconducting materials ranging from the elements [66] to A15 compounds [67] to MgB 2 [68] to high-T c cuprates.…”

Section: Discussionmentioning

confidence: 79%

Effect of orbital relaxation on the band structure of cuprate superconductors and implications for the superconductivity mechanism

Hirsch

2014

Phys. Rev. B

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Where the doped holes reside in cuprate superconductors has crucial implications for the understanding of the mechanism responsible for their high-temperature superconductivity. It has been generally assumed that doped holes reside in hybridized Cu d x 2 −y 2 -O pσ orbitals in the CuO 2 planes, based on results of density functional band-structure calculations. Instead, we propose that doped holes in the cuprates reside in O pπ orbitals in the plane, perpendicular to the Cu-O bond, that are raised to the Fermi energy through local orbital relaxation, that is not taken into account in band-structure calculations that place the bands associated with these orbitals well below the Fermi energy. We use a dynamic Hubbard model to incorporate the orbital relaxation degree of freedom and find in exact diagonalization of a small Cu 4 O 4 cluster that holes will go to the O pπ orbitals for relaxation energies comparable to what is expected from atomic properties of oxygen anions. The bandwidth of this band becomes significantly smaller than predicted by band-structure calculations due to the orbital relaxation effect. Within the theory of hole superconductivity the heavy hole carriers in this almost full band will pair and drive the system superconducting through lowering of their quantum kinetic energy.

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Section: Discussionmentioning

confidence: 79%

Effect of orbital relaxation on the band structure of cuprate superconductors and implications for the superconductivity mechanism

Hirsch

2014

Phys. Rev. B

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“…Alternatively, one may describe the predicted effect as an apparent increase of the atomic number of the ions in the material, Z. It is interesting to note that the theory predicts the largest tendency for superconductivity for materials where conduction occurs through negatively charged ions [17], hence the effective ionic charge Z sensed by the conduction electrons in the normal state is smallest, and the excess negative charge in the conducting substructures is largest. As the sample becomes superconducting it attempts to counter these effects.…”

Section: Discussionmentioning

confidence: 99%

Apparent increase in the thickness of superconducting particles at low temperatures measured by electron holography

Hirsch¹

2013

Ultramicroscopy

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We predict that superconducting particles will show an apparent increase in thickness at low temperatures when measured by electron holography. This will result not from a real thickness increase, rather from an increase in the mean inner potential sensed by the electron wave traveling through the particle, originating in expansion of the electronic wavefunction and resulting negative charge expulsion from the interior to the surface of the superconductor, giving rise to an increase in the phase shift of the electron wavefront going through the sample relative to the wavefront going through vacuum. The temperature dependence of the observed phase shifts will yield valuable new information on the physics of the superconducting state of metals. PACS numbers:FIG. 1: Predicted interference micrograph of a spherical superconducting particle of radius 500nm, mean inner potential V0 = 10V , London penetration depth λL = 40nm and lower critical magnetic field Hc1 = 100G with 300kV electrons, twotimes phase amplified. The full lines indicate the contours of constant phase above Tc, the dashed lines at temperatures well below Tc.

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“…With a high degree of BZ filling the free Fermi surface area becomes small, which is accompanied by an increase in density of state below the k F . The consequences of this FS-BZ configuration are the decrease in reflectivity, increase in resistivity, appearance, and increase in superconductivity, as discussed by Hirsch [38,39].…”

Section: Structure Stability and Correlation With Properties Within Tmentioning

confidence: 98%

Electronic Origin of the Orthorhombic Cmca Structure in Compressed Elements and Binary Alloys

Degtyareva

2013

Crystals

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Formation of the complex structure with 16 atoms in the orthorhombic cell, space group Cmca (Pearson symbol oC16), was experimentally found under high pressure in the alkali elements (K, Rb, Cs) and polyvalent elements of groups IV (Si, Ge) and V (Bi). Intermetallic phases with this structure form under pressure in binary Bi-based alloys (Bi-Sn, Bi-In, Bi-Pb). Stability of the Cmca-oC16 structure is analyzed within the nearly free-electron model in the frame of Fermi sphere-Brillouin zone interaction. A Brillouin-Jones zone formed by a group of strong diffraction reflections close to the Fermi sphere is the reason for the reduction of crystal energy and stabilization of the structure. This zone corresponds well to the four valence electrons in Si and Ge, and leads to assume an spd-hybridization for Bi. To explain the stabilization of this structure within the same model in alkali metals, that are monovalents at ambient conditions, a possibility of an overlap of the core, and valence band electrons at strong compression, is considered. The assumption of the increase in the number of valence electrons helps to understand sequences of complex structures in compressed alkali elements and unusual changes in their physical properties, such as electrical resistance and superconductivity.

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Materials and mechanisms of hole superconductivity

Cited by 16 publications

References 47 publications

Effect of orbital relaxation on the band structure of cuprate superconductors and implications for the superconductivity mechanism

Effect of orbital relaxation on the band structure of cuprate superconductors and implications for the superconductivity mechanism

Apparent increase in the thickness of superconducting particles at low temperatures measured by electron holography

Electronic Origin of the Orthorhombic Cmca Structure in Compressed Elements and Binary Alloys

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