Evidence for Strong-Coupling<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi mathvariant="italic">s</mml:mi></mml:math>-Wave Superconductivity in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>MgB</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>:<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi>B</m

Kotegawa, Hisashi; Ishida, K.; Kitaoka, Y.; Muranaka, Toshiya; Akimitsu, Jun

doi:10.1103/physrevlett.87.127001

Cited by 175 publications

(113 citation statements)

References 11 publications

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“…We measured the T dependence of the 11 B spin-lattice relaxation time, T 1 , in Samples 2 and 3 and found a metallic, T independent value of 1/(T T 1 )= 167±3 s −1 in agreement with Ref. [17] and [18]. It is possible that the difference in morphology and purity at the grain surfaces explains that the CESR signal intensity is almost constant in Sample 1 while it changes strongly in Sample 2 and 3.…”

supporting

confidence: 81%

Magnetic-field-induced density of states inMgB2: Spin susceptibility measured by conduction-electron spin resonance

et al. 2005

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The magnetic field dependence of the spin-susceptibility, χ s was measured in the superconducting state of high purity MgB2 fine powders below 1.3 T. χ s was determined from the intensity of the conduction electron spin resonance spectra at 3.8, 9.4, and 35 GHz. At the lowest magnetic fields (0.14 T), a gap opens in the density of states at the Fermi energy and, accordingly, χ s (T ) is small at low temperatures. Fields above 0.2 T (about 15 % of H c c2 , the minimum upper critical field), destroy the gap. The field induced χ s is much larger than expected from current superconductor models of MgB2. [4] estimate that about half of the density of states (DOS) is on the π band thus the two-gap model predicts that closing the gap at 1 T restores about half of the normal state DOS. Naively, one would expect that properties proportional to the DOS, like the specific heat coefficient or the spin-susceptibility should reach half of their normal state values at about 1 T, whereas the remaining half originating from the σ bands should be restored gradually with further increasing the field to H c2 .In this Letter, we report on the magnetic field dependence of the conduction electron spin-susceptibility, χ s , in the superconducting state of high purity MgB 2 powders. This is the first time χ s is measured using conduction electron spin resonance below T c in a superconductor. We observe an unusually strong increase of χ s with the magnetic field, H, which cannot be reconciled with current models easily. The DOS induced by fields well below H c2 is larger than 50 %, the value expected from closing the gap on the π band FS only.We studied samples from several batches. Sample 1 is made from 99.99 % purity natural isotopic mixture of 10 B and 11 B amorphous boron, Samples 2 and 3 are made from crystalline, isotopically pure 11 B. Chemical analysis, the high normal state conductivities and narrow CESR lines at T c attest the high purity of the samples. Details of sample characterization are discussed in Ref. [14]. Sample 2 was made from the batch used in the CESR work of Ref. [11]. Fine powders with grain sizes less than 1 µm were selected from the starting materials to reduce the inhomogeneity of microwave excitation. The aggregates of small grains of the starting materials were first thoroughly hand-crushed in a borocarbide mortar. The resulting powders were suspended in isopropanol and mixed in a 1:1 weight ratio with pure SnO 2 fine powder. The larger particles were eliminated by sedimentation for long times or in a centifuge and the small grains were extracted from the suspension by filtering. Mixing with SnO 2 is important to separate the MgB 2 particles to avoid eddy-currents. SEM microscopy has confirmed that this procedure results in MgB 2 grains smaller than 0.5 µm size. SQUID magnetometry of the original batch and the final fine powders confirmed that superconducting properties were not affected by this procedure. The powders were finally cast into epoxy for the ESR experiments. We detail CESR experiments at 3.8, ...

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confidence: 81%

Magnetic-field-induced density of states inMgB2: Spin susceptibility measured by conduction-electron spin resonance

et al. 2005

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“…A weak inter-band impurity scat-tering but strong intra-band scattering can suppress the coherence peak. This has been investigated in MgB 2 33 where the coherence peak is also not easily observed experimentally 34 . Since the superconductivity in Fe-based superconductors is created by doping, it is reasonable to assume that disorder is stronger than that in MgB 2 .…”

Section: Fig 10mentioning

confidence: 97%

Experimental consequences of thes-wavecos(kx)cos(ky
Parish
¹
,
Hu
²
,
Bernevig
³

2008
Phys. Rev. B
116
5
105
2
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The experimental consequences of different order parameters in iron-based superconductors are theoretically analyzed. We consider both nodeless and nodal order parameters, with an emphasis on the cos(kx) · cos(ky) nodeless order parameter recently derived by two of us 1 . We analyze the effect of this order parameter on the spectral function, density of states, tunneling differential conductance, penetration depth, and the NMR spin relaxation time. This extended s-wave symmetry has line-zeroes in between the electron and hole pockets, but they do not intersect the two Fermi surfaces for moderate doping, and the superconductor is fully gapped. However, this suggests several quantitative tests: the exponential decay of the penetration depth weakens and the density of states reveals a smaller gap upon electron or hole doping. Moreover, the cos(kx) · cos(ky) superconducting gap is largest on the smallest (hole) Fermi surface. For the 1/T1 NMR spin relaxation rate, the inter-band contribution is consistent with the current experimental results, including a (nonuniversal) T 3 behavior and the absence of a coherence peak. However, the intra-band contribution is considerably larger than the inter-band contributions and still exhibits a small enhancement in the NMR spin relaxation rate right below Tc in the clean limit.Introduction -The recent discovery of iron-based superconductors with a transition temperature as high as 55K has stimulated a flurry of experimental and theoretical activity 2,3,4,5,6,7,8,9,10,11 . However, a conclusive observation of the pairing symmetry still remains elusive, with both nodal and nodeless order parameters reported in experimental observations. Numerical and analytic research suggests that the antiferromagnetic exchange coupling between Fe sites is strong 12,13,14 . Owing to As-mediated hopping, antiferromagnetic exchange exists not only between the nearest neighbor (NN) Fe sites, but also between next nearest neighbor (NNN) sites. Moreover, the NNN coupling strength J 2 is stronger than the NN coupling strength J 1 . The J 1 − J 2 model produces half-filled magnetic physics consistent with experimental neutron data 15 . A nematic magnetic phase transition has been predicted in this model 16,17 , consistent with the experimental observation of a structural transition preceding the spin density wave (SDW) formation. This model suffers, however, from an important deficiency -it is an insulator whereas the real material is an, albeit bad, metal. We, however, believe that the spin-spin interaction insight is important to the physics of the iron-pnictides.In a recent paper 1 , two of us added electron itineracy to the problem and studied a t − J 1 − J 2 model without band renormalization. We found that the singletforming J 1 − J 2 interaction gives rise to four possible pairing symmetries: cos(k x ) ± cos(k y ), sin(k x ) · sin(k y ) and cos(k x ) · cos(k y ). The last two are strongly preferred from an interaction standpoint when J 2 > J 1 , but only cos(k x ) · cos(k y ) matches the symmetry...

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“…The T C is slightly higher than the theoretical upper limit predicted for phonon-mediated superconductivity [2], which had been widely accepted until the discovery of the cuprate superconductors. However, 11 B-NMR studies [3] and the B isotope effect [4] showed that MgB 2 is a conventional BCS superconductor.…”

Section: Introductionmentioning

confidence: 99%

Synthesis and photoemission study of as-grown superconducting MgB2 thin films

Ueda

Yamamoto

Naito

2002

Physica C: Superconductivity

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As-grown superconducting thin films of MgB 2 were prepared by molecular beam epitaxy (MBE), and studied by X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS). Only films prepared at temperatures between 150 and 320 showed superconductivity. The best T C onset of 36 K was obtained with a sharp transition width of 1 K although the film crystallinity was poor. The insitu photoelectron spectra obtained on the surfaces of the MBE grown MgB 2 films were free from dirt peaks. The XP spectra revealed the binding energy of the Mg 2p levels in MgB 2 is close to that of metallic Mg and the binding energy of B 1s is close to that of transition-metal diborides. The valence UP spectra showed a clear Fermi edge although the density of states at E F is low and the major components of the valence band are located between 5 and 11 eV.

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Evidence for Strong-Couplings-Wave Superconductivity inMgB2:B

Cited by 175 publications

References 11 publications

Magnetic-field-induced density of states inMgB2: Spin susceptibility measured by conduction-electron spin resonance

Magnetic-field-induced density of states inMgB2: Spin susceptibility measured by conduction-electron spin resonance

Experimental consequences of thes-wavecos(kx)cos(ky
Parish
¹
,
Hu
²
,
Bernevig
³

2008
Phys. Rev. B
116
5
105
2
View full text Add to dashboard Cite

Synthesis and photoemission study of as-grown superconducting MgB2 thin films

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Evidence for Strong-Couplings-Wave Superconductivity inMgB2:B

Cited by 175 publications

References 11 publications

Magnetic-field-induced density of states inMgB2: Spin susceptibility measured by conduction-electron spin resonance

Magnetic-field-induced density of states inMgB2: Spin susceptibility measured by conduction-electron spin resonance

Experimental consequences of thes-wavecos(kx)cos(kyParish1, Hu2, Bernevig3 2008Phys. Rev. B11651052View full textAdd to dashboardCite

Synthesis and photoemission study of as-grown superconducting MgB2 thin films

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Product

Resources

About

Experimental consequences of thes-wavecos(kx)cos(ky
Parish
¹
,
Hu
²
,
Bernevig
³

2008
Phys. Rev. B
116
5
105
2
View full text Add to dashboard Cite