Optical Conductivity and Penetration Depth in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>Mg</mml:mi><mml:mrow><mml:msub><mml:mrow><mml:mi>B</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

Пронин, А. В.; Pimenov, A.; Loidl, A.; Krasnosvobodtsev, S. I.

doi:10.1103/physrevlett.87.097003

Cited by 80 publications

(54 citation statements)

References 14 publications

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“…A conduction electron spin resonance study in the frequency range from 9 GHz to 225 GHz also reported an excess signal below T c [26]. A similar observation was made from an optical study on MgB 2 thin films with energies ranging from 0.5 meV to 3.7 meV [39] where the complex conductivity,…”

Section: Introductionmentioning

confidence: 51%

Magnetization and microwave study of superconductingMgB2

et al. 2002

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We report magnetic field dependent magnetization and microwave impedance measurements on a MgB 2 superconductor prepared by high pressure synthesis. We find that the upper critical field is linearly dependent on temperature near T c and the dc irreversibility field exponent is ~1.4. The microwave data display an excess surface resistance below T c which is neither observed in low T c nor in high temperature superconductors (HTSC). The real part of the complex conductivity, σ 1 , shows a huge maximum below T c and the imaginary part, σ 2 , is linear for temperatures less than 20 K, which can not be simply accounted for by the weak coupling BCS model with an s-wave superconducting order parameter. We speculate that this may be due to the two gaps reported by other studies.Unlike measurements on the high temperature superconducting cuprates, we find no evidence of weak-links in the superconducting state. By inverting the magnetic field dependent impedance data, we find a vortex depinning frequency that decreases with increasing magnetic field and evidence for an anisotropic upper critical magnetic field.

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Section: Introductionmentioning

confidence: 51%

Magnetization and microwave study of superconductingMgB2

et al. 2002

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“…On the other hand, we note that there are at least six low-T experiments showing or at least suggesting power-law behaviors which seem to indicate gap nodes (Panagopoulos et al 2001, Zhukov et al 2001, Pronin et al 2001, Ohishi et al 2003. [Admittedly, the lowest temperatures in some of these experiments are too high to be convincing.…”

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confidence: 76%

Strongly anisotropic s-wave gaps in exotic superconductors

Brandow

2003

Philosophical Magazine

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The exotic superconductors, defined as those which follow the phenomenological trend of Uemura (T c approximately µ l L -2 ), presently constitute the most broad and general class of superconductors which can reasonably be considered "similar to the high-T c cuprates". It is therefore of much interest to determine the forms of their pairing gap functions. We examine evidence for the gap forms in non-cuprate exotics, and demonstrate some general features. The cubic materials often have highly anisotropic gaps. The planar materials tend to have even stronger gap anisotropy, to the extent that they often have gap nodes, but nevertheless they usually have an s-wave-like gap symmetry. There is good evidence for the latter even in the controversial cases of planar organics and nickel borocarbides. Exceptions to these generalizations are also pointed out and discussed. §1. INTRODUCTIONThere are now many unusual superconductors which are recognized to share some of the novel features of the high-T c cuprate superconductors. The possibility of further similarity to the cuprates, in particular the possibility of pairing by exchange of a virtual spin fluctuation or paramagnon, has motivated suggestions of d-wave gap forms in some of these other superconductors, in cases where there is evidence for gap nodes. This argument has been applied especially to the planar organics and sometimes also to the 2 nickel borocarbides, in addition to the well known cases of heavy-fermion materials. This line of reasoning calls for a careful examination of the evidence for the gap forms in these and other unusual superconductors.In this paper we focus on the exotic superconductors of Uemura and co-workers. 1 These exotics include a number of the unusual superconductors of earlier (pre-cuprate) interest, as well as the cuprates themselves and others discovered after the cuprates, and they include most of the materials previously suggested as being similar to the cuprates. 2 As discussed below, they now constitute the most broad and general class of superconductors that can reasonably be considered similar to the cuprates. (The variety of crystal structures and material chemistries in this class of materials is remarkably extensive.2 ) It is therefore very reasonable to focus on these materials. We now examine many of the non-cuprate exotic materials and material families, those for which there is reasonable gap-form evidence, in order to establish two general conclusions about their superconducting phenomenology: (1) In the cubic and nearly-cubic materials, which are conventionally expected to have nearly isotropic gaps, there is often evidence of very strong gap anisotropy. Sometimes there even appear to be gap nodes, although the gap symmetry remains s-wave-like. (2) In the planar (quasi-two-dimensional) materials there are often gap nodes, but nevertheless they typically still have an s-wave-like gap symmetry.[Here s-wave-like means having the full point-group symmetry of the crystal structure, a condition which does allow the p...

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“…69 From experience with YBCO, however, it seems that it is necessary to reach temperatures well below 0.05T c to differentiate between different symmetries by methods of microwave response. 70 Other data, derived from muon spin rotation 71 and optical conductivity, 72 yield the penetration-depth ϰT 2 while an ac susceptibility experiment fits the penetrationdepth data with a temperature dependence T 2.7 . 73 These works are clearly inconsistent with the exponential falloff expected for a s-wave gap, Eq.…”

Section: Relation Of Our Findings To Existing Literaturementioning

confidence: 99%

Evidence for non-s-wave symmetry of theπgap inMgB2from intermodulation distortion measurements

Agassi

Oates

Moeckly³

2009

Phys. Rev. B

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We present low-temperature low-power intermodulation-distortion ͑IMD͒ measurements of high-quality MgB 2 thin films that are inconsistent with presumed s-wave symmetry of the order parameter. The measurements were carried out in a stripline resonator at approximately 2 GHz between 1.8 K and T c . The IMD arises from the nonlinear Meissner effect in which the penetration depth is dependent on the RF magnetic field. Specifically, the observed IMD vs temperature T for T Ӷ T c / 2 varies as T −2 , while for an s-wave gap symmetry in the clean limit, the low-temperature IMD decreases exponentially with decreasing temperature. We calculate the IMD from first principles for different order-parameter symmetries using a Green's function approach and compare the results with the measured data. We propose that the observed upturn in the low-temperature IMD implies an admixture of an order parameter with nodal lines into the energy gaps of MgB 2 . Most likely, this admixture is prominent for the gap. Within the constraints of the hexagonal crystal symmetry of MgB 2 , the best fit with our IMD measurements is obtained with a gap ⌬͑ , T͒ = ⌬ 0 ͑T͒sin͑6͒, where is the azimuthal angle in the ab plane, and ⌬ 0 ͑T͒ is the amplitude, weakly temperature dependent at low temperatures. This gap symmetry entails six nodal lines. We also present low-temperature penetration-depth measurements that are consistent with the proposed nodal gap symmetry. To relate our proposition with existing literature, we review other low-temperature probes of the order-parameter symmetry. The literature presents conflicting results, some of which are in direct support of the symmetry proposed here.

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Optical Conductivity and Penetration Depth inMgB2

Cited by 80 publications

References 14 publications

Magnetization and microwave study of superconductingMgB2

Magnetization and microwave study of superconductingMgB2

Strongly anisotropic s-wave gaps in exotic superconductors

Evidence for non-s-wave symmetry of theπgap inMgB2from intermodulation distortion measurements

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