Boron Isotope Effect in Superconducting<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>

Bud’ko, Sergey L.; Lapertot, G.; Petrović, C.; Cunningham, Charles E.; Anderson, N. E.; Canfield, Paul C.

doi:10.1103/physrevlett.86.1877

Cited by 925 publications

(624 citation statements)

References 8 publications

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“…Sample 1 and the commercial powder show smooth transitions with some temperature dependence of the zerofield cooled (ZFC) shielded moment, while sample 3 exhibits a step, indicative of non-uniformity in superconducting properties. These transitions are not as sharp as those reported by Bud'ko et al 5 for MgB 2 prepared in sealed Ta and quartz ampules. Figure 2 shows results of the large H-T range VSM examination.…”

contrasting

confidence: 56%

Strongly linked current flow in polycrystalline forms of the superconductor MgB2

Larbalestier

Cooley

Rikel

et al. 2001

Nature

911

533

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The discovery of superconductivity at 39 K in MgB 2 1 raises many issues. One of the central questions is whether this new superconductor resembles a hightemperature-cuprate superconductor or a lowtemperature metallic superconductor in terms of its current carrying characteristics in applied magnetic fields. In spite of the very high transition temperatures of the cuprate superconductors, their performance in magnetic fields has several drawbacks 2 . Their large anisotropy restricts high bulk current densities to much less than the full magnetic field-temperature (H-T) space over which superconductivity is found. Further, weak coupling across grain boundaries makes transport current densities in untextured polycrystalline forms low and strongly magnetic field sensitive 3,4 . These studies of MgB 2 address both issues. In spite of the multi-phase, untextured, nano-scale sub-divided nature of our samples, supercurrents flow throughout without the strong sensitivity to weak magnetic fields characteristic of Josephson-coupled grains 3 . Magnetization measurements over nearly all of the superconducting H-T plane show good temperature scaling of the flux pinning force, suggestive of a current density determine d by flux pinning. At least two length scales are suggested by the magnetization and magneto optical (MO) analysis but the cause of this seems to be phase inhomogeneity, porosity, and minority insulating phase such as MgO rather than by weakly coupled grain boundaries. Our results suggest that polycrystalline ceramics of this new class of superconductor will not be compromised by the weak link problems of the high temperature superconductors, a conclusion with enormous significance for applications if higher temperature analogs of this compound can be discovered.The principal samples were synthesized by direct reaction of bright Mg flakes (Aldrich Chemical) and sub-micron amorphous B powder (Callery Chemical). Starting materials were lightly mixed in half-gram batches, and pressed into pellets. These pellets were placed on Ta foil, which was in turn placed on Al 2 O 3 boats, and fired in a tube furnace under a mixed gas of 95% Ar 5% H 2 for 1 hour at 600 C, 1 hour at 800 C, and 1 hour at 900 C, and then lightly ground. The resulting powders were pressed into pellets and then hot pressed at 10 kbar at temperatures between 650 and 800 °C for periods between 1 and 5.5 hours. Disks ~4 mm in diameter and ~1 mm thick were cut from these pellets for property characterization. As noted later, this process cannot yet be considered optimum.Magnetization properties were examined in SQUID and vibrating sample magnetometers (VSM) in applied fields up to 14 T from 4.2 to above T c . Figure 1 shows onset T c values of 37-38 K for the above samples and for commercial MgB 2 powder (99.5%, ~2 µm diameter by examination, CERAC). Sample 1 and the commercial powder show smooth transitions with some temperature dependence of the zerofield cooled (ZFC) shielded moment, while sample 3 exhibits a step, indicative of non-uniformity in su...

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contrasting

confidence: 56%

Strongly linked current flow in polycrystalline forms of the superconductor MgB2

Larbalestier

Cooley

Rikel

et al. 2001

Nature

911

533

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“…Electron-phonon mechanism of SC has been successfully applied to explain the pairing in a large variety of materials, from Hg and Al to recently discovered and extensively studied MgB 2 with T c as high as 39K [12]. The phonon density of states, obtained by inelastic neutron scattering experiments, and the spectrum of the bosons which mediate pairing, as deduced from tunneling experiments, agree very well in systems like, e.g., Pb [13,14].…”

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confidence: 99%

Superconductivity from repulsive interaction

Maiti¹,

Chubukov²

2014

Novel Superfluids

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Abstract. The BCS theory of superconductivity named electron-phonon interaction as a glue that overcomes Coulomb repulsion and binds fermions into pairs which then condense and super-conduct. We review recent and not so recent works aiming to understand whether a nominally repulsive Coulomb interaction can by itself give rise to a superconductivity. We first discuss a generic scenario of the pairing by electron-electron interaction, put forward by Kohn and Luttinger back in 1965, and then turn to modern studies of the electronic mechanism of superconductivity in the lattice models for the cuprates, the Fe-pnictides, and the doped graphene. We show that the pairing in all three classes of materials can be viewed as lattice version of Kohn-Luttinger physics, despite that the pairing symmetries are different. We discuss under what conditions the pairing occurs and rationalize the need to do parquet renormalization-group analysis. We also discuss the interplay between superconductivity and density-wave instabilities.

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“…Our Raman measurements were made on a polycrystalline MgB 2 sample prepared in sealed Ta tubes as described previously [23]. X-ray powder-diffraction analysis confirmed single-phase purity and an AlB 2 -type structure for our MgB 2 samples.…”

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Role of the E2g phonon in the superconductivity of MgB2: a Raman scattering study

Martinho

Rettori

Pagliuso

et al. 2003

Solid State Communications

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Temperature-dependent Raman scattering studies in polycrystalline MgB2 (10 < T < 300 K) reveal that the E2g phonon does not experience any self-energy renormalization effect across the superconducting critical temperature TC ≈ 39 K, in contrast with most of the current theoretical models. In the presence of our results, those models must be reviewed. The analysis of the temperature dependence of the E2g phonon frequency yields an isobaric Grüneisen parameter of | γE 2g | 1, smaller than the value of 3.9 obtained from isothermal Raman experiments under pressure. It is suggested that this apparent disagreement can be explained in terms of pressureinduced changes of the topology of the Fermi surface. MgB 2 has attracted much recent interest due to its remarkable physical properties such as: i) relatively high superconducting transition, T C = 39 K, for a binary compound with a simple crystal structure, ii) large and anisotropic coherence lengths, critical fields and current densities, and iii) critical currents that are not limited by grain boundaries (absence of weak link effects). [1] MgB 2 forms in a hexagonal structure with space group P6/mmm (D 1 6h ). The B atoms are located on a primitive honeycomb lattice consisting of graphite-type sheets. The B 2 -layers are intercalated with Mg-layers that also form a honeycomb lattice with a Mg atom in the center. For this space group, factor-group analysis predicts four modes at the Γ point: E u + A 2u + E 2g + B 1g , where only the E 2g mode is Raman-active and the B 1g mode is silent. The E 2g phonon is a doubly degenerate in-plane B-B bond-stretching mode[2] with nonvanishing Raman tensor elements (α xx − α yy ) and α xy . First principles lattice dynamics calculations indicate that these modes would be observed at 327 cm −1 (E u ), 405 cm −1 (A 2u ), 572 cm −1 (E 2g ) and 702 cm −1 (B 1g ). [3] In analogy with high T C superconductors (HTS), Hall effect measurements [4] indicate that the charge carriers in MgB 2 are holes with a hole density at 300 K of 1.7 − 2.8×10 23 holes/cm 3 . In fact, hole-mediated su- * Electronic address: hercules@ifi.unicamp.br; URL: http://www.ifi.unicamp.br/gpoms perconductivity has been proposed by An and Pickett for MgB 2 .[5] These authors attribute the relatively high value of T C to the strong coupling between holes and the in-plane boron phonon, E 2g modes. According to this model, the holes originate in the σ (sp 2 orbitals) bands due to charge transfer from the σ bands to the π (p z orbitals) bands. Based on this model, several papers have discussed the possibility of E 2g phonon being a frozen-in mode, strongly coupled to the σ electronic bands near the Fermi level. [2,3,6,7] It is claimed that the E 2g phonon, due to its rather strong coupling to the σ electronic bands, would be highly anharmonic, presenting a very large linewidth. However, Boeri et al [8] pointed out that neither the presence at the Fermi level of the σ bands nor their strong coupling to the E 2g phonon are sufficient to induce such anharmonic effects, and the...

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Boron Isotope Effect in SuperconductingMgB2

Cited by 925 publications

References 8 publications

Strongly linked current flow in polycrystalline forms of the superconductor MgB2

Strongly linked current flow in polycrystalline forms of the superconductor MgB2

Superconductivity from repulsive interaction

Role of the E2g phonon in the superconductivity of MgB2: a Raman scattering study

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