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Cubitt, R.; Levett, S. J.; Bud’ko, Sergey L.; Anderson, N. E.; Canfield, Paul C.

doi:10.1103/physrevlett.90.157002

Cited by 40 publications

(32 citation statements)

References 21 publications

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“…5a, which has been obtained with γ = 3.3, fits the data in the whole range of magnetic fields. However, the value γ = 2.6 is closer to the upper-critical-field anisotropy reported in the literature for MgB 2 at temperatures near T c [5,23,45,46].…”

Section: Field Dependence Of R S In Sample P0supporting

confidence: 82%

“…At low magnetic fields, quasiparticles by π and σ bands are trapped within the vortex core, even if on different spatial scales because of the different coherence lengths ξ π and ξ σ ; in the field range 0.5 ÷ 1 T, though σ-band quasiparticles remain localized, the π-band quasiparticles spread over the sample [22]; on further increasing the field, ∆ π is strongly reduced, the π-quasiparticle contribution remains almost unchanged while the σ-quasiparticle contribution continues to increase with about the same rate up to the macroscopic H c2 . So, a further characteristic field is needed for determining the fluxon-lattice properties of MgB 2 ; the existence of this crossover field, often indicated as H π c2 , has been highlighted in several experiments [1,3,22,23,24,25]. The field-induced evolution of the vortex lattice is expected to affect the vortexvortex and vortex-pinning interactions, making the standard models most likely inadequate to describe the fluxon dynamics.…”

Section: Field Dependence Of R S In Sample P0mentioning

confidence: 99%

“…Scanning tunnelling spectroscopy on MgB 2 single crystals along the c-axis, which probes mainly the π band, has highlighted a core size much larger than the estimates based on the measured H c2 values, as well as a significant core overlap at fields much lower than the macroscopic H c2 [22]. Furthermore, measurements of neutron scattering from the vortex lattice have highlighted a spatial rotation of the vortex lattice for applied magnetic fields in the range 0.5÷1 T [23]. According to point-contact-spectroscopy experiments [24,25,26], these unusual properties have been ascribed to the strong suppression of the superconductivity in the π band, occurring in that field range.…”

Section: Introductionmentioning

confidence: 96%

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Microwave surface resistance of pristine and neutron-irradiated MgB2 samples in magnetic field

et al. 2008

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Abstract. We report on the microwave surface resistance of two polycrystalline Mg 11 B2 samples; one consists of pristine material, the other has been irradiated at very high neutron fluence. It has already been reported that in the strongly irradiated sample the two gaps merge into a single value. The mw surface resistance has been measured in the linear regime as a function of the temperature and the DC magnetic field, at increasing and decreasing fields. The results obtained in the strongly irradiated sample are quite well justified in the framework of a generalized Coffey and Clem model, in which we take into account the field distribution inside the sample due to the critical state. The results obtained in the pristine sample show several anomalies, especially at low temperatures, which cannot be justified in the framework of standard models for the fluxon dynamics. Only at temperatures near Tc and for magnetic fields greater than 0.5Hc2(T ) the experimental data can quantitatively be accounted for by the Coffey and Clem model, provided that the upper-critical-field anisotropy is taken into due account. PACS

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Section: Field Dependence Of R S In Sample P0supporting

confidence: 82%

Section: Field Dependence Of R S In Sample P0mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 96%

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Microwave surface resistance of pristine and neutron-irradiated MgB2 samples in magnetic field

et al. 2008

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“…These are manifested in measurements of the ratios of the upper critical fields γ H = H ⊥c c2 /H c c2 for magnetic fields along the c−axis and the ab−plane and of the London penetration depth γ λ = λ c /λ ab . 1,2,3,4,5,6,7,8,9,10,11,12,13,14 The preponderance of experiments are in agreement with regard to the temperature dependence of γ H , which shows increase from γ H ≈ 2.0 around T c to γ H ≈ 6 at low temperatures. 6,7,14 However, the temperature dependence of γ λ , in particular under applied magnetic field, is not settled yet.…”

Section: Introductionsupporting

confidence: 67%

Magnetic-field-induced orientation of superconductingMgB2crystallites determined by x-ray diffraction

Vaknin

Bud’ko

et al. 2006

Phys. Rev. B

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X-ray diffraction studies of fine polycrystalline samples of MgB2 in the superconducting state reveal that crystals orient with their c-axis in a plane normal to the direction of the applied magnetic field. The MgB2 samples were thoroughly ground to obtain average grain size 5 -10 µm in order to increase the population of free single crystal grains in the powder. By monitoring Bragg reflections in a plane normal to an applied magnetic field we find that the powder is textured with significantly stronger (0,0,l ) reflections in comparison to (h,k,0 ), which remain essentially unchanged. The orientation of the crystals with the ab-plane parallel to the magnetic field at all temperatures below Tc demonstrates that the sign of the torque under magnetic field does not alter, in disagreement with current theoretical predictions.

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“…For these calculations we take λ a (0) = λ c (0) = 1000Å in accord with µSR and neutron studies [19,20,21]. We will discuss the effect of varying λ a (0) and λ c (0) later.…”

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confidence: 99%

Temperature-Dependent Anisotropy of the Penetration Depth and Coherence Length ofMgB2

et al. 2005

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We report measurements of the temperature dependent anisotropies (γ λ and γ ξ ) of both the London penetration depth λ and the upper critical field of MgB2. Data for γ λ = λc/λa was obtained from measurements of λa and λc on a single crystal sample using a tunnel diode oscillator technique. γ ξ = H c c2 /H ⊥c c2 was deduced from field dependent specific heat measurements on the same sample. γ λ and γ ξ have opposite temperature dependencies, but close to Tc tend to a common value (γ λ ≃ γ ξ = 1.75 ± 0.05). These results are in good agreement with theories accounting for the two gap nature of MgB2.The existence of two superconducting gaps in MgB 2 [1] leads to an unusually strong temperature dependence in the anisotropy of the upper critical field, Refs. 2,3,4,5). The maximum value of γ ξ ≃ 6 is found at the lowest temperatures where it is dominated by the anisotropy in the quasi two dimensional σ bands. At higher temperatures, γ ξ is substantially reduced due to the increasing contribution of the more isotropic π band [5]. The anisotropy of the penetration depth, γ λ = λ c /λ a , is predicted to have a markedly different temperature dependence to that of H c2 [4,6]. At zero temperature, in the clean local limit, the anisotropy in the penetration depth is determined only by the anisotropy in the Fermi velocity v F and not by the anisotropy in the gap [7]. Band structure calculations [8] show that the average v F is approximately isotropic suggesting that λ is similarly isotropic at low temperature [4]. As temperature is increased it has been predicted [4,6] that γ λ increases markedly because of the more rapid reduction of the superfluid density on the π sheets (where the gap is smaller) than on the σ sheets.There have been several experimental measurements of the temperature dependence of γ ξ . Although there were some disagreements in early measurements, the most recent data [2, 3, 9] agree well with the theoretical predictions [4,5]. Significantly less experimental data has been reported on γ λ . Measurements of the distortion of the vortex lattice by neutron scattering [10] and scanning tunneling spectroscopy [11] suggest that at low temperature the anisotropy is small, γ λ = 1.2 ± 0.1. γ λ has also been estimated from the anisotropy of H c1 (γ Hc1 = H c c1 /H ⊥c c1 ). There has been some disagreement between different studies (see Ref. 12) but recent data [12] suggest that at low T , γ Hc1 ≃ 1 and there is an upward trend in γ Hc1 near T c . However, γ Hc1 may not be simply related to γ λ in MgB 2 because of two gap effects. In this paper, we present direct measurements of the temperature dependencies of λ a and λ c in single crystal samples of MgB 2 in the Meissner state, using a sensitive radio frequency technique. Our data show that γ λ increases as T → T c , in agreement with theoretical predictions. Measurements of H c2 anisotropy in the same crystals show that the values of γ λ and γ ξ converge as T → T c .Single crystals of MgB 2 were grown using a high pressure technique described elsewhere [13]. Som...

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Experimental Evidence for Anisotropic Double-Gap Behavior inMgB2

Cited by 40 publications

References 21 publications

Microwave surface resistance of pristine and neutron-irradiated MgB2 samples in magnetic field

Microwave surface resistance of pristine and neutron-irradiated MgB2 samples in magnetic field

Magnetic-field-induced orientation of superconductingMgB2crystallites determined by x-ray diffraction

Temperature-Dependent Anisotropy of the Penetration Depth and Coherence Length ofMgB2

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