Superconducting critical fields and anisotropy of a MgB2 single crystal

Perkins, G. K.; Moore, J D; Bugoslavsky, Y.; Cohen, L. F.; Jun, J.; Казаков, С. М.; Karpiński, J.; Caplin, A.D.

doi:10.1088/0953-2048/15/7/330

Cited by 49 publications

(44 citation statements)

References 18 publications

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“…6͑b͒ yielded to be ab (0)ϳ10 nm and c (0)ϳ3 nm. These values are similar to the previous results obtained from magnetic measurements on powder samples 27 or on single crystals [19][20][21][22][23] and the values determined from the calculation using the two band model, 28 but do not agree with the reported ␥-values determined from resistivity measurements on bulk samples [15][16][17][18]10,11,29 and c-axis-oriented films, [12][13][14] which are around 1.1 to 3, as shown in Table II.…”

Section: Anisotropy Of the Upper Critical Fieldsupporting

confidence: 85%

Ambient-pressure synthesis of single-crystalMgB2and their superconducting anisotropy

et al. 2003

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We synthesized single crystalline MgB 2 under ambient pressure by using conventional materials and equipment. The single crystals of MgB 2 were of good quality, where the crystal structure refinements were successfully converged with Rϭ0.020. The measurements of the magnetic properties yielded a sharp superconducting transition at 38 K with transition width ⌬T c ϭ0.8 K. The upper critical field for applied field parallel to the ab plane (H c2 ab ) reveals a positive curvature, while H c2 parallel to the c axis (H c2 c ) increases linearly in temperature dependence, which yields a temperature dependence of the superconducting anisotropy ratio of ␥ ϭH c2 ab /H c2 c with ␥ϳ1 near T c and 4.0 at 25 K.

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Section: Anisotropy Of the Upper Critical Fieldsupporting

confidence: 85%

Ambient-pressure synthesis of single-crystalMgB2and their superconducting anisotropy

et al. 2003

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“…As T decreases, the ratio H c2 || /H c2 ⊥ in both cases decreases, from ≈ 2 at T c down to ≈ 1.5 at 0 K. This behavior is opposite to the usual temperature dependence of H c2 (T) in MgB 2 single crystals for which the anisotropy parameter γ(T) = H c2 || /H c2 ⊥ typically increases from ≈ 3 at T c up to 5-6 at 0 K [8][9][10][11][12] . This unusual dependence of γ(T) for the dirty film is due to the large difference between σ and π electron diffusivities (D π ∼ 0.1D σ ).…”

Section: B) H C2 Anisotropymentioning

confidence: 72%

“…Equations (2) and (3) enable us to address the observed anomalous temperature dependence of the anisotropy of H c2 (θ) [8][9][10][11][12] , as described in detail elsewhere 36 . Here we only use the fact that for the parallel field orientation, H||ab, the effective diffusivities in Equations (2)- (4) …”

Section: Discussion A) Two-gap Dirty Limit Theorymentioning

confidence: 99%

Very high upper critical fields in MgB₂produced by selective tuning of impurity scattering

Gurevich

Patnaik

Braccini

et al. 2003

Supercond. Sci. Technol.

294

175

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We report a significant enhancement of the upper critical field H c2 of different MgB 2 samples alloyed with nonmagnetic impurities. By studying films and bulk polycrystals with different resistivities ρ, we show a clear trend of H c2 increase as ρ increases. One particular high resistivity film had zero-temperature H c2 (0) well above the H c2 values of competing non-cuprate superconductors such as Nb 3 Sn and Nb-Ti. Our high-field transport measurements give record values H c2 ⊥ (0) ≈ 34T and H c2 || (0) ≈ 49 T for high resistivity films and H c2 (0) ≈ 29 T for untextured bulk polycrystals. The highest H c2 film also exhibits a significant upward curvature of H c2 (T), and temperature dependence of the anisotropy parameter γ(T) = H c2 || / H c2⊥ opposite to that of single crystals: γ(T) decreases as the temperature decreases, from γ(T c ) ≈ 2 to γ(0) ≈ 1.5. This remarkable H c2 enhancement and its anomalous temperature dependence are a consequence of the two-gap superconductivity in MgB 2 , which offers special opportunities for further H c2 increase by tuning of the impurity scattering by selective alloying on Mg and B sites. Our experimental results can be explained by a theory of two-gap superconductivity in the dirty limit. The very high values of H c2 (T) observed suggest that MgB 2 can be made into a versatile, competitive high-field superconductor.

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“…As mentioned, for MgB 2 , the ratio (16) this is the magnetization in intermediate fields, M / ð 0 = 2 Þ lnðH c2 =HÞ, the main contribution to anisotropy comes from , however, the H c2 anisotropy contributes as well (being smoothed by the logarithm). The last situation should be taken into account while extracting anisotropy from the torque data in tilted fields, 25,26) the point stressed by Angst. 27) Given the simplifying assumptions about the Fermi surfaces we have made and our results for the H c2 anisotropy which reproduce qualitatively well the measured anisotropy, we believe that the anisotropic properties of H c2 and we have described, are generic for materials with anisotropic gaps and Fermi surfaces.…”

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confidence: 99%

Anisotropy of the Upper Critical Field in Superconductors with Anisotropic Gaps: Anisotropy Parameters of MgB₂

2003

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The upper critical field H c2 is evaluated for weakly-coupled two-band anisotropic superconductors. By modeling the actual bands and the gap distribution of MgB 2 by two Fermi surface spheroids with average parameters of the real material, we show that H c2;ab =H c2;c increases with decreasing temperature in agreement with available data.The anisotropic Ginzburg-Landau (GL) equations, derived for clean superconductors with arbitrary gap anisotropy by Gor'kov and Melik-Barkhudarov, 1) led to a common practice of characterizing materials by a single anisotropy parameter defined as a = c c = a ( is the coherence length, is the penetration depth, and a; c are principal directions of a uniaxial crystal of the interest here). Formally, this came out because in the GL domain, the same ''mass tensor'' determines the anisotropy of both (of the upper critical fields H c2 ) and of .At arbitrary temperatures, however, the ratios of H c2 's and of 's are not necessarily the same. We demonstrate below that in materials with anisotropic Fermi surfaces and anisotropic gaps, not only H c2;a =H c2;c may strongly depend on T, but this ratio might differ considerably from c = a at low T's. Our arguments are based on the weak-coupling model of superconductivity for simple Fermi surfaces and gap anisotropies; as such they are at best qualitative. Still, being applied to MgB 2 , they provide satisfactory description of existing data for the H c2 anisotropy.There are two different approaches in literature in describing macroscopics of the ''two-gap'' superconductivity of MgB 2 . One of these treats the anisotropy of interaction by introducing a coupling matrix ij for intra-and interband pair transfer. Various relations between the matrix elements yield variety of macroscopic consequences. One of the realizations of the model considers two order parameters with two distinct phases which may lead to various static 2,3) and dynamic 4) effects. Certain relations between elements ij provide the experimentally observed gaps and their ratio. The approach adopted in this paper considers the gap on two Fermi sheets just as a particular case of the gap anisotropy, the gap ratio being the experimental input parameter. Within this scheme, there is only one complex order parameter É, a single critical temperature T c is built in, and the number of input parameters needed for calculations is small. The resulting H c2 's of these two approaches are similar. 5) Our choice is dictated by theoretical simplicity, rather than by experimental necessity.Below, the general scheme of calculating H c2 ðTÞ based on the Eilenberger method 6) is outlined for both the gap and the Fermi surface being anisotropic. 7) Then, we estimate the H c2 anisotropy of MgB 2 by modeling the four-sheet Fermi surface as calculated in refs. 8-10, by two distinct Fermi sheets F 1;2 with gaps Á 1;2 , each being constant within its sheet. Since the actual band structure enters macroscopic superconducting parameters via Fermi-surface averages, we further model the sheets F 1;2 by two s...

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Superconducting critical fields and anisotropy of a MgB2 single crystal

Cited by 49 publications

References 18 publications

Ambient-pressure synthesis of single-crystalMgB2and their superconducting anisotropy

Ambient-pressure synthesis of single-crystalMgB2and their superconducting anisotropy

Very high upper critical fields in MgB₂produced by selective tuning of impurity scattering

Anisotropy of the Upper Critical Field in Superconductors with Anisotropic Gaps: Anisotropy Parameters of MgB₂

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Superconducting critical fields and anisotropy of a MgB2 single crystal

Cited by 49 publications

References 18 publications

Ambient-pressure synthesis of single-crystalMgB2and their superconducting anisotropy

Ambient-pressure synthesis of single-crystalMgB2and their superconducting anisotropy

Very high upper critical fields in MgB2produced by selective tuning of impurity scattering

Anisotropy of the Upper Critical Field in Superconductors with Anisotropic Gaps: Anisotropy Parameters of MgB2

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Very high upper critical fields in MgB₂produced by selective tuning of impurity scattering

Anisotropy of the Upper Critical Field in Superconductors with Anisotropic Gaps: Anisotropy Parameters of MgB₂