Pressure effect on the magnetic penetration depth inMgB2

Abstract

A study of the pressure effect on the magnetic penetration depth λ in polycrystalline MgB2 was performed by measuring the temperature dependence of the magnetization under an applied pressure of 0.15 and 1.13 GPa. We found that λ −2 at low temperature is only slightly affected by pressure [ ∆λ −2 λ −2 = 1.5(9)%], in contrast to cuprate superconductors, where, in the same range of pressure, a very large effect on λ −2 was found. Theoretical estimates indicate that most of the pressure effect on λ −2 in MgB2 arises from the electron-phonon interaction.PACS numbers: 74.70. Ad, 74.62.Fj, 63.20.Kr, 74.25.Ha Shortly after the discovery of the superconductivity in MgB 2 at 39 K, several investigations of the pressure dependence of the superconducting critical temperature T c were carried out.1,2,3,4 Indeed, the magnitude and sign of dT c /dp may indicate a way to rise T c at ambient pressure, and moreover, help to understand the superconducting pairing mechanism. So far, all these studies show that T c decreases with increasing pressure, with a rate depending on the method and the pressure medium used. The first hydrostatic measurement of T c (p) up to 0.7 GPa, reveals that T c decreases reversibly under hydrostatic pressure at the rate dT c /dp = − 1.11(2) K/GPa.4 The behavior of T c (p) with pressure in MgB 2 was attributed to the pressure induced lattice stiffening (increase of the phonon frequency), 5,6 rather than to the decrease in the electronic density of states N(E F ), that is only moderately affected by pressure.7 Comparison with theoretical calculations supported the view that MgB 2 is a BCS superconductor with moderately strong electron-phonon coupling. 5Apart from T c , an other relevant superconducting parameter is the magnetic field penetration depth λ. In fact the so-called superfluid density λ −2 is related to the Fermi velocity and to the density of charge carriers, and its temperature dependence gives information on the symmetry and on the magnitude of the superconducting gap. A study of pressure effects on λ −2 (0) can give important informations on how the electronic degrees of freedom are affected by lattice modifications and on the nature of the electron-phonon coupling. Indeed, in cuprates high temperature superconductors (HTS), a huge pressure effect on λ −2 (0) was found, in particular in YBa 2 Cu 4 O 8 (Y124).8 Part of this effect was attributed to the strong renormalization of the Fermi velocity, and therefore to the effective mass, due to the nonadiabatic coupling of the electrons to the lattice. This result is in agreement with the substantial oxygen isotope effect found in cuprates 9,10,11,12,13 and can be interpreted in the framework of non-adiabatic theory of superconductivity.14 In this paper we report measurements of the magnetic penetration depth under pressure in polycrystalline MgB 2 . The temperature dependence of λ −2 (T ) was extracted from the Meissner fraction f measured in a low magnetic field. A small pressure (p) effect on λ −2 (0) was found [ ∆λ −2 λ −2 = 1.5(9)%], for...

Absence of a boron isotope effect in the magnetic penetration depth ofMgB2

“…[13][14][15] This conclusion is in contrast to cuprate superconductors, where a substantial oxygen isotope effect on m * , well above the upper limit stated here, was observed. [19][20][21][22][23][24][25][26] Recent magnetization measurements on MgB 2 under pressure 43 show no substantial pressure effect on the magnetic penetration depth at low temperature, further supporting the main conclusion of the present work.…”

Anomalous electron-phonon coupling probed on the surface of superconductorZrB12

“…There, it was suggested that λ −2 increases under pressure because of two reasons: (i) band structure effects and (ii) renormalization of the electron-phonon coupling [10]. Under the assumption of ellipsoidal or cylindrical Fermi surface the first one can be obtained as [10] d ln λ −2 (0) dp…”

“…Note, that in adiabatic superconductors (or in the superconductors where the nonadiabatic effects are small) the pressure effect (PE) [9,10] as well as the isotope effect (IE) [11] on λ was found to be almost negligible in comparison with substantial PE [12] and IE [13] on λ observed in highly nonadiabatic high-T c cuprates. In this paper we report on PE on T c and λ studies in ZrB 12 superconductor.…”

“…To analyze the observed effect we used a procedure similar to that one described by Di Castro et al [10]. There, it was suggested that λ −2 increases under pressure because of two reasons: (i) band structure effects and (ii) renormalization of the electron-phonon coupling [10].…”

Detailed magnetization study of superconducting properties of YBa2Cu3O7−xceramic spheres

“…Because the magnetic moment of a single sample with r ∼ λ is too small to be measured, powder or grain aligned samples were used. 5,6,7,8,9,10,11,12,13,14 Here, we apply a similar approach to analyze magnetization data collected on ceramic samples.…”