Magnetic-field effects on the density of states of orthorhombic superconductors

Abstract

The quasiparticle density of states in a two-dimensional d-wave superconductor depends on the orientation of the in-plane external magnetic field H. This is because, in the region of the gap nodes, the Doppler shift due to the circulating supercurrents around a vortex core depend on the direction of H. For a tetragonal system the induced pattern is four-fold symmetric and, at zero energy, the density of states exhibits minima along the node directions. But YBa 2 C 3 O 6.95 is orthorhombic because of the chains and the pattern becomes two-fold symmetric with the position of the minima occuring when H is oriented along the Fermi velocity at a node on the Fermi surface. The effect of impurity scattering in the Born and unitary limit are discussed. 74.20.Fg 74.25.Ha 74.72.-h Typeset using REVT E X

Thermodynamics ofd-wave superconductors in a magnetic field

“…Twinning, combined with the orthorhombicity of YBCO is expected to reduce the anisotropy. 17,60 Here we point out another possible reason for the difficulty in extracting the difference between the two directions from the data: the field dependence of the anisotropic term is not simply given by ͱH, as it would be in the absence of the Zeeman term, and as assumed in Ref. 5.…”

“…17 It is also possible that the orthorhombicity, which shifts the minima in the density of states away from the /4 directions, combined with twinning of the crystals used in both experiments reduces the observable anisotropy significantly. 60 However, even in this case, the in-plane anisotropy for the fields of up to 14 T used in the experiments by Wang et al 5 should be within the experimental resolution. A very important observation is that since the anisotropy in the density of states is washed out rapidly as the energy is increased, the in-plane anisotropy of the specific heat is greatly reduced with increased temperature, 17 as seen in Fig.…”

“…In that case the absence of the anisotropy can only be explained ͑somewhat unsatisfactorily͒ by an appeal to the three-dimensionality 17 or a combination of the orthorhombicity and twinning. 17,60 Since part of the experimental difficulty stems from the smallness of the ͱH term with the field in the plane, the analysis for that geometry typically involves assuming a field-dependent contribution of that form. 2,5 However, if E ab is small, the field dependence of the specific heat is modified by the Zeeman splitting, and this splitting has to be taken into account in the analysis.…”

Doppler shift on local density of states and local impurity scattering in the vortex state

“…[12][13][14][15] When the magnetic field is oriented in the copper oxide plane rather than perpendicular to it new anisotropy effects in the specific heat are predicted. 11,[16][17][18] In addition transport properties are also affected by the Doppler shift of a vortex state. 18,19 The thermal conductivity has been particularly widely studied.…”

Angle-dependent magnetothermal conductivity in d-wave superconductors

“…[6] Experimental verification of this prediction has not been possible so far and is hindered by the extrinsic contributions to the specific heat [7] and the orthorhombicity of the material. [8] On the other hand, the angular dependence of the thermal conductivity in the vortex state has already been observed, [9,10] and here we analyse it in the semiclassical framework.…”