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N. Schopohl

Publications and source records attributed to N. Schopohl.

33 records · Page 2Linked to original sources
arXiv2004

Shadow on the wall cast by an Abrikosov vortex

At the surface of a d-wave superconductor, a zero-energy peak in the quasiparticle spectrum can be observed. This peak appears due to Andreev bound states and is maximal if the nodal direction of the d-wave pairing potential is perpendicular to the boundary. We examine the effect of a single Abrikosov vortex in front of a reflecting boundary on the zero-energy density of states. We can clearly see a splitting of the low-energy peak and therefore a suppression of the zero-energy density of states in a shadow-like region extending from the vortex to the boundary. This effect is stable for different models of the single Abrikosov vortex, for different mean free paths and also for different distances between the vortex center and the boundary. This observation promises to have also a substantial influence on the differential conductance and the tunneling characteristics for low excitation energies.

cond-mat.supr-con↗
arXiv2004

High performance magnetic field sensor based on Superconducting Quantum Interference Filters

We have developed an absolute magnetic field sensor using Superconducting Quantum Interference Filter (SQIF) made of high-T_c grain boundary Josephson junctions. The device shows the typical magnetic field dependent voltage response V(B), which is sharp delta-like dip in the vicinity of zero magnetic field. When the SQIF is cooled with magnetic shield, and then the shield is removed, the presence of the ambient magnetic field induces a shift of the dip position from B_0 ~ 0 to a value B ~ B_1, which is about the average value of the earth magnetic field, at our latitude. When the SQIF is cooled in the ambient field without shielding, the dip is first found at B ~ B_1, and the further shielding of the SQIF results in a shift of the dip towards B_0 ~ 0. The low hysteresis observed in the sequence of experiments (less than 5% of B_1) makes SQIFs suitable for high precision measurements of the absolute magnetic field. The experimental results are discussed in view of potential applications of high-T_c SQIFs in magnetometry.

cond-mat.supr-con↗
arXiv2003

Influence of Fermi surface topology on the quasiparticle spectrum in the vortex state

We study the influence of Fermi surface topology on the quasiparticle density of states in the vortex state of type II superconductors. We observe that the field dependence and the shape of the momentum and spatially averaged density of states is affected significantly by the topology of the Fermi surface. We show that this behavior can be understood in terms of characteristic Fermi surface functions and that an important role is played by the number of points on the Fermi surface at which the Fermi velocity is directed parallel to the magnetic field. A critical comparison is made with a broadened BCS type density of states, that has been used frequently in analysis of tunneling data. We suggest a new formula as a replacement for the broadened BCS model for the special case of a cylindrical Fermi surface. We apply our results to the two gap superconductor MgB$_2$ and show that in this particular case the field dependence of the partial densities of states of the two gaps behaves very differently due to the different topologies of the corresponding Fermi surfaces, in qualitative agreement with recent tunneling experiments.

cond-mat.supr-con↗
arXiv2003

Fermi surface topology and the upper critical field in two-band superconductors - application to MgB2

Recent measurements of the anisotropy of the upper critical field Bc2 on MgB2 single crystals have shown a puzzling strong temperature dependence. Here, we present a calculation of the upper critical field based on a detailed modeling of bandstructure calculations that takes into account both the unusual Fermi surface topology and the two gap nature of the superconducting order parameter. Our results show that the strong temperature dependence of the Bc2 anisotropy can be understood as an interplay of the dominating gap on the sigma-band, which possesses a small c-axis component of the Fermi velocity, with the induced superconductivity on the pi-band possessing a large c-axis component of the Fermi velocity. We provide analytic formulas for the anisotropy ratio at T=0 and T=Tc and quantitatively predict the distortion of the vortex lattice based on our calculations.

cond-mat.supr-con↗
arXiv2003

Fermi surface topology and vortex state in MgB2

Based on a detailed modeling of the Fermi surface topology of MgB2 we calculated the anisotropy of the upper critical field Bc2 within the two gap model. The sigma-band is modeled as a distorted cylinder and the pi-band as a half-torus, with parameters determined from bandstructure calculations. Our results show that the unusual strong temperature dependence of the Bc2 anisotropy, that has been observed recently, can be understood due to the small c-axis dispersion of the cylindrical Fermi surface sheets and the small interband pairing interaction as obtained from bandstructure calculations. We calculate the magnetic field dependence of the density of states within the vortex state for field in c-axis direction and compare with recent measurements of the specific heat on MgB2 single crystals.

cond-mat.supr-con↗
arXiv2002

Effects of magnetic field on two-dimensional Superconducting Quantum Interference Filters

We present an experimental study of two-dimensional superconducting quantum interference filters (2D-SQIFs) in the presence of a magnetic field B. The dependences of the dc voltage on the applied magnetic field are characterized by a unique delta-like dip at B=0, which depends on the distribution of the areas of the individual loops, and on the bias current. The voltage span of the dip scales proportional to the number of rows simultaneously operating at the same working point. In addition, the voltage response of the 2D-SQIF is sensitive to a field gradient generated by a control line and superimposed to the homogeneous field coil. This feature opens the possibility to use 2D superconducting quantum interference filters as highly sensitive detectors of spatial gradients of magnetic field.

cond-mat.supr-con↗
arXiv2002

Spectrum of low energy excitations in the vortex state: comparison of Doppler shift method to quasiclassical approach

We present a detailed comparison of numerical solutions of the quasiclassical Eilenberger equations with several approximation schemes for the density of states of s- and d-wave superconductors in the vortex state, which have been used recently. In particular, we critically examine the use of the Doppler shift method, which has been claimed to give good results for d-wave superconductors. Studying the single vortex case we show that there are important contributions coming from core states, which extend far from the vortex cores into the nodal directions and are not present in the Doppler shift method, but significantly affect the density of states at low energies. This leads to sizeable corrections to Volovik's law, which we expect to be sensitive to impurity scattering. For a vortex lattice we also show comparisons with the method due to Brandt, Pesch, and Tewordt and an approximate analytical method, generalizing a method due to Pesch. These are high field approximations strictly valid close to the upper critical field Bc2. At low energies the approximate analytical method turns out to give impressively good results over a broad field range and we recommend the use of this method for studies of the vortex state at not too low magnetic fields.

cond-mat.supr-con↗
arXiv2000

Nonperiodic Flux to Voltage Conversion of an Arithmetic Series Array of dc SQUIDs

A theoretical study on the voltage response function of a series array of dc SQUIDs is presented in which the elementary dc SQUID loops vary in size and, possibly, in orientation. Such series arrays of two-junction SQUIDs possess voltage response functions vs. external magnetic field B that differ substantially from those of corresponding regular series arrays with identical loop-areas, while maintaining a large voltage swing as well as a low noise level. Applications include the design of current amplifiers and quantum interference filters.

cond-mat.supr-con↗
arXiv2000

Transition Radiation of Moving Abrikosov Vortices

We show that Abrikosov vortices moving towards the surface of a superconductor emit electromagnetic radiation into free space. The frequency distribution of the radiated intensity displays a pronounced maximum at microwave frequencies around v_x/lambda, where lambda is the magnetic penetration length. Coherent motion of a lattice of flux lines leads to constructive interference and increases the strength of the radiated power by a large factor.

cond-mat.supr-con↗
arXiv1999

Dynamic electromagnetic response of three-dimensional Josephson junction arrays

We present a theoretical study on the dynamical properties of three-dimensional arrays of Josephson junctions. Our results indicate that such superconducting networks represent highly sensitive 3D-SQUIDs having some major advantages in comparison with conventional planar SQUIDs. The voltage response function of 3D-SQUIDs is directly related to the vector-character of external electromagnetic fields. The theory developed here allows the three-dimensional reconstruction of a detected external field including phase information about the field variables. Applications include the design of novel magnetometers, gradiometers and particle detectors.

cond-mat.supr-con↗
arXiv1998

Transformation of the Eilenberger Equations of Superconductivity to a Scalar Riccati Equation

A new parametrisation of the Eilenberger equations of superconductivity in terms of the solutions to a scalar differential equation of the Riccati type is introduced. It is shown that the quasiclassical propagator, and in particular the local density of states, may be reconstructed, without explicit knowledge of any eigenfunctions and eigenvalues, by solving a simple initial value problem for the linearised Bogoliubov-de Gennes equations. The Riccati parametrisation of the quasiclassical propagator leads to a stable and fast numerical method to solve the Eilenberger equations. For some spatially varying model pair potentials exact solutions to the Eilenberger Equations are found.

cond-mat.supr-con↗