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We present the main features of a home-built scanning tunneling microscope that has been attached to the mixing chamber of a dilution refrigerator. It allows scanning tunneling microscopy and spectroscopy measurements down to the base temperature of the cryostat, T≈30 mK, and in applied magnetic fields up to 13 T. The topography of both highly ordered pyrolytic graphite and the dichalcogenide superconductor NbSe2 has been imaged with atomic resolution down to T≈50 mK as determined from a resistance thermometer adjacent to the sample. As a test for a successful operation in magnetic fields, the flux-line lattice of superconducting NbSe2 in low magnetic fields has been studied. The lattice constant of the Abrikosov lattice shows the expected field dependence ∝1/B and measurements in the scanning tunneling spectroscopy mode clearly show the superconductive density of states with Andreev bound states in the vortex core. [ABSTRACT FROM AUTHOR]

Abstract: Excitations of a solid can be investigated by using ballistic electrons in point contacts as a probe. In particular, point-contact spectroscopy of normal metal–superconductor contacts is an important tool to study the symmetry and the nodal structure of the energy gap of the superconductor. The main mechanism for a current at low bias through a nanosized constriction is Andreev reflection of charge carriers at the interface between a normal metal and a superconductor. Andreev reflection leads to maxima in the differential conductance vs. . The position of the maxima depends not only on the size of the gap along given directions, but also on the order-parameter symmetry, the microscopic details of the contact and the barrier strength. In addition, if the order parameter exhibits a sign change an Andreev surface bound state leads to a conductance anomaly at zero bias. Here, recent investigations of some layered superconductors by point-contact spectroscopy are reviewed. [Copyright &y& Elsevier]

Abstract: Hints at unconventional superconductivity are present in many heavy-fermion superconductors. Among other methods, point-contact spectroscopy is an important tool to study the symmetry and the nodal structure of the energy gap of unconventional superconductors. Andreev reflection of charge carriers at the interface between a normal metal and a superconductor is the main mechanism for a current at low bias through a nanosized constriction, leading to maxima in the differential conductance vs. V. The position of the maxima depends not only on the size of the gap along given directions, but also on the order-parameter symmetry, the microscopic details of the contact and the barrier strength. In addition, if the order parameter exhibits a sign change, an Andreev surface bound state leads to a conductance anomaly at zero bias. We will review recent developments in this field and focus particularly on investigations of , a possible d-wave superconductor. [Copyright &y& Elsevier]