Scanning tunneling microscopy of the flux-line lattice in a superconductor

01.01.2012 - 30.06.2015
Research funding project
Systematic investigation of the real-space vortex matter properties in superconductors by scanning tunneling microscopy Summary: When applying a magnetic field to a (type II) superconductor so-called flux lines (or vortices) are created in the sample. These vortices are roughly cylindrical in field direction, have a normal-conducting core with a radius of some nm and are surrounded by electrical shielding currents. The behavior of the flux line lattice depends on magnetic field, temperature, the defect matrix in the material, and the superconducting parameters and is governed by at least three interaction energies, namely the long-range repulsion between different flux lines (E_VL), the pinning interaction between a flux line and a material defect (E_pin) and thermal fluctuations (E_Th). The interplay of these three energies leads to a very rich phase diagram of the vortex matter ranging from an E_VL dominated hexagonally ordered lattice to an E_pin dominated disordered lattice and an E_Th dominated liquid regime with dramatic effects on the macroscopic behavior. The details of the properties can best be understood by observing the vortex lattice in real space, which can be done by magnetic force microscopy (MFM) and similar methods at low fields but only by scanning tunneling microscopy (STM) at higher magnetic fields. Very few publications are available on this subject, involving usually less than 100 vortices or only small parts of the phase diagram. Basically, it appears that no major systematic study on the real-space properties of the flux line lattice exists. Such a study would be important, because there are still a lot of unresolved problems in vortex matter physics. The main goal of this project is to analyze the vortex matter properties by real-space imaging over a large part of the (field vs. temperature) phase diagram. Doing this for various defect densities in a systematic way will allow to relate the microscopic structure to the corresponding macroscopic properties in the same sample, and to verify the correctness of theoretical descriptions. In detail, we will first determine the macroscopic properties including the superconducting parameters that enter theory and the critical current density and study possible phase transitions. Controlling the defect density is achieved by neutron irradiation, which generates a homogeneously distributed defect matrix. Most of the work will be devoted to imaging the vortex lattice by STM (and partly by MFM) at the same fields and temperatures and in the same samples, where the macroscopic measurements were carried out. In order to achieve high statistical significance a large number of vortices (1000 or more) should be recorded per image, which is possible due to the large maximum scan area of 33 µm x 33 µm available in our instrument. The resulting images will be studied in detail including the quantitative correlation between lattice dislocations and the macroscopic critical current density, order parameters that indicate phase transitions and the corresponding correlation functions, and possible history effects near phase transitions. Finally, microscopic and macroscopic results will be used to verify the correctness of related theories (e.g. the collective pinning theory). Various correlation functions are key parameters of the theory and can directly be compared with results from the evaluation of the vortex images. Most of the study will be carried out on NbSe_2 which meets all necessary requirements. Additional samples will be considered.

People

Project leader

Institute

Grant funds

  • FWF - Österr. Wissenschaftsfonds (National) Stand-Alone Project Austrian Science Fund (FWF)

Research focus

  • Materials Characterization: 100%

Keywords

GermanEnglish
SupraleitungSuperconductivity
FlussliniengitterVortex Matter
RastertunnelmikroskopieScanning Probe Microscopy
PhasenübergängePhase Transitions

Publications