Introduction to our research

The discovery of the high-T_c copper oxide superconductors (HTS) in 1986 renewed the interest in the fundamental physics of superconductivity and in its applications. The magnetic field penetrates into the superconductors in the form of quantum magnetic flux lines or vortices. Due to their repulsive interactions the vortices form a lattice which serves as a unique model system that allows investigation of a wide range of general phenomena in condensed matter physics. In addition, the static and dynamic behavior of vortex matter determines the electrical and magnetic properties of superconductors, comprehension of which is essential for technological applications.
 
The traditional view of type II superconductors was that a periodic vortex lattice exists in the entire field range between the lower critical field, above which vortices penetrate into the superconductor, and the mean-field upper critical field, above which the superconductivity is destroyed. In the case of HTS, however, enhanced role of thermal fluctuations leads to the melting of the vortex lattice and to the existence of a new vortex phase, the vortex liquid. In addition, the presence of intrinsic material disorder results in much more complicated diagram with a number of distinct phases, including a quasi-ordered lattice and a highly disordered solid. These main phases are possibly subdivided into additional regions with different vortex properties. Moreover, due to the highly anisotropic layered structure of the HTS the vortex lines can be considered as stacks of vortex pancakes that give rise to new phenomena including crossing Josephson and pancake lattices and vortex lattice sublimation.
 
The comprehension of the vortex matter properties in HTS is based to a large extent on magnetization studies. Most of the investigations rely on global measurements in which an integrated magnetic response of the entire sample is measured. In recent years we have developed several local techniques. The local studies are particularly important since the distribution of the magnetic field within a superconductor can be highly non-uniform due to disorder, metastability, and geometrical effects.
 
Our first local tool is based on microscopic arrays of Hall sensors fabricated using GaAs/AlGaAs heterostructures and microelectronics technology. It has proven to be an essential tool in the investigation of vortex dynamics. We have used this technique, as well as additional transport methods, to study a number of unique phenomena in HTS and conventional superconductors, such as the first-order melting, disorder-driven transitions, decoupling and sublimation of the vortex pancakes, metastability and memory effects, unconventional distribution of the transport current, surface and geometrical barriers, effects of disorder, and shear-induced decoupling. In addition, by introducing “vortex shaking” by an in-plane ac field, we have discovered new phenomena including inverse melting and crossing first-order melting and second-order glass transition lines. The Hall probe arrays have also allowed observation of quantum tunneling and unique local properties of molecular magnets including magnetic deflagration and propagation of magnetic avalanches.
 
Another important local technique is the magneto-optical imaging, which provides a real-time two-dimensional imaging of the magnetic field at the surface of a superconductor. We have developed a differential MO method that improves the sensitivity of the magneto-optical technique by two to three orders of magnitude. This method allows direct visualization of the first-order melting transition. It provides movies of the nucleation and propagation of the melting process and reveals the local effects of various types of disorder. We have also used the differential MO to discover melting of a porous vortex matter in presence of columnar defects, existence of a Mott insulator phase in presence of periodic potential, large enhancement of local induction in presence of small variations in the lower critical field, and suppression of geometrical barrier by stacks of Josephson vortices.

Finally, we have recently developed the smallest SQUIDs to date. In addition to their record size, the major advantage of these nanoSQUIDs is that they reside on the apex of a sharp tip and hence are ideally suited for scanning probe microscopy. These new devices bring us close to the sensitivity range of detecting single electron spins. By gluing the tip to one tine of a quartz tuning fork we have constructed a scanning SQUID microscope that allows magnetic imaging at distances as small as a few nanometers away from the sample. This novel instrument will serve as a powerful tool for investigation of vortex dynamics and quantum magnetism on a nanoscale.