Shashank Misra

Visualizing nodal heavy fermion superconductivity in CeCoIn5

By means of low-temperature scanning tunnelling spectroscopy, a heavy fermion material in its superconducting and mixed states can be imaged. Besides probing the superconducting gap symmetry, the measurements also reveal a pseudogap.

Design and performance of an ultra-high vacuum scanning tunneling microscope operating at dilution refrigerator temperatures and high magnetic fields

We describe the construction and performance of a scanning tunneling microscope capable of taking maps of the tunneling density of states with sub-atomic spatial resolution at dilution refrigerator temperatures and high (14 T) magnetic fields. The fully ultra-high vacuum system features visual access to a two-sample microscope stage at the end of a bottom-loading dilution refrigerator, which facilitates the transfer of in situ prepared tips and samples. The two-sample stage enables location of the best area of the sample under study and extends the experiment lifetime. The successful thermal anchoring of the microscope, described in detail, is confirmed through a base temperature reading of 20 mK, along with a measured electron temperature of 250 mK. Atomically resolved images, along with complementary vibration measurements, are presented to confirm the effectiveness of the vibration isolation scheme in this instrument. Finally, we demonstrate that the microscope is capable of the same level of performance as typical machines with more modest refrigeration by measuring spectroscopic maps at base temperature both at zero field and in an applied magnetic field.

Failure of scattering interference in the pseudogap state of cuprate superconductors

We calculate scattering interference patterns for various electronic states proposed for the pseudogap regime of the cuprate superconductors. The scattering interference models all produce patterns whose wavelength changes as a function of energy, in contradiction to the energy-independent wavelength seen by scanning tunneling microscopy (STM) experiments in the pseudogap state. This suggests that the patterns seen in STM local density of states measurements are not due to scattering interference, but are rather the result of some form of ordering.

Atomic scale imaging and spectroscopy of a CuO(2) plane at the surface of Bi(2)Sr(2)CaCu(2)O(8+delta).

We have used a scanning tunneling microscope to demonstrate that a single CuO2 plane can form a stable and atomically ordered layer at the surface of Bi(2)Sr(2)CaCu(2)O(8+delta). In contrast to previous studies on high-T(c) surfaces, the CuO2-terminated surface exhibits a strongly suppressed tunneling conductance at low voltages. We consider a number of different explanations for this phenomena and propose that it may be caused by how the orbital symmetry of the CuO2 planes electronic states affects the tunneling process.

Visualizing heavy fermion formation and their unconventional superconductivity in f-electron materials

In solids containing elements with f-orbitals, the interaction between f-electron spins and those of itinerant electrons leads to the development of low-energy fermionic excitations with a heavy effective mass. These excitations are fundamental to the appearance of unconventional superconductivity observed in actinide- and lanthanide-based compounds. We use spectroscopic mapping with the scanning tunneling microscope to detect the emergence of heavy excitations with lowering of temperature in Ce- and U-based heavy fermion compounds. We demonstrate the sensitivity of the tunneling process to the composite nature of these heavy quasiparticles, which arises from quantum entanglement of itinerant conduction and f-electrons. Scattering and interference of the composite quasiparticles is used in the Ce-based compounds to resolve their energy-momentum structure and to extract their mass enhancement, which develops with decreasing temperature. Finally, by extending these techniques to much lower temperatures, we ...

Atomic Scale Imaging and Spectroscopy of aCuO2Plane at the Surface ofBi2Sr2CaCu2O8+δ

We have used a scanning tunneling microscope to demonstrate that a single CuO2 plane can form a stable and atomically ordered layer at the surface of Bi(2)Sr(2)CaCu(2)O(8+delta). In contrast to previous studies on high-T(c) surfaces, the CuO2-terminated surface exhibits a strongly suppressed tunneling conductance at low voltages. We consider a number of different explanations for this phenomena and propose that it may be caused by how the orbital symmetry of the CuO2 planes electronic states affects the tunneling process.

Atomic Scale Imaging and Spectroscopy of a [Formula presented] Plane at the Surface of [Formula presented]

We have used a scanning tunneling microscope to demonstrate that a single CuO2 plane can form a stable and atomically ordered layer at the surface of Bi(2)Sr(2)CaCu(2)O(8+delta). In contrast to previous studies on high-T(c) surfaces, the CuO2-terminated surface exhibits a strongly suppressed tunneling conductance at low voltages. We consider a number of different explanations for this phenomena and propose that it may be caused by how the orbital symmetry of the CuO2 planes electronic states affects the tunneling process.