Unexpected Broken Particle-Hole Symmetry, Physics Research Highlights, University of Illinois at Urbana-Champaign

Unexpected Broken Particle-Hole Symmetry Seen
at Atomically Flat YBCO Surface

Superconductors have remarkable quantum mechanical properties, such as lossless current flow, because electrons form pairs below a critical temperature and these pairs condense into a single quantum mechanical state. In conventional superconductors, a minimum energy must be invested to produce a single excited quasiparticle, and this quantity is known as the energy gap. Studying the excitation spectrum of superconductors has provided important verification of fundamental predictions of theories of superconductivity. In high T_c superconductors (HTS) tunneling experiments using scanning tunneling microscopes (STM) and photoemission experiments have shown the spectrum of states to be different, in that not all of the electrons available for pairing do so in an equal manner. The excitation spectrum obtained in STM measurements at atomic sites within extended CuO2 planes has been approximately particle-hole symmetric. That is, the density of excitation states has been measured to be an approximately symmetric function of voltage around zero, which is the energy of the condensed electron pairs.

Schematic crystal structure at the (100) YBCO/CTO interface, showing oxygen coordination of Cu and Ti atoms In recent work at the University of Illinois, to be published in Phys Rev Lett and now available online, Dr. Bruce Davidson and Prof. Jim Eckstein made HTS tunnel junctions embedded in a single crystal heterostructure. Two theorists, Dr. Revaz Ramazashvili and Dr. Simon Kos, also contributed to this work. They studied quasiparticle tunneling into atomically flat a-axis films of YBa₂Cu₃O_7-d and DyBa₂Cu₃O_7-d through epitaxial CaTiO₃ barriers. The junction heterostructures were grown by oxide molecular beam epitaxy and were carefully optimized using in situ monitoring techniques, resulting in unprecedented crystalline perfection of the superconductor/insulator interface. Below T_c, the tunneling conductance shows the evolution of a large unexpected asymmetrical feature near zero bias. This is evidence that superconducting YBCO crystals, atomically truncated along the lobe direction with a titanate layer, have intrinsically broken particle-hole symmetry over macroscopically large areas.

The figure shown above indicates the arrangement of metal and oxygen atoms right at the interface. The top layer shows one unit cell of CaTiO₃ (CTO). The Ti atoms are contained in the grey octahedra, the Ca atoms are represented by the green spheres and oxygen atoms are at all of the vertices. The HTS layer is below the titanate layer and is an "a-axis" film of the superconductor YBa₂Cu₃O_7-d (YBCO). Transport in the a-axis direction (up-down) as well as in and out of the paper is metallic, while transport in the third direction (left-right) is not. On top of typically five unit cells of CTO a gold film was deposited to form a normal-insulator-superconductor tunnel junction with transport in the superconductor in the a-axis direction.

RHEED images during trilayer deposition The growth of the heterointerface was monitored in real time using reflection high energy electron diffraction (RHEED). Four images recorded during the growth of one sample are shown to the left. In panel a, diffraction from the surface of the YBCO shows 1/3 order reflections. The strong well-defined specular reflection is the signature in diffraction of a smooth surface. Atomic force microscopy of similar samples shows the roughness of this surface to be <4 angstroms rms. The 1/3 order spots are caused by the 12-angstrom periodicity of the film shown in the figure as the separation between, for example, the Y atoms. After growth of one and two unit cells of CTO, the 1/3 order reflections have disappeared, leaving only diffraction peaks from the 4- angstrom periodicity of the CTO. After five unit cells of CTO, the specular or mirror-like reflection is as strong as that of the YBCO surface. Using standard microelectronic techniques, the samples were made into testable trilayer NIS junctions.

In tunneling spectroscopy of superconductors, the differential conductance, dI/dV, is proportional to the density of states of the superconductor. Since superconductors are supposed to have particle-hole symmetric quasiparticle density of states, it was a big surprise when spectra like those shown in the figure were obtained. The interface density of states is strongly modified by superconductivity, as expected, but the resulting excitation spectrum is manifestly not particle-hole symmetric. This finding revealed that at the surface into which the tunneling occurred, superconductivity was very different from what it is like away from the interface. Proximity to the interface dramatically changed the density of states. While the origin of this effect is still being debated, it is clearly dependent on the high degree of crystalline perfection obtained at the insulator-superconductor interface. Since the interface is regular and homogeneous, this result should provide a test for theories of high T_c superconductivity.

Normalized G(V) data for the junction
This figure shows normalized G(V) data for junction. The insert shows extra spectral weight at T = 4K in the empty-state DoS, versus T_c for all junctions (occupied-state DoS is conserved). The extra spectral weight decreases with decreasing T_c, disappearing at T_c —> 50K.