Person: Zeljkovic, Ilija
Loading...
Email Address
AA Acceptance Date
Birth Date
Research Projects
Organizational Units
Job Title
Last Name
Zeljkovic
First Name
Ilija
Name
Zeljkovic, Ilija
7 results
Search Results
Now showing 1 - 7 of 7
Publication Visualizing the Interplay of Structural and Electronic Disorders in High-Temperature Superconductors Using Scanning Tunneling Microscopy(2013-09-26) Zeljkovic, Ilija; Hoffman, Jenny Eve; Sachdev, Subir; Greiner, MarkusThe discovery of high-\(T_c\) superconductivity in 1986 generated tremendous excitement. However, despite over 25 years of intense research efforts, many properties of these complex materials are still poorly understood. For example, the cuprate phase diagram is dominated by a mysterious "pseudogap" state, a depletion in the Fermi level density of states which persists above the superconducting critical temperature \(T_c\). Furthermore, these materials are typically electronically inhomogeneous at the atomic scale, but to what extent the intrinsic chemical or structural disorder is responsible for electronic inhomogeneity, and whether the inhomogeneity is relevant to pseudogap or superconductivity, are unresolved questions. In this thesis, I will describe scanning tunneling microscopy experiments which probe the interplay of structural, chemical and electronic disorder in high-\(T_c\) superconductors. First, I will present the imaging of a picoscale orthorhombic structural distortion in Bi-based cuprates. Based on insensitivity of this structural distortion to temperature, magnetic field, and doping level we conclude that it is an omnipresent background not related to the pseudogap state. I will also present the discovery of three types of oxygen disorder in the high-\(T_c\) superconductor \(Bi_2Sr_2CaCu_2O_{8+x}\) two different interstitials as well as vacancies at the apical oxygen site. We find a strong correlation between the positions of these defects and the nanoscale inhomogeneity in the pseudogap phase, which highlights the importance of chemical disorder in these compounds. Furthermore, I will show the determination of the exact intra-unit-cell positions of these dopants and the effect of different types of intrinsic strain on their placement. I will also describe the identification of chemical disorder in another cuprate \(Y_{1−x}Ca_xBa_2Cu_3O_{7−x}\), and the first observation of electronic inhomogeneity of the spectral gap in this material. Finally, I will present definitive identification of the cleavage surfaces in \(Pr_xCa_{1−x}Fe_2As_2\), and imaging of Pr dopants which exhibit lack of clustering, thus ruling out Pr inhomogeneity as the likely source of the high-\(T_c\) volume fraction. To achieve the aforementioned results, we employ novel analytical and experimental tools such as an average supercell algorithm, high-bias dI/dV dopant mapping, and local barrier height mapping.Publication Fermi Surface and Pseudogap Evolution in a Cuprate Superconductor(American Association for the Advancement of Science (AAAS), 2014) He, Yang; Yin, Yi; Zech, M.; Soumyanarayanan, Anjan; Yee, Michael Manchun; Williams, Tess Lawanna; Boyer, M. C.; Chatterjee, K.; Wise, W. D.; Zeljkovic, Ilija; Kondo, T.; Takeuchi, T.; Ikuta, H.; Mistark, P.; Markiewicz, R. S.; Bansil, A.; Sachdev, Subir; Hudson, E. W.; Hoffman, JennyThe unclear relationship between cuprate superconductivity and the pseudogap state remains an impediment to understanding the high transition temperature (Tc) superconducting mechanism. Here, we used magnetic field–dependent scanning tunneling microscopy to provide phase-sensitive proof that d-wave superconductivity coexists with the pseudogap on the antinodal Fermi surface of an overdoped cuprate. Furthermore, by tracking the hole-doping (p) dependence of the quasi-particle interference pattern within a single bismuth-based cuprate family, we observed a Fermi surface reconstruction slightly below optimal doping, indicating a zero-field quantum phase transition in notable proximity to the maximum superconducting Tc. Surprisingly, this major reorganization of the system’s underlying electronic structure has no effect on the smoothly evolving pseudogap.Publication Nanoscale surface element identification and dopant homogeneity in the high-\(T_{c}\) superconductor \(Pr_xCa_{1−x}Fe_2As_2\)(American Physical Society (APS), 2013) Zeljkovic, Ilija; Huang, Dennis; Song, Can-Li; Lv, Bing; Chu, Ching-Wu; Hoffman, JennyWe use scanning tunneling microscopy to determine the surface structure and dopant distribution in \(Pr_xCa_{1−x}Fe_2As_2\), the highest-Tc member of the 122 family of iron-based superconductors. We identify the cleaved surface termination by mapping the local tunneling barrier height, related to the work function. We image the individual Pr dopants responsible for superconductivity, and show that they do not cluster, but in fact repel each other at short length scales. We therefore suggest that the low volume fraction high-Tc superconducting phase is unlikely to originate from Pr inhomogeneity.Publication Interplay of chemical disorder and electronic inhomogeneity in unconventional superconductors(Royal Society of Chemistry (RSC), 2013) Zeljkovic, Ilija; Hoffman, JennyMany of today's forefront materials, such as high-Tc superconductors, doped semiconductors, and colossal magnetoresistance materials, are structurally, chemically and/or electronically inhomogeneous at the nanoscale. Although inhomogeneity can degrade the utility of some materials, defects can also be advantageous. Quite generally, defects can serve as nanoscale probes and facilitate quasiparticle scattering used to extract otherwise inaccessible electronic properties. In superconductors, non-stoichiometric dopants are typically necessary to achieve a high transition temperature, while both structural and chemical defects are used to pin vortices and increase critical current. Scanning tunneling microscopy (STM) has proven to be an ideal technique for studying these processes at the atomic scale. In this perspective, we present an overview of STM studies on chemical disorder in unconventional superconductors, and discuss how dopants, impurities and adatoms may be used to probe, pin or enhance the intrinsic electronic properties of these materials.Publication Etching of Cr tips for scanning tunneling microscopy of cleavable oxides(AIP Publishing, 2017) Huang, Dennis; Liu, Stephen; Zeljkovic, Ilija; Mitchell, J. F.; Hoffman, JennyWe report a detailed three-step roadmap for the fabrication and characterization of bulk Cr tips for spin-polarized scanning tunneling microscopy. Our strategy uniquely circumvents the need for ultra-high vacuum preparation of clean surfaces or films. First, we demonstrate the role of ex-situ electrochemical etch parameters on Cr tip apex geometry, using scanning electron micrographs of over 70 etched tips. Second, we describe the suitability of the in-situ cleaved surface of the layered antiferromagnet La1.4Sr1.6Mn2O7 to evaluate the spin characteristics of the Cr tip, replacing the UHV-prepared test samples that have been used in prior studies. Third, we outline a statistical algorithm that can effectively delineate closely-spaced or irregular cleaved step edges, to maximize the accuracy of step height and spin-polarization measurements.Publication Nanoscale Surface Element Identification and Dopant Homogeneity in the High-\(T_c\) Superconductor \(Pr_xC_{1-x}Fe_2As_2\)(American Physical Society, 2013) Zeljkovic, Ilija; Huang, Dennis; Song, Can-Li; Lv, Bing; Chu, Ching-Wu; Hoffman, JennyWe use scanning tunneling microscopy to determine the surface structure and dopant distribution in \(Pr_xCa_{1-x}Fe_2As_2\), the highest-\(T_c\) member of the 122 family of iron-based superconductors. We identify the cleaved surface termination by mapping the local tunneling barrier height, related to the work function. We image the individual Pr dopants responsible for superconductivity, and show that they do not cluster, but in fact repel each other at short length scales. We therefore suggest that the low volume fraction high-\(T_c\) superconducting phase is unlikely to originate from Pr inhomogeneity.Publication Scanning Tunnelling Microscopy Imaging of Symmetry-breaking Structural Distortion in the Bismuth-based Cuprate Superconductors(Nature Publishing Group, 2012) Zeljkovic, Ilija; Main, Elizabeth J.; Williams, Tess Lawanna; Boyer, M. C.; Chatterjee, Kamalesh; Wise, W. D.; Yin, Yi; Zech, Martin; Pivonka, Adam Edward; Kondo, Takeshi; Takeuchi, T.; Ikuta, Hiroshi; Wen, Jinsheng; Xu, Zhijun; Gu, G. D.; Hoffman, JennyA complicating factor in unravelling the theory of high-temperature (high-\(T_c\)) superconductivity is the presence of a ‘pseudogap’ in the density of states, the origin of which has been debated since its discovery. Some believe the pseudogap is a broken symmetry state distinct from superconductivity whereas others believe it arises from short-range correlations without symmetry breaking. A number of broken symmetries have been imaged and identified with the pseudogap state, but it remains crucial to disentangle any electronic symmetry breaking from the pre-existing structural symmetry of the crystal. We use scanning tunnelling microscopy to observe an orthorhombic structural distortion across the cuprate superconducting \(Bi_{2}Sr_{2}Ca_{n−1}Cu_{n}O_{2n+4+x}\) (BSCCO) family tree, which breaks two-dimensional inversion symmetry in the surface BiO layer. Although this inversion-symmetry-breaking structure can impact electronic measurements, we show from its insensitivity to temperature, magnetic field and doping, that it cannot be the long-sought pseudogap state. To detect this picometre-scale variation in lattice structure, we have implemented a new algorithm that will serve as a powerful tool in the search for broken symmetry electronic states in cuprates, as well as in other materials.