Subcategory: Physics (not Nanoscience)
Cody Youmans - City College of New York, and The Graduate Center at CUNY
Co-Author(s): Areg Ghazaryan & Pouyan Ghaemi, City College of New York, NY
Fe-pnictides are a class of high-Tc multi-band superconducting materials known to exhibit a rich phase diagram, including both competitive and homeostatic coexistences. For a range of dopings at low temperatures, a coexistence of superconductivity (SC) and spin-density-wave (SDW) ordering is observed in certain pnictides. Theoretical studies show the robustness of the bulk SC band-gap typical to such materials to be enhanced by the concurrent SDW ordering. Meanwhile, experiments on such materials have also shown that there are further enhancements of certain signatures of SC (e.g., superfluid susceptibility) at SDW domain-walls (where the SDW magnitude changes sign, and hence the coexistence is disrupted). This motivated us to look at the possible pairing channels that could be generated as a consequence of symmetry breaking at such defects.
Through numerical and analytical analyses, we investigated the hypothesis that certain pairing amplitudes should be enhanced at defects in the SDW ordering within such a coexistence phase, despite the disruption of coexistence at the defects. This indicates directions for realizing robust forms of unconventional pairing, widely sought-after for generic potential relevance to applications such as realizing Majorana Fermions, as well as its potential for elucidating fundamental questions regarding the physics of high-Tc SC.
In a two-orbital model of Fe-pnictides, starting from a mean-field treatment, we considered the simultaneous presence of extended s-wave (XSW) SC and stripe-antiferromagnetic SDW in candidate materials. We then began our analysis of defects with the simplest type ? surfaces. Exact-diagonalization of a tight-binding Hamiltonian for a finite-lattice model reveals subgap surface states that become gapless for sufficiently large bulk SDW magnitude. The inability of the XSW – an even-frequency, spin-singlet, even-parity pairing that changes sign between separated Fermi pockets, to open a gap in these surface states follows from the emergent ferromagnetic spin-polarization occurring near surfaces, separating states that would otherwise be paired via the inherent SC instability. However, guided by the insight that each of the various forms of symmetry breaking can provide alternative means for pairs to satisfy Pauli?s exclusion principle, as well as the general observation that heterostructures involving ferromagnetism and SC can exhibit pairing amplitudes that are odd in frequency or time, we looked at second-order contributions by calculating anomalous SC correlators using our mean-field solutions. We found that, due to the simultaneous breaking of time-reversal and translational symmetry, surfaces accommodate enhancements of odd-frequency, spin-triplet, even-parity pairing, in addition to odd-frequency, spin-singlet, odd-parity pairing amplitudes. Further research will examine other types of defects in these systems, as well as further potential experimental signatures.
P. Ghaemi and A. Vishwanath, Phys. Rev. B 83, 224513 (2011)
B. Kalisky et al., PRB, 81:184513, May 2010
Y. Tanaka et al., J. Phys. Soc. Jpn. 81, 011013 (2012)Not Submitted
Funder Acknowledgement(s): CREST Center for Interface Design and Engineered Assembly of Low Dimensional systems (IDEALS)
Faculty Advisor: Pouyan Ghaemi, email@example.com
Role: I worked on all aspects of the project outlined above.