Sebastian Hurtado Parra

Education

Ph.D. candidate, Department of Physics and Astronomy, University of Pennsylvania
B.S. in Physics and Mathematics, Saint Joseph’s University, May 2015

Current Research Projects

Ultrafast Optical Spectroscopy of Hybrid Organic-Inorganic Perovskites (HOIPs):
Studied dynamics of photoexcited states in 2D perovskites using ultrafast optical techniques. Extended an existing transient photoluminescence setup to a suite of experiments for fuller optical characterization:

• Developed a novel lock-in detection scheme for transient absorption spectroscopy, using time-based readout of a CCD, which dramatically improves signal-to-noise by 10-100x, while reducing collection time by 10x.
• Increased laser stability for sensitive measurements by 20x with a custom-built active laser noise suppressor at lock-in frequencies, utilizing a multi-channel PID feedback to control an electrically driven optical attenuator.
• Designed and fabricated modifications to the existing cryostat sample mounting to improve cooling efficiency.
• Operated and maintained ultrafast laser (modelocked laser, regenerative amplifier, and optical parametric amplifier), cryogenic (wet and cryogen-free), and vacuum systems.

Transport Measurements of Graphene-Based Materials:
Multilayer graphene holds exciting potential for electronic, spintronic, and valleytronic applications due to its unique band structure. I study properties of this band structure using electronic transport measurements of multilayer graphene devices down to cryogenic temperatures and high magnetic fields. I designed and built electronic transport and magnetometry experiments. Measurements were conducted using Quantum Design’s PPMS and MPMS, often interfacing with other electronic instruments. I also maintained and repaired the QD systems.

Magneto-Optical Imaging of Liquid Crystals:
Collaborated to design and build a polarized optical microscope around an existing 1 T electromagnet, integrating hardware and software into a full measurement platform for various projects:

• Developed imaging software on LabVIEW for instrument use by other users. Software employs real-time image recentering, allowing clean averaging of images while reducing data storage cost by 100x.
• Incorporated a custom-built heated stage for temperature control above room temperature.
• Built and deployed a small 100 mT electromagnet augmentation to an existing cryostat, including custom closed-loop cooling for the magnet coils, for a related project.

Publications

1. D. B. Straus, S. Hurtado Parra, N. Iotov, J. Gebhardt, A. M. Rappe, J. E. Subotnik, J. M. Kikkawa, and C. R. Kagan, JACS (2016) 138, 13798

   Direct Observation of Electron–Phonon Coupling and Slow Vibrational Relaxation in Organic–Inorganic Hybrid Perovskites
   DOI link
   Quantum and dielectric confinement effects in Ruddlesden-Popper 2D hybrid perovskites create excitons with a binding energy exceeding 150 meV. We exploit the large exciton binding energy to study exciton and carrier dynamics as well as electron–phonon coupling (EPC) in hybrid perovskites using absorption and photoluminescence (PL) spectroscopies. At temperatures <75 K, we resolve splitting of the excitonic absorption and PL into multiple regularly spaced resonances every 40–46 meV, consistent with EPC to phonons located on the organic cation. We also resolve resonances with a 14 meV spacing, in accord with coupling to phonons with mixed organic and inorganic character. These assignments are supported by density-functional theory calculations. Hot exciton PL and time- resolved PL measurements show that vibrational relaxation occurs on a picosecond time scale competitive with that for PL. At temperatures >75 K, excitonic absorption and PL exhibit homogeneous broadening. While absorption remains homogeneous, PL becomes inhomogeneous at temperatures <75K, which we speculate is caused by the formation and subsequent dynamics of a polaronic exciton.

2. Z. Gao, S. Wang, J. Berry, Q. Zhang, J. Gebhardt, W. M. Parkin, J. Avila, H. Yi, C. Chen, S. Hurtado-Parra, M. Drndić, A. M. Rappe, D. J. Srolovitz, J. M. Kikkawa, Z. Luo, M. C. Asensio, F. Wang, and A. T. C. Johnson, Nat Commun (2020) 11, 546

   Large-area epitaxial growth of curvature-stabilized ABC trilayer graphene
   DOI link
   The properties of van der Waals (vdW) materials often vary dramatically with the atomic stacking order between layers, but this order can be difficult to control. Trilayer graphene (TLG) stacks in either a semimetallic ABA or a semiconducting ABC configuration with a gate-tunable band gap, but the latter has only been produced by exfoliation. Here we present a chemical vapor deposition approach to TLG growth that yields greatly enhanced fraction and size of ABC domains. The key insight is that substrate curvature can stabilize ABC domains. Controllable ABC yields ~59% were achieved by tailoring substrate curvature levels. ABC fractions remained high after transfer to device substrates, as confirmed by transport measurements revealing the expected tunable ABC band gap. Substrate topography engineering provides a path to large-scale synthesis of epitaxial ABC-TLG and other vdW materials.

3. K. Du, S. D. Zemerov, S. Hurtado-Parra, J. M. Kikkawa, and I. J. Dmochowski, Inorg Chem (2020) 59, 13831

   Paramagnetic Organocobalt Capsule Revealing Xenon Host–Guest Chemistry
   DOI link
   We investigated Xe binding in a previously reported paramagnetic metal–organic tetrahedral capsule, [Co4L6]4–, where L2– = 4,4′-bis[(2-pyridinylmethylene)amino][1,1′-biphenyl]-2,2′-disulfonate. The Xe-inclusion complex, [XeCo4L6]4–, was confirmed by 1H NMR spectroscopy to be the dominant species in aqueous solution saturated with Xe gas. The measured Xe dissociation rate in [XeCo4L6]4–, koff = 4.45(5) × 102 s–1, was at least 40 times greater than that in the analogous [XeFe4L6]4– complex, highlighting the capability of metal–ligand interactions to tune the capsule size and guest permeability. The rapid exchange of 129Xe nuclei in [XeCo4L6]4– produced significant hyperpolarized 129Xe chemical exchange saturation transfer (hyper-CEST) NMR signal at 298 K, detected at a concentration of [XeCo4L6]4– as low as 100 pM, with presaturation at −89 ppm, which was referenced to solvated 129Xe in H2O. The saturation offset was highly temperature-dependent with a slope of −0.41(3) ppm/K, which is attributed to hyperfine interactions between the encapsulated 129Xe nucleus and electron spins on the four CoII centers. As such, [XeCo4L6]4– represents the first example of a paramagnetic hyper-CEST (paraHYPERCEST) sensor. Remarkably, the hyper-CEST 129Xe NMR resonance for [XeCo4L6]4– (δ = −89 ppm) was shifted 105 ppm upfield from the diamagnetic analogue [XeFe4L6]4– (δ = +16 ppm). The Xe inclusion complex was further characterized in the crystal structure of (C(NH2)3)4[Xe0.7Co4L6]·75 H2O (1). Hydrogen bonding between capsule-linker sulfonate groups and exogenous guanidinium cations, (C(NH2)3)+, stabilized capsule–capsule interactions in the solid state and also assisted in trapping a Xe atom (∼42 Å3) in the large (135 Å3) cavity of 1. Magnetic susceptibility measurements confirmed the presence of four noninteracting, magnetically anisotropic high-spin CoII centers in 1. Furthermore, [Co4L6]4– was found to be stable toward aggregation and oxidation, and the CEST performance of [XeCo4L6]4– was unaffected by biological macromolecules in H2O. These results recommend metal–organic capsules for fundamental investigations of Xe host–guest chemistry as well as applications with highly sensitive 129Xe-based sensors.

4. D. B. Straus, S. Hurtado-Parra, N. Iotov, Q. Zhao, M. R. Gau, P. J. Carroll, J. M. Kikkawa, and C. R. Kagan, ACSNano (2020) 14, 3621

   Tailoring Hot Exciton Dynamics in 2D Hybrid Perovskites through Cation Modification
   DOI link
   We report a family of two-dimensional hybrid perovskites (2DHPs) based on phenethylammonium lead iodide ((PEA)2PbI4) that show complex structure in their low-temperature excitonic absorption and photoluminescence (PL) spectra as well as hot exciton PL. We replace the 2-position (ortho) H on the phenyl group of the PEA cation with F, Cl, or Br to systematically increase the cation’s cross-sectional area and mass and study changes in the excitonic structure. These single atom substitutions substantially change the observable number of and spacing between discrete resonances in the excitonic absorption and PL spectra and drastically increase the amount of hot exciton PL that violates Kasha’s rule by over an order of magnitude. To fit the progressively larger cations, the inorganic framework distorts and is strained, reducing the Pb–I–Pb bond angles and increasing the 2DHP band gap. Correlation between the 2DHP structure and steady-state and time-resolved spectra suggests the complex structure of resonances arises from one or two manifolds of states, depending on the 2DHP Pb–I–Pb bond angle (as)symmetry, and the resonances within a manifold are regularly spaced with an energy separation that decreases as the mass of the cation increases. The uniform separation between resonances and the dynamics that show excitons can only relax to the next-lowest state are consistent with a vibronic progression caused by a vibrational mode on the cation. These results demonstrate that simple changes to the cation can be used to tailor the properties and dynamics of the confined excitons without directly modifying the inorganic framework.

5. D Khadka, T. R. Thapaliya, S. Hurtado Parra, X. Han, J. Wen, R. F. Need, P. Khanal, W. Wang, J. Zang, J. M. Kikkawa, L. Wu, S. X. Huang, Science Advances (2020) 6, eabc1977

   Kondo physics in antiferromagnetic Weyl semimetal Mn3+xSn1−x films
   DOI link
   Topology and strong electron correlations are crucial ingredients in emerging quantum materials, yet their intersection in experimental systems has been relatively limited to date. Strongly correlated Weyl semimetals, particularly when magnetism is incorporated, offer a unique and fertile platform to explore emergent phenomena in novel topological matter and topological spintronics. The antiferromagnetic Weyl semimetal Mn3Sn exhibits many exotic physical properties such as a large spontaneous Hall effect and has recently attracted intense interest. In this work, we report synthesis of epitaxial Mn3+xSn1−x films with greatly extended compositional range in comparison with that of bulk samples. As Sn atoms are replaced by magnetic Mn atoms, the Kondo effect, which is a celebrated example of strong correlations, emerges, develops coherence, and induces a hybridization energy gap. The magnetic doping and gap opening lead to rich extraordinary properties, as exemplified by the prominent DC Hall effects and resonance-enhanced terahertz Faraday rotation.

6. D Khadka, T. R. Thapaliya, S. Hurtado Parra, J. Wen, R. Need, J. M. Kikkawa, S. X. Huang, Phys Rev Mat (2020) 4, 084203

   Anomalous Hall and Nernst effects in epitaxial films of topological kagome magnet Fe3Sn2
   DOI link
   The topological kagome magnet (TKM) Fe3Sn2 exhibits unusual topological properties, flat electronic bands, and chiral spin textures, making it an exquisite materials platform to explore the interplay between topological band structure, strong electron correlations, and magnetism. Here we report the synthesis of high-quality epitaxial (0001) Fe3Sn2 films with large intrinsic anomalous Hall effect close to that measured in bulk single crystals. In addition, we measured a large, anisotropic anomalous Nernst coefficient Syx of 1.26μVK−1, roughly 2–5 times greater than that of common ferromagnets, suggesting the presence of Berry curvature sources near the Fermi level in this system. Crucially, the realization of high-quality Fe3Sn2 films opens the door to explore emergent interfacial physics and create novel spintronic devices based on TKMs by interfacing Fe3Sn2 with other quantum materials and by nanostructure patterning.

7. Q. Zhang, S. Wang, Z. Gao, S. Hurtado-Parra, J. Berry, Z. Addison, P. M. Dais, W. M. Parkin, M. Drndic, J. M. Kikkawa, F. Wang, E. J. Mele, Z. Luo, A. T. C. Johnson, submitted (2020)

   Submitted