Yaoju Tarn

All Publications

• Reducing the Strain Required for Ambient-Pressure Superconductivity in Ruddlesden-Popper Bilayer Nickelates. Advanced materials (Deerfield Beach, Fla.) Tarn, Y., Liu, Y., Theuss, F., Li, J., Wang, B. Y., Bhatt, L., Wang, J., Song, J., Thampy, V., Goodge, B. H., Muller, D. A., Shen, Z. X., Yu, Y., Hwang, H. Y. 2026: e20724

  Abstract

  The discovery of high-temperature superconductivity in pressurized bulk Ruddlesden-Popper (RP) bilayer nickelates has prompted the conjecture that epitaxial compressive strain might mimic essential aspects of hydrostatic pressure. The realization of superconductivity in films on SrLaAlO4 (001) (SLAO) supports this correspondence, yet it remains unclear whether the pressure-temperature phase diagram of RP bilayer nickelates can be systematically mapped (and studied at ambient pressure) as a function of epitaxial strain. To this end, experimental access near the elusive edge of the superconducting phase boundary would provide invaluable insight into the nature of the superconducting state and the ground state from which it emerges. Here we report superconducting RP bilayer nickelates grown on LaAlO3 (001) (LAO), where the compressive strain required for ambient-pressure superconductivity is nearly halved to -1.2%. These films exhibit a superconducting onset above 10 K and reach zero resistance at 3 K, with normal-state transport properties differing from those of films grown on SLAO. Our comparative study shows that strain-rather than interfacial structure is the primary factor governing the superconductivity and normal-state properties. This work offers a new opportunity to probe emergent phenomena near the superconducting phase boundary in the strain-temperature phase diagram of RP bilayer nickelates.

  View details for DOI 10.1002/adma.202520724

  View details for PubMedID 41677074

• Superconductivity and normal-state transport in compressively strained La2PrNi2O7 thin films. Nature materials Liu, Y., Ko, E. K., Tarn, Y., Bhatt, L., Li, J., Thampy, V., Goodge, B. H., Muller, D. A., Raghu, S., Yu, Y., Hwang, H. Y. 2025

  Abstract

  The discovery of superconductivity under high pressure in Ruddlesden-Popper phases of bulk nickelates has sparked great interest in stabilizing ambient-pressure superconductivity in the thin-film form using epitaxial strain. Recently, signs of superconductivity have been observed in compressively strained bilayer nickelate thin films with an onset temperature exceeding 40 K, although with broad, two-step-like transitions. Here we report the intrinsic superconductivity and normal-state transport properties in compressively strained La2PrNi2O7 thin films, achieved through a combination of isovalent Pr substitution, growth optimization and precision ozone annealing. The superconducting onset occurs above 48 K, with zero resistance reached above 30 K, and the critical current density at 1.4 K is 100-fold larger than previous reports. The normal-state resistivity exhibits quadratic temperature dependence indicative of Fermi liquid behaviour, and other phenomenological similarities to transport in overdoped cuprates suggest parallels in their emergent properties.

  View details for DOI 10.1038/s41563-025-02258-y

  View details for PubMedID 40442446

  View details for PubMedCentralID 10951943

• Toward Phonon-Limited Transport in Two-Dimensional Transition Metal Dichalcogenides by Oxygen-Free Fabrication. ACS nano Mukherjee, S., Wang, S., Venkatakrishnarao, D., Tarn, Y., Talha-Dean, T., Lee, R., Verzhbitskiy, I. A., Huang, D., Mishra, A., John, J. W., Das, S., Bussolotti, F., Maddumapatabandi, T. D., Teh, Y. W., Ang, Y. S., Johnson Goh, K. E., Lau, C. S. 2025; 19 (9): 9327-9339

  Abstract

  Developing future electronics will require aggressive scaling of the channel material thickness while maintaining device performance. Two-dimensional (2D) semiconductors are promising candidates to sustain further device scaling, but despite more than two decades of intense research, experimental performance continues to lag theoretical expectations. Here, we develop an oxygen-free approach to fabricate 2D field-effect transistors and push the electrical transport toward the theoretical phonon-limited intrinsic mobility. This approach achieves record carrier mobilities of 91 and 132 cm2 V-1 s-1 for mono- and bilayer MoS2 transistors on silicon oxide substrates. Statistical analysis of over 60 MoS2 and WS2 devices confirms that oxygen-free fabrication enhances device performance by more than an order of magnitude across key figures of merit. While previous studies suggest that 2D transition metal dichalcogenides such as MoS2 and WS2 are relatively stable in air, we show that even short-term ambient exposure can degrade their performance. We identify oxygen as a crucial factor in limiting transistor performance through irreversible chemisorption. This study emphasizes the criticality of avoiding oxygen exposure and offers guidance for device manufacturing that impacts fundamental research and practical applications of 2D materials.

  View details for DOI 10.1021/acsnano.5c00995

  View details for PubMedID 39993723

  View details for PubMedCentralID PMC11912576

• Liquid Metal Oxide-Assisted Integration of High-k Dielectrics and Metal Contacts for Two-Dimensional Electronics. ACS nano Venkatakrishnarao, D., Mishra, A., Tarn, Y., Bosman, M., Lee, R., Das, S., Mukherjee, S., Talha-Dean, T., Zhang, Y., Teo, S. L., Chai, J., Bussolotti, F., Goh, K. E., Lau, C. S. 2024; 18 (39): 26911-26919

  Abstract

  Two-dimensional van der Waals semiconductors are promising for future nanoelectronics. However, integrating high-k gate dielectrics for device applications is challenging as the inert van der Waals material surfaces hinder uniform dielectric growth. Here, we report a liquid metal oxide-assisted approach to integrate ultrathin, high-k HfO2 dielectric on 2D semiconductors with atomically smooth interfaces. Using this approach, we fabricated 2D WS2 top-gated transistors with subthreshold swings down to 74.5 mV/dec, gate leakage current density below 10-6 A/cm2, and negligible hysteresis. We further demonstrate a one-step van der Waals integration of contacts and dielectrics on graphene. This can offer a scalable approach toward integrating entire prefabricated device stack arrays with 2D materials. Our work provides a scalable solution to address the crucial dielectric engineering challenge for 2D semiconductor-based electronics.

  View details for DOI 10.1021/acsnano.4c08554

  View details for PubMedID 39350686

• Nanoironing van der Waals Heterostructures toward Electrically Controlled Quantum Dots. ACS applied materials & interfaces Talha-Dean, T., Tarn, Y., Mukherjee, S., John, J. W., Huang, D., Verzhbitskiy, I. A., Venkatakrishnarao, D., Das, S., Lee, R., Mishra, A., Wang, S., Ang, Y. S., Johnson Goh, K. E., Lau, C. S. 2024; 16 (24): 31738-31746

  Abstract

  Assembling two-dimensional van der Waals (vdW)-layered materials into heterostructures is an exciting development that sparked the discovery of rich correlated electronic phenomena. vdW heterostructures also offer possibilities for designer device applications in areas such as optoelectronics, valley- and spintronics, and quantum technology. However, realizing the full potential of these heterostructures requires interfaces with exceptionally low disorder which is challenging to engineer. Here, we show that thermal scanning probes can be used to create pristine interfaces in vdW heterostructures. Our approach is compatible at both the material- and device levels, and monolayer WS2 transistors show up to an order of magnitude improvement in electrical performance from this technique. We also demonstrate vdW heterostructures with low interface disorder enabling the electrical formation and control of quantum dots that can be tuned from macroscopic current flow to the single-electron tunneling regime.

  View details for DOI 10.1021/acsami.4c03639

  View details for PubMedID 38843175

• An algorithm for subtraction of doublet emission lines in angle-resolved photoemission spectroscopy JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA Tarn, Y., Sinha, M., Pasco, C., Schlom, D. G., McQueen, T. M., Shen, K. M., Faeth, B. D. 2023; 265

  View details for DOI 10.1016/j.elspec.2023.147323

  View details for Web of Science ID 000999138100001