Ultrafast dynamics in 2D materials and heterostructures visualized with time- and angle-resolved photoemission

Thomas K. Allison, Zachary H. Withers, Ziling Li, Jin Bakalis, Sergii Chernov, Shuyu Cheng, Gerd Schönhense, Xu Du, Roland Kawakami, Alice Kunin, Proceedings Volume 12884, Ultrafast Phenomena and Nanophotonics XXVIII; 128840A (2024)

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The optical and electronic properties of two-dimensional (2D) materials make them attractive for a variety of applications in quantum information processing, solar-energy harvesting, and catalysis. However, the dynamics of electrons, holes, and excitons formed after photo-excitation are often complex and involve many states that are optically dark, making their characterization with optical spectroscopy alone difficult. We present our recent work imaging the quantum states of 2D materials and their heterostructures in momentum space using time- and angle-resolved photoemission. A unique combination of tunable ultrashort extreme ultraviolet (XUV) pulses with 61 MHz repetition rate and time-of-flight momentum microscopy (ToF k-mic) enables the parallel recording of electron dynamics in all states across the full Brillouin zone after perturbative excitation. We will present results on pseudospin dynamics in graphene, valley circular dichroism in monolayer WS2, and hybridized excitons in MoSe2/WS2 bilayers.

Quantum octets in high mobility pentagonal two-dimensional PdSe2

Yuxin Zhang, Haidong Tian, Huaixuan Li, Chiho Yoon, Ryan A. Nelson, Ziling Li, Kenji Watanabe, Takashi Taniguchi, Dmitry Smirnov, Roland K. Kawakami, Joshua E. Goldberger, Fan Zhang & Chun Ning Lau, Nature Communications volume 15, Article number: 761 (2024)

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Two-dimensional (2D) materials have drawn immense interests in scientific and technological communities, owing to their extraordinary properties and their tunability by gating, proximity, strain and external fields. For electronic applications, an ideal 2D material would have high mobility, air stability, sizable band gap, and be compatible with large scale synthesis. Here we demonstrate air stable field effect transistors using atomically thin few-layer PdSe2 sheets that are sandwiched between hexagonal BN (hBN), with large saturation current > 350 μA/μm, and high field effect mobilities of ~ 700 and 10,000 cm2/Vs at 300 K and 2 K, respectively. At low temperatures, magnetotransport studies reveal unique octets in quantum oscillations that persist at all densities, arising from 2-fold spin and 4-fold valley degeneracies, which can be broken by in-plane and out-of-plane magnetic fields toward quantum Hall spin and orbital ferromagnetism.


Room-Temperature Magnetic Skyrmions in Pt/Co/Cu Multilayers

Shuyu Cheng, Nuria Bagues, Camelia M. Selcu, Jacob B. Freyermuth, Ziling Li, Binbin Wang, Shekhar Das, P. Chris Hammel, Mohit Randeria, David W. McComb, and Roland K. Kawakami, Phys. Rev. B 108, 174433 (2023)

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Magnetic skyrmions are promising for next-generation information storage and processing owing to their potential advantages in data storage density, robustness, and energy efficiency. The magnetic multilayers consisting of Pt, Co, and a third metal element X provide an ideal platform to study skyrmions due to their highly tunable magnetic properties. Here, we report the observation of room-temperature bubblelike Néel skyrmions in epitaxial Pt/Co/Cu multilayers in samples with multidomain states in zero field. The magneto-optic Kerr effect and a superconducting quantum interference device magnetometry are applied to investigate the shapes of the hysteresis loops, the magnetic anisotropy, and the saturation magnetization. By tuning the Co thickness and the number of periods, we achieve perpendicular and in-plane magnetized states and multidomain states that are identified by a wasp-waisted hysteresis loop. Skyrmions are directly imaged by magnetic force microscopy and Lorentz transmission electron microscopy. The development of room-temperature skyrmions in Pt/Co/Cu multilayers may lead to advances in skyrmion-related research and applications.

Tuning the Curie temperature of a two-dimensional magnet/topological insulator heterostructure to above room temperature by epitaxial growth

Wenyi Zhou, Alexander J. Bishop, Xiyue S. Zhang, Katherine Robinson, Igor Lyalin, Ziling Li, Ryan Bailey-Crandell, Thow Min Jerald Cham, Shuyu Cheng, Yunqiu Kelly Luo, Daniel C. Ralph, David A. Muller, and Roland K. Kawakami, Phys. Rev. Materials 7, 104004 (2023)

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The heterostructures of 2D magnets and topological insulators are attractive candidates for quantum anomalous hall effect and highly-efficent spin-orbit torque switching. To better study these phenomena, it will be crucial to synthesize epitaxial heterostructures for scalability while maintaining high-quality interfaces to preserve the topological surface states and having the Curie temperature (TC) above room temperature. We utilize molecular beam epitaxy (MBE) to tune the TC in heterostructures of van der Waals (vdW) magnet Fe3GeTe2 and topological insulator Bi2Te3 to above room temperature by varying growth conditions. Cross-sectional scanning transmission electron microscopy (STEM) imaging reveals existence of thicker vdW compouds in the FemGenTe2 family, as well as some intercalants in between vdW gaps, which are possible origins for the enhanced TC.


Magneto-Optical Detection of the Orbital Hall Effect in Chromium

Igor Lyalin, Sanaz Alikhah, Marco Berritta, Peter M. Oppeneer, and Roland K. Kawakami, Phys. Rev. Lett. 131, 156702 (2023)

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The Hall effect was discovered by a PhD student Edwin Hall and his advisor Henry Rowland in 1879. Since then, the family of Hall effects has grown considerably. The anomalous Hall effect, integer and fractional quantum Hall effects, spin Hall effect, quantum anomalous Hall effect are fundamental physics phenomena of great importance, some of them were recognized by Nobel prizes (1985, 1988, 2016). A new member of the Hall effect family has been theoretically predicted recently [1][2][3][4]. It was coined the orbital Hall effect (OHE), as it relies on the orbital motion of electrons instead of their charge as in the original Hall effect. However, its direct observation is a challenge as it is difficult to separate spin and orbital Hall effects in solids. In our recent study, using magneto-optical Kerr effect with nanoradian sensitivity we detected the current-induced orbital accumulation at the surface of a light 3d transition metal, Cr. We found that the orbital polarization is in-plane, transverse to the current direction, and scales linearly with current density, fully consistent with the orbital Hall effect. Comparing our magneto-optical measurements with theoretical calculations, we found that the measured signal cannot be explained by the small and negative spin Hall effect in Cr but is well explained by the large positive orbital Hall effect. Along with Choi et al. study of the OHE in Ti [5], our work [6] provides strong evidence for the OHE. We believe that the detection of the orbital Hall effect in light metals, such as Ti and Cr, can have important consequences for future spintronics applications that could utilize orbital currents rather than spin currents.


Epitaxial Kagome Thin Films as a Platform for Topological Flat Bands

Shuyu Cheng, M. Nrisimhamurty, Tong Zhou, Núria Bagués, Wenyi Zhou, Alexander J. Bishop, Igor Lyalin, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, David W. McComb, Igor Žutić, and Roland K. Kawakami, Nano Lett. 23, 7107–7113 (2023)

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Systems with flat bands are ideal for studying strongly correlated electronic states and related phenomena. Among them, kagome-structured metals such as CoSn have been recognized as promising candidates due to the proximity between the flat bands and the Fermi level. A key next step will be to realize epitaxial kagome thin films with flat bands to enable tuning of the flat bands across the Fermi level via electrostatic gating or strain. Here, we report the band structures of epitaxial CoSn thin films grown directly on the insulating substrates. Flat bands are observed by using synchrotron-based angle-resolved photoemission spectroscopy (ARPES). The band structure is consistent with density functional theory (DFT) calculations, and the transport properties are quantitatively explained by the band structure and semiclassical transport theory. Our work paves the way to realize flat band-induced phenomena through fine-tuning of flat bands in kagome materials.


An Atomically Tailored Chiral Magnet with Small Skyrmions at Room Temperature

Tao Liu, Camelia M. Selcu, Binbin Wang, Núria Bagués, Po-Kuan Wu, Timothy Q. Hartnett, Shuyu Cheng, Denis Pelekhov, Roland A. Bennett, Joseph Perry Corbett, Jacob R. Repicky, Brendan McCullian, P. Chris Hammel, Jay A. Gupta, Mohit Randeria, Prasanna V. Balachandran, David W. McComb, Roland K. Kawakami, Communications Physics 6, 327 (2023).

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Creating materials that do not exist in nature can lead to breakthroughs in science and technology. Magnetic skyrmions are topological excitations that have attracted great attention recently for their potential applications in low power, ultrahigh density memory. A major challenge has been to find materials that meet the dual requirement of small skyrmions stable at room temperature. Here we meet both these goals by developing epitaxial FeGe films with excess Fe using atomic layer molecular beam epitaxy (MBE) far from thermal equilibrium. Our atomic layer design permits the incorporation of 20% excess Fe while maintaining a non-centrosymmetric crystal structure supported by theoretical calculations and necessary for stabilizing skyrmions. We show that the Curie temperature is well above room temperature, and that the skyrmions have sizes down to 15 nm as imaged by Lorentz transmission electron microscopy (LTEM) and magnetic force microscopy (MFM). The presence of skyrmions coincides with a topological Hall effect-like resistivity. These atomically tailored materials hold promise for future ultrahigh density magnetic memory applications.

Momentum-Resolved Exciton Coupling and Valley Polarization Dynamics in Monolayer WS2

Alice Kunin, Sergey Chernov, Jin Bakalis, Ziling Li, Shuyu Cheng, Zachary H. Withers, Michael G. White, Gerd Schonhense, Xu Du, Roland K. Kawakami, and Thomas K. Allison, Physical Review Letters 130, 046202 (2023).


Scanning Tunneling Microscopy Study of the Antiferromagnetic Topological Insulator MnBi2Se4 

Robert C. Walko, Tiancong Zhu , Alexander J. Bishop, Roland K. Kawakami, Jay A. Gupta, Physica E 143, 115391 (2022). 

Atomic Layer Epitaxy of Kagome Magnet Fe3Sn2 and Sn-modulated Heterostructures

Shuyu Cheng, Binbin Wang, Igor Lyalin, Núria Bagués, Alexander J. Bishop, David W. McComb, Roland K. Kawakami, APL Materials 10, 061112 (2022).

Kinetically-controlled epitaxial growth of Fe3GeTe2 van der Waals ferromagnetic films 

Wenyi Zhou, Alexander J. Bishop, Menglin Zhu, Igor Lyalin, Robert C. Walko, Jay A. Gupta, Jinwoo Hwang, Roland K. Kawakami, ACS Applied Electronic Materials 4, 3190-3197 (2022).

Ultrafast Optical Spin Switching in Ferrimagnetic Nickel Ferrite (NiFe2O4) Studied by XUV Reflection–Absorption Spectroscopy 

Stephen Londo, Somnath Biswas, Igor V. Pinchuk, Alexandra Boyadzhiev, Roland K. Kawakami, L. Robert Baker, J. Phys. Chem. C 126, 2669 (2022).

Extracting weak magnetic contrast from complex background contrast in plan-view FeGe thin films

Binbin Wang, Nuria Bagues, Tao Liu, Roland K. Kawakami, David W. McComb, Ultramicroscopy 232, 113395 (2022).


Atomic-scale visualization of topological spin textures in the chiral magnet MnGe 

Jacob Repicky, Po-Kuan Wu, Tao Liu, Joseph Corbett, Tiancong Zhu, Shuyu Cheng, Adam Ahmed, N. Takeuchi, J. Guerrero-Sanchez, Mohit Randeria, Roland Kawakami, Jay A. Gupta, Science 374, 1484–1487 (2021).


Van der Waals heterostructures for spintronics and opto-spintronics 

Juan F. Sierra, Jaroslav Fabian, Roland K. Kawakami, Stephan Roche, Sergio O. Valenzuela, Nature Nanotechnology 16, 856-868 (2021).


Spin–Orbit Torque in Bilayers of Kagome Ferromagnet Fe3Sn2 and Pt

Igor Lyalin, Shuyu Cheng, Roland K. Kawakami, Nano Letters 21, 6975–6982 (2021).


Epitaxial Growth and Domain Structure Imaging of Kagome Magnet Fe3Sn2

Shuyu Cheng, Igor Lyalin, Alexander J. Bishop, Roland K. Kawakami arXiv:2105.12203 (2021).


Synthesis, Magnetic Properties, and Electronic Structure of Magnetic Topological Insulator MnBi2Se4

Tiancong Zhu, Alexander J. Bishop, Tong Zhou, Menglin Zhu, Dante J. O’Hara, Alexander A. Baker, Shuyu Cheng, Robert C. Walko, Jacob J. Repicky, Tao Liu, Jay A. Gupta, Chris M. Jozwiak, Eli Rotenberg, Jinwoo Hwang, Igor Žutić, Roland K. Kawakami, Nano Letters 21, 5083–5090 (2021).


Coherent growth and characterization of van der Waals 1T-VSe2 layers on GaAs(111)B using molecular beam epitaxy 

Tiancong Zhu, Dante J. O’Hara, Brenton A. Noesges, Menglin Zhu, Jacob J. Repicky, Mark R. Brenner, Leonard J. Brillson, Jinwoo Hwang, Jay A. Gupta, Roland K. Kawakami, Physical Review Materials 4, 084002 (2020).

The 2020 magnetism roadmap

Elena Yu Vedmedenko, Roland Kenji Kawakami, Denis D. Sheka, Pietro Gambardella, Andrei Kirilyuk, Atsufumi Hirohata, Christian Binek, Oksana Chubykalo-Fesenko, Stefano Sanvito, Brian J. Kirby, Julie Grollier, Karin Everschor-Sitte, Tobias Kampfrath, Chun Yeol You, Andreas Berger,  Journal of Physics D: Applied Physics 53, 453001 (2020).

Suppression of magnetic ordering in Fe-deficient Fe3-xGeTe2 from application of pressure

Dante J. O’Hara, Zachary E. Brubaker, Ryan L. Stillwell, Earl F. O’Bannon, Alexander A. Baker, Daniel Weber, Leonardus Bimo Bayu Aji, Joshua E. Goldberger, Roland K. Kawakami, Rena J. Zieve, Jason R. Jeffries, Scott K. McCall, Physical Review B 102, 054405 (2020).

Chemical migration and dipole formation at van der Waals interfaces between magnetic transition metal chalcogenides and topological insulators

Brenton A. Noesges, Tiancong Zhu, Jacob J. Repicky, Sisheng Yu, Fengyuan Yang, Jay A. Gupta, Roland K. Kawakami, Leonard J. Brillson, Physical Review Materials 4, 054001 (2020).

Spatial Frequency Selection in Lorentz 4D-Scanning Transmission Electron Microscopy Reconstruction

Binbin Wang, Nuria Bagues, Tao Liu, Jiaqiang Yan, Roland Kawakami, David McComb, Microscopy and Microanalysis 26, 1902-1905 (2020).


Imaging of Magnetic Textures in Polycrystalline FeGe Thin Films via in-situ Lorentz Transmission Electron Microscopy 

Nuria Bagues, Binbin Wang, Tao Liu, Camelia Selcu, Stephen Boona, Roland Kawakami, Mohit Randeria, and David McComb, Microscopy and Microanalysis 26, 1700-1702 (2020).


Investigation of Antiphase Domain Boundaries in Cobalt Ferrite Thin Films via High Resolution Scanning Transmission Electron Microscopy

Amanda Trout, Igor Pinchuk, Michael Newburger, Roland Kawakami, David McComb, Microscopy and Microanalysis 26, 972-974 (2020).


Ultrafast Spin Crossover in a Room-Temperature Ferrimagnet: Element-Specific Spin Dynamics in Photoexcited Cobalt Ferrite

Stephen Londo, Somnath Biswas, Jakub Husek, Igor V. Pinchuk, Michael J. Newburger, Alexandra Boyadzhiev, Amanda H. Trout, David W. McComb, Roland Kawakami, L. Robert Baker, The Journal of Physical Chemistry C 124, 11368-11375 (2020).

Determining Surface Terminations and Chirality of Noncentrosymmetric FeGe Thin Films via Scanning Tunneling Microscopy 

Joseph P. Corbett, Tiancong Zhu, Adam S. Ahmed, Steven J. Tjung, Jacob J. Repicky, Takahiro Takeuchi, Jonathan Guerrero-Sanchez, Noboru Takeuchi, Roland K. Kawakami, Jay A. Gupta, ACS Applied Materials & Interfaces 12, 9896-9901 (2020).


Spintronics in 2D Materials 

Wei Han and Roland Kawakami, in Spintronics Handbook, Second Edition: Spin Transport and Magnetism : Volume Three: Nanoscale Spintronics and Applications (Editors: E. Y. Tsymbal, I. Žutić, CRC Press, 2019).

Spectroscopic evaluation of charge-transfer doping and strain in graphene/MoS2 heterostructures

Rahul Rao, Ahmad E. Islam, Simranjeet Singh, Rajiv Berry, Roland K. Kawakami, Benji Maruyama, and Jyoti Katoch, Physical Review B 99, 195401 (2019).

Epitaxial growth of cobalt doped TiO2 thin films on LaAlO3 (100) substrate by molecular beam epitaxy and their opto-magnetic based applications

Swaleha Naseem, Igor V. Pinchuk, Yunqiu Kelly Luo, Roland K. Kawakami, Shakeel Khan, Shahid Husain, Wasi Khan, Applied Surface Science 493, 691-702 (2019).


Spin Absorption by In Situ Deposited Nanoscale Magnets on Graphene Spin Valves 

Walid Amamou, Gordon Stecklein, Steven J. Koester, Paul A. Crowell, and Roland K. Kawakami, Phys. Rev. Applied 10, 044050 (2018) pdf

Tuning Spin Dynamics and Localization near the Metal-Insulator Transition in Fe/GaAs Heterostructures

Yu-Sheng Ou, N. J. Harmon, Patrick Odenthal, Roland K. Kawakami, Michael E. Flatté, Ezekiel Johnson-Halperin, Physical Review B 98, 134444 (2018). pdf

Microscopy of Hydrogen and Hydrogen-Vacancy Defect Structures on Graphene Devices

Dillon Wong, Yang Wang, Wuwei Jin, Hsin-Zon Tsai, Aaron Bostwick, Eli Rotenberg, Roland K. Kawakami, Alex Zettl, Arash A. Mostofi, Johannes Lischner and Michael F. Crommie, Physical Review B 98, 155436 (2018). pdf

Transport Spectroscopy of Sublattice-Resolved Resonant Scattering in Hydrogen-Doped Bilayer Graphene

Jyoti Katoch, Tiancong Zhu, Denis Kochan, Simranjeet Singh, Jaroslav Fabian and Roland K. Kawakami, Physical Review Letters 121, 136801 (2018). pdf

Strong and Tunable Spin Lifetime Anisotropy in Dual-Gated Bilayer Graphene

Jinsong Xu, Tiancong Zhu, Yunqiu Kelly Luo, Yuan-Ming Lu and Roland K. Kawakami, Physical Review Letters 121, 127703 (2018). Editors’ Suggestion. Highlighted in Physics

Importance of Paramagnetic Background Subtraction for Determining the Magnetic Moment in Epitaxially Grown Ultrathin van der Waals Magnets

Dante O’Hara, Tiancong Zhu and Roland K. Kawakami, IEEE Magnetics Letters 9, 1405805 (2018). pdf

Spin Inversion in Graphene Spin Valves by Gate-Tunable Magnetic Proximity Effect at One-Dimensional Contacts

Jinsong Xu, Simranjeet Singh, Jyoti Katoch, Guanzhong Wu, Tiancong Zhu, Igor Žutić and Roland K. Kawakami, Nature Communications 9, 2869 (2018). pdf

Topological Dirac Semimetal Na3Bi Films in the Ultrathin Limit via Alternating Layer Molecular Beam Epitaxy

Igor V. Pinchuk, Thaddeus J. Asel, Andrew Franson, Tiancong Zhu, Yuan-Ming Lu, Leonard J. Brillson, Ezekiel Johnston-Halperin, Jay A. Gupta, Roland K. Kawakami, APL Materials 6, 086103 (2018) pdf

Probing Tunneling Spin Injection into Graphene via Bias Dependence

Tiancong Zhu, Simranjeet Singh, Jyoti Katoch, Hua Wen, Kirill Belashchenko, Igor Žutić, Roland K. Kawakami Physical Review B 98, 054412 (2018) pdf

Electronic Structure of Exfoliated and Epitaxial Hexagonal Boron Nitride

Roland J. Koch, Jyoti Katoch, Simon Moser, Daniel Schwarz, Roland K. Kawakami, Aaron Bostwick, Eli Rotenberg, Chris Jozwiak and Søren Ulstrup  Physical Review Materials 2, 074006 (2018) pdf

Modeling the Oblique Spin Precession in Lateral Spin Valves for Accurate Determination of Spin Lifetime Anisotropy: Effect of Finite Contact Resistance and Channel Length

Tiancong Zhu and Roland K. Kawakami, Physical Review B 97, 144413 (2018) pdf

Chiral Bobbers and Skyrmions in Epitaxial FeGe/Si(111) Films

Adam S. Ahmed, James Rowland, Bryan D. Esser, Sarah Dunsiger, David W. McComb, Mohit Randeria, and Roland K. Kawakami, Physical Review Materials (Rapid) 2, 041401(R) (2018)  pdf

Room Temperature Intrinsic Ferromagnetism in Epitaxial Manganese Selenide Films in the Monolayer Limit

Dante J. O’Hara, Tiancong Zhu, Amanda H. Trout, Adam S. Ahmed, Yunqiu (Kelly) Luo, Choong Hee Lee, Mark R. Brenner, Siddharth Rajan, Jay A. Gupta, David W. McComb, and Roland K. Kawakami Nano Letters 18, 3125 (2018) pdf

Magnetic Proximity Effect in Pt/CoFe2O4 Bilayers

Walid Amamou, Igor V. Pinchuk, Amanda Hanks, Robert Williams, Nikolas Antolin, Adam Goad, Dante J. O’Hara, Adam S. Ahmed, Wolfgang Windl, David W. McComb, and Roland K. Kawakami, Physics Review Materials 2, 011401 (R) (2018). pdf  Editors’ Suggestion

Giant Spin-Splitting and Gap Renormalization Driven by Trions in Single-Layer WS2/h-BN Heterostructures

Jyoti Katoch, Søren Ulstrup, Roland J. Koch, Simon Moser, Kathleen M. McCreary, Simranjeet Singh, Jinsong Xu, Berend T. Jonker, Roland K. Kawakami, Aaron Bostwick, Eli Rotenberg, and Chris Jozwiak,  Nature Physics 14, 355-359 (2018).pdf


Imaging Spin Dynamics in Monolayer WS2 by Time-Resolved Kerr Rotation Microscopy

Elizabeth J. McCormick, Michael J. Newburger, Yunqiu (Kelly) Luo, Kathleen M. McCreary, Simranjeet Singh, Iwan B MartinEdward J Cichewicz Jr, Berend T. Jonker, Roland K. Kawakami, 2D Materials 5, 011010 (2018). pdf


J.-P. Wang, S. S. Sapatnekar, C. H. Kim, P. Crowell, S. Koester, S. Datta, K. Roy, A. Raghunathan, X. S. Hu, M. Niemier, A. Naeemi, C.-L. Chien, C. Ross, and R. Kawakami, in Proceedings of the 54th Annual Design Automation Conference 2017 (ACM, Austin, TX, USA, 2017), pp. 1.  pdf

Strontium Oxide Tunnel Barriers for High Quality Spin Transport and Large Spin Accumulation in Graphene

Simranjeet Singh, Jyoti Katoch, Tiancong Zhu, Ryan J. Wu, Adam S. Ahmed, Walid Amamou, Dongying Wang, K Andre Mkhoyan and Roland K. Kawakami, Nano Letters 17, 7578–7585 (2017). pdf

Spatially Resolving Density-Dependent Screening Around a Single Charged Atom in Graphene

Dillon Wong, Fabiano Corsetti, Yang Wang, Victor W. Brar, Hsin-Zon Tsai, Qiong Wu, Roland K. Kawakami, Alex Zettl, Arash A. Mostofi, Johannes Lischner, and Michael F. Crommie, Physical Review B 95, 205419 (2017).

Uniform Large-Area Growth of Nanotemplated High-Quality Monolayer MoS2

Justin R. Young, Michael Chilcote, Matthew Barone, Jinsong Xu, Jyoti Katoch, Yunqiu Kelly Luo, Sara Mueller,  Thaddeus J. Asel,  Susan K. Fullerton-Shirey, Roland Kawakami, Jay A. Gupta, Leonard J. Brillson, and Ezekiel Johnston-HalperinApplied Physics Letters 110, 263103 (2017).

Opto-Valleytronic Spin Injection in Monolayer MoS2/Few-Layer Graphene Hybrid Spin Valves

Yunqiu (Kelly) Luo, Jinsong Xu, Tiancong Zhu, Guanzhong Wu, Elizabeth Joan McCormick, Mahesh R. Neupane, Wenbo Zhan, and Roland K. Kawakami, Nano Letters 17, 3877–3883 (2017). pdf

Molecular Beam Epitaxy Growth of [CrGe/MnGe/FeGe] Superlattices: Toward Artificial B20 Skyrmion Materials with Tunable Interactions

Adam S. Ahmed, Bryan D. Esser, James Rowland, David W. McComb, and Roland K. Kawakami, Journal of Crystal Growth 467, 38-46 (2017). pdf

Growth of Uniform CaGe2 Films by Alternating Layer Molecular Beam Epitaxy

Jinsong Xu, Jyoti Katoch, Adam S. Ahmed, Igor V. Pinchuk, Justin R. Young, Ezekiel Johnston-Halperin, Jonathan Pelz, Roland. K. Kawakami, Journal of Crystal Growth 460, 134-138 (2017). pdf

Molecular Beam Epitaxy of 2D-Layered Gallium Selenide on GaN Substrates

Choong Hee Lee, Sriram Krishnamoorthy, Dante J. O’Hara, Jared M. Johnson, John Jamison, Roberto C. Myers, Roland K. Kawakami, Jinwoo Hwang, Siddharth Rajan, Journal of Applied Physics 121, 094302 (2017). pdf

Strong Modulation of Spin Currents in Bilayer Graphene by Static and Fluctuating Proximity Exchange Fields

Simranjeet Singh, Jyoti Katoch, Tiancong Zhu, Keng-Yuan Meng, Tianyu Liu, Jack T. Brangham, Fengyuan Yang, Michael Flatté and Roland K. Kawakami, Physical Review Letters 118, 187201 (2017).  pdf  Editors’ Suggestion


The Effect of Preparation Conditions on Raman and Photoluminescence of Monolayer WS2

Kathleen M. McCreary, Aubrey T. Hanbicki, Simranjeet Singh, Roland K. Kawakami, Glenn G. Jernigan, Masa Ishigami, Amy Ng, Todd H. Brintlinger, Rhonda M. Stroud, and Berend T. Jonker, Scientific Reports 6, 35154 (2016).  pdf

Spatially Resolved Electronic Properties of Single-Layer WS2 on Transition Metal Oxides

Søren Ulstrup*, Jyoti Katoch*, Roland J. Koch, Daniel Schwarz, Simranjeet Singh, Kathleen M. McCreary, Hyang Keun Yoo, Jinsong Xu, Berend T. Jonker, Roland K. Kawakami, Aaron Bostwick, Eli Rotenberg, and Chris Jozwiak, ACS Nano 10, 10058-10067 (2016). pdf

NaSn2As2: An Exfoliatable Layered van der Waals Zintl Phase

Maxx Q. Arguilla*, Jyoti Katoch*, Kevin Krymowski, Nicholas D. Cultrara, Jinsong Xu, Xiaoxiang Xi, Amanda Hanks, Shishi Jiang, Richard D. Ross, Roland J. Koch, Søren Ulstrup, Aaron Bostwick, Chris Jozwiak, David W. McComb, Eli Rotenberg, Jie Shan, Wolfgang Windl, Roland Kawakami, and Joshua E. Goldberger, ACS Nano 10, 9500-9508 (2016). pdf


Nanosecond spin relaxation times in single layer graphene spin valves with hexagonal boron nitride tunnel barriers

Simranjeet Singh, Jyoti Katoch, Jinsong Xu, Cheng Tan, Tiancong Zhu, Walid Amamou, James Hone, and Roland Kawakami, Applied Physics Letters 109, 122411 (2016). pdf


Molecular beam epitaxy growth of SrO buffer layers on graphite and graphene for the integration of complex oxides

Adam Ahmed, Hua Wen, Taisuke Ohta, Igor Pinchuk, Tiancong Zhu, Thomas Beechem, Roland Kawakami, Journal of Crystal Growth 447, 5 (2016). pdf


Contact induced spin relaxation in graphene spin valves with Al2O3 and MgO tunnel barriers

Walid Amamou, Zhisheng Lin, Jeremiah van Baren, Serol Turkyilmaz, Jing Shi, and Roland K. Kawakami, APL Materials 4, 032503 (2016). pdf

Experimental Demonstration of

Experimental Demonstration of XOR Operation in Graphene Magnetologic Gates at Room Temperature

Hua Wen, Hanan Dery, Walid Amamou, Tiancong Zhu, Zhisheng Lin, Jing Shi, Igor Žutić, Ilya Krivorotov, Lu J. Sham, Roland K. Kawakami, Phys. Rev. Applied 5, 044003 (2016). pdf


Exchange-Driven Spin Relaxation in Ferromagnet-Oxide-Semiconductor Heterostructures

Yu-Sheng Ou, Yi-Hsin Chiu, N. J. Harmon, Patrick Odenthal, Matthew Sheffield, Michael Chilcote, R. K. Kawakami, M. E. Flatté, and E. Johnston-Halperin Phys. Rev. Lett. 116, 107201 (2016). pdf


Ferromagnetic resonance spin pumping and electrical spin injection in silicon-based metal-oxide-semiconductor heterostructures

Yong Pu, Patrick M. Odenthal, Rohan Adur, Jonas Beardsley, Adrian G. Swartz, Denis V. Pelekhov, Michael E. Flatté, Roland K. Kawakami, Jon Pelz, P. Chris Hammel, Ezekiel Johnston-Halperin, Phys. Rev. Lett. 115, 246602 (2015). pdf


Large area epitaxial germanane for electronic devices

Walid Amamou, Patrick M. Odenthal, Elizabeth J. Bushong, Dante J. O’Hara, Yunqiu Kelly Luo, Jeremiah van Baren, Igor Pinchuk, Yi Wu, Adam S. Ahmed, Jyoti Katoch, Marc W. Bockrath, Harry W. K. Tom, Joshua E. Goldberger and Roland K. Kawakami, 2D Materials 2, 035012 (2015)  pdf

Correlating spin transport and electrode magnetization in a graphene spin valve: Simultaneous magnetic microscopy and non-local measurements

Andrew J. Berger, Michael R. Page, Hua Wen, Kathleen M. McCreary, Vidya P. Bhallamudi, Roland K. Kawakami, and P. Chris Hammel, Applied Physics Letters 107, 142406 (2015). pdf

kerr heterostructures

Tuning the dynamic exchange interaction in ferromagnet/semiconductor heterostructures

Yu-Sheng Ou, Yi-Hsin Chiu, N. J. Harmon, Patrick Odenthal, Matthew Sheffield, Michael Chilcote, R. K. Kawakami, M. E. Flatté, E. Johnston-Halperin, SPIE 9551, 95510E (2015).

Spin Amplification by Controlled Symmetry Breaking for Spin-Based Logic

Roland K. Kawakami, 2D Materials 2, 034001 (2015).


Screen Shot 2016-02-29 at 6.56.33 PM

Graphene Spintronics

Wei Han, Roland K. Kawakami, Martin Gmitra, and Jaroslav Fabian, Nature Nanotechnology 9, 794–807 (2014).


Current-Based Detection of Nonlocal Spin Transport in Graphene for Spin-Based Logic Applications

Hua Wen, Tiancong Zhu, Yunqiu Kelly Luo, Walid Amamou, Roland K Kawakami, Journal of Applied Physics 115, 17B741 (2014)


Magnetization dynamics of cobalt grown on graphene

Andrew J. Berger, Walid Amamou, Shane P. White, Rohan Adur, Yong Pu, Roland K. Kawakami, P. Chris Hammel, Journal of Applied Physics 115, 17C510 (2014)


Spin Polarization of Co(0001)/graphene junctions from first principles

Gilherme Sipahi, Igor Zutic, N.Atodiresei, Roland Kawakami, Predrag Lazic, Journal of Physics: Condensed Matter 26, 104204 (2014)


Direct Comparison of Graphene Devices Before and After Transfer to Different Substrates

Raymond Sachs, Zhisheng Lin, Patrick Odenthal, Roland Kawakami, Jing Shi, Applied Physics Letters 104, 033103 (2014)


Epitaxial Co-Deposition Growth of CaGe2 Films by Molecular Beam Epitaxy for Large Area Germanane

Igor V Pinchuk, Patrick M Odenthal, Adam S Ahmed, Walid Amamou, Joshua E Goldberger, Roland K Kawakami, Journal of Materials Research 29, 410-416 (2014)  pdf

2-Dimensional Transition Metal Dichalcogenides with Tunable Direct Band Gaps: MoS2(1-x)Se2x Monolayers

John Mann, Quan Ma, Patrick M Odenthal, Miguel Isarraraz, Duy Le, Edwin Preciado, David Barroso, Koichi Yamaguchi, Gretel von Son Palacio, Andrew Nguyen, Tai Tran, Michelle Wurch, Ariana Nguyen, Velveth Klee, Sarah Bobek, Dezheng Sun, Tony Heinz, Talat S. Rahman, Roland Kawakami, Ludwig Bartels, Advanced Materials 26, 1399-1404 (2014)



A systematic approach to interpreting Hanle spin precession data in non-local spin valves

Adrian G. Swartz, Kathleen M. McCreary, Wei Han, Hua Wen, Roland K. Kawakami, SPIE NanoScience+ Engineering 8813, 881328 (2013)


Correlation of electrical spin injection and non-linear charge-transport in Fe/MgO/Si

Yong Pu, Jonas Beardsley, Patrick M. Odenthal, Adrian G. Swartz, Roland K. Kawakami, P. Chris Hammel, Ezekiel Johnston-Halperin, Jairo Sinova, Jon P. Pelz, Applied Physics Letters 103, 012402 (2013)


Integrating MBE materials with graphene to induce novel spin-based phenomena

Adrian G Swartz, Kathleen M McCreary, Wei Han, Jared JI Wong, Patrick M Odenthal, Hua Wen, Jen-Ru Chen, Roland K Kawakami, Yufeng Hao, Rodney S Ruoff, Jaroslav Fabian, Journal of Vacuum Science and Technology B 31, 04D105 (2013)


Controlled argon beam-induced desulfurization of monolayer molybdenum disulfide

Q. Ma, P. M. Odenthal, J. Mann, D. Le, C. S. Wang, Y. Zhu, T. Chen, D. Sun, K. Yamaguchi, T. Tran, M. Wurch, J. L McKinley, J. Wyrick, K. M. Magnone, T. F. Heinz, T. S. Rahman, R. Kawakami, L. Bartels, Journal of Physics: Condensed Matter 25, 252201 (2013)


Facile Growth of Monolayer MoS2 Film Areas on SiO2

John Mann, Dezheng Sun, Quan Ma, Jen-Ru Chen, Edwin Preciado, Taisuke Ohta, Bogdan Diaconescu, Koichi Yamaguchi, Tai Tran, Michelle Wurch, KatieMarie Magnone, Tony F Heinz, Gary L Kellogg, Roland Kawakami, Ludwig Bartels, The European Physical Journal B 86, 1-4 (2013)


Control of Schottky Barriers in Single Layer MoS2 Transistors with Ferromagnetic Contacts

Jen-Ru Chen, Patrick M. Odenthal, Adrian G. Swartz, George Floyd Christensen, Hua Wen, Kelly Yunqiu Luo, Roland K Kawakami, Nano Letters 13, 3106-3110 (2013)


Observing Atomic Collapse Resonances in Artificial Nuclei on Graphene

Yang Wang, Dillon Wong, Andrey V Shytov, Victor W Brar, Sangkook Choi, Qiong Wu, Hsin-Zon Tsai, William Regan, Alex Zettl, Roland K Kawakami, Steven G Louie, Leonid S Levitov, Michael F Crommie, Science 340, 734-737 (2013)


Effect of in situ deposition of Mg adatoms on spin relaxation in graphene

Adrian G Swartz, Jen-Ru Chen, Kathleen M McCreary, Patrick M Odenthal, Wei Han, and Roland K Kawakami, Phys. Rev. B 87, 075455 (2013)


Measuring the Casimir force gradient from graphene on a SiO2 substrate

A. A. Banishev, H. Wen, J. Xu, R. K. Kawakami, G. L. Klimchitskaya, V. M. Mostepanenko, and U. Mohideen, Physical Review B 87, 205433 (2013)


Comparison of spin lifetimes in n-Ge characterized between three-terminal and four-terminal nonlocal Hanle measurements

L.T. Chang, W. Han, Y. Zhou, J. Tang, I. A. Fischer, M. Oehme, J. Schulze, R. K. Kawakami, and K. L. Wang, Semiconductor Science and Technology 28, 015018 (2013)



Integration of the ferromagnetic insulator EuO onto graphene

Adrian G. Swartz, Patrick M. Odenthal, Yufeng Hao, Rodney S. Ruoff, and Roland K. Kawakami, ACS Nano 6, 10063-10069 (2012)


Magnetic moment formation in graphene detected by scattering of pure spin currents

Kathleen M. McCreary, Adrian G. Swartz, Wei Han, Jaroslav Fabian, and Roland K. Kawakami, Phys. Rev. Lett. 109, 186604 (2012)


Electric field control of the Verwey transition and induced magnetoelectric effect in magnetite

Jared J. I. Wong, Adrian G. Swartz, Renjing Zheng, Wei Han, and Roland K. Kawakami, Phys. Rev. B (Rapid Comm.) 86, 060409 (2012)


Spin relaxation in single-layer graphene with tunable mobility

Wei Han, Jen-Ru Chen, Deqi Wang, Kathleen M. McCreary, Hua Wen, Adrian G. Swartz, Jing Shi, and Roland K. Kawakami, Nano Lett. 12, 3443-3447 (2012)


TiO2 as an electrostatic template for epitaxial growth of EuO on MgO(001) by reactive molecular beam epitaxy

Adrian G. Swartz, Jared J. I. Wong, Igor V. Pinchuk, and Roland K. Kawakami, J. Appl. Phys. 111, 083912 (2012)



Electrical spin injection and transport in germanium

Yi Zhou, Wei Han, Li-Te Chang, Faxian Xiu, Minsheng Wang, Michael Oehme, Inga A. Fischer, Joerg Schulze, Roland K. Kawakami, and Kang L. Wang, Phys. Rev. B 84, 125323 (2011)  Dataset


REVIEW: Spin transport and relaxation in graphene

Wei Han, Kathleen M. McCreary, Kyle Pi, Wei-Hua Wang, Yan Li, Hua Wen, Jen-Ru Chen, and Roland K. Kawakami, J. Magn. Magn. Mater. 324, 369 (2012). Dataset


Thermal annealing effect on spin coherence in ZnO single crystals

Z. Yang, Y. Li, D. C. Look, H. M. Zhou, W. V. Chen, R. K. Kawakami, P. K. L. Yu, and J. L. Liu, J. Appl. Phys. 110, 016101 (2011)


Enhanced spin injection efficiency and extended spin lifetimes in graphene spin valves

Wei Han, Jen-Ru Chen, Kathleen M. McCreary, Hua Wen, and Roland K. Kawakami, Proc. SPIE, Vol. 8100, 81000Q (2011) Dataset


Spin relaxation in single-layer and bilayer graphene

Wei Han, Roland K. Kawakami, Phys. Rev. Lett. 107, 047207 (2011) Dataset


Metallic and insulating adsorbates on graphene

Kathleen M. McCreary, Kyle Pi, and Roland K. Kawakami, Appl. Phys. Lett. 98, 192101 (2011)  Dataset


Epitaxial EuO Thin Films on GaAs

Epitaxial EuO thin films on GaAs

Adrian G. Swartz, John Ciraldo, Jared J. I. Wong, Yan Li, Wei Han, Tao Lin, Shawn Mack, Jing Shi, David D. Awschalom, and Roland K. Kawakami, Appl. Phys. Lett. 97, 112509 (2010)

Tunneling Spin Injection into Single Layer Graphene 

Wei Han, Kyle Pi, , Kathleen M. McCreary, Yan Li, Jared J. I. Wong, Adrian G. Swartz and Roland K. Kawakami, Phys. Rev. Lett. 105, 167202 (2010)

Oscillatory Spin Polarization and Magneto-Optical Kerr Effect in Fe3O4 Thin Films on GaAs(001)

Yan Li, Wei Han, Adrian G. Swartz, Kyle Pi, Jared J. I. Wong, Shawn Mack and David D. Awschalom and Roland K. Kawakami, Phys. Rev. Lett. 105, 167203 (2010)

Room-Temperature Electric-Field Controlled Ferromagnetism in Mn0.05Ge0.95 Quantum Dots

Room-Temperature Electric-Field Controlled Ferromagnetism in Mn0.05Ge0.95 Quantum Dots

Faxian Xiu, Yong Wang, Jiyoung Kim, Pramey Upadhyaya, Yi Zhou, Xufeng Kou, Wei Han, Roland K. Kawakami, Jin Zou and Kang L. Wang, ACS Nano 4, 4948-4954 (2010)

Manipulation of Spin Transport in Graphene by Surface Chemical Doping

Kyle Pi, Wei Han, Kathleen M. McCreary, Adrian G. Swartz, Yan Li, and Roland K. Kawakami, Phys. Rev. Lett. 104, 187201 (2010)

Effect of cluster formation on graphene mobility

Kathleen M. McCreary, Kyle Pi, Adrian G. Swartz, Wei Han, Wenzhong Bao, Chun Ning Lau, Francisco Guinea, Mikhail I. Katsnelson, and Roland K. Kawakami, Phys. Rev. B 81, 115453 (2010)

Tailoring interlayer exchange coupling of ferromagnetic films across MgO with Fe nanoclusters

Jared J. I. Wong, Luciana Ramirez, Adrian G. Swartz, Andrew Hoff, Wei Han, Yan Li, and Roland K. Kawakami, Phys. Rev. B 81, 094406 (2010)

Investigating the origin of Fermi level pinning in Ge Schottky junctions using epitaxially grown ultrathin MgO films

Yi Zhou, Wei Han, Yong Wang, Faxian Xiu, Jin Zou, Roland K. Kawakami, and Kang. L. Wang, Appl. Phys. Lett. 96, 102103 (2010)

Growth of single-crystalline, atomically smooth MgO films on Ge(0 0 1) by molecular beam epitaxy

Wei Han, Yi Zhou, Yong Wang, Yan Li, Jared. J. I. Wong, Kyle Pi, Adrian G. Swartz, Kathleen M. McCreary, Faxian Xiu, Kang L. Wang, Jin Zou, and Roland K. Kawakami, J. Crystal Growth 312, 44 (2010)


Spin transport in graphite and graphene spin valves

Wei Han, Kyle Pi, Wei-Hua Wang, Kathleen M. McCreary, Yan Li, Wenzhong Bao, Peng Wei, Jing Shi, Chun Ning Lau, and Roland K. Kawakami, Proc. SPIE, Vol. 7398, 739819 (2009).Dataset

Electronic doping and scattering by transition metals on graphene

K. Pi, K. M. McCreary, W. Bao, Wei Han, Y. F. Chiang, Yan Li, S.-W. Tsai, C. N. Lau, and R. K. Kawakami, Phys. Rev. B 80, 075406 (2009).Dataset

Engineering of tunnel junctions for prospective spin injection in germanium

Yi Zhou, Masaaki Ogawa, Mingqiang Bao, Wei Han, Roland K. Kawakami, and Kang L. Wang, Appl. Phys. Lett. 94, 242104 (2009)

Electrical detection of spin precession in single layer graphene spin valves with transparent contacts

Wei Han, K. Pi, W. Bao, K. M. McCreary, Yan Li, W. H. Wang, C. N. Lau, and R. K. Kawakami, Appl. Phys. Lett. 94, 222109 (2009)Dataset

Oxidation-induced biquadratic coupling in Co/Fe/MgO/Fe(001)

Y. F. Chiang, Jared J. I. Wong, X. Tan, Yan Li, K. Pi, W. H. Wang, H. W. K. Tom, and R. K. Kawakami, Phys. Rev. B 79, 184410 (2009)

Electron-Hole Asymmetry of Spin Injection and Transport in Single-Layer Graphene

Wei Han, W. H. Wang, K. Pi, K. M. McCreary, W. Bao, Yan Li, F. Miao, C. N. Lau, and R. K. Kawakami, Phys. Rev. Lett. 102, 137205 (2009).Dataset


Fundamentals of spintronics in metal and semiconductor systems 

R. K. Kawakami, K. M. McCreary, Y. Li, “Chapter 5, Fundamentals of Spintronics in Metal and Semiconductor Systems,” in Nanoelectronics and Photonics: From Atoms to Materials, Devices, and Architectures, A. Korkin and F. Rosei, editors, Springer (2008).

Growth of atomically smooth MgO films on graphene by molecular beam epitaxy

W. H. Wang, W. Han, K. Pi, K. M. McCreary, F. Miao, W. Bao, C. N. Lau, and R. K. Kawakami, Appl. Phys. Lett. 93, 183107 (2008)Dataset

Inversion of Ferromagnetic Proximity Polarization by MgO Interlayers

Yan Li, Y. Chye, Y. F. Chiang, K. Pi, W. H. Wang, J. M. Stephens, S. Mack, D. D. Awschalom, and R. K. Kawakami, Phys. Rev. Lett. 100, 237205 (2008)

Magnetotransport properties of mesoscopic graphite spin valves

W. H. Wang, K. Pi, Y. Li, Y. F. Chiang, P. Wei, J. Shi, and R. K. Kawakami, Phys. Rev. B (Rapid Comm.) 77, 020402 (2008)


Spin-polarized transport in magnetically assembled carbon nanotube spin valves

R. Thamankar, S. Niyogi, B. Y. Yoo, Y. W. Rheem, N. V. Myung, R. C. Haddon, and R. K. Kawakami, Appl. Phys. Lett. 89, 033119 (2006).


Magnetically Assembled Multiwalled Carbon Nanotubes on Ferromagnetic Contacts

S. Niyogi, C. Hangarter, R. M. Thamankar, Y. F. Chiang, R. K. Kawakami, N. V. Myung, and R. C. Haddon, Journal of Physical Chemistry B 108, 19818 (2004)