Publications
2025 |
Zhao, Zhiyuan; Lin, Yijie; Avsar, Ahmet Novel spintronic effects in two-dimensional van der Waals heterostructures Journal Article NPJ 2D MATERIALS AND APPLICATIONS, 9 (1), 2025. @article{ISI:001461986000001, title = {Novel spintronic effects in two-dimensional van der Waals heterostructures}, author = {Zhiyuan Zhao and Yijie Lin and Ahmet Avsar}, doi = {10.1038/s41699-025-00546-4}, times_cited = {0}, year = {2025}, date = {2025-04-09}, journal = {NPJ 2D MATERIALS AND APPLICATIONS}, volume = {9}, number = {1}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {Precise engineering of spin interfaces is essential for the development of spintronic devices. Two-dimensional vdW heterostructures enable atomically sharp interfaces that facilitate exploration of fundamental spin phenomena. Moreover, the discovery of two-dimensional magnetic materials has accelerated the field, leading to novel devices and spin effects. This review highlights recent advancements in vdW interfacial spin physics, innovative device structures, and emerging moir & eacute;-induced topological effects, with implications for future spintronic applications.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Precise engineering of spin interfaces is essential for the development of spintronic devices. Two-dimensional vdW heterostructures enable atomically sharp interfaces that facilitate exploration of fundamental spin phenomena. Moreover, the discovery of two-dimensional magnetic materials has accelerated the field, leading to novel devices and spin effects. This review highlights recent advancements in vdW interfacial spin physics, innovative device structures, and emerging moir & eacute;-induced topological effects, with implications for future spintronic applications. |
Shin, Bongki; Ni, Bo; Toh, Chee-Tat; Steinbach, Doug; Yang, Zhenze; Sassi, Lucas M; Ai, Qing; Niu, Kangdi; Lin, Junhao; Suenaga, Kazu; Han, Yimo; Buehler, Markus J; Ozyilmaz, Barbaros; Lou, Jun Intrinsic toughening in monolayer amorphous carbon nanocomposites Journal Article MATTER, 8 (4), 2025, ISSN: 2590-2393. @article{ISI:001462473000001, title = {Intrinsic toughening in monolayer amorphous carbon nanocomposites}, author = {Bongki Shin and Bo Ni and Chee-Tat Toh and Doug Steinbach and Zhenze Yang and Lucas M Sassi and Qing Ai and Kangdi Niu and Junhao Lin and Kazu Suenaga and Yimo Han and Markus J Buehler and Barbaros Ozyilmaz and Jun Lou}, doi = {10.1016/j.matt.2025.102000}, times_cited = {2}, issn = {2590-2393}, year = {2025}, date = {2025-04-02}, journal = {MATTER}, volume = {8}, number = {4}, publisher = {CELL PRESS}, address = {50 HAMPSHIRE ST, FLOOR 5, CAMBRIDGE, MA 02139 USA}, abstract = {Two-dimensional (2D) materials have immense potential to advance flexible electronics, yet they are limited by low fracture toughness. This study addresses the intrinsic toughening of monolayer amorphous carbon (MAC), a 2D nanocomposite, to overcome this challenge. By incorporating both amorphous and nanocrystalline phases, MAC significantly enhances energy absorption during fracture propagation, as evidenced by crack blunting, deflecting, and bridging. Using in situ tensile tests under a scanning electron microscope, our results indicate an 8-fold increase in the energy release rate compared to monolayer graphene, along with improved fracture strain and crack stability. Molecular dynamics simulations demonstrate the impact of phase composition on fracture energy. Our results present a scalable toughening strategy for 2D materials, potentially broadening their applications in fields requiring robust fracture resistance.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Two-dimensional (2D) materials have immense potential to advance flexible electronics, yet they are limited by low fracture toughness. This study addresses the intrinsic toughening of monolayer amorphous carbon (MAC), a 2D nanocomposite, to overcome this challenge. By incorporating both amorphous and nanocrystalline phases, MAC significantly enhances energy absorption during fracture propagation, as evidenced by crack blunting, deflecting, and bridging. Using in situ tensile tests under a scanning electron microscope, our results indicate an 8-fold increase in the energy release rate compared to monolayer graphene, along with improved fracture strain and crack stability. Molecular dynamics simulations demonstrate the impact of phase composition on fracture energy. Our results present a scalable toughening strategy for 2D materials, potentially broadening their applications in fields requiring robust fracture resistance. |
Hota, Mrinal K; Pazos, Sebastian; Lanza, Mario; Alshareef, Husam N A review of MXene memristors and their applications Journal Article MATERIALS SCIENCE & ENGINEERING R-REPORTS, 164 , 2025, ISSN: 0927-796X. @article{ISI:001460474400001, title = {A review of MXene memristors and their applications}, author = {Mrinal K Hota and Sebastian Pazos and Mario Lanza and Husam N Alshareef}, doi = {10.1016/j.mser.2025.100983}, times_cited = {0}, issn = {0927-796X}, year = {2025}, date = {2025-03-31}, journal = {MATERIALS SCIENCE & ENGINEERING R-REPORTS}, volume = {164}, publisher = {ELSEVIER SCIENCE SA}, address = {PO BOX 564, 1001 LAUSANNE, SWITZERLAND}, abstract = {The rapid growth of artificial intelligence (AI) demands efficient management of vast data quantities, a challenge that traditional von Neumann computing struggles to meet due to its power consumption and memory limitations. Memristive devices have emerged as a promising solution to overcome the von Neumann bottleneck through in-memory computing, which is crucial for neuromorphic computing advancements. Among the various materials investigated for memristor development, MXenes have recently gained attention as a highly promising platform. These materials exhibit a wide range of functional behaviors due to their unique electrochemical properties. MXenes offer several advantages, including high electrical conductivity, tunable surface chemistry, and excellent mechanical flexibility, enhancing their potential in advancing memristor technology. This review begins by introducing various MXene-based devices and highlighting switching mechanisms. It then explores the application of MXene memristors in neuromorphic and logic operations. The review concludes by addressing the challenges associated with MXene memristors, examining the obstacles they present, and considering future prospects in this dynamic field.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The rapid growth of artificial intelligence (AI) demands efficient management of vast data quantities, a challenge that traditional von Neumann computing struggles to meet due to its power consumption and memory limitations. Memristive devices have emerged as a promising solution to overcome the von Neumann bottleneck through in-memory computing, which is crucial for neuromorphic computing advancements. Among the various materials investigated for memristor development, MXenes have recently gained attention as a highly promising platform. These materials exhibit a wide range of functional behaviors due to their unique electrochemical properties. MXenes offer several advantages, including high electrical conductivity, tunable surface chemistry, and excellent mechanical flexibility, enhancing their potential in advancing memristor technology. This review begins by introducing various MXene-based devices and highlighting switching mechanisms. It then explores the application of MXene memristors in neuromorphic and logic operations. The review concludes by addressing the challenges associated with MXene memristors, examining the obstacles they present, and considering future prospects in this dynamic field. |
Ho, Yi Wei; Chen, Mingjun; Wong, Cheng Quan; Grzeszczyk, Magdalena; Budniak, Adam K; Viana-Gomes, Jose C; Li, Xinwei; Koperski, Maciej; Eda, Goki Imaging the Neel Vector in Few-Layer CrPS4 with Second-Harmonic Generation Journal Article NANO LETTERS, 25 (14), pp. 5624-5630, 2025, ISSN: 1530-6984. @article{ISI:001453704200001, title = {Imaging the Neel Vector in Few-Layer CrPS_{4} with Second-Harmonic Generation}, author = {Yi Wei Ho and Mingjun Chen and Cheng Quan Wong and Magdalena Grzeszczyk and Adam K Budniak and Jose C Viana-Gomes and Xinwei Li and Maciej Koperski and Goki Eda}, doi = {10.1021/acs.nanolett.4c06237}, times_cited = {0}, issn = {1530-6984}, year = {2025}, date = {2025-03-26}, journal = {NANO LETTERS}, volume = {25}, number = {14}, pages = {5624-5630}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Second-harmonic generation (SHG) is a powerful technique for probing the crystallographic symmetry. Recently, SHG has also been shown to exhibit sensitivity to time-reversal symmetry, allowing studies of magnetic order in layered antiferromagnetic (AFM) materials. Here, we report SHG investigation of chromium thiophosphate (CrPS4), a layered A-type AFM material with a finite/zero net magnetization for odd/even-layer-number crystals, respectively. The SHG intensity of bulk CrPS4 across its Neel temperature reveals that both crystallographic and magnetic symmetry breaking contribute to SHG at low temperatures. From the polarization analysis, we identify the tensor element linked to the magnetic order. Interestingly, the polarization response is sensitive to the history of the magnetic field applied to the even-number-layer crystals, suggesting that the SHG is sensitive to the Neel vector direction. Polarized SHG microscopy of a terraced CrPS4 shows that the AFM order of even-number-layer regions is determined by the ferromagnetic order of the neighboring odd-number-layer regions.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Second-harmonic generation (SHG) is a powerful technique for probing the crystallographic symmetry. Recently, SHG has also been shown to exhibit sensitivity to time-reversal symmetry, allowing studies of magnetic order in layered antiferromagnetic (AFM) materials. Here, we report SHG investigation of chromium thiophosphate (CrPS4), a layered A-type AFM material with a finite/zero net magnetization for odd/even-layer-number crystals, respectively. The SHG intensity of bulk CrPS4 across its Neel temperature reveals that both crystallographic and magnetic symmetry breaking contribute to SHG at low temperatures. From the polarization analysis, we identify the tensor element linked to the magnetic order. Interestingly, the polarization response is sensitive to the history of the magnetic field applied to the even-number-layer crystals, suggesting that the SHG is sensitive to the Neel vector direction. Polarized SHG microscopy of a terraced CrPS4 shows that the AFM order of even-number-layer regions is determined by the ferromagnetic order of the neighboring odd-number-layer regions. |
Pazos, Sebastian; Zhu, Kaichen; Villena, Marco A; Alharbi, Osamah; Zheng, Wenwen; Shen, Yaqing; Yuan, Yue; Ping, Yue; Lanza, Mario Synaptic and neural behaviours in a standard silicon transistor Journal Article NATURE, 640 (8057), 2025, ISSN: 0028-0836. @article{ISI:001453451600001, title = {Synaptic and neural behaviours in a standard silicon transistor}, author = {Sebastian Pazos and Kaichen Zhu and Marco A Villena and Osamah Alharbi and Wenwen Zheng and Yaqing Shen and Yue Yuan and Yue Ping and Mario Lanza}, doi = {10.1038/s41586-025-08742-4}, times_cited = {7}, issn = {0028-0836}, year = {2025}, date = {2025-03-26}, journal = {NATURE}, volume = {640}, number = {8057}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {Hardware implementations of artificial neural networks (ANNs)-the most advanced of which are made of millions of electronic neurons interconnected by hundreds of millions of electronic synapses-have achieved higher energy efficiency than classical computers in some small-scale data-intensive computing tasks1. State-of-the-art neuromorphic computers, such as Intel's Loihi2 or IBM's NorthPole3, implement ANNs using bio-inspired neuron- and synapse-mimicking circuits made of complementary metal-oxide-semiconductor (CMOS) transistors, at least 18 per neuron and six per synapse. Simplifying the structure and size of these two building blocks would enable the construction of more sophisticated, larger and more energy-efficient ANNs. Here we show that a single CMOS transistor can exhibit neural and synaptic behaviours if biased in a specific (unconventional) manner. By connecting one additional CMOS transistor in series, we build a versatile 2-transistor-cell that exhibits adjustable neuro-synaptic response (which we named neuro-synaptic random access memory cell, or NS-RAM cell). This electronic performance comes with a yield of 100% and an ultra-low device-to-device variability, owing to the maturity of the silicon CMOS platform used-no materials or devices alien to the CMOS process are required. These results represent a short-term solution for the implementation of efficient ANNs and an opportunity in terms of CMOS circuit design and optimization for artificial intelligence applications.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Hardware implementations of artificial neural networks (ANNs)-the most advanced of which are made of millions of electronic neurons interconnected by hundreds of millions of electronic synapses-have achieved higher energy efficiency than classical computers in some small-scale data-intensive computing tasks1. State-of-the-art neuromorphic computers, such as Intel's Loihi2 or IBM's NorthPole3, implement ANNs using bio-inspired neuron- and synapse-mimicking circuits made of complementary metal-oxide-semiconductor (CMOS) transistors, at least 18 per neuron and six per synapse. Simplifying the structure and size of these two building blocks would enable the construction of more sophisticated, larger and more energy-efficient ANNs. Here we show that a single CMOS transistor can exhibit neural and synaptic behaviours if biased in a specific (unconventional) manner. By connecting one additional CMOS transistor in series, we build a versatile 2-transistor-cell that exhibits adjustable neuro-synaptic response (which we named neuro-synaptic random access memory cell, or NS-RAM cell). This electronic performance comes with a yield of 100% and an ultra-low device-to-device variability, owing to the maturity of the silicon CMOS platform used-no materials or devices alien to the CMOS process are required. These results represent a short-term solution for the implementation of efficient ANNs and an opportunity in terms of CMOS circuit design and optimization for artificial intelligence applications. |
Dabrowski, Maciej; Haldar, Sumit; Khan, Safe; Keatley, Paul S; Sagkovits, Dimitros; Xue, Zekun; Freeman, Charlie; Verzhbitskiy, Ivan; Griepe, Theodor; Atxitia, Unai; Eda, Goki; Kurebayashi, Hidekazu; Santos, Elton J G; Hicken, Robert J Ultrafast thermo-optical control of spins in a 2D van der Waals semiconductor Journal Article NATURE COMMUNICATIONS, 16 (1), 2025. @article{ISI:001449775100029, title = {Ultrafast thermo-optical control of spins in a 2D van der Waals semiconductor}, author = {Maciej Dabrowski and Sumit Haldar and Safe Khan and Paul S Keatley and Dimitros Sagkovits and Zekun Xue and Charlie Freeman and Ivan Verzhbitskiy and Theodor Griepe and Unai Atxitia and Goki Eda and Hidekazu Kurebayashi and Elton J G Santos and Robert J Hicken}, doi = {10.1038/s41467-025-58065-1}, times_cited = {1}, year = {2025}, date = {2025-03-21}, journal = {NATURE COMMUNICATIONS}, volume = {16}, number = {1}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {Laser pulses provide one of the fastest means of manipulating electron spins in magnetic compounds and pave the way to ultrafast operation within magnetic recording, quantum computation and spintronics. However, effective management of the heat deposited during optical excitation is an open challenge. Layered two-dimensional (2D) van der Waals (vdW) materials possess unique thermal properties due to the highly anisotropic nature of their chemical bonding. Here we show how to control the rate of heat flow, and hence the magnetization dynamics, induced by an ultrafast laser pulse within the 2D ferromagnet Cr2Ge2Te6. Using time-resolved beam-scanning magneto-optical Kerr effect microscopy and microscopic spin modelling calculations, we show that by reducing the thickness of the magnetic layers, an enhancement of the heat dissipation rate into the adjacent substrate leads to a substantial reduction in the timescale for magnetization recovery from several nanoseconds down to a few hundred picoseconds. Finally, we demonstrate how the low thermal conductivity across vdW layers may be used to obtain magnetic domain memory behaviour, even after exposure to intense laser pulses. Our findings reveal the distinctive role of vdW magnets in the ultrafast control of heat conduction, spin dynamics and non-volatile memory.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Laser pulses provide one of the fastest means of manipulating electron spins in magnetic compounds and pave the way to ultrafast operation within magnetic recording, quantum computation and spintronics. However, effective management of the heat deposited during optical excitation is an open challenge. Layered two-dimensional (2D) van der Waals (vdW) materials possess unique thermal properties due to the highly anisotropic nature of their chemical bonding. Here we show how to control the rate of heat flow, and hence the magnetization dynamics, induced by an ultrafast laser pulse within the 2D ferromagnet Cr2Ge2Te6. Using time-resolved beam-scanning magneto-optical Kerr effect microscopy and microscopic spin modelling calculations, we show that by reducing the thickness of the magnetic layers, an enhancement of the heat dissipation rate into the adjacent substrate leads to a substantial reduction in the timescale for magnetization recovery from several nanoseconds down to a few hundred picoseconds. Finally, we demonstrate how the low thermal conductivity across vdW layers may be used to obtain magnetic domain memory behaviour, even after exposure to intense laser pulses. Our findings reveal the distinctive role of vdW magnets in the ultrafast control of heat conduction, spin dynamics and non-volatile memory. |
Verzhbitskiy, Ivan A; Mishra, Abhishek; Mitra, Sanchali; Zhang, Zhepeng; Das, Sarthak; Lau, Chit Siong; Lee, Rainer; Huang, Ding; Eda, Goki; Ang, Yee Sin; Goh, Kuan Eng Johnson Low-Temperature Contacts and the Coulomb Blockade Effect in Layered Nanoribbons with In-Plane Anisotropy Journal Article ACS NANO, 19 (11), pp. 10878-10888, 2025, ISSN: 1936-0851. @article{ISI:001445756900001, title = {Low-Temperature Contacts and the Coulomb Blockade Effect in Layered Nanoribbons with In-Plane Anisotropy}, author = {Ivan A Verzhbitskiy and Abhishek Mishra and Sanchali Mitra and Zhepeng Zhang and Sarthak Das and Chit Siong Lau and Rainer Lee and Ding Huang and Goki Eda and Yee Sin Ang and Kuan Eng Johnson Goh}, doi = {10.1021/acsnano.4c15086}, times_cited = {0}, issn = {1936-0851}, year = {2025}, date = {2025-03-13}, journal = {ACS NANO}, volume = {19}, number = {11}, pages = {10878-10888}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {One-dimensional (1D) nanoribbons (NRs) constitute rapidly advancing nanotechnology with significant potential for emerging applications such as quantum sensing and metrology. TiS3 nanoribbons exhibit strong in-plane crystal anisotropy, enabling robust 1D confinement and resilience to edge disorder. Nevertheless, charge transport in 1D TiS3 remains relatively unexplored, particularly at low temperatures, where high contact resistance impacts device performance and fundamentally limits its applications. Here, we engineer electrical contacts between a bulk metal and a 1D NR and explore the low-temperature characteristics of the 1D field-effect devices. We report ohmic contacts for 1D TiS3 with temperature-independent contact resistances as low as 2.7 +/- 0.3 k Omega keywords = {}, pubstate = {published}, tppubtype = {article} } One-dimensional (1D) nanoribbons (NRs) constitute rapidly advancing nanotechnology with significant potential for emerging applications such as quantum sensing and metrology. TiS3 nanoribbons exhibit strong in-plane crystal anisotropy, enabling robust 1D confinement and resilience to edge disorder. Nevertheless, charge transport in 1D TiS3 remains relatively unexplored, particularly at low temperatures, where high contact resistance impacts device performance and fundamentally limits its applications. Here, we engineer electrical contacts between a bulk metal and a 1D NR and explore the low-temperature characteristics of the 1D field-effect devices. We report ohmic contacts for 1D TiS3 with temperature-independent contact resistances as low as 2.7 +/- 0.3 k Omega |
Chakraborty, Amarnath; Rodin, Aleksandr; Adam, Shaffique; Vignale, Giovanni Insulator-metal transition and magnetic crossover in bilayer graphene Journal Article PHYSICAL REVIEW B, 111 (12), 2025, ISSN: 2469-9950. @article{ISI:001458804200004, title = {Insulator-metal transition and magnetic crossover in bilayer graphene}, author = {Amarnath Chakraborty and Aleksandr Rodin and Shaffique Adam and Giovanni Vignale}, doi = {10.1103/PhysRevB.111.125130}, times_cited = {0}, issn = {2469-9950}, year = {2025}, date = {2025-03-11}, journal = {PHYSICAL REVIEW B}, volume = {111}, number = {12}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {In-plane magnetic fields offer a relatively unexplored opportunity to alter the band structure of stacks of two-dimensional (2D) materials so that they exhibit the desired physical properties. Here we show that an inplane magnetic field combined with a transverse electric field can induce an insulator-metal (IM) transition in bilayer graphene. Our study of the magnetic response reveals that the orbital magnetic susceptibility changes from diamagnetic to paramagnetic around the transition point. We discuss several strategies to observe the IM transition, switch the diamagnetism, and more generally control the band structure of stacked 2D materials at experimentally accessible magnetic fields.}, keywords = {}, pubstate = {published}, tppubtype = {article} } In-plane magnetic fields offer a relatively unexplored opportunity to alter the band structure of stacks of two-dimensional (2D) materials so that they exhibit the desired physical properties. Here we show that an inplane magnetic field combined with a transverse electric field can induce an insulator-metal (IM) transition in bilayer graphene. Our study of the magnetic response reveals that the orbital magnetic susceptibility changes from diamagnetic to paramagnetic around the transition point. We discuss several strategies to observe the IM transition, switch the diamagnetism, and more generally control the band structure of stacked 2D materials at experimentally accessible magnetic fields. |
Liu, Xiongfang; Yang, Tong; Chen, Shanquan; Wu, Jing; Tang, Chi Sin; Ning, Yuanjie; Chen, Zuhuang; Dai, Liang; Sun, Mengxia; Chen, Mingyao; Han, Kun; Zhou, Difan; Zeng, Shengwei; Sun, Shuo; Li, Sensen; Yang, Ming; Breese, Mark B H; Cai, Chuanbing; Venkatesan, Thirumalai; Wee, Andrew T S; Yin, Xinmao Small polarons mediated near-room-temperature metal-insulator transition in vanadium dioxide and their hopping dynamics Journal Article APPLIED PHYSICS REVIEWS, 12 (1), 2025, ISSN: 1931-9401. @article{ISI:001403236800001, title = {Small polarons mediated near-room-temperature metal-insulator transition in vanadium dioxide and their hopping dynamics}, author = {Xiongfang Liu and Tong Yang and Shanquan Chen and Jing Wu and Chi Sin Tang and Yuanjie Ning and Zuhuang Chen and Liang Dai and Mengxia Sun and Mingyao Chen and Kun Han and Difan Zhou and Shengwei Zeng and Shuo Sun and Sensen Li and Ming Yang and Mark B H Breese and Chuanbing Cai and Thirumalai Venkatesan and Andrew T S Wee and Xinmao Yin}, doi = {10.1063/5.0236807}, times_cited = {0}, issn = {1931-9401}, year = {2025}, date = {2025-03-01}, journal = {APPLIED PHYSICS REVIEWS}, volume = {12}, number = {1}, publisher = {AIP Publishing}, address = {1305 WALT WHITMAN RD, STE 300, MELVILLE, NY 11747-4501 USA}, abstract = {Researchers pursuing advanced photoelectric devices have discovered near room-temperature metal-insulator transitions (MIT) in nonvolatile VO2. Despite theoretical investigations suggesting that polaron dynamics mediate the MIT, direct experimental evidence remains scarce. In this study, we present direct evidence of the polaron state in insulating VO2 through high-resolution spectroscopic ellipsometry measurements and first-principles calculations. We illustrate the complementary role of polaron dynamics in facilitating Peierls and Mott transitions, thereby contributing to the MIT processes. Furthermore, our observations and characterizations of conventional metallic and correlated plasmons in the respective phases of the VO2 film offer valuable insight into their electron structures. This investigation enhances comprehension of the MIT mechanism in correlated systems and underscores the roles of polarons, lattice distortions, and electron correlations in facilitating phase transition processes in strongly correlated systems. Additionally, the detailed detection of small polarons and plasmons serves as inspiration for the development of new device functionalities.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Researchers pursuing advanced photoelectric devices have discovered near room-temperature metal-insulator transitions (MIT) in nonvolatile VO2. Despite theoretical investigations suggesting that polaron dynamics mediate the MIT, direct experimental evidence remains scarce. In this study, we present direct evidence of the polaron state in insulating VO2 through high-resolution spectroscopic ellipsometry measurements and first-principles calculations. We illustrate the complementary role of polaron dynamics in facilitating Peierls and Mott transitions, thereby contributing to the MIT processes. Furthermore, our observations and characterizations of conventional metallic and correlated plasmons in the respective phases of the VO2 film offer valuable insight into their electron structures. This investigation enhances comprehension of the MIT mechanism in correlated systems and underscores the roles of polarons, lattice distortions, and electron correlations in facilitating phase transition processes in strongly correlated systems. Additionally, the detailed detection of small polarons and plasmons serves as inspiration for the development of new device functionalities. |
Yudhistira, Indra; Afrose, Ramal; Adam, Shaffique PHYSICAL REVIEW B, 111 (8), 2025, ISSN: 2469-9950. @article{ISI:001448500900003, title = {Nonmonotonic temperature dependence of electron viscosity and crossover to high-temperature universal viscous fluid in monolayer and bilayer graphene}, author = {Indra Yudhistira and Ramal Afrose and Shaffique Adam}, doi = {10.1103/PhysRevB.111.085433}, times_cited = {0}, issn = {2469-9950}, year = {2025}, date = {2025-02-28}, journal = {PHYSICAL REVIEW B}, volume = {111}, number = {8}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {Electrons in quantum matter behave like a fluid when the quantum-mechanical carrier-carrier scattering dominates over other relaxation mechanisms. By combining a microscopic treatment of electron-electron interactions within the random phase approximation with a phenomenological Navier-Stokes-like equation, we predict that in the limit of high temperature and strong Coulomb interactions, both monolayer graphene and bilayer graphene exhibit a universal behavior in dynamic viscosity. We find that the dynamic viscosity to entropy density ratio for bilayer graphene is closer to the holographic bound, suggesting that such a bound might be observable in a condensed matter system. We discuss how this could be observed experimentally using magnetoconductance measurements in a Corbino geometry for a realistic range of temperature and carrier density.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Electrons in quantum matter behave like a fluid when the quantum-mechanical carrier-carrier scattering dominates over other relaxation mechanisms. By combining a microscopic treatment of electron-electron interactions within the random phase approximation with a phenomenological Navier-Stokes-like equation, we predict that in the limit of high temperature and strong Coulomb interactions, both monolayer graphene and bilayer graphene exhibit a universal behavior in dynamic viscosity. We find that the dynamic viscosity to entropy density ratio for bilayer graphene is closer to the holographic bound, suggesting that such a bound might be observable in a condensed matter system. We discuss how this could be observed experimentally using magnetoconductance measurements in a Corbino geometry for a realistic range of temperature and carrier density. |
Loh, Leyi; Ning, Shoucong; Kieczka, Daria; Chen, Yuan; Yang, Jianmin; Wang, Zhe; Pennycook, Stephen J; Eda, Goki; Shluger, Alexander L; Bosman, Michel Electron Ptychography for Atom-by-Atom Quantification of 1D Defect Complexes in Monolayer MoS2 Journal Article ACS NANO, 19 (6), pp. 6195-6208, 2025, ISSN: 1936-0851. @article{ISI:001416560700001, title = {Electron Ptychography for Atom-by-Atom Quantification of 1D Defect Complexes in Monolayer MoS_{2}}, author = {Leyi Loh and Shoucong Ning and Daria Kieczka and Yuan Chen and Jianmin Yang and Zhe Wang and Stephen J Pennycook and Goki Eda and Alexander L Shluger and Michel Bosman}, doi = {10.1021/acsnano.4c14988}, times_cited = {1}, issn = {1936-0851}, year = {2025}, date = {2025-02-07}, journal = {ACS NANO}, volume = {19}, number = {6}, pages = {6195-6208}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Defect complexes can induce beneficial functionalities in two-dimensional (2D) semiconductors. However, understanding their formation mechanism with single-atom sensitivity has proven to be challenging for light elements using conventional transmission electron microscopy (TEM) techniques. Here, we demonstrate the atom-resolved formation of various one-dimensional (1D) defect complexes-consisting of rhenium dopants, sulfur interstitials, and sulfur vacancies-in monolayer MoS2 by applying electron ptychography to our four-dimensional scanning transmission electron microscopy (4D-STEM) data sets. Our image resolution of 0.35 angstrom and a spatial precision of 2 pm allow us to achieve accurate matching between experimental structures and density functional theory (DFT) simulations at the atomic level. Additionally, we utilize out-of-focus ptychography to observe defect formation processes at dose rates comparable to those used in conventional TEM imaging, while maintaining a large field of view. This study demonstrates the systematic application of electron ptychography to extensive 4D-STEM data sets for quantitative defect imaging in 2D materials. We provide direct, atomically precise evidence that critical defect densities govern the formation of extended 1D defect complexes. For instance, we show that sulfur single-vacancy lines form when the vacancy density reaches 5 x 1013 cm-2 and transform into double-vacancy lines beyond 8 x 1013 cm-2. Rhenium-dopant lines emerge at a dopant concentration higher than 3 x 1013 cm-2, where metastable sulfur interstitial-vacancy lines also form as the cumulative electron dose reaches 3 x 105 e/angstrom 2, initiating a local nucleation of the 1T ' phase. Our results highlight the potential of electron ptychography for high-precision defect characterization and engineering in ultrathin 2D materials.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Defect complexes can induce beneficial functionalities in two-dimensional (2D) semiconductors. However, understanding their formation mechanism with single-atom sensitivity has proven to be challenging for light elements using conventional transmission electron microscopy (TEM) techniques. Here, we demonstrate the atom-resolved formation of various one-dimensional (1D) defect complexes-consisting of rhenium dopants, sulfur interstitials, and sulfur vacancies-in monolayer MoS2 by applying electron ptychography to our four-dimensional scanning transmission electron microscopy (4D-STEM) data sets. Our image resolution of 0.35 angstrom and a spatial precision of 2 pm allow us to achieve accurate matching between experimental structures and density functional theory (DFT) simulations at the atomic level. Additionally, we utilize out-of-focus ptychography to observe defect formation processes at dose rates comparable to those used in conventional TEM imaging, while maintaining a large field of view. This study demonstrates the systematic application of electron ptychography to extensive 4D-STEM data sets for quantitative defect imaging in 2D materials. We provide direct, atomically precise evidence that critical defect densities govern the formation of extended 1D defect complexes. For instance, we show that sulfur single-vacancy lines form when the vacancy density reaches 5 x 1013 cm-2 and transform into double-vacancy lines beyond 8 x 1013 cm-2. Rhenium-dopant lines emerge at a dopant concentration higher than 3 x 1013 cm-2, where metastable sulfur interstitial-vacancy lines also form as the cumulative electron dose reaches 3 x 105 e/angstrom 2, initiating a local nucleation of the 1T ' phase. Our results highlight the potential of electron ptychography for high-precision defect characterization and engineering in ultrathin 2D materials. |
Jiang, Yingying; Wong, Zicong Marvin; Yan, Hongwei; Tan, Teck Leong; Mirsaidov, Utkur Revealing Multistep Phase Separation in Metal Alloy Nanoparticles with In Situ Transmission Electron Microscopy Journal Article ACS NANO, 19 (3), pp. 3886-3894, 2025, ISSN: 1936-0851. @article{ISI:001396476700001, title = {Revealing Multistep Phase Separation in Metal Alloy Nanoparticles with \textit{In Situ} Transmission Electron Microscopy}, author = {Yingying Jiang and Zicong Marvin Wong and Hongwei Yan and Teck Leong Tan and Utkur Mirsaidov}, doi = {10.1021/acsnano.4c16095}, times_cited = {0}, issn = {1936-0851}, year = {2025}, date = {2025-01-14}, journal = {ACS NANO}, volume = {19}, number = {3}, pages = {3886-3894}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Phase separation plays a crucial role in many natural and industrial processes, such as the formation of clouds and minerals and the distillation of crude oil. In metals and alloys, phase separation is an important approach often utilized to improve their mechanical strength for use in construction, automobile, and aerospace manufacturing. Despite its importance in many processes, the atomic details of phase separation are largely unknown. In particular, it is unclear how a different crystal phase emerges from the parent alloy. Here, using real-time in situ transmission electron microscopy, we describe the stages of the phase separation in face-centered cubic (fcc) AuRu alloy nanoparticles, resulting in a Ru phase with a hexagonal close-packed (hcp) crystal structure. Our observation reveals that the hcp Ru phase forms in two steps: the spinodal decomposition of the alloy produces metastable fcc Ru clusters, and as they grow larger, these clusters transform into hcp Ru domains. Our calculations indicate that the primary reason for the fcc-to-hcp transformation is the size-dependent competition between the interfacial and bulk energies of Ru domains. These insights into elusive, transient steps in the phase separation of alloys can aid in engineering nanomaterials with unconventional phases.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Phase separation plays a crucial role in many natural and industrial processes, such as the formation of clouds and minerals and the distillation of crude oil. In metals and alloys, phase separation is an important approach often utilized to improve their mechanical strength for use in construction, automobile, and aerospace manufacturing. Despite its importance in many processes, the atomic details of phase separation are largely unknown. In particular, it is unclear how a different crystal phase emerges from the parent alloy. Here, using real-time in situ transmission electron microscopy, we describe the stages of the phase separation in face-centered cubic (fcc) AuRu alloy nanoparticles, resulting in a Ru phase with a hexagonal close-packed (hcp) crystal structure. Our observation reveals that the hcp Ru phase forms in two steps: the spinodal decomposition of the alloy produces metastable fcc Ru clusters, and as they grow larger, these clusters transform into hcp Ru domains. Our calculations indicate that the primary reason for the fcc-to-hcp transformation is the size-dependent competition between the interfacial and bulk energies of Ru domains. These insights into elusive, transient steps in the phase separation of alloys can aid in engineering nanomaterials with unconventional phases. |
Carrio, Juan A G; Donato, Ricardo K; Carvalho, Alexandra; Koon, Gavin K W; Donato, Katarzyna Z; Yau, Xin Hui; Kosiachevskyi, Dmytro; Lim, Karen; Ravi, Vedarethinam; Joy, Josny; Goh, Kelda; Emiliano, Jose Vitorio; Lombardi, Jerome E; Neto, Castro A H From 2D kaolinite to 3D amorphous cement Journal Article SCIENTIFIC REPORTS, 15 (1), 2025, ISSN: 2045-2322. @article{ISI:001396241000050, title = {From 2D kaolinite to 3D amorphous cement}, author = {Juan A G Carrio and Ricardo K Donato and Alexandra Carvalho and Gavin K W Koon and Katarzyna Z Donato and Xin Hui Yau and Dmytro Kosiachevskyi and Karen Lim and Vedarethinam Ravi and Josny Joy and Kelda Goh and Jose Vitorio Emiliano and Jerome E Lombardi and Castro A H Neto}, doi = {10.1038/s41598-024-81882-1}, times_cited = {1}, issn = {2045-2322}, year = {2025}, date = {2025-01-11}, journal = {SCIENTIFIC REPORTS}, volume = {15}, number = {1}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {Kaolinite is a single 2D layer of kaolin or metakaolin (MK), common clays that can be characterized as layered 3D materials. We show that because of its chemical composition, kaolinite can be converted into an amorphous 3D material by chemical means. This dimensional transformation is possible due to the large surface to volume ratio and chemical reactivity of kaolinite. We investigate the formation and influence of quasi- or nanocrystalline phases in MK-based alkali-activated materials (AAM) that are related to the Si/Al ratio. We analyze the formation of an AAM from a MK precursor, which is a 3D bonded network that preserves the layered structure at the nanometer scale. We also exfoliate the remaining layered phase to examine the effects of the alkali-activation in the final sheet structures embedded within the amorphous network. The final material can be used as a cement with no carbon dioxide produced by the transformation reaction.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Kaolinite is a single 2D layer of kaolin or metakaolin (MK), common clays that can be characterized as layered 3D materials. We show that because of its chemical composition, kaolinite can be converted into an amorphous 3D material by chemical means. This dimensional transformation is possible due to the large surface to volume ratio and chemical reactivity of kaolinite. We investigate the formation and influence of quasi- or nanocrystalline phases in MK-based alkali-activated materials (AAM) that are related to the Si/Al ratio. We analyze the formation of an AAM from a MK precursor, which is a 3D bonded network that preserves the layered structure at the nanometer scale. We also exfoliate the remaining layered phase to examine the effects of the alkali-activation in the final sheet structures embedded within the amorphous network. The final material can be used as a cement with no carbon dioxide produced by the transformation reaction. |
Jin, Shangjian; Foo, Darryl C W; Qu, Tingyu; Ozyilmaz, Barbaros; Adam, Shaffique Unified theoretical framework for Kondo superconductors: Periodic Anderson impurities with attractive pairing and Rashba spin-orbit coupling Journal Article PHYSICAL REVIEW B, 111 (1), 2025, ISSN: 2469-9950. @article{ISI:001416427700003, title = {Unified theoretical framework for Kondo superconductors: Periodic Anderson impurities with attractive pairing and Rashba spin-orbit coupling}, author = {Shangjian Jin and Darryl C W Foo and Tingyu Qu and Barbaros Ozyilmaz and Shaffique Adam}, doi = {10.1103/PhysRevB.111.014505}, times_cited = {0}, issn = {2469-9950}, year = {2025}, date = {2025-01-08}, journal = {PHYSICAL REVIEW B}, volume = {111}, number = {1}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {Magnetic superconductors manifest a fascinating interplay between their magnetic and superconducting properties. This becomes evident, for example, in the significant enhancement of the upper critical field observed in uranium-based superconductors, or the destruction of superconductivity well below the superconducting transition temperature Tc in cobalt-doped NbSe2. In this work, we argue that the Kondo interaction plays a pivotal role in governing these behaviors. By employing a periodic Anderson model, we study the Kondo effect in superconductors with either singlet or triplet pairing. In the regime of small impurity energies and high doping concentrations, we find the emergence of a Kondo resistive region below Tc. While a magnetic field suppresses singlet superconductivity, it stabilizes triplet pairing through the screening of magnetic impurities, inducing reentrant superconductivity at high fields. Moreover, introducing an antisymmetric spin-orbital coupling suppresses triplet superconductivity. This framework provides a unified picture to understand the observation of Kondo effect in NbSe2 as well as the phase diagrams in Kondo superconductors such as UTe2 and URhGe.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Magnetic superconductors manifest a fascinating interplay between their magnetic and superconducting properties. This becomes evident, for example, in the significant enhancement of the upper critical field observed in uranium-based superconductors, or the destruction of superconductivity well below the superconducting transition temperature Tc in cobalt-doped NbSe2. In this work, we argue that the Kondo interaction plays a pivotal role in governing these behaviors. By employing a periodic Anderson model, we study the Kondo effect in superconductors with either singlet or triplet pairing. In the regime of small impurity energies and high doping concentrations, we find the emergence of a Kondo resistive region below Tc. While a magnetic field suppresses singlet superconductivity, it stabilizes triplet pairing through the screening of magnetic impurities, inducing reentrant superconductivity at high fields. Moreover, introducing an antisymmetric spin-orbital coupling suppresses triplet superconductivity. This framework provides a unified picture to understand the observation of Kondo effect in NbSe2 as well as the phase diagrams in Kondo superconductors such as UTe2 and URhGe. |
Carrio, Juan A G; Talluri, Vssl Prasad; Toolahalli, Swamy T; Echeverrigaray, Sergio G; Neto, Antonio Castro H Cross-Linked Self-Standing Graphene Oxide Membranes: A Pathway to Scalable Applications in Separation Technologies Journal Article MEMBRANES, 15 (1), 2025. @article{ISI:001404434600001, title = {Cross-Linked Self-Standing Graphene Oxide Membranes: A Pathway to Scalable Applications in Separation Technologies}, author = {Juan A G Carrio and Vssl Prasad Talluri and Swamy T Toolahalli and Sergio G Echeverrigaray and Antonio Castro H Neto}, doi = {10.3390/membranes15010031}, times_cited = {0}, year = {2025}, date = {2025-01-01}, journal = {MEMBRANES}, volume = {15}, number = {1}, publisher = {MDPI}, address = {ST ALBAN-ANLAGE 66, CH-4052 BASEL, SWITZERLAND}, abstract = {The large-scale implementation of 2D material-based membranes is hindered by mechanical stability and mass transport control challenges. This work describes the fabrication, characterisation, and testing of self-standing graphene oxide (GO) membranes cross-linked with oxides such as Fe2O3, Al2O3, CaSO4, Nb2O5, and a carbide, SiC. These cross-linking agents enhance the mechanical stability of the membranes and modulate their mass transport properties. The membranes were prepared by casting aqueous suspensions of GO and SiC or oxide powders onto substrates, followed by drying and detachment to yield self-standing films. This method enabled precise control over membrane thickness and the formation of laminated microstructures with interlayer spacings ranging from 0.8 to 1.2 nm. The resulting self-standing membranes, with areas between 0.002 m2 and 0.090 m2 and thicknesses from 0.6 mu m to 20 mu m, exhibit excellent flexibility and retain their chemical and physical integrity during prolonged testing in direct contact with ethanol/water and methanol/water mixtures in both liquid and vapour phases, with stability demonstrated over 24 h and up to three months. Gas permeation and chemical characterisation tests evidence their suitability for gas separation applications. The interactions promoted by the oxides and carbide with the functional groups of GO confer great stability and unique mass transport properties-the Nb2O5 cross-linked membranes present distinct performance characteristics-creating the potential for scalable advancements in cross-linked 2D material membranes for separation technologies.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The large-scale implementation of 2D material-based membranes is hindered by mechanical stability and mass transport control challenges. This work describes the fabrication, characterisation, and testing of self-standing graphene oxide (GO) membranes cross-linked with oxides such as Fe2O3, Al2O3, CaSO4, Nb2O5, and a carbide, SiC. These cross-linking agents enhance the mechanical stability of the membranes and modulate their mass transport properties. The membranes were prepared by casting aqueous suspensions of GO and SiC or oxide powders onto substrates, followed by drying and detachment to yield self-standing films. This method enabled precise control over membrane thickness and the formation of laminated microstructures with interlayer spacings ranging from 0.8 to 1.2 nm. The resulting self-standing membranes, with areas between 0.002 m2 and 0.090 m2 and thicknesses from 0.6 mu m to 20 mu m, exhibit excellent flexibility and retain their chemical and physical integrity during prolonged testing in direct contact with ethanol/water and methanol/water mixtures in both liquid and vapour phases, with stability demonstrated over 24 h and up to three months. Gas permeation and chemical characterisation tests evidence their suitability for gas separation applications. The interactions promoted by the oxides and carbide with the functional groups of GO confer great stability and unique mass transport properties-the Nb2O5 cross-linked membranes present distinct performance characteristics-creating the potential for scalable advancements in cross-linked 2D material membranes for separation technologies. |