Showing 1–50 of 74 results for author: Tian, S

Quantum fluctuations and the emergence of in-gap Higgs mode in superconductors

Authors: Sida Tian, Naoto Tsuji, Dirk Manske

Abstract: We extend the well-established action of the Higgs mode in $s$-wave superconductors to include quantum fluctuations (QFs). We find that already one-loop quantum corrections to the Higgs propagator shift its eigenfrequency below the superconducting energy gap $2Δ$. Consequently, the Higgs mode appears as an undamped pole below the quasiparticle continuum, leading to drastically sharper experimental… ▽ More We extend the well-established action of the Higgs mode in $s$-wave superconductors to include quantum fluctuations (QFs). We find that already one-loop quantum corrections to the Higgs propagator shift its eigenfrequency below the superconducting energy gap $2Δ$. Consequently, the Higgs mode appears as an undamped pole below the quasiparticle continuum, leading to drastically sharper experimental signatures. We demonstrate this by calculating two characteristic fingerprints of the Higgs mode, namely in Third Harmonic Generation (THG) and inelastic Raman scattering signals. More generally, gaps measured in $s$-wave superconductors with different experimental techniques (such as scanning tunneling microscope and Raman scattering) may be different due to fluctuation corrections. Since already arbitrarily weak QFs lead to the shift and to the new pole, our results shed some light on other amplitude modes even for systems with weak QFs, including charge density waves, (anti-) ferromagnets, or cold atom fermionic condensates. △ Less

Submitted 16 April, 2026; originally announced April 2026.

Comments: 19 pages, 6 figures

Physical properties of RhGe and CoGe single crystals synthesized under high pressure

Authors: Shangjie Tian, Xiangjiang Dong, Bowen Zhang, Zhijun Tu, Runze Yu, Hechang Lei, Shouguo Wang

Abstract: Chiral topological semimetals hosting multifold fermions and exotic surface states represent a frontier in topological materials research. Among them, noncentrosymmetric cubic B20 compounds-notably transition-metal silicides and germanides-offer a unique platform for realizing symmetry-protected topological phases and unconventional optoelectronic responses. Here, we report the physical properties… ▽ More Chiral topological semimetals hosting multifold fermions and exotic surface states represent a frontier in topological materials research. Among them, noncentrosymmetric cubic B20 compounds-notably transition-metal silicides and germanides-offer a unique platform for realizing symmetry-protected topological phases and unconventional optoelectronic responses. Here, we report the physical properties of RhGe and CoGe single crystals with B20 structure in detail. Transport measurements reveal metallic behavior with characteristic Fermi-liquid scaling at low temperatures, while magnetization results confirm paramagnetism in both compounds. In addition, both of materials exhibit low carrier concentrations with small electronic specific heat coefficient, indicating their semimetal feature with weak electronic correlations. Such high-quality CoGe and RhGe single crystals provide a material platform to explore the evolution of multifold fermions and the instability of helicoid-arc surface states with spin-orbit coupling and surface environment in B20 material systems. △ Less

Submitted 6 February, 2026; originally announced February 2026.

Comments: 15 pages, 4 figures, 1 table

Journal ref: Chin. Phys. Lett. 42, 110708 (2025)

Pressure-induced superconductivity in topological insulator Ge2Bi2Te5 and the evolution with Mn doping

Authors: Shangjie Tian, Qi Wang, Yuqing Cao, Ying Ma, Xiao Zhang, Yanpeng Qi, Hechang Lei, Shouguo Wang

Abstract: Introducing superconductivity (SC) or magnetism into topological insulators (TIs) can give rise to novel quantum states and exotic physical phenomena. Here, we report a high-pressure transport study on the TI Ge2Bi2Te5 and its Mn-doped counterparts. The application of pressure induces a SC in Ge2Bi2Te5, which shows a dome-shape phase diagram with the maximum Tc of 7.6 K at 23 GPa. Doping Mn into G… ▽ More Introducing superconductivity (SC) or magnetism into topological insulators (TIs) can give rise to novel quantum states and exotic physical phenomena. Here, we report a high-pressure transport study on the TI Ge2Bi2Te5 and its Mn-doped counterparts. The application of pressure induces a SC in Ge2Bi2Te5, which shows a dome-shape phase diagram with the maximum Tc of 7.6 K at 23 GPa. Doping Mn into Ge2Bi2Te5 introduces an antiferromagnetic order at ambient pressure and strongly weakens the pressure-induced SC, demonstrating that magnetism and SC compete in this material system. Present study provides a new platform for investigating the interplay among band topology, magnetism, and SC. △ Less

Submitted 23 January, 2026; originally announced January 2026.

Comments: 8 pages, 4 figures, 1 table

Z2 Lattice Gauge Theory on Non-trivial Topology and Its Quantum Simulation

Authors: Jiaqi Hu, Shu Tian, Xiaopeng Cui, Rebing Wu, Man-Hong Yung, Yu Shi

Abstract: Wegner duality is essential for Z2 lattice gauge theory, yet the duality on non-trivial topologies has remained implicit. We extend Wegner duality to arbitrary topology and dimension, obtaining a new class of Ising models, in which topology is encoded in non-local domain-wall patterns. Without the overhead of gauge constraints, simulating this model on an L*L torus requires only L*L qubits with tw… ▽ More Wegner duality is essential for Z2 lattice gauge theory, yet the duality on non-trivial topologies has remained implicit. We extend Wegner duality to arbitrary topology and dimension, obtaining a new class of Ising models, in which topology is encoded in non-local domain-wall patterns. Without the overhead of gauge constraints, simulating this model on an L*L torus requires only L*L qubits with two-body couplings, halving the conventional four-body coupled 2L*L qubits, enabling full experimental realization of Z2 lattice gauge theory on near-term devices. △ Less

Submitted 23 January, 2026; originally announced January 2026.

Comments: 8 pages

Three-dimensional topological insulator feature of ternary chalcogenide Ge2Bi2Te5

Authors: Shangjie Tian, Yuchong Zhang, Chenhao Liang, Yuqing Cao, Wenxin Lv, Xingyu Lv, Zhijun Wang, Tian Qian, Hechang Lei, Shouguo Wang

Abstract: The exploration of novel topological insulators (TIs) beyond binary chalcogenides has been accelerated in pursuit of exotic quantum states and device applications. Here, the layered ternary chalcogenide Ge2Bi2Te5 is identified as a three-dimensional TI. The bulk electronic structure of Ge2Bi2Te5 features a hole-type Fermi surface at Fermi level EF, which dominates the transport properties. Moreove… ▽ More The exploration of novel topological insulators (TIs) beyond binary chalcogenides has been accelerated in pursuit of exotic quantum states and device applications. Here, the layered ternary chalcogenide Ge2Bi2Te5 is identified as a three-dimensional TI. The bulk electronic structure of Ge2Bi2Te5 features a hole-type Fermi surface at Fermi level EF, which dominates the transport properties. Moreover, an unoccupied topological surface state with a Dirac point located at 290 meV above EF has been observed. Theoretical calculations confirm a bulk bandgap and a nontrivial Z2 topological invariant (000;1). The present study demonstrates that the material family of layered tetradymite-like ternary compounds is an important platform to explore exotic topological phenomena. △ Less

Submitted 16 January, 2026; originally announced January 2026.

Comments: 7 pages, 4 figures

Cooperative concurrence of 4f and 3d flat bands in kagome heavy-fermion metal YbCr6Ge6

Authors: Wenxin Lv, Pengcheng Ma, Tianqi Wang, Shangjie Tian, Ying Ma, Shouguo Wang, Xiao Zhang, Zhonghao Liu, Hechang Lei

Abstract: Flat-band (FB) systems originating from special lattice geometry like in kagome metals as well as localized orbitals in the materials such as heavy-fermion (HF) compounds have induced intensive interest due to their band topology and strong electron correlation effects, leading to emergent quantum states of matter. However, the question of how these two distinct FBs coexist and interact remains un… ▽ More Flat-band (FB) systems originating from special lattice geometry like in kagome metals as well as localized orbitals in the materials such as heavy-fermion (HF) compounds have induced intensive interest due to their band topology and strong electron correlation effects, leading to emergent quantum states of matter. However, the question of how these two distinct FBs coexist and interact remains unsettled. Here, we report that YbCr6Ge6 hosting both Cr-kagome lattice and Yb-4f electrons exhibits HF behaviors and a robust antiferromagnetic ground state with transition temperature TN = 3 K, significantly higher than other similar kagome metals with Yb ions. Angle-resolved photoemission spectroscopy measurements reveal the coexistence of FBs originating from both Cr-kagome lattice and localized Yb-4f electrons near Fermi energy level EF. More importantly, the clear spectroscopic signatures of a hybridization of Yb-4f FB with kagome-lattice-derived conduction bands and the high density of states of Cr-kagome FB near EF provide the underlying microscopic mechanisms of HF behaviors and enhanced antiferromagnetism in YbCr6Ge6. Our findings demonstrate that the novel kagome HF metals can not only host the cooperative coexistence of two different types of FBs, but also provide a paradigm material platform to explore the exotic correlated topological quantum phenomena. △ Less

Submitted 9 January, 2026; originally announced January 2026.

Comments: 23 pages, 4 figures

Nematic-fluctuation-mediated superconductivity in CuxTiSe2

Authors: Xingyu Lv, Yang Fu, Shangjie Tian, Ying Ma, Shouguo Wang, Cedomir Petrovic, Xiao Zhang, Hechang Lei

Abstract: The interplay among electronic nematicity, charge density wave, and superconductivity in correlated electronic systems has induced extensive research interest. Here, we discover the existence of nematic fluctuations in TiSe2 single crystal and investigate its evolution with Cu intercalation. It is observed that the elastoresistivity coefficient mEg exhibits a divergent temperature dependence follo… ▽ More The interplay among electronic nematicity, charge density wave, and superconductivity in correlated electronic systems has induced extensive research interest. Here, we discover the existence of nematic fluctuations in TiSe2 single crystal and investigate its evolution with Cu intercalation. It is observed that the elastoresistivity coefficient mEg exhibits a divergent temperature dependence following a Curie-Weiss law at high temperature. Upon Cu intercalation, the characteristic temperature T* of nematic fluctuation is progressively suppressed and becomes near zero when the superconductivity is optimized. Further intercalation of Cu leads to the sign change of T* and the suppression of superconductivity. These results strongly indicate that nematic phase transition may play a vital role in enhancing superconductivity in CuxTiSe2. Therefore, CuxTiSe2 provides a unique material platform to explore the nematic-fluctuation-mediated superconductivity. △ Less

Submitted 2 January, 2026; originally announced January 2026.

Comments: 5 pages, 4 figures

High Pressure Growth of Transition-Metal Monosilicide RhGe Single Crystals

Authors: Xiangjiang Dong, Bowen Zhang, Xubin Ye, Peng Wei, Lie Lian, Ning Sun, Youwen Long, Shangjie Tian, Shouguo Wang, Hechang Lei, Runze Yu

Abstract: Transition-metal monosilicide RhGe has been reported to exhibit weak itinerant ferromagnetism, superconductivity, and topological properties. In this study, we report the high-pressure growth of high-quality RhGe single crystals up to millimeter size using flux method. Transport measurements reveal the metallic behavior of RhGe between 2-300 K with Fermi liquid behavior at low temperature region.… ▽ More Transition-metal monosilicide RhGe has been reported to exhibit weak itinerant ferromagnetism, superconductivity, and topological properties. In this study, we report the high-pressure growth of high-quality RhGe single crystals up to millimeter size using flux method. Transport measurements reveal the metallic behavior of RhGe between 2-300 K with Fermi liquid behavior at low temperature region. However, no superconductivity was observed with variations in Ge composition. Magnetic characterizations indicate that RhGe exhibits a paramagnetic behavior between 2-300 K. The high-quality, large-size RhGe single crystals pave the way for further investigation of their topological properties using spectroscopic techniques. △ Less

Submitted 23 June, 2025; originally announced June 2025.

Comments: Chin.Phys.B accepted

Symmetrical bipolar electrobending deformation in acceptor-doped piezoceramics

Authors: Yi Cheng, Shuo Tian, Bin Li, Yejing Dai

Abstract: Since 2022, large apparent strains (>1%) with highly asymmetrical strain-electric field (S-E) curves have been reported in various thin piezoceramic materials, attributed to a bidirectional electric-field-induced bending (electrobending) deformation, which consistently produces convex bending along the negative electric field direction. In this study, we report a novel unidirectional electrobendin… ▽ More Since 2022, large apparent strains (>1%) with highly asymmetrical strain-electric field (S-E) curves have been reported in various thin piezoceramic materials, attributed to a bidirectional electric-field-induced bending (electrobending) deformation, which consistently produces convex bending along the negative electric field direction. In this study, we report a novel unidirectional electrobending behavior in acceptor-doped K0.5Na0.5NbO3 ceramics, where convex bending always occurs along the pre-poling direction regardless of the direction of the applied electric field. This unique deformation is related to the reorientation of the defect dipoles in one surface layer during the pre-poling process, resulting in an asymmetrical distribution of defect dipoles in the two surface layers. The synergistic interaction between ferroelectric domains and defect dipoles in the surface layers induces this unidirectional electrobending, as evidenced by a butterfly-like symmetrical bipolar S-E curve with a giant apparent strain of 3.2%. These findings provide new insights into defect engineering strategies for developing advanced piezoelectric materials with large electroinduced displacements. △ Less

Submitted 4 March, 2025; originally announced March 2025.

MSC Class: 14J60 (Primary) 14F05; 14J26 (Secondary) 14J60 (Primary) 14F05; 14J26 (Secondary)

Observation of topological Anderson Chern insulator phase in MnBi$_4$Te$_7$ monolayer

Authors: Anqi Wang, Bo Yin, Zikang Su, Shangjie Tian, Guoan Li, Xiaofan Shi, Xiao Deng, Yupeng Li, Zhiyuan Zhang, Xingchen Guo, Qinghua Zhang, Lin Gu, Xingjiang Zhou, Bingbing Tong, Peiling Li, Zhaozheng Lyu, Guangtong Liu, Fanming Qu, Ziwei Dou, Yuan Huang, Hechang Lei, Hongming Weng, Zhong Fang, Quansheng Wu, Li Lu , et al. (1 additional authors not shown)

Abstract: The correlation of topology and disorder has attracted great intention due to appropriate disorder could induce the phase transition between trivial and nontrivial topological states. While it is widely recognized that strong disorder can produce rich phase diagrams in topological nontrivial states, moderate disorder has been proposed to induce transitions into topologically nontrivial phases coun… ▽ More The correlation of topology and disorder has attracted great intention due to appropriate disorder could induce the phase transition between trivial and nontrivial topological states. While it is widely recognized that strong disorder can produce rich phase diagrams in topological nontrivial states, moderate disorder has been proposed to induce transitions into topologically nontrivial phases counter-intuitively, leading to the concept of topological Anderson insulators. This phenomenon has been theoretically explored and simulated in various systems, yet experimental realization in solid state systems has remained elusive due to challenges in controlling disorder. Here, we report the experimental observation of Chern insulator state signed by the coexistence of quantized Hall plateau and zero longitudinal resistance in monolayer MnBi$_4$Te$_7$ Hall bar device, which originally hosts a trivial insulating state with Chern number $C$ = 0 in clean limit. We demonstrate that the observed trivial to nontrivial transition in this monolayer device can be attributed to disorder, evidenced by universal conductance fluctuations. Our findings substantiate the existence of a long-sought topological Anderson Chern insulator in real materials, a unique variant of the topological Anderson insulator characterized by broken time-reversal-symmetry. △ Less

Submitted 5 February, 2025; v1 submitted 8 January, 2025; originally announced January 2025.

Comments: 45 pages, 4 main figures, 5 extended data figures, 5 supplementary figures

Interstitial anionic electrons involved superconductivity and T-linear resistivity behavior in electride La3In

Authors: Zhijun Tu, Peihan Sun, Pengcheng Ma, Hongrun Zhen, Shangjie Tian, Shouguo Wang, Tian Cui, Zhonghao Liu, Kai Liu, Hechang Lei

Abstract: Electrides are unique materials because of the existence of interstitial anionic electrons (IAEs). Due to these loosely bound IAEs and their strong interaction with the framework of cations, electrides can host superconductivity with rather high Tc, especially under high pressure, as predicted in theory. However, the experimental observations of superconductivity in electrides are very rare, let a… ▽ More Electrides are unique materials because of the existence of interstitial anionic electrons (IAEs). Due to these loosely bound IAEs and their strong interaction with the framework of cations, electrides can host superconductivity with rather high Tc, especially under high pressure, as predicted in theory. However, the experimental observations of superconductivity in electrides are very rare, let alone the detailed studies on intrinsic properties of single crystals. Here, we report the superconducting and normal-state properties of electride La3In single crystals. La3In shows a type-II superconductivity with Tc ~ 9.4 K and a T-linear resistivity in a wide temperature range. Experimental measurements and theoretical calculations suggest that the relatively high Tc could be ascribed to the high density of states around the Fermi level caused by short flat bands along R-M direction and the strong electron-phonon coupling, partially derived from the IAEs. Meanwhile, the T-linear resistivity may reflect the significant electronic correlation effect in this material. These findings will shed light on understanding the role of IAEs in superconductivity and open a promising way to explore high-temperature superconductors in electrides. △ Less

Submitted 14 October, 2024; originally announced October 2024.

Comments: 7 pages, 4 figures

Gain and Threshold Improvements of 1300 nm Lasers based on InGaAs/InAlGaAs Superlattice Active Regions

Authors: Andrey Babichev, Evgeniy Pirogov, Maksim Sobolev, Sergey Blokhin, Yuri Shernyakov, Mikhail Maximov, Andrey Lutetskiy, Nikita Pikhtin, Leonid Karachinsky, Innokenty Novikov, Anton Egorov, Si-Cong Tian, Dieter Bimberg

Abstract: A detailed experimental analysis of the impact of active region design on the performance of 1300 nm lasers based on InGaAs/InAlGaAs superlattices is presented. Three different types of superlattice active regions and waveguide layer compositions were grown. Using a superlattice allows to downshift the energy position of the miniband, as compared to thin InGaAs quantum wells, having the same compo… ▽ More A detailed experimental analysis of the impact of active region design on the performance of 1300 nm lasers based on InGaAs/InAlGaAs superlattices is presented. Three different types of superlattice active regions and waveguide layer compositions were grown. Using a superlattice allows to downshift the energy position of the miniband, as compared to thin InGaAs quantum wells, having the same composition, being beneficial for high-temperature operation. Very low internal loss (~6$cm^{-1}$), low transparency current density of ~500$ A/cm^2$, together with 46$ cm^{-1}$ modal gain and 53 % internal efficiency were observed for broad-area lasers with an active region based on a highly strained $In_{0.74}Ga_{0.26}As/In_{0.53}Al_{0.25}Ga_{0.22}As$ superlattice. Characteristic temperatures $T_0$ and $T_1$ were improved up to 76 K and 100 K, respectively. These data suggest that such superlattices have also the potential to much improve VCSEL properties at this wavelength. △ Less

Submitted 5 August, 2024; originally announced August 2024.

Journal ref: IEEE J. Quantum Electron. 61 (2025) 2400209

Macroscopic Tunneling Probe of Moiré Spin Textures in Twisted CrI$_3$

Authors: Bowen Yang, Tarun Patel, Meixin Cheng, Kostyantyn Pichugin, Lin Tian, Nachiket Sherlekar, Shaohua Yan, Yang Fu, Shangjie Tian, Hechang Lei, Michael E. Reimer, Junichi Okamoto, Adam W. Tsen

Abstract: Various noncollinear spin textures and magnetic phases have been predicted in twisted two-dimensional CrI$_3$ due to competing ferromagnetic (FM) and antiferromagnetic (AFM) interlayer exchange from moiré stacking - with potential spintronic applications even when the underlying material possesses a negligible Dzyaloshinskii-Moriya or dipole-dipole interaction. Recent measurements have shown evide… ▽ More Various noncollinear spin textures and magnetic phases have been predicted in twisted two-dimensional CrI$_3$ due to competing ferromagnetic (FM) and antiferromagnetic (AFM) interlayer exchange from moiré stacking - with potential spintronic applications even when the underlying material possesses a negligible Dzyaloshinskii-Moriya or dipole-dipole interaction. Recent measurements have shown evidence of coexisting FM and AFM layer order in small-twist-angle CrI$_3$ bilayers and double bilayers. Yet, the nature of the magnetic textures remains unresolved and possibilities for their manipulation and electrical readout are unexplored. Here, we use tunneling magnetoresistance to investigate the collective spin states of twisted double-bilayer CrI$_3$ under both out-of-plane and in-plane magnetic fields together with detailed micromagnetic simulations of domain dynamics based on magnetic circular dichroism. Our results capture hysteretic and anisotropic field evolutions of the magnetic states and we further uncover two distinct non-volatile spin textures (out-of-plane and in-plane domains) at $\approx$ 1° twist angle, with a different global tunneling resistance that can be switched by magnetic field. △ Less

Submitted 12 June, 2024; originally announced June 2024.

Comments: 18 pages, 5 figures

Exploring material compositions for synthesis using oxidation states

Authors: Maung Thway, Andy Paul Chen, Haiwen Dai, Jose Recatala-Gomez, Siyu Isaac Parker Tian, Ruiming Zhu, Wenhao Zhai, Fengxia Wei, D. V. Maheshwar Repaka, Tonio Buonassisi, Pieremanuele Canepa, Kedar Hippalgaonkar

Abstract: Recent advances in machine learning techniques have made it possible to use high-throughput screening to identify novel materials with specific properties. However, the large number of potential candidates produced by these techniques can make it difficult to select the most promising ones. In this study, we develop the oxidation state probability (OSP) method which evaluates ternary compounds bas… ▽ More Recent advances in machine learning techniques have made it possible to use high-throughput screening to identify novel materials with specific properties. However, the large number of potential candidates produced by these techniques can make it difficult to select the most promising ones. In this study, we develop the oxidation state probability (OSP) method which evaluates ternary compounds based on the probability (the OSP metric) of each element to adopt the required oxidation states for fulfilling charge neutrality. We compare this model with Roost and the Fourier-transformed crystal properties (FTCP)-based synthesizability score. Among the top 1000 systems with the most database entries in Materials Project (MP), more than 500 systems exhibit an attested compound among the top 3 compositions when ranked by the OSP metric. We find that the OSP method shows promising results for certain classes of ternary systems, especially those containing nonmetals, s-block, or transition metals. When applied to the Cu-In-Te ternary system, an interesting system for thermoelectric applications, the OSP method predicted the synthesizability of CuIn$_3$Te$_5$ without prior knowledge, and we have successfully synthesized CuIn$_3$Te$_5$ in experiment. Our method has the potential to accelerate the discovery of novel compounds by providing a guide for experimentalists to easily select the most synthesizable candidates from an arbitrarily large set of possible chemical compositions. △ Less

Submitted 14 May, 2024; originally announced May 2024.

Comments: 11 pages, 3 figures

On the Path to High-temperature Josephson Multi-junction Devices

Authors: Xu Wang, Fucong Chen, Zefeng Lin, Changhong Yuan, Shibing Tian, Chunguang Li, Victor Kornev, Nikolay Kolotinskiy

Abstract: We report our progress in the high-temperature superconductor (HTS) Josephson junction fabrication process founded on using a focused helium ion beam damaging technique and discuss the expected device performance attainable with the HTS multi-junction device technology. Both the achievable high value of characteristic voltage $V_c=I_cR_N$ of Josephson junctions and the ability to design a large nu… ▽ More We report our progress in the high-temperature superconductor (HTS) Josephson junction fabrication process founded on using a focused helium ion beam damaging technique and discuss the expected device performance attainable with the HTS multi-junction device technology. Both the achievable high value of characteristic voltage $V_c=I_cR_N$ of Josephson junctions and the ability to design a large number of arbitrary located Josephson junctions allow narrowing the existing gap in design abilities for LTS and HTS circuits even with using a single YBCO film layer. A one-layer topology of active electrically small antenna is suggested and its voltage response characteristics are considered. △ Less

Submitted 5 September, 2024; v1 submitted 19 April, 2024; originally announced April 2024.

Comments: v2: accepted manuscript; 7 pages, 5 figures

Journal ref: EM Science, 2(3), 0070092, 2024

All-magnonic repeater based on bistability

Authors: Qi Wang, Roman Verba, Kristyna Davidkova, Bjorn Heinz, Shixian Tian, Yiheng Rao, Mengying Guo, Xueyu Guo, Carsten Dubs, Philipp Pirro, Andrii V. Chumak

Abstract: Bistability, a universal phenomenon found in diverse fields such as biology, chemistry, and physics, describes a scenario in which a system has two stable equilibrium states and resets to one of the two states. The ability to switch between these two states is the basis for a wide range of applications, particularly in memory and logic operations. Here, we present a universal approach to achieve b… ▽ More Bistability, a universal phenomenon found in diverse fields such as biology, chemistry, and physics, describes a scenario in which a system has two stable equilibrium states and resets to one of the two states. The ability to switch between these two states is the basis for a wide range of applications, particularly in memory and logic operations. Here, we present a universal approach to achieve bistable switching in magnonics, the field processing data using spin waves. As an exemplary application, we use magnonic bistability to experimentally demonstrate the still missing magnonic repeater. A pronounced bistable window is observed in a 1um wide magnonic conduit under an external rf drive characterized by two magnonic stable states defined as low and high spin-wave amplitudes. The switching between these two states is realized by another propagating spin wave sent into the rf driven region. This magnonic bistable switching is used to design the magnonic repeater, which receives the original decayed and distorted spin wave and regenerates a new spin wave with amplified amplitude and normalized phase. Our magnonic repeater is proposed to be installed at the inputs of each magnonic logic gate to overcome the spin-wave amplitude degradation and phase distortion during previous propagation and achieve integrated magnonic circuits or magnonic neuromorphic networks. △ Less

Submitted 19 March, 2024; originally announced March 2024.

Comments: 13 pages, 4 figures

Extreme orbital ab-plane upper critical fields far beyond Pauli limit in 4Hb-Ta(S, Se)2 bulk crystals

Authors: Fanyu Meng, Yang Fu, Senyang Pan, Shangjie Tian, Shaohua Yan, Zhengyu Li, Shouguo Wang, Jinglei Zhang, Hechang Lei

Abstract: Transition metal disulfides 4Hb-Ta(S, Se)2 with natural heterostructure of 1T- and 1H-Ta(S, Se)2 layers have became the focus of correlated materials their unique combinations of Mott physics and possible topological superconductivity. In this work, we study the upper critical fields mu0Hc2 of 4Hb-TaS2 and 4Hb-TaS1.99Se0.01 single crystals systematically. Transport measurements up to 35 T show tha… ▽ More Transition metal disulfides 4Hb-Ta(S, Se)2 with natural heterostructure of 1T- and 1H-Ta(S, Se)2 layers have became the focus of correlated materials their unique combinations of Mott physics and possible topological superconductivity. In this work, we study the upper critical fields mu0Hc2 of 4Hb-TaS2 and 4Hb-TaS1.99Se0.01 single crystals systematically. Transport measurements up to 35 T show that both of ab-plane and c-axis upper critical fields (mu0Hc2,ab and mu0Hc2,c) for 4Hb-TaS2 and 4Hb-TaS1.99Se0.01 exhibit a linear temperature dependent behavior down to 0.3 K, suggesting the three-dimensional superconductivity with dominant orbital depairing mechanism in bulk 4Hb-Ta(S, Se)2. However, the zero-temperature mu0Hc2,ab(0) for both crystals are far beyond the Pauli paramagnetic limit mu0HP. It could be explained by the effects of spin-momentum locking in 1H-Ta(S, Se)2 layers with local inversion symmetry broken and the relatively weak intersublattice interaction between 1H layers due to the existence of 1T layers. △ Less

Submitted 25 December, 2025; v1 submitted 6 December, 2023; originally announced December 2023.

Comments: 6 pages, 3 figures

Journal ref: Phys. Rev. B 109, 134510 (2024)

Large electrobending deformation caused by defect dipoles

Authors: Shuo Tian, Bin Li, Yejing Dai

Abstract: Ultrahigh electrostrains (greater than 1%) in several piezoceramic systems have been reported since 2022, which attract more and more interest in the field of piezoelectricity; however, the mechanism is still unclear. Here, we have directly observed a novel electric field-induced bending (electrobending) phenomenon that visually exhibites as an alternating concave-convex deformation under an elect… ▽ More Ultrahigh electrostrains (greater than 1%) in several piezoceramic systems have been reported since 2022, which attract more and more interest in the field of piezoelectricity; however, the mechanism is still unclear. Here, we have directly observed a novel electric field-induced bending (electrobending) phenomenon that visually exhibites as an alternating concave-convex deformation under an electric field of plus or minus 50 kV cm-1, in nonstoichiometric (K0.48Na0.52)0.99NbO2.995 ceramics, which causes the measured ultrahigh electrostrain. It is demonstrated that the electrobending deformation arises from the different stresses due to the stretching or compression of the defect dipoles on the upper and lower surfaces of the ceramics. As a result of the large electrobending deformation, a giant apparent electrostrain of 11.6% is obtained at room temperature, and it can even reach up to 26.0% at 210 degree Celsius, which far exceeds that of all present piezoelectric materials. Our discovery is an important addition and refinement to the field of condensed matter physics, whilst providing a new strategy and shedding light on the design of future precision actuators or intelligent devices. △ Less

Submitted 5 December, 2023; originally announced December 2023.

Direct observation of the Higgs particle in a superconductor by non-equilibrium Raman scattering

Authors: Tomke E. Glier, Sida Tian, Mika Rerrer, Lea Westphal, Garret Lüllau, Liwen Feng, Jakob Dolgner, Rafael Haenel, Marta Zonno, Hiroshi Eisaki, Martin Greven, Andrea Damascelli, Stefan Kaiser, Dirk Manske, Michael Rübhausen

Abstract: Even before its role in electroweak symmetry breaking, the Anderson-Higgs mechanism was introduced to explain the Meissner effect in superconductors. Spontaneous symmetry-breaking yields massless phase modes representing the low-energy excitations of the Mexican-Hat potential. Only in superconductors the phase mode is shifted towards higher energies owing to the gauge field of the charged condensa… ▽ More Even before its role in electroweak symmetry breaking, the Anderson-Higgs mechanism was introduced to explain the Meissner effect in superconductors. Spontaneous symmetry-breaking yields massless phase modes representing the low-energy excitations of the Mexican-Hat potential. Only in superconductors the phase mode is shifted towards higher energies owing to the gauge field of the charged condensate. This results in a low-energy excitation spectrum governed by the Higgs mode. Consequently, the Meissner effect signifies a macroscopic quantum condensate in which a photon acquires mass, representing a one-to-one analogy to high-energy physics. We report on the direct observation of the Higgs particle in the high-temperature superconductor Bi-2212 by developing an innovative technique to study its symmetries and energies after a "soft quench" of the Mexican-Hat potential. Population inversion of the metastable Higgs particle induced by an initial laser pulse allows identifying the polarization-dependent Higgs modes as an additional anti-Stokes Raman-scattering signal. Within Ginzburg-Landau theory, the Higgs-mode energy is connected to the Cooper-pair coherence length. Within a BCS weak-coupling model we develop a quantitative and coherent description of single-particle and two-particle channels. This opens the avenue for Higgs Spectroscopy in quantum condensates and provides a unique pathway to control and explore Higgs physics. △ Less

Submitted 30 November, 2024; v1 submitted 12 October, 2023; originally announced October 2023.

Comments: Revised version with rewritten text elements and added theory description of the experimental data, new section in SI S.2, S.3, and S.4, experimental data and results unchanged

Journal ref: Nat Commun 16, 7027 (2025)

Classification of Lifshitz invariant in multiband superconductors: an application to Leggett modes in the linear response regime in Kagome lattice models

Authors: Raigo Nagashima, Sida Tian, Rafael Haenel, Naoto Tsuji, Dirk Manske

Abstract: Multiband superconductors are sources of rich physics arising from multiple order parameters, which show unique collective dynamics including Leggett mode as relative phase oscillations. Previously, it has been pointed out that the Leggett mode can be optically excited in the linear response regime, as demonstrated in a one-dimensional model for multiband superconductors[T. Kamatani, et al., Phys.… ▽ More Multiband superconductors are sources of rich physics arising from multiple order parameters, which show unique collective dynamics including Leggett mode as relative phase oscillations. Previously, it has been pointed out that the Leggett mode can be optically excited in the linear response regime, as demonstrated in a one-dimensional model for multiband superconductors[T. Kamatani, et al., Phys. Rev. B 105, 094520 (2022)]. Here we identify the linear coupling term in the Ginzburg-Landau free energy to be the so-called Lifshitz invariant, which takes a form of $\boldsymbol{d}\cdot\left(Ψ^{*}_{i}\nablaΨ_{j} - Ψ_{j}\nablaΨ^{*}_{i}\right)$, where $\boldsymbol{d}$ is a constant vector and $Ψ_{i}$ and $Ψ_{j}$ $(i\neq j)$ represent superconducting order parameters. We have classified all pairs of irreducible representations of order parameters in the crystallographic point groups that allow for the existence of the Lifshitz invariant. We emphasize that the Lifshitz invariant can appear even in systems with inversion symmetry. The results are applied to a model of $s$-wave superconductors on a Kagome lattice with various bond orders, for which in some cases we confirm that the Leggett mode appears as a resonance peak in a linear optical conductivity spectrum based on microscopic calculations. We discuss a possible experimental observation of the Leggett mode by a linear optical response in multiband superconductors. △ Less

Submitted 31 January, 2024; v1 submitted 4 September, 2023; originally announced September 2023.

Comments: 27 pages, 6 figures

Journal ref: Phys. Rev. Research 6, 013120 (2024)

Nearly-room-temperature ferromagnetism and tunable anomalous Hall effect in atomically thin Fe4CoGeTe2

Authors: Shaohua Yan, Hui-Hui He, Yang Fu, Ning-Ning Zhao, Shangjie Tian, Qiangwei Yin, Fanyu Meng, Xinyu Cao, Le Wang, Shanshan Chen, Ki-Hoon Son, Jun Woo Choi, Hyejin Ryu, Shouguo Wang, Xiao Zhang, Kai Liu, Hechang Lei

Abstract: Itinerant ferromagnetism at room temperature is a key ingredient for spin transport and manipulation. Here, we report the realization of nearly-room-temperature itinerant ferromagnetism in Co doped Fe5GeTe2 thin flakes. The ferromagnetic transition temperature TC (~ 323 K - 337 K) is almost unchanged when thickness is down to 12 nm and is still about 284 K at 2 nm (bilayer thickness). Theoretical… ▽ More Itinerant ferromagnetism at room temperature is a key ingredient for spin transport and manipulation. Here, we report the realization of nearly-room-temperature itinerant ferromagnetism in Co doped Fe5GeTe2 thin flakes. The ferromagnetic transition temperature TC (~ 323 K - 337 K) is almost unchanged when thickness is down to 12 nm and is still about 284 K at 2 nm (bilayer thickness). Theoretical calculations further indicate that the ferromagnetism persists in monolayer Fe4CoGeTe2. In addition to the robust ferromagnetism down to the ultrathin limit, Fe4CoGeTe2 exhibits an unusual temperature- and thickness-dependent intrinsic anomalous Hall effect. We propose that it could be ascribed to the dependence of band structure on thickness that changes the Berry curvature near the Fermi energy level subtly. The nearly-room-temperature ferromagnetism and tunable anomalous Hall effect in atomically thin Fe4CoGeTe2 provide opportunities to understand the exotic transport properties of two-dimensional van der Waals magnetic materials and explore their potential applications in spintronics. △ Less

Submitted 22 December, 2025; v1 submitted 24 August, 2023; originally announced August 2023.

Comments: 9 pages, 4 figures, 1 table

Journal ref: Nanoscale 16, 1406 (2024)

Highly Efficient Room-Temperature Nonvolatile Magnetic Switching by Current in Fe3GaTe2 Thin Flakes

Authors: Shaohua Yan, Shangjie Tian, Yang Fu, Fanyu Meng, Zhiteng Li, Hechang Lei, Shouguo Wang, Xiao Zhang

Abstract: Effectively tuning magnetic state by using current is essential for novel spintronic devices. Magnetic van der Waals (vdW) materials have shown superior properties for the applications of magnetic information storage based on the efficient spin torque effect. However, for most of known vdW ferromagnets, the ferromagnetic transition temperatures lower than room temperature strongly impede their app… ▽ More Effectively tuning magnetic state by using current is essential for novel spintronic devices. Magnetic van der Waals (vdW) materials have shown superior properties for the applications of magnetic information storage based on the efficient spin torque effect. However, for most of known vdW ferromagnets, the ferromagnetic transition temperatures lower than room temperature strongly impede their applications and the room-temperature vdW spintronic device with low energy consumption is still a long-sought goal. Here, we realize the highly efficient room-temperature nonvolatile magnetic switching by current in a single-material device based on vdW ferromagnet Fe3GaTe2. Moreover, the switching current density and power dissipation are about 300 and 60000 times smaller than conventional spin-orbit-torque devices of magnet/heavy-metal heterostructures. These findings make an important progress on the applications of magnetic vdW materials in the fields of spintronics and magnetic information storage. △ Less

Submitted 29 September, 2025; v1 submitted 24 August, 2023; originally announced August 2023.

Comments: 18 pages, 5 figures

Journal ref: Small 20, 2311430 (2024)

Revealing intrinsic domains and fluctuations of moiré magnetism by a wide-field quantum microscope

Authors: Mengqi Huang, Zeliang Sun, Gerald Yan, Hongchao Xie, Nishkarsh Agarwal, Gaihua Ye, Suk Hyun Sung, Hanyi Lu, Jingcheng Zhou, Shaohua Yan, Shangjie Tian, Hechang Lei, Robert Hovden, Rui He, Hailong Wang, Liuyan Zhao, Chunhui Rita Du

Abstract: Moiré magnetism featured by stacking engineered atomic registry and lattice interactions has recently emerged as an appealing quantum state of matter at the forefront condensed matter physics research. Nanoscale imaging of moiré magnets is highly desirable and serves as a prerequisite to investigate a broad range of intriguing physics underlying the interplay between topology, electronic correlati… ▽ More Moiré magnetism featured by stacking engineered atomic registry and lattice interactions has recently emerged as an appealing quantum state of matter at the forefront condensed matter physics research. Nanoscale imaging of moiré magnets is highly desirable and serves as a prerequisite to investigate a broad range of intriguing physics underlying the interplay between topology, electronic correlations, and unconventional nanomagnetism. Here we report spin defect-based wide-field imaging of magnetic domains and spin fluctuations in twisted double trilayer (tDT) chromium triiodide CrI3. We explicitly show that intrinsic moiré domains of opposite magnetizations appear over arrays of moiré supercells in low-twist-angle tDT CrI3. In contrast, spin fluctuations measured in tDT CrI3 manifest little spatial variations on the same mesoscopic length scale due to the dominant driving force of intralayer exchange interaction. Our results enrich the current understanding of exotic magnetic phases sustained by moiré magnetism and highlight the opportunities provided by quantum spin sensors in probing microscopic spin related phenomena on two-dimensional flatland. △ Less

Submitted 7 July, 2023; originally announced July 2023.

Pressure-tunable magnetic topological phases in magnetic topological insulator MnSb4Te7

Authors: Lingling Gao, Juefei Wu, Ming Xi, Cuiying Pei, Qi Wang, Yi Zhao, Shangjie Tian, Changhua Li, Weizheng Cao, Yulin Chen, Hechang Lei, Yanpeng Qi

Abstract: Magnetic topological insulators, possessing both magnetic order and topological electronic structure, provides an excellent platform to research unusual physical properties. Here, we report a high-pressure study on the anomalous Hall effect of magnetic TI MnSb4Te7 through transports measurements combined with first-principle theoretical calculations. We discover that the ground state of MnSb4Te7 e… ▽ More Magnetic topological insulators, possessing both magnetic order and topological electronic structure, provides an excellent platform to research unusual physical properties. Here, we report a high-pressure study on the anomalous Hall effect of magnetic TI MnSb4Te7 through transports measurements combined with first-principle theoretical calculations. We discover that the ground state of MnSb4Te7 experiences a magnetic phase transition from the A-type antiferromagnetic state to ferromagnetic dominating state at 3.78 GPa, although its crystal sustains a rhombohedral phase under high pressures up to 8 GPa. The anomalous Hall conductance σxyA keeps around 10 Ω-1 cm-1, dominated by the intrinsic mechanism even after the magnetic phase transition. The results shed light on the intriguing magnetism in MnSb4Te7 and pave the way for further studies of the relationship between topology and magnetism in topological materials. △ Less

Submitted 14 February, 2023; originally announced February 2023.

Comments: 10 pages, 4 figures

Frustrated ferromagnetic transition in AB-stacked honeycomb bilayer

Authors: S. Y. Wang, Y. Wang, S. H. Yan, C. Wang, B. K. Xiang, K. Y. Liang, Q. S. He, K. Watanabe, T. Taniguchi, S. J. Tian, H. C. Lei, W. Ji, Y. Qi, Y. H. Wang

Abstract: In two-dimensional (2D) ferromagnets, anisotropy is essential for the magnetic ordering as dictated by the Mermin-Wagner theorem. But when competing anisotropies are present, the phase transition becomes nontrivial. Here, utilizing highly sensitive susceptometry of scanning superconducting quantum interference device microscopy, we probe the spin correlations of ABC-stacked CrBr3 under zero magnet… ▽ More In two-dimensional (2D) ferromagnets, anisotropy is essential for the magnetic ordering as dictated by the Mermin-Wagner theorem. But when competing anisotropies are present, the phase transition becomes nontrivial. Here, utilizing highly sensitive susceptometry of scanning superconducting quantum interference device microscopy, we probe the spin correlations of ABC-stacked CrBr3 under zero magnetic field. We identify a plateau feature in susceptibility above the critical temperature (Tc) in thick samples. It signifies a crossover regime induced by the competition between easy-plane intralayer exchange anisotropy versus uniaxial interlayer anisotropy. The evolution of the critical behavior from the bulk to 2D shows that the competition between the anisotropies is magnified in the reduced dimension. It leads to a strongly frustrated ferromagnetic transition in the bilayer with fluctuation on the order of Tc, which is distinct from both the monolayer and the bulk. Our observation potentially offers a 2D localized spin system on honeycomb lattice to explore magnetic frustration. △ Less

Submitted 22 July, 2022; originally announced July 2022.

Tackling Data Scarcity with Transfer Learning: A Case Study of Thickness Characterization from Optical Spectra of Perovskite Thin Films

Authors: Siyu Isaac Parker Tian, Zekun Ren, Selvaraj Venkataraj, Yuanhang Cheng, Daniil Bash, Felipe Oviedo, J. Senthilnath, Vijila Chellappan, Yee-Fun Lim, Armin G. Aberle, Benjamin P MacLeod, Fraser G. L. Parlane, Curtis P. Berlinguette, Qianxiao Li, Tonio Buonassisi, Zhe Liu

Abstract: Transfer learning increasingly becomes an important tool in handling data scarcity often encountered in machine learning. In the application of high-throughput thickness as a downstream process of the high-throughput optimization of optoelectronic thin films with autonomous workflows, data scarcity occurs especially for new materials. To achieve high-throughput thickness characterization, we propo… ▽ More Transfer learning increasingly becomes an important tool in handling data scarcity often encountered in machine learning. In the application of high-throughput thickness as a downstream process of the high-throughput optimization of optoelectronic thin films with autonomous workflows, data scarcity occurs especially for new materials. To achieve high-throughput thickness characterization, we propose a machine learning model called thicknessML that predicts thickness from UV-Vis spectrophotometry input and an overarching transfer learning workflow. We demonstrate the transfer learning workflow from generic source domain of generic band-gapped materials to specific target domain of perovskite materials, where the target domain data only come from limited number (18) of refractive indices from literature. The target domain can be easily extended to other material classes with a few literature data. Defining thickness prediction accuracy to be within-10% deviation, thicknessML achieves 92.2% (with a deviation of 3.6%) accuracy with transfer learning compared to 81.8% (with a deviation of 3.6%) 11.7% without (lower mean and larger standard deviation). Experimental validation on six deposited perovskite films also corroborates the efficacy of the proposed workflow by yielding a 10.5% mean absolute percentage error (MAPE). △ Less

Submitted 20 December, 2022; v1 submitted 14 June, 2022; originally announced July 2022.

What Information is Necessary and Sufficient to Predict Materials Properties using Machine Learning?

Authors: Siyu Isaac Parker Tian, Aron Walsh, Zekun Ren, Qianxiao Li, Tonio Buonassisi

Abstract: Conventional wisdom of materials modelling stipulates that both chemical composition and crystal structure are integral in the prediction of physical properties. However, recent developments challenge this by reporting accurate property-prediction machine learning (ML) frameworks using composition alone without knowledge of the local atomic environments or long-range order. To probe this behavior,… ▽ More Conventional wisdom of materials modelling stipulates that both chemical composition and crystal structure are integral in the prediction of physical properties. However, recent developments challenge this by reporting accurate property-prediction machine learning (ML) frameworks using composition alone without knowledge of the local atomic environments or long-range order. To probe this behavior, we conduct a systematic comparison of supervised ML models built on composition only vs. composition plus structure features. Similar performance for property prediction is found using both models for compounds close to the thermodynamic convex hull. We hypothesize that composition embeds structural information of ground-state structures in support of composition-centric models for property prediction and inverse design of stable compounds. △ Less

Submitted 28 June, 2022; v1 submitted 10 June, 2022; originally announced June 2022.

Comments: 18 pages, 8 figures

Ferromagnetic-antiferromagnetic coexisting ground states and exchange bias effects in $\bf{MnBi_4Te_7}$ and $\bf{MnBi_6Te_{10}}$

Authors: Xiaolong Xu, Shiqi Yang, Huan Wang, Roger Guzman, Yaozheng Zhu, Yuxuan Peng, Zhihao Zang, Ming Xi, Shangjie Tian, Yanping Li, Hechang Lei, Zhaochu Luo, Jinbo Yang, Tianlong Xia, Wu Zhou, Yuan Huang, Yu Ye

Abstract: Natural superlattice structures $\rm{(MnBi_2Te_4)(Bi_2Te_3)}$$_n$ ($n$ = 1, 2,...), in which magnetic $\rm{MnBi_2Te_4}$ layers are separated by nonmagnetic $\rm{Bi_2Te_3}$ layers, hold band topology, magnetism and reduced interlayer coupling, providing a promising platform for the realization of exotic topological quantum states. However, their magnetism in the two-dimensional limit, which is cruc… ▽ More Natural superlattice structures $\rm{(MnBi_2Te_4)(Bi_2Te_3)}$$_n$ ($n$ = 1, 2,...), in which magnetic $\rm{MnBi_2Te_4}$ layers are separated by nonmagnetic $\rm{Bi_2Te_3}$ layers, hold band topology, magnetism and reduced interlayer coupling, providing a promising platform for the realization of exotic topological quantum states. However, their magnetism in the two-dimensional limit, which is crucial for further exploration of quantum phenomena, remains elusive. Here, complex ferromagnetic (FM)-antiferromagnetic (AFM) coexisting ground states that persist up to the 2-septuple layers (SLs) limit are observed and comprehensively investigated in $\rm{MnBi_4Te_7}$ ($n$ = 1) and $\rm{MnBi_6Te_{10}}$ ($n$ = 2). The ubiquitous Mn-Bi site mixing modifies or even changes the sign of the subtle inter-SL magnetic interactions, yielding a spatially inhomogeneous interlayer coupling. Further, a tunable exchange bias effect is observed in $\rm{(MnBi_2Te_4)(Bi_2Te_3)}$$_n$ ($n$ = 1, 2), arising from the coupling between the FM and AFM components in the ground state. Our work highlights a new approach toward the fine-tuning of magnetism and paves the way for further study of quantum phenomena in $\rm{(MnBi_2Te_4)(Bi_2Te_3)}$$_n$ ($n$ = 1, 2,...) as well as their magnetic applications. △ Less

Submitted 20 April, 2022; originally announced April 2022.

Comments: 9 pages, 4 figures

Evidence of Noncollinear Spin Texture in Magnetic Moiré Superlattices

Authors: Hongchao Xie, Xiangpeng Luo, Zhipeng Ye, Gaihua Ye, Haiwen Ge, Shaohua Yan, Yang Fu, Shangjie Tian, Hechang Lei, Kai Sun, Rui He, Liuyan Zhao

Abstract: Moiré magnetism, parallel with moiré electronics that has led to novel correlated and topological electronic states, emerges as a new venue to design and control exotic magnetic phases in twisted magnetic two-dimensional(2D) crystals. Here, we report direct evidence of noncollinear spin texture in 2D twisted double bilayer (tDB) magnet chromium triiodide (CrI$_3$). Using magneto-optical spectrosco… ▽ More Moiré magnetism, parallel with moiré electronics that has led to novel correlated and topological electronic states, emerges as a new venue to design and control exotic magnetic phases in twisted magnetic two-dimensional(2D) crystals. Here, we report direct evidence of noncollinear spin texture in 2D twisted double bilayer (tDB) magnet chromium triiodide (CrI$_3$). Using magneto-optical spectroscopy in tDB CrI$_3$, we revealed the presence of a net magnetization, unexpected from the composing antiferromagnetic bilayers with compensated magnetizations, and the emergence of noncollinear spins, originated from the moiré exchange coupling-induced spin frustrations. Exploring the twist angle dependence, we demonstrated that both features are present in tDB CrI$_3$ with twist angles from 0.5$^o$ to 5$^o$, but are most prominent in the 1.1$^o$ tDB CrI$_3$. Focusing on the temperature dependence of the 1.1$^o$ tDB CrI$_3$, we resolved the dramatic suppression in the net magnetization onset temperature and the significant softening of noncollinear spins, as a result of the moiré induced frustration. Our results demonstrate the power of moiré superlattices in introducing novel magnetic phenomena that are absent in natural 2D magnets. △ Less

Submitted 4 April, 2022; originally announced April 2022.

Emergence of electric-field-tunable interfacial ferromagnetism in 2D antiferromagnet heterostructures

Authors: Guanghui Cheng, Mohammad Mushfiqur Rahman, Zhiping He, Andres Llacsahuanga Allcca, Avinash Rustagi, Kirstine Aggerbeck Stampe, Yanglin Zhu, Shaohua Yan, Shangjie Tian, Zhiqiang Mao, Hechang Lei, Kenji Watanabe, Takashi Taniguchi, Pramey Upadhyaya, Yong P. Chen

Abstract: Van der Waals (vdW) magnet heterostructures have emerged as new platforms to explore exotic magnetic orders and quantum phenomena. Here, we study heterostructures of layered antiferromagnets, CrI3 and CrCl3, with perpendicular and in-plane magnetic anisotropy, respectively. Using magneto-optical Kerr effect microscopy, we demonstrate out-of-plane magnetic order in the CrCl3 layer proximal to CrI3,… ▽ More Van der Waals (vdW) magnet heterostructures have emerged as new platforms to explore exotic magnetic orders and quantum phenomena. Here, we study heterostructures of layered antiferromagnets, CrI3 and CrCl3, with perpendicular and in-plane magnetic anisotropy, respectively. Using magneto-optical Kerr effect microscopy, we demonstrate out-of-plane magnetic order in the CrCl3 layer proximal to CrI3, with ferromagnetic interfacial coupling between the two. Such an interlayer exchange field leads to higher critical temperature than that of either CrI3 or CrCl3 alone. We further demonstrate significant electric-field control of the coercivity, attributed to the naturally broken structural inversion symmetry of the heterostructure allowing unprecedented direct coupling between electric field and interfacial magnetism. These findings illustrate the opportunity to explore exotic magnetic phases and engineer spintronic devices in vdW heterostructures. △ Less

Submitted 2 August, 2024; v1 submitted 24 March, 2022; originally announced March 2022.

Journal ref: Nat. Commun. 13 (2022) 7348

Pressure-Induced Superconductivity and Structural Phase Transitions in Magnetic Topological Insulator Candidate MnSb4Te7

Authors: Cuiying Pei, Ming Xi, Qi Wang, Wujun Shi, Lingling Gao, Yi Zhao, Shangjie Tian, Weizheng Cao, Changhua Li, Mingxin Zhang, Shihao Zhu, Yulin Chen, Hechang Lei, Yanpeng Qi

Abstract: The magnetic van der Waals crystals (MnX2Te4)m(X2Te3)n (X = Sb, Bi) have drawn significant attention due to their rich topological properties and the tenability by external magnetic field. In this work, we report on the discovery of superconductivity in magnetic topological insulator candidate MnSb4Te7 (m = 1, n = 1) via the application of high pressure. The antiferromagnetic ordering is robust to… ▽ More The magnetic van der Waals crystals (MnX2Te4)m(X2Te3)n (X = Sb, Bi) have drawn significant attention due to their rich topological properties and the tenability by external magnetic field. In this work, we report on the discovery of superconductivity in magnetic topological insulator candidate MnSb4Te7 (m = 1, n = 1) via the application of high pressure. The antiferromagnetic ordering is robust to pressure until 8 GPa and then fully suppressed. The carrier type converts from hole- to electron-type accompanied with structural phase transition at around 15 GPa. Superconductivity emerges near the critical pressure 30 GPa where MnSb4Te7 converted into a simple cubic phase. Interestingly, MnSb4Te7 shows a dome-like phase diagram with a maximum Tc of 2.2 K at 50.7 GPa. The results demonstrate that MnSb4Te7 with nontrivial topology of electronic states display new ground states upon compression. △ Less

Submitted 19 January, 2022; originally announced January 2022.

Comments: 13 pages, 4 figures

Pressure induced superconductivity in WB2 and ReB2 through modifying the B layers

Authors: Cuiying Pei, Jianfeng Zhang, Chunsheng Gong, Qi Wang, Lingling Gao, Yi Zhao, Shangjie Tian, Weizheng Cao, Changhua Li, Zhong-Yi Lu, Hechang Lei, Kai Liu, Yanpeng Qi

Abstract: The recent discovery of superconductivity up to 32 K in the pressurized MoB2 reignites the interests in exploring high-Tc superconductors in transition-metal diborides. Inspired by that work, we turn our attention to the 5d transition-metal diborides. Here we systematically investigate the responses of both structural and physical properties of WB2 and ReB2 to external pressure, which possess diff… ▽ More The recent discovery of superconductivity up to 32 K in the pressurized MoB2 reignites the interests in exploring high-Tc superconductors in transition-metal diborides. Inspired by that work, we turn our attention to the 5d transition-metal diborides. Here we systematically investigate the responses of both structural and physical properties of WB2 and ReB2 to external pressure, which possess different types of boron layers. Similar to MoB2, the pressure-induced superconductivity was also observed in WB2 above 60 GPa with a maximum Tc of 15 K at 100 GPa, while no superconductivity was detected in ReB2 in this pressure range. Interestingly, the structures at ambient pressure for both WB2 and ReB2 persist to high pressure without structural phase transitions. Theoretical calculations suggest that the ratio of flat boron layers in this class of transition-metal diborides may be crucial for the appearance of high Tc. The combined theoretical and experimental results highlight the effect of geometry of boron layers on superconductivity and shed light on the exploration of novel high-Tc superconductors in borides. △ Less

Submitted 23 November, 2021; originally announced November 2021.

Comments: 17 pages,5 figures

Journal ref: Sci. China-Phys. Mech. Astron. 65, 287412 (2022)

Structures and physical properties of V-based kagome metals CsV$_{6}$Sb$_{6}$ and CsV$_{8}$Sb$_{12}$

Authors: Qiangwei Yin, Zhijun Tu, Chunsheng Gong, Shangjie Tian, Hechang Lei

Abstract: We report two new members of V-based kagome metals CsV$_{6}$Sb$_{6}$ and CsV$_{8}$Sb$_{12}$. The most striking structural feature of CsV$_{6}$Sb$_{6}$ is the V kagome bilayers. For CsV$_{8}$Sb$_{12}$, there is an intergrowth of two-dimensional V kagome layers and one-dimensional V chains and the latter lead to the orthorhombic symmetry of this material. Further measurements indicate that these two… ▽ More We report two new members of V-based kagome metals CsV$_{6}$Sb$_{6}$ and CsV$_{8}$Sb$_{12}$. The most striking structural feature of CsV$_{6}$Sb$_{6}$ is the V kagome bilayers. For CsV$_{8}$Sb$_{12}$, there is an intergrowth of two-dimensional V kagome layers and one-dimensional V chains and the latter lead to the orthorhombic symmetry of this material. Further measurements indicate that these two materials exhibit metallic and Pauli paramagnetic behaviors. More importantly, different from CsV$_{3}$Sb$_{5}$, the charge density wave state and superconductivity do not emerge in CsV$_{6}$Sb$_{6}$ and CsV$_{8}$Sb$_{12}$ when temperature is above 2 K. Small magnetoresistance with saturation behavior and linear field dependence of Hall resistivity at high field and low temperature suggest that the carriers in both materials should be uncompensated with much different concentrations. The discovery of these two new V-based kagome metals sheds light on the exploration of correlated topological materials based on kagome lattice. △ Less

Submitted 22 November, 2021; v1 submitted 21 October, 2021; originally announced October 2021.

Comments: 7 pages, 4 figures, 2 tables

Journal ref: Chin. Phys. Lett. 38, 127401 (2021)

Chirality locking charge density waves in a chiral crystal

Authors: Geng Li, Haitao Yang, Peijie Jiang, Cong Wang, Qiuzhen Cheng, Shangjie Tian, Guangyuan Han, Hechang Lei, Chengmin Shen, Xiao Lin, Wei Ji, Ziqiang Wang, Hong-Jun Gao

Abstract: In Weyl semimetals, charge density wave (CDW) order can spontaneously break the chiral symmetry, gap out the Weyl nodes, and drive the material into the axion insulating phase. Investigations have however been limited since CDWs are rarely seen in Weyl semimetals. Here, using scanning tunneling microscopy/spectroscopy, we report the discovery of a novel unidirectional CDW order on the (001) surfac… ▽ More In Weyl semimetals, charge density wave (CDW) order can spontaneously break the chiral symmetry, gap out the Weyl nodes, and drive the material into the axion insulating phase. Investigations have however been limited since CDWs are rarely seen in Weyl semimetals. Here, using scanning tunneling microscopy/spectroscopy, we report the discovery of a novel unidirectional CDW order on the (001) surface of chiral crystal CoSi - a unique Weyl semimetal with unconventional chiral fermions. The CDW is incommensurate with both lattice momentum and crystalline symmetry directions, and exhibits an intra unit cell π phase shift in the layer stacking direction. The tunneling spectrum shows a particle-hole asymmetric V-shaped energy gap around the Fermi level that modulates spatially with the CDW wave vector. Combined with first-principle calculations, we identify that the CDW is locked to the crystal chirality and is related by a mirror reflection between the two enantiomers of the chiral crystal. Our findings reveal a novel correlated topological quantum state in chiral CoSi crystals and raise the potential for realizing an axion insulator and exploring the unprecedented physical behaviors of unconventional chiral fermions. △ Less

Submitted 19 October, 2021; v1 submitted 14 October, 2021; originally announced October 2021.

Enhancement of spin-orbit coupling and magnetic scattering in hydrogenated graphene

Authors: Shimin Cao, Chuanwu Cao, Shibing Tian, Jian-Hao Chen

Abstract: Spin-orbit coupling (SOC) can provide essential tools to manipulate electron spins in two-dimensional materials like graphene, which is of great interest for both fundamental physics and spintronics application. In this paper, we report the low-field magnetotransport of in situ hydrogenated graphene where hydrogen atoms are attached to the graphene surface in continuous low temperature and vacuum… ▽ More Spin-orbit coupling (SOC) can provide essential tools to manipulate electron spins in two-dimensional materials like graphene, which is of great interest for both fundamental physics and spintronics application. In this paper, we report the low-field magnetotransport of in situ hydrogenated graphene where hydrogen atoms are attached to the graphene surface in continuous low temperature and vacuum environment. Transition from weak localization to weak antilocalization with increasing hydrogen adatom density is observed, indicating enhancing Bychkov-Rashba-type SOC in a mirror symmetry broken system. From the low-temperature saturation of phase breaking scattering rate, the existence of spin-flip scattering is identified, which corroborates the existence of magnetic moments in hydrogenated graphene. △ Less

Submitted 9 October, 2021; originally announced October 2021.

Journal ref: Phys. Rev. B 104, 125422(2021)

Electrically Switchable van der Waals Magnon Valves

Authors: Guangyi Chen, Shaomian Qi, Jianqiao Liu, Di Chen, Jiongjie Wang, Shili Yan, Yu Zhang, Shimin Cao, Ming Lu, Shibing Tian, Kangyao Chen, Peng Yu, Zheng Liu, X. C. Xie, Jiang Xiao, Ryuichi Shindou, Jian-Hao Chen

Abstract: Van der Waals magnets have emerged as a fertile ground for the exploration of highly tunable spin physics and spin-related technology. Two-dimensional (2D) magnons in van der Waals magnets are collective excitation of spins under strong confinement. Although considerable progress has been made in understanding 2D magnons, a crucial magnon device called the van der Waals magnon valve, in which the… ▽ More Van der Waals magnets have emerged as a fertile ground for the exploration of highly tunable spin physics and spin-related technology. Two-dimensional (2D) magnons in van der Waals magnets are collective excitation of spins under strong confinement. Although considerable progress has been made in understanding 2D magnons, a crucial magnon device called the van der Waals magnon valve, in which the magnon signal can be completely and repeatedly turned on and off electrically, has yet to be realized. Here we demonstrate such magnon valves based on van der Waals antiferromagnetic insulator MnPS3. By applying DC electric current through the gate electrode, we show that the second harmonic thermal magnon (SHM) signal can be tuned from positive to negative. The guaranteed zero crossing during this tuning demonstrates a complete blocking of SHM transmission, arising from the nonlinear gate dependence of the non-equilibrium magnon density in the 2D spin channel. Using the switchable magnon valves we demonstrate a magnon-based inverter. These results illustrate the potential of van der Waals anti-ferromagnets for studying highly tunable spin-wave physics and for application in magnon-base circuitry in future information technology. △ Less

Submitted 1 October, 2021; originally announced October 2021.

Comments: 18 pages, 4 figures

Journal ref: Nat. Commun. 12, 6279 (2021)

Pressure-induced superconductivity in itinerant antiferromagnet CrB2

Authors: Cuiying Pei, Pengtao Yang, Chunsheng Gong, Qi Wang, Yi Zhao, Lingling Gao, Keyu Chen, Qiangwei Yin, Shangjie Tian, Changhua Li, Weizheng Cao, Hechang Lei, Jinguang Cheng, Yanpeng Qi

Abstract: The recent discovery of superconductivity up to 32 K in the pressurized MoB2 revives the interests in exploring novel superconductors in transition-metal diborides isostructural to MgB2. Although the Mo 4d-electrons participate in Cooper pairing, the electron-phonon coupling remains the dominant mechanism for the emergence of superconductivity in MoB2. To explore possible superconductivity driven… ▽ More The recent discovery of superconductivity up to 32 K in the pressurized MoB2 revives the interests in exploring novel superconductors in transition-metal diborides isostructural to MgB2. Although the Mo 4d-electrons participate in Cooper pairing, the electron-phonon coupling remains the dominant mechanism for the emergence of superconductivity in MoB2. To explore possible superconductivity driven by unconventional pairing mechanism, we turn our attention to an itinerant antiferromagnet CrB2. Here we report on the discovery of superconductivity up to 7 K in CrB2 via the application of external pressure. Superconductivity is observed after the antiferromagnetic transition at TN ~ 88 K under ambient pressure is completely suppressed. Then, the superconducting Tc increases monotonically with pressure and this evolution takes place without a structural transition in the sample. Since the proximity of superconductivity to an antiferromagnetic order, the quantum criticality and unconventional superconductivity may exist in CrB2. Current study would promote further studies and explorations of novel superconductors in other transition-metal diborides. △ Less

Submitted 30 September, 2021; originally announced September 2021.

Comments: 11 pages, 4 figures

Spin-flip-driven giant magneto-transport in A-type antiferromagnet NaCrTe2

Authors: Junjie Wang, Jun Deng, Xiaowei Liang, Guoying Gao, Tianping Ying, Shangjie Tian, Hechang Lei, Yanpeng Song, Xu Chen, Jian-gang Guo, Xiaolong Chen

Abstract: For anisotropic magneto-resistance (AMR) effect, its value synergistically depends on the magnitudes of magneto-resistance (MR) and magneto-crystalline anisotropy energy (MAE) simultaneously. In a magnetic material, the concurrence of gigantic AMR and MR signals is rather difficult due to weak spin-lattice coupling and small MAE. Here we report the considerable magneto-transport effect in layered… ▽ More For anisotropic magneto-resistance (AMR) effect, its value synergistically depends on the magnitudes of magneto-resistance (MR) and magneto-crystalline anisotropy energy (MAE) simultaneously. In a magnetic material, the concurrence of gigantic AMR and MR signals is rather difficult due to weak spin-lattice coupling and small MAE. Here we report the considerable magneto-transport effect in layered A-type antiferromagnetic (AFM) NaCrTe2 by realigning the spin configurations. By applying H, the antiparallel spins of adjacent layers are flipped to ferromagnetic (FM) coupling either Ising-type along c-axis or XY-type within ab-plane. Theoretical calculations reveal that the energy bandgap narrows from 0.39 eV to 0.11 eV, accompanying a transition from semiconductor (high-R state) and half-semiconductor (low-R state), respectively. Thus, gigantic negative MR ratio of -90% is obtained at 10 K. More importantly, the decrement of R along H//c is far quicker than that of H//ab because the MAE of Ising-FM state is 1017 μeV/Cr3+ lower than that of XY-FM. The distinct trends result in the AMR ratio of 732% at 10 K, which is the record value to our best knowledge. These findings unravel the intrinsic origin of magneto in NaCrTe2 and will stimulate us to exploring the H-sensitive transport property in more AFM materials. △ Less

Submitted 30 September, 2021; originally announced September 2021.

Comments: 16 pages, 4 figures

Journal ref: Phys. Rev. Materials 5, L091401(2021)

Superconductivity in kagome metal YRu3Si2 with strong electron correlations

Authors: Chunsheng Gong, Shangjie Tian, Zhijun Tu, Qiangwei Yin, Yang Fu, Ruitao Luo, Hechang Lei

Abstract: We report the detailed physical properties of YRu3Si2 with the Ru kagome lattice at normal and superconducting states. The results of resistivity and magnetization show that YRu3Si2 is a type-II bulk superconductor with Tc ~ 3.0 K. The specific heat measurement further suggests that this superconductivity could originate from the weak or moderate electron-phonon coupling. On the other hand, both l… ▽ More We report the detailed physical properties of YRu3Si2 with the Ru kagome lattice at normal and superconducting states. The results of resistivity and magnetization show that YRu3Si2 is a type-II bulk superconductor with Tc ~ 3.0 K. The specific heat measurement further suggests that this superconductivity could originate from the weak or moderate electron-phonon coupling. On the other hand, both large Kadawaki-Woods ratio and Wilson ratio indicate that there is a strong electron correlation effect in this system, which may have a connection with the featured flat band of kagome lattice. △ Less

Submitted 10 October, 2025; v1 submitted 23 September, 2021; originally announced September 2021.

Comments: 6 pages, 5 figures, 1 table

Journal ref: Chin. Phys. Lett. 39, 087401 (2022)

Enhanced Thermal Transport across the Interface between Charged Graphene Electrodes and Poly(ethylene oxide) Electrolytes by Non-covalent Functionalization

Authors: Siyu Tian, Dezhao Huang, Zhihao Xu, Shiwen Wu, Tengfei Luo, Guoping Xiong

Abstract: Interfacial thermal transport between electrodes and polymer electrolytes can play a crucial role in the thermal management of solid-state lithium-ion batteries (SLIBs). Modifying the electrode surface with functional molecules can effectively increase the interfacial thermal conductance (ITC) between electrodes and polymers (e.g., electrolytes, separators); however, how they influence the interfa… ▽ More Interfacial thermal transport between electrodes and polymer electrolytes can play a crucial role in the thermal management of solid-state lithium-ion batteries (SLIBs). Modifying the electrode surface with functional molecules can effectively increase the interfacial thermal conductance (ITC) between electrodes and polymers (e.g., electrolytes, separators); however, how they influence the interfacial thermal transport in SLIBs during charge/discharge remains unknown. In this work, we conduct molecular dynamics (MD) simulations to investigate the ITC between charged electrodes and solid-state polymer electrolytes (SPEs) mixed with ionic liquids (ILs). We find that ILs could self assemble at the electrode surface and act as non-covalent functional molecules that could significantly enhance the interfacial thermal transport during charge/discharge because of the formation of a densely packed cationic or anionic layer at the interface. While the electrostatic interactions between the charged electrode and the IL ions are responsible for forming these dense interfacial layers, the enhancement of ITC is mainly contributed by the increased Lennard-Jones (LJ) interactions between the charged electrodes and ILs. This work may provide useful insights into the understanding of interfacial thermal transport between electrodes and electrolytes of SLIBs during charge/discharge. △ Less

Submitted 20 July, 2021; originally announced July 2021.

Structural Monoclinicity and Its Coupling to Layered Magnetism in Few-Layer $\mathrm{CrI_{3}}$

Authors: Xiaoyu Guo, Wencan Jin, Zhipeng Ye, Gaihua Ye, Hongchao Xie, Bowen Yang, Hyun Ho Kim, Shaohua Yan, Yang Fu, Shangjie Tian, Hechang Lei, Adam W. Tsen, Kai Sun, Jia-An Yan, Rui He, Liuyan Zhao

Abstract: Using polarization-resolved Raman spectroscopy, we investigate layer number, temperature, and magnetic field dependence of Raman spectra in one- to four-layer $\mathrm{CrI_{3}}$. Layer-number-dependent Raman spectra show that in the paramagnetic phase a doubly degenerated $E_{g}$ mode of monolayer $\mathrm{CrI_{3}}$ splits into one $A_{g}$ and one $B_{g}$ mode in N-layer (N > 1)… ▽ More Using polarization-resolved Raman spectroscopy, we investigate layer number, temperature, and magnetic field dependence of Raman spectra in one- to four-layer $\mathrm{CrI_{3}}$. Layer-number-dependent Raman spectra show that in the paramagnetic phase a doubly degenerated $E_{g}$ mode of monolayer $\mathrm{CrI_{3}}$ splits into one $A_{g}$ and one $B_{g}$ mode in N-layer (N > 1) $\mathrm{CrI_{3}}$ due to the monoclinic stacking. Their energy separation increases in thicker samples until an eventual saturation. Temperature-dependent measurements further show that the split modes tend to merge upon cooling but remain separated until 10 K, indicating a failed attempt of the monoclinic-to-rhombohedral structural phase transition that is present in the bulk crystal. Magnetic-field-dependent measurements reveal an additional monoclinic distortion across the magnetic-field-induced layered antiferromagnetism-to-ferromagnetism phase transition. We propose a structural change that consists of both a lateral sliding toward the rhombohedral stacking and a decrease in the interlayer distance to explain our experimental observations. △ Less

Submitted 4 June, 2021; originally announced June 2021.

Comments: The article was published in ACS Nano on June 2

Pressure-induced Superconductivity at 32 K in MoB2

Authors: Cuiying Pei, Jianfeng Zhang, Qi Wang, Yi Zhao, Lingling Gao, Chunsheng Gong, Shangjie Tian, Ruitao Luo, Zhong-Yi Lu, Hechang Lei, Kai Liu, Yanpeng Qi

Abstract: Since the discovery of superconductivity in MgB2 (Tc ~ 39 K), the search for superconductivity in related materials with similar structures or ingredients has never stopped. Although about 100 binary borides have been explored, only few of them show superconductivity with relatively low Tc. In this work, we report the discovery of superconductivity up to 32 K in MoB2 under pressure which is the hi… ▽ More Since the discovery of superconductivity in MgB2 (Tc ~ 39 K), the search for superconductivity in related materials with similar structures or ingredients has never stopped. Although about 100 binary borides have been explored, only few of them show superconductivity with relatively low Tc. In this work, we report the discovery of superconductivity up to 32 K in MoB2 under pressure which is the highest Tc in transition-metal borides. Although the Tc can be well explained by theoretical calculations in the framework of electron-phonon coupling, the d-electrons and phonon modes of transition metal Mo atoms play utterly important roles in the emergence of superconductivity in MoB2, distinctly different from the case of well-known MgB2. Our study sheds light on the exploration of high-Tc superconductors in transition metal borides. △ Less

Submitted 27 May, 2021; originally announced May 2021.

Comments: 13 pages,4 figures,1 table

Journal ref: National Science Review, 2023, Volume 10, nwad034

Twist engineering of the two-dimensional magnetism in double bilayer chromium triiodide homostructures

Authors: Hongchao Xie, Xiangpeng Luo, Gaihua Ye, Zhipeng Ye, Haiwen Ge, Suk Hyun Sung, Emily Rennich, Shaohua Yan, Yang Fu, Shangjie Tian, Hechang Lei, Robert Hovden, Kai Sun, Rui He, Liuyan Zhao

Abstract: Twist engineering, or the alignment of two-dimensional (2D) crystalline layers with desired orientations, has led to tremendous success in modulating the charge degree of freedom in hetero- and homo-structures, in particular, in achieving novel correlated and topological electronic phases in moiré electronic crystals. However, although pioneering theoretical efforts have predicted nontrivial magne… ▽ More Twist engineering, or the alignment of two-dimensional (2D) crystalline layers with desired orientations, has led to tremendous success in modulating the charge degree of freedom in hetero- and homo-structures, in particular, in achieving novel correlated and topological electronic phases in moiré electronic crystals. However, although pioneering theoretical efforts have predicted nontrivial magnetism and magnons out of twisting 2D magnets, experimental realization of twist engineering spin degree of freedom remains elusive. Here, we leverage the archetypal 2D Ising magnet chromium triiodide (CrI3) to fabricate twisted double bilayer homostructures with tunable twist angles and demonstrate the successful twist engineering of 2D magnetism in them. Using linear and circular polarization-resolved Raman spectroscopy, we identify magneto-Raman signatures of a new magnetic ground state that is sharply distinct from those in natural bilayer (2L) and four-layer (4L) CrI3. With careful magnetic field and twist angle dependence, we reveal that, for a very small twist angle (~ 0.5 degree), this emergent magnetism can be well-approximated by a weighted linear superposition of those of 2L and 4L CI3 whereas, for a relatively large twist angle (~ 5 degree), it mostly resembles that of isolated 2L CrI3. Remarkably, at an intermediate twist angle (~ 1.1 degree), its magnetism cannot be simply inferred from the 2L and 4L cases, because it lacks sharp spin-flip transitions that are present in 2L and 4L CrI3 and features a dramatic Raman circular dichroism that is absent in natural 2L and 4L ones. Our results demonstrate the possibility of designing and controlling the spin degree of freedom in 2D magnets using twist engineering. △ Less

Submitted 11 April, 2021; v1 submitted 24 March, 2021; originally announced March 2021.

Machine learning and high-throughput robust design of P3HT-CNT composite thin films for high electrical conductivity

Authors: Daniil Bash, Yongqiang Cai, Vijila Chellappan, Swee Liang Wong, Yang Xu, Pawan Kumar, Jin Da Tan, Anas Abutaha, Jayce Cheng, Yee Fun Lim, Siyu Tian, Danny Zekun Ren, Flore Mekki-Barrada, Wai Kuan Wong, Jatin Kumar, Saif Khan, Qianxiao Li, Tonio Buonassisi, Kedar Hippalgaonkar

Abstract: Combining high-throughput experiments with machine learning allows quick optimization of parameter spaces towards achieving target properties. In this study, we demonstrate that machine learning, combined with multi-labeled datasets, can additionally be used for scientific understanding and hypothesis testing. We introduce an automated flow system with high-throughput drop-casting for thin film pr… ▽ More Combining high-throughput experiments with machine learning allows quick optimization of parameter spaces towards achieving target properties. In this study, we demonstrate that machine learning, combined with multi-labeled datasets, can additionally be used for scientific understanding and hypothesis testing. We introduce an automated flow system with high-throughput drop-casting for thin film preparation, followed by fast characterization of optical and electrical properties, with the capability to complete one cycle of learning of fully labeled ~160 samples in a single day. We combine regio-regular poly-3-hexylthiophene with various carbon nanotubes to achieve electrical conductivities as high as 1200 S/cm. Interestingly, a non-intuitive local optimum emerges when 10% of double-walled carbon nanotubes are added with long single wall carbon nanotubes, where the conductivity is seen to be as high as 700 S/cm, which we subsequently explain with high fidelity optical characterization. Employing dataset resampling strategies and graph-based regressions allows us to account for experimental cost and uncertainty estimation of correlated multi-outputs, and supports the proving of the hypothesis linking charge delocalization to electrical conductivity. We therefore present a robust machine-learning driven high-throughput experimental scheme that can be applied to optimize and understand properties of composites, or hybrid organic-inorganic materials. △ Less

Submitted 20 November, 2020; originally announced November 2020.

Comments: 10 pages, 5 figures, includes Supplementary Information

How machine learning can help the design and analysis of composite materials and structures?

Authors: Xin Liu, Su Tian, Fei Tao, Haodong Du, Wenbin Yu

Abstract: Machine learning models are increasingly used in many engineering fields thanks to the widespread digital data, growing computing power, and advanced algorithms. Artificial neural networks (ANN) is the most popular machine learning model in recent years. Although many ANN models have been used in the design and analysis of composite materials and structures, there are still some unsolved issues th… ▽ More Machine learning models are increasingly used in many engineering fields thanks to the widespread digital data, growing computing power, and advanced algorithms. Artificial neural networks (ANN) is the most popular machine learning model in recent years. Although many ANN models have been used in the design and analysis of composite materials and structures, there are still some unsolved issues that hinder the acceptance of ANN models in the practical design and analysis of composite materials and structures. Moreover, the emerging machine learning techniques are posting new opportunities and challenges in the data-based design paradigm. This paper aims to give a state-of-the-art literature review of ANN models in the nonlinear constitutive modeling, multiscale surrogate modeling, and design optimization of composite materials and structures. This review has been designed to focus on the discussion of the general frameworks and benefits of ANN models to the above problems. Moreover, challenges and opportunities in each key problem are identified and discussed. This paper is expected to open the discussion of future research scope and new directions to enable efficient, robust, and accurate data-driven design and analysis of composite materials and structures. △ Less

Submitted 14 October, 2020; originally announced October 2020.

Tunable layered-magnetism-assisted magneto-Raman effect in a two-dimensional magnet $\mathrm{CrI_3}$

Authors: Wencan Jin, Zhipeng Ye, Xiangpeng Luo, Bowen Yang, Gaihua Ye, Fangzhou Yin, Hyun Ho Kim, Laura Rojas, Shangjie Tian, Yang Fu, Shaohua Yan, Hechang Lei, Kai Sun, Adam W. Tsen, Rui He, Liuyan Zhao

Abstract: We use a combination of polarized Raman spectroscopy experiment and model magnetism-phonon coupling calculations to study the rich magneto-Raman effect in the two-dimensional (2D) magnet $\mathrm{CrI_3}$. We reveal a novel layered-magnetism-assisted phonon scattering mechanism below the magnetic onset temperature, whose Raman excitation breaks time-reversal symmetry, has an antisymmetric Raman ten… ▽ More We use a combination of polarized Raman spectroscopy experiment and model magnetism-phonon coupling calculations to study the rich magneto-Raman effect in the two-dimensional (2D) magnet $\mathrm{CrI_3}$. We reveal a novel layered-magnetism-assisted phonon scattering mechanism below the magnetic onset temperature, whose Raman excitation breaks time-reversal symmetry, has an antisymmetric Raman tensor, and follows the magnetic phase transitions across critical magnetic fields, on top of the presence of the conventional phonon scattering with symmetric Raman tensors in $N$-layer $\mathrm{CrI_3}$. We resolve in data and by calculations that the 1st-order $A_g$ phonon of monolayer splits into a $N$-fold multiplet in $N$-layer $\mathrm{CrI_3}$ due to the interlayer coupling ($N$>=2) and that the phonons with the multiple show distinct magnetic field dependence because of their different layered-magnetism-phonon coupling. We further find that such a layered-magnetism-phonon coupled Raman scattering mechanism extends beyond 1st-order to higher-order multi-phonon scattering processes. Our results on magneto-Raman effect of the 1st-order phonons in the multiplet and the higher-order multi-phonons in $N$-layer $\mathrm{CrI_3}$ demonstrate the rich and strong behavior of emergent magneto-optical effects in 2D magnets and underlines the unique opportunities of new spin-phonon physics in van der Waals layered magnets. △ Less

Submitted 23 September, 2020; originally announced September 2020.

Comments: 16 pages, 4 figures + supplementary material

An invertible crystallographic representation for general inverse design of inorganic crystals with targeted properties

Authors: Zekun Ren, Siyu Isaac Parker Tian, Juhwan Noh, Felipe Oviedo, Guangzong Xing, Jiali Li, Qiaohao Liang, Ruiming Zhu, Armin G. Aberle, Shijing Sun, Xiaonan Wang, Yi Liu, Qianxiao Li, Senthilnath Jayavelu, Kedar Hippalgaonkar, Yousung Jung, Tonio Buonassisi

Abstract: Realizing general inverse design could greatly accelerate the discovery of new materials with user-defined properties. However, state-of-the-art generative models tend to be limited to a specific composition or crystal structure. Herein, we present a framework capable of general inverse design (not limited to a given set of elements or crystal structures), featuring a generalized invertible repres… ▽ More Realizing general inverse design could greatly accelerate the discovery of new materials with user-defined properties. However, state-of-the-art generative models tend to be limited to a specific composition or crystal structure. Herein, we present a framework capable of general inverse design (not limited to a given set of elements or crystal structures), featuring a generalized invertible representation that encodes crystals in both real and reciprocal space, and a property-structured latent space from a variational autoencoder (VAE). In three design cases, the framework generates 142 new crystals with user-defined formation energies, bandgap, thermoelectric (TE) power factor, and combinations thereof. These generated crystals, absent in the training database, are validated by first-principles calculations. The success rates (number of first-principles-validated target-satisfying crystals/number of designed crystals) ranges between 7.1% and 38.9%. These results represent a significant step toward property-driven general inverse design using generative models, although practical challenges remain when coupled with experimental synthesis. △ Less

Submitted 15 December, 2021; v1 submitted 15 May, 2020; originally announced May 2020.

Evidence of tunable magnetic coupling in hydrogenated graphene

Authors: Shimin Cao, Chuanwu Cao, Shibing Tian, Jian-Hao Chen

Abstract: A lot of efforts have been devoted to understanding the origin and effects of magnetic moments induced in graphene with carbon atom vacancy, or light adatoms like hydrogen or fluorine. At the meantime, the large negative magnetoresistance (MR) widely observed in these systems is not well understood, nor had it been associated with the presence of magnetic moments. In this paper, we study the syste… ▽ More A lot of efforts have been devoted to understanding the origin and effects of magnetic moments induced in graphene with carbon atom vacancy, or light adatoms like hydrogen or fluorine. At the meantime, the large negative magnetoresistance (MR) widely observed in these systems is not well understood, nor had it been associated with the presence of magnetic moments. In this paper, we study the systematic evolution of the large negative MR of in-situ hydrogenated graphene in ultra-high vacuum (UHV) environment. We find for most combination of electron density ($n_e$) and hydrogen density ($n_H$), MR at different temperature can be scaled to $α=(μ_BB)/[k_B(T-T^*)]$, where $T^*$ is the Curie-Weiss temperature. The sign of $T^*$ indicates the existence of tunable ferromagnetic-like ($T^* >0$) and anti-ferromagnetic-like ($T^* <0$) coupling in hydrogenated graphene. However, the lack of hysteresis of MR or anomalous Hall effect below $|T^*|$ points to the fact that long-range magnetic order did not emerge, which we attribute to the competition of different magnetic orders and disordered arrangement of magnetic moments on graphene. We also find that localized impurity states introduced by H adatoms could modify the capacitance of hydrogenated graphene. This work provides a new way to extract information from large negative MR behavior and can be a key to help understanding interactions of magnetic moments in graphene. △ Less

Submitted 21 March, 2020; originally announced March 2020.

Journal ref: Phys. Rev. B 102, 045402 (2020)

Imaging domain reversal in an ultrathin van der Waals ferromagnet

Authors: David A. Broadway, Sam C. Scholten, Cheng Tan, Nikolai Dontschuk, Scott E. Lillie, Brett C. Johnson, Guolin Zheng, Zhenhai Wang, Artem R. Oganov, Shangjie Tian, Chenghe Li, Hechang Lei, Lan Wang, Lloyd C. L. Hollenberg, Jean-Philippe Tetienne

Abstract: The recent isolation of two-dimensional van der Waals magnetic materials has uncovered rich physics that often differs from the magnetic behaviour of their bulk counterparts. However, the microscopic details of fundamental processes such as the initial magnetization or domain reversal, which govern the magnetic hysteresis, remain largely unknown in the ultrathin limit. Here we employ a widefield n… ▽ More The recent isolation of two-dimensional van der Waals magnetic materials has uncovered rich physics that often differs from the magnetic behaviour of their bulk counterparts. However, the microscopic details of fundamental processes such as the initial magnetization or domain reversal, which govern the magnetic hysteresis, remain largely unknown in the ultrathin limit. Here we employ a widefield nitrogen-vacancy (NV) microscope to directly image these processes in few-layer flakes of magnetic semiconductor vanadium triiodide (VI$_3$). We observe complete and abrupt switching of most flakes at fields $H_c\approx0.5-1$ T (at 5 K) independent of thickness down to two atomic layers, with no intermediate partially-reversed state. The coercive field decreases as the temperature approaches the Curie temperature ($T_c\approx50$ K), however, the switching remains abrupt. We then image the initial magnetization process, which reveals thickness-dependent domain wall depinning fields well below $H_c$. These results point to ultrathin VI$_3$ being a nucleation-type hard ferromagnet, where the coercive field is set by the anisotropy-limited domain wall nucleation field. This work illustrates the power of widefield NV microscopy to investigate magnetization processes in van der Waals ferromagnets, which could be used to elucidate the origin of the hard ferromagnetic properties of other materials and explore field- and current-driven domain wall dynamics. △ Less

Submitted 18 March, 2020; originally announced March 2020.

Comments: includes SI

Journal ref: Adv. Mater. 32, 2003314 (2020)

Observation of the polaronic character of excitons in a two-dimensional semiconducting magnet $\mathrm{CrI_3}$

Authors: Wencan Jin, Hyun Ho Kim, Zhipeng Ye, Gaihua Ye, Laura Rojas, Xiangpeng Luo, Bowen Yang, Fangzhou Yin, Jason Shih An Horng, Shangjie Tian, Yang Fu, Gongjun Xu, Hui Deng, Hechang Lei, Kai Sun, Adam W. Tsen, Rui He, Liuyan Zhao

Abstract: Exciton dynamics can be strongly affected by lattice vibrations through electron-phonon coupling. This is rarely explored in two-dimensional magnetic semiconductors. Focusing on bilayer CrI3, we first show the presence of strong electron-phonon coupling through temperature-dependent photoluminescence and absorption spectroscopy. We then report the observation of periodic broad modes up to the 8th… ▽ More Exciton dynamics can be strongly affected by lattice vibrations through electron-phonon coupling. This is rarely explored in two-dimensional magnetic semiconductors. Focusing on bilayer CrI3, we first show the presence of strong electron-phonon coupling through temperature-dependent photoluminescence and absorption spectroscopy. We then report the observation of periodic broad modes up to the 8th order in Raman spectra, attributed to the polaronic character of excitons. We establish that this polaronic character is dominated by the coupling between the charge-transfer exciton at 1.96 eV and a longitudinal optical phonon at 120.6 cm-1. We further show that the emergence of long-range magnetic order enhances the electron-phonon coupling strength by about 50$\%$ and that the transition from layered antiferromagnetic to ferromagnetic order tunes the spectral intensity of the periodic broad modes, suggesting a strong coupling among the lattice, charge and spin in two-dimensional CrI3. Our study opens opportunities for tailoring light-matter interactions in two-dimensional magnetic semiconductors. △ Less

Submitted 22 September, 2020; v1 submitted 28 February, 2020; originally announced March 2020.

Comments: 21 pages, 4 figures + Supplementary Information

Journal ref: Nature Communications 11, 4780 (2020)