Showing 1–50 of 109 results for author: Fu, Z

Ripple-assisted adsorption of noble gases on graphene at room temperature

Authors: Weilin Liu, Xianlei Huang, Li-Guo Dou, Qianglong Fang, Ang Li, Guowen Yuan, Yongjie Xu, Zhenjia Zhou, Jun Li, Yu Jiang, Zichong Huang, Zihao Fu, Peng-Xiang Hou, Chang Liu, Jinlan Wang, Wu Zhou, Ming-Gang Ju, Shao-Chun Li, Hui-Ming Cheng, Libo Gao

Abstract: Controllable gas adsorption is critical for both scientific and industrial fields, and high-capacity adsorption of gases on solid surfaces provides a significant promise due to its high-safety and low-energy consumption. However, the adsorption of nonpolar gases, particularly noble gases, poses a considerable challenge under atmospheric pressure and room temperature (RT). Here, we theoretically si… ▽ More Controllable gas adsorption is critical for both scientific and industrial fields, and high-capacity adsorption of gases on solid surfaces provides a significant promise due to its high-safety and low-energy consumption. However, the adsorption of nonpolar gases, particularly noble gases, poses a considerable challenge under atmospheric pressure and room temperature (RT). Here, we theoretically simulate and experimentally realize the stable adsorption of noble gases like xenon (Xe), krypton (Kr), argon (Ar), and helium (He) on highly rippled graphene at RT. The elemental characteristics of adsorbed Xe are confirmed by electron energy loss spectroscopy and X-ray photoelectron spectroscopy. The adsorbed gas atoms are crystalized with periodic arrangements. These adsorbed noble gases on graphene exhibit high stability at RT and can be completely desorbed at approximately 350 °C without damaging the intrinsic lattice of graphene. The structural and physical properties of graphene are significantly influenced by the adsorbed gas, and they fully recover after desorption. Additionally, this controllable adsorption could be generalized to other layered adsorbents such as NbSe2, MoS2 and carbon nanotubes. We anticipate that this ripple-assisted adsorption will not only re-define the theoretical framework of gas adsorption, but also accelerate advancements in gas storage and separation technologies, as well as enhance the applications in catalysis, surface modification, and other related fields. △ Less

Submitted 13 November, 2025; originally announced November 2025.

Large-scale stochastic propagation method beyond the sequential approach

Authors: Zhichang Fu, Yunhai Li, Weiqing Zhou, Shengjun Yuan

Abstract: The $O(N)$ stochastic propagation method, which relies on the numerical solution of the time-dependent Schrödinger equation using random initial states, is widely used in large-scale first-principles calculations. In this work, we eliminate the conventional sequential computation of intermediate states by introducing a concurrent strategy that minimizes information redundancy. The new method, in i… ▽ More The $O(N)$ stochastic propagation method, which relies on the numerical solution of the time-dependent Schrödinger equation using random initial states, is widely used in large-scale first-principles calculations. In this work, we eliminate the conventional sequential computation of intermediate states by introducing a concurrent strategy that minimizes information redundancy. The new method, in its state-, moment-, and energy-based implementations, not only surpasses the time step constraint of sequential propagation but also maintains precision within the framework of the Nyquist-Shannon sampling theorem. Systematic benchmarking on one billion atoms within the tight-binding model demonstrates that our new concurrent method achieves up to an order-of-magnitude speedup, enabling the rapid computation of a wide range of electronic, optical, and transport properties. This performance breakthrough offers valuable insights for enhancing other time-propagation algorithms, including those employed in large-scale stochastic density functional theory. △ Less

Submitted 21 October, 2025; v1 submitted 20 October, 2025; originally announced October 2025.

Surface band-selective moiré effect induces flat band in mixed-dimensional heterostructures

Authors: Shuming Yu, Zhentao Fu, Dingkun Qin, Enting Li, Hao Zhong, Xingzhe Wang, Keming Zhao, Shangkun Mo, Qiang Wan, Yiwei Li, Jie Li, Jianxin Zhong, Hong Ding, Nan Xu

Abstract: In this work, we reveal a curious type of moiré effect that selectively modifies the surface states of bulk crystal. We synthesize mixed-dimensional heterostructures consisting of a noble gas monolayer grow on the surface of bulk Bi(111), and determine the electronic structure of the heterostructures using angle-resolved photoemission spectroscopy. We directly observe moiré replicas of the Bi(111)… ▽ More In this work, we reveal a curious type of moiré effect that selectively modifies the surface states of bulk crystal. We synthesize mixed-dimensional heterostructures consisting of a noble gas monolayer grow on the surface of bulk Bi(111), and determine the electronic structure of the heterostructures using angle-resolved photoemission spectroscopy. We directly observe moiré replicas of the Bi(111) surface states, while the bulk states remain barely changed. Meanwhile, we achieve control over the moiré period in the range of 25 Å to 80 Å by selecting monolayers of different noble gases and adjusting the annealing temperature. At large moiré periods, we observe hybridization between the surface band replicas, which leads to the formation of a correlated flat band. Our results serve as a bridge for understanding the moiré modulation effect from 2D to 3D systems, and provide a feasible approach for the realization of correlated phenomena through the engineering of surface states via moiré effects. △ Less

Submitted 8 October, 2025; originally announced October 2025.

Comments: 5 pages, 4 figures

Ultrafast Optical Evidence of Coexisting Density Waves in Bilayer Nickelate La$_3$Ni$_2$O$_7$

Authors: Qi-Yi Wu, De-Yuan Hu, Chen Zhang, Mengwu Huo, Hao Liu, Bo Chen, Ying Zhou, Zhong-Tuo Fu, Chun-Hui Lv, Zi-Jie Xu, Hai-Long Deng, H. Y. Liu, Jun Liu, Yu-Xia Duan, Meng Wang, Jian-Qiao Meng

Abstract: Utilizing ultrafast optical pump-probe spectroscopy, we investigate the coexistence and competition of electronic orders in the bilayer nickelate La$_3$Ni$_2$O$_7$. Our results reveal two coexisting density waves that can be selectively manipulated with light. We directly identify a spin-density wave (SDW) with electronic nematicity emerging below $T_{\rm SDW}$ $\approx$ 140 K by measuring its spi… ▽ More Utilizing ultrafast optical pump-probe spectroscopy, we investigate the coexistence and competition of electronic orders in the bilayer nickelate La$_3$Ni$_2$O$_7$. Our results reveal two coexisting density waves that can be selectively manipulated with light. We directly identify a spin-density wave (SDW) with electronic nematicity emerging below $T_{\rm SDW}$ $\approx$ 140 K by measuring its spin dynamics, and discover a distinct, nonmagnetic charge order appearing below $T_{\rm DW}$ $\approx$ 115 K. The central finding is the demonstration of differential optical control: the charge order is fragile, completely suppressed by a pump fluence of approximately 40 $μ$J/cm$^2$, while the SDW is remarkably robust, persisting to 200 $μ$J/cm$^2$. This work establishes a clear hierarchy in the stability of competing electronic orders and provides a powerful method for disentangling their interplay in quantum materials. △ Less

Submitted 3 December, 2025; v1 submitted 12 August, 2025; originally announced August 2025.

Comments: 8 pages, 4 figures

Journal ref: PHYS REV B 112 (23), 235110 (2025)

Multiple Topological Phases Controlled via Strain in Two-Dimensional Altermagnets

Authors: Zesen Fu, Mengli Hu, Aolin Li, Haiming Duan, Junwei Liu, Fangping Ouyang

Abstract: Altermagnets (AMs) are an emergent class of magnetic materials that combine properties of ferromagnets and antiferromagnets, exhibiting spin-polarized Fermi surfaces and zero net magnetic moment due to combined time-reversal and crystal symmetry. Here, we construct a Kondo-lattice model on a two-dimensional square Lieb lattice to investigate the topological properties of AMs. We identify a type-II… ▽ More Altermagnets (AMs) are an emergent class of magnetic materials that combine properties of ferromagnets and antiferromagnets, exhibiting spin-polarized Fermi surfaces and zero net magnetic moment due to combined time-reversal and crystal symmetry. Here, we construct a Kondo-lattice model on a two-dimensional square Lieb lattice to investigate the topological properties of AMs. We identify a type-II quantum spin Hall state characterized by spin-polarized counterpropagating edge states. Breaking the $C_{4z}\mathcal{T}$ symmetry, which connects magnetic sublattices, induces a transition to a quantum anomalous Hall state. We further establish a strain-induced mechanism to control these topological phase transitions and present the corresponding phase diagram. Finally, we demonstrate the predicted transitions in monolayer CrO, a realistic altermagnetic candidate, using first-principles calculations. Our findings highlight the potential of 2D AMs as a versatile platform for topological spintronics, enabling strain-tunable helical and chiral edge states within a single system. △ Less

Submitted 30 September, 2025; v1 submitted 30 July, 2025; originally announced July 2025.

Comments: 7 pages, 4 figures

Magnetic ground state and persistent spin fluctuations in triangular-lattice antiferromagnet NdZnAl$_{11}$O$_{19}$

Authors: Yantao Cao, Huanpeng Bu, Toni Shiroka, Helen C. Walker, Zhendong Fu, Zhaoming Tian, Jinkui Zhao, Hanjie Guo

Abstract: Rare-earth triangular-lattice magnets serve as an excellent platform for investigating exotic quantum magnetic phenomena. Recently, the hexaaluminate \cmao\ has been proposed to host a $U(1)$ Dirac quantum spin liquid state with dominant Ising anisotropy. Here, we report a systematic study of its analogue, \nzao, employing ac susceptibility, inelastic neutron scattering, and muon spin relaxation m… ▽ More Rare-earth triangular-lattice magnets serve as an excellent platform for investigating exotic quantum magnetic phenomena. Recently, the hexaaluminate \cmao\ has been proposed to host a $U(1)$ Dirac quantum spin liquid state with dominant Ising anisotropy. Here, we report a systematic study of its analogue, \nzao, employing ac susceptibility, inelastic neutron scattering, and muon spin relaxation measurements. Inelastic neutron scattering measurements establish a well-defined $J_\mathrm{eff}$ = 1/2 ground state with moderate Ising anisotropy ($g_c$ = 4.54, $g_\mathrm{ab}$ = 1.42). Muon spin relaxation measurements reveal persistent fluctuations emerging below $\sim$15\,K, and extending down to at least 0.28 K. AC susceptibility data further indicate an absence of magnetic ordering or spin freezing down to 50\,mK, despite an overall antiferromagnetic interaction with the Curie-Weiss temprature of $-0.42$\,K. These results suggest that \nzao\ is a good candidate material for realizing a quantum spin liquid state. △ Less

Submitted 15 July, 2025; originally announced July 2025.

Comments: 8 figures including the Supplementary Materials

Multi-gap and high-Tc superconductivity in metal-atom-free borocarbides: Effects of dimensional confinement and strain engineering

Authors: Hao-Dong Liu, Wei-Yi Zhang, Zhen-Guo Fu, Bao-Tian Wang, Hong-Yan Lu, Hua-Jie Song, Ning Hao, Ping Zhang

Abstract: Pure borocarbides suffer from limited superconducting potential due to intrinsic structural instability, requiring transition/alkali metals as dual-functional stabilizers and dopants. Here, by combining high-throughput screening with anisotropic Migdal-Eliashberg (aME) theory, we identify dynamically stable borocarbides where high-Tc superconductivity predominately originates from E symmetry-selec… ▽ More Pure borocarbides suffer from limited superconducting potential due to intrinsic structural instability, requiring transition/alkali metals as dual-functional stabilizers and dopants. Here, by combining high-throughput screening with anisotropic Migdal-Eliashberg (aME) theory, we identify dynamically stable borocarbides where high-Tc superconductivity predominately originates from E symmetry-selective electron-phonon coupling (EPC). The six distinct superconducting gaps emerge from a staircase distribution or uncoupling of EPC strength across each Fermi surface (FS) sheet, constituting a metal-free system with such high gap multiplicity. Crucially, dimensional reduction from bulk to surface strengthens E-symmetry EPC and enhances Tc from 32 K (3D bulk) to 75 K (2D surface), a result that highlights structural confinement as a key design strategy for observing high Tc. External strain further optimizes the competition between EPC strength and characteristic phonon frequency to achieve Tc > 90 K. This work reveals a systematic correlation between structural dimensionality and gap multiplicity and establishes borocarbide as a tunable platform to engineer both high-Tc and multi-gap superconductivity. △ Less

Submitted 4 July, 2025; originally announced July 2025.

Magnetically disordered ground state in the triangular-lattice antiferromagnets Rb$_3$Yb(VO$_4$)$_2$ and Cs$_3$Yb(VO$_4$)$_2$

Authors: Zhen Ma, Yingqi Chen, Zhongtuo Fu, Shuaiwei Li, Xin-An Tong, Hong Du, Jan Peter Embs, Shuhan Zheng, Yongjun Zhang, Meifeng Liu, Ruidan Zhong, Jun-Ming Liu, Jinsheng Wen

Abstract: Quantum spin liquids~(QSLs) represent a unique quantum disordered state of matter that hosts long-range quantum entanglement and fractional excitations. However, structural disorder resulting from site mixing between different types of ions usually arises in real QSL candidates, which is considered as an obstacle to gain the insight into the intrinsic physics. Here, we have synthesized two new rar… ▽ More Quantum spin liquids~(QSLs) represent a unique quantum disordered state of matter that hosts long-range quantum entanglement and fractional excitations. However, structural disorder resulting from site mixing between different types of ions usually arises in real QSL candidates, which is considered as an obstacle to gain the insight into the intrinsic physics. Here, we have synthesized two new rare-earth compounds Rb$_3$Yb(VO$_4$)$_2$ and Cs$_3$Yb(VO$_4$)$_2$. X-ray diffractions reveal a perfect triangular-lattice structure with no detectable disorder. Magnetic susceptibility measurements do not capture any phase transition or spin freezing down to 1.8~K. A fit to low-temperature data indicates dominant antiferromagnetic interactions with the Curie-Weiss temperature of -1.40~K and -0.43~K for Rb$_3$Yb(VO$_4$)$_2$ and Cs$_3$Yb(VO$_4$)$_2$, respectively. Specific heat results show no sign of long-range magnetic order down to $\sim$0.1~K either, but only a Schottky anomaly that is continuously mediated by the external magnetic fields. Additionally, inelastic neutron scattering is employed to detect low-energy spin excitations in Rb$_3$Yb(VO$_4$)$_2$. The absence of magnetic excitation signals as well as static magnetic order down to 97~mK aligns with the results from magnetic susceptibility and specific heat. Collectively, these findings point to a quantum disordered ground state with persistent spin dynamics, reminiscent of QSL behaviors. Our work provides a promising platform for further exploration of quantum magnetism in this new disorder-free system. △ Less

Submitted 24 April, 2025; originally announced April 2025.

Comments: 11 pages, 5 figures

Journal ref: Phys. Rev. B 111, 155141 (2025)

Complex spin-density-wave ordering in La$_4$Ni$_{3}$O$_{10}$

Authors: Yantao Cao, Andi Liu, Bin Wang, Mingxin Zhang, Yanpeng Qi, Thomas J. Hicken, Hubertus Luetkens, Zhendong Fu, Jason S. Gardner, Jinkui Zhao, Hanjie Guo

Abstract: The discovery of high-temperature superconductivity in layered nickelates under pressure has recently triggered enormous interest. Studies of these compounds have revealed a density-wave-like transition at ambient pressure, though its connection with superconductivity is still not well understood. Here, we report a detailed \msr\ study on single crystals of trilayer nickelate \LNO\ at ambient pres… ▽ More The discovery of high-temperature superconductivity in layered nickelates under pressure has recently triggered enormous interest. Studies of these compounds have revealed a density-wave-like transition at ambient pressure, though its connection with superconductivity is still not well understood. Here, we report a detailed \msr\ study on single crystals of trilayer nickelate \LNO\ at ambient pressure. We have identified a spin-density-wave (SDW) transition at the temperature of $T_\mathrm{N} \sim$130 K, as well as a broad crossover around 70 - 100 K. Based on the temperature dependence of the muon precession amplitudes and magnetic susceptibility, we attribute this additional crossover either to a spin reorientation, or to an inhomogeneous SDW ordering. △ Less

Submitted 6 November, 2025; v1 submitted 18 March, 2025; originally announced March 2025.

Comments: 6 pages, 5 figures

Journal ref: Phys. Rev. B 112, 174423 (2025)

U(1) Dirac quantum spin liquid candidate in triangular-lattice antiferromagnet CeMgAl$_{11}$O$_{19}$

Authors: Yantao Cao, Akihiro Koda, M. D. Le, V. Pomjakushin, Benqiong Liu, Zhendong Fu, Zhiwei Li, Jinkui Zhao, Zhaoming Tian, Hanjie Guo

Abstract: Quantum spin liquid represents an intriguing state where electron spins are highly entangled yet spin fluctuation persists even at 0 K. Recently, the hexaaluminates \textit{R}MgAl$_{11}$O$_{19}$ (\textit{R} = rare earth) have been proposed to be a platform for realizing the quantum spin liquid state with dominant Ising anisotropic correlations. Here, we report detailed low-temperature magnetic sus… ▽ More Quantum spin liquid represents an intriguing state where electron spins are highly entangled yet spin fluctuation persists even at 0 K. Recently, the hexaaluminates \textit{R}MgAl$_{11}$O$_{19}$ (\textit{R} = rare earth) have been proposed to be a platform for realizing the quantum spin liquid state with dominant Ising anisotropic correlations. Here, we report detailed low-temperature magnetic susceptibility, muon spin relaxation, and thermodynamic studies on the CeMgAl$_{11}$O$_{19}$ single crystal. Ising anisotropy is revealed by magnetic susceptibility measurements. Muon spin relaxation and ac susceptibility measurements rule out any long-range magnetic ordering or spin freezing down to 50 mK despite the onset of spin correlations below $\sim$0.8 K. Instead, the spins keep fluctuating at a rate of 1.0(2) MHz at 50 mK. Specific heat results indicate a gapless excitation with a power-law dependence on temperature, $C_m(T) \propto T^α$. The quasi-quadratic temperature dependence with $α$ = 2.28(4) in zero field and linear temperature dependence in 0.25 T support the possible realization of the U(1) Dirac quantum spin liquid state. △ Less

Submitted 26 February, 2025; originally announced February 2025.

Comments: Accepted by Sci. China - Phys. Mech. Astron. 7 pages main text + 8 pages supplementary materials

Journal ref: Sci. China-Phys. Mech. Astron. 68, 267011 (2025)

Nonmagnetic ground state of marcasite FeTe$_{2}$: The competition between crystal field splitting and on-site Coulomb repulsion

Authors: Yue-Fei Hou, Zhibin Shao, Minghu Pan, Shiyang Wu, Fawei Zheng, Zhen-Guo Fu, Ping Zhang

Abstract: The magnetic ground states in crystalline systems are significant for both fundamental condensed matter physics and practical materials engineering. Marcasite FeTe$_{2}$, characterized as a small-gap semiconductor, exhibits anomalous magnetic behaviors in low-temperature experiments. In this study, first-principles density functional theory calculations combined with scanning tunneling microscopy/… ▽ More The magnetic ground states in crystalline systems are significant for both fundamental condensed matter physics and practical materials engineering. Marcasite FeTe$_{2}$, characterized as a small-gap semiconductor, exhibits anomalous magnetic behaviors in low-temperature experiments. In this study, first-principles density functional theory calculations combined with scanning tunneling microscopy/spectroscopy are employed to investigate the magnetic ground state of marcasite FeTe$_{2}$. It is revealed that the competition between crystal field splitting and on-site Coulomb repulsion plays the key role in the formation of localized magnetic moments in FeTe$_{2}$. The ground state of FeTe$_{2}$ bulk is confirmed to be nonmagnetic, while the magnetic responses of FeTe$_{2}$ observed at low temperature are suggested to be related to the magnetic Fe atoms on the crystal surfaces. Our work proposes a straightforward competing mechanism for determining ground-state magnetism of various localized-moment crystalline systems. △ Less

Submitted 17 June, 2025; v1 submitted 7 February, 2025; originally announced February 2025.

Comments: 5 figures, 10 pages. Some revisions were performed. Comments are welcome

Crossover from Conventional to Unconventional Superconductivity in 2M-WS2

Authors: Piumi Samarawickrama, Joseph McBride, Sabin Gautam, ZhuangEn Fu, Kenji Watanabe, Takashi Taniguchi, Wenyong Wang, Jinke Tang, John Ackerman, Brian M. Leonard, Jifa Tian

Abstract: Leveraging reciprocal-space proximity effect between superconducting bulk and topological surface states (TSSs) offers a promising way to topological superconductivity. However, elucidating the mutual influence of bulk and TSSs on topological superconductivity remains a challenge. Here, we report pioneering transport evidence of a thickness-dependent transition from conventional to unconventional… ▽ More Leveraging reciprocal-space proximity effect between superconducting bulk and topological surface states (TSSs) offers a promising way to topological superconductivity. However, elucidating the mutual influence of bulk and TSSs on topological superconductivity remains a challenge. Here, we report pioneering transport evidence of a thickness-dependent transition from conventional to unconventional superconductivity in 2M-phase WS2 (2M-WS2). As the sample thickness reduces, we see clear changes in key superconducting metrics, including critical temperature, critical current, and carrier density. Notably, while thick 2M-WS2 samples show conventional superconductivity, with an in-plane (IP) upper critical field constrained by the Pauli limit, samples under 20 nm exhibit a pronounced IP critical field enhancement, inversely correlated with 2D carrier density. This marks a distinct crossover to unconventional superconductivity with strong spin-orbit-parity coupling. Our findings underscore the crucial role of sample thickness in accessing topological states in 2D topological superconductors, offering pivotal insights into future studies of topological superconductivity. △ Less

Submitted 9 December, 2024; originally announced December 2024.

Comments: 15 pages, 4 figures

Tunneling current-controlled spin states in few-layer van der Waals magnets

Authors: ZhuangEn Fu, Piumi I. Samarawickrama, John Ackerman, Yanglin Zhu, Zhiqiang Mao, Kenji Watanabe, Takashi Taniguchi, Wenyong Wang, Yuri Dahnovsky, Mingzhong Wu, TeYu Chien, Jinke Tang, Allan H. MacDonald, Hua Chen, Jifa Tian

Abstract: Effective control of magnetic phases in two-dimensional magnets would constitute crucial progress in spintronics, holding great potential for future computing technologies. Here, we report a new approach of leveraging tunneling current as a tool for controlling spin states in CrI3. We reveal that a tunneling current can deterministically switch between spin-parallel and spin-antiparallel states in… ▽ More Effective control of magnetic phases in two-dimensional magnets would constitute crucial progress in spintronics, holding great potential for future computing technologies. Here, we report a new approach of leveraging tunneling current as a tool for controlling spin states in CrI3. We reveal that a tunneling current can deterministically switch between spin-parallel and spin-antiparallel states in few-layer CrI3, depending on the polarity and amplitude of the current. We propose a mechanism involving nonequilibrium spin accumulation in the graphene electrodes in contact with the CrI3 layers. We further demonstrate tunneling current-tunable stochastic switching between multiple spin states of the CrI3 tunnel devices, which goes beyond conventional bi-stable stochastic magnetic tunnel junctions and has not been documented in two-dimensional magnets. Our findings not only address the existing knowledge gap concerning the influence of tunneling currents in controlling the magnetism in two-dimensional magnets, but also unlock possibilities for energy-efficient probabilistic and neuromorphic computing. △ Less

Submitted 24 October, 2024; originally announced October 2024.

Comments: 23 pages, 4 figures

Journal ref: Nature Communications 15 (2024) 3630

Anomalous Tunneling Magnetoresistance Oscillation and Electrically Tunable Tunneling Anisotropic Magnetoresistance in Few-layer CrPS4

Authors: ZhuangEn Fu, Hong-Fei Huang, Piumi Samarawickrama, Kenji Watanabe, Takashi Taniguchi, Wenyong Wang, John Ackerman, Jiadong Zang, Jie-Xiang Yu, Jifa Tian

Abstract: Two-dimensional (2D) van der Waals (vdW) magnets with layer-dependent magnetic states and/or diverse magnetic interactions and anisotropies have attracted extensive research interest. Despite the advances, a notable challenge persists in effectively manipulating the tunneling anisotropic magnetoresistance (TAMR) of 2D vdW magnet-based magnetic tunnel junctions (MTJs). Here, we report the novel and… ▽ More Two-dimensional (2D) van der Waals (vdW) magnets with layer-dependent magnetic states and/or diverse magnetic interactions and anisotropies have attracted extensive research interest. Despite the advances, a notable challenge persists in effectively manipulating the tunneling anisotropic magnetoresistance (TAMR) of 2D vdW magnet-based magnetic tunnel junctions (MTJs). Here, we report the novel and anomalous tunneling magnetoresistance (TMR) oscillations and pioneering demonstration of bias and gate voltage controllable TAMR in 2D vdw MTJs, utilizing few-layer CrPS4. This material, inherently an antiferromagnet, transitions to a canted magnetic order upon application of external magnetic fields. Through TMR measurements, we unveil the novel, layer-dependent oscillations in the tunneling resistance for few-layer CrPS4 devices under both out-of-plane and in-plane magnetic fields, with a pronounced controllability via gate voltage. Intriguingly, we demonstrate that both the polarity and magnitude of TAMR in CrPS4 can be effectively tuned through either a bias or gate voltage. We further elucidate the mechanism behind this electrically tunable TAMR through first-principles calculations. The implications of our findings are far-reaching, providing new insights into 2D magnetism and opening avenues for the development of innovative spintronic devices based on 2D vdW magnets. △ Less

Submitted 24 October, 2024; originally announced October 2024.

Comments: 22 pages, 4 figures

Journal ref: Adv. Physics Res. 2024, 3, 2400052

Isolated zero-energy flat-bands and intrinsic magnetism in carbon monolayers

Authors: Chaoyu He, Shifang Li, Yuwen Zhang, Zhentao Fu, Jin Li, Jianxin Zhong

Abstract: Flat-band in twisted graphene bilayer has garnered widespread attention, and whether flat-bands can be realized in carbon monolayer is an interesting topic worth exploring in condensed matter physics. In this work, we demonstrate that, based on the theory of compact localized states, a series of two-dimensional carbon allotropes with flat-bands can be achieved. Two of them named as 191-8-66-C-r567… ▽ More Flat-band in twisted graphene bilayer has garnered widespread attention, and whether flat-bands can be realized in carbon monolayer is an interesting topic worth exploring in condensed matter physics. In this work, we demonstrate that, based on the theory of compact localized states, a series of two-dimensional carbon allotropes with flat-bands can be achieved. Two of them named as 191-8-66-C-r567x-1 and 191-10-90-C-r567x-1 are confirmed to be dynamically stable carbon phases with isolated or weakly overlapped flat-bands at the Fermi-level. The maximum Fermi velocities of the flat-band electrons are evaluated to be 1x10^4 m/s and 0.786x10^4 m/s, both of which are lower than the Fermi velocity of the flat-band electrons in magic-angle graphene (4x10^4 m/s). Furthermore, 191-8-66-C-r567x-1 has been confirmed to be a flat-band related magnetic half-metal with a magnetic moment of 1.854 miuB per cell, while 191-10-90-C-r567x-1 is a flat-band related magnetic normal metal with a magnetic moment of 1.663 miuB per cell. These results not only show that flat-bands can be constructed in carbon monolayer, but also indicate the potential for achieving metal-free magnetic materials with light elements based on flat-band theory. △ Less

Submitted 17 October, 2024; v1 submitted 1 October, 2024; originally announced October 2024.

Comments: 10 pages, 3 figures

Floquet-engineered Emergent Massive Nambu-Goldstone Modes

Authors: Yang Hou, Zhanpeng Fu, Roderich Moessner, Marin Bukov, Hongzheng Zhao

Abstract: We present a general framework to implement massive Nambu-Goldstone quasi-particles in driven many-body systems. The underlying mechanism leverages an explicit Lie group structure imprinted into an effective Hamiltonian that governs the dynamics of slow degrees of freedom; the resulting emergent continuous symmetry is weakly explicitly broken, giving rise to a massive Nambu-Goldstone mode, with a… ▽ More We present a general framework to implement massive Nambu-Goldstone quasi-particles in driven many-body systems. The underlying mechanism leverages an explicit Lie group structure imprinted into an effective Hamiltonian that governs the dynamics of slow degrees of freedom; the resulting emergent continuous symmetry is weakly explicitly broken, giving rise to a massive Nambu-Goldstone mode, with a spectral mass gap scaling linearly with the drive period. We discuss explicit and experimentally implementable realizations, such as Heisenberg-like spin models that support gapped spin-wave excitations. We provide a protocol to certify the existence of the massive Nambu-Goldstone mode from the dynamics of specific observables, and analyse the dispersion spectrum and their lifetime in the presence of weak explicit symmetry breaking. △ Less

Submitted 27 July, 2025; v1 submitted 3 September, 2024; originally announced September 2024.

Journal ref: Phys. Rev. B 112, L020305 (2025)

New-record-Tc and three-gap 2D superconductors with electronic and phononic topology: KB2C2

Authors: Hao-Dong Liu, Xin-Peng Fu, Zhen-Guo Fu, Hong-Yan Lu, Ping Zhang

Abstract: Pursuing higher-temperature superconductors under ambient pressure continues to be a prominent topic in materials discovery. Isomorphic structures like MgB2 exhibit potential for conventional BCS-type superconductivity, but their transition temperatures (Tc) have remained below 100 K based on both experimental findings and theoretical predictions. In this study, two new two-dimensional (2D) superc… ▽ More Pursuing higher-temperature superconductors under ambient pressure continues to be a prominent topic in materials discovery. Isomorphic structures like MgB2 exhibit potential for conventional BCS-type superconductivity, but their transition temperatures (Tc) have remained below 100 K based on both experimental findings and theoretical predictions. In this study, two new two-dimensional (2D) superconductors with sandwich structures,KB2C2,featuring BC layers in AA and AB stacking configurations,are designed, whose Tc can exceed 112 K,setting a new record in 2D superconductors. The analyses suggest that electrons in σ-states covalent bonds and high-frequency E phonon modes dominated by the in-plane vibrations of B/C atoms are predominately responsible for electron-phonon coupling (EPC). An exciting robust three-gap superconducting nature stems from the strong and evident three-region distribution characteristic of electronic EPC parameters λ. When biaxial tensile strain (BTS) is applied, their Tc are boosted above 153 K. The increase in Tc originates from the softening of optical E phonon modes around the Γ point and acoustic modes around the Q point, rather than an increase of electrons at the Fermi level (EF ) as observed in other similar systems. Thus, phonon plays a more beneficial role in the EPC of BTS cases, highlighting its significance as a medium in BCS superconductors. Moreover, we find KB2C2 exhibits interesting topological properties, spin antivortex, and Ising-type spin splitting. This is the first report of the coexistence of nontrivial topology and superconductivity with such a high Tc. Therefore, KB2C2 may offer promising sandwich structures to explore higher-Tc 2D superconductors, alongside present potential avenues for investigating fundamental quantum physics. △ Less

Submitted 22 August, 2024; originally announced August 2024.

Comments: 8 pages, 3 figures

Tunable Fano and Dicke effects in quantum transport of double quantum dots sandwiched between topological insulators

Authors: Yuan Hong, Zhen-Guo Fu, Zhou-Wei-Yu Chen, Feng Chi, Zhigang Wang, Wei Zhang, Ping Zhang

Abstract: We study the quantum transport in double quantum dots (DQD) sandwiched between surfaces of topological insulator (TI) Bi$_{2}$Te$_{3}$, which possess strong spin-orbit coupling (SOC) and $^{d}$C$_{3v}$ double group symmetry. Different from the spin-conserved case with two-dimensional electron gas (2DEG) electrodes, the conductance displays a universal scaling relation for different Fermi energy as… ▽ More We study the quantum transport in double quantum dots (DQD) sandwiched between surfaces of topological insulator (TI) Bi$_{2}$Te$_{3}$, which possess strong spin-orbit coupling (SOC) and $^{d}$C$_{3v}$ double group symmetry. Different from the spin-conserved case with two-dimensional electron gas (2DEG) electrodes, the conductance displays a universal scaling relation for different Fermi energy associated with the topological nature/linear dispersion of topological surface states. The interplay between direct inter-dot tunneling and surface state mediated interaction leads to tunable Dicke and Fano effects with changing the inter-dot distance. We propose nano-rulers with different measurement range and resolution based on the Fano $q$-factor. Furthermore, when applying an in-plane Zeeman field, a crossover from a double-peak shape to a quad-peak shape in conductance curve appears. Moreover, the rotational symmetry of the system could also be revealed from the conductance pattern. Our findings contribute to a better understanding of the quantum transport in the presence of electrode's SOC topological states. △ Less

Submitted 26 March, 2025; v1 submitted 16 June, 2024; originally announced June 2024.

Comments: 7 pages, 6 figures

Three-gap superconductivity with $T_{c}$ above 80 K in hydrogenated 2D monolayer LiBC

Authors: Hao-Dong Liu, Bao-Tian Wang, Zhen-Guo Fu, Hong-Yan Lu, Ping Zhang

Abstract: Although the metalization of semiconductor bulk LiBC has been experimentally achieved, various flaws, including the strong lattice distortion, the uncontrollability of phase transition under pressure, usually appear. In this work, based on the first-principles calculations, we propose a new way of hydrogenation to realize metalization. Using the fully anisotropic Migdal-Eliashberg theory, we inves… ▽ More Although the metalization of semiconductor bulk LiBC has been experimentally achieved, various flaws, including the strong lattice distortion, the uncontrollability of phase transition under pressure, usually appear. In this work, based on the first-principles calculations, we propose a new way of hydrogenation to realize metalization. Using the fully anisotropic Migdal-Eliashberg theory, we investigate the superconducting behaviors in the stable monolayers LiBCH and LiCBH, in which C and B atoms are hydrogenated, respectively. Our findings indicate that the monolayers possess the high $T_{c}$ of 82.0 and 82.5 K, respectively, along with the interesting three-gap superconducting natures. The Fermi sheets showing the obvious three-region distribution characteristics and the abnormally strong electron-phonon coupling (EPC) are responsible for the high-$T_{c}$ three-gap superconductivity. Furthermore, the $T_{c}$ can be dramatically boosted up to 120.0 K under 3.5 \% tensile strain. To a great extent, the high $T_{c}$ is beyond the liquid nitrogen temperature ($77$ K), which is beneficial for the applications in future experiments. This study not only explores the superconducting properties of the monolayers LiBCH and LiCBH, but also offers practical insights into the search for high-$T_{c}$ superconductors. △ Less

Submitted 1 June, 2024; originally announced June 2024.

Comments: 4 figures. Comments are welcome

Magnetic ground state of monolayer CeI$_{2}$: occupation matrix control and DFT+U calculations

Authors: Yue-Fei Hou, Shujing Li, Xinlong Yang, Wei Jiang, Qiuhao Wang, Fawei Zheng, Zhen-Guo Fu, Ping Zhang

Abstract: The magnetic ground state is crucial for the applications of the two-dimension magnets as it decides fundamental magnetic properties of the material, such as magnetic order, magnetic transition temperature, and low-energy excitation of the spin waves. However, the simulations for magnetism of local-electron systems are challenging due to the existence of metastable states. In this study, occupatio… ▽ More The magnetic ground state is crucial for the applications of the two-dimension magnets as it decides fundamental magnetic properties of the material, such as magnetic order, magnetic transition temperature, and low-energy excitation of the spin waves. However, the simulations for magnetism of local-electron systems are challenging due to the existence of metastable states. In this study, occupation matrix control (OMC) and density functional theory plus Hubbard $U$ calculations are applied to investigate the magnetic ground state of monolayer CeI$_{2}$. Following the predicted ferromagnetic (FM) order, the FM ground state and the FM metastable states are identified and found to have different values of the magnetic parameters. Based on the calculated magnetic parameters of the FM ground state, the Curie temperature is estimated to be $128$ K for monolayer CeI$_{2}$. When spin-orbit coupling (SOC) is considered, the FM ground state is further confirmed to contain both off-plane and in-plane components of magnetization. SOC is shown to be essential for reasonably describing not only magnetic anisotropy but also local electronic orbital state of monolayer CeI$_{2}$. △ Less

Submitted 5 December, 2024; v1 submitted 1 June, 2024; originally announced June 2024.

Comments: 4 figures. Accepted by Phys. Rev. B

Synthesis, disorder and Ising anisotropy in a new spin liquid candidate PrMgAl$_{11}$O$_{19}$

Authors: Yantao Cao, Huanpeng Bu, Zhendong Fu, Jinkui Zhao, Jason S. Gardner, Zhongwen Ouyang, Zhaoming Tian, Zhiwei Li, Hanjie Guo

Abstract: Here we report the successful synthesis of large single crystals of triangular frustrated PrMgAl$_{11}$O$_{19}$ using the optical floating zone technique. Single crystal X-ray diffraction measurements unveiled the presence of quenched disorder within the mirror plane, specifically $\sim$7\% of Pr ions deviating from the ideal 2\textit{d} site towards the 6\textit{h} site. Magnetic susceptibility m… ▽ More Here we report the successful synthesis of large single crystals of triangular frustrated PrMgAl$_{11}$O$_{19}$ using the optical floating zone technique. Single crystal X-ray diffraction measurements unveiled the presence of quenched disorder within the mirror plane, specifically $\sim$7\% of Pr ions deviating from the ideal 2\textit{d} site towards the 6\textit{h} site. Magnetic susceptibility measurements revealed an Ising anisotropy with the \textit{c}-axis being the easy axis. Despite a large spin-spin interaction that develops below $\sim$10~K and considerable site disorder, the spins do not order or freeze down to at least 50 mK. The availability of large single crystals offers a distinct opportunity to investigate the exotic magnetic state on a triangular lattice with an easy axis out of the plane. △ Less

Submitted 14 May, 2024; originally announced May 2024.

Comments: 4 figures, 14 pages

Journal ref: Materials Futures 3, 035201 (2024)

Sign-reversal Anomalous Hall effect driven by a magnetic transition in Cr$_{7-δ}$Te$_8$

Authors: Bowen Chen, Xiaokai Wu, Zhiyu Liao, Zhendong Fu, Bing Xu, Meng Wang, Bing Shen

Abstract: The search for exotic spin configurations and related novel transport properties continues to be fueled by the promise of new electronic states and outstanding candidate components for spintronic applications. In layered Cr$_{7-δ}$Te$_8$, the applied field drives a before unreported magnetic transition revealed by the alternating current magnetic susceptibility measurements around room temperature… ▽ More The search for exotic spin configurations and related novel transport properties continues to be fueled by the promise of new electronic states and outstanding candidate components for spintronic applications. In layered Cr$_{7-δ}$Te$_8$, the applied field drives a before unreported magnetic transition revealed by the alternating current magnetic susceptibility measurements around room temperature. This observed magnetic transition results in a sign change for the anomalous Hall effect which exhibits non-monotonous temperature dependence. The prominent topological Hall effect (THE) with a large value of 1$μΩ\cdot cm$ has been observed without breaking the inversion symmetry for Cr$_{7-δ}$Te$_8$. This robust THE can persist up to room temperature attributed to the nonzero fluctuation-driven scalar spin chirality. The complicated interactions of long-range and short-range magnetic orders lead to rich exotic magnetic states with related novel transport properties in Cr$_{7-δ}$Te$_8$. △ Less

Submitted 31 March, 2024; originally announced April 2024.

Topological skyrmions in monolayer multiferroic MoPtGe2S6

Authors: Zuxin Fu, Kuanrong Hao, Min Guo, Jingjing He, Xiaohong Yan, Yangbo Zhou, Lei Shen, Jiaren Yuan

Abstract: Two-dimensional (2D) multiferroic materials with coexisting ferroelectricity and ferromagnetism have garnered substantial attention for their intriguing physical properties and diverse promising applications in spintronics. For example, multiferroic materials with electronically controlled broken central symmetry provide a versatile platform for designing and manipulating topological skyrmions and… ▽ More Two-dimensional (2D) multiferroic materials with coexisting ferroelectricity and ferromagnetism have garnered substantial attention for their intriguing physical properties and diverse promising applications in spintronics. For example, multiferroic materials with electronically controlled broken central symmetry provide a versatile platform for designing and manipulating topological skyrmions and diverse spintronic applications. Here, we investigate the complex magnetic properties of room-temerature multiferroic material MoPtGe2S6 and its electrical control of topological skyrmions using first-principles calculations and atomistic micromagnetic simulations. A sizable Dzyaloshinskii-Moriya interaction (DMI) (2.1 meV) is found in the multiferroic material MoPtGe2S6 with an electrically polarized ground state. The magnetic skyrmions can be stabilized in monolayer MoPtGe2S6 under zero magnetic field, and the chirality of skyrmions can be reversed with electric field-induced flipping of electrical polarization due to the reversed chirality of the DMI. Furthermore, an external magnetic fielc can reverse the magnetization direction and topological charge of the skyrmions as well as tune the size of skyrmions. These results demonstrate that the monolayer MoPtGe2S6 can enrich the 2D skyrmion community and pave the way for electronically controlled spintronic devices. △ Less

Submitted 24 February, 2024; originally announced February 2024.

Comments: 24 pages, 7 figures

Engineering Hierarchical Symmetries

Authors: Zhanpeng Fu, Roderich Moessner, Hongzheng Zhao, Marin Bukov

Abstract: The capacity to custom tailor the properties of quantum matter and materials is a central requirement for enlarging their range of possible functionalities. A particularly promising route is the use of driving protocols to engineer specific desired properties with a high degree of control and flexibility. Here, we present such a program for the tunable generation of sequences of symmetries on cont… ▽ More The capacity to custom tailor the properties of quantum matter and materials is a central requirement for enlarging their range of possible functionalities. A particularly promising route is the use of driving protocols to engineer specific desired properties with a high degree of control and flexibility. Here, we present such a program for the tunable generation of sequences of symmetries on controllable timescales. Concretely, our general driving protocol for many-body systems generates a sequence of prethermal regimes, each exhibiting a lower symmetry than the preceding one. We provide an explicit construction of effective Hamiltonians exhibiting these symmetries, which imprints emergent quasiconservation laws hierarchically, enabling us to engineer the respective symmetries and concomitant orders in nonequilibrium matter. We provide explicit examples, including spatiotemporal and topological phenomena, as well as a spin chain realizing the symmetry ladder $\text{SU(2)}{\rightarrow}\text{U(1)} {\rightarrow} \mathbb{Z}_2{\rightarrow} E$. Our results have direct applications in experiments with quantum simulators. △ Less

Submitted 18 February, 2025; v1 submitted 20 February, 2024; originally announced February 2024.

Journal ref: Phys. Rev. X 14, 041070 (2024)

Sumanene monolayer of pure carbon: a two-dimensional Kagome-analogy lattice with desirable band gap, ultrahigh carrier mobility and strong exciton binding energy

Authors: Xiaoran Shi, Weiwei Gao, Hongsheng Liu, Zhen-Guo Fu, Gang Zhang, Yong-Wei Zhang, Junfeng Gao, Jijun Zhao

Abstract: Design and synthesis of novel two-dimensional (2D) materials that possess robust structural stability and unusual physical properties may open up enormous opportunities for device and engineering applications. Herein we propose a 2D sumanene lattice that be regarded as a derivative of the conventional Kagome lattice. Our tight-binding analysis demonstrates sumanene lattice contains two sets of Dir… ▽ More Design and synthesis of novel two-dimensional (2D) materials that possess robust structural stability and unusual physical properties may open up enormous opportunities for device and engineering applications. Herein we propose a 2D sumanene lattice that be regarded as a derivative of the conventional Kagome lattice. Our tight-binding analysis demonstrates sumanene lattice contains two sets of Dirac cones and two sets of flat bands near the Fermi surface, distinctively different from the Kagome lattice. Using first-principles calculations, we theoretically suggest two possible routines for realization of stable 2D sumanene monolayers (named as a phase and b phase), and a-sumanene monolayer can be experimentally synthesized with chemical vapor deposition using C21H12 as a precursor. Small binding energies on Au(111) surface signify the possibility of their peel-off after grown on the noble metal substrate. Importantly, our GW plus Bethe-Salpeter equation calculations demonstrate both monolayers have moderate band gaps (1.94 eV for a) and ultrahigh carrier mobilities (3.4*104 cm2/Vs for a). In particular, a-sumanene monolayer possesses a strong exciton binding energy of 0.73 eV, suggesting potential applications in optics. △ Less

Submitted 13 November, 2023; originally announced November 2023.

Stability and superconductivity of freestanding two-dimensional transition metal boridene: M4/3B2

Authors: Xiaoran Shi, Junfeng Gao, Shi Qiu, Yuan Chang, Luneng Zhao, Zhen-Guo Fu, Jijun Zhao, Ping Zhang

Abstract: The small atomic mass of boron indicates strong electron-phonon coupling, so it may have a brilliant performance in superconductivity. Recently, a new 2D boride sheet with ordered metal vacancies and surface terminals (Mo4/3B2-x) was realized in experiments (Science 2021, 373, 801). Here, the 2D monolayer freestanding Mo4/3B2is evidenced to be thermodynamically stable. Through electronic structure… ▽ More The small atomic mass of boron indicates strong electron-phonon coupling, so it may have a brilliant performance in superconductivity. Recently, a new 2D boride sheet with ordered metal vacancies and surface terminals (Mo4/3B2-x) was realized in experiments (Science 2021, 373, 801). Here, the 2D monolayer freestanding Mo4/3B2is evidenced to be thermodynamically stable. Through electronic structure, phonon spectrum and electron-phonon coupling, monolayer Mo4/3B2 is found to be an intrinsic phonon-mediated superconductor. The superconducting transition temperature (Tc) is determined to be 4.06 K by the McMillian-Allen-Dynes formula. Remarkably, the Tc of monolayer Mo4/3B2 can be increased to 6.78 K with an appropriate biaxial tensile strain (+5%). Moreover, we predict that other transition metal replacing Mo atoms is also stable and retaining the superconductivity. Such as monolayer W4/3B2 is also a superconductor with the Tc of 2.37 K. Our research results enrich the database of 2D monolayer superconductors and boron-related formed materials science. △ Less

Submitted 13 November, 2023; originally announced November 2023.

Ultra-small topological spin textures with size of 1.3nm at above room temperature in Fe78Si9B13 amorphous alloy

Authors: Weiwei Wu, Huaping Zhang, Hong Wang, Chao Chang, Hongyu Jiang, Jinfeng Li, Zhichao Lv, Laiquan Shen, Hanqiu Jiang, Chunyong He, Yubin Ke, Yuhua Su, Kosuke Hiroi, Zhendong Fu, Zi-An Li, Lin Gu, Maozhi Li, Dong Ma, Haiyang Bai

Abstract: Topologically protected spin textures, such as skyrmions1,2 and vortices3,4, are robust against perturbations, serving as the building blocks for a range of topological devices5-9. In order to implement these topological devices, it is necessary to find ultra-small topological spin textures at room temperature, because small size implies the higher topological charge density, stronger signal of to… ▽ More Topologically protected spin textures, such as skyrmions1,2 and vortices3,4, are robust against perturbations, serving as the building blocks for a range of topological devices5-9. In order to implement these topological devices, it is necessary to find ultra-small topological spin textures at room temperature, because small size implies the higher topological charge density, stronger signal of topological transport10,11 and the higher memory density or integration for topological quantum devices5-9. However, finding ultra-small topological spin textures at high temperatures is still a great challenge up to now. Here we find ultra-small topological spin textures in Fe78Si9B13 amorphous alloy. We measured a large topological Hall effect (THE) up to above room temperature, indicating the existence of highly densed and ultra-small topological spin textures in the samples. Further measurements by small-angle neutron scattering (SANS) reveal that the average size of ultra-small magnetic texture is around 1.3nm. Our Monte Carlo simulations show that such ultra-small spin texture is topologically equivalent to skyrmions, which originate from competing frustration and Dzyaloshinskii-Moriya interaction12,13 coming from amorphous structure14-17. Taking a single topological spin texture as one bit and ignoring the distance between them, we evaluated the ideal memory density of Fe78Si9B13, which reaches up to 4.44*104 gigabits (43.4 TB) per in2 and is 2 times of the value of GdRu2Si218 at 5K. More important, such high memory density can be obtained at above room temperature, which is 4 orders of magnitude larger than the value of other materials at the same temperature. These findings provide a unique candidate for magnetic memory devices with ultra-high density. △ Less

Submitted 29 May, 2023; originally announced May 2023.

Comments: 26 pages, 4 figures

Signatures of a gapless quantum spin liquid in the Kitaev material Na$_3$Co$_{2-x}$Zn$_x$SbO$_6$

Authors: Zhongtuo Fu, Ruokai Xu, Yingqi Chen, Song Bao, Hong Du, Jiahua Min, Shuhan Zheng, Yongjun Zhang, Meifeng Liu, Xiuzhang Wang, Hong Li, Ruidan Zhong, Huiqian Luo, Jun-Ming Liu, Zhen Ma, Jinsheng Wen

Abstract: The honeycomb-lattice cobaltate Na$_3$Co$_2$SbO$_6$ has recently been proposed to be a proximate Kitaev quantum spin liquid~(QSL) candidate. However, non-Kitaev terms in the Hamiltonian lead to a zigzag-type antiferromagnetic~(AFM) order at low temperatures. Here, we partially substitute magnetic Co$^{2+}$ with nonmagnetic Zn$^{2+}$ and investigate the chemical doping effect in tuning the magnetic… ▽ More The honeycomb-lattice cobaltate Na$_3$Co$_2$SbO$_6$ has recently been proposed to be a proximate Kitaev quantum spin liquid~(QSL) candidate. However, non-Kitaev terms in the Hamiltonian lead to a zigzag-type antiferromagnetic~(AFM) order at low temperatures. Here, we partially substitute magnetic Co$^{2+}$ with nonmagnetic Zn$^{2+}$ and investigate the chemical doping effect in tuning the magnetic ground states of Na$_3$Co$_{2-x}$Zn$_x$SbO$_6$. X-ray diffraction characterizations reveal no structural transition but quite tiny changes on the lattice parameters over our substitution range $0\leq x\leq0.4$. Magnetic susceptibility and specific heat results both show that AFM transition temperature is continuously suppressed with increasing Zn content $x$ and neither long-range magnetic order nor spin freezing is observed when $x\geq0.2$. More importantly, a linear term of the specific heat representing fermionic excitations is captured below 5~K in the magnetically disordered regime, as opposed to the $C_{\rm m}\propto T^3$ behavior expected for bosonic excitations in the AFM state. Based on the data above, we establish a magnetic phase diagram of Na$_3$Co$_{2-x}$Zn$_x$SbO$_6$. Our results indicate the presence of gapless fractional excitations in the samples with no magnetic order, evidencing a potential QSL state induced by doping in a Kitaev system. △ Less

Submitted 26 April, 2023; originally announced April 2023.

Comments: 10 pages, 5 figures

Journal ref: Phys. Rev. B 107, 165143 (2023)

Suppression of the antiferromagnetic order by Zn doping in a possible Kitaev material Na$_2$Co$_2$TeO$_6$

Authors: Zhongtuo Fu, Ruokai Xu, Song Bao, Yanyan Shangguan, Xin Liu, Zijuan Lu, Yingqi Chen, Shuhan Zheng, Yongjun Zhang, Meifeng Liu, Xiuzhang Wang, Hong Li, Huiqian Luo, Jun-Ming Liu, Zhen Ma, Jinsheng Wen

Abstract: Very recently, a 3$d$ based honeycomb cobaltate Na$_2$Co$_2$TeO$_6$ has garnered tremendous attention due to the proposed proximity to the Kitaev spin-liquid state as its 4$d$/5$d$ counterparts. Here, we use Zn to substitute Co in a broad range and perform systematic studies on Na$_2$Co$_{2-x}$Zn$_x$TeO$_6$ by structural, magnetic, and thermodynamic measurements, and track the doping evolution of… ▽ More Very recently, a 3$d$ based honeycomb cobaltate Na$_2$Co$_2$TeO$_6$ has garnered tremendous attention due to the proposed proximity to the Kitaev spin-liquid state as its 4$d$/5$d$ counterparts. Here, we use Zn to substitute Co in a broad range and perform systematic studies on Na$_2$Co$_{2-x}$Zn$_x$TeO$_6$ by structural, magnetic, and thermodynamic measurements, and track the doping evolution of its magnetic ground states. Due to the extremely close radii of Zn$^{2+}$ and high-spin Co$^{2+}$ ions, the substitution can be easily achieved. X-ray diffractions reveal no structural transition but only minor changes on the lattice parameter $c$ over a wide substitution range $0 \leq x \leq 1.5$. Magnetic susceptibility and specific heat measurements both suggest an antiferromagnetic ground state which is gradually suppressed with doping. It can survive with $x$ up to $\sim1.0$. Then it evolves into a spin-glass phase with short-range order that is rapidly supplanted by a magnetically disordered state when $x \geq 1.3$. By summarizing all these data, we construct a magnetic phase diagram of Na$_2$Co$_{2-x}$Zn$_x$TeO$_6$. Our results demonstrate that the Zn doping can effectively suppress the magnetic order and induce a possibe quantum paramagnetic state. These may serve as a platform to investigate the Kitaev physics in this system. △ Less

Submitted 1 February, 2023; originally announced February 2023.

Comments: 9 pages, 5 figures

Journal ref: Phys. Rev. Mater. 7, 014407 (2023)

Dipolar Spin Liquid Ending with Quantum Critical Point in a Gd-based Triangular Magnet

Authors: Junsen Xiang, Cheng Su, Ning Xi, Zhendong Fu, Zhuo Chen, Hai Jin, Ziyu Chen, Zhao-Jun Mo, Yang Qi, Jun Shen, Long Zhang, Wentao Jin, Wei Li, Peijie Sun, Gang Su

Abstract: By performing experiment and model studies on a triangular-lattice dipolar magnet KBaGd(BO$_3$)$_2$ (KBGB), we find the highly frustrated magnet with a planar anisotropy hosts a strongly fluctuating dipolar spin liquid (DSL), which originates from the intriguing interplay between dipolar and Heisenberg interactions. The DSL constitutes an extended regime in the field-temperature phase diagram, whi… ▽ More By performing experiment and model studies on a triangular-lattice dipolar magnet KBaGd(BO$_3$)$_2$ (KBGB), we find the highly frustrated magnet with a planar anisotropy hosts a strongly fluctuating dipolar spin liquid (DSL), which originates from the intriguing interplay between dipolar and Heisenberg interactions. The DSL constitutes an extended regime in the field-temperature phase diagram, which gets lowered in temperature as field increases and eventually ends with an unconventional quantum critical point (QCP) at $B_c\simeq 0.75$~T. Based on dipolar Heisenberg model calculations, we identify the DSL as a Berezinskii-Kosterlitz-Thouless (BKT) phase with emergent U(1) symmetry. Due to the tremendous entropy accumulation that can be related to the strong BKT and quantum fluctuations, unprecedented magnetic cooling effects are observed in the DSL regime and particularly near the QCP, making KBGB a superior dipolar coolant to commercial Gd-based refrigerants. We establish the phase diagram for triangular-lattice dipolar quantum magnets where emergent symmetry plays an essential role, and provide a basis and opens an avenue for their applications in sub-Kelvin refrigeration. △ Less

Submitted 16 January, 2023; v1 submitted 9 January, 2023; originally announced January 2023.

Comments: 7 pages and 4 figures in main text, 5 pages and 4 figures in Supplementary Materials

Gapless triangular-lattice spin-liquid candidate in PrZnAl$_{11}$O$_{19}$

Authors: Huanpeng Bu, Malik Ashtar, Toni Shiroka, Helen C. Walker, Zhendong Fu, Jinkui Zhao, Jason S. Gardner, Gang Chen, Zhaoming Tian, Hanjie Guo

Abstract: A quantum spin liquid (QSL) is an exotic state in which electron spins are highly entangled, yet keep fluctuating even at zero temperature. Experimental realization of model QSLs has been challenging due to imperfections, such as antisite disorder, strain, and extra or a lack of interactions in real materials compared to the model Hamiltonian. Here we report the magnetic susceptibility, thermodyna… ▽ More A quantum spin liquid (QSL) is an exotic state in which electron spins are highly entangled, yet keep fluctuating even at zero temperature. Experimental realization of model QSLs has been challenging due to imperfections, such as antisite disorder, strain, and extra or a lack of interactions in real materials compared to the model Hamiltonian. Here we report the magnetic susceptibility, thermodynamic, inelastic neutron scattering (INS), and muon-spin relaxation studies on a polycrystalline sample of PrZnAl$_{11}$O$_{19}$, where the Pr$^{3+}$ ions form an ideal two-dimensional triangular lattice. Our results demonstrate that this system does not order nor freeze, but keep fluctuating down to 50 mK despite large antiferromagnetic couplings ($\sim$ -10 K). Furthermore, the INS and specific-heat data suggest that PrZnAl$_{11}$O$_{19}$ is best described as a gapless QSL. △ Less

Submitted 5 September, 2022; v1 submitted 26 July, 2022; originally announced July 2022.

Comments: 8 pages, 6 figures

Journal ref: Phys. Rev. B 106, 134428 (2022)

Large non-reciprocal charge transport in Pt2MnGe up to room temperature

Authors: K. K. Meng, K. Wang, N. N. Zhang, Z. G. Fu, J. K. Chen, Y. Wu, X. G. Xu, J. Miao, Y. Jiang

Abstract: Non-reciprocal charge transport that is strongly associated with the structural or magnetic chirality of the quantum materials system is one of the most exotic properties of condensed matter physics. Here, using magnetic alloys film Pt2MnGe, we have realized the large non-reciprocal charge transport up to room temperature, which roots in the organic combination of chirality dependent carrier scatt… ▽ More Non-reciprocal charge transport that is strongly associated with the structural or magnetic chirality of the quantum materials system is one of the most exotic properties of condensed matter physics. Here, using magnetic alloys film Pt2MnGe, we have realized the large non-reciprocal charge transport up to room temperature, which roots in the organic combination of chirality dependent carrier scattering and special magnetic configurations. In this framework, the conduction electrons are scattered asymmetrically by the emerging non-zero vector spin chirality under in-plane magnetic field, resulting in robust non-reciprocal response. More astonishingly, the vector spin chirality in Pt2MnGe film can be reversed by a spin-polarized current through spin Hall effect in a junction with Pt layer. Our work resolves the general limitation of non-reciprocal charge transport to cryogenic temperatures, and paves the way for extending its applications in the emerging field of chiral spintronics. △ Less

Submitted 16 June, 2022; originally announced June 2022.

Bulk domain Meissner state in the ferromagnetic superconductor EuFe$_{2}$(As$_{0.8}$P$_{0.2}$)$_{2}$: Consequence of compromise between ferromagnetism and superconductivity

Authors: Wentao Jin, Sebastian Mühlbauer, Philipp Bender, Yi Liu, Sultan Demirdis, Zhendong Fu, Yinguo Xiao, Shibabrata Nandi, Guang-Han Cao, Yixi Su, Thomas Brückel

Abstract: Small-angle neutron scattering (SANS) measurements are performed on the ferromagnetic superconductor EuFe$_{2}$(As$_{0.8}$P$_{0.2}$)$_{2}$ ($T\rm_{sc}=22.5\,$K) to probe the delicate interplay between ferromagnetism and superconductivity. A clear signature of large ferromagnetic domains is found below the ferromagnetic ordering temperature $T\rm_{C}$ = 18.5 K. In a small temperature interval of… ▽ More Small-angle neutron scattering (SANS) measurements are performed on the ferromagnetic superconductor EuFe$_{2}$(As$_{0.8}$P$_{0.2}$)$_{2}$ ($T\rm_{sc}=22.5\,$K) to probe the delicate interplay between ferromagnetism and superconductivity. A clear signature of large ferromagnetic domains is found below the ferromagnetic ordering temperature $T\rm_{C}$ = 18.5 K. In a small temperature interval of $\sim$ 1.5 K below $T\rm_{C}$, additional SANS signal is observed, of which the indirect Fourier transform reveals characteristic length scales in between $\sim$ 80 nm to $\sim$ 160 nm. These nanometer-scaled domain structures are identified to result from an intermediate inhomogeneous Meissner effect denoted domain Meissner state, which was recently observed on the surface of EuFe$_{2}$(As$_{0.79}$P$_{0.21}$)$_{2}$ crystals by means of magnetic force microscopy [V. S. Stolyarov $\mathit{et}$ $\mathit{al.}$, Sci. Adv. 4, 1061 (2018)], ascribing to the competition between ferromagnetism and superconductivity. Our measurements clearly render the domain Meissner state as a bulk phenomenon and provide a key solution to the mystery regarding the intriguing coexistence of strong ferromagnetism and bulk superconductivity in these compounds. △ Less

Submitted 26 April, 2022; originally announced April 2022.

Comments: 6 pages, 4 figures, accepted for publication in Physical Review B as a Letter

Journal ref: Phys. Rev. B 105, L180504 (2022)

Superconductive materials with MgB2-like structures from data-driven screening

Authors: Ze Yu, Tao Bo, Bo Liu, Zhendong Fu, Huan Wang, Sheng Xu, Tianlong Xia, Shiliang Li, Sheng Meng, Miao Liu

Abstract: Finding viable superconducting materials is of interest to the physics community as the superconductors are the playground to manifest many appealing quantum phenomena. This work exemplifies an end-to-end materials discovery towards novel MgB2-like superconductors, starting from the data-driven compound screening all the way to the experimental materialization. In addition to the known superconduc… ▽ More Finding viable superconducting materials is of interest to the physics community as the superconductors are the playground to manifest many appealing quantum phenomena. This work exemplifies an end-to-end materials discovery towards novel MgB2-like superconductors, starting from the data-driven compound screening all the way to the experimental materialization. In addition to the known superconducting compounds, CaB2 (Tc = 9.4 ~ 28.6 K), SrGa2 (Tc = 0.1 ~ 2.4 K), BaGa2 (Tc = 0.3 ~ 3.3 K), BaAu2 (Tc = 0.0 ~ 0.5 K), and LaCu2 (Tc = 0.1 ~ 2.2 K) are newly discovered, out of ~182,000 starting structures, to be the most promising superconducting compounds that share similar atomic structures with MgB2. Moreover, BaGa2 is experimentally synthesized and measured to confirm that the compound is a BCS superconductor with Tc = 0.36 K, in good agreement with our theoretical predictions. This work provides a "once and for all" study for the MgB2-like superconductors and showcases that it is feasible to discover new materials via a data-driven approach. △ Less

Submitted 23 February, 2022; originally announced February 2022.

Journal ref: Phys. Rev. B 105, 214517 (2022)

Selective Trapping of Hexagonally Warped Topological Surface States in a Triangular Quantum Corral

Authors: Mu Chen, Yeping Jiang, Junping Peng, Huimin Zhang, Cui-Zu Chang, Xiao Feng, Zhenguo Fu, Fawei Zheng, Ping Zhang, Lili Wang, Ke He, Xu-Cun Ma, Qi-Kun Xue

Abstract: The surface of a three-dimensional topological insulator (TI) hosts two-dimensional massless Dirac fermions (DFs), the gapless and spin-helical nature of which yields many exotic phenomena, such as the immunity of topological surface states (TSS) to back-scattering. This leads to their high transmission through surface defects or potential barriers. Quantum corrals, previously elaborated on metal… ▽ More The surface of a three-dimensional topological insulator (TI) hosts two-dimensional massless Dirac fermions (DFs), the gapless and spin-helical nature of which yields many exotic phenomena, such as the immunity of topological surface states (TSS) to back-scattering. This leads to their high transmission through surface defects or potential barriers. Quantum corrals, previously elaborated on metal surfaces, can act as nanometer-sized electronic resonators to trap Schrödinger electrons by quantum confinement. It is thus intriguing, concerning their peculiar nature, to put the Dirac electrons of TSS to the test in similar circumstances. Here, we report the behaviors of TSS in a triangular quantum corral (TQC) fabricated by epitaxially growing Bi bilayer nanostructures on the surfaces of Bi2Te3 films. Unlike a circular corral, the TQC is supposed to be totally transparent for DFs. By mapping the electronic structure of TSS inside TQCs through a low-temperature scanning tunneling microscope in the real space, both the trapping and de-trapping behaviors of the TSS electrons are observed. The selection rules are found to be governed by the geometry and spin texture of the constant energy contour of TSS upon the strong hexagonal warping in Bi2Te3. Careful analysis of the quantum interference patterns of quasi-bound states yields the corresponding wave vectors of trapped TSS, through which two trapping mechanisms favoring momenta in different directions are uncovered. Our work indicates the extended nature of TSS and elucidates the selection rules of the trapping of TSS in the presence of a complicated surface state structure, giving insights into the effective engineering of DFs in TIs. △ Less

Submitted 23 January, 2022; originally announced January 2022.

Comments: 31pages, 12figures(including supplementary materials)

Journal ref: Sci. Adv. 5, eaaw398 (2019)

Evolution from helical to collinear ferromagnetic order of the Eu$^{2+}$ spins in RbEu(Fe$_{1-x}$Ni$_{x}$)$_{4}$As$_{4}$

Authors: Qianhui Xu, Yi Liu, Sijie Hao, Jiahui Qian, Cheng Su, Chin-Wei Wang, Thomas Hansen, Zhendong Fu, Yixi Su, Wei Li, Guang-Han Cao, Yinguo Xiao, Wentao Jin

Abstract: The ground-state magnetic structures of the Eu$^{2+}$ spins in recently discovered RbEu(Fe$_{1-x}$Ni$_{x}$)$_{4}$As$_{4}$ superconductors have been investigated by neutron powder diffraction measurements. It is found that as the superconductivity gets suppressed with the increase of Ni doping, the magnetic propagation vector of the Eu sublattice diminishes, corresponding to the decrease of the rot… ▽ More The ground-state magnetic structures of the Eu$^{2+}$ spins in recently discovered RbEu(Fe$_{1-x}$Ni$_{x}$)$_{4}$As$_{4}$ superconductors have been investigated by neutron powder diffraction measurements. It is found that as the superconductivity gets suppressed with the increase of Ni doping, the magnetic propagation vector of the Eu sublattice diminishes, corresponding to the decrease of the rotation angle between the moments in neighboring Eu layers. The ferromagnetic Eu layers are helically modulated along the $\mathit{c}$ axis with an incommensurate magnetic propagation vector in both the ferromagnetic superconductor RbEu(Fe$_{0.95}$Ni$_{0.05}$)$_{4}$As$_{4}$ and the superconducting ferromagnet RbEu(Fe$_{0.93}$Ni$_{0.07}$)$_{4}$As$_{4}$. Such a helical structure transforms into a purely collinear ferromagnetic structure for non-superconducting RbEu(Fe$_{0.91}$Ni$_{0.09}$)$_{4}$As$_{4}$, with all the Eu$^{2+}$ spins lying along the tetragonal (1 1 0) direction. The evolution from helical to collinear ferromagnetic order of the Eu$^{2+}$ spins with increasing Ni doping is supported by first-principles calculations. The variation of the rotation angle between adjacent Eu$^{2+}$ layers can be well explained by considering the change of magnetic exchange couplings mediated by the indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. △ Less

Submitted 16 January, 2022; originally announced January 2022.

Comments: 9 pages, 6 figures, accepted for publication in Physical Review Research

Journal ref: Phys. Rev. Research 4, 013077 (2022)

Electronic confinement in quantum dots of twisted bilayer graphene

Authors: Xiao-Feng Zhou, Yi-Wen Liu, Hong-Yi Yan, Zhong-Qiu Fu, Haiwen Liu, Lin He

Abstract: Electronic properties of quantum dots (QDs) depend sensitively on their parent materials. Therefore, confined electronic states in graphene QDs (GQDs) of monolayer and Bernal-stacked bilayer graphene are quite different. Twisted bilayer graphene (TBG) is distinct from monolayer and Bernal-stacked bilayer graphene because of the new degree of freedom: twist angle. In the past few years, numerous ef… ▽ More Electronic properties of quantum dots (QDs) depend sensitively on their parent materials. Therefore, confined electronic states in graphene QDs (GQDs) of monolayer and Bernal-stacked bilayer graphene are quite different. Twisted bilayer graphene (TBG) is distinct from monolayer and Bernal-stacked bilayer graphene because of the new degree of freedom: twist angle. In the past few years, numerous efforts have been made to realize the GQDs of monolayer and Bernal-stacked bilayer graphene and achieved great success. Thus far, however, strategies for realizing GQDs of TBG have been elusive. Here, we demonstrate a general approach for fabricating stationary GQDs of TBG by introducing nanoscale p-n junctions with sharp boundaries in the TBG. We verify the confinement of low-energy massless Dirac fermions via whispering-gallery modes in the GQDs of TBG. Unexpectedly, electronic states around van Hove singularities of the TBG are also strongly modified around the GQDs. Such a feature has never been reported and is attributed to spatial variation of the interlayer coupling in the TBG induced by the GQDs. △ Less

Submitted 20 October, 2021; originally announced October 2021.

The Observation of Ferroelastic and Ferrielectric Domains in AgNbO3 Single Crystal

Authors: Wei Zhao, Zhengqian Fu, Jianming Deng, Song Li, Yifeng Han, Man-Rong Li, Xueyun Wang, Jiawang Hong

Abstract: Compared to AgNbO3 based ceramics, the experimental investigations on the single crystalline AgNbO3, especially the ground state and ferroic domain structures, are not on the same level. Here in this work, based on successfully synthesized AgNbO3 single crystal using flux method, we observed the coexistence of ferroelastic and ferrielectric domain structures by a combination study of polarized lig… ▽ More Compared to AgNbO3 based ceramics, the experimental investigations on the single crystalline AgNbO3, especially the ground state and ferroic domain structures, are not on the same level. Here in this work, based on successfully synthesized AgNbO3 single crystal using flux method, we observed the coexistence of ferroelastic and ferrielectric domain structures by a combination study of polarized light microscopy and piezoresponse force microscope, this finding may provide a new aspect for studying AgNbO3. The result also suggests a weak electromechanical response from the ferrielectric phase of AgNbO3 which is also supported by the transmission electron microscope characterization. Our results reveal that the AgNbO3 single crystal is in a polar ferrielectric phase at room temperature, clarifying its ground state which is controversial from the AgNbO3 ceramic materials. △ Less

Submitted 5 March, 2021; originally announced March 2021.

Journal ref: Chin. Phys. Lett. 2021, 38 (3): 037701

Impact of the protein composition on the structure and viscoelasticity of polymer-like gluten gels

Authors: Laurence Ramos, Amélie Banc, Ameur Louhichi, Justine Pincemaille, Jacques Jestin, Zhendong Fu, Marie-Sousai Appavou, Paul Menut, Marie-Hélène Morel

Abstract: We investigate the structure of gluten polymer-like gels in a binary mixture of water/ethanol, $50/50$ v/v, a good solvent for gluten proteins. Gluten comprises two main families of proteins, monomeric gliadins and polymer glutenins. In the semi-dilute regime, scattering experiments highlight two classes of behavior, akin to standard polymer solution and polymer gel, depending on the protein compo… ▽ More We investigate the structure of gluten polymer-like gels in a binary mixture of water/ethanol, $50/50$ v/v, a good solvent for gluten proteins. Gluten comprises two main families of proteins, monomeric gliadins and polymer glutenins. In the semi-dilute regime, scattering experiments highlight two classes of behavior, akin to standard polymer solution and polymer gel, depending on the protein composition. We demonstrate that these two classes are encoded in the structural features of the proteins in very dilute solution, and are correlated with the presence of proteins assemblies of typical size tens of nanometers. The assemblies only exist when the protein mixture is sufficiently enriched in glutenins. They are found directly associated to the presence in the gel of domains enriched in non-exchangeable H-bonds and of size comparable to that of the protein assemblies. The domains are probed in neutron scattering experiments thanks to their unique contrast. We show that the sample visco-elasticity is also directly correlated to the quantity of domains enriched in H-bonds, showing the key role of H-bonds in ruling the visco-elasticity of polymer gluten gels. △ Less

Submitted 18 January, 2021; originally announced January 2021.

Comments: accepted for publication in Journal of Physics: Condensed Matter, Special issue on "Glasses and gels: a crossroad of molecular liquids, polymers and colloids"

Journal ref: J. Phys.: Condens. Matter 33 (2021) 144001

Tunable lattice reconstruction and bandwidth of flat bands in magic-angle twisted bilayer graphene

Authors: Yi-Wen Liu, Ying Su, Xiao-Feng Zhou, Long-Jing Yin, Chao Yan, Si-Yu Li, Wei Yan, Sheng Han, Zhong-Qiu Fu, Yu Zhang, Qian Yang, Ya-Ning Ren, Lin He

Abstract: The interplay between interlayer van der Waals interaction and intralayer lattice distortion can lead to structural reconstruction in slightly twisted bilayer graphene (TBG) with the twist angle being smaller than a characteristic angle θc. Experimentally, the θc is demonstrated to be very close to the magic angle (θ ~ 1.05°). In this work, we address the transition between reconstructed and unrec… ▽ More The interplay between interlayer van der Waals interaction and intralayer lattice distortion can lead to structural reconstruction in slightly twisted bilayer graphene (TBG) with the twist angle being smaller than a characteristic angle θc. Experimentally, the θc is demonstrated to be very close to the magic angle (θ ~ 1.05°). In this work, we address the transition between reconstructed and unreconstructed structures of the TBG across the magic angle by using scanning tunnelling microscopy (STM). Our experiment demonstrates that both the two structures are stable in the TBG around the magic angle. By applying a STM tip pulse, we show that the two structures can be switched to each other and the bandwidth of the flat bands, which plays a vital role in the emergent strongly correlated states in the magic-angle TBG, can be tuned. The observed tunable lattice reconstruction and bandwidth of the flat bands provide an extra control knob to manipulate the exotic electronic states of the TBG near the magic angle. △ Less

Submitted 3 July, 2020; originally announced July 2020.

Comments: 4 figures in main text

Hypersonic heat-induced flows of magnons induced by femtosecond laser pulses

Authors: Sergiu Ruta, Zuwei Fu, Thomas Ostler, Alexey Kimel, Roy Chantrell

Abstract: In this work, we present evidence for the existence of a magnonic current on the sub-picosecond time-scale in a ferrimagnetic bilayer and its effect on ultrafast spin dynamics. The ferrimagnet, GdFeCo, is a material known to undergo ultrafast switching within 1-2ps after excitation with femtosecond laser pulses. Here, we show that the strong thermal gradients induced by applying femtosecond laser… ▽ More In this work, we present evidence for the existence of a magnonic current on the sub-picosecond time-scale in a ferrimagnetic bilayer and its effect on ultrafast spin dynamics. The ferrimagnet, GdFeCo, is a material known to undergo ultrafast switching within 1-2ps after excitation with femtosecond laser pulses. Here, we show that the strong thermal gradients induced by applying femtosecond laser pulses and the presence of chemical inhomogeneities lead to local imbalances in the effective temperatures of the spins that produces a rapid transfer of spin angular momentum, which we interpret as an ultrafast spin Seebeck effect. We have quantified the typical magnon propagation in such a system. The results show ballistic magnon propagation with 30nm/ps velocities. The characteristic time scale of such magnon propagation indicates that this magnon transport can play an important role in switching, a crucial piece of understanding towards realising next generation data processing devices that operate at much higher frequencies. △ Less

Submitted 14 June, 2020; originally announced June 2020.

Comments: 6 pages, 4 figures

Breaking whispering-gallery modes of massless Dirac fermions in graphene quantum dots by Coulomb interaction

Authors: Zhong-Qiu Fu, Ke-Ke Bai, Ya-Ning Ren, Jiao-Jiao Zhou, Lin He

Abstract: Coulomb interaction is of central importance in localized energy levels (bound states) or electronic flat bands and could result in many exotic quantum phases, such as magnetic, superconducting, and topological phases in graphene systems1-14. In graphene monolayer, the relativistic massless Dirac fermion nature of the charge carriers enables us to realize unprecedented quasibound states, which are… ▽ More Coulomb interaction is of central importance in localized energy levels (bound states) or electronic flat bands and could result in many exotic quantum phases, such as magnetic, superconducting, and topological phases in graphene systems1-14. In graphene monolayer, the relativistic massless Dirac fermion nature of the charge carriers enables us to realize unprecedented quasibound states, which are trapped temporarily via whispering-gallery modes (WGMs) with the lifetime (trapping time) of ~ 10 fs, in circular graphene quantum dots (GQDs)15-20. Here we show that Coulomb interaction still plays a dominating role in determining the electronic properties of the temporarily-confined quasibound states. Our scanning tunneling microscopy (STM) and spectroscopy (STS) measurements demonstrate that the discrete quasibound state in a GQD will split into two peaks when it is partially filled. The energy separation of the two split peaks increases linear with inverse effective radius of the GQDs, indicating that the splitting arises from the Coulomb interaction. Moreover, we show that the real space distribution of the two split states separates in different regions of the GQD to reduce the Coulomb interaction, leading to the breaking of the WGM of the quasibound states. △ Less

Submitted 8 January, 2020; originally announced January 2020.

Journal ref: PhysRevB.101.235310 (2020)

The three-state Potts model on the centered triangular lattice

Authors: Zhe Fu, Wenan Guo, Henk W. J. Blöte

Abstract: We study phase transitions of the Potts model on the centered-triangular lattice with two types of couplings, namely $K$ between neighboring triangular sites, and $J$ between the centered and the triangular sites. Results are obtained by means of a finite-size analysis based on numerical transfer matrix calculations and Monte Carlo simulations. Our investigation covers the whole $(K, J)$ phase dia… ▽ More We study phase transitions of the Potts model on the centered-triangular lattice with two types of couplings, namely $K$ between neighboring triangular sites, and $J$ between the centered and the triangular sites. Results are obtained by means of a finite-size analysis based on numerical transfer matrix calculations and Monte Carlo simulations. Our investigation covers the whole $(K, J)$ phase diagram, but we find that most of the interesting physics applies to the antiferromagnetic case $K<0$, where the model is geometrically frustrated. In particular, we find that there are, for all finite $J$, two transitions when K is varied. Their critical properties are explored. In the limits $J\to \pm \infty$ we find algebraic phases with infinite-order transitions to the ferromagnetic phase. △ Less

Submitted 11 September, 2019; originally announced September 2019.

Journal ref: Phys. Rev. E 101, 012118 (2020)

Small Energy Gap Revealed in CrBr3 by Scanning Tunneling Spectroscopy

Authors: Dinesh Baral, Zhuangen Fu, Andrei S. Zadorozhnyi, Rabindra Dulal, Aaron Wang, Narendra Shrestha, Uppalaiah Erugu, Jinke Tang, Yuri Dahnovsky, Jifa Tian, TeYu Chien

Abstract: CrBr$_{3}$ is a layered van der Waals material with magnetic ordering down to the 2D limit. For decades, based on optical measurements, it is believed that the energy gap of CrBr$_{3}$ is in the range of 1.68-2.1 eV. However, controversial results have indicated that the band gap of CrBr$_{3}$ is possibly smaller than that. An unambiguous determination of the energy gap is critical to the correct… ▽ More CrBr$_{3}$ is a layered van der Waals material with magnetic ordering down to the 2D limit. For decades, based on optical measurements, it is believed that the energy gap of CrBr$_{3}$ is in the range of 1.68-2.1 eV. However, controversial results have indicated that the band gap of CrBr$_{3}$ is possibly smaller than that. An unambiguous determination of the energy gap is critical to the correct interpretations of the experimental results of CrBr$_{3}$. Here, we present the scanning tunneling microscopy and spectroscopy (STM/S) results of CrBr$_{3}$ thin and thick flakes exfoliated onto pyropytic graphite (HOPG) surfaces and density functional theory (DFT) calculations to reveal the small energy gap (peak-to-peak energy gap to be 0.57 eV $\pm$ 0.04 eV; or the onset signal energy gap to be 0.29 $\pm$ 0.05 eV from dI/dV spectra). Atomic resolution topography images show the defect-free crystal structure and the dI/dV spectra exhibit multiple peak features measured at 77 K. The conduction band - valence band peak pairs in the multi-peak dI/dV spectrum agree very well with all reported optical transitions. STM topography images of mono- and bi-layer CrBr$_{3}$ flakes exhibit edge degradation due to short air exposure (~15 min) during sample transfer. The unambiguously determined small energy gap settles the controversy and is the key in better understanding CrBr$_{3}$ and similar materials. △ Less

Submitted 2 December, 2020; v1 submitted 30 August, 2019; originally announced September 2019.

Scanning tunneling microscope characterizations of a circular graphene resonator realized with p-p junctions

Authors: Ya-Ning Ren, Jiao-Jiao Zhou, Zhong-Qiu Fu, Hai-Xuan Cheng, Si-Yu Li, Zi-Han Guo, Yi-Wen Liu, Chao Yan, Qi-Qi Guo, Jia-Bin Qiao, Yu Zhang, Sheng Han, Hua Jiang, Lin He

Abstract: Using low-temperature high-magnetic-field scanning tunneling microscopy and spectroscopy (STM/STS), we systematically study a graphene quantum dot (GQD) defined by a circular graphene p-p junction. Inside the GQD, we observe a series of quasi-bound states arising from whispering-gallery-mode (WGM) confinement of the circular junction and directly visualize these quasi-bound states down to atomic d… ▽ More Using low-temperature high-magnetic-field scanning tunneling microscopy and spectroscopy (STM/STS), we systematically study a graphene quantum dot (GQD) defined by a circular graphene p-p junction. Inside the GQD, we observe a series of quasi-bound states arising from whispering-gallery-mode (WGM) confinement of the circular junction and directly visualize these quasi-bound states down to atomic dimensions. By applying a strong magnetic field, a large jump in energy of the quasi-bound states, which is about one-half the energy spacing between the quasi-bound states, is observed. Such a behavior results from turning on a π Berry phase of massless Dirac fermions in graphene by a magnetic field. Moreover, our experiment demonstrates that a quasi-bound state splits into two peaks with an energy separation of about 26 meV when the Fermi level crosses the quasi-bound state, indicating that there are strong electron-electron interactions in the GQD. △ Less

Submitted 18 August, 2019; originally announced August 2019.

Relativistic Artificial Molecules Realized by Two Coupled Graphene Quantum Dots

Authors: Zhong-Qiu Fu, Yue-Ting Pan, Jiao-Jiao Zhou, Dong-Lin Ma, Yu Zhang, Jia-Bin Qiao, Haiwen Liu, Hua Jiang, Lin He

Abstract: Coupled quantum dots (QDs), usually referred to as artificial molecules, are important not only in exploring fundamental physics of coupled quantum objects, but also in realizing advanced QD devices. However, previous studies have been limited to artificial molecules with nonrelativistic fermions. Here, we show that relativistic artificial molecules can be realized when two circular graphene QDs a… ▽ More Coupled quantum dots (QDs), usually referred to as artificial molecules, are important not only in exploring fundamental physics of coupled quantum objects, but also in realizing advanced QD devices. However, previous studies have been limited to artificial molecules with nonrelativistic fermions. Here, we show that relativistic artificial molecules can be realized when two circular graphene QDs are coupled to each other. Using scanning tunneling microscopy (STM) and spectroscopy (STS), we observe the formation of bonding and antibonding states of the relativistic artificial molecule and directly visualize these states of the two coupled graphene QDs. The formation of the relativistic molecular states strongly alters distributions of massless Dirac fermions confined in the graphene QDs. Because of the relativistic nature of the molecular states, our experiment demonstrates that the degeneracy of different angular-momentum states in the relativistic artificial molecule can be further lifted by external magnetic fields. Then, both the bonding and antibonding states are split into two peaks. △ Less

Submitted 18 August, 2019; originally announced August 2019.

Machine Learning Forcefield for Silicate Glasses

Authors: Han Liu, Zipeng Fu, Yipeng Li, Nazreen Farina Ahmad Sabri, Mathieu Bauchy

Abstract: Developing accurate, transferable, and computationally-efficient interatomic forcefields is key to facilitate the modeling of silicate glasses. However, the high number of forcefield parameters that need to be optimized render traditional parameterization methods poorly efficient or potentially subject to bias. Here, we present a new forcefield parameterization methodology based on ab initio molec… ▽ More Developing accurate, transferable, and computationally-efficient interatomic forcefields is key to facilitate the modeling of silicate glasses. However, the high number of forcefield parameters that need to be optimized render traditional parameterization methods poorly efficient or potentially subject to bias. Here, we present a new forcefield parameterization methodology based on ab initio molecular dynamics simulations, Gaussian process regression, and Bayesian optimization. By taking the example of glassy silica, we show that our methodology yields a new interatomic forcefield that offers an unprecedented description of the atomic structure of silica. This methodology offers a new route to efficiently parameterize new empirical interatomic forcefields for silicate glasses with very limited need for intuition. △ Less

Submitted 9 February, 2019; originally announced February 2019.

Interface-driven unusual anomalous Hall effect in MnxGa/Pt bilayers: No correlation with chiral spin structures

Authors: Kangkang Meng, Lijun Zhu, Zhenhu Jin, Enke Liu, Xupeng Zhao, Iftikhar Ahmed Malik, Zhenguo Fu, Yong Wu, Jun Miao, Xiaoguang Xu, Jinxing Zhang, Jianhua Zhao, Yong Jiang

Abstract: The effects of spin-orbit coupling and symmetry breaking at the interface between a ferromagnet and heavy metal are particularly important for spin-based information storage and computation. Recent discoveries suggest they can create chiral spin structures (e.g. skyrmions), which have often been identified through the appearance of the bump/dip features of Hall signals, the so-called topological H… ▽ More The effects of spin-orbit coupling and symmetry breaking at the interface between a ferromagnet and heavy metal are particularly important for spin-based information storage and computation. Recent discoveries suggest they can create chiral spin structures (e.g. skyrmions), which have often been identified through the appearance of the bump/dip features of Hall signals, the so-called topological Hall effect (THE). In this work, however, we have present an unusual anomalous Hall effect (UAHE) in MnxGa/Pt bilayers and demonstrated that the features extremely similar to THE can be generated without involving any chiral spin structures. The low temperature magnetic force microscopy has been used to explore the magnetic field-dependent behavior of spin structures, and the UAHE as a function of magnetic field does not peak near the maximal density of magnetic bubbles. The results unambiguously evidence that the UAHE in MnxGa/Pt bilayers shows no correlation with chiral spin structures but is driven by the modified interfacial properties. The bump/dip features of Hall signals cannot be taken as an unambiguous signature for the emergence of chiral spin structures, and a wealth of underlying and interesting physics need explored. △ Less

Submitted 19 July, 2019; v1 submitted 17 January, 2019; originally announced January 2019.

Journal ref: Phys. Rev. B 100, 184410 (2019)

Spiral magnetic ordering of the Eu moments in EuNi$_{2}$As$_{2}$

Authors: W. T. Jin, N. Qureshi, Z. Bukowski, Y. Xiao, S. Nandi, M. Babij, Z. Fu, Y. Su, Th. Brückel

Abstract: The ground-state magnetic structure of EuNi$_{2}$As$_{2}$ was investigated by single-crystal neutron diffraction. At base temperature, the Eu$^{2+}$ moments are found to form an incommensurate antiferromagnetic spiral-like structure with a magnetic propagation vector of $\mathit{k}$ = (0, 0, 0.92). They align ferromagnetically in the $\mathit{ab}$ plane with the moment size of 6.75(6) $μ_{B}$, but… ▽ More The ground-state magnetic structure of EuNi$_{2}$As$_{2}$ was investigated by single-crystal neutron diffraction. At base temperature, the Eu$^{2+}$ moments are found to form an incommensurate antiferromagnetic spiral-like structure with a magnetic propagation vector of $\mathit{k}$ = (0, 0, 0.92). They align ferromagnetically in the $\mathit{ab}$ plane with the moment size of 6.75(6) $μ_{B}$, but rotate spirally by 165.6(1)° around the $\mathit{c}$ axis from layer to layer. The magnetic order parameter in the critical region close to the ordering temperature, $\mathit{T_{N}}$ = 15 K, shows critical behavior with a critical exponent of $β_{Eu}$ = 0.34(1), consistent with the three-dimensional Heisenberg model. Moreover, within the experimental uncertainty, our neutron data is consistent with a model in which the Ni sublattice is not magnetically ordered. △ Less

Submitted 18 November, 2018; originally announced November 2018.

Comments: 6 pages, 5 figures

Journal ref: Phys. Rev. B 99, 014425 (2019)

ADAIS: Automatic Derivation of Anisotropic Ideal Strength via high-throughput first-principles computations

Authors: S. H. Zhang, Z. H. Fu, R. F. Zhang

Abstract: Anisotropic ideal strength is a fundamental and important plasticity parameter in scaling the intrinsic strength of strong crystalline materials, and is a potential descriptor in searching and designing novel hard/superhard materials. However, to the best of our knowledge, an automatic derivation of anisotropic ideal strength has not been implemented in any open-source code available so far. In th… ▽ More Anisotropic ideal strength is a fundamental and important plasticity parameter in scaling the intrinsic strength of strong crystalline materials, and is a potential descriptor in searching and designing novel hard/superhard materials. However, to the best of our knowledge, an automatic derivation of anisotropic ideal strength has not been implemented in any open-source code available so far. In this paper, we present our developed ADAIS code, an automatic derivation of anisotropic ideal strength via high-throughput first-principles computations for both three-dimensional and two-dimensional crystalline materials with any symmetry, as well as for an ideal interface model. Several fundamental mechanical quantities can be automatically derived, including ideal tensile and shear strengths through affine deformation, universal binding energy and generalized stacking fault energy, as well as the ideal cleavage and slide stresses through alias deformation. The implementation of this code has been comprehensively demonstrated and critically validated by a lot of evaluations and tests of various crystalline materials with different symmetry, indicating that our code could provide a high-efficiency solution to quantify the strength of strong solids. △ Less

Submitted 28 October, 2018; originally announced October 2018.