Showing 1–27 of 27 results for author: Yan, T

Nonthermal Dynamics of a One-dimensional Rydberg-atom Chain with Constraint Four-body Interactions

Authors: Tianyi Yan, Weibin Li

Abstract: We investigate dynamics of a linear chain of Rydberg atoms driven by a constrained four-body interaction, where two neighboring atoms are excited simultaneously from the electronic ground state $|0\rangle$ to Rydberg state $|1\rangle$ only when their closest neighbors are in $|0\rangle$ state. By employing an ansatz for the many-body ground state, the low-energy Hamiltonian is given by a tridiagon… ▽ More We investigate dynamics of a linear chain of Rydberg atoms driven by a constrained four-body interaction, where two neighboring atoms are excited simultaneously from the electronic ground state $|0\rangle$ to Rydberg state $|1\rangle$ only when their closest neighbors are in $|0\rangle$ state. By employing an ansatz for the many-body ground state, the low-energy Hamiltonian is given by a tridiagonal form. The many-body ground state energy, which scales linearly with the chain length $L$, is obtained analytically in the thermodynamic limit and agrees with the one of the exact diagonalization. Our model supports quantum many-body scar eigenstates that are nearly equally spaced energetically. The scar states overlap strongly with the basis state $|\mathbf{0}\rangle=|0\cdots 0\rangle$. We show that the overlap distribution is tilted by the state-dependent four-body interaction. This leads to non-ergodic dynamics, evidenced by the revival of the initial state. The four-body constrained model can be realized with Rydberg atoms in a Peierls array with alternating bond lengths, where atoms on the shorter and longer bonds experience Rydberg blockade and antiblockade, respectively. Our study provides a pathway to explore constrained non-ergodic dynamics with four-body interactions by combining the Rydberg blockade and antiblockade. △ Less

Submitted 20 August, 2026; originally announced August 2026.

A framework for separating dephasing from decoherence in matter-wave Bell interferometers

Authors: S. Kannan, Y. S. Athreya, X. T. Yan, S. S. Hodgman, A. G. Truscott

Abstract: Matter-wave Bell interferometers provide a sensitive probe of mass-dependent decoherence in entangled quantum systems. The degree of entanglement is obtained from the Bell-correlation amplitude of this interferometer. For observing potential mass-dependent decoherence, a reliable interpretation of any observed reduction in the Bell correlation amplitude is required, which depends on three factors:… ▽ More Matter-wave Bell interferometers provide a sensitive probe of mass-dependent decoherence in entangled quantum systems. The degree of entanglement is obtained from the Bell-correlation amplitude of this interferometer. For observing potential mass-dependent decoherence, a reliable interpretation of any observed reduction in the Bell correlation amplitude is required, which depends on three factors: geometric dephasing, environmental decoherence, and technical dilution from source and detection statistics. In this work, we present a framework based on the Schwinger SU(2) mapping to separate these contributions into local unitaries or dissipative channels. We show that by evaluating the Bell correlation at zero interferometer path difference, it is possible to extract a source-distribution-independent Bell correlation amplitude reduction. When this framework is extended to involve atoms of different mass, we show that the known differential decoherence channels are negligible at current sensitivity. This yields a concrete bound at which a dual-species Bell interferometer would begin to signal differential decoherence beyond the known systematics, opening the way for such systems to probe new physics, such as mass-dependent decoherence mechanisms. △ Less

Submitted 22 July, 2026; originally announced July 2026.

Phase-Factor-Controlled Interaction and Bonding between a Chiral Bobber and a Skyrmion String in the Conical Phase

Authors: Haijun Zhao, Tingting Yan, Shuai Dong

Abstract: Skyrmion interactions govern the formation of skyrmion lattices, clusters, and particle-like growth patterns. In contrast, the interaction between a chiral bobber and a skyrmion string remains largely unexplored, despite the role of bobbers as intermediate states in skyrmion-string formation and annihilation. Here we show that this interaction is intrinsically phase dependent in the conical phase.… ▽ More Skyrmion interactions govern the formation of skyrmion lattices, clusters, and particle-like growth patterns. In contrast, the interaction between a chiral bobber and a skyrmion string remains largely unexplored, despite the role of bobbers as intermediate states in skyrmion-string formation and annihilation. Here we show that this interaction is intrinsically phase dependent in the conical phase. Using three-dimensional micromagnetic simulations, we compute the locally relaxed constrained energy landscape as a function of the bobber--skyrmion separation $R$ and the surface phase factor $φ_0$. We find that changing $φ_0$ qualitatively reshapes the interaction, producing repulsive, attractive, and bonding-like regimes that cannot be reduced to a conventional distance-dependent potential. Real-space analysis shows that this behavior originates from phase-dependent reconstruction of nonaxisymmetric outer distortion shells. The phase-controlled interaction persists over a finite field range and follows the expected top--bottom phase relation of surface-sensitive conical textures. These results identify the conical phase direction, often hidden in projected or thickness-averaged descriptions, as a previously underappreciated degree of freedom in the interaction between skyrmion strings and finite-length chiral textures, as demonstrated here for chiral bobbers. △ Less

Submitted 5 July, 2026; originally announced July 2026.

Strain-triggered high-temperature superconducting transition in two-dimensional carbon allotrope

Authors: Tian Yan, Ru Zheng, Jin-Hua Sun, Fengjie Ma, Xun-Wang Yan, Miao Gao, Tian Cui, Zhong-Yi Lu

Abstract: Driving non-superconducting materials into a superconducting state through specific modulation is a key focus in the field of superconductivity. Pressure is a powerful method that can switch a three-dimensional (3D) material between non-superconducting and superconducting states. In the two-dimensional (2D) case, strain engineering plays a similar role to pressure. However, purely strain-induced s… ▽ More Driving non-superconducting materials into a superconducting state through specific modulation is a key focus in the field of superconductivity. Pressure is a powerful method that can switch a three-dimensional (3D) material between non-superconducting and superconducting states. In the two-dimensional (2D) case, strain engineering plays a similar role to pressure. However, purely strain-induced superconductivity in 2D systems remains exceedingly scarce. Using first-principles calculations, we demonstrate that a superconducting transition can be induced solely by applying biaxial tensile strain in a 2D carbon allotrope, THO-graphene, which is composed of triangles, hexagons, and octagons. Free-standing THO-graphene is non-superconducting. Surprisingly, the electron-phonon coupling in strained THO-graphene is enhanced strong enough to pair electrons and realize superconductivity, with the highest superconducting transition temperature reaching 45 K. This work not only provides a notable example of controlling metal-superconductor transition in 2D system just via strain, but also sets a new record of superconducting transition temperature for 2D elemental superconductors. △ Less

Submitted 3 January, 2026; originally announced January 2026.

Comments: 7 pages, 5 figures

Journal ref: Front. Phys. 21, 125205 (2026)

Engineer coherent oscillatory modes in Markovian open quantum systems

Authors: Chun Hei Leung, Pak-Tik Fong, Tianyi Yan, Weibin Li

Abstract: We develop a novel framework to engineer persistent oscillatory modes in Markovian open quantum systems governed by a time-independent Lindblad master equation. We show that oscillatory modes can be created when the Hamiltonian and jump operator can be expressed in the same block-diagonal form. A key feature of the framework is that the dissipator of the Lindblad master equation are generally non-… ▽ More We develop a novel framework to engineer persistent oscillatory modes in Markovian open quantum systems governed by a time-independent Lindblad master equation. We show that oscillatory modes can be created when the Hamiltonian and jump operator can be expressed in the same block-diagonal form. A key feature of the framework is that the dissipator of the Lindblad master equation are generally non-zero. We identify the weak and strong conditions, where the onset of the oscillatory modes is dependent and independent of the parameters of the system, respectively. Our method extends beyond the typical decoherence-free subspace approach, in which the dissipator is zero. We demonstrate the applicability of this framework using various models, showing how carefully tailored system-environment interactions can produce sustained coherent oscillations. △ Less

Submitted 19 August, 2026; v1 submitted 10 December, 2025; originally announced December 2025.

Comments: 11 pages, 4 figures

Hilbert space fragmentation in driven-dephasing Rydberg atom array

Authors: Tianyi Yan, Chun Hei Leung, Weibin Li

Abstract: We investigate the onset and mechanism of Hilbert space fragmentation (HSF) in a chain of strongly interacting Rydberg atoms subject to local dephasing. It is found that the emergence of multiple long-lived metastable states is fundamentally tied to HSF of the driven-dephasing Rydberg atom system. We demonstrate that the manifesting HSF is captured by a dephasing PXP model that supports multiple d… ▽ More We investigate the onset and mechanism of Hilbert space fragmentation (HSF) in a chain of strongly interacting Rydberg atoms subject to local dephasing. It is found that the emergence of multiple long-lived metastable states is fundamentally tied to HSF of the driven-dephasing Rydberg atom system. We demonstrate that the manifesting HSF is captured by a dephasing PXP model that supports multiple degenerate zero modes. These modes form disconnected, block-diagonal subspaces of maximally mixed states, which consist of many-body spin states sharing the same symmetry. A key result is the identification of the underlying symmetry in the HSF, where conserved quantities in each subspace are defined by the consecutive double excitation addressing operator. Moreover, we show explicitly that the number of the fragmented Hilbert space grows exponentially with the chain length, following a modified Fibonacci sequence. Our work provides insights into many-body dynamics under dynamical constraints and opens avenues for controlling and manipulating HSF in Rydberg atom systems. △ Less

Submitted 28 November, 2025; originally announced November 2025.

Comments: main text with supplementary materials

Interface-Controlled Antiferromagnetic Tunnel Junctions based on a metallic van der Waals A-type Antiferromagnet

Authors: Wei-Min Zhao, Yi-Lun Liu, Liu Yang, Cheng Tan, Yuanjun Yang, Zhifeng Zhu, Meixia Chen, Tingting Yan, Rong Hu, James Partridge, Guopeng Wang, Mingliang Tian, Ding-Fu Shao, Lan Wang

Abstract: Magnetic tunnel junctions (MTJs) are crucial components in high-performance spintronic devices. Traditional MTJs rely on ferromagnetic (FM) materials but significant improvements in speed and packing density could be enabled by exploiting antiferromagnetic (AFM) compounds instead. Here, we report all-collinear AFM tunnel junctions (AFMTJs) fabricated with van der Waals A-type AFM metal (Fe0.6Co0.4… ▽ More Magnetic tunnel junctions (MTJs) are crucial components in high-performance spintronic devices. Traditional MTJs rely on ferromagnetic (FM) materials but significant improvements in speed and packing density could be enabled by exploiting antiferromagnetic (AFM) compounds instead. Here, we report all-collinear AFM tunnel junctions (AFMTJs) fabricated with van der Waals A-type AFM metal (Fe0.6Co0.4)5GeTe2 (FCGT) electrodes and nonmagnetic semiconducting WSe2 tunnel barriers. The AFMTJ heterostructure device achieves a tunneling magnetoresistance (TMR) ratio of up to 75% in response to magnetic field switching. Our results demonstrate that the TMR exclusively emerges in the AFM state of FCGT, rather than during the AFM-to-FM transition. By engineering FCGT electrodes with either even- or odd-layer configurations, volatile or non-volatile TMR could be selected, consistent with an entirely interfacial effect. TMR in the even-layer devices arose by Néel vector switching. In the odd-layer devices, TMR stemmed from interfacial spin-flipping. Experimental and theoretical analyses reveal a new TMR mechanism associated with interface-driven spin-polarized transport, despite the spin-independent nature of bulk FCGT. Our work demonstrates that collinear AFMTJs can provide comparable performance to conventional MTJs and introduces a new paradigm for AFM spintronics, in which the spin-dependent properties of AFM interfaces are harnessed. △ Less

Submitted 29 December, 2025; v1 submitted 16 July, 2025; originally announced July 2025.

Journal ref: Nature Communications 2025

Topological exciton bands and many-body exciton phases in transition metal dichalcogenide trilayer heterostructures

Authors: Ze-Hong Guo, Tao Yan, Jin-Zhu Zhao, Yuan-Jun Jin, Qizhong Zhu

Abstract: Twisted multilayer transition metal dichalcogenides (TMDs) are a promising platform for realizing topological exciton phases. Here we propose that twisted TMD heterotrilayers WX$_2$/MX$_2$/WX$_2$ with layer symmetry represents a realistic system for realizing topological exciton bands and interesting many-body excitonic phases, simply by tuning the twist angle. These symmetric heterotrilayers form… ▽ More Twisted multilayer transition metal dichalcogenides (TMDs) are a promising platform for realizing topological exciton phases. Here we propose that twisted TMD heterotrilayers WX$_2$/MX$_2$/WX$_2$ with layer symmetry represents a realistic system for realizing topological exciton bands and interesting many-body excitonic phases, simply by tuning the twist angle. These symmetric heterotrilayers form a type-II band alignment, where the electrons are confined in the middle layer and holes are distributed among the outer two layers, for the lowest energy excitons. The outer two layers are then rotated at different centers by opposite angles, forming a helical structure. Interlayer excitons with opposite dipoles are hybridized by the coupling between outer two layers, resulting in topological moiré exciton bands. Furthermore, by constructing a three-orbital tight-binding model, we map the many-body phase diagram of interacting dipolar and quadrupolar excitons at different twist angles and exciton densities and reveal the existence of sublattice-dependent staggered superfluid and Mott insulator phases. The recent experimental observation of quadrupolar excitons in symmetric heterotrilayers brings the intriguing phases predicted in this study within immediate experimental reach. △ Less

Submitted 14 April, 2025; originally announced April 2025.

Comments: 12 pages, 7 figures

Bell correlations between momentum-entangled pairs of $^4\text{He}^*$ atoms

Authors: Y. S. Athreya, S. Kannan, X. T. Yan, R. J. Lewis-Swan, K. V. Kheruntsyan, A. G. Truscott, S. S. Hodgman

Abstract: Nonlocal entanglement between pair-correlated particles is a highly counter-intuitive aspect of quantum mechanics, where measurement on one particle can instantly affect the other, regardless of distance. While the rigorous Bell's inequality framework has enabled the demonstration of such entanglement in photons and atomic internal states, no experiment has yet involved motional states of massive… ▽ More Nonlocal entanglement between pair-correlated particles is a highly counter-intuitive aspect of quantum mechanics, where measurement on one particle can instantly affect the other, regardless of distance. While the rigorous Bell's inequality framework has enabled the demonstration of such entanglement in photons and atomic internal states, no experiment has yet involved motional states of massive particles. Here we report the experimental observation of Bell correlations in motional states of momentum-entangled ultracold helium atoms. Momentum-entangled pairs are generated via $s$-wave collisions. Using a Rarity-Tapster interferometer and a Bell-test framework, we observe atom-atom correlations required for violation of a Bell inequality. This result shows the potential of ultracold atoms for fundamental tests of quantum mechanics and opens new avenues to studying gravitational effects in quantum states. △ Less

Submitted 18 February, 2025; v1 submitted 17 February, 2025; originally announced February 2025.

Journal ref: Nat Commun 17, 2357 (2026)

Proposal for a Bell Test with Entangled Atoms of Different Mass

Authors: X. T. Yan, S. Kannan, Y. S. Athreya, A. G. Truscott, S. S. Hodgman

Abstract: We propose a Bell test experiment using momentum-entangled atom pairs of different masses, specifically metastable helium isotopes 3He* and 4He*, though the method extends to other atom species. Entanglement is generated via collisions, after which the quantum states are manipulated using two independent atom interferometers, enabling precise phase control over each species. Numerical simulations… ▽ More We propose a Bell test experiment using momentum-entangled atom pairs of different masses, specifically metastable helium isotopes 3He* and 4He*, though the method extends to other atom species. Entanglement is generated via collisions, after which the quantum states are manipulated using two independent atom interferometers, enabling precise phase control over each species. Numerical simulations predict a significant violation of Bell's inequality under realistic conditions. This proposal opens a new paradigm to study the intersection of quantum mechanics and gravity. △ Less

Submitted 2 December, 2025; v1 submitted 13 November, 2024; originally announced November 2024.

Measurement of the $s$-wave scattering length between metastable helium isotopes

Authors: S. Kannan, Y. S. Athreya, A. H. Abbas, X. T. Yan, S. S. Hodgman, A. G. Truscott

Abstract: We report the first experimental determination of the interspecies $s$-wave scattering length\,($a_{34}$) between the $2\,^3S_1\,(F=3/2,m_F=3/2)$ state of $^3$He$^*$ and the $2\,^3S_1\,(m_J=1)$ state of $^4$He$^*$. We determine $a_{34}$ by inducing oscillations in a trapped Bose-Einstein condensate of $^4$He$^*$ and measuring the damping rate of these oscillations due to the presence of $^3$He… ▽ More We report the first experimental determination of the interspecies $s$-wave scattering length\,($a_{34}$) between the $2\,^3S_1\,(F=3/2,m_F=3/2)$ state of $^3$He$^*$ and the $2\,^3S_1\,(m_J=1)$ state of $^4$He$^*$. We determine $a_{34}$ by inducing oscillations in a trapped Bose-Einstein condensate of $^4$He$^*$ and measuring the damping rate of these oscillations due to the presence of $^3$He$^*$ atoms. The deduced value of $a_{34}=29\pm3$\,nm is in good agreement with theoretical predictions. The knowledge of this scattering length is important for many fundamental experiments between these helium isotopes. △ Less

Submitted 8 August, 2024; originally announced August 2024.

Journal ref: Phys. Rev. A 110, 063324 (2024)

Exciton-exciton Interaction in Monolayer MoSe$_2$ from Mutual Screening of Coulomb Binding

Authors: Ke Xiao, Tengfei Yan, Chengxin Xiao, Feng-ren Fan, Ruihuan Duan, Zheng Liu, Kenji Watanabe, Takashi Taniguchi, Wang Yao, Xiaodong Cui

Abstract: The potential for low-threshold optical nonlinearity has received significant attention in the fields of photonics and conceptual optical neuron networks. Excitons in two-dimensional (2D) semiconductors are particularly promising in this regard as reduced screening and dimensional confinement foster their pronounced many-body interactions towards nonlinearity. However, experimental determination o… ▽ More The potential for low-threshold optical nonlinearity has received significant attention in the fields of photonics and conceptual optical neuron networks. Excitons in two-dimensional (2D) semiconductors are particularly promising in this regard as reduced screening and dimensional confinement foster their pronounced many-body interactions towards nonlinearity. However, experimental determination of the interactions remains ambiguous, as optical pumping in general creates a mixture of excitons and unbound carriers, where the impacts of band gap renormalization and carrier screening on exciton energy counteract each other. Here by comparing the influences on exciton ground and excited states energies in the photoluminescence spectroscopy of monolayer MoSe$_2$, we are able to identify separately the screening of Coulomb binding by the neutral excitons and by charge carriers. The energy difference between exciton ground state (A-1s) and excited state (A-2s) red-shifts by 5.5 meV when the neutral exciton density increases from 0 to $4\times 10^{11}$ cm$^{-2}$, in contrast to the blue shifts with the increase of either electron or hole density. This energy difference change is attributed to the mutual screening of Coulomb binding of neutral excitons, from which we extract an exciton polarizability of $α_{2D}^{\rm exciton} = 2.55\times 10^{-17}$ eV(m/V)$^2$. Our finding uncovers a new mechanism that dominates the repulsive part of many-body interaction between neutral excitons. △ Less

Submitted 28 August, 2023; originally announced August 2023.

Signatures of quantum chaos of Rydberg dressed bosons in a triple-well potential

Authors: Tianyi Yan, Matthew Collins, Rejish Nath, Weibin Li

Abstract: We study signatures of quantum chaos in dynamics of Rydberg dressed bosonic atoms held in a one dimensional triple-well potential. Long-range nearest-neighbor and next-nearest-neighbor interactions, induced by laser dressing atoms to strongly interacting Rydberg states, affect drastically mean field and quantum many-body dynamics. By analyzing the mean field dynamics, classical chaos regions with… ▽ More We study signatures of quantum chaos in dynamics of Rydberg dressed bosonic atoms held in a one dimensional triple-well potential. Long-range nearest-neighbor and next-nearest-neighbor interactions, induced by laser dressing atoms to strongly interacting Rydberg states, affect drastically mean field and quantum many-body dynamics. By analyzing the mean field dynamics, classical chaos regions with positive and large Lyapunov exponents are identified as a function of the potential well tilting and dressed interactions. In the quantum regime, it is found that level statistics of the eigen-energies gains a Wigner-Dyson distribution when the Lyapunov exponents are large, giving rise to signatures of strong quantum chaos. We find that both the time averaged entanglement entropy and survival probability of the initial state have distinctively large values in the quantum chaos regime. We further show that population variances could be used as an indicator of the emergence of quantum chaos. This might provide a way to directly probe quantum chaotic dynamics through analyzing population dynamics in individual potential wells. △ Less

Submitted 6 June, 2023; v1 submitted 19 April, 2023; originally announced April 2023.

Comments: 13 pages, 7 figures

Observation of van der Waals phonons in the single-layer cuprate (Bi,Pb)$_2$(Sr,La)$_2$CuO$_{6+δ}$

Authors: Y. Y. Peng, I. Boukahil, K. Krongchon, Q. Xiao, A. A. Husain, Sangjun Lee, Q. Z. Li, A. Alatas, A. H. Said, H. T. Yan, Y. Ding, L. Zhao, X. J. Zhou, T. P. Devereaux, L. K. Wagner, C. D. Pemmaraju, P. Abbamonte

Abstract: Interlayer van der Waals (vdW) coupling is generic in two-dimensional materials such as graphene and transition metal dichalcogenides, which can induce very low-energy phonon modes. Using high-resolution inelastic hard x-ray scattering, we uncover the ultra-low energy phonon mode along the Cu-O bond direction in the high-$T_c$ cuprate (Bi,Pb)$_2$(Sr,La)$_2$CuO$_{6+δ}$ (Bi2201). This mode is indepe… ▽ More Interlayer van der Waals (vdW) coupling is generic in two-dimensional materials such as graphene and transition metal dichalcogenides, which can induce very low-energy phonon modes. Using high-resolution inelastic hard x-ray scattering, we uncover the ultra-low energy phonon mode along the Cu-O bond direction in the high-$T_c$ cuprate (Bi,Pb)$_2$(Sr,La)$_2$CuO$_{6+δ}$ (Bi2201). This mode is independent of temperature, while its intensity decreases with doping in accordance with an increasing c-axis lattice parameter. We compare the experimental results to first-principles density functional theory simulations and identify the observed mode as a van der Waals phonon, which arises from the shear motion of the adjacent Bi-O layers. This shows that Bi-based cuprate has similar vibrational properties as graphene and transition metal dichalcogenides, which can be exploited to engineer novel heterostructures. △ Less

Submitted 21 June, 2021; originally announced June 2021.

Journal ref: Physical Review Materials 8, 024804 (2024)

Many-body effect in optical properties of monolayer molybdenum diselenide

Authors: Ke Xiao, Tengfei Yan, Qiye Liu, Siyuan Yang, Chiming Kan, Ruihuan Duan, Zheng Liu, Xiaodong Cui

Abstract: Excitons in monolayer transition metal dichalcogenide (TMD) provide a paradigm of composite Boson in 2D system. This letter reports a photoluminescence and reflectance study of excitons in monolayer molybdenum diselenide (MoSe2) with electrostatic gating. We observe the repulsive and attractive Fermi polaron modes of the band edge exciton, its excited state and the spin-off excitons. Our data vali… ▽ More Excitons in monolayer transition metal dichalcogenide (TMD) provide a paradigm of composite Boson in 2D system. This letter reports a photoluminescence and reflectance study of excitons in monolayer molybdenum diselenide (MoSe2) with electrostatic gating. We observe the repulsive and attractive Fermi polaron modes of the band edge exciton, its excited state and the spin-off excitons. Our data validate the polaronic behavior of excitonic states in the system quantitatively where the simple three-particle trion model is insufficient to explain. △ Less

Submitted 24 August, 2020; originally announced August 2020.

Simulation of Higher-Order Topological Phases and Related Topological Phase Transitions in a Superconducting Qubit

Authors: Jingjing Niu, Tongxing Yan, Yuxuan Zhou, Ziyu Tao, Xiaole Li, Weiyang Liu, Libo Zhang, Song Liu, Zhongbo Yan, Yuanzhen Chen, Dapeng Yu

Abstract: Higher-order topological phases give rise to new bulk and boundary physics, as well as new classes of topological phase transitions. While the realization of higher-order topological phases has been confirmed in many platforms by detecting the existence of gapless boundary modes, a direct determination of the higher-order topology and related topological phase transitions through the bulk in exper… ▽ More Higher-order topological phases give rise to new bulk and boundary physics, as well as new classes of topological phase transitions. While the realization of higher-order topological phases has been confirmed in many platforms by detecting the existence of gapless boundary modes, a direct determination of the higher-order topology and related topological phase transitions through the bulk in experiments has still been lacking. To bridge the gap, in this work we carry out the simulation of a two-dimensional second-order topological phase in a superconducting qubit. Owing to the great flexibility and controllability of the quantum simulator, we observe the realization of higher-order topology directly through the measurement of the pseudo-spin texture in momentum space of the bulk for the first time, in sharp contrast to previous experiments based on the detection of gapless boundary modes in real space. Also through the measurement of the evolution of pseudo-spin texture with parameters, we further observe novel topological phase transitions from the second-order topological phase to the trivial phase, as well as to the first-order topological phase with nonzero Chern number. Our work sheds new light on the study of higher-order topological phases and topological phase transitions. △ Less

Submitted 26 May, 2021; v1 submitted 12 January, 2020; originally announced January 2020.

Comments: 8+8 pages; 4+7 figures

Journal ref: Science Bulletin 66(12), 1168-1175 (2021)

Orbital optimized unitary coupled cluster theory for quantum computer

Authors: Wataru Mizukami, Kosuke Mitarai, Yuya O. Nakagawa, Takahiro Yamamoto, Tennin Yan, Yu-ya Ohnishi

Abstract: We propose an orbital optimized method for unitary coupled cluster theory (OO-UCC) within the variational quantum eigensolver (VQE) framework for quantum computers. OO-UCC variationally determines the coupled cluster amplitudes and also molecular orbital coefficients. Owing to its fully variational nature, first-order properties are readily available. This feature allows the optimization of molecu… ▽ More We propose an orbital optimized method for unitary coupled cluster theory (OO-UCC) within the variational quantum eigensolver (VQE) framework for quantum computers. OO-UCC variationally determines the coupled cluster amplitudes and also molecular orbital coefficients. Owing to its fully variational nature, first-order properties are readily available. This feature allows the optimization of molecular structures in VQE without solving any additional equations. Furthermore, the method requires smaller active space and shallower quantum circuit than UCC to achieve the same accuracy. We present numerical examples of OO-UCC using quantum simulators, which include the geometry optimization of the water and ammonia molecules using analytical first derivatives of the VQE. △ Less

Submitted 19 March, 2020; v1 submitted 25 October, 2019; originally announced October 2019.

Journal ref: Phys. Rev. Research 2, 033421 (2020)

Long valley lifetime of free carriers in monolayer WSe2

Authors: Tengfei Yan, Siyuan Yang, Dian Li, Xiaodong Cui

Abstract: Monolayer transition metal dichalcogenids (TMDs) feature valley degree of freedom, giant spin-orbit coupling and spin-valley locking. These exotic natures stimulate efforts of exploring the potential applications in conceptual spintronics, valleytronics and quantum computing. Among all the exotic directions, a long lifetime of spin and/or valley polarization is critical. The present valley dynamic… ▽ More Monolayer transition metal dichalcogenids (TMDs) feature valley degree of freedom, giant spin-orbit coupling and spin-valley locking. These exotic natures stimulate efforts of exploring the potential applications in conceptual spintronics, valleytronics and quantum computing. Among all the exotic directions, a long lifetime of spin and/or valley polarization is critical. The present valley dynamics studies concentrate on the band edge excitons which predominates the optical response due to the enhanced Coulomb interaction in two dimensions. The valley lifetime of free carriers remains in ambiguity. In this work, we use time-resolved Kerr rotation spectroscopy to probe the valley dynamics of excitons and free carriers in monolayer tungsten diselinide. The valley lifetime of free carriers is found around 2 ns at 70 K, about 3 orders of magnitude longer than the excitons of about 2 ps. The extended valley lifetime of free carriers evidences that exchange interaction dominates the valley relaxation in optical excitation. The pump-probe spectroscopy also reveals the exciton binding energy of 0.60 eV in monolayer WSe2. △ Less

Submitted 5 December, 2016; originally announced December 2016.

Journal ref: Phys. Rev. B 95, 241406 (2017)

Exciton valley dynamics in monolayer WSe2 probed by the two-color ultrafast Kerr rotation

Authors: Tengfei Yan, Xiaofen Qiao, Pingheng Tan, Xinhui Zhang

Abstract: The newly developed two-dimensional layered materials provide perfect platform for valley-spintronics exploration. To determine the prospect of utilizing the valley degree of freedom, it is of great importance to directly detect and understand the valley dynamics in these materials. Here, the exciton valley dynamics in monolayer WSe$_2$ is investigated by the two-color pump-probe magneto-optical K… ▽ More The newly developed two-dimensional layered materials provide perfect platform for valley-spintronics exploration. To determine the prospect of utilizing the valley degree of freedom, it is of great importance to directly detect and understand the valley dynamics in these materials. Here, the exciton valley dynamics in monolayer WSe$_2$ is investigated by the two-color pump-probe magneto-optical Kerr technique. By tuning the probe photon energy in resonance with the free excitons and trions, the valley relaxation time of different excitonic states in monolayer WSe$_2$ is determined. Valley relaxation time of the free exciton in monolayer WSe$_2$ is confirmed to be several picoseconds. A slow valley polarization relaxation process is observed to be associated with the trions, showing that the valley lifetime for trions is one order of magnitude longer than that of free excitons. This finding suggests that trion can be a good candidate for valleytronics application. △ Less

Submitted 8 May, 2016; v1 submitted 16 July, 2015; originally announced July 2015.

Anomalous valley polarization in monolayer MoSe2

Authors: Anmin Zhang, Jiahe Fan, Yusheng Li, Jianting Ji, Guihua Zhao, Tianlong Xia, Tengfei Yan, Xinhui Zhang, Wei Zhang, Xiaoqun Wang, Qingming Zhang

Abstract: Modern electronic devices heavily rely on the accurate control of charge and spin of electrons. The emergence of controllable valley degree of freedom brings new possibilities and presents a promising prospect towards valleytronics. Recently, valley excitation selected by chiral optical pumping has been observed in monolayer MoS2. In this work, we report polarized photoluminescence (PL) measuremen… ▽ More Modern electronic devices heavily rely on the accurate control of charge and spin of electrons. The emergence of controllable valley degree of freedom brings new possibilities and presents a promising prospect towards valleytronics. Recently, valley excitation selected by chiral optical pumping has been observed in monolayer MoS2. In this work, we report polarized photoluminescence (PL) measurements for monolayer MoSe2, another member of the family of transition-metal-dichalcogenides (MX2), and observe drastic difference from the outcomes of MoS2. In particular, we identify a valley polarization (VP) up to 70% for B exciton, while that for A exciton is less than 3%. Besides, we also find a small but finite negative VP for A- trion. These results reveal several new intra- and inter-valley scattering processes which significantly affect valley polarization, hence provide new insights into exciton physics in monolayer MX2 and possible valleytronic applications. △ Less

Submitted 30 March, 2015; originally announced March 2015.

Comments: 19 pages including supplementary information, 4 figures, 30 references

Valley depolarization in monolayer WSe2

Authors: Tengfei Yan, Xiaofen Qiao, Pingheng Tan, Xinhui Zhang

Abstract: We have systematically examined the circular polarization of monolayer WSe2 at different temperature, excitation energy and exciton density. The valley depolarization in WSe2 is experimentally confirmed to be governed by the intervalley electron-hole exchange interaction. More importantly, a non-monotonic dependence of valley circular polarization on the excitation power density has been observed,… ▽ More We have systematically examined the circular polarization of monolayer WSe2 at different temperature, excitation energy and exciton density. The valley depolarization in WSe2 is experimentally confirmed to be governed by the intervalley electron-hole exchange interaction. More importantly, a non-monotonic dependence of valley circular polarization on the excitation power density has been observed, providing the experimental evidence for the non-monotonic dependence of exciton intervalley scattering rate on the excited exciton density. The physical origination of our experimental observations has been proposed, which is in analogy to the D'yakonov-Perel' mechanism that is operative in conventional GaAs quantum well systems. Our experimental results are fundamentally important for well understanding the valley psudospin relaxation in atomically thin transition metal dichalcogenides. △ Less

Submitted 11 March, 2015; v1 submitted 25 February, 2015; originally announced February 2015.

Gate Tuning of High-Performance InSe-Based Photodetectors Using Graphene Electrodes

Authors: Wengang Luo, Yufei Cao, Pingan Hu, Kaiming Cai, Qi Feng, Faguang Yan, Tengfei Yan, Xinhui Zhang, Kaiyou Wang

Abstract: In order to increase the response speed of the InSe-based photodetector with high photoresponsivity, graphene is used as the transparent electrodes to modify the difference of the work function between the electrodes and the InSe. As expected, the response speed of InSe/graphene photodetectors is down to 120 μs, which is about 40 times faster than that of our InSe/metal device. And it can also be… ▽ More In order to increase the response speed of the InSe-based photodetector with high photoresponsivity, graphene is used as the transparent electrodes to modify the difference of the work function between the electrodes and the InSe. As expected, the response speed of InSe/graphene photodetectors is down to 120 μs, which is about 40 times faster than that of our InSe/metal device. And it can also be tuned by the back-gate voltage from 310 μs down to 100 μs. With high response speed, the photoresponsivity can reach as high as 60 AW-1 simultaneously. Meanwhile the InSe/graphene photodetectors possess a broad spectral range at 400-1000 nm. The design of 2D crystal/graphene electrical contacts could be important for high performance optoelectronic devices. △ Less

Submitted 30 June, 2015; v1 submitted 16 January, 2015; originally announced January 2015.

Comments: 27 pages, 10 figures in Advanced optical materials 2015

Strong enhancement of photoresponsivity with shrinking the electrodes spacing in few layer GaSe photodetectors

Authors: Yufei Cao, Kaiming Cai, Pingan Hu, Lixia Zhao, Tengfei Yan, Xinhui Zhang, Xiaoguang Wu, Kaiyou Wang, Houzhi Zheng

Abstract: A critical challenge for the integration of the optoelectronics is that photodetectors have relatively poor sensitivities at the nanometer scale. It is generally believed that a large electrodes spacing in photodetectors is required to absorb sufficient light to maintain high photoresponsivity and reduce the dark current. However, this will limit the optoelectronic integration density. Through spa… ▽ More A critical challenge for the integration of the optoelectronics is that photodetectors have relatively poor sensitivities at the nanometer scale. It is generally believed that a large electrodes spacing in photodetectors is required to absorb sufficient light to maintain high photoresponsivity and reduce the dark current. However, this will limit the optoelectronic integration density. Through spatially resolved photocurrent investigation, we find that the photocurrent in metal-semiconductor-metal (MSM) photodetectors based on layered GaSe is mainly generated from the photoexcited carriers close to the metal-GaSe interface and the photocurrent active region is always close to the Schottky barrier with higher electrical potential. The photoresponsivity monotonically increases with shrinking the spacing distance before the direct tunneling happen, which was significantly enhanced up to 5,000 AW-1 for the bottom contacted device at bias voltage 8 V and wavelength of 410 nm. It is more than 1,700-fold improvement over the previously reported results. Besides the systematically experimental investigation of the dependence of the photoresponsivity on the spacing distance for both the bottom and top contacted MSM photodetectors, a theoretical model has also been developed to well explain the photoresponsivity for these two types of device configurations. Our findings realize shrinking the spacing distance and improving the performance of 2D semiconductor based MSM photodetectors simultaneously, which could pave the way for future high density integration of 2D semiconductor optoelectronics with high performances. △ Less

Submitted 5 August, 2014; originally announced August 2014.

Comments: 25 pages, 4 figures

Journal ref: Scitific Reports 5,8130(2015)

The Kinetics of Chirality Assignment in Catalytic Single Walled Carbon Nanotube Growth

Authors: Ziwei Xu, Tianying Yan, Feng Ding

Abstract: Chirality-selected single-walled carbon nanotubes (SWCNTs) ensure a great potential of building ~1 nm sized electronics. However, the reliable method for chirality-selected SWCNT is still pending. Here we present a theoretical study on the SWCNT's chirality assignment and control during the catalytic growth. This study reveals that the chirality of a SWCNT is determined by the kinetic incorporatio… ▽ More Chirality-selected single-walled carbon nanotubes (SWCNTs) ensure a great potential of building ~1 nm sized electronics. However, the reliable method for chirality-selected SWCNT is still pending. Here we present a theoretical study on the SWCNT's chirality assignment and control during the catalytic growth. This study reveals that the chirality of a SWCNT is determined by the kinetic incorporation of the pentagon formation during SWCNT nucleation. Therefore, chirality is randomly assigned on a liquid catalyst surface. Furthermore, based on the understanding, two potential methods of synthesizing chirality-selected SWCNTs are proposed: i) by using Ta, W, Re, Os, or their alloys as solid catalysts, and ii) by changing the SWCNT's chirality frequently during the growth. △ Less

Submitted 27 March, 2014; originally announced March 2014.

Mechano-freezing of the ambient water

Authors: Xi Zhang, Tingting Yan, Bo Zou, Chang Q Sun

Abstract: Raman spectroscopy examination of the 25 deg-C water freezing under compression revealed transition from 1.35 GPa to 0.86 GPa upon ice being formed at continued volume change. The transition is associated with a slight blue shift of the high-frequency phonon (omiga_H ~ 3120 cm-1) and creation of the low-frequency phonons (Omiga_L ~ 200 cm-1). In the liquid and in the solid phase, the increased pre… ▽ More Raman spectroscopy examination of the 25 deg-C water freezing under compression revealed transition from 1.35 GPa to 0.86 GPa upon ice being formed at continued volume change. The transition is associated with a slight blue shift of the high-frequency phonon (omiga_H ~ 3120 cm-1) and creation of the low-frequency phonons (Omiga_L ~ 200 cm-1). In the liquid and in the solid phase, the increased pressure softens the Omiga_H and stiffens the Omida_L, which indicates the presence of the inter-electron-pair repulsion in both liquid and solid water. △ Less

Submitted 5 October, 2013; originally announced October 2013.

Dynamical generation of phase-squeezed states in a two-component Bose-Einstein condensates

Authors: G. R. Jin, Y. An, T. Yan, Z. S. Lu

Abstract: As an "input" state of a linear (Mach-Zehnder or Ramsey) interferometer, the phase-squeezed state proposed by Berry and Wiseman exhibits the best sensitivity approaching to the Heisenberg limit [Phys. Rev. Lett. 85, 5098 (2000)]. In this paper, we find that it can be generated dynamically with atomic Bose-Einstein condensates confined in a symmetric double well. Similar with the Berry and Wiseman'… ▽ More As an "input" state of a linear (Mach-Zehnder or Ramsey) interferometer, the phase-squeezed state proposed by Berry and Wiseman exhibits the best sensitivity approaching to the Heisenberg limit [Phys. Rev. Lett. 85, 5098 (2000)]. In this paper, we find that it can be generated dynamically with atomic Bose-Einstein condensates confined in a symmetric double well. Similar with the Berry and Wiseman's state, the prepared states show the squeezing along spin operator S_y and the anti-squeezing along S_z, leading to a sub-shot-noise of the phase sensitivity. △ Less

Submitted 22 December, 2010; v1 submitted 14 November, 2010; originally announced November 2010.

Comments: 6.2 pages, 4 figures

Journal ref: Physical Review A 82, 063622 (2010)

On Tsallis Entropy Bias and Generalized Maximum Entropy Models

Authors: Yuexian Hou, Tingxu Yan, Peng Zhang, Dawei Song, Wenjie Li

Abstract: In density estimation task, maximum entropy model (Maxent) can effectively use reliable prior information via certain constraints, i.e., linear constraints without empirical parameters. However, reliable prior information is often insufficient, and the selection of uncertain constraints becomes necessary but poses considerable implementation complexity. Improper setting of uncertain constraints ca… ▽ More In density estimation task, maximum entropy model (Maxent) can effectively use reliable prior information via certain constraints, i.e., linear constraints without empirical parameters. However, reliable prior information is often insufficient, and the selection of uncertain constraints becomes necessary but poses considerable implementation complexity. Improper setting of uncertain constraints can result in overfitting or underfitting. To solve this problem, a generalization of Maxent, under Tsallis entropy framework, is proposed. The proposed method introduces a convex quadratic constraint for the correction of (expected) Tsallis entropy bias (TEB). Specifically, we demonstrate that the expected Tsallis entropy of sampling distributions is smaller than the Tsallis entropy of the underlying real distribution. This expected entropy reduction is exactly the (expected) TEB, which can be expressed by a closed-form formula and act as a consistent and unbiased correction. TEB indicates that the entropy of a specific sampling distribution should be increased accordingly. This entails a quantitative re-interpretation of the Maxent principle. By compensating TEB and meanwhile forcing the resulting distribution to be close to the sampling distribution, our generalized TEBC Maxent can be expected to alleviate the overfitting and underfitting. We also present a connection between TEB and Lidstone estimator. As a result, TEB-Lidstone estimator is developed by analytically identifying the rate of probability correction in Lidstone. Extensive empirical evaluation shows promising performance of both TEBC Maxent and TEB-Lidstone in comparison with various state-of-the-art density estimation methods. △ Less

Submitted 7 April, 2010; originally announced April 2010.

Comments: 29 pages