Showing 1–50 of 80 results for author: Dong, L

Robust quantum-droplet necklace clusters in three dimensions

Authors: Liangwei Dong, Dongshuai Liu, Boris A. Malomed

Abstract: We report the existence of quasi-stable ring-shaped (necklace-shaped) clusters built, in the free space, of 3D quantum droplets (QDs) in a binary Bose-Einstein condensate, modeled by the Gross-Pitaevskii equations with the Lee-Huang-Yang corrections. The QD clusters exhibit diverse dynamical behaviors, including contraction, oscillations, and expansion, depending on the cluster's initial radius. A… ▽ More We report the existence of quasi-stable ring-shaped (necklace-shaped) clusters built, in the free space, of 3D quantum droplets (QDs) in a binary Bose-Einstein condensate, modeled by the Gross-Pitaevskii equations with the Lee-Huang-Yang corrections. The QD clusters exhibit diverse dynamical behaviors, including contraction, oscillations, and expansion, depending on the cluster's initial radius. A phase shift between adjacent QDs imparts net angular momentum to the cluster, inducing its permanent rotation. Through the energy-minimization analysis, we predict equilibrium values of the necklace radius that support persistent rotation with negligible radial pulsations. In this regime, the clusters evolve as robust entities, maintaining the azimuthal symmetry in the course of the evolution, even in the presence of considerable perturbations. Necklace "supervortex" clusters, composed of QDs with inner vorticity 1 and global vorticity M, imprinted onto the cluster, may also persist for a long time. The reported findings may facilitate the experimental realization of complex self-sustained quantum states in the 3D free space. △ Less

Submitted 27 November, 2025; originally announced November 2025.

Comments: 7 pages, 5 figures, to be published in Chaos Solitons & Fractals

Collective Enhancement of Photon Blockade via Two-Photon Interactions

Authors: Lijuan Dong, Aanal Jayesh Shah, Peter Kirton, Hadiseh Alaeian, Simone Felicetti

Abstract: Analogous to Coulomb blockade for electrons, photon blockade is a key quantum optical effect in which the presence of one photon prevents the transmission of subsequent ones through a nonlinear medium. Beyond its fundamental interest, photon and multi-photon blockade are actively studied as mechanisms for generating technologically-relevant quantum states of light. Although photon blockade typical… ▽ More Analogous to Coulomb blockade for electrons, photon blockade is a key quantum optical effect in which the presence of one photon prevents the transmission of subsequent ones through a nonlinear medium. Beyond its fundamental interest, photon and multi-photon blockade are actively studied as mechanisms for generating technologically-relevant quantum states of light. Although photon blockade typically requires achieving strong light-matter coupling, increasing the number of atoms fails to enhance antibunching. Here, we analyze the optical transmission properties of a quantum resonator that embeds a two-photon-coupled ensemble of emitters, combining an approximate analytical approach with full quantum numerical simulations. We show that when light and matter are coupled via a two-photon interaction, both single- and multi-photon blockade can benefit from a collective enhancement. We propose different driving schemes in which the second or third-order correlation functions are strongly suppressed with increasing atom number. Differently from established methods, this collective enhancement of non-classical properties occurs with unitary transmission and is ultimately constrained only by decoherence. This demonstrates that collective two-photon couplings are a powerful mechanism for realizing photon blockade even in platforms where individual strong coupling is not achievable. △ Less

Submitted 14 November, 2025; originally announced November 2025.

Comments: 17 pages, 7 figures

Hopfions in the Lee-Huang-Yang superfluids

Authors: Liangwei Dong, Mingjing Fan, Boris A. Malomed, Yaroslav V. Kartashov

Abstract: It is known that, under appropriate conditions, mean-field interactions can be canceled in binary BEC, leading to the formation of the Lee-Huang-Yang (LHY) superfluid, in which the nonlinearity is solely represented by the quartic LHY term. In this work we systematically investigate the existence, stability and evolution of hopfion states in this species of quantum matter. They are characterized b… ▽ More It is known that, under appropriate conditions, mean-field interactions can be canceled in binary BEC, leading to the formation of the Lee-Huang-Yang (LHY) superfluid, in which the nonlinearity is solely represented by the quartic LHY term. In this work we systematically investigate the existence, stability and evolution of hopfion states in this species of quantum matter. They are characterized by two independent topological winding numbers: inner twist $s$ of the vortex-ring core and overall vorticity $m$. The interplay between the LHY self-repulsion and a trapping harmonic-oscillator potential results in stability of the hopfions with $s = 1$ and $m$ ranging from $0$ to $4$. The hopfions exhibit distinct topological phase distributions along the vertical axis and the radial direction in the horizontal plane. Their effective radius and peak density increase with the chemical potential, along with expansion of the vortex-ring core. Although the instability domain of the hopfion modes broadens with the increase of $m$, stable hopfions persist in a wide range of the chemical potential, up to $m=4$, at least, provided that the norm exceeds a certain threshold value. The predictions are experimentally accessible in currently used BEC setups. △ Less

Submitted 14 November, 2025; originally announced November 2025.

Comments: 8 pages, 5 figures, to be published in Chaos, Solitons & Fractals

UniGenX: a unified generative foundation model that couples sequence, structure and function to accelerate scientific design across proteins, molecules and materials

Authors: Gongbo Zhang, Yanting Li, Renqian Luo, Pipi Hu, Yang Yang, Zeru Zhao, Lingbo Li, Guoqing Liu, Zun Wang, Ran Bi, Kaiyuan Gao, Liya Guo, Yu Xie, Chang Liu, Jia Zhang, Tian Xie, Robert Pinsler, Claudio Zeni, Ziheng Lu, Hongxia Hao, Yingce Xia, Marwin Segler, Maik Riechert, Wei Yang, Hao Jiang , et al. (9 additional authors not shown)

Abstract: Function in natural systems arises from one-dimensional sequences forming three-dimensional structures with specific properties. However, current generative models suffer from critical limitations: training objectives seldom target function directly, discrete sequences and continuous coordinates are optimized in isolation, and conformational ensembles are under-modeled. We present UniGenX, a unifi… ▽ More Function in natural systems arises from one-dimensional sequences forming three-dimensional structures with specific properties. However, current generative models suffer from critical limitations: training objectives seldom target function directly, discrete sequences and continuous coordinates are optimized in isolation, and conformational ensembles are under-modeled. We present UniGenX, a unified generative foundation model that addresses these gaps by co-generating sequences and coordinates under direct functional and property objectives across proteins, molecules, and materials. UniGenX represents heterogeneous inputs as a mixed stream of symbolic and numeric tokens, where a decoder-only autoregressive transformer provides global context and a conditional diffusion head generates numeric fields steered by task-specific tokens. Besides the new high SOTAs on structure prediction tasks, the model demonstrates state-of-the-art or competitive performance for the function-aware generation across domains: in materials, it achieves "conflicted" multi-property conditional generation, yielding 436 crystal candidates meeting triple constraints, including 11 with novel compositions; in chemistry, it sets new benchmarks on five property targets and conformer ensemble generation on GEOM; and in biology, it improves success in modeling protein induced fit (RMSD < 2 Å) by over 23-fold and enhances EC-conditioned enzyme design. Ablation studies and cross-domain transfer substantiate the benefits of joint discrete-continuous training, establishing UniGenX as a significant advance from prediction to controllable, function-aware generation. △ Less

Submitted 26 August, 2025; v1 submitted 9 March, 2025; originally announced March 2025.

Optimization and Characterization of Thermoelectric Properties in Selenium-Doped Bismuth Telluride Ultra Thin Films

Authors: Kien Trung Nguyen, Lan Anh Dong, Hien Thi Dinh, Thi Huyen Trang Bui, Son Truong Chu, Thuat Nguyen-Tran, Chi Hieu Hoang, Hung Quoc Nguyen

Abstract: Thermoelectricity in telluride materials is often improved by replacing telluride with selenium in its crystal. Most work, however, focuses on bulk crystal and leaves the 2D thin films intact. In this paper, we optimize the fabrication of selenium-doped bismuth telluride (Bi$_2$Te$_{3-\rm{x}}$Se$_{\rm{x}}$) thin films using a 3-source thermal co-evaporation. Thermoelectric properties, including th… ▽ More Thermoelectricity in telluride materials is often improved by replacing telluride with selenium in its crystal. Most work, however, focuses on bulk crystal and leaves the 2D thin films intact. In this paper, we optimize the fabrication of selenium-doped bismuth telluride (Bi$_2$Te$_{3-\rm{x}}$Se$_{\rm{x}}$) thin films using a 3-source thermal co-evaporation. Thermoelectric properties, including the Seebeck coefficient and electrical resistivity, are systematically characterized to evaluate the material's performance for thermoelectric applications near room temperature. The thin films were deposited under carefully controlled conditions, with the evaporation rates of bismuth, tellurium, and selenium precisely monitored to achieve the desired stoichiometry and crystalline phase. Finally, thermoelectricity in Bi$_2$Te$_{3-\rm{x}}$Se$_{\rm{x}}$ at the ultra-thin regime is investigated. We consistently obtain films with thickness near 30 nm with a Seebeck coefficient of 400 $μ$V/K and a power factor of 1 mW/mK$^2$. △ Less

Submitted 27 October, 2024; originally announced October 2024.

Fully printed flexible perovskite solar modules with improved energy alignment by tin oxide surface modification

Authors: Lirong Dong, Shudi Qiu, Jose Garcia Cerrillo, Michael Wagner, Olga Kasian, Sarmad Feroze, Dongju Jang, Chaohui Li, Vincent M. Le Corre, Kaicheng Zhang, Heiko Peisert, Felix U Kosasih, Charline Arrive, Tian Du, Fu Yang, Christoph J. Brabec, Hans-Joachim Egelhaaf

Abstract: Fully printed flexible perovskite solar cells (f-PSCs) show great potential for the commercialization of perovskite photovoltaics owing to their compatibility with high-throughput roll-to-roll (R2R) production. However, the challenge remains in the deficiency in controlling interfacial recombination losses of the functional layer, causing remarkable loss of power conversion efficiency (PCE) in ind… ▽ More Fully printed flexible perovskite solar cells (f-PSCs) show great potential for the commercialization of perovskite photovoltaics owing to their compatibility with high-throughput roll-to-roll (R2R) production. However, the challenge remains in the deficiency in controlling interfacial recombination losses of the functional layer, causing remarkable loss of power conversion efficiency (PCE) in industrial production. Here, a fullerene-substituted alkylphosphonic acid dipole layer is introduced between the R2R-printed tin oxide electron transport layer and the perovskite active layer to reduce the energetic barrier and to suppress surface recombination at the buried interface. The resulting f-PSCs exhibit a PCE of 17.0% with negligible hysteresis, retain 95% of their initial PCE over 3000 bending cycles and achieve a T95 lifetime of 1200 h under 1 sun and 65 degreeC in nitrogen atmosphere. Moreover, the fully printed flexible perovskite solar mini-modules (f-PSMs) with a 20.25 cm2 aperture area achieve a PCE of 11.6%. The encapsulated f-PSMs retain 90% of their initial PCE after 500 h damp-heat testing at 65 degreeC and 85% relative humidity (ISOS-D3). This work marks an important progress toward the realization of efficient and stable flexible perovskite photovoltaics for commercialization. △ Less

Submitted 5 June, 2024; originally announced June 2024.

Revealing the Microscopic Mechanism of Elementary Vortex Pinning in Superconductors

Authors: C. Chen, Y. Liu, Y. Chen, Y. N. Hu, T. Z. Zhang, D. Li, X. Wang, C. X. Wang, Z. Y. W. Lu, Y. H. Zhang, Q. L. Zhang, X. L. Dong, R. Wang, D. L. Feng, T. Zhang

Abstract: Vortex pinning is a crucial factor that determines the critical current of practical superconductors and enables their diverse applications. However, the underlying mechanism of vortex pinning has long been elusive, lacking a clear microscopic explanation. Here using high-resolution scanning tunneling microscopy, we studied single vortex pinning induced by point defect in layered FeSe-based superc… ▽ More Vortex pinning is a crucial factor that determines the critical current of practical superconductors and enables their diverse applications. However, the underlying mechanism of vortex pinning has long been elusive, lacking a clear microscopic explanation. Here using high-resolution scanning tunneling microscopy, we studied single vortex pinning induced by point defect in layered FeSe-based superconductors. We found the defect-vortex interaction drives low-energy vortex bound states away from EF, creating a "mini" gap that effectively lowers the system energy and enhances pinning. By measuring the local density-of-states, we directly obtained the elementary pinning energy and estimated the pinning force via the spatial gradient of pinning energy. The results are consistent with bulk critical current measurement. Furthermore, we show that a general microscopic quantum model incorporating defect-vortex interaction can naturally capture our observation. It suggests that the local pairing near pinned vortex core is actually enhanced compared to unpinned vortex, which is beyond the traditional understanding that non-superconducting regions pin vortices. Our study thus unveils a general microscopic mechanism of vortex pinning in superconductors, and provides insights for enhancing the critical current of practical superconductors. △ Less

Submitted 27 September, 2024; v1 submitted 26 March, 2024; originally announced March 2024.

Comments: 32 pages, 18 figures, Supplementary Materials included. Comments are welcome

Journal ref: Phys. Rev. X 14, 041039 (2024)

Stable higher-order vortex quantum droplets in an annular potential

Authors: Liangwei Dong, Mingjing Fan, Boris A. Malomed

Abstract: We address the existence, stability, and evolution of two-dimensional vortex quantum droplets (VQDs) in binary Bose-Einstein condensates trapped in a ring-shaped potential. The interplay of the Lee-Huang-Yang-amended nonlinearity and trapping potential supports two VQD branches, controlled by the radius, width and depth of the potential profile. While the lower-branch VQDs, bifurcating from the sy… ▽ More We address the existence, stability, and evolution of two-dimensional vortex quantum droplets (VQDs) in binary Bose-Einstein condensates trapped in a ring-shaped potential. The interplay of the Lee-Huang-Yang-amended nonlinearity and trapping potential supports two VQD branches, controlled by the radius, width and depth of the potential profile. While the lower-branch VQDs, bifurcating from the system's linear modes, are completely unstable, the upper branch is fully stable for all values of the topological charge $m$ and potential's parameters. Up to $m=12$ (at least), stable VQDs obey the {\it anti-Vakhitov-Kolokolov} criterion. In the limit of an extremely tight radial trap, the modulational instability of the quasi-1D azimuthal VQDs is studied analytically. We thus put forward an effective way to produce stable VQDs with higher vorticity but a relatively small number of atoms, which is favorable for experimental realization. △ Less

Submitted 13 January, 2024; originally announced January 2024.

Comments: 8 pages, 5 figures, to be published in Chaos, Solitons and Fractals

Geodynamics and artificial gravity in space-time crystals under slow perturbation and deformation

Authors: Anzhuoer Li, Liang Dong, Qian Niu

Abstract: We present a theory of geodynamics in a space-time crystal based on an event wave packet constructed from the Floquet-Bloch waves, which not only involve a scalar dispersion function but also a Berry curvature tensor in the phase space manifold of space-time and the reciprocal quasi energy-momentum. In the presence of structural deformation, this theory is naturally extended into a covariant form… ▽ More We present a theory of geodynamics in a space-time crystal based on an event wave packet constructed from the Floquet-Bloch waves, which not only involve a scalar dispersion function but also a Berry curvature tensor in the phase space manifold of space-time and the reciprocal quasi energy-momentum. In the presence of structural deformation, this theory is naturally extended into a covariant form with the introduction of a lattice connection constructed out of the gradients of the local lattice vectors. The geodesic equation for a particle not only involves the lattice connection but also higher-order corrections from space-time inhomogeneity of Berry curvatures and quasi energy-momentum dispersion gradients. These emergent connections and metric terms in the geodesic equations indicate the potential to experimentally realize artificial gravitational effects, thereby establishing a direct conceptual link between general relativity and quantum theory. △ Less

Submitted 8 June, 2025; v1 submitted 25 October, 2023; originally announced October 2023.

Determining Quasi-Equilibrium Electron and Hole Distributions of Plasmonic Photocatalysts using Photomodulated X-ray Absorption Spectroscopy

Authors: Levi D. Palmer, Wonseok Lee, Chung Li Dong, Ru-Shi Liu, Nianqiang Wu, Scott K. Cushing

Abstract: Most photocatalytic and photovoltaic devices operate under broadband, constant illumination. Electron and hole dynamics in these devices, however, are usually measured using ultrafast pulsed lasers in a narrow wavelength range. In this work, we prove that steady-state, photomodulated X-ray spectra from a non-time-resolved synchrotron beamline can be used to estimate electron and hole distributions… ▽ More Most photocatalytic and photovoltaic devices operate under broadband, constant illumination. Electron and hole dynamics in these devices, however, are usually measured using ultrafast pulsed lasers in a narrow wavelength range. In this work, we prove that steady-state, photomodulated X-ray spectra from a non-time-resolved synchrotron beamline can be used to estimate electron and hole distributions. A set of plasmonic metal core-shell nanoparticles is designed to systematically isolate photothermal, hot electron, and thermalized electron-hole pairs in a TiO2 shell. Steady-state changes in the Ti L2,3 edge are measured with and without continuous-wave illumination of the nanoparticle's localized surface plasmon resonance. Ab initio excited-state X-ray theory developed for transient X-ray measurements is then applied to model the experimental spectra in an attempt to extract the resultant steady-state carrier distributions. The results suggest that, within error, the quasi-equilibrium carrier distribution can be determined even from relatively noisy data with mixed excited-state phenomena. △ Less

Submitted 24 January, 2024; v1 submitted 30 August, 2023; originally announced August 2023.

Comments: 41 pages, 18 figures

Journal ref: ACS Nano 2024, 18, 9344-9353

Superconductivity up to 17 K in the high-pressure rhombohedral-I phase of ReO3: a potential oxide analogy of hydride superconductors

Authors: P. F. Shan, T. L. Lu, Z. Y. Liu, Y. Y. Jiao, P. T. Yang, J. Hou, L. Ma, Y. Uwatoko, X. L. Dong, B. S. Wang, M. Liu, J. P. Sun, J. -G. Cheng

Abstract: As an A-site-vacant perovskite-type oxide, ReO3 undergoes sequential pressure-driven structural transitions associated with the rotation of ReO6 octahedra. The rhombohedral-I phase in the pressure range of 12-39 GPa is featured by a lattice of nearly close-packed oxygen layers intercalated with Re cations, in reminiscent of the recently discovered superhydride superconductors. A combined study of… ▽ More As an A-site-vacant perovskite-type oxide, ReO3 undergoes sequential pressure-driven structural transitions associated with the rotation of ReO6 octahedra. The rhombohedral-I phase in the pressure range of 12-39 GPa is featured by a lattice of nearly close-packed oxygen layers intercalated with Re cations, in reminiscent of the recently discovered superhydride superconductors. A combined study of first-principles calculations and transport measurements under high pressures enabled us to discover superconductivity in the rhombohedral-I phase, and it shows a dome-shaped Tc(P) with a maximum Tc of 17 K at about 30 GPa. In addition to the enhanced density of states at Fermi level compared to that of the ambient phase, the vibrations of hexagonal-close-packed oxygen lattice significantly strengthen the electron-phonon coupling, which is responsible for observed superconductivity with a relatively high Tc. The present work thus establishes a rare case among oxide superconductors that the light-element oxygen lattice plays a crucial role in inducing superconductivity. △ Less

Submitted 1 April, 2025; v1 submitted 18 April, 2023; originally announced April 2023.

Journal ref: National Science Review, nwag425 (2026)

Evidence for a robust sign-changing s-wave order parameter in monolayer films of superconducting Fe (Se,Te)/Bi2Te3

Authors: Guannan Chen, Anuva Aishwarya, Mark R. Hirsbrunner, Jorge Olivares Rodriguez, Lin Jiao, Lianyang Dong, Nadya Mason, Dale Van Harlingen, John Harter, Stephen Wilson, Taylor L. Hughes, Vidya Madhavan

Abstract: The Fe-based superconductor Fe (Se,Te) combines non-trivial topology with unconventional superconductivity and may be an ideal platform to realize exotic states such as high-order topological corner modes and Majorana modes. Thin films of Fe (Se,Te) are important for device fabrication, phase sensitive transport measurements and for realizing proposals to engineer higher-order modes. However, whil… ▽ More The Fe-based superconductor Fe (Se,Te) combines non-trivial topology with unconventional superconductivity and may be an ideal platform to realize exotic states such as high-order topological corner modes and Majorana modes. Thin films of Fe (Se,Te) are important for device fabrication, phase sensitive transport measurements and for realizing proposals to engineer higher-order modes. However, while bulk Fe (Se,Te) has been extensively studied with a variety of techniques, the nature of the superconducting order parameter in the monolayer limit has not yet been explored. In this work, we study monolayer films of Fe (Se,Te) on Bi2Te3 with scanning tunneling spectroscopy and Bogoliubov quasiparticle interference (BQPI). We discover that the monolayer Fe (Se,Te)/Bi2Te3 heterostructures host a robust, multigap superconducting state that strongly resembles the bulk. BQPI maps at the gap energies show a strong spatial modulation, oriented 45 degrees to the Fe-Se bond direction. Analysis of the phase-referenced quasiparticle interference signal reveals a sign-changing s-wave order parameter similar to the bulk. Moreover, we observe a unique pattern of sign changes in the BQPI signal which have not been observed in the bulk. Our work establishes monolayer Fe (Se,Te)/Bi2Te3 as a robust multi-band unconventional superconductor and sets the stage for explorations of non-trivial topology in this highly-tunable system. △ Less

Submitted 11 September, 2022; originally announced September 2022.

Comments: 23 pages, 4 figures

Report number: npj Quantum Mater. 7, 110 (2022)

Vortex solitons in twisted circular waveguide arrays

Authors: Liangwei Dong, Yaroslav V. Kartashov, Lluis Torner, Albert Ferrando

Abstract: We address the formation of topological states in twisted circular waveguide arrays and find that twisting leads to important differences of the fundamental properties of new vortex solitons with opposite topological charges that arise in the nonlinear regime. We find that such system features the rare property that clockwise and counter-clockwise vortex states are nonequivalent. Focusing on array… ▽ More We address the formation of topological states in twisted circular waveguide arrays and find that twisting leads to important differences of the fundamental properties of new vortex solitons with opposite topological charges that arise in the nonlinear regime. We find that such system features the rare property that clockwise and counter-clockwise vortex states are nonequivalent. Focusing on arrays with C_{6v} discrete rotation symmetry, we find that a longitudinal twist stabilizes the vortex solitons with the lowest topological charges m=+-1, which are always unstable in untwisted arrays with the same symmetry. Twisting also leads to the appearance of instability domains for otherwise stable solitons with m=+-2 and generates vortex modes with topological charges m=+-3 that are forbidden in untwisted arrays. By and large, we establish a rigorous relation between the discrete rotation symmetry of the array, its twist direction, and the possible soliton topological charges. △ Less

Submitted 9 August, 2022; originally announced August 2022.

Comments: 6 pages, 5 figures, to appear in Physical Review Letters

Journal ref: Phys. Rev. Lett. 129, 123903 (2022)

Electronic and magnetic properties of intermetallic Kagome magnets $R$V$_6$Sn$_6$ ($R$ = Tb - Tm)

Authors: X. X. Zhang, Z. Y. Liu, Q. Cui, N. N. Wang, L. F. Shi, H. Zhang, X. L. Dong, J. P. Sun, Z. L. Dun, J. G. Cheng

Abstract: We present a systematic study of the structure, electronic, and magnetic properties of a new branch of intermetalllic compounds, $R$V$_6$Sn$_6$ ($R$ = Tb - Tm) by using X-ray diffraction, magnetic susceptibility, magnetization, electrical transport, and heat-capacity measurements. These compounds feature a combination of a non-magnetic vanadium kagome sublattice and a magnetic rare-earth triangula… ▽ More We present a systematic study of the structure, electronic, and magnetic properties of a new branch of intermetalllic compounds, $R$V$_6$Sn$_6$ ($R$ = Tb - Tm) by using X-ray diffraction, magnetic susceptibility, magnetization, electrical transport, and heat-capacity measurements. These compounds feature a combination of a non-magnetic vanadium kagome sublattice and a magnetic rare-earth triangular sublattice that supports various spin anisotropies based on different $R$ ions. We find magnetic orders for the $R$ = Tb, Dy, and Ho compounds at 4.4, 3, 2.5 K, respectively, while no ordering is detected down to 0.4 K for the $R$ = Er and Tm compounds with easy-plane anisotropies. Electronically, we found no superconductivity or charge ordering transition down to 0.4 K for any member of this family, while all compounds exhibit multi-band transport properties that originate from the band topology of the vanadium kagome sublattice. △ Less

Submitted 12 June, 2022; originally announced June 2022.

Comments: 9 pages, 5 figures

Pressure-induced superconductivity up to 9 K in the quasi-one-dimensional KMn6Bi5

Authors: Z. Y. Liu, Q. X. Dong, P. T. Yang, P. F. Shan, B. S. Wang, J. P. Sun, Y. Uwatoko, G. F. Chen, X. L. Dong, Z. X. Zhao, J. -G. Cheng

Abstract: The Mn-based superconductor is rare owing to the strong magnetic pair-breaking effect. Here we report on the discovery of pressure-induced superconductivity in KMn6Bi5, which becomes the first ternary Mn-based superconductor. At ambient pressure, the quasi-one-dimensional KMn6Bi5 is an antiferromagnetic metal with TN = 75 K. By measuring resistivity and ac magnetic susceptibility under hydrostatic… ▽ More The Mn-based superconductor is rare owing to the strong magnetic pair-breaking effect. Here we report on the discovery of pressure-induced superconductivity in KMn6Bi5, which becomes the first ternary Mn-based superconductor. At ambient pressure, the quasi-one-dimensional KMn6Bi5 is an antiferromagnetic metal with TN = 75 K. By measuring resistivity and ac magnetic susceptibility under hydrostatic pressures up to 14.2 GPa in a cubic anvil cell apparatus, we find that its antiferromagnetic transition can be suppressed completely at a critical pressure of Pc = 13 GPa, around which bulk superconductivity emerges and displays a superconducting dome with the maximal Tconset = 9.3 K achieved at about 14.2 GPa. The close proximity of superconductivity to a magnetic instability in the temperature-pressure phase diagram of KMn6Bi5 and an unusually large μ0Hc2(0) = 18.9 T exceeding the Pauli limit suggests an unconventional magnetism-mediated paring mechanism. In contrast to the binary MnP, the flexibility of the crystal structure and chemical compositions in the ternary AMn6Bi5 (A = alkali metal) can open a new avenue for finding more Mn-based superconductors. △ Less

Submitted 16 January, 2022; originally announced January 2022.

Comments: 4 figures

Dimension reduction induced anisotropic magnetic thermal conductivity in hematite nanowire

Authors: Qing Xi, Adili Ayiti, Lan Dong, Yuanyuan Wang, Jun Zhou, Xiangfan Xu

Abstract: The thermophysical properties near the magnetic phase transition point is of great importance in the study of critical phenomenon. Low-dimensional materials are suggested to hold different thermophysical properties comparing to their bulk counterpart due to the dimension induced quantum confinement and anisotropy. In this work, we measured the thermal conductivity of $α$-Fe$_2$O$_3$ nanowires alon… ▽ More The thermophysical properties near the magnetic phase transition point is of great importance in the study of critical phenomenon. Low-dimensional materials are suggested to hold different thermophysical properties comparing to their bulk counterpart due to the dimension induced quantum confinement and anisotropy. In this work, we measured the thermal conductivity of $α$-Fe$_2$O$_3$ nanowires along [110] direction (growing direction) with temperature from 100K to 150K and found a dip of thermal conductivity near the Morin temperature. We found the thermal conductivity near Morin temperature varies with the angle between magnetic field and [110] direction of nanowire. More specifically, an angular-dependent thermal conductivity is observed, due to the magnetic field induced movement of magnetic domain wall. The angle corresponding to the maximum of thermal conductivity varies near the Morin transition temperature, due to the different magnetic easy axis as suggested by our calculation based on magnetic anisotropy energy. This angular dependence of thermal conductivity indicates that the easy axis of $α$-Fe$_2$O$_3$ nanowires is different from bulk $α$-Fe$_2$O$_3$ due to the geometric anisotropy. △ Less

Submitted 30 November, 2021; originally announced November 2021.

A density-wave-like transition in the polycrystalline V3Sb2 sample with bilayer kagome lattice

Authors: N. N. Wang, Y. H. Gu, M. A. McGuire, J. Q. Yan, L. F. Shi, Q. Cui, K. Y. Chen, Y. X. Wang, H. Zhang, H. X. Yang, X. L. Dong, K. Jiang, J. P. Hu, B. S. Wang, J. P. Sun, J. -G. Cheng

Abstract: Recently, transition-metal-based kagome metals have aroused much research interest as a novel platform to explore exotic topological quantum phenomena. Here we report on the synthesis, structure, and physical properties of a bilayer kagome lattice compound V3Sb2. The polycrystalline V3Sb2 samples were synthesized by conventional solid-state-reaction method in a sealed quartz tube at temperatures b… ▽ More Recently, transition-metal-based kagome metals have aroused much research interest as a novel platform to explore exotic topological quantum phenomena. Here we report on the synthesis, structure, and physical properties of a bilayer kagome lattice compound V3Sb2. The polycrystalline V3Sb2 samples were synthesized by conventional solid-state-reaction method in a sealed quartz tube at temperatures below 850 Celsius degree. Measurements of magnetic susceptibility and resistivity revealed consistently a density-wave-like transition at Tdw ~ 160 K with a large thermal hysteresis, even though some sample-dependent behaviors are observed presumably due to the different preparation conditions. Upon cooling through Tdw, no strong anomaly in lattice parameters and no indication of symmetry lowering were detected in powder x-ray diffraction measurements. This transition can be suppressed completely by applying hydrostatic pressures of about 1.8 GPa, around which no sign of superconductivity is observed down to 1.5 K. Specific-heat measurements reveal a relatively large Sommerfeld coefficient γ = 18.5 mJ/mol-K2, confirming the metallic ground state with moderate electronic correlations. Density functional theory calculations indicate that V3Sb2 shows a non-trivial topological crystalline property. Thus, our study makes V3Sb2 a new candidate of metallic kagome compound to study the interplay between density-wave-order, nontrivial band topology, and possible superconductivity. △ Less

Submitted 18 December, 2021; v1 submitted 26 October, 2021; originally announced October 2021.

Comments: 22 pages, 8 figures

Journal ref: Chin. Phys. B (2021)

Pressure-induced monotonic enhancement of Tc to over 30 K in the superconducting Pr0.82Sr0.18NiO2 thin films

Authors: N. N. Wang, M. W. Yang, Z. Yang, K. Y. Chen, H. Zhang, Q. H. Zhang, Z. H. Zhu, Y. Uwatoko, L. Gu, X. L. Dong, K. J. Jin, J. P. Sun, J. -G. Cheng

Abstract: The successful synthesis of superconducting infinite-layer nickelate thin films with the highest Tc ~ 15 K has reignited great enthusiasms on this family of potential analogue to high-Tc cuprates. Pursuing a higher Tc is always an imperative task in studying a new superconducting material system. Here we report high-quality Pr0.82Sr0.18NiO2 thin films with Tconset ~ 17 K synthesized by carefully t… ▽ More The successful synthesis of superconducting infinite-layer nickelate thin films with the highest Tc ~ 15 K has reignited great enthusiasms on this family of potential analogue to high-Tc cuprates. Pursuing a higher Tc is always an imperative task in studying a new superconducting material system. Here we report high-quality Pr0.82Sr0.18NiO2 thin films with Tconset ~ 17 K synthesized by carefully tuning the amount of CaH2 in the topological chemical reduction and the effect of pressure on its superconducting properties by measuring electrical resistivity under various pressures in a cubic anvil cell apparatus. We find that the onset temperature of the superconductivity, Tconset, can be enhanced monotonically from ~ 17 K at ambient pressure to ~ 31 K at 12.1 GPa without showing signatures of saturation upon increasing pressure. This encouraging result indicates that the Tc of infinite-layer nickelates superconductors still has room to go higher and it can be further boosted by applying higher pressures or strain engineering in the heterostructure films. △ Less

Submitted 25 February, 2022; v1 submitted 27 September, 2021; originally announced September 2021.

Comments: 16 pages, 4 figures

Journal ref: Nature Communications 13, 4367 (2022)

Detecting Carbon Nanotube Orientation with Topological Data Analysis of Scanning Electron Micrographs

Authors: Liyu Dong, Haibin Hang, Jin Gyu Park, Washington Mio, Richard Liang

Abstract: As the aerospace industry becomes increasingly demanding for stronger lightweight materials, the ultra-strong carbon nanotube (CNT) composites with highly aligned CNT network structures could be the answer. In this work, a novel methodology applying topological data analysis (TDA) to the scanning electron microscope (SEM) images was developed to detect CNT orientation. The CNT bundle extensions in… ▽ More As the aerospace industry becomes increasingly demanding for stronger lightweight materials, the ultra-strong carbon nanotube (CNT) composites with highly aligned CNT network structures could be the answer. In this work, a novel methodology applying topological data analysis (TDA) to the scanning electron microscope (SEM) images was developed to detect CNT orientation. The CNT bundle extensions in certain directions were summarized algebraically and expressed as visible barcodes. The barcodes were then calculated and converted into the total spread function $V(X,θ)$, from which the alignment fraction and the preferred direction could be determined. For validation purposes, the random CNT sheets were mechanically stretched at various strain ratios ranging from $0-40\%$, and quantitative TDA analysis was conducted based on the SEM images taken at random positions. The results showed high consistency ($R^2=0.975$) compared to the Herman's orientation factors derived from the polarized Raman spectroscopy and wide-angle X-ray scattering analysis. Additionally, the TDA method presented great robustness with varying SEM acceleration voltages and magnifications, which might alter the scope in alignment detection. With potential applications in nanofiber systems, this study offers a rapid and simple way to quantify CNT alignment, which plays a crucial role in transferring the CNT properties into engineering products. △ Less

Submitted 9 August, 2021; originally announced August 2021.

Competition between charge-density-wave and superconductivity in the kagome metal RbV3Sb5

Authors: N. N. Wang, K. Y. Chen, Q. W. Yin, Y. N. N. Ma, B. Y. Pan, X. Yang, X. Y. Ji, S. L. Wu, P. F. Shan, S. X. Xu, Z. J. Tu, C. S. Gong, G. T. Liu, G. Li, Y. Uwatoko, X. L. Dong, H. C. Lei, J. P. Sun, J. -G. Cheng

Abstract: The interplay between charge-density-wave (CDW) order and superconductivity (SC) in the Kagome metal RbV3Sb5 is studied by tracking the evolutions of their transition temperatures, T* and Tc, as a function of pressure (P) via measurements of resistivity and magnetic susceptibility under various hydrostatic pressures up to ~ 5 GPa. It is found that the CDW order at T* experiences a subtle modificat… ▽ More The interplay between charge-density-wave (CDW) order and superconductivity (SC) in the Kagome metal RbV3Sb5 is studied by tracking the evolutions of their transition temperatures, T* and Tc, as a function of pressure (P) via measurements of resistivity and magnetic susceptibility under various hydrostatic pressures up to ~ 5 GPa. It is found that the CDW order at T* experiences a subtle modification at Pc1 ~ 1.5 GPa before it is completely suppressed around Pc2 ~ 2.4 GPa. Accordingly, the superconducting transition Tc(P) exhibits a shallow M-shaped double superconducting dome with two extrema of Tconset ~ 4.4 K and 3.9 K around Pc1 and Pc2, respectively, leading to a fourfold enhancement of Tc with respect to that at ambient pressure. The constructed T-P phase diagram of RbV3Sb5 resembles that of CsV3Sb5, and shares similar features as many other unconventional superconducting systems with intertwined competing electronic orders. The strong competition between CDW and SC is also evidenced by the broad superconducting transition width in the coexistent region. Our results shed more light on the intriguing physics involving intertwined electronic orders in this novel topological kagome metal family. △ Less

Submitted 1 July, 2021; originally announced July 2021.

Comments: 19 pages, 5 figures

Journal ref: Phys. Rev. Research 3, 043018 (2021)

Visualizing nematic transition and nanoscale suppression of superconductivity in Fe(Te,Se)

Authors: He Zhao, Hong Li, Lianyang Dong, Binjie Xu, John Schneeloch, Ruidan Zhong, Minghu Fang, Genda Gu, John Harter, Stephen D. Wilson, Ziqiang Wang, Ilija Zeljkovic

Abstract: The interplay of different electronic phases underlies the physics of unconventional superconductors. One of the most intriguing examples is a high-Tc superconductor FeTe1-xSex: it undergoes both a topological transition, linked to the electronic band inversion, and an electronic nematic phase transition, associated with rotation symmetry breaking, around the same critical composition xc where sup… ▽ More The interplay of different electronic phases underlies the physics of unconventional superconductors. One of the most intriguing examples is a high-Tc superconductor FeTe1-xSex: it undergoes both a topological transition, linked to the electronic band inversion, and an electronic nematic phase transition, associated with rotation symmetry breaking, around the same critical composition xc where superconducting Tc peaks. At this regime, nematic fluctuations and symmetry-breaking strain could have an enormous impact, but this is yet to be fully explored. Using spectroscopic-imaging scanning tunneling microscopy, we study the electronic nematic transition in FeTe1-xSex as a function of composition. Near xc, we reveal the emergence of electronic nematicity in nanoscale regions. Interestingly, we discover that superconductivity is drastically suppressed in areas where static nematic order is the strongest. By analyzing atomic displacement in STM topographs, we find that small anisotropic strain can give rise to these strongly nematic localized regions. Our experiments reveal a tendency of FeTe1-xSex near x~0.45 to form puddles hosting static nematic order, suggestive of nematic fluctuations pinned by structural inhomogeneity, and demonstrate a pronounced effect of anisotropic strain on superconductivity in this regime. △ Less

Submitted 14 June, 2021; originally announced June 2021.

Comments: This is the original submitted version. The final version will appear in Nature Physics

Journal ref: Nature Physics 17, 903 (2021)

Highly efficient BiVO4 single-crystal nanosheets with dual modification: phosphorus doping and selective Ag modification

Authors: Can Fu, Baoyun Xu, Lingling Dong, Jinguo Zhai, Xuefei Wang, De-Yi Wang

Abstract: BiVO4, a visible-light response photocatalyst, has shown tremendous potential because of abundant raw material sources, good stability and low cost. There exist some limitations for further applicaitions due to poor capability to separate electron-hole pairs. In fact, a single-component modification strategy is barely adequate to obtain highy efficient photocatalytic performance. In this work, P s… ▽ More BiVO4, a visible-light response photocatalyst, has shown tremendous potential because of abundant raw material sources, good stability and low cost. There exist some limitations for further applicaitions due to poor capability to separate electron-hole pairs. In fact, a single-component modification strategy is barely adequate to obtain highy efficient photocatalytic performance. In this work, P substituted some of the V atoms from VO4 oxoanions, namely P was doped into the V sites in the host lattice of BiVO4 by a hydrothermal route. Meanwhile, Ag as an attractive and efficient electron-cocatalyst was selectively modified on the (010) facet of BiVO4 nanosheets via facile photo-deposition. As a result, the obtained dually modified BiVO4 sheets exhibited enhanced photocatalytic degradation property of methylene blue (MB). In detail, photocatalytic rate constant (k) was 2.285 min-1g-1, which was 2.78 times higher than pristine BiVO4 nanosheets. Actually, P-doping favored the formation of O vacancies, led to more charge carriers, and facilitated photocatalytic reaction. On the other hand, metallic Ag loaded on (010) facet effectively transferred photogenerated electrons, which consequently helped electron-hole pairs separation. The present work may enlighten new thoughts for smart design and controllable synthesis of highly efficient photocatalytic materials. △ Less

Submitted 19 May, 2021; originally announced May 2021.

Bulk superconductivity in FeTe$_{1-x}$Se$_{x}$ via physicochemical pumping of excess iron

Authors: Lianyang Dong, He Zhao, Ilija Zeljkovic, Stephen D. Wilson, John W. Harter

Abstract: The iron-based superconductor FeTe$_{1-x}$Se$_{x}$ has attracted considerable attention as a candidate topological superconductor owing to a unique combination of topological surface states and bulk high-temperature superconductivity. The superconducting properties of as-grown single crystals, however, are highly variable and synthesis dependent due to excess interstitial iron impurities incorpora… ▽ More The iron-based superconductor FeTe$_{1-x}$Se$_{x}$ has attracted considerable attention as a candidate topological superconductor owing to a unique combination of topological surface states and bulk high-temperature superconductivity. The superconducting properties of as-grown single crystals, however, are highly variable and synthesis dependent due to excess interstitial iron impurities incorporated during growth. Here we report a novel physicochemical process for pumping this interstitial iron out of the FeTe$_{1-x}$Se$_{x}$ matrix and achieving bulk superconductivity. Our method should have significant value for the synthesis of high-quality single crystals of FeTe$_{1-x}$Se$_{x}$ with large superconducting volume fractions. △ Less

Submitted 20 April, 2021; originally announced April 2021.

Journal ref: Physical Review Materials 3, 114801 (2019)

Berry curvature effects on quasiparticle dynamics in superconductors

Authors: Zhi Wang, Liang Dong, Cong Xiao, Qian Niu

Abstract: We construct a theory for the semiclassical dynamics of superconducting quasiparticles by following their wave-packet motion and reveal rich contents of Berry curvature effects in the phase-space spanned by position and momentum. These Berry curvatures are traced back to the characteristics of superconductivity, including the nontrivial momentum-space geometry of superconducting pairing, the real-… ▽ More We construct a theory for the semiclassical dynamics of superconducting quasiparticles by following their wave-packet motion and reveal rich contents of Berry curvature effects in the phase-space spanned by position and momentum. These Berry curvatures are traced back to the characteristics of superconductivity, including the nontrivial momentum-space geometry of superconducting pairing, the real-space supercurrent, and the charge dipole of quasiparticles. The Berry-curvature effects strongly influence the spectroscopic and transport properties of superconductors, such as the local density of states and the thermal Hall conductivity. As a model illustration, we apply the theory to study the twisted bilayer graphene with a $d_{x^{2}+y^{2}}+id_{xy}$ superconducting gap function, and demonstrate Berry-curvature induced effects. △ Less

Submitted 18 September, 2020; v1 submitted 26 August, 2020; originally announced August 2020.

Comments: 4 pages, 1 figure

Journal ref: Phys. Rev. Lett. 126, 187001 (2021)

Emergence of superconductivity in strongly correlated hole-dominated Fe1-xSe

Authors: S. L. Ni, J. P. Sun, S. B. Liu, J. Yuan, Li Yu, M. W. Ma, L. Zhang, L. Pi, P. Zheng, P. P. Shen, D. Li, D. E. Shi, G. B. Li, J. L. Sun, G. M. Zhang, K. Jin, J. -G. Cheng, F. Zhou, X. L. Dong, Z. X. Zhao

Abstract: Here we establish a more complete phase diagram for FeSe system, based on experimental results of nonstoichiometric Fe1-xSe single crystals that we have developed recently, as well as nearly stoichiometric FeSe single crystals. The electronic correlation is found to be strongly enhanced in hole-dominated Fe1-xSe, as compared with electron-dominated FeSe, from the magnetic susceptibility and electr… ▽ More Here we establish a more complete phase diagram for FeSe system, based on experimental results of nonstoichiometric Fe1-xSe single crystals that we have developed recently, as well as nearly stoichiometric FeSe single crystals. The electronic correlation is found to be strongly enhanced in hole-dominated Fe1-xSe, as compared with electron-dominated FeSe, from the magnetic susceptibility and electrical transport measurements in the normal state. A superconducting dome is found to emerge starting from the strongly correlated hole-dominated regime with electron doping, while the tetragonal-orthorhombic phase transition at ~90 K is observed only at higher electron-doping levels in the electron-dominated regime. △ Less

Submitted 29 December, 2019; originally announced December 2019.

Comments: 16 pages, 4 figures. First submitted on May 24, 2019

Ultrafast Quasiparticle Dynamics and Electron-Phonon Coupling in (Li0.84Fe0.16)OHFe0.98Se

Authors: Q. Wu, H. X. Zhou, Y. L. Wu, L. L. Hu, S. L. Ni, Y. C. Tian, F. Sun, F. Zhou, X. L. Dong, Z. X. Zhao, Jimin Zhao

Abstract: Distinctive superconducting behaviors between bulk and monolayer FeSe make it challenging to obtain a unified picture of all FeSe-based superconductors. We investigate the ultrafast quasiparticle (QP) dynamics of an intercalated superconductor (Li1-xFex)OHFe1-ySe, which is a bulk crystal but shares a similar electronic structure with single-layer FeSe on SrTiO3. We obtain the electron-phonon coupl… ▽ More Distinctive superconducting behaviors between bulk and monolayer FeSe make it challenging to obtain a unified picture of all FeSe-based superconductors. We investigate the ultrafast quasiparticle (QP) dynamics of an intercalated superconductor (Li1-xFex)OHFe1-ySe, which is a bulk crystal but shares a similar electronic structure with single-layer FeSe on SrTiO3. We obtain the electron-phonon coupling (EPC) constant λA1g (0.22 +/- 0.04), which well bridges that of bulk FeSe crystal and single-layer FeSe on SrTiO3. Moreover, we find that such a positive correlation between λA1g and superconducting Tc holds among all known FeSe-based superconductors, even in line with reported FeAs-based superconductors. Our observation indicates possible universal role of EPC in the superconductivity of all known categories of iron-based superconductors, which is a critical step towards achieving a unified superconducting mechanism for all iron-based superconductors. △ Less

Submitted 22 October, 2019; originally announced October 2019.

Journal ref: Chinese Physics Letters (Express Letter) 37, 097802 (2020)

Phonon renormalization induced by electric field in ferroelectric P(VDF-TrFE) nanofibers

Authors: Lan Dong, Qing Xi, Jun Zhou, Xiangfan Xu, Baowen Li

Abstract: We report phonon renormalization induced by an external electric field E in ferroelectric poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] nanofibers through measuring the E-dependent thermal conductivity. Our experimental results are in excellent agreement with the theoretical ones derived from the lattice dynamics. The renormalization is attributed to the anharmonicity that modifies the… ▽ More We report phonon renormalization induced by an external electric field E in ferroelectric poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] nanofibers through measuring the E-dependent thermal conductivity. Our experimental results are in excellent agreement with the theoretical ones derived from the lattice dynamics. The renormalization is attributed to the anharmonicity that modifies the phonon spectrum when the atoms are pulled away from their equilibrium positions by the electric field. Our finding provides an efficient way to manipulate the thermal conductivity by tuning external fields in ferroelectric materials. △ Less

Submitted 17 October, 2019; originally announced October 2019.

Journal ref: Phys. Rev. Applied 13, 034019 (2020)

Ferroelectric enhancement of superconductivity in compressively strained SrTiO$_3$ films

Authors: Ryan Russell, Noah Ratcliff, Kaveh Ahadi, Lianyang Dong, Susanne Stemmer, John W. Harter

Abstract: SrTiO$_3$ is an incipient ferroelectric on the verge of a polar instability, which is avoided at low temperatures by quantum fluctuations. Within this unusual quantum paraelectric phase, superconductivity persists despite extremely dilute carrier densities. Ferroelectric fluctuations have been suspected to play a role in the origin of superconductivity by contributing to electron pairing. To inves… ▽ More SrTiO$_3$ is an incipient ferroelectric on the verge of a polar instability, which is avoided at low temperatures by quantum fluctuations. Within this unusual quantum paraelectric phase, superconductivity persists despite extremely dilute carrier densities. Ferroelectric fluctuations have been suspected to play a role in the origin of superconductivity by contributing to electron pairing. To investigate this possibility, we used optical second harmonic generation to measure the doping and temperature dependence of the ferroelectric order parameter in compressively strained SrTiO$_3$ thin films. At low temperatures, we uncover a spontaneous out-of-plane ferroelectric polarization with an onset that correlates perfectly with normal-state electrical resistivity anomalies. These anomalies have previously been associated with an enhancement of the superconducting critical temperature in doped SrTiO$_3$ films, directly linking the ferroelectric and superconducting phases. We develop a long-range mean-field Ising model of the ferroelectric phase transition to interpret the data and extract the relevant energy scales in the system. Our results support a long-suspected connection between ferroelectricity and superconductivity in SrTiO$_3$, but call into question the role played by ferroelectric fluctuations. △ Less

Submitted 10 September, 2019; originally announced September 2019.

Journal ref: Physical Review Materials 3, 091401(R) (2019)

Quantized conductance of Majorana zero mode in the vortex of the topological superconductor (Li0.84Fe0.16)OHFeSe

Authors: C. Chen, Q. Liu, T. Z. Zhang, D. Li, P. P. Shen, X. L. Dong, Z. -X. Zhao, T. Zhang, D. L. Feng

Abstract: The Majorana zero mode (MZM), which manifests as an exotic neutral excitation in superconductors, is the building block of topological quantum computing. It has recently been found in the vortices of several iron-based superconductors as a zero-bias conductance peak (ZBCP) in tunneling spectroscopy. In particular, a clean and robust MZM has been observed in the cores of free vortices in (Li0.84Fe0… ▽ More The Majorana zero mode (MZM), which manifests as an exotic neutral excitation in superconductors, is the building block of topological quantum computing. It has recently been found in the vortices of several iron-based superconductors as a zero-bias conductance peak (ZBCP) in tunneling spectroscopy. In particular, a clean and robust MZM has been observed in the cores of free vortices in (Li0.84Fe0.16)OHFeSe. Here using scanning tunneling spectroscopy (STS), we demonstrate that Majorana-induced resonant Andreev reflection occurs between the STM tip and this zero-bias bound state, and consequently, the conductance at zero bias is quantized as 2e2/h. Our results present a hallmark signature of the MZM in the vortex of an intrinsic topological superconductor, together with its intriguing behavior. △ Less

Submitted 9 April, 2019; originally announced April 2019.

Comments: 10 pages, 7 figures, supplementary materials included

Journal ref: Chin. Phys. Lett. 36, 057403 (2019)

Berry-phase effects in dipole density and Mott relation

Authors: Liang Dong, Cong Xiao, Bangguo Xiong, Qian Niu

Abstract: We provide a unified semiclassical theory for thermoelectric responses of any observable represented by an operator $\hat{\boldsymbolθ}$ that is well-defined in periodic crystals. The Einstein and Mott relations are established generally, in the presence of Berry-phase effects, for various physical realizations of $\hat{\boldsymbolθ}$ in electronic systems, including the familiar case of the elect… ▽ More We provide a unified semiclassical theory for thermoelectric responses of any observable represented by an operator $\hat{\boldsymbolθ}$ that is well-defined in periodic crystals. The Einstein and Mott relations are established generally, in the presence of Berry-phase effects, for various physical realizations of $\hat{\boldsymbolθ}$ in electronic systems, including the familiar case of the electric current as well as the currently controversial cases of the spin polarization and spin current. The magnetization current, which has been proven indispensable in the thermoelectric response of electric current, is generalized to the cases of various $\hat{\boldsymbolθ}$. In our theory the dipole density of a physical quantity emerges and plays a vital role, which contains not only the statistical sum of the dipole moment of $\hat{\boldsymbolθ}$ but also a Berry-phase correction. △ Less

Submitted 12 February, 2020; v1 submitted 31 December, 2018; originally announced December 2018.

Comments: 4 pages, 1 figure

Journal ref: Phys. Rev. Lett. 124, 066601 (2020)

Evidence for Undoped Weyl Semimetal Charge Transport in $Y_{2}Ir_{2}O_{7}$

Authors: P. G. LaBarre, L. Dong, J. Trinh, T. Siegrist, A. P. Ramirez

Abstract: Weyl fermions scattering from a random Coulomb potential are predicted to exhibit resistivity versus temperature $ρ\space α\space T^{-4}$ in a single particle model. Here we show that, in closed environment-grown polycrystalline samples of $Y_{2}Ir_{2}O_{7}$, $ρ= ρ_{0} T^{-4}$ over four orders of magnitude in $ρ$. While the measured prefactor, $ρ_{0}$, is obtained from the model using reasonable m… ▽ More Weyl fermions scattering from a random Coulomb potential are predicted to exhibit resistivity versus temperature $ρ\space α\space T^{-4}$ in a single particle model. Here we show that, in closed environment-grown polycrystalline samples of $Y_{2}Ir_{2}O_{7}$, $ρ= ρ_{0} T^{-4}$ over four orders of magnitude in $ρ$. While the measured prefactor, $ρ_{0}$, is obtained from the model using reasonable materials parameters, the $T^{-4}$ behavior extends far beyond the model's range of applicability. In particular, the behavior extends into the low-temperature, high-resistivity region where the Ioffe-Regel parameter, $k_{T} \ell \ll 2π$. Strong on-site Coulomb correlations, instrumental for predicting a Weyl semimetal state in $Y_{2}Ir_{2}O_{7}$, are the possible origin of such "bad" Weyl semimetal behavior. △ Less

Submitted 10 May, 2019; v1 submitted 22 August, 2018; originally announced August 2018.

Comments: 12 pages, 3 figures

Robust and clean Majorana zero mode in the vortex core of high-temperature superconductor (Li0.84Fe0.16)OHFeSe

Authors: Q. Liu, C. Chen, T. Zhang, R. Peng, Y. J. Yan, C. H. P. Wen, X. Lou, Y. L. Huang, J. P. Tian, X. L. Dong, G. W. Wang, W. C. Bao, Q. H. Wang, Z. P. Yin, Z. -X. Zhao, D. L. Feng

Abstract: The Majorana fermion, which is its own anti-particle and obeys non-abelian statistics, plays a critical role in topological quantum computing. It can be realized as a bound state at zero energy, called a Majorana zero mode (MZM), in the vortex core of a topological superconductor, or at the ends of a nanowire when both superconductivity and strong spin orbital coupling are present. A MZM can be de… ▽ More The Majorana fermion, which is its own anti-particle and obeys non-abelian statistics, plays a critical role in topological quantum computing. It can be realized as a bound state at zero energy, called a Majorana zero mode (MZM), in the vortex core of a topological superconductor, or at the ends of a nanowire when both superconductivity and strong spin orbital coupling are present. A MZM can be detected as a zero-bias conductance peak (ZBCP) in tunneling spectroscopy. However, in practice, clean and robust MZMs have not been realized in the vortices of a superconductor, due to contamination from impurity states or other closely-packed Caroli-de Gennes-Matricon (CdGM) states, which hampers further manipulations of Majorana fermions. Here using scanning tunneling spectroscopy, we show that a ZBCP well separated from the other discrete CdGM states exists ubiquitously in the cores of free vortices in the defect free regions of (Li0.84Fe0.16)OHFeSe, which has a superconducting transition temperature of 42 K. Moreover, a Dirac-cone-type surface state is observed by angle-resolved photoemission spectroscopy, and its topological nature is confirmed by band calculations. The observed ZBCP can be naturally attributed to a MZM arising from this chiral topological surface states of a bulk superconductor. (Li0.84Fe0.16)OHFeSe thus provides an ideal platform for studying MZMs and topological quantum computing. △ Less

Submitted 16 December, 2018; v1 submitted 3 July, 2018; originally announced July 2018.

Comments: 32 pages, 15 figures (supplementary materials included), accepted by PRX

Journal ref: Phys. Rev. X 8, 041056 (2018)

Dimensional crossover of heat conduction in amorphous Polyimide nanofibers

Authors: Lan Dong, Qing Xi, Dongsheng Chen, Jie Guo, Tsuneyoshi Nakayama, Yunyun Li, Ziqi Liang, Jun Zhou, Xiangfan Xu, Baowen Li

Abstract: The mechanism of thermal conductivity in amorphous polymers, especially polymer fibers, is unclear in comparison with that in inorganic materials. Here, we report the observation of across over of heat conduction behavior from three dimensions (3D) to quasi-one dimension (1D) in Polyimide(PI) nanofibers at a given temperature. A theoretical model based on the random walk theory has been proposed t… ▽ More The mechanism of thermal conductivity in amorphous polymers, especially polymer fibers, is unclear in comparison with that in inorganic materials. Here, we report the observation of across over of heat conduction behavior from three dimensions (3D) to quasi-one dimension (1D) in Polyimide(PI) nanofibers at a given temperature. A theoretical model based on the random walk theory has been proposed to quantitatively describe the interplay between the inter-chain hopping and the intra-chain hopping in nanofibers. This model explains well the diameter dependence of thermal conductivity and also speculates the upper limit of thermal conductivity of amorphous polymers in the quasi-1D limit. △ Less

Submitted 26 February, 2018; originally announced February 2018.

Comments: this article is published in National Science Review(NSR) Link: https://doi.org/10.1093/nsr/nwy004

Journal ref: National Science Review, Volume 5, Issue 4, July 2018, Pages 500-506

Geometrodynamics of electrons in a crystal under position and time dependent deformation

Authors: Liang Dong, Qian Niu

Abstract: Semiclassical dynamics of Bloch electrons in a crystal under slowly varying deformation is developed in the geometric language of a lattice bundle. Berry curvatures and gradients of energy are introduced in terms of lattice covariant derivatives, with the corresponding connections given by the gradient and rate of strain. A number of physical effects are discussed: an effective post-Newtonian grav… ▽ More Semiclassical dynamics of Bloch electrons in a crystal under slowly varying deformation is developed in the geometric language of a lattice bundle. Berry curvatures and gradients of energy are introduced in terms of lattice covariant derivatives, with the corresponding connections given by the gradient and rate of strain. A number of physical effects are discussed: an effective post-Newtonian gravity at band bottom, polarization induced by spatial gradient of strain, orbital magnetization induced by strain rate, and electron energy stress tensor. △ Less

Submitted 6 February, 2018; originally announced February 2018.

Comments: 24 pages, 3 figures

Journal ref: Phys. Rev. B 98, 115162 (2018)

Matrix-assisted fabrication and exotic charge mobility of (Li,Fe)OHFeSe superconductor films

Authors: Y. L. Huang, Z. P. Feng, J. Yuan, W. Hu, J. Li, S. L. Ni, S. B. Liu, Y. Y. Mao, H. X. Zhou, H. B. Wang, F. Zhou, G. M. Zhang, K. Jin, X. L. Dong, Z. X. Zhao

Abstract: Superconducting (Li1-xFex)OHFe1-ySe films are attractive for both the basic research and practical application. However, the conventional vapor deposition techniques are not applicable in synthesizing the films of such a complex system. So no intrinsic charge transport measurements on the films are available so far to reveal the nature of charge carriers, which is fundamental to understanding the… ▽ More Superconducting (Li1-xFex)OHFe1-ySe films are attractive for both the basic research and practical application. However, the conventional vapor deposition techniques are not applicable in synthesizing the films of such a complex system. So no intrinsic charge transport measurements on the films are available so far to reveal the nature of charge carriers, which is fundamental to understanding the iron-based superconductivity mechanism. Herein we report a soft chemical film technique (matrix-assisted hydrothermal epitaxial growth), by which we have succeeded in growing a series of (Li1-xFex)OHFe1-ySe films covering the whole superconducting regime, with the superconducting transition temperature (Tc) from 4 K up to 42 K. This film technique opens up a new way for fabricating other complex functional materials as well. Furthermore, our systematic transport investigation on the film samples indicates that both the electron and hole carriers contribute to the charge transport, with the scattering rates deviating from the Fermi liquid. We find that the superconductivity occurs upon the electron and hole mobility becoming divergent. And in the high Tc samples, the electron carriers are found much more mobile than the holes, a feature distinct from the low Tc samples. Hence, our transport results provide key insights into the underlying physics for iron-based high-Tc superconductivity. △ Less

Submitted 8 November, 2017; originally announced November 2017.

Comments: 13 pages, 5 figures

Reemergence of high-Tc superconductivity in the (Li1-xFex)OHFe1-ySe under high pressure

Authors: J. P. Sun, P. Shahi, H. X. Zhou, Y. L. Huang, K. Y. Chen, B. S. Wang, S. L. Ni, N. N. Li, K. Zhang, W. G. Yang, Y. Uwatoko, K. Jin, F. Zhou, D. J. Singh, X. L. Dong, Z. X. Zhao, J. -G. Cheng

Abstract: The pressure-induced reemergence of the second high-Tc superconducting phase (SC-II) in the alkali-metal intercalated AxFe2-ySe2 (A = K, Rb, Cs, Tl) remains an enigma and proper characterizations on the superconducting- and normal-state properties of the SC-II phase were hampered by the intrinsic inhomogeneity and phase separation. To elucidate this intriguing problem, we performed a detailed high… ▽ More The pressure-induced reemergence of the second high-Tc superconducting phase (SC-II) in the alkali-metal intercalated AxFe2-ySe2 (A = K, Rb, Cs, Tl) remains an enigma and proper characterizations on the superconducting- and normal-state properties of the SC-II phase were hampered by the intrinsic inhomogeneity and phase separation. To elucidate this intriguing problem, we performed a detailed high-pressure magnetotransport study on the recently discovered (Li1-xFex)OHFe1-ySe single crystals, which have high Tc~40 K and share similar Fermi surface topology as AxFe2-ySe2, but are free from the sample complications. We found that the ambient-pressure Tc~41 K is suppressed gradually to below 2 K upon increasing pressure to Pc ~5 GPa, above which a SC-II phase with higher Tc emerges and the Tc increases progressively to above 50 K up to 12.5 GPa. Interestingly, our high-precision resistivity data enable us to uncover the sharp transition of the normal state from a Fermi liquid for SC-I phase (0 < P < 5 GPa) to a non-Fermi-liquid for SC-II phase (P > 5GPa). In addition, the reemergence of high-Tc SC-II phase is found to accompany with a concurrent enhancement of electron carrier density. Since high-pressure structural study based on the synchrotron X-ray diffraction rules out the structural transition below 10 GPa, the observed SC-II phase with enhanced carrier density should be ascribed to an electronic origin associated with a pressure-induced Fermi surface reconstruction. △ Less

Submitted 20 July, 2017; originally announced July 2017.

Comments: 14 pages, 4 figures

Journal ref: Nat. Commun. 9, 380 (2018)

Probing the superconducting gap structure of (Li$_{1-x}$Fe$_x$)OHFeSe

Authors: M. Smidman, G. M. Pang, H. X. Zhou, N. Z. Wang, W. Xie, Z. F. Weng, Y. Chen, X. L. Dong, X. H. Chen, Z. X. Zhao, H. Q. Yuan

Abstract: We report measurements of the London penetration depth [$Δλ(T)$] of the recently discovered iron-based superconductor (Li$_{1-x}$Fe$_x$)OHFeSe, in order to characterize the nature of the superconducting gap structure. At low temperatures, $Δλ(T)$ displays nearly temperature independent behavior, indicating a fully open superconducting gap. We also analyze the superfluid density $ρ_s(T)$ which cann… ▽ More We report measurements of the London penetration depth [$Δλ(T)$] of the recently discovered iron-based superconductor (Li$_{1-x}$Fe$_x$)OHFeSe, in order to characterize the nature of the superconducting gap structure. At low temperatures, $Δλ(T)$ displays nearly temperature independent behavior, indicating a fully open superconducting gap. We also analyze the superfluid density $ρ_s(T)$ which cannot be well accounted for by a single-gap isotropic $s$-wave model but are consistent with either two-gaps, a model for the orbital selective $s\timesτ_3$ state or anisotropic $s$-wave superconductivity. △ Less

Submitted 14 June, 2017; originally announced June 2017.

Comments: 6 pages, 3 figures

Journal ref: Phys. Rev. B 96, 014504 (2017)

Janus Monolayer Transition Metal Dichalcogenides

Authors: Jing Zhang, Shuai Jia, Kholmanov Iskandar, Liang Dong, Dequan Er, Weibing Chen, Hua Guo, Zehua Jin, Vivek B. Shenoy, Li Shi, Jun Lou

Abstract: A novel crystal configuration of sandwiched S-Mo-Se structure (Janus SMoSe) at the monolayer limit has been synthesized and carefully characterized in this work. By controlled sulfurization of monolayer MoSe2 the top layer of selenium atoms are substituted by sulfur atoms while the bottom selenium layer remains intact. The peculiar structure of this new material is systematically investigated by R… ▽ More A novel crystal configuration of sandwiched S-Mo-Se structure (Janus SMoSe) at the monolayer limit has been synthesized and carefully characterized in this work. By controlled sulfurization of monolayer MoSe2 the top layer of selenium atoms are substituted by sulfur atoms while the bottom selenium layer remains intact. The peculiar structure of this new material is systematically investigated by Raman, photoluminescence and X-ray photoelectron spectroscopy and confirmed by transmission-electron microscopy and time-of-flight secondary ion mass spectrometry. Density-functional theory calculations are performed to better understand the Raman vibration modes and electronic structures of the Janus SMoSe monolayer, which are found to correlate well with corresponding experimental results. Finally, high basal plane hydrogen evolution reaction (HER) activity is discovered for the Janus monolayer and DFT calculation implies that the activity originates from the synergistic effect of the intrinsic defects and structural strain inherent in the Janus structure. △ Less

Submitted 21 April, 2017; originally announced April 2017.

Comments: 22 pages, 12 figures

Thermal conduction across a boron nitride and silicon oxide interface

Authors: Xinxia Li, Yaping Yan, Lan Dong, Jie Guo, Adili Aiyiti, Xiangfan Xu, Baowen Li

Abstract: The needs for efficient heat removal and superior thermal conduction in nano/micro devices have triggered tremendous studies in low-dimensional materials with high thermal conductivity. Hexagonal boron nitride (h-BN) is believed to be one of the candidates for thermal management and heat dissipation due to its novel physical properties, i.e. thermal conductor and electrical insulator. Here we repo… ▽ More The needs for efficient heat removal and superior thermal conduction in nano/micro devices have triggered tremendous studies in low-dimensional materials with high thermal conductivity. Hexagonal boron nitride (h-BN) is believed to be one of the candidates for thermal management and heat dissipation due to its novel physical properties, i.e. thermal conductor and electrical insulator. Here we reported interfacial thermal resistance between few-layer h-BN and its silicon oxide substrate using differential 3 omega method. The measured interfacial thermal resistance is around ~1.6*10-8 m2K/W for monolayer h-BN and ~3.4*10-8 m2K/W for 12.8nm-thick h-BN in metal/h-BN/SiO2 interfaces. Our results suggest that the voids and gaps between substrate and thick h-BN flakes limit the interfacial thermal conduction. This work provides a deeper understanding of utilizing h-BN flake as lateral heat spreader in electronic and optoelectronic nano/micro devices with further miniaturization and integration. △ Less

Submitted 2 March, 2017; originally announced March 2017.

Comments: 9 pages, 6 figures

Journal ref: J. Phys. D: Appl. Phys. 50, 104002 (2017)

Effects of Spin-Orbit Coupling on Jaynes-Cummings and Tavis-Cummings Models

Authors: Chuanzhou Zhu, Lin Dong, Han Pu

Abstract: We consider ultracold atoms inside a ring optical cavity that supports a single plane-wave mode. The cavity field, together with an external coherent laser field, drives a two-photon Raman transition between two internal pseudo-spin states of the atom. This gives rise to an effective coupling between atom's pseudo-spin and external center-of-mass (COM) motion. For the case of a single atom inside… ▽ More We consider ultracold atoms inside a ring optical cavity that supports a single plane-wave mode. The cavity field, together with an external coherent laser field, drives a two-photon Raman transition between two internal pseudo-spin states of the atom. This gives rise to an effective coupling between atom's pseudo-spin and external center-of-mass (COM) motion. For the case of a single atom inside the cavity, We show how the spin-orbit coupling modifies the static and dynamic properties of the Jaynes-Cummings (JC) model. In the case of many atoms in thermodynamic limit, we show that the spin-orbit coupling modifies the Dicke superradiance phase transition boundary and the non-superradiant normal phase may become reentrant in some regimes. △ Less

Submitted 25 November, 2016; v1 submitted 25 September, 2016; originally announced September 2016.

Comments: 11 pages, 6 figures. Comments are welcome

Journal ref: Physical Review A 94, 053621 (2016)

Artificial gravity field, astrophysical analogues, and topological phase transitions in strained topological semimetals

Authors: Shan Guan, Zhi-Ming Yu, Ying Liu, Gui-Bin Liu, Liang Dong, Yunhao Lu, Yugui Yao, Shengyuan A. Yang

Abstract: Effective gravity and gauge fields are emergent properties intrinsic for low-energy quasiparticles in topological semimetals. Here, taking two Dirac semimetals as examples, we demonstrate that applied lattice strain can generate warped spacetime, with fascinating analogues in astrophysics. Particularly, we study the possibility of simulating black-hole/white-hole event horizons and gravitational l… ▽ More Effective gravity and gauge fields are emergent properties intrinsic for low-energy quasiparticles in topological semimetals. Here, taking two Dirac semimetals as examples, we demonstrate that applied lattice strain can generate warped spacetime, with fascinating analogues in astrophysics. Particularly, we study the possibility of simulating black-hole/white-hole event horizons and gravitational lensing effect. Furthermore, we discover strain-induced topological phase transitions, both in the bulk materials and in their thin films. Especially in thin films, the transition between the quantum spin Hall and the trivial insulating phases can be achieved by a small strain, naturally leading to the proposition of a novel piezo-topological transistor device. Possible experimental realizations and analogue of Hawking radiation effect are discussed. Our result bridges multiple disciplines, revealing topological semimetals as a unique table-top platform for exploring interesting phenomena in astrophysics and general relativity; it also suggests realistic materials and methods to achieve controlled topological phase transitions with great potential for device applications. △ Less

Submitted 10 May, 2017; v1 submitted 2 September, 2016; originally announced September 2016.

Comments: 29 pages,6 figures

Journal ref: npj Quantum Materials, 2, 23 (2017)

Superior thermal conductivity in suspended bilayer hexagonal boron nitride

Authors: Chengru Wang, Jie Guo, Lan Dong, Adili Aiyiti, Xiangfan Xu, Baowen Li

Abstract: We reported the basal-plane thermal conductivity in exfoliated bilayer hexagonal boron nitride h-BN that was measured using suspended prepatterned microstructures. The h-BN sample suitable for thermal measurements was fabricated by dry-transfer method, whose sample quality, due to less polymer residues on surfaces, is believed to be superior to that of PMMA-mediated samples. The measured room temp… ▽ More We reported the basal-plane thermal conductivity in exfoliated bilayer hexagonal boron nitride h-BN that was measured using suspended prepatterned microstructures. The h-BN sample suitable for thermal measurements was fabricated by dry-transfer method, whose sample quality, due to less polymer residues on surfaces, is believed to be superior to that of PMMA-mediated samples. The measured room temperature thermal conductivity is around 484 Wm-1K-1(+141 Wm-1K-1/ -24 Wm-1K-1) which exceeds that in bulk h-BN, providing experimental observation of the thickness-dependent thermal conductivity in suspended few-layer h-BN. △ Less

Submitted 27 June, 2016; originally announced June 2016.

Comments: 10 pages, 3 figures

Journal ref: Scientific Reports 6, 25334 (2016)

Signatures of topological quantum phase transitions in driven and dissipative qubit-arrays

Authors: Y. L. Dong, Titus Neupert, R. Chitra, Sebastian Schmidt

Abstract: We study photonic signatures of symmetry broken and topological phases in a driven, dissipative circuit QED realization of spin-1/2 chains. Specifically, we consider the transverse-field XY model and a dual model with 3-spin interactions. The former has a ferromagnetic and a paramagnetic phase, while the latter features, in addition, a symmetry protected topological phase. Using the method of thir… ▽ More We study photonic signatures of symmetry broken and topological phases in a driven, dissipative circuit QED realization of spin-1/2 chains. Specifically, we consider the transverse-field XY model and a dual model with 3-spin interactions. The former has a ferromagnetic and a paramagnetic phase, while the latter features, in addition, a symmetry protected topological phase. Using the method of third quantization, we calculate the non-equilibrium steady-state of the open spin chains for arbitrary system sizes and temperatures. We find that the bi-local correlation function of the spins at both ends of the chain provides a sensitive measure for both symmetry-breaking and topological phase transitions of the systems, but no universal means to distinguish between the two types of transitions. Both models have equivalent representations in terms of free Majorana fermions, which host zero, one and two topological Majorana end modes in the paramagnetic, ferromagnetic, and symmetry protected topological phases, respectively. The correlation function we study retains its bi-local character in the fermionic representation, so that our results are equally applicable to the fermionic models in their own right. We propose a photonic realization of the dissipative transverse-field XY model in a tunable setup, where an array of superconducting transmon qubits is coupled at both ends to a photonic microwave circuit. △ Less

Submitted 16 April, 2016; originally announced April 2016.

Comments: 10 pages, 6 figures

Journal ref: Phys. Rev. B 94, 035441 (2016)

Harmonically Trapped Atoms with Spin-Orbit Coupling

Authors: Chuanzhou Zhu, Lin Dong, Han Pu

Abstract: We study harmonically trapped one-dimensional atoms subjected to an equal combination of Rashba and Dresselhaus spin-orbit coupling induced by Raman transition. We first examine the wave function and the degeneracy of the single-particle ground state, followed by a study of two weakly interacting bosons or fermions. For the two-particle ground state, we focus on the effects of the interaction on t… ▽ More We study harmonically trapped one-dimensional atoms subjected to an equal combination of Rashba and Dresselhaus spin-orbit coupling induced by Raman transition. We first examine the wave function and the degeneracy of the single-particle ground state, followed by a study of two weakly interacting bosons or fermions. For the two-particle ground state, we focus on the effects of the interaction on the degeneracy, the spin density profiles, and the density-density correlation functions. Finally we show how these studies help us to understand the many-body properties of the system. △ Less

Submitted 26 September, 2016; v1 submitted 6 January, 2016; originally announced January 2016.

Comments: 12 pages, 9 figures. Comments are welcome

Journal ref: J. Phys. B: At. Mol. Opt. Phys. 49 (2016) 145301

Photon-Induced Spin-Orbit Coupling in Ultracold Atoms inside Optical Cavity

Authors: Lin Dong, Chuanzhou Zhu, Han Pu

Abstract: We consider an atom inside a ring cavity, where a plane-wave cavity field together with an external coherent laser beam induces a two-photon Raman transition between two hyperfine ground states of the atom. This cavity-assisted Raman transition induces effective coupling between atom's internal degrees of freedom and its center-of-mass motion. In~the meantime, atomic dynamics exerts a back-action… ▽ More We consider an atom inside a ring cavity, where a plane-wave cavity field together with an external coherent laser beam induces a two-photon Raman transition between two hyperfine ground states of the atom. This cavity-assisted Raman transition induces effective coupling between atom's internal degrees of freedom and its center-of-mass motion. In~the meantime, atomic dynamics exerts a back-action to cavity photons. We investigate the properties of this system by adopting a mean-field and a full quantum approach, and show that the interplay between the atomic dynamics and the cavity field gives rise to intriguing nonlinear phenomena. △ Less

Submitted 27 May, 2015; v1 submitted 7 April, 2015; originally announced April 2015.

Comments: 12 pages, 5 figures, and published version

Journal ref: Atoms 2015, 3(2), 182-194

Magnetism and a field-cycling induced effect in staircase Kagomé antiferromagnet PbCu$_3$TeO$_7$ revealed by NMR

Authors: J. Dai, P. S. Wang, S. S. Sun, F. Pang, J. S. Zhang, X. L. Dong, G. Yue, K. Jin, J. Z. Cong, Y. Sun, Weiqiang Yu

Abstract: We report $^{125}$Te and $^{63,65}$Cu nuclear magnetic resonance (NMR) studies on single crystals of staircase Kagomé antiferromagnet PbCu3TeO7 ($T_{N1}\approx$ 36 K). A Curie constant as large as $Θ$~140 K is obtained by a Curie-Weiss fitting of the high-temperature Knight shift. The frustration factor f (= $Θ/T_N $)$\approx$ 4 implies intermediate frustration in this system. From the high-temper… ▽ More We report $^{125}$Te and $^{63,65}$Cu nuclear magnetic resonance (NMR) studies on single crystals of staircase Kagomé antiferromagnet PbCu3TeO7 ($T_{N1}\approx$ 36 K). A Curie constant as large as $Θ$~140 K is obtained by a Curie-Weiss fitting of the high-temperature Knight shift. The frustration factor f (= $Θ/T_N $)$\approx$ 4 implies intermediate frustration in this system. From the high-temperature Knight shift data of $^{125}$Te, its hyperfine coupling constant is estimated to be $^{125}A_{hf}$ = -67 kOe/$μ_B$, which suggests a strong interlayer coupling bridging the neighboring Kagomé layers. At $T$= 2 K, we find two types of zero-field NMR signals. One has no dependence on the history of the field treatment, which is consistent with Cu NMR signals from inequivalent Cu sites with different internal hyperfine fields. The other one is only seen after a field-cycling process, which is consistent with domain wall contributions in this frustrated antiferromagnet. △ Less

Submitted 21 August, 2015; v1 submitted 2 March, 2015; originally announced March 2015.

Comments: Revised argument

Journal ref: Chin.Phys.Lett. 32,127503(2015)

Atomic Control of Strain in Freestanding Graphene

Authors: P. Xu, Y. Yang, S. D. Barber, M. L. Ackerman, J. K. Schoelz, D. Qi, I. A. Kornev, L. Dong, L. Bellaiche, S. Barraza-Lopez, P. M. Thibado

Abstract: In this study, we describe a new experimental approach based on constant-current scanning tunneling spectroscopy to controllably and reversibly pull freestanding graphene membranes up to 35 nm from their equilibrium height. In addition, we present scanning tunneling microscopy (STM) images of freestanding graphene membranes with atomic resolution. Atomic-scale corrugation amplitudes 20 times large… ▽ More In this study, we describe a new experimental approach based on constant-current scanning tunneling spectroscopy to controllably and reversibly pull freestanding graphene membranes up to 35 nm from their equilibrium height. In addition, we present scanning tunneling microscopy (STM) images of freestanding graphene membranes with atomic resolution. Atomic-scale corrugation amplitudes 20 times larger than the STM electronic corrugation for graphene on a substrate were observed. The freestanding graphene membrane responds to a local attractive force created at the STM tip as a highly-conductive yet flexible grounding plane with an elastic restoring force. We indicate possible applications of our method in the controlled creation of pseudo-magnetic fields by strain on single-layer graphene. △ Less

Submitted 6 February, 2015; originally announced February 2015.

Comments: 16 pages, 3 figures

Journal ref: Physical Review B 85, 121406(R) (2012)

Electromechanical properties of freestanding graphene functionalized with tin oxide (SnO2) nanoparticles

Authors: L. Dong, J. Hansen, P. Xu, M. L. Ackerman, S. D. Barber, J. K. Schoelz, D. Qi, P. M. Thibado

Abstract: Freestanding graphene membranes were functionalized with SnO2 nanoparticles. A detailed procedure providing uniform coverage and chemical synthesis is presented. Elemental composition was determined using scanning electron microscopy combined with energy dispersive X-ray analysis. A technique called electrostatic-manipulation scanning tunneling microscopy was used to probe the electromechanical pr… ▽ More Freestanding graphene membranes were functionalized with SnO2 nanoparticles. A detailed procedure providing uniform coverage and chemical synthesis is presented. Elemental composition was determined using scanning electron microscopy combined with energy dispersive X-ray analysis. A technique called electrostatic-manipulation scanning tunneling microscopy was used to probe the electromechanical properties of functionalized freestanding graphene samples. We found ten times larger movement perpendicular to the plane compared to pristine freestanding graphene, and propose a nanoparticle encapsulation model. △ Less

Submitted 2 February, 2015; originally announced February 2015.

Comments: 14 pages, 3 figures

Journal ref: Applied Physics Letters 101, 061601 (2012)

Self-Organized Platinum Nanoparticles on Freestanding Graphene

Authors: Peng Xu, Lifeng Dong, Mehdi Neek-Amal, Matthew L. Ackerman, Jianhua Yu, Steven D. Barber, James Kevin Schoelz, Dejun Qi, Fangfang Xu, Paul M. Thibado, Francois M. Peeters

Abstract: Freestanding graphene membranes were successfully functionalized with platinum nanoparticles (Pt NPs). High-resolution transmission electron microscopy revealed a homogeneous distribution of single-crystal Pt NPs that tend to exhibit a preferred orientation. Unexpectedly, the NPs were also found to be partially exposed to the vacuum with the top Pt surface raised above the graphene substrate, as d… ▽ More Freestanding graphene membranes were successfully functionalized with platinum nanoparticles (Pt NPs). High-resolution transmission electron microscopy revealed a homogeneous distribution of single-crystal Pt NPs that tend to exhibit a preferred orientation. Unexpectedly, the NPs were also found to be partially exposed to the vacuum with the top Pt surface raised above the graphene substrate, as deduced from atomic-scale scanning tunneling microscopy images and detailed molecular dynamics simulations. Local strain accumulation during the growth process is thought to be the origin of the NP self-organization. These findings are expected to shape future approaches in developing Pt NP catalysts for fuel cells as well as NP-functionalized graphene based high-performance electronics. △ Less

Submitted 18 December, 2014; v1 submitted 11 December, 2014; originally announced December 2014.

Comments: 24 pages, 5 figures

Journal ref: ACS Nano 8 2697 (2014)

Dynamical topological phases in quenched spin-orbit coupled degenerate Fermi gas

Authors: Ying Dong, Lin Dong, Ming Gong, Han Pu

Abstract: The spin-orbit coupled degenerate Fermi gas provides a totally new platform to realize topological superfluids and related topological excitations. Previous studies have mainly focused on the properties of the ground state. Here we consider a two-dimensional Fermi gas with Rashba spin-orbit coupling subject to a perpendicular Zeeman field. For this system, we have found that its ground state topol… ▽ More The spin-orbit coupled degenerate Fermi gas provides a totally new platform to realize topological superfluids and related topological excitations. Previous studies have mainly focused on the properties of the ground state. Here we consider a two-dimensional Fermi gas with Rashba spin-orbit coupling subject to a perpendicular Zeeman field. For this system, we have found that its ground state topological structure is captured by the spin texture, which is readily measurable in experiments. We show that, when the Zeeman field is suddenly quenched, dynamical topological phases can be realized. More specifically, three post-quench dynamical phases can be identified according to the asymptotic behavior of the order parameter. In the undamped phase, a persistent oscillation of the order parameter may support a topological Floquet state with multiple edge states. In the Landau damped phase, the magnitude of the order parameter approaches a constant via a power-law decay, and this system can support a dynamical topological phase with a pair of edge states at the boundary. In the over-damped phase, the order parameter decays to zero exponentially although the condensate fraction remains finite. These predictions can be observed in the strong coupling regime of ultracold Fermi gas. △ Less

Submitted 22 January, 2015; v1 submitted 15 June, 2014; originally announced June 2014.

Comments: Nature Communications | Article

Journal ref: Nat. Commun. 6:6103 (2015)