Showing 1–43 of 43 results for author: Hou, B

MBFormer: A General Transformer-based Learning Paradigm for Many-body Interactions in Real Materials

Authors: Bowen Hou, Xian Xu, Jinyuan Wu, Diana Y. Qiu

Abstract: Recently, radical progress in machine learning (ML) has revolutionized computational materials science, enabling unprecedentedly rapid materials discovery and property prediction, but the quantum many-body problem -- which is the key to understanding excited-state properties, ranging from transport to optics -- remains challenging due to the complexity of the nonlocal and energy-dependent interact… ▽ More Recently, radical progress in machine learning (ML) has revolutionized computational materials science, enabling unprecedentedly rapid materials discovery and property prediction, but the quantum many-body problem -- which is the key to understanding excited-state properties, ranging from transport to optics -- remains challenging due to the complexity of the nonlocal and energy-dependent interactions. Here, we propose a symmetry-aware, grid-free, transformer-based model, MBFormer, that is designed to learn the entire many-body hierarchy directly from mean-field inputs, exploiting the attention mechanism to accurately capture many-body correlations between mean-field states. As proof of principle, we demonstrate the capability of MBFormer in predicting results based on the GW plus Bethe Salpeter equation (GW-BSE) formalism, including quasiparticle energies, exciton energies, exciton oscillator strengths, and exciton wavefunction distribution. Our model is trained on a dataset of 721 two-dimensional materials from the C2DB database, achieving state-of-the-art performance with a low prediction mean absolute error (MAE) on the order of 0.1-0.2 eV for state-level quasiparticle and exciton energies across different materials. Moreover, we show explicitly that the attention mechanism plays a crucial role in capturing many-body correlations. Our framework provides an end-to-end platform from ground states to general many-body prediction in real materials, which could serve as a foundation model for computational materials science. △ Less

Submitted 7 July, 2025; originally announced July 2025.

Unraveling Exciton Trap Dynamics and Nonradiative Loss Pathways in Quantum Dots via Atomistic Simulations

Authors: Bokang Hou, Salvatore Gatto, Samuel L. Rudge, Johan E. Runeson, Michael Thoss, Eran Rabani

Abstract: Surface defects in colloidal quantum dots are a major source of nonradiative losses, yet the microscopic mechanisms underlying exciton trapping and recombination remain elusive. Here, we develop a model Hamiltonian based on atomistic electronic calculations to investigate exciton dynamics in CdSe/CdS core/shell QDs containing a single hole trap introduced by an unpassivated sulfur atom. By systema… ▽ More Surface defects in colloidal quantum dots are a major source of nonradiative losses, yet the microscopic mechanisms underlying exciton trapping and recombination remain elusive. Here, we develop a model Hamiltonian based on atomistic electronic calculations to investigate exciton dynamics in CdSe/CdS core/shell QDs containing a single hole trap introduced by an unpassivated sulfur atom. By systematically varying the defect depth and reorganization energy, we uncover how defect-induced excitonic states mediate energy relaxation pathways. Our simulations reveal that a single localized defect can induce a rich spectrum of excitonic states, leading to multiple dynamical regimes, from slow, energetically off-resonant trapping to fast, cascaded relaxation through in-gap defect states. Crucially, we quantify how defect-induced polaron shifts and exciton-phonon couplings govern the balance between efficient radiative emission and rapid nonradiative decay. These insights clarify the microscopic origin of defect-assisted loss channels and suggest pathways for tailoring QD optoelectronic properties via surface and defect engineering. △ Less

Submitted 12 May, 2025; originally announced May 2025.

Direct Observation of Massless Excitons and Linear Exciton Dispersion

Authors: Luna Y. Liu, Steffi Y. Woo, Jinyuan Wu, Bowen Hou, Cong Su, Diana Y. Qiu

Abstract: Excitons -- elementary excitations formed by bound electron-hole pairs -- govern the optical properties and excited-state dynamics of materials. In two-dimensions (2D), excitons are theoretically predicted to have a linear energy-momentum relation with a non-analytic discontinuity in the long wavelength limit, mimicking the dispersion of a photon. This results in an exciton that behaves like a mas… ▽ More Excitons -- elementary excitations formed by bound electron-hole pairs -- govern the optical properties and excited-state dynamics of materials. In two-dimensions (2D), excitons are theoretically predicted to have a linear energy-momentum relation with a non-analytic discontinuity in the long wavelength limit, mimicking the dispersion of a photon. This results in an exciton that behaves like a massless particle, despite the fact that it is a composite boson composed of massive constituents. However, experimental observation of massless excitons has remained elusive. In this work, we unambiguously experimentally observe the predicted linear exciton dispersion in freestanding monolayer hexagonal boron nitride (hBN) using momentum-resolved electron energy-loss spectroscopy. The experimental result is in excellent agreement with our theoretical prediction based on ab initio many-body perturbation theory. Additionally, we identify the lowest dipole-allowed transition in monolayer hBN to be at 6.6 eV, illuminating a long-standing debate about the band gap of monolayer hBN. These findings provide critical insights into 2D excitonic physics and open new avenues for exciton-mediated superconductivity, Bose-Einstein condensation, and high-efficiency optoelectronic applications. △ Less

Submitted 13 March, 2025; v1 submitted 27 February, 2025; originally announced February 2025.

Moiré-Tunable Localization of Simultaneous Type I and Type II Band Alignment in a MoSe2/WS2 Heterobilayer

Authors: Jiaxuan Guo, Zachary H. Withers, Ziling Li, Bowen Hou, Alexander Adler, Jianwei Ding, Victor Chang Lee, Roland K. Kawakami, Gerd Schönhense, Alice Kunin, Thomas K. Allison, Diana Y. Qiu

Abstract: Moiré heterobilayers exhibiting spatially varying band alignment and electron and hole localization that can be precisely controlled through the twist angle have emerged as exciting platforms for studying complex quantum phenomena. While most heterobilayers of transition metal dichalcogenides (TMDs) have a type II band alignment, the introduction of type I band alignment could enable stronger ligh… ▽ More Moiré heterobilayers exhibiting spatially varying band alignment and electron and hole localization that can be precisely controlled through the twist angle have emerged as exciting platforms for studying complex quantum phenomena. While most heterobilayers of transition metal dichalcogenides (TMDs) have a type II band alignment, the introduction of type I band alignment could enable stronger light-matter coupling and enhanced radiative emission. Here, we show through a combination of first-principles GW plus Bethe Salpeter equation (GW-BSE) calculations and time- and angle-resolved photoemission spectroscopy (tr-ARPES) measurements that contrary to previous understanding, the MoSe2/WS2 heterobilayer has a type I band alignment at large twist angles and simultaneous regions of type I and type II band alignment due to the structural reconstruction in different high symmetry regions at small twist angles. In tr-ARPES, consistent with our calculations, a long-lived electron population is only observed in MoSe2 for samples with large twist angles, while in samples with small twist angles, signals from two distinct long-lived excitons are observed. Moreover, despite the near degeneracy of the conduction bands of the two layers, no excitonic hybridization occurs, suggesting that previously observed absorption peaks in this material arise from lattice reconstruction. Our findings clarify the complex energy landscape in MoSe2/WS2 heterostructures, where the coexistence of type I and type II band alignment opens the door to moiré-tunable optoelectronic devices with intrinsic lateral heterojunctions. △ Less

Submitted 19 February, 2025; originally announced February 2025.

Revealing the phonon bottleneck limit in negatively charged CdS quantum dots

Authors: Skylar J. Sherman, Bokang Hou, Matthew J. Coley-O'Rourke, Katherine E. Shulenberger, Lauren Pellows, Eran Rabani, Gordana Dukovic

Abstract: The capture of photoexcited hot electrons in semiconductors before they lose their excess energy to cooling is a long-standing goal in photon energy conversion. Semiconductor nanocrystals have large electron energy spacings that are expected to slow down electron relaxation by phonon emission, but hot electrons in photoexcited nanocrystals nevertheless cool rapidly by energy transfer to holes. Thi… ▽ More The capture of photoexcited hot electrons in semiconductors before they lose their excess energy to cooling is a long-standing goal in photon energy conversion. Semiconductor nanocrystals have large electron energy spacings that are expected to slow down electron relaxation by phonon emission, but hot electrons in photoexcited nanocrystals nevertheless cool rapidly by energy transfer to holes. This makes the intrinsic phonon-bottleneck limited electron lifetime in nanocrystals elusive. We used a combination of theory and experiments to probe the hot electron dynamics of negatively charged Cadmium Sulfide (CdS) colloidal quantum dots (QDs) in the absence of holes. Experiments found that these hot electrons cooled on a 100 ps timescale. Theoretical simulations predicted that pure phonon-bottleneck limited electron cooling occurs on a similar timescale. This similarity suggests that the experimental measurements reflect the upper limit on hot electron lifetimes in these CdS QDs and the lower limit on the rates of processes that can harvest those hot electrons. △ Less

Submitted 21 January, 2025; v1 submitted 25 September, 2024; originally announced September 2024.

The Role of Superlattice Phonons in Charge Localization Across Quantum Dot Arrays

Authors: Bokang Hou, Matthew Coley-O'Rourke, Uri Banin, Michael Thoss, Eran Rabani

Abstract: Understanding charge transport in semiconductor quantum dot (QD) assemblies is important for developing the next generation of solar cells and light-harvesting devices based on QD technology. One of the key factors that governs the transport in such systems is related to the hybridization between the QDs. Recent experiments have successfully synthesized QD molecules, arrays, and assemblies by dire… ▽ More Understanding charge transport in semiconductor quantum dot (QD) assemblies is important for developing the next generation of solar cells and light-harvesting devices based on QD technology. One of the key factors that governs the transport in such systems is related to the hybridization between the QDs. Recent experiments have successfully synthesized QD molecules, arrays, and assemblies by directly fusing the QDs, with enhanced hybridization leading to high carrier mobilities and coherent band-like electronic transport. In this work, we theoretically investigate the electron transfer dynamics across a finite CdSe-CdS core-shell QD array, considering up to seven interconnected QDs in one dimension. We find that, even in the absence of structural and size disorder, electron transfer can become localized by the emergent low-frequency superlattice vibrational modes when the connecting neck between QDs is narrow. On the other hand, we also identify a regime where the same vibrational modes facilitate coherent electron transport when the connecting necks are wide. Overall, we elucidate the crucial effects of electronic and superlattice symmetries and their couplings when designing high-mobility devices based on QD superlattices. △ Less

Submitted 20 January, 2025; v1 submitted 4 August, 2024; originally announced August 2024.

Topological Corner Modes by Composite Wannier States in Glide-Symmetric Photonic Crystal

Authors: Zhenzhen Liu, Xiaoxi Zhou, Guochao Wei, Lei Gao, Bo hou, Jun-Jun Xiao

Abstract: Second-order topological insulators can be characterized by their bulk polarization, which is believed to be intrinsically connected to the center of the Wannier function. In this study, we demonstrate the existence of second-order topological insulators that feature a pair of partially degenerate photonic bands. These arise from the nonsymmorphic glide symmetry in an all-dielectric photonic cryst… ▽ More Second-order topological insulators can be characterized by their bulk polarization, which is believed to be intrinsically connected to the center of the Wannier function. In this study, we demonstrate the existence of second-order topological insulators that feature a pair of partially degenerate photonic bands. These arise from the nonsymmorphic glide symmetry in an all-dielectric photonic crystal. The center of the maximally localized Wannier function (MLWF) is consistently located at the origin but is not equivalent with respect to the sum of constituent polarizations. As a result, topological corner modes can be identified by the distinctly hybridized MLWFs that truncate at the sample boundary. Through full-wave numerical simulations paired with microwave experiments, the second-order topology is clearly confirmed and characterized. These topological corner states exhibit notably unique modal symmetries, which are made possible by the inversion of the Wannier bands. Our results provide an alternative approach to explore higher-order topological physics with significant potential for applications in integrated and quantum photonics. △ Less

Submitted 3 May, 2024; v1 submitted 1 May, 2024; originally announced May 2024.

Unsupervised Learning of Individual Kohn-Sham States: Interpretable Representations and Consequences for Downstream Predictions of Many-Body Effects

Authors: Bowen Hou, Jinyuan Wu, Diana Y. Qiu

Abstract: Representation learning for the electronic structure problem is a major challenge of machine learning in computational condensed matter and materials physics. Within quantum mechanical first principles approaches, Kohn-Sham density functional theory (DFT) is the preeminent tool for understanding electronic structure, and the high-dimensional wavefunctions calculated in this approach serve as the b… ▽ More Representation learning for the electronic structure problem is a major challenge of machine learning in computational condensed matter and materials physics. Within quantum mechanical first principles approaches, Kohn-Sham density functional theory (DFT) is the preeminent tool for understanding electronic structure, and the high-dimensional wavefunctions calculated in this approach serve as the building block for downstream calculations of correlated many-body excitations and related physical observables. Here, we use variational autoencoders (VAE) for the unsupervised learning of high-dimensional DFT wavefunctions and show that these wavefunctions lie in a low-dimensional manifold within the latent space. Our model autonomously determines the optimal representation of the electronic structure, avoiding limitations due to manual feature engineering and selection in prior work. To demonstrate the utility of the latent space representation of the DFT wavefunction, we use it for the supervised training of neural networks (NN) for downstream prediction of the quasiparticle bandstructures within the GW formalism, which includes many-electron correlations beyond DFT. The GW prediction achieves a low error of 0.11 eV for a combined test set of metals and semiconductors drawn from the Computational 2D Materials Database (C2DB), suggesting that latent space representation captures key physical information from the original data. Finally, we explore the interpretability of the VAE representation and show that the successful representation learning and downstream prediction by our model is derived from the smoothness of the VAE latent space, which also enables the generation of wavefunctions on arbitrary points in latent space. Our work provides a novel and general machine-learning framework for investigating electronic structure and many-body physics. △ Less

Submitted 22 April, 2024; originally announced April 2024.

Dissolution of the non-Hermitian skin effect in one-dimensional lattices with linearly varying nonreciprocal hopping

Authors: Bo Hou, Han Xiao, Rong Lü, Qi-Bo Zeng

Abstract: We study the one-dimensional non-Hermitian lattices with linearly varying nonreciprocal hopping, where the non-Hermitian skin effect (NHSE) is found to be dissolved gradually as the strength of nonreciprocity increases. The energy spectrum under the open boundary condition is composed of real and imaginary eigenenergies when the nonreciprocal hopping is weak. Interestingly, the real eigenenergies… ▽ More We study the one-dimensional non-Hermitian lattices with linearly varying nonreciprocal hopping, where the non-Hermitian skin effect (NHSE) is found to be dissolved gradually as the strength of nonreciprocity increases. The energy spectrum under the open boundary condition is composed of real and imaginary eigenenergies when the nonreciprocal hopping is weak. Interestingly, the real eigenenergies form an equally spaced ladder, and the corresponding eigenstates are localized at the boundary with a Gaussian distribution due to NHSE. By increasing the nonreciprocity, the number of real eigenenergies will decrease while more and more eigenenergies become imaginary. Accompanied by the real-imaginary transition in the spectrum, the eigenstates are shifted from the boundary into the bulk of the lattice. When the nonreciprocity gets strong enough, the whole spectrum will be imaginary and the NHSE disappears completely in the system; i.e., all the eigenstates become Gaussian bound states localized inside the bulk. Our work unveils the exotic properties of non-Hermitian systems with spatially varying nonreciprocal hopping. △ Less

Submitted 25 March, 2024; v1 submitted 17 January, 2024; originally announced January 2024.

Comments: 7 pages, 5 figures

Journal ref: Phys. Rev. B 109, 094208 (2024)

Wannier-Stark localization in one-dimensional amplitude-chirped lattices

Authors: Qi-Bo Zeng, Bo Hou, Han Xiao

Abstract: We study the Wannier-Stark (WS) localization in one-dimensional amplitude-chirped lattices with the $j$th onsite potential modulated by a function $Fj\cos(2παj)$, where $F$ is the external field with a period determined by $α=p/q$ ($p$ and $q$ are coprime integers). In the Hermitian (or non-Hermitian) systems with real (or imaginary) fields, we can obtain real (or imaginary) WS ladders in the eige… ▽ More We study the Wannier-Stark (WS) localization in one-dimensional amplitude-chirped lattices with the $j$th onsite potential modulated by a function $Fj\cos(2παj)$, where $F$ is the external field with a period determined by $α=p/q$ ($p$ and $q$ are coprime integers). In the Hermitian (or non-Hermitian) systems with real (or imaginary) fields, we can obtain real (or imaginary) WS ladders in the eigenenergy spectrum. In most cases with $q \geq 2$, there are multiple WS ladders with all the eigenstates localized in the strong field limit. However, in the lattices with $q=4$, the energy-dependent localization phenomenon emerges due to the presence of both spatially periodic and linearly increasing behaviors in the onsite potential. About half the number of eigenstates are gathered at the band center and can extend over a wide region or even the full range of the lattice, even when the field becomes very strong. Moreover, in the non-Hermitian lattices with odd $q$, some of the WS ladders become doubly degenerate, where the eigenstates are evenly distributed at two neighboring sites in a wide regime of field strength. Our work opens an avenue for exploring WS localization in both Hermitian and non-Hermitian amplitude-chirped lattices. △ Less

Submitted 21 September, 2023; v1 submitted 8 June, 2023; originally announced June 2023.

Comments: 8 pages, 9 figures (including supplementary)

Journal ref: Phys. Rev. B 108, 104207 (2023)

Exchange-Driven Intermixing of Bulk and Topological Surface State by Chiral Excitons in Bi2Se3

Authors: Bowen Hou, Dan Wang, Bradford A. Barker, Diana Y. Qiu

Abstract: Topological surface states (TSS) in the prototypical topological insulator (TI) Bi2Se3 are frequently characterized using optical probes, but electron-hole interactions and their effect on surface localization and optical response of the TSS remain unexplored. Here, we use ab initio calculations to understand excitonic effects in the bulk and surface of Bi2Se3. We identify multiple series of chira… ▽ More Topological surface states (TSS) in the prototypical topological insulator (TI) Bi2Se3 are frequently characterized using optical probes, but electron-hole interactions and their effect on surface localization and optical response of the TSS remain unexplored. Here, we use ab initio calculations to understand excitonic effects in the bulk and surface of Bi2Se3. We identify multiple series of chiral excitons that exhibit both bulk and TSS character, due to exchange-driven mixing. Our results address fundamental questions about the degree to which electron-hole interactions can relax the topological protection of surface states and dipole selection rules for circularly polarized light in TIs by elucidating the complex intermixture of bulk and surface states excited in optical measurements and their coupling to light. △ Less

Submitted 21 February, 2023; originally announced February 2023.

Comments: 13 pages, 4 figures

Nonadiabatic to Adiabatic Transition of Electron Transfer in Colloidal Quantum Dot Molecules

Authors: Bokang Hou, Michael Thoss, Uri Banin, Eran Rabani

Abstract: Electron transfer is an important and fundamental process in chemistry, biology and physics, and has received significant attention in recent years. Perhaps one of the most intriguing questions concerns with the realization of the transitions between nonadiabatic and adiabatic regimes of electron transfer, as the coupling (hybridization) energy, $J$, between the donor and acceptor is varied. Here,… ▽ More Electron transfer is an important and fundamental process in chemistry, biology and physics, and has received significant attention in recent years. Perhaps one of the most intriguing questions concerns with the realization of the transitions between nonadiabatic and adiabatic regimes of electron transfer, as the coupling (hybridization) energy, $J$, between the donor and acceptor is varied. Here, using colloidal quantum dot molecules, a new class of coupled quantum dot dimers, we computationally demonstrate how the hybridization energy between the donor and acceptor quantum dots can be tuned by simply changing the neck dimensions and/or the quantum dot size. This provides a handle to tune the electron transfer from the nonadiabatic over-damped Marcus regime to the coherent adiabatic regime in a single system, without changing the reorganization energy, $λ$, or the typical phonon frequency, $ω_c$. We develop an atomistic model to account for several donor and acceptor states and how they couple to the lattice vibrations, and utilize the Ehrenfest mean-field mixed quantum-classical method to describe the charge transfer dynamics as the nonadiabatic parameter, $γ$, is varied. We find that charge transfer rates increase by several orders of magnitude as the system is driven to the coherent, adiabatic limit, even at elevated temperatures, and delineate the inter-dot and torsional acoustic modes that couple most strongly to the charge transfer reaction coordinate. △ Less

Submitted 1 December, 2022; originally announced December 2022.

Atomic-Scale Visualization of Chiral Charge Density Wave States and Their Reversible Transition

Authors: Xuan Song, Liwei Liu, Yaoyao Chen, Han Yang, Zeping Huang, Baofei Hou, Yanhui Hou, Xu Han, Huixia Yang, Quanzhen Zhang, Teng Zhang, Jiadong Zhou, Yuan Huang, Yu Zhang, Hong-Jun Gao, Yeliang Wang

Abstract: Chirality is essential for various amazing phenomena in life and matter. However,chirality and its switching in electronic superlattices, such as charge density wave(CDW) arrays, remain elusive. In this study, we characterize the chirality transition with atom-resolution imaging in a single-layer NbSe2 CDW pattern by technique of scanning tunneling microscopy. The atomic lattice of the CDW array i… ▽ More Chirality is essential for various amazing phenomena in life and matter. However,chirality and its switching in electronic superlattices, such as charge density wave(CDW) arrays, remain elusive. In this study, we characterize the chirality transition with atom-resolution imaging in a single-layer NbSe2 CDW pattern by technique of scanning tunneling microscopy. The atomic lattice of the CDW array is found continuous and intact although its chirality is switched. Several intermediate states are tracked by time-resolved imaging, revealing the fast and dynamic chirality transition. Importantly, the switching is reversibly realized with an external electric-field. Our findings unveil the delicate transition process of chiral CDW array in a 2D crystal down to the atomic scale and may be applicable for future nanoscale devices. △ Less

Submitted 17 March, 2022; originally announced March 2022.

Vortex Dynamics in Amorphous MoSi Superconducting Thin Films

Authors: Zhengyuan Liu, Bingcheng Luo, Labao Zhang, Boyu Hou, Danyang Wang

Abstract: Vortex dynamics in superconductors have received a great deal of attention from both fundamental and applied researchers over the past few decades. Because of its critical role in the energy relaxation process of type II superconductors, vortex dynamics have been deemed a key factor for the emerging superconducting devices, but the effect of irradiation on vortex dynamics remains unclear. With the… ▽ More Vortex dynamics in superconductors have received a great deal of attention from both fundamental and applied researchers over the past few decades. Because of its critical role in the energy relaxation process of type II superconductors, vortex dynamics have been deemed a key factor for the emerging superconducting devices, but the effect of irradiation on vortex dynamics remains unclear. With the support of electrical transport measurements under external magnetic fields and irradiation, photon effect on vortex dynamics in amorphous MoSi (a MoSi) superconducting thin films are investigated in this work. The magnetic field dependent critical vortex velocity v* derived from the Larkin Ovchinnikov model is not significantly affected by irradiation. However, vortex depinning is found to be enhanced by photon-induced reduction in potential barrier, which mitigates the adverse effect of film inhomogeneity on superconductivity in the a MoSi thin films. The thorough understanding of the vortex dynamics in a MoSi thin films under the effect of external stimuli is of paramount importance for both further fundamental research in this area and optimization of future superconducting devices. △ Less

Submitted 17 September, 2021; v1 submitted 2 July, 2021; originally announced July 2021.

Observation of Higher-Order Topological States in Acoustic Twisted Moiré Superlattice

Authors: Shi-Qiao Wu, Zhi-Kang Lin, Bin Jiang, Xiaoxi Zhou, Bo Hou, Jian-Hua Jiang

Abstract: Twisted moiré superlattices (TMSs) are fascinating materials with exotic physical properties. Despite tremendous studies on electronic, photonic and phononic TMSs, it has never been witnessed that TMSs can exhibit higher-order band topology. Here, we report on the experimental observation of higher-order topological states in acoustic TMSs. By introducing moiré twisting in bilayer honeycomb lattic… ▽ More Twisted moiré superlattices (TMSs) are fascinating materials with exotic physical properties. Despite tremendous studies on electronic, photonic and phononic TMSs, it has never been witnessed that TMSs can exhibit higher-order band topology. Here, we report on the experimental observation of higher-order topological states in acoustic TMSs. By introducing moiré twisting in bilayer honeycomb lattices of coupled acoustic resonators, we find a regime with designed interlayer couplings where a sizable band gap with higher-order topology emerges. This higher-order topological phase host unique topological edge and corner states, which can be understood via the Wannier centers of the acoustic Bloch bands below the band gap. We confirm experimentally the higher-order band topology by characterizing the edge and corner states using acoustic pump-probe measurements. With complementary theory and experiments, our study opens a pathway toward band topology in TMSs. △ Less

Submitted 29 May, 2021; originally announced May 2021.

Comments: 4 figures

Observation of non-Abelian topological semimetals and their phase transitions

Authors: Bin Jiang, Adrien Bouhon, Zhi-Kang Lin, Xiaoxi Zhou, Bo Hou, Feng Li, Robert-Jan Slager, Jian-Hua Jiang

Abstract: Topological phases of matter lie at the heart of physics, connecting elegant mathematical principles to real materials that are believed to shape future electronic and quantum computing technologies. To date, studies in this discipline have almost exclusively been restricted to single-gap band topology because of the Fermi-Dirac filling effect. Here, we theoretically analyze and experimentally con… ▽ More Topological phases of matter lie at the heart of physics, connecting elegant mathematical principles to real materials that are believed to shape future electronic and quantum computing technologies. To date, studies in this discipline have almost exclusively been restricted to single-gap band topology because of the Fermi-Dirac filling effect. Here, we theoretically analyze and experimentally confirm a novel class of multi-gap topological phases, which we will refer to as non-Abelian topological semimetals, on kagome geometries. These unprecedented forms of matter depend on the notion of Euler class and frame charges which arise due to non-Abelian charge conversion processes when band nodes of different gaps are braided along each other in momentum space. We identify such exotic phenomena in acoustic metamaterials and uncover a rich topological phase diagram induced by the creation, braiding and recombination of band nodes. Using pump-probe measurements, we verify the non-Abelian charge conversion processes where topological charges of nodes are transferred from one gap to another. Moreover, in such processes, we discover symmetry-enforced intermediate phases featuring triply-degenerate band nodes with unique dispersions that are directly linked to the multi-gap topological invariants. Furthermore, we confirm that edge states can faithfully characterize the multi-gap topological phase diagram. Our study unveils a new regime of topological phases where multi-gap topology and non-Abelian charges of band nodes play a crucial role in understanding semimetals with inter-connected multiple bands. △ Less

Submitted 27 April, 2021; originally announced April 2021.

Comments: 22 + 19 pages, 5+11 figures. With regard to references, please note that this concerns a version submitted to the journal in February 2021

Journal ref: Nature Physics 17,1239--1246 (2021)

Towards Accurate Predictions of Carrier Mobilities and Thermoelectric Performances in 2D Materials

Authors: Yu Wu, Bowen Hou, Ying Chen, Jiang Cao, Congcong Ma, Hezhu Shao, Yiming Zhang, Zixuan Lu, Heyuan Zhu, Zhilai Fang, Rongjun Zhang, Hao Zhang

Abstract: The interactions between electrons and lattice vibrational modes play the key role in determining the carrier transport properties, thermoelectric performance and other physical quantities related to phonons in semiconductors. However, for two-dimensional (2D) materials, the widely-used models for carrier transport only consider the interactions between electrons and some specific phonon modes, wh… ▽ More The interactions between electrons and lattice vibrational modes play the key role in determining the carrier transport properties, thermoelectric performance and other physical quantities related to phonons in semiconductors. However, for two-dimensional (2D) materials, the widely-used models for carrier transport only consider the interactions between electrons and some specific phonon modes, which usually leads to inaccruate predictions of electrons/phonons transport properties. In this work, comprehensive investigations on full electron-phonon couplings and their influences on carrier mobility and thermoelectric performances of 2D group-IV and V elemental monolayers were performed, and we also analyzed in details the selection rules on electron-phonon couplings using group-theory arguments. Our calculations revealed that, for the cases of shallow dopings where only intravalley scatterings are allowed, the contributions from optical phonon modes are significantly larger than those from acoustic phonon modes in group-IV elemental monolayers, and LA and some specific optical phonon modes contribute significantly to the total intravalley scatterings. When the doping increases and intervalley scatterings are allowed, the intervalley scatterings are much stronger than intravalley scatterings, and ZA/TA/LO phonon modes dominate the intervalley scatterings in monolayer Si, Ge and Sn. The dominant contributions to the total intervalley scatterings are ZA/TO in monolayer P, ZA/TO in monolayer As and TO/LO in monolayer Sb. Based on the thorough investigations on the full electron-phonon couplings, we predict accurately the carrier mobilities and thermoelectric figure of merits in these two elemental crystals, and reveal significant reductions when compared with the calculations based on the widely-used simplified model. △ Less

Submitted 11 October, 2020; originally announced October 2020.

Comments: 7 pages

Effects of intervalley scatterings in thermoelectric performance of band-convergent antimonene

Authors: Yu Wu, Bowen Hou, Congcong Ma, Jiang Cao, Ying Chen, Zixuan Lu, Haodong Mei, Hezhu Shao, Yuanfeng Xu, Heyuan Zhu, Zhilai Fang, Rongjun Zhang, Hao Zhang

Abstract: The strategy of band convergence of multi-valley conduction bands or multi-peak valence bands has been widely used to search or improve thermoelectric materials. However, the phonon-assisted intervalley scatterings due to multiple band degeneracy are usually neglected in the thermoelectric community. In this work, we investigate the (thermo)electric properties of non-polar monolayer $β$- and $α$-a… ▽ More The strategy of band convergence of multi-valley conduction bands or multi-peak valence bands has been widely used to search or improve thermoelectric materials. However, the phonon-assisted intervalley scatterings due to multiple band degeneracy are usually neglected in the thermoelectric community. In this work, we investigate the (thermo)electric properties of non-polar monolayer $β$- and $α$-antimonene considering full mode- and momentum-resolved electron-phonon interactions. We also analyze thoroughly the selection rules on electron-phonon matrix-elements using group-theory arguments. Our calculations reveal strong intervalley scattering between the nearly degenerate valley states in both $β$- and $α$-antimonene, and the commonly-used deformation potential approximation neglecting the dominant intervalley scattering gives inaccurate estimations of the electron-phonon scattering and thermoelectric transport properties. By considering full electron-phonon interactions based on the rigid-band approximation, we find that, the maximum value of the thermoelectric figure of merits $zT$ at room temperature reduces to 0.37 in $β$-antimonene, by a factor of 5.7 comparing to the value predicted based on the constant relaxation-time approximation method. Our work not only provides an accurate prediction of the thermoelectric performances of antimonenes that reveals the key role of intervalley scatterings in determining the electronic part of zT, but also showcases a computational framework for thermoelectric materials. △ Less

Submitted 21 September, 2020; v1 submitted 21 January, 2020; originally announced January 2020.

Comments: 7 figures

Twisted quadrupole topological photonic crystals

Authors: Xiaoxi Zhou, Zhi-Kang Lin, Weixin Lu, Yun Lai, Bo Hou, Jian-Hua Jiang

Abstract: Topological manipulation of waves is at the heart of the cutting-edge metamaterial researches. Quadrupole topological insulators were recently discovered in two-dimensional (2D) flux-threading lattices which exhibit higher-order topological wave trapping at both the edges and corners. Photonic crystals (PhCs), lying at the boundary between continuous media and discrete lattices, however, are incom… ▽ More Topological manipulation of waves is at the heart of the cutting-edge metamaterial researches. Quadrupole topological insulators were recently discovered in two-dimensional (2D) flux-threading lattices which exhibit higher-order topological wave trapping at both the edges and corners. Photonic crystals (PhCs), lying at the boundary between continuous media and discrete lattices, however, are incompatible with the present quadrupole topological theory. Here, we unveil quadrupole topological PhCs triggered by a twisting degree-of-freedom. Using a topologically trivial PhC as the motherboard, we show that twisting induces quadrupole topological PhCs without flux-threading. The twisting-induced crystalline symmetry enriches the Wannier polarizations and lead to the anomalous quadrupole topology. Versatile edge and corner phenomena are observed by controlling the twisting angles in a lateral heterostructure of 2D PhCs. Our study paves the way toward topological twist-photonics as well as the quadrupole topology in the quasi-continuum regime for phonons and polaritons. △ Less

Submitted 19 May, 2020; v1 submitted 11 December, 2019; originally announced December 2019.

Journal ref: Laser & Photon. Rev. 14, 2000010 (2020)

Deterministic scheme for two-dimensional type-II Dirac points and experimental realization in acoustics

Authors: Xiaoxiao Wu, Xin Li, Ruo-Yang Zhang, Xiao Xiang, Jingxuan Tian, Yingzhou Huang, Shuxia Wang, Bo Hou, C. T. Chan, Weijia Wen

Abstract: Low-energy electrons near Dirac/Weyl nodal points mimic massless relativistic fermions. However, as they are not constrained by Lorentz invariance, they can exhibit tipped-over type-II Dirac/Weyl cones which provide highly anisotropic physical properties and responses, creating unique possibilities. Recently, they have been observed in several quantum and classical systems. Yet, there is still no… ▽ More Low-energy electrons near Dirac/Weyl nodal points mimic massless relativistic fermions. However, as they are not constrained by Lorentz invariance, they can exhibit tipped-over type-II Dirac/Weyl cones which provide highly anisotropic physical properties and responses, creating unique possibilities. Recently, they have been observed in several quantum and classical systems. Yet, there is still no simple and deterministic strategy to realize them since their nodal points are accidental degeneracies, unlike symmetry-guaranteed type-I counterparts. Here, we propose a band-folding scheme for constructing type-II Dirac points, and we use a tight-binding analysis to unveil its generality and deterministic nature. Through realizations in acoustics, type-II Dirac points are experimentally visualized and investigated using near-field mappings. As a direct effect of tipped-over Dirac cones, strongly tilted kink states originating from their valley-Hall properties are also observed. This deterministic scheme could serve as platform for further investigations of intriguing physics associated with various strongly Lorentz-violating nodal points. △ Less

Submitted 27 January, 2020; v1 submitted 16 July, 2019; originally announced July 2019.

Comments: 4 figures

Journal ref: Phys. Rev. Lett. 124, 075501 (2020)

One-dimensional van der Waals heterostructures

Authors: Rong Xiang, Taiki Inoue, Yongjia Zheng, Akihito Kumamoto, Yang Qian, Yuta Sato, Ming Liu, Devashish Gokhale, Jia Guo, Kaoru Hisama, Satoshi Yotsumoto, Tatsuro Ogamoto, Hayato Arai, Yu Kobayashi, Hao Zhang, Bo Hou, Anton Anisimov, Yasumitsu Miyata, Susumu Okada, Shohei Chiashi, Yan Li, Jing Kong, Esko I. Kauppinen, Yuichi Ikuhara, Kazu Suenaga , et al. (1 additional authors not shown)

Abstract: Property by design is one appealing idea in material synthesis but hard to achieve in practice. A recent successful example is the demonstration of van der Waals (vdW) heterostructures,1-3 in which atomic layers are stacked on each other and different ingredients can be combined beyond symmetry and lattice matching. This concept, usually described as a nanoscale Lego blocks, allows to build sophis… ▽ More Property by design is one appealing idea in material synthesis but hard to achieve in practice. A recent successful example is the demonstration of van der Waals (vdW) heterostructures,1-3 in which atomic layers are stacked on each other and different ingredients can be combined beyond symmetry and lattice matching. This concept, usually described as a nanoscale Lego blocks, allows to build sophisticated structures layer by layer. However, this concept has been so far limited in two dimensional (2D) materials. Here we show a class of new material where different layers are coaxially (instead of planarly) stacked. As the structure is in one dimensional (1D) form, we name it "1D vdW heterostructures". We demonstrate a 5 nm diameter nanotube consisting of three different materials: an inner conductive carbon nanotube (CNT), a middle insulating hexagonal boron nitride nanotube (BNNT) and an outside semiconducting MoS2 nanotube. As the technique is highly applicable to other materials in the current 2D libraries,4-6 we anticipate our strategy to be a starting point for discovering a class of new semiconducting nanotube materials. A plethora of function-designable 1D heterostructures will appear after the combination of CNTs, BNNTs and semiconducting nanotubes. △ Less

Submitted 30 October, 2018; v1 submitted 16 July, 2018; originally announced July 2018.

Journal ref: Science (2020), 367, 537

Type-II Dirac photons at the metasurfaces

Authors: Chuandeng Hu, Zhenyu Li, Rui Tong, Xiaoxiao Wu, Zengzilu Xia, Li Wang, Shanshan Li, Yingzhou Huang, Shuxia Wang, Bo Hou, C. T. Chan, Weijia Wen

Abstract: Topological characteristics of energy bands, such as Dirac/Weyl nodes, have attracted substantial interest in condensed matter systems as well as in classical wave systems. Among these energy bands, the type-II Dirac point is a nodal degeneracy with tilted conical dispersion, leading to a peculiar crossing dispersion in the constant energy plane. Such nodal points have recently been found in elect… ▽ More Topological characteristics of energy bands, such as Dirac/Weyl nodes, have attracted substantial interest in condensed matter systems as well as in classical wave systems. Among these energy bands, the type-II Dirac point is a nodal degeneracy with tilted conical dispersion, leading to a peculiar crossing dispersion in the constant energy plane. Such nodal points have recently been found in electronic materials. The analogous topological feature in photonic systems remains a theoretical curiosity, with experimental realization expected to be challenging. Here, we experimentally realize the type-II Dirac point using a planar metasurface architecture, where the band degeneracy point is protected by the underlying mirror symmetry of the metasurface. Gapless edge modes are found and measured at the boundary between the different domains of the symmetry-broken metasurface. Our work shows that metasurfaces are simple and practical platforms for realizing electromagnetic type-II Dirac points, and their planar structure is a distinct advantage that facilitates applications in two-dimensional topological photonics. △ Less

Submitted 23 March, 2018; originally announced March 2018.

Journal ref: Phys. Rev. Lett. 121, 024301 (2018)

A metasurface with bidirectional hyperbolic surface modes and positionsensing applications

Authors: Chuandeng Hu, Xiaoxiao Wu, Rui Tong, Li Wang, YingZhou Huang, Shuxia Wang, Bo Hou, Weijia Wen

Abstract: We theoretically and experimentally studied resonance-induced hyperbolic metasurfaces, proving that it is an efficient way to introduce Fano-resonance and decreasethe Q-factor in our system in order to create hyperbolic iso-frequency contours (IFCs) along two orthogonal directions. The metasurface with a continuous topological transition for such IFCs has been designed and experimentally implement… ▽ More We theoretically and experimentally studied resonance-induced hyperbolic metasurfaces, proving that it is an efficient way to introduce Fano-resonance and decreasethe Q-factor in our system in order to create hyperbolic iso-frequency contours (IFCs) along two orthogonal directions. The metasurface with a continuous topological transition for such IFCs has been designed and experimentally implemented. In particular, two independent selfcollimation frequencies corresponding to the transition frequencies in orthogonal directions. As a consequence, we experimentally demonstrated that the metasurface can function as a position- sensor by utilizing the bidirectional hyperbolic surface waves, opening a new avenue for position-sensing. △ Less

Submitted 1 December, 2017; originally announced December 2017.

The origin of the turn-on phenomenon in Td-MoTe2

Authors: Q. L. Pei, W. J. Meng, X. Luo, H. Y. Lv, F. C. Chen, W. J. Lu, Y. Y. Han, P. Tong, W. H. Song, Y. B. Hou, Q. Y. Lu, Y. P. Sun

Abstract: We did the resistivity and scanning tunneling microscope/spectroscopy (STM/STS) experiments at different temperatures and magnetic fields to investigate the origin of the turn-on (t-o) phenomenon of Td-MoTe2. There are two interesting observations. Firstly, magnetoresistance (MR) follows the Kohler rule scaling: MR - (H/p0)m with m - 1.92 and the t-o temperature T under different magnetic fields c… ▽ More We did the resistivity and scanning tunneling microscope/spectroscopy (STM/STS) experiments at different temperatures and magnetic fields to investigate the origin of the turn-on (t-o) phenomenon of Td-MoTe2. There are two interesting observations. Firstly, magnetoresistance (MR) follows the Kohler rule scaling: MR - (H/p0)m with m - 1.92 and the t-o temperature T under different magnetic fields can also be scaled by T - (H-Hc)u with u = 1/2. Secondly, a combination of compensated electron-hole pockets and a possible electronic structure phase transition induced by the temperature have been validated in Td-MoTe2 by the STM/STS experiments. Compared with the STS of Td-MoTe2 single crystal under H = 0, the STS hardly changes even when the applied field is up to 7 T. The origins of the t-o phenomenon in Td-MoTe2 are discussed. Meanwhile, we analyzed the universality and applicability of the t-o phenomenon in the extreme MR materials with almost balanced hole and electron densities as well as with other systems where the density of hole or electron is in dominant position. △ Less

Submitted 9 July, 2017; originally announced July 2017.

Comments: Attepted by Phys. Rev. B

Topological Dirac line nodes and superconductivity coexist in SnSe at high pressure

Authors: Xuliang Chen, Pengchao Lu, Xuefei Wang, Yonghui Zhou, Chao An, Ying Zhou, Cong Xian, Hao Gao, Zhaopeng Guo, Changyong Park, Binyang Hou, Kunling Peng, Xiaoyuan Zhou, Yimin Xiong, Jian Sun, Zhaorong Yang, Dingyu Xing, Yuheng Zhang

Abstract: Due to fundamental interest and potential applications in quantum computation, tremendous efforts have been invested to study topological superconductivity. However, bulk topological superconductivity seems to be difficult to realize and its mechanism is still elusive. Several possible routes to induce topological superconductivity have been proposed, including proximity efforts, doping or pressur… ▽ More Due to fundamental interest and potential applications in quantum computation, tremendous efforts have been invested to study topological superconductivity. However, bulk topological superconductivity seems to be difficult to realize and its mechanism is still elusive. Several possible routes to induce topological superconductivity have been proposed, including proximity efforts, doping or pressurizing a topological insulator or semimetal. Among them, the pressurizing is considered to be a clean way to tune the electronic structures. Here we report the discovery of a pressure-induced topological and superconducting phase of SnSe, a material which is highly focused recently due to its superior thermoelectric properties. In situ high-pressure electrical transport and synchrotron X-ray diffraction measurements show that the superconductivity emerges along with the formation of a CsCl-type structural symmetry of SnSe above around 27 GPa, with a maximum critical temperature of 3.2 K at 39 GPa. Based on ab initio calculations, this CsCl-type SnSe is predicted to be a Dirac line nodes (DLN) semimetal in the absence of spin-orbit coupling, whose DLN states are protected by the coexistence of time-reversal and inversion symmetries. These results make CsCl-type SnSe an interesting model platform with simple crystal symmetry to study the interplay of topological physics and superconductivity. △ Less

Submitted 12 January, 2017; v1 submitted 24 August, 2016; originally announced August 2016.

Comments: 32 pages, 7 figures, 2 tables

Journal ref: Phys. Rev. B 96, 165123 (2017)

Distinct itinerant spin-density waves and local-moment antiferromagnetism in an intermetallic ErPd$_2$Si$_2$ single crystal

Authors: Hai-Feng Li, Chongde Cao, Andrew Wildes, Wolfgang Schmidt, Karin Schmalzl, Binyang Hou, Louis-Pierre Regnault, Cong Zhang, Paul Meuffels, Wolfgang Löser, Georg Roth

Abstract: Identifying the nature of magnetism, itinerant or localized, remains a major challenge in condensed-matter science. Purely localized moments appear only in magnetic insulators, whereas itinerant moments more or less co-exist with localized moments in metallic compounds such as the doped-cuprate or the iron-based superconductors, hampering a thorough understanding of the role of magnetism in phenom… ▽ More Identifying the nature of magnetism, itinerant or localized, remains a major challenge in condensed-matter science. Purely localized moments appear only in magnetic insulators, whereas itinerant moments more or less co-exist with localized moments in metallic compounds such as the doped-cuprate or the iron-based superconductors, hampering a thorough understanding of the role of magnetism in phenomena like superconductivity or magnetoresistance. Here we distinguish two antiferromagnetic modulations with respective propagation wave vectors of $Q_{\pm}$ = ($H \pm 0.557(1)$, 0, $L \pm 0.150(1)$) and $Q_\text{C}$ = ($H \pm 0.564(1)$, 0, $L$), where $\left(H, L\right)$ are allowed Miller indices, in an ErPd$_2$Si$_2$ single crystal by neutron scattering and establish their respective temperature- and field-dependent phase diagrams. The modulations can co-exist but also compete depending on temperature or applied field strength. They couple differently with the underlying lattice albeit with associated moments in a common direction. The $Q_{\pm}$ modulation may be attributed to localized 4\emph{f} moments while the $Q_\text{C}$ correlates well with itinerant conduction bands, supported by our transport studies. Hence, ErPd$_2$Si$_2$ represents a new model compound that displays clearly-separated itinerant and localized moments, substantiating early theoretical predictions and providing a unique platform allowing the study of itinerant electron behavior in a localized antiferromagnetic matrix. △ Less

Submitted 3 February, 2015; originally announced February 2015.

Comments: 7 pages, 7 Figures, 1 Table

Journal ref: Scientific Reports 5, Article number: 7968, Pages: 1-7, 2015

Magnetization, crystal structure and anisotropic thermal expansion of single-crystal SrEr2O4

Authors: Hai-Feng Li, Andrew Wildes, Binyang Hou, Cong Zhang, Berthold Schmitz, Paul Meuffels, Georg Roth, Thomas Brückel

Abstract: The magnetization, crystal structure, and thermal expansion of a nearly stoichiometric Sr$_{1.04(3)}$Er$_{2.09(6)}$O$_{4.00(1)}$ single crystal have been studied by PPMS measurements and in-house and high-resolution synchrotron X-ray powder diffraction. No evidence was detected for any structural phase transitions even up to 500 K. The average thermal expansions of lattice constants and unit-cell… ▽ More The magnetization, crystal structure, and thermal expansion of a nearly stoichiometric Sr$_{1.04(3)}$Er$_{2.09(6)}$O$_{4.00(1)}$ single crystal have been studied by PPMS measurements and in-house and high-resolution synchrotron X-ray powder diffraction. No evidence was detected for any structural phase transitions even up to 500 K. The average thermal expansions of lattice constants and unit-cell volume are consistent with the first-order Grüneisen approximations taking into account only the phonon contributions for an insulator, displaying an anisotropic character along the crystallographic \emph{a}, \emph{b}, and \emph{c} axes. Our magnetization measurements indicate that obvious magnetic frustration appears below $\sim$15 K, and antiferromagnetic correlations may persist up to 300 K. △ Less

Submitted 24 October, 2014; originally announced October 2014.

Comments: 6 pages, 5 figure, 2 tables

Journal ref: RSC Adv., 4, 53602-53607 (2014)

Incommensurate antiferromagnetic order in the manifoldly-frustrated SrTb$_2$O$_4$ with transition temperature up to 4.28 K

Authors: Hai-Feng Li, Cong Zhang, Anatoliy Senyshyn, Andrew Wildes, Karin Schmalzl, Wolfgang Schmidt, Martin Boehm, Eric Ressouche, Binyang Hou, Paul Meuffels, Georg Roth, Thomas Brueckel

Abstract: The N$\acute{\rm e}$el temperature of the new frustrated family of Sr\emph{RE}$_2$O$_4$ (\emph{RE} = rare earth) compounds is yet limited to $\sim$ 0.9 K, which more or less hampers a complete understanding of the relevant magnetic frustrations and spin interactions. Here we report on a new frustrated member to the family, SrTb$_2$O$_4$ with a record $T_{\rm N}$ = 4.28(2) K, and an experimental st… ▽ More The N$\acute{\rm e}$el temperature of the new frustrated family of Sr\emph{RE}$_2$O$_4$ (\emph{RE} = rare earth) compounds is yet limited to $\sim$ 0.9 K, which more or less hampers a complete understanding of the relevant magnetic frustrations and spin interactions. Here we report on a new frustrated member to the family, SrTb$_2$O$_4$ with a record $T_{\rm N}$ = 4.28(2) K, and an experimental study of the magnetic interacting and frustrating mechanisms by polarized and unpolarized neutron scattering. The compound SrTb$_2$O$_4$ displays an incommensurate antiferromagnetic (AFM) order with a transverse wave vector \textbf{Q}$^{\rm 0.5 K}_{\rm AFM}$ = (0.5924(1), 0.0059(1), 0) albeit with partially-ordered moments, 1.92(6) $μ_{\rm B}$ at 0.5 K, stemming from only one of the two inequivalent Tb sites mainly by virtue of their different octahedral distortions. The localized moments are confined to the \emph{bc} plane, 11.9(66)$^\circ$ away from the \emph{b} axis probably by single-ion anisotropy. We reveal that this AFM order is dominated mainly by dipole-dipole interactions and disclose that the octahedral distortion, nearest-neighbour (NN) ferromagnetic (FM) arrangement, different next NN FM and AFM configurations, and in-plane anisotropic spin correlations are vital to the magnetic structure and associated multiple frustrations. The discovery of the thus far highest AFM transition temperature renders SrTb$_2$O$_4$ a new friendly frustrated platform in the family for exploring the nature of magnetic interactions and frustrations. △ Less

Submitted 30 June, 2014; originally announced July 2014.

Comments: 19 pages, 8 Figures, 1 Table

Journal ref: Front. Phys. 2, 42 (2014)

Nonmagnetic ordering state of single-crystal SrTm$_2$O$_4$: A polarized and unpolarized neutron-scattering study

Authors: Haifeng Li, Binyang Hou, Andrew Wildes, Anatoliy Senyshyn, Karin Schmalzl, Wolfgang Schmidt, Cong Zhang, Thomas Brueckel, Georg Roth

Abstract: Our single-crystal polarized neutron scattering at 65 mK and powder unpolarized neutron diffraction at 0.5 K show no evidence for a long-range magnetic order and even detect no sign of diffuse magnetic neutron scattering in single-crystal SrTm2O4. The data refinements reveal that the two TmO6 octahedral distortion modes are the same as those of the TbO6 octahedra in SrTb2O4, i.e., one distortion i… ▽ More Our single-crystal polarized neutron scattering at 65 mK and powder unpolarized neutron diffraction at 0.5 K show no evidence for a long-range magnetic order and even detect no sign of diffuse magnetic neutron scattering in single-crystal SrTm2O4. The data refinements reveal that the two TmO6 octahedral distortion modes are the same as those of the TbO6 octahedra in SrTb2O4, i.e., one distortion is stronger than the other one especially at low temperatures, which is attributed probably to different crystal electric fields for the two inequivalent octahedra. Consequently, we conclude that SrTm2O4 has no magnetic order, neither long-ranged nor short-ranged, even down to 65 mK. Therefore, SrTm2O4 is a different compound from its brethren in the new family of frustrated SrRE2O4 (RE = Gd, Tb, Dy, Ho, Er, and Yb) magnets. We propose that crystal field anisotropy may dominate over weak dipolar spin interactions in SrTm2O4, leading to a virtually nonmagnetic ordering state. △ Less

Submitted 31 March, 2014; originally announced April 2014.

Comments: 9 pages, 6 figures, 2 Tables

MSC Class: 61.05.fg; 75.25.-j; 75.47.Lx; 75.50.Ee

Journal ref: Journal of Materials Chemistry C 3 (29), 7658-7668, 2015

Transformation optics with Fabry-Pérot resonances

Authors: M. M. Sadeghi, Sucheng Li, Lin Xu, Bo Hou, Huanyang Chen

Abstract: Transformation optics is a powerful tool to design various novel devices, such as invisibility cloak. Fantastic effects from this technique are usually accompanied with singular mappings, resulting in challenging implementations and narrow bands of working frequencies. Here in this article, we find that Fabry-Pérot resonances can be used to design various transformation optical devices that are no… ▽ More Transformation optics is a powerful tool to design various novel devices, such as invisibility cloak. Fantastic effects from this technique are usually accompanied with singular mappings, resulting in challenging implementations and narrow bands of working frequencies. Here in this article, we find that Fabry-Pérot resonances can be used to design various transformation optical devices that are not only easy to realize but also can work well for a set of resonant frequencies (multiple frequencies). As an example, we fabricate a prototype for a cylindrical concentrator for microwaves. △ Less

Submitted 2 November, 2013; originally announced November 2013.

Comments: 23 pages, 19 figures, submitted to Nature Photonics

Journal ref: Scientific Reports 5: 8680 (2015)

Mean first-passage time for random walks on undirected networks

Authors: Zhongzhi Zhang, Alafate Julaiti, Baoyu Hou, Hongjuan Zhang, Guanrong Chen

Abstract: In this paper, by using two different techniques we derive an explicit formula for the mean first-passage time (MFPT) between any pair of nodes on a general undirected network, which is expressed in terms of eigenvalues and eigenvectors of an associated matrix similar to the transition matrix. We then apply the formula to derive a lower bound for the MFPT to arrive at a given node with the startin… ▽ More In this paper, by using two different techniques we derive an explicit formula for the mean first-passage time (MFPT) between any pair of nodes on a general undirected network, which is expressed in terms of eigenvalues and eigenvectors of an associated matrix similar to the transition matrix. We then apply the formula to derive a lower bound for the MFPT to arrive at a given node with the starting point chosen from the stationary distribution over the set of nodes. We show that for a correlated scale-free network of size $N$ with a degree distribution $P(d)\sim d^{-γ}$, the scaling of the lower bound is $N^{1-1/γ}$. Also, we provide a simple derivation for an eigentime identity. Our work leads to a comprehensive understanding of recent results about random walks on complex networks, especially on scale-free networks. △ Less

Submitted 7 November, 2011; originally announced November 2011.

Comments: 7 pages, no figures; definitive version published in European Physical Journal B

Journal ref: Eur. Phys. J. B 84, 691-697 (2011)

Determinant formula for the partition function of the six-vertex model with a non-diagonal reflecting end

Authors: Wen-Li Yang, Xi Chen, Jun Feng, Kun Hao, Bo-Yu Hou, Kang-Jie Shi, Yao-Zhong Zhang

Abstract: With the help of the F-basis provided by the Drinfeld twist or factorizing F-matrix for the open XXZ spin chain with non-diagonal boundary terms, we obtain the determinant representation of the partition function of the six-vertex model with a non-diagonal reflecting end under domain wall boundary condition. With the help of the F-basis provided by the Drinfeld twist or factorizing F-matrix for the open XXZ spin chain with non-diagonal boundary terms, we obtain the determinant representation of the partition function of the six-vertex model with a non-diagonal reflecting end under domain wall boundary condition. △ Less

Submitted 28 July, 2011; originally announced July 2011.

Comments: Latex file, 24 pages, 4 figures

Journal ref: Nucl. Phys. B 844 (2011), 289-307

Monovalent Ion Condensation at the Electrified Liquid/Liquid Interface

Authors: Nouamane Laanait, Jaesung Yoon, Binyang Hou, Petr Vanysek, Mati Meron, Binhua Lin, Guangming Luo, Ilan Benjamin, Mark L. Schlossman

Abstract: X-ray reflectivity studies demonstrate the condensation of a monovalent ion at the electrified interface between electrolyte solutions of water and 1,2-dichloroethane. Predictions of the ion distributions by standard Poisson-Boltzmann (Gouy-Chapman) theory are inconsistent with these data at higher applied interfacial electric potentials. Calculations from a Poisson-Boltzmann equation that incorpo… ▽ More X-ray reflectivity studies demonstrate the condensation of a monovalent ion at the electrified interface between electrolyte solutions of water and 1,2-dichloroethane. Predictions of the ion distributions by standard Poisson-Boltzmann (Gouy-Chapman) theory are inconsistent with these data at higher applied interfacial electric potentials. Calculations from a Poisson-Boltzmann equation that incorporates a non-monotonic ion-specific potential of mean force are in good agreement with the data. △ Less

Submitted 23 June, 2010; originally announced June 2010.

Comments: 4 pages, 4 figures

Journal ref: J. Chem. Phys. 132, 171101 (2010)

Structure and Depletion at Fluoro- and Hydro-carbon/Water Liquid/Liquid Interfaces

Authors: Kaoru Kashimoto, Jaesung Yoon, Binyang Hou, Chiu-hao Chen, Binhua Lin, Makoto Aratono, Takanori Takiue, Mark L. Schlossman

Abstract: The results of x-ray reflectivity studies of two oil/water (liquid/liquid) interfaces are inconsistent with recent predictions of the presence of a vapor-like depletion region at hydrophobic/aqueous interfaces. One of the oils, perfluorohexane, is a fluorocarbon whose super-hydrophobic interface with water provides a stringent test for the presence of a depletion layer. The other oil, heptane, i… ▽ More The results of x-ray reflectivity studies of two oil/water (liquid/liquid) interfaces are inconsistent with recent predictions of the presence of a vapor-like depletion region at hydrophobic/aqueous interfaces. One of the oils, perfluorohexane, is a fluorocarbon whose super-hydrophobic interface with water provides a stringent test for the presence of a depletion layer. The other oil, heptane, is a hydrocarbon and, therefore, is more relevant to the study of biomolecular hydrophobicity. These results are consistent with the sub-angstrom proximity of water to soft hydrophobic materials. △ Less

Submitted 18 July, 2008; originally announced July 2008.

Comments: 28 pages, 9 figures, main text plus supplementary, to be published in Physical Review Letters

Non-commutative geometry of 4-dimensional quantum Hall droplet

Authors: Yi-Xin Chen, Bo-Yu Hou, Bo-Yuan Hou

Abstract: We develop the description of non-commutative geometry of the 4-dimensional quantum Hall fluid's theory proposed recently by Zhang and Hu. The non-commutative structure of fuzzy $S^{4}$ appears naturally in this theory. The fuzzy monopole harmonics, which are the essential elements in this non-commutative geometry, are explicitly constructed and their obeying the matrix algebra is obtained. This… ▽ More We develop the description of non-commutative geometry of the 4-dimensional quantum Hall fluid's theory proposed recently by Zhang and Hu. The non-commutative structure of fuzzy $S^{4}$ appears naturally in this theory. The fuzzy monopole harmonics, which are the essential elements in this non-commutative geometry, are explicitly constructed and their obeying the matrix algebra is obtained. This matrix algebra is associative. We also propose a fusion scheme of the fuzzy monopole harmonics of the coupling system from those of the subsystems, and determine the fusion rule in such fusion scheme. By products, we provide some essential ingredients of the theory of SO(5) angular momentum. In particular, the explicit expression of the coupling coefficients, in the theory of SO(5) angular momentum, are given. It is discussed that some possible applications of our results to the 4-dimensional quantum Hall system and the matrix brane construction in M-theory. △ Less

Submitted 12 May, 2002; v1 submitted 11 March, 2002; originally announced March 2002.

Comments: latex 22 pages, no figures. some references added. some results are clarified

Journal ref: Nucl.Phys.B638:220-242,2002

q-deformed Supersymmetric t-J Model with a Boundary

Authors: Bo-Yu Hou, Wen-Li Yang, Yao-Zhong Zhang, Yi Zhen

Abstract: The q-deformed supersymmetric t-J model on a semi-infinite lattice is diagonalized by using the level-one vertex operators of the quantum affine superalgebra $U_q[\hat{sl(2|1)}]$. We give the bosonization of the boundary states. We give an integral expression of the correlation functions of the boundary model, and derive the difference equations which they satisfy. The q-deformed supersymmetric t-J model on a semi-infinite lattice is diagonalized by using the level-one vertex operators of the quantum affine superalgebra $U_q[\hat{sl(2|1)}]$. We give the bosonization of the boundary states. We give an integral expression of the correlation functions of the boundary model, and derive the difference equations which they satisfy. △ Less

Submitted 21 February, 2001; originally announced February 2001.

Comments: LaTex file 18 pages

Journal ref: J. Phys. A35 (2002) 2593-2608

The Exact Solution of 1-D SU(n) Hubbard model

Authors: Boyu Hou, Dantao Peng, Ruihong Yue

Abstract: In this paper we derive out the exact solution of the SU(n) Hubbard model through the coordinate and the algebraic Bethe ansatz methods. The energy spectrum and the Bethe ansatz equations are obtained. Furthermore, we analysis the ground state and give out the exact analytic solution of the model. In this paper we derive out the exact solution of the SU(n) Hubbard model through the coordinate and the algebraic Bethe ansatz methods. The energy spectrum and the Bethe ansatz equations are obtained. Furthermore, we analysis the ground state and give out the exact analytic solution of the model. △ Less

Submitted 30 October, 2000; originally announced October 2000.

Comments: 7 pages, RevTex, no figures

The Thermodynamics of the XXZ Heisenberg Chain with Impurities

Authors: Boyu Hou, Kangjie Shi, Ruihong Yue, Shaoyou Zhao

Abstract: In this paper, we discuss the effect of the arbitrary spin impurities to the spin-1/2 and spin-1 XXZ model. The effect of ground state, the free energy, the magnetic susceptibility, the specific heat and the Kondo temperature are given by using the thermodynamic Bethe ansatz equation. In this paper, we discuss the effect of the arbitrary spin impurities to the spin-1/2 and spin-1 XXZ model. The effect of ground state, the free energy, the magnetic susceptibility, the specific heat and the Kondo temperature are given by using the thermodynamic Bethe ansatz equation. △ Less

Submitted 25 October, 1999; originally announced October 1999.

Comments: 13 pages, revtex, no figures

Report number: IMP-10-25-3

Integrability of the Heisenberg Chains with Boundary Impurities and Their Bethe Ansatz

Authors: Boyu Hou, Kangjie Shi, Ruihong Yue, Shaoyou Zhao

Abstract: In this paper, we show the integrability of spin-1/2 XXZ Heisenberg chain with two arbitrary spin boundary Impurities. By using the fusion method, we generalize it to the spin-1 XXZ chain. Then the eigenvalues of Hamiltonians of these models are obtained by the means of Bethe ansatz method. In this paper, we show the integrability of spin-1/2 XXZ Heisenberg chain with two arbitrary spin boundary Impurities. By using the fusion method, we generalize it to the spin-1 XXZ chain. Then the eigenvalues of Hamiltonians of these models are obtained by the means of Bethe ansatz method. △ Less

Submitted 25 October, 1999; originally announced October 1999.

Comments: 13 pages, latex, no figures, to be appeared in J.Phys.A

Report number: IMP-10-25-2

Bethe Ansatz for the Spin-1 XXX Chain with Two Impurities

Authors: Boyu Hou, Kangjie Shi, Ruihong Yue, Shaoyou Zhao

Abstract: By using algebraic Bethe ansatz method, we give the Hamitonian of the spin-1 XXX chain associated with $sl_2$ with two boundary impurities. By using algebraic Bethe ansatz method, we give the Hamitonian of the spin-1 XXX chain associated with $sl_2$ with two boundary impurities. △ Less

Submitted 25 October, 1999; originally announced October 1999.

Comments: 8 pages, latex, no figures, to be appeared in Commun. Theor. Phys

Report number: IMP-99-10-25

The Exact Solution of the SU(3) Hubbard Model

Authors: Buoyu Hou, Dantao Peng, Ruihong Yue

Abstract: The Bethe ansatz equations of the 1-D SU(3) Hubbard model are systematically derived by diagonalizing the inhomogeneous transfer matrix of the XXX model. We first derive the scattering matrix of the SU(3) Hubbard model through the coordinate Bethe ansatz method. Then, with the help quantum inverse scattering method we solve the nested transfer matrix and give the eigenvalues, the eigenvectors an… ▽ More The Bethe ansatz equations of the 1-D SU(3) Hubbard model are systematically derived by diagonalizing the inhomogeneous transfer matrix of the XXX model. We first derive the scattering matrix of the SU(3) Hubbard model through the coordinate Bethe ansatz method. Then, with the help quantum inverse scattering method we solve the nested transfer matrix and give the eigenvalues, the eigenvectors and the Bethe ansatz equations. Finally, we obtain the exactly analytic solution for the ground state. △ Less

Submitted 25 October, 1999; v1 submitted 20 October, 1999; originally announced October 1999.

Comments: 19 pages, latex, no figures

Report number: IMP-99-10-18

Algebraic Bethe ansatz for the supersymmetric $t-J$ model with reflecting boundary conditions

Authors: Heng Fan, Bo-yu Hou, Kang-jie Shi

Abstract: In the framework of the graded quantum inverse scattering method (QISM), we obtain the eigenvalues and eigenvectors of the supersymmetric $t-J$ model with reflecting boundary conditions in FFB background. The corresponding Bethe ansatz equations are obtained. In the framework of the graded quantum inverse scattering method (QISM), we obtain the eigenvalues and eigenvectors of the supersymmetric $t-J$ model with reflecting boundary conditions in FFB background. The corresponding Bethe ansatz equations are obtained. △ Less

Submitted 17 March, 1998; originally announced March 1998.

Comments: Latex file, 23 Pages

Journal ref: Nucl.Phys.B541 (1999) 483-505

Exact diagonalization of the quantum supersymmetric $SU_q(n|m)$ model

Authors: Ruihong Yue, Heng Fan, Boyu Hou

Abstract: We use the algebraic nested Bethe ansatz to solve the eigenvalue and eigenvector problem of the supersymmetric $SU_q(n|m)$ model with open boundary conditions. Under an additional condition that model is related to a multicomponent supersymmetric t-J model. We also prove that the transfer matrix with open boundary condition is $SU_q(n|m)$ invariant. We use the algebraic nested Bethe ansatz to solve the eigenvalue and eigenvector problem of the supersymmetric $SU_q(n|m)$ model with open boundary conditions. Under an additional condition that model is related to a multicomponent supersymmetric t-J model. We also prove that the transfer matrix with open boundary condition is $SU_q(n|m)$ invariant. △ Less

Submitted 4 March, 1996; originally announced March 1996.

Comments: Latex, 30 pages, no figure. to appear in Nucl.Phys.B(FS)

Report number: Bonn-TH-9523