Showing 1–50 of 58 results for author: Qiao, L

Evidence for Clean d-wave Superconductivity in Samarium Nickelates

Authors: Qingming Huang, Xiaofang Fu, Junlong Wu, Laifeng Li, Liang Qiao, Ye Yang

Abstract: The discovery of superconducting nickelates provides a unique opportunity to explore the pairing mechanism of high-temperature superconductivity. Here, we use ultrafast terahertz spectroscopy to probe the temperature-dependent superfluid density in an infinite-layer samarium nickelate film with a Tc of 20 K. The superfluid density decreases linearly with rising temperature, consistent with clean l… ▽ More The discovery of superconducting nickelates provides a unique opportunity to explore the pairing mechanism of high-temperature superconductivity. Here, we use ultrafast terahertz spectroscopy to probe the temperature-dependent superfluid density in an infinite-layer samarium nickelate film with a Tc of 20 K. The superfluid density decreases linearly with rising temperature, consistent with clean limit d-wave pairing. From this linear relation, we extract a superconducting gap of 2.5 meV and a gap-to-Tc ratio of 3, suggesting that this sample lies in the weak-coupling limit. Furthermore, the ratio of the mean free path to the coherence length, is determined to be 1.5, confirming the clean-limit behavior. These findings establish strong parallels between the pairing mechanisms in nickelate and cuprate superconductors. △ Less

Submitted 23 December, 2025; originally announced December 2025.

Facet Specific Electron Conduction in Pentavalent (W5+) WO3 Drives Superior Photocatalytic CO 2 Reduction in (002) Plane

Authors: Muhammad Rizwan Kamal, Mohammad Z. Rahman, Amil Aligayev, Min Liu, Li Zhong, Pengfei Xia, Yueheng Li, Yue Ruan, Xia Xiang, Pir Muhammad Ismail, Qaisar Alam, Ahmed Ismail, Muhammad Zahid, Xiaoqiang Wu, Abdullah N. Alodhayb, Qing Huang, Raj Wali Khan, Fazal Raziq, Sharafat Ali, Liang Qiao

Abstract: This article reports a concept of heat-induced topological modifications of non-layered WO 3 followed by successful synthesis of oxygen-vacant more-porous nanosheets with exposed active (002) facet. Experimental measurements and Density Functional Theory (DFT) calculations have revealed that the photoexcited electrons are found to accumulate preferentially on (002) facet to yield enhanced electron… ▽ More This article reports a concept of heat-induced topological modifications of non-layered WO 3 followed by successful synthesis of oxygen-vacant more-porous nanosheets with exposed active (002) facet. Experimental measurements and Density Functional Theory (DFT) calculations have revealed that the photoexcited electrons are found to accumulate preferentially on (002) facet to yield enhanced electron conduction, and consequently, strengthen the reduction potential as active catalytic sites for photocatalytic CO2 reduction. Owing to these beneficial properties, the more-porous nanosheets of WO 3 with (002) facet have exhibited superior performance than that of less-porous nanosheets of WO3 with (220) facet and bulk WO3 with (205) facet. This study therefore provides a new understanding of regulating physical, optical, and electronic properties through intricate atomic structure modulation of WO3, and may find widespread application in optoelectronics, sensors, and energy conversion. △ Less

Submitted 17 October, 2025; originally announced October 2025.

First-principles approach to ultrafast pump-probe spectroscopy in solids

Authors: Lu Qiao, Ronaldo Rodrigues Pela, Claudia Draxl

Abstract: Pump-probe spectroscopy is a powerful tool to study ultrafast exciton dynamics, revealing the underlying complex interactions on the electronic scale. Despite significant advances in experimental techniques, developing a comprehensive and rigorous theoretical framework for modeling and interpreting the transient response in photoexcited materials remains a challenge. Here, we present a first-princ… ▽ More Pump-probe spectroscopy is a powerful tool to study ultrafast exciton dynamics, revealing the underlying complex interactions on the electronic scale. Despite significant advances in experimental techniques, developing a comprehensive and rigorous theoretical framework for modeling and interpreting the transient response in photoexcited materials remains a challenge. Here, we present a first-principles approach to simulating pump-probe spectroscopy and disentangling the electronic and thermal contributions underlying exciton dynamics. We showcase our method to three materials, representative for different classes of solids: the transition-metal dichalcogenides WSe$_2$, the halide perovskite CsPbBr$_3$, and the transition-metal oxide TiO$_2$, showing remarkable agreement with experimental counterparts. We find that (i) photoinduced Coulomb screening is the primary electronic effect, responsible for a blue shift of exciton resonances, while (ii) Pauli blocking plays a minor role, and (iii) thermal lattice expansion leads to a red shift of the spectra. We further demonstrate how key parameters such as excitation density, pump photon energy, and pump polarization modulate the transient absorption spectra, offering direct control over the exciton-resonance energy. Our approach establishes a quantitative and predictive framework for interpreting pump-probe experiments, providing actionable insights for the design of energy-selective optoelectronic devices through exciton engineering. △ Less

Submitted 9 September, 2025; originally announced September 2025.

Thermoelectricity evidence for quantum criticality in clean infinite-layer nickelate films

Authors: Xu Zhang, Chihao Li, Mingwei Yang, Yan Zhao, Zhitong An, Danfeng Li, Liang Qiao, Haichao Xu, Rui Peng, Donglai Feng, Shiyan Li

Abstract: We investigate the Seebeck coefficient ($S$) in infinite-layer nickelate films with different disorder levels. The disordered NdNiO$_{2}$ film exhibits a flat $S/T$ curve, whereas cleaner samples display a logarithmic divergence with decreasing temperature, followed by a pronounced ``hump'' near 25 K. These distinct behaviors reveal a disorder-driven transition from band-structure-dominated transp… ▽ More We investigate the Seebeck coefficient ($S$) in infinite-layer nickelate films with different disorder levels. The disordered NdNiO$_{2}$ film exhibits a flat $S/T$ curve, whereas cleaner samples display a logarithmic divergence with decreasing temperature, followed by a pronounced ``hump'' near 25 K. These distinct behaviors reveal a disorder-driven transition from band-structure-dominated transport to quantum-critical-dominated transport. Below the ``hump'' temperature, four-fold symmetry breaking is observed in the in-plane angular magnetoresistance, indicating the presence of short-range antiferromagnetic order in parent infinite-layer nickelate films. Furthermore, the logarithmic divergence in $S/T$ is also observed in a clean superconducting Sm$_{0.73}$Ca$_{0.05}$Eu$_{0.22}$NiO$_{2}$ film, where it coexists with linear-in-temperature resistivity over the same temperature range. These findings demonstrate the existence of quantum criticality over a wide doping range in clean infinite-layer nickelate films, similar to cuprates, which highlights the central role of antiferromagnetic spin correlations in their superconducting pairing mechanisms. △ Less

Submitted 18 August, 2025; originally announced August 2025.

Comments: 8 pages, 4 figures

The interplay of ferroelectricity and magneto-transport in non-magnetic moiré superlattices

Authors: Siqi Jiang, Renjun Du, Jiawei Jiang, Gan Liu, Jiabei Huang, Yu Du, Yaqing Han, Jingkuan Xiao, Di Zhang, Fuzhuo Lian, Wanting Xu, Siqin Wang, Lei Qiao, Kenji Watanabe, Takashi Taniguchi, Xiaoxiang Xi, Wei Ren, Baigeng Wang, Alexander S. Mayorov, Kai Chang, Hongxin Yang, Lei Wang, Geliang Yu

Abstract: The coupling of ferroelectricity and magnetic order provides rich tunability for engineering material properties and demonstrates great potential for uncovering novel quantum phenomena and multifunctional devices. Here, we report interfacial ferroelectricity in moiré superlattices constructed from graphene and hexagonal boron nitride. We observe ferroelectric polarization in an across-layer moiré… ▽ More The coupling of ferroelectricity and magnetic order provides rich tunability for engineering material properties and demonstrates great potential for uncovering novel quantum phenomena and multifunctional devices. Here, we report interfacial ferroelectricity in moiré superlattices constructed from graphene and hexagonal boron nitride. We observe ferroelectric polarization in an across-layer moiré superlattice with an intercalated layer, demonstrating a remnant polarization comparable to its non-intercalated counterpart. Remarkably, we reveal a magnetic-field enhancement of ferroelectric polarization that persists up to room temperature, showcasing an unconventional amplification of ferroelectricity in materials lacking magnetic elements. This phenomenon, consistent across devices with varying layer configurations, arises purely from electronic rather than ionic contributions. Furthermore, the ferroelectric polarization in turn modulates quantum transport characteristics, suppressing Shubnikov-de Haas oscillations and altering quantum Hall states in polarized phases. This interplay between ferroelectricity and magneto-transport in non-magnetic materials is crucial for exploring magnetoelectric effects and advancing two-dimensional memory and logic applications. △ Less

Submitted 1 July, 2025; originally announced July 2025.

Journal ref: Nature Communications, volume 16, Article number: 5640 (2025)

Anomalous valley Hall effect in monolayer chromium-based triple-Q magnets

Authors: Xiu-Cai Jiang, Li-Ya Qiao, Yu-Zhong Zhang

Abstract: Using the density functional theory calculations, we predict that several monolayer chromium-based materials exhibit a triple-Q tetrahedral magnetic insulating ground state. By studying the effect of biaxial strain on monolayer CrSi$\rm{_2}$P$\rm{_4}$ under various on-site Coulomb interactions, we reveal that this magnetic insulating state, sandwiched between the itinerant $120^{\circ}$ coplanar n… ▽ More Using the density functional theory calculations, we predict that several monolayer chromium-based materials exhibit a triple-Q tetrahedral magnetic insulating ground state. By studying the effect of biaxial strain on monolayer CrSi$\rm{_2}$P$\rm{_4}$ under various on-site Coulomb interactions, we reveal that this magnetic insulating state, sandwiched between the itinerant $120^{\circ}$ coplanar noncollinear antiferromagnetic and ferromagnetic states, originates from the competition between antiferromagnetic exchange and double exchange interactions of Cr 3$d$ electrons which can also be applied to account for the ground states in other chromium-based materials. Remarkably, anomalous valley Hall effect with giant valley splitting is discovered in the magnetic states of these inversion-asymmetric systems without requiring spin-orbit coupling or net magnetization. Our findings open a new avenue towards exploring monolayer materials for valleytronics. △ Less

Submitted 5 May, 2025; originally announced May 2025.

Comments: 8 pages, 4 figures

Journal ref: Phys. Rev. B 111, L140416 (2025)

Stacking-orientation and twist-angle control on integer and fractional Chern insulators in moiré rhombohedral graphene

Authors: Chushan Li, Chuanqi Zheng, Kai Liu, Ke Huang, Zheng Sun, Lei Qiao, Yifan Wei, Chenyu Zhang, Fan Xu, Kenji Watanabe, Takashi Taniguchi, Hao Yang, Dandan Guan, Liang Liu, Shiyong Wang, Yaoyi Li, Hao Zheng, Canhua Liu, Bingbing Tong, Li Lu, Jinfeng Jia, Zhiwen Shi, Jianpeng Liu, Xiao Li, Guorui Chen , et al. (2 additional authors not shown)

Abstract: Rhombohedral-stacked multilayer graphene aligned with hexagonal boron nitride has emerged as an excellent platform for investigating exotic quantum phenomena arising from the interplay between electron correlations and nontrivial topology. However, the microscopic mechanism governing the emergence of both the integer and fractional Chern insulator states in this system remains an open question. In… ▽ More Rhombohedral-stacked multilayer graphene aligned with hexagonal boron nitride has emerged as an excellent platform for investigating exotic quantum phenomena arising from the interplay between electron correlations and nontrivial topology. However, the microscopic mechanism governing the emergence of both the integer and fractional Chern insulator states in this system remains an open question. In this work, we systematically investigate the electrical transport properties of RMG/hBN moiré devices with controlled alignment orientations and twist angles. We demonstrate that alignment orientation strongly modulates correlated phenomena in the moiré-proximal regime, while having negligible influence on the formation of integer and fractional Chern insulators in the moiré-distant regime. Instead, the moiré periodicity, tuned by the twist angle, serves as the key parameter controlling the stability of these correlated topological states in the moiré-distant regime. Furthermore, in the moiré-proximal regime of one specific alignment, we observe anomalous Hall effect and a variety of competing phases near ν = 1, including integer Chern insulator states, extended Chern insulator states, and trivial insulators, whose stability is highly sensitive to both the applied displacement electric field and magnetic field. Our results underscore the critical role of stacking-alignment and twist-angle engineering in exploring novel quantum states based on rhombohedral-stacked multilayer graphene moiré systems. △ Less

Submitted 11 November, 2025; v1 submitted 3 May, 2025; originally announced May 2025.

Superconductivity Favored Anisotropic Phase Stiffness in Infinite-Layer Nickelates

Authors: Minyi Xu, Dong Qiu, Minghui Xu, Yehao Guo, Cheng Shen, Chao Yang, Wenjie Sun, Yuefeng Nie, Zi-Xiang Li, Tao Xiang, Liang Qiao, Jie Xiong, Yanrong Li

Abstract: In unconventional superconductors such as cuprates and iron pnictides and chalcogenides, phase stiffness - a measure of the energy cost associated with superconducting phase variations - is on the same order of magnitude as the strength of Cooper pairing, translating to superconductivity governed by phase fluctuations. However, due to a lack of a direct experimental probe, there remains a fundamen… ▽ More In unconventional superconductors such as cuprates and iron pnictides and chalcogenides, phase stiffness - a measure of the energy cost associated with superconducting phase variations - is on the same order of magnitude as the strength of Cooper pairing, translating to superconductivity governed by phase fluctuations. However, due to a lack of a direct experimental probe, there remains a fundamental gap in establishing microscopic picture between unconventional superconductivity and phase fluctuations. Here we show a vector current technique that allows for in-situ angle-resolved transport measurements, providing exclusive evidence suggesting an anisotropic nature of phase stiffness in infinite-layer nickelate superconductors. Pronounced anisotropy of in-plane resistance manifests itself in both normal and superconducting transition states, indicating crystal symmetry breaking. Remarkably, the electric conductivity of Nd0.8Sr0.2NiO2 peaks at 125° between the direction of the current and crystal principal axis, but this angle evolves to 160° near zero-resistance temperature. Further measurements reveal that the superconductivity is favored along a direction with minimized phase fluctuations, an orientation strikingly deviating from the symmetric direction imposed by both electronic anisotropy and the underlying crystal lattice. Identical measurements conducted on a prototypical cuprate superconductor yield consistent results, suggesting that this previously unknown behavior could be ubiquitous. By shielding insight into the contrasting anisotropy between electron fluid and superfluid, our findings provide clues for a unified framework for understanding unconventional superconductors △ Less

Submitted 20 February, 2025; originally announced February 2025.

Ultrafast dynamic Coulomb screening of X-ray core excitons in photoexcited semiconductors

Authors: Thomas C. Rossi, Lu Qiao, Conner P. Dykstra, Ronaldo Rodrigues Pela, Richard Gnewkow, Rachel F. Wallick, John H. Burke, Erin Nicholas, Anne-Marie March, Gilles Doumy, D. Bruce Buchholz, Christiane Deparis, Jesus Zuñiga-Pérez, Michael Weise, Klaus Ellmer, Mattis Fondell, Claudia Draxl, Renske M. van der Veen

Abstract: Ultrafast X-ray spectroscopy has been revolutionized in recent years due to the advent of fourth-generation X-ray facilities. In solid-state materials, core excitons determine the energy and line shape of absorption features in core-level spectroscopies such as X-ray absorption spectroscopy. The screening of core excitons is an inherent many-body process that can reveal insight into charge-transfe… ▽ More Ultrafast X-ray spectroscopy has been revolutionized in recent years due to the advent of fourth-generation X-ray facilities. In solid-state materials, core excitons determine the energy and line shape of absorption features in core-level spectroscopies such as X-ray absorption spectroscopy. The screening of core excitons is an inherent many-body process that can reveal insight into charge-transfer excitations and electronic correlations. Under non-equilibrium conditions such as after photoexcitation, however, core-exciton screening is still not fully understood. Here we demonstrate the dynamic Coulomb screening of core excitons induced by photoexcited carriers by employing X-ray transient absorption (XTA) spectroscopy with picosecond time resolution. Our interpretation is supported by state-of-the-art ab initio theory, combining constrained and real-time time-dependent density functional theory with many-body perturbation theory. Using ZnO as an archetypal wide band-gap semiconductor, we show that the Coulomb screening by photoexcited carriers at the Zn K-edge leads to a decrease in the core-exciton binding energy, which depends nonlinearly on both the excitation density and the distribution of photoexcited carriers in reciprocal space. The effect of Coulomb screening dominates over Pauli blocking in the XTA spectra. We show that dynamic core-exciton screening is also observed at other X-ray absorption edges and theoretically predict the effect of core-exciton screening on the femtosecond time scale for the case of ZnO, a major step towards hard X-ray excitonics. The results have implications for the interpretation of ultrafast X-ray spectra in general and their use in tracking charge carrier dynamics in complex materials on atomic length scales. △ Less

Submitted 12 December, 2024; v1 submitted 2 December, 2024; originally announced December 2024.

Comments: 43 pages, 37 figures

In-depth Understanding of the Band Alignment and Interface States Scenario in Bi$_2$O$_2$Se/SrTiO$_3$ Ultrathin Heterojunction

Authors: Ke Zhang, Yusen Feng, Lei Hao, Jing Mi, Miao Du, Minghui Xu, Yan Zhao, Jianping Meng, Liang Qiao

Abstract: Bismuth oxyselenide (Bi$_2$O$_2$Se), a novel quasi-2D charge-carrying semiconductor, is hailed as one of the best emerging platforms for the next generation semiconductor devices. Recent efforts on developing diverse Bi$_2$O$_2$Se heterojunctions have produced extensive potential applications in electronics and optoelectronics. In-depth understanding of the band alignment and especially interface… ▽ More Bismuth oxyselenide (Bi$_2$O$_2$Se), a novel quasi-2D charge-carrying semiconductor, is hailed as one of the best emerging platforms for the next generation semiconductor devices. Recent efforts on developing diverse Bi$_2$O$_2$Se heterojunctions have produced extensive potential applications in electronics and optoelectronics. In-depth understanding of the band alignment and especially interface dynamics is, however, still challenging. In this work, a comprehensive experimental investigation on the band alignment is performed by a high-resolution X-ray photoelectron spectrometer (HRXPS), and the properties of interface states are also fully discussed. The results show that the ultrathin film Bi$_2$O$_2$Se grown on SrTiO$_3$ (TiO$_2$ (001) termination) exhibits Type-I (straddling gap) band alignment with a valence band offset (VBO) of about 1.77\pm0.04 eV and conduction band offset (CBO) of about 0.68\pm0.04 eV. However, further considering the contribution of the interface states, the bands on the interface present a herringbone configuration due to sizable build-in electric fields, which is significantly different from the conventional band alignment. In this sense, our results provide an insightful guidance to the development of high-efficiency electronic and optoelectronic devices, specifically of the devices where the charge transfer is highly sensitive to interface states. △ Less

Submitted 4 August, 2024; originally announced August 2024.

Controllable and Fast Growth of High-Quality Atomically Thin and Atomically Flat Bi$_2$O$_2$Se Films

Authors: Yusen Feng, Pei Chen, Nian Li, Suzhe Liang, Ke Zhang, Minghui Xu, Yan Zhao, Jie Gong, Shu Zhang, Huaqian Leng, Yuanyuan Zhou, Yong Wang, Liang Qiao

Abstract: As a novel and promising 2D material, bismuth oxyselenide (Bi$_2$O$_2$Se) has demonstrated significant potential to overcome existing technical barriers in various electronic device applications, due to its unique physical properties like high symmetry, adjustable electronic structure, ultra-high electron mobility. However, the rapid growth of Bi$_2$O$_2$Se films down to a few atomic layers with p… ▽ More As a novel and promising 2D material, bismuth oxyselenide (Bi$_2$O$_2$Se) has demonstrated significant potential to overcome existing technical barriers in various electronic device applications, due to its unique physical properties like high symmetry, adjustable electronic structure, ultra-high electron mobility. However, the rapid growth of Bi$_2$O$_2$Se films down to a few atomic layers with precise control remains a significant challenge. In this work, the growth of two-dimensional (2D) Bi$_2$O$_2$Se thin films by the pulsed laser deposition (PLD) method is systematically investigated. By controlling temperature, oxygen pressure, laser energy density and laser emission frequency, we successfully prepare atomically thin and flat Bi$_2$O$_2$Se (001) thin films on the (001) surface of SrTiO3. Importantly, we provide a fundamental and unique perspective toward understanding the growth process of atomically thin and flat Bi$_2$O$_2$Se films, and the growth process can be primarily summarized into four steps: i) anisotropic non-spontaneous nucleation preferentially along the step roots; ii) monolayer Bi$_2$O$_2$Se nanosheets expanding across the surrounding area, and eventually covering the entire STO substrate step; iii) vertical growth of Bi$_2$O$_2$Se monolayer in a 2D Frank-van der Merwe (FM) epitaxial growth, and iv) with a layer-by-layer 2D FM growth mode, ultimately producing an atomically flat and epitaxially aligned thin film. Moreover, the combined results of the crystallinity quality, surface morphology and the chemical states manifest the successful PLD-growth of high-quality Bi$_2$O$_2$Se films in a controllable and fast mode. △ Less

Submitted 1 August, 2024; originally announced August 2024.

Mean Field Study of Superconductivity in the Square Lattice $t$-$J$ Model with Three-Site Hopping

Authors: Ke Yang, Qianqian Chen, Lei Qiao, Zheng Zhu

Abstract: It remains an open question whether the two-dimensional single-band pure Hubbard model and its related pure $t$-$J$ model truly capture the superconducting order in cuprates. Recent numerical studies on this issue have raised a notable disparity in superconducting order between the pure Hubbard model and the pure $t$-$J$ model. Inspired by these, we investigate the role of the three-site hopping t… ▽ More It remains an open question whether the two-dimensional single-band pure Hubbard model and its related pure $t$-$J$ model truly capture the superconducting order in cuprates. Recent numerical studies on this issue have raised a notable disparity in superconducting order between the pure Hubbard model and the pure $t$-$J$ model. Inspired by these, we investigate the role of the three-site hopping term in $d$-wave superconductivity, such a term is usually neglected in the effective Hamiltonian of the Hubbard model, though its amplitude is of the same order as the superexchange coupling $J$ in the $t$-$J$ model. Our slave-boson mean-field solution demonstrates the suppression of $d$-wave superconducting order by incorporating the three-site hopping term, consistent with numerical observations by the density matrix renormalization group. This suppression could be understood as a result of competition between superexchange interaction and three-site hopping, the former favors $d$-wave pairing while the latter favors $s$-wave pairing. We also discussed its role in quasiparticle dispersion and boson-condensation temperature. Our findings may offer an alternative understanding of the recent numerical contrasting findings in the strong coupling regime: the absent or weak superconductivity in the pure Hubbard model, while the robust superconductivity in the $t$-$J$ model without including the three-site hopping term. △ Less

Submitted 1 August, 2024; v1 submitted 12 June, 2024; originally announced June 2024.

Comments: 14 pages, 12 figures

Journal ref: Phys. Rev. B 110, 054514 (2024)

Stability and Elasticity of Ultrathin Sphere-Patterned Block Copolymer Films

Authors: Le Qiao, Daniel A. Vega, Friederike Schmid

Abstract: Sphere-patterned ultrathin block copolymers films are potentially interesting for a variety of applications in nanotechnology. We use self-consistent field theory to investigate the elastic response of sphere monolayer films with respect to in-plane shear, in-plane extension and compression deformations, and with respect to bending. The relations between the in-plane elastic moduli is roughly comp… ▽ More Sphere-patterned ultrathin block copolymers films are potentially interesting for a variety of applications in nanotechnology. We use self-consistent field theory to investigate the elastic response of sphere monolayer films with respect to in-plane shear, in-plane extension and compression deformations, and with respect to bending. The relations between the in-plane elastic moduli is roughly compatible with the expectations for two-dimensional elastic systems with hexagonal symmetry, with one notable exception: The pure shear and the simple shear moduli differ from each other by roughly 20%. Even more importantly, the bending constants are found to be negative, indicating that free-standing block copolymer membranes made of only sphere mono-layer are inherently unstable above the glass transition. Our results are discussed in view of experimental findings. △ Less

Submitted 28 April, 2024; originally announced April 2024.

Journal ref: Macromolecules 2024, XXXX, XXX, XXX-XXX

Tunable magnetism in bilayer transition metal dichalcogenides

Authors: Li-Ya Qiao, Xiu-Cai Jiang, Ze Ruan, Yu-Zhong Zhang

Abstract: Twist between neighboring layers and variation of interlayer distance are two extra ways to control the physical properties of stacked two-dimensional van der Waals materials without alteration of chemical compositions or application of external fields, compared to their monolayer counterparts. In this work, we explored the dependence of the magnetic states of the untwisted and twisted bilayer 1T-… ▽ More Twist between neighboring layers and variation of interlayer distance are two extra ways to control the physical properties of stacked two-dimensional van der Waals materials without alteration of chemical compositions or application of external fields, compared to their monolayer counterparts. In this work, we explored the dependence of the magnetic states of the untwisted and twisted bilayer 1T-VX$_2$ (X = S, Se) on the interlayer distance by density functional theory calculations. We find that, while a magnetic phase transition occurs from interlayer ferromagnetism to interlayer antiferromagnetism either as a function of decreasing interlayer distance for the untwisted bilayer 1T-VX$_2$ or after twist, richer magnetic phase transitions consecutively take place for the twisted bilayer 1T-VX$_2$ as interlayer distance is gradually reduced. Besides, the critical pressures for the phase transition are greatly reduced in twisted bilayer 1T-VX$_2$ compared with the untwisted case. We derived the Heisenberg model with intralayer and interlayer exchange couplings to comprehend the emergence of various magnetic states. Our results point out an easy access towards tunable two-dimensional magnets. △ Less

Submitted 18 April, 2024; originally announced April 2024.

Comments: 8 pages, 4 figures

Journal ref: Phys. Rev. Materials 8, 034003 (2024)

Spontaneous superradiant photon current

Authors: Lei Qiao, Jiangbin Gong

Abstract: This work reports the spontaneous emergence of a photon current in a class of spin-cavity systems, where an assemble of quantum emitters interact with distinct photon modes confined in tunneling-coupled cavities. Specifically, with necessary symmetry breaking, photons in a superradiant phase afforded by coherent photon-emitter interaction spontaneously flow from a cavity with a lower resonance fre… ▽ More This work reports the spontaneous emergence of a photon current in a class of spin-cavity systems, where an assemble of quantum emitters interact with distinct photon modes confined in tunneling-coupled cavities. Specifically, with necessary symmetry breaking, photons in a superradiant phase afforded by coherent photon-emitter interaction spontaneously flow from a cavity with a lower resonance frequency to a different cavity with a higher resonance frequency. Theoretical analysis reveals that cavity dissipation is the key to alter spin-cavity coherence, which then makes it possible to extract photons from, and later return photons to the vaccum through the cavities. The interplay between photon loss and emitter coherence hence sustains a counter-intuitive steady current of photons between cavities without an external pumping field. △ Less

Submitted 23 February, 2024; originally announced February 2024.

Comments: 10 pages, 4 figures in total

Journal ref: Physical Review A 110, 063719 (2024)

Phase Diagram of the Square-Lattice $t$-$J$-$V$ Model for Electron-Doped Cuprates

Authors: Qianqian Chen, Lei Qiao, Fuchun Zhang, Zheng Zhu

Abstract: Motivated by significant discrepancies between experimental observations of electron-doped cuprates and numerical results of the Hubbard and $t$-$J$ models, we investigate the role of inter-site interactions $V$ by studying the $t$-$J$-$V$ model on square lattices. Based on large-scale density matrix renormalization group simulations, we identify the ground-state phase diagram across varying inter… ▽ More Motivated by significant discrepancies between experimental observations of electron-doped cuprates and numerical results of the Hubbard and $t$-$J$ models, we investigate the role of inter-site interactions $V$ by studying the $t$-$J$-$V$ model on square lattices. Based on large-scale density matrix renormalization group simulations, we identify the ground-state phase diagram across varying inter-site interactions $V$ and doping concentration $δ$. We find that the phase diagram with finite inter-site interactions $2\lesssim V/J\lesssim3$ offers a more accurate description of electron-doped cuprates than the conventional Hubbard and $t$-$J$ models. Moreover, we reveal the role of inter-site interactions $V$ at varying doping levels: at light doping, inter-site interactions favor Néel antiferromagnetic order, and suppress both superconductivity and charge density wave; around optimal doping, these interactions support a pseudogap-like phase while suppressing superconductivity, and we further perform the slave boson mean-field analysis to understand the numerical results microscopically; at higher doping, the effects of inter-site interactions become insignificant, with our numerical predictions suggesting the emergence of incommensurate spin density wave phase. Our specific focus around optimal doping with various inter-site interactions identifies successive phases including phase separation, uniform $d$-wave SC and a pseudogap-like phase, and reveals a relative insensitivity of charge density wave to superconductivity. Our study suggests the $t$-$J$-$V$ model as the minimal model to capture the essential physics of the electron-doped cuprates. △ Less

Submitted 18 May, 2024; v1 submitted 10 December, 2023; originally announced December 2023.

Comments: 7+9 pages, 6+10 figures

Journal ref: Phys. Rev. B 110, 045134 (2024)

Electronic properties of nickelate superconductor R3Ni2O7 with oxygen vacancies

Authors: Xuelei Sui, Xiangru Han, Xiaojun Chen, Liang Qiao, Xiaohong Shao, Bing Huang

Abstract: The discovery of superconductivity in La3Ni2O7 has attracted significant research interest in the field of nickelate superconductors. Despite extensive studies on pristine La3Ni2O7, the impact of oxygen vacancies (VO), a common type of intrinsic defect in oxides, on electronic structures and superconductivity in La3Ni2O7 remains unclear. In this article, we identify the most energetically favorabl… ▽ More The discovery of superconductivity in La3Ni2O7 has attracted significant research interest in the field of nickelate superconductors. Despite extensive studies on pristine La3Ni2O7, the impact of oxygen vacancies (VO), a common type of intrinsic defect in oxides, on electronic structures and superconductivity in La3Ni2O7 remains unclear. In this article, we identify the most energetically favorable location for VO formation as the oxygen atom connecting the NiO6 bilayer, resulting in a significant reduction in the lattice constant along the c-axis. Interestingly, the electronic structure undergoes notable changes, particularly for the Ni dz2 and Ni dx2-y2 orbitals. The Ni dz2 orbitals change from partially filled in the pristine La3Ni2O7 to completely filled in the presence of VO, leading to a considerable decrease of its proportion near the Fermi level. Conversely, the proportion of Ni dx2-y2 states increases due to the orbital localization and slight upward shift. Additionally, we observe a significant increase in the hopping of intra-bilayer Ni dz2 orbitals when the VO exists, but with an opposite sign, which differs greatly from the previous understanding. The inter-orbital hopping between Ni dz2 and Ni dx2-y2 orbitals also changes its sign in the presence of VO. Our results indicate that the formation of VO may be harmful to the superconductivity in La3Ni2O7, given the general assumption for the critical role of Ni dz2 in generating superconductivity. Furthermore, we suggest that Ce3Ni2O7, which shares similar electronic structures to La3Ni2O7 but has a larger lattice volume, may be a better candidate for nickelate superconductor due to its lower VO concentration. △ Less

Submitted 2 December, 2023; originally announced December 2023.

Mössbauer spectroscopy study of the magnetostructural and spin-state transitions in the breathing pyrochlore LiFeCr$_{4}$O$_{8}$

Authors: Bo Zhang, Wei Ren, Shengyu Yang, Qifeng Kuang, Da Li, Xin Liu, Anmin Zhang, Hua Pang, Liyun Tang, Liang Qiao, Fashen Li, Zhiwei Li

Abstract: We report on investigations of the complex magnetostructural and spin-state transitions in the breathing pyrochlore LiFeCr$_{4}$O$_{8}$ by means of magnetization, Mössbauer spectroscopy, and density functional theory (DFT) calculations. Three transitions corresponding to the ferrimagnetic transition at $T_N\sim94$ K, the spin-gap transition at $T_{SG}\sim50$ K, and the magnetostructural transition… ▽ More We report on investigations of the complex magnetostructural and spin-state transitions in the breathing pyrochlore LiFeCr$_{4}$O$_{8}$ by means of magnetization, Mössbauer spectroscopy, and density functional theory (DFT) calculations. Three transitions corresponding to the ferrimagnetic transition at $T_N\sim94$ K, the spin-gap transition at $T_{SG}\sim50$ K, and the magnetostructural transition at $T_{MS}\sim19$ K were observed from the $χ$(T) curve, whereas only $T_N$ and $T_{MS}$ were evidenced for the Fe site from our Mössbauer measurements, suggesting that the spin-gap transition is absent at the Fe site. This indicates that the spin-gap transition is an effect of the breathing Cr$_4$ lattice, in agreement with our DFT calculations from which we see nearly decoupled electronic states for the FeO$_4$ and CrO$_6$ units. From the temperature dependence of the hyperfine magnetic field we also observed a spin-state transition for the Fe spins at $T_{MS}$ consistent with earlier neutron diffraction measurements. These local characteristics are believed to be important for a complete understanding of the complex magnetostructural coupling effects observed in similar systems. △ Less

Submitted 1 December, 2023; originally announced December 2023.

Comments: 9 pages, 8 figures

Journal ref: Phys. Rev. B 108, 214401(2023)

Charge doping into spin minority states mediates doubling of $T_\mathrm{C}$ in ferromagnetic CrGeTe$_3$

Authors: Liam Trzaska, Lei Qiao, Matthew D. Watson, Monica Ciomaga Hatnean, Igor Marković, Edgar Abarca Morales, Tommaso Antonelli, Cephise Cacho, Geetha Balakrishnan, Wei Ren, Silvia Picozzi, Phil D. C. King

Abstract: The recent discovery of the persistence of long-range magnetic order when van der Waals layered magnets are thinned towards the monolayer limit has provided a tunable platform for the engineering of novel magnetic structures and devices. Here, we study the evolution of the electronic structure of CrGeTe$_3$ as a function of electron doping in the surface layer. From angle-resolved photoemission sp… ▽ More The recent discovery of the persistence of long-range magnetic order when van der Waals layered magnets are thinned towards the monolayer limit has provided a tunable platform for the engineering of novel magnetic structures and devices. Here, we study the evolution of the electronic structure of CrGeTe$_3$ as a function of electron doping in the surface layer. From angle-resolved photoemission spectroscopy, we observe spectroscopic fingerprints that this electron doping drives a marked increase in $T_\mathrm{C}$, reaching values more than double that of the undoped material, in agreement with recent studies using electrostatic gating. Together with density functional theory calculations and Monte Carlo simulations, we show that, surprisingly, the increased $T_\mathrm{C}$ is mediated by the population of spin-minority Cr $t_{2g}$ states, forming a half-metallic 2D electron gas at the surface. We show how this promotes a novel variant of double exchange, and unlocks a significant influence of the Ge -- which was previously thought to be electronically inert in this system -- in mediating Cr-Cr exchange. △ Less

Submitted 1 December, 2023; originally announced December 2023.

Comments: 10 pages including supplementary information

Journal ref: npj 2D Mater Appl 9, 4 (2025)

Observation of unconventional van der Waals multiferroics near room temperature

Authors: Yangliu Wu, Haipeng Lu, Xiaocang Han, Chendi Yang, Nanshu Liu, Xiaoxu Zhao, Liang Qiao, Wei Ji, Renchao Che, Longjiang Deng, Bo Peng

Abstract: The search for two-dimensional (2D) van der Waals (vdW) multiferroics is an exciting yet challenging endeavor. Room-temperature 2D vdW few-layer multiferroic is a much bigger insurmountable obstacle. Here we report the discovery of an unconventional 2D vdW multiferroic with out-of-plane ferroelectric polarization and long-range magnetic orders in trilayer NiI2 device from 10 K to 295 K. The evolut… ▽ More The search for two-dimensional (2D) van der Waals (vdW) multiferroics is an exciting yet challenging endeavor. Room-temperature 2D vdW few-layer multiferroic is a much bigger insurmountable obstacle. Here we report the discovery of an unconventional 2D vdW multiferroic with out-of-plane ferroelectric polarization and long-range magnetic orders in trilayer NiI2 device from 10 K to 295 K. The evolutions of magnetic domains with magnetic field, and the evolutions between ferroelectric and antiferroelectric phase have been unambiguously observed. More significantly, we realize a robust mutual control of magnetism and ferroelectricity at room temperature. The magnetic domains are manipulated by a small voltage ranging from 1 V to 6 V at 0 T and 295 K. This work opens opportunities for exploring multiferroic physics at the limit of few atomic layers. △ Less

Submitted 23 February, 2024; v1 submitted 24 November, 2023; originally announced November 2023.

Comments: 4 figures

MSC Class: 14J60 ACM Class: F.2.2; I.2.7

Multipiezo effect in altermagnetic V2SeTeO monolayer

Authors: Yu Zhu, Taikang Chen, Yongchang Li, Lei Qiao, Xiaonan Ma, Tao Hu, Heng Gao, Wei Ren

Abstract: Inspired by recent theoretical proposal on the interesting piezomagnetism and C-paired valley polarization in V2Se2O monolayer, we predict a stable antiferromagnetic Janus monolayer V2SeTeO with altermagnetic configuration using density functional theory calculations. It exhibits a novel multi-piezo effect combining piezoelectric, piezovalley and piezomagnetism. Most interestingly, the valley pola… ▽ More Inspired by recent theoretical proposal on the interesting piezomagnetism and C-paired valley polarization in V2Se2O monolayer, we predict a stable antiferromagnetic Janus monolayer V2SeTeO with altermagnetic configuration using density functional theory calculations. It exhibits a novel multi-piezo effect combining piezoelectric, piezovalley and piezomagnetism. Most interestingly, the valley polarization and the net magnetization under strain in V2SeTeO exceed these in V2Se2O, along with the additional large piezoelectric coefficient of e31 (0.322*10-10 C m-1). The multi-piezo effect makes antiferromagnetic Janus monolayer V2SeTeO a tantalizing material for potential applications in nanoelectronics, optoelectronics, spintronics and valleytronics. △ Less

Submitted 31 July, 2023; originally announced August 2023.

Comments: 20 pages,9 figures

Reply to "Comment on newly found Charge Density Waves in infinite layer Nickelates''

Authors: Charles C. Tam, Jaewon Choi, Xiang Ding, Stefano Agrestini, Abhishek Nag, Mei Wu, Bing Huang, Huiqian Luo, Peng Gao, Mirian Garcia-Fernandez, Liang Qiao, Ke-Jin Zhou

Abstract: Charge density waves (CDW) have been reported in NdNiO$_2$ and LaNiO$_2$ thin films grown on SrTiO$_3$ substrates using Ni-$L_3$ resonant x-ray scattering in Refs. [1-3]. In their comment [arXiv:2306.15086] on these reports, Pelliciari et al. found no evidence for a CDW in a NdNiO$_2$ film by performing fixed-momentum energy-dependent measurements. Instead, they observed a nearby non-resonant scat… ▽ More Charge density waves (CDW) have been reported in NdNiO$_2$ and LaNiO$_2$ thin films grown on SrTiO$_3$ substrates using Ni-$L_3$ resonant x-ray scattering in Refs. [1-3]. In their comment [arXiv:2306.15086] on these reports, Pelliciari et al. found no evidence for a CDW in a NdNiO$_2$ film by performing fixed-momentum energy-dependent measurements. Instead, they observed a nearby non-resonant scattering peak, attributed to the (101) substrate reflection, made accessible at Ni-$L_3$ due to third harmonic light contamination. Here we present fixed-momentum energy-dependent resonant inelastic x-ray scattering measurements across Ni-$L_3$ on NdNiO$_2$, used in the preceding study [1]. We see intrinsic Ni-$L_3$ energy profiles at all measured \textbf{Q} values, including a strong resonance effect at $\mathbf{Q}_\mathrm{CDW} = (-1/3, 0, 0.316)$ reciprocal lattice units. Attempts to measure the (101) substrate peak using third harmonic light at Ni-$L_3$ at I21, Diamond were unfruitful. Our results clearly demonstrate the electronic origin of the scattering peak published in Ref. [1] and lack of a detectable structural component in the peak. △ Less

Submitted 25 July, 2023; originally announced July 2023.

The curvature-induced magnetization in CrI3 bilayer: flexomagnetic effect enhancement in van der Waals antiferromagnets

Authors: Lei Qiao, Jan Sladek, Vladimir Sladek, Alexey S. Kaminskiy, Alexander P. Pyatakov, Wei Ren

Abstract: The bilayer of CrI3 is a prototypical van der Waals 2D antiferromagnetic material with magnetoelectric effect. It is not generally known, however, that for symmetry reasons the flexomagnetic effect, i.e., the strain gradient-induced magnetization, is also possible in this material. In the present paper, based on the first principle calculations, we estimate the flexomagnetic effect to be 200 μBÅ t… ▽ More The bilayer of CrI3 is a prototypical van der Waals 2D antiferromagnetic material with magnetoelectric effect. It is not generally known, however, that for symmetry reasons the flexomagnetic effect, i.e., the strain gradient-induced magnetization, is also possible in this material. In the present paper, based on the first principle calculations, we estimate the flexomagnetic effect to be 200 μBÅ that is two orders of magnitude higher than it was predicted for the referent antiperovskite flexomagnetic material Mn3GaN. The two major factors of flexomagnetic effect enhancement related to the peculiarities of antiferromagnetic structure of van der Waals magnets is revealed: the strain-dependent ferromagnetic coupling in each layer and large interlayer distance separating antiferromagnetically coupled ions. Since 2D systems are naturally prone to mechanical deformation, the emerging field of flexomagnetism is of special interest for application in spintronics of van der Waals materials and straintronics in particular. △ Less

Submitted 11 July, 2023; originally announced July 2023.

Covalency, correlations, and inter-layer interactions governing the magnetic and electronic structure of Mn$_3$Si$_2$Te$_6$

Authors: Chiara Bigi, Lei Qiao, Chao Liu, Paolo Barone, Monica Ciomaga Hatnean, Gesa-R. Siemann, Barat Achinuq, Daniel Alexander Mayoh, Giovanni Vinai, Vincent Polewczyk, Deepak Dagur, Federico Mazzola, Peter Bencok, Thorsten Hesjedal, Gerrit van der Laan, Wei Ren, Geetha Balakrishnan, Silvia Picozzi, Phil D. C. King

Abstract: Mn$_3$Si$_2$Te$_6$ is a rare example of a layered ferrimagnet. It has recently been shown to host a colossal angular magnetoresistance as the spin orientation is rotated from the in- to out-of-plane direction, proposed to be underpinned by a topological nodal-line degeneracy in its electronic structure. Nonetheless, the origins of its ferrimagnetic structure remain controversial, while its experim… ▽ More Mn$_3$Si$_2$Te$_6$ is a rare example of a layered ferrimagnet. It has recently been shown to host a colossal angular magnetoresistance as the spin orientation is rotated from the in- to out-of-plane direction, proposed to be underpinned by a topological nodal-line degeneracy in its electronic structure. Nonetheless, the origins of its ferrimagnetic structure remain controversial, while its experimental electronic structure, and the role of correlations in shaping this, are little explored to date. Here, we combine x-ray and photoemission-based spectroscopies with first-principles calculations, to probe the elemental-selective electronic structure and magnetic order in Mn$_3$Si$_2$Te$_6$. Through these, we identify a marked Mn-Te hybridisation, which weakens the electronic correlations and enhances the magnetic anisotropy. We demonstrate how this strengthens the magnetic frustration in Mn$_3$Si$_2$Te$_6$, which is key to stabilising its ferrimagnetic order, and find a crucial role of both exchange interactions extending beyond nearest-neighbours and anti-symmetric exchange in dictating its ordering temperature. Together, our results demonstrate a powerful methodology of using experimental electronic structure probes to constrain the parameter space for first-principles calculations of magnetic materials, and through this approach, reveal a pivotal role played by covalency in stabilising the ferrimagnetic order in Mn$_3$Si$_2$Te$_6$. △ Less

Submitted 1 March, 2023; originally announced March 2023.

Journal ref: Phys. Rev. B 108, 054419 (2023)

Non-monotonic Rheology and Stress Heterogeneity in confined Granular suspensions

Authors: Haitao Hu, Yiqiu Zhao, Weiwei Zhao, Ligen Qiao, Qin Xu

Abstract: We systematically investigated the impact of boundary confinement on the shear-thickening rheology of dense granular suspensions. Under highly confined conditions, dense suspensions were found to exhibit size-dependent or even rarely reported non-monotonic ($S$-shaped) flow curves in steady states. By performing in-situ boundary stress microscopy measurements, we observed enhanced flow heterogenei… ▽ More We systematically investigated the impact of boundary confinement on the shear-thickening rheology of dense granular suspensions. Under highly confined conditions, dense suspensions were found to exhibit size-dependent or even rarely reported non-monotonic ($S$-shaped) flow curves in steady states. By performing in-situ boundary stress microscopy measurements, we observed enhanced flow heterogeneities in confined suspensions, where concentrated high-stress domains propagated stably either along or against the shear direction. By comparing the boundary stress microscopy results with macroscopic flow responses, we revealed the connection between non-monotonic rheology and stress heterogeneity in confined suspensions. These findings suggest the possibility of controlling suspension rheology by imposing different boundary confinements. △ Less

Submitted 29 May, 2024; v1 submitted 22 November, 2022; originally announced November 2022.

Rotational symmetry breaking in superconducting nickelate Nd0.8Sr0.2NiO2 films

Authors: Haoran Ji, Yanan Li, Yi Liu, Xiang Ding, Zheyuan Xie, Shichao Qi, Liang Qiao, Yi-feng Yang, Guang-Ming Zhang, Jian Wang

Abstract: The infinite-layer nickelates, isostructural to the high-Tc superconductor cuprates, have risen as a promising platform to host unconventional superconductivity and stimulated growing interests in the condensed matter community. Despite numerous researches, the superconducting pairing symmetry of the nickelate superconductors, the fundamental characteristic of a superconducting state, is still und… ▽ More The infinite-layer nickelates, isostructural to the high-Tc superconductor cuprates, have risen as a promising platform to host unconventional superconductivity and stimulated growing interests in the condensed matter community. Despite numerous researches, the superconducting pairing symmetry of the nickelate superconductors, the fundamental characteristic of a superconducting state, is still under debate. Moreover, the strong electronic correlation in the nickelates may give rise to a rich phase diagram, where the underlying interplay between the superconductivity and other emerging quantum states with broken symmetry is awaiting exploration. Here, we study the angular dependence of the transport properties on the infinite-layer nickelate Nd0.8Sr0.2NiO2 superconducting films with Corbino-disk configuration. The azimuthal angular dependence of the magnetoresistance (R(φ)) manifests the rotational symmetry breaking from isotropy to four-fold (C4) anisotropy with increasing magnetic field, revealing a symmetry breaking phase transition. Approaching the low temperature and large magnetic field regime, an additional two-fold (C2) symmetric component in the R(φ) curves and an anomalous upturn of the temperature-dependent critical field are observed simultaneously, suggesting the emergence of an exotic electronic phase. Our work uncovers the evolution of the quantum states with different rotational symmetries and provides deep insight into the global phase diagram of the nickelate superconductors. △ Less

Submitted 30 October, 2022; originally announced October 2022.

Journal ref: Nat Commun 14, 7155 (2023)

An empirical method to characterize displacement distribution functions for anomalous and transient diffusion

Authors: Le Qiao, Nicholas Ilow, Maxime Ignacio, Gary W. Slater

Abstract: We propose a practical empirical fitting function to characterize the non-Gaussian displacement distribution functions (DispD) often observed for heterogeneous diffusion problems. We first test this fitting function with the problem of a colloidal particle diffusing between two walls using Langevin Dynamics (LD) simulations of a raspberry particle coupled to a lattice Boltzmann (LB) fluid. We also… ▽ More We propose a practical empirical fitting function to characterize the non-Gaussian displacement distribution functions (DispD) often observed for heterogeneous diffusion problems. We first test this fitting function with the problem of a colloidal particle diffusing between two walls using Langevin Dynamics (LD) simulations of a raspberry particle coupled to a lattice Boltzmann (LB) fluid. We also test the function with a simple model of anomalous diffusion on a square lattice with obstacles. In both cases, the fitting parameters provide more physical information than just the Kurtosis (which is often the method used to quantify the degree of anomaly of the dynamics), including a length scale that marks where the tails of the DispD begin. In all cases, the fitting parameters smoothly converge to Gaussian values as the systems become less anomalous. △ Less

Submitted 12 May, 2022; originally announced May 2022.

Coherent Control of Collective Spontaneous Emission through Self-interference

Authors: Lei Qiao, Jiangbin Gong

Abstract: As one of the central topics in quantum optics, collective spontaneous emission such as superradiance has been realized in a variety of systems. This work proposes an innovative scheme to coherently control collective emission rates via a self-interference mechanism in a nonlinear waveguide setting. The self-interference is made possible by photon backward scattering incurred by quantum scatterers… ▽ More As one of the central topics in quantum optics, collective spontaneous emission such as superradiance has been realized in a variety of systems. This work proposes an innovative scheme to coherently control collective emission rates via a self-interference mechanism in a nonlinear waveguide setting. The self-interference is made possible by photon backward scattering incurred by quantum scatterers in a waveguide working as quantum switches. Whether the interference is constructive or destructive is found to depend strongly on the distance between the scatterers and the emitters. The interference between two propagation pathways of the same photon leads to controllable superradiance and subradiance, with their collective decay rates much enhanced or suppressed (also leading to hyperradiance or population trapping). Furthermore, the self-interference mechanism is manifested by an abrupt change in the emission rates in real time. An experimental setup based on superconducting transmission line resonators and transmon qubits is further proposed to realize controllable collective emission rates. △ Less

Submitted 4 April, 2022; originally announced April 2022.

Comments: 10+ pages, 4 figures, including Supplementary Materials

Journal ref: Phys. Rev. Lett. 129, 093602 (2022)

Unusual Hole-doping-dependent Electronic Instability and Electron-Phonon Coupling in Infinite-layer Nickelates

Authors: Xuelei Sui, Jianfeng Wang, Xiang Ding, Ke-Jin Zhou, Liang Qiao, Haiqing Lin, Bing Huang

Abstract: The interplay between superconductivity and charge density waves (CDWs) under hole doping in cuprates is one of the central phenomena in condensed matter physics. Recently, CDWs are also observed in CaCuO$_2$-analogous nickelates RNiO$_2$ (R = La, Nd) but exhibit fundamentally different hole-doping-dependent behaviors compared to that in cuprates, raising a challenging question on its origin. In t… ▽ More The interplay between superconductivity and charge density waves (CDWs) under hole doping in cuprates is one of the central phenomena in condensed matter physics. Recently, CDWs are also observed in CaCuO$_2$-analogous nickelates RNiO$_2$ (R = La, Nd) but exhibit fundamentally different hole-doping-dependent behaviors compared to that in cuprates, raising a challenging question on its origin. In this article, we propose that electronic instability (EI) and moment-dependent electron-phonon coupling (MEPC), mainly contributed by Ni 3dx2-y2 and R 5dz2, respectively, may be the possible reasons for CDW formation in RNiO$_2$. Without hole doping, a strong Fermi surface nesting (FSN) induced by the unique feature of van Hove singularity (VHS) across Fermi level exists in RNiO$_2$ but not in CaCuO$_2$, and the unusual temperature-insensitive feature of EI and MEPC could result in rather high temperature CDWs in RNiO$_2$. Under hole doping, the reduced FSN of Ni 3dx2-y2 by the shift of VHS and decreased occupation of R 5dz2 largely weaken EI and MEPC in RNiO$_2$, respectively, suppressing the CDW formation. Our theory not only offers possible explanations to some puzzling experimental observations, but also establishes a unified understanding on the hole-doping-dependent EI and MEPC in nickelates and cuprates. △ Less

Submitted 24 February, 2022; originally announced February 2022.

Stability of Superconducting Nd0.8Sr0.2NiO2 Thin Films

Authors: Xiang Ding, Shengchun Shen, Huaqian Leng, Minghui Xu, Yan Zhao, Junrui Zhao, Xuelei Sui, Xiaoqiang Wu, Haiyan Xiao, Xiaotao Zu, Bing Huang, Huiqian Luo, Pu Yu, Liang Qiao

Abstract: The discovery of superconducting states in the nickelate thin film with infinite-layer structure has paved a new way for studying unconventional superconductivity. So far, research in this field is still very limited due to difficulties in sample preparation. Here we report on the successful preparation of superconducting Nd0.8Sr0.2NiO2 thin film (Tc = 8.0 - 11.1 K) and study the stability of such… ▽ More The discovery of superconducting states in the nickelate thin film with infinite-layer structure has paved a new way for studying unconventional superconductivity. So far, research in this field is still very limited due to difficulties in sample preparation. Here we report on the successful preparation of superconducting Nd0.8Sr0.2NiO2 thin film (Tc = 8.0 - 11.1 K) and study the stability of such films in ambient environment, water and under electrochemical conditions. Our work demonstrates that the superconducting state of Nd0.8Sr0.2NiO2 is remarkably stable, which can last for at least 47 days continuous exposure to air at 20 degree Celsius and 35% relative humidity. Further we show the superconductivity disappears after being immersed in de-ionized water at room temperature for 5 hours. Surprisingly, it can also survive under ionic liquid gating conditions with applied voltage up to 4 V, which is even more stable than conventional perovskite complex oxides. △ Less

Submitted 31 January, 2022; originally announced January 2022.

Comments: 23 pages, 4 figures

Journal ref: Sci. China-Phys. Mech. Astron. 65, 267411 (2022)

Controllable anomalous Nernst effect in an antiperovskite antiferromagnet

Authors: Yunfeng You, Wenxuan Zhu, Hua Bai, Yongjian Zhou, Lei Han, Leilei Qiao, Tongjin Chen, Feng Pan, Cheng Song

Abstract: Anomalous Nernst effect (ANE), the generation of a transverse electric voltage by a longitudinal temperature gradient, has attracted increasing interests of researchers recently, due to its potential in the thermoelectric power conversion and close relevance to the Berry curvature of the band structure. Avoiding the stray field of ferromagnets, ANE in antiferromagnets (AFM) has the advantage of re… ▽ More Anomalous Nernst effect (ANE), the generation of a transverse electric voltage by a longitudinal temperature gradient, has attracted increasing interests of researchers recently, due to its potential in the thermoelectric power conversion and close relevance to the Berry curvature of the band structure. Avoiding the stray field of ferromagnets, ANE in antiferromagnets (AFM) has the advantage of realizing highly efficient and densely integrated thermopiles. Here, we report the observation of ANE in an antiperovskite noncollinear AFM Mn3SnN experimentally, which is triggered by the enhanced Berry curvature from Weyl points located close to the Fermi level. Considering that antiperovskite Mn3SnN has rich magnetic phase transition, we modulate the noncollinear AFM configurations by the biaxial strain, which enables us to control its ANE. Our findings provide a potential class of materials to explore the Weyl physics of noncollinear AFM as well as realizing antiferromagnetic spin caloritronics that exhibits promising prospects for energy conversion and information processing. △ Less

Submitted 14 January, 2022; originally announced January 2022.

Comments: 20 pages, 4 figures

Charge density waves in infinite-layer NdNiO$_2$ nickelates

Authors: Charles C. Tam, Jaewon Choi, Xiang Ding, Stefano Agrestini, Abhishek Nag, Bing Huang, Huiqian Luo, Mirian García-Fernández, Liang Qiao, Ke-Jin Zhou

Abstract: In materials science, much effort has been devoted to reproduce superconductivity in chemical compositions analogous to cuprate superconductors since their discovery over thirty years ago. This approach was recently successful in realising superconductivity in infinite-layer nickelates. Although differing from cuprates in electronic and magnetic properties, strong Coulomb interactions suggest infi… ▽ More In materials science, much effort has been devoted to reproduce superconductivity in chemical compositions analogous to cuprate superconductors since their discovery over thirty years ago. This approach was recently successful in realising superconductivity in infinite-layer nickelates. Although differing from cuprates in electronic and magnetic properties, strong Coulomb interactions suggest infinite-layer nickelates have a propensity to various symmetry-breaking orders that populate the cuprates. Here we report the observation of charge density waves (CDWs) in infinite-layer NdNiO$_2$ films using Ni-$L_3$ resonant x-ray scattering. Remarkably, CDWs form in Nd 5$d$ and Ni 3$d$ orbitals at the same commensurate wavevector $(0.333, 0)\;r.l.u.$, with non-negligible out-of-plane dependence, and an in-plane correlation length up to $\sim$ 60 Angstrom. Spectroscopic studies reveal a strong connection between CDWs and the Nd 5$d$ - Ni 3$d$ orbital hybridisation. Upon entering the superconducting state at 20\% Sr doping, the CDWs disappear. Our work demonstrates the existence of CDWs in infinite-layer nickelates with a multi-orbital character distinct from cuprates, which establishes their low-energy physics. △ Less

Submitted 16 September, 2022; v1 submitted 8 December, 2021; originally announced December 2021.

Comments: 9 pages, 4 figures, and Supplementary Information

Journal ref: Nature Materials 2022

Revealing the role of interfacial heterogeneous nucleation in metastable thin film growth of rare earth nickelates electronic transition materials

Authors: Fengbo Yan, Zhishan Mi, Jinhao Chen, Haiyang Hu, Lei Gao, Jiaou Wang, Nuofu Chen, Yong Jiang, Lijie Qiao, Jikun Chen

Abstract: Although rare earth nickelates (ReNiO3) exhibit abundant electronic phases and widely adjustable metal to insulator electronic transition properties, their practical electronic applications are largely impeded by their intrinsic meta stability. Apart from elevating oxygen reaction pressures, heterogeneous nucleation is expected as an alternative strategy that enables the crystallization of ReNiO3… ▽ More Although rare earth nickelates (ReNiO3) exhibit abundant electronic phases and widely adjustable metal to insulator electronic transition properties, their practical electronic applications are largely impeded by their intrinsic meta stability. Apart from elevating oxygen reaction pressures, heterogeneous nucleation is expected as an alternative strategy that enables the crystallization of ReNiO3 at low meta stability. In this work, the respective roles of high oxygen pressure and heterogeneous interface in triggering ReNiO3 thin films growth at metastable state are revealed. The ReNiO3 (Re:Nd, Sm, Eu, Gd, and Dy) heterogeneous thin films growth on LaAlO3 single crystal substrate have an effective crystallization at atmosphere without the necessity to apply high oxygen pressures, suggesting the interfacial bonding between the ReNiO3 and substrates can sufficiently reduce the positive Gibbs formation energy of ReNiO3, which is further verified by the first principles calculations. Nevertheless, the abrupt electronic transitions only appear in ReNiO3 thin films grown at high oxygen pressures, in which cases the oxygen vacancies are effectively eliminated via high oxygen pressure reactions as indicated by near edge X ray absorption fine structure (NEXAFS). This work unveils the synergistic effects of heterogeneous nucleation and high oxygen pressure on the growth of high quality ReNiO3 thin films. △ Less

Submitted 25 November, 2021; originally announced November 2021.

Comments: 33 Pages

Experimentally Detecting Quantized Zak Phases without Chiral Symmetry in Photonic Lattices

Authors: Zhi-Qiang Jiao, Stefano Longhi, Xiao-Wei Wang, Jun Gao, Wen-Hao Zhou, Yao Wang, Yu-Xuan Fu, Li Wang, Ruo-Jing Ren, Lu-Feng Qiao, Xian-Min Jin

Abstract: Symmetries play a major role in identifying topological phases of matter and in establishing a direct connection between protected edge states and topological bulk invariants via the bulk-boundary correspondence. One-dimensional lattices are deemed to be protected by chiral symmetry, exhibiting quantized Zak phases and protected edge states, but not for all cases. Here, we experimentally realize a… ▽ More Symmetries play a major role in identifying topological phases of matter and in establishing a direct connection between protected edge states and topological bulk invariants via the bulk-boundary correspondence. One-dimensional lattices are deemed to be protected by chiral symmetry, exhibiting quantized Zak phases and protected edge states, but not for all cases. Here, we experimentally realize an extended Su-Schrieffer-Heeger model with broken chiral symmetry by engineering one-dimensional zigzag photonic lattices, where the long-range hopping breaks chiral symmetry but ensures the existence of inversion symmetry. By the averaged mean displacement method, we detect topological invariants directly in the bulk through the continuous-time quantum walk of photons. Our results demonstrate that inversion symmetry protects the quantized Zak phase, but edge states can disappear in the topological nontrivial phase, thus breaking the conventional bulk-boundary correspondence. Our photonic lattice provides a useful platform to study the interplay among topological phases, symmetries, and the bulk-boundary correspondence. △ Less

Submitted 28 September, 2021; originally announced September 2021.

Comments: 6 pages, 4 figures, 53 references, 1 supplemental materials

Journal ref: Phys. Rev. Lett. 127, 147401 (2021)

Control of active polymeric filaments by chemically-powered nanomotors

Authors: Lilan Qiao, Raymond Kapral

Abstract: Active materials with distinctive nonequilibrium properties have diverse materials science applications. Active systems are common in living matter, such as the filament network in the cell that is activated by molecular motors, and in materials science as exemplified by hydrogels activated by chemical reactions. Here we describe another class of active polymeric filament systems where the filamen… ▽ More Active materials with distinctive nonequilibrium properties have diverse materials science applications. Active systems are common in living matter, such as the filament network in the cell that is activated by molecular motors, and in materials science as exemplified by hydrogels activated by chemical reactions. Here we describe another class of active polymeric filament systems where the filaments are activated by embedded chemically-powered nanomotors that have catalytic and noncatalytic parts. Chemical reactions on the catalytic surfaces produce forces that act on the polymeric filaments. By changing the nonequilibrium conditions these forces can be made to change sign and thereby compress or expand the filaments. The embedded motors provide both the source of activity and the means to control the filament conformational structure. As an example of control, we show that oscillatory variations of the chemical constraints yield gel-like networks that oscillate between expanded or compressed forms, much like those of hydrogels. △ Less

Submitted 13 September, 2022; v1 submitted 26 September, 2021; originally announced September 2021.

Comments: 8 pages, 7 figures

Journal ref: Physical Review Applied,18,024051(2022)

Photo-Induced Ultrafast Symmetry Switch in SnSe

Authors: Yadong Han, Junhong Yu, Hang Zhang, Fang Xu, Kunlin Peng, Xiaoyuan Zhou, Liang Qiao, Oleg V. Misochko, Kazutaka G. Nakamura, Giovanni M. Vanacore, Jianbo Hu

Abstract: Layered tin selenide (SnSe) has recently emerged as a high-performance thermoelectric material with the current record for the figure of merit (ZT) observed in the high-temperature Cmcm phase. So far, access of the Cmcm phase has been mainly obtained via thermal equilibrium methods based on sample heating or application of external pressure, thus restricting the current understanding only to groun… ▽ More Layered tin selenide (SnSe) has recently emerged as a high-performance thermoelectric material with the current record for the figure of merit (ZT) observed in the high-temperature Cmcm phase. So far, access of the Cmcm phase has been mainly obtained via thermal equilibrium methods based on sample heating or application of external pressure, thus restricting the current understanding only to ground-state conditions. Here, we investigate the ultrafast carrier and phononic dynamics in SnSe. Our results demonstrate that optical excitations can transiently switch the point-group symmetry of the crystal from Pnma to Cmcm at room temperature in a few hundreds of femtoseconds with an ultralow threshold for the excitation carrier density. This non-equilibrium Cmcm phase is found to be driven by the displacive excitation of coherent Ag phonons and, given the absence of low-energy thermal phonons, exists in SnSe with the status of 'cold lattice with hot carriers'. Our findings provide important insight for understanding non-equilibrium thermoelectric properties of SnSe. △ Less

Submitted 10 January, 2022; v1 submitted 4 July, 2021; originally announced July 2021.

Comments: 4 figures

Journal ref: J. Phys. Chem. Lett. 2022, 13, 442

Capture and translocation of a rod-like molecule by a nanopore: orientation, charge distribution and hydrodynamics

Authors: Le Qiao, Gary W. Slater

Abstract: We investigate the translocation of rods with different charge distributions using hybrid Langevin Dynamics and Lattice Boltzmann (LD-LB) simulations. Electrostatic interactions are added to the system using the $P^3M$ algorithm to model the electrohydrodynamic interactions (EHI). We first examine the free-solution electrophoretic properties of rods with various charge distributions. Our transloca… ▽ More We investigate the translocation of rods with different charge distributions using hybrid Langevin Dynamics and Lattice Boltzmann (LD-LB) simulations. Electrostatic interactions are added to the system using the $P^3M$ algorithm to model the electrohydrodynamic interactions (EHI). We first examine the free-solution electrophoretic properties of rods with various charge distributions. Our translocation simulation results suggest that the order parameter is asymmetric during the capture and escape processes despite the symmetric electric field lines, while the impacts of the charge distribution on rod orientation are more significant during the capture process. The capture/threading/escape times are under the combined effects of charge screening, rod orientation, and charge distributions. We also show that the mean capture time of a rod is shorter when it is launched near the wall because rods tend to align along the wall and hence with the local field lines. Remarkably, the \textit{orientational capture radius} we proposed previously for uniformly charged rods is still valid in the presence of EHI. △ Less

Submitted 23 May, 2022; v1 submitted 4 June, 2021; originally announced June 2021.

Journal ref: Phys. Chem. Chem. Phys. 24, 6444-6452 (2022)

Coexistence of superconductivity and antiferromagentic order in Er$_{2}$O$_{2}$Bi with anti-ThCr$_{2}$Si$_{2}$ structure

Authors: Lei Qiao, Ning-hua Wu, Tianhao Li, Siqi Wu, Zhuyi Zhang, Miaocong Li, Jiang Ma, Baijiang Lv, Yupeng Li, Chenchao Xu, Qian Tao, Chao Cao, Guang-Han Cao, Zhu-An Xu

Abstract: We investigated the coexistence of superconductivity and antiferromagnetic order in the compound Er$_{2}$O$_{2}$Bi with anti-ThCr$_{2}$Si$_{2}$-type structure through resistivity, magnetization, specific heat measurements and first-principle calculations. The superconducting transition temperature $T_{\rm c}$ of 1.23 K and antiferromagnetic transition temperature $T_{\rm N}$ of 3 K are observed in… ▽ More We investigated the coexistence of superconductivity and antiferromagnetic order in the compound Er$_{2}$O$_{2}$Bi with anti-ThCr$_{2}$Si$_{2}$-type structure through resistivity, magnetization, specific heat measurements and first-principle calculations. The superconducting transition temperature $T_{\rm c}$ of 1.23 K and antiferromagnetic transition temperature $T_{\rm N}$ of 3 K are observed in the sample with the best nominal composition. The superconducting upper critical field $H_{\rm c2}$(0) and electron-phonon coupling constant $λ$$_{e-ph}$ in Er$_{2}$O$_{2}$Bi are similar to those in the previously reported non-magnetic superconductor Y$_{2}$O$_{2}$Bi with the same structure, indicating that the superconductivity in Er$_{2}$O$_{2}$Bi may have the same origin as in Y$_{2}$O$_{2}$Bi. The first-principle calculations of Er$_{2}$O$_{2}$Bi show that the Fermi surface is mainly composed of the Bi 6$p$ orbitals both in the paramagnetic and antiferromagnetic state, implying minor effect of the 4$f$ electrons on the Fermi surface. Besides, upon increasing the oxygen incorporation in Er$_{2}$O$_{x}$Bi, $T_{\rm c}$ increases from 1 to 1.23 K and $T_{\rm N}$ decreases slightly from 3 to 2.96 K, revealing that superconductivity and antiferromagnetic order may compete with each other. The Hall effect measurements indicate that hole-type carrier density indeed increases with increasing oxygen content, which may account for the variations of $T_{\rm c}$ and $T_{\rm N}$ with different oxygen content. △ Less

Submitted 18 May, 2021; v1 submitted 14 May, 2021; originally announced May 2021.

Ratcheting charged polymers through symmetric nanopores using pulsed fields: Designing a low pass filter for concentrating DNA

Authors: Le Qiao, Kai Szuttor, Christian Holm, Gary W. Slater

Abstract: We present a new concept for the separation of DNA molecules by contour length that combines a nanofluidic ratchet, nanopore translocation and pulsed fields. Using Langevin Dynamics simulations, we show that it is possible to design pulsed field sequences to ratchet captured semiflexible molecules in such a way that only short chains successfully translocate, effectively transforming the nanopore… ▽ More We present a new concept for the separation of DNA molecules by contour length that combines a nanofluidic ratchet, nanopore translocation and pulsed fields. Using Langevin Dynamics simulations, we show that it is possible to design pulsed field sequences to ratchet captured semiflexible molecules in such a way that only short chains successfully translocate, effectively transforming the nanopore process into a low pass molecular filter. We also show that asymmetric pulses can significantly enhance the device efficiency. The process itself can be performed with many pores in parallel, and it should be possible to integrate it directly into nanopore sequencing devices, increasing its potential utility. △ Less

Submitted 27 January, 2023; v1 submitted 29 January, 2021; originally announced January 2021.

Enhanced anisotropic superconductivity in the topological nodal-line semimetal InxTaS2

Authors: Yupeng Li, Zhongxiu Wu, Jingang Zhou, Kunliang Bu, Chenchao Xu, Lei Qiao, Miaocong Li, Hua Bai, Jiang Ma, Qian Tao, Chao Cao, Yi Yin, Zhu-An Xu

Abstract: Coexistence of topological bands and charge density wave (CDW) in topological materials has attracted immense attentions because of their fantastic properties, such as axionic-CDW, three-dimensional quantum Hall effect, etc. In this work, a nodal-line semimetal InxTaS2 characterized by CDW and superconductivity is successfully synthesized, whose structure and topological bands (two separated Wely… ▽ More Coexistence of topological bands and charge density wave (CDW) in topological materials has attracted immense attentions because of their fantastic properties, such as axionic-CDW, three-dimensional quantum Hall effect, etc. In this work, a nodal-line semimetal InxTaS2 characterized by CDW and superconductivity is successfully synthesized, whose structure and topological bands (two separated Wely rings) are similar to In0.58TaSe2. A 2 x 2 commensurate CDW is observed at low temperature in InxTaS2, identified by transport properties and STM measurements. Moreover, superconductivity emerges below 0.69 K, and the anisotropy ratio of upper critical field [Gamma = H||ab c2(0)=H||c c2(0)] is significantly enhanced compared to 2H-TaS2, which shares the same essential layer unit. According to the Lawrence-Doniach model, the enhanced Gamma may be explained by the reduced effective mass in kx-ky plane, where Weyl rings locate. Therefore, this type of layered topological systems may offer a platform to investigate highly anisotropic superconductivity and to understand the extremely large upper critical field in the bulk or in the two-dimensional limit. △ Less

Submitted 12 November, 2020; originally announced November 2020.

Comments: Accepted for publication in PRB

Type-I superconductivity in noncentrosymmetric NbGe$_{2}$

Authors: Baijiang Lv, Miaocong Li, Jia Chen, Yusen Yang, Siqi Wu, Lei Qiao, Feihong Guan, Hui Xing, Qian Tao, Guang-Han Cao, Zhu-An Xu

Abstract: Single crystals of NbGe$_{2}$ which crystallize in a noncentrosymmetric hexagonal structure with chirality are synthesized and their superconductivity is investigated. Type-I superconductivity is confirmed by dc magnetization, field-induced second-to first-order phase transition in specific heat, and a small Ginzburg-Landau parameter $κ_{GL}=0.12$. The isothermal magnetization measurements show th… ▽ More Single crystals of NbGe$_{2}$ which crystallize in a noncentrosymmetric hexagonal structure with chirality are synthesized and their superconductivity is investigated. Type-I superconductivity is confirmed by dc magnetization, field-induced second-to first-order phase transition in specific heat, and a small Ginzburg-Landau parameter $κ_{GL}=0.12$. The isothermal magnetization measurements show that there is a crossover from type-I to type-II/1 superconductivity with decreasing temperature and an unusually enhanced surface superconducting critical field ($H_{c3}$) is discovered. The band structure calculations indicate the presence of Kramer-Weyl nodes near the Fermi level. These observations demonstrate that NbGe$_{2}$ is an interesting and rare example involving the possible interplay of type-I superconductivity, noncentrosymmetric structure and topological properties. △ Less

Submitted 25 August, 2020; originally announced August 2020.

Comments: 7 pages,6 figures

Journal ref: Phys. Rev. B 102, 064507 (2020)

Anisotropic gapping of topological Weyl rings in the charge-density-wave superconductor InxTaSe2

Authors: Yupeng Li, Yi Wu, Chenchao Xu, Ningning Liu, Jiang Ma, Baijiang Lv, Gang Yao, Yan Liu, Hua Bai, Xiaohui Yang, Lei Qiao, Miaocong Li, Linjun Li, Hui Xing, Yaobo Huang, Junzhang Ma, Ming Shi, Chao Cao, Yang Liu, Canhua Liu, Jinfeng Jia, Zhu-An Xu

Abstract: Topological materials and topological phases have recently become a hot topic in condensed matter physics. In this work, we report a topological nodal-line semimetal InxTaSe2, in the presence of both charge density wave (CDW) and superconductivity. In the x = 0.58 samples, the 2 * /3 commensurate CDW (CCDW) and the 2 * 2 CCDW are observed below 116 K and 77 K, respectively. Consistent with theoret… ▽ More Topological materials and topological phases have recently become a hot topic in condensed matter physics. In this work, we report a topological nodal-line semimetal InxTaSe2, in the presence of both charge density wave (CDW) and superconductivity. In the x = 0.58 samples, the 2 * /3 commensurate CDW (CCDW) and the 2 * 2 CCDW are observed below 116 K and 77 K, respectively. Consistent with theoretical calculations, the spin-orbital coupling gives rise to two two-fold-degenerate nodal rings (Weyl rings) connected by drumhead surface states, confirmed by angle-resolved photoemission spectroscopy. Our results suggest that the 2 * 2 CCDW ordering gaps out one Weyl ring in accordance with the CDW band folding, while the other Weyl ring remains gapless with intact surface states. In addition, superconductivity emerges at 0.91 K, with the upper critical field deviating from the s-wave behavior at low temperature, implying possibly unconventional superconductivity. Therefore, InxTaSe2 represents an interesting material system to study the interplay between CDW, nontrivial band topology and superconductivity. △ Less

Submitted 7 April, 2020; originally announced April 2020.

Journal ref: Science Bulletin, 2021, 66(3)

Direct Observation of Quantum Percolation Dynamics

Authors: Zhen Feng, Bing-Hong Wu, Hao Tang, Lu-Feng Qiao, Xiao-Wei Wang, Xiao-Yun Xu, Zhi-Qiang Jiao, Jun Gao, Xian-Min Jin

Abstract: Percolation, describing critical behaviors of phase transition in a geometrical context, prompts wide investigations in natural and social networks as a fundamental model. The introduction of quantum-intrinsic interference and tunneling brings percolation into quantum regime with more fascinating phenomena and unique features, which, however, hasn't been experimentally explored yet. Here we presen… ▽ More Percolation, describing critical behaviors of phase transition in a geometrical context, prompts wide investigations in natural and social networks as a fundamental model. The introduction of quantum-intrinsic interference and tunneling brings percolation into quantum regime with more fascinating phenomena and unique features, which, however, hasn't been experimentally explored yet. Here we present an experimental demonstration of quantum transport in hexagonal percolation lattices by successfully mapping such large-scale porous structures into a photonic chip using femtosecond laser direct writing techniques. A quantum percolation threshold of 80% is observed in the prototyped laser-written lattices with up to 1,600 waveguides, which is significantly larger than the classical counterpart of 63%. We also investigate the spatial confinement by localization parameters and exhibit the transition from ballistic to diffusive propagation with the decrease of the occupation probability. Direct observation of quantum percolation may deepen the understanding of the relation among materials, quantum transport, geometric quenching, disorder and localization, and inspire applications for quantum technologies. △ Less

Submitted 1 January, 2020; originally announced January 2020.

Comments: 21 pages, 5 figures, 1 table, comments welcome

Voltage-Driven Translocation: Defining a Capture Radius

Authors: Le Qiao, Maxime Ignacio, Gary W. Slater

Abstract: Analyte translocation involves three phases: (i) diffusion in the loading solution; (ii) capture by the pore; (iii) threading. The capture process remains poorly characterized because it cannot easily be visualized or inferred from indirect measurements. The capture performance of a device is often described by a \textit{capture radius} generally defined as the radial distance $R^*$ at which diffu… ▽ More Analyte translocation involves three phases: (i) diffusion in the loading solution; (ii) capture by the pore; (iii) threading. The capture process remains poorly characterized because it cannot easily be visualized or inferred from indirect measurements. The capture performance of a device is often described by a \textit{capture radius} generally defined as the radial distance $R^*$ at which diffusion-dominated dynamics cross over to field-induced drift. However, this definition is rather ambiguous and the related models are usually over-simplified and studied in the steady-state limit. We investigate different approaches to defining and estimating $R^*$ for a charged particle diffusing in a liquid and attracted to the nanopore by the electric field. We present a theoretical analysis of the Péclet number as well as Monte Carlo simulations with different simulation protocols. Our analysis shows that the boundary conditions, pore size and finite experimental times all matter in the interpretation and calculation of $R^*$. △ Less

Submitted 25 November, 2019; originally announced November 2019.

Comments: The following article has been submitted to The Journal of Chemical Physics

Journal ref: The Journal of Chemical Physics, 151(24), p.244902 (2019)

Parity-Induced Thermalization Gap in Disordered Ring Lattices

Authors: Yao Wang, Jun Gao, Xiao-Ling Pang, Zhi-Qiang Jiao, Hao Tang, Yuan Chen, Lu-Feng Qiao, Zhen-Wei Gao, Jian-Peng Dou, Ai-Lin Yang, Xian-Min Jin

Abstract: The gaps separating two different states widely exist in various physical systems: from the electrons in periodic lattices to the analogs in photonic, phononic, plasmonic systems, and even quasicrystals. Recently, a thermalization gap, an inaccessible range of photon statistics, was proposed for light in disordered structures [Nat. Phys. 11, 930 (2015)], which is intrinsically induced by the disor… ▽ More The gaps separating two different states widely exist in various physical systems: from the electrons in periodic lattices to the analogs in photonic, phononic, plasmonic systems, and even quasicrystals. Recently, a thermalization gap, an inaccessible range of photon statistics, was proposed for light in disordered structures [Nat. Phys. 11, 930 (2015)], which is intrinsically induced by the disorder-immune chiral symmetry and can be reflected by the photon statistics. The lattice topology was further identified as a decisive role in determining the photon statistics when the chiral symmetry is satisfied. Being very distinct from one-dimensional lattices, the photon statistics in ring lattices are dictated by its parity, i.e, odd or even sited. Here, we for the first time experimentally observe a parity-induced thermalization gap in strongly disordered ring photonic structures. In a limited scale, though the light tends to be localized, we are still able to find clear evidence of the parity-dependent disorder-immune chiral symmetry and the resulting thermalization gap by measuring photon statistics, while strong disorder-induced Anderson localization overwhelms such a phenomenon in larger-scale structures. Our results shed new light on the relation among symmetry, disorder, and localization, and may inspire new resources and artificial devices for information processing and quantum control on a photonic chip. △ Less

Submitted 18 March, 2019; v1 submitted 28 March, 2018; originally announced March 2018.

Comments: 20 pages, 19 figures

Journal ref: Phys. Rev. Lett. 122, 013903 (2019)

Dynamical instability of the electric transport in strongly fluctuating superconductors

Authors: Lei Qiao, Dingping Li, Baruch Rosenstein

Abstract: Theory of the influence of the thermal fluctuations on the electric transport beyond linear response in superconductors is developed within the framework of the time dependent Ginzburg - Landau approach. The I - V curve is calculated using the dynamical self - consistent gaussian approximation. Under certain conditions it exhibits a reentrant behaviour acquiring an S - shape form. The unstable reg… ▽ More Theory of the influence of the thermal fluctuations on the electric transport beyond linear response in superconductors is developed within the framework of the time dependent Ginzburg - Landau approach. The I - V curve is calculated using the dynamical self - consistent gaussian approximation. Under certain conditions it exhibits a reentrant behaviour acquiring an S - shape form. The unstable region below a critical temperature $T^{\ast }$ is determined for arbitrary dimensionality ($D=1,2,3$) of the thermal fluctuations. The results are applied to analyse the transport data on nanowires and several classes of 2D superconductors: metallic thin films, layered and atomically thick novel materials. △ Less

Submitted 6 September, 2017; originally announced September 2017.

Comments: 10 pages, 4 figures

Experimental Two-dimensional Quantum Walk on a Photonic Chip

Authors: Hao Tang, Xiao-Feng Lin, Zhen Feng, Jing-Yuan Chen, Jun Gao, Ke Sun, Chao-Yue Wang, Peng-Cheng Lai, Xiao-Yun Xu, Yao Wang, Lu-Feng Qiao, Ai-Lin Yang, Xian-Min Jin

Abstract: Quantum walks, in virtue of the coherent superposition and quantum interference, possess exponential superiority over its classical counterpart in applications of quantum searching and quantum simulation. The quantum enhanced power is highly related to the state space of quantum walks, which can be expanded by enlarging the photon number and/or the dimensions of the evolution network, but the form… ▽ More Quantum walks, in virtue of the coherent superposition and quantum interference, possess exponential superiority over its classical counterpart in applications of quantum searching and quantum simulation. The quantum enhanced power is highly related to the state space of quantum walks, which can be expanded by enlarging the photon number and/or the dimensions of the evolution network, but the former is considerably challenging due to probabilistic generation of single photons and multiplicative loss. Here we demonstrate a two-dimensional continuous-time quantum walk by using the external geometry of photonic waveguide arrays, rather than the inner degree of freedoms of photons. Using femtosecond laser direct writing, we construct a large-scale three-dimensional structure which forms a two-dimensional lattice with up to 49X49 nodes on a photonic chip. We demonstrate spatial two-dimensional quantum walks using heralded single photons and single-photon-level imaging. We analyze the quantum transport properties via observing the ballistic evolution pattern and the variance profile, which agree well with simulation results. We further reveal the transient nature that is the unique feature for quantum walks of beyond one dimension. An architecture that allows a walk to freely evolve in all directions and a large scale, combining with defect and disorder control, may bring up powerful and versatile quantum walk machines for classically intractable problems. △ Less

Submitted 16 May, 2018; v1 submitted 26 April, 2017; originally announced April 2017.

Comments: 7 pages, 4 figures. The experiment has been performed again with heralded single photons instead of the coherent light

Journal ref: Science Advances 4, eaat317 (2018)

Cation nonstoichiometry and its impact on nucleation, structure and defect formation in complex oxide heteroepitaxy : LaCrO3 on SrTiO3(001)

Authors: L. Qiao, K. H. L. Zhang, M. E. Bowden, V. Shutthanandan, R. Colby, Y. Du, B. Kabius, P. V. Sushko, S. A. Chambers

Abstract: Our ability to design and fabricate electronic devices with reproducible properties using complex oxides is critically dependent on our ability to controllably synthesize these materials in thin-film form. Structure-property relationships are intimately tied to film and interface composition. Here we report on the effects of cation stoichiometry in LaCrO3 heteroepitaxial films prepared using molec… ▽ More Our ability to design and fabricate electronic devices with reproducible properties using complex oxides is critically dependent on our ability to controllably synthesize these materials in thin-film form. Structure-property relationships are intimately tied to film and interface composition. Here we report on the effects of cation stoichiometry in LaCrO3 heteroepitaxial films prepared using molecular beam epitaxy. We show that LaCrO3 films grow pseudomorphically on SrTiO3(001) over a wide range of La-to-Cr atom ratios. However, the growth mode and structural quality are sensitive to the La-to-Cr ratio, with La-rich films being of considerably lower structural quality than Cr-rich films. Cation mixing occurs at the interface for all La-to-Cr ratios investigated, and is not quenched by deposition at ambient temperature. Indiffused La atoms occupy Sr sites in the substrate. The presence of defects in the SrTiO3 substrate is implicated in promoting La indiffusion by comparing the properties of LaCrO3/SrTiO3 with those of LaCrO3/Si, both prepared at ambient temperature. Additionally, pulsed laser deposition is shown to result in more extensive interfacial mixing than molecular beam epitaxy for deposition at ambient temperature on Si. △ Less

Submitted 12 November, 2012; originally announced November 2012.

Comments: 33 pages, 9 figures

Graded index and randomly oriented core-shell silicon nanowires with broadband and wide angle antireflection for photovoltaic cell applications

Authors: P. Pignalosa, H. Lee, L. Qiao, M. Tseng, Yasha Yi

Abstract: Antireflection with broadband and wide angle properties is important for a wide range of applications on photovoltaic cells and display. The SiOx shell layer provides a natural antireflection from air to the Si core absorption layer. In this work, we have demonstrated the random core-shell silicon nanowires with both broadband (from 400nm to 900nm) and wide angle (from normal incidence to 60\degre… ▽ More Antireflection with broadband and wide angle properties is important for a wide range of applications on photovoltaic cells and display. The SiOx shell layer provides a natural antireflection from air to the Si core absorption layer. In this work, we have demonstrated the random core-shell silicon nanowires with both broadband (from 400nm to 900nm) and wide angle (from normal incidence to 60\degree) antireflection characteristics within AM1.5 solar spectrum. The graded index structure from the randomly oriented core-shell (Air/SiOx/Si) nanowires may provide a potential avenue to realize a broadband and wide angle antireflection layer. △ Less

Submitted 4 August, 2011; originally announced August 2011.

Journal ref: AIP ADVANCES Vol.1, 032124 (2011)

LaCrO3 heteroepitaxy on SrTiO3(001) by molecular beam epitaxy

Authors: L. Qiao, T. C. Droubay, M. E. Bowden, V. Shutthanandan, T. C. Kaspar, S. A. Chambers

Abstract: Stoichiometric, epitaxial LaCrO3 films have been grown on TiO2-terminated SrTiO3(001) substrates by molecular beam epitaxy using O2 as the oxidant. Film growth occurred in a layer-by-layer fashion, giving rise to structurally excellent films and surfaces which preserve the step-terrace structure of the substrate. The critical thickness is in excess of 500 Å. Near-surface Cr(III) is highly suscepti… ▽ More Stoichiometric, epitaxial LaCrO3 films have been grown on TiO2-terminated SrTiO3(001) substrates by molecular beam epitaxy using O2 as the oxidant. Film growth occurred in a layer-by-layer fashion, giving rise to structurally excellent films and surfaces which preserve the step-terrace structure of the substrate. The critical thickness is in excess of 500 Å. Near-surface Cr(III) is highly susceptible to further oxidation to Cr(V), leading to the formation of a disordered phase upon exposure to atomic oxygen. Recovery of the original epitaxial LaCrO3 phase is readily achieved by vacuum annealing. △ Less

Submitted 21 May, 2011; originally announced May 2011.

Comments: 10 pages, 4 figures