Showing 1–50 of 85 results for author: Zhao, F

Oxygen Reduction Reaction on Platinum Nanocatalysts Produces Long-Lived, Hysteretic Oxygenated Adsorbates

Authors: Jaehyeon Kim, Lalith Krishna Samanth Bonagiri, Fujia Zhao, Yingjie Zhang

Abstract: Aqueous electrocatalysis generates oxygenated intermediates at catalyst surfaces. While intermediate species on single-crystal catalysts have been observed, the nature and evolution of surface oxygenated species on industrially relevant nanoparticle (NP) catalysts remain largely unknown. Here, using in situ Raman spectroscopy, we tracked the formation and potential-dependent evolution of oxygenate… ▽ More Aqueous electrocatalysis generates oxygenated intermediates at catalyst surfaces. While intermediate species on single-crystal catalysts have been observed, the nature and evolution of surface oxygenated species on industrially relevant nanoparticle (NP) catalysts remain largely unknown. Here, using in situ Raman spectroscopy, we tracked the formation and potential-dependent evolution of oxygenated adsorbates in alkaline media on NP catalysts with an active platinum (Pt) surface. By comparing spectroscopic features in Ar- vs O2-saturated electrolytes, we determined three key intermediates produced by the oxygen reduction reaction (ORR): adsorbed OOH, OH, and O2. In contrast to the conventional wisdom that intermediates exist only during catalytic reactions, we found these oxygenated adsorbates to be highly long-lived and hysteretic, and to persist even after the termination of ORR. This adsorbate-retention effect exhibits a modest dependence on the surface oxidation state and the electrolyte cations (K+ vs Li+), and is likely facilitated by the heterogeneous nature of the catalyst surface. The results highlight the complexity of surface adsorption structures on realistic catalysts, which often extends beyond that captured by measurements or simulations on model single-crystal surfaces. △ Less

Submitted 8 August, 2026; originally announced August 2026.

Quadrature magnetoresistance scaling reflects linear field dependence rather than strange metallicity

Authors: D. B. Zhou, Y. Yang, L. F. Feng, M. F. Zhao, Z. Y. Jia, K. H. Gao

Abstract: The quadrature scaling of magnetoresistance has been widely adopted as a hallmark of the strange metal state. However, whether this scaling signals quantum criticality or reflects conventional transport behavior remains controversial. Here, by systematically investigating the magnetotransport properties of NiTe2 nanosheets, we demonstrate that the quadrature scaling is not a unique signature of st… ▽ More The quadrature scaling of magnetoresistance has been widely adopted as a hallmark of the strange metal state. However, whether this scaling signals quantum criticality or reflects conventional transport behavior remains controversial. Here, by systematically investigating the magnetotransport properties of NiTe2 nanosheets, we demonstrate that the quadrature scaling is not a unique signature of strange metallicity. We find that the scaling holds only when the crossover field , marking the transition from quadratic to linear magnetoresistance, is sufficiently small relative to the applied field range. Through controlled simulations, we show that the scaling emerges whenever linear magnetoresistance dominates, irrespective of its origin, and fails when the linear regime is inaccessible. This conclusion is supported by observations in SrTiO3 based heterostructures, where quadrature scaling appears despite the absence of strange metal behavior. Our results establish that the quadrature scaling merely reflects the presence of linear magneto resistance, urging caution in using this scaling as a diagnostic tool for exploring the strange metal state. △ Less

Submitted 21 July, 2026; originally announced July 2026.

Comments: 21 pages,5 figures

Characterization of Exciton-exciton entanglement and correlations

Authors: Fangzhou Zhao, Carlos Mejuto-Zaera, Angel Rubio, Vojtěch Vlček

Abstract: Excitons in the weakly interacting regime can be well-described by many-body perturbation theories such as the Bethe-Salpeter equation formalism. However, for materials such as transition metal dichalcogenides moiré heterostructures under strong illumination, with the emergence of dense excitonic states, the strong correlation and entanglement between electrons and holes may cause the many-body pe… ▽ More Excitons in the weakly interacting regime can be well-described by many-body perturbation theories such as the Bethe-Salpeter equation formalism. However, for materials such as transition metal dichalcogenides moiré heterostructures under strong illumination, with the emergence of dense excitonic states, the strong correlation and entanglement between electrons and holes may cause the many-body perturbation method to fail, and excitons may not be treated in the bosonic picture, but exhibit fermionic behaviors. In our work, we investigate the phase space where excitons, and the electrons and holes which constitute them, are weakly or strongly entangled, as well as their binding for different interaction profiles and the degree of localization of the electrons and holes. We corroborate the validity of using many-body perturbation theory in the exciton with interactions. Our work provides a general way to analyze the correlation and entanglement of multi-particle excitations in many-body systems, and gives a more comprehensive understanding of different phases for exciton entanglement and interactions in 1D systems. △ Less

Submitted 13 March, 2026; originally announced March 2026.

Anisotropic linear magnetoresistance in nanoflakes of Dirac semimetal NiTe2

Authors: Ding Bang Zhou, Kuang Hong Gao, Tie Lin, Yang Yang, Meng Fan Zhao, Zhi Yan Jia, Xiao Xia Hu, Qian Jin Guo, Zhi Qing Li

Abstract: This work investigates the magneto-transport properties of exfoliated NiTe2 nano-flakes with varying thicknesses and disorder levels, unveiling two distinct physical mechanisms governing the observed anisotropic linear magnetoresistance (MR). For the perpendicular magnetic field configuration, the well-defined linear MR in high fields is unambiguously attributed to a classical origin. This conclus… ▽ More This work investigates the magneto-transport properties of exfoliated NiTe2 nano-flakes with varying thicknesses and disorder levels, unveiling two distinct physical mechanisms governing the observed anisotropic linear magnetoresistance (MR). For the perpendicular magnetic field configuration, the well-defined linear MR in high fields is unambiguously attributed to a classical origin. This conclusion is supported by the proportionality between the MR slope and the carrier mobility, and between the crossover field and the inverse of mobility. In stark contrast, the linear MR under parallel magnetic fields exhibits a non-classical character. It shows a pronounced enhancement with decreasing flake thickness, which correlates with an increasing hole-to-electron concentration ratio. This distinctive thickness dependence suggests an origin in the nonlinear band effects near the Dirac point, likely driven by the shift of the Fermi level. Furthermore, the strengthening of MR anisotropic with enhanced inter-layer transport contradicts the prediction of the guiding-center diffusion model for three-dimensional systems. Our findings highlight the critical roles of band topology and structural dimensional in the anomalous magneto-transport of Dirac semi-metals. △ Less

Submitted 6 December, 2025; v1 submitted 1 October, 2025; originally announced October 2025.

Distinguishing dual lattice by strong-pulse matter-wave diffraction

Authors: Fangde Liu, Wei Han, Yunda Li, Feifan Zhao, Liangchao Chen, Lianghui Huang, Pengjun Wang, Zengming Meng, Jing Zhang

Abstract: Dual lattices such as honeycomb and hexagonal lattices typically obey Babinet's principle in optics, which states that the expected interference patterns of two complementary diffracting objects are identical and indistinguishable, except for their overall intensity. Here, we study Kapitza--Dirac diffraction of Bose--Einstein condensates in optical lattices and find that matter waves in dual latti… ▽ More Dual lattices such as honeycomb and hexagonal lattices typically obey Babinet's principle in optics, which states that the expected interference patterns of two complementary diffracting objects are identical and indistinguishable, except for their overall intensity. Here, we study Kapitza--Dirac diffraction of Bose--Einstein condensates in optical lattices and find that matter waves in dual lattices obey Babinet's principle only under the condition of weak-pulse Raman--Nath regimes. In contrast, the Kapitza--Dirac matter-wave diffraction in the strong-pulse Raman--Nath regime (corresponding to the phase wrapping method we developed to generate sub-wavelength phase structures in Sci. Rep. 10, 5870 (2020)) can break Babinet's principle and clearly resolve the distinct interference patterns of the dual honeycomb and hexagonal lattices. This method offers exceptional precision in characterizing lattice configurations and advance the study of symmetry-related phenomena, overcoming the limitations of real-space imaging. △ Less

Submitted 22 July, 2025; originally announced July 2025.

Information fusion strategy integrating pre-trained language model and contrastive learning for materials knowledge mining

Authors: Yongqian Peng, Zhouran Zhang, Longhui Zhang, Fengyuan Zhao, Yahao Li, Yicong Ye, Shuxin Bai

Abstract: Machine learning has revolutionized materials design, yet predicting complex properties like alloy ductility remains challenging due to the influence of processing conditions and microstructural features that resist quantification through traditional reductionist approaches. Here, we present an innovative information fusion architecture that integrates domain-specific texts from materials science… ▽ More Machine learning has revolutionized materials design, yet predicting complex properties like alloy ductility remains challenging due to the influence of processing conditions and microstructural features that resist quantification through traditional reductionist approaches. Here, we present an innovative information fusion architecture that integrates domain-specific texts from materials science literature with quantitative physical descriptors to overcome these limitations. Our framework employs MatSciBERT for advanced textual comprehension and incorporates contrastive learning to automatically extract implicit knowledge regarding processing parameters and microstructural characteristics. Through rigorous ablation studies and comparative experiments, the model demonstrates superior performance, achieving coefficient of determination (R2) values of 0.849 and 0.680 on titanium alloy validation set and refractory multi-principal-element alloy test set. This systematic approach provides a holistic framework for property prediction in complex material systems where quantitative descriptors are incomplete and establishes a foundation for knowledge-guided materials design and informatics-driven materials discovery. △ Less

Submitted 14 June, 2025; originally announced June 2025.

Carbon in GaN as a nonradiative recombination center

Authors: Fangzhou Zhao, Hongyi Guan, Mark E. Turiansky, Chris G. Van de Walle

Abstract: Trap-assisted nonradiative recombination has been shown to limit the efficiency of optoelectronic devices. While substitutional carbon ($\mathrm{C_N}$) has been suggested to be a nonradiative recombination center in GaN devices, a complete recombination cycle including the two charge-state transition levels has not been previously described. In this work, we investigate the trap-assisted recombina… ▽ More Trap-assisted nonradiative recombination has been shown to limit the efficiency of optoelectronic devices. While substitutional carbon ($\mathrm{C_N}$) has been suggested to be a nonradiative recombination center in GaN devices, a complete recombination cycle including the two charge-state transition levels has not been previously described. In this work, we investigate the trap-assisted recombination process due to $\mathrm{C_N}$ in GaN, including multiphonon emission (MPE), radiative recombination, trap-assisted Auger-Meitner (TAAM) recombination, as well as thermal emission of holes. Our study shows the key role of TAAM processes at the high carrier densities relevant for devices. We also reveal the carrier-density regimes where thermal emission and radiative recombination are expected to play an observable role. Our results highlight that carbon concentrations exceeding $\sim$10$^{17}$ cm$^{-3}$ can have a noticeable impact on device efficiency, not just in GaN active layers but also in InGaN and AlGaN. Our comprehensive formalism not only offers detailed results for carbon but provides a general framework for assessing the multiple processes that participate in trap-assisted recombination in semiconductors. △ Less

Submitted 18 February, 2025; originally announced February 2025.

Formation mechanisms and fluorescence properties of carbon dots in coal burning dust from coal fired power plants

Authors: Zhexian Zhao, Weizuo Zhang, Jin Zhang, Yuzhao Li, Han Bai, Fangming Zhao, Zhongcai Jin, Ju Tang, Yiming Xiao, Wen Xu, Yanfei Lü

Abstract: Carbon dots (CDs) shows great application potential with their unique and excellent performances. Coal and its derivatives are rich in aromatic ring structure, which is suitable for preparing CDs in microstructure. Coal burning dust from coal-fired power plants can be utilized as a rich resource to separate and extract CDs. It has been shown in our results that there have two main possible mechani… ▽ More Carbon dots (CDs) shows great application potential with their unique and excellent performances. Coal and its derivatives are rich in aromatic ring structure, which is suitable for preparing CDs in microstructure. Coal burning dust from coal-fired power plants can be utilized as a rich resource to separate and extract CDs. It has been shown in our results that there have two main possible mechanisms for the formation of CDs in coal burning dust. One is the self-assembly of polycyclic aromatic hydrocarbons contained in coal or produced by incomplete combustion of coal. The other mechanism is that the bridge bonds linking different aromatic structures in coal are breaking which would form CDs with different functional groups when the coals are burning at high temperature. Under violet light excitation at 310-340 nm or red light at 610-640 nm, CDs extracted from coal burning dust can emit purple fluorescence around 410 nm. The mechanism of up-conversion fluorescence emission of CDs is due to a two-photon absorption process. The recycling of CDs from coal burning dust from coal-fired power plants are not only good to protect environment but also would be helpful for mass production of CDs. △ Less

Submitted 2 November, 2024; originally announced November 2024.

Journal ref: Advanced Photonics Research 5(10), 2400010 (2024)

Uniaxial plasmon polaritons $\textit{via}$ charge transfer at the graphene/CrSBr interface

Authors: Daniel J. Rizzo, Eric Seewald, Fangzhou Zhao, Jordan Cox, Kaichen Xie, Rocco A. Vitalone, Francesco L. Ruta, Daniel G. Chica, Yinming Shao, Sara Shabani, Evan J. Telford, Matthew C. Strasbourg, Thomas P. Darlington, Suheng Xu, Siyuan Qiu, Aravind Devarakonda, Takashi Taniguchi, Kenji Watanabe, Xiaoyang Zhu, P. James Schuck, Cory R. Dean, Xavier Roy, Andrew J. Millis, Ting Cao, Angel Rubio , et al. (2 additional authors not shown)

Abstract: Graphene is a privileged 2D platform for hosting confined light-matter excitations known as surface plasmon-polaritons (SPPs), as it possesses low intrinsic losses with a high degree of optical confinement. However, the inherently isotropic optical properties of graphene limit its ability to guide and focus SPPs, making it less suitable than anisotropic elliptical and hyperbolic materials as a pla… ▽ More Graphene is a privileged 2D platform for hosting confined light-matter excitations known as surface plasmon-polaritons (SPPs), as it possesses low intrinsic losses with a high degree of optical confinement. However, the inherently isotropic optical properties of graphene limit its ability to guide and focus SPPs, making it less suitable than anisotropic elliptical and hyperbolic materials as a platform for polaritonic lensing and canalization. Here, we present the graphene/CrSBr heterostructure as an engineered 2D interface that hosts highly anisotropic SPP propagation over a wide range of frequencies in the mid-infrared and terahertz. Using a combination of scanning tunneling microscopy (STM), scattering-type scanning near-field optical microscopy (s-SNOM), and first-principles calculations, we demonstrate mutual doping in excess of 10$^{13}$ cm$^{-2}$ holes/electrons between the interfacial layers of graphene/CrSBr heterostructures. SPPs in graphene activated by charge transfer interact with charge-induced anisotropic intra- and interband transitions in the interfacial doped CrSBr, leading to preferential SPP propagation along the quasi-1D chains that compose each CrSBr layer. This multifaceted proximity effect both creates SPPs and endows them with anisotropic transport and propagation lengths that differ by an order-of-magnitude between the two in-plane crystallographic axes of CrSBr. △ Less

Submitted 9 July, 2024; originally announced July 2024.

On-liquid-gallium surface synthesis of ultra-smooth conductive metal-organic framework thin films

Authors: Jinxin Liu, Yunxu Chen, Xing Huang, Yanhan Ren, Mike Hambsch, David Bodesheim, Darius Pohl, Xiaodong Li, Marielle Deconinck, Bowen Zhang, Markus Löffler, Zhongquan Liao, Fengxiang Zhao, Arezoo Dianat, Gianaurelio Cuniberti, Yana Vaynzof, Junfeng Gao, Jingcheng Hao, Stefan C. B. Mannsfeld, Xinliang Feng, Renhao Dong

Abstract: Conductive metal-organic frameworks (MOFs) are emerging electroactive materials for (opto-)electronics. However, it remains a great challenge to achieve reliable MOF-based devices via the existing synthesis methods that are compatible with the complementary metal-oxide-semiconductor technology, as the surface roughness of thus-far synthetic MOF films or pellets is rather high for efficient electro… ▽ More Conductive metal-organic frameworks (MOFs) are emerging electroactive materials for (opto-)electronics. However, it remains a great challenge to achieve reliable MOF-based devices via the existing synthesis methods that are compatible with the complementary metal-oxide-semiconductor technology, as the surface roughness of thus-far synthetic MOF films or pellets is rather high for efficient electrode contact. Here, we develop an on-liquid-gallium surface synthesis (OLGSS) strategy under chemical vapor deposition (CVD) conditions for the controlled growth of two-dimensional conjugated MOF (2D c-MOF) thin films with ten-fold improvement of surface flatness (surface roughness can reach as low as ~2 Å) compared with MOF films grown by the traditional methods. Supported by theoretical modeling, we unveil a layer-by-layer CVD growth mode for constructing flattening surfaces, that is triggered by the high adhesion energy between gallium (Ga) and planar aromatic ligands. We further demonstrate the generality of the as-proposed OLGSS strategy by reproducing such a flat surface over nine different 2D c-MOF films with variable thicknesses (~2 to 208 nm) and large lateral sizes (over 1 cm2). The resultant ultra-smooth 2D c-MOF films enable the formation of high-quality electrical contacts with gold (Au) electrodes, leading to a reduction of contact resistance by over ten orders of magnitude compared to the traditional uneven MOF films. Furthermore, due to the efficient interfacial interaction benifited from the high-quality contacts, the prepared van der Waals heterostructure (vdWH) of OLGSS c-MOF and MoS2 exhibits intriguing photoluminescence (PL) enhancement, PL peak shift and large work function modulation. The establishment of the reliable OLGSS method provides the chances to push the development of MOF electronics and the construction of multicomponent MOF-based heterostructure materials. △ Less

Submitted 17 April, 2024; originally announced April 2024.

Niobium coaxial cavities with internal quality factors exceeding 1.5 billion for circuit quantum electrodynamics

Authors: Andrew E. Oriani, Fang Zhao, Tanay Roy, Alexander Anferov, Kevin He, Ankur Agrawal, Riju Banerjee, Srivatsan Chakram, David I. Schuster

Abstract: Group-V materials such as niobium and tantalum have become popular choices for extending the performance of circuit quantum electrodynamics (cQED) platforms allowing for quantum processors and memories with reduced error rates and more modes. The complex surface chemistry of niobium however makes identifying the main modes of decoherence difficult at millikelvin temperatures and single-photon powe… ▽ More Group-V materials such as niobium and tantalum have become popular choices for extending the performance of circuit quantum electrodynamics (cQED) platforms allowing for quantum processors and memories with reduced error rates and more modes. The complex surface chemistry of niobium however makes identifying the main modes of decoherence difficult at millikelvin temperatures and single-photon powers. We use niobium coaxial quarter-wave cavities to study the impact of etch chemistry, prolonged atmospheric exposure, and the significance of cavity conditions prior to and during cooldown, in particular niobium hydride evolution, on single-photon coherence. We demonstrate cavities with quality factors of Q_int>1.4X10^9 in the single-photon regime, a 15 fold improvement over aluminum cavities of the same geometry. We rigorously quantify the sensitivity of our fabrication process to various loss mechanisms and demonstrate a 2-4X reduction in the two-level system (TLS) loss tangent and a 3-5X improvement in the residual resistance over traditional BCP etching techniques. Finally, we demonstrate transmon integration and coherent cavity control while maintaining a cavity coherence of 11.3ms. The accessibility of our method, which can easily be replicated in academic-lab settings, and the demonstration of its performance mark an advancement in 3D cQED. △ Less

Submitted 16 May, 2025; v1 submitted 1 March, 2024; originally announced March 2024.

Comments: 14 pages, 10 figures v2 edits: -tables in TeX format -Fig 2.C label fixed to Nb2-1 and correctly referenced in text -References added to discussion on wet oxidation and vacancies -Added Nb2O fit to Fig 3B -Deleted duplicate reference -Typo fixes

Superconductivity with Tc 116K discovered in antimony polyhydrides

Authors: K. Lu, X. He, C. L. Zhang, Z. W. Li, S. J. Zhang, B. S. Min, J. Zhang, J. F. Zhao, L. C. Shi, Y. Peng, S. M. Feng, Q. Q. Liu, J. Song, R. C. Yu, X. C. Wang, Y. Wang, M. Bykov, C. Q. Jin

Abstract: Superconductivity (SC) was experimentally observed for the first time in antimony polyhydride. The diamond anvil cell combined with laser heating system was used to synthesize the antimony polyhydride sample at high pressure and high temperature conditions. In-situ high pressure transport measurements as function of temperature with applied magnet are performed to study the SC properties. It was f… ▽ More Superconductivity (SC) was experimentally observed for the first time in antimony polyhydride. The diamond anvil cell combined with laser heating system was used to synthesize the antimony polyhydride sample at high pressure and high temperature conditions. In-situ high pressure transport measurements as function of temperature with applied magnet are performed to study the SC properties. It was found that the antimony polyhydride samples show superconducting transition with critical temperature Tc 116 K at 184 GPa. The investigation of SC at magnetic field revealed that the superconducting coherent length ~40 angstroms based on Ginzburg Landau (GL) equation. Antimony polyhydride superconductor has the second highest Tc in addition to sulfur hydride among the polyhydrides of elements from main group IIIA to VIIA in periodic table. △ Less

Submitted 21 August, 2024; v1 submitted 6 October, 2023; originally announced October 2023.

Journal ref: National Science Review 11, nwad241 (2024)

Superconductivity Observed in Tantalum Polyhydride at High Pressure

Authors: X. He, C. L. Zhang, Z. W. Li, S. J. Zhang, B. S. Min, J. Zhang, K. Lu, J. F. Zhao, L. C. Shi, Y. Peng, X. C. Wang, S. M. Feng, J. Song, L. H. Wang, V. B. Prakapenka, S. Chariton, H. Z. Liu, C. Q. Jin

Abstract: We report experimental discovery of tantalum polyhydride superconductor. It was synthesized at high pressure and high temperature conditions using diamond anvil cell combined with in-situ high pressure laser heating techniques. The superconductivity was investigated via resistance measurements at pressures. The highest superconducting transition temperature Tc was found to be ~30 K at 197 GPa in t… ▽ More We report experimental discovery of tantalum polyhydride superconductor. It was synthesized at high pressure and high temperature conditions using diamond anvil cell combined with in-situ high pressure laser heating techniques. The superconductivity was investigated via resistance measurements at pressures. The highest superconducting transition temperature Tc was found to be ~30 K at 197 GPa in the sample that was synthesized at the same pressure with ~2000 K heating. The transitions are shifted to low temperature upon applying magnetic fields that supports the superconductivity nature. The upper critical field at zero temperature μ0Hc2(0) of the superconducting phase is estimated to be ~20 T that corresponds to GL coherent length ~40 angstroms. Our results suggest that the superconductivity may arise from I-43d phase of TaH3. It is, for the first time to our best knowledge, experimental realization of superconducting hydrides for the VB group of transitional metals. △ Less

Submitted 7 June, 2023; v1 submitted 28 December, 2022; originally announced December 2022.

Comments: Published by CPL

Journal ref: Chinese Physics Letters 40, 057404 (2023) (Express Letter)

Trap-Assisted Auger-Meitner Recombination from First Principles

Authors: Fangzhou Zhao, Mark E. Turiansky, Audrius Alkauskas, Chris G. Van de Walle

Abstract: Trap-assisted nonradiative recombination is known to limit the efficiency of optoelectronic devices, but the conventional multi-phonon emission (MPE) process fails to explain the observed loss in wide-band-gap materials. Here we highlight the role of trap-assisted Auger-Meitner (TAAM) recombination, and present a first-principles methodology to determine TAAM rates due to defects or impurities in… ▽ More Trap-assisted nonradiative recombination is known to limit the efficiency of optoelectronic devices, but the conventional multi-phonon emission (MPE) process fails to explain the observed loss in wide-band-gap materials. Here we highlight the role of trap-assisted Auger-Meitner (TAAM) recombination, and present a first-principles methodology to determine TAAM rates due to defects or impurities in semiconductors or insulators. We assess the impact on efficiency of light emitters in a recombination cycle that may include both TAAM and carrier capture via MPE. We apply the formalism to the technologically relevant case study of a calcium impurity in InGaN, where a Shockley-Read-Hall recombination cycle involving MPE alone cannot explain the experimentally observed nonradiative loss. We find that, for band gaps larger than 2.5 eV, the inclusion of TAAM results in recombination rates that are orders of magnitude larger than recombination rates based on MPE alone, demonstrating that TAAM can be a dominant nonradiative process in wide-band-gap materials. Our computational formalism is general and can be applied to the calculation of TAAM rates in any semiconducting or insulating material. △ Less

Submitted 15 November, 2022; originally announced November 2022.

Vortex gap solitons in spin-orbit-coupled Bose-Einstein condensates with competing nonlinearities

Authors: Xiaoxi Xu, Feiyan Zhao, Yangui Zhou, Bin Liu, Xunda Jiang, Boris A. Malomed, Yongyao Li

Abstract: The formation and dynamics of full vortex gap solitons (FVGSs) is investigated in two-component Bose-Einstein condensates with spin-orbit coupling (SOC), Zeeman splitting (ZS), and competing cubic and quintic nonlinear terms, while the usual kinetic energy is neglected, assuming that it is much smaller than the SOC and ZS terms. Unlike previous SOC system with the cubic-only attractive nonlinearit… ▽ More The formation and dynamics of full vortex gap solitons (FVGSs) is investigated in two-component Bose-Einstein condensates with spin-orbit coupling (SOC), Zeeman splitting (ZS), and competing cubic and quintic nonlinear terms, while the usual kinetic energy is neglected, assuming that it is much smaller than the SOC and ZS terms. Unlike previous SOC system with the cubic-only attractive nonlinearity, in which solely semi-vortices may be stable, with the vorticity carried by a single component, the present system supports stable FVGS states, with the vorticity present in both components (such states are called here full vortex solitons, to stress the difference from the half-vortices). They populate the bandgap in the system's linear spectrum. In the case of the cubic self-attraction and quintic repulsion, stable FVGSs with a positive effective mass exist near the top of the bandgap. On the contrary, the system with cubic self-repulsion and quintic attraction produces stable FVGSs with a negative mass near the bottom of the bandgap. Mobility and collisions of FVGSs with different topological charges are investigated too. △ Less

Submitted 7 October, 2022; originally announced October 2022.

Comments: 14 pages,9 figures, 77 references. Communication in Nonlinear Science and Numerical Simulation, in press

Superconductivity above 80 K in polyhydrides of hafnium

Authors: C. L. Zhang, X. He, Z. W. Li, S. J. Zhang, B. S. Min, J. Zhang, K. Lu, J. F. Zhao, L. C. Shi, Y. Peng, X. C. Wang, S. M. Feng, R. C. Yu, L. H. Wang, V. B. Prakapenka, S. Chariton, H. Z. Liu, C. Q. Jin

Abstract: Studies on polyhydrides are attracting growing attentions recently due to their potential high temperature superconductivity (SC). We here report the discovery of SC in hafnium polyhydrides at high pressures. The hafnium superhydrides are synthesized at high pressure and high temperature conditions using diamond anvil cell in combination with in-situ high pressure laser heating technique. The SC w… ▽ More Studies on polyhydrides are attracting growing attentions recently due to their potential high temperature superconductivity (SC). We here report the discovery of SC in hafnium polyhydrides at high pressures. The hafnium superhydrides are synthesized at high pressure and high temperature conditions using diamond anvil cell in combination with in-situ high pressure laser heating technique. The SC was investigated by in-situ high pressure resistance measurements in applied magnetic fields. A superconducting transition with onset Tc ~83 K was observed at 243 GPa. The upper critical field Hc2(0) was estimated to be 24 Tesla by GL theory and the consequent superconducting coherent length to be ~37 angstrom. Our results suggest that the superconducting phase is from C2/m-HfH14. This is the first 5d transition metal polyhydride superconductor with Tc above the liquid nitrogen temperature. △ Less

Submitted 22 August, 2022; v1 submitted 11 August, 2022; originally announced August 2022.

Comments: Accepted by the journal of Material Today Physics (2022)

Heisenberg's uncertainty principle in the PTOLEMY project: a theory update

Authors: PTOLEMY Collaboration, A. Apponi, M. G. Betti, M. Borghesi, A. Boyarsky, N. Canci, G. Cavoto, C. Chang, V. Cheianov, Y. Cheipesh, W. Chung, A. G. Cocco, A. P. Colijn, N. D'Ambrosio, N. de Groot, A. Esposito, M. Faverzani, A. Ferella, E. Ferri, L. Ficcadenti, T. Frederico, S. Gariazzo, F. Gatti, C. Gentile, A. Giachero , et al. (36 additional authors not shown)

Abstract: We discuss the consequences of the quantum uncertainty on the spectrum of the electron emitted by the $β$-processes of a tritium atom bound to a graphene sheet. We analyze quantitatively the issue recently raised in [Cheipesh et al., Phys. Rev. D 104, 116004 (2021)], and discuss the relevant time scales and the degrees of freedom that can contribute to the intrinsic spread in the electron energy.… ▽ More We discuss the consequences of the quantum uncertainty on the spectrum of the electron emitted by the $β$-processes of a tritium atom bound to a graphene sheet. We analyze quantitatively the issue recently raised in [Cheipesh et al., Phys. Rev. D 104, 116004 (2021)], and discuss the relevant time scales and the degrees of freedom that can contribute to the intrinsic spread in the electron energy. We perform careful calculations of the potential between tritium and graphene with different coverages and geometries. With this at hand, we propose possible avenues to mitigate the effect of the quantum uncertainty. △ Less

Submitted 6 September, 2022; v1 submitted 21 March, 2022; originally announced March 2022.

Comments: 14 pages, 5 figures

Journal ref: Phys.Rev.D 106 (2022) 5, 053002

Josephson effects in twisted nodal superconductors

Authors: Pavel A. Volkov, Shu Yang Frank Zhao, Nicola Poccia, Xiaomeng Cui, Philip Kim, J. H. Pixley

Abstract: Motivated by the recent proposals for unconventional emergent physics in twisted bilayers of nodal superconductors, we study the peculiarities of the Josephson effect at the twisted interface between $d$-wave superconductors. We demonstrate that for clean interfaces with a twist angle $θ_0$ in the range $0^\circ<θ_0<45^\circ$ the critical current can exhibit nonmonotonic temperature dependence wit… ▽ More Motivated by the recent proposals for unconventional emergent physics in twisted bilayers of nodal superconductors, we study the peculiarities of the Josephson effect at the twisted interface between $d$-wave superconductors. We demonstrate that for clean interfaces with a twist angle $θ_0$ in the range $0^\circ<θ_0<45^\circ$ the critical current can exhibit nonmonotonic temperature dependence with a maximum at a nonzero temperature as well as a complex dependence on the twist angle at low temperatures. The former is shown to arise quite generically due to the contributions of the momenta around the gap nodes, which are negative for nonzero twist angles. It is demonstrated that these features reflect the geometry of the Fermi surface and are sensitive to the form of the momentum dependence of the tunneling at the twisted interface. Close to $θ_0=45^\circ$ we find that the critical current does not vanish due to Cooper pair cotunneling, which leads to a transition to a time-reversal breaking topological superconducting $d+id$ phase. Weak interface roughness, quasiperiodicity, and inhomogeneity broaden the momentum dependence of the interlayer tunneling leading to a critical current $I_c\sim \cos(2θ_0)$ with $\cos(6θ_0)$ corrections. Furthermore, strong disorder at the interface is demonstrated to suppress the time-reversal breaking superconducting phase near $θ_0=45^\circ$. Last, we provide a comprehensive theoretical analysis of experiments that can reveal the full current-phase relation for twisted superconductors close to $θ_0=45^\circ$. In particular, we demonstrate the emergence of the Fraunhofer interference pattern near $θ_0=45^\circ$, while accounting for realistic sample geometries, and show that its temperature dependence can yield unambiguous evidence of Cooper pair cotunneling, necessary for topological superconductivity. △ Less

Submitted 30 August, 2021; originally announced August 2021.

Journal ref: Phys. Rev. B 111, 014514 (2025)

Emergent Interfacial Superconductivity between Twisted Cuprate Superconductors

Authors: S. Y. Frank Zhao, Nicola Poccia, Xiaomeng Cui, Pavel A. Volkov, Hyobin Yoo, Rebecca Engelke, Yuval Ronen, Ruidan Zhong, Genda Gu, Stephan Plugge, Tarun Tummuru, Marcel Franz, Jedediah H. Pixley, Philip Kim

Abstract: Twisted interfaces between stacked van der Waals cuprate crystals enable tunable Josephson coupling between in-plane anisotropic superconducting order parameters. Employing a novel cryogenic assembly technique, we fabricate Josephson junctions with an atomically sharp twisted interface between Bi2Sr2CaCu2O8+x crystals. The Josephson critical current density sensitively depends on the twist angle,… ▽ More Twisted interfaces between stacked van der Waals cuprate crystals enable tunable Josephson coupling between in-plane anisotropic superconducting order parameters. Employing a novel cryogenic assembly technique, we fabricate Josephson junctions with an atomically sharp twisted interface between Bi2Sr2CaCu2O8+x crystals. The Josephson critical current density sensitively depends on the twist angle, reaching the maximum value comparable to that of the intrinsic junctions at small twisting angles, and is suppressed by almost 2 orders of magnitude yet remains finite close to 45 degree twist angle. Through the observation of fractional Shapiro steps and the analysis of Fraunhofer patterns we show that the remaining superconducting coherence near 45 degree is due to the co-tunneling of Cooper pairs, a necessary ingredient for high-temperature topological superconductivity. △ Less

Submitted 30 August, 2021; originally announced August 2021.

Spin Splitting of Dopant Edge States in Magnetic Zigzag Graphene Nanoribbons

Authors: Raymond E. Blackwell, Fangzhou Zhao, Erin Brooks, Junmian Zhu, Ilya Piskun, Shenkai Wang, Aidan Delgado, Yea-Lee Lee, Steven G. Louie, Felix R. Fischer

Abstract: Spin-ordered electronic states in hydrogen-terminated zigzag nanographene give rise to magnetic quantum phenomena that have sparked renewed interest in carbon-based spintronics. Zigzag graphene nanoribbons (ZGNRs), quasi one-dimensional semiconducting strips of graphene featuring two parallel zigzag edges along the main axis of the ribbon, are predicted to host intrinsic electronic edge states tha… ▽ More Spin-ordered electronic states in hydrogen-terminated zigzag nanographene give rise to magnetic quantum phenomena that have sparked renewed interest in carbon-based spintronics. Zigzag graphene nanoribbons (ZGNRs), quasi one-dimensional semiconducting strips of graphene featuring two parallel zigzag edges along the main axis of the ribbon, are predicted to host intrinsic electronic edge states that are ferromagnetically ordered along the edges of the ribbon and antiferromagnetically coupled across its width. Despite recent advances in the bottom-up synthesis of atomically-precise ZGNRs, their unique electronic structure has thus far been obscured from direct observations by the innate chemical reactivity of spin-ordered edge states. Here we present a general technique for passivating the chemically highly reactive spin-polarized edge states by introducing a superlattice of substitutional nitrogen-dopants along the edges of a ZGNR. First-principles GW calculations and scanning tunneling spectroscopy reveal a giant spin splitting of the low-lying nitrogen lone-pair flat bands by a large exchange field (~850 Tesla) induced by the spin-polarized ferromagnetically ordered edges of ZGNRs. Our findings directly corroborate the nature of the predicted emergent magnetic order in ZGNRs and provide a robust platform for their exploration and functional integration into nanoscale sensing and logic devices. △ Less

Submitted 1 June, 2021; originally announced June 2021.

Comments: 7 pages, 4 main figures, supplemental information provided

Superconductivity above 200 K Observed in Superhydrides of Calcium

Authors: Z. Li, X. He, C. L. Zhang, X. C. Wang, S. J. Zhang, Y. T. Jia, S. M. Feng, K. Lu, J. F. Zhao, J. Zhang, B. S. Min, Y. W. Long, R. C. Yu, L. H. Wang, M. Y. Ye, Z. S. Zhang, V. Prakapenka, S. Chariton, P. A. Ginsberg, J. Bass, S. H. Yuan, H. Z. Liu, C. Q. Jin

Abstract: Searching for superconductivity with Tc near room temperature is of great interest both for fundamental science & many potential applications. Here we report the experimental discovery of superconductivity with maximum critical temperature (Tc) above 210 K in calcium superhydrides, the new alkali earth hydrides experimentally showing superconductivity above 200 K in addition to sulfur hydride & ra… ▽ More Searching for superconductivity with Tc near room temperature is of great interest both for fundamental science & many potential applications. Here we report the experimental discovery of superconductivity with maximum critical temperature (Tc) above 210 K in calcium superhydrides, the new alkali earth hydrides experimentally showing superconductivity above 200 K in addition to sulfur hydride & rare-earth hydride system. The materials are synthesized at the synergetic conditions of 160~190 GPa and ~2000 K using diamond anvil cell combined with in-situ laser heating technique. The superconductivity was studied through in-situ high pressure electric conductance measurements in an applied magnetic field for the sample quenched from high temperature while maintained at high pressures. The upper critical field Hc(0) was estimated to be ~268 T while the GL coherent length is ~11 angstrom. The in-situ synchrotron X-ray diffraction measurements suggest that the synthesized calcium hydrides are primarily composed of CaH6 while there may also exist other calcium hydrides with different hydrogen contents. △ Less

Submitted 9 June, 2022; v1 submitted 31 March, 2021; originally announced March 2021.

Journal ref: Nature Communications 13, 2863 (2022)

Electric Field Tunable Topological Phases in Graphene Nanoribbons

Authors: Fangzhou Zhao, Ting Cao, Steven G. Louie

Abstract: Graphene nanoribbons (GNRs) possess distinct symmetry-protected topological phases. We show, through first-principles calculations, that by applying an experimentally accessible transverse electric field (TEF), certain boron and nitrogen periodically co-doped GNRs have tunable topological phases. The tunability arises from a field-induced band inversion due to an opposite response of the conductio… ▽ More Graphene nanoribbons (GNRs) possess distinct symmetry-protected topological phases. We show, through first-principles calculations, that by applying an experimentally accessible transverse electric field (TEF), certain boron and nitrogen periodically co-doped GNRs have tunable topological phases. The tunability arises from a field-induced band inversion due to an opposite response of the conduction- and valance-band states to the electric field. With a spatially-varying applied field, segments of GNRs of distinct topological phases are created, resulting in a field-programmable array of topological junction states, each may be occupied with charge or spin. Our findings not only show that electric field may be used as an easy tuning knob for topological phases in quasi-one-dimensional systems, but also provide new design principles for future GNR-based quantum electronic devices through their topological characters. △ Less

Submitted 8 February, 2021; originally announced February 2021.

Journal ref: Phys. Rev. Lett. 127, 166401 (2021)

Quantum battery of interacting spins with environmental noise

Authors: Fang Zhao, Fu-Quan Dou, Qing Zhao

Abstract: A quantum battery is a temporary energy-storage system. We constructed the quantum battery model of an N-spin chain with nearest-neighbor hopping interaction and investigated the charging process of the quantum battery. We obtained the maximum energy in the quantum battery charged by a coherent cavity driving field or a thermal heat bath. We confirmed that for a finite-length spin chain, thermal c… ▽ More A quantum battery is a temporary energy-storage system. We constructed the quantum battery model of an N-spin chain with nearest-neighbor hopping interaction and investigated the charging process of the quantum battery. We obtained the maximum energy in the quantum battery charged by a coherent cavity driving field or a thermal heat bath. We confirmed that for a finite-length spin chain, thermal charging results in a nonzero ergotropy, contradicting a previous result: that an incoherent heat source cannot charge a single-spin quantum battery. The nearest-neighbor hopping interaction induces energy band splitting, which enhances the energy storage and the ergotropy of the quantum battery. We found a critical point in the energy and ergotropy resulting from the ground-state quantum phase transition, after which the energy significantly enhance. Finally, we also found that disorder increased the energy of the quantum battery. △ Less

Submitted 11 December, 2020; originally announced December 2020.

Journal ref: Phys. Rev. A 103, 033715 (2021)

Spatially correlated incommensurate lattice modulations in an atomically thin high-temperature Bi_{2.1}Sr_{1.9}CaCu_{2.0}O_{8+δ} superconductors

Authors: Nicola Poccia, Shu Yang Frank Zhao, Hyobin Yoo, Xiaojing Huang, Hanfei Yan, Yong S. Chu, Ruidan Zhong, Genda Gu, Claudio Mazzoli, Kenji Watanabe, Takashi Taniguchi, Gaetano Campi, Valerii M. Vinokur, Philip Kim

Abstract: Strong variations in superconducting critical temperatures in different families of the cuprate perovskites, even with similar hole doping in their copper-oxygen planes, suggest the importance of lattice modulation effects. The one-dimensional incommensurate lattice modulation (ILM) of Bi_2Sr_2CaCu_2O_{8+y}, with the average atomic positions perturbed beyond the unit cell, offers an ideal test gro… ▽ More Strong variations in superconducting critical temperatures in different families of the cuprate perovskites, even with similar hole doping in their copper-oxygen planes, suggest the importance of lattice modulation effects. The one-dimensional incommensurate lattice modulation (ILM) of Bi_2Sr_2CaCu_2O_{8+y}, with the average atomic positions perturbed beyond the unit cell, offers an ideal test ground for studying the interplay between superconductivity and the long-range incommensurate lattice fluctuations. Here we report Scanning nano X-ray Diffraction (SnXRD) imaging of incommensurate lattice modulations in Bi_{2.1}Sr_{1.9}CaCu_{2.0}O_{8+δ} Van der Waals heterostructures of thicknesses down to two-unit cells. Using SnXRD, we probe that the long-range and short-range incommensurate lattice modulations in bulk sample surface with spatial resolution below 100 nm. We find that puddle-like domains of ILM of size uniformly evolving with dimensionality. In the 2-unit cell thin sample, it is observed that the wavevectors of the long- and short-range orders become anti-correlated with emerging spatial patterns having a directional gradient. The emerging patterns, originated by tiny tuning of lattice strain, induce static mesoscopic charge density waves. Our findings thus demonstrate that the strain can be used to tune and control the electromagnetic properties of two-dimensional high-temperature superconductors. △ Less

Submitted 7 August, 2020; v1 submitted 27 July, 2020; originally announced July 2020.

Journal ref: Phys. Rev. Materials 4, 114007 (2020)

Reply to P. Ao's Comment on "Sign reversing Hall effect in atomically thin high temperature superconductors"

Authors: S. Y. Frank Zhao, Nicola Poccia, Margaret G. Panetta, Cyndia Yu, Jedediah W. Johnson, Hyobin Yoo, Ruidan Zhong, G. D. Gu, Kenji Watanabe, Takashi Taniguchi, Svetlana V. Postolova, Valerii M. Vinokur, Philip Kim

Abstract: We respond to P. Ao's comment in arXiv:1907.09263, which suggests that vortex many-body effects are the origin of Hall sign reversal in few-unit-cell thick Bi-2212 cuprate crystals (Phys. Rev. Lett. 122, 247001 (2019)). Our experimental results are incompatible with the theoretical predictions detailed in Ao's comment. We respond to P. Ao's comment in arXiv:1907.09263, which suggests that vortex many-body effects are the origin of Hall sign reversal in few-unit-cell thick Bi-2212 cuprate crystals (Phys. Rev. Lett. 122, 247001 (2019)). Our experimental results are incompatible with the theoretical predictions detailed in Ao's comment. △ Less

Submitted 7 August, 2019; originally announced August 2019.

Comments: 1 page, 1 figure with inset

Threshold Voltage Improvement and Leakage Reduction of AlGaN/GaN HEMTs Using Dual-Layer SiNx Stressors

Authors: Wei-Chih Cheng, Minghao He, Siqi Lei, Liang Wang, Jingyi Wu, Fanming Zeng, Qiaoyu Hu, Feng Zhao, Mansun Chan, Guangrui, Xia, Hongyu Yu

Abstract: In this work, AlGaN/GaN HEMTs with dual-layer SiNx stressors (composed of a low-stress layer and a high-stress layer) were investigated. The low-stress padding layer solved the surface damage problem caused during the deposition of the high-stress SiNx, and provided a good passivated interface. The HEMTs with dual-layer stressors showed a 1 V increase in the threshold voltage (Vth) with comparable… ▽ More In this work, AlGaN/GaN HEMTs with dual-layer SiNx stressors (composed of a low-stress layer and a high-stress layer) were investigated. The low-stress padding layer solved the surface damage problem caused during the deposition of the high-stress SiNx, and provided a good passivated interface. The HEMTs with dual-layer stressors showed a 1 V increase in the threshold voltage (Vth) with comparable on-current and RF current gain to those without stressors. Moreover, the off-current (I_off) was shown to be reduced by one to three orders of magnitude in the strained devices as a result of the lower electric field in AlGaN, which suppressed the gate injection current. The dual-layer stressor scheme supports strain engineering as an effective approach in the pursuit of the normally-off operation of AlGaN/GaN HEMTs. △ Less

Submitted 31 July, 2019; originally announced August 2019.

Spin-valley locking, bulk quantum Hall effect and chiral surface state in a noncentrosymmetric Dirac semimetal BaMnSb$_2$

Authors: J. Y. Liu, J. Yu, J. L. Ning, H. M. Yi, L. Miao, L. J. Min, Y. F. Zhao, W. Ning, K. A. Lopez, Y. L. Zhu, T. Pillsbury, Y. B. Zhang, Y. Wang, J. Hu, H. B. Cao, F. Balakirev, F. Weickert, M. Jaime, Y. Lai, Kun Yang, J. W. Sun, N. Alem, V. Gopalan, C. Z. Chang, N. Samarth , et al. (3 additional authors not shown)

Abstract: Spin-valley locking in the band structure of monolayers of MoS$_2$ and other group-VI dichalcogenides has attracted enormous interest, since it offers potential for valleytronic and optoelectronic applications. Such an exotic electronic state has sparsely been seen in bulk materials. Here, we report spin-valley locking in a bulk Dirac semimetal BaMnSb$_2$. We find valley and spin are inherently co… ▽ More Spin-valley locking in the band structure of monolayers of MoS$_2$ and other group-VI dichalcogenides has attracted enormous interest, since it offers potential for valleytronic and optoelectronic applications. Such an exotic electronic state has sparsely been seen in bulk materials. Here, we report spin-valley locking in a bulk Dirac semimetal BaMnSb$_2$. We find valley and spin are inherently coupled for both valence and conduction bands in this material. This is revealed by comprehensive studies using first principle calculations, tight-binding and effective model analyses, angle-resolved photoemission spectroscopy and quantum transport measurements. Moreover, this material also exhibits a stacked quantum Hall effect. The spin-valley degeneracy extracted from the plateau height of quantized Hall resistivity is close to 2. This result, together with the observed Landau level spin splitting, further confirms the spin-valley locking picture. In the extreme quantum limit, we have also observed a two-dimensional chiral metal at the side surface, which represents a novel topological quantum liquid. These findings establish BaMnSb$_2$ as a rare platform for exploring coupled spin and valley physics in bulk single crystals and accessing 3D interacting topological states. △ Less

Submitted 4 November, 2020; v1 submitted 14 July, 2019; originally announced July 2019.

Comments: Updated acknowledgement

Report number: NATURE COMMUNICATIONS 12:4062 (2021)

Journal ref: NATURE COMMUNICATIONS 12:4062 (2021)

Evidence for Helical Hinge Zero Modes in an Fe-Based Superconductor

Authors: Mason J. Gray, Josef Freudenstein, Shu Yang F. Zhao, Ryan OConnor, Samuel Jenkins, Narendra Kumar, Marcel Hoek, Abigail Kopec, Takashi Taniguchi, Kenji Watanabe, Ruidan Zhong, G. D. Gu, K. S. Burch

Abstract: Combining topology and superconductivity provides a powerful tool for investigating fundamental physics as well as a route to fault-tolerant quantum computing. There is mounting evidence that the Fe-Based Superconductor FeTe$_{0.55}$Se$_{0.45}$ (FTS) may also be topologically non-trivial. Should the superconducting order be s$^{\pm}$, then FTS could be a higher order topological superconductor wit… ▽ More Combining topology and superconductivity provides a powerful tool for investigating fundamental physics as well as a route to fault-tolerant quantum computing. There is mounting evidence that the Fe-Based Superconductor FeTe$_{0.55}$Se$_{0.45}$ (FTS) may also be topologically non-trivial. Should the superconducting order be s$^{\pm}$, then FTS could be a higher order topological superconductor with Helical Hinge Zero Modes (HHZM).To test the presence of these modes we developed a new method for making normal metal/superconductor junctions via 2D atomic crystal heterostructures. As expected,junctions in contact with the hinge reveal a sharp zero-bias anomaly whose suppression with temperature and magnetic field only along the c-axis are completely consistent with the presence of HHZM. This feature is completely absent when tunneling purely into the c-axis, and its characteristics are also inconsistent with other origins of zero bias anomalies. Furthermore, additional measurements with soft-point contacts in bulk samples with various Fe interstitial contents demonstrate the intrinsic nature of the observed mode. Thus we provide evidence that FTS is indeed a higher order topological superconductor as well as a new method for probing 2D atomic crystals. △ Less

Submitted 10 July, 2019; v1 submitted 27 February, 2019; originally announced February 2019.

Direct observation of corner states in second-order topological photonic crystal slabs

Authors: Xiao-Dong Chen, Wei-Min Deng, Fu-Long Shi, Fu-Li Zhao, Min Chen, Jian-Wen Dong

Abstract: Recently, higher-order topological phases that do not obey the usual bulk-edge correspondence principle have been introduced in electronic insulators and brought into classical systems, featuring with in-gap corner/hinge states. So far, second-order topological insulators have been realized in mechanical metamaterials, microwave circuit, topolectrical circuit and acoustic metamaterials. Here, usin… ▽ More Recently, higher-order topological phases that do not obey the usual bulk-edge correspondence principle have been introduced in electronic insulators and brought into classical systems, featuring with in-gap corner/hinge states. So far, second-order topological insulators have been realized in mechanical metamaterials, microwave circuit, topolectrical circuit and acoustic metamaterials. Here, using near-field scanning measurements, we show the direct observation of corner states in second-order topological photonic crystal (PC) slabs consisting of periodic dielectric rods on a perfect electric conductor (PEC). Based on the generalized two-dimensional (2D) Su-Schrieffer-Heeger (SSH) model, we show that the emergence of corner states roots in the nonzero edge dipolar polarization instead of the nonzero bulk quadrupole polarization. We demonstrate the topological transition of 2D Zak phases of PC slabs by tuning intra-cell distances between two neighboring rods. We also directly observe in-gap 1D edge states and 0D corner states in the microwave regime. Our work presents that the PC slab is a powerful platform to directly observe topological states, and paves the way to study higher-order photonic topological insulators. △ Less

Submitted 22 December, 2018; v1 submitted 18 December, 2018; originally announced December 2018.

Journal ref: Phys. Rev. Lett. 122, 233902 (2019)

Topological Phases in Cove-Edged and Chevron Graphene Nanoribbons: Geometric Structures, Z2 Invariants, and Junction States

Authors: Yea-Lee Lee, Fangzhou Zhao, Ting Cao, Jisoon Ihm, Steven G. Louie

Abstract: Graphene nanoribbons (GNRs) have recently been shown by Cao, Zhao, and Louie [Cao, T.; Zhao, F.; Louie, S. G. Phys. Rev. Lett. 2017, 119, 076401] to possess distinct topological phases in general, characterized by a Z2 invariant. Cove-edged and chevron GNRs moreover are chemically and structurally diverse, quasi-one-dimensional (1D) nanostructures whose structure and electronic properties can be r… ▽ More Graphene nanoribbons (GNRs) have recently been shown by Cao, Zhao, and Louie [Cao, T.; Zhao, F.; Louie, S. G. Phys. Rev. Lett. 2017, 119, 076401] to possess distinct topological phases in general, characterized by a Z2 invariant. Cove-edged and chevron GNRs moreover are chemically and structurally diverse, quasi-one-dimensional (1D) nanostructures whose structure and electronic properties can be rationally controlled by bottom-up synthesis from precursor molecules. We derive the value of the topological invariant of the different types of cove-edged and chevron GNRs, and we investigate the electronic properties of various junctions formed by these GNRs, as well as such GNRs with the more common armchair or zigzag GNRs. We study the topological junction states at the interface of two topologically distinct segments. For an isolated GNR having two ends of different terminations, topological end states are shown to develop only at the topologically nontrivial end. Our work extends the explicit categorization of topological invariants of GNRs beyond the previously demonstrated armchair GNRs and provides new design rules for novel GNR junctions as well as future GNR-based nanoelectronic devices. △ Less

Submitted 11 October, 2018; originally announced October 2018.

Journal ref: Nano Lett., 2018, Article ASAP

Sign reversing Hall effect in atomically thin high temperature superconductors

Authors: S. Y. Frank Zhao, Nicola Poccia, Margaret G. Panetta, Cyndia Yu, Jedediah W. Johnson, Hyobin Yoo, Ruidan Zhong, G. D. Gu, Kenji Watanabe, Takashi Taniguchi, Svetlana V. Postolova, Valerii M. Vinokur, Philip Kim

Abstract: We fabricate van der Waals heterostructure devices using few unit cell thick Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ for magnetotransport measurements. The superconducting transition temperature and carrier density in atomically thin samples can be maintained to close to that of the bulk samples. As in the bulk sample, the sign of the Hall conductivity is found to be opposite to the normal state near the tr… ▽ More We fabricate van der Waals heterostructure devices using few unit cell thick Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ for magnetotransport measurements. The superconducting transition temperature and carrier density in atomically thin samples can be maintained to close to that of the bulk samples. As in the bulk sample, the sign of the Hall conductivity is found to be opposite to the normal state near the transition temperature but with a drastic enlargement of the region of Hall sign reversal in the temperature-magnetic field phase diagram as the thickness of samples decreases. Quantitative analysis of the Hall sign reversal based on the excess charge density in the vortex core and superconducting fluctuations suggests a renormalized superconducting gap in atomically thin samples at the 2-dimensional limit. △ Less

Submitted 5 October, 2018; v1 submitted 18 September, 2018; originally announced September 2018.

Comments: 5 pages, 3 figures. Supplementary information is not included

Journal ref: Phys. Rev. Lett. 122, 247001 (2019)

Superconductivity in a unique type of copper oxide

Authors: W. M. Li, J. F. Zhao, L. P. Cao, Z. Hu, Q. Z. Huang, X. C. Wang, Y. Liu, G. Q. Zhao, J. Zhang, Q. Q. Liu, R. Z. Yu, Y. W. Long H. Wu, H. J. Lin, C. T. Chen, Z. Li, Z. Z. Gong, Z. Guguchia, J. S. Kim, G. R. Stewart, Y. J. Uemura. S. Uchida, C. Q Jin

Abstract: The mechanism of superconductivity in cuprates remains one of the big challenges of condensed matter physics.High Tc cuprates crystallize into layered perovskite structure featuring copper oxygen octahedral coordination. Due to the Jahn Teller effect in combination with the strong static Coulomb interaction, the octahedra in high Tc cuprates are elongated along the c axis, leading to a 3dx2-y2 orb… ▽ More The mechanism of superconductivity in cuprates remains one of the big challenges of condensed matter physics.High Tc cuprates crystallize into layered perovskite structure featuring copper oxygen octahedral coordination. Due to the Jahn Teller effect in combination with the strong static Coulomb interaction, the octahedra in high Tc cuprates are elongated along the c axis, leading to a 3dx2-y2 orbital at the top of the band structure wherein the doped holes reside.This scenario gives rise to two dimensional characteristics in high Tc cuprates that favor d wave pairing symmetry. Here we report superconductivity in a cuprate Ba2CuO4-y wherein the local octahedron is in a very exceptional compressed version.The Ba2CuO4-y compound was synthesized at high pressure at high temperatures, and shows bulk superconductivity with critical temperature Tc above 70 K at ambient conditions. This superconducting transition temperature is more than 30 K higher than the Tc for the isostructural counterparts based on classical La2CuO4. X-ray absorption measurements indicate the heavily doped nature of the Ba2CuO4-y superconductor. In compressed octahedron the 3d3z2-r2 orbital will be lifted above the 3dx2-y2 orbital, leading to significant three dimensional nature in addition to the conventional 3dx2-y2 orbital. This work sheds important new light on advancing our comprehensive understanding of the superconducting mechanism of high Tc in cuprate materials. △ Less

Submitted 2 August, 2019; v1 submitted 28 August, 2018; originally announced August 2018.

Comments: Total 28 pages incuding 9 Figures

Topological Band Engineering of Graphene Nanoribbons

Authors: Daniel J. Rizzo, Gregory Veber, Ting Cao, Christopher Bronner, Ting Chen, Fangzhou Zhao, Henry Rodriguez, Steven G. Louie Michael F. Crommie, Felix R. Fischer

Abstract: Topological insulators (TIs) are an emerging class of materials that host highly robust in-gap surface/interface states while maintaining an insulating bulk. While most notable scientific advancements in this field have been focused on TIs and related topological crystalline insulators in 2D and 3D, more recent theoretical work has predicted the existence of 1D symmetry-protected topological phase… ▽ More Topological insulators (TIs) are an emerging class of materials that host highly robust in-gap surface/interface states while maintaining an insulating bulk. While most notable scientific advancements in this field have been focused on TIs and related topological crystalline insulators in 2D and 3D, more recent theoretical work has predicted the existence of 1D symmetry-protected topological phases in graphene nanoribbons (GNRs). The topological phase of these laterally-confined, semiconducting strips of graphene is determined by their width, edge shape, and the terminating unit cell, and is characterized by a Z2 invariant (similar to 1D solitonic systems). Interfaces between topologically distinct GNRs characterized by different Z2 are predicted to support half-filled in-gap localized electronic states which can, in principle, be utilized as a tool for material engineering. Here we present the rational design and experimental realization of a topologically-engineered GNR superlattice that hosts a 1D array of such states, thus generating otherwise inaccessible electronic structure. This strategy also enables new end states to be engineered directly into the termini of the 1D GNR superlattice. Atomically-precise topological GNR superlattices were synthesized from molecular precursors on a Au(111) surface under ultra-high vacuum (UHV) conditions and characterized by low temperature scanning tunneling microscopy (STM) and spectroscopy (STS). Our experimental results and first-principles calculations reveal that the frontier band structure of these GNR superlattices is defined purely by the coupling between adjacent topological interface states. This novel manifestation of 1D topological phases presents an entirely new route to band engineering in 1D materials based on precise control of their electronic topology, and is a promising platform for future studies of 1D quantum spin physics. △ Less

Submitted 16 May, 2018; originally announced May 2018.

Comments: Contains main manuscript and supplemental information

Heterointerface effects in the electro-intercalation of van der Waals heterostructures

Authors: D. Kwabena Bediako, Mehdi Rezaee, Hyobin Yoo, Daniel T. Larson, Shu Yang Frank Zhao, Takashi Taniguchi, Kenji Watanabe, Tina L. Brower-Thomas, Efthimios Kaxiras, Philip Kim

Abstract: Molecular-scale manipulation of electronic/ionic charge accumulation in materials is a preeminent challenge, particularly in electrochemical energy storage. Layered van der Waals (vdW) crystals exemplify a diverse family of materials that permit ions to reversibly associate with a host atomic lattice by intercalation into interlamellar gaps. Motivated principally by the search for high-capacity ba… ▽ More Molecular-scale manipulation of electronic/ionic charge accumulation in materials is a preeminent challenge, particularly in electrochemical energy storage. Layered van der Waals (vdW) crystals exemplify a diverse family of materials that permit ions to reversibly associate with a host atomic lattice by intercalation into interlamellar gaps. Motivated principally by the search for high-capacity battery anodes, ion intercalation in composite materials is a subject of intense study. Yet the precise role and ability of heterolayers to modify intercalation reactions remains elusive. Previous studies of vdW hybrids represented ensemble measurements at macroscopic films/powders, which do not permit the isolation and investigation of the chemistry at individual 2-dimensional (2D) interfaces. Here, we demonstrate the intercalation of lithium at the level of individual atomic interfaces of dissimilar vdW layers. Electrochemical devices based on vdW heterostructures comprised of deterministically stacked hexagonal boron nitride, graphene (G) and molybdenum dichalcogenide (MoX2; X = S, Se) layers are fabricated, enabling the direct resolution of intermediate stages in the intercalation of discrete heterointerfaces and the extent of charge transfer to individual layers. Operando magnetoresistance and optical spectroscopy coupled with low-temperature quantum magneto-oscillation measurements show that the creation of intimate vdW heterointerfaces between G and MoX2 engenders over 10-fold accumulation of charge in MoX2 compared to MoX2/MoX2 homointerfaces, while enforcing a more negative intercalation potential than that of bulk MoX2 by at least 0.5 V. Beyond energy storage, our new combined experimental and computational methodology to manipulate and characterize the electrochemical behavior of layered systems opens up new pathways to control the charge density in 2D (opto)electronic devices. △ Less

Submitted 26 March, 2018; v1 submitted 9 November, 2017; originally announced November 2017.

Comments: 10 pages, 5 Figures, Methods, and Supplementary Information

Controlled Electrochemical Intercalation of Graphene/h-BN van der Waals Heterostructures

Authors: S. Y. Frank Zhao, Giselle A. Elbaz, D. Kwabena Bediako, Cyndia Yu, Dmitri K. Efetov, Yinsheng Guo, Jayakanth Ravichandran, Kyung-Ah Min, Suklyun Hong, Takashi Taniguchi, Kenji Watanabe, Louis E. Brus, Xavier Roy, Philip Kim

Abstract: Electrochemical intercalation is a powerful method for tuning the electronic properties of layered solids. In this work, we report an electro-chemical strategy to controllably intercalate lithium ions into a series of van der Waals (vdW) heterostructures built by sandwiching graphene between hexagonal boron nitride (h-BN). We demonstrate that encapsulating graphene with h-BN eliminates parasitic s… ▽ More Electrochemical intercalation is a powerful method for tuning the electronic properties of layered solids. In this work, we report an electro-chemical strategy to controllably intercalate lithium ions into a series of van der Waals (vdW) heterostructures built by sandwiching graphene between hexagonal boron nitride (h-BN). We demonstrate that encapsulating graphene with h-BN eliminates parasitic surface side reactions while simultaneously creating a new hetero-interface that permits intercalation between the atomically thin layers. To monitor the electrochemical process, we employ the Hall effect to precisely monitor the intercalation reaction. We also simultaneously probe the spectroscopic and electrical transport properties of the resulting intercalation compounds at different stages of intercalation. We achieve the highest carrier density $> 5 \times 10^{13} cm^{-2}$ with mobility $> 10^3 cm^2/(Vs)$ in the most heavily intercalated samples, where Shubnikov-de Haas quantum oscillations are observed at low temperatures. These results set the stage for further studies that employ intercalation in modifying properties of vdW heterostructures. △ Less

Submitted 21 October, 2017; originally announced October 2017.

Plasmon reflections by topological electronic boundaries in bilayer graphene

Authors: Bor-Yuan Jiang, Guang-Xin Ni, Zachariah Addison, Jing K. Shi, Xiaomeng Liu, Frank Zhao, P. Kim, E. J. Mele, D. N. Basov, M. M. Fogler

Abstract: Domain walls separating regions of AB and BA interlayer stacking in bilayer graphene have attracted attention as novel examples of structural solitons, topological electronic boundaries, and nanoscale plasmonic scatterers. We show that strong coupling of domain walls to surface plasmons observed in infrared nanoimaging experiments is due to topological chiral modes confined to the walls. The optic… ▽ More Domain walls separating regions of AB and BA interlayer stacking in bilayer graphene have attracted attention as novel examples of structural solitons, topological electronic boundaries, and nanoscale plasmonic scatterers. We show that strong coupling of domain walls to surface plasmons observed in infrared nanoimaging experiments is due to topological chiral modes confined to the walls. The optical transitions among these chiral modes and the band continua enhance the local ac conductivity, which leads to plasmon reflection by the domain walls. The imaging reveals two kinds of plasmonic standing-wave interference patterns, which we attribute to shear and tensile domain walls. We compute the electronic structure of both wall varieties and show that the tensile wall contain additional confined bands which produce a structure-specific contrast of the local conductivity. The calculated plasmonic interference profiles are in quantitative agreement with our experiments. △ Less

Submitted 13 July, 2017; originally announced July 2017.

Comments: 14 pages, 5 figures

Andreev Reflection without Fermi surface alignment in High T$_{c}$-Topological heterostructures

Authors: Parisa Zareapour, Alex Hayat, Shu Yang F. Zhao, Michael Kreshchuk, Zhijun Xu, T. S. Liu, G. D. Gu, Shuang Jia, Robert J. Cava, H. -Y. Yang, Ying Ran, Kenneth S. Burch

Abstract: We address the controversy over the proximity effect between topological materials and high T$_{c}$ superconductors. Junctions are produced between Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ and materials with different Fermi surfaces (Bi$_{2}$Te$_{3}$ \& graphite). Both cases reveal tunneling spectra consistent with Andreev reflection. This is confirmed by magnetic field that shifts features via the Dop… ▽ More We address the controversy over the proximity effect between topological materials and high T$_{c}$ superconductors. Junctions are produced between Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ and materials with different Fermi surfaces (Bi$_{2}$Te$_{3}$ \& graphite). Both cases reveal tunneling spectra consistent with Andreev reflection. This is confirmed by magnetic field that shifts features via the Doppler effect. This is modeled with a single parameter that accounts for tunneling into a screening supercurrent. Thus the tunneling involves Cooper pairs crossing the heterostructure, showing the Fermi surface mis-match does not hinder the ability to form transparent interfaces, which is accounted for by the extended Brillouin zone and different lattice symmetries. △ Less

Submitted 9 May, 2017; originally announced May 2017.

Journal ref: New J. Phys. 19 (2017) 043026

Topological Phases in Graphene Nanoribbons: Junction States, Spin Centers and Quantum Spin Chains

Authors: Ting Cao, Fangzhou Zhao, Steven G. Louie

Abstract: Knowledge of the topology of the electronic ground state of materials has led to deep insights to novel phenomena such as the integer quantum Hall effect and fermion-number fractionalization, as well as other properties of matter. Joining two insulators of different topological classes produces fascinating boundary states in the band gap. Another exciting recent development is the bottom-up synthe… ▽ More Knowledge of the topology of the electronic ground state of materials has led to deep insights to novel phenomena such as the integer quantum Hall effect and fermion-number fractionalization, as well as other properties of matter. Joining two insulators of different topological classes produces fascinating boundary states in the band gap. Another exciting recent development is the bottom-up synthesis (from molecular precursors) of graphene nanoribbons (GNRs) with atomic precision control of their edge and width. Here we connect these two fields, and show for the first time that semiconducting GNRs of different width, edge, and end termination belong to different topological classes. The topology of GNRs is protected by spatial symmetries and dictated by the terminating unit cell. We have derived explicit formula for their topological invariants, and show that localized junction states developed between two GNRs of distinct topology may be tuned by lateral junction geometry. The topology of a GNR can be further modified by dopants, such as a periodic array of boron atoms. In a superlattice consisted of segments of doped and pristine GNRs, the junction states are stable spin centers, forming a Heisenberg antiferromagnetic spin 1/2 chain with tunable exchange interaction. The discoveries here are not only of scientific interest for studies of quasi one-dimensional systems, but also open a new path for design principles of future GNR-based devices through their topological characters. △ Less

Submitted 15 February, 2017; v1 submitted 8 February, 2017; originally announced February 2017.

Comments: Corrected typos

Journal ref: Phys. Rev. Lett. 119, 076401 (2017)

Modeling Tunneling for the Unconventional Superconducting Proximity Effect

Authors: Parisa Zareapour, Jianwei Xu, Shu Yang F. Zhao, Achint Jain, Zhijun Xu, T. S. Liu, G. D. Gu, Kenneth S. Burch

Abstract: Recently there has been reinvigorated interest in the superconducting proximity effect, driven by predictions of the emergence of Majorana fermions. To help guide this search, we have developed a phenomenological model for the tunneling spectra in anisotropic superconductor-normal metal proximity devices. We combine successful approaches used in s-wave proximity and standard d-wave tunneling to re… ▽ More Recently there has been reinvigorated interest in the superconducting proximity effect, driven by predictions of the emergence of Majorana fermions. To help guide this search, we have developed a phenomenological model for the tunneling spectra in anisotropic superconductor-normal metal proximity devices. We combine successful approaches used in s-wave proximity and standard d-wave tunneling to reproduce tunneling spectra in d-wave proximity devices, and clarify the origin of various features. Different variations of the pair potential are considered, resulting from the proximity-induced superconductivity. Furthermore, the effective pair potential felt by the quasiparticles is momentum-dependent in contrast to s-wave superconductors. The probabilities of reflection and transmission are calculated by solving the Bogoliubov equations. Our results are consistent with experimental observations of the unconventional proximity effect and provide important experimental parameters such as the size and length scale of the proximity induced gap, as well as the conditions needed to observe the reduced and full superconducting gaps. △ Less

Submitted 4 November, 2016; originally announced November 2016.

Comments: 12 pages, 4 figures

Journal ref: Superconductor Science and Technology,29 125006 (2016)

Macro deformation twins in single-crystal aluminum

Authors: F. Zhao, L. Wang, D. Fan, B. X. Bie, X. M. Zhou, T. Suo, Y. L. Li, M. W. Chen, C. Liu, M. L. Qi, M. H. Zhu, S. N. Luo

Abstract: Deformation twinning in pure aluminum has been considered to be a unique property of nanostructured aluminum. A lingering mystery is whether deformation twinning occurs in coarse-grained or single-crystal aluminum, at scales beyond nanotwins. Here, we present the first experimental demonstration of macro deformation twins in single-crystal aluminum formed under ultrahigh strain-rate ($\sim$10$^6$… ▽ More Deformation twinning in pure aluminum has been considered to be a unique property of nanostructured aluminum. A lingering mystery is whether deformation twinning occurs in coarse-grained or single-crystal aluminum, at scales beyond nanotwins. Here, we present the first experimental demonstration of macro deformation twins in single-crystal aluminum formed under ultrahigh strain-rate ($\sim$10$^6$ s$^{-1}$), large shear strain (200$\%$) via dynamic equal channel angular pressing. Deformation twinning is rooted in the rate dependences of dislocation motion and twinning, which are coupled, complementary processes during severe plastic deformation under ultrahigh strain rates. △ Less

Submitted 15 October, 2015; originally announced October 2015.

Comments: 4 pages and 4 figures

Journal ref: Phys. Rev. Lett. 116, 075501 (2016)

Evidence for a New Excitation at the Interface Between a High-Tc Superconductor and a Topological Insulator

Authors: Parisa Zareapour, Alex Hayat, Shu Yang F. Zhao, Michael Kreshchuk, Yong Kiat Lee, Anjan A. Reijnders, Achint Jain, Zhijun Xu, T. S. Liu, G. D. Gu, Shuang Jia, Robert J. Cava, Kenneth S. Burch

Abstract: High-temperature superconductors exhibit a wide variety of novel excitations. If contacted with a topological insulator, the lifting of spin rotation symmetry in the surface states can lead to the emergence of unconventional superconductivity and novel particles. In pursuit of this possibility, we fabricated high critical-temperature (Tc ~ 85 K) superconductor/topological insulator (Bi2Sr2CaCu2O8+… ▽ More High-temperature superconductors exhibit a wide variety of novel excitations. If contacted with a topological insulator, the lifting of spin rotation symmetry in the surface states can lead to the emergence of unconventional superconductivity and novel particles. In pursuit of this possibility, we fabricated high critical-temperature (Tc ~ 85 K) superconductor/topological insulator (Bi2Sr2CaCu2O8+delta/Bi2Te2Se) junctions. Below 75 K, a zero-bias conductance peak (ZBCP) emerges in the differential conductance spectra of this junction. The magnitude of the ZBCP is suppressed at the same rate for magnetic fields applied parallel or perpendicular to the junction. Furthermore, it can still be observed and does not split up to at least 8.5 T. The temperature and magnetic field dependence of the excitation we observe appears to fall outside the known paradigms for a ZBCP. △ Less

Submitted 17 December, 2014; originally announced December 2014.

Journal ref: Physical Review B 90, 241106 (2014)

Optical evidence of surface state suppression in Bi based topological insulators

Authors: Anjan A. Reijnders, Y. Tian, L. J. Sandilands, G. Pohl, I. D. Kivlichan, S. Y. Frank Zhao, S. Jia, M. E. Charles, R. J. Cava, Nasser Alidoust, Suyang Xu, Madhab Neupane, M. Zahid Hasan, X. Wang, S. W. Cheong, K. S. Burch

Abstract: A key challenge in condensed matter research is the optimization of topological insulator (TI) compounds for the study and future application of their unique surface states. Truly insulating bulk states would allow the exploitation of predicted surface state properties, such as protection from backscattering, dissipationless spin-polarized currents, and the emergence of novel particles. Towards th… ▽ More A key challenge in condensed matter research is the optimization of topological insulator (TI) compounds for the study and future application of their unique surface states. Truly insulating bulk states would allow the exploitation of predicted surface state properties, such as protection from backscattering, dissipationless spin-polarized currents, and the emergence of novel particles. Towards this end, major progress was recently made with the introduction of highly resistive Bi$_2$Te$_2$Se, in which surface state conductance and quantum oscillations are observed at low temperatures. Nevertheless, an unresolved and pivotal question remains: while room temperature ARPES studies reveal clear evidence of TI surface states, their observation in transport experiments is limited to low temperatures. A better understanding of this surface state suppression at elevated temperatures is of fundamental interest, and crucial for pushing the boundary of device applications towards room-temperature operation. In this work, we simultaneously measure TI bulk and surface states via temperature dependent optical spectroscopy, in conjunction with transport and ARPES measurements. We find evidence of coherent surface state transport at low temperatures, and propose that phonon mediated coupling between bulk and surface states suppresses surface conductance as temperature rises. △ Less

Submitted 2 April, 2014; originally announced April 2014.

Comments: 13 pages, 10 figures

Journal ref: Phys. Rev. B 89, 075138 (2014)

The origin of the dead-layer at the La0.67Sr0.33MnO3/SrTiO3 interface and dead-layer reduction via interfacial engineering

Authors: R. Peng, H. C. Xu, M. Xia, J. F. Zhao, X. Xie, D. F. Xu, B. P. Xie, D. L. Feng

Abstract: Transition metal oxide hetero-structure has great potential for multifunctional devices. However, the degraded physical properties at interface, known as dead-layer behavior, present a main obstacle for device applications. Here we present the systematic study of the dead-layer behavior in La0.67Sr0.33MnO3 thin film grown on SrTiO3 substrate with ozone assisted molecular beam epitaxy. We found tha… ▽ More Transition metal oxide hetero-structure has great potential for multifunctional devices. However, the degraded physical properties at interface, known as dead-layer behavior, present a main obstacle for device applications. Here we present the systematic study of the dead-layer behavior in La0.67Sr0.33MnO3 thin film grown on SrTiO3 substrate with ozone assisted molecular beam epitaxy. We found that the evolution of electric and magnetic properties as a function of thickness shows a remarkable resemblance to the phase diagram as a function of doping for bulk materials, providing compelling evidences of the hole depletion in near interface layers that causes dead-layer. Detailed electronic and surface structure studies indicate that the hole depletion is due to the intrinsic oxygen vacancy formation. Furthermore, we show that oxygen vacancies are partly caused by interfacial electric dipolar field, and thus by doping-engineering at the single-atomic-layer level, we demonstrate the dead-layer reduction in films with higher interfacial hole concentration. △ Less

Submitted 21 January, 2013; originally announced January 2013.

Comments: 6 pages, 5 figures

Journal ref: Appl. Phys. Lett. 104, 081606 (2014)

Hybrid high-temperature superconductor-semiconductor tunnel diode

Authors: Alex Hayat, Parisa Zareapour, Shu Yang F. Zhao, Achint Jain, Igor G. Savelyev, Marina Blumin, Zhijun Xu, Alina Yang, G. D. Gu, Harry E. Ruda, Shuang Jia, R. J. Cava, Aephraim M. Steinberg, Kenneth S. Burch

Abstract: We report the demonstration of hybrid high-Tc-superconductor-semiconductor tunnel junctions, enabling new interdisciplinary directions in condensed matter research. The devices were fabricated by our newly-developed mechanical bonding technique, resulting in high-Tc-semiconductor planar junctions acting as superconducting tunnel diodes. Tunneling-spectra characterization of the hybrid junctions of… ▽ More We report the demonstration of hybrid high-Tc-superconductor-semiconductor tunnel junctions, enabling new interdisciplinary directions in condensed matter research. The devices were fabricated by our newly-developed mechanical bonding technique, resulting in high-Tc-semiconductor planar junctions acting as superconducting tunnel diodes. Tunneling-spectra characterization of the hybrid junctions of Bi2Sr2CaCu2O8+δ combined with bulk GaAs, or a GaAs/AlGaAs quantum well, exhibits excess voltage and nonlinearity - in good agreement with theoretical predictions for a d-wave superconductor-normal material junction, and similar to spectra obtained in scanning tunneling microscopy. Additional junctions are demonstrated using Bi2Sr2CaCu2O8+δ combined with graphite or Bi2Te3. Our results pave the way for new methods in unconventional superconductivity studies, novel materials and quantum technology applications. △ Less

Submitted 9 January, 2013; originally announced January 2013.

Journal ref: Phys. Rev. X 2, 041019 (2012)

Wideband trapping of light by edge states in honeycomb photonic crystals

Authors: Chunfang Ouyang, Dezhuan Han, Fangyuan Zhao, Xinhua Hu, Xiaohan Liu, Jian Zi

Abstract: We study theoretically light propagations at the zigzag edge of a honeycomb photonic crystal consisting of dielectric rods in air, analogous to graphene. Within the photonic band gap of the honeycomb photonic crystal, a unimodal edge state may exist with a sharp confinement of optical fields. Its dispersion can be tuned simply by adjusting the radius of the edge rods. For the edge rods with a grad… ▽ More We study theoretically light propagations at the zigzag edge of a honeycomb photonic crystal consisting of dielectric rods in air, analogous to graphene. Within the photonic band gap of the honeycomb photonic crystal, a unimodal edge state may exist with a sharp confinement of optical fields. Its dispersion can be tuned simply by adjusting the radius of the edge rods. For the edge rods with a graded variation in radius along the edge direction, we show numerically that light beams of different frequencies can be trapped sharply in different spatial locations, rendering wideband trapping of light. △ Less

Submitted 23 December, 2012; originally announced December 2012.

Comments: 7 pages, 3 figure

Journal ref: J. Phys.: Condens. Matter 24, 492203 (2012)

Proximity-induced high-temperature superconductivity in topological insulators Bi2Se3 and Bi2Te3

Authors: Parisa Zareapour, Alex Hayat, Shu Yang F. Zhao, Michael Kreshchuk, Achint Jain, Daniel C. Kwok, Nara Lee, Sang-Wook Cheong, Zhijun Xu, Alina Yang, G. D. Gu, Shuang Jia, Robert J. Cava, Kenneth S. Burch

Abstract: Interest in the superconducting proximity effect has been reinvigorated recently by novel optoelectronic applications as well as by the possible emergence of the elusive Majorana fermion at the interface between topological insulators and superconductors. Here we produce high-temperature superconductivity in Bi2Se3 and Bi2Te3 via proximity to Bi2Sr2CaCu2O8+δ, in order to access increasing temperat… ▽ More Interest in the superconducting proximity effect has been reinvigorated recently by novel optoelectronic applications as well as by the possible emergence of the elusive Majorana fermion at the interface between topological insulators and superconductors. Here we produce high-temperature superconductivity in Bi2Se3 and Bi2Te3 via proximity to Bi2Sr2CaCu2O8+δ, in order to access increasing temperature and energy scales for this phenomenon. This was achieved by a new mechanical bonding technique we developed, enabling the fabrication of high-quality junctions between materials, unobtainable by conventional approaches. We observe proximity-induced superconductivity in Bi2Se3 and Bi2Te3 persisting up to at least 80K, a temperature an order of magnitude higher than any previous observations. Moreover, the induced superconducting gap in our devices reaches values of 10mV, significantly enhancing the relevant energy scales. Our results open new directions for fundamental studies in condensed matter physics and enable a wide range of applications in spintronics and quantum computing. △ Less

Submitted 1 November, 2012; originally announced November 2012.

Journal ref: Nature Communications 3, 1056 (2012)

A molecular simulation analysis of producing monatomic carbon chains by stretching ultranarrow graphene nanoribbons

Authors: Zenan Qi, Fengpeng Zhao, Xiaozhou Zhou, Zehui Sun, Harold S Park, Hengan Wu

Abstract: Atomistic simulations were utilized to develop fundamental insights regarding the elongation process starting from ultranarrow graphene nanoribbons (GNRs) and resulting in monatomic carbon chains (MACCs). There are three key findings. First, we demonstrate that complete, elongated, and stable MACCs with fracture strains exceeding 100% can be formed from both ultranarrow armchair and zigzag GNRs. S… ▽ More Atomistic simulations were utilized to develop fundamental insights regarding the elongation process starting from ultranarrow graphene nanoribbons (GNRs) and resulting in monatomic carbon chains (MACCs). There are three key findings. First, we demonstrate that complete, elongated, and stable MACCs with fracture strains exceeding 100% can be formed from both ultranarrow armchair and zigzag GNRs. Second, we demonstrate that the deformation processes leading to the MACCs have strong chirality dependence. Specifically, armchair GNRs first form DNA-like chains, then develop into monatomic chains by passing through an intermediate configuration in which monatomic chain sections are separated by two-atom attachments. In contrast, zigzag GNRs form rope-ladder-like chains through a process in which the carbon hexagons are first elongated into rectangles; these rectangles eventually coalesce into monatomic chains through a novel triangle-pentagon deformation structure under further tensile deformation. Finally, we show that the width of GNRs plays an important role in the formation of MACCs, and that the ultranarrow GNRs facilitate the formation of full MACCs. The present work should be of considerable interest due to the experimentally demonstrated feasibility of using narrow GNRs to fabricate novel nanoelectronic components based upon monatomic chains of carbon atoms. △ Less

Submitted 9 April, 2012; originally announced April 2012.

Comments: 11 pages, 6 figures, Nanotechnology accepted version

Journal ref: Nanotechnology 2010, 21, 265702

Ultrafast laser-induced subwavelength structures towards nanoscale: the significant role of plasmonic effects

Authors: Min Huang, Ya Cheng, Fuli Zhao, Zhizhan Xu

Abstract: Nowadays, via controlling surface plasmons (SPs) on elaborate man-made structures, plasmonics aiming at manipulating light beyond the diffraction limit has aroused great interest. Here, nevertheless, we demonstrate in short-pulse laser ablation ultrafast active plasmonic structures spontaneously generate in virtue of plasmonic effects rather than human ingenuity. First, the splitting of laser-indu… ▽ More Nowadays, via controlling surface plasmons (SPs) on elaborate man-made structures, plasmonics aiming at manipulating light beyond the diffraction limit has aroused great interest. Here, nevertheless, we demonstrate in short-pulse laser ablation ultrafast active plasmonic structures spontaneously generate in virtue of plasmonic effects rather than human ingenuity. First, the splitting of laser-induced subwavelength gratings that is experimentally evidenced on ZnO, Si, and GaAs, is confirmed to originate in the conversion of SP modes from the resonant to the nonresonant mode and further to the inphase or antiphase asymmetric mode. Further, as pulse number increases the universal scaling-down of laser-induced structures is derived from the conversion of physical regimes of plasmonic interaction from the optical to the electrostatic regime, which may arouse quasistatic SPs with interacting scales far beyond the diffraction limit and result in the ultrafast, non-thermal ablation for extraordinary electrostatic enhancement. Generally, the plasmonic mechanisms reveal the link between the deep-subwavelength space-scale and the ultra-short timescale for ultrafast laser-induced nanostructures: basically, "nanoscale" tends to eliminate electromagnetic retardation effects and bring an instant respond to the incident field, and arouse electrostatic interactions with giant local-field enhancement facilitating ultrafast ablation driven by tremendous electrostatic forces. Thus, active plasmonic structures provided with simultaneous "nanoscale" and "ultrafast" is apt to form in ultrafast ablation. In addition, the plasmonic mechanisms can act as a powerful "evolutionary force" acting in multipulse ablation and promoting the growth of resonant nanostructures-various typical plasmonic structures may be self-generated through multipulse evolution, which can be considered as a kind of natural plasmonics. △ Less

Submitted 28 May, 2012; v1 submitted 30 September, 2011; originally announced September 2011.

Comments: 16 pages, 6 figures; submitted to Scientific Reports on April 1st, 2011; revised version submitted to Scientific Reports on November 30th, 2011

Magnetic rogue wave in a perpendicular anisotropic ferromagnetic nanowire with spin-transfer torque

Authors: Fei Zhao, Zai-Dong Li, Qiu-Yan Li, Lin Wen, Guangsheng Fu, W. M. Liu

Abstract: We present the current controlled motion of dynamic soliton embedded in spin wave background in ferromagnetic nanowire. With the stronger breather character we get the novel magnetic rogue wave and clarify its formation mechanism. The generation of magnetic rogue wave is mainly arose from the accumulation of energy and magnons toward to its central part. We also observe that the spin-polarized cur… ▽ More We present the current controlled motion of dynamic soliton embedded in spin wave background in ferromagnetic nanowire. With the stronger breather character we get the novel magnetic rogue wave and clarify its formation mechanism. The generation of magnetic rogue wave is mainly arose from the accumulation of energy and magnons toward to its central part. We also observe that the spin-polarized current can control the exchange rate of magnons between envelope soliton and background, and the critical current condition is obtained analytically. Even more interesting is that the spin-transfer torque plays the completely opposite role for the cases of below and above the critical value. △ Less

Submitted 19 August, 2011; v1 submitted 16 August, 2011; originally announced August 2011.

Comments: 5 figures

Suppression of the Néel temperature in hydrothermally synthesized alpha-Fe2O3 nanoparticles

Authors: Jun Wang, Wei Wu, Fan Zhao, Guo-meng Zhao

Abstract: Magnetic measurements up to 1000 K have been performed on hydrothermally synthesized $α$-Fe$_{2}$O$_{3}$ nanoparticles (60 nm) using a Quantum Design vibrating sample magnetometer. A high vacuum environment (1$\times$10$^{-5}$ torr) during the magnetic measurement up to 1000 K leads to a complete reduction of $α$-Fe$_{2}$O$_{3}$ to Fe$_{3}$O$_{4}$. This precludes the determination of the Néel tem… ▽ More Magnetic measurements up to 1000 K have been performed on hydrothermally synthesized $α$-Fe$_{2}$O$_{3}$ nanoparticles (60 nm) using a Quantum Design vibrating sample magnetometer. A high vacuum environment (1$\times$10$^{-5}$ torr) during the magnetic measurement up to 1000 K leads to a complete reduction of $α$-Fe$_{2}$O$_{3}$ to Fe$_{3}$O$_{4}$. This precludes the determination of the Néel temperature for the $α$-Fe$_{2}$O$_{3}$ nanoparticles. In contrast, coating $α$-Fe$_{2}$O$_{3}$ nanoparticles with SiO$_{2}$ stabilizes the $α$-Fe$_{2}$O$_{3}$ phase up to 930 K, which allows us to determine the Néel temperature of the $α$-Fe$_{2}$O$_{3}$ nanoparticles for the first time. The Néel temperature of the 60-nm $α$-Fe$_{2}$O$_{3}$ nanoparticles is found to be 945 K, about 15 K below the bulk value. The small reduction of the Néel temperature of the $α$-Fe$_{2}$O$_{3}$ nanoparticles is consistent with a finite-size scaling theory. Our current results also show that coating nanoparticles with SiO$_{2}$ can effectively protect nanoparticles from oxidation or reduction, which is important to technological applications. △ Less

Submitted 25 October, 2010; originally announced October 2010.

Comments: 5 pages, 5 figures