Showing 1–29 of 29 results for author: Cai, P

Giant Exfoliation Induced Magnetic Coercivity in Fe$_3$GaTe$_2$

Authors: Lingrui Mei, PeiYu Cai, Sang-Eon Lee, Yue Li, Shyam Raj Karullithodi, Vadym Kulichenko, Charudatta Pathak, Elton J. G. Santos, Luis Balicas

Abstract: Permanent magnets with strong anisotropy and high coercivity underpin modern information and energy technologies, yet rare-earth-free alternatives remain limited. Here, we show that thickness engineering via mechanical exfoliation induces hard magnetic behavior in the van der Waals ferromagnet Fe$_3$GaTe$_2$. Bulk crystals exhibit Curie temperatures above 350 K but negligible room-temperature coer… ▽ More Permanent magnets with strong anisotropy and high coercivity underpin modern information and energy technologies, yet rare-earth-free alternatives remain limited. Here, we show that thickness engineering via mechanical exfoliation induces hard magnetic behavior in the van der Waals ferromagnet Fe$_3$GaTe$_2$. Bulk crystals exhibit Curie temperatures above 350 K but negligible room-temperature coercivity. When thinned below 100 nm, the coercive field is dramatically enhanced, reaching nearly 1 T at room temperature for in-plane fields which is comparable to values of conventional hard magnets. Micromagnetic analysis reveals a crossover in magnetization reversal from domain-mediated processes in bulk samples to quasi-coherent rotation in thin flakes, driven by increased effective anisotropy and suppressed domain formation. This thickness-dependent transition enables tuning of magnetic hardness without chemical modification. Combined with high saturation magnetization and robust room-temperature performance, Fe$_3$GaTe$_2$ emerges as a promising rare-earth-free material for spintronic applications. Its layered structure further allows integration into van der Waals heterostructures, where large in-plane coercivity can stabilize magnetic states against perturbations and interlayer coupling, offering potential for high-density nonvolatile memory and domain-wall-based devices. △ Less

Submitted 30 July, 2026; originally announced July 2026.

Comments: 5 figures, plus Supplementary information, including 6 supplementary figures

Journal ref: Advanced Electronic Materials 2026

Tunneling spectra of $\mathrm{TaO}_x$ junctions for van der Waals superconductors

Authors: Yixuan Niu, Jun Cheng, Shiji Ding, Zhongxin Guo, Shang Wang, Chenglong Li, Meining Zhang, Peng Cai

Abstract: Tunneling spectroscopy and its evolution are crucial for elucidating the intricate electronic structure and emergent phenomena in quantum materials.Nevertheless, high-quality measurements -- specifically those tracking evolution across temperature and external fields -- remain a formidable challenge. We have fabricated a high-quality $\mathrm{TaO}_x$-based planar tunneling junction by using magnet… ▽ More Tunneling spectroscopy and its evolution are crucial for elucidating the intricate electronic structure and emergent phenomena in quantum materials.Nevertheless, high-quality measurements -- specifically those tracking evolution across temperature and external fields -- remain a formidable challenge. We have fabricated a high-quality $\mathrm{TaO}_x$-based planar tunneling junction by using magnetron sputtering for van der Waals (vdW) superconductors. Using the vdW superconductor $\mathrm{Bi}_2\mathrm{Sr}_2\mathrm{CaCu}_2\mathrm{O}_{8+δ}$ (Bi2212) as a benchmark, this platform yields high-quality tunneling spectra, reproducing the electronic signatures obtained from scanning tunneling spectra acquired from atomically clean surfaces under ultra-high vacuum conditions. This architecture enables high-precision spectroscopy across extensive temperature and magnetic field ranges, offering a universal strategy for probing the electronic structures of diverse two-dimensional systems and facilitating future explorations of material properties. △ Less

Submitted 20 May, 2026; originally announced May 2026.

Apparent double-$T_c$ from a single BKT transition in anisotropic phase-only models

Authors: Pei-Yuan Cai, Yi Zhou

Abstract: Transport experiments on two-dimensional superconductors often yield direction-dependent transition temperatures, raising the question of whether such a ``double-$T_c$'' reflects a true thermodynamic splitting or a transport artifact. To establish a baseline, we study a minimal anisotropic phase-only Josephson-junction array in equilibrium and under resistively shunted junction dynamics with fluct… ▽ More Transport experiments on two-dimensional superconductors often yield direction-dependent transition temperatures, raising the question of whether such a ``double-$T_c$'' reflects a true thermodynamic splitting or a transport artifact. To establish a baseline, we study a minimal anisotropic phase-only Josephson-junction array in equilibrium and under resistively shunted junction dynamics with fluctuating twist boundary conditions. The equilibrium model exhibits a single Berezinskii--Kosterlitz--Thouless (BKT) transition. Out of equilibrium, anisotropic Josephson couplings and anisotropic dissipation reshape the linear $R$--$T$ curves in a finite-size, finite-current crossover regime, so that curve-shape criteria such as Halperin--Nelson fits and fixed-resistance thresholds yield an apparent double-$T_c$. In contrast, critical-scaling criteria -- the universal exponent $α=3$ and dynamic finite-size scaling -- remain consistent with the single $T_{\mathrm{BKT}}$. A robust splitting that persists in the nonlinear critical scaling, such as that recently reported at KTaO$_3$ interfaces, therefore points to physics beyond this clean anisotropic baseline. △ Less

Submitted 11 May, 2026; originally announced May 2026.

Journal ref: Phys. Rev. B 114, 034505 (2026)

Local Inversion Symmetry Breaking and Thermodynamic Evidence for Ferrimagnetism in Fe3GaTe2

Authors: Sang-Eon Lee, Yue Li, Yeonkyu Lee, W. Kice Brown, PeiYu Cai, Jinyoung Yun, Chanyoung Lee, Alex Moon, Lingrui Mei, Jaeyong Kim, Yan Xin, Julie A. Borchers, Thomas W. Heitmann, Matthias Frontzek, William D. Ratcliff, Gregory T. McCandless, Julia Y. Chan, Elton J. G. Santos, Jeehoon Kim, Charudatta M. Phatak, Vadym Kulichenko, Luis Balicas

Abstract: The layered compound Fe3GaTe2 is attracting attention due to its high Curie temperature, low dimensionality, and the presence of topological spin textures above room temperature, making Fe$_3$GaTe$_2$ a good candidate for applications in spintronics. Here, we show, through transmission electron microscopy (TEM) techniques, that Fe$_3$GaTe$_2$ single crystals break local inversion symmetry while ma… ▽ More The layered compound Fe3GaTe2 is attracting attention due to its high Curie temperature, low dimensionality, and the presence of topological spin textures above room temperature, making Fe$_3$GaTe$_2$ a good candidate for applications in spintronics. Here, we show, through transmission electron microscopy (TEM) techniques, that Fe$_3$GaTe$_2$ single crystals break local inversion symmetry while maintaining global inversion symmetry according to X-ray diffraction. Coupled to the observation of Néel skyrmions via Lorentz-TEM, our structural analysis provides a convincing explanation for their presence in centrosymmetric materials. Magnetization measurements as a function of the temperature displays a sharp first-order thermodynamic phase-transition leading to a reduction in the magnetic moment. This implies that the ground state of Fe$_3$GaTe$_2$ is globally ferrimagnetic and not a glassy magnetic state composed of ferrimagnetic, and ferromagnetic domains as previously claimed. Neutron diffraction studies indicate that the ferromagnetic to ferrimagnetic transition upon reducing the external magnetic field is associated with a change in the magnetic configuration/coupling between Fe1 and Fe2 moments. We observe a clear correlation between the hysteresis observed in both the skyrmion density and the magnetization of Fe$_3$GaTe$_2$. This indicates that its topological spin textures are affected by the development of ferrimagnetism upon cooling. Observation, via magnetic force microscopy, of magnetic bubbles at the magnetic phase boundary suggests skyrmions stabilized by the competition among magnetic phases and distinct exchange interactions. Our study provides an explanation for the observation of Néel skyrmions in centrosymmetric systems, while exposing a correlation between the distinct magnetic phases of Fe$_3$GaTe$_2$ and topological spin textures. △ Less

Submitted 30 July, 2025; originally announced July 2025.

Comments: 57 pages, 6 figures, and appended Supporting Information file

Journal ref: ACS Nano (2025)

Laser-Induced Topological Toggle Switching at Room Temperature in the van der Waals Ferromagnet Fe3GaTe2

Authors: Charlie W. F. Freeman, Woohyun Cho, Paul S. Keatley, PeiYu Cai, Elton J. G. Santos, Robert J. Hicken, H. Yang, Hidekazu Kurebayashi, Murat Cubukcu, Maciej Dabrowski

Abstract: We demonstrate room-temperature nucleation and manipulation of topological spin textures in the van der Waals (vdW) ferromagnet, Fe3GaTe2, through laser pulse excitation. By leveraging laser-induced heating and subsequent cooling, we access the skyrmion/bubble state at low fields and achieve toggle switching between two topological spin textures - skyrmion/bubble and labyrinth. Micromagnetic simul… ▽ More We demonstrate room-temperature nucleation and manipulation of topological spin textures in the van der Waals (vdW) ferromagnet, Fe3GaTe2, through laser pulse excitation. By leveraging laser-induced heating and subsequent cooling, we access the skyrmion/bubble state at low fields and achieve toggle switching between two topological spin textures - skyrmion/bubble and labyrinth. Micromagnetic simulations reveal that this switching behaviour arises from laser-induced heating and cooling. Our findings highlight the potential of vdW ferromagnets for room temperature laser-controlled non-volatile memory storage applications. △ Less

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

Symmetry-protected topological order identified via Gutzwiller-guided density-matrix-renormalization-group: $\mathrm{SO}(n)$ spin chains

Authors: Pei-Yuan Cai, Hui-Ke Jin, Yi Zhou

Abstract: We present a comprehensive study of topological phases in the SO($n$) spin chains using a combination of analytical parton construction and numerical techniques. For even $n=2l$, we identify a novel SPT$^2$ phase characterized by two distinct topological sectors, exhibiting exact degeneracy at the matrix product state (MPS) exactly solvable point. Through Gutzwiller-projected mean-field theory and… ▽ More We present a comprehensive study of topological phases in the SO($n$) spin chains using a combination of analytical parton construction and numerical techniques. For even $n=2l$, we identify a novel SPT$^2$ phase characterized by two distinct topological sectors, exhibiting exact degeneracy at the matrix product state (MPS) exactly solvable point. Through Gutzwiller-projected mean-field theory and density matrix renormalization group (DMRG) calculations, we demonstrate that these sectors remain topologically degenerate in close chains throughout the SPT$^2$ phase, with energy gaps decaying exponentially with system size. For odd $n=2l+1$, we show that the ground state remains unique in close chains. We precisely characterize critical states using entanglement entropy scaling, confirming the central charges predicted by conformal field theories. Our results reveal fundamental differences between even and odd $n$ cases, provide numerical verification of topological protection, and establish reliable methods for studying high-symmetry quantum systems. The Gutzwiller-guided DMRG is demonstrated to be notably efficient in targeting specific topological sectors. △ Less

Submitted 18 July, 2025; v1 submitted 15 April, 2025; originally announced April 2025.

Journal ref: Phys. Rev. B 112, 045125 (2025)

Gate Tunable Asymmetric Ozone Adsorption on Graphene

Authors: Zhen Qi, Wanlei Li, Jun Cheng, Zhongxin Guo, Chenglong Li, Shang Wang, Zuoquan Tan, Zhiting Gao, Yongchao Wang, Zichen Lian, Shanshan Chen, Yonglin He, Zhiyong Wang, Yapei Wang, Jinsong Zhang, Yayu Wang, Peng Cai

Abstract: Molecular adsorption is pivotal in device fabrication and material synthesis for quantum technology. However, elucidating the behavior of physisorption poses technical challenges. Here graphene with ultrahigh sensitivity was utilized to detect ozone adsorption at cryogenic temperatures. Significant hole doping observed in graphene indicates a strong interaction between ozone and graphene. Interest… ▽ More Molecular adsorption is pivotal in device fabrication and material synthesis for quantum technology. However, elucidating the behavior of physisorption poses technical challenges. Here graphene with ultrahigh sensitivity was utilized to detect ozone adsorption at cryogenic temperatures. Significant hole doping observed in graphene indicates a strong interaction between ozone and graphene. Interestingly, the adsorption exhibits asymmetry with positive and negative gate voltages. The strong affinity of ozone provides a tool to modulate materials and devices, while the gate tunability of adsorption offers new insights into construction and manipulation of oxide quantum materials. △ Less

Submitted 9 May, 2024; originally announced May 2024.

Wave functions for high-symmetry, thin microstrip antennas and two-dimensional quantum boxes

Authors: Joseph R. Rain, PeiYu Cai, Alexander Baekey, Matthew A. Reinhard, Roman I. Vasquez, Andrew C. Silverman, Christopher L. Cain, Richard A. Klemm

Abstract: For a spinless quantum particle in a one-dimensional box or an electromagnetic wave in a one-dimensional cavity, the respective Dirichlet and Neumann boundary conditions both lead to non-degenerate wave functions. However, in two spatial dimensions, the symmetry of the box or microstrip antenna is an important feature that has often been overlooked in the literature. In the high-symmetry cases of… ▽ More For a spinless quantum particle in a one-dimensional box or an electromagnetic wave in a one-dimensional cavity, the respective Dirichlet and Neumann boundary conditions both lead to non-degenerate wave functions. However, in two spatial dimensions, the symmetry of the box or microstrip antenna is an important feature that has often been overlooked in the literature. In the high-symmetry cases of a disk, square, or equilateral triangle, the wave functions for each of those two boundary conditions are grouped into two distinct classes, which are one- and two-dimensional representations of the respective point groups, $C_{\infty v}$, $C_{4v}$, and $C_{3v}$. Here we present visualizations of representative wave functions for both boundary conditions and both one- and two-dimensional representations of those point groups. For the one-dimensional representations, color contour plots of the wave functions are presented. For the two-dimensional representations, the infinite degeneracies are presented as common nodal points and/or lines, the patterns of which are invariant under all operations of the respective point group. The wave functions with the Neumann boundary conditions have important consequences for the coherent terahertz emission from the intrinsic Josephson junctions in the high-temperature superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$: the enhancement of the output power from electromagnetic cavity resonances is only strong for wave functions that are not degenerate. △ Less

Submitted 17 August, 2021; originally announced August 2021.

Comments: 18 pages, 23 figures

Particle-hole asymmetric superconducting coherence peaks in overdoped cuprates

Authors: Changwei Zou, Zhenqi Hao, Xiangyu Luo, Shusen Ye, Qiang Gao, Xintong Li, Miao Xu, Peng Cai, Chengtian Lin, Xingjiang Zhou, Dung-Hai Lee, Yayu Wang

Abstract: To elucidate the superconductor to metal transition at the end of superconducting dome, the overdoped regime has stepped onto the center stage of cuprate research recently. Here, we use scanning tunneling microscopy to investigate the atomic-scale electronic structure of overdoped trilayer Bi-2223 and bilayer Bi-2212 cuprates. At low energies the spectroscopic maps are well described by dispersive… ▽ More To elucidate the superconductor to metal transition at the end of superconducting dome, the overdoped regime has stepped onto the center stage of cuprate research recently. Here, we use scanning tunneling microscopy to investigate the atomic-scale electronic structure of overdoped trilayer Bi-2223 and bilayer Bi-2212 cuprates. At low energies the spectroscopic maps are well described by dispersive quasiparticle interference patterns. However, as the bias increases to the superconducting coherence peak energy, a virtually non-dispersive pattern with sqrt(2)*sqrt(2) periodicity emerges. Remarkably, the position of the coherence peaks exhibits evident particle-hole asymmetry which also modulates with the same period. We propose that this is an extreme quasiparticle interference phenomenon, caused by pairing-breaking scattering between flat anti-nodal Bogoliubov bands, which is ultimately responsible for the superconductor to metal transition. △ Less

Submitted 10 March, 2021; originally announced March 2021.

Comments: 15 pages, 4 figures

Zero-Dimensional Organic-Inorganic Hybrid Material with Ultra-Narrow-Red Emission at Room Temperature

Authors: Peiqing Cai, Song Wang, Tianmou Xu, Ying Tang, Xiaolin Yuan, Mingjie Wan, Qi Ai, Junjie Si, Xin Yao, Yonggang Cao, Maxim K. Rabchinskii, Pavel N. Brunkov, Zugang Liu

Abstract: Recently, low-dimensional organic-inorganic hybrid halide compounds have aroused great attention in the optoelectronic field, due to the unique topology and optical properties. Herein, we report an Mn4+ doped [N(CH3)4]2TiF6 zero-dimensional organic-inorganic hybrid phosphor, which could not only exhibit very narrow and pure red emission, but also maintain efficient emission intensity at room tempe… ▽ More Recently, low-dimensional organic-inorganic hybrid halide compounds have aroused great attention in the optoelectronic field, due to the unique topology and optical properties. Herein, we report an Mn4+ doped [N(CH3)4]2TiF6 zero-dimensional organic-inorganic hybrid phosphor, which could not only exhibit very narrow and pure red emission, but also maintain efficient emission intensity at room temperature. The crystal structure, photoluminescence properties and temperature sensing application are discussed. The excellent temperature dependent luminescent properties are attributed to the rigid structure and isolated MnF62- octahedra in the total crystal framework. These results will help design suitable materials and devices in both warm white light emitting diodes and optical sensors. △ Less

Submitted 22 July, 2020; originally announced July 2020.

Machine Learning Identification of Impurities in the STM Images

Authors: Ce Wang, Haiwei Li, Zhenqi Hao, Xintong Li, Cangwei Zou, Peng Cai, Yayu Wang, Yi-Zhuang You, Hui Zhai

Abstract: In this work we train a neural network to identify impurities in the experimental images obtained by the scanning tunneling microscope measurements. The neural network is first trained with large number of simulated data and then the trained neural network is applied to identify a set of experimental images taken at different voltages. We use the convolutional neural network to extract features fr… ▽ More In this work we train a neural network to identify impurities in the experimental images obtained by the scanning tunneling microscope measurements. The neural network is first trained with large number of simulated data and then the trained neural network is applied to identify a set of experimental images taken at different voltages. We use the convolutional neural network to extract features from the images and also implement the attention mechanism to capture the correlations between images taken at different voltages. We note that the simulated data can capture the universal Friedel oscillation but cannot properly describe the non-universal physics short-range physics nearby an impurity, as well as noises in the experimental data. And we emphasize that the key of this approach is to properly deal these differences between simulated data and experimental data. Here we show that even by including uncorrelated white noises in the simulated data, the performance of neural network on experimental data can be significantly improved. To prevent the neural network from learning unphysical short-range physics, we also develop another method to evaluate the confidence of the neural network prediction on experimental data and to add this confidence measure into the loss function. We show that adding such an extra loss function can also improve the performance on experimental data. Our research can inspire future similar applications of machine learning on experimental data analysis. △ Less

Submitted 29 June, 2020; originally announced June 2020.

Comments: 5 page, 5 figures

Anisotropic transport and quantum oscillations in the quasi-one-dimensional TaNiTe5: Evidence for the nontrivial band topology

Authors: C. Q. Xu, Y. Liu, P. G. Cai, B. Li, W. H. Jiao, Y. L. Li, J. Y. Zhang, W. Zhou, B. Qian, X. F. Jiang, Z. X. Shi, R. Sankar, J. L. Zhang, F. Yang, Zengwei Zhu, P. K. Biswas, Dong Qian, X. Ke, Xiaofeng Xu

Abstract: The past decade has witnessed the burgeoning discovery of a variety of topological states of matter with distinct nontrivial band topologies. Thus far, most of materials studied possess two-dimensional or three-dimensional electronic structures, with only a few exceptions that host quasi-one-dimensional (quasi-1D) topological electronic properties. Here we present the clear-cut evidence for Dirac… ▽ More The past decade has witnessed the burgeoning discovery of a variety of topological states of matter with distinct nontrivial band topologies. Thus far, most of materials studied possess two-dimensional or three-dimensional electronic structures, with only a few exceptions that host quasi-one-dimensional (quasi-1D) topological electronic properties. Here we present the clear-cut evidence for Dirac fermions in the quasi-1D telluride TaNiTe5. We show that its transport behaviors are highly anisotropic and we observe nontrivial Berry phases via the quantum oscillation measurements. The nontrivial band topology is further corroborated by first-principles calculations. Our results may help to guide the future quest for topological states in this new family of quasi-1D ternary chalcogenides. △ Less

Submitted 16 June, 2020; originally announced June 2020.

Comments: 5 figures, 1 table

Journal ref: J. Phys. Chem. Lett. 11, 7782 (2020)

Quasiparticle interference and charge order in a heavily overdoped non-superconducting cuprate

Authors: Xintong Li, Ying Ding, Chaocheng He, Wei Ruan, Peng Cai, Cun Ye, Zhenqi Hao, Lin Zhao, Xingjiang Zhou, Qianghua Wang, Yayu Wang

Abstract: One of the key issues in unraveling the mystery of high Tc superconductivity in the cuprates is to understand the normal state outside the superconducting dome. Here we perform scanning tunneling microscopy and spectroscopy measurements on a heavily overdoped, non-superconducting (Bi,Pb)2Sr2CuO6+x cuprate. Spectroscopic imaging reveals dispersive quasiparticle interferences and the Fourier transfo… ▽ More One of the key issues in unraveling the mystery of high Tc superconductivity in the cuprates is to understand the normal state outside the superconducting dome. Here we perform scanning tunneling microscopy and spectroscopy measurements on a heavily overdoped, non-superconducting (Bi,Pb)2Sr2CuO6+x cuprate. Spectroscopic imaging reveals dispersive quasiparticle interferences and the Fourier transforms uncover the evolution of momentum space topology. More interestingly, we observe nanoscale patches of static charge order with sqrt(2)*sqrt(2) periodicity. Both the dispersive quasiparticle interference and static charge order can be qualitatively explained by theoretical calculations, which reveal the unique electronic structure of strongly overdoped cuprate. △ Less

Submitted 7 May, 2019; originally announced May 2019.

Comments: 16 pages, 4 figures

Journal ref: New Journal of Physics 20, 063041 (2018)

Visualizing the periodic modulation of Cooper pairing in a severely underdoped cuprate

Authors: Wei Ruan, Xintong Li, Cheng Hu, Zhenqi Hao, Haiwei Li, Peng Cai, Xingjiang Zhou, Dung-Hai Lee, Yayu Wang

Abstract: A major obstacle in understanding the mechanism of Cooper pairing in the cuprates is the existence of various intertwined orders associated with spin, charge, and Cooper pairs. Of particular importance is the ubiquitous charge order features that have been observed in a variety of cuprates, especially in the underdoped regime of the phase diagram. To explain the origin of the charge order and its… ▽ More A major obstacle in understanding the mechanism of Cooper pairing in the cuprates is the existence of various intertwined orders associated with spin, charge, and Cooper pairs. Of particular importance is the ubiquitous charge order features that have been observed in a variety of cuprates, especially in the underdoped regime of the phase diagram. To explain the origin of the charge order and its implication to the superconducting phase, many theoretical models have been proposed, such as charge stripes, electronic nematicity, and Fermi surface instability. A highly appealing physical picture is the so-called pair density wave (PDW), a periodic modulation of Cooper paring in space, which may also induce a charge order. To elucidate the existence and nature of the PDW order, here we use scanning tunneling microscopy (STM) to investigate a severely underdoped Bi2Sr2CaCu2O8+δ, in which superconductivity just emerges on top of a pronounced checkerboard charge order. By analyzing the spatial distribution of the spectral features characteristic of superconductivity, we observe a periodic modulation of both the superconducting coherence peak and gap depth, demonstrating the existence of a density wave order of Cooper pairing. The PDW order has the same spatial periodicity as the charge order, and the amplitudes of the two orders exhibit clear positive correlation. These results shed important new lights on the origin of and interplay between the charge order and Cooper pairing modulation in the cuprates. △ Less

Submitted 7 May, 2019; originally announced May 2019.

Comments: 17 pages, 4 figures

Journal ref: Nature Physics 14, 1178-1182 (2018)

Relationship between the parent charge transfer gap and maximum transition temperature in cuprates

Authors: Wei Ruan, Cheng Hu, Jianfa Zhao, Peng Cai, Yingying Peng, Cun Ye, Runze Yu, Xintong Li, Zhenqi Hao, Changqing Jin, Xingjiang Zhou, Zheng-Yu Weng, Yayu Wang

Abstract: One of the biggest puzzles concerning the cuprate high temperature superconductors is what determines the maximum transition temperature (Tc,max), which varies from less than 30 K to above 130 K in different compounds. Despite this dramatic variation, a robust trend is that within each family, the double-layer compound always has higher Tc,max than the single-layer counterpart. Here we use scannin… ▽ More One of the biggest puzzles concerning the cuprate high temperature superconductors is what determines the maximum transition temperature (Tc,max), which varies from less than 30 K to above 130 K in different compounds. Despite this dramatic variation, a robust trend is that within each family, the double-layer compound always has higher Tc,max than the single-layer counterpart. Here we use scanning tunneling microscopy to investigate the electronic structure of four cuprate parent compounds belonging to two different families. We find that within each family, the double layer compound has a much smaller charge transfer gap size ($Δ_{CT}$), indicating a clear anticorrelation between $Δ_{CT}$ and Tc,max. These results suggest that the charge transfer gap plays a key role in the superconducting physics of cuprates, which shed important new light on the high Tc mechanism from doped Mott insulator perspective. △ Less

Submitted 17 January, 2017; v1 submitted 16 January, 2017; originally announced January 2017.

Comments: 4 figures

Journal ref: Science Bulletin 61, 1826 (2016)

Orbital-driven Mottness collapse in 1T-TaS2-xSex transition metal dichalcogenide

Authors: Shuang Qiao, Xintong Li, Naizhou Wang, Wei Ruan, Cun Ye, Peng Cai, Zhenqi Hao, Hong Yao, Xianhui Chen, Jian Wu, Yayu Wang, Zheng Liu

Abstract: The vicinity of a Mott insulating phase has constantly been a fertile ground for finding exotic quantum states, most notably the high Tc cuprates and colossal magnetoresistance manganites. The layered transition metal dichalcogenide 1T-TaS2 represents another intriguing example, in which the Mott insulator phase is intimately entangled with a series of complex charge-density-wave (CDW) orders. Mor… ▽ More The vicinity of a Mott insulating phase has constantly been a fertile ground for finding exotic quantum states, most notably the high Tc cuprates and colossal magnetoresistance manganites. The layered transition metal dichalcogenide 1T-TaS2 represents another intriguing example, in which the Mott insulator phase is intimately entangled with a series of complex charge-density-wave (CDW) orders. More interestingly, it has been recently found that 1T-TaS2 undergoes a Mott-insulator-to-superconductor transition induced by high pressure, charge doping, or isovalent substitution. The nature of the Mott insulator phase and transition mechanism to the conducting state is still under heated debate. Here, by combining scanning tunneling microscopy (STM) measurements and first-principles calculations, we investigate the atomic scale electronic structure of 1T-TaS2 Mott insulator and its evolution to the metallic state upon isovalent substitution of S with Se. We identify two distinct types of orbital textures - one localized and the other extended - and demonstrates that the interplay between them is the key factor that determines the electronic structure. Especially, we show that the continuous evolution of the charge gap visualized by STM is due to the immersion of the localized-orbital-induced Hubbard bands into the extended-orbital-spanned Fermi sea, featuring a unique evolution from a Mott gap to a charge-transfer gap. This new mechanism of orbital-driven Mottness collapse revealed here suggests an interesting route for creating novel electronic state and designing future electronic devices. △ Less

Submitted 10 April, 2017; v1 submitted 27 November, 2016; originally announced November 2016.

Comments: 9 pages, 7 figures

Journal ref: Phys. Rev. X 7, 041054 (2017)

Multiple nodeless superconducting gaps in noncentrosymmetric superconductor PbTaSe2 with topological bulk nodal lines

Authors: M. X. Wang, Y. Xu, L. P. He, J. Zhang, X. C. Hong, P. L. Cai, Z. B. Wang, J. K. Dong, S. Y. Li

Abstract: Low-temperature thermal conductivity measurements were performed on single crystal of PbTaSe$_2$, a noncentrosymmetric superconductor with topological bulk nodal lines in the electronic band structure. It is found that the residual linear term $κ_0/T$ is negligible in zero magnetic field. Furthermore, the field dependence of $κ_0/T$ exhibits a clear "$S$"-shape curve. These results suggest that Pb… ▽ More Low-temperature thermal conductivity measurements were performed on single crystal of PbTaSe$_2$, a noncentrosymmetric superconductor with topological bulk nodal lines in the electronic band structure. It is found that the residual linear term $κ_0/T$ is negligible in zero magnetic field. Furthermore, the field dependence of $κ_0/T$ exhibits a clear "$S$"-shape curve. These results suggest that PbTaSe$_2$ has multiple nodeless superconducting gaps. Therefore, the spin-triplet state with gap nodes does not play an important role in this noncentrosymmetric superconductor with strong spin-orbital coupling. The fully gapped superconducting state also meets the requirement of a topological superconductor, if PbTaSe$_2$ is indeed the case. △ Less

Submitted 9 November, 2015; originally announced November 2015.

Comments: 5 pages, 4 figures

Journal ref: Phys. Rev. B 93, 020503(R) (2016)

Visualizing the evolution from the Mott insulator to a charge ordered insulator in lightly doped cuprates

Authors: Peng Cai, Wei Ruan, Yingying Peng, Cun Ye, Xintong Li, Zhenqi Hao, Xingjiang Zhou, Dung-Hai Lee, Yayu Wang

Abstract: A central question in the high temperature cuprate superconductors is the fate of the parent Mott insulator upon charge doping. Here we use scanning tunneling microscopy to investigate the local electronic structure of lightly doped cuprate in the antiferromagnetic insulating regime. We show that the doped charge induces a spectral weight transfer from the high energy Hubbard bands to the low ener… ▽ More A central question in the high temperature cuprate superconductors is the fate of the parent Mott insulator upon charge doping. Here we use scanning tunneling microscopy to investigate the local electronic structure of lightly doped cuprate in the antiferromagnetic insulating regime. We show that the doped charge induces a spectral weight transfer from the high energy Hubbard bands to the low energy in-gap states. With increasing doping, a V-shaped density of state suppression occurs at the Fermi level, which is accompanied by the emergence of checkerboard charge order. The new STM perspective revealed here is the cuprates first become a charge ordered insulator upon doping. Subsequently, with further doping, Fermi surface and high temperature superconductivity grow out of it. △ Less

Submitted 23 August, 2015; originally announced August 2015.

Comments: 4 figures, with supplementary materials

Journal ref: Nature Physics 12, 1047-1052 (2016)

Structural phase transition and electronic structure evolution in Ir1-xPtxTe2 studied by scanning tunneling microscopy

Authors: Wei Ruan, Peizhe Tang, Aifang Fang, Peng Cai, Cun Ye, Xintong Li, Wenhui Duan, Nanling Wang, Yayu Wang

Abstract: The IrTe2 transition metal dichalcogenide undergoes a series of structural and electronic phase transitions when doped with Pt. The nature of each phase and the mechanism of the phase transitions have attracted much attention. In this paper, we report scanning tunneling microscopy and spectroscopy studies of Pt doped IrTe2 with varied Pt contents. In pure IrTe2, we find that the ground state has a… ▽ More The IrTe2 transition metal dichalcogenide undergoes a series of structural and electronic phase transitions when doped with Pt. The nature of each phase and the mechanism of the phase transitions have attracted much attention. In this paper, we report scanning tunneling microscopy and spectroscopy studies of Pt doped IrTe2 with varied Pt contents. In pure IrTe2, we find that the ground state has a 1/6 superstructure, and the electronic structure is inconsistent with Fermi surface nesting induced charge density wave order. Upon Pt doping, the crystal structure changes to a 1/5 superstructure and then to a quasi-periodic hexagonal phase. First principles calculations show that the superstructures and electronic structures are determined by the global chemical strain and local impurity states that can be tuned systematically by Pt doping. △ Less

Submitted 21 April, 2015; originally announced April 2015.

Comments: 6 figures

Journal ref: Science Bulletin 60, 798-805 (2015)

Strong similarities between the local electronic structure of insulating iron pnictide and lightly doped cuprate

Authors: Cun Ye, Wei Ruan, Peng Cai, Xintong Li, Aifeng Wang, Xianhui Chen, Yayu Wang

Abstract: One of the major puzzles regarding unconventional superconductivity is how some of the most interesting superconductors are related to an insulating phase that lies in close proximity. Here we report scanning tunneling microscopy studies of the local electronic structure of Cu doped NaFeAs across the superconductor to insulator transition. We find that in the highly insulating regime the electroni… ▽ More One of the major puzzles regarding unconventional superconductivity is how some of the most interesting superconductors are related to an insulating phase that lies in close proximity. Here we report scanning tunneling microscopy studies of the local electronic structure of Cu doped NaFeAs across the superconductor to insulator transition. We find that in the highly insulating regime the electronic spectrum develops an energy gap with diminishing density of state at the Fermi level. The overall lineshape and strong spatial variations of the spectra are strikingly similar to that of lightly doped cuprates close to the parent Mott insulator. We propose that the suppression of itinerant electron state and strong impurity potential induced by Cu dopants lead to this insulating iron pnictide. △ Less

Submitted 14 April, 2015; originally announced April 2015.

Comments: 6 figures, to appear in Phys. Rev. X

Journal ref: Physical Review X 5, 021013 (2015)

Drastic pressure effect on the extremely large magnetoresistance in WTe2: quantum oscillation study

Authors: P. L. Cai, J. Hu, L. P. He, J. Pan, X. C. Hong, Z. Zhang, J. Zhang, J. Wei, Z. Q. Mao, S. Y. Li

Abstract: The quantum oscillations of the magnetoresistance under ambient and high pressure have been studied for WTe$_2$ single crystals, in which extremely large magnetoresistance was discovered recently. By analyzing the Shubnikov-de Haas oscillations, four Fermi surfaces are identified, and two of them are found to persist to high pressure. The sizes of these two pockets are comparable, but show increas… ▽ More The quantum oscillations of the magnetoresistance under ambient and high pressure have been studied for WTe$_2$ single crystals, in which extremely large magnetoresistance was discovered recently. By analyzing the Shubnikov-de Haas oscillations, four Fermi surfaces are identified, and two of them are found to persist to high pressure. The sizes of these two pockets are comparable, but show increasing difference with pressure. At 0.3 K and in 14.5 T, the magnetoresistance decreases drastically from 1.25 $\times$ $10^5$\% under ambient pressure to 7.47 $\times$ $10^3$\% under 23.6 kbar, which is likely caused by the relative change of Fermi surfaces. These results support the scenario that the perfect balance between the electron and hole populations is the origin of the extremely large magnetoresistance in WTe$_2$. △ Less

Submitted 27 July, 2015; v1 submitted 29 December, 2014; originally announced December 2014.

Comments: 5 pages, 5 figures

Journal ref: Phys. Rev. Lett. 115, 057202 (2015)

Nodal superconductivity and superconducting dome in new layered superconductor Ta4Pd3Te16

Authors: J. Pan, W. H. Jiao, X. C. Hong, Z. Zhang, L. P. He, P. L. Cai, J. Zhang, G. H. Cao, S. Y. Li

Abstract: We measured the low-temperature thermal conductivity of a new layered superconductor with quasi-one-dimensional characteristics, the ternary telluride Ta$_4$Pd$_3$Te$_{16}$ with transition temperature $T_c \approx$ 4.3 K. The significant residual linear term of thermal conductivity in zero magnetic field and its rapid field dependence provide evidences for nodes in the superconducting gap. By meas… ▽ More We measured the low-temperature thermal conductivity of a new layered superconductor with quasi-one-dimensional characteristics, the ternary telluride Ta$_4$Pd$_3$Te$_{16}$ with transition temperature $T_c \approx$ 4.3 K. The significant residual linear term of thermal conductivity in zero magnetic field and its rapid field dependence provide evidences for nodes in the superconducting gap. By measuring resistivity under pressures, we reveal a superconducting dome in the temperature-pressure phase diagram. The existence of gap nodes and superconducting dome suggests unconventional superconductivity in Ta$_4$Pd$_3$Te$_{16}$, which may relate to a charge-density wave instability in this low-dimensional compound. △ Less

Submitted 16 March, 2015; v1 submitted 1 April, 2014; originally announced April 2014.

Comments: 6 pages, 5 figures

Journal ref: Phys. Rev. B 92, 180505(R) (2015)

Doping dependence of the anisotropic quasiparticle interference in NaFe1-xCoxAs iron-based superconductors

Authors: Peng Cai, Wei Ruan, Xiaodong Zhou, Cun Ye, Aifeng Wang, Xianhui Chen, Dung-Hai Lee, Yayu Wang

Abstract: We use scanning tunneling microscopy to investigate the doping dependence of quasiparticle interference (QPI) in NaFe1-xCoxAs iron-based superconductors. The goal is to study the relation between nematic fluctuations and Cooper pairing. In the parent and underdoped compounds, where four-fold rotational symmetry is broken macroscopically, the QPI patterns reveal strong rotational anisotropy. At opt… ▽ More We use scanning tunneling microscopy to investigate the doping dependence of quasiparticle interference (QPI) in NaFe1-xCoxAs iron-based superconductors. The goal is to study the relation between nematic fluctuations and Cooper pairing. In the parent and underdoped compounds, where four-fold rotational symmetry is broken macroscopically, the QPI patterns reveal strong rotational anisotropy. At optimal doping, however, the QPI patterns are always four-fold symmetric. We argue this implies small nematic susceptibility and hence insignificant nematic fluctuation in optimally doped iron pnictides. Since Tc is the highest this suggests nematic fluctuation is not a prerequistite for strong Cooper pairing. △ Less

Submitted 1 March, 2014; originally announced March 2014.

Comments: 4 figures, to appear in Physical Review Letters

Journal ref: Phys. Rev. Lett. 112, 127001 (2014)

Quantum oscillations in Kondo Insulator SmB$_6$

Authors: G. Li, Z. Xiang, F. Yu, T. Asaba, B. Lawson, P. Cai, C. Tinsman, A. Berkley, S. Wolgast, Y. S. Eo, Dae-Jeong Kim, C. Kurdak, J. W. Allen, K. Sun, X. H. Chen, Y. Y. Wang, Z. Fisk, Lu Li

Abstract: In Kondo insulator samarium hexaboride SmB$_6$, strong correlation and band hybridization lead to an insulating gap and a diverging resistance at low temperature. The resistance divergence ends at about 5 Kelvin, a behavior recently demonstrated to arise from the surface conductance. However, questions remain whether and where a topological surface state exists. Quantum oscillations have not been… ▽ More In Kondo insulator samarium hexaboride SmB$_6$, strong correlation and band hybridization lead to an insulating gap and a diverging resistance at low temperature. The resistance divergence ends at about 5 Kelvin, a behavior recently demonstrated to arise from the surface conductance. However, questions remain whether and where a topological surface state exists. Quantum oscillations have not been observed to map the Fermi surface. We solve the problem by resolving the Landau Level quantization and Fermi surface topology using torque magnetometry. The observed Fermi surface suggests a two dimensional surface state on the (101) plane. Furthermore, the tracking of the Landau Levels in the infinite magnetic field limit points to -1/2, which indicates a 2D Dirac electronic state. △ Less

Submitted 21 June, 2013; originally announced June 2013.

Journal ref: Science 346, 1208 (2014)

Visualizing the microscopic coexistence of spin density wave and superconductivity in underdoped NaFe1-xCoxAs

Authors: Peng Cai, Xiaodong Zhou, Wei Ruan, Aifeng Wang, Xianhui Chen, Dung-Hai Lee, Yayu Wang

Abstract: Although the origin of high temperature superconductivity in the iron pnictides is still under debate, it is widely believed that magnetic interactions or fluctuations play an important role in triggering Cooper pairing. Because of the relevance of magnetism to pairing, the question of whether long range spin magnetic order can coexist with superconductivity microscopically has attracted strong in… ▽ More Although the origin of high temperature superconductivity in the iron pnictides is still under debate, it is widely believed that magnetic interactions or fluctuations play an important role in triggering Cooper pairing. Because of the relevance of magnetism to pairing, the question of whether long range spin magnetic order can coexist with superconductivity microscopically has attracted strong interests. The available experimental methods used to answer this question are either bulk probes or local ones without control of probing position, thus the answers range from mutual exclusion to homogeneous coexistence. To definitively answer this question, here we use scanning tunneling microscopy to investigate the local electronic structure of an underdoped NaFe1-xCoxAs near the spin density wave (SDW) and superconducting (SC) phase boundary. Spatially resolved spectroscopy directly reveal both the SDW and SC gap features at the same atomic location, providing compelling evidence for the microscopic coexistence of the two phases. The strengths of the SDW and SC features are shown to anti correlate with each other, indicating the competition of the two orders. The microscopic coexistence clearly indicates that Cooper pairing occurs when portions of the Fermi surface (FS) are already gapped by the SDW order. The regime TC < T < TSDW thus show a strong resemblance to the pseudogap phase of the cuprates where growing experimental evidences suggest a FS reconstruction due to certain density wave order. In this phase of the pnictides, the residual FS has a favorable topology for magnetically mediated pairing when the ordering moment of the SDW is small. △ Less

Submitted 19 August, 2012; originally announced August 2012.

Comments: 18 pages, 4 figures

Evolution from unconventional spin density wave to superconductivity and a novel gap-like phase in NaFe1-xCoxAs

Authors: Xiaodong Zhou, Peng Cai, Aifeng Wang, Wei Ruan, Cun Ye, Xianhui Chen, Yizhuang You, Zheng-Yu Weng, Yayu Wang

Abstract: Similar to the cuprate high TC superconductors, the iron pnictide superconductors also lie in close proximity to a magnetically ordered phase. A central debate concerning the superconducting mechanism is whether the local magnetic moments play an indispensable role or the itinerant electron description is sufficient. A key step for resolving this issue is to acquire a comprehensive picture regardi… ▽ More Similar to the cuprate high TC superconductors, the iron pnictide superconductors also lie in close proximity to a magnetically ordered phase. A central debate concerning the superconducting mechanism is whether the local magnetic moments play an indispensable role or the itinerant electron description is sufficient. A key step for resolving this issue is to acquire a comprehensive picture regarding the nature of various phases and interactions in the iron compounds. Here we report the doping, temperature, and spatial evolutions of the electronic structure of NaFe1-xCoxAs studied by scanning tunneling microscopy. The spin density wave gap in the parent state is observed for the first time, which shows a strongly asymmetric lineshape that is incompatible with the conventional Fermi surface nesting scenario. The optimally doped sample exhibits a single, symmetric energy gap, but in the overdoped regime another asymmetric gap-like feature emerges near the Fermi level. This novel gap-like phase coexists with superconductivity in the ground state, persists deep into the normal state, and shows strong spatial variations. The characteristics of the three distinct low energy states, in conjunction with the peculiar high energy spectra, suggest that the coupling between the local moments and itinerant electrons is the fundamental driving force for the phases and phase transitions in the iron pnictides. △ Less

Submitted 18 April, 2012; originally announced April 2012.

Comments: 4 figures + supplementary information

Journal ref: Physical Review Letters 109, 037002 (2012)

Visualizing the atomic scale electronic structure of the Ca2CuO2Cl2 Mott insulator

Authors: Cun Ye, Peng Cai, Runze Yu, Xiaodong Zhou, Wei Ruan, Qingqing Liu, Changqing Jin, Yayu Wang

Abstract: Although the mechanism of superconductivity in the cuprates remains elusive, it is generally agreed that at the heart of the problem is the physics of doped Mott insulators. The cuprate parent compound has one unpaired electron per Cu site, and is predicted by band theory to be a half-filled metal. The strong onsite Coulomb repulsion, however, prohibits electron hopping between neighboring sites a… ▽ More Although the mechanism of superconductivity in the cuprates remains elusive, it is generally agreed that at the heart of the problem is the physics of doped Mott insulators. The cuprate parent compound has one unpaired electron per Cu site, and is predicted by band theory to be a half-filled metal. The strong onsite Coulomb repulsion, however, prohibits electron hopping between neighboring sites and leads to a Mott insulator ground state with antiferromagnetic (AF) ordering. Charge carriers doped into the CuO2 plane destroy the insulating phase and superconductivity emerges as the carrier density is sufficiently high. The natural starting point for tackling high Tc superconductivity is to elucidate the electronic structure of the parent Mott insulator and the behavior of a single doped charge. Here we use a scanning tunneling microscope to investigate the atomic scale electronic structure of the Ca2CuO2Cl2 parent Mott insulator of the cuprates. The full electronic spectrum across the Mott-Hubbard gap is uncovered for the first time, which reveals the particle-hole symmetric and spatially uniform Hubbard bands. A single electron donated by surface defect is found to create a broad in-gap electronic state that is strongly localized in space with spatial characteristics intimately related to the AF spin background. The unprecedented real space electronic structure of the parent cuprate sheds important new light on the origion of high Tc superconductivity from the doped Mott insulator perspective. △ Less

Submitted 1 January, 2012; originally announced January 2012.

Comments: 26 pages, 4 figures, supplementary information included

Journal ref: Nature Communications 4, 1365 (2013)

Imaging the coexistence of superconductivity and a charge density modulation in K0.73Fe1.67Se2 superconductor

Authors: Peng Cai, Cun Ye, Wei Ruan, Xiaodong Zhou, Aifeng Wang, Meng Zhang, Xianhui Chen, Yayu Wang

Abstract: We report scanning tunneling microscopy studies of the local structural and electronic properties of the iron selenide superconductor K0.73Fe1.67Se2 with TC = 32K. On the atomically resolved FeSe surface, we observe well-defined superconducting gap and the microscopic coexistence of a charge density modulation with root2*root2 periodicity with respect to the original Se lattice. We propose that a… ▽ More We report scanning tunneling microscopy studies of the local structural and electronic properties of the iron selenide superconductor K0.73Fe1.67Se2 with TC = 32K. On the atomically resolved FeSe surface, we observe well-defined superconducting gap and the microscopic coexistence of a charge density modulation with root2*root2 periodicity with respect to the original Se lattice. We propose that a possible origin of the pattern is the electronic superstructure caused by the block antiferromagnetic ordering of the iron moments. The widely expected iron vacancy ordering is not observed, indicating that it is not a necessary ingredient for superconductivity in the intercalated iron selenides. △ Less

Submitted 13 August, 2011; originally announced August 2011.

Comments: 16 pages, 5 figures

Journal ref: Physical Review B 85, 094512 (2012)

Quasiparticle interference of C2-symmetric surface states in LaOFeAs parent compound

Authors: Xiaodong Zhou, Cun Ye, Peng Cai, Xiangfeng Wang, Xianhui Chen, Yayu Wang

Abstract: We present scanning tunneling microscopy studies of the LaOFeAs parent compound of iron pnictide superconductors. Topographic imaging reveals two types of atomically flat surfaces, corresponding to the exposed LaO layer and FeAs layer respectively. On one type of surface, we observe strong standing wave patterns induced by quasiparticle interference of two-dimensional surface states. The distribut… ▽ More We present scanning tunneling microscopy studies of the LaOFeAs parent compound of iron pnictide superconductors. Topographic imaging reveals two types of atomically flat surfaces, corresponding to the exposed LaO layer and FeAs layer respectively. On one type of surface, we observe strong standing wave patterns induced by quasiparticle interference of two-dimensional surface states. The distribution of scattering wavevectors exhibits pronounced two-fold symmetry, consistent with the nematic electronic structure found in the Ca(Fe1-xCox)2As2 parent state. △ Less

Submitted 16 August, 2010; originally announced August 2010.

Comments: 13 pages, 4 figures

Journal ref: Phys. Rev. Lett. 106, 087001 (2011)