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Search Results: 1 - 10 of 27887 matches for " Xucun Ma "
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Scanning Tunneling Microscopy Studies of Topological Insulators Grown by Molecular Beam Epitaxy
Chen Xi,He Ke,Ma Xucun,Xue Qikun
EPJ Web of Conferences , 2012, DOI: 10.1051/epjconf/20122300020
Abstract: We summarize our recent scanning tunneling microscopy (STM) study of topological insulator thin films grown by molecular beam epitaxy (MBE), which includes the observation of electron standing waves on topological insulator surface and the Landau quantization of topological surface states. The work has provided valuable information to the understanding of intriguing properties of topological insulators, as predicted by theory.
Quantum size effect on the dissociation of O2 molecules on ultrathin Pb(111) films
Ziyu Hu,Yu Yang,Bo Sun,Xiaohong Shao,Wenchuan Wang,Xucun Ma,Qikun Xue,Ping Zhang
Physics , 2010,
Abstract: Using first-principles calculations, we systematically study the dissociation of O$_2$ molecules on different ultrathin Pb(111) films. Based on our previous work revealing the molecular adsorption precursor states for O$_2$, we further explore that why there are two nearly degenerate adsorption states on Pb(111) ultrathin films, but no precursor adsorption states exist at all on the Mg(0001) and Al(111) surfaces. And the reason is concluded to be the different surface electronic structures. For the O$_2$ dissociation, we consider both the reaction channels from gas-like and molecularly adsorbed O$_2$ molecules. We find that the energy barrier for O$_2$ dissociation from the molecular adsorption precursor states is always smaller than from O$_2$ gases. The most energetically favorable dissociation process is found to be the same on different Pb(111) films, and the energy barriers are found to be modulated by the quantum size effects of Pb(111) films.
Quantum and Classical Magnetoresistance in Ambipolar Topological Insulator Transistors with Gate-tunable Bulk and Surface Conduction
Jifa Tian,Cuizu Chang,Helin Cao,Ke He,Xucun Ma,Qikun Xue,Yong P. Chen
Physics , 2014,
Abstract: Weak antilocalization (WAL) and linear magnetoresistance (LMR) are two most commonly observed magnetoresistance (MR) phenomena in topological insulators (TIs) and often attributed to the Dirac topological surface states (TSS). However, ambiguities exist because these phenomena could also come from bulk states (often carrying significant conduction in many TIs) and are observable even in non-TI materials. Here, we demonstrate back-gated ambipolar TI field-effect transistors in (Bi0.04Sb0.96)2Te3 thin films grown by molecular beam epitaxy on SrTiO3(111), exhibiting a large carrier density tunability (by nearly 2 orders of magnitude) and a metal-insulator transition in the bulk (allowing effectively switching off the bulk conduction). Tuning the Fermi level from bulk band to TSS strongly enhances both the WAL (increasing the number of quantum coherent channels from one to peak around two) and LMR (increasing its slope by up to 10 times). The SS-enhanced LMR is accompanied by a strongly nonlinear Hall effect, suggesting important roles of charge inhomogeneity (and a related classical LMR), although existing models of LMR cannot capture all aspects of our data. Our systematic gate and temperature dependent magnetotransport studies provide deeper insights into the nature of both MR phenomena and reveal differences between bulk and TSS transport in TI related materials.
Anisotropic vortex lattice structures in the FeSe superconductor
Hsiang-Hsuan Hung,Can-Li Song,Xi Chen,Xucun Ma,Qi-kun Xue,Congjun Wu
Physics , 2011, DOI: 10.1103/PhysRevB.85.104510
Abstract: In the recent work by Song et al. [Science 332, 1410 (2011)], the scanning tunneling spectroscopy experiment in the stoichiometric FeSe reveals evidence for nodal superconductivity and strong anisotropy. The nodal structure can be explained with the extended s-wave pairing structure with the mixture of the $s_{x^2+y^2}$ and $s_{x^2y^2}$ pairing symmetries. We calculate the anisotropic vortex structure by using the self-consistent Bogoliubov-de Gennes mean-field theory. In considering the absence of magnetic ordering in the FeSe at the ambient pressure, orbital ordering is introduced, which breaks the $C_4$ lattice symmetry down to $C_2$, to explain the anisotropy in the vortex tunneling spectra.
Charge-Stripe Order in a Parent Compound of Iron-based Superconductors
Wei Li,Wei-Guo Yin,Lili Wang,Ke He,Xucun Ma,Qi-Kun Xue,Xi Chen
Physics , 2014,
Abstract: Charge ordering is one of the most intriguing and extensively studied phenomena in correlated electronic materials because of its strong impact on electron transport properties including superconductivity. Despite its ubiquitousness in correlated systems, the occurrence of charge ordering in iron-based superconductors is still unresolved. Here we use scanning tunneling microscopy to reveal a long-range charge-stripe order and a highly anisotropic dispersion of electronic states in the ground state of stoichiometric FeTe, the parent compound of the Fe(Te, Se, S) superconductor family. The formation of charge order in a strongly correlated electron system with integer nominal valence (here Fe$^{2+}$) is unexpected and suggests that the iron-based superconductors may exhibit more complex charge dynamics than originally expected. We show that the present observations can be attributed to the surpassing of the role of local Coulomb interaction by the poorly screened longer-range Coulomb interactions, facilitated by large Hund's rule coupling.
Experimental demonstration of the topological surface states protected by the time-reversal symmetry
Tong Zhang,Peng Cheng,Xi Chen,Jin-Feng Jia,Xucun Ma,Ke He,Lili Wang,Haijun Zhang,Xi Dai,Zhong Fang,Xincheng Xie,Qi-Kun Xue
Physics , 2009, DOI: 10.1103/PhysRevLett.103.266803
Abstract: We report direct imaging of standing waves of the nontrivial surface states of topological insulator Bi$_2$Te$_3$ by using a low temperature scanning tunneling microscope. The interference fringes are caused by the scattering of the topological states off Ag impurities and step edges on the Bi$_2$Te$_3$(111) surface. By studying the voltage-dependent standing wave patterns, we determine the energy dispersion $E(k)$, which confirms the Dirac cone structure of the topological states. We further show that, very different from the conventional surface states, the backscattering of the topological states by nonmagnetic impurities is completely suppressed. The absence of backscattering is a spectacular manifestation of the time-reversal symmetry, which offers a direct proof of the topological nature of the surface states.
KFe_2Se_2 is the parent compound of K-doped iron selenide superconductors
Wei Li,Hao Ding,Zhi Li,Peng Deng,Kai Chang,Ke He,Shuaihua Ji,Lili Wang,Xucun Ma,Jiang-Ping Hu,Xi Chen,Qi-Kun Xue
Physics , 2012, DOI: 10.1103/PhysRevLett.109.057003
Abstract: We elucidate the existing controversies in the newly discovered K-doped iron selenide (KxFe2-ySe2-z) superconductors. The stoichiometric KFe2Se2 with \surd2\times\surd2 charge ordering was identified as the parent compound of KxFe2-ySe2-z superconductor using scanning tunneling microscopy and spectroscopy. The superconductivity is induced in KFe2Se2 by either Se vacancies or interacting with the anti-ferromagnetic K2Fe4Se5 compound. Totally four phases were found to exist in KxFe2-ySe2-z: parent compound KFe2Se2, superconducting KFe2Se2 with \surd2\times\surd5 charge ordering, superconducting KFe2Se2-z with Se vacancies and insulating K2Fe4Se5 with \surd5\times\surd5 Fe vacancy order. The phase separation takes place at the mesoscopic scale under standard molecular beam epitaxy condition.
Phase Separation and Magnetic Order in K-doped Iron Selenide Superconductor
Wei Li,Hao Ding,Peng Deng,Kai Chang,Canli Song,Ke He,Lili Wang,Xucun Ma,Jiang-Ping Hu,Xi Chen,Qi-Kun Xue
Physics , 2011, DOI: 10.1038/nphys2155
Abstract: Alkali-doped iron selenide is the latest member of high Tc superconductor family, and its peculiar characters have immediately attracted extensive attention. We prepared high-quality potassium-doped iron selenide (KxFe2-ySe2) thin films by molecular beam epitaxy and unambiguously demonstrated the existence of phase separation, which is currently under debate, in this material using scanning tunneling microscopy and spectroscopy. The stoichiometric superconducting phase KFe2Se2 contains no iron vacancies, while the insulating phase has a \surd5\times\surd5 vacancy order. The iron vacancies are shown always destructive to superconductivity in KFe2Se2. Our study on the subgap bound states induced by the iron vacancies further reveals a magnetically-related bipartite order in the superconducting phase. These findings not only solve the existing controversies in the atomic and electronic structures in KxFe2-ySe2, but also provide valuable information on understanding the superconductivity and its interplay with magnetism in iron-based superconductors.
Fermi Level Tuning of Epitaxial Sb2Te3 Thin Films on Graphene by Regulating Intrinsic Defects and Substrate Transfer Doping
Yeping Jiang,Y. Y. Sun,Mu Chen,Yilin Wang,Zhi Li,Canli Song,Ke He,Lili Wang,Xi Chen,Qi-Kun Xue,Xucun Ma,S. B. Zhang
Physics , 2011, DOI: 10.1103/PhysRevLett.108.066809
Abstract: High-quality Sb2Te3 films are obtained by molecular beam epitaxy on graphene substrate and investigated by in situ scanning tunneling microscopy/spectroscopy. Intrinsic defects responsible for the natural p-type conductivity of Sb2Te3 are identified to be the Sb vacancies and SbTe antisites in agreement with first-principles calculations. By minimizing defect densities, coupled with a transfer doping by the graphene substrate, the Fermi level of Sb2Te3 thin films can be tuned over the entire range of the bulk band gap. This establishes the necessary condition to explore topological insulator behaviors near the Dirac point.
Landau quantization and the thickness limit of topological insulator thin films of Sb2Te3
Yeping Jiang,Yilin Wang,Mu Chen,Zhi Li,Canli Song,Ke He,Lili Wang,Xi Chen,Xucun Ma,Qi-Kun Xue
Physics , 2011, DOI: 10.1103/PhysRevLett.108.016401
Abstract: We report the experimental observation of Landau quantization of molecular beam epitaxy grown Sb2Te3 thin films by a low-temperature scanning tunneling microscope. Different from all the reported systems, the Landau quantization in Sb2Te3 topological insulator is not sensitive to the intrinsic substitutional defects in the films. As a result, a nearly perfect linear energy dispersion of surface states as 2D massless Dirac fermion system is achieved. We demonstrate that 4 quintuple layers are the thickness limit for Sb2Te3 thin film being a 3D topological insulator. The mechanism of the Landau level broadening is discussed in terms of enhanced quasiparticle lifetime.
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