We have studied the Ge(112)-(4×1)-In reconstruction with scanning tunneling microscopy (STM). Bas... more We have studied the Ge(112)-(4×1)-In reconstruction with scanning tunneling microscopy (STM). Based on our bias dependent STM images and the characteristic electronic structure of stable submonolayer group Ⅲ-metal/group-Ⅳ-semiconductor interfaces, we propose an atomic model of this reconstruction for further investigation. The adsorption sites of In atoms are the same as In atoms' adsorption sites in the Si(112)-(7×1)-In reconstruction, but are different from the adsorption sites of Al and Ga atoms on the Si(112) surface. We suggest that this difference in adsorption sites is mainly due to the longer covalent bond length of In atoms. We further propose that, due to the smaller misfit, Al and Ga atoms may form longer nano wires along the step edges on the Ge(112) surface.
Two-dimensional (2D) palladium diselenide (PdSe2 ) has strong interlayer coupling and a puckered ... more Two-dimensional (2D) palladium diselenide (PdSe2 ) has strong interlayer coupling and a puckered pentagonal structure, leading to remarkable layer-dependent electronic structures and highly anisotropic in-plane optical and electronic properties. However, the lack of high-quality, 2D PdSe2 crystals grown by bottom-up approaches limits the study of their exotic properties and practical applications. In this work, chemical vapor deposition growth of highly crystalline few-layer (≥2 layers) PdSe2 crystals on various substrates is reported. The high quality of the PdSe2 crystals is confirmed by low-frequency Raman spectroscopy, scanning transmission electron microscopy, and electrical characterization. In addition, strong in-plane optical anisotropy is demonstrated via polarized Raman spectroscopy and second-harmonic generation maps of the PdSe2 flakes. A theoretical model based on kinetic Wulff construction theory and density functional theory calculations is developed and described the observed evolution of "square-like" shaped PdSe2 crystals into rhombus due to the higher nucleation barriers for stable attachment on the (1,1) and (1,-1) edges, which results in their slower growth rates. Few-layer PdSe2 field-effect transistors reveal tunable ambipolar charge carrier conduction with an electron mobility up to ≈294 cm2 V-1 s-1 , which is comparable to that of exfoliated PdSe2 , indicating the promise of this anisotropic 2D material for electronics.
Our LEED and STM results confirm that annealed surfaces of the Ga/Ge(001) system, in contrast to ... more Our LEED and STM results confirm that annealed surfaces of the Ga/Ge(001) system, in contrast to those of the Al/Si(001), In/Si(001) and In/Ge(001) systems, reconstruct rather than facet to {103} facets. In the submonolayer range of Ga coverage there appears to be only one Ga-related reconstruction (i.e. Ge(001)-(7 x 5.5)-Ga). On the basis of the information gathered from our high-resolution STM images, and taking into account the three common features of stable surface structures of group III metal-group IV semiconductor systems which have been disclosed very recently, an atomic structure model is proposed for the reconstruction for further investigations. (C) 1998 Elsevier Science B.V. All rights reserved.
The low-temperature scanning tunneling microscope and spectroscopy (STM/STS) are used to visualiz... more The low-temperature scanning tunneling microscope and spectroscopy (STM/STS) are used to visualize superconducting states in the cleaved single crystal of 9% praseodymium-doped CaFe2As2 (Pr-Ca122) with Tc ≈ 30 K. The spectroscopy shows strong spatial variations in the density of states (DOS), and the superconducting map constructed from spectroscopy discloses a localized superconducting phase, as small as a single unit cell. The comparison of the spectra taken at 4.2 K and 22 K (below vs. close to the bulk superconducting transition temperature) from the exact same area confirms the superconducting behavior. Nanoscale superconducting states have been found near Pr dopants, which can be identified using dI/dV conductance maps at +300 mV. There is no correlation of the local superconductivity to the surface reconstruction domain and surface defects, which reflects its intrinsic bulk behavior. We, therefore, suggest that the local strain of Pr dopants is competing with defects induced ...
The electronic inhomogeneities in Co, Ni, and Cr doped BaFe2As2 single crystals are compared with... more The electronic inhomogeneities in Co, Ni, and Cr doped BaFe2As2 single crystals are compared within three bulk property regions: a pure superconducting (SC) dome region, a coexisting SC and antiferromagnetic (AFM) region, and a non-SC region. Machine learning is utilized to categorize the inhomogeneous electronic states: in-gap, L-shape, and S-shape states. Although the relative percentages of the states vary in the three samples, the total volume fraction of the three electronic states is quite similar. This is coincident with the number of electrons (Ni0.04 and Co0.08) and holes (Cr0.04) doped into the compounds. The in-gap state is confirmed as a magnetic impurity state from the Co or Ni dopants, the L-shape state is identified as a spin density wave which competes with the SC phase, and the S-shape state is found to be another form of magnetic order which constructively cooperates with the SC phase, rather than competing with it.
Thin flakes of CrNbS are fabricated successfully via microexfoliation techniques. Temperature-dep... more Thin flakes of CrNbS are fabricated successfully via microexfoliation techniques. Temperature-dependent and field-dependent magnetizations of thin flakes with various thicknesses are investigated. When the thickness of the flake is around several hundred nanometers, the softening and eventual disappearance of the bulk soliton peak is accompanied by the appearance of other magnetic peaks at lower magnetic fields. The emergence and annihilation of the soliton peaks are explained and simulated theoretically by the change in spin spiral number inside the soliton lattice due to dimensional confinement. Compared to the conventional magnetic states in nanoscale materials, the stability and thickness tunability of quantified spin spirals make CrNbS a potential candidate for spintronics nanodevices beyond Moore's law.
We report the synthesis of single-crystal iron germanium nanowires via chemical vapor deposition ... more We report the synthesis of single-crystal iron germanium nanowires via chemical vapor deposition without the assistance of any catalysts. The assembly of single-crystal FeGe2 nanowires with tetragonal C16 crystal structure shows anisotropic magnetic behavior along the radial direction or the growth axial direction, with both antiferromagnetic and ferromagnetic orders. Single FeGe2 nanowire devices were fabricated using e-beam lithography. Electronic transport measurement in these devices show two resistivity anomalies near 250 K and 200 K which are likely signatures of the two spin density wave states in FeGe2.
Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indi... more Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.
Combined scanning tunneling microscopy, spectroscopy and local barrier height (LBH) studies show ... more Combined scanning tunneling microscopy, spectroscopy and local barrier height (LBH) studies show that low-temperature-cleaved optimally-doped Ba(Fe1-xCox)2As2 crystals with x=0.06, with Tc = 22 K, have complicated morphologies. Although the cleavage surface and hence the morphologies are variable, the superconducting gap maps show the same gap widths and nanometer size inhomogeneities irrelevant to the morphology. Based on the spectroscopy and LBH maps, the bright patches and dark stripes in the morphologies are identified as Ba and As dominated surface terminations, respectively. Magnetic impurities, possibly due to cobalt or Fe atoms, are believed to create local in-gap state and in addition suppress the superconducting coherence peaks. This study will clarify the confusion on the cleavage surface terminations of the Fe-based superconductors, and its relation with the electronic structures.
Single crystalline BaMnSb2 is considered as a 3D Weyl semimetal with the 2D electronic structure ... more Single crystalline BaMnSb2 is considered as a 3D Weyl semimetal with the 2D electronic structure containing Dirac cones from the Sb sheet. We report experimental investigation of low-temperature cleaved BaMnSb2 surfaces using scanning tunneling microscopy/spectroscopy and low energy electron diffraction. By natural cleavage, we find two terminations: one is Ba (above the orthorhombically distorted Sb sheet) and another Sb2 (at the surface of the Sb/Mn/Sb sandwich layer). Both terminations show the 2 × 1 surface reconstructions, with drastically different morphologies and electronic properties, however. The reconstructed structures, defect types and nature of the electronic structures of the two terminations are extensively studied. The quasiparticle interference (QPI) analysis is conducted at the energy range between −2 V and 2 V, although no interesting states are observed near the Fermi level, the surface-projected electronic band structures strongly depend on the surface terminat...
We have studied the Ge(112)-(4 x 1) -In reconstruction with scanning tunneling microscopy (STM). ... more We have studied the Ge(112)-(4 x 1) -In reconstruction with scanning tunneling microscopy (STM). Based on our bias-dependent STM images and the characteristic electronic structure of stable submonolayer group-III-metal/group-IV-semiconductor interfaces, we propose an atomic model of this reconstruction for further investigation. The adsorption sites of In atoms are the same as In atoms' adsorption sites in the Si( 112) -(7 x 1) -In reconstruction, but are different from the adsorption sites of Al and Ga atoms on the Si( 112) surface. We suggest that this difference in adsorption sites is mainly due to the longer covalent bond length of In atoms. We further propose that, due to the smaller misfit, Al and Ga atoms may form longer nano-Aires along the step edges on the Ge(112) surface.
ABSTRACT Behaviors such as high Tc superconductivity, colossal magnetoresistivity, and the metal-... more ABSTRACT Behaviors such as high Tc superconductivity, colossal magnetoresistivity, and the metal-insulator transition, have been tied to inherent electronic phases coexisting in a single crystal material. Here we demonstrate a novel approach to induce resistive electric field effect transitions based on the modification of the inherent electronic domain structures in single crystal materials. A phase separated manganite system confined to a scale which isolates a few electronic domains is controlled using laterally gated which give repeatable resistive changes of up to 50%. This technique also makes it possible to create multistate switching devices from a single confined transport channel. These findings provide an avenue to control inherent electronic phases in strongly correlated materials as a means of creating novel nano-electronic devices. Supported by the US DOE Office of Basic Energy Sciences, Materials Sciences and Engineering Division.
The mechanism of magnetic order in iron-oxypnictides has attracted lots of research efforts, but ... more The mechanism of magnetic order in iron-oxypnictides has attracted lots of research efforts, but the conclusions are so distinct: local moment ground state with antiferromagnetic fluctuation vs. itinerant ground state with ferromagnetic fluctuation, although a spin-density-wave (SDW) ground state is generally observed in polycrystalline parent compounds. Because single-crystal growth of iron-oxypnictides seems very tough, the fabrication of epitaxial thin films by pulsed laser deposition (PLD) technique should be an important solution for basic research. Here we report a successfully fabrication of c-axis oriented high quality CeFeAsO(001) heteroepitaxial ultrathin film, which shows an atomic flat surface with (4x1) reconstruction at initial stage. We find a striking in-plane ferromagnetic ground state with a small Fe moment in CeFeAsO ultrathin film, followed by the development of spin glass like order with thickness. Therefore, the robust ferromagnetism in single crystalline parent compound thin film implies that the superconductivity in 1111 system should be mediated by ferromagnetic spin fluctuation.
Two-dimensional materials such as layered transition-metal dichalcogenides (TMDs) are ideal platf... more Two-dimensional materials such as layered transition-metal dichalcogenides (TMDs) are ideal platforms for studying defect behaviors, an essential step towards defect engineering for novel material functions. Here, we image the 3D lattice locations of selenium-vacancy V_{Se} defects and manipulate them using a scanning tunneling microscope (STM) near the surface of PdSe_{2}, a recently discovered pentagonal layered TMD. The V_{Se} show a characterisitc charging ring in a spatially resolved conductance map, based on which we can determine its subsurface lattice location precisely. Using the STM tip, not only can we reversibly switch the defect states between charge neutral and charge negative, but also trigger migrations of V_{Se} defects. This allows a demonstration of direct "writing" and "erasing" of atomic defects and tracing the diffusion pathways. First-principles calculations reveal a small diffusion barrier of V_{Se} in PdSe_{2}, which is much lower than S ...
We have studied the Ge(112)-(4×1)-In reconstruction with scanning tunneling microscopy (STM). Bas... more We have studied the Ge(112)-(4×1)-In reconstruction with scanning tunneling microscopy (STM). Based on our bias dependent STM images and the characteristic electronic structure of stable submonolayer group Ⅲ-metal/group-Ⅳ-semiconductor interfaces, we propose an atomic model of this reconstruction for further investigation. The adsorption sites of In atoms are the same as In atoms' adsorption sites in the Si(112)-(7×1)-In reconstruction, but are different from the adsorption sites of Al and Ga atoms on the Si(112) surface. We suggest that this difference in adsorption sites is mainly due to the longer covalent bond length of In atoms. We further propose that, due to the smaller misfit, Al and Ga atoms may form longer nano wires along the step edges on the Ge(112) surface.
Two-dimensional (2D) palladium diselenide (PdSe2 ) has strong interlayer coupling and a puckered ... more Two-dimensional (2D) palladium diselenide (PdSe2 ) has strong interlayer coupling and a puckered pentagonal structure, leading to remarkable layer-dependent electronic structures and highly anisotropic in-plane optical and electronic properties. However, the lack of high-quality, 2D PdSe2 crystals grown by bottom-up approaches limits the study of their exotic properties and practical applications. In this work, chemical vapor deposition growth of highly crystalline few-layer (≥2 layers) PdSe2 crystals on various substrates is reported. The high quality of the PdSe2 crystals is confirmed by low-frequency Raman spectroscopy, scanning transmission electron microscopy, and electrical characterization. In addition, strong in-plane optical anisotropy is demonstrated via polarized Raman spectroscopy and second-harmonic generation maps of the PdSe2 flakes. A theoretical model based on kinetic Wulff construction theory and density functional theory calculations is developed and described the observed evolution of "square-like" shaped PdSe2 crystals into rhombus due to the higher nucleation barriers for stable attachment on the (1,1) and (1,-1) edges, which results in their slower growth rates. Few-layer PdSe2 field-effect transistors reveal tunable ambipolar charge carrier conduction with an electron mobility up to ≈294 cm2 V-1 s-1 , which is comparable to that of exfoliated PdSe2 , indicating the promise of this anisotropic 2D material for electronics.
Our LEED and STM results confirm that annealed surfaces of the Ga/Ge(001) system, in contrast to ... more Our LEED and STM results confirm that annealed surfaces of the Ga/Ge(001) system, in contrast to those of the Al/Si(001), In/Si(001) and In/Ge(001) systems, reconstruct rather than facet to {103} facets. In the submonolayer range of Ga coverage there appears to be only one Ga-related reconstruction (i.e. Ge(001)-(7 x 5.5)-Ga). On the basis of the information gathered from our high-resolution STM images, and taking into account the three common features of stable surface structures of group III metal-group IV semiconductor systems which have been disclosed very recently, an atomic structure model is proposed for the reconstruction for further investigations. (C) 1998 Elsevier Science B.V. All rights reserved.
The low-temperature scanning tunneling microscope and spectroscopy (STM/STS) are used to visualiz... more The low-temperature scanning tunneling microscope and spectroscopy (STM/STS) are used to visualize superconducting states in the cleaved single crystal of 9% praseodymium-doped CaFe2As2 (Pr-Ca122) with Tc ≈ 30 K. The spectroscopy shows strong spatial variations in the density of states (DOS), and the superconducting map constructed from spectroscopy discloses a localized superconducting phase, as small as a single unit cell. The comparison of the spectra taken at 4.2 K and 22 K (below vs. close to the bulk superconducting transition temperature) from the exact same area confirms the superconducting behavior. Nanoscale superconducting states have been found near Pr dopants, which can be identified using dI/dV conductance maps at +300 mV. There is no correlation of the local superconductivity to the surface reconstruction domain and surface defects, which reflects its intrinsic bulk behavior. We, therefore, suggest that the local strain of Pr dopants is competing with defects induced ...
The electronic inhomogeneities in Co, Ni, and Cr doped BaFe2As2 single crystals are compared with... more The electronic inhomogeneities in Co, Ni, and Cr doped BaFe2As2 single crystals are compared within three bulk property regions: a pure superconducting (SC) dome region, a coexisting SC and antiferromagnetic (AFM) region, and a non-SC region. Machine learning is utilized to categorize the inhomogeneous electronic states: in-gap, L-shape, and S-shape states. Although the relative percentages of the states vary in the three samples, the total volume fraction of the three electronic states is quite similar. This is coincident with the number of electrons (Ni0.04 and Co0.08) and holes (Cr0.04) doped into the compounds. The in-gap state is confirmed as a magnetic impurity state from the Co or Ni dopants, the L-shape state is identified as a spin density wave which competes with the SC phase, and the S-shape state is found to be another form of magnetic order which constructively cooperates with the SC phase, rather than competing with it.
Thin flakes of CrNbS are fabricated successfully via microexfoliation techniques. Temperature-dep... more Thin flakes of CrNbS are fabricated successfully via microexfoliation techniques. Temperature-dependent and field-dependent magnetizations of thin flakes with various thicknesses are investigated. When the thickness of the flake is around several hundred nanometers, the softening and eventual disappearance of the bulk soliton peak is accompanied by the appearance of other magnetic peaks at lower magnetic fields. The emergence and annihilation of the soliton peaks are explained and simulated theoretically by the change in spin spiral number inside the soliton lattice due to dimensional confinement. Compared to the conventional magnetic states in nanoscale materials, the stability and thickness tunability of quantified spin spirals make CrNbS a potential candidate for spintronics nanodevices beyond Moore's law.
We report the synthesis of single-crystal iron germanium nanowires via chemical vapor deposition ... more We report the synthesis of single-crystal iron germanium nanowires via chemical vapor deposition without the assistance of any catalysts. The assembly of single-crystal FeGe2 nanowires with tetragonal C16 crystal structure shows anisotropic magnetic behavior along the radial direction or the growth axial direction, with both antiferromagnetic and ferromagnetic orders. Single FeGe2 nanowire devices were fabricated using e-beam lithography. Electronic transport measurement in these devices show two resistivity anomalies near 250 K and 200 K which are likely signatures of the two spin density wave states in FeGe2.
Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indi... more Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.
Combined scanning tunneling microscopy, spectroscopy and local barrier height (LBH) studies show ... more Combined scanning tunneling microscopy, spectroscopy and local barrier height (LBH) studies show that low-temperature-cleaved optimally-doped Ba(Fe1-xCox)2As2 crystals with x=0.06, with Tc = 22 K, have complicated morphologies. Although the cleavage surface and hence the morphologies are variable, the superconducting gap maps show the same gap widths and nanometer size inhomogeneities irrelevant to the morphology. Based on the spectroscopy and LBH maps, the bright patches and dark stripes in the morphologies are identified as Ba and As dominated surface terminations, respectively. Magnetic impurities, possibly due to cobalt or Fe atoms, are believed to create local in-gap state and in addition suppress the superconducting coherence peaks. This study will clarify the confusion on the cleavage surface terminations of the Fe-based superconductors, and its relation with the electronic structures.
Single crystalline BaMnSb2 is considered as a 3D Weyl semimetal with the 2D electronic structure ... more Single crystalline BaMnSb2 is considered as a 3D Weyl semimetal with the 2D electronic structure containing Dirac cones from the Sb sheet. We report experimental investigation of low-temperature cleaved BaMnSb2 surfaces using scanning tunneling microscopy/spectroscopy and low energy electron diffraction. By natural cleavage, we find two terminations: one is Ba (above the orthorhombically distorted Sb sheet) and another Sb2 (at the surface of the Sb/Mn/Sb sandwich layer). Both terminations show the 2 × 1 surface reconstructions, with drastically different morphologies and electronic properties, however. The reconstructed structures, defect types and nature of the electronic structures of the two terminations are extensively studied. The quasiparticle interference (QPI) analysis is conducted at the energy range between −2 V and 2 V, although no interesting states are observed near the Fermi level, the surface-projected electronic band structures strongly depend on the surface terminat...
We have studied the Ge(112)-(4 x 1) -In reconstruction with scanning tunneling microscopy (STM). ... more We have studied the Ge(112)-(4 x 1) -In reconstruction with scanning tunneling microscopy (STM). Based on our bias-dependent STM images and the characteristic electronic structure of stable submonolayer group-III-metal/group-IV-semiconductor interfaces, we propose an atomic model of this reconstruction for further investigation. The adsorption sites of In atoms are the same as In atoms' adsorption sites in the Si( 112) -(7 x 1) -In reconstruction, but are different from the adsorption sites of Al and Ga atoms on the Si( 112) surface. We suggest that this difference in adsorption sites is mainly due to the longer covalent bond length of In atoms. We further propose that, due to the smaller misfit, Al and Ga atoms may form longer nano-Aires along the step edges on the Ge(112) surface.
ABSTRACT Behaviors such as high Tc superconductivity, colossal magnetoresistivity, and the metal-... more ABSTRACT Behaviors such as high Tc superconductivity, colossal magnetoresistivity, and the metal-insulator transition, have been tied to inherent electronic phases coexisting in a single crystal material. Here we demonstrate a novel approach to induce resistive electric field effect transitions based on the modification of the inherent electronic domain structures in single crystal materials. A phase separated manganite system confined to a scale which isolates a few electronic domains is controlled using laterally gated which give repeatable resistive changes of up to 50%. This technique also makes it possible to create multistate switching devices from a single confined transport channel. These findings provide an avenue to control inherent electronic phases in strongly correlated materials as a means of creating novel nano-electronic devices. Supported by the US DOE Office of Basic Energy Sciences, Materials Sciences and Engineering Division.
The mechanism of magnetic order in iron-oxypnictides has attracted lots of research efforts, but ... more The mechanism of magnetic order in iron-oxypnictides has attracted lots of research efforts, but the conclusions are so distinct: local moment ground state with antiferromagnetic fluctuation vs. itinerant ground state with ferromagnetic fluctuation, although a spin-density-wave (SDW) ground state is generally observed in polycrystalline parent compounds. Because single-crystal growth of iron-oxypnictides seems very tough, the fabrication of epitaxial thin films by pulsed laser deposition (PLD) technique should be an important solution for basic research. Here we report a successfully fabrication of c-axis oriented high quality CeFeAsO(001) heteroepitaxial ultrathin film, which shows an atomic flat surface with (4x1) reconstruction at initial stage. We find a striking in-plane ferromagnetic ground state with a small Fe moment in CeFeAsO ultrathin film, followed by the development of spin glass like order with thickness. Therefore, the robust ferromagnetism in single crystalline parent compound thin film implies that the superconductivity in 1111 system should be mediated by ferromagnetic spin fluctuation.
Two-dimensional materials such as layered transition-metal dichalcogenides (TMDs) are ideal platf... more Two-dimensional materials such as layered transition-metal dichalcogenides (TMDs) are ideal platforms for studying defect behaviors, an essential step towards defect engineering for novel material functions. Here, we image the 3D lattice locations of selenium-vacancy V_{Se} defects and manipulate them using a scanning tunneling microscope (STM) near the surface of PdSe_{2}, a recently discovered pentagonal layered TMD. The V_{Se} show a characterisitc charging ring in a spatially resolved conductance map, based on which we can determine its subsurface lattice location precisely. Using the STM tip, not only can we reversibly switch the defect states between charge neutral and charge negative, but also trigger migrations of V_{Se} defects. This allows a demonstration of direct "writing" and "erasing" of atomic defects and tracing the diffusion pathways. First-principles calculations reveal a small diffusion barrier of V_{Se} in PdSe_{2}, which is much lower than S ...
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