arXiv subjects

Y. G. Shi

Publications and source records attributed to Y. G. Shi.

At least 55 records · Page 3Linked to original sources

arXiv2012

Continuous magnetic phase transition in half-frustrated Ca2Os2O7

We present the specific heat, magnetization, optical spectroscopy measurements and the firstprinciple calculations on the Weberite structure Ca2Os2O7 single crystal/polycrystalline sample. The Ca2Os2O7 shows a Curie-Weiss nature at high temperature and goes into a ferrimagnetic insulating state at 327 K on cooling. A λ-like peak is observed at 327 K in the specific heat implying a second-order phase transition. The vanishing electronic specific heat at low temperature suggests a full energy gap. At high temperature above the transition, small amount of itinerant carriers with short life time τare observed, which is gapped at 20 K with a direct gap of 0:24 eV . Our first principle calculations indicate that the anti-ferromagnetic (AFM) correlation with intermediate Coulomb repulsion U could effectively split Os(4b) t2g bands and push them away from Fermi level(EF). On the other hand, a non-collinear magnetic interaction is needed to push the Os(4c) bands away from EF, which could be induced by Os(4c)-Os(4c) frustration. Therefore, AFM correlation, Coulomb repulsion U and non-collinear interaction all play important roles for the insulating ground state in Ca2Os2O7.

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arXiv2012

Magnetic structure determination of Ca$_3$LiOsO$_6$ using neutron and x-ray scattering

We present a neutron and x-ray scattering investigation of Ca$_3$LiOsO$_6$, a material predicted to host magnetic ordering solely through an extended superexchange pathway involving two anions, an interaction mechanism that has undergone relatively little investigation. This contrasts with the ubiquitous superexchange interaction mechanism involving a single anion that has well defined and long standing rules. Despite the apparent 1D nature and triangular units of magnetic osmium ions the onset of magnetic correlations has been observed at a high temperature of 117 K in bulk measurements. We experimentally determine the magnetically ordered structure and show it to be long range and three dimensional. Our results support the model of extended superexchange interaction.

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arXiv2012

Single crystal growth and optical conductivity of SrPt$_2$As$_2$ superconductors

SrPt$_2$As$_2$ single crystals with CaBe$_2$Ge$_2$-type structure were synthesized by self-melting technique. X-ray diffraction, transmission electron microscopy, electrical resistivity, specific heat and optical spectroscopy measurements were conducted to elucidate the properties of SrPt$_2$As$_2$. SrPt$_2$As$_2$ single crystals exhibit a superconducting transition at 5.2K, experiencing a structural phase transition well above room temperature (about 455K). The superconducting and structural phase transition temperatures are both reduced by 6% Iridium doping. Both pure SrPt$_2$As$_2$ and the doped single crystals are revealed to be highly metallic with rather high plasma frequencies. In particular, the optical spectroscopy measurement revealed two gap-like suppression features. We elaborated that the one at higher energy scale could be ascribed to the correlation effect, while the other one at lower energy scale is related to the structural phase transition, leading to the removal of a small portion of the Fermi surfaces.

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arXiv2012

Two successive field-induced spin-flop transitions in single-crystalline CaCo$_{2}$As$_{2}$

CaCo$_{2}$As$_{2}$, a ThCr$_{2}$Si$_{2}$-structure compound, undergoes an antiferromagnetic transition at \emph{T$_{N}$}=76K with the magnetic moments being aligned parallel to the \emph{c} axis. Electronic transport measurement reveals that the coupling between conducting carriers and magnetic order in CaCo$_{2}$As$_{2}$ is much weaker comparing to the parent compounds of iron pnictide. Applying magnetic field along \emph{c} axis induces two successive spin-flop transitions in its magnetic state. The magnetization saturation behaviors with \emph{\textbf{H}$\parallel$c} and \emph{\textbf{H}$\parallel$ab} at 10K indicate that the antiferromagnetic coupling along \emph{c} direction is very weak. The interlayer antiferromagntic coupling constant \emph{J$_{c}$} is estimated to be about 2 meV.

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arXiv2012

Magnetically driven metal-insulator transition in NaOsO3

The metal-insulator transition (MIT) is one of the most dramatic manifestations of electron correlations in materials. Various mechanisms producing MITs have been extensively considered, including the Mott (electron localization via Coulomb repulsion), Anderson (localization via disorder) and Peierls (localization via distortion of a periodic 1D lattice). One additional route to a MIT proposed by Slater, in which long-range magnetic order in a three dimensional system drives the MIT, has received relatively little attention. Using neutron and X-ray scattering we show that the MIT in NaOsO3 is coincident with the onset of long-range commensurate three dimensional magnetic order. Whilst candidate materials have been suggested, our experimental methodology allows the first definitive demonstration of the long predicted Slater MIT. We discuss our results in the light of recent reports of a Mott spin-orbit insulating state in other 5d oxides.

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arXiv2012

Synthesis and Physical Properties of FeSe1/2Te1/2 Superconductor

One of the most important properties of very recently reported FeSe based superconductors is the robustness of their superconductivity under applied magnetic field. The synthesis and control of superconductivity in FeSe based compounds is rather a difficult task. Synthesis and physical property characterization for optimized superconductivity of FeSe1/2Te1/2 at 13 K is reported here. The compound crystallized in a tetragonal structure with lattice parameters a = 3.8008(10) and c = 6.0187 (15) A. Magnetization measurements indicated bulk superconductivity with lower critical field (Hc1) of around 180 Oe. By applying Ginzburg Landau (GL) theory, the Hc2(0) value is estimated to be = 1840 kOe for the 90% of resistive transition. A heat capacity measurement revealed bulk superconductivity by a hump at Tc near 13 K, and an expected decrease was observed under an applied magnetic field.

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arXiv2011

Optical study of the multiple charge density wave transitions in ErTe$_3$

We present an optical spectroscopy study on singe crystalline ErTe$_3$, a rare-earth-element tri-telluride which experiences two successive charge density wave (CDW) transitions at T$_{c1}$ = 267 K and T$_{c2}$ = 150 K. Two corresponding gap features, centered at 2770 \cm ($\sim$ 343 meV) and 890 \cm ($\sim$ 110 meV) respectively, are clearly seen in ordered state. A pronounced Drude component, which exists at all measurement temperatures, demonstrates the partial gap character of both CDW orders. About half of the unmodulated Fermi surface (FS) remains in the CDW state at the lowest measurement temperature. The study also reveals that fluctuation effect is still prominent in this two-dimensional (2D) material.

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arXiv2011

Infrared spectrum and its implications for the electronic structure of the semiconducting iron selenide K$_{0.83}$Fe$_{1.53}$Se$_2$

We report an infrared spectroscopy study on K$_{0.83}$Fe$_{1.53}$Se$_2$, a semiconducting parent compound of the new iron-selenide system. The major spectral features are found to be distinctly different from all other Fe-based superconducting systems. Our measurement revealed two peculiar spectral structures: a double peak structure between 4000-6000 cm$^{-1}$ and abundant phonon modes much more than those expected for a 122 structure. We elaborate that those features could be naturally explained from the blocked antiferromagnetism due to the presence of Fe vacancy ordering as determined by recent neutron diffraction experiments. The double peaks reflect the coexistence of ferromagnetic and antiferromagnetic couplings between the neighboring Fe sites.

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arXiv2011

Non-magnetic pair-breaking effect on La(Fe_{1-x}Zn_{x})AsO_{0.85} studied by NMR and NQR

$^{75}$As and $^{139}$La NMR and nuclear quadrupole resonance (NQR) studies on Zn-substituted LaFeAsO$_{0.85}$ have been performed to investigate the Zn-impurity effects microscopically. Although superconductivity in LaFeAsO$_{0.85}$ disappears by 3% Zn substitution, we found that NMR/NQR spectra and NMR physical quantities in the normal state are hardly changed, indicating that the crystal structure and electronic states are not modified by Zn substitution. Our results suggest that the suppression of superconductivity by Zn substitution is not due to the change of the normal-state properties, but due to strong non-magnetic pair-breaking effect to superconductivity.

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arXiv2011

Sr3Co2O6: a possible frustrated spin-chain material

A dense phase of a post-layered perovskite Sr3Co2O6 was created by a high-pressure and high-temperature synthetic route and quenched to ambient conditions. The structure of Sr3Co2O6 was determined by synchrotron X-ray powder diffraction. Sr3Co2O6 crystallizes in the K4CdCl6-type structure (an infinite chain-type structure), which is 6% denser than the known, iso-compositional layered perovskite Sr3Co2O7-d (d~1). The K4CdCl6-type Sr3Co2O6 shows complex magnetic behavior reflecting the spin-chain nature as well as the analogue oxides Ca3Co2O6 and Ca3CoRhO6, whose microscopic magnetic origin is still highly debated.

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arXiv2010

Magnetic and electrical properties and carrier doping effects on the Fe-based host compound Sr4Sc2Fe2As2O6

Additional charge carriers were introduced to the iron oxyarsenide Sr4Sc2Fe2As2O6 under a high-pressure condition, followed by measurements of electrical resistivity, Hall coefficient, and magnetic susceptibility. The host compound Sr4Sc2Fe2As2O6 shows metallic conductivity down to ~200 K and turns to show a semiconducting-like conductivity accompanied by a positive magneto-resistance (22% at 70 kOe). Although the carrier density is comparable at 300 K (5.9x1021 cm-3) with that of the other Fe-based superconductors, no superconductivity appears down to 2 K. This is primarily because the net carrier density decreases over 3 orders of magnitude on cooling and additionally a possible magnetic order at ~120 K prevents carriers from pairing. The properties were altered largely by introducing the additional carriers.

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arXiv2010

Large decrease in the critical temperature of superconducting LaFeAsO0.85 compounds doped with 3% atomic weight of nonmagnetic Zn impurities

We observed a large decrease of Tc by no more than 3 at.% of Zn doped to the optimized superconductor LaFeAsO0.85 (Tc = 26 K), confirmed by measurements of electrical resistivity, magnetic susceptibility, specific heat, Mössbauer spectroscopy, Hall coefficient, and an electron probe micro-analysis. The rate ~9 K/% is remarkably higher than observations regarding nonmagnetic impurities. The Tc suppression is likely due to pair-breaking caused by scatterings associated with highly localized electronic state of Zn doped into the Fe2As2 layer. If this is true, the Zn result well accords with the theoretical prediction that suggests a sign reversal s-wave pairing model for the Fe pnictide superconductors, unlike other nonmagnetic impurity results.

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arXiv2009

Drastic suppression of the superconductivity of LaFeAsO0.85 by a nonmagnetic impurity

We observed a drastic Tc suppression caused by no more than 3 at.% of Zn doped to the optimized superconductor LaFeAsO0.85 (Tc = 26 K). The electrical resistivity and magnetic susceptibility measurements suggested that it is likely due to impurity scatterings rather than losing the metallic nature, seeming to support the s+/- pairing model proposed for the Fe pnictide superconductor.

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arXiv2009

Continuous metal-insulator transition of the antiferromagnetic perovskite NaOsO3

Newly synthesized perovskite NaOsO3 shows Curie-Weiss metallic nature at high temperature and suddenly goes into an antiferromagnetically insulating state at 410 K on cooling. Electronic specific heat at the low temperature limit is absent, indicating that the band gap fully opens. In situ observation in electron microscopy undetected any lattice anomalies in the vicinity of the transition temperature. It is most likely that the antiferromagnetic correlation plays an essential role of the gap opening.

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arXiv2009

Superconducting properties of the oxygen-vacant iron oxyarsenide TbFeAsO1-x from underdoped to overdoped compositions

A wide-range doping was achieved by a high-pressure method for TbFeAsO1-x from "under doped" to "over doped" superconducting compositions throughout the optimized superconductivity (Tc of 44 K). Tc vs. lattice constant shows a dome-shaped feature, while Tc vs. the lattice constant likely follows a unique empirical curve over the doping range. The relatively large amount of oxygen vacancies up to 0.3 per the formula unit was introduced possibly because of the smaller replacement Tb than the other Ln (rare-earth element) in the LnFeAsO1-x system.

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arXiv2008

Magnetic and charge transport properties of the Na-based Os oxide pyrochlore

Na-based osmium oxide pyrochlore was synthesized for the first time by an ion-exchange method. KOs2O6 was used as a host compound. Elelectron probe micro-analysis, synchrotron x-ray diffraction analysis, and thermo-gravimetric analysis confirmed its structure not as the beta-type but as the defect-type pyrochlore. The composition was identified as Na1.4Os2O6.H2O. Electrical resistivity, heat capacity, and magnetization measurements of the polycrystalline Na1.4Os2O6.H2O clarified absence of superconductivity above 2 K, being in contrast to what were found for the beta-pyrochlore AOs2O6 (A = Cs, Rb, K). Sommerfeld coefficient of 22 mJ K-2 mol-1 of Na1.4Os2O6.H2O was smallest among those of AOs2O6. A magnetic anomaly at ~57 K and possible associated magnetoresistance (+3.7 % at 2 K in 70 kOe) were found.

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arXiv2008

Synthesis and superconducting properties of the iron oxyarsenide TbFeAsO0.85

The iron oxyarsenide TbFeAsO0.85 was synthesized by a high-pressure method. A high-quality polycrystalline sample was obtained and characterized by measurements of magnetic susceptibility and electrical resistivity. Bulk superconductivity with Tc of 42 K was clearly established without an F doping usually conducted to tune on superconductivity in the iron oxypnictide.

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arXiv2006

Electronic structure and electron energy-loss spectra of Sr0.35CoO2

The electronic structure of Sr0.35CoO2, structurally analogous to the layered NaxCoO2, has been evaluated using the local spin density approximation (LSDA). We find that evident c-dispersions appear in both the e'g and a1g Co-derived bands, demonstrating the existence of a notable interplanar interaction in Sr0.35CoO2. The LSDA+U calculation reveals that the electronic structure, in particular the band splitting between the spin-up and spin-down electrons, changes evidently along with the increase of the effective on-site Coulomb interaction U. Analysis of theoretical and experimental electron energy-loss spectra (EELS) for the oxygen K-edge and the Co L23-edge suggests that the on-site interaction (U) in Sr0.35CoO2 is less than 3eV which is noticeably weaker than the estimated value (from 5eV to 8eV) in the NaxCoO2 materials.

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