arXiv subjects

J. Q. Li

Publications and source records attributed to J. Q. Li.

At least 55 records · Page 3Linked to original sources

arXiv2006

Raman Spectroscopy Study of alpha-, beta-, gamma-NaxCoO2 and gamma-(Ca,Sr)xCoO2

Raman spectroscopy measurements have been performed on alpha-, beta-, gamma-NaxCoO2 phases differing in their stacking of CoO6 octahedra along the c-axis direction. The results demonstrate that, in general, there are five active phonons for gamma-Na0.75CoO2, two Raman active phonons for alpha-NaCoO2, and four Raman active phonons for beta-NaCoO2. We have also performed Raman scattering measurements on several gamma-(Ca,Sr)xCoO2 (0.15 <= x <= 0.35) samples which show well-defined intercalated Ca/Sr-ordering. The experimental data show that the intercalated cation ordering could result in visible alterations on Raman spectral structures. The observations of the spectral changes along with the variation of the CoO6 stacking, as well as the intercalated Sr/Ca ordering suggest that the interlayer interaction plays an important role for understanding the lattice dynamics in this layered system.

cond-mat.supr-con↗

arXiv2006

Correlations among superconductivity, structural instability, and band filling in Nb1-xB2 at the critical point x=0.2

We performed an extensive investigation on the correlations among superconductivity, structural instability and band filling in Nb1-xB2 materials. Structural measurements reveal that a notable phase transformation occurs at x=0.2, corresponding to the Fermi level (EF) in the pseudogap with the minimum total density of states (DOS) as demonstrated by the first-principles calculations. Superconductivity in Nb1-xB2 generally becomes visible in the Nb-deficient materials with x=0.2. Electron energy-loss spectroscopy (EELS) measurements on B K-edge directly demonstrated the presence of a chemical shift arising from the structural transformation. Our systematical experimental results in combination with theoretical analysis suggest that the emergence of hole states in the sigma-bands plays an important role for understanding the superconductivity and structural transition in Nb1-xB2.

cond-mat.supr-con↗

arXiv2006

Influence of water intercalation on the electronic structure of the hydrated Na0.3CoO2.yH2O using a local spin density approximation

The electronic band structures of the parent compound Na0.3CoO2 and the hydrated superconductor Na0.3CoO2.1.3H2O were evaluated using the local spin density approximation (LSDA). Systematic analysis on the bands near the Fermi surface (FS) revealed that the hydration-induced modifications appear, notably in the valence charge density and the orbital hybridization. The most striking changes are the decrease of density in Cobalt's a1g state and the rehybridization between O 2p and Co a1g in the hydrated superconducting phase. These hydration-induced modifications could yield visible differences in interband excitations and dielectric functions. Experimental results of electron energy-loss spectroscopy (EELS) are fundamentally in agreement with the theoretical predictions for the hydrated Na0.3CoO2.yH2O.

cond-mat.supr-con↗

arXiv2005

Charge ordering in charge-compensated $Na_{0.41}CoO_2$ by oxonium ions

Charge ordering behavior is observed in the crystal prepared through the immersion of the $Na_{0.41}CoO_2$ crystal in distilled water. Discovery of the charge ordering in the crystal with Na content less than 0.5 indicates that the immersion in water brings about the reduction of the $Na_{0.41}CoO_2$. The formal valence of Co changes from +3.59 estimated from the Na content to +3.5, the same as that in $Na_{0.5}CoO_2$. The charge compensation is confirmed to arise from the intercalation of the oxonium ions as occurred in the superconducting sodium cobalt oxide bilayer-hydrate.\cite{takada1} The charge ordering is the same as that observed in $Na_{0.5}CoO_2$. It suggests that the Co valence of +3.5 is necessary for the charge ordering.

cond-mat.mtrl-sci↗

arXiv2005

Superconductivity, charge ordering and structural properties of alpha and beta-NaxCoO2.y(H2O, D2O)

Two series of alpha and beta-NaxCoO2 materials have been prepared by means of Na deintercalation from the parent alpha-NaCoO2 and beta-Na0.6CoO2 phases, respectively. The alpha-NaxCoO2 materials undergo a clear phase transition from the hexagonal to the beta-phase like monoclinic structure along with Na deintercalation. Measurements of resistivity and magnetization demonstrated the presence notable charge ordering transitions in both alpha and beta-NaxCoO2 with 0.4 < x < 0.5 below 100K. Bulk Superconductivity has been observed in the hydrated alpha-NaxCoO2.1.3H2O at ~4.5K and in beta-NaxCoO2.1.3H2O at \~4.3K. Intercalation of D2O in alpha and beta-Na0.33CoO2 also yields superconductivity at slightly lower temperatures. It is worthy to note that, despite of the structural difference, the alpha-, beta- and gamma-NaxCoO2 .yH2O materials show up notable commonalities in their essential physical properties, e.g. superconductivity and charge ordering transitions.

cond-mat.supr-con↗

arXiv2005

Structural properties and cation ordering in layered hexagonal CaxCoO2

A series of CaxCoO2 (0.15 <= x <= 0.40) materials have been prepared by means of ion exchange reaction from NaxCoO2. Transmission electron microscopy (TEM) measurements revealed a rich variety of structural phenomena resulting from cation ordering, structural distortion and twinning. Systematical structural analysis, in combination with the experimental data of NaxCoO2 (0.15 <= x <= 0.8) and SrxCoO2 (1.5 <= x <= 0.4) systems, suggests that there are two common well-defined cation ordered states corresponding respectively to the orthorhombic superstructure at around x = 1/2 and the 31/2a31/2a superstructure at around x = 1/3 in this kind of system. Multiple ordered states, phase separation, and incommensurate structural modulations commonly appear in the materials with 0.33 < x < 0.5. The TEM observations also reveals an additional periodic structural distortion with q2 = a* / 2 in materials for x <= 0.35. This structural modulation also appears in the remarkable superconducting phase Na0.33CoO2 1.3H2O.

cond-mat.str-el↗

arXiv2005

Effect of Ti doping on the electrical transport and magnetic properties of layered compound Na0.8CoO2

Effect of Ti doping on the electrical transport and magnetic properties of layered Na0.8Co1-xTixO2 compounds has been investigated. The lattice parameters a and c increase with x. A minor amount of Ti doping results in a metal-insulator transition at low temperatures. For samples with x > 0.03, the variable-range hopping process dominates the transport behavior above a certain temperature. The temperature dependence of magnetization of all the samples is found to obey the Curie-Weiss law. The mechanism of the doping effect is discussed.

cond-mat.str-el↗

arXiv2005

Physical properties, strontium ordering and structural modulation in layered hexagonal Sr0.35CoO2

Layered Sr0.35CoO2 has been synthesized by means of an ion exchange reaction from Na0.7CoO2. Resistivity measurements show that this material can be either metallic or semiconducting depending on the annealing conditions. Magnetic susceptibility, following the Curie-Weiss law, increases with lowering temperature and shows a small anomalous kink at around 30 K. Transmission-electron-microscopy observations reveal the presence of two superstructures arising respectively from the intercalated Sr-ordering (a compositional modulation) with q1 = a* / 3 + b* / 3 and a periodic structural distortion (a transverse structural modulation) with q2 = a* / 2.

cond-mat.str-el↗

arXiv2005

Effects of Mn and Ti doping on superconductivity and charge ordering in NaxCoO2 system

The superconductivity in Na0.3Co1-xMxO2.1.3H2O and the charge ordering in Na0.5Co1-xMxO2 have been investigated for M = Mn and Ti substituting for Co. We have first successfully synthesized the single-phase Na0.7Co1-xMxO2(M= Mn and Ti) materials with 0 < = x < = 0.1, then we obtained Na0.5Co1-xMxO2 (0 < = x < = 0.1, M = Mn and Ti) by Na deintercalation and Na0.3Co1-xMxO2.1.3H2O (0 < = x < = 0.1, M = Mn and Ti) by an additional water intercalations. X-ray diffraction measurements revealed that all samples are single-phase materials, their lattice parameters depend systematically on the Ti and Mn contents. Measurements of physical properties indicate that the superconductivity in Na0.3Co1-xMxO2.1.3H2O is suppressed evidently by Co-site doping and killed at x = 0.02 for Mn doping and x = 0.01 for Ti doping. Charge ordering and magnetic properties in Na0.5Co1-xMxO2 were also influenced by M-atom doping.

cond-mat.supr-con↗

arXiv2004

Electron energy loss spectra of Na0.33CoO2.yH2O (y = 0, 0.6 and 1.3)

Electron energy loss (EEL) spectra have been obtained on materials with the nominal compositions of Na0.33CoO2_yH2O (y = 0, 0.6 and 1.3). Spectral analyses revealed systematic changes in the low-loss energy region (< 10eV) and the core losses(O-K, Co-L3,2) along with the water intercalation. The inter (intra)-band transitions and the relevant plasmon resonance were found to be sensitive to water content. Variations of the electronic structure and the Co valence states during hydration have been discussed based on our experimental results.

cond-mat.supr-con↗

arXiv2004

Phase separation, effects of magnetic field and high pressure on charge ordering in y-Na0.5CoO2

Transmission electron microscopy observations reveal the presence of complex superstructures and remarkable phase separation in association with Na-ordering phenomenon in y-Na0.5CoO2. Resistivity and magnetization measurements indicate that three phase transitions at the temperatures of 25 K, 53 K and 90 K respectively appear commonly in y-Na0.5CoO2 samples. Under a high pressure up to 10 Kbar, the low-temperature transport properties show certain changes below the charge order transition; under an applied magnetic field of 7 Tesla, phase transitions at around 25 K and 53 K, proposed fundamentally in connection with alternations of magnetic structure and charge ordering maintain almost unchanged.

cond-mat.str-el↗

arXiv2004

Structural Phase Transitions and Sodium Ordering in Na0.5CoO2: a Combined Electron Diffraction and Raman Spectroscopy Study

The nonstoichiometric NaxCoO2 system exhibits extraordinary physical properties that correlate with temperature and Na concentration in its layered lattice without evident long-range structure modification when conventional crystallographic techniques are applied. For instance, Na0.7CoO2, a thermodynamically stable phase, shows large thermoelectric power; water-intercalated Na0.33CoO2.1.3H2O is a newly discovered superconductor with Tc ~ 4K, and Na0.5CoO2 exhibits an unexpected charge ordering transition at around Tco ~ 55 K. Recent studies suggest that the transport and magnetic properties in the NaxCoO2 system strongly depend on the charge carrier density and local structural properties. Here we report a combined variable temperature transmission electron microscopy and Raman scattering investigation on structural transformations in Na0.5CoO2 single crystals. A series of structural phase transitions in the temperature range from 80 K to 1000 K are directly identified and the observed superstructures and modulated phases can be interpreted by Na-ordering. The Raman scattering measurements reveal phase separation and a systematic evolution of active modes along with phase transitions. Our work demonstrates that the high mobility and ordering of sodium cations among the CoO2 layers are a key factor for the presence of complex structural properties in NaxCoO2 materials, and also demonstrate that the combination of electron diffraction and Raman spectroscopy measurements is an efficient way for studying the cation ordering and phase transitions in related systems.

cond-mat.str-el↗

arXiv2004

Raman spectroscopy study of NaxCoO2 and superconducting NaxCoO2.yH2O

The Raman spectra of the parent compound NaxCoO2 (x=0.75) and the superconducting oxyhydrates NaxCoO2.yH2O with different superconducting temperatures (Tc) have been measured. Five Raman active phonons around 195 cm-1 (E1g), 482 cm-1, 522 cm-1, 616 cm-1 (3E2g), 663 cm-1 (A1g) appear in all spectra. These spectra change systematically along with the intercalation of H2O and superconducting properties. In particular, the Raman active phonons (A1g and E1g) involving the oxygen motions within the Co-O layers show up monotonous decrease in frequency along with superconducting temperature Tc. The fundamental properties and alternations of other active Raman phonons in the superconducting materials have also been discussed.

cond-mat.supr-con↗

arXiv2004

Off-axis electron holography and microstructure of Ba0.5Sr0.5TiO3 thin film grown on LaAlO3

Epitaxial Ba0.5Sr0.5TiO3 thin films grown on the (001) LaAlO3 substrates with the ferroelectric transition of about 250K have been investigated by TEM and off-axis electron holography. Cross-sectional TEM observations show that the 350nm-thick Ba0.5Sr0.5TiO3 film has a sharp interface with notable misfit dislocations. Off-axis electron holographic measurements reveal that, at low temperatures, the ferroelectric polarization results in systematic accumulations of negative charges on the interface and positive charges on the film surface, and, at room temperature, certain charges could only accumulate at the interfacial dislocations and other defective areas.

cond-mat.mtrl-sci↗

arXiv2004

1S0 Proton and Neutron Superfluidity in beta-stable Neutron Star Matter

We investigate the effect of a microscopic three-body force on the proton and neutron superfluidity in the $^1S_0$ channel in $β$-stable neutron star matter. It is found that the three-body force has only a small effect on the neutron $^1S_0$ pairing gap, but it suppresses strongly the proton $^1S_0$ superfluidity in $β$-stable neutron star matter.

nucl-th↗

arXiv2004

Superstructure, sodium ordering and antiferromagnetism in NaxCoO2 (0.75<x<1.0)

Transmission-electron-microscopy investigation reveals the presence of a superstructure with a doubled period of 2d110 in the NaxCoO2 materials for x ranging from 0.75 to 1.0. Systematic analyses suggest that this superstructure in general appears just below a phase transition which could yield an anomalous kink in resistivity. We herein interpret this superstructure in terms of sodium ordering occurring at low temperatures. Measurements of magnetic susceptibility for all NaxCoO2 (0.75<x<1.0) materials show an antiferromagetic state with Neel temperature TN~4K.

cond-mat.supr-con↗

arXiv2004

Structure and physical properties of NaxCoO2.yH2O superconducting system

The structural properties of NaxCoO2 and NaxCoO2yH2O have been investigated. The NaxCoO2yH2O samples in general show up the superconducting transitions at around 3.5K. EDAX analyses suggest our samples have the average composition of Na0.65CoO2 for the parent compound and Na0.26CoO2yH2O for the superconducting crystals. TEM observation reveals a superstructure with wave vector q=<1/2,0, 0> in the parent Na0.65CoO2 materials. This superstructure becomes very weak in the superconducting oxyhydrates. EELS analyses show that the Co ions have the valence state of around +3.3 in the Na0.65CoO2 materials and around +3.7 in the superconducting materials.

cond-mat.supr-con↗

arXiv2003

Effects of Mn-substitution for Co on the structure and physical properties of NayCoO2

Structure, transport and magnetic properties have been investigated for layered NaCo1-xMnxO2 materials. With the increase of Mn-doping amount, measurements of x-ray diffraction indicate that the in-plane lattice parameter a decreases progressively, and, conversely, the lattice parameter c increases rapidly. Small amount of Mn doping (i.e. 3% Mn in our work) could result in a metal-insulator transition. Electron energy loss spectroscopy (EELS) measurement shows that the Co valence is about 3.3 and remains unchanged with doping. In contrast, the valence of Mn increases rapidly with doping and reaches 3.7 for the sample with x=0.5. The temperature dependence of magnetization of the samples is found to obey the Curie-Weiss law. Some relevant results have been discussed in terms of the disorder effect induced by doping of magnetic ions.

cond-mat.str-el↗