arXiv subjects

Guanghan Cao

Publications and source records attributed to Guanghan Cao.

88 records · Page 5Linked to original sources

arXiv2009

Suppression of spin-density-wave transition and emergence of ferromagnetic ordering of Eu$^{2+}$ moments in EuFe$_{2-x}$Ni$_{x}$As$_{2}$

We present a systematic study on the physical properties of EuFe$_{2-x}$Ni$_{x}$As$_{2}$ (0$\leq$\emph{x}$\leq$0.2) by electrical resistivity, magnetic susceptibility and thermopower measurements. The undoped compound EuFe$_{2}$As$_{2}$ undergoes a spin-density-wave (SDW) transition associated with Fe moments at 195 K, followed by antiferromagnetic (AFM) ordering of Eu$^{2+}$ moments at 20 K. Ni doping at the Fe site simultaneously suppresses the SDW transition and AFM ordering of Eu$^{2+}$ moments. For $x\geq$0.06, the magnetic ordering of Eu$^{2+}$ moments evolves from antiferromagnetic to ferromagnetic (FM). The SDW transition is completely suppressed for $x\geq$0.16, however, no superconducting transition was observed down to 2 K. The possible origins of the AFM-to-FM transition and the absence of superconductivity in EuFe$_{2-x}$Ni$_{x}$As$_{2}$ system are discussed.

cond-mat.supr-con↗

arXiv2009

Spin gap and magnetic resonance in superconducting BaFe$_{1.9}$Ni$%_{0.1}$As$_{2}$

We use neutron spectroscopy to determine the nature of the magnetic excitations in superconducting BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$ ($T_{c}=20$ K). Above $T_{c}$ the excitations are gapless and centered at the commensurate antiferromagnetic wave vector of the parent compound, while the intensity exhibits a sinusoidal modulation along the c-axis. As the superconducting state is entered a spin gap gradually opens, whose magnitude tracks the $T$-dependence of the superconducting gap observed by angle resolved photoemission. Both the spin gap and magnetic resonance energies are temperature \textit{and} wave vector dependent, but their ratio is the same within uncertainties. These results suggest that the spin resonance is a singlet-triplet excitation related to electron pairing and superconductivity.

cond-mat.supr-con↗

arXiv2009

Possible Kondo effect in the iron arsenides

The normal state of the iron arsenides shows the poor metallic behavior mixed with strong magnetic fluctuations. In particular, some FeAs-1111 and FeAs-122 compounds show the linear-T dependence of susceptibility above the spin-density wave (SDW) transition and the logarithmic upturn of resistivity at low temperatures. We suggest that this is due to the spin-flip scattering between the charge carriers and the local moments in the undoped FeAs layer where Kondo effect coexists with the SDW. This scenario is also accounted for the change of the magnetoresistance from positive to negative in the Sr3Sc2O5Fe2As2 compound.

cond-mat.str-el↗

arXiv2009

Superconductivity induced by phosphorus doping and its coexistence with ferromagnetism in EuFe$_{2}$(As$_{0.7}$P$_{0.3}$)$_{2}$

We have studied EuFe$_{2}$(As$_{0.7}$P$_{0.3}$)$_{2}$ by the measurements of x-ray diffraction, electrical resistivity, thermopower, magnetic susceptibility, magnetoresistance and specific heat. Partial substitution of As with P results in the shrinkage of lattice, which generates chemical pressure to the system. It is found that EuFe$_{2}$(As$_{0.7}$P$_{0.3}$)$_{2}$ undergoes a superconducting transition at 26 K, followed by ferromagnetic ordering of Eu$^{2+}$ moments at 20 K. This finding is the first observation of superconductivity stabilized by internal chemical pressure, and supplies a rare example showing coexistence of superconductivity and ferromagnetism in the ferro-arsenide family.

cond-mat.supr-con↗

arXiv2008

Three-dimensional Resonance in superconducting BaFe$_{1.9}$Ni$_{0.1}$As$_2$

We use inelastic neutron scattering to study magnetic excitations of the FeAs-based superconductor BaFe$_{1.9}$Ni$_{0.1}$As$_2$ above and below its superconducting transition temperature $T_c=20$ K. In addition to gradually open a spin gap at the in-plane antiferromagnetic ordering wavevector $(1,0,0)$, the effect of superconductivity is to form a three dimensional resonance with clear dispersion along the c-axis direction. The intensity of the resonance develops like a superconducting order parameter, and the mode occurs at distinctively different energies at $(1,0,0)$ and $(1,0,1)$. If the resonance energy is directly associated with the superconducting gap energy $Δ$, then $Δ$ is dependent on the wavevector transfers along the c-axis. These results suggest that one must be careful in interpreting the superconducting gap energies obtained by surface sensitive probes such as scanning tunneling microscopy and angle resolved photoemission.

cond-mat.str-el↗

arXiv2008

Metamagnetic transition in EuFe$_2$As$_2$ single crystals

We report the measurements of anisotropic magnetization and magnetoresistance on single crystals of EuFe$_2$As$_2$, a parent compound of ferro-arsenide high-temperature superconductor. Apart from the antiferromagnetic (AFM) spin-density-wave transition at 186 K associated with Fe moments, the compound undergoes another magnetic phase transition at 19 K due to AFM ordering of Eu$^{2+}$ spins ($J=S=7/2$). The latter AFM state exhibits metamagnetic transition under magnetic fields. Upon applying magnetic field with $H\parallel c$ at 2 K, the magnetization increases linearly to 7.0 $μ_{B}$/f.u. at $μ_{0}H$=1.7 T, then keeps at this value of saturated Eu$^{2+}$ moments under higher fields. In the case of $H\parallel ab$, the magnetization increases step-like to 6.6 $μ_{B}$/f.u. with small magnetic hysteresis. A metamagnetic phase was identified with the saturated moments of 4.4 $μ_{B}$/f.u. The metamagnetic transition accompanies with negative in-plane magnetoresistance, reflecting the influence of Eu$^{2+}$ moments ordering on the electrical conduction of FeAs layers. The results were explained in terms of spin-reorientation and spin-reversal based on an $A$-type AFM structure for Eu$^{2+}$ spins. The magnetic phase diagram has been established.

cond-mat.supr-con↗

arXiv2008

Antiferromagnetic transition in EuFe$_2$As$_2$: A possible parent compound for superconductors

Ternary iron arsenide EuFe$_2$As$_2$ with ThCr$_2$Si$_2$-type structure has been studied by magnetic susceptibility, resistivity, thermopower, Hall and specific heat measurements. The compound undergoes two magnetic phase transitions at about 200 K and 20 K, respectively. The former was found to be accompanied with a slight drop in magnetic susceptibility (after subtracting the Curie-Weiss paramagnetic contribution), a rapid decrease in resistivity, a large jump in thermopower and a sharp peak in specific heat with decreasing temperature, all of which point to a spin-density-wave-like antiferromagnetic transition. The latter was proposed to be associated with an A-type antiferromagnetic ordering of Eu$^{2+}$ moments. Comparing with the physical properties of the iso-structural compounds BaFe$_2$As$_2$ and SrFe$_2$As$_2$, we expect that superconductivity could be induced in EuFe$_2$As$_2$ through appropriate doping.

cond-mat.supr-con↗

arXiv2008

Superconductivity induced by cobalt doping in iron-based oxyarsenides

Chemical doping has recently become a very important strategy to induce superconductivity especially in complex compounds. Distinguished examples include Ba-doped La$_2$CuO$_4$ (the first high temperature superconductor), K-doped BaBiO$_3$, K-doped C$_{60}$ and Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O. The most recent example is F-doped LaFeAsO, which leads to a new class of high temperature superconductors. One notes that all the above dopants are non-magnetic, because magnetic atoms generally break superconducting Cooper pairs. In addition, the doping site was out of the (super)conducting structural unit (layer or framework). Here we report that superconductivity was realized by doping magnetic element cobalt into the (super)conducting-active Fe$_2$As$_2$ layers in LaFe$_{1-x}$Co$_{x}$AsO. At surprisingly small Co-doping level of $x$=0.025, the antiferromagnetic spin-density-wave transition in the parent compound is completely suppressed, and superconductivity with $T_c\sim $ 10 K emerges. With increasing Co content, $T_c$ shows a maximum of 13 K at $x\sim 0.075$, and then drops to below 2 K at $x$=0.15. This result suggests essential differences between previous cuprate superconductor and the present iron-based arsenide one.

cond-mat.supr-con↗

arXiv2008

Thorium-doping induced superconductivity up to 56 K in Gd1-xThxFeAsO

Following the discovery of superconductivity in an iron-based arsenide LaO1-xFxFeAs with a superconducting transition temperature (Tc) of 26 K[1], Tc was pushed up surprisingly to above 40 K by either applying pressure[2] or replacing La with Sm[3], Ce[4], Nd[5] and Pr[6]. The maximum Tc has climbed to 55 K, observed in SmO1-xFxFeAs[7, 8] and SmFeAsO1-x[9]. The value of Tc was found to increase with decreasing lattice parameters in LnFeAsO1-xFx (Ln stands for the lanthanide elements) at an apparently optimal doping level. However, the F- doping in GdFeAsO is particularly difficult[10,11] due to the lattice mismatch between the Gd2O2 layers and Fe2As2 layers. Here we report observation of superconductivity with Tc as high as 56 K by the Th4+ substitution for Gd3+ in GdFeAsO. The incorporation of relatively large Th4+ ions relaxes the lattice mismatch, hence induces the high temperature superconductivity.

cond-mat.supr-con↗

arXiv2007

Upper critical field of aligned Na_{x}(H_{3}O)_{z}CoO_{2}\cdot yH_{2}O superconductor by magnetization measurement

We present magnetization measurements for the upper critical field $H_{c2}$ on aligned Na$_{x}$(H$_{3}$O)$_{z}$CoO$_{2}\cdot y$H$_{2}$O superconductors with magnetic field parallel to CoO$_2$ layers. The temperature-dependent part of the normal-state magnetization $M(H,T)$ can be well scaled with Brillouin function, suggesting the existence of impurity moments. By subtracting the contribution of the magnetic impurity moments, a well defined onset of superconducting diamagnetism allows the in-field superconducting transition temperature $T_{c}$($H$) accurately determined. No superconducting transition was observed down to 1.9 K under an applied field of 8 Tesla. The result suggests that the $H^{\|ab}_{c2}(0)$ value is just within the Pauli paramagnetic limit, supporting spin-singlet superconductivity in the cobalt oxyhydrate superconductor. Additionally, the upward curvature near $T_{c}$ in the $H^{\|ab}_{c2}-T$ phase diagram was confirmed to be intrinsic. Possible origin of the anomaly was discussed.

cond-mat.supr-con↗

arXiv2007

Tuning Co valence state in cobalt oxyhydrate superconductor by post reduction

We report a successful tuning of Co valence state in cobalt oxyhydrate superconductor via a facile post reduction using NaOH as reducing agent. The change in Co valence was precisely determined by measuring the volume of the released oxygen. The possible hydronium-incorporation was greatly suppressed in concentrated NaOH solution, making the absolute Co valence determinable. As a result, an updated superconducting phase diagram was obtained, which shows that the superconducting transition temperature increases monotonically with increasing Co valence in a narrow range from +3.58 to +3.65.

cond-mat.supr-con↗

arXiv2005

A novel route to lower the dielectric loss in CaCu$_{3}$Ti$_{4}$O$_{12}$ ceramics

Based upon a widely-accepted internal-barrier-layer-capacitance (IBLC) model, the dielectric loss in the giant-dielectric-constant CaCu$_{3}$Ti$_{4}$O$_{12}$ (CCTO) material is expected to decrease as the conductivity of the CCTO grains/subgrains increases. In this letter, we report that this idea was successfully realized through the La-for-Ca substitution in the CCTO ceramics. The impedance spectroscopy analysis gives the evidence.

cond-mat.mtrl-sci↗

arXiv2005

Enhanced thermopower in an intergrowth cobalt oxide Li$_{0.48}$Na$_{0.35}$CoO$_{2}$

We report the measurements of thermopower, electrical resistivity and thermal conductivity in a complex cobalt oxide Li$_{0.48}$Na$_{0.35}$CoO$_{2}$, whose crystal structure can be viewed as an intergrowth of the O3 phase of Li$_{x}$CoO$_{2}$ and the P2 phase of Na$_{y}$CoO$_{2}$ along the c axis. The compound shows large room-temperature thermopower of $\sim$180 $μ$V/K, which is substantially higher than those of Li$_{x}$CoO$_{2}$ and Na$_{y}$CoO$_{2}$. The figure of merit for the polycrystalline sample increases rapidly with increasing temperature, and it achieves nearly 10$^{-4}$ K$^{-1}$ at 300 K, suggesting that Li$_{x}$Na$_{y}$CoO$_{2}$ system is a promising candidate for thermoelectric applications.

cond-mat.mtrl-sci↗

arXiv2004

Proton incorporations and superconductivity in a cobalt oxyhydrate

We report the evidence of proton incorporations in a newly-discovered cobalt oxyhydrate superconductor. During the hydration process for Na$_{0.32}$CoO$_{2}$ by the direct reaction with water liquid, it was shown that substantial NaOH was gradually liberated, indicating that H$^{+}$ is incorporated into the hydrated compound. Combined with the thermogravimetric analysis, the chemical composition of the typical sample is Na$_{0.22}$H$_{0.1}$CoO$_{2}\cdot 0.85$H$_{2}$O, which shows bulk superconductivity at 4.4 K.

cond-mat.supr-con↗

arXiv2003

Variation of metal-insulator transition and formation of bipolarons by the Cd-doping in the thiospinel system of Cu$_{1-x}$Cd$_{x}$Ir$_{2}$S$_{4}$

The pseudo-binary thiospinel system Cu$_{1-x}$Cd$_{x}$Ir$_{2}$S$_{4}$ was investigated by the x-ray diffraction, electrical resistivity, magnetic susceptibility and specific heat measurements. It was shown that the system exhibits a miscibility-gap behavior for the Cd substitution, however, nearly monophasic samples was obtained by quenching at 1373 K, except for $0.4< x \leqslant 0.6$. With increasing the Cd concentration, the room-temperature electrical conductivity and Pauli susceptibility decrease monotonically, consistent with the hole-filling picture. The first-order metal-insulator transition at about 230 K in the parent compound CuIr$_{2}$S$_{4}$ is changed into a second-order transition around 185 K when $x\sim$ 0.25, whereafter the second-order transition disappears at $x\sim 0.8$. No superconductivity was observed down to 1.8 K. The end-member compound CdIr$_{2}$S$_{4}$ is shown as an insulator with a band-gap of 0.3 eV. Analysis for the data of magnetic susceptibility and electrical resistivity suggests the formation of bipolarons below 185 K for $0.25 <x<$ 0.8, which accounts for the absence of superconductivity in terms of the transition from the BCS Cooper pairs to small bipolarons.

cond-mat.str-el↗

arXiv2003

Superconductivity in a layered cobalt oxyhydrate Na$_{0.31}$CoO$_{2}\cdot$1.3H$_{2}$O

We report the electrical, magnetic and thermal measurements on a layered cobalt oxyhydrate Na$_{0.31}$CoO$_{2}\cdot$1.3H$_{2}$O. Bulk superconductivity at 4.3 K has been confirmed, however, the measured superconducting fraction is relatively low probably due to the sample's intrinsic two-dimensional characteristic. The compound exhibits weak-coupled and extreme type-II superconductivity with the average energy gap $Δ_{a}(0)$ and the Ginzburg-Landau parameter $κ$ of $\sim$ 0.50 meV and $\sim$ 140, respectively. The normalized electronic specific heat data in the superconducting state well fit the $T^{3}$ dependence, suggesting point nodes for the superconducting gap structure.

cond-mat.supr-con↗