arXiv subjects

X. G. Luo

Publications and source records attributed to X. G. Luo.

At least 55 records · Page 3Linked to original sources

arXiv2012

From d-wave to s-wave pairing in the iron-pnictide superconductor (Ba,K)Fe2As2

The nature of the pairing state in iron-based superconductors is the subject of much debate. Here we argue that in one material, the stoichiometric iron pnictide KFe2As2, there is overwhelming evidence for a d-wave pairing state, characterized by symmetry-imposed vertical line nodes in the superconducting gap. This evidence is reviewed, with a focus on thermal conductivity and the strong impact of impurity scattering on the critical temperature Tc. We then compare KFe2As2 to Ba0.6K0.4Fe2As2, obtained by Ba substitution, where the pairing symmetry is s-wave and the Tc is ten times higher. The transition from d-wave to s-wave within the same crystal structure provides a rare opportunity to investigate the connection between band structure and pairing mechanism. We also compare KFe2As2 to the nodal iron-based superconductor LaFePO, for which the pairing symmetry is probably not d-wave, but more likely s-wave with accidental line nodes.

cond-mat.supr-con↗

arXiv2012

Pressure effects on superconducting properties of single-crystalline Co doped NaFeAs

Resistivity and magnetic susceptibility measurements under external pressure were performed on single-crystals NaFe1-xCoxAs (x=0, 0.01, 0.028, 0.075, 0.109). The maximum Tc enhanced by pressure in both underdoped and optimally doped NaFe1-xCoxAs is the same, as high as 31 K. The overdoped sample with x = 0.075 also shows a positive pressure effect on Tc, and an enhancement of Tc by 13 K is achieved under pressure of 2.3 GPa. All the superconducting samples show large positive pressure coefficient on superconductivity, being different from Ba(Fe1-xCox)2As2. However, the superconductivity cannot be induced by pressure in heavily overdoped non-superconducting NaFe0.891Co0.109As. These results provide evidence for that the electronic structure is much different between superconducting and heavily overdoped non-superconducting NaFe1-xCoxAs, being consistent with the observation by angle-resolved photoemission spectroscopy.

cond-mat.supr-con↗

arXiv2012

Evolution of band structure from optimally doped to heavily overdoped Co-substituted NaFeAs

Using angle-resolved photoemission spectroscopy, we studied the electronic structure of NaFe$_{1-x}$Co$_x$As from an optimally doped superconducting compound ($x=0.028$) to a heavily overdoped non-superconducting one ($x=0.109$). Similar to the case of "122" type iron pnictides, our data suggest that Co dopant in NaFe$_{1-x}$Co$_x$As supplies extra charge carriers and shifts the Fermi level accordingly. In the $x=0.109$ compound, the hole-like bands around the zone center $Γ$ move to deeper binding energies and an electron pocket appears instead. The overall band renormalization remains basically the same throughout the doping range we studied, suggesting that the local magnetic/electronic correlations are not affected by carrier doping. We speculate that a balance between itinerant properties of mobile carriers and local interactions may play an important role for the superconductivity.

cond-mat.supr-con↗

arXiv2012

Phase Diagram and Calorimetric Properties of NaFe$_{1-x}$Co$_x$As

We measured the resistivity and magnetic susceptibility to map out the phase diagram of single crystalline NaFe$_{1-x}$Co$_x$As. Replacement of Fe by Co suppresses both the structural and magnetic transition, while enhances the superconducting transition temperature ($T_{\rm c}$) and superconducting component fraction. Magnetic susceptibility exhibits temperature-linear dependence in the high temperatures up to 500 K for all the superconducting samples, but such behavior suddenly breaks down for the non-superconducting overdoped crystal, suggesting that the superconductivity is closely related to the T-linear dependence of susceptibility. Analysis on the superconducting-state specific heat for the optimally doped crystal provides strong evidence for a two-band s-wave order parameter with gap amplitudes of $Δ_1(0)/k_{\rm B}T_{\rm c}$= 1.78 and $Δ_2(0)/k_{\rm B}T_{\rm c}$=3.11, being consistent with the nodeless gap symmetry revealed by angle-resolved photoemission spectroscopy experiment.

cond-mat.supr-con↗

arXiv2012

Evidence for nodeless superconducting gap in NaFe$_{1-x}$Co$_x$As from low-temperature thermal conductivity measurements

The thermal conductivity of optimally doped NaFe$_{0.972}$Co$_{0.028}$As ($T_c \sim$ 20 K) and overdoped NaFe$_{0.925}$Co$_{0.075}$As ($T_c \sim$ 11 K) single crystals were measured down to 50 mK. No residual linear term $κ_0/T$ is found in zero magnetic field for both compounds, which is an evidence for nodeless superconducting gap. Applying field up to $H$ = 9 T ($\approx H_{c2}/4$) does not noticeably increase $κ_0/T$ in NaFe$_{1.972}$Co$_{0.028}$As, which is consistent with multiple isotropic gaps with similar magnitudes. The $κ_0/T$ of overdoped NaFe$_{1.925}$Co$_{0.075}$As shows a relatively faster field dependence, indicating the increase of the ratio between the magnitudes of different gaps, or the enhancement of gap anisotropy upon increasing doping.

cond-mat.supr-con↗

arXiv2012

Enhanced Superconductivity by Rare-earth Metal-doping in Phenanthrene

We successfully synthesized La- and Sm-doped phenanthrene powder samples and discovered superconductivity at $T_{\rm c}$ around 6 K in them. The $T_{\rm c}$s are 6.1 K for LaPhenanthrene and 6.0 K for SmPhenanthrene, which are enhanced by about 1 K and 0.5 K compared to those in $A_3$Phenanthrene ($A$=K and Rb) and in $Ae_{1.5}$Phenanthrene ($Ae$ = Sr and Ba) superconductors respectively. The superconductive shielding fractions for LaPhenanthrene and SmPhenanthrene are 46.1%$ and 49.8$%$ at 2 K, respectively. The little effect of the doping of the magnetic ion Sm$^{3+}$ on $T_c$ and the positive pressure dependence coefficient on $T_{\rm c}$ strongly suggests unconventional superconductivity in the doped phenanthrene superconductors. The charge transfer to organic molecules from dopants of La and Sm induces a redshift of 7 cm$^{-1}$ per electron for the mode at 1441 cm$^{-1}$ in the Raman spectra, which is almost the same as those observed in $A_3$Phenanthrene ($A$=K and Rb) and $Ae_{1.5}$Phenanthrene ($Ae$ = Sr and Ba) superconductors.

cond-mat.supr-con↗

arXiv2012

Transport Properties and Electronic Phase Diagram of Single-Crystalline Ca$_{10}$(Pt$_3$As$_8$) ((Fe$_{1-x}$Pt$_x$)$_2$As$_2$)$_5$

Sizable single-crystalline samples of Ca$_{10}$(Pt$_3$As$_8$)((Fe$_{1-x}$Pt$_x$)$_2$As$_2$)$_5$ (the 10-3-8 phase) with 0$\leq x<$0.1 have been grown and systematically characterized via X-Ray diffraction, magnetic, and transport measurements. The undoped sample is a heavily doped semiconductor with no sign of magnetic order down to 2 K. With increasing Pt content, the metallic behavior appears and superconductivity is realized for $x\geq$0.023. $T_{\rm c}$ rises to its maximum of approximately 13.6 K at the doping level of $x\sim$0.06, and then decreases for higher $x$ values. The electronic phase diagram of the 10-3-8 phase was mapped out based on the transport measurements. The mass anisotropy parameter $Γ\sim$10 obtained from resistive measurements in magnetic fields indicates a relatively large anisotropy in the iron-based superconductor family. This strong 2D character may lead to the absence of magnetic order. A linear $T$-dependence of susceptibility at high temperature is observed, indicating that spin fluctuations exist in the underdoped region as in most of the Fe-pnictide superconductors.

cond-mat.supr-con↗

arXiv2012

Transport and magnetic properties of La-doped CaFe$_2$As$_2$

We measured the transport properties and susceptibility of single crystals Ca$_{1-x}$La$_x$Fe$_2$As$_2$(x=0, 0.05, 0.1, 0.15, 0.19 and 0.25). Large in-plane resistivity anisotropy similar to that in Co-doped 122 iron-pnictides is observed although no transition metals were introduced in the FeAs-plane. The in-plane resistivity anisotropy gradually increases with La doping below T$_{SDW}$, being different from the hole-doped 122 superconductors. The susceptibilities of the samples show that La doping leads to suppression of SDW and induces a Curie-Weiss-like behavior at low temperature, which is much stronger than the other 122 iron-based superconductors.

cond-mat.supr-con↗

arXiv2012

Physical properties of $A_x$Fe$_{2-y}$S$_2$ ($A$=K, Rb and Cs) single crystals

We successfully synthesized two new compounds Rb$_x$Fe$_{2-y}$S$_2$ and Cs$_x$Fe$_{2-y}$S$_2$ which were isostructural with K$_x$Fe$_{2-y}$Se$_2$ superconductor. We systematically investigated the resistivity, magnetism and thermoelectric power of $A_x$Fe$_{2-y}$S$_2$ ($A$=K, Rb and Cs) single crystals. High temperature resistivity and magnetic measurements show anomalies above 500 K depending on $A$ which are similar to $A_x$Fe$_{2-y}$Se$_2$. Discrepancy between ZFC and FC curves was observed in K$_x$Fe$_{2-y}$S$_2$ and Rb$_x$Fe$_{2-y}$S$_2$, while it disappears in Cs$_x$Fe$_{2-y}$S$_2$. Our results indicate the similar magnetism between $A_x$Fe$_{2-y}$S$_2$ and $A_x$Fe$_{2-y}$Se$_2$ at high temperature.

cond-mat.str-el↗

arXiv2012

Phase diagram as a function of doping level and pressure in Eu$_{1-x}$La$_x$Fe$_2$As$_2$ system

We establish the phase diagram of Eu$_{1-x}$La$_x$Fe$_2$As$_2$ system as a function of doping level x and the pressure by measuring the resistivity and magnetic susceptibility. The pressure can suppress the spin density wave (SDW) and structural transition very efficiently, while enhance the antiferromagnetic (AFM) transition temperature T$_N$ of Eu$^{2+}$. The superconductivity coexists with SDW order at the low pressure, while always coexists with the Eu$^{2+}$ AFM order. The results suggests that Eu$^{2+}$ spin dynamics is disentangeld with superconducting (SC) pairing taken place in the two-dimensional \emph{Fe-As} plane, but it can strongly affect superconducting coherence along c-axis.

cond-mat.supr-con↗

arXiv2011

Superconductivity in alkali-earth metals doped phenanthrene

We discover superconductivity in alkali-earth metals doped phenanthrene. The superconducting critical temperatures \emph{T}$_c$ are 5.6 K and 5.4 K for Sr$_{1.5}$phenanthrene and Ba$_{1.5}$phenanthrene, respectively. The shielding fraction of Ba$_{1.5}$phenanthrene exceeds 65%. The Raman spectra show 8 cm$^{-1}$/electron and 7 cm$^{-1}$/electron downshifts for the mode at 1441 cm$^{-1}$ due to the charge transfer to organic molecules from the dopants of Ba and Sr. Similar behavior has been observed in A$_3$phenanthrene and A$_3$C$_{60}$(A = K and Rb). The positive pressure effect in Sr$_{1.5}$phenanthrene and Ba$_{1.5}$phenanthrene together with the lower $T_c$ with larger lattice indicates unconventional superconductivity in this organic system.

cond-mat.supr-con↗

arXiv2011

SDW transition of Fe1 zigzag chains and metamagnetic transition of Fe2 in TaFe$_{1+y}$Te$_3$

We systematically study the AFM order of Fe1 zigzag chains and spin-flop of excess Fe2 under high magnetic field H through the susceptibility, magnetoresistance (MR), Hall effect and specific heat measurements in high-quality single crystal TaFe$_{1+y}$Te$_3$. These properties suggest that the high temperature AFM transition of the TaFeTe$_3$ layers should be a SDW-type AFM order. Below T$_N$, Fe1 antiferromangetic zigzag chains will induce a inner magnetic field \textbf{H$_{int}$} to interstitial Fe2 and lead Fe2 also forms an AFM alignment, in which the magnetic coupling strength between Fe1 and Fe2 is enhanced by decreasing temperature. On the other hand, the external magnetic field \textbf{H$_{ext}$} inclines to tune interstitial Fe2 to form FM alignment along \textbf{H$_{ext}$}. When \textbf{H$_{ext}$} arrives at the "coercive" field H$_C$, which is able to break the coupling between Fe1 and Fe2, these interstitial Fe2 atoms take a spin-flop from AFM to FM alignment. The local moment of Fe2 is about 4 $μ_{\textrm{B}}$/Fe. From low field ($<$H$_C$) AFM to high field ($>$H$_C$) FM for Fe2, it also induces sharp drop on resistivity and an anomalous Hall effect. The possible magnetic structure of TaFe$_{1+y}$Te$_3$ is proposed from the susceptibility and MR. The properties related to the spin-flop of Fe2 supply a good opportunity to study the coupling between Fe1 and Fe2 in these TaFe$_{1+y}$Te$_3$ or Fe$_{1+y}$Te with interstitial Fe2 compounds.

cond-mat.str-el↗

arXiv2011

Magnetic properties in the doped spin-1/2 honeycomb-lattice compound In$_3$Cu$_2$VO$_9$

We report the magnetic properties in the Co- and Zn-doped spin-1/2 honeycomb-lattice compound In$_3$Cu$_2$VO$_9$. The magnetic susceptibility and specific heat experiments show no long-range ordering down to 2 K in In$_3$Cu$_2$VO$_9$. In the low temperature range, approximately T$^{2}$-dependent magnetic specific heat and linearly T-dependent spin susceptibility were observed, suggesting a spin liquid candidate with a S = 1/2 honeycomb lattice. When Cu$^{2+}$ ions are partially substituted by Co$^{2+}$ ions, both impurity potential scattering and magnetic impurity scattering induced by magnetic Co$^{2+}$ ions break the homogenous spin-singlet spin liquid state, and lead to an antiferromagnetic(AFM) long-range correlation. While replacing Cu$^{2+}$ with nonmagnetic Zn$^{2+}$ ions, the frustration and antiferromagnetic correlation between Cu$^{2+}$ ions is weakened, leading to breakage of a spin liquid state and suppression of the low-dimensional AFM.

cond-mat.str-el↗

arXiv2011

Electronic and magnetic phase diagram in K$_x$Fe$_{2-y}$Se$_2$ superconductors

The correlation and competition between antiferromagnetism and superconductivity are one of the most fundamental issues in all of high temperature superconductors. The superconductivity in high temperature cuprate superconductors arises from suppressing an antiferromagnetic (AFM) Mott insulator phase by doping1 while that in iron-pnictide high temperature superconductors arises from AFM semimetals and can coexist with AFM orders2-9. This key difference marked in their phase diagrams has raised many intriguing debates about whether the two materials can be placed in the same category to understand the mechanism of superconductivity. Recently, superconductivity at 32 K has been reported in iron-chalcogenide superconductors AxFe2-ySe2 (A=K, Rb, and Cs)10-12, which have the same structure as that of iron-pnictide AFe2As2 (A=Ba, Sr, Ca and K)13-15. Here, we report electronic and magnetic phase diagram of KxFe2-ySe2 system as a function of Fe valence. We find two AFM insulating phases and reveal that the superconducting phase is sandwiched between them, and give direct evidence that the superconductivity in AxFe2-ySe2 originates from the AFM insulating parent compounds. The two insulating phases are characterized by two distinct superstructures caused by Fe vacancy orders with modulation wave vectors of q1=(1/5, 3/5, 0) and q2=(1/4, 3/4, 0), respectively. These experimental results strongly indicate that iron-based superconductors and cuprates share a common origin and mechanism of superconductivity.

cond-mat.supr-con↗

arXiv2011

Structural and Magnetic Properties of the Layered Manganese Oxychalcogenides:(LaO)$_2$Mn$_2$Se$_2$O and (BaF)$_2$Mn$_2$Se$_2$O

The new layered manganese oxychalcogenides (LaO)$_2$Mn$_2$Se$_2$O and (BaF)$_2$Mn$_2$Se$_2$O, isostructural to (LaO)$_2$Fe$_2$Se$_2$O, were synthesized by using solid state reaction. The single crystals of former compound were also successfully grown by using high temperature fusion method. The polycrytalline samples showed the insulating behavior, with the activation energy gaps of about 278 meV and 416 meV for (LaO)$_2$Mn$_2$Se$_2$O and (BaF)$_2$Mn$_2$Se$_2$O, respectively. The magnetic susceptibility indicated an antiferromagnetic (AFM) transition at around 160K and strong anisotropic magnetic properties below T$_N$ for the former compound, which suggests a long-range canted AFM ordering. A broad maximum of the susceptibility can be observed for the two compound at high temperatures, say 360 K and 210 K respectively, suggesting that strong magnetic correlation gives rise to low-dimensional AFM or short-range ordering at high temperatures in these rare transition metal oxychalcogenides with a frustrated AFM checkerboard spin lattice.

cond-mat.mtrl-sci↗

arXiv2011

Superconductivity at 5 K in potassium doped phenanthrene

Organic materials are believed to be potential superconductor with high transition temperature (TC). Organic superconductors mainly have two families: the quasi-one dimensional (TMTSF)2X and two dimensional (BEDT-TTF)2X (Ref. 1 and 2), in which TMTSF is tetramethyltetraselenafulvalene (C10H12Se4) and BEDT-TTF or "ET" is bis(ethylenedithio)tetrathiafulvalene (C10H8S8). One key feature of the organic superconductors is that they have π-molecular orbitals, and the π-electron can delocalize throughout the crystal giving rise to metallic conductivity due to a π-orbital overlap between adjacent molecules. The introduction of charge into C60 solids and graphites with π-electron networks by doping to realize superconductivity has been extensively reported3,4. Very recently, superconductivity in alkali-metal doped picene with π-electron networks was reported5. Here we report the discovery of superconductivity in potassium doped Phenanthrene with TC~5 K. TC increases with increasing pressure, and the pressure of 1 GPa leads to an increase of 20% in TC, suggesting that the potassium doped phenanthrene shows unconventional superconductivity. Both phenanthrene and picene are polycyclic aromatic hydrocarbons, and contain three and five fused benzene rings, respectively. The ribbon of fused benzene rings is part of graphene. Therefore, the discovery of superconductivity in K3Phenanthrene produces a novel broad class of superconductors consisting of fused hydrocarbon benzene rings with π-electron networks. The fact that TC increases from 5 K for KxPhenanthrene with three benzene rings to 18 K for Kxpicene with five benzene rings suggests that such organic hydrocarbons with long benzene rings is potential superconductor with high TC.

cond-mat.supr-con↗

arXiv2011

Coexistence of superconductivity and antiferromagnetism in single crystals $A_{0.8}Fe_{2-y}Se_2$ (A= K, Rb, Cs, Tl/K and Tl/Rb): evidence from magnetization and resistivity

We measure the resistivity and magnetic susceptibility in the temperature range from 5 K to 600 K for the single crystals $A$Fe$_{2-y}$Se$_2$ ($A$ = K$_{0.8}$, Rb$_{0.8}$, Cs$_{0.8}$, Tl$_{0.5}$K$_{0.3}$ and Tl$_{0.4}$Rb$_{0.4}$). A sharp superconducting transition is observed in low temperature resistivity and susceptibility, and susceptibility shows 100% Meissner volume fraction for all crystals, while an antiferromagnetic transition is observed in susceptibility at Neel temperature ($T_N$) as high as 500 K to 540 K depending on A. It indicates the coexistence of superconductivity and antiferromagnetism. A sharp increase in resistivity arises from the structural transition due to Fe vacancy ordering at the temperature slightly higher than $T_{\rm N}$. Occurrence of superconductivity in an antiferromagnetic ordered state with so high $T_{\rm N}$ may suggest new physics in this type of unconventional superconductors.

cond-mat.supr-con↗

arXiv2011

Crystal structure, physical properties and superconductivity in $A_{x}$Fe$_2$Se$_2$ single crystals

We studied the correlation among structure and transport properties and superconductivity in the different $A_x$Fe$_2$Se$_2$ single crystals ($A$ = K, Rb, and Cs). Two sets of (00$l$) reflections are observed in the X-ray single crystal diffraction patterns, and arise from the intrinsic inhomogeneous distribution of the intercalated alkali atoms. The occurrence of superconductivity is closely related to the {\sl c}-axis lattice constant, and the $A$ content is crucial to superconductivity. The hump observed in resistivity seems to be irrelevant to superconductivity. There exist many deficiencies within the FeSe layers in $A_x$Fe$_2$Se$_2$, while their $T_{\rm c}$ does not change so much. In this sense, superconductivity is robust to the Fe and Se vacancies. Very high resistivity in the normal state should arise from such defects in the conducting FeSe layers. $A_x$Fe$_2$Se$_2$ ($A$ = K, Rb, and Cs) single crystals show the same susceptibility behavior in the normal state, and no anomaly is observed in susceptibility at the hump temperature in resistivity. The clear jump in specific heat for Rb$_x$Fe$_2$Se$_2$ and K$_x$Fe$_2$Se$_2$ single crystals shows the good bulk superconductivity in these crystals.

cond-mat.supr-con↗