arXiv subjects

A. F. Wang

Publications and source records attributed to A. F. Wang.

At least 37 records · Page 2Linked to original sources

arXiv2013

Power-law Temperature Dependent Hall Angle in the Normal State and its Correlation with Superconductivity in iron-pnictides

We report Hall measurement of the normal state in K- and Co-doped BaFe$_2$As$_2$, as well NaFe$_{1-x}$Co$_x$As. We found that a power-law temperature dependence of Hall angle, cot$θ_{\rm H}$$\propto$ $T^β$, prevails in normal state with temperature range well above the structural, spin-density-wave and superconducting transitions for the all samples with various doping levels. The power $β$ is nearly 4 for the parent compounds and the heavily underdoped samples, while around 3 for the superconducting samples. The $β$ suddenly changes from 4 to 3 at a doping level that is close to the emergence of superconductivity. It suggests that the $β$ of $\sim 3$ is clearly tied to the superconductivity. Our data suggest that, similar to cuprates, there exists a connection between the physics in the normal state and superconductivity of iron-pnictides. These findings shed light on the mechanism of high-temperature superconductivity.

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arXiv2012

A crossover in phase diagram of NaFe$_{1-x}$Co$_x$As determined by electronic transport measurements

We report electronic transport measurements on single crystals of NaFe$_{1-x}$Co$_x$As system. We found that the cotangent of Hall angle, cot$θ_{\rm H}$, follows $T^4$ for the parent compound with filamentary superconductivity and $T^2$ for the heavily-overdoped non-superconducting sample. While it exhibits approximately $T^3$-dependence in all the superconducting samples, suggesting this behaivor is associated with bulk superconductivity in ferropnictides. A deviation develops below a characteristic temperature $T^*$ well above the structural and superconducting transitions, accompanied by a departure from power-law temperature dependence in resistivity. The doping dependence of $T^*$ resembles the crossover line of pseudogap phase in cuprates.

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arXiv2012

Physical properties in hole-doped SrFe$_{2-x}$Cu$_x$As$_2$ single crystals

We report the structural, magnetic and electronic transport properties of SrFe$_{2-x}$Cu$_x$As$_2$ single crystals grown by self-flux technique. SrCu$_2$As$_2$ and SrFe$_2$As$_2$ both crystallize in ThCr$_2$Si$_2$-type (122-type) structure at room temperature, but exhibit distinct magnetic and electronic transport properties. The x-ray photoelectron spectroscopy(XPS) Cu-2p core line position, resistivity, susceptibility and positive Hall coefficient indicate that SrCu$_2$As$_2$ is an sp-band metal with Cu in the 3d$^{10}$ electronic configuration corresponding to the valence state Cu$^{1+}$. The almost unchanged Cu-2p core line position in SrFe$_{2-x}$Cu$_x$As$_2$ compared with SrCu$_2$As$_2$ indicates that partial Cu substitutions for Fe in SrFe$_2$As$_2$ may result in hole doping rather than the expected electron doping. No superconductivity is induced by Cu substitution on Fe sites, even though the structural/spin density wave(SDW) transition is gradually suppressed with increasing Cu doping.

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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.

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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.

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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.

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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.

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arXiv2012

Metamagnetic transition in Ca$_{1-x}$Sr$_x$Co$_2$As$_2$($x$ = 0 and 0.1) single crystals

We report the magnetism and transport measurements of CaCo$_2$As$_2$ and Ca$_{0.9}$Sr$_{0.1}$Co$_2$As$_2$ single crystals. Antiferromagnetic transition was observed at about 70 K and 90 K for CaCo$_2$As$_2$ and Ca$_{0.9}$Sr$_{0.1}$Co$_2$As$_2$, respectively. Magnetism and magnetoresistance measurements reveal metamagnetic transition from an antiferromagnetic state to a ferromagnetic state with the critical field of 3.5 T and 1.5 T respectively along c-axis for these two materials at low temperature. For the field along ab-plane, spins can also be fully polarized above the field of 4.5 T for Ca$_{0.9}$Sr$_{0.1}$Co$_2$As$_2$. While for CaCo$_2$As$_2$, spins can not be fully polarized up to 7 T. We proposed the cobalt moments of these two materials should be ordered ferromagnetically within the ab-plane but antiferromagnetically along the c-axis(A-type AFM).

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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.

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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.

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arXiv2012

Macroscopic phase segregation in superconducting K0.73Fe1.67Se2 as seen by muon spin rotation and infrared spectroscopy

Using muon spin rotation (\muSR) and infrared spectroscopy we investigated the recently discovered superconductor K0.73Fe1.67Se2 with Tc = 32 K. We show that the combined data can be consistently described in terms of a macroscopically phase segregated state with a matrix of ~88% volume fraction that is insulating and strongly magnetic and inclusions with a ~12% volume fraction which are metallic, superconducting and non-magnetic. The electronic properties of the latter, in terms of the normal state plasma frequency and the superconducting condensate density, appear to be similar as in other iron selenide or arsenide superconductors.

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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.

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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.

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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.

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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.

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arXiv2011

Electronic identification of the actual parental phase of KxFe2-ySe2 superconductor and its intrinsic mesoscopic phase separation

While the parent compounds of the cuprate high temperature superconductors (high-Tc's) are Mott insulators, the iron-pnictide high-Tc's are in the vicinity of a metallic spin density wave (SDW) state, which highlights the difference between these two families. However, insulating parent compounds were identified for the newly discovered KxFe2-ySe2. This raises an intriguing question as to whether the iron-based high-Tc's could be viewed as doped Mott insulators like the cuprates. Here we report angle-resolved photoemission spectroscopy (ARPES) evidence of two insulating and one semiconducting phases of KxFe2-ySe2, and the mesoscopic phase separation between the superconducting/semiconducting phase and the insulating phases. The insulating phases are characterized by the depletion of electronic states over a 0.5 eV window below the chemical potential, giving a compelling evidence for the presence of Mott-like physics. The charging effects and the absence of band folding in the superconducting/semiconducting phase further prove that the static magnetic and vacancy orders are not related to the superconductivity. Instead, the electronic structure of the superconducting phase is much closer to the semiconducting phase, indicating the superconductivity is likely developed by doping the semiconducting phase rather than the insulating phases.

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arXiv2011

Superconducting critical fields of K$_{0.8}$Fe$_{2}$Se$_2$ single crystal

We report the results of an experimental study of dc and low frequencies magnetic properties of K$_{0.8}$Fe$_{2}$Se$_2$ single crystal when the dc magnetic field is applied parallel to the $\bf{ab}$ plane. From the data obtained, we deduce the full H-T phase diagram which consists of all three H$_{c1}$(T), H$_{c2}(T)$ and H$_{c3}(T)$ critical magnetic field plots. The two H$_{c1}$(T) and H$_{c2}$(T) curves were obtained from dc magnetic measurements, whereas the surface critical field H$_{c3}$(T) line was extracted by ac susceptibility studies. It appears that near T$_c$, the H$_{c3}$(T)/H$_{c2}$(T) ratio is $\approx 4.4$ which is much larger than expected.

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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.

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