X. C. Wang, Q. Q. Liu, Y. X. Lv, W. B. Gao, L. X. Yang, R. C. Yu, F. Y. Li, C. Q. Jin
A new iron arsenide superconducting system LiFeAs was found that crystallizes into a tetragonal structure with space group P4/nmm. The superconductivity with Tc up to 18 K was observed in the compounds. This simple 111 type layered iron arsenide superconductor can be viewed as an analogue of the infinite layer structure of copper oxides.
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The thermal conductivity k of the iron-arsenide superconductor LiFeAs (Tc ~ 18K) was measured in single crystals at temperatures down to T~50mK and in magnetic fields up to H=17T, very close to the upper critical field Hc2~18T. For both directions of the heat current, parallel and perpendicular to the tetragonal c-axis, a negligible residual linear term k/T is found as T ->0, revealing that there are no zero-energy quasiparticles in the superconducting state. The increase in k with magnetic field is the same for both current directions and it follows closely the dependence expected for an isot…
The lithium ions in Lithium iron arsenide phases with compositions close to LiFeAs have been located using powder neutron diffraction. These phases exhibit superconductivity at temperatures at least as high as 16 K demonstrating that superconductivity in compounds with [FeAs]- anti-PbO-type anionic layers occurs in compounds with at least three different structure types and occurs for a wide range of As-Fe-As bond angles.
We measured the optical conductivity of superconducting LiFeAs. In the superconducting state, the formation of the condensate leads to a spectral-weight loss and yields a penetration depth of 225 nm. No sharp signature of the superconducting gap is observed. This suggests that the system is likely in the clean limit. A Drude-Lorentz parametrization of the data in the normal state reveals a quasiparticle scattering rate supportive of spin fluctuations and proximity to a quantum critical point.
A combined density functional theory and functional renormalization group method is introduced which takes into account orbital-dependent interaction parameters to derive the effective low-energy theory of weakly to intermediately correlated Fermi systems. As an application, the competing fluctuations in LiFeAs are investigated, which is the main representative of the 111 class of iron pnictides displaying no magnetic order, but superconductivity, for the parent compound. The superconducting order parameter is found to be of s+- type driven by collinear antiferromagnetic fluctuations. They eve…
If strong electron-electron interactions between neighboring Fe atoms mediate the Cooper pairing in iron-pnictide superconductors, then specific and distinct anisotropic superconducting energy gaps Δ_i(k) should appear on the different electronic bands i. Here we introduce intra-band Bogoliubov quasiparticle scattering interference (QPI) techniques for determination of Δ_i(k) in such materials, focusing on LiFeAs. We identify the three hole-like bands assigned previously as γ, α_2 and α_1, and we determine the anisotropy, magnitude and relative orientations of their Δ_i(k). These measurements …
We report fully momentum dependent, self-consistent calculations of the gap symmetry, Fermi surface (FS) anisotropy and Tc of superconducting (SC) LiFeAs using the experimental band structure and a realistic small-q electron phonon interaction within the framework of Migdal-Eliashberg theory. In the stoichiometric regime, we find the exact s++ gap as reported by ARPES. For slight deviations from stoichiometry towards electron doping, we find that a chiral triplet p_x+ip_y state stabilizes near Tc and that at lower temperatures a transition from the triplet to singlet s+- SC takes place. Furthe…
The relationship between unconventional superconductivity, antiferromagnetism and nematic order in iron-based superconductors (FeSCs) is still highly debated. In many FeSCs superconductivity is in proximity of a nematically and magnetically ordered state. LiFeAs is an exceptional stoichiometric FeSC becoming superconducting below 18 K, without undergoing a structural or magnetic transition. However, some recent experimental studies suggested the existence of finite nematic fluctuations and even a nematic superconducting state. In this study, we employ elastoresistance as a measure of nematic f…
To examine the role of magnetism in superconductivity of iron-based superconductors, we first present first-principles optical calculations on three representative parent compounds: LaFeAsO, BaFe2As2 and LiFeAs. Both nonmagnetic (NM) and spin-density wave (SDW) with striped antiferromagnetic order are considered. Due to their layer structure, these compounds in NM states show anomalously large optical anisotropy with a conductivity ratio $σ_{zz}/σ{xx}$ as large as 313% in LaFeAsO and 530% in LiFeAs. Interestingly the giant optical anisotropy is significantly suppressed by SDW. On the other han…
We have performed temperature dependent tunneling spectroscopy on an impurity-free surface area of a LiFeAs single crystal. Our data reveal a highly unusual temperature evolution of superconductivity: at $T_c^*=18$~K a partial superconducting gap opens, as is evidenced by subtle, yet clear features in the tunneling spectra, i.e. particle-hole symmetric coherence peaks, and a dip-hump structure which signals strong-coupling superconductivity. At $T_c=16$~K, these features substantiate dramatically and become characteristic of full superconductivity. Remarkably, this is accompanied by an almost …
A new layered iron arsenide NaFeAs isostructural with the superconducting lithium analogue, displays evidence for the coexistence of superconductivity and magnetic ordering.
In this letter, we reported the results of NMR study on LiFeAs single crystals. We find a strong evidence of the low temperature spin fluctuations; by changing sample preparation conditions, the system can be tuned toward an spin-density-wave (SDW) quantum-critical point. The detection of an interstitial Li(2) ion, possibly locating in the tetrahedral hole, suggests that the off-stoichiometry and/or lattice defect can probably account for the absence of the SDW ordering in LiFeAs. These facts show that LiFeAs is a strongly correlated system and the superconductivity is likely originated from t…
The nonadiabatic Heisenberg model predicts a structural distortion in LiFeAs below a temperature higher than (or at least equal to) the superconducting transition temperature. Within this group-theoretical model, the reduction of the symmetry caused by the distortion is a prerequisite for the superconducting state in this compound and can be realized by a mere displacement of the iron atoms from their positions in the space group P4/nmm.
We use neutron scattering to study vanadium (hole)-doped LiFe$_{1-x}$V$_x$As. In the undoped state, LiFeAs exhibits superconductivity at $T_c=18$ K and transverse incommensurate spin excitations similar to electron overdoped iron pnictides. Upon vanadium-doping to form LiFe$_{0.955}$V$_{0.045}$, the transverse incommensurate spin excitations in LiFeAs transform into longitudinally elongated in a similar fashion as that of potassium (hole) doped Ba$_{0.7}$K$_{0.3}$Fe$_2$As$_2$, but with dramatically enhanced magnetic scattering and elimination of superconductivity. This is different from the su…
Superconducting pnictides are widely found to feature spin-singlet pairing in the vicinity of an antiferromagnetic phase, for which nesting between electron and hole Fermi surfaces is crucial. LiFeAs differs from the other pnictides by (i) poor nesting properties and (ii) unusually shallow hole pockets. Investigating magnetic and pairing instabilities in an electronic model that incorporates these differences, we find antiferromagnetic order to be absent. Instead we observe almost ferromagnetic fluctuations which drive an instability toward spin-triplet p-wave superconductivity.
New iron-arsenide superconductors of REFeAsO1-delta (RE = Ho, Y, Dy and Tb) were successfully synthesized by a high pressure synthesizing method with a special rapid quenching process, with the onset superconducting critical temperatures at 50.3 K, 46.5 K, 52.2K and 48.5 K for RE = Ho, Y, Dy and Tb respectively.
EuFe2As2 is a member of the ternary iron arsenide family. Similar to BaFe2As2 and SrFe2As2, EuFe2As2 exhibits a clear anomaly in resistivity near 200 K. It suggests that EuFe2As2 is another promising parent compound in which superconductivity may be realized by appropriate doping. Here we report the discovery of superconductivity in Eu0.7Na0.3Fe2As2 by partial substitution of the europium site with sodium. ThCr2Si2 tetragonal structure, as expected for EuFe2As2, is formed as the main phase for the composition Eu0.7Na0.3Fe2As2. Resistivity measurement reveals a transition temperature as high as…
The resistivity, Hall effect and transverse magnetoresistance (MR) have been measured in low residual resistivity single crystals of LiFeAs. A comparison with angle resolved photoemission spectroscopy and quantum oscillation data implies that four carrier bands unevenly contribute to the transport. However the scattering rates of the carriers all display the T^2 behavior expected for a Fermi liquid. Near Tc low field deviations of the MR with respect to a H^2 variation permit us to extract the superconducting fluctuation contribution to the conductivity. Though below Tc the anisotropy of super…
We report superconductivity in novel iron-based compounds Ca10(PtnAs8)(Fe2-xPtxAs2)5 with n = 3 and 4. Both compounds crystallize in triclinic structures (space group P-1), in which Fe2As2 layers alternate with PtnAs8 spacer layers. Superconductivity with a transition temperature of 38 K is observed in the n = 4 compound with a Pt content of x ~ 0.36 in the Fe2As2 layers. The compound with n = 3 exhibits superconductivity at 13 K.
Nodal superconductivity is observed in LiFeP while its counterpart LiFeAs with similar topology and orbital content of the Fermi surfaces is a nodeless superconductor. We explain this difference by solving, in the two-Fe Brillouin zone, the frequency-dependent Eliashberg equations with spin-fluctuation mediated pairing interaction. Because of Fermi surface topology details, in LiFeAs all the Fe-$t_{2g}$ orbitals favor a common pairing symmetry. By contrast, in LiFeP the $d_{xy}$ orbital favors a pairing symmetry different from $d_{xz/yz}$ and their competition determines the pairing symmetry a…
The optical properties of LiFeAs with $T_c \simeq$ 18 K have been determined in the normal and superconducting states. The superposition of two Drude components yields a good description of the low-frequency optical response in the normal state. Below $T_c$, the optical conductivity reveals two isotropic superconducting gaps with $Δ_{1} \simeq 2.9$ $\pm$ 0.2 meV and $Δ_{2} \simeq 5.5$ $\pm$ 0.4 meV. A comparison between the superconducting-state Mattis-Bardeen and the normal-state Drude components, in combination with a spectral weight analysis, indicates that the spectral weight associated wi…
In the framework of four-band model of superconductivity in iron arsenides proposed by Barzykin and Gor'kov we analyze the gap ratios on hole - like and electron - like Fermi - surface cylinders. It is shown that experimentally observed (ARPES) gap ratios can be obtained only within rather strict limits on the values of pairing coupling constants. The difference of T_c values in 1111 and 122 systems is reasonably explained by the relative values of partial densities of states. The multiple bands electronic structure of these systems leads to a significant enhancement of effective pairing coupl…
The stoichiometric "111" iron-based superconductor, LiFeAs, has attacted great research interest in recent years. For the first time, we have successfully grown LiFeAs thin film by molecular beam epitaxy (MBE) on SrTiO3(001) substrate, and studied the interfacial growth behavior by reflection high energy electron diffraction (RHEED) and low-temperature scanning tunneling microscope (LT-STM). The effects of substrate temperature and Li/Fe flux ratio were investigated. Uniform LiFeAs film as thin as 3 quintuple-layer (QL) is formed. Superconducting gap appears in LiFeAs films thicker than 4 QL a…
We present extensive 75As NMR and NQR data on the superconducting arsenides PrFeAs0.89F0.11 (Tc=45 K), LaFeAsO0.92F0.08 (Tc=27 K), LiFeAs (Tc = 17 K) and Ba0.72K0.28Fe2As2 (Tc = 31.5 K) single crystal, and compare with the nickel analog LaNiAsO0.9F0.1 (Tc=4.0 K) . In contrast to LaNiAsO0.9F0.1 where the superconducting gap is shown to be isotropic, the spin lattice relaxation rate 1/T1 in the Fe-arsenides decreases below Tc with no coherence peak and shows a step-wise variation at low temperatures. The Knight shift decreases below Tc and shows a step-wise T variation as well. These results ind…
A new layered iron arsenide oxide (Fe2As2)(Ca4(Mg,Ti)3Oy) was discovered. Its crystal structure is tetragonal with a space group of I4/mmm consisted of the anti-fluorite type FeAs layer and blocking layer of triple perovskite cells and is identical with (Fe2As2)(Sr4(Sc,Ti)3O8) discovered in our previous study. The lattice constants of (Fe2As2)(Ca4(Mg,Ti)3Oy) are a = 3.877 A and c = 33.37 A. This compound exhibited bulk superconductivity up to 43 K in susceptibility measurement without intentional carrier doping. A resistivity drop was observed at ~47 K and zero resistance was achieved at 42 K.…
Here we report the synthesis and discovery of superconductivity in a novel ternary iridium-arsenide compound BaIr2As2. The polycrystalline BaIr2As2 sample was synthesized by a high temperature and high pressure method. Crystal structural analysis indicates that BaIr2As2 crystallizes in the ThCr2Si2-type layered tetragonal structure with space group I4/mmm (No. 139), and the lattice parameters were refined to be a = 4.052(9) Å and c = 12.787(8) Å. By the electrical resistivity and magnetic susceptibility measurements we found type-II superconductivity in the new BaIr2As2 compound with a Tc (cri…
The particular shape of the Fermi surface can give rise to a number of collective quantum phenomena in solids, such as density wave orderings or even superconductivity. In many new iron superconductors this shape, the 'nested' Fermi surface, is indeed observed, but its role in the formation of spin-density waves or superconductivity is not clear. We have studied the electronic structure of the non-magnetic LiFeAs (Tc~18K) superconductor using angle-resolved photoemission spectroscopy. We find a notable absence of the Fermi surface nesting, strong renormalization of the conduction bands by a fa…
We report the $^{75}$As-NQR and NMR studies on the iron arsenide superconductor Li$_{x}$FeAs with $T_{\rm c} \sim 17$ K. The spin lattice relaxation rate, $1/T_{1}$, decreases below $T_{\rm c}$ without a coherence peak, and can be fitted by gaps with s$^{\pm}$-wave symmetry in the presence of impurity scattering. In the normal state, both $1/T_{1}T$ and the Knight shift decrease with decreasing temperature but become constant below $T \leq 50 K$. Estimate of the Korringa ratio shows that the spin correlations are weaker than that in other families of iron arsenides, which may account for the l…
The magnetism in SrFe2As2 can be suppressed by electron doping through a small substitution of Fe by Co or Ni, giving way to superconductivity. We demonstrate that a massive substitution of Fe by isovalent ruthenium similarly suppresses the magnetic ordering in SrFe2-xRuxAs2 and leads to bulk superconductivity for 0.6 <= x <= 0.8. Magnetization, electrical resistivity, and specific heat data show Tc up to approx 20K. Detailed structural investigations reveal a strong decrease of the lattice parameter ratio c/a with increasing x. DFT band structure calculations are in line with the observation …
In the electron doped compounds SrFe_(2-x)Co_xAs_2 superconductivity with T_c up to 20 K is observed for 0.2 < x < 0.4. Results of structure determination, magnetic susceptibility, electrical resistivity, and specific heat are reported. The observation of bulk superconductivity in all thermodynamic properties -- despite strong disorder in the Fe-As layer -- favors an itinerant picture in contrast to the cuprates and renders a p- or d-wave scenario unlikely. DFT calculations find that the substitution of Fe by Co (x > 0.3) leads to the suppression of the magnetic ordering present in SrFe_2As_2 …
A crucial step in revealing the nature of unconventional superconductivity is to investigate the symmetry of the superconducting order parameter. Scanning tunneling spectroscopy has proven a powerful technique to probe this symmetry by measuring the quasiparticle interference (QPI) which sensitively depends on the superconducting pairing mechanism. A particularly well suited material to apply this technique is the stoichiometric superconductor LiFeAs as it features clean, charge neutral cleaved surfaces without surface states and a relatively high Tc~18K. Our data reveal that in LiFeAs the qua…
The variations of in- and inter- plane London penetration depths, $Δλ(T)$, were measured using a tunnel diode resonator in single crystals of the intrinsic pnictide superconductor LiFeAs. This compound appears to be in the clean limit with a residual resistivity of 4 ($T\to0$) to 8 ($T_c$) $μΩ\cdot$cm and $RRR$ of 65 to 35, respectively. The superfluid density, $ρ_s(T)=λ^2(0)/λ^2(T)$, is well described by the self-consistent two-gap $γ-$model. Together with the previous data, our results support the universal evolution of the superconducting gap from nodeless to nodal upon departure from optim…
We report neutron inelastic scattering measurements on the stoichiometric iron-based superconductor LiFeAs. We find evidence for (i) magnetic scattering consistent with strong antiferromagnetic fluctuations, and (ii) an increase in intensity in the superconducting state at low energies, similar to the resonant magnetic excitation observed in other iron-based superconductors. The results do not support a recent theoretical prediction of spin-triplet p-wave superconductivity in LiFeAs, and instead suggest that the mechanism of superconductivity is similar to that in the other iron-based supercon…
Precise gap structure in LiFeAs (Tc = 18 K) given by ARPES studies offers us significant information to understand the pairing mechanism in iron-based superconductors. The most remarkable characteristics in LiFeAs gap structure would be that "the largest gap emerges on the tiny hole-pockets around Z point". This result had been naturally explained in terms of the orbital-fluctuation scenario (T. Saito et al., Phys. Rev. B 90, 035104 (2014)), whereas an opposite result is obtained by the spin-fluctuation scenario. In this paper, we study the gap structure in LiFeAs by taking the spin-orbit inte…
Utilizing the high stability of calcium and rare earth hydrides, CaFeAsF1-xHx (x = 0.0-1.0) and SmFeAsO1-xHx (x = 0.0-0.47) have been first synthesized using high pressure to form hydrogen-substituted 1111 type iron-arsenide superconductors. Neutron diffraction and density functional calculations have demonstrated that the hydrogens are incorporated as H- ions occupying F- sites in the blocking layer of CaFeAsF. The resulting CaFeAsF1-xHx is non-superconducting, whereas SmFeAsO1-xHx is a superconductor, with an optimal Tc = 55 K at x 0.2. It was found that up to 40% of the O2- ions can be repl…
In this paper we report the fabrication and superconducting properties of GdO$_{1-x}$F$_{x}$FeAs. It is found that when x is equal to 0.17, GdO$_{0.83}$F$_{0.17}$FeAs is a superconductor with the onset transition temperature T$_{c}^{on}\approx$ 36.6K. Resistivity anomaly near 130K was observed for all samples up to x = 0.17, such a phenomenon is similar to that of LaO$_{1-x}$F$_{x}$FeAs. Hall coefficient indicates that GdO$_{0.83}$F$_{0.17}$FeAs is conducted by electron-like charge carriers.
The discovery of superconducting materials has led to their use in technological marvels such as magnetic-field sensors in MRI machines, powerful research magnets, short transmission cables, and high-speed trains. Despite such applications, the uses of superconductors are not widespread because they function much below room-temperature, hence the costly cooling. Since the discovery of Cu- and Fe-based high-temperature superconductors (HTS), much intense effort has tried to explain and understand the superconducting phenomenon. While no exact explanations are given, several trends are reported …
In conventional s-wave superconductors, only magnetic impurities exhibit impurity bound states, whereas for an s+- order parameter they can occur for both magnetic and non-magnetic impurities. Impurity bound states in superconductors can thus provide important insight into the order parameter. Here, we present a combined experimental and theoretical study of native and engineered iron-site defects in LiFeAs. Detailed comparison of tunneling spectra measured on impurities with spin fluctuation theory reveals a continuous evolution from negligible impurity bound state features for weaker scatter…
Detailed tunneling spectroscopy of vortex core states can provide important insight to the momentum structure of the superconducting order parameter. We present a theoretical study of vortex bound states in iron-based superconductors by use of a realistic five-band model relevant to these systems, and superconductivity stabilized by spin-fluctuation generated pairing vertices yielding an $s\pm$ gap structure. The computed local density of states agrees remarkably well with both the bias dependence of the local conductance and the spatial structure of the low-bias conductance as obtained by sca…
Amongst the iron-based superconductors, LiFeAs is unrivalled in the simplicity of its crystal structure and phase diagram. However, our understanding of this canonical compound suffers from conflict between mutually incompatible descriptions of the material's electronic structure, as derived from contradictory interpretations of the photoemission record. Here, we explore the challenge of interpretation in such experiments. By combining comprehensive photon energy- and polarization- dependent angle-resolved photoemission spectroscopy (ARPES) measurements with numerical simulations, we establish…
Preliminary Tc calculations for iron-based superconductivity in NaFeAs, LiFeAs, FeSe and nanostructured FeSe/SrTiO3 Superconductors
本文结合两种铁基超导模型与双通道xy势模型，纳入反铁磁放大、散射矩阵修正、软声子及界面效应，计算多种铁基材料压力依赖Tc，并解释FeSe/SrTiO3近百K增强。
The palladium-iron-arsenides Ca10(Fe1-xPdxAs)10(Pd3As8) were synthesized by solid state methods and characterized by X-ray powder and single crystal diffraction. The triclinic crystal structure (space group P-1) is isotypic to the homologue platinum 1038 type superconductors with alternating FeAs4/4- and Pd3As8-layers, each separated by layers of calcium atoms. Iron is tetrahedral and palladium is planar coordinated by four arsenic atoms. As2-dimers (dAs-As = 250 pm) are present in the Pd3As8-layer. Even though each layer itself has a fourfold rotational symmetry, the shifted layer stacking ca…
Since the discovery of superconductivity\cite{1} at 26 K in oxy-pnictide $LaFeAsO_{1-x}F_x$, enormous interests have been stimulated in the fields of condensed matter physics and material sciences. Among the five different structures in this broad type of superconductors\cite{2,3,4,5,6}, the ZrCuSiAs structure has received special attention since the $T_c$ has been quickly promoted to 55-56 K\cite{7,8,9,10,11} in fluorine doped oxy-pnictides REFeAsO (RE = rare earth elements). The superconductivity can also be induced by applying a high pressure to the undoped samples\cite{12,13}. The mechanis…
We report 75As-NMR/NQR results on new iron-arsenide compounds (La0.5-xNa0.5+x)Fe2As2. The parent compound x=0 exhibits a stripe-type antiferromagnetic (AFM) order below T_N=130 K. The measurement of nuclear spin relaxation rate at hole-doped x=+0.3 and heavily electron-doped x=-0.5 revealed that the normal-state properties are dominated by AFM spin fluctuations (AFMSFs), which are more significant at x=+0.3 than at x=-0.5. Their superconducting (SC) phases are characterized by unconventional multi-gap SC state, where the smaller SC gaps are particularly weaken in common. The experimental resul…
We report the successful growth of high quality single crystals of LiFeAs with lateral sizes up to 5 x 5 mm2 by the Sn-flux method. Electrical resistivity studies reveal that the superconducting onset temperature is 18.2 K with a transition width less than 1.1 K and the ratio of room temperature to residual resistivity is about 24. Bulk superconductivity is supported by perfect shielding in the magnetic susceptibility and a clear jump in the specific heat Cp, resulting in deltaCp/T ~ 20.0 mJ/mol*K2. Upper critical field slopes of dHc2c/dT ~ -1.39 and dHc2ab/dT ~ -2.99 T/K near Tc predict zero …
The interplay of magnetism and superconductivity (SC) has been a focus of interest in condensed matter physics for decades. EuFe2As2 has been identified as a potential platform to investigate interactions between structural, magnetic, electronic effects as well as coexistence of magnetism and SC with similar transition temperatures. However, there are obvious inconsistencies in the reported phase diagrams of Eu(Fe1-xCox)2As2 crystals grown by different methods. For transition metal arsenide (TMA)-flux-grown crystals, even the existence of SC is open for dispute. Here we re-examine the phase di…
Theory for superconductivity in alkali chromium arsenides A2Cr3As3 (A=K,Rb,Cs)
本文提出扩展Hubbard模型研究A2Cr3As3超导性，发现顺磁振子涨落导致自旋三重态配对，在不同Hubbard U下分别产生带线节点的f波和全能隙p波态。
The ternary iron arsenide BaFe2As2 becomes superconducting by hole doping, which was achieved by partial substitution of the barium site with potassium. We have discovered bulk superconductivity up to Tc = 38 K in (Ba1-xKx)Fe2As2 with x = 0.4. The parent compound BaFe2As2 as well as KFe2As2 both crystallize in the tetragonal ThCr2Si2-type structure, which consists of (FeAs)- iron arsenide layers separated by barium or potassium ions. BaFe2As2 is a poor metal and exhibits a SDW anomaly at 140 K. By substituting Ba2+ for K+ ions we have introduced holes in the (FeAs)- layers, which suppress the …
We investigate the electronic and other properties of the hypothetical compound LiFeSb in relation to superconducting LiFeAs and FeSe using density functional calculations. The results show that LiFeSb in the LiFeAs structure would be dynamically stable in the sense of having no unstable phonon modes, and would have very similar electronic and magnetic properties to the layered Fe based superconductors. Importantly, a very similar structure for the Fermi surface and a spin density wave related to but stronger than that in the corresponding As compound is found. These results are indicative of …
The effect of hydrostatic pressure on the superconductivity in LiFeAs is investigated up to 1.8 GPa. The superconducting transition temperature, T_c, decreases linearly with pressure at a rate of 1.5 K/GPa. The negative pressure coefficient of T_c and the high ambient pressure T_c indicate that LiFeAs is the high-pressure analogue of the isoelectronic SrFe_2As_2 and BaFe_2As_2.
Between the different families of pnictide multiband superconductors, LiFeAs is probably one of the less understood. Indeed, despite the large amount of experiments performed in the last few years on this material, no consensus has been reached yet on the possible pairing mechanism at play in this system. Here we focus on the precursor effects of superconductivity visible in the transport experiments performed above Tc. By analyzing the superconducting fluctuations in a layered multiband model appropriate for this material, we argue that the strong two-dimensional character of the paraconducti…
Non-Fermi-liquid (NFL) phenomena associated with correlation effects have been widely observed in the phase diagrams of unconventional superconducting families. Exploration of the correlation between the normal state NFL, regardless of its microscopic origins, and the superconductivity has been argued as a key to unveiling the mystery of the high-Tc pairing mechanism. Here we systematically investigate the pressure-dependent in-plane resistivity and Hall coefficient (RH ) of a high-quality 112-type Fe-based superconductor Ca1-xLaxFe1-yCoyAs2 (x = 0.2,y = 0.02). With increasing pressure, the no…
We report an investigation of the lattice dynamical properties of LiFeAs using inelastic light scattering. Five out of the six expected phonon modes are observed. The temperature evolution of their frequencies and linewidths is in good agreement with an anharmonic-decay model. We find no evidence for substantial electron-phonon coupling, and no superconductivity-induced phonon anomalies.
Hydrogen is the simplest bipolar element and its valence state can be controlled from +1 to -1. We synthesized the 1111-type CaFeAsH and LnFeAsO1-xHx (Ln = lanthanide) with the ZrCuSiAs type structure by a high-pressure synthesis method. The position and valence state of the substituted H were determined by neutron diffraction and density functional theory calculations. The close similarity in the structural and electrical properties of CaFeAsH and CaFeAsF indicated the formation of the hydride ion (H-), which is isovalent with the fluoride ion (F-), in the 1111-type iron arsenides. When some …
Superconductivity was achieved in Ti-doped iron-arsenide compound Sr4Cr0.8Ti1.2O6Fe2As2 (abbreviated as Cr-FeAs-42622). The x-ray diffraction measurement shows that this material has a layered structure with the space group of \emph{P4/nmm}, and with the lattice constants a = b = 3.9003 A and c = 15.8376 A. Clear diamagnetic signals in ac susceptibility data and zero-resistance in resistivity data were detected at about 6 K, confirming the occurrence of bulk superconductivity. Meanwhile we observed a superconducting transition in the resistive data with the onset transition temperature at 29.2…
Conventional Cooper pairing arises from attractive interaction of electrons in the metallic bands. Recent experiment on Co-doped LiFeAs shows superconductivity in the insulating valence band, which is evolved from a metallic hole band upon doping. Here we examine this phenomenon by studying superconductivity in a three-orbital Hamiltonian relevant to the doped LiFeAs. We show explicitly that Cooper pairing of the insulating hole band requires a finite pairing interaction strength. For strong coupling, the superconductivity in the hole band is robust against the sink of the hole band below the …
Electronic nematic behavior has been identified and studied in iron-based superconductors for some time, particularly in the well-known BaFe$_2$As$_2$ system, where it is well-known to compete with superconductivity. On the other hand, it has been shown recently that FeSe displays a negligible effect of nematicity on superconductivity near the superconducting transition, and actual cooperation between the two orders when the system is doped with S. Recently it has also been proposed that LiFeAs undergoes a nematic transition in the superconducting state itself. Generally, we expect superconduc…
The lack of nesting of the electron and hole Fermi-surface sheets in the Fe-based superconductor LiFeAs, with a critical temperature of 18 K, has led to questions as to whether the origin of superconductivity in this material might be different from other Fe-based superconductors. Both angle-resolved photoemission and quasiparticle interference experiments have reported fully gapped superconducting order parameters with significant anisotropy. The system is also of interest because relatively strong correlations seem to be responsible for significant renormalization of the hole bands. Here we …
Hybrid structures between conventional, s-wave superconductors and two-dimensional topological insulators (2D TIs) are a promising route to topological superconductivity. Here, we investigate planar Josephson junctions fabricated from hybrid structures that use thin films of cadmium arsenide (Cd3As2) as the 2D TI material. Measurements of superconducting interference patterns in a perpendicular magnetic field are used to extract information about the spatial distribution of the supercurrent. We show that the interference patterns are distinctly different in junctions with and without mesa-isol…
We use elastic and inelastic neutron scattering to systematically investigate the evolution of the low-energy spin excitations of the iron arsenide superconductor BaFe2-xNixAs2 as a function of nickel doping x. In the undoped state, BaFe2As2 exhibits a tetragonal-to-orthorhombic structural phase transition and simultaneously develops a collinear antiferromagnetic (AF) order below TN = 143 K. Upon electron-doping of x = 0.075 to induce bulk superconductivity with Tc = 12.3 K, the AF ordering temperature reduces to TN = 58 K.We show that the appearance of bulk superconductivity in BaFe1.925Ni0.0…
Y. M. Dai et al. in their recent work arXiv:1106.4430 presented a reflectivity study of the in-plance optical conductivity of a $\textrm{Ba}_{0.6}\textrm{K}_{0.4}\textrm{Fe}_2\textrm{As}_2$ (BKFA) superconductor with $T_{\textrm{c}}\approx39K$. The single crystals used in this study are of high quality and the measurements are rather accurate. The authors analyzed the optical conductivity of BKFA in the framework of the Mattis-Bardeen theory and the BCS theory of superconductivity. This analysis, however, generates a series of severe contradictions with a large body of experimental data obtain…
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