Coexistence of magnetism and superconductivity in the iron-based compound Cs0.8(FeSe0.98)2

We report on muon-spin rotation and relaxation (μSR), electrical resistivity, magnetization and differential scanning calorimetry measurements performed on a high-quality single crystal of Cs(0.8)(FeSe(0.98))(2). Whereas our transport and magnetization data confirm the bulk character of the superconducting state below T(c)=29.6(2) K, the μSR data indicate that the system is magnetic below T(N)=478.5(3) K, where a first-order transition occurs. The first-order character of the magnetic transition is confirmed by differential scanning calorimetry data. Taken all together, these data indicate in Cs(0.8)(FeSe(0.98))(2) a microscopic coexistence between the superconducting phase and a strong magnetic phase. The observed T(N) is the highest reported to date for a magnetic superconductor.     

Common crystalline and magnetic structure of superconducting A2Fe4Se5 (A=K,Rb,Cs,Tl) single crystals measured using neutron diffraction

Single-crystal neutron diffraction studies on superconductors A(2)Fe(4)Se(5), where A=Rb, Cs, (Tl, Rb), and (Tl, K) (T(c) ～ 30 K), uncover the same Fe vacancy ordered crystal structure and the same block antiferromagnetic order as in K(2)Fe(4)Se(5). The Fe order-disorder transition occurs at T(S)=500-578 K, and the antiferromagnetic transition at T(N) = 471-559 K with an ordered magnetic moment ～3.3μ(B)/Fe at 10 K. Thus, all recently discovered A intercalated iron selenide superconductors share the common crystalline and magnetic structure, which are very different from previous families of Fe-based superconductors, and constitute a distinct new 245 family.     

Large local disorder in superconducting K(0.8)Fe(1.6)Se2 studied by extended x-ray absorption fine structure

We have measured the local structure of superconducting K(0.8)Fe(1.6)Se(2) chalcogenide (T(c) = 31.8 K) by temperature dependent polarized extended x-ray absorption fine structure (EXAFS) at the Fe and Se K-edges. We find that the system is characterized by a large local disorder. The Fe-Se and Fe-Fe distances are found to be shorter than the distances measured by diffraction, while the corresponding mean square relative displacements reveal large Fe-site disorder and relatively large c-axis disorder. The local force constant for the Fe-Se bondlength (k ~ 5.8 eV ?(-2)) is similar to the one found in the binary FeSe superconductor, however, the Fe-Fe bondlength appears to be flexible (k ~ 2.1 eV ?(-2)) in comparison to the binary FeSe (k ~ 3.5 eV ?(-2)), an indication of partly relaxed Fe-Fe networks in K(0.8)Fe(1.6)Se(2). The results suggest a glassy nature for the title system, with the superconductivity being similar to that in the granular materials.     

Electronic structure and nematic phase transition in superconducting multiple-layer FeSe films grown by pulsed laser deposition method

We report comprehensive angle-resolved photoemission investigations on the electronic structure of single crystal multiple-layer FeSe films grown on CaF_2 substrate by pulsed laser deposition(PLD) method. Measurements on FeSe/CaF_2 samples with different superconducting transition temperatures T_c of 4 K, 9 K, and 14 K reveal electronic difference in their Fermi surface and band structure. Indication of the nematic phase transition is observed from temperature-dependent measurements of these samples; the nematic transition temperature is 140-160 K, much higher than ~90 K for the bulk FeSe. Potassium deposition is applied onto the surface of these samples; the nematic phase is suppressed by potassium deposition which introduces electrons to these FeSe films and causes a pronounced electronic structure change. We compared and discussed the electronic structure and superconductivity of the FeSe/CaF_2 films by PLD method with the FeSe/SrTiO_3 films by molecular beam epitaxy(MBE) method and bulk FeSe. The PLD-grown multilayer FeSe/CaF_2 is more hole-doped than that in MBE-grown multiple-layer FeSe films. Our results on FeSe/CaF_2 films by PLD method establish a link between bulk FeSe single crystal and FeSe/SrTiO_3 films by MBE method, and provide important information to understand superconductivity in FeSe-related systems.     

Synthesis of large FeSe superconductor crystals via ion release/introduction and property characterization

Large superconducting Fe Se crystals of(001) orientation have been prepared via a hydrothermal ion release/introduction route for the first time. The hydrothermally derived Fe Se crystals are up to 10 mm×5 mm×0.3 mm in dimension. The pure tetragonal FeSe phase has been confirmed by x-ray diffraction(XRD) and the composition determined by both inductively coupled plasma atomic emission spectroscopy(ICP-AES) and energy dispersive x-ray spectroscopy(EDX). The superconducting transition of the Fe Se samples has been characterized by magnetic and transport measurements. The zero-temperature upper critical field H_(c2) is calculated to be 13.2–16.7 T from a two-band model. The normal-state cooperative paramagnetism is found to be predominated by strong spin frustrations below the characteristic temperature T_(sn), where the Ising spin nematicity has been discerned in the FeSe superconductor crystals as reported elsewhere.     

Synthesis of a new alkali metal-organic solvent intercalated iron selenide superconductor with T(c)?≈?45?K

We report on a new iron selenide superconductor with a T(c) onset of 45?K and the nominal composition Li(x)(C(5)H(5)N)(y)Fe(2-z)Se(2), synthesized via intercalation of dissolved alkaline metal in anhydrous pyridine at room temperature. This superconductor exhibits a broad transition, reaching zero resistance at 10?K. Magnetization measurements reveal a superconducting shielding fraction of approximately 30%. Analogous phases intercalated with Na, K and Rb were also synthesized and characterized. The superconducting transition temperature of Li(x)(C(5)H(5)N)(y)Fe(2-z)Se(2) is clearly enhanced in comparison to those of the known superconductors FeSe(0.98) (T(c)?～?8?K) and A(x)Fe(2-y)Se(2) (T(c)?～?27-32?K) and is in close agreement with critical temperatures recently reported for Li(x)(NH(3))(y)Fe(2-z)Se(2). Post-annealing of intercalated material (Li(x)(C(5)H(5)N)(y)Fe(2-z)Se(2)) at elevated temperatures drastically enlarges the c-parameter of the unit cell (～44%) and increases the superconducting shielding fraction to nearly 100%. Our findings indicate a new synthesis route leading to possibly even higher critical temperatures for materials in this class: by intercalation of organic compounds between Fe-Se layers.     

新型铁基超导体材料的研究进展

文章主要介绍了碱金属插层法合成新型铁基超导体的研究进展.通过采用碱金属K对FeSe层状材料插层的方法,得到了一种新型的铁基超导体K0.8Fe1.7Se2,并对该材料的晶体结构与物性进行了研究.结果表明,该化合物的超导转变温度达到30K,这是FeSe体系在常压下的最高超导转变温度.同时,观察到该体系中存在转变温度为43K的超导相,但未得到纯相.通过磁性元素Co的掺杂研究,进一步加深了对K0.8Fe1.7Se2体系超导演化规律的认识.该超导体的发现对深入认识铁基超导体的超导机理,探索具有更高超导转变温度的铁基超导体具有重要意义.     

Investigation of the structural distortion in FeSex crystals by X-ray absorption near edge structures (XANES) spectroscopy

Crystals of the new superconductor FeSe were investigated by X-ray absorption near edge structures spectroscopy (XANES) in the temperature range 300-77 K. Crystals with Se content varying from x=1-0.8 were used. The Fe K-edge spectra show a structure distortion and increase in hole content as x decreases. The Fe K-edge spectra also show changes when the crystals are cooled to 100 K which may be related to the orthorhombic distortion reported in these crystals. The data is examined in conjunction with XRD measurements on the crystals to seek a correlation.     

Ab initio prediction of pressure-induced structural phase transition of superconducting FeSe

External pressure driven phase transitions of FeSe are predicted using ab initio calculations. The calculations reveal that α-FeSe makes transitions to NiAs-type, MnP-type, and CsCl-type FeSe. Transitions from NiAs-type to MnP-type and CsCl-type FeSe are also predicted. MnP-type FeSe is also found to be able to transform to CsCl-type FeSe, which is easier from α-FeSe than the transition to MnP-type FeSe, but comparable to the transition from NiAs-type FeSe. The calculated electronic structures show that all phases of FeSe are metallic, but the ionic interaction between Fe-Se bonds becomes stronger and the covalent interaction becomes weaker when the structural phase transition occurs from α-FeSe to the other phases of FeSe. The experimentally observed decrease in T(c) of superconducting α-FeSe at high pressure may be due to a structural/magnetic instability, which exists at high pressure. The results suggest an increase of the T(c) of α-FeSe if such phase transitions are frustrated by suitable methods.     

FeSe基超导单晶与薄膜研究新进展:自旋向列序、电子相分离及高临界参数

FeSe基超导体的超导临界温度可大范围调控,物理现象丰富,是非常规超导机理研究的热点.由于较高的超导临界参数及易于加工等特点,FeSe基超导体在超导应用开发方面也日益受到重视.大尺寸高质量的单晶和薄膜形态的FeSe基超导材料,对于相关基础科学研究和应用开发都极为重要.作者近年来先后开发和发明了水热离子交换（ion-exchange）、离子脱插（ion-deintercalation）、基底辅助水热外延生长方法,成功解决了二元FeSe和插层（Li,Fe）OHFeSe超导体高质量单晶和薄膜的生长和物性调控难题.进而在相关物理问题的研究中取得新进展,包括发现二元FeSe中自旋向列序与超导电性密切相关,观测到（Li,Fe）OHFeSe中的电子相分离现象.此外,（Li,Fe）OHFeSe超导薄膜呈现很高的超导临界电流密度和上临界磁场,其应用前景值得关注.     

Synthesis,structure, and properties of Ba_9Co_3Se_(15) with one-dimensional spin chains

A new compound with one-dimensional spin chains, Ba9Co3Se(15), was synthesized under high pressure and high temperature conditions and systematically characterized via structural, transport and magnetic measurements. Ba9Co3Se(15) crystallizes in a hexagonal structure with the space group P-6c2(No. 188) and lattice constants of a = b = 9.6765 ? and c = 18.9562 ?. The structure consists of trimeric face-sharing octahedral CoSe6 chains, which are arranged in a triangular lattice in the ab-plane and separated by Ba atoms. The distance of the nearest neighbor of CoSe6 chains is very large, given by the lattice constant a = 9.6765 ?. The Weiss temperature Tθ associated with the intra-chain coupling strength is about -346 K. However, no long-range magnetic order but a spin glass transition at ~ 3 K has been observed. Our results indicate that the spin glass behavior in Ba9Co3Se(15) mainly arises from the magnetic frustration due to the geometrically frustrated triangular lattice.     

铁基K0.8Fe2-ySe2体系的单晶生长与结构研究

本文报道了采用自助溶剂法对不同组分K0.8Fe2-ySe2单晶生长和结构研究的实验结果.X-射线衍射XRD(Cu靶和同步辐射)结构分析表明样品中存在两套衍射峰(分别对应相1和相2),相1能够用ThCr2Si2结构,空间群I4/mmm(No.139)指标化,对应超导相.而相2能够用单一米勒指数(0 0l)标定,可能对应一个新的物相.相2的存在能够在不同组分的KxFe2-ySe2单晶样品中很好的重复.利用高分辨透射电子显微镜(TEM)研究了K0.8Fe2Se2样品的"结构相分离"特征,结果表明样品中存在明显的位错缺陷,可能与两相结构的竞争有关.对K0.8Fe2Se2单晶样品直流磁化测量表明相2可能为弱铁磁性,从而引起高温Tp～125K的ZFC与FC磁性反常.     

Phase transitions and incommensurability in the layered semiconductor TlInS2--an NMR study

We report on the first NMR study of powder and single crystal samples of thallium indium sulfide, TlInS(2). The crystal under study is a pure single-layer TlInS(2) polytype. Our findings show that transformation from the high temperature paraelectric phase to the low temperature ferroelectric phase occurs via an incommensurate phase that exists in the temperature range from T(c) = 192 K to T(i) = 205 K. On approaching the phase transition at T(i) from above, the crystal exhibits a soft mode behavior. A discrepancy in the literature data on the phase transitions in TlInS(2) is discussed and ascribed to polytypism of the TlInS(2) crystals.     

Single crystal study of the heavy-fermion antiferromagnet CePt?In?

We report the synthesis, structure, and physical properties of single crystals of CePt(2)In(7). Single crystal x-ray diffraction analysis confirms the tetragonal I4/mmm structure of CePt(2)In(7) with unit cell parameters a = 4.5886(6) ?, c = 21.530(6) ? and V = 453.32(14) ?(3). The magnetic susceptibility, heat capacity, Hall effect and electrical resistivity measurements are all consistent with CePt(2)In(7) undergoing an antiferromagnetic order transition at T(N) = 5.5 K, which is field independent up to 9 T. Above T(N), the Sommerfeld coefficient of specific heat is γ ≈ 300 mJ mol(-1) K(-2), which is characteristic of an enhanced effective mass of itinerant charge carriers. The electrical resistivity is typical of heavy-fermion behavior and gives a residual resistivity ρ(0) ～ 0.2 μΩ cm, indicating good crystal quality. CePt(2)In(7) also shows moderate anisotropy of the physical properties that is comparable to structurally related CeMIn(5) (M = Co, Rh, Ir) heavy-fermion superconductors.     

Physical properties and crystal chemistry of Ce2Ga12Pt

Single crystals of the new ternary compound Ce(2)Ga(12)Pt were prepared by the self-flux technique. The crystal structure with the space group P4/nbm was established from single crystal x-ray diffraction data and presents a derivative of the LaGa(6)Ni(0.6) prototype. Magnetic susceptibility measurements show Curie-Weiss behaviour due to local Ce(3+) moments. At high temperatures, the magnetic anisotropy is dominated by the crystal-electric-field (CEF) effect with the easy axis along the crystallographic c direction. Ce(2)Ga(12)Pt undergoes two antiferromagnetic phase transitions at T(N,1) = 7.3 K and T(N,2) = 5.5 K and presents several metamagnetic transitions for the magnetic field along c. Specific-heat measurements prove the bulk nature of these magnetic transitions and reveal a doublet CEF ground state. The 4f contribution to the resistivity shows a broad maximum at T(max) ≈ 85 K due to Kondo scattering off the CEF ground state and excited levels.     

Magnetic behavior of Eu(3)Ni(4)Ga(4): antiferromagnetic order and large magnetoresistance

The results of the magnetic susceptibility, isothermal magnetization, heat capacity, electrical resistivity and magnetoresistance measurements on polycrystalline Eu(3)Ni(4)Ga(4) are presented. Eu(3)Ni(4)Ga(4) forms in Na(3)Pt(4)Ge(4)-type cubic crystal structure (space group [Formula: see text]). The temperature dependence of the magnetic susceptibility of Eu(3)Ni(4)Ga(4) confirms the divalent state (Eu(2+)) of Eu ions with an effective magnetic moment μ(eff)?=?7.98?μ(B). At low fields, e.g.?at 0.01?T, a magnetic phase transition to an antiferromagnetically ordered state occurs at T(N)?=?10.9?K, which is further confirmed by the temperature dependence of the heat capacity and electrical resistivity. The field dependence of isothermal magnetization at 2?K reveals the presence of two field induced metamagnetic transitions at H(c1) and H(c2)?=?0.55 and 1.2?T, respectively and a polarized phase above H(PO)?=?1.7?T. The reduced jump in the heat capacity at the transition temperature, ΔC|(T(N))?=?13.48?J/mol-Eu?K would indicate an amplitude modulated (AM) antiferromagnetic structure. An interesting feature is that a large negative magnetoresistance, MR?=?[ρ(H)?-?ρ(0)]/ρ(0), is observed in the vicinity of magnetic transition even up to 2T(N). Similar large magnetoresistance has been observed in the paramagnetic state in some Gd and Eu based alloys and has been attributed to the magneto-polaronic effect.     

Electrical transport properties of Fe Se single crystal under high magnetic field

Understanding the normal electronic state is crucial for unveiling the mechanism of unconventional superconductivity(SC). In this paper, by applying a magnetic field of up to 37T on FeSe single crystals, we could reveal the normal-state transport properties after SC was completely suppressed. The normal-state resistivity exhibited a Fermi liquid behavior at low temperatures. Large orbital magnetoresistance(MR) was observed in the nematic state with H//c, whereas MR was negligible with H//ab. The magnitude of the orbital MR showed an unusual reduction, and Kohler's rule was severely violated below 10-25 K;these were attributable to spin fluctuations. The results indicated that spin fluctuations played a paramount role in the normalstate transport properties of FeSe albeit the Fermi liquid nature was at low temperature.     

Magnetism and superconductivity in Eu0.2Sr0.8(Fe0.86Co0.14)2As2 probed by 75As NMR

We report bulk superconductivity (SC) in Eu(0.2)Sr(0.8)(Fe(0.86)Co(0.14))(2)As(2) single crystals by means of electrical resistivity, magnetic susceptibility and specific heat measurements with T(c) is approximately equal to 20 K and an antiferromagnetic (AFM) ordering of Eu(2+) moments at T(N) is approximately equal to 2.0 K in zero field. (75)As NMR experiments have been performed in the two external field directions (H is parallel to ab) and (H is parallel to c). (75)As-NMR spectra are analysed in terms of first-order quadrupolar interaction. Spin-lattice relaxation rates (1/T(1)) follow a T(3) law in the temperature range 4.2-15 K. There is no signature of a Hebel-Slichter coherence peak just below the SC transition, indicating a non-s-wave or s(±) type of superconductivity. In the temperature range 160-18 K 1/T(1)T follows the C/(T+θ) law reflecting 2D AFM spin fluctuations.     

Superconductivity at 44.4 K achieved by intercalating EMIM~+ into FeSe

Superconductivity with transition temperature Tc above 40 K was observed in protonated Fe Se(Hy-Fe Se) previously with the ionic liquid EMIM-BF4 used in the electrochemical process. However, the real superconducting phase is not clear until now. And detailed structural, magnetization, and electrical transport measurements are lacking. By using similar protonating technique on Fe Se single crystals, we obtain superconducting samples with Tc above 40 K. We show that the obtained superconducting phase is not Hy-Fe Se but actually an organic-ion(C_6H_(11)N_2~+ referred to as EMIM~+)-intercalated phase(EMIM)x Fe Se. By using x-ray diffraction technique, two sets of index peaks corresponding to different c-axis lattice constants are detected in the obtained samples, which belong to the newly formed phase of intercalated(EMIM)_x Fe Se and the residual Fe Se, respectively. The superconductivity of(EMIM)x Fe Se with Tc of 44.4 K is confirmed by resistivity and magnetic susceptibility measurements. Temperature dependence of resistivity with different applied magnetic fields reveals that the upper critical field H_(c2) is quite high, while the irreversibility field Hirris suppressed quickly with increasing temperature till about 20 K. This indicates that the resultant compound has a high anisotropy with a large spacing between the Fe Se layers.     

Magnetic ordering in semiconducting Tl0.53K0.47Fe1.64Se2 single crystals studied by M?ssbauer spectroscopy

The results of a 57Fe M?ssbauer spectroscopy study between 4.5 and 523.2 K and in external magnetic fields (up to 90 kOe) of semiconducting Tl0.53K0.47Fe1.64Se2 single crystals are reported. Evidence is provided for a possible phase separation into the magnetic majority and minority phases. It is demonstrated that the magnetic moments of the divalent Fe atoms located at the 16i site (space group I4/m) of the majority phase and of the minority phase are antiferromagnetically ordered, with the Néel temperature T(N) = 518.0(3.6) K. The magnetic moments at 5.0 K of 2.09(1) and 2.28(2) μ(B) in these two phases are tilted from the crystallographic c axis by 18(1)° and 32(2)°, respectively. The Debye temperature of Tl0.53K0.47Fe1.64Se2 is found to be 228(4) K.