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.     

Doping effects of transition metals on the superconductivity of (Li,Fe)OHFeSe films

The doping effects of transition metals(TMs = Mn, Co, Ni, and Cu) on the superconducting critical parameters are investigated in the films of iron selenide(Li,Fe)OHFe Se. The samples are grown via a matrix-assisted hydrothermal epitaxy method. Among the TMs, the elements of Mn and Co adjacent to Fe are observed to be incorporated into the crystal lattice more easily. It is suggested that the doped TMs mainly occupy the iron sites of the intercalated(Li,Fe)OH layers rather than those of the superconducting Fe Se layers. We find that the critical current density J_c can be enhanced much more strongly by the Mn dopant than the other TMs, while the critical temperature T_c is weakly affected by the TM doping.     

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.     

铁硒基超导研究新进展:高质量(Li,Fe)OHFeSe单晶薄膜

单晶薄膜形态的高温超导材料对于相关基础科学研究和应用开发都极为重要.多带的铁基高温超导体往往呈现丰富的物理现象,并具有较高的超导临界参数.特别是近年发现的插层（Li,Fe）OHFeSe超导体,无论对高温超导机理还是应用研究而言,都日益受到重视,已成为铁基家族中重要的典型材料.但是,该化合物含有OH键,加热易分解.因此,现有的常规高温成膜技术均不适用于生长该薄膜材料.为解决这一生长难题,我们最近发明了基体辅助水热外延生长法,实现了超导薄膜制备技术上的突破.本文简要介绍用此软化学成膜技术首次成功制备出（Li,Fe）OHFeSe单晶薄膜.该薄膜材料具有优良的结晶质量和较高的超导临界参数,特别是其高的临界电流密度和上临界场对应用开发有实际价值.因此,（Li,Fe）OHFeSe超导单晶薄膜的成功合成,为机理研究和应用开发分别提供了重要的实验载体和备选材料.另外,该薄膜技术也有望应用于其他功能材料的探索与合成,尤其是对常规手段难以获取的材料更具重大价值.     

Crystal orientation in Bi-based superconducting glass-ceramics prepared in high magnetic field

A high magnetic field of 10 T was applied to the crystallization process of Bi₂Sr₂CaCu₂O_x superconducting precursor glasses, and the effect of high magnetic field on crystal grain orientations and superconducting properties were examined from electrical resistivity measurements, X-ray diffraction analyses and scanning electron microscope observations. The glass-ceramics prepared in a high magnetic field show better superconducting properties (higher critical temperature, larger critical current density, and smaller normal-state resistivity) compared with the samples crystallized in a normal heat-treatment with no magnetic field. It was found that Bi2212 crystal grains with a plate-like shape tend to stack to the direction of the magnetic field, i.e., the orientation of the c-axis of the Bi2212 phase to the direction of the magnetic field.     

Magnetic levitation of YBa2Cu3Oy single crystals

By using an ordinary electric balance, we are able to measure the magnetic levitation forces acting on a superconducting YBa₂Cu₃O_y (YBCO) single crystal. It is found that when the external magnetic field is parallel to the c-axis of the single crystal, the hysteretic levitation curve is similar to that of a melt-processed YBCO superconducting sample. However, when the external magnetic field is perpendicular to the c-axis of the YBCO single crystal, the levitation forces are too small to be measured by our equipment. Also, we have introduced a simple model with the Bean's approximations to explain the levitation forces. The critical current density derived from this model by fitting with experimental data is quite close to the value obtained from magnetization measurements.     

112型铁基化合物EuFeAs2的单晶生长与表征

铁基超导体中含有一类特殊的112型结构化合物,其层状结构中含有一层锯齿形的As链构型.本文报道了用CsCl助熔剂法生长新型铁基112型EuFeAs₂母体单晶的具体方法,以及对该单晶的结构和物性的详细表征.通过能量色散X射线能谱扫描对单晶样品进行的化学成分分析,以及单晶X射线衍射的结构解析,确定该单晶样品属于EuFeAs₂相,结构精修得到EuFeAs₂具有空间群为Imm2（No.44）的正交晶体结构,晶格常数分别是a=21.285（9）Å,b=3.9082（10）Å,c=3.9752（9）Å.通过低温电阻测量,发现在110 K附近和46 K附近存在两个异常电阻跳变.进一步分析表明,110 K附近存在两个邻近的相变,这两个相变与铁基母体材料中常见的结构相变和Fe²⁺的反铁磁相变相符合.结合磁化率测量分析,可知46 K附近的相变属于Eu²⁺的反铁磁相变.     

A high-resolution Fourier transform spectrometer for far infrared magneto-optic spectroscopy of magnetic materials

We describe a high-resolution (0.015 cm⁻¹) Fourier transform spectrometer which has been developed to investigate bulk and surface magnetic polaritons in magnetic media by far infrared magneto-optic spectroscopy. The spectrometer uses a novel combination of laser-controlled sampling of the interferogram and phase modulation of the infrared beam to combine very accurate sampling and low signal-to-noise ratio. The spectrometer is coupled to a liquid helium cryostat with a 7 T superconducting magnet, and a liquid helium-cooled silicon bolometer is used as the detector. Samples can be mounted in the cryostat for polarised oblique incidence reflection measurements in the Voigt geometry with the applied magnetic field vertical. Measurements on surface polaritons are made by using attenuated total reflection (ATR) spectroscopy with silicon prisms to obtain the necessary wave vector enhancement. The resolution of the instrument is demonstrated with measurements on the rotational lines of water vapour, and a selection of measurements on a bulk single crystal of FeF₂, a uniaxial antiferromagnet, is presented to illustrate the performance of the instrument as a probe of magnetic excitations.     

Helimagnetism of Fe: high pressure study of an Y2Fe17 single crystal

In this Letter we present direct observation of the Fe helimagnetism in an Y2Fe17 single crystal under pressure. Combined neutron diffraction and magnetization measurements under pressure showed that the collinear ferromagnetic phase of Y2Fe17 is substituted by the pressure induced helical incommensurate phases. The complex pressure-temperature-field behavior of the pressure induced helical magnetic phases is attributed to intrinsic properties of the iron sublattice that gives a valuable contribution to the discussion about dominating theoretical models of magnetism in gamma-Fe.     

Superconductivity of topological matters induced via pressure

By applying pressure on the topological insulator Bi₂Te₃ single crystal, superconducting phase was found without a crystal structure phase transition. The new superconducting phase is under the pressure range of 3 GPa to 6 GPa. The high pressure Hall effect measurements indicated that the superconductivity caused by bulk hole pockets. The high pressure structure investigations with synchrotron X-ray diffraction indicated that the superconducting phase is of similar structure to that of ambient phase structure with only slight change with lattice parameter and internal atomic position. The topological band structures indicate the superconducting phase under high pressure remained topologically nontrivial. The results suggested that topological superconductivity can be realized in Bi₂Te₃ due to the proximity effect between superconducting bulk states and Diractype surface states. We also discussed the possibility that the bulk state could be a topological superconductor.     

Mössbauer study of CaFeO3 under external high—Pressure

⁵⁷Fe Mössbauer and X-ray diffraction measurements have been performed on a perovskite CaFeO₃ under external high pressure upto 50 GPa at room temperature using a diamond anvil cell. Above 29 GPa the⁵⁷Fe magnetic hyperfine splitting appears superimposing with usual paramagnetic pattern of CaFeO₃. Magnitude of hyperfine field is 16 T and much smaller than 33 T of typical Fe⁴⁺ in SrFeO₃ suggesting a transition from high-spin S=2 to low-spin S=1 state in CaFeO₃.     

Zeeman effects on d-wave superconductor and tunneling spectrum in normal-metal/d-wave superconductor tunnel junction

We study the Zeeman effect on the d-wave superconductor and tunneling spectrum in normal-metal(N)/d-wave superconductor(S) junction by applying a Zeeman magnetic field to the S. It is shown that: (1) the Zeeman magnetic field can lead to the S gap decreasing, and with the increase in Zeeman energy, the superconducting state is changed to the normal state, exhibiting a first-order phase transition; (2) the Zeeman energy difference between the two splitting peaks in the conductance spectrum is equal to2h0 (h0 is the Zeeman energy); (3) both the barrier strength of interface scattering and the temperature can lower the magnitudes of splitting peaks, of which the barrier strength can lead to the splitting peaks becoming sharp and the temperature can smear out the peaks,however, neither of them can influence the Zeeman effect.     

Effect of Mn substitution on superconductivity in iron selenide(Li,Fe)OHFeSe single crystals

We synthesize a series of Mn substituted(Li, Fe)OHFeSe superconductor single crystals via a modified ion-exchange method, with the Mn concentration z(the atomic ratio of Mn:Se) ranging from 0 to 0.07. The distribution homogeneity of the Mn element incorporated into the lattice of(Li, Fe)OHFeSe is checked by combined measurements of high-angleannular-dark-field(HAADF) imaging and electron energy-loss spectroscopy(EELS). Interestingly, we find that the superconducting transition temperature T_c and unit cell parameter c of the Mn-doped(Li, Fe)OHFeSe samples display similar V-shaped evolutions with the increasing dopant concentration z. We propose that, with increasing doping level, the Mn dopant first occupies the tetrahedral sites in the(Li, Fe)OH layers before starting to substitute the Fe element in the superconducting Fe Se layers, which accounts for the V-shaped change in cell parameter c. The observed positive correlation between the T_c and lattice parameter c, regardless of the Mn doping level z, indicates that a larger interlayer separation, or a weaker interlayer coupling, is essential for the high-T_c superconductivity in(Li, Fe)OHFeSe. This agrees with our previous observations on powder, single crystal, and film samples of(Li, Fe)OHFeSe superconductors.     

Properties of a large La1.92Sr0.08CuO4+δ single crystal grown by the travelling-solvent floating-zone method

A centimeter size single crystal of La_2−xSr_xCuO_4+δ (volume=1.32 cm³) with x_Sr=0.08 has been grown by the travelling-solvent floating-zone (TSFZ) method using a double ellipsoidal-type optical furnace as the heat source. The crystallised phase was checked solvent free by X-ray powder diffraction experiments, the crystal dimensions and quality being investigated by X-ray and neutron Laüe techniques. Several rocking curves of the Bragg peaks were performed by neutron diffraction giving a full width at half maximum (FWHM) of 0.200° for (006) reflection and clearly showing the presence of twin domains as expected for such an orthorhombic structure. The superconducting critical temperature of the as-grown crystal under 2 oxygen bar was determined by SQUID measurements with T_c=18–20 K. Thermal treatments at different oxygen pressures were carried out showing no significant improvement of the transition sharpness and the T_c value. Normal state susceptibility was also measured from 6 to 800 K for two different field orientations and can be interpreted as an antiferromagnetic insulating state behaviour. The resistivity measurements display an insulating behaviour perpendicular to the CuO₂ planes and a metallic behaviour in the planes, with a high anisotropy ratio R_c/R_b350 at room temperature and a zero resistivity achieved at 27 K in both directions. The specific heat measurements have revealed no anomalies in the temperature range 15–300 K.     

56Fe13+离子引起GaP晶体的微结构损伤

利用X射线衍射（XRD）技术、傅里叶变换红外光谱（FTIR）和Raman光谱对经不同剂量的56Fe13+离子辐照的GaP晶体的微结构进行了表征。结果表明:随着辐照离子剂量的增加,GaP晶体中产生了局部的无序与缺陷。随着56Fe13+离子剂量的增加,Raman光谱展示出振动峰强度逐渐减弱而且一些逐渐消失,但其峰位几乎没有发生变化;XRD显示出GaP晶体的衍射峰的强度逐渐减小;FTIR主要表现为宽化及其强度增加。这表明重离子56Fe13+的辐照使得GaP晶体中的缺陷与无序性增加,导致晶体产生了局部的非晶化。The Misconstructural damage of GaP irradiated with 56Fe13+ to fluences ranging from 1×107 ions/cm2~1×1010 ions/cm2 were analyzed by X-ray diffraction (XRD) techniques, Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. The result shows that, with the increase of irradiation ion fluences, local disorder and defects were produced in GaP crystal. With the increase of ion fluence, Raman spectra reveal the intensity of scattering peaks gradually weakens and some scattering peaks gradually disappear, however no changes in the peak position were found. XRD measurement displays that the intensity of diffraction peaks gradually decreases with an increase in ions fluences. Result from FTIR spectra exhibits that the intensity of reflection peaks gradually increases and the FWHM of reflection peaks broadens. These phenomena indicate that, the irradiation of heavy-ion Fe produces defects and disorder in GaP crystal, leading to a local amorphization.     

Zeeman effects on d-wave superconductor and tunneling spectrum in normal-metal/d-wave superconductor tunnel junction

We study the Zeeman effect on the d-wave superconductor and tunneling spectrum in normal-metal(N)/d-wave superconductor(S) junction by applying a Zeeman magnetic field to the S. It is shown that: (1) the Zeeman magnetic field can lead to the S gap decreasing, and with the increase in Zeeman energy, the superconducting state is changed to the normal state, exhibiting a first-order phase transition; (2) the Zeeman magnetic field may make the zero-bias conductance peak split into two peaks, and the energy difference between the two splitting peaks in the conductance spectrum is equal to 2h ₀ (h ₀ is the Zeeman energy); (3) both the barrier strength of interface scattering and the temperature can lower the magnitudes of splitting peaks, of which the barrier strength can lead to the splitting peaks becoming sharp and the temperature can smear out the peaks, however, neither of them can influence the Zeeman effect.     

GdBa_2Cu_(3-x)Fe_xO_(7-δ)超导体系结构相变及拉曼谱特征

采用固态反应法制备了GdBa2Cu3-xFexO7-δ(x=0.00-0.30)系列样品,利用X射线衍射、拉曼光谱以及电测量技术对体系的晶体结构、拉曼散射谱特征以及电输运特性进行了系统研究。结果表明,当Fe掺杂量在x=0.05-0.10区间时,体系的晶体结构发生了从正交相到四方相的转变。通过对拉曼光谱中典型振动模的指认及振动模随Fe掺杂量的变化规律,得到了拉曼谱随体系正交-四方结构相变的变化特征:对于具有正交相的x=0.00-0.05样品,拉曼谱具有五个与正交相结构相对应的特征峰;而对于具有四方相的x=0.10-0.30样品,随Fe掺杂量增加,振动模强度变弱,且典型振动模发生了不同程度的展宽或频移。电输运测量表明,随Fe掺杂量的增加,超导临界温度Tc降低,正常态电阻率增加且发生了金属-半导体相变。     

Strong magnetic fluctuations in a superconducting state of CeCoIn5

We show results on the vortex core dissipation through current-voltage measurements under applied pressure and magnetic field in the superconducting phase of CeCoIn{5}. We find that as soon as the system becomes superconducting, the vortex core resistivity increases sharply as the temperature and magnetic field decrease. The sharp increase in flux-flow resistivity is due to quasiparticle scattering on critical antiferromagnetic fluctuations. The strength of magnetic fluctuations below the superconducting transition suggests that magnetism is complementary to superconductivity and therefore must be considered in order to fully account for the low-temperature properties of CeCoIn{5}.     

Spin‐freezing in superconducting La2CuO4.03 single crystal

Electron spin‐freezing at T_f=8 K has been detected in superconducting (T_c=12\ K) single crystal La₂CuO_4+y (y\simeq0.03) by ZF‐μSR. According to diffraction data, the crystal is in Bmab phase without any traces of structural phase separation. TF‐μSR experiment has shown that no Abrikosov flux line lattice is formed below T_c. The data allow us to assume that the magnetic and superconducting regions in the crystal are space separated on the microscopic scales ~ 10² Å. The presence of large field induced broadening of the Knight shift distribution k_\sigma > 1000 ppm indicates that the crystal contains micro‐regions possessing enhanced magnetic susceptibility.     

ELECTROCHEMICAL OXIDATION OF La2CuO4 SINGLE CRYSTALS

Bulk superconducting La₂CuO_4+δ single crystals are obtained by using electrochemical intercalation technique from the as-grown insulating samples. Oxidation is carried out by constant current I=10μA at temperature T=70℃ and room temperature, respectively. Structure and magnetic properties are studied by low-temperature X-ray diffraction and susceptibility measurements. A superconducting phase with T_c of 19K and δ-0.12 can be attributed to the formation of oxygen clusters. Room temperature oxidation is inhomogeneous: two superconducting phases with T_c1 of 24K and T_c2 of 8K and an antiferromagnetic phase are coexisting in the crystal. It is found that the appearance of T_c in this system has the "step" tendency.