Raman phonon anomalies in Sr(Fe1-xCox)2As2

Phonon anomalies have been reported in iron-pnictide superconductors indicating a diverse interplay between different orders in the materials.Here,we report Raman scattering measurements on Sr(Fe_1-xCo_x)₂As₂(x=0 and x=0.04)single crystals in the B;symmetry with respect to a 1 Fe unit cell.Upon cooling,we observe a larger split(13 cm;)of Eg Raman phonon modes pertaining to in-plane Fe and As displacements as the crystals undergo the tetragonal-toorthorhombic structural phase transition,although a considerable split(9 cm;)has been reported in BaFe_1-xCo_x)₂As₂.Furthermore,the splitting of E;phonon modes is strongly reduced upon doping.We perform an order-parameter analysis revealing a similar doping dependence of E;phonon splitting as reported in other compounds of the 122 family,indicating these phonon anomalies widely exist in 122 iron-based superconductors and might share the same mechanisms.     

MAGNETIC PROPERTIES OF Pr2Co17-x Mx ( M＝Ga,Si) COMPOUNDS

The influence of Ga or Si substitution for Co on the structural and magnetic properties of Pr₂Co₁₇ compounds is investigated. All samples studied here are single phase and have the rhombohedral Th₂Zn₁₇-type structure. The unit-cell volume is found to increase linearly by the substitution of Ga for Co,but reduce by the substitution of Si for Co in Pr₂Co₁₇ compounds. In Pr₂Co_17-x M_x, the Curie temperature decreases monoto nically with increasing at an approximate rate of 153K per Ga atom and 175K per Si atom. The saturation magnetic moment of Pr₂Co_17-x M_x ( M=Ga,Si) decreases with increasing x. The rates of the decrease are larger than that expected as a simple dilution. For Pr₂Co_17-xSi_x ,the spin reorientation transition is observed above room temperature. The spin-reorientation temperature Tsrfirst decreases with increasing Si content and then increases at higher x values (x>2). The spin reorientat ion behavior is interpreted by the competition between the Pr and Co sublattice anisotropies. The easy magnetization direction in Pr₂Co_17-xGa_x compounds is perpendicular to the c-axis, and no spin reorientation transition is observed.     

MAGNETIC PROPERTIES OF Dy2Fe17-xCrx AND Er2Fe17-xCrx(x=0-3)COMPOUNDS

The crystal structure and magnetic properties of R₂Fe_17-xCr_x(R=Dy,Er,0≤x≤3) compounds have been investigated by me ans of X-ray diffraction and magnetization measurements. These compounds have hexagonal Th₂Ni₁₇-type structure. The unit-cell volumes decrease with the increase of Cr concentration x. The Curie temperature T_c of the Er₂Fe_17-xCr_x compounds increases from 320 K for x=0 to 403 K for x=1.0 and then decreases with further increase of x. The Cur ie temperat ure T_c of Dy₂Fe_17-xCr_x compounds increases from 364 K for x=0 to 435 K for x=1.0 and then decreases with further increase of x. The saturation magnetization of these compounds shows an approximately linear decrease with the increase of x. Spin reorientation transitions occur s in Er₂Fe_17-xCr_x(x=2.0 and 3.0).     

Evolution of the 128-cm-1 Raman phonon mode with temperature in Ba(Fe1-xCox)2As 2 (x = 0.065 and 0.2)

We report electronic Raman scattering measurements on Ba(Fei_1-xCo_x)₂As₂（x = 0.065 and 0.2） single crystals with Raman shifts from 9 cm^-1 up to 600 cm^-1 in the symmetry of B_lg with respect to 1 Fe unit cell.When the crystals are cooled down,the evident quasielastic peaks of Raman spectra occur only in the crystal with x = 0.065,which is due to the contribution of orbital ordering between xz and yz Fe 3d orbitals,as we reported in another work.Here,we analyze the E_g phonon at 128 cm^-1,which has the same function form of its Raman tensors as those of xz and yz Fe 3d orbitals in these two crystals respectively.Unlike their electronic continuums,no anomalies are found in the E_g phonons of these two samples,which simply follows the expressions corresponding to the anharmonic phonon decay into acoustic phonons with the same frequencies and opposite momenta.Our results indicate that the structural and magnetic phase transition might be completely suppressed by chemical doping and there is not any indication of coupling between charge nematicity and E_g phonon mode from our experimental results,which is consistent with the results in our previous work.     

Structural and physical properties of undoped and nickel-doped BaFe2-xNixAs2（x = 0,0.04） single crystals

<正>Single crystals of undoped and nickel-doped BaFe_(2-x)Ni_xAs_2(x=0,0.04) have been grown by FeAs self-flux method.The maximum dimension of the crystal is as large as ~ 1 cm along the ab plane.The crystalline topography of a cleaved crystal surface is examined by scanning electron microscope(SEM).By x-ray powder diffraction(XRD) experiments using pure silicon as an internal standard,precise unit cell parameters(tetragonal at room temperature) are determined:a = 3.9606(4) A(1 A=0.1 nm),c = 13.015(2) A for BaFe_(1.96)Ni_(0.04)As_2 and a = 3.9590(5) A,c = 13.024(1) A for BaFe_2As_2.DC magnetization and transport measurements are performed to check superconducting transition(T_c=15 K for x=0.04) and other subtle anomalies.For BaFe_(0.96)Ni_(0.04)As_2 crystal,the resistance curve at normal state shows two distinct anomalies associated with spin and structure transitions,and its magnetization data above ~ 91 K exhibit a linear temperature dependence due to spin density wave(SDW) instability.     

M1 transition energy and rate in the ground configuration of Ag-like ions with 62 ≤ Z ≤ 94

Systematic calculations and assessments are performed for the magnetic dipole(M1)transition energies and rates between the ²F^o_5/2,7/2 levels in the ground configuration 4d104f along the Ag-like isoelectronic sequence with 62≤Z≤94 based on the second-order many-body perturbation theory implemented in the Flexible Atomic Code.The electron correlations,Breit interaction and quantum electrodynamics effects are taken into account in the present calculations.The accuracy and reliability of our results are evaluated through comprehensive comparisons with available measurements and other theoretical results.For transition energies,our results are in good agreement with the recent experimental data obtained from the electron beam ion traps within 0.18%.The maximum discrepancy between our results and those obtained with the large-scale multiconfiguration Dirac–Hartee–Fock calculations by Grumer et al.[Phys.Rev.A 89062501(2014)]is less than 0.13%along the isoelectronic sequence.Furthermore,the corresponding M1 transition rates are also reported.The present results can be used as the benchmark and useful for spectra simulation and diagnostics of astrophysical and fusion plasmas.     

Density-functional theory investigation of energy gaps and optical properties of Hg1-xCdxTe and In1-xGaxAs

We use a modified Becke-Johnson exchange plus a local density approximation correlation potential within the density functional theory to investigate the electronic structures of Hg_1-xCd_x Te and In_1-xGa_x As with x being 0,0.25,0.5,0.75,and 1.For both of the two series,our calculated energy gaps and dielectric functions （real part ε 1 and imaginary part ε₂） are in agreement with the corresponding experimental results with x being between 0 and 1.The calculated zero-frequency refractive index varies greatly with x for Hg_1-xCd_x Te,but changes little with x for In_1-xGa_x As,which is consistent with the real parts of their dielectric functions.Therefore,this new approach is satisfactory to describe the electronic structures and the optical properties of the semiconductors.     

Physical properties and phase diagram of NaFe1-xVxAs

We grew a series of NaFe_1-xV_x As(0 ≤ x ≤ 0.03) single crystals and performed the measurements of resistance,magnetic susceptibility, and specific heat to study the superconducting phase diagram by doping V into Na Fe As. Both the structural and the spin-density-wave(SDW) transitions are slightly suppressed by V-doping. While superconducting transition temperature is enhanced to the maximum value of 15 K when the optimal doping level x = 0.007 and then is suppressed rapidly with further V-doping, displaying a small superconducting dome. Our results suggest that V-impurities should act as strong magnetic scattering centers which cause the sharp suppression of superconductivity in NaFe_1-xV_x As.     

Effect of impurity scattering on superconductivity in K2Cr3As3

Impurity scattering in a superconductor may serve as an important probe for the nature of superconducting pairing state. Here we report the impurity effect on superconducting transition temperature T c in the newly discovered Cr-based superconductor K₂Cr₃As₃.The resistivity measurements show that the crystals prepared using high-purity Cr metal（99.99%） have an electron mean free path much larger than the superconducting coherence length. For the crystals prepared using impure Cr that contains various nonmagnetic impurities, however, the T c decreases significantly, in accordance with the generalized Abrikosov-Gor’kov pair-breaking theory. This finding supports a non-s-wave superconductivity in K₂Cr₃As₃.     

EFFECT OF THE SUBSTITUTION OF Ga ON THE STRUCTURE AND MAGNETIC PROPERTIES OF Dy2Fe17 COMPOUND

Dy₂Fe_17-xGa_x (x = 0,1,2,3,4, 5 and 6) compounds were prepared by arc melting. These compounds are of single phase, having a hexagonal Th₂Ni₁₇-type structure fox x=0 and rhombohedral Th₂Zn₁₇-type structure for x≥1. The substitution of Ga for Fe in the Dy₂Fe₁₇ leads to a linear increase of unit-cell volumes. The saturation magnetization M_s at 1.5K is found to decrease linearly with increasing Ga concentration, from 65emu/g for x= 0 to 5emu/g for x = 6; and the Fe magnetic moment μ_Fe is almost independent of Ga concentration. The Curie temperature T_C is found first to increase with increasing Ga content x, and goes through a maximum value of 559 K at about x = 3, then decreases. The sharp increase of T_C at lower Ga content may result from the increase of unit cell volumes. Dy₂Fe_17-xGa_x compounds with x≤5 exhibit easy plane anisotropy at room temperature, and those with x = 6 possess easy-axis.     

SUPERCONDUCTING TRANSITION TEMPERATURES AND NORMAL-STATE ELECTRICAL RESISTIVITIES OF Tm1-xPrxBa2Cu3O7-δ

Superconducting transition temperature T_c and normal-state electrical resistivities ρ of the Tm_1-xPr_xBa₂Cu₃O_7-δ system have been measured. The results indicate that T_c remains constant for x = 0 to 0.08 before it drops steadily with higher x. Following the Abrikosov-Gor'kov model, a total suppression of superconductivity occurs at a critical Pr concentration x_cr≈0.56. It is found for the first time, as far as we know, that the logarithmic resistivity at 270 K increases linearly with increasing x, yielding ρ= ρ₀e^ax. Judging from the relation between dρ/dT and x, a metal to insulator transition occurs just beyond x_cr.     

Growth and transport features of electron-doped superconductor Pr1-xLaCexCuO4-δ thin films

<正>T’-phase electron-doped superconductor Pr_1-xLaCe_xCuO_4-δ（PLCCO） thin films are successfully prepared on SrTiO₃（100） substrates by using the dc magnetron sputtering method.It is found that the films each have a highly oriented structure along the c-axis.For optimally doped films with x≈0.10,the superconducting transition temperature T_c is 23.5 K,which is similar to that of a single crystal.The quadratic temperature dependence of the resistivity is observed when T > T_c,which can be attributed to the two-dimensional Fermi liquid behaviour.Besides,the optimal conditions for preparing the T’-phase PLCCO thin films are also discussed in detail.     

A density-functional theory investigation on desorption of O2 on Sn（111） and its comparison with initial oxidation on the X（111） （X=Si,Ge, Sn,Pb） surfaces

<正>The first-principles calculations are performed to investigate the adsorption of O₂ molecules on an Sn（111） 2×2 surface.The chemisorbed adsorption precursor states for O₂ are identified to be along the parallel and vertical channels, and the surface reconstructions of Sn（lll） induced by oxygen adsorption are studied.Based on this,the adsorption behaviours of O₂ on X（111）（X=Si,Ge,Sn,Pb） surfaces are analysed,and the most stable adsorption channels of O₂ on X（111）（X=Si,Ge,Sn,Pb） are identified.The surface reconstructions and electron distributions along the most stable adsorption channels are discussed and compared.The results show that the O₂ adsorption ability declines gradually and the amount of charge transferred decreases with the enhancement of metallicity.     

Anomalous Second Magnetization Peak in 12442-Type RbCa2Fe4As4F2 Superconductors

The second magnetization peak(SMP) appears in most superconductors and is crucial for the understanding of vortex physics as well as the application. Although it is well known that the SMP is related to the type and quantity of disorder/defects, the mechanism has not been universally understood. We selected three stoichiometric superconducting RbCa₂Fe₄As₄F₂ single crystals with identical superconducting critical temperature T_c ～31 K and simi...     

MAGNETIC PROPERTIES AND CRYSTALLIZATION OF THE RAPIDLY QUENCHED (Fe1-xNdx) 81.5B18.5 ALLOYS

Magnetic properties and crystallization behaviors of amorphous (Fe_1-xNd_x) _81.5B_18.5 alloys were studied. The crystallization temperature is found to rise at first and then drop monotonically with x, having a maximum value of 976K at x=0.11 (9at% Nd). The (Fe_1-xNd_x) _81.5B_18.5 alloys prepared at a quenching rate of v_s = 6.6m /s are amorphous, and exhibit good glass formability. Both the coercive field H_c and energy product (BH)_max depend strongly on Nd concentration. Amorphous (Fe_1-xNd_x) _81.5B_18.5 alloys with higher Nd concentration have a high coercive field at low temperature, due to the large random uniaxial anisotropy of Nd. The room-temperature H_c and (BH)_max obtained on optimal annealing con-ditions show two maxima as a function of Nd concentration x. The highest room-temperature coercive field H_c =22 kOe within the Nd concentrations around x=0.368 and the maximum energy product(RH)_max= 13.3 MG·Oe at x =0.055 are observed. The hard magnetic properties of these crystallized samples are related to the presence of the bard magnetic Nd₂Fe₁₄B phase.     

INTER-GRAIN EXCHANGE INTERACTION FOR SINGLE-PHASE NANOCRYSTALLINE Nd-Fe-B MAGNETS

The demagnetization curve as a function of intensity of the inter-grain exchange interaction was calculated for single-phase nanocrystalline Nd-Fe-B magnets by use of the finite-element technique of micromagnetics. Also, the strength of the exchange interaction was estimated as a function of the Nd content x for the nanocrystalline Nd_xFe_94-xB₆ magnets by comparing the above result with the experimental relation between _iH_c and x for the magnets. We found that the inter-grain exchange interaction decreases with the increase of x, are ≈70% and ≈60% of the inter-grain exchange interaction for x=15.5(μ_0iH_c≈2.0T) and x=19(μ_0iH_c≈2.3T), respectively.     

EFFECT OF MANGANESE SUBSTITUTION IN R2AlFe16-x Mnx(R=Y,Ho,Nd AND Pr)COMPOUNDS ON STRUCTURAL AND MAGNETIC PROPERTIES

The structural and magnetic properties of R₂AlFe_16-xMn_x(0≤x≤8 for Y,Ho,0≤x≤10 for Pr, 0≤x≤16 for Nd)compounds have been investigated by means of X-ray diffraction and magnetization measurements.The R₂AlFe_16-xMn_x compounds have a hexagonal Th₂Ni₁₇-type structure for R=Y and Ho, and a rhombohedral Th₂Zn₁₇-type structure for Nd and Pr.With increasing x,the unit-cell volumes have a small increase initially,followed by a greater linear increase.The Curie temperature and the saturation magnetizati on of these compounds show marvelous drop with increasing x.Compensation point was observed in the temperature dependence of the magnetization for Ho₂AlFe₁₂Mn₄ compound.     

Floating-zone growth and property characterizations of high-quality La2-хSrхCuO4 superconductor crystals

We have grown underdoped(x=0.11,0.12) and optimally doped(x=0.16) La2 x SrxCuO4 single crystals by the traveling-solvent floating-zone technique.In order to prepare good quality cuprate crystals,we have made much effort to optimize the preparation procedures.For example,we haveadopted the sol-gel route to prepare a highly fine and homogeneous La2xSrxCuO4 precursor powder for fabricating a very dense ceramic feed rod used for the floating-zone growth,and we have also used quite a slow growth rate.The high quality of the grown crystals has been verified by double-crystal x-ray rocking curves,with the full-width-at-half-maximum being only 113-150 arcseconds,which are the best data reported so far for La2xSrxCuO4 crystals.The superconducting critical temperatures of the grown crystals are 30,31 and 38.5 K for x=0.11,0.12 and 0.16 samples,respectively,according to magnetic measurements.     

Elastic constants and thermodynamic properties of Mg2SixSn1-x from first-principles calculations

<正>This paper stuides the elastic constants and some thermodynamic properties of Mg₂Si_xSn_n-1（x=0,0.25,0.5, 0.75,1） compounds by first-principles total energy calculations using the pseudo-potential plane-waves approach based on density functional theory,within the generalized gradient approximation for the exchange and correlation potential. The elastic constants of Mg₂Si_xSn_n-1 were calculated.It shows that,at 273 K,the elastic constants of Mg₂Si and Mg₂Sn are well consistent with previous experimental data.The isotropy decreases with increasing Sn content.The dependences of the elastic constants,the bulk modulus,the shear modulus and the Debye temperatures of Mg₂Si and Mg₂Si_0.5Sn_0.5 on pressure were discussed.Through the quasi-harmonic Debye model,in which phononic effects were considered,the specific heat capacities of Mg₂Si_xSn_1-x at constant volume and constant pressure were calculated.The calculated specific heat capacities are well consistent with the previous experimental data.     

Growth and Characterization of a New Superconductor GaBa2Ca3Cu4O11+δ

We successfully grow a new superconductor GaBa₂Ca₃Cu₄O_11+δ(Ga-1234) with a transition temperature of 113 K, using the Walker-type high-pressure synthesis apparatus. X-ray diffraction measurements on the powderized samples show a mixture of the Ga-1234 phase and the Ca_0.85CuO₂ phase, and the former is dominant. Under the scanning electron microscope, plate-like crystals of the Ga-based 1234 phase with shiny surfaces can be seen.The...