Common Features in Electronic Structure of the Oxypnictide Superconductors from Photoemission Spectroscopy

High resolution photoemission measurements are carried out on non-superconducting LaFeAsO parent com- pound and various superconducting RFeAs（O1-ZFx） （R=La, Ce and Pr） compounds. It is found that the parent LaFeAsO compound shows a metallic character. By extensive measurements, several common features are identified in the electronic structure of these Fe-based compounds： （1） 0.2 eV feature in the valence band, （2） a universal 13-16 meV feature, （3） near EF spectral weight suppression with decreasing temperature. These uni- versal features can provide important information about band structure, superconducting gap and pseudogap in these Fe-based materials.     

High-Pressure Single-Crystal Neutron Scattering Study of Magnetic and Fe Vacancy Orders in （T1,Rb）：Fe4Se5 Superconductor

The recently discovered metal-intercalated iron selenide superconductors A2Fe4Se5 （A=K, Cs, T1- K, Rb, T1-Rb） （245） compounds, with Tc ～30K, have attracted much interest A high transition- temperature （TN ≈ 470-560K） and large magnetic moment （3.3μB/Fe） block antiferromagnetic （AFM） order exists in the superconducting samples. Mag- netic order-parameter experiences an anomaly when Tc is approached. The superconductors crystal- lize with a highly ordered √5 × √5 superstructure, in which the Fel site of the I4/m structure is only a few percents occupied and the Fe2 site is fully occupied. The non-superconducting samples at low-T also crystallize in the I4/m structure, while both Fe sites are fractionally occupied, since the numbers of the Fe vacancies in the samples and the vacant sites in the √5 × √5 pattern are mismatched. The partially ordered √5× √5 vacancy order becomes one of three competing phases for temperature below room temperature up to ～ 500K, namely, these sam- ples are phase-separated and in the miscibility gap 1lacier the n,mhi~nt condition.     

Electronic and magnetic structures of chain structured iron selenide compounds

Electronic and magnetic structures of iron selenide compounds Ce2O2FeSe2 （2212＊） and BaFe2Se3 （123＊） are studied by the first-principles calculations. We find that while all these compounds are composed of one-dimensional （1D） Fe chain （or ladder） structures, their electronic structures are not close to be quasi-lD. The magnetic exchange couplings between two nearest-neighbor （NN） chains in 2212＊ and between two NN two-leg-ladders in 123＊ are both antiferromagnetic （AFM）, which is consistent with the presence of significant third NN AFM coupling, a common feature shared in other iron-chalcogenides, FeTe （11＊） and KyFe2-xSe2 （122＊）. In magnetic ground states, each Fe chain of 2212＊ is ferromagnetic and each two-leg ladder of 123＊ form a block-AFM structure. We suggest that all magnetic structures in iron-selenide compounds can be unified into an extended J1-J2-J3 model. Spin-wave excitations of the model are calculated and can be tested by future experiments on these two systems.     

Superconducting Properties and Absence of Time Reversal Symmetry Breaking in the Noncentrosymmetric Superconducting Compounds TaxRe1-x(0.1≤x≤0.25)

Unconventional superconductivity,in particular,in noncentrosymmetric systems,has been a long-sought topic in condensed matter physics.Recently,Re-based superconductors have attracted great attention owing to the potential time-reversal symmetry breaking in their superconducting states.We report the superconducting properties of noncentrosymmetric compounds Ta_xRe_1-x with 0.1 ≤x≤0.25,and find that the superconducting transition temperature reaches a maximum of ~8 K at the optimal level x=0.15.Nevertheless,muon-spin rotation and relaxation measurements reveal no time-reversal symmetry breaking existing in its superconducting state,which is in sharp contrast to both centrosymmetric Re metal and many other noncentrosymmetric Re-based superconductors.     

Superconductivity in Hole-Doped （S1-xKx）Fe2As2

A series of layered （Sr1-xKx）Fe2As2 compounds with nominal x = 0-0.40 are synthesized by solid state reaction method. Similar to other parent compounds of iron-based pnictide superconductors, pure SrFe2As2 shows a strong resistivity anomaly near 210 K, which was ascribed to the spin-density-wave instability. The anomaly temperature is much higher than those observed in LaOFeAs and BaFe2As2, the two prototype parent compounds with ZrCuSiAs- and ThCr2Si2-type structures. K-doping strongly suppresses this anomaly and induces superconductivity. Like in the case of K-doped BaFe2As2, sharp superconducting transitions at Tc ～ 38 K is observed. We perform the Hall coefficient measurement, and confirm that the dominant carriers are hole-type. The carrier density is enhanced by a factor of 3 in comparison to F-doped LaOFeAs superconductor.     

Superconductivity and normal state magnetoresistance in superconducting FeSe:Sb

We prepared a series of β-FeSe samples with a nominal composition of Fe1.11Se1-xSbx(0≤x≤0.5).The X-ray diffraction,transport and magnetic measurements were performed on these samples to investigate the structure,the superconducting properties and the normal state transport and magnetic properties.Although the X-ray diffraction data suggested that Sb atoms were not incorporated into the β-FeSe phase,the transport data showed observable changes of superconductivity,normal state resistivity and magnetoresistance.This was represented by the increase in the superconducting transition temperature and the upper critical field.Also,for the samples with a low level of Sb content,a clear decrease of the normal state resistivity and a substantial increase of the residual resistance ratio were observed.Furthermore,the samples showed a significant increase of the normal state magnetoresistance that appeared not to follow the Kohler's rule.The results were discussed in the frame of reduction of excess Fe at interstitial sites of β-FeSe.     

Thermal Conductivity of Ce Doped Bi-2212 Superconductors

The temperature dependence of the thermal conductivity in Bi2Sr2 Ca1-xCexCu2Oy x = 0.1, 0.2, 0.3, 0.4 is presented. With increasing Ce-doping level, the thermal conductivity peak under Tc is suppressed then disappears,while another peak appears at low temperatures for the non-superconducting compounds. The numerical analysis shows that the thermal conductivity peak under Tc can be well described by the normal electron relaxation-time contribution model, and the phonon-induced thermal conductivity peak could be well described within the Debye approximation of the phonon spectrum. The existence and velriation of these two thermal conductivity peaks indicate the adjustability between the superconducting and insulating components in the samples with different Ce-doping levels.     

Single Crystal Growth and Physical Property Characterization of Non-centrosymmetric Superconductor PbTaSe_2

We report on the single crystal growth and superconducting properties of PbTaSe_2 with the non-centrosymmetric crystal structure.By using the chemical vapor transport technique,centimeter-size single crystals are successfully obtained.The measurement of temperature dependence of electrical resistivity p(T) in both normal and superconducting states indicates a quasi-two-dimensional electronic state in contrast to that of polycrystalline samples.Specific heat C(T) measurement reveals a bulk superconductivity with T_c≌3.75 K and a specific heat jump ratio of 1.42.All these results are in agreement with a moderately electron-phonon coupled,type-ⅡBardeen-Cooper-Schrieffer superconductor.     

Magnetic properties and magnetocaloric effects of Mn5Ge3-xGax

We report on the magnetic properties and magnetocaloric effects of Mn5Ge3-xGax compounds with x=0.1,0.2,0.3,0.4,0.6 and 0.9. All samples crystallize in the hexagonal Mn5Si3-type structure with space group P63/mcm and order ferromagnetically.The Curie temperature of these compounds decreases with increasing x, from 306K (x=0.1) to 274K (x=0.9).The average Mn magnetic moments increases with increasing Ga content,reaching a maximum value at x=0.6.The magnetic entropy changes in these compounds are determined from the temperature and field dependence of the magnetization using the thermodynamic Maxwell relation.The Ga substitution has two kinds of influence on the magnetocaloric effect (MCE) of Mn5Ge3.One is that the magnitude of the magnetic entropy change decreases,the other is that the MCE peak becomes broadened.     

A Triplet Resonance in Superconducting Fe_(1.03)Se_(0.4_Te_(0.6)

From heavy fermion compounds and cuprates to iron pnictides and chalcogenides, a spin resonance at hΩ0 ∝ k_BT_c is a staple of nearly magnetic superconductors. Possible explanations include a two-particle bound state or loss of magnon damping in the superconducting state. While both scenarios suggest a central role for magnetic fluctuations,distinguishing them is important to identify the right theoretical framework to understand these types of unconventional superconductors. Using an inelastic neutron scattering technique,we show that the spin resonance in the optimally doped Fe_(1.03) Se_(0.4) Te_(0.6) superconductor splits into three peaks in a high magnetic field,a signature of a two-particle S = 1 triplet bound state.     

Element Substitution Effect in Transition Metal Oxypnictide Re(O1-xFx)TAs (Re=rare earth, T=transition metal)

Different element substitution effects in transition metal oxypnictide Re(O_1-xF_x)TAs, with Re=La, Ce, Nd, Eu, Gd, Tm, T=Fe, Ni, Ru, are studied. Similarto the La- or Ce-based systems, we find that the pure NdOFeAs shows a strong resistivity anomaly near 145K, which is ascribed to the spin-density-wave instability. Electron doping by F increases T_c to about 50K. While in the case of Gd, Tc is reduced below 10K. The tetragonal ZrCuSiAs-type structure could not be formed for Eu or Tm substitution in our preparing process. For the Ni-based case, although both pure and F-doped LaONiAs are superconducting, no superconductivity is found when La is replaced by Ce in both the cases, instead a ferromagnetic ordering transition is likely to form at low temperature in the undoped sample. We also synthesize LaO_1-xF_xRuAs and CeO_1-xF_xRuAs compounds. The metallic behaviour is observed down to 4K.     

The Minimum Model for the Iron-Based Superconductors

A single band t-U-J₁-J₂ model is proposed as the minimum model to describe the superconductivity of the newly discovered iron-based superconductorsR(O_1-xF_x)FeAs and RO_1-xFeAs (R=La, Ce, Sm, Pr, Nd, Gd). With the mean-field approach, it is found that the pairing occurs in the d-wave channel. In the likely parameter region of the real materials, by lowering temperature, the system enters firstly the d_xy superconducting phase with D_4h-symmetryand then enters the time-reversal-symmetry-broken d^_xy+id_x²-y² superconducting phase with C_4h-symmetry.     

Superconductivity at 55K in Iron-Based F-Doped Layered Quaternary Compound Sm[O1-xFx]FeAs

We report the superconductivity in iron-based oxyarsenide Sm[O_1-xF_x]FeAs, with the onset resistivity transition temperature at 55.0K and Meissner transition at 54.6K. This compound has the same crystal structure as LaOFeAs with shrunk crystal lattices, and becomes the superconductor with the highest critical temperature among all materials besides copper oxides up to now.     

Fabrication and Characterization of SmCo5/Fe65Co35 and SmCo5/Fe Composite Spring Exchange Magnets

SmCo5/Fe65Co35 and SmCo5/Fe spring exchange magnets are fabricated by dc magnetron sputtering on MgO substrates and 100-nm-thick Si3N4 membranes, respectively. The base pressure of sputtering chamber is kept below 10^-7 Tort, and Ar pressure is 3 to 8mTorr. The samples are characterized by an x-ray diffractometer, a superconducting quantum interference magnetometer, and high resolution magnetic soft x-ray microscopy. We obtain the complete exchange coupling and single phase behaviour of composite magnets. The （BH）max value achieved is 28.8 MGOe.     

Effect of In-Doping on Electronic Structure and Optical Properties of Sr2TiO4

The effect of In doping on the electronic structure and optical properties of Sr2 TiO4 is investigated by a firstprinciples calculation of plane wave ultrasoft pseudopotentials based on density functional theory. The calculated results reveal that corner-shared TiO6 octahedra dominate the main electronic properties of Sr2TiO4 and the covalency of the Ti-O（1） bond in the ab plane is stronger than that of the Ti-O（2） bond along the c-axis. After In doping, there is a little lattice expansion in Sr2In0.125 Ti0.875 O4 and the interaction between the Ti-O bond near the impurity In atom is weakened. The binding energies of Sr2TiO4 and Sr2In0.125Ti0.875O4 estimated from the electronic structure calculations indicate that the crystal structure of Sr2In0.125 Ti0.875 O4 is still stable after doping, but its stability is lower than that of undoped Sr2TiO4. Moreover, the valence bands （VBs） of the Sr2In0.125Ti0.875O4 system consist of O 2p and In 4d states, and the mixing of O 2p and In 4d states makes the top VBs shift significantly to high energies, resulting in visible light absorption. The adsorption of visible light is of practical importance for the application of St2 TiO4 as a photocatalyst.     

Resistance Switching Characteristic and Charge Carrier Self-Trapping in Epitaxial Pr0.7（Ca1-xSrx）0.3MnO3 Thin Films

Carrier injection performed in Pro.7 Cao.aMnOa junctions demonstrate resistance switching （RS） characteristic with dramatic changes in both resistances and interface barriers, which suggests a charge carrier self-trapping model in strongly correlated electronic framework. Un-stable RS behaviour without electric fields in epitaxial Pr0.7（Ca1-xSrx）0.3MnO3 （PCSMO） films shows dependences on insulator-metal transition temperature, which indicates that RS process is really related to the intrinsic property of carriers. The switched resistance of epitaxial PCSMO films also depends on the amount of current pulses, which shouM be another evidence of the carrier self-trapping model, similarly to the dependence on the amount of self-trapped charge carriers.     

The influence of Exciting Frequency on N2 and N2+ Vibrational Temperature of Nitrogen Capacitively Coupled Plasma

By using optical emission spectroscopy （OES）, N2 and N2^＋ vibrational temperatures in capacitively coupled plasma discharges with different exciting frequencies are investigated. The vibrational temperatures are acquired by comparing the measured and calculated spectra of selected transitions with a least-square procedure. It is found that N2 and N2^＋ vibrational temperatures almost increase linearly with increasing exciting frequency up to 23 MHz, then increase slowly or even decrease. The pressure corresponding to the maximum point of N2 vibrational temperature decreases with the increasing exciting frequency. These experimental phenomena are attributed to the increasing electron density, whereas the electron temperature decreases with exciting frequency rising.     

Room-Temperature Ferromagnetism in Zn1-xMnxO Thin Films Deposited by Pulsed Laser Deposition

Zn1-xMnxO （x = O.Olq3.1） thin films with a Curie temperature above 300K are deposited on Al2O3 （0001） substrates by pulsed laser deposition. X-ray diffraction （XRD）, ultraviolet （UV）-visible transmission and Raman spectroscopy are employed to characterize the microstructural properties of these films. Room temperature ferromagnetism is observed by superconducting quantum interference device （SQUID）. The results indicate that Mn doping introduces the incorporation of Mn^2＋ ions into the ZnO host matrix and the insertion of Mn^2＋ ions increases the lattice defects, which is correlated with the ferromagnetism of the obtained films. The doping concentration is also proven to be a crucial factor for obtaining highly ferromagnetic Zn1-xMnxO films.