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We study the electronic structure of electron-doped Pr$_{1.3-x}$La$_{0.7}$Ce$_{x}$CuO$_{4}$ (PLCCO ; $T_{c}$ = 27 K, x = 0.1) and hole-doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi2212 ; $T_{c}$ = 90 K) cuprate superconductors using x-ray absorption spectroscopy (XAS) and resonant photoemission spectroscopy (Res-PES). From Res-PES across the O K-edge and Cu L-edge, we identify the O 2p and Cu 3d partial density of states (PDOS) and their correlation satellites which originate in two-hole Auger final states. Using the Cini-Sawatzky method, analysis of the experimental O 2p PDOS shows an oxygen on-site Coulomb energy for PLCCO to be $U_{p}$ = 3.3$\pm$0.5 eV and for Bi2212, $U_{p}$ = 5.6$\pm$0.5 eV, while the copper on-site Coulomb correlation energy, $U_{d}$ = 6.5$\pm$0.5 eV for Bi2212. The expression for the Heisenberg exchange interaction $J$ in terms of the electronic parameters $U_{d}$, $U_{p}$, charge-transfer energy $\Delta$ and Cu-O hopping $t_{pd}$ obtained from a simple Cu$_2$O cluster model is used to carry out an optimization analysis consistent with $J$ known from scattering experiments. The analysis also provides the effective one band on-site Coulomb correlation energy $\tilde{U}$ and the effective hopping $\tilde{t}$. PLCCO and Bi2212 are shown to exhibit very similar values of $\tilde{U}$/$\tilde{t}$ $\sim$9-10, confirming the strongly correlated nature of the singlet ground state in the effective one-band model for both the materials., Comment: 13, pages, 11 figures

The rich phenomena in the FeSe and related compounds have attracted great interests as it provides fertile material to gain further insight into the mechanism of high temperature superconductivity. A natural follow-up work was to look into the possibility of superconductivity in MnSe. It was shown that MnP becomes superconducting with Tc ~ 1 K under pressure. We demonstrated in this work that high pressure can effectively suppress the complex magnetic characters of MnSe crystal when observed at ambient condition. MnSe under pressure is found to undergo several structural transformations: the cubic phase first partially transforms to the hexagonal phase at about 12 GPa, the crystal exhibits the coexistence of cubic, hexagonal and orthorhombic phases from 16 GPa to 30 GPa, and above 30 GPa the crystal shows a single orthorhombic phase. Superconductivity with Tc ~ 5 K was first observed at pressure ~12 GPa by magnetic measurements (~16 GPa by resistive measurements). The highest Tc is ~ 9 K (magnetic result) at ~35 GPa. Our observations suggest the observed superconductivity may closely relate to the pressure-induced structural change. However, the interface between the metallic and insulating boundaries may also play an important role to the pressure induced superconductivity in MnSe., Comment: 13 pages, 8 figures. TLH, CHH and LZD contribute equally to this work

The quasi-1D spin chain compound CuBr2 has been found to be multiferroic below TN (73.5K) under ambient pressure, in which the spontaneous electric polarization is induced by emerging spin spiral ordering propagating along b-axis. Herein we studied the hydrostatic pressure effect on the magnetic, dielectric and structural properties of CuBr2. The multiferroic transition temperature is greatly enhanced under hydrostatic pressure. From ambient to about 1 GPa (the limit of our homemade apparatus), TN increases unprecedentedly by more than 20K, and no sign of saturation is observed in our experiments. Meanwhile the corresponding dielectric loss keeps rather low (<<0.1). Further synchrotron-based high pressure X-ray diffraction measurements reveal that there is no pressure-induced structural phase transition in CuBr2 up to 10.2 GPa. Upon increasing pressure, the b-axis (along the spin chain) just changes slowly while transverse a- and c- axis parameters shrink much more greatly. Pressure greatly reduces the separation between spin chains and enhance the inter-chain coupling interactions in CuBr2, which results in the giant increase of multiferroic critical temperate. Our finding suggests a new effective way to improve the known multiferroic systems towards practical high temperature multiferroicity., Comment: 6 figures, submitted

From magnetic susceptibility, dielectric permittivity, electric polarization and specific heat measurements, we discover spin-induced ferroelectricity and magnetoelectric coupling in Mn3TeO6 and observe two successive magnetic transitions at low temperatures. A non-ferroelectric intermediate magnetic state occurs below 23 K and a multiferroic ground state emerges below 21 K. Moreover, Mn3TeO6 is a candidate for a multiferroic material where two types of incommensurate spin structures, cycloidal and helical, coexist. Theoretically, both spin substructures may contribute to the macro electric polarization via different mechanisms., Comment: This is the pre-peer reviewed version of the following article: Phys. Status Solidi RRL,1-5 (2015), which has been published in final form at http://onlinelibrary.wiley.com/doi/10.1002/pssr.201510347/abstract

SmFeO3 has attracted considerable attention very recently due to the reported multiferroic properties above room-temperature. We have performed powder and single crystal neutron diffraction as well as complementary polarization dependent soft X-ray absorption spectroscopy measurements on floating-zone grown SmFeO3 single crystals in order to determine its magnetic structure. We found a k=0 G-type collinear antiferromagnetic structure that is not compatible with inverse Dzyaloshinskii-Moriya interaction driven ferroelectricity. While the structural data reveals a clear sign for magneto-elastic coupling at the N\'eel-temperature of ~675 K, the dielectric measurements remain silent as far as ferroelectricity is concerned.

We used STM to study the tunneling spectra of FeSe0.3Te0.7 films with two orientations of ab-planes and the connection ramp between them. We have discovered that, using pulse laser deposition (PLD) method, the a- and b-axis of the FeSe0.3Te0.7 film deposited on Ar-ion-milled Magnesium Oxide (MgO) substrate are rotated 45 degree with respect to those of MgO, while the a- and b-axis of the film grown on pristine MgO substrate are parallel to those of MgO. With photolithography and this technique, we can prepare FeSe0.3Te0.7 film with two kinds of orientations on the same MgO substrate, and the connection between them forms a ramp at an angle about 25 degree to the substrate plane. In the planar region with either 0 degree or 45 degree orientation, we have observed tunneling spectra with a superconducting gap about 5 meV and 1.78 meV respectively. However, a much larger gap about 18 meV has been observed in the ramp region. Furthermore, we have also observed a small zero-bias conductance peak (ZBCP) inside the large gap at T = 4.3K. The ZBCP becomes smaller and disappear with increasing temperature to 7 K. The possible causes of such large gap and the ZBCP will be discussed., Comment: 13 pages, 4 figures

The electric transport properties of superconducting FeSe0.3Te0.7/Au c-axis (S/N) junctions, fabricated by using Pulsed Laser Deposition technique, have been investigated in the temperature range of 2 K to the superconducting transition temperature TC ~ 13.5 K, and in the presence of applied magnetic fields from 0 to 9 T. A large zero bias conductance peak has always been observed in the conductance spectra of several batches of junctions. In addition, we have also found several gap-like features. Using the extended BTK theory with the currently favored nodeless S+- wave symmetry, our conductance spectrum can be reproduced quite well in the low bias range with {\Delta}1 = 4 meV, and {\Delta}2 = 6 meV at 2 K. However the experimental conductance spectrum is substantially below the calculated one in the high bias range. Furthermore, there is a conductance minimum at about 20 meV, which may be the reason for the discrepancy., Comment: 21 pages, 10 figures

A new multiferroic material, CuBr2, is reported for the first time. CuBr2 has not only a high transition temperature (close to liquid nitrogen temperature) but also low dielectric loss and strong magnetoelectric coupling. These findings reveal the importance of anion effects in the search for the high temperature multiferroics materials among these low-dimensional spin systems., Comment: 3 figures, accepted by Advanced Materials

Based upon the currently predominant theory of Fe-based superconductors, the order parameter symmetry is the nodaless s\pm-wave. Andreev conductance involving normal metals and such superconductors does not have a zero bias conductance peak (ZBCP) except maybe for a very narrow range of a junction-dependent fitting parameter. We have measured differential conductance of the in-plane boundary junctions between Au and superconducting FeSe0.3Te0.7 films. The Au/FeSe0.3Te0.7 ramp-type junctions are fabricated with the bias current direction along the <110> axis of the epitaxial FeSe0.3Te0.7 film. We have always observed a pronounced ZBCP as well as some gap-like features in the conductance spectra of all samples studied, thus our experimental results suggest not the s\pm-wave theory but the possible existence of a nodal point around 45{\deg} in at least one of the gaps in FeSe0.3Te0.7 superconductor., Comment: 17 pages, 7 figures, submitted

We utilize steady-state and transient optical spectroscopies to examine the responses of nonthermal quasiparticles with respect to orbital modifications in normal-state iron-chalcogenide superconductors. The dynamics shows the emergence of gap-like quasiparticles (associated to a ~36 meV energy gap) with a coincident transfer of the optical spectral weight in the visible range, at temperatures above the structural distortion. Our observations suggest that opening of the high-temperature gap and the lattice symmetry breaking are possibly driven by short-range orbital and/or charge orders, implicating a close correlation between electronic nematicity and precursor order in iron-based superconductors.

We report high pressure structural phase transformation sequence in a layered Iron-based superconducting compound FeSe0.3Te0.7 to 31 GPa at room temperature. The ambient pressure PbO type tetragonal phase (Space Group - P4/nmm) transforms to a monoclinic phase (Space group - P21/m) at a pressure of 7.3 \pm 0.9 GPa. This monoclinic phase is similar to the one observed below 100 K at ambient pressure. On further increase of pressure above 12 GPa, a transformation to an amorphous phase is observed that is completed by 20 GPa. The amorphous phase is found to be stable to the highest pressure of 31 GPa. This structural sequence of tetragonal \to monoclinic \to amorphous phase transformation appears to be a common feature of iron-based layered superconductors under compression. The pressure induced structural phase transformations are likely to play a key role in the superconductivity in these materials at high pressures., Comment: 12 pages, 4 figures

The dielectric properties of Fe$_{3}$O$_{4}$ magnetic nanoparticles with an insulating coating layer of SiO$_{2}$ were investigated. At high temperatures, the changes of the dielectric constant and loss induced by the magnetic field are opposite in sign and strongly frequency-dependent, which originates from extrinsic magnetodielectric coupling-the Maxwell-Wagner effect combined with magnetoresistance. And the interface defects leads to the obvious hysteresis phenomena observed in the measurements. On the other hand, the strong coupling of dielectric and magnetic properties at low temperatures contradicts the Maxwell-Wagner model, suggesting the intrinsic magnetodielectric coupling. Our observations are consistent with the recent polarization switching measurements, which confirm the low-temperature multiferroic state existing in highly-lossy Fe$_{3}$O$_{4}$. And the core/shell nanostructure may provide a new route to achieve applicable magnetoelectric materials with low loss., Comment: 6 pages, 6 figures, published in Journal of Applied Physics

We report the magnetic, dielectric and ferroelectric properties of polycrystalline iron vanadate(FeVO4), which has been recently found to exhibit multiferroicity in low temperature with noncollinear magnetic orderings. The influence of external magnetic field up to 9T on these properties is systematically investigated. Besides the suppressing effect on the original ferroelectric transition, the strong magnetic field seems to induce a secondary ferroelectric transition at a slight lower temperature. And the corresponding magnetization measurement reveals a field-induced metamagnetic transition at low temperatures. Our results will help to clarify the complex magnetic structure and microscopic mechanism of multiferroicity in FeVO4., Comment: 5pages,5 figures

X-ray absorption spectroscopy(XAS) Ru L-edge and O K-edge measurements on single crystals of Sr2YRu1-xCuxO6 (x=0.01-0.05) solid solutions show that Cu replaces Ru in the lattice which results in a lattice distortion and itinerant holes that increase with increasing x. Powder X-ray diffraction (XRD) measurements confirm the lattice distortion through changes in the lattice parameters. Magnetic measurements reveal an antiferromagnetic transition at 32 K in the crystals with x=0 and a diamagnetic transition at 32 K for x \geq 0.01indicative of superconductivity in the zero field cooled (ZFC) curves. The field cooled (FC) curves show a weak ferromagnetic character with x=0 and a ferromagnetic transition at 30 K and an antiferromagnetic transition at 23 K in the presence of Cu. The peak at 30 K in the ac susceptibility measurements indicates a spin-glass behavior. The low field part of the M-H curve for x=0.05 crystal indicates weak superconductivity. The shifting of the M-H curve on FC indicates spin-glass state. In conjunction with the XAS results the magnetic behavior is explained in terms of a spontaneous vortex state as a result of the non-freezing Ru5+ spins.

X-ray absorption spectroscopy (XAS) Fe K-edge spectra of the FeSex (x=1-0.8) single crystals cleaved in situ in vacuum reveal characteristic Fe 4sp states, a lattice distortion and the Se K-edge spectra point to a strong Fe 3d-Se 4p hybridization giving rise to itinerant charge carriers. A formal charge of ~1.8+ for Fe and ~2.2- for Se were evaluated from these spectra in the FeSex (x=0.88). The charge balance between Fe and Se is assigned itinerant electrons located in the Fe-Se hybridization bond. As x decreases the 4p hole count increases and a crystal structure distortion is observed that in turn causes the Fe separation in the ab plane change from 4p orbital to varying (modulating) coordination. Powder x-ray diffraction (XRD) measurements also show a slight increase in lattice parameters as x decreases (increasing Se deficiency).

A new approach to grow FeSe1-xTex single crystals with optical zone-melting technique was successfully employed. Crystals with actual composition 0.4 < x < 1.0 all show high crystallinity with no phase separation. The ability to visually observe the locally heated melt, and ease of use and control of the image furnace make this method a promising and time-efficient way for obtaining high-quality FeSe1-xTex crystals. Our results indicate that with adequate heat treatment, the non-uniform distribution of Se and Te atoms in crystal lattice can be effectively eliminated, while the transition width of superconductivity can be reduced to about 2 K, which suggest the crystals are homogeneous in nature., Comment: To appear in Crystal Growth and Design

Superconductivity was recently found in the simple tetragonal FeSe structure. Recent studies suggest that FeSe is unconventional, with the symmetry of the superconducting pairing state still under debate. To tackle these problems, clean single crystals and thin films are required. Here we report the fabrication of superconducting beta-phase FeSe1-x thin films on different substrates using a pulsed laser deposition (PLD) technique. Quite interestingly, the crystal orientation, and thus, superconductivity in these thin films is sensitive to the growth temperature. At 320C, films grow preferably along c-axis, but the onset of superconductivity depends on film thickness. At 500C, films grow along (101), with little thickness dependence. These results suggest that the low temperature structural deformation previously found is crucial to the superconductivity of this material.

Magnetic field dependence of low temperature specific heat of spinel oxide superconductor LiTi_2O_4 has been elaborately investigated. In the normal state, the obtained electronic coefficient of specific heat gamma_n = 19.15 mJ/mol K2, the Debye temperature is 657 K and some other parameters are compared with those reported earlier. The superconducting transition at Tc ~ 11.4 K is very sharp (DeltaTc ~ 0.3 K) and the estimated delataC/gamma_nTc is ~ 1.78. In the superconducting state, the best fit of data leads to the electronic specific heat C_es/Gamma_nTc = 9.87 exp (-1.58 Tc/T) without field and gamma(H) ~ H^0.95 with fields. In addition, Hc2(0) ~ 11.7 T, Hc(0) ~ 0.32 T, xi_GL(0) ~ 5.5 nm, lambda_GL(0) ~ 160 nm, and Hc1(0) ~ 26 mT are estimated from Werthamer-Helfand-Hohenberg (WHH) theory or other relevant relations. All results from the present study indicate that LiTi2O4 can be well described by a typical type-II, BCS-like, moderate coupling, and fully gapped superconductor in the dirty limit. It is further suggested that LiTi2O4 is a moderately electron-electron correlated system., Comment: 6 pages, 2 tables, 6 figures; Physical Review B, in press