Z. J. Xu

Soft x-ray spectroscopy beam line on the NSLS X1 undulator: Optical design and first performance tests

The layout and first data on the performance of the soft x‐ray undulator beam line on the x‐ray ring at Brookhaven are described. The undulator (X1) has 35 periods of 8 cm, resulting in a tuning range for the fundamental of about 200–700 eV at the nominal ring energy of 2.5 GeV. The monochromator is based on the Dragon concept with spherical grating, movable exit slit but with water‐cooled optical elements in order to deal with the maximum flux of ca. 400 W. This system represents one of the next generation of soft x‐ray undulator beam lines which will figure prominently at new synchrotron radiation facilities such as ALS, ELETTRA and BESSY II. During the first performance tests absorption spectra of simple molecules at the C, N, and O K edges have been recorded with resolution and flux so far unattained in the soft x‐ray region.

Isotropic superconducting gaps with enhanced pairing on electron Fermi surfaces in FeTe0.55Se0.45

We used angle-resolved photoemission spectroscopy to reveal directly the momentum distribution of the superconducting gap in FeTe1-xSex, which has the simplest structure of all Fe-based superconductors. We found isotropic superconducting gaps on all Fermi surfaces whose sizes can be fitted by a single gap function derived from a strong coupling approach, promoting local antiferromagnetic exchange interactions as a serious candidate for the pairing origin.

Evidence for unusual superconducting correlations coexisting with stripe order in La 1.875 Ba 0.125 CuO 4

We present new x-ray and neutron-scattering measurements of stripe order in

Electronic correlations and unusual superconducting response in the optical properties of the iron chalcogenide FeTe0.55Se0.45

{\text{La}}_{1.875}{\text{Ba}}_{0.125}{\text{CuO}}_{4}

Recent Advances in Studies of the Electronic Decay of Core Excited States in Small Molecules

, along with low-field susceptibility, thermal conductivity, and specific-heat data. We compare these with previously reported results for resistivity and thermopower. Temperature-dependent features indicating transitions (or crossovers) are correlated among the various experimental quantities. Taking into account recent spectroscopic studies, we argue that the most likely interpretation of the complete collection of results is that an unusual form of two-dimensional superconducting correlations appears together with the onset of spin-stripe order. Recent theoretical proposals for a sinusoidally modulated superconducting state compatible with stripe order provide an intriguing explanation of our results and motivate further experimental tests. We also discuss evidence for one-dimensional pairing correlations that appear together with the charge order. With regard to the overall phenomenology, we consider the degree to which similar behavior may have been observed in other cuprates and describe possible connections to various puzzling phenomena in cuprate superconductors.

Detection of orbital fluctuations above the structural transition temperature in the iron pnictides and chalcogenides

The in-plane complex optical properties of the iron-chalcogenide superconductor FeTe{sub 0.55}Se{sub 0.45} have been determined above and below the critical temperature T{sub c} = 14 K. At room temperature the conductivity is described by a weakly interacting Fermi liquid; however, below 100 K the scattering rate develops a frequency dependence in the terahertz region, signaling the increasingly correlated nature of this material. We estimate the dc conductivity {sigma}{sub dc}(T {ge} T{sub c}) = 3500 {+-} 400 {Omega}{sup -1} cm{sup -1} and the superfluid density {rho}{sub s0} = 9 {+-} 1 x 10{sup 6} cm{sup -2}, which places this material close to the scaling line {rho}{sub s0}/8 = 8.1{sigma}{sub dc}T{sub c} for a BCS dirty-limit superconductor. Below T{sub c} the optical conductivity reveals two gap features at {Delta}{sub 1,2} = 2.5 and 5.1 meV.

Effects of time evolution of coherently excited vibrations in molecular core—hole decay spectra of O2

Electronic decay spectra of neutral core excited states give new insight into charge transfer, screening and electron correlation effects in atoms, molecules and solids. The basic deacy mechanism is an Auger decay but the transition occurs in a natural rather than in a core ionized system. The study of the electronic deacy of neutral core excited states yields information that is complementary to that obtained in both photoemission and Auger spectroscopy. Using synchrotron radiation as the excitation source, deexcitation spectroscopy has been developed over the past decade. Here, these developements are briefly reviewed and prospects, opened up by the availability of high resolution, high intensity undulator radiation, are highlighted using spectra for N2 as an example.

Enhanced charge stripe order of superconducting La2 and #8722;xBaxCuO4 in a magnetic field

We use point-contact spectroscopy (PCS) to probe AEFe2As2 (AE=Ca, Sr, Ba) and Fe1+yTe. For AE=Sr,Ba we detect orbital fluctuations above TS while for AE=Ca these fluctuations start below TS. Co doping preserves the orbital fluctuations while K doping suppresses it. The fluctuations are only seen at those dopings and temperatures where an in-plane resistive anisotropy is known to exist. We predict an in-plane resistive anisotropy of Fe1+yTe above TS. Our data are examined in light of the recent work by Lee and Phillips (arXiv:1110.5917v2). We also study how joule heating in the PCS junctions impacts the spectra. Spectroscopic information is only obtained from those PCS junctions that are free of heating effects while those PCS junctions that are in the thermal regime display bulk resistivity phenomena.

NSLS X1B - an undulator beamline for high resolution soft X-ray spectroscopy

Abstract Lifetime vibrational interference effects modulate the line shapes observed in high-resolution studies of the decay of ultra-short-lived core—hole excited states in molecules. We have analyzed the decay of the 1σ −1 u 1π 3 g core excited state in O 2 , where for the first time subsets of vibronic eigenfunctions could be selected. We demonstrate that this type of spectroscopy can be used to verify the predictions of quantum mechanical interference theory describing the time evolution of coherently excited vibrational states on the femtosecond time scale.

Continuous magnetic and structural phase transitions in Fe1+yTe

The effect of a magnetic field on the charge stripe order in La(2-x)Ba(x)CuO(4) has been studied by means of high energy (100 keV) x-ray diffraction for charge carrier concentrations ranging from strongly underdoped to optimally doped. We find that charge stripe order can be significantly enhanced by a magnetic field applied along the c-axis, but only at temperatures and dopings where it coexists with bulk superconductivity at zero field. The field also increases stripe correlations between the planes, which can result in an enhanced frustration of the interlayer Josephson coupling. Close to the famous x=1/8 compound, where zero field stripe order is pronounced and bulk superconductivity is suppressed, charge stripe order is independent of a magnetic field. The results imply that static stripe order and three-dimensionally coherent superconductivity are competing ground states.