arXiv subjects

J. L. Luo

Publications and source records attributed to J. L. Luo.

At least 73 records · Page 4Linked to original sources

arXiv2008

Band Structure and Fermi Surface of an Extremely Overdoped Iron-Based Superconductor KFe2As2

We have performed high-resolution angle-resolved photoemission spectroscopy on heavily overdoped KFe_2As_2 (transition temperature (Tc = 3 K). We observed several renormalized bands near the Fermi level with a renormalization factor of 2-4. While the Fermi surface (FS) around the Brillouin-zone center is qualitatively similar to that of optimally-doped Ba_{1-x}K_xFe_2As_2 (x = 0.4; Tc = 37 K), the FS topology around the zone corner (M point) is markedly different: the two electron FS pockets are completely absent due to excess of hole doping. This result indicates that the electronic states around the M point play an important role in the high-Tc superconductivity of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ and suggests that the interband scattering via the antiferromagnetic wave vector essentially controls the Tc value in the overdoped region.

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arXiv2008

Doping evoluton of antiferromagnetic order and structural distortion in LaFeAsO$_{1-x}$F$_x$

We use neutron scattering to study the structural distortion and antiferromagnetic (AFM) order in LaFeAsO$_{1-x}$F$_{x}$ as the system is doped with fluorine (F) to induce superconductivity. In the undoped state, LaFeAsO exhibits a structural distortion, changing the symmetry from tetragonal (space group $P4/nmm$) to orthorhombic (space group $Cmma$) at 155 K, and then followed by an AFM order at 137 K. Doping the system with F gradually decreases the structural distortion temperature, but suppresses the long range AFM order before the emergence of superconductivity. Therefore, while superconductivity in these Fe oxypnictides can survive in either the tetragonal or the orthorhombic crystal structure, it competes directly with static AFM order.

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arXiv2008

Magnetic Order of the Iron Spins in NdOFeAs

Polarized and unpolarized neutron diffraction measurements have been carried out to investigate the iron magnetic order in undoped NdOFeAs. Antiferromagnetic order is observed below 141(6) K, which is in close proximity to the structural distortion observed in this material. The magnetic structure consists of chains of parallel spins that are arranged antiparallel between chains, which is the same in-plane spin arrangement as observed in all the other iron oxypnictide materials. Nearest-neighbor spins along the c-axis are antiparallel like LaOFeAs. The ordered moment is 0.25(7) muB, which is the smallest moment found so far in these systems.

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arXiv2008

Low energy spin waves and magnetic interactions in SrFe$_2$As$_2$

We report inelastic neutron scattering studies of magnetic excitations in antiferromagnetically ordered SrFe$_{2}$As$_{2}$ ($T_{N}=220$ K), the parent compound of the FeAs-based superconductors. At low temperatures (T=7 K), the spectrum of magnetic excitations $S(Q,\hbar ω)$ consists of a Bragg peak at the elastic position ($\hbar ω=0$ meV), a spin gap ($ Δ\leq 6.5$ meV), and sharp spin wave excitations at higher energies. Based on the observed dispersion relation, we estimate the effective magnetic exchange coupling using a Heisenberg model. On warming across $T_{N} $, the low temperature spin gap rapidly closes, with weak critical scattering and spin-spin correlations in the paramagnetic state. The antiferromagnetic order in SrFe$_{2}$As$_{2}$ is therefore consistent with a first order phase transition, similar to the structural lattice distortion.

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arXiv2008

Tightly-bound Cooper pair, quasiparticle kinks and clues on the pairing potential in a high Tc FeAs Superconductor

We present a systematic photoemission study of the newly discovered high Tc superconductor class (Sr/Ba)1-xKxFe2As2. By utilizing a unique photon energy range and scattering geometry we resolve the details of the single particle dynamics of interacting electrons on the central Fermi surfaces of this series which shows overall strong coupling behavior (2D/kBTc = 6). Quasiparticle dispersion kinks are observed in a binding energy range of 15 to 50 meV which matches the magnetic excitation energy scales (parameterized by J1,J2). The size of the Cooper pair wavefunction is found to be less than 20A indicating a short in-plane scale uncharacteristic of a BCS-phonon scenario but suggestive of a phase factor in the global order parameter. The kink likely reflects contributions from the strongly frustrated fluctuating spin excitations and the soft phonons around 20-40 meV. Our results provide important clue to the nature of the pairing potential realized in these superconductors.

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arXiv2008

Observation of an Orbital Selective Electron-Mode Coupling in Fe-Based High-$T_c$ Superconductors

We have performed an angle-resolved photoemission spectroscopy study of the new superconductor Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ in the low energy range. We report the observation of an anomaly around 25 meV in the dispersion of superconducting Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ samples that nearly vanishes above $T_c$. The energy scale of the related mode (13$\pm$2 meV) and its strong dependence on orbital and temperature indicates that it is unlikely related to phonons. Moreover, the momentum locations of the kink can be connected by the antiferromagnetic wavevector. Our results point towards an unconventional electronic origin of the mode and the superconducting pairing in the Fe-based superconductors, and strongly support the anti-phase s-wave pairing symmetry.

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arXiv2008

Superconductivity coexisting with phase-separated static magnetic order in (Ba,K)Fe$_{2}$As$_{2}$, (Sr,Na)Fe$_{2}$As$_{2}$ and CaFe$_{2}$As$_{2}$

The recent discovery and subsequent developments of FeAs-based superconductors have presented novel challenges and opportunities in the quest for superconducting mechanisms in correlated-electron systems. Central issues of ongoing studies include interplay between superconductivity and magnetism as well as the nature of the pairing symmetry reflected in the superconducting energy gap. In the cuprate and RE(O,F)FeAs (RE = rare earth) systems, the superconducting phase appears without being accompanied by static magnetic order, except for narrow phase-separated regions at the border of phase boundaries. By muon spin relaxation measurements on single crystal specimens, here we show that superconductivity in the AFe$_{2}$As$_{2}$ (A = Ca,Ba,Sr) systems, in both the cases of composition and pressure tunings, coexists with a strong static magnetic order in a partial volume fraction. The superfluid response from the remaining paramagnetic volume fraction of (Ba$_{0.5}$K$_{0.5}$)Fe$_{2}$As$_{2}$ exhibits a nearly linear variation in T at low temperatures, suggesting an anisotropic energy gap with line nodes and/or multi-gap effects.

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arXiv2008

Fermi Surface and Band Renormalization in (Sr,K)Fe$_2$As$_2$ Superconductor from Angle-Resolved Photoemission Spectroscopy

High resolution angle-resolved photoemission measurements have been carried out on (Sr,K)Fe$_2$As$_2$ superconductor (Tc=21 K). Three hole-like Fermi surface sheets are clearly resolved for the first time around the Gamma point. The overall electronic structure shows significant difference from the band structure calculations. Qualitative agreement between the measured and calculated band structure is realized by assuming a chemical potential shift of -0.2 eV. The obvious band renormalization suggests the importance of electron correlation in understanding the electronic structure of the Fe-based compounds.

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arXiv2008

Spin and Lattice Structure of Single Crystal SrFe2As2

We use neutron scattering to study the spin and lattice structure on single crystals of SrFe2As2, the parent compound of the FeAs based superconductor (Sr,K)Fe2As2. We find that SrFe2As2 exhibits an abrupt structural phase transitions at 220K, where the structure changes from tetragonal with lattice parameters c > a = b to orthorhombic with c > a > b. At almost the same temperature, Fe spins in SrFe2As2 develop a collinear antiferromagnetic structure along the orthorhombic a-axis with spin direction parallel to this a-axis. These results are consistent with earlier work on the RFeAsO (R = rare earth elements) families of materials and on BaFe2As2, and therefore suggest that static antiferromagnetic order is ubiquitous for the parent compound of these FeAs-based high-transition temperature superconductors.

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arXiv2008

Muon spin relaxation studies of magnetic order and superfluid density in antiferromagnetic NdOFeAs, BaFe2As2 and superconducting (Ba,K)Fe2As2

Zero-field (ZF) muon spin relaxation ($μ$SR) measurements have revealed static commensurate magnetic order of Fe moments in NdOFeAs below $T_{N} \sim 135$ K, with the ordered moment size nearly equal to that in LaOFeAs, and confirmed similar behavior in BaFe$_{2}$As$_{2}$. In single crystals of superconducting (Ba$_{0.55}$K$_{0.45}$)Fe$_{2}$As$_{2}$, $μ$SR spectra indicate static magnetism with incommensurate or short-ranged spin structure in $\sim$ 70 % of volume below $T_{N} \sim$ 80 K, coexisting with remaining volume which exhibits superfluid-response consistent with nodeless gap below $T_{c}\sim 30$ K.

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arXiv2008

Probing superconducting energy gap from infrared spectroscopy on a Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ single crystal with T$_c$=37 K

We performed optical spectroscopy measurement on a superconducting Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ single crystal with T$_c$=37 K. Formation of the superconducting energy gaps in the far-infared reflectance spectra below T$_c$ is clearly observed. The gap amplitudes match well with the two distinct superconducting gaps observed in angle-resolved photoemission spectroscopy experiments on different Fermi surfaces. We determined absolute value of the penetration depth at 10 K as $λ\simeq2000 Å$. A spectral weight analysis shows that the Ferrell-Glover-Timkham sum rule is satisfied at low energy scale, less than 6$Δ$.

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arXiv2008

Band-dependent normal-state coherence in Sr$_{2}$RuO$_{4}$: Evidence from Nernst effect and thermopower measurements

We present the first measurement on Nernst effect in the normal state of odd-parity, spin-triplet superconductor Sr$_{2}$RuO$_{4}$. Below 100 K, the Nernst signal was found to be negative, large, and, as a function of magnetic field, nonlinear. Its magnitude increases with the decreasing temperature until reaching a maximum around $T^*$ $\approx$ 20 - 25 K, below which it starts to decrease linearly as a function of temperature. The large value of the Nernst signal appears to be related to the multiband nature of the normal state and the nonlinearity to band-dependent magnetic fluctuation in Sr$_{2}$RuO$_{4}$. We argue that the sharp decrease in Nernst signal below $T^*$ is due to the suppression of quasiparticle scattering and the emergence of band-dependent coherence in the normal state. The observation of a sharp kink in the temperature dependent thermopower around $T^*$ and a sharp drop of Hall angle at low temperatures provide additional support to this picture.

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arXiv2008

Observation of Fermi-surface-dependent nodeless superconducting gaps in Ba0.6K0.4Fe2As2

The recent discovery of superconductivity in iron-arsenic compounds below a transition temperature (Tc) as high as 55K ended the monopoly of copper oxides (cuprates) in the family of high-Tc superconductors. A critical issue in understanding this new superconductor, as in the case of cuprates, is the nature, in particular the symmetry and orbital dependence, of the superconducting gap. There are conflicting experimental results, mostly from indirect measurements of the low energy excitation gap, ranging from one gap to two gaps, from line nodes to nodeless gap function in momentum space. Here we report a direct observation of the superconducting gap, including its momentum, temperature, and Fermi surface (FS) dependence in Ba0.6K0.4Fe2As2 (Tc = 37 K) using angle-resolved photoelectron spectroscopy. We find two superconducting gaps with different values: a large gap (~ 12 meV) on the two small hole-like and electron-like FS sheets, and a small gap (~ 6 meV) on the large hole-like FS. Both gaps, closing simultaneously at the bulk Tc, are nodeless and nearly isotropic around their respective FS sheets. The isotropic pairing interactions are strongly orbital dependent, as the ratio 2Delta/kBTc switches from weak to strong coupling on different bands. The same and surprisingly large superconducting gap due to strong pairing on the two small FS, which are connected by the (pi, 0) spin-density-wave vector in the parent compound, strongly suggests that the pairing mechanism originates from the inter-band interactions between these two nested FS sheets.

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arXiv2008

Scanning tunneling microscopy of the 32 K superconductor (Sr1-xKx)Fe2As2

The discovery of high temperature superconductivity in La[O1-xFx]FeAs at the beginning of this year [1] has generated much excitement and has led to the rapid discovery of similar compounds with as high as 55 K transition temperatures [2]. The high superconducting transition temperatures are seemingly incompatible with the electron-phonon driven pairing of conventional superconductors, resulting in wide speculation as to the mechanism and nature of the superconductivity in these materials. Here we report results of the first scanning tunneling microscopy study of the 32 K superconductor (Sr1-xKx)Fe2As2. We find two distinct topographic regions on the sample, one with no apparent atomic corrugation, and another marked by a stripe-like modulation at double the atomic periodicity. In the latter the stripes appear to modulate the local density of states, occasionally revealing a Delta = 10 mV gap with a shape consistent with unconventional (non-s wave) superconductivity.

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arXiv2008

Superconducting properties of Fe-based layered superconductor LaO$_{0.9}$F$_{0.1-δ}$FeAs

We have employed a new route to synthesize single phase F-doped LaOFeAs compound and confirmed the superconductivity above 20 K in this Fe-based system. We show that the new superconductor has a rather high upper critical field of about 54 T. A clear signature of superconducting gap opening below T$_c$ was observed in the far-infrared reflectance spectra, with 2$Δ/\textit{k}T_c\approx$3.5-4.2. Furthermore, we show that the new superconductor has electron-type conducting carrier with a rather low carrier density.

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arXiv2008

Structural and magnetic phase diagram of CeFeAsO1-xFx and its relationship to high-temperature superconductivity

We use neutron scattering to study the structural and magnetic phase transitions in the iron pnictides CeFeAsO1-xFx as the system is tuned from a semimetal to a high-transition-temperature (high-Tc) superconductor through Fluorine (F) doping x. In the undoped state, CeFeAsO develops a structural lattice distortion followed by a stripe like commensurate antiferromagnetic order with decreasing temperature. With increasing Fluorine doping, the structural phase transition decreases gradually while the antiferromagnetic order is suppressed before the appearance of superconductivity, resulting an electronic phase diagram remarkably similar to that of the high-Tc copper oxides. Comparison of the structural evolution of CeFeAsO1-xFx with other Fe-based superconductors reveals that the effective electronic band width decreases systematically for materials with higher Tc. The results suggest that electron correlation effects are important for the mechanism of high-Tc superconductivity in these Fe pnictides.

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arXiv2008

Superconductivity in hole-doped (Sr$_{1-x}$K$_x$)Fe$_2$As$_2$

A series of layered (Sr$_{1-x}$K$_x$)Fe$_2$As$_2$ compounds with nominal x=0 to 0.40 are synthesized by solid state reaction method. Similar to other parent compounds of iron-based pnictide superconductors, the pure SrFe$_2$As$_2$ 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 BaFe$_2$As$_2$, the two prototype parent compounds with ZrCuSiAs- and ThCr$_2$Si$_2$-type structures. K-doping strongly suppresses this anomaly and induces superconductivity. Like in the case of K-doped BaFe$_2$As$_2$, sharp superconducting transitions at T$_c\sim$38 K was observed. We performed the Hall coefficient measurement, and confirmed that the dominant carriers are hole-type. The carrier density is enhanced by a factor of 3 in comparison to F-doped LaOFeAs superconductor.

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arXiv2008

Raman spectra in iron-based quaternary CeO$_{1-x}$F$_x$FeAs and LaO$_{1-x}$F$_x$FeAs

Raman spectra have been measured on iron-based quaternary CeO$_{1-x}$F$_x$FeAs and LaO$_{1-x}$F$_x$FeAs with varying fluorine doping at room temperatures. A group analysis has been made to clarify the optical modes. Based on the first principle calculations, the observed phonon modes can be assigned accordingly. In LaO$_{1-x}$F$_x$FeAs, the E$_g$ and A$_{1g}$ modes related to the vibrations of La, are suppressed with increasing F doping. However F doping only has a small effect on the E$_g$ and A$_{1g}$ modes of Fe and As. The Raman modes of La and As are absent in rare-earth substituted CeO$_{1-x}$F$_x$FeAs, and the E$_g$ mode of oxygen, corresponding to the in-plane vibration of oxygen, moves to around 450 cm$^{-1}$ and shows a very sharp peak. Electronic scattering background is low and electron-phonon coupling is not evident for the observed phonon modes. Three features are found above 500 cm$^{-1}$, which may be associated with multi-phonon process. Nevertheless it is also possible that they are related to magnetic fluctuations or interband transitions of d orbitals considering their energies.

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