arXiv subjects

G. F. Chen

Publications and source records attributed to G. F. Chen.

At least 91 records · Page 5Linked to original sources

arXiv2010

Evidence for a full energy gap for nickel-pnictide LaNiAsO_{1-x}F_x superconductors by ^{75}As nuclear quadrupole resonance

We report systematic ^{75}As-NQR and ^{139}La-NMR studies on nickel-pnictide superconductors LaNiAsO_{1-x}F_x (x=0, 0.06, 0.10 and 0.12). The spin lattice relaxation rate 1/T_1 decreases below T_c with a well-defined coherence peak and follows an exponential decay at low temperatures. This result indicates that the superconducting gap is fully opened, and is strikingly different from that observed in iron-pnictide analogs. In the normal state, 1/T_1T is constant in the temperature range T_c \sim 4 K < T <10 K for all compounds and up to T=250 K for x=0 and 0.06, which indicates weak electron correlations and is also different from the iron analog. We argue that the differences between the iron and nickel pnictides arise from the different electronic band structure. Our results highlight the importance of the peculiar Fermi-surface topology in iron-pnictides.

cond-mat.supr-con↗

arXiv2010

Superconductivity at 33 K in "111" single crystals at ambient pressure

We have successfully grown single crystalline NaFeAs with cobalt or phosphor doping. Bulk superconductivity occurs in NaFe$_{0.95}$Co$_{0.05}$As at 19 K while much higher transition temperature is observed in NaFeAs$_{0.8}$P$_{0.2}$, in which the Tc of 33 K is even higher than the highest value realized by applying pressure in NaFeAs and all other isoelectronic element substituted samples. We discuss these behaviours by comparison with 122 and 1111 systems. We hope that our findings will help improve our understanding of iron-based superconductivity.

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arXiv2010

Ultrafast pump-probe study of phase separation and competing orders in the underdoped (Ba,K)Fe$_{2}$As$_{2}$ superconductor

We report measurements of quasiparticle relaxation dynamics in the high-temperature superconductor (Ba,K)Fe$_{2}$As$_{2}$ in optimally doped, underdoped and undoped regimes. In the underdoped sample, spin-density wave (SDW) order forms at $\sim$85 K, followed by superconductivity at $\sim$ 28 K. We find the emergence of a normal-state order that suppresses SDW at a temperature $T^{\ast} \sim$ 60 K and argue that this normal-state order is a precursor to superconductivity.

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arXiv2010

Magnetic form factor of SrFe$_2$As$_2$

Neutron diffraction measurements have been carried out to investigate the magnetic form factor of the parent SrFe2As2 system of the iron-based superconductors. The general feature is that the form factor is approximately isotropic in wave vector, indicating that multiple d-orbitals of the iron atoms are occupied as expected based on band theory. Inversion of the diffraction data suggests that there is some elongation of the spin density toward the As atoms. We have also extended the diffraction measurements to investigate a possible jump in the c-axis lattice parameter at the structural phase transition, but find no detectable change within the experimental uncertainties.

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arXiv2009

Observation of intertwined Fermi surface topology, orbital parity symmetries and electronic interactions in iron arsenide superconductors

We present a polarization and topology resolved study of the low energy band structure in optimally doped superconducting Ba0.6K0.4Fe2As2 using angle resolved photoemission spectroscopy. Polarization-contrasted measurements allow us to identify and trace all low energy bands expected in models, revealing unexpected symmetry breaking and a surprisingly intertwined Fermi surface topology of hole-like bands near the Brillouin zone center. Band structure correlations across the Gamma-M spin fluctuation wavevector are compared with the superconducting gap anisotropy which suggest a partial scenario for spin-mediated interband instability contributing to superconductivity in the hole doped regime.

cond-mat.str-el↗

arXiv2009

Point-contact spectroscopic studies on normal and superconducting AFe_2As_2-type iron-pnictide single crystals

Point-contact Andreev reflection spectroscopy (PCARS) is applied to investigate the gap structure in iron pnictide single crystal superconductors of the AFe_2As_2 (A=Ba, Sr) family ("Fe-122"). The observed point-contact junction conductance curves, G(V), can be divided into two categories: one where Andreev reflection is present for both (Ba_{0.6}K_{0.4})Fe_2As_2 and Ba(Fe_{0.9}Co_{0.1})_2As_2, and the other with a V^{2/3} background conductance universally observed extending even up to 100 meV for Sr_{0.6}Na_{0.4}Fe_2As_2 and Sr(Fe_{0.9}Co_{0.1})_2As_2. The latter is also observed in point-contact junctions on the nonsuperconducting parent compound BaFe_2As_2. Mesoscopic phase-separated coexistence of magnetic and superconducting orders is considered to explain distinct behaviors in the superconducting samples. For Ba_{0.6}K_{0.4}Fe_2As_2, double peaks due to Andreev reflection with strongly-sloping background are frequently observed for point-contacts on freshly-cleaved c-axis surfaces. If normalized by a background baseline and analyzed by the Blonder-Tinkham-Klapwijk model, the data show a gap size ~3.0-4.0 meV with 2Δ_0/k_BT_c ~ 2.0-2.6, consistent with the smaller gap size reported in the LnFeAsO family ("Fe-1111"). For the Ba(Fe_{0.9}Co_{0.1})_2As_2, G(V) curves typically display a zero-bias conductance peak.

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arXiv2009

75As NMR study of single crystals of the heavily overdoped pnictide superconductors Ba{1-x}KxFe2As2 (x = 0.7 and 1)

We performed $^{75}$As NMR studies on two overdoped high-quality Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ (x=0.7 and 1.0) single crystals. In the normal states, we found a dramatic increase of the spin-lattice relaxation ($1/^{75}T_1$) from the x=0.7 to the x=1.0 samples. In KFe$_2$As$_2$, the ratio of $1/^{75}T_1TK_n^2$, where $^{75}K_n$ is the Knight shift, increases as temperature drops. These results indicate the existence of a new type of spin fluctuations in KFe$_2$As$_2$ which is accustomed to being treated as a simple Fermi liquid. In the superconducting state, we observe a step-like feature in the temperature dependence of the spin-lattice relaxation of the x=0.7 sample, which supports a two-gap superconductivity as the underdoped materials. However, the temperature scalings of $1/^{75}T_1$ below Tc in the overdoped samples are significantly different from those in the under or optimal doped ones. A power-law scaling behavior $1/^{75}T_1T\sim T^{0.5}$ is observed, which indicates universal strong low energy excitations in the overdoped hole-type superconductors.

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arXiv2009

Possible high temperature superconductivity in Ti-doped A-Sc-Fe-As-O (A= Ca, Sr) system

We report a systematic study on the effect of partial substitution of Sc$^{3+}$ by Ti$^{4+}$ in Sr$_{2}$ScFeAsO$_{3}$, Ca$_{2}$ScFeAsO$_{3}$ and Sr$_{3}$Sc$_{2}$Fe$_{2}$As$_{2}$O$_{5}$ on their electrical properties. High level of doping results in an increased carrier concentration and leads to the appearance of superconductivity with the onset of T$_{c}$ up to 45 K.

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arXiv2009

Observation of Dirac Cone Electronic Dispersion in BaFe2As2

We performed an angle-resolved photoemission spectroscopy study of BaFe2As2, which is the parent compound of the so-called 122 phase of the iron-pnictide high-temperature superconductors. We reveal the existence of a Dirac cone in the electronic structure of this material below the spin-density-wave temperature, which is responsible for small spots of high photoemission intensity at the Fermi level. Our analysis suggests that the cone is slightly anisotropic and its apex is located very near the Fermi level, leading to tiny Fermi surface pockets. Moreover, the bands forming the cone show an anisotropic leading edge gap away from the cone that suggests a nodal spin-density-wave description.

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arXiv2009

Lattice distortion and magnetic quantum phase transition in CeFeAs(1-x)P(x)O

We use neutron scattering to show that replacing the larger arsenic with smaller phosphorus in CeFeAs(1-x)P(x)O simultaneously suppresses the AF order and orthorhombic distortion near x = 0.4, providing evidence for a magnetic quantum critical point. Furthermore, we find that the pnictogen height in iron arsenide is an important controlling parameter for their electronic and magnetic properties, and may play an important role in electron pairing and superconductivity.

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arXiv2009

Angle-Resolved Photoemission Spectroscopy of Iron-Chalcogenide Superconductor Fe1.03Te0.7Se0.3 : Strong-Coupling Superconductivity and Universality of Inter-Band Scattering

We have performed high-resolution angle-resolved photoemission spectroscopy of iron-chalcogenide superconductor Fe1.03Te0.7Se0.3 (Tc = 13 K) to investigate the electronic structure relevant to superconductivity. We observed a hole- and an electron-like Fermi surfaces at the Brillouin zone center and corner, respectively, which are nearly nested by the Q~(pi, pi) wave vector. We do not find evidence for the nesting instability with Q~(pi+delta, 0) reminiscent of the antiferromagnetic order in the parent compound Fe1+yTe. We have observed an isotropic superconducting gap along the hole-like Fermi surface with the gap size Delta of ~4 meV (2Delta/kBTc~7), demonstrating the strong-coupling nature of the superconductivity. The observed similarity of low-energy electronic excitations between iron-chalcogenide and iron-arsenide superconductors strongly suggests that common interactions which involve Q~(pi, pi) scattering are responsible for the superconducting pairing.

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arXiv2009

Optical study on the spin-density wave properties in single crystalline Na$_{1-δ}$FeAs

We report an optical investigation on the in-plane charge dynamics for Na$_{1-δ}$FeAs single crystal. A clear optical evidence for the spin-density wave (SDW) gap is observed. As the structural/magnetic transitions are separated in the Na$_{1-δ}$FeAs case, we find the SDW gap opens in accordance with the magnetic transition. Comparing with the optical response of other FeAs-based parent compounds, both the gap value 2$Δ$ and the energy scale for the gap-induced spectral weight redistribution are smaller in Na$_{1-δ}$FeAs. Our findings support the itinerant origin of the antiferromagnetic transition in the FeAs-based system.

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arXiv2009

Homogeneous vs. inhomogeneous coexistence of magnetic order and superconductivity probed by NMR in Co and K doped iron pnictides

In Ba(Fe0.95Co0.05)2As2 all of the 75As NMR intensity at the paramagnetic resonance position vanishes abruptly below Tonset(SDW)=56 K, indicating that magnetic (spin density wave) order is present in all of the sample volume, despite bulk superconductivity below Tc=15 K. The two phases thus coexist homogeneously at the microscopic scale. In Ba0.6K0.4Fe2As2, on the other hand, the signal loss below Tonset(SDW)~75 K is not complete, revealing that magnetic order is bound to finite-size areas of the sample, while the remaining NMR signal shows a clear superconducting response below Tc=37 K. Thus, the two phases are not homogeneously mixed, at least for this potassium concentration. For both samples, spatial electronic and/or magnetic inhomogeneity is shown to characterize the NMR properties in the normal state.

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arXiv2009

Origin of the structural phase transition at 130 K in BaNi$_2$As$_2$: a combined study of optical spectroscopy and band structure calculations

BaNi$_2$As$_2$ exhibits a first order structural transition at 130 K. Understanding this structural transition is a crucial step towards understanding the electronic properties of the material. We present a combined optical spectroscopy and band structure calculation study on the compound across the transition. The study reveals that BaNi$_2$As$_2$ is a good metal with a rather high plasma frequency. The phase transition leads to a small reduction of conducting carriers. We identify that this reduction is caused by the removal of several small Fermi surface sheets contributed dominantly from the As-As bonding and Ni-As antibonding states.

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arXiv2009

Multiple phase transitions in single-crystalline Na$_{1-δ}$FeAs

Specific heat, resistivity, susceptibility and Hall coefficient measurements were performed on high-quality single crystalline Na$_{1-δ}$FeAs. This compound is found to undergo three successive phase transitions at around 52, 41, and 23 K, which correspond to structural, magnetic and superconducting transitions, respectively. The Hall effect result indicates the development of energy gap at low temperature due to the occurrence of spin-density-wave instability. Our results provide direct experimental evidence of the magnetic ordering in the nearly stoichiometric NaFeAs.

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arXiv2009

Structural and magnetic phase transitions in Na$_{1-δ}$FeAs

We use neutron scattering to study the spin and lattice structures of single crystal and powder samples of Na$_{1-δ}$FeAs ($T_c = 23$ K). On cooling from room temperature, the system goes through a series of phase transitions: first changing the crystal symmetry from tetragonal to orthorhombic at 49 K, then ordering antiferromagnetically with a spin structure similar to that of LaFeAsO and a small moment (0.09$\pm$0.04 $μ_B$), and finally becoming superconducting below about 23 K. These results confirm that antiferromagnetic order is ubiquitous for the parent compounds of the iron arsenide superconductors, and suggest that the separated structural and magnetic phase transition temperatures are due to the reduction in the c-axis exchange coupling of the system.

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arXiv2009

Raman phonons of $α$-FeTe and Fe$_{1.03}$Se$_{0.3}$Te$_{0.7}$ single crystals

The polarized Raman scattering spectra of nonsuperconducting $α$-FeTe and of the newly discovered, As-free superconductor Fe$_{1.03}$Se$_{0.3}$Te$_{0.7}$ are measured at room temperature on single crystals. The phonon modes are assigned by combining symmetry analysis with first-principles calculations. In the parent compound $α$-FeTe, the A$_{1g}$ mode of the Te atom and the B$_{1g}$ mode of the Fe atom are observed clearly, while in superconducting Fe$_{1.03}$Se$_{0.3}$Te$_{0.7}$, only a softened Fe B$_{1g}$ mode can be seen. No electron-phonon coupling feature can be distinguished in the spectra of the two samples. By contrast, the spectra of the superconducting system show a slight enhancement below 300$cm^{-1}$, which may be of electronic origin.

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arXiv2009

Electronic Evidence of Unusual Magnetic Ordering in a Parent Compound of FeAs-Based Superconductors

High resolution angle-resolved photoemission measurements have been carried out on BaFe2As2, a parent compound of the FeAs-based superconductors. In the magnetic ordering state, there is no gap opening observed on the Fermi surface. Instead, dramatic band structure reorganization occurs across the magnetic transition. The appearance of the singular Fermi spots near (pi,pi) is the most prominent signature of magnetic ordering. These observations provide direct evidence that the magnetic ordering state of BaFe2As2 is distinct from the conventional spin-density-wave state. They reflect the electronic complexity in this multiple-orbital system and necessity in involving the local magnetic moment in describing the underlying electron structure.

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