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X. H. Chen

Publications and source records attributed to X. H. Chen.

At least 127 records · Page 7Linked to original sources
arXiv2010

Reentrant spin-glass behavior in $TlFe_{2-x}Se_2$ with the $ThCr_2Si_2$-type structure

We investigated the physical properties of $TlFe_{2-x}Se_2$ single crystals. The resistivity of $TlFe_{2-x}Se_2$ shows typical semiconductor behavior with an activation energy of 25 meV. DC susceptibility indicates an antiferromagnetic transition at about 450 K. Reentrant spin-glass (RSG) behavior was found at about 130 K through DC and AC magnetic measurements. The RSG behavior suggests the existence of a strong competition between ferromagnetic (FM) and antiferromagnetic (AFM) interactions due to Fe deficiencies. Strong electron-electron correlation may exist in this material and it is possibly a candidate of parent compound for high $T_c$ superconductors.

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arXiv2010

Distinct electronic nematicities between electron and hole underdoped iron pnictides

We systematically investigated the in-plane resistivity anisotropy of electron-underdoped $EuFe_{2-x}Co_xAs_2$ and $BaFe_{2-x}Co_xAs_2$, and hole-underdoped $Ba_{1-x}K_xFe_2As_2$. Large in-plane resistivity anisotropy was found in the former samples, while {\it tiny} in-plane resistivity anisotropy was detected in the latter ones. When it is detected, the anisotropy starts above the structural transition temperature and increases smoothly through it. As the temperature is lowered further, the anisotropy takes a dramatic enhancement through the magnetic transition temperature. We found that the anisotropy is universally tied to the presence of non-Fermi liquid T-linear behavior of resistivity. Our results demonstrate that the nematic state is caused by electronic degrees of freedom, and the microscopic orbital involvement in magnetically ordered state must be fundamentally different between the hole and electron doped materials.

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arXiv2010

The orbital characters of bands in iron-based superconductor BaFe1.85Co0.15As2

The unconventional superconductivity in the newly discovered iron-based superconductors is intimately related to its multi-band/multi-orbital nature. Here we report the comprehensive orbital characters of the low-energy three-dimensional electronic structure in BaFe$_{1.85}$Co$_{0.15}$As$_2$ by studying the polarization and photon energy dependence of angle-resolved photoemission data. While the distributions of the $d_{xz}$, $d_{yz}$, and $d_{3z^2-r^2}$ orbitals agree with the prediction of density functional theory, those of the $d_{xy}$ and $d_{x^2-y^2}$ orbitals show remarkable disagreement with theory. Our results point out the inadequacy of the existing band structure calculations, and more importantly, provide a foundation for constructing the correct microscopic model of iron pnictides.

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arXiv2010

Spin dependent electron-phonon interaction in SmFeAsO by low-temperature Raman spectroscopy

The interplay between spin dynamics and lattice vibration has been suggested as an important part of the puzzle of high-temperature superconductivity. Here we report the strong interaction between spin fluctuation and phonon in SmFeAsO, a parent compound of the iron arsenide family of superconductors, revealed by low-temperature Raman spectroscopy. Anomalous zone-boundary-phonon Raman scattering from spin superstructure was observed at temperatures below the antiferromagnetic ordering point, which offers compelling evidence on spin dependent electron-phonon coupling in pnictides.

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arXiv2010

Out-of-plane momentum and symmetry dependent superconducting gap in Ba0.6K0.4Fe2As2

The three-dimensional band structure and superconducting gap of Ba0.6K0.4Fe2As2 are studied with high-resolution angle-resolved photoemission spectroscopy. In contrast to previous results, we have identified three hole-like Fermi surfaces near the zone center with sizable out-of-plane or kz dispersion. The superconducting gap on certain Fermi surface shows significant kz-dependence. Moreover, we found that the superconducting gap sizes are different at the same Fermi momentum for two bands with different spatial symmetries (one odd, one even). Our results provide further information on the rich superconducting gap structure in iron pnictides, and a distinct test for theories.

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arXiv2010

Surface and bulk electronic structures of LaOFeAs studied by angle resolved photoemission spectroscopy

The electronic structure of LaOFeAs, a parent compound of iron-arsenic superconductors, is studied by angleresolved photoemission spectroscopy. By examining its dependence on photon energy, polarization, sodium dosing and the counting of Fermi surface volume, both the bulk and the surface contributions are identified. We find that a bulk band moves toward high binding energies below structural transition, and shifts smoothly across the spin density wave transition by about 25 meV. Our data suggest the band reconstruction may play a crucial role in the spin density wave transition, and the structural transition is driven by the short range magnetic order. For the surface states, both the LaO-terminated and FeAs-terminated components are revealed. Certain small band shifts are verified for the FeAs-terminated surface states in the spin density wave state, which is a reflection of the bulk electronic structure reconstruction. Moreover, sharp quasiparticle peaks quickly rise at low temperatures, indicating of drastic reduction of the scattering rate. A kink structure in one of the surface band is shown to be possibly related to the electron-phonon interactions.

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arXiv2010

$^{77}$Se and $^{63}$Cu NMR studies of the electronic correlations in Cu$_x$TiSe$_2$ ($x=0.05, 0.07$)

We report $^{77}$Se and $^{63}$Cu nuclear magnetic resonance (NMR) investigation on the charge-density-wave (CDW) superconductor Cu$_x$TiSe$_2$ ($x=0.05$ and 0.07). At high magnetic fields where superconductivity is suppressed, the temperature dependence of $^{77}$Se and $^{63}$Cu spin-lattice relaxation rates 1/T_{1}$ follow a linear relation. The slope of $^{77}1/T_{1}$ vs \emph{T} increases with the Cu doping. This can be described by a modified Korringa relation which suggests the significance of electronic correlations and the Se 4\emph{p}- and Ti 3\emph{d}-band contribution to the density of states at the Fermi level in the studied compounds.

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arXiv2010

High-resolution angle-resolved photoemission spectroscopy study of the electronic structure of EuFe2As2

We report the high-resolution angle-resolved photoemission spectroscopy studies of electronic structure of EuFe2As2. The paramagnetic state data are found to be consistent with density-functional calculations. In the antiferromagnetic ordering state of Fe, our results show that the band splitting, folding, and hybridization evolve with temperature, which cannot be explained by a simple folding picture. Detailed measurements reveal that a tiny electron Fermi pocket and a tiny hole pocket are formed near (pi,pi) in the (0,0)-(pi,pi) direction, which qualitatively agree with the results of quantum oscillations, considering kz variation of Fermi surface. Furthermore, no noticeable change within the energy resolution is observed across the antiferromagnetic transition of Eu2+ ordering, suggesting weak coupling between Eu sublattice and FeAs sublattice.

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arXiv2010

Pressure-induced unconventional superconductivity near a quantum critical point in CaFe2As2

75As-zero-field nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements are performed on CaFe2As2 under pressure. At P = 4.7 and 10.8 kbar, the temperature dependences of nuclear-spin-lattice relaxation rate (1/T1) measured in the tetragonal phase show no coherence peak just below Tc(P) and decrease with decreasing temperature. The superconductivity is gapless at P = 4.7 kbar but evolves to that with multiple gaps at P = 10.8 kbar. We find that the superconductivity appears near a quantum critical point under pressures in the range 4.7 kbar < P < 10.8 kbar. Both electron correlation and superconductivity disappear in the collapsed tetragonal phase. A systematic study under pressure indicates that electron correlations play a vital role in forming Cooper pairs in this compound.

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arXiv2010

Quantum criticality and nodal superconductivity in the FeAs-based superconductor KFe2As2

The in-plane resistivity $ρ$ and thermal conductivity $κ$ of FeAs-based superconductor KFe$_2$As$_2$ single crystal were measured down to 50 mK. We observe non-Fermi-liquid behavior $ρ(T) \sim T^{1.5}$ at $H_{c_2}$ = 5 T, and the development of a Fermi liquid state with $ρ(T) \sim T^2$ when further increasing field. This suggests a field-induced quantum critical point, occurring at the superconducting upper critical field $H_{c_2}$. In zero field there is a large residual linear term $κ_0/T$, and the field dependence of $κ_0/T$ mimics that in d-wave cuprate superconductors. This indicates that the superconducting gaps in KFe$_2$As$_2$ have nodes, likely d-wave symmetry. Such a nodal superconductivity is attributed to the antiferromagnetic spin fluctuations near the quantum critical point.

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arXiv2010

Strong correlation and massive spectral-weight redistribution induced spin density wave in a-Fe1.06Te

The electronic structure of a-Fe1.06Te is studied with angle-resolved photoemission spectroscopy. We show that there is substantial spectral weight around Gamma and X, and lineshapes are intrinsically incoherent in the paramagnetic state. The magnetic transition is characterized by a massive spectral-weight transfer over an energy range as large as the band width, which even exhibits a hysteresis loop that marks the strong first order transition. Coherent quasiparticles emerge in the magnetically ordered state due to decreased spin fluctuations, which account for the change of transport properties from insulating behavior to metallic behavior. Our observation demonstrates that Fe1.06Te distinguishes itself from other iron-based systems with more local characters and much stronger interactions among different degrees of freedom, and how a spin density wave is formed in the presence of strong correlation.

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arXiv2010

Electronic structure reconstruction: the driving force behind the magnetic and structural transitions in NaFeAs

The electronic structure of NaFeAs is studied with angle resolved photoemission spectroscopy on high quality single crystals. Large portions of the band structure start to shift around the structural transition temperature, and smoothly evolve as the temperature lowers through the spin density wave transition. Moreover, band folding due to magnetic order emerges around structural transition. Our observation provides direct evidence that the structural and magnetic transitions share the same origin, and are both driven by the electronic structure reconstruction in Fe-based superconductors, instead of Fermi surface nesting.

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arXiv2009

High pressure study of BaFe2As2 - role of hydrostaticity and uniaxial stress

We investigate the evolution of the electrical resistivity of BaFe2As2 single crystals with pressure. The samples used were from the same batch grown from self flux and showed properties that were highly reproducible. Samples were pressurised using three different pressure media: pentane-isopentane (in a piston cylinder cell), Daphne oil (in an alumina anvil cell) and steatite (in a Bridgman cell). Each pressure medium has its own intrinsic level of hydrostaticity, which dramatically affects the phase diagram. An increasing uniaxial pressure component in this system quickly reduces spin density wave order and favours the appearance of superconductivity, similar to what is seen in SrFe2As2.

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arXiv2009

Thermal conductivity study of KFe$_2$As$_2$ single crystal: clear evidence for unconventional superconducting gap with nodes

The in-plane resistivity $ρ$ and thermal conductivity $κ$ of extremely overdoped KFe$_2$As$_2$ ($T_c$ = 3.0 K) single crystal were studied. It is found that $ρ\sim T^{1.5}$ at low temperature, a typical non-Fermi liquid behavior of electrons scattered by antiferromagnetic spin fluctuations. In zero field, we observed a large residual linear term $κ_0/T$, about one third of the normal-state value. In low magnetic fields, $κ_0/T(H)$ increases very fast. Such a behavior of $κ_0/T$ mimics the d-wave cuprate superconductors, therefore provides clear evidence for nodes in the superconducting gap of KFe$_2$As$_2$. Based on the Fermi surface topology of KFe$_2$As$_2$, it is believed that the dominant intraband pairing via antiferromagnetic spin fluctuations results in the unconventional superconducting gap with nodes.

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arXiv2009

Study of Electron Spin Resonance on single crystals $EuFe_{2-x}Co_{x}As_2$

The temperature dependence of electron spin resonance (ESR) was studied in $EuFe_{2-x}Co_{x}As_2 $ (x = 0.0, 0.067, 0.1, 0.2, 0.25, 0.285, 0.35, 0.4 and 0.5). The ESR spectrum of all the samples indicates that the g factor and peak-to-peak linewidth strongly depend on the temperature. Moreover, the peak-to-peak linewidth shows the Korringa behavior, indicating an exchange coupling between the conduction electrons and the $Eu^{2+}$ions. The linewidth, g factor and the integrate ESR intensity show anomalies at the temperature of the spin-density-wave (SDW). The linewidth below the SDW transition does not rely on the temperature. This gives the evidence of the gap opening at the $T_{SDW}$. The slope of the linewidth is closely associated to $T_{SDW}$and $T_C$. This exotic behavior may be related to the nesting of the Fermi surface.

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arXiv2009

Magnetism and superconductivity in single crystals $Eu_{1-x}Sr_xFe_{2-y}Co_{y}As_2$

We systematically studied the transport properties of single crystals of $Eu_{1-x}Sr_xFe_{2-y}$Co$_{y}As_2$. Co doping can suppress the spin-density wave (SDW) ordering and induces a superconducting transition, but a resistivity reentrance due to the antiferromagnetic ordering of $Eu^{2+}$ spins is observed, indicating the competition between antiferromagnetism (AFM) and superconductivity. It is striking that the resistivity reentrance can be completely suppressed by external magnetic field (H) because a metamagnetic transition from antiferromagnetism to ferromagnetism for $Eu^{2+}$ spins is induced by magnetic field. Superconductivity without resistivity reentrance shows up by partial substitution of Eu$^{2+}$ with non-magnetic Sr$^{2+}$ to completely destroy the AFM ordering of $Eu^{2+}$ spins. These results suggest that the antiferromagnetism destroys the superconductivity, while the ferromagnetism can coexist with the superconductivity in the iron-based high-$T_c$ superconductors.

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arXiv2009

Magnetotransport properties in purple bronze Li$_{0.9}$Mo$_6$O$_{17}$ Single Crystal

We have measured resistivity along the a, b and c axes of Li$_{0.9}$Mo$_6$O$_{17}$ single crystal. The anisotropy $ρ$$_c$ / $ρ$$_a$ and $ρ$$_c$ / $ρ$$_b$ is given, confirming the quasi-one-dimensionality of the compound. The sharp decrease in the anisotropy below a certain temperature (T$_M$) indicates dimensional crossover. Superconductivity occurs at 1.8 K well below $T_M$. Negative MRs are observed with magnetic field (H) applied along b axis. This could be ascribed to suppression of energy gap associated with CDW state. While large positive MR is observed with H $\parallel$ c-axis. The MR data can be well fitted by a modified two-band model which has been used in CDW compounds such as quasi-two-dimensional purple bronzes A$_{0.9}$Mo$_6$O$_{17}$ (A = K, Na, Tl) and quasi-one-dimensional conductor NbSe$_3$. The behavior of MR provides a strong evidence for the existence of CDW instability in $Li_{0.9}Mo_6O_{17}$.

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