arXiv subjects

B. P. Xie

Publications and source records attributed to B. P. Xie.

At least 55 records · Page 3Linked to original sources

arXiv2011

Symmetry breaking via orbital-dependent reconstruction of electronic structure in uniaxially strained NaFeAs

The superconductivity discovered in iron-pnictides is intimately related to a nematic ground state, where the C4 rotational symmetry is broken via the structural and magnetic transitions. We here study the nematicity in NaFeAs with the polarization dependent angle-resolved photoemission spectroscopy. A uniaxial strain was applied on the sample to overcome the twinning effect in the low temperature C2-symmetric state, and obtain a much simpler electronic structure than that of a twinned sample. We found the electronic structure undergoes an orbital-dependent reconstruction in the nematic state, primarily involving the dxy- and dyz-dominated bands. These bands strongly hybridize with each other, inducing a band splitting, while the dxz-dominated bands only exhibit an energy shift without any reconstruction. These findings suggest that the development of orbital-dependent spin polarization is likely the dominant force to drive the nematicity, while the ferro-orbital ordering between dxz and dyz orbitals can only play a minor role here.

cond-mat.supr-con↗

arXiv2011

Nodal superconducting gap structure in ferropnictide superconductor BaFe2(As0.7P0.3)2

The superconducting gap is a pivotal character for a superconductor. While the cuprates and conventional phonon-mediated superconductors are characterized by distinct d-wave and s-wave pairing symmetry with nodal and nodeless gap distribution respectively, the superconducting gap distributions in iron-based superconductors are rather diversified. While nodeless gap distributions have been directly observed in Ba1-xKxFe2As2, BaFe2-xCoxAs2, KxFe2-ySe2, and FeTe1-xSex, the signatures of nodal superconducting gap have been reported in LaOFeP, LiFeP, KFe2As2, BaFe2(As1-xPx)2, BaFe2-xRuxAs2 and FeSe. We here report the angle resolved photoemission spectroscopy (ARPES) measurements on the superconducting gap structure of BaFe2(As1-xPx)2 in the momentum space, and particularly, the first direct observation of a circular line node on the largest hole Fermi surface around the Z point at the Brillouin zone boundary. Our data rules out the d-wave pairing origin of the nodal gap, and unify both the nodaland nodeless gaps in iron pnictides under the s\pm pairing symmetry.

cond-mat.supr-con↗

arXiv2011

Electronic identification of the actual parental phase of KxFe2-ySe2 superconductor and its intrinsic mesoscopic phase separation

While the parent compounds of the cuprate high temperature superconductors (high-Tc's) are Mott insulators, the iron-pnictide high-Tc's are in the vicinity of a metallic spin density wave (SDW) state, which highlights the difference between these two families. However, insulating parent compounds were identified for the newly discovered KxFe2-ySe2. This raises an intriguing question as to whether the iron-based high-Tc's could be viewed as doped Mott insulators like the cuprates. Here we report angle-resolved photoemission spectroscopy (ARPES) evidence of two insulating and one semiconducting phases of KxFe2-ySe2, and the mesoscopic phase separation between the superconducting/semiconducting phase and the insulating phases. The insulating phases are characterized by the depletion of electronic states over a 0.5 eV window below the chemical potential, giving a compelling evidence for the presence of Mott-like physics. The charging effects and the absence of band folding in the superconducting/semiconducting phase further prove that the static magnetic and vacancy orders are not related to the superconductivity. Instead, the electronic structure of the superconducting phase is much closer to the semiconducting phase, indicating the superconductivity is likely developed by doping the semiconducting phase rather than the insulating phases.

cond-mat.supr-con↗

arXiv2011

Phosphor induced significant hole-doping in ferropnictide superconductor BaFe2(As(1-x)Px)2

The superconductivity in high temperature superconductors ordinarily arises when doped with hetero-valent ions that introduce charge carriers. However, in ferropnictides, "iso-valent" doping, which is generally believed not to introduce charge carriers, can induce superconductivity as well. Moreover, unlike other ferropnictides, the superconducting gap in BaFe2(As1-xPx)2 has been found to contain nodal lines. The exact nature of the "iso-valent" doping and nodal gap here are key open issues in building a comprehensive picture of the iron-based high temperature superconductors. With angle-resolved photoemission spectroscopy (ARPES), we found that the phosphor substitution in BaFe2(As1-xPx)2 induces sizable amount of holes into the hole Fermi surfaces, while the dxy-originated band is relatively intact. This overturns the previous common belief of "iso-valent" doping, explains why the phase diagram of BaFe2(As1-xPx)2 is similar to those of the holedoped compounds, and rules out theories that explain the nodal gap based on vanishing dxy hole pocket.

cond-mat.supr-con↗

arXiv2011

Electronic structure of BaNi$_2$As$_2$

BaNi$_2$As$_2$, with a first order phase transition around 131 K, is studied by the angle-resolved photoemission spectroscopy. The measured electronic structure is compared to the local density approximation calculations, revealing similar large electronlike bands around $\bar{M}$ and differences along $\barΓ$-$\bar{X}$. We further show that the electronic structure of BaNi$_2$As$_2$ is distinct from that of the sibling iron pnictides. Particularly, there is no signature of band folding, indicating no collinear SDW related magnetic ordering. Moreover, across the strong first order phase transition, the band shift exhibits a hysteresis, which is directly related to the significant lattice distortion in BaNi$_2$As$_2$.

cond-mat.supr-con↗

arXiv2010

Heavily electron-doped electronic structure and isotropic superconducting gap in AxFe2Se2 (A=K,Cs)

The low energy band structure and Fermi surface of the newly discovered superconductor, AxFe2Se2 (A=K,Cs), have been studied by angle-resolved photoemission spectroscopy. Compared with iron pnictide superconductors, AxFe2Se2 (A=K,Cs) is the most heavily electron-doped with Tc~30 K. Only electron pockets are observed with an almost isotropic superconducting gap of ~10.3 meV, while there is no hole Fermi surface near the zone center, which indicates the inter-pocket hopping or Fermi surface nesting is not a necessary ingredient for the unconventional superconductivity in iron-based superconductors. Thus, the sign changed s$_\pm$ pairing symmetry, a leading candidate proposed for iron-based superconductors, becomes conceptually irrelevant in describing the superconducting state here. A more conventional s-wave pairing is a better description.

cond-mat.supr-con↗

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.

cond-mat.supr-con↗

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.

cond-mat.supr-con↗

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.

cond-mat.supr-con↗

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.

cond-mat.supr-con↗

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.

cond-mat.supr-con↗

arXiv2009

High-resolution inelastic X-ray scattering studies of the state-resolved differential cross-section of Compton excitations in helium atoms

The state-resolved differential cross sections for both the 1s2 1S0 - 1s2s 1S0 monopolar transition and the 1s2 1S0 - 1s2p 1P1 dipolar transition of atomic helium have been measured over a large momentum transfer region by the high-resolution inelastic X-ray scattering (IXS) for the first time. The almost perfect match of the present measurement with the theoretical calculations gives a stringent test of the theoretical method and the calculated wavefunctions. Our results demonstrate that high-resolution IXS is a powerful tool for studying the excitations in atoms and molecules.

physics.atom-ph↗

arXiv2008

Primary role of the barely occupied states in the charge density wave formation of NbSe2

NbSe2 is a prototypical charge-density-wave (CDW) material, whose mechanism remains mysterious so far. With angle resolved photoemission spectroscopy, we mapped out the CDW gap and recovered the long-lost nesting condition over a large broken-honeycomb region in the Brillouin zone, which consists of six saddle band point regions with high density of states (DOS), and large regions away from Fermi surface with negligible DOS at the Fermi energy. We show that the major contributions to the CDW come from these barely occupied states rather than the saddle band points. Our findings not only resolve a long standing puzzle, but also overthrow the conventional wisdom that CDW is dominated by regions with high DOS.

cond-mat.str-el↗

arXiv2008

Novel electronic structure induced by a highly strained oxide interface with incommensurate crystal fields

The misfit oxide, Bi$_{2}$Ba$_{1.3}$K$_{0.6}$Co$_{2.1}$O$_{y}$, made of alternating rocksalt-structured [BiO/BaO] layers and hexagonal CoO$_{2}$ layers, was studied by angle-resolved photoemission spectroscopy. Detailed electronic structure of such a highly strained oxide interfaces is revealed for the first time. We found that under the two incommensurate crystal fields, electrons are confined within individual sides of the interface, and scattered by umklapp scattering of the crystal field from the other side. In addition, the high strain on the rocksalt layer raises its chemical potential and induces large charge transfer to the CoO$_{2}$ layer. Furthermore, a novel interface effects, the interfacial enhancement of electron-phonon interactions, is discovered. Our findings of these electronic properties lay a foundation for designing future functional oxide interfaces.

cond-mat.str-el↗

arXiv2008

Superconducting coherence peak in the electronic excitations of a single layer cuprate superconductor $Bi_2 Sr_{1.6} La_{0.4} Cu O_{6+δ}$

Angle resolved photoemission spectroscopy study is reported on a high quality optimally doped Bi2Sr1.6La0.4CuO6+delta high Tc superconductor. In the antinodal region with maximal d-wave gap, the symbolic superconducting coherence peak, which has been widely observed in multi-CuO2-layer cuprate superconductors, is unambiguously observed in a single layer system. The associated peak-dip separation is just about 19 meV, which is much smaller than its counterparts in multi-layered compounds, but correlates with the energy scales of spin excitations in single layer cuprates.

cond-mat.supr-con↗

arXiv2007

Evolution of the Electronic Structure of 1T-CuxTiSe2

The electronic structure of a new charge-density-wave/ superconductor system, 1T-CuxTiSe2, has been studied by photoemission spectroscopy. A correlated semiconductor band structure is revealed for the undoped case. With Cu doping, the charge density wave is suppressed by the raising of the chemical potential, while the superconductivity is enhanced by the enhancement of the density of states. Moreover, the strong scattering at high doping might be responsible for the suppression of superconductivity in that regime.

cond-mat.str-el↗

arXiv2007

A novel mechanism of charge density wave in a transition metal dichalcogenide

Charge density wave, or CDW, is usually associated with Fermi surfaces nesting. We here report a new CDW mechanism discovered in a 2H-structured transition metal dichalcogenide, where the two essential ingredients of CDW are realized in very anomalous ways due to the strong-coupling nature of the electronic structure. Namely, the CDW gap is only partially open, and charge density wavevector match is fulfilled through participation of states of the large Fermi patch, while the straight FS sections have secondary or negligible contributions.

cond-mat.str-el↗

arXiv2006

High-energy scale revival and giant kink in the dispersion of a cuprate superconductor

In the present photoemission study of a cuprate superconductor Bi1.74Pb0.38Sr1.88CuO6+delta, we discovered a large scale dispersion of the lowest band, which unexpectedly follows the band structure calculation very well. The incoherent nature of the spectra suggests that the hopping-dominated dispersion occurs possibly with the assistance of local spin correlations. A giant kink in the dispersion is observed, and the complete self-energy containing all interaction information is extracted for a doped cuprate in the low energy region. These results recovered significant missing pieces in our current understanding of the electronic structure of cuprates.

cond-mat.supr-con↗