arXiv subjects

B. P. Xie

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

At least 37 records · Page 2Linked to original sources

arXiv2014

Drastic electronic structure reconstruction of Ca$_{1-x}$Pr$_x$Fe$_2$As$_2$ in the collapsed tetragonal phase

We report the electronic structure reconstruction of Ca$_{1-x}$Pr$_x$Fe$_2$As$_2$ ($x$ = 0.1 and 0.15) in the low temperature collapsed tetragonal (CT) phase observed by angle-resolved photoemission spectroscopy. Different from Ca(Fe$_{1-x}$Rh$_x$)$_2$As$_2$ and the annealed CaFe$_2$As$_2$ where all hole Fermi surfaces are absent in their CT phases, the cylindrical hole Fermi surface can still be observed in the CT phase of Ca$_{1-x}$Pr$_x$Fe$_2$As$_2$. Furthermore, we found at least three well separated electron-like bands around the zone corner in the CT phase of Ca$_{1-x}$Pr$_x$Fe$_2$As$_2$, which are more dispersive than the electron-like bands in the high temperature tetragonal phase. Based on these observations, we propose that the weakening of correlations (as indicated by the reduced effective mass), rather than the lack of Fermi surface nesting, might be responsible for the absence of magnetic ordering and superconductivity in the CT phase.

cond-mat.supr-con↗

arXiv2014

Orbital selective correlations between nesting/scattering/Lifshitz transition and the superconductivity in AFe1-xCoxAs (A=Li, Na)

The correlations between the superconductivity in iron pnictides and their electronic structures are elusive and controversial so far. Here through angle-resolved photoemission spectroscopy measurements, we show that such correlations are rather distinct in AFe$_{1-x}$Co$_x$As (A=Li, Na), but only after one realizes that they are orbital selective. We found that the superconductivity is enhanced by the Fermi surface nesting, but only when it is between $d_{xz}/d_{yz}$ Fermi surfaces, while for the $d_{xy}$ orbital, even nearly perfect Fermi surface nesting could not induce superconductivity. Moreover, the superconductivity is completely suppressed just when the $d_{xz}/d_{yz}$ hole pockets sink below Fermi energy and evolve into an electron pocket. Our results thus establish the orbital selective relation between the Fermiology and the superconductivity in iron-based superconductors, and substantiate the critical role of the $d_{xz}/d_{yz}$ orbitals. Furthermore, around the zone center, we found that the $d_{xz}/d_{yz}$-based bands are much less sensitive to impurity scattering than the $d_{xy}$-based band, which explains the robust superconductivity against heavy doping in iron-based superconductors.

cond-mat.supr-con↗

arXiv2014

Extraordinary quasiparticle scattering and bandwidth-control by dopants in iron-based superconductors

The diversities in crystal structures and ways of doping result in extremely diversified phase diagrams for iron-based superconductors. With angle-resolved photoemission spectroscopy (ARPES), we have systematically studied the effects of chemical substitution on the electronic structure of various series of iron-based superconductors. In addition to the control of Fermi surface topology by heterovalent doping, we found two more extraordinary effects of doping: 1. the site and band dependencies of quasiparticle scattering; and more importantly 2. the ubiquitous and significant bandwidth-control by both isovalent and heterovalent dopants in the iron-anion layer. Moreover, we found that the bandwidth-control could be achieved by either applying the chemical pressure or doping electrons, but not by doping holes. Together with other findings provided here, these results complete the microscopic picture of the electronic effects of dopants, which facilitates a unified understanding of the diversified phase diagrams and resolutions to many open issues of various iron-based superconductors.

cond-mat.supr-con↗

arXiv2014

Angle-resolved Photoemission Spectroscopy Study on the Surface States of the Correlated Topological Insulator YbB6

We report the electronic structure of YbB6, a recently predicted moderately correlated topological insulator, measured by angle-resolved photoemission spectroscopy. We directly observed linearly dispersive bands around the time-reversal invariant momenta Γ and X with negligible kz dependence, consistent with odd number of surface states crossing the Fermi level in a Z2 topological insulator. Circular dichroism photoemission spectra suggest that these in-gap states possess chirality of orbital angular momentum, which is related to the chiral spin texture, further indicative of their topological nature. The observed insulating gap of YbB6 is about 100 meV, larger than that reported by theoretical calculations. Our results present strong evidence that YbB6 is a correlated topological insulator and provide a foundation for further studies of this promising material.

cond-mat.str-el↗

arXiv2014

Critical role of substrate in the high temperature superconductivity of single layer FeSe on Nb:BaTiO$_3$

In the quest for high temperature superconductors, the interface between a metal and a dielectric was proposed to possibly achieve very high superconducting transition temperature ($T_c$) through interface-assisted pairing. Recently, in single layer FeSe (SLF) films grown on SrTiO$_3$ substrates, signs for $T_c$ up to 65~K have been reported. However, besides doping electrons and imposing strain, whether and how the substrate facilitates the superconductivity are still unclear. Here we report the growth of various SLF films on thick BaTiO$_3$ films atop KTaO$_3$ substrates, with signs for $T_c$ up to $75$~K, close to the liquid nitrogen boiling temperature. SLF of similar doping and lattice is found to exhibit high $T_c$ only if it is on the substrate, and its band structure strongly depends on the substrate. Our results highlight the profound role of substrate on the high-$T_c$ in SLF, and provide new clues for understanding its mechanism.

cond-mat.supr-con↗

arXiv2014

Electronic structure of the BaTi$_2$As$_2$O parent compound of the titanium based oxypnictide superconductor

The electronic structure of BaTi2As2O, a parent compound of the newly discovered titanium-based oxypnictide superconductors, is studied by angle-resolved photoemission spectroscopy. The electronic structure shows multi-orbital nature and possible three-dimensional character. An anomalous temperature-dependent spectral weight redistribution and broad lineshape indicate the incoherent nature of the spectral function. At the density-wave-like transition temperature around 200 K, a partial gap opens at the Fermi patches. These findings suggest that BaTi2As2O is likely a charge density wave material in the strong interaction regime.

cond-mat.supr-con↗

arXiv2013

Electronic Structure Reconstruction across the Antiferromagnetic Transition in TaFe$_{1.23}$Te$_3$ Spin Ladder

With angle-resolved photoemission spectroscopy, we studied the electronic structure of TaFe$_{1.23}$Te$_3$, which is a two-leg spin ladder compound with a novel antiferromagnetic ground state. Quasi-two-dimensional Fermi surface is observed, indicating sizable inter-ladder hopping, which would facilitate the in-plane ferromagnetic ordering through double exchange interactions. Moreover, an energy gap is not observed at the Fermi surface in the antiferromagnetic state. Instead, the shifts of various bands have been observed. Combining these observations with density-functional-theory calculations, we propose that the large scale reconstruction of the electronic structure, caused by the interactions between the coexisting itinerant electrons and local moments, is most likely the driving force behind the magnetic transition. TaFe$_{1.23}$Te$_3$ thus provides a simpler system that contains similar ingredients as the parent compounds of iron-based superconductors, which yet could be readily modeled and understood.

cond-mat.str-el↗

arXiv2013

Enhanced superconductivity and evidence for novel pairing in single-layer FeSe on SrTiO3 thin film under large tensile strain

Single-layer FeSe films with extremely expanded in-plane lattice constant of 3.99A are fabricated by epitaxially growing FeSe/Nb:SrTiO3/KTaO3 heterostructures, and studied by in situ angle-resolved photoemission spectroscopy. Two elliptical electron pockets at the Brillion zone corner are resolved with negligible hybridization between them, indicating the symmetry of the low energy electronic structure remains intact as a free-standing single-layer FeSe, although it is on a substrate. The superconducting gap closes at a record high temperature of 70K for the iron based superconductors. Intriguingly, the superconducting gap distribution is anisotropic but nodeless around the electron pockets, with minima at the crossings of the two pockets. Our results put strong constraints on the current theories, and support the coexistence of both even and odd parity spin-singlet pairing channels as classified by the lattice symmetry.

cond-mat.supr-con↗

arXiv2013

The unique electronic structure of Ca10(Pt4As8)(Fe2-xPtxAs2)5 with metallic Pt4As8 layers

We studied the low-lying electronic structure of the newly discovered iron-platinum-arsenide superconductor, Ca10(Pt4As8)(Fe2-xPtxAs2)5 (Tc=22 K) with angle-resolved photoemission spectroscopy. We found that the Pt4As8 layer contributes to a small electron-like Fermi surface, indicative of metallic charge reservoir layers that are rare for iron based superconductors. Moreover, the electronic structure of the FeAs-layers resembles those of other prototype iron pnictides to a large extent. However, there is only dxy-orbital originated hole-like Fermi surface near the zone center, which is rather unique for an iron pnictide superconductor with relatively high superconducting transition temperature; and the dxz and dyz originated bands are not degenerate at the zone center. Furthermore, all bands near the Fermi energy show negligible kz dependence, indicating the strong two-dimensional nature of this material. Our results indicate this material possesses rather unique electronic structure, which enriches our current knowledge of iron based superconductors.

cond-mat.supr-con↗

arXiv2013

Angle-resolved photoemission spectroscopy study on iron-based superconductors

Angle-resolved photoemission spectroscopy (ARPES) has played an important role in determining the band structure and the superconducting gap structure of iron-based superconductors. Here from the ARPES perspective, we briefly review the main results from our group in the recent years on the iron-based superconductors and their parent compounds, and depict our current understanding on the antiferromagnetism and superconductivity in these materials.

cond-mat.supr-con↗

arXiv2013

Direct observation of the bandwidth control Mott transition in the NiS$_{2-x}$Se$_x$ multiband system

The bulk electronic structure of NiS$_{2-x}$Se$_x$ is studied across the bandwidth-control Mott transition (BCMT) by soft X-ray angle-resolved photoemission spectroscopy. The microscopic picture of the BCMT in this multiband non-half-filled system is revealed for the first time. We show that Se doping does not alter the Fermi surface volume. When approaching the insulating phase with decreasing Se concentration, we observed that the Fermi velocity continuously decreases. Meanwhile, the coherent quasiparticle weight continuously decreases and is transferred to higher binding energies, until it suddenly disappears across the Mott transition. In the insulating phase, there is still finite spectral weight at the Fermi energy, but it is incoherent and dispersionless due to strong correlations. Our results provide a direct observation of BCMT, and unveil its distinct characters in a multiband non-half-filled system.

cond-mat.str-el↗

arXiv2013

Observation of in-gap surface states in the Kondo insulator SmB6 by photoemission

Kondo insulators (KIs) are strongly correlated materials in which the interactions between 4f and conduction electrons lead to a hybridization gap opening at low temperature 1-2. SmB6 is a typical KI, but its resistivity does not diverge at low temperatures, which was attributed to some in-gap states 3-10. However after several decades of research, the nature and origin of the in-gap states remain unclear. Recent band calculation and transport measurements suggest that the in-gap states could actually be ascribed to topological surface states. SmB6 thus might be the first realization of topological Kondo insulator (TKI) 13, the strongly correlated version of topological insulator (TI) 11,12. Here by performing angle-resolved photoemission spectroscopy (ARPES), we directly observed several dispersive states within the hybridization gap of SmB6, which cross the Fermi level and show negligible kz dependence, indicative of their surface origin. Furthermore, the circular dichroism (CD) ARPES results of the in-gap states suggest the chirality of orbital momentum, and temperature dependent measurements have shown that the in-gap states vanish simultaneously with the hybridization gap around 150 K. These strongly suggest their possible topological origin.

cond-mat.str-el↗

arXiv2013

An ARPES Study of the Electronic Structure of the Quantum Spin Liquid EtMe3Sb[Pd(dmit)2]2

The electronic structure of a quantum spin liquid compound, EtMe3Sb[Pd(dmit)2]2, has been studied with angle-resolved photoemission spectroscopy, together with two other Pd(dmit)2 salts in the valence bond solid or antiferromagnetic state. We have resolved several bands that have negligible dispersions and fit well to the calculated energy levels of an isolated [Pd(dmit)2]2 dimer. EtMe3Sb[Pd(dmit)2]2 being a Mott insulator, its lower Hubbard band is identified, and there is a small gap of ~ 50 meV between this band and the chemical potential. Moreover, the spectral features exhibit polaronic behavior with anomalously broad linewidth. Compared with existing theories, our results suggest that strong electron-boson interactions, together with smaller hopping and on-site Coulomb interaction terms have to be considered for a realistic modeling of the organic quantum spin liquid systems like the Pd(dmit)2 salt.

cond-mat.str-el↗

arXiv2013

The origin of the dead-layer at the La0.67Sr0.33MnO3/SrTiO3 interface and dead-layer reduction via interfacial engineering

Transition metal oxide hetero-structure has great potential for multifunctional devices. However, the degraded physical properties at interface, known as dead-layer behavior, present a main obstacle for device applications. Here we present the systematic study of the dead-layer behavior in La0.67Sr0.33MnO3 thin film grown on SrTiO3 substrate with ozone assisted molecular beam epitaxy. We found that the evolution of electric and magnetic properties as a function of thickness shows a remarkable resemblance to the phase diagram as a function of doping for bulk materials, providing compelling evidences of the hole depletion in near interface layers that causes dead-layer. Detailed electronic and surface structure studies indicate that the hole depletion is due to the intrinsic oxygen vacancy formation. Furthermore, we show that oxygen vacancies are partly caused by interfacial electric dipolar field, and thus by doping-engineering at the single-atomic-layer level, we demonstrate the dead-layer reduction in films with higher interfacial hole concentration.

cond-mat.mtrl-sci↗

arXiv2013

Interface-induced superconductivity and strain-dependent spin density wave in FeSe/SrTiO3 thin films

The record of superconducting transition temperature(Tc) has long been 56K for the iron-based high temperature superconductors(Fe-HTS's). Recently, in single layer FeSe films grown on SrTiO3 substrate, signs for a new 65K Tc record are reported. Here with in-situ photoemission measurements, we substantiate the presence of the spin density wave(SDW) in FeSe films, a key ingredient of Fe-HTS that was missed in FeSe before, which weakens with increased thickness or reduced strain. We demonstrate that the superconductivity occurs when the electrons transferred from the oxygen-vacant substrate suppress the otherwise most pronounced SDW in single layer FeSe. Besides providing a comprehensive understanding of FeSe films and directions to further enhance its Tc, we establish the phase diagram of FeSe vs. lattice constant that contains all the essential physics of Fe-HTS's. With the simplest structure, cleanest composition and single tuning parameter, it is ideal for testing theories of Fe-HTS's.

cond-mat.supr-con↗

arXiv2013

Anisotropic but nodeless superconducting gap in the presence of spin density wave in iron-pnictide superconductor NaFe1-xCoxAs

The coexisting regime of spin density wave (SDW) and superconductivity in the iron pnictides represents a novel ground state. We have performed high resolution angle-resolved photoemission measurements on NaFe1-xCoxAs (x = 0.0175) in this regime and revealed its distinctive electronic structure, which provides some microscopic understandings of its behavior. The SDW signature and the superconducting gap are observed on the same bands, illustrating the intrinsic nature of the coexistence. However, because the SDW and superconductivity are manifested in different parts of the band structure, their competition is non-exclusive. Particularly, we found that the gap distribution is anisotropic and nodeless, in contrast to the isotropic superconducting gap observed in an SDW-free NaFe1-xCoxAs (x=0.045), which puts strong constraints on theory.

cond-mat.supr-con↗

arXiv2012

The distinct in-plane resistivity anisotropy in the nematic states of detwinned NaFeAs and FeTe single crystals: evidences for Hund's rule metal

The in-plane resistivity anisotropy has been studied with the Montgomery method on two detwinned parent compounds of the iron-based superconductors, NaFeAs and FeTe. For NaFeAs, the resistivity in the antiferromagnetic (AFM) direction is smaller than that in the ferromagnetic (FM) direction, similar to that observed in BaFe2As2 before. While for FeTe, the resistivity in the AFM direction is larger than that in the FM direction. We show that these two opposite resistivity anisotropy behaviors could be attributed to the strong Hund's rule coupling effects: while the iron pnictides are in the itinerant regime, where the Hund's rule coupling causes strong reconstruction and nematicity of the electronic structure; the FeTe is in the localized regime, where Hund's rule coupling makes hopping along the FM direction easier than along the AFMdirection, similar to the colossal magnetoresistance observed in some manganites.

cond-mat.str-el↗

arXiv2012

Angle-resolved photoemission evidence of s-wave superconducting gap in KxFe2-ySe2 superconductor

Although nodeless superconducting gap has been observed on the large Fermi pockets around the zone corner in KxFe2-ySe2, whether its pairing symmetry is s-wave or nodeless d-wave is still under intense debate. Here we report an isotropic superconducting gap distribution on the small electron Fermi pocket around the Z point in KxFe2-ySe2, which favors the s-wave pairing symmetry.

cond-mat.supr-con↗