arXiv subjects

S. H. Pan

Publications and source records attributed to S. H. Pan.

13 recordsLinked to original sources

arXiv2020

Role of anion in the pairing interaction of iron-based superconductivity

High-temperature iron-based superconductivity develops in a structure with unusual lattice-orbital geometry, based on a planar layer of Fe atoms with 3d orbitals and tetrahedrally coordinated by anions. Here we elucidate the electronic role of anions in the iron-based superconductors utilizing state-of-the-art scanning tunneling microscopy. By measuring the local electronic structure, we find that As anion in Ba0.4K0.6Fe2As2 has a striking impact on the electron pairing. The superconducting electronic feature can be switched off/on by removing/restoring As atoms on Fe layer at the atomic scale. Our analysis shows that this remarkable atomic switch effect is related to the geometrical cooperation between anion mediated hopping and unconventional pairing interaction. Our results uncover that the local Fe-anion coupling is fundamental for the pairing interaction of iron-based superconductivity, and promise the potential of bottom-up engineering of electron pairing.

cond-mat.supr-con↗

arXiv2020

Orbital selectivity of layer resolved tunneling on iron superconductor Ba0.6K0.4Fe2As2

We use scanning tunneling microscopy/spectroscopy (STM/S) to elucidate the Cooper pairing of the iron pnictide superconductor Ba0.6K0.4Fe2As2. By a cold-cleaving technique, we obtain atomically resolved termination surfaces with different layer identities. Remarkably, we observe that the low-energy tunneling spectrum related to superconductivity has an unprecedented dependence on the layer-identity. By cross-referencing with the angle-revolved photoemission results and the tunneling data of LiFeAs, we find that tunneling on each termination surface probes superconductivity through selecting distinct Fe-3d orbitals. These findings imply the real-space orbital features of the Cooper pairing in the iron pnictide superconductors, and propose a new and general concept that, for complex multi-orbital material, tunneling on different terminating layers can feature orbital selectivity.

cond-mat.supr-con↗

arXiv2019

Topological lasers generating and multiplexing topological light

Vortices are topologically stable singularities at the center of a swirl of energy. Optical vortices are conventionally formed using diffractive optics or by bespoke optical elements. We report room temperature integrated lasers directly generating and multiplexing coherent beams carrying arbitrarily large orbital angular momenta (OAM). The OAM beams are created using two-dimensional topological-rings formed by circular boundaries between topologically distinct photonic materials that naturally radiate vortices in the third dimension. We also demonstrate the planar multiplexing of OAM beams using concentric lasers. Our experimental demonstration reveals a subtle connection between topological matter and topological light and provides opportunities in microscopy, metrology, high-capacity communications, and quantum information processing.

physics.optics↗

arXiv2016

Tip Pressure Induced Incoherent Energy Gap in CaFe2As2

In CaFe2As2, superconductivity can be achieved by applying a modest c-axis pressure of several kbar. Here we use scanning tunneling microscopy/spectroscopy (STM/S) to explore the STM tip pressure effect on single crystals of CaFe2As2. When performing STM/S measurements, the tip-sample interaction can be controlled to act repulsive with reduction of the junction resistance, thus to apply a tip pressure on the sample. We find that an incoherent energy gap emerges at the Fermi level in the differential conductance spectrum when the tip pressure is increased. This energy gap is of the similar order of magnitude as the superconducting gap in the chemical doped compound Ca0.4Na0.6Fe2As2 and disappears at the temperature well below that of the bulk magnetic ordering. Moreover, we also observe the rhombic distortion of the As lattice, which agrees with the orthorhombic distortion of the underlying Fe lattice. These findings suggest that the STM tip pressure can induce the local Cooper pairing in the orthorhombic phase of CaFe2As2.

cond-mat.supr-con↗

arXiv2016

Role of Arsenic in Iron-based Superconductivity at Atomic Scale

In iron-based superconductors, a unique tri-layer Fe-As (Se, Te, P) plays an essential role in controlling the electronic properties, especially the Cooper pairing interaction. Here we use scanning tunneling microscopy/spectroscopy (STM/S) to investigate the role of arsenic atom in superconducting Ba0.4K0.6Fe2As2 by directly breaking and restoring the Fe-As structure at atomic scale. After the up-As-layer peeled away, the tunneling spectrum of the exposed iron surface reveals a shallow incoherent gap, indicating a severe suppression of superconductivity without arsenic covering. When a pair of arsenic atoms is placed on such iron surface, a localized topographic feature is formed due to Fe-As orbital hybridization, and the superconducting coherent peaks recover locally with the gap magnitude the same as that on the iron-layer fully covered by arsenic. These observations unravel the Fe-As interactions on an atomic scale and imply its essential roles in the iron-based superconductivity.

cond-mat.supr-con↗

arXiv2016

Cooper Pairing and Phase Coherence in Iron Superconductor Fe1+x(Te,Se)

The Cooper pairing and phase coherence are two fundamental aspects of superconductivity. Due to breaking time reversal symmetry, magnetic impurities are detrimental to superconductivity, yet microscopically how they affect the pairing strength and phase coherence in a real material is less understood. Recently we observed a robust zero-energy bound state at an interstitial Fe impurity (IFI) in superconducting Fe1+x(Te,Se), signifying intense impurity scattering. Here we report a comprehensive study, using scanning tunnelling microscopy/spectroscopy (STM/S) technique, of the global effects of IFIs on the ground state of Fe1+x(Te,Se) over a wide range of IFI concentration x. Our high resolution tunnelling spectroscopy and quasi-particle interference data at very low temperature demonstrate that IFIs hardly affect the electron pairing strength, while they cause significant decoherence of Cooper pairs in precedence of the Coulomb correlation, eventually driving the ground state of the system from strong-coupling-superconductor to diffusive-metal with incoherent electron pairs.

cond-mat.supr-con↗

arXiv2016

Experimental evidence of large-gap two-dimensional topological insulator on the surface of ZrTe5

Two-dimensional (2D) topological insulators (TIs) with a large bulk band-gap are promising for experimental studies of the quantum spin Hall effect and for spintronic device applications. Despite considerable theoretical efforts in predicting large-gap 2D TI candidates, only few of them have been experimentally verified. Here, by combining scanning tunneling microscopy/spectroscopy and angle-resolved photoemission spectroscopy, we reveal that the top monolayer of ZrTe5 crystals hosts a large band gap of ~100 meV on the surface and a finite constant density-of-states within the gap at the step edge. Our first-principles calculations confirm the topologically nontrivial nature of the edge states. These results demonstrate that the top monolayer of ZrTe5 crystals is a large-gap 2D TI suitable for topotronic applications at high temperature.

cond-mat.mtrl-sci↗

arXiv2014

Observation of a Robust Zero-energy Bound State in Iron-based Superconductor Fe(Te,Se)

A robust zero-energy bound state (ZBS) in a superconductor, such as a Majorana or Andreev bound state, is often a consequence of non-trivial topological or symmetry related properties, and can provide indispensable information about the superconducting state. Here we use scanning tunneling microscopy/spectroscopy to demonstrate, on the atomic scale, that an isotropic ZBS emerges at the randomly distributed interstitial excess Fe sites in the superconducting Fe(Te,Se). This ZBS is localized with a short decay length of ~ 10 Å, and surprisingly robust against a magnetic field up to 8 Tesla, as well as perturbations by neighboring impurities. We find no natural explanation for the observation of such a robust zero-energy bound state, indicating a novel mechanism of impurities or an exotic pairing symmetry of the iron-based superconductivity.

cond-mat.supr-con↗

arXiv2013

Experimental investigation of the electronic structure of Ca$_{0.83}$La$_{0.17}$Fe$_2$As$_2$

We performed a combined angle-resolved photoemission spectroscopy and scanning tunneling microscopy study of the electronic structure of electron-doped Ca$_{0.83}$La$_{0.17}$Fe$_2$As$_2$. A surface reconstruction associated with the dimerization of As atoms is observed directly in the real space, as well as the consequent band folding in the momentum space. Besides this band folding effect, the Fermi surface topology of this material is similar to that reported previously for BaFe$_{1.85}$Co$_{0.15}$As$_2$, with $Γ$-centred hole pockets quasi-nested to M-centred electron pockets by the antiferromagnetic wave vector. Although no superconducting gap is observed by ARPES possibly due to low superconducting volume fraction, a gap-like density of states depression of $7.7\pm 2.9$ meV is determined by scanning tunneling microscopy.

cond-mat.supr-con↗

arXiv2001

Discovery of microscopic electronic inhomogeneity in the high-Tc superconductor Bi2Sr2CaCu2O8+x

The parent compounds of the copper oxide high-Tc superconductors are unusual insulators. Superconductivity arises when they are properly doped away from stoichiometry1. In Bi2Sr2CaCu2O8+x, superconductivity results from doping with excess oxygen atoms, which introduce positive charge carriers (holes) into the CuO2 planes, where superconductivity is believed to originate. The role of these oxygen dopants is not well understood, other than the fact that they provide charge carriers. However, it is not even clear how these charges distribute in the CuO2 planes. Accordingly, many models of high-Tc superconductors simply assume that the charge carriers introduced by doping distribute uniformly, leading to an electronically homogeneous system, as in ordinary metals. Here we report the observation of an electronic inhomogeneity in the high-Tc superconductor Bi2Sr2CaCu2O8+x using scanning tunnelling microscopy/spectroscopy. This inhomogeneity is manifested as spatial variations in both the local density of states spectrum and the superconducting energy gap. These variations are correlated spatially and vary on a surprisingly short length scale of ~ 14 Angs. Analysis suggests that the inhomogeneity observed is a consequence of proximity to a Mott insulator resulting in poor screening of the charge potentials associated with the oxygen ions left behind in the BiO plane after doping. Hence this experiment is a direct probe of the local nature of the superconducting state, which is not easily accessible by macroscopic measurements.

cond-mat.supr-con↗

arXiv2001

Interplay of magnetism and high-Tc superconductivity at individual Ni impurity atoms in Bi2Sr2CaCu2O8+d

In conventional superconductors, magnetic interactions and magnetic impurity atoms are destructive to superconductivity. By contrast, in some unconventional systems, e.g. superfluid 3He and superconducting UGe2, superconductivity or superfluidity is actually mediated by magnetic interactions. A magnetic mechanism has also been proposed for high temperature superconductivity (HTSC) in which an electron magnetically polarizes its environment resulting in an attractive pairing-interaction for oppositely polarized spins. Since a magnetic impurity atom would apparently not disrupt such a pairing-interaction, it has also been proposed that the weaker influences on HTSC of magnetic Ni impurity atoms compared to those of non-magnetic Zn are evidence for a magnetic mechanism. Here we use scanning tunneling microscopy (STM) to determine directly the influence of individual Ni atoms on the electronic structure of Bi2Sr2CaCu2O8+d. Two local d-wave impurity-states are observed at each Ni. Analysis of their energies surprisingly reveals that the primary quasiparticle scattering effects of Ni atoms are due to non-magnetic interactions. Nonetheless, we also demonstrate that a magnetic moment coexists with unimpaired superconductivity at each Ni site. We discuss the implications of these phenomena, and those at Zn, for the pairing-mechanism.

cond-mat.supr-con↗

arXiv2000

STM Studies of the Electronic Structure of Vortex Cores in Bi2Sr2CaCu2O8+d

We report on low temperature scanning tunneling microscopy (STM) studies of the electronic structure of vortex cores in Bi2Sr2CaCu2O8+d (BSCCO). At the vortex core center, an enhanced density-of-states (DOS) is observed at energies near 7 meV. Spectroscopic imaging at these energies reveals an exponential decay of these 'core states' with a decay length of 2.2(3) nm. The four-fold symmetry sometimes predicted for d-wave vortices is not seen in spectroscopic vortex images. A locally nodeless order parameter induced by the magnetic field may be consistent with these measurements.

cond-mat.supr-con↗

arXiv1999

Imaging the Effects of Individual Zinc Impurity Atoms on Superconductivity in Bi2Sr2CaCu2O8+delta

Although their crystal structures are complex, all high temperature superconductors contain some crystal planes consisting of only Cu and O atoms in a square lattice. Superconductivity is believed to originate from strongly interacting electrons in these CuO2 planes. Substitution of a single impurity atom at a Cu site creates a simple but powerful perturbation to these interactions. Detailed knowledge of the effects of such an impurity atom on the superconducting order parameter and on the quasi-particle local density of states (LDOS) could allow competing theories of high temperature superconductivity (HTSC) to be tested at the atomic scale. The fundamental implications of results from numerous bulk measurements on samples doped with impurity atoms could also be clarified with such data. Here we describe scanning tunneling microscopy studies of the effects of individual Zn impurity atoms located at the Cu site in the high-Tc superconductor Bi2Sr2CaCu2O8+delta. Tunneling spectroscopy shows intense quasi-particle scattering resonances at the Zn sites, coincident with strong suppression of superconductivity within about 1.5 nm. Imaging of the quasi-particle LDOS at these sites reveals the long sought four-fold symmetric "quasi-particle cloud" aligned with the d-wave gap nodes. Several unexpected phenomena, which can shed new light on the atomic-scale response of HTSC to a probe impurity atom, are also observed.

cond-mat.supr-con↗