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Register a new userDefect states in LiFeAs as seen by low temperature scanning tunneling microscopy and spectroscopy
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We present a microscopic investigation of frequently observed impurity-induced states in stoichiometric LiFeAs using low temperature scanning tunneling microscopy and spectroscopy (STM/STS). Our data reveal seven distinct well defined defects which are discernible in topographic measurements. Depending on their local topographic symmetry, we are able to assign five defect types to specific lattice sites at the Li, Fe and As positions. The most prominent result is that two different defect types have a remarkably different impact on the superconducting state. A specific and quite abundant Fe-defect with $D_2$-symmetry generates significant impurity-induced additional states primarily at positive bias voltage with pronounced peaks in the on-site local density of states (LDOS) at about 4~mV and 12~mV. On the other hand, a $D_4$-symmetric As-defect causes a significantly enhanced LDOS at both positive and negative bias voltages. We expect that these findings provide fresh input for further experimental and theoretical studies on elucidating the nature of superconductivity in LiFeAs.
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Defects in LiFeAs are studied by scanning tunneling microscopy (STM) and spectroscopy (STS). Topographic images of the five predominant defects allow the identification of their position within the lattice. The most commonly observed defect is associated with an Fe site and does not break the local lattice symmetry, exhibiting a bound state near the edge of the smaller gap in this multi-gap superconductor. Three other common defects, including one also on an Fe site, are observed to break local lattice symmetry and are pair-breaking indicated by clear in-gap bound states, in addition to states near the smaller gap edge. STS maps reveal complex, extended real-space bound state patterns, including one with a chiral distribution of the local density of states (LDOS). The multiple bound state resonances observed within the gaps and at the inner gap edge are consistent with theoretical predictions for s$^{pm}$ gap symmetry proposed for LiFeAs and other iron pnictides.
We investigate the vortex lattice and vortex bound states in CsFe$_2$As$_2$ single crystals by scanning tunneling microscopy/spectroscopy (STM/STS) under various magnetic fields. A possible structural transition or crossover of vortex lattice is observed with the increase of magnetic field, i.e., the vortex lattice changes from a distorted hexagonal lattice to a distorted tetragonal one at the magnetic field near 0.5 T. It is found that a mixture of stripelike hexagonal and square vortex lattices emerges in the crossover region. The vortex bound state is also observed in the vortex center. The tunneling spectra crossing a vortex show that the bound-state peak position holds near zero bias with STM tip moving away from the vortex core center. The Fermi energy estimated from the vortex bound state energy is very small. Our investigations provide experimental information to both the vortex lattice and the vortex bound states in this iron-based superconductor.
Tunneling magnetoresistance (TMR) in a vertical manganite junction was investigated by low-temperature scanning laser microscopy (LTSLM) allowing to determine the local relative magnetization M orientation of the two electrodes as a function of magnitude and orientation of the external magnetic field H. Sweeping the field amplitude at fixed orientation revealed magnetic domain nucleation and propagation in the junction electrodes. For the high-resistance state an almost single-domain antiparallel magnetization configuration was achieved, while in the low-resistance state the junction remained in a multidomain state. Calculated resistance $R_mathrm{calc}(H)$ based on the local M configuration obtained by LTSLM is in quantitative agreement with R(H) measured by magnetotransport.
We apply high resolution scanning tunneling microscopy to study intrinsic defect states of bulk FeSe. Four types of intrinsic defects including the type I dumbbell, type II dumbbell, top-layer Se vacancy and inner-layer Se-site defect are extensively analyzed by scanning tunneling spectroscopy. From characterized depression and enhancement of density of states measured in a large energy range, the type I dumbbell and type II dumbbell are determined to be the Fe vacancy and Se$_mathrm{Fe}$ defect, respectively. The top-layer Se vacancy and possible inner-layer Se-site vacancy are also determined by spectroscopy analysis. The determination of defects are compared and largely confirmed in the annular dark-field scanning transmission electron microscopy measurement of the exfoliated FeSe. The detailed mapping of defect states in our experiment lays the foundation for a comparison with complex theoretical calculations in the future.
Exotic quantum phenomena have been demonstrated in recently discovered intrinsic magnetic topological insulator MnBi2Te4. At its two-dimensional limit, quantum anomalous Hall (QAH) effect and axion insulator state are observed in odd and even layers of MnBi2Te4, respectively. The measured band structures exhibit intriguing and complex properties. Here we employ low-temperature scanning tunneling microscopy to study its surface states and magnetic response. The quasiparticle interference patterns indicate that the electronic structures on the topmost layer of MnBi2Te4 is different from that of the expected out-of-plane A-type antiferromagnetic phase. The topological surface states may be embedded in deeper layers beneath the topmost surface. Such novel electronic structure presumably related to the modification of crystalline structure during sample cleaving and re-orientation of magnetic moment of Mn atoms near the surface. Mn dopants substituted at the Bi site on the second atomic layer are observed. The ratio of Mn/Bi substitutions is 5%. The electronic structures are fluctuating at atomic scale on the surface, which can affect the magnetism of MnBi2Te4. Our findings shed new lights on the magnetic property of MnBi2Te4 and thus the design of magnetic topological insulators.
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