Measurement of a Sign-Changing Two-Gap Superconducting Phase in Electron-Doped Ba(Fe_{1-x}Co_x)_2As_2 Single Crystals using Scanning Tunneling Spectroscopy
M. L. Teague, G. K. Drayna, G. P. Lockhart, P. Cheng, B. Shen, H. -H. Wen, N. -C. Yeh
备注：
4 pages, 4 figures. Paper accepted for publication in Physical Review Letters. Contact author: Nai-Chang Yeh (ncyeh@caltech.edu)
Scanning tunneling spectroscopic studies of $Ba(Fe_{1-x}Co_x)_2As_2$ (x = 0.06, 0.12) single crystals reveal direct evidence for predominantly two-gap superconductivity. These gaps decrease with increasing temperature and vanish above the superconducting transition $T_c$. The two-gap nature and the slightly doping- and energy-dependent quasiparticle scattering interferences near the wave-vectors $(\pm π, 0)$ and $(0, \pm π)$ are consistent with sign-changing $s$-wave superconductivity. The excess zero-bias conductance and the large gap-to-$T_c$ ratios suggest dominant unitary impurity scattering.
The tunneling path between the CuO2-layers in cuprate superconductors and a scanning tunneling microscope tip passes through a barrier made from other oxide layers. This opens up the possibility that inelastic processes in the barrier contribute to the tunneling spectra. Such processes cause one or possibly more peaks in the second derivative current-voltage spectra displaced by phonon energies from the density of states singularity associated with superconductivity. Calculations of inelastic processes generated by apical O-phonons show good qualitative agreement with recent experiments report…
We present Scanning Tunneling Spectroscopy measurements at 0.1 K using tips made of Al. At zero field, the atomic lattice and charge density wave of 2HNbSe2 are observed, and under magnetic fields the peculiar electronic surface properties of vortices are precisely resolved. The tip density of states is influenced by the local magnetic field of the vortex, providing for a new probe of the magnetic field at nanometric sizes.
High--quality single crystals of the heavy fermion superconductors CeCoIn$_5$ and CeIrIn$_5$ have been studied by means of low--temperature Scanning Tunneling Microscopy. Methods were established to facilitate \textit{in-situ} sample cleaving. Spectroscopy in CeCoIn$_5$ reveals a gap which persists to above $T_c$, possibly evidencing a precursor state to SC. Atomically resolved topographs show a rearrangement of the atoms at the crystal surface. This modification at the surface might influence the surface properties as detected by tunneling spectroscopy.
We report a tunneling study between Pr_2-xCe_xCuO_{4-delta} (PCCO) and Lead as a function of doping, temperature and magnetic field. The temperature dependence of the gap follows the BCS prediction. Our data fits a nonmonotonic d-wave order parameter for the whole doping range studied. From our data we are able to conclude that the electron-doped cuprate PCCO is a weak coupling, BCS, dirty superconductor.
There is a clear discrepancy among the energy-gap values for different 90 K cuprates, inferred from tunneling measurements. By using the phase diagram for hole-doped cuprates we show that tunneling measurements performed on 90 K cuprates, simply, detect two different energy gaps: the pairing gap and the coherent gap, which are identical in conventional superconductors. We find that there are two reasons why tunneling measurements show in one cuprate exclusively the coherent gap while, in another cuprate, they show the pairing gap: (i) the number of CuO2 planes per unit cell in the cuprate, and…
Using scanning tunneling spectroscopy we examined the local density of states of thin c-axis La2-xSrxCuO4 films, over wide doping and temperature ranges. We found that the pseudogap exists only at doping levels lower than optimal. For x = 0.12, close to the 'anomalous' x = 1/8 doping level, a zero bias conductance peak was the dominant spectral feature, instead of the excepted V- shaped (c-axis tunneling) gap structure. We have established that this surprising effect cannot be explained by tunneling into (110) facets. Possible origins for this unique behavior are discussed.
We report current driven scanning tunneling spectroscopy (CDSTS) measurements at very low temperatures on vortices in 2H-NbSe2. We find that a current produces an increase of the density of states at the Fermi level in between vortices, and a reduction of the zero bias peak at the vortex center. This occurs well below the de-pairing current. We conclude that a supercurrent affects the low energy part of the superconducting gap structure of 2H-NbSe2.
We extract the density of states (DOS) from the scanning tunneling spectroscopy data for Ba$_{1-x}$K$_x$Fe$_2$As$_2$ superconductor. The obtained sample DOS is composed of two ordinary $s$-wave types from the band at $Γ$ point and a linear-like DOS within the $s$-wave gap from the band at M point in the Brillouin zone, and is consistent with the corresponding data from angle-resolved photoemission spectroscopy. We clarify that the major peak of the tunneling conductance is not related to the DOS but is rather the effect of nonequilibrium coherent tunneling including all coherent spins in the t…
We analyze high-resolution scanning tunneling spectra of the electron-doped cuprate Pr0.88LaCe0.12CuO4 (T_c = 24 K). We find that the spectral fine structure below 35 meV is consistent with strong coupling to a bosonic mode at 16 meV, in quantitative agreement with early tunneling spectra of Nd1.85Ce0.15CuO4. Since the energy of the bosonic mode is significantly higher than that (9.5-11 meV) of the magnetic resonance-like mode observed by inelastic neutron scattering, the coupling feature at 16 meV cannot arise from strong coupling to the magnetic mode. The present work thus demonstrates that …
We have imaged the current noise with atomic resolution in a Josephson scanning tunneling microscope with a Pb-Pb junction. By measuring the current noise as a function of applied bias, we reveal the change from single electron tunneling above the superconducting gap energy to double electron charge transfer below the gap energy when Andreev processes become dominant. Our spatially resolved noise maps show that this doubling occurs homogeneously on the surface, also on impurity locations, demonstrating that indeed the charge pairing is not influenced by disruptions in the superconductor smalle…
In this paper, we report on scanning tunneling microscopy and spectroscopy (STM/STS) measurements on iron doped 1$T$-Ta$_{1-x}$Fe$_x$S$_2$. A novel domain structure composed of the domains with localized nature divided by the walls is observed in Ta$_{0.99}$Fe$_{0.01}$S$_2$, where the Mott transition is completely suppressed. This indicates that the melting of the Mott state accompanies the appearance of the domain structure. Since the number of walls increase in superconducting Ta$_{0.98}$Fe$_{0.02}$S$_2$, the domain walls seem to be responsible for superconductivity in iron doped 1$T$-TaS$_2…
The low-energy charge excitations of a doped antiferromagnetic ladder are modeled by a system of interacting spinless fermions that live on the same ladder. A relatively large spin gap is assumed to ``freeze out'' all spin fluctuations. We find that the formation of rung hole pairs coincides with the opening of a single-particle gap for charge excitations along chains and with the absence of coherent tunneling in between chains. We also find that such hole pairs condense into either a crystalline or superconducting state as a function of the binding energy.
In this brief note we repeat an earlier calculation of the Fourier transformed scanning tunneling spectra of the $d$-density wave (DDW) phase using a different band structure, which is more realistic and consistent with the angle resolved photoemission spectroscopy (ARPES) data. We note that four peaks, which used to be located at $(\pm π/4,0)$ and $(0, \pm π/4)$, are still present, but at positive energies their wavevectors shift to the neighborhood of $(\pm 2 π/5,0)$, $(0, \pm 2π/5)$ and slowly disperse with energy. The implications for the sensitivity with respect to the band structure are …
Scanning tunneling microscopy (STM) has made tremendous progress in the study and understanding of both classical and unconventional superconductors. This has motivated a rapidly growing effort to apply the same techniques to the iron-based high-Tc superconductors since their discovery in 2008. Five years have brought exciting advances in imaging and spectroscopic investigation of this new class of materials. In this review, we focus on several recent STM contributions to the identification of the gap symmetry and pairing glue. We highlight the unique capabilities and challenges still ahead fo…
In two dimensions the non-interacting density of states displays a Van Hove singularity (VHS) which introduces an intrinsic electron-hole asymmetry, absent in three dimensions. We show that due to this VHS the strong-coupling analysis of tunneling spectra in high-$T_c$ superconductors must be reconsidered. Based on a microscopic model which reproduces the experimental data with great accuracy, we elucidate the peculiar role played by the VHS in shaping the tunneling spectra, and show that more conventional analyses of strong-coupling effects can lead to severe errors.
In the vortex core of slightly overdoped Bi2Sr2CaCu2Ox, the electron-like and hole-like states have been found to exhibit spatial modulations in anti-phase with each other along the Cu-O bonding direction. Some kind of one-dimensionality has been observed in the vortex core, and it is more clearly seen in differential conductance maps at lower biases below +-9 mV.
We report on a fully nonequilibrium theory of the scanning tunneling microscopy (STM) through resonances induced by impurity atoms adsorbed on metal surfaces. The theory takes into account the effect of the tunneling current and finite bias on the system and is valid for arbitrary intra-adsorbate electron correlation strength. It is thus applicable to the recent STM experiments on Kondo impurities.
The low-energy electronic properties of one-dimensional nanowires formed by Pt atoms on Ge(001) are studied with scanning tunneling microscopy down to the millivolt-regime. The chain structure exhibits various dimerized elements at high tunneling bias, indicative of a substrate bonding origin rather than a charge density wave. Unexpectedly, this dimerization becomes vanishingly small when imaging energy windows close to the Fermi level with adequately low tunneling currents. Evenly spaced nanowire atoms emerge which are found to represent conduction states. Implications for the metallicity of …
We examine theoretically the signatures of magnetic adatoms in graphene probed by scanning tunneling spectroscopy (STS). When the adatom hybridizes equally with the two graphene sublattices, the broadening of the local adatom level is anomalous and can scale with the cube of the energy. In contrast to ordinary metal surfaces, the adatom local moment can be suppressed by the proximity of the probing scanning tip. We propose that the dependence of the tunneling conductance on the distance between the tip and the adatom can provide a clear signature for the presence of local magnetic moments. We …
Motivated by recent observations of charge order in the pseudogap regime of hole-doped cuprates, we show that {\it crisscrossed} stripe order can be stabilized by coherent, momentum-dependent interlayer tunneling, which is known to be present in several cuprate materials. We further describe how subtle variations in the couplings between layers can lead to a variety of stripe ordering arrangements, and discuss the implications of our results for recent experiments in underdoped cuprates.
Thin films of La0.7Sr0.3MnO3 on MgO show a metal insulator transition and colossal magnetoresistance. The shape of this transition can be explained by intrinsic spatial inhomogeneities, which give rise to a domain structure of conducting and insulating domains at the submicrometer scale. These domains then undergo a percolation transition. The tunneling conductance and tunneling gap measured by scanning tunneling spectroscopy were used to distinguish and visualize these domains.
Using a large-N theory in combination with the Keldysh non-equilibrium Greens function formalism, we investigate the current, differential conductance, zero-frequency shot-noise and Fano factor as measured by scanning tunneling shot noise spectroscopy (STSNS) using a scanning tunneling microscope (STM) near single Kondo impurities and in Kondo lattices. We show that the Fano factor $F$ exhibits a characteristic bias dependence arising from Kondo screening that is similar to the Kondo resonance observed in the differential conductance. Moreover, the lineshape of $F$ is strongly dependent on the…
We investigate the role of quantum coherence in tunneling conductance, where quantum criticality turns out to suppress Fano resonance. Based on the nonequilibrium noncrossing approximation, we show that the linear tunneling conductance exhibits weak Fano line-shape with sharp cusp at zero energy in the multichannel Kondo effect, resulting from incoherence associated with quantum criticality of impurity dynamics. In particular, shift of the peak position in the Fano resonance is predicted not to occur for the multichannel Kondo effect, distinguished from the Fermi liquid theory in the single ch…
We describe scanning tunneling spectroscopic signatures of Majorana zero modes (MZMs) in Kitaev spin liquids. The tunnel conductance is determined by the dynamical spin correlations of the spin liquid, which we compute exactly, and by spin-anisotropic cotunneling form factors. Near a $\mathbb{Z}_2$ vortex, the tunnel conductance has a staircase voltage dependence, where conductance steps arise from MZMs and (at higher voltages) from additional vortex configurations. By scanning the probe tip position, one can detect the vortex locations. Our analysis suggests that topological magnon bound stat…
We report experimental observation of Kondo-Fano resonant tunneling in Si-doped GaAs/AlAs multiple quantum wells. The spectrum of differential tunneling conductance at low bias shows a strong temperature dependent resonance, whose peak is split in the presence of a magnetic field, characteristic of a Kondo resonance. The data is well explained as a resonant tunneling of conduction electrons at the Fermi level through the doped silicon impurity states inside the wells by using an impurity Anderson model. The Coulomb blockade resonance of the impurity states induces a Fano dip above the Kondo re…
We report a quantitative low-temperature Scanning Tunneling Spectroscopy (STS)study on the Ag(111) surface state over an unprecedented range of currents (50 pA to 6 $μ$A)through which we can tune the electric field in the tunnel junction of the microscope. We show that in STS a sizeable Stark effect causes a shift of the surface state binding energy $E_{0}$.Data taken are reproduced by a one-dimensional potential model calculation, and are found to yield a Stark-free energy $E_{0}$ in agreement with recent state-of-the-art Photoemission Spectroscopy measurements.
The quest to understand correlated electronic systems has pushed the frontiers of experimental measurements toward the development of new experimental techniques and methodologies. Here we use a novel home-built uniaxial-strain device integrated into our variable temperature scanning tunneling microscope that enables us to controllably manipulate in-plane uniaxial strain in samples and probe their electronic response at the atomic scale. Using scanning tunneling microscopy with spin-polarization techniques, we visualize antiferromagnetic domains and their atomic structure in Fe1+yTe samples, t…
We compute the tunneling conductance of graphene as measured by a scanning tunneling microscope (STM) with a normal/superconducting tip. We demonstrate that for undoped graphene with zero Fermi energy, the first derivative of the tunneling conductance with respect to the applied voltage is proportional to the density of states of the STM tip. We also show that the shape of the STM spectra for graphene doped with impurities depends qualitatively on the position of the impurity atom in the graphene matrix and relate this unconventional phenomenon to the pseudopsin symmetry of the Dirac quasipart…
We report the magnetotransport characteristics of a trilayer ferromagnetic tunnel junction build of an electron doped manganite (La_0.7Ce_0.3MnO_3) and a hole doped manganite (La_0.7Ca_0.3MnO_3). At low temperatures the junction exhibits a large positive tunneling magnetoresistance (TMR), irrespective of the bias voltage. At intermediate temperatures below T_C the sign of the TMR is dependent on the bias voltage across the junction. The magnetoresistive characteristics of the junction strongly suggest that La_0.7Ce_0.3MnO_3 is a minority spin carrier ferromagnet with a high degree of spin pola…
We present a scanning tunneling spectroscopy (STS) study of the local electronic structure of single and bilayer graphene grown epitaxially on a SiC(0001) surface. Low voltage topographic images reveal fine, atomic-scale carbon networks, whereas higher bias images are dominated by emergent spatially inhomogeneous large-scale structure similar to a carbon-rich reconstruction of SiC(0001). STS spectroscopy shows a ~100meV gap-like feature around zero bias for both monolayer and bilayer graphene/SiC, as well as significant spatial inhomogeneity in electronic structure above the gap edge. Nanoscal…
Au-induced atomic wires on the Ge(001) surface were recently claimed to be an ideal 1D metal and their tunneling spectra were analyzed as the manifestation of a Tomonaga-Luttinger liquid (TLL) state. We reinvestigate this system for atomically well-ordered areas of the surface with high resolution scanning tunneling microscopy and spectroscopy (STS). The local density-of-states maps do not provide any evidence of a metallic 1D electron channel along the wires. Moreover, the atomically resolved tunneling spectra near the Fermi energy are dominated by local density-of-states features, deviating …
Scanning tunneling microscopes (STM) are used extensively for studying and manipulating matter at the atomic scale. In spite of the critical role of the STM tip, the control of the atomic-scale shape of STM tips remains a poorly solved problem. Here, we present a method for preparing tips {\it in-situ} and for ensuring the crystalline structure and reproducibly preparing tip structure up to the second atomic layer. We demonstrate a controlled evolution of such tips starting from undefined tip shapes.
We present an extension of the tunneling theory for scanning tunneling microcopy (STM) to include different types of vibrational-electronic couplings responsible for inelastic contributions to the tunnel current in the strong-coupling limit. It allows for a better understanding of more complex scanning tunneling spectra of molecules on a metallic substrate in separating elastic and inelastic contributions. The starting point is the exact solution of the spectral functions for the electronic active local orbitals in the absence of the STM tip. This includes electron-phonon coupling in the coupl…
The ferromagnetic phase in La0.7Sr0.3MnO3 manganite is completely spin polarized (100%) at room temperature. This is demonstrated on high quality epitaxial films by Spin-Polarized Scanning Tunneling Microscopy. The films consist of a highly homogeneous ferromagnetic (FM) phase in which minority paramagnetic (PM) defects are embedded (less than 1% for best films). PM defects exhibit featureless tunneling spectroscopic characteristics and an insulating-like conductance. FM phase exhibit metallic behavior and strongly nonlinear characteristics. Deconvolution of the spectroscopic curves for the FM…
Charge density waves (CDW) have been studied at the surface of a cleaved TTF-TCNQ single crystal using a low temperature scanning tunneling microscope (STM) under ultra high vacuum (UHV) conditions. All CDW phase transitions of TTF-TCNQ have been identified. The measurement of the modulation wave vector along the a direction provides the first evidence for the existence of domains comprising single plane wave modulated structures in the temperature regime where the transverse wave vector of the CDW is temperature dependent, as hinted by the theory more than 20 years ago.
A comprehensive theory is presented for the voltage, temperature, and spatial dependence of the tunneling current between a scanning tunneling microscope (STM) tip and a metallic surface with an individual magnetic adatom. Modeling the adatom by a nondegenerate Anderson impurity, a general expression is derived for a weak tunneling current in terms of the dressed impurity Green function, the impurity-free surface Green function, and the tunneling matrix elements. This generalizes Fano's analysis to the interacting case. The differential-conductance lineshapes seen in recent STM experiments wit…
We investigate the scattering off dilute magnetic impurities placed on the surface of three-dimensional topological insulators. In the low-temperature limit, the impurity moments are Kondo-screened by the surface-state electrons, despite their exotic locking of spin and momentum. We determine signatures of the Kondo effect appearing in quasiparticle interference (QPI) patterns as recorded by scanning tunneling spectroscopy, taking into account the full energy dependence of the T matrix as well as the hexagonal warping of the surface Dirac cones. We identify a universal energy dependence of the…
