Gap-like Feature Research Articles

Unconventional superconductivity induced in Nb films by adsorbed chiral molecules

Motivated by recent observations of chiral-induced magnetization and spin-selective transport we studied the effect of chiral molecules on conventional BCS superconductors. By applying scanning tunneling spectroscopy, we demonstrate that the singlet-pairing s-wave order parameter of Nb is significantly altered upon adsorption of chiral polyalanine alpha-helix molecules on its surface. The tunneling spectra exhibit zero-bias conductance peaks embedded inside gaps or gap-like features, suggesting the emergence of unconventional triplet-pairing components with either d-wave or p-wave symmetry at the Nb organic–molecules interface, as corroborated by simulations. These results may open a way for realizing simple superconducting spintronics devices.

Band structure reconstruction across nematic order in high quality FeSe single crystal as revealed by optical spectroscopy study

We perform an in-plane optical spectroscopy measurement on high quality FeSe single crystals grown by a vapor transport technique. Below the structural transition at Ts ∽90K, the reflectivity spectrum clearly shows a gradual suppression around 400cm−1 and the conductivity spectrum shows a peak at higher frequency. The energy scale of this gap-like feature is comparable to the width of the band splitting observed by ARPES. The low-frequency conductivity consists of two Drude components and the overall plasma frequency is smaller than that of the FeAs based compounds, suggesting a lower carrier density or stronger correlation effect. The plasma frequency becomes even smaller below Ts which agrees with the very small Fermi energy estimated by other experiments. Similar to iron pnictides, a clear temperature-induced spectral weight transfer is observed for FeSe, being indicative of strong correlation effect.

Possible superconducting fluctuation and pseudogap state aboveTcinCsFe2As2

Resistive, magnetization, torque, specific heat and scanning tunneling microscopy measurements are carried out on the hole heavily doped CsFe$_2$As$_2$ single crystals. A characteristic temperature $T^*\sim13$ K, which is several times higher than the superconducting transition temperature $T_c=2.15$ K, is observed and possibly related to the superconducting fluctuation or the pseudogap state. A diamagnetic signal detected by torque measurements starts from the superconducting state, keeps finite and vanishes gradually until a temperature near $T^*$. Temperature dependent resistivity and specific heat also show kinks near $T^*$. An asymmetric gap-like feature with the energy of 8.4 meV and a symmetric superconducting related gap of 2.2 meV on the scanning tunneling spectra are detected, and these pseudogap-related features disappear at temperatures up to at least 9 K. These observations by different experimental tools suggest the possible existence of superconducting fluctuation or pseudogap state in the temperature range up to 4 - 6 times of $T_c$ in CsFe$_2$As$_2$.

Resonant plasmon-axion excitations induced by charge density wave order in a Weyl semimetal

We investigate the charge excitations of a Weyl semimetal in the axionic charge density wave (axionic CDW) state. While it has been shown that the topological response (anomalous Hall conductivity) is protected against the CDW state, we find that the long wavelength plasmon excitation is radically influenced by the dynamics of the CDW order parameter. In the normal state, we show that an undamped collective mode should exist at q~Q_(CDW) if there is an attractive interaction favoring the formation of the CDW state. The undamped nature of this collective mode is attributed to a gap-like feature in the particle-hole continuum at q~Q_(CDW) due to the chirality of the Weyl nodes, which is not seen in other materials with CDW instability. In the CDW state, the long wavelength plasmon excitations become more dispersive due to the additional interband scattering not allowed in the normal state. Moreover, because the translational symmetry is spontaneously broken, Umklapp scattering, the process conserving the total momentum only up to nQ_(CDW) with n an integer and Q_(CDW) the ordering wave vector, emerges in the CDW state. We find that the plasmon excitation couples to the phonon mode of the CDW order via the Umklapp scattering, leading to two branches of resonant collective modes observable in the density-density correlation function at q~0 and q~Q_(CDW). Based on our analysis, we propose that measuring these resonant plasmon-axion excitations around q~0 and q~Q_(CDW) by the momentum-resolved electron energy loss spectroscopy (M-EELS) could serve as a reliable way to detect the axionic CDW state in Weyl semimetals.

Interband optical conductivity of the [001]-oriented Dirac semimetalCd3As2

We measured the optical reflectivity of [001]-oriented $n$-doped Cd$_{3}$As$_{2}$ in a broad frequency range (50 - 22000 cm$^{-1}$) for temperatures from 10 to 300 K. The optical conductivity, $\sigma(\omega) = \sigma_{1}(\omega) + {\rm i}\sigma_{2}(\omega)$, is isotropic within the (001) plane; its real part follows a power law, $\sigma_{1}(\omega) \propto \omega^{1.65}$, in a large interval from 2000 to 8000 cm$^{-1}$. This behavior is caused by interband transitions between two Dirac bands, which are effectively described by a sublinear dispersion relation, $E(k) \propto \lvert k \rvert ^{0.6}$. The momentum-averaged Fermi velocity of the carriers in these bands is energy dependent and ranges from $1.2 \times 10^{5}$ to $3 \times 10^{5}$ m/s, depending on the distance from the Dirac points. We detect a gaplike feature in $\sigma_{1}(\omega)$ and associate it with the Fermi level positioned around $100$ meV above the Dirac points.

D-wave bond-order charge excitations in electron-doped cuprates

We study charge excitation spectra in the two-dimensional model on a square lattice to explore a charge-order tendency recently found in electron-doped cuprates around the carrier density 0.15. The static susceptibility of d-wave charge density, which corresponds to the nematic susceptibility at the momentum transfer , shows two characteristic peaks at momenta of the form and . These two peaks originate from the so-called 2kF scattering processes enhanced by the d-wave character of the bond-charge density. The peak at is much broader, but develops to be very sharp in the vicinity of its instability, whereas the peak at becomes sharper with decreasing temperature, but does not diverge. The equal-time correlation function, which is measured by resonant x-ray scattering, exhibits a momentum dependence similar to the static susceptibility. We also present energy-resolved charge excitation spectra. The spectra show a V-shaped structure around and bend back toward close-to-zero energy due to the charge-order tendency at and . The resulting spectra form gap-like features with a maximal gap at and . We discuss implications for the recent experiments in electron-doped cuprates.

Andreev spectroscopy of FeSexTe1−x

We fabricated two types of junctions on the c-axis plane of FeSe0.2Te0.8 (Fe[Se,Te], Tc∼13K) crystals. In the superconductor/oxide/superconductor heterojunctions with a Pb film as a counter electrode, we found a large conductance peak centered at 0 V, and a gap-like feature, which has a much higher energy scale than those reported as a gap energy of Fe[Se,Te]. In spite of a large energy scale, their temperature dependence approximately follows a BCS curve, indicating they come from superconductivity of Fe[Se,Te]. Since these structures can be understood as the results of Andreev bound state at the S/N boundary and the interference effect in the normal metal [1], we tried to identify the effect of a normal layer by fabricating Fe[Se,Te]/Al/Al2O3/Pb junction. Here, we could realize a clean controlled S/N interface and the tunnel junction to investigate electronic properties of the Al slab. We found similar conductance spectra as those junctions without Al layer, giving a support that the anomalous conductance spectra come from the effect of the S/N interface. As expected, the energy scale of the observed features was reduced when the thickness of the normal metal was increased. The absence of Josephson current and the existence of an Andreev bound state may be a signature of the sign-reversal paring in Fe[Se,Te].

Giant tunnel-electron injection in nitrogen-doped graphene

Scanning tunneling microscopy experiments have been performed to measure the local electron injection in nitrogen-doped graphene on $\text{SiC}(000\overline{1})$ and were successfully compared to ab initio calculations. In graphene, a gaplike feature is measured around the Fermi level due to a phonon-mediated tunneling channel. At nitrogen sites, this feature vanishes due to an increase of the elastic channel that is allowed because of symmetry breaking induced by the nitrogen atoms. A large conductance enhancement by a factor of up to 500 was measured at the Fermi level by comparing local spectroscopy at nitrogen sites and at carbon sites. Nitrogen doping can therefore be proposed as a way to improve tunnel-electron injection in graphene.

Observation of pseudogaplike feature aboveTcin LiFeAs by ultrafast optical spectroscopy

We utilize ultrafast optical spectroscopy to study the quasiparticle relaxation in stoichiometric LiFeAs crystals. According to our temperature-dependent studies, we have observed three electronic phases in LiFeAs. Below the superconducting (SC) temperature ${T}_{c}$ (=15 K), the relaxation time of quasiparticles due to the SC gaps is far longer than 50 ps in our experimental conditions. In addition to SC gaps, we have also found a gaplike feature in the SC state. This gaplike feature is evident up to 40 K, which is above the SC temperature. The quasiparticle relaxation time due to this gaplike feature is in the range of 1--2 ps. We suggest the gaplike feature in LiFeAs is induced by magnetic fluctuations.

Existence of Pseudogap at the Transition Temperature of an Electron-Doped Copper–Oxide Superconductor

We present the scanning tunnelling spectroscopy (STS) studies of the energy gap (Δ) at the Fermi surface of electron-doped high-temperature (high-Tc) cuprate superconductor Pr0.88LaCe0.12CuO4−δ, which is commonly known as PLCCO. We found that the energy gap does not vanish at the transition temperature (Tc). Moreover, it was observed that gap-like features are present at some locations of the sample at temperatures above Tc. The energy gap with magnitude is found consistent with d-wave symmetry. We also found the value of the ratio 2Δ/KBTc higher than 7, which puts electron-doped cuprates in strong coupling regime.

Nonmagnetic impurity resonance states as a test of superconducting pairing symmetry in CeCoIn5

We theoretically study the effect of a nonmagnetic impurity in heavy fermion superconductor CeCoIn$_{5}$ within a realistic band model and the T-matrix approximation approach. By considering two known possible pairing symmetry candidates $d_{x^{2}-y^{2}}$ and $d_{xy}$, we find that although both total density of states exhibit a similar V-shaped gaplike feature, only the $d_{x^{2}-y^{2}}$-wave pairing symmetry gives rise to robust intragap impurity resonance states reflected by the resonance peaks near the Fermi energy in the local density of states. These features can be readily probed by scanning tunneling microscopy experiments, and are proposed to shed light on the pairing symmetry and provide hints on the microscopic mechanism of unconventional superconductivity in the Ce-based heavy fermion superconductors.

Conductance spectrum and superconducting gap structures observed in c-axis FeSe0.46Te0.54/Au junctions

The electric transport properties of superconducting c-axis FeSe0.46Te0.54/Au (S/N) junctions, fabricated using pulsed laser deposition, have been investigated in the temperature range of 2 K to the superconducting transition temperature Tc (∼ 13.5 K) and in the presence of applied magnetic fields from 0 to 9 T. A large zero bias conductance peak has always been observed in every conductance spectrum of several batches of junctions. In addition, we have found several gap-like features. Using the extended BTK theory with the currently favored nodeless S±-wave symmetry, our conductance spectrum can be reproduced quite well in the low bias range with Δ1 = 4 meV and Δ2 = 6 meV at 2 K. However, the experimental conductance spectrum is substantially below the one calculated in the high bias range. Furthermore, there is a conductance minimum at approximately 20 meV, which may be the reason for the discrepancy.

Far-infrared optical conductivity of CeCu2Si2

We investigated the optical reflectivity of the heavy-fermion metal CeCu2Si2 in the energy range 3 meV–30 eV for temperatures between 4 and 300 K. The results for the charge dynamics indicate a behavior that is expected for the formation of a coherent heavy quasiparticle state: upon cooling the spectra of the optical conductivity indicate a narrowing of the coherent response. Below temperatures of 30 K a considerable suppression of conductivity evolves below a peak structure at 13 meV. We assign this gap-like feature to strong electron correlations due to the 4f-conduction electron hybridization.

Gap-like feature in the normal state of X(Fe1−x Cox)2As2, X = Ba, Sr and Fe1+yTe revealed by Point Contact Spectroscopy

Point contact spectroscopy reveals a gap-like feature above the magnetic and structural transition temperatures for underdoped Ba(Fe1−xCox)2As2, SrFe2As2 and Fe1+yTe. The conductance spectrum starts showing an enhancement at temperatures as high as 177 K for BaFe2As2 (TN ∼ 132 K) and 250 K for SrFe2As2 (TN ∼ 192 K). Possible origins for this enhancement are discussed in light of recent experimental claims of nematicity in these materials. We construct a modified phase diagram for Co-doped Ba122 showing a gap-like feature existing above TN and TS for the underdoped regime.

Fermi surface nesting and spin density wave instability in the overdoped superconducting iron pnictides

The nesting of electron Fermi pocket with one of the two hole pockets around the Brillouin zone center has been attributed to the spin density wave (SDW) instability in the parent compound of superconducting iron pnictides. We propose here that the second hole Fermi pocket may be nested with the electron pocket in the doped case, which results in a new SDW instability. Our work is motivated by and may explain the recent scanning tunneling spectroscopy (STM) measurements on NaFe1−xCoxAs, which show an asymmetric gap-like feature near the Fermi level in the overdoped regime (Zhou X. et al., Phys. Rev. Lett., 109 (2012) 037002). We use a multi-band model to examine this feature within random phase approximation to include the coupling between the itinerant electron and the local spins.

Observation of a two-dimensional electron gas at the surface of annealed SrTiO3single crystals by scanning tunneling spectroscopy

An extensive surface characterization of hydrofluoric acid (HF) etched and annealed SrTiO${}_{3}$ single crystals, vacuum-annealed below 300 ${}^{\ensuremath{\circ}}$C, reveals the formation of a two-dimensional electron gas (2DEG). A joint scanning tunneling spectroscopy and low-energy electron diffraction analysis allows us to associate the surface metallic state (characterized by the presence of a nonzero density of states close to the Fermi level) with the low-temperature-annealed highly ordered $1\ifmmode\times\else\texttimes\fi{}1$ reconstructed SrTiO${}_{3}$ surface hosting two-dimensional carriers. Meanwhile, a gap opens in the tunneling spectrum of $2\ifmmode\times\else\texttimes\fi{}1$ reconstructed, high-temperature-annealed surfaces. X-ray photoemission spectroscopy shows that the metallic state is associated with the surface formation of Ti${}^{3+}$. Recently published photoemission data demonstrated the formation of a 2DEG on the surface of cleaved SrTiO${}_{3}$, while scanning tunneling spectroscopy on crystals heated at high temperature revealed gaplike features: Our results can help reconcile this seemingly contradicting phenomenology observed so far by scanning tunneling spectroscopy and photoemission spectroscopy.

Homogeneity of bilayer graphene

We present non-linear transport measurements on suspended, current annealed bilayer graphene devices. Using a multi-terminal geometry we demonstrate that devices tend to be inhomogeneous and host two different electronic phases next to each other. Both of these phases show gap-like features of different magnitude in non-linear transport at low charge carrier densities, as already observed in previous studies. Here, we investigate the magnetic field dependence and find that both features grow with increasing field, the smaller one with 0.6meV/T, the larger one with a 5–10 times higher field dependence. We attribute the larger of the two gaps to an interaction induced broken symmetry state and the smaller one to localization in the more disordered parts of the device.

Evolution from Unconventional Spin Density Wave to Superconductivity and a Pseudogaplike Phase inNaFe1−xCoxAs

We report the doping, temperature, and spatial evolutions of the electronic structure of NaFe(1-x)Co(x)As studied by scanning tunneling microscopy. In the parent state we directly observe the spin density wave gap, which exhibits unconventional features that are incompatible with simple Fermi surface nesting. The optimally doped sample has a single superconducting gap, but in the overdoped regime a novel pseudogaplike feature emerges. The pseudogaplike phase coexists with superconductivity in the ground state, persists well into the normal state, and shows strong spatial variations. The characteristics of the three distinct electronic states revealed here shed important new lights on the microscopic models for the iron-based superconductors.

Planar hybrid superconductor-normal metal-superconductor thin film junctions based on BaFe1.8Co0.2As2

To investigate the transport properties of iron based superconductors, we prepared planar hybrid superconductor-normal metal-superconductor (SNS’) thin film junctions with BaFe1.8Co0.2As2 as base electrode. As counter electrode we used a lead indium alloy, while the normal metal layer was formed by thin gold films. The junction design allows us to characterize the electrodes as well as the junction itself. Temperature dependent Andreev reflection studies of the junction show gap-like features but were also strongly influenced by the properties of the electrodes.

Electrical transport properties of nanostructured ferromagnetic perovskite oxides La0.67Ca0.33MnO3 and La0.5Sr0.5CoO3 at low temperatures (5 K ⩾ T ⩾ 0.3 K) and high magnetic field

We report a comprehensive study of the electrical and magneto-transport properties of nanocrystals of La0.67Ca0.33MnO3 (LCMO) (with size down to 15 nm) and La0.5Sr0.5CoO3 (LSCO) (with size down to 35 nm) in the temperature range 0.3–5 K and magnetic fields up to 14 T. The transport, magneto-transport and nonlinear conduction (I–V curves) were analysed using the concept of spin-polarized tunnelling in the presence of Coulomb blockade. The activation energy of transport, Δ, was used to estimate the tunnelling distances and the inverse decay length of the tunnelling wave function (χ) and the height of the tunnelling barrier (ΦB). The magneto-transport data were used to find the magnetic field dependences of these tunnelling parameters. The data taken over a large magnetic field range allowed us to separate out the magneto-resistance (MR) contributions at low temperatures arising from tunnelling into two distinct contributions. In LCMO, at low magnetic field, the transport and MR are dominated by the spin polarization, while at higher magnetic field the MR arises from the lowering of the tunnel barrier by the magnetic field, leading to an MR that does not saturate even at 14 T. In contrast, in LSCO, which does not have substantial spin polarization, the first contribution at low field is absent, while the second contribution related to barrier height persists. The idea of inter-grain tunnelling has been validated by direct measurements of the nonlinear I–V data in this temperature range, and the I–V data were found to be strongly dependent on magnetic field. We made the important observation that a gap-like feature (with magnitude ∼EC, the Coulomb charging energy) shows up in the conductance g(V ) at low bias for the systems with the smallest nanocrystal size at the lowest temperatures (T ⩽ 0.7 K). The gap closes when the magnetic field and temperature are increased.