Gap-like Feature Research Articles

Evolution of the pseudogap in a polarized Fermi gas

We calculate the single-particle spectral density of a normal (nonsuperfluid) two-component gas of fermions in the BCS-BEC crossover within a $T$-matrix approximation. We review how noncondensed pairs lead to a spectral density reminiscent of the ordered state, and explore how a gaplike feature in the spectrum evolves as one changes the polarization of the gas. As the gas is polarized, we find that this pseudogap becomes more diffuse and moves away from the Fermi level, reflecting the fact that fewer pairs are present but that they still play an important role in the excitations.

Hybridization Wave as the “Hidden Order” inURu2Si2

A phenomenological model for the "hidden order" transition in the heavy-Fermion material URu(2)Si(2) is introduced. The hidden order is identified as an incommensurate, momentum-carrying hybridization between the light hole band and the heavy electron band. This modulated hybridization appears after a Fano hybridization at higher temperatures takes place. We focus on the hybridization wave as the order parameter in URu(2)Si(2) and possibly other materials with similar band structures. The model is qualitatively consistent with numerous experimental results obtained from, e.g., neutron scattering and scanning tunneling microscopy. Specifically, we find a gaplike feature in the density of states and the appearance of features at an incommensurate vector Q(*)∼0.6π/a(0). Finally, the model allows us to make various predictions which are amenable to current experiments.

Dual character of magnetism inEuFe2As2: Optical spectroscopic and density-functional calculation study

We investigate the electronic structure of ${\text{EuFe}}_{2}{\text{As}}_{2}$ using optical spectroscopy and density-functional calculations. At low temperature we observe the evolution of two gaplike features, one having a weak-coupling mean-field behavior and another with strongly nonmean-field behavior. Using band-structure calculations, we identify the former with a spin-Peierls-type partial gap in ${d}_{yz}$ bands and the latter with the transition across the large exchange gap in ${d}_{xz}/{d}_{xy}$ bands. Our results demonstrate that the antiferromagnetism in the ferropnictides is neither fully local nor fully itinerant but contains elements of both.

Study of the pairing symmetry in the electron-doped cuprate [formula omitted] by tunneling spectroscopy

We performed Point Contact Andreev Reflection Spectroscopy experiments on Pr 1 - x LaCe x CuO 4 - y crystals. The variety of the measured spectra are all explained in terms of a modified BTK model considering a d-wave symmetry of the superconducting order parameter. We give an estimation of the energy gap Δ = ( 3.5 ± 0.2 ) meV and of the ratio 2 Δ / k B T C ≃ 3.6 ± 0.3 . We measured the temperature evolution of the dI / dV vs. V characteristics and we extracted a conventional BCS temperature dependence of the superconducting energy gap. Finally, in order to explain the conductance spectra showing higher voltage gap-like features we consider a model in which the formation of a Josephson junction in series with the Point Contact junction is realized due to the tip pressure.

A tunneling spectroscopy study of the pairing symmetry in the electron-dopedPr1−xLaCexCuO4−y

We have performed scanning tunneling spectroscopy and point contactspectroscopy measurements on the electron-doped superconductorPr1−xLaCexCuO4−y (x = 0.12, K). We address the question of the symmetry of the order parameter andof the amplitude of the energy gap. We compare three possible scenarios,i.e. isotropic s-wave, ‘anisotropic’ s-wave, and d-wave. Evidence for a d-wavesymmetry of the order parameter is given. From the temperature evolution of thedI/dV versusV characteristics we extract a BCS-like temperature dependence of the superconducting energy gapΔ. Despitethe variety of measured spectra we give a consistent explanation for the whole set of data, indicatingΔ = (3.6 ± 0.2) meV and a ratio . In particular, point contact characteristics showing gap-like features at higher voltageshave been interpreted by considering the formation of an intergrain Josephson junction inseries with the point contact junction. Further confirmation of the correctness of themodel is given by the behavior of the critical current of the intergrain Josephsonjunction versus temperature which follows the Ambegaokar–Baratoff behavior.

Impurity effect on superconducting gap and pseudogap

Co substituted Bi2212 samples of Bi 2.1Sr 1.9Ca(Cu 1−xCo x)O 8+ δ ( x = 0, 0.01, 0.02, 0.04, 0.06, and Tc = 85,71,52,35 K) were prepared, and their electronic density states were characterized by a low temperature STM. The experimental results revealed that superconducting gap-like features were rapidly destroyed, while pseudogap-like features were enhanced with increasing impurities, suggesting that the origin of the pseudogap is different from that of a superconducting gap. We suggested a two phase-like model to explain the behavior of a gap map and a gap distribution function. On the other hand, spatial gap and pseudogap inhomogeneity persisted in all Co concentrations. This indicates that the origin of inhomogeneity is not related to in-plane disorder or impurities.

Evidence for induced magnetization in superconductor-ferromagnet heterostructures: A scanning tunneling spectroscopy study

We performed scanning tunneling spectroscopy of $c$-axis-oriented ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ films on top of which ferromagnetic ${\text{SrRuO}}_{3}$ islands were grown epitaxially in situ. When measured on the ferromagnetic islands, the density of states exhibits small gaplike features consistent with the expected short-range penetration of the order parameter into the ferromagnet. However, anomalous split-gap structures are measured on the superconductor in the vicinity of ferromagnetic islands. This observation provides evidence for the recently predicted induced magnetization in the superconductor side of a superconductor/ferromagnet junction and demonstrates its manifestation in the density of states of the superconductor. Surprisingly, the length scale of the effect inside the superconductor was found to be an order of magnitude larger than the superconducting coherence length. This is inconsistent with the theoretical prediction of a penetration depth of only a few superconducting coherence lengths. We discuss a possible origin for this discrepancy.

Investigation of superconducting gap structure in TbFeAsO0.9F0.1 using point contact Andreev reflection

Bulk samples of TbFeAsO0.9F0.1 (Tcon = 50 K) were measured by point contact Andreev reflection spectroscopy. The spectra show unambiguous evidence for multiple gap-like features plus the presence of high-bias shoulders. By measuring the spectra as a function of temperature with both gold and superconducting niobium tips, we establish that the gap-like features are associated with the superconducting order parameter in this material. We discuss whether the well-defined zero-bias conductance peak that we observe infrequently is associated with a nodal superconducting order parameter.

Giant phonon-induced conductance in scanning tunnelling spectroscopy of gate-tunable graphene

Scanning tunnelling spectra of a graphene field-effect transistor reveal an unexpected tenfold increase in conductance as a result of phonon-mediated inelastic tunnelling. The honeycomb lattice of graphene is a unique two-dimensional system where the quantum mechanics of electrons is equivalent to that of relativistic Dirac fermions1,2. Novel nanometre-scale behaviour in this material, including electronic scattering3,4, spin-based phenomena5 and collective excitations6, is predicted to be sensitive to charge-carrier density. To probe local, carrier-density-dependent properties in graphene, we have carried out atomically resolved scanning tunnelling spectroscopy measurements on mechanically cleaved graphene flake devices equipped with tunable back-gate electrodes. We observe an unexpected gap-like feature in the graphene tunnelling spectrum that remains pinned to the Fermi level (EF) regardless of graphene electron density. This gap is found to arise from a suppression of electronic tunnelling to graphene states near EF and a simultaneous giant enhancement of electronic tunnelling at higher energies due to a phonon-mediated inelastic channel. Phonons thus act as a ‘floodgate’ that controls the flow of tunnelling electrons in graphene. This work reveals important new tunnelling processes in gate-tunable graphitic layers.

Scanning tunneling spectroscopy of inhomogeneous electronic structure in monolayer and bilayer graphene on SiC

The authors 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 an ∼100meV gaplike feature around zero bias for both monolayer and bilayer graphene/SiC, as well as significant spatial inhomogeneity in electronic structure above the gap edge. Nanoscale structure at the SiC/graphene interface is seen to correlate with observed electronic spatial inhomogeneity. These results are relevant for potential devices involving electronic transport or tunneling in graphene/SiC.

Investigation of the tunneling spectra in HgBr 2-intercalated Bi-2212 single crystals below and above Tc

Interlayer tunneling spectroscopy measurements were performed on mesa arrays of Bi-2212 single crystals, intercalated with HgBr 2. Tunneling conductances were obtained over a wide temperature range to examine the spectral features, especially the behavior of the quasiparticle peaks corresponding to superconducting energy gaps (SGs). Experimental spectra showed that gap-like features are still present even for the temperatures far above the transition temperature, T c. This evidence is consistent with the idea that the SG evolves into a pseudogap above T c for HgBr 2-intercalated Bi-2212 single crystals.

Single intrinsic Josephson junction with double-sided fabrication technique

We make stacks of intrinsic Josephson junctions (IJJs) embedded in the bulk of very thin (d⩽100nm) Bi2Sr2CaCu2O8+x single crystals. By precisely controlling the etching depth during the double-sided fabrication process, the stacks can be reproducibly tailor-made to be of any microscopic height (0−9nm<d), i.e., enclosing a specified number of IJJ (0–6), including the important case of a single junction. We discuss reproducible gaplike features in the current-voltage characteristics of the samples at high bias.

Pairing state in the ruthenocuprate superconductorRuSr2GdCu2O8: A point-contact Andreev reflection spectroscopy study

The results of point contact Andreev reflection spectroscopy on polycrystalline $\mathrm{Ru}{\mathrm{Sr}}_{2}\mathrm{Gd}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8}$ pellets are presented. The wide variety of the measured spectra are all explained in terms of a modified Blonder--Tinkham--Klapwijk model considering a $d$-wave symmetry of the superconducting order parameter. Remarkably low values of the energy gap $\ensuremath{\Delta}=(2.8\ifmmode\pm\else\textpm\fi{}0.2)\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ and of the $2\ensuremath{\Delta}∕{k}_{B}{T}_{c}\ensuremath{\simeq}2$ ratio are inferred. From the temperature evolution of the $dI∕dV$ vs $V$ characteristics we extract a sublinear temperature dependence of the superconducting energy gap. The magnetic field dependence of the conductance spectra at low temperatures is also reported. From the $\ensuremath{\Delta}$ vs $H$ evolution, a critical magnetic field ${H}_{{c}_{2}}\ensuremath{\simeq}30\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ is inferred. To properly explain the curves showing gap-like features at higher voltages, we consider the formation of a Josephson junction in series with the point contact junction, as a consequence of the granularity of the sample.

Transition in the tunneling conductance ofYBa2Cu3O7−δfilms in magnetic fields up to32.4T

We studied the tunneling density of states in YBCO films under strong currents flowing along node directions. The currents were induced by fields of up to 32.4T parallel to the film surface and perpendicular to the $CuO_{2}$ planes. We observed a remarkable change in the tunneling conductance at high fields where the gap-like feature shifts discontinuously from 15meV to a lower bias of 11meV, becoming more pronounced as the field increases. The effect takes place in increasing fields around 9T and the transition back to the initial state occurs around 5T in decreasing fields. We argue that this transition is driven by surface currents induced by the applied magnetic field.

Planar tunneling spectroscopic studies of splitting vs. non-splitting of the zero-bias conductance peak in YBa 2Cu 3O 7−* thin films

Planar tunneling spectroscopy is used to probe the near-surface density of states of YBCO thin films. A gentle solution-growth insulator deposition technique allows us to detect (1) the ZBCP up to the bulk T c of the film; (2) new features and (3) orientation dependence of the gap-like feature. We propose that a narrow tunneling cone and reduced surface disorder contribute to our results.

STM/STS study on Ca2−xNaxCuO2Cl2 single crystals

To explore the evolution of electronic states from the Mott insulator to the high-Tc superconductor, we performed scanning tunneling microscopy/spectroscopy on Ca2−xNaxCuO2Cl2 single crystals which show superior cleavage. Topographic images taken by constant-current scanning at 7 K show clear atomic images superposed on nano-scale patch-like or river-like irregular corrugations. Spectroscopic measurements revealed that low areas are semiconducting and the spectrum at high area is approaching to “metallic” while gap-like feature still remains near the Fermi level. Namely, background corrugations are originated from the electronic inhomogeneity.

Doping dependence of the superconducting gap by Andreev reflection in Au/La 2− xSr xCuO 4 point-contact junctions

We studied the doping dependence of the superconducting gap in La 2− x Sr x CuO 4 (LSCO) by means of Andreev reflection measurements in Au/LSCO point-contact junctions. The Andreev reflection features were found to disappear at T c A close to the bulk T c. The fit of the conductance curves with the BTK–Tanaka–Kashiwaya model gives good results if a (s+d)-wave gap symmetry is used. The low-temperature dominant isotropic gap component Δ s follows very well the T c vs. x curve, while the gap-like features observed by angle-resolved photoemission spectroscopy and tunneling scale with T ∗. This confirms the different origin of these two energy scales at T< T c.

Dephasing effects in superconducting heterojunctions: a crossover from coherent to sequential transmission

We present first experimental and theoretical results demonstrating a crossover from mesoscopic (phase-coherent transmission) to macroscopic (two resistors in series) behavior of conductance spectra for a normal-superconducting heterostructure. Following the lines of the Büttiker's approach to normal double-barrier devices, we show that phase-randomizing events in a mesoscopic interbarrier layer together with Andreev-reflection processes drastically influence corresponding conductance vs. voltage characteristics. An anomalous temperature shift of a gap-like feature in an Ag/LaBa 2Cu 3O 7− x point contact is interpreted as an effect of the partial phase decoherence in the intermediate magnetically active region between the normal-metal tip and the bulk 123-type superconducting cuprate.

Diffraction andtransmission of light in low-refractive index Penrose-tiledphotonic quasicrystals

We report the measurements of the diffraction pattern of atwo-dimensional Penrose-tiled photonic quasicrystal, obtainedby etching air cylinders in a silica substrate, and themodelling of the light propagation and dispersion relations ofphotons inside such a structure. The calculated transmission spectra exhibit dips whose positions are insensitive to thedirection of propagation and whose depth increases withincreasing structure length. An approach is developed for thecalculation of the dispersion relations which is based on a setof reciprocal vectors defined by the diffraction pattern. Thedispersion curves exhibit gap-like features at frequenciescorresponding to the dips in the transmission spectra.

Presence of a pseudo-gap feature in the density of states of disordered W/Si alloys

One of the most striking features of the high-temperature superconducting cuprates is the presence of a gap-like structure in the density of states above the superconductive transition temperature, T c. Analysis of conductance data from point-contact tunneling measurements on disordered W/Si thin films shows the presence of similar gap-like features above T c. Details of the film growth and characterization are presented. A phase diagram which includes T c and a pseudo-gap temperature, T *, are presented.