Gap Amplitude Research Articles

Point contact spectroscopy on electron doped [formula omitted

Point contact spectroscopy has been used to address the question of the symmetry of the order parameter in the superconducting Pr1-xLaCexCuO4-y compound. We recorded a huge variety of data that were consistently interpreted by considering a d-wave symmetry of the superconducting order parameter. Despite the different features appearing in the tunneling conductance spectra, all data inferred a superconducting energy gap amplitude Δ≃3.6±0.2meV, with a conventional BCS temperature dependence and a ratio 2Δ/KBTC≃3.5±0.2. Also for the rare contacts showing a smaller gap value Δ≃2.0meV we found the same BCS ratio confirming that we are probing a sample area with reduced superconductivity (TC≃14K).

Inter-grain Josephson currents in two-gap superconductors

We microscopically study inter-grain Josephson currents in iron–pnictide superconductors. The theoretical examination predicts that the inter-grain Josephson current is significantly reduced by ±s-wave symmetry as the incoherent tunneling process becomes dominant and the density of states and the gap amplitude between two bands are identical. In addition, we suggest important points to improve the inter-grain critical current density.

Doping and temperature dependence of the superconducting energy gap in the electron-doped cuprate Pr2−xCexCuO4−δ

In hole-doped cuprate superconductors at low carrier concentrations two energy scales are identified: the superconducting energy gap and the pseudogap. The relation between these energy scales is still a puzzle. In these compounds a measurement of the energy gap is not necessarily a probe of the order parameter. In the electron-doped cuprates the pseudogap does not obscure the superconducting state. Consequently, the superconducting gap can be studied directly in a tunneling experiment. Here we show that by studying superconductor/insulator/superconductor planar tunnel junctions we are able to map the behavior of the gap amplitude for the entire (doping-temperature) phase diagram of the electron-doped cuprate superconductor Pr2-xCexCuO4-δ. The superconducting gap, Δ, shows a BCS-like temperature dependence even for extremely low carrier concentrations. Moreover, Δ follows the doping dependence of Tc. We can therefore conclude that there is a single superconducting energy scale in the electron-doped cuprates.

Surface-angle dependence of the tunneling spectroscopy in iron-based superconductors: Sign-reversing s-wave scenarios

We discuss the surface Andreev bound states in Fe-based superconductors with the use of an effective five-band model and investigate the surface-angle dependence of the tunneling spectroscopy by a quasiclassical approach for an isotropic and an anisotropic ±s-wave gap superconductivity. We show that information on the normal state is important for the Andreev bound state and its peak positions do not depend on the gap amplitude anisotropy.

Intergrain Josephson Currents in Multigap Superconductors: Microscopic Origin of Low Intergrain Critical Current and Its Recovery Potential in Iron-Pnictide Materials

We microscopically examine the intergrain Josephson current in iron-pnictide superconductors in order to solve the puzzle of why the intergrain current is much lower than the intragrain one. The theory predicts that the intergrain Josephson current is significantly reduced by the $\pm$s-wave symmetry when the incoherent tunneling becomes predominant and the density of states and the gap amplitude between two bands are identical. We find in such a situation that the temperature dependence of the intergrain Josephson current shows an anomalously flat curve over a wide temperature range. Finally, we suggest important points for increasing the intergrain current.

Anisotropy of the pairing gap of FeAs-based superconductors induced by spin fluctuations

We determine the anisotropy of the spin fluctuation induced pairing gap on the Fermi surface of the FeAs based superconductors as function of the exchange and Hund's coupling $J_{H}$. We find that for sufficiently large $J_{H}$, nearly commensurate magnetic fluctuations yield a fully gapped $s^{\pm}$-pairing state with small anisotropy of the gap amplitude on each Fermi surface sheet, but significant variations of the gap amplitude for different sheets of the Fermi surface. In particular, we obtain the large variation of the gap amplitude on different Fermi surface sheets, as seen in ARPES experiments. For smaller values of Hund's coupling incommensurate magnetic fluctuations yield an $s^{\pm}$-pairing state with line nodes. Such a state is also possible once the anisotropy of the material is reduced and three dimensional effects come into play.

Modulated vortex states in Rashba noncentrosymmetric superconductors

Vortex lattice structures to occur in Rashba noncentrosymmetric superconductors under a magnetic field parallel to the basal plane are studied by assuming the $s$-wave pairing and taking account of both the paramagnetic and orbital depairings. A high-field vortex lattice with a modulation perpendicular to the field and of Fulde-Ferrell-Larkin-Ovchinnikov type, occurring in the limit of vanishing $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\zeta}}$ (spin-orbit coupling normalized by Fermi energy), is found to be replaced with increasing $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\zeta}}$ by another vortex lattice with a modulation of the gap amplitude and induced by the absence of inversion symmetry. A correlation between a structural transition and an inflection of the ${H}_{c2}(T)$ curve is discussed in relation to ${H}_{c2}$ data of ${\text{CeRhSi}}_{3}$.

P-WAVE PAIRING IN FERMI SYSTEMS WITH UNEQUAL POPULATION NEAR FESHBACH RESONANCE

We explore p-wave pairing in a single-channel two-component Fermi system with unequal population near Feshbach resonance. Our analytical and numerical study reveal a rich superfluid (SF) ground state structure as a function of imbalance. In addition to the state Δ±1 ∝ Y1±1, a multitude of “mixed” SF states formed of linear combinations of Y1m's give global energy minimum under a phase stability condition; these states exhibit variation in energy with the relative phase between the constituent gap amplitudes. States with local energy minimum are also obtained. We provide a geometric representation of the states. A T = 0 polarization vs. p-wave coupling phase diagram is constructed across the BEC-BCS regimes. With increased polarization, the global minimum SF state may undergo a quantum phase transition to the local minimum SF state.

Field Dependence of π-Band Superconducting Gap in MgB2 Thin Films from Point-Contact Spectroscopy

We present the results of point-contact spectroscopy measurements on high-quality epitaxial MgB2 thin films with injected current along the c-axis. The temperature and field dependences of π-band properties with the field parallel to (H‖) or perpendicular to (H⊥) the c-axis are investigated in detail. When a magnetic field is applied, either parallel or perpendicular to the c-axis, the density of the quasiparticle state (DOS) of the π-band proliferates quickly with increasing field, while the gap amplitude of the π-band decreases slowly, which is different from the recent theoretical calculations, showing a field dependent competition between the interband scattering and the pair-breaking effects.

EMG Burst and Gap Comparisons in Muscles of the Upper and Lower Limbs

Portable electromyography (EMG) equipment enables muscle activity to be measured during activities of daily living. Patterns of EMG activity have been quantified through analysis of frequency and amplitude during periods of EMG quiescence (gaps) and activity (bursts). Research suggests that during daily activity in young adults upper limb muscles burst more frequently, but at lower amplitudes relative to lower limb muscles. PURPOSE: To compare EMG burst and gap activity between the biceps brachii (BB), triceps brachii (TB), vastus lateralis (VL), and biceps femoris (BF) for a typical 8 hour day. METHODS: The young and old men and women (n=15 women, n=16 men) were recreationally active, and questionnaires showed activity scores did not differ (P=0.1). Surface EMG was recorded (Biometrics, Gwent, UK) for the BB, TB, VL, and BF. Signals were sampled at 1000HZ, imported into Spike 2 Version 5 (CED, Cambridge UK) for custom script analysis. Bursts were defined as activation >2% MVE, and gaps as <1%MVE both lasting longer than 0.1s. RESULTS: Burst number was higher in the BB compared with the TB (33%Δ; P=0.011) and higher in the BF compared with the VL (39%Δ P=0.004). Burst number did not differ between upper and lower limb muscles. Burst amplitude recorded in the VL (9.13 + 3.06 %MVE) and the BF (9.45 + 2.89 %MVE) was nearly double the BB (5.48 + 1.39 %MVE) and the TB (4.85 + 0.67 %MVE). The number of gaps did not differ between upper and lower limb muscles, but mean gap duration was higher in the upper limbs (P=0.002), whereas gap amplitude was lower in the upper limb muscles (−7%Δ; P<0.05) relative to the two lower limb muscles. CONCLUSIONS: Burst activity was higher in flexor muscles compared with extensor muscles. Burst and gap number did not differ between upper and lower limb muscles, yet the lower limb muscles relative to the upper limb muscles exhibited greater burst amplitude and gap amplitude.

Nanoscale Modulation of Local Barrier Height on Bi-based Cuprate Superconductors Observed by Scanning Tunneling Microscopy/Spectroscopy

The atomically resolved local barrier height (LBH) of a Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) superconducting single crystal is investigated by using scanning tunneling microscopy/spectroscopy (STM/STS). The LBH distribution is found to be strongly related with the well-known supermodulation structure of the Bi2212 surface with a period of ∼2.7 nm. In particular, the magnitude of the LBH is anticorrelated to the Bi–Bi atomic distance that is affected by supermodulation. A comparison with simultaneously obtained conductance spectra shows that there is a small correlation between the spatial modulation of the gap amplitude (Δ) and the LBH, this indicates that the measured Δ is slightly affected by the topmost BiO surface, but dominantly reflects information on the inner CuO 2 layer, which is responsible for the superconductivity.

Kondo-lattice screening in ad-wave superconductor

We show that local moment screening in a Kondo lattice with d-wave superconducting conduction electrons is qualitatively different from the corresponding single Kondo impurity case. Despite the conduction-electron pseudogap, Kondo-lattice screening is stable if the gap amplitude obeys $\Delta <\sqrt{\tk D}$, in contrast to the single impurity condition $\Delta <\tk$ (where $\tk$ is the Kondo temperature for $\Delta = 0$ and D is the bandwidth). Our theory explains the heavy electron behavior in the d-wave superconductor Nd_{2-x}Ce_{x}CuO_{4}.

Observation of local barrier height and electronic structure on cuprate superconductor by STM/STS

The local barrier height (LBH) of the Bi2212 (Bi2Sr2CaCu2O8+δ, Tc = 93 K) superconducting single crystal is investigated by using scanning tunneling microscopy / spectroscopy (STM/STS). It is found that the atomically resolved LBH distribution is strongly related with the well-known supermodulation structure of Bi2212 surface. The magnitude of LBH is closely related to the Bi-Bi atomic distance affected by the supermodulation. The comparison with simultaneously obtained conductance spectra shows that there are few correlations between the spatial modulation of gap amplitude and LBH.

Investigations of the relationship betweenTcand the superconducting gap under magnetic and nonmagnetic impurity substitutions inYBa2Cu3O7−δ

We report electronic Raman scattering measurements on optimally doped $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ where Zn or Ni impurities have been substituted by Cu. Using Raman selection rules, we have probed the superconducting gap in the nodal and antinodal regions. We show that under impurity substitutions, the energy of the antinodal peak detected in the superconducting state is not related to the critical temperature ${T}_{c}$ and that signatures of superconductivity disappear in the nodal regions. Our experimental findings advocate in favor of gapless arcs around the nodes. The breakdown of the relationship between the antinodal gap amplitude and ${T}_{c}$ is discussed in terms of local superconducting gap and pseudogap.

Effect of Heavy Al Doping on MgB2: A Point-Contact Study of Crystals and Polycrystals

We present the results of point-contact spectroscopy (PCS) measurements in single-phase Mg1−x Al x B2 crystals and polycrystals with x up to 0.32, recently grown at ETH (Zurich) and University of Genova, respectively. The differential conductance curves of our “soft” point contacts were measured as a function of temperature from 4.2 K up to the Andreev critical temperature T , at which the normal-state conductance is recovered. The gap amplitudes, Δ σ and Δ π , were obtained through a two-band BTK fit of the normalized conductance curves. The results extend previous data to the range of heavy Al doping and show that contrary to expectations, no gap merging is observed up to x=0.32 (i.e., down to T =9 K). The analysis of the gap trends within the two-band Eliashberg theory shows that band filling is the main effect of Al doping, even though a small increase in interband scattering might be necessary to account for the experimental data in single crystals.

Local tunneling study of three-dimensional order parameter in theπband of Al-dopedMgB2single crystals

We have performed an extensive study of the temperature and magnetic field dependencies of local tunneling spectra measured by means of variable temperature scanning tunneling spectroscopy on high quality ${\mathrm{Mg}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}{\mathrm{B}}_{2}$ single crystals. Single gap conductance spectra due to $c$-axis tunneling were measured, probing different amplitudes of the three-dimensional ${\ensuremath{\Delta}}_{\ensuremath{\pi}}$ as a function of Al content (i.e., as a function of the critical temperature ${T}_{C}$). We give clear evidence that the locally measured energy gap, as well as the upper critical field, reach maximum values ${\ensuremath{\Delta}}_{\ensuremath{\pi}}=2.4\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ and ${H}_{c2}\ensuremath{\geqslant}3\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ around $x\ensuremath{\simeq}0.1$ $({T}_{C}=33\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ regardless of the monotonous fall of ${T}_{C}$ as a function of doping. Our results strongly suggest that different interband scattering occurs for different gap amplitudes.

Characterization of Muscular Rest Periods (gaps) in Long-term EMG of Older Men and Women

The ability to maintain functional independence in older adults is largely associated with the generation and control of purposeful movement. Laboratory studies indicate that with increasing age there is a decline in muscle strength and a decrease in force control. Limited studies are available on neuromuscular activity outside the laboratory in older adults, but initial data indicate that burst activity is greater in older adults compared with young adults and in women compared with men (Appl Physiol Nut Met 31 (S22), 2006). This difference in burst activity might be due to periods of silence in electromyography (EMG) activity. PURPOSE: To characterize periods of muscle silence (gaps) in young and old men and women for an 8-hour typical weekday. METHODS: EMG was recorded in the biceps brachii, triceps brachii, quadriceps femoris and hamstrings in 16 young (23 + 2.5 yrs; 8 men, 8 women) and 15 older (77 + 3.5 yrs; 8 men, 7 women) adults. A portable EMG device was utilized. Signals were pre-amplified (1000x), band pass filtered (20–450Hz), stored on a 512 MB flashcard and subsequently downloaded to Spike 2 for custom analysis. A gap was denned as an EMG interval <1% of the maximum isometric effort (MVE), and a duration of >0.1s. Total number of gaps, gap duration (sec), gap amplitude (% MVE), mean gap activity (% MVE) and gap rate (gaps/s) were assessed. RESULTS: The number of gaps recorded did not differ as a function of age (10%δ) or gender (14%δ). However, the mean duration (sec) of the gaps were ∼2 times longer in the young compared with old, and during periods of silence the amplitude and area of the gaps were also less (3-10%δ) in the young compared with the old. Gender differences were evident as mean duration of the gaps (sec) and area of the gaps (MVC*s) were longer (∼1.5 times) in men compared with women, yet gap amplitude (%MVC) was greater in women (∼0.8 times) compared with men. CONCLUSIONS: Shorter durations and higher amplitudes of gap activity during periods of EMG silence suggest that muscles of older adults are not as quiescent as muscles of young adults during periods of “rest”. As well, total gaps do not differ between genders but the duration and amplitudes which characterize a gap differ between men and women.

Point-contact spectroscopy in neutron-irradiatedMgB211

We report on recent results of point-contact spectroscopy measurements in Mg^{11}B_2 polycrystals irradiated at different neutron fluences up to 1.4 x 10^{20} cm^{-2}. The point contacts were made by putting a small drop of Ag paint -- acting as the counterelectrode -- on the cleaved surface of the samples. The gap amplitudes were extracted from the experimental conductance curves, showing Andreev-reflection features, through a two-band Blonder-Tinkham-Klapwijk fit and reported as a function of the Andreev critical temperature of the junctions, TcA. The resulting Delta_sigma(TcA) and Delta_pi(TcA) curves show a clear merging of the gaps when TcA ~ 9K that perfectly confirms the findings of specific-heat measurements in the same samples. Anomalous contacts with TcA > Tc (being Tc the bulk critical temperature) were often obtained, particularly in samples irradiated at very high fluences. Their fit gave a different dependence of Delta_pi on TcA. The possible origin of these contacts is discussed in terms of local current-induced annealing and/or nanoscale inhomogeneities observed by STM in the most irradiated samples.

Electronic specific heat of anisotropic superconductors and its doping dependence

The specific heat of electrons in p- and d-symmetry superconducting states ( C S) is comparatively studied within the BCS framework. This study is performed on a square lattice described by a generalized Hubbard model, in which correlated-hopping interactions are included in addition to the repulsive Coulomb ones. The temperature dependences of C S show a power law behavior, instead of the exponential behavior found in the s channel, and for a given gap amplitude the number of nodes in the gap enhances C S in the low temperature regime. Moreover, the normalized discontinuity of C S at the critical temperature ( T c) is smaller than the BCS analytical result of 1.43 for a wide range of electron densities. Finally, the results show that the single-particle van Hove singularity determines the location of the maximum gap, and then the position of the maximum discontinuity of the specific heat at T c.

Enhancement of electronic inhomogeneities due to out-of-plane disorder inBi2Sr2CuO6+δsuperconductors observed by scanning tunneling spectroscopy

Nanoscale electronic inhomogeneity in optimally doped single-layer ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{1.6}{L}_{0.4}\mathrm{Cu}{\mathrm{O}}_{6+\ensuremath{\delta}}$, ($L$-Bi2201, $L=\mathrm{La}$ and Gd) as well as bilayer ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ (Bi2212), superconductors has been investigated by the low-temperature scanning-tunneling-microscopy and spectroscopy. The inhomogeneous gap structures are observed above and below ${T}_{c}$ on Gd-Bi2201. Increasing the out-of-plane disorder which is controlled by mismatch of the ionic radius between Sr and Ln results in the enhancement of pseudogap region with suppressed coherence peaks in the conductance spectrum, making the gap average larger and the gap distribution wider. The degree of electronic inhomogeneity measured by the distribution width of the gap amplitude is shown to have a correlation with ${T}_{c}$ in optimally doped Bi2201 and Bi2212.