Break-junction Tunneling Research Articles

Observation of periodic structures and pseudogaps in pristine compound α-TiNCl by STM/STS and break junction methods

Scanning tunneling microscopy and spectroscopy (STM/STS) and break-junction tunnel spectroscopy (B.TTS) measurements are performed on the pristine layered α-TiNCl semiconductor (pri-TiNCl), being the precursor of superconducting nitride chlorides. The STM topography of pri-TiNCl shows basic crystal structures with the lattice periods of a0≈ 0.38 nm and b0≈ 0.31 nm, ensuring that a clean a–b surface of micro-single crystals is obtained. From the STS measurements, the averaged conductance versus voltage dependence dI/dV(V) reveals kink structures at relatively high bias voltages of |F| ∼ −50 mV and −100 mV. The spatial (r-) conductance distributions dI/dV (V, r) (dI/dV maps) proportional to those of quasiparticle local densities of states (LDOSes) exhibit the bias-independent streak structures with the period of (4.8 ± 0.2) b0, which are predominantly observed within the range of |V| < 95 mV. The temperature dependence of the dI/dV B.TTS spectra shows the gap structure with the energy scale of 4Σ = 180 meV vanishing at Ts ∼ 120 K. Thus, the ratio of the gap Σ to the transition temperature Ts is 2Σ/kBTs ∼ 10. Here, kB is the Boltzmann constant. Such a ratio is typical of the pseudogap features in cuprate superconductors and dielectric gap characteristics in layered chalcogenides with charge-density waves.

Amorphous In-Ga-Mg-O Thin Films Formed by RF Magnetron Sputtering: Optical, Electrical Properties and Thin-Film-Transistor Characteristics

We report on optical and electrical properties of amorphous In-Ga-Mg-O (a-IGMO) films and characteristics of a-IGMO channel thin-film transistors which went through the reductive post-annealing process. Optical band-gap energies of a-IGMO films were larger than that of amorphous In-Ga-Zn-O (a-IGZO) films. Carrier density and Hall mobility of a-IGMO films with the reductive post-annealing were almost the same degree as those of a-IGZO films. Although the reductive annealing with the SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> underlayer makes an a-IGZO film degenerate semiconductor and its TFT inoperative, a-IGMO TFTs successfully operated after this reductive process. Break-junction tunnelling spectroscopy which was applicable not to a-IGMO but to a-IGZO with the reductive process showed a noticeable density of state character in the vicinity of the Fermi level for a-IGZO, which is consistent with its property.

Tunneling STM/STS and break-junction spectroscopy of the Pb-doped Bi2223 superconductor

The combined scanning tunneling microscopy/spectroscopy (STM/STS) and the break-junction tunneling spectroscopy (BJTS) measurements of the three-layered Pb x Bi2-x Sr2Ca2Cu3O10+y cuprate superconductors were carried out. The averaged dI/dV spectrum obtained by the STS shows the gap Δ STS ≈ 45 meV, while that of BJTS shows Δ BJ ≈ 35 meV. In case of the BJTS measurements, we also referred the zero bias (ZB) peak as being due to the maximum Josephson current IJc . The product of IJc and normal resistance RN (IJcRN ) was obtained IJcRN ≈ 3.5 mV at T = 11 K, which is one order lower than that of the BCS-based Ambegaokar-Baratoff theory with Δ BJ =35 meV taken into account. With increasing the temperature, the IJcRN was reduced and vanished just below the Tc (≈ 106 K), indicating the bulk superconductivity. Simultaneously, from the temperature dependence of the gap features from 80 K to 120 K, the gap Δ was shown to persist across the Tc , indicating that the IJcRN product is a significant parameter for understanding the cuprate superconductivity.

Gap features of layered iron-selenium-tellurium compound below and above the superconducting transition temperature by break-junction spectroscopy combined with STS

We studied correlations between the superconducting gap features of Te-substituted FeSe observed by scanning tunnelling spectroscopy (STS) and break-junction tunnelling spectroscopy (BJTS). At bias voltages outside the superconducting gap-energy range, the broad gap structure exists, which becomes the normal-state gap above the critical temperature, Tc. Such behaviour is consistent with the model of the partially gapped density-wave superconductor involving both superconducting gaps and pseudogaps, which has been applied by us earlier to high-Tc cuprates. The similarity suggests that the parent electronic spectrum features should have much in common for these classes of materials.

Interplay between hybridization gaps and antiferromagnetic gap in the hole-doped Kondo semiconductor Ce(Os1-yRey)2Al10

The Kondo semiconductor CeOs2Al10 undergoes an antiferromagnetic (AFM) order at an unexpectedly high temperature 28.5 K. We have performed break junction tunneling measurements for the hole-doped system Ce(Os1-yRey)2Al10 (y ≤ 0.1). The tunneling spectrum dI/dV for y = 0 displays successive openings of a hybridization gap V1, an AFM gap VAF and another hybridization gap V2 in the density of states (DOS). On cooling from 36 K to TN, both the gap value V1 and the DOS at the Fermi level, EF, decrease by 8% of the values at 36 K. This fact indicates that the development of short-range magnetic correlations reduces the c-f hybridization gap. For y = 0.02, a peak appears in dI/dV at V = 0 concurrently with the disappearance of V2. With increasing y further, the in-gap states develop at EF, in good agreement with the increase in the Sommerfeld coefficient of the heat capacity. Thereby, TN, V1 and VAF decrease and disappear at y = 0.05. These facts provide compelling evidence that the presence of V1 is necessary for the AFM order in CeOs2Al10.

Hybridization gaps and antiferromagnetic gap in the Kondo semiconductorsCeT2Al10(T=Feand Os) observed by break-junction tunneling spectroscopy

${\mathrm{CeOs}}_{2}{\mathrm{Al}}_{10}$ exhibits Kondo semiconducting properties, yet orders antiferromagnetically at a rather high temperature, ${T}_{\mathrm{N}}=28.5$ K. We have performed break-junction tunneling measurements on single crystals of ${\mathrm{CeOs}}_{2}{\mathrm{Al}}_{10}$ and the nonmagnetic Kondo semiconductor ${\mathrm{CeFe}}_{2}{\mathrm{Al}}_{10}$. Upon cooling ${\mathrm{CeFe}}_{2}{\mathrm{Al}}_{10}$, two hybridization gaps successively appear in the tunneling spectra, while another gap opens in ${\mathrm{CeOs}}_{2}{\mathrm{Al}}_{10}$ below ${T}_{\mathrm{N}}$. For both compounds, the ratio of the two hybridization gap widths is approximately 4, in agreement with the hybridization gap model for the crystal-field ground state of ${\mathrm{Ce}}^{3+}$ with $|{J}_{z}\ensuremath{\rangle}=|\ifmmode\pm\else\textpm\fi{}3/2\ensuremath{\rangle}$. Furthermore, we found that the gap widths are well scaled by the Kondo temperature among Ce-based Kondo semimetals/semiconductors with orthorhombic structures.

Tunneling STM/STS and break-junction spectroscopy of the layered nitro-chloride superconductors MNCl (M = Ti, Hf, Zr)

The layered superconductors β-MNCl with the critical temperatures Tc = 14 K (M = Zr) − 25 K (M = Hf) were investigated by means of scanning-tunneling microscopy/spectroscopy and break-junction tunneling spectroscopy. The STM/STS was used to investigate the surface electronic structures in nanometer length scale, while the BJTS was employed to precisely determine the gap characteristics. Both techniques consistently clarified the unusually large size of the superconducting gap. Wide gap distributions with large-scale maximum gap values were also revealed in α-KyTiNCl with a different crystal structure.

Tunneling spectroscopy of layered superconductors: intercalated Li0.48(C4H8O)xHfNCl and De-intercalated HfNCl0.7

Tunneling measurements have been carried out on layered superconductors of the β(SmSI)-type -L i0.48(THF)xHfNCl (THF = C4H8O) and HfNCl0.7 - by means of break-junction and scanning tunneling spectroscopy. Break-junction technique reveals Bardeen-Cooper-Schrieffer (BCS) - like gap structures with typical gap values of 2Δ(4. 2K ) =1 1 −12 meV for Li0.48(THF)xHfNCl with the highest Tc = 25.5 K. Some of our measurements revealed multiple gaps and dip-hump structures, the largest gap 2Δ (4.2 K) ≈ 17-20 meV closing at Tc. This was shown both by break-junction and scanning-tunneling spectroscopy. From these experiments it stems that the highest obtained gap ratio 2Δ/kBTc ∼ 8 substantially exceeds the BCS weak-coupling limiting values: ≈3.5 and ≈4.3 for s-wave and d-wave order parameter symmetry, respectively. Such large 2Δ/kBTc ratios are rather unusual for conventional superconductors but quite common to high-Tc cuprates, as well as to organic superconductors. Our studies allowed to collect much more evidence concerning the huge pairing energy in those materials and to investigate in detail the complexity of their superconducting gap spectra. An origin of the observed phenomena still remains to be clarified.

Semiconducting gap of Nd1.85Ce0.15CuO4 revealed by break-junction tunnelling spectroscopy

The quasiparticle energy spectrum of the semiconducting Nd1.85Ce0.15CuO4 was probed by a break-junction technique. The tunnelling conductance exhibits well-defined gap-edge peaks at 4 K with peak-to-peak bias separations 2–2.8 eV, showing the V-shaped gap feature. The gap value 2Δ (4K) = 1–1.4 eV is inferred in view of the very break-junction nature. As temperature, T, increases, the gap gradually disappears to merge with the background at T ∼ 200 K. Possible origins of the gap are discussed on the basis of the break-junction and complementary data obtained using scanning-tunnelling spectroscopy.

Microwave enhancement of superconductivity inBi2Sr2CaCu2O8+δbreak junctions

Superconductor-insulator-superconductor (SIS) tunnel junctions have been used to investigate the enhancement of superconductivity in a single-crystal Bi2212 high-${T}_{c}$ superconductor in the presence of gigahertz-microwave radiation. When the microwave power is increased at constant temperature, the break-junction tunneling spectra show that the energy gap increases several times.

On the relations among the pseudogap, electronic charge order and Fermi-arc superconductivity in Bi2Sr2CaCu2O8+δ

On the basis of STM/STS, break-junction tunneling and electronic Raman scattering experiments on Bi2Sr2CaCu2O8+δ reported so far, we suggest that the static, electronic charge order is associated with inhomogeneous electronic states on antinodal parts of the Fermi surface that are outside the Fermi-arc around the node and responsible for the pseudogap, and coexists with the homogeneous superconductivity caused by the pairing of coherent quasiparticles on the Fermi-arc, the so-called ‘Fermi-arc superconductivity’, in the real space, although the two electronic orders or the corresponding energy gaps compete with each other in the k-space.

Superconducting energy gap from break-junction tunneling spectroscopy in the ternary silicide CaAlSi

Tunneling spectroscopy using a break-junction technique has been conducted to determine the superconducting gap structure of the ternary silicide-layered superconductor CaAlSi with and without superstructure. We find that the tunneling conductance in CaAlSi shows a single gap feature, which can be described by the BCS density of states. The gap size $\ensuremath{\Delta}$ obtained from the tunneling conductance in CaAlSi without superstructure is estimated to be approximately $1.0\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, corresponding to a weak-coupling gap ratio. On the other hand, for a specimen with superstructure, the largest gap, $\ensuremath{\Delta}=1.5\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, significantly deviates from the BCS value, indicating strong-coupling superconductivity. These results demonstrate systematic variations in the pairing interaction with superstructure formations, which are consistent with heat capacity measurements.

Evidence of the Fine Pseudogap at the Fermi Level in Al-based Quasicrystals

Theoretical studies have predicted that quasicrystals (QCs) have conspicuously fine electronic structures near the Fermi level ( E F ); the density of states (DOS) consists of sharp peaks and deep valleys with several 10 meV intervals. These fine electronic structures are believed to explain the unusual sensitivity of the electrical conductivity to slight changes in the chemical composition of QCs. We have investigated the DOS around the E F on single grains of icosahedral ( i -)AlCuFe, i -AlPdMn, and decagonal ( d -)AlCuCo QCs by break-junction tunneling spectroscopy at 4.2 K. Well-defined and reproducible pseudogaps at the E F are observed for these alloys. This is a direct experimental result that confirms the fine electronic structure existing in the vicinity of the E F .

Electron tunneling experiments on La-substituted Kondo-semiconductor CeRhAs

Polycrystalline Ce 1− x La x RhAs is investigated by means of break-junction tunneling. On Ce substituted by La ( x = 0.01 ), a pronounced hump structure is developed at the bias ±0.25 V with a shallow dip on it. The hump emergence is consistent with a drastic reduction in the resistivity. These facts give direct evidence for the appearance of mid-gap states near the Fermi energy by a small La substitution for Ce in CeRhAs.

Tunneling measurements of CeRhAs single crystal

Kondo-semiconducting gap of CeRhAs single crystal has been measured by break-junction tunneling. The observed largest gap at 4.2 K is 2Δ∼0.5 eV, while smaller sizes below 0.3–0.4 eV are also reproducible. We present the temperature evolution of the gap and discuss the gap size in comparison with the other Kondo semiconductors.

Tunneling Spectroscopy of Deintercalated Layered Nitride Superconductor ZrNCl0.7

Break junction tunneling spectroscopy measurements have been carried out on the novel layered nitride superconductor ZrNCl 1- x (\(x\cong 0.3\)) with T c =12–14 K. Two kinds of gap structures are reproducibly observed, both of which can be described by the BCS density of states. Values of the gap parameters at 4 K are obtained to be Δ=1.6–2.3 meV and 4.0–5.1 meV. Upon warming, these gap structures disappear at T c , thereby giving the ratio 2Δ/ k B T c =2.7–3.8 and 6.6–8.4, respectively. The former is within the value of conventional superconductors, while the latter is extremely large in comparison with that of the conventional strong-coupling superconductors. The larger ratio is similar to the value of high- T c cuprates. The origins of such two kinds of gaps are discussed.

Break-junction tunneling spectra of MgB 2: Influence of boron quality

Tunneling spectra of a superconductor–insulator–superconductor (SIS) junction formed by the break-junction technique have been carried out for two different boron quality MgB 2 samples. One is a polycrystalline MgB 2 pellet (RRR ≈ 4) and the other is a high purity MgB 2 wire (RRR ≈ 25) of approximately 150 μm in diameter. Both samples exhibit a multiple-gap feature, which can be expressed by the correlated two-gap model. From the conductance fitting at 4.2 K, the pellet gap parameters are Δ S = 2.2 ± 0.3 meV, Δ M = 6.0 ± 1.5 meV and Δ L = 10 meV, whereas the only clear and very reproducible gap in the wires is the small gap at Δ S = 2.5 ± 0.1 meV. The wires show almost constant values for the fitting parameters of the two-gap model, in contrast with the pellets that show a large variation.

Spatially heterogeneous character of superconductivity in MgB 2 as revealed by local probe and bulk measurements

Controversial tunnel and point-contact experimental data on superconducting gap values in MgB 2 are analyzed and interpreted in terms of the spatial inhomogeneity and the proximity effect appropriate to this layered compound. Existing specific heat measurements are explained in the same way. Break-junction tunnel studies of polycrystalline MgB 2 samples are carried out and a clear-cut multiplicity of gaps is observed, although the shapes of the I– V characteristics vary substantially.

Break-junction tunneling spectroscopy for doped semiconductors in the hopping regime

We present a theory for tunneling spectroscopy in a break-junction semiconductor device for materials in which the electronic conduction mechanism is hopping transport. Starting from the conventional expression for the hopping current we develop an expression for the break-junction tunnel current for the case in which the tunnel resistance is much larger than the effective single-hop resistances. We argue that percolationlike methods are inadequate for this case and discuss in detail the interplay of the relevant scales that control the possibility to extract spectroscopic information from the characteristic of the device.

Multiple-Gap Features from Break-Junction Tunneling in the Superconducting MgB2

Tunneling spectroscopy measurements on polycrystalline MgB 2 have been carried out with a break-junction technique. The observed spectra exhibit the multiple-gap feature, which can be expressed by the correlated two-gap model. From the conductance fittings at 4.2 K, the gap parameters can be classified into Δ S =2.2±0.3 meV, Δ M =6.0±1.5 meV, and \(\Delta_{\text{L}}\cong 10\) meV. All these gaps exist up to the same critical temperature T c . The Δ M undergoes various ways of temperature dependence, which can be attributed to the proximity-induced gap. We have also observed the conductances showing the features of normal conducting and/or low- T c phases. Our tunneling data can be explained by the proximity effect combined with the bulk multiple-gap features.