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Two contradictory phase diagrams have dominated the literature of high-$T_c$ cuprate superconductors. Does the pseudogap line cross the superconducting $T_c$-dome or not? To answer, we have revisited the experimental magnetic susceptibility and knight shift of four different compounds, La$_{1-x}$Sr$_x$CuO$_4$, Bi$_2$Sr$_2$Ca$_{1-x}$Y$_x$Cu$_2$O$_8$, Bi$_2$Sr$_2$CaCu$_2$O$_{8+y}$, and YBa$_2$Cu$_3$O$_{6+y}$, as a function of temperature and doping. The susceptibility can be described by the same function for all materials, having a magnetic and an electronic contributions. The former is the 2D antiferromagnetic (AF) square lattice response, with a characteristic temperature of magnetic correlations $T_{max}$. The latter is the `Pauli' term, revealing the gap opening in the electronic density of states at the pseudogap temperature $T^*$. From precise fits of the data, we find that $T_{max}(p)$ decreases linearly as a function of doping ($p$) over a wide range, but saturates abruptly in the overdoped regime. Concomitantly, $T^*(p)$ is {\it linear and tangent} to the dome, either crossing or approaching $T_{max}(p)$ at the top of the dome, indicating a qualitative change of behavior from underdoped to overdoped regimes. Contrary to the idea that the pseudogap terminates just above optimal doping, our analysis suggests that the gap exists throughout the phase diagram. It is consistent with a pseudogap due to hole pairs, or `pairons', above $T_c$. We conclude that $T_{max}$, reflecting the AF magnetic correlations, has often been misinterpreted as the pseudogap temperature $T^*$., Comment: Solid State Communications (in press, February 2022)

Understanding the thermodynamic properties of high-$T_c$ cuprate superconductors is a key step to establish a satisfactory theory of these materials. The electronic specific heat is highly unconventional, distinctly non-BCS, with remarkable doping-dependent features extending well beyond $T_c$. The pairon concept, bound holes in their local antiferromagnetic environment, has successfully described the tunneling and photoemission spectra. In this article, we show that the model explains the distinctive features of the entropy and specific heat throughout the temperature-doping phase diagram. Their interpretation connects unambiguously the pseudogap, existing up to $T^*$, to the superconducting state below $T_c$. In the underdoped case, the specific heat is dominated by pairon excitations, following Bose statistics, while with increasing doping, both bosonic excitations and fermionic quasiparticles coexist.

ARPES mesurements in cuprates have given key information on the temperature and angle dependence of the gap ($d$-wave order parameter, Fermi arcs and pseudogap). We show that these features can be understood in terms of a Bose condensation of interacting {\it pairons} (preformed hole pairs which form in their local antiferromagnetic environment). Starting from the basic properties of the pairon wavefunction, we derive the corresponding k-space spectral function. The latter explains the variation of the ARPES spectra as a function of temperature and angle up to T*, the onset temperature of pairon formation. While Bose excitations dominate at the antinode, the fermion excitations dominate around the nodal direction, giving rise to the Fermi arcs at finite temperature. This dual role is the key feature distinguishing cuprate from conventional superconductivity., Comment: 8 pages, 7 figures

Recent angle-resolved photoemission electron spectroscopy (ARPES) experiments demonstrate that the momentum dependence of the spectral gap in underdoped cuprates does not follow a pure $d$-wave form [H. Anzai et a., Nat. Comm. {\bf 4}, 1815 (2013)]. This deviation is highly controversial. It has often been interpretated as a proof of the non-superconducting origin of the antinodal gap in the underdoped regime. In this article, we show that the measured angular dependence of the spectral gap can be explained by the basic nature of pairs in high-T$_c$ cuprates. Hole pairs, or {\it pairons}, form as a result of the local antiferromagnetic environment on the scale $\xi_{AF}$, the magnetic coherence length. The spatial extension of the pairon wavefunction beyond first nearest neighbours gives rise to the anomalous angular dependence of the gap, in quantitative agreement with experiments. This simple interpretation strongly indicates a common origin of the nodal and antinodal gaps.

International audience; Two spherical balls are connected by a taught string passing through a small hole in a perfectly planar table: the first ball, subject to a central force, moves without friction on a two-dimensional plane, while the second ball moves only along the vertical axis directly below the hole. The pedagogical aspects of this novel two-body problem are given particular attention: Newton's laws, central force motion, conservation laws, angular momentum, constraints, etc. The dynamics of the system is considered under various initial conditions wherein the ball on the table moves qualitatively in rotating ellipses or hypotrochoids. The conditions for closed or periodic orbits are examined. The more complex case of the inclined plane is then considered, revealing a rich variety of periodic, aperiodic and chaotic solutions as a function of the ballmass ratio and the plane inclination angle. The associated Poincar'e phase-space maps are discussed.

Key properties of the cuprates, such as the pseudogap observed above the critical temperature $T_c$, remain highly debated. Given their importance, we recently proposed a novel mechanism based on the Bose-like condensation of mutually interacting Cooper pairs [W. Sacks, A. Mauger, Y. Noat, Superconduct. Sci. Technol. 28 105014, (2015)]. In this work, we calculate the temperature dependent DOS using this model for different doping levels from underdoped to overdoped. In all situations, due to the presence of excited pairs, a pseudogap is found above $T_c$ while the normal DOS is recovered at $T^*$, the pair formation temperature. A similar behavior is found as a function of magnetic field, crossing a vortex, where a pseudogap exists in the vortex core. We show that the precise DOS shape depends on combined pair (boson) and quasiparticle (fermion) excitations, allowing for a deeper understanding of the SC to the PG transition., Accepted in the European Physical Journal B (EPJB)

We report a microscopic model wherein the unconventional superconductivity emerges from an incoherent `Cooper-pair glass' state. Driven by the pair-pair interaction, a new type of quasi-Bose phase transition is at work. The interaction leads to the unconventional coupling of the quasiparticles to excited pair states, or `super-quasiparticles', with a non-retarded energy-dependent gap. The model describes quantitatively the quasiparticle excitation spectra of both cuprates and pnictides, including the universal `peak-dip-hump' signatures, and for the pseudogap phase above $T_c$. The results show that instantaneous pair-pair interactions account for the SC condensation without a collective mode., Comment: One reference was corrected

International audience; Using Scanning Tunneling Microscope at low temperature we explore the superconducting phase diagram in the pi-band of the two-band superconductor MgB2. In this band the peculiar shape of the local tunneling spectra and their dynamics in the magnetic field reveal the complex character of the quasiparticle density of states (DOS). The gap in the DOS is first rapidly filled with states in raising the magnetic field up to 0.5 T and then slowly approaches the normal state value: the gap is observed up to 2 T. Such a change in the DOS dynamics suggests the existence of two terms in the DOS of the pi-band: a first one, reflecting an intrinsic superconductivity in the band and a second one, originating from an inter-band coupling to the sigma-band. Our findings allow a deeper understanding of the unique phase diagram of MgB2.

We have studied the superconducting state of 2H−NbSe2 by scanning tunneling spectroscopy along two different crystal orientations, the c and the a/b axes. Along the c axis a large gap is dominant in the spectra, while a smaller gap is measured along the a/b axis. We show that these spectra are accurately described by the McMillan model where the small gap is induced through the coupling to the band associated with the large gap. In order to assign the small and large gaps to specific parts of the 2H−NbSe2 Fermi surface, the electronic structure was studied using first-principles calculations. While we cannot exclude the possibility of intrinsic anisotropy of the gaps, we propose that the large gap opens in the Fermi surface cylinders located around the corner K points while the sheets located around Γ are associated with the small gap. An additional component of the Fermi surface, a selenium based pocket, plays an essential role in the tunneling process. The role of the charge density wave occurring in this material is also discussed. Finally, we are able to give a coherent description of the observed characteristics of the tunneling spectra of 2H−NbSe2 as well as the differences with 2H−NbS2 where no charge density wave state is present. Further experimental work, such as high-resolution ARPES, would be very useful to confirm our interpretation. The approach and modeling developed here could also be relevant for other compounds of the dichalcogenide family.

The mutual interaction between Cooper pairs is proposed as a mechanism for the superconducting state. Above $T_c$, pre-existing but fluctuating Cooper pairs give rise to the unconventional {\it pseudogap} (PG) state, well-characterized by experiment. At the critical temperature, the pair-pair interaction induces a Bose-like condensation of these preformed pairs leading to the superconducting (SC) state. Below $T_c$, both the condensation energy and the pair-pair interaction $\beta$ are proportional to the condensate density $N_{oc}(T)$, whereas the usual Fermi-level spectral gap $\Delta_p$ is independent of temperature. The new order parameter $\beta(T)$, can be followed as a function of temperature, carrier concentration and disorder - i.e. the phase diagrams. The complexity of the cuprates, revealed by the large number of parameters, is a consequence of the {\it coupling of quasiparticles to Cooper-pair excitations}. The latter interpretation is strongly supported by the observed quasiparticle spectral function., Comment: 10 pages, 6 figures Higher resolution figures can be provided

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY)., A combined experimental (superconductor-insulator-superconductor tunneling spectra) and theoretical (density functional theory) study of the two-gap superconductor MgB2 is reported. The calculations confirm that the small gap is associated with a π band mostly based on the boron pz orbitals leading to the three-dimensional band component of the Fermi surface. This channel almost completely dominates the tunneling images and spectra for c-axis-oriented samples and not the two-dimensional σ band. The origin of this effect is due to the faster decay of the electronic states associated with the boron px and py orbitals compared to those associated with the boron pz orbitals, together with the symmetry properties of the wave functions. The calculated tunneling channels and partial density of states for each band agree with the values deduced from precise fits of experimental tunneling spectra. The present approach provides a framework for the understanding of tunneling spectra and the nature of superconducting gaps of other multigap superconductors., Work in Bellaterra was supported by Spanish MINECO (Grants No. FIS2012-37549-C05-02 and No. FIS2012-37549-C05-05 with joint financing by FEDER Funds from the European Union, Grants No. CSD2007-00041 and No. CSD2007-00050) and Generalitat de Catalunya (2014SGR301). J.A.S.-G. and P.O. acknowledge support of the Spanish MINECO through the Severo Ochoa Centers of Excellence Program under Grant No. SEV-2013-0295. J.A.S.-G. was supported by an FPI Fellowship from MINECO.

Understanding the physical properties of a large variety of high-T c superconductors (SC), the cuprate family as well as the more recent iron-based superconductors, is still a major challenge. In particular, these materials exhibit the 'peak-dip-hump' structure in the quasiparticle density of states (DOS). The origin of this structure is explained within our pair-pair interaction (PPI) model: The non-superconducting state consists of incoherent pairs, a 'Cooper-pair glass' which, due to the PPI, undergoes a Bose-like condensation below T c to the coherent SC state. We derive the equations of motion for the quasiparticle operators showing that the DOS 'peak-dip-hump' is caused by the coupling between quasiparticles and excited pair states, or 'super-quasiparticles'. The renormalized SC gap function becomes energy-dependent and non retarded, reproducing accurately the experimental spectra of both pnictides and cuprates, despite the large difference in gap value.

The phase diagram of high- T c cuprates can be understood, without resorting to a boson mode, in terms of a single energy scale , the antiferromagnetic (AF) exchange energy at the metal-insulator transition. As a result, holes form a new quantum object, the “pairon”, i.e. , a pair of holes localized within their local antiferromagnetic environment on the scale of the finite AF correlation length, . In the incoherent pseudogap phase, above T c or within the vortex core, the pairon binding energies are distributed statistically, forming a “Cooper-pair glass”. Contrary to conventional superconductors it is the mutual pair-pair interaction that is responsable for their condensation. We give a natural explanation for the ergodic rigidity of the excitation gap, the latter being constant with respect to a perturbation such as temperature or magnetic field, and determined only by the carrier concentration p and J .

Using a low temperature scanning tunneling microscope in the spectroscopic mode, we find that the disorder in a Bi(2)Sr(2)CaCu(2)O(8+delta) thin film modifies dramatically the quasiparticle local density of states. Small, but well-defined superconducting regions, coexisting with dominating semiconducting areas, show well-pronounced gap structures, similar to those observed previously in high-quality single crystals. Surprisingly, between these two regions, the detailed shape of the quasiparticle spectrum is virtually identical to the pseudogap previously observed at temperatures T>T(c), or in the vortex core, at 4.2 K. Thus, the role of the disorder in destroying the superconducting phase is comparable to that of the magnetic field or thermal fluctuations.

We propose a model that connects the quasiparticle spectral function of high-Tc superconductors to the condensation energy. Given the evidence for pair correlations above T c , we consider a coarse-grain Hamiltonian of fluctuating pairs describing the incoherent pseudogap (PG) state, together with a novel pair–pair interaction term that restores long-range superconducting (SC) coherence below T c . A mean-field solution then leads to a self-consistent gap equation containing the new pair–pair coupling. The corresponding spectral function A ( k , E ) reveals the characteristic peak–dip–hump features of cuprates, now observed on iron pnictides (LiFeAs). The continuous transition from SC to PG states is discussed.

We present experimental evidence for a dramatic influence on the aspect of tunneling images of the orientation relative to a [1110] direction of the steps on vicinal copper surfaces. A model is proposed which relates these observations to the mobility of geometrical kinks of the step edges. This model allows one to deduce from the experimental data the mean emission time of an adatom from a kink on a step oriented near the [1110] direction on Cu(100).

International audience; We show how to prepare graphene samples on a glass substrate with the anodic bonding method. In this method, a graphite precursor in flake form is bonded to a glass substrate with the help of an electrostatic field and then cleaved off to leave few layer graphene on the substrate. Now that several methods are available for producing graphene, the relevance of our method is in its simplicity and practicality for producing graphene samples of about 100 µm lateral dimensions. This method is also extensible to other layered materials. We discuss some detailed aspects of the fabrication and results from Raman spectroscopy, local probe microscopy, and transport measurements on these samples.

We considered a two-band superconductor with a nonzero interband quasiparticle coupling and numerically generated partial elementary excitation spectra for each band. These show deviations from the conventional Bardeen, Cooper, and Schrieffer form, resulting in characteristic signatures in the partial tunneling spectra. The total (measurable) tunneling spectra are calculated considering the $\mathbf{k}$ selection in the tunneling process. Due to the thermal smearing, the relevant spectral signatures may not be resolved in superconductor-insulator-normal-metal tunneling while they are clearly revealed in superconductor-insulator-superconductor (SIS) geometry. As an example, the excitation spectrum of $2{\text{H-NbSe}}_{2}$ is considered in the framework of the developed tunneling model. A remarkable agreement obtained with the experimental SIS data suggests the material to be a two-band superconductor rather than an anisotropic one.

We present the synthesis and the tunneling spectroscopy study of superconducting FeSe0.5Te0.5 (Tc = 14 K), SmFeAsO0.85 (Tc = 45 K) and SmFeAsO0.9F0.1 (Tc = 52 K). The samples were characterized by Rietveld refinement of X-ray diffraction patterns and transport measurements. Tunneling experiments on FeSe0.5Te0.5 revealed a single superconducting gap of ~1 meV in BCS-like tunnelling conductance spectra. In SmFeAsO0.85 and SmFeAsO0.9F0.1 however, more complex spectra were observed characterized by two gap-like structures at ~4 meV and ~10 meV. These spectra are qualitatively understood assuming a two-band superconductor with a 's\pm' order parameter. We show that depending on the sign relation between the pairing amplitudes in the two bands, the interband quasiparticle scattering has a crucial effect on the shape of the tunneling spectra. Detailed analysis of the tunneling spectroscopy data supports the 's\pm' model, but single-gap spectra found in FeSe0.5Te0.5 are more compatible with a disorder-induced 's'-wave gap, due to the Se-Te substitution., 11 pages, 7 figures

We have previously derived an expression for the tunneling current between a tip, ending in a single atom, and a sample, which is valid for small tip-surface separations. Here we apply our method to the case of the imaging of a single atom adsorbed on a flat metal surface. The regime approaching contact is discussed, in which it is shown that the tip modifies the surface electronic structure, and therefore affects the STM image. At short distances, the strong interaction between the tip and the surface can cause a distinct bonding—anti-bonding signature in the electronic structure of the coupled system. In this range, electronic effects appear more strongly on spectroscopic measurements ( I , d I /d V , etc.), rather than on the spatial variations of the tunneling probability. The region of contact is also investigated, and it is shown that the contact resistance depends strongly upon the chemical nature of the tip.

We present an analytical method for treating the tunneling current between a tip and a sample in scanning tunneling microscopy (STM) that goes beyond the independent-electrode (Bardeen) approximation and is valid for smaller tip-to-surface separations. The extremity of the tip is represented by a single spherical potential well. This well is strongly coupled to neighboring tip atoms, as well as the sample electrode, both of which we leave in a general form. The wave function for the entire system is obtained by a matching procedure, from which the total current is determined. If the current is associated with s-derived tip orbitals, the result is comparable in simplicity with that of J. Tersoff and D. Hamann [Phys. Rev. B 31, 805 (1985)]. The low-bias tunnel conductance is proportional to the local density of states (LDOS) of the surface, but renormalized to include multiple reflections to all orders: \ensuremath{\sigma}\ensuremath{\propto}${\mathrm{\ensuremath{\rho}}}_{\mathit{s}}$(${\mathbf{r}}_{0}$,${\mathit{E}}_{\mathit{F}}$)/D, where D depends on both the tip and sample electronic structures and on the tip position ${\mathbf{r}}_{0}$. This effect includes the modification of the surface LDOS due to the presence of the tip. A compact expression is also obtained for orbitals of higher angular momenta: p and d states. The current then depends on the gradients of the surface spectral density, and not on the LDOS, and also has a characteristic denominator. We discuss the significance of this effect, both in the interpretation of STM images and related spectroscopies.

As shown recently [Proslier et al., Europhys. Lett. 73, 962 (2006)], it is possible to map the superconducting (SC) condensate by measuring locally the Josephson tunneling current. We apply this technique to image the vortex lattice in ${\text{V}}_{3}\text{Si}$ which is used as the simplest example of a spatially varying quantum condensate. The Josephson scanning tunneling microscope (JSTM) maps revealed the vortex lattice with the Josephson effect being present outside the vortices and disappearing progressively toward the vortex core. The characteristic length scales of a vortex observed in the Josephson regime are compared to the ones obtained in the quasiparticle regime. We demonstrate that the JSTM allows a nanometer scale resolution of the SC condensate that may be applied to inhomogeneous phases such as high-${T}_{c}$ superconductors.

We present local tunneling spectroscopy in the optimally electron-doped cuprate Sm2-xCexCuO4 x=0.15. A clear signature of the superconducting gap is observed with an amplitude ranging from place to place and from sample to sample (Delta~3.5-6meV). Another spectroscopic feature is simultaneously observed at high energy above \pm 50meV. Its energy scale and temperature evolution is found to be compatible with previous photoemission and optical experiments. If interpreted as the signature of antiferromagnetic order in the samples, these results could suggest the coexistence on the local scale of antiferromagnetism and superconductivity on the electron-doped side of cuprate superconductors.

We previously introduced [T. Cren et al., Europhys. Lett. 52, 203 (2000)] an energy-dependant gap function, $\ensuremath{\Delta}(E)$, that fits the unusual shape of the quasiparticle (QP) spectrum for both BiSrCaCuO and YBaCuO. A simple anti-resonance in $\ensuremath{\Delta}(E)$ accounts for the pronounced QP peaks in the density of states, at an energy ${\ensuremath{\Delta}}_{p}$, and the dip feature at a higher energy, ${E}_{\text{dip}}$. Here we go a step further, our gap function is consistent with the $(T,p)$ phase diagram, where $p$ is the carrier density. For large QP energies $(E⪢{\ensuremath{\Delta}}_{p})$, the total spectral gap is $\ensuremath{\Delta}(E)\ensuremath{\simeq}{\ensuremath{\Delta}}_{p}+{\ensuremath{\Delta}}_{\ensuremath{\varphi}}$, where ${\ensuremath{\Delta}}_{\ensuremath{\varphi}}$ is tied to the condensation energy. From the available data, a simple $p$ dependance of ${\ensuremath{\Delta}}_{p}$ and ${\ensuremath{\Delta}}_{\ensuremath{\varphi}}$ is found, in particular, ${\ensuremath{\Delta}}_{\ensuremath{\varphi}}(p)\ensuremath{\simeq}2.3{k}_{B}{T}_{c}(p)$. These two distinct energy scales of the superconducting state are interpreted by comparing to the normal and pseudogap states. The various forms of the QP density of states, as well as the spectral function $A(\mathbf{k},E)$, are discussed.

Superconductivity is a rare example of a quantum system in which the wavefunction has a macroscopic quantum effect, due to the unique condensate of electron pairs. The amplitude of the wavefunction is directly related to the pair density, but both amplitude and phase enter the Josephson current : the coherent tunneling of pairs between superconductors. Very sensitive devices exploit the superconducting state, however properties of the {\it condensate} on the {\it local scale} are largely unknown, for instance, in unconventional high-T$_c$ cuprate, multiple gap, and gapless superconductors. The technique of choice would be Josephson STS, based on Scanning Tunneling Spectroscopy (STS), where the condensate is {\it directly} probed by measuring the local Josephson current (JC) between a superconducting tip and sample. However, Josephson STS is an experimental challenge since it requires stable superconducting tips, and tunneling conditions close to atomic contact. We demonstrate how these difficulties can be overcome and present the first spatial mapping of the JC on the nanometer scale. The case of an MgB$_2$ film, subject to a normal magnetic field, is considered., Comment: 7 pages, 6 figures

Among the methods used to study the vortex state in superconductors, scanning tunneling spectroscopy (STS), is unique in its ability to measure in real space the variations in the local quasiparticle density of states. Thus, as opposed to magnetic imaging, STS gives direct access to the coherence length rather than to the penetration length. Here we discuss two novel methods which enhance the capabilities of STS as a tool for the study of the vortex state. In the first one, called Lazy Fisherman [A. Kohen et al., Appl. Phys. Lett. 86 (2005) 212503], the scanning tunneling microscope's tip is kept fixed at a selected location while the vortices are being moved by varying the applied magnetic field. By continuously acquiring the local tunneling conductance spectra, dI/d V(V), we detect the changes in the local density of states under the tip due to the vortex motion. With no need for scanning, the method permits one to extend the study of vortices to samples in which scanning is difficult or even impossible due to surface non-uniformity and allows one to detect faster vortex dynamics. To illustrate the approach we study single crystal samples of MgB2. In the second STS method, we replace the commonly used normal metal STM tip by a superconducting (SC) tip which we produce either by mechanically breaking a Nb wire under vacuum in the STM chamber [A. Kohen et al., Physica C 49 (2005) 18] or by gluing a piece of a crystal of MgB2 [F. Giubileo et al., Phys. Rev. Lett. 87 (2001) 177008]. The use of a SC tip enhances the energy resolution of STS in comparison to that obtained with a normal metal tip. The method is illustrated by using Nb and MgB2 tips to perform a simultaneous topographic and spectroscopic imaging on 2H-NbSe2. (c) 2006 Published by Elsevier B.V.

We present scanning tunneling microscopy and spectroscopy of the newly discovered superconductor CaC$_6$. The tunneling conductance spectra, measured between 3 K and 15 K, show a clear superconducting gap in the quasiparticle density of states. The gap function extracted from the spectra is in good agreement with the conventional BCS theory with $\Delta(0)$ = 1.6 $\pm$ 0.2 meV. The possibility of gap anisotropy and two-gap superconductivity is also discussed. In a magnetic field, direct imaging of the vortices allows to deduce a coherence length in the ab plane $\xi_{ab}\simeq$ 33 nm.

We address the question of probing the supercurrents in superconducting (SC) samples on a local scale by performing Scanning Tunneling Spectroscopy (STS) experiments with a SC tip. In this configuration, we show that the tunneling conductance is highly sensitive to the Doppler shift term in the SC quasiparticle spectrum of the sample, thus allowing the local study of the superfluid velocity. Intrinsic screening currents, such as those surrounding the vortex cores in a type II SC in a magnetic field, are directly probed. With Nb tips, the STS mapping of the vortices, in single crystal 2H-NbSe_2, reveals both the vortex cores, on the scale of the SC coherence length $\xi$, and the supercurrents, on the scale of the London penetration length $\lambda$. A subtle interplay between the SC pair potential and the supercurrents at the vortex edge is observed. Our results open interesting prospects for the study of screening currents in any superconductor., Comment: 4 pages, 5 figures

We report a simple method for the fabrication of Niobium superconducting (SC) tips for scanning tunnelling microscopy which allow atomic resolution. The tips, formed in-situ by the mechanical breaking of a niobium wire, reveal a clear SC gap of 1.5 meV and a critical temperature Tc=9.2+-0.3 K as deduced from Superconductor Insulator Normal metal (NIS) and Superconductor Insulator Superconductor (SIS) spectra. These match the values of bulk Nb samples. We systematically find an enhanced value of the critical magnetic field in which superconductivity in the tip is destroyed (around 1 T for some tips) up to five times larger than the critical field of bulk Nb (0.21 T). Such enhancement is attributed to a size effect at the tip apex, Comment: to appear in Physica C

Tunneling spectroscopy on various samples of MgB 2 was performed. The first direct evidence for the two-gap superconductivity was given with an inverted junction setup, in which a small crystal of MgB 2 was used as the STM tip and 2H-NbSe 2 as the sample. This technique allowed to show that both gaps close at the critical temperature of the bulk material and thus are intimately related to the superconductivity. The experiments performed in the standard N-I-S geometry evidenced for two strongly coupled gaps Δ L =7.0±1.0 meV and Δ S =3.0±1.0 meV at 4.2 K. STS on as-grown c -axis oriented thin films yielded only small gap which confirmed the identification of this gap as originating from 3D-like π-band and, by exclusion, that of the large gap from 2D-like σ-band. The low gap values Δ S =2.2±0.3 meV were attributed to the degraded film surface. After chemical etching, the gap increased to Δ S =2.8±0.3 meV. The c -axis tunneling spectra are better fitted considering anisotropic superconductivity inside the π-band. The issues of our findings are discussed in terms of two-band superconductivity.

High-speed scanning tunnelling spectroscopy (STS) was used at low temperature to study the quasiparticle excitation spectrum on the surface of c-axis-oriented superconducting thin films of MgB2. The tunnelling spectra measured on as-grown films were compared with those acquired on chemically etched samples. In most cases the STS reveals only one small superconducting gap to be present in the tunnelling spectra, consistent with c-axis tunnelling and the particular electronic band structure of MgB2. We found that the etching leads to the enhancement of the gap energy by 25% from 2.2 +or- 0.3 meV to 2.8 +or- 0.3 meV, and to the modification of the temperature dependence of the superconducting gap which, in both cases, has clearly a non-BCS shape. We argue that the modification of the electronic structure at the surface of the material due to the etching is responsible for these changes and discuss the possible origins of the effect

Topographic and spectroscopic information on GdSr2RuCu2O8 sintered pellets have been obtained by a home built low temperature Scanning Tunneling Microscope (STM) operating at 4.2 K. The topographic image of the surface showed non homogeneous samples with grains of typical size of about 100 nm. In many locations studied, the Tunneling Spectroscopy reveals the presence of charging effects in the current-voltage characteristics over a voltage range up to 100 mV. Two types of charging effects are clearly distinguished: one corresponds to the reduction of the tunneling conductance around zero bias and is attributed to the Coulomb blockade, and another onw, a stepwise increasing of the current as a function of the bias voltage is identified as Coulomb staircase regime. Besides these spurious charging effects, the current-voltage characteristics often show a pronounced non-linearity around 4.0 mV. This non-linearity, disappearing above the critical temperature of the materials, is connected to the superconducting gap in the GdSr 2 RuCu 2 O 8.

Low temperature Scanning Tunneling Microscopy/Spectroscopy (STM/STS) was used to study the superconducting properties of as grown MgB2 thin films. Two different Pulsed Laser Ablation deposition procedures were used to fabricate the samples. Depending on growth conditions, STS/STM analysis revealed either superconducting or normal metallic behavior on the sample surfaces. In the first case the tunneling spectra with well-pronounced superconducting gap of 2.6-3.2 meV were observed at T = 4.2 K. STS spectra were found to be position independent, thus showing a very high spatial homogeneity of the films. No evidence of double gap structures was found. Moreover, in contrast with a bulk TC = 35.7K of the samples, the superconductivity with a damped TC was observed on the surface. In the second case, the superconducting features were completely absent in the tunneling spectra. We associate such a behaviour of the samples of second type with the presence of a thick metallic cap layer all over the surface.

We report on the temperature dependence of the quasiparticle density of states in the simple binary compound ${\mathrm{MgB}}_{2}$ directly measured using scanning tunneling microscope (STM). To achieve high quality tunneling conditions, a small crystal of ${\mathrm{MgB}}_{2}$ is used as a tip in the STM experiment. The ``sample'' is chosen to be a 2H- ${\mathrm{NbSe}}_{2}$ single crystal presenting an atomically flat surface. At low temperature the tunneling conductance spectra show a gap at the Fermi energy followed by two well-pronounced conductance peaks on each side. They appear at voltages ${V}_{S}\ensuremath{\simeq}\ifmmode\pm\else\textpm\fi{}3.8\mathrm{mV}$ and ${V}_{L}\ensuremath{\simeq}\ifmmode\pm\else\textpm\fi{}7.8\mathrm{mV}$. With rising temperature both peaks disappear at the ${T}_{C}$ of the bulk ${\mathrm{MgB}}_{2}$, a behavior consistent with the model of two-gap superconductivity. The possibility of a particular proximity effect is also discussed.

Using scanning tunneling spectroscopy at T = 4.2 K, we perform simultaneously the topographic imaging and the quasiparticle density of states (DOS) mapping in granular MgB2. We observe a new type of spectrum, showing a pronounced double gap, with the magnitudes of Delta_S = 3.9 meV and Delta_L = 7.5 meV, i.e. well below and well above the BCS limit. The largest gap value gives the ratio 2*Delta_L/k_B T_c = 4.5, which implies strong electron-phonon coupling. Other superconducting regions are found to have a characteristic BCS-shaped DOS. However, the variation of the spectral shape and lower gap widths, from 2.0 meV to 6.5 meV, indicate the importance of surface inhomogeneity and proximity effects in previously published tunneling data. Our finding gives no evidence for any important gap anisotropy. Instead, it strongly supports the multiple gap scenario in MgB2 in the clean limit, and the single gap scenario in the dirty limit., 4 pages, 4 figures. 1 figure added, text revised

In this Letter we present new tunneling data on YBa$_2$Cu$_3$O$_7$ thin films by low temperature scanning tunneling spectroscopy. Unusual peak-dip-hump features, previously reported in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$, are also found in YBa$_2$Cu$_3$O$_7$. To analyse these common signatures we propose a new heuristic model in which, in addition to the d-wave symmetry, the gap function is energy dependent. A simple expression for the quasiparticle density of states is derived, giving an excellent agreement with the experiment. The dynamics of the quasiparticle states and the energy scales involved in the superconducting transition are discussed., Comment: 4 page Letter with 3 figures

Using high speed scanning tunneling spectroscopy, we perform a full mapping of the quasiparticle density of states (DOS) in single crystals of BiPbSrCaCuO(2212). The measurements carried out at 5 K showed a complex spatial pattern of important variations of the local DOS on the nanometer scale. Superconducting areas are co-existing with regions of a smooth and larger gap-like DOS structure. The superconducting regions are found to have a minimum size of about 3 nm. The role of Pb-introduced substitutional disorder in the observed spatial variations of the local DOS is discussed., Comment: 4 page Letter with 3 figures (2 color figures)

Self-assembly of nanoparticles is important for many fields of nanotechnology. However, experimental studies such as spin-dependent electron tunneling have been limited by the rapid degradation of the electronic and magnetic properties of nano-objects due to their immediate oxidation in air. Here are presented for the first time scanning tunneling microscopy images at room temperature, under air, of highly oxidizable Co nanocrystals on a dodecanethiol monolayer grafted onto a gold surface. Local organization and electronic properties of the particles are described

In a previous paper we reported an intriguing bias-dependent contrast in the STM images of 2H-NbSe2 taken at low-temperature (4.2 K). This layered material is in a charge density wave (CDW) state below 35 K, with a period nearly commensurate to three times the atomic lattice. The CDW energy gap is 2 Delta approximately=70 meV, but still the material remains metallic, even superconducting below 7.2 K. The bias-dependent phase shift in the STM image, in particular the comparison between occupied and unoccupied states, was shown to be attributable to the CDW, as no phase shift was observed in the atomic pattern. This phase shift does not result in a contrast reversal,which could be expected by analogy to some semi-conductor band gaps. The electron versus hole distribution has not been solved for a CDW gap, which is quite complex due to the many-band situation near the Fermi level. In the present work we write a general expression for the local density of states (LDOS) due to a commensurate CDW. Its amplitude and phase can be related in a simple way to the band structure, if one assumes an approximate form for the surface Bloch functions. We apply the method to the example of NbSe2, where its electronic structure is taken to be two-dimensional and the CDW is exactly commensurate to 3a. The new band structure and Fermi surface in the CDW state is calculated in perturbation theory, with a suitable pseudo-potential. The energy-dependent contrast is due to new states on the order of EF+or- Delta , having characteristic phases. The amplitude of the CDW is largest at the particular energies (or voltages) where tunneling occurs to the high symmetry points of the Brillouin zone. At these energies, the phase of the LDOS varies considerably, which gives a number of possible motifs in the STM image. For a shift in voltage corresponding to the energy gap edges, E=EF+or- Delta , we find that the maxima of the corrugation are shifted along the diagonal of the conventional unit cell

Using scanning tunneling spectroscopy (STS) the local density of states (LDOS) of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{1}$${\mathrm{Cu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ [Bi(2212)] and ${\mathrm{Y}}_{1}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (YBaCuO) monocrystals were studied at low temperature (down to 4.2 K). Due to the high quality of the vacuum tunneling junctions the pair-breaking effects are consequently reduced, and rather fine features of the superconducting LDOS could be analyzed. For Bi(2212) it is shown that, in order to fit properly the experimental data, ${\mathit{d}}_{{\mathit{x}}^{2}\mathrm{\ensuremath{-}}{\mathit{y}}^{2}}$ pairing (or a very anisotropic s pairing) as well as a strong in-plane anisotropy of the Fermi surface are required. Although YBaCuO STS spectra look more ``d like,'' no precise analysis was possible because of the high natural surface contamination of the samples. Both the fundamental problem of pairing symmetry and the influence of experimental conditions on vacuum STS measurements are discussed. \textcopyright{} 1996 The American Physical Society.

The effects of 11 antipsychotic drugs on the pyruvate dehydrogenase complex (PDHC) prepared from bovine heart and rat brain were investigated. All inhibited PDHC to varying extents. With clinically equivalent doses, chlorpromazine and thioridazine inhibited the most and fluphenazine and thiothixene the least. The relationship of degree of inhibition of PDHC by neuroleptics to clinical improvement of 32 outpatients treated with acetazolamide and thiamine (A + T) ancillary therapy for chronic mental illness suggests that patients treated with psychoactive drugs that inhibit PDHC the least are most likely to have a favorable response with A + T treatment.

The design of an RF probe suitable for obtaining proton-decoupled {sup 13}C spectra from a subhuman primate brain is described. Two orthogonal saddle coils, one tuned to the resonant frequency of {sup 13}C and the other to the resonant frequency of 1H, were used to monitor the in vivo metabolism of (1-{sup 13}C)glucose in rhesus monkey brain at 2.1 T. Difference spectra showed the appearance of {sup 13}C-enriched glutamate and glutamine 30 to 40 min after a bolus injection of (1-{sup 13}C)glucose.

The anisotropy of the order parameter in HTSC is a key question on the way to understanding the nature of pairing. A systematic study of the local DOS in monocrystals of BiSrCaCuO and YBaCuO is performed with the help of STM/STS (2.5–90K). The analysis of the data shows that for most of the existing results the “real” LDOS is strongly modified by a number of parasitic effects. However, rather precise spectroscopy may be performed when real metal-vacuum-superconductor junctions are achieved. In such a case, for BiSrCaCuO, the total number of states is conserved, and no slope in the LDOS outside the gap is found. The shape of the LDOS obtained in YBaCuO is different from that of BiSrCaCuO. The pairing anisotropy and the influence of parasitic effects are discussed.

Using low‐temperature scanning tunneling microscopy, detailed aspects of the charge density wave state in 2H–NbSe2 have been investigated by exploiting both the spectroscopic and topographic mode. The images are examined as a function of bias voltage and tip‐to‐surface distance. It is shown that the local density of states in the charge density wave phase is sensitive to the bias voltage, and especially to its sign, revealing the complexity of the electronic structure near the Fermi energy. In particular, for the first time a bias‐dependent contrast in the charge density wave phase of 2H–NbSe2 was observed. This contrast must come from the spatial separation between the occupied and unoccupied electronic states responsible for the charge density wave.