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Presents information on a study which discussed the behavior found in tunneling measurements obtained on two high-T[subc] superconductors using a scanning tunneling microscope. Typical conductance curves for a ceramic sample; Influence of the tunnel barrier on the conductance curves; Existence of smeared conductance curves that display zero-conductance curves.

We describe a three axis vector magnet system for cryogenic scanning probe microscopy measurements. We discuss the magnet support system and the power supply, consisting of a compact three way 100 A current source. We obtain tilted magnetic fields in all directions with maximum value of 5T along z-axis and of 1.2T for XY-plane magnetic fields. We describe a scanning tunneling microscopy-spectroscopy (STM-STS) set-up, operating in a dilution refrigerator, which includes a new high voltage ultralow noise piezodrive electronics and discuss the noise level due to vibrations. STM images and STS maps show atomic resolution and the tilted vortex lattice at 150 mK in the superconductor β-Bi2Pd. We observe a strongly elongated hexagonal lattice, which corresponds to the projection of the tilted hexagonal vortex lattice on the surface. We also discuss Magnetic Force Microscopy images in a variable temperature insert. [ABSTRACT FROM AUTHOR]

We report on several low-temperature experiments supporting the presence of Majorana fermions in superconducting lead nanowires fabricated with a scanning tunneling microscope (STM). These nanowires are the connecting bridges between the STM tip and the sample resulting from indentation-retraction processes. We show here that by a controlled tuning of the nanowire region, in which superconductivity is confined by applied magnetic fields, the conductance curves obtained in these situations are indicative of topological superconductivity and Majorana fermions. The most prominent feature of this behavior is the emergence of a zero bias peak in the conductance curves, superimposed on a background characteristic of the conductance between a normal metal and a superconductor in the Andreev regime. The zero bias peak emerges in some nanowires when a magnetic field larger than the lead bulk critical field is applied. This field drives one of the electrodes into the normal state while the other, the tip, remains superconducting on its apex. Meanwhile a topological superconducting state appears in the connecting nanowire of nanometric size. [ABSTRACT FROM AUTHOR]

We observed chiral charge order in the superconductor 2H-TaS2 using scanning tunneling microscopy and spectroscopy at 0.1 K. Topographic images show hexagonal atomic lattice and charge density wave (CDW) with clockwise and counterclockwise charge modulations. Tunneling spectroscopy reveals the superconducting density of states, disappearing at Tc = 1.75K and showing a wide distribution of values of the superconducting gap, centered around Δ = 0.28 meV. [ABSTRACT FROM AUTHOR]

We describe a method to make scanning tunneling microscopy/spectroscopy imaging at very low temperatures while driving a constant electric current up to some tens of mA through the sample. It gives a new local probe, which we term current driven scanning tunneling microscopy/spectroscopy . We show spectroscopic and topographic measurements under the application of a current in superconducting Al and NbSe2 at 100 mK. Perspective of applications of this local imaging method includes local vortex motion experiments, and Doppler shift local density of states studies. [ABSTRACT FROM AUTHOR]

We describe a scanning tunneling microscope for operation in a dilution refrigerator with a sample stage which can be moved macroscopically in a range up to a cm and with an accuracy down to the tens of nm. The position of the tip over the sample as set at room temperature does not change more than a few micrometers when cooling down. This feature is particularly interesting for work on micrometer sized samples. Nanostructures can be also localized and studied, provided they are repeated over micrometer sized areas. The same stage can be used to approach a hard single crystalline sample to a knife and cleave it, or break it, in situ. In situ positioning is demonstrated with measurements at 0.1 K in nanofabricated samples. Atomic resolution down to 0.1 K and in magnetic fields of 8 T is demonstrated in NbSe2. No heat dissipation nor an increase in mechanical noise has been observed at 0.1 K when operating the slider. [ABSTRACT FROM AUTHOR]

We present a method to produce superconducting tips to be used in scanning tunneling microscopy/spectroscopy experiments. We use these tips to investigate the evolution of the electronic density of states of NbSe2 from 0.3 K up to its critical temperature (7.2 K). The use of a superconducting tip (Pb) as counterelectrode provides an enhancement of the different features related to the DOS of NbSe2 in the tunneling conductance curves, along all the studied thermal range. The analysis of the experimental results gives evidence of the presence of multiband superconductivity in NbSe2. [Copyright &y& Elsevier]

We discuss Scanning Tunneling Microscopy and Spectroscopy (STM/S) measurements at very low temperatures in single crystals of the non magnetic borocarbide superconductors RNi[sub 2]B[sub 2]C (R=Y, Lu, T[sub c]=15.5 and 16.5 K) and in MgB[sub 2]. The tunneling spectra in some regions of the surface show a clear reduction of the anisotropy of the superconducting gap. [ABSTRACT FROM AUTHOR]

Nanosized Pb junctions exhibit superconducting correlations at magnetic fields more than an order of magnitude higher than the zero-temperature critical field of the bulk Pb. The strongly enhanced critical field is a spectacular demonstration of nanosize effect where the Meissner screening currents become ineffective for junction's tip smaller than the London penetration depth (32 nm for Pb). From the enhanced critical field we characterize the geometry of a particular junction using the variable radius pair breaking theoretical model of Suderow et al. [ABSTRACT FROM AUTHOR]

The observation of vortices in superconductors was a major breakthrough in developing the conceptual background for superconducting applications. Each vortex carries a flux quantum, and the magnetic field decreases radially from the center. Techniques used to make magnetic field maps, such as magnetic decoration, give vortex lattice images in a variety of systems. However, strong type II superconductors allow penetration of the magnetic field over large distances, of the order of the magnetic penetration depth λ. Superconductivity survives up to magnetic fields where, for imaging purposes, there is no magnetic contrast at all. Static and dynamic properties of vortices are largely unknown at such high magnetic fields. Reciprocal space studies using neutron scattering have been employed to obtain insight into the collective behavior. But the microscopic details of vortex arrangements and their motion remain difficult to obtain. Direct real-space visualization can be made using scanning tunneling microscopy and spectroscopy (STM/S). Instead of using magnetic contrast, the electronic density of states describes spatial variations of the quasiparticle and pair wavefunction properties. These are of the order of the superconducting coherence length ξ, which is much smaller than λ. In principle, individual vortices can be imaged using STM up to the upper critical field where vortex cores, of size ξ, overlap. In this review, we describe recent advances in vortex imaging made with scanning tunneling microscopy and spectroscopy. We introduce the technique and discuss vortex images that reveal the influence of the Fermi surface distribution of the superconducting gap on the internal structure of vortices, the collective behavior of the lattice in different materials and conditions, and the observation of vortex lattice melting. We consider challenging lines of work, which include imaging vortices in nanostructures, multiband and heavy fermion superconductors, single layers and van der Waals crystals, studying current-driven dynamics and the liquid vortex phases. [ABSTRACT FROM AUTHOR]

We present very low temperature (0.15 K) scanning tunneling microscopy and spectroscopy experiments in the layered superconductor LaSb2. We obtain topographic microscopy images with surfaces showing hexagonal and square atomic size patterns, and observe in the tunneling conductance a superconducting gap. We find well defined quasiparticle peaks located at a bias voltage comparable to the weak coupling s-wave BCS expected gap value (0.17 meV). The amount of states at the Fermi level is however large and the curves are significantly broadened. We find Tc of 1.2 K by following the tunneling conductance with temperature. [ABSTRACT FROM AUTHOR]

Abstract: The vortex electronic structure in the multiband superconductor NbSe2 is studied by means of scanning tunneling spectroscopy (STS) using a superconducting tip. The use of a superconducting tip (Pb) as a probe provides an enhancement of the different features related to the DOS of NbSe2 in the tunneling conductance curves. This use allows the observation of rich patterns of electronic states in the conductance images around the vortex cores in a wide range of temperature, as well as the simultaneous acquisition of Josephson current images in the vortex state. [Copyright &y& Elsevier]

We report a characterization of surfaces of the dichalcogenide TaSe2 using scanning tunneling microscopy and spectroscopy at 150 mK. When the top layer has the 2H structure and the layer immediately below the 1T structure, we find a singular spatial dependence of the tunneling conductance below 1 K, changing from a zero-bias peak on top of Se atoms to a gap in between Se atoms. The zero-bias peak is additionally modulated by the commensurate 3a0×3a0 charge-density wave of 2H-TaSe2. Multilayers of 2H-TaSe2 show a spatially homogeneous superconducting gap with a critical temperature also of 1 K. We discuss possible origins for the peculiar tunneling conductance in single layers. [ABSTRACT FROM AUTHOR]

Abstract: We discuss basics of Scanning Tunneling Microscopy and Spectroscopy (STM/S) of the superconducting state with normal and superconducting tips. We present a new method to measure the local variations in the Andreev reflection amplitude between a superconducting tip and the sample. This method is termed Scanning Andreev Reflection Spectroscopy (SAS). We also briefly discuss vortex imaging with STM/S under an applied current through the sample, and show the vortex lattice as a function of the angle between the magnetic field and sample’s surface. [Copyright &y& Elsevier]

Abstract: We present measurements of the superconducting upper critical field H c2(T) and the magnetic phase diagram of the superconductor ErNi2B2C made with a scanning tunneling microscope (STM). The magnetic field was applied in the basal plane of the tetragonal crystal structure. We have found large gapless regions in the superconducting phase diagram of ErNi2B2C, extending between different magnetic transitions. A close correlation between magnetic transitions and H c2(T) is found, showing that superconductivity is strongly linked to magnetism. [Copyright &y& Elsevier]

We present measurements of the superconducting gap of URu2Si2 made with scanning tunneling microscopy using a superconducting tip of Al. We find tunneling conductance curves with a finite value at the Fermi level. The density of states is V shaped at low energies, and the quasiparticle peaks are located at values close to the expected superconducting gap from weak-coupling BCS theory. Our results point to rather opened gap structures and gap nodes on the Fermi surface. [ABSTRACT FROM AUTHOR]

We observed chiral charge order in the superconductor 2H-TaS2 using scanning tunneling microscopy and spectroscopy at 0.1 K. Topographic images show hexagonal atomic lattice and charge density wave (CDW) with clockwise and counterclockwise charge modulations. Tunneling spectroscopy reveals the superconducting density of states, disappearing at Tc = 1.75K and showing a wide distribution of values of the superconducting gap, centered around Δ = 0.28 meV. [ABSTRACT FROM AUTHOR]

Abstract: We present new scanning tunneling microscopy measurements in the superconductor TmNi2B2C. The topography shows in some areas flat surfaces, where atomic size modulations can be identified. We find a hexagonal vortex lattice between 0.15T and 1.4T, when the magnetic field is applied along the basal plane of the tetragonal crystal structure , and a hexagonal to square transition around 0.15T when the field is applied along the c-axis . Measured intervortex distance are smaller than expected at high field, due to the internal field being larger than the applied field. [ABSTRACT FROM AUTHOR]

Abstract: Interlayer magnetotransport measurements on the highly anisotropic (LaSe)1.14(NbSe2) superconductor with T c ∼1.2K have indicated that this layered compound represents a model system of intrinsic Josephson junctions [P. Szabó et al., Phys. Rev. Lett. 86 (2001) 5990]. Scanning tunneling microscopy at room temperature and tunneling spectroscopy measurements at very low temperatures are presented in this work. STM imaging has revealed the presence of two types of surfaces which can be attributed to the appearance of LaSe or NbSe2 layers on the surface. The use of STM tip made of superconducting lead enabled a precise measurement of the temperature dependence of the superconducting energy gap Δ(T) on the NbSe2 layer. Δ(T) obtained from the surface sensitive STS data support the scenario obtained from our previous interlayer –ergo bulk sensitive magnetotransport measurements. [Copyright &y& Elsevier]

We discuss recent results and advances in very low temperature scanning tunneling microscopy in superconductors, focusing on the experimental developments that have permitted to extend the present applications of this technique to new materials. We detail the experimental apparatus used and discuss the results and the future possibilities of S–I–S tunneling using a superconducting tip and sample. Further on, we present results on the superconductivity of single grains of MgB2 and compare them with other works. We also discuss recent data on the borocarbide material TmNi2B2C. [Copyright &y& Elsevier]