We show that the magnetic ordering of coupled atomic dimers on a superconductor is revealed by their intragap spectral features. Chromium atoms on the superconductor $\beta \text{−}{\mathrm{Bi}}_2\mathrm{Pd}$ surface display Yu-Shiba-Rusinov bound states, detected as pairs of intragap excitations in tunneling spectra. By means of atomic manipulation with a scanning tunneling microscope’s tip, we form Cr dimers with different arrangements and find that their intragap features appear either shifted or split with respect to single atoms. These spectral variations are associated with the magnetic coupling, ferromagnetic or antiferromagnetic, of the dimer, as confirmed by density functional theory simulations. The striking qualitative differences between the observed tunneling spectra prove that intragap Shiba states are extremely sensitive to the magnetic ordering on the atomic scale.

We present very low-temperature scanning tunneling microscopy (STM) experiments on single-crystalline samples of the superconductor $\beta -{\mathrm{Bi}}_2\mathrm{Pd}$. We find a single superconducting gap from the zero-field tunneling conductance. However, the magnetic field dependence of the intervortex tunneling conductance is higher than the one expected in a single-gap superconductor. Such an increase in the intervortex tunneling conductance has been found in superconductors with multiple superconducting gaps. We also find that the hexagonal vortex lattice is locked to the square atomic lattice as expected in crystalline superconductors with anisotropic Fermi surfaces. Moreover, we compare the upper critical field $H_{c2}\left(T\right)$ obtained in our sample with previous measurements and find that $H_{c2}\left(T\right)$ does not increase by reducing the mean free path. We fit $H_{c2}\left(T\right)$ and show that multiband Fermi surface is needed to explain the observed behavior. We propose that $\beta -{\mathrm{Bi}}_2\mathrm{Pd}$ is a single-gap multiband superconductor. We anticipate that single-gap superconductivity might often occur in compounds with anisotropic multiband Fermi surfaces.

The $\beta \text{−}\mathrm{B}{\mathrm{i}}_2\mathrm{Pd}$ compound has been proposed as another example of a multigap superconductor [Imai et al., J. Phys. Soc. Jpn. 81, 113708 (2012)]. Here, we report on measurements of several important physical quantities capable of showing a presence of multiple energy gaps on our superconducting single crystals of $\beta \text{−}\mathrm{B}{\mathrm{i}}_2\mathrm{Pd}$ with the critical temperature $T_c$ close to 5 K. The calorimetric study via a sensitive ac technique shows a sharp anomaly at the superconducting transition, however only a single energy gap is detected. Also other characteristics inferred from calorimetric measurements as the field dependence of the Sommerfeld coefficient and the temperature and angular dependence of the upper critical magnetic field point unequivocally to standard single $s$-wave gap superconductivity. The Hall-probe magnetometry provides the same result from the analysis of the temperature dependence of the lower critical field. A single-gapped BCS density of states is detected by the scanning tunneling spectroscopy measurements. Then, the bulk as well as the surface sensitive probes evidence a standard conventional superconductivity in this system where the topologically protected surface states have been recently detected by angle-resolved photoemission spectroscopy [Sakano et al., Nat. Commun. 6, 8595 (2015).].

We study the spatial distribution of the density of states (DOS) at zero bias $N\left(\mathit{r}\right)$ in the mixed state of single and multigap superconductors. We provide an analytic expression for $N\left(\mathit{r}\right)$ based on deGennes' relationship between DOS and the order parameter that reproduces well scanning tunneling microscopy (STM) data in several superconducting materials. In the single gap superconductor $\beta \text{−}{\mathrm{Bi}}_2\mathrm{Pd}$, we find that $N\left(\mathit{r}\right)$ is governed by a length scale ${\xi }_H=\sqrt{{\varphi }_0/2\pi H}$, which decreases in rising fields. The vortex core size $\mathcal{C}$, defined via the slope of the order parameter at the vortex center, $\mathcal{C}\propto {(d\mathrm{\Delta }/dr{|}_{r\to 0})}^{-1}$, differs from ${\xi }_H$ by a material dependent numerical factor. The new data on the tunneling conductance and vortex lattice of the $2\mathrm{H}\text{−}{\mathrm{NbSe}}_{1.8}{\mathrm{S}}_{0.2}$ show the in-plane isotropic vortices, suggesting that substitutional scattering removes the in-plane anisotropy found in the two-gap superconductor $2\mathrm{H}\text{−}{\mathrm{NbSe}}_2$. We fit the tunneling conductance of $2\mathrm{H}\text{−}{\mathrm{NbSe}}_{1.8}{\mathrm{S}}_{0.2}$ to a two gap model and calculate the vortex core size $\mathcal{C}$ for each band. We find that $\mathcal{C}$ is field independent and has the same value for both bands. We also analyze the two-band superconductor $2\mathrm{H}\text{−}{\mathrm{NbS}}_2$ and find the same result. We conclude that, independently of the magnetic field induced variation of the order parameter values in both bands, the spatial variation of the order parameter close to the vortex core is the same for all bands.

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.

We describe a scanning tunneling microscope (STM) that operates at magnetic fields up to 22 T and temperatures down to 80 mK. We discuss the design of the STM head, with an improved coarse approach, the vibration isolation system, and efforts to improve the energy resolution using compact filters for multiple lines. We measure the superconducting gap and Josephson effect in aluminum and show that we can resolve features in the density of states as small as 8 μeV. We measure the quantization of conductance in atomic size contacts and make atomic resolution and density of states images in the layered material 2H–NbSe₂. The latter experiments are performed by continuously operating the STM at magnetic fields of 20 T in periods of several days without interruption.