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Voltage controlled current source for cryogenic measurements

Electrons can form a two-dimensional electron gas at metal surfaces, where lateral confinement leads to quantum-well states. Such states have been observed for highly itinerant electrons, but it remains an open question whether quantum-well states can be formed from strongly correlated electrons. Here we study atomically flat terraces on surfaces of the heavy-fermion superconductor URu$_2$Si$_2$ using millikelvin scanning tunneling spectroscopy. We observe two-dimensional heavy fermions (2DHF) with an effective mass 17 times the free electron mass that form quantized states separated by a fraction of a meV. Superconductivity at the surface is induced by the bulk into the 2DHF. Our results provide a new route to realize quantum well states in correlated quantum materials.

The superconducting transition leads to a sharp resistance drop in a temperature interval that can be a small fraction of the critical temperature T$_c$. A superconductor exactly at T$_c$ is thus very sensitive to all kinds of thermal perturbations, including the heat dissipated by the measurement current. We show that the interaction between electrical and thermal currents leads to a sizeable imaginary impedance at frequencies of order of tens of Hz at the resistive transition of single crystals of the layered material 2H-NbSe$_2$. We explain the result using models developed for transition edge sensors. By measuring under magnetic fields and at high currents, we find that the imaginary impedance is strongly influenced by the heat associated with vortex motion and out-of-equilibrium quasiparticles.

The magnetoresistance of iron pnictide superconductors is often dominated by electron-electron correlations and deviates from the H$^2$ or saturating behaviors expected for uncorrelated metals. Contrary to similar Fe-based pnictide systems, the superconductor LaRu$_2$P$_2$ (T$_c$ = 4 K) shows no enhancement of electron-electron correlations. Here we report a non-saturating magnetoresistance deviating from the H$^2$ or saturating behaviors in LaRu$_2$P$_2$. We have grown and characterized high quality single crystals of LaRu$_2$P$_2$ and measured a magnetoresistance following H$^{1.3}$ up to 22 T. We discuss our result by comparing the bandstructure of LaRu$_2$P$_2$ with Fe based pnictide superconductors. The different orbital structures of Fe and Ru leads to a 3D Fermi surface with negligible bandwidth renormalization in LaRu$_2$P$_2$, that contains a large open sheet over the whole Brillouin zone. We show that the large magnetoresistance in LaRu$_2$P$_2$ is unrelated to the one obtained in materials with strong electron-electron correlations and that it is compatible instead with conduction due to open orbits on the rather complex Fermi surface structure of LaRu$_2$P$_2$., Comment: 12 pages, 4 figures

Resumen del trabajo presentado a la International conference on: Novel 2D materials explored via scanning probe microscopy & spectroscopy; celebrada en Donostia.San Sebastián (País Vasco, España) del 25 al 29 de junio de 2018., We show that the magnetic ordering of coupled atomic dimers on a superconductor is revealed by their intra-gap spectral features. Chromium atoms on the superconductor Bi2Pd surface display Yu-Shiba-Rusinov bound states, detected as pairs of intra-gap excitations in tunneling spectra. By means of atomic manipulation with a scanning tunneling microscope's tip, we formed Cr dimers with different arrangements and found that their intra-gap features appear either shifted or split with respect to single atoms. The spectral variations reveal that the magnetic coupling of the dimer changes between ferromagnetic and antiferromagnetic depending on its disposition on the surface, in good agreement with density functional theory simulations. The striking qualitative difference between the two magnetic orderings proves that intra-gap state spectroscopy gives an accurate information of exchange interactions on the atomic scale.

Resumen del trabajo presentado a la International conference on: Novel 2D materials explored via scanning probe microscopy & spectroscopy; celebrada en Donostia.San Sebastián (País Vasco, España) del 25 al 29 de junio de 2018., The coupling between spins in transition metal atoms and Cooper pairs in a superconductor gives rise to localized sub-gap excitations known as Yu-Shiba-Rusinov (YSR) states. It has been proposed that the YSR states of proximal atoms hybridize, what can result in the formation of YSR bands. These bands are proposed as a platform to realize topological superconductivity in the presence of spin-orbit coupling or helical magnetic order and to host Majorana Zero-Energy Modes. It is thus crucial to understand the basic mechanisms of YSR state hybridization in atomic nanostructures. In this work we investigate the hybridization of YSR states in atomic-scale structures constructed by lateral manipulation on the surface of β-Bi2Pd, a centrosymmetric s-wave superconductor. We use Mn adatoms as magnetic impurities, which show a single YSR state inside the superconducting gap. We find that Mn dimers show bonding and anti-bonding states as a result of the coupling between the constituent impurities and we track the evolution of these states in longer structures. From our results we conclude that Mn is a good candidate for the construction of atomic chains with extended bands and determine that only along certain directions the magnetic order might result in relevant non-trivial effects.

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 β−Bi2Pd 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., D.-J. C. and J. I. P. thank the European Union for support under the H2020-MSCA-IF2014 Marie-Curie Individual Fellowship program (Proposal No. 654469), Spanish MINECO (MAT2016-78293-C6-1-R), Diputacion Foral de Gipuzkoa for Grant No. 64/15, and the European Regional Development Fund (ERDF). N. L. thanks Spanish MINECO (Grant No. MAT2015-66888-C3-2-R). M. M. U. acknowledges Spanish MINECO (MAT2014-60996-R). E. H., I. G., and H. S. acknowledge FIS2014-54498-R and MDM-2014-0377, COST MP16218 nanocohybri, ERC PNICTEYES Grant Agreement No. 679080, and Departamento Administrativo de Ciencia, Tecnología e Innovación, COLCIENCIAS (Colombia), Programa doctorados en el exterior and convocatoria 568-2012, as well as Comunidad de Madrid through program NanofrontmagCM (Grant No. S2013/MIT-2850).

We show that biaxial strain induces alternating tetragonal superconducting and orthorhombic nematic domains in Co-substituted CaFe2As2. We use atomic force, magnetic force, and scanning tunneling microscopy to identify the domains and characterize their properties, finding in particular that tetragonal superconducting domains are very elongated, more than several tens of micrometers long and about 30 nm wide; have the same Tc as unstrained samples; and hold vortices in a magnetic field. Thus, biaxial strain produces a phase-separated state, where each phase is equivalent to what is found on either side of the first-order phase transition between antiferromagnetic orthorhombic and superconducting tetragonal phases found in unstrained samples when changing Co concentration. Having such alternating superconducting domains separated by normal conducting domains with sizes of the order of the coherence length opens opportunities to build Josephson junction networks or vortex pinning arrays and suggests that first-order quantum phase transitions lead to nanometric-size phase separation under the influence of strain, Work done in Madrid was supported by the Spanish Ministry of Economy and Competitiveness (Grants No. FIS2014-54498-R, No. MDM-2014-0377, No. MAT2014-52405-C2-2-R, No. RYC-2014-16626, and No. RYC-2014-15093), by the Comunidad de Madrid through program Nanofrontmag-CM (S2013/MIT-2850), by European Research Council PNICTEYES Grant Agreement No. 679080, by FP7-PEOPLE-2013-CIG 618321, by the EU Flagship Graphene Core1 under Grant Agreement No. 696656, by COST-EU, Grant No. CA16218 and by Axa Research Fund. SEGAINVEX-UAM is also acknowledged. Work done in Ames Lab was supported by the U.S. Department of Energy, Office of Basic Energy Science, Division of Materials Sciences and Engineering. Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract No. DE-AC02-07CH11358

We show that the magnetic ordering of coupled atomic dimers on a superconductor is revealed by their intra-gap spectral features. Chromium atoms on the superconductor $\beta$-Bi$_2$Pd surface display Yu-Shiba-Rusinov bound states, detected as pairs of intra-gap excitations in the tunneling spectra. We formed Cr dimers by atomic manipulation and found that their intra-gap features appear either shifted or split with respect to single atoms. The spectral variations reveal that the magnetic coupling of the dimer changes between ferromagnetic and antiferromagnetic depending on its disposition on the surface, in good agreement with density functional theory simulations. These results prove that superconducting intra-gap state spectroscopy is an accurate tool to detect the magnetic ordering of atomic scale structures.

We image vortex creep at very low temperatures using Scanning Tunneling Microscopy (STM) in the superconductor Rh$_9$In$_4$S$_4$ ($T_c$=2.25 K). We measure the superconducting gap of Rh$_9$In$_4$S$_4$, finding $\Delta\approx 0.33$meV and image a hexagonal vortex lattice up to close to H$_{c2}$, observing slow vortex creep at temperatures as low as 150 mK. We estimate thermal and quantum barriers for vortex motion and show that thermal fluctuations likely cause vortex creep, in spite of being at temperatures $T/T_c, Comment: 11 pages, 9 figures, 3 videos

Trabajo presentado en la conferencia Fuerzas y Túnel (FyT2016), celebrada en Girona del 5 al 7 de septiembre de 2016., Direct visualization and manipulation of magnetic structures in superconductors with high spatial resolution and sensitivity is at the aim of state-of-the-art research in fundamental and applied physics. The possibility of imaging the evolution of the vortex lattice provides valuable information on microscopic properties such as the symmetry of the superconducting order parameters or the pinning strength. Here we discuss the observation of superconducting vortices in β-Bi2Pd (Tc=5K[1]) up to fields of 600 Oe combining Magnetic Force Microscopy (MFM) and Squid On the Tip Microscopy (SOT). The combination of both techniques allows investigating the vortex lattice in two different regimes, not accessible with each microscopy alone: from very low fields (~1 Oe) up to higher fields (~600 Oe). We use a commercial MFM (Nanomagnetics Ltd.) in a home-made three axis superconducting coil system [2] that cools down to 2K. Experiments at Weizmann are made at 4.2K in a SOT [3]. We discuss in detail the pinning process of the vortex lattice, the evolution of the magnetic contrast with the temperature and the applied magnetic field. We find that the superconducting parameters of β-Bi2Pd are excellent to allow vortex lattice observation in a large field range. Furthermore, in MFM experiments, interesting new vortex shapes have been observed in the images, that will be discussed in terms of the magnetic interaction between tip and sample, and the different magnetic domain configuration of the MFM tip.

The layered rare-earth diantimonides RSb2 are anisotropic metals with generally low electronic densities whose properties can be modified by substituting the rare earth. LaSb2 is a nonmagnetic metal with a low residual resistivity presenting a low-temperature magnetoresistance that does not saturate with the magnetic field. It has been proposed that the latter can be associated to a charge density wave (CDW), but no CDW has yet been found. Here we find a kink in the resistivity above room temperature in LaSb2 (at 355 K) and show that the kink becomes much more pronounced with substitution of La by Ce along the La1−xCexSb2 series. We find signatures of a CDW in x-ray scattering, specific heat, and scanning tunneling microscopy (STM) experiments in particular for x ≈ 0.5. We observe a distortion of rare-earth–Sb bonds lying in-plane of the tetragonal crystal using x-ray scattering, an anomaly in the specific heat at the same temperature as the kink in resistivity and charge modulations in STM. We conclude that LaSb2 has a CDW which is stabilized in the La1−xCexSb2 series due to substitutional disorder, This work was supported by the Spanish MINECO (FIS2014-54498-R, MAT2011-27470-C02-02, and CSD-2009-00013), by the European Union (Graphene Flagship Contract No. CNECT-ICT-604391 and COST MP1201 action), and by the Comunidad de Madrid through programs Nanofrontmag-CM (S2013/MIT-2850) and MAD2D-CM (S2013/MIT-3007).We acknowledge MINECO and CSIC for financial support and for provision of synchrotron radiation facilities and would like to thank the SpLine BM25 staff for assistance in using the beamline

We report an anomalous tunneling conductance with a zero-bias peak in flakes of superconducting 2H-TaS2 detached through mechanical exfoliation. To explain the observed phenomenon, we construct a minimal model for a single unit cell layer of superconducting 2H-TaS2 with a simplified two-dimensional Fermi surface and a sign-changing Cooper-pair wave function induced by Coulomb repulsion. Superconductivity is induced in the central Γ pocket, where it becomes nodal. We show that weak scattering at the nodal Fermi surface, produced by nonperturbative coupling between tip and sample, gives Andreev states that lead to a zero-bias peak in the tunneling conductance. We suggest that reducing dimensionality down to crystals a few atoms thick could drive a crossover from conventional to sign-changing pairing in the superconductor 2H-TaS2., This work was supported by the Spanish MINECO (Consolider Ingenio Molecular Nanoscience CSD2007-00010 program and FIS2011-23488, ACI2009-0905, FIS2011-23713, and MAT2011-22785), by the European Union (ERC Advanced Grants No. 290846 and No. 247384, Graphene Flagship Contract No. CNECT-ICT-604391, Marie Curie Actions FP7-PEOPLE-2013-CIG-618321, and COST MP1201 action), by the Comunidad de Madrid through program Nanobiomagnet, and by the Generalidad de Valencia through programs PROMETEO and ISIC-NANO.

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, This work was supported by the EU (ERC Advanced Grant SPINMOL and COST MP-1201), the Spanish MINECO (Consolider-Ingenio in Molecular Nanoscience, CSD2007-00010 and projects FIS2011-23488, MAT2011-25046, MAT2011-22785 and ACI-2009-0905, co-financed by FEDER), by the Comunidad de Madrid (program Nanobiomagnet) and the Generalitat Valenciana (Programs Prometeo and ISIC-NANO)

Tesis doctoral inédita. Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física de la Materia Condensada. Fecha de lectura: 19-06-08

We use a scanning tunneling microscope to study the superconducting density of states and vortex lattice of single crystals of CaKFe4As4. This material has a critical temperature of Tc=35 K, one of the highest among stoichiometric iron based superconductors (FeBSCs), and is comparable to Tc found near optimal doping in other FeBSCs. We observe quasiparticle scattering from defects with a pattern related to interband scattering between zone centered hole sheets. We measure the tunneling conductance in vortex cores and find a peak due to Caroli-de Gennes-Matricon bound states. The peak is located above the Fermi level, showing that CaKFe4As4 is a clean superconductor with vortex core bound states close to the so-called extreme quantum limit. We identify locations where the superconducting order parameter is strongly suppressed due to pair breaking. Vortices are pinned at these locations, and the length scale of the suppression of the order parameter is of order of the vortex core size. As a consequence, the vortex lattice is disordered up to 8 T., We acknowledge discussions with R. Prozorov and S. Vieira. Work done in Madrid (A.F., I.G. and H.S.) was supported by the Spanish State Agency for Research, AEI (FIS2014-54498-R, MDM-2014-0377), by the Comunidad de Madrid through program Nanofrontmag-CM (S2013/MIT-2850), by the European Research Council PNICTEYES Grant Agreement No. 679080 (IG) and by FP7-PEOPLE-2013-CIG 618321, Cost CA16218 Nanocohybri, and Axa Research Fund. SEGAINVEX-UAM and Banco Santander are also acknowledged. Work done in Ames Lab (P.C.C., S.L.B., V.G.K., W.R.M., and T.K.) was supported by the US Department of Energy, Office of Basic Energy Science, Division of Materials Sciences and Engineering. Ames Laboratory is operated for the US Department of Energy by Iowa State University under Contract No. DE-AC02-07CH11358. W.R.M. was supported by the Gordon and Betty Moore Foundation's EPiQS Initiative through Grant No. GBMF4411