Search

Spin-dependent scattering from magnetic impurities inside a superconductor gives rise to Yu-Shiba-Rusinov (YSR) states within the superconducting gap. They can be modeled by the largely equivalent Kondo or Anderson impurity models. The role of the magnetic and nonmagnetic properties of the impurity in relation to the coupling to the substrate is still under debate. Here, we use a scanning tunneling microscope to make a quantitative connection between the energy of a YSR state and the impurity-substrate hybridization. We corroborate the impurity substrate coupling as a key energy scale for surface derived YSR states using the Anderson impurity model. By combining experimental data from YSR state spectra and additional conductance measurements, we can determine on which side of the quantum phase transition the system resides. We thus provide a crucial step towards a more quantitative understanding of the crucial role of impurity substrate coupling utilizing the Anderson model. The physics of Yu-Shiba-Rusinov states which exist in the superconducting gap are a topical area of research and linked to exotic phenomena such as Majorana fermions. Here, the authors use scanning tunnelling microscopy to investigate the influence of impurities on a superconducting surface and the role impurity-substrate hybridisation has on such states.

In the field of Dirac materials, spin-orbit coupling (SOC) is usually considered disruptive, since it may lift degeneracies that are not protected by high-symmetry elements. Nonsymmorphic symmetries force degenerate points in the band structure at high-symmetry points that are not disrupted by SOC. The degeneracy is, however, often protected along whole high-symmetry lines or faces resulting in highly anisotropic crossings or nodal lines, which can considerably limit the region, in which the bands are linearly dispersed. It has been theoretically suggested that SOC could circumvent this problem. Here, we show experimentally that SOC can lift the extended protection in nonsymmorphic square-net compounds. We compare ZrSiS and CeSbTe , two materials with drastically different SOC, to show the effect of SOC on the band structure by means of angle-resolved photoemission spectroscopy and density functional theory calculations.

Very recent experiments have reported the tunneling between Yu-Shiba-Rusinov (YSR) bound states at the atomic scale. These experiments have been realized with the help of a scanning tunneling microscope where a superconducting tip is functionalized with a magnetic impurity and is used to probe another magnetic impurity deposited on a superconducting substrate. In this way it has become possible to study for the first time the spin-dependent transport between individual superconducting bound states. Motivated by these experiments, we present here a comprehensive theoretical study of the tunneling processes between YSR bound states in a system in which two magnetic impurities are coupled to superconducting leads. Our theory is based on a combination of an Anderson model with broken spin degeneracy to describe the impurities and nonequilibrium Green's function techniques to compute the current-voltage characteristics. This combination allows us to describe the spin-dependent transport for an arbitrary strength of the tunnel coupling between the impurities. We first focus on the tunnel regime and show that our theory naturally explains the experimental observations of the appearance of current peaks in the subgap region due to both the direct and thermal tunneling between the YSR states in both impurities. Then, we study in detail the case of junctions with increasing transparency, which has not been experimentally explored yet, and predict the occurrence of a large variety of (multiple) Andreev reflections mediated by YSR states that give rise to a very rich structure in the subgap current. In particular, we predict the occurrence of multiple Andreev reflections that involve YSR states in different impurities. These processes have no analogue in single-impurity junctions and they are manifested as current peaks with negative differential conductance for subgap voltages., 16 pages, 9 figures. arXiv admin note: text overlap with arXiv:2005.06491

Despite plenty of room at the bottom, there is a limit to the miniaturization of every process. For charge transport this is realized by the coupling of single discrete energy levels at the atomic scale. Here, we demonstrate sequential tunneling between parity protected Yu-Shiba-Rusinov (YSR) states bound to magnetic impurities located on the superconducting tip and sample of a scanning tunneling microscope at 10 mK. We reduce the relaxation of the excited YSR state to the bare minimum and find an enhanced lifetime for single quasiparticle levels. Our work offers a way to characterize and to manipulate coupled superconducting bound states, such as Andreev levels, YSR states, or Majorana bound states., 14 pages, 10 figures incl. supplementary information

The following article appeared in Physical Review B 101.13 (2020): 134507. DOI:10.1103/PhysRevB.101.134507. Open access publication funded by the Max Planck Society, As scanning tunneling microscopy is pushed towards fast local dynamics, a quantitative understanding of tunnel junctions under the influence of a fast ac driving signal is required, especially at the ultralow temperatures relevant to spin dynamics and correlated electron states. We subject a superconductor-insulator-superconductor junction to a microwave signal from an antenna mounted in situ and examine the dc response of the contact to this driving signal. Quasiparticle tunneling and the Josephson effect can be interpreted in the framework of Tien-Gordon theory. The situation is more complex when it comes to higher-order effects such as multiple Andreev reflections. Microwave-assisted tunneling unravels these complex processes, providing deeper insights into tunneling than are available in a pure dc measurement, J.C.C. acknowledges funding from the Spanish Ministry of Economy and Competitiveness (MINECO) (Contract No. FIS2017-84057-P). J.A. acknowledges support from the IQST and the German Science Foundation (DFG) under Grant No. AN336/11-1. This work was funded in part by the ERC Consolidator Grant AbsoluteSpin (Grant No. 681164)

We introduce a van der Waals material that exhibits a very high electronic mobility and antiferromagnetism and can be exfoliated., Van der Waals (vdW) materials with magnetic order have been heavily pursued for fundamental physics as well as for device design. Despite the rapid advances, so far, they are mainly insulating or semiconducting, and none of them has a high electronic mobility—a property that is rare in layered vdW materials in general. The realization of a high-mobility vdW material that also exhibits magnetic order would open the possibility for novel magnetic twistronic or spintronic devices. Here, we report very high carrier mobility in the layered vdW antiferromagnet GdTe3. The electron mobility is beyond 60,000 cm2 V−1 s−1, which is the highest among all known layered magnetic materials, to the best of our knowledge. Among all known vdW materials, the mobility of bulk GdTe3 is comparable to that of black phosphorus. By mechanical exfoliation, we further demonstrate that GdTe3 can be exfoliated to ultrathin flakes of three monolayers.

Transition-metal chalcogenides are a promising family of materials for applications as photocathodes in photoelectrochemical (PEC) H2 generation. A long-standing challenge for chalcopyrite semiconductors is characterizing their electronic structure, both experimentally and theoretically, because of their relatively high-energy band gaps and spin-orbit coupling (SOC), respectively. In this work, we present single crystals of CuInTe2, whose relatively small optically measured band gap of 0.9 ± 0.03 eV enables electronic structure characterization by angle-resolved photoelectron spectroscopy (ARPES) in conjunction with first-principles calculations incorporating SOC. ARPES measurements reveal bands that are steeply dispersed in energy with a band velocity of 2.5-5.4 × 105 m/s, almost 50% of the extremely conductive material graphene. Additionally, CuInTe2 single crystals are fabricated into electrodes to experimentally determine the valence band edge energy and confirm the thermodynamic suitability of CuInTe2 for water redox chemistry. The electronic structure characterization and band edge position presented in this work provide kinetic and thermodynamic factors that support CuInTe2 as a strong candidate for water reduction.

We study the electronic structure of the nodal line semimetal ZrSiTe both experimentally and theoretically. We find two different surface states in ZrSiTe—topological drumhead surface states and trivial floating band surface states, which can be easily distinguished in ARPES experiments. Using the spectra of Wilson loops, we show that a nontrivial Berry phase that exists in a confined region within the Brillouin zone gives rise to the topological drumhead-type surface states. The Z_{2} structure of the Berry phase induces a Z_{2} “modular arithmetic” of the surface states, allowing surface states derived from different nodal lines to hybridize and gap out, which can be probed by a set of Wilson loops. Our findings are confirmed by ab initio calculations and angle-resolved photoemission experiments, which are in excellent agreement with each other and the topological analysis. This work is the first complete characterization of topological surface states in the family of square-net-based nodal line semimetals, and thus it fundamentally increases the understanding of the topological nature of this growing class of topological semimetals.

We study the band dispersion of graphene with randomly distributed structural defects using two complementary methods, exact diagonalization of the tight-binding Hamiltonian and implementing a self-consistent T matrix approximation. We identify three distinct types of impurities resulting in qualitatively different spectra in the vicinity of the Dirac point. First, resonant impurities, such as vacancies or 585 defects, lead to stretching of the spectrum at the Dirac point with a finite density of localized states. This type of spectrum has been observed in epitaxial graphene by photoemission spectroscopy and discussed extensively in the literature. Second, nonresonant (weak) impurities, such as paired vacancies or Stone-Wales defects, do not stretch the spectrum but provide a line broadening that increases with energy. Finally, disorder that breaks sublattice symmetry, such as vacancies placed in only one sublattice, open a gap around the Dirac point and create an impurity band in the middle of this gap. We find good agreement between the results of the two methods and also with the experimentally measured spectra., 11 pages, 5 figures, including supplementary information

New developments in the field of topological matter are often driven by materials discovery, including novel topological insulators, Dirac semimetals and Weyl semimetals. In the last few years, large efforts have been performed to classify all known inorganic materials with respect to their topology. Unfortunately, a large number of topological materials suffer from non-ideal band structures. For example, topological bands are frequently convoluted with trivial ones, and band structure features of interest can appear far below the Fermi level. This leaves just a handful of materials that are intensively studied. Finding strategies to design new topological materials is a solution. Here we introduce a new mechanism that is based on charge density waves and non-symmorphic symmetry to design an idealized Dirac semimetal. We then show experimentally that the antiferromagnetic compound GdSb$_{0.46}$Te$_{1.48}$ is a nearly ideal Dirac semimetal based on the proposed mechanism, meaning that most interfering bands at the Fermi level are suppressed. Its highly unusual transport behavior points to a thus far unknown regime, in which Dirac carriers with Fermi energy very close to the node seem to gradually localize in the presence of lattice and magnetic disorder.

Robust quantum systems rely on having a protective environment with minimized relaxation channels. Superconducting gaps play an important role in the design of such environments. The interaction of localized single spins with a conventional superconductor generally leads to intrinsically extremely narrow Yu-Shiba-Rusinov (YSR) resonances protected inside the superconducting gap. However, this may not apply to superconductors with nontrivial, energy dependent order parameters. Exploiting the Fe-doped two-band superconductor NbSe$_2$, we show that due to the nontrivial relation between its complex valued and energy dependent order parameters, YSR states are no longer restricted to be inside the gap. They can appear outside the gap (i. e. inside the coherence peaks), where they can also acquire a substantial intrinsic lifetime broadening. T-matrix scattering calculations show excellent agreement with the experimental data and relate the intrinsic YSR state broadening to the imaginary part of the host's order parameters. Our results suggest that non-thermal relaxation mechanisms contribute to the finite lifetime of the YSR states, even within the superconducting gap, making them less protected against residual interactions than previously assumed. YSR states may serve as valuable probes for nontrivial order parameters promoting a judicious selection of protective superconductors., Comment: 11 pages, 8 figures, including supporting information

By establishing magnetic order in a square lattice compound, we introduce the first magnetic “new fermion.”, Recent interest in topological semimetals has led to the proposal of many new topological phases that can be realized in real materials. Next to Dirac and Weyl systems, these include more exotic phases based on manifold band degeneracies in the bulk electronic structure. The exotic states in topological semimetals are usually protected by some sort of crystal symmetry, and the introduction of magnetic order can influence these states by breaking time-reversal symmetry. We show that we can realize a rich variety of different topological semimetal states in a single material, CeSbTe. This compound can exhibit different types of magnetic order that can be accessed easily by applying a small field. Therefore, it allows for tuning the electronic structure and can drive it through a manifold of topologically distinct phases, such as the first nonsymmorphic magnetic topological phase with an eightfold band crossing at a high-symmetry point. Our experimental results are backed by a full magnetic group theory analysis and ab initio calculations. This discovery introduces a realistic and promising platform for studying the interplay of magnetism and topology. We also show that we can generally expand the numbers of space groups that allow for high-order band degeneracies by introducing antiferromagnetic order.

A promising route to the realization of Majorana fermions is in non-centrosymmetric superconductors, in which spin-orbit-coupling lifts the spin degeneracy of both bulk and surface bands. A detailed assessment of the electronic structure is critical to evaluate their suitability for this through establishing the topological properties of the electronic structure. This requires correct identification of the time-reversal-invariant momenta. One such material is BiPd, a recently rediscovered non-centrosymmetric superconductor which can be grown in large, high-quality single crystals and has been studied by several groups using angular resolved photoemission to establish its surface electronic structure. Many of the published electronic structure studies on this material are based on a reciprocal unit cell which is not the actual Brillouin zone of the material. We show here the consequences of this for the electronic structures and show how the inferred topological nature of the material is affected., Comment: Originally a comment on 1505.03466/1610.03431 and 1609.08947. 4 pages plus an appendix with literature crystal structure refinements

We present the design and calibration measurements of a scanning tunneling microscope setup in a 3He ultrahigh-vacuum cryostat operating at 400 mK with a hold time of 10 days. With 2.70 m in height and 4.70 m free space needed for assembly, the cryostat fits in a one-story lab building. The microscope features optical access, an xy table, in situ tip and sample exchange, and enough contacts to facilitate atomic force microscopy in tuning fork operation and simultaneous magneto-transport measurements on the sample. Hence, it enables scanning tunneling spectroscopy on microstructured samples which are tuned into preselected transport regimes. A superconducting magnet provides a perpendicular field of up to 14 T. The vertical noise of the scanning tunneling microscope amounts to 1 pmrms within a 700 Hz bandwidth. Tunneling spectroscopy using one superconducting electrode revealed an energy resolution of 120 mueV. Data on tip-sample Josephson contacts yield an even smaller feature size of 60 mueV, implying that the system operates close to the physical noise limit., 12 pages, 11 figures

Layered heavy-metal square-lattice compounds have recently emerged as potential Dirac fermion materials due to bonding within those sublattices. We report quantum transport and spectroscopic data on the layered Sb square-lattice material LaCuSb$_{2}$. Linearly dispersing band crossings, necessary to generate Dirac fermions, are experimentally observed in the electronic band structure observed using angle-resolved photoemission spectroscopy (ARPES), along with a quasi-two-dimensional Fermi surface. Weak antilocalization that arises from two-dimensional transport is observed in the magnetoresistance, as well as regions of linear dependence, both of which are indicative of topologically non-trivial effects. Measurements of the Shubnikov-de Haas (SdH) quantum oscillations show low effective mass electrons on the order of 0.065$m_{e}$, further confirming the presence of Dirac fermions in this material.

We have studied the temporal evolution of a quantum system subjected to strong dissipation at ultra-low temperatures where the system-bath interaction represents the leading energy scale. In this regime, theory predicts the time evolution of the system to follow a generalization of the classical Smoluchowski description, the quantum Smoluchowski equation, thus, exhibiting quantum Brownian motion characteristics. For this purpose, we have investigated the phase dynamics of a superconducting tunnel junction in the presence of high damping. We performed current-biased measurements on the small-capacitance Josephson junction of a scanning tunneling microscope placed in a low impedance environment at milli-Kelvin temperatures. We can describe our experimental findings by a quantum diffusion model with high accuracy in agreement with theoretical predications based on the quantum Smoluchowski equation. In this way we experimentally demonstrate that quantum systems subjected to strong dissipation follow quasi-classical dynamics with significant quantum effects as the leading corrections., 5 pages, 4 figures

In this work, we present a model of the surface states of nonsymmorphic semimetals. These are derived from surface mass terms that lift the high degeneracy imposed in the band structure by the nonsymmorphic bulk symmetries. Reflecting the reduced symmetry at the surface, the bulk bands are strongly modified. This leads to the creation of two-dimensional floating bands, which are distinct from Shockley states, quantum well states or topologically protected surface states. We focus on the layered semimetal ZrSiS to clarify the origin of its surface states. We demonstrate an excellent agreement between DFT calculations and ARPES measurements and present an effective four-band model in which similar surface bands appear. Finally, we emphasize the role of the surface chemical potential by comparing the surface density of states in samples with and without potassium coating. Our findings can be extended to related compounds and generalized to other crystals with nonsymmorphic symmetries., Comment: 8 pages, 5 figures

Materials with strong spin-orbit coupling (SOC) have in recent years become a subject of intense research due to their potential applications in spintronics and quantum information technology. In particular, in systems which break inversion symmetry, SOC facilitates the Rashba-Dresselhaus effect, leading to a lifting of spin degeneracy in the bulk and intricate spin textures of the Bloch wave functions. Here, by combining angular resolved photoemission (ARPES) and low temperature scanning tunneling microscopy (STM) measurements with relativistic first-principles band structure calculations, we examine the role of SOC in single crystals of noncentrosymmetric BiPd. We report the detection of several Dirac surface states, one of which exhibits an extremely large spin splitting. Unlike the surface states in inversion-symmetric systems, the Dirac surface states of BiPd have completely different properties at opposite faces of the crystal and are not trivially linked by symmetry. The spin-splitting of the surface states exhibits a strong anisotropy by itself, which can be linked to the low in-plane symmetry of the surface termination., Comment: 6 page letter, + supplementary material

The Dirac-like electronic structure of graphene originates from the equivalence of the two basis atoms in the honeycomb lattice. We show that the characteristic parameters of the initial state wave function (sublattice symmetry and isospin) can be determined using angle-resolved photoemission spectroscopy (ARPES) with circularly polarized synchrotron radiation. At a photon energy of hν = 52 eV, transition matrix element effects can be neglected allowing us to determine sublattice symmetry and isospin with high accuracy using a simple theoretical model.

Superconductivity is commonly described as a macroscopic quantum phenomenon. However, it arises from microscopic mechanisms occurring at the nanometer scale as illustrated, for example, by the non-trivial pairing in unconventional superconductors. More recently, also local interactions with superconductors in the context of Majorana fermions became of interest. A very direct way to study the atomic scale properties of superconductors is given by the combination of the Josephson effect with scanning tunneling microscopy (STM), also referred to as JSTM. Here, the critical Josephson current serves as a direct local probe of the superconducting ground state and may reveal valuable information that is often inaccessible when studying quasi-particle excitation spectra. We show that we can extract local values of the critical Josephson current from JSTM measurements in the dynamical Coulomb blockade regime. Furthermore, we experimentally determine the regime of sequential Cooper pair tunneling, which is in accordance to theoretical predictions. Our study presents new insights on the tunneling mechanisms in Josephson junctions and lays the basis for the implementation of JSTM as a versatile probe for superconductivity.

Materials harbouring exotic quasiparticles, such as massless Dirac and Weyl fermions, have garnered much attention from physics and material science communities due to their exceptional physical properties such as ultra-high mobility and extremely large magnetoresistances. Here, we show that the highly stable, non-toxic and earth-abundant material, ZrSiS, has an electronic band structure that hosts several Dirac cones that form a Fermi surface with a diamond-shaped line of Dirac nodes. We also show that the square Si lattice in ZrSiS is an excellent template for realizing new types of two-dimensional Dirac cones recently predicted by Young and Kane. Finally, we find that the energy range of the linearly dispersed bands is as high as 2 eV above and below the Fermi level; much larger than of other known Dirac materials. This makes ZrSiS a very promising candidate to study Dirac electrons, as well as the properties of lines of Dirac nodes., The family of topological materials has been growing rapidly but most members bare limitations hindering the study of exotic behaviour of topological particles. Here, Schoop et al. report a Fermi surface with a diamond-shaped line of Dirac nodes in ZrSiS, providing a promising candidate for studying two-dimensional Dirac fermions.

International audience; The charge carrier dynamics of photoexcited bismuth generates a Drude response that evolves over time. Our data show that the plasma frequency of bismuth displays an initial increase and a subsequent decay. We have performed ab initio calculations on bulk bismuth within the density functional theory and show that this peculiar behavior is due to local extrema in the valence and conduction bands. It follows that most of the carriers first accumulate in these extrema and reach the Fermi level only 0.6 ps after the photoexcitation. © 2012 American Physical Society.

We have used s- and p-polarized synchrotron radiation to image the electronic structure of epitaxial graphene near the K-point by angular resolved photoemission spectroscopy (ARPES). Part of the experimental Fermi surface is suppressed due to the interference of photoelectrons emitted from the two equivalent carbon atoms per unit cell of graphene's honeycomb lattice. Here we show that by rotating the polarization vector, we are able to illuminate this 'dark corridor' indicating that the present theoretical understanding is oversimplified. Our measurements are supported by first-principles photoemission calculations, which reveal that the observed effect persists in the low photon energy regime., 5 pages, 4 figures

The application of graphene in electronic devices requires large scale epitaxial growth. The presence of the substrate, however, usually reduces the charge carrier mobility considerably. We show that it is possible to decouple the partially sp3-hybridized first graphitic layer formed on the Si-terminated face of silicon carbide from the substrate by gold intercalation, leading to a completely sp2-hybridized graphene layer with improved electronic properties., 7 pages, 4 figures, 1 table

The tunnelling current in scanning tunnelling spectroscopy (STS) is typically and often implicitly modelled by a continuous and homogeneous charge flow. If the charging energy of a single-charge quantum sufficiently exceeds the thermal energy, however, the granularity of the current becomes non-negligible. In this quantum limit, the capacitance of the tunnel junction mediates an interaction of the tunnelling electrons with the surrounding electromagnetic environment and becomes a source of noise itself, which cannot be neglected in STS. Using a scanning tunnelling microscope operating at 15 mK, we show that we operate in this quantum limit, which determines the ultimate energy resolution in STS. The P(E)-theory describes the probability for a tunnelling electron to exchange energy with the environment and can be regarded as the energy resolution function. We experimentally demonstrate this effect with a superconducting aluminium tip and a superconducting aluminium sample, where it is most pronounced., The tunnelling current in scanning tunnelling spectroscopy has often been treated by a continuous charge flow, which lacks proper treatment of charge quantization. Here, Ast et al. unveil the effects of granularity in the tunnelling current at extremely low temperatures by including P(E) theory, thereby reaching the quantum limit in scanning tunnelling spectroscopy.

Using angle resolved photoemission spectroscopy we studied the evolution of the surface electronic structure of the topological insulator $\text{Bi}_2\text{Se}_3$ as a function of water vapor exposure. We find that a surface reaction with water induces a band bending shifting the Dirac point deep into the occupied states and creating quantum well states with a strong Rashba-type splitting. The surface is thus not chemically inert, but the topological state remains protected. The band bending is traced back to Se-abstraction leaving positively charged vacancies at the surface. Due to the presence of water vapor, a similar effect takes place when $\text{Bi}_2\text{Se}_3$ crystals are left in vacuum or cleaved in air, which likely explains the aging effect observed in the $\text{Bi}_2\text{Se}_3$ band structure., 4 pages, 4 figures

Quantum fluctuations are imprinted with valuable information about transport processes. Experimental access to this information is possible, but challenging. We introduce the dynamical Coulomb blockade (DCB) as a local probe for fluctuations in a scanning tunneling microscope (STM) and show that it provides information about the conduction channels. In agreement with theoretical predictions, we find that the DCB disappears in a single-channel junction with increasing transmission following the Fano factor, analogous to what happens with shot noise. Furthermore we demonstrate local differences in the DCB expected from changes in the conduction channel configuration. Our experimental results are complemented by ab initio transport calculations that elucidate the microscopic nature of the conduction channels in our atomic-scale contacts. We conclude that probing the DCB by STM provides a technique complementary to shot noise measurements for locally resolving quantum transport characteristics., 9 pages, 7 figures, including supplementary material