A crucial step in revealing the nature of unconventional superconductivity is to investigate the symmetry of the superconducting order parameter. Scanning tunneling spectroscopy has proven a powerful technique to probe this symmetry by measuring the quasiparticle interference (QPI) which sensitively depends on the superconducting pairing mechanism. A particularly well suited material to apply this technique is the stoichiometric superconductor LiFeAs as it features clean, charge neutral cleaved surfaces without surface states and a relatively high Tc ∼ 18 K. Our data reveal that in LiFeAs the quasiparticle scattering is governed by a van-Hove singularity at the center of the Brillouin zone which is in stark contrast with other pnictide superconductors where nesting is crucial for both scattering and s±-superconductivity. Indeed, within a minimal model and using the most elementary order parameters, calculations of the QPI suggest a dominating role of the holelike bands for the quasiparticle scattering. Our theoretical findings do not support the elementary singlet pairing symmetries s++, s±, and d-wave. This brings to mind that the superconducting pairing mechanism in LiFeAs is based on an unusual pairing symmetry such as an elementary pwave (which provides optimal agreement between the experimental data and QPI simulations) or a more complex order parameter (e.g. s + id-wave symmetry).The discovery of iron-based superconductors [1] has generated enormous research activities to reveal the nature of superconductivity in these materials. In particular, s ± -pairing, i.e. an s-wave order parameter with alternating sign between almost perfectly nested hole and electron pockets has been suggested to be prevailing for the entire class of iron-based superconductors [2][3][4]. In this regard, the material LiFeAs is of particular interest since experiments have proven an absence of nesting [5] and theoretical works yield contradictory results, i.e., both s ± -wave singlet as well as p-wave triplet pairing has been suggested [6,7].A powerful method to probe the symmetry of the superconducting order parameter is to map out the spatial dependence of the local density of states (DOS) by scanning tunneling spectroscopy (STS). In such experiments, the relation DOS(E) ∝ dI/dV (V bias ) with tunneling voltage V and current I at energy E and bias voltage V bias (E = eV bias ) is exploited. The spatial dependence of the DOS often arises from an impurity scattering of the conduction electrons. In this case the incident and scattered quasiparticle waves interfere and give rise to Friedel-like oscillations in the local density of states (LDOS). Such quasiparticle interference (QPI) effects have first been observed by STS experiments on normal state metal surfaces [8][9][10]. A convenient way to extract the dominating scattering vectors q from a spatially resolved image of the QPI pattern is by analysis of its Fourier transformed image [9,10]. This so-called spectroscopic-imaging scanning tunneling microscopy (SI-STM) has proven to be...

We investigate the influence of the electron-phonon coupling in the one-dimensional spinless Holstein model at half-filling using both a recently developed projector-based renormalization method (PRM) and an refined exact diagonalization technique in combination with the kernel polynomial method. At finite phonon frequencies the system shows a metal-insulator transition accompanied by the appearance of a Peierls distorted state at a finite critical electron-phonon coupling. We analyze the opening of a gap in terms of the (inverse) photoemission spectral functions which are evaluated in both approaches. Moreover, the PRM approach reveals the softening of a phonon at the Brillouin-zone boundary which can be understood as precursor effect of the gap formation.

The discovery of topological quantum materials represents a striking innovation in modern condensed matter physics with remarkable fundamental and technological implications. Their classification has been recently extended to topological Weyl semimetals, i.e., solid state systems which exhibit the elusive Weyl fermions as low-energy excitations. Here we show that the Nernst effect can be exploited as a sensitive probe for determining key parameters of the Weyl physics, applying it to the non-collinear antiferromagnet Mn3Ge. This compound exhibits anomalous thermoelectric transport due to enhanced Berry curvature from Weyl points located extremely close to the Fermi level. We establish from our data a direct measure of the Berry curvature at the Fermi level and, using a minimal model of a Weyl semimetal, extract the Weyl point energy and their distance in momentum-space. arXiv:1902.01647v2 [cond-mat.str-el]

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Using the angle-resolved photoemission spectroscopy (ARPES) data accumulated over the whole Brillouin zone (BZ) in LiFeAs we analyze the itinerant component of the dynamic spin susceptibility in this system in the normal and superconducting state. We identify the origin of the incommensurate magnetic inelastic neutron scattering (INS) intensity as scattering between the electron pockets, centered around the (π, π) point of the BZ and the large two-dimensional hole pocket, centered around the Γ-point of the BZ. As the magnitude of the superconducting gap within the large hole pocket is relatively small and angle dependent, we interpret the INS data in the superconducting state as a renormalization of the particle-hole continuum rather than a true spin exciton. Our comparison indicates that the INS data can be reasonably well described by both the sign changing symmetry of the superconducting gap between electron and hole pockets as well as sign preserving gap, depending on the assumptions made for the fermionic damping.The relation between unconventional superconductivity and magnetism is one of the most interesting topics in condensed-matter physics. For example, in most of the iron-based superconductors superconductivity occurs in close vicinity to an antiferromagnetic (AF) state[1-3]. Moreover, superconductivity emerges when antiferromagnetic order in parent compounds is suppressed, either by electron/hole doping or disorder. In addition, shortrange AF spin excitations are still present in the normal state of the doped systems and also become resonant in the superconducting state at energies below twice the superconducting gap magnitude, 2∆ 0 [4]. This resonant enhancement is believed to be a signature of a certain phase structure of the superconducting gap as the paramagnetic spin response of the Bogolyubov quasiparticles at the antiferromagnetic wave vector Q AF is sensitive to the anomalous coherence factor 1 − ∆ k ∆ k+Q AF |∆ k ||∆ k+Q AF | . Once the superconducting gap at parts of the Fermi surface, connected by Q AF , changes sign, the spin response acquires an additional enhancement at Ω ≤ 2∆ 0 , which is a hallmark of unconventional superconductivity. The observation of the spin resonance in many iron-based superconductors provides strong evidence for the so-called s +− -wave symmetry of the superconducting gap, where the gap structure changes sign between electron and hole pockets [5][6][7]. Note that this does not exclude the gap on each pocket to have a strong angular variation and even accidental nodal lines, allowed by A 1g symmetry [3]. The angular variation of the gap, measured in ARPES [8], is inconsistent with idealized lattice version of s +− , but can be modeled by taking into account interaction effects. While the behavior, described above, is observed in the majority of the iron-based superconductors, there are some notable exceptions. Perhaps the most interesting one is the stoichiometric LiFeAs, which superconducts at T c =17 K without any doping [9][10][11]. In addition, LiFeAs shows ne...