Search

Misfit layer compounds, heterostructures composed by a regular alternating stacking of rocksalt monochalcogenides bilayers and few-layer transition metal dichalchogenides, are an emergent platform to investigate highly doped transition metal dichalcogenides. Among them, (LaSe)$_{1.14}$(NbSe$_2$)$_2$ displays Ising superconductivity, while the presence of a charge density wave (CDW) in the material is still under debate. Here, by using polarized Raman spectroscopy and first-principles calculations, we show that NbSe$_2$ undergoes a doping-driven collapse of the CDW ordering within the misfit, and no signature of the CDW is detected down to 8~K. We provide a complete experimental and theoretical description of the lattice dynamics of this misfit compound. We show that the vibrational properties are obtained from those of the two subunits, namely the LaSe unit and the NbSe$_2$ bilayer, in the presence of a suitable field-effect doping, and then highlight the 2D nature of the lattice dynamics of NbSe$_2$ within the (LaSe)$_{1.14}$(NbSe$_2$)$_2$ 3D structure., Comment: First version of the manuscript (before revisions)

Topological defects are ubiquitous, they manifest in a wide variety of systems such as liquid crystals, magnets or superconductors. The recent quest for nonabelian anyons in condensed matter physics stimulates the interest for topological defects since they can be hosted in vortices in quantum magnets or topological superconductors. In addition to these vortex defects, in this study we propose to investigate edge dislocations in 2D magnets as new building blocks for topological physics since they can be described as vortices in the structural phase field. Here we demonstrate the existence of higher order topological dislocations within the higher order moir\'e pattern of the van der Waals 2D magnet CrCl3 deposited on Au(111). Surprizingly, these higher order dislocations arise from ordinary simple edge dislocations in the atomic lattice of CrCl3. We provide a theoretical framework explaining the higher order dislocations as vortex with a winding Chern number of 2. We expect that these original defects could stabilize some anyons either in a 2D quantum magnet or within a 2D superconductor coupled to it.

The interplay of superconductivity and disorder generates a wealth of complex phenomena. In particular, the peculiar structure of diffusive electronic wavefunctions is predicted to increase the superconducting critical temperature in some range of disorder. In this work, we use an epitaxial monolayer of lead showing a simple band structure and homogenous structural disorder as a model system of a 2D superconductor in the weak-antilocalization regime. Then, we perform an extensive study of the emergent fluctuations of local density of states (LDOS) and spectral energy gap in this material and compare them with both analytical results and numerical solution of the attractive Hubbard model. We show that mesoscopic LDOS fluctuations allow to probe locally both the elastic and inelastic scattering rates which are notoriously difficult to measure in transport measurements., Comment: Accepted for publication in PRB - 13 pages, 6 figures

Transition metal dichalcogenides (TMDs) display a rich variety of instabilities such as spin and charge orders, Ising superconductivity and topological properties. Their physical properties can be controlled by doping in electric double-layer field-effect transistors (FET). However, for the case of single layer NbSe$_2$, FET doping is limited to $\approx 1\times 10^{14}$ cm$^{-2}$, while a somewhat larger charge injection can be obtained via deposition of K atoms. Here, by performing ARPES, STM, quasiparticle interference measurements, and first principles calculations we show that a misfit compound formed by sandwiching NbSe$_2$ and LaSe layers behaves as a NbSe$_2$ single layer with a rigid doping of $0.55-0.6$ electrons per Nb atom or $\approx 6\times 10^{14}$ cm$^{-2}$. Due to this huge doping, the $3\times3$ charge density wave is replaced by a $2\times2$ order with very short coherence length. As a tremendous number of different misfit compounds can be obtained by sandwiching TMDs layers with rock salt or other layers, our work paves the way to the exploration of heavily doped 2D TMDs over an unprecedented wide range of doping.

We study the unconventional superconducting correlations caused by a single isolated magnetic impurity in a conventional s-wave superconductor. Due to the local breaking of time-reversal symmetry, the impurity induces unconventional superconductivity which is even in both space and spin variables but odd under time inversion. We derive an exact proportionality relation between the even-frequency component of the local electron density of states and the imaginary part of the odd-frequency local pairing function. By applying this relation to scanning tunneling microscopy spectra taken on top of magnetic impurities immersed in a Pb/Si(111) monolayer, we show experimental evidence of the occurrence of the odd-frequency pairing in these systems and explicitly extract its superconducting function from the data., Comment: 6 pages, 3 figures, revised version

Majorana zero modes are fractional quantum excitations appearing in pairs, each pair being a building block for quantum computation . Some possible signatures of these excitations have been reported as zero bias peaks at endpoints of one-dimensional semiconducting wires and magnetic chains. However, 1D systems are by nature fragile to a small amount of disorder that induces low-energy excitations, hence obtaining Majorana zero modes well isolated in a hard gap requires extremely clean systems. Two-dimensional systems offer an alternative route to get robust Majorana zero modes. Indeed, it was shown recently that Pb/Co/Si(111) could be used as a platform for generating 2D topological superconductivity with a strong immunity to local disorder. While 2D systems exhibit dispersive chiral edge states, they can also host Majorana zero modes located on local topological defects. According to predictions, if an odd number of zero modes are located in a topological domain an additional zero mode should appear all around the domain's edge. Here we use scanning tunneling spectroscopy to characterize a disordered superconducting monolayer of Pb coupled to underlying Co-Si magnetic islands meant to induce a topological transition. We show that pairs of zero modes are stabilized: one zero mode positioned at a point in the middle of the magnetic domain and its zero mode partner extended all around the domain. The zero mode pair is remarkably robust, it is isolated within a hard superconducting energy gap and it appears totally immune to the strong disorder present in the Pb monolayer. Our theoretical scenario supports the protected Majorana nature of this zero mode pair, highlighting the role of magnetic or spin-orbit coupling textures. This robust pair of Majorana zero modes offers a new platform for theoretical and experimental study of quantum computing.

There is presently a tremendous activity around the field of topological superconductivity and Majorana fermions. Among the many questions raised, it has become increasingly important to establish the topological or non-topological origin of features associated with Majorana fermions such as zero-bias peaks. Here, we compare in-gap features associated either with isolated magnetic impurities or with magnetic clusters strongly coupled to the atomically thin superconductor Pb/Si(111). We study this system by means of scanning tunneling microscopy and spectroscopy (STM/STS). We take advantage of the fact that the Pb/Si(111) monolayer can exist either in a crystal-ordered phase or in an incommensurate disordered phase to compare the observed spectroscopic features in both phases. This allows us to demonstrate that the strongly resolved in-gap states we found around the magnetic clusters in the disordered phase of Pb have a clear topological origin., Comment: 11 pages, 5 figures. To be published in European Physical Journal Special Topics.dedicated to the conference FQMT'17

Just like insulators can host topological Dirac states at their edges, superconductors can also exhibit topological phases characterized by Majorana edge states. Remarkable zero-energy states have been recently observed at the two ends of proximity induced superconducting wires, and were interpreted as localized Majorana end states in one-dimensional (1D) topological superconductor. By contrast, propagating Majorana states should exist at the 1D edges of two-dimensional (2D) topological superconductors. Here we report the direct observation of dispersive in-gap states surrounding topological superconducting domains made of a single atomic layer of Pb covering magnetic islands of Co/Si(111). We interpret the observed continuous dispersion across the superconducting gap in terms of a spatial topological transition accompanied by a chiral edge mode and residual gaped helical edge states. Our experimental approach enables the engineering and control of a large variety of novel quantum phases. This opens new horizons in the field of quantum materials and quantum electronics where the magnetization of the domains could be used as a control parameter for the manipulation of topological states., Comment: 12 pages, 3 figures

The quantum coherent coupling of completely different degrees of freedom is a challenging path towards creating new functionalities for quantum electronics. Usually the antagonistic coupling between spins of magnetic impurities and superconductivity leads to the destruction of the superconducting order. Here we show that a localized classical spin of an iron atom immersed in a superconducting condensate can give rise to new kind of long range coherent magnetic quantum state. In addition to the well-known Shiba bound state present on top of an impurity we reveal the existence of a star shaped pattern which extends as far as 12 nm from the impurity location. This large spatial dispersion turns out to be related, in a non-trivial way, to the superconducting coherence length. Inside star branches we observed short scale interference fringes with a particle-hole asymmetry. Our theoretical approach captures these features and relates them to the electronic band structure and the Fermi wave length of the superconductor. The discovery of a directional long range effect implies that distant magnetic atoms could coherently interact leading to new topological superconducting phases with fascinating properties.

Electrical contacts between nano-engineered systems are expected to constitute the basic building blocks of future nano-scale electronics. However, the accurate characterization and understanding of electrical contacts at the nano-scale is an experimentally challenging task. Here we employ low-temperature scanning tunneling spectroscopy to investigate the conductance of individual nano-contacts formed between flat Pb islands and their supporting substrates. We observe a suppression of the differential tunnel conductance at small bias voltages due to dynamical Coulomb blockade effects. The differential conductance spectra allow us to determine the capacitances and resistances of the electrical contacts which depend systematically on the island--substrate contact area. Calculations based on the theory of environmentally assisted tunneling agree well with the measurements., Comment: 5 pages, 3 figures, to appear in PRL

The two charge-density wave (CDW) transitions in NbSe$_3$ %at wave numbers at $\bm{q_1}$ and $\bm{q_2}$, occurring at the surface were investigated by scanning tunneling microscopy (STM) on \emph{in situ} cleaved $(\bm{b},\bm{c})$ plane. The temperature dependence of first-order CDW satellite spots, obtained from the Fourier transform of the STM images, was measured between 5-140 K to extract the surface critical temperatures (T$_s$). The low T CDW transition occurs at T$_{2s}$=70-75 K, more than 15 K above the bulk T$_{2b}=59$K while at exactly the same wave number. %determined by x-ray diffraction experiments. Plausible mechanism for such an unusually high surface enhancement is a softening of transverse phonon modes involved in the CDW formation.% The large interval of the 2D regime allows to speculate on % %the special Berezinskii-Kosterlitz-Thouless type of the surface transition expected for this incommensurate CDW. This scenario is checked by extracting the temperature dependence of the order % %parameter correlation functions. The regime of 2D fluctuations is analyzed according to a Berezinskii-Kosterlitz-Thouless type of surface transition, expected for this incommensurate 2D CDW, by extracting the temperature dependence of the order parameter correlation functions., Comment: 5 pages, 2 figures