Moiré Magic 3.0

Video Recording

Moiré superlattices have recently emerged as a novel platform where correlated physics and superconductivity can be studied with unprecedented tunability. Although correlated effects have been observed in several other moiré systems, magic-angle twisted bilayer graphene (MATBG) remains the only one where robust superconductivity has been reproducibly measured. In this talk I will present a new moiré superconductor, mirror symmetric magic-angle twisted trilayer graphene (MATTG) with dramatically richer tunability in electronic structure and superconducting properties. Hall effect and quantum oscillations measurements as a function of density and electric field allow us to determine the system's tunable phase boundaries in the normal state. Zero magnetic field resistivity measurements then reveal that the existence of superconductivity is intimately connected to the broken symmetry phase emerging at two carriers per moiré unit cell. Strikingly, we find that the superconducting phase gets suppressed and bounded at the van Hove singularities (vHs) partially surrounding the broken-symmetry phase, which is difficult to reconcile with weak-coupling BCS theory. Moreover, the extensive in situ tunability of our system allows us to achieve the ultra-strong coupling regime, characterized by a Ginzburg-Landau coherence length reaching the average inter-particle distance and very large T_BKT/T_F ratios in excess of 0.1. These observations suggest that MATTG can be electrically tuned close to the two-dimensional BCS-BEC crossover. Our results establish a new generation of tunable moiré superconductors with the potential to revolutionize our fundamental understanding and the applications of strong coupling superconductivity.

Chiral edge modes in the heavy fermion superconductor UTe2

Video Recording

Topological superconductors represent a fundamentally new phase of matter. Similar to topological insulators, the non-trivial topological characteristics of a topological superconductor dictate the presence of a topological edge states composed of Bogoliubov quasiparticles which live inside and span the superconducting gap. The intense interest in these materials stems from the fact that Bogoliubov excitations inside the gap of a topological superconductor are predicted to have all the characteristics of Majorana Fermions. A chiral p-wave superconductor which is topologically non-trivial is a natural platform for realizing these Majorana modes. In this talk I present scanning tunneling microscopy (STM) data on the newly discovered heavy fermion superconductor, UTe2 with a TC of 1.6K. I will show signatures of coexisting Kondo effect and superconductivity which show competing spatial modulations within one unit-cell. STM spectroscopy at step edges show signatures of chiral in-gap states, predicted to exist at the boundaries of a topological superconductor. Combined with existing data indicating triplet pairing, the presence of chiral edge states suggests that UTe2 is a strong candidate material for chiral-triplet topological superconductivity.

Cooper pairing without superconductivity

Video recording

The idea that preformed Cooper pairs could exist in a superconductor above its zero-resistance state has been explored for unconventional, interface, and disordered superconductors, yet direct experimental evidence is lacking. In this talk, I will introduce the use of scanning tunneling noise spectroscopy to unambiguously detect and quantify the number of Cooper pairs in a sample [1-3]. We show that preformed Cooper pairs exist up to temperatures much higher than the zero-resistance critical temperature Tc in the disordered superconductor titanium nitride, by observing a clear enhancement in the shot noise that is equivalent to a change of the effective charge from 1 to 2 electron charges [4]. We further show that spectroscopic gap fills up rather than closes when increasing temperature. Our results thus demonstrate the existence of a novel state above Tc that, much like an ordinary metal, has no (pseudo)gap, but carries charge via paired electrons.

Superconducting Quantum Interference at the Atomic Scale

Video recording

A single spin in a Josephson junction can reverse the flow of the supercurrent. At mesoscopic length scales, such π-junctions are employed in various instances from finding the pairing symmetry to quantum computing. In Yu-Shiba-Rusinov (YSR) states, the atomic scale counterpart of a single spin in a superconducting tunnel junction, the supercurrent reversal so far has remained elusive. Using scanning tunneling microscopy (STM), we demonstrate such a 0 to π transition of a Josephson junction through a YSR state as we continuously change the impurity-superconductor coupling. We detect the sign change in the critical current by exploiting a second transport channel as reference in analogy to a superconducting quantum interference device (SQUID), which provides the STM with the required phase sensitivity. The measured change in the Josephson current is a signature of the quantum phase transition and allows its characterization with unprecedented resolution.

In silico discovery of novel topological materials

Video recording

In my talk, I will focus on our recent efforts directed towards the search of novel topological materials. A large number of diverse topological electronic phases that can be realized in materials have been predicted recently. We have developed a high-throughput computational screening methodology for identifying materials hosting various topological phases among known materials. The entire dataset of results obtained using this high-throughput search is now publicly available via the Materials Cloud platform [1]. Several predictions resulting from this search that have been successfully confirmed by experiments. A new Z2 topological insulator was theoretically predicted and experimentally confirmed in the β-phase of quasi-one-dimensional bismuth iodide Bi4I4 [2]. The electronic structure of β-Bi4I4, characterized by Z2 invariants (1;110), is in proximity of both the weak TI phase (0;001) and the trivial insulator phase (0;000). We further predicted robust type-II Weyl semimetal phase in transition metal diphosphides MoP2 and WP2 characterized by very large momentum-space separation between Weyl points of opposite chirality [3]. Recent experiments on WP2 revealed record magnitudes of magnetoresistance combined with very high conductivity and residual resistivity ratio [4], and many other extraordinary properties. I will discuss in detail the physical mechanism underlying magnetotransport in WP2 as well as in other trivial and topological semimetals [5].

A microscopic view of graphene quantum Hall edge states with STM and AFM measurements

Video recording

Electrostatic pn junction boundaries provide a convenient geometry for the examination of quantum Hall edge states with microscopic probes. In this talk I will review our work in circular and rectangular geometries to examine the quantum Hall states which form in high magnetic field using scanning tunneling microscopy (STM) and atomic force microscopy (AFM) measurements. In circular graphene pn junctions a concentric series of compressible and incompressible rings form due to electron interactions. The compressible rings form from Landau levels at the Fermi level, and show single electron charging of this Landau level quantum dot when probed by scanning tunneling spectroscopy. In a rectangular Hall bar geometry defined by pn junction boundaries, the compressible strips form the topological protected edge states in the quantum Hall effect. For the graphene Hall bar device, we utilize simultaneous AFM, STM, and quantum transport measurements at mK temperatures, which we recently developed for detailed measurements of 2D devices in-operando. The Kelvin probe force microscopy (KPFM) mode of AFM detects the chemical potential transitions when Landau levels are being filled or emptied as a function of back gate potential and show the same fidelity for Landau level spectroscopy as STS measurements. In particular, broken-symmetry states can be resolved at filling factors ν = ±1 inside the N=0 Landau level manifold, showing the lifting of the graphene four-fold degeneracy due to spin and valley. With KPFM we can map the dispersion of the Landau levels across the quantum Hall edge boundary as a function of density and spatial position.

Kitaev Materials and Perspective

Video recording

Fascinating features of two-dimensional van der Waals (vdW) magnetic materials have attracted much attention from theoretical and experimental researchers. Recently spin models for vdW Mott insulators beyond the standard Heisenberg interaction have been developed. It was proposed that alpha-RuCl3 is a Kitaev candidate material described by “bond-dependent” spin interactions; the Kitaev model on honeycomb lattice is a rare example of exactly solvable model, which exhibits non-Abelian anyons under a magnetic field. I will first review a microscopic mechanism to the bond-dependent spin interactions starting from spin, orbitals, and their couplings in Mott insulators. There are two types of bond-dependent interactions named Kitaev and Gamma. A minimal Kitaev-Gamma (KG) model has been investigated by various numerical techniques under a magnetic field, but definite conclusions about field-induced spin liquids remain elusive. The phase diagram of the KG model defined on a two-leg ladder in the presence of magnetic field will be presented. We will then discuss its connection to possible spin liquids in the two-dimensional limit.

Evidence for p-wave pairing and hybridizing Majoranas in artificial finite-size Shiba chains

Video recording

A magnetic chain on an s-wave superconductor hosting a spin spiral or strong spin-orbit coupling can potentially realize a one-dimensional topological superconductor with Majorana bound states on its edges. Here, we investigate artificial spin chains which have been built atom-by-atom [1], with respect to the emergence of such topologically nontrivial electron phases. We vary the substrate and adatom species and the interatomic distances in the chain [2-5]. By this approach we can adjust the energetic positions of the multi-orbital Yu-Shiba-Rusinov (YSR) states induced by the adatoms [2,3], the hybridizations between these YSR states [4], as well as the spin structure in the chain [5]. This eventually enables to tailor the multi-orbital YSR bands emerging in the chain such that a p-wave gap opens in one of the YSR bands [6], and weakly protected, interacting Majorana bound states [7] have been realized.

Nanoscale control of new properties added to graphene: Superconductivity Magnetism and Electronic Gap

Video recording

At present, we have a very high level of understanding of the inherent properties of graphene. This means that we are now in a position to go one-step further and try to add and take advantage of some of the few properties not naturally found in graphene, such as the existence of magnetic moments, gaps in the band structure or superconducting properties. In this talk I will show how we incorporate those properties to graphene, and how we control them at the nanoscale by using STM as main experimental tool. More specifically, we use quantum confinement to selectively introduce gaps in graphene’s electronic band structure [1], atomic H as building blocks to incorporate magnetic moments [2] and Pb islands to induce superconductivity by the proximity effect [3]. In addition, I will show how we use quasiparticle interference patterns to probe graphene topological properties [4].

Probing excited-state lifetimes by means of pump-probe atomic force microscopy

Scanning Tunneling Microscopy (STM) is a powerful tool for the investigation of individual molecules, being able to probe their orbitals with sub-molecular resolution. However, the requirement of a conductive substrate strongly limits the accessible electronic transitions. Conversely, atomic force microscopy (AFM) can be extended to insulating substrates, providing structural and electrostatic information. Recently, the single-electron sensitivity of AFM [1] in detecting electrostatic forces has been exploited to sense individual electron tunneling events between tip and investigated structure. Thereby, electronic states can be investigated with Angstrom resolution in absence of any conductance of the underlying substrate [2-4]. Beyond steady-state spectroscopy, all-electronic pump-probe techniques have opened the door to access dynamic properties on submolecular scales, as demonstrated by measurements of spin lifetimes of individual atoms [5]. Merging these developments, we probe the lifetime of the out-of-equilibrium triplet state of individual molecules by means of novel pump-probe AFM. Combining this with real-space atomic resolution, we observe the quenching of the triplet lifetime by surrounding oxygen molecules in atomistic details [6].

Superconductivity of Sr2RuO4: beyond “unconventional?”

Video recording

After more than 25 years since its discovery, studies of the unconventional superconductivity of Sr2RuO4 have been reactivated in the last couple of years. As the main trigger, it was revealed that previous NMR results had a technical problem of overheating the sample. Now the reduction of the spin susceptibility below Tc consistent with the spin-singlet pairing has been confirmed. As another new development, uniaxial pressure can drive its multi-band Fermi surfaces across a Lifshitz transition, and increases its Tc from 1.5 K to 3.5 K. Correspondingly, muon spin resonance indicates a splitting between the enhanced Tc and the onset of the time-reversal-symmetry breaking at TTRSB that remains at about 1.5 K. In combination with other results such as the unusual jump in a shear elastic constant in ultrasound experiments, one promising pairing symmetry is Eg with the chiral d-wave “d + id” order parameter. However, such a state is controversial since it implies a gap structure that does not allow pairing of electrons within the plane despite the quasi-2-dimensional nature. In order to settle this issue, some promising theories propose the inter-orbital pairing. Superconductivity of Sr2RuO4 may help to establish such a pairing state beyond the traditional “unconventional” pairing. We will also mention scanning tunneling microscopy (STM) and junction experiments to investigate the superconducting state of Sr2RuO4.