Dr.jens.brede

Author(s): Daniel Walkup, Fereshte Ghahari, Steven R. Blankenship, Kenji Watanabe, Takashi Taniguchi, Nikolai B. Zhitenev, and Joseph A. Stroscio

The ability to pattern and simultaneously study a nanometer-scale potential landscape within hBN-based graphene devices using scanning tunneling microscopy (STM) opens a path for detailed investigation of model quantum systems. Here, the authors create a double-well potential and applied a strong magnetic field to induce Landau levels and the formation of quantum dots with single-electron charging characteristics. The system can be broadly tuned form weakly coupled dots to combined single dot while STM provides multiple measurement and manipulation modalities.

[Phys. Rev. B 108, 235407] Published Wed Dec 06, 2023

In this work, we investigate the tunneling characteristics of Majorana zero modes (MZMs) in vortex lattices based on scanning tunneling microscopy measurement. We find that zero bias conductance does not reach the quantized value owing to the coupling between the MZMs. On the contrary, the Fano factor measured in the high voltage regime reflects the local particle-hole asymmetry of the bound states and is insensitive to the energy splitting between them. We propose using spatially resolved Fano factor tomography as a tool to identify the existence of MZMs. In both cases of isolated MZM or MZMs forming bands, there is a spatially resolved Fano factor plateau at one in the vicinity of a vortex core, regardless of the tunneling parameter details, which is in stark contrast to other trivial bound states. These results reveal new tunneling properties of MZMs in vortex lattices and provide measurement tools for topological quantum devices.

Topological insulators (TI) and magnetic topological insulators (MTI) can apply highly efficient spin-orbit torque (SOT) and manipulate the magnetization with their unique topological surface states with ultra-high efficiency. Here, we demonstrate efficient SOT switching of a hard MTI, V-doped (Bi,Sb)2Te3 (VBST) with a large coercive field that can prevent the influence of an external magnetic field. A giant switched anomalous Hall resistance of 9.2 $k\Omega$ is realized, among the largest of all SOT systems. The SOT switching current density can be reduced to $2.8\times10^5 A/cm^2$. Moreover, as the Fermi level is moved away from the Dirac point by both gate and composition tuning, VBST exhibits a transition from edge-state-mediated to surface-state-mediated transport, thus enhancing the SOT effective field to $1.56\pm 0.12 T/ (10^6 A/cm^2)$ and the interfacial charge-to-spin conversion efficiency to $3.9\pm 0.3 nm^{-1}$ (nominal spin Hall angle to $23.2\pm 1.8$). The findings establish VBST as an extraordinary candidate for energy-efficient magnetic memory devices.

Nature, Published online: 29 November 2023; doi:10.1038/s41586-023-06764-4

Ultra-narrow quantum Hall Josephson junctions defined in encapsulated graphene nanoribbons exhibit a chiral supercurrent, visible up to 8  T.

In an extended superconductor-topological insulator-superconductor (S-TI-S) Josephson junction in a magnetic field, localized Majorana bound states (MBS) are predicted to exist at the cores of Josephson vortices where the local phase difference across the junction is an odd-multiple of $\pi$. These states contribute a supercurrent with a $4\pi$-periodic current-phase relation (CPR) that adds to the conventional $2\pi$-periodic sinusoidal CPR. In this work, we present a comprehensive experimental study of the critical current vs. applied magnetic field diffraction patterns of lateral Nb-Bi$_2$Se$_3$-Nb Josephson junctions. We compare our observations to a model of the Josephson dynamics in the S-TI-S junction system to explore what feature of MBS are, or are not, exhibited in these junctions. Consistent with the model, we find several distinct deviations from a Fraunhofer diffraction pattern that is expected for a uniform sin$({\phi})$ CPR. In particular, we observe abrupt changes in the diffraction pattern at applied magnetic fields in which the current-carrying localized MBS are expected to enter the junction, and a lifting of the odd-numbered nodes consistent with a $4\pi$-periodic sin$(\phi/2)$-component in the CPR. We also see that although the even-numbered nodes often remain fully-formed, we sometimes see deviations that are consistent with quasiparticle-induced fluctuations in the parity of the MBS pairs that encodes quantum information.

Quantum sensing is a key component of quantum technology, enabling highly sensitive magnetometry. We combined a nickelocene molecule with scanning tunneling microscopy to perform versatile spin sensing of magnetic surfaces, namely of model Co islands on Cu(111) of different thickness. We demonstrate that atomic-scale sensitivity to spin polarization and orientation is possible due to direct exchange coupling between the Nc-tip and the Co surfaces. We find that magnetic exchange maps lead to unique signatures, which are well described by computed spin density maps. These advancements improve our ability to probe magnetic properties at the atomic level.

Nature Materials, Published online: 27 November 2023; doi:10.1038/s41563-023-01704-z

Monolithic 3D integration of electronics based on fully 2D materials is demonstrated in the performance of artificial intelligence tasks.

The emergence of effective $S=1/2$ spins at the edges of $S=1$ Haldane spin chains is one of the simplest examples of fractionalization. Whereas there is indirect evidence of this phenomenon, direct measurement of the magnetic moment of an individual edge spin remains to be done. Here we show how scanning tunnel microscopy electron-spin resonance (ESR-STM) can be used to map the stray field created by the fractional $S=1/2$ edge spin and we propose efficient methods to invert the Biot-Savart equation, obtaining the edge magnetization map. This permits one to determine unambiguously the two outstanding emergent properties of fractional degrees of freedom, namely, their fractional magnetic moment and their localization length $\xi$.

Cryogenic scanning tunneling microscopy was employed in combination with density-functional theory calculations to explore quantum dots made of In adatoms on the InAs(110) surface. Each adatom adsorbs at a surface site coordinated by one cation and two anions, and transfers one electron to the substrate, creating an attractive quantum well for electrons in surface states. We used the scanning-probe tip to assemble the positively charged adatoms into precisely defined quantum dots exhibiting a bound state roughly 0.1 eV below the Fermi level at an intrinsic linewidth of only ~4 meV, as revealed by scanning tunneling spectroscopy. For quantum-dot dimers, we observed the emergence of a bonding and an antibonding state with even and odd wave-function character, respectively, demonstrating the capability to engineer quasi-molecular electronic states. InAs(110) constitutes a promising platform in this respect because highly perfect surfaces can be readily prepared by cleavage and charged adatoms can be generated in-situ by the scanning-probe tip.

Nature Communications, Published online: 16 November 2023; doi:10.1038/s41467-023-43185-3

The Kondo effect from magnetic impurities has been proposed as a probe of fractionalized excitations in a topological quantum spin liquid. Lee et al. experimentally demonstrate the Kondo effect in a Kitaev candidate material α-RuCl3 with dilute Cr impurities.

Applications in photodetection, photochemistry, and active metamaterials and metasurfaces require fundamental understanding of ultrafast nonthermal and thermal electron processes in metallic nanosystems. Significant progress has been recently achieved in synthesis and investigation of low-loss monocrystalline gold, opening up opportunities for its use in ultrathin nanophotonic architectures. Here, we reveal fundamental differences in hot-electron thermalisation dynamics between monocrystalline and polycrystalline ultrathin (down to 10 nm thickness) gold films. Comparison of weak and strong excitation regimes showcases a counterintuitive unique interplay between thermalised and non-thermalised electron dynamics in mesoscopic gold with the important influence of the X-point interband transitions on the intraband electron relaxation. We also experimentally demonstrate the effect of hot-electron transfer into a substrate and the substrate thermal properties on electron-electron and electron-phonon scattering in ultrathin films. The hot-electron injection efficiency from monocrystalline gold into TiO2, approaching 9% is measured, close to the theoretical limit. These experimental and modelling results reveal the important role of crystallinity and interfaces on the microscopic electronic processes important in numerous applications.

Three-dimensional topological insulators support gapless Dirac fermion surface states whose rich topological properties result from the interplay of symmetries and dimensionality. Their topological properties have been extensively studied in systems of integer spatial dimension but the prospect of these surface electrons arranging into structures of non-integer dimension like fractals remains unexplored. In this work, we investigate a new class of states arising from the coupling of surface Dirac fermions to a time-reversal symmetric fractal potential, which breaks translation symmetry while retaining self-similarity. Employing large-scale exact diagonalization, scaling analysis of the inverse participation ratio, and the box-counting method, we establish the onset of self-similar Dirac fermions with fractal dimension for a symmetry-preserving surface potential with the geometry of a Sierpinski carpet fractal with fractal dimension $D \approx 1.89$. Dirac fractal surface states open a fruitful avenue to explore exotic regimes of transport and quantum information storage in topological systems with fractal dimensionality.

Recently gate-mediated supercurrent suppression in superconducting nano-bridges has been reported in many experiments. This could be either a direct or an indirect gate effect. The microscopic understanding of this observation is not clear till now. Using the quasiclassical Green's function method, we show that a small concentration of magnetic impurities at the surface of the bridges can significantly help to suppress superconductivity and hence the supercurrent inside the systems while applying a gate field. This is because the gate field can enhance the depairing through the exchange interaction between the magnetic impurities at the surface and the superconductor. We also obtain a \emph{symmetric} suppression of the supercurrent with respect to the gate field, a signature of a direct gate effect. Future experiments can verify our predictions by modifying the surface with magnetic impurities.

The weak disorder potential seen by the electrons of a two-dimensional electron gas in high-mobility semiconductor heterostructures leads to fluctuations in the physical properties and can be an issue for nanodevices. In this paper, we show that a scanning gate microscopy (SGM) image contains information about the disorder potential, and that a machine learning approach based on SGM data can be used to determine the disorder. We reconstruct the electric potential of a sample from its experimental SGM data and validate the result through an estimate of its accuracy.

Author(s): Abhishek Banerjee, Max Geier, Md Ahnaf Rahman, Candice Thomas, Tian Wang, Michael J. Manfra, Karsten Flensberg, and Charles M. Marcus

In analogy to conventional semiconductor diodes, the Josephson diode exhibits superconducting properties that are asymmetric in applied bias. The effect has been investigated in a number of systems recently, and requires a combination of broken time-reversal and inversion symmetries. We demonstrate …

[Phys. Rev. Lett. 131, 196301] Published Thu Nov 09, 2023

Nature Communications, Published online: 09 November 2023; doi:10.1038/s41467-023-42989-7

The authors report ultrafast transport measurements on the photo-excited superconducting state in K3C60. They observe characteristic superconducting nonlinear current-voltage responses.

Nodal Superconductivity

Artistic representation of TaS₂, the first monolayer van der Waals material featuring nodal superconductivity in the ultra-clean limit. Experiments with scanning tunneling spectroscopy reveal the appearance of a nodal gap characteristic of strongly correlated superconductors, and the existence of many-body inelastic excitations at finite energies associated with hidden order fluctuations. More details can be found in article number 2305409 by Jose L. Lado, Peter Liljeroth, and co-workers.

Nature Physics, Published online: 07 November 2023; doi:10.1038/s41567-023-02239-5

Annually, the European Research Council (ERC) and the National Science Foundation (NSF) allocate resources to promote research excellence in Europe and the USA. We observe that European Union (EU)-based researchers rely strongly on United States (US) collaborations to secure top EU funding, while the reverse is much less common.

Ultrathin crystalline silver structures of <3 nm in thickness are fabricated by lighographically prepatterning a silicon wafer and subsequently depositing a few atomic layers of metal under ultrahigh vacuum conditions. The method has great flexibility regarding the size and morphology of the structures, which are demonstrated to sustain plasmon resonances with quality factors as high as ten.

Abstract

The ability to confine light down to atomic scales is critical for the development of applications in optoelectronics and optical sensing as well as for the exploration of nanoscale quantum phenomena. Plasmons in metallic nanostructures with just a few atomic layers in thickness can achieve this type of confinement, although fabrication imperfections down to the subnanometer scale hinder actual developments. Here, narrow plasmons are demonstrated in atomically thin crystalline silver nanostructures fabricated by prepatterning silicon substrates and epitaxially depositing silver films of just a few atomic layers in thickness. Specifically, a silicon wafer is lithographically patterned to introduce on-demand lateral shapes, chemically process the sample to obtain an atomically flat silicon surface, and epitaxially deposit silver to obtain ultrathin crystalline metal films with the designated morphologies. Structures fabricated by following this procedure allow for an unprecedented control over optical field confinement in the near-infrared spectral region, which is here illustrated by the observation of fundamental and higher-order plasmons featuring extreme spatial confinement and high-quality factors that reflect the crystallinity of the metal. The present study constitutes a substantial improvement in the degree of spatial confinement and quality factor that should facilitate the design and exploitation of atomic-scale nanoplasmonic devices for optoelectronics, sensing, and quantum-physics applications.

We prove that that if the boundary of a topological insulator divides the plane in two regions containing arbitrarily large balls, then it acts as a conductor. Conversely, we show that topological insulators that fit within strips do not need to admit conducting boundary modes.

We study the microscopic model of electrons in the partially-filled lowest Landau level interacting via the Coulomb potential by the diagrammatic theory within the GW approximation. In a wide range of filling fractions and temperatures, we find a homogeneous non-Fermi liquid (nFL) state similar to that found in the Sachdev-Ye-Kitaev (SYK) model, with logarithmic corrections to the anomalous dimension. In addition, the phase diagram is qualitatively similiar to that of SYK: a first-order transition terminating at a critical end-point separates the nFL phase from a band insulator that corresponds to the fully-filled Landau level. This critical point, as well as that of the SYK model -- whose critical exponents we determine more precisely -- are shown to both belong to the Van der Waals universality class. The possibility of a charge density wave (CDW) instability is also investigated, and we find the homogeneous nFL state to extend down to the ground state for fillings $0.2 \lesssim \nu \lesssim 0.8$, while a CDW appears outside this range of fillings at sufficiently low temperatures. Our results suggest that the SYK-like nFL state should be a generic feature of the partially-filled lowest Landau level at intermediate temperatures.

Author(s): M. Alfonso-Moro, V. Guisset, P. David, B. Canals, J. Coraux, and N. Rougemaille

Under certain experimental conditions, the deposition of C60 molecules onto an atomically flat copper surface gives rise to the formation of corrugated islands. This corrugation, which reflects a molecular displacement perpendicular to the surface plane, presents an astonishing pattern: It is well d…

[Phys. Rev. Lett. 131, 186201] Published Mon Oct 30, 2023

Nature Physics, Published online: 26 October 2023; doi:10.1038/s41567-023-02262-6

Despite the theoretical prediction of spinaron quasiparticles in artificial nanostructures, experimental evidence has not yet been seen. Now it has been observed in a hybrid system comprising Co atoms on a Cu(111) surface.

Nature Communications, Published online: 19 October 2023; doi:10.1038/s41467-023-42287-2

Control of spins down to the atomic scale is a major goal for spin-based information processing. Here, Kot et al. demonstrate electric control over the spin-resonance transitions of a single TiH molecule placed on a surface of MgO by exploiting the electric field between the scanning tunnelling microscopy tip and the sample.

Nature Materials, Published online: 19 October 2023; doi:10.1038/s41563-023-01694-y

Thermally assisted spin–orbit torque is used to switch the edge current chirality in mesoscopic quantum anomalous Hall devices.

Kelvin probe force microscopy (KPFM) has been employed to probe charge carriers in a graphene/hexagonal boron nitride (hBN) heterostructure [Nano Lett, 21, 5013 (2021)]. We propose an approach for operating valley filtering based on the KPFM-induced potential $U_0$ instead of using external or induced pseudo-magnetic fields in strained graphene. Employing a tight-binding model, we investigate the parameters and rules leading to valley filtering in the presence of a graphene quantum dot (GQD) created by the KPFM tip. This model leads to a resolution of different transport channels in reciprocal space, where the electron transmission probability at each Dirac cone ($K_1$= -K and $K_2$ = +K) is evaluated separately. The results show that U0 and the Fermi energy $E_F$ control (or invert) the valley polarization, if electrons are allowed to flow through a given valley. The resulting valley filtering is allowed only if the signs of $E_F$ and $U_0$ are the same. If they are different, the valley filtering is destroyed and might occur only at some resonant states affected by $U_0$. Additionally, there are independent valley modes characterizing the conductance oscillations near the vicinity of the resonances, whose strength increases with $U_0$ and are similar to those occurring in resonant tunneling in quantum antidots and to the Fabry-Perot oscillations. Using KPFM, to probe the charge carriers, and graphene-based structures to control valley transport, provides an efficient way for attaining valley filtering without involving external or pseudo-magnetic fields as in previous proposals.

Author(s): Sergey Trishin, Christian Lotze, Nils Krane, and Katharina J. Franke

Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) are considered highly promising platforms for next-generation optoelectronic devices. Owing to its atomically thin structure, device performance is strongly impacted by a minute amount of defects. Although defects are usually considered t…

[Phys. Rev. B 108, 165414] Published Wed Oct 18, 2023

Over the last decade, the possibility of realizing topological superconductivity (TSC) has generated much excitement, mainly due to the potential use of its excitations (Majorana zero modes) in a fault-tolerant topological quantum computer 1,2. TSC can be created in electronic systems where the topological and superconducting orders coexist3, motivating the continued exploration of candidate material platforms to this end. Here, we use molecular beam epitaxy (MBE) to synthesize heterostructures that host emergent interfacial superconductivity when a non-superconducting antiferromagnet (FeTe) is interfaced with a topological insulator (TI) (Bi, Sb)2Te3 wherein the chemical potential can be tuned through varying the Bi/Sb ratio. By performing in-vacuo angle-resolved photoemission spectroscopy (ARPES) and ex-situ electrical transport measurements, we find that the superconducting transition temperature and the upper critical magnetic field are suppressed when the chemical potential approaches the Dirac point. This observation implies a direct correlation between the interfacial superconductivity and Dirac electrons of the TI layer. We provide evidence to show that the observed interfacial superconductivity and its chemical potential dependence is the result of the competition between the Ruderman-Kittel-Kasuya-Yosida-type ferromagnetic coupling mediated by Dirac surface states and antiferromagnetic exchange couplings that generate the bicollinear antiferromagnetic order in the FeTe layer. The Dirac-fermion-assisted interfacial superconductivity in (Bi,Sb)2Te3/FeTe heterostructures provides a new approach to probe TSC and Majorana physics in hybrid devices and potentially constitutes an alternative platform for topological quantum computation.

Superconductivity emerges from the spatial coherence of a macroscopic condensate characterized by two intrinsic length scales: the coherence length $\xi_0$ and the London penetration depth $\lambda_{\mathrm{L}}$. While their description is well established in weak-coupling Bardeen-Cooper-Schrieffer (BCS) theory and Eliashberg theory, $\xi_0$ and $\lambda_{\mathrm{L}}$ are generally unknown quantities in strongly correlated superconductors. In this work, we establish a framework to calculate these length scales in microscopic theories and from first principles. Central to this idea are Nambu-Gor'kov Green functions under a constraint of finite-momentum pairing and their analysis with respect to the superconducting order parameter and resultant supercurrents. We illustrate with a multi-orbital model of alkali-doped fullerides (A$_3$C$_{60}$) using Dynamical Mean-Field Theory (DMFT) how proximity of superconductivity, Jahn-Teller metallic, and Mott-localized states impact superconducting coherence, order parameter stiffness, and critical temperature. Our analysis reveals a "localized" superconducting regime with robustly short $\xi_0$. Multi-orbital effects cause a domeless rise in the critical temperature as the pairing interaction is increased, setting this system apart from the BCS to Bose-Einstein-Condensate (BEC) crossover phenomenology.

Nature, Published online: 13 October 2023; doi:10.1038/d41586-023-03238-5

The fact that artificial intelligence can do much of the work makes a mockery of the process. It’s time to make it easier for scientists to ask for research funding.