Dr.jens.brede

The interplay between the oxidation state and the optical properties of molecules is important for applications in displays, sensors, and molecular-based memories. The fundamental mechanisms occurring at the level of a single molecule have been difficult to probe. We used a scanning tunneling microscope (STM) to characterize and control the fluorescence of a single zinc-phthalocyanine radical cation adsorbed on a sodium chloride–covered gold (111) sample. The neutral and oxidized states of the molecule were identified on the basis of their fluorescence spectra, which revealed very different emission energies and vibronic fingerprints. The emission of the charged molecule was controlled by tuning the thickness of the insulator and the plasmons localized at the apex of the STM tip. In addition, subnanometric variations of the tip position were used to investigate the charging and electroluminescence mechanisms.

Both the global average per capita consumption of meat and the total amount of meat consumed are rising, driven by increasing average individual incomes and by population growth. The consumption of different types of meat and meat products has substantial effects on people’s health, and livestock production can have major negative effects on the environment. Here, we explore the evidence base for these assertions and the options policy-makers have should they wish to intervene to affect population meat consumption. We highlight where more research is required and the great importance of integrating insights from the natural and social sciences.

Non-collinear spin states in bottom-up fabricated atomic chains

Non-collinear spin states in bottom-up fabricated atomic chains, Published online: 20 July 2018; doi:10.1038/s41467-018-05364-5

Scanning tunnelling microscopes can be used to accurately position atoms and measure emergent behaviour arising from interatomic couplings. Here, the authors fabricate a model spin chain and show the formation of a tunable spiral state due to competing Heisenberg and Dzyaloshinskii-Moriya interactions

The interplay between the oxidation state and the optical properties of molecules plays a key role for applications in displays, sensors or molecular-based memories. The fundamental mechanisms occurring at the level of a single-molecule have been difficult to probe. We used a scanning tunneling microscope (STM) to characterize and control the fluorescence of a single Zn-phthalocyanine radical cation adsorbed on a NaCl-covered Au(111) sample. The neutral and oxidized states of the molecule were identified on the basis of their fluorescence spectra that revealed very different emission energies and vibronic fingerprints. The emission of the charged molecule was controlled by tuning the thickness of the insulator and the plasmons localized at the apex of the STM tip. In addition, sub-nanometric variations of the tip position were used to investigate the charging and electroluminescence mechanisms.

We present controlled growth of c(2$\times$2)N islands on the (100) surface of Cu$_3$Au, which can be used as an insulating surface template for manipulation of magnetic adatoms. Compared to the commonly used Cu(100)/c(2$\times$2)N surface, where island sizes do not exceed several nanometers due to strain limitation, the current system provides better lattice matching between metal and adsorption layer, allowing larger unstrained islands to be formed. We show that we can achieve island sizes ranging from tens to hundreds of nanometers, increasing the potential building area by a factor 10$^3$. Initial manipulation attempts show no observable difference in adatom behaviour, either in manipulation or spectroscopy.

Magic routes: Psilocybin is the natural product of “magic mushrooms” and the prodrug of the psychotropic compound psilocin. Recently, medical interest in psilocybin has re‐emerged. This concept article highlights in vitro and biotechnological approaches for the production of psilocybin, along with a summary of synthetic routes.

Abstract

The fungal genus Psilocybe and other genera comprise numerous mushroom species that biosynthesize psilocybin (4‐phosphoryloxy‐N,N‐dimethyltryptamine). It represents the prodrug to its dephosphorylated psychotropic analogue, psilocin. The colloquial term “magic mushrooms” for these fungi alludes to their hallucinogenic effects and to their use as recreational drugs. However, clinical trials have recognized psilocybin as a valuable candidate to be developed into a medication against depression and anxiety. We here highlight its recently elucidated biosynthesis, the concurrently developed concept of enzymatic in vitro and heterologous in vivo production, along with previous synthetic routes. The prospect of psilocybin as a promising therapeutic may entail an increased demand, which can be met by biotechnological production. Therefore, we also briefly touch on psilocybin's therapeutic relevance and pharmacology.

We propose a universal gate set acting on a qubit formed by the degenerate ground states of a Coulomb-blockaded time-reversal invariant topological superconductor island with spatially separated Majorana Kramers pairs: the "Majorana Kramers Qubit". All gate operations are implemented by coupling the Majorana Kramers pairs to conventional superconducting leads. Interestingly, in such an all-superconducting device, the energy gap of the leads provides another layer of protection from quasiparticle poisoning independent of the island charging energy. Moreover, the absence of strong magnetic fields - which typically reduce the superconducting gap size of the island - suggests a unique robustness of our qubit to quasiparticle poisoning due to thermal excitations. Consequently, the Majorana Kramers Qubit should benefit from prolonged coherence times and may provide an alternative route to a Majorana-based quantum computer.

Author(s): J. L. Lado and M. Sigrist

Two-dimensional topological superconductivity has attracted great interest due to the emergence of Majorana modes bound to vortices and propagating along edges. However, due to its rare appearance in natural compounds, experimental realizations rely on a delicate artificial engineering involving mat...

[Phys. Rev. Lett. 121, 037002] Published Tue Jul 17, 2018

Exchange-biasing topological charges by antiferromagnetism

Exchange-biasing topological charges by antiferromagnetism, Published online: 17 July 2018; doi:10.1038/s41467-018-05166-9

Spin-polarized carriers could show an extra Hall component when moving through certain real-space topological spin textures. Here, He et al. report an exchange bias experienced by the topological spin textures living at the interface between a topological insulator and an adjacent antiferromagnet, suggesting a chiral spin texture is induced.

We present a comprehensive micromagnetic model of isolated axisymmetric skyrmions in magnetic multilayers with perpendicular anisotropy. Most notably, the essential role of the internal dipolar field is extensively considered with a minimum amount of assumptions on the magnetization profiles. The tri-dimensional structure of the multilayered skyrmions is modeled by their radial profiles in each layer. We first compare the results of the model against a full micromagnetic description in Cartesian coordinates. Our model combines information on both layer-dependent size and chirality of the skyrmions. We also provide a convenient criterion in order to characterize the stability of skyrmions against anisotropic elongations that would break their cylindrical symmetry, which allows to confirm the stability of the determined solutions. Because this model is able to treat magnetization configurations twisted through the thickness of multilayered skyrmions, it can provide predictions on any potential hybrid chirality in skyrmions due to the interplay of Dzyaloshinskii-Moriya and dipolar interactions in multilayers. We finally apply the results of our model to the description of the current-driven dynamics of hybrid chiral skyrmions. Using the Thiele formalism, we show that we can predict the forces exerted on the multilayered skyrmions by vertical spin-polarized currents, which provides a method to conform hybrid skyrmion chiralities and spin-current injection geometries in order to optimize skyrmion motion in multilayers, to the aim of maximizing the current-induced velocity, or canceling the skyrmion Hall angle.

Author(s): Romana Baltic, Fabio Donati, Aparajita Singha, Christian Wäckerlin, Jan Dreiser, Bernard Delley, Marina Pivetta, Stefano Rusponi, and Harald Brune

We employed x-ray absorption spectroscopy and x-ray magnetic circular dichroism to study the magnetic properties of single rare-earth (RE) atoms (Nd, Tb, Dy, Ho, and Er) adsorbed on the graphene/Ir(111) surface. The interaction of RE atoms with graphene results for Tb in a trivalent state with 4fn−1...

[Phys. Rev. B 98, 024412] Published Fri Jul 13, 2018

Andreev bound states (ABSs) in hybrid semiconductor-superconductor nanowires can have near-zero energy in parameter regions where band topology predicts trivial phases. This surprising fact has been used to challenge the interpretation of a number of transport experiments in terms of non-trivial topology with Majorana zero modes (MZMs). We show that this ongoing ABS versus MZM controversy is fully clarified when framed in the language of non-Hermitian topology, the natural description for open quantum systems. This change of paradigm allows us to understand topological transitions and the emergence of pairs of zero modes more broadly, in terms of exceptional point (EP) bifurcations of system eigenvalue pairs in the complex plane. Within this framework, we show that some zero energy ABSs are actually non-trivial, and share all the properties of conventional MZMs, such as the recently observed $2e^2/h$ conductance quantization. From this point of view, any distinction between such ABS zero modes and conventional MZMs becomes artificial. The key feature that underlies their common non-trivial properties is an asymmetric coupling of Majorana components to the reservoir, which triggers the EP bifurcation.

Manipulation of spin states at the single-atom scale underlies spin-based quantum information processing and spintronic devices. Such applications require protection of the spin states against quantum decoherence due to interactions with the environment. While a single spin is easily disrupted, a coupled-spin system can resist decoherence by employing a subspace of states that is immune to magnetic field fluctuations. Here, we engineered the magnetic interactions between the electron spins of two spin-1/2 atoms to create a clock transition and thus enhance their spin coherence. To construct and electrically access the desired spin structures, we use atom manipulation combined with electron spin resonance (ESR) in a scanning tunneling microscope (STM). We show that a two-level system composed of a singlet state and a triplet state is insensitive to local and global magnetic field noise, resulting in much longer spin coherence times compared with individual atoms. Moreover, the spin decoherence resulting from the interaction with tunneling electrons is markedly reduced by a homodyne readout of ESR. These results demonstrate that atomically-precise spin structures can be designed and assembled to yield enhanced quantum coherence.

Author(s): Fabian Donat Natterer, Fabio Donati, François Patthey, and Harald Brune

We use spin-polarized scanning tunneling microscopy to demonstrate that Ho atoms on magnesium oxide exhibit a coercive field of more than 8 T and magnetic bistability for many minutes, both at 35 K. The first spontaneous magnetization reversal events are recorded at 45 K, for which the metastable st...

[Phys. Rev. Lett. 121, 027201] Published Tue Jul 10, 2018

Spin-orbit interaction (SOI) plays a key role in creating Majorana zero modes in semiconductor nanowires proximity coupled to a superconductor. We track the evolution of the induced superconducting gap in InSb nanowires coupled to a NbTiN superconductor in a large range of magnetic field strengths and orientations. Based on realistic simulations of our devices, we reveal SOI with a strength of 0.15-0.35 eV$\require{mediawiki-texvc}\AA$. Our approach identifies the direction of the spin-orbit field, which is strongly affected by the superconductor geometry and electrostatic gates.

We predict that a single oscillation of a strong optical pulse can significantly populate the surface conduction band of a three-dimensional topological insulator, Bi2Se3. Both linearly- and circularly-polarized pulses generate chiral textures of interference fringes of population in the surface Brillouin zone. These fringes constitute a self-referenced electron hologram carrying information on the topology of the surface Bloch bands, in particular, on the effect of the warping term of the low-energy Hamiltonian. These electron-interference phenomena are in a sharp contrast to graphene where there are no chiral textures for a linearly-polarized pulse and no interference fringes for circularly-polarized pulse. These predicted reciprocal space electron-population textures can be measured experimentally by time resolved angle resolved photoelectron spectroscopy (TR-ARPES) to gain direct access to non-Abelian Berry curvature at topological insulator surfaces.

Advanced Materials, July 2018.

Although the dynamics of charge transfer (CT) processes can be probed with ultimate lifetime resolution, the helplessness to control CT at the nanoscale constitutes one of the most important road-blocks to revealing some of its deep fundamental aspects. In this work, we present an investigation of CT dynamics in a single iron tetraphenylporphyrin (Fe-TPP) donor/acceptor dyad adsorbed on a CaF2/Si(100) insulating surface. The tip of a scanning tunneling microscope (STM) is used to create local ionic states in one fragment of the dyad. The CT process is monitored by imaging subsequent changes in the neighbor acceptor molecule and its efficiency is mapped revealing the influence of the initial excited state in the donor molecule. In validation of the experiments, simulations based on density functional theory show that holes have a higher donor-acceptor CT rate compared to electrons and highlight a noticeable initial state dependence on the CT process. We leverage the unprecedented spatial resolution achieved in our experiments to show that the CT process in the dyad is governed via molecule-molecule coherent tunneling with negligible surface-mediated character.

Using spin- and angle-resolved spectroscopy and relativistic many-body calculations, we investigate the evolution of the electronic structure of (Bi$_{1-x}$In$_x$)$_2$Se$_3$ bulk single crystals around the critical point of the trivial to topological insulator quantum-phase transition. By increasing $x$, we observe how a surface gap opens at the Dirac point of the initially gapless topological surface state of Bi$_2$Se$_3$, leading to the existence of massive fermions. The surface gap monotonically increases for a wide range of $x$ values across the topological and trivial sides of the quantum-phase transition. By means of photon-energy dependent measurements, we demonstrate that the gapped surface state survives the inversion of the bulk bands which occurs at a critical point near $x=0.055$. The surface state exhibits a non-zero in-plane spin polarization which decays exponentially with increasing $x$, and that persists on both the topological and trivial insulator phases. Its out-of-plane spin polarization remains zero demonstrating the absence of a hedgehog spin texture expected from broken time-reversal symmetry. Our calculations reveal qualitative agreement with the experimental results all across the quantum-phase transition upon the systematic variation of the spin-orbit coupling strength. A non-time reversal symmetry breaking mechanism of bulk-mediated scattering processes that increase with decreasing spin-orbit coupling strength is proposed as explanation.

Vicinal surfaces exhibiting arrays of atomic steps are frequently investigated due to their diverse physical-chemical properties and their use as growth templates. However, surfaces featuring steps with a large number of low-coordinated kink-atoms have been widely ignored, despite their higher potential for chemistry and catalysis. Here, the equilibrium structure and the electronic states of vicinal Ag(111) surfaces with densely kinked steps are investigated in a systematic way using a curved crystal. With scanning tunneling microscopy we observe an exceptional structural homogeneity of this class of vicinals, reflected in the smooth probability distribution of terrace sizes at all vicinal angles. This allows us to observe, first, a subtle evolution of the terrace-size distribution as a function of the terrace-width that challenges statistical models of step lattices, and second, lattice fluctuations around resonant modes of surface states. As shown in angle resolved photoemissi...

A single magnetic atom on a surface epitomizes the scaling limit for magnetic information storage. Indeed, recent work has shown that individual atomic spins can exhibit magnetic remanence and be read out with spin-based methods, demonstrating the fundamental requirements for magnetic memory. However, atomic spin memory has been only realized on thin insulating surfaces to date, removing potential tunability via electronic gating or distance-dependent exchange-driven magnetic coupling. Here, we show a novel mechanism for single-atom magnetic information storage based on bistability in the orbital population, or so-called valency, of an individual Co atom on semiconducting black phosphorus (BP). Distance-dependent screening from the BP surface stabilizes the two distinct valencies and enables us to electronically manipulate the relative orbital population, total magnetic moment and spatial charge density of an individual magnetic atom without a spin-dependent readout mechanism. Furthermore, we show that the strongly anisotropic wavefunction can be used to locally tailor the switching dynamics between the two valencies. This orbital memory derives stability from the energetic barrier to atomic relaxation and demonstrates the potential for high-temperature single-atom information storage.

The exchange scattering at magnetic adsorbates on superconductors gives rise to Yu-Shiba-Rusinov (YSR) bound states. Depending on the strength of the exchange coupling, the magnetic moment perturbs the Cooper pair condensate only weakly, resulting in a free-spin ground state, or binds a quasiparticle in its vicinity, leading to a (partially) screened spin state. Here, we use the flexibility of Fe-porphin molecules adsorbed on a Pb(111) surface to reversibly and continuously tune between these distinct ground states. We find that the FeP moment is screened in the pristine adsorption state. Approaching the tip of a scanning tunneling microscope, we exert a sufficiently strong attractive force to tune the molecule through the quantum phase transition into the free-spin state. We ascertain and characterize the transition by investigating the transport processes as function of tip-molecule distance, exciting the YSR states by single-electron tunneling as well as (multiple) Andreev reflections.