Author(s): Alejandro O. Leon, Adam B. Cahaya, and Gerrit E. W. Bauer
The large spin-orbit interaction in the lanthanides implies a strong coupling between their internal charge and spin degrees of freedom. We formulate the coupling between the voltage and the local magnetic moments of rare-earth atoms with a partially filled 4f shell at the interface between an insul...
[Phys. Rev. Lett. 120, 027201] Published Wed Jan 10, 2018
Realization of spin crossover (SCO) based applications requires studying of spin state switching characteristics of SCO complex molecules at nanostructured environments especially on-surface. Except for a very few cases, the SCO of a surface bound thin molecular film is either quenched or heavily altered due to (i) strong molecule-surface interactions and (ii) differing intermolecular interactions in films relative to the bulk. By fabricating SCO complexes on a weakly interacting surface such as highly oriented pyrolytic graphite (HOPG) and copper nitride (CuN), the interfacial quenching problem has been tackled. However, engineering intermolecular interactions in thin SCO active films is rather difficult. This work proposes a molecular self-assembly strategy to fabricate thin spin switchable surface bound films with programmable intermolecular interactions. Molecular engineering of the parent complex system [Fe(H$_{2}$B(pz)$_{2}$)$_{2}$(bpy)] (pz = pyrazole, bpy = 2,2'-bipyridine) with a dodecyl (C$_{12}$) alkyl chain yielded a classical amphiphile-like functional and vacuum sublimable charge neutral Fe$^{\rm II}$ complex, [Fe(H$_{2}$B(pz)$_{2}$)$_{2}$(C$_{12}$-bpy)] (C$_{12}$-bpy = dodecyl[2,2'-bipyridine]-5-carboxylate). The bulk powder and 10 nm thin film, on quartz glass/SiO$_{\rm x}$ surface, of the complex showed comparable spin state switching characteristics mediated by similar lamellar bilayer like self-assembly/molecular interactions in both bulk and thin film states. This unprecedented observation augurs well for the development of SCO based applications, especially in molecular spintronics.
Author(s): Shuyuan Zhang, Jiaqi Guan, Yan Wang, Tom Berlijn, Steve Johnston, Xun Jia, Bing Liu, Qing Zhu, Qichang An, Siwei Xue, Yanwei Cao, Fang Yang, Weihua Wang, Jiandi Zhang, E. W. Plummer, Xuetao Zhu, and Jiandong Guo
Charge transfer and electron-phonon coupling (EPC) are proposed to be two important constituents associated with enhanced superconductivity in the single unit cell FeSe films on oxide surfaces. Using high-resolution electron energy loss spectroscopy combined with first-principles calculations, we ha...
[Phys. Rev. B 97, 035408] Published Mon Jan 08, 2018
Magnetic adsorbates on superconductors induce local bound states within the superconducting gap. These Yu-Shiba-Rusinov (\YSR) states decay slowly away from the impurity compared to atomic orbitals, even in 3d bulk crystals. Here, we use scanning tunneling spectroscopy to investigate their hybridization between two nearby magnetic Mn adatoms on a superconducting Pb(001) surface. We observe that the hybridization leads to the formation of symmetric and antisymmetric combinations of \YSR states. We investigate how the structure of the dimer wave functions and the energy splitting depend on the shape of the underlying monomer orbitals and the orientation of the dimer with respect to the Pb lattice.
The role of Chemistry in the road towards quantum devices is the design of elementary pieces with a built-in function. A brilliant example is the use of molecular transistors as nuclear spin detectors, which, up to now, has been implemented only on [TbPc$_2$]$^-$. We argue that this is an artificial constraint and critically discuss the limitations of current theoretical approaches to assess the potential of molecules for their use in spintronics. In connection with this, we review the recent progress in the preparation of highly coherent spin qubits based on vanadium dithiolate complexes and argue that the use of vanadyl dithiolates as single molecule transistors to read and control a triple nuclear spin qubit could give rise to new phenomena, notably including a low-current nuclear spin detection scheme by means of a spin valve effect.
On-surface reactions can be performed on catalytic metal surfaces to combine molecular units into extended arrays. Templating the catalyst with a porous self-assembled network gives rise to nanometer-sized pores in which a chemical reaction can occur. Scanning probe microscopy experiments provide single-molecule resolution and show that templating a catalytically active surface, via a supramolecular template, influences the reaction pathway of an on-surface coupling reaction, leading to a different product to that formed on a non-templated surface. For more details, see the Communication by A. Saywell et al. on page 56 ff.
Author(s): Maciej Dendzik, Albert Bruix, Matteo Michiardi, Arlette S. Ngankeu, Marco Bianchi, Jill A. Miwa, Bjørk Hammer, Philip Hofmann, and Charlotte E. Sanders
Low-resistance Ohmic contacts are a challenge for electronic devices based on two-dimensional (2D) materials. We show that an atomically precise in-plane junction between a 2D semiconductor and a metallic contact can lead to a semiconductor-to-metal transition in the 2D material, a finding which poi...
[Phys. Rev. B 96, 235440] Published Tue Dec 26, 2017
We report on the fabrication and performance of a new kind of tip for scanning tunneling microscopy. By fully incorporating a metallic tip on a silicon chip using modern micromachining and nanofabrication techniques, we realize so-called smart tips and show the possibility of device-based STM tips. Contrary to conventional etched metal wire tips, these can be integrated into lithographically defined electrical or photonic circuits, as well as mechanical systems. We experimentally demonstrate that the performance of the smart tips is on par with conventional ones, both in stability and resolution. In situ tip preparation methods are possible and we verify that they can resolve the herringbone reconstruction and Friedel oscillations on Au(111) surfaces. In addition, these devices can be made to accommodate two isolated tips with sub-50 nm apex-to-apex distance to measure electron correlations at the nanoscale using a new type of double-tip experiment described in this letter.
Author(s): Andreas Topp, Raquel Queiroz, Andreas Grüneis, Lukas Müchler, Andreas W. Rost, Andrei Varykhalov, Dmitry Marchenko, Maxim Krivenkov, Fanny Rodolakis, Jessica L. McChesney, Bettina V. Lotsch, Leslie M. Schoop, and Christian R. Ast
While there are many explanations for the variety of material surface states, which can reveal interesting surface details and bulk properties, researchers lack an understanding of how these states arise in nonsymmorphic square-net semimetals. New experiments and calculations reveal that surface states in the compound ZrSiS come about because of nonsymmorphic symmetry breaking at the surface.
[Phys. Rev. X 7, 041073] Published Thu Dec 28, 2017
Magnetic skyrmions are small swirling topological defects in the magnetization texture stabilized by the protection due to their topology. In most cases they are induced by chiral interactions between atomic spins existing in non-centrosymmetric magnetic compounds or in thin films in which inversion symmetry is broken by the presence of an interface. The skyrmions can be extremely small with diameters in the nanometer range and, importantly, they behave as particles that can be moved, created or annihilated, making them suitable for abacus-type applications in information storage, logic or neuro-inspired technologies. Up to the last years skyrmions were observed only at low temperature (and in most cases under large applied fields) but important efforts of research has been recently devoted to find thin film and multilayered structures in which skyrmions are stabilized above room temperature and manipulated by current. This article focuses on these recent advances on the route to devices prototypes.
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 state relaxes in an external field of 8 T. The transverse magnetic anisotropy energy is estimated from magnetic field and bias voltage dependent switching rates at 4.3 K. Our measurements constrain the possible ground state of Ho single atom magnets to either Jz = 7 or 8, both compatible with magnetic bistability at fields larger than 10 mT.
Author(s): Weijiong Chen, Zijun Tian, Ping Li, Weidong Luo, and C. L. Gao
FeSe, as the simplest Fe-based superconductor, invokes tremendous studies on its electronic and magnetic properties. Among these, inconsistent or even contradictory results are often seen in both theory and experiment, especially concerning the magnetism of FeSe. In this paper, the absence of static...
[Phys. Rev. B 96, 214426] Published Wed Dec 20, 2017
We describe the design and performance of a scanning tunneling microscope (STM) which operates at a base temperature of 30 mK in a vector magnetic field. The cryogenics is based on an ultra-high vacuum (UHV) top-loading wet dilution refrigerator that contains a vector magnet allowing for fields up to 9 T perpendicular and 4 T parallel to the sample. The STM is placed in a multi-chamber UHV system, which allows in-situ preparation and exchange of samples and tips. The entire system rests on a 150-ton concrete block suspended by pneumatic isolators, which is housed in an acoustically isolated and electromagnetically shielded laboratory optimized for extremely low noise scanning probe measurements. We demonstrate the overall performance by illustrating atomic resolution and quasiparticle interference imaging and detail the vibrational noise of both the laboratory and microscope. We also determine the electron temperature via measurement of the superconducting gap of Re(0001) and illustrate magnetic field-dependent measurements of the spin excitations of individual Fe atoms on Pt(111). Finally, we demonstrate spin resolution by imaging the magnetic structure of the Fe double layer on W(110).
Author(s): Masato Niwata, Ryuichi Masutomi, and Tohru Okamoto
It is well known that external magnetic fields and magnetic moments of impurities both suppress superconductivity. Here, we demonstrate that their combined effect enhances the superconductivity of a few atomic layer thick Pb films grown on a cleaved GaAs(110) surface. A Ce-doped film, where supercon...
[Phys. Rev. Lett. 119, 257001] Published Mon Dec 18, 2017
A TbIII-phthalocyaninato double-decker ([1]0) single-molecule magnet (SMM) having four 15-crown-5 moieties in one of the ligands was synthesized, and its dimerization and magnetic properties were studied in an attempt to utilize the supramolecular aggregation for enhancing the SMM properties. Aggregation of [1]0 to form [12K4]4+ in the presence of K+ ions was studied by using UV/Vis-NIR absorption and NMR spectroscopies. For the magnetic measurements, [1]0 and [12K4]4+ were dispersed in poly(methyl methacrylate) (PMMA). UV/Vis-NIR absorption measurements on the PMMA dispersed samples were used to track the formation of [12K4]4+. Direct current (DC) magnetic susceptibility measurements revealed that there were ferromagnetic Tb–Tb interactions in [12K4]4+, whereas there was no indication of ferromagnetic interactions in [1]0. Upon the formation of [12K4]4+ from [1]0 and K+ ions, the temperature at which the magnetic hysteresis occurred increased from 7 to 15 K. In addition, the area of magnetic hysteresis became larger for [12K4]4+, meaning that SMM properties of [12K4]4+ are superior to those of [1]0. Alternating current (AC) magnetic measurements were used to confirm this observation. Magnetic relaxation times at 2 K increased 1000-fold upon dimerization of [1]0 to [12K4]4+, demonstrating the effectiveness of using K+ ions to induce dimer formation for the improvement of the SMM properties.
Paired SMMs are better than one! A TbIII-phthalocyaninato double-decker single-molecule magnet (SMM) having four 15-crown-5 moieties on one of the ligands was synthesized, and its dimerization and magnetic properties were studied in an attempt to utilize supramolecular aggregation for enhancing the SMM properties.
Author(s): Aparajita Singha, Romana Baltic, Fabio Donati, Christian Wäckerlin, Jan Dreiser, Luca Persichetti, Sebastian Stepanow, Pietro Gambardella, Stefano Rusponi, and Harald Brune
We report x-ray absorption spectroscopy and x-ray magnetic circular dichroism measurements as well as multiplet calculations for Dy, Ho, Er, and Tm atoms adsorbed on Pt(111), Cu(111), Ag(100), and Ag(111). In the gas phase, all four elements are divalent and we label their 4f occupancy as 4fn. Upon ...
[Phys. Rev. B 96, 224418] Published Wed Dec 13, 2017
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.
We theoretically consider a quantum magnetic impurity coupled to a superconductor, and obtain the local density of states at the position of the impurity taking into account the effect of spin-fluctuations and single-ion magnetic anisotropy. We particularly focus on the spectrum of subgap Yu-Shiba-Rusinov (YSR or Shiba) states induced by a quantum impurity with easy- or hard-axis uniaxial anisotropy. Although this is a relevant experimental situation in, e.g., magnetic adatoms on the surface of clean metals, it is customary that theoretical descriptions assume a classical-spin approximation which is not able to account for single-ion anisotropy and other quantum effects. Here, quantum fluctuations of the spin are taken into account in the equations of motion of the electronic Green's function in the weak-coupling limit, and considerably modify the energy of the Shiba states compared to the classical-spin approximation. Our results point towards the importance of incorporating quantum fluctuations and anisotropy effects for the correct interpretation of scanning tunneling microscopy (STM) experiments.
Author(s): A. Bermudez, X. Xu, R. Nigmatullin, J. O’Gorman, V. Negnevitsky, P. Schindler, T. Monz, U. G. Poschinger, C. Hempel, J. Home, F. Schmidt-Kaler, M. Biercuk, R. Blatt, S. Benjamin, and M. Müller
As small prototype quantum processors progress to large-scale, fault-tolerant computers, it is increasingly necessary to quantitatively assess the performance of quantum error correction. A new benchmark offers just such an assessment of trapped-ion quantum processors.
[Phys. Rev. X 7, 041061] Published Wed Dec 13, 2017
Author(s): Dominic V. Else, Paul Fendley, Jack Kemp, and Chetan Nayak
Topological materials hold much promise for quantum computers that are highly tolerant to errors, but the information can be corrupted by thermally excited quasiparticles. New numerical simulations show that in some materials, quantum information can be protected for much longer than expected.
[Phys. Rev. X 7, 041062] Published Wed Dec 13, 2017