Dr.jens.brede

Authors: Sangmo Cheon, Tae-Hwan Kim, Sung-Hoon Lee, Han Woong Yeom

Recently, there has been substantial interest in realizations of skyrmions, in particular in 2D systems due to increased stability resulting from reduced dimensionality. A stable skyrmion, representing the smallest realizable magnetic texture, could be an ideal element for ultra-dense magnetic memories. Here, we use the most general form of the 2D free energy with Dzyaloshinskii-Moriya interactions constructed from general symmetry considerations reflecting the underlying system. We predict that skyrmion phase is robust and it is present even when the system lacks the in-plane rotational symmetry. In fact, the lowered symmetry leads to increased stability of vortex-antivortex lattices with four-fold symmetry and in-plane spirals, in some instances even in the absence of an external magnetic field. Our results relate different hexagonal and square cell phases to the symmetries of materials used for realizations of skyrmions. This will give clear directions for experimental realizations of hexagonal and square cell phases, and will allow engineering of skyrmions with unusual properties. We also predict striking differences in gyro-dynamics induced by spin currents for isolated skyrmions and for crystals where spin currents can be induced by charge carriers or by thermal magnons. We find that under certain conditions, isolated skyrmions can move along the current without a side motion which can have implications for realizations of magnetic memories.

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Time reversal dictates that nonmagnetic, centrosymmetric crystals cannot be spin-polarized as a whole. However, it has been recently shown that the electronic structure in these crystals can in fact show regions of high spin-polarization, as long as it is probed locally in real and in reciprocal space. In this article we present the first observation of this type of compensated polarization in MoS$_2$ bulk crystals. Using spin- and angle-resolved photoemission spectroscopy (ARPES) we directly observed a spin-polarization of more than 65% for distinct valleys in the electronic band structure. By additionally evaluating the probing depth of our method we find that these valence band states at the $\overline{\text{K}}$ point in the Brillouin zone are close to fully polarized for the individual atomic trilayers of MoS$_2$, which is confirmed by our density functional theory calculations. Furthermore, we show that this spin-layer locking leads to the observation of highly spin-polarized bands in ARPES since these states are almost completely confined within two dimensions. Our findings prove that these highly desired properties of MoS$_2$ can be accessed without thinning it down to the monolayer limit.

Article

Magnetic skyrmions are swirling magnetization textures which are topologically stabilized in helical magnets under an applied magnetic field. Here, the authors use Monte Carlo simulations to explore the stability of skyrmions against a ferromagnetic phase and their potential as single bits.

Nature Communications doi: 10.1038/ncomms9455

Authors: J. Hagemeister, N. Romming, K. von Bergmann, E. Y. Vedmedenko, R. Wiesendanger

A two-qubit logic gate in silicon

Nature 526, 7573 (2015). doi:10.1038/nature15263

Authors: M. Veldhorst, C. H. Yang, J. C. C. Hwang, W. Huang, J. P. Dehollain, J. T. Muhonen, S. Simmons, A. Laucht, F. E. Hudson, K. M. Itoh, A. Morello & A. S. Dzurak

Quantum computation requires qubits that can be coupled in a scalable manner, together with universal and high-fidelity one- and two-qubit logic gates. Many physical realizations of qubits exist, including single photons, trapped ions, superconducting circuits, single defects or atoms in diamond and silicon, and semiconductor quantum dots, with single-qubit fidelities that exceed the stringent thresholds required for fault-tolerant quantum computing. Despite this, high-fidelity two-qubit gates in the solid state that can be manufactured using standard lithographic techniques have so far been limited to superconducting qubits, owing to the difficulties of coupling qubits and dephasing in semiconductor systems. Here we present a two-qubit logic gate, which uses single spins in isotopically enriched silicon and is realized by performing single- and two-qubit operations in a quantum dot system using the exchange interaction, as envisaged in the Loss–DiVincenzo proposal. We realize CNOT gates via controlled-phase operations combined with single-qubit operations. Direct gate-voltage control provides single-qubit addressability, together with a switchable exchange interaction that is used in the two-qubit controlled-phase gate. By independently reading out both qubits, we measure clear anticorrelations in the two-spin probabilities of the CNOT gate.

Article

The ability to electrically control Dirac cones is essential for exploring the physics and applications of Dirac materials. Here, the authors combine ab initio calculations and analytical models to predict that (LaO) 2 (SbSe 2 ) 2 is a Dirac material with multiple electrically-tunable Dirac cones.

Nature Communications doi: 10.1038/ncomms9517

Authors: Xiao-Yu Dong, Jian-Feng Wang, Rui-Xing Zhang, Wen-Hui Duan, Bang-Fen Zhu, Jorge O. Sofo, Chao-Xing Liu

Article

Chiral magnets can support particle-like magnetization textures called skyrmions which form in lattices and can be manipulated for potential device applications. Here, the authors demonstrate the controlled creation and annihilation of a skyrmion lattice in MnSi single crystals using mechanical stress.

Nature Communications doi: 10.1038/ncomms9539

Authors: Y. Nii, T. Nakajima, A. Kikkawa, Y. Yamasaki, K. Ohishi, J. Suzuki, Y. Taguchi, T. Arima, Y. Tokura, Y. Iwasa

Nature Physics. doi:10.1038/nphys3508

Authors: Gerbold C. Ménard, Sébastien Guissart, Christophe Brun, Stéphane Pons, Vasily S. Stolyarov, François Debontridder, Matthieu V. Leclerc, Etienne Janod, Laurent Cario, Dimitri Roditchev, Pascal Simon & Tristan Cren

The quantum coupling of fully different degrees of freedom is a challenging path towards new functionalities for quantum electronics. Here we show that the localized classical spin of a magnetic atom immersed in a superconductor with a two-dimensional electronic band structure gives rise to a long-range coherent magnetic quantum state. We experimentally evidence coherent bound states with spatially oscillating particle–hole asymmetry extending tens of nanometres from individual iron atoms embedded in a 2H–NbSe2 crystal. We theoretically elucidate how reduced dimensionality enhances the spatial extent of these bound states and describe their energy and spatial structure. These spatially extended magnetic states could be used as building blocks for coupling coherently distant magnetic atoms in new topological superconducting phases.

Article

The spins of single molecules and defect centres possess properties which can be strongly influenced by their material contacts in electrical junctions. Here, the authors study the coupling between cobalt hydride complexes and a Rh(111) contact, mediated through a hexagonal boron nitride layer.

Nature Communications doi: 10.1038/ncomms9536

Authors: Peter Jacobson, Tobias Herden, Matthias Muenks, Gennadii Laskin, Oleg Brovko, Valeri Stepanyuk, Markus Ternes, Klaus Kern

##IMG## [http://ej.iop.org/images/0957-4484/26/44/445703/nano519826ieqn1.gif] {${{\rm{C}}}_{60}$} -functionalized tips are used to probe ##IMG## [http://ej.iop.org/images/0957-4484/26/44/445703/nano519826ieqn2.gif] {${{\rm{C}}}_{60}$} molecules on Cu(111) with scanning tunneling and atomic force microscopy. Distinct and complex intramolecular contrasts are found. Maximal attractive forces are observed when for both molecules a [6,6] bond faces a hexagon of the other molecule. Density functional theory calculations including parameterized van der Waals interactions corroborate the observations.

Article

Magnetic skyrmions are particle-like magnetization textures which may be manipulated in thin-film device applications. Here, the authors demonstrate the formation and control of room-temperature artificial skyrmion lattices in Co/Pd multilayers, defined by local ion irradiation and an array of magnetic vortex discs.

Nature Communications doi: 10.1038/ncomms9462

Authors: Dustin A. Gilbert, Brian B. Maranville, Andrew L. Balk, Brian J. Kirby, Peter Fischer, Daniel T. Pierce, John Unguris, Julie A. Borchers, Kai Liu

Universal Fermi liquid crossover and quantum criticality in a mesoscopic system

Nature 526, 7572 (2015). doi:10.1038/nature15261

Authors: A. J. Keller, L. Peeters, C. P. Moca, I. Weymann, D. Mahalu, V. Umansky, G. Zaránd & D. Goldhaber-Gordon

Quantum critical systems derive their finite-temperature properties from the influence of a zero-temperature quantum phase transition. The paradigm is essential for understanding unconventional high-Tc superconductors and the non-Fermi liquid properties of heavy fermion compounds. However, the microscopic origins of quantum phase transitions in complex materials are often debated. Here we demonstrate experimentally, with support from numerical renormalization group calculations, a universal crossover from quantum critical non-Fermi liquid behaviour to distinct Fermi liquid ground states in a highly controllable quantum dot device. Our device realizes the non-Fermi liquid two-channel Kondo state, based on a spin-1/2 impurity exchange-coupled equally to two independent electronic reservoirs. On detuning the exchange couplings we observe the Fermi liquid scale T*, at energies below which the spin is screened conventionally by the more strongly coupled channel. We extract a quadratic dependence of T* on gate voltage close to criticality, and validate an asymptotically exact description of the universal crossover between strongly correlated non-Fermi liquid and Fermi liquid states.

Two-channel Kondo effect and renormalization flow with macroscopic quantum charge states

Nature 526, 7572 (2015). doi:10.1038/nature15384

Authors: Z. Iftikhar, S. Jezouin, A. Anthore, U. Gennser, F. D. Parmentier, A. Cavanna & F. Pierre

Many-body correlations and macroscopic quantum behaviours are fascinating condensed matter problems. A powerful test-bed for the many-body concepts and methods is the Kondo effect, which entails the coupling of a quantum impurity to a continuum of states. It is central in highly correlated systems and can be explored with tunable nanostructures. Although Kondo physics is usually associated with the hybridization of itinerant electrons with microscopic magnetic moments, theory predicts that it can arise whenever degenerate quantum states are coupled to a continuum. Here we demonstrate the previously elusive ‘charge’ Kondo effect in a hybrid metal–semiconductor implementation of a single-electron transistor, with a quantum pseudospin of 1/2 constituted by two degenerate macroscopic charge states of a metallic island. In contrast to other Kondo nanostructures, each conduction channel connecting the island to an electrode constitutes a distinct and fully tunable Kondo channel, thereby providing unprecedented access to the two-channel Kondo effect and a clear path to multi-channel Kondo physics. Using a weakly coupled probe, we find the renormalization flow, as temperature is reduced, of two Kondo channels competing to screen the charge pseudospin. This provides a direct view of how the predicted quantum phase transition develops across the symmetric quantum critical point. Detuning the pseudospin away from degeneracy, we demonstrate, on a fully characterized device, quantitative agreement with the predictions for the finite-temperature crossover from quantum criticality.

Article

Mechanically induced conformational modulation can be used to control the conductance of single molecules junctions, but it is hard to be realized due to broken junctions. Here, the authors probe three-dimensional dynamics of Si/single-molecule/Si junctions, whose conductance shows a binary change.

Nature Communications doi: 10.1038/ncomms9465

Authors: Miki Nakamura, Shoji Yoshida, Tomoki Katayama, Atsushi Taninaka, Yutaka Mera, Susumu Okada, Osamu Takeuchi, Hidemi Shigekawa

Article

Interfaces are essential in organic semiconductor devices, yet the detailed connection between interface geometry and energy level alignment are not fully understood. Here, Cochrane et al . quantify energy level shifts with sub-molecular resolution within a nanoscale model organic semiconductor system.

Nature Communications doi: 10.1038/ncomms9312

Authors: K. A. Cochrane, A. Schiffrin, T. S. Roussy, M. Capsoni, S. A. Burke

Nature advance online publication 05 October 2015. doi:10.1038/nature15263

Authors: M. Veldhorst, C. H. Yang, J. C. C. Hwang, W. Huang, J. P. Dehollain, J. T. Muhonen, S. Simmons, A. Laucht, F. E. Hudson, K. M. Itoh, A. Morello & A. S. Dzurak

Quantum computation requires qubits that can be coupled in a scalable manner, together with universal and high-fidelity one- and two-qubit logic gates. Many physical realizations of qubits exist, including single photons, trapped ions, superconducting circuits, single defects or atoms in diamond and silicon, and semiconductor quantum dots, with single-qubit fidelities that exceed the stringent thresholds required for fault-tolerant quantum computing. Despite this, high-fidelity two-qubit gates in the solid state that can be manufactured using standard lithographic techniques have so far been limited to superconducting qubits, owing to the difficulties of coupling qubits and dephasing in semiconductor systems. Here we present a two-qubit logic gate, which uses single spins in isotopically enriched silicon and is realized by performing single- and two-qubit operations in a quantum dot system using the exchange interaction, as envisaged in the Loss–DiVincenzo proposal. We realize CNOT gates via controlled-phase operations combined with single-qubit operations. Direct gate-voltage control provides single-qubit addressability, together with a switchable exchange interaction that is used in the two-qubit controlled-phase gate. By independently reading out both qubits, we measure clear anticorrelations in the two-spin probabilities of the CNOT gate.

Article

Experimental data from angle-resolved photoemission spectroscopy can be utilized on molecular films to retrieve real-space images of molecular orbitals in two dimensions. Here, by scanning initial states as a function of photon energy, the authors can reconstruct three-dimensional orbital images.

Nature Communications doi: 10.1038/ncomms9287

Authors: S. Weiß, D. Lüftner, T. Ules, E. M. Reinisch, H. Kaser, A. Gottwald, M. Richter, S. Soubatch, G. Koller, M. G. Ramsey, F. S. Tautz, P. Puschnig

Nature Nanotechnology 10, 849 (2015). doi:10.1038/nnano.2015.177

Authors: Ilija Zeljkovic, Daniel Walkup, Badih A. Assaf, Kane L. Scipioni, R. Sankar, Fangcheng Chou & Vidya Madhavan

The unique crystalline protection of the surface states in topological crystalline insulators has led to a series of predictions of strain-generated phenomena, from the appearance of pseudo-magnetic fields and helical flat bands to the tunability of Dirac surface states by strain that may be used to construct ‘straintronic’ nanoswitches. However, the practical realization of this exotic phenomenology via strain engineering is experimentally challenging and is yet to be achieved. Here, we have designed an experiment to not only generate and measure strain locally, but also to directly measure the resulting effects on Dirac surface states. We grew heteroepitaxial thin films of topological crystalline insulator SnTe in situ and measured them using high-resolution scanning tunnelling microscopy to determine picoscale changes in the atomic positions, which reveal regions of both tensile and compressive strain. Simultaneous Fourier-transform scanning tunnelling spectroscopy was then used to determine the effects of strain on the Dirac electrons. We find that strain continuously tunes the momentum space position of the Dirac points, consistent with theoretical predictions. Our work demonstrates the fundamental mechanism necessary for using topological crystalline insulators in strain-based applications.

Nature Nanotechnology 10, 892 (2015). doi:10.1038/nnano.2015.190

Authors: Jakob Bach Knudsen, Lei Liu, Anne Louise Bank Kodal, Mikael Madsen, Qiang Li, Jie Song, Johannes B. Woehrstein, Shelley F. J. Wickham, Maximilian T. Strauss, Florian Schueder, Jesper Vinther, Abhichart Krissanaprasit, Daniel Gudnason, Anton Allen Abbotsford Smith, Ryosuke Ogaki, Alexander N. Zelikin, Flemming Besenbacher, Victoria Birkedal, Peng Yin, William M. Shih, Ralf Jungmann, Mingdong Dong & Kurt V. Gothelf

Author(s): A. V. Matetskiy, S. Ichinokura, L. V. Bondarenko, A. Y. Tupchaya, D. V. Gruznev, A. V. Zotov, A. A. Saranin, R. Hobara, A. Takayama, and S. Hasegawa

A one-atom-layer compound made of one monolayer of Tl and one-third monolayer of Pb on a Si(111) surface having 3×3 periodicity was found to exhibit a giant Rashba-type spin splitting of metallic surface-state bands together with two-dimensional superconducting transport properties. Temperature-depe…

[Phys. Rev. Lett. 115, 147003] Published Fri Oct 02, 2015

Moving beyond the limits of silicon transistors requires both a high-performance channel and high-quality electrical contacts. Carbon nanotubes provide high-performance channels below 10 nanometers, but as with silicon, the increase in contact resistance with decreasing size becomes a major performance roadblock. We report a single-walled carbon nanotube (SWNT) transistor technology with an end-bonded contact scheme that leads to size-independent contact resistance to overcome the scaling limits of conventional side-bonded or planar contact schemes. A high-performance SWNT transistor was fabricated with a sub–10-nanometer contact length, showing a device resistance below 36 kilohms and on-current above 15 microampere per tube. The p-type end-bonded contact, formed through the reaction of molybdenum with the SWNT to form carbide, also exhibited no Schottky barrier. This strategy promises high-performance SWNT transistors, enabling future ultimately scaled device technologies. Authors: Qing Cao, Shu-Jen Han, Jerry Tersoff, Aaron D. Franklin, Yu Zhu, Zhen Zhang, George S. Tulevski, Jianshi Tang, Wilfried Haensch

Author(s): Jingying Wang and Daniel B. Dougherty

Adsorption of the organic semiconductor perylene tetracarboxylic dianhydride onto Cr(001) decreases the metal d-derived surface-state lifetime without causing a shift in its energy. This suggests an indirect electronic interaction that contrasts sharply with expectations of p-d electronic coupling b…

[Phys. Rev. B 92, 161401(R)] Published Thu Oct 01, 2015

A subthermionic tunnel field-effect transistor with an atomically thin channel

Nature 526, 7571 (2015). doi:10.1038/nature15387

Authors: Deblina Sarkar, Xuejun Xie, Wei Liu, Wei Cao, Jiahao Kang, Yongji Gong, Stephan Kraemer, Pulickel M. Ajayan & Kaustav Banerjee

The fast growth of information technology has been sustained by continuous scaling down of the silicon-based metal–oxide field-effect transistor. However, such technology faces two major challenges to further scaling. First, the device electrostatics (the ability of the transistor’s gate electrode to control its channel potential) are degraded when the channel length is decreased, using conventional bulk materials such as silicon as the channel. Recently, two-dimensional semiconducting materials have emerged as promising candidates to replace silicon, as they can maintain excellent device electrostatics even at much reduced channel lengths. The second, more severe, challenge is that the supply voltage can no longer be scaled down by the same factor as the transistor dimensions because of the fundamental thermionic limitation of the steepness of turn-on characteristics, or subthreshold swing. To enable scaling to continue without a power penalty, a different transistor mechanism is required to obtain subthermionic subthreshold swing, such as band-to-band tunnelling. Here we demonstrate band-to-band tunnel field-effect transistors (tunnel-FETs), based on a two-dimensional semiconductor, that exhibit steep turn-on; subthreshold swing is a minimum of 3.9 millivolts per decade and an average of 31.1 millivolts per decade for four decades of drain current at room temperature. By using highly doped germanium as the source and atomically thin molybdenum disulfide as the channel, a vertical heterostructure is built with excellent electrostatics, a strain-free heterointerface, a low tunnelling barrier, and a large tunnelling area. Our atomically thin and layered semiconducting-channel tunnel-FET (ATLAS-TFET) is the only planar architecture tunnel-FET to achieve subthermionic subthreshold swing over four decades of drain current, as recommended in ref. 17, and is also the only tunnel-FET (in any architecture) to achieve this at a low power-supply voltage of 0.1 volts. Our device is at present the thinnest-channel subthermionic transistor, and has the potential to open up new avenues for ultra-dense and low-power integrated circuits, as well as for ultra-sensitive biosensors and gas sensors.

The exchange coupling between magnetic adsorbates and a superconducting substrate leads to Shiba states inside the superconducting energy gap and a Kondo resonance outside the gap. The exchange coupling strength determines whether the quantum many-body ground state is a Kondo singlet or a singlet of the paired superconducting quasiparticles. Here, we use scanning tunneling spectroscopy to identify the different quantum ground states of Manganese phthalocyanine on Pb(111). We observe Shiba states, which are split into triplets by magnetocrystalline anisotropy. Their characteristic spectral weight yields an unambiguous proof of the nature of the quantum ground state.