Dr.jens.brede

A junction between an n- and p-type semiconductor results in the creation of a depletion region whose properties are at the basis of nowadays electronics. If realized using topological insulators as constituent materials, p-n junctions are expected to manifest several unconventional effects with great potential for applications. Experimentally, all these fascinating properties remained unexplored so far, mainly because prototypical topological PNJs, which can be easily realized and investigated, were not readily available. Here, we report on the creation of topological PNJs which can be as narrow as few tenths of nm showing a built-in potential of 110meV. These junctions are intrinsically obtained by a thermodynamic control of the defects distribution across the crystal. Our results make Bi2Te3 a robust and reliable platform to explore the physics of topological p-n junction.

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Author(s): S. K. Mahatha, P. Moras, P. M. Sheverdyaeva, R. Flammini, K. Horn, and C. Carbone

Silicon multilayers on Ag(111) have been suggested to exhibit the structure of silicene, a material that has been heralded as a novel basis for microelectronic applications. However, our angle-resolved photoemission spectra (ARPES) from silicon multilayers on Ag(111) and of the silver-induced recons…

[Phys. Rev. B 92, 245127] Published Tue Dec 22, 2015

Nature Physics. doi:10.1038/nphys3594

Authors: S. V. Borisenko, D. V. Evtushinsky, Z.-H. Liu, I. Morozov, R. Kappenberger, S. Wurmehl, B. Büchner, A. N. Yaresko, T. K. Kim, M. Hoesch, T. Wolf & N. D. Zhigadlo

Spin–orbit coupling is a fundamental interaction in solids that can induce a broad range of unusual physical properties, from topologically non-trivial insulating states to unconventional pairing in superconductors. In iron-based superconductors its role has, so far, not been considered of primary importance, with models based on spin- or orbital fluctuations pairing being used most widely. Using angle-resolved photoemission spectroscopy, we directly observe a sizeable spin–orbit splitting in all the main members of the iron-based superconductors. We demonstrate that its impact on the low-energy electronic structure and details of the Fermi surface topology is stronger than that of possible nematic ordering. The largest pairing gap is supported exactly by spin–orbit-coupling-induced Fermi surfaces, implying a direct relation between this interaction and the mechanism of high-temperature superconductivity.

Author(s): Kyoung-Whan Kim, Kyung-Jin Lee, Hyun-Woo Lee, and M. D. Stiles

We derive an intrinsic contribution to the nonadiabatic spin torque for nonuniform magnetic textures. It differs from previously considered contributions in several ways and can be the dominant contribution in some models. It does not depend on the change in occupation of the electron states due to …

[Phys. Rev. B 92, 224426] Published Mon Dec 21, 2015

Author(s): P. San-Jose, J. L. Lado, R. Aguado, F. Guinea, and J. Fernández-Rossier

Majorana particles, which are their own antiparticles and whose recent detection in solid-state systems remains controversial, are expected to play an important role in future quantum computing. Now, scientists predict that graphene may host Majorana particles.

[Phys. Rev. X 5, 041042] Published Tue Dec 15, 2015

Conventional in-gap Yu-Shiba-Rusinov states require two ingredients: magnetic atoms and a superconducting host that, in the normal phase, has a finite density of states at the Fermi energy. Here we show that hydrogenated graphene can host Yu-Shiba-Rusinov states without any of those two ingredients. Atomic hydrogen chemisorbed in graphene is known to act as paramagnetic center with a weakly localized magnetic moment. Our calculations for hydrogenated graphene in proximity to a superconductor show that individual adatoms induce in-gap Yu-Shiba-Rusinov states with an exotic spectrum whereas chains of adatoms result in a gapless Yu- Shiba-Rusinov band. Our predictions can be tested using state of the art techniques, combining recent progress of atomic manipulation of atomic hydrogen on graphene together with the well tested proximity effect in graphene.

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Author(s): Sebastian Fiedler, Thomas Bathon, Sergey V. Eremeev, Oleg E. Tereshchenko, Konstantin A. Kokh, Evgueni V. Chulkov, Paolo Sessi, Hendrik Bentmann, Matthias Bode, and Friedrich Reinert

The noncentrosymmetric semiconductors BiTeX(X=Cl,Br,I) show large Rashba-type spin-orbit splittings in their electronic structure making them candidate materials for spin-based electronics. However, BiTeI(0001) single-crystal surfaces usually consist of stacking-fault-induced domains of Te and I ter…

[Phys. Rev. B 92, 235430] Published Tue Dec 15, 2015

Abstract

Article

Graphene nanoribbons have potential applications as nanoscale wires, though experimentally studied ribbons display wide bandgaps. Here, the authors synthesise the narrowest armchair graphene nanoribbon predicted to have metallic behaviour and show these ribbons—5 carbon atoms wide—indeed display almost metallic behaviour.

Nature Communications doi: 10.1038/ncomms10177

Authors: Amina Kimouche, Mikko M. Ervasti, Robert Drost, Simo Halonen, Ari Harju, Pekka M. Joensuu, Jani Sainio, Peter Liljeroth

The Zeeman effect, which is usually detrimental to superconductivity, can be strongly protective when an effective Zeeman field from intrinsic spin-orbit coupling locks the spins of Cooper pairs in a direction orthogonal to an external magnetic field. We performed magnetotransport experiments with ionic-gated molybdenum disulfide transistors, in which gating prepared individual superconducting states with different carrier dopings, and measured an in-plane critical field Bc2 far beyond the Pauli paramagnetic limit, consistent with Zeeman-protected superconductivity. The gating-enhanced Bc2 is more than an order of magnitude larger than it is in the bulk superconducting phases, where the effective Zeeman field is weakened by interlayer coupling. Our study provides experimental evidence of an Ising superconductor, in which spins of the pairing electrons are strongly pinned by an effective Zeeman field. Authors: J. M. Lu, O. Zheliuk, I. Leermakers, N. F. Q. Yuan, U. Zeitler, K. T. Law, J. T. Ye

Author(s): Samuel Paul Jarvis, Mohammad Abdur Rashid, Adam Sweetman, Jeremy Leaf, Simon Taylor, Philip Moriarty, and Janette Dunn

Claims that dynamic force microscopy has the capability to resolve intermolecular bonds in real space continue to be vigorously debated. To date, studies have been restricted to planar molecular assemblies with small separations between neighboring molecules. Here we report the observation of interm…

[Phys. Rev. B 92, 241405(R)] Published Wed Dec 09, 2015

Nature Physics. doi:10.1038/nphys3579

Authors: A. W. Tsen, B. Hunt, Y. D. Kim, Z. J. Yuan, S. Jia, R. J. Cava, J. Hone, P. Kim, C. R. Dean & A. N. Pasupathy

Two-dimensional (2D) materials are not expected to be metals at low temperature owing to electron localization. Consistent with this, pioneering studies on thin films reported only superconducting and insulating ground states, with a direct transition between the two as a function of disorder or magnetic field. However, more recent works have revealed the presence of an intermediate quantum metallic state occupying a substantial region of the phase diagram, whose nature is intensely debated. Here, we observe such a state in the disorder-free limit of a crystalline 2D superconductor, produced by mechanical co-lamination of NbSe2 in an inert atmosphere. Under a small perpendicular magnetic field, we induce a transition from superconductor to the quantum metal. We find a unique power-law scaling with field in this phase, which is consistent with the Bose-metal model where metallic behaviour arises from strong phase fluctuations caused by the magnetic field.

Nature Physics. doi:10.1038/nphys3583

Authors: D. K. Efetov, L. Wang, C. Handschin, K. B. Efetov, J. Shuang, R. Cava, T. Taniguchi, K. Watanabe, J. Hone, C. R. Dean & P. Kim

Electrons incident from a normal metal onto a superconductor are reflected back as holes—a process called Andreev reflection. In a normal metal where the Fermi energy is much larger than a typical superconducting gap, the reflected hole retraces the path taken by the incident electron. In graphene with low disorder, however, the Fermi energy can be tuned to be smaller than the superconducting gap. In this unusual limit, the holes are expected to be reflected specularly at the superconductor–graphene interface owing to the onset of interband Andreev processes, where the effective mass of the reflected holes changes sign. Here we present measurements of gate-modulated Andreev reflections across the low-disorder van der Waals interface formed between graphene and the superconducting NbSe2. We find that the conductance across the graphene–superconductor interface exhibits a characteristic suppression when the Fermi energy is tuned to values smaller than the superconducting gap, a hallmark for the transition between intraband retro Andreev reflections and interband specular Andreev reflections.

The Hofstadter energy spectrum provides a uniquely tunable system to study emergent topological order in the regime of strong interactions. Previous experiments, however, have been limited to low Bloch band fillings where only the Landau level index plays a role. We report measurements of high-mobility graphene superlattices where the complete unit cell of the Hofstadter spectrum is accessible. We observed coexistence of conventional fractional quantum Hall effect (QHE) states together with the integer QHE states associated with the fractal Hofstadter spectrum. At large magnetic field, we observed signatures of another series of states, which appeared at fractional Bloch filling index. These fractional Bloch band QHE states are not anticipated by existing theoretical pictures and point toward a distinct type of many-body state. Authors: Lei Wang, Yuanda Gao, Bo Wen, Zheng Han, Takashi Taniguchi, Kenji Watanabe, Mikito Koshino, James Hone, Cory R. Dean

Author(s): Yan-Feng Lv, Wen-Lin Wang, Jun-Ping Peng, Hao Ding, Yang Wang, Lili Wang, Ke He, Shuai-Hua Ji, Ruidan Zhong, John Schneeloch, Gen-Da Gu, Can-Li Song, Xu-Cun Ma, and Qi-Kun Xue

Understanding the mechanism of high transition temperature (Tc) superconductivity in cuprates has been hindered by the apparent complexity of their multilayered crystal structure. Using a cryogenic scanning tunneling microscopy (STM), we report on layer-by-layer probing of the electronic structures …

[Phys. Rev. Lett. 115, 237002] Published Wed Dec 02, 2015

Measuring entanglement entropy in a quantum many-body system

Nature 528, 7580 (2015). doi:10.1038/nature15750

Authors: Rajibul Islam, Ruichao Ma, Philipp M. Preiss, M. Eric Tai, Alexander Lukin, Matthew Rispoli & Markus Greiner

Entanglement is one of the most intriguing features of quantum mechanics. It describes non-local correlations between quantum objects, and is at the heart of quantum information sciences. Entanglement is now being studied in diverse fields ranging from condensed matter to quantum gravity. However, measuring entanglement

Author(s): S. Baumann, F. Donati, S. Stepanow, S. Rusponi, W. Paul, S. Gangopadhyay, I. G. Rau, G. E. Pacchioni, L. Gragnaniello, M. Pivetta, J. Dreiser, C. Piamonteze, C. P. Lutz, R. M. Macfarlane, B. A. Jones, P. Gambardella, A. J. Heinrich, and H. Brune

We report on the magnetic properties of individual Fe atoms deposited on MgO(100) thin films probed by x-ray magnetic circular dichroism and scanning tunneling spectroscopy. We show that the Fe atoms have strong perpendicular magnetic anisotropy with a zero-field splitting of 14.0±0.3  meV/atom. Thi…

[Phys. Rev. Lett. 115, 237202] Published Tue Dec 01, 2015

Article

Magnetic atoms on a surface possess diverse correlated phases under an applied magnetic field due to a balance of exchange interaction and carrier-mediated coupling. Here, the authors use scanning tunnel microscopy to explore the phase diagram of coupled Co atom pairs on the surface of Cu 2 N/Cu(100).

Nature Communications doi: 10.1038/ncomms10046

Authors: A. Spinelli, M. Gerrits, R. Toskovic, B. Bryant, M. Ternes, A. F. Otte

We study the spin waves of the triangular skyrmion crystal that emerges in a two dimensional spin lattice model as a result of the competition between Heisenberg exchange, Dzyalonshinkii-Moriya interactions, Zeeman coupling and uniaxial anisotropy. The calculated spin wave bands have a finite Berry curvature that, in some cases, leads to non-zero Chern numbers, making this system topologically distinct from conventional magnonic systems. We compute the edge spin-waves, expected from the bulk-boundary correspondence principle, and show that they are chiral, which makes them immune to elastic backscattering. Our results illustrate how topological phases can occur in self-generated emergent superlattices at the mesoscale.