Dr.jens.brede

We identify features in the angle-resolved photoemission spectra (ARPES) arising from the periodic pattern characteristic for graphene heterostructure with hexagonal boron nitride (hBN). For this, we model ARPES spectra and intensity maps for five microscopic models used previously to describe moire superlattice in graphene/hBN systems. We show that detailed analysis of these features can be used to pin down the microscopic mechanism of the interaction between graphene and hBN. We also analyze how the presence of a moire-periodic strain in graphene or scattering of photoemitted electrons off hBN can be distinguished from the miniband formation.

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Author(s): Corneliu Nistor, Cornelius Krull, Aitor Mugarza, Sebastian Stepanow, Christian Stamm, Marcio Soares, Svetlana Klyatskaya, Mario Ruben, and Pietro Gambardella

Improving the magnetic stability of single-molecule magnets is a key challenge facing molecular spintronics. We use x-ray magnetic circular dichroism to explore the possibility of magnetically stabilizing TbPc2 molecules by attaching them to ultrathin Fe and FeMn films. We show that TbPc2 deposited …

[Phys. Rev. B 92, 184402] Published Mon Nov 02, 2015

We report controlled formation of sub-100 nm-thin electron channels in SrTiO$_3$ by doping with oxygen vacancies induced by Ar$^+$-ion irradiation. The conducting channels exhibit a consistent high electron mobility (~15,000 cm$^2$V$^{-1}$s$^{-1}$), which enables clear observation of magnetic quantum oscillations, and gate-tunable linear magnetoresistance. Near the onset of electrical conduction, the metal-insulator transition is induced by the mobility suppression. With the high electron mobility and the ease of controlled channel formation, this ion-irradiation doping method may provide an excellent basis for developing oxide electronics.

Nature Materials. doi:10.1038/nmat4456

Authors: He Wang, Huichao Wang, Haiwen Liu, Hong Lu, Wuhao Yang, Shuang Jia, Xiong-Jun Liu, X. C. Xie, Jian Wei & Jian Wang

Three-dimensional (3D) Dirac semimetals, which possess 3D linear dispersion in the electronic structure as a bulk analogue of graphene, have lately generated widespread interest in both materials science and condensed matter physics. Recently, crystalline Cd3As2 has been proposed and proved to be a 3D Dirac semimetal that can survive in the atmosphere. Here, by using point contact spectroscopy measurements, we observe exotic superconductivity around the point contact region on the surface of Cd3As2 crystals. The zero-bias conductance peak (ZBCP) and double conductance peaks (DCPs) symmetric around zero bias suggest p-wave-like unconventional superconductivity. Considering the topological properties of 3D Dirac semimetals, our findings may indicate that Cd3As2 crystals under certain conditions could be topological superconductors, which are predicted to support Majorana zero modes or gapless Majorana edge/surface modes in the boundary depending on the dimensionality of the material.

Nature Materials. doi:10.1038/nmat4455

Authors: Leena Aggarwal, Abhishek Gaurav, Gohil S. Thakur, Zeba Haque, Ashok K. Ganguli & Goutam Sheet

Three-dimensional (3D) Dirac semimetals exist close to topological phase boundaries which, in principle, should make it possible to drive them into exotic new phases, such as topological superconductivity, by breaking certain symmetries. A practical realization of this idea has, however, hitherto been lacking. Here we show that the mesoscopic point contacts between pure silver (Ag) and the 3D Dirac semimetal Cd3As2 (ref. ) exhibit unconventional superconductivity with a critical temperature (onset) greater than 6 K whereas neither Cd3As2 nor Ag are superconductors. A gap amplitude of 6.5 meV is measured spectroscopically in this phase that varies weakly with temperature and survives up to a remarkably high temperature of 13 K, indicating the presence of a robust normal-state pseudogap. The observations indicate the emergence of a new unconventional superconducting phase that exists in a quantum mechanically confined region under a point contact between a Dirac semimetal and a normal metal.

Nature Physics. doi:10.1038/nphys3530

Authors: J. Shiogai, Y. Ito, T. Mitsuhashi, T. Nojima & A. Tsukazaki

Among the recently discovered iron-based superconductors, ultrathin films of FeSe grown on SrTiO3 substrates have uniquely evolved into a high-transition-temperature (Tc) superconducting material. The mechanisms for the high-Tc superconductivity are under debate, with the superconducting gap mainly characterized with in situ analysis for FeSe films grown by molecular beam epitaxy. Here, we investigate the high-Tc superconductivity in ultrathin FeSe using an alternative top-down electrochemical etching technique in a three-terminal transistor configuration. In addition to the high-temperature superconductivity in FeSe on SrTiO3, the electrochemically etched ultrathin FeSe transistor on MgO also exhibits superconductivity around 40 K, implying that the application of an electric field effectively contributes to the high-Tc superconductivity in ultrathin FeSe regardless of substrate material. Moreover, the observable critical thickness for the high-Tc superconductivity is expanded up to 10 unit cells under an applied electric field and the insulator–superconductor transition is electrostatically controlled. The present demonstration implies that the modification of charge imbalance of holes and electrons by the electric-field effect plays a crucial role in inducing high-Tc superconductivity in FeSe-based electric double-layer transistors.

Nature Physics. doi:10.1038/nphys3527

Authors: Miguel M. Ugeda, Aaron J. Bradley, Yi Zhang, Seita Onishi, Yi Chen, Wei Ruan, Claudia Ojeda-Aristizabal, Hyejin Ryu, Mark T. Edmonds, Hsin-Zon Tsai, Alexander Riss, Sung-Kwan Mo, Dunghai Lee, Alex Zettl, Zahid Hussain, Zhi-Xun Shen & Michael F. Crommie

The tunnel junction between tip and sample in a scanning tunneling microscope is an ideal platform to access the local density of states in the sample through the differential conductance. We show that the energy resolution that can be obtained is principally limited by the electromagnetic interaction of the tunneling electrons with the surrounding environmental impedance as well as the capacitative noise of the junction. The parameter tuning the sensitivity to the environmental impedance is the capacitance of the tunnel junction. The higher the junction capacitance, the less sensitive the tunnel junction to the environment resulting in better energy resolution. Modeling this effect within P(E)-theory, the P(E)-function describes the probability for a tunneling electron to exchange energy with the environment and can be regarded as the resolution function of the tunnel junction. We experimentally demonstrate this effect in a scanning tunneling microscope with a superconducting aluminum tip and a superconducting aluminum sample at a base temperature of 15mK, where it is most pronounced.

The Griffiths singularity in a phase transition, caused by disorder effects, was predicted more than 40 years ago. Its signature, the divergence of the dynamical critical exponent, is challenging to observe experimentally. We report the experimental observation of the quantum Griffiths singularity in a two-dimensional superconducting system. We measured the transport properties of atomically thin gallium films and found that the films undergo superconductor-metal transitions with increasing magnetic field. Approaching the zero-temperature quantum critical point, we observed divergence of the dynamical critical exponent, which is consistent with the Griffiths singularity behavior. We interpret the observed superconductor-metal quantum phase transition as the infinite-randomness critical point, where the properties of the system are controlled by rare large superconducting regions. Authors: Ying Xing, Hui-Min Zhang, Hai-Long Fu, Haiwen Liu, Yi Sun, Jun-Ping Peng, Fa Wang, Xi Lin, Xu-Cun Ma, Qi-Kun Xue, Jian Wang, X. C. Xie

Nature advance online publication 28 October 2015. doi:10.1038/nature15723

Authors: Satoru Nakatsuji, Naoki Kiyohara & Tomoya Higo

In ferromagnetic conductors, an electric current may induce a transverse voltage drop in zero applied magnetic field: this anomalous Hall effect is observed to be proportional to magnetization, and thus is not usually seen in antiferromagnets in zero field. Recent developments in theory and experiment have provided a framework for understanding the anomalous Hall effect using Berry-phase concepts, and this perspective has led to predictions that, under certain conditions, a large anomalous Hall effect may appear in spin liquids and antiferromagnets without net spin magnetization. Although such a spontaneous Hall effect has now been observed in a spin liquid state, a zero-field anomalous Hall effect has hitherto not been reported for antiferromagnets. Here we report empirical evidence for a large anomalous Hall effect in an antiferromagnet that has vanishingly small magnetization. In particular, we find that Mn3Sn, an antiferromagnet that has a non-collinear 120-degree spin order, exhibits a large anomalous Hall conductivity of around 20 per ohm per centimetre at room temperature and more than 100 per ohm per centimetre at low temperatures, reaching the same order of magnitude as in ferromagnetic metals. Notably, the chiral antiferromagnetic state has a very weak and soft ferromagnetic moment of about 0.002 Bohr magnetons per Mn atom (refs 10, 12), allowing us to switch the sign of the Hall effect with a small magnetic field of around a few hundred oersted. This soft response of the large anomalous Hall effect could be useful for various applications including spintronics—for example, to develop a memory device that produces almost no perturbing stray fields.

Article

The implementation of topological insulators in spintronics requires the control of the topological spin texture. Here, the authors show that noble metal atoms added to the surface enable this controllability by altering the magnetic anisotropy and energy level alignment.

Nature Communications doi: 10.1038/ncomms9691

Authors: Paolo Sessi, Vyacheslav M. Silkin, Ilya A. Nechaev, Thomas Bathon, Lydia El-Kareh, Evgueni V. Chulkov, Pedro M. Echenique, Matthias Bode

Article

Wrinkles in graphene can restrict the motion of electrons to one dimension. Here, the authors observe a one-dimensional van Hove singularity and bandgap opening in a five nanometre wide wrinkle, which enables them to create a metallic-semiconducting-metallic junction in a single graphene sheet.

Nature Communications doi: 10.1038/ncomms9601

Authors: Hyunseob Lim, Jaehoon Jung, Rodney S. Ruoff, Yousoo Kim

Article

A polaron is a quasiparticle formed through the strong interaction between an electron and the ions in a crystalline solid. Here, the authors observe Fröhlich polarons, formed by the coupling of electrons and long-wavelength optical phonons, in strontium titanate.

Nature Communications doi: 10.1038/ncomms9585

Authors: Chaoyu Chen, José Avila, Emmanouil Frantzeskakis, Anna Levy, Maria C. Asensio

Recently emerging two-dimensional (2D) superconductors in atomically thin layers and at heterogeneous interfaces are attracting growing interest in condensed matter physics. Here, we report that an ion-gated zirconium nitride chloride surface, exhibiting a dome-shaped phase diagram with a maximum critical temperature of 14.8 kelvin, behaves as a superconductor persisting to the 2D limit. The superconducting thickness estimated from the upper critical fields is ≅ 1.8 nanometers, which is thinner than one unit-cell. The majority of the vortex phase diagram down to 2 kelvin is occupied by a metallic state with a finite resistance, owing to the quantum creep of vortices caused by extremely weak pinning and disorder. Our findings highlight the potential of electric-field–induced superconductivity, establishing a new platform for accessing quantum phases in clean 2D superconductors. Authors: Yu Saito, Yuichi Kasahara, Jianting Ye, Yoshihiro Iwasa, Tsutomu Nojima

In a Dirac semimetal, each Dirac node is resolved into two Weyl nodes with opposite “handedness” or chirality. The two chiral populations do not mix. However, in parallel electric and magnetic fields (E||B), charge is predicted to flow between the Weyl nodes, leading to negative magnetoresistance. This “axial” current is the chiral (Adler-Bell-Jackiw) anomaly investigated in quantum field theory. We report the observation of a large, negative longitudinal magnetoresistance in the Dirac semimetal Na3Bi. The negative magnetoresistance is acutely sensitive to deviations of the direction of B from E and is incompatible with conventional transport. By rotating E (as well as B), we show that it is consistent with the prediction of the chiral anomaly. Authors: Jun Xiong, Satya K. Kushwaha, Tian Liang, Jason W. Krizan, Max Hirschberger, Wudi Wang, R. J. Cava, N. P. Ong

We combined the high-energy resolution of conventional spin resonance (here ~10 nano–electron volts) with scanning tunneling microscopy to measure electron paramagnetic resonance of individual iron (Fe) atoms placed on a magnesium oxide film. We drove the spin resonance with an oscillating electric field (20 to 30 gigahertz) between tip and sample. The readout of the Fe atom’s quantum state was performed by spin-polarized detection of the atomic-scale tunneling magnetoresistance. We determine an energy relaxation time of T1 ≈ 100 microseconds and a phase-coherence time of T2 ≈ 210 nanoseconds. The spin resonance signals of different Fe atoms differ by much more than their resonance linewidth; in a traditional ensemble measurement, this difference would appear as inhomogeneous broadening. Authors: Susanne Baumann, William Paul, Taeyoung Choi, Christopher P. Lutz, Arzhang Ardavan, Andreas J. Heinrich

Nature Chemistry 7, 883 (2015). doi:10.1038/nchem.2365

Authors: Tobias C. B. Harlang, Yizhu Liu, Olga Gordivska, Lisa A. Fredin, Carlito S. Ponseca, Ping Huang, Pavel Chábera, Kasper S. Kjaer, Helena Mateos, Jens Uhlig, Reiner Lomoth, Reine Wallenberg, Stenbjörn Styring, Petter Persson, Villy Sundström & Kenneth Wärnmark

Using iron instead of the scarce ruthenium in light-harvesting complexes is challenging because iron complexes generally have short-lived excited states. Now an iron complex has been developed that has a long-lived excited state, which can lead to photo-induced electron injection into nanoporous TiO2 with a yield of 92%.

Nature Chemistry 7, 883 (2015). doi:10.1038/nchem.2365

Authors: Tobias C. B. Harlang, Yizhu Liu, Olga Gordivska, Lisa A. Fredin, Carlito S. Ponseca, Ping Huang, Pavel Chábera, Kasper S. Kjaer, Helena Mateos, Jens Uhlig, Reiner Lomoth, Reine Wallenberg, Stenbjörn Styring, Petter Persson, Villy Sundström & Kenneth Wärnmark

Using iron instead of the scarce ruthenium in light-harvesting complexes is challenging because iron complexes generally have short-lived excited states. Now an iron complex has been developed that has a long-lived excited state, which can lead to photo-induced electron injection into nanoporous TiO2 with a yield of 92%.

Nature advance online publication 21 October 2015. doi:10.1038/nature15759

Authors: B. Hensen, H. Bernien, A. E. Dréau, A. Reiserer, N. Kalb, M. S. Blok, J. Ruitenberg, R. F. L. Vermeulen, R. N. Schouten, C. Abellán, W. Amaya, V. Pruneri, M. W. Mitchell, M. Markham, D. J. Twitchen, D. Elkouss, S. Wehner, T. H. Taminiau & R. Hanson

More than 50 years ago, John Bell proved that no theory of nature that obeys locality and realism can reproduce all the predictions of quantum theory: in any local-realist theory, the correlations between outcomes of measurements on distant particles satisfy an inequality that can be violated if the particles are entangled. Numerous Bell inequality tests have been reported; however, all experiments reported so far required additional assumptions to obtain a contradiction with local realism, resulting in ‘loopholes’. Here we report a Bell experiment that is free of any such additional assumption and thus directly tests the principles underlying Bell’s inequality. We use an event-ready scheme that enables the generation of robust entanglement between distant electron spins (estimated state fidelity of 0.92 ± 0.03). Efficient spin read-out avoids the fair-sampling assumption (detection loophole), while the use of fast random-basis selection and spin read-out combined with a spatial separation of 1.3 kilometres ensure the required locality conditions. We performed 245 trials that tested the CHSH–Bell inequalityS ≤ 2 and found S = 2.42 ± 0.20 (where S quantifies the correlation between measurement outcomes). A null-hypothesis test yields a probability of at most P = 0.039 that a local-realist model for space-like separated sites could produce data with a violation at least as large as we observe, even when allowing for memory in the devices. Our data hence imply statistically significant rejection of the local-realist null hypothesis. This conclusion may be further consolidated in future experiments; for instance, reaching a value of P = 0.001 would require approximately 700 trials for an observed S = 2.4. With improvements, our experiment could be used for testing less-conventional theories, and for implementing device-independent quantum-secure communication and randomness certification.

Nature advance online publication 21 October 2015. doi:10.1038/nature15631

Author: Howard Wiseman

A fundamental scientific assumption called local realism conflicts with certain predictions of quantum mechanics. Those predictions have now been verified, with none of the loopholes that have compromised earlier tests.

Article

Introduction of higher-dimensional structure elements into solids is used to generate unusual materials properties. Here, the authors report how replacing LaO planes with SrO dopants yields space-charge induced superconductivity, showing the potential of two-dimensional doping in this field.

Nature Communications doi: 10.1038/ncomms9586

Authors: F. Baiutti, G. Logvenov, G. Gregori, G. Cristiani, Y. Wang, W. Sigle, P. A. van Aken, J. Maier

Abstract

Author(s): P. P. Aurino, A. Kalabukhov, N. Tuzla, E. Olsson, A. Klein, P. Erhart, Y. A. Boikov, I. T. Serenkov, V. I. Sakharov, T. Claeson, and D. Winkler

The conducting state of a quasi-two-dimensional electron gas (q2DEG), formed at the heterointerface between the two wide-bandgap insulators LaAlO3 (LAO) and SrTiO3, can be made completely insulating by low-energy, 150-eV, Ar+ irradiation. The metallic behavior of the interface can be recovered by hi…

[Phys. Rev. B 92, 155130] Published Mon Oct 19, 2015

Article

Magnetic skyrmions are particle-like configurations which can emerge in the magnetization of materials possessing a chiral exchange interaction. Here, the authors demonstrate how spin-mixing magnetoresistance can allow for the reliable perpendicular detection of single nanoscale skyrmions in Pd/Fe/Ir(111) thin films.

Nature Communications doi: 10.1038/ncomms9541

Authors: Dax M. Crum, Mohammed Bouhassoune, Juba Bouaziz, Benedikt Schweflinghaus, Stefan Blügel, Samir Lounis

Detecting single–photon level signals—carriers of both classical and quantum information—is particularly challenging for low-energy microwave frequency excitations. Here we introduce a superconducting amplifier based on a Josephson junction transmission line. Unlike current standing-wave parametric amplifiers, this traveling wave architecture robustly achieves high gain over a bandwidth of several gigahertz with sufficient dynamic range to read out 20 superconducting qubits. To achieve this performance, we introduce a subwavelength resonant phase-matching technique that enables the creation of nonlinear microwave devices with unique dispersion relations. We benchmark the amplifier with weak measurements, obtaining a high quantum efficiency of 75% (70% including noise added by amplifiers following the Josephson amplifier). With a flexible design based on compact lumped elements, this Josephson amplifier has broad applicability to microwave metrology and quantum optics. Authors: C. Macklin, K. O’Brien, D. Hover, M. E. Schwartz, V. Bolkhovsky, X. Zhang, W. D. Oliver, I. Siddiqi

Author(s): Yun-Mei Li, Jian Li, Li-Kun Shi, Dong Zhang, Wen Yang, and Kai Chang

We propose a light-induced spin Hall effect for interlayer exciton gas in monolayer MoSe2−WSe2 van der Waals heterostructure. By applying two infrared, spatially varying laser beams coupled to the exciton internal states, a spin-dependent gauge potential on the exciton center-of-mass motion is induc…

[Phys. Rev. Lett. 115, 166804] Published Thu Oct 15, 2015