Publications


Antidamping torques from simultaneous resonances in ferromagnet-topological insulator-ferromagnet heterostructures

Journal of Magnetism and Magnetic Materials Elsevier 473 (2018) 470-476

AA Baker, AI Figueroa, T Hesjedal, G Van Der Laan

We studied the magnetodynamics of ferromagnetic films coupling across a topological insulator (TI) Bi2Se3 layer using ferromagnetic resonance (FMR). TIs have attracted much attention across the physics community as they hold the potential for dissipationless carrier transport, extremely high spin-orbit torques, and are host to novel quantum effects. To investigate the coupling between the ferromagnetic (FM) layers, vector network analyzer (VNA)-FMR measurements of the resonance linewidth were performed as a function of bias field angle. By bringing the resonances of the two FM layers into close proximity, it was possible to observe antidamping torques that lead to a narrowing of linewidth, a characteristic of spin pumping. The element- and hence layer-specific technique of x-ray detected ferromagnetic resonance (XFMR) was used to circumvent the difficulty of obtaining accurate fits to the two overlapping resonances in close proximity. Our results confirm that the interaction across the TI is a dynamic exchange mediated by spin pumping, as opposed to a self-coupling of the surface state or similar, more unconventional mechanisms.


Hyperfine interaction of individual atoms on a surface

Science American Association for the Advancement of Science 362 (2018) 336-339

P Willke, Y Bae, K Yang, JL Lado, A Ferron, T Choi, A Ardavan, J Fernández-Rossier, AJ Heinrich, CP Lutz

Taking advantage of nuclear spins for electronic structure analysis, magnetic resonance imaging, and quantum devices hinges on knowledge and control of the surrounding atomic-scale environment. We measured and manipulated the hyperfine interaction of individual iron and titanium atoms placed on a magnesium oxide surface by using spin-polarized scanning tunneling microscopy in combination with single-atom electron spin resonance. Using atom manipulation to move single atoms, we found that the hyperfine interaction strongly depended on the binding configuration of the atom. We could extract atom- and position-dependent information about the electronic ground state, the state mixing with neighboring atoms, and properties of the nuclear spin. Thus, the hyperfine spectrum becomes a powerful probe of the chemical environment of individual atoms and nanostructures.


Molecular electronic spin qubits from a spin-frustrated trinuclear copper complex

Chemical Communications Royal Society of Chemistry 54 (2018) 12934-12937

B Kintzel, M Bohme, J Liu, A Burkhardt, J Mrozek, A Buchholz, A Ardavan, W Plass

The trinuclear copper(II) complex [Cu3(saltag)(py)6]ClO4 (H5saltag = tris(2-hydroxybenzylidene)triaminoguanidine) was synthesized and characterized by experimental as well as theoretical methods. This complex exhibits a strong antiferromagnetic coupling (J = −298 cm−1) between the copper(II) ions, mediated by the N–N diazine bridges of the tritopic ligand, leading to a spin-frustrated system. This compound shows a T2 coherence time of 340 ns in frozen pyridine solution, which extends to 591 ns by changing the solvent to pyridine-d5. Hence, the presented compound is a promising candidate as a building block for molecular spintronics.


The magnetic structures of rare-earth quadruple perovskite manganites RMn7O12

Physical review B: Condensed matter and materials physics American Physical Society 98 (2018) 104423-

R Johnson, D Khalyavin, P Manuel, L Zhang, K Yamaura, AA Belik

We report a neutron powder diffraction study of RMn7O12 quadruple perovskite manganites with R = La, Ce, Nd, Sm, and Eu. We show that in all measured compounds concomitant magnetic ordering of the A and B manganese sublattices occurs on cooling below the N´eel temperature. The respective magnetic structures are collinear, with one uncompensated Mn3+ moment per formula unit as observed in bulk magnetisation measurements. We show that both LaMn7O12 and NdMn7O12 undergo a second magnetic phase transition at low temperature, which introduces a canting of the B site sublattice moments that is commensurate in LaMn7O12 and incommensurate in NdMn7O12. This spin canting is consistent with a magnetic instability originating in the B site orbital order. Furthermore, NdMn7O12 displays a third magnetic phase transition at which long range ordering of the Nd sublattice modifies the periodicity of the incommensurate spin canting. Our results demonstrate a rich interplay between transition metal magnetism, orbital order, and the crystal lattice, which may be fine tuned by cation substitution and rare earth magnetism.


Magnetic and electronic structure of the layered rare-earth pnictide EuCd2Sb2

Physical Review B American Physical Society 98 (2018) 064419-

J Soh, C Donnerer, KM Hughes, E Schierle, E Weschke, D Prabhakaran, A Boothroyd

Resonant elastic X-ray scattering (REXS) at the Eu M5 edge reveals an antiferromagnetic structure in layered EuCd2Sb2 at temperatures below TN = 7.4 K with a magnetic propagation vector of (0, 0, 1/2) and spins in the basal plane. Magneto-transport and REXS measurements with an in-plane magnetic field show that features in the magnetoresistance are correlated with changes in the magnetic structure induced by the field. Ab initio electronic structure calculations predict that the observed spin structure gives rise to a gapped Dirac point close to the Fermi level with a gap of ∆E ∼ 0.01 eV. The results of this study indicate that the Eu spins are coupled to conduction electron states near the Dirac point.


Observation of a crossover from nodal to gapped superconductivity in LuxZr1-xB12

PHYSICAL REVIEW B 98 (2018) ARTN 094505

FKK Kirschner, NE Sluchanko, VB Filipov, FL Pratt, C Baines, NY Shitsevalova, SJ Blundell


Publisher Correction: Magnetic edge states and coherent manipulation of graphene nanoribbons.

Nature (2018)

M Slota, A Keerthi, WILLIAM Myers, E Tretyakov, M Baumgarten, ARZHANG Ardavan, H Sadeghi, CJ Lambert, A Narita, K Müllen, LAPO Bogani

In Fig. 1 of this Letter, there should have been two nitrogen (N) atoms at the 1,3-positions of all the blue chemical structures (next to the oxygen atoms), rather than one at the 2-position. The figure has been corrected online, and the original incorrect figure is shown as Supplementary Information to the accompanying Amendment.


Proposal for a micromagnetic standard problem for materials with Dzyaloshinskii-Moriya interaction

New Journal of Physics Institute of Physics 20 (2018) 113015-

D Cortes-Ortuno, M Beg, V Nehruji, L Breth, R Pepper, T Kluyver, G Downing, T Hesjedal, P Hatton, T Lancaster, R Hertel, O Hovorka, H Fabgohr

Understanding the role of the Dzyaloshinskii-Moriya interaction (DMI) for the formation of helimagnetic order, as well as the emergence of skyrmions in magnetic systems that lack inversion symmetry, has found increasing interest due to the significant potential for novel spin based technologies. Candidate materials to host skyrmions include those belonging to the B20 group such as FeGe, known for stabilising Bloch-like skyrmions, interfacial systems such as cobalt multilayers or Pd/Fe bilayers on top of Ir(111), known for stabilising N´eel-like skyrmions, and, recently, alloys with a crystallographic symmetry where anti-skyrmions are stabilised. Micromagnetic simulations have become a standard approach to aid the design and optimisation of spintronic and magnetic nanodevices and are also applied to the modelling of device applications which make use of skyrmions. Several public domain micromagnetic simulation packages such as OOMMF, MuMax3 and Fidimag already offer implementations of different DMI terms. It is therefore highly desirable to propose a so-called micromagnetic standard problem that would allow one to benchmark and test the different software packages in a similar way as is done for ferromagnetic materials without DMI. Here, we provide a sequence of well-defined and increasingly complex computational problems for magnetic materials with DMI. Our test problems include 1D, 2D and 3D domains, spin wave dynamics in the presence of DMI, and validation of the analytical and numerical solutions including uniform magnetisation, edge tilting, spin waves and skyrmion formation. This set of problems can be used by developers and users of new micromagnetic simulation codes for testing and validation and hence establishing scientific credibility.


Ultrafast and highly sensitive infrared photodetectors based on two-dimensional oxyselenide crystals.

Nature communications 9 (2018) 3311-

J Yin, Z Tan, H Hong, J Wu, H Yuan, Y Liu, C Chen, C Tan, F Yao, T Li, Y Chen, Z Liu, K Liu, H Peng

Infrared light detection and sensing is deeply embedded in modern technology and human society and its development has always been benefitting from the discovery of various photoelectric materials. The rise of two-dimensional materials, thanks to their distinct electronic structures, extreme dimensional confinement and strong light-matter interactions, provides a material platform for next-generation infrared photodetection. Ideal infrared detectors should have fast respond, high sensitivity and air-stability, which are rare to meet at the same time in one two-dimensional material. Herein we demonstrate an infrared photodetector based on two-dimensional Bi2O2Se crystal, whose main characteristics are outstanding in the whole two-dimensional family: high sensitivity of 65 AW-1 at 1200 nm and ultrafast photoresponse of ~1 ps at room temperature, implying an intrinsic material-limited bandwidth up to 500 GHz. Such great performance is attributed to the suitable electronic bandgap and high carrier mobility of two-dimensional oxyselenide.


Giant anomalous Hall effect in a ferromagnetic Kagomé-lattice semimetal.

Nature physics 14 (2018) 1125-1131

E Liu, Y Sun, N Kumar, L Müchler, A Sun, L Jiao, S-Y Yang, D Liu, A Liang, Q Xu, J Kroder, V Süß, H Borrmann, C Shekhar, Z Wang, C Xi, W Wang, W Schnelle, S Wirth, Y Chen, STB Goennenwein, C Felser

Magnetic Weyl semimetals with broken time-reversal symmetry are expected to generate strong intrinsic anomalous Hall effects, due to their large Berry curvature. Here, we report a magnetic Weyl semimetal candidate, Co3Sn2S2, with a quasi-two-dimensional crystal structure consisting of stacked Kagomé lattices. This lattice provides an excellent platform for hosting exotic topological quantum states. We observe a negative magnetoresistance that is consistent with the chiral anomaly expected from the presence of Weyl nodes close to the Fermi level. The anomalous Hall conductivity is robust against both increased temperature and charge conductivity, which corroborates the intrinsic Berry-curvature mechanism in momentum space. Owing to the low carrier density in this material and the significantly enhanced Berry curvature from its band structure, the anomalous Hall conductivity and the anomalous Hall angle simultaneously reach 1130 Ω-1 cm-1 and 20%, respectively, an order of magnitude larger than typical magnetic systems. Combining the Kagomé-lattice structure and the out-of-plane ferromagnetic order of Co3Sn2S2, we expect that this material is an excellent candidate for observation of the quantum anomalous Hall state in the two-dimensional limit.


Erratum: Magnetic ground state and magnon-phonon interaction in multiferroic h−YMnO3 [Phys. Rev. B 97 , 134304 (2018)]

Physical Review B American Physical Society (APS) (2018)

S Holm-Dahlin, A Kreisel, TK Schäffer, A Bakke, M Bertelsen, UB Hansen, M Retuerto, J Larsen, DHARMALINGAM Prabhakaran, PP Deen, Z Yamani, JO Birk, U Stuhr, C Niedermayer, AL Fennell, BM Andersen, K Lefmann

© 2018 American Physical Society. Inelastic neutron scattering has been used to study the magnetoelastic excitations in the multiferroic manganite hexagonal YMnO3. An avoided crossing is found between magnon and phonon modes close to the Brillouin zone boundary in the (a,b) plane. Neutron polarization analysis reveals that this mode has mixed magnon-phonon character. An external magnetic field along the c axis is observed to cause a linear field-induced splitting of one of the spin-wave branches. A theoretical description is performed, using a Heisenberg model of localized spins, acoustic phonon modes, and a magnetoelastic coupling via the single-ion magnetostriction. The model quantitatively reproduces the dispersion and intensities of all modes in the full Brillouin zone, describes the observed magnon-phonon hybridized modes, and quantifies the magnetoelastic coupling. The combined information, including the field-induced magnon splitting, allows us to exclude several of the earlier proposed models and point to the correct magnetic ground state symmetry, and provides an effective dynamic model relevant for the multiferroic hexagonal manganites.


Correlation between spin transport signal and Hensler/semiconductor interface quality in lateral spin-valve devices

PHYSICAL REVIEW B 98 (2018) ARTN 115304

B Kuerbanjiang, Y Fujita, M Yamada, S Yamada, AM Sanchez, PJ Hasnip, A Ghasemi, D Kepaptsoglou, G Bell, K Sawano, K Hamaya, VK Lazarov


Electronic structures and unusually robust bandgap in an ultrahigh-mobility layered oxide semiconductor, Bi2O2Se

Science Advances American Association for the Advancement of Science 4 (2018) eaat8355

C Chen, M Wang, J Wu, H Fu, H Yang, Z Tian, T Tu, H Peng, Y Sun, X Xu, J Jiang, N Schröter, Y Li, D Pei, S Liu, S Ekahana, H Yuan, J Xue, G Li, J Jia, Z Liu, B Yan, H Peng, Y Chen

<p>Semiconductors are essential materials that affect our everyday life in the modern world. Two-dimensional semiconductors with high mobility and moderate bandgap are particularly attractive today because of their potential application in fast, low-power, and ultrasmall/thin electronic devices. We investigate the electronic structures of a new layered air-stable oxide semiconductor, Bi<sub>2</sub>O<sub>2</sub>Se, with ultrahigh mobility (~2.8 × 10<sup>5</sup> cm<sup>2</sup>/V⋅s at 2.0 K) and moderate bandgap (~0.8 eV). Combining angle-resolved photoemission spectroscopy and scanning tunneling microscopy, we mapped out the complete band structures of Bi<sub>2</sub>O<sub>2</sub>Se with key parameters (for example, effective mass, Fermi velocity, and bandgap). The unusual spatial uniformity of the bandgap without undesired in-gap states on the sample surface with up to ~50% defects makes Bi<sub>2</sub>O<sub>2</sub>Se an ideal semiconductor for future electronic applications. In addition, the structural compatibility between Bi<sub>2</sub>O<sub>2</sub>Se and interesting perovskite oxides (for example, cuprate high–transition temperature superconductors and commonly used substrate material SrTiO<sub>3</sub>) further makes heterostructures between Bi<sub>2</sub>O<sub>2</sub>Se and these oxides possible platforms for realizing novel physical phenomena, such as topological superconductivity, Josephson junction field-effect transistor, new superconducting optoelectronics, and novel lasers.</p>


Electrically controlled nuclear polarization of individual atoms

Nature Nanotechnology Nature Publishing Group 13 (2018) 1120–1125-

K Yang, P Willke, Y Bae, A Ferrón, JL Lado, A Ardavan, J Fernández-Rossier, AJ Heinrich, CP Lutz

Nuclear spins serve as sensitive probes in chemistry1 and materials science2 and are promising candidates for quantum information processing3,4,5,6. NMR, the resonant control of nuclear spins, is a powerful tool for probing local magnetic environments in condensed matter systems, which range from magnetic ordering in high-temperature superconductors7,8 and spin liquids9 to quantum magnetism in nanomagnets10,11. Increasing the sensitivity of NMR to the single-atom scale is challenging as it requires a strong polarization of nuclear spins, well in excess of the low polarizations obtained at thermal equilibrium, as well as driving and detecting them individually4,5,12. Strong nuclear spin polarization, known as hyperpolarization, can be achieved through hyperfine coupling with electron spins2. The fundamental mechanism is the conservation of angular momentum: an electron spin flips and a nuclear spin flops. The nuclear hyperpolarization enables applications such as in vivo magnetic resonance imaging using nanoparticles13, and is harnessed for spin-based quantum information processing in quantum dots14 and doped silicon15,16,17. Here we polarize the nuclear spins of individual copper atoms on a surface using a spin-polarized current in a scanning tunnelling microscope. By employing the electron–nuclear flip-flop hyperfine interaction, the spin angular momentum is transferred from tunnelling electrons to the nucleus of individual Cu atoms. The direction and magnitude of the nuclear polarization is controlled by the direction and amplitude of the current. The nuclear polarization permits the detection of the NMR of individual Cu atoms, which is used to sense the local magnetic environment of the Cu electron spin.


Visualizing electronic structures of quantum materials by angle-resolved photoemission spectroscopy

NATURE REVIEWS MATERIALS 3 (2018) 341-353

H Yang, A Liang, C Chen, C Zhang, NBM Schroeter, Y Chen


Imaging orbital-selective quasiparticles in the Hund's metal state of FeSe.

Nature materials 17 (2018) 869-874

A Kostin, PO Sprau, A Kreisel, YX Chong, AE Böhmer, PC Canfield, PJ Hirschfeld, BM Andersen, JCS Davis

Strong electronic correlations, emerging from the parent Mott insulator phase, are key to copper-based high-temperature superconductivity. By contrast, the parent phase of an iron-based high-temperature superconductor is never a correlated insulator. However, this distinction may be deceptive because Fe has five actived d orbitals while Cu has only one. In theory, such orbital multiplicity can generate a Hund's metal state, in which alignment of the Fe spins suppresses inter-orbital fluctuations, producing orbitally selective strong correlations. The spectral weights Zm of quasiparticles associated with different Fe orbitals m should then be radically different. Here we use quasiparticle scattering interference resolved by orbital content to explore these predictions in FeSe. Signatures of strong, orbitally selective differences of quasiparticle Zm appear on all detectable bands over a wide energy range. Further, the quasiparticle interference amplitudes reveal that [Formula: see text], consistent with earlier orbital-selective Cooper pairing studies. Thus, orbital-selective strong correlations dominate the parent state of iron-based high-temperature superconductivity in FeSe.


Discrete and 1D Polymeric Copper(II) Complexes of Tetranuclear Cubane-like Units: Structural and Magnetic Characterization

CHEMISTRYSELECT 3 (2018) 9885-9891

SC Manna, S Manna, S Mistri, A Patra, E Zangrando, H Puschmann, PA Goddard, S Ghannadzadeh


Quantum oscillations of electrical resistivity in an insulator.

Science (New York, N.Y.) 362 (2018) 65-69

Z Xiang, Y Kasahara, T Asaba, B Lawson, C Tinsman, L Chen, K Sugimoto, S Kawaguchi, Y Sato, G Li, S Yao, YL Chen, F Iga, J Singleton, Y Matsuda, L Li

In metals, orbital motions of conduction electrons on the Fermi surface are quantized in magnetic fields, which is manifested by quantum oscillations in electrical resistivity. This Landau quantization is generally absent in insulators. Here, we report a notable exception in an insulator-ytterbium dodecaboride (YbB12). The resistivity of YbB12, which is of a much larger magnitude than the resistivity in metals, exhibits distinct quantum oscillations. These unconventional oscillations arise from the insulating bulk, even though the temperature dependence of the oscillation amplitude follows the conventional Fermi liquid theory of metals with a large effective mass. Quantum oscillations in the magnetic torque are also observed, albeit with a lighter effective mass.


Quantum magnetism in molecular spin ladders probed with muonspin spectroscopy

NEW JOURNAL OF PHYSICS 20 (2018) ARTN 103002

T Lancaster, F Xiao, BM Huddart, RC Williams, FL Pratt, SJ Blundell, SJ Clark, R Scheuermann, T Goko, S Ward, JL Manson, C Ruegg, KW Kramer


Magnetic phases of skyrmion-hosting GaV4S8-ySey (y=0, 2, 4, 8) probed with muon spectroscopy

PHYSICAL REVIEW B 98 (2018) ARTN 054428

KJA Franke, BM Huddart, TJ Hicken, F Xiao, SJ Blundell, FL Pratt, M Crisanti, JAT Barker, SJ Clark, A Stefancic, MC Hatnean, G Balakrishnan, T Lancaster

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