Publications


Hyperfine interaction of individual atoms on a surface.

Science (New York, N.Y.) 362 (2018) 336-339

P Willke, Y Bae, K Yang, JL Lado, A Ferrón, 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 (Cambridge, England) 54 (2018) 12934-12937

B Kintzel, M Böhme, 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.


Measurement of the bulk and surface bands in Dirac line-node semimetal ZrSiS

CHINESE PHYSICS B 27 (2018) ARTN 017105

G-H Hong, C-W Wang, J Jiang, C Chen, S-T Cui, H-F Yang, A-J Liang, S Liu, Y-Y Lv, J Zhou, Y-B Chen, S-H Yao, M-H Lu, Y-F Chen, M-X Wang, L-X Yang, Z-K Liu, Y-L Chen


Magnetic X-ray spectroscopy of two-dimensional CrI3 layers

MATERIALS LETTERS 232 (2018) 5-7

A Frisk, LB Duffy, S Zhang, G van der Laan, T Hesjedal


Extreme Sensitivity of a Topochemical Reaction to Cation Substitution: SrVO2H versus SrV1- xTi xO1.5H1.5.

Inorganic chemistry 57 (2018) 2890-2898

M Amano Patino, D Zeng, SJ Blundell, JE McGrady, MA Hayward

The anion-ordered oxide-hydride SrVO2H is an antiferromagnetic insulator due to strong correlations between vanadium d electrons. In an attempt to hole-dope SrVO2H into a metallic state, a strategy of first preparing SrV1- xTi xO3 phases and then converting them to the corresponding SrV1- xTi xO2H phases via reaction with CaH2 was followed. This revealed that the solid solution between SrVO3 and SrTiO3 is only stable at high temperature. In addition, reactions between SrV0.95Ti0.05O3 and CaH2 were observed to yield SrV0.95Ti0.05O1.5H1.5 not SrV0.95Ti0.05O2H. This dramatic change in reactivity for a very modest change in initial chemical composition is attributed to an electronic destabilization of SrVO2H on titanium substitution. Density functional theory calculations indicate that the presence of an anion-ordered, tetragonal SrMO2H phase is uniquely associated with a d2 electron count and that titanium substitution leads to an electronic destabilization of SrV1- xTi xO2H phases, which, ultimately, drives further reaction of SrV1- xTi xO2H to SrV1- xTi xO1.5H1.5. The observed sensitivity of the reaction products to the chemical composition of initial phases highlights some of the difficulties associated with electronically doping metastable materials prepared by topochemical reactions.


Comparative study of the magnetic properties of La3Ni2B ' O-9 for B ' = Nb, Ta or Sb

JOURNAL OF SOLID STATE CHEMISTRY 258 (2018) 825-834

C-M Chin, PD Battle, SJ Blundell, E Hunter, F Lang, M Hendrickx, RP Sena, J Hadermann


High-Pressure Synthesis, Structures, and Properties of Trivalent A-Site-Ordered Quadruple Perovskites RMn7O12 (R = Sm, Eu, Gd, and Tb).

Inorganic chemistry 57 (2018) 5987-5998

L Zhang, N Terada, RD Johnson, DD Khalyavin, P Manuel, Y Katsuya, M Tanaka, Y Matsushita, K Yamaura, AA Belik

A-site-ordered quadruple perovskites RMn7O12 with R = Sm, Eu, Gd, and Tb were synthesized at high pressure and high temperature (6 GPa and ∼1570 K), and their structural, magnetic, and dielectric properties are reported. They crystallize in space group I2/ m at room temperature. All four compounds exhibit a high-temperature phase transition to the cubic Im3̅ structure at ∼664 K (Sm), 663 K (Eu), 657 K (Gd), and 630 K (Tb). They all show one magnetic transition at TN1 ≈ 82-87 K at zero magnetic field, but additional magnetic transitions below TN2 ≈ 12 K were observed in SmMn7O12 and EuMn7O12 at high magnetic fields. Very weak kinklike dielectric anomalies were observed at TN1 in all compounds. We also observed pyroelectric current peaks near 14 K and frequency-dependent sharp steps in dielectric constant (near 18-35 K)-these anomalies are probably caused by dielectric relaxation, and they are not related to any ferroelectric transitions. TbMn7O12 shows signs of nonstoichiometry expressed as (Tb1- xMn x)Mn7O12, and these samples exhibit negative magnetization or magnetization reversal effects of an extrinsic origin on zero-field-cooled curves in intermediate temperature ranges. The crystal structures of SmMn7O12 and EuMn7O12 were refined from neutron powder diffraction data at 100 K, and the crystal structures of GdMn7O12 and (Tb0.88Mn0.12)Mn7O12 were studied by synchrotron X-ray powder diffraction at 295 K.


Coupling between Spin and Charge Order Driven by Magnetic Field in Triangular Ising System LuFe2O4+delta

CRYSTALS 8 (2018) ARTN 88

L Ding, F Orlandi, DD Khalyavin, AT Boothroyd, D Prabhakaran, G Balakrishnan, P Manuel


THz carrier dynamics and magnetotransport study of topological surface states in thin film Bi<inf>2</inf>Se<inf>3</inf>

Proceedings of SPIE - The International Society for Optical Engineering 10531 (2018)

VS Kamboj, A Singh, T Ferrus, HE Beere, LB Duffy, T Hesjedal, CHW Barnes, DA Ritchie

© 2018 SPIE. The surface of a topological insulator harbors exotic topological states, protected against backscattering from disorder by time reversal symmetry. The study of these exotic quantum states not only provides an opportunity to explore fundamental phenomena in condensed matter physics, such as the spin Hall effect, but also lays the foundation for applications from quantum computing to spintronics. Conventional electrical measurements suffer from substantial bulk interference, making it difficult to clearly distinguish topological surface states from bulk states. Employing terahertz time-domain spectroscopy, we study the temperature-dependent optical behavior of a 23-quintuple-thick film of bismuth selenide (Bi2Se3) allowing for the deconvolution of the surface state response from the bulk. Our measurement of carrier dynamics give an optical mobility exceeding 2100 cm2/V•s at 4 K, indicative of a surface-dominated response, and a scattering lifetime of ∼0.18 ps and a carrier density of 6×1012cm-2at 4 K for the Bi2Se3film. The sample was further processed into a Hall bar device using two different etching techniques, a wet chemical etching and Ar+ion milling, which resulting in a reduced Hall mobility. Even so, the magneto-conductance transport reveals weak antilocalization behavior in our Bi2Se3 sample, consistent with the presence of a single topological surface state mode.


Observation of topological surface states and strong electron/hole imbalance in extreme magnetoresistance compound LaBi

PHYSICAL REVIEW MATERIALS 2 (2018) ARTN 024201

J Jiang, NBM Schroter, S-C Wu, N Kumar, C Shekhar, H Peng, X Xu, C Chen, HF Yang, C-C Hwang, S-K Mo, C Felser, BH Yan, ZK Liu, LX Yang, YL Chen


Ab initio calculation of spin fluctuation spectra using time dependent density functional perturbation theory, planewaves, and pseudopotentials

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

F Giustino, K Cao, P Radaelli


Electronic Structure and Enhanced Charge-Density Wave Order of Monolayer VSe2.

Nano letters 18 (2018) 4493-4499

J Feng, D Biswas, A Rajan, MD Watson, F Mazzola, OJ Clark, K Underwood, I Marković, M McLaren, A Hunter, DM Burn, LB Duffy, S Barua, G Balakrishnan, F Bertran, P Le Fèvre, TK Kim, G van der Laan, T Hesjedal, P Wahl, PDC King

How the interacting electronic states and phases of layered transition-metal dichalcogenides evolve when thinned to the single-layer limit is a key open question in the study of two-dimensional materials. Here, we use angle-resolved photoemission to investigate the electronic structure of monolayer VSe2 grown on bilayer graphene/SiC. While the global electronic structure is similar to that of bulk VSe2, we show that, for the monolayer, pronounced energy gaps develop over the entire Fermi surface with decreasing temperature below Tc = 140 ± 5 K, concomitant with the emergence of charge-order superstructures evident in low-energy electron diffraction. These observations point to a charge-density wave instability in the monolayer that is strongly enhanced over that of the bulk. Moreover, our measurements of both the electronic structure and of X-ray magnetic circular dichroism reveal no signatures of a ferromagnetic ordering, in contrast to the results of a recent experimental study as well as expectations from density functional theory. Our study thus points to a delicate balance that can be realized between competing interacting states and phases in monolayer transition-metal dichalcogenides.


Surface Structure and Reconstructions of HgTe (111) Surfaces

CHINESE PHYSICS LETTERS 35 (2018) ARTN 026802

X-Y Yang, G-Y Wang, C-X Zhao, Z Zhu, L Dong, A-M Li, Y-Y Lv, S-H Yao, Y-B Chen, D-D Guan, Y-Y Li, H Zheng, D Qian, C Liu, Y-L Chen, J-F Jia


Two-gap superconductivity with line nodes in CsCa2Fe4As4F2

PHYSICAL REVIEW B 97 (2018) ARTN 060506

FKK Kirschner, DT Adroja, Z-C Wang, F Lang, M Smidman, PJ Baker, G-H Cao, SJ Blundell


Electrically controlled nuclear polarization of individual atoms.

Nature nanotechnology (2018)

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 processing3-6. 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 silicon15-17. 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.


Observation of magnetic vortex pairs at room temperature in a planar α-Fe2O3/Co heterostructure

Nature Materials Nature Publishing Group (2018)

FP Chmiel, N Waterfield Price, RD Johnson, AD Lamirand, J Schad, G van der Laan, DT Harris, MS Rzchowski, C-B Eom, PG Radaelli


Observation of magnetic vortex pairs at room temperature in a planar α-Fe2O3/Co heterostructure.

Nature materials 17 (2018) 581-585

FP Chmiel, N Waterfield Price, RD Johnson, AD Lamirand, J Schad, G van der Laan, DT Harris, J Irwin, MS Rzchowski, C-B Eom, PG Radaelli

Vortices, occurring whenever a flow field 'whirls' around a one-dimensional core, are among the simplest topological structures, ubiquitous to many branches of physics. In the crystalline state, vortex formation is rare, since it is generally hampered by long-range interactions: in ferroic materials (ferromagnetic and ferroelectric), vortices are observed only when the effects of the dipole-dipole interaction are modified by confinement at the nanoscale1-3, or when the parameter associated with the vorticity does not couple directly with strain 4 . Here, we observe an unprecedented form of vortices in antiferromagnetic haematite (α-Fe2O3) epitaxial films, in which the primary whirling parameter is the staggered magnetization. Remarkably, ferromagnetic topological objects with the same vorticity and winding number as the α-Fe2O3 vortices are imprinted onto an ultra-thin Co ferromagnetic over-layer by interfacial exchange. Our data suggest that the ferromagnetic vortices may be merons (half-skyrmions, carrying an out-of plane core magnetization), and indicate that the vortex/meron pairs can be manipulated by the application of an in-plane magnetic field, giving rise to large-scale vortex-antivortex annihilation.


Folded superstructure and degeneracy-enhanced band gap in the weak-coupling charge density wave system 2H-TaSe2

PHYSICAL REVIEW B 97 (2018) ARTN 115118

YW Li, J Jiang, HF Yang, D Prabhakaran, ZK Liu, LX Yang, YL Chen


Comparative study of the magnetic properties of La<inf>3</inf>Ni<inf>2</inf>B′O<inf>9</inf>for B′ = Nb, Taor Sb

Journal of Solid State Chemistry 258 (2018) 825-834

CM Chin, PD Battle, SJ Blundell, E Hunter, F Lang, M Hendrickx, R Paria Sena, J Hadermann

© 2017 Elsevier Inc. Polycrystalline samples of La 3 Ni 2 NbO 9 and La 3 Ni 2 TaO 9 have been characterised by X-ray and neutron diffraction, electron microscopy, magnetometry and muon spin relaxation (µSR); the latter technique was also applied to La 3 Ni 2 SbO 9 . On the length scale of a neutron diffraction experiment, the six-coordinate sites of the monoclinic perovskite structure are occupied in a 1:1 ordered manner by Ni and a random ⅓Ni/⅔B′ mixture. Electron microscopy demonstrated that this 1:1 ordering is maintained over microscopic distances, although diffuse scattering indicative of short-range ordering on the mixed site was observed. No magnetic Bragg scattering was observed in neutron diffraction patterns collected from La 3 Ni 2 B′O 9 (B′ = Nb or Ta) at 5 K although in each case µSR identified the presence of static spins below 30 K. Magnetometry showed that La 3 Ni 2 NbO 9 behaves as a spin glass below 29 K but significant short-range interactions are present in La 3 Ni 2 TaO 9 below 85 K. The contrasting properties of these compounds are discussed in terms of their microstructure.


Crossover from lattice to plasmonic polarons of a spin-polarised electron gas in ferromagnetic EuO.

Nature communications 9 (2018) 2305-

JM Riley, F Caruso, C Verdi, LB Duffy, MD Watson, L Bawden, K Volckaert, G van der Laan, T Hesjedal, M Hoesch, F Giustino, PDC King

Strong many-body interactions in solids yield a host of fascinating and potentially useful physical properties. Here, from angle-resolved photoemission experiments and ab initio many-body calculations, we demonstrate how a strong coupling of conduction electrons with collective plasmon excitations of their own Fermi sea leads to the formation of plasmonic polarons in the doped ferromagnetic semiconductor EuO. We observe how these exhibit a significant tunability with charge carrier doping, leading to a polaronic liquid that is qualitatively distinct from its more conventional lattice-dominated analogue. Our study thus suggests powerful opportunities for tailoring quantum many-body interactions in solids via dilute charge carrier doping.

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