Publications


Strain in epitaxial MnSi films on Si(111) in the thick film limit studied by polarization-dependent extended x-ray absorption fine structure

PHYSICAL REVIEW B 94 (2016) ARTN 174107

AI Figueroa, SL Zhang, AA Baker, R Chalasani, A Kohn, SC Speller, D Gianolio, C Pfleiderer, G van der Laan, T Hesjedal


Step-Flow Growth of Bi2Te3 Nanobelts

CRYSTAL GROWTH & DESIGN 16 (2016) 6961-6966

P Schonherr, T Tilbury, H Wang, AA Haghighirad, V Srot, PA van Aken, T Hesjedal


Experimental signature of the attractive Coulomb force between positive and negative magnetic monopoles in spin ice

NATURE PHYSICS 12 (2016) 661-+

C Paulsen, SR Giblin, E Lhotel, D Prabhakaran, G Balakrishnan, K Matsuhira, ST Bramwell


Octanuclear Heterobimetallic {Ni4Ln4} Assemblies Possessing Ln4 Square Grid [2 × 2] Motifs: Synthesis, Structure, and Magnetism.

Inorganic chemistry 55 (2016) 8422-8436

S Biswas, J Goura, S Das, CV Topping, J Brambleby, PA Goddard, V Chandrasekhar

Octanuclear heterobimetallic complexes, [Ln4Ni4(H3L)4(μ3-OH)4(μ2-OH)4]4Cl·xH2O·yCHCl3 (Dy(3+), x = 30.6, y = 2 (1); Tb(3+), x = 28, y = 0 (2) ; Gd(3+), x = 25.3, y = 0 (3); Ho(3+), x = 30.6, y = 3 (4)) (H5L = N1,N3-bis(6-formyl-2-(hydroxymethyl)-4-methylphenol)diethylenetriamine) are reported. These are assembled by the cumulative coordination action of four doubly deprotonated compartmental ligands, [H3L](2-), along with eight exogenous -OH ligands. Within the core of these complexes, four Ln(3+)'s are distributed to the four corners of a perfect square grid while four Ni(2+)'s are projected away from the plane of the Ln4 unit. Each of the four Ni(2+)'s possesses distorted octahedral geometry while all of the Ln(3+)'s are crystallographically equivalent and are present in an elongated square antiprism geometry. The magnetic properties of compound 3 are dominated by an easy-plane single-ion anisotropy of the Ni(2+) ions [DNi = 6.7(7) K] and dipolar interactions between Gd(3+) centers. Detailed ac magnetometry reveals the presence of distinct temperature-dependent out-of-phase signals for compounds 1 and 2, indicative of slow magnetic relaxation. Magnetochemical analysis of complex 1 implies the 3d and the 4f metal ions are engaged in ferromagnetic interactions with SMM behavior, while dc magnetometry of compound 2 is suggestive of an antiferromagnetic Ni-Tb spin-exchange with slow magnetic relaxation due to a field-induced level crossing. Compound 4 exhibits an easy-plane single-ion anisotropy for the Ho(3+) ions and weak interactions between spin centers.


Evolution of the Fermi surface of Weyl semimetals in the transition metal pnictide family.

Nature materials 15 (2016) 27-31

ZK Liu, LX Yang, Y Sun, T Zhang, H Peng, HF Yang, C Chen, Y Zhang, YF Guo, D Prabhakaran, M Schmidt, Z Hussain, S-K Mo, C Felser, B Yan, YL Chen

Topological Weyl semimetals (TWSs) represent a novel state of topological quantum matter which not only possesses Weyl fermions (massless chiral particles that can be viewed as magnetic monopoles in momentum space) in the bulk and unique Fermi arcs generated by topological surface states, but also exhibits appealing physical properties such as extremely large magnetoresistance and ultra-high carrier mobility. Here, by performing angle-resolved photoemission spectroscopy (ARPES) on NbP and TaP, we directly observed their band structures with characteristic Fermi arcs of TWSs. Furthermore, by systematically investigating NbP, TaP and TaAs from the same transition metal monopnictide family, we discovered their Fermiology evolution with spin-orbit coupling (SOC) strength. Our experimental findings not only reveal the mechanism to realize and fine-tune the electronic structures of TWSs, but also provide a rich material base for exploring many exotic physical phenomena (for example, chiral magnetic effects, negative magnetoresistance, and the quantum anomalous Hall effect) and novel future applications.


Heterometallic trinuclear {CoLn(III)} (Ln = Gd, Tb, Ho and Er) complexes in a bent geometry. Field-induced single-ion magnetic behavior of the Er(III) and Tb(III) analogues.

Dalton transactions (Cambridge, England : 2003) 45 (2016) 9235-9249

J Goura, J Brambleby, CV Topping, PA Goddard, R Suriya Narayanan, AK Bar, V Chandrasekhar

Through the use of a multi-site compartmental ligand, 2-methoxy-6-[{2-(2-hydroxyethylamino)ethylimino}methyl]phenol (LH3), the family of heterometallic, trinuclear complexes of the formula [CoLn(L)2(μ-O2CCH3)2(H2O)3]·NO3·xMeOH·yH2O has been expanded beyond Ln = Dy(III) to include Gd(III) (), Tb(III) (), Ho(III) () and Er(III) () for , and (x = 1; y = 1) and for (x = 0; y = 2). The metallic core of these complexes consists of a (Co(III)-Ln(III)-Co(III)) motif bridged in a bent geometry resulting in six-coordinated distorted Co(III) octahedra and nine-coordinated Ln(III) monocapped square-antiprisms. The magnetic characterization of these compounds reveals the erbium and terbium analogues to display a field induced single-ion magnetic behavior similar to the dysprosium analogue but at lower temperatures. The energy barrier for the reversal of the magnetization of the CoTb(III) analogue is Ueff ≥ 15.6(4) K, while for the CoEr(III) analogue Ueff ≥ 9.9(8) K. The magnetic properties are discussed in terms of distortions of the 4f electron cloud.


Imaging and manipulation of skyrmion lattice domains in Cu2OSeO3

APPLIED PHYSICS LETTERS 109 (2016) ARTN 192406

SL Zhang, A Bauer, H Berger, C Pfleiderer, G van der Laan, T Hesjedal


Magnetic ordering in Ho-doped Bi2Te3 topological insulator thin films

PHYSICA STATUS SOLIDI-RAPID RESEARCH LETTERS 10 (2016) 467-470

AI Figueroa, SE Harrison, LJ Collins-McIntyre, G van der Laan, T Hesjedal


Making hybrid [n]-rotaxanes as supramolecular arrays of molecular electron spin qubits.

Nature communications 7 (2016) 10240-

A Fernandez, J Ferrando-Soria, EM Pineda, F Tuna, IJ Vitorica-Yrezabal, C Knappke, J Ujma, CA Muryn, GA Timco, PE Barran, A Ardavan, REP Winpenny

Quantum information processing (QIP) would require that the individual units involved--qubits--communicate to other qubits while retaining their identity. In many ways this resembles the way supramolecular chemistry brings together individual molecules into interlocked structures, where the assembly has one identity but where the individual components are still recognizable. Here a fully modular supramolecular strategy has been to link hybrid organic-inorganic [2]- and [3]-rotaxanes into still larger [4]-, [5]- and [7]-rotaxanes. The ring components are heterometallic octanuclear [Cr7NiF8(O2C(t)Bu)16](-) coordination cages and the thread components template the formation of the ring about the organic axle, and are further functionalized to act as a ligand, which leads to large supramolecular arrays of these heterometallic rings. As the rings have been proposed as qubits for QIP, the strategy provides a possible route towards scalable molecular electron spin devices for QIP. Double electron-electron resonance experiments demonstrate inter-qubit interactions suitable for mediating two-qubit quantum logic gates.


Robustness of superconductivity to competing magnetic phases in tetragonal FeS

PHYSICAL REVIEW B 94 (2016) ARTN 134509

FKK Kirschner, F Lang, CV Topping, PJ Baker, FL Pratt, SE Wright, DN Woodruff, SJ Clarke, SJ Blundell


All-in-all-Out Magnetic Order and Propagating Spin Waves in Sm_{2}Ir_{2}O_{7}.

Physical review letters 117 (2016) 037201-

C Donnerer, MC Rahn, MM Sala, JG Vale, D Pincini, J Strempfer, M Krisch, D Prabhakaran, AT Boothroyd, DF McMorrow

Using resonant magnetic x-ray scattering we address the unresolved nature of the magnetic ground state and the low-energy effective Hamiltonian of Sm_{2}Ir_{2}O_{7}, a prototypical pyrochlore iridate with a finite temperature metal-insulator transition. Through a combination of elastic and inelastic measurements, we show that the magnetic ground state is an all-in-all-out (AIAO) antiferromagnet. The magnon dispersion indicates significant electronic correlations and can be well described by a minimal Hamiltonian that includes Heisenberg exchange [J=27.3(6)  meV] and Dzyaloshinskii-Moriya interactions [D=4.9(3)  meV], which provides a consistent description of the magnetic order and excitations. In establishing that Sm_{2}Ir_{2}O_{7} has the requisite inversion symmetry preserving AIAO magnetic ground state, our results support the notion that pyrochlore iridates may host correlated Weyl semimetals.


Quantum Interference in Graphene Nanoconstrictions.

Nano letters 16 (2016) 4210-4216

P Gehring, H Sadeghi, S Sangtarash, CS Lau, J Liu, A Ardavan, JH Warner, CJ Lambert, GAD Briggs, JA Mol

We report quantum interference effects in the electrical conductance of chemical vapor deposited graphene nanoconstrictions fabricated using feedback controlled electroburning. The observed multimode Fabry-Pérot interferences can be attributed to reflections at potential steps inside the channel. Sharp antiresonance features with a Fano line shape are observed. Theoretical modeling reveals that these Fano resonances are due to localized states inside the constriction, which couple to the delocalized states that also give rise to the Fabry-Pérot interference patterns. This study provides new insight into the interplay between two fundamental forms of quantum interference in graphene nanoconstrictions.


Oxidation Effects in Rare Earth Doped Topological Insulator Thin Films.

Scientific reports 6 (2016) 22935-

AI Figueroa, G van der Laan, SE Harrison, G Cibin, T Hesjedal

The breaking of time-reversal symmetry (TRS) in topological insulators is a prerequisite for unlocking their exotic properties and for observing the quantum anomalous Hall effect (QAHE). The incorporation of dopants which exhibit magnetic long-range order is the most promising approach for TRS-breaking. REBiTe3, wherein 50% of the Bi is substitutionally replaced by a RE atom (RE = Gd, Dy, and Ho), is a predicted QAHE system. Despite the low solubility of REs in bulk crystals of a few %, highly doped thin films have been demonstrated, which are free of secondary phases and of high crystalline quality. Here we study the effects of exposure to atmosphere of rare earth-doped Bi2(Se, Te)3 thin films using x-ray absorption spectroscopy. We demonstrate that these RE dopants are all trivalent and effectively substitute for Bi(3+) in the Bi2(Se, Te)3 matrix. We find an unexpected high degree of sample oxidation for the most highly doped samples, which is not restricted to the surface of the films. In the low-doping limit, the RE-doped films mostly show surface oxidation, which can be prevented by surface passivation, encapsulation, or in-situ cleaving to recover the topological surface state.


Direct evidence for charge stripes in a layered cobalt oxide

Nature Communications Nature Publishing Group: Nature Communications (2016)

AT Boothroyd, P Babkevich, PG Freeman, M Enderle, D Prabhakaran


Experimental and Theoretical Electron Density Analysis of Copper Pyrazine Nitrate Quasi-Low-Dimensional Quantum Magnets.

Journal of the American Chemical Society 138 (2016) 2280-2291

LHR Dos Santos, A Lanza, AM Barton, J Brambleby, WJA Blackmore, PA Goddard, F Xiao, RC Williams, T Lancaster, FL Pratt, SJ Blundell, J Singleton, JL Manson, P Macchi

The accurate electron density distribution and magnetic properties of two metal-organic polymeric magnets, the quasi-one-dimensional (1D) Cu(pyz)(NO3)2 and the quasi-two-dimensional (2D) [Cu(pyz)2(NO3)]NO3·H2O, have been investigated by high-resolution single-crystal X-ray diffraction and density functional theory calculations on the whole periodic systems and on selected fragments. Topological analyses, based on quantum theory of atoms in molecules, enabled the characterization of possible magnetic exchange pathways and the establishment of relationships between the electron (charge and spin) densities and the exchange-coupling constants. In both compounds, the experimentally observed antiferromagnetic coupling can be quantitatively explained by the Cu-Cu superexchange pathway mediated by the pyrazine bridging ligands, via a σ-type interaction. From topological analyses of experimental charge-density data, we show for the first time that the pyrazine tilt angle does not play a role in determining the strength of the magnetic interaction. Taken in combination with molecular orbital analysis and spin density calculations, we find a synergistic relationship between spin delocalization and spin polarization mechanisms and that both determine the bulk magnetic behavior of these Cu(II)-pyz coordination polymers.


Muon-spin relaxation study of the double perovskite insulators Sr2 BOsO6 (B  =  Fe, Y, ln).

Journal of physics. Condensed matter : an Institute of Physics journal 28 (2016) 076001-

RC Williams, F Xiao, IO Thomas, SJ Clark, T Lancaster, GA Cornish, SJ Blundell, W Hayes, AK Paul, C Felser, M Jansen

We present the results of zero-field muon-spin relaxation measurements made on the double perovskite insulators Sr2 BOsO6 (B = Fe,Y, In). Spontaneous muon-spin precession indicative of quasistatic long range magnetic ordering is observed in Sr2FeOsO6 within the AF1 antiferromagnetic phase for temperatures below [Formula: see text] K. Upon cooling below T2≈67 K the oscillations cease to be resolvable owing to the coexistence of the AF1 and AF2 phases, which leads to a broader range of internal magnetic fields. Using density functional calculations we identify a candidate muon stopping site within the unit cell, which dipole field simulations show to be consistent with the proposed magnetic structure. The possibility of incommensurate magnetic ordering is discussed for temperatures below TN = 53 K and 25 K for Sr2YOsO6 and Sr2InOsO6, respectively.


de Haas–van Alphen study of role of electrons in antiferromagnetic as compared to its nonmagnetic analog

Physical Review B 94 (2016)

SF Blake, H Hodovanets, A McCollam, SL Bud'ko, PC Canfield, AI Coldea


Bimetallic MOFs (H3O)x[Cu(MF6)(pyrazine)2]·(4 - x)H2O (M = V(4+), x = 0; M = Ga(3+), x = 1): co-existence of ordered and disordered quantum spins in the V(4+) system.

Chemical communications (Cambridge, England) 52 (2016) 12653-12656

JL Manson, JA Schlueter, KE Garrett, PA Goddard, T Lancaster, JS Möller, SJ Blundell, AJ Steele, I Franke, FL Pratt, J Singleton, J Bendix, SH Lapidus, M Uhlarz, O Ayala-Valenzuela, RD McDonald, M Gurak, C Baines

The title compounds are bimetallic MOFs containing [Cu(pyz)2](2+) square lattices linked by MF6(n-) octahedra. In each, only the Cu(2+) spins exhibit long-range magnetic order below 3.5 K (M = V(4+)) and 2.6 K (M = Ga(3+)). The V(4+) spins remain disordered down to 0.5 K.


Anisotropic local modification of crystal field levels in Pr-based pyrochlores: a muon-induced effect modeled using density functional theory.

Physical review letters 114 (2015) 017602-

FR Foronda, F Lang, JS Möller, T Lancaster, AT Boothroyd, FL Pratt, SR Giblin, D Prabhakaran, SJ Blundell

Although muon spin relaxation is commonly used to probe local magnetic order, spin freezing, and spin dynamics, we identify an experimental situation in which the measured response is dominated by an effect resulting from the muon-induced local distortion rather than the intrinsic behavior of the host compound. We demonstrate this effect in some quantum spin ice candidate materials Pr(2)B(2)O(7) (B=Sn, Zr, Hf), where we detect a static distribution of magnetic moments that appears to grow on cooling. Using density functional theory we show how this effect can be explained via a hyperfine enhancement arising from a splitting of the non-Kramers doublet ground states on Pr ions close to the muon, which itself causes a highly anisotropic distortion field. We provide a quantitative relationship between this effect and the measured temperature dependence of the muon relaxation and discuss the relevance of these observations to muon experiments in other magnetic materials.


Universal Magnetic Hall Circuit Based on Paired Spin Heterostructures

ADVANCED ELECTRONIC MATERIALS 1 (2015) ARTN 1400054

S Zhang, AA Baker, J-Y Zhang, G Yu, S Wang, T Hesjedal

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