Publications


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

Chemical communications (Cambridge, England) (2018)

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


Probing α-RuCl_{3} Beyond Magnetic Order: Effects of Temperature and Magnetic Field.

Physical review letters 120 (2018) 077203-

SM Winter, K Riedl, D Kaib, R Coldea, R Valentí

Recent studies have brought α-RuCl_{3} to the forefront of experimental searches for materials realizing Kitaev spin-liquid physics. This material exhibits strongly anisotropic exchange interactions afforded by the spin-orbit coupling of the 4d Ru centers. We investigate the dynamical response at finite temperature and magnetic field for a realistic model of the magnetic interactions in α-RuCl_{3}. These regimes are thought to host unconventional paramagnetic states that emerge from the suppression of magnetic order. Using exact diagonalization calculations of the quantum model complemented by semiclassical analysis, we find a very rich evolution of the spin dynamics as the applied field suppresses the zigzag order and stabilizes a quantum paramagnetic state that is adiabatically connected to the fully polarized state at high fields. At finite temperature, we observe large redistributions of spectral weight that can be attributed to the anisotropic frustration of the model. These results are compared to recent experiments and provide a road map for further studies of these regimes.


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


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


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


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.


Evolution of Magneto-Orbital order Upon B-Site Electron Doping in Na_{1-x}Ca_{x}Mn_{7}O_{12} Quadruple Perovskite Manganites.

Physical review letters 120 (2018) 257202-

RD Johnson, F Mezzadri, P Manuel, DD Khalyavin, E Gilioli, PG Radaelli

We present the discovery and refinement by neutron powder diffraction of a new magnetic phase in the Na_{1-x}Ca_{x}Mn_{7}O_{12} quadruple perovskite phase diagram, which is the incommensurate analogue of the well-known pseudo-CE phase of the simple perovskite manganites. We demonstrate that incommensurate magnetic order arises in quadruple perovskites due to the exchange interactions between A and B sites. Furthermore, by constructing a simple mean field Heisenberg exchange model that generically describes both simple and quadruple perovskite systems, we show that this new magnetic phase unifies a picture of the interplay between charge, magnetic, and orbital ordering across a wide range of compounds.


Evolution of magneto-orbital order upon B-site electron doping in Na1−xCaxMn7O12 quadruple perovskite manganites

Physical Review Letters American Physical Society (2018)

RD Johnson, F Mezzadri, P Manuel, DD Khalyavin, E Gilioli, PG Radaelli


Microscopic effects of Dy doping in the topological insulator Bi2Te3

PHYSICAL REVIEW B 97 (2018) ARTN 174427

LB Duffy, N-J Steinke, JA Krieger, AI Figueroa, K Kummer, T Lancaster, SR Giblin, FL Pratt, SJ Blundell, T Prokscha, A Suter, S Langridge, VN Strocov, Z Salman, G van der Laan, T Hesjedal


Spin-induced multiferroicity in the binary perovskite manganite Mn2O3.

Nature communications 9 (2018) 2996-

J Cong, K Zhai, Y Chai, D Shang, DD Khalyavin, RD Johnson, DP Kozlenko, SE Kichanov, AM Abakumov, AA Tsirlin, L Dubrovinsky, X Xu, Z Sheng, SV Ovsyannikov, Y Sun

The ABO3 perovskite oxides exhibit a wide range of interesting physical phenomena remaining in the focus of extensive scientific investigations and various industrial applications. In order to form a perovskite structure, the cations occupying the A and B positions in the lattice, as a rule, should be different. Nevertheless, the unique binary perovskite manganite Mn2O3 containing the same element in both A and B positions can be synthesized under high-pressure high-temperature conditions. Here, we show that this material exhibits magnetically driven ferroelectricity and a pronounced magnetoelectric effect at low temperatures. Neutron powder diffraction revealed two intricate antiferromagnetic structures below 100 K, driven by a strong interplay between spin, charge, and orbital degrees of freedom. The peculiar multiferroicity in the Mn2O3 perovskite is ascribed to a combined effect involving several mechanisms. Our work demonstrates the potential of binary perovskite oxides for creating materials with highly promising electric and magnetic properties.


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


Coupling of magnetic order and charge transport in the candidate Dirac semimetal EuCd2As2

PHYSICAL REVIEW B 97 (2018) ARTN 214422

MC Rahn, J-R Soh, S Francoual, LSI Veiga, J Strempfer, J Mardegan, DY Yan, YF Guo, YG Shi, AT Boothroyd


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.


Persistence of antiferromagnetic order upon La substitution in the 4d(4) Mott insulator Ca2RuO4

PHYSICAL REVIEW B 98 (2018) ARTN 014429

D Pincini, S Boseggia, R Perry, MJ Gutmann, S Ricco, LSI Veiga, CD Dashwood, SP Collins, G Nisbet, A Bombardi, DG Porter, F Baumberger, AT Boothroyd, DF McMorrow


Spin dynamics and exchange interactions in CuO measured by neutron scattering

PHYSICAL REVIEW B 97 (2018) ARTN 144401

H Jacobsen, SM Gaw, AJ Princep, E Hamilton, S Toth, RA Ewings, M Enderle, EMH Wheeler, D Prabhakaran, AT Boothroyd

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