Publications


Observation of the topological surface state in the nonsymmorphic topological insulator KHgSb

PHYSICAL REVIEW B 96 (2017) ARTN 165143

AJ Liang, J Jiang, MX Wang, Y Sun, N Kumar, C Shekhar, C Chen, H Peng, CW Wang, X Xu, HF Yang, ST Cui, GH Hong, Y-Y Xia, S-K Mo, Q Gao, XJ Zhou, LX Yang, C Felser, BH Yan, ZK Liu, YL Chen


Photoemission study of the electronic structure of valence band convergent SnSe

PHYSICAL REVIEW B 96 (2017) ARTN 165118

CW Wang, YYY Xia, Z Tian, J Jiang, BH Li, ST Cui, HF Yang, AJ Liang, XY Zhan, GH Hong, S Liu, C Chen, MX Wang, LX Yang, Z Liu, QX Mi, G Li, JM Xue, ZK Liu, YL Chen


Quasiparticle Breakdown and Spin Hamiltonian of the Frustrated Quantum Pyrochlore Yb_{2}Ti_{2}O_{7} in a Magnetic Field.

Physical review letters 119 (2017) 057203-

JD Thompson, PA McClarty, D Prabhakaran, I Cabrera, T Guidi, R Coldea

The frustrated pyrochlore magnet Yb_{2}Ti_{2}O_{7} has the remarkable property that it orders magnetically but has no propagating magnons over wide regions of the Brillouin zone. Here we use inelastic neutron scattering to follow how the spectrum evolves in cubic-axis magnetic fields. At high fields we observe, in addition to dispersive magnons, a two-magnon continuum, which grows in intensity upon reducing the field and overlaps with the one-magnon states at intermediate fields leading to strong renormalization of the dispersion relations, and magnon decays. Using heat capacity measurements we find that the low- and high-field regions are smoothly connected with no sharp phase transition, with the spin gap increasing monotonically in field. Through fits to an extensive data set of dispersion relations combined with magnetization measurements, we reevaluate the spin Hamiltonian, finding dominant quantum exchange terms, which we propose are responsible for the anomalously strong fluctuations and quasiparticle breakdown effects observed at low fields.


Low-field spin dynamics of Cr7Ni and Cr7Ni-Cu-Cr7Ni molecular rings as detected by mu SR

PHYSICAL REVIEW B 96 (2017) ARTN 184403

S Sanna, P Arosio, L Bordonali, F Adelnia, M Mariani, E Garlatti, C Baines, A Amato, KPV Sabareesh, G Timco, REP Winpenny, SJ Blundell, A Lascialfari


Proposal of a micromagnetic standard problem for ferromagnetic resonance simulations

Journal of Magnetism and Magnetic Materials 421 (2017) 428

AA Baker, M Beg, G Ashton, M Albert, D Chernyshenko, W Wang, S Zhang, M-A Bisotti, M Franchin, CL Hu, R Stamps, T Hesjedal, H Fangohr

Nowadays, micromagnetic simulations are a common tool for studying a wide range of different magnetic phenomena, including the ferromagnetic resonance. A technique for evaluating reliability and validity of different micromagnetic simulation tools is the simulation of proposed standard problems. We propose a new standard problem by providing a detailed specification and analysis of a sufficiently simple problem. By analyzing the magnetization dynamics in a thin permalloy square sample, triggered by a well defined excitation, we obtain the ferromagnetic resonance spectrum and identify the resonance modes via Fourier transform. Simulations are performed using both finite difference and finite element numerical methods, with OOMMF and Nmag simulators, respectively. We report the effects of initial conditions and simulation parameters on the character of the observed resonance modes for this standard problem. We provide detailed instructions and code to assist in using the results for evaluation of new simulator tools, and to help with numerical calculation of ferromagnetic resonance spectra and modes in general.


Broken rotational symmetry on the Fermi surface of a high-T<inf>c</inf> superconductor

npj Quantum Materials 2 (2017)

PA Goddard

© 2017 The Author(s). Broken fourfold rotational (C4) symmetry is observed in the experimental properties of several classes of unconventional superconductors. It has been proposed that this symmetry breaking is important for superconducting pairing in these materials, but in the high-Tc cuprates this broken symmetry has never been observed on the Fermi surface. Here we report a pronounced anisotropy in the angle dependence of the interlayer magnetoresistance of the underdoped high transition temperature (high-Tc) superconductor YBa2Cu3O6.58, directly revealing broken C4 symmetry on the Fermi surface. Moreover, we demonstrate that this Fermi surface has C2 symmetry of the type produced by a uniaxial or anisotropic density-wave phase. This establishes the central role of C4 symmetry breaking in the Fermi surface reconstruction of YBa2Cu3O6+δ, and suggests a striking degree of universality among unconventional superconductors.


Crystal structure and magnetic modulation in beta-Ce2O2FeSe2

PHYSICAL REVIEW MATERIALS 1 (2017) ARTN 034403

C-H Wang, CM Ainsworth, SD Champion, GA Stewart, MC Worsdale, T Lancaster, SJ Blundell, HEA Brand, JSO Evans


X-ray magnetic circular dichroism study of Dy-doped Bi2Te3 topological insulator thin films

JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS 422 (2017) 93-99

AI Figueroa, AA Baker, SE Harrison, K Kummer, G van der Laan, T Hesjedal


Lifshitz Transitions Induced by Temperature and Surface Doping in Type-II Weyl Semimetal Candidate T-d-WTe2

PHYSICA STATUS SOLIDI-RAPID RESEARCH LETTERS 11 (2017) ARTN 1700209

Q Zhang, Z Liu, Y Sun, H Yang, J Jiang, S-K Mo, Z Hussain, X Qian, L Fu, S Yao, M Lu, C Felser, B Yan, Y Chen, L Yang


Local magnetism and spin dynamics of the frustrated honeycomb rhodate Li2RhO3

PHYSICAL REVIEW B 96 (2017) ARTN 094432

P Khuntia, S Manni, FR Foronda, T Lancaster, SJ Blundell, P Gegenwart, M Baenitz


Strong Coupling of Microwave Photons to Antiferromagnetic Fluctuations in an Organic Magnet.

Physical review letters 119 (2017) 147701-

M Mergenthaler, J Liu, JJ Le Roy, N Ares, AL Thompson, L Bogani, F Luis, SJ Blundell, T Lancaster, A Ardavan, GAD Briggs, PJ Leek, EA Laird

Coupling between a crystal of di(phenyl)-(2,4,6-trinitrophenyl)iminoazanium radicals and a superconducting microwave resonator is investigated in a circuit quantum electrodynamics (circuit QED) architecture. The crystal exhibits paramagnetic behavior above 4 K, with antiferromagnetic correlations appearing below this temperature, and we demonstrate strong coupling at base temperature. The magnetic resonance acquires a field angle dependence as the crystal is cooled down, indicating anisotropy of the exchange interactions. These results show that multispin modes in organic crystals are suitable for circuit QED, offering a platform for their coherent manipulation. They also utilize the circuit QED architecture as a way to probe spin correlations at low temperature.


Adiabatic physics of an exchange-coupled spin-dimer system: Magnetocaloric effect, zero-point fluctuations, and possible two-dimensional universal behavior

PHYSICAL REVIEW B 95 (2017) ARTN 024404

J Brambleby, PA Goddard, J Singleton, M Jaime, T Lancaster, L Huang, J Wosnitza, CV Topping, KE Carreiro, HE Tran, ZE Manson, JL Manson


Direct experimental determination of spiral spin structures via the dichroism extinction effect in resonant elastic soft x-ray scattering

PHYSICAL REVIEW B 96 (2017) ARTN 094401

SL Zhang, G van der Laan, T Hesjedal


Perfect quintuple layer Bi2Te3 nanowires: Growth and thermoelectric properties

APL MATERIALS 5 (2017) ARTN 086110

P Schonherr, D Kojda, V Srot, SF Fischer, PA van Aken, T Hesjedal


Spin Resonance Clock Transition of the Endohedral Fullerene ^{15}N@C_{60}.

Physical review letters 119 (2017) 140801-

RT Harding, S Zhou, J Zhou, T Lindvall, WK Myers, A Ardavan, GAD Briggs, K Porfyrakis, EA Laird

The endohedral fullerene ^{15}N@C_{60} has narrow electron paramagnetic resonance lines which have been proposed as the basis for a condensed-matter portable atomic clock. We measure the low-frequency spectrum of this molecule, identifying and characterizing a clock transition at which the frequency becomes insensitive to magnetic field. We infer a linewidth at the clock field of 100 kHz. Using experimental data, we are able to place a bound on the clock's projected frequency stability. We discuss ways to improve the frequency stability to be competitive with existing miniature clocks.


Topological origin of the type-II Dirac fermions in PtSe2

PHYSICAL REVIEW MATERIALS 1 (2017) ARTN 074202

Y Li, Y Xia, SA Ekahana, N Kumar, J Jiang, L Yang, C Chen, C Liu, B Yan, C Felser, G Li, Z Liu, Y Chen


Topological triplon modes and bound states in a Shastry-Sutherland magnet

NATURE PHYSICS 13 (2017) 736-+

PA McClarty, F Krueger, T Guidi, SF Parker, K Refson, AW Parker, D Prabhakaran, R Coldea


Nontrivial Berry phase and type-II Dirac transport in the layered material PdTe2

PHYSICAL REVIEW B 96 (2017) ARTN 041201

F Fei, X Bo, R Wang, B Wu, J Jiang, D Fu, M Gao, H Zheng, Y Chen, X Wang, H Bu, F Song, X Wan, B Wang, G Wang


Magnetic Field Dependence of Excitations Near Spin-Orbital Quantum Criticality.

Physical review letters 118 (2017) 067205-

A Biffin, C Rüegg, J Embs, T Guidi, D Cheptiakov, A Loidl, V Tsurkan, R Coldea

The spinel FeSc_{2}S_{4} has been proposed to realize a near-critical spin-orbital singlet (SOS) state, where entangled spin and orbital moments fluctuate in a global singlet state on the verge of spin and orbital order. Here we report powder inelastic neutron scattering measurements that observe the full bandwidth of magnetic excitations and we find that spin-orbital triplon excitations of an SOS state can capture well key aspects of the spectrum in both zero and applied magnetic fields up to 8.5 T. The observed shift of low-energy spectral weight to higher energies upon increasing applied field is naturally explained by the entangled spin-orbital character of the magnetic states, a behavior that is in strong contrast to spin-only singlet ground state systems, where the spin gap decreases upon increasing applied field.


Emergence of Dirac-like bands in the monolayer limit of epitaxial Ge films on Au(1 1 1)

2D Materials 4 (2017) 031005

NBM Schröter, MD Watson, LB Duffy, M Hoesch, Y Chen, T Hesjedal, TK Kim

After the discovery of Dirac fermions in graphene, it has become a natural question to ask whether it is possible to realize Dirac fermions in other two-dimensional (2D) materials as well. In this work, we report the discovery of multiple Dirac-like electronic bands in ultrathin Ge flms grown on Au(1 1 1) by angle-resolved photoelectron spectroscopy. By tuning the thickness of the flms, we are able to observe the evolution of their electronic structure when passing through the monolayer limit. Our discovery may signify the synthesis of germanene, a 2D honeycomb structure made of Ge, which is a promising platform for exploring exotic topological phenomena and enabling potential applications.

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