Publications


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


Direct experimental determination of the topological winding number of skyrmions in Cu2OSeO3.

Nature communications 8 (2017) 14619-

SL Zhang, G van der Laan, T Hesjedal

The mathematical concept of topology has brought about significant advantages that allow for a fundamental understanding of the underlying physics of a system. In magnetism, the topology of spin order manifests itself in the topological winding number which plays a pivotal role for the determination of the emergent properties of a system. However, the direct experimental determination of the topological winding number of a magnetically ordered system remains elusive. Here, we present a direct relationship between the topological winding number of the spin texture and the polarized resonant X-ray scattering process. This relationship provides a one-to-one correspondence between the measured scattering signal and the winding number. We demonstrate that the exact topological quantities of the skyrmion material Cu2OSeO3 can be directly experimentally determined this way. This technique has the potential to be applicable to a wide range of materials, allowing for a direct determination of their topological properties.


Combining microscopic and macroscopic probes to untangle the single-ion anisotropy and exchange energies in an S=1 quantum antiferromagnet

PHYSICAL REVIEW B 95 (2017) ARTN 134435

J Brambleby, JL Manson, PA Goddard, MB Stone, RD Johnson, P Manuel, JA Villa, CM Brown, H Lu, S Chikara, V Zapf, SH Lapidus, R Scatena, P Macchi, Y-S Chen, L-C Wu, J Singleton


Terahertz spectroscopy of anisotropic materials using beams with rotatable polarization.

Scientific reports 7 (2017) 12337-

CDW Mosley, M Failla, D Prabhakaran, J Lloyd-Hughes

We introduce a polarization-resolved terahertz time-domain spectrometer with a broadband (0.3-2.5 THz), rotatable THz polarization state, and which exhibits minimal change in the electric field amplitude and polarization state upon rotation. This was achieved by rotating an interdigitated photoconductive emitter, and by detecting the orthogonal components of the generated THz pulse via electro-optic sampling. The high precision (<0.1°) and accuracy (<1.0°) of this approach is beneficial for the study of anisotropic materials without rotating the sample, which can be impractical, for instance for samples held in a cryostat. The versatility of this method was demonstrated by studying the anisotropic THz optical properties of uniaxial and biaxial oxide crystals. For uniaxial ZnO and LaAlO3, which have minimal THz absorption across the measurement bandwidth, the orientations of the eigenmodes of propagation were conveniently identified as the orientation angles that produced a transmitted THz pulse with zero ellipticity, and the birefringence was quantified. In CuO, a multiferroic with improper ferroelectricity, the anisotropic THz absorption created by an electromagnon was investigated, mapping its selection rule precisely. For this biaxial crystal, which has phonon and electromagnon absorption, the polarization eigenvectors exhibited chromatic dispersion, as a result of the monoclinic crystal structure and the frequency-dependent complex refractive index.


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


Coupled commensurate charge density wave and lattice distortion in Na(2)Ti(2)Pn(2)O (Pn = As,Sb) determined by x-ray diffraction and angle-resolved photoemission spectroscopy

PHYSICAL REVIEW B 94 (2016) ARTN 104515

NR Davies, RD Johnson, AJ Princep, LA Gannon, J-Z Ma, T Qian, P Richard, H Li, M Shi, H Nowell, PJ Baker, YG Shi, H Ding, J Luo, YF Guo, AT Boothroyd


Spin resonance in the superconducting state of Li1-xFexODFe1-ySe observed by neutron spectroscopy

PHYSICAL REVIEW B 94 (2016) ARTN 144503

NR Davies, MC Rahn, HC Walker, RA Ewings, DN Woodruff, SJ Clarke, AT Boothroyd


Modulated spin helicity stabilized by incommensurate orbital density waves in a quadruple perovskite manganite

PHYSICAL REVIEW B 93 (2016) ARTN 180403

RD Johnson, DD Khalyavin, P Manuel, A Bombardi, C Martin, LC Chapon, PG Radaelli


Evidence for unidirectional nematic bond ordering in FeSe

Physical Review B - Condensed Matter and Materials Physics American Physical Society (2016)

MD Watson, TK Kim, LC Rhodes, M Eschrig, M Hoesch, AA Haghighirad, AI Coldea

The lifting of $d_{xz}$-$d_{yz}$ orbital degeneracy is often considered a hallmark of the nematic phase of Fe-based superconductors, including FeSe, but its origin is not yet understood. Here we report a high resolution Angle-Resolved Photoemission Spectroscopy study of single crystals of FeSe, accounting for the photon-energy dependence and making a detailed analysis of the temperature dependence. We find that the hole pocket undergoes a fourfold-symmetry-breaking distortion in the nematic phase below 90~K, but in contrast the changes to the electron pockets do not require fourfold symmetry-breaking. Instead, there is an additional separation of the existing $d_{xy}$ and $d_{xz/yz}$ bands - which themselves are not split within resolution. These observations lead us to propose a new scenario of "unidirectional nematic bond ordering" to describe the low-temperature electronic structure of FeSe, supported by a good agreement with 10-orbital tight binding model calculations.


Unconventional Superconductivity in the Layered Iron Germanide YFe(2)Ge(2).

Physical review letters 116 (2016) 127001-

J Chen, K Semeniuk, Z Feng, P Reiss, P Brown, Y Zou, PW Logg, GI Lampronti, FM Grosche

The iron-based intermetallic YFe_{2}Ge_{2} stands out among transition metal compounds for its high Sommerfeld coefficient of the order of 100  mJ/(mol K^{2}), which signals strong electronic correlations. A new generation of high quality samples of YFe_{2}Ge_{2} show superconducting transition anomalies below 1.8 K in thermodynamic, magnetic, and transport measurements, establishing that superconductivity is intrinsic in this layered iron compound outside the known superconducting iron pnictide or chalcogenide families. The Fermi surface geometry of YFe_{2}Ge_{2} resembles that of KFe_{2}As_{2} in the high pressure collapsed tetragonal phase, in which superconductivity at temperatures as high as 10 K has recently been reported, suggesting an underlying connection between the two systems.


Evolution of the Valley Position in Bulk Transition-Metal Chalcogenides and Their Monolayer Limit.

Nano letters 16 (2016) 4738-4745

H Yuan, Z Liu, G Xu, B Zhou, S Wu, D Dumcenco, K Yan, Y Zhang, S-K Mo, P Dudin, V Kandyba, M Yablonskikh, A Barinov, Z Shen, S Zhang, Y Huang, X Xu, Z Hussain, HY Hwang, Y Cui, Y Chen

Layered transition metal chalcogenides with large spin orbit coupling have recently sparked much interest due to their potential applications for electronic, optoelectronic, spintronics, and valleytronics. However, most current understanding of the electronic structure near band valleys in momentum space is based on either theoretical investigations or optical measurements, leaving the detailed band structure elusive. For example, the exact position of the conduction band valley of bulk MoS2 remains controversial. Here, using angle-resolved photoemission spectroscopy with submicron spatial resolution (micro-ARPES), we systematically imaged the conduction/valence band structure evolution across representative chalcogenides MoS2, WS2, and WSe2, as well as the thickness dependent electronic structure from bulk to the monolayer limit. These results establish a solid basis to understand the underlying valley physics of these materials, and also provide a link between chalcogenide electronic band structure and their physical properties for potential valleytronics applications.


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


Ab initio cycloidal and chiral magnetoelectric responses in Cr2O3

PHYSICAL REVIEW B 94 (2016) ARTN 100405

N Tillack, JR Yates, PG Radaelli


Studies of a Large Odd-Numbered Odd-Electron Metal Ring: Inelastic Neutron Scattering and Muon Spin Relaxation Spectroscopy of Cr8 Mn.

Chemistry (Weinheim an der Bergstrasse, Germany) 22 (2016) 1779-1788

ML Baker, T Lancaster, A Chiesa, G Amoretti, PJ Baker, C Barker, SJ Blundell, S Carretta, D Collison, HU Güdel, T Guidi, EJL McInnes, JS Möller, H Mutka, J Ollivier, FL Pratt, P Santini, F Tuna, PLW Tregenna-Piggott, IJ Vitorica-Yrezabal, GA Timco, REP Winpenny

The spin dynamics of Cr8 Mn, a nine-membered antiferromagnetic (AF) molecular nanomagnet, are investigated. Cr8 Mn is a rare example of a large odd-membered AF ring, and has an odd-number of 3d-electrons present. Odd-membered AF rings are unusual and of interest due to the presence of competing exchange interactions that result in frustrated-spin ground states. The chemical synthesis and structures of two Cr8 Mn variants that differ only in their crystal packing are reported. Evidence of spin frustration is investigated by inelastic neutron scattering (INS) and muon spin relaxation spectroscopy (μSR). From INS studies we accurately determine an appropriate microscopic spin Hamiltonian and we show that μSR is sensitive to the ground-spin-state crossing from S=1/2 to S=3/2 in Cr8 Mn. The estimated width of the muon asymmetry resonance is consistent with the presence of an avoided crossing. The investigation of the internal spin structure of the ground state, through the analysis of spin-pair correlations and scalar-spin chirality, shows a non-collinear spin structure that fluctuates between non-planar states of opposite chiralities.


On the temperature dependence of spin pumping in ferromagnet-topological insulator-ferromagnet spin valves

Results in Physics 6 (2016) 293-294

AA Baker, AI Figueroa, G van der Laan, T Hesjedal


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.


Bimetallic MOFs (H3O)x[Cu(MF6)(pyrazine)2]·(4 - x)H2O (M = V4+, x = 0; M = Ga3+, x = 1): co-existence of ordered and disordered quantum spins in the V4+ 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 MF6n- octahedra. In each, only the Cu2+ spins exhibit long-range magnetic order below 3.5 K (M = V4+) and 2.6 K (M = Ga3+). The V4+ spins remain disordered down to 0.5 K.


Building Large-Domain Twisted Bilayer Graphene with van Hove Singularity.

ACS nano 10 (2016) 6725-6730

Z Tan, J Yin, C Chen, H Wang, L Lin, L Sun, J Wu, X Sun, H Yang, Y Chen, H Peng, Z Liu

Twisted bilayer graphene (tBLG) with van Hove Singularity (VHS) has exhibited novel twist-angle-dependent chemical and physical phenomena. However, scalable production of high-quality tBLG is still in its infancy, especially lacking the angle controlled preparation methods. Here, we report a facile approach to prepare tBLG with large domain sizes (>100 μm) and controlled twist angles by a clean layer-by-layer transfer of two constituent graphene monolayers. The whole process without interfacial polymer contamination in two monolayers guarantees the interlayer interaction of the π-bond electrons, which gives rise to the existence of minigaps in electronic structures and the consequent formation of VHSs in density of state. Such perturbation on band structure was directly observed by angle-resolved photoemission spectroscopy with submicrometer spatial resolution (micro-ARPES). The VHSs lead to a strong light-matter interaction and thus introduce ∼20-fold enhanced intensity of Raman G-band, which is a characteristic of high-quality tBLG. The as-prepared tBLG with strong light-matter interaction was further fabricated into high-performance photodetectors with selectively enhanced photocurrent generation (up to ∼6 times compared with monolayer in our device).

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