Publications


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.


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


Surface Monocrystallization of Copper Foil for Fast Growth of Large Single-Crystal Graphene under Free Molecular Flow.

Advanced materials (Deerfield Beach, Fla.) 28 (2016) 8968-8974

H Wang, X Xu, J Li, L Lin, L Sun, X Sun, S Zhao, C Tan, C Chen, W Dang, H Ren, J Zhang, B Deng, AL Koh, L Liao, N Kang, Y Chen, H Xu, F Ding, K Liu, H Peng, Z Liu

Wafer-sized single-crystalline Cu (100) surface can be readily achieved on stacked polycrystalline Cu foils via simple oxygen chemisorption-induced reconstruction, enabling fast growth of large-scale millimeter-sized single-crystalline graphene arrays under molecular flow. The maximum growth rate can reach 300 μm min-1 , several orders of magnitude higher than previously reported values for millimeter-sized single-crystalline graphene growth on Cu foils.


Organic Transistors: Universal Magnetic Hall Circuit Based on Paired Spin Heterostructures (Adv. Electron. Mater. 6/2015)

Advanced Electronic Materials 1 (2016)

S Zhang, AA Baker, JY Zhang, G Yu, S Wang, T Hesjedal


Antiferromagnetism in a Family of S = 1 Square Lattice Coordination Polymers NiX2(pyz)2 (X = Cl, Br, I, NCS; pyz = Pyrazine).

Inorganic chemistry 55 (2016) 3515-3529

J Liu, PA Goddard, J Singleton, J Brambleby, F Foronda, JS Möller, Y Kohama, S Ghannadzadeh, A Ardavan, SJ Blundell, T Lancaster, F Xiao, RC Williams, FL Pratt, PJ Baker, K Wierschem, SH Lapidus, KH Stone, PW Stephens, J Bendix, TJ Woods, KE Carreiro, HE Tran, CJ Villa, JL Manson

The crystal structures of NiX2(pyz)2 (X = Cl (1), Br (2), I (3), and NCS (4)) were determined by synchrotron X-ray powder diffraction. All four compounds consist of two-dimensional (2D) square arrays self-assembled from octahedral NiN4X2 units that are bridged by pyz ligands. The 2D layered motifs displayed by 1-4 are relevant to bifluoride-bridged [Ni(HF2)(pyz)2]EF6 (E = P, Sb), which also possess the same 2D layers. In contrast, terminal X ligands occupy axial positions in 1-4 and cause a staggered packing of adjacent layers. Long-range antiferromagnetic (AFM) order occurs below 1.5 (Cl), 1.9 (Br and NCS), and 2.5 K (I) as determined by heat capacity and muon-spin relaxation. The single-ion anisotropy and g factor of 2, 3, and 4 were measured by electron-spin resonance with no evidence for zero-field splitting (ZFS) being observed. The magnetism of 1-4 spans the spectrum from quasi-two-dimensional (2D) to three-dimensional (3D) antiferromagnetism. Nearly identical results and thermodynamic features were obtained for 2 and 4 as shown by pulsed-field magnetization, magnetic susceptibility, as well as their Néel temperatures. Magnetization curves for 2 and 4 calculated by quantum Monte Carlo simulation also show excellent agreement with the pulsed-field data. Compound 3 is characterized as a 3D AFM with the interlayer interaction (J⊥) being slightly stronger than the intralayer interaction along Ni-pyz-Ni segments (J(pyz)) within the two-dimensional [Ni(pyz)2](2+) square planes. Regardless of X, J(pyz) is similar for the four compounds and is roughly 1 K.


Structural, electronic, and magnetic investigation of magnetic ordering in MBE-grown CrxSb2-xTe3 thin films

EPL 115 (2016) ARTN 27006

LJ Collins-McIntyre, LB Duffy, A Singh, N-J Steinke, CJ Kinane, TR Charlton, A Pushp, AJ Kellock, SSP Parkin, SN Holmes, CHW Barnes, G Van der Laan, S Langridge, T Hesjedal


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 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.


Transverse field muon-spin rotation measurement of the topological anomaly in a thin film of MnSi

Physical Review B: Condensed Matter and Materials Physics American Physical Society 93 (2016) 140412(R)

T Lancaster, F Xiao, Z Salman, IO Thomas, SJ Blundell, F Pratt, SJ Clark, T Prokscha, A Suter, SL Zhang, AA Baker, T Hesjedal


Single crystal growth from separated educts and its application to lithium transition-metal oxides.

Scientific reports 6 (2016) 35362-

F Freund, SC Williams, RD Johnson, R Coldea, P Gegenwart, A Jesche

Thorough mixing of the starting materials is the first step of a crystal growth procedure. This holds true for almost any standard technique, whereas the intentional separation of educts is considered to be restricted to a very limited number of cases. Here we show that single crystals of α-Li2IrO3 can be grown from separated educts in an open crucible in air. Elemental lithium and iridium are oxidized and transported over a distance of typically one centimeter. In contrast to classical vapor transport, the process is essentially isothermal and a temperature gradient of minor importance. Single crystals grow from an exposed condensation point placed in between the educts. The method has also been applied to the growth of Li2RuO3, Li2PtO3 and β-Li2IrO3. A successful use of this simple and low cost technique for various other materials is anticipated.


Free-standing millimetre-long Bi2Te3 sub-micron belts catalyzed by TiO2 nanoparticles.

Nanoscale research letters 11 (2016) 308-

P Schönherr, F Zhang, D Kojda, R Mitdank, M Albrecht, SF Fischer, T Hesjedal

Physical vapour deposition (PVD) is used to grow millimetre-long Bi2Te3 sub-micron belts catalysed by TiO2 nanoparticles. The catalytic efficiency of TiO2 nanoparticles for the nanostructure growth is compared with the catalyst-free growth employing scanning electron microscopy. The catalyst-coated and catalyst-free substrates are arranged side-by-side, and overgrown at the same time, to assure identical growth conditions in the PVD furnace. It is found that the catalyst enhances the yield of the belts. Very long belts were achieved with a growth rate of 28 nm/min. A ∼1-mm-long belt with a rectangular cross section was obtained after 8 h of growth. The thickness and width were determined by atomic force microscopy, and their ratio is ∼1:10. The chemical composition was determined to be stoichiometric Bi2Te3 using energy-dispersive X-ray spectroscopy. Temperature-dependent conductivity measurements show a characteristic increase of the conductivity at low temperatures. The room temperature conductivity of 0.20 × 10(5) S m (-1) indicates an excellent sample quality.


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.


One-Step SnO2 Nanotree Growth.

Chemistry (Weinheim an der Bergstrasse, Germany) 22 (2016) 13823-13825

P Schönherr, T Hesjedal

A comparison between Au, TiO2 and self-catalysed growth of SnO2 nanostructures using chemical vapour deposition is reported. TiO2 enables growth of a nanonetwork of SnO2 , whereas self-catalysed growth results in nanoclusters. Using Au catalyst, single-crystalline SnO2 nanowire trees can be grown in a one-step process. Two types of trees are identified that differ in size, presence of a catalytic tip, and degree of branching. The growth mechanism of these nanotrees is based on branch-splitting and self-seeding by the catalytic tip, facilitating at least three levels of branching, namely trunk, branch and leaf.


Photonic topological insulator with broken time-reversal symmetry.

Proceedings of the National Academy of Sciences of the United States of America 113 (2016) 4924-4928

C He, X-C Sun, X-P Liu, M-H Lu, Y Chen, L Feng, Y-F Chen

A topological insulator is a material with an insulating interior but time-reversal symmetry-protected conducting edge states. Since its prediction and discovery almost a decade ago, such a symmetry-protected topological phase has been explored beyond electronic systems in the realm of photonics. Electrons are spin-1/2 particles, whereas photons are spin-1 particles. The distinct spin difference between these two kinds of particles means that their corresponding symmetry is fundamentally different. It is well understood that an electronic topological insulator is protected by the electron's spin-1/2 (fermionic) time-reversal symmetry [Formula: see text] However, the same protection does not exist under normal circumstances for a photonic topological insulator, due to photon's spin-1 (bosonic) time-reversal symmetry [Formula: see text] In this work, we report a design of photonic topological insulator using the Tellegen magnetoelectric coupling as the photonic pseudospin orbit interaction for left and right circularly polarized helical spin states. The Tellegen magnetoelectric coupling breaks bosonic time-reversal symmetry but instead gives rise to a conserved artificial fermionic-like-pseudo time-reversal symmetry, Tp ([Formula: see text]), due to the electromagnetic duality. Surprisingly, we find that, in this system, the helical edge states are, in fact, protected by this fermionic-like pseudo time-reversal symmetry Tp rather than by the bosonic time-reversal symmetry Tb This remarkable finding is expected to pave a new path to understanding the symmetry protection mechanism for topological phases of other fundamental particles and to searching for novel implementations for topological insulators.


Tuning Chemical Potential Difference across Alternately Doped Graphene p-n Junctions for High-Efficiency Photodetection.

Nano letters 16 (2016) 4094-4101

L Lin, X Xu, J Yin, J Sun, Z Tan, AL Koh, H Wang, H Peng, Y Chen, Z Liu

Being atomically thin, graphene-based p-n junctions hold great promise for applications in ultrasmall high-efficiency photodetectors. It is well-known that the efficiency of such photodetectors can be improved by optimizing the chemical potential difference of the graphene p-n junction. However, to date, such tuning has been limited to a few hundred millielectronvolts. To improve this critical parameter, here we report that using a temperature-controlled chemical vapor deposition process, we successfully achieved modulation-doped growth of an alternately nitrogen- and boron-doped graphene p-n junction with a tunable chemical potential difference up to 1 eV. Furthermore, such p-n junction structure can be prepared on a large scale with stable, uniform, and substitutional doping and exhibits a single-crystalline nature. This work provides a feasible method for synthesizing low-cost, large-scale, high efficiency graphene p-n junctions, thus facilitating their applications in optoelectronic and energy conversion devices.


Incommensurate counterrotating magnetic order stabilized by Kitaev interactions in the layered honeycomb alpha-Li2IrO3

PHYSICAL REVIEW B 93 (2016) ARTN 195158

SC Williams, RD Johnson, F Freund, S Choi, A Jesche, I Kimchi, S Manni, A Bombardi, P Manuel, P Gegenwart, R Coldea


La2SrCr2O7F2: A Ruddlesden-Popper Oxyfluoride Containing Octahedrally Coordinated Cr(4+) Centers.

Inorganic chemistry 55 (2016) 3169-3174

R Zhang, G Read, F Lang, T Lancaster, SJ Blundell, MA Hayward

The low-temperature fluorination of the n = 2 Ruddlesden-Popper phase La2SrCr2O7 yields La2SrCr2O7F2 via a topochemical fluorine insertion reaction. The structure-conserving nature of the fluorination reaction means that the chromium centers of the initial oxide phase retain an octahedral coordination environment in the fluorinated product, resulting in a material containing an extended array of apex-linked Cr(4+)O6 units. Typically materials containing networks of octahedrally coordinated Cr(4+) centers can only be prepared at high pressure; thus, the preparation of La2SrCr2O7F2 demonstrates that low-temperature topochemical reactions offer an alternative synthesis route to materials of this type. Neutron diffraction, magnetization, and μ(+)SR data indicate that La2SrCr2O7F2 undergoes a transition to an antiferromagnetic state below TN ≈ 140 K. The structure-property relations of this phase and other Cr(4+) oxide phases are discussed.


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).


Control of the third dimension in copper-based square-lattice antiferromagnets

PHYSICAL REVIEW B 93 (2016) ARTN 094430

PA Goddard, J Singleton, I Franke, JS Moeller, T Lancaster, AJ Steele, CV Topping, SJ Blundell, FL Pratt, C Baines, J Bendix, RD McDonald, J Brambleby, MR Lees, SH Lapidus, PW Stephens, BW Twamley, MM Conner, K Funk, JF Corbey, HE Tran, JA Schlueter, JL Manson


Nanoscale depth-resolved polymer dynamics probed by the implantation of low energy muons

POLYMER 105 (2016) 516-525

FL Pratt, T Lancaster, PJ Baker, SJ Blundell, T Prokscha, E Morenzoni, A Suter, HE Assender

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