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Search Results: 1 - 10 of 325390 matches for " Sarnjeet S. Dhesi "
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Momentum-dependent snapshots of a melting charge density wave
Jesse C. Petersen,Stefan Kaiser,Nicky Dean,Alberto Simoncig,Haiyun Liu,Adrian L. Cavalieri,Cephise Cacho,I. C. Edmond Turcu,Emma Springate,Fabio Frassetto,Luca Poletto,Sarnjeet S. Dhesi,Helmuth Berger,Andrea Cavalleri
Physics , 2010, DOI: 10.1103/PhysRevLett.107.177402
Abstract: Charge density waves (CDWs) underpin the electronic properties of many complex materials. Near-equilibrium CDW order is linearly coupled to a periodic, atomic-structural distortion, and the dynamics is understood in terms of amplitude and phase modes. However, at the shortest timescales lattice and charge order may become de-coupled, highlighting the electronic nature of this many-body broken symmetry ground state. Using time and angle resolved photoemission spectroscopy with sub-30-fs XUV pulses, we have mapped the time- and momentum-dependent electronic structure in photo-stimulated 1T-TaS2, a prototypical two-dimensional charge density wave compound. We find that CDW order, observed as a splitting of the uppermost electronic bands at the Brillouin zone boundary, melts well before relaxation of the underlying structural distortion. Decoupled charge and lattice modulations challenge the view of Fermi Surface nesting as a driving force for charge density wave formation in 1T-TaS2.
Spectroscopy of orbital ordering in La0.5Sr1.5MnO4 : A many-body cluster calculation
Alessandro Mirone,S. S. Dhesi,G. van der Laan
Physics , 2006, DOI: 10.1140/epjb/e2006-00340-5
Abstract: We have studied the orbital ordering (OO) in La0.5Sr1.5MnO4 and its soft x-ray resonant diffraction spectroscopic signature at the Mn L2, L3 edges. We have modelled the system in second quantization as a small planar cluster consisting of a central Mn atom, with the first neighbouring shells of oxygen and Mn atoms. For the effective Hamiltonian we consider Slater-Koster parameters, charge transfer and electron correlation energies obtained from previous measurements on manganites. We calculate the OO as a function of oxygen distortion and spin correlation used as adjustable parameters. Their contribution as a function of temperature is clearly distinguished with a good spectroscopic agreement.
Random Projection Trees Revisited
Aman Dhesi,Purushottam Kar
Computer Science , 2010,
Abstract: The Random Projection Tree structures proposed in [Freund-Dasgupta STOC08] are space partitioning data structures that automatically adapt to various notions of intrinsic dimensionality of data. We prove new results for both the RPTreeMax and the RPTreeMean data structures. Our result for RPTreeMax gives a near-optimal bound on the number of levels required by this data structure to reduce the size of its cells by a factor $s \geq 2$. We also prove a packing lemma for this data structure. Our final result shows that low-dimensional manifolds have bounded Local Covariance Dimension. As a consequence we show that RPTreeMean adapts to manifold dimension as well.
Magnetic Coupling and Single-Ion Anisotropy in Surface-Supported Mn-based Metal-Organic Networks
L. Giovanelli,A. Savoyant,M. Abel,F. Maccherozzi,Y. Ksari,M. Koudia,R. Hayn,F. Choueikani,E. Otero,P. Ohresser,J. -M. Themlin,S. S. Dhesi,S. Clair
Physics , 2013, DOI: 10.1021/jp502209q
Abstract: The electronic and magnetic properties of Mn coordinated to 1,2,4,5-tetracyanobenzene (TCNB) in the Mn-TCNB 2D metal-ligand networks have been investigated by combining scanning tunneling microscopy and X-ray magnetic circular dichroism (XMCD) performed at low temperature (3 K). When formed on Au(111) and Ag(111) substrates the Mn-TCNB networks display similar geometric structures. Magnetization curves reveal ferromagnetic (FM) coupling of the Mn sites with similar single-ion anisotropy energies, but different coupling constants. Low-temperature XMCD spectra show that the local environment of the Mn centers differs appreciably for the two substrates. Multiplet structure calculations were used to derive the corresponding ligand field parameters confirming an in-plane uniaxial anisotropy. The observed interatomic coupling is discussed in terms of superexchange as well as substrate-mediated magnetic interactions.
Magnetic fan structures in Ba$_{0.5}$Sr$_{1.5}$Zn$_2$Fe$_{12}$O$_{22}$ hexaferrite revealed by resonant soft X-ray diffraction
Alexander J. Hearmon,R. D. Johnson,T. A. W. Beale,S. S. Dhesi,X. Luo,S. -W. Cheong,P. Steadman,Paolo G. Radaelli
Physics , 2013, DOI: 10.1103/PhysRevB.88.174413
Abstract: The hexaferrites are known to exhibit a wide range of magnetic structures, some of which are connected to important technological applications and display magnetoelectric properties. We present data on the low field magnetic structures stabilised in a Y-type hexaferrite as observed by resonant soft X-ray diffraction. The helical spin block arrangement that is present in zero applied magnetic field becomes fan-like as a field is applied in plane. The propagation vectors associated with each fan structure are studied as a function of field, and a new magnetic phase is reported. Mean field calculations indicate this phase should stabilise close to the boundary of the previously reported phases.
Optically induced superconductivity in striped La2-xBaxCuO4 by polarization-selective excitation in the near infrared
D. Nicoletti,E. Casandruc,Y. Laplace,V. Khanna,C. R. Hunt,S. Kaiser,S. S. Dhesi,G. D. Gu,J. P. Hill,A. Cavalleri
Physics , 2014, DOI: 10.1103/PhysRevB.90.100503
Abstract: We show that superconducting interlayer coupling, which coexists with and is depressed by stripe order in La1.885Ba0.115CuO4, can be enhanced by excitation with near-infrared laser pulses. For temperatures lower than Tc = 13 K, we observe a blue-shift of the equilibrium Josephson plasma resonance, detected by terahertz-frequency reflectivity measurements. Key to this measurement is the ability to probe the optical properties at frequencies as low as 150 GHz, detecting the weak interlayer coupling strengths. For T > Tc a similar plasma resonance, absent at equilibrium, is induced up to the spin-ordering temperature TSO = 40 K. These effects are reminiscent but qualitatively different from the light-induced superconductivity observed by resonant phonon excitation in La1.675Eu0.2Sr0.125CuO6.5. Importantly, enhancement of the below-Tc interlayer coupling and its appearance above Tc are preferentially achieved when the near-infrared pump light is polarized perpendicular to the superconducting planes, likely due to more effective melting of stripe order and the less effective excitation of quasiparticles from the Cooper pair condensate when compared to in-plane excitation.
RASOR: An advanced instrument for soft x-ray reflectivity and diffraction
T. A. W. Beale,T. P. A. Hase,T. Iida,K. Endo,P. Steadman,A. R. Marshall,S. S. Dhesi,G. van der Laan,P. D. Hatton
Physics , 2010, DOI: 10.1063/1.3458004
Abstract: We report the design and construction of a novel soft x-ray diffractometer installed at Diamond Light Source. The beamline endstation RASOR is constructed for general users and designed primarily for the study of single crystal diffraction and thin film reflectivity. The instrument is comprised of a limited three circle ({\theta}, 2{\theta}, {\chi}) diffractometer with an additional removable rotation ({\phi}) stage. It is equipped with a liquid helium cryostat, and post-scatter polarization analysis. Motorised motions are provided for the precise positioning of the sample onto the diffractometer centre of rotation, and for positioning the centre of rotation onto the x-ray beam. The functions of the instrument have been tested at Diamond Light Source, and initial test measurements are provided, demonstrating the potential of the instrument.
Imaging of coherent magneto-elastic domains in multiferroic BiFeO3 films
N. Waterfield Price,R. D. Johnson,W. Saenrang,F. Maccherozzi,S. S. Dhesi,A. Bombardi,F. P. Chmiel,C. -B. Eom,P. G. Radaelli
Physics , 2015,
Abstract: A current challenge presented in thin film physics is imaging and controlling antiferromagnetic domains at the nanoscale. By employing a combination of non-resonant x-ray magnetic scattering, neutron diffraction and vector-mapped x-ray magnetic linear dichroism photoemission electron microscopy, we have directly visualised the sub-micron scale antiferromagnetic domain structure of epitaxial (111) BiFeO3 films. We find that these domains are coherently coupled to crystallographic domain structure, which comprises an array of monoclinic twins (space group Cc). This magneto-elastic coupling stabilises the antiferromagnetic domain structure, providing a pathway towards strain-engineering multiferroic domains.
Modified Brownian Motion Approach to Modelling Returns Distribution
Gurjeet Dhesi,Muhammad Bilal Shakeel,Ling Xiao
Quantitative Finance , 2015,
Abstract: An innovative extension of Geometric Brownian Motion model is developed by incorporating a weighting factor and a stochastic function modelled as a mixture of power and trigonometric functions. Simulations based on this Modified Brownian Motion Model with optimal weighting factors selected by goodness of fit tests, substantially outperform the basic Geometric Brownian Motion model in terms of fitting the returns distribution of historic data price indices. Furthermore we attempt to provide an interpretation of the additional stochastic term in relation to irrational behaviour in financial markets and outline the importance of this novel model.
Probing photo-induced melting of antiferromagnetic order in La0.5Sr1.5MnO4 by ultrafast resonant soft X-ray diffraction
H. Ehrke,R. I. Tobey,S. Wall,S. A. Cavill,M. F?rst,V. Khanna,Th. Garl,N. Stojanovic,D. Prabhakaran,A. T. Boothroyd,M. Gensch,A. Mirone,P. Reutler,A. Revcolevschi,S. S. Dhesi,A. Cavalleri
Physics , 2010, DOI: 10.1103/PhysRevLett.106.217401
Abstract: Photo-excitation in complex oxides1 transfers charge across semicovalent bonds, drastically perturbing spin and orbital orders2. Light may then be used in compounds like magnetoresistive manganites to control magnetism on nanometre lengthscales and ultrafast timescales. Here, we show how ultrafast resonant soft x-ray diffraction can separately probe the photo-induced dynamics of spin and orbital orders in La0.5Sr1.5MnO4. Ultrafast melting of CE antiferromagnetic spin order is evidenced by the disappearance of a (1/4,1/4,1/2) diffraction peak. On the other hand the (1/4,1/4,0) peak, reflecting orbital order, is only partially reduced. Cluster calculations aid our interpretation by considering different magnetically ordered states accessible after photo-excitation. Nonthermal coupling between light and magnetism emerges as a primary aspect of photo-induced phase transitions in manganites.
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