Publications (199)278.08 Total impact
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Article: High-pressure study of transport properties in Co_ {0.33} NbS_ {2}
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ABSTRACT: This is the first study of the effect of pressure on transition metal dichalcogenides (TMDs) intercalated by atoms that order magnetically. Co0.33NbS2 is a layered system where the intercalated Co atoms order antiferromagnetically at TN = 26 K at ambient pressure. We have conducted a detailed study of dc-resistivity (ρ), thermoelectric power (S), and thermal conductivity (κ). At ambient pressure the magnetic transition corresponds to a well-pronounced peak in dS/dT, as well as to a kink in the dc-resistivity. The effect of ordering on the thermal conductivity is rather small but, surprisingly, more pronounced in the lattice contribution than in the electronic contribution to κ. Under pressure the resistivity increases in the high-temperature range, contrary to all previous measurements in other layered TMDs. In the low-temperature range the strong dependences of thermopower and resistivity on pressure are observed below TN, which, in turn, also depends on pressure at rate of dTN/dp ∼ −1 K/kbar. Several possible microscopic explanations of the reduction of the ordering temperature and the evolution of the transport properties with pressure are discussed.Phys. Rev. B. 06/2013; 84(7). -
Article: Tunable Polaronic Conduction in Anatase TiO 2
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ABSTRACT: Oxygen vacancies created in anatase TiO 2 by UV photons (80–130 eV) provide an effective electron-doping mechanism and induce a hitherto unobserved dispersive metallic state. Angle resolved photo-emission reveals that the quasiparticles are large polarons. These results indicate that anatase can be tuned from an insulator to a polaron gas to a weakly correlated metal as a function of doping and clarify the nature of conductivity in this material.Physical Review Letters 05/2013; 110(19):196403. · 7.37 Impact Factor -
Article: Pressure dependence of the large-polaron transport in anatase TiO 2 single crystals
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ABSTRACT: EPL, 99 (2012) 57005 Please visit the new website www.epljournal.orgEPL (Europhysics Letters) 09/2012; 96(5):57005. · 2.17 Impact Factor -
Article: Pressure dependence of the large-polaron transport in anatase TiO 2 single crystals
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ABSTRACT: EPL, 99 (2012) 57005 Please visit the new website www.epljournal.orgEPL (Europhysics Letters) 09/2012; 99(5):57005. · 2.17 Impact Factor -
Article: Site-selective quantum correlations revealed by magnetic anisotropy inthe tetramer system SeCuO3
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ABSTRACT: We present the investigation of a monoclinic compound SeCuO3 using x-ray powder di?raction, magnetization, torque and electron-spin-resonance (ESR). Structurally based analysis suggests that SeCuO3 can be considered as a 3D network of tetramers. The values of intra-tetramer exchange interactions are extracted from the temperature dependence of the susceptibility and amount to ~200 K. The inter-tetramer coupling leads to the development of long-range antiferromagnetic order at TN = 8 K. An unusual temperature dependence of the effective g-tensors is observed, accompanied with a rotation of macroscopic magnetic axes. We explain this unique observation as due to site-selective quantum correlations.Physical Review B 08/2012; 86(05):054405. · 3.69 Impact Factor -
Article: Mapping of Electron-Hole Excitations in the Charge-Density-Wave System 1T-TiSe 2 Using Resonant Inelastic X-Ray Scattering
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ABSTRACT: In high-resolution resonant inelastic x-ray scattering at the Ti L edge of the charge-density-wave system 1T-TiSe 2 , we observe sharp low energy loss peaks from electron-hole pair excitations developing at low temperature. These excitations are strongly dispersing as a function of the transferred momentum of light. We show that the unoccupied bands close to the Fermi level can effectively be probed in this broadband material. Furthermore, we extract the order parameter of the charge-density-wave phase from temperature-dependent measurements. A detailed knowledge of the band structure of crystalline solids near their Fermi energy is essential for describing and understanding their low energy electronic properties. For this purpose, momentum ~ k-resolved spectroscopic probes are necessary and angle-resolved photoemission spectroscopy (ARPES), which measures the one-particle spectral function, is particularly appropriate, given its excellent energy resolution. However, ARPES provides information only on the occupied band structure, espe-cially deficient in the case of gapped systems. Inverse photoemission is a possible candidate for directly probing the unoccupied band structure but offers only a poor energy resolution. More insight in the band structure around the Fermi energy can be gained from two-particle spectroscopic techniques, like electron energy loss spec-troscopy, which can measure electron-hole excitations across the gap. However, this technique is demanding in sample preparation, due to the limited escape depth of electrons. Resonant inelastic x-ray scattering (RIXS) is a good answer to this issue, given its rapid technical progress and its great potential [1–6]. As a photon-in and photon-out technique, it does not require a complicated sample preparation, and it is bulk sensitive. Similarly to electron energy loss spectroscopy, it measures electron-hole exci-tations and has early on been shown to give access to the band structure of insulating and semimetallic materials by varying the incident energy of the photons [7,8]. In these experiments, band mapping was done by assigning peaks in the x-ray absorption spectra (XAS) to high symmetry points in the unoccupied band structure and linking them to particular ~ k-resolved positions in the occupied band struc-ture, invoking momentum conservation in the scattering process. However, this procedure has been applied on a typical energy scale of 5 to 10 eV with an energy resolution of about 0.5 to 1 eV. In this Letter, we demonstrate that electron-hole excita-tions in a charge-density-wave (CDW) material can di-rectly be mapped with RIXS by changing the momentum of light. With this method, sharp electron-hole excitations at low energy loss and with a substantial dispersion are measured in the CDW system 1T-TiSe 2 . Furthermore, a detailed temperature dependence of these excitations allows us to directly follow the evolution of the CDW gap and its closing over a wide temperature range. This observation of sharp momentum-dispersive peaks in a broadband semimetallic material opens new perspectives for probing the unoccupied states with RIXS. RIXS experiments were performed at the ADRESS beam line [9] of the Swiss Light Source, Paul Scherrer Institut, using the SAXES spectrometer [10]. RIXS spectra were recorded in typically 4 h acquisition time, achieving a statistics of 150–200 photons in the peak maxima of inter-est (see below). A scattering angle of 130 was used, and all the spectra were measured with linearly -polarized incident light [perpendicular to the scattering plane; see Fig. 1(a)]. The combined energy resolution was 90 meV at the Ti L edge ($ 460 eV). 1T-TiSe 2 single crystals were cleaved [with a surface in the (001) direction] in situ at a pressure of about 1 Â 10 À8 mbar. The crystals were ori-ented so that the transferred momentum of light lies in the ÀML plane [see Fig. 1(a)] and that the polarization of light lies completely in the Ti plane. 1T-TiSe 2 is a quasi-two-dimensional transition metal dichalcogenide, which undergoes at T c ¼ 200 K a second-order phase transition towards a CDW phase [11], the origin of which is still debated [12–15]. Its normal phase band structure is at the edge between a semimetallic and aPhysical Review Letters 07/2012; 109(12):047401. · 7.37 Impact Factor -
Article: Magneto-Electric Coupling in Single Crystal Cu2OSeO3 Studied by a Novel Electron Spin Resonance Technique
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ABSTRACT: The magneto-electric (ME) coupling on spin-wave resonances in single-crystal Cu2OSeO3 was studied by a novel technique using electron spin resonance combined with electric field modulation. An external electric field E induces a magnetic field component \mu_0 H^i = \gamma E along the applied magnetic field H with \gamma=0.7(1) \mu T/(V/mm) at 10 K. We found that ME coupling strength \gamma is temperature dependent and highly anisotropic. \gamma(T) nearly follows that of spin susceptibility J(T) and rapidly decreases above the Curie temperature Tc. The ratio \gamma/J monotonically decreases with increasing temperature without an anomaly at Tc.04/2012; -
Article: BaVS3 probed by V L edge x-ray absorption spectroscopy.
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ABSTRACT: Polarization dependent vanadium L edge x-ray absorption spectra of BaVS(3) single crystals are measured in the four phases of the compound. The difference between signals with the polarizations E perpendicular to c and E is parallel to c (linear dichroism) changes with temperature. Besides increasing the intensity of one of the maxima, a new structure appears in the pre-edge region below the metal-insulator transition. More careful examination brings to light that the changes start already with pretransitional charge density wave fluctuations. Simple symmetry analysis suggests that the effect is related to rearrangements in the E(g) and A(1g) states, and is compatible with the formation of four inequivalent V-sites along the V-S chain.Journal of Physics Condensed Matter 02/2012; 24(4):045503. · 2.55 Impact Factor -
Article: BaVS3 probed by V L edge x-ray absorption spectroscopy
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ABSTRACT: Polarization dependent vanadium L edge x-ray absorption spectra of BaVS3 single crystals are measured in the four phases of the compound. The difference between signals with the polarizations E ? c and E k c (linear dichroism) changes with temperature. Besides increasing the intensity of one of the maxima, a new structure appears in the pre-edge region below the metal–insulator transition. More careful examination brings to light that the changes start already with pretransitional charge density wave fluctuations. Simple symmetry analysis suggests that the effect is related to rearrangements in the Eg and A1g states, and is compatible with the formation of four inequivalent V-sites along the V–S chain.Journal of Physics Condensed Matter 01/2012; 24(4):045503. · 2.55 Impact Factor -
Article: Unified one-band Hubbard model for magnetic and electronic spectra of the parent compounds of cuprate superconductors
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ABSTRACT: Using low-energy projection of the one-band t -t �-t �� Hubbard model we derive an effective spin Hamiltonian and its spin-wave expansion to order 1/S. We fit the spin-wave dispersion of several parent compounds to the high-temperature superconducting cuprates La2CuO4, Sr2CuO2Cl2, andBi2Sr2YCu2O8.Our accurate quantitative determination of the one-band Hubbard model parameters allows prediction and comparison to experimental results. Among those we discuss the two-magnon Raman peak line shape, the K-edge resonant inelastic x-ray scattering 500-meV peak, and the high-energy kink in the angle-resolved photoemission spectroscopy quasiparticle dispersion, also known as the waterfall feature.Physical Review B 01/2012; 100508(R)(85). · 3.69 Impact Factor -
Article: BaVS$_3$ probed by V L edge X-ray absorption spectroscopy
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ABSTRACT: Polarization dependent vanadium L edge X-ray absorption spectra of BaVS$_3$ single crystals are measured in the four phases of the compound. The difference between signals with the polarization \textbf{E}$\perp$\textbf{c} and \textbf{E}$\parallel$\textbf{c} (linear dichroism) changes with temperature. Besides increasing intensity of one of the maxima, a new structure appears in the pre-edge region below the metal-insulator transition. More careful examination brings to light that the changes start already with pretransitional charge density wave fluctuations. Simple symmetry analysis suggests that the effect is related to rearrangements in $E_{g}$ and $A_{1g}$ states, and is compatible with the formation of four inequivalent V sites along the V-S chain.11/2011; -
Article: Slow magnetic dynamics and hysteresis loops of the bulk ferromagnet Co_ {7}(TeO_ {3}) _ {4} Br_ {6}
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ABSTRACT: Magnetic dynamics of a bulk ferromagnet, a new single crystalline compound Co7(TeO3)4Br6, was studied by ac susceptibility and related techniques. Very large Arrhenius activation energy of 17.2 meV (201 K) and long attempt time (2×10−4 s) span the full spectrum of magnetic dynamics inside a convenient frequency window, offering a rare opportunity for general studies of magnetic dynamics. Within the experimental window, the ac susceptibility data build almost ideally semicircular Cole-Cole plots. A comprehensive study of experimental dynamic hysteresis loops of the compound is presented and interpreted within a simple thermal-activation-assisted spin-lattice relaxation model for spin reversal. Quantitative agreement between the experimental results and the model’s prediction for dynamic coercive field is achieved by assuming the central physical quantity, the Debye relaxation rate, to depend on frequency, as well as on the applied field strength and sample temperature. Crossover between minor to major hysteresis loops is carefully analyzed. Low-frequency limitations of the model, relying on domain-wall pinning effects, are experimentally detected and appropriately discussed.Phys. Rev. B. 08/2011; 84(6). -
Article: Slow magnetic dynamics and hysteresis loops of a bulk ferromagnet
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ABSTRACT: Magnetic dynamics of a bulk ferromagnet, a new single crystalline compound Co7(TeO3)4Br6, was studied by ac susceptibility and the related techniques. Very large Arrhenius activation energy of 17.2 meV (201 K) and long attempt time (2x10^(-4)s) span the full spectrum of magnetic dynamics inside a convenient frequency window, offering a rare opportunity for general studies of magnetic dynamics. Within the experimental window the ac susceptibility data build almost ideally semicircular Cole-Cole plots. Comprehensive study of experimental dynamic hysteresis loops of the compound is presented and interpreted within a simple thermal-activation-assisted spin lattice relaxation model for spin reversal. Quantitative agreement between the experimental results and the model's prediction for dynamic coercive field is achieved by assuming the central physical quantity, the Debye relaxation rate, to depend on frequency, as well as on the applied field strength and sample temperature. Cross-over between minor- to major hysteresis loops is carefully analyzed. Low-frequency limitations of the model, relying on domain wall pinning effects, are experimentally detected and appropriately discussed.Physical Review B 07/2011; 84(06):064441. · 3.69 Impact Factor -
Article: Ground state of the quasi-1D compound BaVS3 resolved by resonant magnetic x-ray scattering.
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ABSTRACT: Resonant magnetic x-ray scattering near the vanadium L2,3-absorption edges has been used to investigate the low temperature magnetic structure of high quality BaVS3 single crystals. Below T(N)=31 K, the strong resonance revealed a triple-incommensurate magnetic ordering at the wave vector (0.226 0.226 ξ) in hexagonal notation, with ξ=0.033. The azimuthal-angle dependence of the scattering signal and time-dependent density functional theory simulations indicate an antiferromagnetic order within the ab plane with the spins polarized along a in the monoclinic structure.Physical Review Letters 04/2011; 106(16):167203. · 7.37 Impact Factor -
Article: Unified quantitative model for magnetic and electronic spectra of the undoped cuprates
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ABSTRACT: Using low-energy projection of the one-band t-t'-t"-Hubbard model we derive an effective spin-Hamiltonian and its spin-wave expansion to order 1/S. We fit the spin-wave dispersion of several parent compounds to the high-temperature superconducting cuprates: La2CuO4, Sr2CuO2Cl2 and Bi2Sr2YCu2O8. Our accurate quantitative determination of the one-band Hubbard model parameters allows prediction and comparison to experimental results of measurable quantities such as staggered moment, double occupancy density, spin-wave velocity and bimagnon excitation spectrum and density of states, which is discussed in relation to K-edge RIXS and Raman experiments.04/2011; -
Article: Energy and symmetry of dd excitations in undoped layered cuprates measured by Cu L3 resonant inelastic x-ray scattering
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ABSTRACT: We measured the high-resolution Cu L3 edge resonant inelastic x-ray scattering (RIXS) of undoped cuprates La2CuO4, Sr2CuO2Cl2, CaCuO2 and NdBa2Cu3O6. The dominant spectral features were assigned to dd excitations and we extensively studied their polarization and scattering geometry dependence. In a pure ionic picture, we calculated the theoretical cross sections for those excitations and used these to fit the experimental data with excellent agreement. By doing so, we were able to determine the energy and symmetry of Cu-3d states for the four systems with unprecedented accuracy and confidence. The values of the effective parameters could be obtained for the single-ion crystal field model but not for a simple two-dimensional cluster model. The firm experimental assessment of dd excitation energies carries important consequences for the physics of high-Tc superconductors. On the one hand, we found that the minimum energy of orbital excitation is always ≥1.4 eV, i.e. well above the mid-infrared spectral range, which leaves to magnetic excitations (up to 300 meV) a major role in Cooper pairing in cuprates. On the other hand, it has become possible to study quantitatively the effective influence of dd excitations on the superconducting gap in cuprates.New Journal of Physics 04/2011; 13(4):043026. · 4.18 Impact Factor -
Article: Energy and symmetry of dd excitations in undoped layered cuprates measured by Cu L3 resonant inelastic x-ray scattering
New Journal of Physics 04/2011; · 4.18 Impact Factor -
Article: Ground State of the Quasi-1D \bvs\ resolved by Resonant Magnetic X-ray Scattering
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ABSTRACT: Resonant-magnetic x-ray scattering (RMXS) near the vanadium $L_{2,3}$-absorption edges has been used to investigate the low temperature magnetic structure of high quality \bvs\ single crystals. Below $T_N$ = 31 K, the strong resonance revealed a triple-incommensurate magnetic ordering at wave vector (0.226 0.226 $\xi$) in the hexagonal notation, with $\xi$ = 0.033. The simulations of the experimental RMXS spectra with a time-dependent density functional theory indicate an antiferromagnetic order with the spins polarized along $a$ in the monoclinic structure.02/2011; -
Article: High pressure study of transport properties in Co$_{1/3}$NbS$_2$
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ABSTRACT: This is the first study of the effect of pressure on transition metal dichalcogenides intercalated by atoms that order magnetically. Co$_{1/3}$NbS$_2$ is a layered system where the intercalated Co atoms order antiferromagnetically at T$_N$ = 26 K at ambient pressure. We have conducted a detailed study of dc-resistivity ($\rho$), thermoelectric power (S) and thermal conductivity ($\kappa$). We found that at ambient pressure the magnetic transition corresponds to a well pronounced peak in dS/dT, as well as to a kink in the dc-resistivity. The effect of ordering on the thermal conductivity is rather small but, surprisingly, more pronounced in the lattice contribution than in the electronic contribution to $\kappa$. Under pressure, the resistivity increases in the high temperature range, contrary to all previous measurements in other layered transition metal dichalcogenides (TMD). In the low temperature range, the strong dependences of thermopower and resistivity on pressure are observed below TN, which, in turn, also depends on pressure at rate of dT$_N$/dp $\approx$ -1 K/kbar. Several possible microscopic explanations of the reduction of the ordering temperature and the evolution of the transport properties with pressure are discussed.Physical Review B 12/2010; 84(7):075157. · 3.69 Impact Factor -
Article: Measurement of magnetic excitations in the two-dimensional antiferromagnetic Sr₂CuO₂Cl₂ insulator using resonant x-ray scattering: evidence for extended interactions.
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ABSTRACT: We measured the momentum dependence of magnetic excitations in the model spin-1/2 2D antiferromagnetic insulator Sr2CuO2Cl2 (SCOC). We identify a single-spin-wave feature and a multimagnon continuum, with different polarization dependences. The spin waves display a large (70 meV) dispersion between the zone-boundary points (π, 0) and (π/2, π/2). Employing an extended t-t'-t''-U one-band Hubbard model, we find significant electronic hopping beyond nearest-neighbor Cu ions, indicative of extended magnetic interactions. The spectral line shape at (π, 0) indicates sizable quantum effects in SCOC and probably more generally in the cuprates.Physical Review Letters 10/2010; 105(15):157006. · 7.37 Impact Factor
Top Journals
Institutions
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1993–2013
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École Polytechnique Fédérale de Lausanne
- • Laboratoire de spectroscopie ultrarapide
- • Institut de physique de la matière condensée
Lausanne, VD, Switzerland
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2012
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Université Pierre et Marie Curie Paris 6
Paris, Ile-de-France, France
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2001–2009
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ETH Zurich
- Laboratory for Solid State Physics
Zürich, ZH, Switzerland
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2006
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Budapest University of Technology and Economics
- Department of Physics
Budapest, Budapest fovaros, Hungary
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2005
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Universität Stuttgart
Stuttgart, Baden-Wuerttemberg, Germany -
University of Toronto
- Department of Obstetrics and Gynaecology
Toronto, Ontario, Canada
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2004
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Mount Sinai Hospital
New York City, NY, USA
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2000
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University of Bayreuth
- Chair of Theoretical Physics I
Bayreuth, Bavaria, Germany
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1995
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École Polytechnique
Paris, Ile-de-France, France
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