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Helmuth Berger

Cette personne n’est plus à l’EPFL

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Publications associées (311)

High-field magnetization of a two-dimensional spin frustration system, Ni-5(TeO3)(4)X-2 (X = Br, Cl)

Helmuth Berger

The high-field magnetization, M(H), of Ni-5(TeO3)(4)X-2 (X = Br, Cl) was measured by using a pulse magnet. These compounds have a two-dimensional crystal structure and a distorted Kagome spin frustrated system which is built from the Ni2+ ions (S = 1). The ...

2009

Interplay of electronic correlations and lattice instabilities in BaVS3

Helmuth Berger

The quasi-one-dimensional metallic system BaVS3 with a metal-insulator transition at T-MI=70 K shows large changes in the optical phonon spectrum, a central peak, and an electronic Raman scattering continuum that evolve in a three-step process. Motivated b ...

2009

Giant Kohn Anomaly and the Phase Transition in Charge Density Wave ZrTe3

Helmuth Berger

A strong Kohn anomaly in ZrTe3 is identified in the mostly transverse acoustic phonon branch along the modulation vector q(P) with polarization along the a(*) direction. This soft mode freezes to zero frequency at the transition temperature T-P, and the te ...

2009

Pressure Induced Superconductivity in Pristine 1T-TiSe2

Helmuth Berger, László Forró, Eduard Tutis, Balazs Sipos

The interplay between superconductivity and the charge-density wave (CDW) state in pure 1T-TiSe2 is examined through a high-pressure study extending up to pressures of 10 GPa between sub-Kelvin and room temperatures. At a critical pressure of 2 GPa a super ...

2009

Two Energy Gaps and Fermi-Surface "Arcs" in NbSe2

Helmuth Berger

Using angle-resolved photoemission spectroscopy, we report on the direct observation of the energy gap in 2H-NbSe2 caused by the charge-density waves (CDW). The gap opens in the regions of the momentum space connected by the CDW vectors, which implies a ne ...

2009

Spontaneous exciton condensation in 1T-TiSe2: BCS-like approach

Helmuth Berger, László Forró, Philipp Aebi, Corsin Battaglia

Recently we found strong evidence in favor of a BCS-like condensation of excitons in 1T-TiSe2 [Cercellier , Phys. Rev. Lett. 99, 146403 (2007)]. Theoretical photoemission intensity maps have been generated by the spectral function calculated within the exc ...

2009

Negative plasmon dispersion in the transition-metal dichalcogenide 2H-TaSe2

Helmuth Berger

We report about an electron energy-loss study on the dispersion of the charge-carrier plasmon in the charge-density-wave system 2H-TaSe2 above and below the metal-insulator transition. In both cases we find a negative slope, which is in remarkable contrast ...

2009

Nonmonotonic pseudogap in high-T-c cuprates

Helmuth Berger

The mechanism of high-temperature superconductivity has not been resolved for so long because the normal state of cuprates, which exhibits enigmatic pseudogap phenomena, is not yet understood. We performed careful temperature- and momentum-resolved photoem ...

2009

Metastability of two-dimensional vortex glass in Bi2Sr2CaCu2O8+delta

Helmuth Berger, László Forró

The decoupled layer quasi-two-dimensional glass phase of c-axis vortices in optimally doped Bi2Sr2CaCu2O8+delta (BSCCO) exhibits low-temperature metastability, the onset temperature of which is coincident with the peak in depinning current for zero-field-c ...

2009

Electronic Raman scattering in magnetite: Spin versus charge gap

Helmuth Berger

We report Raman-scattering data of single crystals of magnetite (Fe3O4) with Verwey transition temperatures (T-v) of 123 and 117 K, respectively. Both single crystals reveal broad electronic background extending up to 900 wave numbers (similar to 110 meV). ...

2009

Electronic structure of pyrochlore Cd2Re2O7

Helmuth Berger

Detailed band structure calculations have been performed for Cd2Re2O7 in high-, middle- and low-temperature (T) phases. The calculations are based on the observed lattice structures from x-ray diffraction measurements. The spin-orbit interaction is incorpo ...

2009

Raman studies of doped magnetite above and below the Verwey transition

Helmuth Berger

The nature of the Verwey transition in magnetite (Fe3O4) remains an unsolved puzzle after more than 60 years. Impurities and dopants present in magnetite can alter the Verwey transition temperature T-v as well as other physical characteristics of magnetite ...

American Institute of Physics2009

Crystal field and low energy excitations measured by high resolution RIXS at the L-3 edge of Cu, Ni and Mn

Helmuth Berger, Marco Grioni, Xiaoqiang Wang, Thorsten Schmitt, Xiao Wang

Resonant inelastic x-ray scattering in the soft x-ray regime has been profiting much from technical advances that have lowered considerably the instrumental linewidth. At the ADRESS beam line of the Swiss Light Source the SAXES spectrometer can be used to ...

Springer Verlag2009

Observation of Two Nondispersive Magnetic Excitations in NiO by Resonant Inelastic Soft-X-Ray Scattering

Helmuth Berger, Marco Grioni, Xiaoqiang Wang, Thorsten Schmitt, Xiao Wang

We present high resolution (Delta E=120 meV) resonant inelastic x-ray scattering data measured at the Ni L-3 edge (2p(3/2)-> 3d) on the paradigmatic antiferromagnetic oxide NiO. Spectra reveal clear signatures of magnetic excitations at similar to 95 and s ...

American Physical Society2009

Anomalous Magnetic Excitations of Cooperative Tetrahedral Spin Clusters

Helmuth Berger, Krunoslav Prsa, Henrik Moodysson Rønnow, Joël Mesot, Johan Juul Chang

An inelastic neutron scattering study of Cu2Te2O5X2 (X=Cl, Br) shows strong dispersive modes with large energy gaps persisting far above T-N, notably in Cu2Te2O5Br2. The anomalous features: a coexisting unusually weak Goldstone-like mode observed in Cu2Te2 ...

2009

Magnetic and resonance properties of the two-dimensional S=1 compound Ni-5(TeO3)(4)Cl-2 with frustrated geometry

Helmuth Berger

The magnetic and magnetoresonance properties of a new single-crystal compound Ni-5(TeO3)(4)Cl-2 are studied. Measurements of the magnetization and magnetic susceptibility of the crystal in a wide temperature range (5-300 K) make it possible to conclude tha ...

2008

Excitation energy map of high-energy dispersion anomalies in cuprates

Helmuth Berger

The anomalous high-energy dispersion of the conductance band in the high-T-c superconductor Bi(Pb)(2)Sr2CaCu2O8+delta (Pb-Bi2212) has been extensively mapped by angle-resolved photoemission spectroscopy as a function of excitation energy in the range from ...

2008

Differential-thermal analysis around and below the critical temperature T-c of various low-T-c superconductors: A comparative study

Helmuth Berger

We present specific-heat data on the type-II superconductors V3Si, LuNi2B2C and NbSe2 which were acquired with a low-temperature thermal analysis (DTA) technique. We compare our data with available literature data on these superconductors. In the first par ...

2008

Transport and magnetic properties of BaVSe3

Helmuth Berger, László Forró, Neven Barisic, Ana Akrap, Vladan Stevanovic

We report a comprehensive study of transport, magnetotransport, and magnetic properties of single crystals of BaVSe3. The paramagnetic metal-ferromagnetic metal transition at 43 K was followed as a function of pressure by measuring the electrical resistivi ...

2008

Magnetic crossover and complex excitation spectrum of the ferromagnetic/antiferromagnetic spin-1/2 chain system alpha-TeVO4

Helmuth Berger

We report magnetic-susceptibility, specific-heat, and Raman-scattering investigations of alpha-TeVO4 containing V-O edge-sharing chains. These chains promote a system of ferromagnetic/antiferromagnetic spin-1/2 Heisenberg alternating exchange chains with p ...

2008

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