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Helmuth Berger

Cette personne n’est plus à l’EPFL

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Publications associées (311)

Magnetodielectric coupling of infrared phonons in single-crystal Cu2OSeO3

Helmuth Berger

Reflection and transmission as a function of temperature (5-300 K) have been measured on a single crystal of the magnetoelectric ferrimagnetic compound Cu2OSeO3 utilizing light spanning the far infrared to the visible portions of the electromagnetic spectr ...

2010

Colossal electroresistance and colossal magnetoresistance in spinel multiferroic CdCr2S4

Helmuth Berger

Colossal magnetoresistance (CMR) and colossal electroresistance (CER) induced by the electric field in spinel multiferroic CdCr2S4 are reported. It is found that a metal-insulator transition in CdCr2S4 is triggered by the electrical field. In magnetic fiel ...

AIP American Institute of Physics2010

Weak magnetism in insulating and superconducting cuprates

Helmuth Berger

X-ray magnetic circular dichroism provides evidence of an out-of-plane spin moment in undoped and doped cuprates. In La2CuO4 this moment is related to the canting of the antiferromagnetically ordered Cu2+ spins caused by the Dzyaloshinskii-Moriya interacti ...

2010

Self-energy effects and electron-phonon coupling in Fe-As superconductors

Helmuth Berger

Doping and temperature dependent studies of optical phonon modes in Fe-122 pnictides are performed using Raman scattering experiments and compared with model calculations to elucidate the role of electron-phonon and spin-phonon interaction in this family o ...

2010

Charge ordering in substituted and non-stoichiometric BaVS3

Helmuth Berger, László Forró, Ana Akrap

Among the correlated electronic systems, BaVS3 which exhibits both itinerant and localized states as well as a subtle interplay betwen charge, orbital, spin and lattice degrees of freedom, is a model system. Its electronic structure consists in a broad qua ...

2010

Probing the exciton condensate phase in 1T-TiSe2 with photoemission

Helmuth Berger, Philipp Aebi

We present recent results obtained using angle-resolved photoemission spectroscopy performed on 1T-TiSe2. Emphasis is put on the peculiarity of the bandstructure of TiSe2 compared to other transition metal dichalcogenides, which suggests that this system i ...

2010

Measurement of Magnetic Excitations in the Two-Dimensional Antiferromagnetic Sr2CuO2Cl2 Insulator Using Resonant X-Ray Scattering: Evidence for Extended Interactions

Helmuth Berger, Marco Grioni, Bastien Dalla Piazza, Ping-Hui Lin, Peng Xu, Henrik Moodysson Rønnow, Marco Guarise, Thorsten Schmitt, Dirk Van der Marel

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 ...

American Physical Society2010

Observation of a High-Field Magnetic Transition of the Two-Dimensional Spin-Frustrated System Ni-5(TeO3)(4)Br-2 up to 120 T

Helmuth Berger

2010

High-pressure study of anatase TiO2

Helmuth Berger, László Forró, Richard Gaal, Arnaud Magrez, Jacim Jacimovic, George Cristian Vaju

We report resistivity and thermo-electric power measurements of the anatase phase of TiO2 under pressure up to 2.3 GPa. Despite its transparent appearance, the single crystal of anatase exhibits a metallic-like resistivity above 60 K, at all pressures. The ...

2010

Snapshots of cooperative atomic motions in the optical suppression of charge density waves

Helmuth Berger

Macroscopic quantum phenomena such as high-temperature superconductivity, colossal magnetoresistance, ferrimagnetism and ferromagnetism arise from a delicate balance of different interactions among electrons, phonons and spins on the nanoscale(1). The stud ...

2010

Superconducting phase in the layered dichalcogenide 1T-TaS_{2} upon inhibition of the metal-insulator transition

Helmuth Berger, László Forró, Marco Grioni, Balazs Sipos, Julian Piatek, Ping-Hui Lin, Peng Xu, Henrik Moodysson Rønnow

When a Mott metal-insulator transition is inhibited by a small amount of disorder in the layered dichalcogenide 1T-TaS2, an inhomogeneous superconducting state arises below T=2.1 K and coexists with a nearly commensurate charge-density wave. By angle-resol ...

2010

Magnetic phase diagram of the two-dimensional antiferromagnet Ni-5(TeO3)(4)Br-2

Helmuth Berger

Phase diagram of the two-dimensional antiferromagnet Ni-5(TeO3)(4)Br-2 with triangular arrangement of Ni2+ (S=1) magnetic moments within the [Ni5O17Br2] subunits has been investigated by temperature and magnetic field dependent heat-capacity, magnetization ...

2009

Ferromagnetism in Co-7(TeO3)(4)Br-6: A byproduct of complex antiferromagnetic order and single-ion anisotropy

Helmuth Berger, Ivica Zivkovic

Pronounced anisotropy of magnetic properties and complex magnetic order of a new oxi-halide compound Co-7(TeO3)(4)Br-6 has been investigated by powder and single-crystal neutron-diffraction, magnetization, and ac susceptibility techniques. Anisotropy of su ...

2009

Anomalous frequency and intensity scaling of collective and local modes in a coupled-spin tetrahedral system Cu2Te2O5Cl2

Helmuth Berger

We report on the magnetic excitation spectrum of the coupled-spin tetrahedral system Cu2Te2O5Cl2 using Raman scattering on single crystals. The transition to an ordered state at T-N(Cl)=18.2 K evidenced from thermodynamic data leads to the evolution of dis ...

2009

Field-dependent dielectric and magnetic properties in multiferroic CdCr2S4

Helmuth Berger

Based on the results of field-dependent dielectric and magnetic measurements, we observe several interesting behaviors and phase transitions in this geometrically frustrated spinel system CdCr2S4. (1) A glassy dipolar state occurs near T-C similar to 85 K, ...

2009

Infrared investigation of the phonon spectrum in the frustrated spin cluster compound FeTe2O5Cl

Helmuth Berger, László Forró

We present our optical investigations on the frustrated spin cluster compound FeTe2O5Cl, which develops a long-range antiferromagnetic order below 10 K. We measure the optical reflectivity from the far-infrared to the ultraviolet with polarized light. We f ...

2009

Separation of the Oxide and Halide Part in the Oxohalide Fe3Te3O10Cl Due to High Lewis Acidity of the Cations

Helmuth Berger, Dong Zhang

A new iron tellurite halide Fe3Te3O10Cl has been identified that crystallizes in the monoclinic space group P2(1)/c. The crystal structure comprises channels where the Cl ions and also the stereochemically active lone-pair electrons on Te4+ are located. Th ...

2009

Spin Amplitude Modulation Driven Magnetoelectric Coupling in the New Multiferroic FeTe2O5Br

Helmuth Berger

The magnetic and ferroelectric properties of the layered geometrically frustrated cluster compound FeTe2O5Br were investigated with single-crystal neutron diffraction and dielectric measurements. An incommensurate transverse amplitude modulated magnetic or ...

2009

Temperature-dependent Fermi surface of 2H-TaSe2 driven by competing density wave order fluctuations

Helmuth Berger

Temperature evolution of the 2H-TaSe2 Fermi surface (FS) is studied by high-resolution angle-resolved photoemission spectroscopy. High-accuracy determination of the FS geometry was possible after measuring electron momenta and velocities along all high-sym ...

2009

Splitting in the Fermi surface of ZrTe3: A surface charge density wave system

Helmuth Berger, Corsin Battaglia

The electronic band structure and Fermi surface of ZrTe3 was determined by angle-resolved photoemission spectroscopy. Several bands and a large part of the Fermi surface are found to be split by 100-200 meV into two parallel dispersions. Calculations of th ...

2009

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