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Helmuth Berger

Cette personne n’est plus à l’EPFL

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Publications associées (311)

Magnetic anisotropy of paramagnetic and ferromagnetically ordered state of single crystal BaVSe3

Helmuth Berger, László Forró, Ana Akrap

Anisotropic magnetism of single crystal BaVSe3 was Studied for the first time by the combination of the susceptibility and torque methods. As reported previously for polycrystalline sample, BaVSe3 undergoes ferromagnetic ordering below 43 K. Well above fer ...

2008

Fermi surface nesting in several transition metal dichalcogenides

Helmuth Berger

By means of high-resolution angle-resolved photoelectron spectroscopy (ARPES), we have studied the fermiology of 2H transition metal dichalcogenide polytypes TaSe2, NbSe2 and Cu0.2NbS2. The tight-binding model of the electronic structure, extracted from AR ...

2008

Nonlinear magnetic response in ruthenocuprates

Helmuth Berger, Ivica Zivkovic

We have performed an investigation of the nonlinear magnetic response in ruthenocuprates. A negative, diverging-like peak at the main magnetic transition T-N in RuSr2 RECu2O8 (RE = Gd, Y) indicates a possible canted antiferromagnetic order. Another well de ...

EDP Sciences, Springer2008

Muon spin spectroscopy evidence of a charge density wave in magnetite below the Verwey transition

Helmuth Berger

We present muon spectroscopy data on a Fe3O4 single crystal, revealing different spin precession patterns in five distinct temperature ranges. A careful analysis of the local field and its straightforward modeling obtains surprisingly good agreement with e ...

2008

Lattice anomalies and magnetic excitations of the spin web compound Cu3TeO6

Helmuth Berger

We report on the magnetic susceptibility and Raman scattering measurements of the S = 1/2 three-dimensional (3D) spin web compound Cu3TeO6. The magnetic susceptibility shows an antiferromagnetic ordering at T-N similar to 61 K and a deviation from the Curi ...

2008

Pseudogap-driven sign reversal of the Hall effect

Helmuth Berger

We present a calculation of the Hall coefficient in 2H -TaSe2 and 2H-Cu0.2NbS2 based on their electronic structure extracted from angle-resolved photoemission spectra. The well-known semiclassical approach, based on the solution of the Boltzmann equation, ...

2008

Temperature dependence of antiferromagnetic resonance mode in two-dimensional system Ni-5(TeO3)(4)Br-2

Helmuth Berger

Antiferromangentic resonance (AFMR) in layered Ni-5(TeO3)(4)Br-2 system with a geometrically frustrated Ni2+ (S =1) spin arrangement has been investigated. Temperature dependence of the lowest mode resonant field could be described with a simple two sublat ...

2008

Collective charge excitations below the metal-to-insulator transition in BaVS3

Helmuth Berger, László Forró, Ana Akrap

The charge response in barium vanadium sulfide (BaVS3) single crystals is characterized by dc resistivity and low-frequency dielectric spectroscopy. A broad relaxation mode in the megahertz range with a huge dielectric constant of approximate to 10(6) emer ...

2008

Atomic origin of the scanning tunneling microscopy images of charge-density-waves on 1T-TaSe2

Helmuth Berger

We show atomically resolved scanning tunneling microscopy (STM) images of charge density waves (CDWs) at room temperature together with angle-resolved photoelectron band-mapping of 1T-TaSe2. By comparing the results of these two techniques, we demonstrate ...

2008

The magnetic state of the low dimensional CuTe2O5 compound below 20 K

Helmuth Berger

We report low field studies of the static magnetic susceptibility and torque measurements in the temperature range 2-330 K for the powder and crystalline CuTe2O5 compound. We show that the observed extraordinarily large magnetic axis rotation below 20 K an ...

2008

Pseudogap and charge density waves in two dimensions

Helmuth Berger

Using angle-resolved photoemission spectroscopy we demonstrate that a normal-state pseudogap exists above TN-IC in one of the most studied two-dimensional charge-density wave (CDW) dichalcogenides 2H-TaSe2. The initial formation of the incommensurate CDW i ...

2008

From Mott state to superconductivity in 1T-TaS2

Helmuth Berger, László Forró, Eduard Tutis, Ana Akrap, Balazs Sipos

The search for the coexistence between superconductivity and other collective electronic states in many instances promoted the discovery of novel states of matter. The manner in which the different types of electronic order combine remains an ongoing puzzl ...

2008

Influence of chemical substitutions on the charge instability of BaVS3

Helmuth Berger, László Forró, Ana Akrap

We study the influence of Sr substitutions on the structural counterpart of the MI transition of BaVS3. When Sr content increases, the commensurate CDW modulation of pure BaVS3 is changed into an incommensurate short range modulation that we attribute to a ...

2008

Temperature dependent phonon dispersion in 2H-NbSe2 investigated using inelastic neutron scattering

Helmuth Berger

The longitudinal low frequency acoustic and optic phonon modes of 2H-NbSe2 have been determined for the Gamma - M direction with inelastic neutron scattering at 50 K close to the charge density wave transition temperature. ...

2008

Influence of point defects on the metal-insulator transition in BaVS3

Helmuth Berger, László Forró, Aurore Rudolf, Ana Akrap

2008

Femtosecond dynamics of electronic states in the Mott insulator 1T-TaS2 by time resolved photoelectron spectroscopy

Helmuth Berger, Luca Perfetti, Matthias Wolf

Photoexcitation of the Mott insulator 1T-TaS2 by an intense laser pulse leads to an ultrafast transition toward a gapless phase. Besides the collapse of the electronic gap, the sudden excitation of the charge density wave (CDW) mode results in periodic osc ...

2008

Magnetic structure of the S=1Ni(5)(TeO3)(4)Br-2 layered system governed by magnetic anisotropy

Helmuth Berger

Magnetization, neutron diffraction, and antiferromagnetic resonance measurements were employed to investigate the magnetic ground state of Ni-5(TeO3)(4)Br-2 single crystal. Despite the layered topology of the Ni2+ sites, typical for frustrated antiferromag ...

2007

Experimental evidence for a two-gap structure of superconducting NbSe2: A specific-heat study in external magnetic fields

Helmuth Berger

To resolve the discrepancies of the superconducting order parameter in quasi-two-dimensional NbSe2, comprehensive specific-heat measurements have been carried out. By analyzing both the zero-field and mixed-state data with magnetic fields perpendicular (H ...

2007

High-pressure infrared spectroscopy: Tuning of the low-energy excitations in correlated electron systems

Helmuth Berger, László Forró, Richard Gaal, Balazs Sipos

We have extended the range of the high-pressure optical spectroscopy to the far-infrared region keeping the accuracy of ambient-pressure experiments. The developed method offers a powerful tool for the study of pressure-induced phase transitions and electr ...

2007

Photoemission and optical studies of ZrSe3, HfSe3, and ZrS3

Helmuth Berger, Marco Grioni, Daniela Pacilè

Angle-resolved photoemission spectroscopy (ARPES) and optical measurements were performed on single crystal samples of ZrSe3, HfSe3, and ZrS3, which belong to the class of low-dimensional band insulators. By ARPES, we traced the dispersion of the (S 3p, Se ...

2007

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