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Helmuth Berger

Cette personne a quitté l’EPFL

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Publications associées (298)

Page 4 sur 15

Helical fluctuations in the Raman response of the topological insulator Bi2Se3

Helmuth Berger

The topological insulator Bi2Se3 shows a Raman-scattering (RS) response related to topologically protected surface states amplified by a resonant interband transition. Most significantly this signal has a characteristic Lorentzian lineshape and spin-helica ...

2011

Incommensurate magnetic ordering in Cu2Te2O5X2 (X=Cl,Br) studied by single crystal neutron diffraction

Helmuth Berger, Henrik Moodysson Rønnow, Joël Mesot

Polarized and unpolarized neutron-diffraction studies have been carried out on single crystals of the coupled spin tetrahedra systems Cu2Te2O5X2 (X=Cl, Br). A model of the magnetic structure associated with the propagation vectors k(') (Cl) approximate to ...

2006

Vanishing of the metal-insulator Peierls transition in pressurized BaVS3

Helmuth Berger

BaVS3 presents a metal-to-insulator (MI) transition at ambient pressure due to the stabilization of a 2k(F) commensurate charge density wave (CDW) Peierls ground state built on the dz(2) V orbitals. The MI transition vanishes under pressure at a quantum cr ...

Amer Physical Soc2012

Plasmon evolution and charge-density wave suppression in potassium intercalated 2H-TaSe2

Helmuth Berger

We have investigated the influence of potassium intercalation on the formation of the charge-density wave (CDW) instability in 2H-TaSe2 by means of electron energy-loss spectroscopy and density functional theory. Our observations are consistent with a fill ...

IOP Publishing - EPL Association2012

Revealing the role of electrons and phonons in the ultrafast recovery of charge density wave correlations in 1T-TiSe2

Matthias Wolf, Helmuth Berger, Arnaud Magrez

Using time-and angle-resolved photoemission spectroscopy with selective near-and midinfrared photon excitations, we investigate the femtosecond dynamics of the charge density wave ( CDW) phase in 1T-TiSe2 as well as the dynamics of CDW fluctuations at 240 ...

Amer Physical Soc2016

Unusual Shubnikov-de Haas oscillations in BiTeCl

Helmuth Berger, Arnaud Magrez, Philippe Bugnon

We report measurements of Shubnikov-de Haas oscillations in single crystals of BiTeCl at magnetic fields up to 31 T and at temperatures as low as 0.4 K. Two oscillation frequencies were resolved at the lowest temperatures, F-1 = 65 +/- 4 T and F-2 = 156 +/ ...

Amer Physical Soc2014

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

From nanotubes to single crystals: Co doped TiO2

Helmuth Berger, László Forró, Arnaud Magrez, Bálint Náfrádi, Endre Horvath, Richard Gaal, Jacim Jacimovic, Neda Nikseresht Ghanepour

Millimeter-sized cobalt doped rutile crystals with a bi-pyramidal shape are obtained by chemical vapour transport using scroll-type H2Ti3O7 nanotubes as a precursor in which Co2+ ions are introduced by a simple ion exchange method prior to the growth. Desp ...

Amer Inst Physics2013

Archetypal Soft-Mode-Driven Antipolar Transition in Francisite Cu3Bi(SeO3)(2)O2Cl

Helmuth Berger

Model materials are precious test cases for elementary theories and provide building blocks for the understanding of more complex cases. Here, we describe the lattice dynamics of the structural phase transition in francisite Cu3Bi(SeO3)(2)O2Cl at 115 K and ...

2020

Origin and Magnitude of 'Designer' Spin-Orbit Interaction in Graphene on Semiconducting Transition Metal Dichalcogenides

Helmuth Berger, Zhe Wang

We use a combination of experimental techniques to demonstrate a general occurrence of spin-orbit interaction (SOI) in graphene on transition metal dichalcogenide (TMD) substrates. Our measurements indicate that SOI is ultrastrong and extremely robust, des ...

American Physical Society2016

Transport and magnetic properties of BaVSe3

Helmuth Berger, László Forró, Ana Akrap, Neven Barisic

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

Two-step transition in a magnetoelectric ferrimagnet Cu2OSeO3

Helmuth Berger, Ivica Zivkovic

We report a detailed single-crystal investigation of a magnetoelectric ferrimagnet Cu2OSeO3 using dc magnetization and ac susceptibility along the three principal directions [100], [110], and [111]. We have observed that in small magnetic fields two magnet ...

2012

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

Helmuth Berger, Marco Grioni, Thorsten Schmitt, Xiaoqiang 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

Elementary excitation in the spin-stripe phase in quantum chains

Helmuth Berger, Jonathan White

Elementary excitations in condensed matter capture the complex many-body dynamics of interacting basic entities in a simple quasiparticle picture. In magnetic systems the most established quasiparticles are magnons, collective excitations that reside in or ...

Springer2019

Nonuniform and coherent motion of superconducting vortices in the picometer-per-second regime

Helmuth Berger

We investigated vortex dynamics in a single-crystal sample of type-II superconductor NbSe2 using scanning tunneling microscopy at 4.2 K. The decay of the magnetic field at a few nT/s in our superconducting magnet induced the corresponding motion of vortice ...

2011

Local resilience of the 1T-TiSe2 charge density wave to Ti self-doping

Helmuth Berger

In Ti-intercalated self-doped 1T-TiSe2 crystals, the charge density wave (CDW) superstructure induces two nonequivalent sites for Ti dopants. Recently, it has been shown that increasing Ti doping dramatically influences the CDW by breaking it into phase-sh ...

Amer Physical Soc2017

Time-resolved ARPES at LACUS: Band Structure and Ultrafast Electron Dynamics of Solids

Majed Chergui, Helmuth Berger, Arnaud Magrez, Marco Grioni, Frank van Mourik, Alberto Crepaldi, Christopher Alexander Arrell, Philippe Bugnon, Gianmarco Gatti, José Javier Pancracio Ojeda Andara, Silvan Dino Marcos Roth

The manipulation of the electronic properties of solids by light is an exciting goal, which requires knowledge of the electronic structure with energy, momentum and temporal resolution. Time- and angle-resolved photoemission spectroscopy (tr-ARPES) is the ...

2017

Influence of Dimensionality on the Charge Density Wave Phase of 2H-TaSe2

Helmuth Berger

Metallic transition metal dichalcogenides like tantalum diselenide (TaSe2) exhibit exciting behaviors at low temperatures, including the emergence of charge density wave (CDW) states. In this work, density functional theory (DFT) is used to catalog influen ...

WILEY-V C H VERLAG GMBH2022

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

Giant Ambipolar Rashba Effect in the Semiconductor BiTeI

Klaus Kern, Helmuth Berger, Marco Grioni, Oleg Yazyev, Philippe Bugnon, Alberto Crepaldi, Luca Moreschini, Gabriel Albert Autes, Simon Karl Moser, Naunidh Singh Virk, Cédric Tournier-Colletta

We observe a giant spin-orbit splitting in the bulk and surface states of the noncentrosymmetric semiconductor BiTeI. We show that the Fermi level can be placed in the valence or in the conduction band by controlling the surface termination. In both cases, ...

Amer Physical Soc2012