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Helmuth Berger

Cette personne n’est plus à l’EPFL

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Publications associées (311)

Raman study of the Verwey transition in magnetite at high-pressure and low-temperature: Effect of Al doping

Helmuth Berger

We employed Raman spectroscopy to investigate how the hydrostatic pressure affects the temperature of the Verwey transition in pure (Fe3O4) and Al-doped (Fe2.8Al0.2O4) magnetite. In both samples, pressure suppresses the transition. The Al-doped sample disp ...

Amer Inst Physics2012

Infrared phonon anomaly and magnetic excitations in single-crystal Cu3Bi(SeO3)(2)O2Cl

Helmuth Berger

Infrared reflection and transmission as a function of temperature have been measured on single crystals of Cu3Bi(SeO3)(2)O2Cl. The complex dielectric function and optical properties along all three principal axes of the orthorhombic cell were obtained via ...

Amer Physical Soc2012

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

Giant Ambipolar Rashba Effect in the Semiconductor BiTeI

Helmuth Berger, Philippe Bugnon, Klaus Kern, Marco Grioni, Oleg Yazyev, Luca Moreschini, Alberto Crepaldi, Simon Karl Moser, Gabriel Albert Autes, 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

Persistent Spin Dynamics Intrinsic to Amplitude-Modulated Long-Range Magnetic Order

Helmuth Berger

An incommensurate elliptical helical magnetic structure in the frustrated coupled-spin-chain system FeTe2O5Br is surprisingly found to persist down to 53(3) mK (T/T-N similar to 1/200), according to neutron scattering and muon spin relaxation. In this stat ...

Amer Physical Soc2012

Quantitative Determination of the Band Gap of WS2 with Ambipolar Ionic Liquid-Gated Transistors

Helmuth Berger

We realized ambipolar field-effect transistors by coupling exfoliated thin flakes of tungsten disulfide (WS2) with an ionic liquid dielectric. The devices show ideal electrical characteristics, including very steep subthreshold slopes for both electrons an ...

Amer Chemical Soc2012

Low-dimensional magnetism of spin-1/2 chain systems of alpha- and beta-TeVO4: A comparative study

Helmuth Berger

We present a comparative study of the low-dimensional compounds alpha- and beta-TeVO4. Our data clearly show that the change in the local coordination geometry of V4+ ions between alpha- and beta-TeVO4 leads to drastic differences in their magnetic propert ...

Amer Inst Physics2012

Multiferroic FeTe2O5Br: Alternating spin chains with frustrated interchain interactions

Helmuth Berger

A combination of density functional theory calculations, many-body model considerations, and magnetization and electron-spin-resonance measurements shows that the multiferroic FeTe2O5Br should be described as a system of alternating antiferromagnetic S = 5 ...

Amer Physical Soc2012

Ultrafast photodoping and effective Fermi-Dirac distribution of the Dirac particles in Bi2Se3

Helmuth Berger, Philippe Bugnon, Klaus Kern, Marco Grioni, Alberto Crepaldi

We exploit time- and angle-resolved photoemission spectroscopy to determine the evolution of the out-of-equilibrium electronic structure of the topological insulator Bi2Se3. The response of the Fermi-Dirac distribution to ultrashort IR laser pulses has bee ...

Amer Physical Soc2012

Pressure dependence of the large-polaron transport in anatase TiO2 single crystals

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

We have measured the pressure and temperature dependences of the resistivity and the thermoelectric power of anatase, TiO2. The resistivity varies with T-3 at high temperatures, and its absolute value is in the 1 Omega cm range. Below 60 K, the resistivity ...

IOP Publishing - EPL Association2012

Three-Dimensional Electron Realm in VSe2 by Soft-X-Ray Photoelectron Spectroscopy: Origin of Charge-Density Waves

Helmuth Berger, Xiaoqiang Wang, Thorsten Schmitt, Vladimir N. Strocov

The resolution of angle-resolved photoelectron spectroscopy (ARPES) in three-dimensional (3D) momentum k is fundamentally limited by ill defined surface-perpendicular wave vector k(perpendicular to) associated with the finite photoelectron mean free path. ...

Amer Physical Soc2012

Crystal Structure and Magnetic Properties of Two New Antiferromagnetic Spin Dimer Compounds; FeTe3O7X (X = Cl, Br)

Helmuth Berger, Dong Zhang, Jia Liu

Two new isostructural layered oxohalides FeTe3O7X (X = Cl, Br) were synthesized by chemical vapor transport reactions, and their crystal structures and magnetic properties were characterized by single-crystal X-ray diffraction, Raman spectroscopy, magnetic ...

2011

Low-temperature magnetic and thermal properties of the frustrated two-dimensional S=1 compound Ni-5(TeO3)(4)Cl-2

Helmuth Berger

The temperature and magnetic field dependent magnetization of the two-dimensional quantum spin system Ni-5(TeO3)(4)Cl-2 has been investigated using single crystals in temperature range 1.8-100 K and in magnetic fields up to 14 T. The magnetization below th ...

2011

Helical fluctuations in the Raman response of the topological insulator Bi2Se3

Helmuth Berger, Kazimierz Conder

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

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

Helmuth Berger, Hui Wang

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

Slow magnetic dynamics and hysteresis loops of the bulk ferromagnet Co-7(TeO3)(4)Br-6

Helmuth Berger, Ivica Zivkovic

Magnetic dynamics of a bulk ferromagnet, a new single crystalline compound Co-7(TeO3)(4)Br-6, was studied by ac susceptibility and related techniques. Very large Arrhenius activation energy of 17.2 meV (201 K) and long attempt time (2 x 10(-4) s) span the ...

2011

Magnetic phase diagram of the antiferromagnetic cobalt tellurate Co3TeO6

Helmuth Berger, Wei Li

In this study, we measured the temperature-dependent magnetic susceptibility [chi(T)], ac magnetic susceptibility [chi(ac)(T)], specific heat [C-p(T)], and high-field magnetization [M(H)] of a newly synthesized single-crystalline material, Co3TeO6. Two tra ...

2011

Singlet-Triplet Excitations and High-Field Magnetization in CuTe2O5

Helmuth Berger, Zhe Wang

By measuring the THz electron spin resonance (ESR) transmission spectra and high-field magnetization on the spin-gapped system CuTe2O5, we identified the singlet-triplet excitations in the dimerized non-magnetic ground state. The determined spin-gap value ...

2011

Energy and symmetry of dd excitations in undoped layered cuprates measured by Cu L-3 resonant inelastic x-ray scattering

Helmuth Berger, Marco Grioni, Marco Guarise, Thorsten Schmitt

We measured the high-resolution Cu L-3 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 polarizat ...

Institute of Physics (IoP) and Deutsche Physikalische Gesellschaft2011

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