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Helmuth Berger

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Related publications (311)

THz Generation and Detection on Dirac Fermions in Topological Insulators

Helmuth Berger, Hua Chen

Wiley-V C H Verlag Gmbh2013

Evidence of reduced surface electron-phonon scattering in the conduction band of Bi2Se3 by nonequilibrium ARPES

Helmuth Berger, Philippe Bugnon, Klaus Kern, Marco Grioni, Alberto Crepaldi, Jens Christian Johannsen

The nature of the Dirac quasiparticles in topological insulators calls for a direct investigation of the electron-phonon scattering at the surface. By comparing time-resolved ARPES measurements of the topological insulator Bi2Se3 with different probing dep ...

Amer Physical Soc2013

High-Pressure Phase Transitions and Structures of Topological Insulator BiTel

Helmuth Berger, Feng Peng, Hui Wang

Being a giant bulk Rashba semiconductor, the ambient-pressure phase of BiTeI was predicted to transform into a topological insulator under pressure at 1.7-4.1 GPa [Nat. Commun. 2012, 3, 679]. Because the structure governs the new quantum state of matter, i ...

Amer Chemical Soc2013

Complex magnetic couplings in Co3TeO6

Helmuth Berger

We report powder and single-crystal neutron diffractionmeasurements, combinedwith x-ray powder diffraction data, to unravel the complex magnetic phase diagram and exchange coupling in Co3TeO6. The magnetic structures of the various phases differ markedly f ...

Amer Physical Soc2013

Evolution of magnetic and crystal structures in the multiferroic FeTe2O5Br

Helmuth Berger

Neutron diffraction and nuclear quadrupole resonance (NQR) measurements were employed to investigate magnetic order in the nonferroelectric phase preceding the low-temperature multiferroic state in FeTe2O5Br. Refinement of the neutron diffraction data and ...

American Physical Society2013

Electronic Instability in a Zero-Gap Semiconductor: The Charge-DensityWave in (TaSe4)(2)I

Helmuth Berger, Marco Grioni, Oleg Yazyev, Luca Moreschini, Alberto Crepaldi, Simon Karl Moser, Gabriel Albert Autes, Cédric Tournier-Colletta

We report a comprehensive study of the paradigmatic quasi-1D compound (TaSe4)(2)I performed by means of angle-resolved photoemission spectroscopy (ARPES) and first-principles electronic structure calculations. We find it to be a zero-gap semiconductor in t ...

American Physical Society2013

Analysis of the low-frequency spectrum of the cubic noncentrosymmetric ferrimagnet Cu2OSeO3

Helmuth Berger

The IR reflectance spectra of the cubic noncentrosymmetric ferrimagnet Cu2OSeO3 (T-C approximate to 60 K) were investigated in the temperature range 10-80 K. Detailed analysis of the symmetry and composition of vibrational modes was conducted on the basis ...

2012

Magnetoelectric effects in single crystals of the cubic ferrimagnetic helimagnet Cu2OSeO3

Jean-Philippe Ansermet, Helmuth Berger

We present magnetodielectric measurements in single crystals of the cubic spin-1/2 compound Cu2OSeO3. A magnetic-field-induced electric polarization (P) and a finite magnetocapacitance (MC) is observed at the onset of the magnetically ordered state (T-c = ...

2012

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

Interplay between the magnetic and electric degrees of freedom in multiferroic Co3TeO6

Helmuth Berger, Yang Zhao

Neutron diffraction, magnetic susceptibility, specific heat, and dielectric constant measurements of single-crystal Co3TeO6 have been measured to study the interplay between the ferroelectricity and magnetic order. Long-range incommensurate magnetic order ...

2012

Anomalously enhanced photoemission from the Dirac point and other peculiarities in the self-energy of the surface-state quasiparticles in Bi2Se3

Helmuth Berger

Line-shape analysis of the photoemission intensity from the surface states of Bi2Se3 reveals two unusual features: spectral line asymmetry and anomalously enhanced photoemission from the Dirac point. The former can be described by the one-particle spectral ...

2012

Magnetoelectric Coupling in Single Crystal Cu2OSeO3 Studied by a Novel Electron Spin Resonance Technique

Helmuth Berger

The magnetoelectric (ME) coupling on spin-wave resonances in single-crystal Cu2OSeO3 was studied by a novel technique using electron spin resonance combined with electric field modulation. An external electric field E induces a magnetic field component mu ...

2012

Phonon dynamics in CuxBi2Se3 (x=0, 0.1, 0.125) and Bi2Se2 crystals studied using femtosecond spectroscopy

Helmuth Berger, Jia Liu

Phonon dynamics in Bi, Bi2Se2, and CuxBi2Se3 (x=0, 0.1, 0.125) single crystals was investigated using femtosecond pump-probe spectroscopy. Two damped oscillations in measured transient reflectivity changes were observed, respectively, attributed to coheren ...

Amer Inst Physics2012

Persistence of singlet fluctuations in the coupled spin tetrahedra system Cu2Te2O5Br2 revealed by high-field magnetization, Br-79 NQR, and Te-125 NMR

Helmuth Berger

We present high-field magnetization and Br-79 nuclear quadrupole resonance (NQR) and Te-125 nuclear magnetic resonance (NMR) studies in the weakly coupled Cu2+ (S = 1/2) tetrahedral system Cu2Te2O5Br2. The field-induced level crossing effects were observed ...

Amer Physical Soc2012

Momentum dependence of the excitons in pentacene

Helmuth Berger

We have carried out electron energy-loss investigations of the lowest singlet excitons in pentacene at 20 K. Our studies allow to determine the full exciton band structure in the a*, b* reciprocal lattice plane. The lowest singlet exciton can move coherent ...

2012

Mapping of Electron-Hole Excitations in the Charge-Density-Wave System 1T-TiSe2 Using Resonant Inelastic X-Ray Scattering

Helmuth Berger, Philipp Aebi, Thorsten Schmitt

In high-resolution resonant inelastic x-ray scattering at the Ti L edge of the charge-density-wave system 1T-TiSe2, we observe sharp low energy loss peaks from electron-hole pair excitations developing at low temperature. These excitations are strongly dis ...

2012

BaVS3 probed by V L edge x-ray absorption spectroscopy

Helmuth Berger, László Forró, Ana Akrap

Polarization dependent vanadium L edge x-ray absorption spectra of BaVS3 single crystals are measured in the four phases of the compound. The difference between signals with the polarizations E perpendicular to c and E parallel to c ( linear dichroism) cha ...

2012

Unified one-band Hubbard model for magnetic and electronic spectra of the parent compounds of cuprate superconductors

Helmuth Berger, Marco Grioni, Bastien Dalla Piazza, Martin Mourigal, Henrik Moodysson Rønnow, Marco Guarise, Thorsten Schmitt

Using low-energy projection of the one-band t-t′-t′′ Hubbard model we derive an effective spin Hamiltonian and its spin-wave expansion to order 1/S. We fit the spin-wave dispersion of several parent compounds to the high-temperature superconducting cuprate ...

American Physical Society2012

Long-Wavelength Helimagnetic Order and Skyrmion Lattice Phase in Cu2OSeO3

Helmuth Berger

We report a long-wavelength helimagnetic superstructure in bulk samples of the ferrimagnetic insulator Cu2OSeO3. The magnetic phase diagram associated with the helimagnetic modulation inferred from small-angle neutron scattering and magnetization measureme ...

American Physical Society2012

Ultrafast Dynamics of Charge Density Waves in 4H(b)-TaSe2 Probed by Femtosecond Electron Diffraction

Helmuth Berger

The dynamics of the photoinduced commensurate-to-incommensurate charge density wave (CDW) phase transition in 4H(b)-TaSe2 are investigated by femtosecond electron diffraction. In the perturbative regime, the CDW re-forms on a 150-ps time scale, which is tw ...

Amer Physical Soc2012

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