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Helmuth Berger

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Related publications (311)

Coexistence of localized and collective magnetism in the coupled-spin-tetrahedra system Cu4Te5O12C14

Helmuth Berger

We report high-field magnetization, electron spin resonance (ESR), and Raman scattering measurements of the coupled spin-tetrahedra system Cu4Te5O12C14 with magnetic ordering at T-N = 13.6 K. We find thermodynamic and spectroscopic signatures for the conco ...

Amer Physical Soc2014

Microscopic evidence for strong periodic lattice distortion in two-dimensional charge-density wave systems

Helmuth Berger, Ji Dai

In quasi-two-dimensional electron systems of layered transition metal dichalcogenides (TMDs) there is still controversy about the nature of the transitions to charge-density wave (CDW) phases, i.e., whether they are described by a Peierls-type mechanism or ...

Amer Physical Soc2014

Scanning photocurrent microscopy reveals electron-hole asymmetry in ionic liquid-gated WS2 transistors

Helmuth Berger

We perform scanning photocurrent microscopy on WS2 ionic liquid-gated field effect transistors exhibiting high-quality ambipolar transport. By properly biasing the gate electrode, we can invert the sign of the photocurrent showing that the minority photoca ...

Amer Inst Physics2014

Electric-Field-Induced Skyrmion Distortion and Giant Lattice Rotation in the Magnetoelectric Insulator Cu2OSeO3

Helmuth Berger, Arnaud Magrez, Krunoslav Prsa, Henrik Moodysson Rønnow, Ivica Zivkovic, Arash Alahgholipour Omrani, Jonathan White, Ping Huang

Uniquely in Cu2OSeO3, the Skyrmions, which are topologically protected magnetic spin vortexlike objects, display a magnetoelectric coupling and can be manipulated by externally applied electric (E) fields. Here, we explore the E-field coupling to the magne ...

American Physical Society2014

Dissipation processes in the insulating skyrmion compound Cu2OSeO3

Helmuth Berger, Ivica Zivkovic

We present a detailed study of the phase diagram surrounding the skyrmion lattice (SkL) phase of Cu2OSe2O3 using high-precision magnetic ac susceptibility measurements. An extensive investigation of transition dynamics around the SkL phase using the imagin ...

Amer Physical Soc2014

Magnetic anisotropy of the spin tetramer system SeCuO3 studied by torque magnetometry and ESR spectroscopy

Helmuth Berger

We present an experimental study of macroscopic and microscopic magnetic anisotropy of a spin tetramer system SeCuO3 using torque magnetometry and ESR spectroscopy. Large rotation of macroscopic magnetic axes with temperature observed from torque magnetome ...

Amer Physical Soc2014

Pressure tuning the Fermi level through the Dirac point of giant Rashba semiconductor BiTeI

Helmuth Berger

We report measurements of Shubnikov-de Haas oscillations in the giant Rashba semiconductor BiTeI under applied pressures up to similar to 2GPa. We observe one high frequency oscillation at all pressures and one low frequency oscillation that emerges betwee ...

Iop Publishing Ltd2014

From nanotubes to single crystals: Co doped TiO2

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

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

Dynamics of charge density wave order in the quasi one dimensional conductor (TaSe4)(2)I probed by femtosecond optical spectroscopy

Helmuth Berger

Carrier and collective mode dynamics in the quasi one-dimensional charge density wave (CDW) system (TaSe4)(2)I have been investigated by means of time-resolved optical pump-probe spectroscopy. In the low excitation, linear, regime we focus on the temperatu ...

Springer Heidelberg2013

Magnetic order and low-energy excitations in the quasi-one-dimensional antiferromagnet CuSe2O5 with staggered fields

Helmuth Berger

Ground-state and low-energy excitations of the quasi-one-dimensional antiferromagnet CuSe2O5 where finite interchain interactions promote long-range antiferromagnetic order below T-N = 17 K, were experimentally studied using bulk magnetization, neutron dif ...

Amer Physical Soc2013

Signatures of a Pressure-Induced Topological Quantum Phase Transition in BiTeI

Helmuth Berger

We report the observation of two signatures of a pressure-induced topological quantum phase transition in the polar semiconductor BiTeI using x-ray powder diffraction and infrared spectroscopy. The x-ray data confirm that BiTeI remains in its ambient-press ...

Amer Physical Soc2013

Infrared phonon modes in multiferroic single-crystal FeTe2O5Br

Helmuth Berger

Reflection and transmission as a function of temperature (7-300 K and 5-300 K respectively) have been measured on single crystals of the multiferroic compound FeTe2O5Br utilizing light spanning from the far infrared to the visible. The complex dielectric f ...

Amer Physical Soc2013

Exciton properties of selected aromatic hydrocarbon systems

Helmuth Berger

We have examined the singlet excitons in two representatives of acene-type (tetracene and pentacene) and phenacene-type (chrysene and picene) molecular crystals, respectively, using electron energy-loss spectroscopy at low temperatures. We show that the ex ...

EDP Sciences, Springer2013

Multiferroicity in the geometrically frustrated FeTe2O5Cl

Helmuth Berger

The layered FeTe2O5Cl compound was studied by specific-heat, muon-spin relaxation, nuclear magnetic resonance, and dielectric as well as neutron and synchrotron x-ray diffraction measurements, and the results were compared to isostructural FeTe2O5Br. We fi ...

Amer Physical Soc2013

Tunable Polaronic Conduction in Anatase TiO2

Helmuth Berger, László Forró, Marco Grioni, Arnaud Magrez, Luca Moreschini, Jacim Jacimovic, Simon Karl Moser

Oxygen vacancies created in anatase TiO2 by UV photons (80–130 eV) provide an effective electron-doping mechanism and induce a hitherto unobserved dispersive metallic state. Angle resolved photoemission reveals that the quasiparticles are large polarons. T ...

Amer Physical Soc2013

Quantum oscillations and optical conductivity in Rashba spin-splitting BiTeI

Helmuth Berger

We report the observation of Shubnikov-de Haas oscillations in single crystals of the Rashba spin-splitting compound BiTeI, from both longitudinal [R-xx(B)] and Hall [R-xy(B)] magnetoresistance. Under magnetic fields up to 65 T, we resolved unambiguously o ...

Amer Physical Soc2013

Opening of a Peierls gap in BaVS3 probed by V L-3 edge resonant inelastic x-ray scattering

Helmuth Berger, Marco Grioni, Marco Guarise, Thorsten Schmitt

V L-3 edge resonant inelastic x-ray scattering measurements performed on high quality BaVS3 single crystals reveal that the intra-t(2g) dd excitations close to the elastic peak are suppressed below the metal-insulator transition induced by the Peierls inst ...

Iop Publishing Ltd2013

Bulk Fermi surface and electronic properties of Cu0.07Bi2Se3

Helmuth Berger

The electronic properties of Cu0.07Bi2Se3 have been investigated using Shubnikov-de Haas and optical reflectance measurements. Quantum oscillations reveal a bulk, three-dimensional Fermi surface with anisotropy k(F)(c)/k(F)(ab) approximate to 2 and a modes ...

Amer Physical Soc2013

Sample preparation methods for femtosecond electron diffraction experiments

Helmuth Berger

Exploring the dynamics of Charge density wave system 1T-TaS2 via femtosecond electron diffraction demonstrated the power of this technique for studying ultrafast structural phenomena in strongly correlated electron materials [1]. The results revealed first ...

Elsevier Science Bv2013

Doping dependence of the plasmon dispersion in 2H-TaSe2

Helmuth Berger

Inelastic electron scattering is applied to investigate the impact of potassium intercalation on the charge-carrier plasmon energy and dispersion in the charge- density wave (CDW) bearing compound 2H-TaSe2. We observe an unususal doping dependence of the p ...

Amer Physical Soc2013

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