Publications de Helmuth Berger

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Polycrystalline (Cu1-xZnx)(2)OSeO3 (0

Proximity-induced superconductivity in a ferromagnet can induce Cooper pairs with a finite center-of-mass momentum and stabilize Josephson junctions (JJs) with pi phase difference in superconductor-ferromagnet-superconductor heterostructures. The emergence ...

We investigate magnetic excitations in the frustrated zigzag spin-1/2 chain compound beta-TeVO4 by inelastic neutron scattering. In the magnetically ordered ground state, the excitation spectrum exhibits coexisting magnon dispersion, characteristic of long ...

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 ...

1T-TiSe2 is a quasi-two-dimensional transition metal dichalcogenide, which exhibits a charge density wave transition at a critical temperature of similar to 200 K as well as low-temperature superconductivity induced by pressure or intercalation. The electr ...

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 ...

The infrared reflectance of the transition-metal chalcogenide 1T-TaS2 has been measured at temperatures from 30 K to 360 K over 30-45 000 cm(-1) (4 meV-5.5 eV). The optical conductivity was obtained by Kramers-Kronig analysis. At 360 K only modest traces of the phonon lines are noticeable. The phonon modes are followed by a pseudo-gap-like increase of the optical conductivity, with direct optical transitions observed at frequencies above 1 eV. As the temperature decreases, the low-frequency conductivity als

The recent discovery of Skyrmions in Cu2OSeO3 has established a new platform to create and manipulate Skyrmionic spin textures. We use high-field electron spin resonance with a terahertz free-electron laser and pulsed magnetic fields up to 64 T to probe an ...

Low-dimensional electron systems, as realized in layered materials, often tend to spontaneously break the symmetry of the underlying nuclear lattice by forming so-called density waves(1); a state of matter that at present attracts enormous attention(2-6). ...

A review is given of recent angle-resolved photoemission (ARPES) experiments and analyses on a series of layered charge density wave materials. Important aspects of ARPES are recalled in view of its capability for bulk band, Fermi surface and spectral func ...

We characterize experimentally and theoretically the collective electronic excitations in two prototypical layered transition-metal dichalcogenides, NbSe2 and Cu0.2NbS2. The energy- and momentum-dependent dynamical structure factor was measured by inelasti ...

We have studied electronic properties of (YBa2Cu3O6.9)1-xAgx compounds with 5, 10, 15, 25 and 50 wt% Ag. In our samples, the silver fills the intergranular space and the superconductivity is preserved even at 50 wt% Ag(Tc=87 K). These samples can be safely dropped in water without loss of superconductivity. The magnitude of electrical resistivity at 300 K rapidly decreases from 103 μΩcm for 5 wt% Ag down to approximately 30 μΩcm for 50 wt% Ag. The superconducting transition is always around 92 K. Preliminar

In clean metallic superconductors, 100% of the mobile carriers participate ill the condensate, so that the London penetration depth (which measures the electromagnetic screening by the superconductor) indicates charge densities comparable to those inferred from the free-carrier plasma frequency. In the cuprates, this is not the case, even though penetration depth measurements have shown a good correlation between superfluid density and superconducting transition temperature in the underdoped-to-optimally-do

The transition metal tellurium oxychloride, Ni-5(TeO3)(4)Cl-2, has been investigated by high-field electron-spin resonance for frequencies up to 3 THz, at temperatures well below the magnetic ordering at 23 K. At zero external field several resonance modes ...

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 ...

The capability to perform absorption spectroscopy with hard x-rays was added to the synchrotron imaging spectromicroscopy technique. We present several successful test cases. First, the EXAFS (extended x-ray absorption fine structure) analysis was implemented in areas as small as few mu m(2) for transition-metal K edge absorption spectra. The corresponding structural information was complemented by chemical information from the near-edge region. Specifically, in the case of a cross-sectional cut steel diffe

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 ...

We report the behavior of the charge carrier plasmon of 2H-transition metal dichalcogenides (TMDs) as a function of intercalation with alkali metals. Intercalation and concurrent doping of the TMD layers have a substantial impact on plasmon energy and disp ...

The prediction of Weyl fermions in the low-temperature noncentrosymmetric 1T' phase of MoTe2 still awaits clear experimental confirmation. Here, we report angle-resolved photoemission (ARPES) data and ab initio calculations that reveal a surface state arc ...

Luminescence properties for C-60 single crystal, the temperature dependence of the luminescence spectra and decay kinetics were measured for a wide range of excitation energies. At low temperatures the emission spectra with well resolved structure were detected. There were not observed any changes in the luminescence spectra under different excitation energy. With careful measurements of the luminescence intensity as function of temperature we were able to detect the particularity on the curve at T similar

Helmuth Berger

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Unexpected observation of splitting of skyrmion phase in Zn doped Cu2OSeO3

van der Waals pi Josephson Junctions

Coexisting spinons and magnons in the frustrated zigzag spin-1/2 chain compound beta-TeVO4

High-Pressure Phase Transitions and Structures of Topological Insulator BiTel

Three-dimensional momentum-resolved electronic structure of 1T-TiSe2: A combined soft-x-ray photoemission and density functional theory study

Sample preparation methods for femtosecond electron diffraction experiments

Phonon anomaly at the charge ordering transition in 1T-TaS2

Establishing the Fundamental Magnetic Interactions in the Chiral Skyrmionic Mott Insulator Cu2OSeO3 by Terahertz Electron Spin Resonance

Orbital textures and charge density waves in transition metal dichalcogenides

Fermi surface of layered compounds and bulk charge density wave systems

High-energy collective electronic excitations in layered transition-metal dichalcogenides

Electronic properties of superconducting (YBa2 Cu3O6.9)1-xAgx compounds

Superfluid and normal-fluid densities in the high-T-c superconductors

Spin resonance in the ordered magnetic state of Ni-5(TeO3)(4)Cl-2

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

The development and application of imaging EXAFS spectromicroscopy

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

Doping dependent plasmon dispersion in 2H-transition metal dichalcogenides

Persistence of a surface state arc in the topologically trivial phase of MoTe2

Temperature dependence of luminescence intensity and decay times in C-60 single crystal