Publications de Helmuth Berger

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We studied the temperature dependence of heat capacity C-P(T) of a quasi-one-dimensional magnetic system beta-TeVO4, in a zero magnetic field and a temperature region of 0.1 K

Using superconducting quantum interference device magnetometry techniques, we have studied the change in magnetization versus applied ac electric field, i.e. the magnetoelectric (ME) susceptibility dM/dE, in the chiral-lattice ME insulator Cu2OSeO3. Measur ...

TaSe3 belongs to a class of low-dimensional materials characterized by the interplay and competition between dimensionality crossover and broken symmetry ground states. A comprehensive study by dc-transport, optical, and angle-resolved photoemission (ARPES) experiments shows that the electronic properties of this compound are strongly anisotropic between the chain and the transverse crystallographic direction. Even though TaSe3 fails to undergo a charge-density-wave (CDW) phase transition, we found evidence

We have measured the resonant inelastic x-ray scattering (RIXS) spectra at the Cu L-3 edge in a variety of cuprates. Exploiting a considerably improved energy resolution (0.8 eV) we recorded significant dependencies on the sample composition and orientation, on the scattering geometry, and on the incident photon polarization. The RIXS final states correspond to two families of electronic excitations, having local (dd excitations) and nonlocal (charge-transfer) character. The dd energy splitting can be estim

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 ...

We present a combined angle-resolved photoemission spectroscopy and resistivity study of the model Fermi liquid system 1T-TiTe2. From the analysis of the quasiparticle spectral line shape we identify and separately evaluate the electron-electron. electron-phonon, and impurity scattering contributions to the quasiparticle lifetime. There is a clear correspondence between spectroscopic and transport data. A residual low-temperature spectral linewidth (Gamma (o)=17 meV) indicates that the three-dimensional nat

We have studied by high resolution ARPES the spectroscopic properties of the prototype charge density wave system (TaSe4)(2)I and of the closely related insulating compound (NbSe4)(3)I. We observe features dispersing with the lattice periodicity, including commensurate (in (NbSe4)(3)I) and incommensurate (in (TaSe4)(2)I) superlattice shadow bands. The spectral lineshape is anomalous in the normal metallic state, and the intensity is strongly suppressed near the Fermi surface. In the insulating CDW state the

Optical and photoemission experiments reveal unexpected spectral signatures of one-dimensional band insulators. In the model compound (NbSe4)(3)I the optical conductivity decays as a power law sigma(1)(omega) similar to omega(-4.25) above a sharp gap edge. Photoemission observes both the valence and a shadow band, produced w by a commensurate superstructure. We identify an optical and photoemission band gap consistent with other measurements but much smaller than the energy scale defined by the dispersion o

Photoexcitation of the Mott insulator 1T-TaS2 by an intense laser pulse leads to an ultrafast transition toward a gapless phase. Besides the collapse of the electronic gap, the sudden excitation of the charge density wave (CDW) mode results in periodic osc ...

Using angle-resolved photoemission, we report the first observation of an incomplete charge-density-wave (CDW) gap opening for the quasi-two-dimensional material Mo4O11. Our data indicate that the gap opening occurs over a range of 0.5 (A)ngstrom(-1) in the Brillouin zone. The maximum gap opening (Delta(max)) we observe is 60 +/- 10 meV, which leads to a ratio of 2 Delta(max)/kT(CDW) = 14 +/- 6 (T-CDW = transition temperature), well above the weak coupling value of 3.5. Our data suggest that both 1-dimensio

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 ...

We report transport measurements on single crystals of Sr2RuO4-δ, grown by the flux technique. The temperature dependence of the Hall coefficient is similar to the one measured in cuprates, and the linear resistivity persists up to ~1000 K, while the superconductivity remains confined below 1 K. This suggests that the linear temperature dependence of resistivity is not an exclusive signature of the anomalous normal state of high-Tc cuprates but rather of layered oxides in general, especially single-layer pe

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 ...

GalnSe(2) single crystals have been grown and characterized by experimental techniques such as high-resolution transmission electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy and optical and electrical measurements. The samples were prepared in single-crystal form from a melt. The structural analysis indicates that GalnSe(2) has a hexagonal structure, and confirms the high quality of the produced single crystals. Quantitative information on electrical and optical properties of single-c

We report on angle-resolved photoemission and superconductor-insulator-superconductor tunneling measurements of the normal and superconducting states of Bi2Sr2CaCu2O8+x single crystal samples and Bi2Sr2CaCu2O8+x - native oxide- lead (S-I-S) tunnel junctions.

We studied using x-ray photoelectron spectroscopy the deposition of Au, In, Ag and Al overlayers onto clean-cleaved GaTe(001) surfaces at room temperature. The first steps of the Schottky barrier formation on Au and In showed no interface reactions. Free Te release was observed after the deposition of 3.5 Angstrom of silver. On the other hand, the deposition of Al leads to an exchange chemical reaction where clustering dominates at high metal coverage.

Superconductivity in van der Waals materials, such as NbSe2 and TaS2, is fundamentally novel due to the effects of dimensionality, crystal symmetries, and strong spin-orbit coupling. In this work, we perform tunnel spectroscopy on NbSe2 by utilizing MoS2 o ...

Studying the nodal quasiparticles in superconducting cuprates by photoemission with highly improved momentum resolution, we show that a new "kink" feature in the scattering rate is a key to uncover the nature of electron correlations in these compounds. Our data provide evidence that the main doping independent contribution to the scattering can be well understood in terms of the conventional Fermi liquid model, while the additional doping dependent contribution has a magnetic origin. This sheds doubt on ap

We find that even very low Ni doping levels of high-quality Bi2Sr2Ca1Cu2O8 single crystals strongly affect the transition temperature T(c). We also observed that T(c) is not related to the total Ni concentration, but only to that of Ni engaged in NiO-type bonds. By controlling the temperature during crystal growth, one can modify the relative weight of Ni in NiO-type bonds with respect to other configurations-and therefore T(c).

We have measured two structurally similar superconducting systems, RuSr2GdCu2O8 and GdBa2Cu3O7 by means of high-resolution ac susceptibility. The real and the imaginary part of ac susceptibility of both bulk-ceramic and powdered samples have been studied down to the very small magnetic-field levels. We show that there are significant differences in the evolution of superconductivity in the two studied superconducting systems. In particular, we show that the superconducting transition in the grains in RuSr2G

Helmuth Berger

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Heat capacity properties of quasi-one-dimensional magnet beta-TeVO4

Exploration of the helimagnetic and skyrmion lattice phase diagram in Cu2OSeO3 using magnetoelectric susceptibility

Spectroscopic and dc-transport investigations of the electronic properties of TaSe3

Low energy electronic excitations in the layered cuprates studied by copper L-3 resonant inelastic X-ray scattering

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

High-resolution angle-resolved photoemission investigation of the quasiparticle scattering processes in a model Fermi liquid: 1T-TiTe2

The spectroscopic properties of 1D Peierls systems

Dynamical properties of the one-dimensional band insulator (NbSe4)(3)I

Femtosecond dynamics of electronic states in the Mott insulator 1T-TaS2 by time resolved photoelectron spectroscopy

Incomplete charge-density-wave gap opening in orthorhombic Mo4O11

Multiferroic FeTe2O5Br: Alternating spin chains with frustrated interchain interactions

On linear resistivity from ~1 to 103 K in Sr2RuO4-δ single crystals grown by flux technique

Multiferroicity in the geometrically frustrated FeTe2O5Cl

The growth and characterization of GaInSe2 single crystals

Doping dependence of electronic structure in Bi2Sr2CaCu2O8+x

Interface formation and chemical reactions on metal-GaTe(001) surfaces

Origin of Subgap States in Normal-Insulator-Superconductor van der Waals Heterostructures

Manifestation of the magnetic resonance mode in the nodal quasiparticle lifetime of the superconducting cuprates

7Ni-O Chemical Interaction and the Transition-Temperature of Ni-Doped Bi2Sr2Ca1Cu2O8

Superconducting transition in ruthenocuprate RuSr2GdCu2O8 viewed from the studies of the imaginary part of ac susceptibility