Publications by Helmuth Berger

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The authors present the percolation and electronic properties of (Y1Ba2Cu3O7-δ)1-xAgx compounds in which silver fills the intergranular space without reducing Tc, which remains at 92±1 K. Normal-state resistivity is decreased by up to two orders of magnitude when adding up to 50 wt.% Ag (Tc=87 K), and samples exhibit improved contact resistance, better mechanical properties, and resistance to water. They analyzed the percolation properties of these compounds and found that the critical indices t, s are in a

Core-level photoemission peaks taken on several insulators with spatial resolution of 30-300 mu m rigidly shift in energy from place to place, most likely because of local charging effects. Such phenomena can affect the linewidth and the general lineshape of conventional, spatially integrated spectra even when a flood gun is used, which is one of the standard remedies against charging. This may require a critical revision of photoemission linewidths and lineshapes for many past experiments on insulators.

We report a single-crystal neutron diffraction and inelastic neutron scattering study on the spin 1/2 cuprate Cu3Bi(SeO3)(2)O2Cl, complemented by dielectric and electric polarization measurements. The study clarifies a number of open issues concerning this ...

We measured the momentum dependence of magnetic excitations in the model spin-1/2 2D antiferromagnetic insulator Sr2CuO2Cl2 (SCOC). We identify a single-spin-wave feature and a multimagnon continuum, with different polarization dependences. The spin waves ...

We have exploited angle-resolved photoelectron spectroscopy to study changes in the quasiparticle excitations of CuO2 planes as a function of the carrier density, in optimally doped and underdoped Bi2Sr2CaCuO8+x and in Sr2CuO2Cl2. We have observed large changes in the E versus k dispersion of the quasiparticle bands, and in the topology of the Fermi surface away from optimal doping. We have also found that the carrier density has a nontrivial effect on the symmetry properties of these states.

Improved experimental conditions enabled us to increase the signal-to-noise ratio of the photoemission spectra for the superconducting state of Bi2Ca2SrCu2O8, taken with high angular and energy resolution. This also enabled us to reveal a pronounced minimum that separates the two basic features of the spectrum, the narrow quasiparticle excitation peak and the still controversial broad band at lower kinetic energies. The minimum is approximately 3-DELTA below the Fermi level.

Iodine intercalation in Bi2Sr2CaCu2Oy crystals enables one to modify the critical temperature, T(c), of the superconducting phase, and to obtain a semiconductor phase. This doping drastically changes the size of the unit cell. A photoemission spectroscopy study enables us to see that the decrease of the critical temperature is not related only to the change in the c-axis lattice parameter, but also to an over-doping of copper planes (hole doping).

In nodal-line semimetals, linearly dispersing states form Dirac loops in the reciprocal space with a high degree of electron-hole symmetry and a reduced density of states near the Fermi level. The result is reduced electronic screening and enhanced correla ...

In the cubic chiral magnet Cu2OSeO3 a low-temperature skyrmion state (LTS) and a concomitant tilted conical state are observed for magnetic fields parallel to h100i. Here, we report on the dynamic resonances of these novel magnetic states. After promoting ...

CuO2SeO3 is an insulating material that hosts topologically nontrivial spin whirls, so-called skyrmions, and exhibits magnetoelectric coupling allowing to manipulate these skyrmions by means of electric fields. We report magnetic force microscopy imaging o ...

The transition-metal dichalcogenide 1T-TiSe2 is a quasi-two-dimensional layered material with a charge density wave (CDW) transition temperature of T-CDW approximate to 200 K. Self-doping effects for crystals grown at different temperatures introduce struc ...

We have extended the range of the high-pressure optical spectroscopy to the far-infrared region keeping the accuracy of ambient-pressure experiments. The developed method offers a powerful tool for the study of pressure-induced phase transitions and electr ...

One of the most unique and robust experimental facts about iron-based superconductors is the renormalization of the electronic band dispersion by factor of 3 and more near the Fermi level. Obviously related to the electron pairing, this prominent deviation ...

The nature of the Verwey transition in magnetite (Fe3O4) remains an unsolved puzzle after more than 60 years. Impurities and dopants present in magnetite can alter the Verwey transition temperature T-v as well as other physical characteristics of magnetite ...

New temperature-dependent angle resolved photoemission (ARPES) data on the quasi-one-dimensional Peierls compound (TaSe4)(2)I suggest a new interpretation of the spectral lineshapes, which removes discrepancies with the transport and thermodynamic results. We argue that the carriers are heavily dressed by the interaction with the lattice, and that the ARPES spectra are dominated by incoherent features. Signatures of the Peierls transitions are observed at the correct energy scale, but only with vanishingly

We briefly review some recent photoelectron spectroscopy (PES) studies of quasi one- and two-dimensional metallic systems. We discuss the spectral features of a metallic system, the expected fingerprints of the metal-insulator transition and some experimental and conceptual limitations of the single particle framework. This approach gives a satisfactory interpretation of PES experiments in 2D charge-density-wave materials. The single particle picture is however incompatible with experiments on 1D systems.

We have exploited angle-resolved photoelectron spectroscopy (ARPES) to study the electronic structure of single crystal samples of high T-c material Bi2Sr2CaCu2O8+delta and of the antiferromagnetic insulator Sr2CuO2Cl2. As one moves away from optimal doping into underdoped region (T-c as low as 30K), the large Fermi surface sheet centered around the (pi,pi) point evolves into a small pocket centered around (pi/2,pi/2), and the saddle-point van Hove singularity is lost This evolution can be extrapolated to S

Angle-resolved photoemissions studies were conducted on single-crystal (TaSe4)(2)I samples above and below the charge-density-wave transition temperature (T-CDW) of 260 K. We observed a shift of the leading photoemission edge between 300 and 60 K consistent with resistivity measurements performed on the same sample: a band gap opens throughout the Brillouin zone below T-CDW. However, several aspects of the data are difficult to reconcile with any standard model.

Random defects destroy long-range phase coherence, and inhibit the metal-insulator transition in the charge density wave (CDW) system 1T-TaS2. Photoelectron spectroscopy with high resolution reveals corresponding changes in the electronic structure. Core level spectra indicate a temperature-dependent limited growth of commensurate CDW domains. Screening by free carriers from the domain boundaries reduces the Coulomb repulsion in the Ta d band by similar to 1/3, compared to pure samples, yielding a finite sp

Raman and optical absorption spectra of FeTe2O5Cl at pressures (P) up to 5.9 GPa were measured in order to identify the point of pressure-induced structural phase transition and to compare the vibrational and optical properties of FeTe2O5Cl to those of FeT ...

Helmuth Berger

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Percolation and electronic properties of superconducting (YBa2Cu3O7-δ)1-xAgx ceramics and thick films

Spatially Localized Energy Shifts in the Photoemission Spectroscopy of Insulators

Magnetic and dielectric order in the kagomelike francisite Cu3Bi(SeO3)(2)O2Cl

Measurement of Magnetic Excitations in the Two-Dimensional Antiferromagnetic Sr2CuO2Cl2 Insulator Using Resonant X-Ray Scattering: Evidence for Extended Interactions

Doping effects on the electronic structure of CuO2 planes

Electronic-Spectrum of the High-Temperature Superconducting State

Study of Iodine-Intercalated Bi2Sr2CaCu2Oy Single-Crystals

Light-Induced Renormalization of the Dirac Quasiparticles in the Nodal-Line Semimetal ZrSiSe

Microwave Spectroscopy of the Low-Temperature Skyrmion State in Cu2OSeO3

Heuristic Description of Magnetoelectricity of Cu2OSeO3

Doping Nature of Native Defects in 1T-TiSe2

High-pressure infrared spectroscopy: Tuning of the low-energy excitations in correlated electron systems

High-energy electronic interaction in the 3d band of high-temperature iron-based superconductors

Raman studies of doped magnetite above and below the Verwey transition

Evidence for strong correlations in a 1D Peierls system

Photoemission in low-dimensional systems

Doping dependence of the Fermi surface in the cuprates: A photoemission investigation

Temperature-Dependence of Electronic States in (Tase4)(2)I

Spectral consequences of broken phase coherence in 1T-TaS2

High pressure phase transition in FeTe2O5Cl