Publications by Helmuth Berger

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Compared to the previous report on Cu3Bi(SeO3)(2)O2X (X = Cl, Br) [V. Gnezdilov et al., (arXiv: 1604.04249)], where no dielectric anomaly was observed without magnetic field near T-N similar to 25.6K, we further present the dielectric behaviors without and ...

Chiral magnets with topologically nontrivial spin order such as Skyrmions have generated enormous interest in both fundamental and applied sciences. We report broadband microwave spectroscopy performed on the insulating chiral ferrimagnet Cu2OSeO3. For the ...

Insulating helimagnetic Cu2OSeO3 shows sizeable magnetoelectric effects in its skyrmion phase. Using magnetization measurements, magneto-current analysis and dielectric spectroscopy, we provide a thorough investigation of magnetoelectric coupling, polariza ...

High-resolution angle-resolved photoemission measurements were performed on single crystals of Bi2Sr2Ca1Cu2O8+delta with different oxygen stoichiometries. The data establish that the gap anisotropy (ratio of the gap along T-M to the gap along T-X) can be reversibly changed from similar to 20:1 (optimal or underdoped) to similar to 2:1 (overdoped). Differences in sample doping explain the conflicting reports on gap anisotropy in the literature. Possible effects of this change in gap anisotropy on the symmetr

We report high resolution angle-scanned photoemission and Fermi surface (FS) mapping experiments on the layered transition-metal dichalcogenide 1T-TaS2 in the quasicommensurate (metallic) and the commensurate (insulating) charge-density-wave (CDW) phase. Instead of a nesting induced partially removed FS in the CDW phase we find a pseudogap over large portions of the FS. This remnant FS exhibits the symmetry of the one-particle normal state FS. Possibly, this Mott localization induced transition represents t

We report that Cu9O2(SeO3)(4)Cl-6 is a new multiferroic compound. Comprehensive studies of this compound have been carried out on single crystals as well as polycrystalline samples. Magnetic susceptibility chi and specific heat C with H parallel to c axis ...

Shubnikov-de Haas (SdH) oscillations, Hall effect, and optical reflectance (R(omega)) measurements have been performed on single crystals of BiTeBr. Under magnetic fields up to 32 tesla and at temperatures as low as 0.4K, the SdH data shows a single oscill ...

Angle-resolved photoemission (ARPES) on the quasi-one-dimensional conductor (TaSe4)(2)I shows a hidden Fermi-surface crossing in its metallic state and the opening of a Peierls gap at low temperatures, The underlying quasiparticles have vanishing spectral weight and extremely short coherence lengths. They are interpreted as polarons in the strong-coupling adiabatic Lin-Lit, and almost all their ARPES weight is incoherent. These observations suggest a scenario where the long-standing contradictions between A

We explored the reactivity of gold on cleaved 2212 BiCaSrCuO by synchrotron-radiation photoemission spectroscopy. We found no evidence for reactivity when the substrate was at room temperature, similar to what has been reported for very low temperatures (20 K). At intermediate temperatures (100 K), however, clear evidence was found of a chemical reaction with formation of a localized nonmetallic phase. These findings have potentially important implications for the manufacturing of devices based on BiCaSrCuO

We identified the nature of the chemical reaction between silver and the cleaved surface of BiCaSrCuO at room temperature as a Bi-Ag reaction with Bi segregation, accompanied by interaction with the Sr atoms. Our synchrotron-radiation photoemission spectra also reveal an interaction of the adatoms with the O atoms, not detected by previous studies.

We investigated the effect of disorder on the spectral properties of the high temperature superconductor Bi2Sr2CaCu2O8+x. We find that small defect densities, in the low 10(-3) range, already suppress the characteristic spectral signature of the superconducting state, while new excitations appear within the gap. We conclude that, due to defect-induced pair breaking, superconducting pairs and normal carriers coexist below T-c. At higher levels of disorder the normal state is also strongly affected, and the q

The chiral magnet Cu2OSeO3 hosts a Skyrmion lattice that may be equivalently described as a superposition of plane waves or a lattice of particlelike topological objects. A thermal gradient may break up the Skyrmion lattice and induce rotating domains, rai ...

We report a study of the lateral band bending at the Ge-GaSe interface. Spectromicroscopy measurements with synchrotron radiation were performed at the ESCA microscopy line of ELETTRA in Ge patterned films on GaSe substrates. Preliminary results of lateral band bending, chemical reactions and the beam stimulated Ge surface migration are presented. These studies allowed us to seek on the microscopical equivalent of the semiconductor Debye length and to probe the capabilities of scanning-focused spectromicros

We investigated the effects of disorder in Bi2Sr2CaCu2O8+x high temperature superconductor. We found that small defect densities already suppress the characteristic spectral signature of the superconducting state. The spectral line shape clearly reflects new excitations within the gap, as expected for defect-induced pair breaking.

Using high energy resolution angle-resolved photoemission, we have measured both the superconducting gap and the photoemission superconducting condensate spectral area of Bi2Sr2Ca1Cu1O8+x as a function of temperature in two different locations of the Brillouin zone. We found that the size of superconducting gap along the Gamma - X direction decreases to values indistinguishable from zero for which the gap retains virtually full value along the Gamma - (M) over bar direction. The increased gap anisotropy wit

We present scanning tunneling microscopy (STM) measurements at the TaSe2 surface showing a first order process leading to a surface Mott transition. This metal to insulator transition is triggered by the commensurate charge density wave (CDW) which set up at 475 K. The electronic structure rearrangement due to the new periodicity gives rise to a half filled sub-band prone to the localization phenomena. This transition results fully developed below 250K in agreement with previous photoemission Measurements.

Angle-resolved photoemission spectroscopy (ARPES) and optical measurements were performed on single crystal samples of ZrSe3, HfSe3, and ZrS3, which belong to the class of low-dimensional band insulators. By ARPES, we traced the dispersion of the (S 3p, Se ...

Recent photoemission evidence for non-zero gap in highly overdoped Bi2Sr2-CaCu2O8+x, along the directions in the Brillouin zone where the d-wave gap exhibits nodes, is in accordance with more conventional behavior in strongly overdoped high-T-c cuprates. This differs from underdoped regime, where departures from Fermi-liquid (FL) behavior are found. Observed non-FL linear resistivity up to similar to 1050 K in low-T-c perovskite Sr2RuO4-delta is reminiscent of cuprates and implies some similarities between

The authors present electronic properties and critical current densities (jc at 4.2 K, up to 18 T) of (Y1Ba2Cu3O6.9)1-xAgx compounds in which silver fills the intergranular space without reducing Tc which remains at ~92 K. Normal state resistance is decreased by two orders of magnitude by addition of 50 wt.% Ag (Tc=87 K) and samples exhibit improved contact resistance and resistance to water. jc estimated from magnetisation attains almost 105 Acm-2 (4.2 K, H=0) and is practically unchanged by presence of 10

The authors have produced YBaCuO thin films by mono-target ion beam sputtering. Post annealing in O2 at 850°C results in superconducting films on (100) SrTiO3 with R=0 at ~60 K. Preliminary results of chemical analysis of the films by X-ray photoelectron spectroscopy (XPS), as well as combined secondary ion mass (SIMS) and Auger electron spectroscopy (AES) indicate formation of hydroxides and carbonates in the surface of samples and somewhat lower concentration of oxygen than the optimal value of 6.9

Helmuth Berger

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Anisotropic spin-flip-induced multiferroic behavior in kagome Cu3Bi(SeO3)(2)O2Cl

Low spin wave damping in the insulating chiral magnet Cu2OSeO3

Magnetoelectric effects in the skyrmion host material Cu2OSeO3

Doping-induced change of superconducting gap anisotropy in Bi2Sr2Ca1Cu2O8+delta

Remnant fermi surface in the presence of an underlying instability in layered 1T-TaS2

Antiferroelectric antiferromagnetic type-I multiferroic Cu9O2(SeO3)(4)Cl-6

Experimental determination of the bulk Rashba parameters in BiTeBr

Spectroscopic indications of polaronic carriers in the quasi-one-dimensional conductor (TaSe4)(2)I

Evidence for Surface Chemical-Reactions between Gold and Bicasrcuo

Substitutional Reactions in the Surface-Chemistry of Bicasrcuo

Spectroscopic signatures of defect-induced pair breaking in Bi2Sr2CaCu2O8+x

Dynamical Defects in Rotating Magnetic Skyrmion Lattices

Spectromicroscopy study of lateral band bending of the Ge-GaSe heterostructure

Defect-induced changes in the spectral properties of Bi2Sr2CaCu2O8+x: Angle-resolved photoemission study

Temperature-Dependence of the Superconducting Condensate and Gap in Bi2sr2ca1cu2o8+X

Scanning tunneling microscopy observation of a mott-insulator phase at the 1T-TaSe2 surface

Photoemission and optical studies of ZrSe3, HfSe3, and ZrS3

On some common features in high and low-Tc superconducting perovskites

Electronic properties and critical current densities of superconducting (Y1Ba2Cu3O6.9)1-xAgx compounds

Ion beam sputtering and properties of YBaCuO superconducting thin films