Publications by Helmuth Berger

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We report the study of the skyrmion state near the surface of Cu2OSeO3 using soft resonant elastic x-ray scattering (REXS) at the Cu L-3 edge. Within the lateral sampling area of 200 x 200 mu m(2), we found a long-range-ordered skyrmion lattice phase as we ...

We report the in-plane (rho (ab)) and out-of-plane (rho (c)) resistivities in the 4-800 K range on single crystals of Tl2Ba2CaCu2O8 high-T-c cuprate with T-c = 111 K (rho (c)/rho (ab) approximate to 1000). rho (ab)(T) exhibits "usual" linear behavior and rho (c)(T) follows generally metallic-like, positive slope in the 150-400 K temperature range. There is a clear crossover to semiconductor-like behavior close to T-c. However, for the first time in a chainless high-T-c cuprate we observe a crossover above 5

Zirconium pentatelluride was recently reported to be a 3D Dirac semimetal, with a single conical band, located at the center of the Brillouin zone. The cone's lack of protection by the lattice symmetry immediately sparked vast discussions about the size an ...

We present a comparative study of the optical properties-reflectance, transmission, and optical conductivity- and Raman spectra of two layered bismuth-tellurohalides BiTeBr and BiTeCl at 300 and 5 K, for light polarized in the a-b planes. Despite different ...

Using angle-resolved photoemission spectroscopy, we probe the electronic band structure of TaTe4 single crystal samples. Resistivity and optical measurements were also performed on the same samples. The experimental data indicate a combination of quasi-one-dimensional and three-dimensional electronic properties. We observe a finite spectral intensity, but no dispersing quasiparticle states near the chemical potential. [S0163-1829(99)06611-4].

When a Mott metal-insulator transition is inhibited by a small amount of disorder in the layered dichalcogenide 1T-TaS2, an inhomogeneous superconducting state arises below T=2.1 K and coexists with a nearly commensurate charge-density wave. By angle-resol ...

We report Raman-scattering data of single crystals of magnetite (Fe3O4) with Verwey transition temperatures (T-v) of 123 and 117 K, respectively. Both single crystals reveal broad electronic background extending up to 900 wave numbers (similar to 110 meV). ...

We performed angle-resolved photoelectron spectroscopy (ARPES) and resistivity measurements on the layered metal 1T-TiTe2. We determined the quasiparticle dispersion and lifetime. In particular, we could characterize and separately evaluate the effects of the electron-electron and electron-phonon interactions on the experimental spectra. The good correlation between transport and spectroscopic scattering rates indicates that this material behaves as a model Fermi liquid system.

We report on the temperature dependence of the ZrTe5 electronic properties, studied at equilibrium and out of equilibrium, by means of time and angle resolved photoelectron spectroscopy. Our results unveil the dependence of the electronic band structure ac ...

We discuss the spectral signatures of strong electronic correlations and electron-phonon interactions in two representative low-dimensional systems. In the 2D compound 1T-TaSe2 a charge-density-wave (CDW) induces a narrowing of the conduction band, which triggers a surface metal-insulator (M-I) transition and the disappearance of the quasiparticle (QP) features. In the 1D blue bronze K0.3MoO3 strong interaction with the lattice lead to pseudogapped polaronic lineshapes, and the large mass renormalization ma

Detailed band structure calculations have been performed for Cd2Re2O7 in high-, middle- and low-temperature (T) phases. The calculations are based on the observed lattice structures from x-ray diffraction measurements. The spin-orbit interaction is incorpo ...

We investigate the evolution of both the occupied and unoccupied electronic structure in representative compounds of the infinitely adaptive superlattice series (Sb2)m−Sb2Te3 (m=0–3) by means of angle-resolved photoemission spectroscopy and time-delayed tw ...

An incommensurate elliptical helical magnetic structure in the frustrated coupled-spin-chain system FeTe2O5Br is surprisingly found to persist down to 53(3) mK (T/T-N similar to 1/200), according to neutron scattering and muon spin relaxation. In this stat ...

Over the past decades, investigations of the anomalous low-energy electronic properties of ZrTe5 have reached a wide array of conclusions. An open question is the growth method's impact on the stoichiometry of ZrTe5 samples, especially given the very small ...

High-resolution angle-resolved (ARPES) photoemission in the quasi-one dimensional (1D) Peierls systems (TaSe4)(2)I and K0.3MoO3 reveals spectral features with clear 1D character. Their dispersion and polarization dependence is consistent with open Fermi surfaces and with the symmetry properties of the lattice. However the measured spectral lineshapes are unusual in various respects, as shown also by a comparison with the quasiparticle spectra of a 'good' metal. A consistent description of the normal metalli

We present a comparative study of the low-dimensional compounds alpha- and beta-TeVO4. Our data clearly show that the change in the local coordination geometry of V4+ ions between alpha- and beta-TeVO4 leads to drastic differences in their magnetic propert ...

We measured, by angle-resolved photoemission spectroscopy, the electronic structure of LiCu2O2, a mixed-valence cuprate where planes of Cu(I) (3d(10)) ions are sandwiched between layers containing one-dimensional edge-sharing Cu(II) (3d(9)) chains. We find ...

Collective modes are responsible for the emergence of novel quantum phases in topological materials. In the quasi-one dimensional (1D) Weyl semimetal (TaSe4)(2)I < i , a charge density wave (CDW) opens band gaps at the Weyl points, thus turning the system ...

Disorder has a strong influence on the spectral properties of the Bi2Sr2CaCu2O8+x high-T-c superconductor, both in the normal and in the superconducting state. High-resolution photoemission reveals a progressive quasiparticle suppression with increasing disorder in electron-irradiated optimally doped single crystals. The spectral Line shapes of the disordered samples show striking analogies with those of underdoped samples specifically the widely discussed pseudogap at the chemical potential. Disorder provi

Interfacial interactions allow the electronic properties of graphene to be modified, as recently demonstrated by the appearance of satellite Dirac cones in graphene on hexagonal boron nitride substrates. Ongoing research strives to explore interfacial inte ...

Helmuth Berger

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Resonant elastic x-ray scattering from the skyrmion lattice in Cu2OSeO3

Crossovers in the out-of-plane resistivity of superconducting Tl2Ba2CaCu2O8 single crystals

Two-Dimensional Conical Dispersion in ZrTe5 Evidenced by Optical Spectroscopy

Optical properties of BiTeBr and BiTeCl

Coexisting one-dimensional and three-dimensional spectral signatures in TaTe4

Superconducting phase in the layered dichalcogenide 1T-TaS_{2} upon inhibition of the metal-insulator transition

Electronic Raman scattering in magnetite: Spin versus charge gap

Quasiparticle scattering processes in a model Fermi liquid: 1T-TiTe2

Ultrafast Optical Control of the Electronic Properties of ZrTe5

Strong electron-electron and electron-phonon interactions in one- and two-dimensional solids

Electronic structure of pyrochlore Cd2Re2O7

Engineering the topological surface states in the (Sb2)m−Sb2Te3(m=0–3) superlattice serie

Persistent Spin Dynamics Intrinsic to Amplitude-Modulated Long-Range Magnetic Order

Nature of native atomic defects in ZrTe5 and their impact on the low-energy electronic structure

Band features and strong correlations in 1D Peierls systems

Low-dimensional magnetism of spin-1/2 chain systems of alpha- and beta-TeVO4: A comparative study

Electronic Phase Separation and Dramatic Inverse Band Renormalization in the Mixed-Valence Cuprate LiCu2O2

Optically induced changes in the band structure of the Weyl charge-density-wave compound (TaSe4)(2)I

Alternative pseudogap scenario: Spectroscopic analogies between underdoped and disordered Bi2Sr2CaCu2O8+x

Strong interface-induced spin-orbit interaction in graphene on WS2