Publications by Helmuth Berger

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We report on the ab-plane polarized reflectance of an untwinned (Bi1.57Pb0.43)Sr2CaCu2O8+delta single crystal over the frequency range from 80 to 32 000 cm(-1) (10 meV-4 eV) at temperatures between 10 and 300 K. We find a clear anisotropy in the ab-plane optical conductivity above and below T-c, which is very similar to that formerly published data of Bi2Sr2CaCu2O8 (M.A. Quijada et al., Z. Phys. B 94, 255 (1994)). We employ both the one-component and two-component analyses to the optical data, which suggest

Terahertz and far-infrared techniques are combined with complementary Raman and inelastic neutron scattering results to probe the low-energy phonon dynamics of archetypal antiferroelectric candidate - francisite (Cu3Bi(SeO3)(2)O2Cl). The combined results c ...

Cu2OSeO3 is an insulating skyrmion-host material with a magnetoelectric coupling giving rise to an electric polarization with a characteristic dependence on the magnetic-field (H) over right arrow. We report a magnetic force microscopy imaging of the helic ...

Photoemission spectromicroscopy was used to investigate the electronic structure of TiO2 anatase single crystals and polycrystalline thin films. The stoichiometry and the degree of oxidation of as-grown crystals, as-deposited films, as well as of thermally annealed samples in different atmospheres, were analyzed, based on the Ti 2p and O 1s core levels, with an energy resolution of 0.4 eV. The experimental density of states (DOS) was found to be in agreement with the theoretical DOS reported in the literatu

In quasi-two-dimensional electron systems of layered transition metal dichalcogenides (TMDs) there is still controversy about the nature of the transitions to charge-density wave (CDW) phases, i.e., whether they are described by a Peierls-type mechanism or ...

Very recently, multiferroic behavior close to the boiling temperature of liquid nitrogen was reported in oxohalide-based Cu2OCl2. In this study, we have established yet another novel mechanism for the origin of charge ordering in multiferroic Cu2OCl2. The ...

We report on a distortion of Bi2Sr2CanCun+1 O6+2n+x (n = 0,1) single crystals, perpendicular to the CuO2 planes (the c-axis), for non-stoichiometric samples containing an excess of Bi. The distortion involves two parts: (a) symmetric displacements in the SrO and CuO2 planes along the c-axis, and (b) an antisymmetric longitudinal compressive wave. The latter is revealed by observing odd harmonics in the X-ray diffraction (XRD) data. Such odd harmonics are typically extinguished for the 14/mmm space group of

The reaction of FePS3 single crystals with a 1.6M n-butyl lithium solution in n-hexane has been followed through de electrical conductivity measurements as a function of the intercalation time. In order to investigate the influence of lithium insertion on the FePS3 electronic properties we have also measured as a function of both temperature and intercalation time the de conductivity of equilibrated LixFePS3 single crystals. Upon lithium intercalation we have observed the following main effects: (i) an incr

We employed Raman spectroscopy to investigate how the hydrostatic pressure affects the temperature of the Verwey transition in pure (Fe3O4) and Al-doped (Fe2.8Al0.2O4) magnetite. In both samples, pressure suppresses the transition. The Al-doped sample disp ...

Polytypism in transition metal dichalcogenides (TMDs) introduces an additional degree of freedom for tailoring the electronic properties of layered van der Waals materials. Polytypes with larger unit cells, spanning four or six layers, can be viewed as natural homostructures, since their atomic composition remains identical across the layers. The resultant crystalline environments can potentially give rise to exotic electronic states, earning these materials recent attention. In this study, we examine struc

The antiferromagnetically ordered state of the monoclinic quasi-one-dimensional S = 1/2 Heisenberg antiferromagnet CuSb2O6 was studied combining torque magnetometry with a phenomenological approach to magnetic anisotropy. This system is known to have a num ...

Magnetic structure of the Co ions in monoclinic Co3TeO6 in the antiferroelectric state at 16 K has been determined by neutron powder together with single-crystal diffractions. The indices of the magnetic reflections that appear at the incommensurate positi ...

When two different periodic potentials are present at the same time in a solid, the electron wavefunctions must conform to the resulting overall periodicity. It is the case of the broken-symmetry phases which are often observed in low-dimensional systems. ...

We report on the ab-plane optical reflectance of an iodine-intercalated Bi1.9Pb0.1Sr2CaCu2O8+delta single crystal in the 80-40000 cm(-1) (10 meV-5 eV) frequency range and at temperatures between 10 and 300 K. As compared to the iodine-free Bi2Sr2CaCu2O8+delta, we find that the visible-ultraviolet interband transitions are strongly modified after intercalation. Estimates of the low-frequency spectral weight indicate that then is an increase of hole concentration in the CuO2 planes. This behavior is a consequ

Polarized and unpolarized neutron-diffraction studies have been carried out on single crystals of the coupled spin tetrahedra systems Cu2Te2O5X2 (X=Cl, Br). A model of the magnetic structure associated with the propagation vectors k(') (Cl) approximate to ...

Using the X-ray spectromicroscopy technique, we studied the primary and secondary electron photoemission of cesium iodide (CsI) films deposited on different substrates, such as copper, aluminum, stainless-steel, and gold-coated quartz. We found an increase of more than a factor of 2 in the secondary emission after exposing, for a few seconds in ultra-high-vacuum (3 X 10(-10) mbar), the sample surface to an X-ray dose-rate of about 5 X 10(11) photons/s cm2 at the Al Kalpha photon energy (1486.7 eV). The acti

BaVS3 presents a metal-to-insulator (MI) transition at ambient pressure due to the stabilization of a 2k(F) commensurate charge density wave (CDW) Peierls ground state built on the dz(2) V orbitals. The MI transition vanishes under pressure at a quantum cr ...

We have investigated the influence of potassium intercalation on the formation of the charge-density wave (CDW) instability in 2H-TaSe2 by means of electron energy-loss spectroscopy and density functional theory. Our observations are consistent with a fill ...

Using time-and angle-resolved photoemission spectroscopy with selective near-and midinfrared photon excitations, we investigate the femtosecond dynamics of the charge density wave ( CDW) phase in 1T-TiSe2 as well as the dynamics of CDW fluctuations at 240 ...

We report measurements of Shubnikov-de Haas oscillations in single crystals of BiTeCl at magnetic fields up to 31 T and at temperatures as low as 0.4 K. Two oscillation frequencies were resolved at the lowest temperatures, F-1 = 65 +/- 4 T and F-2 = 156 +/ ...

Helmuth Berger

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Optical study of an untwinned (Bi1.57Pb0.43)Sr2CaCu2O8+delta single crystal: ab-plane anisotropy

Soft-mode dynamics and sublattice polarization in an archetypal antiferroelectric

Field-induced reorientation of helimagnetic order in Cu2OSeO3 probed by magnetic force microscopy

Electronic-Structure of Anatase TiO2 Oxide

Microscopic evidence for strong periodic lattice distortion in two-dimensional charge-density wave systems

Observation of charge-transfer-driven antiferroelectricity in 3d-pyrochlore multiferroic Cu2OCl2

Periodic c-axis modulation and crystallographic Fourier analysis of Bi2Sr2Cun+1O6+2n+x (n=0,1) single crystals with excess Bi

Effects of lithium intercalation on the electronic properties of FePS3 single crystals

Raman study of the Verwey transition in magnetite at high-pressure and low-temperature: Effect of Al doping

Towards atomically-thin regime in bulk 4H-NbSe2 with interlayer disorder

Torque magnetometry study of magnetically ordered state and spin reorientation in the quasi-one-dimensional S=1/2 Heisenberg antiferromagnet CuSb2O6

Charge transfer enhanced magnetic correlations in type-II multiferroic Co3TeO6

Photoemission as a probe of coexisting and conflicting periodicities in low-dimensional solids

ab-plane optical spectra of iodine-intercalated Bi1.9Pb0.1Sr2CaCu2O8+delta: Normal and superconducting properties

Incommensurate magnetic ordering in Cu2Te2O5X2 (X=Cl,Br) studied by single crystal neutron diffraction

Spatially-Resolved X-Ray Spectroscopy of CsI Deposits

Vanishing of the metal-insulator Peierls transition in pressurized BaVS3

Plasmon evolution and charge-density wave suppression in potassium intercalated 2H-TaSe2

Revealing the role of electrons and phonons in the ultrafast recovery of charge density wave correlations in 1T-TiSe2

Unusual Shubnikov-de Haas oscillations in BiTeCl