Publications de Helmuth Berger

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The strong Ising spin-orbit coupling in certain two-dimensional transition metal dichalcogenides can profoundly affect the superconducting state in few-layer samples. For example, in NbSe2, this effect combines with the reduced dimensionality to stabilize ...

In the frame of the RD26 collaboration at CERN we have built and tested several prototypes of fast-RICH detectors suitable for the construction of large systems. The detectors used a solid photocathode consisting of a 500 nm CsI layer evaporated on a conventional pad readout board of a MWPC operated with a methane/isobutane mixture at atmospheric pressure. The results indicate that such a detector may be used for long periods of time in stable operating conditions provided the gas mixture is continuously fl

Charge density wave (CDW) is a startling quantum phenomenon, distorting a metallic lattice into an insulating state with a periodically modulated charge distribution. Astonishingly, such modulations appear in various patterns even within the same family of ...

The band structure of many semiconducting monolayer transition metal dichalcogenides (TMDs) possesses two degenerate valleys with equal and opposite Berry curvature. It has been predicted that, when illuminated with circularly polarized light, interband tr ...

Infrared reflection and transmission as a function of temperature have been measured on single crystals of Cu3Bi(SeO3)(2)O2Cl. The complex dielectric function and optical properties along all three principal axes of the orthorhombic cell were obtained via ...

The ab-plane optical reflectance of single crystal of Bi2Sr2Ca(Cu1-xNix)(2)O8+d, for x = 0.0215, has been measured from 80 to 40 000 cm(-l) (10 meV-5 eV) and at temperatures between 20 and 300 K. Through Ni substitution, the optical conductivity shows a broad peak at about 400 cm(-1), which grows in intensity and shifts to lower frequency as the temperature is reduced. We suggest that this peculiar low-frequency feature is associated with the significant disorder induced by Ni impurities. Analysis of the co

The layered transition metal dichalcogenides which show an inherent tendency towards charge density wave order are traditionally considered to realize rather two-dimensional electronic systems. However, recent theoretical and experimental results suggest t ...

Nearly seven decades of research on microwave excitations of magnetic materials have led to a wide range of applications in electronics(1-4). The recent discovery of topological spin solitons in chiral magnets, so-called skyrmions(5-9), promises high-frequ ...

beta-TeVO4 is a frustrated spin-1/2 zig-zag chain system, where spin-density-wave (SDW), vector-chiral (VC) and an exotic dynamic spin-stripe phase compete at low temperatures. Here we use torque magnetometry to study the anisotropy of these phases in magn ...

Polarized and unpolarized neutron diffractions have been carried out to investigate the nature of the magnetic structures and transitions in monoclinic Co3TeO6. As the temperature is lowered below 26 K long range order develops, which is fully incommensura ...

Tantalum diselenide (TaSe2) is a metallic transition metal dichalcogenide whose structure and vibrational behavior strongly depend on temperature and thickness, and this behavior includes the emergence of charge density wave (CDW) states at very low temper ...

Metallic transition metal dichalcogenides, such as tantalum diselenide (TaSe2), display quantum correlated phenomena of superconductivity and charge density waves (CDWs) at low temperatures. Here, the photophysics of 2H-TaSe2 during CDW transitions is reve ...

The resolution of angle-resolved photoelectron spectroscopy (ARPES) in three-dimensional (3D) momentum k is fundamentally limited by ill defined surface-perpendicular wave vector k(perpendicular to) associated with the finite photoelectron mean free path. ...

Angle-resolved photoemission (ARPES) and optical measurements were performed on single crystal samples of LiCu2O2, an antiferromagnetic S=1/2 spin-chain compound. The ARPES spectra show several dispersive branches associated with hybrid copper-oxygen state ...

In materials with nearly commensurate band filling the electron liquid may spontaneously separate into components with distinct properties, yielding complex intra- and interunit cell ordering patterns and a reduced dimensionality. Polarization-dependent an ...

Applying elastic deformation can tune a material's physical properties locally and reversibly. Spatially modulated lattice deformation can create a bandgap gradient, favoring photogenerated charge separation and collection in optoelectronic devices. These ...

Topological quantum materials have stimulated growing attention because they reveal novel aspects of condensed matter physics and point to new opportunities in materials science, in particular for thermoelectrics. Here, we experimentally study thermoelectr ...

We report on ah-plane resistivity (rho) and angle-reserved photoemission (ARPES) spectra for Bi2Sr2CaCu2O8+x single crystals irradiated with neutrons or electron-beam irradiation. Both the normal and superconducting states were measured with angle-resolved photoemission. Electron-beam irradiation leads to an increase in the residual resistivity, and a decrease in the superconducting transition temperature (T-c). The resistivity data does not indicate any pseudogap; the resistivity is linear from T-c to 300K

Quantum magnets display a wide variety of collective excitations, including spin waves (magnons), coherent singlet-triplet excitations (triplons), and pairs of fractional spins (spinons). These modes differ radically in nature and properties, and in all co ...

The search for and understanding of low-dimensional magnetic materials is essential for both fundamental and technological purposes. Here we propose a combined experimental and theoretical investigation of such a system, namely the monoclinic phase of SeCu ...

Helmuth Berger

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Two-fold symmetric superconductivity in few-layer NbSe2

Development of Large-Area Fast-Rich Prototypes with Pad Readout and Solid Photocathodes

Patterns and driving forces of dimensionality-dependent charge density waves in 2H-type transition metal dichalcogenides

Microscopic Origin of the Valley Hall Effect in Transition Metal Dichalcogenides Revealed by Wavelength-Dependent Mapping

Infrared phonon anomaly and magnetic excitations in single-crystal Cu3Bi(SeO3)(2)O2Cl

ab-plane optical properties of Ni-doped Bi2Sr2CaCu2O8+delta

Stacking-driven gap formation in layered 1T-TaS2

Universal helimagnon and skyrmion excitations in metallic, semiconducting and insulating chiral magnets

Magnetic-field-induced reorientation in the spin-density-wave and the spin-stripe phases of the frustrated spin-1/2 chain compound beta-TeVO4

Complex magnetic incommensurability and electronic charge transfer through the ferroelectric transition in multiferroic Co3TeO6

Examining Experimental Raman Mode Behavior in Mono- and Bilayer 2H-TaSe2 via Density Functional Theory: Implications for Quantum Information Science

Phonon origin and lattice evolution in charge density wave states

Three-Dimensional Electron Realm in VSe2 by Soft-X-Ray Photoelectron Spectroscopy: Origin of Charge-Density Waves

Electronic structure of one-dimensional copper oxide chains in LiCu2O2 from angle-resolved photoemission and optical spectroscopy

Polarization dependence of angle-resolved photoemission with submicron spatial resolution reveals emerging one-dimensionality of electrons in NbSe3

Structural Phase Transition and Bandgap Control through Mechanical Deformation in Layered Semiconductors 1T-ZrX2 (X = S, Se)

Large magnetothermopower and anomalous Nernst effect in HfTe5

Disorder effects in cuprates

Triplons, magnons, and spinons in a single quantum spin system: SeCuO3

Strong decoupling between magnetic subsystems in the low-dimensional spin-1/2 antiferromagnet SeCuO3