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Helmuth Berger

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Related publications (298)

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Axial-Bonding-Driven Dimensionality Effect on the Charge-Density Wave in NbSe2

László Forró, Helmuth Berger

2H-NbSe2 is a prototypical charge-density-wave (CDW) system, exhibiting such a symmetry-breaking quantum ground state in its bulk and down to a single-atomic-layer limit. However, how this state depends on dimensionality and what governs the dimensionality ...

AMER CHEMICAL SOC2022

Two-fold symmetric superconductivity in few-layer NbSe2

László Forró, Helmuth Berger

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 ...

NATURE RESEARCH2021

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

László Forró, Helmuth Berger

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 ...

Nature Research2020

Crystal Structure and Magnetic Properties of Two New Antiferromagnetic Spin Dimer Compounds; FeTe3O7X (X = Cl, Br)

Jia Liu, Helmuth Berger, Dong Zhang

Two new isostructural layered oxohalides FeTe3O7X (X = Cl, Br) were synthesized by chemical vapor transport reactions, and their crystal structures and magnetic properties were characterized by single-crystal X-ray diffraction, Raman spectroscopy, magnetic ...

2011

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

Helmuth Berger

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 ...

Amer Chemical Soc2017

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

Helmuth Berger

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 ...

Amer Physical Soc2012

Stacking-driven gap formation in layered 1T-TaS2

Helmuth Berger

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 ...

2018

Dirac nodal lines and flat-band surface state in the functional oxide RuO2

Helmuth Berger, Arnaud Magrez, Simon Karl Moser, Philippe Bugnon

The efficiency and stability of RuO2 in electrocatalysis has made this material a subject of intense fundamental and industrial interest. The surface functionality is rooted in its electronic and magnetic properties, determined by a complex interplay of la ...

AMER PHYSICAL SOC2018

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

Helmuth Berger, Dirk Grundler

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 ...

Nature Publishing Group2015

Magnetic phase diagram of the two-dimensional antiferromagnet Ni-5(TeO3)(4)Br-2

Helmuth Berger

Phase diagram of the two-dimensional antiferromagnet Ni-5(TeO3)(4)Br-2 with triangular arrangement of Ni2+ (S=1) magnetic moments within the [Ni5O17Br2] subunits has been investigated by temperature and magnetic field dependent heat-capacity, magnetization ...

2009

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

Helmuth Berger

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 ...

2020

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

Helmuth Berger, Yang Zhao

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 ...

Springer Nature2017

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

Helmuth Berger

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 ...

2021

Phonon origin and lattice evolution in charge density wave states

Helmuth Berger

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 ...

AMER PHYSICAL SOC2019

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

Helmuth Berger, Thorsten Schmitt, Xiaoqiang Wang

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. ...

Amer Physical Soc2012

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

Miguel Angel Valbuena, Helmuth Berger, Marco Grioni, Emmanouil Frantzeskakis

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 ...

AMER PHYSICAL SOC2019

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

Helmuth Berger, Marco Grioni

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 ...

2006

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

Helmuth Berger, Oleg Yazyev, Ana Akrap, Edoardo Martino, Michele Pizzochero, Kristians Cernevics, Konstantin Semeniuk

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 ...

AMER CHEMICAL SOC2020

Ferromagnetism in Co-7(TeO3)(4)Br-6: A byproduct of complex antiferromagnetic order and single-ion anisotropy

Ivica Zivkovic, Helmuth Berger

Pronounced anisotropy of magnetic properties and complex magnetic order of a new oxi-halide compound Co-7(TeO3)(4)Br-6 has been investigated by powder and single-crystal neutron-diffraction, magnetization, and ac susceptibility techniques. Anisotropy of su ...

2009

Large magnetothermopower and anomalous Nernst effect in HfTe5

Helmuth Berger, Jean-Philippe Ansermet, Arnaud Magrez, Jan Hugo Dil, Haiming Yu, Edoardo Martino, Junfeng Hu, Sa Tu, Marco Caputo

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 ...

AMER PHYSICAL SOC2019