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Helmuth Berger

Publications and source records attributed to Helmuth Berger.

7 recordsLinked to original sources
PubMed2006

Monoclinic Cu3(SeO3)2Cl2: an oxohalide with an unusual CuO4Cl trigonal-bipyramidal coordination.

In single crystals of a new monoclinic (C2/m) form of tricopper(II) diselenium(IV) dichloride hexaoxide, Cu3(SeO3)2Cl2, the Se atom is in the 4i position, while the two Cu atoms are in 2a and 4i positions. The structure is based on layers of CuO4Cl trigonal bipyramids, CuO4 square planes and SeO3E tetrahedra. The Cu polyhedra are connected by edge- and corner-sharing to form [010] chains and these chains are bridged by the Se atoms to form (001) layers. The compound is isostructural with Cu3(TeO3)2Br2.

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PubMed2006

Spin-lattice interaction in the quasi-one-dimensional helimagnet LiCu2O2.

The field dependence of the electron spin resonance in a helimagnet LiCu2O2 was investigated for the first time. In the paramagnetic state, a broad resonance line was observed corresponding to a g factor of 2.3. In the critical regime, around the paramagnetic to helimagnetic phase transition the resonance broadens and shifts to higher frequencies. A narrow signal is recovered at a low temperature, corresponding to a spin gap of 1.4 meV in zero field. A comprehensive model of the magnons is presented, using exchange parameters from neutron scattering [T. Masuda Phys. Rev. B 72, 014405 (2005)10.1103/PhysRevB.72.014405] and the spin anisotropy determined here. The role of the quantum fluctuations is discussed.

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PubMed2005

Mott phase at the surface of 1T-TaSe2 observed by scanning tunneling microscopy.

In this Letter we report the observation, by scanning tunneling microscopy, of a Mott metal to insulator transition at the surface of 1T-TaSe2. Our spectroscopic data compare considerably well with previous angle-resolved photoemission spectroscopy measurements and confirm the presence of a large hysteresis related to a first order process. The local character of the tunneling spectroscopy technique allows a direct visualization of the surface symmetry and provides spectroscopic measurements on the defect-free region of the sample. It follows that the electronic localization is driven purely by the enhancement of the charge density wave amplitude which drives a bandwidth controlled metal-insulator transition.

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PubMed2004

Superconductors: time-reversal symmetry breaking?

One of the mysteries of modern condensed-matter physics is the nature of the pseudogap state of the superconducting cuprates. Kaminski et al. claim to have observed signatures of time-reversal symmetry breaking in the pseudogap regime in underdoped Bi2Sr2CaCu2O8+delta (Bi2212). Here we argue that the observed circular dichroism is due to the 51 superstructure replica of the electronic bands and therefore cannot be considered as evidence for spontaneous time-reversal symmetry breaking in cuprates.

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PubMed2003

Ambipolar Nernst effect in NbSe2.

The first study of the Nernst effect in NbSe2 reveals a large quasiparticle contribution with a magnitude comparable and a sign opposite to the vortex signal. Comparing the effect of the charge density wave (CDW) transition on Hall and Nernst coefficients, we argue that this large Nernst signal originates from the thermally induced counterflow of electrons and holes and indicates a drastic change in the electron scattering rate in the CDW state. The results provide new input for the debate on the origin of the anomalous Nernst signal in high-T(c) cuprates.

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PubMed2003

One-dimensional instability in BaVS3.

The 3d(1) system BaVS3 undergoes a series of remarkable electronic phase transitions. We show that the metal-insulator transition at T(MI)=70 K is associated with a structural transition announced by a huge regime of one-dimensional (1D) lattice fluctuations, detected up to 170 K. These 1D fluctuations correspond to a 2k(F)=c(*)/2 charge-density wave (CDW) instability of the d(z(2)) electron gas. We discuss the formation below T(MI) of an unconventional CDW state involving the condensation of the other V4+ 3d(1) electrons of the quasidegenerate e(t(2g)) orbitals. This study stresses the role of the orbital degrees of freedom in the physics of BaVS3 and reveals the inadequacy of current first principle band calculations to describe its electronic ground state.

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