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Search Results: 1 - 10 of 2224 matches for " Helmuth Berger "
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Giant Kohn anomaly and the phase transition in charge density wave ZrTe_3
Moritz Hoesch,Alexey Bosak,Dmitry Chernyshov,Helmuth Berger,Michael Krisch
Physics , 2009, DOI: 10.1103/PhysRevLett.102.086402
Abstract: A strong Kohn anomaly in ZrTe_3 is identified in the mostly transverse acoustic phonon branch along the modulation vector q_P with polarization along the a* direction. This soft mode freezes to zero frequency at the transition temperature T_P and the temperature dependence of the frequency is strongly affected by fluctuation effects. Diffuse x-ray scattering of the incommensurate superstructure shows a power law scaling of the intensity and the correlation length that is compatible with an order parameter of dimension n = 2.
Interplay of electronic correlations and lattice instabilities in BaVS3
Kwang-Yong Choi,Dirk Wulferding,Helmuth Berger,Peter Lemmens
Physics , 2009, DOI: 10.1103/PhysRevB.80.245108
Abstract: The quasi-one-dimensional metallic system BaVS3 with a metal-insulator transition at T_MI=70 K shows large changes of the optical phonon spectrum, a central peak, and an electronic Raman scattering continuum that evolve in a three-step process. Motivated by the observation of a strongly fluctuating precursor state at high temperatures and orbital ordering and a charge gap at low temperatures we suggest a concerted action of the orbital, electronic, and lattice subsystems dominated by electronic correlations.
Electrostatically Induced Superconductivity at the Surface of WS$_2$
Sanghyun Jo,Davide Costanzo,Helmuth Berger,Alberto F. Morpurgo
Physics , 2015, DOI: 10.1021/nl504314c
Abstract: We investigate transport through ionic liquid gated field effect transistors (FETs) based on exfoliated crystals of semiconducting WS$_2$. Upon electron accumulation, at surface densities close to -or just larger than- 10$^{14}$ cm$^{-2}$, transport exhibits metallic behavior, with the surface resistivity decreasing pronouncedly upon cooling. A detailed characterization as a function of temperature and magnetic field clearly shows the occurrence of a gate-induced superconducting transition below a critical temperature $T_c \approx 4$ K, a finding that represents the first demonstration of superconductivity in tungsten-based semiconducting transition metal dichalcogenides. We investigate the nature of superconductivity and find significant inhomogeneity, originating from the local detaching of the frozen ionic liquid from the WS$_2$ surface. Despite the inhomogeneity, we find that in all cases where a fully developed zero resistance state is observed, different properties of the devices exhibit a behavior characteristic of a Berezinskii-Kosterlitz-Thouless transition, as it could be expected in view of the two-dimensional nature of the electrostatically accumulated electron system.
Gate-induced Superconductivity in atomically thin MoS2 crystals
Davide Costanzo,Sanghyun Jo,Helmuth Berger,Alberto F. Morpurgo
Physics , 2015,
Abstract: When thinned down to the atomic scale, many layered van der Waals materials exhibit an interesting evolution of their electronic properties, whose main aspects can be accounted for by changes in the single-particle band structure. Phenomena driven by interactions are also observed, but identifying experimentally systematic trends in their thickness dependence is challenging. Here, we explore the evolution of gate-induced superconductivity in exfoliated MoS2 multilayers ranging from bulk-like to individual monolayers. We observe a clear transition for all the thicknesses down to the ultimate atomic limit, providing the first demonstration of superconductivity in atomically thin exfoliated crystals. Additionally, we characterize the superconducting state by measuring the critical temperature (TC) and magnetic field (BC) in a large number of multilayer devices, upon decreasing their thickness. The superconducting properties change smoothly down to bilayers, and a pronounced reduction in TC and BC is found to occur when going from bilayers to monolayers, for which we discuss possible microscopic mechanisms. Finding that gate-induced superconductivity persists in individual monolayers, which form the basic building blocks of more sophisticated van der Waals heterostructures, opens new possibilities for the engineering of the electronic properties of materials at the atomic scale.
Superconducting Gap and Pseudogap in Bi-2212
Matthias Opel,Francesca Venturini,Rudi Hackl,Helmuth Berger,Laszlo Forro
Physics , 1999, DOI: 10.1016/S0921-4526(99)02344-3
Abstract: We present results of Raman scattering experiments in differently doped Bi-2212 single crystals. Below Tc the spectra show pair-breaking features in the whole doping range. The low frequency power laws confirm the existence of a $d_{x^2-y^2}$-wave order parameter. In the normal state between Tc and T* = 200K we find evidence for a pseudogap in B2g symmetry. Upon doping its effect on the spectra decreases while its energy scale appears to be unchanged.
Torque magnetometry study of magnetically ordered state and spin reorientation in the quasi-one-dimensional $S=1/2$ Heisenberg antiferromagnet CuSb$_2$O$_6$
Mirta Herak,Dijana ?ili?,Dubravka Matkovi? ?alogovi?,Helmuth Berger
Physics , 2015, DOI: 10.1103/PhysRevB.91.174436
Abstract: We present an experimental study of macroscopic and microscopic magnetic anisotropy of a spin tetramer system \cso using torque magnetometry and ESR spectroscopy. Large rotation of macroscopic magnetic axes with temperature observed from torque magnetometry agrees reasonably well with the rotation of the $\mathbf{g}$ tensor above $T \gtrsim 50$~K. Below 50~K, the $\mathbf{g}$ tensor is temperature independent, while macroscopic magnetic axes continue to rotate. Additionally, the susceptibility anisotropy has a temperature dependence which cannot be reconciled with the isotropic Heisenberg model of interactions between spins. ESR linewidth analysis shows that anisotropic exchange interaction must be present in \csos. These findings strongly support the presence of anisotropic exchange interactions in the Hamiltonian of the studied system. Below $T_N=8$~K, the system enters a long - range antiferromagnetically ordered state with easy axis along the $<\bar{1} 0 1>^*$ direction. Small but significant rotation of magnetic axes is also observed in the antiferromagnetically ordered state suggesting strong spin-lattice coupling in this system.
High-Pressure Study of Anatase TiO2
Ja?im Ja?imovi?,Cristian Vaju,Richard Gaál,Arnaud Magrez,Helmuth Berger,László Forró
Materials , 2010, DOI: 10.3390/ma3031509
Abstract: We report resistivity and thermo-electric power measurements of the anatase phase of TiO 2under pressure up to 2.3 GPa. Despite its transparent appearance, the single crystal of anatase exhibits a metallic-like resistivity above 60 K, at all pressures. The rather high value of the thermo-electric power at room temperature points to complex transport mechanism in this phase.
Nonuniform and coherent motion of superconducting vortices in the picometer-per-second regime
Jonghee Lee,Hui Wang,Michael Dreyer,Helmuth Berger,Barry I. Barker
Physics , 2009, DOI: 10.1103/PhysRevB.84.060515
Abstract: We investigated vortex dynamics in a single-crystal sample of type-II superconductor NbSe$_{2}$ using scanning tunneling microscopy at 4.2 K. The decay of the magnetic field at a few nT/s in our superconducting magnet induced the corresponding motion of vortices at a few pm/s. Starting with an initial magnetic field of 0.5 T, we continued to observe motion of vortices within a field of view of 400$\times$400 nm$^2$ subject to decay of the magnetic field over a week. Vortices moved collectively, and maintained triangular lattices due to strong vortex-vortex interactions during the motion. However, we observed two peculiar characteristics of vortex dynamics in this superconductor. First, the speed and direction of the vortex lattice motion were not uniform in time. Second, despite the non-uniform motion, we also found that there exists an energetically favored configuration of the moving vortices in the single-crystal sample of NbSe$_{2}$ based on the overlaid trajectories and their suppressed speeds. We model the system with weak bulk pinning, strong bulk pinning, and edge barrier effects.
Splitting in the Fermi surface of ZrTe_3: a surface charge density wave system
Moritz Hoesch,Xiaoyu Cui,Kenya Shimada,Corsin Battaglia,Shin-ichi Fujimori,Helmuth Berger
Physics , 2009, DOI: 10.1103/PhysRevB.80.075423
Abstract: The electronic band structure and Fermi surface of ZrTe_3 was precisely determined by linearly polarized angle-resolved photoelectron spectroscopy. Several bands and a large part of the Fermi surface are found to be split by 100-200 meV into two parallel dispersions. Band structure calculations reveal that the splitting is due to a change of crystal structure near the surface. The agreement between calculation and experiment is enhanced by including the spin-orbit potential in the calculations, but the spin-orbit energy does not lead to a splitting of the bands. The dispersion of the highly nested small electron pocket that gives rise to the charge density wave is traceable even in the low-temperature gapped state, thus implying that the finite correlation length of the long-wavelength modulation leads to a smearing of the band back-folding.
Momentum dependence of the excitons in pentacene
Friedrich Roth,Roman Schuster,Andreas K?nig,Martin Knupfer,Helmuth Berger
Physics , 2012, DOI: 10.1063/1.4723812
Abstract: We have carried out electron energy-loss investigations of the lowest singlet excitons in pentacene at 20 K. Our studies allow to determine the full exciton band structure in the a*,b* reciprocal lattice plane. The lowest singlet exciton can move coherently within this plane, and the resulting exciton dispersion is highly anisotropic. The analysis of the energetically following (satellite) features indicates a strong admixture of charge transfer excitations to the exciton wave function.
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