Tuning the spin dynamics of kagome systems

Despite the conceptional importance of realizing spin liquids in solid states only few compounds are known. On the other side the effect of lattice distortions and anisotropies on the magnetic exchange topology and the fluctuation spectrum are an interesting problem. We compare the excitation spectra of the two s=1/2 kagome lattice compounds volborthite and vesignieite using Raman scattering. We demonstrate that even small modifications of the crystal structure may have a huge effect on the phonon spectrum and low temperature properties.Comment: 3 pages, 2 figure

Interplay of electronic correlations and lattice instabilities in BaVS3

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.Comment: 4 pages, 4 figure

Dynamical Dzyaloshinsky-Moriya interaction in KCuF3: Raman evidence for an antiferrodistortive lattice instability

In the orbitally ordered, quasi-one dimensional Heisenberg antiferromagnet KCuF3 the low-energy Eg and B1g phonon modes show an anomalous softening (25% and 13%) between room temperature and the characteristic temperature T_S = 50 K. In this temperature range a freezing-in of F ion dynamic displacements is proposed to occur. In addition, the Eg mode at about 260 cm-1 clearly splits below T_S. The width of the phonon lines above T_S follows an activated behavior with an activation energy of about 50 K. Our observations clearly evidence a reduction of the structural symmetry below T_S and indicate a strong coupling of lattice and spin fluctuations for T>T_S.Comment: 7 pages, 9 figure

Lichtstreuung in Antiferromagneten mit konkurrierenden Wechselwirkungen - von Spin Ketten zu Kagome Gittern

Within this thesis we discuss excitations in spin systems with competing interactions and orbital degrees of freedom. In particular, materials with frustration effects are investigated where interactions of similar strength compete. This can lead to a macroscopic degeneracy of ground states as well as strongly enhance quantum fluctuations. Promising candidates for a high degree of spin frustration are transition metal oxides with low dimensionality, a low spin quantum number and a low coordination number. Restricting the spin degrees of freedom geometrically can lead to an intricate interplay between spin, lattice and electronic degrees of freedom as well as a suppression of magnetic order due to strong quantum fluctuations down to lowest temperatures. The kagome lattice, with a triangular motif, is of particular importance as it combines geometric frustration with a coordination number lower than in conventional triangular lattices. It may thus allow for spin liquid ground states with a vast ground state degeneracy. These states host exotic, fractional spinon excitations. One of the main results of this work is the first experimental evidence of a gapless continuum of spinon excitations in the structurally perfect kagome lattice herbertsmithite. A comparison with the two slightly distorted kagome structures volborthite and vesignieite allow to determine the influence of the structure on the ground state properties. Further correlated electron systems that have been studied within the thesis are the 1D spin chain TiPO4 with a spin-Peierls instability, the isostructural dimer systems Sr3Cr2O8 and Ba3Cr2O8, which are characterized by fundamentally different dynamic behaviour, as well as the distorted triangular lattice CaCr2O4.Die vorliegende Arbeit befasst sich mit Anregungen frustrierter Spinsysteme. Typischerweise wird Frustration durch konkurrierende Wechselwirkungen vergleichbarer Stärke ausgelöst. Dies führt zu einer hohen Entartung des Grundzustandes sowie zu starken Fluktuationen. Ein hoher Grad an Spinfrustration kann in Übergangsmetalloxiden mit niedriger Dimensionalität, niedriger Spinquantenzahl und niedriger Koordinationszahl erreicht werden. Die geometrische Einschränkung der Spinfreiheitsgrade führt oftmals zu einem komplexen Wechselspiel zwischen Spin-, Gitter- und elektronischen Freiheitsgraden, sowie zu einer völligen Unterdrückung der magnetischen Ordnung durch starke Quantenfluktuationen bis hin zu niedrigsten Temperaturen. Das Kagome Gitter mit seiner Dreieckstruktur ist von besonderer Bedeutung, da es einen hohen Grad an geometrischer Frustrationmit einer Koordinationszahl verbindet, die niedriger ist als in herkömmlichen Dreieckgittern. Somit ist es ein guter Kandidat für Spinflüssigkeitsgrundzustände. Diese Zustände zeichnen sich durch exotische, fraktionelle Spinon Anregungen aus. Ein zentrales Ergebnis der Arbeit ist der erste experimentelle Nachweis eines Spinonkontinuums in dem strukturell perfekten Kagome Gitter Herbertsmithite. Durch Vergleiche mit den leicht verzerrten Kagome Strukturen Volborthite und Vesignieite konnte der Einfluss des Gitters auf den Grundzustand bestimmt werden. Weitere korrelierte Elektronensysteme, die im Rahmen der Arbeit studiert wurden, sind die 1D Spinkette TiPO4 mit einer Spin-Peierls-Instabilität, die isostrukturellen Dimersysteme Sr3Cr2O8 und Ba3Cr2O8, die sich durch fundamental unterschiedliches dynamisches Verhalten auszeichnen, sowie das verzerrte Dreiecksgitter CaCr2O4

Dynamical lattice instability versus spin liquid state in a frustrated spin chain system

The low-dimensional s=1/2 compound (NO)[Cu(NO3)3] has recently been suggested to follow the Nersesyan-Tsvelik model of coupled spin chains. Such a system shows unbound spinon excitations and a resonating valence bond ground state due spin frustration. Our Raman scattering study demonstrates phonon anomalies as well as the suppression of a broad magnetic scattering continuum for temperatures below a characteristic temperature, T<T*=100K. We interpret these effects as evidence for a dynamical interplay of spin and lattice degrees of freedom that might lead to a further transition into a dimerized or structurally distorted phase at lower temperatures.Comment: 5 pages, 6 figure

Enhanced quasiparticle dynamics of quantum well states: the giant Rashba system BiTeI and topological insulators

In the giant Rashba semiconductor BiTeI electronic surface scattering with Lorentzian linewidth is observed that shows a strong dependence on surface termination and surface potential shifts. A comparison with the topological insulator Bi2Se3 evidences that surface confined quantum well states are the origin of these processes. We notice an enhanced quasiparticle dynamics of these states with scattering rates that are comparable to polaronic systems in the collision dominated regime. The Eg symmetry of the Lorentzian scattering contribution is different from the chiral (RL) symmetry of the corresponding signal in the topological insulator although both systems have spin-split surface states.Comment: 6 pages, 5 figure