Muon spin spectroscopy: magnetism, soft matter and the bridge between the two

LS would like to acknowledge financial support from the Swiss National Science Foundation, grant numbers PBFRP2-138632 and PBFRP2-142820. AD would like to acknowledge financial support from the UK Engineering and Physical Sciences Research Council, grant number EP/G054568/1, the European Union Seventh Framework Programme project NMP3-SL- 2011-263104 ‘HINTS’ and the European Research Council project ‘Muon Spin Spectroscopy of Excited States (MuSES)’ proposal number 307593LS would like to acknowledge financial support from the Swiss National Science Foundation, grant numbers PBFRP2-138632 and PBFRP2-142820. AD would like to acknowledge financial support from the UK Engineering and Physical Sciences Research Council, grant number EP/G054568/1, the European Union Seventh Framework Programme project NMP3-SL- 2011-263104 ‘HINTS’ and the European Research Council project ‘Muon Spin Spectroscopy of Excited States (MuSES)’ proposal number 307593LS would like to acknowledge financial support from the Swiss National Science Foundation, grant numbers PBFRP2-138632 and PBFRP2-142820. AD would like to acknowledge financial support from the UK Engineering and Physical Sciences Research Council, grant number EP/G054568/1, the European Union Seventh Framework Programme project NMP3-SL- 2011-263104 ‘HINTS’ and the European Research Council project ‘Muon Spin Spectroscopy of Excited States (MuSES)’ proposal number 30759

Spectral flow of monopole insertion in topological insulators

Inserting a magnetic flux into a two-dimensional one-particle Hamiltonian leads to a spectral flow through a given gap which is equal to the Chern number of the associated Fermi projection. This paper establishes a generalization to higher even dimension by inserting non-abelian monopoles of the Wu-Yang type. The associated spectral flow is then equal to a higher Chern number. For the study of odd spacial dimensions, a new so-called `chirality flow' is introduced which, for the insertion of a monopole, is then linked to higher winding numbers. This latter fact follows from a new index theorem for the spectral flow between two unitaries which are conjugates of each other by a self-adjoint unitary.Comment: title changed; final corrections before publication; to appear in Commun. Math. Phy

Electron spin relaxation in organic semiconductors probed through muSR

Muon spin spectroscopy and in particular the avoided level crossing technique is introduced, with the aim of showing it as a very sensitive local probe for electron spin relaxation in organic semiconductors. Avoided level crossing data on TMS-pentacene at different temperatures are presented, and they are analysed to extract the electron spin relaxation rate, that is shown to increase on increasing the temperature from 0.02 MHz to 0.33 MHz at 3 K and 300 K respectively.Comment: International Conference TSN2010 "Trends in spintronics and nanomagnetism

Collective Diffusion and a Random Energy Landscape

Starting from a master equation in a quantum Hamiltonian form and a coupling to a heat bath we derive an evolution equation for a collective hopping process under the influence of a stochastic energy landscape. There results different equations in case of an arbitrary occupation number per lattice site or in a system under exclusion. Based on scaling arguments it will be demonstrated that both systems belong below the critical dimension $d_c$ to the same universality class leading to anomalous diffusion in the long time limit. The dynamical exponent $z$ can be calculated by an $\epsilon = d_c-d$ expansion. Above the critical dimension we discuss the differences in the diffusion constant for sufficient high temperatures. For a random potential we find a higher mobility for systems with exclusion.Comment: 15 pages, no figure

Valley polarization effects on the localization in graphene Landau levels

Effects of disorder and valley polarization in graphene are investigated in the quantum Hall regime. We find anomalous localization properties for the lowest Landau level (LL), where disorder can induce wavefunction delocalization (instead of localization), both for white-noise and gaussian-correlated disorder. We quantitatively identify the contribution of each sublattice to wavefunction amplitudes. Following the valley (sublattice) polarization of states within LLs for increasing disorder we show: (i) valley mixing in the lowest LL is the main effect behind the observed anomalous localization properties, (ii) the polarization suppression with increasing disorder depends on the localization for the white-noise model, while, (iii) the disorder induces a partial polarization in the higher Landau levels for both disorder models.Comment: 5 pages, 6 figures, extended version, with 2 new figures adde

Weak disorder expansion for localization lengths of quasi-1D systems

A perturbative formula for the lowest Lyapunov exponent of an Anderson model on a strip is presented. It is expressed in terms of an energy-dependent doubly stochastic matrix, the size of which is proportional to the strip width. This matrix and the resulting perturbative expression for the Lyapunov exponent are evaluated numerically. Dependence on energy, strip width and disorder strength are thoroughly compared with the results obtained by the standard transfer matrix method. Good agreement is found for all energies in the band of the free operator and this even for quite large values of the disorder strength

Adittional levels between Landau bands due to vacancies in graphene: towards a defect engineering

We describe the effects of vacancies on the electronic properties of a graphene sheet in the presence of a perpendicular magnetic field: from a single defect to an organized vacancy lattice. An isolated vacancy is the minimal possible inner edge, showing an antidotlike behaviour, which results in an extra level between consecutive Landau levels. Two close vacancies may couple to each other, forming a vacancy molecule tuned by the magnetic field. We show that a vacancy lattice introduce an extra band in between Landau levels with localization properties that could lead to extra Hall resistance plateaus.Comment: 6 pages, 4 figures, few comments added after referees - accepted to publication in Phys. Rev.