10,252 research outputs found

    Emerging magnetism and electronic phase separation at titanate interfaces

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    The emergence of magnetism in otherwise nonmagnetic compounds and its underlying mechanisms have become the subject of intense research. Here we demonstrate that the nonmagnetic oxygen vacancies are responsible for an unconventional magnetic state common for titanate interfaces and surfaces. Using an effective multiorbital modelling, we find that the presence of localized vacancies leads to an interplay of ferromagnetic order in the itinerant t2g band and complex magnetic oscillations in the orbitally-reconstructed eg-band, which can be tuned by gate fields at oxide interfaces. The magnetic phase diagram includes highly fragmented regions of stable and phase-separated magnetic states forming beyond nonzero critical defect concentrations.Comment: 5 pages, 4 figure

    How Large is the Intrinsic Flux Noise of a Magnetic Flux Quantum, of Half a Flux Quantum and of a Vortex-Free Superconductor?

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    This article addresses the question whether the magnetic flux of stationary vortices or of half flux quanta generated by frustrated superconducting rings is noisy. It is found that the flux noise generated intrinsically by a superconductor is, in good approximation, not enhanced by stationary vortices. Half flux quanta generated by π\pi-rings are characterized by considerably larger noise.Comment: 11 pages, 3 figures. in: A. Bussmann-Holder, H. Keller (Eds.) High Tc Superconductors and Related Transition Metal Oxides, Springer, 237-242; also to be published in: Journal of Superconductivity (2007

    Dynamic Freeze-In: Impact of Thermal Masses and Cosmological Phase Transitions on Dark Matter Production

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    The cosmological abundance of dark matter can be significantly influenced by the temperature dependence of particle masses and vacuum expectation values. We illustrate this point in three simple freeze-in models. The first one, which we call kinematically induced freeze-in, is based on the observation that the effective mass of a scalar temporarily becomes very small as the scalar potential undergoes a second order phase transition. This opens dark matter production channels that are otherwise forbidden. The second model we consider, dubbed vev-induced freeze-in, is a fermionic Higgs portal scenario. Its scalar sector is augmented compared to the Standard Model by an additional scalar singlet, SS, which couples to dark matter and temporarily acquires a vacuum expectation value (a two-step phase transition or `vev flip-flop'). While S0\langle S \rangle \neq 0, the modified coupling structure in the scalar sector implies that dark matter production is significantly enhanced compared to the S=0\langle S \rangle = 0 phases realised at very early times and again today. The third model, which we call mixing-induced freeze-in, is similar in spirit, but here it is the mixing of dark sector fermions, induced by non-zero S\langle S \rangle, that temporarily boosts the dark matter production rate. For all three scenarios, we carefully dissect the evolution of the dark sector in the early Universe. We compute the DM relic abundance as a function of the model parameters, emphasising the importance of thermal corrections and the proper treatment of phase transitions in the calculation.Comment: 26 pages, 11 figures, v2: matches journal version, change to the value of a benchmark coupling in section II, impact of thermal masses increase

    Large and Small Polaron Excitations in La2/3(Sr/Ca)1/3MnO3 Films

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    We present detailed optical measurements of the mid-infrared (MIR) excitations in thin films of La2/3Sr1/3MnO3 (LSMO) and La2/3Ca1/3MnO3 (LCMO) across the magnetic transition. The shape of the excitation at about 0.2 eV in both samples is analyzed in terms of polaron models. We propose to identify the MIR resonance in LSMO as the excitation of large polarons and that in LCMO as a small polaron excitation. A scaling behavior for the low-energy side of the polaronic MIR resonance in LSMO is established

    On the Consistency of Perturbativity and Gauge Coupling Unification

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    We investigate constraints that the requirements of perturbativity and gauge coupling unification impose on extensions of the Standard Model and of the MSSM. In particular, we discuss the renormalization group running in several SUSY left-right symmetric and Pati-Salam models and show how the various scales appearing in these models have to be chosen in order to achieve unification. We find that unification in the considered models occurs typically at scales below M^{min}_{B violation} = 10^16 GeV, implying potential conflicts with the non-observation of proton decay. We emphasize that extending the particle content of a model in order to push the GUT scale higher or to achieve unification in the first place will very often lead to non-perturbative evolution. We generalize this observation to arbitrary extensions of the Standard Model and of the MSSM and show that the requirement of perturbativity up to M^{min}_{B violation}, if considered a valid guideline for model building, severely limits the particle content of any such model, especially in the supersymmetric case. However, we also discuss several mechanisms to circumvent perturbativity and proton decay issues, for example in certain classes of extra dimensional models.Comment: LaTeX, 20 pages, 8 figures, 1 tabl
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