ESR of YbRh2Si2 and 174YbRh2Si2 : local and itinerant properties

Below the Kondo temperature the heavy Fermion compound YbRh$_{2}$Si$_{2}$ shows a well defined Electron Spin Resonance (ESR) with local Yb$^{3+}$ properties. We report a detailed analysis of the ESR intensity which gives information on the number of ESR active centers relative to the ESR of well localized Yb$^{3+}$ in YPd$_3$:Yb. The ESR lineshape is investigated regarding contributions from itinerant centers. From the ESR of monoisotopic $^{174}$YbRh$_{2}$Si$_{2}$ we could exclude unresolved hyperfine contributions to the lineshape.Comment: 3 Figure

Spin dynamics of YbRh2Si2Yb Rh_2 Si_2 observed by Electron Spin Resonance

Below the Kondo temperature $T_{\rm K}$ electron spin resonance (ESR) usually is not observable from the Kondo-ion itself because the characteristic spin fluctuation energy results in a huge width of the ESR line. The heavy fermion metal YbRh$_{2}$Si$_{2}$ seems to be an exceptional case where definite ESR spectra show characteristic properties of the Kondo-ion Yb$^{3+}$ well \textit{below} $T_{\rm K}$. We found that the spin dynamics of YbRh$_{2}$Si$_{2}$, as determined by its ESR relaxation, is spatially characterized by an anisotropy of the zero temperature residual relaxation only.Comment: Presented at NanoRes 2004, Kazan; 4 pages, 3 Figure

Relevance of ferromagnetic correlations for the Electron Spin Resonance in Kondo lattice systems

Electron Spin Resonance (ESR) measurements of the ferromagnetic Kondo lattice system CeRuPO show a well defined ESR signal which is related to the magnetic properties of the Ce3+ moment. In contrast, no ESR signal could be observed in the antiferromagnetic homologue CeOsPO. Additionally, we detect an ESR signal in a further ferromagnetic Yb compound, YbRh, while it was absent in a number of Ce or Yb intermetallic compounds with dominant antiferromagnetic exchange, independently of the presence of a strong Kondo interaction or the proximity to a (quantum) critical point. Thus, the observation of an ESR signal in a Kondo lattice is neither specific to Yb nor to the proximity of a quantum critical point, but seems to be connected to the presence of ferromagnetic fluctuations. These conclusions not only provide a basic concept to understand the ESR in Kondo lattice systems even well below the Kondo temperature as observed in the heavy fermion metal YbRh2Si2 but point out ESR as a prime method to investigate directly the spin dynamics of the Kondo ion.Comment: 5 pages, 2 figures, 1 tabl

Memory Effect and Triplet Pairing Generation in the Superconducting Exchange Biased Co/CoOx/Cu41Ni59/Nb/Cu41Ni59 Layered Heterostructure

We fabricated a nanolayered hybrid superconductor-ferromagnet spin-valve structure, the resistive state of which depends on the preceding magnetic field polarity. The effect is based on a strong exchange bias (about -2 kOe) on a diluted ferromagnetic copper-nickel alloy and generation of a long range odd in frequency triplet pairing component. The difference of high and low resistance states at zero magnetic field is 90% of the normal state resistance for a transport current of 250 {\mu}A and still around 42% for 10 {\mu}A. Both logic states of the structure do not require biasing fields or currents in the idle mode.Comment: 9 pages, 4 figures, Accepted to Applied Physics Letter

Coupling of localized moments and itinerant electrons in EuFe2As2 single crystals studied by Electron Spin Resonance

Electron spin resonance measurements in EuFe2As2 single crystals revealed an absorption spectrum of a single resonance with Dysonian lineshape. Above the spin-density wave transition at T_SDW = 190 K the spectra are isotropic and the spin relaxation is strongly coupled to the CEs resulting in a Korringa-like increase of the linewidth. Below T_SDW, a distinct anisotropy develops and the relaxation behavior of the Eu spins changes drastically into one with characteristic properties of a magnetic insulating system, where dipolar and crystal-field interactions dominate. This indicates a spatial confinement of the conduction electrons to the FeAs layers in the SDW state.Comment: 4 pages, 4 figure

Poly-MTO, {(CH_3)_{0.92} Re O_3}_\infty, a Conducting Two-Dimensional Organometallic Oxide

Polymeric methyltrioxorhenium, {(CH_{3})_{0.92}ReO_{3}}_{\infty} (poly-MTO), is the first member of a new class of organometallic hybrids which adopts the structural pattern and physical properties of classical perovskites in two dimensions (2D). We demonstrate how the electronic structure of poly-MTO can be tailored by intercalation of organic donor molecules, such as tetrathiafulvalene (TTF) or bis-(ethylendithio)-tetrathiafulvalene (BEDT-TTF), and by the inorganic acceptor SbF$_3$. Integration of donor molecules leads to a more insulating behavior of poly-MTO, whereas SbF$_3$ insertion does not cause any significant change in the resistivity. The resistivity data of pure poly-MTO is remarkably well described by a two-dimensional electron system. Below 38 K an unusual resistivity behavior, similar to that found in doped cuprates, is observed: The resistivity initially increases approximately as $\rho \sim$ ln$(1/T$) before it changes into a $\sqrt{T}$ dependence below 2 K. As an explanation we suggest a crossover from purely two-dimensional charge-carrier diffusion within the \{ReO$_2$\}$_{\infty}$ planes at high temperatures to three-dimensional diffusion at low temperatures in a disorder-enhanced electron-electron interaction scenario (Altshuler-Aronov correction). Furthermore, a linear positive magnetoresistance was found in the insulating regime, which is caused by spatial localization of itinerant electrons at some of the Re atoms, which formally adopt a $5d^1$ electronic configuration. X-ray diffraction, IR- and ESR-studies, temperature dependent magnetization and specific heat measurements in various magnetic fields suggest that the electronic structure of poly-MTO can safely be approximated by a purely 2D conductor.Comment: 15 pages, 16 figures, 2 table

Frustration on a centered pyrochlore lattice in metal-organic frameworks

Geometric frustration inhibits magnetic systems from ordering, opening a window to unconventional phases of matter. The paradigmatic frustrated lattice in three dimensions to host a spin liquid is the pyrochlore, although there remain few experimental compounds thought to realize such a state. Here, we go beyond the pyrochlore via molecular design in the metal-azolate framework [Mn(II)(ta)2], which realizes a closely related centered pyrochlore lattice of Mn spins with S=5/2. Despite a Curie-Weiss temperature of −21 K indicating the energy scale of magnetic interactions, [Mn(II)(ta)2] orders at only 430 mK, putting it firmly in the category of highly frustrated magnets. Comparing magnetization and specific-heat measurements to numerical results for a minimal Heisenberg model, we predict that this material displays distinct features of a classical spin liquid with a structure factor reflecting Coulomb physics in the presence of charges

Field cooling memory effect in Bi2212 and Bi2223 single crystals

A memory effect in the Josephson vortex system created by magnetic field in the highly anisotropic superconductors Bi2212 and Bi2223 is demonstrated using microwave power absorption. This surprising effect appears despite a very low viscosity of Josephson vortices compared to Abrikosov vortices. The superconductor is field cooled in DC magnetic field H_{m} oriented parallel to the CuO planes through the critical temperature T_{c} down to 4K, with subsequent reduction of the field to zero and again above H_{m}. Large microwave power absorption signal is observed at a magnetic field just above the cooling field clearly indicating a memory effect. The dependence of the signal on deviation of magnetic field from H_{m} is the same for a wide range of H_{m} from 0.15T to 1.7T

Low temperature properties of the Electron Spin Resonance in YbRh2Si2

We present the field and temperature behavior of the narrow Electron Spin Resonance (ESR) response in YbRh2Si2 well below the single ion Kondo temperature. The ESR g factor reflects a Kondo-like field and temperature evolution of the Yb3+ magnetism. Measurements towards low temperatures (>0.5K) have shown distinct crossover anomalies of the ESR parameters upon approaching the regime of a well defined heavy Fermi liquid. Comparison with the field dependence of specific heat and electrical resistivity reveal that the ESR parameters can be related to quasiparticle mass and cross section and, hence, contain inherent heavy electron properties.Comment: 4 pages, 6 figures; Manuscript for Proceedings of the International Conference on Quantum Criticality and Novel Phases (QCNP09, Dresden); subm. to pss(b

Memory effect and triplet pairing generation in the superconducting exchange biased Co/CoOx/Cu41Ni59/Nb/Cu 41Ni59 layered heterostructure

We fabricated a nanolayered hybrid superconductor-ferromagnet spin-valve structure, the resistive state of which depends on the preceding magnetic field polarity. The effect is based on a strong exchange bias (about -2 kOe) on a diluted ferromagnetic copper-nickel alloy and generation of a long range odd in frequency triplet pairing component. The difference of high and low resistance states at zero magnetic field is 90% of the normal state resistance for a transport current of 250 μA and still around 42% for 10 μA. Both logic states of the structure do not require biasing fields or currents in the idle mode. © 2013 AIP Publishing LLC