Search

While several experiments claim that two-level system (TLS) defects in amorphous surfaces/interfaces are responsible for energy relaxation in superconducting resonators and qubits, none can provide quantitative explanation of their data in terms of the conventional noninteracting TLS model. Here a model that interpolates between the interacting and noninteracting TLS loss tangent is proposed to perform numerical analysis of experimental data and extract information about TLS parameters and their distribution. As a proof of principle, the model is applied to TESLA cavities that contain only a single lossy material in their interior, the niobium/niobium oxide interface. The best fits show interacting TLSs with a sharp modulus of electric dipole moment for both thin (5 nm) and thick (100 nm) oxides, indicating that the TLSs are "atomic" instead of "glassy". The proposed method can be applied to other devices with multiple material interfaces and substrates, with the goal of elucidating the nature of TLSs causing energy loss in resonators and qubits., Comment: 7 pages, 2 figures

The main feature of amorphous materials is the presence of excess vibrational modes at low energies, giving rise to the so called "boson peak" in neutron and optical spectroscopy. These same modes manifest themselves as two level systems (TLSs) causing noise and decoherence in qubits and other sensitive devices. Here we present an experiment that uses the spin relaxation of dangling bonds at the Si/(amorphous)SiO$_2$ interface as a probe of TLSs. We introduce a model that is able to explain the observed non-exponential electron spin inversion recovery and provides a measure of the degree of spatial localization and concentration of the TLSs close to the interface, their maximum energy and its temperature dependence., Comment: 4 pages, 2 figures, regular paper

Optical spectroscopy has been combined with computational and theoretical techniques to show how the spin dynamics in the model multiferroic BiFeO3 responds to the application of hydrostatic pressure and its corresponding series of structural phase transitions from R3c to the Pnma phases. As pressure increases, multiple spin excitations associated with non-collinear cycloidal magnetism collapse into two excitations, which show jump discontinuities at some of the ensuing crystal phase transitions. Effective Hamiltonian approach provides information on the electrical polarization and structural changes of the oxygen octahedra through the successive structural phases. The extracted parameters are then used in a Ginzburg-Landau model to reproduce the evolution with pressure of the spin waves excitations observed at low energy and we demonstrate that the structural phases and the magnetic anisotropy drive and control the spin excitations., Comment: 5 pages, 4 figures

To face the challenges lying beyond current CMOS-based technology, new paradigms for information processing are required. Magnonics proposes to use spin waves to carry and process information, in analogy with photonics that relies on light waves, with several advantageous features such as potential operation in the THz range and excellent coupling to spintronics. Several magnonic analog and digital logic devices have been proposed, and some demonstrated. Just as for spintronics, a key issue for magnonics is the large power required to control/write information (conventionally achieved through magnetic fields applied by strip lines, or by spin transfer from large spin-polarized currents). Here we show that in BiFeO3, a room-temperature magnetoelectric material, the spin wave frequency (>600 GHz) can be tuned electrically by over 30%, in a non-volatile way and with virtually no power dissipation. Theoretical calculations indicate that this effect originates from a linear magnetoelectric effect related to spin-orbit coupling induced by the applied electric field. We argue that these properties make BiFeO3 a promising medium for spin wave generation, conversion and control in future magnonics architectures., Comment: 3 figures

Recently, oxide multiferroics have attracted much attention due to their large magnetoelectric effect which allows the tuning of magnetic properties with electric field and vice versa and open new venues for future spintronic applications such as multiple-state memory devices with dual magnetic and electric control. BiFeO$_3$ (BFO) belongs to this new class of materials and shows both ferroelectric and antiferromagnetic orders at room temperature with a large electric polarizationassociated with a cycloidal spiral magnetic ordering. The incommensurate magnetic order induces magnon zone folding and allows investigations by optical probes of unusual spin waves which couples to optical phonons, the so called "`electromagnons"'. Here, we unravel for the first time the electromagnon spectra of BFO by means low energy inelastic light scattering technique. We show the existence of two species of electromagnons corresponding to spin wave excitations in and out of the cycloidal plane. The present observations present an unique opportunity to study the interplay between ferroelectric and magnetic orders.

In this work, we have calculated the vibrational modes and frequencies of the crystalline PPP (in both the Pbam and Pnnm symmetries) and PPV (in the P21/c symmetry). Our results are in good agreement with the available experimental data. Also, we have calculated the temperature dependence of their specific heats at constant volume, and of their vibrational entropies. Based on our results, at high temperatures, the PPP is more stable in the Pnnm structure than in the Pbam one., Comment: 5 pages, 7 figures, accepted for publication in Braz. J. Phys., special number, Proceedings of BWSP-12, 12th Brazilian Workshop on Semiconductor Physics

We implanted ultra low doses (2x10^11 cm-2) of 121Sb ions into isotopically enriched 28Si and find high degrees of electrical activation and low levels of dopant diffusion after rapid thermal annealing. Pulsed Electron Spin Resonance shows that spin echo decay is sensitive to the dopant depths, and the interface quality. At 5.2 K, a spin decoherence time, T2, of 0.3 ms is found for profiles peaking 50 nm below a Si/SiO2 interface, increasing to 0.75 ms when the surface is passivated with hydrogen. These measurements provide benchmark data for the development of devices in which quantum information is encoded in donor electron spins.

The transverse Meissner effect (TME) in the highly layered superconductor $Bi_2Sr_2CaCu_2O_{8+y}$ with columnar defects is investigated by transport measurements. We present detailed evidence for the persistence of the Bose glass phase for $H_{\perp}

The transverse Meissner effect (TME) in the highly layered superconductor Bi2Sr2CaCu2O(8+y) with columnar defects is investigated by transport measurements. We present detailed evidence for the persistence of the Bose-glass phase when H is tilted at an angle theta < theta_c (T) away from the column direction: (i) the variable-range vortex hopping process for low currents crosses over to the half-loops regime for high currents; (ii) in both regimes near theta_c(T) the energy barriers vanish linearly with tan(theta) ; (iii) the transition temperature is governed by T_{BG}(0) -T_{BG}(theta) sim |tan(theta)|^{1/\nu_{\perp}} with \nu_{\perp}=1.0 +/- 0.1. Furthermore, above the transition as theta->\theta_c+, moving kink chains consistent with a commensurate-incommensurate transition scenario are observed. These results thereby clearly show the existence of the TME for theta < theta_c(T)., Comment: 4 pages, RevTeX, 5 EPS figures

We study the effect of hole hopping in a doped antiferromagnet described by the `t-J' model, using exact analytic solutions for small clusters. In spite of the small size, they reveal interesting details about the magnetic order, which are not apparent in Mean Field treatments or in numerical calculations. The 4-site cluster with one hole yields the most interesting physics, displaying different behaviors for the ground state: i) an antiferromagnetic phase for t<>J, we obtain strong ferromagnetic correlations (maximum spin) and no density waves, with quantum fluctuations precluding the saturation of the magnetic moment. This behavior shows traces of a metal-insulator transition as the hole kinetic energy competes with the antiferromagnetic interactions., Comment: 12 pages and 4 figures, standard LaTex file of 30 KB, figures in postscript format

We report on transport measurements in a Bi-2212 single crystal with columnar defects parallel to the c-axis. The tilt of the magnetic field away from the direction of the tracks is studied for filling factors f=B_z/B_phi<1. Near the Bose Glass transition temperature T_BG, the angular scaling laws are verified and we find the field independent critical exponents nu'=1.1 and z'=5.30. Finally, above H_perpC we evidence the signature of a smectic-A like vortex phase. These experimental results provide support for the Bose Glass theory., Comment: 2 pages LaTeX, 2 EPS figures, uses fleqn and espcrc2 style macros. Submitted to Proceedings of M2S-HTSC-VI

We report on vortex transport in Bi_{2}Sr_{2}Ca_{1-x}Y_{x}Cu_{2}O_{8} crystals irradiated at different doses of heavy ions. We show evidence of a flux-creep resistivity typical of a variable-range vortex hopping mechanism as predicted by Nelson and Vinokur., Comment: 5 pages LaTeX2e (uses elsart.cls), 1 Encapsulated PostScript figure