Fast magnetization reversal of nanoclusters in resonator

An effective method for ultrafast magnetization reversal of nanoclusters is suggested. The method is based on coupling a nanocluster to a resonant electric circuit. This coupling causes the appearance of a magnetic feedback field acting on the cluster, which drastically shortens the magnetization reversal time. The influence of the resonator properties, nanocluster parameters, and external fields on the magnetization dynamics and reversal time is analyzed. The magnetization reversal time can be made many orders shorter than the natural relaxation time. The reversal is studied for both the cases of a single nanocluster as well as for the system of many nanoclusters interacting through dipole forces.Comment: latex file, 21 pages, 7 figure

Transition state method and Wannier functions

We propose a computational scheme for materials where standard Local Density Approximation (LDA) fails to produce a satisfactory description of excitation energies. The method uses Slater's "transition state" approximation and Wannier functions basis set. We define a correction to LDA functional in such a way that its variation produces one-electron energies for Wannier functions equal to the energies obtained in "transition state" constrained LDA calculations. In the result eigenvalues of the proposed functional could be interpreted as excitation energies of the system under consideration. The method was applied to MgO, Si, NiO and BaBiO$_3$ and gave an improved agreement with experimental data of energy gap values comparing with LDA.Comment: 13 pages, 6 figures, 1 tabl

Field Effect Transistor Based on KTaO3 Perovskite

An n-channel accumulation-type field effect transistor (FET) has been fabricated utilizing a KTaO3 single crystal as an active element and a sputtered amorphous Al2O3 film as a gate insulator. The device demonstrated an ON/OFF ratio of 10^4 and a field effect mobility of 0.4cm^2/Vs at room temperature, both of which are much better than those of the SrTiO3 FETs reported previously. The field effect mobility was almost temperature independent down to 200K. Our results indicate that the Al2O3 / KTaO3 interface is worthy of further investigations as an alternative system of future oxide electronics.Comment: 3 pages, 3 Postscript figures, submitted to Appl.Phys.Let

Low temperature metallic state induced by electrostatic carrier doping of SrTiO3_3

Transport properties of SrTiO$_3$-channel field-effect transistors with parylene organic gate insulator have been investigated. By applying gate voltage, the sheet resistance falls below $R_{\Box}$ $\sim$ 10 k$\Omega$ at low temperatures, with carrier mobility exceeding 1000 cm$^2$/Vs. The temperature dependence of the sheet resistance taken under constant gate voltage exhibits metallic behavior ($dR$/$dT$ $>$ 0). Our results demonstrate an insulator to metal transition in SrTiO$_3$ driven by electrostatic carrier density control.Comment: 3 pages, 4 figure

Field-Effect Transistor on SrTiO3 with sputtered Al2O3 Gate Insulator

A field-effect transistor that employs a perovskite-type SrTiO3 single crystal as the semiconducting channel is revealed to function as n-type accumulation-mode device with characteristics similar to that of organic FET's. The device was fabricated at room temperature by sputter-deposition of amorphous Al2O3 films as a gate insulator on the SrTiO3 substrate. The field-effect(FE) mobility is 0.1cm2/Vs and on-off ratio exceeds 100 at room temperature. The temperature dependence of the FE mobility down to 2K shows a thermal-activation-type behavior with an activation energy of 0.6eV

muSR study of the Cu-spin dynamics in the electron-doped high-Tc cuprate of Pr0.86LaCe0.14Cu1-y(Zn,Ni)yO4

Effects of the Zn- and Ni-substitution on the Cu-spin dynamics in the electron-doped Pr0.86LaCe0.14Cu1-y(Zn,Ni)yO4+a-d with y = 0, 0.01, 0.02, 0.05 and different values of the reduced oxygen content d have been studied using zero-field muon-spin-relaxation (muSR) measurements at temperatures down to 2 K. For the as-grown sample (d = 0, y = 0) and the sample with a very small d value (d < 0.01, y = 0), a muon-spin precession due to long-range antiferromagnetic order has been observed. On the other hand, no precession has been observed for moderately oxygen-reduced samples (0.01 < d < 0.09). It has been found that for all the samples of 0.01 < d < 0.09 the asymmetry A(t) (muSR time spectrum) in the long-time region increases with decreasing temperature at low temperatures, suggesting possible slowing-down of the Cu-spin fluctuations. On the other hand, no significant difference between Zn- and Ni-substitution effects on the slowing down of the Cu-spin fluctuations has been observed.Comment: 4 pages, 2 figures, Proceeding of 10th muSR conference 2005, to be published in Physica

Effects of Fermi surface and superconducting gap structure in the field-rotational experiments: A possible explanation of the cusp-like singularity in YNi2_2B2_2C

We have studied the field-orientational dependence of zero-energy density of states (FODOS) for a series of systems with different Fermi surface and superconducting gap structures. Instead of phenomenological Doppler-shift method, we use an approximate analytical solution of Eilenberger equation together with self-consistent determination of order parameter and a variational treatment of vortex lattice. First, we compare zero-energy density of states (ZEDOS) when a magnetic field is applied in the nodal direction ($\nu_{node}(0)$) and in the antinodal direction ($\nu_{anti}(0)$), by taking account of the field-angle dependence of order parameter. As a result, we found that there exists a crossover magnetic field $H^*$ so that $\nu_{anti}(0) > \nu_{node}(0)$ for $H \nu_{anti}(0)$ for $H > H^*$, consistent with our previous analyses. Next, we showed that $H^*$ and the shape of FODOS are determined by contribution from the small part of Fermi surface where Fermi velocity is parallel to field-rotational plane. In particular, we found that $H^*$ is lowered and FODOS has broader minima, when a superconducting gap has point nodes, in contrast to the result of the Doppler-shift method. We also studied the effects of in-plane anisotropy of Fermi surface. We found that in-plane anisotropy of quasi-two dimensional Fermi surface sometimes becomes larger than the effects of Doppler-shift and can destroy the Doppler-shift predominant region. In particular, this tendency is strong in a multi-band system where superconducting coherence lengths are isotropic. Finally, we addressed the problem of cusp-like singularity in YNi$_2$B$_2$C and present a possible explanation of this phenomenon.Comment: 13pages, 23figure

Fulde-Ferrell-Larkin-Ovchinnikov state in a perpendicular field of quasi two-dimensional CeCoIn5

A Fulde-Ferrell-Larkin-Ovchinnkov (FFLO) state was previously reported in the quasi-2D heavy fermion CeCoIn5 when a magnetic field was applied parallel to the ab-plane. Here, we conduct 115^In NMR studies of this material in a PERPENDICULAR field, and provide strong evidence for FFLO in this case as well. Although the topology of the phase transition lines in the H-T phase diagram is identical for both configurations, there are several remarkable differences between them. Compared to H//ab, the FFLO region for H perpendicular to the ab-plane shows a sizable decrease, and the critical field separating the FFLO and non-FFLO superconducting states almost ceases to have a temperature dependence. Moreover, directing H perpendicular to the ab-plane results in a notable change in the quasiparticle excitation spectrum within the planar node associated with the FFLO transition.Comment: 5 pages, 3 figure