Photon and spin dependence of the resonance lines shape in the strong coupling regime

We study the quantum dynamics of a spin ensemble coupled to cavity photons. Recently, related experimental results have been reported, showing the existence of the strong coupling regime in such systems. We study the eigenenergy distribution of the multi-spin system (following the Tavis-Cummings model) which shows a peculiar structure as a function of the number of cavity photons and of spins. We study how this structure causes changes in the spectrum of the admittance in the linear response theory, and also the frequency dependence of the excited quantities in the stationary state under a probing field. In particular, we investigate how the structure of the higher excited energy levels changes the spectrum from a double-peak structure (the so-called vacuum field Rabi splitting) to a single peak structure. We also point out that the spin dynamics in the region of the double-peak structure corresponds to recent experiments using cavity ringing while in region of the single peak structure, it corresponds to the coherent Rabi oscillation in a driving electromagnetic filed. Using a standard Lindblad type mechanism, we study the effect of dissipations on the line width and separation in the computed spectra. In particular, we study the relaxation of the total spin in the general case of a spin ensemble in which the total spin of the system is not specified. The theoretical results are correlated with experimental evidence of the strong coupling regime, achieved with a spin 1/2 ensemble

MnAs dots grown on GaN(0001)-(1x1) surface

MnAs has been grown by means of MBE on the GaN(0001)-(1x1) surface. Two options of initiating the crystal growth were applied: (a) a regular MBE procedure (manganese and arsenic were delivered simultaneously) and (b) subsequent deposition of manganese and arsenic layers. It was shown that spontaneous formation of MnAs dots with the surface density of 1$\cdot 10^{11}$ cm$^{-2}$ and $2.5\cdot 10^{11}$ cm$^{-2}$, respectively (as observed by AFM), occurred for the layer thickness higher than 5 ML. Electronic structure of the MnAs/GaN systems was studied by resonant photoemission spectroscopy. That led to determination of the Mn 3d - related contribution to the total density of states (DOS) distribution of MnAs. It has been proven that the electronic structures of the MnAs dots grown by the two procedures differ markedly. One corresponds to metallic, ferromagnetic NiAs-type MnAs, the other is similar to that reported for half-metallic zinc-blende MnAs. Both system behave superparamagnetically (as revealed by magnetization measurements), but with both the blocking temperatures and the intra-dot Curie temperatures substantially different. The intra-dot Curie temperature is about 260 K for the former system while markedly higher than room temperature for the latter one. Relations between growth process, electronic structure and other properties of the studied systems are discussed. Possible mechanisms of half-metallic MnAs formation on GaN are considered.Comment: 20+ pages, 8 figure

Role of defects on the electronic and magnetic properties of CrAs/InAs and CrAs/CdSe half-metallic interfaces

We present an extended study of single impurity atoms at the interface between the half-metallic ferromagnetic zinc-blende CrAs compound and the zinc-blende binary InAs and CdSe semiconductors in the form of very thin multilayers. Contrary to the case of impurities in the perfect bulk CrAs studied in [I. Galanakis and S.G. Pouliasis, J. Magn. Magn. Mat. 321 (2009) 1084] defects at the interfaces do not alter in general the half-metallic character of the perfect systems. The only exception are Void impurities at Cr or In(Cd) sites which lead, due to the lower-dimensionality of the interfaces with respect to the bulk CrAs, to a shift of the $p$ bands of the nearest neighboring As(Se) atom to higher energies and thus to the loss of the half-metallicity. But Void impurities are Schottky-type and should exhibit high formation energies and thus we expect the interfaces in the case of thin multilayers to exhibit a robust half-metallic character

Influence of defects on the electronic and magnetic properties of half-metallic CrAs, CrSe and CrSb zinc-blende compounds

We present an extended study of single impurity atoms and atomic swaps in half-metallic CrAs, CrSb and CrSe zinc-blende compounds. Although the perfect alloys present a rather large gap in the minority-spin band, all defects under study, with the exception of void impurities at Cr and sp sites and Cr impurities at sp sites (as long as no swap occurs), induce new states within the gap. The Fermi level can be pinned within these new minority states depending on the lattice constant used for the calculations and the electronegativity of the sp atoms. Although these impurity states are localized in space around the impurity atoms and very fast we regain the bulk behavior, their interaction can lead to wide bands within the gap and thus loss of the half-metallic character

Detection of Phase Jumps of Free Core Nutation of the Earth and their Concurrence with Geomagnetic Jerks

We detected phase jumps of the Free Core Nutation (FCN) of the Earth directly from the analysis of the Very Long Baseline Interferometer (VLBI) observation of the Earth rotation for the period 1984-2003 by applying the Weighted Wavelet Z-Transform (WWZ) method and the Short-time Periodogram with the Gabor function (SPG) method. During the period, the FCN had two significant phase jumps in 1992 and 1998. These epochs coincide with the reported occurrence of geomagnetic jerks.Comment: 8 pages, 4 figure

Ab initio study on the magneto-structural properties of MnAs

The magnetic and structural properties of MnAs are studied with ab initio methods, and by mapping total energies onto a Heisenberg model. The stability of the different phases is found to depend mainly on the volume and on the amount of magnetic order, confirming previous experimental findings and phenomenological models. It is generally found that for large lattice constants the ferromagnetic state is favored, whereas for small lattice constants different antiferromagnetic states can be stabilized. In the ferromagnetic state the structure with minimal energy is always hexagonal, whereas it becomes orthorhombically distorted if there is an antiferromagnetic component in the hexagonal plane. For the paramagnetic state the stable cell is found to be orthorhombic up to a critical lattice constant of about 3.7 Angstrom, above which it remains hexagonal. This leads to the second order structural phase transition between paramagnetic states at about 400 K, where the lattice parameter increases above this critical value with rising temperature due to the thermal expansion. For the paramagnetic state an analytic approximation for the magnitude of the orthorhombic distortion as a function of the lattice constant is given. Within the mean field approximation the dependence of the Curie temperature on the volume and on the orthorhombic distortion is calculated. For orthorhombically distorted cells the Curie temperature is much smaller than for hexagonal cells. This is mainly due to the fact that some of the exchange coupling constants in the hexagonal plane become negative for distorted cells. With these results a description of the susceptibility as function of temperature is given

Fe XI emission lines in a high resolution extreme ultraviolet spectrum obtained by SERTS

New calculations of radiative rates and electron impact excitation cross sections for Fe XI are used to derive emission line intensity ratios involving 3s^23p^4 - 3s^23p^33d transitions in the 180-223 A wavelength range. These ratios are subsequently compared with observations of a solar active region, obtained during the 1995 flight Solar EUV Research Telescope and Spectrograph (SERTS). The version of SERTS flown in 1995 incorporated a multilayer grating that enhanced the instrumental sensitivity for features in the 170 - 225 A wavelength range, observed in second-order between 340 and 450 A. This enhancement led to the detection of many emission lines not seen on previous SERTS flights, which were measured with the highest spectral resolution (0.03 A) ever achieved for spatially resolved active region spectra in this wavelength range. However, even at this high spectral resolution, several of the Fe XI lines are found to be blended, although the sources of the blends are identified in the majority of cases. The most useful Fe XI electron density diagnostic line intensity ratio is I(184.80 A)/I(188.21 A). This ratio involves lines close in wavelength and free from blends, and which varies by a factor of 11.7 between N_e = 10^9 and 10^11 cm^-3, yet shows little temperature sensitivity. An unknown line in the SERTS spectrum at 189.00 A is found to be due to Fe XI, the first time (to our knowledge) this feature has been identified in the solar spectrum. Similarly, there are new identifications of the Fe XI 192.88, 198.56 and 202.42 A features, although the latter two are blended with S VIII/Fe XII and Fe XIII, respectively.Comment: 21 pages, 9 gigures, accepted for publication in the Astrophysical Journa

Discovery Potential for Low-Scale Gauge Mediation at Early LHC

Low-scale gauge-mediated supersymmetry(SUSY)-breaking (GMSB) models with gravitino mass m_{3/2}<16 eV are attractive, since there are no flavor and cosmological problems. In this paper, we thoroughly study the collider signal in the case that the next-to-lightest SUSY particle is the bino or slepton and investigate the discovery potential of the LHC. Our result is applicable to a wider class of GMSB models other than the minimal GMSB models and we pay particular attention to realistic experimental setups. We also apply our analysis to the minimal GMSB models with a metastable SUSY-breaking vacuum and we show, by requiring sufficient stability of the SUSY-breaking vacuum, these models can be tested at an early stage of the LHC.Comment: 21 pages, 7 figures.Texts in section 3.2.2 and 3.2.4 are revised. Captions change

Surface Half-Metallicity of CrAs in the Zinc-Blende Structure

The development of new techniques such as the molecular beam epitaxy have enabled the growth of thin films of materials presenting novel properties. Recently it was made possible to grow a CrAs thin-film in the zinc-blende structure. In this contribution, the full-potential screened KKR method is used to study the electronic and magnetic properties of bulk CrAs in this novel phase as well as the Cr and As terminated (001) surfaces. Bulk CrAs is found to be half-ferromagnetic for all three GaAs, AlAs and InAs experimental lattice constants with a total spin magnetic moment of 3 $\mu_B$. The Cr-terminated surface retains the half-ferromagnetic character of the bulk, while in the case of the As-termination the surface states destroy the gap in the minority-spin band.Comment: 4 pages, 2 figures, new text, new titl

General moments of the inverse real Wishart distribution and orthogonal Weingarten functions

Let $W$ be a random positive definite symmetric matrix distributed according to a real Wishart distribution and let $W^{-1}=(W^{ij})_{i,j}$ be its inverse matrix. We compute general moments $\mathbb{E} [W^{k_1 k_2} W^{k_3 k_4} ... W^{k_{2n-1}k_{2n}}]$ explicitly. To do so, we employ the orthogonal Weingarten function, which was recently introduced in the study for Haar-distributed orthogonal matrices. As applications, we give formulas for moments of traces of a Wishart matrix and its inverse.Comment: 29 pages. The last version differs from the published version, but it includes Appendi