2,037 research outputs found
Determination of the copper layer thickness in spin valves by grazing incidence x-ray fluorescence
We show that at the standard laboratory wavelength
of CuKα the scattering factors of Cu and Ni(-0.8)Fe(-0.2) are
identical, thereby making it impossible to distinguish the
boundary of the Cu spacer layer in a Cdpermalloy spin valve
structure from grazing incidence x-ray reflectivity curves. Use of grazing incidence fluorescence, in conjunction with x-ray reflectivity provides sufficient information to control the Cu layer thickness. We demonstrate the technique on two spin valves with Cu spacer layers differing in thickness by a factor
of 2.5
Ultrafast dephasing of coherent optical phonons in atomically controlled GeTe/SbTe superlattices
Femtosecond dynamics of coherent optical phonons in GeTe/SbTe
superlattices (SLs), a new class of semiconductor SLs with three different
states, have been investigated by using a reflection-type pump-probe technique
at various lattice temperatures. The time-resolved transient reflectivity (TR)
obtained in as-grown SLs exhibits the coherent A optical modes at 5.10
THz and 3.78 THz, while only the single A mode at 3.68 THz is observed in
annealed SLs. The decay rate of the A mode in annealed SLs is strongly
temperature dependent, while that in as-grown SLs is not temperature dependent.
This result indicates that the damping of the coherent A phonons in
amorphous SLs is governed by the phonon-defect (vacancy) scattering rather than
the anharmonic phonon-phonon coupling.Comment: 5 pages, 5 figure
Spin Defects in Spin-Peierls Systems
We examine spin-Peierls systems in the presence of spin defects which are
introduced by replacing magnetic ions with non-magnetic ones
in . By using the action for the bosonized Hamiltonian, it
is shown directly that the antiferromagnetic state induced by the spin defects
coexists with the spin-Peierls states. Further the doping dependences of both
transition temperature of spin-Peierls state and the spin gap have been
calculated. The transition temperature of the present estimation shows good
agreement quantitatively with that observed in Cu_{1-\de} Zn_\de O_3 for the
region of the doping rate, \de<0.02.Comment: jpsj style, 11 pages, 2 figure
Separation of the magnetic phases at the N\'{e}el point in the diluted spin-Peierls magnet CuGeO3
The impurity induced antiferromagnetic ordering of the doped spin-Peierls
magnet Cu(1-x)Mg(x)GeO(3) was studied by ESR technique. Crystals with the Mg
concentration x<4% demonstrate a coexistence of paramagnetic and
antiferromagnetic ESR modes. This coexistence indicates the separation of a
macroscopically uniform sample in the paramagnetic and antiferromagnetic
phases. In the presence of the long-range spin-Peierls order (in a sample with
x=1.71%) the volume of the antiferromagnetic phase immediately below the
N\'{e}el point T_N is much smaller than the volume of the paramagnetic phase.
In the presence of the short-range spin-Peierls order (in samples with x=2.88%,
x= 3.2%) there are comparable volumes of paramagnetic and antiferromagnetic
phases at T=T_N. The fraction of the antiferromagnetic phase increases with
lowering temperature. In the absence of the spin-Peierls dimerization (at
x=4.57%)the whole sample exhibits the transition into the antiferromagnetic
state and there is no phase separation. The phase separation is explained by
the consideration of clusters of staggered magnetization located near impurity
atoms. In this model the areas occupied by coherently correlated spins expand
with decreasing temperature and the percolation of the ordered area through a
macroscopic distance occurs.Comment: 7pages, 10 figure
Elementary excitations, exchange interaction and spin-Peierls transition in CuGeO
The microscopic description of the spin-Peierls transition in pure and doped
CuGeO_3 is developed taking into account realistic details of crystal
structure. It it shown that the presence of side-groups (here Ge) strongly
influences superexchange along Cu-O-Cu path, making it antiferromagnetic.
Nearest-neighbour and next-nearest neighbour exchange constants and
are calculated. Si doping effectively segments the CuO_2-chains
leading to or even slightly ferromagnetic. Strong
sensitivity of the exchange constants to Cu-O-Cu and (Cu-O-Cu)-Ge angles may be
responsible for the spin-Peierls transition itself (``bond-bending mechanism''
of the transition). The nature of excitations in the isolated and coupled
spin-Peierls chains is studied and it is shown that topological excitations
(solitons) play crucial role. Such solitons appear in particular in doped
systems (Cu_{1-x}Zn_xGeO_3, CuGe_{1-x}Si_xO_3) which can explain the
phase diagram.Comment: 7 pages, revtex, 7 Postscript figure
First proof of concept of remote attendance for future observation strategies between Wettzell (Germany) and Concepción (Chile)
Current VLBI observations are controlled and attended locally at the radio telescopes on the basis of pre-scheduled session files. Operations have to deal with system specific station commands and individual setup procedures. Neither the scheduler nor the correlator nor the data-analyst gets real-time feedback about system parameters during a session. Changes in schedules after the start of a session by remote are impossible or at least quite difficult. For future scientific approaches, a more flexible mechanism would optimize the usage of resources at the sites. Therefore shared-observation control between world-wide telescope s, remote attendance/control as well as completely unattended-observations could be useful, in addition to the classic way to run VLBI observations. To reach these goals, the Geodetic Observatory Wettzell in cooperation with the Max-Planck-Institute for Radio Astronomy (Bonn) have developed a software extension to the existing NASA Field System for remote control. It uses the principle of a remotely accessible, autonomous process cell as server extension to the Field System on the basis of Remote Procedure Calls (RPC). Based on this technology the first completely remote attended and controlled geodetic VLBI session between Wettzell, Germany and Concepción, Chile was successfully performed over 24 hours. This first test was extremely valuable for gathering information about the differences between VLBI systems and measuring the performance of internet connections and automatic connection re-establishments. During the 24h-session, the network load, the number of sent/received packages and the transfer speed were monitor ed and captured. It was a first reliable test for the future wishes to control several telescopes with one graphical user interface on different data transfer rates over large distances in an efficient way. In addition, future developments for an authentication and user role management will be realized within the upcoming NEXPReS project
Critical neutron scattering study of the compositional phase transition in Mg-doped CuGeO3
Cu1-xMgxGeO3 undergoes a first-order phase transition at a critical
concentration xc between an antiferromagnetic (AF) state on dimerized lattice
(D-AF) and an AF Neel state on undistorted uniform lattice (U-AF). Previous
magnetic susceptibility measurements showed xc = 0.023 while a recent neutron
scattering study reported xc = 0.027 +- 0.001. The present critical scattering
due to antiferromagnetic fluctuations near the superlattice reflection
(0,1,1/2) unambiguously determines xc = 0.028 +- 0.001 at TN = 3.4 ~ 4 K. Also
at T = 1.3 K, the phase boundary was determined as xc = 0.028 +- 0.001 by
observation of a jump of an effective magnetic moment across xc.Comment: 8 pages,3 figures, Post Scripts, PACS numbers: 75.30.Kz, 75.40.-s,
61.12, 75.30.K. submitted to Journal of Physical Society of Japa
Infrared study of spin-Peierls compound alpha'-NaV2O5
Infrared reflectance of alpha'-NaV2O5 single crystals in the frequency range
from 50 cm-1 to 10000 cm-1 was studied for a, b and c-polarisations. In
addition to phonon modes identification, for the a-polarised spectrum a broad
continuum absorption in the range of 1D magnetic excitation energies was found.
The strong near-IR absorption band at 0.8 eV shows a strong anisotropy with
vanishing intensity in c-polarisation. Activation of new phonons due to the
lattice dimerisation were detected below 35K as well as pretransitional
structural fluctuations up to 65K.Comment: 3 pages, 2 figures, 1 table. Contributed paper for the SCES'98 (15-18
July 1998, Paris). To be published in Physica
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