57 research outputs found
Nonlogarithmic magnetization relaxation at the initial time intervals and magnetic field dependence of the flux creep rate in Bi2Sr2Ca(sub I)Cu2Ox single crystals
At the initial time intervals, preceding the thermally activated flux creep regime, fast nonlogarithmic relaxation is found. The fully magnetic moment Pm(t) relaxation curve is shown. The magnetic measurements were made using SQUID-magnetometer. Two different relaxation regimes exist. The nonlogarithmic relaxation for the initial time intervals may be related to the viscous Abrikosov vortices flow with j is greater than j(sub c) for high enough temperature T and magnetic field induction B. This assumption correlates with Pm(t) measurements. The characteristic time t(sub O) separating two different relaxation regimes decreases as temperature and magnetic field are lowered. The logarithmic magnetization relaxation curves Pm(t) for fixed temperature and different external magnetic field inductions B are given. The relaxation rate dependence on magnetic field, R(B) = dPm(B, T sub O)/d(1nt) has a sharp maximum which is similar to that found for R(T) temperature dependences. The maximum shifts to lower fields as temperature goes up. The observed sharp maximum is related to a topological transition in shielding critical current distribution and, consequently, in Abrikosov vortices density. The nonlogarithmic magnetization relaxation for the initial time intervals is found. This fast relaxation has almost an exponentional character. The sharp relaxation rate R(B) maximum is observed. This maximum corresponds to a topological transition in Abrikosov vortices distribution
Optical spectra, crystal-field parameters, and magnetic susceptibility of the new multiferroic NdFe3(BO3)4
We report high-resolution optical absorption spectra for NdFe3(BO3)4 trigonal
single crystal which is known to exhibit a giant magnetoelectric effect below
the temperature of magnetic ordering TN = 33 K. The analysis of the
temperature-dependent polarized spectra reveals the energies and, in some
cases, symmetries and exchange splittings of Nd3+ 84 Kramers doublets. We
perform crystal-field calculations starting from the exchange-charge model,
obtain a set of six real crystal-field parameters, and calculate wave functions
and magnetic g-factors. In particular, the values g(perpendicular) = 2.385,
g(parallel) = 1.376 were found for the Nd3+ ground-state doublet. We obtain
Bloc=7.88 T and |JFN|= 0.48 K for the values of the local effective magnetic
field at liquid helium temperatures at the Nd3+ site and the Nd - Fe exchange
integral, respectively, using the experimentally measured Nd3+ ground-state
splitting of 8.8 cm-1. To check reliability of our set of crystal field
parameters we model the magnetic susceptibility data from literature. A dimer
containing two nearest-neighbor iron ions in the spiral chain is considered to
partly account for quasi-one-dimensional properties of iron borates, and then
the mean-field approximation is used. The results of calculations with the
exchange parameters for Fe3+ ions Jnn = -6.25 K (intra-chain interactions) and
Jnnn = -1.92 K (inter-chain interactions) obtained from fitting agree well with
the experimental data.Comment: 13 pages, 8 figures, 2 table
Magnetic Resonance of the Intrinsic Defects of the Spin-Peierls Magnet CuGeO3
ESR of the pure monocrystals of CuGeO3 is studied in the frequency range 9-75
GHz and in the temperature interval 1.2-25 K. The splitting of the ESR line
into several spectral components is observed below 5 K, in the temperature
range where the magnetic susceptibility is suppressed by the spin-Peierls
dimerization. The analysis of the magnetic resonance signals allows one to
separate the signals of the S=1/2- and S=1 defects of the spin-Peierls phase.
The value of g-factor of these signals is close to that of the Cu-ion. The
additional line of the magnetic resonance is characterized by an anomalous
value of the g-factor and by the threshold-like increase of the microwave
susceptibility when the microwave power is increasing. The ESR signals are
supposingly attributed to two types of the planar magnetic defects, arising at
the boundaries of the domains of the spin-Peierls state with the different
values of the phase of the dimerization.Comment: LATEX-text, 12 PS-figures, typos corrected, LATEX-style change
ab-plane tunneling and Andreev spectroscopy of superconducting gap and pseudogap in (Bi,Pb)2Sr2Ca2Cu3O10 and Bi2Sr2CaCu2O8
We have measured the temperature dependence of gap features revealed by
Andreev reflection Delta_s and by tunneling Delta in the ab-plane of optimal
and slightly overdoped microcrystals of (BiPb)2Sr2Ca2Cu3O10 (Bi2223) with
critical temperature Tc=110-115 K, and Bi2Sr2CaCu2O8 (Bi2212) with Tc=80-84 K.
The tunneling conductance of Bi2223-Insulator-Bi2223 junction shows peaks at
the 2Delta gap voltage, as well as dips and broad humps at other voltages. In
Bi2223, similarly to the well known Bi2212 spectra, the energies corresponding
to 2Delta, to the dip, and to the hump structure are in the ratio of 2:3:4.
This confirms that the dip and hump features are generic to the high
temperature superconductors, irrespective of the number of CuO2 layers or the
BiO superstructure. On the other hand, in both compounds Delta(T) and
Delta_s(T) dependences are completely different, and we conclude that the two
entities have different nature.Comment: LaTeX 2e, 17 pages, 7 figures in .eps forma
Folded modes in the infrared spectra of the spin-Peierls phase of CuGeO_3
Polarized far-infrared transmittance spectra of CuGeO_3 single crystals were
measured at different temperatures (6K < T < 300K). Two spectral lines, at
284.2 cm-1 in E||c polarization and at 311.7 cm-1 in E||b polarization, appear
at the temperature of the spin-Peierls transition and grow in intensity with
decreasing temperature. Both of them are, most probably, folded modes of the
dimerized lattice. We discuss a possible role of the spin-phonon interaction in
the formation of the 311.7 cm-1 feature.Comment: 4 pages, 5 figures, 1 table; Submitted to Phys.Rev.B Second revision.
Figures and text were slightly change
Analytical Bethe Ansatz for closed and open gl(n)-spin chains in any representation
We present an "algebraic treatment" of the analytical Bethe Ansatz. For this
purpose, we introduce abstract monodromy and transfer matrices which provide an
algebraic framework for the analytical Bethe Ansatz. It allows us to deal with
a generic gl(n)-spin chain possessing on each site an arbitrary
gl(n)-representation. For open spin chains, we use the classification of the
reflection matrices to treat all the diagonal boundary cases. As a result, we
obtain the Bethe equations in their full generality for closed and open spin
chains. The classifications of finite dimensional irreducible representations
for the Yangian (closed spin chains) and for the reflection algebras (open spin
chains) are directly linked to the calculation of the transfer matrix
eigenvalues. As examples, we recover the usual closed and open spin chains, we
treat the alternating spin chains and the closed spin chain with impurity
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