403 research outputs found
Spin-resonance modes of the spin-gap magnet TlCuCl_3
Three kinds of magnetic resonance signals were detected in crystals of the
spin-gap magnet TlCuCl_3.
First, we have observed the microwave absorption due to the excitation of the
transitions between the singlet ground state and the excited triplet states.
This mode has the linear frequency-field dependence corresponding to the
previously known value of the zero-field spin-gap of 156 GHz and to the closing
of spin-gap at the magnetic field H_c of about 50 kOe.
Second, the thermally activated resonance absorption due to the transitions
between the spin sublevels of the triplet excitations was found. These
sublevels are split by the crystal field and external magnetic field.
Finally, we have observed antiferromagnetic resonance absorption in the
field-induced antiferromagnetic phase above the critical field H_c. This
resonance frequency is strongly anisotropic with respect to the direction of
the magnetic field.Comment: v.2: typo correction (one of the field directions was misprinted in
the v.1
Linear dependence of peak width in \chi(\bq, \omega) vs T_c for YBCO superconductors
It is shown that the momentum space width of the peak in the spin
susceptibility, Im, is linearly proportional to the
superconducting : with \AA. This relation is similar to the linear relation between incommensurate
peak splitting and in LaSrCuO superconductors, as first proposed by
Yamada et al. (Phys. Rev. B 57, 6165, (1998)). The velocity is
smaller than Fermi velocity or the spin-wave velocity of the parent compound
and remains the same for a wide doping range. This result points towards strong
similarities in magnetic state of YBCO and LaSrCuO.Comment: 5 pages, 3 figures, latex fil
Theoretical Aspects of the Fractional Quantum Hall Effect in Graphene
We review the theoretical basis and understanding of electronic interactions
in graphene Landau levels, in the limit of strong correlations. This limit
occurs when inter-Landau-level excitations may be omitted because they belong
to a high-energy sector, whereas the low-energy excitations only involve the
same level, such that the kinetic energy (of the Landau level) is an
unimportant constant. Two prominent effects emerge in this limit of strong
electronic correlations: generalised quantum Hall ferromagnetic states that
profit from the approximate four-fold spin-valley degeneracy of graphene's
Landau levels and the fractional quantum Hall effect. Here, we discuss these
effects in the framework of an SU(4)-symmetric theory, in comparison with
available experimental observations.Comment: 12 pages, 3 figures; review for the proceedings of the Nobel
Symposium on Graphene and Quantum Matte
Effect of Nonmagnetic Impurities on the Magnetic Resonance Peak in YBa2Cu3O7
The magnetic excitation spectrum of a YBa_2 Cu_3 O_7 crystal containing 0.5%
of nonmagnetic (Zn) impurities has been determined by inelastic neutron
scattering. Whereas in the pure system a sharp resonance peak at E ~ 40 meV is
observed exclusively below the superconducting transition temperature T_c, the
magnetic response in the Zn-substituted system is broadened significantly and
vanishes at a temperature much higher than T_c. The energy-integrated spectral
weight observed near q = (pi,pi) increases with Zn substitution, and only about
half of the spectral weight is removed at T_c
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
Hypercoagulability resulting from opposite effects of lupus anticoagulants is associated strongly with thrombotic risk
Tuning the spin Hamiltonian of NENP by external pressure: a neutron scattering study
We report an inelastic neutron scattering study of antiferromagnetic spin
dynamics in the Haldane chain compound Ni(C2H8N2)2NO2ClO4 (NENP) under external
hydrostatic pressure P = 2.5 GPa. At ambient pressure, the magnetic excitations
in NENP are dominated by a long-lived triplet mode with a gap which is split by
orthorhombic crystalline anisotropy into a lower doublet centered at
1.2meV and a singlet at 2.5meV.
With pressure we observe appreciable shifts in these levels, which move to
1.45 meV and
2.2meV. The dispersion of these modes in the crystalline c-direction
perpendicular to the chain was measured here for the first time, and can be
accounted for by an interchain exchange J'_c approximately 3e-4*J which changes
only slightly with pressure. Since the average gap value 1.64
meV remains almost unchanged with P, we conclude that in NENP the application
of external pressure does not affect the intrachain coupling J appreciably, but
does produce a significant decrease of the single-ion anisotropy constant from
D/J = 0.16(2) at ambient pressure to D/J = 0.09(7) at P = 2.5 GPa.Comment: LaTeX file nenp_p.tex, 10 pages, 1 table, 5 figures. Submitted to
Phys. Rev.
Field-induced structural evolution in the spin-Peierls compound CuGeO: high-field ESR study
The dimerized-incommensurate phase transition in the spin-Peierls compound
CuGeO is probed using multifrequency high-resolution electron spin
resonance (ESR) technique, in magnetic fields up to 17 T. A field-induced
development of the soliton-like incommensurate superstructure is clearly
indicated as a pronounced increase of the ESR linewidth (magnon
excitations), with a at 13.8 T. The anomaly is
explained in terms of the magnon-soliton scattering, and suggests that the
soliton-like phase exists close to the boundary of the dimerized-incommensurate
phase transition. In addition, magnetic excitation spectra in 0.8% Si-doped
CuGeO are studied. Suppression of the anomaly observed in the
doped samples suggests a collapse of the long-range-ordered soliton states upon
doping, that is consistent with high-field neutron scattering experiments.Comment: Accepted to Phys. Rev.
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