1,759 research outputs found
Wilson ratio of a Tomonaga-Luttinger liquid in a spin-1/2 Heisenberg ladder
Using micromechanical force magnetometry, we have measured the magnetization
of the strong-leg spin-1/2 ladder compound (CHN)CuBr at
temperatures down to 45 mK. Low-temperature magnetic susceptibility as a
function of field exhibits a maximum near the critical field H_c at which the
magnon gap vanishes, as expected for a gapped one-dimensional antiferromagnet.
Above H_c a clear minimum appears in the magnetization as a function of
temperature as predicted by theory. In this field region, the susceptibility in
conjunction with our specific heat data yields the Wilson ratio R_W. The result
supports the relation R_W=4K, where K is the Tomonaga-Luttinger-liquid
parameter
Effects of Disorder on the Competition between Antiferromagnetism and Superconductivity
Motivated by the observation of unusual magnetism in Ce_xCu_2Si_2 (), we study the effect of disorder, such as Ce vacancy, on the competition
between superconductivity (SC) and antiferromagnetism (AF) on the basis of the
phenomenological Ginzburg-Landau theory. Assuming that the AF-SC transition is
of first order in clean system, we show that a single impurity in the SC state
can induce staggered magnetization by suppressing the SC around it. For finite
concentration of impurities, the first-order AF-SC boundary in the clean case
is replaced by a finite region where the SC and the induced AF moments coexist
microscopically with spatially varying order parameters. We argue that spin
excitation spectrum in the coexistent state has a dual structure of SC gapped
mode and the low-energy spin-wave mode. In accordance with the emergence of AF
out of SC ground state, the spectral weight will be transferred from the former
mode to the latter, keeping the structure of both modes basically unchanged.Comment: 5 pages, 1 figure, submitted to J. Phys. Soc. Japa
Neutron Scattering Study of Temperature-Concentration Phase Diagram of (Cu1-xMgx)GeO3
In doped CuGeO3 systems, such as (Cu1-xZnx)GeO3 and Cu(Ge1-xSix)O3, the
spin-Peierls (SP) ordering (T<Tsp) coexists with the antiferromagnetic (AF)
phase (T<TN<Tsp). Tsp decreases while TN increases with increasing x in low
doping region. For higher x, however, the SP state disappears and only the AF
state remains. These features are common for all the doped CuGeO3 systems so
far studied, indicating the existence of universal T-x phase diagram. Recently,
Masuda et al. carried out comprehensive magnetic susceptibility (chi)
measurements of (Cu1-xMgx)GeO3, in which doping concentration can be controlled
significantly better than the Zn doped systems. They found that TN suddenly
jumps from 3.43 to 3.98K at the critical concentration xc sim 0.023 and that a
drop in chi corresponding to the SP ordering also disappears at x>xc. They thus
concluded that there is a compositional phase boundary between two distinct
magnetic phases. To clarify the nature of two phases, we performed
neutron-scattering measurements on (Cu1-xMgx)GeO3 single crystals with various
x. Analysis of the data at fixed temperature points as a function of doping
concentration has revealed sudden changes of order parameters at the critical
concentration xc=0.027 +- 0.001. At finite temperatures below TN, the drastic
increase of the AF moment takes place at xc. The spin-Peierls order parameter
delta associated with lattice dimerization shows a precipitous decrease at all
temperature below Tsp. However, it goes to zero above xc only at the low
temperature limit.Comment: 9 pages, 9 figure
Thermal conductivity of Mg-doped CuGeO_3 at very low temperatures: Heat conduction by antiferromagnetic magnons
Thermal conductivity \kappa is measured at very low temperatures down to 0.28
K for pure and Mg-doped CuGeO_3 single crystals. The doped samples carry larger
amount of heat than the pure sample at the lowest temperature. This is because
antiferromagnetic magnons appear in the doped samples and are responsible for
the additional heat conductivity, while \kappa of the pure sample represents
phonon conductivity at such low temperatures. The maximum energy of the magnon
is estimated to be much lower than the spin-Peierls-gap energy. The result
presents the first example that \kappa at very low temperatures probes the
magnon transport in disorder-induced antiferromagnetic phase of spin-gap
systems
Site-Dilution-Induced Antiferromagnetic Long-Range Order in Two-Dimensional Spin-Gapped Heisenberg Antiferromagnet
Effects of the site dilution on spin-gapped Heisenberg antiferromagnets with
and S=1 on a square lattice are investigated by means of the quantum
Monte Carlo method. It is found that effective magnetic moments induced around
the diluted sites exhibit the antiferromagnetic long-range order in the medium
of spin-singlet pairs. Their microscopic structure is examined in detail and
important roles of the higher dimensionality than one on the phenomenon are
discussed.Comment: RevTeX, 4 pages, 6 figure
Bond-Dilution-Induced Quantum Phase Transitions in Heisenberg Antiferromagnets
Bond-dilution effects on the ground state of the square-lattice
antiferromagnetic Heisenberg model, consisting of coupled bond-alternating
chains, are investigated by means of the quantum Monte Carlo simulation. It is
found that, when the ground state of the non-diluted system is a non-magnetic
state with a finite spin gap, a sufficiently weak bond dilution induces a
disordered state with a mid gap in the original spin gap, and under a further
stronger bond dilution an antiferromagnetic long-range order emerges. While the
site-dilution-induced long-range order is induced by an infinitesimal
concentration of dilution, there exists a finite critical concentration in the
case of bond dilution. We argue that this essential difference is due to the
occurrence of two types of effective interactions between induced magnetic
moments in the case of bond dilution, and that the antiferromagnetic
long-range-ordered phase does not appear until the magnitudes of the two
interactions become comparable.Comment: 7 pages, 13 figure
Angle-resolved photoemission study of MX-chain compound [Ni(chxn)Br]Br
We report on the results of angle-resolved photoemission experiments on a
quasi-one-dimensional -chain compound [Ni(chxn)Br]Br (chxn =
1,2-cyclohexanediamine), a one-dimensional Heisenberg system with
and K, which shows a gigantic non-linear optical effect. A "band"
having about 500 meV energy dispersion is found in the first half of the
Brillouin zone , but disappears at . Two
dispersive features, expected from the spin-charge separation, as have been
observed in other quasi-one-dimensional systems like SrCuO, are not
detected. These characteristic features are well reproduced by the -
chain model calculations with a small charge-transfer energy compared
with that of one-dimensional Cu-O based compounds. We propose that this smaller
is the origin of the absence of clear spin- and charge-separation in
the photoemission spectra and strong non-linear optical effect in
[Ni(chxn)Br]Br.Comment: 4 pages, 3 figure
Structural Critical Scattering Study of Mg-Doped CuGeO3
We report a synchrotron x-ray scattering study of the diluted spin-Peierls
(SP) material Cu_(1-x)Mg_xGeO_3. We find that for x>0 the temperature T_m at
which the spin gap is established is significantly higher than the temperature
T_s at which the SP dimerization attains long-range order. The latter is
observed only for xx_c the SP correlation length
quickly decreases with increasing x. We argue that impurity-induced competing
interactions play a central role in these phenomena.Comment: 5 pages, 4 embedded eps figures, to appear in PR
Approach to the semiconductor cavity QED in high-Q regimes with q-deformed boson
The high density Frenkel exciton which interacts with a single mode
microcavity field is dealed with in the framework of the q-deformed boson. It
is shown that the q-defomation of bosonic commutation relations is satisfied
naturally by the exciton operators when the low density limit is deviated. An
analytical expression of the physical spectrum for the exciton is given by
using of the dressed states of the cavity field and the exciton. We also give
the numerical study and compare the theoretical results with the experimental
resultsComment: 6 pages, 2 figure
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