164 research outputs found
Novel Crossover in Coupled Spin Ladders
We report a novel crossover behavior in the long-range-ordered phase of a
prototypical spin- Heisenberg antiferromagnetic ladder compound
. The staggered order was previously evidenced
from a continuous and symmetric splitting of N NMR spectral lines on
lowering temperature below mK, with a saturation towards
mK. Unexpectedly, the split lines begin to further separate away
below mK while the line width and shape remain completely
invariable. This crossover behavior is further corroborated by the NMR
relaxation rate measurements. A very strong suppression reflecting
the ordering, , observed above , is replaced by
below . These original NMR features are indicative of
unconventional nature of the crossover, which may arise from a unique
arrangement of the ladders into a spatially anisotropic and frustrated coupling
network.Comment: 5 pages, 3 figure
Field Induced Staggered Magnetization and Magnetic Ordering in
We present a D NMR investigation of the gapped spin-1/2 compound . Our measurements reveal the presence of a magnetic
field induced transverse staggered magnetization (TSM) which persists well
below and above the field-induced 3D long-range magnetically ordered (FIMO)
phase. The symmetry of this TSM is different from that of the TSM induced by
the order parameter of the FIMO phase. Its origin, field dependence and
symmetry can be explained by an intra-dimer Dzyaloshinskii-Moriya interaction,
as shown by DMRG calculations on a spin-1/2 ladder. This leads us to predict
that the transition into the FIMO phase is not in the BEC universality class.Comment: 4 page
Quantum-critical spin dynamics in quasi-one-dimensional antiferromagnets
By means of nuclear spin-lattice relaxation rate 1/T1, we follow the spin
dynamics as a function of the applied magnetic field in two gapped
one-dimensional quantum antiferromagnets: the anisotropic spin-chain system
NiCl2-4SC(NH2)2 and the spin-ladder system (C5H12N)2CuBr4. In both systems,
spin excitations are confirmed to evolve from magnons in the gapped state to
spinons in the gapples Tomonaga-Luttinger-liquid state. In between, 1/T1
exhibits a pronounced, continuous variation, which is shown to scale in
accordance with quantum criticality. We extract the critical exponent for 1/T1,
compare it to the theory, and show that this behavior is identical in both
studied systems, thus demonstrating the universality of quantum critical
behavior
Incipient charge order observed by NMR in the normal state of YBa2Cu3Oy
The pseudogap regime of high-temperature cuprates harbours diverse
manifestations of electronic ordering whose exact nature and universality
remain debated. Here, we show that the short-ranged charge order recently
reported in the normal state of YBa2Cu3Oy corresponds to a truly static
modulation of the charge density. We also show that this modulation impacts on
most electronic properties, that it appears jointly with intra-unit-cell
nematic, but not magnetic, order, and that it exhibits differences with the
charge density wave observed at lower temperatures in high magnetic fields.
These observations prove mostly universal, they place new constraints on the
origin of the charge density wave and they reveal that the charge modulation is
pinned by native defects. Similarities with results in layered metals such as
NbSe2, in which defects nucleate halos of incipient charge density wave at
temperatures above the ordering transition, raise the possibility that
order-parameter fluctuations, but no static order, would be observed in the
normal state of most cuprates if disorder were absent.Comment: Updated version. Free download at Nature Comm. website (doi below
Uniform and staggered magnetizations induced by Dzyaloshinskii-Moriya interactions in isolated and coupled spin 1/2 dimers in a magnetic field
We investigate the interplay of Dzyaloshinskii-Moriya interactions and an
external field in spin 1/2 dimers. For isolated dimers and at low field, we
derive simple expressions for the staggered and uniform magnetizations which
show that the orientation of the uniform magnetization can deviate
significantly from that of the external field. In fact, in the limit where the
vector of the Dzyaloshinskii-Moriya interaction is parallel to the
external field, the uniform magnetization actually becomes {\it perpendicular}
to the field. For larger fields, we show that the staggered magnetization of an
isolated dimer has a maximum close to one-half the polarization, with a large
maximal value of in the limit of very small Dzyaloshinskii-Moriya
interaction. We investigate the effect of inter-dimer coupling in the context
of ladders with Density Matrix Renormalization Group (DMRG) calculations and
show that, as long as the values of the Dzyaloshinskii-Moriya and of the
exchange interaction are compatible with respect to the development of a
staggered magnetization, the simple picture that emerges for isolated dimers is
also valid for weakly coupled dimers with minor modifications. The results are
compared with torque measurements on
Cu(CHN)Cl.Comment: 8 pages, 9 figure
Antiferromagnetic Spin Fluctuations in the Metallic Phase of Quasi-Two-Dimensional Organic Superconductors
We give a quantitative analysis of the previously published nuclear magnetic
resonance (NMR) experiments in the k-(ET)2X family of organic charge transfer
salts by using the phenomenological spin fluctuation model of Moriya, and
Millis, Monien and Pines (M-MMP). For temperatures above T_nmr ~ 50 K, the
model gives a good quantitative description of the data in the metallic phases
of several k-(ET)2X materials. These materials display antiferromagnetic
correlation lengths which increase with decreasing temperature and grow to
several lattice constants by T_nmr. It is shown that the fact that the
dimensionless Korringa ratio is much larger than unity is inconsistent with a
broad class of theoretical models (such as dynamical mean-field theory) which
neglects spatial correlations and/or vertex corrections. For materials close to
the Mott insulating phase the nuclear spin relaxation rate, the Knight shift
and the Korringa ratio all decrease significantly with decreasing temperature
below T_nmr. This cannot be described by the M-MMP model and the most natural
explanation is that a pseudogap, similar to that observed in the underdoped
cuprate superconductors, opens up in the density of states below T_nmr. Such a
pseudogap has recently been predicted to occur in the dimerised organic charge
transfer salts materials by the resonating valence bond (RVB) theory. We
propose specific new experiments on organic superconductors to elucidate these
issues. For example, measurements to see if high magnetic fields or high
pressures can be used to close the pseudogap would be extremely valuable.Comment: 11 pages, 2 figures. Accepted for publication in Phys. Rev.
Superconductivity and spin correlation in organic conductors: a quantum Monte Carlo study
The d-wave pairing correlations along with spin correlation are calculated
with quantum Monte Carlo method for the two-dimensional Hubbard model on
lattice structures representing organic superconductors
-(BEDT-TTF)X and (TMTSF)X. In both cases the pairing
correlations for superconducting order parameters with nodes are found to be
enhanced. The symmetry and the enhancement of the pairing is systematically
correlated with the spin structure factor, suggesting a spin-fluctuation
mediated pairing. We have further found that, as we deform the Fermi surface to
make the system approach the half-filled square lattice, the coherence of the
pairing saturates while the local pairing amplitude continues to increase.Comment: 5 pages, RevTeX, uses epsf.sty and multicol.st
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