84 research outputs found
31P NMR study of Na2CuP2O7: a S=1/2 two-dimensional Heisenberg antiferromagnetic system
The magnetic properties of Na2CuP2O7 were investigated by means of 31P
nuclear magnetic resonance (NMR), magnetic susceptibility, and heat capacity
measurements. We report the 31P NMR shift, the spin-lattice 1/T1, and spin-spin
1/T2 relaxation-rate data as a function of temperature T.
The temperature dependence of the NMR shift K(T) is well described by the
S=1/2 square lattice Heisenberg antiferromagnetic (HAF) model with an
intraplanar exchange of J/k_B \simeq 18\pm2 K and a hyperfine coupling A =
(3533\pm185) Oe/mu_B. The 31P NMR spectrum was found to broaden abruptly below
T \sim 10 K signifying some kind of transition. However, no anomaly was noticed
in the bulk susceptibility data down to 1.8 K. The heat capacity appears to
have a weak maximum around 10 K. With decrease in temperatures, the
spin-lattice relaxation rate 1/T1 decreases monotonically and appears to agree
well with the high temperature series expansion expression for a S = 1/2 2D
square lattice.Comment: 12 pages, 8 figures, submitted to J. Phys.: Cond. Ma
Precise calculation of the two-step process for K- d -> pi Sigma N in the Lambda (1405) resonance region
The reaction K- d -> pi Sigma N is investigated taking into account single
scattering and the two-step process due to Kbar N -> pi Sigma rescattering. The
influence of some common approximations are examined. It is found that the
treatment of the kinematics in the Green's function that appears in the loop
integral of the rescattering process has a rather strong impact on the
resulting lineshape of the pi Sigma invariant mass spectrum. Specifically, a
calculation with correct kinematics where the three-body unitarity cut due to
the nK- p threshold occurs at the physical value yields a pronounced peak in
the invariant mass spectrum at this threshold and, at the same time, suppresses
the signal in the region of the Lambda(1405) resonance. On the other hand, an
approximation applied in past calculations shifts that threshold down and,
consequently, leads to an accidental and therefore erroneous enhancement of the
signal of the Lambda(1405) in the pi Sigma invariant mass spectrum.Comment: 11 pages, 13 figure
Short-range repulsion and isospin dependence in the KN system
The short-range properties of the KN interaction are studied within the
meson-exchange model of the Juelich group. Specifically, dynamical explanations
for the phenomenological short-range repulsion, required in this model for
achieving agreement with the empirical KN data, are explored. Evidence is found
that contributions from the exchange of a heavy scalar-isovector meson
(a_0(980)) as well as from genuine quark-gluon exchange processes are needed.
Taking both mechanisms into account a satisfactory description of the KN phase
shifts can be obtained without resorting to phenomenological pieces.Comment: 26 pages, 5 figure
Towards a better understanding of the combustion of oxygenated aromatic hydrocarbons. Comparing benzene, toluene, phenol and anisole with ignition delay times in a rapid compression machine
Ignition delay times (IDTs) of the oxygenated aromatic hydrocarbons (OAHCs) anisole (C6H5OCH3) and phenol
(C6H5OH) and the analogues non-oxygenated aromatic hydrocarbons (AHCs) toluene (C6H5CH3) and benzene (C6H6)
have been measured in the PCFC rapid compression machine (RCM) at stoichiometric, fuel-in-air conditions. With
the two targeted compression pressures () of 1 and 2 MPa a temperature range of 870 to 1100 K was covered. The
IDTs of all four molecules revealed an Arrhenius behavior. The different reactivity can be ranked as the following,
starting with the lowest reactivity: benzene < toluene < phenol < anisole. Literature available models containing
anisole and phenol have been used to simulate the IDTs of this study highlighting discrepancies in both, model to
experiment and model to model accordance. Finally, the CRECK mechanism was used to conduct rate-of-production
(ROP) and sensitivity analysis to gain insight into the combustion of OAHCs and highlight interconnections and
shortcomings of OAHCs
Structural, thermodynamic, and local probe investigations of a honeycomb material AgLiMnO
The system Ag[LiMn]O belongs to a quaternary
3R-delafossite family and crystallizes in a monoclinic symmetry with space
group and the magnetic Mn() ions form a honeycomb
network in the -plane. An anomaly around 50 K and the presence of
antiferromagnetic (AFM) coupling (Curie-Weiss temperature
K) were inferred from our magnetic susceptibility data. The magnetic specific
heat clearly manifests the onset of magnetic ordering in the vicinity of 48\,K
and the recovered magnetic entropy, above the ordering temperature, falls short
of the expected value, implying the presence of short-range magnetic
correlations. The (ESR) line broadening on approaching the ordering temperature
could be described in terms of a Berezinski-Kosterlitz-Thouless
(BKT) scenario with K. Li NMR line-shift probed as a
function of temperature tracks the static susceptibility (K) of
magnetically coupled Mn ions. The Li spin-lattice relaxation rate
(1/) exhibits a sharp decrease below about 50 K. Combining our bulk
and local probe measurements, we establish the presence of an ordered ground
state for the honeycomb system AgLiMnO.Our ab-initio
electronic structure calculations suggest that in the -plane, the nearest
neighbor (NN) exchange interaction is strong and AFM, while the next NN and the
third NN exchange interactions are FM and AFM respectively. In the absence of
any frustration the system is expected to exhibit long-range, AFM order, in
agreement with experiment.Comment: 11 pages, 13 figures, accepted in Phys Rev
Unconventional magnetism in the 4d based () honeycomb system AgLiRuO
We have investigated the thermodynamic and local magnetic properties of the
Mott insulating system AgLiRuO containing Ru
(4) for novel magnetism. The material crystallizes in a monoclinic
structure with RuO octahedra forming an edge-shared
two-dimensional honeycomb lattice with limited stacking order along the
-direction. The large negative Curie-Weiss temperature ( = -57
K) suggests antiferromagnetic interactions among Ru ions though magnetic
susceptibility and heat capacity show no indication of magnetic long-range
order down to 1.8 K and 0.4 K, respectively. Li nuclear magnetic
resonance (NMR) shift follows the bulk susceptibility between 120-300 K and
levels off below 120 K. Together with a power-law behavior in the temperature
dependent spin-lattice relaxation rate between 0.2 and 2 K, it suggest dynamic
spin correlations with gapless excitations. Electronic structure calculations
suggest an description of the Ru-moments and the possible importance of
further neighbour interactions as also bi-quadratic and ring-exchange terms in
determining the magnetic properties. Analysis of our SR data indicates
spin freezing below 5 K but the spins remain on the borderline between static
and dynamic magnetism even at 20 mK.Comment: 10 pages, 11 figures. accepted in Phys. Rev.
New high magnetic field phase of the frustrated chain compound LiCuVO
Magnetization of the frustrated chain compound LiCuVO, focusing
on high magnetic field phases, is reported. Besides a spin-flop transition and
the transition from a planar spiral to a spin modulated structure observed
recently, an additional transition was observed just below the saturation
field. This newly observed magnetic phase is considered as a spin nematic
phase, which was predicted theoretically but was not observed experimentally.
The critical fields of this phase and its dM/dH curve are in good agreement
with calculations performed in a microscopic model (M. E. Zhitomirsky and H.
Tsunetsugu, preprint, arXiv:1003.4096v2).Comment: 5 pages, 4 figure
Strong frustration due to competing ferromagnetic and antiferromagnetic interactions: magnetic properties of M(VO)2(PO4)2 (M = Ca and Sr)
We present a detailed investigation of the magnetic properties of complex
vanadium phosphates M(VO)2(PO4)2 (M = Ca, Sr) by means of magnetization,
specific heat, 31P NMR measurements, and band structure calculations.
Experimental data evidence the presence of ferro- and antiferromagnetic
interactions in M(VO)2(PO4)2 resulting in a nearly vanishing Curie-Weiss
temperature \theta_{CW} < 1 K that contrasts with the maximum of magnetic
susceptibility at 3 K. Specific heat and NMR measurements also reveal weak
exchange couplings with the thermodynamic energy scale J_c = 10-15 K.
Additionally, the reduced maximum of the magnetic specific heat indicates
strong frustration of the spin system. Band structure calculations show that
the spin systems of the M(VO)2(PO4)2 compounds are essentially
three-dimensional with the frustration caused by competing ferro- and
antiferromagnetic interactions. Both calcium and strontium compounds undergo
antiferromagnetic long-range ordering at T_N = 1.5 K and 1.9 K, respectively.
The spin model reveals an unusual example of controllable frustration in
three-dimensional magnetic systems.Comment: 14 pages, 13 figures, 3 tables; published versio
Comparison of and Quasielastic Scattering
We formulate -nucleus quasielastic scattering in a manner which closely
parallels standard treatments of -nucleus quasielastic scattering. For
scattering, new responses involving scalar contributions appear in
addition to the Coulomb (or longitudinal) and transverse responses
which are of vector character. We compute these responses using both nuclear
matter and finite nucleus versions of the Relativistic Hartree Approximation to
Quantum Hadrodynamics including RPA correlations. Overall agreement with
measured responses and new quasielastic scattering data for
Ca at |\qs|=500 MeV/c is good. Strong RPA quenching is essential for
agreement with the Coulomb response. This quenching is notably less for the
cross section even though the new scalar contributions are even more
strongly quenched than the vector contributions. We show that this
``differential quenching'' alters sensitive cancellations in the expression for
the cross section so that it is reduced much less than the individual
responses. We emphasize the role of the purely relativistic distinction between
vector and scalar contributions in obtaining an accurate and consistent
description of the and data within the framework of our nuclear
structure model.Comment: 26 pages, 5 uuencoded figures appended to end of this fil
Heavy-Fermion Formation at the Metal-to-Insulator Transition in GdSrTiO
The perovskite-like transition-metal oxide GdSrTiO is
investigated by measurements of resistivity, specific-heat, and electron
paramagnetic resonance (EPR). Approaching the metal-to-insulator transition
from the metallic regime (), the Sommerfeld coefficient of
the specific heat becomes strongly enhanced and the resistivity increases
quadratically at low temperatures, which both are fingerprints of strong
electronic correlations. The temperature dependence of the dynamic
susceptibility, as determined from the Gd-EPR linewidth, signals the
importance of strong spin fluctuations, as observed in heavy-fermion compounds.Comment: 4pages, 3 figure
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