49 research outputs found
Observation of a Transient Magnetization Plateau in a Quantum Antiferromagnet on the Kagome Lattice
The magnetization process of an S=1/2 antiferromagnet on the kagome lattice,
[Cu_3(titmb)_2(OCOCH_3)_6]H_2O {titmb= 1,3,5-tris(imidazol-1-ylmethyl)-2,4,6
trimethylbenzene} has been measured at very low temperatures in both pulsed and
steady fields. We have found a new dynamical behavior in the magnetization
process: a plateau at one third of the saturation magnetization appears in the
pulsed field experiments for intermediate sweep rates of the magnetic field and
disappears in the steady field experiments. A theoretical analysis using exact
diagonalization yields J_1=-19K and J_2=6K, for the nearest neighbor and second
nearest neighbor interactions, respectively. This set of exchange parameters
explains the very low saturation field and the absence of the plateau in the
thermodynamic equilibrium as well as the two-peak feature in the magnetic heat
capacity. Supported by numerical results we argue that a dynamical order by
disorder phenomenon could explain the transient appearance of the 1/3 plateau
in pulsed field experiments.Comment: 7 pages, 5 figure
Chirality and vortices in an Heisenberg spin model on the kagom\'e lattice
The phase diagram of the classical \jj model on the \kag lattice is
investigated using extensive \mc simulations. In a realistic range of
parameters, this model has a low-temperature chiral-ordered phase without
long-range spin order. We show that the critical transition marking the
destruction of chiral order is preempted by the first order proliferation of
\Zdeux point defects. The core energy of these vortices appears to vanish when
approaching the T=0 phase boundary, where both \Zdeux defects and gapless
magnons contribute to disordering the system at very low temperature. This
situation might be typical of a large class of frustrated magnets. Possible
relevance for real materials is also discussed.Comment: 4 pages, 4 figure
Two Step Restoration of SU(2) Symmetry in a Frustrated Ring-Exchange Magnet
We demonstrate the existence of a spin-nematic, moment-free phase in a
quantum four-spin ring exchange model on the square lattice. This unusual
quantum state is created by the interplay of frustration and quantum
fluctuations which lead to a partial restoration of SU(2) symmetry when going
from a four-sublattice orthogonal biaxial Neel order to this exotic uniaxial
magnet. A further increase of frustration drives a transition to a fully gapped
SU(2) symmetric valence bond crystal.Comment: 4 pages, 5 figure
Properties and Detection of Spin Nematic Order in Strongly Correlated Electron Systems
A spin nematic is a state which breaks spin SU(2) symmetry while preserving
translational and time reversal symmetries. Spin nematic order can arise
naturally from charge fluctuations of a spin stripe state. Focusing on the
possible existence of such a state in strongly correlated electron systems, we
build a nematic wave function starting from a t-J type model. The nematic is a
spin-two operator, and therefore does not couple directly to neutrons. However,
we show that neutron scattering and Knight shift experiments can detect the
spin anisotropy of electrons moving in a nematic background. We find the mean
field phase diagram for the nematic taking into account spin-orbit effects.Comment: 13 pages, 11 figures. (v2) References adde
Novel spin-liquid states in the frustrated Heisenberg antiferromagnet on the honeycomb lattice
Recent experiment on a honeycomb-lattice Heisenberg antiferromagnet (AF)
BiMnO(NO) revealed a novel spin-liquid-like behavior down to
low temperature, which was ascribed to the frustration effect due to the
competition between the AF nearest- and next-nearest-neighbor interactions
and . Motivated by the experiment, we study the ordering of the
- frustrated classical Heisenberg AF on a honeycomb lattice both by
a low-temperature expansion and a Monte Carlo simulation. The model has been
known to possess a massive degeneracy of the ground state, which, however,
might be lifted due to thermal fluctuations leading to a unique ordered state,
the effect known as 'order-by-disorder'. We find that the model exhibits an
intriguing ordering behavior, particularly near the AF phase boundary. The
energy scale of the order-by-disorder is suppressed there down to extremely low
temperatures, giving rise to exotic spin-liquid states like a "ring-liquid" or
a "pancake-liquid" state accompanied by the characteristic spin structure
factor and the field-induced antiferromagnetism. We argue that the recent
experimental data are explicable if the system is in such exotic spin-liquid
states
Randomized trial of neoadjuvant chemotherapy in oropharyngeal carcinoma
The objective of the study was to evaluate the effect of neoadjuvant chemotherapy on the survival of patients with oropharyngeal cancer. Patients with a squamous cell carcinoma of the oropharynx for whom curative radiotherapy or surgery was considered feasible were entered in a multicentric randomized trial comparing neoadjuvant chemotherapy followed by loco-regional treatment to the same loco-regional treatment without chemotherapy. The loco-regional treatment consisted either of surgery plus radiotherapy or of radiotherapy alone. Three cycles of chemotherapy consisting of Cisplatin (100 mg/m2) on day 1 followed by a 24-hour i.v. infusion of fluorouracil (1000 mg/m2/day) for 5 days were delivered every 21 days. 2â3 weeks after the end of chemotherapy, local treatment was performed. The trial was conducted by the Groupe d'Etude des Tumeurs de la TĂȘte Et du Cou (GETTEC). A total of 318 patients were enrolled in the study between 1986 and 1992. Overall survival was significantly better (P = 0.03) in the neoadjuvant chemotherapy group than in the control group, with a median survival of 5.1 years versus 3.3 years in the no chemotherapy group. The effect of neoadjuvant chemotherapy on event-free survival was smaller and of borderline significance (P = 0.11). Stratification of the results on the type of local treatment, surgery plus radiotherapy or radiotherapy alone, did not reveal any heterogeneity in the effect of chemotherapy. © 2000 Cancer Research Campaign http://www.bjcancer.co
High-field Phase Diagram and Spin Structure of Volborthite Cu3V2O7(OH)2/2H2O
We report results of 51V NMR experiments on a high-quality powder sample of
volborthite Cu3V2O7(OH)2/2H2O, a spin-1/2 Heisenberg antiferromagnet on a
distorted kagome lattice. Following the previous experiments in magnetic fields
below 12 T, the NMR measurements have been extended to higher fields up to
31 T. In addition to the two already known ordered phases (phases I and II), we
found a new high-field phase (phase III) above 25 T, at which a second
magnetization step has been observed. The transition from the paramagnetic
phase to the antiferromagnetic phase III occurs at 26 K, which is much higher
than the transition temperatures from the paramagnetic to the lower field
phases I (B < 4.5 T) and II (4.5 < B < 25 T). At low temperatures, two types of
the V sites are observed with different relaxation rates and line shapes in
phase III as well as in phase II. Our results indicate that both phases II and
III exhibit a heterogeneous spin state consisting of two spatially alternating
Cu spin systems, one of which exhibits anomalous spin fluctuations contrasting
with the other showing a conventional static order. The magnetization of the
latter system exhibits a sudden increase upon entering into phase III,
resulting in the second magnetization step at 26 T.We discuss the possible spin
structure in phase III.Comment: 9 pages, 12 figure
A non-randomized comparison of gemcitabine-based chemoradiation with or without induction chemotherapy for locally advanced squamous cell carcinoma of the head and neck
<p>Abstract</p> <p>Background</p> <p>Concomitant chemotherapy and radiotherapy (chemoradiation; CRT) is the standard treatment for locoregionally advanced squamous cell carcinoma of the head and neck (LA-SCCHN). CRT improves local control and overall survival (OS) when compared to radiotherapy (RT) alone. Induction chemotherapy (IC) reduces the risk of distant metastases (DM) and improves OS by 5% with the use of cisplatin/infusional 5 fluorouracil (PF) in meta-analysis. Adding a taxane to PF in the IC regimen confers a better outcome. Sequential treatment (ST) of IC followed by CRT is therefore under active investigation in multiple phase III trials.</p> <p>Methods</p> <p>We compared the outcome of two cohorts of patients (pts) with LA-SCCHN treated at our institution with CRT (n = 27) or ST (n = 31), respectively. CRT consisted of GEM 100 mg/m<sup>2 </sup>weekly + conventional RT (70 Gy); ST consisted of the same CRT preceded by platinum-based IC.</p> <p>Results</p> <p>Response to IC: complete 8 (26%), partial 20 (65%), stable 1, progressive 1, not evaluable 1. Median follow up of the surviving pts: for CRT 73 months, for ST 51 months. Median time to distant metastasis (TDM) was for CRT 23.6 months, for ST not reached. Median OS was for CRT 20.2 months, for ST 40.2 months. Cox regression analysis, taking into account age, T and N stage and tumor site, showed a hazard ratio with ST of 1.190 for time to locoregional failure (p = 0.712), 0.162 for TDM (p = 0.002), and 0.441 for overall survival (OS) (p = 0.026).</p> <p>Conclusion</p> <p>TDM and OS were found significantly longer in the ST cohort without a reduced locoregional control. Notwithstanding the limitations of a non-randomized single-center comparison, the results are in line with very preliminary data of randomized comparisons suggesting an improved outcome with ST.</p
A phase II study of docetaxel in patients with metastatic squamous cell carcinoma of the head and neck
This study was designed to evaluate the activity, safety and tolerance of docetaxel (D) in a selected population with metastatic squamous cell carcinoma of the head and neck (SCCHN). Twenty-four patients with no prior palliative therapy were enrolled and received D 100 mg mâ2 by 1 h of infusion, every 3 weeks. All but two patients had been evaluated for efficacy on lung metastatic sites. No prophylactic administration of anti-emetics or growth factors was given. A pharmacokinetic study was performed in 22 patients. Twenty-one patients were assessable for response and 24 for toxicity. One hundred and four cycles were administered with a median of 4.5 (range 1â9) per patient. The median cumulative dose was 449 mg mâ2. Partial responses were achieved in five patients with a median duration of 18.7 weeks (range 13.1â50.3). The overall response rate was 20.8% with a median duration of 11.0 weeks (range 2.4â52.6). The most frequent side-effect was neutropenia (79.2% grade IV) but with a short duration (median 4 days) and no febrile neutropenia. The incidence of moderate/severe fluid retention was 29.2% with one treatment discontinuation. Other toxicities (all grades) were common (skin 75%, asthenia 50%, infection 29.2%, nausea 16.7%, diarrhoea 12.5%, stomatitis 16.7%, vomiting 8.3% and HSR 8.3%). A mean clearance of 19.6 l hâ1 mâ2 and an area under the curve of 6.00 ÎŒg mlâ1 hâ1 was found in the pharmacokinetic analysis. Docetaxel is active in this selected population with metastatic SCCHN, with a good tolerance. © 1999 Cancer Research Campaig