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D. J. J. Farnell J. Richter R. Zinke R. F. Bishop 《Journal of statistical physics》2009,135(1):175-198
In this article, we prove that exact representations of dimer and plaquette valence-bond ket ground states for quantum Heisenberg
antiferromagnets may be formed via the usual coupled cluster method (CCM) from independent-spin product (e.g. Néel) model
states. We show that we are able to provide good results for both the ground-state energy and the sublattice magnetization
for dimer and plaquette valence-bond phases within the CCM. As a first example, we investigate the spin-half J
1–J
2 model for the linear chain, and we show that we are able to reproduce exactly the dimerized ground (ket) state at J
2/J
1=0.5. The dimerized phase is stable over a range of values for J
2/J
1 around 0.5, and results for the ground-state energies are in good agreement with the results of exact diagonalizations of
finite-length chains in this regime. We present evidence of symmetry breaking by considering the ket- and bra-state correlation
coefficients as a function of J
2/J
1. A radical change is also observed in the behavior of the CCM sublattice magnetization as we enter the dimerized phase. We
then consider the Shastry-Sutherland model and demonstrate that the CCM can span the correct ground states in both the Néel
and the dimerized phases. Once again, very good results for the ground-state energies are obtained. We find CCM critical points
of the bra-state equations that are in agreement with the known phase transition point for this model. The results for the
sublattice magnetization remain near to the “true” value of zero over much of the dimerized regime, although they diverge
exactly at the critical point. Finally, we consider a spin-half system with nearest-neighbor bonds for an underlying lattice
corresponding to the magnetic material CaV4O9 (CAVO). We show that we are able to provide excellent results for the ground-state energy in each of the plaquette-ordered,
Néel-ordered, and dimerized regimes of this model. The exact plaquette and dimer ground states are reproduced by the CCM ket
state in their relevant limits. Furthermore, we estimate the range over which the Néel order is stable, and we find the CCM
result is in reasonable agreement with the results obtained by other methods. Our new approach has the dual advantages that
it is simple to implement and that existing CCM codes for independent-spin product model states may be used from the outset.
Furthermore, it also greatly extends the range of applicability to which the CCM may be applied. We believe that the CCM now
provides an excellent choice of method for the study of systems with valence-bond quantum ground states. 相似文献
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Bonalde I Yanoff BD Salamon MB Van Harlingen DJ Chia EM Mao ZQ Maeno Y 《Physical review letters》2000,85(22):4775-4778
We report measurements of the magnetic penetration depth in single crystals of Sr2RuO4 down to 0.04 K using a tunnel-diode based, self-inductive technique. We observe a power law temperature dependence below 0.8 K, with no sign of a second phase transition nor of a crossover predicted for a multiband superconductor. A power law dependence suggests that the gap function has nodes, inconsistent with candidate p-wave states. We argue that nonlocal effects, rather than impurity scattering, can explain the observed T2 dependence instead of the T-linear behavior expected for line nodes. 相似文献
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DJ Smith KW Ledingham RP Singhal T McCanny P Graham HS Kilic P Tzallas C Kosmidis AJ Langley PF Taday 《Rapid communications in mass spectrometry : RCM》1999,13(14):1366-1373
With the development of high intensity femtosecond lasers, the ionisation and dissociation dynamics of molecules has become an area of considerable interest. Using the technique of femtosecond laser mass spectrometry (FLMS), the molecules carbon disulphide, pyrimidine, toluene, cyclohexanone and benzaldehyde are studied with pulse widths of 50 fs in the near infrared (IR) wavelength region (790 nm). Results are presented and contrasted for laser beam intensities around 10(15) and 10(16) W cm(-2). For the lower intensities, the mass spectra yield dominant singly charged parent ions. Additionally, the appearance of doubly charged parent ions is evident for carbon disulphide, toluene and benzaldehyde with envelopes of doubly charged satellite species existing in these local regions. Carbon disulphide also reveals a small triply charged component. Such atomic-like features are thought to be a strong fingerprint of FLMS at these intensities. However, upon increasing the laser intensity to approximately 10(16) W cm(-2), parent ion dominance decreases and the appearance of multiply charged atomic species occurs, particularly carbon. This phenomenon has been attributed to Coulomb explosions in which the fast absorption of many photons may produce transient highly ionised parent species which can subsequently blow apart. Copyright 1999 John Wiley & Sons, Ltd. 相似文献
36.
X Fang KW Ledingham P Graham DJ Smith T McCanny RP Singhal AJ Langley PF Taday 《Rapid communications in mass spectrometry : RCM》1999,13(14):1390-1397
The potential of femtosecond laser time-of-flight mass spectrometry (FLMS) for uniform quantitative analysis of molecules has been investigated. Various samples of molecular gases and vapours have been studied, using ultra-fast ( approximately 50 fs) laser pulses with very high intensity (up to 1.6 x 10(16) Wcm(-2)) for non-resonant multiphoton ionisation/tunnel ionisation. Some of these molecules have high ionisation potentials, requiring up to ten photons for non-resonant ionisation. The relative sensitivity factors (RSF) have been determined as a function of the laser intensity and it has been demonstrated that for molecules with very different masses and ionisation potentials, uniform ionisation has been achieved at the highest laser intensities. Quantitative laser mass spectrometry of molecules is therefore a distinct possibility. Copyright 1999 John Wiley & Sons, Ltd. 相似文献
37.
MR Eskildsen AB Abrahamsen PL Gammel DJ Bishop NH Andersen K Mortensen PC Canfield 《Pramana》2002,58(5-6):903-905
We compare the results of small angle neutron scattering on the flux line lattice (FLL) obtained in the borocarbide superconductor
LuNi2B2C with the applied field along the c- and a-axes. For H‖c the temperature dependence of the FLL structural phase transition from square to hexagonal symmetry was investigated. Above
10 K the transition onset field. H
2(T), rises sharply, bending away from H
c2(T) in contradiction to theoretical predictions of the two merging. For H‖a a first order FLL reorientation transition is observed at H
tr=3–3.5 kOe. Below H
tr the FLL nearest neighbor direction is parallel to the b-axis, and above H
tr to the c-axis. This transition cannot be explained using nonlocal corrections to the London model. 相似文献
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The relative rates of H-transfer between partners in ion-neutral complexes were compared with those in intramolecular rearrangements using results of first differential photoionization mass spectrometry measurements. Complex-mediated H-transfers are inferred to have rates of the same order as those for intramolecular hydrogen rearrangements, suggesting a similar range of motion of the reactive sites in both types of reactions. It is also concluded that at their fastest H-transfers take place between the partners in ion-neutral complexes within at most the time of several rotations of the partners in the complexes. Copyright 1999 John Wiley & Sons, Ltd. 相似文献