3,997 research outputs found
Random-Singlet Phase in Disordered Two-Dimensional Quantum Magnets
We study effects of disorder (randomness) in a 2D square-lattice
quantum spin system, the - model with a 6-spin interaction
supplementing the Heisenberg exchange . In the absence of disorder the
system hosts antiferromagnetic (AFM) and columnar valence-bond-solid (VBS)
ground states. The VBS breaks symmetry, and in the presence of
arbitrarily weak disorder it forms domains. Using QMC simulations, we
demonstrate two kinds of such disordered VBS states. Upon dilution, a removed
site leaves a localized spin in the opposite sublattice. These spins form AFM
order. For random interactions, we find a different state, with no order but
algebraically decaying mean correlations. We identify localized spinons at the
nexus of domain walls between different VBS patterns. These spinons form
correlated groups with the same number of spinons and antispinons. Within such
a group, there is a strong tendency to singlet formation, because of
spinon-spinon interactions mediated by the domain walls. Thus, no long-range
AFM order forms. We propose that this state is a 2D analog of the well-known 1D
random singlet (RS) state, though the dynamic exponent in 2D is finite. By
studying the T-dependent magnetic susceptibility, we find that varies, from
at the AFM--RS phase boundary and larger in the RS phase The RS state
discovered here in a system without geometric frustration should correspond to
the same fixed point as the RS state recently proposed for frustrated systems,
and the ability to study it without Monte Carlo sign problems opens up
opportunities for further detailed characterization of its static and dynamic
properties. We also discuss experimental evidence of the RS phase in the
quasi-two-dimensional square-lattice random-exchange quantum magnets
SrCuTeWO.Comment: 31 pages, 29 figures; substantial additions in v2; additional
analysis in v
Random-singlet phase in disordered two-dimensional quantum magnets
We study effects of disorder (randomness) in a 2D square-lattice S=1/2 quantum spin system, the J-Q model with a 6-spin interaction Q supplementing the Heisenberg exchange J. In the absence of disorder the system hosts antiferromagnetic (AFM) and columnar valence-bond-solid (VBS) ground states. The VBS breaks Z4 symmetry, and in the presence of arbitrarily weak disorder it forms domains. Using QMC simulations, we demonstrate two kinds of such disordered VBS states. Upon dilution, a removed site leaves a localized spin in the opposite sublattice. These spins form AFM order. For random interactions, we find a different state, with no order but algebraically decaying mean correlations. We identify localized spinons at the nexus of domain walls between different VBS patterns. These spinons form correlated groups with the same number of spinons and antispinons. Within such a group, there is a strong tendency to singlet formation, because of spinon-spinon interactions mediated by the domain walls. Thus, no long-range AFM order forms. We propose that this state is a 2D analog of the well-known 1D random singlet (RS) state, though the dynamic exponent z in 2D is finite. By studying the T-dependent magnetic susceptibility, we find that z varies, from z=2 at the AFM--RS phase boundary and larger in the RS phase The RS state discovered here in a system without geometric frustration should correspond to the same fixed point as the RS state recently proposed for frustrated systems, and the ability to study it without Monte Carlo sign problems opens up opportunities for further detailed characterization of its static and dynamic properties. We also discuss experimental evidence of the RS phase in the quasi-two-dimensional square-lattice random-exchange quantum magnets Sr2CuTe1−xWxO6.Accepted manuscrip
Anomalous quantum-critical scaling corrections in two-dimensional antiferromagnets
We study the N\'eel-paramagnetic quantum phase transition in two-dimensional
dimerized Heisenberg antiferromagnets using finite-size scaling of
quantum Monte Carlo data. We resolve the long standing issue of the role of
cubic interactions arising in the bond-operator representation when the dimer
pattern lacks a certain symmetry. We find non-monotonic (monotonic) size
dependence in the staggered (columnar) dimerized model, where cubic
interactions are (are not) present. We conclude that there is an irrelevant
field in the staggered model that is not present in the columnar case, but, at
variance with previous claims, it is not the leading irrelevant field. The new
exponent is and the prefactor of the correction
is large and comes with a different sign from that of the
formally leading conventional correction with exponent .
Our study highlights the possibility of competing scaling corrections at
quantum critical points.Comment: 6 pages, 6 figure
Quantum Non-Demolition Bell State Measurement and N-party GHZ State Preparation in Quantum Dot
By exploiting the fermionic qubit parity measurement, we present a scheme to
realize quantum non-demolition (QND) measurement of Bell-states and generate
n-party GHZ state in quantum dot. Compared with the original protocol, the
required electron transfer before and after parity measurement can be
nonadiabatic, which may speed up the operation speed and make the omitting of
spin-orbit interaction more reasonable. This may help us to construct CNOT gate
without highly precise control of coupling as the way of D. Gottesman and I. L.
Chuang.Comment: some modification to introduction and some details are adde
A new topology of the HK97-like fold revealed in Bordetella bacteriophage by cryoEM at 3.5 A resolution.
Bacteriophage BPP-1 infects and kills Bordetella species that cause whooping cough. Its diversity-generating retroelement (DGR) provides a naturally occurring phage-display system, but engineering efforts are hampered without atomic structures. Here, we report a cryo electron microscopy structure of the BPP-1 head at 3.5 Å resolution. Our atomic model shows two of the three protein folds representing major viral lineages: jellyroll for its cement protein (CP) and HK97-like ('Johnson') for its major capsid protein (MCP). Strikingly, the fold topology of MCP is permuted non-circularly from the Johnson fold topology previously seen in viral and cellular proteins. We illustrate that the new topology is likely the only feasible alternative of the old topology. β-sheet augmentation and electrostatic interactions contribute to the formation of non-covalent chainmail in BPP-1, unlike covalent inter-protein linkages of the HK97 chainmail. Despite these complex interactions, the termini of both CP and MCP are ideally positioned for DGR-based phage-display engineering. DOI: http://dx.doi.org/10.7554/eLife.01299.001
Possible Way to Synthesize Superheavy Element Z=117
Within the framework of the dinuclear system model, the production of
superheavy element Z=117 in possible projectile-target combinations is analyzed
systematically. The calculated results show that the production cross sections
are strongly dependent on the reaction systems. Optimal combinations,
corresponding excitation energies and evaporation channels are proposed in this
letter, such as the isotopes ^{248,249}Bk in ^{48}Ca induced reactions in 3n
evaporation channels and the reactions ^{45}Sc+^{246,248}Cm in 3n and 4n
channels, and the system ^{51}V+^{244}Pu in 3n channel.Comment: 10 pages, 4 figures, 1 tabl
Electronic Structure of KFeSe from First Principles Calculations
Electronic structure and magnetic properties for iron-selenide KFeSe
are studied by first-principles calculations. The ground state is stripe-like
antiferromagnetic with calculated 2.26 magnetic moment on Fe atoms; and
the , coupling strengths are calculated to be 0.038 eV and 0.029 eV.
The states around are dominated by the Fe-3d orbitals which hybridize
noticeably to the Se-4p orbitals. While the band structure of KFeSe is
similar to a heavily electron-doped BaFeAs or FeSe system, the Fermi
surface of KFeSe is much closer to \fs11 system since the electron
sheets around is symmetric with respect to - exchange. These
features, as well as the absence of Fermi surface nesting, suggest that the
parental KFeSe could be regarded as an electron over-doped 11 system
with possible local moment magnetism.Comment: accepted by Chinese Physics Letter, to appear as Chinese Physics
Letter, Vol 28, page 057402 (2011
- …