4,448 research outputs found
AC Hopping Magnetotransport Across the Spin Flop Transition in Lightly Doped La_2CuO_4
The weak ferromagnetism present in insulating La_{2}CuO_4 at low doping leads
to a spin flop transition, and to transverse (interplane) hopping of holes in a
strong external magnetic field. This results in a dimensional crossover 2D
3D for the in-plane transport, which in turn leads to an increase of the
hole's localization length and increased conduction. We demonstrate
theoretically that as a consequence of this mechanism, a frequency-dependent
jump of the in-plane ac hopping conductivity occurs at the spin flop
transition. We predict the value and the frequency dependence of the jump.
Experimental studies of this effect would provide important confirmation of the
emerging understanding of lightly doped insulating La_{2-x}Sr_xCuO_4.Comment: 4 pages, 1 figur
Single hole dynamics in dimerized spin liquids
The dynamics of a single hole in quantum antiferromagnets is influenced by
magnetic fluctuations. In the present work we consider two situations. The
first one corresponds to a single hole in the two leg t-J spin ladder. In this
case the wave function renormalization is relatively small and the
quasiparticle residue of the S=1/2 state remains close to unity. However at
large t/J there are higher spin (S=3/2,5/2,..) bound states of the hole with
the magnetic excitations, and therefore there is a crossover from
quasiparticles with S=1/2 to quasiparticles with higher spin.
The second situation corresponds to a single hole in two coupled
antiferromagnetic planes very close to the point of antiferromagnetic
instability. In this case the hole wave function renormalization is very strong
and the quasiparticle residue vanishes at the point of instability.Comment: 12 pages, 3 figure
Bound states of magnons in the S=1/2 quantum spin ladder
We study the excitation spectrum of the two-leg antiferromagnetic S=1/2
Heisenberg ladder. Our approach is based on the description of the excitations
as triplets above a strong-coupling singlet ground state. The quasiparticle
spectrum is calculated by treating the excitations as a dilute Bose gas with
infinite on-site repulsion. We find singlet (S=0) and triplet (S=1)
two-particle bound states of the elementary triplets. We argue that bound
states generally exist in any dimerized quantum spin model.Comment: 4 REVTeX pages, 4 Postscript figure
A study of the temperature dependence of bienzyme systems and enzymatic chains
It is known that most enzyme-facilitated reactions are highly temperature dependent processes. In general, the temperature coefficient, Q10, of a simple reaction reaches 2.0-3.0. Nevertheless, some enzyme-controlled processes have much lower Q10 (about 1.0), which implies that the process is almost temperature independent, even if individual reactions involved in the process are themselves highly temperature dependent. In this work, we investigate a possible mechanism for this apparent temperature compensation: simple mathematical models are used to study how varying types of enzyme reactions are affected by temperature. We show that some bienzyme-controlled processes may be almost temperature independent if the modules involved in the reaction have similar temperature dependencies, even if individually, these modules are strongly temperature dependent. Further, we show that in non-reversible enzyme chains the stationary concentrations of metabolites are dependent only on the relationship between the temperature dependencies of the first and last modules, whilst in reversible reactions, there is a dependence on every module. Our findings suggest a mechanism by which the metabolic processes taking place within living organisms may be regulated, despite strong variation in temperature
New quantum phase transitions in the two-dimensional J1-J2 model
We analyze the phase diagram of the frustrated Heisenberg antiferromagnet,
the J1-J2 model, in two dimensions. Two quantum phase transitions in the model
are already known: the second order transition from the Neel state to the spin
liquid state at (J_2/J_1)_{c2}=0.38, and the first order transition from the
spin liquid state to the collinear state at (J_2/J_1)_{c4}=0.60. We have found
evidence for two new second order phase transitions: the transition from the
spin columnar dimerized state to the state with plaquette type modulation at
(J_2/J_1)_{c3}=0.50(2), and the transition from the simple Neel state to the
Neel state with spin columnar dimerization at (J_2/J_1)_{c1}=0.34(4). We also
present an independent calculation of (J_2/J_1)_{c2}=0.38 using a new approach.Comment: 3 pages, 5 figures; added referenc
On the nature of the transition from the spontaneously dimerized to the Neel phase in the two-dimensional J1-J2 model
We analyze the spectrum of the 2D S=1/2 frustrated Heisenberg model near the
transition from the spontaneously dimerized spin-liquid phase into the Neel
ordered phase. Two excitation branches: the triplet magnon, and the collective
singlet mode, both become gapless at the transition point. However we find that
the length scales associated with these modes are well separated at the quantum
transition. While in the quantum disordered phase the singlet excitation has
finite spectral weight and reflects the existence of spontaneous dimer order,
near the transition point the size of the singlet bound state grows
exponentially with the correlation length, and hence the quasiparticle residue
is exponentially small. Therefore the critical dynamics remains in the O(3)
universality class in spite of the four gapless modes.Comment: 5 pages, 3 figure
Soliton binding and low-lying singlets in frustrated odd-legged S=1/2 spin tubes
Motivated by the intriguing properties of the vanadium spin tube Na2V3O7, we
show that an effective spin-chirality model similar to that of standard
Heisenberg odd-legged S=1/2 spin tubes can be derived for frustrated inter-ring
couplings, but with a spin-chirality coupling constant alpha that can be
arbitrarily small. Using density matrix renormalization group and analytical
arguments, we show that, while spontaneous dimerization is always present,
solitons become bound into low-lying singlets as alpha is reduced. Experimental
implications for strongly frustrated tubes are discussed.Comment: 4 pages, 4 figure
An antibody raised against a pathogenic serpin variant induces mutant-like behaviour in the wild-type protein
A monoclonal antibody (mAb) that binds to a transient intermediate may act as a catalyst for the corresponding reaction; here we show this principle can extend on a macro molecular scale to the induction of mutant-like oligomerization in a wild-type protein. Using the common pathogenic E342K (Z) variant of α1-antitrypsin as antigen-whose native state is susceptible to the formation of a proto-oligomeric intermediate-we have produced a mAb (5E3) that increases the rate of oligomerization of the wild-type (M) variant. Employing ELISA, gel shift, thermal stability and FRET time-course experiments, we show that mAb5E3 does not bind to the native state of α1-antitrypsin, but recognizes a cryptic epitope in the vicinity of the post-helix A loop and strand 4C that is revealed upon transition to the polymerization intermediate, and which persists in the ensuing oligomer. This epitope is not shared by loop-inserted monomeric conformations. We show the increased amenity to polymerization by either the pathogenic E342K mutation or the binding of mAb5E3 occurs without affecting the energetic barrier to polymerization. As mAb5E3 also does not alter the relative stability of the monomer to intermediate, it acts in a manner similar to the E342K mutant, by facilitating the conformational interchange between these two states
Critical Dynamics of Singlet Excitations in a Frustrated Spin System
We construct and analyze a two-dimensional frustrated quantum spin model with
plaquette order, in which the low-energy dynamics is controlled by spin
singlets. At a critical value of frustration the singlet spectrum becomes
gapless, indicating a quantum transition to a phase with dimer order. This T=0
transition belongs to the 3D Ising universality class, while at finite
temperature a 2D Ising critical line separates the plaquette and dimerized
phases.
The magnetic susceptibility has an activated form throughout the phase
diagram, whereas the specific heat exhibits a rich structure and a power law
dependence on temperature at the quantum critical point.
We argue that the novel quantum critical behavior associated with singlet
criticality discussed in this work can be relevant to a wide class of quantum
spin systems, such as antiferromagnets on Kagome and pyrochlore lattices, where
the low-energy excitations are known to be spin singlets, as well as to the
CAVO lattice and several recently discovered strongly frustrated square-lattice
antiferromagnets.Comment: 5 pages, 5 figures, additional discussion and figure added, to appear
in Phys. Rev.
- …