12 research outputs found
Dynamical response functions in the quantum Ising chain with a boundary
We determine dynamical response functions in the scaling limit of the quantum Ising chain on the half line in
the presence of a boundary magnetic field. Using a spectral representation in
terms of infinite volume form factors and a boundary state, we derive an
expansion for the correlator that is found to be rapidly convergent as long as
|\frac{x_1+x_2}{\xi}|\agt 0.2 where is the correlation length. At
sufficiently late times we observe oscillatory behaviour of the correlations
arbitrarily far away from the boundary. We investigate the effects of the
boundary bound state that is present for a range of boundary magnetic fields.Comment: 32 page
Modelling Magnetic Fluctuations in the Stripe Ordered State
The nature of the interplay between superconductivity and magnetism in the
cuprates remains one of the fundamental unsolved problems in high temperature
superconductivity. Whether and how these two phenomena are interdependent is
perhaps most sharply seen in the stripe phases of various copper-oxide
materials. These phases, involving a mixture of spin and charge density waves,
do not yet admit a complete, overarching theoretical treatment. However aspects
of this problem can be analyzed. In this work, we focus on the magnetic side of
stripe physics. To this end, we study a simple model of a stripe-ordered phase
consisting of an array of alternating coupled doped and undoped two-leg
Hubbard-like ladders. To obtain the magnetic response, we employ already
available dynamical susceptibilities of the individual two-leg ladders and
treat the interladder coupling in a random phase approximation. Strikingly, we
find two possible scenarios for the ordered state induced by the coupling
between ladders: the spin modulation can occur either along or perpendicular to
the direction of the stripes. These two scenarios are differentiated according
to different microscopic realizations of the component doped ladders. However
inelastic neutron scattering experiments on the two stripe ordered cuprates,
La_{1.875}Ba_{0.125}CuO_4 and La_{2-x}Sr_xCuO_4, do not readily distinguish
between these two scenarios due to manner in which stripes form in these
materials.Comment: 24 pages, 8 figure
Doped Spin Liquid: Luttinger Sum Rule and Low Temperature Order
We analyze a model of two-leg Hubbard ladders weakly coupled by interladder
tunneling. At half filling a semimetallic state with small Fermi pockets is
induced beyond a threshold tunneling strength. The sign changes in the single
electron Green's function relevant for the Luttinger Sum Rule now take place at
surfaces with both zeroes and infinities with important consequences for the
interpretation of ARPES experiments. Residual interactions between electron and
hole-like quasi-particles cause a transition to long range order at low
temperatures. The theory can be extended to small doping leading to
superconducting order.Comment: 4 pages, 3 figure
Finite Temperature Dynamical Structure Factor of Alternating Heisenberg Chains
We develop a low-temperature expansion for the finite temperature dynamical
structure factor of the spin half Heisenberg chain with alternating nearest
neighbour exchange in the limit of strong alternation of the exchange
constants. We determine both the broadening of the low lying triplet lines and
the contribution of the thermally activated intraband scattering.Comment: 11 pages, 4 eps-figure
Form factor expansion for thermal correlators
We consider finite temperature correlation functions in massive integrable
Quantum Field Theory. Using a regularization by putting the system in finite
volume, we develop a novel approach (based on multi-dimensional residues) to
the form factor expansion for thermal correlators. The first few terms are
obtained explicitly in theories with diagonal scattering. We also discuss the
validity of the LeClair-Mussardo proposal.Comment: 41 pages; v2: minor corrections, v3: minor correction
Quantum Quench in the Transverse Field Ising chain I: Time evolution of order parameter correlators
We consider the time evolution of order parameter correlation functions after
a sudden quantum quench of the magnetic field in the transverse field Ising
chain. Using two novel methods based on determinants and form factor sums
respectively, we derive analytic expressions for the asymptotic behaviour of
one and two point correlators. We discuss quenches within the ordered and
disordered phases as well as quenches between the phases and to the quantum
critical point. We give detailed account of both methods.Comment: 65 pages, 21 figures, some typos correcte
Energy Level Distribution of Perturbed Conformal Field Theories
We study the energy level spacing of perturbed conformal minimal models in finite volume, considering perturbations of such models that are massive but not necessarily integrable. We compute their spectrum using a renormalization group improved truncated conformal spectrum approach. With this method we are able to study systems where more than 40000 states are kept and where we determine the energies of the lowest several thousand eigenstates with high accuracy. We find, as expected, that the level spacing statistics of integrable perturbed minimal models are Poissonian while the statistics of non-integrable perturbations are GOE-like. However by varying the system size (and so controlling the positioning of the theory between its IR and UV limits) one can induce crossovers between the two statistical distributions