Effects of thermal phase fluctuations in a 2D superconductor: an exact result for the spectral function

We consider the single particle spectral function for a two-dimensional clean superconductor in a regime of strong critical thermal phase fluctuations. In the limit where the maximum of the superconducting gap is much smaller than the Fermi energy we obtain an exact expression for the spectral function integrated over the momentum component perpendicular to the Fermi surface.Comment: 4 pages, 3 figures. References added, figures improve

A Field Theory for Fermionic Ladder with Generic Intrachain Interactions

An effective low energy field theory is developed for a system of two chains. The main novelty of the approach is that it allows to treat generic intrachain repulsive interactions of arbitrary strength. The chains are coupled by a direct tunneling and four-fermion interactions. At low energies the individual chains are described as Luttinger liquids with an arbitrary ratio of spin $v_s$ and charge $v_c$ velocities. A judicious choice of the basis for the decoupled chains greatly simplifies the description and allows one to separate high and low energy degrees of freedom. In a direct analogy to the bulk cuprates the resulting effective field theory distinguishes between three qualitatively different regimes: (i) small doping ($v_c << v_s$), (ii) optimal doping ($v_s \approx v_c$) and (iii) large doping ($v_s << v_c$). I discuss the excitation spectrum and derive expressions for the electron spectral function which turns out to be highly incoherent. The degree of incoherence increases when one considers an array of ladders (stripe phase).Comment: 32 pages, 4 figures. A section explaining adiabatic approximation is modified. Typos correcte

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

On the origin of the Fermi arc phenomena in the underdoped cuprates: signature of KT-type superconducting transition

We study the effect of thermal phase fluctuation on the electron spectral function $A(k,\omega)$ in a d-wave superconductor with Monte Carlo simulation. The phase degree of freedom is modeled by a XY-type model with build-in d-wave character. We find a ridge-like structure emerges abruptly on the underlying Fermi surface in $A(k,\omega=0)$ above the KT-transition temperature of the XY model. Such a ridge-like structure, which shares the same characters with the Fermi arc observed in the pseudogap phase of the underdoped cuprates, is found to be caused by the vortex-like phase fluctuation of the XY model.Comment: 5 page

A strange metal with a small Fermi surface and strong collective excitations

We develop a theory of a hybrid state, where quasi-particles coexist with strong collective modes, taking as a starting point a model of infinitely many 1D Mott insulators coupled by a weak interchain tunneling. This state exists at an intermediate temperature range and undergoes an antiferromagnetic phase transition at temperatures much smaller than the Mott-Hubbard gap. The most peculiar feature of the hybrid state is that the volume of the Fermi surface is unrelated to the electron density. We present a self-consistent derivation of the low energy effective action for our model.Comment: 12 pages, 7 figure

On the spin-liquid phase of one dimensional spin-1 bosons

We consider a model of one dimensional spin-1 bosons with repulsive density-density interactions and antiferromagnetic exchange. We show that the low energy effective field theory is given by a spin-charge separated theory of a Tomonaga-Luttinger Hamiltonian and the O(3) nonlinear sigma model describing collective charge and spin excitations respectively. At a particular ratio of the density-density to spin-spin interaction the model is integrable, and we use the exact solutions to provide an independent derivation of the low energy effective theory. The system is in a superfluid phase made of singlet pairs of bosons, and we calculate the long-distance asymptotics of certain correlation functions.Comment: 17 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

Finite temperature spectral function of Mott insulators and CDW States

We calculate the low temperature spectral function of one-dimensional incommensurate charge density wave (CDW) states and half-filled Mott insulators (MI). At $T=0$ there are two dispersing features associated with the spin and charge degrees of freedom respectively. We show that already at very low temperatures (compared to the gap) one of these features gets severely damped. We comment on implications of this result for photoemission experiments.Comment: 4 pages, 2 figures, published versio

The Supersymmetric t-J Model with a Boundary

An open supersymmetric t-J chain with boundary fields is studied by means of the Bethe Ansatz. Ground state properties for the case of an almost half-filled band and a bulk magnetic field are determined. Boundary susceptibilities are calculated as functions of the boundary fields. The effects of the boundary on excitations are investigated by constructing the exact boundary S-matrix. From the analytic structure of the boundary S-matrices one deduces that holons can form boundary bound states for sufficiently strong boundary fields.Comment: 23 pages of revtex, discussion on analytic structure of holon S-matrix change

The one-dimensional Hubbard model with open ends: Universal divergent contributions to the magnetic susceptibility

The magnetic susceptibility of the one-dimensional Hubbard model with open boundary conditions at arbitrary filling is obtained from field theory at low temperatures and small magnetic fields, including leading and next-leading orders. Logarithmic contributions to the bulk part are identified as well as algebraic-logarithmic divergences in the boundary contribution. As a manifestation of spin-charge separation, the result for the boundary part at low energies turns out to be independent of filling and interaction strength and identical to the result for the Heisenberg model. For the bulk part at zero temperature, the scale in the logarithms is determined exactly from the Bethe ansatz. At finite temperature, the susceptibility profile as well as the Friedel oscillations in the magnetisation are obtained numerically from the density-matrix renormalisation group applied to transfer matrices. Agreement is found with an exact asymptotic expansion of the relevant correlation function.Comment: 30 pages, 8 figures, reference adde