Variational description of the dimensional cross-over in the array of coupled one-dimensional conductors

Variational wave function is proposed to describe electronic properties of an array of one-dimensional conductors coupled by transverse hopping and interaction. For weak or intermediate in-chain interaction the wave function has the following structure: Tomonaga-Luttinger bosons with momentum higher then some variational quantity \tilde\Lambda are in their ground state while other bosons (with |k|<\tilde\Lambda) form kinks -- fermion-like excitations of the Tomonaga-Luttinger boson field. Nature of the ground state for this quasiparticles can be determined by solving three dimensional effective hamiltonian. Since the anisotropy of the effective hamiltonian is small the use of the mean field theory is justified. For repulsive interaction possible phases are density wave and p-wave superconductivity. Our method allows us to calculate the low-energy part of different electronic Green's functions. In order to do that it is enough to apply standard perturbation theory technique to the effective hamiltonian. When the in-chain interaction is strong \tilde\Lambda vanishes and no fermionic excitation is present in the system. In this regime the dynamics is described by transversally coupled Tomonaga-Luttinger bosons

Half-metallicity in NiMnSb: a Variational Cluster Approach with ab-initio parameters

Electron correlation effects in the half-metallic ferromagnet NiMnSb are investigated within a combined density functional and many-body approach. Starting from a realistic multi-orbital Hubbard-model including Mn and Ni-d orbitals, the many-body problem is addressed via the Variational Cluster Approach. The density of states obtained in the calculation shows a strong spectral weight transfer towards the Fermi level in the occupied conducting majority spin channel with respect to the uncorrelated case, as well as states with vanishing quasiparticle weight in the minority spin gap. Although the two features produce competing effects, the overall outcome is a strong reduction of the spin polarisation at the Fermi level with respect to the uncorrelated case. This result emphasizes the importance of correlation in this material.Comment: 8 pages, 6 figure

Half-metallic ferromagnetism induced by dynamic electron correlations in VAs

The electronic structure of the VAs compound in the zinc-blende structure is investigated using a combined density-functional and dynamical mean-field theory approach. Contrary to predictions of a ferromagnetic semiconducting ground state obtained by density-functional calculations, dynamical correlations induce a closing of the gap and produce a half-metallic ferromagnetic state. These results emphasize the importance of dynamic correlations in materials suitable for spintronics.Comment: Published in Phys. Rev. Lett. 96, 197203 (2006

Electron transport in coupled chains of interacting fermions with impurities

We study the low-temperature transport of a doped two-chain ladder system of interacting fermions in the presence of a barrier or of a low concentration of impurities. Above a certain value of the interaction, the conductance is suppressed, like for a single chain, despite the presence of dominant superconducting correlations. There is, however, a region of repulsive interaction where perfect transmission across the barrier occurs unlike the single-chain case. We provide a possible explanation for the temperature maximum of the resistivity in the normal state of \srca.Comment: 4 pages, 2 figures, to be published in Phys. Rev. Let

Ultrasound-Guided Percutaneous Tenotomy of the Long Head of Biceps Tendon in Patients with Symptomatic Complete Rotator Cuff Tear: In Vivo Non-contRolled Prospective Study

Background: We prospectively tested technical feasibility and clinical outcome of percutaneous ultrasound-guided tenotomy of long head of biceps tendon (LHBT). Methods: We included 11 patients (6 women; age: 73 \ub1 8.6 years) with symptomatic full-thickness rotator cuff tear and intact LHBT, in whom surgical repair was not possible/refused. After ultrasound-guided injection of local anesthetic, the LHBT was cut with a scalpel under continuous ultrasound monitoring until it became no longer visible. Pain was recorded before and at least six months after procedure. An eight-item questionnaire was administered to patients at follow-up. Results: A median of 4 tendon cuts were needed to ensure complete tenotomy. Mean procedure duration was 65 \ub1 5.7 s. Mean length of skin incision was 5.8 \ub1 0.6 mm. Pre-tenotomy VAS score was 8.2 \ub1 0.7, post-tenotomy VAS was 2.8 \ub1 0.6 (p &lt; 0.001). At follow-up, 5/11 patients were very satisfied, 5/11 satisfied and 1/11 neutral. One patient experienced cramping and very minimal pain in the biceps. Six patients had still moderate shoulder pain, 1/11 minimal pain, 2/11 very minimal pain, while 2/11 had no pain. No patients had weakness in elbow flexion nor limits of daily activities due to LHBT. One patient showed Popeye deformity. All patients would undergo ultrasound-guided tenotomy again. Conclusion: ultrasound-guided percutaneous LHBT tenotomy is technically feasible and effective

New Chiral Universality Class in a Frustrated Three-Leg Spin Ladder

We study a model of three $S=1/2$ antiferromagnetic Heisenberg spin chains weakly coupled by on-rung and plaquette-diagonal interchain interactions. It is shown that the model exhibits a critical phase with central charge C=2 and belongs to the class of ``chirally stabilized'' liquids recently introduced by Andrei, Douglas, and Jerez. By allowing anisotropic interactions in spin space, we find an exact solution at a Toulouse point which captures all universal properties of the model, including the SU(2) symmetric case. At the new critical point the massless degrees of freedom are described in terms of an effective $S = 1/2$ Heisenberg spin chain and two critical Ising models. We discuss the spectral properties of the model, compute spin-spin correlation functions and estimate the NMR relaxation rate.Comment: 4 page

Mean field approach to antiferromagnetic domains in the doped Hubbard model

We present a restricted path integral approach to the 2D and 3D repulsive Hubbard model. In this approach the partition function is approximated by restricting the summation over all states to a (small) subclass which is chosen such as to well represent the important states. This procedure generalizes mean field theory and can be systematically improved by including more states or fluctuations. We analyze in detail the simplest of these approximations which corresponds to summing over states with local antiferromagnetic (AF) order. If in the states considered the AF order changes sufficiently little in space and time, the path integral becomes a finite dimensional integral for which the saddle point evaluation is exact. This leads to generalized mean field equations allowing for the possibility of more than one relevant saddle points. In a big parameter regime (both in temperature and filling), we find that this integral has {\em two} relevant saddle points, one corresponding to finite AF order and the other without. These degenerate saddle points describe a phase of AF ordered fermions coexisting with free, metallic fermions. We argue that this mixed phase is a simple mean field description of a variety of possible inhomogeneous states, appropriate on length scales where these states appear homogeneous. We sketch systematic refinements of this approximation which can give more detailed descriptions of the system.Comment: 14 pages RevTex, 6 postscript figures included using eps

Lightly Doped t-J Three-Leg Ladders - an Analog for the Underdoped Cuprates

The three-leg ladder has one odd-parity and two even-parity channels. At low doping these behave quite differently. Numerical calculations for a t-J model show that the initial phase upon hole doping has two components - a conducting Luttinger liquid in the odd-parity channel, coexisting with an insulating (i.e. undoped) spin liquid phase in the even-parity channels. This phase has a partially truncated Fermi surface and violates the Luttinger theorem. This coexistence of conducting fermionic and insulating paired bosonic degrees of freedom is similar to the recent proposal of Geshkenbein, Ioffe, and Larkin for the underdoped spin-gap normal phase of the cuprates. A mean field approximation is derived which has many similarities to the numerical results. One difference however is an induced hole pairing in the odd-parity channel at arbitrary small dopings, similar to that proposed by Geshkenbein, Ioffe, and Larkin for the two-dimensional case. At higher dopings, we propose that a quantum phase transition will occur as holes enter the even-parity channels, resulting in a Luther-Emery liquid with hole pairing with essentially d-wave character. In the mean field approximation a crossover occurs which we interpret as a reflection of this quantum phase transition deduced from the numerical results.Comment: RevTex, 36 pages with 16 figure

Impurity-induced stabilization of Luttinger liquid in quasi-one-dimensional conductors

It is shown theoretically that the Luttinger liquid phase in quasi-one-dimensional conductors can exist in the presence of impurities in a form of a collection of bounded Luttinger liquids. The conclusion is based upon the observation by Kane and Fisher that a local impurity potential in Luttinger liquid acts, at low energies, as an infinite barrier. This leads to a discrete spectrum of collective charge and spin density fluctuations, so that interchain hopping can be considered as a small parameter at temperatures below the minimum excitation energy of the collective modes. The results are compared with recent experimental observation of a Luttinger-liquid-like behavior in thin NbSe$_3$ and TaS$_3$ wires.Comment: 11 pages, revtex, final version published in JETP Lett

Theory of the density fluctuation spectrum of strongly correlated electrons

The density response function $N(q,\omega)$ of the two-dimensional $t-J$ model is studied starting from a mixed gauge formulation of the slave boson approach. Our results for $N(q, \omega)$ are in remarkable agreement with exact diagonalization studies, and provide a natural explanation of the anomalous features in the density response in terms of the spin polaron nature of the charge carriers. In particular we have identified unexplained low energy structures in the diagonalization data as arising from the coherent polaron motion of holes in a spin liquid.Comment: 4 pages with 4 figures, to be published in Physical Review B (RC