Entangled spin clusters: some special features

In this paper, we study three specific aspects of entanglement in small spin clusters. We first study the effect of inhomogeneous exchange coupling strength on the entanglement properties of the S=1/2 antiferromagnetic linear chain tetramer compound NaCuAsO_{4}. The entanglement gap temperature, T_{E}, is found to have a non-monotonic dependence on the value of $\alpha$, the exchange coupling inhomogeneity parameter. We next determine the variation of T_{E} as a function of S for a spin dimer, a trimer and a tetrahedron. The temperature T_{E} is found to increase as a function of S, but the scaled entanglement gap temperature t_{E} goes to zero as S becomes large. Lastly, we study a spin-1 dimer compound to illustrate the quantum complementarity relation. We show that in the experimentally realizable parameter region, magnetization and entanglement plateaus appear simultaneously at low temperatures as a function of the magnetic field. Also, the sharp increase in one quantity as a function of the magnetic field is accompanied by a sharp decrease in the other so that the quantum complementarity relation is not violated.Comment: 17 pages, 6 figures. Accepted in Phys. Rev.

Polaritonic characteristics of insulator and superfluid phases in a coupled-cavity array

Recent studies of quantum phase transitions in coupled atom-cavity arrays have focused on the similarities between such systems and the Bose-Hubbard model. However, the bipartite nature of the atom-cavity systems that make up the array introduces some differences. In order to examine the unique features of the coupled-cavity system, the behavior of a simple two-site model is studied over a wide range of parameters. Four regions are identified, in which the ground state of the system may be classified as either a polaritonic insulator, a photonic superfluid, an atomic insulator, or a polaritonic superfluid.Comment: 7 pages, 9 figures, 1 table, REVTeX 4; published versio

Dynamics in a coupled-cavity array

The dynamics of a system composed of two coupled optical cavities, each containing a single two-level atom, is studied over a wide range of detuning and coupling values. A description of the field in terms of delocalized modes reveals that the detuning between the atoms and these modes is controlled by the coupling between the cavities; this detuning in turn governs the nature of the dynamics. If the atoms are highly detuned from both delocalized field modes, the dynamics becomes dispersive and an excitation may be transferred from the first atom to the second without populating the field. In the case of resonance between the atoms and one of the delocalized modes, state transfer between the atoms requires intermediate excitation of the field. Thus the interaction between the two atoms can be controlled by adjusting the coupling between the cavities.Comment: 11 pages, 3 figure

Single hole doped strongly correlated ladder with a static impurity

We consider a strongly correlated ladder with diagonal hopping and exchange interactions described by $t-J$ type hamiltonian. We study the dynamics of a single hole in this model in the presence of a static non-magnetic (or magnetic) impurity. In the case of a non-magnetic (NM) impurity we solve the problem analytically both in the triplet (S=1) and singlet (S=0) sectors. In the triplet sector the hole doesn't form any bound state with the impurity. However, in the singlet sector the hole forms bound states of different symmetries with increasing $J/t$ values. Binding energies of those impurity-hole bound states are compared with the binding energy of a pair of holes in absence of any impurity. In the case of magnetic impurity the analytical eigenvalue equations are solved for a large (50 X 2) lattice. In this case also, with increasing $J/t$ values, impurity-hole bound states of different symmetries are obtained. Binding of the hole with the impurity is favoured for the case of a ferromagnetic (FM) impurity than in the case of antiferromagnetic (AFM) impurity. However binding energy is found to be maximum for the NM impurity. Comparison of binding energies and various impurity-hole correlation functions indicates a pair breaking mechanism by NM impurity.Comment: 15 Pages, 6 figure

Quantum communication via a continuously monitored dual spin chain

We analyze a recent protocol for the transmission of quantum states via a dual spin chain [Burgarth and Bose, Phys. Rev. A 71, 052315 (2005)] under the constraint that the receiver's measurement strength is finite. That is, we consider the channel where the ideal, instantaneous and complete von Neumann measurements are replaced with a more realistic continuous measurement. We show that for optimal performance the measurement strength must be "tuned" to the channel spin-spin coupling, and once this is done, one is able to achieve a similar transmission rate to that obtained with ideal measurements. The spin chain protocol thus remains effective under measurement constraints.Comment: 5 pages, revtex 4, 3 eps figure

Superconductivity in Boron under pressure - why are the measured Tc_c's so low?

Using the full potential linear muffin-tin orbitals (FP-LMTO) method we examine the pressure-dependence of superconductivity in the two metallic phases of Boron: bct and fcc. Linear response calculations are carried out to examine the phonon frequencies and electron-phonon coupling for various lattice parameters, and superconducting transition temperatures are obtained from the Eliashberg equation. In both bct and fcc phases the superconducting transition temperature T$_c$ is found to decrease with increasing pressure, due to stiffening of phonons with an accompanying decrease in electron-phonon coupling. This is in contrast to a recent report, where T$_c$ is found to increase with pressure. Even more drastic is the difference between the measured T$_c$, in the range 4-11 K, and the calculated values for both bct and fcc phases, in the range 60-100 K. The calculation reveals that the transition from the fcc to bct phase, as a result of increasing volume or decreasing pressure, is caused by the softening of the X-point transverse phonons. This phonon softening also causes large electron-phonon coupling for high volumes in the fcc phase, resulting in coupling constants in excess of 2.5 and T$_c$ nearing 100 K. We discuss possible causes as to why the experiment might have revealed T$_c$'s much lower than what is suggested by the present study. The main assertion of this paper is that the possibility of high T$_c$, in excess of 50 K, in high pressure pure metallic phases of boron cannot be ruled out, thus substantiating the need for further experimental investigations of the superconducting properties of high pressure pure phases of boron.Comment: 16 pages, 8 figures, 1 Tabl

Entanglement in a molecular three-qubit system

We study the entanglement properties of a molecular three-qubit system described by the Heisenberg spin Hamiltonian with anisotropic exchange interactions and including an external magnetic field. The system exhibits first order quantum phase transitions by tuning two parameters, $x$ and $y$, of the Hamiltonian to specific values. The three-qubit chain is open ended so that there are two types of pairwise entanglement : nearest-neighbour (n.n.) and next-nearest-neighbour (n.n.n.). We calculate the ground and thermal state concurrences, quantifying pairwise entanglement, as a function of the parameters $x$, $y$ and the temperature $T$. The entanglement threshold and gap temperatures are also determined as a function of the anisotropy parameter $x$. The results obtained are of relevance in understanding the entanglement features of the recently engineered molecular $Cr_{7}Ni$-$Cu^{2+}$-$Cr_{7}Ni$ complex which serves as a three-qubit system at sufficiently low temperatures.Comment: 9 pages, 13 figures, revtex

Fibre Insulation Refractories in Reheating Furnaces

Reheating furnaces are the heart of a rolling and forging shop. Primary steel Ingot In changing shapes Into blooms, billets, bars, rods, slabs, plates, sheets, strips, rails, angles, channels and tubes has to be heated into pyro-plastic stage at 1200-1320°C in a reheating furnace. The temperature Is dependent upon the steel composition and rolling/forging technique. Hardy and Titteringtonl has dealt with the refractories of reheating furnaces. At this temperature enough iron oxide scale formation takes place due to which the hearth of the furnace has to bear the corrosive action of molten iron oxide and the walls, oxide atmosphere. In the heat treatment furnaces for annealing, normalising, hardening or stress relieving, the temperature Is never more than 1020°C and Is generally around 780°C. But unlike reheating furnaces the atmosphere Is either reducing or neutral without any suspension of inorganic material. Except for its hearth which has to bear the load of the work pieces, all the walls and roof has to withstand and conserve the heat only

Role of pseudospin in quasiparticle interferences in epitaxial graphene probed by high-resolution scanning tunneling microscopy

Pseudospin, an additional degree of freedom related to the honeycomb structure of graphene, is responsible of many of the outstanding electronic properties found in this material. This article provides a clear understanding of how such pseudospin impacts the quasiparticle interferences of monolayer (ML) and bilayer (BL) graphene measured by low temperature scanning tunneling microscopy and spectroscopy. We have used this technique to map, with very high energy and space resolution, the spatial modulations of the local density of states of ML and BL graphene epitaxialy grown on SiC(0001), in presence of native disorder. We perform a Fourier transform analysis of such modulations including wavevectors up to unit-vectors of the reciprocal lattice. Our data demonstrate that the quasiparticle interferences associated to some particular scattering processes are suppressed in ML graphene, but not in BL graphene. Most importantly, interferences with 2qF wavevector associated to intravalley backscattering are not measured in ML graphene, even on the images with highest resolution. In order to clarify the role of the pseudospin on the quasiparticle interferences, we use a simple model which nicely captures the main features observed on our data. The model unambiguously shows that graphene's pseudospin is responsible for such suppression of quasiparticle interferences features in ML graphene, in particular for those with 2qF wavevector. It also confirms scanning tunneling microscopy as a unique technique to probe the pseudospin in graphene samples in real space with nanometer precision. Finally, we show that such observations are robust with energy and obtain with great accuracy the dispersion of the \pi-bands for both ML and BL graphene in the vicinity of the Fermi level, extracting their main tight binding parameters