arXiv subjects

Indrani Bose

Publications and source records attributed to Indrani Bose.

66 records · Page 4Linked to original sources

arXiv2000

Incommensurate structure factor in a hole-doped spin-1 system

The nickelate compound Y_{2}BaNiO_{5} is a spin-1 Haldane-gap antiferromagnet. The compound is doped with holes on replacing the off-chain Y^{3+} ions by Ca^{2+} ions. Inelastic neutron scattering (INS) experiments reveal the existence of sub-gap states on doping. A recent INS experiment provides evidence for an incommensurate double-peaked structure factor S(q) corresponding to the sub-gap states. In this paper, we formulate a microscopic theory for the origin of the incommensurate peak.

cond-mat.str-el↗

arXiv2000

Low-dimensional quantum spin systems

Low-dimensional quantum spin systems are interacting many body systems for which several rigorous results are known. Powerful techniques like the Bethe Ansatz provide exact knowledge of the ground state energy and low-lying excitation spectrum. A large number of compounds exists which can effectively be described as low-dimensional spin systems. Thus many of the rigorous results are also of experimental relevance. In this Lecture Note, an introduction is given to quantum spin systems and the rigorous results known for such systems. Some general theorems and exactly-solvable models are discussed. An elementary introduction to the Bethe Ansatz technique is given.

cond-mat.str-el↗

arXiv1999

Punctuated equilibrium in an evolving bacterial population

Recently, Lenski et al have carried out an experiment on bacterial evolution. Their findings support the theory of punctuated equilibrium in biological evolution. We show that the M=2 Bak-Sneppen model can explain some of the experimental results in a qualitative manner.

cond-mat.stat-mech↗

arXiv1999

Exact ground and excited states of a t-J ladder doped with two holes

A two chain ladder model is considered described by the strong coupling $t-t^\prime-J-J^\prime$ Hamiltonian. For the case of two holes moving in a background of antiferromagnetically interacting spins, exact, analytical results are derived for the ground state energy and low-lying excitation spectrum. The ground state is a bound state of two holes with total spin S=0. The charge excitation is gapless and the spin excitation has a gap. The corresponding wavefunctions are also exactly determined. The bound hole pair is found to have symmetry of the d-wave type. In the limit of strong rung coupling, the model maps onto an effective hard core boson model which exhibits dominant superconducting pairing correlations.

cond-mat.str-el↗

arXiv1999

Mixed-spin systems: coexistence of Haldane gap and antiferromagnetic long range order

Recent experiments on the quasi-1D antiferromagnets $R_{2}BaNiO_{5}$ (R = rare earth) have shown the existence of purely 1D Haldane gap excitations propagating on the Ni chains. Below an ordering temperature, the gap excitations survive and coexist with the conventional spin waves in the ordered phase. We construct a model mixed-spin system in 2D for which the ground state can be exactly specified. Using the Matrix Product Method, we show the existence of Haldane gap excitations in the ordered phase. We consider different cases of ordering to study the effect of ordering on the degeneracy of the Haldane gap excitations.

cond-mat.str-el↗

arXiv1998

Percolation-like phase transition in a non-equilibrium steady state

We study the Gierer-Meinhardt model of reaction-diffusion on a site-disordered square lattice. Let $p$ be the site occupation probability of the square lattice. For $p$ greater than a critical value $p_c$, the steady state consists of stripe-like patterns with long-range connectivity. For $p < p_c$, the connectivity is lost. The value of $p_c$ is found to be much greater than that of the site percolation threshold for the square lattice. In the vicinity of $p_c$, the cluster-related quantities exhibit power-law scaling behaviour. The method of finite-size scaling is used to determine the values of the fractal dimension $d_f$, the ratio, $\fracγν$, of the average cluster size exponent $γ$ and the correlation length exponent $ν$ and also $ν$ itself. The values appear to indicate that the disordered GM model belongs to the universality class of ordinary percolation.

cond-mat.stat-mech↗

arXiv1998

Collinear Néel-type ordering in partially frustrated lattices

We consider two partially frustrated S = 1/2 antiferromagnetic spin systems on the triangular and pentagonal lattices. In an elementary plaquette of the two lattices, one bond has exchange interaction strength $α$ ($α\leq 1$) whereas all other bonds have exchange interaction strength unity. We show that for $α$ less than a critical value $α_{c}$, collinear Néel-type ordering is possible in the ground state. The ground state energy and the excitation spectrum have been determined using linear spin wave theory based on the Holstein-Primakoff transformation.

cond-mat.str-el↗

arXiv1997

Exact ground and excited states of frustrated antiferromagnets on the CaV_4O_9 lattice

The experimental observation of a spin-gap in the antiferromagnetic spin-1/2 compound $CaV_4O_9$ has been attributed to the formation of a plaquette resonating-valence-bond (PRVB) state in the underlying 1/5-depleted square lattice. We construct a spin-1/2 model on the 1/5-depleted lattice for which the PRVB state can be shown to be the exact ground state in a particular parameter regime. In a subspace of this parameter regime, the first excited state can be calculated exactly. In a different parameter regime, the dimer state is shown to be the exact ground state.

cond-mat.str-el↗

arXiv1997

Lattice animals and the Percolation model under rotational constraint

The effect of rotational constraint on the properties of lattice models like the self-avoiding walk, lattice animals and percolation is discussed. The results obtained so far, using a variety of exact and approximate techniques, are described. Examples of the rotational constraint in real systems are also given.

cond-mat.stat-mech↗

arXiv1997

Effect of randomness and anisotropy on Turing patterns in reaction-diffusion systems

We study the effect of randomness and anisotropy on Turing patterns in reaction-diffusion systems. For this purpose, the Gierer-Meinhardt model of pattern formation is considered. The cases we study are: (i)randomness in the underlying lattice structure, (ii)the case in which there is a probablity p that at a lattice site both reaction and diffusion occur, otherwise there is only diffusion and lastly, the effect of (iii) anisotropic and (iv) random diffusion coefficients on the formation of Turing patterns. The general conclusion is that the Turing mechanism of pattern formation is fairly robust in the presence of randomness and anisotropy.

cond-mat.stat-mech↗

arXiv1996

Antiferromagnetic systems with spin gap: exact results

We describe some antiferromagnetic systems which exhibit spin gaps. We also discuss the effect of doping one such system, namely, the spin-ladders, with holes. Some model antiferromagnetic systems with spin gap are reviewed for which exact results are available. Exact results for a doped spin-ladder model are also mentioned.

cond-mat.str-el↗

arXiv1996

Antiferromagnetic Spin Ladders with Odd and Even number of Chains

We construct frustrated antiferromagnetic spin ladders with m chains for which the exact ground state can be determined in a particular parameter regime. The excitation spectrum is shown rigorously to be gapless ( with gap ) for odd ( even ) m. In a general parameter regime, the four-chain and periodic ladders are studied using a mean-field theory based on the bond operator formalism for spin $S=1/2$. The excitation spectrum and the spin-gap are calculated in both the cases. The spin-gap of the frustrated ladder system has a larger magnitude than in the case of non-frustrated ladders. For the frustrated periodic ladder, the spin-gap vanishes at a critical value of the interladder coupling strength which is larger than the critical value in the case of non-frustrated periodic ladder.

cond-mat.str-el↗