Spin susceptibilities for the Hubbard model in a band approach

The properties of spin excitations superposed on a uniform ground state with antiferromagnetic (or spiral) spin structure are studied in a 2D Hubbard model. Expressions are derived for the spin susceptibility in the random phase approximation (RPA) using split Hubbard bands as a zeroth approximation. The calculated collective modes with dispersion ω(Q)=c|Q−(π, π)| near Q∼(π, π) reproduce well the characteristics of the spin excitations observed in undoped cuprates. For doped systems with an antiferromagnetic structure of the ground state, calculating X″(Q,ω→0) gives the same mode with a peak at Q∼(π, π), regardless of the type of Fermi surface. It is shown that in doped systems with a spiral ground state spin structure, X″(Q,ω→0) peaks occur with incommensurate quasimomenta Q that are coupled to the spirality vector. Zh. éksp. Teor. Fiz. 116, 1058–1080 (September 1999)     

Dynamics of spinor condensates and stochastic approach

The dynamics of the collective spin for Bose-Einstein condensates with nonlinear interactions, is studied within the framework of the two-component spinor. We discuss the spin resonance when the system is submitted to a periodically-modulated magnetic field at the zero temperature. In this case, the nonlinearity parameter controls the critical change between a localized and a homogeneous spin state. When the temperature is finite – or a random magnetic field is considered – the movement of the collective spin is governed by the Landau-Lifshitz-Gilbert equation, from which the complete Fokker-Planck equation is derived. This equation is the essential tool to describe the time-evolution of the probability distribution function for the collective spin. The functional integral approach is used to solve analytically examples of BEC spin behavior in a static magnetic field at finite temperature. We show how such a method can lead effectively to the complete solution of the Fokker-Planck equation for this kind of problems.     

Effect of Dzyaloshinskii−Moriya interactions on Kagome anti-ferromagnetic clusters

The low energy and low temperature behavior of a few finite size Kagome clusters, including mixed spin systems of S=1/2 and S=1, with the nearest neighbor Heisenberg antiferromagnetic model is studied under the influence of out-of-plane Dzyaloshinskii?Moriya interactions (DMI) within the exact diagonalization formalism. The ground state of all the finite size systems is found to be present in the lowest spin sector with a finite gap to the lowest magnetic excitation irrespective of the strength of out-of-plane DMI. The energy level structures within the non-magnetic ground state and the lowest magnetic state have been studied for all the systems as a function of DMI. The characteristic signature of such low-lying non-magnetic excitations is reflected in the low temperature behavior of the specific heat. It is also found that the ground state chiral structure (characterized by the vector chiral order of the system) in the xy-plane shows sharp changes as a function of out-of-plane DMI at level crossing or avoided crossing regions. The in-plane spin ordering for each system is also studied with the estimation of static structure factor as a response to the varying strength of DMI.     

Recent studies in heavy ion induced fission reactions

Nuclear fission process involves large scale shape changes of the nucleus, while it evolves from a nearly spherical configuration to two separated fission fragments. The dynamics of these shape changes in the nuclear many body system is governed by a strong interplay of the collective and single particle degrees of freedom. With the availability of heavy ion accelerators, there has been an impetus to study the nuclear dynamics through the investigations of nucleus-nucleus collisions involving fusion and fission process. From the various investigations carried out in the past years, it is now well recognized that there is large scale damping of collective modes in heavy ion induced fission reactions, which in other words implies that nuclear motion is highly viscous. In recent years, there have been many experimental observations in heavy ion induced fission reactions at medium bombarding energies, which suggest possible occurrence of various non-equilibrium modes of fission such as quasi-fission, fast fission and pre-equilibrium fission, where some of the internal degrees of freedom of the nucleus is not fully equilibrated. We have carried out extensive investigations on the fission fragment angular distributions at near barrier bombarding energies using heavy fissile targets. The measured fragment anisotropies when compared with the standard saddle point model (SSPM) calculations show that for projectile-target systems having zero or low ground state spins, the angular anisotropy exhibits a peak-like behaviour at the sub barrier energies, which cannot be explained by the SSPM calculations. For projectiles or targets with large ground state spins, the anomalous peaking gets washed out due to smearing of the K-distribution by the intrinsic entrance channel spins. Recently studies have been carried out on the spin distributions of fission fragments through the gamma ray multiplicity measurements. The fission fragments acquire spin mainly from two sources: (i) due to rigid rotation of the nascent fragments at scission and (ii) due to statistical excitation of the spin bearing collective modes in the fissioning nucleus. One of the collective modes — the tilting mode depends on the K quantum number and is responsible for the emission angle dependence of fragment spin. In our studies, we have shown conclusively that the collective statistical spin modes get strongly suppressed for high K values corresponding to large rotational frequencies along the fission axis. These results bring out the importance of the dynamical effects in the heavy ion induced fusion-fission reactions. The present article will review the work carried out on the above aspects in heavy ion fission reactions as well as on the fission time scales, and some of the recent studies on the mass-energy correlations of fission fragments at near-barrier bombarding energies.     

Vertex cover problem studied by cavity method: Analytics and population dynamics

We study the vertex cover problem on finite connectivity random graphs by zero-temperature cavity method. The minimum vertex cover corresponds to the ground state(s) of a proposed Ising spin model. When the connectivity c > e = 2.718282, there is no state for this system as the reweighting parameter y, which takes a similar role as the inverse temperature β in conventional statistical physics, approaches infinity; consequently the ground state energy is obtained at a finite value of y when the free energy function attains its maximum value. The minimum vertex cover size at given c is estimated using population dynamics and compared with known rigorous bounds and numerical results. The backbone size is also calculated. Received 11 November 2002 Published online 1st April 2003 RID="a" ID="a"e-mail: zhou@mpikg-golm.mpg.de     

Spin‐Polarized Symmetric Electron‐Hole Quantum Bilayers: Finite width Effect

In this paper, the effect of finite width on ground‐state properties of a spin‐polarized symmetric electron‐hole quantum bilayers (EHBL) system is investigated at zero temperature. The quantum self‐consistent mean‐field approximation of Singwi, Tosi, Land and Sjölander (qSTLS) is adopted to explore intra‐ and interlayer properties such as the pair‐correlation function, the static density susceptibility, the local‐field corrections and the ground‐state energy. Interestingly, we noticed that due to the inclusion of finite width, the critical density for the onset of Wigner crystal (WC) phase is now lowered as compared to the recent spin‐polarized EHBL system without finite width and unpolarized EHBL system with finite width. Further, spin‐polarization effect is seem to introduce a marked change in the ground‐state energy of EHBL system as compared to that of unpolarized system. Results of ground‐state energy are also compared with the recent EHBL system without finite width (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Broken symmetry in antiferromagnets

We discuss limitations of the conventional ‘broken symmetry’ picture of the Heisenberg antiferromagnet. The exact results on the ground state of the linear chain and of the three-dimensional Hamiltonian do not show a ‘degeneracy of the vacuum’. With the help of a solvable model it is shown that the correlations in the ground state may have the Néel character, as revealed by the neutron experiments, even though the ground state is quite different from the Néel states. There is no Goldstone mode in the linear chain. The spin of the antiferromagnetic spin wave is 1/2. But the physical states have a doublet of the spin waves which could be regarded as degenerate states of spin 1 and spin 0. The fermionic character is suppressed and the bosonic character revealed, as in the decolouring phenomena in quantum field theory. It is plausible that in the three-dimensional case also there is no Goldstone mode.     

Decoherence-free generation of many-particle entanglement by adiabatic ground-state transitions

We discuss a decoherence insensitive method to create many-particle entanglement in a spin system with controllable collective interactions and propose an implementation in an ion trap. An adiabatic change of parameters allows a transfer from separable to a large variety of entangled eigenstates. We show that the Hamiltonian can have a supersymmetry permitting an explicit construction of the ground state at all times. Of particular interest is a transition in a nondegenerate ground state with a finite energy gap since here the influence of collective as well as individual decoherence mechanisms is substantially reduced. A lower bound for the energy gap is given.     

Ground-state long-range order in quasi-one-dimensional Heisenberg quantum antiferromagnets: high-order coupled-cluster calculations

We investigate the ground-state magnetic long-range order of quasi-one-dimensional quantum Heisenberg antiferromagnets for spin quantum numbers s = 1/2 and s = 1. We use the coupled cluster method to calculate the sublattice magnetization and its dependence on the inter-chain coupling J_⊥. We find that for the unfrustrated spin-1/2 system, an infinitesimal inter-chain coupling is sufficient to stabilize magnetic long-range order, in agreement with results obtained by other methods. For s = 1, we find that a finite inter-chain coupling is necessary to stabilize magnetic long-range order. Furthermore, we consider a quasi one-dimensional spin-1/2 system, where a frustrating next-nearest neighbor in-chain coupling is included. We find that for stronger frustration as well, a finite inter-chain coupling is necessary to have magnetic long-range order in the ground state, and that the strength of the inter-chain coupling necessary to establish magnetic long-range order is related to the size of the spin gap of the isolated chain.     

Segmental dynamics of poly(methyl phenyl siloxane) confined to nanoporous glasses

The effect of a nanometer confinement on the molecular dynamics of poly(methyl phenyl siloxane) (PMPS) was studied by dielectric spectroscopy (DS), temperature modulated DSC (TMDSC) and neutron scattering (NS). Nanoporous glasses with pore sizes of 2.5–20 nm have been used. DS and TMDSC experiments show that for PMPS in 7.5 nm pores the molecular dynamics is faster than in the bulk which originates from an inherent length scale of the underlying molecular motions. For high temperatures the temperature dependence of the relaxation rates for confined PMPS crosses that of the bulk state. Besides finite states effects also the thermodynamic state of nano-confined PMPS is different from that of the bulk. At a pore size of 5 nm the temperature dependence of the relaxation times changes from a Vogel/Fulcher/Tammann like to an Arrhenius behavior where the activation energy depends on pore size. This is in agreement with the results obtained by NS. The increment of the specific heat capacity at the glass transition depends strongly on pore size and vanishes at a finite length scale between 3 and 5 nm which can be regarded as minimal length scale for glass transition to appear in PMPS.     

Entanglement in a valence-bond solid state

We study entanglement in a valence-bond solid state, which describes the ground state of an Affleck-Kennedy-Lieb-Tasaki quantum spin chain, consisting of bulk spin-1's and two spin-1/2's at the ends. We characterize entanglement between various subsystems of the ground state by mostly calculating the entropy of one of the subsystems; when appropriate, we evaluate concurrences as well. We show that the reduced density matrix of a continuous block of bulk spins is independent of the size of the chain and the location of the block relative to the ends. Moreover, we show that the entanglement of the block with the rest of the sites approaches a constant value exponentially fast, as the size of the block increases. We also calculate the entanglement of (i) any two bulk spins with the rest, and (ii) the end spin-1/2's (together and separately) with the rest of the ground state.     

Magnons versus free spinons in finite quantum frustrated antiferromagnets

We have investigated the validity of doping with a vacancy the J₁–J₃ frustrated Heisenberg model on a finite square lattice as a way to test the existence of fractional spin excitations. Using a generalized t–J₁–J₃ model we have computed the vacancy spectral functions in the self-consistent Born approximation. We have found that by including spiral fluctuations in the magnetic ground state, the spectral functions on finite systems agree very well with the unbiased exact ones. In contrast to the recent proposal that the quasiparticle weight reduction could be a signal of a spinon free excitation in finite systems, we have found strong evidence that such a reduction is due to the existence of spiral fluctuations.     

Quantum leakage of collective excitations of atomic ensemble induced by spatial motion

We generalize the conception of quantum leakage for the atomic collective excitation states. By making use of the atomic coherence state approach, we study the influence of the atomic spatial motion on the symmetric collective states of 2-level atomic ensemble due to inhomogeneous coupling. In the macroscopic limit, we analyze the quantum decoherence of the collective atomic state by calculating the quantum leakage for a very large ensemble at a finite temperature. Our investigations show that the fidelity of the atomic system will not be good in the case of atom numberN→∞. Therefore, quantum leakage is an inevitable problem in using the atomic ensemble as a quantum information memory. The detailed calculations shed theoretical light on quantum processing using atomic ensemble collective qubit.     

Magnetic state of the bulk, surface and nanoclusters of CaMnO3: A DFT study

The electronic structures of bulk, 2D slabs and clusters of CaMnO₃ in various magnetic configurations are presented. The obtained results including optimized cell constant, band-gap, Mn magnetic moment, on-site Coulomb repulsion potential and p-d charge separation potential are in good agreement with experiment data. The energetically most preferable configuration was an insulating charge-transfer ground state with G-type antiferromagnetic (AF) configuration (classified according to Wollan and Koehler, Phys. Rev. 100 (1955) 545). For the finite 2D layers the C-type AF ground state was found to be most stable. The surface effect on magnetism of finite quasi 2D systems appeared to originate in the pyramidal field splitting of Mn 3d levels, which induced the formation of ferromagnetic (FM) regions within the AF matrix and the extension of FM correlation deep through 7 subsurface layers (2.7 nm from the surface). All finite systems (clusters and slabs) were found non-conducting due to the localization of electrons and the cancellation of surface excess carriers (holes) after surface relaxation, although the band-gaps of 2D systems were sufficiently reduced in comparison with that of the bulk.     

Interlayer transport in bilayer quantum Hall systems

Bilayer quantum Hall systems have a broken symmetry ground state at a filling factor which can be viewed either as an excitonic superfluid or as a pseudospin ferromagnet. We present a theory of interlayer transport in quantum Hall bilayers that highlights remarkable similarities and critical differences between transport in Josephson junction and ferromagnetic metal spin-transfer devices. Our theory is able to explain the size of the large but finite low-bias interlayer conductance and the voltage width of this collective transport anomaly.     

A Multi-Dimensional Lieb-Schultz-Mattis Theorem

For a large class of finite-range quantum spin models with half-integer spins, we prove that uniqueness of the ground state implies the existence of a low-lying excited state. For systems of linear size L, with arbitrary finite dimension, we obtain an upper bound on the excitation energy (i.e., the gap above the ground state) of the form (C log L)/L. This result can be regarded as a multi-dimensional Lieb-Schultz-Mattis theorem [14] and provides a rigorous proof of the main result in [8].     

Intersubband collective excitations in quasi-two-dimensional systems in a strong magnetic field

The spectrum of intersubband collective excitations of spin and charge density in a system of quasi-two-dimensional electrons is calculated in the strong magnetic field limit for filling factors υ≤4. For υ≤2 two new closely spaced modes of collective excitations are obtained. The modes obtained make it possible to give a new interpretation of the experimentally observed line, which is usually interpreted as being due to single-particle excitations. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 8, 543–547 (25 April 1999)     

Extremal optimization for Sherrington-Kirkpatrick spin glasses

Extremal Optimization (EO), a new local search heuristic, is used to approximate ground states of the mean-field spin glass model introduced by Sherrington and Kirkpatrick. The implementation extends the applicability of EO to systems with highly connected variables. Approximate ground states of sufficient accuracy and with statistical significance are obtained for systems with more than N=1000 variables using ±J bonds. The data reproduces the well-known Parisi solution for the average ground state energy of the model to about 0.01%, providing a high degree of confidence in the heuristic. The results support to less than 1% accuracy rational values of ω=2/3 for the finite-size correction exponent, and of ρ=3/4 for the fluctuation exponent of the ground state energies, neither one of which has been obtained analytically yet. The probability density function for ground state energies is highly skewed and identical within numerical error to the one found for Gaussian bonds. But comparison with infinite-range models of finite connectivity shows that the skewness is connectivity-dependent.     

Stimulated Electron Spin Polarization in Strongly Coupled Triplet–Doublet Spin Pairs

The possibility of stimulating electron spin polarization in a system consisting of a stable paramagnetic center and a chromophore that can be excited into its triplet state is discussed. In such systems, the doublet state of the paramagnetic center couples to the excited triplet state of the chromophore and if the coupling is larger than the difference in the precession frequencies of the doublet and triplet, the eigenstates of the coupled system are quartet and doublet states. The quartet state is usually the lowest energy excited state. Following light excitation, the initial electronic relaxation to the quartet state generates strong multiplet polarization if it is governed by the spin–orbit coupling that follows the molecular symmetry. It is shown that application of a selective π-pulse to the ±3/2 ↔ ±1/2 transitions of the quartet converts this multiplet polarization into net polarization. The magnitude and orientation dependence of the generated polarization is estimated on the basis of a simple analytical model. The experimental conditions required for this net polarization to be retained in the ground state after decay of the quartet state are discussed. The viability of using this as a method to enhance the signal strength of a spin label or metal center in selective excitation experiments is considered.