Spin Ladder System Research Articles

Coherent magnetic oscillation in the spin ladder system alpha(')- NaV2O5.

We report the first observation of coherent magnetic excitations in a spin ladder system NaV2O5 by using femtosecond time-domain spectroscopy. A pronounced coherent oscillation is observed at 127 cm(-1) (nearly twice the spin gap energy) and assigned to a two-magnon bound state, based on the temperature dependence of the intensity below the charge ordering phase transition at T(C) = 34 K. This mode can be observable only when circularly polarized light is used as a pump or a probe beam, suggesting that it corresponds to a spin-flip excitation from the singlet ground state. A phonon mode strongly coupled to the spin state is also found at 303 cm(-1).

Soliton in an anisotropic spin ladder with site and spin-dependent Dzyaloshinsky–Moriya interaction

We investigate the nature of nonlinear spin excitations in an anisotropic Heisenberg ferromagnetic spin ladder with ferromagnetically coupled rungs in the presence of a site and spin-dependent Dzyaloshinsky–Moriya type interaction in the semiclassical limit. The spin ladder system is found to exhibit soliton spin excitations for specific parameter values. In the general case we construct perturbed solitons.

Universality class of anS=12quantum spin ladder system with four-spin exchange

We study a $s=\frac{1}{2}$ Heisenberg spin ladder with four-spin exchange. Combining numerical results with conformal field theory, we find a phase transition with central charge $c=\frac{3}{2}.$ Since this system has an SU(2) symmetry, we can conclude that this critical theory is described by $k=2$ SU(2) Wess-Zumino-Witten model with ${Z}_{2}$ symmetry breaking.

Magnetic properties of organic two-leg spin–ladder systems with S=1/2 and S=1

Magnetic properties of spin systems on the two-leg ladder made of organic radicals are studied. The magnetization measurements below 4.2 K in pulsed magnet fields up to about 50 T were done for BIP-BNO as an S=1/2 ladder and for BIP-TENO as an S=1 ladder. An energy gap above the singlet ground state is observed for each compound. Experimental results of magnetizations and susceptibilities are understood from the analysis using the results of the numerical diagonalizations.

Magnetic Properties of a Random-bond Ladder Cu 2 (C 5 H 12 N 2 ) 2 (Cl 1− x Br x ) 4 at Low Temperatures

Magnetic measurements have been performed to investigate random-bond effects on a spin-ladder system. We synthesize a random mixture compound Cu 2 (C 5 H 12 N 2 ) 2 (Cl 1− x Br x ) 4 which is expected to form a spin ladder with random-exchange legs and rungs. The magnetic properties of the compound are rather different from those of the pure systems. Paramagnetic divergence is significantly observed for the susceptibility at low temperatures and the magnetization appears even at low fields. The results suggest that the ground state of the random-bond ladder is a Griffiths phase.

Dynamics ofS=1/2 Two-Leg Spin Ladder Systems

We calculate the dynamical structure factor of the S =1 /2 two-leg spin ladder system, using a continued fraction method based on Lanczos algorithm. Characteristics of the lowest excited states are discussed from strong to weak interchain-coupling regions. S =1 /2 two-leg spin ladder systems with antiferromagnetic interchain interactions have attracted great attention both theoretically and experimentally. When the interchain coupling (J⊥) is stronger than the intrachain coupling (J� ), the ground state consists of the product of rung singlets, and the elementary excitation is described on the basis of the rung triplet. Using several theoretical methods, it was shown that a singlet and a triplet two-magnon bound-states exist below the twomagnon continuum in addition to the elementary one-triplet excitation. 1) - 7) Twomagnon singlet bound-state was observed recently by phonon-assisted optical absorption. 8),9) From the theoretical analysis of the experimental findings, it was confirmed that the above picture of the elementary excitation holds even in J⊥/J� ∼ 1. 7),9) Furthermore, momentum dependence of the one-triplet and two-magnon spectral densities was calculated for J⊥/J � ≥ 1. 7),9) In this paper, we investigate dynamical properties of S =1 /2 two-leg spin ladder systems from strong to weak interchain-coupling regions. Using a continued fraction method based on Lanczos algorithm, 10) the dynamical structure factor (DSF) is calculated. On the basis of the finite size effects, characteristics of the lowest excited states are discussed. The S =1 /2 two-leg spin ladder is described by the following Hamiltonian,

Magnetism in New Classes of TTF-Based Charge Transfer Complexes

Charge transfer (CT) complexes are promising for developing molecule-based magnets, where ~ -electrons play an important role. New classes of molecular magnets are presented in the TTF-based CT complexes from points of unconventional magnetism. ~ -d interaction in d-electron-spin-incorporated CT complexes give anomalous effect of magnetic transition on the electron transport. The coexistence of metallic conduction and localized spin feature appears in the system located in the metal-insulator boundary. 1D metal having ferromagnetic interaction and triangle-based spin ladder system are also obtained from CT complexes. organic metal molecular magnet ~ -d interaction metal-insulator transition spin ladder ferromagnet

Magnon hole scattering in (Sr,Ca,La)14Cu24O41

We have investigated the one-dimensional magnon heat transport of the spin ladder system (Sr,Ca,La)14Cu24O41. Along the ladder direction in all compounds heat conduction is dominated by magnon heat conduction which in some cases exceeds the phonon heat conduction by almost two orders of magnitude. Isovalent as well as trivalent doping strongly affect the magnon heat transport: Large magnon heat conduction is observed as long as charges are localized whereas it is effectively suppressed if charges become mobile as a function of temperature and/or doping.

Hole-doping effects on an S=1 ladder system

Some zero-temperature properties of doped S=1 spin ladder systems are reported here. We study the low-energy states for small hole concentrations by means of the series expansion method . One-hole doping generates two kinds of the low-energy states distinguished by their spin, S=12 or 32, and the characteristic dispersion relation. In particular, we show that the one-hole state with S=32 is a composite particle, i.e. a bound state of an S=12 hole and a spin triplet excitation.

The real-time observation of a coherent magnetic oscillation in NaV 2O 5

We studied the quantum spin ladder system, sodium vanadate, by using a femtosecond impulsive excitation technique and report on the first observation of the lattice and spin dynamics. We found a coherent magnetic oscillatory mode (magnon) at 127 cm −1(twice the spin gap energy) below T C (=34 K) with circularly polarized light, which means that the magnetic mode corresponds to a spin-flip excitation from the singlet ground state.

Generalized antiferromagnetic Heisenberg spin ladders

We utilize the path-integral technique to derive the non-linear Sigma model (NLσM) for generalized antiferromagnetic spin-ladder systems on the square lattice with diagonal (next-nearest neighbor) interactions in addition to the nearest neighbor interaction. The model Hamiltonian is: H=∑ a=1 n l ∑ i J a S a(i)· S a(i+1)+J a,a+1′ S a(i)· S a+1(i)+K a,a+1 S a(i)· S a+1(i+1)+M a,a+1 S a(i+1)· S a(i) . The topological term of the NLσM is absent for the spin- s ladder with an even number of legs and is equal to 2 πs for the ladder with an odd number of legs. The spin wave velocity is s[∑ a ( J a − M a, a+1 − K a, a+1 )/∑ b, c L b, c −1] 1/2 where L a, b =4 J a + J a, a+1 ′+ J a, a−1 ′− M a, a+1 − M a, a−1 − K a, a+1 − K a, a−1 when a= b, and L a, b = J a, b ′+ K a, b + M a, b , when | a− b|=1. The spin gaps are predicted for spin ladders with an even number of chains. We also consider a two-leg ladder with spin s̃ and s, in which diagonal interactions occur only in the even (or odd) cells. The Berry phase is found to be dependent on the coupling constants. The expressions of the spin-wave velocity and spin gap are also given for even-leg ladders. The operator approach to the generalized spin-ladder problem is also presented. Finally we address the finite-size NLσM treatment of this system.

Upper critical field of the spin ladder system Sr 2.5Ca 11.5Cu 24O 41

We present transport measurements under pressure of the upper critical field of the superconducting spin ladder system Sr 2.5Ca 11.5Cu 24O 41. The temperature dependence and anisotropy of H c2 at low field is compared to results in high- T c superconductors and in 1D organic superconductors, as well as to simple theoretical predictions.

Magnetic properties of organic spin ladder systems

Novel organic polyradicals, BIP-BNO and BIP-TENO, are synthesized and crystallized to form ladder systems, where BIP-BNO and BIP-TENO denote 3,5'-bis(N-tert-butylaminoxyl)-3',5'-dibromobiphenyl and 3,3',5,5'-tetrakis(N-tert-butylaminoxyl)biphenyl, respectively. The BIP-BNO crystals form a two-leg ladder of S=1/2 with antiferromagnetic legs and rungs. The BIP-TENO crystals can be regarded as a two-leg ladder of S=1 species. The magnetic measurements revealed that both compound have singlet ground states.

Magnetic properties of mixed crystal (C 1TET-TTF)(FeBr 4) 1− x(FeCl 4) x

The magnetic properties of triangle-based spin ladder system are investigated for (C 1 TET-TTF)(FeBr 4 ) 1-x (FeCl 4 ) x , in which the ratio of the strengths of two constituent interactions varies depending on x. A spin-canting weak ferromagnetic state is stabilized for x=0 and 1 where the spin easy axis is oriented along different directions between x=0 and 1. In the intermediate range x=0.18, a spiral spin arrangement appears, consistent with theoretical suggestion for the triangle-based spin ladder systems.

Quantum spin ladder systems associated withsu(2|2)

Two integrable quantum spin ladder systems will be introduced associated with the fundamental su(2 \2) solution of the Yang-Baxter equation. The first model is a generalized quantum Ising system with Ising rung interactions. In the second model the addition of extra interactions allows us to impose Heisenberg rung interactions without violating integrability. The existence of a Bethe ansatz solution for both models allows us to investigate the elementary excitations for antiferromagnetic rung couplings. We find that the first model does not show a gap whilst in the second case there is a gap for all positive values of the rung coupling.

ONE KIND OF THE GAUGE FIELD THEORY OF SPIN LADDERS

We consider a massless fermion interacting with the gauge field by Pauli interaction in which the coupling constant is dimensionless. The model is equivalent to the s=1/2 antiferromagnetic XXZ spin chain. The two flavours massless fermion of such model can describe the two-leg spin ladder system qualitatively, and there is no gapless exitation in such a system.

A Density Functional Study of the Highly Adaptable Molecular Structure of Mo(V) and W(V) Dithiolene Complexes: From Three-Dimensional Antiferromagnet to Spin Ladder

Cp2M(dithiolene) (M = MoIV, WIV) d2 complexes exhibit a folding (theta) of the MS2C2 metallacycle along the S-S axis upon oxidation to the d1 paramagnetic cation. The evolution of the peculiar unfolded structure (theta = 0 degree) of Cp2Mo(dmit).+ (dmit2-, 2-thioxo-1,3-dithiole-4,5-dithiolate) in its AsF6- salt toward the introduction of heavy atoms (W, Se) is examined here by substituting dsit2- (dsit2-, 2-thioxo-1,3-dithiole-4,5-diselenolate) for dmit2- and/or W for Mo in those AsF6- salts. While [Cp2Mo(dsit).+][AsF6-] (orthorhombic, Cmcm, a = 9.071(2), b = 20.868(4), c = 10.243(2) A, V = 1938.9(7) A3, Z = 4) and [Cp2W(dmit).+][AsF6-] (orthorhombic, Cmcm, a = 9.0295(15), b = 20.568(2), c = 10.2641(12) A, V = 1906.2(4) A3, Z = 4) crystallize with an unfolded geometry and are isostructural with [Cp2Mo(dmit).+][AsF6-], the introduction of both W and dsit2- in Cp2W(dsit) stabilizes a fully different structure for [Cp2W(dsit).+][AsF6-] (monoclinic, P2(1)/c, a = 6.8442, b = 15.9923(13), c = 17.6594(16) A, beta = 90.934(11) degrees, V = 1932.6(3) A3, Z = 4) with the Cp2W(dsit).+ cation in a folded geometry (theta = 30.1(1) degrees). DFT calculations show that the variety of folding angles and hence of molecular structures found in those open-shell d1 Cp2M(dithiolene).+ complexes is attributable to a very low energy cost (< 1 kcal mol-1) for folding around the minimal energy folding angle. The molecular geometry of those complexes and hence their frontier orbitals are therefore highly related to the actual crystal structure they adopt. The unfolded complexes exhibit, in their AsF6- salts, a three-dimensional set of intermolecular interactions in the solid state, confirmed by the presence of an antiferromagnetic ground state (TNéel = 3.5(5) K in [Cp2W(dmit).+][AsF6-]). On the other hand, the folding of Cp2W(dsit).+ in [Cp2W(dsit).+][AsF6-] leads to the formation of a rare spin-ladder system with a spin gap of 90-100 K, as deduced from the temperature dependence of its magnetic susceptibility.

Raman Scattering Study of Sr14-xCaxCu24O41

Polarized Raman scattering spectra of Sr 14- x Ca x Cu 24 O 41 have been measured for x =0 and 11.5 at ambient pressure. In the spectra, the intra-molecular vibrations due to the unit structure of (Sr/Ca) 2 Cu 2 O 3 and CuO 2 have been clearly observed below 700 cm -1 . A comparison of the number of the observed peaks with the results of a factor group analysis indicates that the crystallographic symmetry of the ladder is F 222 for x =0 and 11.5, instead of centro-symmetric F m m m , while that of the chain is A m m a for both compositions. However the symmetry of the chain for x =11.5 is close to F 222 or F m m m . That is, the structure of the chains changes from an incomplete staggered structure to almost complete staggered one with increasing x . In the ( c , c ) and ( a , a ) spectra, the broad two-magnon peaks have been observed at around 3000 cm -1 . From the analysis of the recent theory of the magnetic scattering in the 2-leg spin ladder system done by Natsume et al. , the following magnetic para...

Low-temperature properties of the [formula omitted] spin ladder system A 10Cu 17O 29 (A=Sr,Ca,Y,Bi,Pb)

Electron paramagnetic resonance (EPR) measurements were performed on the S= 1 2 spin ladder system A 10Cu 17O 29 (where A=Sr,Ca,Y,Bi,Pb). The EPR signal intensity shows a broad peak around 70 K which indicates the presence of an energy gap in the spin excitation spectrum originating from simple (dimerized) chains. EPR spectra of Cu 2+ ions in chains are described in terms of Wannier excitons. It is also shown that these ions are in a strong bottleneck regime due to the coupling to current carriers (holes) by the exchange interaction.

Solution of a two-leg spin ladder system

A new model for a spin 1/2 ladder system with two legs is introduced. It is demonstrated that this model is solvable via the Bethe ansatz method for arbitrary values of the rung coupling J. This is achieved by a suitable mapping from the Hubbard model with appropriate twisted boundary conditions. We determine that a phase transition between gapped and gapless spin excitations occurs at the critical value J_c=1/2 of the rung coupling.