Spin Ladder System Research Articles

Ultrafast Dynamics of Photoinduced Phenomena in the Spin Ladder System NaV2O5

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Dynamical correlations and quantum phase transition in the quantum Potts model

We present a detailed study of the finite temperature dynamical properties of the quantum Potts model in one dimension.Quasiparticle excitations in this model have internal quantum numbers, and their scattering matrix {\gf deep} in the gapped phases is shown to take a simple {\gf exchange} form in the perturbative regimes. The finite temperature correlation functions in the quantum critical regime are determined using conformal invariance, while {\gf far from the quantum critical point} we compute the decay functions analytically within a semiclassical approach of Sachdev and Damle [K. Damle and S. Sachdev, Phys. Rev. B \textbf{57}, 8307 (1998)]. As a consequence, decay functions exhibit a {\em diffusive character}. {\gf We also provide robust arguments that our semiclassical analysis carries over to very low temperatures even in the vicinity of the quantum phase transition.} Our results are also relevant for quantum rotor models, antiferromagnetic chains, and some spin ladder systems.

Quantum Melting of the Hole Crystal in the Spin Ladder ofSr14−xCaxCu24O41

We have used resonant soft x-ray scattering to study the effects of discommensuration on the hole Wigner crystal (HC) in the spin ladder Sr(14-x)CaxCu24O41 (SCCO). As the hole density is varied the HC forms only with the commensurate wave vectors L(L) = 1/5 and L(L) = 1/3; for incommensurate values it "melts." A simple scaling between L(L) and temperature is observed, tau1/3/tau1/5 = 5/3, indicating an inverse relationship between the interaction strength and wavelength. Our results suggest that SCCO contains hole pairs that are crystallized through an interplay between lattice commensuration and Coulomb repulsion, reminiscent of the "pair density wave" scenario.

Photo-induced phenomena in NaV 2O 5 studied by time-resolved Raman spectroscopy

Time-resolved Raman measurements were performed in a spin ladder system α′-NaV 2O 5 in picoseconds time range. We observed the rapid decrease and gradual recovery of the peak intensity at 66 cm −1 due to the superlattice structure in the low-temperature phase by photoexcitation. The decay and recovery time constants from a fitting by the double exponential function are 5 and 800 ps, respectively. The transient changes of the local temperature of the sample surface were determined from the intensity ratio of the anti-Stokes and Stokes scattering. The result shows that the highest temperature of 23 K at 20 ps is far below the phase transition temperature of 34 K and the phase change does not occur by laser heating. The temporal changes of the Raman intensities are ascribed to the decrease of the order parameter in the superlattice structure induced by optical pumping in the low-temperature phase.

Hole redistribution inSr14−xCaxCu24O41(x=0,12)spin ladder compounds:Cu63andO17NMR studies under pressure

We report the results of a ^63Cu and ^17O NMR study of the nuclear quadrupole interaction tensor, ^(17,63)nu\_{Q,alpha}, in the hole doped spin ladder system Sr\_(14-x)Ca\_xCu\_24O\_41 (x = 0 and 12) performed under ambient and high pressures. NMR data show that the hole density in the Cu\_2O\_3 ladder layer grows with temperature, Ca content and an applied pressure. We have derived the hole occupation of Cu 3d and O 2p orbitals at the different ion sites in the Cu\_2O\_3 ladder as a function of the temperature, Ca substitution and pressure. We also suggest that the most important role of high pressure for the stabilization of a superconducting ground state in Ca-rich two-leg ladders is an increase of the hole concentration in the conducting Cu\_2O\_3 planes. We have obtained an estimate of 0.10 hole per Cu1 for the hole concentration at low temperature in Ca12 under 32 kbar when this compound undergoes a superconducting transition at 5K. Such a value fits fairly well with the doping phase diagram of cuprate superconductors.

Quantum information storage and state transfer based on spin systems

The idea of quantum state storage is generalized to describe the coherent transfer of quantum information through a coherent data bus. In this universal framework, we comprehensively review our recent systematical investigations to explore the possibility of implementing the physical processes of quantum information storage and state transfer by using quantum spin systems, which may be an isotropic antiferromagnetic spin ladder system or a ferromagnetic Heisenberg spin chain. Our studies emphasize the physical mechanisms and the fundamental problems behind the various protocols for the storage and transfer of quantum information in solid state systems.

Field-induced incommensurate order in spin ladder with ring exchange

The exact diagonalization study on the magnetization process of the S=1/2 spin ladder system revealed that the incommensurate spin correlation parallel to H would be possibly dominant around half the saturation magnetization, with the ring exchange. In such a situation, an interladder coupling would yield the field-induced long-range incommensurate order. Several phase diagrams including the magnetization plateau are also presented.

Energy gap of spin nanotube

Recently some quantum spin systems on tube lattices, so-called spin nanotubes, have been synthesized. They are expected to be interesting low-dimensional systems like the carbon nanotubes. As a first step of theoretical study on the spin nanotube, we investigate the S = 1 2 three-leg spin tube, which is the simplest one, using numerical analyses of finite clusters and a finite-size scaling technique. The spin gap, which is one of the most interesting quantities reflecting the macroscopic quantum effect, was revealed to be open for any finite rung exchange couplings, in contrast to the three-leg spin ladder system which is gapless. We also found a quantum phase transition caused by an asymmetric rung interaction. When one of the three rung coupling constants is changed, the spin gap vanishes very rapidly.

Phase diagram ofS=12two-legXXZspin-ladder systems

We investigate the ground state phase diagram of the S=1/2 two-leg $XXZ$ spin ladder system with an isotropic interchain coupling. In this model, there is the Berezinskii-Kosterlitz-Thouless transition which occurs at the XY-Haldane and the XY-rung singlet phase boundaries. It was difficult to determine the transition line using traditional methods. We overcome this difficulty using the level spectroscopy method combined with the twisted boundary condition method, and we check the consistency. We find out that the phase boundary between XY phase and Haldane phase lies on the $\Delta=0$ line. And we show that there exist two different XY phases, which we can distinguish investigating a $XX$ correlation function.

Raman resonance in spin-Stwo-leg spin-ladder systems

We argue that the Raman intensity in a spin-$S$ two-leg spin ladder has a pseudoresonance peak, whose width is very small at large $S$. The pseudoresonance originates from the existence of a local minimum in the magnon excitation spectrum, and is located slightly below twice the magnon energy at the minimum. The physics behind the peak is similar to the excitonic scenario for the neutron and Raman resonances in a $d$-wave superconductor.

Organic Spin Ladders from Tetrathiafulvalene (TTF) Derivatives

Starting from the first organic spin ladder reported, a dithiophene-tetrathiafulvalene salt ((DT-TTF)2[Au(mnt)2]) (mnt = maleonitriledithiolate), two different approaches to enlarge the family of organic spin-ladder systems are described. The first approach consists of a molecular variation of the donor; to that purpose, the new TTF derivative ethylenethiothiophene-tetrathiafulvalene (ETT-TTF, 3), is synthesized and structurally characterized. From this donor a new ladder-like structure compound, (ETT-TTF)2[Au(mnt)2] (4), isostructural with (DT-TTF)2[Au(mnt)2], is obtained. However, the magnetic properties of 4 do not follow the known spin-ladder behavior owing to orientational disorder exhibited by the ETT-TTF molecules in the crystal structure. In the second approach, the acceptor complex is changed, either in the nature of the ligand or in the metal. With the [Au(i-mnt)2]– salt (i-mnt = iso-maleonitriledithiolate), the new ladder-like compound (DT-TTF)2[Au(i-mnt)2] (5), isostructural with 4, is obtained, but only as a minority product. Two other compounds with a different anion generated in situ, bearing a Au(I) dimeric core, were also isolated; (DT-TTF)9[Au2(i-mnt)2]2 (6) as the most abundant phase and (DT-TTF)2[Au2(i-mnt)2] (7) as another minority phase. Salt 7 is characterized by X-ray crystallography as a chiral compound, due to the torsion of the ligands around the central Au–Au bond. The magnetic properties of (DT-TTF)2[Au(i-mnt)2] (5) indicate that it follows a spin-ladder behavior and the electron paramagnetic resonance (EPR) data is fitted to the Troyer and Barnes and Riera equations with the parameters Δ/kB = 71 K, J∥/kB = 86 K, and J⟂/kB = 142 K, indicating a J⟂/J∥ ratio of 1.65. The change of the gold complex [Au(mnt)2] for its copper analogue, [Cu(mnt)2] also leads to a ladder-like structure, (DT-TTF)2[Cu(mnt)2] (8), which is isostructural with the gold analogue and with salts 4 and 5. The fully ionic salt (DT-TTF)[Cu(mnt)2] (9) is also obtained. The magnetic properties demonstrated that compound 8 is the third organic spin-ladder system of this family, and the values found by a fitting to the ladder equations were Δ/kB = 123 K, J∥/kB = 121 K, and J⟂/kB = 218 K, corresponding to a J⟂/J∥ ratio of 1.75, similar to that of 5 and close to that of an ideal spin ladder.

Structural, Magnetic, and Electrical Characterization of New Polycrystalline Phases of Nickel- and Platinum-Doped [(DT-TTF)n][Au(mnt)2] (n = 1, 2)

Doping of spin-ladder systems by isostructural paramagnetic complexes was attempted. Despite the close isostructural nature of the pure (DT-TTF)2[M(mnt)2] (M = Au, Ni, Pt) end-members, which present a ladder structure, doping of the spin-ladder (DT-TTF)2[Au(mnt)2] with either 5% or 25% [M(mnt)2]- (M = Ni, Pt) generates two (metrically) new phases. Their markedly different crystal structures have been determined using laboratory X-ray powder diffraction data. (DT-TTF)2[Au0.75Ni0.25(mnt)2] consists of a mixed-valence compound (of triclinic symmetry), which was only detected, pure or in a mixture of phases, when [Ni(mnt)2]- was used as a dopant. Differently, the stoichiometric 1:1 [DT-TTF][Au0.75Pt0.25(mnt)2] monoclinic phase was found when [Pt(mnt)2]- (in 5% and 25%) was employed as the doping agent. Remarkably, only in the 5% Pt doping experiment, the major component of the mixture was the ladder structure compound (DT-TTF)2[Au(mnt)2] doped with minor amounts of Pt. This 5% Pt-doped specimen shows an EPR signal (g = 2.0115, DeltaHpp = 114 G at 300 K) wider than the pure compound (DT-TTF)2[Au(mnt)2], denoting exchange between the donor spins and Pt(mnt)2- centers. The electrical transport properties of the 5% Pt-doped composition at high temperatures are comparable to those of (DT-TTF)2[Au(mnt)2] with room-temperature conductivity sigma300K = 13 S/cm and thermopower S300K = 46 microV/K, with a sharp transition at 223 K similar to that previously observed in the Cu analogue at 235 K.

Quantum-state transmission via a spin ladder as a robust data bus

We explore the physical mechanism to coherently transfer the quantum information of spin by connecting two spins to an isotropic antiferromagnetic spin ladder system as data bus. Due to a large spin gap existing in such a perfect medium, the effective Hamiltonian of the two connected spins can be archived as that of Heisenberg type, which possesses a ground state with maximal entanglement. We show that the effective coupling strength is inversely proportional to the distance of the two spins and thus the quantum information can be transferred between the two spins separated by a longer distance, i.e., the characteristic time of quantum-state transferring linearly depends on the distance.

Ultrafast dynamics of photoinduced phenomena in the spin ladder system NaV2O5

Recent developments in the study of ultrafast dynamics are reviewed concerning the spin ladder system, α'-NaV2O5, which is known to have a charge order phase transition accompanying spin dimerization at 34 K. The low energy excitations specific to the low temperature phase is observed by coherent excitation method and their origins are discussed, comparing with the Raman and infrared absorption data. Photoinduced phenomena are studied by transient reflectance and time-resolved Raman spectroscopy and the thermal and nonthermal effects are distinguished by their response time.

Time-resolved Raman spectroscopy applied to the photoinduced phenomena in NaV2O5

Picosecond time resolved Raman scattering measurements were performed in a spin ladder system α'−NaV2O5. We observed the decrease and recovery of the superstructure peak at 66 cm−1 in the low temperature phase by photoexcitation. The transient local temperature of the sample by photoexcitation was obtained from the intensity ratio of the anti-Stokes and Stokes scattering. It is found that the highest temperature of 23 K induced by the optical pumping is far below the phase transition temperature of 34 K. The temporal changes of the Raman scattering intensities are ascribed to the decrease of the order parameter in the superlattice structure induced by optical pumping in the low temperature phase. We demonstrate a high potentiality of time-resolved Raman scattering spectroscopy for investigating the photoinduced phenomena.

Bond-AlternatingS=1 Spin Ladder in Magnetic Field

The magnetization process of the S =1 spin ladder system with bond-alternation due to the static lattice distortion is investigated by the numerical diagonalization of finite clusters and the level spectroscopy method. We focus our attention mainly on the magnetization plateau caused by field-induced spin gap. The plateau phase diagram is presented at M s /4, where M s is the saturated magnetization. In addition we discuss the magnetic curves and the mechanisms of the plateau at M s /4 observed in the high field magnetization measurement of organic S =1 ladder compound BIP-TENO.

Magnetization Plateaus and Magnetic Excitations in theS= 1/2 Spin Ladder Antiferromagnet NH4CuCl3

Field-induced magnetic phases are investigated in one-dimensional S = 1/2 generalized spin ladder systems having the lattice modulation along the leg with the wavevector π/2a, a being the lattice constant, using numerical calculations. We point out that there exist cases where the 1/4- as well as the 3/4-magnetization plateaus appear but the 1/2-plateau does not, for an appropriate set of the parameters. Triplet, excitations in the plateau regions are discussed in connection with the experimental result, which provides possible implications for the important role of the interaction between triplets.

Magnetization Plateau ofS= 1 Spin Ladder BIP-TENO

A mechanism of the magnetization plateau observed in the high-field magnetizatiom measurement of the organic S = 1 spin ladder system BIP-TENO is investigated. Assuming possible lattice distortion, we propose a possible scenario of the plateau formation on the basis of the bond-alternating ladder. Some unsolved problems are also discussed.

Infrared Optical Absorption Spectra in Cuprate Two-Leg Spin-Ladder Systems with a Cyclic Four-Spin Interaction

On the basis of a resonating valence-bond description, we investigate the excitation spectrum caused by the Raman process in cuprate two-leg spin ladders with a cyclic four-spin interaction. Owing to a cyclic four-spin interaction, the S = 0 three-triplet excitation energy is reduced and becomes smaller than the S = 0 two-triplet excitation energy around the end of the Brillouin zone. Possible interpretations for the change in the lowest-lying mode are discussed. We show that such a change in the lowest-lying mode yields the shoulder structure at the lower edge in the phonon-assisted optical absorption spectrum.

Photo-induced phase transition in the 2D spin ladder compound NaV 2O 5

Transient reflectance changes are studied for a spin ladder system α′-NaV 2O 5 in a temperature range from 4 K to room temperature, by means of femtosecond pump-and-probe measurements. A large response with a slow rise (3 ps) and a slow decay (>400 ps) is found only below 34 K, the phase transition temperature of charge ordering. The observed phenomena are ascribed to a photo-induced transition from the low-temperature phase to a state similar to the high-temperature phase.