spin-Peierls Phase Research Articles

Magnetoelastic coupling in the spin-Peierls ground state of hydrogenated and deuterated (TMTTF)2PF6salts

We report an ultrasonic study of the magneto-elastic coupling of the hydrogenated and deuterated (TMTTF)$_2$PF$_6$ organic salts. For both salts the temperature dependence of the longitudinal velocity along the c* axis displays a monotonic stiffening of the $C_{33}$ compressibility modulus upon cooling. Below the characteristic temperature scale 40 K the modulus stiffening becomes markedly enhanced, in concomitance with the reduction of spin degrees of freedom previously seen in magnetic measurements as low dimensional precursors of the spin-Peierls transition. The magneto-elastic coupling appears to be much weaker in the hydrogenated salt due to the highly inhomogeneous elastic behavior induced by the proximity of the charge ordering transition to the spin-Peierls phase. For the deuterated salt, an important anomaly in the ultrasound velocity is observed below the spin-Peierls transition temperature $T_{\rm SP}$ in agreement with scaling of the elastic deformation with the spin-Peierls order parameter. In spite of the weakly inhomogeneous character of the spin-Peierls phase transition, the magnetic field dependence of $T_{\rm SP}$ is well captured with the mean-field prediction for the lattice distorted Heisenberg spin chain.

Two-Spinon and Orbital Excitations of the Spin-Peierls System TiOCl

We combine high-resolution resonant inelastic x-ray scattering with cluster calculations utilizing a recently derived effective magnetic scattering operator to analyze the polarization, excitation energy, and momentum-dependent excitation spectrum of the low-dimensional quantum magnet TiOCl in the range expected for orbital and magnetic excitations (0-2.5eV). Ti 3d orbital excitations yield complete information on the temperature-dependent crystal-field splitting. In the spin-Peierls phase we observe a dispersive two-spinon excitation and estimate the inter- and intradimer magnetic exchange coupling from a comparison to cluster calculations.

Quantum phase transitions, frustration, and the Fermi surface in the Kondo lattice model

The quantum phase transition from a spin-Peierls phase with a small Fermi surface to a paramagnetic Luttinger-liquid phase with a large Fermi surface is studied in the framework of a one-dimensional Kondo-Heisenberg model that consists of an electron gas away from half filling, coupled to a spin-1/2 chain by Kondo interactions. The Kondo spins are further coupled to each other with isotropic nearest-neighbor and next-nearest-neighbor antiferromagnetic Heisenberg interactions which are tuned to the Majumdar-Ghosh point. Focusing on three-eighths filling and using the density-matrix renormalization-group (DMRG) method, we show that the zero-temperature transition between the phases with small and large Fermi momenta appears continuous, and involves a new intermediate phase where the Fermi surface is not well defined. The intermediate phase is spin gapped and has Kondo-spin correlations that show incommensurate modulations. Our results appear incompatible with the local picture for the quantum phase transition in heavy fermion compounds, which predicts an abrupt change in the size of the Fermi momentum.

Synchrotron X-ray Study of Lattice Vibrations in CdCr2O4

Using inelastic x-ray scattering we have investigated lattice vibrations in a geometric frustrated system CdCr2O4 that upon cooling undergoes a spin-Peierls phase transition at TN = 7.8 K from a cubic and paramagnetic to a tetragonal and Neel state. Phonon modes measured around Brillouin zone boundaries show energy shifts when the transition occurs. Our analysis shows that the shifting can be understood as the ordinary effects of the lowering of the crystal symmetry.

Singlet-Triplet Excitations in the Unconventional Spin-Peierls TiOBr Compound

We have performed time-of-flight neutron scattering measurements on powder samples of the unconventional spin-Peierls compound TiOBr using the fine-resolution Fermi chopper spectrometer (SEQUOIA) at the Spallation Neutron Source at Oak Ridge National Laboratory. These measurements reveal two branches of magnetic excitations within the commensurate and incommensurate spin-Peierls phases, which we associate with n=1 and n=2 triplet excitations out of the singlet ground state. These results represent the first direct measurement of the singlet-triplet energy gap in TiOBr, which has a value of E(g)=21.2±1.0 meV.

Structure, Hydrogen Bonding and Other Exciting Effects in Low Dimensional TCNQ Salts. Recent Advances

Organic compounds have long been considered the most natural representatives of the insulating state. It started to change about fifty years ago, after the synthesis of an extraordinary organic molecule: tetracyano-p-quinodimethane (TCNQ). This molecule displays unique chemical and physical properties and give rise to a very rich class of organic conducting materials. The common feature of this conductors is high ordering of TCNQ molecules, very often in linear one-dimensional chains or columns. The physical properties of TCNQ salts are determined by two major points: 1) a restricted electronic dimensionality and 2) electronic correlations and electronphonon interactions. The aim of the paper is to review the influence of the properties and spatial structure of the cations on selected physical properties of adequate TCNQ complex salts. Some uncommon effects such as thin layers molecular organization and anions orientation, multidimensionality of hydrogen bonded networks of the salts – their types and strengths, melting of organic composites based on TCNQ and photoinduced melting of spin-Peierls phase in CT compounds are discussed. Possible applications of TCNQ salts are also mentioned. Keywords: Organic conductors, TCNQ salts, thin films, hydrogen bonding, melting

Syntheses, Structures and Solid-State Properties of MMX Mixed-Valence Chains, [NiII/III2(RCS2)4I]∞ (R = Et, n-Pr and n-Bu): Evidence of a Spin-Peierls Transition

The partial oxidation of [Ni(II/II)(2)(RCS(2))(4)] (R = Et (1), n-Pr (2), and n-Bu (3)) with iodine affords the MMX chain compounds [Ni(II/III)(2)(RCS(2))(4)I](infinity) (R = Et (4), n-Pr (5), and n-Bu (6)), respectively. The crystal structures of 4-6 consist of neutral one-dimensional (1-D) chains with a repeating -Ni-Ni-I- unit. The room-temperature (RT) structure of 4 indicates a charge-polarization (CP) state {-Ni((2.5-delta)+)-Ni((2.5+delta)+)-I(-)-Ni((2.5-delta)+)-Ni((2.5+delta)+)-I(-)- (delta << 0.5)} close to an averaged valence state judged by the Ni-I distances. In contrast, 5 and 6 exhibit a 3-fold periodicity of a -Ni-Ni-I- unit due to the disorder of the dithiocarboxylato ligands. Compounds 4-6 show typical semiconducting behavior and exhibit an intense sharp absorption band centered at 5400 cm(-1), which is attributed to a Mott-Hubbard gap due to a relatively large on-site Coulomb repulsion energy U of the nickel atoms. The high-temperature magnetic susceptibilities of 4-6 can be described by a 1-D Heisenberg antiferromagnetic chain model with |J|/k(B) ranging from 898(2) to 939(3) K. Compounds 4 and 5 undergo a spin-Peierls (SP) transition at relatively high T(sp) = 47 and 36 K, respectively, which are accompanied by superlattice reflections corresponding to a 2-fold -Ni-Ni-I- period below T(sp). By determining the superstructure of 4 at 26 K, we conclude that the valence-ordered state changes from the CP in the RT phase to the alternate charge-polarization (ACP) state of -Ni((2.5-delta)+)-Ni((2.5+delta)+)-I(-)-Ni((2.5+delta)+)-Ni((2.5-delta)+)-I(-)- in the SP phase. Such a spin-Peierls transition could not be observed for 6.

Ultrafast photoinduced melting of spin-Peierls phase in potassium-tetracyanoquinodimethane

Ultrafast dynamics of photoinduced melting of spin-Peierls (SP) phase was investigated in K-tetracyanoquinodimethane by using fs pump-probe reflection spectroscopy. It was revealed that the photocarriers are localized with ∼70 fs and behave as nonmagnetic impurities, which lead to the melting of the SP phase within ∼ 400 fs. In the melting process of the SP phase, several kinds of coherent oscillations were observed, suggesting that the corresponding several phonon modes contribute to the stabilization of the SP phase.

Photoinduced spin-Peierls phase transition

An equation is obtained for the order parameter ξ of the spin-Peierls phase transition in a light field. This equation is used to derive the expressions for the frequency ν of the photogenerated longitudinal optical phonon mode and the time τ of the photoinduced phase transition. Numerical values of the quantities ν and τ calculated in terms of the proposed model agree well with the experimental data for the quasi-one-dimensional compound potassium tetracyanoquinodimethane (K TCNQ) irradiated by a laser pulse.

Spin-Peierls transition in low-dimensional quantum spin systems: a Green’s function approach

We study the spin-Peierls (SP) transition of one-dimensional chain polymeric complexes coupled to lattice by means of many-body Green's function theory. The chain effective elastic constant is an intrinsic factor that determines the order of SP transition. It is found that the SP transition temperature T(SP) and the susceptibility-maximum temperature T(max) are in agreement with the experimental results. When an external magnetic field is applied to the chain, it makes T(SP) and T(max) decrease, and drives the SP transition from the second order to the first order. Besides, we show that the two-site thermal entanglement entropy is a good indicator of SP transition. Further considering the effect of interchain coupling on SP transition, with weak coupling of double-chain, the theoretical values are closer to the experimental results. We also calculate the density of states and spectral functions, which show that the energy gap vanishes at a critical temperature lower than T(SP), indicating a gapless SP phase lies in the gapped dimerized phase. The interchain coupling can drive the SP transition from the second order to the first order, while the SP dimerization may collapse for large interchain couplings.

Ti oxohalides: A playground for electron correlation physics

Abstract Low-dimensional S = 1 2 systems of the form TiO X ( X = Cl , Br ) have been investigated using photoemission spectroscopy. In their pristine form they display unusual spin-Peierls phases and their behavior under pressure suggests a possible metallic phase. While upon electron-doping photoemission spectra reveal the filling of the upper Hubbard band, clear spectroscopic evidence for a metal–insulator transition is difficult due to correlation and coupling effects in these compounds. However, these effects qualify them as almost ideal model systems for the study of strong electron correlations away from half-filling.

Theory of the spin-nematic to spin-Peierls quantum phase transition in ultracold spin-1 atoms in optical lattices

We present a theory of the anisotropy-tuned quantum phase transition between spin-nematic and spin-Peierls phases in $S=1$ systems with significant biquadratic exchange interactions. Based on quantum Monte Carlo studies on finite-size systems, it has been proposed that this phase transition is second order with new deconfined fractional excitations that are absent in either of the two phases. The possibility of a weak first-order transition, however, cannot be ruled out. To elucidate the nature of the transition, we construct a large-$N$ $\text{SO}(3N)$ model for this phase transition and find in the $N\ensuremath{\rightarrow}\ensuremath{\infty}$ limit that the transition is generically of first-order. Furthermore, we find a critical point in the one-dimensional (1D) limit, where two transition lines, separating spin-nematic, ferromagnetic, and spin-Peierls phases, meet. Our study indicates that the spin-nematic phase is absent in 1D, while its correlation length diverges at the critical point. Predictions for $^{23}\text{N}\text{a}$ atoms trapped in an optical lattice, where the nematic to spin-Peierls quantum phase transition naturally arises are discussed.

Suppression of the commensurate spin-Peierls state in Sc-dopedTi1−xScxOCl(x=0.0, 0.01, and 0.03)

We have performed x-ray scattering measurements on single crystals of the doped spin-Peierls compound Ti(1-x)Sc(x)OCl (x = 0, 0.01, 0.03). These measurements reveal that the presence of non-magnetic dopants has a profound effect on the unconventional spin-Peierls behavior of this system, even at concentrations as low as 1%. Sc-doping suppresses commensurate fluctuations in the pseudogap and incommensurate spin-Peierls phases of TiOCl, and prevents the formation of a long-range ordered spin-Peierls state. Broad incommensurate scattering develops in the doped compounds near Tc2 ~ 93 K, and persists down to base temperature (~ 7 K) with no evidence of a lock-in transition. The width of the incommensurate dimerization peaks indicates short correlation lengths on the order of ~ 12 angstroms below Tc2. The intensity of the incommensurate scattering is significantly reduced at higher Sc concentrations, indicating that the size of the associated lattice displacement decreases rapidly as a function of doping.

Mott-Hubbard gap closure and structural phase transition in the oxyhalides TiOBr and TiOCl under pressure

Pressure-dependent transmittance and reflectance spectra of TiOBr and TiOCl single crystals at room temperature suggest the closure of the Mott-Hubbard gap, i.e., the gap is filled with additional electronic states extending down to the far-infrared range. According to pressure-dependent x-ray powder-diffraction data, the gap closure coincides with a structural phase transition. The transition in TiOBr occurs at slightly lower pressure $(p=14\text{ }\text{GPa})$ compared to TiOCl $(p=16\text{ }\text{GPa})$ under hydrostatic conditions, which is discussed in terms of the chemical pressure effect. The results of pressure-dependent transmittance measurements on TiOBr at low temperatures reveal similar effects at 23 K, where the compound is in the spin-Peierls phase at ambient pressure.

Structural phase transitions in geometrically frustrated antiferromagnets

We study geometrically frustrated antiferromagnets with magnetoelastic coupling. Frustration in these systems may be relieved by a structural transition to a low-temperature phase with reduced lattice symmetry. We examine the statistical mechanics of this transition and the effects on it of quenched disorder using Monte Carlo simulations of the classical Heisenberg model on the pyrochlore lattice with coupling to uniform lattice distortions. The model has a transition between a cubic paramagnetic high-temperature phase and a tetragonal N\'eel ordered low-temperature phase. It does not support the spin-Peierls phase, which is predicted as an additional possibility within Landau theory, and the transition is first order for reasons unconnected with the symmetry analysis of Landau theory. Quenched disorder stabilizes the cubic phase, and we find a phase diagram as a function of temperature and disorder strength similar to that observed in ${\text{Zn}}_{1\ensuremath{-}x}{\text{Cd}}_{x}{\text{Cr}}_{2}{\text{O}}_{4}$.

A theory of spin-Peierls transitions in chains of exchange clusters

A theoretical approach to analysis of magnetic structural phase transitions of chain polymer heterospin complexes is proposed. The approach is based on the model for spin-Peierls phase transition of chain exchange clusters. The type of the phase transition depends on the elastic constant of the chain.

Density functional study of the electronic and vibrational properties of TiOCl

We present the phonon spectrum of TiOCl computed using hybrid density functional theory DFT .A complete analysis of the spectrum is performed for the space group Pmmn high-symmetry phase and the space group P21 /m low-symmetry phase, which is the symmetry of the spin-Peierls phase. We show that the nonlocal correlations present in the hybrid DFT approach are important for understanding the electron-lattice interactions in TiOCl. The computed frequencies compare well with those observed in Raman and infrared spectroscopy experiments, and we identify the origin of an anomalous phonon observed in Raman spectroscopy. The relationship between relevant zone boundary phonons in the high-symmetry phase and the zone center counterparts in the P21 /m symmetry allow us to speculate about the origin of the spin-Peierls phonon.

Neutron and x-ray scattering studies of the lightly doped spin–Peierls systemCu1−xCdxGeO3

Single crystals of the lightly doped spin–Peierls systemCu1−xCdxGeO3 have been studied using bulk susceptibility, x-ray diffraction, and inelasticneutron scattering techniques. We investigate the triplet gap in the magneticexcitation spectrum of this quasi-one-dimensional quantum antiferromagnet, andits relation to the spin–Peierls dimerization order parameter. We employ twodifferent theoretical forms to model the inelastic neutron scattering cross section andχ′′(Q,ω), and show the sensitivity of the gap energy to the choice of ). We find that a finite gap exists at the spin–Peierls phase transition.

Ultrafast photoinduced melting of spin-Peierls phase in the organic charge-transfer compounds alkali-tetracyanoquinodimethane

Photoinduced phase transitions in spin-Peierls (SP) systems of alkali $(M=\mathrm{K},\mathrm{Na})$-tetracyanoquinodimethane (TCNQ) have been studied by a reflection-type femtosecond (fs) pump-probe spectroscopy. The SP phase is destabilized by the generation of photocarriers through the breaking of the spin-singlet states in dimers. It results in the decrease of the dimeric molecular displacements within a few hundred of femtoseconds over several tens of TCNQ molecules. It is accompanied by the displacive-type coherent oscillations, which consist mainly of three modes with the frequencies of 20, 49, and $90\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ in K-TCNQ and of two modes with the frequencies of 49 and $99\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ in Na-TCNQ. By taking into account the temperature dependence of the Raman scattering spectra, the mode with $20\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ in K-TCNQ and the modes with 49 and $99\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ in Na-TCNQ are assigned to the phonon modes in the SP ground state, while the modes with 49 and $90\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ in K-TCNQ are assigned to the local modes originating from the photoexcited states. Polarization dependence of the Raman scattering signals shows that the $20\text{\ensuremath{-}}{\mathrm{cm}}^{\ensuremath{-}1}$ mode of K-TCNQ and the $49\text{\ensuremath{-}}{\mathrm{cm}}^{\ensuremath{-}1}$ mode of Na-TCNQ are longitudinal optical (LO) modes, whereas the $99\text{\ensuremath{-}}{\mathrm{cm}}^{\ensuremath{-}1}$ mode of Na-TCNQ is a transverse optical (TO) mode. Namely, the LO mode plays an important role on the stabilization of the SP phase in K-TCNQ, while both the LO and TO modes in Na-TCNQ. Such a difference is discussed by scrutinizing the difference of the crystal structures and the nature of the SP transitions in the two compounds.

Symmetry disquisition on theTiOXphase diagram(X=Br,Cl)

The sequence of phase transitions and the symmetry of, in particular, the low temperature incommensurate and spin-Peierls phases of the quasi-one-dimensional inorganic spin-Peierls system $\mathrm{Ti}\mathrm{O}X$ ($X=\mathrm{Br}$ and Cl) have been studied using inelastic light scattering experiments. The anomalous first-order character of the transition to the spin-Peierls phase is found to be a consequence of the different symmetries of the incommensurate and spin-Peierls $(P{2}_{1}∕m)$ phases. The pressure dependence of the lowest transition temperature strongly suggests that magnetic interchain interactions play an important role in the formation of the spin-Peierls and the incommensurate phases. Finally, a comparison of Raman data on VOCl to the $\mathrm{Ti}\mathrm{O}X$ spectra shows that the high energy scattering previously observed has a phononic origin.