spin-Peierls Phase Transition Research Articles

Orbital disorder and ordering in NaTiSi2O6 : Si29 and Na23 NMR study

${\mathrm{NaTiSi}}_{2}{\mathrm{O}}_{6}$ is an exemplary compound, showing an orbital assisted spin-Peierls phase transition at ${T}_{c}=210$ K. We present the results of $^{29}\mathrm{Si}$ and $^{23}\mathrm{Na}$ nuclear magnetic resonance (NMR) measurements of ${\mathrm{NaTiSi}}_{2}{\mathrm{O}}_{6}$. The use of magic angle spinning (MAS) techniques unambiguously shows that only one dynamically averaged silicon site can be seen at $T$ $>\phantom{\rule{4pt}{0ex}}{T}_{c}$. At cooling, the $^{29}\mathrm{Si}$ MAS NMR spectrum shows interesting changes. Immediately below ${T}_{c}$, the spectrum gets very broad. Cooling further, it shows two broad lines of unequal intensities which become narrower as the temperature decreases. Below 70 K, two narrow lines have chemical shifts that are typical for diamagnetic silicates. The hyperfine couplings for the two sites are $^{29}\mathrm{H}$ ${}_{\text{hf}}=7.4$ kOe/${\ensuremath{\mu}}_{B}$ and 4.9 kOe/${\ensuremath{\mu}}_{B}$. In the paramagnetic state at high temperature, the spin-lattice relaxation of $^{29}\mathrm{Si}$ was found to be weakly temperature dependent. Below ${T}_{c}$ the Arrhenius-type temperature dependence of the relaxation rate indicates an energy gap $\mathrm{\ensuremath{\Delta}}$/${\mathrm{k}}_{B}=1000(50)$ K. In the temperature region from 120 to 300 K, the relaxation rate was strongly frequency dependent. At room temperature, we found a power law dependence ${T}_{1}^{\ensuremath{-}1}\phantom{\rule{3.33333pt}{0ex}}\ensuremath{\propto}\phantom{\rule{3.33333pt}{0ex}}{\ensuremath{\omega}}_{L}^{\ensuremath{-}0.65(5)}$. For 70 K $<$ $T$ $<$ 120 K, the relaxation appeared to be nonexponential, which we assigned to a relaxation due to fixed paramagnetic centers. Simulation of the magnetization recovery curve showed activation type temperature dependence of the concentration of these centers. The NMR spectrum of $^{23}\mathrm{Na}$ shows the line with typical shape for the central transition of a quadrupolar nucleus. A small frequency shift of $^{23}\mathrm{Na}$ resonance corresponds to a very small hyperfine coupling $^{23}\mathrm{H}$ ${}_{\text{hf}}=0.32$ kOe/${\ensuremath{\mu}}_{B}$. In addition, at $T$ $>\phantom{\rule{4pt}{0ex}}{T}_{c}$ the $^{23}\mathrm{Na}$ spectrum shows another Lorentzian shaped resonance which we attribute to the Na sites where the quadrupolar coupling is partly averaged by ionic motion.

Inverse pressure-induced Mott transition in TiPO4

TiPO$_4$ shows interesting structural and magnetic properties as temperature and pressure are varied, such as a spin-Peierls phase transition and the development of incommensurate modulations of the lattice. Recently, high pressure experiments for TiPO$_4$ reported two new structural phases appearing at high pressures, the so-called phases IV and V [M. Bykov et al., Angew. Chem. Int. Ed. 55, 15053]. The latter was shown to include the first example of 5-fold O-coordinated P-atoms in an inorganic phosphate compound. In this work we characterize the electronic structure and other physical properties of these new phases by means of ab-initio calculations, and investigate the structural transition. We find that the appearance of phases IV and V coincides with a collapse of the Mott insulating gap and quenching of magnetism in phase III as pressure is applied. Remarkably, our calculations show that in the high pressure phase V, these features reappear, leading to an antiferromagnetic Mott insulating phase, with robust local moments.

Spin-driven symmetry breaking in the frustrated fcc pyrite MnS2

We report the characterisation of natural samples of the cubic pyrite mineral MnS2 using very high resolution synchrotron x-ray diffraction techniques. At low temperatures we find a new low temperature polymorph, which results from coupling between magnetic and lattice degrees of freedom. Below the magnetic ordering temperature TN = 48 K, we detect a pseudo-tetragonal distortion with a tiny c/a ratio of 1.0006. The structure can be refined in the space group Pbca The symmetry lowering reduces magnetic frustration in the fcc Mn2+ lattice and is likely responsible for the previously reported lock-in of the magnetic propagation vector. This behaviour is similar to the spin-Peierls phase transitions reported in other three-dimensional Heisenberg magnets like the chromate spinels.

The time of a photoinduced spin-Peierls phase transition

The time τ of the spin-Peierls phase transition is analyzed theoretically as a function of the duration τp of the exciting light pulse and the average number x0 of absorbed photons per magnetic ion after the transmission of the pulse. It is shown that the phase transition occurs at x0 > xc. The critical value xc is determined as a function of the duration τp of the light pulse. A photoinduced variation in the optical reflection coefficient R is calculated as a function of time t. The results of calculation are compared with experimental data on ultrafast photoinduced melting of the low-temperature spin-Peierls phase into potassium tetracyanoquinodimethan (K-TCNQ).

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.

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.

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.

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.

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.

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.

Orbital and lattice dynamics in pyroxenes

We have studied with infrared reflectivity and Raman scattering techniques the orbital and lattice dynamics, and the origin of the phase transitions in the pyroxene compounds NaMSi 2O 6 (with M=Ti, V, and Cr). In the quasi one-dimensional S=1/2 system NaTiSi 2O 6 we observe the anomalous high-temperature phonon broadenings and large changes of the phonon energies and line-widths across the phase transition at 210 K. The phonon anomalies originate from an orbital order–disorder type of the phase transition, which we describe as an orbital analogue of the spin-Peierls phase transition. In the S=1 NaVSi 2O 6 system orbital degrees of freedom are strongly suppressed since both of the active t 2g orbitals are occupied and the magnetic excitations are well described within the Heisenberg model, indicating that at T N=19 K this system orders antiferromagnetically. The NaCrSi 2O 6 has a fully polarized t 2g core, no orbital degrees of freedom, and no anomalous phonon broadening in the Raman spectra.

Redistribution of Electronic Charges in Spin-Peierls State in (TMTTF)2AsF6Observed by13C NMR

We report ^{13}C NMR spectra and nuclear spin lattice relaxation rate 1/T_1 for a quasi-one-dimensional quarter-filled organic material (TMTTF)_{2}AsF_{6}, which undergoes charge ordering (T_{{CO}} = 102 K) and spin-Peierls phase transitions (T_{{SP}} = 14 K). The ratio of two 1/T_1 for the charge accepting and donating TMTTF sites which grows from T_{CO} finally saturates in approaching T_{{SP}}, indicating one spin correlation function even in the charge ordered state. Below T_{{SP}}, however, the doubly split NMR lines from inequivalently charged molecules merge into one line, originated from the variation in charge densities. This shows that a rearrangement of the charge configuration occurs at T_{{SP}}.

13C NMR spectral study of Q1D organic conductor (TMTTF)2AsF6

We report 13 C NMR spectra of a quasi-one-dimensional quarter-filled organic material (TMTTF) 2 AsF 6 , which undergoes charge ordering (T CO = 102 K) and spin-Peierls phase transitions (T SP = 14 K). We found that the doubly split NMR lines originating from charge ordering again merge into one line at T SP . This shows redistribution of charge densities or suppression of charge ordering across the spin-Peierls transition.

Spin-Peierls lattice fluctuations and disorders in CuGeO$_\mathsf{3}$ and its solid solutions

The inorganic quasi-one dimensional (1D) S = 1/2 antiferromagnetic (AF) system CuGeO3 undergoes a 2nd order spin-Peierls (SP) phase transition at T SP = 14.2 K. In this study we present an X-ray synchrotron radiation investigation which confirms that the SP instability is announced by an important regime of pretransitional structural fluctuations which have been detected until 36 K. Furthermore we show that these fluctuations are 1D above 24 K, a feature expected for a structural instability triggered by the Cu2 + chains of spin 1/2. By extrapolating the thermal dependence of the correlation length in the chain direction, we show that formation of singlet dimers begins at about 50 K, a temperature that we identify as the mean field temperature of the SP chain. The critical nature of the pretransitional fluctuations does not change when low amounts (<1%) of non-magnetic dopants substitute either the Cu site (case of Zn and Mg) or the Ge site (case of Si and Al) of CuGeO3. However, the spatial extension of the fluctuations is considerably reduced when the magnetic dopant Ni substitutes the Cu site. In the SP ground state of doped materials we have been able to detect, in addition to the superlattice SP reflections previously observed, a very weak anisotropic diffuse scattering. We give evidences that this scattering originates from dopant-induced quasi-1D domains in which the dimerisation is perturbed. If we assume that each domain is limited by a soliton-antisoliton pair, pinned either on the substituent of the Cu site or by the deformation field induced by the substituent of the Ge site, we deduce that the soliton and antisoliton are separated by a distance of about $L_{0}\sim 28{-}45$ A, and that the soliton half width amounts to about $\xi_{SP}\sim 16{-}20$ A. With these numbers we are able to account for the rate of decrease of T SP as a function of the dopant concentration, and to deduce the critical concentration above which the long-range SP order vanishes. The overall size of the perturbed domains thus obtained, $L_{0} + 2\xi_{SP}\sim 70$ A, is comparable with the size of the magnetic inhomogeneities determined by muon spin spectroscopy in the AF phase of doped CuGeO3.

Orbital dimerization in NaTiSi2O6. An orbital analogue of the spin-Peierls phase transition

Abstract The phonon dynamics in NaTiSi2O6 is studied using the Raman scattering technique. The observed phonon anomalies around T=210 K combined with theoretical considerations show that the high-temperature dynamical Jahn–Teller phase of NaTiSi2O6 exhibits a spontaneous breaking of translational symmetry into an orbital dimerized state. We describe this phase instability as an orbital analogue of the spin-Peierls phase transition.

Raman-scattering study on the revival of the spin-Peierls transition in heavily Mg-doped CuGeO 3 under high pressures

Raman-scattering study has been carried out in 3.5% Mg-doped CuGeO 3 crystals under high pressures at low temperatures. This sample does not undertake the spin-Peierls (SP) transition at ambient pressure. The folded phonon modes and the two-magnetic-excitation bound state appear under high pressures. The SP gap mode is also observed. These facts give a clear evidence that the SP phase transition is revived by the application of high pressure. A new phonon peak is observed above 2.8 GPa , indicating a pressure-induced structural phase transition. It remains observable far above the SP transition temperature. We propose a P– T phase diagram for this sample.

XANES study of charge ordering on the spin-Peierls phase transition of alpha'-NAV2O5.

alpha'-NaV2O5 transforms at Tc = 34K, and the origin of the phase transition seems to be caused by the charge order and spin-Peierls mechanism. However the experiments which reveal the existence of charge order are very little. We measured XANES of V atoms which relates to the valence of V. It was found that the valences of V atoms in NaV2O5 were V4.5+ in the high temperature phase by comparing pre-edge and XANES of NaV2O5 with that of LiV2O5 CaV2O5 and V2O5. The analysis of EXAFS also shows the environment of the V4.5+ state.

Optical spectroscopy of pure and dopedCuGeO3

We investigated in detail the optical properties of several Cu(1-delta)Mg(delta)GeO3 (with delta=0,0.01), and CuGe(1-x)B(x)O3 with B=Si (x=0,0.007,0.05,0.1), and Al (x=0,0.01) single crystals, in the frequency range 20-32000 cm^-1. We report temperature dependent reflectivity and transmission measurements, performed with polarized light in order to probe the anisotropy of the crystals along the b and c axes, and optical conductivity spectra obtained by Kramers-Kronig transformation or direct inversion of the Fresnel formula. Special emphasis is given to the far-infrared phonon spectra. The temperature dependence of the phonon parameters is presented and discussed in relation to the soft mode issue in CuGeO3. For T<Tsp we could detect zone boundary folded modes activated by the spin-Peierls phase transition. Following the temperature dependence of these modes, which shows the second order character of the phase transition, we were able to study the effect of doping on Tsp. Moreover, in transmission experiments we detected a direct singlet-triplet excitation at 44 cm^-1, across the magnetic gap, which is not understandable on the basis of the magnetic excitation spectrum so far assumed for CuGeO3. The optical activity of this excitation and its polarization dependence confirm the existence of a second (optical) magnetic branch, recently suggested on the basis of inelastic neutron scattering data. The anisotropy in the magnetic exchange constants along the b axis, necessary for the optical triplet mode to gain a finite intensity, and the strong effect of Si substitution on the phonon spectra are discussed in relation to the alternative space group P2(1)2(1)2(1), recently proposed for CuGeO3 in the high temperature uniform phase.

Tricritical to mean-field crossover at the spin-Peierls transition inCuGeO3

The spin-Peierls phase transition in ${\mathrm{CuGeO}}_{3}$ has been extensively studied utilizing a variety of experimental techniques. Interpretations of the phase transition behavior vary from tricritical to mean field to Ising critical to $\mathrm{XY}$ critical. We show that the behavior in the vicinity of the phase transition of each of the order parameters, the magnetic energy gap, and the heat capacity can be quantitatively fitted with few adjustable parameters with a mean-field model incorporating a tricritical to mean-field crossover in the transition region.

Critical phenomena at the spin-Peierls transition inMEM(TCNQ)2

We have performed a detailed x-ray scattering study of the critical phenomena associated with the spin-Peierls phase transition in the organic compound $\mathrm{MEM}(\mathrm{TCNQ}{)}_{2}.$ Analysis of the superlattice reflection intensity indicates an order parameter with an associated critical exponent $\ensuremath{\beta}=0.35\ifmmode\pm\else\textpm\fi{}0.06$ consistent with three-dimensional behavior, as seen in the inorganic compound ${\mathrm{CuGeO}}_{3},$ and clearly inconsistent with mean field behavior, as indicated in previous studies. Measurements of lattice constants indicate the presence of spontaneous strains below the transition temperature. The measured critical scattering is not well described by an Ornstein-Zernike, Lorentzian line shape. An adequate description is obtained with a ${\mathrm{Lorentzian}}^{x}$ with x varying or with a ${\mathrm{L}\mathrm{o}\mathrm{r}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{z}\mathrm{i}\mathrm{a}\mathrm{n}+\mathrm{L}\mathrm{o}\mathrm{r}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{z}\mathrm{i}\mathrm{a}\mathrm{n}}^{2}$ line shape. The latter descriptor is reminiscent of previous high-resolution x-ray scattering studies where a two-component line shape has been observed. Analysis using such a line shape indicates critical exponents $\ensuremath{\gamma}$ and $\ensuremath{\nu},$ obtained from the Lorentzian component, consistent with the results obtained for the order parameter.