spin-Peierls Phase Research Articles

Destabilization of spin-Peierls phase via a charge-spin modulated Floquet state induced by intramolecular vibrational excitation

The electronic state control using a periodic light field is one of the central subjects in photophysics. In molecular solids, intramolecular vibrations sometimes couple to intermolecular electron transfer, thus modulating electron and spin densities of each molecule. Here, we show that in a quasi-one-dimensional molecular solid K-tetracyanoquinodimethane (TCNQ) in which TCNQ molecules are dimerized by the spin-Peierls mechanism, an intramolecular vibrational excitation with a phase-locked mid-infrared pulse induces a charge-spin modulated Floquet state, which destabilizes the spin-Peierls phase. By detecting reflectivity changes of the intramolecular transition band along the mid-infrared electric field with 6.6-fs probe pulses, we detected high-frequency oscillations reflecting electron- and spin-density modulations synchronized with intramolecular vibrations. More significantly, we observed an oscillation of ~110 cm−1 due to a dimeric mode driven by a decrease in spin-Peierls dimerization. This dimerization reduction was confirmed by measuring transient reflectivity changes of the Mott-gap transition band. These results demonstrate the effectiveness of intramolecular vibrational excitation as a method for Floquet engineering in molecular solids.

A tetrathiafulvalene salt of the nitrite (NO2−) anion: investigations of the spin-Peierls phase

The first nitrite salt of a tetrathiafulvalene derivative exhibits a rare chessboard-like organization of the conducting stacks, with a spin-Peierls transition at 70 K.

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.

Magnetic field driven transition between valence bond solid and antiferromagnetic order in a distorted triangular lattice

A molecular Mott insulator $\kappa$-(ET)$_2$B(CN)$_4$ [ET = bis(ethylenedithio)tetrathiafulvalene] with a distorted triangular lattice exhibits a quantum disordered state with gapped spin excitation in the ground state. $^{13}$C nuclear magnetic resonance, magnetization, and magnetic torque measurements reveal that magnetic field suppresses valence bond order and induces long-range magnetic order above a critical field $\sim 8$ T. The nuclear spin-lattice relaxation rate $1/T_1$ shows persistent evolution of antiferromagnetic correlation above the transition temperature, highlighting a quantum spin liquid state with fractional excitations. The field-induced transition as observed in the spin-Peierls phase suggests that the valence bond order transition is driven through renormalized one-dimensionality and spin-lattice coupling.

Uncovering the Functional Role of Coherent Phonons during the Photoinduced Phase Transition in a Molecular Crystal

The atomic motions that make up phonons and molecular vibrations in molecular crystals influence their photophysical and electronic properties, including polaron formation, carrier mobility, and phase transitions. Discriminating between spectator and driving motions is a significant challenge hindering optimization. Unlocking this information and developing fine-tuned controls over actively participating phonon modes would not only lead to a stronger understanding of photochemistry but also provide a significant new tool in controlling solid state chemistry. We present a strategy using rationally designed double pulses to unveil the unique function of specific excited state phonon modes. Using ultrafast spectroscopy, we identified 50 and 90 cm-1 phonons involved in modulating the photoinduced spin-Peierls melting of potassium tetracyanoquinodimethane crystals. We show that the 50 cm-1 phonon specifically corresponds to the coherent nuclear wavepacket involved in the charge transfer component of the overall spin-Peierls phase melting process, while the 90 cm-1 phonon facilitates the phase transition component.

Coherent spin dynamics of solitons in the organic spin chain compounds (o-DMTTF)2X(X=Cl,Br)

We studied the magnetic properties, in particular dynamics, of the correlated spins associated with natural defects in the organic spin chain compounds ($o$-DMTTF)$_2X$ ($X$ = Br, Cl) by means of electron spin resonance (ESR) spectroscopy. Both materials exhibit spin-Peierls transitions at temperatures around 50 K [P. Foury-Leylekian et al. Phys. Rev. B 84, 195134 (2011)], which allow a separation of the properties of defects inside the chains from the magnetic response of the spin chains. Indeed, continuous wave ESR measurements performed over a wide temperature range evidence the evolution of the spin dynamics from being governed by the spins in the chains at elevated temperatures to a low-temperature regime which is dominated by defects within the spin-dimerized chains. Such defects polarize the antiferromagnetically coupled spins in their vicinity, thereby leading to a finite local alternating magnetization around the defect site which can be described in terms of a soliton, i.e. a spin 1/2 quasiparticle built of many correlated spins, pinned to the defect. In addition, contributions of triplon excitations of the spin-dimerized state to the ESR response below the transition temperature were observed which provides a spectroscopic estimate for the spin-gap of the studied systems. Moreover, details of spin dynamics deep in the spin-Peierls phase were investigated by pulse ESR experiments which revealed Rabi-oscillations as signatures of coherent spin dynamics. From a comparison of the characteristic damping times of the Rabi oscillations with measurements of the spin relaxation times by means of primary-echo decay and CPMG methods it becomes evident that inhomogeneities in local magnetic fields strongly contribute to the soliton decoherence.

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.

Enhancement and destruction of spin-Peierls physics in a one-dimensional quantum magnet under pressure

The application of pressure can induce transitions between unconventional quantum phases in correlated materials. The inorganic compound TiOCl, composed of chains of S=1/2 Ti ions, is an ideal realization of a spin-Peierls system with a relatively simple unit cell. At ambient pressure, it is an insulator due to strong electronic interactions (a Mott insulator). Its resistivity shows a sudden decrease with increasing pressure, indicating a transition to a more metallic state which may coincide with the emergence of charge density wave order. Therefore, high pressure studies of the structure with x-rays are crucial in determining the ground-state physics in this quantum magnet. In ambient pressure, TiOCl exhibits a transition to an incommensurate nearly dimerized state at $T_{c2}=92$ K and to a commensurate dimerized state at $T_{c1}=66$ K. Here, we discover a rich phase diagram as a function of temperature and pressure using x-ray diffraction on a single crystal in a diamond anvil cell down to $T=4$ K and pressures up to 14.5 GPa. Remarkably, the magnetic interaction scale increases dramatically with increasing pressure, as indicated by the high onset temperature of the spin-Peierls phase. At $\sim$7 GPa, the extrapolated onset of the spin-Peierls phase occurs above $T=300$ K, indicating a quantum singlet state exists at room temperature. Further comparisons are made with the phase diagrams of related spin-Peierls systems that display metallicity and superconductivity under pressure.

Successive Dimensional Transition in (TMTTF)_{2}PF_{6} Revealed by Synchrotron X-ray Diffraction.

A quasi-one-dimensional organic charge-transfer salt (TMTTF)_{2}PF_{6} undergoes a multistep phase transition as the temperature decreases. One of these transitions is called a "structureless transition," and these detailed structures were unknown for many years. With synchrotron x-ray diffraction, we observed a slight structural difference owing to the effect of charge-order transition between two TMTTF molecules in a dimer, which corresponds to the charge transfer δ_{CO}=0.20e. The two-dimensional Wigner crystallization was determined from an electron density analysis using core differential Fourier synthesis. Furthermore, we found that the ground state due to tetramerization, called the spin Peierls phase, is a three-dimensional transition with interchain correlation.

Inelastic neutron scattering investigation of magnetostructural excitations in the spin-Peierls organic system (TMTTF)2PF6

One-dimensional (1D) conductors such as Bechgaard and Fabre salts are a prototypal example of correlated systems where the phase diagram is controlled by sizable electron-electron repulsions. In deuterated ${(\mathrm{TMTTF})}_{2}\mathrm{P}{\mathrm{F}}_{6}$, where this interaction achieves charge localization at ambient pressure on donor stacks, magnetostructural coupling plays a decisive role to stabilize a spin-Peierls (SPs) ground state at ${T}_{\mathrm{SP}}=13\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. In this paper, we present the first inelastic neutron scattering investigation of SP magnetic excitations in organics. Our paper reveals the presence above ${T}_{\mathrm{SP}}$ of sizable critical fluctuations leading to the formation of a pseudogap in the 1D antiferromagnetic (AF) $S=1/2$ magnetic excitation spectrum of the donor stack, concomitant with the local formation of singlet of paired spins into dimers below ${T}_{\mathrm{SP}}^{\mathrm{MF}}\ensuremath{\approx}40\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. In addition, the inelastic neutron scattering investigation allows us also to probe the SP critical lattice dynamics and to show that at ambient pressure these dynamics are of relaxation or order-disorder type. Below ${T}_{\mathrm{SP}}$, our paper reveals the emergence of a two gap SP magnetic excitation spectrum towards a well-defined $S=1$ magnon mode and a continuum of two excitations, as theoretically predicted. Our measurements allow us to locate the ambient pressure SP phase of ${(\mathrm{TMTTF})}_{2}\mathrm{P}{\mathrm{F}}_{6}$ in the classical (adiabatic) limit close to the classical/quantum crossover line. Then we provide arguments suggesting that pressurized ${(\mathrm{TMTTF})}_{2}\mathrm{P}{\mathrm{F}}_{6}$ shifts to the quantum (antiadiabatic) SP gapped phase, which ends in a quantum critical point allowing the stabilization of an AF phase that competes with superconductivity at higher pressure. Finally, we propose that the magnetostructural coupling mechanism in the Fabre salts is caused by dimer charge/spin fluctuations driven by the coupling of donors with anions.

Pressure-dependent optical investigations of Fabre salts in the charge-ordered state

In a comprehensive infrared study, the molecular vibrational features of (TMTTF)2SbF6, (TMTTF)2AsF6 and (TMTTF)2PF6 single crystals have been measured down to temperatures as low as 7 K by applying hydrostatic pressure up to 11 kbar. We follow the charge disproportionation below the critical temperatures TCO as pressure increases, and determine the critical pressure values pCO at which the charge-ordered phase is suppressed. The coexistence of the spin-Peierls phase with charge order is explored at low temperatures, and the competition of these two phases is observed. Based on our measurements we construct a generic phase diagram of the Fabre salts with centrosymmetric anions. The pressure-dependent anion and methyl-group dynamics in these quasi-one-dimensional charge transfer compounds yields information about the interplay of the organic molecules in the stacks and the anions, and how this interaction varies upon the transition to the charge-ordered state.

Magnetic ordering of defects in a molecular spin-Peierls system

With interest in charge transfer compounds growing steadily, it is important to understand all aspects of the underlying physics of these systems, including the properties of the defects and interfaces that are universally present in actual experimental systems. For the study of these defects and their interactions a spin-Peierls (SP) system provides a useful testing ground. This work presents an investigation within the SP phase of potassium TCNQF4 where anomalous features are observed in both the magnetic susceptibility and ESR spectra for temperatures between 60 K and 100 K. Muon spin spectroscopy measurements confirm the presence of these anomalous magnetic features, with low temperature zero-field data exhibiting the damped oscillatory form that is a characteristic signature of static magnetic order. This ordering is most likely due to the interaction between structurally correlated magnetic defects in the system. The critical behaviour of the temperature dependent muon spin rotation frequency indicates that a 2D Ising model is applicable to the magnetic ordering of these defects. We show that these observations can be explained by a simple model in which the magnetic defects are located at stacking faults, which provide them with a 2D structural framework to constrain their interactions.

Tuneability and criticality in a three-dimensional stacked molecular system

With the changing of physical properties through chemical substitution in mind, this work shows theapplicability of this approach to organic based spin-Peierls (SP) systems. To demonstrate this we have useda well known system, potassium TCNQ, that undergoes an SP transition atTSP=396 K. Simply substitutingprotons for fluorine, using TCNQF4, shows a decrease in the coupling strength between TCNQ anions where thespin-Peierls transition is dramatically reduced in temperature, with KTCNQF4showing aTSPat approximately160 K, which is due to changes of the electron spin density across the molecule. Muon spin relaxation is a moresuitable technique for determining the magnetic properties of these systems and measurements were conducted onboth KTCNQ and KTCNQF4in order to study the behavior of the SP transition. This has highlighted and shownthat both transitions exhibit the same behavior and so are indeed similar; however, we have also succeeded inbeing able to tune the transition. Estimates of the dynamic critical exponents from both samples are obtained fromthe muon data and are found to be∼0.33, corresponding to a three-dimensional (3D) antiferromagnetic systemimplying that spin fluctuations associated with the SP state are correlated not just along stacks of dimerizedTCNQ anions. This result extends the understanding of the SP phase transition in the KTCNQ system beyondthat determined from purely structural studies.

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.

Photoinduced complete melting of spin-Peierls phase in Na-tetracyanoquinodimethane revealed by frequency doubling of coherent molecular oscillations

Photoinduced melting of the spin-Peierls phase was investigated in Na-tetracyanoquinodimethane by pump-probe reflection spectroscopy with a time resolution of 49 fs. Photoirradiation generates coherent molecular oscillation, which indicates that the spin-Peierls dimerization decreased. Increasing the excitation photon density to $g0.02$ photon/molecule doubles the oscillation frequency attributable to recovery of the original symmetry, that is, complete melting of the spin-Peierls phase. We observed a complete photoinduced transition with symmetry recovery revealed by frequency doubling of coherent oscillations.

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).

Sub-10 fs spectroscopy of K-TCNQ crystal for observation of intramolecular vibration modulation in melting of the Peierls dimer

K-TCNQ crystals show photoinduced phase transition (PIPT) between a Mott insulator and a spin Peierls (SP) phase, having ${T}_{c}=395$ K (= and the crystal has ${T}_{c}=395$ K of the PIPT). We report the intra-molecular vibrations in the excited K-TCNQ crystal after impulsive excitation using a sub 9.4 fs pulse laser in the reflection spectrum. The frequencies of the vibrations are close to the Raman frequencies of molecules. The frequencies increase with melting of the SP phase, and then decrease with rearrangement of the dimerization. In addition, the frequencies of the vibrations are time-resolved to be modulated by the intermolecular optical phonon with 510 fs period corresponding to 65 $\mathrm{cm}{}^{\ensuremath{-}1}$. This modulation is associated with the periodical change of coupling strengths of intermolecular charge transfer and/or the dipolar coupling between the counterpart molecules in the neighboring chains.

Pressure-induced normal-incommensurate-commensurate phase transitions in TiPO4

The complex interplay between spin, charge, orbital, and lattice degrees of freedom has made low-dimensional quantum spin magnets with strong antiferromagnetic (AF) spin-exchange coupling prime candidates for studying unusual magnetic phenomena. A progressive spin-lattice dimerization in one-dimensional AF Heisenberg chains, which occurs below a critical temperature and induces a singlet ground state with a magnetic gap, is commonly referred to as spin-Peierls (SP) transition. Recently, the compounds TiOX (X = Cl, Br) and TiPO4have been intensively investigated due to their unconventional behavior [1,2]. Unlike standard SP systems, TiOX and TiPO4undergo a sequence of normal-incommensurate-commensurate phase transitions on cooling at remarkably high transition temperatures. The transition temperatures are related to the direct exchange interactions between Ti ions, which increases strongly with decreasing the distance between the Ti ions, and therefore is very sensitive to the applied hydrostatic pressure. We have performed pressure-dependent single-crystal X-ray diffraction of TiPO4using synchrotron radiation. TiPO4undergoes a pressure-induced pahse transiton towards an incommensurate phase already below 10 GPa. This transformation is followed by the lock-in phase transition to the dimerized SP phase. Both structures are analogous to those at low temperatures, but reveal significantly larger modulation amplitudes. In this contribution we will present the detailed discussion of the high-pressure structures of TiPO4and their behavior on compression. Furthermore, similarities and differences of high-pressure phase diagrams of TiOCl and TiPO4and discrepancies between predicted and observed structures will be considered.

Lattice and orbital fluctuations in TiPO4

In the $s=1/2$ antiferromagnetic spin chain material TiPO${}_{4}$, the formation of a spin gap takes place in a two-step process with two characteristic temperatures, ${T}^{*}=111$ K and ${T}_{\mathrm{SP}}=74$ K. We observe an unusual lattice dynamics over a large temperature regime as well as evidence for an orbital instability preceding the spin-Peierls transition. We relate different intrachain exchange interactions of the high temperature compared to the spin-Peierls phase to a modification of the orbital ordering pattern. In particular, our observation of a high-energy excitation of mixed electronic and lattice origin suggests an exotic dimerization process different from other spin-Peierls materials.

Charge ordering and antiferromagnetism in (TMTTF)2SbF6

Carbon-13 nuclear magnetic resonance (${}^{13}$C NMR) measurements have been carried out in the quasi-one-dimensional organic conductor (TMTTF)${}_{2}$SbF${}_{6}$, which exhibits charge ordering (CO) transition and antiferromagnetic (AF) transition at 157 K and 6 K at ambient pressure, respectively. We estimated the degree of charge disproportionation, $\ensuremath{\Delta}\ensuremath{\rho}={\ensuremath{\rho}}_{\mathrm{rich}}\ensuremath{-}{\ensuremath{\rho}}_{\mathrm{poor}}$, in the CO state to be 0.25$e$ from the analysis of the NMR shift and found the broadening of the NMR spectral peaks caused by the inhomogeneous distribution in association with the AF fluctuation at charge rich site below 20 K in the CO state. Moreover, we determined the magnetic structure with two kinds of staggered moments in the AF phase of (TMTTF)${}_{2}$SbF${}_{6}$, which suggests that the CO state remains even in the AF phase. Consequently, we found the difference between two AF phases appearing on both sides of the spin-Peierls phase in the universal $P\ensuremath{-}T$ phase diagram.