spin-Peierls Transition Research Articles

Quasi-2D spin-Peierls transition through interstitial anionic electrons in K(NH3)2

Electron–phonon interactions and electron–electron correlations represent two crucial facets of condensed matter physics. For instance, in a half-filled spin-1/2 anti-ferromagnetic chain, the lattice dimerization induced by electron-nucleus interaction can be intensified by onsite Coulomb repulsion, resulting in a spin-Peierls state. Through first-principles calculations and crystal structure prediction methods, we have identified that under mild pressures, potassium and ammonia can form stable compounds: R3¯m K(NH3)2, Pm3¯m K(NH3)2, and Cm K2(NH3)3. Our predictions suggest that the R3¯m K(NH3)2 exhibits electride characteristics, marked by the formation of interstitial anionic electrons (IAEs) in the interlayer space. These IAEs are arranged in quasi-two-dimensional triangular arrays. With increasing pressure, the electronic van-Hove singularity shifts toward the Fermi level, resulting in an augmented density of states and the onset of both Peierls and magnetic instabilities. Analyzing these instabilities, we determine that the ground state of the R3¯m K(NH3)2 is the dimerized P21/m phase with zigzag-type anti-ferromagnetic IAEs. This state can be described by the triangular-lattice antiferromagnetic Heisenberg model with modulated magnetic interactions. Furthermore, we unveil the coexistence and positive interplay between magnetic and Peierls instability, constituting a scenario of spin-Peierls instability unprecedented in realistic 2D materials, particularly involving IAEs. This work provides valuable insights into the coupling of IAEs with the adjacent lattice and their spin correlations in quantum materials.

Role of V–V dimerization on electronic and magnetic properties of ilmenite-type CoVO3

Structural, electronic and magnetic properties of ilmenite-type CoVO3 have been explored via the generalized gradient approximation + effective Hubbard U eff correction, in the framework of density functional theory. Our results indicate that high temperature rhombohedral phase is metallic with oxidation states and electronic configurations Co2+ (), V4+ (), respectively, while low temperature triclinic phase, induced from spin-Peierls transition in the V–V dimerization manner, is insulating, maintaining charge and electronic states unchanged. Furthermore, the A-type antiferromagnetic ordering, where the ferromagnetic honeycomb layers are anti-aligned along the stacking axis, is identified to be the magnetic ground state for the low temperature phase, in nice agreement with experimental findings, analogous to CoTiO3. The unexpected intralayer ferromagnetic couplings can be attributed to the intraorbital t2g −t2g exchange coupling, which was assumed to be small earlier and ignored, but actually large in honeycomb cobaltates with 3d 7 electronic configuration. In addition, the computed magnetic moment on Co2+ ion ranges from 2.5 to 2.7 μ B, Hubbard U eff dependent, close to ideal S = 3/2 state, rather than the anticipated J eff = 1/2 state. Furthermore, the supplemental calculations, taking spin-orbit coupling (SOC) into account, uncover faint orbital moments of 0.21–0.27 μ B at the Co site, illustrating the insignificance of SOC. Except for the inevitable trigonal distortions, the excessive structural distortion triggered by the formation V–V dimerization, i.e. the breaking of trigonal symmetry around Co2+, further lifts the degeneracy of t 2g orbitals and increases crystal field splitting, driving it away from potential candidates for realizing Kitaev model physics.

Driving a Molecular Spin-Peierls System into a Short Range Ordered State through Chemical Substitution

Chemically altering molecules can have dramatic effects on the physical properties of a series of very similar molecular compounds. A good example of this is within the quasi-1D spin-Peierls system potassium TCNQ (TCNQ = 7,7,8,8-tetracyanoqunidimethane), where substitution of TCNQF4 for TCNQ has a dramatic effect on the 1D interactions, resulting in a drop in the corresponding spin-Peierls transition temperature. Within this work, we extend the investigation to potassium TCNQBr2, where only two protons of TCNQ can be substituted with bromine atoms due to steric constraints. The new system exhibits evidence for a residual component of the magnetism when probed via magnetic susceptibility measurements and muon spin spectroscopy. The observations suggest that the system is dominated by short range, and potentially disordered, correlations within the bulk phase.

Spin-Peierls transition to a Haldane phase

We present an organic compound exhibiting a spin-Peierls (SP) transition to an effective spin-1 antiferromagnetic uniform chain, that is, the Haldane chain. The clear disappearance of magnetization, accompanied by a structural phase transition, is well explained by the deformation to an effective spin-1 Haldane chain. The flexibility of the molecular orbitals in the organic radical compound allows the transformation of the exchange interactions into the Haldane state with different topologies. The SP transition in the present compound demonstrates a mechanism different from that of the conventional systems, paving another path for research in quantum phenomena originating from spin-lattice couplings.

Stable Crystalline Nanohoop Radical and Its Self-Association Promoted by van der Waals Interactions.

A stable nanohoop radical (OR3) combining the structures of cycloparaphenylene and an olympicenyl radical is synthesized and isolated in the crystalline state. X-ray crystallographic analysis reveals that OR3 forms a unique head-to-tail dimer that further aggregates into a one-dimensional chain in the solid state. Variable-temperature NMR and concentration-dependent absorption measurements indicate that the π-dimer is not formed in solution. An energy decomposition analysis indicates that van der Waals interactions are the driving force for the self-association process, in contrast with other olympicenyl derivatives that favor π-dimerization. The physical properties in solution phase have been studied, and the stable cationic species obtained by one-electron chemical oxidation. This study offers a new molecular design to modulate the self-association of organic radicals for overcoming the spin-Peierls transition, and to prepare novel nanohoop compounds with spin-related properties.

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.

Charge-orbital Ordering, Magnetic State, and Exchange Couplings in Quasi-One-Dimensional Vanadate V6O13

Charge and orbital ordering, magnetic state, and exchange couplings in quasi-one-dimensional vanadate V6O13, a potential cathode material for Li-ion batteries, are investigated using the density functional theory with Coulomb interaction correction method (DFT + U). While the difference between {{t}_{{2g}}} orbital occupancies of V4+ (with a nominal 3{{d}^{1}} electronic configuration) and V5+ ions is large and gives direct evidence for charge and orbital ordering, the screening is so effective that the total 3d charge disproportionation is rather small. Our results show that the occupied {{t}_{{2g}}} states of V4+ ions in the single V–V layer form a spin-singlet molecular orbital, while the rest half of V4+ ions in the structurally distinct double V–V layers order antiferromagnetically in the low-temperature insulating phase of V6O13. We conclude that the metal-insulator transition and low-temperature magnetic properties of V6O13 involve the spin-Peierls transition assisted by orbital ordering and concomitant distortions of the crystal structure.

Spin-Peierls transition of J1−J2 and extended models with ferromagnetic J1 : Sublattice dimerization and thermodynamics of zigzag chains in β−TeVO4

The spin$-1/2$ chain with ferromagnetic exchange $J_1 < 0$ between first neighbors and antiferromagnetic $J_2 > 0$ between second neighbors supports two spin-Peierls (SP) instabilities depending on the frustration $\alpha = J_2/\vert J_1\vert$. Instead of chain dimerization with two spins per unit cell, $J_1-J_2$ models with $\alpha > 0.65$ and linear spin-phonon coupling are unconditionally unstable to sublattice dimerization with four spins per unit cell. Unequal $J_1$ to neighbors to the right and left extends the model to gapped ($\gamma > 0$) chains with conditional SP transitions at $T_{SP}$ to dimerized sublattices and a weaker specific heat $C(T)$ anomaly. The spin susceptibility $\chi(T)$ and $C(T)$ are obtained in the thermodynamic limit by a combination of exact diagonalization of small systems with $\alpha > 0.65$ and density matrix renormalization group (DMRG) calculations of systems up to $N \sim 100$ spins. Both $J_1-J_2$ and $\gamma > 0$ models account quantitatively for $\chi(T)$ and $C(T)$ in the paramagnetic phase of $\beta$-TeVO$_{4}$ for $T > 8$ K, but lower $T$ indicates a gapped chain instead of a $J_1-J_2$ model as previously thought. The same parameters and $T_{SP} = 4.6$ K generate a $C(T)/T$ anomaly that reproduces the anomaly at the $4.6$ K transition of $\beta$-TeVO$_{4}$, but not the weak $\chi(T)$ signature.

Comprehensively understanding the natures of crystal structure and magnetic behavior of a radical salt with regular mixed anion-cation stacks

In this study, the ionic crystals of [TTF][Au(mnt)2] (1; mnt2− ​= ​maleonitriledithiolate) have been grown using solvent diffusion method, and characterized by microanalysis, IR and Raman spectra, TG and powder X-ray diffraction (PXRD) techniques. Crystal of 1 belongs to triclinic space P−1 with Z ​= ​1 and both planar TTF+ and [Au(mnt)2]− locate at an inversion center, respectively. Cation (C) and anion (A) form uniform … CACA … mixed stack along [1 0 0] direction, and the charge-assisted H-bonds form between neighboring mixed stacks. 1 features an S ​= ​½ Heisenberg uniform antiferromagnetic (AFM) chain with J/kB ​= ​−2.08(1) K with g ​= ​2.0085 fixed, and absent spin-Peierls transition above 1.9 ​K, however, with long-range AFM ordering below 1.9 ​K. Additionally, 1 displays different style of mixed stack from [TTF][M(mnt)2] (M ​= ​Pd or Pt; … C2A2C2A2 … manner mixed stack) owing to strong AFM couplings (−J/kB ​> ​1000 ​K) in a [M(mnt)2]22− dimer.

Triplon current generation in solids

A triplon refers to a fictitious particle that carries angular momentum S=1 corresponding to the elementary excitation in a broad class of quantum dimerized spin systems. Such systems without magnetic order have long been studied as a testing ground for quantum properties of spins. Although triplons have been found to play a central role in thermal and magnetic properties in dimerized magnets with singlet correlation, a spin angular momentum flow carried by triplons, a triplon current, has not been detected yet. Here we report spin Seebeck effects induced by a triplon current: triplon spin Seebeck effect, using a spin-Peierls system CuGeO3. The result shows that the heating-driven triplon transport induces spin current whose sign is positive, opposite to the spin-wave cases in magnets. The triplon spin Seebeck effect persists far below the spin-Peierls transition temperature, being consistent with a theoretical calculation for triplon spin Seebeck effects.

Dimerization and spin decoupling in a two-leg Heisenberg ladder with frustrated trimer rungs

We study the antiferromagnetic spin-half Heisenberg ladder in the presence of an additional frustrating rung spin that is motivated and relevant also for the description of real two-dimensional materials such as the two-dimensional trimer magnet ${\mathrm{Ba}}_{4}{\mathrm{Ir}}_{3}{\mathrm{O}}_{10}$. We study the zero-temperature phase diagram, where we combine numerical and analytical methods into an overall consistent description. All numerical simulations are also accompanied by studies of the dynamical spin structure factor obtained via the density matrix renormalization group. Overall, we find in the regime of strong rung coupling a gapped dimerized phase related to competing symmetry sectors in Hilbert space that ultimately results in frustration-driven spin-Peierls transition. In the weak rung-coupling regime, the system is uniform yet shows a gapped spinon continuum together with a sharp coherent low-energy branch that renders the system critical overall. In either case, the additional rung spin quickly gets sidelined and nearly decouples once its bare coupling to the ladder drops somewhat below the direct Heisenberg coupling of the legs.

Crystal structure and magnetic properties of the magnetically isolated zigzag chain in KGaCu(PO4)2.

Magnetism of any material depends on its crystal structure. However, two isostructural compounds such as MCuMoO4(OH) (M = Na, K) can have markedly different magnetic properties. Herein, we introduce a new method to describe the linkages between the O-atoms and their bridged Cu2+ ions in order to clearly illustrate the structure-magnetic property relationships. This new method can account for magnetic differences between the two isostructural MCuMoO4(OH) and is further confirmed by the rational design and development of a new compound KGaCu(PO4)2 with different linkages. The title compound crystalized in a space group of P21/c adopts a one-dimensional (1D) magnetically isolated S = 1/2 zigzag chain composed of elongated [CuO6] octahedra via sharing alternately equatorial and skew edges. O atoms at the skew edges bridge the equatorial and axial orbitals of neighbouring Cu2+ ions (denoted EOA), while those at the equatorial edges bridge the equatorial orbitals of Cu2+ ions (EOE). The nearest-neighbour (NN) magnetic coupling of Cu2+ ions with the EOA linkage at 2.821 Å in the title compound is negligible, whereas the NN magnetic coupling of Cu2+ ions with the EOE linkage at 2.974 Å is essential. Therefore, the zigzag chain containing alternating spin-exchange dimers and no-spin-exchange ones is similar in electronic configuration to the dimerization of the quasi-one-dimensional antiferromagnet. Magnetic investigation of analogous compounds with a 'trans-cis-trans-cis' configuration observed in the title compound may shed light on structural evolutions associated with spin-Peierls (SP) transition.

Coupled dimer and bond-order-wave order in the quarter-filled one-dimensional Kondo lattice model

Motivated by the experiments on the organic compound $(Per)_{2}[Pt(mnt)_{2}]$, we study the ground state of the one-dimensional Kondo lattice model at quarter filling with the density matrix renormalization group method. We show a coupled dimer and bond-order-wave (BOW) state in the weak coupling regime for the localized spins and itinerant electrons, respectively. The quantum phase transitions for the dimer and the BOW orders occur at the same critical coupling parameter $J_{c}$, with the opening of a charge gap. The emergence of the combination of dimer and BOW order agrees with the experimental findings of the simultaneous Peierls and spin-Peierls transitions at low temperatures, which provides a theoretical understanding of such phase transition. We also show that the localized spins in this insulating state have quasi-long ranged spin correlations with collinear configurations, which resemble the classical dimer order in the absence of a magnetic order.

Variational wave functions for the spin-Peierls transition in the Su-Schrieffer-Heeger model with quantum phonons

We introduce variational wave functions to evaluate the ground-state properties of spin-phonon coupled systems described by the Su-Schrieffer-Heeger model. Quantum spins and phonons are treated on equal footing within a Monte Carlo sampling, and different regimes are investigated. We show that the proposed variational Ansatz yields good agreement with previous density-matrix renormalization group results in one dimension and is able to accurately describe the spin-Peierls transition. This variational approach is neither constrained by the magnetoelastic-coupling strength nor by the dimensionality of the systems considered, thus allowing future investigations in more general cases, which are relevant to spin-liquid and topological phases in two spatial dimensions.

Spinon excitations in the quasi-one-dimensional S=12 chain compound Cs4CuSb2Cl12

The spin-1/2 Heisenberg antiferromagnetic chain is ideal for realizing one of the simplest gapless quantum spin-liquids (QSLs), supporting a many-body ground state whose elementary excitations are fractional fermionic excitations called spinons. Here we report the discovery of such a 1D QSL in Cs4CuSb2Cl12. Compared to previously reported S = 1/2 1D chains, this material possesses a wider temperature range over which the QSL state is stabilized. We identify spinon excitations extending at T > 0.8 K, with a large T-linear contribution to the specific heat, gamma = 31.5(2) mJ mol-1 K-2 which contribute itinerantly to thermal transport up to temperatures as high as T = 35 K. At T = 0.7 K, we find a second-order phase transition, suggesting a weak spin-Peierls transition that is unchanged by a 5 T magnetic field. Cs4CuSb2Cl12 reveals new phenomenology deep in the 1D QSL regime, supporting a gapped QSL phase over a wide temperature range compared to many other experimental realizations.

Modeling the spin-Peierls transition of spin- 12 chains with correlated states: J1−J2 model, CuGeO3 , and TTF–CuS4C4(CF3)4

The spin-Peierls transition at $T_{SP}$ of spin-$1/2$ chains with isotropic exchange interactions has previously been modeled as correlated for $T > T_{SP}$ and mean field for $T < T_{SP}$. We use correlated states throughout in the $J_1-J_2$ model with antiferromagnetic exchange $J_1$ and $J_2 = \alpha J_1$ between first and second neighbors, respectively, and variable frustration $0 \leq \alpha \leq 0.50$. The thermodynamic limit is reached at high $T$ by exact diagonalization of short chains and at low $T$ by density matrix renormalization group calculations of progressively longer chains. In contrast to mean field results, correlated states of 1D models with linear spin-phonon coupling and a harmonic adiabatic lattice provide an internally consistent description in which the parameter $T_{SP}$ yields both the stiffness and the lattice dimerization $\delta(T)$. The relation between $T_{SP}$ and $\Delta(\delta,\alpha)$, the $T = 0$ gap induced by dimerization, depends strongly on $\alpha$ and deviates from the BCS gap relation that holds in uncorrelated spin chains. Correlated states account quantitatively for the magnetic susceptibility of TTF-CuS$_4$C$_4$(CF$_3$)$_4$ crystals ($J_1 = 79$ K, $\alpha = 0$, $T_{SP} = 12$ K) and CuGeO$_3$ crystals ($J_1 = 160$ K, $\alpha = 0.35$, $T_{SP} = 14$ K). The same parameters describe the specific heat anomaly of CuGeO$_3$ and inelastic neutron scattering. Modeling the spin-Peierls transition with correlated states exploits the fact that $\delta(0)$ limits the range of spin correlations at $T = 0$ while $T > 0$ limits the range at $\delta= 0$.

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.

The gapless energy spectrum and spin-Peierls instability of 1D Heisenberg spin systems in polymeric complexes of transition metals and hypothetical carbon allotropes

We investigate the spin-Peierls instability of some periodic 1D Heisenberg spin systems having a gapless energy spectrum at different values of coupling J between the unit cells. Using the density-matrix renormalization group method we numerically study the dependence of critical exponents p of spin-Peierls transition of above spin systems on the value of J. In contrast to chain systems, we find significantly non-monotonous dependence p (J) for three-legs ladder system. In the limit of weak coupling J we derive effective spin s chain Hamiltonians describing the low-energy states of the system considered by means of perturbation theory. The value of site spin s coincides with the value of the ground-state spin of the isolated unit cell of the system considered. This means that at small J values all the systems with the singlet ground state and the same half-integer value of s should have a similar critical behavior which is in agreement with our numerical study. The presence of gapped excitations inside the unit cells at small values of J should give, for our spin systems, at least one intermediate plateau in field dependence of magnetization at low temperatures. The stability of this plateau against the increase of the values of J and temperature is studied using the quantum Monte-Carlo method.

Charge and Bonding in CuGeO3 Nanorods.

We combine infrared and Raman spectroscopies to investigate finite length scale effects in CuGeO3 nanorods. The infrared-active phonons display remarkably strong size dependence whereas the Raman-active features are, by comparison, nearly rigid. A splitting analysis of the Davydov pairs reveals complex changes in chemical bonding with rod length and temperature. Near the spin-Peierls transition, stronger intralayer bonding in the smallest rods indicates a more rigid lattice which helps to suppress the spin-Peierls transition. Taken together, these findings advance the understanding of size effects and collective phase transitions in low-dimensional oxides.