Polar Dynamics at the Jahn-Teller Transition in Ferroelectric<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>Ga</mml:mi><mml:msub><mml:mrow><mml:mi mathvariant="normal">V</mml:mi></mml:mrow><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mrow><mml:mi mathvariant="normal">S</mml:mi></mml:mrow><mml:mrow><mml:mn>8</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

Wang, Zhe; Ruff, E.; Schmidt, Michael; Tsurkan, V.; Kézsmárki, I.; Lunkenheimer, P.; Loidl, A.

doi:10.1103/physrevlett.115.207601

Cited by 47 publications

(91 citation statements)

References 37 publications

(90 reference statements)

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“…At high T , GVS has a rock-salt like structure composed of alternating GaS 4 and V 4 S 4 clusters. The highest-energy unpaired electron of the V 4 S 4 cluster occupies a triply degenerate molecular orbital which drives a cubic (F 43m) to rhombohedral (R3m) Jahn-Teller transition at T JT =42 K [15][16][17][18][19]. Since this phase transition breaks the 4 symmetry [15], four structural domains develop below T JT .…”

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Direct evidence for cycloidal modulations in the thermal-fluctuation-stabilized spin spiral and skyrmion states of GaV4S8

et al. 2018

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We report small-angle neutron scattering studies of the lacunar spinel GaV4S8, which reveal the long-wavelength magnetic states to be cycloidally modulated. This provides direct support for the formation of Néel-type skyrmions recently claimed to exist in this compound. In striking contrast with all other bulk skyrmion host materials, upon cooling the modulated magnetic states transform into a ferromagnetic state. These results indicate all of the modulated states in GaV4S8, including the skyrmion state, gain their stability from thermal fluctuations, while at lower temperature the ferromagnetic state emerges in accord with the strong easy-axis magnetic anisotropy. In the vicinity of the transition between the ferromagnetic and modulated states, both a phase coexistence and a soliton-like state are also evidenced by our study. PACS numbers:Particle-like magnetic skyrmions with topologically non-trivial spin textures receive continued attention since their creation, stability and annihilation, either individually or in skyrmion lattices (SkLs), raise fundamental questions related to topology in physics [1][2][3]. Moreover, the observations of metastable skyrmions [4,5] and their current-driven dynamics [6,7] inspires new ideas for using skyrmions in data storage and logic devices [8][9][10].The spin texture of a skyrmion can be described by a vector field S=S(cos(Φ(ϕ))sin(Θ(ρ)), sin(Φ(ϕ))sin(Θ(ρ)), cos(Θ(ρ)), where ϕ and ρ are the azimuth and radial spatial coordinates, respectively [8,11]. The non-trivial skyrmion topology is evidenced when its spin pattern is mapped on to a sphere, and the mapping wraps the surface of the sphere entirely. Two parameters classify the skyrmion type: the vorticity m which is a non-zero integer, and the helicity, γ where Φ(ϕ) = mϕ + γ.The bulk cubic chiral helimagnets with B20 structure, like MnSi and FeGe, host whirlpool-like Bloch-type skyrmions described by m=+1 and γ=±π/2 [4,11,12]. In these compounds the competition between a dominant ferromagnetic (FM) exchange and weaker DzyaloshinskiiMoriya interactions (DMIs) stabilize a long-wavelength (∼10-100 nm) helical ground state in zero magnetic field (B). The SkL phase exists under a finite B, where it competes with either a spin-flop conical phase or a field polarized state. Since the SkL is stable typically only over the narrow temperature (T ) range 0.9 ≤ T ≤ T C , where T C is the ordering T , thermal fluctuations are proposed to be crucial for the phase stability [11][12][13].Recently a new bulk SkL phase was reported in the lacunar spinel GaV 4 S 8 (GVS) [14]. At high T , GVS has a rock-salt like structure composed of alternating GaS 4 and V 4 S 4 clusters. The highest-energy unpaired electron of the V 4 S 4 cluster occupies a triply degenerate molecular orbital which drives a cubic (F 43m) to rhombohedral (R3m) Jahn-Teller transition at T JT =42 K [15][16][17][18][19]. Since this phase transition breaks the 4 symmetry [15], four structural domains develop below T JT . Each domain carries an electric polarization along its r...

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Direct evidence for cycloidal modulations in the thermal-fluctuation-stabilized spin spiral and skyrmion states of GaV4S8

et al. 2018

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“…As predicted in the seminal works of Bogdanov et al [3][4][5] the important prerequisite for the appearance of this very unusual spin texture (Néel-type SkL) is the polar macroscopic symmetry of the underlying crystal lattice. In GVS, the suitable C 3v symmetry emerges together with the ferroelectric polarization below the JahnTeller phase transition T JT = 42 K [6,7]. This unusual phase transition was shown to be driven by a strong electron-phonon coupling between the unpaired electron in the highest-energy, orbitally degenerate electronic state of the V 4 cluster and two corresponding Jahn-Teller-active zone-center modes of the noncentrosymmetric paraelectric structure of GVS [8].…”

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“…Therefore, it is very likely that the critically slowing down THz-range relaxational dynamics, observed above T JT in Ref. [7], is simply related to the stochastic jumps of the local Jahn-Teller coordinate among the four equivalent Jahn-Teller potential minima. This is what is actually expected in the canonical picture of the dynamical Jahn-Teller effect.…”

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Lattice modes and the Jahn-Teller ferroelectric transition ofGaV4S8

et al. 2016

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Crystal of GaV 4 S 8 , a multiferroic system hosting a Néel-type skyrmion lattice phase, has been investigated by polarized Raman and IR spectroscopy above and below the ferroelectric phase transition. Counts of the observed IR and Raman-active modes belonging to distinct irreducible representations agree quite well with group-theory predictions. Phonon spectra are assigned and interpreted with the aid of ab initio calculations of the phonon spectra in the ferroelectric phase. Results allow appreciation of phonon frequencies of the modes involved in Jahn-Teller distortion and their contribution to the spontaneous polarization. DOI: 10.1103/PhysRevB.94.060104 The ternary chalcogenide GaV 4 S 8 (GVS) has recently attracted considerable attention due to its multiferroic properties. First of all, the low-temperature zero-field phase of GVS is simultaneously ferroelectric and ferromagnetic up to about T FM ≈ 5 K. Its magnetization corresponds to S = 1/2 per formula unit and its electric polarization is also appreciable (P s ≈ 1 μC cm −2 ). Moreover, clear experimental evidence has been found for a sizable magnetoelectric coupling [1]. Secondly, GVS exhibits an interesting magnetic cycloidal order between T FM and T N = 13 K, which can be transformed by a moderate magnetic field (of about 40 mT) into a Néel-type skyrmion lattice (SkL) phase [2]. As predicted in the seminal works of Bogdanov et al. [3][4][5] the important prerequisite for the appearance of this very unusual spin texture (Néel-type SkL) is the polar macroscopic symmetry of the underlying crystal lattice. In GVS, the suitable C 3v symmetry emerges together with the ferroelectric polarization below the JahnTeller phase transition T JT = 42 K [6,7]. This unusual phase transition was shown to be driven by a strong electron-phonon coupling between the unpaired electron in the highest-energy, orbitally degenerate electronic state of the V 4 cluster and two corresponding Jahn-Teller-active zone-center modes of the noncentrosymmetric paraelectric structure of GVS [8].The aim of this Rapid Communication is to explore the role of the zone-center phonon modes in the phase transition using Raman and IR spectroscopy and the ab initio approach. In particular, we have (i) determined the spectrum of zonecenter phonon modes of GVS, (ii) identified two phonon modes primarily involved in the phase transition, and (iii) related these findings to the Jahn-Teller phase transition and dielectric properties of GVS.GVS is a lacunar spinel, i.e., it can be presented as a spinel structure lacking every second Ga atom. It can be viewed as a face-centered cubic (fcc) lattice formed of * hlinka@fzu.cz tetrahedral GaS 4 and cubane V 4 S 4 clusters [see Fig. 1(a)]. The paraelectric phase of GaV 4 S 8 has a noncentrosymmetric cubic (T d ) symmetry [6]. The hybridization of the valence states of the vanadium metal ions within the V 4 S 4 magnetic building blocks leads to one unpaired localized electron occupying a triply degenerate t 2 cluster orbital with an overall spin 1/2. The Ga...

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“…GaV 4 S 8 is a semiconductor with a noncentrosymmetric cubic crystal structure at room temperature characterized by the space group F43m and the corresponding T d point symmetry group. The compound undergoes a cooperative Jahn-Teller distortion at T S = 42 K through the stretching of the lattice along one of the four cubic body diagonals reducing the crystal symmetry to polar rhombohedral, R3m [21][22][23][24]. As a result a sizable ferroelectric polarization develops along the rhombohedral axis [25,26].…”

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Relaxation dynamics of modulated magnetic phases in the skyrmion host GaV4S8 : An ac magnetic susceptibility study

et al. 2017

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We report on the slow magnetization dynamics observed upon the magnetic phase transitions of GaV 4 S 8 , a multiferroic compound featuring a long-ranged cycloidal magnetic order and a Néel-type skyrmion lattice in a relatively broad temperature range below its Curie temperature. The fundamental difference between GaV 4 S 8 and the chiral helimagnets, the prototypical skyrmion host compounds, lies within the polar symmetry of GaV 4 S 8 , promoting a cycloidal instead of a helical magnetic order and rendering the magnetic phase diagram essentially different from that in the cubic helimagnets. Our ac magnetic susceptibility study reveals slow relaxation dynamics at the field-driven phase transitions between the cycloidal, skyrmion lattice and field-polarized states. At each phase boundary, the characteristic relaxation times were found to exhibit a strong temperature dependence, starting from the minute range at low temperatures, decreasing to the micro-to millisecond range at higher temperatures.

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Polar Dynamics at the Jahn-Teller Transition in FerroelectricGaV4S8

Cited by 47 publications

References 37 publications

Direct evidence for cycloidal modulations in the thermal-fluctuation-stabilized spin spiral and skyrmion states of GaV4S8

Direct evidence for cycloidal modulations in the thermal-fluctuation-stabilized spin spiral and skyrmion states of GaV4S8

Lattice modes and the Jahn-Teller ferroelectric transition ofGaV4S8

Relaxation dynamics of modulated magnetic phases in the skyrmion host GaV4S8 : An ac magnetic susceptibility study

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