Polar Dynamics at the Jahn-Teller Transition in Ferroelectric GaV₄S₈

Hybrid electrons in the trimerized GaV4O8

Mixed-valent transition-metal compounds display complex structural, electronic and magnetic properties, which often intricately coexist. Here, we report the new ternary oxide GaV₄O₈, a structural sibling of skyrmion-hosting lacunar spinels. GaV₄O₈ contains a vanadium trimer and an original spin-orbital-charge texture that forms upon the structural phase transition at T_S = 68 K followed by the magnetic transition at T_N = 35 K. The texture arises from the coexistence of orbital molecules on the vanadium trimers and localized electrons on the remaining vanadium atoms. Such hybrid electrons create opportunities for novel types of spin, charge, and orbital order in mixed-valent transition-metal compounds.

Giant conductivity of mobile non-oxide domain walls

Atomically sharp domain walls in ferroelectrics are considered as an ideal platform to realize easy-to-reconfigure nanoelectronic building blocks, created, manipulated and erased by external fields. However, conductive domain walls have been exclusively observed in oxides, where domain wall mobility and conductivity is largely influenced by stoichiometry and defects. Here, we report on giant conductivity of domain walls in the non-oxide ferroelectric GaV₄S₈. We observe conductive domain walls forming in zig-zagging structures, that are composed of head-to-head and tail-to-tail domain wall segments alternating on the nanoscale. Remarkably, both types of segments possess high conductivity, unimaginable in oxide ferroelectrics. These effectively 2D domain walls, dominating the 3D conductance, can be mobilized by magnetic fields, triggering abrupt conductance changes as large as eight orders of magnitude. These unique properties demonstrate that non-oxide ferroelectrics can be the source of novel phenomena beyond the realm of oxide electronics.

Cooperative Cluster Jahn-Teller Effect as a Possible Route to Antiferroelectricity

We report the observation of an antipolar phase in cubic GaNb_{4}S_{8} driven by an unconventional microscopic mechanism, the cooperative Jahn-Teller effect of Nb_{4}S_{4} molecular clusters. The assignment of the antipolar nature is based on sudden changes in the crystal structure and a strong drop of the dielectric constant at T_{JT}=31  K, also indicating the first-order nature of the transition. In addition, we found that local symmetry lowering precedes long-range orbital ordering, implying the presence of a dynamic Jahn-Teller effect in the cubic phase above T_{JT}. Based on the variety of structural polymorphs reported in lacunar spinels, also including ferroelectric phases, we argue that GaNb_{4}S_{8} may be transformable to a ferroelectric state, which would further classify the observed antipolar phase as antiferroelectric.

Establishing magneto-structural relationships in the solid solutions of the skyrmion hosting family of materials: GaV4S8-ySey

The GaV4S8-ySey (y = 0 to 8) family of materials have been synthesized in both polycrystalline and single crystal form, and their structural and magnetic properties thoroughly investigated. Each of these materials crystallizes in the F[Formula: see text][Formula: see text]3m space group at ambient temperature. However, in contrast to the end members GaV4S8 and GaV4Se8, that undergo a structural transition to the R3m space group at 42 and 41 K respectively, the solid solutions (y = 1 to 7) retain cubic symmetry down to 1.5 K. In zero applied field the end members of the family order ferromagnetically at 13 K (GaV4S8) and 18 K (GaV4Se8), while the intermediate compounds exhibit a spin-glass-like ground state. We demonstrate that the magnetic structure of GaV4S8 shows localization of spins on the V cations, indicating that a charge ordering mechanism drives the structural phase transition. We conclude that the observation of both structural and ferromagnetic transitions in the end members of the series in zero field is a prerequisite for the stabilization of a skyrmion phase, and discuss how the absence of these transitions in the y = 1 to 7 materials can be explained by their structural properties.

Discovery of twin orbital-order phases in ferromagnetic semiconducting VI3 monolayer

Discovery of twin orbital-order ferromagnetic semiconducting phases in VI₃-like 2D systems.

Optically Driven Collective Spin Excitations and Magnetization Dynamics in the Néel-type Skyrmion Host GaV_{4}S_{8}

GaV_{4}S_{8} is a multiferroic semiconductor hosting magnetic cycloid (Cyc) and Néel-type skyrmion lattice (SkL) phases with a broad region of thermal and magnetic stability. Here, we use time-resolved magneto-optical Kerr spectroscopy to show the coherent generation of collective spin excitations in the Cyc and SkL phases. Our micromagnetic simulations reveal that these are driven by an optically induced modulation of uniaxial anisotropy. Our results shed light on spin dynamics in anisotropic materials hosting skyrmions and pave a new pathway for the optical manipulation of their magnetic order.

Architecture of nanoscale ferroelectric domains in GaMo4S8

Local-probe imaging of the ferroelectric domain structure and auxiliary bulk pyroelectric measurements were conducted at low temperatures with the aim to clarify the essential aspects of the orbitally driven phase transition in GaMo₄S₈, a lacunar spinel crystal that can be viewed as a spin-hole analogue of its GaV₄S₈ counterpart. We employed multiple scanning probe techniques combined with symmetry and mechanical compatibility analysis to uncover the hierarchical domain structures, developing on the 10-100 nm scale. The identified domain architecture involves a plethora of ferroelectric domain boundaries and junctions, including primary and secondary domain walls in both electrically neutral and charged configurations, and topological line defects transforming neutral secondary walls into two oppositely charged ones.

Infrared nanoscopy down to liquid helium temperatures

We introduce a scattering-type scanning near-field infrared microscope (s-SNIM) for the local scale near-field sample analysis and spectroscopy from room temperature down to liquid helium (LHe) temperature. The extension of s-SNIM down to T = 5 K is in particular crucial for low-temperature phase transitions, e.g., for the examination of superconductors, as well as low energy excitations. The low temperature (LT) s-SNIM performance is tested with CO₂-IR excitation at T = 7 K using a bare Au reference and a structured Si/SiO₂-sample. Furthermore, we quantify the impact of local laser heating under the s-SNIM tip apex by monitoring the light-induced ferroelectric-to-paraelectric phase transition of the skyrmion-hosting multiferroic material GaV₄S₈ at T_c = 42 K. We apply LT s-SNIM to study the spectral response of GaV₄S₈ and its lateral domain structure in the ferroelectric phase by the mid-IR to THz free-electron laser-light source FELBE at the Helmholtz-Zentrum Dresden-Rossendorf, Germany. Notably, our s-SNIM is based on a non-contact atomic force microscope (AFM) and thus can be complemented in situ by various other AFM techniques, such as topography profiling, piezo-response force microscopy (PFM), and/or Kelvin-probe force microscopy (KPFM). The combination of these methods supports the comprehensive study of the mutual interplay in the topographic, electronic, and optical properties of surfaces from room temperature down to 5 K.

Equilibrium Skyrmion Lattice Ground State in a Polar Easy-plane Magnet

The skyrmion lattice state (SkL), a crystal built of mesoscopic spin vortices, gains its stability via thermal fluctuations in all bulk skyrmion host materials known to date. Therefore, its existence is limited to a narrow temperature region below the paramagnetic state. This stability range can drastically increase in systems with restricted geometries, such as thin films, interfaces and nanowires. Thermal quenching can also promote the SkL as a metastable state over extended temperature ranges. Here, we demonstrate more generally that a proper choice of material parameters alone guarantees the thermodynamic stability of the SkL over the full temperature range below the paramagnetic state down to zero kelvin. We found that GaV₄Se₈, a polar magnet with easy-plane anisotropy, hosts a robust Néel-type SkL even in its ground state. Our supporting theory confirms that polar magnets with weak uniaxial anisotropy are ideal candidates to realize SkLs with wide stability ranges.

Characteristics of ferroelectric-ferroelastic domains in Néel-type skyrmion host GaV4S8

GaV₄S₈ is a multiferroic semiconductor hosting Néel-type magnetic skyrmions dressed with electric polarization. At T_s = 42 K, the compound undergoes a structural phase transition of weakly first-order, from a non-centrosymmetric cubic phase at high temperatures to a polar rhombohedral structure at low temperatures. Below T_s, ferroelectric domains are formed with the electric polarization pointing along any of the four 〈111〉 axes. Although in this material the size and the shape of the ferroelectric-ferroelastic domains may act as important limiting factors in the formation of the Néel-type skyrmion lattice emerging below T_C = 13 K, the characteristics of polar domains in GaV₄S₈ have not been studied yet. Here, we report on the inspection of the local-scale ferroelectric domain distribution in rhombohedral GaV₄S₈ using low-temperature piezoresponse force microscopy. We observed mechanically and electrically compatible lamellar domain patterns, where the lamellae are aligned parallel to the (100)-type planes with a typical spacing between 100 nm–1.2 μm. Since the magnetic pattern, imaged by atomic force microscopy using a magnetically coated tip, abruptly changes at the domain boundaries, we expect that the control of ferroelectric domain size in polar skyrmion hosts can be exploited for the spatial confinement and manipulation of Néel-type skyrmions.

Exchange anisotropy in the skyrmion host GaV4S8

Using ferromagnetic resonance spectroscopy at 34 GHz we explored the magnetic anisotropy of single-crystalline GaV₄S₈ in the field-polarized magnetic state. We describe the data in terms of an easy-axis type uniaxial anisotropy with an anisotropy constant [Formula: see text] erg cm^-3 at 2 K, corresponding to a relative exchange anisotropy [Formula: see text]%, and about [Formula: see text]erg cm^-3 near 11 K, i.e. at temperatures where the skyrmion-lattice phase was recently discovered. The relatively large value of K ₁ explains the confinement of the skyrmion tubes to the [Formula: see text] easy axes. A distinct set of resonances in the spectra is attributed to the co-existence of different rhombohedral domains. Complementary broadband spectroscopy demonstrates that non-collinear spin states may sensitively be detected by electron spin resonance techniques.