Session G10: Symmetry Broken States in Layered Materials

Scanning tunneling microscopy/spectroscopy studies of monolayer FeTe on SrTiO3(001) - √13⨯√13

Bulk FeTe shows a bicollinear antiferromagnetic order under 65 K [1], but recent angle-resolved photoemission and scanning tunneling spectroscopy research also revealed a Kondo-like behavior in FeTe [2], showing that the ground state of FeTe is complicated. Our previous research showed that SrTiO₃(001)-√13⨯√13 substrate leads to a C₂ symmetry on monolayer (ML) FeSe, a cousin of FeTe; furthermore, we found that the electron doping from the SrTiO₃ surface locally modulates the superconducting gap size of ML FeSe [3]. These prompt several questions: 1. Can the SrTiO₃-√13⨯√13 substrate lower the rotational symmetry on the isostructural ML FeTe? 2. Can the Kondo behavior be confirmed on the ML FeTe? 3. Can the SrTiO₃ substrate locally modulate the ground state of ML FeTe?

To clarify the above points, we performed low-temperature scanning tunneling microscopy/spectroscopy (STM/STS, T: 5 K ~ 77 K) experiments on the ML FeTe/SrTiO₃-√13⨯√13.

Results [4]:

1. The STM data indicates a substrate-induced C₂ rotational symmetry on ML FeTe, similar to ML FeSe on SrTiO₃-√13⨯√13.

2. The STS spectra obtained on ML FeTe show Fano-line shape near the Fermi level, illustrating a Kondo scenario that localized Fe spins are screened by the conduction electrons.

3. Two types of Fano-line spectra were confirmed, and their spatial dependence follows the √13⨯√13 periodicities, indicating that the SrTiO₃ substrate can modulate the interaction between the localized spin and conduction electrons mentioned above.

Details will be given in the presentation.   

Scanning SQUID microscopy on van der Waals devices with in situ uniaxial strain

Van der Waals (vdW) materials, such as graphene and transition metal dichalcogenides, are atomically-thin, crystalline materials that exhibit a rich variety of electronic phenomena. Application of uniaxial strain can be a powerful tool to alter the properties of materials by breaking symmetry and modulating electronic structure. Scanning superconducting quantum interference device (SQUID) microcopy probes local magnetization, electronic transport and magnetic response, which can give insights into the electronic phenomena observed in vdW materials. In this talk, we  report on  local magnetic measurements using a scanning SQUID on uniaxially strained vdW samples. We fabricate vdW devices on a polyimide substrate, and use a commercial piezoelectric strain cell to apply in-situ tunable strain to the device. We present the design of our straining platform and discuss  measurements on few layer NbSe2 samples, where we locally observe changes in the superconducting transition temperature as a function of strain. Our approach is compatible with a wide variety of vdW heterostructures.   

Structural properties of plastically deformed SrTiO3 and KTaO3

Strontium titanate (SrTiO3, STO) and potassium tantalate (KTaO­3, KTO) are considered incipient ferroelectrics. These perovskites are also remarkably ductile at ambient temperatures, making them excellent candidates for plastic deformation. While this property has been known for over two decades in the case of STO [1], it is worth noting that ductility in KTO has not been previously documented. Here, we present a comprehensive analysis involving diffuse x-ray and neutron scattering data as well as nuclear magnetic resonance (NMR) results aimed to investigate and compare the formation of strain-induced self-organized dislocation structures in plastically deformed STO and KTO. Additionally, we present Raman scattering measurements on plastically deformed KTO, which indicate the emergence of ferroelectric fluctuations induced by severe strain, similar to prior observations in STO [2].

[1] P. Gumbsch et. al., Phys. Rev. Lett. 87, 085505 (2001)

[2] S. Hameed et al., Nat. Mater. 21, 54 (2022).   

Electromagnon excitation modes in the two-dimensional multiferroic van der Waals insulator NiI2

NiI₂ is a two-dimensional van der Waals magnetic insulator, which exhibits spiral magnetic ordering below the Neel temperature of T_N=59.5 and simultaneously forms multiferroic domains. Magnetic excitons associated with Zhang-Rice entangled states has been reported in the multiferroics phase [1]. Using terahertz time-domain spectroscopy, we performed transmission and reflection experiments on a NiI₂ single crystal and observed two strong absorption modes in the terahertz region. By tracking the temperature and magnetic field dependences, it was confirmed that the modes are magnetic excitations only observed in the multiferroic phase. Additionally, through optical model analysis of the reflectance spectra, we confirmed that the modes are electro-dipole-active, i.e. electromagnon modes [2].   

Orthorhombic distortion drives orbital ordering in antiferromagnetic 3d1 Mott insulator

The orbital, which represents the shape of the electron cloud, very often strongly influences the manifestation of various exotic phenomena, e.g., magnetism, metal-insulator transition, colossal magnetoresistance, unconventional superconductivity, etc. in strongly correlated transition metal oxides. The observation of the antiferromagnetism in RETiO₃ (RE = rare-earth) series has been puzzling since no Jahn-Teller distortion was observed and therefore, the celebrated Kugel-Khomskii model of spin-orbital superexchange predicts ferromagnetism in an orbitally degenerate d¹ system. Further, the existence of the orbitally ordered vs. orbital liquid phase in both antiferromagnetic (AFM) and paramagnetic phases remains highly debated. To address these longstanding questions, we investigate single crystalline film of PrTiO₃, a prototypical AFM Mott insulator (3d¹). Our synchrotron x-ray diffraction measurements confirm the retention of bulklike orthorhombic (D_2h) symmetry in the thin film geometry. We observe similar x-ray linear dichroism signals in both paramagnetic and antiferromagnetic phases that originate due to robust ferro-orbital ordering (FOO). While the presence of D_2h crystal field does not always guarantee the lifting of orbital degeneracy, we find it strong enough in these rare-earth titanates, leading to the FOO state. Thus, our work demonstrates that orthorhombic distortion is the driving force for the orbital ordering of antiferromagnetic RETiO₃.   

Role of Fe intercalation on resistively switchable antiferromagnet FexNbS2

Among 3d transition metal intercalated transition-metal dichalcogenides (TMDCs), Fe_xNbS₂ has been found to possess unique current pulse-induced resistive switching behaviors and tunable antiferromagnetically ordered states, both of which show an abrupt change across x=1/3 doping. Currently, how the low-energy conduction electrons and local magnetic moment in this system contribute to such coupled effects remains unclear. Using angle-resolved photoemission spectroscopy (ARPES), we report several doping and orbital selective correlation effects across the ⅓ doping level. There is massive electronic structure reconstruction originating from the Fe sub-lattice, together with a doping sensitive charge order and spectral decoherence. What’s more, the electronic structure suggests a mixed dimensionality of this system as opposed to the common notion of a "2D" magnet in intercalated TMDCs.  Finally, we discuss the possibility for revealing potential novel magnetic and electronic properties in other intercalated TMDC systems from a strongly correlated point of view, complementing existing weak coupling descriptions.   

From Stoner to Local Moment Magnetism in Atomically Thin Cr2Te3

Here, we report a thickness-dependent ferromagnetism in epitaxially grown Cr₂Te₃ thin films and investigate the evolution of the underlying electronic structure by synergistic angle-resolved photoemission spectroscopy, scanning tunneling microscopy, x-ray absorption spectroscopy, and first-principle calculations. A conspicuous ferromagnetic transition from Stoner to Heisenberg-type is directly observed in the atomically thin limit, indicating that dimensionality is a powerful tuning knob to manipulate the novel properties of 2D magnetism. Our results establish atomically thin Cr₂Te₃ as an excellent platform to explore the dual nature of localized and itinerant ferromagnetism in 2D magnets.     

Band Structure Evolution through Electron Doping in Cr2Ge2Te6

Van Hove singularities (VHSs) in the electronic structure of 2D materials produce a divergent density of states that is prone to electronic instabilities when tuned to the Fermi level, and therefore often associated with a precursor of new ordered phases. However, despite the previous fruitful studies on this topic, it is still challenging to understand the effects of VHSs and directly compare them to theoretical predictions, often due to a mixing of VHSs with other coexisting bands as well as their existence in 3D bulk crystals. Here, we have investigated a clean single-band system with isolated VHSs in a ferromagnetic semiconducting material Cr₂Ge₂Te₆ (CGT). By surface alkali metal doping, we were able to tune the VHS to near the Fermi level, hence creating a genuine 2D system suitable for investigating VHS-induced novel states and their interplay with magnetism. In this talk, we will present our angle-resolved photoemission spectroscopy measurement via a step-by-step in situ potassium doping experiments on pristine CGT crystals, tuning the Fermi level to go across the VHS level and beyond, and discuss our observations of electron-electron correlations and electron-boson coupling effects.   

Manganese Chromium Nitride Thin Film Synthesis via Molecular Beam Epitaxy

Transition metal nitrides have exceptional properties and are used in a wide variety of electrochemical, structural, photochemical, and plasmonic applications. Among these compounds Mn- and Cr- nitrides have shown exceptional potential for magnetic sensing and spintronics. The Mn_xN_y system is complex with several different metastable phases both predicted and experimentally realized. Cr_xN_y has two primary phases, cubic (CrN) and hexagonal (Cr₂N), which exhibit desirable mechanical, thermal, wear, anti-corrosion, thermoelectric properties. Recent studies have provided valuable insights into the growth and formation of phases of both materials using various vapor deposition techniques. However, there are conflicting reports on the electrical and magnetic properties of Cr_xN_y which could be attributed to impurities, nitrogen vacancies, substrate effects, and strain. This controversy calls for a more detailed study and preparation of high-quality monocrystalline CrN to investigate he intrinsic physical properties. This study uses molecular beam epitaxy to synthesize epitaxial thin films of different Mn-N and Cr-N phases. The electrical and magnetic properties of these films are investigated with the rocksalt MnN and CrN both showing metallic behavior, with the latter showing a magnetic transition ~280K. However, when combining these materials at similar growth conditions, instead of maintaining the rocksalt structure, a new ternary cubic phase of Mn_xCr_yN is obtained which potentially shows a narrow-gap semiconducting behavior. This work presents an avenue for the epitaxial integration of metallic, magnetic, and semiconductor materials.   

Bipolar Wigner crystals and valley-dependent exciton Umklapp scattering in monolayer WSe2

We observe exciton Umklapp scattering signatures of electron and hole Wigner crystals in monolayer WSe₂; they exhibit melting temperatures of ~27K and ~22K, respectively. By introducing a magnetic field to lift the valley degeneracy, we observe markedly enhanced scattering between excitons and Wigner crystal when they occupy the same valley and band. Our theoretical calculations show that the valley-dependent Umklapp scattering arises from different exchange interactions between excitons and Wigner crystals in different valleys. Our research demonstrates intriguing interplay among Wigner crystals, excitons, and valleys in monolayer WSe₂.   

Investigating structural and magnetic properties of FeTiO3

Ilmenite-type FeTiO₃ antiferromagnet is a well-known triangular lattice material explored for various practical applications in the field of spintronics, optoelectronics, and high-temperature integrated circuits. Structural analysis of FeTiO₃ was done through high-temperature X-ray diffraction measurements coupled with Pair distribution function analysis across a wide temperature range (11 K – 300 K). The thermal evolution of the lattice parameter ‘c’ shows an interesting behavior where it starts expanding below phase transition 58 K exhibiting a negative thermal coefficient of the studied material. It was evident from the results that TiO₆ octahedra becoming less distorted as Ti atom move towards the centroid of the octahedra while distortion in FeO₆ become more prominent as Fe atom move away from the center with decreasing temperature. The lattice distortion effects the Fe-Fe distance which play a crucial role in the magnetic properties of FeTiO₃. Furthermore, experiments done in the presence of an external magnetic field at a fixed temperature of around 50 K exhibiting metamagnetic phase transition. i.e., inducing an AFM to FM transition. In this presentation I will address how structural distortions effect the magnetism in FeTiO₃.   

Noise spectroscopic study of resistive switchings in NbO2 thin films

Transition metal oxides showcase several phase transitions accessible by external parameters such as temperature, voltage, and stress. NbO₂, exhibiting a voltage-driven insulator to metal transition (IMT) at room temperature, is of great interest as a relaxation oscillator in neuromorphic computing applications. Transport and ultra-low frequency conductance noise spectroscopy measurements are employed to investigate voltage-driven Poole-Frenkel type instability accompanied by a Mott transition in nanoscale thin films of NbO₂. The power spectral density of fluctuations shows a significant deviation from Gaussian behavior in the two transition regions pointing to a possible dynamic coexistence of multiple conducting phases. The microscopic transport mechanisms in NbO₂ can be understood by the Mott-correlated and inhomogeneous transport signatures from transport and noise spectroscopy measurements. Relaxation oscillators built using these nanostructures display oscillation parameters that can be tuned through a wide range by external parameters. The transport measurements are supported by NSF-MRI award 1726303.   

Spin-orbit coupling induced Van Hove singularity in proximity to a Lifshitz transition in Sr4Ru3O10

The physics of correlated electron systems can be strongly influenced by Van Hove singularities (VHSs) in the vicinity of the Fermi energy (E_F). In the Ruddlesden-Popper (RP) series of strontium ruthenates, Sr_n+1Ru_nO_3n+1, near- E_F VHSs are thought to drive a host of complex emergent phenomena including enhanced superconductivity and metamagnetism. The tri-layer compound, Sr₄Ru₃O₁₀, hosts a ferromagnetic ground state, with magnetisation aligned parallel to the c-axis. Interestingly, an in-plane metamagnetic transition has been observed whose physical origin is not well understood, although it has been postulated that the transition is related to VHSs situated close to E_F. I will show our recent work combining angle-resolved photoemission spectroscopy and scanning tunnelling microscopy to identify and classify a rich hierarchy of VHSs located within 30 meV of the Fermi level in Sr₄Ru₃O₁₀. ^[1] Most importantly, we find that a spin-orbit induced hybridisation between a heavy spin-majority and light spin-minority band mediates the formation of a saddle point VHS right at the Fermi level. We further identify a delicate interplay between the magnetisation direction and spin-orbit coupling, demonstrating a new mechanism through which field-driven Lifshitz transitions may arise.

 [1] Carolina A. Marques, Philip A.E. Murgatroyd, et al., “Spin-orbit coupling induced Van Hove singularity in proximity to a Lifshitz transition in Sr₄Ru₃O₁₀”,  arXiv:2303.05587, Oct. 2023.