Session G07: Measuring and Controlling Correlated Insulators

Tuning the insulator metal transition in rare earth nickelates through dynamic electrochemical ion insertion

The rare earth nickelates have received renewed attention due to the discovery of superconductivity in infinite layered structures under substitutional doping[1] and the observation of widely tunable electronic behavior in perovskite structures for use in analog memory devices[2]. Recent work has demonstrated that interstitial dopants (H, alkali metals) can be introduced into nickelates to change the room temperature resistance by 10⁶ -10⁸. However, the evolution of the bond disproportionation transition as a function of interstitial dopants has not been reported and the doping fraction leading to rich correlated electronic behavior is often unknown. Therefore, the electronic phase diagram in nickelate compounds as a function of interstitial doping is of interest. Here, we carried out lithium doping of PrNiO₃ using a dynamic electrochemical process. We constructed electrochemical cells using epitaxial thin films as electrodes and then insert lithium using an electrolyte. For Li_xPrNiO₃, we find that increased lithium doping interrupts bond disproportionation causing a reduction in the ground state resistivity at small fractions 00.25 we observe the disproportionation transition to be destroyed and fully insulating type behavior is observed over T= 5-300K. Raman spectroscopy reveals that lithium introduces structural changes that affect A1g modes. Density functional theory calculations confirm the disruption to bond disproportionation with an initial reduction in the bandgap at small fractions and an increase at larger fractions. The results point to interstitial doping as a powerful method to synthesize new phases in strongly correlated systems.

[1] D. Li et al., "Superconductivity in an infinite-layer nickelate," Nature, vol. 572, no. 7771, pp. 624-627, Aug 2019, doi: 10.1038/s41586-019-1496-5.

[2] H. T. Zhang et al., "Reconfigurable perovskite nickelate electronics for artificial intelligence," Science, vol. 375, no. 6580, pp. 533-539, Feb 4 2022, doi: 10.1126/science.abj7943.   

Crystal Structure and Bonding Patterns Leading to High Temperature Magnetic Order and Spin Canting in the Ternary Nitride MnSiN2

Ternary nitride materials possess a wide range of structures, compositions, and properties, making them viable candidates for novel correlated electron states. Even though the N³⁻ ion creates a distinctive chemical bond with open d-shell transition metals, magnetic compounds rich in nitrogen are still relatively unexplored. Here, we discuss the magnetic ternary nitride MnSiN₂, a canted antiferromagnet with a high T_N=443K. Neutron diffraction, symmetry group analysis, density functional theory calculations, lead to a model where the spins are rotated 10^o away from the crystallographic axis, and have a 0.6⁰  canting relative to each other. We also discuss how the sp³ bonding characterizing the SiN₄ tetrahedra, and crystal structure of this material, leads to the spin physics observed in this material. [1]

[1] Linus Kautzsch, Alexandru B. Georgescu, Danilo Puggioni, Greggory Kent, Keith M. Taddei, Aiden Reilly, Ram Seshadri, James M. Rondinelli, and Stephen D. Wilson, Physical Review Materials (Editors' Suggestion), Phys. Rev. Materials 7, 104406, 2023   

Dynamic Spectral Weight Transfer in the Multi-Orbital Hatsugai-Kohmoto Model

The Hatsugai-Kohmoto (HK) model provides a rich testbed for Mott physics due to its exact diagonalizability in momentum space [1,2]. Extending HK to multiple orbitals introduces non-trivial commutators between the interaction and kinetic terms. We show that the orbital HK model exhibits Dynamical Spectral Weight Transfer under doping away from the Mott insulating phase, which indicates the presence of delocalized modes. We postulate that the orbital HK model provides a bridge between band HK and the Hubbard model due to the introduction of momentum state mixing.

1) Phillips, P.W., Yeo, L. & Huang, E.W. Exact theory for superconductivity in a doped Mott insulator. Nat. Phys. 16, 1175–1180 (2020)

2) Mai, P., Zhao, J., Feldman, B.E. et al. 1/4 is the new 1/2 when topology is intertwined with Mottness. Nat Commun 14, 5999 (2023)   

Probing the susceptibility of the excitonic insulator candidate Ta2NiSe5 through the elastocaloric effect

The excitonic insulator, a theorized phase in which electron-hole pairs form a condensate, analogous to the Cooper pairs condensate in superconductivity, has long been searched for in bulk materials, yet scant experimental evidence exists of its appearance. Ta₂NiSe₅, a near zero-gap semiconductor at high temperatures, exhibits a phase transition at 328K for which both transport and spectroscopic measurements indicate a gap forms which is consistent with a semiconductor to excitonic insulating phase transition. However, a structural orthorhombic to monoclinic q=0 phase transition occurs simultaneously, raising questions as to whether the driving mechanism for the phase is from electronic excitonic fluctuations or from purely structural degrees of freedom. Here we present elastocaloric measurements of the material, which use strain as a tuning parameter to change the entropy of the system and hence probe order parameters which couple to strain (and their associated susceptibilities). This technique effectively isolates the electronic fluctuations which drives the phase transition, and we measure the corresponding susceptibility which can be described by a Curie-Weiss form with a high Weiss temperature (T*=300K), consistent with non-structural modes being the driving mechanism of the phase transition.   

A Time-Resolved Second Harmonic Generation Study of a Cuprate Mott Antiferromagnet

Here we report a study of the cuprate antiferromagnetic Mott insulator Ba2Cu3O4Cl2 using rotational anisotropy second harmonic generation (RA-SHG), which is sensitive to the underlying lattice and magnetic symmetries of a crystal. Our data reveal a four-fold symmetric RA-SHG pattern above the Neel temperature dominated by electric quadrupole radiation. Below the Neel temperature, a magnetic-dipole SHG process emerges that breaks the four-fold symmetry of the underlying lattice, consistent with earlier reports of a canted antiferromagnetic state. Pump-probe time-resolved SHG experiments reveal that photo-generated carriers, populated by optically exciting the crystal with a photon energy above the Mott gap, rapidly quench the antiferromagnetic order. The far-from-equilibrium dynamics as a function of temperature and pump fluence will be discussed.   

Mott insulating negative thermal expansion perovskite TiF3

We characterize perovskite TiF₃, a material which displays significant negative thermal expansion at elevated temperatures above its cubic-to-rhombohedral structural phase transition at 330 K. We find the optical response favors an insulating state in both structural phases, which we show can be produced in density functional theory calculations only through the introduction of an on-site Coulomb repulsion. Analysis of the magnetic susceptibility data gives a S=1/2 local moment per Ti⁺³ ion and an antiferromagnetic exchange coupling. Together, these results show that TiF₃ is a strongly correlated electron system, a fact which constrains possible mechanisms of strong negative thermal expansion in the Sc_1-xTi_xF₃ system. We consider the relative strength of the Jahn-Teller and electric dipole interactions in driving the structural transition.   

Order and strain dynamics during electrical melting of charge density waves in TaS2 visualized by ultrafast electron microscopy

Electrically induced transitions in charge density waves (CDWs) have proven valuable for understanding the underlying electron and lattice interactions as well as accessing novel transition pathways and non-equilibrium states. However, the transition mechanisms are often debated as electric field, current, carrier injection, heat, and strain can all contribute and play varying roles across length and time scales. I will present our recent experiments which visualize and disentangle these factors for transitions in the room temperature, nearly commensurate CDW state in 1T-TaS₂. We employed a unique, new ultrafast electron microscopy capability at the Center for Nanoscale Materials at Argonne National Laboratory to record atomic and mesoscopic structural dynamics during electrical pulse stimulation with nanosecond-nanometer spatiotemporal resolution. Recording the order parameter following voltage pulses down to 20 ns duration, we find the transition thresholds and dynamics are consistent with a self-heating mechanism, showing robustness to applied fields and injected currents. In addition, time-resolved imaging reveals heterogeneous strain dynamics, including emergence of coherent acoustic resonances for sub-100 ns pulses.   

Evolution of elastocaloric signature across x of cantidate bulk excitonic insulator Ta_2Ni(Se_(1-x)Sx)_5

The near zero-gap semiconductor Ta2NiSe5 is a quasi 1-d van der Waals layered material that undergoes a structural transition at Tc~328K, which lowers the crystal symmetry from orthorhombic to monoclinic. It has been argued that this structural transition is driven by an excitonic instability, which binds the electrons and holes whose wavefunctions can only be hybridized in the monoclinic phase. Sulfur doping on the Se site is expected to increase the gap, which also suppresses the lattice instability, leading to a reduction of Tc. In this work, we report the study of elastocalroic effect of Sulfer doped Ta2NiSe5, which probes the excitonic susceptibility via the change of temperature induced by strain in the adiabatic condition. This work is intended to disentangle the lattice and electronic contributions to the symmetry breaking phase transition in this system.   

Exploring strongly correlated 2D insulators via millikelvin far-infrared spectroscopy

Quantum materials associated with strongly correlated gaps may show unusual electronic, magnetic and optical properties, whose carefully examinations can help reveal the nature of the underlying phases. The energy scale of these gaps is often in the far-infrared (THz) regime and many phenomena are optimized at sub-kelvin temperatures. In this talk, we explore the characterization of strongly correlated gaps and low energy excitations in selected strongly correlated 2D insulators, using our recently developed milli-kelvin FIR spectroscopy instrument. We focus on 2D transition metal chalcogenides, such as thin layers of 1T-TaSe2, where a correlated gap depends strongly on the number of layers.    

Ultrafast control of the electronic structure of TiSe2 with light

TiSe₂ is a prototypical charge density wave (CDW) material, featuring a complex interplay of many-body interactions such as electron-electron, electron-lattice and electron-hole. Despite being studied for decades, it is still not clear what role these different degrees of freedoms play in stabilizing the CDW phase and how they are related to the pressure and doping induced superconductivity in this material. The ideal tool to study such a strongly correlated material is time and angular resolved photoelectron spectroscopy (trARPES), as it allows to not only map the response of the CDW order parameter to optical excitation, but also to directly visualize changes in many body interactions in the form of band renormalizations. In this work, we will show how the electronic structure of TiSe₂ can be precisely controlled by optical excitation as a function of time and fluence. Our results are then interpreted with the aid of DFT calculations.    

Resistive switching localization and tuning via selective ion irradiation

Various materials exhibit resistive switching (RS), a useful feature which lends well to the development of novel bioinspired electronic devices, notably artificial neurons and synapses for neuromorphic computing. This effect often manifests itself through the percolation of conducting filaments or the formation of transverse barriers. The location and switching parameters of RS are often impacted by inherent material defects which pose a challenge for scalability. By selectively engineering defects in VO_x and LSMO using a focused ion beam, we report a novel method for locally tuning the electronic properties (i.e. conductivity and metal-insulator transition temperature) of a material and by extension, controlling the location and geometry of RS. In addition to confining the conducting filament to the irradiated region, we observe a greater than 3 orders of magnitude reduction in RS power. Our work demonstrates that local ion irradiation impacts the electronic distribution and structure of a material, and is an efficient tool for fine-tuning material properties related to RS. This offers promising avenues for new energy-efficient biomimetic circuitry.   

Role of magnetic ions in the thermal Hall effect of the paramagnetic insulator TmVO4

In a growing number of materials, phonons have been found to generate a thermal Hall effect, but the underlying mechanism remains unclear. Inspired by previous studies that revealed the importance of Tb³⁺ ions in generating the thermal Hall effect of Tb₂Ti₂O₇, we investigated the role of Tm³⁺ ions in TmVO₄, a paramagnetic insulator with a different crystal structure. We observe a negative thermal Hall conductivity in TmVO₄ with a magnitude such that the Hall angle, |K_xy / K_xx|, is approximately 1 x 10^-3 at H = 15 T and T = 20 K, typical for a phonon-generated thermal Hall effect. In contrast to the negligible K_xy found in Y₂Ti₂O₇, we observe a negative K_xy in YVO₄ with a Hall angle of magnitude comparable to that of TmVO₄. This shows that the Tm³⁺ ions are not essential for the thermal Hall effect in this family of materials. Interestingly, at an intermediate Y concentration of 30 % in Tm_1-xY_xVO₄, 

K_xy was found to have a positive sign, pointing to the possible importance of impurities in the thermal Hall effect of phonons.   

Role of nanoscale heterogeneties on the metal to insulator transition in rare earth nickelates

Rare earth nickelates RNiO3 display an insulator to metal transition (MIT) which is accompanied by a magnetic transition, charge ordering, and a crystal structure change from orthorhombic low temperature to monoclinic high temperature state. While this system has been widely studied, the nature of the fluctuation across the transition, and the associated time- and length-scales is not known. Spontaneous fluctuations are important components for stabilizing topological magnetic structures such as skyrmions in quantum materials. However, the dynamic susceptibility of the nickelates remains relatively unexplored, and the role played by nanoscale phenomena such as domain-wall formation and motion, local strain fields and phase separation in underlying pathways of MIT is not well-understood. In our work, we focus on understanding the role of nanoscale heterogeneities and their fluctuations in rare earth nickelates by employing x-ray photon correlation spectroscopy (XPCS). Our XPCS measurements on NdNiO3 thin films, show complex evolution of fluctuations dependent upon temperature and wavevector q. We also observe unexpected non-equilibrium dynamics and suggests a new approach to understanding these materials.   

Thermal conductivity study of charge-neutral excitations in topological Kondo insulator YbB12

YbB₁₂ is a topological Kondo insulator candidate with recent reports of charge-neutral excitations and quantum oscillations at the lowest temperatures [1-3]. These reports of Fermi-like behavior are unconventional in this bulk insulator, motivating further study of such exotic bulk insulating phenomena. In particular, the experimental study of charge-neutral excitations in YbB₁₂ assists in characterizing these itinerant excitations in order to reliably realize them in real materials. Thermal conductivity is a useful tool for measuring the mobility of such excitations in an insulating bulk.

 

We report thermal conductivity measurements of two YbB₁₂ single crystals at temperatures from 0.1 K to 50 K and fields up to 12 T. A comparison of thermal conductivity measurements suggests that novel excitations may be obscured by phonon scattering of a non-magnetic origin. Therefore, a careful analysis of phonon scattering sources is required to accurately identify these charge-neutral excitations in thermal transport measurements.

 

[1] H. Liu et al., J. Phys: Condens. Matter 30, 16LT01 (2018).

[2] Y. Sato et al., Nat. Phys. 15, 954-959 (2019).

[3] Xiang et al., Science 362, 65–69 (2018).