Session F63: Metal-Insulator Transitions: Experiment

The insulator-metal transition of alkali-metal-doped fullerene

Quantum many-body interactions trigger new and unexpected emergent phenomena, such as superconductivity and magnetism, even in weak correlation materials, as recently exploited in twisted bilayer graphene. One classical example is superconductivity in Alkali-doped fullerides (e.g. K₃C₆₀) where conventional BCS theory and unconventional Mott physics meet. Despite a large number of works, the origin and the detailed mechanism of such a state remain still elusive. Here, by using angle-resolved photoemission spectroscopy on fullerene films grown on Bi₂Se₃ substrate, we study the insulator-metal transition derived by doping. With a controllable potassium doping mechanism, doping dependence of the electronic band structures is investigated.   

Role of grain boundaries in the insulator-to-metal transition in VO2

Vanadium dioxide (VO₂) undergoes an insulator-to-metal transition (IMT) at 340K that can be triggered by Joule heating from applied voltage. The large change in resistivity of up to five orders of magnitude and high-speed switching time make VO₂ an attractive material for nanoelectronics. In the voltage-triggered IMT, grain boundaries in a polycrystalline VO₂ film can affect the IMT properties at the nanoscale. Here we use COMSOL Multiphysics, a finite element analysis software, to simulate the temperature and IV-characteristics of a range of polycrystalline films with different grain boundary characteristics. We determine how the voltage required to trigger the IMT and temperature distribution depend on grain size, grain boundary thickness, and grain boundary conductivity. We find that tuning the properties of grain boundaries can reduce the IMT voltage by up to 50%.   

Interplay between Photoinduced Charge Injection and Material Defects in VOx/CdS Mott/Photoconducting Heterostructures

The Metal-Insulator Transition (MIT) exhibited by a number of transition metal oxides has garnered much interest for various technological applications, including hardware-level implementation of neuromorphic computing architectures, as it can induce several orders of magnitude change in a material's resistivity. Various electronic degrees of freedom can influence this transition, including the presence of defects and doping. In our study, we couple two forms of doping: selectively engineered defects via ion irradiation and photoinduced carrier injection, to tune and suppress the MIT. We fabricated ion-irradiated VOx thin film devices with a layer of an archetypal photoconductor, Cadmium Sulfide, to study the interplay between photodoping and material defects.  We show that ion irradiation is a controllable mode of selectively engineering defects within vanadium oxides and can be used for tuning transport properties, resistive switching, and the MIT in VOx/CdS heterostructures. We further analyze the influence material defects have on altering the photosensitivity and dynamic response of optoelectronic Mott/Hybrid heterostructures.    

Insulator-Metal-Transition of VO2 with Modified Orbital Occupancy by Octahedral Symmetry

Vanadium dioxide (VO₂) has received much attention due to its insulator-metal-transition (IMT) accompanied by the structural phase transition from an asymmetric to a symmetry octahedral structure near room temperature (~68 ^oC). In the structural aspect, the variation of the asymmetric octahedral structure is known to tune the IMT temperature effectively. However, most studies regarding the effect of the octahedral symmetry carried out on the rutile-like VO₂ having a symmetrical octahedral structure.

In this presentation, we show the correlation between the asymmetry octahedral structure, orbital occupancy, and the IMT temperature of monoclinic VO₂ films with different in-plane compressive strains using x-ray diffraction and x-ray spectroscopy techniques with the theoretical calculation. The octahedral structure with low asymmetry, caused by the in-plane compressive strain, increased the splitting between d_// and d_//^* orbitals and the bandwidth of π^*. The modified orbitals suppressed the hybridization of V 3d - O 2p and subsequently increased interdimer hopping energy, lowering the energy barrier for IMT. As a result, the VO₂ with the low asymmetric octahedral structure has a lower IMT temperature than the VO₂ with a high asymmetric one. These results provide the role of octahedral symmetry in tuning the IMT temperature of VO₂.   

Insulator-to-metal transition in BaCoS2 via chlorine substitution

Our previous work highlighted the role of Jahn-Teller-associated displacements in the insulator-to-metal transition (IMT) in the BaCo_1-xNi_xS₂ system¹. Substitution of chlorine onto the sulfur site provides an alternate route to increasing the electron concentration past the IMT critical point. We leverage density functional theory to predict the charge, spin, and lattice behavior of BaCoS_2-xCl_x. In particular, we examine the influence of chlorine doping on the first-order Jahn-Teller effect arising from partial occupancy of nominally degenerate Co²⁺ d_xz and d_yz orbitals which is responsible for the insulating state in the undoped end member. Analysis of the resulting Jahn-Teller distortion and long-range orbital ordering allows us to compare the doping-induced metallization between the well-studied BaCo_1-xNi_xS₂ system and the novel BaCoS_2-xCl_x system.

¹ Schueller, E. C. et al., “Structural signatures of the insulator-to-metal transition in BaCo_1−xNi_xS₂,” Phys. Rev. Mater. 2020, 4 (10), 104401. 10.1103/PhysRevMaterials.4.104401   

T→0 Metal-Insulator Transition in Bulk (La,R)NiO3 (R = rare-earth) Nickelates: A Bond-Percolation Transition?

We report x-ray diffraction, electrical resistivity, and other data on bulk polycrystalline La_1-xR_xNiO₃ (R=Y, Nd; x = 0-1) in efforts to test the prediction^[1] of a universal T→0 Metal-Insulator (M-I) transition for all (La,R)NiO₃ nearly-perovskite materials. The samples were prepared using a sol-gel precursor that was then subject to high oxygen pressure (150-200 bar), high temperature (950°C-1050°C) conditions, or high quasi-hydrostatic pressures using a hot piston-cylinder apparatus (2.0-2.5 GPa) at high temperatures (900°C-1000°C) for La_1-xY_xNiO₃ (x > 0.2). A bond percolation model for a simple cubic lattice predicts a threshold doping concentration of only x = 0.29 to reduce the bond fraction to the percolation threshold, p_c ≈ 0.249. The concentration-dependent conductivity trends lower than the theoretical predictions^[2]. The correlated bond-breaking considered here may only slightly shift the percolation threshold, but the decrease in the conductance is more dramatic.

[1] Gregorio Ponti, et al. 2021 APS March Meeting, X43.14 (2021), Virtual.

[2] Scott Kirkpatrick, Rev. Mod. Phys. 45, 574 (1973).   

Metal-insulator transition in ultrathin rutile RuO2 films

Emergence of superconductivity in anisotropically strained rutile RuO₂ thin films has shown strain as an effective control knob to induce electronic phases that are absent in the bulk. By controlling film thickness, and thereby epitaxial strain and dimensionality, we reveal a metallic to insulating transition in RuO₂/TiO₂ (110) films with decreasing thicknesses. Atomically-smooth, epitaxial films were grown using solid-source metal-organic molecular beam epitaxy approach. Hall bar devices with current channels along two in-plane directions [001] and [10] revealed anisotropic transport. Quantitative analysis of the temperature-dependent magnetotransport data revealed, the transition from metallic to insulating state is driven via strong electron-electron interactions. By combining transport and spectroscopy measurements, we discuss the interplay between dimensionality, and electron correlation and their influence on the origin of metal-to-insulator transition in RuO₂ films.   

Volatile and Non-volatile insulating state assisted by photoreaction on the hybrid CdS/Nickelates

Rare-earth nickelates host a complex entanglement of various degrees of freedoms which offers a high tunability over several emerging noble phases, realizing a perfect playground for developing hybridized functionalities. Among these nickelates, the PrNiO₃ (PNO) possess a sharp metal to insulator transition (MIT) at ~120K concomitant with paramagnetic (PM) to antiferromagnetic (AFM) phase transition. These transitions are highly tunable upon application of substrate induced strain and growth symmetry. Here we study PNO thin films grown on NdGaO₃ substrate with two different orthorhombic crystal plane orientation, i.e., (101) and (110); PNO(101) and PNO(110), respectively. The optical functionalities of these films are realized with engineering photoconductor (CdS)/PNO hybrid Bi-layers. Depending on the growth symmetry CdS/PNO(101) and CdS/PNO(110) films exhibit two very different low temperature photoinduced non-volatile and volatile changes in the resistivity, respectively. The photoinduced non-volatile state is highly stable and stays intact over 10 hours. We assume that at low temperature AFM insulating phase of PNO, the light induced chemical reactions and photodoping is highly influenced by the direction of spin order which is along orthorhombic (101) direction.   

Imaging stripe phases in Mn- and Ti-doped bilayer calcium ruthenates

As a prototypical transition-metal oxide compound, the bilayer calcium ruthenate Ca₃Ru₂O₇ exhibits exotic electronic phases under various temperatures and magnetic fields. Upon doping with a small concentration of 3d metals on the Ru sites, the quasi-two-dimensional metallic state of Ca₃Ru₂O₇ could transition into a Mott insulating state with a G-type antiferromagnetic order. Using microwave impedance microscopy, we report the nanoscale imaging of metal-insulator phase transition in Mn- and Ti-doped Ca₃Ru₂O₇. For the Ti-doped single crystal, a new stripe phase different from the two terminal phases emerges within a narrow window across the transition [1]. Interestingly, while stripe-like metallic features are present in the Mn-doped sample, their orientation, evolution across the phase transition, and electronic properties are distinct from the Ti-counterparts. Our work highlights the doping effect in complex oxides that critically impacts phase transitions in the parent compound.   

Electron Spin Resonance near the Metal Insulator Transition in Phosphorous doped Si

Phosphorus doped Si offers a platform to study the quantum phase transition from an Anderson localized insulating state to a metallic state from an impurity band. Localized electrons interact through dipolar and exchange interactions while their individual energies are disordered due to the random nature of the doping process. As a result, this system can be used to raise & answer fundamental questions about the nature of thermalization and decoherence in strongly-interacting disordered systems. 

Here, we utilize an ion-implanted layer of phosphorus donors in high-resistivity Si doped near the critical doping density. We fabricate coplanar microwave resonators out of NbTiN. The high-critical field of NbTiN enables electron spin resonance (ESR) measurements in static fields of about 200 mT at mK temperatures. We observe resonant dissipation from the impurity band as well as hyperfine-split localized electrons in a single sample. The impurity band ESR line is motionally narrowed from 25 mK to 1 K. Using Hahn echo techniques, we measure non-exponential energy and phase relaxation from the isolated donors. While these observations can be partly attributed to the inhomogeneities in doping density, a complete picture requires incorporating many-body dynamics of the interacting spins.   

Photo-assisted volatile insulating state in hybrid CdS/Mott heterostructures

The manipulation of the insulating to metal transition (IMT) of strongly correlated oxides is of major interest due to its potential applications in optoelectronics and neuromorphic computing. We have recently showed that the IMT of VO₂, V₂O₃ and V₃O₅ can largely be manipulated through the use of photodoping. We discovered recently large effects in heterostructures which incorporate a photoconducting material (CdS) and a strongly correlated Mott oxide. In this work, we have extended it to a heterostructures containing manganites, e.g., La_0.3Sr_0.7MnO₃ (LSMO), whose insulating state resistance drops notably when exposed to light. We will discuss the possible physical origin of this interesting room temperature, volatile effect.   

Insulator-metal transition in the rutile structure of ultrathin VO2 films on (001) TiO2

The origin of the insulator-metal transition (IMT) in VO₂ is not well understood due to the concurrent monoclinic-rutile transition (MRT). The long-standing dilemma is whether the IMT occurs due to the breakdown of Mott electron-electron correlations or due to the melting of Peierls charge density wave under the influence of controllable parameters. To solve the dilemma, we investigate ultrathin VO₂ films on rutile (001) TiO₂ substrates by using synchrotron XRD, electrical transport, and optical techniques. We reveal that below the critical thickness of 7.5 nm, the VO₂ films persist in the rutile structure before and after the IMT avoiding Peierls ordering and, thus, the MRT. The observed pure electronic Mott IMT holds the key to understanding VO₂ and other phase-change materials and to boosting their application potential.   

Filming the metal-insulator transition - from Tcmaps to machine learning pattern recognition

Over recent years, phase separation has been revealed from the nanometer to micron scales in various transition metal oxides. These maps have offered the possibility to extract and analyze cluster size, critical exponents etc… at snapshot temperatures across the transition. Tracking fine changes in the cluster dynamics at slow temperature sweep has, however, not yet been possible. To do so we have developed a 77K-600K variable temperature autofocus microscope operating in the visible range. Up to 1000 images can be typically measured crossing T_c. We will first review the experimental setup and image analysis developed to complete this study. We will then review key results obtained on Vanadium Dioxide, VO₂: critical temperature maps (T_c maps), ΔT_c hysteretic maps, memory maps, switching maps, resistor network and pattern recognition of clusters using machine learning.   

Electronic structure modification of rare-earth nickelates by hole doping

RENiO3 is a negative charge transfer energy system which exhibits a temperature-driven metal-insulator transition (MIT), which is also accompanied by a bond disproportionation (BD) transition. The insulating phase is gradually suppressed with hole doping. We have investigated the underlying microscopic changes of the electronic structure parameters by employing several techniques on epitaxial thin films. We have found that the doped holes are localized on Ni around the dopant in the doping range where it is insulating. While, as expected the effective charge transfer energy increases with hole doping, our analysis suggests that it still remains negative and hence a BD is expected [1]. Above a critical concentration, the BD phase cannot be sustained and the sample becomes metallic. This study firmly demonstrates that the insulating state that emerges upon hole doping has polaronic distortions that localize the doped hole, while the BD state still exists.

 

[1]. B. Mandal et al., arXiv 1701.06819.   

Spatial Mapping of Ramp Reversal Memory in VO2

We use optical microscopy to image spatial structure of metal and insulator patches as a thin film of VO₂ is repeatedly driven partway through its temperature-driven insulator-to-metal transition. The location and shape of accumulated memory was tracked after each subloop, revealing for the first time the internal structure of the memory effect. A combination of insulator domains appearing at cluster boundaries, as well as large insulator nucleation sites, was identified. Two large temperature sweeps were subsequently completed to reset the ramp reversal memory. Transition temperature maps reveal that memory is surprisingly also stored deep in the insulating and metallic puddles throughout the entire sample surface. We discuss two possible diffusion models to explain these 2D surface memory maps. These results pave the way to enhancing, controlling and manipulating the memory in this material in the near future.