Field Induced Metal Insulator Research Articles

Quantum linear magnetoresistance and magnetic-field-induced metal–insulator transition in the Weyl semimetal candidate HgSe

In ultrapure single HgSe crystal, a large linear magnetoresistance with a record magnitude of 15 000% in mercury chalcogenides was discovered in a quantum limit at the magnetic field of 12 T in the temperature range of 20–40 K. The effect was described in terms of Abrikosov's theory of quantum linear magnetoresistance, thus providing its first experimental confirmation. In the temperature range of 0.4–4.2 K, a magnetic field-induced metal–insulator transition appeared in the ultrapure HgSe. Estimates show that the critical magnetic field of this transition of 6.8 T satisfies the criterion for stabilization of a condensed Wigner phase in the quantum limit.

Field-induced Insulator–Metal Transition in EuTe2

Field-induced Insulator–Metal Transition in EuTe₂

Tunable electrical field-induced metal-insulator phase separation in LiCoO2 synaptic transistor operating in post-percolation region

While mixed ionic-electronic conductors with metal-insulator transition (MIT) are promising candidates for designing neuromorphic computing hardware, the fundamentals of resistive switching in these materials are yet to be well understood. This work studies the switching mechanism of the three-terminal nonvolatile redox transistor (NVRT) containing the LiCoO2 (LCO) channel layer with tunable preferred crystallographic orientation. We used atomic force microscope nanotomography to reconstruct the 3D conductance map of NVRTs, that reveals the applied gate electric-field induces the MIT via reversible phase separation in the LCO channel layer, with the nonequilibrium thermodynamics analytical model providing validation to this mechanism. By operating in the post-percolation region, the memory properties can continuously adjust the conductance states of NVRTs. The percolation conductance mechanism via the metallic LCO phase ensures the exceptional linearity and reproducibility of conductance modulation, whereas the field-, rather than current-, induced transition results in the low energy consumption replicating key features of the living neural cells.

Large magneto-thermoelectric effect on the verge of metal–insulator and topological transitionsin pyrochlore iridates

We investigate the Seebeck and Nernst effects of pyrochlore iridium oxides (Nd1−xPrx)2Ir2O7 across the thermally induced and magnetic field-induced metal–insulator and topological transitions. Nd2Ir2O7 exhibits the salient temperature dependence of the Seebeck coefficient accompanied by the enhancement of the Nernst effect in the vicinity of the thermal magnetic transitions. Moreover, the Seebeck coefficient shows a remarkable magnetic field hysteresis with the differential magnitude reaching as large as 110 μV/K, as the conductive magnetic domain walls are generated/annihilated by the external field. For x = 0.5, the Nernst signal increases rapidly across the field-induced metal–insulator transitions, exceeding the values reported in existing ferromagnetic oxides. These findings indicate that the thermoelectric effects increase significantly near the topological electronic phase transitions in strongly correlated systems, providing a new guideline for thermoelectric material design.

Grain-Size-Induced Collapse of Variable Range Hopping and Promotion of Ferromagnetism in Manganite La0.5Ca0.5MnO3

Among transition metal oxides, manganites have attracted significant attention because of colossal magnetoresistance (CMR)—a magnetic field-induced metal–insulator transition close to the Curie temperature. CMR is closely related to the ferromagnetic (FM) metallic phase which strongly competes with the antiferromagnetic (AFM) charge ordered (CO) phase, where conducting electrons localize and create a long range order giving rise to insulator-like behavior. One of the major open questions in manganites is the exact origin of this insulating behavior. Here we report a dc resistivity and magnetization study on manganite La1−xCaxMnO3 ceramic samples with different grain size, at the very boundary between CO/AFM insulating and FM metallic phases x=0.5. Clear signatures of variable range hopping (VRH) are discerned in resistivity, implying the disorder-induced (Anderson) localization of conducting electrons. A significant increase of disorder associated with the reduction in grain size, however, pushes the system in the opposite direction from the Anderson localization scenario, resulting in a drastic decrease of resistivity, collapse of the VRH, suppression of the CO/AFM phase and growth of an FM contribution. These contradictory results are interpreted within the standard core-shell model and recent theories of Anderson localization of interacting particles.

Electric Field-Induced Metal-Insulator Phase Separation in Lithium Titanates: Tuning Symmetry By Lithiation

Specialized hardware for neural networks requires materials with tunable symmetry, retention and speed at low power consumption. The vast majority of memristor are based on two types of ions: either oxygen vacancy migration, in the so-called Valence Change Memories (VCM), or a metal cation, usually Ag+ and Cu2+, in the so-called Electrochemical Metallization Cells (ECM). Despite their excellent performance, their widespread implementation in today’s integrated circuits is delayed due to the need to address cycle-to-cycle and device-to-device variabilities while circumventing electroforming and asymmetry, which are inherent issues associated to the filamentary nature of the switching mechanism. Recently, Li-ion is emerging as an alternative, given the higher diffusivity of Li+ when compared to oxygen, and the ability of Li-oxides solid state conductors to accumulate and deplete lithium at the interfaces and bulk. We have recently proposed lithium titanates, originally developed as Li-ion battery anode materials, as promising candidates for memristive-based neuromorphic computing hardware[1]. In this presentation, we will discuss the non-volatile, non-filamentary bipolar resistive switching characteristics of lithium titanates compounds, Li4+3xTi5O12, as a function of the lithiation degree. We have employed a recently proposed strategy to overcome lithium loss during thin film deposition and finely control the final lithiation degree of the films[2] to create stoichiometrically lithiated Li4Ti5O12 spinel phase and a highly lithiated Li7Ti5O12 rock- salt phase memristive devices. By using ex- and in-operando spectroscopy to monitor the Lithium filling and emptying of structural positions during electrochemical measurements, we investigate the controlled formation of a metallic phase (Li7Ti5O12) percolating through an insulating medium (Li4Ti5O12) with no volume changes under voltage bias, thereby controlling the spatially averaged conductivity of the film device. We present a theoretical model to explain the observed hysteretic switching behavior based on electrochemical nonequilibrium thermodynamics, in which the metal-insulator transition results from electrically driven phase separation of Li4Ti5O12 and Li7Ti5O12. Permittivity enhancement drives lithium ions to regions of high electric field intensity, which become metallic filaments above a critical applied bias, and the ions relax back to their low-conductivity initial state at lower voltages. One of the most striking outcomes is that the metal-insulator transition of llithium titanate can be uniquely modulated for neuromorphic computing purposes, such as control of the neural pulse train symmetry in conductance and the resistance on-to-off ratio, simply by adjusting the lithium stoichiometry and phase pattern of the films. We report ability of highly lithiated phase of Li7Ti5O12 for Deep Neural Network applications, given the large retentions and symmetry, and opportunity for the low lithiated phase of Li4Ti5O12 towards Spiking Neural Network applications, due to the shorter retention and large resistance changes. Our findings pave the way for lithium oxides to enable thin-film memristive devices with adjustable symmetry and retention.

Field-Induced Metal–Insulator Transition in β-EuP3*Supported by the National Natural Science Foundation of China (Grant Nos. U1832214 and 11774007), the National Key R&D Program of China (Grant No. 2018YFA0305601) and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB28000000). The experimental and theoretical work at Princeton University was supported by the Gordon and Betty Moore Foundation

Metal–insulator transition (MIT) is one of the most conspicuous phenomena in correlated electron systems. However such a transition has rarely been induced by an external magnetic field as the field scale is normally too small compared with the charge gap. We present the observation of a magnetic-field-driven MIT in a magnetic semiconductor β-EuP3. Concomitantly, we find a colossal magnetoresistance in an extreme way: the resistance drops billionfold at 2K in a magnetic field less than 3T. We ascribe this striking MIT as a field-driven transition from an antiferromagnetic and paramagnetic insulator to a spin-polarized topological semimetal, in which the spin configuration of Eu2+ cations and spin-orbital coupling play a crucial role. As a phosphorene-bearing compound whose electrical properties can be controlled by the application of field, β-EuP3 may serve as a tantalizing material in the basic research and even future electronics.

Fermi surface topology and large magnetoresistance in the topological semimetal candidate PrBi

We report a detailed magnetotransport study on single crystals of PrBi. The presence of $f$-electrons in this material raises the prospect of realizing a strongly correlated version of topological semimetals. PrBi shows a magnetic field induced metal insulator transition below $T \sim 20$ K and a very large magnetoresistance ($\approx 4.4 \times 10^4~$) at low temperatures ($T= 2$ K). We have also probed the Fermi surface topology by de Haas van Alphen (dHvA) and Shubnikov de Haas (SdH) quantum oscillation measurements complimented with density functional theory (DFT) calculations of the band structure and the Fermi surface. Angle dependence of the SdH oscillations have been carried out to probe the possible signature of surface Dirac fermions. We find three frequencies corresponding to one electron ($\alpha$) and two hole ($\beta$ and $\gamma$) pockets in experiments, consistent with DFT calculations. The angular dependence of these frequencies is not consistent with a two dimensional Fermi surface suggesting that the transport is dominated by bulk bands. Although the transport properties of this material originate from the bulk bands, the high mobility and small effective mass are comparable to other compounds in this series proposed as topologically nontrivial.

High-field magnetoresistance of graphite revised

A detailed magnetoresistance study of bulk and microflake samples of highly oriented pyrolytic graphite with a thickness of 25 $\mu$m to 23~nm reveals that the usually observed field-induced metal-insulator and electronic phase transitions vanish in thinner samples. The observed suppression is accompanied by orders of magnitude decrease of the magnetoresistance and of the amplitude of the Shubnikov-de-Haas oscillations. The overall behavior is related to the decrease in the quantity of two-dimensional interfaces between crystalline regions of the same and different stacking orders present in graphite samples. Our results indicate that these field-induced transitions are not intrinsic to the ideal graphite structure and, therefore, a relevant portion of the published interpretations should be reconsidered.

Insulator–metal transition induced by electric voltage in a ruthenate Mott insulator

We report the observation of electric-voltage induced insulator–metal phase transition in a ruthenate Mott insulator Ca3(Ru0.9Ti0.1)2O7. We show that the electric field effect dominates and leads to a sharp phase transition at measurement temperatures far below the Mott transition, whereas the thermal effect becomes more significant and broadens the phase transition as the measurement temperature approaches the insulator–metal transition. The electric field induced insulator–metal transition is presumably attributed to the avalanche breakdown of the correlated insulating state when driven out of equilibrium. This work highlights the strategy of using electric voltage to control the phase transition of this system in addition to other nonthermal parameters such as magnetic field and pressure reported previously.

Magnetic field induced metal–insulator transition in single nodal ring topological semimetals

Our theoretical investigation indicates that an applied magnetic field can open a gap between the conduction and valence bands of a nodal line semimetal (NLSM), though it is a kind of gapless material in the absence of a magnetic field. The emerging bandgap depends sensitively on the strength and orientation of the magnetic field which implies a tunable and large anisotropy of magnetoresistance in such kinds of topological materials. Following such a theoretical finding, we predict that in some candidates of NSLMs with a single nodal ring, such as the materials of CaP3 family, a transition between metallic and insulating states driven by a magnetic field is possibly observed experimentally. Consequently, a magnetic field can be viewed as a novel mechanism for metal–insulator transition of solid materials, in additional to the well-known conventional ones such as the Anderson and Mott transitions.

Giant positive magnetoresistance and field-induced metal insulator transition in Cr2NiGa

We report the magneto-transport properties of the newly synthesized Heusler compound Cr2NiGa which crystallizes in a disordered cubic B2 structure belonging to the Pmm space group. The sample is found to be paramagnetic down to 2 K with metallic characteristics. On application of a magnetic field, a significantly large increase in resistivity is observed which corresponds to magnetoresistance as high as 112% at 150 kOe of field at the lowest temperature. Most remarkably, the sample shows a negative temperature coefficient of resistivity below about 50 K under the application of field ⩾80 kOe, signifying a field-induced metal to ‘insulating’ transition. The observed magnetoresistance follows Kohler’s rule below 20 K indicating the validity of the semiclassical model of electronic transport in metals with a single relaxation time. A multi-band model for electronic transport, originally proposed for semimetals, is found to be appropriate to describe the magneto-transport behavior of the sample.

Electric field induced metal–insulator transition in VO2 thin film based on FTO/VO2/FTO structure

A VO2 thin film has been prepared using a DC magnetron sputtering method and annealing on an F-doped SnO2 (FTO) conductive glass substrate. The FTO/VO2/FTO structure was fabricated using photolithography and a chemical etching process. The temperature dependence of the I–V hysteresis loop for the FTO/VO2/FTO structure has been analyzed. The threshold voltage decreases with increasing temperature, with a value of 9.2V at 20°C. The maximum transmission modulation value of the FTO/VO2/FTO structure is 31.4% under various temperatures and voltages. Optical modulation can be realized in the structure by applying an electric field.

Magnetic field induced insulator-metal transition in nanocrystalline Pr0.67Ca0.33MnO3 compounds: Evidence of large temperature co-efficient of resistance

We report the electronic transport, magneto-transport, and magnetic properties of nanocrystalline Pr0.67Ca0.33MnO3 compound. A magnetic field induced insulator—metal transition appears for the external magnetic field higher than 50 kOe. We have obtained large value of the temperature coefficient of resistance (TCR) along with magnetoresistance and field coefficient of resistance (FCR). The value of TCR is 135%/K at 48 K. The calculated magnetoresistance is about −9.8 × 107% for 70 kOe and maximum FCR is about 320%/kOe around 75 K. Due to the application of the external magnetic field, charge ordered state of the compound is destabilized leading to such large values of TCR and FCR. Large values of TCR and FCR along with the large magnetoresistance exhibited by the material is interesting from the application point of view.

Magnetic-Field-Induced Insulator–Metal Transition in (Pr1−yYy)0.7Ca0.3CoO3 at Ultrahigh Magnetic Fields

The (Pr1−yYy)0.7Ca0.3CoO3 (y ≥ 0.0625) compounds show a first-order metal–insulator (MI) transition and simultaneous spin-state (SS) transition upon a change in the temperature. We have measured the electrical resistivity of the samples with y = 0.075 (the transition temperature, TMI = 64 K) and 0.10 (TMI = 93 K) in ultrahigh magnetic fields of up to 120 T using a contactless radio-frequency transmission technique. We observed a magnetic-field-induced insulator–metal (IM) transition with large hysteresis below TMI and obtained the temperature–magnetic field phase diagrams. A possible mechanism of the field-induced IM and SS transitions is discussed.

Switching mechanism in two-terminal vanadium dioxide devices

Two-terminal thin film VO2 devices show an abrupt decrease of resistance when the current or voltage applied exceeds a threshold value. This phenomenon is often described as a field-induced metal–insulator transition. We fabricate nano-scale devices with different electrode separations down to 100 nm and study how the dc switching voltage and current depend on device size and temperature. Our observations are consistent with a Joule heating mechanism governing the switching. Pulsed measurements show a switching time to the high resistance state of the order of one hundred nanoseconds, consistent with heat dissipation time. In spite of the Joule heating mechanism which is expected to induce device degradation, devices can be switched for more than 1010 cycles making VO2 a promising material for nanoelectronic applications.

Unravelling the switching mechanisms in electric field induced insulator–metal transitions in VO2 nanobeams

We studied insulator–metal transitions in VO2 nanobeams for both abrupt and gradual changes in applied electric fields. Based on the observations, the Poole–Frenkel effect explained the abrupt transition, while the gradual case is found to be dominated by the Joule heating phenomenon. We also carried out power model and finite element method based simulations which supported the Joule heating phenomena for gradual transition. An in-principle demonstration of the Poole–Frenkel effect, performed using a square voltage pulse of 1 µs duration, further confirms the proposed insulator–metal transition mechanism with a switching time in the order of 100 ns. Finally, conductivity variations introduced via rapid thermal annealing at various temperatures validate the roles of both Joule heating and Poole–Frenkel mechanisms in the transitions.

Nanosize effects on the magnetic field induced transitions in La0.67−xEuxCa0.33MnO3 perovskite manganite

The nanosize effects on magnetic field induced transitions in La0.67−xEuxCa0.33MnO3 (x=0.25 and 0.27) system are presented in this paper. The reduction in the particle size of the system shows drastic effects on the electrical transport properties leading to robustness of the charge ordering phenomenon. The metal–insulator transition found in bulk materials at low magnetic fields disappeared in nanoparticles of the same material and a high field induced metal–insulator transition emerged at lower temperatures. These results manifest a strong correlation between the chemical pressures induced by doping of various ions at A-site and nanosize related phenomenon.

Field-induced insulator—metal—insulator transitions in low-energy H+2 ion-irradiated epitaxial La2/3Ca1/3MnO3 thin films

Epitaxial La2/3Ca1/3MnO3 thin films grown on LaAlO3 (001) substrates were irradiated with low-energy 120-keV H+2 ions over doses ranging from 1012 ions/cm2 to 1017 ions/cm2. The irradiation suppresses the intrinsic insulator—metal (I—M) transition temperature and increases the resistance by reducing the crystallographic symmetry of the films. No irradiation-induced columnar defects were observed in any of the samples. The specific film irradiated at a critical dose around 8 × 1015 ions/cm2 is in a threshold state of the electric insulator where the I—M transition is absent. In an external field of 4 T or higher, the I—M transition is restored and thus an enormous magnetoresistance is observed, while a negative temperature coefficient resumes as the temperature is reduced further. Magnetic relaxation behavior is confirmed in this and other heavily irradiated samples. The results are interpreted in terms of the displacement of oxygen atoms provoked by ion irradiation and the resulting magnetic glassy state, which can be driven into a phase coexistence of metallic ferromagnetic droplets and the insulating glass matrix in a magnetic field.

Suppression of itinerant ferromagnetism and a spin-glass-like state in Zn substitutedLa0.67Ca0.33MnO3

We examine the magneto-transport, static and dynamic magneticresponses of the single phase glassy insulating compoundLa0.67Ca0.33Mn1 − xZnxO3 (x = 0.10). Thiscompound undergoes a field induced metal–insulator transition and exhibits colossal magnetoresistancefor H ≥ 2 T. Many experimental features, such as a frequency dependent cusp inthe in-phase component of linear ac susceptibility at a temperatureT′f, a broadmaximum (at Ta) in the ZFC cooled thermo-magnetization close toT′f, large thermo-magneticirreversibility below Ta, non-saturation of magnetization at 5 K even under 12 T with a characteristic S shape in the virgincurve, monotonic increase of the coercivity at low fields under ZFC condition as one approaches theT′f, logarithmic relaxation of thermo-magnetization and ageing effects belowTa and a characteristic maximum in the magnetic viscosity below the spin-glass transition temperature(Tg) indicate a spin-glass-like state at low temperatures. This is further substantiated by theabsence of second order non-linear ac susceptibility, a characteristic negative maximum atTg in third order non-linearac susceptibility (χ′3), thecritical divergence of χ′3 as a function of temperature and ac probe field and an unusually large linear term in thelow temperature specific heat.