Field-induced Metal-insulator Transition Research Articles

Field-induced Insulator–Metal Transition in EuTe2

Field-induced Insulator–Metal Transition in EuTe₂

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.

Field-induced valence fluctuations in YbB12

We performed high-magnetic-field ultrasonic experiments on YbB$_{12}$ up to 59 T to investigate the valence fluctuations in Yb ions. In zero field, the longitudinal elastic constant $C_{11}$, the transverse elastic constants $C_{44}$ and $\left( C_{11} - C_{12} \right)/2$, and the bulk modulus $C_\mathrm{B}$ show a hardening with a change of curvature at around 35 K indicating a small contribution of valence fluctuations to the elastic constants. When high magnetic fields are applied at low temperatures, $C_\mathrm{B}$ exhibits a softening above a field-induced insulator-metal transition signaling field-induced valence fluctuations. Furthermore, at elevated temperatures, the field-induced softening of $C_\mathrm{B}$ takes place at even lower fields and $C_\mathrm{B}$ decreases continuously with field. Our analysis using the multipole susceptibility based on a two-band model reveals that the softening of $C_\mathrm{B}$ originates from the enhancement of multipole-strain interaction in addition to the decrease of the insulator energy gap. This analysis indicates that field-induced valence fluctuations of Yb cause the instability of the bulk modulus $C_\mathrm{B}$.

Giant Seebeck effect across the field-induced metal-insulator transition of InAs

Lightly doped III–V semiconductor InAs is a dilute metal, which can be pushed beyond its extreme quantum limit upon the application of a modest magnetic field. In this regime, a Mott-Anderson metal–insulator transition, triggered by the magnetic field, leads to a depletion of carrier concentration by more than one order of magnitude. Here, we show that this transition is accompanied by a 200-fold enhancement of the Seebeck coefficient, which becomes as large as 11.3 mV K−1approx 130frac{{k}_{B}}{e} at T = 8 K and B = 29 T. We find that the magnitude of this signal depends on sample dimensions and conclude that it is caused by phonon drag, resulting from a large difference between the scattering time of phonons (which are almost ballistic) and electrons (which are almost localized in the insulating state). Our results reveal a path to distinguish between possible sources of large thermoelectric response in other low-density systems pushed beyond the quantum limit.

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 (Grant Nos. GBMF4547 and GBMF9461/Hasan)

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.

Multi-terminal Transistor-like Devices Based on Strongly Correlated Metallic Oxides for Neuromorphic Applications.

Memristive devices are attracting a great attention for memory, logic, neural networks, and sensing applications due to their simple structure, high density integration, low-power consumption, and fast operation. In particular, multi-terminal structures controlled by active gates, able to process and manipulate information in parallel, would certainly provide novel concepts for neuromorphic systems. In this way, transistor-based synaptic devices may be designed, where the synaptic weight in the postsynaptic membrane is encoded in a source-drain channel and modified by presynaptic terminals (gates). In this work, we show the potential of reversible field-induced metal-insulator transition (MIT) in strongly correlated metallic oxides for the design of robust and flexible multi-terminal memristive transistor-like devices. We have studied different structures patterned on YBa2Cu3O7−δ films, which are able to display gate modulable non-volatile volume MIT, driven by field-induced oxygen diffusion within the system. The key advantage of these materials is the possibility to homogeneously tune the oxygen diffusion not only in a confined filament or interface, as observed in widely explored binary and complex oxides, but also in the whole material volume. Another important advantage of correlated oxides with respect to devices based on conducting filaments is the significant reduction of cycle-to-cycle and device-to-device variations. In this work, we show several device configurations in which the lateral conduction between a drain-source channel (synaptic weight) is effectively controlled by active gate-tunable volume resistance changes, thus providing the basis for the design of robust and flexible transistor-based artificial synapses.

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.

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 and Reentrant Metallic Behaviors in Kish Graphite

The metal-insulator transition on electronic transport properties and the extraordinarily large magnetoresistances (MR) at low temperature had been found in Kish graphite under low magnetic field. Along with the large ordinary magnetoresistances (MR), Kish graphite had clearly shown the reentrant metallic behavior at low temperature under 600 and 900 Oe applied magnetic fields. Besides a dominating ordinary MR (OMR) mechanism in the free-electron mode, the Coulomb interacting quasiparticles within graphite sheet play some role to extraordinarily large OMR.

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.

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.

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.

Research on Charge Transport in One-Dimensional Organic Semiconductors Material

1D conjugated polymers belong to the family of organic semiconductor materials, in which the charge carriers are polarons or bipolarons. Charge transport in 1D organic semiconductors in the presence of high electric fields is studied within the SSH model. It is found that under a sufficiently high electric field, the polaron is dissociated into free-like electron. The electron performs Bloch oscillation (BO) in the organic semiconductors. By enhancing the electric field, BO will be destroyed and electrons can transit from the valence band to the conduction band, which is Zener tunneling in organic semiconductors. The results also indicate a field-induced insulator-metal transition.

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.

Field-induced crossover from phonon to field assisted hopping conductivity in organic materials

Recently the change in temperature dependence of conductivity observed in polythiophene field-effect transistors has been attributed to field-induced metal-insulator transition. Under a high source-drain voltage, the conductivity becomes independent from temperature and depends exponentially on the inverse of the square root of electric field. In this paper we present a theoretical interpretation of this behavior in the framework of hopping mechanism. The ingredient of this model, is the energy gained from the electrical field is always larger than the energy difference between the localized states involved into the charge transitions, which induces a crossover from phonon-assisted hopping to field-assisted hopping.

Dielectric permittivity and conductivity spectra of La 0.5Ca 0.5MnO 3 thin films presenting electric field induced metal–insulator transition

Electric behavior of polycrystalline La 0.5Ca 0.5MnO 3 thin films, pulsed laser deposited on a (100) silicon substrate, is reported and discussed. An electrically induced metal-insulating transition around 150 K is found, which is voltage and thickness dependent. At low temperatures, the film conductivity is non-Ohmic and moderate electric fields lead to resistivity switching towards metaestable low-resistive states. Impedance spectroscopy measurements were also performed in order to determine the film dielectric permittivity and conductivity and to estimate the characteristic metal–semiconductor interface parameters. The obtained results show that the fraction of ferromagnetic metallic regions does not change when a voltage is applied, and that the mechanism responsible for the low temperature metal insulator transition and the conduction behavior is the appearance of connective paths due to the enhanced mobility of carriers activated by the electric field.

Paring instability in the mixed state of ad-wave superconductor

We propose that an excitonic gap can be generated along nodal directions by Coulomb interaction in the mixed state of $d$-wave cuprate superconductors. In a superconductor, the Coulomb interaction usually cannot generate any fermion gap since its strength is weakened by superfluidity. It becomes stronger as superfluid density is suppressed by external magnetic field, and is able to generate a gap for initially gapless nodal quasiparticles beyond some critical field ${H}_{c}$. By solving the gap equation, it is found that the nodal gap increases with growing field $H$, which leads to a suppression of thermal conductivity at zero temperature. This mechanism naturally produces the field-induced thermal metal-insulator transition observed in transport experiments.