Ohmic Regime Research Articles

Simulation of an ohmic regime in the T-15MD tokamak based on the canonical profile transport model

mail: nrcki@nrcki.ru The canonical profiles transport model (CPTM), whose coefficients were determined from the T-10 tokamak database with a standard magnetic field BT = 2.3—2.5 T, has shown its robustness in ohmic regimes with a reduced magnetic field BT = 1.55—2.1 T. We used the CPTM for predictions of radial profiles and dependences of the electron and ion temperatures and the energy confinement time on the average plasma density for the T-15MD tokamak at the initial stage of its operation: the ohmic regime in a circular limiter configuration with BT = 1.0—2.0 T and plasma current Ip 1 MA.

Coulomb drag induced non-local resistance in double graphene layers.

We study the effect of Coulomb drag between graphene layers in presence of viscosity term. To do this, we use the simple model of Stokes equations for drift velocities in active and passive layers, known as Pogrebinskii's approach. The solution to these equations allows us to find the potential distribution, and thus the non-local drag resistance of passive layer. It is shown that in viscous regime the non-local resistance may take negative values, in contrast, the ohmic regime results in positive non-local resistance for all drag strengths. Additionally, we discuss the influence of magnetic field on the non-local drag magnetoresistance.

Study of impurity C transport and plasma rotation in negative triangularity on the TCV tokamak

Carbon impurity transport is studied in the TCV tokamak using a charge exchange recombination diagnostic. TCVs flexible shaping capabilities were exploited to extend previous impurity transport studies to negative triangularity (δ < 0). A practical way of studying light impurity transport (like C, TCVs main impurity species due to graphite tiled walls) is to investigate the correlations between the impurity ion gradients that, in this study, highlighted significant differences between positive (PT) and negative δ (NT) plasma configurations. δ scans ( −0.6<δ<+0.6 ) were performed in limited configurations, but displayed little correlation between C temperature, rotation and density gradients for positive δ. This stiff response for δ > 0 changes for negative δ, where the evolution of ∇vtor was accompanied by variations of ∇nC over a range of negative δ, showing that transport, in NT, is affected by velocity gradients. Similar δ scans were performed with additional NBH (Neutral Beam Heating), with power steps ranging from 0.25 MW to 1.25 MW, highlighting increased momentum confinement in negative δ. Finally, the evolution of intrinsic plasma toroidal rotation across linear to saturated ohmic confinement regime (LOC/SOC) transitions was explored at δ < 0, expanding previous studies performed in TCV for δ> 0 (Bagnato et al 2023 Nucl. Fusion 63 056006). Toroidal rotation reversal was not observed for δ < 0, despite clear LOC/SOC transitions, confirming that these two phenomena occur concomitantly only in a restricted number of cases and under specific conditions.

Impact of Surface States in Graphene/p-Si Schottky Diodes.

Graphene-silicon Schottky diodes are intriguing devices that straddle the border between classical models and two-dimensional ones. Many papers have been published in recent years studying their operation based on the classical model developed for metal-silicon Schottky diodes. However, the results obtained for diode parameters vary widely in some cases showing very large deviations with respect to the expected range. This indicates that our understanding of their operation remains incomplete. When modeling these devices, certain aspects strictly connected with the quantum mechanical features of both graphene and the interface with silicon play a crucial role and must be considered. In particular, the dependence of the graphene Fermi level on carrier density, the relation of the latter with the density of surface states in silicon and the coupling between in-plane and out-of-plane dynamics in graphene are key aspects for the interpretation of their behavior. Within the thermionic regime, we estimate the zero-bias Schottky barrier height and the density of silicon surface states in graphene/type-p silicon diodes by adapting a kown model and extracting ideality index values close to unity. The ohmic regime, beyond the flat band potential, is modeled with an empirical law, and the current density appears to be roughly proportional to the electric field at the silicon interface; moreover, the graphene-to-silicon electron tunneling efficiency drops significantly in the transition from the thermionic to ohmic regime. We attribute these facts to (donor) silicon surface states, which tend to be empty in the ohmic regime.

Breakdown of Ohm's Law in Molecular Junctions with Electrodes of Single-Layer Graphene.

For sufficiently low biases, Ohm's law, the cornerstone of electricity, stating that current I and voltage V are proportional, is satisfied at low biases for all known systems ranging from macroscopic conductors to nanojunctions. In this study, we predict theoretically and demonstrate experimentally that in single-molecule junctions fabricated with single-layer graphene as electrodes the current at low V scales as the cube of V, thereby invalidating Ohm's law. The absence of the ohmic regime is a direct consequence of the unique band structure of the single-layer graphene, whose vanishing density of states at the Dirac points precludes electron transfer from and to the electrodes at low biases.

Simulation of an Ohmic Regime in the T-15MD Tokamak Based on the Canonical Profile Transport Model

Simulation of an Ohmic Regime in the T-15MD Tokamak Based on the Canonical Profile Transport Model

Back‐End‐of‐Line Integration of Synaptic Weights using HfO2/ZrO2Nanolaminates

AbstractIn artificial neural networks, the “synaptic weights” connecting the neurons are adjusted during the training. Beyond silicon, functionalizing the back‐end‐of‐line (BEOL) of CMOS circuits with novel materials is a key enabler for deploying neural network accelerators. The hardware implementation of the synaptic weights requires linear and reprogrammable resistive elements. In ferroelectric tunnel junctions, the resistance is programmed by controlling the configuration of the ferroelectric domains with electrical pulses. From ferroelectric HfZrO4to HfO2–ZrO2nanolaminates (NL), the crystallization temperature lowers below the upper limit of 400 °C required for CMOS BEOL. The device footprint is reduced, and the maximum‐to‐minimum conductance ratio increases from 7 to 32. Operated with pulses in the ultra‐fast (20 ns) and biological (500 µs) timescales, the synaptic plasticity exhibits several regimes. Dynamic hysteresis mode characterization after up to 1011switching cycles indicates the coexistence of ferroelectric and non‐ferroelectric effects such as defect rearrangement. Temperature‐dependent transport measurements in the Ohmic (linear) regime support these conclusions. Multi‐level resistive switching is achieved in HfO2–ZrO2NL co‐integrated to CMOS in the BEOL. 1T‐1R operation is demonstrated, paving the way for hardware implementation of synaptic weights for in‐memory neuromorphic computing.

Role of isotopes in microturbulence from linear to saturated Ohmic confinement regimes

The first-principle gyrokinetic numerical experiments investigating the isotopic dependence of energy confinement achieve a quantitative agreement with experimental empirical scalings, particularly in Ohmic and L-mode tokamak plasmas. Mitigation of turbulence radial electric field intensity |δEr|2 and associated poloidal δE×B fluctuating velocity with the radial correlation length lcr∝Mi0.11 strongly deviating from the gyro-Bohm scaling is identified as the principal mechanism behind the isotope effects. Three primary contributors are classified: the deviation from gyro-Bohm scaling, zonal flow, and trapped electron turbulence stabilization. Zonal flow enhances isotope effects primarily through reinforcing the inverse dependence of turbulence decorrelation rate on isotope mass with ωc∝Mi−0.76, which markedly differs from the characteristic linear frequency. The findings offer insights into isotope effects, providing critical implications for energy confinement optimization in tokamak plasmas. Published by the American Physical Society 2024

Characterization of Quantum and Classical Critical Points for an Integrable Two-Qubit Spin–Boson Model

The class of two-interacting-qubit spin–boson models with vanishing transverse fields on the spin-pair is studied. The model can be mapped exactly into two independent standard single-impurity spin–boson models where the role of the tunneling parameter is played by the spin–spin coupling. The dynamics of the magnetization are analyzed for different levels of (an)isotropy. The existence of a decoherence-free subspace, as well as of different classical regimes separated by a critical temperature, and symptoms of quantum (first-order and Kosterlitz–Thouless type) phase transitions in the Ohmic regime are brought to light.

Working Regime Criteria for Microscale Electrohydrodynamic Conduction Pumps.

We investigated the microscale electrohydrodynamic (EHD) conduction pumps in a wide range of working regimes, from the saturation regime to the ohmic regime. We showed that the existing macro- and microscale theoretical models could not accurately predict the electric force of microscale EHD conduction pumps, especially for the cases of a strong diffusion effect. We clarified that the failure is caused by a rough estimate of the heterocharge layer thickness. We revised the expression of heterocharge layer thickness by considering the diffusion effect and developed a new theoretical model for the microscale EHD conduction pumps based on the revised expression of heterocharge layer thickness. The results showed that our model can accurately predict the dimensionless electric force of the microscale EHD conduction pumps even for the cases of a strong diffusion effect. Furthermore, we developed a working regime map of microscale EHD conduction pumps and found that the microscale EHD conduction pumps more easily fall into the saturation regime compared with the macroscale EHD conduction pumps due to the enhanced diffusion effect; in other words, the microscale EHD conduction pumps have a wider saturation regime. We showed that the conduction number C0 could not distinguish the working regime of the microscale EHD conduction pumps because it does not take the diffusion effect into account. By employing the revised expression of heterocharge layer thickness, we proposed a new dimensionless number, C0D to distinguish the working regimes of microscale EHD conduction pumps.

(Invited) In-Depth DC and Low Frequency Noise Characterization of Nanosheet FETs at Room and Cryogenic Temperatures

In this work, an in-depth static and low frequency noise characterization of nanosheet FETs, consisting of two vertically stacked silicon channels per device, is performed. A comparison of the performances from n- versus p-channel FETs operated at 300 K and 78 K, in terms of static and low frequency noise parameters variability, is also discussed.The studied devices were fabricated at imec. The geometrical dimensions of the devices are the following: each fin presents a width of 15 nm and each nanosheet thickness is around 11 nm. The effective channel width is calculated as Weff = 2·N·(2·Wfin + 2·Hfin), where N is the number of fins in parallel in the layout and per device. The equivalent oxide thickness is around 0.9 nm. Two structures having different vertical distance between the stacked nanosheets: 7.5 nm (wafer#1) and 4.7 nm (wafer#2) are investigated. Overview results focus on devices having a fixed gate length (LG) of 100 nm and 22 fins in parallel (N = 22) [1,2] but preliminary results on devices having LG = 70 nm and N = 2 are also presented. More information about the device fabrication and the experimental setup used for the various types of measurements covered in this work can be found in [3,4].Typical drain current and transconductance transfer characteristics for n- and p-channel devices from wafer#1 operated at 78 K are illustrated in Figure 1. The main static parameter extraction of the devices operated in ohmic regime is made comparing two methodologies which are not affected by the access resistance impact, as Y-function [5,6] and the McLarty equations [7] methodologies. Good agreement between the extracted parameters using these two methods is obtained in particular when the simple criteria proposed in [1] are employed. Indeed, criteria in [1] allow to properly choose the applied gate voltage interval for which the parameter extraction using Y-function should be made when the impact of the surface roughness collisions in the mobility expression cannot be neglected.An example of input-referred noise spectra, at different applied gate voltages, for n-channel devices from wafer#1 operated at 78 K is shown in Figure 2. White noise, flicker noise and depending on the polarization one or more Lorentzian noise contributions may be observed on the total input-referred noise spectra for all investigated devices. The noise parameters are estimated using methodologies presented in [4,8]. It is demonstrated that the 1/f noise mechanism may be explained by the carrier number fluctuations correlated to the mobility fluctuations mechanism [9] from weak to moderate inversion operation, while in strong inversion operation the 1/f noise behavior is dominated by the access resistances noise [10] contribution. It is shown that the access resistance parameter is the key parameter in order to not underestimate the Coulomb scattering coefficient value.Overall, we have found that the wafer#1 devices present good electrical properties in terms of mobility, threshold voltage, access resistance and even from a flicker noise level point of view, for both n- and p-channel nanosheet FETs, while devices from wafer#2 with a smaller separation between the nanosheets suffer more from mobility variability and high access resistances due to a non-optimized process flow.References :[1] Cretu et al., accepted in IEEE Trans. Electron Dev.[2] Cretu et al., Solid-State Electron, 2022, 194, pp. 102-108, 108209, DOI: 10.1016/j.sse.2022.108360.[3] Veloso et al., SSDM Tech. Dig., 2019, pp. 559-560, DOI: 10.7567/SSDM.2019.N-1-03.[4] Boudier et al., Solid-State Electron, 2017, 128, pp. 102-108, DOI: 10.1016/j.sse.2016.10.012.[5] Ghibaudo, Electronics Letters, vol.24, no. 9, p. 543, 1988, DOI: 10.1109/ICMTS.2000.844428.[6] Mourain et al., ICMTS 2000, pp. 181-186, 2000, DOI: 10.1109/ICMTS.2000.844428.[7] K. McLarty et al., Solid-State Electron., 1995, 38 (6), pp. 1175-1177, DOI: 10.1016/0038-1101(94)00248-E.[8] Boudier et al., Solid-State Electronics, vol. 168, DOI : 10.1016/j.sse.2019.[9] Ghibaudo et al., Phys. Status Solidi (a), 1991;124 (2), pp. 571-581, DOI : 10.1002/pssa.2211240225.D..107732.[10] F.N. Hooge, IEEE Trans. Electron Dev. 41, pp. 1926-1935, 1994. DOI: 10.1109/16.333808. Figure 1

The evaluation of the Al/ZnO/Si/Al heterojunction diode current-conducting mechanisms

This work focuses on the electrical transport and conduction mechanisms — inside a sprayed — made zinc oxide-on-p-type silicon junction device. A fabricated Al/ZnO/Si/Al heterojunction is characterized at 300 and 380[Formula: see text]K in the dark. The aforementioned theories predict that the current density (J) is an exponential function of measured voltage (V) and work temperature (T). Exponential equations are linearized as a result of a current conduction mechanism (CCM) kind. Useful parameters like barrier height [Formula: see text] of trap are extracted from the slope B. Ohmic and space charge limited current (SCLC) regimes inside are studied at 300 and 380[Formula: see text]K recording a shift in exponent as a result of temperature. Fowler–Nordheim (FN) conduction mechanism inside Al/ZnO/Si/Al device is considered slope and [Formula: see text] values are determined for two cited work temperatures. Our device presents a thermionic emission (TE) and exhibits an effect of temperature on slope and intercept values of (ln[Formula: see text]/A*[Formula: see text]) function. The Poole–Frenkel (PF) emission in terms of 1/V data are presented for 300 and 380[Formula: see text]K. Besides, PF emission mechanism in terms of 1/[Formula: see text] is exhibited and parameters are influenced by temperature. Behavior of conductance as a result of temperature is detailed and evidenced.

Induced-charge membrane capacitive deionization enables high-efficient desalination with polarized porous electrodes

Exposure of a conducting porous material to an electric field in electrolytes induces an electric dipole, which results in capacitive charging of cations and anions at opposite poles. In this letter, we investigate a novel desalination method using this induced-charge capacitive deionization (ICCDI). To do this, we devise a microscale ICCDI platform that can visualize in situ ion concentrations, pH shifts, and fluid flows, and study ion transport dynamics and desalination performances compared to conventional CDI with unipolar / bipolar connections. Similar ion concentration and fluid flow characteristics were observed in Ohmic, limiting, and over-limiting regimes, but variations in desalination performance trends were noted based on the number of stacks. In a single cell, ICCDI generates a higher electric field at the opposite poles of porous electrodes than simple conducted electrodes in CDIs with unipolar/bipolar connections, leading to superior salt removal and/or lower ionic current at a given applied voltage. This marks a clear contrast from CDI with bipolar connection, which lacks any advantage over CDI with unipolar connection in a single cell. These metrics of ICCDI however deteriorated as the stack number increased, likely due to short-circuiting between the dipoles. As a result, ICCDI in current form shows higher desalination efficient than conventional CDIs with low stack numbers (< 6), so we offer the scale-up module by repeating 4-stack ICCDI units. Our study enhances comprehension of ion transport dynamics and desalination performance in ICCDI, and the results could aid in the development of ICCDI for energy/cost-efficient desalination.

Signatures of Dissipation Driven Quantum Phase Transition in Rabi Model.

By using the worldline MonteCarlo technique, matrix product state, and a variational approach àla Feynman, we investigate the equilibrium properties and relaxation features of the dissipative quantum Rabi model, where a two level system is coupled to a linear harmonic oscillator embedded in a viscous fluid. We show that, in the Ohmic regime, a Beretzinski-Kosterlitz-Thouless quantum phase transition occurs by varying the coupling strength between the two level system and the oscillator. This is a nonperturbative result, occurring even for extremely low dissipation magnitude. By using state-of-the-art theoretical methods, we unveil the features of the relaxation towards the thermodynamic equilibrium, pointing out the signatures of quantum phase transition both in the time and frequency domains. We prove that, for low and moderate values of the dissipation, the quantum phase transition occurs in the deep strong coupling regime. We propose to realize this model by coupling a flux qubit and a damped LC oscillator.

Full-f gyrokinetic simulations of Ohmic L-mode plasmas in linear and saturated Ohmic confinement regimes

Two time slices in linear and saturated Ohmic confinement (LOC and SOC) regimes in a Tore Supra Ohmic L-mode discharge are analyzed using nonlinear global full-f gyrokinetic simulations, and qualitative features of the LOC–SOC transition are reproduced. The exhaust of carbon impurity ions is caused by ion mixing, which is driven by the toroidal field stress. The intrinsic rotation develops in the opposite direction between the LOC and SOC phases, which is characterized by different features of the mode asymmetry between trapped electron modes (TEMs) in the LOC phase and ion temperature gradient-driven modes in the SOC phase, leading to the change of the profile shear stress. Here, the mode asymmetry or the ballooning angle depends not only on the profile shear and the Er shear but also on the radial electric field Er itself. The energy fluxes of electrons and deuterium ions are dominant in the LOC and SOC phases, respectively, and the ratio of the energy confinement time between the two phases agree with the experimental value. Turbulent frequency spectra are characterized by quasi-coherent modes (QCMs) and broad-band spectra in the LOC and SOC phases, respectively. The QCMs are produced by a split of the toroidal mode number spectra of TEMs induced by the Doppler shift of poloidal E × B rotation due to Er, which is sustained in the electron diamagnetic direction by the ripple induced counter-current rotation, and enhanced by higher temperature in the LOC phase.

Hot Ion Mode in the Globus-M2 Spherical Tokamak

NBI-assisted plasma heating with one or two injectors of fast neutral atoms was studied at the Globus-M2 spherical tokamak at the toroidal magnetic fields of 0.8–0.9 T and plasma currents of 0.35–0.4 MA. Measurements of the spatial temperature and electron density distributions, performed using the Thomson scattering diagnostics, showed a twofold increase in heating of plasma electrons during the injection of neutral particles with energies of up to 45 keV at the beam power of 0.75 MW, as compared to the ohmic heating regime. Switching on the second additional beam with the particle energy of up to 30 keV and power of up to 0.5 MW resulted in obtaining the hot ion mode in the range of mean plasma densities of (1.6–10) × 1019 m−3. According to the data of active spectroscopy and neutral particle analyzer diagnostics, in the hot zone, the ion temperature reached 4 keV at the plasma density of 8 × 1019 m−3, which is more than 2.5 times higher than the electron temperature.

First-order transitions in spin chains coupled to quantum baths

We show that tailoring the dissipative environment allows us to change the features of continuous quantum phase transitions and even induce first-order transitions in ferromagnetic spin chains. In particular, using a numerically exact quantum Monte Carlo method for the paradigmatic Ising chain of one-half spins in a transverse magnetic field, we find that spin couplings to local quantum boson baths (in the Ohmic regime) can drive the transition from the second to the first order even for a low dissipation strength. Moreover, using a variational mean-field approach for the treatment of spin-spin and spin-boson interactions, we point out that phase discontinuities are ascribable to a dissipation-induced effective magnetic field which is intrinsically related to the bath quantum fluctuations and vanishes for classical baths. The effective field is able to switch the sign of the magnetization along the direction of spin-spin interactions. The results can be potentially tested in recent quantum simulators and are relevant for quantum sensing since the spin system could not only detect the properties of nonclassical baths, but also the effects of weak magnetic fields.

Isotope effects on intrinsic rotation in hydrogen, deuterium and tritium plasmas

The isotope effect on intrinsic rotation was studied at the Joint European Torus (JET) tokamak. With the unique capability of JET to operate with tritium (T), for the first time, experiments in hydrogen (H), deuterium (D) and T in Ohmic plasmas were compared. Two rotation reversals per isotope type are observed in plasma density scans spanning the linear and the saturated Ohmic confinement regimes. A clear isotope mass dependence is observed at the higher densities. The magnitude of the core rotation was found to depend on isotope mass, with stronger co-current rotation observed in H. Change on intrinsic rotation characteristics coexist with a stronger thermal energy confinement in T.

Mesoscopic Klein-Schwinger effect in graphene

Strong electric field annihilation by particle–antiparticle pair creation, also known as the Schwinger effect, is a non-perturbative prediction of quantum electrodynamics. Its experimental demonstration remains elusive, as threshold electric fields are extremely strong and beyond current reach. Here, we propose a mesoscopic variant of the Schwinger effect in graphene, which hosts Dirac fermions with an approximate electron–hole symmetry. Using transport measurements, we report on universal one-dimensional Schwinger conductance at the pinchoff of ballistic graphene transistors. Strong pinchoff electric fields are concentrated within approximately 1 μm of the transistor’s drain and induce Schwinger electron–hole pair creation at saturation. This effect precedes a collective instability towards an ohmic Zener regime, which is rejected at twice the pinchoff voltage in long devices. These observations advance our understanding of current saturation limits in ballistic graphene and provide a direction for further quantum electrodynamic experiments in the laboratory.

Hall effect in Poiseuille flow of two-dimensional electron fluid

The hydrodynamic regime of charge transport has been recently realized in high-quality conductors. In the hydrodynamic as well as in the Ohmic regimes the main part of the Hall resistance of a long sample is determined by the balance between the Lorentz force and the electric force, acting on conduction electrons. Experimentally observed deviations of the Hall resistance in hydrodynamic samples from such ``standard'' value are usually associated with the Hall viscosity term in the Navier-Stokes equation. In this work we theoretically study the Hall effect in a Poiseuille flow of a two-dimensional electron fluid. We show that the near-edge semiballistic layers with the width of the order of the interparticle mean-free path, which inevitably appear near sample edges, give the contribution to the Hall resistance which is comparable with the bulk contribution from the Hall viscosity. In this way, the measured deviations of the Hall resistance from the ``standard'' one in hydrodynamic samples by the usual contact techniques should be associated with both the Hall viscosity in the bulk and the semiballistic effects in the near-edge layers.