scholarly journals Gate-tunable charge carrier electrocaloric effect in trilayer graphene

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Natalia Cortés ◽  
Oscar Negrete ◽  
Francisco J. Peña ◽  
Patricio Vargas
Keyword(s):  
Charge Carrier ◽  
Tight Binding ◽  
Field Potential ◽  
Thermal Response ◽  
The Novel ◽  
Layered Systems ◽  
Electrocaloric Effect ◽  
Electronic Density ◽  
Graphene Layers ◽  
Trilayer Graphene

AbstractThe electrocaloric (EC) effect is the change in temperature and entropy of a material driven by the application of an electric field. Our tight-binding calculations linked to Fermi statistics, show that the EC effect can be produced in trilayer graphene (TLG) structures connected to a heat source, triggered by changes in the electronic density of states (DOS) at the Fermi level when external gate fields are applied on the outer graphene layers. We demonstrate that entropy changes are sensitive to the stacking arrangement in TLG systems. The AAA-stacked TLG presents an inverse EC response (cooling) regardless of the temperature value and gate field potential strength, whereas the EC effect in ABC-stacked TLG remains direct (heating) above room temperature. We reveal otherwise the TLG with Bernal-ABA stacking generates both the direct and inverse EC response within the same sample, associated with gate-dependent electronic transitions of thermally excited charge carriers from the valence band to the conduction band in the band structure. The novel charge carrier electrocaloric effect we propose in quantum layered systems may bring a wide variety of prototype van der Waals materials that could be used as versatile platforms to controlling the thermal response in nanodevices.

scholarly journals Gate-tunable Charge Carrier Electrocaloric Effect in Trilayer Graphene

2021 ◽  
Author(s):  
Natalia Cortés ◽  
Oscar Negrete ◽  
Francisco J. Peña ◽  
Patricio Vargas
Keyword(s):  
Charge Carrier ◽  
Tight Binding ◽  
Field Potential ◽  
Thermal Response ◽  
The Novel ◽  
Layered Systems ◽  
Electrocaloric Effect ◽  
Electronic Density ◽  
Graphene Layers ◽  
Trilayer Graphene

Abstract The electrocaloric (EC) effect is the change in temperature and entropy of a material driven by the application of an electric field. Our tight-binding calculations linked to Fermi statistics, show that the EC effect can be produced in trilayer graphene (TLG) structures connected to a heat source, triggered by changes in the electronic density of states (DOS) at the Fermi level when external gate fields are applied on the outer graphene layers. We demonstrate that entropy changes are sensitive to the stacking arrangement in TLG systems. The AAA-stacked TLG presents an inverse EC response (cooling) regardless of the temperature value and gate field potential strength, whereas the EC effect in ABC-stacked TLG remains direct (heating) above room temperature. We reveal otherwise the TLG with Bernal-ABA stacking generates both the direct and inverse EC response within the same sample, associated with gate-dependent electronic transitions of thermally excited charge carriers from the valence band to the conduction band in the band structure. The novel charge carrier electrocaloric effect we propose in quantum layered systems may bring a wide variety of prototype van der Waals materials that could be used as versatile platforms to controlling the thermal response in nanodevices.

Thermal Properties of Divalent Metal Oxides: CaO as a Prototype

2013 ◽  
Vol 209 ◽  
pp. 190-193
Author(s):  
Nisarg K. Bhatt ◽  
Brijmohan Y. Thakore ◽  
P.R. Vyas ◽  
A.Y. Vahora ◽  
Asvin R. Jani
Keyword(s):  
Density Functional ◽  
Tight Binding ◽  
Mean Field ◽  
Field Potential ◽  
Theoretical Data ◽  
Test Case ◽  
Generalized Gradient ◽  
Core Electrons ◽  
Vibrational Free Energy ◽  
Semi Empirical

Commonly employed quasiharmonic approximation (QHA) is inadequate to account for intrinsic anharmonism such as phonon-phonon interaction, vacancy contribution, etc. Though anharmonic contributions are important at high temperatures and low pressure, complete ab initio calculations are scanty due largely to laborious computational requirements. Nevertheless, some simple semi-empirical schemes can be used effectively to incorporate the anharmonism. In this regards, in the present study we have proposed a simple computational scheme to include the effect of vacancy directly into the description within the mean-field potential approach, which calculates vibrational free energy of ions. Validity of the scheme is verified by taking calcium oxide as a test case. Equilibrium properties at (T,P) = (0,0) condition is obtained within the tight-binding second-moment approximation (TB-SMA), whose parameters were determined through first principles density functional theory. Kohn-Sham equations for core electrons were solved using ultrasoft plane-wave pseudopotential employing the generalized gradient approximation for exchange and correlation. Present findings for thermal expansion and high-T EOS clearly show perceptible improvement over the case when vacancy contribution was not included. Some related thermodynamic properties are also calculated and compared with the available experimental and theoretical data.

Tuning the electrical properties of exfoliated graphene layers using deep ultraviolet irradiation

2014 ◽  
Vol 2 (27) ◽  
pp. 5404-5410 ◽  
Author(s):  
M. Z. Iqbal ◽  
M. F. Khan ◽  
M. W. Iqbal ◽  
Jonghwa Eom
Keyword(s):  
Electrical Properties ◽  
Ultraviolet Irradiation ◽  
Single Layer ◽  
Single Layer Graphene ◽  
Graphene Bilayer ◽  
Graphene Layers ◽  
Exfoliated Graphene ◽  
Deep Ultraviolet ◽  
Trilayer Graphene ◽  
Unique Band

Deep ultraviolet irradiation tunes the electronic properties of mechanically exfoliated single-layer graphene, bilayer graphene, and trilayer graphene while maintaining their unique band structure and electrical properties.

scholarly journals Optical scatterings in layered systems

2020 ◽  
Author(s):  
Chang-Ting Liu ◽  
Chih-Wei Chiu ◽  
Chiun-Yan Lin ◽  
Ming-Fa Lin
Keyword(s):  
Tight Binding ◽  
State Transitions ◽  
Finite Width ◽  
Bulk Materials ◽  
Layered Systems ◽  
Binding Model ◽  
Reflection And Transmission ◽  
Atomic Interactions ◽  
Dynamic Charge ◽  
Electro Magnetic

Abstract Optical properties, reflectance, absorbance and transmittance spectra, are fully investigated for the layered structures through the development of theoretical framework. The transverse dielectric function, which characterizes the dynamic charge screenings, can cover all the intralayer and interlayer atomic interactions under the electro-magnetic wave perturbation. By the continuous reflection and transmission scatterings at two surfaces, their analytic formulas are established from the vertical valence-state transitions and the boundary conditions. They are also suitable for finite-width bulk materials. Most important, this study is fully combined with the generalized tight-binding model with all the intrinsic interactions and external field.

Extraction of Preformed Mixed Phase Graphene Sheets from Graphitized Coal by Fungal Leaching

2017 ◽  
pp. 287-299
Author(s):  
Manoj Balachandran
Keyword(s):  
Graphene Quantum Dots ◽  
Primary Source ◽  
Mixed Phase ◽  
Low Grade ◽  
The Novel ◽  
Carbon Nanostructure ◽  
Nano Structure ◽  
Graphene Layers ◽  
Bio Imaging ◽  
Potential Use

The potential use of coal as source of carbon nano structure is seldom investigated. Herein we report a facile fungal solubilization method to extract mixed phase carbon structure from low grade coal. Coal had been used as a primary source for the production of carbon nanostructure with novel property, in addition to its main utility as a fuel. The major hurdle in its application is the inherent mineral embedded in it. An environmentally benign demineralization procedure make coal as a widely accepted precursor for the novel carbon materials. With Aspergiilus niger leaching, the randomly oriented preformed crystalline mixed phase nanocarbon in coal can be extracted. Raman studies revealed the presence of E2g scattering mode of graphite. The sp3 domains at ~1355 cm-1 (D band) is an indication of diamond like structure with disorder or defect. In the 2D region, multilayer stacking of graphene layers is noticed. The ratio of the defect to graphitic bands was found to be decreasing with increasing rank of coal. Bio leaching of coal enhances the carbon content in coal while eliminating the associated minerals in it. These defected carbon is an ideal material for graphene quantum dots and carbon dots, which are useful in drug delivery and bio imaging applications.

scholarly journals The electronic thickness of graphene

2020 ◽  
Vol 6 (11) ◽  
pp. eaay8409 ◽  
Author(s):  
Peter Rickhaus ◽  
Ming-Hao Liu ◽  
Marcin Kurpas ◽  
Annika Kurzmann ◽  
Yongjin Lee ◽  
...  
Keyword(s):  
Graphene Layer ◽  
Finite Thickness ◽  
Tight Binding ◽  
Single Layer ◽  
Energy Difference ◽  
Quantum Capacitance ◽  
Density Range ◽  
Dielectric Thickness ◽  
Graphene Layers ◽  
Fabry Perot

When two dimensional crystals are atomically close, their finite thickness becomes relevant. Using transport measurements, we investigate the electrostatics of two graphene layers, twisted by θ = 22° such that the layers are decoupled by the huge momentum mismatch between the K and K′ points of the two layers. We observe a splitting of the zero-density lines of the two layers with increasing interlayer energy difference. This splitting is given by the ratio of single-layer quantum capacitance over interlayer capacitance Cm and is therefore suited to extract Cm. We explain the large observed value of Cm by considering the finite dielectric thickness dg of each graphene layer and determine dg ≈ 2.6 Å. In a second experiment, we map out the entire density range with a Fabry-Pérot resonator. We can precisely measure the Fermi wavelength λ in each layer, showing that the layers are decoupled. Our findings are reproduced using tight-binding calculations.

scholarly journals Moire Is Different

2021 ◽  
Vol 13 (1) ◽  
pp. 50
Author(s):  
Wenyuan Shi
Keyword(s):  
Electronic Structures ◽  
Tight Binding ◽  
Physical Property ◽  
Large Change ◽  
Bilayer Graphene ◽  
Dirac Fermion ◽  
Dirac Fermions ◽  
Graphene Layers ◽  
Flat Bands ◽  
Twisted Bilayer Graphene

Graphene, as the thinnest material ever found, exhibits unconventionally relativistic behaviour of Dirac fermions. However, unusual phenomena (such as superconductivity) arise when stacking two graphene layers and twisting the bilayer graphene. The relativistic Dirac fermion in graphene has been widely studied and understood, but the large change observed in twisted bilayer graphene (TBG) is intriguing and still unclear because only van der Waals force (vdW) interlayer interaction is added from graphene to TBG and such a very weak interaction is expected to play a negligible role. To understand such dramatic variation, we studied the electronic structures of monolayer, bilayer and twisted bilayer graphene. Twisted bilayer graphene creates different moiré patterns when turned at different angles. We proposed tight-binding and effective continuum models and thereby drafted a computer code to calculate their electronic structures. Our calculated results show that the electronic structure of twisted bilayer graphene changes significantly even by a tiny twist. When bilayer graphene is twisted at special “magic angles”, flat bands appear. We examined how these flat bands are created, their properties and the relevance to some unconventional physical property such as superconductivity. We conclude that in the nanoscopic scale, similar looking atomic structures can create vastly different electronic structures. Like how P. W. Anderson stated that similar looking fields in science can have differences in his article “More is Different”, similar moiré patterns in twisted bilayer graphene can produce different electronic structures.

scholarly journals Electric field–tunable superconductivity in alternating-twist magic-angle trilayer graphene

Science ◽  
2021 ◽  
Vol 371 (6534) ◽  
pp. 1133-1138 ◽  
Author(s):  
Zeyu Hao ◽  
A. M. Zimmerman ◽  
Patrick Ledwith ◽  
Eslam Khalaf ◽  
Danial Haie Najafabadi ◽  
...  
Keyword(s):  
Displacement Field ◽  
Weak Coupling ◽  
Twist Angle ◽  
Magic Angle ◽  
Van Hove Singularity ◽  
Displacement Fields ◽  
Graphene Layers ◽  
Vdw Heterostructure ◽  
Quantum Phenomena ◽  
Trilayer Graphene

Engineering moiré superlattices by twisting layers in van der Waals (vdW) heterostructures has uncovered a wide array of quantum phenomena. We constructed a vdW heterostructure that consists of three graphene layers stacked with alternating twist angles ±θ. At the average twist angle θ ~ 1.56°, a theoretically predicted “magic angle” for the formation of flat electron bands, we observed displacement field–tunable superconductivity with a maximum critical temperature of 2.1 kelvin. By tuning the doping level and displacement field, we found that superconducting regimes occur in conjunction with flavor polarization of moiré bands and are bounded by a van Hove singularity (vHS) at high displacement fields. Our findings display inconsistencies with a weak coupling description, suggesting that the observed moiré superconductivity has an unconventional nature.

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