scholarly journals Rippling ultrafast dynamics of suspended 2D monolayers, graphene

2016 ◽  
Vol 113 (43) ◽  
pp. E6555-E6561 ◽  
Author(s):  
Jianbo Hu ◽  
Giovanni M. Vanacore ◽  
Andrea Cepellotti ◽  
Nicola Marzari ◽  
Ahmed H. Zewail
Keyword(s):  
Time Scale ◽  
Negative Thermal Expansion ◽  
Early Time ◽  
Optical Excitation ◽  
Atomic Scale ◽  
Monolayer Graphene ◽  
Phonon Modes ◽  
Ultrafast Optical ◽  
Out Of Plane ◽  
Deformation Dynamics

Here, using ultrafast electron crystallography (UEC), we report the observation of rippling dynamics in suspended monolayer graphene, the prototypical and most-studied 2D material. The high scattering cross-section for electron/matter interaction, the atomic-scale spatial resolution, and the ultrafast temporal resolution of UEC represent the key elements that make this technique a unique tool for the dynamic investigation of 2D materials, and nanostructures in general. We find that, at early time after the ultrafast optical excitation, graphene undergoes a lattice expansion on a time scale of 5 ps, which is due to the excitation of short-wavelength in-plane acoustic phonon modes that stretch the graphene plane. On a longer time scale, a slower thermal contraction with a time constant of 50 ps is observed and associated with the excitation of out-of-plane phonon modes, which drive the lattice toward thermal equilibrium with the well-known negative thermal expansion coefficient of graphene. From our results and first-principles lattice dynamics and out-of-equilibrium relaxation calculations, we quantitatively elucidate the deformation dynamics of the graphene unit cell.

scholarly journals Ultrafast Modulation of Magnetization Dynamics in Ferromagnetic (Ga, Mn)As Thin Films

2018 ◽  
Vol 8 (10) ◽  
pp. 1880 ◽  
Author(s):  
Hang Li ◽  
Xinhui Zhang ◽  
Xinyu Liu ◽  
Margaret Dobrowolska ◽  
Jacek Furdyna
Keyword(s):  
Magnetic Anisotropy ◽  
Spin Dynamics ◽  
Optical Excitation ◽  
Laser Pulses ◽  
Time Resolved ◽  
Ultrafast Optical ◽  
Optical Effects ◽  
Out Of Plane ◽  
Linearly Polarized ◽  
Magnetization Precession

Magnetization precession induced by linearly polarized optical excitation in ferromagnetic (Ga,Mn)As was studied by time-resolved magneto-optical Kerr effect measurements. The superposition of thermal and non-thermal effects arising from the laser pulses complicates the analysis of magnetization precession in terms of magnetic anisotropy fields. To obtain insight into these processes, we investigated compressively-strained thin (Ga,Mn)As films using ultrafast optical excitation above the band gap as a function of pulse intensity. Data analyses with the gyromagnetic calculation based on Landau-Lifshitz-Gilbert equation combined with two different magneto-optical effects shows the non-equivalent effects of in-plane and out-of-plane magnetic anisotropy fields on both the amplitude and the frequency of magnetization precession, thus providing a handle for separating the effects of non-thermal and thermal processes in this context. Our results show that the effect of photo-generated carriers on magnetic anisotropy constitutes a particularly effective mechanism for controlling both the frequency and amplitude of magnetization precession, thus suggesting the possibility of non-thermal manipulation of spin dynamics through pulsed laser excitations.

scholarly journals Temperature and interlayer coupling induced thermal transport across graphene/2D-SiC van der Waals heterostructure

2022 ◽  
Vol 12 (1) ◽  
Author(s):  
Md. Sherajul Islam ◽  
Imon Mia ◽  
A. S. M. Jannatul Islam ◽  
Catherine Stampfl ◽  
Jeongwon Park
Keyword(s):  
High Temperatures ◽  
Thermal Transport ◽  
Van Der Waals ◽  
Thermal Conductance ◽  
Interlayer Coupling ◽  
Phonon Modes ◽  
Out Of Plane ◽  
System Temperature ◽  
Non Equilibrium Molecular Dynamics ◽  
Increasing Temperature

AbstractGraphene based two-dimensional (2D) van der Waals (vdW) materials have attracted enormous attention because of their extraordinary physical properties. In this study, we explore the temperature and interlayer coupling induced thermal transport across the graphene/2D-SiC vdW interface using non-equilibrium molecular dynamics and transient pump probe methods. We find that the in-plane thermal conductivity κ deviates slightly from the 1/T law at high temperatures. A tunable κ is found with the variation of the interlayer coupling strength χ. The interlayer thermal resistance R across graphene/2D-SiC interface reaches 2.71 $$\times$$ × 10–7$${\text{Km}}^{2} /{\text{W}}$$ Km 2 / W at room temperature and χ = 1, and it reduces steadily with the elevation of system temperature and χ, demonstrating around 41% and 56% reduction with increasing temperature to 700 K and a χ of 25, respectively. We also elucidate the heat transport mechanism by estimating the in-plane and out-of-plane phonon modes. Higher phonon propagation possibility and Umklapp scattering across the interface at high temperatures and increased χ lead to the significant reduction of R. This work unveils the mechanism of heat transfer and interface thermal conductance engineering across the graphene/2D-SiC vdW heterostructure.

THz-bandwidth coherent phonon emission by supported monolayer graphene in the out-of-plane direction

2012 ◽  
Author(s):  
I-Ju Chen ◽  
Pierre-Adrien Mante ◽  
Cheng-Kai Chang ◽  
Chun-Chiang Kuo ◽  
Kuei-Hsien Chen ◽  
...  
Keyword(s):  
Monolayer Graphene ◽  
Coherent Phonon ◽  
Phonon Emission ◽  
Out Of Plane ◽  
Plane Direction

scholarly journals Single-atom vibrational spectroscopy in the scanning transmission electron microscope

Science ◽  
2020 ◽  
Vol 367 (6482) ◽  
pp. 1124-1127 ◽  
Author(s):  
F. S. Hage ◽  
G. Radtke ◽  
D. M. Kepaptsoglou ◽  
M. Lazzeri ◽  
Q. M. Ramasse
Keyword(s):  
Electron Microscope ◽  
Vibrational Spectroscopy ◽  
Materials Science ◽  
Scanning Transmission Electron Microscope ◽  
Atomic Scale ◽  
Single Atom ◽  
Phonon Modes ◽  
Resonant States ◽  
Resolution Electron ◽  
Scanning Transmission

Single-atom impurities and other atomic-scale defects can notably alter the local vibrational responses of solids and, ultimately, their macroscopic properties. Using high-resolution electron energy-loss spectroscopy in the electron microscope, we show that a single substitutional silicon impurity in graphene induces a characteristic, localized modification of the vibrational response. Extensive ab initio calculations reveal that the measured spectroscopic signature arises from defect-induced pseudo-localized phonon modes—that is, resonant states resulting from the hybridization of the defect modes and the bulk continuum—with energies that can be directly matched to the experiments. This finding realizes the promise of vibrational spectroscopy in the electron microscope with single-atom sensitivity and has broad implications across the fields of physics, chemistry, and materials science.

Effect of high-optical excitation on the ultrafast electron dynamics in stacked-monolayer graphene samples

2016 ◽  
Author(s):  
Juan A. Castañeda ◽  
Henrique Guimarães Rosa ◽  
José C. V. Gomes ◽  
Eunezio A. Thoroh de Souza ◽  
Carlos H. de Brito-Cruz ◽  
...  
Keyword(s):  
Optical Excitation ◽  
Monolayer Graphene ◽  
Electron Dynamics ◽  
Ultrafast Electron Dynamics

scholarly journals Simulated Nanoscale Peeling Process of Monolayer Graphene Sheet: Effect of Edge Structure and Lifting Position

2010 ◽  
Vol 2010 ◽  
pp. 1-12 ◽  
Author(s):  
Naruo Sasaki ◽  
Hideaki Okamoto ◽  
Shingen Masuda ◽  
Kouji Miura ◽  
Noriaki Itamura
Keyword(s):  
Graphene Sheet ◽  
Atomic Scale ◽  
Monolayer Graphene ◽  
Lattice Period ◽  
Force Curve ◽  
Stick Slip ◽  
Edge Structure ◽  
Sliding Motion ◽  
Peeling Process ◽  
Surface Contact

The nanoscale peeling of the graphene sheet on the graphite surface is numerically studied by molecular mechanics simulation. For center-lifting case, the successive partial peelings of the graphene around the lifting center appear as discrete jumps in the force curve, which induce the arched deformation of the graphene sheet. For edge-lifting case, marked atomic-scale friction of the graphene sheet during the nanoscale peeling process is found. During the surface contact, the graphene sheet takes the atomic-scale sliding motion. The period of the peeling force curve during the surface contact decreases to the lattice period of the graphite. During the line contact, the graphene sheet also takes the stick-slip sliding motion. These findings indicate the possibility of not only the direct observation of the atomic-scale friction of the graphene sheet at the tip/surface interface but also the identification of the lattice orientation and the edge structure of the graphene sheet.

Mechanism of Thermal Conductivity Reduction From Suspended to Supported Graphene: A Quantitative Spectral Analysis of Phonon Scattering

2011 ◽  
Author(s):  
Bo Qiu ◽  
Xiulin Ruan
Keyword(s):  
Thermal Conductivity ◽  
Spectral Analysis ◽  
Phonon Scattering ◽  
Md Simulations ◽  
Mean Free Path ◽  
Spectral Energy ◽  
Phonon Modes ◽  
Suspended Graphene ◽  
Out Of Plane ◽  
Predicted Values

In this work, we perform molecular dynamics (MD) simulations together with phonon spectral analysis to predict the thermal conductivity of both suspended and supported graphene. We quantitatively address the relative importance of different types of phonon in thermal transport and explain why thermal conductivity is significantly reduced in supported graphene compared to that in suspended graphene. Within the framework of equilibrium MD, we perform spectral energy density analysis to obtain the phonon mean free path of each individual phonon mode. The contribution of each mode to thermal conductivity is then calculated and summed to obtain the lattice thermal conductivity in the temperature range 300–650 K. Our predicted values and temperature dependence for both suspended and supported graphene agree with experimental data well. In contrast to prior studies, our results suggest that the contribution from out-of-plane acoustic (ZA) branch to thermal conductivity is around 25–30% in suspended graphene at room temperature. The thermal conductivity of supported graphene is predicted to be largely reduced, which is consistent with experimental observations. Such reduction is shown to be due to stronger scattering of all phonon modes rather than only the ZA mode in the presence of the substrate.

Photo-modulated dynamic competition between metallic and insulating phases in a layered manganite

2014 ◽  
Vol 1636 ◽  
Author(s):  
Yuelin Li ◽  
Donald Walko ◽  
Qing’an Li ◽  
Yaohua Liu ◽  
Stephan Rosenkranz ◽  
...  
Keyword(s):  
Optical Excitation ◽  
Superlattice Reflection ◽  
Metallic Phase ◽  
Local Competition ◽  
Ultrafast Optical ◽  
Laser Fluences ◽  
Show Evidence ◽  
Jahn Teller ◽  
Insulating Phase ◽  
Reflection Intensity

ABSTRACTWe show evidence that the competition between the antiferromagnetic metallic phase and the charge- and orbital-ordered insulating phase at the reentrant phase boundary of a layered manganite, LaSr2Mn2O7, can be manipulated using ultrafast optical excitation. The time-dependent evolution of the Jahn-Teller superlattice reflection, the indicator of the formation of charge and orbital order, was measured at different laser fluences. The laser-induced change in the Jahn-Teller reflection intensity shows a reversal of sign between earlier (∼10 ns) and later (∼150 ns) times during the relaxation of the sample. This is consistent with a physics picture whereby the laser excitation modulates the local competition between the metallic and the insulating phases.

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