arXiv subjects

B. Cheng

Publications and source records attributed to B. Cheng.

At least 19 recordsLinked to original sources

arXiv2024

Self-induced light emission in solid-state memristors replicates neuronal biophotons

Key pre-synaptic and post-synaptic biological functions have been successfully implemented in various hardware systems. A noticeable example are neuronal networks constructed from memristors, which are emulating complex electro-chemical biological dynamics such a neuron's efficacy and plasticity. Neurons are highly active cells, communicating with chemical and electrical stimuli, but also emit light. These photons are suspected to be a complementary vehicle to transport information across the brain. Here, we show that a memristor also releases photons akin to the production of neuronal light. Critical attributes of so-called biophotons such as self-generation, origin, stochasticity, spectral coverage, sparsity and correlation with the neuron's activity are replicated by our solid-state approach. Our findings further extend the emulating capability of a memristor to encompass neuronal biophoton emission and open the possibility to construct a bimodal electro-optical platform with the assistance of atomic-scale devices capable of handling electrons and photons as information carriers.

cond-mat.mes-hall↗

arXiv2022

Overbias Light Emission From Memristive Nanojunctions

A nanoscale dielectric gap clamped between two metal electrodes may undergo a large resistance change from insulating to highly conducting upon applying an electrical stress. This sudden resistive switching effect is largely exploited in memristors for emulating synapses in neuromorphic neural networks. Here, we show that resistive switching can be accompanied by a release of electromagnetic radiation spanning the visible spectral region. Importantly, we find that the spectrum is characterized by photon energies exceeding the maximum kinetic energy of electrons provided by the switching voltage. This so-called overbias emission can be described self-consistently by a thermal radiation model featuring an out-of-equilibrium electron distribution generated in the device with an effective temperature exceeding 2000~K. The emitted spectrum is understood in terms of hot electrons radiatively decaying to resonant optical modes occurring in a nanoscale \ch{SiO2} matrix located between two \ch{Ag} electrodes. The correlation between resistive switching and the onset of overbias emission in atomic-scale photonic memristor brings new venues to generate light on chip and their exploitation in optical interconnects. Photons emitted during memristive switching can also be monitored to follow the neural activation pathways in memristor-based networks.

cond-mat.mes-hall↗

arXiv2022

A light-induced Weyl semiconductor-to-metal transition mediated by Peierls instability

Elemental tellurium is a strongly spin-orbit coupled Peierls-distorted semiconductor whose band structure features topologically protected Weyl nodes. Using time-dependent density functional theory calculations, we show that impulsive optical excitation can be used to transiently control the amplitude of the Peierls distortion, realizing a mechanism to switch tellurium between three states: Weyl semiconductor, Weyl metal and non-Weyl metal. Further, we present experimental evidence of this inverse-Peierls distortion using time-resolved optical second harmonic generation measurements. These results provide a pathway to multifunctional ultrafast Weyl devices and introduce Peierls systems as viable hosts of light-induced topological transitions.

cond-mat.str-el↗

arXiv2020

Calibration of a superconducting gravimeter with an absolute atom gravimeter

We present a 27-days long common view measurement of an absolute cold atom gravimeter (CAG) and a relative iGrav superconducting gravimeter, which we use to calibrate the iGrav scale factor. This allowed us to push the CAG long-term stability down to the level of 0.5~nm.s$^{-2}$. We investigate the impact of the duration of the measurement on the uncertainty in the determination of the correlation factor and show that it is limited to about 3\textperthousand~by the coloured noise of our cold atom gravimeter. A 3-days long measurement session with an additional FG5X absolute gravimeter allows us to directly compare the calibration results obtained with two different absolute meters. Based on our analysis, we expect that with an improvement of its long term stability, the CAG will allow to calibrate the iGrav scale factor to better than the per mille level (1$\sigma$ level of confidence) after only one-day of concurrent measurements for maximum tidal amplitudes.

physics.ins-det↗

arXiv2020

Sub-THz momentum drag and violation of Matthiessen's rule in an ultraclean ferromagnetic SrRuO$_3$ metallic thin film

SrRuO$_3$, a ferromagnet with an approximately 160\,K Curie temperature, exhibits a $T^2$ dependent dc resistivity below $\approx$ 30 K. Nevertheless, previous optical studies in the infrared and terahertz range show non-Drude dynamics at low temperatures which seem to contradict a Fermi-liquid picture with long-lived quasiparticles. In this work, we measure the low-frequency THz range response of thin films with residual resistivity ratios, $\rho_{300K}/ \rho_{4K} \approx$ 74. Such low disorder samples allow an unprecedented look at the effects of electron-electron interactions on low-frequency transport. At temperatures below 30 K, we found both a very sharp zero-frequency mode which has a width narrower than $k_BT/\hbar$ as well as a broader zero frequency Lorentzian that has at least an order of magnitude larger scattering rate. Both features have temperature dependencies consistent with a Fermi-liquid with the wider feature explicitly showing a T$^2$ scaling. Such two -Drude transport sheds light on previous reports of the violation of Mathielssen's rule and extreme sensitivity to disorder in metallic ruthenates. We consider a number of possibilities for the origin of the two feature optical conductivity including multiband effects that arise from momentum conserving interband scattering and the approximate conservation of a pseudo-momentum that arises from quasi-1D Fermi surfaces.

cond-mat.str-el↗

arXiv2019

Reinforcement Learning Based Orchestration for Elastic Services

Due to the highly variable execution context in which edge services run, adapting their behavior to the execution context is crucial to comply with their requirements. However, adapting service behavior is a challenging task because it is hard to anticipate the execution contexts in which it will be deployed, as well as assessing the impact that each behavior change will produce. In order to provide this adaptation efficiently, we propose a Reinforcement Learning (RL) based Orchestration for Elastic Services. We implement and evaluate this approach by adapting an elastic service in different simulated execution contexts and comparing its performance to a Heuristics based approach. We show that elastic services achieve high precision and requirement satisfaction rates while creating an overhead of less than 0.5% to the overall service. In particular, the RL approach proves to be more efficient than its rule-based counterpart; yielding a 10 to 25% higher precision while being 25% less computationally expensive.

cs.DC↗

arXiv2018

Quantum Wires and Waveguides Formed in Graphene by Strain

Confinement of electrons in graphene to make devices has proven to be a challenging task. Electrostatic methods fail because of Klein tunneling, while etching into nanoribbons requires extreme control of edge terminations, and bottom-up approaches are limited in size to a few nanometers. Fortunately, its mechanical flexibility raises the possibility of using strain to alter graphene's properties and create novel straintronic devices. Here, we report transport studies of nanowires created by linearly-shaped strained regions resulting from individual folds formed by layer transfer onto hexagonal boron nitride. Conductance measurements across the folds reveal Coulomb blockade signatures, indicating confined charges within these structures, which act as quantum dots. Along folds, we observe sharp features in traverse resistivity measurements, attributed to an amplification of the dot conductance modulations by a resistance bridge incorporating the device. Our data indicates ballistic transport up to ~1 um along the folds. Calculations using the Dirac model including strain are consistent with measured bound state energies and predict the existence of valley-polarized currents. Our results show that graphene folds can act as straintronic quantum wires.

cond-mat.mes-hall↗

arXiv2016

Tuning and Stabilizing Topological Insulator Bi2Se3 in the Intrinsic Regime by Charge Extraction with Organic Overlayers

In this work, we use charge extraction via organic overlayer deposition to lower the chemical potential of topological insulator Bi2Se3 thin films into the intrinsic (bulk-insulating) regime. We demonstrate the tuning and stabilization of intrinsic topological insulators at high mobility with low-cost organic films. With the protection of the organic charge extraction layers tetrafluorotetracyanoquinodimethane(F4TCNQ) or tris(acetylacetonato)cobalt(III) (Co(acac)3), the sample is stable in the atmosphere with chemical potential ~135 meV above the Dirac point (85 meV below the conduction band minimum, well within the topological insulator regime) after four months, which is an extraordinary level of environmental stability. The Co complex represents the first use of an organometallic for modulating TI charge density. The mobility of surface state electrons is enhanced as high as ~2000 cm2/Vs. Even at room temperature, a true topologically insulating state is realized and stabilized for months' exposure to the atmosphere.

cond-mat.mes-hall↗

arXiv2015

Influence of chirping the Raman lasers in an atom gravimeter: phase shifts due to the Raman light shift and to the finite speed of light

We present here an analysis of the influence of the frequency dependence of the Raman laser light shifts on the phase of a Raman-type atom gravimeter. Frequency chirps are applied to the Raman lasers in order to compensate gravity and ensure the resonance of the Raman pulses during the interferometer. We show that the change in the Raman light shift when this chirp is applied only to one of the two Raman lasers is enough to bias the gravity measurement by a fraction of $\mu$Gal ($1~\mu$Gal~=~$10^{-8}$~m/s$^2$). We also show that this effect is not compensated when averaging over the two directions of the Raman wavevector $k$. This thus constitutes a limit to the rejection efficiency of the $k$-reversal technique. Our analysis allows us to separate this effect from the effect of the finite speed of light, which we find in perfect agreement with expected values. This study highlights the benefit of chirping symmetrically the two Raman lasers.

physics.atom-ph↗

arXiv2014

Coexistence and competition of multiple charge-density-wave orders in rare-earth tri-telluride RTe3

The occurrences of collective quantum states, such as superconductivity (SC) and charge- or spin-densitywaves (CDWs or SDWs), are among the most fascinating phenomena in solids. To date much effort has been made to explore the interplay between different orders, yet little is known about the relationship of multiple orders of the same type. Here we report optical spectroscopy study on CDWs in the rare-earth tri-telluride compounds RTe3 (R = rare earth elements). Besides the prior reported two CDW orders, the study reveals unexpectedly the presence of a third CDW order in the series which evolves systematically with the size of R element. With increased chemical pressure, the first and third CDW orders are both substantially suppressed and compete with the second one by depleting the low energy spectral weight. A complete phase diagram for the multiple CDW orders in this series is established.

cond-mat.str-el↗

arXiv2012

Continuous magnetic phase transition in half-frustrated Ca2Os2O7

We present the specific heat, magnetization, optical spectroscopy measurements and the firstprinciple calculations on the Weberite structure Ca2Os2O7 single crystal/polycrystalline sample. The Ca2Os2O7 shows a Curie-Weiss nature at high temperature and goes into a ferrimagnetic insulating state at 327 K on cooling. A \lambda-like peak is observed at 327 K in the specific heat implying a second-order phase transition. The vanishing electronic specific heat at low temperature suggests a full energy gap. At high temperature above the transition, small amount of itinerant carriers with short life time \tau are observed, which is gapped at 20 K with a direct gap of 0:24 eV . Our first principle calculations indicate that the anti-ferromagnetic (AFM) correlation with intermediate Coulomb repulsion U could effectively split Os(4b) t2g bands and push them away from Fermi level(EF). On the other hand, a non-collinear magnetic interaction is needed to push the Os(4c) bands away from EF, which could be induced by Os(4c)-Os(4c) frustration. Therefore, AFM correlation, Coulomb repulsion U and non-collinear interaction all play important roles for the insulating ground state in Ca2Os2O7.

cond-mat.str-el↗

arXiv2012

Electronic properties of 3d transitional metal pnictides : A comparative study by optical spectroscopy

Single-crystalline KFe2As2 and CaT2As2 (T = Fe, Co, Ni, Cu) are synthesized and investigated by resistivity, susceptibility and optical spectroscopy. It is found that CaCu2As2 exhibits a similar transition to the lattice abrupt collapse transitions discovered in CaFe2(As1-xPx)2 and Ca1-xRexFe2As2 (Re-rare earth element). The resistivity of KFe2As2 and CaT2As2 (T = Fe, Co, Ni, Cu) approximately follows the similar T^2 dependence at low temperature, but the magnetic behaviors vary with different samples. Optical measurement reveals the optical response of CaCu2As2 is not sensitive to the transition at 50 K, with no indication of development of a new energy gap below the transition temperature. Using Drude-Lorentz model, We find that two Drude terms, a coherent one and an incoherent one, can fit the low-energy optical conductivity of KFe2As2 and CaT2As2 (T = Fe, Co, Ni) very well. However, in CaCu2As2, which is a sp-band metal, the low-energy optical conductivity can be well described by a coherent Drude term. Lack of the incoherent Drude term in CaCu2As2 may be attributed to the weaker electronic correlation than KFe2As2 and CaT2As2 (T = Fe, Co, Ni). Spectral weight analysis of these samples indicates that the unconventional spectral weight transfer, which is related to Hund's coupling energy J_H, is only observed in iron pnictides, supporting the viewpoint that J_H may be a key clue to seek the mechanism of magnetism and superconductivity in pnictides.

cond-mat.supr-con↗

arXiv2012

Single crystal growth and optical conductivity of SrPt$_2$As$_2$ superconductors

SrPt$_2$As$_2$ single crystals with CaBe$_2$Ge$_2$-type structure were synthesized by self-melting technique. X-ray diffraction, transmission electron microscopy, electrical resistivity, specific heat and optical spectroscopy measurements were conducted to elucidate the properties of SrPt$_2$As$_2$. SrPt$_2$As$_2$ single crystals exhibit a superconducting transition at 5.2K, experiencing a structural phase transition well above room temperature (about 455K). The superconducting and structural phase transition temperatures are both reduced by 6% Iridium doping. Both pure SrPt$_2$As$_2$ and the doped single crystals are revealed to be highly metallic with rather high plasma frequencies. In particular, the optical spectroscopy measurement revealed two gap-like suppression features. We elaborated that the one at higher energy scale could be ascribed to the correlation effect, while the other one at lower energy scale is related to the structural phase transition, leading to the removal of a small portion of the Fermi surfaces.

cond-mat.supr-con↗

arXiv2012

Two successive field-induced spin-flop transitions in single-crystalline CaCo$_{2}$As$_{2}$

CaCo$_{2}$As$_{2}$, a ThCr$_{2}$Si$_{2}$-structure compound, undergoes an antiferromagnetic transition at \emph{T$_{N}$}=76K with the magnetic moments being aligned parallel to the \emph{c} axis. Electronic transport measurement reveals that the coupling between conducting carriers and magnetic order in CaCo$_{2}$As$_{2}$ is much weaker comparing to the parent compounds of iron pnictide. Applying magnetic field along \emph{c} axis induces two successive spin-flop transitions in its magnetic state. The magnetization saturation behaviors with \emph{\textbf{H}$\parallel$c} and \emph{\textbf{H}$\parallel$ab} at 10K indicate that the antiferromagnetic coupling along \emph{c} direction is very weak. The interlayer antiferromagntic coupling constant \emph{J$_{c}$} is estimated to be about 2 meV.

cond-mat.str-el↗

arXiv2011

Optical study of the multiple charge density wave transitions in ErTe$_3$

We present an optical spectroscopy study on singe crystalline ErTe$_3$, a rare-earth-element tri-telluride which experiences two successive charge density wave (CDW) transitions at T$_{c1}$ = 267 K and T$_{c2}$ = 150 K. Two corresponding gap features, centered at 2770 \cm ($\sim$ 343 meV) and 890 \cm ($\sim$ 110 meV) respectively, are clearly seen in ordered state. A pronounced Drude component, which exists at all measurement temperatures, demonstrates the partial gap character of both CDW orders. About half of the unmodulated Fermi surface (FS) remains in the CDW state at the lowest measurement temperature. The study also reveals that fluctuation effect is still prominent in this two-dimensional (2D) material.

cond-mat.str-el↗

arXiv2010

Optical spectroscopy study on CeTe$_3$: evidence for multiple charge-density-wave orders

We performed optical spectroscopy measurement on single crystal of CeTe$_3$, a rare-earth element tri-telluride charge density wave (CDW) compound. The optical spectra are found to display very strong temperature dependence. Besides a large and pronounced CDW energy gap being present already at room temperature as observed in earlier studies, the present measurement revealed the formation of another energy gap at smaller energy scale at low temperature. The second CDW gap removes the electrons near E$_F$ which undergo stronger scattering. The study yields evidence for the presence of multiple CDW orders or strong fluctuations in the light rare-earth element tri-telluride.

cond-mat.str-el↗

arXiv2010

Three dimensionality of band structure and a large residual quasiparticle population in Ba$_{0.67}$K$_{0.33}$Fe$_2$As$_2$ as revealed by the c-axis polarized optical measurement

We report on a c-axis polarized optical measurement on a Ba$_{0.67}$K$_{0.33}$Fe$_2$As$_2$ single crystal. We find that the c-axis optical response is significantly different from that of high-T$_c$ cuprates. The experiments reveal an anisotropic three-dimensional optical response with the absence of the Josephson plasma edge in R($\omega$) in the superconducting state. Furthermore, different from the ab-plane optical response, a large residual quasiparticle population down to $T\sim\frac{1}{5}T_c$ was observed in the c-axis polarized reflectance measurement. We elaborate that there exist nodes for the superconducting gap in regions of the 3D Fermi surface that contribute dominantly to the c-axis optical conductivity.

cond-mat.supr-con↗

arXiv2010

Formation of partial energy gap below the structural phase transition and the rare-earth element substitution effect on infrared phonons in ReFeAsO (Re=La, Nd, and Sm)

Single crystals of LaFeAsO, NdFeAsO, and SmFeAsO have been prepared by means of a NaAs flux growth technique and studied by optical spectroscopy measurements. We show that the spectral features corresponding to the partial energy gaps in the spin-density-wave (SDW) state are present below the structural phase transition. This indicates that the electronic state below the structural phase transition is already very close to that in the SDW state. We also show that in-plane infrared phonon modes display systematic shifts towards high frequency upon rare-earth element substitutions for La, suggesting a strong enhancement of the bonding strength. Furthermore, an asymmetric line-shape of the in-plane phonon mode is observed, implying the presence of an electron-phonon coupling effect in Fe-pnictides.

cond-mat.supr-con↗