arXiv subjects

Jiaqi Guan

Publications and source records attributed to Jiaqi Guan.

26 records · Page 2Linked to original sources

arXiv2018

Lattice Dynamics of Ultrathin FeSe Films on SrTiO3

Charge transfer and electron-phonon coupling (EPC) are proposed to be two important constituents associated with enhanced superconductivity in the single unit cell FeSe films on oxide surfaces. Using high-resolution electron energy loss spectroscopy combined with first-principles calculations, we have explored the lattice dynamics of ultrathin FeSe films grown on SrTiO3. We show that, despite the significant effect from the substrate on the electronic structure and superconductivity of the system, the FeSe phonons in the films are unaffected. The energy dispersion and linewidth associated with the Fe- and Se-derived vibrational modes are thickness- and temperature-independent. Theoretical calculations indicate the crucial role of antiferromagnetic correlation in FeSe to reproduce the experimental phonon dispersion. Importantly, the only detectable change due to the growth of FeSe films is the broadening of the Fuchs-Kliewer (F-K) phonons associated with the lattice vibrations of SrTiO$_3$(001) substrate. If EPC plays any role in the enhancement of film superconductivity, it must be the interfacial coupling between the electrons in FeSe film and the F-K phonons from substrate rather than the phonons of FeSe.

cond-mat.supr-con↗

arXiv2017

Energy-efficient Amortized Inference with Cascaded Deep Classifiers

Deep neural networks have been remarkable successful in various AI tasks but often cast high computation and energy cost for energy-constrained applications such as mobile sensing. We address this problem by proposing a novel framework that optimizes the prediction accuracy and energy cost simultaneously, thus enabling effective cost-accuracy trade-off at test time. In our framework, each data instance is pushed into a cascade of deep neural networks with increasing sizes, and a selection module is used to sequentially determine when a sufficiently accurate classifier can be used for this data instance. The cascade of neural networks and the selection module are jointly trained in an end-to-end fashion by the REINFORCE algorithm to optimize a trade-off between the computational cost and the predictive accuracy. Our method is able to simultaneously improve the accuracy and efficiency by learning to assign easy instances to fast yet sufficiently accurate classifiers to save computation and energy cost, while assigning harder instances to deeper and more powerful classifiers to ensure satisfiable accuracy. With extensive experiments on several image classification datasets using cascaded ResNet classifiers, we demonstrate that our method outperforms the standard well-trained ResNets in accuracy but only requires less than 20% and 50% FLOPs cost on the CIFAR-10/100 datasets and 66% on the ImageNet dataset, respectively.

cs.LG↗

arXiv2017

Large Magnetic Anisotropy of an Iron-Porphyrin Complex on Metal Substrate

Single magnetic atoms or molecules with large single-ion magnetic anisotropy are highly desired for future applications in high-density data storage and quantum computation. Here we have synthesized an Fe-porphyrin complex on Au(111) substrate in a controlled way, and revealed large magnetic anisotropy energy of more than 15 meV by low-temperature scanning tunneling microscopy and spectroscopy. Two magnetic states with opposite spin directions are discriminated by inelastic electron tunneling spectroscopy in varied magnetic field, and are found to have long spin lifetimes. First-principle calculations reveal that the weak ligand filed in this complex keeps the Fe atom in a high-spin state and preserves its large orbital angular momentum, which gives rise to an easy-axis perpendicular to the molecular plane and large magnetic anisotropy energy by spin-orbit coupling.

cond-mat.mes-hall↗

arXiv2017

Superstructure evolution of tellurium atoms on Au(111) surface at different coverages

We systematically investigated the superstructure evolution of Te atoms on Au(111) substrate at different coverages. As revealed by low temperature scanning tunneling microscopy and spectroscopy, Te atoms form one-dimensional root3 R30° chains near 0.10 monolayer (ML). Two two-dimensional chiral structures, (root111*root111)R4.7° and (3root21*3root21)R10.9°, can be formed and their stability can be tuned by slightly adjusting the Te coverge near 1/3 ML. A honeycomb-like superstructure is observed by further increasing the coverage to 4/9 ML. An interfacial state emerges at ~-0.65 eV due to Te adsorption on Au(111). The formation of these Te-induced high-order superstructures is accompanied by relaxation of gold atoms in the surface layer, indicating the strong Te-Au interaction.

cond-mat.mes-hall↗

arXiv2017

Superconducting transition of FeSe/SrTiO3 induced by adsorption of semiconducting organic molecules

We prepared superconducting and non-superconducting FeSe films on SrTiO3(001) substrates (FeSe/STO) and investigated the superconducting transition induced by charge transfer between organic molecules and FeSe layers by low temperature scanning tunneling microscopy and spectroscopy. At low coverage, donor- and acceptor-type molecules adsorbed preferentially on the non-superconducting and superconducting FeSe layers, respectively. Superconductivity was induced by donor molecules on non-superconducting FeSe layer, while the superconductivity was suppressed near acceptor molecules. The corresponding evolutions of electronic states and work function were also resolved by scanning tunneling microscopy. These results illustrate the important role played by local electron concentration in the superconducting transition of FeSe/STO.

cond-mat.supr-con↗

arXiv2017

Epitaxial Growth and Band Structure of Te Film on Graphene

Tellurium (Te) films with monolayer and few-layer thickness are obtained by molecular beam epitaxy on a graphene/6H-SiC(0001) substrate and investigated by in situ scanning tunneling microscopy and spectroscopy (STM/STS). We reveal that the Te films are composed of parallel-arranged helical Te chains flat-lying on the graphene surface, exposing the (1x1) facet of (10-10) of the bulk crystal. The band gap of Te films increases monotonically with decreasing thickness, reaching ~0.92 eV for the monolayer Te. An explicit band bending at the edge between the monolayer Te and graphene substrate is visualized. With the thickness controlled in atomic scale, Te films show potential applications of in electronics and optoelectronics.

cond-mat.mtrl-sci↗

arXiv2016

The Role of SrTiO3 Phonon Penetrating into thin FeSe Films in the Enhancement of Superconductivity

The significant role of interfacial coupling on the superconductivity enhancement in FeSe films on SrTiO3 has been widely recognized. But the explicit origination of this coupling is yet to be identified. Here by surface phonon measurements using high resolution electron energy loss spectroscopy, we found electric field generated by Fuchs-Kliewer (F-K) phonon modes of SrTiO3 can penetrate into FeSe films and strongly interact with electrons therein. The mode-specific electron-phonon coupling (EPC) constant for the ~92 meV F-K phonon is ~0.25 in the single-layer FeSe on SrTiO3. With increasing FeSe thickness, the penetrating field intensity decays exponentially, which matches well the observed exponential decay of the superconducting gap. It is unambiguously shown that the SrTiO3 F-K phonon penetrating into FeSe is essential in the interfacial superconductivity enhancement.

cond-mat.supr-con↗

arXiv2015

Detection of a superconducting phase in a two-atom layer of hexagonal Ga film grown on semiconducting GaN(0001)

The recent observation of superconducting state at atomic scale has motivated the pursuit of exotic condensed phases in two-dimensional (2D) systems. Here we report on a superconducting phase in two-monolayer crystalline Ga films epitaxially grown on wide band-gap semiconductor GaN(0001). This phase exhibits a hexagonal structure and only 0.552 nm in thickness, nevertheless, brings about a superconducting transition temperature Tc as high as 5.4 K, confirmed by in situ scanning tunneling spectroscopy, and ex situ electrical magneto-transport and magnetization measurements. The anisotropy of critical magnetic field and Berezinski-Kosterlitz-Thouless-like transition are observed, typical for the 2D superconductivity. Our results demonstrate a novel platform for exploring atomic-scale 2D superconductor, with great potential for understanding of the interface superconductivity.

cond-mat.supr-con↗