arXiv subjects

K. Gao

Publications and source records attributed to K. Gao.

12 recordsLinked to original sources

arXiv2023

Machine-learning the phase diagram of a strongly-interacting Fermi gas

We determine the phase diagram of strongly correlated fermions in the crossover from Bose-Einstein condensates of molecules (BEC) to Cooper pairs of fermions (BCS) utilizing an artificial neural network. By applying advanced image recognition techniques to the momentum distribution of the fermions, a quantity which has been widely considered as featureless for providing information about the condensed state, we measure the critical temperature and show that it exhibits a maximum on the bosonic side of the crossover. Additionally, we back-analyze the trained neural network and demonstrate that it interprets physically relevant quantities.

cond-mat.quant-gas↗

arXiv2023

Detecting the phase transition in a strongly-interacting Fermi gas by unsupervised machine learning

We study the critical temperature of the superfluid phase transition of strongly-interacting fermions in the crossover regime between a Bardeen-Cooper-Schrieffer (BCS) superconductor and a Bose-Einstein condensate (BEC) of dimers. To this end, we employ the technique of unsupervised machine learning using an autoencoder neural network which we directly apply to time-of-flight images of the fermions. We extract the critical temperature of the phase transition from trend changes in the data distribution revealed in the latent space of the autoencoder bottleneck.

cond-mat.quant-gas↗

arXiv2021

A compact and fast magnetic coil for the interaction manipulation of quantum gases with Feshbach resonances

Cold atom experiments commonly use broad magnetic Feshbach resonances to manipulate the interaction between atoms. In order to induce quantum dynamics by a change of the interaction strength, rapid ($\sim\mu s$) magnetic field changes over several tens of Gauss are required. Here we present a compact design of a coil and its control circuit for a change of the magnetic field up to $36G$ in $3\mu s$. The setup comprises two concentric solenoids with minimal space requirements, which can be readily added to existing apparatuses. This design makes the observation of non-equilibrium physics with broad Feshbach resonances accessible.

cond-mat.quant-gas↗

arXiv2019

Higgs mode in a strongly interacting fermionic superfluid

Higgs and Goldstone modes are possible collective modes of an order parameter upon spontaneously breaking a continuous symmetry. Whereas the low-energy Goldstone (phase) mode is always stable, additional symmetries are required to prevent the Higgs (amplitude) mode from rapidly decaying into low-energy excitations. In high-energy physics, where the Higgs boson has been found after a decades-long search, the stability is ensured by Lorentz invariance. In the realm of condensed--matter physics, particle--hole symmetry can play this role and a Higgs mode has been observed in weakly-interacting superconductors. However, whether the Higgs mode is also stable for strongly-correlated superconductors in which particle--hole symmetry is not precisely fulfilled or whether this mode becomes overdamped has been subject of numerous discussions. Experimental evidence is still lacking, in particular owing to the difficulty to excite the Higgs mode directly. Here, we observe the Higgs mode in a strongly-interacting superfluid Fermi gas. By inducing a periodic modulation of the amplitude of the superconducting order parameter $Δ$, we observe an excitation resonance at frequency $2Δ/h$. For strong coupling, the peak width broadens and eventually the mode disappears when the Cooper pairs turn into tightly bound dimers signalling the eventual instability of the Higgs mode.

cond-mat.quant-gas↗

arXiv2017

Estimation and Inference for Very Large Linear Mixed Effects Models

Linear mixed models with large imbalanced crossed random effects structures pose severe computational problems for maximum likelihood estimation and for Bayesian analysis. The costs can grow as fast as $N^{3/2}$ when there are N observations. Such problems arise in any setting where the underlying factors satisfy a many to many relationship (instead of a nested one) and in electronic commerce applications, the N can be quite large. Methods that do not account for the correlation structure can greatly underestimate uncertainty. We propose a method of moments approach that takes account of the correlation structure and that can be computed at O(N) cost. The method of moments is very amenable to parallel computation and it does not require parametric distributional assumptions, tuning parameters or convergence diagnostics. For the regression coefficients, we give conditions for consistency and asymptotic normality as well as a consistent variance estimate. For the variance components, we give conditions for consistency and we use consistent estimates of a mildly conservative variance estimate. All of these computations can be done in O(N) work. We illustrate the algorithm with some data from Stitch Fix where the crossed random effects correspond to clients and items.

stat.ME↗

arXiv2016

Precision Measurement of the Quadrupole Transition Matrix Element in a Single Trapped $^{40}$Ca$^{+}$

We report the first experimental determination of the $4s \ ^{2}S_{1/2} $ $\leftrightarrow $ $3d \ ^{2}D_{5/2}$ quadrupole transition matrix element in $^{40}$Ca$^+$ by measuring the branching ratio of the $3d \ ^{2}D_{5/2} $ state decaying into the ground state $4s \ ^{2}S_{1/2} $ and the lifetime of the $3d \ ^{2}D_{5/2} $ state, using a technique of highly synchronized measurement sequence for laser control and highly efficient quantum state detection for quantum jumps. The measured branching ratio and improved lifetime are, respectively, 0.9992(80) and 1.1652(46) s, which yield the value of the quadrupole transition matrix element (in absolute value) 9.737(43)~$ea_{0}^{2}$ with the uncertainty at the level of 0.44\%. The measured quadrupole transition matrix element is in good agreement with the most precise many-body atomic structure calculations. Our method can be universally applied to measurements of transition matrix elements in single ions and atoms of similar structure.

physics.atom-ph↗

arXiv2015

Bandgap narrowing in Mn doped GaAs probed by room-temperature photoluminescence

The electronic band structure of the (Ga,Mn)As system has been one of the most intriguing problems in solid state physics over the past two decades. Determination of the band structure evolution with increasing Mn concentration is a key issue to understand the origin of ferromagnetism. Here we present room temperature photoluminescence and ellipsometry measurements of Ga_{100%-x}Mn_{x}As alloy. The up-shift of the valence-band is proven by the red shift of the room temperature near band gap emission from the Ga_{100%-x}Mn_{x}As alloy with increasing Mn content. It is shown that even a doping by 0.02 at.% of Mn affects the valence-band edge and it merges with the impurity band for a Mn concentration as low as 0.6 at.%. Both X-ray diffraction pattern and high resolution cross-sectional TEM images confirmed full recrystallization of the implanted layer and GaMnAs alloy formation.

cond-mat.mtrl-sci↗

arXiv2015

High Curie temperature and perpendicular magnetic anisotropy in homoepitaxial InMnAs films

We have prepared the dilute magnetic semiconductor (DMS) InMnAs with different Mn concentrations by ion implantation and pulsed laser melting. The Curie temperature of the In1-xMnxAs epilayer depends on the Mn concentration x, reaching 82 K for x=0.105. The substitution of Mn ions at the Indium sites induces a compressive strain perpendicular to the InMnAs layer and a tensile strain along the in-plane direction. This gives rise to a large perpendicular magnetic anisotropy, which is often needed for the demonstration of electrical control of magnetization and for spin-transfer-torque induced magnetization reversal.

cond-mat.mtrl-sci↗

arXiv2015

Hyperdoping silicon with selenium: solid vs. liquid phase epitaxy

Chalcogen-hyperdoped silicon shows potential applications in silicon-based infrared photodetectors and intermediate band solar cells. Due to the low solid solubility limits of chalcogen elements in silicon, these materials were previously realized by femtosecond or nanosecond laser annealing of implanted silicon or bare silicon in certain background gases. The high energy density deposited on the silicon surface leads to a liquid phase and the fast recrystallization velocity allows trapping of chalcogen into the silicon matrix. However, this method encounters the problem of surface segregation. In this paper, we propose a solid phase processing by flash-lamp annealing in the millisecond range, which is in between the conventional rapid thermal annealing and pulsed laser annealing. Flash lamp annealed selenium-implanted silicon shows a substitutional fraction of around 70% with an implanted concentration up to 2.3%. The resistivity is lower and the carrier mobility is higher than those of nanosecond pulsed laser annealed samples. Our results show that flash-lamp annealing is superior to laser annealing in preventing surface segregation and in allowing scalability.

cond-mat.mtrl-sci↗

arXiv2013

Electronic transport properties of indolyl spirooxazine/merooxazine-based light-driven molecular switch: The effect of amino/nitro substituents

By applying non-equilibrium Green's function formulation combined with first-principles density functional theory,we explore the electronic transport properties of indolinospironaphthoxazine (SO)/indolinomeronaphthoxazine (MO).The results indicate that the MO allows a far larger current than the SO.The substituent group can cause the shift of the energy levels.Higher ON/OFF current ratio can be obtained if either amino or nitro substituent is placed at the position of naphthalene moiety.Our results suggest that such molecular wires can generally display perfect switching function and the efficiency can be tuned flexibly by adding certain substituent groups to the molecules.

cond-mat.mes-hall↗

arXiv2012

Investigation on organic magnetoconductance based on polaron-bipolaron transition

We explore the magnetoresistance (MC) effect in an organic semiconductor device based on the magnetic field related bipolaron formation. By establishing a group of dynamic equations, we present the transition among spin-parallel, spin-antiparallel polaron pairs and bipolarons. The transition rates are adjusted by the external magnetic field as well as the hyperfine interaction of the hydrogen nuclei. The hyperfine interaction is addressed and treated in the frame work of quantum mechanics. By supposing the different mobility of polarons from that of bipolarons, we obtain the MC in an organic semiconductor device. The theoretical calculation is well consistent to the experimental data. It is predicated that a maximum MC appears at a suitable branching ratio of bipolarons. Our investigation reveals the important role of hyperfine interaction in organic magnetic effect.

cond-mat.mtrl-sci↗

arXiv2011

Hertz-level Measurement of the 40Ca+ 4s 2S1/2-3d 2D5/2 Clock Transition Frequency With Respect to the SI Second through GPS

We report a frequency measurement of the clock transition of a single ^40Ca^+ ion trapped and laser cooled in a miniature ring Paul trap with 10^-15 level uncertainty. In the measurement, we used an optical frequency comb referenced to a Hydrogen maser, which was calibrated to the SI second through the Global Positioning System (GPS). Two rounds of measurements were taken in May and June 2011, respectively. The frequency was measured to be 411 042 129 776 393.0(1.6) Hz with a fractional uncertainty of 3.9{\times}10^-15 in a total averaging time of > 2{\times}10^6 s within 32 days.

physics.atom-ph↗