arXiv subjects

Qianli Yu

Publications and source records attributed to Qianli Yu.

5 recordsLinked to original sources

arXiv2020

Probing Excitons in Ultrathin PbS Nanoplatelets with Enhanced Near-Infrared Emission

Strongly quantum-confined 2D colloidal PbS nanoplatelets (NPLs) are highly interesting materials for near-infrared optoelectronic applications. Here, we use ultrafast transient optical absorption spectroscopy to study the characteristics and dynamics of photoexcited excitons in ultrathin PbS NPLs with a cubic (rock-salt) structure. The NPLs are synthesized at near room temperature from lead oleate and thiourea precursors and show an optical absorption onset at 680 nm (1.8 eV) as well as photoluminescence at 720 nm (1.7 eV). By treating PbS NPLs with CdCl2 in a post-synthetic step, their photoluminescence quantum yield is strongly enhanced from 1.4 % to 19.4 %. The surface treatment leads to an increased lead to sulfur ratio in the structures and associated reduced non-radiative recombination. Exciton-phonon interactions in pristine and CdCl2 treated PbS NPLs at frequencies of 1.8 and 2.2 THz are apparent from coherent oscillations in the measured transient absorption spectra. This study is an important step forward in unraveling and controlling the optical properties of IV-VI semiconductor NPLs.

physics.optics↗

arXiv2017

Observation of Dynamical Super Efimovian Expansion in a Unitary Fermi Gas

We report an observation of a dynamical super Efimovian expansion in a two-component strongly interacting Fermi gas by engineering time dependent external harmonic trap frequencies. When trap frequency is followed as $[1/4t^2+1/t^2\lambda\log(t/t_*)]^{1/2}$, where $t_*$ and $\lambda$ are two control parameters, and the change is faster than a critical value, the expansion of such the quantum gas shows a novel dynamics due to its spatial and dynamical scaling symmetry. A clear double-log periodicity, which is a hallmark of the super Efimov effect, is emergent for the cloud size in the expansion. The universality of such scaling dynamics is verified both in the non-interacting limit and in the unitarity limit. Observing super-Efmovian evolution represents a paradigm in probing universal properties and allows in a new way to study many-body nonequilibrium dynamics with experiments.

cond-mat.quant-gas↗

arXiv2016

Shortcuts to adiabaticity in the strongly-coupled regime: nonadiabatic control of a unitary Fermi gas

Coherent control of complex quantum systems is a fundamental requirement in quantum information processing and engineering. Recently developed notion of shortcut to adiabaticity (STA) has spawned intriguing prospects. So far, the most experimental investigations of STA are implemented in the ideal thermal gas or the weakly interacting ultracold Bose gases. Here we report the first demonstration of a many-body STA in a 3D anisotropically trapped unitary Fermi gas. A new dynamical scaling law is demonstrated on such a strongly interacting quantum gas. By simply engineering the frequency aspect ratio of a harmonic trap, the dynamics of the gas can be manipulated and the many-body state can be transferred adiabatically from one stationary state to another one in short time scale without the excitation. The universal scaling both for non-interacting and unitary Fermi gas is also verified. This could be very important for future many-body quantum engineering and the exploration of the fundamental law of the thermodynamics.

cond-mat.quant-gas↗

arXiv2015

Observation of the Efimovian Expansion in Scale Invariant Fermi Gases

Scale invariance emerges and plays an important role in strongly correlated many-body systems such as critical regimes nearby phase transitions and the unitary Fermi gases. Discrete scaling symmetry also manifests itself in quantum few-body systems such as the Efimov effect. Here we report both theoretical predication and experimental observation of a novel type expansion dynamics for scale invariant quantum gases. When the frequency of the harmonic trap holding the gas decreases continuously as the inverse of time $t$, surprisingly, the expansion of cloud size exhibits a sequence of plateaus. Remarkably, the locations of these plateaus obey a discrete geometric scaling law with a controllable scale factor and the entire expansion dynamics is governed by a log-periodic function. This striking expansion of quantum Fermi gases shares similar scaling laws and same mathematical description as the Efimov effect. Our work demonstrates the first expansion dynamics of a quantum many-body system with the temporal discrete scaling symmetry, which reveals the underlying spatial continuous scaling symmetry of the many-body system.

cond-mat.quant-gas↗

arXiv2015

All-Optical Production of quantum degeneracy and molecular BEC of $^6$Li

We achieve a highly degenerate and strongly interacting Fermi gas in a mixture of the two lowest hyperfine states of $^6$Li by direct evaporative cooling in a high power crossed optical dipole trap. The trap is loaded from a large atom number magneto-optical trap (MOT) which is realized by a laser system of 2.5-watts intracavity-frequency-doubled light output at 671 nm. With this system, we also demonstrate the production of a molecular Bose-Einstein condensates (mBEC), and observe the anisotropic expansion of Fermi gases in the so-called BEC-BCS crossover regime.

cond-mat.quant-gas↗