arXiv subjects

Chen-Yen Lai

Publications and source records attributed to Chen-Yen Lai.

At least 19 recordsLinked to original sources

arXiv2020

Time-resolved Photoluminescence in Terahertz-driven Hybrid Systems of Plasmons and Excitons

Ultrafast pump-probe technique is a powerful tool to understand and manipulate properties of materials for designing novel quantum devices. An intense, single cycle terahertz pulse can change the intrinsic properties of semiconductor quantum dots to have different luminescence. In a hybrid system of plasmon and exciton, the coherence and coupling between these two degrees of freedom play an important role on their optical properties. Therefore, we consider a terahertz pump optical probe experiment in the hybrid systems where the terahertz pump pulse couples to the exciton degrees of freedom on the quantum dot. The time resolved photoluminescence of the hybrid system shows that the response of the characteristic frequency shifts according to the overlap between the pump and probe pulses. Furthermore, the resonance between the exciton and plasmons can be induced by the terahertz pump pulse in some parameter regimes. Our results show the terahertz driven hybrid system can be a versatile tool for manipulating the material properties and open a new route to design modern optical devices.

cond-mat.mes-hall↗

arXiv2020

Absence of diffusion and fractal geometry in the Holstein model at high temperature

We investigate the dynamics of an electron coupled to dispersionless optical phonons in the Holstein model, at high temperatures. The dynamics is conventionally believed to be diffusive, as the electron is repeatedly scattered by optical phonons. In a semiclassical approximation, however, the motion is not diffusive. In one dimension, the electron moves in a constant direction and does not turn around. In two dimensions, the electron follows and then continues to retrace a fractal trajectory. Aspects of these nonstandard dynamics are retained in more accurate calculations, including a fully quantum calculation of the electron and phonon dynamics.

cond-mat.str-el↗

arXiv2019

Ultrafast X-ray Absorption Spectroscopy of Strongly Correlated Systems: Core Hole Effect

In recent years, ultrafast pump-probe spectroscopy has provided insightful information about nonequilibrium dynamics of excitations in materials. In a typical experiment of time-resolved x-ray absorption spectroscopy, the systems are excited by a femtosecond laser pulse (pump pulse) followed by an x-ray (probe pulse) after a time delay to measure the absorption spectra of the photoexcited systems. We present a theory for nonequilibrium x-ray absorption spectroscopy in one-dimensional strongly correlated systems. The core hole created by x-ray is modeled as an additional effective potential of the core hole site which changes the spectrum qualitatively. In equilibrium, the spectrum reveals the charge gap at half-filling and the metal-insulator transition in the presence of the core hole effect. Furthermore, a pump-probe scheme is introduced to drive the system out of equilibrium before the x-ray probe. The effects of the pump pulse with varying frequencies, shapes and fluences are discussed for the dynamics of strongly correlated systems in and out of resonance. The spectrum indicates that the driven insulating state has a metallic droplet around the core hole. The rich structures of the nonequilibrium x-ray absorption spectrum give more insight into the dynamics of electronic structures.

cond-mat.str-el↗

arXiv2019

Optical absorption spectroscopy in hybrid systems of plasmons and excitons

Understanding the physics of light emitter in quantum nanostructure regarding scalability, geometry, structure of the system and coupling between different degrees of freedom is important as one can improve the design and further provide controls of quantum devices rigorously. The couplings between these degrees of freedom, in general, depends on the external field, the geometry of nano particles, and the experimental design. An effective model is proposed to describe the plasmon-exciton hybrid systems and its optical absorption spectrums are studied in details by exact diagonalization. Two different designs are discussed: nano particle planet surrounded by quantum dot satellites and quantum dot planet surrounded by nano particle satellites. In both setups, details of many quantum dots and nano particles are studied, and the spectrums are discussed in details regarding the energy of transition peaks and the weight distribution of allowed transition peaks. Also, different polarization of external fields are considered which results in anisotropic couplings, and the absorption spectrums clearly reveal the difference qualitatively. Finally, the system will undergo a phase transition in the presence of attractive interaction between excitons. Our work sheds the light on the design of nano scale quantum systems to achieve photon emitter/resonator theory in the plasmon-exciton hybrid systems.

cond-mat.mes-hall↗

arXiv2018

Many-body multi-valuedness of particle-current variance in closed and open cold-atom systems

The quantum variance of an observable is a fundamental quantity in quantum mechanics, and the variance provides additional information other than the average itself. By examining the relation between the particle-current variance $(δJ)^2$ and the average current $J$ in both closed and open interacting fermionic systems, we show the emergence of a multi-valued Lissajous curve between $δJ$ and $J$ due to interactions. As a closed system we considered the persistent current in a benzene-like lattice enclosing an effective magnetic flux and solved it by exact diagonalization. For the open system, the steady-state current flowing through a few lattice sites coupled to two particle reservoirs was investigated using a Lindblad equation. In both cases, interactions open a loop and change the topology of the corresponding $δJ$-$J$ Lissajous curve, showing that this effect is model-independent. We finally discuss how the predicted phenomena can be observed in ultracold atoms, thus offering an alternative way of probing the dynamics of many-body systems.

cond-mat.quant-gas↗

arXiv2018

Tunable current circulation in triangular quantum-dot metastructures

Advances in fabrication and control of quantum dots allow the realization of metastructures that may exhibit novel electrical transport phenomena. Here, we investigate the electrical current passing through one such metastructure, a system composed of quantum dots placed at the vertices of a triangle. The wave natural of quantum particles leads to internal current circulation within the metastructure in the absence of any external magnetic field. We uncover the relation between its steady-state total current and the internal circulation. By calculating the electronic correlations in quantum transport exactly, we present phase diagrams showing where different types of current circulation can be found as a function of the correlation strength and the coupling between the quantum dots. Finally, we show that the regimes of current circulation can be further enhanced or reduced depending on the local spatial distribution of the interactions, suggesting a single-particle scattering mechanism is at play even in the strongly-correlated regime. We suggest experimental realizations of actual quantum-dot metastructures where our predictions can be directly tested.

cond-mat.mes-hall↗

arXiv2017

Quantification of the memory effect of steady-state currents from interaction-induced transport in quantum systems

Dynamics of a system in general depends on its initial state and how the system is driven, but in many-body systems the memory is usually averaged out during evolution. Here, interacting quantum systems without external relaxations are shown to retain long-time memory effects in steady states. To identify memory effects, we first show quasi-steady state currents form in finite, isolated Bose and Fermi Hubbard models driven by interaction imbalance and they become steady-state currents in the thermodynamic limit. By comparing the steady state currents from different initial states or ramping rates of the imbalance, long-time memory effects can be quantified. While the memory effects of initial states are more ubiquitous, the memory effects of switching protocols are mostly visible in interaction-induced transport in lattices. Our simulations suggest the systems enter a regime governed by a generalized Fick's law and memory effects lead to initial-state dependent diffusion coefficients. We also identify conditions for enhancing memory effects and discuss possible experimental implications.

cond-mat.quant-gas↗

arXiv2017

Protocols for dynamically probing topological edge states and dimerization with fermionic atoms in optical potentials

Topological behavior has been observed in quantum systems including ultracold atoms. However, background harmonic traps for cold-atoms hinder direct detection of topological edge states arising at the boundary because the distortion fuses the edge states into the bulk. We propose experimentally feasible protocols to probe localized edge states and dimerization of ultracold fermions. By confining cold-atoms in a ring lattice and changing the boundary condition from periodic to open using an off-resonant laser sheet to cut open the ring, topological edge states can be generated. A lattice in a topological configuration can trap a single particle released at the edge as the system evolves in time. Alternatively, depleting an initially filled lattice away from the boundary reveals the occupied edge states. Signatures of dimerization in the presence of contact interactions can be found in selected correlations as the system boundary suddenly changes from periodic to open and exhibit memory effects of the initial state distinguishing different configurations or phases.

cond-mat.quant-gas↗

arXiv2016

Challenges and constraints of dynamically emerged source and sink in atomtronic circuits: From closed-system to open-system approaches

While batteries offer electronic source and sink in electronic devices, atomic analogues of source and sink and their theoretical descriptions have been a challenge in cold-atom systems. Here we consider dynamically emerged local potentials as controllable source and sink for bosonic atoms. Although a sink potential can collect bosons in equlibrium and indicate its usefulness in the adiabatic limit, sudden switching of the potential exhibits low effectiveness in pushing bosons into it. This is due to conservation of energy and particle in isolated systems such as cold atoms. By varying the potential depth and interaction strength, the systems can further exhibit averse response, where a deeper emerged potential attracts less bosonic atoms into it. To explore possibilities for improving the effectiveness, we investigate what types of system-environment coupling can help bring bosons into a dynamically emerged sink, and a Lindblad operator corresponding to local cooling is found to serve the purpose.

cond-mat.quant-gas↗

arXiv2016

Hysteresis of noninteracting and spin-orbit coupled atomic Fermi gases with relaxation

Hysteresis can be found in driven many-body systems such as magnets and superfluids. Rate-dependent hysteresis arises when a system is driven periodically while relaxing towards equilibrium. A two-state paramagnet driven by an oscillating magnetic field in the relaxation approximation clearly demonstrates rate-dependent hysteresis. A noninteracting atomic Fermi gas in an optical ring potential, when driven by a periodic artificial gauge field and subjected to dissipation, is shown to exhibit hysteresis loops of atomic current due to a competition of the driving time and the relaxation time. This is in contrast to electronic systems exhibiting equilibrium persistent current driven by magnetic flux due to rapid relaxation. Universal behavior of the dissipated energy in one hysteresis loop is observed in both the magnetic and atomic systems, showing linear and inverse-linear dependence on the relaxation time in the strong and weak dissipation regimes. While interactions in general invalidate the framework for rate-dependent hysteresis, an atomic Fermi gas with artificial spin-orbit coupling exhibits hysteresis loops of atomic currents. Cold-atoms in ring-shape potentials are thus promising in demonstrating rate-dependent hysteresis and its associated phenomena.

cond-mat.quant-gas↗

arXiv2016

Superconducting circuit simulator of Bose-Hubbard model with a flat band

Recent advance in quantum simulations of interacting photons using superconducting circuits offers opportunities for investigating the Bose-Hubbard model in various geometries with hopping coefficients and self-interactions tuned to both signs. Here we investigate phenomena related to localized states associated with a flat-band supported by the saw-tooth geometry. A localization-delocalization transition emerges in the non-interacting regime as the sign of hopping coefficient is changed. In the presence of interactions, patterns of localized states approach a uniform density distribution for repulsive interactions while interesting localized density patterns can arise in strongly attractive regime. The density patterns indicate the underlying inhomogeneity of the simulator. Two-particle correlations can further distinguish the nature of the localized states in attractive and repulsive interaction regimes. We also survey possible experimental implementations of the simulator.

cond-mat.supr-con↗

arXiv2016

Geometry-induced memory effects in isolated quantum systems: Observations and applications

Memory effects can lead to history-dependent behavior of a system, and they are ubiquitous in our daily life and have broad applications. Here we explore possibilities of generating memory effects in simple isolated quantum systems. By utilizing geometrical effects from a class of lattices supporting flat-bands consisting of localized states, memory effects could be observed in ultracold atoms in optical lattices. As the optical lattice continuously transforms from a triangular lattice into a kagome lattice with a flat band, history-dependent density distributions manifest quantum memory effects even in noninteracting systems, including fermionic as well as bosonic systems in the proper ranges of temperatures. Rapid growth in ultracold technology predicts a bright future for quantum memory-effect systems, and here two prototypical applications of geometry-induced quantum memory effects are proposed: An accelerometer recording the mechanical change rate in a coupled system and a rate-controlled memvalve where the rate of ramping the lattice potential acts as a control of the remnant density in the lattice.

cond-mat.quant-gas↗

arXiv2015

The effective U(1)-Higgs theory at strong coupling on optical lattices?

We discuss the U(1)-Higgs model in two dimensions in the strongly coupled regime. If we neglect the plaquette interactions, we generate an effective theory where link variables are integrated out, producing 4-field operators. Plaquette interactions can be restored order by order as in recent calculations with staggered fermions. In the case of a SU(2) gauge theory with fermions, this strong coupling expansion can be related to the strong coupling expansion of Fermi-Hubbard models possibly implementable on optical lattice. We would like to provide a similar construction relating the U(1)-Higgs model to some Bose-Hubbard model. As a first step in this direction, we discuss a recent proposal to implement the O(2) model on optical lattices using a 87Rb and 41K Bose-Bose mixture of cold atoms.

hep-lat↗

arXiv2014

Progress towards quantum simulating the classical O(2) model

We connect explicitly the classical $O(2)$ model in 1+1 dimensions, a model sharing important features with $U(1)$ lattice gauge theory, to physical models potentially implementable on optical lattices and evolving at physical time. Using the tensor renormalization group formulation, we take the time continuum limit and check that finite dimensional projections used in recent proposals for quantum simulators provide controllable approximations of the original model. We propose two-species Bose-Hubbard models corresponding to these finite dimensional projections at strong coupling and discuss their possible implementations on optical lattices using a $^{87}$Rb and $^{41}$K Bose-Bose mixture.

hep-lat↗

arXiv2014

d_{xy}-Density wave in fermion-fermion cold atom mixtures

We study density wave instabilities in a doubly-degenerate Fermi-Fermi mixture with $SU(2)\times SU(2)$ symmetry on a square lattice. For sufficiently large on-site inter-species repulsion, when the two species of fermions are both at half-filling, two conventional ($s$-wave) number density waves are formed with a $π$-phase difference between them to minimize the inter-species repulsion. Upon moving one species away from half-filling, an unconventional density wave with $d_{xy}$-wave symmetry emerges. When both species are away from the vicinity of half-filling, superconducting instabilities dominate. We present results of a functional renormalization-group calculation that maps out the phase diagram at weak couplings. Also, we provide a simple explanation for the emergence of the $d_{xy}$-density wave phase based on a four-patch model. We find a robust and general mechanism for $d_{xy}$-density-wave formation that is related to the shape and size of the Fermi surfaces. The density imbalance between the two species of fermions in the vicinity of half-filling leads to phase-space discrepancy for different inter-species Umklapp couplings. Using a phase space argument for leading corrections in the one-loop renormalization group approach to fermions, we show that the phase-space discrepancy in our system causes opposite flows for the two leading intra-species Umklapp couplings and that this triggers the $d_{xy}$-density-wave instability.

cond-mat.quant-gas↗

arXiv2013

Unconventional superconducting phases on a two-dimensional extended Hubbard model

We study the phase diagram of the extended Hubbard model on a two-dimensional square lattice, including on-site (U) and nearest-neighbor (V) interactions, at weak couplings. We show that the charge-density-wave phase that is known to occur at half-filling when 4V > U gives way to a d_{xy} -wave superconducting instability away from half-filling, when the Fermi surface is not perfectly nested, and for sufficiently large repulsive and a range of on-site repulsive interaction. In addition, when nesting is further suppressed and in presence of a nearest-neighbor attraction, a triplet time-reversal breaking (p_x + ip_y)-wave pairing instability emerges, competing with the d_{x2+y2} pairing state that is known to dominate at fillings just slightly away from half. At even smaller fillings, where the Fermi surface no longer presents any nesting, the (p_x +ip_y)-wave superconducting phase dominates in the whole regime of on-site repulsions and nearest-neighbor attractions, while d_{xy}-pairing occurs in the presence of on-site attraction. Our results suggest that zero-energy Majorana fermions can be realized on a square lattice in the presence of a magnetic field. For a system of cold fermionic atoms on a two-dimensional square optical lattice, both an on-site repulsion and a nearest-neighbor attraction would be required, in addition to rotation of the system to create vortices. We discuss possible ways of experimentally engineering the required interaction terms in a cold atom system.

cond-mat.supr-con↗

arXiv2013

Correlated Phases of Population Imbalanced Fermi-Fermi Mixtures on an Optical Lattice

We study a two species fermion mixture with different populations on a square lattice modeled by a Hubbard Hamiltonian with on-site inter-species repulsive interaction. Such a model can be realized in a cold atom system with fermionic atoms in two different hyperfine states loaded on an optical lattice and with tunable inter-species interaction strength via external fields. For a two-dimensional square lattice, when at least one of the fermion species is close to half-filling, the system is highly affected by lattice effects. With the majority species near half-filling and varying densities for the minority species, we find that several correlated phases emerge as the ground state, including a spin density wave state, a charge density wave state with stripe structure, and various p-wave BCS pairing states for both species. We study this system using a functional renormalization group method, determine its phase diagram at weak coupling, discuss the origin and characteristics of each phase, and provide estimates for the critical temperatures.

cond-mat.str-el↗

arXiv2010

Field Induced Spin Supersolidity in Frustrated Spin-1/2 Spin-Dimer Models

By means of the recently developed algorithm based on the tensor product states, the magnetization process of frustrated spin-1/2 spin-dimer models on a square lattice is investigated. Various field-induced quantum phases are discovered. In particular, clear evidence of a supersolid phase over a finite regime of magnetic field is observed. Critical fields at various field-induced transitions are determined accurately. Our work not only implies that the employed numerical technique can be an efficient algorithm even for the frustrated spin systems, but also shed light on the search of the supersolid phase in real frustrated spin-dimer compounds.

cond-mat.str-el↗