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Gang Chen

Publications and source records attributed to Gang Chen.

At least 703 records · Page 39Linked to original sources
arXiv2012

BCS-BEC crossover in spin-orbit coupled two-dimensional Fermi gases

The recent experimental realization of spin-orbit coupling for ultra-cold atoms has generated much interest in the physics of spin-orbit coupled degenerate Fermi gases. Although recently the BCS-BEC crossover in three-dimensional (3D) spin-orbit coupled Fermi gases has been intensively studied, the corresponding two-dimensional (2D) crossover physics has remained unexplored. In this paper, we investigate, both numerically and analytically, the BCS-BEC crossover physics in 2D degenerate Fermi gases in the presence of a Rashba type of spin-orbit coupling. We derive the mean field gap and atom number equations suitable for the 2D spin-orbit coupled Fermi gases and solve them numerically and self-consistently, from which the dependence of the ground state properties (chemical potential, superfluid pairing gap, ground state energy per atom) on the system parameters (e.g., binding energy, spin-orbit coupling strength) is obtained. Furthermore, we derive analytic expressions for these ground state quantities, which agree well with our numerical results within a broad parameter region. Such analytic expressions also agree qualitatively with previous numerical results for the 3D spin-orbit coupled Fermi gases, where analytic results are lacked. We show that with an increasing SOC strength, the chemical potential is shifted by a constant determined by the SOC strength. The superfluid pairing gap is enhanced significantly in the BCS limit for strong SOC, but only increases slightly in the BEC limit.

cond-mat.quant-gas↗
arXiv2011

Tunable Spin-orbit Coupling and Quantum Phase Transition in a Trapped Bose-Einstein Condensate

Spin-orbit coupling (SOC), the intrinsic interaction between a particle spin and its motion, is responsible for various important phenomena, ranging from atomic fine structure to topological condensed matter physics. The recent experimental breakthrough on the realization of SOC for ultra-cold atoms provides a completely new platform for exploring spin-orbit coupled superfluid physics. However, the SOC strength in the experiment, determined by the applied laser wavelengths, is not tunable. In this Letter, we propose a scheme for tuning the SOC strength through a fast and coherent modulation of the laser intensities. We show that the many-body interaction between atoms, together with the tunable SOC, can drive a \textit{quantum phase transition} (QPT) from spin-balanced to spin-polarized ground states in a harmonic trapped Bose-Einstein condensate (BEC). This transition realizes the long-sought QPT in the quantum Dicke model, and may have important applications in quantum optics and quantum information. We characterize the QPT using the periods of collective oscillations (center of mass motion and scissors mode) of the BEC, which show pronounced peaks and damping around the quantum critical point.

cond-mat.quant-gas↗
arXiv2011

Geometric phase in the Kitaev honeycomb model and scaling behavior at critical points

In this paper a geometric phase of the Kitaev honeycomb model is derived and proposed to characterize the topological quantum phase transition. The simultaneous rotation of two spins is crucial to generate the geometric phase for the multi-spin in a unit-cell unlike the one-spin case. It is found that the ground-state geometric phase, which is non-analytic at the critical points, possesses zigzagging behavior in the gapless $B$ phase of non-Abelian anyon excitations, but is a smooth function in the gapped $A$ phase. Furthermore, the finite-size scaling behavior of the non-analytic geometric phase along with its first- and second-order partial derivatives in the vicinity of critical points is shown to exhibit the universality. The divergent second-order derivative of geometric phase in the thermodynamic limit indicates the typical second-order phase transition and thus the topological quantum phase transition can be well described in terms of the geometric-phase.

cond-mat.str-el↗
arXiv2011

SU(N) magnetism in chains of ultracold alkaline-earth-metal atoms: Mott transitions and quantum correlations

We investigate one dimensional SU$(N)$ Hubbard chains at zero temperature, which can be emulated with ultracold alkaline earth atoms, by using the density matrix renormalization group (DMRG), Bethe ansatz (BA), and bosonization. We compute experimental observables and use the DMRG to benchmark the accuracy of the Bethe ansatz for $N>2$ where the BA is only approximate. In the worst case, we find a relative error $ε\lesssim 4%$ in the BA ground state energy for $N \leq 4$ at filling 1/N, which is due to the fact that BA improperly treats the triply and higher occupied states. Using the DMRG for $N \leq 4$ and the BA for large $N$, we determine the regimes of validity of strong- and weak-coupling perturbation theory for all values of $N$ and in particular, the parameter range in which the system is well described by a SU$(N)$ Heisenberg model at filling 1/N. We find this depends only weakly on $N$. We investigate the Berezinskii-Kosterlitz-Thouless phase transition from a Luttinger liquid to a Mott-insulator by computing the fidelity susceptibility and the Luttinger parameter $K_ρ$ at 1/N filling. The numerical findings give strong evidence that the fidelity susceptibility develops a minimum at a critical interaction strength which is found to occur at a finite positive value for $N>2$.

cond-mat.quant-gas↗
arXiv2011

Heat transport in silicon from first principles calculations

Using harmonic and anharmonic force constants extracted from density-functional calculations within a supercell, we have developed a relatively simple but general method to compute thermodynamic and thermal properties of any crystal. First, from the harmonic, cubic, and quartic force constants we construct a force field for molecular dynamics (MD). It is exact in the limit of small atomic displacements and thus does not suffer from inaccuracies inherent in semi-empirical potentials such as Stillinger-Weber's. By using the Green-Kubo (GK) formula and molecular dynamics simulations, we extract the bulk thermal conductivity. This method is accurate at high temperatures where three-phonon processes need to be included to higher orders, but may suffer from size scaling issues. Next, we use perturbation theory (Fermi Golden rule) to extract the phonon lifetimes and compute the thermal conductivity $κ$ from the relaxation time approximation. This method is valid at most temperatures, but will overestimate $κ$ at very high temperatures, where higher order processes neglected in our calculations, also contribute. As a test, these methods are applied to bulk crystalline silicon, and the results are compared and differences discussed in more detail. The presented methodology paves the way for a systematic approach to model heat transport in solids using multiscale modeling, in which the relaxation time due to anharmonic 3-phonon processes is calculated quantitatively, in addition to the usual harmonic properties such as phonon frequencies and group velocities. It also allows the construction of accurate bulk interatomic potentials database.

cond-mat.mtrl-sci↗
arXiv2011

Quantum resonant effect of the strongly-driven spin-boson model

In this paper we discuss both analytically and numerically the rich quantum dynamics of the spin-boson model driven by a time-independent field of photon. Interestingly, we predict a new Rabi oscillation, whose period is inversely proportional to the driving amplitude. More surprisingly, some nonzero resonant peaks are found for some special values of the \emph{strong} driving regime. Moreover, for the different resonant positions, the peaks have different values. Thus, an important application of this resonance effect is to realize the precision measurement of the relative parameters in experiment. We also illustrate that this resonant effect arises from the interference of the nontrivial periodic phase factors induced by the evolution of the coherent states in two different subspaces. Our predictions may be, in principle, observed in the solid-state cavity quantum electrodynamics with the ultrastrong coupling if the driving magnitude of the photon field is sufficiently large.

quant-ph↗
arXiv2011

Light-shift-induced quantum phase transitions of a Bose-Einstein condensate in the optical cavities

In this paper we reveal the rich ground-state properties induced by the strong nonlinear atom-photon interaction, which has been found in the recent experiment about a Bose-Einstein condensate coupling with a high-finesse cavity [Nature (London) 464, 1301 (2010)]. Two detuning-dependent phase diagrams are revealed by investigating the experimentally-measurable atomic population. In particular, two new quantum phase transitions from the superradiant phase or the normal phase to a dynamically-unstable phase are predicted in the blue detuning, and moreover, the three-phase coexistence points are found. It is also demonstrated that these predicted quantum phase transitions are the intrinsic transitions governed only by the second-order derivative of the ground-state energy. Finally, we also point out that the region involving coexistence of the superradiant phase and the normal phase predicted before can not happen in the ground state.

quant-ph↗
arXiv2011

Analytical ground state for the Jaynes-Cummings model with the ultrastrong coupling

We present a generalized variational method to analytically obtain the ground-state properties of the unsolvable Jaynes-Cummings model with the ultrastrong coupling. An explicit expression for the ground-state energy, which agrees well with the numerical simulation in a wide range of the experimental parameters, is given. In particular, the introduced method can successfully solve this Jaynes-Cummings model with the positive detuning (the atomic resonant level is larger than the photon frequency), which can not be treated in the adiabatical approximation and the generalized rotating-wave approximation. Finally, we also demonstrate analytically how to control the mean photon number by means of the current experimental parameters including the photon frequency, the coupling strength, and especially the atomic resonant level.

quant-ph↗
arXiv2011

Light anti-nuclei production in pp collisions at $\sqrt{s}$=7 and 14 TeV

A dynamically constrained coalescence model based on the phase space quantization and classical limit method was proposed to investigate the production of light nuclei (anti-nuclei) in non-single diffractive (NSD) pp collisions at $\sqrt{s}$=7 and 14 TeV. This calculation was based on the final hadronic state in the PYTHIA and PACIAE model simulations, the event sample consisted of 1.2$\times 10^8$ events in both simulations. The PACIAE model calculated $\bar D$ yield of 6.247$\times 10^{-5}$ in NSD pp collisions at $\sqrt{s}$=7 TeV is well comparing with the ALICE rough datum of 5.456$\times 10^{-5}$. It indicated the reliability of proposed method in some extent. The yield, transverse momentum distribution, and rapidity distribution of the $\bar D$, $^3{\bar{He}}$, and $_{\barΛ} ^3{\bar H}$ in NSD pp collisions at $\sqrt{s} $=7 and 14 TeV were predicted by PACIAE and PYTHIA model simulations. The yield resulted from PACIAE model simulations is larger than the one from PYTHIA model. This might reflect the role played by the parton and hadron rescatterings.

nucl-th↗
arXiv2011

Spin-orbit coupling in $d^{2}$ ordered double perovskites

We construct and analyze a microscopic model for insulating rock salt ordered double perovskites, with the chemical formula A$_2$BB'O$_6$, where the magnetic ion B' has a 4d$^2$ or 5d$^2$ electronic configuration and forms a face centered cubic (fcc) lattice. For these B' ions, the combination of the triply-degenerate antisymmetric two-electron orbital states and strong spin-orbit coupling forms local quintuplets with an effective spin moment $j=2$. Moreover, due to strongly orbital-dependent exchange, the effective spins have substantial biquadratic and bicubic interactions (fourth and sixth order in the spins, respectively). This leads, at the mean field level, to a rich ground state phase diagram which includes seven different phases: a uniform ferromagnetic phase with an ordering wavevector ${\bf p} = {\bf 0}$ and uniform magnetization along $[111]$ direction, four two-sublattice phases with an ordering wavevector ${\bf p} = 2π(001)$ and two four-sublattice antiferromagnetic phases. Amongst the two-sublattice phases there is a quadrupolar ordered phase which preserves time reversal symmetry. Extending the mean field theory to finite temperatures, we find ten different magnetization processes with different magnetic thermal transitions. In particular, we find that thermal fluctuations stabilize the two-sublattice quadrupolar ordered phase in a large portion of phase diagram. Existing and possible future experiments are discussed in light of these theoretical predictions.

cond-mat.str-el↗
arXiv2011

Recursion relations for the general tree-level amplitudes in QCD with massive dirac fields

QCD amplitudes with many external fields have been studied for a long time. At tree-level, the amplitudes can be obtained effectively by the BCFW recursion relations. In this article, we extend the Britto-Cachazo-Feng-Witten (BCFW) relations to the QCD amplitude of which the external fields are all massive or include only one massless line. We find such amplitude can be split into two parts and each part of the amplitude is of some correlated spin configuration between the two shifted lines. After choosing proper momentum shift scheme, we can show that each part is constructible directly. Hence, we can obtain a general procedure for the amplitudes in QCD by the BCFW recursion relations. We apply the procedure to several amplitudes as examples. We find such methods are very efficient when there are many massive external fields in the amplitudes.

hep-th↗
arXiv2011

Quantum Magnetism with Polar Alkali Dimers

We show that dipolar interactions between ultracold polar alkali dimers in optical lattices can be used to realize a highly tunable generalization of the t-J model, which we refer to as the t-J-V-W model. The model features long-range spin-spin interactions J_z and J_perp of XXZ type, long-range density-density interaction V, and long-range density-spin interaction W, all of which can be controlled in both magnitude and sign independently of each other and of the tunneling t. The "spin" is encoded in the rotational degree of freedom of the molecules, while the interactions are controlled by applied static electric and continuous-wave microwave fields. Furthermore, we show that nuclear spins of the molecules can be used to implement an additional (orbital) degree of freedom that is coupled to the original rotational degree of freedom in a tunable way. The presented system is expected to exhibit exotic physics and to provide insights into strongly correlated phenomena in condensed matter systems. Realistic experimental imperfections are discussed.

cond-mat.quant-gas↗
arXiv2011

Tunable Superfluidity and Quantum Magnetism with Ultracold Polar Molecules

By selecting two dressed rotational states of ultracold polar molecules in an optical lattice, we obtain a highly tunable generalization of the t-J model, which we refer to as the t-J-V-W model. In addition to XXZ spin exchange, the model features density-density interactions and novel density-spin interactions; all interactions are dipolar. We show that full control of all interaction parameters in both magnitude and sign can be achieved independently of each other and of the tunneling. As a first step towards demonstrating the potential of the system, we apply the density matrix renormalization group method (DMRG) to obtain the 1D phase diagram of the simplest experimentally realizable case. Specifically, we show that the tunability and the long-range nature of the interactions in the t-J-V-W model enable enhanced superfluidity. Finally, we show that Bloch oscillations in a tilted lattice can be used to probe the phase diagram experimentally.

cond-mat.quant-gas↗
arXiv2011

Median statistics and the Hubble constant

Following Gott et al. (2001), we use Huchra's final compilation of 553 measurements of the Hubble constant ($H_0$) to determine median statistical constraints on $H_0$. We find $H_0=68 \pm 5.5$ (or $\pm 1$) $\kmsmpc$, where the errors are the 95% statistical and systematic (or statistical) errors. With about two-third more measurements, these results are close to what Gott et al. found a decade ago, with smaller statistical errors and similar systematic errors.

astro-ph.CO↗
arXiv2010

Enhancement of two photon processes in quantum dots embedded in subwavelength metallic gratings

We show a large enhancement of two-photon absorption processes in nanocrystal quantum dots and of light upconversion efficiency from the IR to the near-IR spectral regime, using a hybrid optical device in which near-IR emitting InAs quantum dots were embedded on top a metallic nanoslit array. The resonant enhancement of these nonlinear optical processes is due to the strong local electromagnetic field enhancements inside the nanoslit array structure at the extraordinary transmission resonances. A maximal two-photon absorption enhancement of more than 20 was inferred. Different high field regions were identified for different polarizations, which can be used for designing and optimizing efficient nonlinear processes in such hybrid structures. Combining nanocrystal quantum dots with subwavelength metallic nanostructures is therfore a promising way for a range of possible nonlinear optical devices.

physics.optics↗
arXiv2010

Exotic phases induced by strong spin-orbit coupling in ordered double perovskites

We construct and analyze a microscopic model for insulating rock salt ordered double perovskites, with the chemical formula A$_2$BB'O$_6$, where the B' atom has a 4d$^1$ or 5d$^1$ electronic configuration and forms a face centered cubic (fcc) lattice. The combination of the triply-degenerate $t_{2g}$ orbital and strong spin-orbit coupling forms local quadruplets with an effective spin moment $j=3/2$. Moreover, due to strongly orbital-dependent exchange, the effective spins have substantial biquadratic and bicubic interactions (fourth and sixth order in the spins, respectively). This leads, at the mean field level, to three main phases: an unusual antiferromagnet with dominant octupolar order, a ferromagnetic phase with magnetization along the $[110]$ direction, and a non-magnetic but quadrupolar ordered phase, which is stabilized by thermal fluctuations and intermediate temperatures. All these phases have a two sublattice structure described by the ordering wavevector ${\boldsymbol Q} =2π(001)$. We consider quantum fluctuations and argue that in the regime of dominant antiferromagnetic exchange, a non-magnetic valence bond solid or quantum spin liquid state may be favored instead. Candidate quantum spin liquid states and their basic properties are described. We also address the effect of single-site anisotropy driven by lattice distortions. Existing and possible future experiments are discussed in light of these results.

cond-mat.str-el↗
arXiv2009

Dyadic Green's functions and electromagnetic local density of states

A formal proof to relate the concept of electromagnetic local density of states (LDOS) to the electric and magnetic dyadic Green's functions is provided. The expression for LDOS is obtained by relating the electromagnetic energy density at any location in a medium at uniform temperature $T$ to the electric and magnetic dyadic Green's functions. With this the concept of LDOS is also extended to material media. The LDOS is split into two terms -- one that originates from the energy density in an infinite, homogeneous medium and the other that takes into account scattering from inhomogenieties. The second part can always be defined unambiguously, even in lossy materials. For lossy materials, the first part is finite only if spatial dispersion is taken into account.

cond-mat.other↗