Quantum phases of Bose gases on a lattice with pair-tunneling

We investigate the strongly interacting lattice Bose gases on a lattice with two-body interaction of nearest neighbors characterized by pair tunneling.The excitation spectrum and the depletion of the condensate of lattice Bose gases are investigated using the Bogoliubov transformation method and the results show that there is a pair condensate as well as a single particle condensate.The various possible quantum phases,such as the Mott-insulator phase(MI),the superfluid phase(SF) of an individual atom,the charge density wave phase(CDW),the supersolid phase(SS),the pair-superfluid(PSF) phase,and the pair-supersolid phase(PSS) are discussed in different parametric regions within our extended Bose-Hubbard model using perturbation theory.     

自旋-轨道耦合作用下玻色-爱因斯坦凝聚在量子相变附近的朗道临界速度

各向异性超流体中的朗道临界速度并非简单地由运动方向的元激发能谱决定.在自旋-轨道耦合作用下的双分量玻色-爱因斯坦凝聚中,当系统跨过平面波相与零动量相之间的量子相变时,尽管超流声速连续变化,但垂直于自旋-轨道耦合方向的朗道临界速度会出现跳变,跳变幅度随自旋相互作用强度单调增加.根据线性响应理论,计算了凝聚体中运动杂质在不同速度下的能量耗散率,提出可以通过能量耗散观测临界速度在量子相变处的不连续性.     

Many-body exceptional points in colliding condensates

ABSTRACT

Exceptional points describe the coalescence of the eigenmodes of a non-Hermitian matrix. When an exceptional point occurs in the unitary evolution of a many-body system, it generically leads to a dynamical instability with a finite wavevector [N. Bernier et al., Phys. Rev. Lett. 113, 065303 (2014)]. Here, we study exceptional points in the context of the counterflow instability of colliding Bose–Einstein condensates. We show that the instability of this system is due to an exceptional point in the Bogoliubov spectrum. We further clarify the connection of this effect to the Landau criterion of superfluidity and to the scattering of classical particles. We propose an experimental set-up to directly probe this exceptional point, and demonstrate its feasibility with the aid of numerical calculations. Our work fosters the observation of exceptional points in nonequilibrium many-body quantum systems.     

2003 Nobel Prize in physics for theoretical work on superfluid 3He.

The element helium comes in two (stable) forms, 4He and 3He; at low temperatures and pressures both form liquids rather than solids. The liquid phase of the common isotope, 4He, was realized nearly a century ago, and since 1938 has been known to show, at temperatures below about 2K, the property of superfluidity--the ability to flow through the narrowest capillaries without apparent friction. The light isotope, 3He, is believed to be of quite a different nature; however, because of its similarity to the electrons in metals, which at low temperatures sometimes form "Cooper pairs" and thereby become superconducting, theorists in the 1960s and early 1970s had speculated that something similar might happen in liquid 3He, which would then also show superfluidity though for reasons rather different than 4He. In 1972 nuclear magnetic resonance (NMR) experiments at Cornell University revealed the existence, below 3 millidegrees, if two new phases, one of which displayed extraordinary NMR properties. Anthony Leggett is one of the theorists who succeeded in fitting the experimental properties into the "Cooper-pairing" scenario; in particular, he explained the NMR behavior and predicted further novel NMR phenomena which were subsequently found.     

同位旋非对称核物质性质与扩展的BHF方法(Ⅶ)中子星物质中质子和中子的1S0态超流性

利用Brueckner Hartree Fock和BCS理论方法,计算了β稳定中子星物质中处于¹S₀ 态的质子和中子的对关联能隙,着重研究和讨论了三体核力的影响.结果表明三体核力对β稳定中子星物质中¹S₀态中子超流性的影响相对较小,但对¹S₀态质子超流性具有重要影响,而且其效应随核子数密度增大而迅速增强.三体核力的主要作用是强烈地抑制了高密度β稳定中子星物质中的1S0态质子超流性.     

3PF2 neutron superfluidity in neutron stars and three-body force effect

We investigate the ^3PF2 neutron superfluidity in H-stable neutron star matter and neutron stars by using the BCS theory and the Brueckner-Hartree-Fock approach. We adopt the Argonne V18 potential supplemented with a microscopic three-body force as the realistic nucleon-nucleon interaction. We have concentrated on studying the threebody force effect on the ^3PF2 neutron pairing gap. It is found that the three-body force effect is to enhance remarkably the ^3PF2 neutron superfluidity in neutron star matter and neutron stars.     

Superfluidity of Paired Bosons from Correlated Tunneling

We study superfluidity of paired Bosonic atoms in optical lattices. The atoms have strong repulsive on-site energy. Single atom tunneling is severely suppressed while the atom-pair may co-tunnel by the second order quantum transition, which induces paired superfluidity as repulsive nearest-neighbor interactions are included. The mean-field phase diagram and low energy excitations are explored for a square lattice system.     

偶极玻色-爱因斯坦凝聚体中的各向异性耗散

针对偶极相互作用的玻色-爱因斯坦凝聚体,解析计算了点状杂质沿平行极化轴和垂直极化轴运动的能量耗散率,证明了在超流临界速度更大的方向上耗散率也更高.该结论为最近在162Dy原子气体中观测到的实验现象提供了理论支持.对于一般的运动方向,给出了耗散率在高速极限下以及临界速度附近的渐近形式.结合数值计算的结果,论证了耗散率随方向角的变化总是表现出与临界速度一致的各向异性.     

p‐Wave Superfluid Phases of Fermi Molecules in a Bilayer Lattice Array

The superfluid $\mathbf{p}=p_x+{\mathit{ip}}_y$ phases in an ultracold gas of dipolar Fermi molecules lying in two parallel square lattices in 2D are investigated. As shown by a two‐body study, dipole moments oriented in opposite directions in each layer are the key ingredients in our mean‐field analysis from which unconventional superfluidity is predicted. The $T=0$ phase diagram summarizes our findings: stable and metastable superfluid phases appear as a function of both, the dipole–dipole interaction coupling parameter and filling factor. A first‐order phase transition, and thus a mixture of superfluid phases at different densities, is revealed from the coexistence curves in the metastable region. The model predicts that these superfluid phases can be observed experimentally at 10 nK in molecules of NaK confined in optical lattices of size $a=532$ nm. Other routes to reach higher temperatures require the use of subwavelength confinement technique .     

3He Impurities in Nearly Frictionless Transport of Solid 4He at Low Temperatures

Glassy matter, when subjected to high shear rates exhibit shear thinning, that is, the viscosity diminishes with increasing shear rate. One possible outcome for the almost vanishing viscosity is nearly frictionless transport, which is possible in solid ⁴He due to the presence of minute concentrations of ³He. The glassy state of solid ⁴He is also relevant to the possible onset of superuidity in solid ⁴He. By treating the solid ⁴He locally as an amorphous matter and using the transition‐rate dependent model together with the specific activation energy, we observe a series of sudden changes of the shearing stresses (which directly relates to the resistance) at corresponding onset temperatures of supersolid ⁴He (ranging from 175 to 1200 mK) for different activation volumes of ³He. Even at higher concentrations of ³He than previous reported (around 1700 ppm), the supersolidity of ⁴He still occurs.