2016 Hangzhou Symposium on Degenerate Fermi Gaseszimp.zju.edu.cn/~fermigassymposium/Book.pdf ·...

2016 Hangzhou Symposium 2016 Hangzhou Symposium on Degenerate Fermi Gaseson Degenerate Fermi Gases

PROGRAM

June 27-30, 2016

Runrun Shaw Science Building, Rm 211Zhejiang University

Organized by

Zhejiang Universityand

University of Science and Technology of China

Sponsored by

International Collaborative Center for Quantum Matter

Zhejiang Institute of Modern Physics

and

Department of Physics, Zhejiang University

2016 Hangzhou Symposium on Degenerate Fermi Gases page 1/23

About the Symposium

Atomic Fermi gases have been a rapidly developing interdisciplinary field. Evolvingfrom the primary focus on superfluid pairing in BCS-BEC crossover in the early days,this field has transformed and diversified dramatically, as our understanding andknowledge become deeper and broader, with an ever growing toolbox and parametricspace. The numerous experimentally tunable parameters have made the physics muchricher than one can ever imagine. The 2016 Hangzhou Symposium on DegenerateFermi Gases will focus on the frontiers in this field, and will cover a broad spectrumof topics, including, but not limited to, BCS-BEC crossover, dipolar molecules andatoms, Fermi-Fermi and Fermi-Bose mixtures, mixed dimensions, synthetic gaugefields and spin-orbit coupling, alkaline-earth atoms, few-body physics, Fulde-Ferrell-Larkin-Ovchinnikov states, topological phases and other exotic phases.

The Symposium will be organized by the International Collaborative Center forQuantum Matter (ICCQM), Zhejiang Institute of Modern Physics, the Department ofPhysics, Zhejiang University, and by the Key Laboratory of Quantum Information, theUniversity of Science and Technology of China.

Organizing CommitteeQijin ChenWei Yi

SponsorsZhejiang University

VenueRunrun Shaw Science Building, Rm 211Zhejiang University

Accommodation Yuquan Hotel (玉泉饭店，玉古路 138 号，Tel: 0571-87982678)

Lingfeng Hotel (灵峰山庄，玉古路 140 号，Tel: 0571-87979298)

Contacts: Qijin Chen [email protected] Tel: 13868051460Wei Yi [email protected] Tel: 15715694695

Website:http://zimp.zju.edu.cn/~fermigassymposium/

1

http://zimp.zju.edu.cn/~fermigassymposium/

mailto:[email protected]

mailto:[email protected]

http://lqcc.ustc.edu.cn/

http://physics.zju.edu.cn/

http://physics.zju.edu.cn/

http://zimp.zju.edu.cn/

http://zimp.zju.edu.cn/~iccqm/

http://zimp.zju.edu.cn/~iccqm/


Scientific Program

June 26th：Registration (玉泉饭店)

June 27th: 邵科馆，浙大玉泉校区

08:50–09:00 Welcome speech：Qijin Chen (Zhejiang University）

Chair: Hongwei Xiong (Zhejiang University of Technology)09:00–10:00 Hui Hu (Swinburne University of Technology)

Beyond the Gaussian fluctuation theory for two- and three-

dimensional Fermi gases

10:00–10:30 Tea Break

10:30–11:00 Chao Gao (Zhejiang Normal University)

Atom-Dimer Scattering in Bose-Fermi Mixture

11:00–12:00 Chih-Chun Chien (UC,Merced)

Quantum Transport in Ultracold Atoms

12:00–14:00 LunchChair: Hui Hu14:00–15:00 Congjun Wu (UCSD)

The p-orbital BEC and fermions

15:00–15:30 Wei Zhang (Remin University of China)

Polaron and Molecule transition in Fermi gases with Orbital

Feshbach Resonance

15:30–16:00 Tea Break

16:00–17:00 Bo Yan (Zhejiang University)

Ultracold polar molecules in an 3D optical lattice

18:00–20:00 Dinner


Chair：Yan He09:00–10:00 Yu-Ao Chen (USTC)

Observation of two-species vortex lattices in a mixture of

mass-imbalance Bose and Fermi superfluids

10:00–10:30 Tea Break

10:30–11:00 Hao Guo (Southeastern University)

Algebraic properties of the vacua and thermal vacua of BCS

and BEC superfluids

11:00–12:00 Xiaoling Cui (Institute of Physics, CAS)

One-dimensional Fermi gases with “p”-wave interaction

12:00–14:00 Lunch

2


Chair: Yu-Ao Chen14:00–15:00 Yan He (Sichuan University)

Establishing the Gauge Invariant Linear Response of

Fermionic Superfluids with Pair Fluctuations

15:00–15:30 Yu Chen (Capital Normal University)

Cavity assited polaron of Fermi gases in a lossy ring cavity

15:30–16:00 Tea Break

16:00–17:00 Lianyi He (Tshinghua University)

Dynamic density and spin responses in the BCS-BEC

crossover: Toward a theory beyond RPA

18:00–20:00 Dinner


Chair: Xianlong Gao (Zhejiang Normal University)09:00–10:00 Han Pu (Rice University)

Novel quantum states in spin-orbit coupled quantum gases

10:00–10:30 Tea Break

10:30–11:00 Wei Yi (USTC)

Universal trimer emerging from a spin-orbit coupled Fermi gas

11:00–12:00 Pengjun Wang (Shanxi University)

2D spin orbit coupling in ultracold Fermi gas

12:00–14:00 LunchChair: Han Pu14:00–15:00 Haibin Wu (East China Normal University)

Universal dynamics in strongly interacting Fermi gases

15:00–15:30 Ran Qi (Renmin University of China)

Efimovian Expansion in Scale Invariant Fermi Gases

15:30–16:00 Tea Break

16:00–17:00 Lan Yin (Peking University)

Topological quantum phases of dipolar Fermi gases

18:00–20:00 Dinner (山外山)

June 30th:邵科馆，浙大玉泉校区

Chair: Xingcan Yao (University of Science and Technology of China)09:00–10:00 Qijin Chen (Zhejiang University)

Superfluidity, pairing, and other exotic quantum states in

ultracold atomic Fermi gases

10:00–10:45 Tea Break

10:45–11:45 Lei Wang (Institute of Physics, CAS)

Simulation of Hubbard Models in the Era of Synthetic Gauge

Field

12:00–14:00 Lunch

3


Chair: Yu-Ao Chen14:00–15:00 Gentaro Watanabe (Zhejiang University)

Non-linear phenomena in superfluid Fermi gases in an optical

lattice --- swallowtails and period doubling

15:00–15:30 Tea Break

15:30–16:30 Yu Wang (Wuhan University)

Thermal valence-bond-solid transition and cooling of SU(2N)

ultra-cold Dirac fermions

16:30-17:00 Closing/Panel Discussion

18:00–20:00 Dinner

4


AbstractsAbstracts

5


Imbalanced superfluidity in tunable atomic Fermi gases

Hui Hu (胡辉)Swinburne University of Technology

The unprecedented controllability of ultracold atomic Fermi gases makes them an ideal platformto explore exotic fermionic superfluidity with mass or spin-population imbalance. Here, inspiredby the recent experimental advances, we discuss possible Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) superfluidity in some new cold-atom setups, for example, a Fermi gas withspin-orbit coupling, in two dimensions or near an orbital Feshbach resonance. In the presence ofspin-orbit coupling, we show the existence of a novel gapless topological FF superfluid. In twodimensions, we explore the enlarged parameter space of the LO superfluid. Finally, near anorbital Feshbach resonance (described a two-band model Hamiltonian), we examine the co-existence of FF pairing and Sarma pairing in different bands.

Atom-dimer scattering in Bose-Fermi mixtures

Chao Gao (高超)

Zhejiang Normal University

we considered the atom-dimer scattering problem in Bose-Fermi mixtures where the dimer isheteronuclear and the atom is a boson. We found that the boson-boson interaction, thoughnaturally weak, and was neglected in a previous study, can significantly change the atom-dimerscattering length, and even drive resonances. Such property can be manifested in the many-bodysystem.

6


Quantum transport in ultracold atoms

Chih-Chun Chien (简志均)University of California, Merced

Recent advance in cooling and trapping atoms has brought exciting quantum phenomena,including atomic Bose-Einstein condensate (BEC), superfluid, Mott insulator, and topologicalphases. One realistic yet ambitious goal is to use cold-atoms and optical potentials to simulateand complement electronic devices, and this leads to a thriving field called atomtronics. Incontrast to electronic systems, cold-atoms are charge neutral and isolated from theirenvironment. Moreover, strong quantum effects drastically alter transport properties of cold-atoms. I will first review breakthroughs in the study of quantum transport using various cold-atom systems and contrast theoretical descriptions from open- vs. closed- system approaches.Then I will discuss the question of whether quantum memory effects can arise in simple,isolated, and noninteracting systems. Although a lack of competing time scales commonly resultsin a negative answer, I will show that an affirmative answer can be found in cold-atom systemsby utilizing geometrical effects of optical lattices.

References: C. C. Chien, S. Peotta, and M. Di Ventra, “Quantum transport in cold atoms”, Nat. Phys. 11,998 (2015).C. Y. Lai and C. C. Chien, “Geometry-induced memory effects in isolated quantum systems:Observations and applications”, Phys. Rev. Applied 5, 034001 (2016).

The p-orbital BEC and fermions

Congjun Wu (吴从军)

University of California, San Diego

The first part is to introduce the idea of unconventional symmetry to BEC condensation. The

second part particularly focuses on fermions – actually the long-stand problem of itinerant

ferromagnetism. It is a traditional strong correlation physics problem.

7


Polaron and molecule transitions in Fermi gases with orbital Feshbachresonance

Wei Zhang (张威)Renmin University of China

The study of the impurity problem in the presence of a background Fermi sea is a long-lastingtopic in condensed matter physics. In this talk, we discuss the property of such a system indegenerate Fermi gases with orbital Feshbach resonance. We identify a polaron-moleculetransition, and investigate various properties within both regimes.

Ultracold polar molecules in an 3D optical lattice

Bo Yan (颜波)

Zhejiang University

Long-range dipolar interactions are expected to facilitate an understanding of strongly

correlated many-body quantum systems such as quantum magnets and high Tc

superconductors. Ultracold polar molecules in an optical lattice are an excellent experimental

platform for studying interactions, since they possess a large electric dipole moment and can

be created at ultracold temperatures and manipulated with exquisite precision. Here we will

talk about the observation of long-range dipolar spin exchange interactions for ultracold polar

molecules confined in a deep three-dimensional optical lattice. With spin encoded in

rotational states, the interactions manifest as a density dependent and oscillating decay of the

spin coherence of the system. These results motivated and are consistent with a new many-

body theoretical calculation.

8


Observation of two-species vortex lattices in a mixture of mass-imbalance Boseand Fermi superfluids

Yu-Ao ChenUniversity of Science and Technology of China

Algebraic properties of the vacua and thermal vacua of BCS and BECsuperfluids

Hao Guo (郭昊)Southeastern University

We first present the algebraic properties of the family of ground states of BCS and BECsuperfluids. These ground states share some similar properties, such as saturating theRobertson-Schrodinger inenquality, having theta-vacuum structures and being classified by theq-deformed Hopf algebra. However, they acquire different Berry phases when the correspondingsystem evolves in the parameter space. This implies that there is some topological change when aFermi system undergoes BCS-BEC crossover. Secondly, we study the thermal vacuumformalism for BCS and BEC superfluids. The equations of states of the corresponding system canbe easily obtained within this formalism.

9


One-dimensional Fermi gases with p-wave interactions

Xiaoling Cui (崔晓玲)Institute of Physics, CAS

In this talk, I will discuss two significant effects induced by p-wave (or odd-wave) interaction inone-dimensional Fermi gases. One is to enable a bran-new universal system near the p-waveresonance; the other is to engineer quantum magnetisms that have been long sought in solidstates and cold atoms systems. These results suggest fascinating few-body and many-bodyphysics to be explored in 1D atomic gases with p-wave interaction.

References:1. X.Cui, arxiv: 1605.043632. L.Yang, X-W. Guan and X.Cui, Phys.Rev.A 93, 051605 (Rapid Communication) (2016) (arxiv: 1511.09377)

Establishing the gauge invariant linear response of fermionic superfluids withpair fluctuations

Yan He (贺言)Sichuan University

We present a gauge invariant linear response theory for ultra-cold Fermi gases undergoing BCS-BEC crossover with pair fluctuation effect. Below Tc, the construction of a gauge invariant vertexis non-trivial, because the effects of Nambu-Goldstone modes (collective modes) intertwineswith that of the pairing fluctuation. We focus on the G0G t-matrix approach to the pairfluctuations, although our formalism should also works equally well for the G0G0 t-matrixtheory.

1


Cavity assited polaron of Fermi gases in a lossy ring cavity

Yu Chen (陈宇)Capital Normal University

The combination of strong coupling cavity and quantum degenerate gases gives us an idealplatform to study superradiance, emergent lattices and nonequilibrium phenomena. Manyprevious theoretical studies assumed the system could reach a steady state after long timedynamics and the atomic excitations are well defined quasi-particles. However, the validity ofthis assumption is not fully justified yet. In this talk we will present a single particle excitationspectrum study of degenerate Fermi gases in a lossy ring cavity. We find that the single particleexcitation is strongly modified by the presence of polariton particle-hole resonance, causing a“scar" in single particle spectrum when cavity decay rate is small. The influence of the cavityloss on the single particle spectrum is non-monotonic, where the most unstable mode reach amaximal decay rate in an intermediate cavity loss rate then decreases to zero in large cavity losslimit.

Dynamic density and spin responses in the BCS-BEC crossover: Toward atheory beyond RPA

Lianyi He (何联毅)Tsinghua University

We present a field theoretical derivation of the dynamic density and spin linear responsefunctions of a dilute superfluid Fermi gas in the BCS-BEC crossover in both three and twodimensions. The derivation of the response functions is based on the elegant functional pathintegral approach which allows us to calculate the density-density and spin-spin correlationfunctions by introducing the external sources for the density and the spin density. Since thegenerating functional cannot be evaluated exactly, we consider two gapless approximationswhich ensures a gapless collective mode (Goldstone mode) in the superfluid state: the BCS-Leggett mean-field theory and the Gaussian-pair-fluctuation (GPF) theory. In the mean-fieldtheory, our results of the response functions agree with the known results from the random phaseapproximation. We further consider the pair fluctuation effects and establish a theoreticalframework for the dynamic responses within the GPF theory. We show that the GPF responsetheory naturally recover three kinds of famous diagrammatic contributions: the Self-Energycontribution, the Aslamazov-Lakin contribution, and the Maki-Thompson contribution. We alsoshow that unlike the equilibrium state, in evaluating the response functions, the linear (first-order) terms in the external sources as well as the induced order parameter perturbations shouldbe treated carefully. In the superfluid state, there is an additional order parameter contributionwhich ensures that in the static and long wavelength limit, the density response function recoversthe result of the compressibility (compressibility sum rule). We expect that the f-sum rule ismanifested by the full number equation which includes the contribution from the Gaussian pairfluctuations.

1


Novel quantum states in spin-orbit coupled quantum gases

Han Pu (浦晗)Rice University

Spin-orbit coupling (SOC) in cold atoms is a single-particle effect. SOC dramatically changesthe single-particle dispersion. In this talk, I will show that the modification of the single-particledispersion can have interesting effects in many-body settings. I will present a couple ofexamples, one is a repulsive Fermi gas subject to Rashba SOC which exhibits itinerantferromagnetism, and the other is an attractive condensate with 3D SOC which forms a self-trapped soliton-like state in free space.

Universal trimers emerging from a spin-orbit coupled Fermi gas

Wei Yi (易为)Universit of Science and Technology of China

We report the existence of a universal trimer state induced by an impurity interacting with atwo-component spin-orbit coupled Fermi gas. In the zero-density limit with a vanishing Fermisea, the trimer is stabilized by the symmetry of the single-particle spectrum under spin-orbitcoupling, and is therefore universal against the short-range details of the interaction potential.When the Fermi energy increases, we show that the trimer is further stabilized by particle-holefluctuations over a considerable parameter region. We map out the phase diagram consisting oftrimers, molecules, and polarons, and discuss the detection of these states using radio-frequencyspectroscopy. The universal trimer revealed in our work is exemplary of intriguing three-bodycorrelations emerging from a many-body environment, which, in our case, is cooperativelysupported by the single-particle spectral symmetry and the collective particle-hole excitations.

1


2D spin-orbit coupled in ultracold Fermi gases

Pengjun Wang (王鹏军)Shanxi University

Universal dynamics in strongly interacting Fermi gases

Haibin Wu (武海斌)East China Normal University

1


Efimovian expansion in scale invariant Fermi gases

Ran Qi (齐燃)Renmin University of China

Scale invariance plays an important role in unitary Fermi gases. Discrete scaling symmetry manifests itself in quantum few-body systems such as the Efimov effect. Here we report the theoretical prediction and experimental observation of a distinct type of 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, the expansion of the cloud size exhibits a sequence of plateaus. The locations of these plateaus obey a discrete geometric scaling law with a controllable scale factor and the expansion dynamics is governed by a log-periodic function. This striking expansion shares the same scaling law and mathematical description as the Efimov effect.

Topological quantum phases of dipolar Fermi gases

Lan Yin (尹澜)Peking University

Ultracold quatum gases with electric and magnetic dipole moments have been realized inexperiments, which may offer opportunities to create topological quantum phases in thesesystems. Under a rotating DC electric or magnetic fields, a chiral p-wave superfluid phase canbe generated in a square optical lattice and a Weyl superfluid can be created in threedimensional. Fractional Chern states can also be created in optical lattices with topologicalnotrivial bands at fractional fillings.

1


Exotic quantum states in ultracold atomic Fermi gases

Qijin Chen (陈启谨)Zhejiang University

Ultracold atomic Fermi gases have been a rapidly growing field over the past 15 years, and

have attracted enormous attention from various fields including condensed matter, AMO,

nuclear matter, astrophysics and particle physics. Owing to the large tunability of various

parameters, esp. the pairing interaction strength and population imbalance, atomic Fermi

gases have become a prototype for studying and engineering many existing and non-existing

manybody quantum systems. In this talk, I will present some of our latest work on atomic

Fermi gases in the high density regime or in the presence of anisotropic long-range dipole-

dipole interactions, where we find emergent pair density wave states when inter-pair

interactions dominate. In addition, I will discuss the instability of the Fulde-Ferrel-Larkin-

Ovchinnikov states at large population imbalances, and show that the FFLO states are

intrinsically unstable in 3D.

Simulation of Hubbard models in the era of synthetic gauge field

Lei Wang (王磊)Institute of Physics, CAS

Interacting fermions dressed by artificial gauge fields exhibit intriguing ground state phases dueto the interplay of strong correlation, band topology and conventional long range orders. Usingunbiased quantum Monte Carlo (QMC) simulations we address several questions related to thecurrent experimental efforts: topological phase transition from a quantum-spin-Hall insulator toa superfluid state; phase diagram and fermionic quantum criticality of mass-imbalancedHubbard models.

The last study is enabled by very recent advances in solving the fermion sign problem ofparticular models in QMC. I will mention some relevant ones to the ultracold atom community.

1


Non-linear phenomena in superfluid Fermi gases in an optical lattice :swallowtails and period doubling

Gentaro Watanabe (渡边元太郎)Zhejiang University

Interplay between the non-linearity due to the emergence of the superfluid order parameter andthe effect of a periodic potential is one of the most important issues of ultracold atomic gases inan optical lattice. However, the study of non-linear phenomena of superfluid Fermi gases is at avery infant stage unlike that for Bose gases. In this talk, we will discuss two types of non-linearphenomena in this system, which we have found recently: 1) the loop structure in the energyband called "swallowtail" and 2) multiple period states whose period does not coincide with thatof the lattice, instead, a multiple of it. Throughout this talk, we will shed light on uniqueproperties of superfluid Fermi gases which are absent in the bosonic counterpart.

Thermal valence-bond-solid transition and cooling of SU(2N) ultra-cold Diracfermions

Yu Wang (王宇)Wuhan University

We have investigated the effects of the SU(2N) symmetries on the system of interacting Diracfermions at finite temperatures, including the thermal valence-bond-solid transition, thePomeranchuk cooling effect, the density compressibility and the uniform spin susceptibility, byperforming, for the first time, the determinant quantum Monte Carlo simulations of the half-filledSU(2N) Hubbard model on a honeycomb lattice. Owing to the discrete symmetry breaking in theSU(2N) Dirac system, a finite-temperature phase transition between a disordered state and acolumnar valence-bond-solid state is realized in the two-dimensional SU(2N) Hubbard model,whereas the broken continuous symmetry prohibits a thermal phase transition in a two-dimensional SU(2) Hubbard model according to the Mermin-Wagner theorem. The SU(2N)symmetry also significantly facilitates the interaction-induced adiabatic cooling in the Diracsystem, an analogue of Pomeranchuk effect originally proposed in the 3He systems, which startswith even relatively high entropies and drives the fermionic system to lower temperature regime.Our work sheds new light on future explorations of novel states of matter with ultra-cold atomexperiments.

1


1


2


Symposium Agenda

2016/6/27 2016/6/28 2016/6/29 2016/6/30

9:00 - 10:00 Hui Hu Yuao Chen Han Pu Qijin Chen

10:00 – 10: 30 Tea Break

10:30 – 11: 00 Chao Gao Hao Guo Wei Yi

11:00 – 12:00 Chih-Chun Chien Xiaoling Cui Pengjun Wang Lei Wang

Lunch

14:00 – 15:00 Congjun Wu Yan He Haibin Wu Gentaro Watanabe

15:00 – 15:30 Wei Zhang Yu Chen Ran Qi

15:30 – 16:00 Tea Break

16:00 – 17:00 Bo Yan Lian Yi He Lan Yin Yu Wang

Venue: Runrun Shaw Science Building (邵毅夫科学馆，简称邵科馆），

Yuquan Campus, Zhejiang University

Hotels: Yuquan Hotel (玉泉饭店，玉古路138号，Tel: 0571-87982678)

Lingfeng Hotel (灵峰山庄，玉古路140号，Tel: 0571-87979298)

Dinner will be served at the restaurant of Yuquan Hotel on June 26. Please arrive early to catch the dinner.

Registration desk will be available at Yuquan Hotel on Sunday afternoon and evening.

Contacts:

Qijin Chen 13868051460

Wei Yi 15715694695

2

2016 Hangzhou Symposium on Degenerate Fermi Gaseszimp.zju.edu.cn/~fermigassymposium/Book.pdf ·...

Documents

Transcript of 2016 Hangzhou Symposium on Degenerate Fermi Gaseszimp.zju.edu.cn/~fermigassymposium/Book.pdf ·...