At the time of the first workshop in this series in 1993, the only experimentally realized Bose condensate (at least in the simple sense conjectured by Einstein) was liquid 4 He. In the intervening twenty-plus years, much has happened in the world of Bose-Einstein condensation (BEC). Probably the most exciting development has been the attainment of condensation in ultracold bosonic atomic gases such as 87 Rb and 23 Na in 1995, followed a few years later by the achievement of degeneracy and eventually Bardeen-Cooper-Schrieffer (BCS) pairing in their fermionic counterparts, and the experimental realization of the theoretically long-anticipated “BEC-BCS crossover” by using the magnetic field degree of freedom to tune the system through a Feshbach resonance. One particularly fascinating aspect of the latter has been the realization of a “unitary gas” at the resonance itself – a system which prima facie has no characteristic length scale other than the interparticle separation, and is therefore a major challenge to theorists. Other systems in which BEC has been realized, sometimes transiently, include exciton-polariton complexes in semiconducting microcavities and, at least in a formal sense, the magnons in a magnetic insulator, as well as ultracold gases with a nontrivial and sometimes large “spin” degree of freedom. As compared with our “traditional” Bose condensate, liquid 4 He, these new systems typically have many more (and more rapidly adjustable) control parameters, and have therefore permitted qualitatively new types of experiment. One particularly fascinating development has been the use of optical techniques to generate “synthetic gauge fields” and thus mimic some of the topologically nontrivial systems which have recently been of such intense interest in a condensed-matter setting. At the same time, there remain long-standing issues from helium physics, such as the nature and consequences of “spontaneously broken U(1) symmetry,” the “Kibble-Zurek” mechanism, and more generally the relaxation of strongly nonequilibrium states to equilibrium; in some cases, the new systems have been used to address these more quantitatively than was possible with 4 He. The chapters in this volume address all of these questions and more, and should be of intense interest to both the experimental and the theoretical sides of the BEC community.
At the time of the first workshop in this series in 1993, the only experimentally realized Bose condensate (at least in the simple sense conjectured by Einstein) was liquid 4 He. In the intervening twenty-plus years, much has happened in the world of Bose-Einstein condensation (BEC). Probably the most exciting development has been the attainment of condensation in ultracold bosonic atomic gases such as 87 Rb and 23 Na in 1995, followed a few years later by the achievement of degeneracy and eventually Bardeen-Cooper-Schrieffer (BCS) pairing in their fermionic counterparts, and the experimental realization of the theoretically long-anticipated “BEC-BCS crossover” by using the magnetic field degree of freedom to tune the system through a Feshbach resonance. One particularly fascinating aspect of the latter has been the realization of a “unitary gas” at the resonance itself – a system which prima facie has no characteristic length scale other than the interparticle separation, and is therefore a major challenge to theorists. Other systems in which BEC has been realized, sometimes transiently, include exciton-polariton complexes in semiconducting microcavities and, at least in a formal sense, the magnons in a magnetic insulator, as well as ultracold gases with a nontrivial and sometimes large “spin” degree of freedom. As compared with our “traditional” Bose condensate, liquid 4 He, these new systems typically have many more (and more rapidly adjustable) control parameters, and have therefore permitted qualitatively new types of experiment. One particularly fascinating development has been the use of optical techniques to generate “synthetic gauge fields” and thus mimic some of the topologically nontrivial systems which have recently been of such intense interest in a condensed-matter setting. At the same time, there remain long-standing issues from helium physics, such as the nature and consequences of “spontaneously broken U(1) symmetry,” the “Kibble-Zurek” mechanism, and more generally the relaxation of strongly nonequilibrium states to equilibrium; in some cases, the new systems have been used to address these more quantitatively than was possible with 4 He. The chapters in this volume address all of these questions and more, and should be of intense interest to both the experimental and the theoretical sides of the BEC community.
The year 2015 marks the 20th anniversary of the achievement of Bose–Einstein condensation (BEC) in atomic vapors. Atomic BEC, realized by Eric Cornell, Carl Wieman and Wolfgang Ketterle in 1995, commenced an exciting wave to explore quantum few- and many-body physics. Different from solid state matter and helium superfluids, atomic condensates in the gas phase come with drastically lower densities and temperatures, which place the systems in a unique regime where the atomic interactions are simple and precisely known. Furthermore, the condensates are typically weakly interacting: the interaction length scale of few nanometers is much shorter than the interatomic separation of hundreds of nanometers. Thus many early experimental observations can be well understood based on mean-field theory. One celebrated model is the Gross-Pitaevskii equation which provides a precise and quantitative description of weakly interacting Bose condensates [1]. Opportunities to extend cold atom research beyond the weak interaction regime emerged in the early 2000s. Several experimental schemes were developed to prepare atoms in the strong correlation and interaction regimes. One approach is loading atoms into an optical lattice, the interference pattern of laser beams. The lattice potential enhances the interactions of localized atoms and quenches their kinetic energy. A variety of lattice configurations are realized that promise novel quantum phases of ultracold atoms. Another route to strong interaction is based on Feshbach resonance, typically induced by magnetically shifting a molecular state near the scattering continuum of atoms. The resonant coupling between scattering atoms and the molecular state leads to a divergence of the atomic interaction length scale. Exactly on Feshbach resonance, the collision cross section reaches the quantum mechanical limit, and the system becomes strongly hydrodynamic with mean-free path as short as interparticle separation. Fascinating observations in the strong coupling regime soon followed: prominent examples include the superfluid-Mott insulator transition in optical lattices [2], resonantly interacting Fermi gases in the BEC–BCS (Bardeen–Cooper–Schieffer superfluid) crossover [3] and Efimov three-body states [4], and they lead to tremendous research interest into novel quantum states and quantum dynamics. One of the key motivations in these explorations is to uncover new, universal principles that underpin the physics of strongly correlated quantum systems. A well-studied universal behavior in many-body physics manifests in the critical phenomena near a continuous phase transition. Near the critical point, the system develops a diverging correlation length, which suppresses the dependence on other microscopic length scales. The systems are thus scale invariant and can be universally characterized by a few critical exponents. In cold atoms, divergence of correlation lengthes has been observed near the superfluid phase transition [5,6], as well as the superfluid-Mott insulator quantum phase transition in optical lattices. In the lattice experiments, scale invariance can be tested based on the equation of state measurements [7]. These works complement the transport measurements in condensed matter experiments. Has a new class of universal physics been discovered in atomic quantum gases? Yes, it turns out that a special class of strongly interacting quantum gas with ‘intrinsic’ scaling symmetry emerges near a Feshbach resonance [8]. Near the resonance, the scattering length scale diverges a → ∞ [9], unimaginable in classical systems, and the interaction length reaches the maximum value allowed by quantum mechanics as 1/k, called the ‘unitarity limit’ and k is the momentum of the scattering state. A famous example is the strongly interacting Fermi gas in the BEC–BCS crossover regime, where the resonant interactions lead to strong pairing and and universal hydrodynamic behavior. An exciting development in the study of the BEC–BCS crossover focuses on the Fermi gas exactly on Feshbach resonance. Here the system, also called a ‘unitary Fermi gas’, is the most complex and strongly correlated. The resonant interaction, however, implies a special scaling symmetry of the system. To understand the symmetry, we consider a scale transformation: x → Λx that stretches the coordinates of all atoms by a factor of Λ, and thus reduces the atomic density n by a factor of Λ3 in a 3D gas and the kinetic energy Ek ∝ −∇2 by Λ2. One can see that both kinetic energy Ek and interaction energy in the unitarity limit na → n/k are reduced by the same factor Λ2. The system is thus hypothesized to be invariant under the scale transformation [8]. Remarkably, similar scaling symmetry also holds for 2D quantum gas with a constant coupling g = const. [10]. Here stretching the size of the 2D gas by a factor of Λ also reduces both the kinetic Ek and interaction energy gσ by Λ2, where σ is the 2D density. Since the scaling symmetry in these systems comes directly from the form of the interactions, intrinsic scale invariance is robust at all temperatures T and chemical potentials μ and does not rely on the proximity to a phase transition. The scaling symmetry suggests that the only independent variables are the ratios of the length scales or the equivalent energy scales. In a grand canonical ensemble, the independent ratio can be chosen as μ/kBT. This realization greatly simplifies theoretical description as well as the experimental characterization of the system. Theoretically, a new set of universal thermodynamic relations are expected to be insensitive to the microscopic details; experimentally, measurements are expected to collapse into universal functions that only depend on μ/kBT. Measurements on unitary Fermi gas and the 2D Bose gas are consistent with the scaling hypothesis, shown in Fig. 1(a) and (b). Remarkable quantitative agreement between measurements and calculations has greatly advanced our understanding of the ‘unitary Fermi gas’ and 2D atomic gas. Ultracold atomic gases which are intrinsically scale invariant: (a) unitary Fermi gas, (b) 2D Bose gas and (c) unitary Bose gas. The scaling symmetry suggests that thermodynamic observables only depend on the ratio of chemical potential and thermal energy |μ~=μ/kBT|. In (a), thermodynamic pressure of a unitary Fermi gas is compared with calculations and other measurements. Figure courtesy of M. Zwierlein [11]. In (b), the collapse of the density measurement at various temperatures (inset) confirms the scaling symmetry. Figure adopted from [12]. Panel (c) shows the three-body loss spectrum in a 6Li133Cs mixture. The geometric progression of the Efimov resonance positions at B = B1, B2 and B3 toward the Feshbach resonance at B0 results from the discrete scaling symmetry of Efimov physics. Figure adopted from [13]. We may expect the same scaling symmetry for a resonantly interacting Bose gas. Here, however, the scaling symmetry is more subtle because of the presence of three-body physics. Predicted by Vitaly Efimov in the context of nuclear physics [14], an infinite number of three-body bound states exist for resonantly interacting bosons and their energies follow a discrete scaling law, breaking the otherwise continuous scaling symmetry. Identical fermions do not form such three-body states due to Pauli's exclusion principle. Theoretically the three-body bound states are supported by the ‘Efimov potential’ V(R)∝ − 1/R2, where R is the characteristic length scale of the three interacting atoms. While the Efimov potential does not break the continuous scaling symmetry, the presence of discrete three-body states does break the symmetry. An alternative picture based on renormalization group limited cycles [15] also explains the discrete scaling symmetry of the system, a scenario that rarely happens in electronic systems. Experimentally, Efimov states have been observed in cold gases of alkali atoms, as well as in helium-4. The geometric scaling symmetry of Efimov states is reported in a recent experiment, shown in Fig. 1(c). The same discrete scaling symmetry is expected for unitary quantum gas of bosons, which, if confirmed, will establish a unique and fascinating class of strongly interacting quantum systems. While in this article, we focus on atoms in optical lattices and unitary atomic gases as primary examples of universal physics that have been actively investigated, efforts in many new research frontiers have shown promising prospects to uncover new universal physics in the strongly correlated and interacting systems. Non-equilibrium dynamics is one of the hottest directions. Ultracold atoms are particularly suited for the study of quantum dynamics due to their slow evolution time scales of miniseconds to seconds. Dynamics near a quantum phase transition and the thermalization of isolated quantum systems are two research topics with many unanswered questions. Cold atom experiments may provide new universal insights that go beyond current theoretical framework. Another exciting direction is quantum many-body systems with long range or very strong interactions. Examples include trapped ions, and condensates of exotic atoms with huge magnetic dipole moments, polar molecules, as well as those with Rydberg excitations. These systems possess various long range interactions. Novel universal dynamics, for example, propagation of information and entanglement, are predicted to depend solely on the asymptotic form of the interaction and the dimension of the systems. The author thanks Yijun Wang for discussion and careful reading of the article. This work was supported by National Science Foundation (PHY-0747907) and Army Research Office-Multiple University Research Initiative (W911NF-14-1-0003).
In opening the Levico BEC 2003 meeting, the directors thought it would be useful to say a little about the first BEC workshop held in the same hotel, in the same lecture room, almost exactly ten years ago. I want to give you a personal view of what led up to the historic BEC 93 workshop at Levico and contrast the situation then with the fantastic developments which have occurred over the last decade. BEC 93 was organized by Griffin, Snoke, Stringari, Laloe and Baym. The purpose of the workshop was to bring together people working in different fields of physics but with a common interest in the phenomenon of Bose–Einstein condensation. It is useful to briefly recall the history of BEC studies before the BEC 93 workshop, to understand why it was organized. In 1938 Fritz London suggested that the mysterious phase transition which occurred in liquid 4 He at T = 2.17 K might be related to the formation of a Bose–Einstein condensate (BEC) of 4 He atoms, a phenomenon first predicted by Einstein in 1925 in an ideal gas. After Tisza suggested that the spectacular superfluid effects discovered in 1938 were related to the motion of the Bose condensate, BEC became a hot topic in the years before World War II. In 1941, Landau developed a very successful two-fluid theory of superfluid 4 He, based on the relatively new concept of elementary excitations of a many-body system to describe the 'normal fluid' component. The Landau picture was spectacularly successful in explaining various thermodynamic and transport properties, albeit in a phenomenological manner. However, Landau's magnificent theory made no use of the concept of BEC, or even the fact that the 4 He atoms obeyed Bose statistics. Interest in BEC faded from low temperature conferences in the period 1945–1955, in spite of Bogoliubov's seminal work in 1947 showing that a dilute weakly interacting Bose gas still exhibited BEC. Quite separate from the study of BEC in superfluid 4 He, interest in finding a BEC in a dilute, weakly interacting Bose gas started to develop in the early 1960s. The first system was optically excited excitons in semiconductors (electron-hole pairs, with zero total spin, played the role of the bosons). However, the short lifetime of the excitons in most semiconductors frustrated the early experiments. A second major effort developed in the 1980s with attempts to produce an atomic BEC in spin-polarized hydrogen gas. The initial experiments were very promising, but various problems ultimately stopped people from ever producing (until 1998) high enough densities needed for BEC. However, this work revived interest in the theory of a dilute, weakly interacting Bose gas, and also led to several workshops on 'spin-polarized' quantum gases in the 1980s in which BEC was a major topic. The preceding summary sets the stage for why BEC 93 was organized, namely, to bring together people (both theorists and experimentalists!) interested in BEC in quite different systems, for encouragement as well as stimulation. The topic was largely ignored in the large international low temperature conferences. The organizers were mainly from the condensed matter and low temperature physics communities. Fortunately Franck Laloe was involved, and he was aware of the attempts to produce a BEC in trapped alkali gases using the novel technique of laser-cooling. The titles of the review talks given at BEC 93 show that the popular Bose systems in 1993 were liquid 4 He, excitons, spin-polarized hydrogen and small Cooper pairs in high-temperature superconductors. Work on laser-cooled alkali gases was reviewed by Yvan Castin, but people seemed pessimistic at the time about producing a stable BEC in these very dilute systems in the near future. We also had several excellent talks about Bose condensates involving mesons, as well as Cooper pairing in nuclei. The main reason for this was that crucial financial support for BEC 93 came from the newly formed ECT* Institute in Trento, thanks to Stringari. As a result, several theorists interested in condensates in nuclear and high energy physics were incorporated into the schedule of BEC 93. To my surprise, their talks, relating to Bose condensates in fairly exotic settings, fitted in very well with the other speakers. It was decided to hold BEC 93 in Levico-Terme at the Grand Hotel Bellavista. Stringari scouted many hotels in the area around Trento and Sandro clearly chose wisely, since we returned ten years later for Levico BEC 2003. Because of the interdisciplinary quality of the BEC 93 workshop and the slightly controversial nature of BEC in real systems, however, we found it very difficult to obtain funding for BEC 93. Many of the funding agencies turned down requests for funding so the BEC 93 workshop was funded by the ECT*, a small grant from CNRS in France, a payment from Cambridge University Press (in lieu of royalties for the conference proceedings) and registration fees. We were therefore overjoyed when we received some late applications from people who said they would pay the full registration fee. One of these was Eric Cornell, a young AMO physicist from 'someplace' called JILA in Boulder, Colorado. We gladly accepted his money and his participation. The opening session of BEC 93 in the Grand Hotel Bellavista was a very historic occasion, since almost all the people in the world interested in BEC were present. The excellent review talks and energetic discussions quickly made us realize that the workshop was going to be a success. People heard about the search for BEC in systems quite different from what they were familiar with. It probably was the first time that the CMP and AMO communities were in the same room together. Eric Cornell later told me that it was the first meeting where he met a lot of well known CMP theorists, and heard them arguing about the role of interactions, broken symmetry and order parameters, Goldstone modes, phase rigidity, etc. It dawned on Eric that the physics of a Bose condensate had not been completely settled in Einstein's 1925 paper! One of the star invited speakers was Nozières but at the last moment, he could not attend because of family reasons. However, he gave a lecture a month later at the 60th birthday party for Cohen-Tannoudji in Paris, based on his planned talk for BEC 93. I convinced him that his lecture 'belonged' to BEC 93 so he wrote it up for the BEC book we were preparing. His insightful article became one of the most quoted articles in the book, and many of his phrases (fragmentation of the condensate, phase locking, enemies of Bose–Einstein condensation, etc.) have entered our language. At the conference dinner, we gave out small prizes to some of the best speakers and participants who had made a significant contribution. One of the themes of BEC 93 was to pin down what an unambiguous signature of a Bose condensate was, and the term 'smoking gun' became very popular in discussions. Shlyapnikov received the coveted 'Smoking Gun' award, the prize being a package of cigarettes. The 'Prix d'Or' (1000 Lire), for giving the most convincing evidence for a Bose condensate, was awarded to Wolfe for his group's work on exciton gases. Some years later, in 1999, further work by his group at Urbana showed that an unexpected rapid exciton decay process had not been included in the original data analysis. As a result, the exciton densities had been over-estimated and they had not crossed into the BEC region. Shortly after, I received a letter from Wolfe telling me about the new results, and also the original 1000 Lire banknote he had received as a prize at BEC 93. It made me proud having colleagues like this. Brown, who argued that kaon condensates played a crucial role in the collapse of large stars, received the very special 'Smoking Universe' award. He gave the most impressive 'thank you' speech, worthy of the Academy Awards. At the end of BEC 93, we had an open discussion about whether we should organize another BEC workshop two years later, in 1995. Many people felt that, while BEC 93 had been very stimulating, there probably would not be enough progress to have another BEC meeting 'so soon', but a committee was set up to monitor developments. Walraven took the initiative to organize BEC 95, which was held in Mt St Odile near Strasbourg in France, in June 1995. As luck would have it, a few weeks before BEC 95 convened, rumours circulated that the JILA group at Boulder had clear evidence for a Bose condensate in a trapped gas of 87 Rb atoms. When Eric arrived in Mt St Odile from a meeting in Capri and presented the JILA data 'hot off the press', we all gave him a standing ovation. The earth had moved and BEC was about to move to centre stage. Another piece of luck was that our BEC 93 book ( Bose–Einstein Condensation 1995 ed A Griffin, D W Snoke and S Stringari (Cambridge: Cambridge University Press)) finally appeared in March 1995. The discovery of BEC in atomic gases a few months later made the book a modest 'bestseller' and it was quickly re-issued as a paperback (almost unheard of for this type of conference proceedings). Several people later congratulated us on producing such an excellent book on BEC so quickly after the discovery in June 1995! The BEC book (often called the 'Green Book') did fill a vacuum in the early days, providing carefully written reviews about many aspects of BEC. Several of the articles were well ahead of their time, such as the discussion of the BCS–BEC crossover in Fermi gases by Randeria. We are only now seeing a lot of references to his review article, as the search for BCS pairing in trapped Fermi gases has developed over the last year. Probably over half of all publications in the area of BEC and ultracold quantum gases since 1995 have made one or more references to the Green Book. Beginning with BEC 95, these biennial meetings have now become a regular affair, generously financed and sponsored by the European Science Foundation (ESF). BEC 99, 2001 and 2003 have all been held in San Feliu de Guixols, in Spain. Since BEC 95, these workshops have been dominated by people working on trapped atomic gases, for obvious reasons. However, it is nice to see that work has picked up recently in the search for BEC in exciton gases (especially in quantum wells), and the study of trapped Fermi gases has stimulated new connections with people working in nuclear physics as well as highly correlated electron systems. So one can see that the exciting interdisciplinary atmosphere of BEC 93 may return in the near future. The papers given at Levico BEC 2003 already show strong evidence of this trend. In their 2001 Nobel Prize lectures, Cornell, Wieman and Ketterle all emphasized how 'surprised' they were on how rich BEC physics turned out to be. Initially, they expected that these atomic condensates would be small and somewhat delicate, certainly only of minor interest to the larger physics community. In fact, as we all know, interest in ultracold gases has exploded since 1995 like a 'Bosenova' and, like the universe, there is no sign of this expansion stopping. It is rapidly producing a whole new field of research which is an amalgamation of AMO and CMP physics. This in turn is producing a major re-organization of all major physics departments, in terms of the hiring of new faculty and resource allocations. International conferences in more traditional research areas in CMP and AMO have been trying to incorporate this growing subject, but it is probably time to start a new 'International Conference on Ultracold Quantum Gases' which is open to all. Of course, the young physicists who have organized and are the main speakers at Levico BEC 2003 illustrate this on-going revolution. I predict that Bose matter waves, atomic optics and, more generally, phenomena in ultracold quantum gases, will be the basis for all really new technology in the 21st century. I say this with confidence, since I don't see any real competition from developing technologies based on electrons (discovered about 1900) and laser beams (discovered about 1960). As with these older technologies, it will take decades to see how cold quantum gases can be used, in ways that we cannot even imagine today. A crucial reason for the exciting future of ultracold gases is based on the fact that it involves a creative interaction between two major areas of physics, CMP and AMO. Each has their own well developed paradigms for thinking about physics, and this difference surfaces in dealing with cold quantum gases. This creates a certain tension but ultimately it is the basis of a new creativity. Many CMP theorists who had previously worked in the area of quantum fluids and superconductors have entered this new field. This is shown by the recent publication of two excellent books on cold quantum gases (Pethick C and Smith H 2002 Bose–Einstein Condensation in Dilute Gases (Cambridge: Cambridge University Press); Pitaevskii L and Stringari S 2003 Bose–Einstein Condensation (Oxford: Oxford University Press)), both by CMP theorists. This is easy to understand since trapped atomic gases (Bose or Fermi) are beautiful examples of interacting, highly coherent, quantum many-body systems, precisely the subject that CMP theorists have been struggling to understand since the 1950s. The new concepts and techniques needed to understand such many-body systems were developed and fought over in the period 1955–1965, when it was realized that there was more to the quantum theory of matter than solving Schrödinger's wave equation. A major contribution was made by the Landau school in Moscow. This battle of ideas was focused on the attempts to understand both superconductivity and superfluid 4 He. While the BCS theory of superconductivity in metals which appeared in 1957 clearly had the right physics to explain the key properties of superconductors, it still took some time for most theorists to really understand the BCS theory at a deeper level, in terms of broken symmetry, order parameters and Goldstone modes. The BCS theory gave CMP physics a simple microscopic model for superfluidity in a real system, in terms of the coherent motion of Cooper pairs. Very quickly, the close analogy was made between Bogoliubov's theory of a Bose-condensed gas and the BCS theory of superconductors. Strangely enough, however, it was only in the 1980s that pioneering work by Leggett, Nozières and others re-emphasized that one should think of the BCS theory as a Bose condensate of Cooper pairs. This led to an understanding of the BCS–BEC crossover, and also that the Bogoliubov theory of a weakly interacting Bose-condensed gas was 'buried' in the full BCS theory. It is fitting that, in 2003, this BCS–BEC crossover is becoming a central topic in the field of ultracold atoms. I say this because it represents a beautiful synthesis of both AMO physics (the formation of diatomic molecules using Feshbach resonances) and CMP physics (many-body effects giving rise to Cooper pairs) in a novel setting. As Levico BEC 2003 and the articles published in this special section show, we can expect more exciting advances of this kind in the near future. Acknowledgments I would like to thank the organizers of the Levico BEC 2003 workshop for inviting me to participate as a senior lecturer and also to commemorate the tenth anniversary of the first BEC workshop held in Levico-Terme. My research is funded by NSERC of Canada.
Since the realization of Bose-Einstein condensation in 1995, experimentalists in ultracold atomic gas community have developed a set of entirely new methods to study quantum gases. In ultracold atomic gases, implementation of synthetic gauge fields, the spin-orbit coupling, optical lattices and Feshbach resonances, provides a clean and highly controllable platform to tune the parameters of the system, which are typically fixed for real materials. Motivated by the rapid developments of experimental progresses, I investigate the ultracold atomic gases with synthetic gauge fields. \n \nIn this thesis, a bosonic gas in optical lattices in the presence of an effective staggered magnetic flux will be studied, using a generalized Bose-Hubbard model. Phase transition from the Mott insulating phase to the superfluid phase will be discussed. Novel superfluid phases including the stripe phase and the plane wave phase will be discussed. Furthermore, the existence of a tricritical point in the phase diagram is predicted. \n \nTo study Fermi systems with synthetic gauge fields, a spin-1=2 Fermi gas with Rashba spin-orbit coupling is investigated by studying the structure of vortex line through the Bardeen-Cooper-Schrieffer to Bose-Einstein condensation (BCSBEC) crossover. Due to the spin-orbit coupling, the s-wave interacting Fermi gas behaves like an effective p-wave paired system. A discussion about the vortex line structure will be provided by calculating the spatial distribution of the gap function, the density, the particle current, and also the elementary excitations. It has been found that the most robust superfluid occurs around the unitary point, indicated by the smallest healing length and the largest critical current. \n \nFinally, motivated by the recent progress of p-wave resonantly Fermi gas in experiment, an extension of Nozières-Schmitt-Rink method to the p-wave case will be given. An investigation of the normal state properties will be given by calculating the equation of state and the p-wave contacts, which have been measured by the experiment. Furthermore, the superfluid transition temperature with realistic parameters appropriate for experiment is calculated and is found to be within experimental reach.
In 1995 two different research groups observed for the first time the Bose-Einstein condensation (BEC) in ultracold gases. When the confining magnetic trap was turned off the gas was left free to expand, and the velocity of the particles showed a clear peak: most of the particles were occupying the same single particle state, the one of lowest energy. The Bose-Einstein condensation had been predicted in 1925 by Einstein, written by inspiration of a work on the statistic of the photons by Bose (1924). In this work Bose described the behavior of an ensemble of photons, treating them as massless particles, with no number conservation associated. Einstein extended this approach to particles with a mass and with fixed number, creating what is now called the Bose-Einstein distribution. The particles that follow such a description are called ``bosons'', as opposed to the ``fermions'' of the Fermi-Dirac statistics. Einstein predicted that in a gas of bosons - under a critical temperature - a finite fraction of the total number of particles would have been in the ground state, and act as a single entity. This amusing theoretical discovery found its utility a few years later. In the late thirties, new techniques allowed to cool Helium-4 at few Kelvins above the absolute zero. The properties of the resulting liquid were a puzzlement to the scientific community: among others, it could flow without experiencing friction. The liquid was called a ``superfluid''. A first explanation was given by London in 1938, which linked the superfluid behavior to the presence of a BEC among the bosonic Helium particles. The fermions cannot condense by themselves. On the other hand, they can form bound pairs and act as bosons, as it happens in a metal at low temperature. Using this approach, in 1957 Bardeen, Cooper and Schrieffer created a successful model of superconductivity by describing a superconductor as a superfluid in a charged system. During the course of these years we explored the superfluid properties of Bosons and Fermions in different settings. The original contributions of the thesis are described starting from the third chapter, where we speak about the generation and stability of solitons in a periodic optical lattices, both fixed or in motion. In the fourth chapter we study the generation of giant vortices in cold fermions, by using a generalized hydrodynamical approach. In chapter 5 we study the effect of a quasiperiodic lattice and the glassy phase it produces on a gas of bosons. Finally, we study the interaction of normal matter and superfluids with gravitational waves. While this interaction is seen to be extremely small, we believe that the resulting formalism is interesting by itself.
Bose-Einstein condensation is a unique phase transition in that it is not driven by interparticle interactions, but can theoretically occur in an ideal gas, purely as a consequence of quantum statistics. This chapter addresses the question, ‘ How is this ideal Bose gas condensation modified in the presence of interactions between the particles?’ This seemingly simple question turns out to be surprisingly difficult to answer. Here we outline the theoretical background to this question and discuss some recent measurements on ultracold atomic Bose gases that have sought to provide some answers. Introduction Unlike the vast majority of phase transitions, Bose-Einstein condensation (BEC) is not driven by interparticle interactions but can theoretically occur in an ideal (noninteracting) gas, purely as a consequence of quantum statistics. However, in reality, interactions are needed for a Bose gas to remain close to thermal equilibrium. It is thus interesting to discuss if something close to ideal gas BEC can be observed in a real system and what happens in the vicinity of the BEC transition in the presence of interparticle interactions. These simple questions have not been easy to answer, either theoretically or experimentally. The theoretical foundations for studying the effect of interactions on Bose condensed systems were laid over half a century ago by Bogoliubov [1], Penrose and Onsager [2], and Belieav [3], among others. These works initially focused on zerotemperature properties and were extended to nonzero temperature in the pioneering papers of Lee, Huang, and Yang [4, 5, 6, 7]. At that time, the main experimental system was liquid He-4 in which the inter-particle interactions are strong, making connections with theory difficult. The realisation in 1995 of BEC in weakly interacting ultracold atomic gases [8, 9] thus opened up the possibility to experimentally revisit some of these long-discussed questions. This was further aided by, among other advances, the use of Feshbach resonances to tune the interaction strength in atomic gases [10, 11]. Thus, in the last twenty years the study of ultracold Bose gases has been very successful (see Chapter 3 for an insightful account of selected topics and surprising developments over this exciting period). However, the fact that, until recently, ultracold atoms were confined using harmonic potentials has hindered the study of the BEC transition itself.
The study of Bose-Einstein condensation has undergone a remarkable expansion during the last twenty years. Observations of this phenomenon have been reported in a number of diverse atomic, optical, and condensed matter systems, facilitated by remarkable experimental advances. The synergy of experimental and theoretical work in this broad research area is unique, leading to the establishment of Bose-Einstein condensation as a universal interdisciplinary area of modern physics. This chapter reviews the broad expansion of Bose-Einstein condensation physics in the past two decades. Introduction The field of Bose-Einstein condensation (BEC) has undergone an explosive expansion during the past twenty years. Newcomers to this field are now often introduced to this as a universal phenomenon, which nonetheless exhibits diverse (and sometimes strikingly different) manifestations. Despite such differences, the common underlying theme creates a unique identity across many different energy and length scales. The study of BEC as a universal phenomenon was highlighted in a focused conference in 1993, leading to the publication of the well-known “green book” [1], which surveyed the breadth of the field of condensate physics at that time. The success of the conference led to a second meeting in 1995, at which Eric Cornell and Carl Wieman announced the achievement of Bose-Einstein condensation of ultracold 87 Rb atoms in a harmonic trap. That work began an explosion of new research in the field of cold atoms, which has continued to this day, and this very success inevitably led many of those studying cold atoms to pay less attention to other types of condensates. Wolfgang Ketterle gives a historical overview of this exciting period of time in Chapter 3. Recently, various scientific meetings have worked to re-establish the physical connections across different BEC systems, and in 2013 a workshop was held with the focused goal of improving communications across disciplines. This present book is an outgrowth of that meeting. The Situation Before the Revolution Because of the great success of the cold atom BEC and other BEC systems in the past twenty years, it may be hard for young scientists to understand the climate of BEC research in the early 1990s. At that time, there was only one known example of BEC, namely liquid helium-4, and there was a small but vocal minority of scientists who questioned whether BEC was established even in that system.