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Determine the range of k for stability of a unity feedback controlWe employ ultracold atomic Fermi gases to study quantum many-body physics. Often, our work is motivated by unresolved problems from condensed matter physics, which we emulate with trapped atomic gases. Moreover, we strive to realize new quantum phases not encountered in the solid state. Quantum gases of interacting fermionic atoms in optical lattices promise to shed new light on the low-temperature phases of Hubbard-type models, such as spin-ordered phases or possible d-wave superconductivity.

Cabsida badan maxaa keenaWe emulate the physics of the Hubbard model by loading a quantum degenerate two-component Fermi gas of 40K atoms into a three-dimensional optical lattice geometry. Using high-resolution absorption imaging combined with radio-frequency spectroscopy we can resolve the in-situ distribution of singly and doubly occupied lattice sites within a single two-dimensional layer.

This allows the observation of the fermionic Mott insulator and a measurement of the equation of state for the repulsive Hubbard model in two dimensions. The physics of an interacting quantum gas in the optical lattice can often be described by the Hubbard model which plays a key role in the description of many intriguing phenomena in modern condensed matter physics.

In the Hubbard model the physics of the particles is determined by two parameters: the hopping rate between lattice sites J and the onsite interaction strength U.

The unique versatility of atoms in optical lattices makes researchers optimistic to study a whole range of phenomena linked to solid-state physics. A particular tantalizing prospect is that fermionic atoms in optical lattices may provide solutions to unanswered questions, such as high-temperature superconductivity. The challenge here is twofold. One central requirement is to reach extremely low temperatures inside the optical lattice. The second challenge is how to extract the information on the quantum many-body state from the experiment.

Daniel Pertot, Dr. Skip to navigation Search Site. Advanced Search…. Ultracold Fermi gases. Ultracold Fermi gases II. Contact Prof. Secretary Tina Naggert Raum 5. Bibliography Search for bibliography. Local probing of interacting fermions in optical lattices Quantum gases of interacting fermionic atoms in optical lattices promise to shed new light on the low-temperature phases of Hubbard-type models, such as spin-ordered phases or possible d-wave superconductivity.

Optical lattices The physics of an interacting quantum gas in the optical lattice can often be described by the Hubbard model which plays a key role in the description of many intriguing phenomena in modern condensed matter physics.

Team Dr. Document Actions Print this.One of the most dynamic directions in ultracold atomic gas research is the study of low-dimensional physics in quasi-low-dimensional geometries, where atoms are confined in strongly anisotropic traps.

**Quantum Materials Simulation: Realizing Richard Feynman's Vision**

Recently, interest has significantly intensified with the realization of synthetic spin—orbit coupling SOC. As a first step toward understanding the SOC effect in quasi-low-dimensional systems, the solution of two-body problems in different trapping geometries and different types of SOC has attracted great attention in the past few years. In this review, we discuss both the scattering-state and the bound-state solutions of two-body problems in quasi-one and quasi-two dimensions.

We show that the degrees of freedom in tightly confined dimensions, in particular with the presence of SOC, may significantly affect system properties. Specifically, in a quasi-one-dimensional atomic gas, a one-dimensional SOC can shift the positions of confinement-induced resonances whereas, in quasitwo- dimensional gases, a Rashba-type SOC tends to increase the two-body binding energy, such that more excited states in the tightly confined direction are occupied and the system is driven further away from a purely two-dimensional gas.

The effects of the excited states can be incorporated by adopting an effective low-dimensional Hamiltonian having the form of a two-channel model. With the bare parameters fixed by two-body solutions, this effective Hamiltonian leads to qualitatively different many-body properties compared to a purely low-dimensional model.

Download to read the full article text. Lin, R.

Compton, A. Perry, W. Phillips, J. Porto, and I. Spielman, Bose—Einstein condensate in a uniform light-induced vector potential, Phys. Lin, K. Spielman, Spin—orbit-coupled Bose—Einstein condensates, Nature83 Zhang, S.

Ji, Z. Chen, L. Zhang, Z. Du, B. Yan, G. Pan, B. Zhao, Y. Deng, H. Zhai, S.

## Crossing a Quantum Fluid Divide

Chen, and J. Pan, Collective dipole oscillations of a spin—orbit coupled Bose—Einstein condensate, Phys. Qu, C.Thank you for visiting nature. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser or turn off compatibility mode in Internet Explorer.

In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript. The dynamics of a single impurity in an environment is a fundamental problem in many-body physics.

In the solid state, a well known case is an impurity coupled to a bosonic bath such as lattice vibrations ; the impurity and its accompanying lattice distortion form a new entity, a polaron. This quasiparticle plays an important role in the spectral function of high-transition-temperature superconductors, as well as in colossal magnetoresistance in manganites 1. More recently, mobile impurities have moved into the focus of research, and they have been found to form new quasiparticles known as Fermi polarons 4567.

It has been proposed that such quantum phases and other elusive exotic states might become realizable in Fermi gases confined to two dimensions 10 Their stability and observability are intimately related to the theoretically debated 1213141516 properties of the Fermi polaron in a two-dimensional Fermi gas. Here we create and investigate Fermi polarons in a two-dimensional, spin-imbalanced Fermi gas, measuring their spectral function using momentum-resolved photoemission spectroscopy 1718 For attractive interactions, we find evidence for a disputed pairing transition between polarons and tightly bound dimers, which provides insight into the elementary pairing mechanism of imbalanced, strongly coupled two-dimensional Fermi gases.

Additionally, for repulsive interactions, we study novel quasiparticles—repulsive polarons—the lifetime of which determines the possibility of stabilizing repulsively interacting Fermi systems. Devreese, J.

Anderson, P. Infrared catastrophe in Fermi gases with local scattering potentials. Kondo, J. Resistance minimum in dilute magnetic alloys. Prokof'ev, N. Fermi-polaron problem: diagrammatic Monte Carlo method for divergent sign-alternating series. B 77 Schirotzek, A. Observation of Fermi polarons in a tunable Fermi liquid of ultracold atoms. Collective oscillations of an imbalanced Fermi gas: axial compression modes and polaron effective mass.

Kohstall, C. Metastability and coherence of repulsive polarons in a strongly interacting Fermi mixture. Chevy, F. Ultra-cold polarized Fermi gases. Duine, R. Itinerant ferromagnetism in an ultracold atom Fermi gas. Chubukov, A. B 48— Conduit, G. A 77 Parish, M. Polaron-molecule transitions in a two-dimensional Fermi gas.

A 83 Polarons and molecules in a two-dimensional Fermi gas. Klawunn, M. Fermi polaron in two dimensions: Importance of the two-body bound state.JavaScript is disabled for your browser. Some features of this site may not work without it. Author Revelle, Melissa C.

### Attractive and repulsive Fermi polarons in two dimensions

Date Advisor Hulet, Randall G. Degree Doctor of Philosophy. Abstract Ultracold atoms have become an essential tool in studying condensed matter phenomena. The advantage of atomic physics experiments is that they provide an easily tunable system. This experiment uses the lowest two ground state hyperfine levels of fermionic lithium.

In our experiment, we can control the ratio between these two states resulting in either a spin-balanced or a spin-imbalanced gas.

Imposing an imbalance is analogous to applying a magnetic field to a superconductor which causes the electrons in the material to align to the field thus breaking the electron pairs which cause superconductivity.

This motivates us to understand the phases created when a spin-imbalance is created and the effect of changing the atomic interactions. Using phase separation as a guide, we probe the dimensional crossover between 1D and 3D. The phase separation in 1D is inverted from that in 3D, which provides a unique characteristic to distinguish between the dimensions. By varying the tunneling between tubes and the atomic interactions in a 2D optical lattice, we control whether the system is 1D, 3D, or in between.

Using the properties of a 3D gas as a guide, we directly observe when the gas has crossed over from being dominated by 1D-like behavior to 3D. In this way, we have found a universal value for the dimensional crossover. While most superconductors do not coexist with magnetism, the FFLO phase requires large magnetic fields to support its pairing mechanism. Additionally, this phase is more likely to be found in lower dimensional systems.

However, at low dimensions, the effect of temperature fluctuations on the phase is destabilizing, but these temperature effects are reduced with higher dimensionality. Thus, the quasi-1D regime is the optimal region of parameter space to find this phase.

Fosroc lebanonThe search for direct evidence of FFLO continues in this regime. Keyword ultracold; atomic gas; Fermi gas; bundled tubes; dimensional crossover; More FFLO Less Citation Revelle, Melissa C. Metadata Show full item record. Searching scope Search: Go Search search. Search the archive. This Collection. View Usage Statistics.Understanding superfluidity and superconductivity in two dimensions 2D has long challenged and intrigued physicists.

In the early s, 2D quantum fluids provided a preliminary model for the motion of electrons in layered high-temperature superconductors. Since then, alternative theories have been put forth, but there is growing interest in understanding how confinement to two dimensions can influence the flow of quantum particles.

An exciting arena for studying 2D quantum fluids is atomic physics, where ultracold atoms have the advantage that their interactions can be fairly easily manipulated with electromagnetic fields. Writing in Physical Review LettersVasiliy Makhalov and colleagues from the Russian Academy of Sciences in Nizhniy Novgorod report measurements of the ground-state pressure of a 2D cloud of atoms as the binding between the atoms is increased.

By monitoring the gas pressure, they were able to observe how pairs of these fermionic atoms transform into tightly bound bosonic molecules [1]. The properties of this fermion-to-boson crossover will help refine theories of strongly correlated quantum fluids in 2D.

One of the main motivations for studying quantum fluids is that they behave in many ways like superconductors. Below a certain transition temperature T c the electrons in a superconductor conduct electricity without resistance, just as certain fluids can flow without viscosity below a transition temperature.

Generally speaking, superconductivity occurs when electrons in a metal feel an attractive interaction and bind into pairs, as described by the Bardeen-Cooper-Schrieffer BCS theory of superconductivity. A similar pairing occurs in superfluids composed of interacting fermionic atoms. While the superconducting transition temperature is quite low in typical metals, much stronger binding and larger values of T c are reached in special materials such as cuprates and pnictides.

The physics underlying these high-temperature superconductors has not been fully resolved, but atomic physics may provide some insight into this solid-state physics problem. Recent advances have made it possible to realize very strong and tunable binding of trapped fermionic atoms, leading to high- T c superfluids that may share important properties with high- T c superconductors.

Below the transition temperature, the binding between atoms can affect their statistical distribution, which can be monitored by measuring the pressure of the gas. By the Pauli principle, fermions cannot occupy the same quantum state, and as such they try to keep a certain distance from each other. A gas of fermionic atoms therefore exhibits a finite ground-state pressure already by statistics, even without interaction. Once a weak attraction between the atoms is introduced, however, it becomes favorable for two distinct fermions for instance, atoms in different spin states to form a bound state, a so-called Cooper pair.

Bing checkerAn increase in the attraction causes the pairs to become more tightly bound, until they eventually form diatomic molecules, which are much smaller than the average separation between molecules. A molecule of two fermions behaves as a boson, the type of particle that prefers to occupy the same quantum state, hence the molecules exhibit the much lower ground-state pressure of a Bose gas.

The smooth crossover from a fermionic BCS superfluid state to a molecular Bose-Einstein condensate BEC at low temperature has been studied extensively in recent years using ultracold atoms in three dimensions [2].

But how would such a crossover appear if the atoms are constrained to move in 2D flatland [3]?Our primary research is in the area of atom cooling and trapping.

### Two-body physics in quasi-low-dimensional atomic gases under spin–orbit coupling

The central feature of our program is the study of strongly interacting Fermi gases near a Feshbach resonance, using all-optical methods to achieve quantum degeneracy. Our experiments employ an optical trap consisting of a single focused beam from a high-power ultrastable CO 2 laser.

Atoms are attracted to the highest intensity region near the focal point, where a potential well is formed. By employing an ultrahigh vacuum, trap lifetimes of seconds are achieved. Forced evaporation in the optical trap is used to achieve quantum degeneracy. The all-optical approach is ideally suited to exploring atomic gases with magnetically tunable interactions, as used in our experiments.

Thus we can produce a Fermi gas with an interaction strength which can be tuned from zero to very strongly attractive or repulsive, creating the most strongly interacting, nonrelativistic system known. Surprisingly, this strongly interacting atomic gas shares similarities to many other system in nature, such as high-temperature superconductors, neutron stars, and the quark-gluon plasma of the Big Bang, which has been reproduced in heavy ion collisions.

Our research program strives to make precise and model-independent measurements to aid in the theoretical understanding of these systems. Our group was the first to produce a strongly interacting degenerate Fermi gas in the so-called BEC-BCS crossover region and to observe its elliptic flow.

Science, pp. This data has been featured in many places, including the poster for the first workshop on ultracold Fermi gases. We were also the first group to observe evidence for superfluid hydrodynamics, Phys. We measure radio-frequency spectra bottom for a two-component mixture of a 6 Li atomic Fermi gas, confined in quasi-two-dimensional pancake traps, which are created by a CO 2 laser standing wave top. We study the regime where the transverse Fermi energy is comparable to the energy level spacing in the tightly confining direction.

Near the Feshbach resonance, we find that the observed positions of the resonances do not correspond to transitions between confinement-induced dimers. The spectral shifts can be fit by assuming transitions between noninteracting polaron states in two dimensions. We investigate a new paradigm for nonlinear hydrodynamics in quantum matter, by colliding two strongly interacting atomic Fermi gas clouds to observe traveling shock waves.

In contrast to previous investigations into the dispersive properties of weakly interacting Bose-Einstein condensates, our Fermi gas system enables investigation of shock-waves with tunable interactions over a wide range of temperatures, to explore dissipative as well as dispersive hydrodynamics.

John Thomas. Atom Cooling and Trapping Our primary research is in the area of atom cooling and trapping. Our funding:.Quantum gases - collections of atoms cooled to the lowest known temperatures in the universe, have sparked a revolution in atomic physics. Behaviours, usually only found at the microscopic level, become prominent at the macroscopic level.

Taking advantage of the precise control and purity available in atomic systems, quantum gases are now being used to address a range of questions in many-body quantum physics. In the ultracold Fermi gas experimental group we use gases of Li-6 atoms, cooled to temperatures below nK, to investigate strongly correlated fermions in the crossover from a Bardeen-Cooper-Schrieffer BCS superfluid of Cooper pairs to a Bose-Einstein condensate BEC of molecules.

The ability to control the interparticle interactions using Feshbach resonances, as well as the confining potential and dimensionality of the system, enable us to characterise these Fermi systems with unprecedented accuracy.

Our recent work has focussed on i measuring excitations and universal properties using Bragg spectroscopy, and, ii understanding Fermi gases confined to move in two spatial dimensions.

Tyson Peppler PhD. We have mapped out the low-lying excitation spectra of strongly interacting Fermi gases using low-momentum Bragg spectroscopy.

Using tightly focussed Bragg beams we directly obtain the speed of sound and pairing gap at near-homogeneous density.

Paper: S. Hoinka et al. We experimentally measure the thermodynamic equation of state of a 2D Fermi gas with tunable interactions. Paper: K.

Albert aretz as a kidFenech et al. Here we measure how increasing the strength of interactions in a 2D Fermi gas leads to a departure from strict 2D behaviour in regimes where an ideal gas would remain kinematically 2D. Paper: P. Dyke et al. A 93R This paper reviews experiments with Bragg spectroscopy of two-component Fermi gases.

Paper: M. Lingham et al. We have developed a technique to obtain local measurements of homogeneous parameters in a harmonically trapped quantum gas and used this to make in situ observations of pair condensation in a unitary Fermi gas. We have made a high precision measurement of Tan's universal contact parameter and compared this with the latest QMC results.

Our data have now reached a level of accuracy that allows us to distinguish between several of the established theoretical predictions. A new type of Bragg spectroscopy is presented in which we measure the dynamic spin susceptibility of a strongly interacting Fermi gas. This allows independent measurement of the spin-parallel and spin-antiparallel response functions which become universal at high energy. Here we present a detailed review of the contact in strongly interacting Fermi gases using Bragg spectroscopy.

Both the interaction and temperature dependence are studied and two methods for Bragg spectrocscopy are described.

S4u make face plugin download freePaper: E. Kuhnle et al. The contact is a recently introduced parameter to characterise universal properties of strongly interacting quantum gases. In a Fermi gas, the contact depends on the strength of the interparticle interactions and the temperature relative to the Fermi temperature. In this paper we present the first measurements of the temperature dependence of the contact in a unitary Fermi gas.

A weakly interacting Fermi gas is prepared in an oblate trapping potential to study the 2D-3D dimensional crossover.

Shell structure, associated with the filling of discrete transverse states becomes apparent in the density profile of the gas. The dynamic structure factor of a strongly interacting Fermi gas was measured and compared to theoretical calculations based on the Random Phase Approximation RPA.

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