Publications - Publications

Mixed-dimensional Bose polaron

Loft, N. J. S., Wu, Z., Bruun, G. — Tue, 19 Sep 2017 14:11:53 +0200

A new generation of cold atom experiments trapping atomic mixtures in species-selective optical potentials opens up the intriguing possibility to create systems in which different atoms live in different spatial dimensions. Inspired by this, we investigate a mixed-dimensional Bose polaron consisting of an impurity particle moving in a two-dimensional (2D) layer immersed in a 3D Bose-Einstein condensate (BEC), using a theory that includes the mixed-dimensional vacuum scattering between the impurity and the bosons exactly. We show that similarly to the pure 3D case, this system exhibits a well-defined polaron state for attractive boson-impurity interaction that evolves smoothly into a mixed-dimensional dimer for strong attraction, as well as a well-defined polaron state for weak repulsive interaction, which becomes overdamped for strong interaction. We furthermore find that the properties of the polaron depend only weakly on the gas parameter of the BEC as long as the Bogoliubov theory remains a valid description for the BEC. This indicates that higher-order correlations between the impurity and the bosons are suppressed by the mixed-dimensional geometry in comparison to a pure 3D system, which led us to speculate that the mixed-dimensional polaron has universal properties in the unitarity limit of the impurity-boson interaction.

Time-reversal-invariant topological superfluids in Bose-Fermi mixtures

Midtgaard, J. M., Wu, Z., Bruun, G. M. — Tue, 05 Sep 2017 14:11:53 +0200

A mixed dimensional system of fermions in two layers immersed in a Bose-Einstein condensate (BEC) is shown to be a promising setup to realize topological superfluids with time-reversal symmetry (TRS). The induced interaction between the fermions mediated by the BEC gives rise to a competition between p-wave pairing within each layer and s-wave pairing between the layers. When the layers are far apart, intralayer pairing dominates and the system forms a topological superfluid either with or without TRS. With decreasing layer separation or increasing BEC coherence length, interlayer pairing sets in. We show that this leads either to a second-order transition breaking TRS where the edge modes gradually become gapped or to a first-order transition to a topologically trivial s-wave superfluid. Our results provide a realistic road map for experimentally realizing a topological superfluid with TRS in a cold atomic system.

Finite-temperature behavior of the Bose polaron

Levinsen, J., Parish, M. M., Christensen, R. S., Arlt, J. J., Bruun, G. M. — Thu, 28 Dec 2017 14:11:53 +0100

We consider a mobile impurity immersed in a Bose gas at finite temperature. Using perturbation theory valid for weak coupling between the impurity and the bosons, we derive analytical results for the energy and damping of the impurity for low and high temperatures, as well as for temperatures close to the critical temperature $T_c$ for Bose-Einstein condensation. These results show that the properties of the impurity vary strongly with temperature. The energy exhibits an intriguing non-monotonic behavior close to $T_c$, and the damping rises sharply close to $T_c$. We finally discuss how these effects can be detected experimentally.

Goldstone mode and pair-breaking excitations in atomic Fermi superfluids

Hoinka, S., Dyke, P., Lingham, M., Kinnunen, J., Bruun, G., Vale, C. — Mon, 26 Jun 2017 14:11:53 +0200

Spontaneous symmetry breaking is a central paradigm of elementary particle physics ¹ , magnetism ² , superfluidity ³ and superconductivity ⁴ . According to Goldstone's theorem, phase transitions that break continuous symmetries lead to the existence of gapless excitations in the long-wavelength limit. These Goldstone modes can become the dominant low-energy excitation, showing that symmetry breaking has a profound impact on the physical properties of matter. Here, we present a comprehensive study of the elementary excitations in a homogeneous strongly interacting Fermi gas through the crossover from a Bardeen-Cooper-Schrieffer (BCS) superfluid to a Bose-Einstein condensate (BEC) of molecules using two-photon Bragg spectroscopy. The spectra exhibit a discrete Goldstone mode, associated with the broken-symmetry superfluid phase, as well as pair-breaking single-particle excitations. Our techniques yield a direct determination of the superfluid pairing gap and speed of sound in close agreement with strong-coupling theories.

Long-range mediated interactions in a mixed-dimensional system

Suchet, D., Wu, Z., Chevy, F., Bruun, G. — Fri, 28 Apr 2017 14:11:53 +0200

We present a mixed-dimensional atomic gas system to unambiguously detect and systematically probe mediated interactions. In our scheme, fermionic atoms are confined in two parallel planes and interact via exchange of elementary excitations in a three-dimensional background gas. This interaction gives rise to a frequency shift of the out-of-phase dipole oscillations of the two clouds, which we calculate using a strong-coupling theory taking the two-body mixed-dimensional scattering into account exactly. The shift is shown to be easily measurable for strong interactions and can be used as a probe for mediated interactions.

Topological superfluidity of lattice fermions inside a Bose-Einstein condensate

Midtgaard, J. M., Wu, Z., Bruun, G. — Thu, 22 Dec 2016 14:11:53 +0100

We calculate the phase diagram of identical fermions in a two-dimensional lattice immersed in a three-dimensional Bose-Einstein condensate (BEC). The fermions exchange density fluctuations in the BEC, which gives rise to an attractive induced interaction. The resulting zero-temperature phase diagram exhibits topological px+ipy superfluid phases as well as a phase separation region. We show how to use the flexibility of the Bose-Fermi mixture to tune the induced interaction, so that it maximizes the pairing between nearest-neighbor sites, whereas phase separation originating from long-range interactions is suppressed. Finally, we calculate the Berezinskii-Kosterlitz-Thouless critical temperature of the topological superfluid in the lattice and discuss experimental realizations.

Topological Superfluid in a Fermi-Bose Mixture with a High Critical Temperature

Wu, Z., Bruun, G. — Fri, 09 Dec 2016 14:11:53 +0100

We show that a 2D spin-polarized Fermi gas immersed in a 3D Bose-Einstein condensate constitutes a very promising system for realizing a px+ipy superfluid. The fermions attract each other via an induced interaction mediated by the bosons, and the resulting pairing is analyzed with retardation effects fully taken into account. This is further combined with Berezinskii-Kosterlitz-Thouless (BKT) theory to obtain reliable results for the superfluid critical temperature. We show that both the strength and the range of the induced interaction can be tuned experimentally, which can be used to make the critical temperature approach the maximum value allowed by general BKT theory. Moreover, this is achieved while keeping the Fermi-Bose interaction weak so that three-body losses are small. Our results show that realizing a topological superfluid with atomic Fermi-Bose mixtures is within experimental reach.

Mixed parity pairing in a dipolar gas

Bruun, G., Hainzl, C., laux, m. — Fri, 10 Jun 2016 14:11:53 +0200

We show that fermionic dipoles in a two-layer geometry form Cooper pairs with both singlet and triplet components when they are tilted with respect to the normal of the planes. The mixed parity pairing arises because the interaction between dipoles in the two different layers is not inversion symmetric. We use an efficient eigenvalue approach to calculate the zero-temperature phase diagram of the system as a function of the dipole orientation and the layer distance. The phase diagram contains purely triplet as well as mixed singlet and triplet superfluid phases. We show in detail how the pair wave function for dipoles residing in different layers smoothly changes from singlet to triplet symmetry as the orientation of the dipoles is changed. Our results indicate that dipolar quantum gases can be used to unambiguously observe mixed parity pairing.

Observation of Attractive and Repulsive Polarons in a Bose-Einstein Condensate

Jørgensen, N. B., Wacker, L., Skalmstang, K. T., et al. — Thu, 28 Jul 2016 14:11:53 +0200

The behavior of a mobile impurity particle interacting with a quantum-mechanical medium is of fundamental importance in physics. Due to the great flexibility of atomic gases, our understanding of the impurity problem has improved dramatically since it was realized experimentally in a particularly pure form using degenerate Fermi gases. However, there has not been such a realization of the impurity problem in a bosonic reservoir so far. Here, we use radio frequency spectroscopy of ultracold bosonic $^{39}$K atoms to experimentally demonstrate the existence of a well-defined quasiparticle state for an impurity interacting with a Bose-Einstein condensate (BEC). We measure the energy of the impurity both for attractive and repulsive interactions with the BEC, and find excellent agreement with theories that incorporate three-body correlations, both in the weak-coupling limits and across unitarity. Our results show that the spectral response consists of a well-defined quasiparticle peak at weak coupling and a continuum of excited many-body states. For increasing interaction strength, the spectrum is strongly broadened and becomes dominated by the many-body continuum, but no significant effects of three-body decay are observed. Our results open up intriguing prospects for studying mobile impurities in a bosonic environment, as well as strongly interacting Bose systems in general.

Few-Body Precursor of the Higgs Mode in a Fermi Gas

Bjerlin, J., Reimann, s., Bruun, G. — Wed, 13 Apr 2016 14:11:53 +0200

We demonstrate that an undamped few-body precursor of the Higgs mode can be investigated in a harmonically trapped Fermi gas. Using exact diagonalization, the lowest monopole mode frequency is shown to depend nonmonotonically on the interaction strength, having a minimum in a crossover region. The minimum deepens with increasing particle number, reflecting that the mode is the few-body analogue of a many-body Higgs mode in the superfluid phase, which has a vanishing frequency at the quantum phase transition point to the normal phase. We show that this mode mainly consists of coherent excitations of time-reversed pairs, and that it can be selectively excited by modulating the interaction strength, using, for instance, a Feshbach resonance in cold atomic gases.

Impurity in a Bose-Einstein Condensate and the Efimov Effect

Levinsen, J., Parish, M. M., Bruun, G. M. — Fri, 18 Sep 2015 14:11:53 +0200

Mean-field yrast spectrum and persistent currents in a two-component Bose gas with interaction asymmetry

Tue, 29 Sep 2015 14:11:53 +0200

Liquid crystal phases of two-dimensional dipolar gases and Berezinskii-Kosterlitz-Thouless melting

Wu, Z., Block, J. K., Bruun, G. — Mon, 11 Jan 2016 14:11:53 +0100

Liquid crystals are phases of matter intermediate between crystals and liquids. Whereas classical liquid crystals have been known for a long time and are used in electro-optical displays, much less is known about their quantum counterparts. There is growing evidence that quantum liquid crystals play a central role in many electron systems including high temperature superconductors, but a quantitative understanding is lacking due to disorder and other complications. Here, we analyse the quantum phase diagram of a two-dimensional dipolar gas, which exhibits stripe, nematic and supersolid phases. We calculate the stiffness constants determining the stability of the nematic and stripe phases, and the melting of the stripes set by the proliferation of topological defects is analysed microscopically. Our results for the critical temperatures of these phases demonstrate that a controlled study of the interplay between quantum liquid and superfluid phases is within experimental reach for the first time, using dipolar gases.

Using superlattice potentials to probe long-range magnetic correlations in optical lattices

Pedersen, K. G. L., Andersen, B., Bruun, G., Sørensen, A. — Mon, 28 Dec 2015 14:11:53 +0100

Quasiparticle Properties of a Mobile Impurity in a Bose-Einstein Condensate

Christensen, R. S., Levinsen, J. F., Bruun, G. — Thu, 01 Jan 2015 14:11:53 +0100

Strong-coupling ansatz for the one-dimensional Fermi gas in a harmonic potential

Levinsen, J., Massignan, P., Bruun, G., Parish, M. — Fri, 24 Jul 2015 14:11:53 +0200

Decoherence of Impurities in a Fermi Sea of Ultracold Atoms

Cetina, M., Jag, M., Lous, R., et al. — Tue, 22 Sep 2015 14:11:53 +0200

Induced interactions in a superfluid Bose-Fermi mixture

Kinnunen, J., Bruun, G. — Thu, 30 Apr 2015 14:11:53 +0200

We analyze a Bose-Einstein condensate (BEC) mixed with a superfluid two-component Fermi gas in the whole BCS-BEC crossover. Using a quasiparticle random-phase approximation combined with Beliaev theory to describe the Fermi superfluid and the BEC, respectively, we show that the single-particle and collective excitations of the Fermi gas give rise to an induced interaction between the bosons, which varies strongly with momentum and frequency. It diverges at the sound mode of the Fermi superfluid, resulting in a sharp avoided crossing feature and a corresponding sign change of the interaction energy shift in the excitation spectrum of the BEC. In addition, the excitation of quasiparticles in the Fermi superfluid leads to damping of the excitations in the BEC. Besides studying induced interactions themselves, we can use these prominent effects to systematically probe the strongly interacting Fermi gas.

Quasiparticle scattering rate in a strongly polarized Fermi mixture

Christensen, R. S., Bruun, G. — Fri, 03 Apr 2015 14:11:53 +0200

Coexistence of density wave and superfluid order in a dipolar Fermi gas

Wu, Z., Block, J. K., Bruun, G. M. — Fri, 05 Jun 2015 14:11:53 +0200

We analyse the coexistence of superfluid and density wave (stripe) order in a quasi-two-dimensional gas of dipolar fermions aligned by an external field. Remarkably, the anisotropic nature of the dipolar interaction allows for such a coexistence in a large region of the zero temperature phase diagram. In this region, the repulsive part of the interaction drives the stripe formation and the attractive part induces the pairing, resulting in a supersolid with $p$-wave Cooper pairs aligned along the stripes. From a momentum space perspective, the stability of the supersolid phase is due to the fact that the stripe order renders the Fermi surface only partially gapped, leaving gapless regions that are most important for $p$-wave pairing. We finally demonstrate how this supersolid phase can be detected in time-of-flight experiments.

Anisotropic relaxation dynamics in a dipolar Fermi gas driven out of equilibrium

Aikawa, K., Frisch, A., Mark, M., et al. — Tue, 23 Dec 2014 14:11:53 +0100

We report on the observation of a large anisotropy in the rethermalization dynamics of an ultracold dipolar Fermi gas driven out of equilibrium. Our system consists of an ultracold sample of strongly magnetic $^{167}$Er fermions, spin-polarized in the lowest Zeeman sublevel. In this system, elastic collisions arise purely from universal dipolar scattering. Based on cross-dimensional rethermalization experiments, we observe a strong anisotropy of the scattering, which manifests itself in a large angular dependence of the thermal relaxation dynamics. Our result is in very good agreement with recent theoretical predictions. Furthermore, we measure the rethermalization rate as a function of temperature for different angles and find that the suppression of collisions by Pauli blocking is not influenced by the dipole orientation.

Properties of the density-wave phase of a two-dimensional dipolar Fermi gas

Block, J. K., Bruun, G. — Fri, 03 Oct 2014 14:11:53 +0200

Long-lived Higgs mode in a two-dimensional confined Fermi system

Bruun, G. — Thu, 14 Aug 2014 14:11:53 +0200

Quantum hexatic order in two-dimensional dipolar and charged fluids

Bruun, G., Nelson, D. — Mon, 31 Mar 2014 14:11:53 +0200

Polarons, dressed molecules and itinerant ferromagnetism in ultracold Fermi gases

Massignan, P., Zaccanti, M., Bruun, G. — Wed, 19 Feb 2014 14:11:53 +0100

Self-bound many-body states of quasi-one-dimensional dipolar Fermi gases: Exploiting Bose-Fermi mappings for generalized contact interactions

Thu, 10 Oct 2013 14:11:53 +0200

XYZ Quantum Heisenberg Models with p-Orbital Bosons

Pinheiro, F., Bruun, G., Martikainen, J., Larson, J. — Tue, 12 Nov 2013 14:11:53 +0100

We demonstrate how the spin-1/2 XYZ quantum Heisenberg model can be realized with bosonic atoms loaded in the p band of an optical lattice in the Mott regime. The combination of Bose statistics and the symmetry of the p-orbital wave functions leads to a nonintegrable Heisenberg model with antiferromagnetic couplings. Moreover, the sign and relative strength of the couplings characterizing the model are shown to be experimentally tunable. We display the rich phase diagram in the one-dimensional case and discuss finite size effects relevant for trapped systems. Finally, experimental issues related to preparation, manipulation, detection, and imperfections are considered.

Dipolar fermions in 1D and 2D geometries

Bruun, G. — Mon, 09 Sep 2013 14:11:53 +0200

Quasiparticle Lifetime in Ultracold Fermionic Mixtures with Density and Mass Imbalance

Lan, Z., Bruun, G., Lobo, C. — Wed, 02 Oct 2013 14:11:53 +0200

We show that atomic Fermi mixtures with density and mass imbalance exhibit a rich diversity of scaling laws for the quasiparticle decay rate beyond the quadratic energy and temperature dependence of conventional Fermi liquids. For certain densities and mass ratios, the decay rate is linear, whereas in other cases, it exhibits a plateau. Remarkably, this plateau extends from the deeply degenerate to the high temperature classical regime of the light species. Many of these scaling laws are analogous to what is found in very different systems, including dirty metals, liquid metals, and high temperature plasmas. The Fermi mixtures can in this sense span a whole range of seemingly diverse and separate physical systems. Our results are derived in the weakly interacting limit, making them quantitatively reliable. The different regimes can be detected with radio-frequency spectroscopy.

Collective modes of a two-dimensional spin-1/2 Fermi gas in a harmonic trap

Baur, S., Vogt, E., Köhl, M., Bruun, G. — Tue, 09 Apr 2013 14:11:53 +0200

We derive analytical expressions for the frequency and damping of the lowest collective modes of a two-dimensional Fermi gas using kinetic theory. For strong coupling, we furthermore show that pairing correlations overcompensate the effects of Pauli blocking on the collision rate for a large range of temperatures, resulting in a rate which is larger than that of a classical gas. Our results agree well with experimental data, and they recover the observed crossover from collisionless to hydrodynamic behavior with increasing coupling for the quadruple mode. Finally, we show that a trap anisotropy within the experimental bounds results in a damping of the breathing mode which is comparable to what is observed, even for a scale-invariant system.

Itinerant Ferromagnetism in a Polarized Two-Component Fermi Gas

Massignan, P., Yu, Z., Bruun, G. — Tue, 04 Jun 2013 14:11:53 +0200

We analyze when a repulsively interacting two-component Fermi gas becomes thermodynamically unstable against phase separation. We focus on the strongly polarized limit, where the free energy of the homogeneous mixture can be calculated accurately in terms of well-defined quasiparticles, the repulsive polarons. Phase diagrams as a function of polarization, temperature, mass imbalance, and repulsive polaron energy, as well as scattering length and range parameter, are provided. We show that the lifetime of the repulsive polaron increases significantly with the interaction range and the mass of the minority atoms, raising the prospects of detecting the transition to the elusive itinerant ferromagnetic state with ultracold atoms.