Thermodynamics provides powerful constraints on physical and chemical systems in equilibrium. However, non-equilibrium dynamics depends explicitly on microscopic properties, requiring an understanding beyond thermodynamics. Remarkably, in dilute gases, a set of universal relations is known to connect thermodynamics directly with microscopic properties. So far, these "contact" relations have been established only for interactions with s-wave symmetry, i.e., without relative angular momentum.We report measurements of two new physical quantities, the "p-wave contacts", and present evidence that they encode the universal aspects of p-wave interactions through recently proposed relations. Our experiments use an ultracold Fermi gas of 40 K, in which s-wave interactions are suppressed by polarising the sample, while p-wave interactions are enhanced by working near a scattering resonance. Using time-resolved spectroscopy, we study how correlations in the system develop after "quenching" the atoms into an interacting state. Combining quasi-steady-state measurements with new contact relations, we infer an attractive p-wave interaction energy as large as half the Fermi energy. Our results reveal new ways to understand and characterise the properties of a resonant p-wave quantum gas. 1 arXiv:1505.08151v4 [cond-mat.quant-gas] 13 Apr 2016 A fundamental question provoked by observation of natural systems is how macroscopic and collective properties depend on microscopic few-body interactions. Ultracold neutral atoms provide a model system in which to explore this question, since in certain conditions,few-body interactions can be tuned and characterised precisely. Over the last decade, a direct link has been made in these systems between thermodynamic properties and the underlying isotropic (s-wave) interactions. At the centre stage is a quantity called the "contact" [1][2][3][4][5][6], which describes how the energy of a system changes when the interaction strength is changed. Surprisingly, the contact is also the pivot of a set of universal relations, that constrain numerous microscopic properties, including the two-particle correlation function at short range. These relations apply regardless of temperature, density, or interaction strength [1-3, 5], to fermions and bosons [7][8][9], and in one-, two-, and three-dimensional systems [7,[10][11][12]. Contact relations have also been extended to Coulomb gases [13] and neutron-proton interactions [14]. Despite the breadth of this discussion, measurements of the contact have so far been restricted to systems with s-wave interactions.In general, the relative wave function of any pair of particles can be decomposed into components with angular momentum equal to an integer multiple of quanta. In a spinpolarised Fermi gas, quantum statistics forbids short-range interactions with even values of . Therefore, the first allowed scattering channel has = 1 (p-wave), which is typically weak due to the centrifugal barrier (see Fig. 1): the scattering cross section decreases with the square of...

Ultracold fermions provide a new testbed for dynamical phases of quantum matter forbidden by equilibrium thermodynamics.

We observe that the diffusive spin current in a strongly interacting degenerate Fermi gas of 40 K precesses about the local magnetization. As predicted by Leggett and Rice, precession is observed both in the Ramsey phase of a spin-echo sequence, and in the nonlinearity of the magnetization decay. At unitarity, we measure a Leggett-Rice parameter γ = 1.08(9) and a bare transverse spin diffusivity D ⊥ 0 = 2.3(4) /m for a normal-state gas initialized with full polarization and at one fifth of the Fermi temperature, where m is the atomic mass. One might expect γ = 0 at unitarity, where two-body scattering is purely dissipative. We observe γ → 0 as temperature is increased towards the Fermi temperature, consistent with calculations that show the degenerate Fermi sea restores a non-zero γ. Tuning the scattering length a, we find that a sign change in γ occurs in the range 0 < (kF a) −1 1.3, where kF is the Fermi momentum. We discuss how γ reveals the effective interaction strength of the gas, such that the sign change in γ indicates a switching of branch, between a repulsive and an attractive Fermi gas.Transport properties of unitary Fermi gases have been studied extensively in the past few years. Due to strong inter-particle interactions at unitarity, various transport coefficients like viscosity and spin diffusivity are bounded [1-3] by a conjectured quantum minimum [4][5][6], in three dimensions. On the other hand, transport in twodimensional unitary Fermi gases shows anomalous behavior, apparently violating a quantum limit [7]. This remains to be understood.In the case of spin diffusion, experiments so far [2, 3, 7] have been interpreted with a spin current proportional to the magnetization gradient, J j = −D∇ j M , where D is the diffusion constant [8], and M = M x , M y , M z is the local magnetization. Bold letters indicate vectors in Bloch space and the subscript j ∈ {1, 2, 3} denotes spatial direction. In general, J j has both a longitudinal component J j M and a transverse component J ⊥ j ⊥ M . Longitudinal spin currents are purely dissipative, and the standard diffusion equation applies [5,6,9,10]. However, as Leggett and Rice pointed out [11], the transverse spin current followswhereis the effective transverse diffusivity and γ is the Leggett-Rice (LR) parameter [12] (see Fig. 1a). Physically, the second term describes a reactive component of the spin current that precesses around the local magnetization. This precession has been observed in weakly interacting Fermi gases [7,13,14] and is a manifestation of the so-called identical spinrotation effect [15], which is intimately related to the LR effect [16]. In a unitary Fermi gas, however, neither the existence of the LR effect nor the value of γ has been measured. In this Letter, we provide the first evidence for LR effects in a unitary Fermi gas, and measure γ using a spin-echo technique. Our experiments are carried out in a trapped cloud of 40 K atoms using the two lowest-energy Zeeman states |± z of the electronic ground-state manifold [17]. Interaction...

We measure the transport properties of two-dimensional ultracold Fermi gases during transverse demagnetization in a magnetic field gradient. Using a phase-coherent spin-echo sequence, we are able to distinguish bare spin diffusion from the Leggett-Rice effect, in which demagnetization is slowed by the precession of spin current around the local magnetization. When the two-dimensional scattering length is tuned to be comparable to the inverse Fermi wave vector k −1 F , we find that the bare transverse spin diffusivity reaches a minimum of 1.7(6) /m, where m is the bare particle mass. The rate of demagnetization is also reflected in the growth rate of the s-wave contact, observed using time-resolved spectroscopy. At unitarity, the contact rises to 0.28(3)k 2 F per particle, measuring the breaking of scaling symmetry. Our observations support the conjecture that in systems with strong scattering, the local relaxation rate is bounded from above by kBT / .Conjectured quantum bounds on transport appear to be respected and nearly saturated by quark-gluon plasmas [1, 2], unitary Fermi gases [3][4][5][6][7][8][9][10][11], and bad metals [12,13]. For many modalities of transport these bounds can be recast as an upper bound on the rate of local relaxation to equilibrium 1/τ r k B T / , where k B is the Boltzmann constant and T is temperature [14,15]. Systems that saturate this "Planckian" bound do not have well defined quasiparticles promoting transport [1,[12][13][14][15]. A canonical example is the quantum critical regime, where one expects diffusivity D ∼ /m, a ratio of shear viscosity to entropy density η/s ∼ /k B , and a conductivity that is linear in T [4, 12, 13]. These limiting behaviors can be understood by combining τ r with a propagation speed v ∼ k B T /m, for example D ∼ v 2 τ r . This argument applies to ultracold three-dimensional (3D) Fermi gases, whose behavior in the strongly interacting regime is controlled by the quantum critical point at divergent scattering length, zero temperature, and zero density [4,16,17]. In such systems, one observes D 2 /m [6-8] and η/s 0.4 /k B [3], compatible with conjectured quantum bounds.However in attractive two-dimensional (2D) Fermi gases, scale invariance is broken by the finite bound-state pair size, so the strongly interacting regime is no longer controlled by a quantum critical point [16,[18][19][20][21][22][23]. Strikingly, an extreme violation of the conjectured D /m bound has been observed in an ultracold 2D Fermi gas: a spin diffusivity of 6.3(8) × 10 −3 /m near ln(k F a 2D ) = 0 [24], where k F is the Fermi momentum and a 2D is the 2D s-wave scattering length. No similarly dramatic effect of dimensionality is observed in charge conductivity [12] or bulk viscosity [25], and such a low spin diffusivity is unexplained by theory [11,19].In this work, we recreate the conditions of Ref. [24], and study the demagnetization dynamics of ultracold 2D Fermi gases using both a coherent spin-echo sequence [8] and time-resolved spectroscopy [7]. We find a modification of th...

Using Triumf's neutral atom trap, Trinat, for nuclear β decay, we have measured the β asymmetry with respect to the initial nuclear spin in ^{37}K to be A_{β}=-0.5707(13)_{syst}(13)_{stat}(5)_{pol}, a 0.3% measurement. This is the best relative accuracy of any β-asymmetry measurement in a nucleus or the neutron, and is in agreement with the standard model prediction -0.5706(7). We compare constraints on physics beyond the standard model with other β-decay measurements, and improve the value of V_{ud} measured in this mirror nucleus by a factor of 4.